Orientation-induced crystallization of isotactic polypropyle

Feature article

Orientation-induced crystallization of isotactic polypropylene Qi Liu,Xiaoli Sun*,Huihui Li,Shouke Yan*

State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing100029,China

a r t i c l e i n f o

Article history:

Received17January2013 Received in revised form

7April2013

Accepted27April2013 Available online17May2013

Keywords:

Orientation-induced Polypropylene

Crystallization a b s t r a c t

In polymer processing operations,the molten polymer chains are frequently subjected to shear or/and elongation?ow?elds,which will produce molecular chain orientation of the melt.This leads to the orientation-induced crystallization has been an important subject in the?eld of polymer physics.Sys-tematic studies indicated that the chain orientation in?uences the crystallization kinetics,the?nal morphology as well as the polymorphic behavior of the polymers.In this article,the effects of pre-orientation on the crystallization of isotactic polypropylene(iPP)concerning the above mentioned as-pects have been reviewed.In particular,the formation mechanism of orientation-induced b-iPP crystallization has been discussed according to the recent experimental results.It is suggested that the local order of the macromolecular chain segments in the melt is most important for b-nucleation of iPP. The formation of b-iPP nuclei may be restricted in a certain chain orientation window of the iPP melts. Chain orientation outside of this window results in the formation of a-iPP.

ó2013Elsevier Ltd.All rights reserved.

1.Introduction

Polymeric materials have been widely used in the?elds from packaging to aerospace.The reason is that the polymeric materials offer a great potential to meet the requirements for different ap-plications.This rests on the fact that the physical and chemical properties of polymeric materials can be effectively modulated by controlling their structures at different scales.Therefore,it is of great importance to get a fully understanding on the solidi?cation dependant microstructures,which determines the product prop-erties[1e3].In most polymer processing operations such as in-jection molding,?lm blowing,and?ber spinning,the molten polymer chains are subjected to shear or/and elongation?ow?elds. The shear?ow produces chain orientation and affects the crystal-lization kinetics as well as the crystal structure and morphology of semicrystalline polymers[4e15].This leads the study of shear-induced polymer crystallization to be an interesting subject for several decades.For this purpose,various devices for imposing shear to the polymer melts have been developed to study the crystallization of polymers under or after shear[10,16e39].

There are a vast body of literature that describe the effects of ?ow on the crystallization of various polymers as summarized previously in Refs.[39e41].Among them,isotactic polypropylene (iPP)has been most frequently chosen as model system since(i)its wide applications as conventional plastics;(ii)its lower nucleation density and crystal growth rate at relatively high temperatures, which enable the in situ rheo-X-ray[42e50]and rheo-optical [4,6,8,10,29,45,51e55]studies;and(iii)its persi?ed structure and morphology,which are sensitive to the changes in processing parameters and molecular parameters such as molecular weight, molecular weight distribution and chain branching etc.[20,46,56]. This makes it ideal to illustrate the in?uence of shear?ow on the solidi?cation behavior of polymeric material in several different aspects.

It is well accepted that external?ow causes orientation or even extension of macromolecules in the melt[5,57,58].Therefore,the ?ow-induced crystallization of polymers can be referred to as orientation-induced crystallization.Due to the chain orientation, the crystallization behavior of the sheared polymer melt differs notably from its melt crystallization under quiescent con-dition.First of all,the crystallization rate of a polymer melt with pre-ordered molecular chains can be substantially accelerated [8,9,59e64].For example,the quiescent melt of isotactic poly-styrene crystallizes very slowly.The crystallization of its sheared melt takes place immediately.Moreover,it is well demonstrated that the existence of pre-oriented polymer chains in the melt can in?uence the crystallization manner and the resultant morphology [9,10,54,65,66].Generally,oriented structure will form instead of spherulites grown from bulk melt.For polymorphic polymers, shear-?ow can also in?uence the crystal structure.As an example, sheared or strained iPP melt encourages the formation of b-iPP

*Corresponding authors.Tel./fax:t861064455928.

E-mail addresses:xiaolisun@6bb30d507fd5360cba1adb31(X.Sun),skyan@6bb30d507fd5360cba1adb31 (S.

Yan).

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Polymer

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0032-3861/$e see front matteró2013Elsevier Ltd.All rights reserved.

6bb30d507fd5360cba1adb31/10.1016/j.polymer.2013.04.066

Polymer54(2013)4404e4421

crystals rather than its thermodynamically most stable a -iPP crystals,which are always obtained under quiescent condition [10,31,67e 69].The mechanism of iPP b -crystallization in ?ow ?eld is,however,a long-standing puzzle [70e 73].Taking all these into account,in this article,the effects of chain orientation on the crystallization of iPP concerning the crystallization kinetics,crys-talline morphology and crystal structure are reviewed.Particular attention is paid to the formation mechanism of orientation-induced b -iPP crystallization.

The rest of this feature article is organized as follows.In Section 2,a quite brief introduction about the crystallization kinetics of iPP in the ?ow ?eld is given to show the great progress in this ?eld.Henceforth,the common structural and morphological features of iPP together with their formation conditions are presented in Sections 3for a better comparison with the orientation-induced structures.The in ?uence of shear ?ow on the crystalline mor-phologies of iPP is discussed in Section 4,while Section 5focuses mainly on the in ?uence of chain orientation on the polymorphic behavior of iPP and the mechanism of iPP b -crystallization.After that the article ends up with a brief summary and future challenges in this subject.

2.Crystallization kinetics of pre-oriented iPP melt

It is well established that polymer chains can be oriented or even stretched by shear ?ow and consequently crystallize with different kinetics compared to those encountered under quiescent conditions.Therefore,the crystallization kinetics of polymers un-der shear or elongation ?ow has been intensively studied [16e 18,21e 24,28].We here give a brief introduction to illustrate the great progress in the understanding of the in ?uence of chain orientation on the crystallization kinetics of iPP.For an extensive review please see Ref.[74].The early studies in this ?eld were focused on the measurements of overall global crystallization ki-netics [18,20,25,63,75,76].The experimental results show that the crystallization process is affected remarkably by the shear ?ow.An enhancement in the overall crystallization kinetics is seen when-ever crystallization takes place during shear or even after shear.Fig.1presents an example showing the corresponding Avrami analysis and the increase in the Avrami constant of iPP under isothermal conditions at 136 C as a function of the shear rate [76].The change in the Avrami constant k as a function of the imposed

shear has been described by K shear ?2:4_g

3:6.It indicates that a change of a factor of 2in the imposed shear rate (_g

)could result in a ten-fold increase in the crystallization rate.Undoubtedly,shear rate or shear stress is the most important parameters in shear-induced crystallization of polymers.The shear strain,which is equivalent to the shearing time for a constant shear rate,shows also evident effect on the crystallization kinetics.It was well established that for

a constant shear rate,a higher shear strain results in a larger in-crease in the crystallization kinetics,even though the effect of shear strain (or shear time)is limited at low shear rates.

For characterizing the crystallization kinetics of polymers,a quite frequently used parameter is the half crystallization time s 1=2,de ?ned as the time taken to achieve a 50%degree of crystallinity.The in ?uence of ?ow on the crystallization kinetics of iPP is man-ifested by a tremendous decrease in the half crystallization time as compared to that for quiescent condition.Due to very fast crystal-lization of polymers under ?ow,intensity measurement of trans-mitted light through a rheo-optical device provided an excellent tool to follow the rapid crystallization kinetics of polymers [51].In this connection,a turbidity half time (s 1=2),is introduced [8,77].It is de ?ned as the time where the normalized transmitted light in-tensity has decayed to its half initial value.Fig.2shows a repre-sentative result demonstrating the crystallization process of iPP under varying shear conditions.It can be can clearly seen that the s 1=2for quiescent crystallization was on the order of hours,while it decreases by approximately 2orders of magnitude upon shearing the melt for 4s.With the increase of shearing time,the crystalli-zation time ?rst decreases rapidly and then reaches a plateau at a shearing time of about 5s.

It was suggested that shear ?ow in ?uences both the nucleation and crystal growth steps.However,the ?ow-enhanced crystalli-zation rate is predominately attributed to a signi ?cant acceleration of the nucleation rate and an increase of nucleus density [30,78].The common physical intuition is that ?ow induces local orienta-tion of polymer chains and thus enhances the nucleation ability.In this regard,temperature is very important since it affects the relaxation of the polymer chains or chain segments.For example,a decrease of 20 C in the crystallization temperature generates a two-order of magnitude acceleration in the crystallization kinetics of iPP.Janeschitz-Kriegl and coworkers have isolated the effects

of

Fig.1.Avrami analysis:(a)Effect of shear rate at constant shear duration (t s ?30s)and induction time (t 0?0s).(b)Relationship between the kinetic rate constants,k shear /k static ,as a function of shear rate for iPP under isothermal conditions at 136 C.Reproduced with permission from Ref.[76],copyright ó2006,Elsevier Science

Ltd.

Fig.2.Turbidity of iPP crystallized following a “pulse ”of shear at Tc ?141 C and s w ?0.06MPa for various shearing times.The half crystallization time (s 1/2)reaches a plateau after a shearing time of about 5s.Reproduced with permission from Ref.[8],copyright ó1999,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 44214405

?ow from thermal effects and separated simultaneously the nucleation and crystal growth evens by counting the formed nuclei [29,36,37,79e 83].Their results have demonstrated that the in ?u-ence of ?ow on the nucleus density is overwhelming;especially at high temperatures when the number density of nucleus in quies-cent melt is extremely small.Moreover,it was found that the number of nuclei increases continuously with increasing duration of the shearing.

It should be pointed out that counting the number of nuclei could only be done at relatively low shear rate and high tempera-ture.At high shear rate,thread like structure forms,which leads to the counting of nucleus number impossible.Therefore,the char-acterization of the nucleation process of polymer melts under ?ow is most frequently based on induction time,i.e.,the time required for the onset of crystallization,which is suggested to be inversely proportional to the nucleation rate [16,18,20e 23,26,28,84].The determination of induction time is relatively easy through rheology measurement since an abrupt increase in shear viscosity takes place whenever crystallization starts [18,20,21,84].It was found that the higher the shear rate,the shorter the induction time.Fig.3presents a typical dependence of the induction time of sheared iPP melt on shear rate.It shows several features.A most important feature is that they show a zero initial slope,which means the existence of a low shear region where the melt behaves like the quiescent melt.Second,the critical shear rate increases with increasing shear temperature.Another important feature is that the induction time decreases more quickly with increasing shear rate at high temperature.This demonstrates that the effect of ?ow on the nucleation of iPP is more ef ?cient at high crystallization tempera-ture.The high ef ?ciency of shear on the induced crystallization at high crystallization temperature has been explained in the following way.With increasing temperature the quiescent crys-tallization rate tends to zero when the temperature closes to its thermodynamic melting point,but the ?ow-induced orientation effect survives at this temperature.Moreover,Janeschitz-Kriegl and coworkers [79e 83]suggested that there is a huge reservoir of poorly organized aggregates (local alignments)of molecular chains.These aggregates can become effective nuclei only at rather low temperatures in a quiescent melt.The ?ow action can,however,promote the growth of them.In this way,a large amount of poor quality dormant nuclei will be transformed into nuclei with better quality,which are active at a higher temperature under shear.

Therefore,the effect of ?ow on the crystallization kinetics is relative more pronounced as the degree of supercooling decreases.

The dependence of crystallization rate of sheared iPP melt on the molecular weight and molecular weight distribution supports the above explanations.As reported by several research groups,such as,Janeschitz-Kriegl [37],Winter [51,85],Korn ?eld [53],and Hsiao [86e 88],just to name a few,an increase in M W results in a larger increase of the ?ow-induced crystallization rate under the same shear rate.It is widely accepted that for a broad molecular weight distribution sample,only polymer chains above a “critical molecular weight ”(M*)can become suf ?ciently oriented at a given shear rate [41,46].In this case,the long molecular chains in a broad molecular weight distributed polymer will be more sheared or stretched with respect to the shorter molecular chains [37,46,51,53].As a consequence,a small amount of high molecular weight tail leads to an absolutely faster crystallization of a sheared iPP melt.

The above discussion has qualitatively explained the observed phenomena of the ?ow effect on the iPP crystallization.Actually,phenomenological models based on the fundamental work of quiescent isothermal crystallization kinetics have also been pro-posed to describe the ?ow-induced polymer crystallization [5,75,89e 96].McHugh has made the ?rst attempt for extending the theory for quiescent condition to ?ow-induced crystallization [90].In their case,the same expression for quiescent nucleation rate was chosen with the addition of a driving force term [97].Grizzuti and coworkers [75,95,96]have considered the coupling between temperature and ?ow under isothermal conditions and modi ?ed the ?ow-induced crystallization model for describing the early stage of isothermal crystallization of sheared polymer melts.Further improvement of their model has been made by the calculation of free energy change of a polymer chain during ?ow in terms of the Doi e Edwards micro-rheological theory [98].Based on the assumptions that (i)the ?ow mainly affects the nucleation process [29],and (ii)the effect of ?ow on the nucleation rate is merely additive and leads to an increase in the thermodynamic driving force for nucleation,they have modi ?ed the free energy difference between liquid and crystalline phase.It should be emphasized that the proposed theoretical model is successful not only for a qualitatively explanation of the observed experimental phenomena,but also for a quantitative prediction of the induction time of iPP under steady shear ?ow at isothermal conditions since the related parameters can be obtained from independent exper-iments [75,96].

3.Basic structure and morphology of iPP

The last paragraph has brie ?y summarized the great progress in the study of crystallization kinetics of pre-oriented iPP melt.Henceforth,the in ?uence of shear ?ow on the crystalline morphology and crystal modi ?cation of iPP will be discussed.To this end,the common structural and morphological features of iPP together with their formation conditions are presented ?rst for a better comparison with the orientation-induced structures.IPP is a semicrystalline polymer with pronounced structures and mor-phologies depending on the processing and molecular parameters [20,46,56].It is clear that the iPP chains possess always a 3/1helical chain conformation in the crystal.However,the existence of left-or right-handed helical chains made the crystalline architecture and morphology of iPP multifaceted.Depending on the tacticity,mo-lecular weight,thermal treatments and mechanical handling,the iPP can pack in three different crystalline structures,designated as a ,b and g forms [99e 107].Moreover,a so-called smectic meso-phase,which is sometimes referred to as a conformational disor-dered phase,has also been reported [108,109]

.

Fig.3.The induction time of iPP as a function of shear rate.Solid lines indicate the quiescent induction time,while the dashed lines are the linear ?ts of the high shear rate data.Reproduced with permission from Ref.[139],copyright ó2004,Elsevier Science Ltd.

Q.Liu et al./Polymer 54(2013)4404e 4421

4406

The a-iPP,which can be simply produced by melt crystallization of commercial grade iPP under static condition,exists in a mono-clinic lattice with alternate layers of left-or right-handed helices along its b-axis direction[2,100].Different types of a-iPP spheru-lites depending on growth condition have further been classi?ed by Padden and Keith on the basis of their optical birefringence [101,110].As listed in Table1,spherulites grown below134 C have a slightly positive birefringence(the type referred to as a I),whereas spherulites grown above138 C have a negative birefringence(the type referred to as a II).When the crystallization temperature is set between134and138 C,spherulites often display a mixed bire-fringence and are therefore referred to as‘mixed’type(a m).The temperature-dependent optical behavior of a-iPP spherulites is quite complex.There exist discrepancies in the temperature ranges for a-iPP spherulites grown by different researchers.This may be associated to the used materials.Moreover,the optical characters of a-iPP spherulites could change by thermal treatment.It was found that high temperature annealing can lead to a transformation from positive or mixed a-iPP spherulites into negative ones.The struc-tural origin of the peculiar birefringence feature was explained using the unique wide angle lamellar branching of a-iPP[73,102e 104].The wide angle lamellar branching creates a cross-hatched bimodal lamellar orientation with the radial lamellae initiating the overgrowth of the nearly tangential oriented lamellae.It is an intrinsic property of a-iPP and happens under many crystallization conditions,e.g.,growth from thin molten layer(see Fig.4a)[111] and in solution cast thin?lms(see Fig.4b)[112].Even though the phenomenon was?rst observed in the early1960s,an under-standing of it on a molecular level was achieved nearly20years later[73,102,104,105].Taking the molecular subgroups,i.e.,the side methyl groups,and the handedness of the helices into account,Lotz et al.[73]has explained that the branching takes place whenever two successive ac layers are made of chains with the same hand (see Fig.5),whereas the crystallographic unit cell requires opposite handed helices.

According to the“cross-hatching”model,the peculiar positive and mixed birefringence was associated to the existence of tangential oriented lamellae in the spherulites.For a better understanding,Fig.6shows a polarized optical micrograph,using a primary red?lter(l-plate),of a-iPP spherulites.The inner and outer parts of each spherulite are optically positive and negative, respectively[113].Scanning electron microscopy observations illustrate the?ne lamellar structures of both inner and outer parts of the a-iPP spherulites.As presented in Fig.7,it is obvious that the growth character of the central part of the spherulite is different from the outer part.In the outer part,most of the lamellae grow along the radial direction of the spherulites,re?ecting the con-ventional spherulite growth character of most polymer spherulites. On the other hand,the inner part of the spherulite exhibits an interwoven structure with the lamellae oriented preferentially both in the radial and tangential directions.The adjacent lamellae lay approximately orthogonal(ca.80 )to each other,forming a cross-hatched structure.This provides the direct evidence that the negative a-iPP spherulites are characterized by predominantly radial lamellae while the positive spherulites are characterized by a cross-hatched lamellar morphology.Based on the above descrip-tion,one can easily imagine that the‘mixed’spherulites are the consequence of inhomogeneity in a given spherulite with the coexistence of radial and tangential predominant domains.

The b-iPP is thermodynamically metastable.Even though it was ?rst observed in1959[109],its crystal structure has been a puzzle for a long time and was just solved in1994[71,72].The reason for the delay rests on the uncertainty of the unit cell dimensions and symmetry.Dif?culty in solving the b-iPP crystal structure stems from the fact that its?ber pattern cannot be obtained since ba conversion takes place during stretching.This leads to the sug-gestion of various hexagonal unit-cells with the a-axis parameter ranging from1.1to2.2nm[106,114e116].A sophisticated under-standing of b-iPP structure has been realized by detailed electron diffraction analysis of its single crystals.It is now clear that,in the b phase,the iPP molecular chains of three-fold isochiral helices build up trigonal unit cell with different azimuthal orientations.The helices thus have different environments and at least one of these environments is less favorable,i.e.,the formation of frustrated packing of helices[21,72].The spherulites of b-iPP exhibit very strong negative birefringence in comparison with its a-counter-parts and have been pided into two categories as also listed in Table1,i.e.,the radial b III and ring banded b IV.It was con?rmed that the banded b-spherulites contain lamellae twisted around their longitudinal axes.In the dark bands,lamellae lying?at-on are dominating,while the bright bands represent the regions where the lamellae stand on their edges[22,23,117,118].Moreover,hedritic structures of b-iPP could form at high crystallization temperature, which exhibit regular polygonal forms[24e26,119e121].The appearance of the hedrites depends remarkably on the direction of view and the maturity of the structure.Several forms have been recognized.They are mainly:(i)hexagonites in hexagonal shape with weak birefringence,(ii)axialites with strong negative

Table1

Types of iPP spherulites.

Spherulite type Banding Birefringence(D n)Crystallization

temperature

a I No0.003?0.001<134 C

a II Noà0.002?0.0005>138 C

a m No134e138 C

b III Noà0.013<128 C

b

IV

Yesà0.023(bright)

&à0.002(dark)

128e132

C

Fig.4.Cross-hatched lamellar structures of iPP formed under different conditions.(a)An AFM phase image taken during isothermal crystallization of iPP from melt at146 C.(b) A phase contrast TEM micrograph taken from a solution cast iPP thin?lm at110 C.Part a is reproduced with permission from Ref.[111],copyrightó2005Elsevier Science Ltd.

Q.Liu et al./Polymer54(2013)4404e44214407

birefringence,(iii)ovalites in oval form developed from the axialites in later stage of crystallization,and (iv)quasi-spherulites developed either from hexagonites or axialites.These hedrites are regarded as the precursors of b -spherulites since transformation of them into spherulites generally takes place in the later stage of growth [24,120].

The g phase of iPP has received much less attention.This leads to the early work being not well continued for a long time [31,32,122,123].The breakthrough in the understanding of g -iPP crystal structure has been realized in the 1990s after Meille and Bruckner reported a non-parallel chain packing in an orthorhombic unit cell [124].The non-parallelism of iPP chains in the g -phase has actually a close resemblance with the lamellar branching of a -iPP in a way that the chain axis rotation of lamellar branching in a -iPP has become a crystallographic element of symmetry in the g -iPP unit-cell.Actually,the g -iPP crystals usually co-crystallize with their a

counterparts under special conditions,such as,low M w specimen,crystallization of conventional iPP under high pressure or incor-poration of a few percent of comonomer at atmospheric pressure [125e 131].The achievement in producing well-de ?ned iPP co-polymers with high stereoregularity and large content of co-monomers has revived the studies on chemical and physical requirements for the formation of g -iPP [130e 132].Morphological analyses show that the g -iPP spherulites can be optically positive or negative depending upon the crystallization temperature [17,19,56,109].

The smectic phase or conformational disordered phase of iPP re ?ects a well-de ?ned phase with an extent of order intermediate between the crystalline and amorphous phases [2,100,107,108].It has also been less frequently named as paracrystal [35,133],microcrystal [36],condis crystal [37,134],or mesomorphic phase [38,135].In this phase,the chains have undergone a conformational ordering from random coil to 3/1helix,but the packing of helices are not as that in the crystalline phase.It is often characterized by parallel helices with some degree of positional correlation along the chain axis between adjacent helices but disorder in the

lateral

Fig.5.A schematic representation of the wide angle lamellar branching of a -iPP and the interactions between facing methyl groups of parallel ac planes made of either isochiral or antichiral helices.(a)Illustration of parallel deposition of the i-PP helices on the ac -plane of the a -phase giving rise to a continuous a -phase growth.The methyl groups of the substrate growth face (a 1c 1-plane)are shown as red balls.The yellow methyl groups of the depositing antichiral stems in the a 2c 2-plane face and are interdigitated with the red methyl groups which are the stem orientation in the new layer is parallel to in the original a -phase crystal (a 1is parallel to a 2,and c 1is parallel to c 2).(b)Molecular deposition of the i-PP helices on the ac -plane of the a -phase leading to the lamellar branching in i-PP.The yellow methyl groups are now of helices isochiral to the substrate ones.A pattern of interactions similar to (a)is generated when the stem orientation in the a 3c 3-plane is either 80 or 100 away from that in the original a -phase crystal (a 3is parallel to c 1,and c 3is parallel to a 1).Adapted from Ref.[73]

.

Fig.6.A polarized optical micrograph,using a primary red ?lter (l -plate),of a -iPP spherulites.The sample was prepared by crystallizing the iPP from melt at 138 C for 30min,then heated again to ca.174 C,at which the birefringence disappears completely,and ?nally cooled with a cooling rate of 30 C/min to 138 C for 40min isothermal recrystallization.The inner parts of the spherulites correspond to the parts ?rst crystallized at 138 C,melt at 174 C and then recrystallized at 138 C again,while the outer parts are those grown directly from the melt at 138 C after cooling from 174 C to 138 C.Reproduced with permission from Ref.[113],copyright ó2006,

Wiley-VCH.

Fig.7.An SEM micrograph shows a boundary area including the ?ne structures of both inner and outer parts of the a -iPP spherulites presented Fig.2.The right side corre-sponds to the inner part,while the left side corresponds to the outer part.Reproduced with permission from Ref.[113],copyright ó2006,Wiley-VCH.

Q.Liu et al./Polymer 54(2013)4404e 4421

4408

packing.In general,this phase can be produced by cold-drawing and quenching the melt at a drastic cooling rate,which is consid-ered as a“frozen”intermediate ordering state during crystallization pathway[37,134].

4.Orientation-induced morphology of iPP

When polymers crystallize from melt subjected to?ow,the semicrystalline morphology changes due to the existence of chain orientation[136].This is re?ected by the change from typical spherulite shape encountered under quiescent condition to the highly oriented structure.Fig.8presents a transmission electron micrograph and its corresponding electron diffraction pattern of the uniaxially oriented iPP thin?lms crystallized under elongation ?ow.It was prepared according to the melt-draw technique intro-duced by Petermann and Gohil[137].In the bright?eld image,one can see the highly oriented crystalline lamellae parallel aligned perpendicular to the?ow direction.The sharp and well de?ned electron diffraction spots demonstrate the excellent chain orien-tation of the iPP.All of the observed electron diffraction spots can be accounted for by a monoclinic unit cell with lattice constants a?0.666,b?2.078,c?0.650nm and b?99.62 [100],indicating the formation of oriented a-iPP crystals.

Another most frequently encountered morphology formed un-der?ow condition is the“shish e kebab”structure[3,41e 44,46,49,75,138e161].It is generally accepted that molecular chains of a polymer could be stretched under?ow?eld.The stretched molecular chains form extended chain“shish”crystals, which initiate the overgrowth of folded chain“kebab”crystals.The ?rst theory dealing with chain dynamics of polymers in dilute so-lution under?ow has been developed by de Gennes[162].The core of his theory was the abrupt coil e stretch transition;i.e.polymer chains undergo a sharp transition from random coil to a fully extended-chain conformation.Keller et al.[138]have provided the experimental evidence for the coil e stretch transition of polymer crystallization in dilute solutions under?ow and proposed a mo-lecular model of the“shish e kebab”structure.Taking this into ac-count,the“shish e kebab”morphology can be used as a signature of the occurrence coil e stretch transition.Similar to those observed in dilute solutions,the coil e stretch transition was also found in entangled iPP melts,which leads to the formation of“shish e kebab”structure of iPP under shear?ow.

To understand the origin of“shish e kebab”structure of iPP,Han et al.[155e158]have investigated the shear-induced crystallization of iPP at low shear rate(shear rate<1sà1)and low temperature. They have observed the shear-induced cylindrite structures.As shown in Fig.9,the core of the cylindrites formed rapidly after shear cessation.The length of the core did not change after shear cessation.However,it initiates the growth of the crystals laterally.At late stage quantity of the newly formed crystals impinged into each other.From the AFM images of the etched iPP samples,the shish-like structure was found to be much 6bb30d507fd5360cba1adb31pared with the radius of gyration of the used sample,it is suggested that the process of shear-induced shish crystals may involve a large number of entangled molecules under a low shear rate and low tempera-ture.According to these results,a schematic illustration of the shish e kebab structure under shear at low shear rate is presented in Fig.10.

Shear induced shish e kebab morphology has also been achieved and monitored by AFM,as presented in Fig.11.The shear force was introduced by controlling the value of setpoint and moving the cantilever a certain distance under the lift mode.Based on the re-sults,several points have been identi?ed.First of all,the morphology of shish e kebab depends on the shear stress.Under low shear stress,the distance of two adjacent kebabs is large (z16nm)and the thickness of the kebabs is much thinner than that of shish structure.With increasing shear stress,the kebabs pack much more densely.Second,the relaxation time at shear temperature determines the length of shish crystals.With the in-crease of relaxation time,the overall length of the shish structure shortens through fracturing into several parts.When the relaxation time is longer than480s,only very short shish structure can be observed.At the same time,the ability of the shish for inducing the kebab lamellae weakens.Third,the in situ melting experiment suggests that the unoriented lamellae melt?rst at the nominal melting temperature(about167 C),followed by the melting of kebab lamellae at a relatively higher temperature(174 C),and ?nally the shish structures melt at an even higher temperature (180 C).This indicates that the shear induces the formation of shish e kebab crystals of iPP in its a-phase.

Hsiao et al.[43e46,144e150,163]have studied the formation of shish e kebab structure of entangled iPP melt by in situ rheo-X-ray and revealed the early stage of the shear-induced crystallization. Through combined SAXS and WAXD experiments,they found that scaffold(network)of precursor structures forms at the early stage, which may correspond to the density?uctuation prior to crystal-lization.This was also found by the in situ rheo-optical investiga-tion of the?ow-or shear-induced crystallization of iPP under oscillatory shear[51,164e166].The scaffold containing shish crys-tals with extended chain conformation can exist in amorphous[45], mesomorphic[148]or crystalline[164]states while the kebabs with fold chain conformation exist only in the crystalline state.The chains in the shish crystals were entangled with the neighboring chains.The inpidual chains do not disentangle completely in the deformed melt and undergo abrupt unwinding to fully extended chain conformation.It was further found that the core of the cylindrites comes from stretched bundles of the entangled network strands rather than from the extended chains in stretched

melt. Fig.8.(a)A bright?eld transmission electron micrograph and(b)its corresponding electron diffraction pattern of the highly oriented iPP thin?lms crystallized under elongation ?ow through melt-draw at130 C.The arrow indicates the drawing direction.

Q.Liu et al./Polymer54(2013)4404e44214409

Moreover,the average length of shish-like structure decreases instead of increasing with time after shear.

5.Orientation-induced polymorphic behavior of iPP

As mentioned in the third section,iPP is one of the polymers exhibiting pronounced polymorphism depending upon the ther-mal and mechanical treatments.The a modi ?cation can be easily produced by melt crystallization of commercial grade iPP under static condition [100].The b form is dif ?cult to be obtained under normal processing conditions and therefore has been considered as

laboratory curiosities.Nevertheless,it attracts due attention owing to its different performance characteristics,including low crystal density,low melting temperature,and markedly improved impact strength and toughness [167e 188].It was found that crystallization from sheared or strained iPP melt encourages the crystallization of iPP in its b -phase [9e 11,42e 44,65e 70,170,189].Studies in this ?eld concern mostly the effect of shear rate on the b -crystallization of iPP.The mechanism of its b -crystallization remains a long-standing problem.

Steady shear experiments with rotational plates show that the b -iPP crystals can be created at very low shear rate.Fig.12presents the optical micrographs of an iPP sample prepared by shearing the iPP supercooled melt at 134 C for 5s at a shear rate of only 1s à1[156].Selective melting at 156 C demonstrates the existence of the b -iPP crystals.It was found that the amount of b -iPP crystals de-pends evidently on the shear rate.Hsiao et al.[44,190]found

that

Fig.9.Optical micrographs of the shish-kebab-like structure grown from sheared iPP melt.The thermal history of the iPP was melted at 200 C for 5min,then quenched to 140 C and sheared for 2s at shear rate of 0.5s à1.After shear cessation,the sample was isothermally crystallized at 140 C for different times.The arrow indicates the ?ow direction.Reproduced with permission from Ref.[156],copyright ó2007,Elsevier Science

Ltd.

Fig.10.Illustration of shish e kebab structure formed under shear at low shear

rate.

Fig.11.An AFM image shows the shear induced shish e kebab morphology (indicated by an blue arrow)achieved under AFM.The sample has been sheared by the AFM tip and relaxed for 120s after shear.The white arrow indicates the shear direction.

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the amount of b -phase reached a plateau value at a shear rate of 57s à1,while a 70%?nal contribution of b -iPP crystals to the total crystalline phase was reported at the shear rate of 102s à1.An et al.[191]have studied the development of both a and b iPP crystals in the iPP melt after shear as a function of shear rate.They found that the total crystallinity of iPP is not affected by the shear rate,while the portion of the b crystals depends remarkably on the shear rate.With increasing shear rate,the contribution of b -iPP crystals to the total crystalline phase increases rapidly and reaches a maximum at a shear rate of ca.20s à1.Henceforth it decreases slightly with further increase of the shear rate.This is somewhat different from the result reported by Hsiao et al.The inconformity may be caused by different materials used.On the other hand,WAXD experiments demonstrated that the intensity of the (300)b re ?ection did not show noticeable azimuthal dependence.This means that the b -crystals were primarily unoriented.From this point,the increase in the crystallization kinetics of unoriented crystals concluded from the SAXS study may be attributed to the b -crystallization of the iPP.Moreover,the oriented a -crystals form immediately after shear and then the b -crystals grow subsequently suggests that the surface of the oriented a -crystal entities may provide nucleation sites for the b -crystals.These results just provide us some speculations about the formation mechanism of b -iPP crystals.Detailed information about shear-induced b crystallization of iPP has been obtained by ?ber pulling procedure,in which the shear-induced crystallization of iPP was performed by pulling the ?ber embedded in iPP melt along the ?ber axis [33,66,192].

It was attested that the ?ber-pulling process was ef ?cient in producing samples rich in b -modi ?cation,regardless of the nucle-ation abilities of the used ?bers toward the iPP matrix.For example,the unsized glass ?ber shows no nucleation capability towards iPP in static melt but induces column structures of iPP by pulling the ?ber during crystallization of iPP matrix [30,65].Even for the ?bers which can nucleate the iPP,the supermolecular structures of iPP induced by shear created through ?ber pulling are different from those in the transcrystalline zone formed in static melt.When crystallizing from static melt,the monoclinic a -iPP was the observed morphology in the vicinity of the ?bers as well as in the areas of bulk crystallization.In the shear-induced crystallization of iPP through ?ber pulling,mainly b -iPP crystals were produced in the vicinity of the ?bers.To distinguish the column structure generated by ?ber pulling from the transcrystalline structure trig-gered by surface induced heterogeneous nucleation,Varga et al.[66]have de ?ned the shear-induced column structure as “cylin-drite ”.Through selective melting of the b -iPP crystals at tempera-tures above the melting point of b -iPP but below that of a -iPP (e.g.160 C),they found that the shear-induced supermolecular struc-ture consisted of two crystalline forms,namely,a thin a -iPP layer

directly connected to the pulled ?ber with a zigzag outline and the later grown b -iPP layer away from the ?ber.Based on this fact,it was concluded that shearing the iPP melt by ?ber pulling yielded primary a -row nuclei along the ?ber.As sketched in Fig.13,on the surface of these a -row nuclei,a a -to-b growth transition or ab -bifurcation took place during crystal growth,which led to the for-mation of randomly dispersed point like b nuclei.These b nuclei induce a layer with b -phase in rich along the pulled ?ber as long as a higher growth rate of the b -phase (G b )than a -phase (G a )was achieved [10,70,193,194].Otherwise,the quickly growing a -iPP crystals would embed the generated b -nuclei.A faster growth rate of b -iPP crystals than its a -counterparts can only be achieved in the temperature range of 100e 140 C.Therefore,the ab -growth tran-sition is also limited in this temperature window.The fact that the oriented a -crystals always form ?rst and then the b -crystals grow afterward seems to support the present explanation since the ex-istence of a row nuclei in shear-induced crystallization has been proved by optical microscopy and AFM observations [10,66,70,121].Also the b crystallization of iPP in a temperature gradient can be well explained in terms of ab -growth transformation along the oriented a growth front [171,172].There are,however,two aspects which remain dif ?cult to be understood,namely,(i)the

occurrence

Fig.12.(a)An optical micrograph of iPP crystallized under shear.The sample was prepared by melting at 200 C for 5min,then quenched to 134 C and sheared 5s at the shear rate ?1s à1,followed by isothermal crystallization at 134 C for 500s.(b)The selective melting of the b -form iPP at 156 C (indicated by the arrows),indicating the formation b -iPP under used shear condition.Reproduced with permission from Ref.[155],copyright ó2005,Elsevier Science

Ltd.

Fig.13.Schematic diagram of a cylindrite structure with ab bifurcation.Reproduced with permission from Ref.[202],copyright ó2006,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 44214411

of the ab -growth transition on the surface of the in situ formed a -row nuclei instead of the growth of a -iPP crystals and (ii)the original morphological difference on the a -row nuclei surface be-tween the places where the ab -growth transition taking place and the continuous growth of a -iPP crystals,i.e.the bottom edges of the a -iPP triangles shown in Fig.13.In other words,the origin behind the ab -growth transition,if it exists,is still not quite clear.

To disclose the origin of shear-induced b -crystallization of iPP,our group has adopted another way to follow the orientation-induced crystallization of iPP [195e 203].In our procedure,we check the orientation-induced crystallization of iPP by recrystall-izing its highly oriented ?bers after incomplete melting of them.Here the orientation of the iPP molecular chains is controlled by the melting temperature and relaxation time of the ?bers.Considering that the morphological features of the single ?ber are dif ?cult to be monitored,self-induced crystallization of iPP melt by its homoge-neous ?ber with varying melting extent was studied.The ?ber/matrix single iPP composites were produced by a procedure sketched in Fig.14.The iPP matrix thin ?lm was ?rst heated to

200 C for 10min to erase possible effects of thermal history on the subsequent crystallization of the sample and then moved to a preheated hot plate,where the iPP matrix was kept in the molten or supercooled molten state at the moment of ?ber introduction.As the iPP molten or supercooled molten thin layer reached equilib-rium at the desired temperature,homogeneous iPP ?bers tightly ?xed on a metal frame (see upper part of Fig.14)were introduced into the iPP matrix.After introduction of the ?bers,the prepared ?ber/matrix single iPP composites were subsequently moved quickly to another hot plate set at desired temperature for isothermal crystallization.In the henceforth paragraphs,we review our recent results and discuss their implications for the orientation-induced b -iPP crystallization mechanism.

We ?rst check the importance of crystallization rate for the b crystallization of iPP.During our experimental procedure,we do ?nd that by melting the iPP ?ber at temperatures slightly over its nominal melting point for short time,mainly b -iPP cylindrical structures are observed after crystallization in the temperature window of 100e 138 C (see Fig.15a,b and c),while a -iPP cylindrical structures are generated at crystallization temperatures above 140 C (see Fig.15d)[197].This is,however,not necessary to indi-cate that a higher G b is suf ?cient for the b crystallization of iPP.The interfacial morphology created by crystallization at 139 C under otherwise unchanged parameters is particularly helpful for eluci-dating the effect of crystal growth rate on the b -iPP crystallization.As presented in Fig.16,now both a -iPP and fan shaped b -iPP crystals are observed in the transcrystalline layers of 6bb30d507fd5360cba1adb31paring the growth fronts of the a -and b -iPP crystals,one can ?nd that the b -iPP crystals possess an evident higher growth rate than its a -counterpart.The formation of b -iPP transcrystalline structure is,however,suppressed.This clearly implies that a higher G b may be necessary but not suf ?cient for promoting b -iPP crys-tallization.The nucleation of b -iPP crystals is the prerequisite for b -iPP crystallization.

According to the above experimental results,we focus our attention on the nucleation of b -iPP crystals by adjusting the ?ber introduction temperature.For this purpose,the crystallization temperatures in all experiments were set in the temperature

range

Fig.14.A sketch shows the sample preparation procedure of single iPP ?ber/matrix composites.Reproduced with permission from Ref.[195],copyright ó2003,American Chemical

Society.

Fig.15.Optical micrographs of iPP single-polymer composites with a and b iPP crystals indicated.The ?ber introduction temperature was 173 C and the isothermal crystallization temperatures were (a)126 C,(b)130 C,(c)133 C and (d)141 C.Reproduced with permission from Ref.[197],copyright ó2004,Elsevier Science Ltd.

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for a higher G b .Optical microscopy observations show that if the iPP ?bers were introduced into the supercooled iPP matrix at temperatures far below the melting point of iPP ?ber (T m z 170 C),e.g.,below 160 C,the iPP ?ber is kept in the solid state without any melting.In this case,it illustrates simply the crystallization of iPP matrix induced by its homogeneous ?ber.As shown in Fig.17,the morphological difference between iPP in the vicinity of its homo-geneous ?bers and in the areas away from the ?bers unambigu-ously indicates that the iPP matrix was indeed kept in supercooled molten state at the moment of ?ber introduction.Two different morphologies of the sample were produced.One is the column structure of iPP matrix surrounding its single polymer ?bers,while the other one is the spherulitic structure of iPP matrix away from the ?bers.Melting test demonstrates that both spherulitic and column structures are composed of monoclinic a -iPP.This has a close resemblance with what obtained by Loos et al.[204]The same negative optical character of the interface layers surrounding the iPP ?bers as well as the matrix spherulites indicates a similar lamellar arrangement in both regions.Moreover,the density of the iPP nuclei generated on its own ?ber surface is too high to resolve each inpidual one on an optical level,even in the enlarged optical micrograph,Fig.17b.

Scanning electron microscopy (SEM)helps to disclose the ?ne interfacial structure.As shown in Fig.18,the smooth surface and well-de ?ned boundary of the iPP ?ber,which is located near the left side edge of the picture,indicates that the iPP ?ber is not molten.From the ?ber surface,one sees parallel-aligned a -iPP crystalline lamellae grown transversely,indicating the high nucleation ability of the iPP ?ber toward its single polymer matrix.The discontinuity of the crystalline lamellae allows the crystalli-zation to occur in other inaccessible regions.The characteristic lamellar branching of a -iPP appears between the parallel aligned lamellae [205,206].Unlike crystallization from the bulk,the daughter lamellae are thinner than the mother ones and further branching on the daughter lamellae has been suppressed.It implies that the rejected materials produce the daughter lamellae during the cooling process of the sample.The optical character of the iPP spherulites depends on the relative proportion of the two pop-ulations of radial lamellae and tangential lamellae and it relies also on the orientation of the radial lamellae relative to the path of light [113,206].The ?ne structure disclosed by SEM supports the conclusion made by optical microscopy observation.

With increasing ?ber introduction temperature,fan-shaped b -iPP domains sporadically inlaid in the a -iPP column layers have been occasionally observed.When the ?bers were put into the matrix at temperatures close to its nominal melting point,e.g.,168 C,substantive b -iPP crystals can be created.Fig.19shows the representative optical micrographs with b -iPP crystals distributed unevenly along the ?bers.Around the top part of the left ?ber (see Fig.19a),the iPP grows mainly in its crystalline b phase,whereas the ?ber on the right side of the picture is predominantly sur-rounded by the a -iPP crystals.In the middle of the picture,different supermolecular structures of iPP matrix are generated on different sides of the same iPP ?ber,i.e.the a -iPP and b -iPP on the right and left sides of the same iPP ?ber,respectively.The different crystal-lization manners may be associated with either the different local thermal condition of the sample or the different local nature of the ?ber.Anyway,this is quite helpful for exploring the different growth mechanisms of the a -iPP and b -iPP crystals.With careful comparison,one can ?nd the different responses of the iPP ?bers in different areas during thermal treatment.The ?bers in the areas where b -iPP crystals are generated seem to be molten,see upper left corner of Fig.19a.The ?ber located at the middle of the picture becomes thinner and thinner from its bottom to top ends,sug-gesting a partial melting of the iPP ?bers.This phenomenon is more clearly seen from the magni ?ed optical micrograph as illustrated in Fig.19b.At the right side of the bottom end of the iPP ?ber,the existence of well de ?ned boundary line between the iPP ?ber and the induced crystalline a -iPP (as indicated by an “a ”)indicates that the iPP ?ber remains intact during the thermal treatment.On

the

Fig.16.Optical micrographs of an iPP single-polymer composite with a and b iPP crystals indicated.The ?ber introduction temperature was 173 C and the isothermal crystal-lization temperature was 139 C.Reproduced with permission from Ref.[197],copyright ó2004,Elsevier Science

Ltd.

Fig.17.Optical micrographs of an iPP single-polymer composite,which show only a -iPP crystals.The ?ber introduction and the isothermal crystallization temperatures were 138 C.Reproduced with permission from Ref.[195],copyright ó2003,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 44214413

contrary,the top part of the iPP ?ber surrounded by b -iPP crystals can hardly be distinguished from the iPP matrix,indicating the inosculation of the iPP ?ber with its homogeneous molten matrix through melting or at least surface melting.This implies that melting or at least partial melting of the iPP ?ber is in favor of the formation of b -iPP in the ?ber/matrix interfacial layer.Therefore,as expected,interfacial layers composed of purely b -iPP crystals were obtained when the ?bers were introduced into the matrix melt at their nominal melting point,see Fig.20.

SEM observations provide further evidence for the occurrence of ?ber melting and illustrate the growth process of b -iPP crystalline lamellae [199].As shown in Fig.21b,the ?ber consists of edge-on lamellae with most of them well arranged in the direction perpendicular to the ?ber axis.A few of the iPP lamellae aligned more or less in the axial direction of the ?ber,as indicated by a small white arrow,indicates the occurrence of the unique lamellar

branching of a -iPP.All these should result from the melting and recrystallization of iPP ?ber during sample preparation.Parts c and d of Fig.21provide detailed structural information about b -iPP crystal growth at initial stage.It can be seen that the growth of b -iPP crystals starts from several single lamella embedded in the rich a -iPP lamellar region,as indicated by the white arrows.At the early stage,the b -iPP lamellae are loosely packed with some a -iPP in-clusion,as indicated by the ellipses.These a -and b -lamellae propagate for few micrometers without interference.Subsequently,the b -iPP lamellae start to branch and splay out leading to the formation of fan-shaped structures,which stop the growth of a -iPP crystals,and ?nally pure b -iPP crystalline lamellae are observed with non-periodic cracks as indicated by white arrows (Fig.21e).Further propagation of the b -iPP crystals results in the occurrence of the typical lamellae twisting,see Fig.21f.

The single ?ber induced crystallization of iPP indicates that melt recrystallization of the iPP ?ber is in favor of initiating b crystalli-zation of the matrix.It is,however,uncertain whether this is caused by orientation-induced recrystallization or shear-induced crystal-lization caused by ?ber introduction.The result of b recrystalliza-tion of the neat molten a -iPP ?bers helps to exclude the possibility of shear-induced crystallization.Also the crystallization of iPP matrix in the heterogeneous PET/iPP ?ber/matrix system prepared in exactly the same way as the single polymer composite con ?rms that the b -iPP crystallization in the single polymer composite originates from the oriented iPP chain segments in the molten ?ber [201].As presented in Fig.22,by introducing the PET ?ber into the iPP matrix at 170 C and then crystallized isothermally at 130 C for 90min (Fig.22a),well-developed column structures of iPP in a -modi ?cation surrounding the PET ?ber are observed.On the con-trary,b -iPP column structures become the solely observed inter-facial morphology along the whole PET ?ber when introducing PET ?ber into the supercooled iPP matrix at temperatures below 150 C,see Fig.22b.This is totally different from the case of single iPP ?ber/matrix system,and therefore re ?ects a different crystallization mechanisms of iPP around the PET ?ber compared with iPP ?ber induced crystallization.It is con ?rmed that PET ?ber exhibits only

a

Fig.18.A SEM micrograph shows the a -iPP edge-on lamellae grown from the surface of the a -iPP ?ber.The introduction temperature of iPP ?ber,located at the left side edge of the picture,was 158 C.The sample was isothermally crystallized at 135 C for 1h after the ?ber introduction.Reproduced with permission from Ref.[199],copy-right ó2004,American Chemical

Society.

Fig.19.Optical micrographs of an iPP single-polymer composite with a and b iPP crystals indicated.The ?ber introduction temperature was 168 C and the isothermal crystal-lization temperature was 138 C.Reproduced with permission from Ref.[195],copyright ó2003,American Chemical

Society.

Fig.20.Optical micrographs show the morphologies of an iPP ?ber/matrix single polymer composite.The ?ber introduction temperature was 173 C.The crystallization tem-perature was 138 C.Reproduced with permission from Ref.[195],copyright ó2003,American Chemical Society.

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weak a nucleating capacity toward iPP in the quiescent condition [66,70,201,207].In this case,the in ?uence of sample preparation procedure on the crystallization of iPP overwhelms the heteroge-neous nucleation of iPP on the surface of PET ?ber.The in ?uence of sample preparation procedure on the crystallization of iPP can be easily removed by keeping the sample at the ?ber introduction temperature for a certain period of time.To ?nd out the validity of the above hypothesis,another experiment was performed with the PET ?ber being introduced into the supercooled iPP melt at 145 C,then kept at this temperature for different time and ?nally moved to another hot plate at 130 C for isothermal crystallization.As shown in Fig.23a,a 5s placement of the sample at 145 C before crystallization shows no evident effect on the interfacial morphology compared with that formed by direct cooling.A 30s placement of the sample at 145 C,as shown in Fig.23b,leads to the formation of well developed b -iPP column layers with some triangular a -iPP crystals as indicated by white arrows in the picture.When the sample was kept at 145 C for 10min,see Fig.23c,the cylindrites are composed of purely a -iPP crystals.This reveals that the initial state of the melt in ?uences the crystallization of b -iPP in the PET/PP system.

The iPP single polymer system follows another scenario.The existence of perfect lattice matching between the ?ber and matrix encourages the occurrence of polymer homoepitaxy.The homo-epitaxy is found to be quite ef ?cient in improving the crystalliza-tion kinetics of a polymer [208].Moreover,the better surface

wettability of the iPP matrix on its own ?ber will also enhance the adsorbability of iPP chains on the surface of the solid ?ber and accelerate the secondary nucleation of the iPP chain on its ho-mogenous ?ber.All those have enabled a very high nucleation ability of iPP ?ber toward its homogeneous matrix.Consequently,the homoepitaxial crystallization of iPP on the solid surface of its highly oriented single polymer ?ber caused by heterogeneous nucleation (or secondary nucleation)overwhelms the effect of ?ber introduction.As a result,transcrystallization of iPP in its a -form driven by the homoepitaxial crystallization is generated at low ?ber introduction temperature since lattice matching possesses a very strong controlling power on the crystal structure of the epitaxial polymer [209e 214].On the other hand,if the iPP ?ber was to some extent molten during ?ber introduction at high temperature,the molten iPP ?ber would not relax much due to the high viscosity and short ?ber introduction time (about tens of second).The surviving extended iPP chains or chain segments will serve as self-nuclei and quickly induce the oriented recrystallization of the matrix during the subsequent cooling process [43,44,46,56,215e 217].

From the above discussion,the chain orientation status in molten iPP ?ber is expected to play an important role in producing b -iPP crystallization.The results obtained from iPP ?ber/matrix single polymer composites with iPP ?bers of varying molecular weight show indeed different induced crystallization behavior owing to the different relaxation of molecular chains [196].Fig.24shows three representative optical micrographs of iPP

single

Fig.21.SEM micrographs of an iPP ?ber/matrix single polymer composite prepared by introducing the iPP ?ber into its supercooled homogeneity matrix at 168 C and isothermally crystallized at 135 C for 1h.(a)An overall view at low magni ?cation.(b)Surface lamellar structure of the iPP ?ber with its axis being indicated by a big white arrow;a small white arrow labels the crosshatched lamellar structure.(c)Interfacial structure with the b -iPP lamellae indicated by white arrows.(d)Growth of b -iPP at early stage.(e)Continuous growth of edge-on b -iPP lamellae with the nonperiodic cracks being indicated by white arrows.(f)Twisting of the b -iPP lamellae.Reproduced with permission from Ref.[199],copyright ó2004,American Chemical

Society.

Fig.22.Optical micrographs of iPP/PET composites,which were prepared by introducing the PET into the supercooled iPP melt at (a)170 C and (b)150 C,respectively.The isothermal crystallization of the samples were performed at 130 C for 90min.Reproduced with permission from Ref.[201],copyright ó2007,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 44214415

polymer composites with same matrix material and crystallization temperature but different iPP ?bers in molecular weight.For high molecular weight ?ber system,see Fig.24a,pure b -iPP crystalline interfacial layers can be produced in a wide temperature range near its nominal melting point.On the contrary,in the sample with low molecular weight ?ber,see Fig.24c,only a -iPP crystalline interfa-cial layers have been observed at all chosen temperatures.Inter-facial morphologies with coexistence of both a -and b -iPP crystals were always observed for the systems with the iPP ?bers having moderate molecular weights,see Fig.24b.These discrepant inter-facial morphologies should originate from the different relaxation behavior of the used ?bers,which depends strongly on the mo-lecular weight [216,217].The following equivalent experiments have con ?rmed the validity of the above conclusion.Fig.25shows the optical micrographs of the iPP matrix/?ber (same as that used in Fig.24a)samples prepared by introducing the ?ber into the matrix at 175 C.After holding at 175 C for different times,the samples were subsequently cooled to 135 C for isothermal crys-tallization.It can be seen that a 5min maintenance of the sample at 175 C after ?ber introduction (Fig.25a)induces still the crystalli-zation of iPP in its b -form.Holding the sample at 175 C for 10min,not only the total nucleation ability but also the ability in gener-ating b -iPP crystals have decreased obviously,see Fig.25b.For a 15min placement of the sample at 175 C,see Fig.25c,the nuclei formed along the molten ?ber becomes sparse.Inpidual nucleus along the iPP ?ber is now recognizable.Moreover,the appearance of fully a -iPP crystals indicates undoubtedly that the more relaxed molten iPP ?ber has lost its ability in triggering its b -crystallization.These experimental results indicate that the relaxation extent of the originally highly oriented iPP chains in the molten iPP ?ber is an important parameter for controlling the formation of b -iPP trans-crystalline layers.This leads to the conclusion that the formation of b -crystals is associated with the oriented or stretched macromo-lecular chains survived during incomplete melting of the highly oriented iPP ?ber.From this point of view,the melting and recrystallization process of the iPP ?ber should be concerned to get a better understanding of the orientation-induced b -iPP crystallization.

The effect of molecular mass of iPP matrix on the crystallization of b -iPP was also studied by introducing iPP ?bers with M w ?1.94?105g/mol and draw ratio ?4into the matrixes with different molecular weight [200].As shown in Fig.26a,it is found that b -cylindrites formed around the ?ber in the matrix with M w ? 1.94?105g/mol.On the other hand,the interfacial morphology for the matrix with M w ?4.46?105g/mol consists of mainly a -crystals (Fig.26b).There are a few of fan-shaped b -iPP regions (indicated by the arrows).It has to be noted that both iPP matrixes can crystallize in b -form at the existence of b

-nucleating

Fig.23.Optical micrographs of iPP/PET composites.The PET ?ber introduction temperature was 145 C.After ?ber introduction,the samples were kept at 145 C for (a)5s,(b)30s,and (c)10min and then cooled to 130 C for isothermal crystallization.Reproduced with permission from Ref.[201],copyright ó2007,American Chemical

Society.

Fig.24.Optical micrographs of iPP ?ber/matrix single polymer composites.The iPP ?bers with molecular weight (a)1.94?105,(b)1.85?105and (c)1.15?105were introduced into the same iPP melts at (a)178 C,(b)168 C and (c)165 C.The samples were isothermally crystallized at 135 C.Reproduced with permission from Ref.[196],copyright ó2006,American Chemical

Society.

Fig.25.Optical micrographs of iPP ?ber/matrix single polymer composites.The iPP ?bers with molecular weight of 1.94?105were introduced into iPP melts at 175 C,then kept at that temperature for (a)5,(b)10,and (c)15min and ?nally cooled to 135 C for isothermal crystallization.Reproduced with permission from Ref.[196],copyright ó2006,American Chemical Society.

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agent.This indicates that the iPP chains in the matrix participate in the nucleation process of b -iPP.A phenomenological hypothesis for the nucleation of the b -phase is that the necessary condition for the formation of b -nuclei requires the participation of iPP chains in the matrix with the oriented iPP chains at the partially relaxed ?ber surface.In this case,some extent of iPP chain relaxation at the ?ber surface is necessary for the penetration of the matrix chains.The chain orientation of iPP in the molten ?ber should also maintain above a critical level for creating b 6bb30d507fd5360cba1adb31cking such relaxation and interdiffusion,the solid ?ber surface will act as a substrate for homoepitaxy of the a -phase.Considering that the diffusivity is inversely proportional to the square of the molecular mass [218,219],the diffusivity of matrix with M w ?1.94?105is esti-mated ca.5times greater than that of the matrix with M w ?4.46?105.Therefore,the required chain interpenetration and relaxation for b -nucleation could be easily realized in the lower molecular weight matrix.

It is well known that the melting behavior of macromolecules is different from that of the low molecular weight compounds.Due to the long chain character,the macromolecular chains should expe-rience a “recoiling ”or “relaxation ”process during melting.The recoiling process may take quite long time depending on melting temperature and the molecular weight.Extensive studies on the relaxation and crystallization of sheared polymer melt indicated that metastable oriented structures in sheared polymer melt might resist relaxation upon cessation of shearing and could act as “pre-nuclei ”from which fold chain lamellae grow with the chain axis highly oriented along the shear direction [43,44,46,215].These “prenuclei ”can remain active only for a certain period of time after cessation of shear ?ow at the temperatures above its melting point.Azzurri and Alfonso reported that the lifetime of the nucleation precursors is very sensitive to both relaxation temperature and molecular weight [216,217].The melting and recrystallization processes of highly oriented iPP ?ber are actually similar to the sheared iPP melt.Considering that the ?ber introduction was per-formed in a quite short time and at temperatures close to or slightly above its nominal melting point,a complete recoiling of the molten iPP ?ber could hardly be attained before recooling of the samples.Varga et al.[56]found that some local order of the molecular chains previously included in a crystal lattice could be preserved as “prenuclei ”or sometime referred to “nucleation precursors ”through studying the effect of melting history on the recrystalli-zation of iPP by optical microscopy.Taking this into account,one may argue that the partially or incompletely molten iPP ?bers exist actually in the form of amorphous domains with oriented or stretched macromolecular chain segments as in the sheared melt.These oriented molecular segments or chains,in turn,serve as nucleation precursors and initiate the b -iPP crystallization during cooling process just like the case of shear-induced crystallization.Selective melting of the b -crystals displays,however,a scenario different from the shear-induced iPP crystallization by ?ber pulling.

Fig.27shows the optical micrographs of an iPP single polymer composite before and after melting of the b -iPP crystals.In the interface layer,some randomly dispersed leaf-shaped a -iPP in-clusions are clearly 6bb30d507fd5360cba1adb31paring parts a and b of Fig.27,one may notice that a transition of a -iPP spherulites from mixed type (a m )into negative radial type (a II )has been realized by annealing at 158 C for a short time [68].From Fig.27b,the similar diameter and relatively smooth surface of the recrystallized iPP ?ber compared with the used original one implies that the growth of the b -iPP transcrystals starts directly from the ?ber surface.This is different from what observed in heterogeneous ?ber/matrix composites produced by ?ber pulling,where a -iPP layers with zigzag edges on both sides of the heterogeneity ?ber were observed after melting of the b -iPP cylindrites.Moreover,it was occasionally observed that the iPP ?bers could be broken off by selective melting

at

Fig.26.Optical micrographs of the iPP ?ber/matrix composites prepared by introducing the same iPP ?ber into molten iPP matrix with molecular weight (a)M w ?1.94?105and (b)M w ?4.46?105at 180 C and subsequently isothermally crystallized at 138 C for 2h.Reproduced with permission from Ref.[200],copyright ó2006,American Chemical

Society.

Fig.27.Polarized optical micrographs of an iPP ?ber/matrix composite crystallized isothermally at 116 C for 30min.The temperature of ?ber introduction was 173 C.(a)As-prepared sample and (b)After melting of the b -iPP crystals at 158 C.Reproduced with permission from Ref.[196],copyright ó2006,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 44214417

temperatures above the melting point of b -iPP but below that of a -iPP,see the circled part in Fig.27b.Since the used iPP ?bers are originally in their a -form,the breakage of the iPP ?ber unambig-uously implies that some local parts of the original a -iPP ?ber have transformed into b -form through melting and recrystallization.From this point,one may conclude that the melting status,or in other words,the degree of local chain orientation of the molten iPP ?ber plays a very important role in generating transcrystalline b -iPP crystals.This has been further con ?rmed by ?ber pulling ex-periments with controlled pulling rate and time.

It was found that the interfacial morphologies of iPP on Kevlar ?bers,which exhibit no nucleation ability to iPP in quiescent melt,could vary from pure a -cylindrites to pure b -cylindrites in the pulled system depending upon ?ber pulling rate and duration [202].At a very low ?ber pulling rate of 17m m/s,the enhanced nucleation ability of the ?ber to the iPP matrix leads to the for-mation of a -iPP cylindrites and indicates the existence of shear exerted on the iPP melt.As in the steady shear-experiments,an increase of b -iPP content was seen at high ?ber-pulling rate with increasing pulling time.These results indicate that a certain extent of chain orientation is required to initiate b -crystallization of iPP,or the existence of a lower threshold of chain orientation for initiating b -crystallization of iPP.Therefore,at lower ?ber pulling rate,since chain orientation and relaxation take place simultaneously during ?ber pulling,the iPP chains in the molten state can hardly be ori-ented well enough for triggering b -crystallization of iPP.At higher ?ber pulling rate,chain orientation is faster than relaxation and high degree of chain orientation for initiating b -iPP crystallization can be achieved in a short time.From this viewpoint,it is expected to obtain pure b -iPP crystals at appropriate shear conditions in steady state shear experiments.This is,however,never realized.On the contrary,a reduction in the b -iPP crystal content was seen with

further increase of the shear rate [191].This result implies the ex-istence of an upper threshold of chain orientation for b -iPP crys-tallization,which is con ?rmed by our selective melting experiments.Fig.28a and b show the optical micrographs of sam-ples prepared by pulling the ?bers at 131 C along the ?ber axis at different pulling rate for different duration,and then isothermally crystallized for suf ?cient time.It can be seen that b -iPP cylindrites are produced in both cases.After selective melting of the b -iPP crystals at 158 C,the morphologies of the remaining a -iPP crystals are quite interesting.As shown in parts c and d of Fig.28,while smooth ?ber surface is seen at lower ?ber pulling rate,residual a -iPP crystal layers surrounding the ?ber have been observed at higher ?ber pulling rate.This indicates that a better chain orien-tation of the iPP directly in contact with the ?ber achieved at higher ?ber pulling rate leads to the formation of thin a -iPP transcrystal-line layers around the ?ber.From the above discussion,it is concluded that there exists a chain orientation window,in which nucleation of b -iPP crystals is preferred.As a result,poor chain orientation outside of this window results in only a -iPP cylindrites,while highly oriented molecular chains beyond this window initiate ?rst the oriented a -iPP crystals and then the b -iPP crystals.This may be the reason for the formation of oriented a -iPP crystals in melt-drawn thin ?lm (Fig.8)and the a -iPP shish-kebab structure shown in Fig.11.

Combing the results presented in this section,it was found that a higher crystal growth rate of b -iPP compared with its a -coun-terpart is a necessary condition for b -iPP crystallization.Otherwise the fast growing a -iPP crystals will embed the generated b -nuclei.The local order and environment of the macromolecular chains just before nucleation are important for b -nucleation of iPP.It seems that the formation of b -iPP nuclei is restricted in a certain chain orientation window of the iPP melt.At presented stage,the

exact

Fig.28.Optical micrographs present the interface morphologies of the iPP/Kevlar matrix/?ber systems.The samples were heat-treated at 210 C for 5min and then quickly cooled to and kept at 131 C for ?ber pulling and isothermal crystallization.The ?bers were pulled (a)at a rate of 40m/s for 10s and (b)at a rate of 515m/s for 1s.Parts c and d highlight the morphologies of the residual a -crystals after selective melting of the b -iPP crystals at 158 C of the samples shown in a and b,respectively.Reproduced with permission from Ref.[202],copyright ó2006,American Chemical Society.

Q.Liu et al./Polymer 54(2013)4404e 4421

4418

origin of the nucleation process of b-iPP is not quite clear.Consid-ering that isochiral chains are involved in the b-iPP crystals,one may suggest that microdomains with isochiral chains are probably produced through chains rotation in the viscous melt during relaxation.These isochiral chain aggregates transform into b-iPP nuclei during cooling process and initiate the growth of b-iPP crystals.Further experimental and simulation work are needed to clarify this.

6.Concluding remarks

During polymer processing,the polymer melts are frequently subjected to shear or/and elongation?ow?elds,which produce molecular chain orientation in the melt.The oriented molecular chains crystallized in a different way as that encountered under quiescent conditions.Therefore,the orientation-induced crys-tallization has long been an object of intense interests.This leads to a vast body of researches about the effects of preorientation on the crystallization of various polymers.Among them,iPP is most frequently studied since its persi?ed structure and morphology are very sensitive not only to the changes in processing conditions but also to its molecular parameters.For example, depending on different molecular structure and thermal and mechanical treatments,three different crystalline structures,i.e. the monoclinic a,the hexagonal b and the orthorhombic g,can be produced.It was well demonstrated that a sheared or strained melt encouraged the b-crystallization of iPP.Taking this into account,the study on orientation-induced crystallization is also expected to be helpful for a better understanding of the b-iPP crystallization.

Systematical studies on the crystallization kinetics of iPP under ?ow?eld,which produces oriented or even stretched molecular chains,show that the crystallization process is affected remark-ably by the preorientation of the molecular chains.This is man-ifested by an evident enhancement in the overall crystallization rates.As an example,it is found that a change of a factor of2in the imposed shear rate(_g)could result in a ten-fold increase in the crystallization rate of iPP.First of all,?ow affects the crystal growth speed.This rests on the fact that the organization of pre-ordered polymer chain segments into the crystal lattice rather easily since they are to some extent closer to their state in the crystal phase,which exhibit a less kinetic barrier to overcome compared with the random coiled chains.The contribution of crystal growth to enhanced overall global kinetics is,however, rather limited as compared with the signi?cant acceleration of nucleation.It was widely accepted that the effect of?ow on polymer crystallization is mainly in the change of the nucleation process and the nucleation density.This is well supported by the experimental results obtained from the earliest stage of crystal-lization process under?ow.It is also well explained theoretically under the help of phenomenological models.Actually,the pro-posed theoretic model can be used not only to explain the observed experimental phenomena qualitatively but also to pre-dict the in?uence of?ow on crystallization kinetics precisely.

Studies on the?nal morphology of iPP indicate that shear?ow encourages the formation of highly oriented structures.At appropriate conditions,shish-kebab structures have been observed even at a low shear rate.It is found that the iPP mo-lecular chains undergo a sharp transition from random coil to an extended-chain conformation under?ow,i.e.,coil e stretch tran-sition.The stretched molecular chains form extended chain “shish”crystals,which initiate the overgrowth of folded chain “kebab”crystals.Many experimental results suggested that the shear-induced iPP shish crystals may involve a large number of entangled molecules.It is somewhat different from the traditional scenario about the shish-kebab structures.Therefore,new models about the shear-induced shish-kebab structure of iPP have been proposed based on the new experimental results.A fully under-standing about this aspect should be one of the most important challenges in this?eld.

Studies on the crystal structure of iPP obtained from its pre-oriented melt indicate that both a and b iPP crystals can be pro-duced depending on the orientation status and crystallization conditions.It is found that a faster b-iPP crystal growth rate is necessary but not suf?cient for promoting b-iPP crystallization,or in other words,the formation of b-iPP crystalline nuclei is the prerequisite for b-iPP crystallization.Based on the recent experi-mental results,it is suggested that the formation of the b-iPP nuclei may be restricted in a certain chain orientation window of the iPP melts.Poorly oriented iPP chains in the melt crystallize in the a-iPP form cylindrites,while highly oriented molecular iPP chains beyond this window initiate?rst the oriented a-iPP crystals and then b-iPP crystals.At presented stage,the exact origin of the b-iPP nucleation process is not quite clear.A better understanding of the orientation-induced b-iPP crystallization should be another chal-lenge of this?eld.

Acknowledgments

A portion of the used materials is based on the researches supported by the National Natural Science Foundation of China under Grant No.50833006,21274009,51221002and50973008. References

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.

Qi Liu completed her B.S from Qufu Normal University in

2008.She then started an academic project involving

postgraduate and doctoral study at the college of Materials

Science and Engineering of Beijing University of Chemical

Technology under the guidance of Prof.Shouke Yan in the

State Key Laboratory of Chemical Resource Engineering.

For her postgraduate study,she focused on the orientation

induced crystallization of isotactic polypropylene and

preparation of microporous membrane from oriented iPP

?lm by

stretching.

Xiaoli Sun is currently Associate Professor in the College of

Material Sciences and Engineering at Beijing University of

Chemical Technology(BUCT).She obtained her Ph.D.in

Polymer Physics and chemistry at the Institute of Chem-

istry,the Chinese Academy of Sciences(ICCAS),in2009

under the guidance of Prof.Shouke Yan.Thereafter she

spent two years as a Postdoctoral fellow at the Kwansei

Gakuin University and focused her study in the?eld of X-

ray scattering from soft-matter thin?lms.In2011,she

became Associate Professor in BUCT.Her current research

focuses on the surface and interface effect on the polymer

structures and morphologies,crystallization and phase

transition behavior of polymer in con?ned

spaces.

Huihui Li is currently an Associate Professor in the College

of Material Sciences and Engineering at Beijing University

of Chemical Technology(BUCT).She earned her M.S.from

Shandong University in1998and her Ph.D.in Polymer

Science at the Institute of Chemistry,the Chinese Academy

of Sciences(ICCAS),in2004under the guidance of Prof.S.

Yan.After completing her graduate studies,she became an

Assistant Professor at the ICCAS.She joined BUCT in2009.

Her current research focuses on the crystallization and

multiscale structure manipulation of crystalline/amor-

phous,crystalline/crystalline polymer

blends.

Shouke Yan is Professor in the College of Material Sciences

and Engineering at Beijing University of Chemical Tech-

nology(BUCT).After receiving a B.S.from Qufu Normal

University in1985,he completed his M.S.in Polymer Sci-

ence at the Changchun Institute of Applied Chemistry,The

Chinese Academy of Sciences(CIAC-CAS).After completing

his M.S.study,he joined the CIAC-CAS as an Assistant Pro-

fessor and then earned his Ph.D.in Polymer Science at the

CIAC-CAS under the joint guidance of Prof.Decai Yang and

Prof.J.Petermann(Dortmund University,Germany)

through a sandwich program between the CAS and the

Max-Planck-Society.He then took a position on the

research staff at Dortmund University.In2001,he returned

to China through the Hundred Talents Program,to become full Professor at the Institute of Chemistry,The Chinese Academy of Sciences(ICCAS).

He has been recognized with several honors,including the Excellent Hundred Talents

Award and an NSFC Outstanding Youth Fund.His current research involves surface-

induced polymer crystallization,orientation-induced polymer crystallization,phase

transition of crystalline polymers and structure-property relationship of polymers. Q.Liu et al./Polymer54(2013)4404e44214421

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