effects of the phase-separated melt on crystallization behavior and morphology in short chain...

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Effects of the phase-separated melton crystallization behavior andmorphology in short chain branchedmetallocene polyethylenesQiang Fu a d , Fang-Chyou Chiu a , Kevin W. McCreight a ,Mingming Guo a , Wen W. Tseng a , Stephen Z. D. Cheng a , MimiY. Keating b , Eric T. Hsieh c & Paul J. DesLauriers ca Maurice Morton Institute and Department of Polymer Science,The University of Akron, Akron, Ohio, 44325-3909b Central Research and Development Department, ExperimentalStation E. I. du Pont de Nemours and Company, Wilmington,Delaware, 19880c Research and Development Phillips Petroleum Company,Bartlesville, Oklahoma, 74004d Department of Polymer Sciences and Materials, Sichun UnionUniversity, Chengdu, Sichun, P.R. China

Version of record first published: 19 Aug 2006

To cite this article: Qiang Fu, Fang-Chyou Chiu, Kevin W. McCreight, Mingming Guo, Wen W.Tseng, Stephen Z. D. Cheng, Mimi Y. Keating, Eric T. Hsieh & Paul J. DesLauriers (1997): Effectsof the phase-separated melt on crystallization behavior and morphology in short chain branchedmetallocene polyethylenes, Journal of Macromolecular Science, Part B: Physics, 36:1, 41-60

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J. MACROMOL. SCI. -PHYS., B36(1), 41-60 (1997)

Effects of the Phase-Separated Melt on Crystallization Behavior and Morphology in Short Chain Branched Metallocene Polyethylenes

QIANG FU,* FANG-CHYOU CHIU, KEVIN W. McCREIGHT, MINGMING GUO, WEN W. TSENG, and STEPHEN Z. D. CHENGt Maurice Morton Institute and Department of Polymer Science The University of Akron, Akron, Ohio 44325-3909

MIMI Y. KEATING Central Research and Development Department Experimental Station E. I. du Pont de Nemours and Company Wilmington, Delaware 19880

ERIC T. HSIEH and PAUL J. DEsLAURIERS Research and Development Phillips Petroleum Company Bartlesville, Oklahoma 74004

*Permanent address: Department of Polymer Sciences and Materials, Sichun Union Univer- sity, Chengdu, Sichun, P.R. China. ?To whom correspondence should be addressed.

41

Copyright 0 1997 by Marcel Dekker, Inc.

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42 FU ET AL.

ABSTRACT

Some metallocene catalyst synthesized short chain branched polyethylene (SCBPE) samples have been found to possess at least intermolecular heterogeneity in the SCB. As-received SCBPE Sam- ples are molecularly homogeneous in the isotropic melt. However, phase separation due to the intermolecular heterogeneity can be found via molecular segregation processes induced by multiple-step isothermal crystallization experiments. When the phase-separated SCBPE samples are reheated above their melting temperatures, the phase-separated (heterogeneous) melt is maintained for an extended period of time (at least 20 h at 150OC). Neither phase mix- ing in the melt nor significant changes in molecular weight, molecular weight distribution, comonomer content, or sequence have been found during the high-temperature treatment. Comparisons of overall crystallization kinetics and morphology of the SCBPE Sam- ples obtained from the homogeneous and heterogeneous melts exhibit substantially different behavior which indicates that the phase-separated melt exists and can be identified.

INTRODUCTION

Metallocene polyethylene (PE) was developed as the latest step in a long history in the development of PE, which was driven by special de- mands on properties. The polymerization of branched PE was discovered by Imperial Chemical Industries, Ltd., in 1933 and consisted of the polymerization of ethylene under high pressure (> 124 MPa) and high temperature ( 1OO0-3OO0C) with free-radical catalysts [ 1,2]. With the discovery of Ziegler-Natta catalysts, the ability to polymerize ethylene at atmospheric pressure and room temperature became a reality. New catalysts for the polymerization of linear polyethylene (LPE) were independently developed by Phillips Petroleum [3] and Hoechst [4] in the early 1950s, while the polymerization procedure for branched PE underwent a drastic change in the 1970s. At this time Union Carbide developed a low-pressure, gas-phase, fluidized-bed process for the production of short chain branched PE (SC- BPE) [ S ] . However, the catalyst sites for Ziegler-Natta copolymerization had different activities and poor control of comonomer incorporation, and as a result, produced a mixture of polymers differing greatly in both chain length and comonomer sequence distribution [6-121. Mitsui, Exxon, Dow, and other companies concentrated their efforts on the development of single-site metallocene catalysts, and as a result, a new generation of SCBPE was introduced [ 13-24]. Compared with Ziegler-Natta copolymers, these

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SHORT CHAIN BRANCHED METALLOCENE POLYETHYLENES 43

SCBPEs set a new standard for polymer purity with narrower molecular weight and more uniform comonomer sequence distributions. Many industrial applications refer to this class of SCBPE as a homogeneous product. Dow has recently introduced another type of product which is referred to as homogeneous ethylene/cw-olefins with appended homogeneous long chain branches.

Extensive studies have been carried out in order to understand structure, morphology, and property relationships of LPE and SCBPE. These studies were stimulated by the industrial improvement of PE materials and applications. Although substantial progress has been made in almost every aspect of structure and property characterization, it is clear that we do not yet have a full understanding in the area of structure and morphology of these materials. A recent example is the discussion of phase-separated melts in SCBPE in a certain range of molecular weights and SCB contents [25-291.

In the last few decades, differential scanning calorimetry (DSC) has been used to study molecular segregation during crystallization from the melt based on molecular weight and chemical composition during crystallization. One example is the molecular segregation of linear PE during crystallization from the melt based on different molecular weights [30]. Ex- tensive studies of semicrystalline polymer blends and copolymers have provided examples of the chemical composition effect on molecular and/ or segmental segregation during crystallization. Recently, a few reports indicated that one may use DSC to conduct multiple-step isothermal crystallization experiments from high to low temperatures with a small decre- ment (e.g., a few degrees per step) to separate chemical compositions of SCBPEs [3 1-36]. When this method was utilized for SCBPEs synthesized via multiple-site catalysts, heterogeneity in the systems was obvious. This method seems to yield results corresponding to those from other methods such as I3C-nuclear magnetic resonance (NMR) and temperature-raising elution fractionation (TREF) experiments [37]. This process has been called a “thermal fractionation” in some literature, but we would like to call it a “thermal segregation” to distinguish the difference between “fractionation,” which is often under a condition of thermodynamic equilibrium, and segregation, which is a nonequilibrium, kinetic process.

It is important to recognize that two kinds of heterogeneity may exist in a SCBPE system: intra-and intermolecular heterogeneity. The former concept implies that within one macromolecule, SCB distribution is not uniform along the chain backbone, while all the molecules in the system possess the same SCB distribution. The latter concept implies that among the molecules, the SCB distribution is not uniform, namely, the SCB content is higher in some molecules than others. However, each molecule possesses a uniform SCB distribution. As a result, one may have four possible

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44 FU ET AL.

combinations: a system which is both inter-and intramolecularly heterogeneous; both inter-and intramolecularly homogeneous; intermolecularly heterogeneous but intramolecularly homogeneous; or finally, intramolecularly heterogeneous but intermolecularly homogeneous. These classifications are more finely detailed compared with the simple definition of heterogeneous or homogeneous systems commonly used in industrial applications. To dif- ferentiate these combinations, it is necessary to carefully analyze the experimental results observed via thermal segregation, combined with crystal and phase morphologies.

In this report, we attempt to show that some metallocene-synthesized SCBPEs exhibit at least intermolecular heterogeneity. The overall crystallization kinetics obtained via DSC combined with morphological observations may provide additional information regarding phase separation in the SCBPE melt after thermal segregation,

EXPERIMENTAL SECTION Materials

Metallocene SCBPE samples were purchased from Exxon. Two SCBPE samples were chosen in this study. First was a hexane comonomer branched SCBPE(H) having a weight average molecular weight (M,) of 97,800 and a polydispersity (M,/M,) of 2.2. The average short chain branch (SCB) content was 7.8 per 1000 carbon atoms. The second sample, SCBPE(B), con- tained butene comonomers. The M, was 106,000 and the M,/M,, was 1.8. The SCB content was 20.7 per 1000 carbon atoms. The samples were whole polymers and no fractionation was attempted. The molecular weight and polydispersity were measured via gel permeation chromatography (GPC) and the SCB contents were measured via infrared (IR) and solution I3C- NMR methods. Note that the SCB content only represents an average value of the SCB content and does not reflect intra-and/or intermolecular SCB sequence distributions. In order to monitor molecular changes of the materials after prolonged annealing at high temperature, SCBPE samples were held at 150°C for 20 h in order to determine their xylene solubility, molecular weight, and molecular weight distribution (by GPC); thermooxidative degradation (by FTIR); and comonomer content and sequence (by I3C- NMR).

Equipment and Experiments

DSC experiments were carried out on a Perkin-Elmer DSC-7 coupled with an intercooler to obtain thermal segregation. Aluminum DSC plans

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with approximately 2-mg samples of as-received SCBPE were first heated to 15OOC under a dry nitrogen atmosphere and then held for 2 min. The samples were then stepwise cooled to each preset temperature. The experimentally chosen temperatures for the SCBPE(H) were 1 1 5 O , llOo, 1 0 5 O , and 100OC. At each temperature, the samples were held for 12 h in order to achieve complete crystallization. For SCBPE( B), crystallization temperatures of looo, 95O, 90°, and 85OC were chosen, and the same isothermal time was used. To observe detailed molecular segregation, decrements narrower than 5OC were also used as specifically indicated in the text. The isothermal crystallization temperatures chosen were based on the SCB content. In SCBPE(B) the SCB content is almost three times higher than that of SCBPE(H). The equilibrium heat of fusion of PE which was used in order to calculate the crystallinity was 293 J/g [38].

Overall crystallization kinetics were studied following two different experimental designs. First, regular isothermal crystallization kinetics experiments were performed by quenching the samples from 15OoC to a preset crystallization temperature. The crystallinity development in the samples was recorded with respect to time. Secondly, multiple-step crystallization kinetics experiments were conducted. In this case, the crystallinity development with each step in the samples was monitored with time. Note that in these experiments each subsequent step in the crystallization might be nucleated by the existing crystals formed at higher temperatures. Moreover, crystallization kinetics were studied for the SCBPE samples before and after thermal segregation. In order to study the heterogeneity of the melt, different residence times from 2 min to 20 h at 15OoC were used under a carefully controlled dry nitrogen atmosphere.

Polarized light microscopy (PLM) experiments were carried out via an Olympus BH-2 using an attached 35-mm camera and a Mettler FP-80 hot stage. Thin film samples were prepared by casting SCBPE/xylene solutions [2(w/v)%] on clean glass slides. After solvent evaporation, the film thick- ness was around 10 pm. Both single-and multiple-step crystallization processes were also conducted on samples with the same thermal histories as used in the DSC experiments. The crystal morphology under PLM was observed during and after crystallization at different temperatures.

A JEOL JEM-1200EXII transmission electron microscope (TEM) was used to examine morphology in ultrathin film samples on a microscopic scale. The thermal histories of the samples were the same as those in the PLM experiments. Films with thicknesses of about 0.1 pm were cast from SCBPE/xylene solutions [O.~(W/W)%] and subjected to conventional Pt shadowing and carbon backing in a vacuum evaporator. The films were then transferred to 200-mesh Cu TEM grids for morphological observations.

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46

RESULTS AND DISCUSSION

Thermal Segregation

FU ET AL.

Figures 1 and 2 show typical DSC heating curves of multiple-step- crystallized samples cooled at different decrements for both SCBPE( H) and SCBPE(B), respectively. Multiple melting peaks may be seen which are formed during each step of isothermal crystallization. The onset melting temperatures are close to the isothermal crystallization temperatures and the width at half-height of each endotherm is narrow ( < 3 "C). Generally speaking, each endotherm represents a population of crystals with almost the same thermodynamic stability and the melting temperature and difference among the endotherms is mainly caused by different crystal sizes (thicknesses). Although both molecular weight and molecular weight distribution may obscure this phenomenon, this effect may not be dominant in the study here since the PE molecular segregation occurs mainly when the molecular weight is less than 20,000 during crystallization at low undercoolings [30]. The molecular weights of these SCBPEs are high enough and their polydispersities are narrow enough to prevent major molecular segregation due to low molecular weight species.

If a system has only intermolecular heterogeneity but uniform SCB distribution within each chain, molecular segregation is prominent when crystallization takes place at low undercoolings. Therefore, the DSC obser-

Temperature (T)

FIG. 1. DSC melting trace of SCBPE(H) crystallized stepwise from the homogeneous melt. The crystallization temperatures were 1 1 6 O , 1 1 3 O , l l O o , 1 0 7 O , 1 0 3 O , looo, 9 5 O , 8 5 O , SOo, and 7OOC.

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40 60 120

Temperature ("C)

FIG. 2. DSC melting trace of SCBPE(B) crystallized stepwise from the homogeneous melt. The crystallization temperatures were looo, 9 7 O , 9 3 O , 90°, 8 7 O , 8 3 O , 80°, 1 5 O , 70°, 6 5 O , 60°, and 55OC.

vations are an indication that each melting endotherm may be attributed to molecules having the same, or at least very similar, SCB content. On the other hand, if a system had only intramolecular heterogeneity, but all the chains possessed the same SCB content and sequence distribution, the major segregation process would be segmental. In this case, different DSC endotherms represent the crystals formed by chains of equal or very similar segmental length. A more complicated situation involves the combination of both intra-and intermolecular heterogeneity. Unfortunately, the cases described above are very difficult to distinguish by DSC multiple-step- crystallization experiments alone. Detailed structural formation kinetics and morphological observations are necessary to illustrate the difference caused by molecular or segmental segregation (see below).

Crystallization Kinetics from the Melt Before and After Thermal Segregation

Figures 3(a) and 3(b) show crystallinity changes of both SCBPEs with time at different isothermal temperatures using a regular (single temperature) crystallization procedure. It is evident that the crystallinity increases monotonically with time and exhibits an induction time stage (the formation of primary nuclei). This is followed by a major increase of the crystallinity and, finally, a gradual approach to a maximum. The Avrami exponents of these SCBPEs are between 2 and 4 in the initial stage of

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48

0.25

0.20 -

0.15 -

0.10 -

0.05 -

0.00 -

2

FU ET AL.

A A A

A 0 0 0

0 rp

a 105°C A 1 10°C A n

A

0 0 0

A

t I I I I

0.1 1 10 100 1000

Time (min) (b)

0.14 I

I 1 I I

1 10 100 1000

Time (min)

FIG. 3. Crystallinity increases during the single-step crystallization for (a) SCBPE(H) and (b) SCBPE(B) at three different temperatures from the homogeneous melt (before thermal segregation).

crystallization. This phenomenon has been widely reported in literature [39-441. As the crystallization temperature approaches the melting point, the overall crystallization rate decreases.

Figures 4(a) and 4(b) show the Avrami treatments of both samples when the samples were crystallized via multiple-step experiments. The crystallization kinetic parameters of the first step are the same as those in the regular crystallization at the same temperature (undercooling). The Avrami

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0

-5 I I I I

-1 0 1 2 3 4

- -2-

7 v

c - & -3 - 0 -

-4 - 0 T,=lOO"C 0 Tc=95"C

-5 I -1 0 1 2 3 4

log time (min)

FIG. 4. Avrami treatments of (a) SCBPE(H) and (b) SCBPE(B) at three different temperatures via a multiple-step crystallization from the homogeneous melt.

exponent of this step is 4 for SCBPE(H) and 2 for SCBPE(B). This may be due to the different SCB contents and lengths in these two samples. How- ever, from the second step on, the Avrami exponents of the initial crystallization stage are all fractional and below one-half. Note that the crystallization kinetics monitored in the second step were carried out after the first step of crystallization had been completed. This indicates that the crystals formed in the first step may serve as primary nuclei for the later crystallization steps . Furthermore, the crystals formed after the first step of crystalli-

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50 FU ET AL.

zation may also change the dimensionality in their morphologies due to different SCB contents, and/or crystallization rates may deviate from lin- earity at constant temperatures, thereby substantially decreasing the Avrami exponents [45].

After the SCBPE samples had undergone the multiple-step crystallization, the samples were reheated to 15OOC and held for 2 min. Next, both single-and multiple-step-crystallization experiments were once again carried out. This experiment was designed to investigate whether the multiple-step crystallization generates molecular and/or segmental segregation. If molecular segregation occurs, macroscopic phase separation should take place in the multiple-step experiments on the as-received samples, and the subsequent crystallization after the thermal segregation should occur more or less independently in each of the phase-separated domains. Therefore, the kinetics must be different from those crystallized from a homogeneous melt (before the thermal segregation). If any segmental segregation occurs during the multiple-step crystallization on the as-received samples, no macroscopic phase separation should be observed and only lamellar crystals with different thicknesses may stack together. The crystals formed in the previous step may therefore still serve as primary nuclei for the subsequent crystallization steps. The Avrami exponents for the second and third steps should not be very different from those obtained in the multiple-step- crystallization experiment on the as-received samples.

Figures 5(a) and 5(b) show crystallinity increases during regular isothermal crystallization from SCBPE melts with and without thermal segregation. It is clear that at T, = l l 5 O C for SCBPE(H) and T, = 100°C for SCBPE(B) crystallized from the heterogeneous melt after the thermal segregation, the crystallization rates are faster than those from the homogeneous melt before the thermal segregation. This reveals that molecules having similar SCB content and sequence distribution have undergone phase separation induced by the multiple-step crystallization. As a result, in the subsequent regular crystallization processes at the highest temperature, molecules do not need to go through the molecular segregation and disentanglement which decrease the crystallization rate. In Figures 6(a) and 6(b), the Avrami kinetic treatment for the multiple-step process shows that at each subsequent crystallization temperature, the Avrami exponents range from 1.5 to 2, indicating that each crystallization step may depend on its own primary nucleation, not on the previously formed crystals. Such phenomena can only be observed when each isothermal crystallization step occurs within separated domains since in this case, no sufficiently large crystal surfaces are provided by the previously formed crystals to allow surface nucleation in the subsequent crystallization processes. It thus be-

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(a)

0.04

so 0.03 0.02

0.01

0.00

300 1000 2000 Time (min)

(b) 0.05

0.04

0.03

3" 0.02

0.01

0.00

0

0

0 lrom heterogeneous melt

0 from homogeneous melt 0

0 0

0 0 c1

0

100 1000

Time (min)

FIG. 5. Crystallinity increases during the crystallization of (a) SCBPE(H) crystallized at 115OC and (b) SCBPE(B) crystallized at 100°C from the heterogeneous melt. Their counterparts crystallized from the homogeneous melt are also included for comparison [see Figs. 3(a) and 3(b)].

comes evident that the phase separation is mainly formed due to molecular segregation in the multiple-step crystallization of as-received samples.

We also investigated the residence time effect on the heterogeneous melt by keeping samples at 15OOC for different periods of time between 2 min and 20 h under a dry nitrogen atmosphere. The SCBPE(H) crystallization rates at a given temperature (1 15 "C) increase when the residence time

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52

$ -2- c v

C - & -3 - 0 -

-4 -

FU ET AL.

-4 -31 0 Tc=llO"C

d) 8 0

00 0

-5 -/ I I I I

-1 0 1 2 3 4 log time (min)

(b)

U A

0 Tc=lOO"C

A Tc=90"C

-5 I -1 0 1 2 3 4

log time (min)

FIG. 6. Avrami treatments of (a) SCBPE(H) and (b) SCBPE(B) at three different temperatures via a multiple-step crystallization from the heterogeneous melt.

is increased (Fig. 7). This indicates that at 1 5 O O C the SCBPE samples are further phase separated with increasing residence time. Furthermore, the induction times in the initial stage of isothermal crystallization in the second and third steps [shown in Figs. 6(a) and 6(b)] are longer than their counterparts crystallized from the homogeneous melt [and therefore, the slopes in the Avrami plots are larger; see Figs. 4(a) and 4(b)]. This may indicate an energy barrier difference between the primary and the surface nucleation

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0.07

0 200 min.

0.04

% 0.02 0.03 i 0.00 0-01 i - L L L - -

200 500 1000

Time (min)

FIG. 7. Crystallinity increases of the SCBPE(H) crystallized at 115OC with respect to isothermal times after different residence times at 1 5OoC.

processes. However, the final crystallinity of each step crystallized from the heterogeneous melt is slightly higher than that from the homogeneous melt, possibly due to disentanglement during the crystallization and phase separation.

Since the SCBPE samples had been held at 150OC for a prolonged time, it is possible that the samples underwent chemical changes due to crosslinking and/or chain scission which certainly affected the experimental observations reported. In order to identify any chemical changes, both as-received resins and the samples annealed by 15OOC for 20 h under dry nitrogen were placed into xylene; they completely dissolved at almost the same rate. This indicates that no crosslinking reaction occurred during the high-temperature annealing. Molecular weights and molecular weight distributions of these two samples measured via GPC were virtually identi- cal. The FTIR experiments for examining carbonyl moieties and terminal vinyl groups did not show any chain scission. Moreover, solution 13C-NMR experiments showed that the chemical compositions did not vary in these two samples. We may thus conclude that after the 150OC annealing, the samples did not show significant changes in chemical composition, molecular weight, or molecular weight distribution. As a result, the difference in crystallization kinetics experimentally observed is a true reflection of a heterogeneous melt in the samples.

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Crystal Morphology Observations

FU ET AL.

Crystallization kinetics have shown that after thermal segregation in the melt and/or during crystallization, the samples become phase separated and reheating the samples leads to a heterogeneous melt. One expects that the crystal or phase morphologies should exhibit substantial differences under PLM and TEM. Figure 8 shows a set of PLM micrographs after a three-step crystallization from the homogeneous melt. It is clear that during the first step of crystallization, isolated crystalline domains (in a broad definition, they may belong to irregular spherulites) are formed. In the second step, crystals fill in between these domains, which serve as centers of crystal texture (primary nuclei). Further reduction in temperature leads to an enhanced birefringence due to in-filling lamellar crystal growth, indicating that the crystallization occurs on a smaller scale, which cannot be identified due to limits in the resolution of PLM. Figure 9 is another set of PLM micrographs for the same sample and the same three-step crystallization procedure but after thermal segregation. Although the first step seems to be similar to Fig. 8(a), the crystal morphology developed in the second and third steps shows that the majority of the crystals are independent from the crystals formed in the first step. This reveals that they possess their

FIG. 8. PLM observations of the crystal morphology for SCBPE(H) crystallized at (a) 115OC, (b) llO°C, and (c) lO5OC from the homogeneous melt.

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FIG. 8. Continued.

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56 FU ET AL.

FIG. 9. PLM observations of the crystal morphology for SCBPE(H) crystallized at (a) 11S0C, (b) llO°C, and (c) 105OC from the heterogeneous melt.

own primary nuclei during crystallization. This morphological observation clearly supports our conclusion derived from the overall crystallization kinetics, namely, that each crystalline domain formed at different temperatures is representative of the phase morphology caused by the phase separation.

On the micrometer scale, different crystal morphologies of these multiple-step-crystallized samples (before and after thermal segregation) can be observed under TEM. Figures 10(a) and 10(b) show two TEM microphoto- graphs which illustrate the morphologies of the SCBPE samples stepwise crystallized from the heterogeneous melt. Although thermal segregation is a process of crystallization-induced phase separation, it is still necessary to distinguish two different kinds of phase-separation morphologies: macroscopic and microscopic. The former possesses a larger size of phase- separated domains (more than 1 pm) compared with the latter, which occurs between two lamellae or lamellar stacks (around 20-100 nm). Figure 10(a) is a clear indication of the macroscopically phase-separated domains crystallized at different temperatures. Thicker lamellar crystals are formed at 115OC, and others are formed during the subsequent crystallization process at lower temperatures. Figure 10(b) illustrates that within the thicker lamellar domains, some minor thinner lamellae can also be found. They are

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FIG. 9. Continued.

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FIG. 10. TEM crystal morphological observations of (a) macroscopic and (b) microscopic phase separation for SCBPE(H) crystallized in three steps (1 IS0, 1 loo, and 105 "C) from the heterogeneous melt.

presumably formed at relatively low crystallization temperatures as in- filling lamellar crystal growth.

It is speculated that either the molecules do not have enough time to diffuse away from the crystal growth front and, therefore, are trapped in between lamellae, or that there is segmental segregation due to nonuniform SCB sequences in the molecules. However, this microscopic phase segregation induced by the multiple-step-crystallization process is minor, and macroscopic phase separation is predominant.

CONCLUSION

It is evident that some SCBPEs sythesized via metallocene catalysts exhibit at least intermolecular heterogeneity in the comonomer content. This heterogeneity can be found through a multiple-step-crystallization process and macroscopic phase separation is induced by molecular segregation

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during crystallization. It is surprising that the phase-separated SCBPE melt can be held for an extended time at 15OOC (up to 20 h) without homogeniz- ing; instead, the phase separation continues and the size scale over which segregation occurs increases. Multiple-step isothermal crystallization kinetics of the SCBPEs from the homogeneous and heterogeneous melts are different. This may serve as a useful method to study phase separation behavior in semicrystalline polymer blends and copolymers.

ACKNOWLEDGMENT

This work was supported by S.Z.D.C.’s Presidential Young Investiga- tor Award from the National Science Foundation (DMR 91-577381, Du- pont Company and Phillips Petroleum Company.

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Received January 5 , 1996 Revised February 6, 1996 Accepted February 7, 1996

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effects of the phase-separated melt on crystallization behavior and morphology in short chain...

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