Shear-induced crystallization in phase-separated blend of isotactic polypropylene and poly (ethylene-co-octene)

Isothermal crystallization after shear in a blend of isotactic polypropylene (iPP) and poly (ethylene-co-octene) (PEOc) was investigated by in situ optical microscopy and shear hot stage under various thermal and shear histories. Crystalline cylindrites during growth were observed in phase-separated iPPPEOc blends for the first time. According to our results, the very long cylindrites are formed which are much longer than the dimensions of the liquid-liquid phase-separated domains under shear, and the cylindrites appear to grow through noncrystallizable domains, as well as through crystallizable ones. Obviously, the nuclei ("shish") come from the oriented and entangled network strands instead of pulled-out long chains. The number of cylindrites and the distortion and breakup of the cylindrites are related to the shear rate and shear time. On the other hand, the number of spherulites increases not only with shear rate but also with liquid-liquid phase separation time. Spherulites always form with longer induction time than cylindrites due to the different nucleation mechanism. The shish is nucleated through the shear-induced mechanism, and most of the spherulites are nucleated through liquid-liquid spinodal decomposition and crossover after the cessation of shear. During the process of experiments, we also found three kinds of shish-kebab structures, which provide further physical insights into the mechanism of the shish formation in polymer blend after liquid-liquid phase separation under shear.     

Solvent‐ and interface‐induced surface segregation in blends of isotactic polypropylene with poly(ethylene‐co‐propylene) rubber

The surface compositions and morphologies of melt‐quenched blends of isotactic polypropylene (iPP) with aspecific poly(ethylene‐co‐propylene) rubber (aEPR) were characterized by atomic force microscopy, optical microscopy, and X‐ray photoelectron spectroscopy. The surface morphologies and compositions formed in the melt are frozen‐in by crystallization of the iPP component and, depending on the processing conditions, are enriched in iPP or aEPR or contain a phase‐separated mix of iPP and aEPR. Enrichment of iPP is observed for blends melted in open air, in agreement with earlier work showing the high surface activity of atactic polypropylene at open interfaces. Surface segregation of iPP is suppressed at confined interfaces. Blends melt‐pressed between hydrophilic and hydrophobic substrates have phase‐separated iPP and aEPR domains present at the surface, which grow in size as the melt time increases. Surface enrichment of aEPR is observed after exposing melt‐pressed blends to n‐hexane vapor, which preferentially solvates aEPR and draws it to the surface. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 421–432, 2004     

Poly(p‐phenylene sulfide) nonisothermal cold‐crystallization

The poly(p‐phenylene sulfide) (PPS) nonisothermal cold‐crystallization behavior was investigated in a wide heating rate range. The techniques employed were the usual Differential Scanning Calorimetry (DSC), and the less conventional FT‐IR spectroscopy and Energy Dispersive X‐ray Diffraction (EDXD). The low heating rates (Φ) explored by EDXD (0.1 K min^?1) and FT‐IR (0.5–10 K min^?1) are contiguous and complementary to the DSC ones (5–30 K min^?1). The crystallization temperature changes from 95 °C at Φ = 0.05 K min^?1 to 130 °C at Φ = 30 K min^?1. In such a wide temperature range the Kissinger model failed. The model is based on an Arrhenius temperature dependence of the crystallization rate and is widely employed to evaluate the activation energy of the crystallization process. The experimental results were satisfactorily fit by replacing in the Kissinger model the Arrhenius equation with the Vogel–Fulcher–Tamann function and fixing U* = 6.28 k J mol^?1, the activation energy needed for the chains movements, according to Hoffmann. The temperature at which the polymer chains are motionless (T_∞ = 42 °C) was found by fitting the experimental data. It appears to be reasonable in the light of our previously reported isothermal crystallization results, which indicated T_∞ = 48 °C. Moreover, at the lower heating rate, mostly explored by FT‐IR, a secondary stepwise crystallization process was well evidenced. In first approximation, it contributes to about 17% of the crystallinity reached by the sample. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2725–2736, 2005     

Isothermal Crystallization of Isotactic Poly(propylene) Studied by Superfast Calorimetry

The isothermal crystallization behavior and the structure and morphology of isotactic poly(propylene) (iPP) and iPP/hydrogenated hydrocarbon resin (HR) 90/10 blend were analyzed. To cover the entire temperature range, isothermal crystallizations were studied using superfast calorimetry at a high cooling rate in the range 0 to 110 °C, and by conventional DSC at a low cooling rate in the range 120 to 140 °C. Structural and morphological changes due to the different thermal treatments were also analyzed. The complete crystallization curve ranging from T_g to T_m showed bimodal crystallization behaviors for both iPP and iPP/HR 90/10 blend. This result is explained by taking into consideration the polymorph properties of iPP. It is in fact assumed that the curve from T_g to 60 °C referred mainly to the crystallization kinetics of the iPP mesomorphic form by homogeneous nucleation, whereas the curve from 60 °C to T_m mainly represented the crystallization kinetic curve for the monoclinic α form by heterogeneous nucleation. This hypothesis is confirmed by the analysis of the structures obtained using wide angle X‐ray experiments. Moreover, the addition of HR to iPP causes a drastic reduction in the crystallization rate of iPP in both regions due to the diluent effect of the miscible resin.

     

Phase behavior and structure development in extruded poly(ethylene terephthalate)/poly(ethylene‐2,6‐naphthalate) blend

Liquid–liquid phase separation and subsequent homogenization during annealing in an extruded poly(ethylene terephthalate) (PET)/poly(ethylene‐2,6‐naphthalate) (PEN) blend were investigated with time‐resolved light scattering and optical microscopy. In the initial stage, the domain structure was developed by demixing via spinodal decomposition. In the later stage, the blend was homogenized by transesterification between the two polyesters. The crystallization rate depended on the sequence distribution of polymer chains, which was determined by the level of transesterification rather than the composition change of separated phases. When the crystallization of PEN preceded that of PET, PEN showed a higher melting point. However, when the crystallization rate of PEN was slower than that of PET, the previously formed PET crystals suppressed the crystallization of PEN, causing the coarse crystalline structure of PEN to have a lower melting point. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2625–2633, 2000     

Complex phase behavior and state of miscibility in Poly(ethylene glycol)/Poly(l‐lactide‐co‐ε‐caprolactone) Blends

A miscibility and phase behavior study was conducted on poly(ethylene glycol) (PEG)/poly(l ‐lactide‐ε‐caprolactone) (PLA‐co‐CL) blends. A single glass transition evolution was determined by differential scanning calorimetry initially suggesting a miscible system; however, the unusual T_g bias and subsequent morphological study conducted by polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) evidenced a phase separated system for the whole range of blend compositions. PEG spherulites were found in all blends except for the PEG/PLA‐co‐CL 20/80 composition, with no interference of the comonomer in the melting point of PEG (T_m = 64 °C) and only a small one in crystallinity fraction (X_c = 80% vs. 70%). However, a clear continuous decrease in PEG spherulites growth rate (G) with increasing PLA‐co‐CL content was determined in the blends isothermally crystallized at 37 °C, G being 37 µm/min for the neat PEG and 12 µm/min for the 20 wt % PLA‐co‐CL blend. The kinetics interference in crystal growth rate of PEG suggests a diluting effect of the PLA‐co‐CL in the blends; further, PLOM and AFM provided unequivocal evidence of the interfering effect of PLA‐co‐CL on PEG crystal morphology, demonstrating imperfect crystallization in blends with interfibrillar location of the diluting amorphous component. Significantly, AFM images provided also evidence of amorphous phase separation between PEG and PLA‐co‐CL. A true T_g vs. composition diagram is proposed on the basis of the AFM analysis for phase separated PEG/PLA‐co‐CL blends revealing the existence of a second PLA‐co‐CL rich phase. According to the partial miscibility established by AFM analysis, PEG and PLA‐co‐CL rich phases, depending on blend composition, contain respectively an amount of the minority component leading to a system presenting, for every composition, two T_g's that are different of those of pure components. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 111–121     

Crystallization studies of isotactic polypropylene containing nanostructured polyhedral oligomeric silsesquioxane molecules under quiescent and shear conditions

Crystallization studies at quiescent and shear states in isotactic polypropylene (iPP) containing nanostructured polyhedral oligomeric silsesquioxane (POSS) molecules were performed with in situ small‐angle X‐ray scattering (SAXS) and differential scanning calorimetry (DSC). DSC was used to characterize the quiescent crystallization behavior. It was observed that the addition of POSS molecules increased the crystallization rate of iPP under both isothermal and nonisothermal conditions, which suggests that POSS crystals act as nucleating agents. Furthermore, the crystallization rate was significantly reduced at a POSS concentration of 30 wt %, which suggests a retarded growth mechanism due to the molecular dispersion of POSS in the matrix. In situ SAXS was used to study the behavior of shear‐induced crystallization at temperatures of 140, 145, and 150 °C in samples with POSS concentrations of 10, 20, and 30 wt %. The SAXS patterns showed scattering maxima along the shear direction, which corresponded to a lamellar structure developed perpendicularly to the flow direction. The crystallization half‐time was calculated from the total scattered intensity of the SAXS image. The oriented fraction, defined as the fraction of scattered intensity from the oriented component to the total scattered intensity, was also calculated. The addition of POSS significantly increased the crystallization rate during shear compared with the rate for the neat polymer without POSS. We postulate that although POSS crystals have a limited role in shear‐induced crystallization, molecularly dispersed POSS molecules behave as weak crosslinkers in polymer melts and increase the relaxation time of iPP chains after shear. Therefore, the overall orientation of the polymer chains is improved and a faster crystallization rate is obtained with the addition of POSS. Moreover, higher POSS concentrations resulted in faster crystallization rates during shear. The addition of POSS decreased the average long‐period value of crystallized iPP after shear, which indicates that iPP nuclei are probably initiated in large numbers near molecularly dispersed POSS molecules. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2727–2739, 2001     

Experimental study of flow-induced crystallization in the blends of isotactic polypropylene and poly(ethylene-co-octene)

The crystallization behavior of the blends of isotactic polypropylene (iPP) and poly(ethylene-co-octene) (PEOc) under quiescent condition and shear flow were studied by differential scanning calorimetry (DSC) and rheology, respectively. The DSC curves of the iPP phase in the blends showed similar crystallization exothermic peaks to that of pure iPP itself, indicating that the addition of PEOc up to a percentage of 30 in weight almost had no influence on the crystallization behavior of iPP at quiescent condition. The rheological results of isothermal flow-induced crystallization (FIC) of iPP in the blends showed that the crystallization kinetics of iPP was enhanced with the increase of shear rate, similar to that of pure iPP, but the presence of PEOc enhanced the effect of shear on the crystallization kinetics of iPP significantly in the cases of shear rates larger than 0.2 s⁻¹, which was due to that PEOc played an important role to promote the nucleation of iPP. The rheological results also implied that the characteristic relaxation times of blends were longer than that of pure iPP during the FIC process, indicating a different relaxation mechanism which might be related to the occurrence of interface relaxation and chain relaxation of the PEOc phase in the blends.     

Morphology and nonisothermal crystallization of in situ microfibrillar poly(ethylene terephthalate)/polypropylene blend fabricated through slit‐extrusion,hot‐stretch quenching

This study describes the morphology and nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ micro‐fiber‐reinforced blends (MRB) obtained via slit‐extrusion, hot‐stretching quenching. For comparison purposes, neat PP and PET/PP common blends are also included. Morphological observation indicated that the well‐defined microfibers are in situ generated by the slit‐extrusion, hot‐stretching quenching process. Neat iPP and PET/iPP common blends showed the normal spherulite morphology, whereas the PET/iPP microfibrillar blend had typical transcrystallites at 1 wt % PET concentration. The nonisothermal crystallization kinetics of three samples were investigated with differential scanning calorimetry (DSC). Applying the theories proposed by Jeziorny, Ozawa, and Liu to analyze the crystallization kinetics of neat PP and PET/PP common and microfibrillar blends, agreement was found between our experimental results and Liu's prediction. The increases of crystallization temperature and crystallization rate during the nonisothermal crystallization process indicated that PET in situ microfibers have significant nucleation ability for the crystallization of a PP matrix phase. The crystallization peaks in the DSC curves of the three materials examined widened and shifted to lower temperature when the cooling rate was increased. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 374–385, 2004     

Side‐chain crystallization behavior of graft copolymers consisting of an amorphous main chain and crystalline side chains. II. Poly(n‐hexyl methacrylate)‐graft‐poly(ethylene glycol)

The phase behavior and crystallization of graft copolymers consisting of poly(n‐hexyl methacrylate) (PHMA) as an amorphous main chain and poly(ethylene glycol) (PEG) as crystallizable side chains (HMAx with 15 ≤ x ≤ 73, where x represents the weight percentage of PEG) were investigated. Small‐angle X‐ray scattering profiles measured above the melting temperature of PEG suggested that a microdomain structure with segregated PHMA and PEG domains was formed in HMA40 and HMA46. This phase behavior was qualitatively described by a calculated phase diagram based on the mean‐field theory. Because of the segregation of PEG into microdomains, the crystallization temperature of the PEG side chains in HMAx was higher than that in poly(methyl acrylate)‐graft‐poly(ethylene glycol) having a similar value of x, which was considered to be in a disordered state above the melting temperature. In HMAx with x ≤ 40, PEG crystallization was strongly restricted, probably because the PEG microdomains were isolated in the PHMA matrix. As a result, the growth of PEG spherulite was not observed because the PEG crystallization occurred after vitrification of the PHMA segregated domains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 129–137, 2007     

Crystallization behavior of isotactic propylene‐1‐hexene random copolymer revealed by time‐resolved SAXS/WAXD techniques

The crystallization behavior of isotactic propylene‐1‐hexene (PH) random copolymer having 5.7% mole fraction of hexene content was investigated using simultaneous time‐resolved small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For this copolymer, the hexene component cannot be incorporated into the unit cell structure of isotactic polypropylene (iPP). Only α‐phase crystal form of iPP was observed when samples were melt crystallized at temperatures of 40 °C, 60 °C, 80 °C, and 100 °C. Comprehensive analysis of SAXS and WAXD profiles indicated that the crystalline morphology is correlated with crystallization temperature. At high temperatures (e.g., 100 °C) the dominant morphology is the lamellar structure; while at low temperatures (e.g., 40 °C) only highly disordered small crystal blocks can be formed. These morphologies are kinetically controlled. Under a small degree of supercooling (the corresponding iPP crystallization rate is slow), a segmental segregation between iPP and hexene components probably takes place, leading to the formation of iPP lamellar crystals with a higher degree of order. In contrast, under a large degree of supercooling (the corresponding iPP crystallization rate is fast), defective small crystal blocks are favored due to the large thermodynamic driving force and low chain mobility. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 26–32, 2010     

Shear‐Controlled Micro/Nanometer‐Scaled Super‐Hydrophobic Surfaces with Tunable Sliding Angles from Single Component Isotactic Poly(propylene)

Summary: With the proper selection of shear and thermal conditions, super‐hydrophobic polymeric surfaces (contact angle > 150°) with tunable sliding angles (from less than 1° to higher than 90°) can be prepared from pure isotactic poly(propylene) (iPP) without any further modification with low‐surface‐energy components under ambient atmosphere. The formed surfaces have naturally good thermal properties, chemical and moisture resistance, low density, and potentially low manufacturing cost.

SEM images of formed super‐hydrophobic surfaces and related two extreme sliding angles (contact angles of these surfaces are higher than 150°).     

Effect of in situ poly(ethylene terephthalate) (PET) microfibrils on the morphological structure and crystallization behavior of isotactic polypropylene (iPP) under an intensive shear rate

The morphological structure and crystallization behavior of in situ poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) microparts prepared through micro‐injection molding are investigated using a polarized light microscope, differential scanning calorimeter, scanning electron microscope, and two‐dimensional wide‐angle X‐ray. Results indicate that both the shear effect and addition of PET fibers greatly influence the morphologies of the iPP matrix. Typical “skin‐core” and oriented crystalline structures (shish‐kebab) may simultaneously be observed in neat iPP and iPP/PET microparts. The presence of PET phases reveals significant nucleation ability for iPP crystallization. High concentrations of PET phases, especially long PET fibers, correspond to rapid crystallization of the iPP matrix. The occurrence of PET microfibrils decreases the content and size of β‐crystals; by contrast, the orientation degree of β‐crystals increases with increasing PET content in the microparts. This result suggests that the existence of the microfibrillar network can retain the ordered clusters and promote the development of oriented crystalline structures to some extent. Copyright © 2015 John Wiley & Sons, Ltd.     

Side‐chain crystallization behavior of graft copolymers consisting of amorphous main chain and crystalline side chains: Poly(methyl methacrylate)‐graft‐poly(ethylene glycol) and poly(methyl acrylate)‐graft‐poly(ethylene glycol)

Graft copolymers consisting of amorphous main chain, poly(methyl methacrylate) (PMMA), or poly(methyl acrylate) (PMAc), and crystalline side chains, poly(ethylene glycol) (PEG), have been prepared by copolymerization of PEG macromonomers with methyl methacrylate or methyl acrylate (MMAx or MACx, respectively). Because of the compatibility of PMMA/PEG and PMAc/PEG, from small‐angle X‐ray scattering results, the main and side chains in graft copolymers were suggested to be homogeneous in the molten state. Differential scanning calorimetry (DSC) cooling scans revealed that PEG side chains for graft copolymers with large PEG fractions were crystallized when the sample was cooled, with a cooling rate of 10 °C/min. The spherulite pattern observed by a polarized optical microscope suggested the growth of PEG crystalline lamellae. Crystallization of PEG in MMAx was more restrained than in MACx. From these results, we have concluded that the crystallization behavior of the grafted side chains is strongly influenced by the glass transition of a homogeneously molten sample as well as dilution of the crystallizable chains. Domain spacings for isothermally crystallized graft copolymers were described by interdigitating chain packing in crystalline–amorphous lamellar structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 79–86, 2005     

Sphere‐to‐Rod Transition of Micelles formed by the Semicrystalline Polybutadiene‐block‐Poly(ethylene oxide) Block Copolymer in a Selective Solvent

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)‐block‐poly(ethylene oxide), in the selective solvent n‐heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5 kg · mol⁻¹, respectively. In this solvent, micellization into a liquid PEO‐core and a corona of PB‐chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at −30 °C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (−196 °C; liquid nitrogen) leads to the transition from spherical to rod‐like micelles. With time these rod‐like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO‐cores of the rod‐like micelles increases. The transition from a spherical to a rod‐like morphology can be explained by a decrease of solvent power of the solvent n‐heptane for the PB‐corona chains: n‐Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.

     

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

Herein, we reported the formation mechanism of hybrid crystalline (cylindrite) in isotactic polypropylene (iPP)/carbon fiber (CF) via pulling a CF within the iPP melt. The α‐row nuclei layer closely attached to the surface of CF acts as a self‐nucleation site, rather than a heterogeneous nucleation one, to grow cylindrites. As a result, the polymorphic feature of iPP/CF cylindrite is significantly influenced by the microstructure of α‐row nuclei. With decreasing crystallization temperature (T_c), the polymorphic cylindrite changes from pure α‐form to mixed α‐/β‐form and to β‐rich form. The main characteristics of this change include: (a) the outlines of α‐row nuclei layer correspond to wave‐like, saw‐like, and straight lines; (b) the orientation level of iPP molecules in the α‐row nuclei layer become higher; (c) the α‐lamellae rearrange from loose to compact; and (d) the distance between the growth sites of β‐sectors and the surface of CF is evidently longer than in the case of α‐sectors. Moreover, this study provides a guideline for developing the interfacial enhanced iPP/CF composites through manipulation of polymorphic structure in cylindrites. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 368–377     

Epitaxial crystallization and oriented structure of linear low‐density polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding

In this work, as a part of a long‐term project aimed at controlling of crystal structure and phase morphology for a injection molded product, we investigated the oriented structure and possible epitaxial growth of polyolefin blend (low‐density polyethylene (LLDPE)/isotatic polypropylene (iPP)), achieved by dynamic packing injection molding, which introduced strong oscillatory shear on the gradually‐cooled melt during the packing process. The crystalline and oriented structures of the prepared blends with different compositions were estimated in detail through 2D X‐ray diffraction, calorimetry, and optical microscopy. As iPP was the dominant phase (its content was more than 50 wt%), our results indicated that it could be highly oriented in the blends. In such case, it was interesting to find that LLDPE epitaxially crystallized on the oriented iPP through a crystallographic matching between (100)_LLDPE and (010)_iPP, resulting in an inclination of LLDPE chains, about 50° to the iPP chain axis. On the other hand, as iPP was the minor phase, iPP was less oriented and no epitaxial growth between iPP and LLDPE was observed; even LLDPE remained oriented. The composition‐dependent epitaxial growth of LLDPE on oriented iPP could be understood as due to: (1) the effect of crystallization sequence, it was found that iPP always crystallized before LLDPE for all compositions; (2) the dependence of oriented iPP structure on the blend composition; (3) the “mutual nucleation” between LLDPE and iPP due to their partial miscibility. Copyright © 2009 John Wiley & Sons, Ltd.     

Crystallization‐Induced Phase Segregation Based on Double‐Crystalline Blends of Poly(3‐hexylthiophene) and Poly(ethylene glycol)s

Crystallization‐induced vertical stratified structures were constructed based on double‐crystalline poly(3‐hexylthiophene) (P3HT)/poly(ethylene glycol)s (PEG) systems at room temperature, in which the P3HT crystallinity and the mechanism were investigated. Vertical stratified microstructures with highly crystalline P3HT network on the surface were formed when depositing from marginal solvents, while lateral phase‐separated structures or low P3HT crystallinity were observed for good solvents. The morphological differences came from the solvent effect. In marginal solvents, p‐xylene and dichloromethane, P3HT large‐scale microcrystallites were generated in solution, which ensured the priority of P3HT crystalline sequence, and phase separation began in the liquid states. When the PEG matrix began to crystallize, great energy from which the second phase separation was induced drove P3HT crystallites to the surface, resulting in the formation of vertical stratified microstructures with highly crystalline P3HT network on the surface. The method, crystallization‐induced phase segregation of crystalline–crystalline blends in marginal solvent, provides a facile way to construct vertically stratified structures, in which P3HT highly crystalline network is favored.

     

The effect of phase‐separated morphology on the rheological properties of polystyrene/poly(vinyl methyl ether) blend

The effect of phase‐separated morphology on the rheological properties of polystyrene/poly(vinyl methyl ether) (PS/PVME) blend was investigated by optical microscopy (OM), light scattering (LS) method, and rheology. The blend had a lower critical solution temperature (LCST) of 112°C obtained by turbidity experiment using LS at a heating rate of 1°C/h. Three different blend compositions (critical 30/70 PS/PVME by weight) and two off‐critical (50/50 and 10/90)) were prepared. The rheological properties of each composition were monitored with phase‐separation time after a temperature jump from a homogeneous state to the preset phase‐separation temperature. For the 30/70 and 50/50 blends, it was found that with phase‐separation time, the storage and loss moduli (G′ and G″) increased at shorter times due to the formation of co‐continuous structures resulting from spinodal decomposition. Under small oscillatory shearing, shear moduli gradually decreased with time at longer phase‐separation times due to the alignment of co‐continuous structures toward the flow direction, as verified by scanning electron microscopy. However, for the 10/90 PS/PVME blend, the rheological properties did not change with phase‐separation times. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 889–906, 1999     

Epitaxial and graphoepitaxial growth of isotactic polypropylene (iPP) from the melt on highly oriented high density polyethylene (HDPE) substrates

Although under normal conditions only the crystallization behavior of PE on oriented iPP substrates can be studied due to the higher melting point of iPP, the faster crystallization rate of a molten, oriented HDPE film compared to a nonoriented iPP layer was used to study the crystallization of iPP on the oriented HDPE film by means of transmission electron microscopy (TEM) and electron diffraction (ED). Besides the known epitaxial relationship of HDPE/iPP with their chains 50° apart, two new orientation relationships with (a) chains of both polymers parallel and (hk0)_iPP in contact with the HDPE substrate, and (b) the a‐axis of iPP crystals parallel to the chain direction of HDPE but (001)_iPP in contact with the HDPE substrate were observed. Both orientations are assumed as graphoepitaxy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1893–1898, 1999