Directional solidification of isotactic and atactic polypropylene blends

The directional solidification of polypropylene (PP) films results in an oriented semicrystalline microstructure and may offer a method to improve the properties of a product. The directional solidification of isotactic PP samples blended with 0% to 50% atactic PP, by mass, was therefore studied. The effects of composition and processing conditions were monitored to determine how they affect the quality and microstructure of the directionally solidified films. Difficulty was encountered in reproducing testable samples with a unidirectional crystal microstructure. Tensile testing of directionally solidified films was used to quantify the yield strength and elastic modulus of the films. These properties were compared with those of other PP films. The tensile test results do not support the hypothesis that enhanced mechanical properties were produced by directional solidification of the PP films. Improving the sample fabrication method and optimizing the processes involved may, however, lead to directionally solidified PP films with enhanced mechanical properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1516–1528, 2000     

Ultradrawing and crystallization of isotactic polystyrene and blends with atactic polystyrene

Pure isotactic polystyrene (iPS, M_w = 8.89 × 10⁴, M_w/M_n = 4.89) and its blends with an atactic polystyrene (aPS, M_w = 3.9 × 10⁵, M_w/M_n < 1.13) were subjected to draw by solid state coextrusion at 127°C within polyethylene. The content of amorphous iPS in these blends was varied from 100 to 24.4 wt %. The extent of draw-induced crystallization was found to depend on the draw ratio and on iPS concentration. The blend with 24.4% iPS was coextruded in two stages. The highest effective draw ratio (EDR) was 7.6 and 13.7 for one- and two-stage draw, respectively. The highest crystallinity of 33.2% was obtained for pure iPS at the maximum EDR of 7.6. Considerable crystallinity was induced in blends, requiring successively higher draw ratio to reach similar crystallinity with increased aPS content. The tensile modulus increased from 1.5 to 3.2 GPa, independent of iPS concentration. Thermal shrinkage results indicate that the elastic recovery of draw in the blends is near quantitative for an EDR < 8. For pure iPS, extrudate elastic recovery was dramatically altered by the draw-induced crystallinity.     

The atactic polystyrene molecular weight effect on the thermal properties and crystal structure of syndiotactic polystyrene/atactic polystyrene blends

This work examined how the molecular weight of atactic polystyrene (aPS) affects the thermal properties and crystal structure of syndiotactic polystyrene (sPS)/aPS blends using differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction (WAXD) technique. For comparative purposes, the structure and properties of the parent sPS was also investigated. The experimental results indicated that these blends showed single glass transition temperatures (T_gs), implying the miscibility of these blends in the amorphous state regardless of the aPS molecular weight. The non-isothermal and isothermal melt crystallization of sPS were hindered with the incorporation of aPSs. Moreover, aPS with a lower molecular weight caused a further decrease in the crystallization rate of sPS. Complex melting behavior was observed for parent sPS and its blends as well. The melting temperatures of these blends were lower than those of the parent sPS, and they decreased as the molecular weight of aPS decreased. Compared with the results of the WAXD study, the observed complex melting behavior resulted from the mixed polymorphs (i.e. the α and β forms) along with the melting-recrystallization-remelting of the β form crystals during the heating scans. The degree of melting-recrystallization-remelting phenomenon for each specimen was dependent primarily on how fast the sPS crystals were formed instead of the incorporation of aPSs. Furthermore, the existence of aPS in the blends, especially the lower molecular weight aPS, apparently reduced the possibility of forming the less stable α form in the sPS crystals.     

Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene

The crystallization kinetics and morphology development of pure isotactic polypropylene (iPP) homopolymer and iPP blended with atactic polypropylene (aPP) at different aPP contents and the isothermal crystallization temperatures were studied with differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscopy. The spherulitic morphologies of pure iPP and larger amounts of aPP for iPP blends showed the negative spherulite, whereas that of smaller amounts of aPP for the iPP blends showed a combination of positive and negative spherulites. This indicated that the morphology transition of the spherulite may have been due to changes the crystal forms of iPP in the iPP blends during crystallization. Therefore, with smaller amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends increased with increasing aPP and presented a lower degree of perfection of the γ form coexisting with the α form of iPP during crystallization. However, with larger amounts of aPP, the spherulitic density and overall crystallinity of the iPP blends decreased and reduced the γ‐form crystals with increasing aPP. These results indicate that the aPP molecules hindered the nucleation rate and promoted the molecular motion and growth rate of iPP with smaller amounts of aPP and hindered both the nucleation rate and growth rate of iPP with larger amounts of aPP during isothermal crystallization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1093–1104, 2007     

High modulus of isotactic polypropylene attained by coextrusion with atactic polystyrene

Orientation-induced crystallization of crystallizable polymer melts can occur, under certain conditions, during flow through converging channels. Attempts have been made to achieve this phenomenon in a two-phase system, i.e., during simultaneous extrusion of a continuous concentric core of polypropylene within a polystyrene matrix through a conical duct. On one occasion, using Carlona P SY6100 (MFI = 11.0) with Hostyren N2000-V-01 (MFI = 25.0), a highly oriented polypropylene thread with a modulus of 14.6 GPa and a melting point of 178°C was extruded at a die temperature of approximately 170°C and a pressure lower than 40 MPa. It is, in principle, possible to form highly oriented, fiber-like structures as reinforcing elements in a polymer matrix.     

Thermal and structural properties of blends of isotactic with atactic polystyrene

Blends of isotactic polystyrene (iPS) with non-crystallizable atactic polystyrene (aPS) were studied by differential scanning calorimetry and small angle X-ray scattering. The iPS/aPS blends, prepared by solution casting, were found to be miscible in the melt over the entire composition range. Both quenched amorphous and semicrystalline blends exhibit a single, composition-dependent glass transition temperature, depressed from that of either of the homopolymer components. Addition of aPS causes a decrease in crystallinity and in the rigid amorphous fraction, and suppression of the reorganization/recrystallization of iPS during thermal scanning: only one melting peak is observed for blends with larger aPS content. Formation and devitrification of the rigid amorphous fraction of iPS are also affected by aPS addition. The annealing peak, which is due to the relaxation of rigid amorphous fraction in parallel with melting of a tiny amount of crystals, is retarded with an increase of the composition of aPS, resulting in the slow devitrification of RAF in parallel with the melting of large amount of crystals. X-ray scattering shows that the long period in the iPS/aPS blends is greater than in the iPS homopolymer, and long period increases slightly as aPS content increases. Comparison of the volume fraction of phase 1 with the volume fraction crystallinity from DSC suggests that more and more amorphous phase is rejected outside the lamellar stacks as aPS content increases. The effect of aPS addition is to reduce the confinement of the amorphous phase chains. The cooperativity length, ξ_A, which is calculated from thermal analysis of the T_g region, increases with aPS addition. The interlamellar and extra-lamellar amorphous chains both contribute to the glass transition relaxation process.     

Syndiotactic polystyrene‐b‐atactic polypropylene block copolymer alloy as a compatibilizer for syndiotactic polystyrene/isotactic polypropylene blends

A polymeric alloy (SP–A) containing syndiotactic polystyrene (sPS), atactic polypropylene (aPP), and about 66 wt % sPS‐b‐aPP diblock copolymer, was prepared by the sequential feed of monomers in the presence of the half‐titanocene Cp*Ti(OBz)₃ (where Cp* is C₅Me₅ and Bz is PhCH₂), modified methylaluminoxane, and external triisobutylaluminum. The effects of the SP–A alloy as a compatibilizer for sPS and isotactic polypropylene (iPP) blends were evaluated. The blending of sPS and iPP, with and without SP–A, was performed in a single‐screw miniextruder with a side channel that allowed the continuous recycling of materials. The influence of SP–A on the mechanical and thermal properties of the immiscible sPS/iPP blends was investigated over a range of composition. The presence of the SP–A alloy resulted in a significant improvement of the impact strength of the blends compared with that of pure sPS and their pure blends. This improvement was particularly obvious in the sPS/iPP (90/10 wt %) blend containing 5 wt % SP–A. Morphological analysis of the impact‐fractured surface of the ternary blends indicated that the sPS‐b‐aPP diblock copolymer contained in the SP–A alloy acted as an efficient compatibilizer by decreasing the dispersed‐phase iPP particle size, improving the interfacial adhesion, and generating a stable microphase‐separated state. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1596–1605, 2003     

Miscibility, crystallization and morphologies of syndiotactic polystyrene blends with isotactic polystyrene and with atactic polystyrene

Two-component blends of differing polystyrene (PS), one syndiotactic (sPS) and the other isotactic (iPS) or atactic (aPS), were discussed. The phase behavior, crystallization and microstructure of binary polystyrene blends of sPS/iPS and sPS/aPS with a specific composition of 5/5 weight ratio were investigated using optical microscopy (OM), differential scanning calorimetry, wide-angle X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Based on the kinetics of enthalpy recovery, complete miscibility was found for the sPS/aPS blends where a single recovery peak was obtained, whereas phase separation was concluded for the sPS/iPS blends due to the presence of an additional recovery shoulder indicating the heterogeneity in the molten state. These findings were consistent with OM and SEM observations; sPS/iPS exhibits the dual interconnectivity of phase-separated phases resulting from spinodal decomposition.Both iPS and aPS have the same influence on the sPS crystal structure, i.e., dominant β-form sPS and mixed α-/β-form sPS obtained for melt-crystallization at high and low temperatures respectively, but imperfect α-form sPS developed when cold-crystallized at 175 °C. Co-crystallization of iPS and sPS into the common lattice was not observed regardless the thermal treatments, either cold or melt crystallization. Due to its slow process, crystallization of iPS was found to commence always after the completion of sPS crystallization in one-step crystallization kinetics. Segregation of rejected iPS component during sPS crystallization was extensively observed from TEM and SEM images which showed iPS pockets located between sPS lamellar stacks within spherulites, leading to the interfibrillar segregation, which was similar with that observed in the sPS/aPS blends. The addition of iPS (or aPS) component will reduce the overall crystallization rate of the sPS component and the retardation of crystal growth rates can be simply accounted by a dilution effect, keeping the surface nucleation intact. The phase-separated structure in the sPS/iPS blend shows a negligible effect on sPS crystallization and the signature of phase separation disappears after sPS crystallization. Depending on the relative dimensions of the segregated domains and iPS lamellar nucleus, subsequent crystallization of iPS can proceed to result in a crystalline/crystalline blend, or be inhibited to give a crystalline/amorphous blend morphology similar with that of sPS/aPS blends.     

Preparation of isotactic polypropylene/polystyrene blends by diffusion and subsequent polymerization of styrene in isotactic polypropylene pellets

Isotactic polypropylene (iPP) pellets were used to prepare isotactic polypropylene/polystyrene (iPP/PS) blends by diffusion and subsequent polymerization of styrene in water medium, with initiator benzoyl peroxide (BPO) added after diffusion of styrene. Two methods, differencing in whether the excess monomer was removed after diffusion, were used and parameters influencing PS contents were investigated. Diffusion kinetic study showed that the diffusion coefficient at 90 °C is 2.8 times that at 80 °C due to the α relaxation of iPP segments. Investigation on the distribution state of styrene in the mixture before diffusion revealed that most styrene adsorbed on the surface of iPP pellets, and thus the diffusion behavior of styrene into iPP pellets in water medium is similar to that in bulk styrene. Phase morphology of a typical iPP/PS blend showed an average particle size of about 90 nm in the inner part of the iPP pellets. The diametrical distributions of PS showed that styrene can diffuse up to the center of the pellets at 90 °C. It is important to note that the depth of styrene is limited when the initiator BPO is added along with styrene, providing a good explanation for the previously reported ineffective solid-state modification of melt-extruded iPP pellets.     

Residual solvent removal and n-hexane sorption in blends of atactic and isotactic polystyrene

Atactic polystyrene, isotactic polystyrene, and intermediate polyblend films of the homopolymers were cast from solutions of 1,3,5-trimethylbenzene (mesitylene) or o-chlorotoluene. Residual solvent content, glass transition temperatures, and densities of the polyblends were determined. Consequent to these preliminary characterizations, n-hexane absorption kinetics and equilibria in unannealed blends were determined; maxima in the plots of weight change versus time were apparent for all compositions. The maximum is a consequence of the ‘unlocking’ of previously trapped residual casting solvent consequent to plasticizing invasion by the n-hexane in the unannealed film specimens. Neutron activation analysis for chlorine confirmed that the o-chlorotoluene content in the films was reduced dramatically consequent to n-hexane absorption. Residual solvent content was also reduced by thermally annealing the films at 110°C for 48 h under a vacuum and, therefore, no maximum in the sorption/time behaviour was observed in thermally annealed samples nor in films previously contacted with n-hexane vapour and subsequently vacuum stripped. Both the rate of sorption and the apparent sorption equilibrium are greatly reduced as the isotactic polystyrene content is increased. Predominantly Case II or relaxation controlled absorption kinetics were observed in the polyblends consequent to thermal annealing and/or ‘solvent’ annealing. Although the form of the sorption kinetics was quite similar for absorption of n-hexane in thermally annealed or ‘solvent-annealed’ films, the sorption rate and amount of penetrant sorbed at apparent equilibrium were both larger for the thermally annealed films. These results suggest that the history dependence, which so frequently affects polymeric glasses, appears to confound a simple interpretation of the sorption experiments reported here. Consistent with observations in related systems, Super Case II kinetics were observed for the more slowly sorbing films which, in this case, contained isotactic polystyrene. Moreover, film whitening related to micro-voiding accompanied n-hexane sorption in blends containing atactic polymer; however, the whitening in the isotactic polymer was less pronounced and densification, presumably related to solvent induced crystallization, occurred consequent to long term exposure of the isotactic polymer to n-hexane.     

Plastic deformation and subsequent crystallization of thin films of isotactic/atactic polystyrene (iPS/aPS) blends

Thin films of crystallized and non crystallized isotactic polystyrene (iPS) and its blends with atactic polystyrene (aPS) were deformed below their glass transition temperature T_g. Deformation occurs in a very narrow ‘deformation zone’ (λ = 4 nm) and the deformed material exhibits long range order independent of the crystallinity of the films till concentrations of aPS up to 15%. Films containing more than 20% aPS do not show long range order within the deformed material even after subsequent annealing above T_g. From these results, which were obtained by transmission electron microscopy and electron diffraction, the conclusion is drawn that the molecular processes of crazing in amorphous polymers and the high local deformation of polymer single crystals are obeying similar mechanisms.     

Thermal degradation kinetics of isotactic and atactic polypropylene

The thermal degradation of isotactic and atactic polypropylene was investigated in bulk and in solution. The degradation in bulk was studied with thermogravimetric analysis, and the degradation in solution was studied by the dissolution of the polymer in paraffin oil. The degradation in solution was investigated from 230 to 350°C. The effect of the hydrogen donor on the degradation of the polymer in solution was also studied at 350°C. Continuous distribution kinetics were employed to model the degradation kinetics for the degradation in solution. The rate coefficients were obtained, and the activation energy was calculated from an Arrhenius plot. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2206–2213, 2003     

Fabrication and morphology development of isotactic polypropylene nanofibers from isotactic polypropylene/polylactide blends

The excellent characteristics of polymeric nanofibers with diameters less than 1 μm such as the enormous specific surface result in a dramatic increase in a variety of functional applications. In this article, polymer blends of isotactic polypropylene (iPP) and polylactide (PLA) were fabricated through a twin‐screw extruder. The extrudates were prepared at various processing conditions and the iPP nanofibers were obtained by removal of the PLA matrix from the drawn samples. The influences of drawing ratio, the processing temperature, and the blend ratio of iPP/PLA on the morphology development of iPP phase were investigated by scanning electron microscopy. It was found that the uniformed iPP nanofibers with averaged diameters less than 500 nm were fabricated by the suitable processing parameters. Otherwise, the processing immiscibility and rheological behavior of iPP/PLA blends were studied by means of dynamic mechanical analysis and capillary rheometer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

间规聚苯乙烯/等规聚丙烯共混物的热性能

用差示扫描量热法(DSC)、动态力学分析(DMA)和热重法(TGA)等研究了间规聚苯乙烯/等规聚丙烯(sPS/iPP)、sPS与聚-1-丁烯(B30)组成的嵌段共聚物和sPS/iPP/B30共混体系的热性能。结果表明sPS/iPP是不相容体系,B30与iPP可相容,B30可作为sPS/iPP共混物的相容剂。DSC和DMA分析结果表明,加入适量的B30时,sPS/iPP/B30共混物中iPP的玻璃化转变温度随B30加入量的增加而逐渐升高,而sPS的玻璃化转变温度则随B30加入量的增加而逐渐降低;TGA分析结果表明共混体系的热失重温度高于300℃,比iPP的高。     

Physical characterization of isotactic polypropylene/ethylene-propylene copolymer blends

The properties of solution-mixed blends of isotactic polypropylene (iPP) and an amorphous ethylene-propylene copolymer (EPM), containing 40 wt.-% of ethylene, were studied throughout the composition range. The techniques used were differential scanning calorimetry (DSC), dynamic mechanical spectroscopy, and optical microscopy. Morphological examination revealed a heterogeneous structure. The dynamic mechanical technique at isochronous conditions (110 Hz) between -120 and 160°C confirmed the heterophase nature of the blends and revealed limited phase mixing of the amorphous components of the two constituents. DSC measurements of specimens quenched at 0°C, indicated that EPM rubber promotes crystallization of the iPP phase. Appropriate mechanics models tested in the complete composition range led to the conclusion that the system is incompatible, with rubber spherical inclusion strongly adhering to the iPP matrix, up to an EPM volume fraction Φ (EPM) ? 0.70. At higher EPM compositions, matrix reversal occurs — a fact corroborated by the available mechanical data for this blend.     

Stereoblock polypropylene/isotactic polypropylene blends. II. Mechanical behavior

The mechanical properties of stereoblock polypropylene/isotactic polypropylene blends have been analyzed at different temperatures and at large deformations. The samples at a lower content of isotactic polypropylene show good elastic properties, both in terms of deformation reversibility and in terms of energy dissipation in the hystersis cycles. Considering the dependence of the elastic behavior on the temperature, the model that can be suggested is based on the presence of a physical network in which the cross-linking is due to cocrystallization between the matrix (the stereblock polypropylene) and the crystalline domains of isotactic polypropylene. © 1995 John Wiley & Sons, Inc.     

Stereoblock polypropylene/isotactic polypropylene blends. I. Phase organization

The phase organization of stereoblock polypropylene/isotactic polypropylene blends has been analyzed. The different samples were prepared by a two-stage process, that is, solvent casting followed by pressure molding. The analysis was carried out using techniques such as X-ray diffraction, differential scanning calorimetry (DSC), and thermomicroscopy. The experimental results show that both the components segregate a crystalline phase and that the overall crystallinity, as well as the crystallinity of each component, is affected by the sample composition. Some evidence of co-crystallization, at least for samples at low content in isotactic polypropylene, has been found. © 1994 John Wiley & Sons, Inc.     

Compatibilizing effect of a diblock copolymer of isotactic polystyrene and isotactic polypropylene in blends of the corresponding homopolymers

A high molecular weight polymer mixture was obtained from a sequential Ziegler–Natta polymerization of styrene and propylene. After removing unwanted homopolymers from the reaction product, the remaining copolymer was subjected to extensive molecular and morphological characterization. The results of these experiments indicate that the purified material is a diblock copolymer of isotactic polystyrene and isotactic polypropylene, with each block having a molecular weight in the range of 225,000 g/mol. This block copolymer was incorporated in varying amounts into blends of isotactic polystyrene and isotactic polypropylene homopolymers. At low concentrations of copolymer (<20 wt %), the diblock functions as a dispersing agent, significantly reducing the size of domains in the heterogeneous blends. At higher copolymer concentrations, a dramatic improvement in the adhesion across the domain boundaries is also observed. The favorable effects of the copolymer on mechanical properties are demonstrated in the results of tensile impact experiments.     

Characterization of EPDM/atactic polypropylene blends by high-resolution solid-state NMR

The study of EPDM/atactic polypropylene blend compatibility was investigated by cross-polarization/magic angle spinning carbon-13 NMR spectra, variation contact time experiment analysis, proton spin-lattice relaxation time in the rotating frame, carbon-13 spin-lattice relaxation time, and a dipolar dephasing experiment. The data are discussed in terms of mobility and compatibility of polymeric blends. © 1996 John Wiley & Sons, Inc.