Crystallization and orientation behaviors in isotactic polystyrene and poly(2,6-dimethylphenylene oxide) blends

The crystallization and orientation behavior in the miscible iPS/PPO blends were studied aiming at producing oriented materials consisting of iPS crystals and amorphous PPO chains. Oriented films of iPS/PPO blends were prepared by drawing the melt-quenched blend films. The films were heat-treated under constraint at the drawing temperature so as to crystallize the molecular chains of iPS in the oriented state. The crystallinity and the crystal orientation in the drawn annealed films were studied by the wide-angle X-ray diffraction (WAXD), and the orientation behaviors of molecular chains were analyzed by polarized FTIR spectroscopy. WAXD diagrams show the presence of the highly oriented crystalline structure of iPS in the drawn annealed films of pure iPS and iPS/PPO=7/3 blend. The polarized FTIR spectra of drawn annealed films suggest that the molecular orientation of the amorphous chains of PPO and iPS is markedly relaxed by the heat treatment, although the orientation of iPS with 3₁ helical structure was retained during the oriented crystallization. It was concluded that the drawn annealed samples of the iPS/PPO=7/3 blend consist of highly oriented iPS crystals and nearly isotropic amorphous materials. The mechanical properties of the oriented iPS/PPO blends were measured not only in the stretching direction but also perpendicular to the stretching direction. It was shown that the ultimate strength in the perpendicular direction is 4-5 times higher in the drawn annealed film of iPS/PPO=7/3 blend than in the drawn annealed iPS. The improvement in the vertical strength in the blend is discussed in relation to the structural characteristics of the iPS/PPO blend.     

Unique orientation textures formed in miscible blends of poly(vinylidene fluoride) and poly[(R)-3-hydroxybutyrate]

The oriented crystallization of poly[(R)-3-hydroxybutyrate] (PHB) in the miscible blends with poly(vinylidene fluoride) (PVDF) was investigated with various compositions. The PVDF/PHB blend films were prepared by solution casting and subsequent melt-quenching in ice water. Oriented films of the blends were prepared by uniaxially stretching the melt-quenched film at 0 °C in ice water using a hand-operated stretching apparatus. The oriented blend films were heat-treated at a fixed length in order to crystallize PHB in the oriented state. The crystal orientation and the lamellar textures of the obtained samples were studied with wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS), respectively. The SAXS measurements showed that a considerable amount of molecular chains of PHB are excluded from the lamellar stacks of PVDF and exist in the interfibrillar regions in the oriented films of the blends. The cold crystallization of PHB in the interfibrillar region results in the orientation of PHB crystals, and the type of crystal orientation depends upon the composition of the blends. For the PVDF/PHB=4/6-7/3 blends, the crystal a-axis of PHB is highly oriented parallel to the drawing direction and the crystal c-axis (molecular chain axis) in PHB crystals is perpendicular to the drawing direction, i.e. orthogonal to the chain axis of the crystals of PVDF. It is considered that the a-axis orientation is induced by the confinement of crystal growth in the interfibrillar nano-domains. For the PVDF/PHB=2/8-3/7 blends, however, the crystal c-axis of PHB is primarily oriented in the drawing direction, suggesting that the stressed molecular chains of PHB are crystallized with the molecular orientation retained.     

Structure and mechanical properties of uniaxially oriented films of syndiotactic polystyrene and poly(2,6‐dimethyl‐1,4‐phenylene oxide) blends

The structure and mechanical properties of highly oriented films of a miscible blend of syndiotactic polystyrene and poly(2,6‐dimethylphenylene‐1,4‐oxide) (sPS/PPO) were studied in the composition range of sPS/PPO = 10/0 to 5/5. The oriented films were prepared by stretching the amorphous films of the blends. Wide‐angle X‐ray diffraction and polarized FTIR spectroscopy were used to analyze the amount of mesophase and molecular orientation. Drawing of the amorphous films of sPS and sPS/PPO blend induced a highly oriented mesophase. The mesophase content increases with increasing draw ratio and becomes nearly constant above a draw ratio of 3. Under the same draw ratio, the mesophase content decreases with increasing PPO content. The orientation function in the mesophase is as high as 0.95–0.99 irrespective of the composition and draw ratio. On the other hand, the orientation of molecular chains in the amorphous phase and mesophase increases with increasing draw ratio, and it decreases with increasing PPO content. The drawn films of pure sPS show high strength and high modulus in the drawing direction, but exhibit low strength in the direction perpendicular to the drawing. In the case of sPS/PPO = 7/3 blend, however, the ultimate strength in the perpendicular direction was dramatically improved compared with that of pure sPS and the ultimate strength in the parallel direction was similar to that for the oriented pure sPS. The improved mechanical properties in the sPS/PPO blends were discussed in relation to the structural characteristics of the sPS/PPO blend system. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:2789–2797, 2004     

Chain orientation resulting from the crystallization under strain of poly(ϵ‐caprolactone) in miscible blends with poly(styrene‐co‐maleic anhydride)

Amorphous miscible blends of poly(?‐caprolactone) (PCL) with a random copolymer of styrene–maleic anhydride (SMA) were stretched at room temperature and the crystallization of PCL was allowed to develop under strain. The crystallization of PCL, from oriented amorphous chains, leads to macroscopic crystalline chain orientation. Using infrared dichroism and wide‐angle X‐ray scattering techniques, it was found that the resultant crystalline orientation of PCL is always parallel to the strain direction, regardless of the experimental conditions, which is in contrast with the PCL/poly(vinyl chloride) blend, where both parallel and perpendicular chain orientations can be induced. Furthermore, the degree of crystalline orientation of PCL is influenced by the initial stretching‐induced amorphous chain orientation of the system. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1683–1690, 2001     

Mechanical properties of oriented poly(vinyl chloride)–poly(caprolactone) blends

Blends of poly(vinyl chloride) (PVC) with polycaprolactone (PCL) of different compositions were prepared from solutions in tetrahydrofuran (THF). The dried blends were stretched at different temperatures above the glass transition, and the birefringence and mechanical properties were studied. It is shown that the birefringence of PVC and the 75/25 PVC/PCL blend follows an affine deformation scheme with a decreasing number of segments with deformation. The 50/50 PVC/PCL blend shows a complex orientation behavior because of the presence of crystallinity in the PCL phase. The mechanical properties of the blends are shown to increase with orientation, and the aggregate model is acceptably followed by the amorphous oriented blends.     

Micro-phase separation and crystallization behavior of amorphous oriented PLLA/PVAc blends during heat treatment under strain

Amorphous oriented poly(l-lactide) (PLLA)/poly(vinyl acetate) (PVAc) 50/50 films were prepared by uniaxial drawing of melt-mixed blends at 65 °C. The morphology development and crystal organization of the blends during heat treatment under strain were investigated using small angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Equatorial scattering maxima in the SAXS patterns for samples annealed at 75 °C were observed before the appearance of crystal reflections. Further annealing of the samples at higher temperature induced two further discrete meridian scattering maxima. The observations indicated that homogenous oriented PLLA/PVAc film undergoes micro-phase separation first, followed by crystallization of PLLA in the PLLA-rich phase. The micro-phase separated PVAc nanodomains are aligned parallel to the stretching direction, whereas the crystallized PLLA lamellae are oriented perpendicular to the stretching direction (crystal c-axis along the stretching direction). Micro-phase separation was not observed when films were annealed at 120 °C, at which temperature the high crystallization rate of PLLA overwhelmed the micro-phase separation process.     

Comparison of crystallization kinetics of miscible blends of syndiotactic polystyrene with atactic polystyrene or poly(1,4-dimethyl-p-phenylene oxide)

Crystallization kinetics were characterized by using isothermal differential scanning calorimetry (DSC) analysis on neat semicrystalline syndiotactic polystyrene (s-PS) and its miscible blends with atactic (amorphous) polystyrene (a-PS) or with poly (1,4-dimethyl-p-phenylene oxide) (PPO). The crystallization behavior of neat s-PS polymer was compared to its miscible blends. The results suggest that the crystal growth in melt-crystallized s-PS is nearly of homogeneous nucleation and that the growth pattern of the spherulites is 3-D spherical. The s-PS component in its miscible blends was found to crystallize with similar behavior to that of neat s-PS, but with lower Avrami exponents, suggesting that the nucleation mechanism is likely more heterogeneous with blending.     

Comparative study on development of structural hierarchy in constrained annealed simultaneous and sequential biaxially stretched polylactic acid films

Structural evolution during constrained annealing of PLA films biaxially stretched in simultaneous and sequential biaxial stretching was compared. Annealing of simultaneous biaxially stretched films yields films with in-plane isotropy with (100) crystallographic planes parallel to the surface. The first stage of sequential stretching where the films are stretched in Uniaxial constrained mode was found to yield films exhibiting transverse isotropy. The transverse stretching of these films lead to formation of a distinct second population of chains primarily oriented in transverse direction, generating bimodal orientation texture. When the extent of stretching in two directions are balanced, constrained annealed samples were found to exhibit uniplanar axial (100)[001] texture with the primary chain axial direction now switched to transverse direction. Two new superstructures, along [110] and [100] respectively were discovered in annealed PLA films.     

Shear induced shish-kebab structure in PP and its blends with LLDPE

In order to better understand the effect of shear stress on the crystal morphology and orientation of polyolefins, dynamic packing injection molding was used to prepare oriented pure polypropylene (PP) and its blends with linear low density polyethylene (LLDPE). The obtained samples were characterized via 2d-SAXS, 2d-WAXD and AFM. Macroscopically, shear induced morphology with surface skin, central core and oriented layer between the skin and the core was observed in the cross-section areas of the samples. For pure PP, a highly oriented structure was seen in the sheared layer but much less oriented structure exists in the core. The orientation in the skin lies in between. The shish-kebab structure, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), was found in the sheared layer. Shish structure exists mainly in the skin layer and oriented spherulits dominates in the core. For PP/LLDPE (50/50) blends, a change of phase morphology from less-phase-separated structure (homogeneous) in the skin, to co-continuous structure in the sheared layer and sea-island structure in the core was observed. PP formed a shish-kebab structure in all the three layers. And on the other hand, a very unique crystal morphology and lamellar orientation of LLDPE were obtained, with the lamellar stack oriented either perpendicularly or 45-50° away from the shear flow direction.     

Investigation of the effect of chain rigidity on orientation of polymer blends: the case of poly(vinyl phenol)/poly(ethylene terephthalate) blends

Orientation of amorphous, miscible poly(vinyl phenol) (PVPh)-poly(ethylene terephthalate) (PET) blends is studied using experimental and modelling techniques. Up to 50 wt% PVPh, the blends are semi-crystalline and were therefore not studied. At 60 wt% PVPh, no crystallisation was observed using either differential scanning calorimetry or X-ray diffraction. For the 60wt% PVPh blend, FTIR dichroism determination showed that orientation was relatively high (0.09 at a λ=3.4) and similar for both polymers. Above 60 wt% PVPh, however, no appreciable orientation was detected. In order to gain insights about the deformation phenomena in polymer blends, atomistic models of the 60 wt% PVPh composition were built using the Meirovitch approach. These were found in good agreement with X-ray diffraction, and exhibit a fair degree of interpenetration as estimated visually and by comparing intermolecular pair distribution functions. Significant hydrogen bonding was found: 8% of carbonyl oxygens and 1% of carboxylate oxygens of PET are bound to PVPh. Deformation simulations were performed using the Parrinello-Rahman deformation scheme. Orientation of PVPh and PET in the blend was found equivalent to that observed in pure polymers simulations. PET orientation followed the aggregate model and was more oriented than PVPh by a factor of two. It was concluded that the similarity in orientation of the two polymers in the blend, which was observed experimentally on quenched samples, could be due to a combination of different deformation-induced orientation followed by distinct relaxation mechanism and relaxation times for both polymers.     

Impact of different die draw ratio on crystalline and oriented properties of polypropylene cast films and annealed films

Four kinds of polypropylene (PP) cast films with different die draw ratios (DDR) were prepared. The impact of different DDR on the crystalline and oriented properties of PP cast films and annealed films was explored herein. Wide angle X-ray diffraction (WAXD) and fourier transform infrared (FTIR) methods were adopted to examine the orientation degree of crystalline and amorphous phases. Long period distance (L_p) of the crystalline structure was tested by small angle X-ray scattering (SAXS). Crystallization was determined by differential scanning calorimeter (DSC). The oriented and crystalline behaviors of the samples were carried out by the elastic recovery (ER) testing. Then, samples after being annealed were examined by the same methods. The influence of annealing process on the films’ structures and properties was explored. Besides, the final stretched microporous membranes manufactured via stretching the annealed films along machine direction were examined by scanning electronic microscope (SEM). No matter for cast films or for annealed films, it is found that the films’ orientation degree of crystalline and amorphous phases, as well as L_p and crystallinity are larger at higher DDR and relatively lower at lower DDR. When the DDR is overly high (DDR?=?170), both the oriented and crystalline properties will decline. Elastic recovery testing indicates that a film with better orientation of the crystalline and the amorphous phases as well as with higher crystallinity can be obtained at an appropriate DDR. SEM images show that stretched membranes with better microporous structure can be obtained when the precursor film is prepared at a proper DDR.     

Properties and morphology of oriented ternary blends of poly(ethylene terephthalate), high density polyethylene,and compatibilizing agent

Oriented blends of poly(ethylene terephthalate) (PET) and high density polyethylene (HDPE) with and without compatibilizing agent have been studied with regard to orientation temperature, stretch rate, extension ratio, mode of orientation, and blend composition. These oriented blends have been characterized using infrared spectroscopy and differential scanning calorimetry. The tensile and tensile impact properties were also investigated. The results show that blends with compatibilizer show strain hardening upon orientation, whereas the blend without compatibilizer does not strain harden upon orientation. The blends with less PET content have been difficult to orient. The morphology of these blends show fibril structure, highly oriented in the direction of stretch. Infrared measurements show that PET within the blend has undergone strain induced crystallization upon orientation. It has also been observed that the mechanical properties, such as the modulus and ultimate stress, show improvement upon orientation. Simultaneously stretched blends show better physical properties than sequentially oriented blends.     

Conformational motions in immiscible blends of polycarbonate and styrene-acrylonitrile copolymers

Blends of commercial bisphenol-A -polycarbonate and styrene-acrylonitrile copolymers were prepared by precipitation in ethanol from the solution in methylene chloride in order to eliminate the low molecular weight substances contained in the commercial polymers, specially the oligomers contained in commercial SAN copolymers. Two glass transitions appear in the DSC thermograms of the blend at the same temperatures as in the pure components which, in principle, indicates that the blend consists of two phases formed by pure PC and pure SAN. In order to detect small changes in the glass transition process that could be indicative of different mobility of the polymer chains in the blend with respect to the pure polymers, blends of different compositions were subjected to different thermal treatments that included annealing at temperatures below both glass transitions, and then the DSC thermograms were recorded. A broadening in the peaks shown by the c_p(T) curves measured on annealed samples in the zone of the PC transition is detected while no significant differences are shown by the glass transition of the San phase of the blend with respect to pure SAN copolymer. Dielectric relaxation experiments in the frequency domain (from 100 to 3·10⁶Hz) were carried out on the blends. The dielectric relaxation spectrum in the zone of the SAN main relaxation process was fitted with the stretched exponential equation showing no significant difference between the blends and the pure SAN copolymer. The region of the main relaxation process of PC was not analyzed due to the small polar activity of PC and the overlapping with the relaxation of the SAN phase.     

Structure and properties of biaxial‐oriented crystalline polymers by solid‐state crossrolling

The effect of biaxial orientation by solid‐state crossrolling on the morphology of crystalline polymers including polypropylene (PP), high density polyethylene (HDPE) and Nylon 6/6 was investigated with polarized optical microscopy, atomic force microscopy, wide‐angle X‐ray scattering, and small‐angle X‐ray scattering techniques. It was found that crossrolling gradually changed the initial spherulitic structure into a biaxially oriented crystal texture with chain axis of crystals becoming parallel to the rolling direction for all three polymers. The effect of microstructure change on the macromechanical properties was studied in tension at both ambient temperature and ?40°C. In tension at room temperature, the localized necking deformation of HDPE and PP control changed upon orientation into homogeneous deformation for the entire sample length. This was attributed to that the oriented crystal morphology eliminated the stress concentration, which existed in the original spherulitic structure from lamellae orientation in the polar and equatorial regions. At ambient conditions, the elastic moduli of HDPE and PP were found to decrease slightly with orientation whereas the modulus of Nylon 6/6 increased with increasing orientation. This was due to the fact that the amorphous chains of HDPE and PP are in a rubbery state and orientation increased the shear relaxation in the orientation direction but the amorphous chains of Nylon 6/6 are in the glassy state inhibited the shear relaxation. Both the yield stress and strain hardening exponent increased with increasing orientation for all three polymers. In tension at ?40°C, orientation changed the failure mechanism of all three polymers from brittle fracture into ductile failure, as the original spherulitic structure was changed into an oriented structure with chain axis of crystals becoming parallel to the tension direction, which allowed chain slip deformation of crystals and resulted in oriented samples showing ductile failure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphological and infra-red studies on highly oriented poly(vinylidene fluoride)/poly(methyl methacrylate) blends

Highly oriented melt drawn films of poly(vinylidene fluoride) (PVDF) and blends of poly(vinylidene fluoride) and poly(methyl methacrylate) (PMMA) have been studied by transmission electron microscopy, electron diffraction and infra-red spectroscopy. Infra-red spectra show the second moment of the orientation function for PVDF samples to be greater than 0.94. Using such a sample, the transition dipole directions relative to the chain axis have been calculated. Electron microscopic studies of the PVDF/PMMA blends show a transformation for pure PVDF from a lamellar morphology to a mixture of lamellar and needle-like crystals for the 80/20 blend. The 60/40 blend shows a pure needle-like morphology. The β phase content for this blend is dependent upon the composition and thermal history. An increase in the β phase content is observed with the addition of PMMA. After annealing at 110°C, the 50/50 blend shows a lamellar β phase morphology. A significant increase in the segmental orientation of PVDF is also observed.     

Polymorphism and mechanical properties of syndiotactic polystyrene films

The dynamic-mechanical behaviour and the tensile moduli of unstretched and stretched semicrystalline s-PS films, presenting different polymorphic forms (α, γ, δ and clathrate) but similar crystallinity and orientation, have been compared. The main aim is to elucidate the possible influence of different crystalline phases, being largely different in chain conformation and density, on mechanical properties of s-PS semicrystalline samples. For unstretched films presenting a preferential perpendicular orientation of the chain axes, the highest elastic modulus is observed for films with the high density γ phase while for uniaxially oriented films the highest modulus is observed for films with the trans-planar α phase. As for the clathrate films, the guest molecules when only included into the crystalline clathrate phase, have no plasticizing effect.     

Crystal structure transformations and orientation in crosslinked elastic fibers of an ethylene‐octene copolymer in response to deformation and heat treatment

Crosslinked elastic fibers, made from a low density (0.875 g/cc) ethylene‐octene copolymer, were studied after constrained at 300% elongation and annealed at different temperatures (40–80°C) to simulate conditions encountered in yarn and textile processing. It is surprisingly found that the transition from pseudo hexagonal to orthorhombic structure is much faster under simultaneously constraining and annealing than that without strain. Almost a neat orthorhombic structure can be produced when the fiber is annealed at 60°C. Annealing above 60°C leads to mixed orthorhombic and pseudo‐hexagonal structures. The average melting point increases with an increase in the fraction of orthorhombic phase. It is also surprisingly noted that the simultaneously constraining and annealing of the fiber can produce highly oriented crystals, even annealed at 80°C (above the average melting point of 65°C). The unique effect of annealing under large strain can be attributed to the crosslinking of the fiber, which makes it possible for the fiber to have strong chain orientation (even in molten state) under large strain. The strong chain orientation in melt leads to a faster structural transition from pseudo hexagonal to more stable orthorhombic structure. The strong chain orientation is also very likely the reason why highly oriented crystal and amorphous phases are formed, including the case where the fiber is annealed above melting point. These findings could be leveraged for improving thermal and mechanical properties of the fabrics made with such fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3565–3573, 2013     

Doubly oriented β-form films of poly(vinylidene fluoride)

PVDF sheets, rapidly quenched, were (1) two-step transversely stretched at various temperatures and (2) stretched at various temperatures, rolled at room temperature and then annealed. The orientation patterns of the β-form crystal (which contains the polar b-axis) in these films were analysed on the basis of X-ray diffraction photographs taken with flat and cylindrical cameras. In the case of (1), when both of the two-step transversely stretching temperatures were below 100°C, a doubly oriented film with the plar b-axis oriented parallel to the film surface was obtained. In the case of (2), when the stretching temperature was below 100°C, the sheets then rolled without annealing, another doubly oriented film with the polar b-axis preferentially oriented at 30° to the film surface was obtained. On the other hand, when these films were annealed above 100°C, or the stretching temperatures were above 100°C, orientation patterns in which the polar b-axis was partially rotated through 60° were obtained. The orientation mechanisms of these films are discussed using the measurements of the lattice spacings of the β-form crystal.     

Chain orientation in melt-extruded samples of vectra A,vectra B,and blends in relation to the mechanical properties

The profile of molecular orientation within injection-molded tensile bars of the liquid crystalline copolyesters Vectra A, Vectra B, and Vectra C, as well as blends of these polymers, was investigated by means of wide-angle X-ray scattering (WAXS) using synchrotron radiation (HASYLAB, Hamburg). The local variation of chain orientation was resolved into steps of 100 μm. An even higher resolution was obtained by using the microfocus camera (focal spot 2 μm) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. In Vectra A and in the blend of Vectra A and Vectra B, a smooth variation of the orientation was found being almost zero at the surface and showing its maximum at a distance of 0.6 mm from the surface. The orientation in Vectra B was rather fluctuating. The average chain orientation in the blend samples processed under the same conditions was higher than in samples of the pure liquid crystalline copolyesters. The mechanical properties of the different layers within the injection moldings were determined by cutting the samples into slices and measuring the stress-strain curves. For specimens of comparable orientation, it turned out that the blend samples had the largest values of Young's modulus and tensile strength. The synergism of orientation and mechanical strength was also found for different blend compositions, as well as in blends of Vectra B and Vectra C. Annealing the injection moldings above the melting point resulted in a rapid relaxation of the orientation, whereas the chain alignment persisted at lower temperatures. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 531–545, 1998     

Molecular orientation and relaxation in poly(butylene terephthalate)/polycarbonate blends

Rheo-optical Fourier-transform infrared (FTIR) spectroscopy is based on the simultaneous acquisition of stress-strain data and FTIR spectra on-line to the mechanical treatment of polymers and is frequently applied for the characterization of transient structural changes during deformation and stress-relaxation. In the present communication, this technique has been employed in order to investigate the distribution of molecular orientation and its relaxation in uniaxially drawn solution-cast films of semicrystalline partial miscible blends of poly(butylene terephthalate) (PBT) with polycarbonate (PC) containing 10, 30 and 50 wt% PC. The uniaxial deformation of these blend films having a PBT-crystallinity degree ranging from 31 to 12%, in unstretched blends, leads to a appreciable high segmental orientation for the crystalline PBT due to a structural transformation from lamellae to microfibrils. The formation of this fibrillar structure is attributed to non-reversibility of an extended phase with all-trans conformational sequence of the aliphatic segments of PBT, occurring during elongation. The rate of relaxation of this conformational transition, however, increases with increasing amorphous content in the blends. Also it is observed that even with increasing amorphous content in the PBT/PC blends the crystalline PBT shows significant orientation. In such cases, apart from the few lamellae which transform to microfibrils, it is suggested that a stress induced transformation of PBT chains in amorphous PBT-component to irreversible all-trans extended crystalline form also contributes to PBT crystalline orientation. In contrast with this high crystalline orientation, the amorphous PBT located in the interlamellar regions inside the PBT-spherulites show a lower orientation in blends as compared in pure PBT.On the other hand, an overall segmental orientation of PC chains in blends is of lower order which is attributed mainly to low stretching temperature compared to T_g of pure PC. The results are discussed in terms of the resulting spherulitic morphology and the temporary network formed by the elongated PBT and PC chains inside the interlamellar regions, in blends.