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.     

Melt flow crystallization of ultrahigh molecular weight polyethylene under curvilinear flow conditions

Multiaxially oriented ultrahigh molecular weight polyethylene films were prepared recently under curvilinear flow conditions generated by the combined effects of compression and rotation. The films were prepared by melt flow crystallization in an optical apparatus specially designed for rheo-optical observations. Morphological studies indicate that the films were fibrillar with the fibrils oriented along the circular flow lines. The fibrils were lamellar, and their lamellar thickness (150–450 Å) and spacing (400–2000 Å) depended on the processing conditions. The film exhibited strong birefringence (1.4 × 10^?2) and had a Young modulus of 15–25 GPa along the flow lines and 3–5 GPa in the transverse direction.     

Poly(p-phenylene terephthalamide) films formed from extrusion and coagulation of liquid crystalline sulphuric acid solutions: Characterization of orientation and void structure,annealing, and upgrading of film mechanical properties

Poly(p-phenylene terephthalamide) (PPD-T) films have been prepared by continuous extrusion of liquid crystalline 17 percent PPD-T/sulphuric acid solutions through an annular die followed by coagulation, Films extruded without drawdown exhibit some polymer chain orientation in the machine direction. This is increased by uniaxially drawing down films. Films produced with a lubricated conical mandrel sitting between the die and the coagulation bath exhibit an equal biaxial orientation. The uniaxially oriented films exhibit highly anisotropic mechanical properties, while the mandrel-produced film exhibits balanced properties. Heat treatment at 350°C results in significant enhancement of the tensile strength of the mandrel film. Void structures in the films have been investigated by mass density, scanning electron microscopy (SEM), and small-angle X-ray scattering (SAXS). Density measurement indicate a void content decreasing with decreasing film thickness and heat treatment. SEM locates micron-size voids in the thickest films, apparently caused by rapid coagulation. SAXS indicates much smaller void sixes which are roughly prolate ellipsoids (long axis in machine direction) for uniaxial films and oblate ellipsoids (short axis in thickness direction) for the mandrel produced films. Various techniques are used to estimate mean void size.     

The role of interlamellar chain entanglement in deformation-induced structure changes during uniaxial stretching of isotactic polypropylene

In-situ small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) were carried out to investigate the deformation-induced structure changes of isotactic polypropylene (iPP) films during uniaxial stretching at varying temperatures (room temperature, 60 °C and 160 °C). From the WAXD data, mass fractions of amorphous, mesomorphic and crystal phases were estimated. Results indicate that at room temperature, the dominant structure change is the transformation of folded-chain crystal lamellae (monoclinic α-form) to oriented mesomorphic phase; while at high temperatures (>60 °C); the dominant change is the transformation of amorphous phase to oriented folded-chain crystal lamellae. This behavior may be explained by the relative strength between the interlamellar entangled network of amorphous chains, which probably directly influence the tie chain distribution, and the surrounding crystal lamellae. It appears that during stretching at low temperatures, the interlamellar entanglement network is strong and can cause lamellar fragmentation, resulting in the formation of oriented mesomorphic phase. In contrast, during stretching at high temperatures, the chain disentanglement process dominates, resulting in the relaxation of restrained tie chains and the formation of more folded-chain lamellae.     

Crystal structure and orientation of uniaxially and biaxially oriented PLA and PP nanoclay composite films

Cast films of poly(lactic acid) (PLA) and polypropylene (PP) with 2.5 and 5 wt % organo modified nanoclay were prepared and then uniaxially and biaxially hot drawn at T = 90 and 155°C, respectively, using a biaxial stretcher. The orientation of PLA and PP crystal unit cells, alignment of clay platelets, as well as the extent of intercalation and exfoliation were studied using wide angle X‐ray diffraction (WAXD). The measurement of d‐spacing of the 001 plane (normal to platelets plane) of the clay tactoids indicated the intercalation of the silicate layers for the PLA nanocomposite films, whereas the PP nanofilled films showed only dispersion of the nanoparticles (i.e., neither intercalation nor exfoliation were observed). The intercalation level of the clay platelets in PLA was almost identical for the uniaxially and biaxially drawn films. Our finding showed that the crystallite unit cell alignments are appreciably dependent on uniaxial and biaxial stretching. Moreover, the incorporation of clay to some extent influenced the orientation of the crystal unit cell axes (a, b, and c) of the oriented films. The silicate layers revealed a much higher orientation into the flow direction in the uniaxially stretched films compared to the biaxially drawn samples. In addition, the orientation of the 001 plane of nanoclays was significantly greater in the PLA compared to the PP nanoclay composite films probably due to a better intercalation and stress transfer in the former. Morphological pictograms illustrating the effects of uniaxial and biaxial stretching on the clay orientation are proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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.     

Morphological studies of oriented ultra-high molecular weight polyethylene with enhanced planar mechanical properties

Ultra-high molecular weight polyethylene films prepared by melt flow crystallization under torsional flow conditions were characterized by wide angle (WAXS) and small angle (SAXS) diffraction techniques and scanning electron microscopy (SEM). The films had a fibrillar morphology in which lamellae having an average fold period of 650Å were stacked with their c-axis along the circular flow lines. X-ray analysis showed that the a and b-axes were preferentially oriented along the thickness and radial directions of the sample.     

Nanoscale fibrillar crystals of PET from dilute quiescent solution

The crystallization behavior of poly(ethylene terephthalate) (PET, IV∼2 dL/g) from solution in biphenyl-diphenyl ether mixed solvent is examined. Reversible gelation of the polymer solution is observed during cooling of the solutions. Light scattering and DSC analysis are used to follow the heating and cooling processes, thus determining the crystallization temperature and the melting point, which are found to be nearly independent of the polymer concentration (0.25-5%). High degree of crystallization (>50%) is observed in the PET crystallized from the solution at 170 °C. Morphological characteristics of the crystals obtained after solvent removal are determined by WAXD, FTIR, SEM and TEM examination. The crystallization of PET into unique high aspect ratio fibrillar morphology during cooling of the solutions explains their gelation even at low PET concentration. Thin films made from the thus obtained PET could be drawn five times at 250 °C, resulting in only moderate values of modulus and strength.     

Processing and properties of polyimide melt blends containing a thermotropic liquid crystalline polymer

Several polymer blend compositions of LaRC-TPI 1500 and New TPI 450 (Mitsui Toatsu) with Xydar SRT 900 LCP (Amoco Performance Products) were extrusion processed. In addition to binary blends containing one TPI with an LCP, ternary blends consisting of an alloy containing both TPIs as the matrix were also processed. By varying the ratio of the polyimides in the matrix, the blends' thermal behavior could be tailored. This paper addresses both processing issues and film properties of these blends. Rheological and thermal studies were conducted on both blends made in a torque rheometer and on biaxially oriented film produced with a counter-rotating annular die. These biaxial blend films were further characterized by measuring tensile and electrical properties. For 70/30 New TPI/Xydar equal biaxial films of 50 μm thickness, a modulus of 3.8 GPa and a stress at break of 100 MPa were measured. For near uniaxial blend films (±3°) a modulus of 14.5 GPa and a strength of 220 MPa in the machine direction (MD) were measured. The transverse direction (TD) properties were still higher than the neat New TPI. The electrical properties of these blends were outstanding. The dissipation factor was typically less than 0.01 for most blend compositions. Similarly, the dielectric constant was typically less than 3 up to temperatures as high as 300°C.     

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.     

Structure development during crystallization and solid‐state processing of poly(glycolic acid)

DSC and time‐resolved WAXS and SAXS are used to study the structure development during isothermal crystallization of poly(glycolic acid) (PGA) in the temperature range 180–195°C. It is shown that the crystallization rate increases with degree of supercooling in the temperature range of consideration. WAXS and DSC crystallinity measurements agree well and a final crystallinity of 50% is found independently of the crystallization temperature. In‐situ SAXS measurements indicate that for PGA the final crystal thickness approaches a limiting value of 70 Å independent of the crystallization temperature in the range 195–180°C. The material develops a well‐defined lamellar structure during crystallization at the highest crystallization temperature under study (195°C). We show that by increasing the degree of supercooling it is possible to hinder the formation of the lamellar structure and crystals, resulting in a less ordered structure. We report that PGA fibers with elastic modulus in the range 20–25 GPa can be prepared by adequate control of the structure before solid‐state plastic deformation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dual lamellar crystal structure in poly(vinylidene fluoride)/acrylic rubber blends and its biaxial orientation behavior

The miscibility for melt-mixed poly(vinylidene fluoride) (PVDF)/acrylic rubber (ACM) blends and the crystal morphology of PVDF in the blends were investigated over the whole composition ranges by dynamic mechanical analysis (DMA), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). DMA measurements revealed that PVDF is miscible with ACM in ACM-rich system, and partially miscible in PVDF-rich system. Two kinds of PVDF lamellar structures with different long periods were detected by SAXS and TEM for the partially miscible blends. In the miscible system, only one kind of crystal lamellae with enlarged long period is found. The two kinds of lamellar structures in the blend show different orientation behavior during the uniaxial stretching to result in a biaxial orientation. The lamellae with short long period are oriented vertical to the stretching direction, while those with large long period were found to be oriented parallel to the stretching direction.     

Higher-order structural analysis of bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanote] highly oriented films

Bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBH) highly oriented films were prepared by the combination of roll and uniaxial drawing processes. The change in the higher-order structure of PHBH films was investigated by wide-angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Extended films, which show superior mechanical properties and high ductility, have complicated structures. By roll extension, both deformed and undeformed spherulites co-exist, the former inclined to the direction perpendicular to the film surface. The latter were destroyed by further uniaxial extension. The tie-molecules between uniaxially oriented lamellae were extended and transformed to the β-form with a planar zig-zag conformation. Three kinds of structures, c-axis parallel to the uniaxial drawing direction, c-axis inclining to the normal vector of the film surface, and the β-form between lamellae, are intermingled in the film.     

The effect of heat treatment on the microfibrillar network of poly(p-phenylene benzobisthiazole)

Materials exhibiting exceptional mechanical properties can be fabricated from rigid polymers. In a spinning process, an oriented solution is solidified by the action of a nonsolvent, which induces crystallization of an oriented rigid polymer. Drying and heat treatment result in the final material having the desired properties. The effect of heat treatment on the morphology of poly(p-phenylene benzobisthiazole) (PBZT) films was studied by measurements of small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Films were formed by coagulation in water. The wet films were dried with supercritical CO₂ to maintain an open network structure. Coagulation in water resulted in formation of microfibrils having diameter of ∼9 nm. Heat-treatment at 600°C caused significant coalescence of microfibrils. The crystallite size was increased by heat treatment from a value that is smaller than the microfibrillar diameter indicating defects in chain packing, to a value comparable to the microfibrillar diameter.     

Structural deformation behavior of isotactic polypropylene with different molecular characteristics during hot drawing process

Structural evolution in hot drawing process of isotactic polypropylene (iPP) films with different molecular weight distribution (MWD) and isotacticity (IT) was investigated by in situ time-resolved measurements of synchrotron-sourced wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Any significant difference was not recognized among different molecular characteristics as for the changes in the WAXD patterns, indicating that the deformation behavior viewed at the crystal lattice scale was almost the same among these samples. On the other hand, the deformation behavior of lamellar stacking structure was found to be significantly dependent on the molecular characteristics of the sample used. For iPP sample with narrower MWD and higher IT, only the lamellar stacking structure with c-axis crystallites oriented along the drawing direction was detected at the deformation stage after necking, but the oriented fibrillar structure was observed in addition to the lamellar stacking structure for the iPP sample with broader MWD and IT distribution. The structural deformation models were presented for both the samples with different molecular characteristics, and these models were reasonably related with the difference in the stress-strain curve.     

Structure and properties of MDO stretched polypropylene

Two polypropylene cast films of different crystalline structures (one with coexisting small rows of lamellae and spherulites and the other with only a spherulitic structure) were prepared by extrusion. The produced cast films were uniaxially hot drawn at T = 120 °C using a machine direction orientation (MDO) unit and the changes in structure and morphology were examined and related to barrier as well as tear and puncture properties. Structural changes in terms of the degree of crystallinity and crystal size distribution, orientation of the amorphous and crystalline phases, and the deformation behavior at the crystal lattice and lamellae scales were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), respectively. A significant effect of the original crystal morphology on the alignment of the amorphous and crystalline phases was observed from FTIR and WAXD. The results also revealed that the deformation behavior of the crystal structure was dependent on the draw ratio (DR). Our findings showed that by increasing DR the crystal lamellae first broke up and oriented along the drawing direction and then, at large DR, they were deformed and created a fibrillar structure. Morphological pictograms illustrating the effects of original morphology and draw ratio on the stretched film microstructure are proposed. The tear resistance along the machine direction (MD) decreased significantly with increasing DR whereas the puncture resistance increased drastically. Finally, the oxygen transmission rate (OTR) of the MDO stretched films could be correlated with the orientation parameters as well as the β-relaxation peak magnitude of the amorphous tie chains.     

双轴取向聚乙烯薄膜晶体取向的二维X光衍射分析

聚乙烯膜的晶体取向决定着薄膜的多种力学性能和热力学性能。因此,针对薄膜晶体取向的表征显得非常重要,尤其是具有双轴取向的聚乙烯薄膜。通过二维广角衍射研究了单轴和双轴高取向聚乙烯薄膜的取向度。建立表征取向度的三种方法,包括Herman取向分析法、局部积分法和环向积分法。结果发现,上述方法均适用于所有的双轴取向的高分子薄膜,包括非晶态高分子薄膜。Herman取向分析法可以通过取向因子定量计算简单取向材料取向度;局部积分的方法能分析出衍射较弱方向晶体信息,更适用于取向度较复杂的样品;环向积分法能更直观地分析薄膜的取向特点、取向强度。     

Gelation of poly(vinyl alcohol)/water/dimethylformamide solutions and properties of dried films

The properties of solutions for syndiotacticity-rich poly(vinyl alcohol) (s-PVA)/dimethylformamide (DMF)/water systems, the gelation of the s-PVA solutions, and the properties of the dried s-PVA gel films were examined. From the results of the dissolution temperature of the polymer, the gelation temperature of the solutions, the melting temperature of the gels, and the compressive modulus of the gels, the solubility of the polymer was the highest at DMF contents of 10–20 vol %. The maximum dynamic tensile modulus of the drawn (×18) films obtained from the dried gel films with a DMF content of 10 vol % was 54.9 GPa at 20°C. The orientation of the polymer chains in the amorphous regions was higher than that in the crystalline regions. The orientation of the polymer chains in the amorphous regions for the drawn films with a DMF content of 10 vol % was higher than that for the drawn films with a DMF content of 60 vol %. The orientation of the polymer chains in the amorphous regions was considered to play an important role in the properties of high strength and high modulus. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1661–1667, 1998     

Correlation between different microstructural parameters and tensile modulus of various polyethylene blown films

The objective in this study is to establish a model for the prediction of tensile properties using various types of polyethylene films (LLDPE, HDPE, and LDPE). A series of blown films were produced by varying three process parameters: take‐up ratio, blow‐up ratio, and frost line height. The tensile properties of the resulting films were investigated in relation to their microstructural characteristics. The microstructural parameters were determined by differential scanning calorimetry, wide angle X‐ray diffraction (WAXD) pole figures, SAXS (small angle X‐ray diffraction), and birefringence measurements. The orientation parameters of the films were measured by WAXD and birefringence. They were determined for both crystalline and amorphous phases. The crystalline content, lamellar thickness, and crystal sizes were obtained from DSC and WAXD. The SAXS technique was used to find the average length of the crystalline and amorphous layers. A model for the tensile modulus is proposed and correlated to some structural parameters including crystallinity, orientation factors for crystalline c‐axis and amorphous phase, lamellar thickness, crystal size, the average length of the crystal layer, and long spacing period. The measured modulus and the calculated one were compared and a reasonable agreement was found between them for all series of films. POLYM. ENG. SCI., 47:1430–1440, 2007. © 2007 Society of Plastics Engineers     

Crystallization and orientation behaviors of poly(vinylidene fluoride) in the oriented blend with nylon 11

Oriented films of nylon 11/poly(vinylidene fluoride) (PVDF) blend were prepared by uniaxially stretching the melt-mixed blends. The drawn films of fixed length were heat-treated at 170 °C for 5 min to melt the PVDF component, followed by quenching in ice water or isothermal crystallization at various temperatures. The crystal forms and orientation textures of the obtained samples were studied using wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). It was found that PVDF can crystallize into both and β forms in the nylon 11/PVDF blends, and that the content of the β form increases with increasing crystallization temperature above 120 °C. The orientation behavior of the -form PVDF was observed to be dependent on the crystallization conditions: c-axis orientation to the stretching direction was produced for the sample crystallized below 50 °C; the a-axis of crystals was tilted from the stretching direction when PVDF was crystallized at about 75 °C; the parallel orientation of the a-axis to the stretching direction becomes dominant at higher crystallization temperatures (above 100 °C). In contrast, the β crystalline form maintains the c-axis orientation irrespective of crystallization temperature. It was shown by the confocal laser scanning microscopy that cylindrical domains of PVDF were dispersed in the oriented matrix of nylon 11. The mechanism for the formation of the unique orientation textures is discussed in detail. It was proposed that the a-axis orientation is a result of the trans-crystallization of PVDF in the cylindrical domains confined by the oriented matrix of nylon 11. The crystallization kinetics, WAXD analysis, and morphology studies preferred the trans-crystallization mechanism. The mechanical properties of the as-drawn and heat-treated samples were measured not only in the stretching direction but also in the direction perpendicular to it. It was found that the heat-treated samples show slightly lower tensile strength, but more elongation at the break in the two directions than the as-drawn samples.