High‐tensile‐strength polyvinyl alcohol films prepared from freeze/thaw cycled gels

The mechanical properties and molecular structure of a poly(vinyl alcohol) (PVA) film, which was obtained by eliminating water from a PVA hydrogel using repeated freeze/thaw cycles, were investigated by tensile tests, thermal analysis, and X‐ray diffraction measurements. The mechanical properties of PVA with 99.9% saponification were measured as a function of the number of freeze/thaw cycles performed. The tensile strength and Young's modulus increased and the elongation at break decreased with increasing freeze/thaw cycles. The tensile strength and Young's modulus of PVA films obtained after seven freeze/thaw cycles were as high as 255 MPa and 13.5 GPa after annealing at 130°C. Thermal analysis and X‐ray diffraction measurements revealed that this is because of a high crystallinity and a large crystallite size. A good relationship between the tensile strength and the glass transition temperature was obtained, regardless of the degree of saponification and annealing conditions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40578.     

Structural Evolution of UHMWPE Fibers during Prestretching Far and Near Melting Temperature: An In Situ Synchrotron Radiation Small‐ and Wide‐Angle X‐Ray Scattering Study

Combining a homemade extension apparatus and the in situ synchrotron radiation small‐ and wide‐angle X‐ray scattering methods for measurement, the structural evolutions of gel‐spun ultrahigh molecular weight polyethylene (UHMWPE) fibers during prestretching at temperatures of 25 and 100 °C are investigated, respectively. Lamellar rotation toward the stretching direction occurs before strain hardening, while the folded‐chain crystal destruction and extended‐chain fibril formation processes occur in the strain hardening zone at 25 °C. While at 100 °C, stretching induced crystal melting before the stress plateau region and formation of fibrous crystals at the onset of the stress plateau are observed. Further stretching results in shear displacement of crystal blocks and, finally, destruction of the folded‐chain crystals and formation of extended‐chain fibrils. Prestretching UHMWPE fibers at 100 °C within a certain strain range can produce highly oriented fibrous crystals, which may provide an ideal precursor structure for the poststretching process.     

Crystal structure of uniaxially stretched bio-based polyamide 510 films

The crystal structure changes of PA510 films during uniaxially stretching at 80°C, 110°C, 140°C and 170°C had been investigated as a function of stretching ratio and stretching rate. The stress–strain relationship curves showed that the stress of the PA510 films gradually increased when the stretching ratio increased. The wide-angle X-ray diffraction results verified that only one distinct equator reflection of stretched films was clearly identified at 80°C, 110°C and 140°C, namely γ(100) at 2θ = 20.6°. However, when the stretching temperature reached 170°C, the γ(004), γ(006) and γ(008) crystal form appeared in the meridional direction at λ = 12. Combined with differential scanning calorimeter analysis, it was found that the Xc increased from 7% to 40% as a result of the strain induced crystallization phenomenon and the stretching promoted the appearance of γ crystal form. In addition, the increase in the crystallinity and the molecular chain orientation increased the strength of the PA510 films in the tensile direction. And it also found that the microcracks occurred in the stretched films at high stretching ratio (λ = 12).     

Poly (vinyl alcohol)/polylactic acid blend film with enhanced processability,compatibility, and mechanical property fabricated via melt processing

Poly (vinyl alcohol)/polylactic acid (PVA/PLA) blend film, which is environment friendly and has potential applications in food and electronic packaging fields, was fabricated by melt extrusion casting. Fourier transform infrared spectroscopy analysis confirmed the formation of the hydrogen bonding between PLA and PVA, which improved the compatibility of PLA with PVA, making PLA uniformly dispersed in PVA matrix as small spheres, even when PLA content increase to 15 wt%. In this way, the original hydrogen bond network among PVA was disturbed and the chain mobility of PVA was activated, endowing PVA/PLA blends with lower melt viscosity than bot modified PVA and PLA, and the blend films with the increased crystallinity, mechanical property, and water resistance. Compared with PVA film, the crystallinity, tensile strength and Young's modulus of the blend film with 15 wt% PLA, respectively, increased by 15.1%, 9 and 51 MPa, and the water contact angle enlarged from 23° to 60°.     

Structure and orientation development in biaxial stretching of ultrahigh molecular weight polyethylene gel films

Ultrahigh molecular weight polyethylene (UHMWPE) gel films were prepared by gelation crystallization from decalin solutions. According to wide- and small-angle X-ray scattering (WAXD and SAXS) studies, single crystal mat texture with crystal c-axis oriented normal to film surface generally develops. However, randomly oriented structure can also develop if an external force is applied to gel films during gelation crystallization. Both textures invariably yield high drawability in uniaxial mode with outstanding modulus and strength. Biaxial films, typically 5 × 5, 6 × 6, 8 × 8, and 10 × 10 times the original dimensions, were prepared at 130°C–135°C in a biaxial stretcher. Optical microscopic Investigations reveal the development of interwoven fibrillar structure in all specimens. WAXD and SAXS studies show that lamellar structure transforms to fibrillar texture during biaxial stretching. Crystal orientation is characterized by WAXD pole figure and infrared dichroic methods. Mechanical studies suggest that tensile modulus and strength appear uniform. In comparison with uniaxial deformation, significant improvement in the lateral strength is seen in the biaxially stretched films.     

Biodegradable high oxygen barrier membrane for chilled meat packaging

“Chilled” meat is more nutritional, healthy and hygienic than the meat kept at ambient temperature. “Poly(propylene carbonate) (PPC) and poly(vinly alcohol) (PVA) were used to prepare biodegradable three‐layer PPC/PVA/PPC films with high barrier and tensile properties. The potential benefits of the developed films were also evaluated on the shelf life of chilled meat products. Compared to PPC film, using 20 wt % PVA as an intermediate layer in PPC/PVA/PPC film remarkably enhanced oxygen barrier performance at 0 and 50 RH % by about 500 times, tensile strength by about 8 times, and Young's modulus by nine times, but no beneficial effect on water vapor barrier performance has been observed. A new “sandwich” type of completely biodegradable material with high barrier was obtained. The application of PPC/PVA20/PPC film as the packaging material of chilled meat was effectively kept the total viable count (TVC) and total volatile basic nitrogen (TVB‐N) to acceptable levels in chilled meats until 19th day of storage at 4°C, however, the spoilage occurred within 11th and 14th days of refrigerated storage in term of TVC and TVB‐N, respectively, in the chilled meats packed with only PPC. Herein, we report that PPC/PVA/PPC three‐layer film can be a promising well‐defined biodegradable material with excellent potential in chilled meat packaging. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41871.     

Effect of uniaxial stretching on thermal,oxygen barrier,and mechanical properties of polyamide 6 and poly(m‐xylene adipamide) nanocomposite films

In this study, the effect of uniaxial stretching on the thermal, oxygen barrier and mechanical properties of aliphatic polyamide 6 (PA6) and aromatic Poly(m‐xylene adipamide) (MXD6) nylon films as well as their in‐situ polymerized nanocomposites with 4 wt% clay were studied. Cast films were prepared by extrusion process and rapidly cooled using an air knife. The precursor films were uniaxially stretched at 110°C with draw ratios varying from 1.5 to 5. DSC results showed that the cold crystallization temperature (T_cc) of the uniaxially stretched MXD6 and MXD6/clay films drastically shifted to the lower temperatures when draw ratio increased. The aromatic nylon films had lower oxygen permeability than those of the aliphatic films, due to more rigidity and chain packing. However, the oxygen permeability of the stretched films increased with draw ratio (DR) up to a critical value for each sample, while further stretching resulted in a reduction in the oxygen permeation. This phenomenon was related to the changes in free volume upon uniaxial stretching. The ability of different geometrical models to describe the experimental relative permeability data was investigated. The Bharadwaj model that took into account clay orientation was the most successful one to predict the oxygen barrier characteristics of the stretched nanocomposites at high draw ratios. The Young's modulus and tensile strength of the aliphatic and aromatic nylons increased with uniaxial deformation, while the flexibility and elongation at break of the former decreased with increasing DR. A larger increase in the Young's modulus of the uniaxially stretched nanocomposite films compared with the neat samples was observed and could be related to the improvement in the clay orientation as well as a better alignment of the crystalline phase due to incorporating the clay platelets in the polymer matrix. In contrast, the flexibility of the stretched MXD6 improved remarkably (ca., 25 times) compared with the precursor film (DR = 1) when the draw ratio increased to 1.5. This could be related to the effect of hot stretching on the enhancement of polymer chains relaxation and mobility at low draw ratios. POLYM. ENG. SCI., 55:1113–1127, 2015. © 2014 Society of Plastics Engineers     

Effects of heat treatment on tensile properties of high‐strength poly(vinyl alcohol) fibers

The tensile properties of high‐strength poly(vinyl alcohol) (PVA) fibers after heat treatment in air, water, and engine oil were studied. The results show that heat treatment in air, water, and engine oil have a different influence on the tensile properties of high‐strength PVA fibers. After heat treatment in air, the fibers possess excellent heat stability of the tensile properties. But in water, especially in hot water, the tenacity, Young's modulus, and specific work of rupture of the fibers decrease, while the elongation at break of the fibers increases. Similarly, engine oil has a significant influence on the tensile properties of the fibers. When the temperature of engine oil is above 120°C, the tensile properties of the fibers decrease drastically. We also discuss the influence of heat, water, and engine oil on the tensile properties of high‐strength PVA fibers in relation to the structure of the fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 237–242, 2000     

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.     

Freeze–thaw resistance of unidirectional‐fiber‐reinforced epoxy composites

The freeze–thaw resistance of unidirectional glass‐, carbon‐, and basalt‐fiber‐reinforced polymer (GFRPs, CFRPs, and BFRPs, respectively) epoxy wet layups was investigated from ?30 to 30°C in dry air. Embedded optic‐fiber Bragg grating sensors were applied to monitor the variation of the internal strain during the freeze–thaw cycles, with which the coefficient of thermal expansion (CTE) was estimated. With the CTE values, the stresses developed in the matrix of the FRPs were calculated, and CFRPs were slightly higher than in the BFRP and GFRP cases. The freeze–thaw cycle showed a negligible effect on the tensile properties of both GFRP and BFRP but exhibited an adverse effect on CFRP, causing a reduction of 16% in the strength and 18% in the modulus after 90 freeze–thaw cycles. The susceptibility of the bonding between the carbon fibers and epoxy to the freeze–thaw cycles was assigned to the deterioration of CFRP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Properties of isocyanate‐reactive waterborne polyurethane adhesives: Effect of cure reaction with various polyol and chain extender content

Three series of isocyanate‐reactive waterborne polyurethane adhesives were prepared with various contents of chain extender (4.25/8.25/12.50 mol %) and polyol (20.75/16.75/12.50 mol %). Each series had a fixed amount of excess (residual) NCO group (0.50–2.00 mol %). FTIR and ¹H‐NMR spectroscopy identified the formation of urea crosslink structure mainly above 80°C of various cure temperatures (20–120°C) with excess diisocyanate. The molecular weight, tensile strength, Young's modulus, and adhesive strength depend on excess NCO content and cure temperature and also varied with polyol and chain extender content. The optimum cure temperature was 100°C for all the samples. The tensile strength, Young's modulus, and adhesive strength increased with increasing cure temperature above 60°C up to the optimum temperature) (100°C) and then almost leveled off. Among all the samples, the maximum values of tensile strength, Young's modulus, and adhesive strength were found with 63.22 wt % polyol, 0.93 wt % chain extender, and 1.50 mol % excess (residual) NCO content at 100°C optimum cure temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation and characterization of syndiotacticity-rich ultra-high molecular weight poly(vinyl alcohol)/imogolite blend film

To enhance the physical properties of syndiotacticity-rich (syndiotactic diad content 63·4%) ultra-high molecular weight (UHMW) (number-average degree of polymerization 12300) poly(vinyl alcohol) (PVA) film, it was solution blended with rigid-rod imogolite in dimethyl sulphoxide. In addition, the blend film prepared was stretched using a high-temperature zone drawing technique for effective orientation of the film. Through a series of experiments, it was found that imogolite caused significant changes in the structure and properties of syndiotacticity-rich UHMW PVA film, i.e. imogolite acted as an important agent which increased crystal orientation of syndiotacticity-rich UHMW PVA, resulting in enhanced tensile strength of the film. However, imogolite played a hindering role in raising the amorphous orientation of syndiotacticity-rich UHMW PVA. The maximum tensile modulus of 19·8GPa and maximum tensile strength of 1·8GPa could be obtained at the maximum draw ratio of 7·45 for PVA/imogolite blend film. In the case of PVA homo film, the highest tensile modulus and strength were 25·2GPa and 1·4GPa, respectively. © 1998 Society of Chemical Industry     

Improving the dielectric performance of poly(vinylidene fluoride)/polyaniline nanorod composites by stretch‐induced crystal transition

The introduction of conductive polyaniline (PANI) can significantly improve the dielectric constant of polymer‐based materials. However, there is a drawback of high dielectric loss. Herein, a simple and efficient stretching process was applied to improve the dielectric performance of poly(vinylidene fluoride)/PANI (PVDF/PANI) nanorod films through the stretch‐induced crystal transition from non‐polar α‐crystal to polar β‐crystal in PVDF and the oriented distribution of PANI nanorods. XRD, DSC and Fourier transform IR analyses indicate that the stretched PVDF and stretched PVDF/PANI films possess a high content of β‐crystal at the stretching temperature of 135 °C under a stretching ratio of 200%–400%. Furthermore, the stretched PVDF/PANI film with 10 wt% PANI displays a high dielectric constant of 338 at 100 Hz, which is increased by 20% compared to non‐stretched PVDF/PANI film (281). More importantly, the corresponding dielectric loss is reduced from 0.31 for the non‐stretched film to 0.17 for the stretched film. © 2018 Society of Chemical Industry     

Relationship between crystalline structure and mechanical behavior in isotropic and oriented polyamide 6

Polyamide 6 (PA6) isotropic films and oriented cables were prepared by compression molding or by consecutive extrusion and cold‐drawing. These samples were isothermally annealed in the 120–200°C range and were then subjected to tensile tests at room temperature. Synchrotron wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) patterns were obtained before and after mechanical failure. These data were related with the mechanical properties of the respective PA6 samples. The annealing of isotropic PA6 resulted in an increase in the Young's modulus (E) and yield stress (σ_y) values, which was attributed to the observed proportional reduction of the d‐spacings of the intersheet distances in both the α‐PA6 and γ‐PA6 polymorphs. Analysis of the WAXS and SAXS patterns of isotropic PA6 after break allowed the supposition of structural changes in the amorphous phase, with these being better pronounced with increasing annealing temperature; this made the samples less ductile. In oriented PA6 samples, annealing resulted in a drastic increase in the E and σ_y values accompanied by a phase transition from γ‐PA6 to α‐PA6 and a well‐pronounced reduction in the intersheet distances of both polymorphs. The stretching of the oriented samples led to an additional γ‐to‐α transition, whose extent was also related to structural changes in the amorphous phase. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2242–2252, 2007     

Uniaxial tensile and structural properties of poly(vinyl alcohol) films: The influence of heating and film thickness

Poly(vinyl alcohol) as pure or composite materials is widely used in the food and textile industries, andbiomedical applications due to some important properties such as uniaxial tensile, biocompatibility, and noncarcinogenicity. Investigation of the influence of the film thickness and heating on the uniaxial tensile, spectroscopic, and surface properties of PVA films investigated in this study is quite important for improving the properties of such materials and their applicability in different conditions. In this study, with the influence of heating, a necking behavior was observed at around 2% for thin films and 4–9% strain for thicker PVA films for which a kind of transition point at around 1–2% strain was observed. The mechanical strength of PVA films, strain at break, and Young's modulus were enhanced greatly as the temperature increased from 80 to around 110 °C, and then most of them decreased. The degree of crystallinity increased linearly with the heat temperature from around 36–40%. Although PVA thin films obtained a very smooth surface structure after being heated at 80 °C, with increasing heat temperature, the surface roughness of both thin and thick PVA films increased and the PVA thin films obtained more degraded film surface. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44915.     

Study of cryostructuration of polymer systems. XX. Foamed poly(vinyl alcohol) cryogels

Foamed poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze–thaw treatment of whipped PVA water solutions (polymer with MW of 69,000 Da and DD ～99 mol % was used), were obtained and their properties were studied. The rheological characteristics and macrostructure of these gel materials were controlled by the same factors as for the ordinary nonfoamed PVA cryogels (initial polymer concentration and freezing–thawing regimes) and also by the conditions of generation of fluid PVA foams. The study of the kinetics of the freeze–thaw‐induced gel formation of these foams revealed that the temperature dependence of the efficiency of cryotropic gelation showed a maximum at about ?1.5°C. The presence of low molecular weight admixtures in the initial polymer solution appears to be a rather important factor because the admixtures were capable of decreasing the stability of fluid PVA foams and weakening both foamed and nonfoamed cryogel samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1609–1619, 2001     

Relationship between structure and dynamic mechanical properties of a carbon nanofiber reinforced elastomeric nanocomposite

The tensile and dynamic mechanical properties of a nanocomposite, containing modified carbon nanofibers (MCNFs) homogenously dispersed in an elastomeric ethylene/propylene (EP) copolymer semicrystalline matrix (84.3 wt% P), have been correlated with the structure development. These properties were characterized by in situ synchrotron X-ray diffraction during stretching, dynamic mechanical analysis and X-ray analysis techniques over a wide temperature range. Upon sequential drawing, the tensile strength of the nanocomposite film was notably higher than that of the unfilled polymer even though both samples exhibited a similar amount of crystal fraction and the same degree of crystal orientation, revealing the effect of nanofiller reinforcement in the semicrystalline matrix. The mechanical spectra of the 10 wt% MCNF filled samples in both stretched and non-stretched states showed broadening of the elastic modulus at high temperatures, where the corresponding crystallinity index also decreased. It is conceivable that a significant fraction of chain orientation is induced in the vicinity of the nanofillers during stretching, and these stretched chains with reduced mobility significantly enhance the thermal mechanical properties.     

Mechanical properties of films of poly(vinyl alcohol) derived from vinyl trifluoroacetate

In order to study the influence of the stereoreguralities of polymer chains on the mechanical properties of films of poly(vinyl alcohol) (PVA)_(VTFA) derived from vinyl trifluoroacetate, the strength of the film was measured. In the case of undrawn PVA_(VTFA) films, Young's modulus and strength at break were the smallest at the annealing temperature of about 100°C. It is considered to be due to the melt of small microcrystals and the increase in mobility of chains in amorphous parts. Young's moduli of undrawn PVA_(VTFA) films were in the range of 1.50–3.75 GPs and the values were higher than that (0.17–0.36 GPa) of undrawn film of commercial PVA with the low concentration of syndiotacticity and the high concentration of head-to-head bounds. In the case of drawn, annealed PVA_(VTFA) films, the maximum Young's modulus was about 20 GPa.     

Effects of water on drawability,structure, and mechanical properties of poly(vinyl alcohol) melt‐spun fibers

Poly(vinyl alcohol) (PVA) melt‐spun fibers with circular cross‐section and uniform structure, which could support high stretching, were prepared by using water as plasticizer. The effects of water content on drawability, crystallization structure, and mechanical properties of the fibers were studied. The results showed that the maximum draw ratio of PVA fibers decreased with the increase of water content due to the intensive evaporation of excessive water in PVA fibers at high drawing temperature. Hot drying could remove partially the water content in PVA as‐spun fibers, thus reducing the defects caused by the rapid evaporation of water and enhancing the drawability of PVA fibers at high drawing temperature. The decreased water content also improved the orientation and crystallization structure of PVA, thus producing a corresponding enhancement in the mechanical properties of the fibers. When PVA as‐spun fibers with 5 wt % water were drawn at 180 °C, the maximum draw ratio of 11 was obtained and the corresponding tensile strength and modulus reached ～0.9 GPa and 24 GPa, respectively. Further drawing these fibers at 215 °C and thermal treating them at 220 °C for 1.5 min, drawing ratio of 16 times, tensile strength of 1.9 GPa, and modulus of 39.5 GPa were achieved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45436.     

Moisture‐induced effects on the tensile mechanical properties and glass‐transition temperature of poly(vinyl alcohol) films

In this study, the effect of sorbed water on the tensile mechanical properties of noncrosslinked, thermally treated poly(vinyl alcohol) (PVA) films was studied. The Young's modulus, elongation at break, and tensile strength of the PVA films equilibrated at different relative humidities (0–86%) are reported, together with the depression of the glass transition of the polymer at each equilibrating humidity, as determined by temperature‐modulated differential scanning calorimetry. The results indicate that drastic changes in the tensile properties were correlated with the transition of the hydrated polymer from the glassy to the rubbery state. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011