Studies on the stretching of polypropylene film. II. Polyaxial stretching

Polypropylene film was stretched polyaxially at 100–160°C., and the orientation behavior was studied by means of optical and x-ray method. The molecular chains oriented progressively to the film surface with an increase in stretching area v_A in the range 1–16, and the (040) selective uniplanar orientation developed at the extreme stretching. The plot of orientation versus v_A was less steep when the stretching was carried out at higher temperature, but the final degree of orientation was independent of the temperature, because the final v_A increased with temperature. At 160°C. premelting occured to such a degree that the high stretching and, consequently, the high orientation could not be obtained. The orientation of the amorphous chains was always behind that of the crystalline region. In the initial stage the polyaxial stretching was not as effective in attaining high biaxial orientation as the two-step biaxial stretching, but the final orientation was the same in both types of stretching because v_A reached a value of 16 in the polyaxial stretching while it was only 2 in biaxial stretching.     

Effect of stretching temperature on proton spin-lattice and spin-spin relaxation times of isotactic polypropylene film

Structural properties of isotactic polypropylene film during stretching were investigated mainly by the measurements of proton spin-lattice, T₁ and spin-spin, T₂, relaxation times. Both T₁ and T_2a, T₂ of the most mobile amorphous regions, of the sample stretched at 150°C are longer than those of the sample stretched at 130°C. These results indicate that in the sample stretched at 150°C the proportion of mobile amorphous region decreases, while the amorphous region achieves enhanced molecular mobility.     

Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. II. Effects of stretching ratio and temperature

Proton spin–lattice, T₁, and spin–spin, T₂, relaxation times of uniaxially stretched polypropylene film were measured at 40°C using a wide line pulse spectrometer operating at 19.8 MHz. T_1l, the longer T₁, increases almost linearly with increasing stretching ratio, and T_2a, T₂ of the amorphous region, decreases gradually as the stretching ratio is increased. These results can be interpreted in terms of the increased constraints to molecular motion in the amorphous region. The fraction of the rigid protons in the sample, F_c, increases with increasing stretching ratio, while the crystallinity calculated from the density, X_d, does not change largely. The difference between F_c and X_d, therefore, increases as the stretching ratio is increased. This indicates that the physical structure of the highly stretched sample is far from the ideal two-phase model. The influence of the stretching temperature was also investigated. There are only slight increases in T_1l and in F_c for the samples stretched in a temperature range from 80°C to 150°C, whereas the considerable increase in T_2a occurs. The most notable change introduced at a high temperature stretching is the increase in the chain mobility in the amorphous region.     

Structure variation and puncture resistance of stretched crosslinked polyethylene film: Effects of stretching temperature

Crosslinked polyethylene films were prepared and stretched at different temperatures (below and above melting temperature). The stretching temperature exhibited pronounced effects on the structure and puncture resistance of the films. Combining the results of differential scanning calorimetry, 1D X-ray diffraction, 2D X-ray diffraction, and scanning electron microscope, it was demonstrated that the film stretched above T _m has lower crystallinity, thinner lamellar, and larger crystal grain size than that stretched below but near T _m. All the stretched films showed highly oriented lamellae in the bulk and arranged perpendicular to the stretching direction. The crystals of the film stretched below T _m were arranged in highly aligned microfibrils with distinct interlamellar boundary and the crystals of the film stretched above T _m were arranged in a progressively less orderly manner with increased interlamellar entanglement. Due to this structure difference, the puncture resistances of the films stretched above T _m were much larger than that stretched below T _m. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47542.     

Untersuchung der ursachen für das auftreten mehrerer schmelzpeaks in den DSC-thermogrammen von hochverstreckten und getemperten folien aus isotaktischem polypropylen

Isotactic polypropylen films were highly stretched and annealed during 20 s under stretching tension at temperatures up to 206°C at maximum. The melting behaviour of these films was investigated by using a differential scanning calorimeter (DSC — 1 B). Two melting peaks at about 165 and 174°C respectively, were observed. The heat of fusion of the former peak predominates, if the film temperature during annealing exceeds 190°C. Its position is independent of the stretching ratio. This melting peak is assigned to crystalline domains, which show a long identity period of 140 to 160 Å and corresponds to an amorphous orientation factor of 0 to ?0.2. The higher melting peak appears during stretching. This peak is assigned to crystalline domains, which show a long identity period longer than 200 Å and corresponds to an amorphous orientation factor of 0.4 to 0.5. This fraction refers obviously to the paracrystalline building blocks of the microfibrils, which are transformed at a film temperature higher than 190°C to a greater part into smaller lamellaforming paracrystallites. The transformation of the fibrillar into a lamellar texture was visualised by transmission electronmicroscopy.     

Studies on the stretching of polypropylene film. I. Two-step biaxial stretching

A polypropylene film was stretched at 100–160°C., quenched to room temperature, and then restretched at the same temperature perpendicularly to the first stretching. The reorientation behavior was investigated by using optical and x-ray methods. During the restretching the monoaxial orientation caused by the stretching is converted into a new monoaxial orientation through a balanced state, where n_pp = n_ps < n_ss. The more or less parallel orientation to the film surface of the polypropylene molecules, brought about by the first stretching, proceeds further on restretching. n_ss is a linear function of the degree of stretching in area v_A. The inclination of this line is independent of the type of deformation, stretching, or restretching, provided the temperature is kept constant. At 160°C. the plot of n_ss versus thickness is less steep than it is at 100 or 130°C. The overall reorientation apparently proceeds according to Kratky's first deformation law. The x-ray pattern of a res?tretched film is a four-point diagram which indicates the existence of a pair of reorientation axes inclined symmetrically against the stretching axis. The inclination grows larger with restretching, and the axes merge into the restretching axis at extreme restretching. This phenomenon is less pronounced when the restretching is carried out at higher temperatures. The density of the restretched film is determined mainly by the stretching temperature, but extreme restretching has a tendency to lower it very slightly.     

Change of molecular aggregation state in amorphous region with uniaxial extension of preoriented polypropylene

Preoriented isotactic polypropylene was uniaxially drawn at various testing directions and testing temperatures. Change of the molecular aggregation state in amorphous region with extension was elucidated by measurements of melting temperature, enthalpy of fusion, and birefringence at each stage of extension. Melting temperature depends on both crystallite thickness and orientation function of amorphous chains. It is assumed that the enthalpy change of amorphous region takes place when oriented amorphous chains are transformed into random state by heating. The ratio of the enthalpy change of amorphous region in the sample after extension to that in the sample before extension monotonously increased with increasing orientation function of amorphous chains, f_a, independent of testing direction and testing temperature. Increase of true stress with drawing led to increase of f_a. Increase of f_a with extension depended on the testing angle θ between the testing direction and the direction of the crystal c-axis of the preoriented sample, and f_a most remarkably increased in extension at θ = 45°.     

Study of heat shrinkability of crosslinked low‐density polyethylene/poly(ethylene vinyl acetate) blends

In this study, the heat‐shrinkage property in polymer was induced by first compounding low‐density polyethylene/poly(ethylene vinyl acetate) (LDPE/EVA) blends with various amounts of peroxide in a twin‐screw extruder at about 130°C. The resulting granules were molded to shape and chemically crosslinked by compression molding. A process of heating–stretching–cooling was then performed on the samples while on a tensile machine. Shrinkability and effective parameters were also investigated using thermal mechanical analysis. The results showed that the gel fraction was higher for the sample of higher EVA content with the same amount of dicumyl peroxide (DCP). A decrease in the melting point and heat of fusion (ΔH_f), as determined from DSC, was observed with an increase in the DCP content. Studies on the heat shrinkability of the samples showed that samples stretched above the melting point had a higher shrinkage temperature than those stretched around the crystal transition temperature. The results showed that by increasing the peroxide content, the shrinkage temperature was decreased. These could be attributed to the formation of new spherulites as well as changes in the amount and the size of crystals. Furthermore, in samples elongated at 120°C (above the melting point), the rate of stretching had no effect on the shrinkage temperature. The results showed that the extent of strain had no effect on the temperature of shrinkage, but rather on the ultimate shrinkage value. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1389–1395, 2004     

Photoinitiated crosslinking of low density polyethylene. V: Orientation in stretched samples

Low density polyethylene (LDPE) is extruded, stretched, and photocrosslinked at different temperatures with different stretch rations in a continuous production line. Three different sequences are adopted: crosslinking before drawing, crosslinking after drawing, and crosslinking during drawing. The effect of drawing is studied by measuring the tensile properties. The increase of tensile strength and the decrease of elongation at break after stretching are related to chain orientation in crystalline morphology measured by X-ray diffraction. Thermal analysis by differential scanning calorimetry (DSC) supports these results. Owing to chain relaxation, the effects of orientation by stretching are significant only when the samples are drawn at a temperature not much higher than the melting point. Annealing of the drawn samples at a temperature above the melting point shows that the orientation in a crosslinked sample is retained for a much longer time than in an uncrosslinked sample. For the crosslinked samples, it is found that crosslinking after or during drawing gives a material with lower shrinkage upon heating; however, crosslinking before drawing gives a heat-shrinkable material.     

Intrinsic birefringence of amorphous poly(bisphenol-A carbonate)

The birefringence of uniaxially oriented poly(bisphenol-A carbonate) (PC) samples stretched over a wide range of temperatures has been measured accurately with a combination of the compensator and the wedge methods. The Hermans' orientation function of anisotropic PC was calculated from the measured dichroic ratio of the infrared absorption band at 1364 cm^-1. Measurements using differential scanning calorimetry, X-ray diffraction, or infrared spectroscopy indicated no stress-induced crystallinity in stretched amorphous PC. At each state having a defined molecular orientation, samples stretched below the glass transition temperature (T_g) always exhibited excess birefringence and slightly higher density. This phenomenon is attributed to bond distortion during stretching, a result of the suppression of large-scale segmental motions of polymer chains below the T_g. The birefringence of samples stretched above the T_g arises exclusively from the orientation effect as a result of greater chain mobility. These measured birefringence values are proportional to Hermans' orientation functions, yielding a linear relationship which allows precise determination of the intrinsic birefringence of amorphous PC as 0.192 ± 0.006.     

Morphology and texture development of uniaxially stretched poly(ethylene naphthalene‐2,6‐dicarboxylate)

The texture development of PEN films with different semicrystalline morphologies have been studied by X‐ray diffraction. These different structures have been obtained by uniaxially stretching PEN amorphous films at 100 and 160°C (below and above T_g) at different drawing ratios. Samples have also been characterized by DSC to determine the crystallinity ratios, the crystallization, and melting temperatures. To define the orientation of crystallites in the oriented samples, pole figures have been constructed, as a function of temperature and drawing ratio (DR) in the range 1.5–4. In the range from DR = 2 to 4 the orientation is clearly uniplanar‐axial. At T_draw = 100°C the crystallinity shown by DSC analysis is higher than the sample stretched at 160°C. The orientation is also higher when samples are stretched at 100°C. The naphthalene rings mainly stay in the plane of the film with a lower fraction perpendicular to the plane of the film. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 395–401, 2007     

The deformation behavior of polypropylene film under high strain rates

The relationship between stress and strain for polypropylene film was studied under strain rates from 0.13 to 5.21 s^?1 in order to study the deformation behavior of film under higher strain rates than previous studies. Uniform thickness was obtained in the strain rates from 2.08 to 5.21 s^?1 at 435 K, or from 2.08 to 3.13 s^?1 at 437 K. The temperature rise of film due to the generation of heat from plastic strain influenced the relationship between stress and strain, in particular, at high strain rates and low temperature. Material constants for the constitutive equation of film were determined using the measurements from 2.08 to 5.21 s^?1 at 435 K and from 2.08 to 3.13 s^?1 at 437 K. Film thicknesses during and after transverse direction stretching were successfully predicted by applying the material constants obtained. The authors concluded that the material constants should be determined by applying the stretching conditions, under which there is little or no effect from heat generation and under which film can be stretched uniformly in thickness. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Study on hot air aging and thermooxidative degradation of peroxide prevulcanized natural rubber latex film

The air‐aging process at 120°C and the thermooxidative degradation of peroxide prevulcanized natural rubber latex (PPVL) film were studied with FTIR and thermal gravity (TG) and differential thermal gravity (DTG) analysis, respectively. The result of FTIR shows that the ? OH and ? COOH absorption of the rubber molecules at IR spectrum 3600–3200 cm^?1, the ? C?O absorption at 1708 cm^?1, and the ? C? OH absorption of alcohol at 1105 and 1060 cm^?1 increased continuously with extension of the aging time, but the ? CH₃ absorption of saturated hydrocarbon at 2966 and 2868 cm^?1, the ? CH₃ absorption at 1447 and 1378 cm^?1, and the C?C absorption at 835 cm^?1 decreased gradually. The result of TG‐DTG shows that the thermal degradation reaction of PPVL film in air atmosphere is a two‐stage reaction. The reaction order (n) of the first stage of thermooxidation reaction is 1.5; the activation energy of reaction (E) increases linearly with the increment of the heating rate, and the apparent activation energy (E₀) is 191.6 kJ mol^?1. The temperature at 5% weight loss (T_0.05), the temperature at maximum rate of weight loss (T_p), and the temperature at final weight loss (T_f) in the first stage of degradation reaction move toward the high temperature side as the heating rate quickened. The weight loss rate increases significantly with increment of heating rate; the correlation between the weight loss rate (α_p) of DTG peak and the heating rate is not obvious. The weight loss rate in the first stage (α_f1) rises as the heating rate increases. The final weight loss rate in second stage (α_f2) has no reference to heating rate; the weight loss rate of the rubber film is 99.9% at that time. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3196–3200, 2004     

Characterization of poly(lactic acid) multifilament yarns. I. The structure and thermal behavior

The structure and thermal behavior of poly(lactic acid) (PLA) multifilament yarns were studied by complementary techniques of differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and wide angle X‐ray diffraction (WAXD). As for PLA filaments, notable differences in the WAXD patterns, DSC curves, and FTIR spectra were observed. The combination of the WAXD and FTIR results showed that PLA samples with different crystallinity contain α‐form crystal structure. The FTIR spectra of the filaments were analyzed to study their crystallinity and crystal structure. The total crystallinity of the PLA filaments was obtained from the percent area loss of the skeletal amorphous band at 955 cm^?1. Crystalline fraction from FTIR and DSC were comparable with each other. The C?O stretching region, which is sensitive to crystallization and dipole–dipole interactions, was evaluated to provide information about chain conformers and crystallinity of the samples. Depending on the processing conditions, double melting peaks were observed in the DSC curves of the samples. This exhibited the structural reorganization of the crystal phase during heating affected by heating and cooling rate. In the DSC curves of the nearly amorphous multifilament yarn, the exothermic peak observed right above the glass transition temperature (T_g) indicated two relaxed and deformed amorphous regions. However, the multifilament yarn with higher crystallinity showed just endothermic melting peak after its glass transition. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Biaxially stretched in comparison with conventionally blown coextruded composite plastic films

Multilayer composite films consisting of polyolefins, either polypropylene, linear low density or medium density polyethylene, a thin adhesive layer, and a polyamide in various ratios were prepared using two basically different processes. The first, a conventional blown film process in which the extrudate is stretched while in the molten state; and the second, a two-stage process in which the quenched extrudate is stretched at a temperature below the polymers' melting point. The films so prepared, having identical composition and similar extents of stretching, were compared on the basis of their tensile properties, thermoelastic shrinkage, oxygen transmission rate, and thermal behavior. The effects of stretching temperature, rate, and extent on the behavior of the composite films and control single layer films were investigated. The stretching temperature was found to be the dominating single process parameter in determining the films' behavior. The overall performance including tensile properties, barrier, and shrink properties of the “cold” stretched films was found markedly superior to that of the conventionally blown films. The stretched composite films possess physical properties that cannot be attained by the conventional process.     

Poly(lactic acid)-Based Film with Excellent Thermal Stability for High Energy Density Capacitor Applications

Dielectric capacitors play a key role in high power electronics, electric vehicles, and medical devices. Due to the biodegradability, high thermal stability, and ease of processing, poly(lactic acid)-based (PLA) films are widely used in applications of tissue engineering, actuators, and sensors. In this investigation, PLA films are prepared using a solution casting method for high energy density capacitor applications. It is found that the stretched poly(d -lactic acid) (PDLA) film exhibits an energy density and charge discharge efficiency of 18.5 J cm⁻³ and 95%, respectively, at room temperature. Furthermore, after a heating time of 30 min at 100 °C, it is found that the energy density and chargedischarge efficiency of the stretched PDLA films are maintained at 11.2 J cm⁻³ and 76%, respectively. In comparison, the energy density and efficiency of the stretched PDLA film are 1.03-fold and 53% higher, respectively, than those of the stretched poly(vinylidene fluoride-hexafluoropropylene) film at 100 °C with the same heat treatment. Compared with a high temperature polyimide material, the energy density of the stretched PDLA film is 1.98-fold higher at 100 °C. The PLA-based material shows great promise for application in high energy density capacitors operating at a high temperature (100 °C).     

Processing characteristics,structure development,and properties of uni and biaxially stretched poly(ethylene 2,6 naphthalate) (PEN) films

Poly(ethylene 2,6, naphthalene dicarboxilate), PEN, is very similar to poly(ethylene terephthalate), PET, in its chemical structure and was, therefore, expected to exhibit similar processing characteristics. We, however, observed a few problems during stretching of PEN, the most important of which was necking behavior at 145°C, which is between T_g (117°C) and T^cc (195°C). This is usually observed in PET only when it is stretched close to or below T_g. At temperatures between T_g and T_cc (cold crystallization temperature) PET stretches rather uniformly. The temperature window for film stretching appears to be rather wide, but our results indicate that this is not the case. Films stretched to high stretch ratios become uniform due to propagation and final disappearance of necks as a result of stress hardening. Our attempts at stretching these films at higher temperatures indicated that necking is eliminated, but so is stress induced crystallization, which causes stress hardening (unless high stretching rates are employed). The presence of stress hardening is essential for obtaining high quality, uniform films of these polymers. In addition, at high temperatures thermally activated crystallization which starts dominating the structure development, detrimentally affects the general appearance of the films. In brief, the PEN films we investigated have a narrower processing window than was anticipated based on their thermal behavior alone. At elevated temperatures the films are sensitive to the rate of stretching even more than typical PET processed at comparable conditions. The uniformity of the films depends on the stretch ratio, stretching mode, ratio(s) and rates and temperature. WAXS studies on the films indicate that the macromolecules packed into the low temperature crystal modification. In addition, WAXS pole figure studies suggest that naphthalene planes preferentially orient parallel to the film surface during biaxial stretching. The biaxially stretched films were observed to exhibit a bimodal chain orientation as evidenced by pole figure analysis of the (010) planes.     

Growth of tetrahedral amorphous carbon film: Tight-binding molecular dynamics study

Molecular dynamics simulation using tight-binding potential has been performed to examine the growth and performance of tetrahedral amorphous carbon during ion deposition. The sp³ hybrid atom content, density, and compressive stress of the tetrahedral amorphous carbon film depend on the growing conditions such as substrate temperature, ion energy, ion dose, and annealing temperature. The critical temperature for sp³ transition to sp² decreases with ion energy (40, 80, and 120 eV). At low temperatures (<300 K) and low ion energies, the sp³ fraction increases up to 82%. At the annealing temperature less than 1200 K or with a few ions (<20) implanted into the film, its sp³ content and density have only slight changes while the compressive stress has a large reduction with the annealing temperature and the number of implanted ions. This large reduction in the compressive stress is due to a structural relaxation.     

Reinforced via the insulative boric oxide in the BaTiO3 amorphous thin film with high energy storage capability and stability

Dielectric film capacitors are considered as a potential candidate for advanced power electronics technology due to their fast charging and discharging rate and stability. However, the further improvement of energy storage density is still a major challenge. Herein, a reasonable amorphous structure is applied to the preparation of dielectric film capacitors to improve the dielectric and energy storage properties. The high breakdown strength and energy storage density in the amorphous film are assigned by the disordered structure and intrinsic high insulative for B₂O₃. As a result, a high discharge energy storage density of 68.64 J cm^?3 and an efficiency of 85% can be achieved in the BaTiO₃-5wt%B₂O₃ amorphous thin film at 7.3 MV cm^?1, together with excellent thermal stability (20–200 °C) and cyclic stability (up to 10⁵ times) This work provides a paradigmatic method to achieve high energy storage density and stability.