Rheo-optical Raman study of microscopic deformation in high-density polyethylene under hot drawing

In situ observation of the microscopic structural changes in high-density polyethylene during hot drawing was performed by incorporating a temperature-controlled tensile machine into a Raman spectroscopy apparatus. It was found that the load sharing and molecular orientation during elongation drastically changed at 50°C. The microscopic stress of the crystalline chains decreased with increasing temperature and diminished around 50°C. Moreover, the orientation of the crystalline chains was greatly promoted above 50°C. These microscopic structural changes were caused by the thermal activation of the molecular motion within lamellar crystalline chains owing to the onset of relaxation of the crystalline phase.     

Physical and mechanical properties of ultra-oriented high-density polyethylene fibers

Ultra-oriented high-density polyethylene fibers (HDPE) have been prepared by solid-state extrusion over 60–140°C range using capillary draw ratios up to 52 and extrusion pressures of 0.12 to 0.49 GPa. The properties of the fibers have been assessed by birefringence, thermal expansivity, differential scanning calorimetry, x-ray analysis, and mechanical testing. A maximum birefringence of 0.0637 ± 0.0015 was obtained, greater than the calculated value of 0.059 for the intrinsic birefringence of the orthorhombic crystal phase. The maximum modulus obtained was 70 GPa. The melting point, density, crystallinity, and negative thermal expansion coefficient parallel to the fiber axis all increase rapidly with draw ratio and at draw ratios of 20–30 attain limiting values comparable with those of a polyethylene single crystal. The properties of the fibers have been analyzed using the simple rule of mixtures, assuming a two-phase model of crystalline and noncrystalline microstructure. The orientation of the noncrystalline phase with draw ratio was determined by birefringence and x-ray measurements. Solid-state extrusion of HDPE near the ambient melting point produced a c-axis orientation of 0.996 and a noncrystalline orientation function of 0.36. Extrusion 50°C below the ambient melting point produced a decrease in crystallinity, c-axis orientation, melting point, and birefringence, but the noncrystalline orientation increased at low draw ratios and was responsible for the increased thermal shrinkage of the fibers.     

High-temperature morphological transition in a styrene-butadiene-styrene block copolymer

Electron microscopy reveals a high-temperature morphological transition in a styrene-butadiene-styrene block copolymer of 7000 polystyrene block molecular weight and 43,000 polybutadiene block molecular weight (7S-43B-7S). Samples quenched in liquid nitrogen from temperatures above 150°C show no structure, whereas those quenched from temperatures below 140°C clearly show a multiphase structure. We previously reported that the 7S-43B-7S polymer exhibits a relatively sharp melt rheological transition in the temperature region between 140 and 150°C from highly viscoelastic and nonlinear viscous behavior to linear viscous behavior with insignificant elasticity. The dynamic viscoelastic properties are measured at different strain amplitudes in this study, and the results show that the melt rheological transition behavior is not influenced by the strain amplitude. This study clearly shows that the melt rheological transition in the 7S-43B-7S results from a morphological transition from a multiphase structure below about 140°C to a single-phase structure above about 150°C.     

Apparatus for infrared measurement of sorption/desorption in strained polymeric films

A new apparatus has been developed for optical measurement of sorption/desorption in transparent polymer films at a given strain or stress. The technique utilizes a chosen infrared absorption frequency of the diffusing vapor in a spectral region where the film has negligible absorption. From the time dependence of the IR absorption at this frequency the sorption/desorption behavior of the film may be determined at any strain or stress. The simultaneous measurement of mechanical relaxation as a function of the amount of sorbed vapor is also possible. The results presented here show the applicability of the apparatus for determining the transport and mechanical properties of a low-density polyethylene film in ethyl acetate vapor at 30°C.     

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

The influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion^® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion^®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to ?0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148     

Residual thermal birefringence in freely quenched plates of amorphous polymers: Simulation and experiment

Free quenching experiments were performed on thin plates of polystyrene (PS) and polycarbonate (PC). The thermal birefringence distribution along the thickness direction of the plates was measured. The birefringence data were compared with the results of a numerical simulation based on the linear viscoelastic and photoviscoelastic constitutive equations for the mechanical and optical properties, respectively, and the first‐order rate equation for volume relaxation. The effects of the initial temperature, quenching temperature, and quenching media on the development of residual thermal stresses and birefringence were evaluated. At higher initial temperatures (>105 °C), the thermal birefringence in quenched PS plates was negative at the center and positive at the surface, whereas at lower temperatures (close to the glass‐transition temperature), the birefringence became positive at the core and negative at the surface or positive through the entire cross section of the plate. The birefringence in freely quenching PC plates was positive at the center and negative at the surface at any initial temperature. These observations were in fair agreement with predicted data. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1850–1867, 2003     

Epitaxial recrystallization of HDPE-quenched ultrathin films on oriented iPP substrates

The recrystallization behavior of high-density polyethylene (HDPE) on the highly oriented isotactic polypropylene (iPP) substrates at temperatures below the melting temperature of HDPE has been investigated by means of transmission electron microscopy. The results obtained by the bright-field observation and the electron diffraction show that upon annealing the HDPE-quenched films on the oriented iPP substrates at temperatures below 125°C, only a small amount of HDPE recrystallizes on the iPP substrate with [001]_HDPE//[001]_iPP, while annealing the HDPE-quenched films at temperatures above 125°C, all of the HDPE crystallites recrystallize epitaxially on the iPP substrate with [001]_HDPE//[101]_iPP. © 1997 John Wiley & Sons, Inc. J Polym Sci B: 35 : 1415–1421, 1997     

Radiation-induced grafting in the polyethylene–styrene system and differential thermal analysis of the grafted samples

The graft polymerization of styrene onto high-density polyethylene films was carried out by γ-irradiation in the vapor phase. Two methods were used for grafting in these experiments: a preirradiation method and a simultaneous irradiation method. The effects of these grafting methods on the reaction mechanism of grafting and on the properties of the grafted samples were investigated. The amounts of styrene homopolymer in the grafted samples is under 2% in the case of the preirradiation method and above 10% in the case of the simultaneous irradiation method. The activation energies were calculated to be 18 kcal/mole for grafting in the preirradiation method and 15 kcal/mole for weight increase of polyethylene films in styrene vapor. The difference in the dimensional expansion between in the direction of stretching and the direction prependicular to it is smaller with preirradiation grafting than with grafting by the simultaneous irradiation method. Differential thermal analysis of the grafted films shows an endothermic peak due thermal decomposition which decreases gradually from 450°C to 415°C with increase in degree of grafting from 30 to 60%. The lowering of this peak temperature appears at a lower degree of grafting when the preirradiation method is used. On the basis of these results, it is concluded that the reaction rate of radiation-induced grafting in the vapor phase depends closely upon the processes of adsorption, dissolution, and diffusion of styrene monomer in polyethylene films; in the case of simultaneous irradiation method, the reaction proceeds comparatively uniformly in the amorphous region, while in the case of the preirradiation method, the reaction proceeds mainly at the boundary of the crystalline and amorphous regions.     

Rheo-optical studies of high polymers. XVIII. Significance of the vertical shift in the time-temperature superposition of rheo-optical and viscoelastic properties

To determine the true reason for the increase in birefringence and the decrease in relaxation modulus for high-density polyethylene with rising temperature, changes in crystalline structure as well as in thermal, viscoelastic, and rheo-optical properties with temperature were measured, by several techniques, including DSC, DLI, infrared dichroism, x-ray diffraction, and NMR. The values for degree of crystallinity obtained from the DSC fusion curve, density, and infrared absorbances coincide very well and show almost no divergence till about 80°C. The optical vertical shift factor pT can be related to the ratio of the orientation function for the crystal c axis at an arbitrary temperature to that at the references temperature, f_ε/f_ε0. The mechanical vertical shift factor b_T, on the other hand, is associated with the temperature dependence of the mobile fraction, as determined by NMR measurements, but not with variations in degree of crystallinity.     

Structure of ultrathin polyethylene layers in multilayer films

The crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high-density polyethylene and polystyrene. Transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and wide-angle x-ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row-nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row-nucleated morphology imparts highly anisotropic mechanical properties to the nanolayer polyethylene.     

Rheo-optical studies on the deformation mechanism of semicrystalline polymers. IV. On the nature of alpha mechanical dispersion of low density polyethylene in relation to the mechanism of spherulite deformation

The dynamic mechanical and birefringence behavior of three kinds of test specimens, prepared from a low-density polyethylene by different heat treatments, was investigated over the frequency range 0.008 to 4.3 Hz at various temperatures from 20 to 80°C. Reduction of both kinds of data to the common reference temperature of 50°C revealed rather broad frequency dispersions, corresponding to the α dispersion, with activation energies for the mechanical and optical relaxation processes of around 25 kcal/mole for all specimens. An annealed specimen was further investigated by dynamic x-ray diffraction to determine the dispersions of crystal orientation and of lattice deformation. The mechanical and birefringence dispersions are associated with the dispersion of crystal orientation and with the relatively elastic nature of the lattice deformation. Thus, the mechanical dispersion could be assigned to an α₁ mechanism arising from rotation of crystallites within the spherulites, i.e., a type of grain-boundary phenomena, but not to an α₂ mechanism resulting from a crystal disordering transition or premelting. Combining dynamic birefringence with dynamic crystal orientation, the mechanical α dispersion is further discussed in terms of dynamic orientation behavior of noncrystalline chain segments in order to elucidate the nature of the grain-boundary phenomena in detail in relation to the dynamic deformation mechanism of the spherulitic crystalline texture, i.e., reorientation of “crystal grains” within orienting lamellae due to intralamellar shearing resulting from the paracrystalline nature of the lamellae. A mechanical model characterizing the grain-boundary relaxation in association with the reorientation of the crystal grain is also proposed.     

An instrument for measuring the oscillatory electric birefringence properties of polymer solutions

An instrument for measuring the oscillatory electric birefringence properties of synthetic polymer dissolved in organic solvents has been designed and constructed. Novel features of the design include an in situ variable inter-electrode spacing Kerr cell and a double-beam optical train. The accessible frequency range extends from below 1 Hz to at least 100 kHz, with electric fields variable up to approximately 6000 V cm^?1 (peak-to-peak). Measurements are made with a powerful computerized data acquisition and processing system, based on an approach previously used for viscoelastic and oscillatory flow birefringence experiments. Preliminary results on a viscous liquid, Aroclor-1248, indicate that time-temperature superposition holds to reduced frequencies of at least 100 MHz. Comparison with theoretical predictions for rigid rod suspensions suggests that this liquid exhibits relaxation behavior with a time constants of ca. 6 ns at 25.00°C.     

Extensional flow-induced crystallization of a polyethylene melt

Measurements of flow-induced orientation and crystallization have been made on a high-density polyethylene melt (HDPE) undergoing a planar extensional flow in a four-roll mill. The HDPE was suspended as a cylindrical droplet at the flow stagnation point in a linear low density polyethylene (LLDPE) carrier phase. Deformation and crystallization of the HDPE droplet phase were monitored using video imaging in conjunction with measurement of the birefringence and dichroism to quantify the in-situ transformation kinetics. Planar deformation rates along the symmetry axis of the molten HDPE phase were on the order of 0.03 s^?1. Measurements of the initial transformation rate following flow cessation at 131.5°C and 133.2°C show a dependence on initial amorphous phase orientation and the total Hencky strain achieved during flow. The flow-induced crystallization rate is enhanced over the quiescent transformation rate by orders of magnitude, however, the dependence on temperature is less dramatic than expected for a nucleation-controlled growth mechanism. Analysis demonstrates that the melting point elevation model cannot account either qualitatively or quantitatively for the phenomena observed, suggesting that alternative explanations for the strong orientation dependence of the transformation rate are needed.     

β Relaxations in polyethylenes and their anisotropy

Measurements have been made of the anisotropy of viscoelastic behavior in specially oriented sheets of low-density polyethylene. The results for the cold-drawn sheets show a β relaxation process of very characteristic anisotropy. The annealed sheets show two relaxations in this region of temperature. The lower relaxation (about ?10°C) is identified as an interlamellar shear process. The higher relaxation (about 70°C) has a very similar anisotropy to the β relaxation in cold-drawn samples. Isotropic sheets of low-density polyethylene have been also investigated. Two β relaxations are found in these materials.     

Stress in polyimide coatings

The effect of film thickness on in-plane molecular orientation and stress in polyimide films prepared from pyromellitic dianhydride with 4,4′-oxydianline was investigated using a prism coupling technique to measure the refractive index. Film thickness was controlled by varying both solution concentration and spinning conditions. Birefringence, the difference between the in-plane and out-of-plane refractive indices, was used to characterize the in-plane molecular orientation. The observed birefringence is a combination of the birefringence resulting from molecular orientation and the birefringence induced by the residual stress present in the films. The birefringence decreases with increasing film thickness over the range of thicknesses studied (3–20 μm) indicating that the molecular orientation decreases with increasing film thickness. The in-plane coefficient of linear thermal expansion (CTE), controlled by the level of orientation in the film, increases from 18 to 32 × 10^?6/°C over the same thickness range. The birefringence of free-standing films was lower than that of adhered films due to the release of residual stress in the film once the film is removed from the substrate. The residual film stress arises primarily from the mismatch in CTEs between the polyimide film and the substrate to which the film is adhered. Thus, since the film anisotropy decreases with increasing thickness, the film stress increases with increasing thickness. Residual stress calculated by integrating the product of the film modulus and the CTE mismatch assuming temperature-dependent properties is comparable to experimentally measured film stress. Ignoring the temperature dependence of the film properties leads to an overestimation of stress. Moisture uptake was used to study the stress dependence of the optical properties. Moisture uptake increases both the in-plane and out-of-plane refractive indices by equal amounts in free-standing films due to an isotropic increase in the polarizability. In adhered films, an increase in moisture uptake leads to a decrease in the birefringence due to a swelling-induced decrease in the residual film stress. © 1994 John Wiley & Sons, Inc.     

Use of carbon suboxide to obtain block and graft copolymers. II. Grafting of carbon suboxide on polyethylene

Experiments have been carried out on grating of carbon suboxide on nonactivated polyethylene films and on films previously activated by ultraviolet irradiation and by γ-irradiation. The experiments gave a grafted copolymer. A grafted copolymer was also obtained on grafting carbon suboxide in solution on polyethylene films preactivated by means of ozonization at 70°C. Examination of the copolymer indicated its structure to be cross linked. It has been proved that below 50°C single molecules of carbon suboxide react with polyethylene. The polyethylene thus modified is then easily surfacedyed.     

Effect of birefringence on light scattering from deformed spherulitic polymer films

Light scattering from oriented samples of crystalline polymers is affected by the birefringence of the sample. An extension of the theory for scattering from uniaxially deformed two-dimensional and three-dimensional spherulites is made so as to include the retardation of the incident and scattered beams in passing through the birefringent sample. Strain influences scattering, in that it changes the birefringence of the sample and it also changes the anisotropy and shape of the spherulites. Scattering intensities are calculated for both crossed and parallel polarizers as a function of Ω, χ, and Φ, where Ω is the angle between the stretching direction of the sample and the horizontal direction, and χ and Φ are the angles between the stretching direction and the polarization directions of the polarizer and analyzer, respectively. It is shown that for crossed polarizers with Φ = 45° and χ = 45° birefringence changes largely influence the results but that for the polarizers parallel at Φ = 0° and χ = 0° or crossed at Φ = 90° and χ = 0° the birefringence effect is minimized. The intensity distributions for crossed polarizers at Φ = 45° and χ = 45° from polyethylene films stretched to give retardations up to several wavelengths, are found to be in good agreement with the calculated results.     

Temperature dependence of photoelastic properties of crosslinked amorphous polyethylenes

Crosslinked samples of polyethylene were prepared by electron irradiation of both high- and low-density polymers in the crystalline state. A further crosslinked sample was obtained by curing a high-density polyethylene by reaction with dicumyl peroxide at 180°C. The stress–strain–birefringenece relations were obtained on specimens cut from these samples at temperatures between 130 and 250°C. All samples showed a substantial decrease in stress-optical coefficient with increasing degree of crosslinking and with increasing temperature. The stress-optical properties at each temperature were extrapolated to zero degree of crosslinking to give quantities characteristic of the Gaussian network. Comparison of these properties with the theory of networks of rotational isomeric chains with both independent and interdependent rotation allows estimates to be obtained for (1) the trans–gauche energy differences in rotation around skeletal bonds and (2) the difference in principal optical polarizabilities for the CH₂ group in the elastomeric state. This latter quantity is shown to be more nearly given by Denbigh's than by Bunn and Daubeny's bond polarizability values.     

Pre-irradiation grafting of span 60-IAH onto polyethylene to improve dripping properties of water on polyethylene films

A reactive type dripping anti-condensation agent, Span 60-IAH, was grafted onto linear low density polyethylene (LLDPE) by β-ray pre-irradiation and reactive extrusion. Effects of total dose, monomer concentration and extrusion temperature and rate on the degree of grafting were studied in detail. It was shown that the optimum conditions for grafting were the extrusion temperature of 130–220°C, screw run speed of 90 rpm and total β-ray dose of 12.5 kGy. The structure of the LLDPE-g-(Span 60-IAH) (LS) was characterized by Fourier-transform infrared spectroscopy (FT-IR). The tensile properties and light transmission properties of extruded films were determined. The thermal behavior of the LS was investigated by using differential scanning calorimetry (DSC). Compared with pure LLDPE, the crystallization temperature (T_c) of LS increased about 3°C. Accelerated dripping properties of film samples were investigated. The dripping duration of the LS film and a commercial anti-fog dripping film at 60°C were 45 days and 17 days, respectively, indicating a significant improvement.