Behavior Of Crystalline Polymers Research Articles

Differences in the Residual Behavior of a Bumetrizole-Type Ultraviolet Light Absorber during the Degradation of Various Polymers.

The alteration of an ultraviolet light absorber (UVA: UV-326) in polymers (PP, HDPE, LDPE, PLA, and PS) over time during degradation was studied using an enhanced degradation method (EDM) involving sulfate ion radicals in seawater. The EDM was employed to homogeneously degrade the entire polymer samples containing the UVA. The PP and PS samples containing 5-phr (phr: per hundred resin) UVA films underwent rapid whitening, characterized by the formation of numerous grooves or crushed particles. Notably, the UVA loss rate in PS, with the higher glass transition temperature (Tg), was considerably slower. The behavior of crystalline polymers, with the exception of PS, was analogous in terms of the change in UVA loss rate over the course of degradation. The significant increase in the initial loss rate observed during EDM degradation was due to microplasticization. A similar increase in microplasticization rate occurred with PS; however, the intermolecular interaction between UVA and PS did not result in as pronounced an increase in loss rate as observed in other polymers. Importantly, the chemical structure of UVA remained unaltered during EDM degradation. These findings revealed that the primary cause of UVA loss was leaching from the polymer matrix.

Photoplasticization Behavior and Photoinduced Pressure-Sensitive Adhesion Properties of Various Polymers Containing an Azobenzene-Doped Liquid Crystal

Abstract Photochemical control of the hardness and fluidity of polymeric materials is applicable to the development of photoreversible adhesives. Here, we report that common polymers show photoplasticization behavior, which is the photoinduced reduction of the glass transition temperature, just by adding an azobenzene-doped liquid crystal (azo-doped LC). We found that the photoplasticization effect that occurred in amorphous polymers with an azo-doped LC was due to a microscopic phase separation of the azo-doped LC from the polymers. In contrast, we did not observe any photoplasticization behavior in crystalline polymers because of the macroscopic segregation of the azo-doped LC from the polymers and high rigidity of the crystalline regions of the polymers. Based on the photoplasticization effect, we successfully achieved photochemical induction of tackiness, which is a pressure-sensitive adhesion property. Furthermore, the maximum tack strength was enhanced upon photoirradiation of the polymers. We inferred that this behavior originated from an increase in the interaction between the samples and the test probe based on the change in polarity of the samples by the photoisomerization of azobenzene. We conclude that “photoplastic” polymers composed of common materials can be used for photoresponsive pressure-sensitive adhesives.

Crystallization Behavior of Linear Low Density Polyethylene in its Blend with a Rubber Polymer as Revealed by Synchrotron SAXS/WAXS/Hv-SALS Simultaneous Measurements

This study concerns crystallization behavior of crystalline polymers in a phase-separated polymer blend. It might be generally considered that spherulites cannot grow bigger than phase-separated domains. However, it has been confirmed experimentally that it is possible. We found that the same phenomena for blends of LLDPE (linear low density polyethylene) with a rubber polymer. In this study, effects of phase-separated domains on the spherulite growth and the crystallization behavior of LLDPE were studied. For quantitative discussion, we employed a simultaneous measurement technique of synchrotron SAXS/WAXS/Hv-SALS. It was found that the spherulites were 10~20 times lager than the phase-separated domains. It was also found that the long period of crystalline lamellae was smaller than phase-separated domains. Therefore, we suppose that the phase-separated structure has no influence on the fibril growth, which is the fundamental step of spherulites growth. Namely, the fibril growth through a channel of phase-separated crystalline domains enables the overall growth of spherulites. In the meantime, it was found that the index of degree of crystallinity in the LLDPE phase decreased slightly with the blend composition and induction periods of SAXS, WAXS, Hv-SALS in the phase-sepatared blend of LLDPE with a rubbery polymer were longer than those in a neat LLDPE. It was also suggested that nuclear growth was obstructed to some extent by the phase-separated domains. We suppose that nuclei which were not obstructed grew so that spherulites in the phase-sepatared blend sample became bigger than in a neat LLDPE sample.

Crystallization and melting behavior of amorphous poly(iminosebacoyl iminodecamethylene)

Multiple melting behavior was observed in the differential scanning calorimetry (DSC) scans for the isothermally crystallized poly(iminosebacoyl iminodecamethylene) (PA1010) samples. Coexistence of crystal populations with different lamellar thickness in PA1010 was discussed by means of DSC, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering techniques. During crystallization of the polymer, a major lamellar crystal population developed first, which possessed a higher melting temperature. However, a small fraction of the polymer formed minor crystal population with thinner lamellae, which was metastable and, upon post-annealing, could grow into more stable and thicker lamellae through melting and recrystallization process. Lamellae insertion or stacks would develop during the post-annealing at a lower temperature for the isothermally crystallized samples; thus, multiple crystal populations with different thickness could be produced. It is the multiple distribution of lamella thickness that gives rise to multiple melting behavior of crystalline polymers. (C) 2000 John Wiley & Sons, Inc.

Interaction of supercritical carbon dioxide with polymers. II. Amorphous polymers

In continuation of our goal to implement supercritical fluid (SCF) technology for various applications such as precision cleaning, foaming, and impregnation of materials, a systematic study has been performed involving amorphous polymers. Eleven different polymers of amorphous nature have been subjected to supercritical carbon dioxide (SC CO2) treatment under a wide pressure and temperature range (1000–3000 psi and 25–70°C, respectively). The influence and impact of such treatment on the appearance, weight change, and thermal and mechanical properties were followed systematically. In addition, the effect of treatment conditions and dimension of the samples on weight changes were also monitored. It has been found that amorphous polymers can absorb carbon dioxide to a greater extent than crystalline polymers and, in turn, the phenomenon of plasticization was also very high. In addition to morphology, the polarity of the polymer is also crucial in determining the solubility in carbon dioxide. Comparison was also made with the behavior of crystalline polymers. © 1996 John Wiley & Sons, Inc.

Analysis of small angle x‐ray scattering intensity of hard elastic polypropylene under extension

The influence of the changes in density and the extension distribution function on the profiles of the small angle X-ray scattering of a hard elastic polypropylene film under extension was investigated. So far the scattering behavior of crystalline polymers, consisting of nearly parallel lamellae and intervening layers staked perpendicular to the fiber axis, has been described based on the lattice factor and the structure factor of a constant density difference. In this work it could be shown that the structure factor depends not only on the thickness, decided by the extension distribution function, but also on the density changing with the extension of each layer. Good agreement with the observation that the profile greatly changes in shape and intensity with extension could be obtained by introduction of an extension distribution function and a density function for the intervening layers.

Molecular Organization of Amorphous Polymers

The rapid advance in materials in modern times has resulted largely from the detailed comparison of properties and behavior with the microstructure. For polymers we usually know the atomic sequence along the chain, the bond distances, and, roughly, the bond anglcs. But since it is the nature of the amorphous state to lack regularity, the rotational conformations about backbone bonds and the nature of the packing of the chains vary from point to point and are largely unknown. One would expect that if more were known about the molecular arrangements, better control could be exerted over the behavior and properties of polymers. This review is confined to mainly single phase homopolymers in equilibrium conformations. This means, for example, that the amorphous content of crystalline polymers will not be dealt with, though considerable information concerning the structure of the amorphous content has come to light recently, and its structure is very important for the properties and behavior of crystalline polymers. We are concerned with the molecular organization in undeformed polymer melts, lightly crosslinked elastomers, and unoriented polymer glasses. Aside from the crosslinks, polymer melts and elastomers are virtually the same. The molecules in polymer glasses, though not in conformational equilibrium at the ambient tempetature, are thought to have conformations characteristic of equilibrium at some higher temperature near that of the glass transition. Thus the structures of the glass and melt are thought to be only infinitesmally different. The plan of this review is to examine the data concerning molecular organization that has been obtained from various sources and attempt to evaluate its significance. The more direct or traditional measurements are considered first, followed by those that are less so. The review concludes by attempting to draw the information together for a best current estimate of the nature of the organization.

General equation of stress-strain behavior in crystalline polymers

Crystalline polymers show a remarkable strain hardening in its plastic deformation compared with metals. In the case of metals, the power law holds between true stress and true strain, the power law index of which is lower than one (1). True stress tends to saturate with increasing true strain. Polymer chain shows very high modulus and high bonding energy along molecular axis, whereas intermolecular force is weak and the modulus perpendicular to the molecular axis is about one hundredth of that along molecular axis. With the progress of unfolding of molecular chains from folded structure in lamellar crystals, the deformation stress increases rapidly. To describe such a deformation process of crystalline polymers, it is especially necessary to employ true stress and true strain instead of nominal stress and nominal strain. In our previous paper (2), we have presented the equation to describe the true stress-strain relationship in a very simplified form, which was verified for linear polyethylene (HDPE) and nylon 6. In this paper, we have examined the applicability of our equation to isotactic polypropylene (PP), polyoxymethylene (POM) and polytetrafluoroethylene (PTFE).

Effect of external pressure on the physicochemical and mechanical properties of polymers

The behavior of crystalline polymers obtained by molding at high pressures has been investigated. External pressure can promote or retard polymer crystallization processes, depending on its intensity. Comparison of various physicochemical and mechanical properties of specimens obtained at different external pressures shows that the corresponding curves have two maxima: one at 200–800 kgf/cm2 and another on the narrower interval between 1300 and 1500 kgf/cm2. The position of the first maximum is associated with the molecular weight distribution and the flexibility of the polymer chain, and position of the second is determined by the chemical nature of the polymer. The data obtained help to make possible a rational choice of polymer processing methods in which pressure is involved.

Effect of hydrostatic pressure on the tensile strength of polymer materials

The effect of hydrostatic pressure on the tensile strength of amorphous, crystalline, and thermosetting polymers has been investigated. The maximum hydrostatic pressure was 2000 kgf/cm2. The surfaces of some specimens were protected from the ambient medium (oil). The tests showed that hydrostatic pressure improves the strength or high-elastic limit and Young's modulus of all the materials investigated. In the case of brittle materials, the increase in strength is greater if the surface is protected, whereupon the plasticity is also improved. Hydrostatic pressure produces important changes in the deformation behavior of crystalline polymers.