Changes In Polymer Structure Research Articles

Effect of monochromatic radiation on change in physical structure of polymers

Effect of monochromatic radiation on change in physical structure of polymers

Structural changes in polymers of diethyl succinylsuccinate(1,4‐diethoxycarbonyl‐2,5‐dihydroxy‐1,4‐cyclohexadiene) with diamines at high temperatures

AbstractThe structural changes of the polymers having the following structures: magnified image at high temperatures were studied by the spectroscopic and thermal analysis. When the polymers were heated in air, they were oxidized predominantly to polymers having the structure: magnified image The rate of oxidation was fast and the reaction was almost completed within 2 hr at 180°C. In the case of polymer 3, an intramolecular cyclization to form acridone was detected as a minor reaction.

Linear viscoelastic properties of partly to completely formylated poly(vinyl alcohols)

Abstract Linear viscoelastic behavior of partly to completely formylated poly(vinyl alcohols), including stereoregular poly(vinyl formates), was investigated at various temperatures above the glass-transition temperature to obtain the effect of successive changes of polymer structure from semi- to quasi-crystalline to amorphous upon the mechanical behavior and its temperature dependence. The following two anomalies in the temperature dependence of the mechanical behavior of semi- to quasi-crystalline polymers, in contrast to that of amorphous polymers, were obtained: a deviation in the temperature dependence of the shift factor from the WLF equation used for composing the master relaxation or complex dynamic modulus curve and the appearance of another maximum of apparent activation energy for the relaxation process, both in a range of temperayures corresponding to the leathery plateau of the viscoelastic functions. The above anomalies suggest the existence of another type of inter-molecular relaxation mec...

Ziegler-natta catalysisIV. The stereospecificity in the polymerization of olefins and conjugated dienes in relation to the crystal structure of TiCl3(α,β)

The principles on which our recent theory of stereospecific propene polymerization with α-TiCl3/AlR3 was based, appear applicable to a much wider field. Not only can the molecular structures of diene polymers prepared with the same catalyst system be explained but also the change in molecular structure of polymers on variation of the crystal structure of the solid phase of the catalyst.

The possibility of applying the method of electrical recording of electron diffraction to amorphous polymers

IN THE electron diffraction analysis of the structure of amorphous polymers it is usual to measure the intensity of scatter by the photometric method. I t is well known that this method is laborious and does not distinguish between coherent and incoherent scattering from the polymer specimen. Incoherent scattering is particularly marked when sin 0/~ is small, with the result that it is practically impossible to bring out the details of the first diffraction maxima in the diffraction patterns. The use of sector devices smoothes out the incoherent and atomic background but does not remove the incoherently scattered electrons. In view of this the method of electrical recording of electron diffraction patterns is promising. This method, with the use of a retarding field, was suggested by Bagdykyants and Alekseyev [1]. The essential feature of the method is that the diffraction pattern from the specimen is displaced in the plane perpendicular to the optical axis of the apparatus by means of deflecting plates. A radial section of the pattern is passed through a diaphragm and falls on a fixed, sensitive receiver (secondary electron multiplier), the output of which is connected through a constant current amplifier to the recorder. B y means of an electron filter, which is given a retarding potential, a considerable proportion of the incoherently scattered electrons are cut off. There are reports in the literature of the use of a CdS crystal [2] and a Faraday cylinder [3] as receivers. These have been used mainly for s tudy of electron energy losses in solid bodies and also for s tudy of elastic and non-elastic scattering of electrons [4-6]. The purpose of the present work was to assess the possibility of using the electrical method of recording the intensity of electron diffraction for the s tudy of amorphous polymers, because for these systems photographic recording does not enable the weak reflections to be measured with sufficient accuracy. In addition it was of interest to discover the applicability of the electrical method for s tudy of the various processes that accompany structural changes in polymers.

Neutralization of residual catalyst in polydimethylsiloxane. effect of neutralization on the thermal stability of the polymer

AbstractIt is known that the thermal stability of polydimethylsiloxane is adversely affected by the presence in the siloxane polymer of active chain ends (i.e., by residual catalyst). The presence of catalyst causes the degradation reactions to proceed through an ionic mechanism (as opposed to the free‐radical mechanism typical of the “pure” polymer). Since ionic reactions take place at temperatures at least 100°C. below those at which free‐radical reactions occur, the thermal stability is thus strongly impaired. Degradation reactions occurring in polydimethylsiloxanes exposed to elevated temperatures fall roughly into two groups: (1) depolymerization (i.e., reactions involving the cleavage of the siloxane bonds) and (2) degradation proper, which results in profound changes in the chemical structure of the polymer (i.e., reactions involving carbon substituents on the Si atom, their splitting off, oxidation, etc.). Depolymerization of polydimethylsiloxane at temperatures of 150°C. and higher has been studied in vacuo and in air. The rate of depolymerization was followed quantitatively by means of weight losses; changes in the molecular weight of the polymer were taken as indicative of changes in polymer structure. The dependence of the rate of depolymerization of polydimethylsiloxane on the catalyst (KOH) concentration in the polymer was measured. It was found that the residual basic catalyst can be neutralized by acids. The neutralized polymer is characterized by a high resistance to depolymerization. The kind of acid used for the neutralization has a strong influence on the resistance of substituents toward high temperature and oxidative atmosphere. Benzoic acid and H2SO4, suppress the depolymerization almost completely, but strong mineral acids considerably decrease the resistance of substituents toward oxidation. It was found that very advantageous stabilizing properties are displayed by hydroxides of the Al(OH)3 type. The amphoteric nature of these hydroxides makes them suitable for stabilization of polydimethylsiloxane produced by anionic as well as cationic polymerization.

The sorption and desorption kinetics of water in a regenerated cellulose

AbstractThe sorption and desorption kinetics for different parts of the hysteresis loop of a water–cellulose system have been determined. Tentative explanations of these kinetics are advanced in terms of changes of polymer structure. It is proposed that during sorption the water first clusters in the more accessible regions of the polymer structure, and that this is followed by a dissolution of network bonds. During desorption the reformation of interchain bonds is hindered by the presence of water and the rigidity of the structure. The proposed structural changes which occur during sorption are somewhat similar to those advanced to explain the viscoelastic properties of the polymer.

Stereoregulated polymerization in the free propagating species. I. Theory

AbstractThe theoretical aspects of a new method of regulating the steric structure in addition polymers are treated. The basic principle utilized is the directing force at the free end of the propagating species, as contrasted to the influence of a bound initiator. Only small differences in the free energies of activation of the isotactic and syndiotactic propagations are required for relatively significant changes in polymer structure, particularly at low polymerization temperatures. Both the relative amounts of the two configurations in the polymer and the distribution of individual sequence lengths bear a simple relationship to the two propagation constants. In the analogous rotational isomerism, the free energy difference between the gauche and trans conformations is known to be influenced by both steric and electrostatic factors. For molecules of the type ACH2CH2A, the difference in heat content is increased by an increase in the size or the polarity of the substituents and, for polar substituents, by a decrease in the dielectric constant of the medium. Steric and electrostatic factors at the free end of the growing polymer chain can, in theory, influence the structure in polymers. This conclusion is based on an estimate of steric and electrostatic energy components of the most probable conformational arrangements for the isotactic and syndiotactic configurations in representative polyvinyl chloride with theoretical structure. The refinement introduced by bond angle deformation can also be approximated for the two polymer configurations. With respect to each of the three energy components the syndiotactic configuration is the preferred structure, in agreement with known experimental results that polyvinyl chloride prepared under free radical conditions does contain syndiotactic sequences sufficiently long for x‐ray detection. The combined, alternate use of the directing force at the free end of the propagating species and that of a bound initiator to form syndiotactic–isotactic block copolymers is suggested.

The sorption and desorption kinetics of water in a regenerated cellulose

The sorption and desorption kinetics for different parts of the hysteresis loop of a water–cellulose system have been determined. Tentative explanations of these kinetics are advanced in terms of changes of polymer structure. It is proposed that during sorption the water first clusters in the more accessible regions of the polymer structure, and that this is followed by a dissolution of network bonds. During desorption the reformation of interchain bonds is hindered by the presence of water and the rigidity of the structure. The proposed structural changes which occur during sorption are somewhat similar to those advanced to explain the viscoelastic properties of the polymer.

The viscosity slope constant k′—ternary systems: Polymer–polymer–solvent

AbstractThe viscosity slope constant k′, i.e., \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\lim }\limits_{c \to 0} \frac{{d\left( {{{\eta _{sp} } \mathord{\left/ {\vphantom {{\eta _{sp} } c}} \right. \kern-\nulldelimiterspace} c}} \right)}}{{dc}}\frac{1}{{[\eta ]^2 }} $\end{document}, is shown to be of increasing significance in polymer science as a molecular‐weight‐independent criterion of solvent power and as a parameter sensitive to various changes in polymer structure, such as long‐chain branching. Ideally, it is a dimensionless parameter, independent of molecular size, which arises only from the mutual hydrodynamic interaction of polymer molecules and depends, therefore, on the intrinsic flexibility of the polymer chain and on the polymer density in the coiled molecule. In real systems, however, other interactions may contribute, sometimes very significantly, to k′. For such real systems, the general expression: is suggested. In ternary systems, polymer–polymer–solvent, ideal expressions for ηsp/c and k′ are developed, that for k′ being: It is proposed that deviations from ideal behavior so defined are due to nonhydrodynamic polymer‐polymer interactions and might be used to detect and measure the strength of such interactions. Some preliminary data for the system polystyrene–poly(methyl methacrylate)–m‐xylene are presented and discussed.

A theoretical investigation of the influence of molecular relaxation and internal stress on diffusion in polymers

AbstractRecent experiments have indicated that the diffusion properties of a penetrant‐polymer system may change with time as diffusion proceeds. It is thought that two possible explanations of these time effects are slow changes of polymer structure accompanying diffusion and internal stresses exerted by one part of the polymer sheet on another as it swells. Two theoretical models are set up in order to express these effects quantitatively. The first model is essentially that in terms of which mechanical and other properties of polymers are commonly discussed, involving the concept of an instantaneous change of diffusion coefficient when the concentration changes, followed by a slow drift toward an equilibrium value. The second model expresses the effect of stresses set up between the outer swollen layers and the unattacked center of a polymer sheet during sorption. These stresses are slowly relieved as the diffusion proceeds, leading to a time‐dependent diffusion coefficient. The main features of diffusion behavior are established by calculation and the models are shown to account for the various types of sorption and desorption curves observed experimentally under different conditions, e.g., for sheets of different thicknesses and for different ranges of penetrant concentration. As examples, sorption and desorption curves calculated from the first model are shown to agree reasonably well with experimental curves for the methylene chloride‐polystyrene system. The rate of sorption and of the associated change in area of the sheet are accounted for by the second model. The presence of internal stresses also provides an explanation for the observation that there may be an appreciable interval of time during which a thin sheet takes up more penetrant than a thicker one under corresponding conditions. On the other hand, it is the slow structural changes which cause the rate of sorption to be not always inversely proportional to the square of the thickness of the sheet. It is concluded that both polymer relaxation and internal stresses play a part in determining diffusion behavior. The particular experimental results examined can be accounted for if the half‐life of the slow structural changes is comparable with that of the sorption experiment itself, and if the internal stresses change the diffusion coefficient by a factor of two or three.

Structure of neoprene. V. Viscosity‐conversion relationships in sulfur‐modified polychloroprenes

AbstractThe solution properties, i.e., gel content and intrinsic viscosity, of sulfur‐modified polychloroprenes were examined at monomer conversions ranging from 5 to 95% to obtain information about the mechanism of action of the sulfur modifier and other factors influencing the polymerization. Sulfur‐modified polymers isolated at conversions beyond 10–40%, depending upon the polymerization temperature, were found to be insoluble before treatment with tetraethylthiuram disulfide. These results, in confirmation of earlier results with radiosulfur, indicate that sulfur in chloroprene polymerization does not act as a chain transfer agent. The intrinsic viscosity‐conversion relationships for sulfur‐modified, tetraethylthiuram disulfide‐treated polychloroprenes show two viscosity maxima. The first maximum, at 15–20% conversion, appears to be the result of a change in polymer structure and, therefore, a change in the molecular weight‐intrinsic viscosity relationship. The second maximum appears at high conversion, 85–90%, and is caused by the formation of relatively tight gel that cannot be solubilized by the tetraethylthiuram disulfide scission reaction. The less pronounced decrease in viscosity beyond this peak is caused by removal of high molecular weight polymer by gelation, leaving relatively lower molecular weight sol polymer.

Influence of Structure on Polymer-Liquid Interaction. III. Swelling and Mechanical Properties of Some Partly Crystallized Polymers

Abstract We have shown in the first part of this work the influence of association on liquid-polymer equilibrium, while a study of nitrile copolymers revealed that mechanical properties are much more sensitive to certain changes in polymer structure (such as the introduction of methyl groups) than are swelling equilibria. In partly crystallized polymers, however, a close correlation between liquid-polymer interaction and mechanical properties exists, as is seen from the present paper.