Journal of Polymer Science: Polymer Symposia最新文献

The self-diffusion constant and solution viscosity are calculated for polymers in semidilute solution using the reptation concept. A space-filling constraint (rather than binary contacts) is used to construct the tube in the reptation model. In good solvents, scaling predictions are recovered, but in marginal systems unexpected effective power-law exponents are predicted for the concentration dependence of the self-diffusion constant, relative viscosity, and reptation time.

Glassy polymers are nonequilibrium solids whose properties may change slowly due to time-dependent variation in the excess volume and enthalpy functions of these materials. Measurable changes in the gas sorption behavior of samples exposed to different time–temperature histories below the polymer's glass transition temperature can be explained primarily in terms of differences between the excess volume of different samples. A simple physical and mathematical model will be described which relates excess sorption in glasses, compared to rubbery polymers, to sorption into excess volume frozen into the quenched matrix. The model will be explained for pure component gases and then generalized to treat multicomponent sorption data for the CO₂/C₂H₄ and CO₂/N₂O systems in poly(methyl methacrylate). The rather complex mixed gas behavior can be predicted quantitatively using only pure component sorption data in conjunction with the generalized model for multicomponent gases. The model, based on the concept of competition of sorbing penetrants for the excess volume present in the glass, is successful for the two systems described above for total pressures up to 300 psia.

The rate constants and the equilibrium constants for end-to-end cyclization have been determined in seven solvents for a polystyrene chain of M_n - 2900 and narrow molecular weight distribution. These values were obtained by measuring the kinetics of intramolecular excimer formation between pyrene groups attached to the chain ends. Values were plotted versus the Hildebrand solubility parameter h: It was found that not only did the rate of cyclization decrease in good solvents, but that the rate of ring opening was substantially faster in good solvents than in poor solvents. Consequently, the cyclization equilibrium constant is far more sensitive to the quality of the solvent than is the mean-squared radius of gyration. The results are interpreted in terms of excluding volume effects on the end-to-end distance distribution function of the polymer.

Spontaneous addition and polymerization reactions between olefins of different electron densities result in a wide variety of small molecules and polymers. The small molecules include cyclobutanes, 1-butenes, and pyrans; the polymers can be homopolymers and/or alternating copolymers. Tetramethylenes are proposed as key intermediates in most of these reactions. The tetramethylenes, generated by bond-forming initiation, are hybrids of 1,4-biradical and zwitterionic forms. The character is mostly determined by the substituents at the terminals. Zwitterionic character is favored by strong donors such as alkoxy and dialkylamino groups at the carbenium ion end, and strong acceptors such as two cyano groups at the carbanion end. Biradical character is favored by acceptor groups such as diester and cyanoester at the acceptor end, and aryl and vinyl as donor groups. Zwitterionic tetramethylenes initiate ionic homopolymerization, while biradical tetramethylenes initiate alternating copolymerization.

This unifying concept of bond-forming initiation is extended to spontaneous addition and polymerization reactions of hetero-atom acceptor molecules and 7,7,8,8-tetrasubstituted quinodimethanes. Finally, compounds possessing labile σ-bonds (mainly halogens and peroxides) fit the general scheme. In a few cases, coupling of tetramethylenes may contribute to polymer formation. Ion-radical pairs, charge-transfer complexes, and adventitious impurities are excluded as significant initiators.

The bond-forming initiation has been thoroughly discussed in a recent paper by Hall [1].

Items fabricated from thermoplastics containing short glass fibers are anisotropic and the anisotropy varies from point to point. Guidance as to how such materials should be evaluated and what data are required for the design of end products molded from them is provided by a combination of the theories of viscoelasticity and anisotropic elasticity, whereby the stiffness and compliance coefficients of the latter are functions of time, temperature, and strain. A methodology then emerges, similar in overall nature to that pertaining to thermosetting resins containing long aligned fibers, but rather more complex because of the variations in fiber alignment. However, an adequately comprehensive evaluation in accordance with the formal rationale tends to be prohibitively expensive and, on the other hand, a curtailed evaluation can yield misleading data. Recent work aimed at resolving that impasse seems promising. The central principle is the use of special test specimens chosen because their flow geometries and possible flow irregularities are similar to those commonly found in commercial end products. The data that can be derived from such specimens can be related quantitatively to those derived from the conventional tests, and therefore the presentation and use of the new class of data entail no interpretation difficulties. Examples of what has been achieved and speculations about what might further be achieved are presented in the paper.

Dr. Turner Alfrey was a scientific pioneer in the area of copolymerization. Copolymerization is a polymerization process in which more than the minimum number of monomers required to form a polymer are polymerized together to give a product which has incorporated in it the repeat units corresponding to all the monomers employed, and the products thus obtained are called copolymers. Copolymers can also be produced by employing the mutual reactions of functional polymers or oligomers or their reactions with the appropriate monomers. Both step-growth and chain-growth processes, the latter involving a free-radical, ionic, coordination, or ring-opening mechanism, can be used to make copolymers.

Within the past several years considerable interest has developed in the practice of retrofitting of plasticating extruders with gear pumps. The primary motivation for this is to reduce surges and fluctuations in the extrusion rate, thereby improving product quality and reducing waste. It is shown that the effectiveness of a gear pump for this purpose is directly related to its volumetric efficiency. The volumetric efficiency depends upon the pump dimensions, the flow behavior of the polymer, and the operating variables of temperature, pressure, and pump speed. Experimental data on a variety of thermoplastic materials has been obtained using a pump with gear wheels of 3.6 cm diameter. These data have been correlated with an empirical equation. Some tentative conclusions are drawn concerning the sizing of gear pumps for specific applications.

Methods to measure the plane strain fracture toughness (K_1c) of PVC pipe material, valid according to linear elastic fracture mechanics (LEFM), have been developed. As in metals practice, the crack tip plastic zone must be small compared to initial crack length, section thickness, and other specimen dimensions to achieve validity. Other conditions of test rate and temperature also must be fulfilled. K_1c values vary from 2.8 to 4.0 MN m^−3/2. Quantitative micromechanical analyses of the plastic flow regions on the fracture surfaces account for only a fraction of the measured toughness. Other deformation mechanisms (crazing) below but near the surfaces have been observed; apparently they absorb the balance of the work required to drive the stable crack. Under catastrophic, rapid crack propagation, only a single craze exists ahead of the crack tip; the multiple crazes disappear. Thus the dynamic fracture toughness may be several factors less than the static or stable value.

We assume that the polyelectrolyte moves through a “tube” in a neutral gel under the influence of the electrical field. The tube is random except for possible bias due to the effects of the field. We also assume a frictional coefficient proportional to contour length for longitudinal motion in the tube. When the field is small, we easily recover the inverse-length dependence of the mobility found previously by de Gennes and by Doi and Edwards. At higher fields a new effect appears; the tube becomes oriented because the field biases the direction of the leading end of the chain as it moves to form an extension of the tube. This leads to an increase of the mobility with increasing field.

The solution, diffusion, and permeation of low molecular weight substances in polymeric materials are governed by the relative chemical compositions and physical structures of the penetrant molecule and the polymer. Those factors determine the chain segmental mobility, defect structures, and interactions which control the sorption magnitude and penetrant molecular mobility within the polymer. The transport of relatively noninteracting penetrant molecules in a polymer almost always follows the classical behavior predicted by Fick's law relationships with a constant (or nearly so) diffusion coefficient. An increase in generalized interactions (van der Waals, etc.) leads to increased sorption of the penetrant, with consequent increase in plasticization of the polymer, such that the diffusion process often becomes concentration-dependent. Then, depending upon the relative rates of polymer relaxation processes concurrent with the sorption-diffusion process, the overall transport process may exhibit Fickian behavior with a simple concentration-dependent diffusion coefficient or it may deviate significantly from that behavior due to complicating relaxation effects. Systems in which there may be more specific interactions (hydrogen-bonding, polar, ionic) often show a pronounced dependence of transport behavior on compositional, temporal, and spatial parameters which relate to changes in sorption modes. In these present investigations, we have used copolymer systems, prepared under controlled conditions, to gain some further insight into the dependence of polymer structure on variations in composition and how these variations affect or are reflected by transport and relaxation properties.