Journal of Polymer Science: Polymer Symposia最新文献

Semirigid IPN foams were prepared by simultaneous polymerization (SIN technique) of urethane-isocyanurate and epoxy resin systems. Significantly higher compressive strength was obtained with the IPN foams as compared to the corresponding urethane-isocyanurate foams. Due to the relatively low functionality of the foam components, and hence, resulting low crosslink density of the IPN foams, the oxygen indices and the friability were relatively low but increased with increasing isocyanate index and epoxy content. Although these studies were preliminary in nature, this type of IPN foams could find applications in sound and noise attenuation as well as for energy absorption such as shock and vibration damping.

An understanding of the transmittal of force or stress through the molecular (or atomic) network of a body should not only be helpful in predicting the load carrying capability of materials but might also provide information on how structure can be altered to enhance properties. A molecular understanding of mechanical properties is particularly difficult in polymers where such factors as molecular weight, tacticity, crystallinity, presence of networks, orientation, etc., may have an influence. In no class of materials are intricacies of physical structure more important than in polymers. Polymers with very similar chemical structures but differing physical structure can have physical properties differing by orders of magnitude. It would obviously be helpful to have means to probe molecular and atomic events and occur during the loading and destruction of the polymer structure. Instruments that have been used for this purpose include: (1) electron spin resonance (ESR) to monitor free radical production resulting from homolytic chain scission, (2) Fourier transform infrared spectroscopy (FTIR) to measure new end groups resulting from molecular chain rupture, and (3) intrinsic viscosity or gel permeation chromotography (GPC) to detect molecular weight changes (MWC) accompanying molecular degradation. This presentation will discuss and compare research results from these methods. Particular emphasis will be placed on recent studies in which computer modeling has been used to identify ESR spectra and a comparison of the results obtained by ESR, FTIR, and MWC for polystyrene.

In 1963 Boyer proposed the existence of a liquid-liquid transition, T_ll, in polystyrene above the glass transition T_g. Since that time there has been considerable discussion and disagreement on the existence of this transition. Detailed shear modulus data as a function of temperature in the range 165–215°C will be presented for a series of polystyrenes of various molecular weights. In each case, as the temperature is increased, the modulus gradually decreases until a specific molecular weight dependent temperature is reached and then suddenly the modulus decreases rapidly over a few °C and then returns to its previous gradual decrease with increasing temperature. The response is reminiscent of the modulus behavior going through T_g. It is suggested that the temperature at which this sharp decreased in modulus takes place is T_ll. The molecular weight effects on T_ll will be discussed in terms of several rheological properties that measure the temperature of this transition.

Investigations have been carried out in this laboratory over the past four years on the structure—property relationship in main chain, thermotropic liquid crystal polymers containing a variety of different types of mesogenic groups and flexible spacers. Our goal has been to relate the effect of polymer structure to the type of mesophase formed in the melt and to the melting and clearing temperatures of the mesophases. Many interesting, unusual, and unexpected results have been obtained in our survey, and this review is concerned with our observations on the molecular factors which determine the types of mesophases formed, nematic or smectic, and their transition temperatures and behaviors.

Various molecular expressions for the stress tensor for polymeric liquids are given and interrelated. Particular attention is paid to restrictions associated with constraints in the molecular models, hydrodynamic interaction, and molecular confinement in concentrated systems. A recently derived generalization of the Kramers-Kirkwood stress-tensor formula can be taken as the starting point for many polymer calculations.

Latest injection molding machines are often equipped with process control computers monitoring and controlling operating sequence, injection speed, holding pressure, clamping motion, clamping force, and all temperatures. This provides for the possibility to overlay process-related optimization programs for adaptation of the machine to the respective material condition and programs for the machine control. The following pages inform about such optimization programs.

The dynamics of dense model systems of pearl-necklace polymer chains, consisting of up to N = 98 hard spheres each, has been investigated using Monte Carlo methods. Excluded volume conditions as well as entanglement constraints have been taken into account. The time-dependent displacement of a single monomer on the chain follows the classic Rouse equation gr(t) α t^1/2 until the monomer equilibrates, whereas the diffusion constant for the center-of-mass motion is in agreement with the reptation law D α N^−2±0.2. The equilibration time for conformational fluctuations is given by the Rouse equation T_e α N², whereas the disengagement time, after which the motion of the monomers is dominated by the center-of-mass motion, is given by T_d, α N^3.4±0.2.

We define six kinetic growth models that have witnessed an explosion of recent activity:

We also describe briefly some of the approaches used to study these models, with emphasis on the re normalization group approach being developed by us.

Recent advances in the area of molecular diffusion in polymer-solvent systems will be reviewed. Alfrey's classification scheme for the diffusion of solvent molecules in polymers will be used as a starting point. It will be shown that the various regions on Alfrey's diagram are distinguished by the ratio of two characteristic times, a characteristic relaxation time for the polymer-solvent system and a characteristic diffusion time. Fickian diffusion is realized when this characteristic ratio is a small number, and anomalous behavior occurs when these two characteristic times are the same order of magnitude. Alfrey's “null” region corresponds to those conditions where this dimensionless ratio is much greater than one. The different manifestations of anomalous diffusion with polymer-solvent systems will be discussed, including case II transport as defined by Alfrey, Gurnee, and Lloyd. Finally, diffusion in polymer-solvent systems above the glass transition temperature where classical diffusion theory is applicable will be considered. Emphasis will be placed on the demonstration of the ability of theoretical methods based on free volume concepts to predict the concentration and temperature dependencies of diffusion coefficients. The extension of this free volume theory to describe diffusion in glassy polymers will also be demonstrated. Experimental results for several polymer-solvent systems which exhibit different types of transport behavior will be used to illustrate the correlative and predictive capabilities of these recently developed theories.