The molecular structure of macromolecules determines to a very large extent the macroscopic properties of the polymers they make up, and delimits what variations in properties can potentially be effected by processing. It is, therefore, important to understand and to be able to reliably predict the relevant properties of polymers and the limits of these properties upon treatment from the knowledge of the molecular structure of the constituent chains. For many properties of amorphous polymeric glasses correlations have been introduced in the last decades; some of them have been very useful [1–3], but they lack basic understanding of the processes and mechanisms operative in the generation of the properties in question. Their application is, by definition, limited to interpolation in that part of “chemical structure space” used as basis for the correlation. Prediction implies the power of extrapolation and, consequently, some use of first-principle methods. Remarkably little such work [1, 4–6] has been done to date on amorphous polymeric glasses. We have recently begun to investigate the atomistic-level modeling of structure and properties of amorphous, “fully relaxed” polymeric glasses [7–9].

A stochastic Monte-Carlo approach, based on the kinetic theory of fracture, has been used to study the axial maximum tensile strength of polymer fibers. The approach is entirely microscopic and the inhomogenous distribution of the external stress among atomic bonds near the chain ends is explicitly taken into account. Both primary and secondary bonds are assumed to break during fracture of the polymer fiber. The approach has been applied to perfectly oriented and ordered polyethylene and poly(p-phenylene-terephthalamide) fibers. The influence of the molecular weight, temperature ard strain rate on the axial tensile properties are presented.

We have developed a model of polymeric materials which includes many of the features of condensed-phase polymer chain dynamics, central among them chain relaxation by conformational motion. The model consists of a number of chains of particles which are connected by bonds with double-welled potentials to approximate the energetics of conformational motion. Interactions between particles on adjacent chains are modeled by shortrange repulsive potentials. We have examined the stress-strain behavior of the model using molecular dynamics simulations and find qualitative agreement with the observed experimental behavior of polymeric materials.

Successive stress relaxation testing was used to investigate the strain hardening of polypropylene and polystyrene in the stage of deformation before yielding. By combining this information with that of a stress relaxation test it was possible to measure the change in flow stress with plastic strain or “workhardening” parameter K. K has been associated with the nucleation of “defects” of some sort which slow down the kinetics of the deformation process.

Both polymers were found to strain harden in this deformation region. In polystyrene, the amount of time need to relax through a fixed stress increment reached a plateau at a point corresponding with visible crazing in the gage section of the sample. The workhardening parameter K was determined and found to decrease with stress. By plotting the rate of change of flow stress with total strain plots were obtained which avoided the use of strain, an ill-defined parameter in materials which change state during deformation. From these plots it was seen that polystyrene exhibits a well-defined linear region at small strain whereas polypropylene deviates from linearity immediately. Hardening of polystyrene was observed even in the linear response regime.

Understanding the dynamics of the microscopic mixing of polymer across an interface is crucial in determining the dependence of the fracture yield stress on healing time, and on the dynamics of polymer blending. Previous theoretical analyses of healing by Prager, Tirrell and Adolf [1,2,3] and Kim and Wool [4] have been helpful in interpreting the results of crack healing experiments. These models have asserted strict reptation dynamics and then postulated simple relationships between microstructure and fracture stress. The underlying microstructure can be probed directly by fluorescence measurements on interdiffusing chromophore-labeled polymers.

We present an asymototic analysis of small angle neutron scattering data of a comb-shaped charged copolymer, poly (1-octadecene-co-maleic anhydride), in D20. It is shown that at full ionization, the coRolymer self-asso iate to form cylindrical shape micelles with a radius 26Å and length 101 Å. The number of repeating units per micelle is calculated to be 200 and the hydration number 15 per repeating unit. The dimension of the cylinder remains approximately constant over the concentration range studied.

The stress-induced crystalline α⇌β phase transition found in poly(butylene terephthalate) and its copolymers with poly(tetramethylene oxide) has been studied by Fourier transform infrared spectroscopy coupled with mechanical measurements. The phase transformation behavior was explained in terms of a cooperative model which considered both intermolecular as well as intramolecular interactions within the crystal. It was shown that the strength of the intramolecular interactions increased with length of the hard segments and that the strength of the intermolecular interactions increased with perfection and lateral size of the crystals. The intermolecular interaction was assumed to be dominated by the interaction between neighboring terephthalate groups. The “mean” intramolecular energy was estimated at 0.40 Kcal/mole. This calculation was based on the potential energy of rotations of a carbonyl group about a benzene-carbonyl bond. Cooperativity between chains diminished when the surface to volume ratio increased above 2 x 10-2 Å-1.