It is shown that diffusion in the hard-square and hard-octahedron lattice gases at high particle concentration has cooperative properties resembling molecular relaxation in undercooled liquids near the glass transition. For these models a characteristic length of cooperativity is introduced by an underlying percolation problem, which determines whether permanently blocked particles exist in lattices of finite size. The percolation problem belongs to a general class of bootstrap percolation models. Salient Monte Carlo results for the concentration and size dependence of self diffusion in the hard-square lattice gas are presented. Similarities with the n-spin facilitated kinetic Ising models are also pointed out.

Positron annihilation spectroscopy (PAS) has been developed to characterize the free-volume properties of polymers. Positron annihilation lifetime measurements give direct information about the dimension, content, and hole-size distributions of free-volume in amorphous materials. The angular correlation of positron annihilation radiation measurements give additional information about the shape of the free-volume holes in oriented polymeric materials. The unique capability of PAS to probe free-volume properties is from the fact that positronium atom is preferentially trapped in the atomic-scale holes which have a size ranging from 1 to 10 Å.

The free volume behaviour of polycarbonates annealed above and below the glass transition temperature, Tg, is studied. The free volume behaviour as a function of temperature and as a function of time during isothermal relaxation is compared for the different heat treatments. The relaxations which occur during contraction experiments performed after cooling from the heat treatment temperature result in a decrease in free volume for both sets of samples; however, the general decrease is much greater in the polycarbonate annealed below Tg. For the polycarbonate annealed below Tg, the free volume is partially recoverable if the samples are reheated to the original annealing temperature. Differences in the free volume relaxation behaviour during contraction experiments are discussed.

Molecular dynamics simulation has been performed with dense systems of alkane-like chain molecules. Distribution of occupied and unoccupied space in the system was then evaluated by the technique of Voronoi tessellation of space and by enumeration of the cavities formed when hard spheres of diameter D were assumed placed on atomic centers. The distribution of cavity volumes is extremely broad and often exhibits a bimodal or trimodal character. These cavities change shape and size with time even at temperatures much below Tg.

Dielectric and mechanical relaxation spectroscopy of thermoplastics and thermosets show that, in the former, ageing is related only to the spontaneous structural relaxation towards a lower energy, but still disordered, structure and decreases the strength of the β- relaxation process. Ageing of a thermoset initially increases the strength of its (dielectric) β-relaxation as a result of diffusionallowed chemical reactions, which increase the number of dipolar groups, and later decreases it towards a limiting value. The decrease in the strength of β-relaxation on ageing of polymers and other amorphous solids suggests that structural relaxation causes a collapse of low-density, high-entropy, regions or “soft-sites” randomly distributed in the structure of a glassy material. The increase in the strength of β-relaxation observed in thermosets is higher than calculated from the extent of chemical reactions that occur during their structural relaxation at T<Tg. It is suggested that these effects should be included in the phenomenological theories for both the physical ageing and “memory behaviour” - the latter can be conveniently studied by both dielectric and mechanical spectroscopy and by differential scanning calorimetry. Structural relaxation also decreases the magnitude of the specific heat anomaly in amorphous solids and can be used to experimentally test the postulate that tunneling centres involved in the low-temperature phonon properties are related to the presence of “islands of mobility” or “soft-sites” in an otherwise rigid glassy matrix.

Solubility and diffusivity measurements of CO2 have been used to measure amount of residual porosity (excess volume) and the kinetics of its removal in latex films cast from a terpolymer of butyl acrylate, vinyl acetate, and vinyl chloride. The pore volume fraction is very small, ranging from 0.5–0.05% for latex films aged from 62 hrs to 480 hrs, respectively. Long-term aging kinetics can be followed by sorbing to equilibrium at constant CO2 pressure and temperature, and following the weight loss as the sample relaxes and CO2 diffuses out. Relaxation data obtained at temperatures ranging from 25–50°C could be superimposed on a single master curve using the WLF equation. The implications these results have concerning the morphology of the residual porosity in latex films are discussed.

Films of a poly(imidc siloxane) thermoplastic copolymer with a thickness of 0.5 mm have been characterized using thermal analytic techniques including DSC, TMA, and DMA. It was found that the glass transition temperature depends not only on heating rates but also on loading conditions. The mechanism of the glass transition in this copolymer is shown to involve two interconnected steps, namely volume change and phase ordering.

Room temperature free volume relaxation of polycarbonate as a function of aging time at 393K has been evaluated via positron annihilation lifetime spectroscopy. Free volume response data were normalized and modeled using an expression of the Williams-Watts form. The process of physical aging is discussed in terms of the trends associated with the resultant characteristic relaxation time constants and distribution of relaxation times.

Relaxation in the glass transition region is very important to the understanding of the mechanical properties of polymers. Both (i) traditional viscoelastic behavior resulting from application of a mechanical stress or strain and (ii) volume relaxation due to the temperature or pressure change. Structural relaxation near the glass transition region has mostly been probed via isothermal volume recovery after a step change in temperature. Key features of the isothermal volume recovery are well known and include (i) nonlinearity of the kinetics in terms of the magnitude of the initial departure from equilibrium, (ii) asymmetry of the relaxational behavior with respect to the sign of the temperature perturbation and (iii) memory effects related to the thermal history and physical aging.

Ab initio calculations were used to characterize the inter- and intramolecular energy surfaces for polyvinylchloride (PVC) model compounds and a classical force field was fit to these energy surfaces. This new force field will be used in molecular mechanics and stochastic dynamics simulations to analyze the atomic level morphology of PVC.