A comprehensive elastic-plastic constitutive model is employed in a finite element analysis of fatigue crack closure. An improved node release scheme is used to simulate crack growth during cyclic loading, which eliminates the associated numerical difficulties. New definitions of crack opening and closing stresses are presented in this paper. Special attention is paid to a discussion of some basic concepts of fatigue crack growth and crack closure behaviour. Residual tensile deformation and residual compressive stress are found to be two major factors in determining the crack opening stress. A comparison of crack tip node release at the maximum or minimum load of each cycle is made and the disadvantage of releasing crack tip node at the minimum load are pointed out.

Dynamic mechanical-loss spectra at 1 Hz, and dipolar rotational spin-echo 13C NMR spectra at 15.1 MHz, have been obtained for a series of copolymers of polycarbonate made from monodisperse oligomers of Bisphenol A polycarbonate (B X ), alternating (via connecting carbonate linkages) with single units of 3,3‘,5,5‘-tetramethylbisphenol A (T). The mechanical-loss relaxation of B X T suggests cooperative motions in the copolymer glass at T = −100 and −20 °C. The NMR results indicate that no phenylene rings undergo π flips in T units and that most but not all of the B rings flip faster than 10 kHz at 300 K. A necessary but not sufficient condition to constrain a B ring from flipping in B X T is the presence of both interchain and intrachain T-unit nearest neighbors. These results are interpreted in terms of a packing model for polycarbonate-like systems that emphasizes the importance for chain dynamics of the proximity and cooperative motion of nearest-neighbor chains having similar local orientational order.

Positronium annihilation lifetime spectroscopy (PALS) was employed to study the structure and dynamic properties of glassy BPA-PC and TMBPA-PC containing tris(2-ethylhexyl) phosphate (TOP) as diluent molecules. For BPA-PC/TOP blends, the results showed that the total hole volume fraction is insensitive to the addition of TOP up to 10 wt % but increases at higher TOP concentration. However, in the case of TMBPA-PC/TOP blends, a different trend was observed. The total hole volume fraction decreases monotonically with TOP concentration up to 15% and thereafter increases slightly from 15 to 20%. Dynamic mechanical relaxation was also used to monitor the dynamics of the blends. This part of the work was motivated by a prediction of the coupling model (made before the experimental measurement was carried out) that addition of TOP should have opposite effects on the γ-relaxation times of BPA-PC and TMBPA-PC. Indeed, it was found by dynamic mechanical measurement at constant frequency that, while the γ-relaxation peak temperature of BPA-PC was shifted to higher temperature with addition of TOP, the opposite effect was found in TMBPA-PC. The γ-relaxation peak temperature of TMBPA-PC was shifted to lower temperatures upon addition of TOP with little change in the width of the peak as predicted by the coupling model. The predictions of the coupling model and the dynamic mechanical results are supported by (1) dielectric relaxation measurements of the γ-relaxation performed on the same TMBPA-PC/TOP blends by Rizos and co-workers [accompanying paper] and also (2) the phosphorus-31 nuclear magnetic resonance data on diluent dynamics in the same BPA-PC/TOP and the TMBPA-PC/TOP blends obtained by Bergquist et al. [accompanying paper], which is shown by them to be also consistent with the lattice model for diluent dynamics.

The sub-Tg relaxations of Bisphenol-A polycarbonate (BPA-PC) can be selectively altered by appropriate chemical modification. It is established that these secondary relaxations are responsible for the in-chain cooperative motions of BPA-PC and have profound effects on the deformation behavior of the bulk material. Through investigations of the microdeformation behavior of BPA-PC and alternating block copolymers based on BPA-PC, it is found that the extent of cooperative motion is also influential in activating the disentanglement crazing mechanism at elevated temperatures.

Dipolar-rotational spin−echo 13C NMR spectra at 15.1 MHz have been obtained for 12 homopolymers, copolymers, and blends of polycarbonate, poly(ether carbonate), poly(ether sulfone), dimethyl polycarbonate, and tetramethyl polycarbonate. The spectra were used to determine the fraction of nonmethylated phenylene rings that are blocked from undergoing π flips faster than 10 kHz at 300 K. This fraction is interpreted in terms of a simple two-chain bundle packing model in which empirically determined steric parameters specify the interchain interactions between flexible chain linkers and rigid chain extenders. The success of this bundle description suggests that the glassy state for all of these polycarbonate-like systems has local mainchain orientational order.