Deuterium NMR has been employed to determine the average orientation of chain segments in poly(butadiene) networks. It is shown that the free induction decay separates the contribution to the orientation arising from the network constraint to that from chain interactions. The NMR spectrum line shape reveals the orientational distribution of network vectors due to the cross-links, whereas the observed splitting gives information about the orientation due to segmental interactions. Both the line shape and splitting have been fitted simultaneously for a range of deformed poly(butadiene) networks. From the fitting parameters, the separate contributions to the average orientation of the chain segments arising from the network constraint and from the interactions are calculated. These in turn are used to determine the molecular weight between cross-links and the size of the segmental interactions, which we choose to express in terms of the Edwards' screening length. This work is also important to computer modeling of the stress−optical coefficient where the interchain interactions are ignored.

Solid-state molecular chain diffusion in linear high-density polyethylene (HDPE) is established as the dominant mechanism for the crystalline 13C longitudinal relaxation at 60 °C, confirming previous work of Schmidt-Rohr and Spiess. A progressive saturation NMR experiment was undertaken on several samples of HDPE, where different degrees of cross-linking were introduced by electron irradiation. A decrease in the rate of recovery of the crystalline signal was observed with an increase in cross-link density, as measured by gel fraction. As irradiation forms cross-links on the fold surfaces of the lamellae, this behavior cannot be explained through a conventional dipolar spin−lattice mechanism or 13C spin diffusion. Transport of magnetization via chain diffusion to the amorphous region, where it experiences an efficient relaxation process, is consistent with the relaxation data and the Overhauser enhancement. As the gel fraction increased from zero to 87%, the effective diffusion coefficients almost halved from the nonirradiated sample (0.033 nm2·s-1) to 0.018 nm2·s-1.

Polybutadiene star polymers, of nominal functionality 3, 4, 8 and 12, all with arm lengths of approximately 30 000 g mol -1 , have been synthesized using chlorosilane coupling agents. Dynamic mechanical studies have been carried out using parallel-plate rheometry at various temperatures, and the data frequency-temperature shifted to produce master curves. Fits to the Ball-McLeish constraint release theory indicate an effective entanglement molecular weight somewhat higher than the literature value for a linear polymer. This has been interpreted by considering a dilution of constraints effect caused by the Rouse diffusion of the terminal section of the star arm, leading to a predicted modified entanglement molecular weight similar to that found experimentally. Evidence for the Rouse-like behavior of the terminal section has been obtained from deuterium NMR spectra of a selectively deuterated sample. The presence of small amounts of residual linear chains is shown to be responsible for a further dilution of entanglements. An extension of the theoretical fits to include the high-frequency Rouse mode spectrum shows a divergence between theory and experiment which is indicative of the onset of the glass transition.

Two very different high‐modulus polyethylene fiber samples, a low molecular weight melt‐spun and drawn fiber, and a high molecular weight gel‐spun and drawn fiber, have been subjected to electron beam irradiation to various doses in vacuum and in the presence of acetylene. The gel content after irradiation in acetylene was found to be much greater than for an equivalent dose in vacuum. The gel content–dose relationship could not be described by either Charlesby–Pinner analysis or the Inokuti equation. This is attributed to the polydispersity and the complications introduced by the unique morphologies of highly drawn fibers. Following previous studies, the tensile creep behavior was interpreted in terms of a model comprising two thermally activated processes in parallel, a low stress process relating to the amorphous network, and a high stress process relating to the continuous crystal fraction. Analysis of the creep behavior of the melt‐spun, low molecular weight fiber irradiated in vacuum revealed crosslinking in the amorphous regions and chain scission in the crystal. Chain scission was found to be much reduced when irradiating in acetylene, for which a mechanism has been proposed. The creep rates and activation volumes of the high molecular weight, gel‐spun fiber were found to be significantly lower, probably due to the unique morphology. In this case the dominant effect of irradiation on the mechanical properties can be attributed to chain scission rather than crosslinking.