MONTE-CARLO SIMULATIONS OF THE ORIENTATIO NAL ORDER IN A STRAINED POLYMER NETWORK

Deuterium nuclear magnetic resonance experiments performed in rubber n etworks under uniaxial stress display a characteristic doublet structu re, both for network chains and for free chains diffusing into the net work. This doublet structure can be explained in a mean-field model by introducing orientational segmental interactions in the network syste m. Monte Carlo simulations on a cubic lattice are performed and compar ed to the model. Segmental interactions are simply introduced into the simulations by applying the condition of mutually avoiding chain segm ents. It is confirmed that the segmental interactions induce a uniaxia l orientation in stretched networks, which means that each segment sho ws orientational behavior as being under the influence of a uniaxial f ield. The mean-field description is shown to be well reproduced by the simulations. The calculation of the nuclear magnetic resonance spectr um for the model network in the simulation gives a doublet structure o nly for the case where steric interactions are present. The effect of network dilution is studied in the simulations by varying the volume f raction in the system. It is shown that the uniaxial segmental orienta tion decreases when the system is diluted, which is in accordance with the mean-field model as well as with experimental observations.