Molecular aspects of polymer network deformation ‐ small angle neutron scattering and NMR studies

Abstract

networks obtained by a 4functional random cross-linking reaction over a broad range of polymer concentration were studied by small angle neutron scattering(SANS),ZH NMR and Monte Carlo(MC) simulation in the isotropic and uniaxially deformed state. The defect structure of the networks has been characterized by MC simulation of the cross-linking reaction. The anisotropy of the radius of gyration in deformed networks determined from SANS has been analyzed by the theory of Ullman. It was found that the number of active cross-links per chain is in agreement with MC and that the chain deformation follows phantom behaviour. The local orientation as measured by 2H NMR is related to the global anisotropy of the network by a MC calculation of oriented chains. The NMR line shape of the deformed network is analyzed in terms of two relaxation processes arising from interior parts of the chains and from segments at chain ends. The mobility of both decrease with strain. It was found that the orientation connected to the first process shows the classical strain dependence of rubber elasticity,whereas the second exhibits a weaker dependence on strain.

Effect of Precursor Molar Mass on the 2H NMR Line Shapes of End-Linked PDMS Elastomers

“…These observations have been corroborated by Monte Carlo (MC) simulations of rubbery networks, where the mean field was introduced as an excluded volume effect between chain segments. , Further MC work demonstrated identical Δν values upon stretching for both elastic chains and free chains dissolved in a network, but a higher average total order for the elastic chains . These analyses and experimental results led to several models that attempted to describe the entire spectrum in terms of contributions from two different segment populations. − In their computer simulation study of NMR line shapes of perfect networks (no defects) under increasing strain, Yong and Higgs observed that the central split of the spectra correlates with chain interaction and packing whereas the outer wings result from highly extended chains with initially large end-to-end distances . Networks of noninteracting (“phantom”) chains exhibited only a widening of the spectral line under strain and no split.…”

“…22 These analyses and experimental results led to several models that attempted to describe the entire spectrum in terms of contributions from two different segment populations. [23][24][25][26] In their computer simulation study of NMR line shapes of perfect networks (no defects) under increasing strain, Yong and Higgs observed that the central split of the spectra correlates with chain interaction and packing whereas the outer wings result from highly extended chains with initially large end-to-end distances. 27 Networks of noninteracting ("phantom") chains exhibited only a widening of the spectral line under strain and no split.…”