Structural properties of end -linked polymer networks: Monte Carlo simulation and neutron scattering studies

In this work, computational and experimental approaches were taken to elucidate the structural behavior of end-linked polymer networks. Monte Carlo simulations with the bond fluctuation model were used to study the formation and structure of 10-, 20-, and 50-mer networks cured with various ratios of cross-link sites to chain ends, r. The simulations confirmed experimental results that optimum r values for structural properties are found at non-stoichiometric ratios, increase with increasing precursor polymer chain length, and increase with side reaction. The same structural properties were determined for 50-mer networks formed in the presence of a series of concentrations of 50-mer unreactive linear chain diluent. A slight maximum in the fraction of elastic material was observed at a small degree of dilution and was explained in terms of entanglement effects. The conformational behavior of a small fraction of 50-mer linear probe chains was studied in various polymer network mesh sizes, where mesh size is the number of monomers between cross-links. The radius of gyration, R g, of the probe chains decreased by less than 6% for all mesh sizes studied.;Uniaxial network deformation was studied using constant pressure simulations. A new methodology to perform volume change moves was developed. The equation of state of 10-mer athermal network system was determined and found to be similar to the corresponding athermal linear polymer equation of state at high volume fractions, but approached a region of negative pressures at low volume fractions. The chain segment orientational correlation was determined for probe free chains trapped in isotropic and deformed networks. Increased correlation was observed at small distances, whereas no orientational preference was evident at larger distances.;The study of probe free chains trapped in polymer networks was also performed using small-angle neutron scattering. The conformational behavior of 70K poly(dimethylsiloxane) (PDMS) probe chains dissolved in 10K, 35K, 70K PDMS networks was studied and it was found that any variation in Rg as a function of mesh size was too small to be detected. It was also determined that network compression induced stiffness in the probe chains at short distances while maintaining Gaussian behavior at long distances.