Investigation of network heterogeneities in filled, trimodal, highly functional PDMS networks by ¹H multiple quantum NMR

The segmental order and dynamics of polymer network chains in a filled, trimodal silicone foam network have been studied by static ¹H multiple quantum (MQ) NMR methods to gain insight into the structure property relationships. The foam materials were synthesized with two different types of cross-links, with functionalities, φ, of 4 and near 60. The network chains were composed of distributions of high, low, and medium molecular weight chains. Cross-linking was accomplished by standard acid-catalyzed reactions. MQ NMR methods have detected domains with residual dipolar couplings (〈Ω _d〉) of near 4 and 1 krad/s assigned to (a) the shorter polymer chains and chains near the multifunctional (φ = 60) cross-linking sites and to (b) the longer polymer chains far from these sites. Three structural variables were systematically varied and the mechanical properties via compression and distributions of residual dipolar couplings measured in order to gain insight into the network structural motifs that contribute significantly to the composite properties. The partitioning and average values of the residual dipolar couplings for the two domains were observed to be dependent on formulation variables and provided increased insight into the network structure of these materials which are unavailable from swelling and spin-echo methods. The results of this study suggest that the domains with high cross-link density contribute significantly to the high strain modulus, while the low cross-link density domains do not. This is in agreement with theories and experimental studies on silicone bimodal networks over the last 20 years. In situ MQ-NMR of swollen samples suggests that the networks deform heterogeneously and nonaffinely. The heterogeneity of the deformation process was observed to depend on the amount of the high functionality cross-linking site PMHS. The NMR experiments shown here provide increased ability to characterize multimodal networks of typical engineering silicone foam materials and to gain significant insight into structure-property relationships.