Molecular Mobility in Semi-IPNs of Linear Polyurethane and Heterocyclic Polymer Networks

A study of the thermal and mechanical properties of new semi-interpenetrating polymer networks (IPN's) based on linear polyurethane (PU) and crosslinked trimerized dicyanate (TDC) reveals the existence of structures characterized by the absence of chemical interactions. Two distinct glass transitions are observed in the thermograms, as an indication of the fact that the two polymeric components preserve their molecular structure. The interpenetration affects markedly the glass transition temperatures revealed in the pure components in consequence of modifications in the local environments of the relaxing molecular units in the two phases. The primary and secondary relaxations of these systems show features which can be explained by accounting for the free-volume decrease due to the inclusion of PU in the network of TDC. Below the glass transition two molecular relaxations have been observed which have been ascribed to the secondary relaxation motions characterizing each polymeric component. Both relaxations exhibit a marked non-exponentiality which has been well accounted for in terms of a Gaussian distribution of relaxation times. The results of this analysis suggest that the local motions of TDC, ascribed to phenylene groups in the crosslinks between the cyanate units, are severely restricted by the inclusion of PU, while those of PU are slightly influenced by the presence of TDC.