DIRECT COMPARISON OF THE SEGMENTAL ORIENTATIONS OF FREE OLIGOMER AND NETWORK CHAINS IN POLYBUTADIENE MODEL NETWORKS

The segmental orientation in a semi-interpenetrating polymer network b ased on crosslinked polybutadiene swollen to different degrees by free butadiene oligomers (of molar masses below the entanglement limit) is studied as a function of uniaxial strain for both the oligomer chains and the network chains by using H-2 nuclear magnetic resonance (n.m.r .) spectroscopy. While the second moment of the orientation distributi on function, P-2, is usually obtained from the quadrupolar splitting o f the maxima of the spectra, herein we obtain the averaged P-2 from th e whole spectrum. For high volume fractions of the network chains (phi > 0.8) the untangled oligomer shows the same degree of orientation as the network, while for larger degrees of swelling the oligomer orient ation is smaller than that of the network chains. Strain birefringence measurements performed on the same materials show a similar strain de pendence as the orientation parameter obtained from H-2 n.m.r. spectro scopy. (C) 1996 Elsevier Science Ltd.