Direct comparison of the segmental orientations of free oligomer and network chains in polybutadiene model networks

Abstract

The segmental orientation in a semi-interpenetrating polymer network based 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 2H nuclear magnetic resonance (n.m.r.) spectroscopy. While the second moment of the orientation distribution function, P2, is usually obtained from the quadrupolar splitting of the maxima of the spectra, herein we obtain the averaged P2 from the whole spectrum. For high volume fractions of the network chains (φ > 0.8) the untangled oligomer shows the same degree of orientation as the network, while for larger degrees of swelling the oligomer orientation is smaller than that of the network chains. Strain birefringence measurements performed on the same materials show a similar strain dependence as the orientation parameter obtained from 2H n.m.r. spectroscopy.