Segmental Orientation in Model Networks of Poly(dimethylsiloxane): Fourier Transform Infrared Dichroism Measurements and Theoretical Interpretation

Segmental orientation in model networks of poly(dimethylsiloxane) (PDMS) in uniaxial tension is measured by infrared dichroism. Measurements are made on four tetrafunctional end-linked networks having molecular weights of 2 × 10³, 5 × 10³, 10 × 10³, and 23 × 10³ between junctions. Results of experiments are compared with predictions of calculations based on (i) the widely used Kuhn expression and (ii) the rotational isomeric state formalism. The Kuhn expression is found to considerably overestimate the segmental orientation of the PDMS chains. The rotational isomeric state approach leads to values of segmental orientation that fall between predictions of the affine and phantom network models. This indicates that nematic-like intermolecular contributions to orientation are not significant.