Role of Structural Heterogeneities on Segmental Orientation in Deformed Chains: Application to Alternating Copolymers

Effects of intrinsic structural and conformational properties on segmental orientation in uniaxially deformed copolymers are considered. Depedence of segmental orientation on equilibrium values of bond angles, torsional states, and probability distribution of rotameric states is studied. Calculations are carried out for chains with independent as well as pairwise interdependent rotameric states for neighboring bonds using the matrix generation technique of rotational isomeric state formalism. Results invite attention to the importance of specific energy and geometry parameters in prescribing the level of molecular orientation in the two diierent components A and B of AB type copolymers. Results are interpreted with reference to polarized Fourier transform infrared spectroscopy measurements in which the orientation of transition moment vectors is detected. The consequences of certain assumptions in data interpretation such as the choice of cylindrically symmetric reference axes along the chain contour are pointed out. The orientations of vectors along the backbone exhibit strong nonlinear dependence on the conformational characteristics of the component A or B to which they are appended. Thus, the bond vectors of the two monomeric units may exhibit quite distinct orientations, arising only from slight perturbation in bond angles of one of the units. Vectors perpendicular to chain backbone were less sensitive to monomeric structure. Calculations carried out independently by Monte Carlo simulations showed that this method yields an adequate qualitative description of the orientational behavior of chain segments while precise quantitative determination requires the use of the exact matrix generation technique.