Molecular Orientation and Relaxation in Uniaxially Stretched Segmented PTMO Zwitterionomers by Polarization Modulation Infrared Linear Dichroism

ABSTRACT: The orientation and relaxation behavior of segmented poly(tetramethylene oxide) (PTMO) zwitterionomers was studied under conditions where no strain-induced crystallization occurs, using polarization modulation infrared linear dichroism. Samples of three different PTMO segment lengths, designated Z67, Z52, and Z20 for PTMO blocks with molar masses of 6700, 5200, and 2000 g/mol, respectively, were used. During deformation to a draw ratio of 3.2, the PTMO chains progressively orient along the stretch direction, the maximum induced orientation decreasing with increase in temperature. In contrast, the orientation function of the sulfonate units increases only at the beginning of the deformation process, reaching a maximum of about 0.015 at a draw ratio of about 1.5, and then decreases to become negative before the end of the stretch period. During the subsequent relaxation period, the PTMO orientation decreases as a function of time and the negative orientation function of the ionic groups further decreases slightly with time. By analogy with segmented copolymers characterized by a crystalline hard phase and an elastomeric soft phase, the following model is proposed. Before deformation, the ionic aggregates, which have previously been shown to have lamellar (Z20) or hexagonal (Z52, Z67) morphology, are like rigid anisotropic entities randomly oriented throughout the sample. Upon deformation to low draw ratios, the flexible PTMO chains orient along the stretch direction and the ionic rigid entities orient perpendicular to the stretch direction, the latter entailing orientation of the ionic side chains parallel to the stretch direction. At higher draw ratios, the applied stress causes progressive disruption of the ionic aggregates, resulting in a reorientation of the “freed ” ionic side chains perpendicular to the deformation direction