Conformational change and orientation fluctuations of isotactic polystyrene prior to crystallization

In order to clarify the preparation process of crystal nucleation of polymers, we have studied the conformational changes and orientation fluctuations of isotactic polystyrene (iPS) in the induction period of crystallization using time-resolved Fourier transform infrared (FT-IR) spectroscopy and depolarized light scattering (DPLS) techniques, respectively. Their measurements were carried out for the so-called cold crystallization, when it was crystallized somewhat above the glass transition temperature T_g = 100 °C from the glassy states. It is found from the time-resolved FT-IR measurements that the polymer chains begin to assume alternate conformations of trans (T) and gauche (G) in the induction period, which are characteristic to the 3/1 helix in the crystalline state. On the other hand, the time-resolved DPLS observations reveal that orientation fluctuations begin to grow in the induction period after a temperature jump to the crystallization temperature, which means that some rod-like segments of polymer chains actually start to orient parallel to one another prior to crystallization. These results are discussed in terms of the Doi's theory on the isotropic-to-nematic transition of liquid crystals where we assumed that the 3/1 helical segments are stiff and have a persistence length being almost equal to the length of the helix axis. Thus, the growth of the helices involves the increase in persistence length and hence in excluded volume, to cause the parallel ordering of the helix parts as stiff segments.