In this work, we explore how melt memory, induced by a previous crystalline state, can be used to tune the polymorphs of poly(vinylidene fluoride) during the recrystallization process. In situ experiments were conducted to monitor the melting and chain relaxation of highly oriented α-PVDF crystals during the heating process. Accordingly, a series of heterogeneous amorphous states (induced by self-nucleating the sample) can be made by controlling the thermal treatment temperature (i.e., the self-nucleation temperature, T s ), and the effect of the relaxation state of oriented crystals on the subsequent polymorphic crystallization behavior is studied in depth. A complete α → γ phase transition occurs during the cooling process if the sample is thermally treated in the self-nucleation temperature range of 181−186 °C within the Domain IIa region (i.e., within the melt memory Domain). Only if the oriented molecular chains relax to a certain degree and the 3D structures of the α lamellae are molten, can the retained ordered segments develop γ nuclei in the subsequent crystallization process. If the samples are heated to T s within Domain I (i.e., isotropic melt) or Domain IIb (self-seeding Domain), an α phase is preferred upon subsequent cooling. Hence, a temperature region of 181−186 °C leads to a special heterogeneous self-nucleated melt that can cultivate the rich γ nuclei developed from the relaxation along with segmental flip-flop and inversion motions of oriented molecular chains with a TGTG′ conformation. As a result, the rich γ nuclei enable the formation of pure γ crystals during the cooling process.

The effects of crystallization temperature and cooling rate on the polymorphism of trans-1,4-polyisoprene vulcanizate (TPI-V) with different sulfur dosages were monitored by WAXD in real time. The results showed that both the crystallization temperature and the cooling rate play decisive roles in determining the polymorphism and their relative amounts, whereas their effects are interfered by the crosslinking density which restrains the crystallization kinetics. Only α crystal is formed when crystallization is conducted at 40 C or greater and mixed α/β crystals are obtained in the temperature range of 10-30 C. β crystal is predominate when crystallized at 10 C while α crystal is limited at 30 C. Both the lower temperature and increased crosslinking density are beneficial to β crystal formation. A high crosslinking density causes a prolonged time is needed for pure α crystal formation, and hinders the TPI-Vs from forming pure α crystal even if crystallization is conducted at 45 C. A lower cooling rate favors the formation of α crystal while a higher cooling rate favors β crystal formation. The β spherulites are much brighter than α spherulites under the observation of polarized microscopy.

Poly(L-lactic acid) (PLLA) thin films with a highly oriented structure, successfully prepared by a fast friction transfer technique, were investigated mainly on the basis of synchrotron radiation wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR). The crystalline structure of the highly oriented PLLA film was remarkably affected by friction transfer temperatures, which exhibited various crystal forms in different friction temperature regions. Interestingly, metastable β-form was generated at all friction transfer temperatures (70–140 °C) between Tg and Tm, indicating that fast friction transfer rate was propitious to the formation of β-form. Furthermore, the relative content among β-, α′-, and α-forms at different friction temperatures was estimated by WAXD as well as FTIR spectroscopy. In situ temperature-dependent WAXD was applied to reveal the complicated phase transition behavior of PLLA at a friction transfer temperature of 100 °C. The results illustrated that the contents of β- and α′-forms decreased in turn, whereas the α-form increased in content due to partially melt-recrystallization or crystal perfection. Moreover, by immersing into a solvent of acetone, β-, α′-form were transformed into stable α-crystalline form directly as a consequence. The highly oriented structure was maintained with the chain perfectly parallel to friction transfer direction after acetone treatment, evidenced by polarized FTIR and polarized optical microscopy (POM) measurements.