Unusual Phase Transition of Poly(di(alkyl) vinylterephthalates)

An unusual phase behavior, showing the disordered phase at low temperatures and the ordered phase at high temperatures, which is known as the isotropic phase reentry, has been observed in mesogen-jacketed liquid crystalline polymers and other side-chain jacketed polymers, for example, in some poly(di(alkyl) vinylterephthalates) (PDAVTs). In general, such a phase behavior is considered entropy dominant. However, the mechanism underneath is not fully addressed yet. Here, we study in detail the feature of phase transition of the PDAVT samples with the alkyl group of butyl, hexyl and octyl (denoted as P4, P6 and P8, respectively) by differential scanning calorimetry (DSC), X-ray diffraction (XRD), rheology, and solid-state NMR. Undetectable in DSC experiment, the columnar liquid crystalline (Col) phase formation is demonstrated by XRD upon heating the PDAVT samples from isotropic state. It is also evidenced by rheology measurement, showing that, after glass transition, the samples have their shear storage modulus increased by two orders of magnitude at high temperature. Upon cooling, P4 retains its Col phase probably due to the molecular motion frozen by glass transition, while P6 and P8, which have quite low glass transition temperature (T-g), can fully return to the isotropic state, exhibiting the typical behavior of isotropic phase reentry. It is found that the Col phase formation is nucleation-limited, with the feature of one-dimensional growth. At higher temperature, the nucleation barrier is drastically reduced, resulting in a much faster growth rate of Col phase. Sufficient development of the Col phase at the temperature above Tg makes the samples solid-like. Solid-sate NMR experiment reveals that increasing the temperature activates the alkyl tail motion first, which in fact triggers the Col phase formation. Effects of shearing and stretching on the Col phase formation are further examined. It is found that the external fields applied cannot induce the isotropic-to-Col transition of PDAVT when the temperature is close to the transition temperature, although the PDAVT chains can be oriented to some extent. On the basis of the experimental data obtained using these techniques, we conclude that the maximization of side-chain entropy is the driving force for the isotropic-to-Col phase transition. Namely, only at sufficiently high temperature can the strong side-chain motion enhance the side-chain jacketing effect, making the chains more rod-like which will pack parallelly to form the Col phase.