On the Stability of the Nematic Order Observed During the Cold-crystallization of PET

Abstract

The rotational isomeric state analysis of poly(ethylene terephthalate) (PET) was carried out by using parameters adjusted so as to reproduce the experimental results of 1 H NMR and dipole moment measurements on ethylene glycol dibenzoate. The central OC-CO bond of the ethyleneglycol unit prefers to be in the gauche form at lower temperatures, suggesting that the PET chains should contain many kink conformations in the amorphous state. In the following part, the nematic conformation of the same C-C bond was studied by preparing a model compound which is capable of forming an enantiotropic nematic liquid crystalline (LC) phase. IR studies indicate that the trans fraction of the C-C bond increases from $15% in the isotropic liquid to $50% in the nematic LC state at the phase boundary. For this drastic change in the conformation at the transition, the C-C bond should be required to pay certain entropy penalty. These results are however compatible with the recent observations by SAXS and DPLS that the formation of nematic LC-like domains proceeds prior to the evolution of primary crystal nucleation during the cold-crystallization of PET. The PET chain is known to take the planar all-trans conformation in the crystalline state. Finally a possible mechanism leading to the nematic order in the amorphous PET is presented. Growth of the nematic LC-like domains in the amorphous phase may lead to a spinodal microphase separation, as suggested by Kaji et al. The spatial configurations and intermolecular packing of polymer chains in the amorphous state are largely affected by the rotational characteristics of the constituent bonds. Upon crystallization, the polymer chains are required to uniformly take an ordered configuration to attain a dense packing. In most polymers, the crystalline form is found to coincide with one of the lowest energy arrangements of the free chain.1-3 The poly(ethylene terephthalate) (PET) chain is an exception, in which the chain goes through a large transformation to a highenergy conformation during the crystallization process.

Spatial Configuration and Thermodynamic Characteristics of Main‐Chain Liquid Crystals

“…Alternatively, the polymer molecules may cooperatively transform from the coil to a more ordered form to gain higher stability in the nematic LC domain. The latter scheme is compatible with the framework elucidated above for the LC formation of main-chain compounds [Abe et al, 2008].…”