SUPERMOLECULAR STRUCTURE DEVELOPMENT–MELTING BEHAVIOR RELATIONSHIP IN SOLID-STATE POLYMERIZED POLY(ETHYLENE TEREPHTHALATE)

Abstract

Low-molecular-weight poly(ethylene terephthalate) (PET) (viscosity-average molecular weight was subjected to solid-state polymerization (SSP) in the temperature range between 230 and 250°C for up to 20 hr under high vacuum The procedure of SSP allowed us to obtain a number of PETs ranging in M η from 43 to 300 kg/mol. It was found that the reaction of SSP in PET follows a second-order law for the degree of polymerization and is characterized by an apparent activation energy of 205 kJ/mol. The differential scanning calorimetry (DSC) scans of the SSP-PET were measured over a range of heating rates from 0.5 to 64°C/min. The increase in molecular weight from 43 to 300 kg/mol was accompanied by a marked increase in the melting point T m, by 26°C. However, such an increase in the T m value was not due to build-up of fully extended chains crystals since the thickness of the lamella in the PET with the highest value of as observed by atomic force microscopy, was found to be 200–400 Å (the chain length for this molecular weight is ∼8400 Å). A model for the morphology development in the course of SSP of the PET is proposed. In the framework of this model, the substantial increase in T m, beside the lamella thickening, may be mainly explained by two factors: (i) by the creation of a large number of intercrystalline tie molecules via both ester interchange between chain ends and the redistribution reaction between a chain end and a fold, and (ii) by a decrease in the crystal surface free energy as a result of the redistribution reaction. On the other hand, the model also demonstrates why the molecular weight of SSP-PET did not exceed the value of 300 kg/mol.