Thermodynamic studies on the helix-sense inversion of polyaspartates in the solid state

The Clapeyron analysis has been performed on polyaspartate samples (-[NH-CH(CH2C(O)OX)C(O)]n-) carrying side chains such as X = benzyl (1PLA), phenethyl (2PLA), phenylpropyl (3PLA), and phenoxyethyl (PPOLA). In all cases examined, the unwinding and rewinding process of the helical backbone is consistent with the zippering mechanism previously elucidated for the concentrated LC solution. Variation of the PVT properties such as the volume and enthalpy changes at the transition was carefully examined. In the bulk solid state, contribution from the thermodynamic quantities associated with the redistribution of molecules in the configurational space (molecular packing) becomes substantial. It has been concluded that the characteristic features of the ℓ-α|ℓ-ω transition of 1PLA (140 °C) are inconsistent with the Clapeyron concept, suggesting that the phase transition may involve a non-equilibrium step. The r-α|ℓ-ω transitions of the other polymers were found to conform to the Clapeyron relation. The screw-sense inversion seems to be reversible only when the crystalline structure (hexagonal) remains unaltered before and after the transition. The purpose of this work is to elucidate the key factor responsible for the reversibility of the solid-sate transition, thus providing a basis to understand the role of flanking side chains.

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