Thermal Properties and Thermodynamics of Poly(l-lactic acid)

Abstract

Thermal properties and thermodynamics of poly(l-lactic acid) PLLA at nonequilibrium and equilibrium states and during reversing and nonreversing processes are presented, based on the apparent heat-flow and heat capacity (C p ). The experimental, apparent heat capacity results from measurements by adiabatic calorimetry, standard differential scanning calorimetry, and temperature-modulated differential scanning calorimetry are interpreted in terms of microscopic molecular motion in the entire temperature range. The low-temperature, below the glass transition, experimental heat capacity of solid state is linked to the vibrational motion. The heat capacity of the liquid state of PLLA is linked additional to the vibrational, also to the conformational, and anharmonic motions or estimated from an empirical addition scheme based on contributions of the constituent chain-segments of polymers. Once calculated, solid C p (vibration) and liquid C p (liquid) heat capacities are established so they can serve as two reference baselines for the quantitative thermal analysis of nonequilibrium semicrystalline poly(lactic acid). Knowing heat capacities (C p (vibration), C p (liquid)) and transitions parameters, the integral functions such as the enthalpy (H), entropy (S) and free enthalpy (Gibbs function) (G) for equilibrium conditions are calculated and used as a reference for analysis. All recommended results for PLLA, are collected and organized as part of the ATHAS Data Bank. Examples of the qualitative and quantitative thermal analysis of amorphous and semicrystalline poly(lactic acid) are presented to characterize phases and phase transitions such as glass transition, enthalpy relaxation, cold crystallization/cystallization, reorganization and melting, as well as amount of phase: crystallinity, mobile and rigid amorphous fraction on the ATHAS scheme (Advanced Thermal Analysis System).