(579e) Water Transport Mechanisms in Polylactide by Quartz Crystal Microbalance/Heat Conduction Calorimetry
Amongst all bio-based polymers, polylactide (PLA) is well-known for its renewability and being able to biodegraded. PLA therefore is favored in packaging and medical applications. Water transport in PLA has been studied on several commercial and synthesized films of PLA prepared by spin coating and heat treatment to control crystallinity. Morphology of PLA was investigated by SEM and DSC methods. Water sorption in all PLA samples was studied by the quartz crystal microbalance/heat conduction calorimetry (QCM/HCC) technique. Sorption isotherms were constructed at 35oC for different water activity levels. Despite the difference in morphology, all PLA samples showed a negligible affect of thermal treatment on the amount of water sorption.
Amorphous PLA was used for the evaluation of mathematical models predicting water sorption mechanisms in PLA. Experimental results were fitted to Henry's law, Flory-Huggins theory, dual mode sorption model, and the engaged species induced clustering (ENSIC) model. ENSIC model accounting for the probability of addition of water in water/polymer system provided the best fit amongst the four models. By DSC experiment, loosely bound water was also found by a melting peak at near -0.62oC. Therefore, both DSC and curve fitting results showed a potential of water clustering existence in PLA.
Additional evidence of water clustering in PLA comes from sorption enthalpy which was measured with the QCM/HCC instrument by two different methods. The first method is to apply the Van't Hoff's expression relating sorption enthalpy to the solubility measured at a series of different temperatures. The second method uses the thermal power signals from the HCC. By integrating the area under a sharp peak in thermal power caused by a change in moisture content, the sorption enthalpy can be estimated. Sorption enthalpy of water in commercial PLA was found to be close to -40 kJ/mol, which is very similar to the heat of condensation of water (-44 kJ/mol).