Polylactide is a bio-based and biodegradable polymer well-known for its renewable origins. Water sorption and clustering behavior in both a homopolymer polylactide and a graft copolymer of polylactide was studied using the quartz crystal microbalance/heat conduction calorimetry (QCM/HCC) technique. The graft copolymer, poly(1,5-cyclooctadiene-co-5-norbornene-2-methanol- graft- d,l-lactide), contained polylactide chains (95 wt%) grafted onto a hydrophobic rubbery backbone (5 wt%). Clustering is an important phenomenon in the study of water transport properties in polymers since the presence of water clusters can affect the water diffusivity. The HCC method using the thermal power signals and Van't Hoff's law were both employed to estimate the water sorption enthalpy. Sorption enthalpy of water in both polymers was determined to be approximately -40 kJ/mol for all water activity levels. Zimm-Lundberg analysis showed that water clusters start to form at a water activity of 0.4. The engaged species induced clustering (ENSIC) model was used to curve fit sorption isotherms and showed that the affinity among water molecules is higher than that between water molecules and polymer chains. All the methods used indicate that clustering of water molecules exists in both polymers.
Bibliographical noteFunding Information:
This research was supported by USDA Biomass Research and Development Initiative , funding number DE-PS36-06GO96002P . Partial support was provided by the Center for Sustainable Polymers, a National Science Foundation Center for Chemical Innovation ( CHE-1136607 ). Access to experimental instruments in the laboratory of Giuseppe Palmese (Chemical and Biological Eng), Christopher Li (Materials Sci. & Eng) and the Drexel Centralized Research Facility is acknowledged. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company and Northwestern University. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract no. DE-AC02-06CH11357 . Discussions with Shri Ramaswamy (U. Minnesota, Bio-based products Dept.) and Yossef Elabd (Chemical Eng.) were also helpful for this research.
- Water sorption