The project pursues fundamental enabling scientific work regarding the rheological and permeation properties of a family of copolymers based on lactic acid to ultimately lead to improved properties and applications. Unlike current packaging materials that are made from nonrenewable fossil fuels and often involve the use of noxious and toxic materials in their manufacture, these materials are fermentation based. Lactic acid polymers (PLA) are available from the fermentation of plant based materials including agricultural wastes; their use leads to a closed loop with respect to the greenhouse effect of CO2 by using renewable agricultural sources. In addition, these materials are compostable, freeing up landfill space and may be recycled to a lactic acid monomer source through simple hydrolysis. For these reasons, PLA is receiving a tremendous amount of commercial interest, though little fundamental understanding of the properties most important for successful commercialization is available. Furthermore, synthetic variations in structure are required to improve upon the properties for application in many areas.
Building on the results of some preliminary studies, the chemistry of PLA is manipulated in order to vary the L to D ratio within the chain. The characteristic ratio is determined as a function of the chiral ratio and correlated against the entanglement molecular weight, Me. The exact relationship between characteristic ratio and Me is not known for the general class of linear polymers but the PLA system provides an ideal experimental candidate for elucidating this very fundamental issue. Control of chirality implies control of the characteristic ratio, which can then be correlated against experimental values for Me. Furthermore, the relationship between Me and gas permeability will be examined; this is an intriguing possibility because Me is normally a material property for a given polymer, however, the PLA system provides the opportunity to fix the chemical groups present but to change Me. As gas permeability is a product of solubility and diffusivity, the PLA system allows the solubility to be held constant and the effect of Me on diffusivity to be examined. The unique stereochemical features of PLA perhaps provide a molecular feature capable of controlling a highly desirable packaging property, namely, the permeation of important gases like oxygen, nitrogen, carbon dioxide, and water.
In addition to the work on linear PLA, branched structures are being synthesized and evaluated as additives to the linear material. These materials are important for their ability to improve the melt strength of PLA which is an important property in the blow molding operation used to produce bottles and other hollow bodied objects. We are developing new techniques for the formation of branched material. A variety of branched monomers are being investigated for their copolymerizability with lactide and lactic acid.
Polymerization of hyperbranched polyesters through the use of ring-opening initiating or ring-opening terminating systems would allow for increased control in the overall molecular weight between branches as well as overall size to the polymers. In a ring-opening intiator system, each of the cyclic initiators contains one initiating group, a primary hydroxyl group, and the overall number of polymer chains generated in a reaction is controllable. Branched polymerizations using this method proceed in a divergent manner. Distance between branch points of polymers made in this manner are varied through the stoichiometric ratio of the initiating ring species to the lactide monomer. Similarly, in a ring-opening and terminating system, the number of branches can be controlled by the number of added terminating groups and the method of addition.
These approaches permit the processing of these environmentally benign materials into recyclable, value added, consumer products. The net societal and environmental benefits are extraordinary and include a reduction in the consumption of fossil resources, substitution for noxious and toxic based vinyl polymerizations, improvement of America’s farm economy, the elimination of a net source of the greenhouse gas CO2, and finally a reduction in municipal landfill volumes.