Eric Hansen and Michael Schilling
Abstract
The properties of thermoplastic “amorphous” polymers, resulting from application from a solution, are affected by the solvent or solvent mixtures in several ways, including the amount of retained solvent and the thermodynamic quality of the solvent(s). Retention of significant amounts of solvent in a polymer film following formation usually results in a lowering of the glass transition temperature (Tg) along with a decrease in hardness and an increase in pliability or deformability. Such effects from retained solvent decrease with time as the solvent continues to diffuse through the film and evaporate. In contrast, the thermodynamic quality of the solvent determines the polymer conformation in solution and subsequently in a film. Polymer chains in a “good” solvent are strongly interpenetrating and result in a dense structure in a film, whereas a “poor” solvent promotes aggregate chains and a porous structure. The effects of solvent quality on the physical properties (Tg and tensile properties) have been demonstrated for poly(vinyl acetate)s and Acryloid B-72. Films of poly(vinyl acetate)s exhibit an increase in Tg in comparison to the bulk form as supplied, whereas films of Acryloid B-72 exhibit a decrease in Tg. In contrast, films of both of these polymers exhibit lower strength and significantly greater elongation when deposited from a solution made with a good solvent in contrast to a poor solvent. These results indicate that a polymer property, including adhesion to a substrate, cannot be easily predicted based upon the thermodynamic quality of the solvent alone and must be experimentally determined following full solvent release. Recognizing these effects may allow polymer performance to be optimized, explain unexpected failures, and allow suggestions to be made for additional testing of specific properties of polymers used in conservation.