Effect of long-term and short-term dynamic mechanical evaluation of networks based on urethane and soybean oil.

The intermixing between urethane acrylate (UA) prepolymer and acrylated epoxidized soybean oil (AESO) by thermal polymerization results in a product with promising properties for biomedical application. The results of dynamic mechanical analysis (DMA) performed on the polymerized UA, in a limited range of frequencies (0.5-10 Hz), have raised questions related to the origin of relaxations. Whether a molecular motion can be associated with a relaxation process was ascertained by performing experiments in a much larger frequency range, in a step-scan DMA experiment. The particularities of this experiment made possible to evidence unequivocally the α-relaxations for the urethane domains, organized in a hydrogen bond physical network, and to register the transformations associated with the breaking of hydrogen bonds. Incorporation of AESO has gradually loosened the physical network and transformed it into a mixed urethane-AESO chemical network. The properties of urethane-AESO networks are ruled by the double function of AESO: plasticizer and crosslinker. The evolution of secondary relaxations reflected also the changes that occur in the urethane-AESO samples. During the step-scan procedure supplementary changes were induced in the samples with high AESO content, at high temperature. The morphology of the networks was investigated by electronic microscopy studies.