Differential scanning calorimetry study of the impact of annealing conditions on poly(ε-caprolactone) thermogels

A sample of poly(ε-caprolactone) was synthesized and mixed with dimethylformamide to form thermogels. Most of these gels show two endothermic melting events as measured by differential scanning calorimetry. Increasing the annealing temperature and annealing time or decreasing the cooling rate results in the appearance of the low-melting endotherm shifting to higher temperatures and growing in size and the high-melting endotherm shrinking until the two endotherms merge into a single peak. Mathematical simulations of the two endothermic melting events and an exothermic recrystallization event show that the high-melting endotherm remains constant, but a shift to higher temperatures of the low-melting endotherm and exothermic recrystallization results in the appearance of the low-melting endotherm growing in size and the high-melting endotherm shrinking. These results suggest two distinct crystallite populations. Additional evidence of the different populations of crystallites was provided through partial melting experiments. The melting behavior of a given set of annealing parameters is reproducible, but changing the annealing parameters can result in a very different melting behavior for the thermogels.