Heat of fusion of polymer crystals by fast scanning calorimetry

Knowledge of the specific equilibrium heat of fusion of polymer crystals, Δhf0 [J/g], is an essential thermal property of polymers which permits the degree of crystallinity to be obtained from thermal measurements. We describe an approach to evaluate Δhf0(Tm) and implement this method using fast scanning calorimetry (FSC). Our method uses the measured enthalpy of melting plotted against the product of the sample mass times its crystallinity for samples with variable masses and/or crystallinities. Then, Δhf0 is obtained from the slope of the entire data set, reducing errors in the measurement. To demonstrate the method and give proof of principle, we measure Δhf0(Tm) of samples of a narrow fraction of linear polyethylene (PE) with a weight average molecular weight of 60,700 g/mol, whose thermal properties are already known in the literature. For PE, we obtain Δhf0(PE) = (281 ± 6) J/g at Tm = 136 °C, in close agreement with literature values. Then, we apply the method to determine Δhf0(Tm) of silk fibroin, a fibrous protein, yielding a first estimate of the heat of fusion of silk crystals, Δhf0(Silk) ∼ (137 ± 7) J/g. Advantages include: reduction of error, applicability to all types of polymers, copolymers, and blends regardless of degree of crystallinity, and applicability to biomaterials which may require fast scanning rates of FSC to prevent degradation.