Determination of the Structural Relaxation Enthalpy Using a Mathematical Approach

Structural relaxation is a well-known phenomenon in amorphous Materials such as amorphous solid dispersions. It is generally understood as a measure for molecular mobility and has been shown to impact certain material properties such as the dissolution rate. Several quantification methods to evaluate structural relaxation using differential scanning calorimetry have been proposed in the past, but all approaches exhibit disadvantages. In this work, a mathematical model was developed and fitted to calorimetric data enabling the analysis of the structural relaxation enthalpy by separating the structural relaxation peak from the underlying glass transition. The proposed method was validated using a parameter sensitivity analysis. Differently stressed amorphous samples were analyzed applying the new model and the results were compared to commonly applied quantification methods in literature. The proposed method showed high robustness and accuracy and overcame the observed disadvantages of the established methods. The heating rate dependence of the calculated structural relaxation enthalpy was in accordance with theoretical considerations of previous studies, supporting the validity of the results. Thus, the proposed model is suitable to accurately quantify the degree of structural relaxation and should be a valuable tool for further investigations regarding the impact of structural relaxation on material properties.

amorphism; amorphous solid dispersion (ASD); differential scanning calorimetry (DSC); glass transition; mathematical model; physical characterization; polymer.