Effects of calcium salts on thermal characteristics of hydroxypropyl methylcellulose films

Cast films comprising hydroxypropyl methylcellulose (HPMC) and calcium chloride (CaCl2) have higher flexibility than those with HPMC and calcium lactate pentahydrate (CLP). The aim of the present study was to investigate the relationship between the molecular behaviour and the film flexibility of HPMC cast films. In differential scanning calorimetry (DSC) measurements, the HPMC-only cast films exhibited a glass transition temperature (Tg) of 142.1–143.9 °C, which is similar that of HPMC/CLP cast films. In contrast, HPMC/CaCl2 cast films exhibited Tg of 76.1–77.3 °C, which is lower than that of HPMC-only and HPMC/CLP films. Thermal mechanical analysis (TMA) results indicated that the HPMC-only and HPMC/CLP cast films contracted strongly around the Tg calculated using DSC. In contrast, the cast films comprising HPMC/CaCl2 blends gradually contracted as the temperature increased; this behaviour is significantly different from that observed in the HPMC-only and HPMC/CLP films. The most probable mechanism for the film flexibility of HPMC/calcium salt blends was clarified through attenuated total reflection Fourier transform-infrared (ATR FT-IR) and thermogravimetric (TG) analysis. The analysis results suggest that the difference in the flexibility of the cast films in the presence of CaCl2 or CLP depends on the difference affinity between calcium salts and water molecules.

Research highlights
► The cast films comprising HPMC and CaCl2 blends had higher flexibility than those comprising HPMC and CLP blends.
► The HPMC/CaCl2 cast films had a lower Tg and a higher water content than the HPMC/CLP cast films.
► Unlike the ATR FT-IR spectra of the HPMC-only and HPMC/CLP cast films, those of the HPMC/CaCl2 cast films showed peaks at 1641 cm−1 and 3385 cm− 1 corresponding to water adsorption and hydrogen bonding, respectively.
► The difference in the flexibility of the cast films in the presence of CaCl2 or CLP depends on the affinity between calcium salts in the HPMC polymer phase and water molecules.