Controllable Synthesis and Rheological Characterization of Hydroxypropyl Methyl Cellulose

Hydroxypropyl methyl cellulose (HPMC) with controllable viscosity average molecular mass (M_η) and degree of substitution had been successfully synthesized in this article. α-cellulose was firstly methylated to be methyl cellulose (MC) by dimethyl sulphate (DMS). Then MC was hydroxypropylated to be HPMC by propylene oxide (PO). In this way, the end capping structure that methoxy group connected to terminal hydroxyl group of hydroxypropyl had been avoided. FT-IR,¹H NMR and ¹³C NMR showed the successful synthesis of products with expected structure. The dilute hydrochloric acid was used to degrade HPMC to obtain a range of products with different M_η measured by ubbelohde viscometer. The rheological properties of HPMC solution with different molecular mass and concentrations were studied, including flow behavior index, thixotropy, entanglement concentration (C^**), gelation temperature (T_gel), degelation temperature (T_sol). The results showed that the flow behavior index of HPMC solutions decreased with the increase of molecular mass and concentration, and gradually changed from Newtonian fluid to pseudoplastic fluid. In this article, T_gel and T_sol of HPMC both increased with the increase of molecular mass. When wt = 14%, M_η = 140 kDa, the maximum T_gel and T_sol was separately up to 70.18 °C and 46.81 °C. The effect of concentration on T_gel and T_sol was not noticeable. T_gel changed within 60 ± 2 °C and T_sol decreased from 40.56 °C to 35.72 °C as the concentration increased from 14 to 20%. These rheological studies are expected to provide data for subsequent processing and molding of HPMC capsules.