A thermorheological investigation into the gelation and phase separation of hydroxypropyl methylcellulose aqueous systems

The thermorheological properties of a range of hydroxypropyl methylcellulose (HPMC) solutions have been studied with a view to determining the concentration and substitution dependence of the gelation process. Solutions containing up to 20% w/v HPMC were prepared using three grades of material (METHOCEL1 E4M, F4M, K4M). Rheological studies were performed using a TA AR1000-N Rheometer. Temperature sweeps were performed at a rate of 2°C/min between 20 and 90°C at 1.0Hz and at 4.7Pa, while frequency sweeps were performed at 4.7Pa. A series of thermal transitions were observed for the E4M systems with a minimum seen at approximately 60°C followed by an increase in moduli at approximately 70°C, while on cooling an increase in moduli is seen over a wide range of temperatures, commencing at approximately 70°C and plateauing at 50°C. Comparison with frequency sweep data for the 2% E4M solutions indicated liquid-like behaviour at 25 and 55°C with a lower frequency dependence and considerably higher moduli seen at 85°C. Macroscopic examination of the 2% gels indicated that clouding was seen at ca. 42°C, while phase separation was apparent at 55°C. Comparison with F4M and K4M systems indicated a similar behaviour pattern, although the decrease in moduli and phase separation occurred at a higher temperature for the K4M systems. It is suggested that on heating HPMC solutions, the transitions indicated by the thermorheological studies relate to phase separation causing a decrease in moduli, followed by an increase in moduli which may correspond to gelation of the polymer rich phase. This process of phase separation has not been previously considered in the context of the rheology of HPMC thermogelation and may have implications for the behaviour of solutions of this material in a practical environment