The influence of HPMC substitution pattern on solid-state properties
ABSTRACT The solid-state properties were studied for different batches of hydroxypropyl methylcellulose (HPMC). The batches had similar chemical composition, but different degree of heterogeneity with regard to the distribution of the substituents along the polymer chains. The glass transition temperature, Tg, was analysed using a new developed method where dynamic mechanic analysis, DMA, was performed in compression mode on compacts, utilizing a wedge-shaped probe. The method was verified by conventional DMA on films. Molecular interactions were studied using FT-IR. In addition, the water vapour sorption was determined by gravimetric measurements and the plasticization by water vapour was studied on film samples using DMA. The results revealed a linear relationship between increasing Tg and increasing percent glucose liberated after enzyme hydrolysis. The percent glucose liberated can in turn be considered to account for both the heterogeneity of the substituents and the total degree of substitution. The results indicated that more heterogeneously substituted cellulose derivatives and derivates with a lower degree of substitution had stronger interactions between polymer chains. As expected from these results, some small difference in the plasticization by water vapour could be detected. However, no significant differences were found in molecular interactions using FT-IR or in the sorption of water vapour. The correlation between heterogeneity in the distribution of the substituents and Tg is of much interest as heterogeneously substituted batches of HPMC have been previously shown to exhibit very different behaviour in solution and in gelling tablets.
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ABSTRACT: Voltage-time characteristics of SF<sub>6</sub> gas and an SF<sub>6 </sub>-N<sub>2</sub> gas mixture with a mixing ratio of 50/50 were measured in a 50/20-mm-diameter coaxial electrode system with and without a spacer under both lightning impulse and steep-fronted impulse voltages. A nonoscillating steep-fronted impulse with a risetime as short as 50 ns was derived from a 1.2/60-μs lightning impulse by means of a wave-front-steepening gap, and was measured using a compact resistive type voltage divider, with the measuring system response time being less than 5 ns. Experiments showed that the breakdown voltage under the steep-fronted impulse was lower than that under the lightning impulse for the same time to breakdown in both SF<sub>6</sub> and the SF <sub>6</sub>-N<sub>2</sub> gas mixture, but it was still higher than the 50% lightning impulse breakdown voltage used for insulation co-ordination. For the case with a spacer, breakdowns in both SF<sub>6 </sub> and SF<sub>6</sub>-N<sub>2</sub> were greatly scattered compared with gas insulation without any spacerElectrical Insulation, Conference Record of the 1992 IEEE International Symposium on; 07/1992