Structural and kinetic modification of aqueous hydroxypropylmethylcellulose(HPMC) induced by electron beam irration
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概要
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Electron beam was irradiated on 10% and 20% hydroxypropylmethylcellulose (HPMC) aqueous solutions with different doses to make gel films. As increasing dose, the gel fraction of the film increased sharply above a critical dose and then decreased gradually after passing a maximum. The scission/cross-linking ratio and the critical dose were determined using the Charlesby-Rosiak equation as 0.52 and 9 kGy for 10% gel and 0.43 and 14 kGy for 20% gel, respectively. The gel fraction for 20% HPMC film was lower at low dose and higher at high dose than that for 10% film. The behavior of the swelling ratio of the gel film was just opposite to that of the gel fraction. The cross-linking density of the gel estimated from the Flory theory linearly increased with irradiation dose at low dose, passed a maximum around 100 and 160 kGy for 10% and 20% films, respectively, and decreased at high dose. These results suggest the competition of scission and cross-linking induced by indirect effect of irradiation. Dielectric relaxation measurement by time domain reflectometry and RF impedance/material analyzer revealed two characteristic relaxations of chain motions around 100MHz and of orientation of free water around 20GHz. From the dose dependence of the relaxation parameters determined by fitting to a combined equation of Cole-Cole type and KWW type, a coupling of motions of HPMC molecules and water molecules was strongly suggested. The critical dose for gelation was coincident with the dose for the maximum of t h and the minimum of Deh together with the minimum of t m and the maximum of Dem, where t h and Deh denote the relaxation time and the relaxation strength for free water molecular motion and t m and Dem the corresponding ones for HPMC molecular motion. The characteristic behavior was discussed in terms of the increase of affinity between HPMC and water and the constrained molecular motion in the gel network.
- 2005-08-01
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