Application of syndiotacticity-rich PVA hydrogels prepared at a low temperature to thermo- and pH-responsive release devices

Three kinds of physically cross-linked syndiotacticity-rich poly(vinyl alco hol) (s-PVA) hydrogels were prepared at 0 degrees C with use of the buffer solutions (BS) of pHs 4.0, 7.4, and 9.0. Three gels swelled at first and th en began to shrink after 12 h when they were dipped in the same BS for prep aration at higher temperature than 0 degrees C. The release of Brilliant Bl ue (3 mg/l mt) from the cylindrical gels prepared using BS of pH 7.4 was st udied at 27, 37, and 47 degrees C. Brilliant Blue has been released spendin g 4-12 h almost completely. The rate of release from the gel at temperature s of 27, 37, and 47 degrees C became large with increasing temperature. The main factor on release of Brilliant Blue is not the contraction of gel, bu t swelling, because the degree of swelling (DS) became large with increasin g temperatures for 27, 37, and 47 degrees C. The rate of release from the g el (pH 4.0) was larger than that (pH 7.4) due to the increased DS of the hy drogel in early step at pH of 4.0. The apparent diffusion exponents of thes e releases at pH 7.4 evaluated from first 60% of the fractional release wer e lower than 0.45 due to the swelling during release. The exponent at pH 4. 0 was 0.45 due to immediate swelling. The on-off of shrinking behavior of a tactic PVA (a-PVA) hydrogel was observed under several temperature changes. The rate of release of Brilliant Blue at 5 degrees C was lower than that a t 27 degrees C and no change was observed at 5 degrees C after one on-off c ycle. (C) 2000 John Wiley & Sons, Inc.