The spatial effect of hydrophobic groups in pendant amino acid monomers on the thermal volume phase transition of hydrogels

Abstract

A series of hydrogels containing each of the regioisomers, methacryloyl l-valine methyl ester (MA-ValOMe) and methacryloyl l-alanine isopropyl ester (MA-AlaOiPr), were prepared by radiation-induced polymerization. The regioisomers have both methyl and isopropyl groups but at different positions in the side chain of the hydrogels. We estimated the effect of the hydrophobic position in the side chain (spatial effect) on the volume phase transition temperatures (VPTT) of the hydrogels. When methacryloyl l-alanine methyl ester (MA-AlaOMe) hydrogel, showing a VPTT of 23°C, was used as a co-monomer, the VPTTs of copolymer hydrogels exhibited a prominent decrease when the MA-ValOMe and MA-AlaOiPr compositions were increased. In this case, the MA-ValOMe component, which has a hydrophobic group near the main chain of the hydrogels, decreased the VPTT more efficiently than Ma-AlaOiPr. The relationship between the VPTT and the co-monomer composition exhibited a linear correlation at temperatures above 0°C. Thus, the apparent VPTTs of homopolymer hydrogels, MA-ValOMe and MA-AlaOiPr, were estimated to be −78 and −25°C, respectively, by extrapolating 100% of the composition of the monomers from the linear VPTT versus the composition plot.