Effects of Carboxyls Attached at Alkyl Side Chain Ends on the Lamellar Structure of Hydrogels

Two kinds of water-swollen hydrogels composed of three monomers&sbd;poly(stearyl acrylate (C18A)-co-n-alkyl acrylates (CnA)-co-acrylic acid (AA)) and poly(16-acryloyloxyhexadecanoic acid (AHA)-co-CnA-co-AA) (n is the number of carbon atoms of side chains:  n = 14, 16, 18, 20, 22)&sbd;were synthesized, and their structures were investigated by wide-angle X-ray diffraction (WAXD) and small-angle X-ray diffraction (SAXD). Both gels formed the crystalline structure with the lamellae. In the case of poly(C18A-co-CnA-co-AA) gels, both the lamellar thickness, d<INF>2</INF>, and the melting temperature, T<INF>m</INF>, increased with the increase in n, suggesting that C18A and CnA form molecularly mixed crystalline domains. On the other hand, d<INF>2</INF> of poly(AHA-co-CnA-co-AA) gels was much smaller than that of poly(CnA-co-AA) gels, and T<INF>m</INF> was constant regardless of n. An abrupt decrease in T<INF>m</INF> was observed for poly(AHA-co-C18A-co-AA) gels at an AHA composition of 0.8. These results indicate that carboxyls attached at alkyl side chain end of AHA significantly shorten and stabilize the lamellae of the hydrogel due to the cooperative hydrogen bonding and rearrangement of the lamellar structure occurred.