Poly(ethylene oxide) (PEO) chains are introduced as graft chains maintaining freely mobile
ends in thermo-responsive cross-linked poly(N-isopropylacrylamide) (PIPAAm) hydrogels by copolymerization of IPAAm with α-acryloyl-ω-methoxy-PEO. The deswelling response on raising the temperature
of this gel above the gel phase transition temperature (T
P) takes place within 10 min, whereas a
conventionally cross-linked PIPAAm gel of the same dimensions requires 1 month for deswelling. This
difference is due to the formation of water release channels within the skin layer by the hydrophilic PEO
graft chains. The rapid deswelling of the grafted gel is compared with the deswelling changes of random
copolymer gels composed of IPAAm and hydrophilic acrylic acid (AAc), which also accelerates gel
deswelling. Deswelling is fastest in copolymers containing 1.3 wt % AAc and in grafted gels containing
13 wt % PEO. These results were interpreted as reflecting the gel structure.
Two types of thermo-responsive hydrogels are synthesized to obtain comb-type grafted gels with different lower critical solution temperatures (LCSTs) between graft chains and cross-linked backbone networks: these are poly(N-isopropylacrylamide) (PIPAAm) cross-linked hydrogels grafted with poly(N-isopropylacryl amide-co-N,N-dimethylacrylamide) (poly(IPAAm-co-DMAAm)) maintaining a freely mobile end and poly(IPAAm-co-DMAAm) cross-linked hydrogels grafted with PIPAAm chains. The effect of graft chain hydrophilic/hydrophobic balance as well as its mobility on deswelling kinetics of these grafted gels are investigated through the polymer LCST modulation and external temperature changes. The deswelling rate of poly(IPAAm-co-DMAAm)-grafted PIPAAm gel increases with increasing in temperature. This gel shows a discontinuous increase of the deswelling rate when the temperature is applied from below to above the graft chain LCST (37 degrees C). The deswelling rate of PIPAAm-grafted poly(IPAAm-co-DMAAm) gel increases continuously when the temperature is applied from below to above the graft chain LCST (31 degrees C). Due to the strong hydrophilicity of backbone network, the hydrophobic aggregation force weak. In contrast to the graft-type gels, normal-type poly(IPAAm-co-DMAAm) cross-linked gel without graft chains demonstrates the discontinuous decrease for the deswelling rate when the temperature is applied from below to above the polymer LCST (36 degrees C), entrapping water inside the gel due to the formation of an impermeable dense skin layer at the gel surface. These gel deswelling mechanisms are discussed in terms of gel structures.
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