Hydrophilic–hydrophobic copolymer nano-sized particle gels: Swelling behavior and dependence on crosslinker chain length

Abstract

Poly N-isopropylacrylamide-co-ethylacrylate [P(NIPAM-co-EA)], Poly N-isopropylacrylamide-co-2-hydroxyethylmethacrylate [P(NIPAM-co-HEMA)], and Poly N-isopropylacrylamide-co-2-hydroxyethylacrylate [P(NIPAM-co-HEA)] nano-sized particle copolymer hydrogels were synthesized to investigate their volume phase transition behavior. Increasing the hydrophobic or hydrophilic monomer content of hydrogels led to uniform changes in transition temperature and swelling ratio. Different statistical thermodynamic models are presented to describe the swelling behavior of copolymer gels. The classical interaction energy parameter in the thermodynamic model, which is not suitable for accurately representing the equilibrium swelling of a copolymer hydrogel, was modified semi-empirically in order to reflect the individual contribution of each hydrogel constituent. In addition, we investigated the volume phase transition of PNIPAM gels as a function of crosslinker chain length. The crosslinkers used in this study were N,N′-methylenebisacrylamide (BIS) and two different molecular weights of Poly(ethyleneglycol) diacrylate (PEGDA, 575 and 700). Interestingly, the use of a long chain crosslinker resulted in a decreased volume change compared to that of a short chain crosslinker without any significant changes in transition temperature. To understand this unexpected result, we used a molecular dynamics simulation for BIS and PEGDA 575 to demonstrate the micro-scale conformation change of crosslinker chain mixed with water or ethanol. The proposed thermodynamic models represent the swelling behavior of the copolymer hydrogels and PNIPAM gels with different length crosslinkers.