Compressive moduli and network parameters of N-isopropylacrylamide hydrogels copolymerized by monoesters of itaconic acid and crosslinked with tetraallylammonium bromide

The present study focuses on the mechanical properties of hydrophilically or hydrophobically modified poly(N-isopropylacrylamide) (PNIPAAm) hydrogels, and all discussions on their improved mechanical strengths are based on the conformational effects of hydrophobic side chains attached to the comonomers and the structural differences between the crosslinkers. Three different types of monoalkyl itaconates, bearing octyl (Oc), cetyl (Ce), and cyclohexyl (CH) groups as comonomers, were used to prepare the copolymeric PNIPAAm hydrogels crosslinked with N,N′-methylenebisacrylamide (BIS) and tetraallylammonium bromide (TAB) as neutral tetrafunctional and ionic octafunctional crosslinkers, respectively. The most striking result is the compressive E modulus of TAB-crosslinked PNIPAAm hydrogel containing 10 mol % of mOcI. It reaches nearly 1.0 MPa and is independent of the temperature and pH of the swelling/shrinking medium. The result was discussed in terms of the inter/intramolecular interactions between hydrophobic octyl groups adopting a rod-like conformation in the case of 25 °C/distilled deionized water (DDW) and 37 °C/DDW combinations. Further, it was observed that the electrostatic repulsive forces between the carboxylate groups on mOcI units could be suppressed even at 37 °C and pH 9 due to the rod-like conformations of C₈H₁₇ groups. Its micrographs under bright-field and polarized light supported the presence of an ordered anisotropic phase and multiple associations of extended, hydrophobic side chains.