Multicompartment thermoresponsive gels: does the length of the hydrophobic side group matter?

Abstract

Multicompartment thermoresponsive gels are novel materials with fascinating self-assembly and interesting applications. The aim of this study was to investigate for the first time the effect of the length of the alkyl side group of a hydrophobic monomer on the thermoresponsive and self-assembly behaviour of terpolymers. Specifically twelve well-defined terpolymers based on the hydrophilic monomers 2-(dimethylamino)ethyl methacrylate (DMAEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA), and on the hydrophobic monomer ethyl-, n-butyl or n-hexyl methacrylate (EtMA, BuMA or HMA) of varying architectures (ABC, ACB, BAC and statistical) were synthesised using Group Transfer Polymerisation. The A, B and C blocks were based on PEGMA, the alkyl containing methacrylate monomer, and DMAEMA, respectively. The molecular weights (MWs) and compositions of the polymers were kept the same. The polymers and their precursors were characterised in terms of their MWs, MW distributions and compositions. Aqueous solutions of the polymers were studied by turbidimetry, hydrogen ion titration, light scattering and rheology to determine their cloud points, pK a s, hydrodynamic diameters and thermoresponsive behaviour and investigate the effect of the architecture and the hydrophobic alkyl side group of the terpolymers. It was found that the pK a s and the T g s were mostly affected by the hydrophobicity of the side groups and not by the architecture, while the cloud points and the sol-gel transition of the polymers were affected by both the length of the alkyl side group and the polymer architecture. Interestingly the sharpest sol-gel transitions and stable multicompartment hydrogels were observed for the ABC triblock copolymers with the short alkyl-side groups even though the sol-gel transition occurred at higher temperatures.

Tuning the gelation of thermoresponsive gels

“…61 The only study on triblock bipolymers involved ABA triblock copolymers where the A and B blocks were based on BuMA and DMAEMA, respectively, and the observation was similar; the block structure forms gels more easily than the random structure. 54 All the studies on terpolymers were performed by our group and were based on PEGMA 45,54 (A block), DMAEMA (C block) 45,54 and either EtMA, 54 BuMA 45,54 or HMA 54 (B block) and they are in agreement with previous observations. When the random copolymers did form a gel it was either because the MM was relatively high 45 or because the most hydrophobic, HMA 54 comonomer was used.…”

“…45,54 This effect was observed for both bipolymers [59][60][61] and terpolymers 45,47,54 (two and three component based polymers, respectively). For bipolymers these were observed for diblock copolymers based on 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-(tertbutyldimethylsilyloxy)ethyl vinyl ether (BMSiVE) 59 and EOEOVE and 2-methoxyethyl vinyl ether (MOVE).…”

“…54 Specifically A and C were based on PEGMA and DMAEMA, respectively and were always kept the same while the B monomer was varied, specifically; ethyl methacrylate (EtMA), BuMA and n-hexyl methacrylate (HMA) were used. For all architectures the viscosity increased by increasing the length of the hydrophobic monomer and where a sol-gel transition was observed the T gel decreased by increasing the length of the side group.…”

“…Therefore, the elasticity increased upon increasing the hydrophobic alkyl side chain, leading to reduced stability of the gels. 54 Two studies on ABA triblock copolymers were carried out. 55,56 In the first one the A blocks consisted of EO and the hydrophobicity of the B block was varied by using hexamethylene adipate (HA), ethylene adipate (EA) or ethylene succinate (ESc) units.…”

“…Specifically in many occasions where random and block copolymers were compared the random polymer either did not form a gel 47,54,[59][60][61] or formed a less stable gel at higher T gel . 45,54 This effect was observed for both bipolymers [59][60][61] and terpolymers 45,47,54 (two and three component based polymers, respectively).…”