Thermoresponsive amphiphilic symmetrical triblock copolymers with a hydrophilic middle block made of poly(N-isopropylacrylamide): synthesis, self-organization, and hydrogel formation

Abstract

Several series of symmetrical triblock copolymers were synthesized by the reversible addition fragmentation chain transfer method. They consist of a long block of poly(N-isopropylacrylamide) as hydrophilic, thermoresponsive middle block, which is end-capped by two small strongly hydrophobic blocks made from five different vinyl polymers. The association of the amphiphilic polymers was studied in dilute and concentrated aqueous solution. The polymer micelles found at low concentrations form hydrogels at high concentrations, typically above 30-35 wt.%. Hydrogel formation and the thermosensitive rheological behavior were studied exemplarily for copolymers with hydrophobic blocks of polystyrene, poly(2-ethylhexyl acrylate), and poly(n-octadecyl acrylate). All systems exhibited a cloud point around 30 A degrees C. Heating beyond the cloud point initially favors hydrogel formation but continued heating results in macroscopic phase separation. The rheological behavior suggests that the copolymers associate into flower-like micelles, with only a small share of polymers that bridge the micelles and act as physical cross-linkers, even at high concentrations

Rheology of associative polymer solutions

“…The polyelectrolyte character of the central block renders these systems sensitive to the ionic strength and the pH [68]. In some cases very hydrophobic blocks were used such as PMMA [65,69], PS [58,61,70], PtBA [71], and PnBMA [60]. These hydrophobic blocks are known to exchange on timescales much larger than the duration of the experiments unless they are very short.…”

“…Remarkably, the same elastic behavior was found relatively quickly after cessation of the flow before significant exchange of the hydrophobic blocks could occur. Bivigou-Koumba et al [70] studied solutions of PNIPAM end-functionalized with PS, poly(2-ethylhexylacrylate), and poly(n-octadecyl acrylate). At temperatures below the LCST of PNIPAM, transparent gels were formed at high concentrations that showed shear-thinning, sometimes preceded by weak shear-thickening.…”

Temperature induced phase separation of luminescent silica nanoparticles in Triton X-100 solutions

“…Smart colloidal systems, which are temperature, pH, or substrate responsive, gain much attention due to their possible application in bioanalysis, medicine and technology [1][2][3][4][5][6][7][8]. The thermoresponsive nanosized aggregates and polymeric particles represent very important type of smart materials [3][4][5][6][7][8].…”

Living Radical Polymerization by the RAFT Process – A Third Update

“…S S S A* [331,335] CMS [336] St [182,[337][338][339] tBS [337] AN [331] BA [182,274] DMAEA [221] MA [182] iBoA [182] tBA [182] ODA [337] 2EHA [337] PEGA [340] NVP [248] (VAc) [274] B/AN [312] DVE/MAH [341,342] PEGA-b-CIPEA [340] ODA-b-NIPAm [337] 2EHA-b-NIPAm [337] St-bNIPAm [337] tBS-b-NIPAm [337] (BA-b-VAc) [274] DVE/MAH-bSt [341,342] …”