Thermoresponsive Copolymer Hydrogels Based on N-Isopropylacrylamide and Cationic Surfactant Monomers Prepared from Micellar Solution and Microemulsion in a One-Step Reaction

Abstract

Thermoresponsive hydrogels were prepared upon radiation-induced copolymerization of aqueous micellar solutions containing N-isopropylacrylamide (NiPAAm) and a cationic surfactant monomer (surfmer), and of microemulsions containing NiPAAm, surfmer, and styrene. Three surfmer compounds were used: (11-(acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB), (11-(methacryloyloxy)undecyl)trimethylammonium bromide (MUTMAB), and (2-(methacryloyloxy)ethyl)dodecyldimethylammonium bromide (MEDDAB). Comonomer solutions were studied on their phase behavior and structure using small-angle neutron scattering (SANS). The presence of surfmers increased the solubility of NiPAAm in the aqueous phase. SANS studies indicate that the surfmers form spherical micelles, which in the presence of styrene are increased and in the presence of NiPAAm are decreased in size. Styrene is incorporated in the core, and NiPAAm is incorporated in the shell of the micelles. If styrene and NiPAAm are present, the effects of both compensate each other, the micelle size remains unchanged, and only small amounts of styrene are solubilized. Evaluation of scattering curves indicated remarkable changes in headgroup dissociation of surfmers in the presence of NiPAAm in the micellar solutions. If exposed to (60)Co-gamma irradiation (dose: 80 kGy), stable, transparent, and thermoresponsive hydrogels were directly obtained. The lower critical solution temperature (LCST) of gels containing surfmer in low concentration was higher than that for pure NiPAAm gels, whereas in gels with high surfmer concentration it was lower. The lowest LCST was observed if MEDDAB was present in the gel. 1 % (w/w) was already sufficient to lower the LCST from 33.2 to 28.5 degrees C. Gels with low surfmer concentration (< or = 1 wt %) exhibited a strong, rapid swelling in water at 20 degrees C and a rapid and reversible shrinking at 50 degrees C. For a gel containing 1% AUTMAB, the swelling ratio was 2.4 times higher (MUTMAB, 2.8; MEDDAB, 1.5) than that for a pure NiPAAm gel. Copolymer gels containing more than 1 wt % surfmer exhibited a strong and rapid swelling below and above the LCST, because the copolymerized ionic surfmer induced an osmotic pressure in the gel. The effects of a variation of NiPAAm and surfmer concentration were studied, and the origins of the thermoresponsive properties are discussed.

Facile fabrication method of hydrophobic-associating cross-linking hydrogel with outstanding mechanical performance and self-healing property in the absence of surfactants

“…Recently, Friedrich [24,25] has focused on the synthesis of a dual cross-linking (containing hydrophobic-associating physical and chemical cross-linking) hydrogel via copolymerization of NIPAAm and surfactant monomer (surfmer) under g irradiation in the absence of surfactants. The synthesized hydrogel displayed good mechanical performance.…”

Photoisomerizable and Thermoresponsive N‐isopropylacrylamide–Surfmer Copolymer Hydrogels Prepared upon Electrostatic Self‐Assembly of an Azobenzene Bolaamphiphile

“…In the first step, hydrogels consisting of 10 wt% NIPAM, 1 wt% AUTMAB, and 89 wt% water were prepared upon 60 Co‐gamma radiation polymerization: A comprehensive characterization of structure and properties of gels prepared according to this method can be found in the literature 17–20. In a second step, the hydrogel was heated to 50 °C, i.e., above the LCST, in order to exclude the water from the gel.…”

“…( 11‐(Acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB) was synthesized upon esterfication of 11‐bromoundecanol‐1 with acryloyl chloride followed by reaction with trimethylamine as described in the literature 17 . N ‐Isopropylacrylamide (NIPAM) (Acros) was recrystallized from toluene and n‐hexane (volume ratio: 1:2).…”

“…Monomeric solutions containing 10 wt% NIPAM and 1 wt% AUTMAB in degassed Milli‐Q‐water were homogenised using a Vortex mixer, aged for 24 h and subsequently copolymerized in a one‐step reaction using 60 Co‐gamma radiation, the dose rate being 128.3 Gy h −1 . All of the samples were exposed to a radiation dose of 80 kGy because previous studies indicated that this dose is sufficient to cause nearly complete conversion into the hydrogel 17–21…”

“…We now succeeded in the preparation of thermoresponsive hydrogels containing an azobenzene derivative, which upon light irradiation undergoes a reversible photoisomerization, but does not induce a phase transition of the gel. The hydrogels were prepared upon γ‐ray‐induced copolymerization of N ‐isopropylacrylamide (NIPAM) and the surfactant monomer (surfmer) (11‐(acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB) in an aqueous micellar solution 17–21. Subsequent to gel preparation, the water‐soluble azo compound 4,4′‐di(6‐sulfatohexyloxy)azobenzene22 (DSHA, see Scheme ) was incorporated in the gel via electrostatic adsorption following a method recently described in the literature 23.…”