This study focuses on improving the speed of actuation of poly(N-isopropylacrylamide)-based photoresponsive gels containing copolymerised spiropyran as the photoswitch and acrylic acid as the proton source. The improvement is realised by introducing pores into the material. For this purpose, polyethylene glycol (PEG) of two molecular weights (2,000 and 20,000 g mol -1 ) has been used as the porophore in the polymerisation mixture. Upon removal of the PEG porophore post polymerisation, hydrogels of different pore sizes were obtained. This impacts the diffusion of water molecules moving in/out of the hydrogel matrix, thus improving the swelling and shrinking kinetics of the hydrogel due to reduction of the average diffusion pathlength. Most significant improvement was observed in the re-swelling step and is demonstrated with optical microscopy combined with kinetic ultraviolet-visible spectroscopy (UV-Vis) analysis. Scanning Electron Microscopy reveals the PEG-induced pores to be in the range of 0.1 -2 µm. Moreover, the mechanical stability of the gels is confirmed with rheometry. Lastly, the presented photoresponsive porous gels exhibit an order of magnitude faster reswelling rate compared to the non-PEG produced control sample.
The toluene-P-sulphonates of the methyl and benzyl esters of some amino-acids have been prepared by the action of dialkyl sulphites on the acid in the presence of toluene-p-sulphonic acid.WE have obtained high yields of the toluene-9-sulphonates of the methyl and the benzyl esters of certain amino-acids by the action of the dialkyl sulphite on the amino-acid in the presence of just over one equivalent of anhydrous toluene-9-sulphonic acid.2 Voss and Blanke prepared alkyl esters of simple carboxylic acids by the use of alkyl sulphites, but Voss and Wulkan found that the methanesulphonates of betaine esters were formed when glvcine, alanine, and tyrosine were heated with dimethyl sulphite for some hours at 130" (without added acid). We have not observed any N-alkylation under our conditions. Iselin, Rittel, Sieber, and Schwyzer obtained high yields of the P-nitrophenyl esters of acylamido-acids by the use of di-p-nitrophenyl sulphite in the presence of pyridine, but this reaction does not proceed with alkyl sulphites.In those cases reported in the Tables high yields were obtained of products which were shown by paper chromatography to contain at most traces of amino-acid toluene-p-sulphonate (a common impurity in the product from other methods), an exception being the methylation of isoleucine, from which the crude product required purification by liberation of the free aniino-ester and reprecipitation by toluene-p-sulphonic acid. Optical activity is retained; the leucine and phenylalanine methyl ester toluene-9-sulphonates were converted into optically active hydrochlorides, the proline methyl ester was converted into fully active 9-nitrobenzoyl-L-proline methyl ester, and the optical rotations of the benzyl ester toluene-P-sulphonates agree reasonably well with those reported in the literature. The methylation of alanine, valine, and lysine could not be completed, nor could the benzylation of alanine, The success of the reaction depends largely on the nature of the amino-acid.
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