Thermo-responsive Properties Of P(N-isopropylacrylamide)-based Microgels

The thermal-responsive hydrogels have great potential applications in the area of fluorescence probe, drug delivery, sensor etc. Poly(N-isopropylacrylamide)(PNIPAM) hydrogels with the low critical solution temperature(LCST) have been intensively studied by scientists up to date. Poly(N-isopropylacryl amide-co-N-(2-aminoethyl)acrylamide) [P(NIPAM-co-NAEAM)] was synthesized in this work, the relationship between structure and thermosensitivity was discussed from the nanocomposite hydrogels of the as-synthesized copolymer and CdTe nanocrystals, along with the effect of salts on the cloud point of copolymer in water. The main contents are as follows:1. The P(NIPAM-co-AA) hydrogel was synthesized by copolymerizting N-isopropylacrylamide(NIPAM) and acrylic acid(AA) by free radical solution copolymerization, using potassium persulfate(KPS) as the initiator and N,N’-methylenebis(acrylamide) as the corss-linking agent. P(NIPAM-co-NAEAM) copolymer hydrogel was prepared by the amidation reaction of ethylenediamine with carboxyl group in the copolymer. The nanocomposite microgels with hollow structure were prepared by mixing copolymer hydrogel with CdTe NCs. The structure and properties of microgel were analyzed through X-ray photoelectron spectroscopy, infrared spectroscopy, transmission electron microscope, fluorescence spectrometer.2. The changes of the fluorescence intensity of nanocomposite microgels with temperature are significantly effected by the polymer concentration, composition and the content of CdTe nanocrystals. The experimental data indicate that:(i) the nanocomposite hydrogels not only perform high sensitivity to the slight thermo-stimuli, but also presents good reversibility in the temperature range of 32℃-41℃;(ii) when polymer composition and the molar ratio of CdTe nanocrystals/NAEAM hold constant in microgels, the transition temperature declines with increasing the polymer concentration;(iii) when polymer concentration and composition hold constant, the hydrogels with the high loaded CdTe NCs are prone to respond a slight thermo-stimulation at the low temperatures;(iv) because the introduction of hydrophilic NAEAM into the hydrogel affects the interconversion between hydrogen-bonding and hydrophobic interactions of PNIPAM segments, the transition temperature rises with increasing the molar fraction of NAEAM in the polymer.3. The additional salts exert the great effects on the cloud point(Tcp) of P(NIPAM-co- NAEAM) polymer, depending on the salt species and concentration.(i) Tcp declines with increasing NaCl or NaAc concentration, and the polymer would precipitate at the high salt concentration;(ii) below 0.01 M of polymer in water, Tcp exhibits a abrupt decline with increasing the concentration of imidazolium, which is related to the alkyl side chain in imidazolium, and the imidazolium with longer alkyl side chain implies lower Tcp;(iii) it is shown that these salt effects follow the order of Hofmeister sequence.