Thermoresponsive poly(vinyl alcohol) derivatives: preparation, characterization and their capability of dispersing gold nanoparticles

Three amino acid derivatives, GI, AI and VI resulted from the corresponding isobutyryl chloride modified Gly, Ala and Val, were each conjugated with some of the hydroxyl groups of poly(vinyl alcohol) (PVA) through a one-step esterification reaction, resulting in PVA-GI, PVA-AI and PVA-VI, respectively. FTIR and H-1 NMR verified the successful conjugation of these amino-acid derivative units onto PVA. X-ray diffraction characterization demonstrated that the introduction of these units led to a decrease in the degree of crystallinity. Turbidity measurement showed that GI, AI and VI were all effective molecular units to make such PVA-derivatives thermoresponsive, and the phase transition temperature could be modulated in a wide range by varying the degree of substitution or altering the type of amino-acid derivative. The efficiency of these three molecular units to make PVA thermoresponsive was as follows: VI > AI similar to GI. Dynamic light scattering (DLS) measurement demonstrated that these thermoresponsive PVA derivatives experienced a conformation transition from the loose coil to the compact and crumbled state, till the aggregated state was attained following the temperature increase. Such thermoresponsive PVA derivatives were good dispersing agents for colloidal gold nanoparticles (AuNPs) in water when enough polymers were used. A core-shell structure was proved by DLS and TEM measurements where the AuNP was covered by a thick organic shell formed by the thermoresponsive PVA derivatives. Turbidity measurement showed that the composites of such PVA derivatives and AuNPs were also thermoresponsive, and they had a little lower phase transition temperature than those of the corresponding neat thermoresponsive PVA derivatives. DLS measurement demonstrated that such thermoresponsive AuNP composites experienced a shell shrinkage and subsequent aggregation of nanoparticles during the temperature increase. Moreover, AuNPs stabilized by such thermoresponsive PVA derivatives showed better salt-resistance than those stabilized by neat PVAs.