No abstract

Fluorescence techniques, including time-resolved (fluorescence) anisotropy (TRAMS), have been used to study the effects of hydrophobic modification upon the thermoresponsive behavior of NIPAMbased polymers. Incorporation of styrene, through statistical free radical copolymerization, changes the hydrophobic/hydrophilic balance of the macromolecule and lowers the lower critical solution temperature (LCST) of the system. Unfortunately, although simple copolymerization with styrene can be used to manipulate the system's LCST characteristics, the polymer loses its ability to release solubilized hydrophobic guests below the critical point. This results from the formation of intramolecular aggregates between the styryl residues of the polymer chain, which can accommodate guest solutes. This is a serious limitation to this form of chemical modification if the aim is to produce smart materials for controlled solubilization and release at specific temperatures.

The size-dependent physicochemical and optical properties of silica nanoparticles have been studied. Significant increase in the specific surface area (SSA), concentration of silanol groups (δOH) and apparent density (Da) were observed as the particle size reduced from ∼130 to ∼7 nm. The decrease in the silanol number (αOH) and Si-O-Si bond angle in smaller particle size suggest that the silica structure, especially the surface has been significantly altered at nanoscale. This finding is supported by the presence of defect sites such as E′ centers and oxygen deficient centers (OCD). The stability of E′ centers (UV-vis analysis) increase linearly with the increase in particle size. The increase in the intensity of blue and green bands (PL analysis) with the decrease in the particle size are attributed to the higher silanol concentration and increased in the number of self-trapped exciton (STE)/OCD, respectively. The green band was blue-shifted with the decrease in the particle size. Overall, the silica nanoparticles have shown distinctive properties relative to the bulk silica.

Grafting of free maleimide and epoxide pendant groups onto the surface of approximately 7-nm silica nanoparticles was investigated. Glycidyloxypropyl groups (3-glycidyloxypropyltrimethoxysilane and 3-aminopropyltrimethoxysilane) that carried epoxide groups and aminopropyl groups were grafted to the silica surface with the help of condensation reactions. Maleimide groups [1,1(')-(methylenedi-4,1-phenelene) bismaleimide] were introduced to the silica surface via nucleophilic addition reaction with the aminopropyl groups pre-grafted onto the surface. The grafted silica samples were characterized using CHN, FTIR, DSC, TGA-FTIR, and 13C and 29Si CP/MAS NMR spectroscopy. NMR analyses revealed that all the functional groups were covalently bonded to the silica surface and most of the maleimide and epoxide rings remained intact on surface. DSC analysis showed that the epoxide groups were more reactive than the maleimide groups.