Pickering emulsion has attracted the attention of many researchers because of its good stability. In some cases, such as catalyst separation and drug release, control of emulsion stability and demulsification under mild temperature is required. In this work, temperature-sensitive PS@PNIPA (polystyrene@polyisopropylacrylamide) with controlled PNIPA thickness was applied as the Pickering solid emulsifier . The effect of PNIPA thickness on the Pickering emulsion formation was studied by Polarizing optical microscope (POM). PS@PNIPA concentration was also found to be related to the size of the emulsion and the droplet size could be controlled from 17 μm to 39 μm. By changing the temperature, the emulsification/demulsification process of Pickering emulsion is reversible and repeatable for several cycles. Using ibuprofen as a chemical model, the maximum ibuprofen release rate reaches 93% and can be tuned by the temperature. Therefore, this opens up more potential applications for temperature-induced controlled demulsification for chemicals release and related fields.
Nanocrystalline cellulose (NCC) whisker obtained from acid hydrolysis of cotton was incorporated into the freezing polymerized PNIPA/clay hydrogels to prepare inorganic–organic hybrid nanocomposite hydrogels (named as C-NC gels). The influence of NCC on the properties of C-NC gels was investigated systematically. It was found that all C-NC gels exhibit similar lower critical solution temperature as that of NCC-free gels, being independent of the NCC content. However, with the increase of NCC content in C-NC gels, the swelling ability of gels decreases slightly while the response rate of gels increases gradually, the gels with high content of NCC exhibit an ultrarapid deswelling rate due to the amount of interconnected micropores appeared inside the gels. Moreover, the enhancement effect of increased NCC on the gels is significant, which is also determined by the swelling degree of gels directly. Comparably, for the gels with the same content of NCC, higher strength was found when the gels were kept in lower swelling ratio due to the stronger interaction of NCC through hydrogen bond in the gels.
To prepare hydrogels with ultrarapid response rate and excellent mechanical properties, the poly(N-isopropylacrylamide)/clay nanocomposite hydrogels were synthesized by freezing polymerization technique. The start freezing time, as an important parameter determining the properties of gels, was designed and investigated. The results showed that the properties of gels including mechanical properties, swelling ratio, and swelling/deswelling rate were closely dependent on the freezing polymerization time. Comparably, the gels synthesized with earlier freezing time exhibit a faster swelling rate and an ultrarapid deswelling rate due to the integral interconnecting porous structure, while the swelling ratio, tensile strength and modulus decrease considerably. With the delay of start freezing time, the response rate decreases while the mechanical properties improve. Through the analysis of scanning electron microscope, differential scanning calorimetry, x-ray diffraction, dynamic rheological tests, and mechanical tests, the relevance of gels' structure with the freezing time was explored. It is reasonably presumed that freezing process impacts the effective crosslink of polymer chains by clay significantly, the earlier the freezing started, the more chains with free end existed in gels.
The preparation of a novel fast response thremosensitive hydrogel was investigated by incorporating nanosized octavinyl polyhedral oligomeric silsesquioxane (OvPOSS) particles into inorganic clay-crosslinked poly(N-isopropylacrylamide) (PNIPA) nanocomposites hydrogels (NC gels).
The resulting hydrogels named as P-NC gels involving both the strategies of NC gels and double network gels (DN gels) were successfully synthesized via a two-step technique. The second PNIPA network crosslinked by OvPOSS were polymerized in the presence of the first clay-PNIPA network. The
P-NC gels were characterized by FTIR, X-ray, DSC, UV/vis spectra, SEM, mechanical properties and swelling behaviors measurements. The key factor for preparing homogenous P-NC gels with good transparency was the low concentration of OvPOSS (c
OvPOSS) dispersed uniformly in
polymer as a cross-linker. On the contrary, the high c
OvPOSS led to the significant aggregation of the OvPOSS particles and thus resulted in the heterogeneity of gels. The SEM images of freezedried P-NC gels exhibited a highly interconnected microporous network structure,
which could be adjusted by varying the amount of OvPOSS. The special porous morphology brought about an attractive faster swelling/deswelling rate than that of normal NC gels. All properties of P-NC gels displayed an obvious dependence on the concentration of incorporated OvPOSS. The excellent
mechanical properties, tunable LCST, especially the fast deswelling rate make these hydrogels potential candidates for applications in drug release and other biological fields.
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