This article reports on the synthesis of thermo- and light-sensitive hydrophilic block copolymers, poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate), and the study of their micellization/dissociation transitions in water in response to temperature changes and UV irradiation. The block copolymers with controlled molecular weights and narrow polydispersities were synthesized by atom transfer radical polymerization of a mixture of ethoxytri(ethylene glycol) acrylate and o-nitrobenzyl acrylate with a molar ratio of 100:10 from a PEO macroinitiator. Dynamic light scattering and fluorescence spectroscopy studies showed that these copolymers were molecularly dissolved in water at lower temperatures and self-assembled into micelles with the thermosensitive block associating into the core and the PEO block forming the corona when the temperature was above the lower critical solution temperature (LCST) of the thermosensitive block. Upon UV irradiation, the o-nitrobenzyl group was cleaved and the LCST of the thermosensitive block was increased, causing the dissociation of micelles into unimers and the release of encapsulated fluorescent dye Nile Red into water. Further increasing the temperature induced the formation of micelles again and the re-encapsulation of Nile Red. The thermo-induced formation and dissociation of micelles were reversible.

This article presents a systematic study of the effect of pH on the rheological properties of aqueous micellar gels formed from 10.0 wt % aqueous solutions of a thermo- and pH-sensitive ABA triblock copolymer, poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethylene oxide)-b-poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid) (P(DEGEA-co-AA)-b-PEO-b-P(DEGEA-co-AA)). The block copolymer was synthesized by atom transfer radical polymerization of DEGEA and tert-butyl acrylate with a molar ratio of 100:5 from a difunctional PEO macroinitiator and subsequent removal of tert-butyl groups using trifluoroacetic acid. PDEGEA is a thermosensitive water-soluble polymer with a cloud point of 9 °C in water. The thermo-induced sol-gel transition temperature (T(sol-gel)) of the 10.0 wt % aqueous solution of P(DEGEA-co-AA)-b-PEO-b-P(DEGEA-co-AA) can be continuously and reversibly tuned over a wide temperature range by varying the solution pH. The sol-gel transition became broader with the increase of pH, which stemmed from the weaker and broader LCST transition of P(DEGEA-co-AA) blocks at higher pH values. The maximum value of dynamic storage modulus, obtained from heating ramp, and the plateau storage moduli (G(N)), evaluated from frequency sweeps at three normalized temperatures (T/T(sol-gel) = 1.025, 1.032, and 1.039), decreased with the increase of pH from 3.00 to 5.40 with the sharpest drop observed at pH = ∼4.7. The decrease in G(N) reflects the reduction of the number of bridging polymer chains and simultaneously the increase of the numbers of loops and dangling polymer chains. The ionization of carboxylic acid groups at higher pH values introduced charges onto the thermosensitive blocks and made the polymer chains more hydrophilic, facilitating the formation of loops and dangling chains in the gels. The increase in the number of dangling polymer chains with the increase of pH was supported by fluorescence spectroscopy studies, which showed that the critical micelle concentration of P(DEGEA-co-AA)-b-PEO-b-P(DEGEA-co-AA) at a temperature corresponding to T(sol-gel) was higher at a higher pH. The results reported in this article showed that both T(sol-gel) and gel strength can be tuned by varying the solution pH, providing greater design flexibility for potential applications.

Cellulose nanocrystals (CNCs) have great potential as sustainable reinforcing materials for polymers, but there are a number of obstacles to commercialization that must first be overcome. High levels of water absorption, low thermal stabilities, poor miscibility with nonpolar polymers, and irreversible aggregation of the dried CNCs are among the greatest challenges to producing cellulose nanocrystal-polymer nanocomposites. A simple, scalable technique to modify sulfated cellulose nanocrystals (Na-CNCs) has been developed to address all of these issues. By using an ion exchange process to replace Na with imidazolium or phosphonium cations, the surface energy is altered, the thermal stability is increased, and the miscibility of dried CNCs with a nonpolar polymer (epoxy and polystyrene) is enhanced. Characterization of the resulting ion exchanged CNCs (IE-CNCs) using potentiometry, inverse gas chromatography, dynamic vapor sorption, and laser scanning confocal microscopy reveals that the IE-CNCs have lower surface energies, adsorb less water, and have thermal stabilities of up to 100 °C higher than those of prepared protonated cellulose nanocrystals (H-CNCs) and 40 °C higher than that of neutralized Na-CNC. Methyl(triphenyl)phosphonium exchanged cellulose nanocrystals (MePhP-CNC) adsorbed 30% less water than Na-CNC, retained less water during desorption, and were used to prepare well-dispersed epoxy composites without the aid of a solvent and well-dispersed polystyrene nanocomposites using a melt blending technique at 195 °C. Predictions of dispersion quality and glass transition temperatures from molecular modeling experiments match experimental observations. These fiber-reinforced polymers can be used as lightweight composites in transportation, infrastructure, and renewable energy applications.

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