A set of high grafting density mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes with PtBA number-average molecular weight (M n ) fixed at 18.6 kDa and PS M n ranging from 8.7 to 28.0 kDa was synthesized from 172 nm asymmetric difunctional initiator (Y-initiator)-functionalized silica particles by sequential surface-initiated atom transfer radical polymerization of tert-butyl acrylate and nitroxidemediated radical polymerization (NMRP) of styrene. The Y-initiator-functionalized particles were prepared by the immobilization of a triethoxysilane-terminated Y-initiator onto the surface of bare silica particles via a hydrolysis/condensation process. The overall grafting densities of the obtained mixed brushes were 0.9-1.2 chains/nm 2 , which were significantly higher than those of mixed brushes prepared from silica particles that were surface functionalized by a monochlorosilane-terminated Y-initiator (0.6-0.7 chains/nm 2 ). Differential scanning calorimetry analysis showed that all high density mixed brushes exhibited two distinct glass transitions, suggesting that the two grafted polymers were microphase separated in the brush layer. TEM showed that with the increase of PS M n from 8.7 to 28.0 kDa the morphology of the mixed brushes changed from mostly isolated PS nanodomains buried in the PtBA matrix to a nearly cocontinuous nanostructure and two-layered nanostructures composed of a laterally microphase-separated bottom layer and a thin PS top layer. These morphologies were similar to the asymmetric mixed brushes with grafting densities of 0.6-0.7 chains/nm 2 . However, the feature sizes of the patterns formed from the microphase separation were much smaller. The observed grafting density effect on the pattern feature size was further confirmed from the study of a high grafting density mixed brush sample with PtBA M n of 23.7 kDa and PS M n of 25.7 kDa.

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.

We report in this article the synthesis of a series of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes with PtBA number-average molecular weight (M n) being fixed at 24.5 kDa and PS M n ranging from 14.8 to 30.4 kDa on 160 nm silica particles and the study of their microphase separation behaviors using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The samples were synthesized by a two-step “grafting from” process from asymmetric difunctional initiator (Y-initiator)-functionalized silica particles using two different “living”/controlled radical polymerization techniques. PtBA brushes were grown first from Y-initiator-functionalized particles by surface-initiated atom transfer radical polymerization of tert-butyl acrylate at 75 °C in the presence of a free initiator, followed by nitroxide-mediated radical polymerization (NMRP) of styrene at 120 °C. The “living” nature of NMRP allowed the synthesis of mixed PtBA/PS brushes with different PS molecular weights (14.8, 18.7, 24.9, and 30.4 kDa) in a one-pot polymerization. DSC studies showed that all thermally annealed mixed brush samples exhibited two glass transitions with the middle points located at ∼47 and ∼90 °C, suggesting that the two tethered polymers microphase separated in the brush layer. For TEM studies, the samples were dispersed in CHCl3, a good solvent for both PtBA and PS, drop-cast onto carbon films, thermally annealed in vacuum at 120 °C for 3 h, and then stained with RuO4 vapor. All samples showed clear microphase separation, consistent with the DSC results. With increasing PS M n from 14.8, to 18.7, and 24.9 kDa, the morphology of mixed brushes evolved from isolated, nearly spherical PS microdomains buried inside the PtBA matrix, to short PS cylindrical microdomains in the PtBA matrix, and to nearly bicontinuous nanostructures. Further increasing the molecular weight of PS to 30.4 kDa resulted in the formation of isolated PtBA microdomains which were covered by PS chains.

Owing to the intriguing transitions between free-flowing liquids and free-standing gels and the associated changes in rheological properties, stimuli-induced reversible formation of aqueous micellar gels of block copolymers has received considerable interest. 1À3 Compared with chemically cross-linked hydrogels, these responsive micellar gels, especially those triggered by temperature changes, can be more advantageous for certain applications because of the in situ solÀgel transition. 1À3 For example, Jeong et al. reported injectable drug delivery systems based on aqueous solutions of block copolymers of poly(ethylene glycol) (PEO) and polylactide that can undergo cooling-induced solÀgel transitions. 3 The polymer solutions were loaded with a model drug in the sol state at an elevated temperature. Upon subcutaneous injection and cooling to the body temperature, the polymer solutions formed gels instantaneously that subsequently acted as matrices for sustained release of drug molecules.Generally, there are two types of stimuli-responsive aqueous block copolymer micellar gels: 3-dimensional network gels, in which one block, e.g., the central block of an ABA triblock copolymer, forms bridges among micellar cores of other blocks, 1a,4 and physically jammed micellar gels, in which discrete spherical micelles of block copolymers are packed into an ordered structure. 1À3,5,6 Representative examples of the latter include aqueous gels of PEOb-poly(propylene oxide)-b-PEO (PEO-b-PPO-b-PEO) triblock ABSTRACT: This article reports on the synthesis of a hydrophilic diblock copolymer composed of two distinct thermosensitive polymers with one block containing a small amount of carboxylic acid groups, poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate) (P(TEGMA-co-AA)-b-PDEGEA), and the study of thermo-induced solÀgelÀ sol transitions of its moderately concentrated aqueous solutions at various pH values. The diblock copolymer was obtained by the removal of tert-butyl groups of P(TEGMA-co-tert-butyl acrylate)-b-PDEGEA, which was synthesized by reversible additionÀfragmentation chain transfer polymerization. PTEGMA and PDEGEA are thermosensitive polymers with lower critical solution temperatures (LCSTs) of 58 and 9°C, respectively, in water. The incorporation of a small amount of carboxylic acid groups into PTEGMA allowed the LCST of the P(TEGMA-co-AA) block to be tuned by changing the solution pH. We found that a 20 wt % aqueous solution of P(TEGMA-co-AA)-b-PDEGEA with pH of 3.11 (measured at 0°C) underwent multiple phase transitions upon heating, from a clear, free-flowing liquid (<19°C) to a clear, free-standing gel (19 to 39°C), to a clear, free-flowing hot liquid (40 to 55°C), and to a cloudy mixture (g56°C). With the increase of pH, the gel-to-sol transition (T gelÀsol ) and the clouding temperature (T clouding ) of the sample shifted to higher values, while the sol-to-gel transition temperature (T solÀgel ) remained the same. These transitions and the tunability of T gelÀsol...

Hybrid agarose hydrogels embedded with pH-responsive diblock copolymers micelles were developed to achieve functional hydrogels capable of stimulus-triggered drug release. Specifically, a well-defined poly(ethylene oxide) (PEO)-based diblock copolymer, PEO-bpoly(2-(N,N-diisopropylamino)ethyl methacrylate) (PEO113-b-PDPAEMA31, where the subscripts represent the degrees of polymerization of two blocks), was synthesized by atom transfer radical polymerization. PDPAEMA is a pH-responsive polymer with a pKa value of 6.3. The PEO113-b-PDPAEMA31 micelles were formed by a solvent-switching method, and their pH-dependent dissociation behavior was investigated by dynamic light scattering and fluorescence spectroscopy. Both studies indicated that the micelles were completely disassembled at pH = 6.40. The biocompatibility of PEO113-b-PDPAEMA31 micelles was demonstrated by in vitro primary cortical neural culture. Hybrid agarose hydrogels were made by cooling agarose solutions that contained various amounts of PEO113-b-PDPAEMA31 micelles at either 2 or 4 °C. Rheological measurements showed that the mechanical properties of gels were not significantly adversely affected by the incorporation of diblock copolymer micelles with a concentration as high as 5.0 mg/g. Using Nile Red as a model hydrophobic drug, its incorporation into the core of diblock copolymer micelles was demonstrated. Characterized by fluorescent spectroscopy, the release of Nile Red from the hybrid hydrogel was shown to be controllable by pH due to the responsiveness of the block copolymer micelles. Based on the prominent use of agarose gels as scaffolds for cell transplantation for neural repair, the hybrid hydrogels embedded with stimuli-responsive block copolymer micelles could allow the controlled delivery of hydrophobic neuroprotective agents to improve survival of transplanted cells in tune with signals from the surrounding pathological environment.