Hetero-network hydrogels crosslinked with silica nanoparticles for strategic control of thermal responsive property

Two thermoresponsive copolymers with different lower critical solution temperatures (LCSTs) were crosslinked using silica nanoparticles to afford hybrid hydrogels exhibiting two distinct thermo-responsivities. The thermo-responsive copolymers were synthesised by free radical polymerisation from a monomer with a reactive side chain (3-methacryloxypropyl trimethoxysilane (S)) and water-soluble monomers with different thermo-responsivities (N-isopropyl acrylamide (N) or N-(3-methoxy propyl)acrylamide (M)). The obtained reactive copolymers, poly(N-isopropyl acrylamide-co-3-methacryloxypropyl trimethoxysilane) (pNS) and poly(N-(3-methoxy propyl acrylamide-co-3-methacryloxypropyl trimethoxysilane)) (pMS), were characterized by multiple techniques including 1H NMR and FTIR spectroscopy. The hetero-network hybrid hydrogels were easily prepared by mixing aqueous solutions of the copolymer with an aqueous colloidal silica suspension; their gelation properties could be tuned by varying the amounts of pNS, pMS, and Si. Differential scanning calorimetric analysis showed that the hetero-network hydrogel exhibited a critical two-step phase transition at temperatures around the LCST of each copolymer (33 °C for pNS, 73 °C for pMS), indicating that each polymer does not disturb the phase transitions of the other. The deswelling of the hetero-network hydrogel could be controlled with respect to temperature and time.

Molecular Mass Affects the Phase Separation of Aqueous PEG-Polycation Block Copolymer

Mechanisms of the phase separation and remixing of cationic PEG-containing block copolymers have been investigated in aqueous lithium triflate solutions. The polycation was poly(vinylbenzyl trimethylammonium triflate). We have previously reported on one such block copolymer, which upon cooling of a hot clear solution first underwent phase separation into a turbid colloid and, later, partially cleared again with further cooling. To better understand the balance of various interactions in the solutions/dispersions, a series of polymers with varying DP of the cationic block was synthesized. From one of the polymers, the alkyl end group (a fragment of the chain transfer agent) was removed. The length of the cationic block affected critically the behavior, but the hydrophobic end group had a minimal effect. Polymers with a short cationic block turn cloudy and partially clear again during a temperature decrease, whereas those with a long cationic block phase separate and slowly precipitate and remix only when heated. Phase separation takes place via particle formation, and we suggest different mechanisms for colloidal stabilization of particles composed of short or long chains.

Soluble-Insoluble-Soluble Transitions of Thermoresponsive Cryptand-Containing Graft Copolymers

Cryptand-containing alternative copolymers were first made from copolymerization of styrenic derivatives and maleic anhydride and then chemically modified in this work by grafting methoxy poly(ethylene glycol) (MPEG) onto the maleic functional groups. These graft copolymers show interesting multistep soluble-insoluble-soluble (S-I-S) transitions in acidic aqueous media at a cloud point (T cp) and a subsequent mixing temperature (T mix). Turbidity measurements and dynamic light scattering studies indicate that such complex transitions may be attributed to the entropic contribution associated with the dehydration and aggregation of the MPEG groups and then the enthalpic contribution associated with the hydrogen bonding between ethylene glycol and carboxylic acid groups. More importantly, the phase transition temperatures and insoluble temperature ranges are very sensitive to changes in subtle hydrophobic-hydrophilic balance of the copolymers, such as the variation of pH, the cryptand size, and the length of the MPEG graft. The understanding of the S-I-S transition in relation to the structure of the copolymers and the external conditions may be useful in the design of smart materials and sensors.

Study of theThermo-/pH-Sensitivity of Stereo-Controlled Poly(N-isopropylacrylamide-co-IAM) Copolymers via RAFT Polymerization

In this work, a smart copolymer, Poly(nipam-co-IAM) was synthesized by copolymerization of N-isopropylacrylamide (nipam) and itaconamic acid (IAM) through reversible addition-fragmentation chain-transfer (RAFT) polymerization. Poly(nipam-co-IAM) has been studied previously synthesized via radical polymerization without stereo-control, and this work used cumyl dithiobenzoate and Ytterbium(III) trifluoromethanesulfonate as RAFT and stereo-control agents, respectively. The stereo-control result in this work shows that tacticity affects the lower critical solution temperature (LCST) and/or the profile of phase separation of Poly(nipam-co-IAM). In the pH 7 and pH 10 buffer solutions, the P(nipam-co-IAM) copolymer solutions showed soluble–insoluble–soluble transitions, i.e., both LCST and upper critical solution temperature (UCST) transitions, which had not been found previously, and the insoluble to soluble transition (redissolved behavior) occurred at a relatively low temperature. The insoluble to soluble transition of P(nipam-co-IAM) in alkaline solution occurred at a temperature of less than 45 °C. However, the redissolved behavior of P(nipam-co-IAM) was found only in the pH 7 and pH 10 buffer solutions and this redissolved behavior was more prominent for the atactic copolymers than in the isotactic-rich ones. In addition, the LCST results under our experimental range of meso content did not show a significant difference between the isotactic-rich and the atactic P(nipam-co-IAM). Further study on the soluble-insoluble-soluble (S-I-S) transition and the application thereof for P(nipam-co-IAM) copolymers will be conducted.

Double thermoresponsive pentablock copolymers: synthesis by one-pot RAFT polymerization and self-assembly in aqueous solutions

Pentablock copolymers synthesized by one-pot successive RAFT polymerization are double thermoresponsive and exhibit block sequence dependent aggregation in aqueous solutions.

Recent Advances in Dual Temperature Responsive Block Copolymers and Their Potential as Biomedical Applications

The development of stimuli responsive polymers has progressed significantly with novel preparation techniques, which has allowed access to new materials with unique properties. Dual thermoresponsive (double temperature responsive) block copolymers are particularly of interest as their properties can change depending on the lower critical solution temperature (LCST) or upper critical solution temperature (UCST) of each segment. For instance, these block copolymers can change from being hydrophilic, to amphiphilic or to hydrophobic simply by changing the solution temperature without any additional chemicals and the block copolymers can change from being fully solubilized to self-assembled structures to macroscopic aggregation/precipitation. Based on the unique solution properties, these dual thermo-responsive block copolymers are expected to be suitable for biomedical applications. This review is divided into three parts; LCST-LCST types of block copolymers, UCST-LCST types of block copolymers, and their potential as biomedical applications.

Smart Macroporous IPN Hydrogels Responsive to pH, Temperature, and Ionic Strength: Synthesis, Characterization, and Evaluation of Controlled Release of Drugs

Fast responsive macroporous interpenetrating polymer network (IPN) hydrogels were fabricated in this work by a sequential strategy, as follows: the first network, consisting of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEM) cross-linked with N,N'-methylenebisacrylamide (BAAm), was prepared at -18 °C, the second network consisting of poly(acrylamide) (PAAm) cross-linked with BAAm, being also generated by cryogelation technique. Both single network cryogels (SNC) and IPN cryogels were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and water uptake. The presence of weak polycation PDMAEM endows the SNCs and the IPNs cryogels with sensitivity at numerous external stimuli such as pH, temperature, ionic strength, electric field, among which the first three were investigated in this work. It was found that the initial concentration of monomers in both networks was the key factor in tailoring the properties of IPN cryogels such as swelling kinetics, equilibrium water content (EWC), phase transition temperature and the response at ionic strength. The pore size increased after the formation of the second network, the swelling kinetics in pure water being comparable with that of the SNC, phase transition temperature being situated in the range 35-36 °C for IPN cryogels. The water uptake at equilibrium (WUeq) abruptly increased at pH < 3.0 in the case of SNCs, whereas the response of IPN cryogels at the decrease of pH from 6.0 to 1.0 was strongly dependent on the gel structure, the values of WUeq being lower at a higher concentration of DMAEM in the first network, the monomer concentration in the second network being about 10 wt %. The pH response was very much diminished when the monomer concentration was high in both networks (15 wt % in the first network, and 21 wt % in the second network). The increase of the ionic strength from 0 up to 0.3 M NaCl led to the decrease of the WUeq, for all cryogels, the level of dehydration being higher and faster for the SNC than for the corresponding IPN cryogel. The release of diclofenac sodium (DS), as a model acidic drug, triggered by pH, temperature, and ionic strength from the IPN cryogels was evaluated. A pulsatile release of DS from the IPN cryogels was presented, with a slower release at 34 °C (below VPTT) and a faster release at 37 and 40 °C (above the VPTT).

pH-modulated double LCST behaviors with diverse aggregation processes of random-copolymer grafted silica nanoparticles in aqueous solution

Thermo-responsive hybrid nanoparticles composed of silica-core and poly(N,N-dimethylaminoethyl methacrylate-co-N-isopropylacrylamide) P(DMAEMA-co-NIPAM) copolymer-shell were prepared through a one-pot ATRP technique.

Thermo and pH dual-controlled charge reversal amphiphilic graft copolymer micelles for overcoming drug resistance in cancer cells

A novel thermo and pH dual-controlled charge reversal PSMA₈₉-g-P(DMA₁₆-co-SD₅₆) graft copolymer micelle was developed with effectively enhanced cellular uptake for overcoming multi-drug resistance in cancer cells.

Unique multiple soluble-insoluble phase transitions in aqueous two-phase copolymerization of acrylamide and a weakly charged comonomer

A unique phenomenon of multiple soluble-insoluble phase transitions was found in the two-phase copolymerization of acrylamide (AM) and a weakly charged comonomer, N,N-dimethylaminoethyl methacrylate (DMAEMA), in the aqueous solution of poly(ethylene glycol) (PEG). As the DMAEMA molar fraction increased from 0 to 0.30, the insoluble-soluble (I-S) phase transition first appeared and then disappeared. Varying the PEG concentration, the salt concentration, or the pH of reaction mixture, the phase transitions were tuned dependently. The volume fractions and refractive indices of continuous and disperse phases, as well as the viscosity change in the phase transitions were investigated, and the results showed that phase reentrance had occurred in the I-S phase transition. The transitional conversion for the first S-I phase transition increased with the DMAEMA molar fraction, indicating the solubility enhancement of charged polyelectrolytes. The content of DMAEMA in the resulting copolymer first increased and then decreased as the polymerization progressed. Accordingly, the droplet size increased in the two S-I phase transitions and decreased in the I-S phase transition. And it was proved that the copolymers were molecularly solubilized after the I-S phase transition. The multiple soluble-insoluble phase transitions were ascribed to the synergistic effect of polymer concentration, solubility enhancement of charged copolymers, and the salting-out effect of ionic comonomers. A generalized mechanism for the multiple soluble-insoluble phase transitions was proposed, which showed that the effects of polymer concentration were dominant in the two S-I phase transitions, while the effects of solubility enhancement played a key role in the I-S phase transition.

Modeling swelling behavior of thermoresponsive polymer brush with lattice density functional theory

A key problem in designing thermoresponsive polymer brushes on a solid surface is to find a relation between the targeted thermoresponsive properties and controllable conditions. Usually, a temperature-thickness curve showing the heating-induced swelling behavior of polymer brushes is chosen as the relation by either experimental or theoretical investigation. In this work, a lattice density functional theory (LDFT) developed previously is employed to investigate the temperature-thickness curves for five different types of polymer brushes, where the density profiles of polymer brushes calculated by LDFT are compared directly with simulation. It is found that the thermoresponsive behavior of a polymer brush can be characterized by the bulk phase behaviors of its corresponding polymer solution, including UCST, LCST, both UCST and LCST, closed LOOP and hourglass-shaped, which implies that the bulk phase diagram of polymer solutions can help us to find an appropriate polymer brush for a targeted thermoresponsive behavior. As an example, we show that the swelling behavior of a thermoresponsive polymer brush found in the experiment could be predicted by our LDFT results with the bulk phase diagram of real polymer solution only.

Complex thermoresponsive behavior of diblock polyacrylamides

A phase diagram showing complex thermoresponsive transitions of diblock copolymers from unimers to micellar clusters, micelles and aggregates.