Diverse thermoresponsive behaviors of uncharged UCST block copolymer micelles in physiological medium.

Three amphiphilic diblock copolymers, representative of three types of block copolymer (BCP) design, were synthesized using reversible addition-fragmentation chain-transfer (RAFT) polymerization. All of them have a same uncharged block of a random copolymer of commercially available acrylamide and acrylonitrile, P(AAm-co-AN), and exhibit a composition-tunable upper critical solution temperature (UCST). We show that by coupling a common P(AAm-co-AN) block with either hydrophobic polystyrene (PS) or hydrophilic poly(dimethylacrylamide) (PDMA) or the lower critical solution temperature (LCST) polymer of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), the BCP micelles formed in water or in phosphate buffered saline (PBS) can display diverse and UCST-dictated changes in response to temperature variations, such as the reversible dispersion-aggregation of micelles, dissolution-formation of micelles, and reversal of micelle core and corona. The results point out that P(AAm-co-AN) is a robust UCST polymer that can be introduced into controlled polymer architectures producible by RAFT, the same way as using the extensively studied LCST counterparts like poly(N-isopropylacrylamide) (PNIPAM). This ability should make the door wide open to exploring new thermosensitive polymers based on the thermosensitivity opposite to the LCST.

Light polarization-controlled shape-memory polymer/gold nanorod composite.

It is demonstrated that light polarization can be used to control photothermal effect-based shape-memory polymers (SMPs). Gold nanorods (AuNRs) are embedded in poly(vinyl alcohol) (PVA) and aligned by stretching the composite film. By changing the polarization direction of the incident laser at 785 nm with respect to the film stretching direction, the magnitude of the longitudinal surface plasmon resonance of AuNRs can be varied continuously, which determines the amount of heat generated upon laser exposure and thus the local temperature rise in the composite relative to the glass transition of the PVA matrix. Consequently, the temporary-to-permanent shape recovery process of the composite can be made to occur to different extents by tuning the polarization of laser while keeping all other conditions unchanged. This finding enhances the toolbox for controlling light-triggered SMPs.

Block copolymer micelles with a dual-stimuli-responsive core for fast or slow degradation.

We report the design and demonstration of a dual-stimuli-responsive block copolymer (BCP) micelle with increased complexity and control. We have synthesized and studied a new amphiphilic ABA-type triblock copolymer whose hydrophobic middle block contains two types of stimuli-sensitive functionalities regularly and repeatedly positioned in the main chain. Using a two-step click chemistry approach, disulfide and o-nitrobenzyle methyl ester groups are inserted into the main chain, which react to reducing agents and light, respectively. With the end blocks being poly(ethylene oxide), micelles formed by this BCP possess a core that can be disintegrated either rapidly via photocleavage of o-nitrobenzyl methyl esters or slowly through cleavage of disulfide groups by a reducing agent in the micellar solution. This feature makes possible either burst release of an encapsulated hydrophobic species from disintegrated micelles by UV light, or slow release by the action of a reducing agent, or release with combined fast-slow rate profiles using the two stimuli.

Cyclic peptide-polymer complexes and their self-assembly.

The efficient synthesis of novel chiral cyclic peptides cyclo[NHCHX-CH=CHCH(2)CO(NHCH(2)CH=CHCH(2)CO)(2)] designed to develop hydrogen-bonding interactions with suitable polymers is described. Complexation of a carboxylic acid derivatized cyclic peptide 2 (X = CH(2)OCOCH(2)CH(2)CO(2)H) capable of self-assembling as "endless" tubes, with poly(vinyl alcohol) (PVA) led to a vast weak-interaction network, in which the cyclopeptide developed extensive hydrogen-bonding interactions with the hydroxyl groups of PVA through not only the carboxylic acid, but also its ester carbonyl and amide groups. In aqueous solution, the peptide/PVA complexes self-assemble into long-grain ricelike aggregates compatible with the stacking of cyclic peptides through intercycle hydrogen bonds. Upon casting on silicon wafer, the anisotropic aggregates can coalesce to form filaments tens of micrometers long. The study demonstrates that complexing functionalized cyclic peptides with polymers through hydrogen bonding is a useful approach for using polymers to mediate the self-assembly and self-organization of cyclic peptides.

Anisotropic optical properties of oriented silver nanorice and nanocarrots in stretched polymer films.

Stretching-induced orientation of both silver nanorice and silver nanocarrots dispersed in or deposited on the surface of poly(vinyl alcohol) (PVA) films was investigated using polarized UV-visible-near-infrared (NIR) spectroscopy and atomic force microscopy (AFM). The results show that the film stretching not only aligns the long axis of individual nanorice or nanocarrots preferentially along the stretching direction, but also induces assembly of these nanostructures into oriented arrays of random lengths in the deformation process. Consequently, the longitudinal surface plasmon resonance peaked at 820 nm for nanorice and 1050 nm for nanocarrots before stretching can be replaced by continuous extinction over the entire 800-1800 nm NIR spectral range after stretching. Stretched PVA films containing either silver nanorice or nanocarrots thus display polarization-dependent transmission of NIR light.

Photodegradable and size-tunable single-chain nanoparticles prepared from a single main-chain coumarin-containing polymer precursor.

A polyester bearing coumarin moieties in the main chain was used to prepare photodegradable single-chain nanoparticles (SCNPs) of variable sizes. While the intra-chain photodimerization of the chromophore determines the size of SCNPs, the photocleavage occurring under UV irradiation at a different wavelength breaks down the ultra-small nanoparticles.

A CO2-switchable polymer brush for reversible capture and release of proteins.

We report on a polymer brush that can be switched between extended (hydrated) and collapsed (dehydrated) chain conformational states just by passing CO(2) and an inert gas like N(2) in solution alternately. This conformational change allows for reversible adsorption and release of a protein. In contrast to adding acids and bases for pH change, using gases as the trigger makes it possible to repeat the switching cycle many times without salt accumulation.

A new design for light-breakable polymer micelles.

A new and general design strategy is presented for amphiphilic block copolymers whose micellar aggregates can be dissociated by light. A diblock copolymer composed of hydrophilic poly(ethylene oxide) (PEO) and a hydrophobic polymethacrylate bearing pyrene pendant groups (PPy) was synthesized using ATRP. Upon UV light irradiation of polymer micellar solutions, the photosolvolysis of pyrene moieties results in their detachment from the polymer and converts the hydrophobic PPy block into hydrophilic poly(methacrylic acid). This effect leads to complete dissociation of polymer micelles.