Programed Thermoresponsive Polymers with Cleavage-Induced Phase Transition

A new programed upper critical solution temperature-type thermoresponsive polymer was developed using water-soluble anionic polymer conjugates derived from polyallylamine and phthalic acid with cleavage-induced phase transition property. Intrinsic charge inversion from anion to cation of the polymer side chain is induced through a side chain cleavage reaction in acidic aqueous media. With the progress of side chain cleavage under fixed external conditions, the polymer conjugates express a thermoresponsive property, followed by shifting a phase boundary due to the change in polymer composition. When the phase transition boundary eventually reached the examined temperature, phase transition occurs under fixed external conditions. Such new insight obtained in this study opens up the new concept of time-programed stimuli-responsive polymer possessing a cleavage-induced phase transition.

Poly(sulfobetaine)-Based Diblock Copolymer Thin Films in Water/Acetone Atmosphere: Modulation of Water Hydration and Co-nonsolvency-Triggered Film Contraction

The water swelling and subsequent solvent exchange including co-nonsolvency behavior of thin films of a doubly thermo-responsive diblock copolymer (DBC) are studied via spectral reflectance, time-of-flight neutron reflectometry, and Fourier transform infrared spectroscopy. The DBC consists of a thermo-responsive zwitterionic (poly(4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate)) (PSBP) block, featuring an upper critical solution temperature transition in aqueous media but being insoluble in acetone, and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, featuring a lower critical solution temperature transition in water, while being soluble in acetone. Homogeneous DBC films of 50-100 nm thickness are first swollen in saturated water vapor (H₂O or D₂O), before they are subjected to a contraction process by exposure to mixed saturated water/acetone vapor (H₂O or D₂O/acetone-d6 = 9:1 v/v). The affinity of the DBC film toward H₂O is stronger than for D₂O, as inferred from the higher film thickness in the swollen state and the higher absorbed water content, thus revealing a pronounced isotope sensitivity. During the co-solvent-induced switching by mixed water/acetone vapor, a two-step film contraction is observed, which is attributed to the delayed expulsion of water molecules and uptake of acetone molecules. The swelling kinetics are compared for both mixed vapors (H₂O/acetone-d6 and D₂O/acetone-d6) and with those of the related homopolymer films. Moreover, the concomitant variations of the local environment around the hydrophilic groups located in the PSBP and PNIPMAM blocks are followed. The first contraction step turns out to be dominated by the behavior of the PSBP block, whereas the second one is dominated by the PNIPMAM block. The unusual swelling and contraction behavior of the latter block is attributed to its co-nonsolvency behavior. Furthermore, we observe cooperative hydration effects in the DBC films, that is, both polymer blocks influence each other's solvation behavior.

Cononsolvency of Poly[2-(methacryloyloxy)ethyl phosphorylcholine] in Ethanol-Water Mixtures: A Neutron Reflectivity Study

Molecular mechanisms underlying the cononsolvency, a re-entrant coil-to-globule-to-coil conformational transition of polymer chains in mixtures of two good solvents, of poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC) in ethanol-water binary mixtures were complementarily investigated. This was accomplished by following a statistical mechanical model for competitive hydrogen bonding combined with the cooperative solvation concept as well as neutron reflectivity (NR) experiments employing contrast variation in the cononsolvents. The experimental re-entrant aggregation of the PMPC chains in ethanol-water mixed solvents, obtained on the basis of turbidity was accurately reproduced by theoretical calculations. The calculation proved the relatively strong cooperativity of ethanol and the preferential interaction of water, while the total coverage of solvents was the lowest at an ethanol volume fraction (f_ethanol) of 0.90. At this level, the cononsolvency was the most significant, and the collapsed PMPC chains were solvated with more water than the bulk mixed solvent. The ethanol-water cononsolvency for the PMPC brushes on a planar silicon wafer was investigated by NR experiments, and the solvent composition involved in the collapsed PMPC brush was addressed according to the contrast variation study with mixed solvents of water, deuterium oxide, ethanol-d₅, and ethanol-d₆. The collapsed PMPC brushes at f_ethanol = 0.90 contained more water than the bulk solvent. The preferential distribution of water in the collapsed PMPC brush was consistent with the simulation results. Therefore, the molecular mechanism for the cononsolvency of PMPC in ethanol-water mixed solvents based on competitive hydrogen bonding coupled with cooperative solvation was experimentally rationalized.

Sulfobetaine-based polydisulfides with tunable upper critical solution temperature (UCST) in water alcohols mixture, depolymerization kinetics and surface wettability

HYPOTHESIS

Synthesis of a new family of polymers having a polydisulfide structure can be conducted from sulfobetaine-based derivative of natural (R)-lipoic acid. A polydisulfide backbone of polymer can be depolymerized by response to external stimuli and sulfobetaine pendant groups ensure the upper critical solution temperature (UCST) behaviour temperatures that can be modulated according to the nature of the solvent and concentration.

EXPERIMENTS

Sulfobetaine-bearing polydisulfides were synthesized from dithiolane derivatives and then characterized. UCST behavior of the polymers in water and in mixtures containing different alcohols (methanol, ethanol, isopropanol) was investigated. The regeneration of monomers from the polymers in response to external stimuli was examined using UV-vis and circular dichroism (CD) spectroscopy. Tunable surface wettability were shown on the grafted polymers.

FINDINGS

Decreasing polarity and/or increasing alcohol percentage in the water mixtures induced an increase in the cloud points of the polymers in the solutions. Thermoresponsive behaviour were repeatable and fully reversible with negligible hysteresis from aggregate to unimer state. The regeneration of monomers by depolymerization was tunable by temperature and sunlight. A thickness dependence on surface wettability was observed on wafers covalently modified with polydisulfides. This is the first report of sulfobetaine-based polydisulfides showing tunable UCST behavior and surface wettability.

UCST-Type Thermosensitive Hairy Nanogels Synthesized by RAFT Polymerization-Induced Self-Assembly

While lower critical solution temperature (LCST)-type thermosensitive nanogels have been intensively studied, the upper critical solution temperature (UCST)-type versions are much less explored. This communication reports a method for the synthesis of zwitterionic UCST nanogels by reversible addition-fragmentation chain transfer (RAFT) polymerization-induced self-assembly in water-organic solvent mixtures. The nanogels were prepared by RAFT polymerization of 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate, whose polymer is known to exhibit UCST behavior in water, conducted in ethanol-water mixtures at 70 °C using poly(poly(ethylene glycol) methyl ether methacrylate) as a macro-chain transfer agent (CTA) and a difunctional monomer as cross-linker. At a sufficiently high ethanol content in reaction media, spherical hairy nanogels with a single size distribution were obtained. These nanogels exhibited reversible heating-induced swelling and cooling-induced shrinking, consistent with the expected UCST behavior. The hydrodynamic size, volume changing ratio, and transition temperature of nanogels can be tuned by varying ethanol content in solvent mixtures, molar ratio of monomer-to-macro-CTA, and amount of cross-linker. Hairy nanogels were also successfully synthesized using a water-THF mixture as medium. The use of water-organic solvent mixtures as reaction media allowed for facile incorporation of a hydrophobic fluorescent monomer to make functional UCST nanogels.

Effect of brush thickness and solvent composition on the friction force response of poly(2-(methacryloyloxy)ethylphosphorylcholine) brushes

The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

Co-nonsolvency effects for surface-initiated poly(2-(methacryloyloxy)ethyl phosphorylcholine) brushes in alcohol/water mixtures

Surface-initiated atom transfer radical polymerization (SI-ATRP) has been used to grow brushes of poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) from silicon wafers using a polyelectrolytic macroinitiator on planar silicon wafers. Film thicknesses of up to 450 nm were possible within 21 h, and the effect of adding activator and deactivator species on the brush growth rate was studied. The solvation of PMPC brushes in mixed alcohol/water solvents was investigated using in situ ellipsometry. Co-nonsolvency (a re-entrant swelling transition) behavior was observed in water/ethanol binary mixtures; that is, the PMPC brushes were highly swollen in either pure ethanol or water but became deswollen at specific ethanol-rich solvent compositions. A similar effect was obtained with water/2-propanol mixtures, except that in this case pure 2-propanol was not a particularly good solvent for the PMPC chains. However, co-nonsolvency was not observed for water/methanol binary mixtures, since the brushes remained well swollen at all solvent compositions. This is consistent with prior reports of co-nonsolvency effects in both PMPC gels and linear PMPC chains. However, this is the first report of this phenomenon for PMPC brushes and one of the first examples of co-nonsolvency observed for any polymer brush system. A direct comparison of brush and gel swelling reveals an approximate power-law relationship between the equilibrium volumes of these two systems at various solvent compositions, which is interpreted by treating the brush layer as a surface-attached gel. We believe this to be the first quantitative comparison of brush and gel swelling using the same polymer under the same conditions. The kinetics of the PMPC brush response to adjustment of the alcohol/water composition is relatively fast, with the brush volume change occurring on time scales of less than 1 min as judged by in situ ellipsometry.

Synthesis of biocompatible sterically-stabilized poly(2-(methacryloyloxy)ethyl phosphorylcholine) latexes via dispersion polymerization in alcohol/water mixtures

Poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) is soluble in either 2-propanol or water but becomes insoluble in certain alcohol-rich 2-propanol/water mixtures. We have exploited this unusual cononsolvency behavior in order to prepare new biocompatible sterically stabilized PMPC latexes via nonaqueous dispersion polymerization in 2-propanol/water mixtures. All polymerizations were conducted in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) as a reactive stabilizer, with some formulations including ethylene glycol dimethacrylate (EGDMA) as a cross-linker. Under optimized conditions, unimodal size distributions could be obtained with a mean latex diameter of approximately 1 microm, as judged by laser diffraction and DLS. The mean latex diameter depended on both the PEGMA and initiator concentration but was almost independent of the cross-linking density. Smaller PMPC latexes were obtained by increasing the alcohol content of the dispersion medium. On dilution with water, these latexes acquired microgel character. The microgel solution viscosity was insensitive to added salt due to the so-called "antipolyelectrolyte" effect, which is characteristic of polyzwitterions. Finally, copolymerization of MPC with a fluorescein-based methacrylic comonomer produced fluorescently labeled PMPC latexes, which may have potential biomedical applications.

Dimensions of a free linear polymer and polymer immobilized on silica nanoparticles of a zwitterionic polymer in aqueous solutions with various ionic strengths

The dimensions and intermolecular interactions of a surface-grafted and unbound free polyampholyte, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), were estimated in aqueous solutions with various ionic strengths. Free linear PMPC was synthesized by atom-transfer radical polymerization (ATRP), and static light scattering (SLS) and dynamic light scattering (DLS) were carried out for the PMPC solutions with various concentrations of NaCl, c s. The hydrodynamic radius R H and the second virial coefficient A 2 of PMPC were independent of c s (0-0.5 M), though both R H and A 2 of polyampholytes usually strongly depend on the ionic strength. PMPC-immobilized silica nanoparticles (PMPC-SiNP) were also synthesized by surface-initiated ATRP, and DLS was carried out as for the solutions of linear PMPC to investigate the dependence of the dimensions of PMPC immobilized on a solid surface on the ionic strength. The molecular weight and surface density of PMPC immobilized on SiNP were estimated from the results obtained by GPC, NMR, and thermogravimetric analysis. The independence of R H of PMPC-SiNP was also observed, but its magnitude was larger than that of linear PMPC, although the molecular weight of PMPC immobilized on SiNP was smaller than that of linear PMPC. The larger dimension of PMPC immobilized on SiNP was explained by the excluded volume effect between the immobilized polymer chains.