Heat-Induced Flower Nanogels of Both Cholesterol End-Capped Poly(N-isopropylacrylamide)s in Water.

Thermoresponsive self-assembled nanogels were conveniently prepared by cholesterol end-capped poly(N-isopropylacrylamide) (PNIPAM) in water. Both cholesterol end-capped PNIPAMs (telelchelic cholesterol PNIPAM, tCH-PNIPAM) formed flower-like nanogels by the self-assembling of four to five polymer chains with multiple domains of cholesterol in water at 20 °C. Meanwhile, one end-group cholesterol-capped PNIPAM (semitelechelic cholesterol PNIPAM, stCH-PNIPAM) was also formed as a nanogel by the self-assembling of 15-20 polymer chains with 3 to 4 cholesterol domains. The hydrophobic cholesterol domains of tCH-PNIPAM nanogels were maintained above the lower critical solution temperature (LCST) of PNIPAM (>32 °C). Differently, the hydrophobic domains of stCH-PNIPAM were disrupted by cholesterol-free PNIPAM chain ends and formed large mesoglobules above the LCST. These transition controls of hydrophilic end-capped smart polymers may open new methodologies to design thermoresponsive nanosystems.

Temperature-dependent adsorption and adsorption hysteresis of a thermoresponsive diblock copolymer.

A nonionic-cationic diblock copolymer, poly(2-isopropyl-2-oxazoline)60-b-poly((3-acrylamidopropyl)trimethylammonium chloride)17, (PIPOZ60-b-PAMPTMA17), was utilized to electrostatically tether temperature-responsive PIPOZ chains to silica surfaces by physisorption. The effects of polymer concentration, pH, and temperature on adsorption were investigated using quartz crystal microbalance with dissipation monitoring and ellipsometry. The combination of these two techniques allows thorough characterization of the adsorbed layer in terms of surface excess, thickness, and water content. The high affinity of the cationic PAMPTMA17 block to the negatively charged silica surface gives rise to a high affinity adsorption isotherm, leading to (nearly) irreversible adsorption with respect to dilution. An increase in solution pH lowers the affinity of PIPOZ to silica but enhances the adsorption of the cationic block due to increasing silica surface charge density, which leads to higher adsorption of the cationic diblock copolymer. Higher surface excess is also achieved at higher temperatures due to the worsening of the solvent quality of water for the PIPOZ block. Interestingly, a large hysteresis in adsorbed mass and other layer properties was observed when the temperature was cycled from 25 to 45 °C and then back to 25 °C. Possible causes for this temperature hysteresis are discussed.

The Two Phase Transitions of Hydrophobically End-Capped Poly(N-isopropylacrylamide)s in Water.

High-sensitivity differential scanning calorimetry (HS-DSC) thermograms of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions present a sharp unimodal endotherm that signals the heat-induced dehydration/collapse of the PNIPAM chain. Similarly, α,ω-di-n-octadecyl-PNIPAM (C18-PN-C18) aqueous solutions exhibit a unimodal endotherm. In contrast, aqueous solutions of α,ω-hydrophobically modified PNIPAMs with polycyclic terminal groups, such as pyrenylbutyl (Py-PN-Py), adamantylethyl (Ad-PN-Ad), and azopyridine- (C12-PN-AzPy) moieties, exhibit bimodal thermograms. The origin of the two transitions was probed using microcalorimetry measurements, turbidity tests, variable temperature 1H NMR (VT-NMR) spectroscopy, and 2-dimensional NOESY experiments with solutions of polymers of molar mass (M n) from 5 to 20 kDa and polymer concentrations of 0.1 to 3.0 mg/mL. The analysis outcome led us to conclude that the difference of the thermograms reflects the distinct self-assembly structures of the polymers. C18-PN-C18 assembles in water in the form of flower micelles held together by a core of tightly packed n-C18 chains. In contrast, polymers end-tagged with azopyridine, pyrenylbutyl, or adamantylethyl form a loose core that allows chain ends to escape from the micelles, to reinsert in them, or to dangle in surrounding water. The predominant low temperature (T 1) endotherm, which is insensitive to polymer concentration, corresponds to the dehydration/collapse of PNIPAM chains within the micelles, while the higher temperature (T 2) endotherm is attributed to the dehydration of dangling chains and intermicellar bridges. This study of the two phase transitions of telechelic PNIPAM homopolymer highlights the rich variety of morphologies attainable via responsive hydrophobically modified aqueous polymers and may open the way to a variety of practical applications.

Theoretical Study of the Wavelength Selection for the Photocleavage of Coumarin-caged D-luciferin.

The equilibrium structures and optical properties of the photolabile caged luciferin, (7-diethylaminocoumarin-4-yl)methyl caged D-luciferin (DEACM-caged D-luciferin), in aqueous solution were investigated via quantum chemical calculations. The probable conformers of DEACM-caged D-luciferin were determined by potential energy curve scans and structural optimizations. We identified 40 possible conformers of DEACM-caged D-luciferin in water by comparing the Gibbs free energy of the optimized structures. Despite the difference in their structures, the conformers were similar in terms of assignments, oscillator strengths and energies of the three low-lying excited states. From the concentrations of the conformers and their oscillator strengths, we obtained a theoretical UV/Vis spectrum of DEACM-caged D-luciferin that has two main bands of shape nearly identical to the experimental UV/Vis spectrum. The absorption bands with maxima ~ 384 and 339 nm were attributed to the electronic excitations of the caged group and the luciferin moiety, respectively, by analysis of the theoretical UV/Vis spectrum. Furthermore, the analysis showed that DEACM-caged D-luciferin is excited in the caged group only by light of wavelength ranging within 400-430 nm, which is in the long-wavelength tail of the 384 nm band. This should be tested to lower damage upon photocleavage.

pH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water.

A series of azopyridine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (∼5000 < M w < 20 000 g mol-1, polydispersity index 1.25 or less) were prepared by reversible addition-fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an n-dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm < R h < 10.9 nm) that were characterized by light scattering (LS), 1H NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV-vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life τ = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (τ = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (τ > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, 1H NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy+ end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.

Influence of Confinement on the Chain Conformation of Cyclic Poly(N-isopropylacrylamide).

In thin and ultrathin supported films, the conformations of flexible linear polymer chains might be considerably confined, in particular, for film thicknesses smaller than a few times the radius of gyration. For ring polymers in solution or in melt, the radii of gyration are significantly reduced as compared to those of their linear counterparts. We study here the influence of geometrical confinement on the chain conformation of cyclic PNIPAM in silicon-supported films. Measurements are performed by grazing incidence small angle neutron scattering (GISANS). For all films, the component of the radius of gyration parallel to the substrate, Rgc∥, is significantly higher than the unperturbed Rgc determined under theta solvent or melt conditions. We attribute this effect to a preferential selection of stretched PNIPAM ring conformations in thin films and a preferential orientation of macromolecules parallel to the film interfaces with the substrate and air.

Temperature-responsive telechelic dipalmitoylglyceryl poly(N-isopropylacrylamide) vesicles: real-time morphology observation in aqueous suspension and in the presence of giant liposomes.

Telechelic α,ω-di(twin-tailed poly(N-isopropylacrylamides)) form polymersomes in water that increase in size by fusion when the water temperature exceeds the polymers cloud point temperature. Hybrid vesicles form in mixed suspensions of giant phospholipid liposomes and polymersomes by adsorption/fusion, and undergo further transformations, such as fission.

Synthesis and characterization of phosphorylcholine-substituted chitosans soluble in physiological pH conditions.

A polymer analogous synthesis involving the reductive amination of phosphorylcholine (PC)-glyceraldehyde with primary amines of deacetylated chitosan (M(w) approximately 57000 g mol(-1)) was used to prepare phosphorylcholine-substituted chitosans (PC-CH) with a degree of substitution (DS) ranging from approximately 11 to approximately 53 mol % PC-substituted glucosamine residues. The PC-CH derivatives were characterized by (1)H NMR spectroscopy, FTIR spectroscopy, and multiangle laser light scattering gel permeation chromatography (MALLS-GPC). The pK(a) of the PC-substituted amine groups (pK(a) approximately 7.20) was determined by (1)H NMR titration. The PC-CH samples (1.0 g L(-1)) were shown to be nontoxic using an MTT assay performed with human KB cells. Aqueous solutions of PC-CH samples (4.0 g L(-1)) of DS >or= 22 mol % PC-substituted glucosamine residues remained clear, independently of pH (4.0 < pH < 11.0). The remarkable water solubility and nontoxicity displayed by the new PC-CH samples open up new opportunities in the design of chitosan-based biomaterials and nanoparticles.