We present a new and easy method for preparing polymer single chain nanoparticles (SCNPs). It uses the photodimerization of coumarin groups located on the same chain to obtain the intrachain cross-linking required for chain collapse in solution. To demonstrate the approach, samples of a random copolymer composed of N,N-dimethylaminoethyl methacrylate (DMAEMA) and 4-methyl-[7-(methacryloyl)oxy-ethyl-oxy]coumarin (CMA), with 7 or 13 mol% of CMA, were synthesized via the reversible addition-fragmentation chain transfer (RAFT) polymerization. We show that well-defined SCNPs could be obtained by the intrachain photodimerization of coumarin groups upon l > 310 nm UV irradiation in a dilute copolymer solution. The coil-to-globule transition induced by intrachain photo-cross-linking was investigated by means of 1 H NMR spinspin relaxation time (T 2 ). The result indicates that the photoinduced chain collapse is accompanied by a sharp increase of the fraction of chain segments having reduced mobility. SCNPs were further used as a nanoreactor to synthesize gold nanoparticles (AuNPs) in situ. In tetrahydrofuran, the rate of AuNP formation was found to be dependent on the polymer chain conformation and mobility determined by the dimerization degree of coumarin. This provides a means to optically control the kinetics of AuNP formation.
1H nuclear magnetic resonance (NMR) relaxation studies of the temperature-induced micellization of a poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide), PEO−PPO−PEO, block copolymer in aqueous solution were performed in the range 280−345 K. 1H NMR spectra of the block copolymer were well resolved, thus allowing us to probe specifically the methyl and methylene relaxation processes in the PPO and PEO blocks, respectively. Interpretation of the relaxation data in terms of the Hall−Helfand correlation function leads to four distinct correlation times for the PPO and PEO blocks. The slower correlation time in the PPO block was identified as the Zimm−Rouse first normal mode of the copolymer and served to determine the hydrodynamic radius, R H, of the unimers and the micelles. On a more local scale, the behavior of the correlation time for segmental motions in the PPO block indicates an extension of the PPO chains in micelles relative to the unimers. This conformational change is related to the formation of a water insoluble liquid-like core created by the PPO chains in the micelle where the trans isomers are favored. The rotational isomeric states model used to interpret the faster correlation time in the PEO chains yields an activation energy of 14.6 kJ/mol for the correlated transitions in the PEO blocks, in agreement with previous theoretical calculations. The slower correlation time in the PEO blocks shows a marked increase upon micellization attributed in part to the polymer−polymer interactions between the different PEO blocks constituting the hydrophilic moiety of the micelles. A power law relating this relaxation time to the micelle hydrodynamic radius is predicted and observed experimentally. The concentration dependence of the critical micellization temperature, inferred from the methyl spin−lattice relaxation time in the PPO block, was found to be adequately described by a closed association model. The standard free energy, enthalpy, and entropy of micellization obtained by NMR are in close agreement with the recent experimental thermodynamic studies of P. Alexandridis et al. (Macromolecules 1994, 27, 2414).
Multiple quantum ͑MQ͒ nuclear magnetic resonance ͑NMR͒ spin dynamics are investigated analytically in infinite one-dimensional ͑1D͒ chains of spins 1/2. The representation of spin 1/2 operators with fermion field operators allows to calculate exactly the spin density operator, and hence NMR observables, under a variety of different conditions for 1D spin systems. The exact expressions are valid for all times and for a macroscopic number of coupled spins. The calculations for a 1D spin system initially at thermal equilibrium, and evolving under a 2-quantum/2-spin average dipolar Hamiltonian, in the presence of nearest-neighbor dipolar interactions yield MQ NMR spectra with 0-and 2-quantum coherences only. For a nonequilibrium initial condition with transverse magnetization, the analogous spin dynamics calculations produce MQ NMR spectra with all possible coherences of odd orders. Calculations at the level of perturbation theory, which include next-nearest-neighbor dipolar interactions, generate MQ spectra with higher even order coherences for equilibrium initial condition and evolution under a 2-quantum/2-spin propagator. Consideration of multiple spin correlations, 0-quantum coherences, and rf pulse imperfections are also presented. The relevance and implications of these theoretical results for comparison with the recent MQ NMR experiments of Yesinowski et al. on materials with quasi-one-dimensional distributions of spins, and for MQ NMR of solids in general are discussed.
A new approach for amplifying the effect of high-intensity focused ultrasound (HIFU) in disassembling amphiphilic block copolymer (BCP) micelles in aqueous solution was investigated. The diblock copolymer is comprised of a water-soluble poly(ethylene oxide) (PEO) block and a block of poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO(2)MA) that is hydrophobic at temperatures above its lower critical solution temperature (LCST). We show that by introducing a small amount of HIFU-labile 2-tetrahydropyranyl methacrylate (THPMA) comonomer units into the PMEO(2)MA that forms the micelle core at T > LCST, an ultrasound irradiation of a micellar solution could induce the hydrolysis of THPMA groups. As a result, the LCST of the thermosensitive polymer increases due to the conversion of hydrophobic THPMA comonomer units onto hydrophilic methacrylic acid. Consequently, the BCP micelles disassemble without actually changing the solution temperature. In addition to the characterization results of transmittance measurements, variable-temperature (1)H NMR, SEM, and DLS, a (13)C NMR spectral analysis provided critical evidence for the hydrolysis reaction of THPMA groups under HIFU irradiation.
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