Serge Lacelle

Preparation of polymer single chain nanoparticles using intramolecular photodimerization of coumarin

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 λ > 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 1H NMR spin–spin relaxation time (T2). 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.

Multiple quantum nuclear magnetic resonance in one-dimensional quantum spin chains

Multiple quantum (MQ) nuclear magnetic resonance (NMR) spin dynamics are investigated analytically in infinite one-dimensional (1D) 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...

Multiple quantum nuclear magnetic resonance of solids: A cautionary note for data analysis and interpretation

The conventional method of data analysis and interpretation of time‐resolved multiple quantum (MQ) nuclear magnetic resonance (NMR) spectra of solids is closely examined. Intensity profiles of experimental 1H MQ NMR spectra of polycrystalline adamantane and hexamethylbenzene serve to test the Gaussian statistical model approach. Consequences of this model are explored with a least‐squares fitting procedure, transformation of data to yield linear plots, and a scaling analysis. Non‐Gaussian behavior of the MQ NMR spectral intensity profiles, as a function of order of coherences, is demonstrated with all these methods of analysis. A heuristic argument, based on the multiplicative properties of dipolar coupling constants in the equation of motion of the density operator, leads to the prediction of exponentially decaying MQ NMR spectral intensity profiles. Scaling analysis and semilog plots of experimental time‐resolved MQ NMR spectra of adamantane and hexamethylbenzene support this deduction. Dynamical scale ...

Ultrasound-responsive block copolymer micelles based on a new amplification mechanism.

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.

Multiple quantum NMR spin dynamics in one-dimensional quantum spin chains

Abstract Multiple quantum NMR spin dynamics arising from a two-quantum/two-spin non-secular average dipolar Hamiltonian are solved exactly for an infinite one-dimensional (1-D) quantum spin 1 2 chain with nearest-neighbor dipole-dipole interactions and initially at thermal equilibrium in a Zeeman field. Using fermion field-theoretic representations of spin 1 2 operators, the calculations yield analytical expressions valid for all times for the spin density operator, multiple quantum NMR spectral intensities, and multiple spin correlations. Implications and perspectives for multiple quantum NMR of materials with 1-D distributions of spins are briefly discussed.

Configurational averaging of dipolar interactions in magnetically diluted spin networks

Some analytical results, concerning the disorder average of magnetic dipolar interactions in dilute spin networks and valid in the thermodynamic limit, are derived within a mean field approximation. In particular, it is shown that the quenched substitutional disorder average of the free induction decay of the nuclear magnetization in such a homonuclear spin system yields, 〈G(t)〉c ∼ exp[−ρ(γ2ℏt)D/3], where ρ is the concentration of magnetic nuclei embedded in a D‐dimensional space and γ the nuclear magnetogyric ratio. Furthermore, the relative fluctuations about the disorder average, {[〈G2(t)〉c−〈G(t)〉2c]1/2/〈G(t)〉 ic}, scale as [ρ1/2(γ2ℏt)D/6/N1/2], when ρ(γ2ℏt)D/3≪1, and as N−1/2 exp[ρ(γ2ℏt)D/3] [1+ρ−3(γ2ℏt)−D]1/2 when ρ(γ2ℏt)D/3≫1; N is the number of spins in the lattice. The distinction between the role of the arithmetic mean and the most probable value of the distribution of dipolar coupling constants in the configurational averages of the free induction decay and its relative fluctuations is pointed o...

How can photoisomerization of azobenzene induce a large cloud point temperature shift of PNIPAM

We present a comprehensive study of the photo-induced phase transition of azobenzene-containing poly(N-isopropylacrylamide) (PNIPAM) in block copolymers (BCPs) upon the isomerization of azobenzene in the mixed solvent of water and dioxane. The BCP is composed of a poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) block and a PNIPAM block decorated with ∼2 mol% of azobenzene groups (P(NIPAM-co-MAzo)). In the mixed solvent of water and dioxane, the cloud point temperature (CPT) for the phase transition of the PNIPAM block displays a large shift >10 °C induced by the photoisomerization of azobenzene under UV or visible light. The CPT shift of PNIPAM is highly dependent on the solvent composition and the large shift only occurs at high dioxane concentration (>30%). The mechanism of the photo-induced large CPT shift by the isomerization of azobenzene was investigated by means of 1H NMR spin relaxation studies. With the addition of dioxane, the exchange of hydrogen bonding from PNIPAM–water to PNIPAM–dioxane was detected in the neighborhood of the CPT, thereby demonstrating the better solvation of the BCP chains and the azobenzene moieties by dioxane. We further demonstrate that the spin relaxation of azobenzene in the trans–cis isomerization only displayed a notable change at higher dioxane concentrations where the large shift of the CPT was observed. An improved understanding of the mechanism of photoinduced phase transition offers new insights into the rational design, synthesis and application of photoresponsive polymer materials.

Surface fractal dimensions of some industrial minerals from gas-phase adsorption isotherms

A high precision gravimetric method was used to investigate the adsorption of nitrogen (N 2 ) and carbon tetrachloride (CCl 4 ), respectively, at 77 and 298 K, onto various industrial minerals. The solids investigated include silica isomorphs, blast furnace slags, an Y-zeolite, and several naturally occurring fibrous minerals. The adsorption isotherms were analyzed to derive surface areas, BET constants ( C ), and surface fractal parameters ( D ). The latter were obtained through an approach recently suggested by Avnir and Jaroniec, 1 using multilayer adsorption data; D values obtained for the various gas-solid systems investigated cover the range 2.1–3.0. A systematic comparison between D values inferred from N 2 and CCl 4 adsorption data shows that the CCl 4 molecule probes the surface roughness in a more discriminate fashion than N 2 . In most cases, the differences between D (N 2 ) and D (CCl 4 ) appear to reflect changes due to sample preparation more than intrinsic differences amongst the various solids.

Low temperature multiple quantum nuclear magnetic resonance spin dynamics in one-dimensional quantum spin chains

Time-resolved multiple quantum nuclear magnetic resonance spectral intensity profiles as a function of the order of coherences, n, are calculated analytically for an infinite one-dimensional quantum spin chain with nearest-neighbor dipolar interactions and shown to be symmetric about n=0 for temperatures above tens of μK.

Nonergodic quasiequilibrium in nuclear magnetic resonance of infinite linear spin-1/2 chains

The long-term behavior of longitudinal spin order is investigated analytically in infinite one-dimensional (1-D) chains of nuclear spins 1/2 driven by a multiple pulse sequence of nonresonant rf pulses in the presence of a strong magnetic field. We consider a chain of spins with only nearest-neighbor dipolar interactions, initially at thermal equilibrium in the rotating frame, and evolving under the Hamiltonian, H=ΔIZ+H+2+H−2, (where Δ is a resonance offset of the carrying frequency of the pulses from the Larmor frequency of the spins, and H+2+H−2 is a two-spin/two-quantum nonsecular average dipolar Hamiltonian). The corresponding spin density operator and longitudinal magnetization are calculated exactly with fermion field operator representations of spin-1/2 operators. The spin density operator is determined by an additional integral of motion, beyond the energy, reflecting inherent nonergodic behavior. The establishment of the spin thermodynamic equilibrium between the Zeeman and the nonsecular dipolar...