Spectral time moment analysis of microgel deswelling. Effect of the heating rate

Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through the self-association of amphiphilic HPC molecules and the subsequent cross linking at room temperature. Dynamic Light Scattering was used to study the transport properties of HPC microgels below and above the volume phase transition. Highly nonexponential, multimodal microgel spectra were observed and successfully analyzed by spectral time moment analysis. This article expands earlier results and focuses on the effect of the heating rate on microgel deswelling. During the fast heating two identified microgel modes with apparent hydrodynamic radii (R_H) of 25–30 nm and 400–650 nm collapse into one mode with R_H = 100–150 nm. This indicates the shrinkage of microgel size distribution and an apparent decrease in the radius of larger microgels. During the slow heating, however, both microgel-identified modes remain present above T_c. Although equally represented below the transition, the dominance of larger microgels' mode increases almost two fold with rising temperature above 40°C. Moreover, R_H for this mode increases from 250–300 nm to about 800–850 nm with a multi-step temperature change from 40 to 42.5°C, indicating the growth (and not shrinkage) of microgels. The second mode is represented by the temperature independent R_H, but its contribution goes down from about 50% to less than 10%. © 2008Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2792–2802, 2008     

Interfacial structure and properties in polystyrene/epoxy bilayer films

The interfacial structure and properties of immiscible deuterated polystyrene (dPS)/epoxy bilayer films were investigated with neutron reflectivity as functions of the composition of the epoxy layer, the thickness of the dPS layer, and the annealing time. We have found that the interfacial width and its growth rate depend strongly on the compositions of the epoxy layer but only weakly on the thickness of the dPS layer. The effect of the resin/crosslinker composition on the interfacial width and its growth rate is likely due to the different near‐surface structures that result for different epoxy stoichiometries. For an ultra‐thin dPS film (thickness = 2R_g), the data suggest a slight suppression of the growth of the interfacial width that could be due to confinement effects for the long‐chain molecules such as have been previously reported for a thickness of less than approximately 4R_g, where R_g is the radius of gyration of polymer molecules. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2653–2660, 2002     

Mode-specific pressure effects on the relaxation of an excited nitromethane molecule in an argon bath

The vibrational and rotational mode-specific relaxations of CH₃NO₂ with 50 kcal/mol of initial internal energy in an argon bath is computed at 300 K at pressures of 10-400 atm. This work uses archived information from our previously published [J. Chem. Phys. 142, 014303 (2015)] molecular dynamics simulations and employs our previous published [J. Chem. Phys. 151, 034303 (2019)] method for projecting time-dependent Cartesian velocities onto normal mode eigenvectors. The computed relaxations cover three types of energies: vibrational, rotational, and Coriolis. In general, rotational and Coriolis relaxations in all modes are initially fast followed by an orders of magnitude slower relaxation. For all modes, that slower relaxation rate is approximately comparable to the vibrational relaxation rate. For all three types of energies, there are small-scale mode-to-mode variations. Of particular prominence is the exceptionally fast relaxation shared in common by the external rotation about the C N axis, the internal hindered rotation of the CH₃ group relative to the NO₂ group, and the symmetric stretch of the CH₃ group.     

Continuous phase viscosity influence on maximum diameters of poly(styrene‐divinylbenzene) beads prepared by suspension polymerization

The influence of the continuous phase viscosity (μ_c) on the diameter of styrene‐divinylbenzene polymer beads was studied over a large range of viscosities at constant dissipated power. This study was based on the inertial breakup and viscous shear breakup theories for a stirred dispersion. These two theoretical models were compared with an experiment for the two highly viscous agents sucrose and acacia gum [dispersed and continuous phase viscosities (μ_d, μ_c) = 10⁻³ < μ_d/μ_c < 1]. We found that the maximum diameters of the polymer beads could not be described by an inertial breakup. The maximum diameters were in good agreement with a viscous shear breakup model for the two viscous agents in turbulent and semilaminar flows. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 201–210, 2001     

Surface plasmon resonance as a tool for the estimation of adsorbed polymeric layer characteristics: Theoretical considerations and experiment

The potential application of Surface Plasmon Resonance (SPR) spectroscopy in evaluating the thickness and volume fraction of adsorbed macromolecular layers is discussed in this work. The sensitivity of SPR spectroscopy to different layer concentration and to the layer extension normal to the surface (thickness) is theoretically illustrated. A new approach for the interpretation of SPR data is presented, which is applicable whenever the functional form of the density profile is known. The use of the proposed procedure for the fitting of experimental results from PS‐PEO brush self‐assembly on alumina surface has allowed the determination of the layer parameters, which have been found to be in accordance with theoretical mean‐field and scaling predictions, being also in good agreement with previous results from neutron reflectivity experiments. Furthermore, it has been confirmed that the dependence of the brush layer thickness d on the molecular weight M_w obeys the scaling law d ～ M_w^0.63. Since surface plasmon measurements can be acquired quite fast, it is suggested that under the present analysis scheme, the technique may be implemented to probe the average conformational properties of adsorbed macromolecular layers during their formation or under external stimuli. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2060–2070, 2007     

Conductivity relaxation behavior of interpenetrating polymer network composites of polypyrrole and poly(styrene‐co‐butyl acrylate)

Conductivity relaxation spectra of interpenetrating network conducting composites of polypyrrole (PPy) and poly(styrene‐co‐butyl acrylate) (SBA) have been analysed on the basis of coupling model developed by Ngai. The macroscopic activation energy obtained from coupling model using the stretch exponent β compares favourably with the tunnel energy estimated from the overlapping large polaron tunnelling (OLPT) model. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1193–1200, 2000     

Length scale hierarchy in sol,gel, and coacervate phases of gelatin

Length scale hierarchy in gelatin sol, gel, and coacervate (induced by ethanol) phases, having same concentration of gelatin in aqueous medium (13% w/v), has been investigated through small angle neutron scattering and rheology measurements. The static structure factor profile, I(q) versus wave vector q, was found to be remarkably similar for all these samples. This data could be split into three distinct q‐regimes: the low‐q regime, I_ex(q) = I_ex(0)/(1+q²ζ²)² valid for q < 3R_g^?1; the intermediate q‐regime, I(q) = I(0)/(1+q²ξ²) for 3R_g^?1 < q < ξ^?1; and the asymptotic regime, I(q) = (c/q) exp(?R_c²q²/2) for q > ξ^?1. Consequently, three distinct length scales could be deduced from structure factor data: (a) inhomogeneity of size, ζ = 20 ± 1 nm for all the three phases; (b) average mesh size, ξ₀ = 2.6 ± 0.2 nm for sol and gel, and smaller mesh size, ξ_os = 1.2 ± 0.2 nm for coacervate; and (c) cross section of gelatin chains, R_c = 0.35 ± 0.04 nm. In addition, the structure factor data obtained from coacervating solution analyzed in the Guinier region, I(q) = exp(?q²R_g²/3), yielded value of typical radius of gyration of clusters, R_g ≈ 69 nm that indicated existence of triple‐helices of length, L ≈ 239 nm; (d) Frequency and temperature sweep measurements conducted on coacervate samples revealed two other length scales: (e) viscoelastic length, ξ_ve = 14 ± 2 nm and (f) correlation length at melting, ξ_T = 500 ± 70 nm. Thus, existence of six distinct length scales, (a–f), ranging from 1.2 to 500 nm has been established in the coacervate phase of gelatin–ethanol–water system. Results are discussed within the framework of Landau‐Ginzburg treatment of dynamically asymmetric systems (Prog Theor Phys 1977, 57, 826; Phys Rev A 1991, 44, R817; J Phys II (France) 1992, 2, 1631). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1653–1667, 2006     

Double‐helical network in amylose as seen by slow calorimetry and FTIR

The phase content and crystallinity of initially amorphous amylose–water mixtures (70/30 W/W) have been changed by slow cycles of dissolution and recrystallization from T_max with 50 °C < T_max < 120 °C. Analysis of the treatment‐induced changes is made by X‐ray diffraction, FTIR, fast T‐ramp DSC and slow calorimetry. Our interest was to follow the relaxation of the network phase and its consequence on the growth of crystallinity. The DSC technique, which gives the temperature of disappearance of long‐range order, is unable to quantitatively follow the growth of crystallinity achieved by treating the samples. In highly interactive polymer–solvent systems, order is unmeltable in a fast T‐ramp due to strain developed during the ramp. In a 6 K/h T‐ramp, the order becomes meltable and grows from 21 J/g to 147 J/g when T_max increases. The other conclusion is that strain‐melting and the network phase, characterized first in polyolefins has a more prominent role in the characterization of H‐bonded polysaccharide–water mixtures. Correlation is achieved between the concentration of bands in the C O stretching region, the fraction of single and double helices, and the three endotherms found on the slow T‐ramp dissolution traces. FTIR spectra show that chains in the network cannot be disentangled by quenching but can be organized during a slow cooling. The B and V crystalline modifications are observed in the treated samples. Quenched treated amylose and enzyme‐resistant amylose seem to contain a comparable amount of double‐helical/strainable fraction. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1662–1677, 2000     

Dynamic light scattering of semidilute hydrophobically modified alkali‐soluble emulsion solutions with different lengths of poly(ethylene oxide) spacer chain

Semidilute solutions of hydrophobically modified alkali‐soluble emulsion (HASE) were examined by dynamic light scattering and rheological techniques. For the model polymer without associative macromonomer, two q² dependent diffusional modes were detected in the decay time distributions. With increasing hydrophobicity of the associative macromonomer, the narrow fast peak was substituted by a shallow broad peak and only one q² dependent slow mode could be accurately detected, which indicated that the heterogeneity of these associated clusters increases with increasing carbon number. The bulk steady‐shear viscosity exhibits similar results to the diffusion coefficients of the aggregate observed from light scattering measurements. The length of poly(ethylene oxide) (PEO) spacer chain alters the solution properties as well as the associative cluster structure. With increasing length of PEO spacer chain, intramolecular association was substituted by intermolecular association. For EO segment larger than 32 units, intramolecular association dominates, where the formation of HASE aggregates is controlled by the balance of electrostatic repulsion and hydrophobic attraction. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3288–3298, 2005     

Molecular probe techniques for studying diffusion and relaxation in thin and ultrathin polymer films

Two optically based, molecular probe techniques are employed to study relaxation and small-molecule translational diffusion in thin and ultrathin (thicknesses < ∼200 nm) polymer films. Second harmonic generation (SHG) is used to study the reorientational dynamics of a nonlinear optical chromophore, Disperse Red 1 (DR1) (previously shown to be an effective probe of α-relaxation dynamics) either covalently attached or freely doped in polymer films. Our studies on films ranging in thickness from 7 nm to 1 μm show little change in T_g with film thickness; however, a substantial broadening of the relaxation distribution is observed as film thickness decreases below approximately 150 nm. Experimental guidelines are given for using fluorescence nonradiative energy transfer (NRET) to study translational diffusion in ultrathin polymer films. Appropriate choice of a fluorescence donor species is important along with ensuring that diffusion is slow enough to be measured appropriately. Initial results on the diffusion of a small-molecule probe, lophine, in poly(isobutyl methacrylate) indicates that there is little change in probe diffusion coefficients in films as thin as 90 nm as compared to bulk films. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2795–2802, 1997     

Dilute solutions of compositionally heterogeneous κ-carrageenan studied by combined dynamic and static light scattering

Dilute water–salt solutions of compositionally heterogeneous κ-carrageenan (κ-CG) containing 3% of ι-CG units are studied by combined static and dynamic (DLS) light scattering in 0.1M NaCl at 25°C. The structure-sensitive ratio R_g/R_h?=?2.2 of the fast DLS mode corresponds to the coils of κ-CG homopolymer. The irreversible core–shell aggregates (R_g/R_h?=?0.4) give rise to a slow mode. The optical rotation reveals that the aggregates originate from helical fragments of contaminating multi-block κ/ι-copolymers. Comparison with capillary viscometry shows that the weight fraction of aggregates is as small as the fraction of contaminating κ/ι-hybrids.     

Formation and structure of pachyman aggregates in dimethyl sulfoxide containing water

The aggregation of pachyman, β-(1 → 3)-D -glucan (M_w = 1.68 × 10⁵) from the Poria cocos mycelia, was investigated using static and dynamic laser light scattering (LLS) in dimethyl sulfoxide (DMSO) containing about 15% water, which leads to large aggregates. Both the time dependence of hydrodynamic radius and the angle dependence of the scattering intensity were used to calculate the fractal dimension (d_f) of the aggregates. The aggregation rate and average size of aggregates increase dramatically with increasing the polymer concentration from 1.7 × 10⁻⁴ g/mL to 8.6 × 10⁻⁴ g/mL, and with the decrease of the solvent quality, that is, water content from 13 to 15%. In the cases, the fractal dimensions change from 1.94 to 2.43 and from 1.92 to 2.54, respectively, suggesting that transforms of aggregation processes: a slow process called reaction-limited cluster aggregation (RLCA) to a fast process called diffusion-limited cluster aggregation (DLCA) in different polymer concentrations and water content. The fractal dimensions above 2 of the fast aggregation is larger than the 1.75 predicted for the ideal DLCA model, suggesting that the aggregation involves a restructuring process through the interchain hydrogen bonding interaction. There are no aggregates of pachyman in DMSO without water, but aggregates formed in the DMSO containing 15% water at 25°C as a compact structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3201–3207, 1999     

Effect of the degree of deacetylation and the substitution of carboxymethyl chitosan on its aggregation behavior

The aggregation behavior of carboxymethyl chitosan (CM‐chitosan) with various degrees of deacetylation (DD) and substitution (DS) was characterized with viscometry, gel permeation chromatography (GPC), and GPC coupled with laser light scattering (GPC‐LLS). The results indicate that CM‐chitosan has a strong tendency to form aggregates in aqueous solution and the aggregation behavior depends on DD and DS values. The apparent aggregation number (N_ap), the gyration radius (R_g), and the weight fraction of the aggregates (F_a) reached maximum at a DD value of 50%, then decreased, with the DD value deviating from 50%. A higher DS value helped to form aggregates; when the DS value increased from 0.65 to above 1.0, N_ap and R_g increased sharply. The dependence of the refractive index increment (dn/dc) on the DD and DS values was related to variation of the charge density and the hydrophobic interaction along the molecular chains. The conformations of CM‐chitosan aggregates were studied by the LLS method. The aggregates showed a spherical shape, and the chain stiffness increased with introduction of the acetyl groups. The DS value had no clear influence on the chain conformation that was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 296–305, 2005     

Stress overshoot of polymer solutions at high rates of shear

Overshoot of shear stress, σ, and the first normal stress difference, N₁, in shear flow were investigated for polystyrene solutions. The magnitudes of shear corresponding to these stresses, γ_σm and γ_Nm, for entangled as well as nonentangled solutions were universal functions of γ˙τ_eq, respectively, and γ_Nm was approximately equal to 2γ_σm at any rate of shear, γ˙. Here τ_eq = τ_R for nonentangled systems and τ_eq = 2τ_R for entangled systems, where τ_R is the longest Rouse relaxation time evaluated from the dynamic viscoelasticity at high frequencies. Only concentrated solutions exhibited stress overshoot at low reduced rates of shear, γ˙τ_eq < 1. The behavior at very low rates, γ˙τ_eq < 0.2, was consistent with the Doi–Edwards tube model theory for entangled polymers. At high rates, γ˙τ_eq > 1, γ_σm and γ_Nm were approximately proportional to γ˙τ_eq. At very high rates of shear, the peak of σ is located at t = τ_R, possibly indicating that the polymer chain shrinks with a characteristic time τ_R in dilute solutions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1917–1925, 2000     

Controlling the thermomechanical properties of polymer nanocomposites by tailoring the polymer–particle interface

We show that the thermomechanical properties of polymer nanocomposites are critically affected by polymer-particle wetting behavior. Silica nanoparticles grafted with dense polystyrene brushes of degree of polymerization 1050 are blended with polystyrene melts to form nanocomposites. It was found that low molecular weight (MW) polystyrene melts with lengths <880 wet these particles. Concurrently, the glass transition temperature (T_g) of the nanocomposite increases. At higher MW, the matrix does not wet the particles and the T_g decreases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2944–2950, 2006     

Reentrant behavior of poly(vinyl alcohol)–borax semidilute aqueous solutions

 The reentrant behavior of Poly(vinyl alcohol) (PVA)–borax aqueous semidilute solutions with a PVA concentration of 20 g/l and borax concentrations varies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS) and dynamic viscoelastic measurements. Two (fast and slow modes) and three (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solutions without and with the presence of borax, respectively, were observed from DLS measurements. The fast and middle relaxation modes were q ²-dependent (q is the scattering vector) characteristic of diffusive behavior; however, the slow modes were q ³-dependent, characteristic of intraparticle dynamics. The experimental results showed that the slow relaxation mode dominates the DLS relaxation. The DLS slow mode relaxation time, τ_s, and the viscoelastic modulus G′(ω) and G′′(ω) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased from 0.0 to 0.20 M, i.e. τ_s, G′(ω), and G′′(ω) fluctuated with increasing borax concentration. The excluded-volume effect of polymers, charge repulsion among borate ions bound on PVA molecules, and intermolecular cross-linking didiol–borate complexation caused an expansion of the polymer chain; however, the screening effect of free Na⁺ ions on the negative charge of the borate ions bound on PVA and intramolecular cross-linking didiol–borate complexation led to a shrinkage of the polymer chain. The reentrant behavior was the consequence of the balance between expansion and shrinkage of the PVA–borate complex. Received: 26 March 1999/Accepted in revised form: 3 September 1999     

Aggregation behavior of polystyrene‐b‐poly(ethylene/butylene)‐b‐polystyrene triblock copolymer in N‐methylpyrrolidone

Solution property of hydrogenated polystyrene‐b‐poly(ethylene/butylene)‐b‐polystyrene triblock copolymer (SEBS copolymer) was studied by using static light scattering and dynamic light scattering for cyclohexane and N‐methylpyrrolidone (NMP) solutions. From the values of dimensionless parameters ρ, defined as the ratio of radius of gyration 〈S²〉^1/2 to hydrodynamic radius R_H, and solubility parameters, SEBS copolymer proved to exist as single chain close to random coil in nonpolar cyclohexane, whereas aggregate into the core‐shell micelle consisting of poly(ethylene/butylene) (PEB) core surrounded by PS shell in polar NMP. The core‐shell micelle formed in NMP is composed of 65 polymer chains, having three times larger average chain density (d = 0.12 g cm^?3) than a single polymer chain (d = 0.04 g cm^?3) in cyclohexane. The comparison with the aggregation behaviors in other solvents demonstrated that the aggregate compactness of the copolymer depended largely on solvent polarity, resulting in formation of the highly dense PEB core (R_c = 4.5 nm) and the thick PS shell (ΔR = 22.9 nm) in high‐polar NMP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 588–594, 2010     

Dependence of dielectric,ferroelectric, and piezoelectric properties on crystalline properties of p(VDF‐co‐TrFE) copolymers

Thermal processing at various temperatures has been used to fabricate poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐co‐TrFE)] films with varied crystalline properties in an attempt to improve their piezoelectric properties. Although the dielectric constant of the films annealed at higher temperature is smaller than that of cooled and quenched ones, it has been shown that the annealed films possess larger crystallinity and stacked lamellar crystal grain size. The ferroelectric domains deriving from crystal region in all the samples are effectively improved by hot polarization. As a result, the remnant polarizations (P_r) and coercive electric field (E_c) of the corresponding films are improved at a low frequency due to the response of dipoles in crystal phase, and the largest piezoelectric constant in the longitudinal thickness mode (d₃₃=?25 pC/N) is obtained in an annealed copolymer film. The results illustrate improving the crystal structure of P(VDF‐co‐TrFE) is an effective way to realize high electromechanical properties, which provides broadly applied scenery for this kind of copolymer in piezoelectric components. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Hydrosilylation of acetophenone with diphenylsilane in the presence of rhodium(I) complexes with chiral amines

New chiral rhodium complexes cis-[Rh(CO)₂(RNH₂)Cl] [RNH₂ = (R)-(−)-cis-MyrtNH₂, (R)-(−)-MenthylNH₂, (R)-(+)-BornylNH₂] were synthesized and their catalytic properties in reactions of hydrosilylation of acetophenone with diphenylsilane were studied. It was shown that the reaction products were diphenyl-1-phenylethoxysilane, diphenyl-1-phenylvinyloxysilane and 1,1,3,3-tetraphenyldisiloxane. The best catalytic activity displayed (−)-cis-[Rh(CO)₂(MenthNH₂)Cl]. The hydrosilylation of acetophenone with diphenylsilane in the presence of [Rh(CO)₂(μ-Cl)]₂ and [Rh(cod)Cl]₂ and amines in situ was studied. The best ratio amine:complex = 5:1 was established. With the catalytic systems based on [Rh(cod)Cl]₂ or [Rh(CO)₂(μ-Cl)]₂ the activity increased in the series of amines: (R)-(−)-cis-MyrtNH₂ < (R)-(−)-MenthylNH₂ < (R)-(+)-BornylNH₂, and (R)-(−)-MenthylNH₂ < (R)-(+)-BornylNH₂ < (R)-(−)-cis-MyrtNH₂, respectively. The chemoselectivity maximum was observed in the presence of [Rh(cod)Cl]₂ with (R)-(−)-MenthylNH₂ and [Rh (CO)₂(μ-Cl)]₂ with (R)-(+)-BornylNH₂; maximum asymmetric induction was 43.5% ee at the use of [Rh(CO)₂ (μ-Cl)]₂ and (R)-(+)-BornylNH₂.     

Dynamic light scattering and rheology studies of aqueous solutions of amphiphilic sodium maleate containing copolymers

Light scattering techniques, video particle‐tracking microrheology, and bulk rheology were employed to examine the structure and dynamics of a series of alternating sodium maleate copolymers with moderately hydrophobic comonomers (diisobutylene, styrene, and isobutylene) in aqueous solutions. The scaling dependence of the specific viscosity (η_sp) on the polyelectrolyte concentration (c) was studied with and without added salt; similar trends were found in both conventional rheology and particle‐tracking microrheology measurements, showing good performance of the technique with flexible polyelectrolytes. Furthermore, with dynamic light scattering performed in high added salt conditions, we examined the behavior of the amplitude of the fast mode, which is in agreement with scaling predictions. In contrast, the slow modes are not understood and display three separate behaviors for the wavevector q dependence of the decay rate (Γ), depending on the comonomer; superdiffusive (Γ ～ q^2.7, isobutylene) possibly because of sticky aggregates, wavevector independent (Γ ～ q⁰, styrene) most likely because of coupled polyion‐ion diffusion and diffusive (Γ ～ q^2.0, diisobutylene) presumably because these aggregates are not sticky. The hydrophobicity of the comonomer appears to switch the aggregation process between “open,” “closed,” and “non” association for isobutylene, diisobutylene, and styrene respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 774–785, 2007