H NMR study of thermotropic phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(vinyl methyl ether) mixtures

¹H NMR spectroscopy was used to investigate thermotropic phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(vinyl methyl ether) (PVME) mixtures. In all studied solutions (polymer concentrations c=0.1-10 wt%) two phase transitions were detected at temperatures roughly corresponding to different lower critical solution temperatures of PIPMAm and PVME. While the phase transition of PVME component (located at lower temperatures) is not affected by the presence of PIPMAm in the mixture, the phase transition temperatures of PIPMAm component (located at higher temperatures) are affected by the phase separation of the PVME component. Measurements of ¹H spin-spin relaxation of residual water (HDO) molecules revealed that above the phase transition, a certain portion of water molecules is bound to polymer globular structures. A major part of bound water is present in globular structures of predominating polymer component in the mixture.     

Electrospinning fabrication of partially crystalline bisphenol A polycarbonate nanofibers: The effects of molecular motion and conformation in solutions

Partially crystalline bisphenol A polycarbonate (BPAPC) nanofibers were successfully fabricated using a combination of a centrifugal field (1800 rpm) and an electrostatic field (25 kV). The BPAPC solution properties are key factors for adequately electrospinning the partially crystalline BPAPC nanofibers. The correlation times (τ_c) of methyl (τ_c = 9.3 ns) and of benzene-ring (τ_c = 15.3 and 15.8 ns) motions in the 14 wt.% BPAPC/THF solution were longer than in CH₂Cl₂ and CHCl₃, as determined by NMR. The distribution-peak maximum of the hydrodynamic radius of BPAPC in the 14 wt.% THF solution (R_h = 15 Å) was higher than in CH₂Cl₂ (R_h = 9.2 Å) and CHCl₃ (R_h = 7.9 Å), as evidenced by DLS data. We conclude that the BPAPC assumed a denser, more worm-like chain conformation in THF solvation.     

Effect of time on the hydration and temperature-induced phase separation in aqueous polymer solutions. H NMR study

Dehydration during temperature-induced phase separation in D₂O solutions of poly(vinyl methyl ether) (PVME), poly(N-isopropylmethacrylamide) (PIPMAm) and poly(N-isopropylacrylamide) (PIPAAm) was followed from time dependences of NMR spin-spin relaxation times T₂ of HDO. Both the time characterizing the exclusion of the water from mesoglobules (manifested by the increase in T₂ values) and the induction period which precedes the increase in T₂ values, increased in the order PVME < PIPMAm < PIPAAm. For D₂O solutions of PIPMAm/PVME (or PIPMAm/PIPAAm) mixtures a direct connection between the state of the mesoglobules (hydrated or dehydrated) formed by the component with lower LCST (PVME, PIPAAm) and the temperatures of the phase transition of the PIPMAm component was established by NMR spectroscopy.     

Structure-property relationships in novel poly(imide-amide)-poly(ethylene glycol) hybrid networks

Broadband dielectric relaxation spectroscopy (DRS), thermally stimulated depolarisation currents (TSDC), differential scanning calorimetry (DSC) and to a lesser extent water uptake measurements, were employed to investigate molecular mobility, morphology and crystallization/melting events of PEG in poly(imide-amide)-polyethylene glycol hybrid networks (PIA-PEG) with short (M_n=1000 g/mol) and long (M_n=3400 g/mol) PEG crosslinks. The results obtained suggest long range connectivity of the PEG component in the hybrids with short PEG crosslinks at PEG content higher than 40 wt% and in these with long PEG crosslinks at PEG content higher than 20 wt%. Crystallization of the PEG component is observed by DSC in the hybrids with the longer crosslinks at sufficiently high content of PEG, only. The glass transition temperature, T_g, of PEG component in the hybrids with the shorter PEG crosslinks is shifted to higher temperatures compared to that of the hybrids with longer PEG crosslinks, while suppression of the glass transition of the PEG component is observed in the hybrids with the shorter PEG crosslinks at PEG content lower than 40 wt%. The results are discussed in terms of constraints to segmental motion of the PEG crosslinks, imposed by fixed PEG chain ends on the rigid PI chains.     

Determination of molecular weight and molecular sizes of polymers by high temperature gel permeation chromatography with a static and dynamic laser light scattering detector

Flow-mode static and dynamic laser light scattering (SLS/DLS) studies of polymers, including polystyrene, polyethylene, polypropylene and poly(dimethylsiloxane) (PDMS), in 1,2,4-trichlorobenzene (TCB) at 150 °C were performed on a high temperature gel permeation chromatography (GPC) coupled with a SLS/DLS detector. Both absolute molecular weight (M) and molecular sizes (radius of gyration, R_g and hydrodynamic radius, R_h) of polymers eluting from the GPC columns were obtained simultaneously. The conformation of different polymers in TCB at 150 °C were discussed according to the scaling relationships between R_g, R_h and M and the ρ-ratio (ρ=R_g/R_h). Flow-mode DLS results of PDMS were verified by batch-mode DLS study of the same sample. The presented technique was proved to be a convenient and quick method to study the shape and conformation of polymers in solution at high temperature. However, the flow-mode DLS was only applicable for high molecular weight polymers with a higher refractive index increment such as PDMS.     

Cylindrical polyelectrolyte-comb-surfactant complexes

Quaternized polymer combs based on poly(2-vinylpyridine-macromonomers) and the surfactant sodium dodecylsulfate are employed in the synthesis of a novel cylindrical polyelectrolyte-comb-surfactant complex (PECSC). The complex formed has 1:1 stoichiometry with respect to the ratio of dodecylsulfate to pyridinium units. It is soluble in organic solvents such as 2-butanol or chloroform. Characterization of single particle properties of the complex in organic solution is possible and yields a radius of gyration of 〈R_g〉_z = 78.4 nm, a hydrodynamic radius of 〈1/R_h〉_z⁻¹ = 51.4 nm and a cross-sectional radius of R_g,cross = 3.9 nm in chloroform. The characteristic ratio γ = 〈R_g〉_z/〈1/R_h〉_z⁻¹ decreases from γ = 1.73 for the original quaternized polymer comb, indicating the semi-flexible, cylindrical nature of the macromolecules in aqueous solution, to γ = 1.53 for the complex in chloroform. The effect of the main-chain stretching accompanied by the increase of the volume of the comb by introduction of the surfactant is smaller as compared to the electrostatic interactions in the parent comb. This is also reflected in the persistence length l_p, which is determined by SANS, and found to be 21.3 nm for the complex and 24.4 nm for the polyelectrolyte comb. In addition, AFM investigations of the polymers adsorbed onto mica showed a 2D-equilibrium adsorption for the complex and a kinetically dominated adsorption process in case of the polyelectrolyte comb.     

Revisiting of dimensional scaling of linear chains in dilute solutions

The dimensions of linear polymer chains are scaled to their molar mass (M) as R = kM^α with α = 1/2 and 3/5 in a theta and an athermal solvent, respectively. In a good solvent, both k and α are a function of the solvent quality and chain length range. A high-temperature laser light-scattering spectrometer was used to measure the average radius of gyration (〈R_g〉) and hydrodynamic radius (〈R_h〉) of a set of narrowly distributed linear polystyrene chains in decalin over a wide temperature range. k and α in the scaling experimentally varying with T over a chain length range was analyzed. The results reveal that for 〈R_g〉, α = 0.59 − 0.09exp(−τ/0.066) and k = 0.60τ^2α−1, reasonably agreeing with the thermal blob theory. For 〈R_h〉, α = 0.59 − 0.09exp(−τ/0.106), but k deviates from the relationship of k ∝ τ^2α−1, reflecting that the hydrodynamic interaction and chain draining are not considered in the thermal blob theory.     

An observation of the coexistence of multimers and micelles in a nonionic surfactant C10E4 solution by dynamic light scattering

The bulk phase of nonionic surfactant C₁₀E₄ solution was monitored by a dynamic light scattering (DLS) system at 20 °C in a narrow range of concentration near the cmc. Two particle aggregations were observed. The DLS data show (i) there exist premicellar multimers (or called sub-micelles) and (ii) micelles coexist with multimers. The C₁₀E₄ sub-micelles have a narrow size distribution with an averaged hydrodynamic diameter (D_h) of 1.35 nm. The D_h of the micelles is around 10.5 nm at 1.0 × 10⁻⁶ mol/mL and increases slightly with C₁₀E₄ concentration. It is illustrated from the DLS data that (i) at C = 0.78-0.82 μmol/mL, monomers and premicellar multimers coexist and (ii) at C = 0.84-0.92 μmol/mL, monomers + submicellar multimers + micelles coexist. At more elevated concentrations, only the signals from the micelles are detected by DLS.     

Dependence of particle fluctuation velocity on gas flow, and particle diameter in gas fluidized beds for monodispersed spheres in the Geldart B and A fluidization regimes

In this paper we present new experimental data on the steady-state, mean squared, fluctuation velocity, or granular temperature, of Geldart B polymer, glass, nickel, and stainless steel monodispersed spheres averaged over the wall of a gas fluidized bed, as a function of gas flow and sphere diameter. The granular temperature is obtained by Acoustic Shot Noise technology—namely power spectral analysis of the steady state vibrational energy of the wall excited by random sphere impact, and calibrated by hammer excitation over the wall. The new data extends to polymer and metallic spheres the experimental discovery of a 1996 paper of Cody et al. that the fluctuation velocity of Geldart B glass spheres when scaled to the gas superficial velocity, U_s, is inversely proportional to sphere diameter, directly proportional to a fundamental length scale, D_o^B, and is a universal function of U = (U_s / U_mf). We also demonstrate that the new data is consistent with the diameter dependence of the fluctuation velocity that can be derived from both the 1997 paper of Menon and Durian, who measured random sphere motion near the wall through the spectroscopy of scattered laser light, and the 1992 paper of Rahman and Campbell, who measured the average granular pressure of random sphere impact on a porous steel membrane. While the inverse scaling of the fluctuation velocity with sphere diameter, and the existence of a fundamental length scale for gas fluidization, D_o^B, had not been a feature of any published fundamental model, or computer simulation, of the steady state granular temperature of spheres in gas fluidized beds, we show that it is a feature of two recent dense kinetic fluidization models published in 1999, by Buyevich and Kapbasov, and Koch and Sangani. Both theories implicitly define a fundamental length scale for the fluctuation velocity, D^? = (μ_f² / ρ_p²g)^1 / 3, where ρ_p is the sphere density, μ_f is the gas viscosity, and g is the laboratory gravitational field. The new data for polymer, glass, nickel and stainless steel spheres presented in this paper, defines D_o^B = (56 ± 2)D^?. We use the Anderson-Jackson stability model to show that the length scale D_o^B, also defines a stability length scale, such that for D < D_o^B(D > D_o^B), the uniform dense phase of the fluidized bed is stable (unstable), against one dimensional, first order fluctuations in sphere concentration. The length scale, D_o^B is thus the theoretical equivalent to the empirical scaling length introduced by Geldart, D_B/A, to distinguish spheres (D > D_B/A) that bubble at fluidization, from spheres (D < D_B/A) that fluidize before bubbling. Finally, we present new experimental data, on the remarkable changes in the granular temperature, bed expansion, and bed collapse time, between Geldart B and Geldart A monodispersed glass spheres, and compare that data to granular temperature, and bed expansion, for Geldart A rough, non-spherical, log-normal dispersed diameter catalytic particles.     

Self-assembly of C60 containing poly(methyl methacrylate) in ethyl acetate/decalin mixtures solvent

[60]Fullerene (C₆₀) was mono-substituted with well-defined poly(methyl methacrylate) (PMMA-b-C₆₀) using the atom transfer radical polymerization (ATRP) technique. The self-assembly behaviors of PMMA-b-C₆₀ in ethyl acetate (EA) and decalin mixtures were studied using laser light scattering (LLS) and transmission electron microscopy (TEM). Homogeneous solutions of PMMA-b-C₆₀ can be obtained in the solvent mixtures containing more than 40 wt% EA, where the molar ratio of decalin to EA is close to 1. For each solvent mixture, unimers coexist with micelles and large aggregates. The sizes of PMMA-b-C₆₀ micelles and aggregates are independent of polymer concentration, confirming that they are produced via the closed association mechanism. For the various solvent mixtures, the weight-averaged molecular weights, M_w of the PMMA-b-C₆₀ aggregates range from 4.1×10⁷ to 12.5×10⁷ g/mol. The hydrodynamic radii of the large aggregate, R_h, vary from 90 to 136 nm, while the z-averaged radii of gyration, R_g, range from 210 to 311 nm. The R_g/R_h value for each solvent mixture is ∼2.3, which is independent of decalin contents in the mixed solvents. The morphological study using the transmission electron microscope suggests that the large aggregates are composed of porous large compound micelles (LCM) in solution.