Glass transition in ionic polymers: The acrylates

It is shown that the glass transition temperatures of the ionic acrylates are subject to the same correlation as found for the phosphates, silicates, and ionenes; the equation is T_g = 730 (q/a) ? 67, where q is the cation charge and a the separation between centers of charge for the cation and anion.     

Diffusion-controlled transitions in high polymers

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Glass transitions in hydrogen-bonded polymer complexes

Mutual precipitates of poly (N, N-dimethyl acrylamide) and poly (4-hydroxystyrene) were collected from dioxane, methanol, or acetone. The glass transition (T_g) temperatures of the precipitates are higher than the weight-average values. Clear films cast from dimethylformamide solutions have lower T_g values. Complexation also occurred between poly (ethyl oxazoline) and poly (4-hydroxystyrene) in dioxane and between poly (vinyl pyrrolidone) and poly (4-hydroxystyrene) in methanol. Again, the glass transition temperatures of the precipitates are higher than the values for the blend films. The ΔC_p values associated with the glass transitions of the complexes are smaller than those of the blends having the same compositions. Negative excess heat capacities of mixing have been observed for several precipitates.     

Glass transitions in crosslinked epoxy networks

Epoxy resins of DGEBA type were thermally cured with diaminodiphenylmethane as crosslinking agent, and then analysed by Differential Scanning calorimetry (DSC) at various heating rates in order to determine the glass transition temperatureT _g of the final networks. First it was shown that during cyclingT _g is shifted towards higher values up to a maximum or . Such a change is attributed to an increasing extent of cure which develops during the thermal analysis, and also to relaxation processes thermally activated inside the polymeric matrix. Then the dependence of on the heating rateq imposed by the DSC apparatus was presented forq changing from 0.1 to10C min^–1. At heating rates exceeding 3C min^–1 only the classical temperatureT _g was detected, but at smallerq values, an additional endothermic transition was revealed, located at higher temperature and linked to a physical aging-like phenomenon, which takes place at low heating rates. The plot of against logq is divided into two quasi-linear parts on each side ofq=3C min^–1. In conclusions, an equation was given to describe the vs. logq function.     

Glass transitions of epoxide prepolymers

Glass transitions of four solid epoxide prepolymers are described. The rigidity of their molecular chains are measured. With them an equation is developed which relates the glass transition temperatures to the molecular parameters of these polymers.     

Wetting transitions of ionic solutions

Cahn's phenomenological theory of wetting of a solid substrate by a saturated vapor is generalized to the case where the substrate is charged and the wetting film contains counterions, with or without added salt. The electrostatic contribution to the grand potential associated with these ions is calculated within a nonlinear Poisson-Boltzmann theory. In the salt-free case, when the wetting film includes only counterions released by the substrate, the wetting transition is always first order, regardless of its nature in a neutral system. When salt is present, other wetting scenarios may arise, depending on the salt concentration and substrate surface charge. Over a restricted range of salt concentrations, a wetting scenario similar to that of prewetting, is predicted to occur along the liquid-vapor coexistence line. This scenario includes a discontinuous wetting transition between microscopic and mesoscopic film thicknesses, followed by a continuous divergence of the film thickness at higher temperatures.     

Positron annihilation in ionic conductivity polymers

Positron annihilation lifetime spectra and ionic conductivity have been measured for poly(etherurethane)-LiClO₄ as a function of temperature. The effects of Li salt on glas transition free volume and ionic conductivity have been discussed. A correlation between fractional free volume and ionic conductivity was first experimentally confirmed by using the free volume theory.Supported by the National Natural Science Foundation of China.     

Structural ordering transitions in ionic liquids mixtures

Differential capacitance curves for the electrical double layer (EDL) of mixtures of imidazolium-based ionic liquids (ILs) with a common cation (1-ethyl-3-methylimidazolium, [C₂MIM]⁺) and two different anions (bis(trifuoromethylsulfonyl)imide, [Tf₂N]⁻) and tris(pentafluoroethyl)trifluorophosphate [FAP]⁻) were obtained. Sharp peaks in the differential capacitance curves were observed for a small range of mixtures compositions at positive charge densities. The appearance and position on the potential scale of the peaks were found to be dependent on the mixture composition and temperature. The occurrence of these phenomena is interpreted as corresponding to an abrupt change in the EDL structure arrangement as a result of a complex interplay of electrostatic interactions and steric effects. The use of the non-structured mercury electrode allowed to decouple the eventual potential induced restructuring occurring at the double layer from the well-known surface reconstruction effects often reported for ionic liquids in contact with single crystal face electrodes.     

Polyphosphonates as ionic conducting polymers

Polyphosphonates, a class of polymers with the generic formula –[P(R)(X)–OR'O]_n–, exhibit a high degree of modularity due to the range of R, R', and X groups that can be incorporated. As such, these polymers may be designed with a polyethylene oxide (PEO) backbone (R' group) and employed as solid polymer electrolytes (SPEs). Two PEO-containing polyphosphonate analogs (R = Ph; X = S or Se) were doped with LiPF₆ and their conductivities were measured. Conductivities were similar (X = S) to or exceeding (X = Se) those of standard PEO systems (just below 10⁻⁴ S/cm at 100°C). Binding models for Li⁺ were generated using ³¹P{¹H}NMR titration experiments. Binding of Li⁺ by these polyphosphonates followed a positive cooperativity model, and varying the X group (S or Se) affected the observed cooperativity (Hill coefficient = 1.73 and 4.16, respectively). The presence of Se also leads to an increase in conductivity as temperature is raised above the T_g, which is likely an effect of reduced Columbic interactions. Because of their modularity and ease with which cation binding can be evaluated using ³¹P{¹H} NMR titration experiments, polyphosphonates offer a unique approach for the modification of Li⁺ ion battery technology.     

Cooperative relaxations/transitions in ferroelectric polymers

Dielectric relaxation and thermal transitions in β-PVDF have been investigated by Thermo-Stimulated Current spectroscopy and Differential Scanning Calorimetry respectively. A comparative study of spectra and thermograms has been performed. The relaxation mode associated with the glass transition of the true amorphous phase is characterized by relaxation times obeying a compensation law due to cooperative molecular movements. A conformationally disordered structure is proposed for β-PVDF to explain thermal events occurring around 60°C. Ageing of ferroelectric properties of β-PVDF has been associated with cooperative molecular movements liberated largely below the melting point.     

Glass transitions of ethyl acrylate-based ionomers

The glass-transition temperatures of a series of copolymers of ethyl acrylate and acrylic acid neutralized with various cations were investigated. It was found that a plot of T_g as a function of ion content, for every type of ion investigated here, gives an unusual sigmoidal curve, which can be correlated with the onset of the failure of time–temperature superposition in viscoelastic studies, as will be shown in a future publication. Also, all of the T_g versus concentration curves for the various counterions can be superposed if the plots are made against cq/a, where c is the metal acrylate content, q the cation charge, and a the distance between centers of charge. Furthermore, in one region of water content, a linear relation is obtained between the glass transition and the water content (in weight-%) independent of the ion concentration over wide ranges of ion content. Finally, above an ion concentration of 12 mole-%, the rate of change in T_g per water molecule per ion pair at constant ion content, (?T_g/?n)_c is linear but with different slopes above and below two water molecules per ion pair.     

Glass transitions and mixed phases in block SBS

Differential scanning calorimetry (DSC) does not allow for easy determination of the glass‐transition temperature (T_g) of the polystyrene (PS) block in styrene–butadiene–styrene (SBS) block copolymers. Modulated DSC (MDSC), which deconvolutes the standard DSC signal into reversing and nonreversing signals, was used to determine the (T_g) of both the polybutadiene (PB) and PS blocks in SBS. The T_g of the PB block was sharp, at ?92 °C, but that for the PS blocks was extremely broad, from ?60 to 125 °C with a maximum at 68 °C because of blending with PB. PS blocks were found only to exist in a mixed PS–PB phase. This concurred with the results from dynamic mechanical analysis. Annealing did not allow for a segregation of the PS blocks into a pure phase, but allowed for the segregation of the mixed phase into two mixed phases, one that was PB‐rich and the other that was PS‐rich. It is concluded that three phases coexist in SBS: PB, PB‐rich, and PS‐rich phases. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 276–279, 2005     

Carbon-13 NMR relaxation and transitions in polymers

High-resolution proton-decoupled carbon-13 nuclear magnetic resonance relaxation parameters have been obtained as a function of temperature for a set of completely amorphous polymers, semicrystalline polymers, and a series of ethylene–vinyl acetate copolymers. With these samples the nature of the glass temperature, other postulated amorphous transitions, and the β transition were investigated. For the completely amorphous polymers, the average correlation times depend on temperature according to the Williams–Landel–Ferry relation. Spectral collapse occurs at temperatures whose ratio to T_g is in the range 1.2–1.4 and corresponds to a correlation time of about 10^?7s. The loss of resolvable spectra is demonstrated to be a consequence of experimental methods and is not due to the occurrence of another amorphous transition. Both the methylene and methine carbons can be resolved for the ethylenevinyl acetate copolymers. Although the correlation time for the methylene carbon is continuous and resolvable through the β transition region, the methine branch-point resonance is lost. The implication of these results to the molecular nature of the β transition is discussed.     

Modelling of water transport in ionic hydrophilic polymers

Water and ion transport in thin sheets of initially dry, ionic, hydrophilic crosslinked polymers was modelled throughout the dynamic swelling process. The water transport was expressed in terms of a non-Fickian equation with a diffusion term containing a Fujitatype concentration-dependent diffusion coefficient coupled with a pseudoconvective term arising from the reasonable assumption that the stress in ionic polymers is proportional to the total number of ionized pendant groups in the polymer. Ion transport was expressed in terms of generalized Fickian equations with water concentration-dependent diffusion coefficients. These equations were solved with appropriate boundary conditions to establish the water uptake as a function of time, pH and ionic strength in a citrate-phosphate-borate buffer solution. A new dimensionless number, the Stress Swelling number, A, was defined to quantify the relative importance of stress in the overall swelling process. Water uptake was a strong function of A. © 1994 John Wiley & Sons, Inc.     

Glass transition temperatures of several fluorine-containing polymers

The glass transition temperatures T_g of several fluorine-containing polymers were determined by use of the differential scanning calorimeter. Values between ?3 and 230°C were obtained. In polymers of α-olefins, T_g increases with the fluorine content of the backbone and the length of the n-perfluoroalkyl branch. In styrene polymers T_g also is higher if the backbone contains fluorine but nearly the same T_g's are found for polymers with phenyl and pentafluorophenyl groups. Saturated polymers of perfluoro-α,ω-dienes have lower T_g's than polyperfluoro-α-olefins. The T_g's of chloroperfluoropolymers are higher than those of perfluoropolymers. Polyperfluoropentadiene-1,3 has the lowest T_g of the polymers examined. Polyperfluoropentadiene-1,3 forms by 1,4-addition.