An explicit mechanism is described for the anomalous increase in dielectric constant and dielectric loss at low frequencies and high temperatures for poly(vinylidene fluoride) containing ionic impurities. Relations are proposed for the ionic contributions, εi″ and εi″, to the dielectric constant and dielectric loss: where v0 and D0 are the concentration and the diffusion coefficient of the mobile ions at infinite temperature, q is the charge of an ion (in cgs electrostatic units), l is the distance between electrodes, k is the Boltzmann constant, T is the absolute temperature, Ed is the apparent activation energy for diffusion of the ions, and W is the dissociation energy of the ionic impurities. From the slopes of curves of log εT′ versus 1/T and log ε″T versus 1/T for poly(vinylidene fluoride), energies Ed = 34 kcal/mole and W = 342 kcal/mole were obtained. 相似文献
Surface pressure-area isotherms have been determined for both a pure lecithin (L, -dipalmitoyl phosphatidyl choline) and an impure lecithin (soya bean lecithin) at the water/air and water/oil interfaces. Equations of state have been applied and an equation of Gaines was found to be particularly successful in describing the isotherms. Mixed monolayers with an ABA nonionic block copolymer surfactant (A is poly(12-hydroxystearic) acid and B is poly(ethylene oxide)) were also investigated. The additivity rule was obeyed only at high surface pressures; inefficient packing was observed at low surface pressures. The polymer may promote a horizontal headgroup orientation in the lecithin, which gives rise to this effect. The presence of electrolyte up to very high concentrations in the aqueous phase (8.75 mol dm–3 NH4NO3) was shown to expand the lecithin monolayer.Glossary of symbols W/A
Water-air interface
- W/O
Water-oil interface
- E/A
Electrolyte-air interface
- L-C
Liquid-condensed
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Ac
Area per molecule obtained by conventional extrapolation of the -A isotherm at close-packing
-
Ae
Experimentally determined area per molecule
-
At
Theoretically predicted area per molecule
-
Av
Area per molecule obtained by vertical extrapolation of the -A isotherm at close-packing
-
A0
Head group area term
-
fi
Activity coefficient of water in surface region
-
i
Constant
-
xi
Mol fraction of componenti
-
Z
Compressibility factor=A/kT
-
Interfacial tension
-
Surface pressure
- i
Partial molar area of component i 相似文献
Segmented poly(ether-ester-amide)s, (PEEA)s, of controlled hydrophilicity degree, based on poly(ε-caprolactone) (PCL), were synthesized according to a facile two-step procedure using α,ω-dihydroxy oligomeric PCL, 4,7,10-trioxa-1,13-tridecanediamine and macromers prepared from poly(ethylene glycol)s and adipoyl chloride. The PEEAs showed Mn values in the range 5–11.5 kDa. A PCL-type crystallinity was found by WAXS. DSC indicated Tm values (49–51 °C) close to that of PCL macromer. Single glass transitions were observed both by DSC and DMTA techniques and the Tg values (−58–−50 °C by DSC) were slightly higher than that of PCL. The water uptake was in the range 4.8–26.0 wt.-% depending on the length of the poly(ethylene glycol) segment.
The specific isobaric heat capacities of poly (methylmethacrylates) (PMMA) having various tacticities were measured by the DSC method within a broad range of temperatures including the glass transition. Glasses with uniform thermal history were used in the measurements and the data were treated by employing a procedure which provided the thermodynamic Tg independent of the experimental conditions. The semiquantitative validity of Boyer's empirical relationTg×cp=const. was confirmed; also it was found that within the limits of experimental accuracy the cp,g values at 298 K andCp,l values at 400 K are independent of the tacticity of the sample.Using the data thus measured and linearized equations representing the dependence ofTg on the content of iso-, syndio- and heterotriads, the Tg values of pure isotactic PMMA and pure syndiotactic PMMA were found to be respectively 315 K and 397 K.Dedicated to Professor Dr. F. H. Müller. 相似文献
Initial decomposition temperature (Ti), apparent activation energy of degradation (Ea) and glass transition temperature (Tg) of some low molar mass (Mn ≈ 8000 g mol−1) sulfonated poly(arylene ethersulfone)s s-(PAES)s were determined to check their dependence on sulfonation degree (SD). The results obtained were compared with those for unsulfonated poly(arylene ethersulfone) PAES. In order to have an accurate control of the chemical structure, a pre-sulfonation route was followed for the preparation of sulfonated compounds. The thermal behaviour of the investigated s-(PAES)s as well as that of PAES appears not to be influenced by the environment (flowing nitrogen or static air atmosphere) of degradation. Both Ti and Tg values of s-(PAES)s were higher than those of PAES and increased quite linearly as a function of sulfonation degree. An analogous linear trend was observed for the apparent degradation energy of s-(PAES)s, but the values found were largely lower than those of unsulfonated homopolymer. The results are discussed and interpreted. 相似文献
Poly(vinyl phosphonic acid) (PVPA) as obtained by free radical polymerization of aqueous vinyl phosphonic acid was studied by light scattering (SLS, DLS) and size exclusion chromatography (SEC) in dilute aqueous solutions containing sufficient salt in order to screen long range electrostatic interactions. Samples of 37< < 110 × 103 were studied. The polymers showed positive A2‐values in aqueous NaH2PO4 solution (0.04 M ), and self‐diffusion behavior and RH/RG—ratios indicative of the structure of random coiled chains. A comparison of the SEC‐elugrams of the PVPA‐samples with those of commercially available standards of poly(acrylic acid) sodium salt gave a fit to the same calibration curve described by log Pn(PVPA) = −0.21νe + 7.0(+0.1) which correlates the number average degree of polymerization (Pn) with the elution volume νe. This indicates that PVPA and PAA have the same hydrodynamic structure under given solution conditions.
Summary: Well‐defined poly[(ethylene oxide)‐block‐(sodium 2‐acrylamido‐2‐methyl‐1‐propane sulfonate)] diblock copolymers [P(EOm‐b‐AMPSn)], have been obtained by water‐based ATRP using α‐methoxy‐ω‐(2‐methylbromoisobutyrate) poly(ethylene oxide)s (MeO‐P[EO]m‐BriB with m ranging from 12 to 113) and CuBr · 2Bpy (Bpy for 2,2′‐bipyridyl) as macroinitiator and catalytic complex, respectively. Compared to direct polymerization in water, it has been demonstrated that the water/methanol (3:1, v/v) mixture is better suited for predicting the final number‐average molar mass from the initial monomer‐to‐macroinitiator molar ratio and achieving a quite narrow polydispersity, even at high monomer conversion ( ≈ 1.4 at 80% conversion). The effect of temperature, solvent mixture composition and addition of NaCl salt on the polymerization rate and extent of control over the copolymer molecular parameters have been highlighted as well.
Summary: Quartz crystal microbalance with dissipation monitoring (QCM‐D) is employed to determine the effect of salt on the volume phase transition of thermoresponsive polymer brushes. Changes in mass and viscoelasticity of poly(N‐isopropylacrylamide) (PNIPAM) layers grafted from a QCM‐D crystal are measured as a function of temperature, upon contact with aqueous solutions of varying salt concentrations. The phase‐transition temperature of PNIPAM brushes, TC,graft, quantified from the QCM‐D measurements is found to decrease as the concentration of salt is increased. This phenomenon is explained by the tendency of salt ions to affect the structure of water molecules (Hofmeister effect). However, in contrast to the linear decrease in phase‐transition temperature upon increasing salt concentration observed for free PNIPAM, the trend in TC,graft for PNIPAM brushes is distinctively non‐linear.
Schematic representation of the effect of salt concentration on the phase transition behavior of thermoresponsive polymer brushes. 相似文献