Unified approach to ion transport and structural relaxation in amorphous polymers and glasses

Kinetic data for structural relaxation in silver iodomolybdates at the glass transition temperature (Tg) are obtained by high-pressure differential scanning calorimetry (HP-DSC) and are compared with activation energies (EA) and volumes (VA) obtained earlier from conductivities below Tg. The results are fitted to an empirical equation, EA = MVA, and displayed in the form of a master plot of EA versus VA, an approach previously applied to strongly coupled systems, including polymer electrolytes and molten salts above their glass transition temperatures. The parameter M emerges as a localized modulus, expressive of interatomic forces within the medium, linking together EA,sigma, VA,sigma and EA,s, VA,s, the "apparent" activation parameters for ionic conductivity and structural relaxation, respectively. The VA and EA values for ion transport are much smaller than the corresponding values for structural relaxation. However, remarkably close agreement emerges between the "process parameters", Ms and Msigma, both close to 8 GPa, thus establishing a quantitative link between ion transport and structural relaxation in this highly decoupled system. A new EA versus VA master plot is constructed, which points the way to a unified approach to ion transport in polymers and glasses.     

Gas transport properties of polyphenylene ethers

Gas transport properties of the polyphenylene ethers poly(2,6-dimethyl-1,4-phenylene oxide)PDMPO, and poly(2,6-diphenyl-1,4-phenylene oxide), PDPPO, and the thioether poly(1,4-phenylene sulfide), PPS, have been measured as a function of pressure and temperature. The PPS material and free volume correlations were used to estimate the behavior of the unavailable poly(1,4-phenylene oxide), PPO. The results show that symmetrical substitution of phenyl groups on the backbone of polyphenylene ether, PDPPO, increases the gas transport properties by one order of magnitude relative to the unsubstituted material, PPO. Symmetrical methyl substitution, PDMPO, however, increase the permeability, apparent diffusion and sorption coefficients even further. The gas transport coefficients correlate with the fractional free volume of the polymers. PDMPO has the largest fractional free volume and gas transport coefficients followed by PDPPO and the PPS. The results show that substitution of phenyl groups, which leads to polymers that have better thermal and oxidative stability than methyl substituted ones, can be a useful means for increasing free volume and gas permeability coefficients. While methyl groups appear to be more effective for the latter, the enhanced chemical stability of phenyl rings may be useful when gas separation membranes are to be used in harsh environments. © 1993 John Wiley & Sons, Inc.     

Frequency and temperature dependence of molecular and micromechanical transitions in PPO/PMMA/P(S-g-EO) blends

The frequency and temperature dependence of molecular and micromechanical transitions were studied in polymer blends with an interphase. The viscoelastic properties of poly(2,6-dimethyl-p-phenylene oxide) (PPO) and poly(methyl methacrylate) (PMMA) blends that were compatibilized by a poly(styrene-graft-ethylene oxide) (P(S-g-EO)) copolymer were studied by dynamic mechanical spectroscopy (DMS) and the experimental data were compared with an interlayer model. The addition of the copolymer resulted in a micromechanical transition, and the relation between the volume fraction of interphase, the activation energy of the micromechanical transition, and the micromechanical transition temperature was studied. A qualitative agreement between experiments and theory was achieved. The quantitative difference was explained by partial mixing of PPO and/or PMMA with the copolymer in the interphase. © 1996 John Wiley & Sons, Inc.     

Apparent Specific Volume Measurements of Poly(ethylene oxide), Poly(butylene oxide), Poly(propylene oxide), and Octadecyl Chains in the Micellar State as a Function of Temperature

Apparent specific densities of aqueous solutions of the diblock copolymers C18(EO)100, C18(EO)20, and (EO)92(BO)18 and the triblock copolymers (EO)25(PO)40(EO)25 and (EO)21(PO)47(EO)21 in the micellar state have been measured over a temperature range from 10 to 90 degrees C at concentrations between 1% and 5%, using an oscillating tube densitometer. From these measurements, apparent specific volumes of poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), poly(butylene oxide) (PBO), and octadecane in the micellar state have been determined. The composition of the block copolymers was checked by NMR spectroscopy. Results were compared with published data for the polymers and bulk values for octadecane, respectively. The apparent specific density of PEO chains in the dissolved state was also measured for PEG4600 solutions at different concentrations and compared with results in the micellar state. The results presented in the paper are crucial in connection with analysis and modeling of small-angle X-ray scattering (SAXS) data from polymer and block copolymer micellar systems. PEO and PPO have a relatively low apparent partial specific volume in water at low temperatures. It is associated with water molecules making strong hydrogen bonds with the oxygen atoms on the polymer backbone. These water molecules gradually become disordered when the temperature is increased and the polymer apparent specific volume increases. For PBO in the micellar cores of PBO-PEO block copolymer micelles and in PNiPAM microgels, pronounced temperature dependence with the same origin is also found. The application of the derived results for the apparent specific volume of PEO for deriving contrast factors is demonstrated and the results are used in the analysis of SAXS data for semidilute solutions of PEG4600 in a broad temperature range.     

Chain Architecture Dependent 3‐Dimensional Supramolecular Assembly of Rod‐Coil Molecules with a Conjugated Hexa‐p‐phenylene Rod

Summary: We have prepared hexa‐p‐phenylene based rod‐coil molecules with identical coil volume fractions, but different poly(propylene oxide) (PPO) coil architectures (linear versus dibranched), and investigated their self‐assembling behavior in the solid state by small angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Rod‐coil molecules with a linear PPO coil showed a honeycomb‐like lamellar assembly of rod segments with hexagonally arrayed PPO coil perforations. In contrast, the rod‐coil molecules with dibranched PPO coils self‐organized into rod bundles with a body centered tetragonal symmetry surrounded by a PPO coil matrix. These results demonstrate that the steric hindrance at the rod/coil interface arising from coil architectural variation is a dominant parameter governing supramolecular rod assembly in the rod‐coil system.

TEM images and schematic illustrations of the self‐assembled structures of rod‐coil molecules with linear (left) and dibranched (right) PPO coils, respectively.     

Simulated Dilatometry and Static Deformation Prediction of Glass Transition and Mechanical Properties of Polyacetylene and Poly(para‐phenylene vinylene)

Thermophysical and mechanical properties of two conjugated polymers, poly(p‐phenylene vinylene) (PPV) and polyacetylene (PA), are predicted using molecular dynamics simulations and compared with results obtained from differential scanning calorimetry, nanoindentation, and dynamic mechanical analysis experiments. Glass transition temperature (T_g) is calculated from the changes in the slopes of the specific volume versus temperature and cohesive energy density versus temperature plots, obtained from constant pressure and constant temperature simulations (NPT ensemble). The effects of temperature on the torsion angle distributions and characteristic ratio are analyzed. PPV is found to have a T_g of 416 ± 8 K. PA does not exhibit a glass transition in the temperature range of 120 to 500 K. Using the static deformation method, the values of Young's modulus are calculated to be 1.81 ± 0.34 GPa for PA and 9.20 ± 0.57 GPa for PPV at 298 K. These values are in good agreement with the experimental measurements, validating the suitability of these techniques in the prediction of the polymer properties.

     

马来酸酐接枝氢化苯乙烯-丁二烯嵌段共聚物增韧聚苯醚体系的韧性、亚微相态和流变性能

考察了氢化苯乙烯-丁二烯嵌段共聚弹性体(SEBS)及其马来酸酐接枝共聚物(SEBS-g-MA)增韧聚苯醚(PPO)体系。DSC谱图显示,PPO与SEBS的共混物仅有一个T_g,两者完全相容;PPO与SEBS-g-MA的共混物存在两个T_g,只能达到部分相容。力学性能研究表明,在PPO/SEBS体系中,基体中分散相SEBS的相界面模糊,无法引发基体银纹和剪切屈服,增韧PPO的效果有限;而部分相容的PPO/SEBS-g-MA共混物显示了增韧剂良好的相界面引发基体银纹和剪切屈服的作用,其缺口冲击强度在SEBS-g-MA质量分数为20%时达到1260J/m的超韧性。亚微相态显示,SEBS在PPO中呈现条形分散相的"海岛"结构;而SEBS-g-MA在基体中呈现网络结构。流变性能研究显示,PPO/SEBS共混物的表观粘度均高于PPO,并随SEBS的含量增加而变大;而PPO/SEBS-g-MA则完全相反。     

Computer simulation study of thermodynamic scaling of dynamics of 2Ca(NO3)2·3KNO3

Molecular dynamics (MD) simulations of the glass-former 2Ca(NO(3))(2·3KNO(3), CKN, were performed as a function of temperature at pressures 0.1 MPa, 0.5 GPa, 1.0 GPa, and 2.0 GPa. Diffusion coefficient, relaxation time of the intermediate scattering function, and anion reorientational time were obtained as a function of temperature and densitiy ρ. These dynamical properties of CKN scale as ρ(γ)∕T with a common value γ = 1.8 ± 0.1. The scaling parameter γ is consistent with the exponent of the repulsive part of an effective intermolecular potential for the repulsion between the atoms at shortest distance in the equilibrium structure of liquid CKN, Ca(2+), and oxygen atoms of NO(3)(-). Correlation between potential energy and virial is obeyed for the short-range terms of the potential function, but not for the whole potential including coulombic interactions. Decoupling of diffusion coefficient and reorientational relaxation time from relaxation time take place at a given ρ(γ)∕T value, i.e., breakdown of Stokes-Einstein and Debye-Stokes-Einstein equations result from combined thermal and volume effects. The MD results agree with correlations proposed between long-time relaxation and short-time dynamics, lnτ ∝ 1∕, where the mean square displacement concerns a time window of 10.0 ps. It has been found that scales as ρ(γ)∕T above and below the glass transition temperature, so that thermodynamic scaling of liquid dynamics can be thought as a consequence of theories relating short- and long-time dynamics, and the more fundamental scaling concerns short-time dynamical properties.     

Novel Gas Separation Membranes Containing Covalently Bonded Fullerenes

Summary: In this work, we report superior mass transport properties of polymers prepared by the covalent coupling of supermolecular carbon cages (e.g., fullerenes, bucky balls) to a poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) polymer. Dispersing the bucky balls into the polymer reduces gas permeability, whereas covalent bonding enhances permeability up to 80% in comparison to the pure PPO. Gas pair selectivity, however, is not compromised and stays constant.

Schematic representation of the PPO polymer membrane and the PPO‐covalently bonded C₆₀ polymer membrane.     

Microcanonical analysis of the kinetic method. the meaning of the "apparent entropy"

A rigorous analysis of the kinetic method is carried out using Rice-Ramsperger-Kassel-Marcus (RRKM) theory of microcanonical statistical unimolecular dissociation rates. The model employs a kinetics treatment appropriate for metastable ion dissociation. Proton-bound alkoxide dimer anions are used as model systems, with realistic vibrational and rotational parameters calculated by ab initio methods for the cluster ion and transition states leading to the competitive dissociation channels. The numerical simulations show that the kinetic method plots of ln(I2/I1) versus AAH are nearly linear but can exhibit significant curvature. The apparent entropy obtained in the extended kinetic method is not approximately equal to the thermodynamic entropy difference for dissociation, AAS(T), or for activation, deltadeltaS++(T), either at the effective temperature or at any fixed equilibrium temperature. Instead, the apparent entropy term can be related to the ratio of the microcanonical sum of states of the dissociation transition states for the kinetically selected internal energy of the dissociating ions.     

Thermodynamics of temperature-sensitive polyether-modified poly(acrylic acid) microgels

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.     

The phase behaviour of poly(styrene-co-methacrylic acid)/poly(2,6-dimethyl-1,4-phenylene oxide) by inverse gas chromatography

The miscibility behaviour of blends of poly(styrene-co-methacrylic acide) (PSMA-12) containing 12% of methacrylic acid with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) at five different compositions has been studied by inverse gas chromatography (IGC). The adequacy of two types of solvents (solvents and precipitants) has been tested in order to detect the glass transition temperature, T(g). A single T(g) has been observed in PSMA-12/PPO blends containing less than 67wt% of PPO. Two glass transition temperatures appeared, however, in blends containing higher PPO content. The polymer-polymer interaction parameters have been calculated in the molten state (260-280 degrees C), their values being in good agreement with the observed phase behaviour. Moreover, the methods proposed by Farooque-Deshpande and Huang for obtaining the true polymer-polymer interaction parameters have been compared. Finally, the ability of IGC in order to determine T(g)s has been extended to a ternary system, PSMA-12/PPO/EMV4P-8 poly(ethylmethacrylate-co-4-vinylpyridine) which, depending on the composition, gives a single, two and even three T(g)s.     

Ring Shuttling Controls Macroscopic Motion in a Three‐Dimensional Printed Polyrotaxane Monolith

Amplification of molecular motions into the macroscopic world has great potential in the development of smart materials. Demonstrated here is an approach that integrates mechanically interlocked molecules into complex three‐dimensional (3D) architectures by direct‐write 3D printing. The design and synthesis of polypseudorotaxane hydrogels, which are composed of α‐cyclodextrins and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO‐PPO‐PEO) triblock copolymers, and their subsequent fabrication into polyrotaxane‐based lattice cubes by 3D printing followed by post‐printing polymerization are reported. By switching the motion of the α‐cyclodextrin rings between random shuttling and stationary states through solvent exchange, the polyrotaxane monolith not only exhibits macroscopic shape‐memory properties but is also capable of converting the chemical energy input into mechanical work by lifting objects against gravity.     

Electron free energy levels in oxidic solutions: relating oxidation potentials in aqueous and non-aqueous systems

We provide background to the problem of describing the state of redox couples in different types of solvent media ranging from acidic aqueous solutions to high temperature molten silicates, pointing out the essential similarity between these solvent media in Lewis acid–base terms. We review the adaptation of the Gurney proton energy level diagram approach to the case of electron transfer processes. Using data from various spectroscopic and analytical chemistry sources, we review the construction of electron free energy level diagrams for redox couples in aqueous and non-aqueous systems using, as a common reference, the potential of the oxygen gas (1 atm)/oxide ion couple in the solution of interest. We emphasize the anomalous effect of “oxide ion activity” (mean ionic activity of alkali oxide) on the state of equilibrium and interpret this in terms of oxide ion transfers that accompany electron transfers. After showing the essential agreement between recent direct electrochemical assessments of the energy levels and those deduced in our original analysis of oxidic melts of different glass formers, we provide an interpretation of the apparent “oxide ion transfer” in terms of the differential medium polarization by the two redox species involved in the equilibrium. We anticipate the extension of these ideas to redox chemistry in the currently burgeoning field of “ionic liquids” in its recent ambient temperature liquid incarnation. Dedicated to the 85th birthday of John O’M. Bockris.

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Thermosensitive sol–gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions

The thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)–polypropylene oxide (PPO)–PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO–water and PPO–water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol–gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic–hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO–PHA–PEO and PEO–PEA–PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO–PESc–PEO aqueous solution showed gelation at a low polymer concentration. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 772–784, 2004     

一种研究聚合物非等温结晶动力学的方法

作者基于多年对聚合物结晶动力学方面研究的工作积累,联合Avrami方程和Ozawa方程,提出了一种研究聚合物非等温结晶动力学的新方法.该方法既克服了使用Ozawa方程所获得的数据点过少,常常出现非线性,不能获得可靠的动力学参数的缺点,又克服了使用经Jeziorny修正的Avrami方程所获得的表观Avrami指数无法准确预测非等温过程成核生长机理的缺点.该方法已成功用于多种聚合物体系,被国内外学者引用数百次,已成为研究聚合物非等温结晶动力学一种有效方法.     

The analysis of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO® resin) in mixtures of PPO resin,polystyrene, and copolymers of PPO resin and polystyrene

A method of determining the quantity of ungrafted poly(2,6-dimethyl-1,4-phenylene oxide) (PPO®

1 Trademark of General Electric Company.

resin) in mixtures of such polyphenylene oxide or PPO resin, polystyrene (PS), and graft copolymers of PPO resin and PS is described. The technique is a combination of physical and chemical separations and gel permeation chromatographic (GPC) analysis. The extent of grafting on PPO resin and the quantity of ungrafted PPO resin were calculated from molecular weight data and compositional analysis.     

EFFECTS OF TEMPERATURE ON THE VISCOSITY OF NONIONIC MJCELLAR SOLUTION IN THE PRESENCE OF ADDED SALTS

ABSTRACT

The effects of temperature on the viscosity of nonionic surfactant, Triton X-100, in the presence of added salts are presented. It was found that the plots of logarithm of relative viscosity versus 1/T were obtained from the slopes of these plots. The activation free energies (ΔG?) of the viscous flow were obtained from the slopes of these plots. The activation enthalpies ( ΔH?) and the entropies ( ΔS?) for the viscous flow were calculated and the entropic contribution was found to be negligible. The ΔG? and ΔH? were highly dependent on the nature of the positive ions. The greater the effect on the energy of the sphere-to-rod transition, the more strongly ions are bound. The ΔG? is interpreted in terms of the energy required to activate the transition of micellar structure capable of being transformed into rod micelle.     

Nanoscale inhomogeneities in thermoresponsive triblock copolymers

We present continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy data of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in aqueous solutions of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic or Poloxamer). TEMPO is notably smaller than the spin probes conventionally used in the context of polymer science and reveals the early emergence of small hydrophobic cavities when PPO strands of several molecules aggregate and collapse upon temperature increase. The occurrence of hydrophobic cavities appears independent of the overall molecular weight of the Pluronics, but clearly depends on the relative PPO/PEO contents. In all the cases studied, the volume fraction of hydrophobic cavities increases in a broad temperature range of ≥40 °C. The appearance of the hydrophobic regions does not seem to be directly correlated to micellization of the polymers. A decrease of the relative PPO amount in the polymers not only hinders collapse of the PPO strands, it can also be described as a site exchange of the spin probes between hydrophobic cavities and the surrounding medium. On the other hand, in cases of high PPO contents, spin probe exchange could not be observed. This suggests that one may potentially control the diffusion of small molecules between the micellar cores and the surrounding medium by adjusting the PEO/PPO ratio of the used Pluronics.     

Pressure-induced phase transitions in PbTiO3: a query for the polarization rotation theory

Our first-principles computations show that the ground state of PbTiO3 under hydrostatic pressure transforms discontinuously from P4mm to R3c at 9 GPa. Spontaneous polarization decreases with increasing pressure so that the R3c phase transforms to the centrosymmetric Rc phase at around 27 GPa. The first-order phase transition between the tetragonal and rhombohedral phases is exceptional since there is no evidence for a bridging phase. The essential feature of the R3c and Rc phases is that they allow the oxygen octahedron to increase its volume VB at the expense of the cuboctahedral volume VA around a Pb ion. This is further supported by the fact that neither the R3m nor Cm phase, which keep the VA/VB ratio constant, is a ground state within the pressure range between 0 and 40 GPa. Thus, tetragonal strain is dominant up to 9 GPa, whereas at higher pressures, efficient compression through oxygen octahedra tilting plays the central role for PbTiO3. Previously predicted pressure induced colossal enhancement of piezoelectricity in PbTiO3 corresponds to unstable Cm and R3m phases. This suggests that the phase instability, in contrast to the polarization rotation, is responsible for the large piezoelectric properties observed in systems like Pb(Zr,Ti)O3 in the vicinity of the morphotropic phase boundary.