Frank Rittig

The local free volume, glass transition, and ionic conductivity in a polymer electrolyte: A positron lifetime study

The size of free-volume holes in neat poly[(ethylene glycol)23dimethacrylate] [poly((EG)23DMA)] and in the same polymer doped with 0.6 mol/kg LiCF3SO3 have been studied as a function of temperature in the range between 100 and 370 K using positron annihilation lifetime spectroscopy. The results are compared with differential scanning calorimetry and ionic conductivity measurements. In both systems, the hole volume νh shows a typical glass-transition behavior, i.e., a small linear increase with temperature below the glass transition temperature Tg and a steeper increase above Tg. From these measurements Tg was estimated to be 233 K (neat polymer) and 240 K (polymer with salt) and the coefficients of the thermal expansion of the hole volume were determined. The fractional free volume (f=0.080) and the number density of holes (Nh=0.6 nm−3) were also estimated. Below Tg the average hole volume of the polymer electrolyte is larger than in the neat polymer. This is consistent with the bulky character of the CF3...

Molecular Dynamics in Poly(N-vinylpyrrolidone)-Poly(ethylene glycol) Blends Investigated by the Pulsed-Field Gradient NMR Method: Effects of Aging, Hydration and PEG Chain Length

Full Paper: The self-diffusion in PVP-PEG blends has been studied as a function of PEG molecular weight, water content and temperature. The blend corresponding to a stoichiometric PVP-PEG complex with 36 wt.-% PEG400 results from hydrogen bonding of the PEG terminal hydroxyl groups to the carbonyls of the PVP repeating units. Changes in diffusion parameters during the time of storage of the samples up to 1.5-2 months after preparation have been established. The attainment of an equilibrium value is characterized by a redistribution of the PEG molecules bonded to the comparatively long PVP chains, resulting in the formation of a supramolecular network. The spin echo attenuation, S inc (q, t d ), has been found to depend on the temperature, PEG chain length and water content. Water acts as a mediator in the complex formation, accelerating the PVP-PEG complexation. The PVP blend with PEG200 retains about 21 wt.-% of water after evacuation at 378 K. This amount of water is believed to be incorporated into the network. The longer the PEG chain length, i.e. the larger the flexibility of the chain, and the lower the OH group concentration in the blend, the smaller the number of PEG molecules bound to PVP. This has been established by comparing diffusion parameters of PVP blends containing PEG400 and PEG600, respectively.

Emulsification of poly(ethylene glycol) in thermoplastic elastomers by using polybutadiene-block-poly(ethylene oxide) diblock copolymers as stabilisers. Determination of the liquid phase mobility by pulsed field gradient NMR

Polymeric film materials comprising a continuous elastomer phase of a styrene-hydrogenated diene thermoplastic elastomer (SEBS) and a dispersed liquid phase of poly(ethylene glycol) (PEG) were prepared by starting from an emulsion of PEG dispersed as small droplets in a solution of SEBS in methylcyclohexane. A polybutadiene-block-poly(ethylene oxide) block copolymer is used as emulsifier and stabiliser for these emulsions. The particle size of the dispersed PEG phase, in the emulsion, and in the final film obtained by solvent evaporation could be adjusted as a function of the block copolymer concentration. The morphology of the material was studied by scanning electron microscopy (SEM) and pulsed field gradient (PFG) NMR. From the diffusion coefficients determined by PFG NMR it is demonstrated that a part of the block copolymer, in addition to its stabilisation effect, has a tendency to form in the rubber matrix micelles and aggregates (polymolecular micelles) which are able to solubilize a given fraction of PEG.

Molecular dynamics of glass-forming liquids in confining geometries

Pulsed field gradient NMR and dielectric spectroscopy are applied to investigate the rates of molecular reorientation and translation of the glass-forming liquids propanediol and salol in porous glasses. Though there is a general tendency for the confinement in the pore system to reduce both the rotational and translational mobility in comparison with the liquid phase, in particular cases also an enhancement of the molecular mobility is observed. As a possible explanation of this enhancement it is assumed that the spatial confinement within the pore system excludes the formation of long-range molecular aggregates which dominate molecular dynamics in the bulk liquid.

Self-diffusion investigations on a series of PEP-PDMS diblock copolymers with different morphologies by pulsed field gradient NMR

We report on temperature-dependent self-diffusion measurements of compositionally different and non-entangled poly(ethylene-co-propylene)-b-poly(dimethylsiloxane) PEP-PDMS diblock copolymers in the melt above and below the order-to-disorder transition temperature. Depending on the dimensionality of the structures (lamellar: two-dimensional, hexagonal: one-dimensional, spherical: zero-dimensional) characteristic diffusion mechanisms are observed: in the two- and one-dimensional cases anisotropic single-chain diffusion proceeds along the geometries, i.e. parallel and perpendicular to the interfaces. The shape of the echo attenuations provides information on the anisotropy of motion. Motion perpendicular to the interfaces is markedly suppressed. In the spherical morphology composed of micelle-like structures, however, the diblock copolymers are locally confined to the micelles, and only the collective diffusion of clusters of micelles is observed. In none of the cases is a dramatic change of the self-diffusivity observed at the order-to-disorder transition temperature. The self-diffusion in the disordered state is influenced by concentration fluctuations over a broad temperature range.

SELF-DIFFUSION OF AN ASYMMETRIC DIBLOCK COPOLYMER ABOVE AND BELOW THE ORDER-TO-DISORDER TRANSITION TEMPERATURE

The self-diffusivity of an asymmetric poly(ethylenepropylene)–poly(dimethylsiloxane) diblock copolymer (PEP–PDMS) was investigated using pulsed field gradient NMR both in the ordered and the disordered state. In a temperature range below the order-to-disorder transition temperature, the diblock copolymer forms micelles which order in a simple cubic structure, and at lower temperatures, a noncubic structure is formed. We have found a broad distribution of self-diffusivities. The distribution width increases with decreasing temperature. The self-diffusivity as well as its distribution width changes only slightly at the order-to-disorder transition. In contrast to self-diffusion in symmetric diblock copolymers where single chains diffuse independently, our results are interpreted as a collective diffusion of chains confined to micelles which further cluster to larger aggregates. This behavior cannot be described by existing models of chain dynamics in the melt. The disordered state is characterized by a larg...

Influence of associations on self‐diffusion of styrene–methylmethacrylate random copolymers in semidilute acetone solution

The self-diffusion of four styrene-methylmethacrylate random copolymers dissolved in acetone, a thermodynamically bad solvent for the styrene sequences in the copolymer, were investigated by pulsed-field gradient NMR. The echo attenuations S inc (q,t) are dominated by diffusion of clusters. At higher concentrations, physical gelation sets in (formation of a transient network), which is manifested in an anomalous, restricted diffusion at short observation times. The measured fluctuations of the positions of the chain segments in the transient network have an amplitude of about 100 nm. The free long-range diffusion attained at long observation times shows a much stronger concentration dependence than in the nonassociating solvent benzene. The results are in accord with light-scattering investigations.

Separation of non-UV-absorbing synthetic polyelectrolytes by CE with contactless conductivity detection

A specific method for the separation and detection of non‐UV‐absorbing polyelectrolytes has been developed. The analysis of such polyelectrolytes by liquid chromatography is nearly impossible due to strong ionic interactions and charge density effects. CE makes use of these charge density effects and thus enables for proper separation. A capacitively coupled contactless conductivity detector has been applied for the detection in CE. A low molar mass poly(acrylic acid) sodium salt standard (PAA1.3k) was separated in free solution CE and detected with the contactless conductivity detector. Different amphoteric electrolytes have been tested for their applicability as BGE for the separation of polyelectrolytes with conductivity detection. It has been shown that the best detection results are obtained with an arginine‐sorbate buffer.

Liquid polyethylene glycol dispersed in a poly(styrene)-b-poly(ethylene/butylene)-b-poly(styrene) elastomer: determination of morphology and molecular mobility by light and electron microscopy as well as nuclear magnetic resonance self-diffusion andT2 measurements

An emulsion of liquid polyethylene glycol (PEG) in a rubbery poly(styrene)-b-poly(ethylene/butylene-b-poly-(styrene) (SEBS) matrix was prepared using a poly-(butadiene-b-poly(ethylene oxide) (PB-b-PEO) diblock copolymer as emulsifier. In addition to its emulsifying effect, it was shown that a block copolymer containing a crystalline PEO sequence has a tendency to form micelles as well as supramolecular structures in form of square platelets when they are solubilized in nonpolar solvent such as methylcyclohexane. The filmification of the emulsion by solvent evaporation and the film morphology were examined by light and electron microscopy and by nuclear magnetic resonance (NMR) T2 and self-diffusion measurements. Microscopic evaluations have shown cavities having an average size, depending on the copolymer content, of a few micrometers and filled with liquid PEG. Additionally, smaller structures typically in the range of 30–50 nm have been observed by transmission electron microscopy and are attributed to crystalline lamellae formed by the copolymer in the matrix. The T2 measurements reveal three contributions to the transverse magnetization decay: one of the SEBS matrix, one of the liquid within the cavities and one of the liquid dissolved in the smaller structures formed by the copolymer in the matrix. In the pulsed field gradient NMR self-diffusion experiments two self-diffusion coefficients were found, one of the liquid in the cavities and one of the liquid dissolved in the matrix within the smaller structures. Generally, a restricted diffusion is observed in that latter case owing to confining structures for the diffusing liquid molecules.

Influence of the morphology on the translational and collective dynamics in ordered diblock copolymer melts

Diblock copolymers in the melt state form a variety of mesoscopically ordered morphologies, depending on their composition and on temperature. They can thus serve as a model system for studying the dynamics in different ordered morphologies with the disordered state as a reference state. In this review, the methods for studying the dynamics are compiled. The dynamics in the disordered, the lamellar, the gyroid, the hexagonal and the body-centred cubic phase are described with particular emphasis on the polymer self-diffusion as well as on the collective dynamics. The dimensionality of the morphology has a strong influence on the dynamics: the copolymer diffusion along the interfaces is anisotropic, in contrast to the isotropic disordered phase, and collective motions such as undulations of lamellar interfaces or fluctuation of the micellar distance in the body-centred cubic state become possible.