James Runt

Modeling electrode polarization in dielectric spectroscopy: Ion mobility and mobile ion concentration of single-ion polymer electrolytes

A novel method is presented whereby the parameters quantifying the conductivity of an ionomer can be extracted from the phenomenon of electrode polarization in the dielectric loss and tan delta planes. Mobile ion concentrations and ion mobilities were determined for a poly(ethylene oxide)-based sulfonated ionomer with Li(+), Na(+), and Cs(+) cations. The validity of the model was confirmed by examining the effects of sample thickness and temperature. The Vogel-Fulcher-Tammann (VFT)-type temperature dependence of conductivity was found to arise from the Arrhenius dependence of ion concentration and VFT behavior of mobility. The ion concentration activation energy was found to be 25.2, 23.4, and 22.3+/-0.5 kJmol for ionomers containing Li(+), Na(+), and Cs(+), respectively. The theoretical binding energies were also calculated and found to be approximately 5 kJmol larger than the experimental activation energies, due to stabilization by coordination with polyethylene glycol segments. Surprisingly, the fraction of mobile ions was found to be very small, <0.004% of the cations in the Li(+) ionomer at 20 degrees C.

Human foetal osteoblastic cell response to polymer-demixed nanotopographic interfaces

Nanoscale cell–substratum interactions are of significant interest in various biomedical applications. We investigated human foetal osteoblastic cell response to randomly distributed nanoisland topography with varying heights (11, 38 and 85 nm) produced by a polystyrene (PS)/polybromostyrene polymer-demixing technique. Cells displayed island-conforming lamellipodia spreading, and filopodia projections appeared to play a role in sensing the nanotopography. Cells cultured on 11 nm high islands displayed significantly enhanced cell spreading and larger cell dimensions than cells on larger nanoislands or flat PS control, on which cells often displayed a stellate shape. Development of signal transmitting structures such as focal adhesive vinculin protein and cytoskeletal actin stress fibres was more pronounced, as was their colocalization, in cells cultured on smaller nanoisland surfaces. Cell adhesion and proliferation were greater with decreasing island height. Alkaline phosphatase (AP) activity, an early stage marker of bone cell differentiation, also exhibited nanotopography dependence, i.e. higher AP activity on 11 nm islands compared with that on larger islands or flat PS. Therefore, randomly distributed island topography with varying nanoscale heights not only affect adhesion-related cell behaviour but also bone cell phenotype. Our results suggest that modulation of nanoscale topography may be exploited to control cell function at cell–biomaterial interfaces.

Electrical breakdown and ultrahigh electrical energy density in poly(vinylidene fluoride-hexafluoropropylene) copolymer

This paper investigates the electrical breakdown of a polar fluoropolymer, poly(vinylidene fluoride-hexafluoropropylene) which exhibits an exceptionally high discharged electrical energy density (>25 J/cm3). It is shown that above room temperature, the breakdown strength decreases with temperature. It is further shown that such a temperature dependence of breakdown strength is consistent with the electromechanical breakdown model by taking into consideration of the plastic deformation of semicrystalline polymers.

Phase separation of diamine chain-extended poly(urethane) copolymers: FTIR spectroscopy and phase transitions

Abstract As part of our continuing effort to understand microphase separation of poly(urethane urea) block copolymers, FTIR spectroscopy and thermal techniques (DSC and DMA) were used to investigate the phase behavior of two series of MDI–polytetramethylene oxide soft segment copolymers, chain-extended with ethylene diamine or a diamine mixture. Due to the complex nature and multiple absorbances in the carbonyl and N–H regions of the FTIR spectra, quantitative analysis was not possible. However, qualitative trends could be discerned, and the spectral changes were found to be in excellent agreement with our previous quantitative analysis of the same copolymers using small-angle X-ray scattering. DSC and DMA experiments both indicate that the soft phase Tg decreases with increasing hard segment content. This is contrary to increased hard segment mixing in the soft phase, but can be rationalized by taking into consideration soft segment crystallinity and the concentration of ‘lone’ MDI units in the soft phase.

Molecular mobility and Li+ conduction in polyester copolymer ionomers based on poly(ethylene oxide)

We investigate the segmental and local dynamics as well as the transport of Li(+) cations in a series of model poly(ethylene oxide)-based single-ion conductors with varying ion content, using dielectric relaxation spectroscopy. We observe a slowing down of segmental dynamics and an increase in glass transition temperature above a critical ion content, as well as the appearance of an additional relaxation process associated with rotation of ion pairs. Conductivity is strongly coupled to segmental relaxation. For a fixed segmental relaxation frequency, molar conductivity increases with increasing ion content. A physical model of electrode polarization is used to separate ionic conductivity into the contributions of mobile ion concentration and ion mobility, and a model for the conduction mechanism involving transient triple ions is proposed to rationalize the behavior of these quantities as a function of ion content and the measured dielectric constant.

Crystallization and structure formation of poly(L-lactide-co-meso-lactide) random copolymers: a time-resolved wide- and small-angle X-ray scattering study

Abstract Time-resolved synchrotron simultaneous wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) experiments were used to investigate the crystallization behavior and microstructure development of poly( l -lactide), PLLA, and two random copolymers containing l -lactide (predominately) and randomly placed R stereochemical defects. The general features of crystallization of PLLA and the copolymers are similar except that the copolymers crystallize much more slowly and to a lesser extent than PLLA, as expected. Lamellar thicknesses derived from SAXS experiments are in very good agreement with mean thicknesses determined in a tapping mode AFM study of the same materials. The reduction in lamellar thickness and crystallinity with increasing meso -lactide content supports significant exclusion of the R stereoisomer from crystalline lamellae. In a separate series of time-resolved WAXD/SAXS experiments, each (co)polymer was crystallized for a fixed time, then heated to above its melting point. The observed behavior suggests a model for crystallization of the copolymers in which thinner lamellae form between ‘primary’ lamellar stacks during crystallization, with an average lamellar thickness that decreases with increasing R stereoisomer content.

In vitro oxidation of high polydimethylsiloxane content biomedical polyurethanes: correlation with the microstructure.

The resistance to in vitro metal ion oxidation of a polydimethylsiloxane (PDMS)-containing thermoplastic polyurethane elastomer (Elast-Eon) is compared with that of a polyurethane consisting of the same hard segment chemistry and content, but with aliphatic polycarbonate soft segments (PCU). Scanning electron microscopy and attenuated total reflectance Fourier transform infrared spectroscopy were used to assess changes in surface morphology and chemistry. The extent of bulk degradation was assessed indirectly by dynamic mechanical analysis and small-angle X-ray scattering experiments. The findings indicate that Elast-Eon is more resistant to oxidation than the PCU, because of the presence of the PDMS soft segments as well as its phase separated microstructure. The PCU exhibits a rather high degree of intermixing between hard and soft segments, rendering the hard segments dissolved or trapped in the soft phase more susceptible to oxidative conditions. By contrast, we propose that the existence of a completely phase separated PDMS soft phase in Elast-Eon protects the remainder of the segments from oxidation.

Observation of a fast dielectric relaxation in semi-crystalline poly(ethylene oxide)

Abstract Our broadband dielectric study has revealed for the first time a relaxation (termed γ′) between the low temperature β and γ-processes of semi-crystalline poly(ethylene oxide) (PEO). The activation energy of the γ′-transition is indicative of a local process, and this relaxation becomes broader with increasing temperature. We propose that this process is due to the motion of PEO segments in the transition region between PEO lamellae and disordered interlamellar amorphous segments. The unusual local character of the γ′-process is explained by analogy to the fast process or primitive α-relaxation in nanoconfined glass forming systems. This mechanism is supported by comparison of the dielectric behavior of samples with different thermal histories.

FT i.r. and thermal analysis studies of blends of poly(ε-caprolactone) with homo- and copolymers of poly(vinylidene chloride)

Abstract FT i.r. and thermal analysis studies have been performed on blends of poly(e-caprolactone) (PCL) with vinylidene chloride copolymers containing the comonomers vinyl chloride and acrylonitrile. The results indicate that the blends are compatible over the whole composition range in the amorphous state. Attempts were also made to obtain coherent blends of PCL and the homopolymer of vinylidene chloride. Evidence for the presence of specific intermolecular interactions has been obtained and the origin of these interactions is discussed.

Thin film 0–3 polymer/piezoelectric ceramic composites: Piezoelectric paints

Abstract In this paper we summarize our investigation of the electrical properties of thin-film 0–3 polymer-ceramic composites. The main objective of this study was to demonstrate the principle that a piezoelectric paint can indeed be prepared. Two polymers, an acrylic copolymer and a polyurethane, were utilized in our research. Both were loaded with 60–70 volume percent PZT and a coprecipitated PbTiO3. The addition of various surfactants and dispersing agents to the acrylate-based composites was necessary to aid in dispersing the ceramic particles in the polymer matrix. The hydrostatic strain and voltage coefficients, along with the ‘figures of merit’ for PZT-filled acrylic and polyurethane composites were found to be significantly larger than values reported previously for other 0–3 polymer-PZT composites. For acrylic copolymer-coprecipitated PbTiO3 composites, the hydrostatic coefficients were found to be roughly 50% larger than the comparable PZT-filled materials. For example, gh dh ranged from about ...