Christer Svanberg

Poly(methyl methacrylate)-based protonic gel electrolytes: a spectroscopic study

We present the first vibrational spectroscopic investigation of a novel non-aqeous proton conducting polymer gel electrolyte consisting of a PMMA matrix and a solvent mixture (ethylene carbonate (EC)/propylene carbonate (PC) or EC/PC/-N,Ndimethylformamide (DMF)) with a dissolved organic acid (benzoic or salicylic acid). The protonic conductivity of the gels is of the order ∼ 10 -4 - 10 -5 S/cm at room temperature. We show that the conductivity is proportional to the degree of dissociation of the acid, the latter determined from Raman spectroscopic data, and that the degree of dissociation depends on the properties of the solvent mixture. Finally, we comment on the relation between the proton conductivity and the solvent diffusion dynamics as studied by PCS.

Fabrication of reverse symmetry polymer waveguide sensor chips on nanoporous substrates using dip-floating

Reverse symmetry waveguide biosensors employ substrates with a refractive index less than the analyzed aqueous cover sample (1.33). This design offers higher sensitivities for detecting micron scale biological objects such as bacteria and living cells. In the present paper, the fabrication details of reverse symmetry polymer waveguide sensors using nanoporous silica substrates with refractive index 1.2 are presented. Using nanoporous substrates the direct spin-coating deposition of polymer films is not feasible, since the solvent used to dissolve the polymer fills up the nanopores leading to a substrate RI more than 1.33. Instead, a technique—referred to as dip-floating—was applied to create freely floating 150-nm thick polymer films on the surface of water. The films were transferred to the nanoporous substrates simply by pressing the substrates through the air-floating film–water interface. A heat molding technique using a PDMS grating was used to create integrated gratings on the polymer films for light coupling.

Relaxational and vibrational dynamics of poly(propylene glycol)

Abstract Photon correlation spectroscopy has been used to study structural relaxation dynamics near the glass transition temperature for glass-forming linear chain molecules differing in the number of repeat units. Complementary investigations of the low-frequency vibrational dynamics were performed by means of Raman spectroscopy. The systems studied range from the low molecular weight liquid propylene glycol (PG, molecular weight (MW)=76) to the polymer, poly(propyleneglycol) (PPG, MW=4000). Differences are observed in terms of the fragility of the systems. The findings are attributed to bonding between the hydroxy endgroups which decrease in density as the MW increases.

Diffusive and segmental dynamics in polymer gel electrolytes

Dynamic light scattering data on a polymer gel electrolyte with a complex relaxation behavior is presented. The electrolyte consists of lithium perchlorate dissolved in an ethylene carbonate and propylene carbonate solution that is immobilized with poly(methyl methacrylate). We attribute the observed relaxation processes to two diffusive and one segmental relaxation processes based on the form of the time decay of the intermediate scattering function and the corresponding temperature and wave vector dependencies. The dynamic light scattering results are compared with the ionic conductivity, which reveals a close connection between the fast diffusive motion of the low molecular weight solvent within the gel and the ionic conductivity. This motion is strongly decoupled from and considerably faster than the segmental motion of the polymer matrix. The results indicate that the ionic transport occurs mainly within the low molecular weight solvent.

Interplay between Hydration Water and Headgroup Dynamics in Lipid Bilayers

In this study, the interplay between water and lipid dynamics has been investigated by broadband dielectric spectroscopy and modulated differential scanning calorimetry (MDSC). The multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been studied over a broad temperature range at three different water contents: about 3, 6, and 9 water molecules per lipid molecule. The results from the dielectric relaxation measurements show that at temperatures <250 K the lipid headgroup rotation is described by a super-Arrhenius temperature dependence at the lowest hydration level and by the Arrhenius law at the highest hydration level. This difference in the temperature dependence of the lipid headgroup rotation can be explained by the increasing interaction between the headgroups with decreasing water content, which causes their rotational motion to be more cooperative in character. The main water relaxation shows an anomalous dependence on the water content in the supercooled and glassy regime. In contrast to the general behavior of interfacial water, the water dynamics is fastest in the driest sample and its temperature dependence is best described by a super-Arrhenius temperature dependence. The best explanation for this anomalous behavior is that the water relaxation becomes more determined by fast local lipid motions than by the intrinsic water dynamics at low water contents. In support for this interpretation is the finding that the relaxation time of the main water process is faster than that in most other host systems at temperatures below 180 K. Thus, the dielectric relaxation data show clearly the strong interplay between water and lipid dynamics; the water influences the lipid dynamics and vice versa. In the MDSC data, we observe a weak enthalpy relaxation at 203 K for the driest sample and at 179 K for the most hydrated sample, attributed to the freezing-in of the lipid headgroup rotation observed in the dielectric data, since this motion reaches a time scale of about 100 s at about the same temperatures.

Polymer concentration dependence of the dynamics in gel electrolytes

A polymer gel electrolyte sample with a gradually varying concentration of polymer have been examined by Raman and photon correlation spectroscopy. The gel consisted of poly(methyl methacrylate) complexed with a liquid electrolyte of lithium perchlorate salt in an ethylene carbonate and propylene carbonate solution. The mole ratio composition was determined throughout the sample by means of Raman spectroscopy, which reveal a gradual vertical decrease of the polymer concentration. The photon correlation experiments show three dynamical processes of which two are attributed to diffusive processes, related to the solvent, and one to segmental motion, due to the relaxation of the polymer matrix. As the polymer concentration is increased the fast diffusive process gets slower, while there is no or a very small effect on the segmental motion. The implications for the overall performance in applications concerning ionic conductivity is also briefly discussed.

Diffusion of solvent:salt and segmental relaxation in polymer gel electrolytes

Dynamic light scattering and broad band impedance spectroscopy have been performed on polymer gel electrolytes consisting of a mixture of propylene carbonate/lithium perchlorate (PC/LiClO4) stabilised with a polymer matrix, poly(methyl methacrylate) (PMMA). The salt concentration and temperature dependences of the conductivity are compared to the dynamical behaviour of the system. Complex relaxation dynamics is observed in the dynamic light scattering experiments with multiple relaxation processes with different temperature and wave vector dependencies. In focus here is a fast exponential decay attributed to diffusion of low molecular compounds and a slower and very broad relaxation process attributed to the segmental motion of the polymer backbone. The relation between the relaxation processes and the ionic conductivity is investigated, with special emphasis on the relation to the fast diffusive process. We show that the slowing down of this diffusion process is a more important limiting factor for ionic conductivity than the possible creation of ion pairs at higher salt-concentrations. The physical picture that emerges from the results is that the conductivity mechanism is a diffusion of ions moving together with the solvent molecules, both of which are essentially decoupled from the segmental motion of the polymer backbone.

Structural relaxations of phospholipids and water in planar membranes

We have used dielectric spectroscopy and temperature modulated differential scanning calorimetry (TMDSC) to investigate the structural relaxation processes and phase transitions of water and lipids in multilamellar, planar phospholipids. At low hydration levels we observe the main structural relaxation related to the glass transition of the phospholipids. With increasing water content a more pronounced pretransition, attributed to a gel to ripple phase transition, is observed in the TMDSC data. In the proximity of this pretransition, a distinct change in the temperature dependence or alternatively a bifurcation into two processes is observed in the dielectric data. Around this temperature a crossover in the long-range ionic conductivity across the membranes is also observed, which is one of the key parameters for biological membranes. Thus, the major dynamical changes do not occur at the main, i.e., the gel to liquid structural phase transition, but at a pretransition that occurs roughly 20 K below the main transition.

Diffusive dynamics in polymer gel electrolytes investigated by quasi-elastic neutron scattering

Abstract Quasi-elastic neutron scattering has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). Two relaxational processes are observed, and their momentum transfer dependencies are studied. From this we attribute the faster process to a rotational motion and the slower process to diffusive motion of the solvent. The diffusive process was modelled with a jump diffusion model, giving a diffusion constant of 1.6×10 −10 m 2 / s , a mean residence time of 34 ps and mean jump length of 1.8 A . A jump rotation model analysis of the rotational motion yields a rotation radius of 1.5 A , which is compatible with the size of the solvent molecules.

Susceptibility functions for disordered materials: the full width at half maximum for different frequency and time representations

Abstract We here propose relations that provide a convenient way to obtain, from curve-fit parameters, the full width at half maximum (FWHM), which is one of the fundamental parameters in the scaling procedure for relaxations in disordered materials proposed by Dixon et al. We derive an analytical expression for the FWHM for symmetric loss peaks using a newly proposed response function. Since the response function contains e.g. the Cole–Cole and the Fouss–Kirkwood expressions as special cases, exact analytical expressions of the FWHM is also obtained for these functions. For asymmetric loss peaks, approximations are introduced and we show that the errors for the obtained functions are less than 6%. Furthermore, we propose an approximate relation between the stretching parameter, β , of the Kohlrausch–Williams–Watts function and the FWHM.