Positron annihilation lifetime spectroscopy as a probe of free volume in plasticized solid polymer electrolytes

A recent report on the correlation between enhanced polymer mobility and ionic conductivity at room temperature in plasticized polyether-urethane solid polymer electrolytes (Forsyth et al.[1]), has prompted the present investigation. Positron annihilation lifetime spectroscopy (PALS) has been used to study the effect of plasticizer addition on the room temperature free volume characteristics of the crosslinked polyether-urethane. The addition of low molecular weight plasticizers to the polyether-urethane results in a constant or decreasing mean free volume cavity radius, as measured by the orthoPositronium lifetime τ₃, and a decreasing relative concentration of free volume cavities as measured by the ortho-Positronium intensity, I₃. It is postulated that the plasticizers interrupt polymer-polymer interactions by occupying the inter- and intra-chain free volume. The plasticizer structure influences the polymerplasticizer interactions which affect inter- and intra-chain separation and hence the free volume of the system. The decrease in polymer-polymer interaction and the increase in polymer-plasticizer interaction in turn influence the glass transition temperature behaviour.

Plasticizer effects in polyether electrolytes studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) has been used to probe the effects on free volume of the addition of either propylene carbonate or tetraglyme to a polyether-based electrolyte. Despite their very different behaviour as observed by NMR and conductivity measurement, the PALS sensitive free volume of the samples changes in a similar way on addition of the two plasticizers. It is concluded that the effect of these plasticizers on conductivity is determined more by their effect on ion-polymer and ion-ion interactions than by their effect on the PALS free volume.

Positron annihilation lifetime spectroscopy of mechanically milled protein fibre powders and their free volume aspects

The present study reports the fabrication of ultra-fine powders from animal protein fibres such as cashmere guard hair, merino wool and eri silk along with their free volume aspects. The respectively mechanically cleaned, scoured and degummed cashmere guard hair, wool and silk fibres were converted into dry powders by a process sequence: Chopping, Attritor Milling, and Spray Drying. The fabricated protein fibre powders were characterised by scanning electron microscope, particle size distribution and positron annihilation lifetime spectroscopy (PALS). The PALS results indicated that the average free volume size in protein fibres increased on their wet mechanical milling with a decrease in the corresponding intensities leading to a resultant decrease in their fractional free volumes.

Graphene networks and their influence on free-volume properties of graphene-epoxidized natural rubber composites with a segregated structure: rheological and positron annihilation studies

Epoxidized natural rubber-graphene (ENR-GE) composites with segregated GE networks were successfully fabricated using the latex mixing combined in situ reduced technology. The rheological behavior and electrical conductivity of ENR-GE composites were investigated. At low frequencies, the storage modulus (G′) became frequency-independent suggesting a solid-like rheological behavior and the formation of GE networks. According to the percolation theory, the rheological threshold of ENR-GE composites was calculated to be 0.17 vol%, which was lower than the electrical threshold of 0.23 vol%. Both percolation thresholds depended on the evolution of the GE networks in the composites. At low GE concentrations (<0.17 vol%), GE existed as individual units, while a "polymer-bridged GE network" was constructed in the composites when GE concentrations exceeded 0.17 vol%. Finally, a "three-dimensional GE network" with percolation conductive paths was formed with a GE concentration of 0.23 vol%, where a remarkable increase in the conductivity of ENR-GE composites was observed. The effect of GE on the atom scale free-volume properties of composites was further studied by positron annihilation lifetime spectroscopy and positron age momentum correlation measurements. The motion of ENR chains was retarded by the geometric confinement of "GE networks", producing a high-density interfacial region in the vicinity of GE nanoplatelets, which led to a lower ortho-positronium lifetime intensity and smaller free-volume hole size.

Formation of defects in the graphite oxidization process: a positron study

High-resolution positron annihilation lifetime (PAL) and two-detector coincidence Doppler broadening of annihilation radiation (2D-DBAR) measurements on graphite and its oxide derivatives for defect information, differing in oxidization agents, are reported. Positron measurements were found to be very effective in the investigation of defects in graphite and its derivatives. Positrons are mainly annihilated in vacancy-like defects on the particle surface and in large open-volume holes associated with the interface of graphite and graphite oxide. Different types of defects have been detected for unexfoliated graphite oxide and exfoliated graphene oxide based on 2D-DBAR measurements, namely the vacancy cluster and vacancy-oxygen complexes. It is also interesting to observe that the calculated large open-volume diameter of graphene oxide coincides with the distance between the layers from the XRD investigation, which indicates that the annihilation of the long-lived lifetime component τ3 might take place in the area between the graphene layers; no large open-volume hole has been detected.

Sequential 123-I-iododexetimide scans in temporal lobe epilepsy: comparison with neuroimaging scans (MR imaging and 18-F-FDG-PET imaging)

Purpose Muscarinic acetylcholine receptors (mAChRs) play an important role in the generation of seizures. Single-photon emission computed tomography (SPECT) with ¹²³I-iododexetimide (IDEX) depicts tracer uptake by mAChRs. Our aims were to: (a) determine the optimum time for interictal IDEX SPECT imaging; (b) determine the accuracy of IDEX scans in the localisation of seizure foci when compared with video EEG and MR imaging in patients with temporal lobe epilepsy (TLE); (c) characterise the distribution of IDEX binding in the temporal lobes and (d) compare IDEX SPECT and ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in identifying seizure foci.
Methods We performed sequential scans using IDEX SPECT imaging at 0, 3, 6 and 24 h in 12 consecutive patients with refractory TLE undergoing assessment for epilepsy surgery. Visual and region of interest analyses of the mesial, lateral and polar regions of the temporal lobes were used to compare IDEX SPECT, FDG PET and MR imaging in seizure onset localisation.
Results The 6-h IDEX scan (92%; κappa=0.83, p=0.003) was superior to the 0-h (36%; kappa=0.01, p>0.05), 3-h (55%;κappa=0.13, p>0.05) and 24-h IDEX scans in identifying the temporal lobe of seizure origin. The 6-h IDEX scan correctly predicted the temporal lobe of seizure origin in two patients who required intracranial EEG recordings to define the seizure onset. Reduced ligand binding was most marked at the temporal pole and mesial temporal structures. IDEX SPECT was superior to interictal FDG PET (75%; κappa=0.66, p=0.023) in seizure onset localisation. MR imaging was non-localising in two patients in whom it was normal and in another patient in whom there was bilateral symmetrical hippocampal atrophy.
Conclusion The 6-h IDEX SPECT scan is a viable alternative to FDG PET imaging in seizure onset localisation in TLE.

The effect of nano-particle TiO2 fillers on structure and transport in polymer electrolytes

Nano-particle oxide fillers including TiO₂, SiO₂ and Al₂O₃ have previously been shown to have a significant affect on the properties of polymer electrolytes, especially those based on polyether–lithium salt systems. In some cases, conductivity increases of more than one order of magnitude have been reported in crystalline PEO-based complexes. In this work, we report on the effects of TiO₂ on a completely amorphous polyether-based system to remove the complication of multiple phases presented by the semi-crystalline nature of PEO. Multinuclear magnetic resonance spectroscopy has shown that the lithium ion environment is changed by the addition of filler. Vibrational spectroscopy shows that the filler influences the disordered-longitudinal acoustic modes (DLAM) in the case of an amorphous polyether and suggests an interaction between the filler surface and the polymer. Positron annihilation lifetime spectroscopy indicates an increase in free volume upon addition of filler to an amorphous polyether–salt complex, coinciding with an apparent increase in polymer mobility as determined from ¹H T₂ NMR measurements. Impedance spectroscopy has shown clear evidence of an inter-phase region that may be more or less conductive than the bulk polymer electrolyte itself. The data support a model which includes conduction through an interfacial region in addition to the bulk polymer

Glass transition and free volume behaviour of poly(acrylonitrile)/LiCF3SO3 polymer-in-salt electrolytes compared to poly(ether urethane)/LiClO4 solid polymer electrolytes

Measurements of the glass transition temperature (T_g) and free volume behaviour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with varying salt composition from 10 to 66 wt% LiTf, were made by positron annihilation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiTf resulted in an increase in the mean free volume cavity size at room temperature (r.t.) as measured by the orthoPositronium (oPs) pickoff lifetime, τ₃, with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, I₃. The region from 45 to 66 wt% salt displayed no variation in relative free volume cavity size and concentration. This salt concentration range (45 wt%<[LiTf]<66 wt%) corresponds to a region of high ionic conductivity of order 10⁻⁵ to 10⁻⁶ S cm⁻¹ at T_g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower the T_g as measured by PALS; T_g was 115°C for PAN and 85°C for PAN/66 wt% LiTf. The T_g and free volume behaviour of this polymer-in-salt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO₄ where the polymer is the major component, i.e. traditional solid polymer electrolyte (SPE). In contrast to the PISE, the T_g of the SPE was shown to increase with increasing salt concentration from 5.3 to 15.9 wt%. The relative free volume cavity size and concentration at r.t. were shown to decrease with increasing salt concentration. Ionic conductivity in this SPE was of order 10⁻⁵ S cm⁻¹ at r.t., which is over 60°C above T_g, 10⁻⁸ S cm⁻¹ at 25°C above T_g, and conductivity was not measurable at T_g.

Microstructural and molecular level characterisation of plastic crystal phases of pyrrolidinium trifluoromethanesulfonyl salts

Ambient temperature conductive plastic crystal phases of alkylmethylpyrrolidinium trifluoromethanesulfonyl amide (TFSA) salts are studied using positron annihilation lifetime spectroscopy (PALS) to examine the role of vacancy size and concentration in conductivity. The ethyl methylpyrrolidinium TFSA salt (P₁₂ TFSA) has larger vacancies and a greater concentration of vacancies than the dimethylpyrrolidinium TFSA salt (P₁₁ TFSA) over the temperature range investigated. The relative vacancy size and concentration vary with temperature and reflect the solid–solid transitions as measured by differential scanning calorimetry (DSC). P₁₂ TFSA has greater conductivity than P₁₁ TFSA and has furthermore been observed to exhibit slip planes at room temperature. P₁₂ TFSA has greater entropy changes associated with solid–solid phase transitions below the melting point than P₁₁ TFSA possibly indicating greater rotational freedom in P₁₂ TFSA. These results support the notion that the diffusion, conduction, and plastic flow properties of the pyrrolidinium TFSA salts are derived from the lattice vacancies.

Nanoparticle enhanced conductivity in organic ionic plastic crystals : space charge versus strain induced defect mechanism

High conductivity in solid-state electrolytes is a critical requirement for many advanced energy and other electrochemical applications. Plastic crystalline materials have shown promise in this regard, and the inclusion of nanosized inorganic particles in both amorphous and crystalline materials has indicated order of magnitude enhancements in ion transport induced by space charge or other defect enhancement. In this paper we present conductivity enhancements in the plastic crystal N,N‘-ethylmethylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C₂mpyr][NTf₂]) induced by nanosized SiO₂ particles. The addition of the nanoparticles dramatically increases plasticity and ion mobility. Positron annihilation lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion. The scaling of the conductivity with size suggests that a “trivial space charge” effect is operable, although a strain induced enhancement of defects (in particular extended defects) is also likely given the observed increase in plasticity.

Surprising effect of nanoparticle inclusion on ion conductivity in a lithium doped organic ionic plastic crystal

Doping lithium bis(trifluoromethanesulfonyl)amide (Li[NTf₂]) into the N-ethyl,N′-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C₂mpyr][NTf₂]) plastic crystal material has previously indicated order of magnitude enhancements in ion transport and conductivity over pure [C₂mpyr][NTf₂]. Recently, conductivity enhancements in this ionic plastic crystal induced by SiO₂ nanoparticles have also been reported. In this work the inclusion of SiO₂ nanoparticles in Li ion doped [C₂mpyr][NTf₂] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, NMR spectroscopy and scanning electron microscopy (SEM). Solid state ¹H NMR indicates that the addition of the nanoparticles increases the mobility of the [C₂mpyr] cation and positron lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion, especially with 10 wt% SiO₂. Thus, the substantial drop in ion conductivity observed for this doped nanocomposite material was surprising. This decrease is most likely due to the decrease in mobility of the [NTf₂] anion, possibly by its adsorption at the SiO₂/grain boundary interface and concomitant decrease in mobility of the Li ion.