Solid-state Transition Research Articles

Carrier-Multiplication-Induced Structural Change during Ultrafast Carrier Relaxation and Nonthermal Phase Transition in Semiconductors.

While being extensively studied as an important physical process to alter exciton population in nanostructures at the fs time scale, carrier multiplication has not been considered seriously as a major mechanism for phase transition. Real-time time-dependent density functional theory study of Ge_{2}Sb_{2}Te_{5} reveals that carrier multiplication can induce an ultrafast phase transition in the solid state despite that the lattice remains cold. The results also unify the experimental findings in other semiconductors for which the explanation remains to be the 30-year old phenomenological plasma annealing model.

On-line monitoring of vacuum drying of theophylline using NIR spectroscopy: solid-state transitions, water content and semi-empirical modeling

The aim of this work was to monitor in-line and at a real time, the solid-state forms during pharmaceuticals manufacturing. It concerns the dehydration behavior and the solid-state transitions of theophylline in an agitated vacuum contact dryer. First, a near infrared spectroscopy (NIRS) method was performed using a reflectance diffuse probe to measure the in-line and in-situ exact composition of the mixture of different forms of theophylline and water content during drying. A multivariate modeling has been investigated to build a robust model which can predict four components at the same time during drying process. The XRPD analysis was used as a reference method in the process of calibration of NIRS. The indicators of the accuracy in quantitative spectral analysis confirm the robustness of the model and the efficiency of the method of calibration. Second, the kinetics of solid state transformations were investigated. It was shown that the dehydration advanced first by the formation of the metastable anhydrate and after a lag time of the stable one. Once the stable form appeared, formation of the metastable form came to an end. The temperature was found out to be the main factor controlling the overall process rate but also the final contents of the stable and metastable anhydrates for the considered dryer and operating conditions range. Finally, a semi-empirical drying model was proposed and significant quantitative differences were found, particularly at the product temperature which was probably caused by the excessive simplicity of the model.

Effect of thermal history on nucleation and crystallization of poly(lactic acid)

In this paper, a successive heating and cooling protocol was designed to investigate the self-nucleation behavior of poly(lactic acid), PLA. The main objective of this investigation was to study the efficiency of the α and α′ crystalline modifications of PLA. This was carried out by comparing crystallization temperatures upon cooling after self-nucleation of samples previously crystallized at various isothermal temperatures ranging from 80 to 130 °C. During heating to the partial melting range, three different mechanisms were observed for crystallized samples. For samples crystallized below 100 °C, an exothermic peak was detected prior the main melting peak which is ascribed to the α′–α solid-state transition. For samples crystallized between 100 and 120 °C, a melt recrystallization mechanism was observed. Finally, for samples crystallized above 120 °C, only melting of the α phase was detected. Upon cooling after partial melting, it was found that samples comprising a mixture of α and α′ exhibited the highest crystallization temperature, the highest nuclei density, and the smallest spherulite size. Moreover, it was observed that samples that were isothermally crystallized between 100 and 120 °C, heated up to partial melting, and then cooled back to room temperature exhibited two peculiar crystallization peaks at 100 and 120° C. This phenomenon was ascribed to the formation of α and α′ crystalline phases as revealed by X-ray diffraction. In addition, by slightly changing the temperature within the self-nucleation temperature range, a change of the proportion of each peak was observed.

Characterization, thermal stability, and solid-state phase transition behaviors of gestodene polymorphs and amorphous phase

In the current work, a systematic investigation on the thermal behavior of gestodene was carried out to understand temperature induced solid-state transitions between its polymorphs and amorphous phase. The focus was on polymorph identification, thermal stability analysis, and the nature of the phase transitions. These characteristics were compared to the complexity of the phase transitions, studied by differential scanning calorimetry (DSC) and variable temperature X-ray powder diffraction (VT-XRD) techniques. DSC studies indicated that the form II was enantiotropically related to form I, which melted at about 470 K. The temperature of polymorphic transition was 309 K, and form II was the more stable form between room temperature and the transition temperature. A schematic Gibbs free energy-temperature diagram was subsequently constructed to describe the thermal stability of the two forms. Amorphous phase converted exothermically to form I at 368 K on heating and was shown by VT-XRD to be accompanied by diffraction pattern changes. In addition, the crystallization kinetics studied by DSC heating technique followed by analysis using the Kissinger–Akahira–Sunose (KAS) method where values of apparent activation energy (E a) were estimated as a function of extent of conversion (α). The variations in E a with α on kinetic analysis from α = 0.10 to 0.88 for the amorphous to form I conversion suggested more complex processes, possibly liquid–solid and solid–solid transformations prior to formation of the form I.

(Invited) Conductivity and Relaxation Phenomena in Ion Conducting Materials By Broadband Electric Spectroscopy

Ionically conducting materials (ICM) are of great importance for the fabrication of portable batteries for electronic devices such as computers, tools, video and still cameras, and for the development of fuel cell and battery-powered electric vehicles, dye-sensitized solar cells, supercapacitors and sensors.1 It has been suggested that conductivity in ICMs occurs via a number of different processes. The predominant conductivity processes are attributed to: a) the charge migration of ions between coordination sites in the host materials;2-5 and b) the increase of conductivity due to relaxation phenomena involving the dynamics of the host materials.2-5 Ions “hopping” to new chemical environments can lead to successful charge migration only if ion-occupying domains relax via reorganizational processes,2-5 which generally are coupled with relaxation events associated with the host matrix. Here, it will be described in a concise fashion, the instruments used to comprehensively study the electric response of ionic conductors. To provide the reader with the basic tools necessary for understanding broadband electric spectroscopy,6-8 the first part will review the general phenomena and basic theory behind each type of electric response that materials may exhibit when they are subjected to static or dynamic electric fields. This will be achieved by focusing on the practical use of equations, while referring to specialized texts for detailed explanations of the equations. The second part of this presentation will describe in detail the strategies of data analysis, which is accomplished using specific empirical or theoretical models (Fig. 1). The third part will detail the methodologies for accurate data collection. Acknowledgements The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this activity. References Polymer Electrolytes: Fundamentals and Applications; Sequeira, C.; Santos, D., Eds.; Woodhead Publishing Limited: Oxford, 2010.Di Noto, V. J. Phys. Chem. B 2000, 104, 10116.Di Noto, V.; Vittadello, M.; Lavina, S.; Fauri, M.; Biscazzo, S. J. Phys. Chem. B 2001, 105, 4584.Di Noto, V. J. Phys. Chem. B 2002, 106, 11139.Di Noto, V.; Vittadello, M.; Greenbaum, S. G.; Suarez, S.; Kano, K.; Furukawa, T. J. Phys. Chem. B 2004, 108, 18832.Di Noto, V.; Giffin, G. A.; Vezzù, K.; Piga, M.; Lavina S. Broadband Dielectric Spectroscopy: A Powerful Tool for the Determination of Charge Transfer Mechanisms in Ion Conductors, in Solid State Proton Conductors: Properties and Applications in Fuel Cells, P. Knauth, M. L. Di Vona, Eds., John Wiley & Sons Weinheim, Germany, 2012.Runt, J. P.; Fitzgerald, J. J. Dielectric Spectroscopy of Polymeric Materials: Fundamentals and Applications; American Chemical Society: Washington, D.C., 1997.Schoenhals, A.; Kremer, F. Broadband Dielectric Spectroscopy; Springer-Verlag: Berlin, 2003.Di Noto, V.; Guinevere, G.A.; Vezzù, K.; Nawn, G.; Bertasi, F.; Tsai, T.-H.; Maes, A.; Seifert, S. ; Coughlin, B.; Herring, A. Interplay between solid state transitions, conductivity mechanism, and electrical relaxations in a [PVBTMA][Br]-b-PMB diblock copolymer membrane for electrochemical applications. Phys Chem Chem Phys 2015, 17, 31125-311139. Figure 1

Fabrication and characterisation of mixed oxide-covered mesh electrodes of nominal composition Ni(x)Co(1 − x)Oy supported on stainless-steel prepared by thermal decomposition using the slow cooling rate method

Mixed oxide-covered mesh electrodes (MOME) containing Co and Ni of nominal composition MOME-Ni(x)Co(1−x)Oy were prepared by thermal decomposition on a stainless-steel fine-mesh support using the slow cooling rate method in order to obtain fluid-permeable electrodes (FPE) for environmental applications in solid polymer electrolyte (SPE) cells. The surface morphology and true composition of the oxide layers were investigated using SEM and EDS techniques, respectively. The electrochemical characterisation study was carried out in electrolyte-free water using a filter-press cell in the zero-gap configuration with Nafion® 117 membrane as the SPE. SEM images revealed that the mixed oxide coatings exhibited a compact surface morphology. A very good adhesion of the oxide layers to the stainless-steel fine-mesh support was achieved after the polishing of the latter using emery paper. EDS data revealed that the Co:Ni ratios were in agreement with the nominal compositions obtained from the precursor salt solutions. The cyclic voltammetry study revealed that a reversible pseudo-capacitive behaviour can be attributed to the solid-state surface redox transitions occurring at the three-phase interfacial regions in an acidic environment. Polarisation curves obtained for the oxygen evolution reaction (OER) in electrolyte-free water revealed the overpotential is strongly affected by the Co percentage, the highest overpotentials being observed for the electrodes containing 0, 10 and 70 mol% Co. The stability tests carried out in electrolyte-free water during the OER (j=400mAcm−2 and T=24°C) revealed that the Ni(x)Co(1−x)Oy mixed oxide electrodes are more stable than the pure oxides (e.g., NiO and Co3O4). The use of an SPE resulted in good stabilisation of the Co+Ni mixed oxide electrodes during electrolysis of the electrolyte-free water.

Processing-induced-transformations (PITs) during direct compression: Impact of tablet composition and compression load on phase transition of caffeine

In the pharmaceutical field, solid-state transitions that may occur during manufacturing of pharmaceuticals are of great importance. The phase transition of a model API, caffeine Form I (CFI), was studied during direct compression process by analysing the impacts of the operating conditions (process and formulation). This work is focused on two formulation parameters: nature of the diluent and impact of the caffeine dilution, and one process parameter: the compression pressure that may impact the phase transition of CFI. Tablets were made from pure CFI and from binary mixture of CFI/diluent (microcrystalline cellulose or anhydrous dicalcium phosphate). A kinetic study performed during six months helped to highlight the influence of these parameters on the CFI transition degree. Results showed a triggering effect of the direct compression process, transformation was higher in tablets than in uncompressed powders. Whatever the pressure applied, CFI transition degree was almost constant and uniformly occurring throughout the tablet volume. Nevertheless, several differences on the evolution of the CFI transition degree were observed between binary mixtures of CFI/diluent. An analysis of the transition mechanism with a stretched exponential law of the Johnson-Mehl-Avrami model shows that tableting accelerates the polymorphic transition without modifying its mechanism controlled by nucleation only.

Ab initio molecular dynamic study of solid-state transitions of ammonium nitrate.

High-pressure polymorphism and phase transitions have wide ranging consequences on the basic properties of ammonium nitrate. However, the phase diagram of ammonium nitrate at high pressure and high temperature is still under debate. This study systematically investigates the phase transitions and structural properties of ammonium nitrate at a pressure range of 5–60 GPa and temperature range of 250–400 K by ab initio molecular dynamics simulations. Two new phases are identified: one corresponds to the experimentally observed phase IV’ and the other is named AN-X. Simultaneously, the lattice strains play a significant role in the formation and stabilization of phase IV’, providing a reasonable explanation for experimental observation of phase IV-IV’ transition which only appears under nonhydrostatic pressure. In addition, 12 O atoms neighboring the NH (N atom in ammonium cation) atom are selected as reference system to clearly display the tanglesome rotation of ammonium cation.

Spectroscopic-Based Chemometric Models for Quantifying Low Levels of Solid-State Transitions in Extended Release Theophylline Formulations

Variations in the solid state form of a pharmaceutical solid have profound impact on the product quality and clinical performance. Quantitative models that allow rapid and accurate determination of polymorphic changes in pharmaceutical products are essential in ensuring product quality throughout its lifecycle. This study reports the development and validation of chemometric models of Raman and near infrared spectroscopy (NIR) for quantifying the extent of pseudopolymorphic transitions of theophylline in extended release formulations. The chemometric models were developed using sample matrices consisting of the commonly used excipients and at the ratios in commercially available products. A combination of scatter removal (multiplicative signal correction and standard normal variate) and derivatization (Savitzky-Golay second derivative) algorithm were used for data pretreatment. Partial least squares and principal component regression models were developed and their performance assessed. Diagnostic statistics such as the root mean square error, correlation coefficient, bias and Q2 were used as parameters to test the model fit and performance. The models developed had a good fit and performance as shown by the values of the diagnostic statistics. The model diagnostic statistics were similar for MSC-SG and SNV-SG treated spectra. Similarly, PLSR and PCR models had comparable performance. Raman chemometric models were slightly better than their corresponding NIR model. The Raman and NIR chemometric models developed had good accuracy and precision as demonstrated by closeness of the predicted values for the independent observations to the actual TMO content hence the developed models can serve as useful tools in quantifying and controlling solid state transitions in extended release theophylline products.

Mechanochromism triggered fluorescent color switching among polymorphs of a natural fluorescence pigment.

A naturally occurring pigment emodin is reported here as an example to extend mechanochromic luminescent material systems. Three polymorphs, two hydrates and the amorphous state of emodin, are successfully synthesized. These solid forms exhibit significantly different fluorescence properties, which indicates its potential application in mechano-sensors and security papers.

Impact of formulation and process variables on solid-state stability of theophylline in controlled release formulations

Understanding the impact of pharmaceutical processing, formulation excipients and their interactions on the solid-state transitions of pharmaceutical solids during use and in storage is critical in ensuring consistent product performance. This study reports the effect of polymer viscosity, diluent type, granulation and granulating fluid (water and isopropanol) on the pseudopolymorphic transition of theophylline anhydrous (THA) in controlled release formulations as well as the implications of this transition on critical quality attributes of the tablets. Accordingly, 12 formulations were prepared using a full factorial screening design and monitored over a 3 month period at 40°C and 75%. Physicochemical characterization revealed a drastic drop in tablet hardness accompanied by a very significant increase in moisture content and swelling of all formulations. Spectroscopic analysis (ssNMR, Raman, NIR and PXRD) indicated conversion of THA to theophylline monohydrate (TMO) in all formulations prepared by aqueous wet granulation in as early as two weeks. Although all freshly prepared formulations contained THA, the hydration–dehydration process induced during aqueous wet granulation hastened the pseudopolymorphic conversion of theophylline during storage through a cascade of events. On the other hand, no solid state transformation was observed in directly compressed formulations and formulations in which isopropanol was employed as a granulating fluid even after the twelve weeks study period. The transition of THA to TMO resulted in a decrease in dissolution while an increase in dissolution was observed in directly compressed and IPA granulated formulation. Consequently, the impact of pseudopolymorphic transition of theophylline on dissolution in controlled release formulations may be the net result of two opposing factors: swelling and softening of the tablets which tend to favor an increase in drug dissolution and hydration of theophylline which decreases the drug dissolution.

Monitoring the Tetraethyl Orthosilicate (TEOS)-Based Sol–Gel Process with Cu(II) Ions as a Spin Probe

The various stages of the tetraethyl orthosilicate-based sol–gel process were investigated using electron paramagnetic resonance (EPR) spectroscopy with Cu(II) cations as a spin probe. The latter were introduced to the starting reaction mixture in the form of various copper(II) salts containing anions of different basicity (CuCl2, Cu(ac)2, or CuSO4). At the various defined stages of the sol–gel process, the experimental EPR spectra, recorded at both ambient and liquid nitrogen temperature, were found to be a superimposition of three main types of individual subspectra (Γ 1, Γ 2, and Φ), which reflect the different local environment in which the Cu(II) ions were located. The spin Hamiltonian parameters of each individual subspectrum remained identical, within experimental error, throughout the various stages of the sol–gel process. In contrast, the relative proportion of the individual subspectra varied significantly as the sol–gel process proceeded, from which the liquid-state to solid-state transition could be monitored as it occurred in the sol–gel reaction medium. Identical results were obtained, irrespective of the nature of the copper(II) salt employed. The results demonstrate that the EPR method provides an effective means with which to monitor the sol-to-gel transition from the viscous, colloid suspension to the final viscoelastic gel.

Mechanisms of Reversible Phase Transitions in Molecular Crystals: Case of Ciclopirox

The detailed characterization of several subambient solid state transitions occurring in the pharmaceutical ingredient Ciclopirox between −20 °C and −85 °C was performed by using a combination of DSC, cold-stage optical microscopy, vibrational spectroscopies, solid state NMR at controlled temperature, structural analyses by single crystal X-ray diffraction, and temperature-resolved X-ray powder diffraction. The global analysis of the available data reveals that the mechanisms of these reversible transitions involve a subtle compromise between phenomena related to molecular disorder, cooperative release of strains induced by cooling, and structural reorganization associated with topotactic changes in crystal lattice and symmetry. However, no major change in the main features of crystal packings is observed during the successive single crystal-to-single crystal transitions, which highlights the difficulty to classify such transitions in the frame of conventional theoretical frameworks. The successive therma...

Morphology and phase transitions of n-alkyl alcohol microcrystals

Phase transition behaviors and morphology of thin crystals of 1-heptadecanol and 1-octadecanol with sample masses from 3 μg to 1 mg were investigated by the high-sensitive DSC and AFM. The solid transition and the melting temperatures depressed with the decrease in sample mass. The phase transition behaviors were divided into three groups, the transitions were same to the bulk (more than 0.1 mg), the solid-state transition temperature depressed (from 0.1 to 0.03 mg), both the melting and the solid-state transition temperature depressed (<0.03 mg). The crystal morphologies of 0.1, 0.01, 0.005 mg sample were observed by AFM for the representative of each groups. The morphologies of crystals were classified into three groups, the large droplets with the curved shape, the middle droplets with flat and round shape and the polygonal shape. The crystal morphologies correlated with the phase transition behaviors.

The UC2−x – Carbon eutectic: A laser heating study

The UC2−x – carbon eutectic has been studied by laser heating and fast multi-wavelength pyrometry under inert atmosphere.The study has been carried out on three compositions, two of which close to the phase boundary of the UC2−x – C miscibility gap (with C/U atomic ratios 2 and 2.1), and one, more crucial, with a large excess of carbon (C/U = 2.82). The first two compositions were synthesised by arc-melting. This synthesis method could not be applied to the last composition, which was therefore completed directly by laser irradiation. The U – C – O composition of the samples was checked by using a combustion method in an ELTRA® analyser. The eutectic temperature, established to be 2737 K ± 20 K, was used as a radiance reference together with the cubic – tetragonal (α → β) solid state transition, fixed at 2050 K ± 20 K. The normal spectral emissivity of the carbon-richer compounds increases up to 0.7, whereas the value 0.53 was established for pure hypostoichiometric uranium dicarbide at the limit of the eutectic region. This increase is analysed in the light of the demixing of excess carbon, and used for the determination of the liquidus temperature (3220 K ± 50 K for UC2.82). Due to fast solid state diffusion, also fostered by the cubic – tetragonal transition, no obvious signs of a lamellar eutectic structure could be observed after quenching to room temperature. The eutectic surface C/UC2−x composition could be qualitatively, but consistently, followed during the cooling process with the help of the recorded radiance spectra. Whereas the external liquid surface is almost entirely constituted by uranium dicarbide, it gets rapidly enriched in demixed carbon upon freezing. Demixed carbon seems to quickly migrate towards the inner bulk during further cooling. At the α → β transition, uranium dicarbide covers again the almost entire external surface.

Optimizing thermoelectric property of antimony telluride nanowires by tailoring composition and crystallinity

In this work, an effect of the composition and microstructure changes on the electrical and thermoelectric properties of antimony telluride (SbxTey) nanowires was investigated to determine the optimum conditions that give the highest Seebeck coefficient. For a systematic study, a facile template-directed electrodeposition technique was utilized to prepare amorphous SbxTey nanowires with a well-controlled dimension and composition. The composition-dependent phase transition behavior of the nanowires, corresponding to the amorphous-to-crystalline solid-state transition, was investigated by measuring their electrical resistivity changes, where the electrical resistivity variation ratio (Δρ/ρ0 = (ρ − ρ0)/ρ0) was highly dependent on the Sb content. Temperature-dependent electrical properties (electrical resistivity, thermal activation energy (Ea), and field effect transistor (FET) mobility) of the amorphous and crystalline nanowires were also characterized before/after the phase transition. Material properties of the nanowires, such as crystallinity, grain size, and phase segregation as a function of the Sb content, were tailored by a post-annealing process. The tailored material properties successfully altered the electrical and thermoelectric properties of the nanowires. The highest Seebeck coefficient of 318 μV K−1 at the carrier concentration of 1.1 × 1019 cm−3 was observed in the Te-segregated Sb2Te3 (i.e., Sb32Te68) nanowires.

Formulation and Pharmacokinetic Evaluation of Polymeric Dispersions Containing Valsartan.

Valsartan exhibits poor aqueous solubility and dissolution rate limited absorption. The lower solubility in the upper part of gastrointestinal tract (pH-dependant solubility) where its absorption window exists further contributes to the low oral bioavailability of valsartan. The present work was aimed to improve the in vivo pharmacokinetics of valsartan by preparing amorphous polymeric dispersions using Eudragit E 100 as carrier. Eudragit E 100 is a cationic polymer soluble in gastric fluid up to pH 5.0 and exhibits pH-dependent release. Hence, the dispersions prepared using Eudragit E 100 rapidly dissolves at lower pH presenting drug in molecularly dispersed and soluble form at its absorption site. Polymeric solid dispersions were prepared in different drug-to-carrier ratios. The prepared dispersions were evaluated for drug-carrier interactions, solid-state transitions and drug-release properties with the help of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and in vitro dissolution studies. The optimized formulation containing valsartan was tested in rats for bioavailability and pharmacokinetic parameters and compared with that of valsartan pure drug. The results from FTIR studies indicated no interactions between drug and excipients. DSC studies confirmed reduction in crystallinity of drug. The dissolution studies performed in 0.1N HCl showed significant improvement (p<0.05) in the dissolution of valsartan. In vivo pharmacokinetic studies showed 199% relative bioavailability with significant improvement (p<0.05) in area under the curve compared to valsartan pure drug. Eudragit E 100 can be used to improve the dissolution of drugs that show low solubility at lower pH and thereby enhancing the bioavailability.

Dissolved Gases and Ice Fracturing During the Freezing of a Multicellular Organism: Lessons from Tardigrades.

Three issues are critical for successful cryopreservation of multicellular material: gases dissolved in liquid, thermal conductivity of the tissue, and localization of microstructures. Here we show that heat distribution is controlled by the gas amount dissolved in liquids and that when changing the liquid into solid, the dissolved gases either form bubbles due to the absence of space in the lattice of solids and/or are migrated toward the concentrated salt and sugar solution at the cost of amount of heat required to be removed to complete a solid-state transition. These factors affect the heat distribution in the organs to be cryopreserved. We show that the gas concentration issue controls fracturing of ice when freezing. There are volumetric changes not only when changing the liquid into solid (volume increases) but also reduction of the volume when reaching lower temperatures (volume decreases). We discuss these issues parallel with observations of the cryosurvivability of multicellular organisms, tardigrades, and discuss their analogy for cryopreservation of large organs.

Interaction and release kinetics study of hybrid polymer blend nanoparticles for pH independent controlled release of an anti-viral drug

Incompatible interactions of polymers with active chemical entity offer variable challenges in drug delivery kinetics. This study focuses on development of hybrid polymer blend nanoparticles with amphiphilic (Poly Vinyl Pyrrolidone) and hydrophobic (Ethyl cellulose and Eudragit RSPO) polymers to encapsulate maximum drug without significant incompatibility. Optimized nanoparticles developed using Acyclovir model drug exhibited 80% entrapment with size and surface charge of 100 nm and +26 mV, respectively. Spherical morphology and solid state transition of drug from crystalline to amorphous in nanoparticles was confirmed by SEM and XRD analysis. pH independent in-vitro drug release was observed in four different media with initial burst release followed by sustained release for >12 h. Statistically significant difference (P < 0.05) was observed in percentage of drug release from each formulation in different media. H-bonding and hydrophobic interactions between drug and polymer (FTIR, TG-DTA analysis and Makoid–Banakar Kinetics) and diffusion of drug from matrix nanospheres followed by pore transport were the key factors for effective drug release kinetics. Uptake of nanoparticles by corneal epithelial cells was more prominent within 30 min, however viability of cells was not altered significantly. Release kinetics and interactions studies revealed the suitability of polymer blend nanoparticles for better encapsulation and sustained release of the drug.

Interplay between solid state transitions, conductivity mechanisms, and electrical relaxations in a [PVBTMA] [Br]-b-PMB diblock copolymer membrane for electrochemical applications

Understanding the structure-property relationships and the phenomena responsible for ion conduction is one of the keys in the design of novel ionomers with improved properties. In this report, the morphology and the mechanism of ion exchange in a model anion exchange membrane (AEM), poly(vinyl benzyl trimethyl ammonium bromide)-block-poly(methylbutylene) ([PVBTMA][Br]-b-PMB), is investigated with small angle X-ray scattering, high-resolution thermogravimetry, modulated differential scanning calorimetry, dynamic mechanical analysis, and broadband electrical spectroscopy. The hyper-morphology of the material consists of hydrophilic domains characterized by stacked sides of [PVBTMA][Br] which are sandwiched between "spaghetti-like" hydrophobic cylindrical parallel domains of the PMB block. The most important interactions in the hydrophilic domains occur between the dipoles of ammonium bromide ion pairs in the side chains of adjacent chains. A reordering of the ion pair dipoles is responsible for a disorder-order transition (Tδ) at high temperature, observed here for the first time in AEMs, which results in a dramatic decrease of the ionic conductivity. The overall mechanism of long range charge transfer, deduced from a congruent picture of all of the results, involves two distinct ion conduction pathways. In these pathways, hydration and the motion of the ionic side groups are crucial to the conductivity of the AEM. Unlike the typical perfluorinated sulfonated proton-conducting polymer, the segmental motion of the backbone is negligible.