Priya Maheshwari

Free volumes and structural relaxations in diglycidyl ether of bisphenol-A based epoxy–polyether amine networks

Two types of polyether diamines were used to prepare model rubbery and glassy epoxy–amine networks with diglycidyl ether of bisphenol-A; α,ω-diamino terminated polyoxypropylene (POP) diamines and α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene)s (POP-POE-POP). The structural relaxations in the glassy and rubbery epoxy–amine networks at segmental (α relaxation) and local (β relaxation) levels were investigated by modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The characteristic length of glass transition of the networks ξ(Tg) was evaluated from MDSC using Donths thermal fluctuation approach. While the POP diamine networks showed ξ(Tg) values of 2.0 and 2.07 nm, for POP diamine molecular weights of 230 and 400, respectively, the corresponding values for POP-POE-POP diamine networks were found to be 1.41 and 1.58 nm for POP-POE-POP diamine molecular weights of 600 and 900. This implied diminishing size of the cooperatively rearranging regions with decreasing crosslink density. DMA measurements were used to evaluate the crosslink density of the networks, characteristic features of the α and β transitions in terms of the width, intensity of transitions, and activation energy of the β relaxation. The studies revealed highly cooperative sub-Tg β relaxations for the glassy networks and a truncated but pronounced β relaxation for the rubbery networks. Positron annihilation lifetime spectroscopy (PALS) was used to characterize the molecular topology of the networks in terms of the free volume nanohole sizes and their distribution. The difference of the average distance between crosslink points and the free volume nanohole size was seen to increase with the chain length of the diamines, indicating the fluctuational nature of the networks influenced by the sub Tg relaxation.

Revealing the Nano‐Level Molecular Packing in Chitosan–NiO Nanocomposite by Using Positron Annihilation Spectroscopy and Small‐Angle X‐ray Scattering

Chitosan-NiO nanocomposite (CNC) is shown to be a potential dielectric material with promising properties. CNCs containing NiO nanoparticles (0.2, 0.6, 1, 2, 5 wt %) are prepared through chemical methods. The inclusion of NiO nanoparticles in the chitosan matrix is confirmed by scanning electron microscopy (SEM) and X-ray diffraction. The morphology of the NiO nanoparticles and the nanocomposites is investigated by transmission electron microscopy and SEM, respectively. Positron annihilation lifetime spectroscopy (PALS) and the coincidence Doppler broadening (CDB) technique are used to quantify the free volume and molecular packing in the nanocomposites. The triplet-state positronium lifetime and the corresponding intensity show the changes in nanohole size, density, and size distribution as a function of NiO loading. Small-angle X-ray scattering indicates that the NiO aggregates are identical in all the CNCs. The momentum density distribution obtained from CDB measurements excludes the possibility of a contribution of vacant spaces (pores) available in NiO aggregates to the free volume of nanocomposites upon determination by using PALS. The results show systematic variation in free-volume properties and nano-level molecular packing as a function of NiO loading, which is presumed to play a vital role in determining the various properties of the nanocomposites.

Investigation of Nanoscopic Free Volume and Interfacial Interaction in an Epoxy Resin/Modified Clay Nanocomposite Using Positron Annihilation Spectroscopy

Epoxy/clay nanocomposites are synthesized using clay modified with the organic modifier N,N-dimethyl benzyl hydrogenated tallow quaternary ammonium salt (Cloisite 10A). The purpose is to investigate the influence of the clay concentration on the nanostructure, mainly on the free-volume properties and the interfacial interactions, of the epoxy/clay nanocomposite. Nanocomposites having 1, 3, 5 and 7.5 wt. % clay concentrations are prepared using the solvent-casting method. The dispersion of clay silicate layers and the morphologies of the fractured surfaces in the nanocomposites are studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The observed XRD patterns reveal an exfoliated clay structure in the nanocomposite with the lowest clay concentration (≤1 wt. %). The ortho-positronium lifetime (τ(3)), a measure of the free-volume size, as well as the fractional free volume (f(v)) are seen to decrease in the nanocomposites as compared to pristine epoxy. The intensity of free positron annihilation (I(2)), an index of the epoxy-clay interaction, decreases with the addition of clay (1 wt. %) but increases linearly at higher clay concentrations. Positron age-momentum correlation measurements are also carried out to elucidate the positron/positronium states in pristine epoxy and in the nanocomposites. The results suggest that in the case of the nanocomposite with the studied lowest clay concentration (1 wt. %), free positrons are primarily localized in the epoxy-clay interfaces, whereas at higher clay concentrations, annihilation takes place from the intercalated clay layers.

Probing molecular packing at engineered interfaces in organic field effect transistor and its correlation with charge carrier mobility.

Surface engineering of SiO2 dielectric using different self-assembled monolayer (SAM) has been carried out, and its effect on the molecular packing and growth behavior of copper phthalocyanine (CuPc) has been studied. A correlation between the growth behavior and performance of organic field effect transistors is examined. Depth profiling using positron annihilation and X-ray reflectivity techniques has been employed to characterize the interface between CuPc and the modified and/or unmodified dielectric. We observe the presence of structural defects or disorder due to disorientation of CuPc molecules on the unmodified dielectric and ordered arrangement on the modified dielectrics, consistent with the high charge carrier mobility in organic field effect transistors in the latter. The study also highlights the sensitivity of these techniques to the packing of CuPc molecules on SiO2 modified using different SAMs. Our study also signifies the sensitivity and utility of these two techniques in the characterization of buried interfaces in organic devices.

Development of NTD Ge sensors for low temperature thermometry

The development of NTD Ge sensors for use in cryogenic bolometric detector to search for neutrinoless double beta decay (Ovββ) in 124Sn is reported. The samples made from device grade Ge wafers are irradiated with thermal neutrons at Dhruva reactor, Bhabha Atomic Research Centre (BARC), Mumbai. The carrier concentration in irradiated Ge samples is estimated by Hall effect measurement at 77K. The fast neutron induced defects are studied using Positron Annihilation Lifetime Spectroscopy and Channeling. It is found that vacuum annealing of the samples at 600°C for 2 hours is necessary to cure the defects. Sensors are made from annealed NTD samples using Au-Ge Ohmic contact. Preliminary measurements have shown a significantly large dR/dT ~ 2.3 kΩ/mK at 100 mK. Details of these measurements are presented.

Freezing of Nanodroplets: Phase Transition of Organic Liquids Confined in Nanopores Studied by Positron Annihilation Spectroscopy

Behavior of solids and liquids confined in nano domain has great relevance in fundamental research as well as applications in nanotribology, nanofabrication, membrane separation, interfacial adhesion and lubrication. In this work, we have studied phase transition behavior of different organic liquids confined in nanopores of ZSM-5 zeolite and silica using temperature dependent positron annihilation Doppler broadening and lifetime spectroscopy. It is observed that the freezing and melting properties of liquids confined in nanopores are different from their bulk behavior. The liquid molecules (isopropanol) that feel more attractive interaction with the pore wall show an increase in freezing temperature. Similarly, a liquid like benzene which is weakly attractive with pore wall as compared to its solid phase show a decrease in its freezing point under confinement. The confining pore diameters are in the range of 10-60 Å. It was observed that the shift in phase transition temperature does not follow classical Gibbs-Thomson relation.

Phase Transition of Water Confined in Saponites Using Positron Annihilation Spectroscopy

The temperature dependent Positron lifetime and Doppler broadening spectroscopy have been carried out to study the phase transition of water confined in interlamellar spaces of saponite clay in the temperature range 300K-200K. The change in slope/discontinuity in S-parameter and Ps lifetime as a function of temperature is an indication of phase transition. Two phase transition temperatures viz. above and below the bulk freezing temperature have been observed for water confined in saponite clay. This indicates the presence of two regimes of water confined in interlamellar spaces. The freezing-melting cycle is also marked by significant hysteresis.

Depth Profile of Chemical Composition and Free Volume of Polyurethane-Urea/Clay Nanocomposite

Depth profile of subsurface chemical composition and free volume in segmented polyurethane-urea/clay nanocomposites was studied by angle resolved X-ray photoelectron spectroscopy (ARXPS) and Doppler broadening energy spectroscopy (DBES) using slow positron beam. The ARXPS studies revealed increasing N/C atomic ratio (hard segment to soft segment ratio) at any given depth for the clay loaded samples compared to the neat polymer. DBES study revealed significant microstructure modification with clay loading. Self segregation of hard and soft segments in neat polymer and their interspersing with clay loading was observed from DBES measurements.

Microstructural studies of poly (perfluorosulfonic acid) membrane doped with silver nanoparticles using positron annihilation spectroscopy

The Nafion -117 (H+ form) was converted to Ag+ form by ion exchange. The silver nanoparticles were formed in the membrane by reduction of Ag+ (of Ag+ form membrane) using an ionic reducing agent, NaBH4, and a neutral reducing agent, formamide. The free volume hole sizes in the nanoparticle doped and undoped Nafion were measured by positron annihilation lifetime spectroscopy (PALS). The surface modifications in the membrane by incorporation of nanoparticles were investigated using slow positron beam. The changes in the bulk free volume of Nafion with presence of nanoparticles were found to be nominal. In contrast, the surface microstructure appears to have undergone significant change.

Evidence for confinement induced phase separation in ethanol–water mixture: a positron annihilation study

We report an experimental evidence for the phase separation of ethanol-water mixture confined in mesoporous silica with different pore size using positron annihilation lifetime spectroscopy (PALS). A bulk-like liquid in the core of the pore and a distinct interfacial region near the pore surface have been identified based on ortho-positronium lifetime components. The lifetime corresponding to the core liquid shows similar behavior to the bulk liquid mixture while the interfacial lifetime shows an abrupt rise within a particular range of ethanol concentration depending on the pore size. This abrupt increase is attributed to the appearance of excess free-volume near the interfacial region. The excess free-volume is originated due to microphase separation of confined ethanol-water primarily at the vicinity of the pore wall. We envisage that probing free-volume changes at the interface using PALS is a sensitive way to investigate microphase separation under nanoconfinement.