M. Patri

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.

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.

Sequential interpenetrating polymer network based on nitrile–phenolic blend and poly(alkyl methacrylate)

Interpenetrating polymer networks (IPNs) based on a nitrile rubber (NBR)–phenolic resin (PH) blend and poly(alkyl methacrylates) were synthesized by a sequential method. The cured blends were swollen in a methacrylate monomer containing a crosslinker and initiator. The swollen rubber sheets were cured at 60°C. From the swelling study of the monomer, it was found that IPN formation in the blend is in between the rubber and poly(alkyl methacrylate) phases only. The IPNs thus formed were characterized for their tensile, dynamic mechanical, and solvent-resistance characteristics. The tensile strength of the IPNs are dependent on the PH content; at a lower content of PH (up to 20 parts), IPNs have a higher strength compared to their corresponding blends, whereas at a higher content of PH (beyond 30 parts), the strength decreases. But for every NBR/PH-fixed composition, the strength of IPNs was found to be increasing in the order of PBuMA < PEMA < PMMA. The dynamic property results showed that NBR/PH blends are incompatible. The storage modulus of IPNs are always higher than their corresponding blends at all temperatures. The tan δ peaks of IPNs are broad, indicating the presence of microphase-separated domains. The IPNs show superior solvent-resistance characteristics compared to the blends.

Studies on IPNs based on nitrile rubber and polyalkyl methacrylates

Sequential interpenetrating polymer networks (IPNs) based on nitrile rubber and various types of polyalkyl methacrylates such as poly(n-butyl methacrylate), poly(ethyl methacrylate), and poly(methyl methacrylate) were synthesized. The compositions of the IPNs could be varied by varying the reaction parameters such as swelling time and concentration of crosslinker. The tensile properties of the IPNs show that with increase in bulkiness of the ester group of the acrylates the tensile strength decreases, whereas elongation at break increases because of decreased stiffness of the acrylate phase. The dynamic modulus and loss tangent of the IPNs also show similar trend because of the above reason. All the IPNs were also tested for dynamic properties under multifrequency mode, and with the help of the WLF equation, the behavior of these IPNs in the frequency range of 1-10 5 Hz were evaluated. The results showed reasonably high tan δ with good storage modulus in the entire frequency range for all the IPNs.

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.

Structural Investigations of Polyurethane‐Urea/Clay Nanocomposites

Structural features of polyurethane‐urea (PUU)/organoclay nanocomposites have been investigated by FTIR and small angle x‐ray scattering techniques. It is found that the clay has significant influence on morphology of the hard segments (HS) of the PUU. The HS domains which have rough surface, becomes more compact with mass fractal morphology with increase of clay content. FTIR suggests the clay interacts with PUU through hydrogen bonding network in HS domains leading to improvement of the mechanical properties.