Morphology and mechanical response of S–B star block copolymer – Layered silicate nanocomposites

The influence on the mechanical response by incorporation of oligostyrene-modified montmorillonite (MMT) and oligostyrene-modified bentonite (BET) into star shaped styrene–butadiene block copolymer has been investigated. The modified silicates are highly intercalated with a gallery distance of more than 9 nm and partly exfoliated. The array of tactoids consisting of 1–8 layers showing uniform state of distribution as revealed from TEM. The layers of the modified MMT and BET are observed to undergo nano-confinement, i.e. restricted to the PS-domains of the styrene–butadiene star block copolymer with impinging/bridging effects of silicate layers through the SB soft phase. DMA studies have showed an appreciable shift of the glass transition temperatures of PB- and PS-rich phases towards higher temperatures in addition to an increase of the storage modulus due to nanoclay reinforcement. Generally the Young’s modulus and yield stress was strongly increased with the incorporation of modified nanoparticles whereas at the same time the strain at break reduces slightly. The elastic–plastic hysteresis–stress and the hysteresis–work are largely increased due to effective interfacial effect; an effect that is largely attributed to the presence of highly intercalated and partially exfoliated silicate layers. The extent of increase was more in the modified MMT than in the modified BET based nanocomposites. The stress-decay and the strain-recovery aspects have also been critically analyzed in relation to their micro-structural attributes. Our study fundamentally demonstrates two critical aspects related to mechanical properties and particularly with regard to elastic–plastic hysteresis response. Firstly, partial confinement of the silicate layers is promoted by PS-aided surface modification facilitating enhancement in mechanical properties and secondly, the nano-confinement of modified MMT seems to be more effective in improving the hysteresis performance when compared to BET with higher charge density.     

Anomalous amorphous SiO2 films

Anomalous SiO₂ films have been prepared by sputtering Si in a mixture of Ar-10% O₂ at 77 K. The same sputtering conditions at room temperature yield normal SiO₂ which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO₂ is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO₂ is only slightly different from bulk SiO₂ but esr experiments reveal about 3 × 10¹⁸ spins cm which do not exist in bulk SiO₂. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10⁵) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO₂ films consist of essentially bulk like SiO₂ clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties.