Anubhav Saxena

Switching in molecular shapes: main chain length driven rod-circle transition of isolated helical polysilanes

Unique conformations such as rod, semicircle, and circle structures of isolated semi-flexible helical polysilanes were observed by atomic force microscopy (AFM); the chain topology was significantly related to the chain length (molecular weight) on the surfaces.

Cooperative C–F⋯Si interaction in optically active helical polysilanes

Cooperative amplification of the C-F...Si weak interaction between side chains and the main chain was found to afford rigid rodlike helical polysilanes with a preferential screw sense.

First observation of a smectic A–cholesteric phase transition in a thermotropic liquid crystal consisting of a rigid-rod helical polysilane

A smectic A–cholesteric phase transition for a rigid-rod helical polymer, poly[n-decyl-(S)-2-methylbutylsilane] (PD2MBS), with a narrow molecular weight distribution, has been observed for the first time. Polarizing optical microscopy showed that the fan-shaped texture of the smectic A phase turned into the characteristic planar texture of the cholesteric phase upon heating. The positive CD band, which corresponds to the reflection band of the cholesteric phase, gradually decreased in intensity within a range 30°C below the transition temperature on cooling, while the peak maximum shifted towards shorter wavelengths. It was concluded that the system has a very wide temperature region over which the cholesteric–smectic A phase transition occurs and in which the cholesteric pitch varies with temperature.

Ambidextrous optically active copper(II) phthalocyanine supramolecules induced by peripheral group homochirality

Self-assembling features of the disk-shaped copper(II) phthalocyanine (Pc) molecules carrying four enantiopure (S)- and (R)-1-(p-tolyl)ethylaminocarbonyl groups at their peripheral positions, CuPc-(S) and CuPc-(R), were studied. For comparison, an analogous CuPc carrying racemic 1-(p-tolyl)ethylaminocarbonyl groups, CuPc-(RS), was used. From circular dichroism and visible/infrared spectroscopic studies with wide-angle-X-ray diffraction data, CuPc-(S) and CuPc-(R) were shown to generate optically active π−π stacked assemblies formed in less-polar solvents. CuPc-(RS) produced similar assemblies with a loss of optical activity in the less-polar solvents. Plural interactions of Pcπ/Pcπ stacking, quadruple hydrogen bonding, homochiral stereocenters at the peripheral groups, bulky aralkyl entities and weak coordinating nature of copper(II) of the CuPcs were thought to be responsible for the formation of the optically active CuPc assemblies. The solvent effect was evident: less polar chloroform and tetrahydrofuran helped to generate the optically active CuPc assembly while highly polar N,N-dimethylformamide gave a monomeric Pc species due to the loss of Pc–Pc intermolecular hydrogen-bonding interaction. The self-assembling ability of the CuPcs was similar to NiPc analogues carrying the same chiral peripherals reported previously. Atomic force microscope study revealed that by casting dilute chloroform solutions of CuPc-(S), CuPc-(R) and CuPc-(RS) onto mica surface, worm-like supramolecular polymers were observed while the NiPc-(R) analogue provided needle-like supramolecular polymers.

Room-temperature one-step immobilization of rod-like helical polymer onto hydrophilic substrates

A facile, one-pot immobilization method for a rigid rod-like helical polysilane, poly(n-decyl-i-butylsilane), was developed onto hydrophilic surfaces at room temperature in the presence of triethylamine as a catalyst.

Small-Angle X-Ray Analysis of Smectic a Cholesteric Liquid Crystal Phase Transition in Rigid-Rod Helical Polysilane

The smectic A Cholesteric liquid crystal phase transition of rigid-rod helical polysilane was studied by means of synchrotron radiation small-angle X-ray scattering (SR-SAXS). The information on the smectic trans-layer electron density distribution was clarified by fitting the model of electron density distribution to the diffraction data. Each of the layers was approximated by the boxlike function to introduce the molecular form factor of the polymer. The distribution of the molecular center of gravity within the layers due to the thermal motion was taken into consideration by convolution with the Gaussian function. The elucidated smectic layer structure was not like the conventional structure with their ends packed into the interlayer space. The layers with bumpy surfaces interdigitate in a key-and-keyhole manner to pack the molecules with different lengths into layers due to their finite molecular weight distribution.

Air-stable poly(3,3,3-trifluoropropylsilyne) homo- and copolymers

A soluble Si–Si bonded polymer (polysilyne, SNP) bearing n-butyl groups (BSNP) has been reported to exhibit a green photoluminescence (PL) at 540 nm (2.30 eV). However, BSNP gradually changed to a colorless solid because of auto-oxidation within a few weeks when the polymer was stored in the absence of light without special care. Herein, we demonstrate the photophysical properties of SNP carrying 3,3,3-trifluoropropyl (TFP) side groups (FSNP), which remains unaffected by auto-oxidation. This stability against oxidation may be possibly attributable to intra- and inter-molecular CF–Si interactions between the electron-donating Si–Si main chain and the electron withdrawing TFP side groups. FSNP in polar tetrahydrofuran (THF) exhibited an almost pure-blue PL peak at 450 nm (2.76 eV), whereas in non-polar n-octane, it emitted a near-UV peak at 337 nm (3.68 eV), possibly, because of the CF–Si interactions. Additionally, thin films of FSNP exposed to air and in THF exhibited excellent resistance to air oxidation for at least one month, as determined by the lack of any changes in its PL, IR, and Raman spectra. From Gaussian03 calculations (TD-DFT, 6-31G* basis set, B3LYP) of trans-perhydrosiladecaline partially substituted with TFP and n-butyl groups as models of FSNP and BSNP, the most essential roles of TFP groups suggested that (i) the lowest Siσ–Siσ* transition state becomes the allowed transition by introducing strong polarity to the SNP skeleton and (ii) both the HOMO and LUMO energy levels are significantly stabilized, which provides the observed stability toward air oxidation. The air stability was effective in copolymers carrying TFP and n-butyl groups (FBSNP). However, FSNP and FBSNP that had been pyrolysed at temperatures greater than 500 °C exhibited no such air stability because of the loss of the TFP groups.

Liquid Silicone Rubber Vulcanizates: Network Structure - Property Relationship and Cure Kinetics

Liquid silicone rubber (LSR) vulcanizates have been prepared from vinyl-end capped polydimethylsiloxane (PDMS) and polymethylhydrogenosiloxane (PMHS) in presence of platinum catalyst using solution casting technique. The science involving synthesis of usable silicone rubber by curing with polymethylhydrogenosiloxane has been studied in details. The effects of vinyl content of PDMS and the ratio of hydride crosslinker to vinyl concentration on crosslink density, mechanical, dynamic-mechanical and thermal properties have been discussed. It has been found that at optimum level of crosslinker, the LSR vulcanizate shows a maximum crosslink density with highest improvement in mechanical and thermal properties. The amount of crosslinker required, depends upon the number of vinyl functionality per unit length of the polymer chain and the chemical composition of the hydride crosslinker. This study makes a novel attempt to determine cure kinetics of PDMS by using Fourier Transform Infrared spectroscopy. Measurements have been done at three different temperatures and found to follow the first order reaction with respect to consumption of vinyl (C=C) and silyl hydride groups (Si-H). The activation energy and kinetic parameters have been determined as well.

Rubber nanocomposites with polyhedral oligomeric silsesquioxanes (POSS) as the nanofiller

Abstract This chapter provides a comprehensive review on the role of polyhedral oligomeric silsesquioxanes (POSS) as reinforcing fillers for polymer and rubber nanocomposites. POSS, having an Si O Si framework, provides the high thermal stability, while the peripheral organic functionality offers the better polymer–filler interface adhesion. The peripheral organic functionalities of POSS were reported to play a key role in controlling the POSS dispersion in fumed silica based addition cured liquid silicone rubber. The radical grafting of organo-functionalized POSS was reported to reinforce the ethylene propylene diene monomer (EPDM) based rubbers, whilst causing a detrimental effect on radically curable polysiloxane elastomers. On the other hand, the matrix–filler incompatibility and the preferential crystallization of POSS segments were effectively leveraged as tunable linkages to fabricate thermoplastic elastomeric materials. The use of POSS based molecular units were also explored for developing functional structures like radiation protective coatings, nanoporous organic–inorganic frameworks for gas storage, or in low dielectric composites.