Hiren V. Shah

Perfluorocyclobutyl Copolymers for Microphotonics

The copolymerization of aryl bis- and tris-trifluorovinyl other monomers aromatic perfluorocyclobutyl (PFCB) polymers, via thermally initiated stepgrowth cycloaddition chemistry. PFCB polymers and their copolymers enjoya unique combination of attributes well suited for applications in photonic technologies, such as broad tailorability of refractive indices and thermo-optic coefficients, low transmission losses as 1300 and 1550 nm, high thermal, mechanical, and optical stability, and excellent melt and solution processability. Planar PFCB structures lif can be processed by direct micro-transfer molding, which is a first step towards rapid soft-lithographic fabrication of polymer planar lightwave circuits. Copolymerization chemistry and processing parameters and characterization, including thermal (T g ≃120-350°C) and optical properties trefractive indices from 1.443 to 1.308 at 1550 nm, thermooptic coefficients dn/dT-7×10 - 5 K - 1 to -1.5×10 - 6 K - 1 ), birefringence (< 0.003), and temporal stability of refractive index, are described.

Perfluorocyclobutane (PFCB) polyaryl ethers: versatile coatings materials

Abstract The cyclopolymerization of aromatic trifluorovinyl ether (TFVE) monomers offers a versatile route to a unique class of linear and network fluoropolymers containing the perfluorocyclobutyl (PFCB) linkage. Polymerization proceeds by a thermal — radical mediated — step-growth mechanism and provides well-defined polymers containing known fluoroolefin end groups. PFCB polymers combine the engineering thermoplastic nature of polyaryl ethers with fluorocarbon segments and exhibit excellent processability, optical transparency, high temperature performance, and low dielectric constants. An intermediate strategy utilizing Grignard and aryllithium reagents has been developed which offers access to a wide variety of hybrid materials amenable to coatings applications. Liquid crystalline examples have recently been achieved in addition to tailoring optical properties by co-polymerization.

Bergman cyclopolymerization kinetics of bis-ortho-diynylarenes to polynaphthalene networks. A comparison of calorimetric methods

Bis-ortho-diynylarene (BODA) monomers undergo radical mediated Bergman-type cycloaddition polymerization to yield polynaphthalene networks. This report describes the use of conventional and modulated temperature differential scanning calorimetry to compute the cure kinetic parameters of BODA monomers with different spacer groups. Three different kinetic methods employed here utilize non-isothermal dynamic thermal profiles to estimate activation energiesOEaa 28:7‐33:5 kcal=mol [120‐140 kJ/mol]) and the first order rate constantsOk , 10 25 s 21 at 2108C) of polymerization. The results obtained from these methods show surprisingly good mutual agreement and also reveal that varying the spacer group has marginal effect on the cure kinetics. It has also been shown that the reaction kinetic information obtained from the dynamic methods must be corrected, when applied under isothermal conditions, to account for sample vitrification. The correction factor has been estimated using modulated temperature differential scanning calorimetry which is capable of monitoring the sample heat capacity in real time. q 2000 Elsevier Science Ltd. All rights reserved.

Substituent effects on Bergman cyclopolymerization kinetics of bis-ortho-diynylarene (BODA) monomers by dynamic scanning calorimetry

Abstract Bis- ortho -diynylarene (BODA) monomers polymerize thermally via the Bergman cyclization and diradical intermediates. The structure reactivity relationships of eight BODA monomers and their kinetics were studied by differential scanning calorimetry (DSC). The spacer (X=C(CF 3 ) 2 , C(CH 3 ) 2 , O, a bond) groups and alkyne terminal (R=Ph, Si(CH 3 ) 3 ,  iso -propanol, pyridine, thiophene) groups were varied and the effect on polymerization rate was determined. Among the phenyl-terminated monomers, only the isopropylidene spacer group imparts a significant increase on the cure onset temperature and the activation energy. However, the terminal (R) groups play a significant role in polymerization rate of BODA monomers.

Processable high-carbon-yielding polymer for micro- and nanofabrication

Bis-ortho-Diynyl Arene (BODA) monomers polymerize to network polynapthalene by the thermally-driven Bergman cyclization and subsequent radical polymerization via oligomeric intermediates that can be melt or solution processed. Further heating of the network to 1000 °C affords a high-yield glassy carbon structure that retains the approximate size and dimensions of the polymer precursor. The higher carbon-yield for BODA networks (75- 80 % by mass) is significantly greater than that of traditional phenol-formaldehyde resins and other carbon precursor polymers leading to its greater dimensional stability. Phenyl terminated BODA derived polymers were fabricated using microprocessing such as the micromolding in capillaries (MIMIC) technique, direct microtransfer molding, and molding in quartz capillary tubes. Nano-scale fabrication using closed packed silica spheres as templates was demonstrated with an hydroxy-terminated monomer which exhibits greatly enhanced compatibility for silica surfaces. After pyrolysis to glassy carbon, the silica is chemically etched leaving an inverse carbon opal photonic crystal which is electrically conductive. The wavelength of light diffracted is a function of the average refractive index of the carbon/ filler composite, which can be modified for use as sensitive detector elements.