Yiwang Chen

Ultra-low-κ materials based on nanoporous fluorinated polyimide with well-defined pores via the RAFT-moderated graft polymerization process

Thermally-initiated living radical graft polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ozone-pretreated fluorinated polyimide (FPI) via the reversible addition–fragmentation chain transfer (RAFT)–moderated process was carried out. The chemical composition and structure of the copolymers were characterized by nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and molecular weight measurements. The “living” character of the grafted PEGMA side chains was ascertained in the subsequent extension of the PEGMA side chains. Nanoporous ultra-low dielectric constant (κ) fluorinated polyimide films were prepared by solution casting of the graft copolymers, followed by thermal decomposition of the labile PEGMA side chains in air. The nanoporous FPI films obtained from the RAFT-moderated graft copolymers had well-preserved FPI backbones, porosity in the range of 2–10% and pore size in the range of 20–50 nm. These films also had more well-defined pores, more uniform pore size distribution, and better-retained mechanical properties than those of the corresponding nanoporous FPI films obtained from the graft copolymers prepared by the conventional free-radical process. Dielectric constants approaching 2.0 were obtained for the nanoporous FPI films prepared from the RAFT-moderated graft copolymers.

Thermotropic aromatic/lactide copolyesters with solubilizing side chains on aromatic rings

Liquid-crystalline polyesters with solubilizing side chains on aromatic rings have been synthesized by melt polycondensation of diacid dichlorides and silylated hydroquinones. Corresponding copolyesters incorporated with lactide units were obtained by addition of oligolactides. The molecular structures of these polymers were verified by spectroscopy techniques. Molecular weights were characterized by gel permeation chromatography. X-ray diffraction studies confirmed that all polyesters are partially crystalline and have self-organized ordered stacking structures. The polyesters can form smectic melts upon heating above the melting temperature. Contact angle measurements and water absorption data of solution cast films showed that the polyesters have relatively high hydrophilicity and decreasing contact angle with increasing amount of lactide moieties.

Antimicrobial Hydantoin-grafted Poly(ε-caprolactone) by Ring-opening Polymerization and Click Chemistry

Novel degradable and antibacterial polycaprolactone-based polymers are reported in this work. The polyesters with pendent propargyl groups are successfully prepared by ring-opening polymerization and subsequently used to graft antibacterial hydantoin moieties via click chemistry by a copper(I)-catalyzed azide-alkyne cycloaddition reaction. The well-controlled chemical structures of the grafted copolymers and its precursors are verified by FT-IR spectroscopy, NMR spectroscopy, and GPC characterizations. According to the DSC and XRD results, the polymorphisms of these grafted copolymers are mostly changed from semicrystalline to amorphous depending on the amount of grafted hydantoin. Antibacterial assays are carried out with Bacillus subtilis and two strains of Escherichia coli and show fast antibacterial action.

Antimicrobial Hydantoin-Containing Polyesters

A new N-hydantoin-containing biocompatible and enzymatically degradable polyester with antibacterial properties is presented. Different polyesters of dimethyl succinate, 1,4-butanediol, and 3-[N,N-di(β-hydroxyethyl)aminoethyl]-5,5-dimethylhydantoin in varying molar ratios are prepared via two-step melt polycondensation. The antibacterially active N-halamine form is obtained by subsequent chlorination of the polyesters with sodium hypochlorite. Chemical structures, thermal properties, and spherulitic morphologies of the copolymers are studied adopting FT-IR, NMR, TGA, DSC, WAXD, and POM. The polyesters exhibit antibacterial activity against Escherichia coli. The adopted synthetic approach can be transferred to other polyesters in a straightforward manner.

Semi-Empirical VIV Analysis of Full-Scale Deepwater Composite Risers

A global-local analysis methodology based on fluid-structure coupling is used to investigate the mechanical responses of both composite and steel risers. Since the design of the riser system can be a daunting task, involving hundreds of load cases for global analysis, semi-empirical fluid load models are considered for the reduced order computations of full-scale riser models. The structural performance of composite risers under real sea current conditions is investigated systematically and discussed with regard to the practical concerns in full-scale settings. The failure envelops of internal liners are found to be within that of the composite layers, which reveals that the liner is the weakest link for composite riser design. Results show that the composite risers can be more prone to vortex-induced-vibration (VIV) due to their lower structural frequencies. In the present study, the composite riser yields 25.5% higher RMS strains than the steel riser. Placement of buoyancy modules along the riser may be critical for the design against VIV, and our results show that the modules are not recommended at the top region of the riser, especially if a top-sheared current is expected. Instead, it is preferable to implement them at the bottom-half portion of the riser and as a continuously buoyed region rather than short discrete buoys separated with gap spaces.© 2014 ASME