J. L. Hedrick

Chemical modification of matrix resin networks with engineering thermoplastics: 1. Synthesis, morphology, physical behaviour and toughening mechanisms of poly(arylene ether sulphone) modified epoxy networks

Abstract Bisphenol A-based epoxy resins were modified with either phenolic hydroxyl or aromatic amine functionally-terminated poly(arylene ether sulphone) oligomers and thermally cured with 4,4′ diaminodiphenyl sulphone. The resulting networks displayed significantly improved fracture toughness, with little sacrifice in modulus. The bisphenol A-based polysulphones were molecularly miscible with the epoxy precursors over the entire range of compositions and molecular weights investigated, but developed a two phase structure upon network formation. The molecular weights and composition of polysulphone chemically incorporated into the network were varied and their effect on several important physical properties was investigated. The dynamic mechanical analysis and scanning electron microscopy (SEM) studies showed that it is possible to generate a two-phase morphology in the cured networks wherein polysulphone composite particles are dispersed in the epoxy matrix. Despite the two-phase structure, the modified crosslinked systems are nearly transparent, due to a similarity in component refractive index values. The fracture toughness of these modified networks under plane strain conditions improved significantly with minimal sacrifice of the flexural modulus.

Sn(OTf)2 and Sc(OTf)3: Efficient and versatile catalysts for the controlled polymerization of lactones

A general and novel method for the controlled synthesis of aliphatic polyesters is presented. The evaluation of stannous (II) trifluoromethane sulfonate [Sn(OTf)2] and scandium (III) trifluoromethane sulfonate [Sc(OTf)3] as catalysts for the ring-opening polymerization (ROP) of various lactones is described as a route to polyesters under mild and highly selective polymerization conditions. Size exclusion chromatograms of poly(ϵ-caprolactone) initiated from ethanol in the presence of either Sn(OTf)2 or Sc(OTf)3 demonstrate the facile synthesis of narrowly dispersed products. Predictable molecular weights, typical of a living or controlled polymerization, were obtained with high yields. These catalysts are versatile and applicable toward the ROP of other cyclic (di)esters, including β-butyrolactone, which produces the synthetic analogue of the biopolymer poly(β-hydroxybutyrate).

High temperature polymer foams

Abstract A means of generating high temperature polymer foams with pore sizes in the nanometre range has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block as the matrix and a thermally labile material as the dispersed phase. Upon thermal treatment the thermally unstable block underwent thermolysis, leaving pores with a size and shape dictated by the initial copolymer morphology. Nanopore foam formation is shown for triblock copolymers composed of a poly(phenylquinoxaline) (PPQ) matrix with either poly(propylene oxide) (PO) or poly(methyl methacrylate) (PMMA) as the thermally labile coblocks. Upon decomposition of these blocks, a 10–20% reduction in density was observed, consistent with the initial PO or PMMA composition, and the resulting PPQ foams showed dielectric constants of about 2.4, substantially lower than that of PPQ (2.8). Small-angle X-ray scattering and transmission electron microscopy showed pore sizes of approximately 100 A.

Pore size distributions in nanoporous methyl silsesquioxane films as determined by small angle x-ray scattering

Small angle x-ray scattering (SAXS) measurements were performed on nanoporous methyl silsesquioxane films that were generated by the incorporation of a sacrificial polymeric component into the matrix and subsequently removed by thermolysis. The average pore radii ranged from 1 to 5 nm over a porosity range of ∼5–50%. The distribution in pore size was relatively broad and increases in breadth with porosity. The values and observations obtained by SAXS are in good agreement with field emission scanning electron microscopy.

High-temperature polyimide nanofoams for microelectronic applications

Abstract Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term ‘nanofoam’. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block, leaving pores of the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials were investigated as well as from a number of thermally labile polymers. The foams were characterized by a variety of experiments including TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and low dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

High temperature nanofoams derived from rigid and semi-rigid polyimides

A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology provides a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon a thermal treatment the thermally unstable block undergoes thermolysis, leaving pores where the size and shape are dictated by the initial copolymer morphology. Multiblock and triblock copolymers, comprising rigid, semi-rigid and flexible polyimide matrices with either poly(propylene oxide) or poly(methyl methacrylate) as the thermally labile coblocks, were prepared. The copolymer synthesis was carried out through the poly(amic alkyl ester) precursor to the polyimide since this precursor is stable and allows for isolation and characterization prior to imidization. Microphase-separated morphologies were observed for all copolymers irrespective of block type or length by both dynamic mechanical and small-angle X-ray scattering techniques. Upon decomposition of the propylene oxide or methyl methacrylate coblock, reductions in the film thickness and the total integrated scattering were found for those copolymers derived from rigid and semi-rigid polyimide matrices, thus indicating a collapse of the foam as it was being formed. Conversely, copolymers based on the flexible polyimide produced stable foams upon decomposition of the labile coblocks.

Chemically induced phase separation: a new technique for the synthesis of macroporous epoxy networks

Abstract We have developed a new technology, based on chemically induced phase separation, that allows for the synthesis of porous epoxies with a closed cell morphology and a narrow pore size distribution in the micrometre range. The potential of this technique for the synthesis of new types of porous thermosets is enlightened by the comparison with the current state of the art on technologies for the preparation of macroporous polymeric materials. The strategy of the chemically induced phase separation technique, as a general approach for the synthesis of macroporous thermosets with controlled morphology is presented. The particular system reported is a diglycidylether of bisphenol-A cured with 2,2′-bis(4-amino-cyclohexyl)-propane in the presence of hexane or cyclohexane. Depending on the hexane concentration, the morphology can be varied ranging from a monomodal to bimodal distribution. By regarding the kinetics and the development of a bimodal distribution, we surmise that the phase separation proceeds via a nucleation and growth mechanism. The influence of internal and external reaction parameters, such as chemical nature of the solvent, solvent concentration and curing temperature on the final morphology are reported. These porous materials are characterized by a significantly lower density without any loss in thermal stability compared to the neat matrix.

High temperature polymer nanofoams based on amorphous, high Tg polyimides

Abstract A means of generating high temperature polymer foams which leads to pore sizes in the nanometre regime has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block and a thermally labile material, such that the morphology consists of a matrix of the thermally stable material with the thermally labile material as the dispersed phase. Upon thermal treatment, the thermally unstable block undergoes thermolysis leaving pores of which the size and shape are dictated by the initial copolymer morphology. Triblock and graft copolymers comprising high glass transition temperature amorphous polyimide matrices with poly(propylene oxide), as the thermally decomposable coblock, were prepared. The copolymer synthesis was carried out through either the poly(amic alkyl ester) or poly(amic acid) precursor and subsequent cyclodehydration to the polyimide by either thermal or chemical means, respectively. Microphase-separated morphologies were observed for all copolymers, irrespective of the block lengths surveyed, by dynamic mechanical analysis. Upon decomposition of the thermally labile coblock, a 5–15% reduction in density was observed, consistent with the generation of a foam.

Transport properties of hyperbranched and dendrimer-like star polymers

Moisture transport properties were assessed by sorption and desorption measurements on hydroxyl-functional hyperbranched polyesters based on 2,2-bis(methylol) propionic acid (bis-MPA) as AB(2)-mono ...

Structures and dielectric properties of thin polyimide films with nano‐foam morphology

Thin polyimide films with dispersed nano‐foam morphology have been prepared for the purpose of obtaining low dielectric polymer insulators for microelectronic applications. They were obtained by utilizing micro phase‐separated triblock copolymers where the thermally stable polyimide matrix component was derived from pyromellitic dianhydride (PMDA) with 1,1‐bis(4‐aminophenyl)‐1‐phenyl‐2,2,2‐trifluoroethane (3F) and a thermally labile poly(propylene oxide)(PO) component comprised the outside block of the ABA triblock architecture. TEM studies show that the initial irregular nanoscale phase‐separated morphology of polyimide triblock copolymers are mostly maintained in the final nano‐foam films upon thermal decomposition of the dispersed PO component. The nano‐foam polyimide films exhibit significantly lower dielectric constants e′ (e.g., 2.3 at 19% porosity) as compared with e′≊2.9 for the homopolymer, as predicted by Maxwell–Garnett theory, with the nano‐pore structures remaining stable at 350 °C.