Garth L. Wilkes

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.

Structure-property behaviour of hybrid materials incorporating tetraethoxysilane with multifunctional poly(tetramethylene oxide)

Abstract Polytetramethylene oxide based polyurethane oligomers with multiple triethoxysilane groups have been successfully incorporated into a TEOS based silicate network. It is observed that the incorporation of these oligomeric compounds was enhanced by the presence of additional functional groups along their backbone. The materials produced showed significant improvement in both tensile modulus and strength over previous organic-inorganic hybrid systems. As the number of functional groups per molecule increased, the stiffness of the material increased and the oligomeric T g shifted to higher temperatures. Also, the SAXS correlation distance decreased systematically. A schematic model suggested previously can still be used to interpret these results. Other effects of TEOS content and ageing are also reported and discussed.

Some investigations on the fiber formation by utilizing a side-by-side bicomponent electrospinning approach

Abstract Simultaneous electrospinning of two polymer solutions in a side-by-side fashion is conducted for two bicomponent polymer systems—poly (vinyl chloride)/segmented polyurethane (PVC/Estane®) and poly(vinyl chloride)/poly(vinylidiene fluoride) (PVC/PVDF). The new experimental device is described and suitable process conditions to electrospin bicomponent fibers are described. Field emission scanning electron microscopy and energy dispersive spectroscopy were utilized to interpret the results regarding local structure and chemical composition, respectively.

Solid-state 29Si NMR of TEOS-based multifunctional sol-gel materials

Abstract Solid-state 29 Si NMR experiments were carried out on multifunctional TEOS-based sol-gel materials to determine the types of silicate structures formed as well as the degree of reaction attained under previously established acid-catalyzed conditions. The degree of reaction attained and the final functionality of the gels were found to correlate with a flexible/brittle transition which was observed to be a function of the gel composition. Both FT/MAS and CP/MAS experiments were conducted which allowed a comparison of the two methods in terms of their quantitative results. It was found that the CP/MAS experiment run under the proper conditions provides (with the exception of pure TEOS gels) quantitatively similar spectra to those obtained by FT/MAS experiments.

Chemical modification of matrix Resin networks with engineering thermoplastics

SummaryFunctionally terminated bisphenol-A polysulfone oligomers were used in the modification of Epon Resin 828/4,4′-diamino-diphenylsulfone (DDS) network system. Phenolic hydroxyl terminated PSF oligomers were first capped with a large excess of bisphenol-A diglycidyl ether or Epon Resin 828 at both ends and then the resulting system was cured with DDS, in a two-step process. During these studies molecular weight and the amount of PSF oligomers incorporated into the network were varied and their effect on the overall properties of the resulting systems were investigated. The capping and curing reactions were followed by using FT-IR and NMR spectroscopy, GPC, HPLC and DSC techniques. As a function of the oligomer molecular weight, SEM studies showed the formation of two-phase structures with ductile PSF particles dispersed in the continuous epoxy matrix. Mechanical characterization and fracture toughness measurements showed a remarkable increase in KIC or gIC values of the modified networks over that of control, without significant loss in the modulus. This work would appear to be one of the first studies where well bonded ductile glassy modifiers have significantly improved the fracture toughness of highly crosslinked networks.

Ceramers: Hybrid materials incorporating polymeric/oligomeric species with inorganic glasses by a sol-gel process

A sol-gel process has been successfully utilized to produce hybrid materials incorporating polymeric/oligomeric components of polydimethyl siloxane(PDMS) with silicon glasses. All the samples made were transparent and flexible. Dynamic mechanical studies indicate that a portion of the siloxane species is phase-separated while the remainder is well dispersed. The effect of acid content were proven to be significant on the dispersion of the siloxane components and on the structure and properties of final products.

Non-isothermal crystallization kinetics of poly(p-phenylene sulphide)

Abstract The non-isothermal crystallization kinetics of poly( p -phenylene sulphide) (PPS) were studied. Linear PPS of selected molecular weights and branched PPS were investigated. The theory of Ozawa for non-isothermal crystallization was used to analyse the differential scanning calorimetry data. All the samples studied followed this theory and allowed the calculation of the Avrami exponent under non-isothermal conditions. The values of the Avrami exponent determined by the Ozawa equation were in good agreement with our earlier reported values obtained by means of isothermal methods.

Crystallization kinetics of poly(p-phenylene sulphide): effect of molecular weight

Abstract The crystallization behaviour of poly( p -phenylene sulphide) (PPS) has been studied in terms of linear crystal growth rates and overall rates of bulk crystallization as functions of molecular weight and temperature. In addition, nucleation densities were estimated for PPS crystallized from the melt. The overall rate of bulk crystallization was described by the Avrami equation. In the range of molecular weights studied (24 000–63 000), crystal growth rates and overall rates of bulk crystallization decreased as the molecular weight increased. However, the effect was not particularly large. The estimated nucleation densities indicated a decrease by a factor of 32 as the molecular weight decreased. The linear crystal growth rate data were analysed in terms of several proposed models. The data seemed to conform very well to an ‘inverse’ logarithmic function of the number-average molecular weight recently proposed by Cheng and Wunderlich.

Solid state structure–property behavior of semicrystalline poly(ether-block-amide) PEBAX® thermoplastic elastomers

Abstract The solid state structure–property behavior was investigated of a series of poly(ether- block -amide) PEBAX ® thermoplastic elastomers based on nylon 12 and poly(tetramethylene oxide) with varying hard segment content. Particular emphasis was placed on better defining the morphological features of this entire series of commercially available materials. Compression molded and solution cast samples were studied by the techniques of DMA, DSC, WAXS, SAXS, AFM, SALS and stress–strain response. The strain-induced crystallization behavior of the soft polyether (PE) segments was also investigated. All samples exhibited a microphase separated morphology over a broad temperature range. As expected, an increase in the interconnectivity of the polyamide hard phase was greatly controlled by the polyamide (PA) content. Due to the crystallization of the PA hard segment, the formation of PA lamellar crystals was noted in both melt and solution cast films. At the higher PA contents, a distinct spherulitic superstructure was also observed but this form of morphological texture was diminished as the PE soft segment content increased. Limited studies of the deformation/recovery behavior of the spherulitic superstructure provided further information concerning the interaction between the hard and soft segments.

Segmented organosiloxane copolymers: 2 Thermal and mechanical properties of siloxane—urea copolymers

Abstract The structure-property behaviour of new siloxane-urea containing segmented copolymers has been investigated. Amino-propyl terminated poly(dimethylsiloxane) oligomers of from 900–3660 〈 M n 〉 were reacted with various diisocynates to form segmented copolymers with urea linkages. The length of the hard segments in these copolymers corresponds approximately to the length of the diisocynate unit employed. A number of mechanical and thermal properties were investigated for these phase separated materials. It was found that the performance of these copolymers was effected by varying the hard segment type and/or content and that high strength necessitates a microphase texture. The two phase nature of these copolymers was verified by dynamic mechanical, thermal and SAXS studies. The phase separation was found to occur in these copolymers even with 6% hard segment by weight. In conclusion, these materials displayed a behaviour similar to the segmented polyurethanes and were found to be superior to the unfilled silicone elastomers.