D.R.G. Williams

Water in methacrylates—I. Sorption and desorption properties of poly(2-hydroxyethyl methacrylate-co-glycol dimethacrylate) networks

Sorption and desorption kinetics of water in copolymers of 2-hydroxyethyl methacrylate with ethylene glycol dimethacrylate and oligo (ethylene glycol) dimethacrylates are reported and analysed in terms of Ficks and Frischs laws. Sorption kinetics range from Fickian to Alfrey Case II with the Frisch parameter n ranging from 0.5–0.9 In desorption, n was less than 0.5 in non-Fickian cases. Diffusivities calculated according to Ficks law are reported. The results are consistent with the dual mode sorption mechanism, i.e. penetration into voids or low density domains followed by redistribution as the high density domains soften and relax.

Properties of dimethacrylate copolymers of varying crosslink density

Abstract Homopolymers and copolymers of tetrafunctional oligo-ethylene dimethacrylate monomers of widely varying chain lengths are investigated by thermal and fracture techniques. Differential scanning calorimetry of catalysed monomer shows that the polymerization process is inhomogeneous. Quantitative analysis of these results suggest that the mechanism of cure is complex and may be ascribed to complex copolymerization, partial phase separation or a form of interpenetrating network. Dynamic mechanical spectra of the copolymers reveals that their mechanical properties vary monotonically between those of the constituent homopolymers. In some systems, double-peak behaviour is noted in the glass transition region. This is ascribed to possible phase separation. Short rod fracture, a fracture toughness technique suitable for glassy polymers, is also used on a copolymer system to obtain information about the copolymer morphology. The transition from ‘stick-slip’ to a smoother fracture mode with higher concentrations of the longer, flexible monomer is accompanied by a concomitant decrease in fracture toughness. This behaviour is explained in terms of the inhomogeneity of both homo- and copolymer dimethacrylate systems and failure occurring through points of weakness in the system, the regions of lower crosslink density.

Characterization of elastomer compounds by thermal analysis

Abstract Thermal analysis provides very useful tools for the characterization and identification of both, elastomer compound and finished product. This investigation focuses on the use of different thermal techniques for compositional analysis, characterization of thermal, oxidative stability and glass-transition temperature ( T g ) of different components present in the elastomeric systems. Thermogravimetry (TG) is critical for identification of composition. TG analysis of three compositions ( S 1 , S 2 and S 3 ) shows S 1 and S 2 are soft-oil extended compounds and S 3 is a hard compound, all comprising ethylene propylene diene rubber (EPDM). Energy dispersive X-ray analysis (EDAX) of the residues from TGA reveals the presence of mineral filler clay in sample ( S 3 ) besides carbon black. The use of high-resolution TG has been found to give better resolution between overlapping weight loss steps leading to better quantification of various components compared to conventional TG. The coefficient of expansion of the rubber compound (from thermo-mechanical analysis – TMA) above T g is correlated to the hardness of the samples. The use of differential scanning calorimetry (DSC) to determine oxidative stability identifies also the presence of similar antioxidants. Dynamic mechanical analysis (DMA) is found to be very sensitive for characterization of glass-transition temperature, visco-elastic properties and, in particular, the adhesion between the elastomer compound and coating.

The radical polymerization of dimethacrylate monomers—the use of NMR spectrometry to follow the development of network structure

Abstract 13 C NMR spectrometry has been used to identify and assay residual unsaturation in the cure of ethylene glycol dimethacrylate, and distinguish between monomer molecules and pendant double-bonds. Direct evidence of heterogeneity is provided by the distinction between constrained and unconstrained monomer. The predictions of a simulation using a kinetic gelation model coincide with the experimental estimations. Cure estimated by differential scanning calorimetry does not correlate linearly with residual unsaturation.

Some physical and mechanical properties of isotactic-atactic poly(methyl methacrylate) blends and stereoblocks

Abstract Dynamic-mechanical, DSC, and fracture-toughness measurements are reported for blends of it-PMMA with at-PMMA and syndiotactic-rich PMMA, both in the amorphous state and after solvent and thermally induced crystallization. Crystalline stereocomplex forms even in the blends of 74% isolactic with commercial at-PMMA under SIC. Some stereoblock PMMA shows two melting points and two distinct crystalline forms; one corresponds to it-PMMA the other probably to stereocomplex. Microcrystallinity in the blends greatly enhances stiffness. The decline of the storage shear modulus with temperature involves several processes. Blending of it-PMMA (even low mol. wt) into at-PMMA greatly improves its fracture toughness.

Changes of proton-enhanced, magic-angle-spinning 13C-NMR dynamic parameters during cure of tetra(ethylene glycol)dimethacrylate

Abstract Dynamic NMR and mechanical properties TSL, T1p(C) at 60 kHz, Tg at ≈1 Hz and its half width, log decrement (Δmax) and compression modulus (E) were measured for poly[tetra(ethylene glycol)dimethacrylate] (PTEGDMA) at different extents of cure. The β, γ and water-induced transitions were also noted. The spin-lock, cross-polarization time-constant, TSL, declined to its limiting value before the vitrification point. T1p(C), relaxation time in the rotating 13C field, increased sharply at or just after vitrification. The change is greatest at quaternary C, decreasing through CH2, CH2O and CH3, being barely significant in the latter. The observed changes affirm that T1p(C) is sufficiently influenced by spin-lattice relaxations to provide a monitor of the damping of mid kHz components during cure. The small change observed at CH3 shows that this group is not the origin of changes in relaxation rates at other groups. Correlations with E and Tg show the importance of mid kHz components of group motion to the cooperative motions determining the response to macroscopic strain at lower frequencies. MAS 13C-NMR showed that PTEGDMA contains residual unsaturation at limiting cure and that two types, constrained and unconstrained, can be distinguished.

Fracture behaviour of accelerated aged solid rocket propellants

The effect of temperature, strain-rate and ageing on the crack growth mechanism in a composite propellant has been examined to obtain an understanding of the fracture process under service life conditions. Both as-received and aged specimens were tested at each of three strain-rates and temperatures. The materials were aged by subjecting them to various thermal loads (accelerated ageing, thermal cycle and thermal shock) designed to expose them to conditions similar to that experienced by a rocket motor during its service life. The fracture behaviour of the propellant specimens were affected by changes in temperature whilst strain-rate had only a marginal effect over the range studied. It was found that as the material temperature decreased from 60 to − 40 °C the stiffening of the propellant caused increased hysteresis ratios and decreased crack velocities. The deterioration of the mechanical properties of the propellant depended on the severity of the thermal loads. In each case the accelerated aged specimens became harder and more brittle whilst the thermally cycled and thermally shocked specimens were only marginally affected. A distinct difference in the mechanism of crack growth was observed for the accelerated aged specimens, along with a marked decrease in hysteresis ratio, critical stress and critical strain and an increase in crack velocity.

Size analysis of poly(3-hydroxybutyric acid) granules produced in recombinant Escherichia coli

SummarySize distributions of PHB granules synthesized in recombinant Escherichia coli are determined by photosedimentation. Mean granule Stokes diameters are in the range 1.13–1.25 μm, which is larger than reported values for wild type microorganisms. Treatment with 1.5% hypochlorite and mild centrifugation did not affect granule size distribution. Treatment with 10% hypochlorite led to a significant reduction in mean diameter and total PHB.

Dynamic-mechanical properties and cross-polarized, proton-enhanced, magic angle spinning 13C-NMR time constants of urethane acrylates—1. Homopolymer networks

Abstract Dynamic-mechanical properties have been measured for a series of urethane acrylates containing polyether and polyester soft segments. Polymer glass transition temperatures predicted from a crosslinking relation showed better agreement with experimental T g values than those obtained from applying the Fox equation to hard and soft segments in the polymer. Proton-enhanced, magic angle spinning 13 C-NMR time constants were obtained for a polyether soft segment urethane acrylate series. Increasing soft segment molecular weight resulted in a release of both near static and high frequency (mid-kHz) components of motion in the soft segment.

The influence of salt type in the environmental fracture of nylons

The fracture surfaces of nylons strained in the presence of inorganic salt solutions have been analysed using energy dispersive X-rays. Correlations are made between surface morphology and ion distribution so that the role of salts in craze formation and breakdown could be determined. Differences between cations binding directly to the polymer chain from those in which solvent molecules form a bridge, are reflected in the ion distribution. Initial studies of the nature of salt uptake into the polymer are also described.