Richard J. Farris

Influence of processing conditions on the thermal and mechanical properties of SU8 negative photoresist coatings

The thermal and mechanical properties of a new negative photoresist, SU8, were characterized. The influence of curing conditions, such as baking temperature, baking time and UV dosage, on the thermal and mechanical properties of the resultant coatings was studied in detail. It was found that the glass-transition temperature (Tg) of the coatings was coincident with the baking temperature over the temperature range of 25 °C–220 °C for coatings being baked for just 20 min. However, the Tg reached a limiting value (about 240 °C) once the cross-linking reaction was complete, and would not increase further with the baking temperature. The peak temperature of the dimension versus temperature plots, where heat shrinkage occurred, was about a factor of 1.16 times higher than the baking temperature for the temperature range studied. Both the Tg and the shrinkage temperature were affected by the baking time. The thermal expansion coefficients (TEC), including the volumetric TEC (αv), the in-plane TEC (α1) and the out-of-plane TEC (α2), were measured by a pressure–volume–temperature (PVT) apparatus and thermal–mechanical analyzer (TMA). Great residual stress could be generated during the process, and the change in residual stress with the environmental humidity was investigated using vibrational holographic interferometry.

AMINE-TERMINATED POLY(ARYL ETHER KETONE)-EPOXY/AMINE RESIN SYSTEMS AS TOUGH HIGH PERFORMANCE MATERIALS

Abstract A series of tough thermosetting resins based on the incorporation of amine-terminated poly(aryl ether ketone) oligomeric derivatives into commercial epoxy/amine resins, through a solventless process, have been evaluated. It is shown that when the amine-terminated oligomer is loaded at a level sufficient to form a material consisting of a lightly crosslinked thermoplastic continuous phase with epoxy/amine-rich inclusions, the fracture energy of the system increases by 350–750%, with either no loss or a slight increase in modulus, with respect to the unmodified commercial epoxy/amine resin, while possessing glass transition temperatures greater than the neat thermoplastic material. These tough resins are processable by conventional thermoset technology, as the system is initially a miscible low viscosity ternary melt consisting of a commercial epoxy, an amine-terminated oligomeric modifier, and an amine curing agent. Room temperature B-stage tack and drape is also maintained to allow for conventional prepreg lay-up procedures. The toughest systems combine the toughness characteristics of thermoplastic systems with the processing characteristics of thermosetting systems into a unique hybrid material. Phase separation occurs with curing and results in a lightly-crosslinked thermoplastic-rich phase and a highly-crosslinked epoxy/amine-rich phase. The thermal and mechanical properties are shown to be dependent on the morphology of the system which is determined by the loading level of oligomer for a given curing cycle. Three morphologically distinct systems are discussed. The synthesis, processing, solvent, thermal and mechanical properties of resins formulated with amine-terminated tertiary-butylhydroquinone, methylhydroquinone and bisphenol A based poly(aryl ether ketone) oligomers with various commercial epoxy resins are reported, and the properties are related to the observed phase separated morphology. The materials represent conventionally processable high performance thermosetting resins with significantly increased fracture energies and equivalent moduli, with respect to the unmodified materials.

Structure–property relation in poly(p-phenylene terephthalamide) (PPTA) fibers

Abstract Structure parameters of various poly(p-phenylene terephthalamide fibers have been investigated using WAXD and correlated with mechanical properties. The mechanical properties examined were modulus E and strength σ; the pertinent structural parameters include orientation angle φ200, lattice constants a, b, c, paracrystalline parameter gII,apparent crystal sizes ACS110, ACS200, ACS00l, intensity ratio I110/I200 and transverse crystallinity X. The parameters, c, gII and I110/I200 are found to be interrelated and to provide indications of nonreversible chain conformational changes due to post-treatment. It is concluded that the fiber modulus is determined by the combination of the orientation of the crystallites and the paracrystalline parameter through the following equation: (10) 1/E f =(1/E 0 +D 1 g II 2 )+A〈 sin 2 φ〉 in which Ef is the fiber modulus; gII the paracrystalline parameter; φ the orientation angle; and E0, D1 and A the material constants. This relationship is derived from our proposed morphological model in which crystallites are: (a) formed from chains have nonlinear conformations; and (b) packed with an orientation distribution. The correlation of structure with strength has also been studied. In addition, different types of Kevlar® fibers, Kevlar® 119, Kevlar® 29, Kevlar® 49 and Kevlar® 149 show slight, systematic, variations in structure. In particular, all Kevlar® fibers except Kevlar® 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences.

Effect of moisture absorption on the thermal properties of Bombyx mori silk fibroin films

Films of regenerated Bombyx mori silk are strongly affected by absorbed moisture, a phenomenon studied here by differential scanning calorimetry (DSC). Exposure of previously dried films to environments of controlled relative humidity produces test samples of well-defined equilibrium moisture content. Ultimate moisture uptake is as high as 20–23% (by weight) at 75% relative humidity. The glass transition temperature, Tg, drops by 40°C at moisture uptakes as low as 2%, and Tg depressions as large as 140°C are observed at higher relative humidity. The moisture-induced decrease of Tg is completely reversible, as a film remoistened and then redried possesses an unchanged Tg. Trends in Tg with water uptake correspond reasonably well to predictions of a classical thermodynamic theory, indicating that the plasticization effect of moisture on the combined silk-water system can be satisfactorily explained from macroscopic properties of the constituents without any reference to specific interactions.

Compressive and torsional behaviour of Kevlar 49 fibre

The mechanical anisotropy of an aromatic polyamide fibre, Kevlar 49, was studied in tension, compression and torsion. A new technique involved applying small and defined compressive strains to filaments by bonding them to one side of a beam which is subsequently bent to compress the fibres. Using scanning electron and optical microscopy, fibres were shown to form regularly-spaced helical kink bands at 50 to 60° to the fibre axis after the application of small axial compressive strains. Tensile tests of previously-compressed fibres revealed only a 10% loss in tensile strength, after application of as much as 3% compressive strain. A torsion pendulum apparatus was used to measure the shear modulus and an apparent shear strength of fibres. A loss of tensile strength after the application of large (> 10%) torsional shear strains coincided with a loss in recoverable shear strain due to longitudinal fibre splitting. Ratios of tensile-to-compressive strength, tensile-to-shear strength and tensile-to-shear moduli of 5∶1, 17∶1, and 70∶1, respectively, were measured for Kevlar 49.

Evolution of residual stresses in three-dimensionally constrained epoxy resins

Residual stresses arising from the curing process in a three-dimensionally constrained epoxy resin have been determined experimentally by a strain-gauge method. The results were analysed using the theory of incremental elasticity for an ageing, linear thermoelastic material subjected to dimensional constraints. It was found that large residual stresses developed in three-dimensionally constrained epoxy resins and that, by modifying the cure schedule, the internal stresses could be reduced. The bulk behaviour of the epoxy resin under volumetric constraints was also investigated. Values for the bulk modulus of the epoxy resin were calculated from the results at temperatures above and below the glass transition temperature of the resin.

Experimental verification of a microbuckling model for the axial compressive failure of high performance polymer fibres

A previously derived theoretical compressive strength for fibres composed of uniaxially oriented and extended polymer chains was compared with the measured strengths of several high performance fibres. For failure initiated by elastic microbuckling of polymer chains or fibrils, the maximum fibre strength is predicted to be equal to the minimum longitudinal shear modulus of the fibre. An excellent linear correlation between measured strengths and torsion moduli was obtained for four liquid-crystalline polymer fibres and high modulus graphite fibres. The correlation shows that measured strengths are 30% of the corresponding torsion moduli for all these fibres. A high modulus, high strength polyethylene fibre exhibited a compressive strength-torsion modulus ratio that was lower than the value 0.3 obtained for the other fibres examined in this study.

The characterization of thermal and elastic constants for an epoxy photoresist SU8 coating

All of the thermal and elastic constants of the high-aspect ratio, negative, UV resist, SU8 coating were carried out, and the compliance matrix of the coating was obtained. DSC, TGA, TMA, and DMTA techniques were utilized to study the thermal properties of the material. The in-plane thermal expansion coefficient (TEC) (α1) was determined by TMA, and the glass transition behavior was studied by DMTA. The TGA study provided information about the thermal stability of the material, and DSC was applied to study the thermal calorimetric properties of the material. The in-plane Youngs modulus (E1) was measured by tensile tests. The residual stresses of a 1D stretched ribbon sample and a 2D stretched membrane sample were measured by vibrational holography tests, and the in-plane Poissons ratio (ν1) was also determined by holography. The out-of-plane Poissons ratio (ν2) was obtained by high pressure gas dilatometry measurements. The bulk compressibility (κ) and the volumetric TEC (αv) of the material were measured by a pressure-volume-temperature (PVT) apparatus. Finally, the out-of-plane properties, including the out-of-plane TEC (α2) and the out-of-plane Youngs modulus (E2), were calculated from the measured in-plane properties and the volumetric properties. Therefore, the compliance matrix of the studied SU8 coating could be obtained.

Chemical modification of bacterial elastomers: 1. Peroxide crosslinking

Abstract An enhancement in the elastic response of two bacterial thermoplastic elastomers, poly(β-hydroxyoctanoate), a saturated copolymer, and poly(β-hydroxyoctanoic-co-undecylenic acid), an unsaturated copolymer, was attempted using peroxide crosslinking both with and without multifunctional co-agents. Sol-gel analysis verified that crosslinking had occurred and that crosslinking varied with peroxide type and concentration. A peroxide efficiency model suggested that the olefin group improved the peroxide efficiency and reduced the probability of chain scission. Differential scanning calorimetry showed that crosslinking could eliminate all crystallinity. The elastic response was improved as indicated by a reduced tensile set. In general, the crosslinked materials exhibited a decrease in tensile modulus, and a very low tensile strength and tear resistance.

A model for the compressive buckling of extended chain polymers

A model for the compressive buckling of an extended polymer chain is presented. The application of classical elastic instability analysis to an idealized polymer chain reveals that the bending rigidity and critical buckling loads for a chain are proportional to the force constants for valence bond angle bending and torsion. Highly oriented polymer fibres are treated as a collection of elastic chains that interact laterally. The critical stresses to buckle this collection of chains are calculated following a procedure developed to predict the compressive strengths of fibre-reinforced composites. This buckling stress is predicted to be equal to the shear modulus of the fibres and is the limiting value of compressive strength. Comparison of experimental and predicted values shows that the theory overestimates the compressive strength, but that there is a correlation of shear modulus with axial compressive strength. Consideration of flaws in both the theory and the material indicate that the compressive strength should be proportional to either the shear modulus or shear strength of the fibres.