T. C. Ward

Thermal characteristics of the self-healing response in poly(ethylene-co-methacrylic acid) copolymers

A class of poly(ethylene-co-methacrylic acid) (EMAA) copolymers and ionomers has shown the unique ability to instantaneously self-heal following ballistic puncture. It is noteworthy that the thermomechanical healing process active in these materials appears to be significantly different in capability and mechanism than any of the other self-repairing systems studied. To better understand this phenomenon, the thermal response during EMAA self-healing was examined. Tests of various damage types, including sawing, cutting and puncture, revealed high-energy transfer damage modes to produce heat and store energy favourable to healing. DSC probed healed specimens revealing they had reached the viscoelastic melt believed requisite to healing response. Low-temperature ballistic experiments demonstrated films continue healing even when punctured at −30°C; analysis showed healing efficacy comparable to room temperature, holding significant pressures of approximately 3 MPa. At the lowest temperature, brittle fracture occurred in one material indicating insufficient heat transfer to store recoverable energy. In total, the results supported the defined healing model and provided additional information on the healing process in both its thermal dependence and general mechanism. Finally, a new DSC method was developed for probing the thermal history of healed films which may lead to a more complete mechanistic model.

Self-Healing of Poly(Ethylene-co-Methacrylic Acid) Copolymers Following Projectile Puncture

Several poly(ethylene-co-methacrylic acid) copolymers have shown the unique ability to self-heal following projectile puncture, with the punctured site subsequently holding pressures in excess of 3 MPa. Four specific materials (ionomers and non-ionomer copolymers) were examined to determine if ionic interactions are responsible for puncture reversal. A range of novel experiments including peel, projectile, and quantitative post-puncture testing were used to determine that ionic content is not necessary for the healing response. Mechanistically, it was concluded that healing occurs through at least a two-stage process of melt elastic recovery followed by sealing and polymer chain interdiffusion at the damaged site.

Synthesis and characterization of organosiloxane modified segmented polyether polyurethanes

Abstract Segmented polyurethanes derived from a 1000 M n hydroxyl terminated polytetramethylene oxide soft segment, 4,4′-methylene diphenyl diisocyanate, MDI, and 1,4-butanediol were modified with a 1200 M n secondary aminoalkyl functional polydimethylsiloxane (PDMS) oligomer via solution polymerization in tetrahydrofuran (THF)/dimethylacetamide (DMAC). Various compositions were studied using FTIR and NMR spectroscopy, thermal analysis, quantitative size exclusion chromatography (SEC), cone calorimetry, transmission electron microscopy, XPS and mechanical testing. The results suggest that, with as little as 15% of the polydimethylsiloxane one may reduce the cone calorimetry heat release rate by a factor of about 2/3 and hence improve fire resistance, while maintaining mechanical behavior. It is suggested that the low surface energy characteristics of PDMS promote migration to the air–polymer interface to form a predominately PDMS enriched surface. The latter is oxidized at elevated temperatures in air to a silicate-like material and this serves as a protective layer, which further reduces burning of the underlying polyurethane. Dynamic mechanical behavior and electron microscopy suggest that a complex mutiphase structure is produced, particularly at low PDMS weight fractions.

The single-fibre pull-out test. 1: Review and interpretation

Abstract For the first time, the load versus extension trace generated by the single-fibre pull-out test is thoroughly interpreted and mathematically modelled. The single-fibre pull-out test is employed experimentally to model the failure of fibre-reinforced composite materials. The interpretation of this model, however, varies between laboratories. In this paper, the test methodologies and the experimental and mathematical interpretations of various scientists are presented and discussed, as is some preliminary work employing optical fibres embedded in various neat resins. Also, a more complete description of the experimental events is presented and described mathematically via the critical strain energy release rate for crack initiation and propagation, the interfacial shear stress of the bond and the coefficient of friction.

Effect of thermoplastic modifier variables on toughening a bismaleimide matrix resin for high-performance composite materials

Abstract A series of engineering thermoplastic toughness modifiers were used to modify the fracture toughness properties of a bismaleimide resin. The effects of thermoplastic loading, molecular weight and functionality were examined. Substantial improvements in K Ic stress intensity values were found when modifiers possessing reactive end-groups were used. Increases in modifier molecular weight and weight per cent loading produced steady increases in K Ic values and a limiting value at high concentrations and molecular weights. These observations were attributed to the thermoset network being the limiting material with respect to fracture toughness. By controlling molecular weight and adding reactive end-groups to the toughener, the fracture toughness properties of the commercial bismaleimide resin were successfully improved without sacrificing its desirable hot-melt processing characteristics. A unidirectional carbon-fibre prepreg was prepared and mode I and II fracture toughness tests were performed using double cantilever beam and end notch flexure specimens. Thermoplastic loadings of 15 and 20% of maleimide-terminated poly(ether sulphone) ( M n = 12 800 g mol −1 ) yielded composite G Ic values of 489 ± 25 and 734 ± 1 J m −2 , respectively—a substantial improvement over the unmodified composite value of 359 ± 17 J m −2 .

Environmental aging of high-performance polymeric composites: Effects on durability

Abstract The effect of sub-Tg environmental aging on the durability of two high-performance polymeric composites has been investigated. The material systems under study were a thermoplastic-toughened cyanate ester resin (Fiberite 954-2) and a semicrystalline thermoplastic resin (Fiberite ITX), and their respective carbonfiber composites, IM8/954-2 and IM8/ITX. Specimens were aged for periods of up to 9 months in environmental chambers at 150 °C and in one of three different gas environments: nitrogen, a reduced air pressure of 13·8 kPa (2psi air) or atmospheric ambient air (14·7psi air). The glass transition temperatures, Tg, of the two resin systems were monitored as a function of aging time and environment. The changes in Tg showed effects of both physical aging and chemical degradation; the latter appeared to be sensitive to the oxygen concentration in the aging environment. Flexure tests were performed on 8-ply unidirectional (90 °) IM8/954-2 and IM8/ITX composites, aged up to 6 months in the three gas environments at 150 °C. The samples showed a 30–40% loss in the bending strength after aging. These strength reductions were sensitive to the oxygen concentrations in the aging environment. Stress/strain tests were also conducted on the same composites to measure the ultimate properties of the materials before and after aging in the three different environments at 150 °C. The results showed a decrease of 40–60% in the ultimate strain to failure with aging. The modulus of both composite systems on the other hand increased by up to 20 % after aging for 6 months, possibly as a consequence of the physical aging phenomena. In both systems greatest reduction in ‘useful’ mechanical properties occurred in the ambient air environment, while the least reduction occurred in nitrogen. Weight loss in the plain resin and composite samples was monitored as a function of aging time and environment. Typically, all of the samples showed 1–2 % weight loss after 9 months of aging at 150 °C, and the composite samples lost much more weight (on a polymer basis) than unreinforced resin specimens over the same aging period. The weight loss data as well as all the above-mentioned observations were indicative of an oxidation process in the composites.

Synthesis and Characteristics of Novel Poly(Imide Siloxane) Segmented Copolymers

Abstract New poly(imide siloxane) copolymers for possible use as tough environmentally stable structural matrix resins and structure adhesives have been prepared. Thus, 3,3′-4,4′-benzophenone tertracarboxylic dianhydride was reacted with various Mn aminopropyl-terminated polydimethylsiloxane oligomers and a meta-substituted diamine “chain-extender” such as 3,3′-diaminodiphenyl sulfone or 3,3′-diaminobenzophenone to produce the siloxane-modified poly(amic acid). Thin films were cast from the reaction mixtures and subsequent thermal dehydration produced the poly(imide siloxane) block or segmented copolymers. Upper “cure” temperatures of 300°C were used to insure complete imidization. By varying the amount and molecular weight of the siloxane oligomer, a variety of novel copolymers of controlled composition have been synthesized. Tough, transparent, flexible soluble films were produced by this method. The thermal and bulk properties of films having low to moderate siloxane content closely resemble those of t...

Engineering Plastics from Lignin XIV. Characterization of Chain-Extended Hydroxypropyl Lignins

Abstract Three series of chain-extended hydroxypropyl lignins (CEEQLs), prepared fran oqanosolv and kraft lignin, were examined regarding their chemical, molecular weight and them1 characteristics. Results showed that the molar substitution (MS) of propylene oxide, which was defined as the number of propoxy repeat units which comprise the chain attached to a single reactive site on lignin, varied and affected copolymer properties. As the MS increased from 1 to 7.2, the number average molecular weight (Mg) increased while the glass transition temperature (Tg) decreased. The actual Mg observed by GPC exceeded however that expected on the basis of mass gain by derivatization. This was attributed to changes in the apparent hydrodynamic volume in relation to MS. The change in Tg with increasing MS followed the Gordon-Taylor relationship. Differences in the chemical composition of the original lignin (organosolv or Kraft) were not obvious as the lignin content of the copolymer decreased below 50%.

Effect of penetrant size and shape on its transport through a thermoset adhesive: I. n-alkanes

Abstract The diffusion behaviors of a series of n-alkanes, ranging from C6 to C17, through a polyamide-type polymeric matrix have been investigated by means of mass uptake measurements. Since n-alkanes are known to display negligible interactions with the polymer matrix, this study serves to isolate the effects of penetrant size and shape on the transport process without undue interference from polymer–penetrant interactions. It is established that the diffusion of the n-alkanes through the polymer matrix studied is Fickian and proceeds via a Henrys law-type mechanism. The diffusion coefficients, D, are evaluated based on a thin-film approximation of the Fickian equation. The activation energies of diffusion, Ed, are determined from the temperature dependence of D, using the Arrhenius equation. Correlations between the Arrhenius terms, Ed and D0, are also established which enable the prediction of diffusion coefficients for similar polymer–penetrant systems. It is also demonstrated by means of activation energy calculations and molecular simulations that the n-alkanes assume a linear geometry within the polymer matrix and diffuse along their long axes.

Production of controlled networks and morphologies in toughened thermosetting resins using real-time, in situ cure monitoring

Based on knowledge of the chemical reactions and morphology, significant changes can be made in the morphology of a toughened dicyanate thermosetting resin through the intelligent manipulation of the cure cycle and real-time knowledge of the conversion of the system. Fourier transform near infra-red spectroscopy using fibre-optic sensors was employed to follow such reactions. Various cure cycle changes resulted in similar degree of cure, thermal stability and solvent resistance, but yielded a 20% change in neat resin toughness associated with the morphologies. The morphological variety was shown not only to occur within reasonable cure cycle variations for neat resin, but were also induced through a processing change in a graphite-reinforced composite containing this resin. Design of custom or gradient morphologies to provide specific mechanical properties is now feasible with this technology. These same approaches could be adapted to the custom manufacture of optical and/or damping properties. This manipulation is not limited to the processing of toughened thermosetting resins.