Richard P. Wool

A theory crack healing in polymers

A theory of crack healing in polymers is presented in terms of the stages of crack healing, namely, (a) surface rearrangement, (b) surface approach, (c) wetting, (d) diffusion, and (e) randomization. The recovery ratio R of mechanical properties with time was determined as a convolution product, R = Rh (t)*φ(t), where Rh (t) is an intrinsic healing function, and φ(t) is a wetting distribution function for the crack interface or plane in the material. The reptation model of a chain in a tube was used to describe self‐diffusion of interpenetrating random coil chains which formed a basis for Rh (t). Applications of the theory are described, including crack healing in amorphous polymers and melt processing of polymer resins by injection or compression molding. Relations are developed for fracture stress σ, strain e, and energy E as a function of time t, temperature T, pressure P, and molecular weight M. Results include (i) during healing or processing at t

Self-healing materials: a review

The ability of materials to self-heal from mechanical and thermally induced damage is explored in this paper and has significance in the field of fracture and fatigue. The history and evolution of several self-repair systems is examined including nano-beam healing elements, passive self-healing, autonomic self-healing and ballistic self-repair. Self-healing mechanisms utilized in the design of these unusual materials draw much information from the related field of polymer-polymer interfaces and crack healing. The relationship of material damage to material healing is examined in a manner to provide an understanding of the kinetics and damage reversal processes necessary to impart self-healing characteristics. In self-healing systems, there are transitions from hard-to-soft matter in ballistic impact and solvent bonding and conversely, soft-to-hard matter transitions in high rate yielding materials and shear-thickening fluids. These transitions are examined in terms of a new theory of the glass transition and yielding, viz., the twinkling fractal theory of the hard-to-soft matter transition. Success in the design of self-healing materials has important consequences for material safety, product performance and enhanced fatigue lifetime.

Composites from Natural Fibers and Soy Oil Resins

The goal of this project is to develop new composites using fibers and resins from renewable resources. The ACRES (Affordable Composites from Renewable Sources) group at the University of Delaware has developed new chemistries to synthesize rigid polymers from plant oils. The resins produced contain at least 50% plant triglycerides and have mechanical properties comparable to commercially available synthetic resins such as vinyl esters, polyesters and epoxies. This project explores the development of all-natural composites by using natural fibers such as hemp and flax as reinforcements in the ACRES resins. Replacing synthetic fibers with natural fibers has both environmental and economic advantages. Unlike carbon and glass fibers, natural fibers are abundantly available from renewable resources. In terms of cost, natural fibers are cheaper than the synthetic alternatives. The natural fibers and plant-based resins have been shown to combine to produce a low cost composite with good mechanical properties. Tensile strength in the 30 MPa range has been obtained for a composite containing about 30 wt% Durafibre Grade 2 flax. The tensile modulus was found to be 4.7 GPa for a 40 wt% flax composite. Similar numbers where obtained for the hemp composites obtained from Hemcore Inc. Composites from renewable resources offer significant potential for new high volume, low cost applications.

All natural composite sandwich beams for structural applications

Abstract As part of developing an all natural composite roof for housing application, structural panels and unit beams were manufactured out of soybean oil based resin and natural fibers (flax, cellulose, pulp, recycled paper, chicken feathers) using vacuum assisted resin transfer molding (VARTM) technology. Physical and chemical investigations and mechanical testing of the beams yielded good results in line with desired structural performance. Room temperature curing of an acrylated epoxidized soybean oil (AESO) resin gave a flexural modulus of 1 GPa. Natural fiber reinforcement of 20–55 wt.% fiber increased the flexural modulus to 2–6 GPa. The same resin reinforced with woven E-glass gave a flexural modulus of 17 GPa. Using this type of composite in building construction introduces many advantages such as high strength and stiffness to weight, survivability in severe weather conditions, desired ductility, fatigue resistance, and design flexibility (three-dimensional forms, molded in place, easy to install and structure replacement). A bio-based (natural and biodegradable) matrix reinforced by natural fibers also provides an important environmental advantage, as renewable resources are used instead of petroleum-based materials. Five different structural beams were successfully manufactured and mechanically tested giving good results. Combinations of two different fibers were also introduced to give processing and strength advantages.

Miniemulsion polymerization of acrylated methyl oleate for pressure sensitive adhesives

The focus of this work was to improve the aqueous emulsion polymerization of a highly water-insoluble monomer derived from plant oil, acrylated methyl oleate. Conventional emulsion polymerization requires excessive amounts of surfactant and long reaction time. Miniemulsion polymerization improved the polymerization significantly. Only a fraction of the surfactant and reaction time that was necessary in the conventional emulsion polymerization is required. The resulting polymers have properties comparable to petroleum-based polymers commonly used in pressure sensitive adhesive applications.

Lignin Model Compounds as Bio-Based Reactive Diluents for Liquid Molding Resins

Lignin is a copious paper and pulping waste product that has the potential to yield valuable, low molecular weight, single aromatic chemicals when strategically depolymerized. The single aromatic lignin model compounds, vanillin, guaiacol, and eugenol, were methacrylated by esterification with methacrylic anhydride and a catalytic amount of 4-dimethylaminopyridine. Methacrylated guaiacol (MG) and methacrylated eugenol (ME) exhibited low viscosities at room temperature (MG: 17 cP and ME: 28 cP). When used as reactive diluents in vinyl ester resins, they produced resin viscosities higher than that of vinyl ester-styrene blends. The relative volatilities of MG (1.05 wt% loss in 18 h) and ME (0.96 wt% loss in 18 h) measured by means of thermogravimetric analysis (TGA) were considerably lower than that of styrene (93.7 wt% loss in 3 h) indicating the potential of these chemicals to be environmentally friendly reactive diluents. Bulk polymerization of MG and ME generated homopolymers with glass transition temperatures (T(g)s) of 92 and 103 °C, respectively. Blends of a standard vinyl ester resin with MG and ME (50 wt % reactive diluent) produced thermosets with T(g)s of 127 and 153 °C, respectively, which are comparable to vinyl ester-styrene resins, thus demonstrating the ability of MG and ME to completely replace styrene as reactive diluents in liquid molding resins without sacrificing cured-resin thermal performance.

Molecular Aspects of Tack

Abstract In this paper, we examined strength development at a polymer-polymer interface in terms of the dynamics and statics of random-coil chains. Interdiffusion of chain segments across the inter...

Vanillin-based resin for use in composite applications

Lignin is an abundant, renewable material that has the potential to yield valuable, low molecular weight, single aromatic chemicals when strategically depolymerized. In order to generate a highly bio-based thermoset for use in polymer composites, a lignin-derived chemical, vanillin, was methacrylated in a two-step, one-pot synthesis to produce a vinyl ester resin (87 cP at 25 °C) with a 1 : 1 mole ratio of a mono-functional monomer, methacrylated vanillin, to cross-linking agent, glycerol dimethacrylate. The synthetic scheme was solventless, required little catalyst and moderate reaction temperatures while generating no by-products. Upon resin curing, a hard, transparent thermoset with a broad glass transition, Tg = 155 °C (based on the tan δ maximum), and a temperature of maximum decomposition rate, Tmax, of 426 °C was produced. Overall, a potentially 100% bio-based thermoset was synthesized possessing comparable thermo-gravimetric and thermo-mechanical properties to commercial vinyl ester-based thermosets.

Biodegradation dynamics of polymer–starch composites

The dynamics of starch biodegradation in polyethylene–starch (PE–S) composites was investigated by aerobic biodegradation methods and computer simulations, with the starch fraction p above and below the percolation threshold pc. Two models for starch degradation were considered: (i) microbial invasion through the composite and (ii) macromolecular (enzyme) diffusion which results in the back-diffusion of small molecules to the surface for further assimilation by microorganisms. The microbial-invasion model was based on scanning electron microscopy (SEM) studies of PE–S composites that contained a 1–15-micron distribution of starch particles. Following exposure to soil test conditions, micrographs of thin films clearly showed the colonization of microorganisms within channels of the matrix that were initially occupied by starch. The enzymatic diffusion was based on hydrolytic experiments of PE–S composites. Following exposure of a composite to a hydrolytic test condition, small molecules were produced. The starch accessed by microbes and enzymes was computed by simulating degradation of a monodisperse and polydisperse (starch grains of 1–10-micron diameter) composite. Aerobic degradation studies in a biometer indicate that the starch accessibility. A follows a power-law dependence with time A ∼ tn, where the exponent n depends on the fractal dimension of the accessed starch clusters and pathways and approaches unity when p > pc. Microbial invasion simulations indicate that the average power-law exponent near pc is approximately 0.5 and approaches 1.0 at p > pc, whereas the enzymatic diffusion simulations indicate that the average power-law exponent near pc is about 0.25 and approaches 0.5 at p > pc. The observed exponent for the aerobic degradation study suggests that for composites with a starch fraction less than and greater than pc the starch is predominantly accessed by microbial invasion.

Interdiffusion of polymers across interfaces.

Neutron Reflection (NR) and Dynamic Secondary Ion Mass Spectroscopy (DSIMS) experiments were conducted on symmetrically deuterated polystyrene triblock bilayers (HDH/DHD) which directly probed the interdiffusion dynamics of the chains during welding. The HDH chains had their centers deuterated 50%, the DHD chains had their ends deuterated (25% at each end) such that each chain contained approximately 50% D. During welding, anisotropic motion of the chains produces a time-dependent oscillation (ripple) in the H and D concentration at the interface, which bears the characteristic signature of the polymer dynamics. These oscillations were compared with those predicted by Rouse, polymer mode coupling (PMC), and reptation dynamics. The following conclusions can be made from this study. (a) During the interdiffusion of high molecular weight HDH/DHD pairs, higher mobility of the chain ends caused a concentration oscillation which increased to a maximum amplitude, and eventually vanished at times, t > τD. The amplitude, or excess enrichment found, was appreciably more than that predicted by Rouse and PMC simulations, and was only slightly less than that predicted from reptation simulations. (b) The oscillations were completely missing in the 30 and 50K HDH/DHD polymers, which are only weakly entangled. The lack of oscillations for the 30 and 50K pairs may be due to a combination of surface roughness and fluctuations of order 30 A. (c) It was found that the position of the maximum in this ripple stayed at the interface during its growth. This is also consistent with reptation and has not been explained by other theories. (d) All dynamics models for linear polymers produce ripples, many of which are qualitatively similar to that predicted for reptation. However, each ripple bears the fingerprint of the dynamics in terms of its time-dependent shape, position, and magnitude, and the models are clearly distinguishable. Our results, in summary, support reptation as a candidate mechanism of interdiffusion at polymer(SINGLEBOND) polymer interfaces and its uniqueness is being further pursued.