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Micromechanical deformation processes responsible for toughening mechanisms in ultrafine monospherical inorganic particle-filled polyethylene were investigated in situ by a field-emission gun-environmental scanning electron microscope (FEG-ESEM) with low-voltage techniques. In general, the ultimate properties of polymer composites are largely dependent on the degree of dispersion of filler particles into the matrix. Very often, the agglomeration is one of inevitable occurrences in polymer composites, mixed with ultrafine filler particles. In the present work, the effects of agglomerates, consisting of ultrafine monospherical filler particles, were reexamined in polymer composites on the toughening mechanism. The results show that the dominant micromechanical deformation processes are the multiple debonding processes inside agglomerates, in which the ratio of the matrix strand and the size of agglomerate plays a great role of matrix yielding. In the specimen, where the agglomerates are isolated in the matrix, deformation begins at the equatorial region of agglomerates and propagates through them. However, in the case of closely placed agglomerates, deformation occurs homogeneously within the whole area inside the agglomerates. In both cases, in conjunction with the multiple debonding processes, the major part of energy during the deformation dissipates through the shear-flow processes of the matrix material. In particular, the micromechanical deformation processes observed in this work confirm that the agglomerates do not always have negative effects on the mechanical properties-at least, in the shear deformable semicrystalline polymer matrices. The agglomerates may be effectively used for the improvement of toughness. Furthermore, the FEG-ESEM with low-voltage techniques offers an extremely promising and efficient alternative method to study the morphology as well as in situ micromechanical deformation processes in nonconducting polymer systems.

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 ε, 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<t∞, σ,ε∼t1/4, E∼t1/2; (ii) at constant t<t∞, σ,ε∼M−1/4, E∼M−1/2; (iii) in the fully interpenetrated healed state at t = t∞, σ,ε∼M1/2, E∼M; (iv) the time to achieve complete healing, t∞ ∼M3, ∼exp P, ∼exp 1/T. Chain fracture, creep, and stress relaxation are also discussed. New concepts for strength predictions are introduced.

Crack healing in polymers has several stages, namely, surface rearrangement, surface approach, wetting, diffusion, and randomization. We present a microscopic theory of the diffusion and randomization stages based on the reptation model of chain dynamics by de Gennes. The theory analyzes motion of chains at the interface and calculates the average interpenetration distance of polymer segments as a function of time t and molecular weight M. The theory predicts that, when fracture stress is proportional to , (i) / " ~t1/4Ai~3/4 for t < Tr, where T, is the tube renewal time of the reptation model and " is the virgin fracture stress, and (ii) the t1/4 dependence persists for a wide range of M and also for time t until t becomes comparable to Tv These results are in agreement with healing experiments.

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Kraft lignins from hardwood and softwood were esterified with several anhydrides to alter their solubility behavior in nonpolar solvents, such as styrene-containing thermoset resins. The esterification reaction was facile, it reduced the amount of waste products, and can be readily scaled up. Increasing the carbon chain length on the ester group improved the solubility of kraft lignin in nonpolar solvents, with butyrated lignin being completely soluble in styrene. Esterification with unsaturated groups such as methacrylic anhydride, improved the solubility to a lesser extent than the saturated analogues. The solubility behavior of the modified lignin was described using the Flory-Huggins solubility theory, combined with the predictive method of Hoy. The main goal to obtain a styrene soluble kraft lignin that could be used in unsaturated polyesters and vinyl esters was achieved with fully butyrated kraft lignin and a butyrated/methacrylated kraft lignin. The solubility of the latter is governed by the butyrate/methacrylate ratio. The reaction rate constants for the butyration and methacrylation reactions were also determined and the aromatic hydroxyl groups were found to be consistently three times more reactive than the aliphatic ones.

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Studies of strength development at polymer-polymer interfaces are examined and applications to welding of similar and dissimilar polymers are considered. The fracture properties of the weld, namely, fracture stress, u, fracture energy, GrC, fatigue crack propagation rate daldN, and microscopic aspects of the deformation process are determined using compact tension, wedge cleavage, and double cantilever beam healing experiments. The mechanical properties are related to the structure of the interface via microscopic deformation mechanisms involving disentanglement and bond rupture. The time dependent structure of the welding interface is determined in terms of the molecular dynamics of the polymer chains, the chemical compatibility, and the fractal nature of diffuse interfaces. Several experimental methods are used to probe the weld structure and compare with theoretical scaling laws. Results are given for symmetric amorphous welds, incompatible and compatible asymmetric amorphous welds, incompatible semicrystalline and polymer-metal welds. The relevance of interface healing studies to thermal, friction, solvent and ultrasonic welds is discussed.