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Thermally reversible cross-links in a healable polymer: Estimating the quantity, rate of formation, and effect on viscosity The conversion behavior of 2MEP4FS, a polymer with thermally reversible Diels-Alder crosslinks, is modeled. A processing method is developed to create small, homogeneous prepolymer samples. The glass transition temperature of the prepolymer is estimated using temperature modulated differential scanning calorimetry and equated with the conversion of the polymer.Comparing the measured energy with the literature and computational estimates, the fully cured polymer appears to have a large portion of its moieties unreacted. An autocatalytic model is considered to approximate the reaction rate of 2MEP4FS as a function of conversion and temperature. Outside of the fitted temperature range, the model underpredicts the reaction rates at room temperature and 100 °C. Manual mixing of the monomers is determined to be inadequate to obtain a maximum level of cross-linking. Viscosity measurements made at room temperature and elevated temperatures are correlated with the conversion of the prepolymer.

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Cross-ply composites with healable polymer matrices are characterized using dynamic mechanical analysis (DMA). The [90,0] s samples are prepared by embedding layers of unidirectional glass or carbon fibers in 2MEP4FS, a polymer with thermally reversible covalent cross-links previously shown to be capable of healing internal cracks and fully recovering fracture toughness under ideal conditions. After fabrication, cracks in the composites' transverse plies are observed and attributed to residual thermal stresses introduced during processing.Single cantilever bending DMA measurements show the samples exhibit periods of increasing storage moduli with increasing temperature. These results are accurately modeling using simple one-dimensional composite and beam analyses. The effect of cracks on the measured stiffness is considered using a shear lag model, and the predicted crack density of the glass fiber composite falls within a range estimated by microscopy observations. Crack healing is assumed to occur as a function of temperature, and rationales for the onset and conclusion of healing are given.

The double cleavage drilled compression (DCDC) geometry is useful for creating large cracks in a material in a controlled manner. Several models for estimating fracture toughness from DCDC measurements have been proposed, but each is suitable for a subset of geometries and material properties. In this work, a series of finite element fracture simulations are performed over a range of sample widths, hole sizes, heights, Young's moduli, Poisson's ratios, critical stress intensity factors, and boundary conditions. Analyzing the simulation results, fracture toughness is found to be a simple function of sample width, hole size, and an extrapolated stress at zero crack length obtained from a linear fit of the data. Experimental results in the literature are found to agree with this simple relationship. DCDC, finite element modeling, fracture toughness