Ryan Toivola

Highly Sensitive Built-In Strain Sensors for Polymer Composites: Fluorescence Turn-On Response through Mechanochemical Activation.

A new class of rationally designed mechanophores is developed for highly sensitive built-in strain sensors in polymer composites. These mechanophores are designed to regenerate the π-conjugation pathway between the electron donor and electron acceptor by force-induced cleavage of the covalent bond to form a fluorescent dipolar dye.

Influence of aging on flexural strength of translucent zirconia for monolithic restorations

Statement of problem Concern has been raised with regard to the low‐temperature degradation (LTD) of translucent yttria‐stabilized tetragonal zirconia polycrystalline (Y‐TZP) for monolithic zirconia restorations. Purpose The purpose of this in vitro study was to assess the LTD behavior of 4 commercially available translucent Y‐TZP materials by accelerated aging specimens in steam at 134°C, 0.2 MPa. Material and Methods Thin bars (22×3×0.2 mm) of Y‐TZP, including Katana ML (Kuraray Noritake Dental Inc), Katana HT13 (Kuraray Noritake Dental Inc), Prettau (Zirkonzahn), and BruxZir (Glidewell Laboratories) (n=30 for each group), were machined from sintered blocks. Control specimens were assessed in the nonaged condition. Artificially ageing (n=5 per group at 5, 50, 100, 150, and 200 hours) was conducted in steam at 134°C at 0.2 MPa. The specimens were characterized, tested in 4‐point flexure, and the fracture surfaces were analyzed. The monoclinic‐to‐tetragonal (m/t) peak intensity ratio measured by x‐ray diffraction was used to calculate the monoclinic phase fraction and monitor LTD. Linear regression with heteroscedasticity‐consistent robust standard errors was used to test for the effect of LTD (aging time) on (&sgr;f) and m/t. The Spearman rank correlation coefficient was used to assess the relationship between &sgr;f and monoclinic phase fraction (&agr;=.05). Results Artificial aging resulted in LTD as shown by an increase in the monoclinic phase fraction for all specimens. After aging for 200 hours, the mean ±SD monoclinic phase fraction increased from 2.90 ±0.34% to 76.1 ±0.64% for Prettau, 2.69 ±0.18% to 76.0 ±0.26% for BruxZir, 4.6 ±0.19% to 35.8 ±0.80% for Katana HT13, and 3.57 ±0.35% to 33.2 ±1.1% for Katana ML (all P<.001). Flexural strength changed from a mean ±SD of 1612 ±197 MPa to all fractured during aging for Prettau (P<.001); 1248 ±73.5 MPa to all fractured during aging for BruxZir (P<.001); 1052 ±84.2 to 1099 ±70 MPa ±130 for Katana HT13 (P=.45); and from 875 ±130 to 909 ±70 MPa (P=.82) for Katana ML. The mean flexural strength values of Prettau and BruxZir decreased with an increase in the monoclinic phase with Spearman rank correlation coefficients of −0.80 (P=.001) for Prettau and −0.63 (P=.022) for BruxZir. No significant changes in flexural strength were measured for Katana ML or Katana HT13 (P>.05). Conclusions The LTD of Y‐TZP resulted in a significant decrease in flexural strength of Prettau and BruxZir, whereas Katana ML and Katana HT13 exhibited less LTD and no significant decrease in flexural strength.

Thermochromic Polymer Film Sensors for Detection of Incipient Thermal Damage in Carbon Fiber–Epoxy Composites

Carbon fiber–epoxy composites have become prevalent in the aerospace industry where mechanical properties and light weight are at a premium. The significant non-destructive evaluation challenges of composites require new solutions, especially in detecting early-stage, or incipient, thermal damage. The initial stages of thermal damage are chemical rather than physical, and can cause significant reduction in mechanical properties well before physical damage becomes detectable in ultrasonic testing. Thermochromic fluorescent probe molecules have the potential to sense incipient thermal damage more accurately than traditional inspection methods. We have designed a molecule which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature–time combinations that can cause damage in composites. Moreover, this molecule can be dispersed in a polymer film and attached to composite parts as a removable sensor. This work presents an evaluation of the sensor performance of this thermochromic film in comparison to ultrasonic C-scan as a method to detect incipient thermal damage in one of the most widely used carbon fiber–epoxy composite systems. Composite samples exposed to varying thermal exposures were used to evaluate the fluorescent thermal sensor films, and the results are compared to the results of ultrasonic imaging and short-beam shear tests for interlaminar shear strength.

Mechanochemical changes in absorption and fluorescence of DDM-containing epoxies

Photochemical changes in the optical characteristics of solid polymers form the basis for many important technologies, but few examples are demonstrated in diamine-cured epoxy, one of the most important structural polymer systems. We have observed that diamine-cured epoxies containing the 4,40-diaminodiphenyl methane (DDM) framework display changes in both absorption and fluorescence in response to UV light. The change from original “blue” to photo-activated “red” emission can be accomplished by illuminating samples of DDM-containing epoxy with photons of 254 nm or 370 nm wavelengths, followed by excitation of the newly-generated red-emitting fluorophore with 350e400 nm light. Studies of the monomer constituents and of many formulations of diamine-cured epoxy identify the DDM structure as the responsive moiety, both from the epoxy or diamine monomer. The fluorescence change is accompanied by the formation of orange and green chromophores; the orange chromophore is the fluorophore, with a broad 410 nm excitation and 607 nm emission. Our work shows the “blue-to-red” transition to be irreversible and independent of atmospheric oxygen; the fluorophore’s impermanence with time and its radical spectral signature identify it as a reactive intermediate rather than a photooxidation product. The central methylene radical of the benzoidal DDM structure is proposed as a redemitting fluorophore/orange chromophore, with the green chromophore being a quinoidal methine resulting from the DDM radical.

The preparation and properties of carbon inverse opal papers using carbon fiber sheets as a framework

Carbon inverse opal monoliths store energy, sense small molecules, and catalyse reactions, but can be limited by their rigid form because it prevents the mechanical flexure that would be needed to embed this material within devices that bend during use. We report a method that uses carbon fiber papers to impart partial mechanical flexibility and greater electronic conductivity to carbon inverse opals. Our approach deposits colloidal crystals between the fibers in carbon fiber paper and uses the colloidal crystal templates to form inverse opal carbon from phenolic resin. The resulting carbon inverse opal papers (CIOPs) comprise both three-dimensionally interconnected macropores and aperiodic meso- and micropores, the combination of which establishes hundreds of square meters of interfacial area per gram that can store and discharge electrochemical capacitance while planar, while flexed around radii of curvature as small as 5 mm, and after 150 reversible flexures. Carbon inverse opal papers offer at least 3 times greater electronic conductivity than reported carbon inverse opals because the network of carbon fibers doubly serves as pliable framework and conductive conduit.