calibration-procedures-for-orthogonal-superposition-rheology

Calibration Procedures for Orthogonal Superposition Rheology

Orthogonal superposition (OSP) rheology is an advanced rheological technique that involves superimposing a small-amplitude oscillatory shear deformation ...

National Institute of Standards and Technology

Multicomponent formulations coupled with complex processing conditions govern the final properties of photopolymerizable dental composites. In this study, a single test substrate was fabricated to support multiple formulations with a gradient in degree of conversion (DC), allowing the evaluation of multiple processing conditions and formulations on one specimen. Mechanical properties and damage response were evaluated as a function of filler type/content and irradiation. DC, surface roughness, modulus, hardness, scratch deformation and cytotoxicity were quantified using techniques including near-infrared spectroscopy, laser confocal scanning microscopy, depth-sensing indentation, scratch testing and cell viability. Scratch parameters (depth, width, percent recovery) were correlated to composite modulus and hardness. Total filler content, nanofiller and irradiation time/intensity all affected the final properties, with the dominant factor for improved properties being a higher DC. This combinatorial platform accelerates the screening of dental composites through the direct comparison of properties and processing conditions across the same sample.

Effects of filler type and content on mechanical properties of photopolymerizable composites measured across two-dimensional combinatorial arrays.

Thermoplastic olefins (TPOs) are polymeric materials utilized for interior and exterior automotive parts. These materials are often painted to protect and enhance their appearance. TPO materials possess a low surface energy because of the aliphatic backbone, and adhesion promoters (APs), such as chlorinated polyolefins, are used to increase paint adhesion and improve paint performance. The impact of the AP structure and processing conditions on the mechanical properties and degree of interpenetration was investigated on cross sectioned coatings. The AP structure was varied by the chlorine content and molecular weight for the same TPO and polyester acrylic, melamine-cured paint system. Two different processing conditions were investigated. Mechanical properties were measured using depth-sensing indentation. Long-range diffusion of coating components across each interface was verified with confocal Raman microscopy. The AP interfaces, AP/base coat and AP/TPO, were chemically and mechanically sharp at the 1 mum lateral resolution of both techniques. Depth-sensing indentation measured a modulus gradient across the top coating cross section. The free air surface of the top coat had a higher modulus than the interior. Processing conditions and polishing to prepare the cross section for measurements were found to affect only the control AP.

Modulus and chemical mapping of multilayer coatings.

Cubic silsesquioxanes as a green, high-performance mold material for nanoimprint lithography.

Soft elastomeric materials that mimic real soft human tissues are sought to provide realistic experimental devices to simulate the human body's response to blast loading to aid the development of more effective protective equipment. The dynamic mechanical behavior of these materials is often measured using a Kolsky bar because it can achieve both the high strain rates (>100s(-1)) and the large strains (>20%) that prevail in blast scenarios. Obtaining valid results is challenging, however, due to poor dynamic equilibrium, friction, and inertial effects. To avoid these difficulties, an inverse method was employed to determine the dynamic response of a soft, prospective biomimetic elastomer using Kolsky bar tests coupled with high-speed 3D digital image correlation. Individual tests were modeled using finite elements, and the dynamic stiffness of the elastomer was identified by matching the simulation results with test data using numerical optimization. Using this method, the average dynamic response was found to be nearly equivalent to the quasi-static response measured with stress-strain curves at compressive strains up to 60%, with an uncertainty of ±18%. Moreover, the behavior was consistent with the results in stress relaxation experiments and oscillatory tests although the latter were performed at lower strain levels.

Identifying the dynamic compressive stiffness of a prospective biomimetic elastomer by an inverse method.

As carbon nanotube (CNT) infused hybrid composites are increasingly identified as next-generation aerospace materials, it is vital to evaluate their long-term structural performance under aging environments. In this work, the durability of hierarchical, aligned CNT grafted aluminoborosilicate microfiber-epoxy composites (CNT composites) are compared against baseline aluminoborosilicate composites (baseline composites), before and after immersion in water at 25 °C (hydro) and 60 °C (hydrothermal), for extended durations (90 d and 180 d). The addition of CNTs is found to reduce water diffusivities by approximately 1.5 times. The mechanical properties (bending strength and modulus) and the damage sensing capabilities (DC conductivity) of CNT composites remain intact regardless of exposure conditions. The baseline composites show significant loss of strength (44 %) after only 15 d of hydrothermal aging. This loss of mechanical strength is attributed to fiber-polymer interfacial debonding caused by accumulation of water at high temperatures.  acoustic and DC electrical measurements of hydrothermally aged CNT composites identify extensive stress-relieving micro-cracking and crack deflections that are absent in the aged baseline composites. These observations are supported by SEM images of the failed composite cross-sections that highlight secondary matrix toughening mechanisms in the form of CNT pullouts and fractures which enhance the service life of composites and maintain their properties under accelerated aging environments.

Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites.

Roma Plastilina No. 1 (RP1), an artist modeling clay that has been used as a ballistic clay, is essential for evaluation and certification in standards-based ballistic resistance testing of body armor. It serves as a ballistic witness material (BWM) behind the armor, where the magnitude of the plastic deformation in the clay after a ballistic impact is the figure of merit (known as "backface signature"). RP1 is known to exhibit complex thermomechanical behavior that requires temperature conditioning and frequent performance-based evaluations to verify that its deformation response satisfies requirements. A less complex BWM formulation that allows for room-temperature storage and use as well as a more consistent thermomechanical behavior than RP1 is desired, but a validation based only on ballistic performance would be extensive and expensive to accommodate the different ballistic threats. A framework of lab-scale metrologies for measuring the effects of strain, strain rate, and temperature dependence on mechanical properties are needed to guide BWM development. The current work deals with rheological characterization of a candidate BWM, i.e., silicone composite backing material (SCBM), to understand the fundamental structure⁻property relationships in comparison to those of RP1. Small-amplitude oscillatory shear frequency sweep experiments were performed at temperatures that ranged from 20 °C to 50 °C to map elastic and damping contributions in the linear elastic regime. Large amplitude oscillatory shear (LAOS) experiments were conducted in the non-linear region and the material response was analyzed in the form of Lissajous curve representations with the values of perfect plastic dissipation ratio reported to identify the degree of plasticity. The results show that the SCBM exhibits dynamic properties that are similar in magnitude to those of temperature-conditioned RP1, but with minimal temperature sensitivity and weaker frequency dependence than RP1. Both SCBM and RP1 are identified as elastoviscoplastic materials, which is particularly important for accurate determination of backface signature in body armor evaluation. The mechanical properties of SCBM show some degree of aging and work history effects. The results from this work demonstrate that the rheological properties of SCBM, at small and large strains, are similar to RP1 with substantial improvements in BWM performance requirements in terms of temperature sensitivity and thixotropy.

Rheological Characterization of Next-Generation Ballistic Witness Materials for Body Armor Testing.

Carbon nanotube (CNT) grafted glass fiber reinforced epoxy nanocomposites (GFRP) present a range of stiffnesses (MPa to GPa) and length scales (m to nm) at the fiber-matrix interface. The contribution of functionalized CNT networks to the local and bulk polymer dynamics is studied here by using a combination of torsion dynamical mechanical thermal analysis (DMTA), positron annihilation lifetime spectroscopy (PALS), and neutron scattering (NS) measurements. DMTA measurements highlight a reduction in the storage modulus (') in the rubbery region and an asymmetric broadening of the loss modulus (″) peak in the -transition region. NS measurements show a suppressed hydrogen mean-square displacement (MSD) in the presence of glass fibers but a higher hydrogen MSD after grafting functionalized CNTs onto fiber surfaces. PALS measurements show greater free volume characteristics in the presence of the functionalized CNT modified composites, supporting the view that these interface layers increase polymer mobility. While NS and DMTA are sensitive to different modes of chain dynamics, the localization of functionalized nanotubes at the fiber interface is found to affect the distribution of polymer relaxation modes without significantly altering the thermally activated relaxation processes.

Article	Year
Effects of filler type and content on mechanical properties of photopolymerizable composites measured across two-dimensional combinatorial arrays. Acta biomaterialia\| PubMed ID: 19282260	2009
Modulus and chemical mapping of multilayer coatings. ACS applied materials & interfaces\| PubMed ID: 20355981	2009
Cubic silsesquioxanes as a green, high-performance mold material for nanoimprint lithography. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 20715068	2011
Identifying the dynamic compressive stiffness of a prospective biomimetic elastomer by an inverse method. Journal of the mechanical behavior of biomedical materials\| PubMed ID: 22982958	2012
Enhanced durability of carbon nanotube grafted hierarchical ceramic microfiber-reinforced epoxy composites. Carbon\| PubMed ID: 29170562	2017
Rheological Characterization of Next-Generation Ballistic Witness Materials for Body Armor Testing. Polymers\| PubMed ID: 30960431	2019
Multiscale Polymer Dynamics in Hierarchical Carbon Nanotube Grafted Glass Fiber Reinforced Composites. ACS applied polymer materials\| PubMed ID: 32166225	2019

Aaron M. Forster

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