Aaron M. Forster

The addition of hard fillers to polymeric networks allows for enhancement of mechanical properties, generally at the expense of extensibility. In the case of filled elastomers (such as tires), the hard particles cause damage to the underlying network when strained, resulting in severe mechanical hysteresis in cyclic loading experiments (the Mullins effect). As such, dynamic networks, which are able to heal damage through exchange reactions, are a promising candidate for composite matrices. This work investigates the influence of tunable dynamic bonds at the surface of silica particles in the presence of a fixed, complementary dynamic network matrix. The surface chemistry, composed of benzalcyanoacetamide Michael acceptors, undergoes room temperature, catalyst-free dynamic exchange with thiols with equilibrium constants (K_eq) that can be manipulated by the electronic nature of the acceptor. Increasing the K_eq of the particle surface relative to the dynamic matrix was found to promote the overall reinforcement of the composites, while also influencing the phase separation behavior of the matrix. Critically, tensile experiments reveal that ambient dynamic exchange allows for the recovery of network damage as a function of waiting time between loading cycles.

Hyperbranched polymers are commonly used as surface and cure modifiers, along with nanoparticles, of epoxy resins used in medical and infrastructure applications; however, their impact on structure and performance of the epoxy resins is not fully understood. Here, a commercial epoxy-aromatic amine resin (EP-AA) system is blended with off-stoichiometric amounts of hyperbranched amine (HA), and the cure kinetics, structure, polymer dynamics, and mechanical properties of the different compositions are evaluated. Cure kinetic studies indicate that the curing of the hyperbranched amine is nearly complete prior to that of the aromatic amines. At its highest concentration (1% by mass), the hyperbranched amine is found to autocatalyze the epoxy and the aromatic amine cure, but the bulky HA groups slow the diffusion kinetics during the final stages of the cure process. This results in a heterogeneous, cocontinuous network structure consisting of unreacted secondary amines that increase the side-group/pendant contribution and increase the free volume within the polymer network. The restricted cooperative segmental mobility of the polymer network in the 1% by mass HA blend results in poor energy dissipative properties, as evidenced by reduced strength and strain to failure.

A suite of phase separated dynamic covalent networks based on highly tunable dynamic benzalcyanoacetate (BCA) thia-Michael acceptors are investigated. In situ kinetic studies on small molecule model systems are used in conjunction with macroscopic characterization of phase stability and stress relaxation to understand how the molecular dynamics relate to relaxation modes. Electronic modification of the BCA unit strongly impacts the exchange dynamics (particularly the rate of dissociation) and the overall equilibrium constant (K_eq) of the system, with electron-withdrawing groups leading to decreased dissociation rate and increased K_eq. Critically, below a chemistry-defined temperature cutoff (related to the stability of the hard phase domains), the stress relaxation behavior of these phase separated materials is dominated by the molecular exchange dynamics, allowing for networks with a tailored thermomechanical response.

The availability of materials data for impact-mitigating materials has lagged behind applications-based data. For example, data describing on-field helmeted impacts are available, whereas material behaviors for the constituent impact-mitigating materials used in helmet designs lack open datasets. Here, we describe a new FAIR (findable, accessible, interoperable, reusable) data framework with structural and mechanical response data for one example elastic impact protection foam. The continuum-scale behavior of foams emerges from the interplay of polymer properties, internal gas, and geometric structure. This behavior is rate and temperature sensitive, therefore, describing structure-property characteristics requires data collected across several types of instruments. Data included are from structure imaging via micro-computed tomography, finite deformation mechanical measurements from universal test systems with full-field displacement and strain, and visco-thermo-elastic properties from dynamic mechanical analysis. These data facilitate modeling and design efforts in foam mechanics, e.g., homogenization, direct numerical simulation, or phenomenological fitting. The data framework is implemented using data services and software from the Materials Data Facility of the Center for Hierarchical Materials Design.

Cure kinetics of epoxy molding compounds (EMCs) is a fundamental material property that affects the molding process of semiconductor chips and final package performance. However, due to measurement challenges related to the small polymer fraction, only the main reaction is considered in the current industry practice for EMC cure kinetics evaluation. In this work, we show that a second reaction exotherm is clearly identified in commercial multi-resin EMC systems upon dynamic heating scan using differential scanning calorimetry (DSC). The second reaction completes the overall reaction from about 0.8 to 1.0 fractional conversion, which is critical for designing cure schedules to achieve ultimate properties, especially the post-mold curing (PMC) procedure during manufacturing.

Scratches in polymer coatings and barrier layers negatively impact optical properties (haze, light transmission, etc.), initiate routes of degradation or corrosion (moisture permeability), and nucleate delamination of the coating. Detecting scratches in coatings on advanced materials systems is an important component of structural health monitoring but can be difficult if the defects are too small to be detected by the naked eye. The primary focus of the present work is to investigate scratch damage using fluorescence lifetime imaging microscopy (FLIM) and mechanical activation of a mechanophore (MP)-containing transparent epoxy coating. The approach utilizes a Berkovich tip to scratch MP-epoxy coatings under a linearly increasing normal load. The goal is to utilize the fluorescent behavior of activated MPs to enable the detection of microscale scratches and molecular scale changes in polymeric systems. Taking advantage of the amine functionality present in a polyetheramine/bisphenol A epoxy network, a modified rhodamine dye is covalently bonded into a transparent, thermoset polymer network. Following instrumented scratch application, subsequent fluorescence imaging of the scratched MP-epoxy reveals the extent of fluorescence activation induced by the mechanical deformation. In this work, the rhodamine-based mechanophore is used to identify both ductile and fracture-dominated processes during the scratch application. The fluorescence intensity increases linearly with the applied normal load and is sensitive to fracture dominated processes. Fluorescence lifetime and hyperspectral imaging of damage zones provide additional insight into the local (nanoscopic) environment and molecular structure of the MP around the fracture process zone, respectively. The mechanophore/scratch deformation approach allows a fluorescence microscope to probe local yielding and fracture events in a powerful way that enhances the optical characterization of damage zones formed by standard scratch test methods and leads to novel defect detection strategies.

In this work, different formulations of a room-temperature silicone composite backing material (SCBM) composed of polydimethylsiloxane (PDMS), fumed silica