Randy Mrozek

Impact of precursor size on the chain structure and mechanical properties of solvent-swollen epoxy gels

The thermal and mechanical properties of solvent-swollen epoxy gels were investigated as a function of pre-cursor size in a model end-linked network. The epoxy networks were formed by cross-linking diepoxide and diamine precursors in the presence of a low volatility solvent, dibutylphthalate (DBP). Both precursors were end functionalized and contained a poly(propylene glycol) (PPG) spacer between the epoxy and amine functionality, respectively. The lengths of both precursors were controlled by varying the molecular weight of the PPG spacer between functional groups. The glass transition temperature for the gels as a function of solvent loading was well predicted by the Fox equation. The scaling factors of shear storage modulus versus solvent loading increased with increasing diamine precursor molecular weight to values much larger (3.91) than the theoretical value of 2.3, for entanglement dominated network formation in a theta solvent. In contrast, the scaling factor increased with decreasing epoxy precursor molecular weight to values near 4.5. The large, molecular weight dependent scaling factors are attributed to loop defect formation as the result of the amine cross-linker architecture, which consists of two difunctional reactive end groups separated by a spacer. Rather than equal spacing of all four reactive sites, the amine end groups contain two reactive hydrogens that increase the local concentration of reactive species, and facilitates loop formation. We anticipate that this work will aid in the development of non-aqueous gels and provide enhanced tailoring of the gel properties over a broad range of stiffness.

Developing an Alternative to Roma Plastilina #1 as a Ballistic Backing Material for the Ballistic Testing of Body Armor

Ballistic clay (Roma Plastilina #1; RP1) is currently used as a backing material that is meant to simulate the penetration resistance of the human body during the ballistic testing of body armor. RP1 is a modeling clay with a primary market in the artistic community. Over time, RP1’s formulation and performance have changed to meet the demands of the artistic community. As a result, RP1 must now be heated to 100 °F to obtain the desired response and exhibits a strong temperature-dependent performance such that the backing material is considered out of calibration after 45 min. This presentation will focus on our efforts to develop a replacement for RP1 that exhibits the desired backing material response at room temperature with minimal temperature-dependence. Specifically, the challenges of designing a viscoplastic material with a controlled response that exhibits dimensional stability while providing minimal elastic recovery from deformation even at high strain rates and linking the quasistatic mechanical response with the ballistic performance.

Carbon nanofiber-filled conductive silicone elastomers as soft, dry bioelectronic interfaces

Soft and pliable conductive polymer composites hold promise for application as bioelectronic interfaces such as for electroencephalography (EEG). In clinical, laboratory, and real-world EEG there is a desire for dry, soft, and comfortable interfaces to the scalp that are capable of relaying the μV-level scalp potentials to signal processing electronics. A key challenge is that most material approaches are sensitive to deformation-induced shifts in electrical impedance associated with decreased signal-to-noise ratio. This is a particular concern in real-world environments where human motion is present. The entire set of brain information outside of tightly controlled laboratory or clinical settings are currently unobtainable due to this challenge. Here we explore the performance of an elastomeric material solution purposefully designed for dry, soft, comfortable scalp contact electrodes for EEG that is specifically targeted to have flat electrical impedance response to deformation to enable utilization in real world environments. A conductive carbon nanofiber filled polydimethylsiloxane (CNF-PDMS) elastomer was evaluated at three fill ratios (3, 4 and 7 volume percent). Electromechanical testing data is presented showing the influence of large compressive deformations on electrical impedance as well as the impact of filler loading on the elastomer stiffness. To evaluate usability for EEG, pre-recorded human EEG signals were replayed through the contact electrodes subjected to quasi-static compressive strains between zero and 35%. These tests show that conductive filler ratios well above the electrical percolation threshold are desirable in order to maximize signal-to-noise ratio and signal correlation with an ideal baseline. Increasing fill ratios yield increasingly flat electrical impedance response to large applied compressive deformations with a trade in increased material stiffness, and with nominal electrical impedance tunable over greater than 4 orders of magnitude. EEG performance was independent of filler loading above 4 vol % CNF (< 103 ohms).

Mode I Fracture Response of Polymer Based Gelatins as a Function of Loading Rate

Traditional collagen-based ballistic gelatin has been used extensively to determine the effectiveness of bullets and firearms because of its similar density to natural tissue. In these investigations, the gel is subjected to a projectile penetration experiment, during which temporary and permanent cavities are observed in the gel medium. The tensile failure mode dominates the damage mechanisms in the formation of these cavities. However, the mechanical response of ballistic gelatin is sensitive to temperature and the gel has a limited shelf life. Synthetic polymer-based gelatins alleviate these issues and thus are good candidates for an alternative tissue simulant. The ability to closely control the synthesis of these polymer gels gives them a distinct material structure, which governs mechanical characteristics on the macro-scale (i.e. strength, compliance). This ability to tune mechanical properties deems them suitable for many other applications such as in sensors, robotics, chemical, and biomedical uses. In order to determine the tensile failure properties of these materials, a Mode I experimental method was developed. The present work builds upon prior studies to incorporate styrene-ethylene-butylene-styrene (SEBS) gelatins at low and high loading rates. Using a digital image correlation (DIC) technique to quantify the full-field surface strains around a crack tip, the critical strain required for initiation of failure and crack growth was determined as a function of loading rate. The experimental methods are presented with results from Mode I fracture experiments, including energy and strain based criteria for failure initiation and growth and their corresponding loading rate dependence.

Challenges for Implementing Polymer Gels in Defense Applications

Polymer gels are soft, lightly crosslinked polymers that are highly swollen with solvent. The gel properties can be tuned by manipulating the polymer and solvent chemistry, solvent loading, polymer and solvent chain architecture, and the incorporation of various fillers and additives. This tunability provides broad utility in military applications including electronic devices, sensors, robotics, multi-functional textiles, responsive coatings, combat medical care, and tissue surrogates for ballistic testing. While potentially useful, a number of challenges can hinder gel utility for the Army. This paper describes recent efforts that offer promise to overcome these obstacles, including improving operational temperature performance and gel toughness.