Zhanzhan Jia

Milled Glass Reinforced Polyurea Composites: The Effect of Surface Treatment

Polyurea is commonly utilized in blast-mitigating applications due to its excellent thermo-mechanical properties. In this work, we seek to develop polyurea-based composite materials capable of enhanced blast-induced stress-wave management through material design. Typically the matrix, the filler, and the interfacial surface chemistry comprise the basic structure of a composite material. Here we evaluate the effect of the matrix-filler interfacial properties through the integration of surface treated milled glass fibers into polyurea. The milled glass fibers are connected to the matrix via weak (van der Waals force), intermediate (hydrogen bonding), or strong (covalent bonding) interactions using a variety of surface treatments. Although a complete understanding of the interfacial relationships is exceedingly complex, experimental studies are essential in providing a basic understanding to support simulations and ultimately guide the optimal design of the polyurea-based composites. The properties of the resultant composite materials are thermo-mechanically characterized using dynamic mechanical analysis. Additionally, the surface treatments are also applied to glass slides in order to allow for water droplet contact angle measurements to assess the hydrophobicity/hydrophilicity. Furthermore, interfacial adhesion tests are conducted on samples fabricated by casting polyurea on surface treated glass slides. These efforts are part of an ongoing initiative to develop elastomeric composites with optimal properties to manage blast-induced stress-wave energy.

Determining the Shear Relaxation Modulus and Constitutive Models for Polyurea and Polyurea-Based Composite Materials from Dynamic Mechanical Testing Data

Polyurea and polyurea-based composite materials are widely used due to their excellent mechanical properties. In order to facilitate large-scale computational studies for this group of materials, a robust and standard method is needed to extract their viscoelastic constitutive parameters. In this study, frequency-domain master curves which cover a wide range of frequencies are developed using the data of dynamic mechanical analysis through time-temperature superposition (TTS). The quality of the master curves is assessed both by Kramers-Kronig relations and by comparing with the ultrasonic wave testing data. Then the time-domain relaxation modulus is obtained by the high-resolution Prony series approximated from the relaxation spectrum. To reduce computational cost, 4 to 8-term Prony series are then fitted from the time-domain relaxation modulus for a limited frequency range of interest. Both the high and low-resolution Prony series are converted back to frequency domain to compare with the master curves developed by TTS and show good agreements. This method is not limited to polyurea and polyurea-based composites and it can be applied to other similar polymer systems as well.

Polyurea-Based Composites: Ultrasonic Testing and Dynamic Mechanical Properties Modeling

Many scientists and researchers study polyurea due to its excellent blast-mitigating properties. In this work, we have studied two polyurea composite systems with filler materials intended to improve dynamic mechanical properties. The two filler materials are milled glass and fly ash. The shape and quantity of filler significantly affect the dynamic mechanical properties of the composite. Ultrasonic tests were conducted on samples with both fillers. The volume fraction of the inclusions was varied to study the effect of filler quantity on mechanical properties. Moreover, computational models based on the methods of dilute-randomly-distributed inclusions and periodically-distributed inclusions were created to improve our understanding of polyurea-based composites and serve as tools for estimating the dynamic mechanical properties of similar composite material systems. The experimental and computational results were compared and show good agreement. The experiments and modeling have been conducted to facilitate the design of new elastomeric composites with desirable impact- and blast-mitigating properties.

The Effect of Stoichiometric Ratio on Viscoelastic Properties of Polyurea

Polyurea is a commonly utilized elastomer due to its excellent thermo-mechanical properties. In this study, the polyurea is synthesized using Versalink P-1000 (Air Products) and Isonate 143 L (Dow Chemicals). The diisocynate blocks generally assemble into hard domains embedded in the soft matrix, creating a lightly cross-linked heterogenous nano-structure. We seek to evaluate the effect of the stoichiometric ratio of the two components on the viscoelastic properties of the resultant polyurea. By altering the ratio, polyurea samples with different stoichiometric variations are made. In order to approximate the mechanical properties of polyurea for a wide frequency range, master curves of storage and loss moduli are developed. This is achieved by time-temperature superposition of the dynamic mechanical analysis (DMA) data, which is conducted at low frequencies and at temperatures as low as the glass transition. Furthermore, in order to access the effect of the stoichiometric ratio on the relaxation mechanisms in the polyurea copolymer system, continuous relaxation spectra of all the stoichiometric variations are calculated and compared.

Tailored Polyurea-Glass Interfaces and the Characterization by the Single-Fiber Fragmentation

Polyurea is an elastomer that has been intensively researched due to its excellent thermal and mechanical properties. Polyurea based composite material has recently become a research interest to further explore what this polymer has to offer. In order to better understand the overall static or dynamic mechanical properties of the polyurea based composites, how to tailor and characterize the polyurea-filler interface has become a crucial problem. This study focuses on one of the filler materials, glass. Three types of polyurea-glass interfaces are studied by using silane reagents that have similar molecular structures but with different end functional groups to modify the glass surfaces. Accordingly, bonds with different strengths are formed between the glass and the polyurea through the different chemical character of the reagent molecules. The polyurea-glass interfacial properties are tested by the single-fiber fragmentation, which is a widely used method to test the shear properties of the interface between the fiber and the polymer. Single-fiber fragmentation samples are fabricated by casting a single glass fiber along the axial direction of the dogbone-shaped polyurea tension test sample. Tension tests are conducted and the continuous photoelastic videos are taken to observe the single fiber fragmentation process until the fragmentation reaches its saturation state. Meanwhile, stress-strain data are recorded. By analyzing the single-fiber fragmentation data, the polyurea-glass interfacial shear strengths are calculated. The observation of the debonding zones at the interface is used to find the approximate models for the interfacial shear adhesion of polyurea-glass interfaces for different reagents, hence proving the potential for tailoring of the interfacial strength using surface treatment.© 2013 ASME

Storage and Loss Moduli of Low-Impedance Materials at kHz Frequencies

Standard Dynamic Mechanical Analysis (DMA) is generally used to measure the mechanical properties of polymers at frequencies around and below 100 Hz. Ultrasonic (US) techniques measure wave speeds and impedances at higher frequencies. However, both approaches run into issues between the two regimes. DMA systems become less reliable due to the dynamic response of the frames and load path as one tries increasing the frequency. On the other hand, the internal multiple reflections in the wave propagation techniques introduce challenges in clean measurements and require careful analysis. In this presentation, we introduce a robust procedure for determining the storage and loss moduli of low-impedance materials, where a cylindrical sample is placed between two long metal bars, similar to SHPB technique. However, unlike SHPB, the incidence signal is created by a very light impact, to ensure that the sample does not experience permanent or large deformation. Furthermore, due to the length of the specimen, dynamic equilibrium is neither guaranteed nor intended. The reflected and transmitted pulses are measured using semi-conductor strain gages. The wave speed may be determined using a phase spectral analysis of the time-resolved signals. Determination of the material loss requires a more thorough transfer matrix analysis. The method was applied to a soft polyurea elastomer that was tested in a temperature-control chamber and results were compared with DMA and US data using time-temperature superposition (TTS). While the predictions of the storage modulus using DMA and TTS matched very well with the direct measurements, the DMA/TTS predictions generally underestimate the material loss at higher frequencies. We expect that this method may be applied successfully to other low impedance materials including foams and metamaterials.

9 – Modification and Engineering of HSREP to Achieve Unique Properties

Polyurea is a microphase-separated block copolymer that exhibits unique properties as a result of its phase-separated morphology. Experimental results suggest and computational simulations support that judicious arrangement of steel and polyurea can enhance shock mitigation. Schlieren imaging techniques can be used to qualitatively investigate the behavior of shock waves. The properties and performance of polyurea can be significantly modified by the integration of additives, and micromechanical modeling can aid in the design of such composites with specific properties. For polymers and their composites, temperature, pressure, and strain rate usually affect the mechanical properties of the material at the same time. However, direct measurements of the mechanical properties for the high-rate loadings are often difficult to perform. Thus, alternative methods are used to predict and estimate the mechanical properties. Full-scale head–neck manikins were fabricated in order to assess comparative intracranial pressure and acceleration from explosive blast. A range of standoffs to the explosive were used. Three helmet variations were examined: the standard advanced combat helmet (ACH), an ACH helmet with a polymer coating, and a standard World War II, M1 helmet. Use of a helmet decreased the pressure at different locations within the brain. All of the helmets resulted in similar peak pressure and resultant acceleration values. However, the ACH helmet with a polymer coating decreased the time of pressure application significantly, lowering the applied impulse at the closest standoffs. The lower impulse would also contribute to a decrease in applied power density in these portions of the brain. Use of a polymer coating provides a basis to explore other helmet variations to provide protection from traumatic brain injury.

Low-Density, Polyurea-Based Composites: Dynamic Mechanical Properties and Pressure Effect

In this study, we explore the fabrication, characterization and modeling of low-density polymeric composites to understand their acoustic responses. Polyurea is chosen as the matrix of the composites due to its excellent properties and advantages, i.e. blast mitigation, easy casting, corrosion protection, abrasion resistance, and various uses in current military and civilian technology. Two low-mass-density filler materials of interest are phenolic and glass microballoons. They have significant differences in their mechanical properties and chemical interactions with the matrix. Ultrasonic tests are conducted on samples with different volume fractions of fillers and variable pressure. Computational models based on the methods of dilute randomly distributed and periodically distributed inclusions are created to improve our understanding of low-density polymer-based composites and serve as tools for estimating the dynamic mechanical properties of similar composite material systems. The experimental and computational results are compared. The results are expected to facilitate the design of new elastomeric composites with desirable densities and acoustic impedances. These new composites will be useful in developing layered metamaterial structures. Furthermore, we seek to find out whether such inclusions may substantially affect the time-dependent response of the composite by introducing new resonant modes.

Pressure-invariant non-reflective and highly dissipative acoustic metamaterials

Metamaterials have shown great potential to transform the design of acoustic components in many applications. Many composites with extreme properties have been envisioned, designed, fabricated, and experimentally verified. The next step involves testing such composites in realistic application environments. Oceans are one such environment in which the mechanical effects of water pressure and flow become important factors in any acoustic design, particularly for soft dissipative shells. We have designed a layered metamaterial composite that not only shows very high dissipation but also matches the acoustic impedance of water. Furthermore, we have experimentally verified that the relevant properties of the constituents of this layered design do not change under pressure levels that exist down to significant depths. We are in the process of fabricating this composite to test its acoustic properties under pressure. The metamaterial composites lend themselves naturally to multi-component designs, examples of w...

Modifying the acoustic impedance of polyurea-based composites

Acoustic impedance is a material property that depends on mass density and acoustic wave speed. An impedance mismatch between two media leads to the partial reflection of an acoustic wave sent from one medium to another. Active sonar is one example of a useful application of this phenomenon, where reflected and scattered acoustic waves enable the detection of objects. If the impedance of an object is matched to that of the surrounding medium, however, the object may be hidden from observation (at least directly) by sonar. In this study, polyurea composites are developed to facilitate such impedance matching. Polyurea is used due to its excellent blast-mitigating properties, easy casting, corrosion protection, abrasion resistance, and various uses in current military technology. Since pure polyurea has impedance higher than that of water (the current medium of interest), low mass density phenolic microballoon particles are added to create composite materials with reduced effective impedances. The volume fraction of particles is varied to study the effect of filler quantity on the acoustic impedance of the resulting composite. The composites are experimentally characterized via ultrasonic measurements. Computational models based on the method of dilute-randomly-distributed inclusions are developed and compared with the experimental results. These experiments and models will facilitate the design of new elastomeric composites with desirable acoustic impedances.