High Strain Rate Failure Behavior of Polycarbonate Plates due to Hypervelocity Impact

Developing a fundamental understanding of how polymers respond during hypervelocity impact (HVI), a high strain rate process, is crucial for the potential use of polymers in HVI protection systems. Here, we present the high-strain-rate impact behavior of commercially available polycarbonates with two different molecular weights, 42 and 21 kg/mol. A powder gun and a two-stage light-gas gun were used to achieve the velocities (v0) of a 4 mm projectile impacting the polycarbonate targets over the range of 400–6,500 m/s. The deformation behavior of the polymer during the impact event was captured using high-speed cameras. The impact caused a variety of responses, ranging from deflection of the projectile to complete perforation for higher v0, leading to major debris cloud formation. These debris clouds consist of both fluid- and dust-like ejecta. The fluid-like debris cloud indicates the melting of polymer caused by the conversion of kinetic energy to thermal energy and subsequent adiabatic heating. The dust-like response can likely be attributed to the brittle failure behavior of polymers, as the polymer became brittle at a high strain rate. Dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) on the polycarbonate samples used here indicate an approximately 40 °C increase of Tg with increasing frequency from 1 Hz to 106 Hz, and such an increase has likely led to brittle behavior. While the leading-edge velocity of the debris clouds and perforation diameters scale linearly with v0, we found negligible differences in the HVI response for the two molecular weights of polycarbonate tested here. This study displays the importance of v0 and thickness contributing to responses of polycarbonates undergoing HVI

High Strain Rate Failure Behavior of Polycarbonate Plates due to Hypervelocity Impact

Developing a fundamental understanding of how polymers respond during hypervelocity impact (HVI), a high strain rate process, is crucial for the potential use of polymers in HVI protection systems. Here, we present the high-strain-rate impact behavior of commercially available polycarbonates with two different molecular weights, 42 and 21 kg/mol. A powder gun and a two-stage light-gas gun were used to achieve the velocities (v0) of a 4 mm projectile impacting the polycarbonate targets over the range of 400–6,500 m/s. The deformation behavior of the polymer during the impact event was captured using high-speed cameras. The impact caused a variety of responses, ranging from deflection of the projectile to complete perforation for higher v0, leading to major debris cloud formation. These debris clouds consist of both fluid- and dust-like ejecta. The fluid-like debris cloud indicates the melting of polymer caused by the conversion of kinetic energy to thermal energy and subsequent adiabatic heating. The dust-like response can likely be attributed to the brittle failure behavior of polymers, as the polymer became brittle at a high strain rate. Dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) on the polycarbonate samples used here indicate an approximately 40 °C increase of Tg with increasing frequency from 1 Hz to 106 Hz, and such an increase has likely led to brittle behavior. While the leading-edge velocity of the debris clouds and perforation diameters scale linearly with v0, we found negligible differences in the HVI response for the two molecular weights of polycarbonate tested here. This study displays the importance of v0 and thickness contributing to responses of polycarbonates undergoing HVI

Modeling Enzymatic Kinetic Pathways for Ring-Opening Lactone Polymerization

A unified kinetic pathway for the enzyme-catalyzed polymerization and degradation of poly(ε-caprolactone) was developed. This model tracks the complete distribution of individual chain lengths, both enzyme-bound and in solution, and successfully predicts monomer conversion and the molecular mass distribution as a function of reaction time. As compared to reported experimental data for polymerization reactions, modeled kinetics generate similar trends, with ring-opening rates and water concentration as key factors to controlling molecular mass distributions. Water is critically important by dictating the number of linear chains in solution, shifting the molecular mass distribution at which propagation and degradation equilibrate. For the enzymatic degradation of poly(ε-caprolactone), the final reaction product is also consistent with the equilibrium dictated by the propagation and degradation rates. When the modeling framework described here is used, further experiments can be designed to isolate key reaction steps and provide methods for improving the efficiency of enzyme polymerization

Increasing Molecular Mass in Enzymatic Lactone Polymerizations

Using a model developed for the enzyme-catalyzed polymerization and degradation of poly­(caprolactone), we illustrate a method and the kinetic mechanisms necessary to improve molecular mass by manipulating equilibrium reactions in the kinetic pathway. For these polymerization/degradation reactions, a water/linear chain equilibrium controls the number of chains in solution. Here, we control the equilibrium by adding water-trapping molecular sieves in the batch polymerization reactions of ε-caprolactone. While ring-opening rates were mostly unaffected, the molecular mass shifted to higher molecular masses after complete conversion was reached, and a good agreement between the experimental and modeling results was found. These results provide a framework to improve the molecular mass for enzyme-catalyzed ring-opening polymerization of lactone

High Strain Rate Failure Behavior of Polycarbonate Plates due to Hypervelocity Impact

Developing a fundamental understanding of how polymers respond during hypervelocity impact (HVI), a high strain rate process, is crucial for the potential use of polymers in HVI protection systems. Here, we present the high-strain-rate impact behavior of commercially available polycarbonates with two different molecular weights, 42 and 21 kg/mol. A powder gun and a two-stage light-gas gun were used to achieve the velocities (v0) of a 4 mm projectile impacting the polycarbonate targets over the range of 400–6,500 m/s. The deformation behavior of the polymer during the impact event was captured using high-speed cameras. The impact caused a variety of responses, ranging from deflection of the projectile to complete perforation for higher v0, leading to major debris cloud formation. These debris clouds consist of both fluid- and dust-like ejecta. The fluid-like debris cloud indicates the melting of polymer caused by the conversion of kinetic energy to thermal energy and subsequent adiabatic heating. The dust-like response can likely be attributed to the brittle failure behavior of polymers, as the polymer became brittle at a high strain rate. Dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) on the polycarbonate samples used here indicate an approximately 40 °C increase of Tg with increasing frequency from 1 Hz to 106 Hz, and such an increase has likely led to brittle behavior. While the leading-edge velocity of the debris clouds and perforation diameters scale linearly with v0, we found negligible differences in the HVI response for the two molecular weights of polycarbonate tested here. This study displays the importance of v0 and thickness contributing to responses of polycarbonates undergoing HVI

Design and Implementation of Two-Dimensional Polymer Adsorption Models: Evaluating the Stability of Candida antarctica Lipase B/Solid-Support Interfaces by QCM‑D

A two-dimensional model of a solid-supported enzyme catalyst bead is fabricated on a quartz crystal microbalance with dissipation monitoring (QCM-D) sensor to measure in situ interfacial stability and mechanical properties of Candida antarctica Lipase B (CAL B) under varied conditions relating to ring-opening polymerization. The model was fabricated using a dual photochemical approach, where poly­(methyl methacrylate) (PMMA) thin films were cross-linked by a photoactive benzophenone monolayer and blended cross-linking agent. This process produces two-dimensional, homogeneous, rigid PMMA layers, which mimic commercial acrylic resins in a QCM-D experiment. Adsorption of CAL B to PMMA in QCM-D under varied buffer ionic strengths produces a viscoelastic enzyme surface that becomes more rigid as ionic strength increases. The rigid CAL B/PMMA interface demonstrates up to 20% desorption of enzyme with increasing trace water content. Increased polycaprolactone (PCL) binding at the enzyme surface was also observed, indicating greater PCL affinity for a more hydrated enzyme surface. The enzyme layer destabilized with increasing temperature, yielding near complete reversible catalyst desorption in the model

Quantifying the Stress Relaxation Modulus of Polymer Thin Films via Thermal Wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally induced instability, termed thermal wrinkling, to measure viscoelastic properties of polystyrene films as a function of geometric confinement via changes in film thickness. With application of the appropriate buckling mechanics model for incompressible and geometrically confined films, we estimate the stress-relaxation modulus of polystyrene films by measuring the time-evolved wrinkle wavelength at fixed annealing temperatures. Specifically, we use time−temperature superposition to shift the stress relaxation curves and generate a modulus master curve for polystyrene films investigated here. On the basis of this master curve, we are able to identify the rubbery plateau, terminal relaxation time, and viscous flow region as a function of annealing time and temperatures that are well-above its glass transition. Our measurement technique and analysis provide an alternative means to measure viscoelastic properties and relaxation behavior of geometrically confined polymer films

Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties

Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower than Tgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties

Effects of Salt on Phase Behavior and Rheological Properties of Alginate–Chitosan Polyelectrolyte Complexes

Oppositely charged polyelectrolytes often form polyelectrolyte complexes (PECs) due to the association through electrostatic interactions. Obtaining PECs using natural, biocompatible polyelectrolytes is of interest in the food, pharmaceutical, and biomedical industries. In this work, PECs were prepared from two biopolymers, positively charged chitosan and negatively charged alginate. We investigate the changes in the structure and properties of PECs by adding sodium chloride (salt doping) to the system. The shear modulus of PECs can be tuned from ∼10 to 104 Pa by changing the salt concentration. The addition of salt led to a decrease in the water content of the complex phase with increasing shear modulus. However, at a very high salt concentration, the shear modulus of the complex phase decreased but did not lead to the liquid coacervate formation, typical of synthetic polyelectrolytes. This difference in phase behavior has likely been attributed to the hydrophobicity of chitosan and long semiflexible alginate and chitosan chains that restrict the conformational changes. Large amplitude oscillatory shear experiments captured nonlinear responses of PECs. The compositions of the PECs, determined as a function of salt concentration, signify the preferential partitioning of salt into the complex phase. Small-angle X-ray scattering of the salt-doped PECs indicates that the Kuhn length and radius of the alginate–chitosan associated structure qualitatively agree with the captured phase behavior and rheological data. This study provides insights into the structure–property as a function of salt concentration of natural polymer-based PECs necessary for developing functional materials from natural polyelectrolytes