Session S49: Polymers in Reactive Conditions

Capturing shock-driven dissociation of polymers and foams

Polymers, and polymer-based composites and foams are used extensivey in engineering and defense applications in a range of components including structural bodies and supports, insulating layers, and vibration and shock mitigation parts. In several applications, they may be subjected to impact and shockwave compression. Under these loading conditions, polymers have several features that differ from those of metals, including compression of network volume at low pressures, viscoelastic behaviors, and shock-driven dissociation. A novel equation of state methodology has been developed for describing the shock-driven decomposition of solid and porous polymers at moderate shock input conditions. Examples of the shockwave response of several polymers, and a comparison of historical metals-based equation of state approaches to the new methodology will be presented. Furthermore, this modeling approach has been extended to porous polymer foams, and revealed, through a large series of plate impact experiments, a phenomena we have termed "reactive anomalous compression." A striking feature of our results is the shift from densification to expansion of shock-driven reaction products as a function of initial porosity. Expansion with increasing shock pressure is most commonly associated either with shock heating due to pore collapse in chemically inert materials such as metals or exothermic decomposition (detonation) of energetic materials. We describe an unexpected admixture of these two effects: shock heating due to pore collapse, but expansion in conjunction with chemical reaction. Lastly, efforts to measure reactive wave structures in shock-dissociated polymers, and new x-ray based diagnostic techniques will be described.   

Dynamic Response of Polymers Characterized using Ultrafast Laser Compression

Polymer degradation kinetics and reaction mechanisms are of a great interest for many applications. Reports about chemistry related aging effects found in the literature are, however, rarely correlated to the performance of these materials under dynamic compression conditions. One longstanding question is how much degradation must occur before a polymer-based component is significantly affected? Answering this question requires high-throughput testing under a broad spectrum of aging conditions. In this talk, we describe a path forward using an ultrafast laser compression platform. First, we demonstrate validation results from PDMS-based polymers, reproducing available gas-gun shock Hugoniot data and further, show Hugoniot slope changes with polymer filler and radiation-induced damage. Lastly, we present shock results from a set of CVD deposited Kapton and Parylene films cured at different thermal/humidity aging conditions. Discussion of our main findings and future development plans are presented.   

Predicted pathways for chemical degradation in siloxane polymers

Chemical degradation can result in undesirable changes in the performance of functional materials over the service lifetime, but the underlying atomic-scale processes are often subtle to probe with experiments. We use ensembles of quantum molecular dynamics (QMD) simulations to predict the chemical reactions that follow radiation-induced excitations of phenyl groups in a model copolymer of polydimethylsiloxane and polydiphenylsiloxane. Initial benzene-forming proton transfer reactions are predicted to induce subsequent reactions including intrachain cyclization and chain scission reactions. Water is found to play a crucial role in chain scission reactions, which indicates a possible synergistic effect between environmental humidity and radiation that could promote alterations of a larger polymer network. An approach for obtaining probability distributions of reaction intermediates and products by coupling high throughput semiempirical QMD simulations with automated graph-based structure recognition tools is discussed.   

Perturbing effects on the structure and properties of physically and chemically crosslinked phenolic resins

Crosslinked polymers may consist of physical crosslinks such as hydrogen bonds, or covalent bonds or both. The network of crosslinked phenolic resins involves the both. The formation of each type of crosslinking has not been studied in detail. Their contributions to the physical properties are also unclear. Utilizing a combination of infrared spectroscopy, thermal analysis, low field NMR and a cantilever deflection technique, the relative contributions of two types of crosslinks have been characterized as a function of temperature and moisture. The results show that the increased chemical crosslinks reduced the formation of hydrogen bonds. As expected, the covalent bonds dominate at elevated temperatures. However, the contribution of hydrogen bonds is unexpectedly important at low temperatures. As expected, the hydrogen bonding contribution of crosslinked structures is significantly decreased by the moisture absorbed. However, segmental relaxation behavior measured under different humidity levels depends on not only the two types of crosslinks achieved but also the morphological features affecting the moisture absorption process.
  

Photothermally-driven oxidative degradation of LDPE nanocomposites containing plasmonic nanoparticles

Novel approaches to control and drive polymer degradation are interesting because of the multiple challenges to remediating plastic waste including minimizing its effect when unconfined in the environment and potentially recovering value from discarded material. Photothermal heating from metal nanoparticles, where light is focused and converted into heat at the nanoscale, can be used to drive chemical reactions, such as oxidative degradation within low density polyethylene (LDPE), and results in a high temperature, high electric field environment in the particles' immediate vicinity within which reactions could be manipulated. It also provides a mechanism to convert sunlight to internal heat. We present an LDPE nanocomposite system incorporating silver nanoparticles and cobalt ions where the cobalt-mediated oxidative process is driven by heat resulting from illumination with blue light. In this configuration, the observed degradation (characterized by FTIR, absorption spectroscopy, electron microscopy and mechanical testing) under photothermal heating is almost identical to that due to heating conventionally, as expected due to the non-local oxidative process. We discuss these results and next steps.   

Phase field model for reactive blending of a symmetric binary polymer blend

Reactive blending is a process for forming polymer alloys where mutually reactive homopolymers are mixed together to form compatibilizers in-situ. The dynamics of the problem is replete with rich nonequilibrium physics stemming from reactions between the polymers, spatiotemporal transport, thermodynamics of mixing, and externally imposed processing conditions, and has hence been poorly understood. To study this process, we develop a coarse-grained phase-field model designed to capture all of the above physical effects. We then implement the model using pseudospectral collocation and semi-implicit time stepping to solve for temporal evolution of concentration distributions. In the case of reaction-diffusion dynamics, we find that the reaction typically progresses with the formation of an interfacial, diblock-rich layer. Eventually, the diblock diffuses out of the interface and the homopolymers diffuse in, as the reaction proceeds toward completion. The reaction rates in each stage depend on the incompatibility of the species, the Damkohler number and the initial distribution.   

Integrating Particle and Field-Theoretic Simulations: A Multiscale Approach to Complex Polymeric Solutions

Our work investigates the phase-behavior of complex polymeric solutions leveraging the strengths of both particle and polymer field-theoretic simulations. Mesostructured polymeric solutions are difficult to simulate using traditional particle-explicit approaches due to the disparate time and length scales, while the predictive capability of field-theoretic simulations is hampered by the need to specify emergent parameters (e.g., chi parameters) with nonobvious connections to molecular architecture. To overcome the weaknesses of both, we discuss an original way to use small-scale, atomistic simulations to parameterize statistical field theory models. Subsequently, field-theoretic simulations can probe behavior at larger length scales in polymeric solutions efficiently while maintaining a connection to the underlying polymer chemistry. This synergistic approach to polymer simulations opens the door to explore–de-novo–a wide variety of polymeric solution phase behavior. In particular, we will show how the above machinery can predict complex inter-colloidal potentials-of-mean force as modulated by the surrounding polymeric solution.   

Hydration of the polymer block in globular protein-polymer bioconjugates

Protein-polymer bioconjugates are a type of AB block copolymer where one block is a globular protein. The self-assembly of these systems in solution differ significantly from that of synthetic coil block copolymers. The chemistry of the polymer block and the resulting interactions with the solvent (water) can tune the phase behavior of bioconjugates. Previously, our group has observed that changing the hydrogen bonding of the polymer can shift phase boundaries, create new phases, and change order-disorder transitions. Using small angle neutron scattering (SANS), we have devised a contrast matching method to quantify the hydration number, the number of water molecules associated with a repeat unit, of polymers that can be conjugated to globular proteins. This generalizable method can be applied to polymer solutions above the overlap concentration and does not require any underlying assumptions. The SANS data show that there are differences in the hydration of zwitterionic polymers, hydrogen bond donors, and hydrogen bond acceptors. Moreover, the hydration number correlates with a polymer’s tendency to order and the polymer concentration. These results suggest that water has a significant impact on self-assembly and thus material design.   

Controlled Crystallization of Small Organic Molecules using Polymers: the Role of Hidden Liquid-Liquid Phase Domain

We investigated the crystallization behaviors of model organic compound pyrene mixed with polystyrene using differential scanning calorimetry. The thermal characterization of the pyrene and polystyrene mixtures provided extensive thermodynamic information regarding the phase states and transition temperatures. Using the glass transition temperatures, the chemical compositions of phase-separated phases are elucidated. Remarkably, the crystallization temperature curve of pyrene of the mixtures shows a clear signature of upper critical solution temperature (UCST) with the composition. We attribute the UCST signature to the hidden liquid-liquid phase domain of pyrene and polystyrene in which the nucleation barrier of crystallization is significantly depressed by the spinodal decomposition of liquid phases. This result suggests that polymer additives can be effectively utilized to control the crystallization behaviors.
  

Dynamics of photoresponsive molecules in glassy solids

The photoresponse of azobenzene and azobenzene-like molecules undergoing the trans → cis photoisomerization transition has been primarily explored through simulation and experiment in solution or vacuum. The response and behavior of these photoactive molecules in solids, where barriers to rearrangement are significantly higher, is less well characterized. Here we use molecular dynamics simulations to study the behavior of azobenzene and disperse red one during and after photoactivation in glassy samples. Data show that the dynamics of photoactivated molecules and the surrounding solid depends little on the sample temperature or measures of local stability such as density and energy. On the other hand, the height of transition barriers, which depends strongly on intermolecular interactions, cooling rate and temperature, is reflected in the waiting time between photoactivation and a local transition. The timescale for flipping is compared with the timescale for rotational and diffusive motion. The rotation of azobenzene or disperse red one around the long axis of the molecule is correlated with the wait time or barrier to flipping while the translational motion and the rotation of the long-axis itself is less correlated with the flipping wait time.