Reactive molecular dynamics simulation on the disintegration of Kapton, POSS polyimide, amorphous silica, and teflon during atomic oxygen impact using the ReaxFF reactive force-field method

The formation mechanism of Sc-based metallofullerenes: a molecular dynamics simulation study

The practical applications of endohedral metallofullerenes (EMFs) are mainly constrained by their low yields. Understanding the formation mechanisms is therefore crucial for developing methods for high-yield and selective synthesis. To address this, a novel force-field parameter set, "CSc.ff", was created using a single-parameter search optimization method, then molecular dynamics simulations of various systems with a carbon-to-scandium atomic ratio of 12.5 were carried out. The simulations were run under a constant atomic number, volume, and energy (NVE) ensemble. The influence of the working gas, helium, as well as temperature gradients on the formation process was examined. Our simulations reveal that the cage growth patterns of Sc-based EMFs (Sc-EMFs) closely resemble those of hollow fullerenes, evolving from free carbon atoms to chains, rings, and, ultimately, to cage-shaped clusters. Importantly, the Sc-EMFs formed in the simulation frequently exhibit structural defects or under-coordinated carbon atoms. Scandium atoms, whether at the periphery or on the surface of these cages, can be incorporated into the cages, forming Sc-EMFs. Helium was found to not only promote the formation of carbon cages but also facilitate the encapsulation of scandium atoms, playing a crucial role in the formation of cluster fullerenes. Moreover, cooling effectively inhibits the uncontrollable growth of the carbon cage and is essential for forming stable, appropriate-sized cages. This study enhances our understanding of the formation of Sc-EMFs and provides valuable insights for developing more efficient synthetic methods.

Reactive Molecular Dynamics Study of the Mechanism and Effect of Various Protective Coatings on the Protection of Polyimide Antierosion from Atomic Oxygen

Polyimide (PI), due to its exceptional performance, is commonly utilized in spacecraft. However, when such polymers are used in spacecraft navigating low Earth orbit, they are exposed to atomic oxygen (AO) that can cause the polymer to decompose. A protective coating method is a more effective way to safeguard the polymer from erosion caused by AO. This study employs the molecular dynamics simulation based on the reaction force field to investigate the protective effects of various coatings, including polydimethylsiloxane (PDMS), graphene (Gr), polytetrafluoroethylene (PTFE), and the (0 0 1), (0 1 1), and (1 1 1) surfaces of SiO2. The results indicate that the protective performance of the (0 1 1) surface is superior to that of the (0 0 1) and (1 1 1) surfaces. Moreover, protective coatings are classified into three categories based on different protective mechanisms: rebound, absorption, and sacrificial. The protective effectiveness of coatings depends on their anti-AO performance and ability to combine with the substrate. Gr displays exceptional anti-AO properties and can effectively shield the substrate from AO erosion. Silicone-based coatings have a superior ability to adhere to PI substrates, and PDMS is an excellent choice for protective coatings. This paper offers guidance for the protective coating method of PIs against AO erosion.

Mesoscale Simulation of Polymer Pyrolysis by Coarse-Grained Molecular Dynamics: A Parametric Study

Full comprehension of the pyrolysis of polymer materials is crucial for the design and application of thermal protection systems; however, it involves complex phenomena at different spatial and temporal scales. To bridge the gap between the abundant atomistic simulations and continuum modeling in the literature, we perform a novel mesoscale study of the pyrolysis process using coarse-grained molecular dynamics (CG MD) simulations. Polyethylene (PE) consisting of united atoms including implicit hydrogen is considered a model polymer, and the configurational change of PE in thermal degradation is modeled by applying the bond-breaking phenomenon based on bond energy or bond length criteria. A cook-off simulation is implemented to optimize the heuristic protocol of bond dissociation by comparing the reaction products with a ReaxFF simulation. The aerobic hyperthermal pyrolysis under oxygen bombardment is simulated at a large scale of hundreds of nanometers to observe the intricate phenomena occurring from the surface to the depth inside the material. The intrinsic thermal durability of the model polymer at extreme conditions with and without oxygen environment can be effectively simulated from the proposed mesoscale simulation to predict important thermal degradation properties required for continuum-scale pyrolysis and ablation simulations. This work serves as an initial investigation of polymer pyrolysis at the mesoscale and helps understand the concept at a larger scale.

Molecular Dynamic Simulation of Ni-Al Alloy-H2O Reactions Using the ReaxFF Reactive Force Field

Hydrogen as clean energy can effectively solve the problems of fossil energy shortage and environmental pollution. However, traditional methods of H₂ production are generally lacking in application value. The procedure for manufacturing H₂ by a reaction between active metals and H₂O has received wide attention due to its high efficiency. Profound insights into the mechanism and influencing factors of H₂ production from active metals are insufficient. The ReaxFF reaction force field module of the Amsterdam Modeling Suite (AMS) is applied in this paper to simulate the reaction of Ni-Al alloys with H₂O. It reveals the reaction route of H₂ production at the atomic level. The calculation results show that Al is the most critical active site. Moreover, the H₂ production capacity of the alloy varies with the crystal structure and atomic ratio. The H₂ production rate decreases due to the influence of the water solvation layer and surface coverage. Oxygen reduces the H₂ production capacity because oxygen reduces the active sites for H₂O adsorption by forming a stable oxide layer with Al.

A Machine Learning-Combined Flexible Sensor for Tactile Detection and Voice Recognition

Intelligent sensors have attracted substantial attention for various applications, including wearable electronics, artificial intelligence, healthcare monitoring, and human-machine interactions. However, there still remains a critical challenge in developing a multifunctional sensing system for complex signal detection and analysis in practical applications. Here, we develop a machine learning-combined flexible sensor for real-time tactile sensing and voice recognition through laser-induced graphitization. The intelligent sensor with a triboelectric layer can convert local pressure to an electrical signal through a contact electrification effect without external bias, which has a characteristic response behavior when exposed to various mechanical stimuli. With the special patterning design, a smart human-machine interaction controlling system composed of a digital arrayed touch panel is constructed to control electronic devices. Based on machine learning, the real-time monitoring and recognition of the changes of voice are achieved with high accuracy. The machine learning-empowered flexible sensor provides a promising platform for the development of flexible tactile sensing, real-time health detection, human-machine interaction, and intelligent wearable devices.

Dependence of Incidence Angle and Flux Density in the Damage Effect of Atomic Oxygen on Kapton Film

Kapton film is a polymeric material widely used on low-Earth-orbit (LEO) spacecraft surfaces. In the LEO environment, atomic oxygen (AO) is spaceflight materials' most destructive environmental factor. The erosion mechanism of AO on Kapton films has long been an important issue, where the parameter dependence of the AO effect has received increasing attention. Studies of AO energy and cumulative flux have been extensively carried out, while the influence mechanism of the incidence angle and flux density is not fully understood. The AO incidence angle and flux density in space are diverse, which may cause different damage effects on aerospace materials. In this paper, the dependence of the incidence angle and flux density in the damaging effect of AO on Kapton films was investigated using ground-based AO test technology and the reactive molecular dynamics (ReaxFF MD) simulation technique. Firstly, the ground-based experiment obtained the mass loss data of Kapton films under the action of AO with a variable incidence angle and flux density. Then, the mass loss, temperature rise, product, and erosion yield of Kapton during AO impact with different incidence angles and dose rates were calculated using the ReaxFF MD method. The influences of the incidence angle and flux density on the damage mechanism of the AO effect were discussed by comparing the simulation and test results. The results show that the AO effect in the lower incidence angle range (0-60°) is independent of the incidence angle and depends only on the amount of impacted atomic oxygen. AO in the higher incidence angle range (60-90°) has a surface stripping effect, which causes more significant mass loss and a temperature rise while stripping raised macromolecules from rough surfaces, and the erosion effect increases with the increasing incidence angle and amount of impacted atomic oxygen. There is a critical value for the influence of flux density on the AO effect. Above this critical value, AO has a reduced erosive capacity due to a lower chance of participating in the reaction. The amount of each main product from the AO effect varies with the incidence angle and flux density. Nonetheless, the total content of the main products is essentially constant, around 70%. This work will contribute to our understanding of the incidence angle and flux density dependence of the AO effect and provide valuable information for the development of standards for ground simulation tests.

Mechanically Robust Irradiation, Atomic Oxygen, and Static-Durable CrOx/CuNi Coatings on Kapton Serving as Space Station Solar Cell Arrays

The polymers that served for solar cell arrays are constantly subject to various hazards, such as atomic oxygen (AO), ion irradiation, or electrostatic discharge (ESD) events. To address these issues, we fabricated and sifted CrO_0.16/CuNi-coated Kapton with a gradient structure with the goal of reaching an equilibrium between AO durability and resistance. The resulting material exhibits an impressively low E_y of 6.61 × 10^-26 cm³ atom^-1, 2.20% of which was detected as pristine Kapton. Self-evolution of the CrO_0.16 coating under 525.4 displacement per atom (dpa) Fe⁺ ion irradiation indicated that it can still maintain a good state of ultrafine nanocrystalline in addition to local amorphization. Its AO-based degradation and irradiation evolution are demonstrated by molecular dynamics (MD) simulations. It is mechanically robust enough to endure the cyclic folding treatments attributed to its gradient structure fabrication. Moreover, the CrO_0.16/CuNi-coated Kapton exhibits alleviated electrostatic accumulation capability and sufficient conductivity. Our strategy has promising potential for creating surface protection on flexible polymers operating in the low Earth orbit (LEO).

How Polytetrafluoroethylene Lubricates Iron: An Atomistic View by Reactive Molecular Dynamics

The tribochemistry and transfer film formation at the metal/polymer interface plays an essential role in surface protection, wear reduction, and lubrication. Although the topic has been studied for decades, challenges persist in clarifying the nanoscale mechanism and dynamic evolution of tribochemical reactions. To investigate the tribochemistry between iron and polytetrafluoroethylene (PTFE) in ambient and cryogenic environments, we have trained and expanded a ReaxFF reactive force field to describe iron-oxygen-water-PTFE systems (C/H/O/F/Fe). Using ReaxFF molecular dynamics simulations, we find that mechanical shearing of single asperity induced the degradation of PTFE molecules and radicals, showing subsequent oxidation and hydroxylation reactions of the radicals initiated by C-C bond cleavage, in agreement with previous experimental observations. Furthermore, we studied mechanisms of interfacial tribochemical reactions and formation of transfer films. We found that tribochemical wear and Fe-C and Fe-F bonding networks are important mechanisms for anchoring molecular chains to form a transfer film on the iron countersurface. Hydroxyl groups can dehydrogenate to form short and strong chelation bonds with the Fe₂O₃ countersurface. A friction-induced oriented molecular layer plays a key role in reducing friction, which is responsible for the excellent lubrication property. By varying temperatures in the range of 10-300 K, we found a nonmonotonic change in friction with a maxima at 100 K. At cryogenic temperatures, the molecular mobility was obviously suppressed, while the chain rigidity was enhanced, resulting in the less oriented interface and brittle-like shear interface, which is responsible for nonmonotonic friction. This work elucidates mechanisms of tribochemical reactions and transfer film formation between iron and PTFE at the atomistic level, facilitating design and development of self-lubricating materials, especially under harsh conditions.

Evaluation of Radiation Resistance of Polystyrene Using Molecular Dynamics Simulation

The scission rates of polystyrene and fluorinated polystyrene irradiated in an irradiation facility with Co-60 γ-rays were determined using molecular dynamics simulation and gel permeation chromatography (GPC) molecular weight distributions. The prediction was based on the assumption that γ-ray energy is transferred to the initial velocity of the primary knock-on atom. We employed a molecular dynamics simulation procedure to compute the changes in bond length between the connections for selected values of the absorbed dose and compared the calculated values with measurements made on the irradiated samples. The samples were exposed to four different absorbed doses of 25, 50, 75, and 100 kGy. The scission process and scission ratio were simulated with LAMMPS with ReaxFF potential for each bond, and we compared the simulation results with the experimental data especially measuring average molecular weight to evaluate the effect of fluorination on radiation enhancement.

Molecular Dynamics Simulation of Silicon Dioxide Etching by Hydrogen Fluoride Using the Reactive Force Field

In this study, we develop a reactive force field (ReaxFF) for a Si/O/H/F system to perform etching simulations of SiO₂ with an HF etchant. Quantum mechanical (QM) training sets from density functional theory calculations, which contain structures of reactant/product and energies with bond dissociation, valence angle distortions, and reactions between SiO₂ clusters and SiO₂ slab with HF gases, are used to optimize the ReaxFF parameters. Structures and energies calculated using the ReaxFF match well with the QM training sets. Using the optimized ReaxFF, we conduct molecular dynamics simulations of the etching process of SiO₂ substrates with active HF molecules. The etching yield and number of reaction products with different incident energies of the HF etchant are investigated. These simulations show that the developed ReaxFF offers insights into the atomistic surface reaction of the SiO₂ etching process.

ReaxFF/AMBER-A Framework for Hybrid Reactive/Nonreactive Force Field Molecular Dynamics Simulations

Combined quantum mechanical/molecular mechanical (QM/MM) models using semiempirical and ab initio methods have been extensively reported on over the past few decades. These methods have been shown to be capable of providing unique insights into a range of problems, but they are still limited to relatively short time scales, especially QM/MM models using ab initio methods. An intermediate approach between a QM based model and classical mechanics could help fill this time-scale gap and facilitate the study of a range of interesting problems. Reactive force fields represent the intermediate approach explored in this paper. A widely used reactive model is ReaxFF, which has largely been applied to materials science problems and is generally used as a stand-alone (i.e., the full system is modeled using ReaxFF). We report a hybrid ReaxFF/AMBER molecular dynamics (MD) tool, which introduces ReaxFF capabilities to capture bond breaking and formation within the AMBER MD software package. This tool enables us to study local reactive events in large systems at a fraction of the computational costs of QM/MM models. We describe the implementation of ReaxFF/AMBER, validate this implementation using a benzene molecule solvated in water, and compare its performance against a range of similar approaches. To illustrate the predictive capabilities of ReaxFF/AMBER, we carried out a Claisen rearrangement study in aqueous solution. In a first for ReaxFF, we were able to use AMBER's potential of mean force (PMF) capabilities to perform a PMF study on this organic reaction. The ability to capture local reaction events in large systems using combined ReaxFF/AMBER opens up a range of problems that can be tackled using this model to address both chemical and biological processes.

Effect of 172-nm UV irradiation on polyimide and its application in surface modification by grafting

Polyimides (PIs) have a wide range of industrial and scientific applications due to their excellent thermal and mechanical stability and chemical resistance. Their response to ultraviolet (UV) irradiation is of further interest in high-value applications such as spacecraft technology and electronics packaging. In this work, we investigated the effect of 172-nm UV xenon excimer lamp irradiation on samples of pyromellitimido-oxydianiline (PMDA-ODA) commercial films in the absence of oxygen. The average irradiance received at the sample position was 90 mW/cm2, and the total radiation dosage varied from 0 to 64 J/cm2. X-Ray photoelectron spectroscopy, time-of-flight-secondary ion mass spectrometry, atomic force microscopy, and contact angle measurements were used to characterize the effect. Calculated UV-visible spectroscopy absorption spectra were obtained using the ZINDO//B3LYP/3-21G method to give an indication of which orbitals are involved in the transitions near 172 nm. The reactivity of the different UV-treated PI samples toward nitrogen-borne heptafluorodecene vapor was then investigated using the above techniques. Grafting reactions occurred on the surface of the photochemically activated polymer. This study explored the potential for modification of PI surfaces using UV-light-assisted grafting to impart valuable functionalities.

Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion

Metal combustion reaction is highly exothermic and is used in energetic applications, such as propulsion, pyrotechnics, powering micro- and nano-devices, and nanomaterials synthesis. Aluminum (Al) is attracting great interest in those applications because of its high energy density, earth abundance, and low toxicity. Nevertheless, Al combustion is hard to initiate and progresses slowly and incompletely. On the other hand, ultrathin carbon nanomaterials, such as graphene, graphene oxide (GO), and graphene fluoride (GF), can also undergo exothermic reactions. Herein, we demonstrate that the mixture of GO and GF significantly improves the performance of Al combustion as interactions between GO and GF provide heat and radicals to accelerate Al oxidation. Our experiments and reactive molecular dynamics simulation reveal that GO and GF have strong chemical and thermal couplings through radical reactions and heat released from their oxidation reactions. GO facilitates the dissociation of GF, and GF accelerates the disproportionation and oxidation of GO. When the mixture of GO and GF is added to micron-sized Al particles, their synergistic couplings generate reactive oxidative species, such as CF_x and CF_xO_y, and heat, which greatly accelerates Al combustion. This work demonstrates a new area of using synergistic couplings between ultrathin carbon nanomaterials to accelerate metal combustion and potentially oxidation reactions of other materials.

Advances in Polyimide-Based Materials for Space Applications

The space environment raises many challenges for new materials development and ground characterization. These environmental hazards in space include solar radiation, energetic particles, vacuum, micrometeoroids and debris, and space plasma. In low Earth orbits, there is also a significant concentration of highly reactive atomic oxygen (AO). This Progress Report focuses on the development of space-durable polyimide (PI)-based materials and nanocomposites and their testing under simulated space environment. Commercial PIs suffer from AO-induced erosion and surface electric charging. Modified PIs and PI-based nanocomposites are developed and tested to resist degradation in space. The durability of PIs in AO is successfully increased by addition of polyhedral oligomeric silsesquioxane. Conductive materials are prepared based on composites of PI and either carbon nanotube (CNT) sheets or 3D-graphene structures. 3D PI structures, which can expand PI space applications, made by either additive manufacturing (AM) or thermoforming, are presented. The selection of AM-processable engineering polymers in general, and PIs in particular, is relatively limited. Here, innovative preliminary results of a PI-based material processed by the PolyJet technology are presented.

Thermal Boundary Characteristics of Homo-/Heterogeneous Interfaces

The interface of two solids in contact introduces a thermal boundary resistance (TBR), which is challenging to measure from experiments. Besides, if the interface is reactive, it can form an intermediate recrystallized or amorphous region, and extra influencing phenomena are introduced. Reactive force field Molecular Dynamics (ReaxFF MD) is used to study these interfacial phenomena at the (non-)reactive interface. The non-reactive interfaces are compared using a phenomenological theory (PT), predicting the temperature discontinuity at the interface. By connecting ReaxFF MD and PT we confirm a continuous temperature profile for the homogeneous non-reactive interface and a temperature jump in case of the heterogeneous non-reactive interface. ReaxFF MD is further used to understand the effect of chemical activity of two solids in contact. The selected Si/SiO 2 materials showed that the TBR of the reacted interface is two times larger than the non-reactive, going from 1 . 65 × 10 - 9 to 3 . 38 × 10 - 9 m 2 K/W. This is linked to the formation of an intermediate amorphous layer induced by heating, which remains stable when the system is cooled again. This provides the possibility to design multi-layered structures with a desired TBR.

INFLUENCE OF NANOFILLER ON THERMAL DEGRADATION RESISTANCE OF HYDROGENATED NITRILE BUTADIENE RUBBER

Studies on the degradation of elastomers and their prevention have become increasingly important in recent years because of stringent environmental conditions in many industrial applications. The reactive atomistic simulation was executed on a hydrogenated acrylonitrile-butadiene rubber (HNBR40) model compound composed of 40 monomer units. The reactive simulation was used to study the decomposition behavior of HNBR40, to visualize different pyrolysis products, and also to analyze the degradation mechanism of HNBR40. Ethylene, propylene, and acrylonitrile were observed as dominant products at lower temperature, and 1-butene was found at higher temperature. Pyrolysis–gas chromatography–mass spectrometry was used to verify the decomposition products obtained from the prediction of atomistic simulation. In this study, nanofillers, especially nanoclays and nanosilicas, were used to prevent degradation significantly. Restricted degradation by the nanofiller-reinforced rubber prolonged the durability. Furthermore, the reactive simulation was performed to understand thermal decomposition characteristics of the model compound in the presence of the nanofiller. The initial decomposition temperature, the final degradation temperature, and the rate of degradation improved to a great extent on the addition of the model nanosilica compound as obtained from the simulation studies. Moreover, the lifetime of nanoclay- and nanosilica-reinforced hydrogenated acrylonitrile–butadiene rubber was calculated by using thermogravimetric analysis, and its useful lifetime was compared with that of the pristine polymer in the application temperature range of 150 °C.

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Study of thermal conductivity of ice clusters after impact deposition on the silica surfaces using the ReaxFF reactive force field

During aircraft or spacecraft missions, ice accumulates on different parts of their surface elements. An important parameter affecting the ability to remove this ice from the surface is the heat transfer characteristics of the accumulated ice. The ice heat transfer is related to the process of ice formation and its density and internal structure. In this study we investigate the effects of the ice and silica structure and the ice cluster attachment mechanism to the silica surface on the thermal conductivity (TC) of the attached ice cluster using the ReaxFF reactive force field. The purpose of this study is to investigate the thermal transport in amorphous and crystalline ice after high-velocity deposition on the silica surfaces. A dual thermostat method has been applied for the calculation of TC values. The validity of this method has been verified by comparing the calculated values of TC for crystal and amorphous ice with available experimental values. Our calculations show that the TC values of both crystal and amorphous ice drop after deposition on the silica surfaces. This decrease in the TC is more significant for the ice deposition on suboxide silica surfaces. Furthermore, crystal ice shows higher TC values than amorphous ice after accumulation. However, when crystal ice impacts on the silica surface at 1 km s(-1) impact speed, the crystalline shape of the ice cluster is lost to a considerable level and the TC values obtained for the ice clusters in such cases are closer to amorphous ice TC values. We observed a decrease in the TC values when ionic species are added inside the ice clusters.

Reactive Molecular Dynamics Simulations on the Disintegration of PVDF, FP-POSS, and Their Composite during Atomic Oxygen Impact

Poly(vinylidene fluoride) (PVDF) is a kind of important piezoelectric polymer used in spacecraft industry. But the atomic oxygen (AO) is the most abundant element in the low Earth orbit (LEO) environment. AO collision degradation is an important issue in the application of PVDF on spacecrafts. To investigate the erosion behaviors of PVDF during AO impacts and how to improve the stability of PVDF against AO impacts, the temperature evolution, mass loss, and erosion yields of neat PVDF, neat polyhedral oligomeric silsesquioxanes compound (3,3,3-trifluoropropyl)8Si8O12 (FP-POSS) and the PVDF/FP-POSS composite under AO impacts, as well as some key disintegrated structures and separated chemical compositions, were researched using the molecular dynamics (MD) simulations and the reactive ReaxFF force field. The simulation erosion yield result of PVDF is very close to the experiment results, which shows our simulations are reliable. The results of the temperature evolution, mass loss, and erosion yield of three materials show that the antierosion performance of PVDF is not outstanding. However, incorporating FP-POSS into PVDF matrix enhances the stability of PVDF against AO impact greatly and reduces the temperature rise, mass loss, and the erosion yield of PVDF rapidly. A detailed analysis on the flight chemical compositions and key snapshots of the structures reveals that the erosion process on PVDF and PVDF/FP-POSS is continuous and should be derived from the same PVDF matrix in two materials. In contrast, the erosion process on FP-POSS is stepped. The erosion will not take place until the number of AO reaches a specific value. There is a barrier for the erosion of high-energy AO because of the stable cagelike Si-O frame in FP-POSS molecules. This should be chiefly responsible for the high stability of FP-POSS and the reinforcement mechanism of FP-POSS on PVDF against AO impacts. This work is helpful for people to understand the erosion details of PVDF and POSS and provides valuable information to design effective protective structure for PVDF against AO impacts in LEO environment.

Photoactive fluoropolymer surfaces that release sensitizer drug molecules

We describe a physical-organic study of two fluoropolymers bearing a photoreleasable PEGylated photosensitizer that generates (1)O2((1)Δg) [chlorin e6 methoxy tri(ethylene glycol) triester]. The surfaces are Teflon/poly(vinyl alcohol) (PVA) nanocomposite and fluorinated silica. The relative efficiency of these surfaces to photorelease the PEGylated sensitizer [shown previously to be phototoxic to ovarian cancer cells (Kimani, S. et al. J. Org. Chem 2012, 77, 10638)] was slightly higher for the nanocomposite. In the presence of red light and O2, (1)O2 is formed, which cleaves an ethene linkage to liberate the sensitizer in 68-92% yield. The fluoropolymers were designed to deal with multiple problems. Namely, their success relied not only on high O2 solubility and drug repellency but also on the C-F bonds, which physically quench little (1)O2, for singlet oxygen's productive use away from the surface. The results obtained here indicate that Teflon-like surfaces have potential uses in delivering sensitizer and singlet oxygen for applications in tissue repair and photodynamic therapy (PDT).

A novel structural polyimide material with synergistic phosphorus and POSS for atomic oxygen resistance

A series of phosphorus-containing polyimide–POSS nanocomposites were synthesized, phosphorus and POSS synthetic self-passivating layers were formed to protect the underlying material from AO attack.