A Review of Polysiloxanes in Terms of Their Application in Explosives

Synthesis and Properties of Modified Biodegradable Polymers Based on Caprolactone

In this paper, the synthesis and characterization of two polycaprolactone-polydimethylsiloxane (PDMS-CL) copolymers with biodegradable properties are reported. A comparative study was carried out using an aminopropyl-terminated polydimethylsiloxane macro-initiator (APDMS) with two different molecular weights. The copolymers (PDMS-CL-1 and PDMS-CL-2) were obtained by ring-opening polymerization of ɛ-caprolactone using APDMS as initiators and stannous 2-ethylhexanoate as a catalyst. The copolymer's structures were confirmed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (1H-NMR) spectra, and energy dispersion spectroscopy (EDX). Surface morphology was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The hydrophobic properties of the copolymers were demonstrated by the water contact angle and water vapor sorption capacity. Additionally, biological tests were conducted on San Marzano type tomato plants (Lypercosium esculentum) to assess the synthesized copolymers' susceptibility to the environment in terms of biological stability and metabolic activity. The biodegradation of PDMS-CL-1 and PDMS-CL-2 copolymers does not have a dangerous effect on the metabolic activity of plants, which makes it a convenient product in interaction with the environment.

Multi-aspect and comprehensive atomic insight: the whole process of thermolysis of HMX/Poly-NIMMO-based plastic bonded explosive

Based on the reactive molecular dynamics, the whole process of thermolysis of HMX/Poly-NIMMO-based PBX was studied in detail at the micro scale, which provided a novel atomic insight into the thermolysis mechanism of HMX/Poly-NIMMO. Further, it was compared with the HMX single substance system to explore the influence of binder on thermolysis of HMX. According to the findings, the activation energy required by pyrolysis of HMX in the mixed system is much less than that required by the pure HMX system at both phases. From the point of view of reaction energy, poly-NIMMO promoted the thermolysis of HMX. Especially, the mechanism analysis confirmed this point. The nitro and hydroxyl groups detached from Poly-NIMMO will react with HMX, and the generated HNO2 will further accelerate the decomposition process of nitrogen heterocycles. In addition, the number of the final products H2O and H2 in the two-component system increased greatly, but the number of CO2 and N2 molecules had not changed much, and C clusters were formed in the system. The evolution trend of bond number further verified the above analysis. While the maximum cluster number does drop with increasing temperature at first, after a particular temperature threshold is reached, it remains unchanged. In a nutshell, poly-NIMMO will hasten HMX's thermolysis and reduce the system's stability when subjected to heat.

“Green” PBX Formulations Based on High Explosives (RDX and HMX) and Water-Soluble pH-Sensitive Polymeric Binders

The increasingly harsher and more complex international and European environmental legislation drives the current development of “greener” energetics materials and munitions. The aerospace and defense industries rely on extensive research in the formulation and scale-up production of polymer-bonded explosives (PBX). In this context, this paper aims to present a versatile method for obtaining “green” PBX formulations based on two high explosives (hexogen (RDX) and octogen (HMX)) and acrylic acid—ethyl acrylate copolymeric binders. This study developed an innovative “eco-friendly” technology for coating the RDX and HMX crystals, allowing straightforward and safer manufacture of PBX, avoiding the use of traditional organic solvents. At the same time, these polymeric binders are soluble in water at a slightly alkaline pH and insoluble at acidic or neutral pH, thus ensuring a safer manipulation of the energetic materials during their entire life cycle and a facile recovery of the explosive in its original shape and morphology in demilitarization. The PBX formulations were characterized via specific analytical tools to evaluate the influence of their composition on the safety and performance characteristics: scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), alkaline pH solubility tests, differential thermal analysis (DTA), impact sensitivity test (BAM Fall Hammer Test), friction sensitivity test (BAM Friction Test), electrostatic sensitivity test (ESD), vacuum stability test, small scale shock reactivity test (SSRT), detonation velocity test. The “green” PBX formulations obtained through a simple and innovative coating method, based on the polymeric binders’ adjustable water solubility, demonstrated remarkable energetic performances and a facile recovery of the explosive crystals by the dissolution of the polymeric binder at pH 11 and 30 °C.

Polydimethylsiloxane/Magnesium Oxide Nanosheet Mixed Matrix Membrane for CO2 Separation Application

Carbon dioxide (CO₂) concentration is now 50% higher than in the preindustrial period and efforts to reduce CO₂ emission through carbon capture and utilization (CCU) are blooming. Membranes are one of the attractive alternatives for such application. In this study, a rubbery polymer polydimethylsiloxane (PDMS) membrane is incorporated with magnesium oxide (MgO) with a hierarchically two-dimensional (2D) nanosheet shape for CO₂ separation. The average thickness of the synthesized MgO nanosheet in this study is 35.3 ± 1.5 nm. Based on the pure gas separation performance, the optimal loading obtained is at 1 wt.% where there is no observable significant agglomeration. CO₂ permeability was reduced from 2382 Barrer to 1929 Barrer while CO₂/N₂ selectivity increased from only 11.4 to 12.7, and CO₂/CH₄ remained relatively constant when the MMM was operated at 2 bar and 25 °C. Sedimentation of the filler was observed when the loading was further increased to 5 wt.%, forming interfacial defects on the bottom side of the membrane and causing increased CO₂ gas permeability from 1929 Barrer to 2104 Barrer as compared to filler loading at 1 wt.%, whereas the CO₂/N₂ ideal selectivity increased from 12.1 to 15.0. Additionally, this study shows that there was no significant impact of pressure on separation performance. There was a linear decline of CO₂ permeability with increasing upstream pressure while there were no changes to the CO₂/N₂ and CO₂/CH₄ selectivity.

A Fusion of Molecular Imprinting Technology and Siloxane Chemistry: A Way to Advanced Hybrid Nanomaterials

Molecular imprinting technology is a well-known strategy to synthesize materials with a predetermined specificity. For fifty years, the "classical" approach assumed the creation of "memory sites" in the organic polymer matrix by a template molecule that interacts with the functional monomer prior to the polymerization and template removal. However, the phenomenon of a material's "memory" provided by the "footprint" of the chemical entity was first observed on silica-based materials nearly a century ago. Through the years, molecular imprinting technology has attracted the attention of many scientists. Different forms of molecularly imprinted materials, even on the nanoscale, were elaborated, predominantly using organic polymers to induce the "memory". This field has expanded quickly in recent years, providing versatile tools for the separation or detection of numerous chemical compounds or even macromolecules. In this review, we would like to emphasize the role of the molecular imprinting process in the formation of highly specific siloxane-based nanomaterials. The distinct chemistry of siloxanes provides an opportunity for the facile functionalization of the surfaces of nanomaterials, enabling us to introduce additional properties and providing a way for vast applications such as detectors or separators. It also allows for catalyzing chemical reactions providing microreactors to facilitate organic synthesis. Finally, it determines the properties of siloxanes such as biocompatibility, which opens the way to applications in drug delivery and nanomedicine. Thus, a brief outlook on the chemistry of siloxanes prior to the discussion of the current state of the art of siloxane-based imprinted nanomaterials will be provided. Those aspects will be presented in the context of practical applications in various areas of chemistry and medicine. Finally, a brief outlook of future perspectives for the field will be pointed out.

Mesoscale Modeling of Phase Separation Controlled by Hydrosilylation in Polyhydromethylsiloxane (PHMS)-Containing Blends

Controlling morphology of polysiloxane blends crosslinked by the hydrosilylation reaction followed by pyrolysis constitutes a robust strategy to fabricate polymer-derived ceramics (PDCs) for a number of applications, from water purification to hydrogen storage. Herein, we introduce a dissipative particle dynamics (DPD) approach that captures the phase separation in binary and ternary polymer blends undergoing hydrosilylation. Linear polyhydromethylsiloxane (PHMS) chains are chosen as preceramic precursors and linear vinyl-terminated polydimethylsiloxane (v-PDMS) chains constitute the reactive sacrificial component. Hydrosilylation of carbon-carbon unsaturated double bonds results in the formation of carbon-silicon bonds and is widely utilized in the synthesis of organosilicons. We characterize the dynamics of binary PHMS/v-PDMS blends undergoing hydrosilylation and ternary blends in which a fraction of the reactive sacrificial component (v-PDMS) is replaced with the non-reactive sacrificial component (methyl-terminated PDMS (m-PDMS), polyacrylonitrile (PAN), or poly(methyl methacrylate) (PMMA)). Our results clearly demonstrate that the morphology of the sacrificial domains in the nanostructured polymer network formed can be tailored by tunning the composition, chemical nature, and the degree of polymerization of the sacrificial component. We also show that the addition of a non-reactive sacrificial component introduces facile means to control the self-assembly and morphology of these nanostructured materials by varying the fraction, degree of polymerization, or the chemical nature of this component.

Fabrication, Thermal Conductivity, and Mechanical Properties of Hexagonal-Boron-Nitride-Pattern-Embedded Aluminum Oxide Composites

As electronics become more portable and compact, the demand for high-performance thermally conductive composites is increasing. Given that the thermal conductivity correlates with the content of thermally conductive fillers, it is important to fabricate composites with high filler loading. However, the increased viscosity of the composites upon the addition of these fillers impedes the fabrication of filler-reinforced composites through conventional methods. In this study, hexagonal-boron-nitride (h-BN)-pattern-embedded aluminum oxide (Al₂O₃) composites (Al/h-BN/Al composites) were fabricated by coating a solution of h-BN onto a silicone-based Al₂O₃ composite through a metal mask with square open areas. Because this method does not require the dispersion of h-BN into the Al₂O₃ composite, composites with high filler loading could be fabricated without the expected problems arising from increased viscosity. Based on the coatability and thixotropic rheological behaviors, a composite with 85 wt.% Al₂O₃ was chosen to fabricate Al/h-BN/Al composites. The content of the Al₂O₃ and the h-BN of the Al/h-BN/Al-1 composite was 74.1 wt.% and 12.8 wt.%, respectively. In addition to the increased filler content, the h-BN of the composite was aligned in a parallel direction by hot pressing. The in-plane (k_x) and through-plane (k_z) thermal conductivity of the composite was measured as 4.99 ± 0.15 Wm^-1 K^-1 and 1.68 ± 0.2 Wm^-1 K^-1, respectively. These results indicated that the method used in this study is practical not only for increasing the filler loading but also for achieving a high k_x through the parallel alignment of h-BN fillers.

Effects of Hybrid Polymeric Material Based on Polycaprolactone on the Environment

Polymers are of great interest in areas such as agriculture, medicine and pharmacy, the food and cosmetic industries, and the chemical and construction industries. However, many polymers are nonbiodegradable and are not environmentally friendly. They are highly resistant to degradation and therefore can lead to waste disposal problems. In recent years, the interest in the microbial degradation of polymeric materials has grown due to the desire for less waste pollution in the environment. In this study, the biodegradable polymer that was obtained by the ring-opening polymerization of ε-caprolactone (CL) using an aminopropyl-polydimethylsiloxane (APDMS) oligomer and the effects of the polymer towards the growth and development of tomato plants (Lypercosium esculentum) were investigated. The obtained product was characterized using FTIR spectroscopy, NMR spectroscopy, and energy dispersion spectroscopy (EDX) analysis, and the effects of this compound on the evolution of tomato plants (Lypercosium esculentum) were studied. We also studied the biological stability of the product by identifying some of the microorganisms that developed on the surface, given its susceptibility to biodegradation.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

Ring-Opening Polymerization (ROP) and Catalytic Rearrangement as a Way to Obtain Siloxane Mono- and Telechelics, as Well as Well-Organized Branching Centers: History and Prospects

PDMS telechelics are important both in industry and in academic research. They are used both in the free state and as part of copolymers and cross-linked materials. At present, the most important, practically used, and well-studied method for the preparation of such PDMS is diorganosiloxane ring-opening polymerization (ROP) in the presence of nucleophilic or electrophilic initiators. In our brief review, we reviewed the current advances in the field of obtaining polydiorganosiloxane telechelics and monofunctional PDMS, as well as well-organized branching centers by the ROP mechanism and catalytic rearrangement, one of the first and most important reactions in the polymer chemistry of silicones, which remains so at the present time.

A Novel Approach for the Creation of Electrically Controlled LC:PDMS Microstructures

This work presents research on unique optofluidic systems in the form of air channels fabricated in PDMS and infiltrated with liquid crystalline material. The proposed LC:PDMS structures represent an innovative solution due to the use of microchannel electrodes filled with a liquid metal alloy. The latter allows for the easy and dynamic reconfiguration of the system and eliminates technological issues experienced by other research groups. The paper discusses the design, fabrication, and testing methods for tunable LC:PDMS structures. Particular emphasis was placed on determining their properties after applying an external electric field, depending on the geometrical parameters of the system. The conclusions of the performed investigations may contribute to the definition of guidelines for both LC:PDMS devices and a new class of potential sensing elements utilizing polymers and liquid crystals in their structures.

Simple Preparation of Polydimethylsiloxane and Polyurethane Blend Film for Marine Antibiofouling Application

A key way to prevent undesirable fouling of any structure in the marine environment, without harming any microorganisms, is to use a polymer film with high hydrophobicity. The polymer film, which was simply prepared from a blend of hydrophobic polydimethylsiloxane elastomer and hydrophilic polyurethane, showed improved properties and economic viability for antifouling film for the marine industry. The field emission scanning electron microscope and energy dispersive X-ray spectrometer (FESEM and EDX) results from the polymer blend suggested a homogenous morphology and good distribution of the polyurethane disperse phase. The PDMS:PU blend (95:5) film gave a water contact angle of 103.4° ± 3.8° and the PDMS film gave a water contact angle of 109.5° ± 4.2°. Moreover, the PDMS:PU blend (95:5) film could also be modified with surface patterning by using soft lithography process to further increase the hydrophobicity. It was found that PDMS:PU blend (95:5) film with micro patterning from soft lithography process increased the contact angle to 128.8° ± 1.6°. The results from a field test in the Gulf of Thailand illustrated that the bonding strength between the barnacles and the PDMS:PU blend (95:5) film (0.07 MPa) were lower than the bonding strength between the barnacles and the carbon steel (1.16 MPa). The barnacles on the PDMS:PU blend (95:5) film were more easily removed from the surface. This indicated that the PDMS:PU blend (95:5) exhibited excellent antifouling properties and the results indicated that the PDMS:PU blend (95:5) film with micro patterning surface could be employed for antifouling application.

Siloxane Matrix Molecular Weight Influences the Properties of Nanocomposites Based on Metal Complexes and Dielectric Elastomer

Siloxane-based elastomers are some of the most sought-after materials for the construction of actuators and equipment for energy harvesting devices. This article focuses on changes of the mechanical (breaking stress, breaking strain, Young's modulus) and dielectric properties for elastomers prepared with silicones, induced by the variation of molecular weight of the matrix, with three different silicone polymers having 60,000 g/mol, 150,000 g/mol, and 450,000 g/mol (from GPC measurements). Multiple siloxane elastomers were crosslinked with methyltriacetoxysilane using the sol-gel route. The dielectric permittivity values of the elastomers were also enhanced with two different complex structures containing siloxane bond and 3d transition metals as filler materials for polydimethylsiloxane polymers with various molecular weights. The dielectric spectroscopy tests demonstrated a small decrease (5%) for the values of the dielectric permittivity in relation to increased molecular weight of the siloxane polymer, both for samples prepared with pure polymer and for samples with metal complexes. The samples of nanocomposites showed a >50% increase of dielectric permittivity values relative to samples prepared of pure siloxane elastomer. The thermal tests demonstrated that the nanocomposites retained thermal stability similar with samples prepared of pure siloxane elastomer. The behavior under controlled conditions of humidity showed a trend of increased water vapor sorption with increasing molecular weight but an overall hydrophobic stable character of nanocomposites.