Reversible modulus reinforcement of end-linked polydimethylsiloxane ionomer networks

We report the mechanical behavior and swelling properties of covalently end-linked networks of polydimethylsiloxane with tailored number of monomers between side carboxyl groups and number of carboxyl groups along the chain. Imperfect carboxyl networks are reinforced by neutralization of the carboxyl groups with gallium ions via conversion of pendent chains into elastically active strands due to formation of inter-molecular ionic cross-links. These networks swell in non-polar solvents to a similar degree as unmodified end-linked PDMS networks of comparable moduli. For unannealed samples, the modulus reinforcement is reversed by a polar THF:water solution that breaks the ionic cross-links. Neutralization with transition metal cations such as cobalt causes no initial reinforcement due to weak intra-molecular interactions. Reinforcement in annealed gallium, cobalt or carboxyl networks is not readily reversible.     

Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

The products of the thermal degradation of polydimethylsiloxane (PDMS) are determined by the heating conditions, since two competing mechanisms are involved.Cyclic oligomers are formed in the low degradation temperature range and during slow heating in programmed degradation. This involves molecular splitting of oligomers from loop conformations of the PDMS chain favoured by its flexibility, and assistance on the part of empty silicon d-orbitals.Methane and oligomers are formed in the high temperature range and during fast heating. This shows that homolytic scission of Si-CH₃ also takes place and is followed by hydrogen abstraction.     

聚硅氧烷/聚醚聚氨酯弹性体的合成与表征

以聚二甲基硅氧烷(PDM S)与聚丁醚(PTM G)为混合软段与4,4'-二苯甲基二异氰酸酯(M D I)、1,4-丁二醇(1,4-BD)制备出一系列的聚氨酯共聚物。用FT IR、DM A、TGA以及AFM分别对共聚物的结构、热性质以及表面形态进行了表征。结果表明,共聚物含有复杂的微相分离结构,而且与纯聚氨酯相比,含硅氧烷的聚氨酯比纯聚氨酯具有更好的热稳定性。     

Flame retardant mechanism of polydimethylsiloxane material containing platinum compound studied by analytical pyrolysis techniques and alkaline hydrolysis gas chromatography

The flame retardant mechanism for silicone rubber induced by adding ppm order of a platinum compound, platinum (0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex was studied by analytical pyrolysis techniques and alkaline hydrolysis gas chromatography. The thermal degradation measurement of the flame retardant silicone rubber sample (FR-SR) indicated that a considerable amount of methane was evolved from the FR-SR sample at around 400-800 °C while formations of cyclic siloxanes were fairly suppressed in comparison with that from the control SR sample. On the other hand, on the gas chromatogram of the hydrolysis products from the FR-SR residue samples thermally treated over 400 °C, significant amounts of characteristic products reflecting the cross-linking structures generating from methylene-bridge and trifunctional siloxane structures in the polymer chains were observed. These results suggest that the thermal decomposition of the FR-SR material is strongly suppressed by the formation of the cross-linking structures induced by the platinum compound during combustion, which leads to its flame retardancy.     

聚酯型可生物降解弹性体的表征及其降解

用丙三醇和癸二酸通过熔融共缩聚反应合成了具有一定热塑性、可生物降解的弹性体（tp-PGS）,并对其结构进行了表征,同时讨论了该弹性体的力学性能、亲水性能和降解性能。结果表明,tp-PGS属于一种交联网络型聚酯,具有以非晶相为软区、晶相为硬区的微观相分离结构。tp-PGS具有一定程度的热塑性,可模压成型。tp-PGS分子结构中所具有的羟基和酯基赋予其良好的生物降解性能。     

Structure-mechanical property correlations of model siloxane elastomers with controlled network topology

We review our recent studies towards the molecular understanding of mechanical properties-structure relationships of elastomers using model polydimethylsiloxane (PDMS) networks with controlled topology. The model elastomers with controlled lengths of the network strands and known amounts of cross-links and dangling chains are obtained by end-linking the functionally terminated precursor PDMS with known molecular weights using multi-functional cross-linkers. Several modern entanglement theories of rubber elasticity are assessed in an unambiguous manner on the basis of the nonlinear stress-strain behavior of the model elastomers under general biaxial strains. The roles of cross-links and entanglements in the large-scale structure of the swollen state are revealed from small angle X-ray scattering spectra. A remarkably stretchable elastomer with the ultimate strain over 3000% is obtained by optimizing the network topology for high extensibility, i.e., by reducing the amounts of trapped entanglements and the end-to-end distance of the network strands. The model elastomers with unattached chains exhibit a pronounced viscoelastic relaxation originating from the relaxation by reptative motion of the guest chains. The relaxation spectra provide a definite basis to discuss the dynamics of guest linear chains trapped in fixed polymer networks. The temperature- and frequency-insensitive damping elastomers are made by introducing intentionally many dangling chains into the networks.     

Piezoresistive effects of copper-filled polydimethylsiloxane composites near critical pressure

Piezoresistive properties of polydimethylsiloxane (PDMS) filled with copper particles in volume fraction above the electrical percolation threshold (25.3–50.7 vol%) were investigated. Piezoresistive behavior of the PDMS-copper composites under compressive pressure showed not only a change in resistance by approximately six orders of magnitude (∼1.5 MPa), but also a change in the critical pressure due to variations in the hardness of the composites. Resistivity relaxation was observed near the critical pressure and was explained through a stress relaxation and percolation mechanism. The mean tunneling distance was calculated by using a theoretical equation for percolation under compression. When the gauge factors of the composites were plotted versus strain, a universal curve was obtained regardless of the copper contents. Finally, the PDMS-copper composite demonstrated good repeatability, showing only small differences in the relative resistance after five successive tests.     

Plasma treatment of polydimethylsiloxane

Plasma treatment of silicone surfaces is a useful way of increasing wettability to improve adhesion and a first step in producing various organosilicon thin-film composites. Despite numerous earlier studies, there is no consensus on the effect of plasma treatment nor on the mechanism of the subsequent hydrophobic recovery. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to study the effect of plasma treatments of polydimethylsiloxane elastomer using four different plasma gases: argon, helium, oxygen, and nitrogen. In each case, the surface was oxidized to produce a thin, wettable, brittle silica-like layer. These surfaces progressively recover their hydrophobicity by diffusion of untreated polymer chains through cracks in the treated layer. Angle-resolved XPS detected the untreated, diffused layer and SEM revealed the common occurrence of cracks in the treated layer, although conditions could be found for each gas where the surface becomes completely wettable by water but is free from cracks.     

Designing of cradle-to-cradle loops for elastomer products

ABSTRACT

Reuse of devulcanisates in virgin rubber compounds is the shortest loop in cradle-to-cradle cycles for elastomers. Due to the enormous complexity of these loops, the technology still stands in its infancy. Two elastomer types, EPDM (Ethylene–Propylene-Diene Terpolymer) and SBR (Styrene–Butadiene Rubber), are compared as to their ease of devulcanisation with an optimized devulcanisation aid. EPDM is relatively easy to devulcanise into a quality which allows for reuse of large amounts till 50% in virgin compounds for applications like roofing sheets and others. This is already being done on industrial scale. SBR is more difficult to devulcanise due to re-crosslinking under the influence of oxygen. Furthermore, tyre scrap is a mix of compounds and elastomers, which are impossible to segregate before devulcanisation. A lot more research and development is needed to bring this to a success.     

Surface properties of polyamides modified with reactive polydimethylsiloxane oligomers and copolymers

Surface modification of polyamide-6 (PA-6) was investigated by melt blending with silicone-urea copolymers or organofunctionally terminated polydimethylsiloxane oligomers. Blends were prepared in a laboratory scale high-shear melt mixer. Surface characteristics of the blends were determined by static water contact angle measurements and X-ray photoelectron spectroscopy. Effect of the type, composition, average molecular weight and amount of the silicone additive in the blends, on the surface properties were determined. Influence of the thermal history of the samples on the surface properties was also investigated. All of the blends showed formation of silicone rich surfaces. Surface modification was permanent due to either the formation of chemical bonds between additive and polyamide and/or very strong hydrogen bonding between urea and amide groups.     

碱水易溶聚酯热降解动力学研究

利用热重分析法(TGA)对自制的碱水易溶聚酯(ASHPET)的热降解动力学进行了研究,并与常规PET和国外某公司碱水易溶聚酯(K-COP)进行了比较。结果表明:几种ASHPET的热稳定性略低于常规PET;三种自制的ASHPET的热稳定性不低于国外某公司的K-COP。     

交联聚丙烯腈纳米微球的制备与水解研究

通过乳液聚合制备了交联聚丙烯腈纳米微球,对其进行碱性水解,得到了含酰胺基等亲水基团的亲水性微球。用动态光散射、红外光谱、透射电镜对水解前后微球的结构、大小和形态进行了表征,发现:水解后,微球中的腈基大多转变为酰胺基或羧基亲水基团,且微球的尺寸大幅增加。     

Analysis of nanoindentation creep of polydimethylsiloxane

Abstract

The scope of the present work is to present a modified model extracted by finite element method (FEM) analysis for the nanoindentation of polydimethylsiloxane (PDMS). The suggested model, based on Sneddon’s equation, takes into account the specimen finite dimensions and the indenter curvature. Load–displacement curves from FEM analysis are obtained and compared with the experimental and those based on Sneddon’s analysis. Sneddon’s solution and FEM analysis deviate from the experiment. Thus, a new analysis is applied considering the finite tip radius effect, which results in the prediction of the elastic modulus of PDMS with 2% accuracy. A fitting analysis of a typical nanoindentation creep curve is performed using the Maxwell two-element model in order to obtain information about the time dependent properties of untreated and ultraviolet treated PDMS. The aforementioned method describes well the creep curve, especially the part attributed to viscous flow.     

An explicit polymer and node network model to compute micromechanical properties of silica-filled polydimethylsiloxane

Two important aspects of filled polymer systems that can influence elasticity are the random position of filler particles and the nonuniformity of polymer chain lengths that form the chain/particle network. Historically, most network elasticity models have been based on idealized assumptions of uniform chain length constrained to highly symmetric orientations. We present a novel, three-dimensional explicit polymer and node network model (EPnet) that includes both randomly distributed filler particles (nodes) and polymer lengths taken from a Gaussian distribution. The molecular level polymer forces that produce elasticity are assumed to operate between pairs of connected network nodes. The numerical model is amenable to any molecular force that depends on the distance between two nodes, however, for this paper, we assume that the polymer chain segments that connect the filler particles obey a simple two-force model, i.e. a constant force required to stretch a single polymer chain and a force arising from the binding energy between a polymer chain and a filler particle surface. Free ends, i.e. polymer segments connected to only one particle, do not contribute to the elasticity. With these assumptions, the model contains intrinsic mechanisms that appear to predict the phenomena of yield stress, tensile failure, permanent set and stress hysteresis. The model is applied to a mesoscale volume element (∼1 μm³) of silica-filled polydimethylsiloxane to study the micromechanical stress in response to various strains, e.g. tensile, compressive, shear and swell. Model predictions are in quantitative agreement with tensile stress/strain experiments.     

Regulatable pervaporation performance of Zn-MOFs/polydimethylsiloxane mixed matrix pervaporation membranes

Pervaporation (PV) is an emerging separation technique for liquid mixture. Mixed matrix membranes (MMMs) often demonstrate trade-off relationship between separation factor and flux. In this study, by changing the organic linkers (2-methyl imidazolate, imidazole-2-carboxaldehyde, 2-ethyl imidazolate), ZIF-8, ZIF-90 and MAF-6 were prepared and filled in polydimethylsiloxane (PDMS) membranes for dealcoholization of 5% (mass) n-butanol solution, and the membranes properties and pervaporation performances were adjusted. Compared with the pure PDMS membrane, the addition of ZIF-8 resulted in a 9% increase in flux (1136 g·m^-2·h^-1) and a 22.5% increase in separation factor (28.3), displaying anti-trade-off effect. For the MAF-6/PDMS MMMs (2.0% mass loading), the pervaporation separation index (PSI) and separation factor were 32347 g·m^-2·h^-1 and 58.6 respectively (increased by 34% and 154% in contrast with that of the pure PDMS membrane), and the corresponding permeation flux was 552 g·m^-2·h^-1, presenting great potential in the removal butanol from water. It was deduced that the large aperture size combined with moderate hydrophobicity of metal-organic frameworks (MOFs) favor the concurrent increase in permeability and selectivity.     

Study on effect of polydimethylsiloxane in intumescent flame retardant polypropylene

The effect of polydimethylsiloxane (PDMS) in intumescent flame retarded polypropylene (PP) containing melamine phosphate (MP) was studied using limiting oxygen index (LOI), UL-94 test, thermogravimetric analysis (TG), real time Fourier transform infrared (FTIR) and pyrolysis/gas chromatogram/mass spectrometry (Py–GC/MS). It was found that the addition of PDMS into the PP/MP composites leads to a decrease in the LOI values of the PP/MP/PDMS composites, and the values increase gradually with the increase of the PDMS content. The UL-94 test results indicate that the PP/MP/PDMS composite with 30% PDMS reaches the V-0 rating compared with no rating of the PP/MP composite at the same MP loading. The water resistance of the PP/MP/PDMS composites was investigated, and the improved water leaching of MP was observed. The TG and RTFTIR results illustrate the thermal stability of the PP/MP composites containing PDMS is improved. Moreover, the pyrolysis of the PP/MP/PDMS composites was studied by the Py–GC/MS method.     

Energy optimization of bioethanol production via hydrolysis of switchgrass

The optimal flowsheet for the production of bioethanol from switchgrass via hydrolysis is proposed in this work. A superstructure embedding a number of alternatives is proposed. Two technologies are considered for switchgrass pretreatment, dilute acid and ammonia fiber explosion so that the structure of the grass is broken down. Next, enzymatic hydrolysis follows any of the pretreatments to obtain fermentable sugars, mainly xylose and glucose. Ethanol is obtained by fermentation of the sugars. To obtain fuel quality, ethanol and water must be removed from the water–ethanol mixture. A number of dehydration technologies is considered including rectification, adsorption in corn grits, molecular sieves, and pervaporation. The problem is formulated as a mixed‐integer nonlinear programming (MINLP). The superstructure is optimized by decomposing the MINLP for each of the pretreatments. Then, multieffect columns and heat integration are used to reduce the energy consumption and cooling needs. Finally, an economic evaluation is performed. The optimal flowsheet consists of using dilute acid hydrolysis followed by molecular sieves as dehydration technology, which requires less energy and cooling and yielding a promising production price of 0.8 $/gal. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Stress softening experiments in silica-filled polydimethylsiloxane provide insight into a mechanism for the Mullins effect

Under repeated tensile strain, many particle-filled polymers such as silica-filled polydimethylsiloxane (PDMS), exhibit a reduction in stress after the initial extension, the so-called Mullins effect, and the mechanism(s) responsible for this is considered to be a major unsolved mystery of polymer physics. We report here the first observation of the absence of this effect in cross linked, silica-filled PDMS when the second strain axis is perpendicular to the initial strain axis. This result poses a challenge for existing theories. We propose a mechanism to account for the Mullins effect that is consistent with our experimental observations.     

厌氧水解的新解读：快速水解和慢速水解

挥发性脂肪酸（VFA）具有可生化性好、附加价值高等优点而得到广泛应用,厌氧消化产VFA是目前学术界的研究热点,但对有机物经过水解后的生物降解性能则少有论及,厌氧发酵过程中的水解产物存在“数量”和“质量”之间的权衡问题。为了进一步提高厌氧发酵过程中微生物对有机物的利用效率,突破水解对厌氧消化的限制性作用,从基质碳源释放快慢及厌氧发酵过程中碳源降解性能不同,本文将厌氧水解分为快速水解和慢速水解,分别阐述了两种水解方式的含义、分类以及优缺点,指出细胞内外碳源的释放速率和释放方式的不同是影响厌氧产酸和生物降解性能的决定性因素。最后指出快速水解与慢速水解相结合的分阶段联合处理方式,是今后厌氧消化产VFA的主要研究方向。     

Physicochemical and mechanical degradation of polyamide 11 induced by hydrolysis and thermal aging

This article deals with the effect of both temperature and water activity on polyamide 11 physicochemical and mechanical properties. The purpose of this work is to describe the ductile–brittle transition of polyamides during aging in a wet environment using a mechanical behavior model. For that it is necessary to make physicochemical analyzes (DSC, FTIR, GPC, viscosity) and mechanical tests (uniaxial tensile test and dynamic mechanical thermal analysis). All of these tests were performed on unaged samples and on aged samples for until 60 days. Two types of aging conditions were performed: In water in an acidic medium at temperature, and in purely thermal conditions in a neutral environment. Changes in mechanical properties with aging were observed in the polymer and were correlated to morphological changes deduced from the physicochemical characterizations. An increase of the second yield stress related to the recrystallization and a decrease of the strain at break due to the decrease of the molecular weight were observed during aging. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47628.