Improving breakdown strength and energy storage efficiency of poly(vinylidene fluoride-co-chlorotrifluoroethylene) and polyurea blend films by double layer structure design

All-organic composites are widely used in energy storage application due to the high breakdown strength performance, but the improvement of energy storage was limited by the relatively low dielectric constant. Therefore, to satisfy the high demands of dielectric materials, energy storage properties of polymer composites should be further enhanced. In this article, poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) and polyurea (PUA), which are known as high dielectric ferroelectric material and linearly high energy storage efficiency material respectively, are composited through double layer (DL) casting method for the first time. The properties of DL structured composite film is contrasted with solution blending structure especially in energy storage efficiency, and the results demonstrate that DL structure design can make great use of advantages of two materials and also can avoid the influence of phase separation between P(VDF-CTFE) and PUA efficiently. Moreover, high breakdown strength (6180 kV/cm) and high energy storage efficiency (77%) of DL composites can be realized simultaneously by incorporating PUA as an insulating layer, and the mechanism is discussed in detail. This work provides an effective route to improve the energy storage properties of polymer dielectric materials and shows great application potential.     

Strain rate sensitivity of polyurea coatings: Viscous and elastic contributions

Polyurea, a reaction product of isocyanates and an amine blend has been reported to be highly strain rate sensitive, which responds in its own unique way when subjected to increasing strain rates. The amine blend comprise of a long chain amine, forming the soft segments, chain extender, which brings the urea linkages closer and a crosslinker, which serves as a chemical bond between the chains. In this work, we attempt to tune the viscous contribution in polyurea, by increasing crosslinking density, while keeping the amount of chain extender more or less constant. The viscoelastic behavior and its effect on the time dependant behavior was established by extensive dynamic studies. Quasi-static tests and Split Hopkinson testing was performed over large deformation rates (10⁻⁴ to 10³ s⁻¹). It was observed that polyureas with larger viscous contribution (tan δ), were capable of exhibiting larger strain rate sensitivity.     

Polyurea based aerogel for a high performance thermal insulation material

Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO₂) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications and as a thermal insulation component of spacesuits.     

聚氰丙基甲基硅氧烷-聚脲嵌段共聚物的NMR表征及研究

用高分辨NMR对聚甲基硅氧烷-聚脲嵌段共聚物以及聚氰丙基甲基硅氧烷-聚脲嵌段共聚物结构进行表征,对NH形成的各种氢键进行详细的研究,同时观察了氢键在力学性能中的作用,结果发现氢键强烈地影响着力学性能。用固体¹H宽线研究这类材料的相分离状况,并总结影响相分离的因素。保持硬段含量不变,增加软段分子量使相分离程度增加;保持软段分子量不变,增加硬段含量使相分离程度降低。软、硬段之间的相互作用(如氢键)使相分离程度降低。     

以聚脲为囊壁薄荷素油微胶囊的制备及表征

采用界面聚合法,以薄荷素油为芯材,以异佛尔酮二异氰酸酯为壁材单体,在催化剂四甲基乙二胺作用下和水反应形成聚脲外壳,制备出了薄荷素油微胶囊.通过扫描电镜、激光粒度分析仪、傅里叶红外光谱仪及热重分析仪分别对香精微胶囊的表面形貌、粒径分布、单体反应情况和热稳定性进行了分析表征.通过紫外可见分光光度计对香精微胶囊包覆率进行了测定.并分析了均质化速率和微胶囊平均粒径的关系以及不同乳化剂种类和芯壁比条件下微胶囊的形貌特征.结果表明,微胶囊平均粒径随均质化速率的增大而减小,下降到1μm左右时趋于平稳,当乳化剂采用聚乙烯醇且芯壁比为4∶1时,微胶囊形貌最佳,为规整球形.最终测得微胶囊芯材包覆率为84.09 wt%,粉末状微胶囊样品含油率为72.64 wt%,并且微胶囊芯材具有良好的热稳定性.     

A micromechanical damage and fracture model for polymers based on fractional strain-gradient elasticity

We formulate a simple one-parameter macroscopic model of distributed damage and fracture of polymers that is amenable to a straightforward and efficient numerical implementation. We show that the macroscopic model can be rigorously derived, in the sense of optimal scaling, from a micromechanical model of chain elasticity and failure regularized by means of fractional strain-gradient elasticity. In particular, we derive optimal scaling laws that supply a link between the single parameter of the macroscopic model, namely, the critical energy-release rate of the material, and micromechanical parameters pertaining to the elasticity and strength of the polymer chains and to the strain-gradient elasticity regularization. We show how the critical energy-release rate of specific materials can be determined from test data. Finally, we demonstrate the scope and fidelity of the model by means of an example of application, namely, Taylor-impact experiments of polyurea 1000 rods.     

Synthesis and Characterization of Microcapsules with Chlorpyrifos Cores and Polyurea Walls

Microcapsules with chlorpyrifos cores and polyurea walls were synthesized with 2,4-tolylene diisocyanate as an oil-soluble monomer and ethylenediamine as a water-soluble monomer via an interracial polycondensation reaction. The products were characterized by means of Fourier transform infrared spectrometry, ^13C NMR spectrometry and ^31p NMR spectrometry. The morphology, the particle size and the particle size distribution, and the thermal properties were also evaluated. The prepared microcapsules exhibit clear and smooth surfaces and have a mean diameter of 28. 13 μm. These microcapsules also have a good thermal stability for long-term use, and have potential applications in minimizing the toxicity of chlorpyrifos through controlled release.     

Hybrid resins from polyisocyanate/vinyl ester/water glass systems: Structure and properties

Polysilicate modified polyurea/vinyl ester hybrid resins were produced by dispersing water glass (WG) in a mixture of vinyl ester (VE) and polyisocyanate in presence of a liquid phosphate as emulsifier. As styrene-crosslinkable VE resins bisphenol A (BA) and novolac types (N), whereas as polyisocyanate a polymeric methylene diphenyl isocyanate (PMDI) were used. The structure and selected properties of the hybrid resins were determined and compared to those of the neat VEs and polysilicate filled polyurea (denoted as 3P resin). Using VE for resin hybridization, which worked as an additional emulsifier for the WG/PMDI/phosphate system, resulted in a fine particle dispersion of the polysilicate. It was found that the type of VE affected not only the dispersion of WG (and thus of the polysilicate) but also the network formation of the polyurea/VE hybrids and their properties. Information about the structure of the polysilicate filled hybrid resins was gained from dynamic-mechanical thermal analysis (DMTA), scanning electron and atomic force microscopic measurements. It was argued that the resin hybridization yielded a conetwork instead of an interpenetrating one. The properties of the hybrid systems were determined by DMTA, fracture mechanical tests, thermogravimetric analysis and flammability measurements. It was established that the stiffness and resistance to thermal degradation of the initial 3P resin was strongly improved by hybridization with VEs. The fracture toughness (K_c) proved to be less sensitive to the formulation of the hybrid resins. On the other hand, the fracture energy (G_c) and limiting oxygen index experienced a positive deviation from the additivity as a function of the 3P/VE composition, at least in a given range.     

不同硬段含量溶液聚合和RIM嵌段聚脲热性质和动态力学性质的研究

用溶液聚合法和RIM制备了软段为胺端基聚环氧丙烷,硬段为4,4′-二苯甲烷二异氰酸酯(含量为30％,50％,70％)经二乙基甲苯二胺扩链的热塑性嵌段聚脲。用动态力学温度谱(DMS)和示差扫描量热计法(DSC)对比研究了这些聚脲的性质。结果表明,溶液聚脲比RIM聚脲的枢分离情况好。DMS和DSC均未观察到聚脲中硬段的玻璃化转变。     

不同硬段含量嵌段聚脲合成及性质的研究

用两步溶液聚合法合成了硬段含量不同的一系列热塑性嵌段聚脲.研究了影响反应的各种因素.所得聚脲是线性和无定形的.用DSC和DMS法初步研究了这些聚脲的性质,随硬段含量增加.聚脲从软弹性体转变为高模量的硬塑料.