Effect of morphology on the dynamic mechanical properties of a semicrystalline polysiloxane

The dynamic mechanical properties of semicrystalline poly(tetramethyl-p-silphenylene siloxane) in three morphological preparations were measured over the wide frequency range of about 0.002 Hz to 500 Hz and the temperature range of about ? 190°C to 100°C. The three samples were all isothermally crystallized at 125°C. Two samples had a spherulite size of 25 μ diameter but differed in the time allowed for secondary crystallization. The other sample had a smaller spherulite size. By assuming compliance additivity, the viscoelastic behavior could be separated into five relaxation processes with an indication that a sixth existed at low temperature. Two processes called γ₁ and γ₂ could be resolved at low temperatures. The γ₁ process was associated with the amorphous region since the peak strength was affected by the rate of cooling through the glass transition region; the γ₂ peak, unaffected by cooling rate, is attributed to the crystalline part. In the high-temperature region, the β peak is associated with the glass transition and has a shape and location that is essentially independent of the morphology. The highest temperature α₂ process, whose maximum was not observed in the experimental range covered, is attributed to the crystalline region and is sensitive to changes in crystallization history. The strength of the α₁ process unlike that of the other processes was found to be a function of temperature; it was associated with the noncrystalline region.     

Viscoelastic and structural properties of a phenyl-modified polysiloxane system with a three-dimensional structure

The relationships between the viscoelastic and structural properties of glass-forming materials with polysiloxane bonds, which serve as network formers, and phenyl groups, which act as network terminators, are examined based on shear viscoelasticity, (29)Si MAS NMR, and GPC measurements during the early stages of the network-forming process. The viscosities of the present samples do not depend on the frequency at temperatures up to 200 degrees C, suggesting that the origin of the viscous flow does not include intermolecular entanglement. According to the results of the strain dependence of the elastic modulus, the bridging-oxygen number, and molecular weight, the present polysiloxane system has a complex structure, or distribution of various-sized molecules composed of a polysiloxane network with various dimensionalities, and furthermore an elementary process of the viscosity is simple flow of these molecules. The structural factors that determine the viscosity and its temperature dependence are categorized into the molecular size and the intramolecular structure by using a theory based on the free-volume model. The relationship between the viscosity and the structure around the glass transition temperature is quantitatively examined and it is concluded that introducing larger numbers of Ph groups makes the viscosity less sensitive to structural factors.     

Effects of organic groups on structure and viscoelastic properties of organic-inorganic polysiloxane hybrid system

The structure and viscoelastic properties of an organic-inorganic hybrid system composed of an organically modified polysiloxane network were examined, and the influence of organic groups on elastic-modulus variation by heat treatment was studied. The increase in the number of phenyl (Ph) groups per silicon decelerates the increase in elastic modulus; the substitution of the Ph group for a methyl (Me) group accelerates it. The system basically consists of R4-mSi[O-]m/2 units, where R is the organic group. The 29Si magic-angle spinning (MAS) NMR and gel permeation chromatography (GPC) measurements classified the structure related with the viscoelastic behavior into two factors: the number of bridging oxygens and the distribution of molecular weight. The elastic modulus was expressed by these structural factors through a simple empirical formula, irrespective of the type and number of the organic groups. The effects of the organic groups on the variation in elastic modulus by heat treatment were found to work mostly only through the molecular-weight change, and such effects can be controlled by the type or the number of organic groups.     

Heterogeneity of structure and mechanical properties of polymers

Specific features of development of micro- and macrofibrils as well as the structure of their interfaces are considered for oriented filaments of high-density polyethylene with different initial supermolecular structures. As evidenced by SAXS, WAXS, EPR, Raman spectroscopy and electron microscopy, the melt-crystallized samples contain a greater amount of tie molecules connecting macro- and microfibrils than the samples crystallized from solution. This hampers slippage of fibrils past each other and does not allow high draw ratios to be achieved. It was found that the density of macrofibrillar ends in the drawn melt-crystallized samples is nearly an order of magnitude greater than that in the drawn samples crystallized from solution. This leads to generation of kink bands (dangerous large-scale defects) and, as a result, the sample, being oriented, fractures long before high draw ratios and a perfect fibrillar structure are reached. The ultraoriented samples produced from solution have a more perfect intrafibrillar structure, and the density of intrabrillar disordered regions is close to that of crystalline ones. Nevertheless, they do contain clusters of defects which limit their mechanical properties. The analysis of the Raman and X-ray data shows that these defects are localized at crystallite boundaries in the long periods. Possible routes for improvement of the parameters of the fibrillar structure and their relation with mechanical properties are discussed.     

Improving mechanical and biological properties of macroporous HA scaffolds through composite coatings

Interconnected porous hydroxyapatite (HA) scaffolds are widely used for bone repair and replacement, owing to their ability to support the adhesion, transfer, proliferation and differentiation of cells. In the present study, the polymer impregnation approach was adopted to produce porous HA scaffolds with three-dimensional (3D) porous structures. These scaffolds have an advantage of highly interconnected porosity (≈85%) but a drawback of poor mechanical strength. Therefore, the as-prepared HA scaffolds were lined with composite polymer coatings in order to improve the mechanical properties and retain its good bioactivity and biocompatibility at the same time. The composite coatings were based on poly(d,l-lactide) (PDLLA) polymer solutions, and contained single component or combination of HA, calcium sulfate (CS) and chondroitin sulfate (ChS) powders. The effects of composite coatings on scaffold porosity, microstructure, mechanical property, in vitro mineralizing behavior, and cell attachment of the resultant scaffolds were investigated. The results showed that the scaffolds with composite coatings resulted in significant improvement in both mechanical and biological properties while retaining the 3D interconnected porous structure. The in vitro mineralizing behaviors were mainly related to the compositions of CS and ChS powders in the composite coatings. Excellent cell attachments were observed on the pure HA scaffold as well as the three types of composite scaffolds. These composite scaffolds with improved mechanical properties and bioactivities are promising bone substitutes in tissue engineering fields.     

X-ray CT microtomography and mechanical response of foamed polysiloxane elastomers

3D X-ray computer microtomography (CT) experiments have been performed to assess the microstructure of scaled cellular polysiloxane elastomers and to predict how key morphological features alter as a function of compressive loading. In the work reported here, full scale (nominally 600 μm pore size) and half scale (nominally 300 μm pore size) polydimethylsiloxane foams (M97) were prepared using extractable urea particles, and tested. CT test methodology was developed to image foam microstructure at different levels of compression. 1D magnetic resonance imaging (MRI) experiments have also been performed on full scale foams for baseline characterisation. Material porosity, bulk density and dynamic mechanical analysis (DMA) stress/strain responses as a function of compression were recorded. Our results show that undesirable engineering stress responses are evident when the material microstructure (cell size and shape) is non-uniform and complex. This is particularly evident when non-spherical urea particles are used, leading to undesirable scaled foam microstructures with mechanical responses that do not match that shown by ‘full scale’ versions. Through the use of X-ray CT and MRI, our studies have provided insights into the link between manufacturing, polymer architecture (cell size/shape) and mechanical response of scaled M97 cellular materials. The data collected will support materials FEA (finite element model) code development activities, as well as help identify how the material architecture can be modified to achieve more controlled and uniform mechanical responses.     

Relationship of hierarchical structure to mechanical properties

The mechanical properties in complex systems are explained based on the hierarchical structures present in the system. Hierarchical structures designed for specific mechanical responses are best exemplified by examples from biology. Collagen, a main component in soft connective tissues, is organized into hierarchical structures in the form of tendons or intervertebral discs as examples. Understanding these structures is vital in relating the structures to the intended properties. This approach is also used to characterize organic/inorganic natural composites such as human bone, reindeer antler and nacre. Another example of a hierarchical structure in biology with excellent mechanical properties is that of cellulose, when organized into wood. The importance of hierarchical structures also applies to synthetic polymers for a clearer understanding of the structure-property relationships. Solid-state biaxially oriented polypropylene has excellent tensile and impact properties, which are explained by the hierarchical structure induced during the processing. Thermotropic liquid crystalline polymers develop a hierarchical structure during injection molding that influence the final properties. Furthermore, the impact modification of polycarbonate is more easily understood when the system is explained in a hierarchical manner. It is also now possible to create or force hierarchical structures in synthetic polymers by microlayering technology. Several systems are outlined in which a hierarchical structure is created to enhance specific properties. SAN, a brittle polymer, can be microlayered with PC to create tough materials due to the scale, interaction and architecture of the microlayered composite. Another example is the effect of microlayered composite of PC/SAN on the interfacial adhesion mechanisms. Furthermore, toughening mechanisms in filled microlayers are examined based on the hierarchical structure.     

聚苯基硅氧烷/聚丙烯酸酯复合乳液的合成与性能

本文以二苯基二甲氧基硅烷(DPDMS)和γ-甲基丙烯酰氧基丙基三甲氧基硅烷(MAPTMS)合成了具有微交联结构的聚硅氧烷乳液,并以此为种子乳液再与丙烯酸酯类单体聚合,合成了稳定且性能优良的聚硅氧烷/聚丙烯酸酯复合乳液.用激光粒度分析仪、FTIR、DMA等进行了表征.结果表明,硅氧烷被有效的接枝到了聚丙烯酸酯分子链中;硅氧烷的加入,使聚合物的储能模量,损耗模量增大,玻璃化温度提高,加入量为10％时,Tg由50.5℃升高到57.2℃;加入20％的硅氧烷涂膜吸水率由13.8％降低到6.8％.     

Network structure and mechanical properties of allophanate-cured polyurethane rubber

An allophanate-cured polyurethane (PU) has been prepared. The relationship between trifunctional network structure and mechanical properties is discussed. Mooney-Rivlin constants C₁ and C₂ are determined from stress-strain isotherms. C₂ is approximately constant. The average value of structure factor A₃ in the limit at large deformation is 0.432, in fair agreement with the theoretical prediction of 1/3. The structure factor A′₃ of all PU samples becomes roughly unity in the limit at small deformation. The experimental results are interpreted by the recent Flory theory (a fluctuation-constraint model of junctions). The κ parameter decreases with increasing crosslink density; the ξ parameter is small and approaches zero. Increasing crosslink density tends to cause phantom deformation for extended PU networks. However, the absolute value of the reduced stress in the limit at large deformation (or for the equivalent phantom network) [f] is believed to be overestimated for PU networks with high crosslink density. The origin of this behavior is discussed in detail.     

Chemical structure and physical properties in polyester based segmented polyurethanes—I: Dynamic mechanical properties

Nine polyester based segmented polyurethanes and one polyurea have been synthesised, wet spun into fibres and cast into films. Both forms have been found to have melting points and densities strongly dependent on soft segment (polyester) concentration, the films having higher values in each case. Electron microscope studies have revealed the presence of crystallinity within the hard domains. Dynamic mechanical measurements showed several transitions: an α transition above 200° complete in several film samples but incomplete in fibre, an α' transition between 47 and 80° and a β transition in the region of ?20°. The α transitions occurred in the film but not in fibre, the α' transition in the fibre but not the film and the β transition in both. The significances of the transitions are discussed in terms of physical and chemical structure.     

Synthesis and surface properties of polyurethane modified by polysiloxane

A series of polyurethanes modified by polysiloxane (Si-PU) were synthesized based on 2,4-toluene diisocyanate (TDI), dihydroxybutyl-terminated polydimethylsiloxane (DHPDMS), polytetramethylene glycol (PTMG) and 1,4-butanediol (BDO). Fourier transform infrared spectroscopy analysis showed that DHPDMS had been incorporated into the polyurethane chains. With the increase of DHPDMS content, the water contact angle increased while the surface tension decreased. As the DHPDMS content increases above 5%, both the contact angle and the surface tension tend to approach a constant. The contact angle increases with increasing temperature, and it tends to approach a constant when the temperature is higher than 50°C. The result indicates that Si-PU exhibits good surface and mechanical properties when the DHPDMS content is 5%. __________ Translated from Polymer Materials Science and Engineering, 2007, 23(3): 47–50 [译自: 高分子科学与工程]     

Synthesis and properties of polysiloxane side chain cholesteric elastomers

The synthesis of new side chain cholesteric liquid crystalline elastomers containing the flexible non-mesomorphic crosslinking agent M-1 and the cholesteric monomer M-2 by a one-step hydrosilylation reaction is described. The chemical structures of the obtained monomers and network polymers were confirmed by ¹H NMR and FTIR spectroscopy. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The glass transition temperatures and isotropic temperatures of the mesomorphic elastomers decreased as the concentration of crosslinking units increased; in the mesomorphic region the liquid crystalline elastomers showed elasticity, reversible phase transitions and Grandjean texture. The flexible crosslinking agent did not disturb the cholesteric structure; moreover, it was beneficial for adjusting the helix of the cholesteric liquid crystalline polymers, and cholesteric elastomers P-6, P-7, show reversible selective reflection of visible light.     

Chemical composition,structure, and properties of electrochemically deposited cobalt-boron coatings

Polycrystalline and amorphous cobalt-boron coatings were electrochemically deposited. The effect of the boron content and heating temperature on the physicomechanical properties of the coatings was studied. A relationship between the composition, structure, and properties of the coatings was established.     

Theoretical investigation of polymer molecular structure influence on dielectric properties and mechanical properties

Triboelectric nanogenerator (TENG) technologies have explosive development in the field of energy harvesting and self-powered sensing. As the key element of triboelectric devices, dielectric polymers have obtained much attention in recent years. The dielectric properties of polymer determine the output performance of TENG. In this paper, we take silicone rubber as an example of dielectric polymers, to study the properties of molecular structure influence on the dielectric properties and mechanical properties by the molecular dynamics simulation method. The free volume fraction, dielectric constant, and mechanical properties of silicone rubbers with different branch chains were calculated. The dielectric constant is highly related to the free volume distribution and the dipole moments of silicone rubbers with different amounts of branch chains. For fewer branch chains silicone rubber, the free volume distribution contributes most to the dielectric constant; for more branch chains silicone rubber, the dipole moment dominates the dielectric constant. Therefore, the silicone rubber ratio has a great influence on the dielectric constant of silicone rubber. With the increase of temperature, the dielectric constant of 2-chain silicone rubber increases at first and then decreases, and the maximum value is obtained near 300 K. Therefore, it is necessary to control the temperature when silicone rubber is used as a dielectric material. This work can be a guide for improving the dielectric properties of silicone rubber, and it provides a new approach to the optimal design of high-performance triboelectric nanogenerators.     

Phenolic materials via ring-opening polymerization of benzoxazines: Effect of molecular structure on mechanical and dynamic mechanical properties

A series of benzoxazine compounds have been prepared from different bifunctional phenols and primary amines and the molecular compositions of the precursors are characterized by nuclear magnetic resonance, Fourier transform infrared, and size exclusion chromatography. It has been demonstrated that by design of the molecular structures of the benzoxazine precursors, therefore by tailoring of the network molecular architectures, new phenolic materials with superior mechanical and thermal properties, and ease-of-processing characteristics can be obtained. The characteristics of dynamic mechanical spectra of the phenolic materials are studied and related to their structural features. © 1994 John Wiley & Sons, Inc.     

Synthesis and properties of polysiloxane xerogels containing tetrasulfide groups

Polysiloxane xerogels with a functional group content of 1.1?C1.9 mmol/g have been obtained by the hydrolytic condensation of the alkoxysilanes Si(OC₂H₅)₄ and [(C₂H₅O)₃Si(CH₂)₃S₂]₂ in the 2 : 1, 4 : 1, and 8 : 1 ratios. It has been demonstrated by ¹³C and ²⁹Si CPMAS NMR spectroscopy that the xerogels have a polysiloxane framework with dipropyl tetrasulfide bridges, silanol groups, unhydrolyzed ethoxyl groups, and hydrogen-bonded water molecules on the surface. The xerogels have a porous structure. As the molar ratio of the reacting alkoxysilanes is increased in the above-specified range, the specific surface area of the xerogel increases (from 89 to 312 m²/kg) and the same is valid for other structure-adsorption characteristics. The synthesized polysiloxane xerogels readily sorb Hg²⁺ from acidified solutions. Their static sorption capacity can be as high as 1.5 g Hg per gram of sorbent. However, in the course of time, the 1 : 1 complexes forming on the xerogel surface undergo transformations accompanied by the release of mercury sulfide and/or Hg²⁺ reduction to mercury metal.     

Aggregation structure and mechanical properties of functionally graded polyurethane elastomers

Functionally graded polyurethane elastomers (FGPUEs) were prepared with two molds fixed at different temperatures (30 and 150 °C). The effects of the molar ratio of the curing agent (60/40, 75/25, or 97/3 1,4‐butane diol/1,1,1‐trimethylol propane) and the molecular weight of the polymer glycol (number‐average molecular weight = 2000 or 3000) on the molecular aggregation state and mechanical properties of the FGPUEs were investigated with differential scanning calorimetry, polarized optical microscopy, dynamic viscoelastic measurements, and tensile tests. The aggregation state of the FGPUEs was changed continuously from the one side (lower temperature side) to the other side (higher temperature side); for example, the glass‐transition temperature gradually increased in this direction. Also, the number of spherulites formed in the FGPUEs increased in the same manner. In the mechanical tests, the tensile strength and elongation at break of the lower temperature side were higher than those of the higher temperature side. This was correlated with the strong phase separation of the lower temperature side. The poly(oxytetramethylene glycol)‐based FGPUE with a chain extender of 75 wt % showed the largest degree of the temperature gradient. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2355–2363, 2003     

Correlations between structure and some mechanical properties of carbon fibres

Three types of carbon materials were studied by X-ray analysis in order to get correlations between structure and some mechanical properties. Both X-ray and mechanical measurements were done in various conditions.