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.     

Tris-cis-tris-trans-dodeca[organo(dimethylorganosiloxy)]cyclododecasiloxanes {RSi(O)[OSiMe2R']}12. Self-ordering features

The molecular order and thermotropic transitions of tris-cis-tris-trans-dodeca- [organo(dimethylorganosiloxy)]cyclododecasiloxanes {RSi(O)[OSiMe(2)R']}(12) (R = Ph, R' = Me, CH(2)Cl, Vi; R = Me, Et, Vi, R' = Me) have been investigated using differential scanning calorimetry, thermogravimetric analysis, and X-ray scattering. The cyclododecasiloxanes with phenyl side groups (R = Ph) can form mesomorphic structures within a very wide temperature range. Compounds with R = Me and Vi are liquids and exhibit microphase separation above their glass transition temperature because of the different nature and structure of the organic R and trimethylsiloxy OSiMe(3) side groups. When the side group R = Et, a mesomorphic structure is formed in a substantially more narrow temperature region than that for cycles containing phenyl groups. Thus, the type of side group R in organocyclododecasiloxanes determines their ability for self-ordering into mesomorphic structures and the thermal stability of the mesomorphic state.     

Properties of epoxy networks derived from the reaction of diglycidyl ether of bisphenol A with polyhedral oligomeric silsesquioxanes bearing OH‐functionalized organic substituents

A polyhedral oligomeric silsesquioxane (POSS), consisting mainly of a mixture of octahedra, nonahedra, and decahedra with bulky and flexible organic substituents, with three secondary hydroxyls per organic group, was used to modify epoxy networks produced by the homopolymerization of diglycidyl ether of bisphenol A in the presence of benzyldimethylamine. Several physical, thermal, and mechanical properties of the cured materials containing 0, 10, 30, and 50 wt % POSS were determined. The addition of POSS increased the elastic modulus and the yield stress measured in uniaxial compression tests, mainly because of the increase in the cohesive energy density produced by hydrogen bonding through the hydroxyl groups. A constant yield stress/elastic modulus ratio equal to 0.03 was observed for different POSS concentrations and test temperatures. The glass‐transition temperature decreased with POSS addition because of the flexibility of organic branches present in the POSS structure and the decrease in the crosslink density (determined from the rubbery modulus). Although a combination of a reduction in the glass‐transition temperature (plasticization) with an increase in the glassy modulus (antiplasticization) is a well‐known phenomenon, what is original is that in this case it was not the result of the suppression (or reduction in intensity) of subglass relaxations but was produced by an increase in the cohesive energy density. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1451–1461, 2003     

Effects of Ta content on the phase stability and elastic properties of β Ti–Ta alloys from first-principles calculations

The effects of Ta content on the phase stability, the elastic property and the electronic structure of β type Ti–Ta alloys were studied from first-principles calculations based on the density functional theory. It is found that the phase stability, tetragonal shear constant C′, bulk modulus, elastic modulus and shear modulus of β type Ti–Ta alloys increase with the Ta content increasing monotonously. The lowest elastic modulus of the alloys is realized when the valence electron number (e/a) is around 4.25. Moreover, the phase stability of the alloys was discussed based on the calculated density of state (DOS).     

Polypropylene-polysilsesquioxane blends

This paper deals with the preparation of PP/polysilsesquioxane blends and their study to investigate silsesquioxane dispersion, mechanical properties, thermal stability and combustion properties by means of a number of techniques, such as SEM, XRD, Rheology, TGA, DSC, Cone Calorimeter tests and LOI.Polysilsesquioxane with different organic groups (methyl, vinyl or phenyl) were used; both dispersion and final properties were found to be dependent on the silsesquioxane organic fraction type.The PP/polysilsesquioxane blends showed an increased thermoxidative stability and combustion resistance, in terms of lower rate of heat release.Higher mechanical performances were also achieved with PP/vinyl polysilsesquioxane, with higher elastic modulus as well as higher elongation at break.     

Influence of Mg2+ Contents on the Thixotropy of HTlc/MT Suspensions

In this article, we found that the variation of Mg²⁺ content in the Fe-Al-Mg hydrotalcite-like compounds (HTlc) and the change of R values (R_defined = W_HTlc /W_MT) will influence the thixotropic and viscoelastic properties of HTlc/sodium montmorillonite (MT) suspensions. With increasing the content of Mg²⁺ in the HTlc samples, the storage modulus (G′) and the loss modulus (G″) of the HTlc/MT suspensions will increase gradually, which means the structure strength of the HTlc/MT suspension increases with increasing of Mg²⁺ content in the HTlc samples. In addition, the structure strength of the suspensions will increase with increasing of R values. Special emphasis has been laid on the phenomenon of thixotropy. As the gradual increase of the structure strength in the HTlc/MT suspension, the thixotropic types will change accordingly. The mechanism has been discussed emphatically.     

Viscoelastic properties of adsorbed and cross-linked polypeptide and protein layers at a solid-liquid interface

The real-time changes in viscoelasticity of adsorbed poly(L-lysine) (PLL) and adsorbed histone (lysine rich fraction) due to cross-linking by glutaraldehyde and corresponding release of associated water were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR). The kinetics of PLL and histone adsorption were measured through changes in mass adsorbed onto a gold-coated quartz surface from changes in frequency and dissipation and using the Voigt viscoelastic model. Prior to cross-linking, the shear viscosity and shear modulus of the adsorbed PLL layer were approximately 3.0 x 10(-3) Pas and approximately 2.5 x 10(5) Pa, respectively, while after cross-linking, they increased to approximately 17.5 x 10(-3) Pas and approximately 2.5 x 10(6) Pa, respectively. For the adsorbed histone layer, shear viscosity and shear modulus increased modestly from approximately 1.3 x 10(-3) to approximately 2.0 x 10(-3) Pas and from approximately 1.2 x 10(4) to approximately 1.6 x 10(4) Pa, respectively. The adsorbed mass estimated from the Sauerbrey equation (perfectly elastic) and the Voigt viscoelastic model differ appreciably prior to cross-linking whereas after cross-linking they converged. This is because trapped water molecules were released during cross-linking. This was confirmed experimentally via ATR/FTIR measurements. The variation in viscoelastic properties increased substantially after cross-linking presumably due to fluctuation of the randomly cross-linked network structure. An increase in fluctuation of the viscoelastic properties and the loss of imbibed water could be used as a signature of the formation of a cross-linked network and the amount of cross-linking, respectively.     

Modelling physical properties of highly crystallized polyester reinforced with multiwalled carbon nanotubes

The effects of carbon nanotubes dispersion into thermoplastic polymers are complex and strongly dependent upon their aggregation state. A poly(ethylene terephthalate) (PET) matrix has been reinforced through addition of multiwalled carbon nanotubes (MWCNTs). Such an addition has generated an increase in flexural modulus and a decrease in flexural strength at room temperature, and an increase in both properties above the glass transition temperature (at 100 °C). These different behaviours, dictated by temperature, have been investigated through two different micromechanical models that have permitted to put forward hypothesis on failure mechanisms and to shed light on the role played by crystalline phase. The results of thermal analyses have shown that the heat capacity of PET nanocomposites varies according to the MWCNTs content as the flexural modulus. Such a similarity has suggested to modify the Halpin-Tsai equations (H-T), typically used to predict elastic properties of short fibres reinforced composites, in order to determine the relationships occurring between PET specific heat and aspect ratio of dispersed MWCNT. The analyses performed by means of either classical H-T (elastic modulus) or modified H-T (heat capacity) equations, provided very similar estimation of the MWCNT aspect ratios. In addition, a simple elaboration of the modified H-T equations permitted the calculation of rigid amorphous fraction (RAF) into PET. The obtained values were slightly higher than those evaluated by means of a procedure based on the loss tangent peak variation measured through dynamic mechanical experiments. The detected strength decrease at 25 °C have been attributed to crack propagation through a percolative path between crystalline coating layer of MWCNTs and PET (favoured by matrix brittleness), while at 100 °C the crack propagation is hampered by rubbery behaviour of the matrix.     

Elastic and viscoelastic properties of sugarcane bagasse-filled poly(vinyl chloride) composites

Elastic and viscoelastic properties of sugarcane bagasse-filled poly(vinyl chloride) were determined by means of three-point bending flexural tests and dynamic mechanical and thermal analysis. The elastic modulus, storage modulus, loss modulus, and damping parameter of the composites at fibre contents of 10, 20, 30, and 40% in mass were determined, as well as those of the unfilled matrix. There was a correlation between the elastic modulus and storage modulus of the composites. Moreover, the elastic and viscoelastic properties of the composites were highly influenced by fibre content.     

Dispersions of polymer ionomers: I

The principal subject discussed in the current paper is the effect of ionic functional groups in polymers on the formation of nontraditional polymer materials, polymer blends or polymer dispersions. Ionomers are polymers that have a small amount of ionic groups distributed along a nonionic hydrocarbon chain. Specific interactions between components in a polymer blend can induce miscibility of two or more otherwise immiscible polymers. Such interactions include hydrogen bonding, ion-dipole interactions, acid-base interactions or transition metal complexation. Ion-containing polymers provide a means of modifying properties of polymer dispersions by controlling molecular structure through the utilization of ionic interactions. Ionomers having a relatively small number of ionic groups distributed usually along nonionic organic backbone chains can agglomerate into the following structures: (1) multiplets, consisting of a small number of tightly packed ion pairs; and (2) ionic clusters, larger aggregates than multiplets. Ionomers exhibit unique solid-state properties as a result of strong associations among ionic groups attached to the polymer chains. An important potential application of ionomers is in the area of thermoplastic elastomers, where the associations constitute thermally reversible cross-links. The ionic (anionic, cationic or polar) groups are spaced more or less randomly along the polymer chain. Because in this type of ionomer an anionic group falls along the interior of the chain, it trails two hydrocarbon chain segments, and these must be accommodated sterically within any domain structure into which the ionic group enters. The primary effects of ionic functionalization of a polymer are to increase the glass transition temperature, the melt viscosity and the characteristic relaxation times. The polymer microstructure is also affected, and it is generally agreed that in most ionomers, microphase-separated, ion-rich aggregates form as a result of strong ion-dipole attractions. As a consequence of this new phase, additional relaxation processes are often observed in the viscoelastic behavior of ionomers. Light functionalization of polymers can increase the glass transition temperature and gives rise to two new features in viscoelastic behavior: (1) a rubbery plateau above T(g) and (2) a second loss process at elevated temperatures. The rubbery plateau was due to the formation of a physical network. The major effect of the ionic aggregate was to increase the longer time relaxation processes. This in turn increases the melt viscosity and is responsible for the network-like behavior of ionomers above the glass transition temperature. Ionomers rich in polar groups can fulfill the criteria for the self-assembly formation. The reported phenomenon of surface micelle formation has been found to be very general for these materials.     

Linear Viscoelastic Behavior of Multiphase Dispersions

The linear viscoelastic behavior of polymer-thickened oil-in-water emulsions, polymer-thickened solids-in-liquid suspensions, and their blends is investigated using a controlled-stress rheometer. The emulsions exhibit a predominantly viscous behaviour at low values of oil concentration in that the loss modulus (G") exceeds the storage modulus (G') over most of the frequency range. At high values of oil concentration, the emulsions exhibit a predominantly elastic behavior. The ratio of storage modulus to loss modulus (G'/G") increases with the increase in oil concentration. Emulsions follow the theoretical model of J. F. Palierne (1990, Rheol. Acta 29, 204) only at low values of oil volume fraction (/=G' over most of the frequency range. The ratio G'/G" varies only slightly with the increase in solids volume fraction. The Palierne model describes the linear viscoelastic properties of suspensions accurately only at low values of solids volume fraction. At high values of solids concentration, the Parlierne model underpredicts the linear viscoelastic properties of suspensions and the deviation increases with the increase in solids concentration. The blends of emulsions and suspensions exhibit strong synergistic effects at low to moderate values of frequencies; the plots of blend modulus versus emulsion content exhibit a minimum. However, at high values of frequency, the blend modulus generally falls between the moduli of pure suspension and pure emulsion. The high-frequency modulus data of blends of emulsions and suspensions are successfully correlated in terms of the modulus ratio versus volume fraction of solids, where modulus ratio is defined as the ratio of blend modulus to pure emulsion modulus at the same frequency. Copyright 2000 Academic Press.     

Viscoelastic properties of ionic polymer composites reinforced by soy protein isolate

Both linear and nonlinear viscoelastic properties of ionic polymer composites reinforced by soy protein isolate (SPI) were studied. Viscoelastic properties were related to the aggregate structure of fillers. The aggregate structure of SPI is consisted of submicron size of globule protein particles that form an open aggregate structure. SPI and carbon black (CB) aggregates characterized by scanning electron microscope and particle size analyzer indicate that CB aggregates have a smaller primary particle and aggregate size than SPI aggregates, but the SPI composites have a slightly greater elastic modulus in the linear viscoelastic region than the CB composites. The composite containing 3–40 wt % of SPI has a transition in the shear elastic modulus between 6 and 8 vol % filler, indicating a percolation threshold. CB composites also showed a modulus transition at <6 vol %. The change of fractional free volume with filler concentration as estimated from WLF fit of frequency shift factor also supports the existence of a percolation threshold. Nonlinear viscoelastic properties of filler, matrix, and composites suggested that the filler‐immobilized rubber network generated a G′ maximum in the modulus‐strain curves and the SPI formed a stronger filler network than the CB in these composites. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3503–3518, 2005     

Oscillating adhesive contacts between micron-scale tips and compliant polymers

Adhesion of micron-scale probes with model poly(dimethylsiloxane), PDMS, elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. For contacts between diamond indenters (radius R = 5 or 10 microm) and PDMS, force-displacement curves were highly reversible and consistent with Johnson-Kendall-Roberts (JKR) behavior. However, our experiments have revealed striking differences between the experimental measurements of tip-sample interaction stiffness and the theoretical JKR stiffness. The measured stiffness was always greater than zero, even in the reflex portion of the curve (between the maximum adhesive force and release), where the JKR stiffness is negative. This apparent paradox can be resolved by considering the effects of viscoelasticity of PDMS on an oscillating crack tip in a JKR contact. Under well described conditions determined by oscillation frequency, sample viscoelastic properties, and the Tabor parameter (with variables R, reduced elastic modulus, E*, and interfacial energy, deltagamma), an oscillating crack tip will neither advance nor recede. In that case, the contact size is fixed (like that of a flat punch) at any given point on the load-displacement cycle, and the experimentally measured stiffness is equal to the equivalent punch stiffness. For a fixed oscillation frequency, a transition between JKR and punch stiffness can be brought about by an increase in radius of the probe or a decrease in PDMS modulus. Additionally, varying the oscillation frequency for a fixed E*, R, and deltagamma also resulted in transition between JKR and punch stiffness in a predictable manner. Comparisons of experiments and theory for an oscillating viscoelastic JKR contact are presented. The storage modulus and surface energy from nanoscale JKR stiffness measurements were compared to calculated values and those measured with conventional nanoindentation and JKR force-displacement analyses.     

国内外PVB材性研究

主要介绍了国内外研究PVB(聚乙烯醇缩丁醛)材性的现状。国内外研究表明,PVB是应变率及温度敏感材料。应变率增加,弹性模量变大;温度升高,弹性模量和剪切模量均下降。同时,国内外进行了少量的实验,研究PVB的本构模型。总结发现,PVB的本构模型可描述为线弹性、弹塑性、线性粘弹性和非线性粘弹性四种,但本质上PVB是非线性粘弹性材料,不同的环境条件与计算要求可选择不同的本构模型。目前,国内外学者比较认可的是用超弹性考虑其非线性,用Maxwell模型考虑其粘弹性。     

Photochemical control of network structure in gels and photo-induced changes in their viscoelastic properties

Poly(amide acid) gels containing photosensitive azobenzene groups in the main chain have been synthesized and their viscoelastic properties before and after light irradiation have been investigated by dynamic viscoelasticity measurements. It was found that 405 nm light induced a local volume change and a two-fold increase in the storage modulus of the gels. We discuss the change in storage modulus upon light irradiation quite simply in terms of classical rubber elasticity theory, which cannot explain this large increase in storage modulus. The photo-induced increase in storage modulus may result from an increase in entanglement interactions of topological constraints fixed in the network structure, caused by photoisomerization of the azobenzene moieties. We suggest that topological constraints in the network structure of the gels were realized by light irradiation and calculate the resulting slip link ratio (index of the topological constraints) in the gel network.     

On the evolution of the viscoelastic properties and its microstructural/chemical origin in filled NBR subjected to coupled thermal and mechanical loads

This study deals with the effect of coupled thermal and cyclic mechanical loadings on the viscoelastic response of carbon black filled nitrile rubber. For this purpose, cyclic loading tests were performed at different temperatures by means of Dynamic Mechanical and Thermal Analysis (DMTA). The type and level of the thermomechanical loadings applied were chosen in order to determine the relative contribution of each of the mechanical and thermal loadings (and their coupling) to the viscoelastic response during the cyclic tests. X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed Infrared spectroscopy (FTIR) analyses were also carried out to track the change in the chemical structure corresponding to the evolution in the viscoelastic response. First, results obtained show that due to the crosslink increase, the storage modulus increases with the number of cycles. It is also observed that temperature amplifies this phenomenon. Second, the cyclic mechanical loading is found to significantly amplify the effect of temperature.     

Nanoindentation creep of nonlinear viscoelastic polypropylene

Uniaxial tensile creep tests at various applied stresses were carried out to demonstrate that PP is nonlinear viscoelastic. A novel phenomenological model consisting of springs, dashpots, stress-locks and sliders was proposed to describe the nonlinear viscoelasticity. Indentation creep tests at different applied load levels were also performed on nonlinear viscoelastic PP. It was found that the shear creep compliance varies with the applied load level when the applied load is less than 5 mN, which means the indentation creep behavior was nonlinear. To find the real reason for the nonlinearity in indentation creep tests, the elastic modulus at various indentation depths was measured using continuous stiffness measurements (CSM). By analyzing the variation of elastic modulus with indentation depth, the nonlinearity of indentation creep behavior was proved to be caused by the non-uniform properties in the surface of the specimen rather than nonlinear viscoelasticity.     

New organotin(IV) ascorbates: synthesis, spectral characterization, biological and potentiometric studies

New organotin(IV) ascorbates of the general formulae R(3)Sn(HAsc) (where R = Me , n-Pr, n-Bu and Ph) and R(2)Sn(Asc) (where R = n-Bu and Ph) have been synthesized by the reaction of R(n)SnCl(4-n) (where n = 2 or 3) with monosodium-l-ascorbate. The bonding and coordination behaviour in these complexes are discussed on the basis of UV-Vis, IR, Far-IR, (1)H and (13)C NMR, and (119)Sn Mossbauer spectroscopic studies. L-Ascorbic acid acts as a monoanionic bidentate ligand in R(3)Sn(HAsc) coordinating through O(1) and O(3). The Mossbauer studies together with IR and NMR studies suggest that for these polymeric derivatives, the polyhedron is trigonal bipyramidal around tin with three organic groups in the equatorial positions. In R(2)Sn(Asc), L-ascorbic acid acts as dianionic tetradentate ligand and a polymeric structure with octahedral geometry around tin with trans organic groups has been tentatively proposed. The complexes have been assayed for their anti-inflammatory and cardiovascular activity. Ph(2)Sn(Asc) has been found to show the highest activity among the studied complexes. It is suggested on the basis of potentiometric studies of Me(2)Sn(IV) and Me(3)Sn(IV) systems with L-ascorbic acid that under physiological conditions (pH = 7.0) Me(2)Sn(HAsc)(OH) (approximately 60%), Me(2)Sn(OH)(2) (approximately 40%) and Me(3)Sn(HAsc) (approximately 60%), Me(3)Sn(OH) (approximately 40%), respectively, are existing, which may be responsible for their biological activities.     

有机过渡金属五卤苯基化合物的研究XI.(Ph2PR)2Co(C6Cl5)2在苯溶液中的构型转化平衡

测定了化合物(Ph2PR)3Co(C5Cl5)2(R=Et,n-Pr)在苯溶液中型转化的热力学和动力学参数,讨论了这种涉及附加配位的新型转化平衡.初始和平衡的八面体构型有不同的附加配位,苯基邻位氢配位比R基端位氢配位更为稳定.四面体构型的稳定性较低,构型转化主要受R基的推动.     

Viscoelastic and fractal characteristics of a supramolecular hydrogel hybridized with clay nanoparticles

The supramolecular hydrogels derived from low-molecular-mass gelators represent a unique class of soft matters and have important potential applications in biomedical fields, separation technology and cosmetic science. However, they suffer usually from weak mechanical and viscoelastic properties. In this work, we carry out the in situ hybridization of clay nanoparticles (Laponite RD) into the supramolecular hydrogel formed from a low-molecular-mass hydrogelator, 2,6-di[N-(carboxyethyl carbonyl)amino]pyridine (DAP), and investigate the viscoelastic and structural characteristics of resultant hybrid hydrogel. It was found that a small concentration of Laponite RD could lead to a significant increase in the storage modulus, loss modulus or complex viscosity. Compared with neat DAP hydrogel, the hybrid hydrogel has a greater hydrogel strength and a lower relaxation exponent. In particular, the enhancement of the clay nanoparticles to the viscoelastic properties of the DAP hydrogel is more effective in the case of higher DAP concentration. By relating its macroscopic elastic properties to a scaling fractal model, such a hybrid hydrogel was confirmed to be in the strong-link regime and to have a more complex network structure with a higher fractal dimension when compared with neat DAP hydrogel.