Crosslinking effect on polydimethylsiloxane elastic modulus measured by custom‐built compression instrument

A macroscopic compression test utilizing a simple custom‐built instrument was employed to measure polydimethylsiloxane (PDMS) elastic modulus. PDMS samples with varying crosslinking density were prepared with the elastomer base to the curing agent ratio ranging from 5 : 1 to 33 : 1. The PDMS network elastic modulus varied linearly with the amount of crosslinker, ranging from 0.57 MPa to 3.7 MPa for the samples tested. PDMS elastic modulus in MPa can be expressed as 20 MPa/PDMS base to curing agent ratio. This article describes a simple method for measuring elastic properties of soft polymeric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41050.     

Influence of solvent size on the mechanical properties and rheology of polydimethylsiloxane-based polymeric gels

Soft polymeric gels have utility in a broad range of medical, industrial, and military applications, which has led to an extensive research investment over the past several decades. While most gel research exploits a cross-linked polymer network swollen with small molecule solvents, this article systematically investigates the impact of the solvent molecular weight on the resulting gel mechanical properties. The model polymer gel was composed of a chemically cross-linked polydimethylsiloxane (PDMS) network loaded with a non-reactive PDMS solvent. In addition to investigating the impact of solvent loading, the solvent molecular weight was varied from 423,000 g/mol to 1250 g/mol, broadly spanning the molecular weight of entanglement for PDMS (MW_ENT ∼29,000 g/mol). The gels exhibited a strong frequency dependent mechanical response when the solvent molecular weight >MW_ENT. In addition, scaling factors of shear storage modulus versus solvent loading displayed a distinct decrease from the theoretical value for networks formed in a theta solvent of 2.3 with increasing measurement frequency and solvent molecular weight. The frequency dependent shear storage modulus could be shifted by the ratio of solvent molecular weights to the 3.4 power to form a master curve at a particular solvent loading indicating that mobility of entangled solvent plays a critical role for the mechanical response. In addition, the incorporation of entangled solvent can increase the toughness of the PDMS gels.     

Nanoindentation study of polydimethylsiloxane elastic modulus using Berkovich and flat punch tips

This article explores polydimethylsiloxane (PDMS) mechanical properties, and presents nanoindentation experiments with Berkovich and flat punch indenters. In the Berkovich tip quasi‐static nanoindentation test, there are pull‐in and pull‐off events observed during the initial tip contact, and when withdrawing from the surface, respectively. The pull‐in interaction needs to be accounted for to properly determine the initial contact point, and thus the accurate contact area. Once accounted for the pull‐in event, the Berkovich and flat punch tips quasi‐static nanoindentation tests give comparable results of about 1.5 MPa for the PDMS elastic modulus (5 : 1 elastomer base to the curing agent ratio). However, PDMS unloading stiffness is higher than the loading stiffness, and dynamic PDMS testing yields higher elastic modulus of about 3.6 MPa. While these results are comparable with the large strain macroscopic compression test results, the difference underscores the complexity of elastomer mechanical characterization and illustrates the discrepancies typical of the reported values. This article describes nanoindentation methods and critical aspects of interpreting results to assess PDMS mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41384.     

Molecular modeling approach to the prediction of mechanical properties of silica‐reinforced rubbers

Recently, we have suggested a nanomechanical model for dissipative loss in filled elastomer networks in the context of the Payne effect. The mechanism is based on a total interfiller particle force exhibiting an intermittent loop, due to the combination of short‐range repulsion and dispersion forces with a long‐range elastic attraction. The sum of these forces leads, under external strain, to a spontaneous instability of “bonds” between the aggregates in a filler network and attendant energy dissipation. Here, we use molecular dynamics simulations to obtain chemically realistic forces between surface modified silica particles. The latter are combined with the above model to estimate the loss modulus and the low strain storage modulus in elastomers containing the aforementioned filler‐compatibilizer systems. The model is compared to experimental dynamic moduli of silica filled rubbers. We find good agreement between the model predictions and the experiments as function of the compatibilizer's molecular structure and its bulk concentration. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40806.     

Rheological behavior and network dynamics of silica filled vinyl‐terminated polydimethylsiloxane suspensions

Present article reports the rheological properties and network dynamics of fumed silica filled vinyl‐terminated polydimethylsiloxane suspensions. The results reveal that as filler loading increases, the span of the linear viscoelastic region with constant dynamic storage modulus is narrowed with increase in strain amplitude while the relaxation time of the compounds get shifted to longer time scales. The dynamics of filler‐network indicated significant Payne effect due to fumed silica incorporation into the PDMS matrix. Further, strain‐induced agglomeration of fumed silica particles, characterized by a peak in the dynamic loss modulus curve could also be observed. High loss‐tangent was observed for lower contents of filler in the suspension, an effect with an apparent relationship to the loosely formed filler‐network. The formation of a saturated network structure of fumed silica particles was evident from the dynamic modulus and complex viscosity data, that remained unaffected with frequency till a critical amount of fumed silica loading. Han plots (storage modulus versus loss modulus) revealed the microstructural changes for various filled systems that was attributed to build‐up of the filler‐network causing an apparent evolution of yielding phenomenon. Van Gurp‐Palmen plots (complex modulus versus phase lag) showed that flow behavior of the filled PDMS suspensions resembled to that of typical viscous fluids. POLYM. ENG. SCI., 57:973–981, 2017. © 2016 Society of Plastics Engineers     

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.     

Linear‐nonlinear dichotomy of the rheological response of particle‐filled polymers

Novel nanoparticles, polymer‐particle coupling agents, and functionalized polymers are being developed to enhance the performance of particle‐reinforced polymer systems such as advanced rubber compounds for automobile tires. Understanding the complex rheological behavior of rubber is critical to providing insights into both processability and end‐use properties. One unique aspect of the rheology of filled elastomers is that the incorporation of particles introduces a hysteretic softening (Payne effect) at small dynamic strains. This study demonstrates that this nonlinear viscoelastic behavior needs to be considered when attempting to correlate steady shear response (Mooney viscosity) to oscillatory shear measurements from test equipment such as the Rubber Process Analyzer (RPA). While a wide array of unfilled gum elastomers show good correlation between Mooney viscosity and dynamic torque from the RPA at all of the strain amplitudes used, rubber compounds containing silica and carbon black particles only exhibit good agreement between the two measures of processability when the oscillatory strain amplitude is high enough to sufficiently break up the filler network. Other features of the filler network and its influence on nonlinear rheology are considered in this investigation, including the effects of polymer–filler interactions on filler flocculation and the use of Fourier transform rheometry to illustrate the “linear‐nonlinear dichotomy” of the Payne effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40818.     

Beneficial role of “sol–gel” synthesized mesoscale silica networks on the performance of liquid silicone elastomer

This piece of contribution highlights the profound effect of unique mesoscale morphology of tailor made nanosilica assembly (SS‐Silica), synthesized by sol–gel route, on the mechanical and dynamic rheological properties of platinum catalyzed addition‐cured silicone elastomers. While commercial colloidal nanosilica (CS Silica) is used as the control nanofiller representing particulate morphology, the tailor‐made SS‐Silica having highly percolated network structure offers 10‐fold increase in storage modulus of the uncured reactive PDMS precursor nanocomposite with stable dynamic rheological behavior and more than 180% enhancement in tensile strength of resulting liquid silicone rubber (LSR) produced on curing, as compared to colloidal silica of commercial origin. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40125.     

Dynamic Mechanical Properties of Extruded Rods of Poly(dimethylsilylene) and Polysilane Copolymers having Methyl and Ethyl Substituents

The oriented rods of poly(dimethylsilylene) (PDMS), poly(dimethylsilylene‐co‐methylethylsilylene) (PMMME) and poly(diethylsilylene‐co‐methylethylsilylene) (PEEME) were prepared by extruding the polymers through a circular tube die in the temperature range where the hexagonal mesophase forms. In general PDMS cannot be processed into materials below its decomposition temperature, but it was found that PDMS could be compression molded at 250°C under nitrogen atmosphere prior to the extrusion. The extruded polysilanes were characterized in detail by dynamic viscoelasticity and wide‐angle X‐ray diffraction in order to clarify the structure‐property relationship. The temperature dependence of wide‐angle X‐ray diffraction showed that the crystalline phases of PDMS, PMMME, and PEEME were transformed into the hexagonal mesophase at elevated temperatures. The dynamic storage modulus of PDMS amounts to 7.6 GPa at room temperature and 11.2 GPa at liquid nitrogen temperature. The dynamic storage modulus increases with increasing extrusion ratio, and the increase of modulus with extrusion ratio is well correlated with the change of the crystal orientation function. The dynamic storage moduli of the extruded polysilanes were lowered with the rise in temperature owing to the structure relaxation processes, which lie in the lower temperature range. The decrease of modulus with temperature was more marked in PEEME than in PDMS, suggesting that the molecular motion of the ethyl substituents lowered the modulus at room temperature.     

Rheological behavior of nanofibrillated cellulose/acrylic polymer nanocomposites: Effect of melt extrusion

Nanocomposite films based on nanofibrillated cellulose (NFC) and acrylic latex were prepared by film casting and their melt rheology was investigated under dynamic conditions in both the linear and the nonlinear regimes. The addition of cellulose nanofiller increased the storage modulus G′ and the dynamic viscosities η* of the nanocomposites monotonically, with NFC contents up to 6wt%. In addition, a transition from liquid‐like to solid‐like viscoelastic behavior was observed up to 1wt% of the added NFC with a terminal plateau in the low‐frequency range. This was explained in terms of the formation of an interconnected network involving the filler. After melt extrusion, a considerable change in the rheological properties was observed, with a major downward shift in the magnitude of G′ vs. the frequencies along with an upward shift to higher strain in the linear viscoelastic range. Such a transition was attributed to the irreversible break‐down and disruption of the NFC network. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Viscoelastic properties of poly(ε‐caprolactone)/clay nanocomposites in solid and in melt state

Viscoelastic properties in solid and in melt state of poly(ε‐caprolactone), PCL, nanocomposites with organomodified clays (Cloisite30B and Cloisite15A) are thoroughly investigated. Although WAXD is insensitive to the difference in the nanocomposites structure, the melt rheology reveals pronounced differences between the two series. Melt yield stress values, obtained from fittings by the Carreau–Yasuda model, are used as a measure of partial exfoliation of the clay. Temperature dependence of the shift factors, used for time–temperature superposition of the modulus curves, yields similar values of the flow activation energies for all the samples. Temperature dependences of the dynamic modulus and loss factor of solid nanocomposites were correlated to the structural differences deduced from the melt rheology. The increase in the storage modulus is compared to the theoretical predictions from the Halpin–Tsai model. The effective aspect ratio obtained from this comparison agrees reasonably with the value estimated from the melt rheology. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42896.     

Modeling tensile modulus of (polyamide 6)/nanoclay composites: Response surface method vs. taguchi‐optimized artificial neural network

Tensile modulus is an important mechanical property of polymer/nanoclay nanocomposites. In this study, response surface method (RSM) and Taguchi‐optimized artificial neural network (Taguchi‐optimized ANN) were used to model tensile modulus as a function of nanoclay content, melt temperature, screw speed, and feeding rate for (polyamide 6)/nanoclay nanocomposites prepared in a twin‐screw extruder. The comparison between Taguchi‐optimized‐ANN‐ and RSM‐generated plots showed that predictions made by both models were in agreement in general. Coefficient of determination, R², showed that the RSM model can explain the variation with the accuracy of 0.768, indicating there was no strong correlation. However, from ANOVA, the p value for the RSM model was less than 0.05, signifying that the obtained model could be considered statistically significant. In addition, further assessment in terms of data fitting and prediction capabilities demonstrated the superiority of a properly trained Taguchi‐optimized ANN model in characterizing the nonlinear behavior of a response‐factors relationship. The Taguchi‐optimized ANN model R² for training data and testing data were 0.965 and 0.902, respectively. Also, the Taguchi‐optimized ANN model was developed by using 20% less data in comparison to the RSM model. J. VINYL ADDIT. TECHNOL., 22:29–36, 2016. © 2014 Society of Plastics Engineers     

Organic solvent nanofiltration with a Poly(dimethylsiloxane) membrane: Parameters affecting its sieving properties

In this study, retention experiments were performed to characterize the variable sieving properties of a poly(dimethylsiloxane) (PDMS) membrane in relation with operating parameters. The swelling, transmembrane pressure, and temperature are all known to impact the physicochemical properties and morphology of PDMS polymer and were therefore varied for the purposes of our retention experiments which assessed them with the homologous series of polyethylene glycols (PEGs; 200–1500 g mol^?1). The objectives were twofold—first, to evaluate the capacity to induce a targeted molecular weight cutoff (MWCO) by selecting appropriate filtration conditions and second to better understand the mechanisms involved during solvent‐resistant nanofiltration with PDMS. The selected solvents or solvent/solvent mixtures used throughout this study were found to induce swelling ratios of 1.16 (ethanol/ethyl acetate: 25/75), 1.26 (ethyl acetate), 1.33 (ethyl acetate/toluene: 50/50), and 1.41 (toluene), respectively. Linear correlations were obtained between the MWCO and the swelling ratio induced by each solvent and between the MWCO and the transmembrane pressure. Pore size calculations using solvent flux and retention data confirmed the variable sieving properties of the PDMS membrane in relation to the solvent‐induced swelling and applied transmembrane pressure. In addition, the study of the solute‐transfer rate through several operating conditions showed that both diffusive and convective transports occurred for the PEG solutes and that their respective contributions appeared dependent on the variable pore size of the PDMS membrane. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41171.     

A two‐step method for the synthesis of a hydrophilic PDMS interpenetrating polymer network

A hydrophilic PDMS (polydimethylsiloxane) surface was formed by the synthesis of an interpenetrating polymer network (IPN) in a two‐step process. In the first step, PDMS was loaded with crosslinker and initiator using a solvent that swells the PDMS. In the second step, the PDMS sample was submerged into a solution containing the hydrophilic monomer followed by a UV‐polymerization step. The choice of solvent in the second step is critical to obtain a hydrophilic surface. It can be concluded that the solubility parameter of the solvent should be above a threshold value. Hence, in the second step only sufficiently polar solvents will result in hydrophilic PDMS‐IPNs. These principles are illustrated by using N‐vinyl‐2‐pyrrolidone as the hydrophilic monomer forming PVP/PDMS‐IPNs. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008.     

Properties of new nanocomposite triblock copolymer gels based on expandable graphite

In this work, we investigated the effect of expandable graphite (EG) on the property of triblock copolymer prepared from a poly(styrene‐b‐(ethylene‐co‐butylene)‐b‐styrene) (SEBS) imbibed with an EB‐compatible hydrocarbon oil. The rheological properties showed that at a temperature between 30 and 40°C below the gel point, the triblock copolymer gels had a dynamic storage modulus (G′) greater than loss modulus (G‵), thereby indicating that at ambient temperature, a physical network is still present in spite of the addition of nanoparticles. Dynamic rheological measurements of the resultant nanocomposite triblock copolymer gels confirmed that the addition of EG affects the linear viscoelastic properties and maximum operating temperature of the parent triblock copolymer gels. The mechanical properties showed only marginal increase, which can be attributed to the poor dispersion that leads to agglomeration of particle into micrometer size stacks, and thus the particles behave only as inorganic fillers. The morphology and X‐ray diffraction revealed that the EG used to generate nanocomposite triblock copolymer gels is dispersed generally within the swollen copolymer and/or solvent. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Expanded graphite/polydimethylsiloxane composites with high thermal conductivity

Expanded graphite (EG)/polydimethylsiloxane (PDMS) composites with high thermal conductivity and high flexibility are prepared in this work. EG derived from natural graphite flake is infiltrated in PDMS prepolymer solution and then hot pressed in a steel mould at 80 °C for 2 h. Optical microscope and scanning electron microscope investigation reveals the interpenetrating network structures in the EG/PDMS composites. When mass fraction of EG increases to 10.0 wt %, the thermal conductivity of EG/PDMS reaches to 4.70 W/m K which should be attributed to the conductive path of graphite platelets. Meanwhile, the composites have excellent flexibility (the compressive modulus is 0.68 Mpa) because of its continuous PDMS network. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44843.     

Reinforcement of poly(dimethylsiloxane) networks by montmorillonite platelets

A PDMS network, synthesized from a vinyl‐terminated precursor, was reinforced by plate‐like montmorillonite (volclay) particles with different surface cations. The optimal ratio of crosslinker‐to‐PDMS precursor was ascertained from the mechanical properties of networks prepared with different crosslinker concentrations. The elastic modulus of the polymer was enhanced by the montmorillonite particles. The increase in modulus was higher in the Li– than in the Na–volclay composites. The ultimate strength of the composites was also strongly enhanced by the small platelets, especially in presence of surface Li⁺. The stronger influence of Li–volclay on the mechanical properties of the composites can be attributed to the partial formation of an intercalated structure, which leads to thinner particles with a high aspect ratio. Both composite strength and modulus were proportional to the filler‐volume‐fraction, but the increase in strength was limited by rising particle agglomeration at high loading. In contrast to organic‐modified montmorillonite, the inorganic surface of volclay catalyzed the thermal degradation of PDMS. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2175–2183, 2002     

Formation and evolution of the carbon black network in polyethylene/carbon black composites: Rheology and conductivity properties

We report a detailed investigation on the effect of carbon black (CB) morphology on network formation and evolution in high‐density polyethylene/CB composites. There were three types of networks in our study, the electrical network in the solid state and the electrical and rheological networks in the melt state. The evolution of the network in the polymer melt was traced by simultaneous electrical resistivity (R) and dynamic rheology testing. An oscillation strain sweep was used to investigate the network stability with a large strain. We found that with high‐structure CB with a branched morphology, it was easier to form a filler–polymer or filler–filler network than with low‐structure CB with a spherical morphology in the composite melt. The high‐structure CB network was more stable with a large strain compared to the low‐structure one. Meanwhile, the low‐structure CB aggregates had stronger capability of movement and re‐aggregation in the polymer melt. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39953.     

Mechanical,morphological, and thermal properties of nanotalc reinforced PA6/SEBS‐g‐MA composites

Ternary butylene‐styrene‐g‐maleic anhydride (SEBS‐g‐MA) (100/20 w/w) blend with varying content of nanotalc (1, 3, and 5 wt %) were prepared by melt compounding followed by injection molding. Thermal properties were investigated by thermogravimetric analysis (TGA) and the results show that the thermal properties of nanocomposites are slightly improved by the addition of nanotalc content. The morphology of nanocomposites using wide angle X‐ray diffraction (WAXD) and transmission electron microscopy (TEM) revealed the delamination of talc layers in the ternary nanocomposites. The dynamic mechanical properties of the samples were analyzed by using dynamic mechanical thermal analyzer (DMTA). The results show that the storage modulus of the blend monotonically increased while tan δ curve show the diffuse pattern with the nanotalc content. The mechanical properties of PA6/SEBS‐g‐MA nanocomposites were studied by tensile, flexural, and impact tests. The tensile and flexural properties continuously increased while izod impact and elongation‐at‐break decreased with nanotalc content. Various theoretical predictive models were used to correlate tensile modulus with the experimental data. The experimental data shows the positive deviation with the applied models. Bela Pukanszky model has been used to calculate the value of parameter B by employing tensile strength data. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41381.