Equation of state for star polymers in good solvents

A novel method for static equation-of state-development: Equation of state of a cross-linked poly(dimethylsiloxane) (PDMS) network to 10 GPa

Pressure-volume-temperature (PVT) equation-of-state (EOS) information for polymers and polymeric composites is valuable for predicting their response to extreme conditions. An obstacle in determining equations of state for polymeric materials is the lack of a simple, static experimental method for acquiring PVT data for solid networks and liquids at pressures greater than several kilobars. Here, we report a novel approach in determining static EOS for polymers using high-pressure diamond-anvil cells coupled with optical microscopy and image analysis. Results are presented for a cross-linked poly(dimethylsiloxane) polymer, Sylgard 184. Static isothermal results were fitted to empirical and semiempirical equations of state, including the Tait, Birch-Murnaghan, and Vinet forms. Static PV data were also converted to pseudoshock velocity-pseudoparticle velocity (U(s)-u(p)) for comparison to dynamic Hugoniot data. A linear Rankine-Hugoniot fit U(s)=s(T)u(p)+c(T) gives c(T)=1.572 km/s and s(T)=1.703. s(T) is related to the pressure derivative of the bulk modulus B(0) (') by s(T)=(B(0) (')+1)/4 and B(0) (')=5.8. A comparison of the static and shock data is given, along with an estimate of the Grüneisen parameter, and a discussion of the free volume content in the polymer network, and limitations of this novel method.