A real chain-like molecule can be approximately described as a square-well chain.The molecular thermodynamic model for chain-like molecule fluids has been established by adopting the molecular thermodynamic model for hard-sphere-chain fluids as a reference and a perturbation term contributed by the square-well potential.It has a very simple form.For pure normal fluidswith no association

good agreement with experimental data can be obtained by using three tem-perature independent molecular parameters:chain length r

segment diameter σ and interaction energy ε/κ between segments

estimated from saturated vapor pressures and liquid volumes.Excluding the critical region

the deviation of vapor pressures is less than 2% for most substances

the deviation of liquid volume is less than 3%.With the same three temperature independent parameters

the specific volume of polymers with a wide range of temperature and pressure can be satisfactorily correlated.The deviations of specific volume with pressures up to 200MPa are less than 0.2%.