THE EQUATION OF STATE OF SOLID AND MOLTEN POLY(BUTYLENE TEREPHTHALATE) TO 300-DEGREES-C AND 200 MPA

The volumetric behavior of poly(butylene terephthalate) (PBT) was studied from 30 to 303-degrees-C and pressures from atmospheric to 200 MPa. The pressure-volume-temperature (PVT) data show behavior typical of other semicrystalline materials. The empirical Tait equation was used to fit the measured volumes in the solid and melt regions. The maximum deviations between the Tait fits and the measured volumes were 0.001 and 0.002 cm3/g in the semicrystalline and melt regions, respectively. The theoretical equation of Simha-Somcynsky was used to represent the PVT behavior of this material in the equilibrium melt. Fitting the theory to the data involves a determination of the reducing parameters of the theory. We find P* = 1139 MPa, V* = 0.7838 cm3/g, and T* = 11133 K. From these parameters we calculate that the chemical repeat unit of PBT has 6.37 external degrees of freedom, in comparison with 4.97 degrees of freedom to the closely related poly(ethylene terephthalate), with two less(-CH2-) units in the backbone. The difference of 1.4 degrees of freedom may be compared with the 0.88 degrees of freedom of the polyethylene (-C2H4-) repeat unit itself.