Pressure-volume-temperature (P-V-T) relationships were determined for polymer liquid crystals (PLC) solids and melts up to 400 C and 240 J cm-3 . We have studied a series of longitudinal PLC copolymers with the formula PET/xPHB, where PET=poly(ethylene terephthalate), PHB=p-hydroxyben-zoic acid (the LC component), with the mole fractions of the LC component 0 >/ x > 0.8. The P-V-T results are represented by the Hartmann equation of state and its characteristic parameters v*, T* and P* evaluated. In both solid and liquid phases the v*(x) and T*(x) curves show minima near the concentration qLC limit above which islands of the LC-rich phase are formed. Surface tensions of melts are calculated as a function of x from the P-V-T data for the liquid state using the theory of Prigogine and Patterson. Increasing x first causes lowering of the cohesion of the single-phase structure dominated by the flexible PET. Above qLC limit the islands appear to impart their orientation to both phases; this is the channeling effect predicted earlier from the Flory statistical-mechanical theory of PLCs (Blonski et al., Macromolecules, 1993, 26, 84) Copyright 1998 Elsevier Science Ltd. All right reseved.