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Polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF) and polychlorotrifluoroethylene (PCTFE) were studied by differential calorimetry at normal pressure in the temperature interval 130-555 K, and by pressure dilatometry in the temperature interval 303-545 K and in the pressure range 14-70 MPa. Pressure derivatives of the melting temperature, ClTm/ClP, were used to calculate the "true" meking entropy A 8~ from the Clausius-Clapeyron equation, while equilibrium values of the mek specific volume were treated according to the Simha-Somcynsky cell theory. Comparison of the experimental data for PVF, PVDF and PCTFE with pertinent published data for polyethylene (PE) and polytetrafluoroethylene (PTFE) showed that a steady decrease of A S~ and of the conformational contribution to A Sm reflects the increase of the equilibrium chain stiffness as the hydrogen atoms of the hydrocarbon chain (PE) are substituted with bulkier fluorine and/or chlorine atoms (PCTFE, PTFE), while a concomitant drop of the pressure reducing parameter from the cell model and an increase of melt compressibility and the derivati'ce, clT,~lclP, should be attributed to the increase of the free volume fraction of the melt in the series, PE to I~FE.

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