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Electron microscopy, together with wide- and small-angle x-ray diffraction studies, indicates that copolymers of tetrafluorocthylene and hexafluoropropylene are lamellar. The lamellar development is extensive; the lamellae can be broad and extend for many microns. The perfluoromethyl groups are incorporated within the lamellae as point defects.

X-ray diffraction has been used to measure the separation of the molecular axes as a function of temperature, lamella thickness, and comonomer concentration in copolymers of tetrafluoroethylene and hexafluoropropylene. These data show that the increase of separation with increasing concentration of perfluoromethyl groups is a consequence of inclusion of the groups in the crystals and not an artifact associated with lamella thickness or crystal transition temperature. This conclusion is supported by the fact that diffuseness of some of the x-ray reflections in the copolymers indicates the presence of molecular disorder which might be expected from inclusion of the perfluoromethyl groups in the crystals. The lamellas in the copolymers are thinner than those in the typical homopolymer and this aspect of the structure is the primary cause of the lower density in the copolymers. Analysis of the available data indicates that this is also the situation in copolymers of ethylene and propylene. For both copolymer series, the density of the lamellas is apparently increased by the inclusion of the methyl groups in the crystals as defects.

A theory of crystallization is formulated for random copolymers which crystallize with the non-crystallizable co-units incorporated into the crystalline lattice as defects. The appropriate melting point equation and other associated thermodynamic properties are derived for this model as a function of crystal thickness and comonomer concentration. The formation of lamellar type morphology is assumed to be a kinetically determined phenomena and nucleation theory is utilized accordingly. The isothermal lamella thickness is predicted to increase in a definitive manner as the noncrystallizable comonomer concentration X increases, while the associated isothermal growth rate is predicted to decrease. The variation of lamella thickness with X when the copolymer is quenched or cooled at a uniform rate is also qualitatively predicted. Under these conditions lamella thickness decreases with increasing X , which is in accord with previous experimental observations on random copolymers of tetrafluoroethylene and hexafluoropropylene as well as other random copolymers. Theory also suggests how the surface free energy parameters σ e and σ can be determined from isothermal crystallization experiments for a series of random copolymers of varying composition.

Between approximately 20 and 30 °C at atmospheric pressure, polytetrafluoroethylene exhibits a phase (denoted as IV), which has not been observed in P-V-T measurements at pressures above 10 8 Nm -2 (1 Kilobar). Data are presented to resolve this phase in the temperature range 0 to 50 °C and the pressure range 0 to 0.686 × 10 8 Nm -2 . The II-IV (20 °C) transition pressure increases with temperature according to the equation P = (-820 + 26.9 t + 0.68 t 2 ) × 10 5 and the IV-I (30 °C) transition according to the equation P = (- 91 - 57.8 t + 1.99 t 2 ) × 10 5 suggesting a possible triple point near 2.6 × 10 8 Nm -2 and 54 °C. However, the IV-I transition probably cannot be resolved by volume measurements at pressures above 2 × 10 8 Nm -2 because its volume of transition is small and the two transitions overlap. The enthalpies of the transitions are calculated for different pressures and some thermodynamic properties of phase IV are determined indirectly. It is shown that, if the effect of volume change is subtracted, random reversals of the hand of the helical conformation of the molecule can account for appreciable fractions of the enthalpy and corresponding entropy change for the combined transitions at atmospheric pressure.