Relationship between the size of the latex beads and the solid-solid phase transitions in emulsion polymerized poly(tetrafluoroethylene)

Ten virgin powder samples of poly(telrafluoroethylene) (PTFE) were examined by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The samples, prepared by emulsion or nanoemulsion polymerization, differed by average molecular weight and particle size. DSC was used to study the trends of the temperature and enthalpy of solid-solid transitions around room temperature. The samples with smaller particle size and lower molecular weight departed heavily from the behavior reported in the literature. The major role of particle size in determining this behavior was established by experiments of crystallization from the melt. WAXS was employed to study the crystalline phases of PTFE as a function of temperature and to confirm the hypotheses put forward in the DSC analysis. WAXS data showed that the degree of order of the crystallites in the virgin powders is controlled by particle size. Therefore, the confinement of the as-polymerized PTFE crystals in small entities (tens of nanometers) has a similar effect on the crystal correlation length and on the solid-state transitions, while it appears that the molecular weight per se is not able to induce such a behavior in the melt-crystallized polymers.