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The Hermans orientation factor is a scalar giving a simple assessment of molecular or crystal orientation in a polymer. It is usually determined from X‐ray diffraction measurements. The evaluation of the Hermans orientation factor is proposed from the diffraction analysis of only a single diffracting plane. The estimation relies on (a) the assumption that the polymer crystals possess a cylindrical symmetry with respect to the chain direction and (b) that the processing conditions induce a transverse isotropy of the polymer chains. A practical consequence of the proposed analysis is that the Hermans orientation factor can be computed from a single 1D diffractogram. This enables fast evaluations since even low signal‐to‐noise ratio signals can be processed with optimal noise reduction. An illustration of this method for uniaxially compacted polytetrafluoroethylene (PTFE) powder is presented. The crystalline orientation is followed during in situ crystallization experiments with a fine time resolution. [ABSTRACT FROM AUTHOR]

Abstract Cold uniaxial pressed PTFE compacts were subjected to thermal cycles of increasing maximum temperature below melting temperature, and thermal expansion was followed in both the compaction and transverse directions. The measurements reveal that thermal expansion consists of two distinct contributions: i) an irreversible part that is controlled only by the maximum temperature experienced by the sample, and ii) a reversible, or thermo-elastic, part that is observed while cycling below the maximum temperature. Moreover, both of these thermal strains exhibit marked anisotropy, resulting from the cold oedometric compaction of the sample. A quantitative analysis of these eigenstrains is proposed, following a formal analogy with elasto-plasticity. Finally, it is shown that such behavior is specific to cold-pressed compacts as sintered samples exhibit isotropic and purely reversible thermo-elastic behavior. Highlights • Thermal expansion of cold pressed PTFE compacts contains two anisotropic contributions, one reversible and one irreversible. • Dilatometric measurements during repeated cycles of increasing amplitude allows these two contributions to be distinguished. • The irreversible part of the thermal expansion is controlled solely by the maximum temperature experienced by the sample. [ABSTRACT FROM AUTHOR]

PTFE presents an original secondary crystallization that appears to be independent of the primary one. The conjunction of differential scanning calorimetry and dilatometry experiments has offered complementary data to study both crystallization melted compacted powder and of recrystallized samples. The primary crystallization is responsible for the crystallinity content variation with the cooling rate. It preserves the anisotropy induced by previous compaction that can be observed by dilatometry. Conversely, the secondary crystallization appears to be instantaneous, reversible, kinetic-free and isotropic. It occurs over a wide temperature range between 250°C and 310°C. • Two independent crystallization mechanisms for PTFE are proposed. • The rate-limited primary crystallization sets the final crystallinity content. • It inherits from uniaxial pressing of an anisotropy revealed in dilatometry. • In contrast, the secondary crystallization is instantaneous and fully reversible. • The secondary crystallization appears to be isotropic. [ABSTRACT FROM AUTHOR]