Structure of Negative Spherulites of Even–Even Polyamides. Introducing a Complex Multicomponent Spherulite Architecture

Even–even polyamides are known to grow as positive spherulites, which implies that the radial, fastest growth direction is parallel to the a-axis and hydrogen bond direction. However, after annealing/self-seeding close to T_m, crystallization in a limited T_c window (down to ≈20 °C below T_m) yields profuse negative spherulites and, frequently stemming from the latter, less frequent and ill-defined entities named “spherulitic aggregates”. The detailed structure and origin of these two entities, and especially of the negative spherulites, are still not clearly established although they were first observed some 70 years ago. The recent recognition that polymer spherulites (specifically, spherulites of PVDF in its γ phase) are made of scrolled, radiating lamellae and the observation and analysis of solution grown, scrolled nylon-66 single crystals provide useful guidelines for a renewed analysis of this structural puzzle. The present analysis relies heavily on the approach and on the detailed diffraction data obtained by Lovinger in the late 1970s. It strongly supports the contention that negative spherulites of even–even polyamides are made of scrolled lamellae. The hydrogen bonds are oblique to the spherulite radius. Twinning parallel to the hydrogen-bonded sheets generates two different orientations of the unit cell that helically wind around the scroll axis. These two cell orientations plus a contribution of aggregate-like lamellae that grow inside the radial scrolls account for the apparent lack of orientation of the unit cell in these negative spherulites. This model explains also the birefringence variation of negative spherulites with T_c and their melting point identical to that of aggregates. Negative even–even PA spherulites thus illustrate an original spherulite architecture in which one population of lamellae generates a scaffold within which a second population develops in a confined but oriented frame. It appears to be applicable, perhaps with variants, to the spherulite structure of other types of polyamides.