Abstract: Crystalline polymer fibrils having a highly ordered crystalline core surrounded by less ordered crystalline polymer are produced by causing a solution of the polymer to flow through a gauze. The solution is at a temperature below the dissolution temperature of the polymer but above the temperature at which the polymer will crystallize spontaneously in an unstirred solution. The flow rate is selected in relation to the gauze dimensions to impart a sufficiently high longitudinal velocity gradient to the solution flowing through the gauze, preferably about 10.sup.2 sec.sup..sup.-1 or greater.

Abstract: Molten crystallizable polymer is extruded through an orifice. Opposite the orifice on the upstream side is a fixed surface arranged so as to provide in the molten polymer a line of uniaxial extension or a plane of pure shear passing through the orifice. The polymer is extruded at a temperature and flow rate such as to cause a core of crystalline polymer to form in the melt along said line or plane. It is particularly suitable for extruding a continuous filament having a highly crystalline core in a generally less crystalline matrix of the polymer.