Fine Structure of Poly (p-phenylene terephthalamide) Fibers

Abstract

In order to clarify the fine structure of poly (p-phenylene terephthalamide) fibers, hereafter refered to as PPTA fibers, the absolute value of refractive index was determined and the orientations of b and c axes and chain axis of molecules in an amorphous region were evaluated quantitatively. Keviar 49R was adopted as the model sample of PPTA fibers. An interference optical microscopy and an electron microscopy were mainly adopted. By analyzing the interference fringe pattern, the refractive indexes along a, b and c axes were evaluated as follows : nα, c≤1.579, nβ, c≤1.764, nγ, c≥2.13. The refractive indexes along a molecular chain axis (nγ, α) and along the axis perpendicular to the molecular chain axis (n (α+β) /2, a) in the amorphous region were evaluated to be as nγ, a≥2.13 and n (α+β) /2, a≤1.498 respectively. The results obtained were : (1) The local orientation function of c axis changed from place to place ranging from ca. 1.0 at periphery of a fiber to 0.60.8 at the central part of a fiber. (2) b axis oriented to the radial direction of a fiber, and the local orientation function of this axis changed from 0.8 (periphery part) to 0.2∼0.4 (central part). (3) The orientation of chain axis in an amorphous region at the periphery was higher than that at the central part. (4) The volume fractions of relaxed chains, tie chains, and stretched chains in an amorphous region were evaluated to be 0.10, 0.02, and 0.17 respectively. (5) The pleated lamella structure with the period of 300500 nm, and also the micro-fibril structure of 10 nm in width were observed in the freezed fracture surface. (6) The refractive index changed periodically by the period of 1 μm along the fiber axis. By taking the above resits of (1) (6) into consideration, a new fine structure model, called “fibrious block model”, has been proposed which was constructed with the fibrious structure with many blocks of 5-4.0 nm in width and 300∼500 nm in length, for representing the fine structure of PPTA fibers.