Transcrystallinity in Surface Modified Aramid Fiber Reinforced Nylon 66 Composites Alexander Feldman,* M. Fernanda Gonzalez, Gad Marom Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel Fax: 00972 2 6586068; E-mail: sashka@pob.huji.ac.il Received: June 4, 2003; Revised: August 26, 2003; Accepted: September 3, 2003; DOI: 10.1002/mame.200300151 Keywords: aramid fibers; fiber surface treatment; nylon 66 composites; transcrystallinity Introduction Composite systems of semicrystalline thermoplastic matri- ces can develop a highly oriented crystalline domain around the fibers, known as the transcrystalline (tc) layer. Tc morphology depends on the physical and chemical nature of the constituents as well as on the processing conditions of the composite material. [1] In the tc layer the crystal nucle- ation is produced at the fiber surface and the lamellae grow away from the fiber to the bulk. The lamellar orientation, which depends on the fiber/polymer system and on the distance from the fiber surface, affects in turn the mecha- nical properties of the composites. Nylon 66 is a semicrystalline polyamide that crystallizes in two known triclinic crystal structures named a and b. At room temperature both structures can be found in chain- folded lamellae, usually composed of hydrogen-bonded sheets stacked together via Van der Waals interactions. [2] The a structure is the most probable one when the polymer crystallizes from the melt. In comparison, transcrystallinity in aramid fiber rein- forced nylon 66 composites was reported to comprise preferential orientation of the a structure based lamellae [3] and a model was proposed, which presented the orientation of the chain axis as a function of the distance from the fiber surface. [4] According to the model, the nucleation is produ- ced with the c-axis of the unit cell (the chain axis) parallel to the fiber axis and the lamellae grow outwardly forming a sheaf-like structure. Correspondingly, using the proposed structural model, the effect of the transcrystalline layer on the composite material was studied for a range of properties such as the thermal expansion coefficient [5] and the Full Paper: Aramid (kevlar-49) fibers were surface treated by two different methods to induce roughness and then used to produce unidirectional nylon 66 based composites. The transcrystallinity generated around the treated fibers was characterized by SEM and polarized light microscopy and compared with the regular transcrystalline layers produced by pristine aramid under the same processing conditions. The treated fibers generated a double transcrystalline layer, the inner layer being thinner and more compact than the regular nylon 66 transcrystallinity. In addition, mechanical testing of the composites showed the longitudinal Young’s modulus of the treated fiber composites to be significantly higher than the control in a wide range of fiber volume fractions. Polarized light microscopy picture of double transcrystalli- nity in Br/NH 3 treated aramid fiber reinforced nylon 66. Macromol. Mater. Eng. 2003, 288, 861–866 861 Macromol. Mater. Eng. 2003, 288, No. 11 DOI: 10.1002/mame.200300151 ß 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim