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The elastic modulus ([E.sub.1]) of crystalline regions in the direction parallel to the chain axis in a new semiaromatic polyamide 9-T (PA9-T) containing a long aliphatic chain unit was measured by X-ray diffraction and the derived [E.sub.1], value was discussed in terms of its molecular conformation and mechanical properties. For low and high molecular weight PA9-Ts, the [E.sub.1] value of 40 GPa was obtained. The low [E.sub.1] value for PA9-T is due to the presence of aromatic groups with long crankshaft arms, which result in a greater moment of force during deformation compared with those polymers with planner zigzag molecular conformation like polyethylene and nylon 6 α-form. Another factor for the low [pounds sterling] is due to the contraction of its molecular chains that is likely originated from the existence of a torsional conformation. The presence of contraction causes some internal rotation in the polymer chain during extensional deformation, which results in low Eh POLYM. ENG. SCI., 52:331-337, 2012. © 2011 Society of Plastics Engineers, INTRODUCTION Polyamide 9-T (PA9-T) is a new semiaromatic polyamide recently industrialized as a high-performance engineering plastic in the name of Genestar (TM) by Kuraray Co. (1). In PA9-T, a long, [...]