’ Solid State Communications, Vol. 80, NO. 9, pp. 687-689, 1991. Printed in Great Britain. 0038-1098/91$3.00+.00 Pergamon Press plc MOLECULAR STRUCTURE OF STRETCH ORIENTED POLY(3-HEXYLTHIOPHENE) STUDIED BY AN EXTENDED X-RAY DIFFRACTION MAPPING Jostein M%rdalen and Emil J.Samuelsen Institutt for Fysikk, Universitetet i Trondheim NTH, N-7034 Trondheim, Norway and Odd R.Gautun and Per H.Carlsen Organisk Kjemi, Universitetet i Trondheim NTH, N-7034 Trondheim, Norway (Received 19 august 1991 by P.Burlet) Detailed x-ray diffraction intensity mapping of the reciprocal space of stretched poly(3- hexylthiophene) films enabled us for the first time to reveal reflections associated with the stretch direction (1=1-7). The reflections are located at finite angles to the stretch axis and can therefore only be indexed on a monoclinic (or triclinic) unit cell. The monoclinic cell parameters are a=16.90& b=4.85& c=7.84A, cc=50.6”, (a=4.85A, b=16.90& c=7.84A and 8=129.4” by crystallographic conventions) and with two monomer units per cell. The values of a and c are directly measured whereas b and CY are refined from structure factor calculations. Existence of hOO but absence of WtO, h01 and hkl reflections indicate layer disorder between oblique bc planes, whereas structure factor calculations indicate side chain disorder. Introduction Alkyl substituted polythiophenes (P3AT’s) are conductive polymers fusible and soluble in several organic solvents, and therefore of potential interest for industrial applications. Solution cast P3AT films with alkyl side chains longer than n=4 (butyl-) can to some extent be oriented by uniaxial stretching. The polymer backbone is assumed to align along the stretch direction with rotational averaged isotropy around this direction. This gives a cylindrical symmeuy where the structural information is accessible in a reciprocal plane containing the stretch axis, i.e. a plane intersecting the cylinder through its symmetry axis. Previous structural studies of P3AT’s were. based on x-ray diffraction profiles mainly taken perpendicular to the stretch direction’.’ (equatorial scans with I=@, or along it4 (assumed to be 001). In this communication we report on further detailed x-ray diffraction study of poly(3-hexylthiophene) (P3HT). For the first time accurate positions of reflections with f=l- _?and 1=5-7 are determined, leading to a new model for the crystalline phase of undoped P3HT. As the present work will clarify, the previously observed periodicity zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB cv=7.4A4 does not represent the chain direction repetition unit. This periodicity was deduced from axial scans along the stretch direction (c-axis) which intersects the mosaic spread “reflection tails” of non-axial reflections. The new periodicity c=7.84A deduced from the projection of the diffraction peaks onto the c-axis makes the cell doubling model’ superfluous. Experimental Polymerization, casting, stretching and density measurements were carried out as previously reported4. One or two layers of solution cast 1OOpm thick P3HT films stretched up to 4 times initial length were used for this study. Diffraction profiles were measured with automatic diffractometer LOFlE using graphite monochromatized and slightly focused CuK, radiation; x=1.5418& a 15OOW tube and a point detector. A plane intersecting the cylinder of symmetry along the rotational axis (c-axis) was carefully mapped on a fine masked grid by measuring several scans parallel to c. The intensity profiles were transformed to an x-ray intensity contour map giving the position and cross sectional shape of the diffraction “doughnuts” (Fig. 1). Intensity profiles were calculated for a new structure model for planes perpendicular to the crystallographic c-axis with integer values of I. Non-standard intensity correction factors, corresponding to the polarization and Lorenz’ factors for powder diffraction, were calculated for this geometry. Results Diffraction peak positions and cross sectional shape for I-values 23 and 5-7 were recorded by the mapping technique as shown in Fig. 1. The peak for I=4 was not observed. Most peaks are rather flat, with increased position uncertainty especially in the R-direction. Experimentally determined positions of the diffraction peaks are listed in Table 1. Only the 1=6 peak is found on or near the symmetry axis. This implies a reciprocal c--axis non-parallel with the c-axis and thus a monoclinic (or triclinic) unit cell. (Fig. 2) The shape of the reflection halos deviates considerably from being arcs of circles, unlike the equatorial /ZOO and Ok0 reflections shown in previously reported wide angle x-ray exposure?. This is clearest for the 1=2 peak. 687