X-RAY STRUCTURAL STUDIES OF VARIOUS OCTYL-SUBSTITUTED POLYTHIOPHENES

The structure of poly(octylthiophene)s and alternating octylthiophene- thiophene-copolymers with a varying degree of intrachain regularity wa s studied by means of X-ray diffraction and optical absorption measure ments. Although these polymers show a varying degree of crystallinity, evidence is found that they all tend to pack as relatively straight p arallel chains. The lateral packing distance depends strongly on the r egularity and the relative amount of alkyl side chains. Generally poly (thiophene)s with a larger relative amount and higher stereoregularity of the octyl side chains tend to pack more ordered. On the other hand , a large conjugation length (high degree of chain planarity), as reve aled by optical absorption spectroscopy, and a crystalline packing of the polymer chains are not found to be interdependent. For the polymer with highest degree of crystallinity two surprisingly different cryst alline structures were found for the two versions with different regul arity, viz. poly(3-octylthiophene) and poly(3,3'-dioctyl-2,2'-bithioph ene). For the latter we find evidence of a polymer chain twist between adjacent head-to-head coupled thiophene rings, and we propose a struc tural model for its crystalline packing. Our new model can explain pre viously measured differences in spectroscopic and transport-related pr operties for these two stoichiometrically identical polymers.