Structure of Poly-3-butylthiophene in

Since the discovery that Poly-n-alkylthiophenes can be solubilized (15), they have been the subject Of intensive studies. The stability of the polymer together with the possibility to study the doped polymer in solution are die origins of the interest Besides, poly-n alkylthiophene films exhibit drastic change as shown with the thermochromism which varies widi the size of the substituent (Salaneck et al, this book). Moreover poly-n alkylthiophene provides an unique oppcntunity to investigate the chain conformation of a conjugal polymer with a conjugated backbone maiicedly different from the substituted polydiacetylene or polyacetylene. 

Before investigating the properties of the doped polymer solutions, a good knowledge of the chain statistics in the neutral state in solution is needed. SANS experiments were first performed on poly-butylthiophene in deuterated nitrobenzene (16), then in deuterated THF(17). Two types of samples were used, one with Mw = 49000 and a weight distribution = 1-5 used for die neutral solutions in nitrobenzene, and 

Inset in figure 2 illustrates the difference between the scattering curve measured at 6S°C and the calculated structure factor. A residual interchain interaction is still observed in a range of scattering vector q lower than 0.035 A l. Nevertheless, the single chain conformation can be obtained with the help of the constraints given by the contour length 

L = 1400 A (Mw = 49000) and the mass per unit length Mi = 35.4 g mole l A. Incidentally the absolute measurements made with SANS do not allow to discriminate between die aromatic and quinold structure. A M.N.D.O calculation gives a monomer unit length a = 3.91 A for the aromatic stracture and a = 3.89 A for the quinold structure. This difference is too small for the accuracy of SANS (typically 5 to 10%). The best fit obtained with the wormlike chain model (see chapter 3) gives Mi = 35 g mole i A l, a statistical length b = 55 A, and a mean square radial radius of gyration = 3.5 A. The 

In this part, we are dealing with Ae properties of doped polymers in solution. Here we report experiments measuring the screening effect on Ae electrostatic repulsion between polarons, then SANS experiments giving the chain conformation in the doped state. 

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