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The chain shape of polymers affects many aspects of theirbehavior and is governed by their intramolecular interactions. Delocalizationof electrons along the backbone of conjugated polymers has been shownto lead to increased chain rigidity by encouraging a planar conformation.Poly(3-hexylthiophene) and other poly(3-alkylthiophenes) (P3ATs) areinteresting for organic electronics applications, and it is clearthat a hierarchy of structural features in these polymers controlscharge transport. While other conjugated polymers are very rigid,the molecular structure of P3AT allows for two different planar conformationsand a significant degree of torsion at room temperature. It is unclear,however, how their chain shape depends on variables such as side chainchemistry or regioregularity, both of which are key aspects in themolecular design of organic electronics. Small-angle neutron scatteringfrom dilute polymer solutions indicates that the chains adopt a randomcoil geometry with a semiflexible backbone. The measured persistencelength is shorter than the estimated conjugation length due to thetwo planar conformations that preserve conjugation but not backbonecorrelations. The persistence length of regioregular P3HT has beenmeasured to be 3 nm at room temperature and decreases at higher temperatures.Changes in the regioregularity, side chain chemistry, or syntheticdefects decrease the persistence length by 60–70%. [ABSTRACT FROM AUTHOR]