Poly(3-alkylthiophenes) homopolymers

Of the many example side-chain substituted hosts synthesized so far, one of the most heavily studied families of these compounds has been the poly(3-alkythiophenes) or P3AT's. This popularity is, in part, based on its versatility in terms of available synthesis procedures, variety of molecular designs, and the range of potential applications. Much of the structural behavior seen in the P3AT's extends to all other model systems. It is also worth noting that in most cases the molecular weights and molecular number are rather modest and so these results pertain to P3AT samples in which the polymer chain length is relatively limited.

Diffraction studies[,,,,,,,,,,] of the longer side chain containing P3AT's (butyl, pentyl, hexyl, etc.) almost universally find at least one well defined peak at smaller scattering angle or, equivalently, wave vector (according to the formula $k=\frac{4\pi}{\lambda}\sin \theta$ where the x-ray wavelength is fixed at $\lambda=1.542$Å). Scattering spectra from three representative P3AT homopolymers are displayed in Fig. 15. In many hosts, this low angle scattering feature has a relatively narrow $2\theta$ width implying scattering coherence over a significant distance (up to $\sim 300$Å using the Scherrer relationship, $L=\frac{0.9 \lambda}{\Delta 2 \theta \cos{\theta}}$ where L is the coherence length). This intense feature often appears in combination with two to four weaker intensity, higher order reflections located at integer multiples of the fundamental wavevector. These so called (h00) reflections are indicative of the lamellar structural phase alluded to previously. At somewhat larger scattering angles there is always a very broad, modest intensity peak centered near $2\theta$ values of 22$^\circ$(1.55 Å^-1) and, often, sharper features superimposed on this scattering at angles close to 24$^\circ$ (1.7 Å^-1). Scans out to much higher angles[110] are able to resolve additional weak, broad scattering features at $2\theta$ angles near 40$^\circ$ (2.8 Å^-1), 64$^\circ$(4.3Å^-1) and 77$^\circ$ (5.1Å^-1). The broad amorphous peak near 22$^\circ$ is primarily correlated with inter- and intra-chain molecular disorder of the side chains while the broad higher angle features are considered to to be representative of the short-range intrachain structure of the polymer main chains. The presence of a sharper scattering peak near 24$^\circ$ implies the possibility of additional long-range order both along the polymer main chains (at half the nominal 7.8Å chain axis repeat) and perpendicular to the main chains within individual lamellae. In some P3AT samples there are a number of well-defined steps on the high angle side of this 24$^\circ$ scattering feature which give evidence for even further structural ordering. Finally, in the best uniaxially oriented samples the non-equatorial scattering can be distinguished out to very high order[] (c^* Miller indices of $\ell=6$ and beyond).

 

Figure 15: Typical $\theta$-$2\theta$ P3AT x-ray diffraction spectra for unoriented (powder) samples of (a) poly(3-hexylthiophene), (b) poly(3-octylthiophene) and (d) poly(3-dodecylthiophene). The two additional scans are spectra from a stretch-oriented (6:1 draw ratio) P3DDT sample (c) in the (hk0) equatorial plane (perpendicular to the chain axis) and, (e) along the $(00\ell)$ meridional direction (parallel to the polymer chain axis).

The interlayer d-spacing is directly correlated with the length of the side chains in accordance to the models of Fig. 14. However, as shown in Fig. 16, this functionality is somewhat less (or more) than would be expected if the side chains were oriented normal to the main chain axis and to the layers themselves assuming no main chain tilt and no interdigitation or (full interdigitation). This indicates that these side chains are either conformationally disordered, strongly tilted away from the normal, or experience significant overlap with the side chains from the neighboring stack. In contrast, the $2\theta$ position of the sharper features located near $24^\circ$ are essentially independent of side chain length. Studies of uniaxially oriented samples, as seen if Fig. 15 curves c and e, find scattering in this vicinity both parallel and perpendicular to the draw (chain c-axis) direction. Hence this scattering is correlated with an extended main chain conformation along the c-axis and an intrastack chain-to-chain repeat of approximately 3.8Å.

 

Figure 16: The large interlayer d-spacing as a function of alkyl chain size, CH₃(CH₂)_n-1, using the data of Table II in combination with the estimated distances from two specific structural models as shown in Figs. 14(a) and 14(b).

These qualitative features can be complimented by more rigorous analysis of the experimental data. A variety of planar orthorhombic[,,] and monoclinic unit cells[,] have been proposed. Table II contains a partial list of reported room temperature lattice constants for some of the more common P3AT compounds. One current structural model[111], which is found to be consistent with the full range of experimental data (e.g., infrared and UV-vis absorption, x-ray and electron diffraction, etc.) and detailed modelling calculations[,,114] (crystal structure factor refinements, molecular simulations, etc.), requires an average tilting the side chain away from the layer normal with a minimum of side chain interdigitation between stacks as shown in Fig. 13(c). Other proposed P3AT models invoke variations in the chain structure including a periodic up-up-down-down doubling of the trans-planar repeat[], a non-planar conformation of the main chain[], and even a helical structure[](specifically for poly(3-methyl thiophene)).