refining the crystal structure of a semicrystalline polymer lies in the limited size of
single crystalline domains, which precludes the use of structural methods based on
single crystal X-ray diffraction. As demonstrated in the work by Lotz and
coworkers, electron diffraction in a TEM is a powerful alternative method for
unraveling the structure of polyolefins [62–64] because it can be applied to
micron-sized single crystalline domains. However, although numerous polyolefins
can be grown in the form of lamellar single crystals, this is more difficult for P3ATs
and has been reported only very recently for form II P3HT [61].
Accordingly, in the absence of single crystals of form I, the crystal structure of
this polymorph was investigated by electron diffraction (ED) on highly oriented
and crystalline films of P3HT (7.9 kDa) grown by slow-DEC [47]. A rotation-tilt
sample holder is used in a TEM to acquire representative projections in reciprocal
space, as shown in Fig. 7. The use of a rotation-tilt sample holder makes it possible
to tilt the sample around well-defined directions, e.g., around the in-plane direction
of the P3HT chains. The ED patterns were analyzed by a trial-and-error method to
determine a structural model. Figure 8 depicts the resulting structure of form I
P3HT. Of interest is the fact that this methodology makes it possible to determine
the reflection rules (the systematic extinctions of some reflections), which are
necessary to identify the space group of the crystal structure (P21/c in the present
case). It is worth noting that space group identification is usually very difficult when
only aligned fiber patterns are obtained and analyzed by X-ray diffraction.
The structural model of form I obtained by ED analysis is displayed in Fig. 8. It
shows the following characteristic features: (1) the polythiophene backbone adopts
a trans planar conformation, (2) n-hexyl side chains are not interdigitated, (3) the
polythiophene backbones are separated by 0.38 nm along the b-axis with relatively
short interatomic contacts of 0.34 nm between successive polythiophene backbones
due to a tilting of the backbone plane to the stacking direction, and (4) the n-hexyl
side chains are arranged in a rectangular subcell. Although this model accounts for a
large number of characteristic features in the ED patterns, it needs further improvements from a crystallographic point of view. In particular, the use of an all-trans
side chain conformation results in non-optimal torsional angles at the junction
between the thiophene ring and the n-hexyl side chain. Further work to improve
this model is currently in progress.
This structure has been recently challenged by a few alternative models. In
particular, Hansen and coworkers determined a model using a combination of
solid state NMR, X-ray diffraction and molecular modeling analysis on powders
[65]. This model of form I proposes that the planes of the polythiophene backbones
are perpendicular to the stacking direction (b-axis), resulting in the absence of short
interchain contacts as observed in the model of Kayunkid et al. [47]. To explain the
discrepancy between the two models, it was suggested that the structure in the DEC
films may correspond to a different polymorph, i.e., form I
0 . However, the fiber
pattern obtained for epitaxied P3HT films on TCB (M w ¼ 17 kDa) is identical to
that reported by Tashiro et al. on stretch-oriented films, demonstrating that the
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M. Brinkmann et al.
single crystalline domains, which precludes the use of structural methods based on
single crystal X-ray diffraction. As demonstrated in the work by Lotz and
coworkers, electron diffraction in a TEM is a powerful alternative method for
unraveling the structure of polyolefins [62–64] because it can be applied to
micron-sized single crystalline domains. However, although numerous polyolefins
can be grown in the form of lamellar single crystals, this is more difficult for P3ATs
and has been reported only very recently for form II P3HT [61].
Accordingly, in the absence of single crystals of form I, the crystal structure of
this polymorph was investigated by electron diffraction (ED) on highly oriented
and crystalline films of P3HT (7.9 kDa) grown by slow-DEC [47]. A rotation-tilt
sample holder is used in a TEM to acquire representative projections in reciprocal
space, as shown in Fig. 7. The use of a rotation-tilt sample holder makes it possible
to tilt the sample around well-defined directions, e.g., around the in-plane direction
of the P3HT chains. The ED patterns were analyzed by a trial-and-error method to
determine a structural model. Figure 8 depicts the resulting structure of form I
P3HT. Of interest is the fact that this methodology makes it possible to determine
the reflection rules (the systematic extinctions of some reflections), which are
necessary to identify the space group of the crystal structure (P21/c in the present
case). It is worth noting that space group identification is usually very difficult when
only aligned fiber patterns are obtained and analyzed by X-ray diffraction.
The structural model of form I obtained by ED analysis is displayed in Fig. 8. It
shows the following characteristic features: (1) the polythiophene backbone adopts
a trans planar conformation, (2) n-hexyl side chains are not interdigitated, (3) the
polythiophene backbones are separated by 0.38 nm along the b-axis with relatively
short interatomic contacts of 0.34 nm between successive polythiophene backbones
due to a tilting of the backbone plane to the stacking direction, and (4) the n-hexyl
side chains are arranged in a rectangular subcell. Although this model accounts for a
large number of characteristic features in the ED patterns, it needs further improvements from a crystallographic point of view. In particular, the use of an all-trans
side chain conformation results in non-optimal torsional angles at the junction
between the thiophene ring and the n-hexyl side chain. Further work to improve
this model is currently in progress.
This structure has been recently challenged by a few alternative models. In
particular, Hansen and coworkers determined a model using a combination of
solid state NMR, X-ray diffraction and molecular modeling analysis on powders
[65]. This model of form I proposes that the planes of the polythiophene backbones
are perpendicular to the stacking direction (b-axis), resulting in the absence of short
interchain contacts as observed in the model of Kayunkid et al. [47]. To explain the
discrepancy between the two models, it was suggested that the structure in the DEC
films may correspond to a different polymorph, i.e., form I
0 . However, the fiber
pattern obtained for epitaxied P3HT films on TCB (M w ¼ 17 kDa) is identical to
that reported by Tashiro et al. on stretch-oriented films, demonstrating that the
94
M. Brinkmann et al.