Organizing Semiconductor Block Copolymers
3. Crystalline‐crystalline donor acceptor block copolymers 1 Synthesis and characterization
3.3 X‐ray diffraction
X‐ray diffraction (XRD) measurements give additional insight into the crystallinity of the double‐crystalline donor acceptor block copolymers P3HT‐b‐PPerAcr. The powder X‐ray diffraction patterns of the homo‐ and block copolymers are shown in figure 13. The thermally annealed macroinitiators P3HT 9 and P3HT 17 both show the well‐known (100) reflection at 2θ= 5.36° and 5.25°, respectively, due to the lamellar packing of the P3HT chains, and the slight shift of the peak with increasing molecular weight is in accordance with the literature (figure 13a,b).74 Furthermore, a peak at 2θ= 23.4° is observed in both macroinitiator patterns. Both are broad and lie between the (020) and the (002) reflections of P3HT73, and therefore are assumed to be a superposition of the (020) and the (002) peak (figure 13c). Here, only the XRD pattern of P3HT 9 is shown for clarity.
PPerAcr, on the other hand, exhibits a different diffraction pattern. The observed reflections arise from stacks of perylene bisimide moieties which are separated by the alkyl side groups.
Within one stack, perylene bisimide units are stacked with a π‐π distance of 0.35 nm, as indicated by the reflection at 2θ= 25.6°.58 We assume the amophous halo at 2θ= 19.6° to arise from the disordered alkyl substituents of PBI. A graphic illustration of the proposed supramolecular structure of PPerAcr and the lamellar packing of P3HT is depicted in figure 13d and 13e, respectively. Similarly, low molecular weight PBIs with branched alkyl side groups were reported to form such stacks or columns.80 The XRD patterns of BC 16, BC 17 and BC 30 (the PPerAcr weight fraction is ~ 55‐60 %) are a superposition of the individual homopolymer reflections, and the contributions from the two homopolymers change with composition and molecular weight. Figure 13b shows the most intense reflection of PPerAcr and the 100 reflection of P3HT, normalized to the former. Obviously, the 100P3HT reflection becomes less intensive with increasing PPerAcr fraction in the series from BC 16 to BC 25.
The 100P3HT reflection cannot be seen anymore in BC 21 and BC 25, which resemble the PPerAcr homopolymer pattern to a large extent. This is in accordance with the results from DSC (figure 12a), where only one melting point is observed for BC 21 and BC 25, but two for BC 16, BC 17 and BC 30. Note that the broadening of the reflections around 2θ~ 5° of BC 16, BC 17, and BC 30 is due to underlaying peaks of PPerAcr. Interestingly, the comparison of the curves of BC 16 and BC 30 reveals that the 100P3HT reflection is more intensive in BC 30, indicating a higher crystallinity of P3HT. This result is in agreement with the results from
calorimetry, where the higher degree of P3HT crystallinity was already observed. Figure 13c shows the 2θ region between 15 and 30° of all samples. Here, the 002P3HT reflection and the π‐π stacking of PPerAcr are seen. Again, the 002P3HT peak becomes less intensive with increasing PPerAcr fraction, and no distinct 002P3HT peaks are observed for block copolymers BC 21 and BC 25 with the higher PPerAcr weight fractions. Obviously, these patterns of BC 21 and 25 are dominated by the reflections of the PPerAcr homopolymer in this region. BC 30 exhibits a slightly more intensive 002P3HT peak than BC 16, again indicating the higher P3HT crystallinity of BC 30.
S Figure 13. Supramolecular arrangement of double crystalline block copolymers P3HT‐b‐PPerAcr, as derived by powder X‐ray diffraction measurements of thermally annealed homo‐ and block copolymer samples. The color code in b) is equal in all plots. a) shows the entire range measured and b) shows the normalized intensity between 2 theta= 1‐7 °, and c) the region between 2 theta= 15 ‐30°. d) Schematic drawing of supramolecular arrangement of PPerAcr. Note that the unit cell is not determined and that the drawing might not reflect the exact geometry of the stacks. Rather, the separation of PBI units and alkyl groups is depicted. e) Lamellar packing of P3HT according to the literature.81
Furthermore, chloroform vapor annealed powder samples are measured. For this purpose, powders are exposed to chloroform vapor during several days, dried, milled, and measured.
The resulting XRD patterns are shown in figure 14.
15 20 25 30
020 002
in te n sit y ct s/ s
2 [°]
a) b)
5 10 15 20 25 30
PPerAcr BC 25 BC 21 BC 17 BC 16 BC 30 P3HT9
100 200 300 002020
in te n sit y ct s/ s
2 [°]
Figure 14. Powder X‐ray diffraction measurements of chloroform vapor annealed homo‐ and block copolymer samples. a) full range between 2 theta= 1 ‐30° is shown, b) enlarged region of 2 theta = 15‐ 30°. Legend is valid for both plots.
Chloroform vapor annealing influences the XRD patterns off all samples to a large extend, and the signal intensity of all patterns is lower compared to the thermally annealed samples.
Instead of the sharp reflections of PPerAcr in the thermally annealed sample, only one broad peak appears at 2θ= 3.87°, corresponding to a d spacing of 2.3 nm (figure 14a). The reflection arising from π‐π stacking is broader as well, but the peak position remains at 2θ=
25.6° (corresponding to 0.35 nm) compared to the thermally annealed sample. This suggests that the PPerAcr stacks are disordered after vapor annealing, with a mean distance of 2.3 nm. In contrast to this, the reflections of P3HT9 are resolved better compared to the thermally annealed sample, indicating a rearrangement and a higher order of the chains.
Especially the 002 and 020 reflections are now clearly resolved. From these observations we conclude that chloroform vapor annealing breaks up the π‐π interactions of P3HT as well as PPerAcr. However, after drying of the samples, P3HT9 exhibits a higher degree of order, whereas the PPerAcr sample is less ordered. These two findings have a large effect on the
XRD patterns of the block copolymers: Here, chloroform vapor leads to a high chain mobility in PPerAcr, enabling rearrangement of P3HT chains. Therefore, the 100P3HT is more intense in BC 30, 16, and 17, compared to the reflection of PPerAcr at 2θ= 3.87°, which was inverse in the thermally annealed samples. BC 21 and 25 now also show the 100P3HT reflection, whereas the corresponding thermally annealed samples almost revealed the PPerAcr homopolymer pattern. Qualitatively the same behaviour is observed in figure 14b. The
002P3HT reflection becomes visible in all block copolymer patterns, even in BC 21 and 25 with
the small P3HT fractions, where only PPerAcr features were observed in the thermally annealed samples (see also figure 13c). The π‐π stacking peak of PPerAcr is only weak and very broad in the block copolymers.
We conclude that thermal annealing of PPerAcr leads to sharp reflections suggesting defined and ordered stacks. The π‐π distance of two PBI units inside one stack is 0.35 nm. In the thermally annealed block copolymer samples, these features dominate over the P3HT signals, as a consequence of order of crystallization (PPerAcr crystallizes first). When chloroform vapor annealing is applied, the π‐π interactions in PPerAcr break up which gives rise to rearrangement of P3HT chains, and therefore, the P3HT reflections dominate the XRD patterns.