525, 555, and 610 nm gradually emerge, which is indicative of the formation of
highly ordered structures. The appearance of an isosbestic point at 475 nm clearly
suggests the presence of two different states in the polymer solution. This is
explained by a transition of completely dissolved polymer chains into aggregated
stacks without intermediate states.
A combination of centrifugation and filtration was used to isolate the fibers in the
form of a crystalline powder that forms stable suspensions in p-xylene (1 wt%). The
filtrate mainly comprised well-dissolved polymer chains, as can be seen from
Fig. 5c. Size-exclusion chromatography further demonstrated that the nanofibrils
were mainly composed of high molecular weight chains (M w % 46 kg/mol,
PDI ¼ 2.3), whereas shorter chains remain dissolved in solution (M w % 38 kg/mol,
PDI ¼ 2.8).
The mixed-solvent method is based on use of a combination of good and poor
solvents [52, 54–56]. Thereby, unfavorable interactions between the polymer and
the poor solvent drive the aggregation of chains into nanostructures. Kiriy
et al. showed that highly aggregated 1D nanostructures of P3HT with a helical
conformation of the backbone can be obtained by adding hexane to a solution of
P3HT in CHCl 3 . Hexane acts as a selective solvent for the side chains, but is a rather
poor solvent for the conjugated backbone and, hence, favors π–π-interactions and
aggregation [54]. In contrast, Sun et al. started with a solution of P3HT in a
marginal solvent and demonstrated that, by adding small amounts of a good solvent,
the crystallinity of nanofibrils could be improved as a result of an increased
flexibility and organization of the chains within the aggregates [52]. Self-assembly
can further be triggered by ultrasonication of the solvent mixture, as reported by
Kim and coworkers [57].
Zhai and coworkers showed that P3HT nanofibrils can grow into highly ordered
2D nanoribbons, whose structure is shown in Fig. 6 [49]. However, this requires a
well-defined nanofibril surface, which is present only in P3HT of low molecular
weight for which chain folding is unlikely. Nanofibril growth was achieved by
cooling a solution of P3HT in a marginal solvent from 90
C to room temperature.
For M n > 10 kg/mol, anisole (0.05 mg/mL) was used as a marginal solvent, whereas
crystallization of low molecular weight P3HT (M n < 10 kg/mol) required the use of
a co-solvent of poor dissolving power (anisole/dimethylformamide (DMF),
1:2 vol%, 0.2 mg/mL) to reduce the solubility of the short chains. At first,
crystallization from solution results in the formation of 1D nanofibrils, which
then act as nucleating points for further growth along the [100] direction via alkyl
chain interactions. For high molecular weight P3HT with M n above a critical value
of 10 kg/mol, chain folding results in nanofibrils with more defects, which prevents
further growth of the nanofibrils into 2D nanoribbons.
A serious consequence of polymer crystallization in solution is gel formation,
which is a severe issue regarding, for example, inkjet printing of polymeric semiconductors because gelation dramatically shortens the lifetime of electronic inks.
The gelation mechanism has been intensively studied by Nandi and coworkers, who
describe the gel formation of P3HT as a two-step process [58, 59]. In phase I, single
isolated chains undergo a coil-to-rod transformation, and phase II involves the
50
K. Tremel and S. Ludwigs
highly ordered structures. The appearance of an isosbestic point at 475 nm clearly
suggests the presence of two different states in the polymer solution. This is
explained by a transition of completely dissolved polymer chains into aggregated
stacks without intermediate states.
A combination of centrifugation and filtration was used to isolate the fibers in the
form of a crystalline powder that forms stable suspensions in p-xylene (1 wt%). The
filtrate mainly comprised well-dissolved polymer chains, as can be seen from
Fig. 5c. Size-exclusion chromatography further demonstrated that the nanofibrils
were mainly composed of high molecular weight chains (M w % 46 kg/mol,
PDI ¼ 2.3), whereas shorter chains remain dissolved in solution (M w % 38 kg/mol,
PDI ¼ 2.8).
The mixed-solvent method is based on use of a combination of good and poor
solvents [52, 54–56]. Thereby, unfavorable interactions between the polymer and
the poor solvent drive the aggregation of chains into nanostructures. Kiriy
et al. showed that highly aggregated 1D nanostructures of P3HT with a helical
conformation of the backbone can be obtained by adding hexane to a solution of
P3HT in CHCl 3 . Hexane acts as a selective solvent for the side chains, but is a rather
poor solvent for the conjugated backbone and, hence, favors π–π-interactions and
aggregation [54]. In contrast, Sun et al. started with a solution of P3HT in a
marginal solvent and demonstrated that, by adding small amounts of a good solvent,
the crystallinity of nanofibrils could be improved as a result of an increased
flexibility and organization of the chains within the aggregates [52]. Self-assembly
can further be triggered by ultrasonication of the solvent mixture, as reported by
Kim and coworkers [57].
Zhai and coworkers showed that P3HT nanofibrils can grow into highly ordered
2D nanoribbons, whose structure is shown in Fig. 6 [49]. However, this requires a
well-defined nanofibril surface, which is present only in P3HT of low molecular
weight for which chain folding is unlikely. Nanofibril growth was achieved by
cooling a solution of P3HT in a marginal solvent from 90
C to room temperature.
For M n > 10 kg/mol, anisole (0.05 mg/mL) was used as a marginal solvent, whereas
crystallization of low molecular weight P3HT (M n < 10 kg/mol) required the use of
a co-solvent of poor dissolving power (anisole/dimethylformamide (DMF),
1:2 vol%, 0.2 mg/mL) to reduce the solubility of the short chains. At first,
crystallization from solution results in the formation of 1D nanofibrils, which
then act as nucleating points for further growth along the [100] direction via alkyl
chain interactions. For high molecular weight P3HT with M n above a critical value
of 10 kg/mol, chain folding results in nanofibrils with more defects, which prevents
further growth of the nanofibrils into 2D nanoribbons.
A serious consequence of polymer crystallization in solution is gel formation,
which is a severe issue regarding, for example, inkjet printing of polymeric semiconductors because gelation dramatically shortens the lifetime of electronic inks.
The gelation mechanism has been intensively studied by Nandi and coworkers, who
describe the gel formation of P3HT as a two-step process [58, 59]. In phase I, single
isolated chains undergo a coil-to-rod transformation, and phase II involves the
50
K. Tremel and S. Ludwigs