The effect of alkyl side chain length on the formation of two semi-crystalline phases in low band gap conjugated polymers

Abstract

The length of solubilizing alkyl chains affects the formation, structure, and properties of two semi-crystalline phases in conjugated polymers.

Polymorphism of a semi‐crystalline diketopyrrolopyrrole‐terthiophene polymer

“…This originates from the large differences in boiling point (214 vs. 61°C) and vapor pressure (0.46 vs. 197 mm Hg at 25°C) between TCB and CF, which causes that in the final stages of film formation only TCB remains, irrespective of the initial CF:TCB ratio. [ 14,15 ]…”

“…The difference in lattice parameters observed for the β 1 and β 2 polymorphs of D‐PDPP3T‐EH are consistent with our previous study on PDPP4T polymers. [ 14,15 ]…”

“…It is further intriguing that d lam = 2.1–2.2 nm for D‐PDPP3T‐EH is close to the values of d lam = 2.1 and 2.3 nm found for N‐PDPP4T‐HD and D‐PDPP4T‐HD, while those of H‐PDPP4T‐HD (1.9 nm) and P‐PDPP4T‐HD (2.5 nm) are much shorter or much larger, respectively. [ 15 ] It thus appears that the PDPP n T polymers that have d lam close to 2.2 nm readily provide the β 1 → α → β 2 polymorphic transition in solution, while those with a shorter or longer d lam have too low or too high solubility. [ 15 ] For constant d π , d lam gauges the amount of side chain per unit length of main chain and hence for semi‐crystalline PDPP n T polymers d lam is related to solubility.…”

“…[ 15 ] It thus appears that the PDPP n T polymers that have d lam close to 2.2 nm readily provide the β 1 → α → β 2 polymorphic transition in solution, while those with a shorter or longer d lam have too low or too high solubility. [ 15 ] For constant d π , d lam gauges the amount of side chain per unit length of main chain and hence for semi‐crystalline PDPP n T polymers d lam is related to solubility.…”

“…By varying the length of linear alkyl chains on the DPP units, we found that a β 1 → α → β 2 polymorphic transition proceeds readily for alkyl side chains of intermediate length (i.e., nonyl and dodecyl), but that for either short or long (i.e., hexyl or pentadecyl) side chains it is more difficult to generate the β 1 or β 2 polymorph, respectively, due to too low or too high solubility. [ 15 ]…”