Composition-Temperature Phase Diagrams and Crystal Growt in Solution-Crystallized Poly(3-hexylthiophene):N,N'-Alkylated Naphthtalene Diimide Blends

Abstract

Architecture of alkyl substituents and π-π interactions between aromatic parts are the key factors governing self-assembly and crystallization of alkyl-bearing aromatic molecules or macromolecules [1,2]. It was demonstrated that interactions between alkyl chains of N-substituted aromatic diimide derivatives (ADI) and poly(3-hexylthiophene) (P3HT) may drive formation of different crystal structures in P3HT:ADI blends [3,4]. In this work we discuss in detail the influence of composition of P3HT:ADI blends and temperature on phase behavior and crystal growth in the blends.We focused here on the blends of P3HT with N,N’-alkylated naphthalene diimides (NDI). Crystal structure and morphology of the blends were studied by X-ray diffraction (XRD). In order to gain insight into phase transitions we used differential scanning calorimetry (DSC). These studies were supplemented with scanning electron microscopic (SEM) imaging to provide information about morphology of the blends. Results of our experiments indicated that packing of molecules in crystalline phases as well as phase transition temperatures were related to molecular architecture of NDI and compositions of the blends. Based on DSC and XRD data we have plotted the phase diagrams for the blends of P3HT with NDI. In all the P3HT:NDI blends melting (isotropisation) points forming the liquidus line depend on their composition. The liquidus lines in different blends reveal characteristic minima located at different P3HT-to-NDI ratios. These characteristic minima are referred to as pseudoeutectic points. Our study revealed that extending the alkyl chains of the NDI molecules caused a shift of the pseudoeutectic points towards the NDI component in the phase diagram (i.e. towards the NDI weght fraction = 1). At the pseudoeutectic composition of the P3HT:NDInC8 blend, we have observed additional transition occurring neither in pure P3HT nor pure NDI. Based on our diffraction data we attributed this point to the formation of new crystalline phase resulting probably from co-crystallization of P3HT and NDI molecules. References[1] J. P. Sun et al., Org. Electron., 2016, 35, 151.[2] S. Y. Son et al., J. Am. Chem. Soc., 2016, 138, 8096.[3] D. Chlebosz et al., Dyes and Pigments, 2017, 140, 491.[4] L. Bu et al., ACS Nano, 2015, 9, 1878.AcknowledgementThe work was supported by National Science Centre, Poland through the grant DEC-2016/22/E/ST5/00472. Variable-temperature and grazing-incidence XRD measurements were partly performed using the 10 keV beam at the BL9 beamline at DELTA synchrotron facility in Dortmund (Germany). The Authors thank Dr. Christian Sternemann for support during the experiments