Capturing the Phase Transformation and Thermal Behavior of P(NDI2OD-T2) with In Situ Grazing Incidence WAXS

Abstract

The thermal behavior of the well-known electron transporting polymer P(NDI2OD-T2) is investigated using in situ temperature-dependent grazing incidence wide-angle X-ray scattering (GIWAXS). Careful study of the evolution of lattice parameters, peak widths, and scattering patterns enables us to observe the phase transition from form I to form II, with these two polymorphs exhibiting distinct thermal expansion and crystal growth behavior. Over the range of 30–230 °C, form I is observed to be dominant, characterized by thermal expansion in both lamellar and π–π stacking directions with comparable thermal expansion coefficients of 2 × 10–4 K–1. Form II is observed with heating from 230 °C to 320 °C (and persists with subsequent cooling) and is characterized by negative thermal expansion in the lamellar stacking direction and accelerated thermal expansion (5-fold increase in thermal expansion coefficient) in the π–π stacking direction. For both forms, no noticeable lattice expansion or contraction is found in the backbone direction. The growth of P(NDI2OD-T2) crystals with annealing is also found to be anisotropic. In particular, the size of crystallites in the lamellar stacking direction increases throughout the heating process from room temperature to 320 °C with a faster rate of growth in form II than in form I. In the backbone direction, crystallite size increases in form I but decreases in form II; in the π–π direction no systematic change in coherence length is observed. With regard to texture, the face-on orientation is found to remain dominant throughout the heating and cooling cycle, with a minority edge-on population found to slightly increase with heating to 320 °C and subsequent cooling. This work provides new insights into the thermal behavior and polymorphism of P(NDI2OD-T2) and the understanding of the structure–property relationship.

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