Influence of molecular weight on polymorphs and temperature-induced structure evolution of regioregular poly(3-dodecylthiophene)

Abstract

Formation of form I and form II polymorphs and their phase transitions were investigated for poly(3-dodecylthiophene) (P3DDT) films with molecular weights of 5300, 14 800, and 59 000 respectively, using temperature-dependent FTIR spectroscopy in conjugation with X-ray diffraction and differential scanning calorimetry. For the P3DDTs with the higher molecular weights (14 800 and 59 000), the form I polymorph was obtained by fast drying a solution at ambience or crystallization from the melt, whereas for the low molecular weight one (5300), the form II structure was produced, the amount of which can be increased by slowing down the process. Temperature-induced phase transitions of the three P3DDTs were explored. The results show that form II is thermodynamically more stable at room temperature, but its formation is kinetically hindered, and possible reasons are discussed.