Semiconducting Liquid Crystalline Block Copolymers of Poly(3-Hexylthiophene)
Kekunawela Pathiranage, Taniya Madhushani Sashika
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Poly(3-hexylthiophene) (P3HT) is one of the most studied semiconducting polymers among polythiophenes. Grignard metathesis polymerization (GRIM) has enabled the synthesis of P3HT with well-defined molecular weights and high regioregularity. Furthermore, the synthesis of block copolymers of P3HT has allowed the generation of electronic materials in which polymer blocks with different properties were attached to the end functionalized P3HT. Improvement of alignment of P3HT by synthesizing both main chain and side chain liquid crystalline block copolymers has been used to enhance the charge transport within the active layers of photovoltaics and organic field effect transistors (OFETs). In chapter 1, an introduction to the field of polythiophenes is included and describes the synthesis, end group functionalization, and the synthesis of the block copolymer. Some recent studies of liquid crystalline block copolymers synthesized with P3HT are also included in this chapter. In chapter 2, the incorporation of an azobenzene nematic liquid crystalline mesogen, from 6-(4-((4-methoxyphenyl)diazenyl)phenoxy)hexyl methacrylate (MMAZO) into a series of block copolymers with regioregular poly(3-hexylthiophene) (P3HT) is discussed. The synthesis was performed by a combination of Grignard metathesis (GRIM) and atom transfer radical polymerization (ATRP). The composition of synthesized copolymers was determined from 1H-NMR analysis. The P3HT block/random copolymers containing thermotropic liquid crystalline segments have demonstrated both nanofibrillar and granular morphological features and good opto-electronic properties. The surface morphology of the P3HT copolymer films upon annealing was investigated by tapping mode atomic force microscopy (TMAFM). Field effect mobility in the order of 10-3 cm2/Vs was measured in OFETs with a bottom gate bottom contact configuration. In chapter 3, methoxybiphenyl, a liquid crystalline mesogen from 6-((4'-methoxy-[1,1'-biphenyl]-4-yl)oxy)hexyl methacrylate (MeOBP) was incorporated into diblock copolymers and triblock copolymers with P3HT. Switching the mesogen from azobenzene (nematic) to biphenyl (smectic) has produced improved morphology resulting in increased field-effect mobilities in the order of 10-2 cm2/Vs. The reduced π−π stacking distance between P3HT chains upon annealing resulted in enhanced packing of the smectic mesophase accomplished with π−π stacked biphenyl mesogens. In chapter 4, the synthesis and characterization of a block copolymer of 6-(pyren-1-yloxy)hexyl methacrylate (PyMA) with regioregular poly(3-hexylthiophene) (rr-P3HT) by a combination of Grignard metathesis polymerization and Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization (ICAR ATRP) is described. Introducing pyrene as discotic pendent groups enhanced the ordering of the block copolymer with microdomain formation after annealing above the mesophase transition. Enhancement of electronic properties due to the p-type semiconducting behavior of pyrene resulted in relatively high field-effect mobilities of ~10-2 cm2/Vs in bottom gate top contact organic thin film transistors (OTFTs) when annealed above the mesophase transition. Enhanced long range ordering with π−π stacking was observed in thin film X-ray diffraction studies (XRD). The thermotropic liquid crystalline behavior in discotic the phase of the block copolymer was confirmed with polarized optical microscopy (POM).