(381b) Manipulation of P3AT Crystallization Behavior

In order to improve the performance of organic optoelectronic devices both the nanometer scale morphology and the crystalline structure must be optimized.  Simultaneous control of these two length scales may be obtained with a block copolymer in which one component is a crystalline conjugated polymer such as poly(3-alkylthiophene) (P3AT). While it is true that the self-assembly of these systems requires balancing the driving forces of crystallization and microphase separation, in many systems, crystallinity dominates resulting in significant distortion or destruction of the melt phase structure. Indeed, the most popular polythiophene derivative, P3HT, has a high melting temperature, 220 °C, such that crystallization occurs quickly as the material is cast into a thin film from solvent. Furthermore, when P3HT is incorporated into block copolymers, thermal annealing above the P3HT Tm cannot be utilized due to degradation that occurs at such high temperatures. In contrast to flexible polymer chains (such as the archetypical polyethylene) which upon crystallization form layer-like crystallites separated by disordered regions, poly(3-alkylthiophene) (P3AT) polymers such as poly(3-ethylhexy thiophene) (P3EHT) and poly(3-hexyl thiophene) form fibrils with relatively monodisperse diameters.  Consequently, fibrillar structures identical to those of the homopolymer are observed as the self-assembly process of the block copolymer is precluded by P3HT crystallization.

        However, it has been shown that judicious selection of the alkyl side chain in P3ATs results in melting transitions which can be controlled over a range of 150 °C.  Specifically, P3EHT has a multimodal melting transition that occurs between 35 and 85 °C, below potential degradation temperatures. This depression of the melting transition leads to regions of phase space for which the block copolymer self-assembly is unhindered by the P3EHT crystallization. Thus, by lowering of the Tm in P3EHT, block copolymer assembly occurs, creating a wide variety of morphologies with tunable domain sizes when samples are annealed between Tm and ODT. Upon cooling from the melt to room temperature, the P3EHT crystallization may be confined within the preexisting microdomains.

        We have recently shown that the fractional crystallinity is intimately tied to charge mobility with a critical point that result in a change in mobility by two orders of magnitude.  Here, the effects of thermal history on the crystallization of P3EHT of varying molecular weights and the resulting transient absorption, photoluminescence, and charge mobility is explored further.  As mentioned above, the melting endotherms of P3ATs are multimodal in nature such that upon the quenched annealing of P3EHT at room temperature—as was done to demonstrate the relationship between fractional crystallinity and charge mobility—three distinct melting peaks are observed upon heating. Interestingly, the relative peak sizes of these peaks may be manipulated as a function of undercooling (annealing temperature) allowing for suppression of the higher temperature melting peak, suggesting a change in crystalline texture and potentially material properties; an idea we are currently testing.

This work was generously supported by National Science Foundation (DMR-1206296).