The physical properties of conjugated semiconducting polymers are mainly influenced by their complex structural dynamics and solid-state organization. Detailed characterization of the morphology and structure of these materials is extremely challenging. This can be attributed to the variation in their chemical structure and increased complexity due to intensive synthetic efforts to optimize the device performance. To this end, various material modeling approaches like molecular dynamics (MD) are widely used for in-depth characterization of these materials.

Among the available p-type semiconducting polymers, poly(3-n-hexylthiophene) (P3HT) is one of the most studied and used as the reference for studying similar materials. P3HT consists of crystalline and amorphous domains and exhibits good flexibility, high carrier mobility, low-temperature processability and cost-effective fabrication. Recent findings have grouped crystal structures of various poly(3-alkylthiophenes), including P3HT, into two families: the often disordered form I, packed with noninterdigitated side chains, and the more ordered form II with interdigitated side chains. The relevance of P3HT and its structural dynamics, organization and thermal behavior is well highlighted in the literature. While the two polymorphs may coexist at room temperature depending on the processing conditions, form II can experimentally be converted readily into form I whereas the reverse transformation hardly occurs.

Unbiased MD has proved useful in studying the form II-form I phase transition for P3AT polymers to elucidate the resulting structural reorganization. Unsurprisingly, temperatures at which this transition as well as melting of the two forms is observed in simulations are substantially higher than experimental values. The deviations are attributed to many reasons, including extended-chain morphology, infinite molecular weight and infinite crystal size in models used in the simulations, which also lack the molecular and packing defects, ubiquitous in experimental polymer crystals.

Herein, Mosè Casalegno, Antonino Famulari and Stefano Valdo Meille from Polytechnic University of Milan (Politecnico di Milano) explored the polymorphism of P3HT via molecular dynamics modeling of selected 3-hexylthiophene (3HT) oligomers. For better evaluation of the reliability and insights emanating from simulation studies, the authors expanded their work in two directions. First, they assessed the applicability of three force fields, all specifically developed for P3ATs, in modeling form II-form I transitions for P3HT. Second, the behavior of 3HT oligomers was investigated using MD simulations to compare the ability of the three force fields to reproduce experimental trends. Their work is currently published in the journal Macromolecules.

The authors demonstrated that the P3HT polymer form II crystal structure was a potential energy minimum with clearly defined features identifiable with all the investigated force fields as well as with solid-state DFT calculations. Similar observations were made for form II (3HT)_n oligomer crystal structures. The three force fields predicted solid-solid form II-form I transition for P3HT polymers. The transition in all the cases involved a substantial disordering, high-energy stage followed by rapid re-establishment of the main chain stacking and improved packing energy, although extensive disorder persisted. Such disordering transitions occur in atomistic modelling at rates comparable to melting, at variance with crystallization which requires times beyond present possibilities. The resulting form I may be described as mesomorphic instead of 3D crystalline, with features which differ significantly for the three investigated force fields.

More reliable results, showing that relatively short oligomers are likely to undergo melting-recrystallization processes in the form II-form I transformation, were obtained with two force fields, while the third suggests for the oligomers the same solid state transition, at the same temperature, observed for the polymer. The perceived identical behavior of the studied oligomers and polymers was attributed to the high main-chain rigidity characterizing the third force field.

In summary, the study by Mosè Casalegno and colleagues utilized different force fields to model the structures and thermal behaviors of P3HT and its oligomers. Despite the high transition temperatures compared to those reported experimentally, the results indicated the ability of the force fields to capture the transition between the two forms. Overall, MD is generally an effective tool for adequately describing the relative stability and key features of various crystal phases and to provide plausible interconversion mechanisms for both melting and solid-solid transitions. Differences among the force fields become however important when highly disordered structures are investigated, which require extreme care. In a joint statement to Advances in Engineering, the authors said their study will advance our understanding of the features and behaviors of a wide range of conjugated polymers.