# Mapping Microstructural Dynamics up to the Nanosecond of the Conjugated Polymer P3HT in the Solid State

## TL;DR

This study combines neutron scattering, molecular dynamics, and quantum chemistry to elucidate the nanosecond-scale microstructural dynamics of P3HT, revealing insights into its vibrational modes and phase behavior relevant for optoelectronic applications.

## Contribution

It provides a comprehensive multi-technique analysis of P3HT's structure and dynamics, validating assumptions about amorphous phases and emphasizing the role of neutron scattering in conjugated polymer research.

## Key findings

- MD accurately models structural features and slow motions.
- Neutron scattering reveals differences in collective dynamics between stacking types.
- Vibrational analysis distinguishes intra- and inter-molecular modes.

## Abstract

We present a detailed study of the structure-dynamics relationship of regio-regular and regio-random PEHT using different neutron scattering techniques. Deuteration is employed to modulate the coherent and incoherent cross-sections, allowing particularly to access both self-motions and collective dynamics of the materials. The measurements are underpinned by extensive quantitative calculations using classical MD, as well as first principles quantum chemistry. MD reproduced well the main structural features and slow motions, and shed light on differences in collective dynamics between Q-values linked with the $\pi-\pi$ stacking and the lamellar stacking, with the crystalline phase being the most impacted. On the other hand MD led to a limited description of molecular vibrations. In this context, first principles molecular calculations described well the high-energy vibrational features ( $>$ 900 cm$^{-1}$ ), while periodic calculations allowed to better describe the low- and mid-energy vibrational ranges ( 200-900 cm$^{-1}$ ). The mid-energy range is predominantly associated with both intra-molecular and inter-molecular mode coupling, which encloses information about both the polymer conformation and the polymer packing at short range. One of the outcomes of this study is the validation of the common assumption made that RRa-P3HT is a good approximation for the amorphous phase of RR-P3HT at the macroscopic level. The present work helps to clarify unambiguously the latter point which has been largely overlooked in the literature. We highlight the importance to complement optical spectroscopy techniques with inelastic neutron scattering. The latter offering the advantage of being insensitive to the delocalized $\pi$-electron system, and thus enabling to infer relevant quantities like conjugation lengths, for instance, impacting properties of conjugated polymer.