  The dynamics of the conjugated polymers poly(9,9-dioctylfluorene) (PF8) and
poly(9,9-didodecylfluorene) (PF12), differing by the length of their side
chains, is investigated in the amorphous phase using the quasielastic neutron
scattering (QENS) technique. The measurements are synergistically underpinned
by molecular dynamics (MD) simulations. The probe is focused on the picosecond
time scale, where the structural dynamics of both PF8 and PF12 would mainly be
dominated by motions of their side chains. The measurements highlighted
temperature-induced dynamics, reflected in the broadening of the QENS spectra
upon heating. The MD simulations reproduced well the observations; hence
validating the MD force fields, the adopted amorphous model structures, and the
numerical procedure. As the QENS spectra are dominated by the signal from the
hydrogens on the backbones and side chains of PF8 and PF12, extensive analysis
of the MD simulations allowed: (i) tagging these hydrogens, (ii) estimating
their contributions to the self-part of the van Hove functions and hence to the
QENS spectra, and (iii) determining the activation energies of the different
motions involving the tagged hydrogens. PF12 is found to exhibit QENS spectra
broader than those of PF8, indicating a more pronounced motion of the didodecyl
chains of PF12 as compared to dioctyl chains of PF8. This is in agreement with
the outcome of our MD analysis: (i) confirming a lower glass transition
temperature of PF12 compared to PF8, (ii) showing PF12 having a lower density
than PF8, and (iii) highlighting lower activation energies of the motions of
PF12 in comparison with PF8. This study helped to gain insights into the
temperature-induced side-chain dynamics of the PF8 and PF12 conjugated
polymers, influencing their stability, which could potentially impact the
performance of the associated optoelectronic active layer.
