  Exciton-exciton annihilation is a ubiquitous nonlinear dynamical phenomenon
in materials hosting Frenkel excitons, that has been employed to probe exciton
diffusion processes in conjugated polymeric materials. In this work, we
investigate the nonlinear exciton dynamics of an electron push-pull conjugated
polymer by fluence-dependent transient absorption and excitation-correlation
photoluminescence spectroscopy, where we can quantitatively show the latter
technique to be a more selective probe of the nonlinear dynamics. Simulations
based on an exciton annihilation model that implements a simple (\textit{i.e.}\
time-independent) bimolecular rate constant decreasing trend for the extracted
annihilation rates with excitation fluence. However, further investigation of
the fluence-dependent transients suggests that the exciton-exciton annihilation
bimolecular rate parameters are not constant in time, displaying a $t^{-1/2}$
time dependence, which we rationalize as reflective of anisotropic exciton
diffusion. At ambient temperature, we estimate the exciton diffusion length to
be $9 \pm 2$\,nm. In addition, exciton annihilation gives rise to a long-lived
species that recombines on a nanosecond timescale. Our conclusions shed broad
light onto nonlinear exciton dynamics in push-pull conjugated polymers.
