  We explore the shape and internal structure of diblock copolymer (di-BCP)
nanoparticles (NPs) by using the Ginzburg-Landau free-energy expansion. The
self-assembly of di-BCP lamellae confined in emulsion droplets can form either
ellipsoidal or onion-like NPs. The corresponding inner structure is a lamellar
phase that is either perpendicular to the long axis of the ellipsoids
(L$_\perp$) or forms a multi-layer concentric shell (C$_\parallel$),
respectively. We focus on the effects of the interaction parameter between the
A/B monomers $\tau$, and the polymer/solvent $\chi$, as well as the NP size on
the nanoparticle shape and internal morphology. The aspect ratio ($l_{\rm AR}$)
defined as the length ratio between the long and short axes is used to
characterize the overall NP shape. Our results show that for the solvent that
is neutral towards the two blocks, as $\tau$ increases, the $l_{\rm AR}$ of the
NP first increases and then decreases, indicating that the NP becomes more
elongated and then changes to a spherical NP. Likewise, decreasing $\chi$ or
increasing the NP size can result in a more elongated NP. However, when the
solvent has a preference towards the A or B blocks, the NP shape changes from
striped ellipsoid to onion-like sphere by increasing the A/B preference
parameter strength. The critical condition of the transition from an L$_\perp$
to C$_\parallel$ phase has been identified. Our results are in good agreement
with previous experiments, and some of our predictions could be tested in
future experiments.
