  Thermoplastic starch (TPS) is an excellent film-forming material, and adding
fillers such as tetramethylammonium-montmorillonite (TMA-MMT) clay has
significantly expanded its use in packaging applications. We first used
all-atom (AA) simulation to predict several macroscopic (Young modulus, glass
transition temperature, density) and microscopic (conformation along 1-4 and
1-6 glycosidic linkages, composite morphology) properties of TPS melt and
TPS-TMA-MMT composite. The interplay of polymer-surface, plasticizer-surface,
and polymer-plasticizer interactions leads to conformational and dynamic
properties distinct from those in systems with either attractive or repulsive
polymer-surface interactions. A subset of AA properties was used to
parameterize the MARTINI coarse-grained (CG) force field (FF) for the melt and
composite systems. Specifically, we determined the missing bonded parameters of
amylose and amylopectin and rationalized the bead types for 1-4 and 1-6 linked
alpha-D glucose using two-body excess entropy, density, and bond and angle
distributions in AA TPS melt. The MARTINI CG model for TPS was combined with an
existing parameter set for TMA-MMT. The liquid-liquid partitioning-based
MARTINI-2 FF shows freezing and compaction of polymer chains near the sheet
surface, further accentuated by lowering of dispersive interactions between
pairs of high covalent coordination ring units of TPS polymers and MMT sheet. A
rescaling of the dispersive component of TPS MMT cross-interactions was used to
optimize the FF for the composite system, with structural, thermodynamic, and
dynamic properties obtained from long AA simulations forming the constraints
for optimization. The obtained CG FF parameters provided excellent estimates
for several other properties of the melt and composite systems not used in
parameter estimation, thus establishing the robustness of the developed model.
