It
is now well-accepted that hydrophilic nanoparticles (NPs) lightly
grafted with polymer chains self-assemble into a variety of superstructures
when placed in a hydrophobic homopolymer matrix or in a small molecule
solvent. Currently, it is thought that a given NP sample should only
assemble into one kind of superstructure depending on the relative
balance between favorable NP core–core attractions and steric
repulsion between grafted polymer chains. Surprisingly, we find that
each sample shows the simultaneous formation of a variety of NP-assemblies,
e.g., well-dispersed particles, strings, and aggregates. We show through
the generalization of a simple geometric model that accounting for
the distributions of the NP core size and the number of grafted chains
on each NP (which is especially important at low coverages) allows
us to quantitatively model the aggregate shape distribution. We conclude
that, in contrast to molecular surfactants with well-defined chemistries,
the self-assembly of these NP analogues is dominated by such fluctuation
effects.
Fused
filament fabrication
(FFF) three-dimensional (3D) printing of semicrystalline polymers
such as high-density polyethylene (HDPE) is challenging because crystallization-induced
shrinkage of the filament, as it cools, results in stresses that warp
the printed part and debond it from the print substrate. Here, we
demonstrate that waste-derived HDPE can be successfully 3D printed
by (i) blending with a small fraction (<0.5% by weight) of dimethyl
dibenzylidene sorbitol (DMDBS) and (∼10%) linear low density
polyethylene (LLDPE) and (ii) printing the object with a thin “brim”
around it that is adhered to the print substrate using common polyvinyl
acetate-based glue. We match our experimental results with FEM simulations
that provide insight into the origin of the stresses developed during
printing. Because HDPE forms a significant fraction of the plastic
waste stream, conversion of waste-derived HDPE to 3D printing filament
has important technological implications.
In polymer-grafted nanoparticles
(PGN), covalent tethering of apolar
polymer chains to a polar inorganic nanoparticle core induces the
formation of self-assembled aggregates. Since the nature of these
aggregates determines bulk mechanical and transport properties, it
is of importance to understand the factors that determine the underlying
assembly processes. In the literature, the solution assembly of PGNs
has been understood in analogy to small-molecule amphiphiles. However,
in any experimental realization, PGNs are invariably characterized
by additional structural complexity, such as the distributions in
the inorganic core size and in the grafted chains (both in their
length and grafting density). These strongly influence the assembly
of amphiphilic PGNs. We have previously demonstrated that dispersity
in core size qualitatively affects the structure of PGN aggregates,
and Jayaraman et al. demonstrated the effect of grafted chain-length
dispersity. The combined effects of dispersity in the size of the
core and grafted chains have not been explored previously. Here, we
develop a model that builds on the work of Daoud and Cotton to explore
a wide parameter space of PGN with dispersity simultaneously in core
size and grafted chain length. We demonstrate that dispersity in core
size is the dominant factor affecting the self-assembled solution
structure of PGN aggregates. Our work suggests the importance of focusing
on synthetic strategies for control of core-size dispersity to control
aggregate structure in PGN.
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