Previously, our group developed a novel technique for measuring diffusion coefficients
of bare nanoparticles in polymer melts using time-of-flight secondary ion mass
spectrometry (ToF-SIMS). We have investigated nanoparticles of various sizes, as well
as strongly or moderately attractive interactions with the polymer, and have mapped
how these characteristics impact nanoparticle diffusion. And we, naively, expected
nanoparticles with grafted bottlebrush polymers to behave similarly. Then, we did the
experiments and had quite a surprise.
Using ToF-SIMS, we measured the diffusion coefficients of polymer grafted
nanoparticles wherein the polymers are bottlebrushes with backbones of
poly(norbornene) and side chains of polystyrene grafted, and the nanoparticles are
large silica nanoparticles (80 nm radius). These nanoparticles were synthesized by Prof.
Rob Hickey’s Group at the Pennsylvania State University. When the bottlebrush
molecular weights are large relative to the matrix polymer, the nanoparticles display
core-shell diffusion behavior – this was our expectation. The surprise came when the
bottlebrush molecular weight is small relative to the matrix polymer, when the
nanoparticle diffusion is 10–100 times faster than predicted. We attribute this fast
diffusion to a local decrease in viscosity associated with the dewetting of the matrix near
the surface of these large nanoparticles. This is evidence of autophobic dewetting at the
surface of the nanoparticle.
And now there are more experiments to do and more questions. Could this happen with
a linear-polymer grafted nanoparticle? How large does a nanoparticle need to exhibit
this effect? Stay tuned!
Read the article in Macromolecules at https://doi.org/10.1021/acs.macromol.5c02957
A surprise from bottlebrush-polymer grafted nanoparticles
