Interaction of Poly(methyl acrylate) with Carbon Nanotubes as a Function of CNT Diameter, Chirality, and Temperature

The
use of carbon nanotubes (CNTs) is promising for ultrahigh-strength
composite materials with tunable electrical and thermal properties.
Noncovalent polymer wrapping onto CNTs has proven effective to reduce
bundling and improve the dispersion in solvents and polymer matrices.
However, the mechanisms of CNT–polymer wrapping are unclear.
In this study, extensive replica exchange molecular dynamics simulations
in atomic resolution (interface force field (IFF)–polymer consistent
force field (PCFF)) were used to characterize the interaction of syndiotactic
poly­(methyl acrylate) (PMA) with 13 different CNTs of diverse diameter
and chirality across a temperature range of −50 to 100 °C
in the absence of solvent. The resulting polymer conformations on
the CNTs were classified into extended, random coil, folded, and wrapped
states. Binding free energies were favorable in a range of 0 to −2.7
kcal/mol monomer. The binding strength increased for larger CNT diameter
and origins of conformation transitions and coexistence are analyzed.
Extended conformations were least stable in all cases, and the CNT
diameter as well as the chiral angle determined the driving forces
toward random coil, folded, or wrapped structures. The findings agree
with experimental tests of PMA/CNT films in solution and X-ray data.
Follow-on studies may screen a wide range of polymers in melts and
solutions to tailor the properties of CNT dispersions and composites.