Recent experimental studies have
demonstrated that the introduction
of oxygen-containing functional groups in graphene sheets can greatly
enhance the mechanical properties of their nanocomposites with polar
polymers even at extremely low loadings. Motivated by these reports,
we determine here the elastic constants of syndiotactic poly(methyl
methacrylate) (sPMMA) at small wt % loadings of graphene sheets through
atomistic modeling. To carry out a comparative study of the effect
of graphene functionalization on the degree of mechanical reinforcement,
we address both pure (i.e., unfunctionalized) and functionalized graphene
sheets bearing epoxy and hydroxyl groups randomly bound on both sides
of their surface in the host sPMMA matrix. The calculation of elastic
constants (which involves no adjustable parameters) follows the methodology
originally proposed by Theodorou and Suter [Macromolecules198619139], and has been
based on the use of the Dreiding all-atom force-field. Our predictions
for the elastic constants (which for the pure sPMMA matrix are within
the error bars of experimentally computed values) suggest a substantial
increase in the elastic constants, especially in the case of functionalized
graphene sheets. For example, at just 5.67 wt % loading of the host
matrix in functionalized graphene sheets, they indicate an improvement
in Young’s modulus E by ∼74%, in the
bulk modulus B by ∼19%, and in the shear modulus G by 83%. Our results fully corroborate recent experimental
measurements about the unique opportunities that functionalized graphene
sheets offer for the design of new, very strong multifunctional materials
at low nanofiller content.
Molecular dynamics simulations are performed for two model pressure-sensitive adhesive (PSA) materials, atactic poly(n-butyl acrylate) [poly(n-BA)] and atactic poly(nbutyl acrylate-co-acrylic acid) [poly(n-BA-co-AA)] at a very low concentration in acrylic acid (one acrylic acid monomer per fifty butyl acrylate monomers plus three acrylic acids at each one of the two chain ends) in the bulk and confined between three crystalline substrates, silica (SiO 2 ) represented as α-quartz, α-ferric oxide (α-Fe 2 O 3 ), and metallic α-ferrite (α-Fe), over a range of temperatures. The simulations are carried out with the accurate, all-atom Dreiding force-field and provide important information for the distribution of local mass density of the two polymers at the three crystalline substrates, and their adsorption in conformations that lead to the formation of loop, train, and tail structures. By analyzing potential energy interactions between the two polymers and each one of the three substrates through their van der Waals contact area or by computing the diagonal elements of the local stress tensor in the vicinity of the substrate, we have calculated the effect of the type of substrate on the work of adhesion of the two polymers. Our calculations reveal a considerably stronger adsorption on α-quartz and α-ferric oxide compared to α-ferrite, which we attribute to strong attractive oxygen interactions of the two polymers with the oxygen atoms of SiO 2 and Fe 2 O 3 . Detailed calculations of the local number density distribution of individual atomic species above the three substrates, of the local variation of the bond order parameter for skeletal and side group bonds or chords, of the distribution of dihedral angles, and of the histograms of chain ends support that the two PSAs adsorb on the three substrates with both their skeletal and pendant C−C and C−O groups lying practically parallel to the surface and in such a way that the CO bond of the pendant butyl acrylate groups points toward the surface. The detailed MD simulations indicate that adding a small amount (7.1% wt. here) of acrylic acid to poly(n-BA) significantly improves its wetting characteristics on oxygen-containing substrates. This is in full support of the findings of Kisin et al. [Chem. Mater. 2007, 19, 903−907] that introducing oxygen atoms to a metallic surface or to polymer molecules increases considerably the work of adhesion.
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