Molecular Structure and Work of Adhesion of Poly(n-butyl acrylate) and Poly(n-butyl acrylate-co-acrylic acid) on α-Quartz, α-Ferric Oxide, and α-Ferrite from Detailed Molecular Dynamics Simulations

Abstract

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 CO 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.

Atomistic Modelling of Confined Polypropylene Chains between Ferric Oxide Substrates at Melt Temperature

“…For less favorable interactions, these peaks would be reduced significantly. Such observations have been shown before for the interfacial systems of acrylic chains on α-quartz, α-Fe 2 O 3 , and α-ferrite where the strong adsorption of acrylic chains on α-quartz and α-Fe 2 O 3 was evidenced by the higher intensities of the local mass density peaks compared with the less pronounced peaks for α-ferrite [26]. These peaks are, on the other hand, influenced by the available space on the surfaces.…”

“…Figure 3 displays the mass density profile, ρ, across the film thickness. Due to the symmetry of the density profiles with respect to the film midplane, one can conclude that the number of polymer configurations embedded in the film thickness was sufficient to provide adequate predictions of the relevant properties studied here [26]. Near the substrates, the density profile shows a profound peak at a distance equal to ~4 Å away from the solid surface.…”

“…In order to study the cross-linked structure of epoxy near an alumina surface, Kacar et al [25] developed interfacial MD models which were then coarse-grained into DPD models. Anastassiou and Mavrantzas [26] performed MD simulations for poly( n -butyl acrylate) and poly( n -butyl acrylate- co -acrylic acid) on α-quartz, α-ferric oxide, and α-ferrite. Liu et al [27] studied via MD the interfacial region of surface modified cadmium sulfide in contact with mercaptopropyltrimethoxysilane.…”

Entropic Effects in Polymer Nanocomposites

“…Many systems of biophysical and technological interest consist of nanoparticles and polymer chains in a confined volume, where not only interactions between polymers and particles but also the volume limitations need to be taken into considerations [ 51 , 78 , 79 , 80 ]. One key study [ 68 ] relating confinement to diblock copolymer nanocomposite is that reported by Lee et al, where they prepared blends of AB diblocks and nanoparticles that are confined between two hard walls separated by a distence using SCFT/DFT calculations.…”

“…For example, the importance of the relationship between dispersion and orientation of graphene nanoribbon and electrical properties has been highlighted by the research group of Sundarara [ 17 , 18 ], and was supported by DPD simulations [ 75 , 76 , 77 ]. Confinement effects in highly-filled polymer composites and nanocomposites are also an important field where energy/entropy considerations play a key role in determining the final structures and properties [ 78 , 79 , 80 ]. For instance, the interactions and conformational characteristics of confined molten polypropylene (PP) chains between ferric oxide (Fe 2 O 3 ) substrates were investigated by molecular dynamics (MD) simulations, showing the influence of the confinement on the buildup imbalance of normal and tangential pressures [ 78 ].…”