Effect of Molecular Weight on the Capillary Absorption of Polymer Droplets

Abstract

We study capillary absorption of small polymer droplets into nonwettable capillaries using coarse-grained molecular dynamics simulations and a simple analytical model. Studies of droplets of simple fluids have revealed that the capillary process depends on the ratio of tube-to-droplet radii [Willmott Faraday Discuss., 2010, 146, 233; Marmur J. Colloid Interface Sci. 1988, 122, 209]. Here we consider the absorption of droplets of polymers and study the effect of polymer chain length on the capillary absorption process. Our simulations reveal that for droplets of the same size (radius), the critical tube radius, below which there is no absorption, increases with the length of the polymer chains that constitute the droplets. We propose a model to explain this effect, which incorporates an entropic penalty for polymer confinement and find that this model agrees quantitatively with the simulations. We also find that the absorption dynamics is sensitive to the polymer chain length. In some cases during the capillary uptake transient partial absorption states, where the droplet is partially in and partially out of the tube, were observed. Such dynamics cannot be explained by a generalized Lucas-Washburn approach.

Wetting Behavior of Polymer Droplets: Effects of Droplet Size and Chain Length

“…This plot is presented in Figure , where it is observed for both adsorption energies that droplets of n > 2 share a common plateau value of R g z / R g xy , while droplets of n ≤ 2 gradually attain lower values of R g z / R g xy . The plateau of the n > 2 region is also supported by data, which were reported in the literature, as discussed in the Introduction. ,,, …”

“…The plateau of the n > 2 region is also supported by data, which were reported in the literature, as discussed in the Introduction. 25,29,31,32 Further to the discussion above, which was aimed at contrasting single chains with multichain droplets, one can separately discuss single chain droplets. It was seen in Figure 2 that the R gz /R gxy ratio for N T = N = 1000 is larger than that for N T = N = 5000 and N T = N = 10 000.…”

“…While the focus was on the non-equilibrium dynamics, one could still extract important information regarding the equilibrium state, namely the similarity in the wetting state among large droplets (~200000 monomers) consisting of different molecular weight chains. Dhondi, et al 32 studied the capillary absorption of smaller (~10000 monomers) polymer droplets into non-wettable capillaries. Their main finding, via both theory and simulation, was that for droplets of the same size (radius), the critical capillary radius (below which there was no absorption) increased with the length of the polymer chains that constituted the droplets and that the capillary dynamics were sensitive to polymer chain length.…”

“…However, in characterizing the equilibrium properties of those droplets, it was observed that droplets of the same number of total monomers but made up of chains of different length shared similar dimensions. Subsequently, Dhondi, et al 32 placed these droplets on a solid substrate; after equilibration, they measured adsorbed heights and extracted contact angles, which were roughly constant, independent of the length of the chains contained within the droplet. These investigations allow drawing a hypothesis that the adsorbed droplet dimensions are independent of the length of the constituent chains when one compares droplets of equal total size.…”

Filling of carbon nanotubes and nanofibres

“…This process has been modelled to determine the extent of the capillary action [ 90 ]. In addition, the effect of the molecular weight of a polymer on the capillary action has also been investigated [ 91 ]. Microscopy has been used to visualize the filling of a MWCNT with gold via Joule heating and capillary action [ 92 ].…”