High Polymer Mass Densities at the Mouths of Carbon Nanotubes (CNTs) Control the Diffusion of Small Molecules through CNT-Based Polymer Nanocomposite Membranes

Abstract

Detailed molecular dynamics (MD) simulations of model single-walled carbon nanotube (CNT) membranes based on atactic poly­(methyl methacrylate) (aPMMA) indicate that PMMA chains significantly penetrate nanotubes through their faces. They predict very high-density values of the polymer in the interfacial area around the CNT mouths that can exceed by 50% the density of the bulk polymer at the same thermodynamic conditions. This dramatically decreases the diffusivity of relatively small penetrants (in our study, water molecules) in the nanocomposite membrane, because of the exceedingly long times needed by these small molecules to diffuse through such a dense interfacial layer before accessing the interior of the nanotubes where they can travel really fast. According to our simulations, the escape time of a confined water molecule from the blocked mouths of a CNT can exceed by several orders of magnitude the time needed by the same molecule to move through the CNT pore. Our work indicates the importance of completely avoiding (or at least minimizing) penetration of polymer chains into the CNT pores through the mouths of the tubes in enabling the efficient transport of small- to moderate-size molecules in model CNT-based polymer membranes, since this provides the highest resistance to their mobility through the membrane.

Geometric Analysis of Clusters of Free Volume Accessible to Small Penetrants and Their Connectivity in Polymer Nanocomposites Containing Carbon Nanotubes

“…Our purpose in the present study is to carry out a systematic analysis of the cavities of free volume accessible to a small penetrant molecule in a polymer nanocomposite containing CNTs, of their clusters, and of the connectivity of these clusters. Our work is motivated by the findings of the recent MD study already mentioned above (ref ), which indicated significant penetration of PMMA chains inside CNTs in model CNT-PMMA nanocomposites. It involves a geometric analysis of free volume in these model CNT-polymer configurations, which allows one to determine the holes accessible to water molecules and identify their clustering as well as the network of pathways through which a water molecule can diffuse from one such cavity to another, inside the nanocomposite.…”

“…The model CNT-PMMA nanocomposite structures analyzed have already been described and characterized in two previous contributions. , They comprise a pure atactic poly­(methyl methacrylate) matrix (system 1) and four single-walled carbon nanotube-based nanocomposites (systems 2–5). System 1 consists of 280 45 monomer units long (molecular weight 4500 g/mol) PMMA chains in an orthorhombic cell subject to periodic boundary conditions and edge lengths approximately equal to 125 Å.…”

“…Following previous, well-established approaches, , such a problem is typically broken down to three subproblems: (a) generation of realistic microstructures through detailed simulations using accurate force-fields fully accounting for all possible interatomistic interactions in the polymeric system; (b) geometric analysis of sites of free volume (where the spherical guest molecule can fit), and their connectivity; and (c) detailed calculation of transition rates of the guest molecule from one cluster of accessible volume to another in the polymer structure accounting explicitly for the coupling of degrees of freedom between the penetrant and the polymer matrix. Step (a) was addressed in refs and , step (b) is addressed here, and step (c) will be addressed in a forthcoming contribution. We further note that the sites or clusters of free volume accessible to a penetrant (such as water) that will be computed in the present study (step 2 of the overall methodology) will be the most likely sorption sites for the penetrant, even if these were computed based purely on geometric criteria and the assumption of spherical symmetry for the penetrant, as we will see below.…”

“…Because of their unique properties (intrinsic smoothness and hydrophobicity of graphitic walls, extremely high aspect ratios, and ultranarrow inner diameters), − carbon nanotubes (CNTs) allow for the ultrafast flow of small molecules through their pores, a feature that in the last few years has been systematically and persistently exploited for the design of polymer–matrix CNT membranes with enhanced or controlled permeability and selectivity properties to specific molecules. − Simultaneously, with the development of this new class of materials, unique opportunities emerged for fundamental theoretical and simulation studies − related to molecular transport and flow at the nanoscale, and how these can be exploited in order to guide new membrane designs exhibiting higher efficiency and selectivity. Farimani and Aluru showed that the diffusion coefficient of water molecules in small diameter CNTs decreases compared to that in bulk water because of confinement.…”

“…Farimani and Aluru showed that the diffusion coefficient of water molecules in small diameter CNTs decreases compared to that in bulk water because of confinement. However, for larger diameters, the smooth interior surface of CNTs and the fact that less hydrogen bonds are formed between the confined water molecules enhance water mobility. ,, On the other hand, a recent molecular dynamics (MD) study showed that nanotube size (diameter) can have a dramatic effect on small molecule diffusion in CNT-polymer matrices and thus on transport efficiency because for large enough CNT diameters, polymer chains can deeply penetrate the tubes through their mouths. Large penetration depths block the entrance and exit faces of CNTs, thus substantially inhibiting molecular transport.…”