Enhanced mobility of confined polymers

Abstract

Non-classical behaviour, brought about by a confinement that imposes spatial constraints on molecules, is opening avenues to novel applications. For example, carbon nanotubes, which show rapid and selective transport of small molecules across the nanotubes, have significant potential as biological or chemical separation materials for organic solvents or gaseous molecules. With polymers, when the dimensions of a confining volume are much less than the radius of gyration, a quantitative understanding of perturbations to chain dynamics due to geometric constraints remains a challenge and, with the development of nanofabrication processes, the dynamics of confined polymers have significant technological implications. Here, we describe a weak molecular-weight-dependent mobility of polymers confined within nanoscopic cylindrical pores having diameters smaller than the dimension of the chains in the bulk. On the basis of the chain configuration along the pore axis, the measured mobility of polymers in the confined geometry is much higher than the mobility of the unconfined chain. With the emergence of nanofabrication processes based on polymer flow, the unexpected enhancement in flow and reduction in intermolecular entanglements are of significant importance in the design and execution of processing strategies.

Direct Observation of Confined Single Chain Dynamics by Neutron Scattering

“…Observing a significantly enhanced flow of large polystyrene molecules into the pores of anodic alumina oxide, Russell and co-workers concluded on a reduced viscosity that would follow from a decreasing entanglement density due to lateral chain compression [4]. Performing very delicate studies on the extension ratio of nanoscopic polymer films under shear, Si et al also conjectured on a reduced density of interchain entanglements which, according to them, should be compensated by a corresponding increase of intrachain entanglements [10].…”

“…The dominant motional mechanisms in this model are (i) a curvilinear version of the Rouse motion (local reptation) followed by (ii) the escape of the whole molecule from the tube at long times, the reptation process (see, e.g., [6,7]). The important question that is addressed now both by simulations [2,3,8,9] as well as by a variety of experiments on a macroscopic level [4,[10][11][12] is how these dynamics change under confinement.Basically all simulations available indicate that confinement reduces chain mobility independent of the adhesive potential of the wall. An analysis of the Rouse modes of unentangled chains under confinement reveals a uniform slowing down of all modes which was interpreted by an effective increase of monomeric friction [8].…”

“…The combined effects of increased effective friction and entanglement distances were even found to lead to a nonmonotonic behavior of the chain relaxation with confinement size [2]. Also, heterogeneous scenarios like a repulsion of entanglements away from the confining surface were proposed [9].Observing a significantly enhanced flow of large polystyrene molecules into the pores of anodic alumina oxide, Russell and co-workers concluded on a reduced viscosity that would follow from a decreasing entanglement density due to lateral chain compression [4]. Performing very delicate studies on the extension ratio of nanoscopic polymer films under shear, Si et al also conjectured on a reduced density of interchain entanglements which, according to them, should be compensated by a corresponding increase of intrachain entanglements [10].…”

“…DOI: 10.1103/PhysRevLett.104.197801 PACS numbers: 61.41.+e, 62.25.Àg, 78.70.Nx, 82.35.Lr Confinement effects in polymer melts may lead to unusual properties. This concerns both the chain conformation, which may be distorted, as well as chain dynamics, which may be altered due to surface interactions and changes of topology and chain self-density [1][2][3][4]. The understanding of such behavior is not only a scientific challenge but is also important for knowledge-based applications in nanotechnology, such as nanocomposites, coatings, adhesives, etc.…”

Molecular composition distribution of polycarbonate/polystyrene blends in cylindrical nanopores

“…1 For amorphous polystyrene (PS) confined in cylindrical alumina nanopores, an unexpected enhancement of flow and a reduction in intermolecular entanglement have been observed, leading to higher mobility of polymer in the confined geometry than that of unconfined chains. 2 In the case of semicrystalline polymers under nanocylindrical geometry, the polymers exhibit novel orientation, [5][6][7][8][9][10][11][12][13][14][15][16][17] polymorphism 18 and segmental dynamic behavior. 5,17 The crystals that form in nanorods at low supercooling show perpendicular orientation; that is, the c-axes of the polymer crystals that develop in cylindrical nanopores preferentially orient perpendicular to the long axis of the nanopore.…”