Conflicting Effects of Extreme Nanoconfinement on the Translational and Segmental Motion of Entangled Polymers

Abstract

Physically confining polymers into nanoscale pores induces significant changes in their dynamics. Although different results on the effect of confinement on the dynamics of polymers have been reported, changes in the segmental mobility of polymers typically are correlated with changes in their chain mobility due to increased monomeric relaxation times. In this study, we show that translational and segmental dynamics of polymers confined in disordered packings of nanoparticles (NPs) can exhibit completely opposite behavior. We monitor the capillary rise dynamics of entangled polystyrene (PS) in disordered packings of silica NPs of 7 and 27 nm diameter. The effective viscosity of PS in 27 nm SiO2 NP packings, inferred based on the Lucas–Washburn equation, is significantly smaller than the bulk viscosity, and the extent of reduction in the translational motion due to confinement increases with the molecular weight of PS, reaching 4 orders of magnitude reduction for PS with a molecular weight of 4M g/mol. The glass transition temperature of entangled PS in the packings of 27 nm SiO2 NPs, however, increases by 45 K, indicating significant slowdown of segmental motion. Interestingly, confinement of the polymers into packings made of 7 nm SiO2 NPs results in molecular weight-independent effective viscosity. The segmental dynamics of PS in 7 nm SiO2 NP packings are slowed down even further, as evidenced by a 65 K increase in glass transition temperature. These seemingly disparate effects are explained by the microscopic reptation-like transport controlling the translational motion and the physical confinement affecting the segmental dynamics under extreme nanoconfinement. Although we do not fully understand the origin of the molecular weight-independent effective viscosity of PS in the 7 nm SiO2 NP packings, nonlinear flow and hyperconfinement effects may be playing a role. The novel flow behavior that we observe opens new ground for further theoretical exploration of the dynamics of extremely confined polymer melts.

Interfacial Friction Controls the Motion of Confined Polymers in the Pores of Nanoparticle Packings

“…The concentration of the liquid in the unfilled region is non-zero due to the presence of capillary condensed water in the pores of the packing. It is important to note that this mode of spontaneous transport of PDMS in the SiO 2 nanoparticle packing is fundamentally different from the meniscus-driven motion of polymers in nanoporous media as has been previously studied in multiple reports. ,,,− The motion in this case is driven by gradient in concentration within the porous media (i.e., nanoparticle packing) without any contact with a reservoir of polymer melt.…”

“…This entropic barrier model predicts stronger scaling of diffusivity in the higher MW regime over bulk reptation, D ∼ N –α (α > 2). − Polymers undergoing capillary rise infiltration into the nanopores of a nanoparticle packing also exhibit significant deviations from bulk dynamics. As the degree of confinement increases, unentangled polymers show slowdown in segmental and chain-level motion, whereas entangled chains exhibit faster-than-bulk chain motion. − At extremely high levels of confinement, the chain motion becomes independent of the MWa surprising observation for polymeric chains . Perturbations to the nature of dissipative processes active in nanoscale transport of polymers could arise from physical confinement and interfacial friction.…”

Role of Polymer–Nanoparticle Interactions on the Fracture Toughness of Polymer-Infiltrated Nanoparticle Films

“…While this paper focuses on the fracture behavior of PINFs, we also measure the T g of the polymer in PINFs which provides information on polymer segmental relaxation and is often affected by interfacial interactions . Spectroscopic ellipsometry measurements on bilayer films composed of a fully infiltrated PINF with a neat polymer layer on top are performed as previously described. , The T g values of the confined polymer in NP packings are determined based on refractive index change of the PINF layer with temperature on a controlled cooling ramp and compared to the bulk T g values measured using the polymer layer atop the PINF (Figure S4). To estimate the degree of confinement, we define a CR as the ratio between the radius of gyration of the unperturbed polymer chain ( R g ) to average pore radius (i.e., CR = R g / R pore ) .…”

“…Segmental dynamics of polymers in PINFs: Δ T g ( T g,confined – T g,bulk ) as a function of polymer MW and CR for PINFs made of PS/P2VP and (a) 27 nm NPs and (b) 9 nm NPs obtained in this study (circular symbols) and from reference (square symbols) . The lines are to guide the eye.…”

“…The glass transition temperature ( T g ) of the polymer confined in NP packings is measured using a J.A. Woollam M-2000V spectroscopic ellipsometer as previously described. ,,,, A bilayer film composed of a polymer layer atop a PINF is mounted onto a temperature-controlled stage (Linkam THMS 600) that is attached to the ellipsometer and enclosed in a nitrogen-filled chamber. The in situ ellipsometry sampling rate is 1 s with high accuracy zone-averaging.…”