Enhanced Polymer Segment Exchange Kinetics Due to an Applied Shear Field

Abstract

Introduction. Fundamental knowledge about the behavior of polymer melts near solid surfaces is important to many technologies such as electronics packaging, adhesion, coatings, and polymer processing. When polymers are placed under flow, surface effects can be critical in widely used polymer processing methods such as die extrusion and injection molding. Changes in the extrusion die material can suppress or induce instabilities that lead to unsatisfactory products. 1 These results show that the short-ranged interactions between the polymer and the substrate (coupled with the general viscoelastic properties of polymers) have macroscopic impacts. These processing instabilities have been correlated with "wall slip" at the polymer/solid interface where the apparent fluid velocity near the solid surface is no longer zero. Much work has been performed to determine the important physical parameters for wall slip and to understand the underlying mechanisms of the behavior of polymers at solid boundaries with the goal of better designing processing routes. [2][3][4][5][6][7][8][9][10][11][12] However, the effect of flow on the microscopic behavior of polymer chains directly at a solid surface is not well understood. Few experimental data are available for the behavior of polymer melt chains at the solid interface because of the inherent difficulty of distinguishing between surfacebound chains and the melt matrix. [13][14][15] In this communication, neutron reflectometry was used to study the effect of a shear deformation on the adsorption/desorption kinetics of an entangled polymer melt at an attractive solid substrate. The high spatial resolution of neutron reflectivity, O (Å), provides information directly at the polymer/solid interface. A labeled (deuterated) polymer layer was formed at the surface of a silicon wafer and was welded with a hydrogenated polymer matrix. The fraction of deuterated polymer segments at the silicon surface was monitored with different annealing times with no applied shear and with varying shear rates after a set annealing time. Increases in the shear rate resulted in an increased rate of desorption of the surface-bound polymer chains (or adsorption of the matrix polymers). To the best of our knowledge, this shear-induced adsorption/desorption of an entangled polymer melt at an attractive solid surface was directly observed for the first time. These results provide important experimental evidence of molecular mechanisms which should be incorporated into future models of flow effects on polymer dynamics at surfaces.

Segmental Dynamics of Interfacial Poly(methyl acrylate)-d3 in Composites by Deuterium NMR Spectroscopy

“…Using isotope labeling, neutron reflectivity can, in principle, be used to get similar information. Lin et al , found that the translational interdiffusion coefficients for films less than 5 times the radius of gyration ( R g ) of poly(methyl methacrylate) on silicon were decreased compared to bulk. At less that 3 R g , the effect was particularly dramatic and as much as 2 orders of magnitude slower than bulk.…”

“…At the air−polymer−solid interface, many techniques have been used to characterize the interfacial polymers such as elipsometry − and X-ray and neutron reflectometry. − These techniques basically characterize the structure (often the thickness) of the polymer layer and are typically used to estimate the glass transition temperature through a break in the thickness−temperature curve. Some structural techniques can also be followed as a function of time to yield information on changes, such as those due to diffusion .…”

Neutron confinement cell for investigating complex fluids

“…It will be interesting to compare our results with those reported previously with more conventional solvent flow shear cells as well as to investigate the effects of oscillatory shear motion. 10,16 …”