Abstract

Polymeric products are an integrated component of our everyday life and the importance of optimizing manufacturing processes for higher output and lower waste is a necessity. A crucial factor here is our knowledge of the liquid-solid boundary condition in the flow of molten polymers. In contrast to small molecule fluids, polymer melts slip on solid substrates sometimes to significant extents. Polymer slip has been under investigation over the past few decades, however detailed knowledge of the extent of slip of polydisperse polymers typical of industrial applications has not yet been fully achieved. This study brings into focus the effect of molecular weight distribution on polymer slip.
We perform a systematic study of the effect of molecular weight distribution on polymer slip by examining binary and ternary mixtures of monodisperse polymers. Three different experimental techniques are used to cover different aspects of our objective.
Steady state slip of polybutadiene mixtures on a silicon wafer in planar Couette flow is first examined. Using a microfluidic geometry, confocal microscopy and particle image velocimetry are used to measure unidirectional velocities across the flow channel. It is shown that slip dynamics at the onset and within the transition to the strong slip regime are significantly affected by the incorporation of weakly entangled polymer chains. Both size and content of short chains are substantially important in slip dynamics. Among ternary mixtures with a fixed weight average molecular weight (Mw), those with the minimum content of intermediate chains (the lowest number average molecular weight, Mn) exhibit the largest slip lengths. This means that a binary mixture of short and long chains of two distinct separate molar modes are most effective in promoting slip.
Polymer slip has long been related to the surface enrichment of short chains. Enrichment theories include thermodynamic segregation mechanisms as well as the shear induced fractionation mechanism. In line with our previous experiments, we examine the short chain surface enrichment of polybutadiene mixtures after strong slip in simple shear. After strong slip, a hazy debris of polymer remains behind on the steel substrates. Experiments show that the debris from all binary and ternary polybutadiene mixtures undergoing strong slip is enriched with the short chain constituent. We develop a simple disentanglement induced enrichment model in agreement with previous slip theories that predicts our experimental data very well.
We also examine slip of thin polystyrene (PS) films dewetting from fluorinated and silanized silicon wafers. We first show that the sacrificial mica substrate, used very often in a two-step film preparation procedure in thin film studies, significantly affects the polymer slip length after the film is transferred onto a nonwetting substrate. We show that a highly entangled monodisperse PS exhibits different flow dynamics and slip length depending on whether its air or buried interface with mica is placed on the nonwetting substrate, and whether the film has been pre-annealed. We find that the buried interface shows reduced slip length compared to the air interface. The discrepancy between the two interfaces is molecular weight dependent and almost undetectable in unentangled polymers.
We next show that slip of highly entangled PS chains on nonwetting substrates is affected by the incorporation of short chains of molecular weights, M, below the polymer critical molecular weight, Mc. Among PS mixtures of the same Mw those with a higher content of short chains (lower Mn) are likely to exhibit larger slip lengths. We show that such slip enhancement effect in binary mixtures of long and short chains is only effective when the long chain content, ϕL, is dominant (ϕL > 0.5).
While the polymer slip mechanism on different substrates can be different depending on the density of adsorbed chains – disentanglement versus detachment – the consequence of the incorporation of short chains seems to be similar, i.e., slip is promoted when small amounts of short chains are added to a highly entangled polymer. In this aspect, both size and content of short chains matter. This finding has practical implications in polymer processing and is useful for developing new slip models. Furthermore, the disentanglement induced surface enrichment phenomenon is a new approach in the investigation of extrudate surface instabilities and die drool.