A lattice model for the surface segregation of polymer chains due to molecular weight effects

Abstract

The effects of polydispersity on the behavior of a polymer melt near a surface are considered in this work. To study these effects we have modeled an a thermal polymer melt with a bimodal molecular weight distribution in the vicinity of a neutral surface in the spirit of the mean-field lattice treatments of Sheutjens and Fleer20 and Theodorou.23 The results of the calculations show that purely entropic effects cause the shorter chains in the system to partition preferentially to the surface. This partitioning becomes more pronounced with increases in the difference between the molecular weights of the chains. The concentration profile of the segments of any one species in the vicinity of the wall can be described by a hyperbolic tangent function with a single correlation length that is a simple function of the mixture composition and the radii of gyration of the two chains. Implications of these results on the surface tension and orientation of chain segments in the interface of a bimodal polymer melt are also examined.

Entropic segregation of short polymers to the surface of a polydisperse melt

“…Furthermore, it reproduces the simple inverse dependence on N n , first observed in the lattice SCFT of Hariharan et al [27]. In a previous study for monodisperse melts [33], we showed that the reduction in surface tension obeys…”

“…The surface enrichment of short polymers was first demonstrated by Hariharan et al [27] by applying SCFT to a lattice model of an incompressible bidisperse melt of short and long chains. They investigated both the amplitude and the range of the enrichment.…”

Surface segregation of conformationally asymmetric polymer blends

“…This finding is in qualitative agreement with the result predicted in Ref. [34] and observed in Refs. [14] and [35,36,37].…”

Reduction of polymer surface tension by crystallized polymer nanoparticles

“…As such, it would be expected to disperse more uniformly in the poly mer melt 20 and would be better modeled as a star polymer or at least a linear polymer with greater contour volume (longer) than the host polymer (it should be of equal or greater contour volume since the crystallization could join several polymers together). The groups of Russell, 21 Fredrickson, 22 Archer, 23,24 and Sauer 25 have discussed theo retical and experimental results for polydisperse polymer melts and linear/star mixtures. In such cases, these authors have shown that the longer polymers (here representing loosely crystallized particles) would avoid the surface and, since the average molecular weight would be higher than a monodisperse polymer system (noncrystallized system), the surface tension would increase rather than decrease.…”

“…10 This makes sense since in this picture the "particles" (longer poly mers or star polymers) would have more conformational en tropy than the noncrystallized polymers-they would have fewer free ends after crystallizing together. It is known that the architecture of a polymer affects its localization with re spect to a surface with molecules having fewer free ends avoiding the surface; [21][22][23][24][25][26] free ends have less configurational entropy to lose near a surface. The experimental situation of Wei et al 2 is modeled in this paper by nanoparticles that are an extreme limit of this effect: a single segment "polymer" that is purely an end-segment and that therefore has a strong entropic affinity to the surface.…”