Reversal of the isotopic effect in the surface behavior of binary polymer blends

Abstract

We report neutron reflectivity measurements on the surface behavior of isotopic polystyrene blends of symmetric and disparate molecular weights near an air surface. For the symmetric blends we find, in agreement with past findings, that the segments of the deuterated polymer always partition to the air surface. These results, which are driven purely by energetic effects, can then be modeled in the framework of a mean-field lattice theory with a constant surface energy difference parameter. In contrast, for the asymmetric blends we find that the segments of either polymer can partition to the surface, and the controlling variable is the disparity in the molecular weights of the two components. These new results, which cannot be predicted with the constant density lattice models utilized for symmetric blends, can be modeled if one balances the energetic preference of placing the deuterated segments near the surface with entropic effects, which are caused by the presence of a density gradient at the air surface, preferring the surface segregation of the short chains. These findings emphasize the need for the inclusion of free-volume effects when modeling the segregation to a free surface, and we show that a recent mean-field compressible lattice model does capture these effects adequately.

Investigation of the Effects of Isotopic Labeling at a PS/PMMA Interface Using SIMS and Mean-Field Theory

“…20 This is also evidenced in Figure 11 with the surface excess of dPS-70, which is clearly larger for the measured profile than the simulated profile using ∆ s ) 0.005. Therefore, there is an apparent increase in ∆ p and ∆ s from dPS-130 to dPS-70.…”

“…20 The value of ∆ s for an hPS:dPS surface was shown to deviate by as much as 50% from the nominal value of 0.005 over a range of dPS molecular weights of ∼30-500 h at 138°C for initial concentrations of (a) 10% dPS (system C), (b) 20% dPS (system D), and (c) 30% dPS (system E). The PS/PMMA interface is shown with the vertical line.…”

“…13,19,20 Similarly where kT∆ p is the net energetic preference per segment for dPS over hPS at an hPMMA "substrate", and λ 1 is a lattice weighting factor (1/6 for cubic). 18 As shown in eq 3, ∆ p should be proportional to a simple difference in values for dPS/ hPMMA and hPS/hPMMA.…”

“…A constant value of ∆ s ) 0.005, as reported previously for dPS surface segregation at 160°C, 20 was used to account for dPS segregation to the hPS:dPS surface. To determine the effective depth resolution, system H was fit to a Gaussian convoluted step function (error function) with a measured full width at half-maximum (FWHM) of 15 nm, as shown in Figure 5.…”

“…13,20 The reported values for hPS:dPS are nominally 2.0 × 10 -4 over the range of temperatures employed in this investigation (128-166°C). 10,32 Figure 4 shows a theoretical comparison of dPS segregation using ) 0.0 and ) 2.0 × 10 -4 for N hPS ) N dPS ) 1200 and ∆ p ) 0.01.…”