Effect of Deuterium Substitution on Surface Segregation

The staining of individual molecules, obtained by a replacement of hydrogen for deuterium (used to create a contrast required in experiments with condensed matter) may lead to drastic changes in the phase behavior of the studied systems. It has been widely recognized that this effect influences bulk interactions in polymer mixtures, as is discussed in Sect. 2.2.3. Here we describe first experimental results [145] on the role of deuterium labeling on surface interactions in blends which are not isotopic mixtures. 

We have studied binary blends dxx/hx2 of random olefinic copolymers x=(Ex x EEx)n, with one blend constituent protonated (hx) and the other deuterated (dx). The blends examined were grouped in four pairs of structurally identical mixtures xx/x2 but with a swapped isotope labeled component (dxx/hx2 and hxx/dx2). For such blend pairs the bulk interaction parameter % (and hence also the critical point Tc) has been found (see Sect. 2.2.3 and references therein) to be higher when the more branched (say xx x2) component is deuterated, i.e., X(dx /hx2) x(hx /dx2) or Tc(dxx/hx2) Tc(hxx/dx2) (see Fig. 9). An identical pattern is exhibited here by the force driving the segregation at the free surface. This is illustrated in Fig. 26a,b where the composition vs depth profiles of the more branched (xx) component are shown for blend pairs with swapped isotope 

Our further analysis is based on the relative difference of the surface tension -Ay/y between blend components, evaluated at a reference temperature Tref-100 °C and presented in Fig. 23. Determined -Ay/y values (see Fig. 23) are always higher for the blends with deuterated more branched components (denoted by open symbols) than for their counterparts with isotope labeled more linear constituents (denoted by solid symbols). This pattern is almost undetectable for the 52/38 blend pair with the -Ay/y values at Tref equal to 1.31(19)% and 1.23(24)% for d52/h38 and h52/d38 blends, respectively. The change in -Ay/y(Tref), caused by isotopic swapping, is the highest for the 66/52 blend pair (symbols O and in Fig. 23) and decreases through 86/75 (symbols A and ) and 75/66 (symbols  

V and ) to the 52/38 blend pair. The same sequence gives a monotonic decrease in the difference of critical temperature Tc(dx1/hx2) - Tc(hx1/dx2) (yielding values equal to 116,84,68, and 27 °C, respectively). 

A more detailed insight into this relation might be obtained when comparing surface %s and bulk % interaction parameters both evaluated for all the examined blend pairs at the same reference temperature Tref=100 °C (see Fig. 27). The %s values are just recalculated (with Eqs. 45,34, and 35) from -Ay/y(Tref) values described above. The % values are basing on critical point data %(Tref)=%cTc/Tref (see Sect. 2.2.3). This is based on the assumption that only enthalpic contribution (Eq. 16) is relevant for bulk interactions. 

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