Neutron Scattering

6
Û
ô 4
3 .0
2 .5
; 0 1 .5
0 .0
0 .0 0 .3 0 .6 0 .9 1 .2 1 .5 2 .32 2 .33 2 .34 2 .35 2 .36
Qz [10
4 X
2 ]
1/T [10'/I-,]
Figure 14.10a : Structurefactor S Q) in Zimm representation ofa critical mixture dPS/PVME
polymer blend at various temperatures.
Figurel4.10b : S- '(0) plotted versus IIT. The solid lines describe the critical behavior ofthe
thermal composition fluctuations. Near the critical temperature one observes deviationsfrom
the mean field behavior. The spinodal and critical temperatures are determined frorn the
extrapolation S - '(0) =0; S - '(0) =0 means an infinite susceptibily.
approximation being a mean field approximation as well . According to RPA the slope A in
E«14 .43) is related to the square of the radius of gyration of both chains assumed as being
undisturbed .
Next we discuss an experimental example of a binary blend of a deuterated polystyrene (d-
PS) und polyvinylmethylether (PVME) . This mixture shows the specialty being miscible at
low temperatures and decomposes in two macroscopic phases at high temperatures .
reasons are a prefen ed interaction between PS und PVME (Fh < 0) and an increase of the
total free volume during decomposition (n < 0 ) . The free volume is related with the entropy
F. and in case of F, > F c becomes dominant and the driving force for the process of
decomposition at high temperatures .
0 .5
The
The SANS experiments were exclusively performed
within the homogeneous one -phase region . In Figure 14.10 S(Q) is plotted in Zimm
representation (a) and l/S(0) versus 1/T in (b) . One clearly realizes the increasing scattering at
higher temperatures (the inverse S(Q) is of course decreasing), from which one can conclude
to stronger thermal composition fluctuations . The inverse susceptibility is linearly
proportional to 1/T, it is zero at the spinodal respectively at the critical point, and its slope
directly gives Fh . The observed linear shape of S -'(0) with 1/T is representative for the
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