Neutron Scattering

500
450
-400
U
0
;, 350
42,
300
s~.
250
200
d-Ps/ps
VN- 1 .42106cni /mo
--------- ----------------- -----
Binodal
Spinodal
150
0.0 0.2 0.4 0.6 0.8 0.0 0.5 1.0 1 .5 2.0 2.5
Composition
1- [l0~mol/cmb]
Figure 14 .9 : Phase diagram and Flory-Huggins interaction parameter ofan isotopic d-PS/PS
blend
the structure factor of an ideal mixture . For small Q Eq. (14 .42) can be approximated in Zimm
approximation according to
S-' (Q)= S-'(0)+AQ2 (14 .43)
wich the inverse structure factor S -'(0) = 2[FS - F] at Q=0 and the FH-parameter at the
spinodal temperature, 2F, = + , being inversely proportional to both chain
VD
(1- 1»Vx
molar volumes and being related to the translatorial entropy of mixing . In experimental
reality one tries to measure at sufficiently small Q in order to be able to use Eq .(14 .43) for
analysis of the scattering data . As shown in Figure 14 .9, the spinodal temperature represents
the phase boundaiy between the metastable und unstable two-phase regions and the unstable
region touches the stable one-phase region at the critical point .
phase at high temperatures the FH-parameter is
In the single homogeneous
smaller than F, = 2 / V in accordance with
the Gibbs condition of stability of a positive S(0) ; S(0) represents a susceptibility which
according to the fluctuation-dissipation theorem is related with the free enthalpy of mixing
AG according to
7 2 (AG/RT)
aq) 2
(14 .44)
In case of F > F, the System decomposes with the mechanism of spinodal decomposition in
two macroscopically large phases which one polymer component dominating . The free
enthalpy of mixing of polymer blends was originally formulated within the
approximation by Flory and Huggins, it gives the saure result as the random-phase
mean field
1445