Neutron Scattering

3 .0
ô 2 .5
U
2 .0
O
1 .5
a
1 .0
0 .5
PEP-PDMS
T=170°C;P=1365 bar
0 .0 0 2 4 6 8 10
Q [10-2X1]
Figure 14 .8 :
Structurfactor ofa PEP-PDMS diblock copolymer
14 .6 . Binary Melt with different Polymers
The structure factor of an ideal binary melt is evaluated within the "random phase"
approximation (RPA) according to
\'_ 1 1
S D , (Q/
(14 .40)
~VDPDb(Q)+(1-1»VHPDb(Q)
The inverse structure factor is obtained from the sum of the inverse form factors of both
ideal" chains weighted with their molar volumes and volume fractions . Eq . (14 .40)
corresponds to an ideal solution of two components with mixing energy being zero and
therefore no phase transition phenomena . Those ideal solution are usually not found in reality ;
as demonstrated in Figure14.9 even isotopic mixtures of chemically identical polymers show
phase decomposition at low temperatures because of a small but finite mixing interaction
energy . For polymer blends such interaction is described by the Floly-Huggins (F-H)
parameter
rh - rcr .
The FH-parameter bas the meaning of a
free enthalpy of mixing with the enthapy and
entropie terras hh
and F'6 , respectively . In the RPA approximation the interaction parameter
added accordingto
S-' (Q) = S o ' (Q)- 2F (14 .42)
1444