Chemistry Reference
In-Depth Information
stronger position in the market place than the bare plastic. Attention has been
especially focused on blends since the commercialization of General Electric's
Noryl, a blend of HIPS with poly(2,6-dimethyl-1,4-phenylene oxide). We are now
in a period of investigating both the new blends and related compound systems and
the scientific principles underlying blend characteristics [
1
,
2
].
When the two (or more) blend components are compatible, the performance of
the final product is straightforwardly controlled by the properties of the individual
materials and their mixing ratio. In the most frequently encountered situation of
immiscible polymer/polymer dispersions, however, one is faced with the problem
of controlling the morphology (phase structure) and the interfacial adhesion
between the phases in order to obtain an optimized product [
1
,
2
]. The phase
structure (e.g., the dispersed particle size) in such systems is controlled by the
chemical character of the individual components and their rheological properties
[
5
] as well as by the deformation and/or thermal history; these factors affect how the
phase morphology evolves. A number of experimental investigations have clearly
shown that the characteristic size of the dispersed phase in incompatible polymer
blends is directly proportional to the interfacial tension [
6
], whereas the equilibrium
adhesive bond strength between the two phases depends strongly on the interfacial
tension. For example, the characteristic size of the dispersed phase obtained during
melt extrusion of an incompatible polymer blend is related to the interfacial tension
between the two phases (
g
,), the viscosities of the dispersed phase and the matrix (
d
and
m
, respectively), and the process characteristics (shear rate,
g
) by the empirical
_
relationship [
7
]:
0
:
84
g
_
m
d
n
g
4
d
m
¼
(1)
where
d
n
is the number-average particle diameter. The plus (+) sign applies for
p
¼
d
/
m
>
) sign for
p
<
1. Moreover, the rate of phase growth
during the later stages of phase separation increases with increasing interfacial
tension [
8
]. It is noted that the size of the dispersed phase is an important factor that
influences the mechanical properties of incompatible polymer blends.
Therefore, interfacial tension is an important, if not overriding, factor in the
formation of a phase boundary and in the development of phase morphology in
incompatible polymer blends. Interfacial tension,
g
, is defined as the reversible
work required to create a unit of interfacial area at constant temperature,
T
,
pressure,
P
, and composition,
n
, i.e., [
9 18
]:
1 and the minus (
@
G
g ¼
(2)
@
A
T
;
P
;
n
where
G
is the Gibbs free energy of the system and
A
the interfacial area. Interfacial
tension is, thus, a thermodynamic property of the system and can be calculated
directly from statistical thermodynamic theories. Experimental investigation of
interfacial tension is, therefore, a straightforward means for evaluating the validity
of such theories.
