Chemistry Reference
In-Depth Information
the particle size distribution after successive polymerizations is a simple expedi-
ent to ensure that later stages have produced polymer that is attached to the previ-
ously existing particles.
Simple sequential processes frequently do not yield particles with the planned
architectures. This is because of the complexity of emulsion polymerizations and
because a system in which different polymers coexist with water will tend to rear-
range toward the composition with the lowest overall surface energy. Theoretical
descriptions of such phenomena
[17
19]
are based on the concept that the final
state of the system consisting of polymer 1, polymer 2, and water (labeled phase 3)
depends on the three interfacial tensions
γ
12
,
γ
23
, and
γ
23
, and the corresponding
interfacial areas
A
ij
. The equilibrium state of the three phases is determined by the
minimum value of the surface free energy,
G
s
:
X
G
s
5
y
ij
A
ij
(10-9)
On a less mathematical, but still very useful, format, various research groups
have presented guidelines for determining which configuration will be the most
stable, in multistage emulsion polymerizations
[20
22]
. These are primarily in
terms of the relative hydrophilicities of the various polymers. The more hydro-
philic polymer will tend to be situated at the interface with water, if it is mobile
enough to get there. Thus, if the first-stage polymer is the more hydrophilic, the
final two-stage structure is more likely to be inverted if the second-stage mono-
mer can dissolve in the first-stage polymer and plasticize it. Also, with persulfate
initiation by ion radicals, many of the polymer chain ends will be ionic [reaction
(10-7)] and can be expected to be located preferentially at the water interface.
Although the thermodynamics of the products of multistage polymerization
determine which structure will be the most stable, it is possible, and frequently
necessary, to produce particle structures that appear nominally to be thermody-
namically forbidden. This is achieved either by changing the surface characteris-
tics of a polymer phase from those of the bulk material or by employing kinetic
factors to produce and anchor energetically unfavored morphologies. Both types
of methods are summarized below.
10.2.5.1
Alteration of Particle Surface Hydrophilicity
Note that the nature of the particle surface need not necessarily be the same as
that of the bulk polymer. (In fact, it seldom is.) This makes it possible to produce
thermodynamically stable species in which the more polar (hence, more hydro-
philic) polymers form the encapsulated phases. The most direct approach is to use
surfactants to modify polymer surface characteristics. This is a preferred method,
when it works, but several cautions must be observed. Remember that the surfac-
tant that is added to the reaction mixture is not the only one that is present, as
pointed out in
Section 10.2.3
. The
surfactant competes with added surfac-
tants for adsorption sites on polymer surfaces and may augment or detract from
the effectiveness of the latter soaps. The surfaces of earlier and subsequent stage
in situ
