Chemistry Reference
In-Depth Information
polymerization of an acrylonitrile/alkyl acrylate copolymer. High acrylonitrile
contents are used in the first-stage polymerization, and the higher acrylate compo-
sition of the second-stage reaction gives an NAD with superior film properties,
since the acrylate-rich copolymer on the particle surfaces fuses readily.
Early work on production of stable polymer dispersions in hydrocarbon dilu-
ents is summarized in
[1]
. More recently, publications in this field have been
directed at polymerizations of nonpolar monomers in polar media, with the aim
of producing large particles with narrow size distributions for use such as toners
and packings for solid phase peptide syntheses
[2,3]
.
Stabilizers are generally polymers and are added as preformed graft or block
polymers or as a precursor polymer that grafts
during the polymerization.
The polymerization of styrene in ethanol, with poly(vinyl pyrrolidone) (PVP) sta-
bilizer, serves as a useful example here. The starting mixture, containing an initia-
tor like azoisobutyronitrile, is a single-phase solution. When the reaction mixture
is heated to decompose the initiator both PS homopolymer and PVP
in situ
PS graft
copolymers are formed. The growing PS polymers soon reach their solubility
limit in ethanol and associate into unstable nuclei, which collapse together to
reduce the polymer/alcohol interfacial area. This coagulation process continues
until the graft copolymer has been adsorbed in sufficient surface concentration to
inhibit further coalescence. This marks the end of the nucleation period in the
reaction. Once the initial crop of particles precipitates, the reaction can be contin-
ued so that the particles grow without fresh nucleation. This is accomplished by
control of the dispersant concentration at the level needed for the particle size
that is wanted. Insufficient stabilizer results in particle coalescence. In practice,
such control is achieved by establishing empirical relations for particular poly-
merization systems of interest. The end result is particles with insoluble (in the
diluent) PS cores, shielded by anchored surface layers of PVP
PS graft copoly-
mers in which the PVP segments are dissolved in the ethanol medium.
Coagulation of the polymer particles is hindered by
.The
nature of the soluble segment of the stabilizer polymer is not important as long as
it is miscible with the reaction medium, while the counterpart of the stabilizer
and the polymer particle are effectively insoluble. Polymer particles are then sur-
rounded by a shroud of attached, solvated polymeric fibrils. When two particles
with attached soluble segments on their surfaces approach, the number of confor-
mations available to the soluble dispersant polymer is reduced and their entropy
decreases (
steric stabilization
for this compression is negative). Since the corresponding change
in Gibbs free energy is positive (recall that
Δ
S
), the close approach
of sterically shielded polymer particles is energetically unprofitable. In effect, the
higher concentration of dissolved polymer segments in the interparticle solvent
region generates an osmotic pressure (cf. Eq. 3-16), which tends to force the parti-
cles apart. The same phenomenon operates in emulsion polymerization systems,
which are the next topic in this chapter. In the latter case, however, the dispersion
medium is usually water, and steric stabilization typical of NAD reactions is sup-
plemented by ionic repulsions.
Δ
G5
Δ
H2T
Δ
S
