Semi-Batch Emulsion Copolymerization of Vinyl Acetate and Butyl ...

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2617
Figure 1. The effect of surfactant structure on (a) the instantaneous
monomer conversion and (b) the average particle size
(surfactant concentration 1 wt.-%; initiator addition policy
0.25–0.25 wt.-%).
presence of polymeric stabilizers with a high PEO content
(e.g., a high ratio of hydrophilic/hydrophobic moieties)
due to the decrease of surface coverage. On the
other hand, particle destabilization was also observed at a
low PEO content of polymeric stabilizers and was attributed
to the insufficient amount of stabilizing ethylene
oxide segments per adsorbed surface area. [15] Furthermore,
an increase in the surfactant molecular weight
resulted in a shift in the optimum value of the hydrophilic/hydrophobic
chain length ratio and gave rise to a
slight increase of the average particle size and polymerization
rate.
Figure 1a and 1b illustrate the effect of the molecular
structure of the surfactant on the instantaneous monomer
conversion and average particle size of latexes produced
using the standard recipe. As can be seen in Figure 1a,
the polymerization initially proceeds under non-starved
conditions. After an approximate polymerization time of
90–120 min, depending on the surfactant type, the polymerization
continues at an almost maximum rate (e.g.,
the instantaneous monomer conversion is higher than
80%). It should be noted that surfactants S2 and S3
resulted in higher polymerization rates than that of surfactant
S1 (Figure 1a). The lower polymerization rate
observed for surfactant S1 was attributed to the reduced
oligomer radical entry rate through the “viscous layer” of
the adsorbed nonionic surfactants. [4] A similar behavior
has been observed for mixed surfactant systems in which
the monomer conversion was found to be lower than that
obtained with simple anionic surfactants even though the
former provided better stabilization and an increased particle
number. [4]
The effect of the surfactant molecular structure on the
latex particle size was found to be significant (Figure
1b). The final average particle size varied by more than
100 nm depending on the surfactant structure (e.g., the
values of N C and N S ). It is noteworthy to notice that surfactant
S1, despite having a shorter adsorbing chain segment,
N C , than surfactant S2 and a shorter stabilizing
chain, N S , than surfactant S3, provided the most effective
particle stabilization. Assuming that surfactants S1 and
S2 have similar surface coverages, the observed increase
in particle stabilization in the presence of surfactant S1
can be attributed to the larger number of stabilizing
chains per unit adsorbed area. On the other hand, because
of the substantially different values of the hydrophobic/
hydrophilic segment ratios for surfactants S1 and S3
(e.g., 4.5 and 2.3, respectively) the surface coverage of
surfactant S3 will be lower than that of surfactant S1,
resulting in a reduced particle stabilization ability.
The above arguments were confirmed by comparison
of the experimental data to the predictions of a comprehensive
mathematical emulsion copolymerization model
that included the effect of steric stabilization. [16] The
values of the saturated surface coverages were obtained
on the basis of best fit of theoretical stabilization model
predictions to experimental data. It was found that the
surfactant volume fraction of the adsorbed layer, f, varied
with the hydrophobic/hydrophilic segment ratio, N C /N S ,
according to the following expression:
u ¼ N C
N S
0:107 0:0111 N
C
0:135
N S
ð1Þ
Notice that for a value of (N C /N S ) ratio of 4.8, Equation
(1) exhibits a maximum value of 0.123 with respect to the
volume fraction u. Furthermore, it was found that the diffusional
radical entry rate to the particles increased with a
decrease of the volume fraction of the adsorbed layer, u. [16]
Based on the above observations, surfactant S1 will provide
the best stabilization, for it has the largest value of u
and exhibits the lowest polymerization rate due to the largest
“viscous layer” resistance to the radical entry rate.
Effect of Surfactant Concentration
Figure 2a and 2b illustrate the effect of concentration of
surfactant S1 on the instantaneous conversion and the