This work is an extension of a communication reported by two of the authors [Carro and Herrera-Ordoñ ez, Macromol Rapid Commun 2006, 27, 274], where bimodal particle size distributions (PSD), obtained by asymmetric flow-field flow fractionation (AFFF, AF 4 ), were taken as evidence of certain degree of stability of primary particles. Now, emulsion polymerizations of styrene were performed under conditions employed before by other researchers, intending to examine if the behavior observed is general. The number of particles (N) and PSD were studied by means of dynamic light scattering and AF 4 . By the later, bimodal PSDs were detected in all cases, where the population corresponding to primary particles (diameter <20 nm) depends on reaction conditions. Regarding N, AF 4 results show that it is constant during interval II, in contrast to DLS results. Primary particle coagulation was evidenced as minimums in N evolution and the rate of polymerization curves, monitored by calorimetry and gravimetry, which are enhanced when higher particle number is generated and/or the ionic strength is increased. These results suggest that particle coagulation is not as extensive as it would be expected according to the coagulative theory. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: [3152][3153][3154][3155][3156][3157][3158][3159][3160] 2010
Summary: The number (N) and size distribution of particles (PSD) of a styrene emulsion polymerization above the CMC were studied by means of asymmetric flow‐field flow fractionation (AF4). Bimodal PSDs were obtained, suggesting that coagulation of the primary particles is not as extensive as would be expected, according to the coagulative mechanism. AF4 allowed it to be demonstrated that N is constant during interval II, and that the resolution limit of other particle sizing techniques can lead to erroneous mechanistic inferences, from the evolution of N.Particle size distribution measured at low conversion for the emulsion polymerization of styrene, obtained by AF4 and DLS. The initial surfactant (S0), initiator (I0) and monomer (M0) concentrations are indicated in the figure.magnified imageParticle size distribution measured at low conversion for the emulsion polymerization of styrene, obtained by AF4 and DLS. The initial surfactant (S0), initiator (I0) and monomer (M0) concentrations are indicated in the figure.
Results
on the shear flow of telechelic associative polymers using
nonequilibrium molecular dynamics (NEMD) are presented. The particle
stream velocities can be calculated from the peculiar velocities and
the imposed velocity profile using a novel approach. The constitutive
relationship stress–shear rate becomes nonmonotonic
when the interaction force between hydrophobic sites is increased.
This condition induces a steady banding flow, which arises under transient
conditions as a local instability originated from the breakage of
micellar aggregates, thus promoting the migration of these aggregates
to regions of low velocity gradients. Here, for the first time, NEMD
simulations predict the banding flow of associative polymers.
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