Non-equilibrium particle morphology development in seeded emulsion polymerization. 1: penetration of monomer and radicals as a function of monomer feed rate during second stage polymerization (original) (raw)

Journal of Colloid and Interface Science

Journal of Colloid and Interface Science

The interfacial tension of three different binary polymer blends has been measured as function of time by means of a pendent drop apparatus, at temperatures ranging from 24 • C to 80 • C. Three grades of polybutene (PB), differing in average molecular weight and polydispersity, are used as dispersed phase, the continuous phase is kept polydimethylsiloxane (PDMS), ensuring different asymmetry in molecular weight across the interface. The interfacial tension changes with time and, therefore, this polymer blends can not be considered fully immiscible. Changes in interfacial tension are attributed to the migration of low-molecular weight components from the source phase into the interphase and, from there, into the receiving phase. In the early stages of the experiments, just after the contact between the two phases has been established, the formation of an interphase occurs and the interfacial tension decreases with time. As time proceeds, the migration process slows down given the decrease in driving force which is the concentration gradient and, at the same time, molecules accumulated in the interphase start to migrate into the "infinite" matrix phase. A quasi-stationary state is found before depletion of the lowmolecular weight fraction in the drop occurs and causes the interfacial tension σ (t) to increase. The time required to reach the final stationary value, σ stat , increases with molecular weight and is a function of temperature. Higher polydispersity leads to lower σ stat and a weaker dependence of σ stat on temperature is found. A model coupling the diffusion equation in the different regimes is applied in order to interpret the experimental results. Numerical solutions of the diffusion equation are proposed in the cases of a constant and a changing interphase thickness. In the latter case, the interphase is defined by tracking with time a fixed limiting concentration in the transient concentration profiles and the variations found in σ (t) are attributed to the changes in the interphase thickness. A discrete version of this continuous model is proposed and scaling arguments are reported in order to compare the results obtained with the predictions of the continuous model. The kinetic model as proposed by Shi et al. [T. Shi, V.E. Ziegler, I.C. Welge, L. An, B.A. Wolf, Macromolecules 37 -1599] appears as a special case of the discrete model, when depletion is not taken into account. Using the models, time scales for the diffusion process can be derived, which fit the experimental results quite well.

Fundamental properties of colloidal unimolecular polymer particles

2013

Colloidal Unimolecular Polymer (CUP) particles are a new genre of material formed by self-assembly into spheroidal particles due to the effect of hydrophilic/hydrophobic interactions of the polymer with a change in the solvent. The particle sizes were characterized by means of Dynamic Light Scattering with corrected the distribution of molecular weight proving that the polymer had undergone unimolecular collapse. The rheology study shows the presence of surface water and its significant effect on the rheology. The primary and secondary electroviscous effects were also found to play roles when the suspension was in dilute to semi-dilute regime. When the volume fraction of particles were higher than 0.15, the rheology behavior fit well with Krieger-Dougherty equation. The thickness and the density of surface water were calculated to be 0.57nm and 1.0688 g/cm 3 respectively. When small amounts of external electrolytes were added, (<2% by weight), the viscosity of the suspensions (<0.06 by volume) dropped due to screening effect of the added electrolytes. When the volume fraction of the suspension increased or the addition of electrolytes increased, the viscosities of suspension increased sharply at different critical points. The CUPs showed remarkable surface activities, which increase with molecular weights. The calculated average equivalent area occupied by each particle was much smaller than that of the largest cross-section of the particle indicating that the liquid-solid interface contact angle of the particle was quite low, close to 10. v ACKNOWLEDGMENTS I wish to express my gratitude and appreciation to my advisor, Dr. Michael Van De Mark, for his continuous guidance and support during my pursuit of graduate studies at Missouri S&T. I admire his creative thinking, critical attitude, and passion for science and research which benefit me in conducting research, learning and creating knowledge for science.

Fundamentals of Interface and Colloid Science

Studies in Interface Science, 2010

Microemulsions are macroscopically isotropic mixtures of at least three components: water, oil and surfactant. They are single thermodynamically stable phases, different from ordinary emulsions (chapter 8). Microscopically, the surfactant molecules form an extended interfacial monolayer separating the water from the oil molecules. The preferential adsorption of the surfactant reduces the interfacial tension between the polar and non-polar solvent effectively to zero, which, in turn, permits thermal energy to disperse the two incompatible solvents into each other. The general features of the phase behaviour of microemulsions are best introduced by considering the following simple experiment. A simple experiment: We take a test tube with equal amounts of water and oil. As water and oil do not mix, we see two phases, water (A) forming the bottom phase, oil (B) forming the phase on top. This situation is shown by the test tube furthest on the left in fig. 5.1. When we add a surfactant, it has, in principle, three options. It can dissolve in the water phase (fig. 5.1, tube I), it can dissolve In the oil phase (fig. 5.1, tube II) or it can make up its own phase (fig. 5.1, tube III). These situations are frequently observed and are denoted by the three Winsor states (I, II, III), after Winsor, who was the first to study this behaviour systematically ^\ The surfactant rich phase is called the microemulsion. We will explain further how to select the components to achieve a desired microemulsion type, which structures and properties to expect and provide hints for applications of microemulsions. As illustrated in fig. 5.1, the first observation dealing with mixtures of water, oil and surfactant is the spontaneous appearance of different phases. Therefore studying the phase behaviour and the construction of phase diagrams is the first step. A phase diagram may be viewed as the road map for the researcher and helps him to reach his destination or goal. For example, the Gibbs triangle on the left-hand side of fig. 5.1 indicates that at intermediate temperatures a hydrophilic surfactant system is over wide composition regions in the Winsor I (2)-state. The Gibbs triangle on the right-' PA. Winsor, Solvent Properties of Amphiphilic Compounds, Butherworth & Co. (1954).

Trends in Colloid and Interface Science XXIV

2011

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