Emulsion Polymerisation Intervals

During Interval II, the existing particles continue to polymerise and consume the monomer contained in the large monomer droplets. The monomer is transported through the aqueous phase, as the result of a concentration gradient, to the site of polymerisation (i.e., the growing polymer particles). At the end of Interval II, the monomer in the droplets is depleted, and no monomer droplets are present thereafter. 

During Interval III, the monomer already contained in the polymer particles polymerises until it is fully converted to polymer. The concentration of monomer in the polymer particles ([A/]p) drops to essentially zero. 

There may be an autoacceleration in the rate of polymerisation as the result of a Tromsdorff effect near the end of the polymerisation. 

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The polymerisation process, during which polymer particles are nucleated and then grow in size, is theoretically divided into three intervals (as in Figure 2) according to Harkins (a. 12) model of emulsion polymerisation. During Interval 1, new particles are nucleated by radical entry into micelles... 

The kinetics of polymerisation of styrene-in-water microemulsions is investigated using dilatometry. From plots of percentage conversion versus time, the rate of polymerization, Rp, is determined. From log-log plots of Rp versus styrene and initiator (2,2 -azobis(isobutyronitrile), AIBN) concentrations a relationship is estabhshed. The exponents are similar to those predicted by the theory of emulsion polymerisation. The results also show a rapid conversion in the initial period (interval 1) followed by a slower rate at longer times (interval 2). It is suggested that in interval 1, the main process in nncleation of the microemnlsion droplets, whereas in interval 2 propagation is the more dominant factor. The rapid polymerisation of microemnlsions is consistent with their strncture, whereby very small droplets with flexible interfaces are prodnced. 4 refs. 

Venkatesan and Silebi [6] used capillary hydrodynamic fractionation to monitor an emulsion polymerisation of styrene monomer as a model system. A sample taken from the reactor at different time intervals is injected into the capillary hydrodynamic fractionation system to follow the evolution of the particle size distribution of the polymer particles formed in the emulsion polymerisation. After the colloidal particles have been fractionated by capillary hydrodynamic fractionation they pass through a photodiode array detector which measures the turbidity at a number of wavelengths instantaneously, thereby enabling the utilisation of turbidimetric methods to determine the particle size distribution. The particle size measurement is not hindered by the presence of monomer-swollen particles. The shrinkage effect due to the monomer swelling phenomenon is found to be accurately reflected in the particle size measurements. 

Figure 1.1 Cartoon of an emulsion polymerisation b lsed on the Harkins theory. Ingredients are monomer, surfactant, and initiator. The surfactant forms micelles and the initiator is soluble in water. This snapshot is taken during Interval I, when particles are being formed and monomer is present both as free droplets, in aqueous solutions, in micelles and in already formed polymer particles. The surfactant is distributed as dissolved molecules, in micelles, adsorbed on polymer particles and on monomer droplets (to a lesser degree).

In a typical ab initio emulsion polymerisation, the starting emulsion is opaque. After initiator is added, particle formation commences (sometimes the suspension then has a bluish sheen for a few minutes if the newly formed particles are sufficiently small to give rise to Mie-scattering of incident light). As the polymerisation proceeds, the dispersion turns milky white. The number of particles increases in Interval 1, as does the rate of polymerisation. Interval 2 commences when particle formation is finished, thus the particle number is constant and frequently (although not invariably) the polymerisation rate is also constant. 

During the Intervals 1 and 11 of a batch emulsion polymerisation, monomers are divided, that is, partitioned, over the monomer droplets, the aqueous phase and the polymer particles. The monomer that is consumed by polymerisation in the polymer particles is replaced by monomer that is transferred from the monomer droplets through the aqueous phase into the particle phase. In Interval 111, there are no droplets and the monomer is mosdy located in the polymer particles. In the semi-batch processes, monomers are continuously fed into the reactor, usually under starved conditions, namely, at high instantaneous conversions, for example, polymer/monomer ratios close to 90/10 on weight bases. Under these circumstances, only the newly fed monomer droplets are present in the reactor and the life-time of these droplets is short because the monomers are transferred through the aqueous phase to the polymer particles where they are consumed by polymerisation. 

Vl stands for the molar volume of pure monomer 1. Note that molar volume changes of the monomers due to mixing with monomer 2 and/or the polymer have been neglected on going from Equations 4.9 to 4.10. For the mass balance of monomer 2 over the three phases analogous equations as 4.8-4.10 can be derived. Note that in the absence of monomer droplets, for example. Interval III for a batch emulsion polymerisation process. Equation 4.10 reduces to Equation 4.11 ... 

Several studies reported the successful application of reversible chain transfer techniques in water-borne systems. All of these studies apply RCTA species with low Qx constants to control the polymerisation. The alkyl iodides (degenerative transfer) used by several groups (Lansalot et aL, 1999 Butte et al., 2000) have a transfer constant only shghtly higher than unity. The ab initio emulsion polymerisation of styrene using CsFbI was carried out at 70°C (Lansalot et aL, 1999). It was found that the rate of polymerisation was not affected by the presence of CeFial. However, the evolution of Mn with conversion was not in accord with the Qx value. The authors postulated that due to the hydrophobic character of CsFbI, its transfer from droplets to particles was slower than the rate of consumption of CeF I within the particles. To overcome slow diffusion of CsFbI to the particles, the authors carried out miniemulsion (essentially Interval III kinetics), in which polymerisation takes... 

Sequential polymerisation may be operated by producing particles of the first polymer by either emulsion or dispersion polymerisation and by adding the monomer for the second polymer together with free-radical initiator at a slow and controlled rate such that the rate of addition and the rate of polymerisation are equal. The amount of free monomer remains at a low level throughout the process. This monomer-starved process (5) has been used with an aqueous continuous phase in many studies. Samples may be taken at regular intervals to... 

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