Particle polymers, measuring

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

Particle-size measurements were accomplished with light-scattering techniques. Typical particle-size distributions were established for each polymer by electron microscopy (5). All polymerizations were conducted at 60.0° C in a thermostated bath having a temperature fluctuation of less than 0.03°C. 

The most convenient of these methods is viscosity measurement of a liquid in which particles coated with a polymer are dispersed, or measurement of the flow rate of a liquid through a capillary coated with a polymer. Measurement of diffusion coefficients by photon correlation spectroscopy as well as measurement of sedimentation velocity have also been used. Hydrodynamically estimated thicknesses are usually considered to represent the correct thicknesses of the adsorbed polymer layers, but it is worth noting that recent theoretical calculations52, have shown that the hydrodynamic thickness is much greater than the average thickness of loops. 

There are various kinds of polytetrafluoroethylene. One is granular polymer consisting of spongy, white particles having a median size of about 600/l The specific surface of this polymer is on the order of 2 m2/g (determined by nitrogen adsorption and calculations by the method of Brunauer, Emmett, and Teller). Since this specific surface area is about 1700 times the observed outer surface of the particles, these measurements confirm the porous, spongelike structure that can be seen in the photomicrograph of a cross section of several particles in Fig. la. 

Figures 6, 7 and 8 show experimental verification of Eq.(40) in batch emulsion polymerization of styrene ( 14). The number of polymer particles was measured by electron micrscopy, not at finite but at 1 hour after the start of polymerization. Figure 6 represents the effect of lowering the initial monomer concentration, Mq on the number of polymer particles formed at fixed initial initiator and emulsifier concentrations. The number of polymer particles formed is constant even if M is lowered to the critical value Mc. This is because normal°condition that micelles disappear before the disappearance of monomer droplets is satisfied in the range of monomer concentration above Mc. The value of Mc can be calculated by the following equation obtained by equating XMc, the monomer conversion where micelles disappear, to XMc2, the monomer conversion where monomer droplets disappear.

The reason why the experimental values of particle number are somewhat lower than the theoretical values seems to be that the time where the number of polymer particles was measured is not at infinite but at only 1 hour after the start of polymerization. Figure 9 shows that the number of polymer particles increases with reaction time. The solid lines represent the theoretical values predicted by the Nomura and Harada model. However, since Nt= 0 when Mq= 0, there would be an optimum value of MQ where the number of polymer particles formed becomes maximum. Unfortunately, it is difficult at present to predict the optimum value of MQ theoretically because any reaction model cannot yet explain perfectly the kinetic behavior at high monomer-conversion range. Therefore, one cannot help determining, at present, the optimum value of MQ experimentally. Figures 7 and 8 also show that Eq.(40) roughly satisfies the experimental results. 

The preservation of particle character and size throughout polymerization itself is very hard to determine. The size of the final polymer particles is easily determined by light scattering or microscopic methods since the dispersions can be diluted without changing the particle size. Measurements of the emulsion droplets in concentrated media on the other hand are a very difficult task and have already been discussed above. 

Two major entry models - the diffusion-controlled and propagation-controlled models - are widely used at present. However, Liotta et al. [28] claim that the collision entry is more probable. They developed a dynamic competitive growth model to understand the particle growth process and used it to simulate the growth of two monodisperse polystyrene populations (bidisperse system) at 50 °C. Validation of the model with on-line density and on-line particle diameter measurements demonstrated that radical entry into polymer particles is more likely to occur by a collision mechanism than by either a propagation or diffusion mechanism. 

Experimental investigations that deal in detail with particle formation in emulsion copolymerization are scarce. Nomura et al. [78] studied the kinetics of particle formation and growth in the emulsion copolymerization of VDC and MMA using NaLS as the emulsifier and KPS as the initiator. The number of polymer particles produced was determined using particle diameters measured by both electron microscopy (TEM) and dynamic light scattering (DLS) for comparison. They found that where Sq and Iq are the initial... 

The number of polymer particles was determined from the monomer conversion Xj and the volume average diameter of the polymer particles dp measured with an electron microscope. 

The growth of gold, palladium, and platinum nanoparticles in the nanostructured matrix of poly(octadecylsiloxane) (PODS) was demonstrated by Bronstein et al.102 PODS, an amphiphilic polymer with nonpolar alkyl chains and hydrophilic silanol moieties (—SiOl OH) forms a bilayer nanostructure composed of these two moieties in its hydrated form. In the presence of an aqueous solution of metal salt (e.g., Na2PdCl4, K2PtCl4, AuC13), the siloxy layer absorbed metal anions by hydrogen bonding (Scheme 5.9). After reduction, the metal ions were reduced into nanoparticles, and TEM results revealed that the nanoparticles were distributed in layer structures. The sizes of the particles were measured to be about 1-2 nm, and the narrow size distribution was attributed to the volume availability within the ordered bilayers of PODS. 

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