Ostwald process polymers

Because the onset of monomer-polymer equilibrium can occur before the filaments achieve their own equilibrium concentration behavior, these filaments will undergo polymer length redistribution. This is a slow process in vitro that in many respects resembles crystallization (See Ostwald Ripening). 

Catalysis refers to the phenomenon by which the rate of a chemical reaction is accelerated by a snbstance (the catalyst) not appreciably consnmed in the process. The term catalysis was coined by Berzelins in 1835 and scientifically defined by Ostwald in 1895, but applications based on catalysis can be traced back to thousands of years ago with the discovery of fermentation to produce wine and beer. Nowadays, catalysts are used in 80% of all chemical industrial processes, and create annual global sales of about 1500 billion dollars and contribute directly or indirectly to approximately 35% of the world s GDP. Catalysis is central to a myriad of applications, including the manufacture of commodity, fine, specialty, petro-, and agro- chemicals as well as the production of pharmaceuticals, cosmetics, foods, and polymers. Catalysis is also an important component in new processes for the generation of clean energy, and in the protection of the enviromnent both by abating environmental pollutants and by providing alternative cleaner chemical synthetic procedures. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

Laplace pressure than the larger bubbles, but as the gas solubility increases with pressure the gas molecules will difTuse from the smaller to the larger bubbles. This process only occurs with spherical foam bubbles, and may be opposed by the Gibbs elasticity effect. Alternatively, rigid films produced using polymers may resist Ostwald ripening as a result of their high surface viscosity. 

This process continues until the crystal reaches a thickness of several thousand A. Crystal growth does not stop even after equilibrium conversion (polymer-monomer equilibrium) is reached. This is explained in terms of the Ostwald ripening. Solubility of particles depends on their size thus smaller particles tend to redissolve (by depropagation) in favour of further growth of already larger ones. This is illustrated by the data shown in Table 7.7. 

Mechanisms of liquid-liquid phase separation were studied in the binary styrene-acrylonitrile copolymer/poly(methyl methacrylate) system. Evidence is presented which suggests that spinodal decomposition occurs in this system. The Cahn theory provides an interpretation of key experimental results. Both a dispersed, two-phase structure and a highly interconnected, two-phase structure can be formed. These two structures coarsen at significantly different rates. The dispersed-phase structure coarsens by Ostwald ripening, an extremely slow process in the polymer-polymer system. The interconnected structure coarsens more rapidly. Data suggest that the mechanism of coarsening is viscous flow driven by interfacial tension. 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

However, it is difficult to control the micelle formation during microemulsion polymerization, hi general, polymerization process is kinetically and thermodynamically unstable because of Ostwald ripening, the growth by collision between monomer droplets and monomer consumption during polymerization [154,155]. It is noteworthy that precise control of the micelle is essential to produce monodisperse and nano-sized conducting polymer nanomaterials. 

The different influence on the two elementary processes leads us to a conclusion that the polyelectrolytes should not be regarded as catalysts, if we follow earlier definition by Ostwald [49] that a catalyst is a substance which influences the backward and forward processes in the same proportioa It seems to be the recent usual practice to use the terminology polymer catalyst , when polymer shows a rate-enhancing ability. To our knowledge, however, no work was reported on the influence of polymers on equilibrium reactions, or in other words, on polymer effect on elementary processes. Thus, the use of the term polymer catalyst is not yet justified. 

Microsuspension polymerization is a process used in the PVC industry to produce resins for plastisols [125], In this process, which resembles miniemulsion polymerization, a mixture of monomer and an oil-soluble initiator are dispersed in an aqueous solution of surfactants using intensive shear. The monomer droplets are polymerized yielding particles usually <2 pm, which are normally isolated by spray drying as they cannot be separated by centrifuging or filtering. These particles are solid and nonporous. The polymer particles are larger than the monomer droplets (0.1-2 pm) because the combined effect of the Ostwald ripening (as no costabilizer is used in the formulation) and droplet/particle coalescence. 

In most cases AAyi2 TAS , which means that AG ° is positive, i.e. the formation of emulsions is nonspontaneous and the system is thermodynamically unstable. In the absence of any stabilization mechanism, the emulsion will break by flocculation, coalescence, Ostwald ripening or a combination of all these processes. This is illustrated in Fig. 3.29 which shows several paths for emulsion breakdown processes. In the presence of a stabilizer (surfactant and/or polymer), an energy barrier is created between the droplets and therefore the reversal from state II to state I becomes non-continuous as a result of the presence of these energy barriers this is illustrated in Fig. 3.30. In the presence of the above energy barriers, the system becomes kinetically stable [85]. As discussed before, the energy barrier can be created by electrostatic and/or steric repulsion which will overcome the everlasting van der Waals attraction. 

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