Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Diffusion copolymerization

Nadareishvili L.I. / Obtaining of multi-canal light-focusing matrix by diffusion-copolymerization of monomers // Proceedings of the academy of sciences of Georgia. Chemical Series. 2000, vol. 26, No 3-4, pp. 157-161 [In Georgian]. [Pg.111]

Of special importance for realization of the controlled gradient formation is an understanding of the reaction mechanism (polymer-analogous transformation /diffusion copolymerization) so that the dmation of the process can be determined in order to control the reaction product. Specifically, the refractive index change with time is investigated, i.e. the function n =J[t) is determined, where n is the refractive index, r is the duration of chemical reaction/diffusion. [Pg.34]

The starting quantitative characteristics for determining the regime of polymer-analogous/diffusion copolymerization are ... [Pg.34]

Value of the refractive index of the process product (polymer-analogous transformation/diffusion copolymerization) ... [Pg.34]

It should be specially noted that when radial distribution of the refractive index is formed from one and the same process (polymer-analogous transformation /diffusion copolymerization), two principally different results may be achieved - obtaining of a polymeric medium with the properties of a convex or concave plate lens. [Pg.35]

Actually, if ri (the initial polymer/homopolymer - the product of prepolymerization of gel-polymer) is greater than (the product of polymer-analogous transformation/diffusion copolymerization), then decrease of the process duration with the radius of the polymeric/gel-polymeric film/plate from its periphery to center gives a medium with properties of a convex lens and, vice versa, at the increase of the process duration with sample radius from periphery to center, a medium possessing properties of a concave lens may be obtained. [Pg.35]

The opposite results are obtained, when polymer-analogous transformation /diffusion copolymerization is accompanied by an increase of refractive index, i.e. when ri > 2 In this case, decrease of the process duration by the sample radius from the periphery to the center produces a medium with properties of a concave lens, and increase gives a medium with properties of a convex lens. In this process, the results mentioned are achieved under conditions of injection of an active medium/diffusate and an inert liquid into the reactor under different regimens. [Pg.35]

To set the required duration of the process (polymer-analogous transformation/ diffusion copolymerization) in given directions on the surface of the gradient carrier a device is used that allows use of a diaphragm mask in the contact zone of the gradient former with the gradient earrier. Several teehnical solutions of application of such devices have been... [Pg.35]

Describing conditions of the equal shrinkage of monomers, equations (84) and (85) give the possibility for simultaneous regulation of changes of monomer overall sizes Q12, S2 and S3) and, consequently, overall size of light focusing elements. The method of diffusion copolymerization of monomers includes positive aspects of the above-mentioned methods of... [Pg.81]

The water solubilities of the functional comonomers are reasonably high since they are usually polar compounds. Therefore, the initiation in the water phase may be too rapid when the initiator or the comonomer concentration is high. In such a case, the particle growth stage cannot be suppressed by the diffusion capture mechanism and the solution or dispersion polymerization of the functional comonomer within water phase may accompany the emulsion copolymerization reaction. This leads to the formation of polymeric products in the form of particle, aggregate, or soluble polymer with different compositions and molecular weights. The yield for the incorporation of functional comonomer into the uniform polymeric particles may be low since some of the functional comonomer may polymerize by an undesired mechanism. [Pg.216]

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. [Pg.495]

The values of these ratios change appreciably by passing from the heterogeneous (suspension) to the homogeneous (DMF) system. In the case of copolymerization in suspension in the presence of the K2S208—AgN03 oxidation-reduction system at 30—40 °C, the ratios were found to be ry = 0,77 0,2 and r2 = 1,09 0,04, whereas in the case of the copolymerization in solution they are = 0,52 and r2 = 1,7. The difference in these values seems to be the result of the different solubility of the monomers in water and of the different rate of diffusion of the monomers to the surface of the precipitated copolymer20. From this it follows that 4 is the more reactive monomer in this binary system. [Pg.103]

Values of 0 required to fit the rate of copolymerization by the chemical control model were typically in the range 5-50 though values <1 are also known. In the case of S-MMA copolymerization, the model requires 0 to be in the range 5-14 depending on the monomer feed ratio. This "chemical control" model generally fell from favor wfith the recognition that chain diffusion should be the rate determining step in termination. [Pg.368]

More complex models for diffusion-controlled termination in copolymerization have appeared.1 tM7j Russo and Munari171 still assumed a terminal model for propagation but introduced a penultimate model to describe termination. There are ten termination reactions to consider (Scheme 7.1 1). The model was based on the hypothesis that the type of penultimate unit defined the segmental motion of the chain ends and their rate of diffusion. [Pg.369]

Surface composition and morphology of copolymeric systems and blends are usually studied by contact angle (wettability) and surface tension measurements and more recently by x-ray photoelectron spectroscopy (XPS or ESCA). Other techniques that are also used include surface sensitive FT-IR (e.g., Attenuated Total Reflectance, ATR, and Diffuse Reflectance, DR) and EDAX. Due to the nature of each of these techniques, they provide information on varying surface thicknesses, ranging from 5 to 50 A (contact angle and ESCA) to 20,000-30,000 A (ATR-IR and EDAX). Therefore, they can be used together to complement each other in studying the depth profiles of polymer surfaces. [Pg.69]

A detailed description of AA, BB, CC step-growth copolymerization with phase separation is an involved task. Generally, the system we are attempting to model is a polymerization which proceeds homogeneously until some critical point when phase separation occurs into what we will call hard and soft domains. Each chemical species present is assumed to distribute itself between the two phases at the instant of phase separation as dictated by equilibrium thermodynamics. The polymerization proceeds now in the separate domains, perhaps at differen-rates. The monomers continue to distribute themselves between the phases, according to thermodynamic dictates, insofar as the time scales of diffusion and reaction will allow. Newly-formed polymer goes to one or the other phase, also dictated by the thermodynamic preference of its built-in chain micro — architecture. [Pg.175]

The effect of hydrophobicity of the polymer on the permeability of poly(2-hydroxyethyl methacrylate (HEMA)-co-methacrylic acid (MAAc) hydrogels was studied [12], The hydrophobicity was controlled by copolymerization with butyl methacrylate (BMA). The dependence of permeability on pH increased as the hydrophobicity increased even though the rate of diffusion decreased. Cross-link density of the hydrogel also contributed to pH-dependent permeability. [Pg.560]

Similar results were obtained by Hunke and Matheson [59], They found that copolymerization of polyurethane with PEG of different molecular weights yielded polymers with different properties than those of polyurethane. Percent hydration values increased with PEG as did the diffusion coefficients of the model drugs (Fig. 19). [Pg.613]

By using lipophilic initiators, such as 2,2 -azobis(isobutyronitrile) (AIBN), in the micro-ECP, diffusion of monomers is too slow compared with the reaction rate. Therefore, copolymerization is confined to the incoherent, lipophilic phase [112,113] and very small microgel particles with a rather uniform size result. [Pg.160]

In non-crosslinking ECP, monomers are supplied to the growing polymer species by diffusion of monomer from droplets. In crosslinking ECP, however, the gel effect increases the copolymerization rate in the droplets as well as in the growing microgel particles. As the diffusion rate of lipophilic monomers in the aqueous phase is lower than the copolymerization rate, monomer droplets may... [Pg.166]

Because the copolymerization of the components of micelles is very rapid, the microgel particles scarcely grow by intermicellar diffusion of the comonomers or by diffusion from the microemulsion droplets. This has been confirmed by the microgel composition [112] which remains constant over the whole reaction time (Fig. 25), even when using different ratios of EUP/comonomer [113,116]. [Pg.171]


See other pages where Diffusion copolymerization is mentioned: [Pg.89]    [Pg.38]    [Pg.75]    [Pg.81]    [Pg.81]    [Pg.89]    [Pg.38]    [Pg.75]    [Pg.81]    [Pg.81]    [Pg.192]    [Pg.508]    [Pg.537]    [Pg.732]    [Pg.366]    [Pg.371]    [Pg.401]    [Pg.525]    [Pg.603]    [Pg.56]    [Pg.22]    [Pg.53]    [Pg.54]    [Pg.869]    [Pg.203]    [Pg.531]    [Pg.600]    [Pg.144]    [Pg.12]    [Pg.169]    [Pg.173]    [Pg.154]    [Pg.176]    [Pg.162]   
See also in sourсe #XX -- [ Pg.34 , Pg.35 , Pg.38 , Pg.81 ]




SEARCH



© 2024 chempedia.info