Polymer concentration, monomer

Figure 1 Is a flow sheet showing some significant aspects of the Iterative analysis. The first step In the program Is to Input data for about 50 physical, chemical and kinetic properties of the reactants. Each loop of this analysis Is conducted at a specified solution temperature T K. Some of the variables computed In each loop are the monomer conversion, polymer concentration, monomer and polymer volume fractions, effective polymer molecular weight, cumulative number average molecular weight, cumulative weight average molecular weight, solution viscosity, polymerization rate, ratio of polymerization rates between the current and previous steps, the total pressure and the partial pressures of the monomer, the solvent, and the nitrogen.

During Stage I the number of polymer particles range from 10 to 10 per mL. As the particles grow they adsorb more emulsifier and eventually reduce the soap concentration below its critical micelle concentration (CMC). Once below the CMC, the micelles disappear and emulsifier is distributed between the growing polymer particles, monomer droplets, and aqueous phase. 

To maintain a high polymerization rate at high conversions, reduce the residual amount of the monomer, and eliminate the adverse process of polyacrylamide structurization, polymerization is carried out in the adiabatic mode. An increase in temperature in the reaction mixture due to the heat evolved in the process of polymerization is conductive to a reduction of the system viscosity even though the polymer concentration in it rises. In this case, the increase in flexibility and mobility of macromolecules shifts the start of the oncoming gel effect into the range of deep transformation or eliminates it completely. 

Polymerizations Above Tg. Let the polymerization begin in pure monomer. As the concentration of polymer chains increases initially one observes a relatively small increase in the termination rate constant. This is related to the effect of polymer concentration on coil size. A reduction in coil size increases the probability of finding a chain end near the surface and hence causes an increase in k-. Soon thereafter at conversions 15-20 polymer chains begin to entangle causing a dramatic reduction in radical chain translational mobility giving a rapid drop in k-j. ... 

Low Conversion Reactors. The major problem in temperature control in low conversion reactors is the orders cf magnitude increase in viscosity as the conversion increases. Fig.8 shows the viscosity of a polystyrene solution as the function of percent PS. The data are for polystyrene with a Staudinger molecular weight of 60,000 at 100 C and 150 C in a cumene solution, a satisfactory analog for styrene monomer solutions. As the polymer concentration increases from 0 to 60%, viscosity increases from about 1 cp to 10 cp. 

N umerical simulations of reactor start-up were programmed, predicting monomer and initiator concentrations, total polymer concentration, weight and number average molecular weights, viscosity and population density distribution dynamics. The following two relationships obtained from steady state observations were utilized in the simulation. 

This is a free-radical polymerization with short chain lives. The first molecules formed contain nearly 58 mol% styrene when there is only 50% styrene in the monomer mixture. The relative enrichment of styrene in the polymer depletes the concentration in the monomer mixture, and both the polymer and monomer concentrations drift lower as polymerization proceeds. If the reaction went to completion, the last 5% or so of the polymer would be substantially pure polyacrylonitrile. 

The large viscosity increases that accompany increased polymer concentrations have a strong effect on reactor performance. This phenomenon is illustrated through a simplified yet realistic example (also used in Reference 1 to study the effects of radial convection). In this case the polymerization rate is first order in monomer concentration and the physical properties are constant, except for viscosity, which is given by the following expression ... 

Fractionation by GPC was sessed by changing injected concentrations and by GPC analysis of polymer before and after fractionation. Efficiency of fractionation of polymer from monomer did not appear to be a source of error. In fact, an advantage of this method over others is that the separation of monomer from the polymer can be clearly monitored and controlled,... 

Initiator s concentration Monomer s concentration Chain radical concentration Dead polymer chain of x units Growing polymer chain of y units Initiator free radical Absolute temperature Reactor volume... 

Species present during polymerization Monomer, polymer and propagating chains (at very low concentration) Monomer, dimer, trimer, oligomer, polymer... 

Figure 2. Reduced transition temperatures as a function of PPDA monomer and polymer concentration for the isotropic to smectic A transition before (O) and after polymerization ( ) and for the smectic A to smectic C transition transition before ( ) and after polymerization ( ).

At the end of the polymerisation, when species IA has disappeared and ions are present, the addition of styrene makes the ions vanish instantaneously and they remain absent whilst polymerisation proceeds. Moreover, this polymerisation has the same rate constant as the first. This means that it cannot have been initiated only by the acid that was free at that time and that the acid bound as ions must also have become available. These facts are represented by the reaction paths leading to esters IB and IC, which complete the cycle whereby eventually ions are formed again, and can be destroyed again by addition of more monomer. Of course, reaction of the freshly added monomer with the then free acid leads to formation of ester IA. The maximum concentration of carbonium ions increases after each addition because of the increasing double bond concentration, as the polymer concentration increases. Thus the final value of the equivalent conductance and... 

Auto-acceleration was observed in the homopolymerization of methacrylic acid solutions over limited concentration ranges in methanol and in water. Perhaps under such conditions swelling of the polymer favors monomer diffusion leading to a larger amount of pre-oriented structures III. Alternatively, a monomer-solvent complex may arise which favors a pre-oriented structure and thus, may be responsible for the onset of a matrix effect (9). 

It should be noted that the rate of change of fluorescence intensity and the fluorescence enhancement factor depend on the polymerization conditions (temperature, initiator concentration, monomer reactivity) and on the nature of the polymer formed. 

On expressing both the polymer and monomer concentrations in terms of the fraction polymerised (or extent of reaction, p = )> they reduced Eq. 5.27 to... 

The gel was prepared by mixing 66 ml of gel buffer (0.01 M Cd(C104)2, 0.01 M sodium polyphosphate, pH 11) 7.4 ml acrylamide solution (44.4 g of acrylamide, 1.2 g of methylene-bisacrylamide in 100 ml water) and 62.4 ml water in round bottom flask and deaerated for 5 min with a water pump. Then 1.2 ml of freshly prepared ammonium persulfate solution (150 mg in 10 ml water) and 0.1 ml of N,N,N, N tetramethylethylenediamine (TEMED) were added and mixed carefully to avoid the introduction of air. Finally, 100 ml of the resulting solution was poured carefully, to minimize the introduction of air, into a 25 mm-diameter tube. The monomer solution was overlayered with water to remove the meniscus and reduce the entry of air. The polymerization was allowed to proceed overnight at room temperature. The resulting gel, having a polymer concentration of 2.5% and a cross-link density of 2.6%, was used in a home made apparatus. ... 

Relative fluorescence intensities and spectra at a series of pH values are shown in Fig. 12, and some pertinent parameters are summarized in Table IX. At a specific pH the fluorescence intensity as a function of wavelength has been expressed as relative to the intensity at the maximum at the same pH. The maximum intensities were different at different PH values. Nevertheless, the ratio of intensities of dimer and monomer, ID//M, being independent of polymer concentration, may be compared for all the samples and pH values. 

---