Weight of Polymers

MOLECULAR WEIGHT DISTRIBUTION AND AVERAGE MOLECULAR WEIGHT 

The average diameter Dy based on volume (three dimensions) is 

Although Dl reflects the preponderant number of small balls, the 1-in balls represent only about 0.1% of the total volume. Dy reflects the importance of the few large balls, which represent 99% of the volume (and weight) of the system. Incidentally, each 

As in the case of the steel balls, is very sensitive to the concentration of low-molecular-weight species. The weight-average molecular weight is defined as 

In correlating such important polymer properties as viscosity or toughness, M is often a more useful parameter than M . Higher averages are defined as 

The size of macromolecules gives them their unique and useful properties. Size allows polymers to act more as a group so that when one polymer chain moves, surrounding chains are affected by that movement. Size also allows polymers to be nonvolatile since the secondary attractive forces are cumulative (e.g., the London dispersion forces are about 8 kJ/mol of repeat units), and, because of the shear size, the energy necessary to volatilize them is greater than the energy to degrade the polymer. 

FIGURE 3.2 Relationship between molecular weight and polymer physical properties. 

The value of TMWV is dependent on the cohesive energy density (CED) of amorphous polymers, the extent of crystallinity in crystalline polymers, and the effect of reinforcements in polymeric composites. Thus, while a low molecular weight amorphous polymer may be satisfactory for use as a coating or adhesive, a chain length generally above 100 is often required if the polymer is to be used as an elastomer or plastic. 

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The molecular weights obtained by this method are averages. This is particularly evident from the situations where additives are present. In these cases, two different kinds of chains result, with those terminated by the same end group being stunted in growth compared to the normal polycaprolactam. Yet it is the total weight of polymer and the total number of ends that are... 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Molecular weights of polymers are determined by the weight—average molecular weight, and the number—average molecular weight, M. The... 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Molecular weights of polymers that function as bridging agents between particles are ca 10 —10. Ionic copolymers of acrylamide are the most significant commercially (see Acrylamide POLYMERS). Cationic comonomers include (2-methacryloyloxyethyl)trimethylammonium salts, diethyl aminoethyl methacrylate [105-16-8], and dimethyldiallylammonium chloride [7398-69-8], anionic comonomers include acryUc acid [79-10-7] and its salts. Both types of polyacrylamides, but especially the anionic, can be more effective in the presence of alum (10,11). Polyetbylenimine and vinylpyridine polymers, eg, po1y(1,2-dimethy1-5-viny1pyridiniiim methyl sulfa te) [27056-62-8] are effective but are used less frequentiy. 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

Polymers. The molecular weights of polymers used in high energy electron radiation-curable coating systems are ca 1,000—25,000 and the polymers usually contain acryUc, methacrylic, or fumaric vinyl unsaturation along or attached to the polymer backbone (4,48). Aromatic or aUphatic diisocyanates react with glycols or alcohol-terrninated polyether or polyester to form either isocyanate or hydroxyl functional polyurethane intermediates. The isocyanate functional polyurethane intermediates react with hydroxyl functional polyurethane and with acryUc or methacrylic acids to form reactive p olyurethanes. 

Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

FIGURE 6.8 Guidelines for Shodex column selection depending on the molecular weight of polymer. Column Shodex GPC KF-800 series, 8 mm i.d. x 300 mm. Eluent THE. Flow rate 1.0 mUmin. Detector Shodex Rl. Column temp. 40°C. Sample EPIKOTE 828... 

Recently, interesting composite materials incorporating polymeric materials into the sol-gel glasses have been reported by Wilkes and his co-workers [9]. These materials are named ceramers . The properties of ceramers strongly depend on the reaction conditions, i.e., acidity, water content, reaction temperature, the amount of organic polymer, the molecular weight of polymer, solvent, and so on. 

Optimized molecular weight-conversion relationship is related to the system heat transfer coefficient. The degree of conversion improvement from improved heat transfer depends on the average molecular weights of polymer being produced for a given initiator system. 

The development of a rechargeable polymer battery is being pursued worldwide. Its attraction lies in the specific weight of polymers, which is considerably lower than that of ordinary inorganic materials, as well as potential environmental benefits. In principle there are three different types of battery. The active polymer electrode can be used either as cathode (cell types 1, 2), or as anode (cell type 3), or as both cathode and anode (cell type 4) (Fig. 14). As the most common polymer materials are usually only oxidizable, recent research has concentrated on developing cells with a polymer cathode and a metal anode. 

After cooling, the microspheres were washed by decantation with petroleum ether to give a free-flowing powder. They were then sieved, dried, and stored in a freezer. Size distribution can be controlled by the stirring rate the yield is 70-90%. The process was quite reproducible with respect to yield, size, and loading distribution, if the same molecular weight of polymer was used. Less than 5% error was observed (5). 

In the view of molecular weight and molecular weight distribution we found that, the molecular weight of polymer obtained fix>m MMAO system p.ve the larger munber of molecular weight than the MAO system. This phenomenon was attributed to the present of TMA and TIBA in MAO and MMAO respectively. The molecular wei t distribution was independent with any almninoxane type. However, it essentially depended on typ i of cocatalyst (aluminoxane and borate)... 

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