Molecular mass rubbers

As with polybut-l-ene and many other vinyl monomers that contain an asymmetric carbon, isotactic, syndiotactic and atactic stmctures may be drawn. Using co-ordination catalysts such as mixtures of cobalt chlorides, aluminium alkyls, pyridine and water high-1,2 (high vinyl) polymers may be obtained. One product marketed by the Japan Synthetic Rubber Company (JSR 1,2 PBD) is 91% 1,2, and 51-66% of the 1,2 units are in the syndiotactic state. The molecular mass is said to be several hundred thousand and the ratio MJM is in the range 1.7-2.6. 

Both polyethers and polyesters may be used as polyols. For example, Du Pont use polytetrahydrofuran for Lycra whilst US Rubber originally used a polyester of molecular mass of about 2000 obtained by condensing adipic acid with a mixture of ethylene and propylene glycols. A polyether-based mixture was used for Vyrene 2 introduced in 1967. All the polyols have terminal hydroxyl groups. 

Along with such materials as plastics, adhesives, fibers, and coatings. rubber is polymeric in nature. Such materials consist of long chains, with molecular masses generally of the order of 50,000 to 500,000 g/mol. Common rubbery materials—often called elastomers— include automotive tires and rubber bands. 

The effective molecular mass Mc of the network strands was determined experimentally from the moduli of the polymers at temperatures above the glass transition (Sect. 3) [11]. lVlc was derived from the theory of rubber elasticity. Mc and the calculated molecular mass MR (Eq. 2.1) of the polymers A to D are compared in Table 3.1. 

Polymers are examples of organic compounds. However, the main difference between polymers and other organic compounds is the size of the polymer molecules. The molecular mass of most organic compounds is only a few hundred atomic mass units (for reference, atomic hydrogen has a mass of one atomic mass unit). The molecular masses of polymeric molecules range from thousands to millions of atomic mass units. Synthetic polymers include plastics and synthetic fibers, such as nylon and polyesters. Naturally occurring polymers include proteins, nucleic acids, polysaccharides, and rubber. The large size of a polymer molecule is attained by the repeated attachment of smaller molecules called monomers. 

Pol5miers are high molecular mass substances consisting of large numbers of repeating structural units. They are also called as macromolecules. Some examples of pol)miers are pol5rthene, bakelite, rubber, nylon 6, 6, etc. 

Synthetic polymers are man-made high molecular mass macromolecules. TTiese include synthetic plastics, fibres and rubbers. The two specific examples are polythene and dacron. 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

Determining the Molecular Mass of Carbon Dioxide. Carbon dioxide can be prepared in a Kipp gas generator or taken from a cylinder in which it is stored under pressure. To determine the molecular mass of carbon dioxide, take a dry flat-bottomed 500-ml flask with a tightly fitting rubber stopper. Use a rubber band or a pencil for writing... 

Polymers are very soft materials which are not easily milled under normal conditions. The mastication of natural rubber is the reduction of molecular weight by milling or cutting, the process being invented by Thomas Hancock as early as 1820. Even today this is a major industrial process in the tire and rubber industry. Other polymers are milled under reduced temperature or even in liquid nitrogen to achieve a controlled molecular mass. The degradation of several polymers has been investigated by Dimitrov et al. [104]. 

Chain entanglements are the cause of rubber-elastic properties in the liquid. Below the "critical" molecular mass (Mc) there are no indications of a rubbery plateau. The length of the latter is strongly dependent on the length of the molecular chains, i.e. on the molar mass of the polymer. From the shear modulus of the pseudo rubber plateau the molecular weight between entanglements may be calculated ... 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

Summary Comprehensive investigation of the influence of nine modifiers on the properties of fluoro- and phenylsiloxane rubber was carried out. The synthesized modifiers were analyzed by DTA, TGA, and Si NMR methods. Modifiers polydispersivity was evaluated in terms of molecular-mass distribution (MMD). The modifiers were found to produce a selective effect on the performance of phenyl- and fluorosiloxane-based rubbers, and at their optimum content an improvement in most rubbCT properties was observed. The optimum amount of the introduced modifier depends on its structure. 

Table 3. Effect of modifier s molecular mass on fluorosiloxane rubber properties.

The effect of synthesized modifiers on fluorosiloxane rubbers is determined on the basis of fluorosiloxane rubber CKTFT-100 As seen fix)m data presented in Table 3, characteristics of fluorosiloxane rubber depend on quantity of introduced modifier and its molecular mass Properties of rubber compounds are highly dependent on modifier concentrations. Optimal modifier content amounts to 6 - 7%. Increase in molecular mass of modifier on transition from modifier II to modifier VI leads to increase in tear strength and relative elongation in comparison to the control specimen (modifier concentration = 0). Also, after thermal treatment at 250 °C for 24 h in the presence of modifier VI, improvement of all fluorosiloxane rubber compound properties is observed. 

Figure 2. Product map (average molecular mass vs. copolymer composition) and applications. EPR = ethylene/propylene rubber PE = polyethylene HD = high-density ...

In addition to titanium-based Ziegler-Natta catalysts, vanadium-based systems have also been developed for PE and ethylene-based co-polymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow molecular mass distribution PE, whereas supported V catalysts give broad molecular mass distribution.422 Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl co-catalyst.422,423... 

The molecular mass of a compound used to prepare silicone rubber is 296.6 amu. The empirical formula of the compound is C2H6SiO. What is its molecular formula ... 

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