Molecular weight of nylon

Why is acetic acid used to control the molecular weight of nylons during polymerization ... 

Table I shows the effect of the extent of reaction p on the molecular weight of nylon 66.

Figure 3, which shows the effect of nonfunctional end groups on the molecular weight of nylon 66.with equal amine and carboxyl end-group concentrations at 50 eq/10° g, illustrates the necessity for the absence of monofunctional impurities in condensation polymer intermediates if maximum molecular weight is to be obtained. 

By playing around with Equation 6.14 we see how the degree of polymerization and hence the molecular weight of the product is influenced by a proper control of the purity of reactants and prevention of extraneous reactions. In practice the molecular weight of nylons can be stabilized by the deliberate addition of a predetermined amount of a monofunctional monomer like acetic acid. 

The molecular weights of nylon-6 are generally in the same range as nylon-6,6. Its molecular weight distribution as prepared under the above conditions is also the same as linear nylon-6,6 and other linear condensation polymers with M /M = +p,or about two. However, nylon-6 and AB polyamides are unique in that a narrower molecular weight distribution can be attained by adding a bifunctional stabilizer such as the Bb-type dicarboxylic add [37]. An AB polymer chain can only pick up one BB unit and becomes terminated with B end groups at both ends. For example, if 40 equiv. 10 g. of BB units are added and the concentration of A ends [NH2] is reduced to 10 equiv. 10 g, M jM = 1.6. It would be possible to increase by 25% in that case. 

The molecular weight of nylon polymers is determined from viscosity measurements by use of the Mark-Houwink equation, given by n- KM (3)... 

During this cold-drawing process, vegetable oil is applied to the yarn as a lubricant and is washed off afterwards. In the case of nylon 6 melt-spinning is also used. The filaments are normally drawn to 350 00%. If the molecular weight of nylon 6 is between 20 000 and 25 000, hot-drawing is used. Unlike PET, both nylon fibers are fairly crystalline as spun. The crystal structures of nylon 66 and 6 fall into the categories of a, ji and 7. Almost all the commercially important nylons exist in either the a or the /3 form. 

The analyses just outlined show that the most reliable relationship which can be recommended for the determination of the viscosity-averagB molecular weight of nylon 6 and 6,6 is in m-cresol If it is necessary to make the... 

In order to determine the molecular weight of nylon-6 polymer in the nanocomposite, it was assumed that the viscosity of nanocomposite solution had been altered by the carbon nanotubes according to the theory of dilute Brownian rods. " The intrinsic viscosity of nylon-6 polymer in the nanocomposite was determined by excluding the effect of carbon nanotubes " ... 

Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerisation process. The third, the soHd-phase polymerisation process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

Oxidation. AH polyamides are susceptible to oxidation. This involves the initial formation of a free radical on the carbon alpha to the NH group, which reacts to form a peroxy radical with subsequent chain reactions leading to chain scission and yellowing. As soon as molten nylon is exposed to air it starts to discolor and continues to oxidize until it is cooled to below 60°C. It is important, therefore, to minimize the exposure of hot nylon to air to avoid discoloration or loss of molecular weight. Similarly, nylon parts exposed to high temperature in air lose their properties with time as a result of oxidation. This process can be minimized by using material containing stabilizer additives. 

Caprolactam is an amide and, therefore, undergoes the reactions of this class of compounds. It can be hydrolyzed, Ai-alkylated, O-alkylated, nitrosated, halogenated, and subjected to many other reactions (3). Caprolactam is readily converted to high molecular weight, linear nylon-6 polymers. Through a complex series of reactions, caprolactam can be converted to the biologically and nutritionally essential amino acid L-lysine (10) (see Amino acids). 

Since the molecular weight of this nylon-6,6, produced by Carothers, was higher than he desired, 1% acetic acid (a monofunctional acid that acted to terminate chain growth) was added, resulting in a product with a lower molecular weight and a melting point of about 200°C. 

Calculate the intrinsic viscosity for these two polymers. The Mark-Houwink constants for this system are known to be a = 0.72 and k = 11 x 10 4 dl g-1 calculate the molecular weights of the two nylon fractions. 

Polyolefins. Ultra-High-Molecular-Weight Polyethylene.TIus is easily made by conventional low-pressure coordination polymerization. Hercules Hi-fax 1900 has a molecular weight of 2.5-5.0 million. Its outstanding properties are low coefficient of friction (0.11) abrasion-resistance superior to nylon, polyurethane, and steel unbreakable in the Izod notched impact test and high resistance to most inorganic and many organic chemicals. 

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