Nylon crystalline melting point

Table 14.4 Approximate Crystalline Melting Points of Selected Polyamides (Nylons)...

PolyBCMO is a light-coloured thermoplastic with a crystalline melting point of 181 °C. Its mechanical properties are close to those of Nylon 6 (impact strength is poorer) as well as to those of low density polyethylene. 

In terms of polymer matrices for composite materials, there will be a compromise between solvent and water resistance. Thus non-polar resins are likely to be less resistant to hydrocarbon solvents, which have low polarity, but more resistant to moisture absorption. Polar resins behave in the opposite way. Strongly polar solvents, such as dimethyl sulphoxide or similar, can interact with polar structures in the resin and are difficult to resist. Crystalline thermoplastic polymers are often better for such applications. For example, polyethene will only dissolve in hydrocarbon solvents (of similar solubility parameter) at temperatures above the crystalline melting point. Polar semi-crystalline polymers such as the polyamides or nylons can be dissolved in highly polar solvents, such as cresol, because of a stronger interaction than that between molecules within the crystallites. High performance thermoplastic polymers such as polyether ether ketone (PEEK) have been promoted for their resistance to organic solvents (see Table 3.5) [12], The chemical resistance of unsaturated polyester and vinyl ester and urethane resins is indicated in Table 3.6 [15]. 

Like their nylon counterparts, aromatic polyesters are polar polymers with relatively high crystalline melting points and are soluble in only two classes of solvents, high-temperature solvent blends and HFIP blends. The high-temperature solvents or blends are w-cresol, o-chlorophenol/chloroform, and nitrobenzene/tetrachloroethane. The fluorinated solvents or blends include HFIP, HFIP/methylene chloride (30 70), HFIP/pentafluorophenol, and HFIP/chloroform (1 9). 

Example 6.4 Discuss how the crystalline melting point varies with n in the nylon ... 

Solution. Increasing values of n dilute the nylon linkages that are responsible for interchain hydrogen bonding, and thus should lower AH and the crystalline melting point. As n goes to infinity, the structure approaches that of linear polyethylene. This should represent the asymptotic minimum T, with the chains held together only by van der Waal s forces. 

The glass transition temperatures of the nylons appear to be below room temperature so that the materials have a measure of flexibility in spite of their high crystallinity under general conditions of service. The polymers have fairly sharply defined melting points and above this temperature the homopolymers have low melt viscosities. Some thermal properties of the nylons are given in Table 18.4. 

Crystalline polar polymers and solvents It has already been pointed out that at temperatures well below their melting point crystalline non-polar polymers will not interact with solvents, and similar considerations can apply to a large number of polar crystalline polymers. It has, however, been possible to find solvents for some polar, crystalline polymers such as the nylons, polyvinyl chloride and the polycarbonates. This is because of specific interactions between polymer and solvent that may often occur, e.g. by hydrogen bonding. 

Many aliphatic poly(amides), more commonly known as nylons, exhibit an unusual phase transition below their melting point. First noted by Brill [122], the phenomenon has been studied extensively [128-131]. It is observed for instance in nylon (6,6) at a temperature of about 210°C, when the stable low-temperature triclinic oc phase of the crystalline polymer changes to a pseudo-hexagonal phase. 

ADIPIC ACID. [CAS 124-04-9], Adipic acid, hexanedioic acid, 1,4-bulanedicarboxylic acid, mo] wL 146.14, HOOCCH2CH2CH2CH2COOH, is a white crystalline solid with a melting point of about 152°C. Little of this dicarhoxylic acid occurs naturally, but it is produced on a very large scale at several locations around the world. The majority of this material is used in the manufacture of nylon-6,6 polyamide, which is prepared by reaction wilh 1,6-hexanediamine. 

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