Point of Copolymers

Figure 4.J2. Melting points of copolymers of hexamethyleneadipamide and -terephthalamide, and of hexamethylenesebacamide and -terephthalamide. (After Edgar and Hill )...

Statistical copolymers of the types described in Chapter 8 tend to have broader glass transition regions than homopolymers. The two comonomers usually do not fit into a common crystal lattice and the melting points of copolymers will be lower and their melting ranges will be broader, if they crystallize at all. Branched and linear polyethylene provide a case in point since the branched polymer can be regarded as a copolymer of ethylene and higher 1-olefins. 

Figure 4.10 Melting points of copolymers of hexamethylene adipamide and terephthalamide, and of hexame-thylene sebacamide and terephthalamide. (From Edgar, D.B. and Hill, R.J., J. Polym. Sd 8(1), 1, 1952. With...

Figure 13.3. Melting points of copolymers of hexamethy-lene adipamide and terephthalamide, and of hexamethylene sebacamide and terephthalamide (Billmeyer, 1962, pp. 213-214 Edgar and Hill, 1952). Adipamide copolymer with terephthalamide forms isomorphous crystals, whereas seba-camide-terephthalamide copolymers result in more normal melting-point depression curves, which superficially resemble curves characteristic of eutectic behavior.

If there is a tendency to block formation (rj rj > 1), the melting points of copolymers will be higher, and if there is a tendency to alternating addition (r, rj < 1), they will be lower than for random copolymers. 

The classical Flory-Huggin s theory can predict the equilibrium melting point of copolymers. 

One remaining possibility that is less costly from an energy point of view but needs to be carefully controlled is to incorporate additives called flow improvers. These materials favor the dispersion of the paraffin crystals and in doing so prevent them from forming the large networks which cause the filter plugging. The conventional flow improvers essentially change the CFPP and pour point, but not the cloud point. They are usually copolymers, produced, for example, from ethylene and vinyl acetate monomers ... 

In Chap. 4 we discussed the crystallizability of polymers and the importance of this property on the mechanical behavior of the bulk sample. Following the logic that leads to Eq. (4.17), the presence of a comonomer lowers T for a polymer. Carrying this further, we can compare a copolymer to an alloy in which each component lowers the melting point of the other until a minimummelting eutectic is produced. Similar trends exist in copolymers. 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

Peifluorinated ethylene—piopjiene (FEP) lesin [25067-11-2] is a copolymer of tetiafluoioethylene [116-14-3] (TFE) and hexafluoiopiopylene [116-15-4] (HEP) thus its blanched stmctuie contains units of —CF2—CF2— and units of —CF2—CF(CF2)—. It retains most of the desirable characteristics of polytetrafluoroethylene (PTFE) but with a melt viscosity low enough for conventional melt processing. The introduction of hexafluoropropylene lowers the melting point of PTFE from 325°C to about 260°C. 

In order to increase the solubiUty parameter of CPD-based resins, vinyl aromatic compounds, as well as other polar monomers, have been copolymerized with CPD. Indene and styrene are two common aromatic streams used to modify cyclodiene-based resins. They may be used as pure monomers or contained in aromatic steam cracked petroleum fractions. Addition of indene at the expense of DCPD in a thermal polymerization has been found to lower the yield and softening point of the resin (55). CompatibiUty of a resin with ethylene—vinyl acetate (EVA) copolymers, which are used in hot melt adhesive appHcations, may be improved by the copolymerization of aromatic monomers with CPD. As with other thermally polymerized CPD-based resins, aromatic modified thermal resins may be hydrogenated. 

The projected equHibrium melting point of completely linear PE is 146—147°C (5) its highest actual melting point is 133—138°C. In the case of ethylene copolymers with a uniform compositional distribution, the melting point decreases almost linearly with copolymer composition for instance. 

PPO and EOPO copolymers are low hazard—low vapor pressure hquids. Contact with skin, eyes, or inhalation cause irritation. There are no known acute or chronic affects associated with polyols. First aid for contact with polyols involves washing the affected area with water. The flash point of PPO is greater than 93°C. 

Epichlorohydrin (ECO) has exceUent resistance to fuel and oil sweU. The ECOs show a volume sweU of 35% at room temperature compared to 70% for a medium ACN—nitnle mbber in ASTM Reference Fuel C. The copolymer has a low temperature britde point of —40° C and the homopolymer,... 

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