Crosslinked polymers 830 INDEX

Usually the chiroptical properties of highly crosslinked polymers cannot be measured. The asymmetry of the empty cavities can be analyzed by the excellent racemate resolution ability. By a new method it can now also be directly detected by measuring the optical activity [133], This is measured by suspending the polymer in a solvent which has the same refractive index as the polymer, a technique which was developed for other types of insoluble polymers [141]. The molar optical rotation values thus measured are shown in Table 2-4. 

A variety of techniques have been used to monitor the course of the polymeri zation of diallyl o-phtfaalate. The traditional methods of precipitating the polymer from the pol)mierizing solutions lend themselves only to that portion of the process in which gelation has not yet taken place. Methods of studying postgelation kinetics are few. Generally such studies are made on bulk polymers. Starkweather and Eirich [89] determined the refiactive index of the crosslinked polymer by floating cylindrical samples on solvent mixtures which were adjusted imtil the composition had the same refiactive index as the polymer. Then flie refiactive index of solution was measured with an Abbe... 

Anisotropic mechanical properties are exhibited by ordered, highly crosslinked polymers obtained by bulk polymerization of oriented monomers [117, 119]. These networks show a high degree of ordering and behave anisotropically in a number of physical properties, sueh as the refractive index, the thermal expansion coeffieient, and the modulus of elasticity. Figure 31 shows the tensile moduli in the two major directions for uniaxially oriented sample, and also for an isotropic sample. As can be observed, the modulus in the direetion of moleeular orientation is much higher than in the perpendicular direction. 

The Index Guide gives no information on how crosslinked polymers are indexed, because CAS treats them as copolymers. 

Aryloxyphosphazene copolymers can also confer fireproof properties to flammable materials when blended. Dieck [591] have used the copolymers III, and IV containing small amounts of reactive unsaturated groups to prepare blends with compatible organic polymers crosslinkable by the same mechanism which crosslinks the polyphosphazene, e.g. ethylene-propylene and butadiene-acrylonitrile copolymers, poly(vinyl chloride), unsaturated urethane rubber. These blends were used to prepare foams exhibiting excellent fire retardance and producing low smoke levels or no smoke when heated in an open flame. Oxygen index values of 27-56 were obtained. 

Table I shows a comparison of free film swell Index results as a function of crosslinker at a constant comonomer level (0.3 moles/kg of polymer). ABDA and AEP gave crosslinking performance Identical to a conventional crosslinker, BNMA. However, derivatives which cannot cycllze, either because the amide has an additional substituent, as In 13, or the chain connecting the amide to the blocked aldehyde Is too short, as In 14, did not exhibit efficient crosslinking. They also showed significant discoloration, presumably due to Increased aldol condensation relative to 1, 2, or BNMA.

Cross-linked pre-polymers having fractionalized molecular weights between 35,000 and 70,000 daltons with polydispersity indexes of less than 3.4 have been prepared by the free radical addition of 2-hydroxethyl methacrylate, HEMA, with 2% methacrylic acid or glycerol methacrylate. Once crosslinked, these materials are particularly useful as contact lenses because of their limited shrinkage and expansion. 

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