Degree of cross-linking

The physical properties of the final foam can be varied broadly by controlling the degree of cross-linking in the final polymer as well as the stmcture... 

Macroporous resins are also called macroreticular. Macroporous resins have a measurable porosity. It does not disappear when the resin is dry. Porosity is more dependent on the solvent used when manufacturing the copolymer than on the degree of cross-linking. 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

In the case of phenoHcs, it is possible to make linear thermoplastic polymers called novolaks, but this is done by reaction of less than one mole of formaldehyde with one mole of phenol the resulting resin has a large excess of free phenol. Usually in appHcation hexamethylene tetramine (HEXA) is added to the novolak. When heated, the HEXA breaks down into ammonia and formaldehyde and enters the reaction to form a light degree of cross-links in the final product. 

Combination techniques such as microscopy—ftir and pyrolysis—ir have helped solve some particularly difficult separations and complex identifications. Microscopy—ftir has been used to determine the composition of copolymer fibers (22) polyacrylonitrile, methyl acrylate, and a dye-receptive organic sulfonate trimer have been identified in acryHc fiber. Both normal and grazing angle modes can be used to identify components (23). Pyrolysis—ir has been used to study polymer decomposition (24) and to determine the degree of cross-linking of sulfonated divinylbenzene—styrene copolymer (25) and ethylene or propylene levels and ratios in ethylene—propylene copolymers (26). 

The extent of swelling is inversely related to the degree of cross-linking. 

Polyester Resins. Reinforced polyester resins are thermosets based on unsaturated polyesters from glycols and dibasic acids, either or both of which contain reactive double bonds. The ratio of saturated to unsaturated components controls the degree of cross-linking and thus the rigidity of the product (see Polyesters, unsaturated). Typically, the glycols and acids are esterified until a viscous Hquid results, to which an inhibitor is added to prevent premature gelation. Addition of the monomer, usually styrene, reduces the viscosity to an easily workable level. 

The use of stabilisers (antioxidants) may, however, have adverse effects in that they inhibit cross-linking of the rubber. The influence of phenolic antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C has been studied. It was found that alloys containing 1% of certain phenolic antioxidants were gel-deficient in the rubber phase.The gel-deficient blends were blotchy in appearance, and had lower flow rates compared with the normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added later in the mixing cycle after the rubber had reached a moderate degree of cross-linking. 

In order to obtain cured products with higher heat distortion temperatures from bis-phenol epoxy resins, hardeners with higher functionality have been used, thus giving a higher degree of cross-linking. These include pyromellitic dianhydride IV, and trimellitic anhydride V. 

As a consequence the resins are rather brittle. The high degree of cross-linking does, however, lead to higher heat distortion temperatures than obtained with the normal diglycidyl ether resins. 

The thermal properties of the resin are dependent on the degree of cross-linking, the flexibility of the resin molecule and the flexibility of the hardener molecule. Consequently the rigid structures obtained by using cycloaliphatic resins or hardeners such as pyromellitic dianhydride will raise the heat distortion temperatures. 

The flexible foams discussed in the previous section have polymer stmctures with a low degree of cross-linking. If polyols of higher functionality, i.e. more hydroxyl groups per molecule, are used, tougher products may be obtained and in the case of material with a sufficiently high functionality rigid foams will result. 

As with the flexible foams there has been a shift to the use of polyethers. These are largely adducts based either on trifunctional hydroxy compounds, on tetrafunctional materials such as pentaerythritol or a hexafunctional material such as sorbitol. Ethylene diamine and, it is understood, domestic sugar are also employed. Where trifunctional materials are used these are of lower molecular weight (-500) than with the polyethers for flexible foams in order to reduce the distance between hydroxyl groups and hence increase the degree of cross-linking. 

When the temperamre is lowered, rubbers become stiff and brittle. All rubbers eventually stiffen to a rigid, amorphous glass at the glass transition temperature (Tg). This temperature also indicates the low-temperature service limit of the rubber. Tg values are dependent on the structure, degree of cross-linking (vulcanization) and isomeric composition of the rubber. 

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