Crosslinked hydrocarbon Polymer

In addition to the vulcanization of diene hydrocarbon polymers using sulfur, other methods of crosslinking hydrocarbon polymers, which do not require a double bond and which do not use sulfur have been developed. Thus, saturated hydrocarbon polymers and, in particular, PE, are crosslinked by reactions resulting from the addition of a peroxide to the polymer at elevated temperatures (6). 

Automobile tires are the largest-volume mbber material manufactured, and they are formed from natural or synthetic monomers with sulfur compounds added to crosslink the polymer within the tire mold. Tires also contain large amounts (typically 35%) of carbon black particles, which are made by hydrocarbon combustion in excess fuel, as we have considered previously in Chapters 9 and 10. 

Polyisobutylene. The polymers which we have dealt with until now are of the type which mainly crosslink under the influence of radiation in vacuo. Polyisobutylene with one tetrasubstituted carbon in each repeat unit can be considered as the simplest hydrocarbon polymer of the other type—i.e., the polymers degraded by radiation. 

Therefore, the improvement obtained in the solid state properties of a hydrocarbon polymer by crosslinking have been highly appreciated. However, crosslinking causes a tolerable loss in fabricabil-ity. Crosslinking reduces the crystallinity of saturated hydrocarbon polymers, thereby decreasing the stiffness and rigidity of the product. 

With the exception of elastomeric hydrocarbon polymers, other crosslinked polymers besides of ionomers have found little commercial success as compared to the uncrosslinked hydrocarbon polymers. 

The efficient light-initiated decomposition of azides has been the basis for commercially important photoresist formulations for the semiconductor industry. A common approach is to mix a diazide, such as diazadibenzylidenecyclohexanone (I), with an unsaturated hydrocarbon polymer. Excitation of the difunction-al sensitizer produces highly reactive nitrenes which crosslink the polymer by a variety of paths including insertion into both carbon-carbon double bonds and carbon-hydrogen bonds, and by generation of radicals. The polymer component in the most widely used resists is polyisoprene which has been partially eye Iized by reaction with p-toluenesulfonic acid G). Other polymers used include polycyclopentadiene and the copolymer of cyclopentadiene and a-methyI styrene ( ). 

Photocrosslinking of glycidyl azide polymers, supposedly via nitrene intermediates, has been studied. Addition of an unsaturated hydrocarbon polymer accelerates the crosslinking process, and this is thought to be due to the formation of aziridine rings by nitrene addition to the carbon-carbon double bonds. 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

Saturated hydrocarbon polymers are also crosslinked by the action of organic peroxides, though branching reduces the efficiency. Polyethylene is crosslinked by dicumyl peroxide at an efficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubber cannot be cured at all. For polyethylene, the reaction scheme is similar to that of the unsaturated elastomers. 

A very common reaction during processing is the polymerization or crosslinking reaction of thermoset materials to form a final polymeric product. This is a common, heat- induced reaction in the epoxy resins and phenol/methanal polymers discussed in this book. It is also common in mbbers undergoing vulcanization and in isocyanates undergoing transformation to urethanes. These reactions form molecules structurally similar to the crosslinked hydrocarbon shown in Figure 4. 

In principle, termination of two alkoxy radicals would give a peroxide cross-link, whilst termination of an alkoxy radical with an alkyl radical would give an ether crosslink. It is very difficult to get direct experimental evidence for either of these reactions, since both dialkyl peroxides and ethers are difficult to detect, however, there is no doubt that chain scission is dominant in saturated hydrocarbon polymers. 

Chapters 10-12 are three chapters that address special areas of interpretation. Chapter 10 is focused on the interpretation of polymer spectra. Exercise Sections 1 and II have three exercises that involve the identification of relatively simple polymer spectra. These spectra were introduced to demonstrate to the reader that the extension of the group frequencies approach to the interpretation of polymer spectra is, in general, straightforward. However because of the importance of polymer spectra, we now consider this area in some detail in Chapter 10. Section I of the chapter builds on the interpretation of the spectra of hydrocarbon polymers started in exercise sections I and II. In section II the problem of the presence of plasticizers is examined and in addition the polymerization of hetero-atom monomers is explored. The sampling of polymers to acquire infrared and Raman spectra often requires specialized techniques. A short introduction to a few of these techniques is given in Section III. The chemistry involved in the formation of polymers is reviewed in part IV with examples of condensation (nylon) and addition (polyethylene) polymerization presented. Copolymers are examined next (V) with methylmethacrylate-stryene used as an example. The effects on the spectra of block and random copolymerization are also noted. Next crosslinked polymerization is studied (VI) with phenol-formaldehyde. Tacticity (VII) is then explored with evidence for its presence in the spectra of polypropylene. This discussion leads to a concise examination of conformational isomerism (VIII) and the impact of this... 

In plasma polymerization, monomer vapors are crosslinked to form a polymer either in the plasma or on a surface in contact with the plasma. The process can occur with either organic or inorganic monomers. Examples are the formation of amorphous silicon (a-Si H) from SiH4 and hydrocarbon polymer films from gaseous hydrocarbon species. 

Manufacture of highly water-absorbent polymers with uniform particle size and good flowability can be carried out by reverse phase suspension polymerization of (meth)acrylic acid monomers in a hydrocarbon solvent containing crosslinker and radical initiator. Phosphoric acid monoester or diester of alka-nole or ethoxylated alkanole is used as surfactant. A polymer with water-absorbent capacity of 78 g/g polymer can be obtained [240]. 

Small-particle-size cement has found a number of uses in production and injection well casing repair jobs [440]. Oil-based cement is particularly useful for water shutoff jobs, because the hydrocarbon slurry sets only in the presence of water, so the oil-producing sections of a reservoir remain relatively damage free after water shutoff. The selective water shutoff with oil-based cement also has been used with polymers crosslinked by metal crosslinkers [442,1178]. 

R. D. Sydansk. Hydrocarbon recovery process utilizing a gel prepared from a polymer and a preformed crosslinking agent. Patent US 5415229,1995. 

---