AB Crosslinked Copolymers

A new synthetic technique for making AB crosslinked copolymers systematically was reported by Bamford et al (1971, 1974). His synthesis involves crosslinking of a suitable polymer (I) with chains of vinyl polymer (II) by reaction of halogen-containing groups in I with an organometallic derivative, e.g., Mo(CO)6 or Mn2(CO)io, in the presence of monomer II. The reaction scheme may be written as 

---

Direction substitution of element notation (Equation 10) representing the grafting of polystyrene side chains onto polybutadiene backbone. An AB-crosslinked copolymer of Bamford and Eastmond (14a)... 

Polymerization of monomer II in the presence of polymer I definitely leads to increases in molecular weight, increased viscosity, and often true gel formation. This latter is actually a form of joined IPN or AB crosslinked copolymer. Using the cellular model of phase separation, we may predict that gelation, if it occurs, will be most extensive near the cell wall. In Section 3.1.1.3, we concluded that polymer II was unable to pass easily through polymer I even in the highly swollen state, because of polymer incompatibility. Obviously, this transport difficulty is augmented by a surrounding gel network. 

Although the term joined IPN was introduced by Sperling and Sarge (1972), the term AB crosslinked copolymer is now preferred. 

Both paints and adhesives are commonly formulated as polymer blends or grafts. In fact, some compositions resemble semi-IPN s or AB crosslinked copolymers (Section 8.7). For example, epoxy adhesive resins are often cured with polyamides (Bikerman, 1968). The product is tougher than materials cured with low-molecular-weight amines, possibly because of a separate amide phase in this AB crosslinked copolymer. A more complex molecular architecture is exhibited by the alkyd resins common in oil-based paints (Martens, 1968, Chapters 3 and 4). The major component is a polyester, which often forms a network structure on drying. The polyester component is reacted with various drying oils, such as linseed oil or tung oil (Martens, 1968, Chapters 3 and 4). These oils form an ester link to the polyester structures and also polymerize through their multiple double bonds. Latex paints always contain thickeners, such as cellulosics, poly(acrylic acid), casein. 

Thermoplastic elastomers AB crosslinked copolymers Sequential IPNs Latex lENs... 

Some recent research in the area of IPN-related materials must be noted before proceeding. Eastmond and Smith studied the morphology of AB crosslinked copolymers (ABCPs). Note structure (f) in Figure 1.3. They found that the domain diameters appeared to be proportional to the square root of the degree of polymerization of the domain-forming chains. 

The AB crosslinked copolymers of Bamford et are not strictly IPNs, since the polymer II chains are part of the same network as the polymer I chains. However, like graft copolymers and IPNs, they do phase separate into distinct domains. The polymer II chains usually form the discontinuous phase, because, similarly to the sequential IPNs, they are not stirred during polymerization. 

In both the block copolymer case and the AB crosslinked copolymer case, the phase domain dimensions depend upon the molecular weight of the polymer in the same domain. Use of equation (6.1) above, employing either polymer I or polymer II, leads to an altogether incorrect analysis for IPNs. In the case of IPNs, presented below, the phase domain sizes of polymer II are shown to depend upon the molecular weight between crosslinks (Me) of polymer I. 

Figure 6.20. Dilatomeric data for poly(vinyl trichloroacetate)/poly(methyl methacrylate) AB crosslinked copolymers. Networks IV and V show two second-order transitions, while network VI shows only one transition. ...

Unsaturated polyesters, dissolved in greater or lesser quantities of styrene monomer, form the matrix material widely used for glass-fiber-reinforced thermoset sheet molding and bulk molding compounds. Upon polymerization of the styrene and concomitant conterminous grafting, an AB crosslinked copolymer network forms (see Section 5.6). 

F. P. Baldwin and I. J. Gardner, in Chemistry and Properties of Crosslinked Polymers, S. S. Labana, ed.. Academic, New York (1977). Graft Curing with Modified Butyl Rubber and AB Crosslinked Copolymers of Butyl Rubber with Polystyrene. Crosslinking of the polystyrene. Physical and mechanical Properties. Swelling and extraction. 

H. Parriss and R. Orr, Improvements in Casting Resins, Br. Pat. 786,102 (1957). Styrene-polyester AB-crosslinked copolymer as network I. Epoxy Resin as network II. SIN compositions. 

From a topological point of view, the IPN s are closely related to polymer blends, block and graft copolymers, AB-crosslinked copolymers " and ionomeric blends. Some interesting hybrids exist between the IPN s and other polymer materials. The thermoplastic IPN s contain physical crosslinks rather than chemical (covalent) crosslinks. Physical crosslinks can be formed from block copolymers, ionomers or semi-crystalline polymers. When the temperature is raised above the softening point of the respective components, the material flows like a thermoplastic. At service temperatures, it behaves like an IPN, with thermoset behavior. Table 1 summarizes some of the terminology used to describe IPN structure and morphology. 

---