On the Nature of a Crosslink

Polymers may be synthesized as linear, branched, or crosslinked entities, or a mixture containing all three. In many cases, the complete gamut between a strictly linear chain and a densely crosslinked network can be covered systematically. As extreme examples, high-density polyethylene serves as the model for a strictly linear polymer, and the diamond as the most densely crosslinked polymer imaginable. 

A word should be said about physical crosslinks. A physical crosslink is a bond of a physical nature, which joins two or more chains together. Three types of physical crosslinks can be readily identified. They arise from (1) crystalline portions of a semicrystalline polymer, (2) the glassy or crystalline portion of a block copolymer, and (3) the ionic portion of an ionomer. Frequently, the physical crosslink forces can be reduced or overcome by raising the temperature. Thus such materials are thermoplastic at high temperatures, and thermoset at room temperature. 

While most of the materials considered herein are crosslinked chemically, several sections are devoted to physically crosslinked systems. This becomes all the more important when one notes that several recent patents and papers utilize the latter to synthesize IPN-like materials having a thermoplastic nature. 

Entanglements between chains also serve as a type of crosslink. In a linear or branched polymer, entanglements can slip or move, and so are very impermanent. However, chemical (or physical) crosslinking limits their motion, and increases their effect on bulk properties. At this time, the phantom network theory is calling into question the reality of entanglements. While a monograph such as this cannot of itself resolve the controversy, some of the properties of interpenetrating polymer networks described in later chapters bear on the problem. 

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