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Co-polymerization is the chemical modification process in which two homopolymers are co-polymerized to make an entirely new polymer. When two homopolymers are co-polymerized, the two sets of polymer chains are actually linked together, resulting in an entirely different polymer, composed of an entirely different polymer chain. When three homopolymers are chemically linked together, you get a ter-polymer. A common ter-polymer is ABS (Acrylnitrile-Butadiene-Stryrene).
Blending is the combining of homopolymers to take advantage of the properties of each polymer. Combining homopolymers to create a resin blend also referred to as alloys, can result in a new or unique product with the best properties of each homopolymer. An example of a blend would be combining a polycarbonate with a polyester like PBT to a get a unique plastic that offers something more than what each individual polymer would have offered. This PC/PBT blend would offer good impact resistance, better than a PBT homopolymer, and very good chemical resistance, much better than PC homoploymer.
The process of blending significantly expands the variety of materials available to better meet the designers or part manufacturer’s specific performance requirements. To recap, blending is the result of a physical combination of polymers, co-polymerization is the chemical combination of two polymers.
What is the difference between a physical combination and a chemical combination? When two homopolymers are blended, they are physically mixed together. A polycarbonate and a PBT polymer are merely stirred together to make a PC/PBT blend or alloy. Within blends, there are various levels of blend miscibility, which determines the degree of mixing in a blend. Some materials have a greater capacity to dissolve in each other.
An example of a highly miscible blend is salt in water. The salt and water will actually break apart into smaller and smaller particles, eventually particles of salt will dissolve with particles of water creating a single-phase blend. In a single phase system, the materials are mixed together and dissolve, creating a single, continuous phase. Salt and water are soluble, when mixed, they dissolve, creating a single-phase system.
Single phase polymers appear homogeneous when examined under magnification. An example of a single phase blend is the polymer blend of PPO (Polyphenylene Oxide) with polystyrene.
If the level of miscibility is low, then what typically results is discrete phases. This is because the parent homopolymers are not miscible or soluble in each other and each maintains a discrete phase in melt and solid states. Low miscibility results in a multiple-phase system. Fruit in Jello is an example of a multiple phase system. The fruit does not dissolve in the Jello; when mixed they remain separate in the mixture, creating a two phase system.
