|
A heteropolymer or copolymer is a polymer derived from two (or more) monomeric species, as opposed to a homopolymer where only one monomer is used.[1] Copolymerization refers to methods used to chemically synthesize a copolymer.
Commercially relevant copolymers include ABS plastic, SBR, Nitrile rubber, styrene-acrylonitrile, styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate.
Types of copolymers
Since a copolymer consists of at least two types of constitutional units (not structural units), copolymers can be classified based on how these units are arranged along the chain.[2] These include:
- Alternating copolymers with regular alternating A and B units (2)
- Periodic copolymers with A and B units arranged in a repeating sequence (e.g. (A-B-A-B-B-A-A-A-A-B-B-B)n
- Random copolymers (or Statistical Copolymers) with random sequences of monomer A and B (3)
- Block copolymers comprised of two or more homopolymer subunits linked by covalent bonds (4). The union of the homopolymer subunits may require an intermediate non-repeating subunit, known as a junction block. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.
Copolymers may also be described in terms of the existence of or arrangement of branches in the polymer structure. Linear copolymers consist of a single main chain whereas branched copolymers consist of a single main chain with one or more polymeric side chains.
Other special types of branched copolymers include star copolymers, brush copolymers, and comb copolymers.
A terpolymer is a copolymer consisting of three distinct monomers. The term is derived from ter (Latin), meaning thrice, and polymer.
Graft copolymers
Graft copolymers are a special type of branched copolymer in which the side chains are structurally distinct from the main chain. The illustration (5) depicts a special case where the main chain and side chains are composed of distinct homopolymers. However, the individual chains of a graft copolymer may be homopolymers or copolymers. Note that different copolymer sequencing is sufficient to define a structural difference, thus an A-B diblock copolymer with A-B alternating copolymer side chains is properly called a graft copolymer.
For example, suppose we perform a free-radical polymerization of styrene in the presence of polybutadiene, a synthetic rubber, which retains one reactive C=C double bond per residue. We get polystyrene chains growing out in either direction from some of the places where there were double bonds, with a one-carbon rearrangement. Or to look at it the other way around, the result is a polystyrene backbone with polybutadiene chains growing out of it in both directions. This is an interesting copolymer variant in that one of the ingredients was a polymer to begin with.
As with block copolymers, the quasi-composite product has properties of both "components". In the example cited, the rubbery chains absorb energy when the substance is hit, so it is much less brittle than ordinary polystyrene. The product is called high-impact polystyrene, or HIPS.
Block Copolymers
A special kind of copolymer is called a "block copolymer". Block copolymers are made up of blocks of different polymerized monomers[3]. For example, PS-b-PMMA is short for polystyrene-b-poly(methyl methacrylate) and is made by first polymerizing styrene, and then subsequently polymerizing MMA from the reactive end of the polystyrene chains. This polymer is a "diblock copolymer" because it contains two different chemical blocks. You can also make triblocks, tetrablocks, multiblocks, etc. Diblock copolymers are made using living polymerization techniques, such as atom transfer free radical polymerization (ATRP), reversible addition fragmentation chain transfer (RAFT), ring-opening metathesis polymerization (ROMP), and living cationic or living anionic polymerizations. Recent research in block copolymers suggests that they may be useful in creating self-constructing fabrics with potential utility in semiconductor arrays (for example, computer memory devices) by assembling fine details atop a structured base created using conventional microlithography methods.[4]
Phase separation
Sbs block copolymer in TEM
Block copolymers are interesting because they can "microphase separate" to form periodic nanostructures, as in the styrene-butadiene-styrene block copolymer shown at right. The polymer is known as Kraton and is used for shoe soles and adhesives. Owing to the microfine structure, the transmission electron microscope or TEM was needed to examine the structure. The butadiene matrix was stained with osmium tetroxide to provide contrast in the image. The material was made by living polymerization so that the blocks are almost monodisperse, so helping to create a very regular microstructure. The molecular weight of the polystyrene blocks in the main picture is 102,000; the inset picture has a molecular weight of 91,000, producing slightly smaller domains.
SBS block copolymer schematic microstructure
Microphase separation is a situation similar to that of oil and water. Oil and water are immiscible - they phase separate. Due to incompatibility between the blocks, block copolymers undergo a similar phase separation. Because the blocks are covalently bonded to each other, they cannot demix macroscopically as water and oil. In "microphase separation" the blocks form nanometer-sized structures. Depending on the relative lengths of each block, several morphologies can be obtained. In diblock copolymers, sufficiently different block lengths lead to nanometer-sized spheres of one block in a matrix of the second (for example PMMA in polystyrene). By using less different block lengths, a hexagonally-packed-cylinder geometry can be obtained. Blocks of similar length form layers (often called lamellae in the technical literature). Between the cylindrical and lamellar phase is the gyroid phase. The nanoscale structures created from block copolymers could potentially be used for creating devices for use in computer memory, nanoscale-templating and nanoscale separations.
Polymer scientists use thermodynamics to describe how the different blocks interact. The product of the degree of polymerization, N, and the Flory-Huggins interaction parameter, ?, gives an indication of how incompatible the two blocks are and whether or not they will microphase separate. For example, a diblock copolymer of symmetric composition will microphase separate if the product ?N is greater than 10.5. If ?N is less than 10.5, the blocks will mix and microphase separation is not observed.
Copolymer equation
An alternating copolymer has the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)n-. The molar ratios of the monomer in the polymer is close to one, which happens when the reactivity ratios r1 & r2 are close to zero, as given by the Mayo-Lewis equation also called the copolymerization equation:[5]
![\frac {d\left [M_1 \right]}{d\left [M_2\right]}=\frac{\left [M_1\right]\left (r_1\left[M_1\right]+\left [M_2\right]\right)}{\left [M_2\right]\left (\left [M_1\right]+r_2\left [M_2\right]\right)}](http://upload.wikimedia.org/math/d/e/a/dea8c560dd0363f25dc92f414a5b64f6.png)
where r1 = k11/k12 & r2 = k22/k21
Copolymer engineering
Copolymerization is used to modify the properties of man-made plastics to specific needs, for example to reduce crystallinity, modify glass transition temperature or to improve solubility. It is a way of improving mechanical properties, in a technique known as rubber toughening. Elastomeric phases within a rigid matrix act as crack initiators, and so increase the energy absorption when the material is impacted for example. Acrylonitrile butadiene styrene is a common example.
External links
See also
References
- ^ Odian, G. Principles of Polymerization, 4th Ed., Wiley-Interscience, Hoboken, NJ 2004, Ch. 6. ISBN 0-471-27400-3
- ^ IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311
- ^ [http:http://www.princeton.edu/~polymer/block.html Polymer Research Laboratory] (Princeton.edu accesed Aug 15, 2008)
- ^ Self -growing material promises chip, storage advances (NetworkWorld accesed Aug 15, 2008)
- ^ Copolymerization. I. A Basis for Comparing the Behavior of Monomers in Copolymerization; The Copolymerization of Styrene and Methyl MethacrylateFrank R. Mayo and Frederick M. Lewis J. Am. Chem. Soc.; 1944; 66(9) pp 1594 - 1601; doi:10.1021/ja01237a052
| |
