User:Stepagco/Copolymer

Properties
Since copolymers are polymers made from different types of monomers, it allows for easier fine tuning of properties. For application purposes, it is common to mix many different monomers to obtain specific properties like a high melting point or flexibility. A common example is Poly(ethylene-co-vinyl acetate) (PEVA) which is a random copolymer consisting of ethylene and vinyl acetate. By changing the ratios of the monomers, the properties of the resulting copolymer will vary. Since vinyl acetate is more polar and larger than ethylene, the more vinyl acetate in PEVA, the higher polarity, lower crystallinity, more flexibility, and more adhesion. So a copolymer with a high percentage of vinyl acetate compared to ethylene can be used as a glue, but if a material needed more rigidity and less adhesive properties, PEVA is made with more ethylene.

Glass Transition and Melting Temperatures
Copolymers are also used to greatly change density of a material. Small monomers or really polar monomers aggregate closer to each other in a material, and decrease the free space resulting in a material with higher density. Monomers with large groups attached to the backbone like styrene, or large grafts will cause more free space and therefore decrease the density of the material. This property is advantageous in copolymers because to increase density, a small amount of monomers with high density can be added to make a polymer with similar properties as before but less dense. To reference the earlier example, the more vinyl acetate is in PEVA, the less dense the material is because vinyl acetate creates more free space. Density and therefore free volume affect the glass transition temperature and the melting temperature of the material. The more free space in a material, the lower the glass transition and the melting temperature.

Thermoplastics
Another property often sought out in material is thermoplasticity. Some polymers after heating will soften, and can be molded into various shapes. After cooling the polymer will stiffen and retain its new shape, which can be reversed again and again after heating. Polymers like polyethylene, polystyrene, and polyvinyl acetate are all thermoplastics, and if combined into a copolymer like PEVA, will retain that thermoplastic property which is why PEVA is a common material: it is very easy to work with and mold. Different polymers "soften" at different temperatures depending on their structure, and if a copolymer contains monomers that have different softening temperatures, then the softening temperature is influenced by the percent of each monomer in the copolymer. Polyethylene has a really low glass transition temperature compared to poly(vinyl-acetate), and when they combine into a copolymer PEVA, the glass transition temperature is somewhere in between their two values, and it can be further fine tuned by decreasing or increasing the amounts of the monomers. This has major industrial applications, since some material need to have lower glass transition states to be molded and worked with easier, but some materials need high glass transition temperatures to withstand higher temperatures.

Thermosets and Elastomers
Thermoset copolymers differ from thermoplastics by being moldable at high temperatures, but when cooled, their shape is irreversible. They're also generally more rigid, less flexible, and brittle in comparison to thermoplastics. When thermoplastics set, they undergo an irreversible crosslinking process where the polymers link to each other via covalent bonds, which creates a very rigid network that has less freedom of movement. Poly(urea-formaldehyde) is a common thermoset copolymer made from condensation of urea and formaldehyde, and since one urea can undergo multiple condensation reactions, the many copolymer strands can crosslink within a setting material and it becomes more rigid and inflexible. The more monomers in a copolymer that has the ability to crosslink, the more rigid, brittle, and inflexible the material is. Bakelite is a thermostat polymer that can have many more crosslinks per monomer than poly(urea-formaldehyde) and therefore the overall properties of Bakelite are more rigid, and inflexible. Some materials can have thermoset and elastic properties. Elastomers are slightly crosslinked polymers that allow for some flexibility and are viscoelastic, but after small physical strain can return back to their original shape due to the crosslinks. When an elastomer stretches, the crosslinks stretch with it, and when the strain is gone, the crosslinks move the polymer chains back into the original conformation. If the material is stretched too much, the crosslinks may break, and the material can no longer go back to its original shape. Mixing different monomers and creating copolymers is useful when fine-tuning elasticity of a material, because it directly influences the amount of crosslinks which determines rigidity and flexibility or the material. For example, poly(styrene-butadiene) is an elastomer and the butadiene monomer crosslinks, while the styrene doesn't, causing only a few crosslinks resulting in a flexible material that is often used in place of rubber.

Characterization
Characterization techniques for copolymers are similar to those for other polymeric materials. These techniques can be used to determine the average molecular weight, molecular size, chemical composition, molecular homogeneity, and physiochemical properties of the material. However, given that copolymers are made of base polymer components with heterogenous properties, this may require multiple characterization techniques to accurately characterize these copolymers.

Spectroscopic techniques, such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, and UV spectroscopy, are often used to identify the molecular structure and chemical composition of copolymers. In particular, NMR can indicate the tacticity and configuration of polymeric chains while IR can identify functional groups attached to the copolymer.

Scattering techniques, such as static light scattering, dynamic light scattering, and small-angle neutron scattering, can determine the molecular size and weight of the synthesized copolymer. Static light scattering and dynamic light scattering use light to determine the average molecular weight and behavior of the copolymer in solution whereas small-angle neutron scattering uses neutrons to determine the molecular weight and chain length.

Differential scanning calorimetry is a thermoanalytical technique used to determine the thermal events of the copolymer as a function of temperature. It can indicate when the copolymer is undergoing a phase transition, such as crystallization or melting, by measuring the heat flow required to maintain the material and a reference at a constantly increasing temperature.

Thermogravimetric analysis is another thermoanalytical technique used to access the thermal stability of the copolymer as a function of temperature. This provides information on any changes to the physicochemical properties, such as phase transitions, thermal decompositions, and redox reactions.

Size-exclusion chromatography can separate copolymers with different molecular weights based on their hydrodynamic volume. From there, the molecular weight can be determined by deriving the relationship from its hydrodynamic volume. Larger copolymers tend to elute first as they do not interact with the column as much. The collected material is commonly detected by light scattering methods, a refractometer, or a viscometer to determine the concentration of the eluted copolymer.

Block copolymers
A common application of block copolymers is to develop thermoplastic elastomers (TPEs). Early commercial TPEs were developed from polyurethranes (TPUs), consisting of alternating soft segments and hard segments, and are used in automative bumpers and snowmobile treads. Styrenic TPEs entered the market later, and are used in footwear, bitumen modification, thermoplastic blending, adhesives, and cable insulation and gaskets. Modifying the linkages between the blocks resulted in newer TPEs based on polyesters (TPES) and polyamides (TPAs), used in hose tubing, sport goods, and automative components.

Amphiphilic block copolymers have the ability to form micelles and nanoparticles. Due to this property, amphiphilic block copolymers have garnered much attention in research on vehicles for drug delivery. Similarly, amphiphilic block copolymers can be used for the removal of organic contaminants from water either through micelle formation or film preparation.

Alternating copolymers
The styrene-maleic acid (SMA) alternating copolymer displays amphiphilicity depending on pH, allowing it to change conformations in different environments. Some conformations that SMA can take are random coil formation, compact globular formation, micelles, and nanodiscs. SMA has been used as a dispersing agent for dyes and inks, as drug delivery vehicles, and for membrane solubilization.