Polymers

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Overview

Polymer Atomistic Research. Movie showing deformation of an amorphous polyethylene structure with 20 chains of 1000 monomers length. The strain rate is 1010 s-1 and the temperature is 100 K[1][2].

In our cyberinfrastructure, we provide a material database (a database from the literature can also be found at CAMPUS) and constitutive models of various polymers for the community to use.

What is a polymer?

Polymers are made up of many many molecules all strung together to form really long chains ( and sometimes more complicated structures, too ). What makes polymers so fun is that how they act depends on what kinds of molecules they're made up of and how they're put together. The properties of anything made out of polymers really reflect what's going on at the ultra-tiny ( molecular ) level. So, things that are made of polymers look, feel, and act depending on how their atoms and molecules are connected, as well as which ones we use to begin with! Some are rubbery, like a bouncy ball, some are sticky and gooey, and some are hard and tough, like a skateboard.

The term, polymer, comes from the two Greek words poly- (many) and -meros (parts). A polymer is a macromolecule formed by the union of many groups of atoms called monomeric units that are covalently bonded. The simplest hydrocarbon chain polymer is polyethylene, which has the general formula: A1—[—CH2—CH2—]n—A2 This polymer is obtained from the monomer ethylene: H2C=CH2. The number of ethylene monomeric units, n, depend on the degree of polymerization and can reach values between 103 and 106, giving a range or distribution of molecular weights. For polymer chains with high molecular weight, terminal groups A1 and A2 are present in low concentrations and will therefore have no effect on the mechanical properties of the polymer. Terminal groups influence mainly the chemical stability of the polymer; with unstable terminal groups, heating or irradiation can provoke the degradation of the polymer.

The monomers in a polymer can be arranged in a number of different ways. Polymer chains can be linear, branched, cross-linked, or a combination of cross-linked and branched. Polymer chains with only one type of monomer or repeating unit are known as homopolymers. If two or more different type monomers are involved, the resulting copolymer can have several configurations or arrangements of the monomers along the chain. Segments of polymer molecules can exist in two distinct physical structures or morphologies. Polymers can be composed by crystalline or/and amorphous regions. Crystalline polymers are characterized by a regular chemical structure (e.g., homopolymers or alternating copolymers), and the chains possess a highly ordered arrangement of their segments. Amorphous polymers, on the other hand, do not show an order. The molecular segments in amorphous polymers or the amorphous domains of semi-crystalline polymers are randomly oriented and entangled.

Polymers Classifications

In general, polymers are commonly classified into three main families or groups:

• Thermosets: are rigid materials. They are characterized by a specific structure of chain networks where the chain motions are severely restricted due to the high density of the chemical cross-linking points. Thermosets cannot be transformed once obtained, nor do they flow under the action of heat.

• Thermoplastics: are linear or branched polymers becoming fluid when heat is applied. Thermoplastics can be crystalline and/or amorphous.

• Elastomers: are cross-linked polymers with a low density of cross-linking points that can be easily deformed, reaching extensions up to ten times their original dimension and rapidly recovering their original size when the loading is released.

Mechanical Interest

Polymers can also be categorized broadly as plastics, rubbers, fibers, foams, adhesives, ... In many of their uses, as in plastics and fibers, this is the combination of properties such as high strength-to-weight or stiffness-to-weight ratio and high resistance to chemical attack that give them their importance. In other uses, it is flexibility combined with toughness. These capabilities of tuning polymers properties by modifying their microstructures, in addition to their low weight, make polymers an important candidate for a wide variety of applications, particularly in the automobile manufacturing industry.


Research Interest

As energy consumption becomes a larger concern for both industry and consumers, a significant amount of research is currently being carried out to save fuel and reduce production costs. Developing new and/or optimizing the use of lightweight materials, such as polymers, without compromising safety standards and performance criteria is one method of cutting fuel consumption, particularly in industries associated with transportation.

Although most of, if not all, the polymeric materials found in industry are well characterized in terms of their chemical and physical structures as well many of their properties (mechanical, electrical, etc), there is still a need for a comprehensive database capturing the structure-property relationship of specific types of polymers that can be used for material model development. This is particularly important for polymers used in structural components of automotive parts, as their mechanical response, in particular, high strain rate response, will affect the overall vehicle performance in crash scenarios. In addition, as the mechanical response is strongly dependent on a number of structural factors (molecular weight, cross linking, crystallizations, etc), understanding the microstructure behavior and its relation to the macroscopic properties is fundamental for development of physics-based constitutive models for these materials.

In our cyber infrastructure, we provide material database (a database from the literature can also be found at CAMPUS) and constitutive models of various polymers for the community to use.

Related Research

Polymer Modeling

Polymer Systems

Acrylonitrile Butadiene Styrene (ABS)
Polycarbonate
Polyethylene
Polypropylene

Related Reading

References

  1. Hossain, D., Tschopp, M.A., Ward, D.K., Bouvard, J.L., Wang, P., Horstemeyer, M.F., “Molecular dynamics simulations of deformation mechanisms of amorphous polyethylene,” Polymer, 51 (2010) 6071-6083. (http://dx.doi.org/10.1016/j.polymer.2010.10.009)
  2. Tschopp, M.A., Ward, D.K., Bouvard, J.L., Horstemeyer, M.F., “Atomic Scale Deformation Mechanisms of Amorphous Polyethylene under Tensile Loading,” TMS 2011 Conference Proceedings, accepted.
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