linear and branched polymers overview

    Linear polymer is a polymer in which the molecules form long chains without branches or cross-linked structures. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus can be weakened by energy applied in the form of heat. Such linear polymers are thermoplastics. The simplest polymer is a linear polymer. A linear polymer is simply a chain in which all of the monomers exist in a single line. An example of a linear polymer is Teflon, which is made from tetrafluoroethylene. It is a single strand of units made from two carbon atoms and four fluorine atoms. When formed, these linear polymers can create strands of fibers or form a mesh that can be very strong and hard to break through.

    These linear polymers are well packed and have high magnitude of intermolecular forces of attraction and therefore have high densities, high tensil (pulling) strength and high melting points. Some common example of linear polymers are high density polyethylene nylon, polyester, PVC, PAN etc.

Linear, Branched, and Cross-linked Polymers:

 

Polyethylene is called a linear or straight-chain polymer because it consists of a long string of carbon-carbon bonds. These terms are misleading because the geometry around each carbon atom is tetrahedral and the chain is neither linear nor straight, as shown in the figure

As the polymer chain grows, it folds back on itself in a random fashion to form structures such as the one shown in the figure below on the left.  Straight chains can sometimes fold tightly enough to make crystal structures (on the right below) even though the molecules are very long!

Neatly packed straight chains can make crystals!

Randomly oriented straight chains can be pretty messy

      

   Polymers with branches at irregular intervals along the polymer chain are called branched polymers (see figure to the right).   These branches make it difficult forthe polymer molecules to pack in a regular array, and therefore make the polymer less crystalline and less dense.  The amount and type of branching also affects physical properties such as viscosity and elasticity (see below).  Branches often prevent chains from getting close enough together for intermolecular forces to work effectively. they have low tensile strength, low density, boiling point and melting points than linear polymers. Some common examples are low density polythene, glycogen, starch etc. (Amylopectin).

         Cross-linked polymers contain short side chains (cross links) that connect different polymer chains into a “network” as shown in the figure to the right.  At first, adding cross-links between polymer chains makes the polymer more elastic (they can stretch and return to their original form.)    The links can “pull” the chains back together when they are stretched!  The vulcanization of rubber, for example, results from the introduction of short chains of sulfur atoms that link the polymer chains in natural rubber.  Cross-linking also decreases the viscosity (the resistance to flow) of polymers.  In order for polymers to flow, the chains must move past each other and cross-linking prevents this.  Elastomers are elastic polymers created by limited cross-linking.  As the number of cross-links increases, however, the polymer becomes more rigid and cannot stretch as much; the polymer will become less viscous and less elastic and might even become brittle. e.g., Bakelite, malamine formaldehyde resin etc.

cross linking

Cross-link is a bond formed between polymer chains, either between different chains or between different parts of the same chain. Cross-links can be covalent bonds or ionic bonds. Polyethylene consists of many random molecular

chains with no particular orientation and no chemical bonds existing between chains. When heat is applied to such a material, the chains are free to slip and flow under relatively small outside force. Such a material is called a thermoplastic. If we are able to introduce cross-linking bonds between adjacent molecular chains, this adds form stability at higher temperatures. There will still be some loss of strength at elevated temperatures, but the cross-linked molecular chains are much more resistant to flow when stress is applied. The chemical process of vulcanization is a type of cross-linking and it changes the property of rubber to the hard, durable material we associate with car and bike tires. This process is often called sulfur curing, and the term vulcanization comes from Vulcan, the Roman god of fire. However, this is a slow process taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalyzing the addition of sulfur chains to the rubber molecules. The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials. Cross-links are the characteristic property of thermosetting plastic materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave solutions, for example, break and re-form naturally occurring cross-links (disulfide bonds) between protein chains in hair. When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a "solid" or "gel" by cross-linking the chains together. Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car tires cannot be recycled easily. A class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains which act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.