Hair is a complex biological fiber, and the simple answer to whether it is a polymer is a definitive yes. Understanding hair’s physical properties, from its strength to its ability to be styled, requires examining its chemical structure. This structure is built on a foundation of chemistry, making hair a prime example of a naturally occurring polymer.
Defining Polymers and Macromolecules
Polymers are substances made up of very large molecules, known as macromolecules, composed of many repeating subunits called monomers. Think of a polymer like a long chain, where each link represents a monomer. Examples of natural polymers include DNA, cellulose, and proteins.
While all polymers are macromolecules, not all macromolecules are polymers; a polymer specifically requires the structure to be built from multiple repetitions of smaller units. This repetition gives polymers unique physical properties, such as toughness and elasticity, a structure hair fits perfectly.
Keratin: The Primary Polymer of Hair
The specific polymer that constitutes approximately 80% of hair’s weight is a structural protein called keratin. Keratin is a fibrous protein built from chains of amino acids, which act as the monomers. These amino acids are chemically linked by peptide bonds to form long polypeptide chains.
The primary structure of these chains is the alpha-helix, a coiled shape. Two individual helices twist around each other to form a coiled-coil structure, which then aligns to build larger filaments. Hair is primarily made of alpha-keratin, classified as “hard” keratin due to its high sulfur content.
This high sulfur content distinguishes hair keratin from the “soft” keratin found in skin. The hard keratin forms robust, insoluble intermediate filaments that provide the hair fiber with immense structural integrity.
How Chemical Bonds Determine Hair’s Characteristics
The ultimate strength and shape of the hair fiber are determined by how the keratin polymer chains are cross-linked. The most powerful connections are the disulfide bonds, which form between the sulfur atoms of the amino acid cysteine. These covalent bonds are incredibly strong, giving hair resilience and stability, and locking in the hair’s natural texture.
Because disulfide bonds are so strong, they can only be broken and reformed using chemical processes. This principle is the basis behind permanent treatments like perms and chemical relaxers, allowing for a lasting change in the hair’s shape.
In addition to these permanent links, hair contains numerous weaker, temporary bonds, primarily hydrogen bonds and ionic bonds (or salt linkages). Hydrogen bonds are easily broken by water or heat, allowing for temporary styling changes. When hair gets wet, these bonds break, and they reform in the new configuration when the hair dries. Ionic bonds are also temporary and are affected by changes in the hair’s pH balance. The interplay between the permanent disulfide bonds and the temporary bonds gives hair its unique combination of durability, elasticity, and responsiveness to styling.