Polymers form the foundation of most modern adhesives and sealants. They promote cohesion within the adhesive to hold it together, give it flexibility or rigidity, and help determine how it responds to heat, moisture, chemical, and mechanical forces. Polymers are made from smaller molecular building blocks known as monomers.
Monomers and polymers are related, but they are not interchangeable terms. A monomer is a type of molecule that can participate in polymerization and become a part of a polymer. A polymer is a large macromolecule made from many interconnected structural units. The way monomers react and transform into polymers is central to how many adhesive technologies work.
Not all adhesives begin as a collection of individual monomers. Some adhesives already start as polymers when they are manufactured and applied. Other adhesives contain oligomers, prepolymers, resins, or mixtures of materials that undergo further reactions during curing.
What is a Monomer?
A monomer is a molecule that can participate in polymerization and become a part of a polymer. The term ‘mono’ means one, which reflects the role of monomers as molecular building blocks.
Monomers contain chemical structures that enable them to react with other molecules. These reactive structures, which include functional groups, are capable of participating in polymer forming reactions.
For example, many acrylic adhesives are based on acrylic or methacrylic monomers containing polymerizable carbon-carbon double bonds. When these double bonds undergo polymerization, they participate in reactions that connect them into polymer chains.
A monomer is not just a small molecule, but it possesses the ability to become a structural unit of a polymer. There are many small molecules that cannot undergo polymerization, and those are not classified as monomers.
What is a Polymer?
A polymer is a large molecule or macromolecule which is composed of many interconnected structural units. These units are created from monomers that have undergone polymerization.
A polymer can contain hundreds, thousands, or even more atoms connected through covalent bonds. Instead of performing like a collection of independent small molecules, the resulting macromolecule can have significantly different physical and mechanical properties than what it started out as.
Polymers can take several forms. Some are long-molecular chains, while others contain branches extending from the main chains. Polymers that are highly interconnected can form three-dimensional networks.
These polymer structures are vitally important in adhesive science. The polymer is responsible for providing much of the adhesive’s internal strength, flexibility, toughness, elasticity, and resistance to environmental conditions. Changes in the chemical architecture of the polymer can radically change the behavior of the adhesive.
How Monomers Become Polymers
The process through which monomers and other suitable reactants form polymers is called polymerization.
Polymerization occurs when chemical reactions create bonds that connect molecular building blocks into larger structures. Polymerization can produce polymers that have long chains, branched structures, or interconnected networks.
For example, in acrylic chemistry, suitable acrylic and methacrylic monomers contain carbon-carbon double bonds. When these functional groups undergo polymerization, these reactive double bonds are converted into new bonds between monomer-derived units, allowing polymer chains to grow.
Polymerization works differently for other types of adhesives. For example, polyurethanes can form through reactions between isocyanate groups and hydroxyl groups. Likewise, epoxy systems form networks through reactions between epoxide groups and curing agents.
Polymerization does not represent a single chemical reaction. It is a broad category of processes through which smaller molecular building blocks become connected into macromolecular structures.
Repeating Units in Polymers
Many polymers contain structural patterns that repeat along their chains, known as repeating units. Repeating units are derived from monomers, but they are not chemically identical to the original free monomer. Polymerization changes the bonding arrangement of the monomer as it becomes a part of the polymer.
In polymers that are produced from monomers containing carbon-carbon double bond functional groups, the double bond participates in polymerization, and the monomer-derived unit becomes connected to neighboring units through the polymer backbone. The structure is incorporated into the polymer and is different than the original isolated monomer.
Polymer Chains and Molecular Structure
Polymers can have different molecular architectures, and these structures significantly impact their properties.
A linear polymer contains mainly long chains without complex branching. The individual polymer chains can interact with one another through intermolecular attractions and physical entanglements.
A branched polymer contains a main polymer chain with additional branches extending from it. Branching can affect the closeness, mobility, and behavior of the polymer.
A network polymer has extensive chemical connections between polymer chains or multifunctional molecular structures. These connections create three-dimensional networks throughout the material.
Most adhesives will have one of these polymer structures, and the type of architecture will influence the usability of the adhesive. For example, a highly crosslinked network may provide better dimensional stability and chemical resistance, while a branched polymer may provide greater mobility and flexibility.
Homopolymers and Copolymers
Polymers can be classified according to the type of monomers or structural unit from which they are produced.
A homopolymer is a polymer which is made from just a single type of monomer. The polymer consists of chains containing repeating units derived from that monomer.
A copolymer consists of two or more different monomers. The properties of the resulting polymer change when multiple different types of monomers are added and react with each other.
Different monomers can provide a polymer with different types of characteristics. One monomer type may provide flexibility, while another contributes stiffness, polarity, heat resistance, or other useful characteristics. The arrangement of the monomers inside the polymer can also influence its properties.
Copolymers are desirable in adhesive formulations because multiple monomers provide an adhesive with multiple properties. For example, copolymers can help an adhesive wet surfaces, remain flexible, resist environmental degradation, maintain cohesive strength, and perform over a particular temperature range.
How Polymer Structure Affects Adhesive Properties
The properties of an adhesive such as curing ability, viscosity, thermal resistance, and dimensional stability depend not only on what polymer is present, but also on the structure of that polymer.
Longer polymer chains can become physically entangled with one another, which can contribute to the mechanical behavior of the material. Branching can contribute to molecular packing and mobility or flexibility. Crosslinking restricts the movement of polymer chains and creates a more rigid, interconnected structure.
Polarity is another important feature of polymers that matters. Polar functional groups can affect intermolecular attractions and compatibility with other materials. Likewise, crosslink density can impact stiffness, dimensional stability, heat resistance, and chemical resistance. Too much crosslinking can cause the adhesive to become brittle.
Effective adhesive formulation depends on making the right trade-offs and balancing molecular characteristics.
Why Polymers Are Important in Adhesives
Polymers are a vital component of adhesive science because they usually form the repeating material that remains with a bond after the adhesive has been applied and cured.
The adhesive must possess sufficient cohesive strength to remain intact, meaning there cannot be a tear in the adhesive layer itself. The polymer structure helps contribute internal strength to an adhesive.
Polymers can also impact the flexibility or rigidity of an adhesive. For example, some adhesive polymers may deform while remaining intact, making them suitable for flexible applications. Other polymers resist deformation and are comparatively rigid.
Polymer structure may also impact toughness, viscosity, elasticity, thermal behavior, moisture resistance, chemical resistance, and long-term durability. These properties matter because adhesive joints are exposed to different types of stress and environmental conditions.
The polymer is a major factor in determining how the adhesive performs during the application and throughout the life cycle of the bonded assembly.
