Meaning
Intermolecular forces create a web of attraction between polar groups in certain polymer chains. Hydrogen bonding networks are particularly prevalent in polyamides and polyurethanes, where they greatly influence the mechanical and thermal properties of the material. These connections act as physical cross-links that increase the melting point and the tensile strength of the resin.
The strength of these networks is sensitive to the presence of water molecules which can disrupt the bonds.
Thermal Stability
Resistance to heat-induced deformation is a direct result of the energy required to break the intermolecular attractions. Strong hydrogen bonding networks prevent polymer chains from sliding past each other at elevated temperatures. This characteristic allows parts made from nylon to maintain their shape in automotive under-hood applications.
When the temperature exceeds the breaking point of these bonds, the material undergoes a sharp drop in viscosity.
Moisture Absorption
Polar polymers attract water molecules which penetrate the structure and compete for bonding sites. This absorption expands the hydrogen bonding networks and pushes the polymer chains further apart, leading to dimensional swelling. The plastic becomes tougher and more ductile as the water acts as a plasticizer.
Moulders must account for this change in properties when designing parts that will be exposed to varying humidity levels.
Mechanical Strength
Tensile and flexural modulus values are heavily dependent on the density of the intermolecular connections. A dense system of hydrogen bonding networks results in a stiff, hard material suitable for structural components. Processing conditions, such as the cooling rate in the mould, can affect the final arrangement of these bonds.
Achieving the optimal balance of strength and impact resistance requires precise control over the morphology of the finished part. These networks define the performance limits of high-performance engineering plastics.
