Basic Factors Affecting the Performance of Polyurethane Structure

1. Effect of Molecular Weight

The performance of most polymers improves with increasing molecular weight. Properties such as tensile strength, elongation, hardness, and glass transition temperature all tend to rise as molecular weight increases. Conversely, solubility decreases as molecular weight increases. This relationship becomes less pronounced once the molecular weight reaches a certain threshold.

For polyurethanes, the polymer's molecular weight is generally sufficiently high. Especially in polyurethane foams, the molecular structure is predominantly cross-linked, unlike the linear structure of typical polyurethane fibers. Therefore, molecular weight is not the primary factor affecting the performance of polyurethane foams.

2. Effect of Intermolecular Forces

Intermolecular forces in large molecules usually arise from interactions between molecular dipoles. The strength of these forces depends on hydrogen bonds, polarizability, and dipole moments within the molecules.

These intermolecular forces, known as secondary forces or Van der Waals forces, are different from the primary forces (covalent bonds) that involve chemical bonding. Secondary forces are generally much weaker than primary forces and are more affected by temperature and stress. For example, the bond energy of a C-C bond in a macromolecular chain is 347.4 kJ/mol, while a C-H bond is 414.4 kJ/mol. Secondary forces are usually weaker; the strongest electrostatic force, hydrogen bonding, ranges from 20.9 to 41.8 kJ/mol, while the electrostatic force of polar molecules ranges from 8.4 to 20.9 kJ/mol, and the induction force of non-polar molecules ranges from 6.3 to 12.6 kJ/mol. Dispersion forces are even weaker, typically between 0.8 and 8.3 kJ/mol.

Despite their relatively small size, secondary forces play a significant role in polymers due to their large molecular weight. If the secondary force generated by each structural unit of a polymer chain equals that of a single monomer, the total secondary force of a large polymer molecule composed of hundreds of structural units will be close to the primary force.

Factors such as molecular charge repulsion, bulky side chains or groups, geometric alignment of attractive groups, medium to high cross-link density, and added plasticizers can influence the intermolecular forces in polymers. Secondary forces are also significantly affected by temperature.

Intermolecular forces can greatly influence the physical and chemical properties of polyurethanes, such as strength, elasticity, and chemical reactivity.

3. Rigidity of Chain Units

Rigid chain units in polymers, such as benzene rings and other aromatic heterocycles, lead to stiffer polymer chains. Introducing these rigid chain units increases the polymer's glass transition temperature, hardness, and strength while decreasing elasticity and solubility. Conversely, introducing flexible chain units lowers the glass transition temperature and increases softness, elasticity, and flexibility. Typical flexible chain units include ether bonds (~~~~O~~~~) and thioether bonds (~~~~S~~~~), followed by aliphatic hydrocarbon bonds (~~~~CH2-CH2-CH2-CH2~~~~). Polyurethane foams are usually composed of various flexible and rigid chain units, forming soft or hard plastic products as needed.

4. Crystallinity

The crystallinity of a polymer largely depends on its linearity, the tightness and effectiveness of chain unit packing, the magnitude of intermolecular forces, and the rigidity of the chain units. Higher linearity, more regular arrangement, and effective geometric alignment between chain units increase crystallinity. Greater intermolecular forces and higher chain unit rigidity also promote crystallization. Increased crystallinity enhances polymer strength, hardness, and melting point but reduces softness, elasticity, elongation, and solubility. Since the binding forces between molecules in crystalline regions differ from the chemical bonding in truly cross-linked polymers, crystalline regions often temporarily disintegrate upon heating and melting. Thus, in polyurethane foam manufacturing, chemical cross-linking is generally preferred over increasing crystallinity.

5. Degree of Cross-linking

Polymers are generally classified into linear, branched, and cross-linked types. Linear structures in polyurethanes result from the condensation of two or more difunctional reactants. Branched structures have side chains attached to the main chain. Both types are soluble, meltable thermoplastic polymers. Cross-linked polymers, formed by two or more polyfunctional reactants, create a network structure. The degree of cross-linking can be measured by the number of cross-linked molecules per unit volume or the molecular weight between cross-linking points (MC). For instance, if 0.25 mol of a trifunctional reactant is added to 1000 g of a difunctional reactant under equimolar reactive conditions, the relative molecular weight between cross-linking points (MC) would be 1000/0.25 = 4000.

Most polyurethane foams are cross-linked polymers. As with other polymers, the degree of cross-linking is a crucial factor determining performance. High cross-linking in amorphous polymers increases hardness, softening temperature, and elastic modulus while reducing elongation and solvent swelling. For highly crystalline polymers, slight cross-linking decreases crystallinity and molecular chain orientation, transforming hard, high-melting crystalline polymers into more elastic and soft amorphous polymers. Further increasing cross-linking yields effects similar to those in amorphous polymers.

Generally, highly cross-linked (low MC) polyurethane foams are hard, while less cross-linked (high MC) polyurethane foams are mostly soft and elastic.

From these general principles, it can be seen that the primary molecular structural factors affecting the performance of polyurethane foams are intermolecular forces, the rigidity of chain units, and the degree of cross-linking.