The influence of the polyurethane soft segment structure on its properties

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The soft segment of polyurethane is composed of oligomeric polyols (usually polyether or polyester diols), while the hard segment is made up of polyisocyanate or it combined with small molecular chain extenders. ～Soft segment～Hard segment～Soft segment～Hard segment～Soft segment. Due to the thermodynamic incompatibility of the two types of segments, microphase separation occurs, forming phase regions or microphase regions within the polymer. Polyurethanes contain urethane groups, as well as urea, ester, ether and other functional groups that can form extensive hydrogen bonds. Among these, the hydrogen bonds formed by urethane and urea groups play a significant role in the formation of the hard segment regions; these hard segments enhance the polyurethane structure by providing multi-functional physical cross-links, with the soft segment matrix being cross-linked by the hard segment regions. The excellent properties of polyurethanes are primarily due to the formation of microphase regions, rather than solely to hydrogen bonds between hard and soft segments. The molecular weight of the oligomeric polyols that make up the soft segments is usually between 600 and 3000. Generally, the soft segments constitute the majority of the PU, and the properties of the PU prepared from different oligomeric polyols and isocyanates vary accordingly. Polyester-based PU possesses higher strength and hardness than polyether-based PU, as the ester groups are highly polar and thus provide a stronger adhesive force to polar materials ； The cohesive energy (12.2 KJ/mol) is higher than that of the ether group (4.2 KJ/mol); the intermolecular forces in the soft segments are stronger, resulting in greater cohesion strength and higher mechanical strength. It also exhibits better antioxidant properties compared to polyether types. To achieve better adhesion strength, polyester is commonly used as the soft segment in PU. Of course, PU with a polyether soft segment exhibits good flexibility due to the ease of rotation of the ether groups, as well as superior low-temperature performance. Moreover, since there are no ester groups in polyether that are prone to hydrolysis, such PU has better resistance to hydrolysis compared to polyester-based PU. The molecular weight and crystallinity of the soft segment also have a significant impact on the properties of PU. The molecular weight of the soft segment affects the mechanical properties of polyurethanes. Generally speaking, assuming that the PU molecular weight remains constant, if the soft segment is a polyester, then the strength of the PU increases as the molecular weight of the polyester diol increases ； If the soft segment is a polyether, the strength of PU decreases as the molecular weight of the polyether diol increases, whereas the elongation rate increases. This is because polyester soft segments are inherently highly polar; a larger molecular weight leads to greater structural regularity, which helps to improve strength ； The polyether soft segments, on the other hand, have weaker polarity; as the molecular weight increases, the relative proportion of the hard segments in PU decreases, resulting in a reduction in strength. Crystallization can multiply the cohesion and adhesion of the adhesive layer. Adhesives made from high-molecular-weight linear PU using highly crystalline butylene adipate diol as the soft segment can achieve high-strength adhesion even without a curing agent, and exhibit good initial performance. In contrast, polyesters made from pentaerythritol or similar compounds containing side groups have poor crystallinity; however, these side groups protect the ester bonds, thereby improving the PU’s resistance to thermal oxidation, water, and mold. Polyester-based PUs prepared from long-chain aromatic dihydroxy acids and similar compounds exhibit improved hydrolysis resistance and heat resistance. For linear thermoplastic PUs, higher molecular weight results in greater strength. It has good heat resistance. However, for most reactive PU adhesive systems, the effect of PU molecular weight on the adhesive’s bonding strength should be considered in terms of comprehensive factors such as molecular diffusion ability prior to curing, functional group density, as well as the toughness and cross-linking density of the cured product. A low molecular weight results in greater molecular mobility and stronger wetting capacity of the adhesive, which are conditions necessary for achieving good adhesion; if the molecular weight does not increase sufficiently during curing, the adhesion strength will be poor. In adhesives, a higher molecular weight of the prepolymer results in better initial bonding strength, while a lower molecular weight leads to weaker initial adhesion. A certain degree of cross-linking can improve the adhesive’s bonding strength, heat resistance, hydrolysis resistance, and solvent resistance. Excessive cross-linking affects crystallization and microphase separation. It may damage the cohesion strength of the adhesive layer.