Understanding the Melting and Crystallization of Polyurethanes: A Deep Dive

Polyurethanes are versatile polymers with unique thermal and mechanical properties that make them suitable for various applications, particularly in biomedical fields. Their behavior under heat is a complex interplay of microphase separation and crystallization processes. Research indicates that the melting endotherm observed in these materials is linked to a transition towards a disordered state, as the microphase structure dissolves. This phenomenon emphasizes the importance of sample conditioning in influencing the crystalline nature of polyurethanes.

In a study utilizing differential scanning calorimetry and small-angle X-ray scattering, two high-temperature endotherms were identified during the melting process. The first endotherm corresponds to partial disordering of the microdomain structure, while the second signifies complete disordering. Such findings highlight how different thermal cycles impact molecular organization and crystallization behavior. Conditioning the samples in various temperature regions can result in a more crystalline structure, affecting the material's overall properties.

The thermal treatment of polyurethanes profoundly affects their crystalline structure and mechanical properties. For instance, quenching a polyurethane sample from the melt and subsequently reheating it can facilitate crystallization at higher temperatures, ultimately yielding structures with significantly higher melting points than those achieved through direct cooling. This behavior can be attributed to the formation of crystal nuclei from prior thermal cycles, which serve as seeds for crystallization at elevated temperatures.

Different thermal cycles can also lead to polymorphic transitions, as demonstrated by researchers studying MDI/BD-based polyurethanes. Initial crystallization typically leads to contracted crystal structures, while processes like annealing and stretching encourage the formation of extended crystal polymorphs. These variations not only influence the melting points but also affect the mechanical properties of the materials, illustrating the critical role of thermal history in defining the characteristics of polyurethanes.

When evaluating the mechanical properties of biomedical polyurethanes, it becomes evident that their tensile strength and elasticity vary significantly when compared to other biomedical polymers. Polyurethanes exhibit tensile strength ranging from 25 to 62 MPa and elongation percentages between 355% and 800%. In contrast, other polymers in the biomedical market often display tensile strengths as high as 3000 MPa. Such differences underscore the unique attributes of polyurethanes, particularly their enhanced elasticity, which is paramount in applications requiring materials to closely mimic biological tissue.

Despite the advancements in understanding the thermal and mechanical properties of polyurethanes, there appears to be a gap in the literature regarding the consideration of thermal history in the selection of biomaterials. This oversight could lead to mismatches between the mechanical requirements of biological replacements, such as arterial prostheses, and the materials used. A careful evaluation of thermal history alongside mechanical characteristics is crucial for optimizing the performance of polyurethane-based biomedical applications.