Side-Group Effect of Poly(ε-caprolactone)-Based Polyurethane on Thermal, Surface-Wetting and Mechanical Properties

Abstract

To effectively optimize the physicochemical properties of polyurethanes for practical applications, we have designed and synthesized a series of poly(ε-caprolactone)-based polyurethanes (PCL-based PUs) functionalized with five different side groups of similar atomic number but distinct rigidity and hydrophilicity. These PUs are prepared via a two-step route that the controlled ring-opening polymerization (ROP) is initially employed to obtain the azido-functionalized PCL diols, followed by the polyaddition with hexamethylene diisocyanate (HDI) and subsequent copper-catalyzed azide-alkyne cycloaddition (CuAAC) to introduce the target side groups. Successful incorporation of side groups is confirmed by SEC, 1H NMR spectrum, and FT-IR spectroscopy. The 1D and 2D wide-angle X-ray diffraction (WAXD) results suggest the formation of a liquid crystalline mesophase structure in the samples with a short side-group spacing. Additionally, the variable-temperature polarized optical microscopy (POM) and WAXD measurements are also performed on the 10-mer PCL-based PUs to further characterize the liquid crystalline mesophase structure. Systematic evaluation of the mechanical, thermal, and surface-wetting properties reveals that the PUs exhibit glass transition temperatures ranging from −60.9 to −31.6 ℃, close to neat PCL. They behave as thermoplastic elastomers with Young's moduli between 0.88 and 25.3 MPa and elongation at break from 744% to 1380%. The results about the contact angles of distilled water on the resulting PU films demonstrate that the wetting properties are closely related to the hydrophilicity of the side group itself but have almost no correlation with the side group spacing. This work provides a facile and cost-effective strategy for the side-chain modification of polyurethanes, allowing precise structural characterization of the resulting polymers. Using structurally comparable modifiers, the stiffness, wettability, and overall performance can be finely tuned, showing great potential in the design of functional PU materials.