Understanding the Interactions between Soft Segments in Polyurethanes: Structural Synergies in Blends of Polyester and Polycarbonate Diol Polyols

Abstract

There are no previous studies on the interactions between polyols of different nature as a model for understanding the interactions between soft segments in PUs. In this study, different blends of two polyols of different natures (polyester—PE, and polycarbonate diol—CD) and similar molecular weights were prepared and their structural, thermal, surface, viscoelastic, and self-adhesion properties were assessed. Different experimental techniques were used: infrared spectroscopy (ATR-IR), differential scanning calorimetry (DSC), X-ray diffraction, thermal gravimetric analysis (TGA), and plate–plate rheology. PE showed a larger number of structural repeating units and a higher number of polar groups than CD, but the carbonate–carbonate interactions in CD were stronger than the ester–ester interactions in PE. The blending of CD and PE imparted synergic structural properties, particularly in the blends containing less than 50 wt.% PE, they were associated with the disrupt of the carbonate–carbonate interactions in CD and the formation of new ester–carbonate and hydroxyl–carbonate interactions. CD + PE blends with less than 50 wt.% PE exhibited higher glass transition temperatures, a new diffraction peak at 2θ = 24°, one additional thermal degradation at 426–436 °C, and a less-steep decline of the storage moduli. Furthermore, the different interactions between the polyol chains in the blends were also evidenced on their surface properties, and all CD + PE blends showed self-adhesion properties which seemed related to the existence of ester–carbonate and carbonate–carbonate interactions.

Dynamic Non-Covalent Exchange Intrinsic Self-Healing at 20 °C Mechanism of Polyurethane Induced by Interactions among Polycarbonate Soft Segments

“…In fact, the polyols (CD and PE) show diffraction peaks at the same 2θ values as their corresponding PUs (Figure 12), and, therefore, the crystallinity of YCD and YPE should derive from the interactions between the soft segments. The diffraction peaks at 2θ values of 21 • and 22 • in PE polyol have been ascribed to ester-ester interactions and the ones at 20 • and 23 • to carbonate-carbonate interactions in CD polyol [84]. On the other hand, the intensities of the diffraction peaks of YPE are higher than the ones of YCD (Table 3) and they are lower than the ones of the corresponding polyols (Table 4), to a greater extent in YPE.…”

“…The assignment of the thermal degradations is based on the DTGA curves of the polyols (Figure 14) that only show one main thermal degradation at 340 • C and 352 • C, respectively. These thermal degradations have ascribed to carbonate-carbonate and ester-ester interactions, respectively [84]. Previous studies have shown that the TGA curves of PUs show differenced thermal degradations of the hard and soft segments [66,80].…”

“…The assignment of the thermal degradations is based on the DTGA curves of the polyols (Figure 14) that only show one main thermal degradation at 340 °C and 352 °C, respectively. These thermal have been ascribed to carbonatecarbonate and ester-ester interactions, respectively [84]. The main thermal degradations of YCD and YPE at 311 °C and 300 °C can be ascribed to the interactions between the soft segments, which are more important/stronger in YCD (higher weight loss is obtained).…”

“…The chemical differences between YCD and YPE rely in the lower percentage of urethane species, the higher percentage of free urea, and the existence of free carbonate groups in YCD with respect to YPE. In fact, a previous study has established that PE polyol had 88% free ester species at 1730 cm −1 and 8% bonding by dipole-dipole interactions of C=O groups at 1712 cm −1 , whereas the CD polyol had 36% free carbonate species at 1741 cm −1 , and 64% bonding by dipole-dipole interactions of C=O groups at 1730 cm −1 [84]. The structure of the PUs was also assessed by DSC.…”