# Poly-(3-ethyl-3-hydroxymethyl)oxetanes--Synthesis and Adhesive Interactions with Polar Substrates
## 3 Results and Discussion
The supposition is confirmed by the work of adhesion (\(W_{a}\)) values 101-105 mJ/m\({}^{2}\), which were just ca. 20% lower than the work of adhesion of water on natural wood surfaces (126-133 mJ/m\({}^{2}\)) [36]. Thus, the abovementioned surface characteristics of POXs' films are a sufficient ground to hypothesize that POXs are efficient hot melt adhesives for polar substrate bonding. \begin{table}
\begin{tabular}{c c c c} \hline
**TMP/EHO Molar Ratio** & \(M_{\rm n}\) & \(M_{\rm w}\) & \(D\) \\ \hline
1:5 & 1240 & 1690 & 1.77 \\
1:10 & 1220 & 1650 & 2.18 \\
1:20 & 1160 & 1390 & 2.52 \\
1:50 & 1310 & 1850 & 3.57 \\ \hline \end{tabular}
\end{table} 
Table 2: Molar mass and dispersity (D) of POXs determined by gel permeation chromatography (GPC). Figure 6: MALDI TOF (DCTB, Na+) spectrum of POX with 1,1,1-tris(hydroxymethyl)propane (TMP) core; a-a distance \(\Delta m/\!\!z=116\). In order to empirically verify the hypothesis, the softening and flow points of the polymers were determined (Table 3). The \(T\)s values were comparable to those of the commercial hot melt adhesives (88-105 \({}^{\circ}\)C) used in furniture manufacturing. Bonding experiments performed on birch veneers revealed apparent differences in joints shear strength (Figure 7). One can see that the POX 1:20 and 1:50 series exhibited much higher values (1.32 MPa and 1.05 MPa, respectively) than those of the 1:5 and 1:10 series (0.39 MPa and 0.86 MPa, respectively). The phenomenon is associated with both the abundancy of hydroxyl groups and the ability of hydrogen bond formation, which increases with the degree of branching as well as with the mechanical properties of POXs. Neither of them exhibited cohesive failures in wood, whereas cohesive failure in the POX layer was found in each case (Figure 8). Such behavior resulted from an adhesion to polar substrate that was higher than the cohesion within POXs. As shown in Figure 9, in all cases, a brittle fracture occurred at a low extension during the shear test (0.18-0.97 mm). It is known from the literature that brittle materials fracture under relatively low stress [48], and, moreover, the performance of high modulus adhesives on low modulus substrates is not optimal [49]. However, the brittle character of the investigated POXs cannot be associated with a highly crystalline structure since [PERSON] et al. demonstrated that the degree of crystallinity decreases with increasing degree of branching in POX and is close to 0% for the degree of branching ca. 40% [11]. The highest extension to fracture was observed for the 1:20 POX, which indicates its abilities for higher deformation and stress dissipation than the other investigated POXs. Subsequently, the increased shear strength of the bond-line was yielded. The mechanical properties of the bond-lines confirmed our assumptions regarding the strong adhesive interactions of POXs with polar substrates. It is noteworthy that the observed shear strength of the 1:20 series exceeded the minimum required for plywood (\(\geq\)1.0 MPa) [50] and were comparable to those reported for poly(lactide)-poly(\(\varepsilon\)-caprolactone)-based and ethylene-vinyl acetate based hot-melt adhesives (0.6-1.5 MPa [51]) but still lower than those reported for FeCl\({}_{3}\)-curable polyoxetanes [30]. [PERSON] and [PERSON] investigated plywood bonding with HDPE and demonstrated that shear strengths of the bond-lines greatly depended on the time, pressure, and temperature of the process [52]. The authors obtained the highest strengths for a 160 \({}^{\circ}\)C temperature and a 3 min press time, which were necessary for proper penetration of the molten adhesive into the substrate. Similar observations were made by [PERSON] and co-workers for polypropylene-based hot-melts [53]. As it is commonly agreed that the quality of bonding strongly depends on processing parameters such as temperature, time and pressure [54], further studies on POX bonding parameters are required.