3.2.1. Vicat Softening Temperature (VST)
The VST of a material is closely related to its heat tolerance, which can reflect the movement ability of chain segments. VST cannot be directly used to evaluate the actual temperature of a material, but it can be employed to guide the quality control of the material and is regarded as an evaluation standard for the heat resistance of a material [
40].
Figure 12 shows that the VST of polyester-based TPUs was higher than that of the polyether-based TPUs in the same circumstance; however, the polyester-based TPUs exhibited a similar variation trend as polyether-based TPUs. When the hard segment content was identical, the VST of the TPUs gradually increased with increasing
r. At a constant
r, the VST of the TPUs gradually increased with the increase in the hard segment content, i.e. the hard segment content showed a more important effect on the heat resistance of TPUs as compared to
r [
41]. When the hard segment content was increased by 10%, the VST of the TPUs increased more than 45%, which meant that the TPUs exhibited a weak flexibility at high temperature and could not move freely under a constant force. Therefore, the rigidity of the TPUs increased with increasing content of the hard segment, and the VST also increased because deformation tended to be difficult.
In addition, it was found that the hard segment content remained unchanged; both the relative molecular weight and the VST tended to increase with r increasing. Otherwise, when the r was a fixed value, as the hard segment content increased, the relative molecular weight of the TPUs decreased, while the VST results still showed an upward growth trend, indicating that the relative molecular weight had a weak effect on the heat resistance of TPUs.
3.2.2. Thermogravimetric (TG)
TGA and Derivation Thermogravimetry (DTA) were employed to investigate the thermal stability of polyester-based and polyether-based TPUs. As shown in
Figure 13, the TG curves of all samples showed one thermal decomposition process. The weight loss of the polyester-based TPUs accounted for about 20%, 30%, and 40% of the total mass in the temperature range of 290–410 °C when the hard segment content was 20%, 30%, and 40%, respectively. The weight loss stage at 410–500 °C was attributed to the thermal cracking of the soft segment (polyester) to generate small molecular gases and macromolecular volatile components; therefore, the weight loss was most obvious at this stage. Furthermore, the residue of each sample was slowly decomposed after 500 °C. The test results show that the thermal mass loss ratio of the soft and hard segments to the total mass was consistent with the total mass ratio of the soft and hard segments of TPUs [
42]. This meant that the decomposition of the weight loss temperature for TPUs occurred at about 290–410 °C owing to the hard segment thermolysis. Detailed thermal degradation parameters are shown in
Table 7.
In addition, as shown in
Table 7, for the identical hard segment content,
T5% and
T10% decreased with the increase of
r. Similarly, as the hard segment content increased, when the
r was constant,
T5% and
T10% decreased. Within a certain range of the hard segment content,
T5% and
T10% decreased with the increase of the relative molecular weight of the polyester-based TPUs. It was found through the comparison of the hard segment content and
r that the relative molecular weight showed a slight effect on the thermal weight loss of polyester-based TPUs.
The TG curves of the polyether-based TPUs are shown in
Figure 14, and the ratio of thermal decomposition temperature of polyether-based TPUs to residual mass is shown in
Table 8. All polyether-based TPUs displayed distinct weight loss at two stages. The polyether-based TPUs with higher hard segment content showed lower initiation temperature and lower thermal stability [
43]. At the first stage of pyrolysis, the temperatures of maximum weight loss were around 278–380 °C, which was similar to the curves of polyester-based TPUs, and this stage almost ended at 410 °C. The second stage of weight loss happened from 420 to 440 °C in all polyether-based TPUs regardless of soft segment content and molecular weight, which was caused by the thermal cracking of soft segment polyether to generate small molecular gases and macromolecular volatile components. Finally, the residue slowly decomposed after 450 °C, and then remained invariable up to 800 °C.
Table 8 shows that the thermal stability of polyester-based TPUs was obviously better than that of polyether-based TPUs. In addition,
T5%,
T10% tD, and
Tmax of the polyether-based and polyester-based TPUs seemed to have an identical variation tendency. Nevertheless, the residual mass fraction of polyether-based TPUs was evidently smaller than that of polyester-based TPUs, which indicated that polyether-based TPUs burned more fully than polyester-based TPUs.
3.2.3. Differential Scanning Calorimetric (DSC)
The crystallization of polymer is a process in which macromolecular chains transform from irregular arrangement to compact packing [
44,
45], based on which the crystallization status of a material can be judged by the position of the endothermic or exothermic peak in the DSC curve [
46]. Generally, TPUs display a structural characteristic of separate block polymers containing soft and hard segments. The DSC test results can not only characterize the microphase separation behavior of block copolymers, but also demonstrate the glass transition temperature (
Tg) of TPUs. Therefore, it is of great significance to investigate the crystallization behavior of TPUs to understand the relationship between their structures and properties since the crystallization of TPUs directly affects the mixing and separation of microphases.
Figure 15 and
Figure 16 show the cooling and second heating curves of polyester-based and polyether-based TPUs, respectively; their thermal transitions and relative crystallinities are shown in
Table 9 and
Table 10. The peak temperature of the melting peak is defined as the melting point (
Tm), the peak temperature of the crystallization peak is defined as the crystallization temperature (
Tp), and the peak areas in curves represent the crystallization enthalpy (Δ
Hc) and the melting enthalpy (Δ
Hm), respectively. In this study, both synthesized TPUs contained identical hard segment composition (MDI/BDO). To analyze the effects of the hard segment content on their crystallization, the hard segment crystallinity (
Xhs) of both TPUs was calculated according to the relative crystallinity equation:
Xc = (Δ
Hm − Δ
Hmax)/Δ
Hmax × 100% (where Δ
Hmax is the melting enthalpy by 100%, with the melting enthalpy by 100% of hard segment being 150.6 J·g
−1 [
6]). Generally, the more orderly are the molecular chains in the synthesized TPU, the better are the symmetry and corresponding crystallinity, and the faster is the crystallization rate [
47].
As shown in
Figure 15 and
Table 9, Δ
Hm of the polyester-based TPUs increased apparently with the increase of the hard segment content. When
r = 1, the hard segment crystallinity raised from 35.21% to 67.98%. This is because the molecular weight of the hard segment increased, and the hard segment phase exhibited high purity and preferable phase separation. With regard to the same hard segment content, the Δ
Hm of polyester-based TPUs increased first and then decreased, the peak shape became wider, and the
Tm and peak area became smaller with increasing
r. Meanwhile the crystallization peak area decreased, the
Tp decreased distinctly, and the peak shape widened slightly. This is mainly because the molecular weight gradually increased with the increase of
r and the molecular weight distribution became wider.
After the introduction of many –NCO, the symmetry and regularity in the polyester-based TPU molecular chains were disrupted, the crystallization ability of TPUs was gradually weakened, and thus the crystallization ability of the material was strengthened and then weakened. In addition, the
Tg gradually increased, which was mainly related to the irregular arrangement of –CH
2 in the molecular chains. The synthesized TPUs indicated a lower
Tg from −40.74 to −47.92 °C within the range of the hard segment content used.
Tg exhibited a slight increase with increasing content of the hard segment; similar results were reported by Illinger et al. [
48].
As can be seen clearly in
Figure 16, when the hard segment content was greater than 30%, a shift peak appeared at 75–120 °C in the DSC curves; as the hard segment content increased, the shift peak gradually became apparent and toward the direction of higher temperature. Hewitt et al. [
49] attributed this to the glass transition and plasticization of the hard segment phase. The occurrence of this phenomenon may also be the reason that the hard segment phase transitioned from a certain ordered state to a disordered state. As the hard segment content increased, the average length of the hard segment gradually increased, aggregation between hard segments gradually strengthened, and the ordered level increased, which suggested that, as the hard segment content in the TPUs increased the degree of both two-phase miscibility and microphase separation gradually increased. When the temperature was increased enough to destroy the aggregation force between the hard segments, the ordered state dissociated into the disordered state. It was found that the shift peak tended to be obvious and the dissociation temperature drifted to high temperature in the DSC curves. In summary, with an increase in the hard segment content, the structure of the polyester-based TPUs became more regular and easier to crystallize, and the degree of crystallinity presented basically no effect on
Tg, which was consistent with the findings of Seefried et al. [
50].
As shown in
Figure 16 and
Table 10, the crystalline character of polyether-based TPUs were stronger, which had the sharp melting peak and crystallization peak, and then the rate of its crystallization was faster, which was attributed to the regular arrangement of –CH
2 in chains of molecules inside polyether-based TPUs. When
r = 1, the hard segment crystallinity increased from 56.28% to 59.46%, because of the increased molecular weight of the hard segment that had higher purity and better phase separation. Both Δ
Hm and
Xhc increased and then decreased with increasing
r; concurrently, the backbone could also maintain a better symmetry and regularity in regard to the identical hard segment content.
In this study, the molecular weights of PBA and PTMEG were both 2000, and, even if some hard segment phase was dissolved into the soft segment phase, it had little influence on the movement ability of molecules in the whole long soft segment. In other words, the anchoring effect of the hard segment on the soft segment was not obvious. For a certain content of r, the melting peak strength decreased with the increase of the hard segment content, the peak shape widened, and both Tm and Tp increased, whereas the area of the melting peak and the crystalline peak tended to be smaller. Xhs gradually increased with an increase in the hard segment content, and Tg gradually decreased accordingly. PBA was used as the soft segment in the polyester-based TPUs with more hydrogen bonds, which cannot effectively form a microphase separation structure; hence, the melting shift temperature of the hard segment was lower. The incorporation of PTMEG destroyed the original orderly arrangement of molecular chains; nevertheless, it can effectively form a microphase separation structure, and thus the hard segment exhibited the highest melting transition temperature, which was consistent with the TGA findings. In addition, as the content of the hard segment increased, the Tg of polyester-based and polyether-based TPUs showed an upward variation trend as result of the higher hard segment content, the lower molecular weight, and the greater rigidity of the molecular chains. The relative crystallinity of the hard segment of the polyester-based TPUs in the same conditions was significantly higher than that of the polyether-based TPUs, and the Tg of the polyether-based TPUs was evidently lower than that of the polyester-based TPUs. This meant that the thermal behavior of polyester-based TPUs was better than that of the polyether-based TPUs. However, the low temperature performance of polyether-based TPUs was superior.
Thermal characteristics analysis results show that the hard segment content and r of the polyester-based and polyether-based TPUs had a significant effect on the heat resistance. When the hard segment content was increased by 10%, the VST of the TPUs grew by more than 45%. However, the relative molecular weight exhibited a weak effect on the heat resistance of TPUs. Moreover, the thermal stability of polyester-based TPUs was obviously better than that of polyether-based TPUs, hence the polyether-based TPUs burned fully more than the polyester-based TPUs. The higher was the hard segment content, the lower was the molecular weight, and the greater was the rigidity of the molecular chains. The relative crystallinity of the hard segment of the polyester-based TPUs under the same conditions was significantly higher than that of the polyether-based TPUs, and the polyether-based TPUs had a distinctly lower Tg than the polyester-based TPUs.