## 2 Results and Discussion
### Sol-Gel-Sol Phase Transition and Rheological Properties  
The sol-gel-sol transition of thermosensitive PCEC hydrogels was investigated using the tube inverting method. According to the data in Table 1, when the \(\mathrm{M_{n}}\) of the PEG block was below 1000 g/mol, the PCEC copolymers were insoluble in an aqueous environment and did not form a homogeneous sol; the \(\mathrm{M_{n}}\) of the copolymers was too low to form stable micelles. Furthermore, the hydrophobic/hydrophilic moieties ratio was relatively high, limiting the copolymer's water solubility. For hydrogels obtained from PEG 1500 and PEG 2000, the gel-sol transition was not observed; the mixtures remained undissolved or formed a transparent sol up to the temperature of precipitation. The temperature-dependent gelation behavior of hydrogels was observed when PEG 1000 was used as the polymerization co-initiator, and the LCST value was close to physiological temperature. Specifically, when the temperature reached LCST, the sol became opaque and formed a stable gel with good mechanical properties (Figure 7). After analysing the PCEC structure, it was determined that the LCST is primarily determined by the PCL/PEG ratio and the \(\mathrm{M_{n}}\) of the copolymer. In brief, increasing the hydrophobic/hydrophilic balance decreases the gelation temperature and limits the system's water solubility. When the hydrophobic block size increases, the hydrogel is hydrated less effectively, allowing the sol-gel transition to occur more easily; the LCST decreases. On the other hand, it was determined that increasing the copolymer \(\mathrm{M_{n}}\) increases the LCST. The PCEC micelles became more stable and exhibited the sol-gel transition at higher temperatures, or did not perform the transition at all. It is suspected that for higher \(\mathrm{M_{n}}\) values there have fewer PCEC macromolecules in a solution of a given concentration than for lower \(\mathrm{M_{n}}\) copolymers. In the described experiment, the \(\mathrm{M_{n}}\) of PEG remained constant, thus an increase in PCL/PEG ratio was correlated with an increase in the copolymer \(\mathrm{M_{n}}\). The effects of these two parameters are opposite to each other, therefore, it is difficult to define the LCST's dependence on these parameters unanimously. However, based on the experimental data, it is possible to conclude that the optimal CL/PEG ratio is 1.7 or 2.0 [m/m], and these ratios were chosen for further investigation. The relationship between the copolymer concentration and the LCST and UCST was evaluated for PCEC-A1.7 and PCEC-A2.0; the concentration range was 5-30 wt%. The phase transition diagrams were presented in Figure 8.  
Figure 7: The example of thermosensitive hydrogel phase transition (PCEC-A2.0, concentration 20 wt%). In general, it was discovered that as concentration increases, the gel window expands for Sample A; for Sample B the effect was not clearly observed (Figure 3). At higher concentrations, there are more PCEC macromolecules per volume unit, and the intermolecular interactions are stronger; micelles expand and can form a non-flowing gel at lower temperatures. Furthermore, at the same concentration, the LCST values of Hydrogel B are lower than those of Hydrogel A. This phenomenon could be explained by the fact that the M\({}_{\text{n}}\) of the PCEC copolymer that forms Hydrogel B is slightly higher than that of Hydrogel A (3400 g/mol versus 3000 g/mol, respectively). It is well known that increasing the M\({}_{\text{n}}\) of PCEC causes the LCST of the hydrogel to increase. The same phenomenon is seen with UCST, but it is less noticeable [17]. However, no significant relationships were found between copolymer concentration and UCST values. The gel-sol transition of PCEC hydrogels is thought to be caused by micelle disintegration, which leads to copolymer concentration. Figure 8: Phase transition diagrams of PCEC-A1.7 (**A**) and PCEC-A2.0 (**B**). precipitation.