## 3 Results and Discussion
### Rheological Behavior  
Figure 5 presents the gelation kinetics of Plx-\(g\)-CMP at a concentration of 13% (\(w/v\)) when the sample is heated in situ at the physiological temperature. Firstly, the sample was introduced into the rheometer geometry at 4 \({}^{\circ}\)C (Figure 5a) or 20 \({}^{\circ}\)C (Figure 5b), heated at 37 \({}^{\circ}\)C and then the evolution of the viscoelastic parameters was followed in time at a constant oscillation frequency (1 rad/s) and strain amplitude (1%). Depending on the rest temperature before the test, the gelation started after 990 s or 280 s, when \(G^{\prime}\) and \(G^{\prime\prime}\)  
Figure 4: Influence of the polymer concentration on the gelation temperature of Plx (\(\bullet\)) and Plx-\(g\)-CMP (\(\blacklozenge\)) in phosphate buffer 0.05 M at pH = 7.4 (**a**). Schematic representation of sol–gel transition of the Plx-\(g\)-CMP solution (**b**). increase very fast, more than two decades, and then the viscoelastic parameters change more slowly in time as the tridimensional structure evolves (tan \(\delta<1\)) to a stationary state that is independent of the rest temperature or Plx-g-CMP concentration. For higher concentrations (22 and 30%, \(w/v\)), the gelation time is tenths of seconds for samples kept at 4 \({}^{\circ}\)C, whereas for samples kept at 20 \({}^{\circ}\)C, the gelation occurs nearly instantaneous when the temperature is raised to 37 \({}^{\circ}\)C, making these samples not appropriate for an injectable hydrogel. In the case of a concentration of 13% (\(w/v\)), a gelation time of 280 s obtained for the sample stored at 20 \({}^{\circ}\)C can be considered a too short a time to develop a network structure; moreover, 990 s could be an optimum gelation time for an injectable hydrogel. Another observation is the fact that for all samples it was observed that there are two steps during the gelation process (as, for example, after 990 s and 2500 s in Figure 5a or 280 s and 2000 s in Figure 5b), which is probably due to the competition between different types of interactions (hydrogen bonds, hydrophobic interactions) that develop in time. For samples kept 24 h at 4 \({}^{\circ}\)C, the viscoelastic parameters were also followed as a function of temperature for a heating rate of 0.5 \({}^{\circ}\)C/min, \(\omega=1\) rad/s, \(\gamma=1\%\). An increase of the viscoelastic moduli was registered at the gelation temperature that is located around 37 \({}^{\circ}\)C for 13% Plx-g-CMP (Figure 5c) and below 20 \({}^{\circ}\)C for higher concentrations. In the gel state, tan \(\delta\cong\) 0.08-0.1 regardless of the polymer concentration. Another observation concerns the behavior at a decreasing temperature, when the sample is submitted to cooling with a rate of 0.5 \({}^{\circ}\)C/min. Between 50 \({}^{\circ}\)C and 28 \({}^{\circ}\)C, there is a delay in the answer to the thermal stimulus, whereas the complete and fast structure recovery was observed at low temperatures (below 28 \({}^{\circ}\)C), when the hydrophobic interactions diminished considerably. Figure 5: The evolution of the viscoelastic parameters in time for 13% (\(w/v\)) Plx-g-CMP after 24 h of rest at (a) 4 \({}^{\circ}\)C; (b) 20 \({}^{\circ}\)C (\(\omega=1\) rad/s, \(\gamma=1\%\)), raising the sample temperature at 37 \({}^{\circ}\)C inside of the rheometer; as a function of temperature at increasing (full symbols) and decreasing the temperature (empty symbols) after 24 h of rest at 4 \({}^{\circ}\)C (heating/cooling rate of 0.5 \({}^{\circ}\)C/min, \(\omega=1\) rad/s, \(\gamma=1\%\)) (c); as a function of the oscillation frequency after 2 h of rest at 37 \({}^{\circ}\)C for the sample previously kept at 4 \({}^{\circ}\)C (\(\gamma=1\%\)) (d). As the network structure is well formed at 37 \({}^{\circ}\)C (after about 2 h of rest at 37 \({}^{\circ}\)C), the viscoelastic parameters are independent on the oscillation frequency (\(\gamma\) = 1%) as it is shown in Figure 5d; \(G^{\prime}\) > \(G^{\prime\prime}\) and tan \(\delta\cong\) 0.1.