Effects of Silica Nanospheres on the Sol-Gel Transition of Thermo-Responsive Polymers Based on poly(N-vinylcaprolactam)

The insertion of nanoparticles in smart polymers allows the addition of new functionalities to the material, as well as the improvement of some of its properties. However, the design of these materials needs to be done with care so that the stimulus-response property is not lost. An example of a responsive polymer is poly(N-vinylcaprolactam) (PNVCL), which suffers a transition when heated above the lower critical solution temperature (LCST). This transition occurs due to a decrease in the mixing entropy that leads to the breaking of hydrogen bonds between the polymer and the water molecules. As a consequence, the polymer self-aggregates through predominant hydrophobic interactions. This polymer is used in biomedical applications, mainly in tissue engineering and drug delivery systems, making its mechanical properties essential to support the daily movements of the human body. One way to enhance this property is through the insertion of nanoparticles of silica that can improve the rheological properties of polymers. Thus, nanocomposites of PNVCL and silica nanospheres were synthesized in situ by radical polymerization. Nanoparticles, of two different sizes (80 and 330 nm) were added at the proportion of 5% and the reaction was carried out for 4h at 70 °C in an inert atmosphere. The LCST and the energies involved during the transition were calculated from the analysis of a polymer aqueous suspension (1% m/v) by UV-vis spectrophotometry. The material changes from a transparent to opaque system when heated, allowing to be studied by its transmittance at different temperatures. All three synthesized polymers showed a LCST close to 34 °C, which results in gel formation in the body temperature. In addition, the pure polymer presented constant transmittance before the transition. On the other hand, the nanocomposites presented a diffuse decay of transmittance with increasing temperature not previously observed for pure polymer, indicating a diffuse transition. This behavior is related to changes in the solvation of PNVCL chains. The presence of nanoparticles disrupts the interactions between water molecules and polymer chains, facilitating phase separation [1]. This phenomenon was confirmed by the dynamic light scattering technique, in which the hydrodynamic diameter of the pure polymer remained constant at 4 nm during heating, while the mean globule size of the nanocomposites increased. The insertion of nanoparticles in the PNVCL can affect not only the transition profile but also its thermodynamics. The calculated enthalpy change during the sol-gel transition for the pure polymer was 9752 kJ/mol. However, the nanocomposites presented lower values, close to 6400 kJ/mol. This decrease was previously reported for crosslinked systems of thermosensitive polymers, indicating that in addition to interfering with the solvation of polymer chains, these nanoparticles also act as crosslinking agents [2]. Furthermore, the size of the nanoparticles did not interfere in the variation of the transition enthalpy. Considering the application of these biocompatible sensitive polymers as injectable systems, the effect of the nanospheres on the rheological properties of PNVCL was analyzed. When the solutions were heated to 37 °C, they still presented the loss modulus higher than the storage modulus, confirming the transition of the materials to the agglomerated state, however with no hydrogel formation. In addition, the nanocomposites presented higher values of G′ and G″ when compared to the pure polymer, confirming that the addition of the nanoparticles can improve the rheological properties after the transition. Thereby, the insertion of silica nanoparticles, despite altering the transition profile of PNVCL, manages to improve rheological properties without affecting their temperature sensitivity.<br/><br/>[1] - Ribeiro, L. S., et al., Langmuir, 2021, 37 (24), 7373-7379.<br/>[2] - Alf, M. E., et al., Langmuir, 2011, 27 (17), 10691-10698.