Investigation of the luminescence, mechanical, and thermal properties of ZnO-entrapped poly(N-isopropyl acrylamide) gels

ZnO nanoparticles were synthesized in the pores that are formed inside the PNIPAm (poly(N-isopropyl acrylamide)) gels during polymerization. Different pore regions were obtained with changing the cross-linker concentrations of gels. We concluded from the SEM results that when the cross-linker concentration of the gels is increased, the average size of the pores becomes narrower. Thus, depending on the internal porosity of the gel, the size and distribution of ZnO nanoparticles were changed and accordingly it ranged in the fluorescence spectra. It is observed that fluorescence spectra of ZnO nanoparticles released from PNIPAm gel with low cross-linker concentration have only UV emission, while the nanoparticles released from PNIPAm gel with high cross-linker concentration have both the UV and the green emissions of ZnO. The size distributions of these nanoparticles were measured by Stabino-Nanoflex analyzer and it clearly indicates that the width of the size distribution decreases with increasing cross-linker concentration. To see the effect of ZnO nanoparticles on the properties of the gel, we investigated the luminescence, mechanical, and thermal properties of these ZnO/PNIPAm composites and plain gels of which the internal morphology was set by changing the cross-linker concentration. The nature of interactions between the nanoparticles and polymer strands affects both mechanical and thermal behaviors. Compared to plain gels, entrapping ZnO nanoparticles into the gels cause an increased compressive elastic modulus and strength. We proved that ZnO nanoparticles improve considerably the mechanical properties of PNIPAm gels.