Tuning the surface microstructure and gradient properties of polymers with photopolymerizable polysiloxane-modified nanogels

This paper reports a series of photopolymerizable polysiloxane-modified nanogels for regulating the surface microstructure and gradient properties of polymers, which were synthesized by solution polymerization with different feed ratios of methacrylate-modified polysiloxane, urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of a thiol chain transfer agent. The nanogel structures and compositions were characterized by proton nuclear magnetic resonance (H-1-NMR), Fourier transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The dispersion of these nanogels in triethylene glycol dimethacrylate (TEGDMA) can delay the onset and reduce the magnitude of shrinkage stress during polymerization without compromising the mechanical properties of the resulting polymers. Most importantly, as demonstrated by elemental analysis and X-ray photoelectron spectroscopy (XPS), the nanogels exhibit good self-floating ability in the monomer-polymer matrix and the increase in polysiloxane content in the nanogel can enhance the self-floating capability due to the lower surface tension and energy associated with the polysiloxane component. As a result, the polysiloxane-modified nanogels can spontaneously form a concentration gradient that can be locked in upon photopolymerization, leading to a well-controlled heterogeneous polymer that presents a gradient change in thermal stability. With the increase in polysiloxane content, the thermal stability of the polymer was improved significantly. Furthermore, the enrichment of the nanogel on the surface resulting from the good self-floating ability can reduce the dispersion surface energy of the gradient polymer film and generate a more hydrophobic surface with an altered surface microstructure. These photopolymerizable polysiloxane-modified nanogels are demonstrated to have potentially broad applications in the preparation of gradient polymers with controlled surface properties.