Study Of Shrinkage Stress Reduction And Control Of Microstructure And Gradient Property Of Polymer Network By Silicone-Containing Nanogel

Nanogels, the structures between branched polymers and macroscopic crosslinking networks, are defined as internally physically or chemically cross-linked and cyclized single or multi-chain polymeric particles with three-dimensional polymer networks and nanoscale size (10-1,000 nm). In recent years, it has been demonstrated that nanogels can be used to reduce volumetric shrinkage and shrinkage stress during polymerization. As is well known, volumetric shrinkage and shrinkage stress in the polymerization process, which contribute to internal and interfacial defects, warped structures as well as inferior mechanical properties, remain as critical problems for a wide array of polymeric materials applications. Polysiloxanes have many unique properties such as good resistance to high or low temperatures, excellent weatherability, low surface tension and energy, low dielectric constant, high resistance to an array of chemical substances and physiological inert due to their unusual molecular structures. This stimulated our interest to introduce polysiloxane into nanogels to engender some new desirable properties, as well as to examine the possibility of preparing functionally and structurally gradated polymers with controlled surface properties. The research on polysiloxane-modified nanogels could provide the theory and technology basis for application of photopolymerization materials, broaden polysiloxane application fields. Besides, the research also provided a new way to prepare gradient polymers and control structures as well as properties of polymers and polymer surface. The main research contents and conclusions are described as follows:1. A series of nanogel compositions were synthesized with different ratios of the methacrylate-modified polysiloxanes with different chain lengths, urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of 2-mercaptoethanol (ME) as a chain transfer agent. Their structures were characterized by proton nuclear magnetic resonance (1H NMR), silicon nuclear magnetic resonance (29Si NMR), Fourier transform-infrared spectroscopy (FTIR), gel permeation chromatography (GPC), transmission electron microscopy (TEM), dynamic light scattering (DLS) and differential scanning calorimetry (DSC). The results show that the synthesized polysiloxane-modified nanogels have similar structures and appeared as microspheres. Molecular weight (Mw), polydispersity index (PDI), Tg and size of the nanogels are related to the polysiloxane content and the length of polysiloxane chain in nanogels. With the addition of nanogels at 10-20 wt%, the effect of the dispersed nanogel on TEGDMA viscosity is minimal. With the addition of nanogels at 30-40 wt%, the viscosity increase in an exponential manner.2. The kinetics of photopolymerization, shrinkage stress and mechanical properties of nanogel-modified UV-cured materials were investigated. The results indicate that with the addition of nanogels at modest loading level, the nanogel-modified systems presented a significant delay in the onset of stress and the photopolymerization stress was obviously decreased without compromising the mechanical properties of the resulting polymers.3. The influences of polysiloxane-modified nanogels on properties of UV-cured films were also investigated. The results show that the nanogels have relatively good compatibility with TEGDMA and the invistigated UV-cured film have only one Tg. With the increase of nanogel loading levels and the polysiloxane content as well as the length of polysiloxane chain in nanogels, Tg and tensile strength of UV-cured films were decreased and elongations at break were increased. With the addition of polysiloxane-modified nanogels, the thermal stability of UV-cured films was increased and UV-cured films became more compact and hydrophobic.4. It is demonstrated by elemental analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis and hardness measurement that polysiloxane with the particularly low surface tension and energy offers the nanogels relatively good self-floating ability. The nanogels can spontaneously form concentration gradient in a photopolymerization system, thereby controlling gradient polymerization to obtain a polymer with gradient in structures and properties that involve thermal stability and hardness. The increase of polysiloxane content and polysiloxane chain length in the nanogel can enhance self-floating ability of the nanogel. Besides, polysiloxane chain length is the main driver for the self-floating of the nanogel.