Thermoplastic Processing, Blends And Composites Of Cellulose Plasticized By Ionic Liquids

Cellulose, the most abundant natural polymer on the Earth with many advantages such as outstanding mechanical properties, biocompatibility, degradability and renewability, is considered to be one of the most promising substitutes for petroleum based synthetic polymers. The utilization of cellulose is limited due to the strong and complex hydrogen bonding networks formed by numerous hydroxyls make cellulose not to melt, and very difficult to dissolve. The development of green and efficient dissolving systems like N-methylmorpholine-N-oxide (NMMO), NaOH/urea and ionic liquids has led to breakthrough in wet processing of cellulose. However, the thermoplastic processing is much more preferred by modern plastic industry, and this important way is still lack of progress so far. The dissertation focused on the plasticizing effect of ionic liquids on thermalplastic processing of cellulose, structures and properties of ionic liquid plasticized cellulose (IPC) and its blends and composites were also studied.Firstly, IPC was prepared by 1-butyl-3-methylimidazolium chloride (BmimCl) and microcrystal cellulose (MCC) using direct thermalplastic processing. The effects of BmimCl content on morphological structure, mechanical properties, rheological characteristics and glass transition behaviors of IPC were investigated systematically. The results showed that BmimCl could break hydrogen bonding networks, swell the amorphous region of cellulose and destroy the crystal one. With increasing the BmimCl content, the processability of IPC increased, while its crystallinity, strength and modulus decreased. Based on the free volume transition and the percolation of continuous hydrogen bonding networks, the effects of free volume and hydrogen bonding interactions on IPC glass transition behaviors were differentiated. The glass transition temperature (Tg) of IPC was related to the free volume transition at high BmimCl content, and related to the percolation of continuous hydrogen bonding networks at low BmimCl content, where the critical mass fraction of BmimCl was 30%. Furthermore, the phase diagram with four regions was plotted for IPC, and Tg of cellulose was extrapolated to be 456 K, which was useful to optimize its processing and modulate its properties.Secondly, ionic liquid plasticized cellulose electrolyte (IPCE) was prepared by thermal compression. The effects of BmimCl content on ionic conductivity, compressing properties and thermal stability of IPCE were studied. The results indicated that the IPCE exhibited satisfactory ionic conductivity and compressing resistance. The thermal stability of IPCE declined comparing with MCC because the condensed structure of cellulose was partially destroyed, but the IPCE with 40 wt% BmimCl still showed a high conductivity of 5.6 × 10-4 S·cm-1 even kept at 160? for 40 days. The interactions between the cations of BmimCl and the hydroxyl groups of cellulose at elevated temperature promoted the dissociation of BmimCl, which led to a further increasing in conductivity. The ion conducting ability of IPCE increased with the addition of BmimCl, which could be originated from the improved mobility of ions at low BmimCl contents and the increased quantity of ions at high BmimCl contents, respectively.Thirdly, poly(propylene carbonate) (PPC) was added into IPC by molten blending. The effects of component content on the phase separation behaviors, morphological structure, mechanical properties, glass transition temperature and hygroscopicity of cellulose/PPC/BmimCl blends were studied. The blends showed a sea-island morphology. The phase separation of the blends, which caused by the migration of BmimCl from cellulose into PPC, was enhanced by increasing PPC content. The migration of BmimCl also reduced the plasticizing effect on cellulose and prevented BmimCl from moisture, so the glass transition behaviors of IPC disappeared and hygroscopicity of the blends were depressed. With appropriate PPC contents, the cellulose/PPC/BmimCl blends showed good mechanical properties.Finally, multi-walled carbon nanotubes (MWNTs) were used as fillers to reinforce IPC, resulting in cellulose/MWNTs/BmimCl composites with high strength, modulus and thermal stability. The strength and modulus of composites with acid-treated MWNTs were higher than those with pristine MWNTs at low content, because the acid treatment of MWNTs enhanced their interfacial strength with cellulose matrix. However, acid-treated MWNTs badly dispersed in BmimCl and composites because of the interruption of cation-? interaction, leading to decreasing mechanical properties with the increasing of MWNTs content. The short aspect ratio of MWNTs cut by excess acid treatment reduced the reinforcement but dramatically improved the toughness, because the short MWNTs had good bridging effect during the fracture of composites. Glass transition temperature, electric resistance and capacitance values of composites decreased with increasing pristine MWNTs content. However, they did not show strong regularity with acid-treated MWNTs content due to the comprehensive influence of interfacial strength, dispersity, and aspect ratio.