Study Of Vinyl Acetate Pickering High Internal Phase Emulsions Stabilized By Cellulose Nanocrystals

Colloidal particles with a size in the range of 100 nanometers to 10 microns can act as emulsifiers to stabilize emulsions,which is called Pickering emulsions.Compared with emulsions stabilized by traditional surfactants,Pickering emulsions have abundant advantages,such as low toxicity and environmental responsiveness.Pickering high internal phase emulsions(HIPE)are a special type of emulsion system with a high internal phase volume ratio(greater than or equal to 74%).They are not only widely used in different fields such as food,cosmetics,pharmaceuticals,coatings,etc.,but also have good research prospects in emerging fields such as3 D printing.However,most of the colloidal particles currently used for HIPE are non-degradable or bio-incompatible,which leads to potential risks to the environment or human health.Therefore,it is very significant and meaningful to study colloidal particles with wide sources,good biocompatibility and low toxicity to stabilize Pickering emulsion and HIPE.The cellulose nanocrystals(CNCs)have high crystallinity and small size,and are easily adsorbing at the oil/water interface to form a thick coating layer,which is a promising Pickering emulsifier.In this paper,CNCs were selected as Pickering emulsifier and vinyl acetate(VAc)was used as oil phase.First,the synergistic effect of oil-water ratio and CNCs concentration on the micro-morphology and macro-state of Pickering emulsion was studied.The ternary phase diagram was used to predict emulsions polymerization;Subsequently,in order to solve the problem of increasing the concentration of CNCs that slightly improved its emulsifying property and made the emulsion system gelation which greatly limited its fluidity,this study planned to find a pathway that CNCs could stabilize homogeneous Pickering emulsions and even HIPE at low concentrations.By means of physical blending,methylcellulose(MC)with high viscosity and some hydrophobic groups was entangled with CNCs through hydrogen bonding to synergistically prepare HIPE and its polymerization product;Finally,for purpose of reducing the viscosity of the HIPE system,the liquid state HIPE polymerization product could be obtained and the application scope could be broadened.Using sodium periodate and dodecylamine to hydrophobically modify CNCs(12-CNCs)by first oxidation and then grafting twelve long carbon chains,the physicochemical properties of the 12-CNCs were studied,and the stability of the 12-CNCs in HIPE was further explored.And the adhesive properties of the polymerized products were also analyzed.The results are as follows:(1)When the CNCs concentration was low and the oil-water ratio was 90/10(v/v),the phase inversion of the emulsion system occurred,so the homogeneous water-in-oil Pickering emulsion was prepared.Its polymerization products were composed of polyvinyl acetate(PVAc)as the main structure,and there were a large number of micro-pores,which could be used as adsorbents for some organic solvents;In the case of low oil-to-water ratio and high concentration of CNCs,the CNCs were sufficient to completely coat and stabilize the limited oil phase,resulting in a homogeneous oil-in-water Pickering emulsion.On the premise that the CNCs could be uniformly dispersed into the water phase,and with the increase of the CNCs concentration,the oil-water ratio range that could form a homogeneous oil-in-water Pickering emulsion increased.In its polymerized emulsion,PVAc particles and CNCs formed a solid three-dimensional network structure in the continuous phase with shear-thinning properties.Adjusting the oil-water ratio could significantly change the rheological properties of the polymerized emulsion.(2)In order to rapidly and efficiently improve the emulsifying ability of CNCs,MC with high viscosity and partial hydrophobic groups was introduced to obtain stable HIPE.The results exhibited that there was no chemical reaction after blending CNCs and MC and only connected to each other through hydrogen bonding.The introduction of MC did not destroy the cellulose type I crystal structure of CNCs.The entanglement of MC improved the stability of the entire emulsion system,because some of the hydrophobic groups strengthened the emulsifying properties of CNCs,and the high viscosity of the system also contributed to the stability of the emulsion.In addition,with the gradual increase of the proportion of the internal phase,the emulsion droplets continuously participated in the process of gel formation,which constituted a stable gel network structure.It also had the shear thinning property.Under high shear force,the network structure was destroyed and the fluidity was restored;Under low shear action or static state,the viscosity was higher,and the shape and storage stability were better.Besides,the viscosity of the HIPE was higher,and the increase of the internal phase ratio made the HIPEs’ viscosity further increased,and the force between the droplets was stronger,so a tightly connected PVAc host structure was further formed in the polymerization process,which could keep the porous structure intact after polymerization.(3)On the basis of obtaining high viscosity HIPE,in order to reduce the excessive force between the emulsion droplets,the HIPE polymer product in liquid form was obtained to broaden its application range.Starting from the body of CNCs,through simple and safe hydrophobic modification,the emulsification ability was improved,and the HIPE system was controlled to stabilize in a suitable viscosity range.The results revealed that the hydrophobic modification had no significant effect on the crystal structure of CNCs,and the oxidation and grafting reactions mainly occurred in the amorphous structure region of CNCs.The 12-CNCs’ average width increased.The introduction of twelve carbon chains only changed the hydrophobicity of CNCs,but had no effect on the viscosity of the emulsion system.In this part,the restriction effect of emulsion droplets and CNCs’ random motion was quite weak,so its capacity of stabilizing HIPE was relatively poor,but these created conditions for the polymerization to obtain a HIPE polymer product in a liquid state.Moreover,the hydrogen bonding of the CNCs after hydrophobic modification was weakened,and the network structure constructed by the 12-CNCs was relatively loose and fragile.During high temperature stirring and polymerization,the HIPE droplets were in an unstable state and could not be in close contact.Finally,the low viscosity polymeric emulsions composed of the 12-CNCs were partially wrapped in PVAc particles were obtained.Due to the partial substitution of hydroxyl groups,the wettability with the surface of the wood specimens decreased,so the liquid polymer product had lower adhesive properties.