Preparing And Application Of Cellulose ? Nanomaterials

Cellulose nanocrystalline(CNC)has a high production cost and a low yield(10-30%)by traditional methods.If efficient production technology can be used to achieve mass production and obtain CNC with controllable properties,it will be conducive to the further promotion and application of CNC-based products.In this paper,the change of cellulose supramolecular structure during sulfuric acid hydrolysis was studied,revealling the essential reason why the yield,structure and properties of CNC changed with the preparation conditions.Thus,the yield of CNC prepared by acid hydrolysis would be improved and the controllable properties could also be obtained.The effects of cellulose nanowhiskers with different structures and properties on the reinforcement of polyvinyl alcohol(PVA)were further studied,providing a possible way for its utilization and promotion.(1)Polymorphism is an important factor associated with the cellulose nanomaterial properties.In this paper,cellulose fibers(CF)were efficiently isolated from waste Tetra pak packages,and cellulose ? and ? nanocrystals were produced by treatment of CF with 64%sulfuric acid and controlling the reaction time from 15 to 30 min in 45°C.Cellulose ?(C?)was partially converted to cellulose ?(C?)within 15 min and the resulting cellulose nanocrystal product(i.e.CNC15)contained 93.2%C?.Further extending the hydrolysis time decreased the C? content of CNC20 to 25.5%and CNC30 was completely C? without C?.CNC15 has a twisted strip structure,while CNC20 and CNC30 has a rod or spindle structure.The size of CNC15(104.4±32.2 nm and 10.2±2.3 nm)was similar to that of CNC20(107.7±29.7 nm and 10.8±3.2 nm).But the size of CNC30 was almost two times of CNC15 and CNC20,with an average length and width of 192.8±75.5 nm and 28.3±9.2nm.The aspect ratios of CNC15,CNC20 and CNC30 are 10.2,10.0 and 9.5,respectively.The thermal stability of CNC15(0.64%S)is worse than that of CNC30(0.34%S),because it contains more sulfate groups or cellulose ? components.After the same process,cellulose nanocrystals(ECNC)extracted from eucalyptus wood have similar results in the trend of crystalline transformation and micro-morphology.The results showed that the transformation of crystalline phases(cellulose ?-cellulose ?-cellulose ?)existed in the early stage of acid hydrolysis,and the crystalline form of CNC could be regulated by controlling hydrolysis time,which may provide potential for the diversification of CNC applications.(2)Cellulose nanocrystals(CNC)with cellulose ? and ? allomorph were prepared by controlling the hydrolysis conditions,including acid concentration,reaction temperature and time.The crystalline structures of cellulose ? and ? nanocrystal were explored by XRD and ¹³C-NMR.The results showed that the CNC yield had a turning point at 58%acid concentration.And the maximum CNC yield was obtained under this acid concentration in60°C instead of 45°C.This is because in 45°C,when sulfuric acid concentration was less than 58%,cellulose would not dissolve during the hydrolyze process,the decomposition rate of non-crystalline region was faster than that of the crystalline region,it finally producted CNC-?.the same phenomenon would happen when the hydrolysis temperture was 60°C under 58-64%H₂SO₄,When acid concentration was 60-62%and the hydrolysis temperture was 45°C,disorered region and paracrystalline region in cellulose was dissolved,CNC-?(recrystallization of dissolved partial)and CNC-?(cellulose ? crystal cores)can be obtained by controlling the hydrolysis time.Under this condition,due to dissolved effective,the depolymerization rate is accelerated,hence CNC yield fell sharply.When low temperature(45°C)and high sulfuric acid concentration(?64%)is used,cellulose may become completely soluble in the first few minutes.The homogeneous hydrolysis would occur,and CNC yield decline further.TEM showed that CNC-? was rod shape,while CNC-? showed a twisted ribbon appearance.In addition,CNC-? was smaller in size and more concentrated in distribution,owing to that CNC-? was recrystallized from dissolved cellulose molecular chains.In conclusion,the inner connection between cellulose allomorph changes and CNC yield,as well as the supramolecular transitions of cellulose in hydrolysis procedure,was detailly discussed.This work provided knowledge for the preparation of CNC with different allomorph and morphology adjusted for various specific applications,promoting to commercialization.(3)Using Eucalyptus cellulose(EC)and mercerized cellulose(MC)as cellulose substrates,three kinds of cellulose nanowhiskers were prepared by sulfuric acid hydrolysis combineted with mechanical treatment.They were recorded as RNW-?(EC,64%H₂SO₄,10 min,45°C),CNW-I(EC,58%H₂SO₄,30 min,45°C)and MNW-?(MC,58%H₂SO₄,30 min,45°C).The yield of RNC-?,CNW-? and MNW-? were 50.8,74.8 and 81.1%,respectively.And the Cr ? of RNW-?,CNW-? and MNW-? were 80.4,73.7 and 84.8%.The average lengths of CNW-? and MNW-? were 228 and 196 nm,respectively,showing spindle or rod structure,while RNC-? showed twisted banded or starfish appearance with large size(average length was 416 nm).The addition of CNWs could promote the glass transition temperature(T_g)of PVA nanocomposite,while the melting temperature(T_m)and melting enthalpy(?H_m)decreased.Otherwise,adding CNWs to PVA film could significantly improve its mechanical properties.With the increase of CNWs content,the tensile strength and young's modulus firstly increased and then decreased.The tensile strength and young's modulus of CNW-?/PVA composite films were better than those of RNW-?/PVA and MNW-?/PVA composite films,and the enhancement effect of RNW-? was the lowest.The maximum increment of tensile strength of CNW-I/PVA,RNW-?/PVA and MNW-?/PVA composite films was 42.9,27.7 and 20.3%,respectively.When the content of cellulose nanowhisker was up to 6%,the tensile strength and young's modulus of the nanocomposite film would not continue to increase,or even began to decline.