Research On Technology Of Polymorphic Transformation In Cellulose Nanocrystals Preparation

Cellulose is a plentiful renewable natural resource.Its high value-added utilization is a long-term goal to pursuit.As a kind of important cellulose nano-material,cellulose nanocrystals(CNCs)has broad application foreground.It has a higher crystallinity compare to other nanocellulose materials.Thus,the polymorphic structure of CNCs has an important influence on its preparation and application.In this work,the mechanism and approach of polymorphic transformation of cellulose were studied for CNCs preparation.Polymorphic transformation technology on cellulose was applied in CNCs preparation,in order to provide a novel idea for increasing CNCs yield,improving CNCs properties,as well as preparation of variety CNCs.The sulfuric acidic hydrolysis characteristics of raw cellulose materials with various polymorphs in the hydrolysis process of CNCs preparation was studied,including their characteristics of polymorphic transformation and hydrolysis efficiency.The results showed that,cellulose with various allomorphs were different in hydrolytic characteristics.At the beginning of the sulfuric acid hydrolysis,the raw materials cellulose I and cellulose II were swelled and dissolved at different levels.Their polymorphic structure of cellulose I and cellulose II,which had not been swelled or dissolved were not affected by sulfuric acid hydrolysis,and their original structure were maintained,during the sulfuric acid hydrolysis.While for cellulose III,there was a remarkable swelling and dissolving at the beginning of the sulfuric acid hydrolysis.Its polymorphic structure which had not been swelled or dissolved was converted back to the original structure,cellulose I.Their sulfuric acid hydrolysis efficiency interrelation was like the following: amorphous cellulose?cellulose III?cellulose II?cellulose I.The properties of CNCs obtained from cellulose with various allomorphs by sulfuric acid hydrolysis were studied,including their polymorphic structure,crystallinity,morphology,particle size,and thermal stability.The results showed that,CNCs obtained from cellulose with various polymorph were significantly different in the morphology and particle size,as well as polymorphic structure.In a conventional sulfuric acid hydrolysis condition(acid concentration was 64 wt%,temperature was 45 °C,time was more than 30 min),CNCs obtained from cellulose I and cellulose III,exhibited a “rod-like” structure with a large aspect ratio.Both of their polymorphs were cellulose I,meanwhile CNCs obtained from cellulose II exhibited a “round-like” or “rod-like” structure with short lengths,and its polymorph was cellulose II.The concentration of sulfuric acid was important to CNCs preparation and its properties.The properties of CNCs obtained from cellulose III,including morphology,particle size and thermal stability,were more susceptible to the acid concentration compared with those of CNCs obtained from cellulose I.For cellulose I and cellulose II,their crystallinity were increased with the increase of sulfuric acid concentration.The yields of CNCs obtained from cellulose II and cellulose III with 61 wt% sulfuric acid hydrolysis were very high.Cellulose II nanocrystals and cellulose III nanocrystals were obtained via polymorphic transformation treatment of CNCs.Then,above CNCs samples with various allomorphs were redispersed by TEMPO-mediated oxidation treatment.Their properties,including polymorphic structure,crystallinity,morphology,particle size,thermal stability,and redispersion ability were studied.The results showed that,cellulose II nanocrystals with a short length and cellulose III nanocrystals with a classical “rod-like” structure were successfully obtained,respectively,via mercerizing and EDA treatment of CNCs.CNCs after polymorphic transformation could be well redispersed by TEMPO-mediated oxidation treatment.The polymorph of cellulose II nanocrystals was maintained during the TEMPO-mediated oxidation redispersion process.Whereas in case of cellulose III nanocrystals,an excessive TEMPO-mediated oxidation treatment would bring a destruction of cellulose III structure.CNCs that have been treated with alkaline polymorphic transformation decreased in redispersion ability,but their thermal stability were significantly increased.