Study On Preparation,Characterizations And Crystalline Stability Of Cellulose Ⅲ Nanocrystals

The relationship between structure and chemistry activity of cellulose has been obtained increased attention,due to the intricate supramolecular structure and complex hydrogen bond networks in cellulose.There are six interchangeable polymorphs,named cellulose I,II,III_I,III_II,IV_I and IV_II,respectively.Among them,cellulose III has a special crystalline structure,which shows a better accessibility and chemistry activity.However,the research on cellulose III has been mainly focused on the macroscopic or submicroscopic level,but the nanoscale is significantly insufficient.Here,the stability and performance of crystalline structure of cellulose III nanocrystals（CNC-III）in different reaction environments are mainly discussed.It would offer some new insights for understanding the allomorphs of cellulose.CNC-III were obtained from cellulose nannocrystals（CNC-I）prepared by the traditional sulfuric acid hydrolysis through the ethylenediamine（EDA）treatment.The variations of CNC-III and CNC-I in the crystalline property and morphology were compared and analyzed.The results showed that CNC-III could maintan the initially rod-like morphology,but displayed a shorter size and lower crystallinity.CNC-III were used to investigate the crystalline stability of cellulose III in the hydrothermal condition and a lower concentrated Na OH-water system.It was shown that H₂O or Na⁺played an important role in the process of polymorphic transformation.The CNC-III allomorph turned back to cellulose I with an extremely low crystallinity（～4.18%）during hydrothermal process at 95?C,or cellulose II when the temperature excessed boiling point（≥100?C）.Furthermore,CNC-III could be mostly and rapidly dissolved in an aqueous Na OH（～7.0 wt.%）without a pre-cooled treatment to obtain its stably concentration solution.Then cellulose II was acquired after the regeneration process via dialysis with distilled water.CNC-III were further investigated after some typical modification routes,including alkylation,TEMPO-mediated oxidation,cationization,polymer grafting and silylation.After acetylation,hydrophobic cationization,polymer grafting and surface alkylation,CNC-III kept the original crystalline morphology but the crystallinity（53.9%）decreased to 31.0%,42.5%,43.1%and 43.8%,respectively.However,after TEMPO-mediated oxidation and hydrophilic cationization,the crystalline structure of CNC-III transformed to the cellulose polymorph I with a crystallinity（10.2%）and polymorph II with a crystallinity（68.3%）,respectively.After modification,the dispersibility and thermal stability of CNC-III samples had been changed.