Exploration Of Strategies Towards The Heterogeneous Preparation Of Thermoplastic Cellulose Derivatives

In the last decades,plastics fabricated by fossil-based polymers have been widely used in various fields in our daily life.However,along with the depletion of petroleum resources and the aggravation of“white pollution”,it has become one of the most important subjects in scientific and industrial researches to explore alternatives for fossil-based materials.Biopolymers are ideal substitutes for petroleum-based polymers,which have drawn intensive attention and interest due to their renewability and degradability.Among numerous bio-based polymers,cellulose is the most abundant,which represents a potential candidate for preparing polymer products in the future.However,cellulose is insoluble in conventional solvents and also not melt-processable under the decomposition temperature because of the close packing of cellulose chains through numerous hydrogen bonds.The poor processability leads to inefficient utilization of cellulose.To date,the industrial processing and practical applications of cellulose materials are generally required for plenty of solvents,which inevitably lead to environmental pollution or the recovery of expensive solvents.In contrast to solvent-based methods,thermoplastic processing is solvent-free and efficient.Thus,it is of great importance to explore thermoplastic processing methods for the preparation of cellulose materials and study the common characteristics of thermoplastic cellulose-based products.Besides,designing appropriate technologies for developing novel melt-processable cellulose-based products have great application prospects in the future.In this thesis,new strategies towards heterogeneous preparation of thermoplastic cellulose grafted polyurethane(RCP-g-PU)and cellulose oleate(COE)were proposed with the assistance of traditional thermal reaction,sonochemistry,and mechanochemistry based on previous researches,respectively.Besides,the effects of reaction conditions on the structures and thermoplasticity of the obtained RCP-g-PU and COEs were systematically studied.Moreover,a“one-step”protocol was proposed for the efficient synthesis of thermoplastic fatty acid cellulose esters(FACEs)without the pre-activation of cellulose with the assistance of ball-milling mechanochemistry.Based on the proposed strategy,a series of FACE with different degrees of substitution(DS)and side chains were obtained by regulated the ball-milling reaction conditions.Furthermore,the effect of mechanochemical conditions on the chemical structures and thermoplasticity of FACEs were carefully discussed.The main research contents of this thesis are as follows:(1)Pristine cellulose was activated via dissolving in sodium hydroxide(Na OH)aqueous solution and regenerating in polyethylene glycol(PEG),and amorphous regenerated cellulose paste(RCP)was fabricated.Subsequently,thermoplastic RCP-g-PU was synthesized heterogeneously by the addition polymerization of cellulose,PEG,and hexamethylene diisocyanate(HDI)without using other organic solvents when HDI was added.Based on this concept,a series of thermoplastic RCP-g-PUs with different DS(0.67～1.75)and molar substitutions(MS,1.61～2.72)were achieved by tuning the reaction temperature and the feeding ratio of PEG/anhydroglucose(AGU).Furthermore,the thermoplasticity of RCP-g-PU was characterized by an optical microscope(OM).In this research,the prepared RCP-g-PU with MS values beyond 1.95 can be melt during the heating process,and the melting flow temperature(T_f)of RCP-g-PU was descended as the MS increased,and T_f of RCP-g-PU was 160°C when its MS was 2.72.Eventually,the prepared RCP-g-PU with high MS values(MS≥1.95)can be directly processed into transparent films by hot-pressing at 180°C,and the transparency of the hot-pressed film can reach 81.7%under optimal conditions.(2)After the activation of cellulose by a similar dissolving/regenerated process,a heterogeneous method was provided to prepare cellulose oleate(COE)by the esterification of wet regenerated cellulose gelation and oleic acid(OA)via sonochemistry in the presence of 4-dimethylaminopyridine(DMAP)and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride(EDC·HCl).Due to the“cavitation effect”and dispersion of ultrasonic,the DS value of COE was increased by 281%compared to traditional thermal reactions.Moreover,based on the provided strategy,a series of COEs with different DS(0.74～1.71)were synthesized by tuning the intensity of ultrasound,the molar ratio of OA/AGU,ultrasonic time,and reaction media.Furthermore,a glass transition of cellulose backbone in COE can be detected according to differential scanning calorimetry(DSC)analysis when the DS of COE was more than 1.23.Unfortunately,the COEs prepared by sonochemistry cannot be processed by the hot-pressing technique(180°C,10MPa)because there were still hydrogen-bonding networks among the macromolecules.(3)After similar activation,an amorphous regenerated cellulose/OA cake(RCC)was achieved by solvent exchange.Subsequently,a sustainable esterification strategy was developed to efficiently synthesize COE by ball milling mechanochemistry to endow cellulose with thermoplasticity in the presence of DMAP and EDC·HCl.In this strategy,a low dosage of oleic acid(6 mol per AGU)and no solvent were used to obtain thermoplastic COE during the ball-milling process.Moreover,ball-milling time and rotational speed were optimized to synthesized COE with different DS(1.29～2.55).According to DSC analysis,the synthesized COEs possessed two major glass transitions at 25.6°C～57.3°C and153.8°C～72.5°C corresponding to the motion of oleic side chains and cellulose backbones,respectively.Besides,the prepared COEs with DS beyond 2.12 can be readily processed into homogeneous films by hot-pressing at 160°C,and the hot-pressing films fabricated by COEs with high DS values(DS≥2.34)held excellent flexibility,transparency,and hydrophobicity.(4)An“one-step”strategy towards the heterogeneous synthesis of thermoplastic fatty acid cellulose stearate(SCE)was developed through directly mechanochemical esterification of microcrystalline cellulose(MCC)and stearoyl chloride(Sty Cl)using pyridine as acid-acceptor without the pre-activation of cellulose.Based on the proposed protocol,a series of SCE with DS ranged from 1.30to 2.37 was synthesized by tuning rotational speed,ball-milling time,and the molar ratio of Sty Cl/AGU.Besides,the DS value of the prepared SCE was elevated by accelerating the rotational speed,prolonging the balling-milling time,and concentrating the molar ratio of Sty Cl/AGU.Moreover,the thermoplasticity of the obtained SCE was regulated by the ball-milling conditions.The prepared SCEs were melt-processable when the rotational speed was beyond 400 rpm and DS values of the products were beyond 1.62 simultaneously.In this chapter,when the rotational speed was 500 rpm and the molar ratio of Sty Cl/AGU was 6/1,SCE with DS of 2.37and T_f of 152°C can be achieved by mechanochemical reaction for 6 h.Eventually,cellulose-based films with excellent mechanical properties and high transparency can be fabricated by directly hot-pressing the thermoplastic SCEs at 160°C.This strategy is advantageous over traditional methods considering simplified process,reduced reactant dosage,enhanced efficiency,and improved environmental friendliness,which have great potential for the scalable fabrication of thermoplastic cellulose derivatives in industry.(5)A similar protocol was used to obtain FACE with different side chains(C10～C18)by mechanochemical esterification of MCC and fatty acyl chloride in the presence of pyridine.By tuning the molar ratio of fatty acyl chloride/AGU,a series of FACEs with various DS were synthesized.In this chapter,the reactivity of mechanochemical esterification was regulated by the length of fatty acyl chloride,which was confirmed by the different DS of FACEs prepared by the same molar ratio of fatty acyl chloride/AGU.Specifically,fatty acyl chloride with short chains held high reactivity with MCC,so FACE with high DS values can be achieved at the same conditions.In this study,the glass transition temperature(T_g)corresponding to cellulose backbones in cellulose decanoates(C10)was decreased from 169.5°C to63.4°C as the DS values elevated from 1.85 to 2.60,which were confirmed by DSC analysis,and the similar phenomenon was also observed in cellulose laurates(C12).However,the same T_g appeared at 165.1°C～172.8°C when the side chains of FACE were myristoyl(C14),palmitoyl(C16),and stearyl(C18).Furthermore,the T_f of FACEs prepared by the same molar ratio of fatty acyl chloride/AGU was elevated as the length of fatty acyl chloride increased from C10 to C18.The work in this part systematically studied the effect of the length of fatty acyl chloride on the activity of esterification during mechanochemical preparation of FACE,and also carefully evaluated the properties of the obtained FACE hot-pressing films considering the mechanical properties,transparency,and hydrophobicity,respectively.This research provided a potential theoretical fundamental for the mechanochemical synthesis of thermoplastic FACE in industry.