Study On Preparation, Properties And Structures Of Cellulose Internal Plasticization

Cellulose, a linear homo-polymer composed of D-anhydroglucopyranose units (AGU), linked together byβ(1→4)-glycosidic bonds, is the most abundant polymer raw material available today worldwide. Esterification of cellulose represents one of the most versatile transformations as it provides access to a variety of bio-based materials with valuable properties. The internal plasticization of cellulose through grafting or chemical modification is an effective way to eliminate the change of material performance resulted from exudation or volatilization of low molecular weight plasticizers in processing and application. One potentially feasible solution would be the synthesis of esters of cellulose with longer chain acids than the currently commercial acetate, propionate and butyrate.The idea of the present paper is to synthesize long-chain cellulose esters in a homogeneous phase by microwave irradiation for preparing internally plasticized materials, which are modified by blending with other polymers, and to discover the correlation of its preparation, properties and structure, and as well as to provide access to a variety of cellulose-based materials with valuable properties. The main research works and results are summarized as follows:1. Preparation of cellulose solution by ultrasonic treatmentThe insolubility of cellulose in water and in most organic solvents was caused by areas of both high order (crystalline) and low order (amorphous). And the mixed solvent N,N'-dimethyl acetamide with lithium chloride (LiCl/DMAc) was used to dissolve cellulose after being activated. One-step procedure of activation and dissolution of cellulose in LiCl/DMAc irradiated by ultrasonic was studied. The effect of the ultrasonic on the supermolecular structure of cellulose was assessed by means of FT-IR and Wide-angle X-ray diffractometry, respectively. And ultrasonic irradiation exerted a clear influence on the morphological character of cellulose measured by scanning electron microscopy. The results of analysis showed that ultrasonic treatment affected the crystallization of the cellulose examined. The multiple parameters during the solvation process such as ultrasonic power, treatment time and degree of polymerization shown in the paper were evaluated. It was found that cellulose could be easily solubilized in LiCl/DMAc after being treated for 7 min by ultrasound at 400 w, with no degradation of the cellulose chain during the process. A study of the rheological property of the cellulose solution indicated that it was non-Newtonian fluids.13C-NMR spectrum proved that no derivatives of cellulose were formed during the solvation process.2. Esterification of cellulose with long-chain fatty acid irradiated by ?microwaveAcylation of cellulose in a homogeneous phase by microwave irradiation was essential to get internally plasticized cellulose derivatives. The reaction system was cellulose, fatty acid and LiCl/DMAc with p-toluenesulfonyl chloride as an activating reagent. The cellulose esters of different degree of substitution (DS) were directly synthesized, and the correlation of properties, structure and DS, and as well substituents were investigated.The effect of the reaction conditions on esterification was evaluated and the results showed that microwave power and irradiation time were critical factors. For reducing reaction time and utilizing the cellulose sufficiently, the reaction time of 60-90s at 300 w was chosen in the experiment. The reagent molar ratio has significant impact on weight yields (Rmass ), molar yields(Rmol), and the DS, when irradiation power and time was kept constant. The feasible molar ratio of reagent was 1:4:4 to obtain products with desired DS.The—OH band in FT-IR spectra of cellulose laureates with different DS did not disappear thoroughly due to partial substitution and the remaining hydroxyl groups from crystalline regions of the cellulose, which were in good agreement with those observation from 1H-NMR for all DS < 3. Concerning 13C-NMR data of cellulose laurate with 2.6, it was observed that C-6 position was substituted, preferentially to C-3 and then to C-2 in the study. And the changes in the WAXD pattern of starting cellulose, caused by esterification reaction, were attributed to the long-chain ester bonded to cellulose. These cellulose esters had glass-transition temperature (Tg) in the 0-80°C range. However, further increase in molecular weight and DS had very little effect on Tg although the Mn and Mw of cellulose lauroyl esters increased with the DS increased.Cellulose laurate, cellulose caprylate, and cellulose caproate are all soluble in DMSO, CHCl3 and THF when the DS was around 2.1. At cellulose caproate,the endotherm was barely visible in the DSC thermograms, while a clear glass transition was found for cellulose caprylate and cellulose laurate in the 0-80°C range. The Mn and Mw of these cellulose esters increased with the ester chain length increasing, but thermal stability was decreased because of the weakening of extensive hydrogen bond.3. Structure and properties of cellulose ester plasticsThese longer chain esters (LCCEs) show promise as internally plasticized cellulose esters because the supermolecular structure of cellulose was significantly changed with the introduction of longer substituents. The internally plasticized cellulose films were obtained by casting and the influences of the DS and substitutents on the properties of these films, such as water resistance, thermal stability, glass transition temperature, tensile strength and elongation, were investigated.Contact angle of cellulose laurate films with different DS was nearly 90°. TEM images and XRD diffraction patterns all showed that their crystalline state was similar. DMTA in tension mode revealed separate relaxations, denotedαandβ, corresponding to the glass transition and side-chain melting processes, respectively. For the three cellulose laurate samples, two main peaks separated were readily distinguished in the tanδcurves. The relaxation at the higher temperature (αransition), which varied between 120°C and 200°C, was accompanied by the most important decrease of G′, and this indicated that it corresponded to the glass transition of the polymer. At a lower temperature, the peak corresponded toβtransition, accompanying lateral substituent melting in the vicinity of -30°C.The films properties of long-chain cellulose esters were also compared to those of cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. The results showed that the water absorption and tensile strength of short-chain cellulose ester plastic films were higher than the long-chain ones with preferable failure strain level. In addition to this, thermal decomposition process was similar.4. Compatibility Characterization of cellulose ester matrixBlending is a practical way to improve the performance of a single polymer. Cellulose laurate(CE) was blended with polycaprolactone (PCL) in order to improve its mechanical property. The result indicated that the elongation of the CE was improved significantly through blending with PCL, and a synergism was observed for certain compositions (CE/PCL = 90/10). The tensile strength rose from around 12MPa to 30MPa, and tensile strain increased from 70% to 103%. All the blend films showed better water resistibility regardless of mass ratio of CE and PCL. FT-IR spectroscopy revealed that there were several specific interactions between the chains of CE and PCL, such as the interaction between C—H and O=C—and C=O…O=C or C=O…O—C dipole–dipole interactions. TEM and SEM images showed that there was somewhat compatibility between CE and PCL.5. Elementary investigation on the enzyme degradation of cellulose ester filmsBiodegradable polymers featuring ecological advantages for sustainable development have attracted great commercial interest because of growing environmental concerns. A different assay was developed based on the utilization of ester enzyme and cellulolytic enzymes rather than dynamic mixed cultures found in the natural environment.The extent of biodegradation was followed by monitoring glucose production using HPLC. As the DS and the acyl chain length increased, the rate of biodegradation rates decreased. Preferential attack of the microorganisms was observed on the face of the cellulose laurate film in SEM images. Collectively, this work indicated that the cellulose ester films with lower DS and shorter side-chain showed better biodegradability.