Preparation Of UCST Thermo - Sensitive Hydrogel And Its Properties

Bagasse pith (BP) konwn as a residue by-product after the extraction of juice from the crushed stalks of sugar cane, dominantly consist of parenchyma cells. It is a fact that the annual output of BP are about 40 million tons only in Yunnan Province, which means the amounts of this biomass resource is substantial. Unlike the depithed bagasse, BP is hardly utilized in fiber-manufacturing field due to the limitation of cell types and fiber length. So far BP has been underutilized, and most of it is simply burned for applying energy to the sugar or papermaking mill. Its utilization has the potential to become more valuable than the simple burning. The reasonable and effective application of BP would significantly improve its add-value, and thus enlarge the applying scope.Environmentally sensitive hydrogel, also called intelligent polymers, is an important and newly-developed kind of functional macromolecular materials. Such smart polymers can respond to the slightly external stimuli, resulting in the sharp changes in their structures, physical and chemical properties. Therefore, these polymers have extensive applications in such fields as medical and sanitary, agriculture and forestry, horticulture, food processing, and biotechnology, etc. According to the popular study on developing environmental-friendly hydrogel based on nature polymers and the application blank of intelligent polymers in farming irrigation at present time, the preparation of thermo-responsive hydrogel based on BP modified cellulose and its mechanism were systematically studied in this dissertation. The main contents and results of this research paper were shown as follows:(1) The chemical composition and internal structure of bagasse pith, as a starting material, was systematically researched in order to provide a evidence for BP utilization. It was revealed that the BP material was constituted of considerable parenchyma cells, moderate vessel cells and marginal short fibrous cells. The typical morphology of parenchyma cells is the flakiness-like shape with obviously round piths on it, and the majority of BP are fragile and have the large specific surface area. Parenchyma cellulose has a much more loose structure than fiber’s, indicating that cellulose isolated from parenchyma cells has the comparatively more incompact super-molecular structure, and thus results in its optimal reactive properties for further derivative modification. It was also found that specific surface area of BP prepared at the range of 80-120 meshes was 1.9 m2/g, which means that its cellulose is facilitative to be isolated and subsequently modified.(2) Based on preparation of cellulose with high content from BP, degradation of lignin and hemicellulose by the oxygen-containing agents was sysmatically researched. And an effective and convenient process of oxygen treatment (O) and peroxide treatment with oxygen and anthraquinone as a catalyst (Po-AQ) sequence has been developed for cellulose isolation from BP. The optimal conditions were:20% alkali dosages with 12% BP consistency at 120℃ for 3h under 0.6 MPa initial oxygen pressure for the O step; then 8% H2O2,20% NaOH, and 0.3% AQ charges with 8% BP consistency at 120℃ for 6h under the same oxygen pressure for the Po-AQ step. Under the conditions above, the lignin of the BP raw material was extensively removed, and yield of extractive sample and cellulose content were reached at 39.7% and 91.52%, respectively. Compared with traditional methods for isolating cellulose from fibrous biomass by hydrolyzing, pulping and multistage bleaching, the O/Po-AQ process is proved to be environmental-benign and effective.The extractive mainly consisted of two different tissues:parenchyma cells and epidermal fibers, and the crystallinity of the samples and surface roughness of preparations were both enhanced with the increasing cellulose content in the extractives.(3) BP cellulose, pretreated by ultrasonic wave, was regioselectively oxidized in the 4-acetamide-TEMPO/NaClO/NaClO2 systems. The oxidized derivative obtained from parenchyma cellulose under the optimal conditions was prepared for the modification of thermo-sensitive hydrogel. The result shows that the morphological structure of the cellulose has been changed by ultrasound wave, mainly reflecting both fibrillation and fragmentation, while crystallinity has not been greatly affected. On the other hand, the oxidized products obtained from this 4-acetamide-TEMPO-mediated reactive system were comprised of water-insoluble (IOC) and soluble samples (SOC). And the polymerization degree (DP) of the SOC, so-called β-(1→4)-linked cellouronic acid sodium salt, is much lower than BP cellulose, while the oxidization degree of the SOC is remarkably high. Meanwhile, it is explicated that ultrasound treatment can not only enhance the oxidization capability of cellulose, but also affect the morphology of oxidized samples. Hence, the SOC is chosen as a linear polymer for the subsequently synthetic reaction in order to realize the thermo-responsive semi-IPN.On the basis of an orthogonal analysis, the optimum conditions were ultrasonic power of 550 w, reaction time of 3 h, and cellulose consistency of 2.0%. Ultrasonic power and duration were found to have significant impact on the selective oxidation reactivity of cellulose. Additionally, an optimal SOC yield of 71.0% with 32.92% carboxyl content and 262 DPv was found under the conditions of NaC102 dosage of 16 mmol/g,4-acetamide-TEMPO charge of 0.20 mmol/g, and oxidization temperature of 50℃ in acetate buffer at pH 5.5 for 72 h. Moreover, both oxidization temperature and amount of NaClO2 were found to have the crucial impact on the yield of SOC, followed by 4-acetamide-TEMPO charge, then initial pH and reaction time.(4) Based on the two-step method, SOC/poly(acrylamide-co-dimethyl diallyl ammonium chloride) (SOC/P(AM-co-DAC)) semi-IPN hydrogel was synthesized using free radical polymerization in aqueous solution. The results show that the crystalline structure and morphology of BP oxidized cellulose are changed after the semi-IPN modification. It is also proved that the hydrogen bond and (or) electrovalent bond are existed between the SOC and P(AM-co-DAC). The micrographs showed that the open macroporous architecture was observed in SOC/P(AM-co-DAC) hydrogel and typical-IPN incontinuous phase separation was viewed. Moreover, the swelling behavior and thermo-responsive ability of the hydrogel were dependant on preparation conditions and the properties of aqueous solutions.The ideal thermo-sensitive hydrogel with notable upper critical solution temperature (UCST) and favorable swelling reversibility can be obtained under the conditions of DAC:AM 0.03:1 mol/mol, SOC:AM+DAC 0.0006:1 mol/mol, potassium persulphate (KPS):AM+DAC 0.013:1 mol/mol, and N,N-methylene diacrylamide (MABM): AM+DAC 0.00015:1 mol/mol at 70 ℃ for 20 h. The thermo-sensitive point (TSP) and the theoretical equilibrium swelling capacity (Qmax) of the optimal semi-IPN gel were 20-25 ℃ and 5828 g/g in deionized water. And the Qmax of the hydrogel in 0.1 mol/L KH2PO4 and 0.1 mol/L MgSO4 were 1032 g/g and 897 g/g, respectively, while TSP in the both salt solutions was 10～15 ℃. The Qmax of the absorbent polymer was 2032 g/g, but the thermo-responsive capability was disappeared in 0.1 mol/L CO(NH2)2 aqueous solution.According to the analysis of swelling kinetics and thermodynamics, the swelling exponent’n’for the gel sample in various aqueous solutions, lies between 0.6 and 0.9, thus suggesting the anomalous or non-Fickian type diffusion. The sequence of swelling exponent in different aqueous solutions was as follows:n deionized water> n urea> n potassium dihydrogen phosphate> n magnesium suifate, and the tendencies of D and Qmax in these four solutions were both the same as the above’n’value. However, the activation energy showed the opposite order, that is Ea deionized water< Ea urea< Ea potassium dihydrogen phosphate< Ea magnesium sulfete.