A series of chemically modified starches were synthesized in the present study: (1) a starch modified with an anionic group and benzoyl group; (2) a starch modified with an anionic group and hexanoyl group; (3) a starch modified with a cationic group and benzoyl group; and (4) a starch modified with a cationic group and hexanoyl group. The glass transition temperatures of the modified starches were found to be lower than that of natural starch. The modified starches synthesized in the present study are found to be hydrophobic and can biodegrade in composting. Blends of a modified starch and poly(ethylene- co-vinyl alcohol) (EVOH) were prepared by melt blending. Phase diagrams of the blends exhibited a lower critical solution temperature. The DSC thermograms of the blends showed a single glass transition temperature and melting point depression. The crystallization of EVOH in the blends was interrupted by the hydrogen bonds formed between the hydroxyl groups in the modified starch and EVOH. The crystalline structure of EVOH in the blends had an orthorhombic lattice in the quenched specimen and evolved into a monoclinic lattice when the annealing temperature was increased. The nanocomposites based on a modified starch and EVOH were prepared. Natural clay (montmorillonite, MMT) and two commercial organoclays (Cloisite 30B and Cloisite 15A) were employed to investigate how the functional groups in the modified starch influence the dispersion characteristics of nanocomposites. Anionically modified starch was found to be very effective in exfoliating organoclay aggregates due to the presence of ionic interaction, as determined by Fourier transmission infrared (FTIR) spectroscopy, between the anionic group in the modified starch and positively charged N+ in the surfactant residing at the surface of an organoclay. For the nanocomposites based on a cationically modified starch and EVOH, the preparation methods had a large influence on the dispersion characteristics of the nanocomposites. The aggregates of MMT have a very high degree of dispersion characteristics in the nanocomposites prepared by solution blending but poor dispersion characteristics in the nanocomposites prepared by melt blending. FTIR spectroscopy has indicated that the ionic interaction between the cationic group in a modified starch and negatively charged surface of silicate sheets of MMT could be formed in the nanocomposites prepared by solution blending but not in the nanocomposites prepared by melt blending. Wide-angle X-ray diffraction (WAXD) has indicated that the crystalline structure of EVOH in the nanocomposites containing exfoliated aggregates of organoclay Cloisite 30B takes orthorhombic lattice in both quenched and annealed specimens, which is different from that of neat EVOH. We ascribe the difference in the crystalline structure of EVOH in the EVOH-containing nanocomposites and in neat EVOH to the formation of hydrogen bonds between the hydroxyl groups in EVOH and the hydroxyl groups in the surfactant residing at the surface of Cloisite 30B. We have found that an improvement in the tensile properties of nanocomposites can only be obtained if the nanocomposites have a very high degree of dispersion of the aggregates of clay, and there is strong attractive interaction between the clay and the matrix.
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