| Summary: | 碩士 === 國立中興大學 === 化學工程學系所 === 96 === In this research, we used tetraethyl orthosilicate (TEOS) as the precursor to fabricate SiO2, and then we used the SiO2 modifier - GT to perform surface modification on SiO2; further, we used solution blend method and in-situ method to produce the nanocomposites of PVA/ SiO2 from PVA. In this study, we varied the content of SiO2 to discuss how SiO2 affected the PVA’s physical properties and after modifying SiO2 by GT to investigate the effects to PVA/SiO2 composites.
We analyzed the fabricated PVA/SiO2 composites via DSC and found PVA’s glass transition temperatures enhanced with the increase of the content of SiO2. When the content of SiO2 in the composites reached to 20 wt%, the glass transition temperature of the composites produced by in-situ in acid condition enhanced by 6.9 ℃, and it enhanced by 6.1 ℃ and 2.9 ℃ respectively in the in-situ in basic condition and solution blend method. Due to SiO2 and PVA in the composites brought the hydrogen bonding interaction to restrain PVA’s chains movemwnt. When the content of SiO2 increased, polymer chains needed more energy to mobilize, so we observed the increase of glass transition temperature. After modifying SiO2 in the composites by GT, due to the OH groups on SiO2 increased, we could enhance the hydrogen bonding interaction between SiO2 and PVA to lead the glass transition temperature of the composites to increase again. In the composites made by in-situ in acid condition, the modified SiO2 had 1.0 ℃ more than the unmodified SiO2 in the comparison of the glass transition temperature, and it increased by 5.3 ℃ and 2.7 ℃ respectively in the in-situ in basic condition and solution blend method.
In the TGA analysis, we found that PVA’s thermal decomposition temperature at 5 % weight loss increased with the content of SiO2. When the SiO2 in the composites reached to 20wt%, the thermal decomposition temperature at 5 % weight loss of the composites made by in-situ in acid condition increased by 17.8 ℃and it increased by 11.7 ℃ and 8.9 ℃ respectively for the in-situ basic condition and solution blend method. Because the SiO2 in the composites inhibited thermal motion in the chains of PVA, the thermal decomposition temperature at 5 % weight loss enhanced. After modifying the SiO2 in the composites by GT, the thermal decomposition temperature at 5 % weight loss of the composites also increased again. In the composites made by in-situ acid condition, the thermal decomposition temperature of the one had modified SiO2 increased by 5.8 ℃ at 5 % weight loss, in the comparison with the one had unmodified SiO2. In the in-situ in basic condition, the enhancement of the thermal decomposition temperature was 14.7℃. In solution blend method, the thermal decomposition temperature at 5 wt% loss decreased by 4.0℃.
In the analysis of XRD, the degree of crystallinity of PVA decreased with the increase of the content of SiO2. When the SiO2 in the composites reached 30wt%, the degree of crystallinity of the composites made by in-situ in acid condition decreased by 18.8% and decreased by 16.9% and 12.7% respectively in in the in-situ in basic condition and solution blend method. Due to the hydrogen bonds between SiO2 and PVA, it reduced the interaction among the chains of PVA; hence, PVA’s degree of crystallinity decreased. After modifying the SiO2 in the composites by GT, because the OH groups on the SiO2 surface increased and interacted with the hydroxyl groups of PVA, the degree of crystallinity for PVA decreased again. In the composites made by in the in-situ in basic condition and solution blend method, the degree of crystallinity of the modified SiO2 and that without SiO2 decreased by 3.7% and 1.4% respectively. However, it increased 3.3% for the in-situ at acid condition.
In the FESEM analysis of fracture surfaces, when the content of SiO2 in the composites reached 30 wt%, the particles of SiO2 still well-dispersed in the matrix of PVA and there was no aggregation. SiO2 had good dispersion in PVA, so there was no apparent effect on the dispersion of SiO2 after we modified the SiO2 in the composites by GT. The diameter of the particles of SiO2 was around 5 nm to 20 nm.
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