| Poly(vinyl alcohol)(PVA) is manufactured mainly through saponification of poly(vinyl acetate)(PVAc), and can be used as a raw material for production of poly(vinyl butyral), dispersant of suspension polymerization, adhesive and so on, depending on the degree of alcoholysis. The structure of PVA has a significant impact on the liquid-liquid dispersion, droplet coalescence behavior of polymerization system, and particle properties of the resulting resin when it was used as dipersant. In this thesis, PVAc was prepared through solution polymerization, and then alcoholysed to obtain PVA. Influences of alcoholysis condition on the degree of alcoholysis, sequence distribution of PVA, and relationships between PVA structure and application properties were investigated.Firstly, PVAc were prepared by solution polymerization, using methanol as solvent, and AIBN as initiator. It was found that the molecular weight of PVAc was significantly increased with the decrease of polymerization temperature, the decrease of the amount of initiator and the increase of monomer concentration. PVAc was alcoholysed under different conditions and it was found that PVAc alcoholysis reaction rate was proportional to the concentration of PVAc at temperature of30-45℃, alkali molar ratio of10%-25%, and water content of0.25%-2.5%. The alcoholysis reaction rate was significantly increased with the increase of temperature and alkali molar ratio. The activation energies of PVAc alcoholysis reaction were63.84,62.55,60.73and60.08kJ/mol when the alkali molar ratios were10%,15%,20%and25%, respectively. The catalytic effect of NaOH to PVAc alcoholysis was not significant in the absence of water, while the presence of a greater concentration of water (i.e.2.5%) would cause the decrease of NaOH concentration and alcoholysis reaction rate due to the increased hydrolysis of NaOH.Secondly, a series of PVA samples with the closed degree of alcoholysis were prepared by controlling of reaction time according to the previous studies. Influences of alcoholysis temperature, molar ratio between NaOH and vinyl acetate unit (alkali molar ratio), and concentration of water on structural parameters reflecting the sequence distribution in PVA were investigated through analysis of PVA samples by1H NMR and13C NMR. It was found that the alcoholysis conditions had no obvious influence on the polymerization degree and molecular weight distribution of PVA. The average sequence length of vinyl alcohol unit was increased with the increase of alcoholysis temperature, alkali molar ratio and water content, while the average sequence length of vinyl acetate unit had a maximum value when the alcoholysis temperature was35℃and alkali molar ratio was15%. Correspondingly, PVA prepared at35℃,15%alkali molar ratio and low water content exhibited chains structure more closed to block copolymer.Finally, the application properties of PVA were analyzed and related to the structure of PVA. It was found that the surface tension of PVA aqueous solution and interfacial tension between PVA aqueous solution and trichloroethylene were both decreased with the increase of the sum of sequence length, and the decrease of the block character. Dispersion and colloid-protection abilities of PVA were increased with the increase of the sum of sequence length, and the decrease of the block character. It is concluded that PVA with longer chain segments and obvious block distribution, had better surface activity. The cloud point of PVA was decreased with the increase of the block character. PVC resin prepared by suspension polymerizstion using lower block character PVA had more regular particle morphology, smaller particle size and greater porosity.The above results would provide instruction for synthesis and application of PVA with different molecular weight, degree of alcoholysis and sequence distribution of vinyl alcohol and vinyl acetate units. |
PDF Full Text Request