| Degradable poly (propylene carbonate)(PPC) is derived from carbon dioxide and propylene oxide through ring-opening copolymerization. Its wide use would be an effective method to prevent "white pollution". However, low thermal stability and mechanical property have greatly limited its application. In this thesis, we try to build polymer/layered silicate nanocomposite, and organic cation was utilized to modify montmorillonite (MMT) to improve thermal stability and mechanical property of PPC.In order to illustrate interactions among organic modifier, MMT and PPC matrix which have an effect on performance of nanocomposite, Three organic cations with various structure and volume, i.e. imidazole quaternary ammonium salt (HHMIB), hydrochloric-amino acid salt (AUA), polystyrene oligomer quaternary ammonium salt (COPS) were adopted to modify MMT respectively. MMT modification was based on cation-exchange and actual amount of intercalated cation was calculated through change of C content or thermal gravimetric analysis (TG). It were indicated that intercalating ability of HHMIB was weakest, actual cation exchange capacity was only64.6mmol/100g; AUA was92.3mmol/100g; COPS was 97.6mmol/100g due to improved lipophilicity of MMT caused by silane-modification before intercalation. X-ray diffraction (XRD) showed that interlayer spacing of MMT was enlarged from1.52nm to2.60nm,1.62nm and over3.53nm after being intercalated by HHMIB, AUA and COPS respectively. The interlayer spacing of MMT modified by AUA is smallest because of its smallest volume and that of MMT modified by COPS is largest because of its largest volume.Three kinds of PPC/MMT nanocomposites with different mass fractions of MMT were prepared by melt blending using organic montmorillonites mentioned above. Structure, thermal stability, mechanical property and medium resistance of the composites were studied systematically. It was illustrated that poly (propylene carbonate)/imidazole salt modified-montmorillonite (PPC/H-MMT) and poly (propylene carbonate)/hydrochloric-amino acid salt modified-montmorillonite (PPC/A-MMT) were all intercalated nanocomposites with enlarged interlayer spacing of3nm and2nm respectively through XRD and transmission electron microscope (TEM) characterizations. It was similarly indicated that poly (propylene carbonate)/silane and polystyrene-modified-montmorillonite (PPC/A-C-MMT) was exfoliated nanocomposite. TG results demonstrated that thermal stabilities of the three composites were all improved. Maximum increments of ronset of PPC/H-MMT, PPC/A-MMT and PPC/A-C-MMT were64℃,41℃and82℃; maximum increments of T10%were64℃,46℃and76℃; maximum increments of T50%were47℃,27℃and47℃, respectively. Nano-effect derived from exfoliated structure made thermal stability of PPC/A-C-MMT to be significantly improved compared to PPC and to be highest of the three composites. Static tensile test demonstrated that mechanical properties of the composites changed. Maximum increments of tensile strength of PPC/H-MMT, PPC/A-MMT and PPC/A-C-MMT were34.6%,78.4%and549%; maximum increments of elastic modulus were69.7%,1425%和4382%. In addition the maximum decrements of elongation at break were34%,52.3%and99.3%, respectively. However, PPC/H-MMT and PPC/A-MMT still kept excellent toughness because of their large absolute value of900%and650%. Elongation at break of PPC/A-C-MMT was10.18%, its toughness decreased and brittleness increased. Significant reinforcement in strength and hardness of PPC/A-C-MMT was also owed to nano-effect caused by exfoliated structure. Dynamic mechanical property analyses (DMA) showed that rigidity of the three composites were all enhanced for their strengthened storage modulus. It was also showed by DMA that state transition of the composites lagged than pure PPC with increasing temperature:transition temperatures from glass state to glass-high elastic mixed state of PPC/H-MMT, PPC/A-MMT and PPC/A-C-MMT increased about5℃,16℃and24℃, glass temperatures increased2.4℃,10℃and18.5℃respectively. Medium resistances of the composites were characterized through detecting changes in quality after being soaked in water or oil for a certain time. Water absorption of PPC was4.4%, maximum water absorption of PPC/H-MMT, PPC/A-MMT, and PPC/A-C-MMT were about13%,3%and2%respectively. Accordingly, water resistance of PPC/H-MMT was wakened and that of PPC/A-MMT, PPC/A-C-MMT were enhanced. Oil absorption of all composites kept within0.9%and they had good oil resistance.Molecular weight of PPC was detected by gel permeation chromatography. Degradability was characterized through changes in molecular weight of PPC when the composites were placed in natural environment or buried in soil for180days. It was founded that photodegradation ratio and biodegradation ratio of pure PPC were83.9%and74.9%, maximum photodegradation ratio of PC/H-MMT, PPC/A-MMT, PPC/A-C-MMT were82.6%,82.0%,65.4%, maximum biodegradation ratio of the three composites were78.1%,64.7%,28.4%, respectively. The degradability of composites was all decreased in addition to improved biodegradability of PC/H-MMT. Results indicated that PPC/H-MMT, PPC/A-MMT have better degradability, and although biodegradability of PPC/A-C-MMT is low, its photodegradation ratio can reached65.4%which make it still a good photodegradable material.Influence of PPC/MMT nanocomposite on soil quality after biodegradation was not reported. Total numbers of microbial colonies in soil were determined by adopting dilution method of plate counting, microbial biomass carbon was detected through chloroform fumigation-extraction method in this work. Results indicated that the greater degradability of the composite was, the more number of microbial colonies and microbial biomass carbon in soil were, and the higher bioactivity and soil fertility were. Fertility of soil filled with composites was better than virgin soil, the composites can amend soil quality and make soil ecosystem in beneficial cycle. Therefore, the three composites are eco-materials and using them will not cause environmental pollution. Furthermore, because of good degradability and better thermal stability and mechanical property than PPC, we can predict that their wide use especial PPC/A-MMT and PPC/A-C-MMT will effectively mitigate "white pollution" caused by use of conventional plastics. |
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