Structure And Properties Of Biodegradable Polyester/Polyethylene Glycol Blends

Compared with the traditional petroleum-based plastics,the aliphatic polyesters have the advantages of good degradation ability and biocompatibility,excellent processability and mechanical properties.However,the aliphatic polyesters are hydrophobic polymers with long degradation cycle,which limits their application in some specific fields.Therefore,it is of great significance to modify the degradation and hydrophilicity of aliphatic polyesters.In this thesis,two common aliphatic polyesters,poly(L-lactic acid)(PLLA)and polybutylene succinate(PBS),were selected to study the effects of poly(ethylene glycol)(PEG)blending on the physical properties of PLLA and PBS.Based on the results obtained in laboratory,we further prepared the degradable diverting agents in industrial scale for the applications in oilfield fracturing process.First,several common degradable polyesters were selected to compare their degradation behavior in the same pH value of acid and alkali solution.Compared with poly(butylene adipate-co-terephthalate)and poly(ethylene terephthalate),PLLA and PBS have higher content of ester bond and are easier to be degraded hydrolytically.The degradable polyesters with different crystallinities were prepared by melting and crystallizing PLLA and PBS at different temperatures.The effect of crystallinity on the degradation behavior of materials was studied.The results indicated that the degradation rate of PLLA and PBS decreased with the increase of crystallinity,due to the faster degradation of amorphous region.A series of PLLA/PEG and PBS/PEG blends were prepared by melt blending.The melt viscosity was moderate during extrusion,indicating the materials were suitable for melt processing.PEG can improve the fluidity of PLLA and PBA in melt processing.In the process of non-isothermal crystallization,no significant crystallization peak was observed in the cooling scan of neat PLLA and two distinct crystallization exotherms were observed as the content of PEG increased.In the process of isothermal crystallization at 120?,the addition of PEG led to the decrease of the t1/2 and the increase of the spherulite growth rate of PLLA.The results show that PEG can enhance the mobility of PLLA chains and accelerate the crystallization rate.For the PBS/PEG blends,there are two crystallization and exothermic peaks during the non-isothermal crystallization and subsequent heating scan.During isothermal crystallization at 70? Avrami model was used to analyze the isothermal crystallization kinetics of the blends.Degradation behavior of PLLA/PEG and PBS/PEG blends in acid or base solutions was also investigated.The results showed that the addition of PEG was beneficial to the hydrolytic degradation of PLLA and PBS.As a hydrophilic substance,PEG can improve the hydrophobicity of PLLA and PBS;PEG can be easily soluble in water to form holes in the material and thus increased the contact area with water.The changes in microstructure,crystallinity and other properties of the material during degradation were studied by SEM,DSC,and GPC.It was observed by SEM that the addition of PEG strong influenced the degradation behavior of PLLA and PBS.After degradation,the crystallinity and crystallization temperature of the material gradually increased;the corresponding crystallinity of PBS in the PBS/PEG blend increased more remarkably than that of neat PBS.Meanwhile,during the degradation process,Mn of PBS decreased gradually and the molecular weight distribution became wider.The effect of PEG on the hydrophilicity of PLLA and PBS was also investigated.The contact angle of PLLA/PEG blends decreased from 74.3°to 47.7° and that of PBS/PEG blends decreased from 80.4° to 52.9° with the increase of PEG content from 0 to 0.5.