Investigation Of Formation Mechanism Of Multiple Endothermic Peaks In Isomorphous Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

The properties of crystalline polymer materials are largely controlled by the polymer aggregation state structure,especially for the isomorphic random copolymer,which the complicated crystallization behavior is closely related to the chain structure.,and the exist of isomorphism caused the diversity of the crystal structure and morphology,therefore by adjusting the chemical structure,content and distribution of comonomer,it can exactly control the aggregation state structure,especially for crystal structure,to improve the properties of isomorphic copolymer,and expand its application in differnet fleid.In this paper,the isomorphous random copolymer P(HB-co-HV)is chosen as an demonstrative polymer,which investigate the effect of comonomer units to crystal structure and thermal behavior by synchrotron radiation wide angle X-ray diffraction(WAXD),small angle X-ray scattering(SAXS)and differential scanning calorimeter(DSC).The main contents are as following:1.Through investigating the double endothermic peaks in the melting process of P(HB-co-HV)with HV content less than 10 mol%,we found that the P(HB-co-HV)samples with low HV content had different lamellar structures,namely the uniform lamellae with HV total exclusion and sandwich lamellae with HV partial inclusion.As the temperature increases,the thinner uniform lamellae melted first,and the thicker sandwich lamellae melted later,revealing the origin of double melting peaks.Moreover,the number of sandwich lamellae and the HV content in crystal increased with the HV content.2.Through studying the thermal behavior of double endothermic peaks in P(HB-co-36.2%HV)with high HV content,we found a huge temperature difference more than50.0 ? in double endothermic peaks without discernible recrystallization exothermic peak,indicating that the different lamellar structures in P(HB-co-HV)with high HV content was differ to that in P(HB-co-HV)with low HV content.It can be seen in 1D WAXD curves that the double diffraction peaks of(020)and(110)crystallographic planes disappear at different temperature ranges,corresponding to the different lamellar crystals melt at different temperature ranges.Combining the SAXS and WAXD analysis,it was clear that the thinner unstable uniform lamellae with HV total inclusion melted first,and the thicker sandwich lamellar with HV partial inclusion melt later.Meanwhile,it was also found that sandwich lamellae and uniform lamellae were formed during the cooling and subsequent isothermal crystallization processes,respectively.3.Through studying the phenomenon of multiple endothermic peaks of P(HB-co-HV)with HV content between 10 mol% and 30 mol%,we found that the triple melting peaks was caused by the thinner unstable uniform lamellae with HV total inclusion,thinner sandwich lamellae with HVpartial inclusion and thicker sandwich lamellae with HV partial inclusion melted at different range of temperature,which also confirmed that the crystal structure of HV content before and after 10 mol% was different.Combining WAXD and SAXS analysis,we found that P(HB-co-19.4%HV)with a lower HV content had a larger second melting peak,and the thinner sandwich lamellae contains the large unit cell parameters,indicating that the exist of transtion structure from sandwich lamellae to the unifom lamellae with HV total inclusion.With the HV content increasing,the temperature difference between the high and low melting peaks became larger,the HV content in the crystal increases,and when the HV content close to 30 mol%,the transition structure less exised in P(HB-co-HV)sample.