In-situ investigation of multiple endothermic peaks in isomorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with low HV content by synchrotron radiation

Origin of the double melting peaks of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) with a high HV content as revealed by in situ synchrotron WAXD/SAXS analyses

“…On the one hand, at the same temperature, the parameters a and b of P(HB‐ co ‐HV) crystals both enlarge with the increase of HV content. For example at 150 °C, the values of a and b with HV content of 4.9, 9.4, and 36.2 mol % are about 0.584, 0.584, 0.588 nm, and 1.326, 1.327, 1.333 nm, respectively . This enlargement effect again supports that HV units are packed into the PHB unit cell in the second P(HB‐ co ‐36.2%HV) crystals.…”

“…). Taking the isomorphism of random copolyesters into account, it is therefore reasonable to expect that the change of lamellar structures perhaps play a profound influence on the thermal behavior of P(HB‐ co ‐HV), and we recently have confirmed that two distinct kinds of lamellae formed in the P(HB‐ co ‐HV) with low HV contents . Herein, the nature of the double melting behaviors of high HV content's P(HB‐ co ‐36.2%HV) deserves further in‐depth investigation.…”

“…A coupling of the bigger unit cell, thinner thickness and weak expansion well support the crystals with the low melting point possessing the structure with HV units total inclusion. In addition, by comparing with the first structure of P(HB‐ co ‐4.9%HV) and P(HB‐ co ‐9.4%HV), one can find that the values of L 1 and d c1 are similar but the related melting temperature is different. The point again affirms that the first periodic structure of P(HB‐ co ‐36.2%HV) is formed by the uniform lamellae with HV units total inclusion, totally differing to the uniform lamellar crystals with HV units total exclusion in the low HV contents' P(HB‐ co ‐HV).…”

“…As presented in Figure , a nearly linear dependence of both the a 2 and b 2 of the second P(HB‐ co ‐36.2%HV) crystals on temperature from 30.0 to 170.2 °C is evident, and the slopes of the developing lines of a 2 and b 2 are 6.76 × 10 −5 and 3.68 × 10 −5 , respectively. It has been reported that the corresponding slopes of P(HB‐ co ‐4.9%HV) crystals are 6.88 × 10 −5 and 6.07 × 10 −5 , and those of P(HB‐ co ‐9.4%HV) are 6.87 × 10 −5 and 5.27 × 10 −5 , respectively . On the one hand, at the same temperature, the parameters a and b of P(HB‐ co ‐HV) crystals both enlarge with the increase of HV content.…”

“…Based on the ex situ and in situ synchrotron X‐ray experiments, we recently proved that the different lamellar structures play an important role on the dual endothermic peaks in P(HB‐ co ‐HV) with HV contents lower than 10 mol %, for the HV units packing into or out of the crystalline phase influences strongly the melting temperature . When the HV content changes from 10 to 40 mol %, the amount and sequence length of HB units become much less and shorter, consequently the HV units should be included in the crystalline phase .…”