Different thermal behaviors of microbial polyesters poly(3-hydroxybutyrate-co-3- hydroxyvalerate-co-3-hydroxyhexanoate) and poly(3-hydroxybutyrate-co-3- hydroxyhexanoate) Hai-Mu Ye a,1 , Zhen Wang b,1 , Hong-Hui Wang b , Guo-Qiang Chen c, ** , Jun Xu a, * a Dept Chemical Engineering, Key Laboratory of Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, PR China b Multidisciplinary Research Center, Shantou University, Shantou 515063, PR China c Dept Biology, School of Life Science, Tsinghua University, Beijing 100084, PR China article info Article history: Received 8 February 2010 Received in revised form 22 September 2010 Accepted 13 October 2010 Available online 20 October 2010 Keywords: Poly(3-hydroxybutyrate-co-3- hydroxyhexanoate) Poly(3-hydroxybutyrate-co-3- hydroxyvalerate-co-3-hydroxyhexanoate) Multiple melting behavior abstract Various methods were employed to study the thermal behaviors of a novel microbial polyhydroxyalkanoate (PHA) terpolyester, namely, poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (PHBVHHx) compared with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx). PHBVHHx showed higher crystallization rate and degree of crystallinity. PHBVHHx exhibited also different multiple melting behaviors from PHBHHx. The WAXD results demonstrated that the crystal lattice of PHBVHHx was more compact than that of PHBHHx, suggesting stronger interaction between chain stems. DSC and in-situ heating WAXD studies revealed that PHBVHHx showed a partial melting-lamellar thickening-remelting process during heating, while PHBHHx demonstrated a melting-rapid formation of new crystals-remelting process. It is proposed that the simultaneous introduction of 3-hydroxyvalerate and 3-hydroxyhexanoate monomers into poly(3-hydroxybutyrate) improves the mobility of chain stems along the chain direction, leading to easier intralamellar slip during heating or drawing, further resulting in improvement of mechanical properties, which was supported by the DMA tests. Consequently, we establish a relationship between the thermal behavior and the mechanical properties of biodegradable plastics, which we believe is applicable to other polymers as well. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Polyhydroxyalkanoates (PHA) are microbial polyesters produced by many microorganisms. They have attracted much attention for their biodegradability, biocompatibility and the comparable mec- hanical properties with commercial non biodegradable plastics [1e3]. Poly(3-hydroxybutyrate) (PHB) is the first member of PHA family; which shows high melting point (wc.a. 180  C) and tensile strength, but brittleness and low elongation at break (3 b < 5%) at room temperature due to its higher degree of crystallinity limits its applications. Comonomers, such as 3-hydroxyvalerate (3HV) and 3- hydroxyhexanoate (3HHx), have been introduced into PHB, resulting in formation of PHBV and PHBHHx, which display a broad range of thermal and mechanical properties varying with comonomer content. For instance, the elongation at break (3 b ) and decomposition temperature of PHBV and PHBHHx increase with increasing como- nomer content in a certain range, widening their processing window and application areas. However, aging is a serious problem for the semi-crystalline PHA. PHB, PHBV and PHBHHx age rather quickly over time even at room temperature, exhibiting reduction of 3 b due to increase of brittleness [4e7]. Furthermore, the introduction of 3HV or 3HHx monomers results in significant reduction on the crystallization rate and on the degree of crystallinity, which is disadvantageous for their processing including injection molding, film blowing and fiber spinning. Recently, a microbial terpolyester PHBVHHx containing 3HB, 3HV and 3HHx monomers was synthe- sized by recombinant Aeromonas hydrophila 4AK4 harboring phbA and phbB (phaAB) genes encoding b-ketothiolase and acetoacetyl- CoA reductase of Ralstonia eutropha [8,9]. It was reported that simultaneous incorporation of 3HV and 3HHx improved the ductility and rigidity of the polyester compared with PHBV and PHBHHx with similar comonomer contents. Mechanical properties of these PHAs from literatures are listed in Table 1 . For example, the cast film of poly (3HB-co-5.4 mol% 3HV-co-11.7 mol% 3HHx) terpolyester showed a rather high elongation at break up to 340%, and the tensile strength was still maintained at about 15.7 MPa. In contrast, PHBHHx12 film * Corresponding author. Tel.: þ86 10 62784740; fax: þ86 10 62784550. ** Corresponding author. Tel.: þ86 10 62783844; fax: þ86 10 62794217. E-mail addresses: chengq@mail.tsinghua.edu.cn (G.-Q. Chen), jun-xu@mail. tsinghua.edu.cn (J. Xu). 1 Both authors contributed equally to this study. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.10.030 Polymer 51 (2010) 6037e6046