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PART II PROPERTIES

6 STRUCTURES AND PHASE TRANSITIONS OF PLA AND ITS RELATED POLYMERS

      Hai Wang and Kohji Tashiro

6.1 INTRODUCTION

      This chapter focuses on the structure and phase transition behavior of PLA and related polymers. The crystal modifications of PLA revealed so far are the mesophase [1–5], the δ (α′) phase [5–10], the α phase [11–16], the β phase [17–20], the γ phase [21, 22], the ε phase [23–26], and the stereocomplex (SC) between PLLA and PDLA [27–37]. The relationship between the chain conformation and aggregation state of chains in these crystalline modifications and the resulting intrinsic physical properties has to be understood for the improvement of the physicochemical property of PLA products. Besides, the unknowns about the crystal structure information make it difficult to fully understand the phase transition behaviors between the various crystal forms. In this way, the establishment of reliable and precise structures of PLA crystalline forms and the clarification of their phase transition behaviors are indispensable for the development of PLA.

      The similar situation is seen also for the crystal structures of poly(3‐hydroxybutyrate) (PHB). By adding one CH₂ unit to the basic chemical formula of PLA (─[─C*H(CH₃)OCO─] _n ─), PHB (─[─CH₂C*H(CH₃)OCO─] _n ─) is obtained. One skeletal C atom is optically asymmetric and so the PHB polymer chain may take also the L and D enantiomeric species similarly to those of PLLA and PDLA. This apparently slight difference of chemical structure between PLA and PHB, however, provides the remarkably different characters with respect to the helical chain conformation and the chain packing mode, as well as the crystallization behavior itself. Additionally, there are many kinds of aliphatic polyesters, which can show relatively high biodegradability comparable with that of PLA and PHB. For example, poly(ethylene adipate) (PEA) has the linear chemical formula without CH₃ side groups, ─[─OCH₂CH₂OCO(CH₂)₄CO─] _n ─. In some cases, PEA is blended with PLA or PHB to control the biodegradability. To understand the crystallization behavior of PEA and PLA in these blends, it is indispensable to know the structure of PEA at the starting point.

      In the present chapter, the crystal structures and phase transition behaviors of PLA are mainly discussed, but, at the same time, such related polyesters as PHB and PEA are also reviewed to extract the similarity and differences between these polyester compounds, from the structural points of view.

      Although many review articles have been published about the structures, the phase transition behaviors and the relation between structure and property of these polymers [38–41], we need to realize the most reliable and accurate information, which were derived mainly from the quantitative analyses of the X‐ray diffraction and vibrational spectroscopic data, are introduced in the present chapter.

6.2 STRUCTURAL STUDY OF PLA

      6.2.1 Preparation of Crystal Modifications of PLA

To elucidate the detailed crystal structures and phase transition behaviors of PLLA, X‐ray diffraction method of highly oriented and highly crystalline samples is most useful. Figure 6.1 shows that the various crystal modifications are obtained depending on the sample preparation conditions. When the sample in the molten state is quenched into ice water, predominantly amorphous‐phase PLA is obtained [4, 5]. Casting PLA films using chloroform