Polymorphism and isomorphism in biodegradable polyesters

https://doi.org/10.1016/j.progpolymsci.2009.01.003Get rights and content

Abstract

Aliphatic polyesters, which can be obtained from bacterial or chemical synthesis, are emerging as an important class of biodegradable and biocompatible polymers for applications as biomedical and conventional materials. The last two decades have witnessed explosive growth in this subject. Most biodegradable polyesters are crystallizable. The crystallization and thermal behavior of biodegradable polyesters have been extensively studied in recent years. The physical properties, such as the mechanical, thermal properties, and biodegradability (of biodegradable polymers) of a semicrystalline polymer generally depend on the crystalline structure. The polymorphic crystallization of biodegradable polyesters, including poly(3-hydroxybutyrate) [P(3HB)], poly(lactide) (PLA), poly(3-hydroxypropionate) (PHP), and poly(butylene adipate) (PBA), as well as isomorphic crystallization in many kinds of random copolyesters, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], have been drawing more and more attention from both academic and industrial viewpoints. This article aims to provide a comprehensive review of the current understanding on (i) polymorphic crystallization and phase transition of biodegradable polyesters, (ii) isomorphic crystallization of biodegradable random copolyesters, where the effects of comonomer composition and crystallization conditions are highlighted. The relationships between the solid-state structure and physical properties as well as the biodegradability of biodegradable polyesters are also addressed.

Introduction

Biodegradable polymers have attracted increasing interest in fundamental research as well as in technology, due to their potential in addressing environmental concerns and biomedical applications. Biodegradable polymers break down in physiological environments by macromolecular chain scission into smaller fragments, and ultimately into simple, stable end-products. The degradation may be due to aerobic or anaerobic microorganisms, biologically active processes (e.g., enzyme reactions) or passive hydrolytic cleavage. The last two decades of polymer technology have seen a sharp rise in the development and commercial marketing of such new materials [1], [2]. Aliphatic polyesters are amongst the most important biocompatible and biodegradable materials that have received increasing attention. Their applications in conventional fields, such as agriculture, packaging, and fiber, and biomedical fields, e.g., tissue engineering, surgical suture, gene therapy, and controlled drug delivery, have grown significantly due to the availability of novel products with better performance characteristics [1], [2], [3], [4], [5], [6].

The chemical structures of several typical biodegradable polyesters are summarized in Table 1. Biodegradable polyesters can be divided into two groups with regard to the mode of bonding of the constituent monomers: (i) poly(hydroxy acid)s with –O–R–CO– as repeating monomeric units such as poly(3-hydroxybutyrate) [P(3HB)], poly(lactide) (PLA), poly(glycolic acid) (PGA), poly(ɛ-caprolactone) (PCL), etc., and (ii) poly(alkylene dicarboxylate)s, which are synthesized by the polycondensation reaction of diols (HO–R1–OH) and diacids (HOOC–R1–COOH), for example, poly(ethylene succinate) (PESu), poly(butylene succinate) (PBSu), poly(ethylene adipate) (PEA), poly(butylene adipate) (PBA), and so forth [2]. With regard to resources, biodegradable polyesters are classified into two groups: (i) biomass-based polyesters such as PLA, P(3HB) and its copolymers, and (ii) petroleum-based aliphatic polyesters such as PGA, poly(3-hydropropionate) (PHP), PCL, PBSu, PBA, and so on (Table 1). Renewable resource-derived polymers have been considered as candidate materials to overcome energy shortage and environmental problems [7].

Poly(hydroxyalkanoate)s (PHAs) are a group of naturally occurring biodegradable and biocompatible polyesters synthesized intracellularly as a carbon and energy reserve by a wide range of bacteria [8], [9], [10], [11]. The monomer composition of PHAs is variable and can be manipulated by means of the carbon sources used or by changing the fermentation conditions. At present, more than 100 different known types of PHA monomers with various structures have been discovered [11]. Depending on the number of carbon atoms in the monomers, PHAs are classified as short-chain-length PHAs (scl-PHA, 3–5 C-atoms) and medium-chain-length PHAs (mcl-PHA, 6 or more C-atoms) [10]. A representative member of PHA family is P(3HB) homopolymer, which is a semicrystalline polyester with a fast crystallization rate, high crystallinity, and high-melting point (Tm) (∼180 °C). However, P(3HB) is brittle and thermally unstable during melt processing because its decomposition temperature is very close to Tm. In order to overcome these drawbacks, a variety of copolymers of P(3HB) such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)] have been developed. The thermal and mechanical properties of these copolymers can be well manipulated by changing the chemical structure, content, and distribution of the comonomer units [10].

PLA has been considered as one of the most promising bioplastics because its raw material, l- or d-lactic acid, can be efficiently produced by fermentation from renewable resources such as starchy materials and sugars [12], [13], [14], [15], [16]. PLA can be chemically synthesized either by the condensation polymerization of lactic acid or the ring-opening polymerization of lactide, that is, a cyclic dimer of lactic acid. Lactide (or lactic acid) is a chiral molecule and has two optically active isomers: l- and d-lactide (or lactic acid). The polymerization of optically pure monomers leads to the formation of stereoregular poly(l-lactide) (PLLA) or poly(d-lactide) (PDLA). The polymerization of racemic lactide (or lactic acid) or meso-lactide, however, results in the formation of amorphous poly(d,l-lactide) (PDLLA). Both stereoregular PLLA or PDLA are semicrystalline, with a Tm of ∼175 °C and a glass transition temperature (Tg) of ∼60 °C. Stereoregular PLA has a high strength and modulus, comparable to that of polypropylene and polystyrene [16]. Moreover, PLLA (or PDLA) can be processed by versatile methods such as injection molding, stretch blow molding, foaming, fiber spinning, and so forth [16]. Stereoregular PLA has been considered as an ideal biomaterial for load bearing devices and a potential substitution to traditional petroleum-based thermoplastic polymers.

PDLLA is an amorphous polymer due to the random distribution of l- and d-lactic acid units. Due to its amorphous nature, the polymer shows a faster degradation rate than its stereoregular counterparts, making it a preferred candidate for developing drug delivery vehicles and as a low-strength scaffolding material for tissue regeneration [1]. Moreover, a wide range of physical properties such as biodegradation and mechanical properties of PLA can be manipulated by changing the stereoregularity, i.e., the content of l- or d-lactic acid units in the molecular chains. On the other hand, a very promising PLA-related material is the PLLA/PDLA stereocomplex, which has improved thermal stability and mechanical properties than PLLA or PDLA. This material has attracted much attention from both academic and practical viewpoints in the last two decades [17].

Among petroleum-based biodegradable polyesters, PCL and PBSu have been extensively studied as biomedical, packing, and fiber materials. PCL has low Tm (∼60 °C) and Tg (ca. −60 °C). It is highly processible, flexible, and biocompatible, making it a good candidate for drug delivery and scaffolds used in tissue engineering [1]. PBSu has a relatively higher Tm (∼120 °C), good flexibility, and toughness. PBSu and its copolymer poly(butylene succinate-co-adipate) [P(BSu-co-BA)] have been commercially produced under the trade name Bionolle. Bionolle has excellent processability and can be molded into a variety of products such as films, injection-molded products, filaments, and nonwoven fabrics [2].

Almost all biodegradable polyesters are semicrystalline, and therefore crystallization is a key process affecting their physical properties. The crystallization behavior of biodegradable polyesters has been extensively investigated in recent decades. Generally, the physical properties of thermoplastic polymers such as thermal, mechanical properties, and biodegradability (of biodegradable polymers) are considerably influenced by the crystalline structure and morphology that can be manipulated by changing the crystallization conditions. Accordingly, studies on the relationships between structure, morphology, and properties are of fundamental importance for controlling the final properties of polymeric materials. As described in the following sections, polymorphic crystallization has been observed in many aliphatic polyesters, including P(3HB), PLA, PHP, and PBA, and also the isomorphic phenomenon has been detected in some biodegradable random copolyesters such as P(3HB-co-3HV) and poly(hexamethylene sebacate-co-hexamethylene adipate) [P(HSe-co-HA)]. The syntheses [8], [10], [11], [15], [18], physical properties [6], [9], [10], [13], applications [1], [2], [4], [5], [14], and processing [14], [16] of biodegradable polyesters have been summarized in several recent reviews. In this contribution, we will make a comprehensive review on polymorphic and isomorphic crystallization, as well as the structure–properties relationships for biodegradable polyesters.

This paper reviews the polymorphic and isomorphic crystallization of biodegradable homo- and copolyesters, emphasizing the published research in this field in the last one or two decades. For polymorphic polyesters, attention has focused on crystalline structures, phase transition, formation mechanism, and preparation conditions of the different crystal modifications. With regard to isomorphic copolyesters, the effects of chemical composition and crystallization conditions on crystalline structure are emphasized. The relationships between crystalline structure and physical properties of biodegradable polyesters are highlighted. In addition, the preliminary theories concerning the macromolecular polymorphism and isomorphism are briefly addressed.

Section snippets

Basic theory of macromolecular polymorphism

Polymorphism in material science represents the existence of more than one form or crystalline structure in a solid material with the same chemical composition. It describes different crystal packing arrangements of the same molecular species in its crystal unit cells. If only one chemical element is present, the forms are called allotropes. Graphite and diamond are allotropes of carbon, whereas quartz and cristobalite are polymorphs of silica. Polymorphism can potentially be found in any

Basic theory of macromolecular isomorphism

In general, isomorphism represents that two or more distinct substances with similar crystalline structure crystallize together in a single crystal unit cell. Isomorphic crystallization has been extensively observed in minerals, metals, inorganic substances, and organic compounds. For example, sodium nitrate and calcium sulfate are isomorphic, as are the sulfates of barium, strontium, and lead. These isomorphic substances are usually analogous in size and chemical structure. Macromolecular

Crystal structure

Because bacterial P(3HB) contains asymmetric centers with only the R configuration, it is optically active and has a perfectly isotactic structure. The bacterial P(3HB) can crystallize in two modifications, the α- and β-forms. The α-form crystals are generally produced under the most common conditions such as melt, cold, or solution crystallization. A typical X-ray diffraction pattern (XRD) of the α-form P(3HB) is shown in Fig. 1(a). The α-form is characterized by two antiparallel chains in the

P(3HB-co-3HV)

P(3HB-co-3HV) is a typical bacterial copolyester, exhibiting isomorphic behavior. Since early studies on the cocrystallization behavior of P(3HB-co-3HV) have been summarized in our previous reviews [9], [55] this paper will focus mainly on recent researches. The P(3HB-co-3HV) random copolymers display several unusual thermal properties, for example, high crystallinity (∼60%) at all compositions [156], [157], and the presence of pseudoeutectic point in the composition-dependent Tm curve between

Poly(3-hydroxybutyrate)

The P(3HB) films or fibers produced by the cold, melt, or solution crystallization are usually too brittle, and their mechanical properties further deteriorate due to a secondary crystallization at ambient condition. Several research groups have attempted to improve the mechanical performance of P(3HB) materials by changing the crystallization and processing conditions. Schmack et al. [211] obtained P(3HB) fibers with a tensile strength of 330 MPa and a tensile modulus of 7.7 GPa by a high-speed

Concluding remarks

The crystallization behavior of biodegradable polyesters has been extensively studied in recent years. Most of the aliphatic polyesters show polymorphic behavior, depending on a variety of factors such as MW, Tc, stress, and processing conditions. The polymorphic phase transition between the different crystal forms can occur under the thermal treatments or stress. Because of the similarity in the conformation and chemical and crystalline structure of the parent homopolymers, the isomorphic

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