The morphology of crystallisation of PHBV/PHBV copolymer blends
Graphical abstract
Introduction
Polyhydroxyalkanoates (PHAs) are a family of microbially synthesised and biocompatible polyesters [1]. They can be processed using conventional equipment and are fully biodegradable under ambient conditions in soil and aquatic environments [2]. As a result they have attracted significant commercial attention [3], [4].
Poly(3-hydroxybutyrate) (PHB) is the most common homopolymer of the PHAs. But it typically crystallises into large, dense, radially orientated lamellar spherulites, which make the material exceptionally crystalline and too brittle for many commercial applications without the use of additives and additional processing [1]. This can be addressed through copolymerization or blending. Copolymers of PHB have been developed by incorporating other monomers such as 3-hydroxyvalyrate (3HV) [5], 3-hydroxyhexanoate (3HHx) [6], [7], 3-hydroxypropionate (3HP) [6], and 4-hydroxybutyrate (4HB) [6], [8], [9], though only 3HV can be co-crystallised within the 3-hydroxybutyrate (3HB) crystal lattice [1], [9], [10], [11], [12]. In such 3HB lattice crystallisation, the bulkier 3HV units lead to a gradual expansion of the a-axis in the unit cell [13], reducing packing density within the unit cell due to internal stresses, and in turn reducing packing density of the lamellae in the crystals. This inclusion of foreign monomer units into the unit cell is responsible for the improved mechanical properties of these copolymeric materials, as the perfect crystalline structure of PHB crystals is disturbed, leading to larger amorphous regions with better flexibility. By contrast, when 3HB units are incorporated into the 3HV lattice, a decrease in the length of the b- and c-axes occurs, leading to a more compact unit cell [1], [14]. In some crystals, this leads to a characteristic ‘eye-like’ region in the crystal (Fig. 1), thought to be due to shift of direction of lamellar twisting [14]. As a result, PHBV copolymers exhibit a minimum crystallinity of 45% at 47–52% 3HV content (for carefully fractionated materials of narrow compositional distribution) [15], [16], a pseudoeutectic which also represents the transition from the 3HB to the 3HV lattice crystals. Pure PHB and PHV homopolymers have higher crystallinities of 60–68% [1], [17] and ∼64%, respectively [15], although there can be very significant variability depending on production method, processing history, degree of secondary crystallisation, etc. [18].
PHB and low 3HV content PHBVs are miscible, provided that the 3HV content of the PHBV is less than ∼15 mol% [19]. However, blends of the PHBV copolymers are immiscible if the compositional distribution difference is larger than 15 mol% [19]. As a result, PHBV blends containing both higher- and lower-HV content copolymers represent a system of polymers which may exhibit “interpenetrating crystallisation” where the spherulites of the component with slower crystal growth rate intrude in the spherulites of the one with faster growth [20]. While Jungnickel et al. [20], [21] and Yoshi and Inoue et al. [19], [22] have developed useful frameworks for studying the crystallisation of blends, there have to our knowledge been no reports on the crystallisation of blends of PHBV with very wide compositional distributions (i.e. >50% differences in 3HV contents).
The crystallisation behaviour of the fractionated poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymeric polyhydroxyalkanoates with narrow compositional distributions are well studied. In carefully fractionated materials, these copolymers are understood to undergo isodimorphic crystallisation at a range of temperatures, producing crystals of regular ring-banded spherulitic morphology, with increasing rejection of 3HV from the 3HB crystal and increases in band spacing at higher temperatures (lower degrees of undercooling) when the 3HV content is<50%, and a transition to the 3HV crystal lattice unit above 50% 3HV content [15], [16], [23], [24]. It is also known that the rate of crystallisation from the melt decreases with an increase in 3HV content up to the pseudoeutectic, then increases again [1], [25].
In reality, however, much of the PHBV material as-produced comprises blends of copolymers with different degrees of compositional distribution, particularly when using a mixed culture production process [26], but even when using pure culture production [25], [26]. The direct use of the as-produced PHBV blends without further fractionation is attractive as it offers cost-advantages.
This work builds on the studies of Inoue and co-workers [19], [22] and Organ and co-workers [27], [28], [29] who studied PHB blends with PHBV copolymers with 3HV contents of up to 27%, and Jungnickel and co-workers who extended the understanding of the behaviour of crystalline-crystalline blends [20], [21]. It further develops the previous studies of PHBV blends by using as-produced materials with 3HV contents up to 82% and a broad compositional distribution. The fractionated components of one of these materials, with narrow compositional distributions, are also included for comparison. The crystallisation properties of these materials were studied at a range of temperatures using polarized optical microscopy (POM), with the material properties being further characterized using DSC and WAXS to understand the behaviour of these PHBV blends in the melt and after crystallisation. SAXS is also used to probe the lamellar structure and the so-called linear crystallinity of the crystallised polymer [30].
Section snippets
Materials
HPLC grade chloroform and laboratory grade hexane were obtained from Sigma-Aldrich and used as received (99.9% and >99% purity respectively). Fermented whey permeate waste that was used as a carbon source was obtained from a cheese production site near Lund, Sweden. Acetic and propionic acids, which were used as carbon sources, were obtained from Merck and used as-is.
Samples
Samples used in this study were produced in an activated sludge-based Aerobic Dynamic Feeding (ADF) pilot plant operated by
Thermal properties of the as-produced polymer
Fig. 3a–c shows the DSC thermograms of the first heating scan (melt transition), first cooling scan (crystallisation transition), and final heating scan after quench cooling (glass and melt transitions), respectively, of the as-produced PHBV materials. The as-determined thermal properties are presented in Table 2.
As can be seen from Fig. 3a, sample A1 (15% 3HV random copolymer) only exhibits endotherms in the high temperature range (at 152 and 166 °C), consistent with high-3HB copolymer
Conclusion
The crystallisation morphology and growth kinetics of as-produced mixed culture PHBV blends produced using different combinations of feeding sequences were characterised. POM images indicate that the crystal morphology of the samples was governed by the compositional distribution of the blend and the temperature of crystallisation. PHBV with a narrow, low 3HV content composition (Sample A1) showed only single-phase 3HB crystals, where the 3HV monomer units predominantly co-crystallised within
Acknowledgement
This work is supported by the Australian Research Council for funding through ARC linkage grant LP0990917. The ARC had no role in the study design, collection, analysis, or interpretation of the data. The authors also thank A. Werker, L. Karabegovic, P. Johansson, and P. Magnusson from AnoxKaldnes for their valuable assistance with pilot plant operation.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The data will be made available on request.
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