Biosynthesized poly(hydroxyalkanoate)s (PHAs) are biodegradable and renewable.¹ As a result, they have attracted much attention recently as candidates to replace petroleum-derived plastics. Among them is the copolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate), commonly known as P(3HB-co-4HB). Its processing properties are superior to those of pure poly(3-hydroxybutyrate), and its mechanical properties are adjustable.^{1, 2} As well as having environmentally friendly applications such as food packaging and films, P(3HB-co-4HB) has gained interest in a wide variety of medical fields, such as drug delivery and tissue engineering.³ Currently, over 10 kilotons a year of P(3HB-co-4HB) is produced industrially.⁴ Nevertheless, its high cost and other properties such as strength, thermal stability, gas permeability, solvent resistance, and flame retardance restrict its use for food packaging and tissue engineering.

Adding nanoclays to PHA-related materials to form intercalated nanocomposites improves their thermal and mechanical performance.⁵However, to the best of our knowledge, no previous study of P(3HB-co-4HB)/organoclay nanocomposites has been reported in the literature. We prepared P(3HB-co-4HB)/organoclay nanocomposites with 1–5wt% organoclay loadings by simple solution casting. We used wide-angle x-ray diffraction (WAXD) and transmission electron microscopy (TEM) to characterize the nanocomposites' morphologies. In addition, we studied the effect of incorporating organoclay on the thermal behavior of P(3HB-co-4HB) using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). We also used tensile testing to evaluate the effect of incorporating organoclay on the mechanical properties of P(3HB-co-4HB).⁶

WAXD patterns corresponding to P(3HB-co-4HB), organoclay, and their nanocomposites show that the organoclay presents a diffraction peak around 2θ=4.6^°, while no diffraction peak is presented in neat P(3HB-co-4HB): see Figure 1. For the nanocomposites with low organoclay loadings (≤3wt%), no diffraction peak is observed in the relevant region, suggesting that the structure may be highly exfoliated. The nanocomposite with 5wt% organoclay shows a weak diffraction peak at 2θ=4.6^°, indicating that some organoclays are exfoliated, but some unexfoliated organoclay agglomerates remain in the polymer matrix. A representative TEM image of P(3HB-co-4HB) nanocomposite with 3wt% organoclay further confirms the exfoliation structure of layered organoclay in the polymer matrix: see Figure 2.

We used DSC to evaluate the thermal behavior of P(3HB-co-4HB) nanocomposites melt-quenched from the glassy state: see Figure 3. Pure P(3HB-co-4HB) exhibits a cold crystallization peak at around 60^°C, and it shifts to significantly lower temperatures with increasing organoclay content, suggesting that organoclay fillers accelerate the cold crystallization process of the P(3HB-co-4HB) matrix.

We used TGA to assess the thermal stability of P(3HB-co-4HB) and its nanocomposites: see Figure 4. The 50% weight loss of pure P(3HB-co-4HB) takes place at 279^°C, while for nanocomposites with 1, 3, and 5wt% organoclay, it shifts to 296, 282, and 323^°C, respectively. The drop from 296 to 282^°C can be attributed to the interplay of layered organoclay and organomodifier on the polymer matrix. We believe the silicate layers of organoclay act as a barrier to oxygen, which increases the thermal stability of the polymer matrix, while the organomodifiers can accelerate thermal degradation of polymer. This indicates that, in general, the layered structure of organoclay improves the thermal stability of the polymer matrix.

Figure 5 shows the representative stress-strain curves of neat P(3HB-co-4HB) and its nanocomposites. It is clear that all the materials present yield behavior, that is, all the curves present a yield point, showing the materials are ductile. We have further evaluated quantitative data about the modulus, tensile strength, and elongation at break of the materials. Adding 3wt% organoclay improves the modulus of the nanocomposite by 34% compared to that of neat P(3HB-co-4HB). However, the modulus decreases with a further increase in organoclay content to 5wt%. Compared to that of neat P(3HB-co-4HB), the tensile strength does not change when the amounts of organoclay are less than 3wt%, while it decreases by 27% at organoclay loading of 5wt%. The elongation at break of the nanocomposites increases slightly with organoclay loading of 1wt% and reduces dramatically when the content is 3wt% and more.

In summary, we successfully used solution casting to prepare novel nanocomposites based on biodegradable P(3HB-co-4HB) and organoclay. As demonstrated by WAXD and TEM, the nanostructures are exfoliated when the organoclay content is up to 3wt%. Organoclay particles act as a strong nucleating agent for the cold crystallization of P(3HB-co-4HB), and in general, adding organoclay improved the thermal stability of P(3HB-co-4HB). We observed a considerable increase in tensile modulus for the nanocomposite with 3wt% organoclay in comparison to neat P(3HB-co-4HB). These P(3HB-co-4HB) nanocomposites show potential for various applications such as food packaging and tissue engineering. As our next steps, we plan to prepare the nanocomposites by melt intercalation, which, unlike solution casting, is suitable for industrial use. It is also solvent-free, and so less damaging to the environment.