The effect of preparation methods on the morphological and rheological properties of PHBH/CNC nanocomposites

Abstract

The growing use of conventional plastics has become a serious environmental problem. Most of these materials are produced from petroleum and remain in the environment for many years after use. As a result, plastic waste accumulates in landfills, soil, and oceans, causing long-term pollution and ecological damage. At the same time, the dependence on fossil resources increases carbon emissions and limits sustainability. For these reasons, there is a strong need to develop alternative materials that are both environmentally friendly and derived from renewable sources. Biodegradable polymers offer an important solution to this problem. Polyhydroxyalkanoates (PHAs) have attracted great interest because they are produced by microorganisms and can fully degrade under natural conditions. Poly(3-hydroxybutyrate) (PHB) is a key member of the PHA family and has attracted strong industrial interest because it is biodegradable and hydrophobic. To overcome some of its limitations, a copolymer known as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) was developed. The presence of 3-hydroxyhexanoate (3HH) units in the polymer chain increases flexibility and toughness compared with PHB. These units also reduce brittleness and improve processability, which makes PHBH more suitable for applications such as packaging, agricultural films, and medical products. Despite these advantages, PHBH still has some limitations. Its mechanical strength is lower than that of many conventional plastics, and it shows slow crystallization and weak melt stability. These properties make processing difficult and limit its use in more demanding applications. Therefore, improving the performance of PHBH while keeping its biodegradable nature is an important research goal. One effective way to enhance polymer properties is the preparation of nanocomposites. In this approach, small amounts of nanoparticles are added to the polymer matrix. Even at low concentrations, these fillers can significantly improve stiffness, strength and thermal stability. Among the available nanofillers, cellulose nanocrystals (CNCs) are especially attractive. CNCs are obtained from natural cellulose, which is abundant, renewable, and biodegradable. They are lightweight, stiff, and have a large surface area, which allows strong interactions with polymer chains. When CNCs are well dispersed in a polymer, they can greatly improve its mechanical and thermal properties. They can also influence the crystallization behavior of the polymer by acting as nucleation agents. However, CNCs tend to form agglomeration because of strong hydrogen bonding between their surfaces. This makes it difficult to achieve a uniform dispersion, especially in hydrophobic polymers such as PHBH. Therefore, the processing method and the choice of solvent play a crucial role in determining the final properties of the nanocomposites. In this study, PHBH/CNC nanocomposites were prepared using the solution casting method. This technique allows better control over CNC dispersion compared to melt processing, particularly for hydrophilic fillers. Four different solvents were used: tetrahydrofuran (THF), chloroform (CHL), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). These solvents have different polarities and solvating abilities, which can strongly affect how PHBH and CNCs interact with each other. First, the influence of solvent type on CNC dispersion and melt behavior was examined. For this purpose, nanocomposites containing 3 wt.% CNC were prepared using each solvent. Rheological measurements were then carried out to study the internal structure of the materials. Rheological behavior is very sensitive to filler dispersion. When CNCs are poorly dispersed, the material behaves similarly to neat PHBH. When CNCs are well distributed and interact with each other, the melt becomes more elastic and more resistant to flow. The results showed that the solvent had a strong effect on composite structure. Samples prepared with THF and CHL showed only small changes in rheological behavior, indicating limited CNC dispersion. DMF performed better, leading to higher viscosity and elasticity. The best results were obtained with DMSO. In this case, the nanocomposites showed a strong increase in elastic behavior at low frequencies, which suggests the formation of a well-connected CNC network inside the PHBH matrix. The superior performance of DMSO can be explained by its strong ability to interact with both PHBH and CNCs. DMSO can effectively weaken the hydrogen bonds between CNC particles, reducing their tendency to agglomeration. After identifying DMSO as the most suitable solvent, nanocomposites with different CNC contents (0.1, 1, 3, and 5 wt.%) were prepared. Rheological analysis showed that increasing the CNC content gradually changed the melt behavior from liquid-like to more solid-like. At a very low CNC concentration, a continuous filler network was formed, known as the percolation threshold. This value was found to be extremely low, showing that CNCs are very effective when they are well dispersed. Thermal analysis also revealed that CNCs improved the crystallization behavior of PHBH. The crystallization temperature increased and the crystallization process became faster, which is beneficial for industrial processing. These effects are related to the nucleating ability of CNCs, which help polymer chains organize more easily into crystalline regions. Overall, this study shows that the properties of PHBH can be greatly enhanced by adding small amounts of cellulose nanocrystals. The effectiveness of this improvement strongly depends on the solvent used during processing. Among the solvents examined, DMSO led to the best CNC dispersion and the most noticeable gains in rheological and thermal performance. These results demonstrate that PHBH/CNC nanocomposites have strong potential as sustainable, high-performance materials. By carefully adjusting the solvent and CNC content, the properties of PHBH can be tailored for more demanding uses while still preserving its biodegradable and environmentally friendly nature. This strategy supports the development of advanced bioplastics that can reduce dependence on conventional petroleum-based polymers and promote more sustainable material.