Abstract
This research focuses on the preparation of poly (3-hydroxy butyrate) (PHB) nanocomposites using the melt mixing method. Two types of organically modified nanoclay, Cloisite 93A (C93A), and Cloisite 30B (C30B), were incorporated at various weight percentages into the PHB matrix to create the nanocomposites. Comparative analyses were conducted between PHB/C93A and PHB/C30B to assess their tensile and impact properties in relation to the matrix polymer. Between the nanocomposites, the PHB/C93A nanocomposites shows an optimum tensile modulus of 949 Mpa with a 3 wt% clay loading, while PHB/C30B nanocomposites demonstrated improved percentage elongation at break of 5.33 % and enhanced Izod impact strength of 39.67 J/m at 3 wt% of clay load. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) signifies the thermal behavior of both the matrix and nanocomposite. The degree of crystallinity is observed to be 47 % in case of the PHB/C30B nanocomposites as compared to the PHB/C93A nanocomposites as 38 %. Again in case of thermogravimetric analysis (TGA), the maximum % char of 5.198 is observed for the PHB/C30B nanocomposites. The enhanced viscoelastic behavior of the PHB/C93A nanocomposites was attributed at a peak of approx. 55–60 °C due to the incorporation of C93A nanoclay into the matrix in the study of dynamic mechanical analysis (DMA). The morphological investigation using WAXD analysis showcased particle clay intercalation and dispersion within the PHB matrix, indicating effective clay-matrix interactions. Overall, this study sheds light on the enhanced properties of PHB nanocomposites with the incorporation of organoclay, offering potential applications in various industries.
1 Introduction
The development of newer materials is a sustainable challenge in the today’s scenario. Many researchers work on various materials viz. metals, plastics, elastomers, ceramics, wood, etc. with respect to its requirements and applications. Among all the materials the polymeric materials plays a vital role for the development of many newer materials due to its many advantages and versatility. The realm of polymer science has witnessed remarkable progress over the years, with polymers serving as useful foundational materials across diverse industries [1], [2], [3]. In addition, the introduction of polymer composite and blend materials was crucial in the modern science and engineering, offering a unique blend of strength, durability, and versatility. Composites, created by combining different materials, outperform traditional ones in applications demanding lightweight yet robust solutions [4], [5], [6], [7]. They find extensive use in aerospace, automotive, construction, and sports industries, contributing to fuel efficiency, durable infrastructure, and innovative consumer goods. As technology advances, composites play a pivotal role in driving progress across various sectors and providing sustainable solutions [8, 9].
However, the global pursuit of sustainable practices has fueled the exploration of biopolymers, such as polylactic acid, microbial bio-polymer [poly(3-hydroxy butyrate) (PHB) – a family of polyhydroxy alkanoates], bacterial cellulose, chitosan, etc. are offering a green alternative to traditional polymers. Derived from renewable resources, biopolymers present a compelling solution to the ecological concerns associated with polymer production and disposal. New environmentally friendly materials were developed by incorporating biopolymers into conventional polymers as a part of ecological awareness, particularly for single-use plastic goods. Because of its benefits, biopolymers have received significant interest recently.
One of the most promising biopolymer is poly (3-hydroxy butyrate, or PHB), which is quickly gaining prominence after polylactic acid [10]. Poly (3-hydroxy butyrate) belongs to the polyhydroxy alkanoates family and synthesized by various organisms. This is straight, optically active aliphatic polyester. This eco-friendly PHB plastic have been manufactured with the adoption of diversified methods to generate either from the source of plants or produced by bacteria [11]. PHB became a substitute to the petroleum-based plastics in the domains of packaging, agriculture, and biomedical applications due to its inherent characteristics of environment friendly and benign degradation behaviour [12, 13]. Nevertheless, the utilization of PHB is limited with respect to its strength, thermal stability, barrier properties, molecular weight, structural crystallinity and amorphous phase, narrow range of processing temperature, solvent resistance, and flame retardance for the end use applications [14, 15]. Blending PHB with other biodegradable plastics, such as polylactides, could result in enhanced mechanical and thermal properties [16, 17]. The mechanical properties of PHB can improved through copolymerization with valerate. This polymerisation results in development of poly (β-hydroxybutyrate-co-valerate) (PHB-HV) copolymer [18, 19].
Moreover, the integration of nanoparticles into polymers represents a revolutionary stride in material design. Among the myriad of nanoparticles, including carbon-based materials, carbon nano fibers, graphene, and metal oxides, each brings forth unique properties that can be harnessed for specific applications. Their significance lies in unique properties at the nanoscale, such as enhanced strength and tailored mechanical properties [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Within the spectrum of nanoparticles, nano clay-based polymer nanocomposites have emerged as a focal point of interest, showcasing unparalleled enhancements in mechanical strength, barrier properties, and flame retardancy [32]. The concept of preparation of bio-polymer nanocomposites is also became a predominant effect of research due to the enhanced property resolutions observed in the study of nanoscience and nanotechnology and also in the preparation of polymer nanocomposites. Likewise, the addition of nanoclays into PHB is an alternate way to improve the material performance by forming biopolymer nanocomposites [33], [34], [35], [36]. The barrier properties are enhanced with the incorporation of filler and nanoclays into the PHB, by preserving the flexibility and optical clarity of the pure polymer [37, 38].
Herein, we successfully prepared the PHB nanocomposites with the incorporation of organically modified nanoclays viz. Cloisite 30B and Cloisite C93A by the melt blending technique. An extensive study is made with the preparation of said nanocomposites, where it is observed that the presence of nanoclays enhanced the thermal stability and viscoelastic properties leading to improved mechanical performance and higher storage and loss moduli as compared to the matrix biopolymer PHB.
2 Materials and methods
2.1 Materials
Poly (3-hydroxybutyrate) (PHB) (Biomer P226): the PHB polymer was procured from Biomer, Germany, and had a density of 1.25 g/cc. For the preparation of nanocomposites two types of organically modified nanoclays namely Closite 93A (C93A) and Closite 30B (C30B) supplied by southern clay product (Gonzalez Inc., Tx) were used for its incorporation into the polymer matrix by melt mixing method.
2.2 Preparation of PHB nanocomposites
The PHB underwent a drying process at 50 °C for 6–8 h before processing, while the nanoclay was dried at 80 °C for 8–10 h. The PHB/nano clay were prepared by the melt mixing method by using extruder (DSM Xplore 15, Netherlands) of 15 cc conical co-rotating twin-screw in it.
For melting and mixing the matrix and nanoclays the temperatures were maintained at the range between 170 °C and 180 °C at the different zones of the barrel. The process parameters of the extruder were optimised for the preparation of the nanocomposite with a 40 rpm of screw speed and the acceleration rate of 50 rpm/min. The testing samples/specimens were prepared subsequently with the transfer of melt into specimen molds using a micro-injection molding machine. The process parameters included a temperature of 180 °C, an injection pressure of 7–7.5 bar, and a cycle time of 10 s. The specimens were fabricated in agreement with the ASTM D standard.
2.3 Characterization methods
2.3.1 Mechanical testing
The double type specimens of the measuring dimensions 167 × 12.7 × 3 mm were used for the tensile testing having gauge length of 50 mm in an universal testing machine (UTM) [Make & Model: Instron 3386, UK]. The tests were performed as per the standard test method of ASTM D638 at a crosshead speed of 5 mm/min. In case of Izod impact strength, the notched specimens were used with the ‘V’notch depth of 2.54 mm and at an angle of 45°. The impact specimens sized 63.5 × 12.7 × 3 mm with the defined notch size were tested as per the standard test method ASTM D 256. The Tinius Olsen Model 899 specimen notcher used for the preparation of the ‘V’ notch and the Tinius Olsen Model IT504 Plastic Impact apparatus from the USA used for the impact testing.
2.3.2 Thermal characterization
The thermal characterizations of the matrix and the nanocomposites were performed through DSC [Make and Model: Perkin-Elmer Diamond DSC instrument] and the thermogravimetric analyzer [Make and Model: Perkin-Elmer Pyris-7 TGA instrument] from the USA.
The thermal scanning was performed by DSC with a sample mass of 5 mg or less for the investigation of the glass transition temperature and melting behavior. The testing of the samples was performed under a nitrogen atmosphere with a rate of heating of 5 °C/min ranging from 30 to 200 °C. Subsequently the sample was kept hold for 1 min. The values of glass transition and melting temperatures of the both pure PHB and PHB nanocomposite samples were obtained through the analysis of DSC data corresponding to the respective temperature peaks of the graph.
The values of ∆Hm (heat of fusion of the sample), ∆Hf (heat of fusion for a fully crystalline material, set as 146 J/g), and Wf (mass fraction of the filler) were used in the following formula to calculate the degree of crystallinity.
With respect to the investigation of thermal stability of both PHB and its nanocomposite samples by TGA, the sample weight of 5–10 mg was used for testing under the nitrogen atmosphere from room temperature to 500 °C with a heating rate of 10 °C/min. The weight loss for all the samples was observed corresponding to the initial and final degradation temperatures of the heating process.
2.3.3 Heat distortion temperature (HDT)
The sample dimensions of 125 mm × 12.7 mm × 3 mm were used for the determination of heat deflection temperature (HDT) of the matrix and its nanocomposites by using the HDT tester [Make and Model: M/s GOTECH, model HV-200-C3], originating from Taiwan. The test was performed at a heating rate of 2 °C/min with the application of 66 psi as per the standard test method ASTM D648.
2.3.4 Dynamic mechanical properties
The viscoelastic properties of the materials were evaluated at a consistent frequency of 1 Hz, heating rate of 3 °C/min with a temperature ranging from 30 °C to 120 °C by using the dynamic mechanical analyzer (Make and Model: DMA Q 800, TA Instruments) from USA. The tests were performed in the three-point bending mode on the specimens having dimensions of 40 mm × 10 mm × 3 mm.
2.3.5 X-ray diffraction
X-ray diffraction (XRD) analysis was carried out by using the XRD instrument (Make and Model: XRD-7000, M/s Shimadzu) from Japan. The instrument is operated at 40 kV and 30 mA and having the monochromator. It employs a Cu radiation source with a wavelength of 1.54 Å. The d-spacing (d001) reflection of the nanoclay gallery and the PHB nanocomposites were evaluated by using the Bragg’s law equation (nλ = 2dsinθ). The experiment was performed at a scanning rate of 2 °/min for the diffraction angle 2θ in the range of 2°–10°.
2.3.6 FE-SEM analysis
The JSM-7001F field-emission scanning electron microscopy (FE-SEM) instrument was operated at appropriate accelerating voltages and working distances to achieve optimal imaging conditions. Samples were carefully prepared with a thin layer of conductive material by the method of sputter coating prior to imaging to ensure accurate and reliable results without any charging effects during electron beam exposure.
3 Results and discussion
3.1 Mechanical properties
The mechanical properties viz. tensile and impact properties of virgin PHB and its nanocomposites are enumerated in Table 1. From Table 1, it is observed that the preparation of PHB nanocomposites with the loading of organoclays in the range of 1–5 wt%, have an effect on the mechanical properties as compared to the pure PHB polymer matrix. Further the loading of two different types of organoclay into the PHB matrix resulted differently with respect to the tensile and impact properties.
Mechanical properties of PHB and its nanocomposites.
| Sample type | Tensile strength, MPa | Tensile modulus, MPa | Elongation at break, % | Impact strength, J/m |
|---|---|---|---|---|
| PHB | 21.56 ± 1.11 | 861.82 ± 19.61 | 4.96 ± 0.05 | 27.18 ± 0.73 |
| PHB+1 wt% C93A | 21.37 ± 0.23 | 929.18 ± 58.48 | 4.92 ± 0.07 | 27.64 ± 0.91 |
| PHB+3 wt% C93A | 22.20 ± 0.50 | 948.50 ± 36.76 | 4.00 ± 0.24 | 27.31 ± 0.50 |
| PHB+5 wt% C93A | 21.85 ± 0.25 | 917.69 ± 55.33 | 4.98 ± 0.32 | 24.81 ± 1.08 |
| PHB+1 wt% C30B | 21.01 ± 0.46 | 684.60 ± 23.54 | 5.76 ± 0.30 | 29.59 ± 2.24 |
| PHB+3 wt% C30B | 21.90 ± 0.20 | 744.08 ± 45.37 | 5.33 ± 0.98 | 39.67 ± 1.14 |
| PHB+5 wt% C30B | 21.09 ± 0.44 | 603.60 ± 57.09 | 5.23 ± 0.59 | 26.56 ± 0.50 |
At the different compositions of PHB/C93A nanocomposites, the increased tensile modulus is observed in all the cases of nanoclay loading from 1 to 5 wt%, but the optimum tensile modulus value of 949 MPa is obtained at 3 wt% of clay loading. This indicates that the nanoclay content at 3 wt% offers the best enhancement of tensile modulus for this particular composite system. Similar trends were observed for PHB/C30B nanocomposites, with increased tensile moduli of 744 MPa for 3 wt% of clay loading. These results suggest that the judicious incorporation of organoclay can significantly enhance the mechanical properties of PHB nanocomposites, and an optimal clay loading exists to achieve the highest improvement in the tensile modulus for both PHB/C93A and PHB/C30B nanocomposites. Again, in the case of PHB/C30B nanocomposites, decreased tensile modulus with optimized value of 744 MPa at 3 wt% loading of C30B is observed as compared to the matrix and the loading of C93A nanoclays. Due to the interfacial intercalation of 2 – hydroxyl ethyl groups present in the C30B nanoclay which was organically modified with methyl bis – hydroxyl ethyl tallow groups (MT2EtOH), a quaternary ammonium at a concentration of 90 meq/100 g. With the matrix polymer, this may be attributed to decrease in the tensile modulus of PHB/C30B nanocomposites. This interaction could lead to changes in the molecular arrangement and mechanical properties of the nanocomposite.
The values obtained for the tensile strength with the loading of nanoclay at 1 wt% and 5 wt% into the polymer matrix show the decrease in tensile strength as compared to the virgin PHB. The decrease in tensile strength may be due to the improper mixing and generation of phase separation between the virgin PHB and the organically modified nanoclays at 1 wt% loading. However in case of 5 wt% of nanoclay loading the decreased phenomenon is observed, which may be attributed to the agglomeration of nanoparticles. The slight increased value of tensile strength of 21.90 MPa and 22.20 MPa observed at 3 wt% loading of C30B and C93A nanoclay, respectively, as compared to the tensile strength value of PHB polymer matrix as 21.56 MPa. At 3 wt% clay loading, an optimal value of tensile strength is observed, indicating the establishment of better interfacial properties possibly due to the exfoliation of nanoparticles.
With respect to the elongation at break with the loading of 1 wt% and 5 wt% of C93A nanoclay into the polymer matrix, no significant change is observed. However, a decreased value is observed for the 3 wt% clay loading, suggesting that intermediate clay content affects the elongation properties for PHB/C93A nanocomposite. Contrastingly, the loading of C30B nanoclay show an increased percentage elongation compared to the pure polymer matrix, implying improved ductility and flexibility for PHB/C30B nanocomposite.
The stress–strain behavior of the polymer matrix and its nanocomposites observed during the study of tensile properties is represented in the Figure 1. The graph provides insights into the effect of organoclays on the preparation of nanocomposites and the type of failure exhibited by each system. From the graph, it is evident that the PHB nanocomposite system with C30B nanoclay shows a higher degree of strain compared to the virgin PHB. This increase in strain suggests that the incorporation of C30B clay enhances the ductility and flexibility of the nanocomposite, making it more capable of undergoing deformation before failure. On the other hand, the PHB nanocomposite system with C93A nanoclay undergoes brittle failure after reaching its ultimate tensile strength. The brittle failure phenomenon indicates the sudden failure in the nanocomposite without significant deformation upon reaching its maximum load-carrying capacity. Both stiffness and strength was improved in an analogous manner with the incorporation of C93A clay into PHB. This indicates that the nanocomposite becomes stiffer and exhibits higher resistance to deformation, leading to improved tensile strength.
A typical stress-strain curve of PHB and its nanocomposites.
In the Izod impact tests, a similar trend of optimization of clay loading at 3 wt% was observed for impact strength in both PHB/C30B and PHB/C93A nanocomposites. The PHB/C30B nanocomposites showed a remarkable increase in the impact strength by 45.95 %, whereas PHB/C93A nanocomposites showed slightly higher impact properties at 3 wt% clay loading compared to the PHB polymer matrix.
From the data, it is evident that PHB/C30B nanocomposites exhibit optimum impact properties, while PHB/C93A nanocomposites show optimal tensile properties. This phenomenon highlights the significance of commercially available organoclays of various types and their mixing with the matrix polymer to tailor the performance of nanocomposites for different applications. These findings emphasize the importance of selecting appropriate types and concentrations of organoclays for specific desired properties in polymer nanocomposites. Depending on the application requirements, the different polymer nanocomposites were prepared by using the types of organoclays to enhance specific mechanical properties, such as tensile strength and impact strength depending on the requirements of the applications with improved performance characteristics.
3.2 Thermal properties
3.2.1 Differential scanning calorimetry
Figure 2A and B presents the second heating thermal scan of the virgin PHB, PHB/C93A and PHB/C30B nanocomposites in this study. The samples were tested by DSC, where the corresponding values represents in Table 2 for glass transition temperature (Tg), melting temperature (Tm), crystallization temperature (Tc) including the enthalpy of fusion. In melt quenching scan, the materials were subjected to an operation aimed at completely erasing any previous thermal history. This step ensures that the subsequent thermal analysis accurately reflects the material’s intrinsic properties, free from any influence of prior thermal treatment.
Differential scanning calorimetry (DSC) curves of the samples. (A) Glass transition temperature (Tg) and melting temperature (Tm). (B) Crystallization temperature (Tc).
DSC data of PHB and its nanocomposites.
| Sample | Tg, °C | Tc, °C | Tm, °C | ∆Hm, J/g | Xc, % |
|---|---|---|---|---|---|
| PHB | −20.49 | 119.05 | 168.28 | 62.55 | 43 |
| PHB/C93A | −20.72 | 119.22 | 167.78 | 56.66 | 38 |
| PHB/C30B | −21.63 | 118.38 | 167.27 | 66.65 | 47 |
The observed changes in Tg indicate the influence of the clay nanoparticles on the polymer’s mobility, which can have implications for the material’s overall properties and performance. The glass transition temperatures (Tg) of the virgin PHB and its nanocomposites were represented in the Figure 2A. It is evident that the peaks observed at −20.49°C, −20.72°C, and −21.63°C correspond to the glass transition temperatures (Tg) of PHB, PHB/C93A, and PHB/C30B nanocomposites, respectively. The value of Tg in the PHB nanocomposites shows a small increase, which may be due to the restriction of the mobility of the polymer chains based on their bonding or adsorption on the clay surface. In Figure 2A the Tg is evaluated from the step-like shape which indicates the occurrence of physical aging in the samples. Physical aging refers to the phenomenon where the properties of the material change with time at a constant temperature. The step-like shape indicates a gradual increase in stiffness over time as the material undergoes physical aging.
During cooling in the DSC analysis, the second transition peak observed around 119.05 °C indicates the crystallization temperature of the pure PHB polymer matrix. It is observed that with the incorporation of C30B organoclay, the exotherms reveal the decrease in crystallization temperature (Tc) to 118.38 °C, which is nearly 1 °C lower than the polymer matrix. However, no significant change in Tc was observed for PHB/C93A nanocomposites. This decrease in crystallization temperature can be attributed to the facilitation of the nanoclays for the crystallization process of PHB. The nanoclays are having the high aspect ratio and large surface area to exhibit the effective nucleating sites to promote the formation of crystalline structures in the polymer matrix. The presence of nanoclays provides more sites for the PHB molecules to align and organize into a crystalline structure during the cooling process, leading to a slight reduction in Tc. From the DSC data, it is observed that the overall process of crystallization in the materials thermal behaviour changes with the incorporation of nanoclays into the polymer matrix due to their nucleating effect.
The third transition peaks were represented in the DSC thermograms of Figure 2A as the melting peaks of the virgin PHB, PHB/C93A, and PHB/C30B nanocomposites. Both the virgin matrix and its nanocomposites exhibit a melting peak and a shoulder on the melting peak. The melting peat and its shoulder were observed at around 168 °C and 174 °C, respectively, for the Virgin PHB. The two phases in this melting transition is likely observed due to the endothermic effect of melting of the original crystallites. The first phase of melting peak observed at lower temperature due to the imperfect crystals melt and the subsequent phase of shoulder on the melting peak is mainly due to the afterward formation of more perfect crystals by the process of recrystallization at a higher temperature, which gives rise to the main melting peak. Similarly, both nanocomposite systems, PHB/C93A and PHB/C30B, display a melting endotherm peak along with a shoulder. This indicates that there is no significant effect on the melting behavior of PHB with the addition of nanoclays. The Tm of PHB remains relatively unchanged with the addition of nanoclays. Furthermore, a melting peak around 50 °C is observed in Figure 2A in all the cases. This melting endotherm peak is primarily due to the presence of low molecular weight additive with the polymer matrix. This additive plays a role to facilitate its better processing during manufacturing.
It is evident from Table 2 that the PHB/C30B nanocomposite shows increased degree of crystallinity (Xc) among all other materials. The C30B nanoclay acts as a nucleating agent and facilitates to form more crystalline structures in the PHB polymer matrix during its solidification process which leads to increase in crystallinity. But the surfactant present in C93A nanoclay at the interface with the polymer matrix may act as a hindrance to the crystallization process and the formation of crystallites in PHB. As a result, the degree of crystallinity of PHB/C93A is reduced compared to both PHB and PHB/C30B nanocomposites. Due to the different interactions of C30B and C93A nanoclays with the PHB matrix, the variation in the degree of crystallinity observed among the nanocomposites. While C30B enhances crystallization, C93A may hinder the crystallization process, leading to different levels of crystallinity in the respective nanocomposites. The degree of crystallinity is an important factor influencing the mechanical and thermal properties of polymer materials. The variation in Xc observed in the nanocomposites demonstrates the potential of organoclays to influence the crystallization characteristics of the PHB matrix, which in turn affects the overall performance of the nanocomposite materials.
3.2.2 Thermogravimetric analysis
The virgin PHB and its nanocomposites exhibit thermal degradation behavior with two decomposition steps due to the chemical composition of the PHB matrix and the same is represented in Figure 3A and B). The first or initial step of degradation temperature for V-PHB, PHB/C93A, and PHB/C30B is observed at 287.29 °C, 286.20 °C, and 285.12 °C, respectively, as seen in the differential thermogravimetry curve. Similarly, the second or final step of degradation temperature for V-PHB, PHB/C93A, and PHB/C30B is observed at 392.78 °C, 381.96 °C, and 392.78 °C, respectively, as observed in the differential thermogravimetry curve. The lower values of temperatures in the initial degradation step observed for the nanocomposites may be attributed to the early decomposition of organically modified clays (C93A and C30B). This early decomposition of clays subsequently accelerates the degradation of PHB. Furthermore, PHB/C93A exhibits a lower temperature in the final degradation step compared to the virgin PHB and PHB/C30B. This may be due to the chemical structure of C93A organoclay (methyl dehydrogenated tallow) intercalate with more hydrophobic characteristics, which acts as a deformation accelerator at high temperatures. Notably, all the nanocomposites show a higher percentage of char compared to the virgin PHB. This indicates improved flame retardancy for the PHB nanocomposites. The char structure is formed on the surface of the nanocomposite during burning and serves as a protective barrier against further combustion. The interaction between the clay platelets and the polymer matrix plays an important role in promoting the formation of char during thermal degradation (see Table 3). Here in the nanocomposites, the dispersed clay platelets provide a high surface area and act as a physical barrier, limiting the movement and diffusion of volatile decomposition products during heating or burning. Indeed, the reassembly of dispersed silicate layers in polymer nanocomposites contributes to the improved mass transport barrier properties, thereby enhancing the thermal stability of the material. The presence of nanoclays in the polymer matrix creates a network of clay platelets that act as barriers to the movement of decomposition products and heat. This barrier effect slows down the degradation process and prevents the rapid release of volatile components. The enhanced thermal stability observed in nanocomposites is a result of the synergistic effects of the nanoclay. As the matrix degrades, the clay platelets reassemble, forming a more interconnected structure that hinders the diffusion of degradation products. This reassembly also leads to the generation of a protective char layer, which further contributes in the improvement of nanocomposites with respect to thermal stability and flame retardancy [39, 40].
Thermogravimetric analysis (TGA) curves. (A) PHB and its nanocomposites. (B) Derivative TGA curve of PHB and its nanocomposites.
Percentage char results of PHB nanocomposites.
| Sample name | Ti, °C | Tf, °C | Percentage char |
|---|---|---|---|
| PHB | 287.29 | 392.78 | 2.893 |
| PHB/C93A | 286.20 | 381.96 | 3.692 |
| PHB/C30B | 285.20 | 392.78 | 5.198 |
3.2.3 Heat distortion temperature
In this study, the heat deflection temperature (HDT) values were determined for virgin PHB, PHB/C93A, and PHB/C30B nanocomposites, yielding respective values of 131.9 °C, 137 °C, and 127.2 °C. Notably, the introduction of nanoclay into PHB results in an increased HDT in the case of PHB/C93A. However, the PHB/C30B composite exhibits a decreased HDT compared to pure PHB. This trend suggests a robust interaction between PHB and C93A within the PHB/C93A nanocomposites.
The examination of melting thermograms, as discussed in preceding sections, indicates that the incorporation of nanoclays does not significantly alter the Tm of the PHB matrix. Consequently, the enhanced HDT observed can be attributed to the improved mechanical stability of the nanocomposites. This enhancement stems from the reinforcing effect of the dispersed nanoparticles. Moreover, the PHB/C93A nanocomposite demonstrates superior strength and modulus, as highlighted in the earlier mechanical properties section.
3.3 Dynamic mechanical analysis (DMA)
In Figure 4A and B, three distinct regions were outlined in response to the temperature fluctuations as represented for the storage modulus (E′) and tanδ, respectively, for virgin PHB and its nanocomposites. These visuals distinctly outline three distinct regions. Firstly, the “Glassy State Region” manifests at temperatures below −25 °C, characterizing the material as a solid with limited molecular mobility. Secondly, the “glass transition region”, spanning roughly from −20 °C to 55 °C, signifies the phase shift from a glassy to a rubbery state, marked by heightened molecular mobility. Lastly, the “rubbery plateau region”, reflected from approximately 60 °C–115 °C, showcases the maintenance of flexible, rubber-like attributes, underscoring notable molecular mobility. These distinct zones effectively illuminate the dynamic mechanical behavior of both PHB and its nanocomposites across temperature variations, thus granting insights into molecular structural alterations and interactions within the material. In the glassy state, the material exhibits high rigidity and low energy dissipation, as indicated by the high E′ and close-to-zero tanδ values. As the temperature rises, the material undergoes the glass transition, with a notable drop in E′ and a peak in tanδ, signifying the shift from a rigid to a flexible state. For virgin PHB, the Tg is determined as 15 °C from the tanδ curve. Subsequently, in the rubbery plateau region, from around 80 °C–110 °C, the matrix polymer becomes remarkably soft, displaying high flexibility and energy dissipation, with a constant E′ and elevated tanδ value. Moreover, throughout the temperature region the substantial effect on the viscoelastic behavior is observed for the nanocomposites due to the incorporation of nanoclays. The effect of organically modified nanoclays upsurges the storage modulus in the glassy region, which may be clearly evident from Figure 4. This observation points to the reinforcing impact facilitated by C93A and C30B. Noteworthy is the fact that nanocomposites crafted using C93A nanoclay exhibited a notably higher E′ magnitude in comparison to both PHB/C30B nanocomposites and the virgin PHB counterpart. This behavior underscores the potent reinforcing influence of C93A in tandem with the polymer matrix. This robust effect could be attributed to a more favorable dispersion and an augmented aspect ratio of C93A. Another plausible explanation might stem from the potential occurrence of hydrolytic degradation of the polymer due to the existence of C30B nanoclay.
Dynamic mechanical analysis (DMA). (A) Storage modulus of PHB and its nanocomposites. (B) Tan delta of PHB and its nanocomposites.
The viscoelastic characteristics of the PHB nanocomposites are significantly affected by the type and dispersion of the organoclays into the PHB polymer matrix. The improved reinforcing effect observed with C93A suggests that it has a more beneficial interaction with the polymer matrix, leading to enhanced mechanical performance. On the other hand, in the glassy region the presence of C30B nanoclay may have led to hydrolytic degradation, potentially disturbing the mechanical characteristics of the nanocomposite [41].
In proximity to the glass transition region, the PHB/C30B and PHB/C93A nanocomposites shows effective improvement in the modulus due to the reinforcing effect of the fillers as compared to the notable decrease in the PHB matrix. This compensation mechanism is ascribed to the segmental restriction of the polymer chains, resulting in an elevation of the E′ and a concurrent attenuation of tanδ intensity (depicted in Figure 4B). This segmental immobilization phenomenon emerges from the exchanges established between the nanoclays and the polymer matrix. These exchanges curtail the mobility of polymer chains, thereby heightening the overall rigidity of the nanocomposite structure.
Furthermore, it was discerned that the Tg of PHB exhibited an elevation from 15 °C to 21.81 °C and 20.30 °C with the addition of type of nanoclays and the preparation of the PHB/C93A and PHB/C30B nanocomposites, respectively. This observation can be attributed to the heightened compatibility among the nanoclays and the bulk PHB matrix. The existence of well-dispersed and strongly interacting nanoclays within the polymer matrix contributes to an augmented Tg, signifying a more robust and enduring nanocomposite architecture.
Within the rubbery region, a substantial reinforcing outcome emerged due to the presence of organoclays. As evidenced by the tanδ curves, a peak in relaxation marked beyond 55 °C. Unlike the PHB matrix, which lacked such a relaxation peak, the nanocomposite revealed a distinct step approximately around 55–60 °C, particularly evident in the case of the PHB/C93A nanocomposite. This relaxation peak denotes an enhanced viscoelastic behavior attributed to the incorporation of nanoclays. These nanoclays assume the role of reinforcing agents, impeding the mobility of polymer chains and engendering superior mechanical properties and viscoelastic behavior within the rubbery region.
3.4 X-ray diffraction
The X-ray diffraction patterns are displayed in Figure 5A and B for both the nanoclays (C30B and C93A) and their respective PHB nanocomposites. The d001 spacing, which signifies the interlayer distance of the clay, was determined by analyzing peak places using Bragg’s law. In the case of C30B and C93A nanoclays, the primary silicate reflections were observed at 2θ = 4.8° (with an approximate d-spacing of 1.76 nm) and 3.8° (with an approximate d-spacing of 2.32 nm), as outlined in Table 4.
XRD dffractograms of (a) PHB/C93A, (b) PHB/C30B and (c) PHB.
Diffraction peaks and d-spacing of nanoclays and their nanocomposites.
| Composition | XRD peak position, 2θ | d-spacing, nm, d001 |
|---|---|---|
| Closite 93A | 3.8 | 2.32 |
| PHB/C93A, 97:3 | 2.61 | 3.38 |
| Closite 30B | 4.8 | 1.76 |
| PHB/C30B, 97:3 | 4.16 | 2.12 |
For the PHB/C93A nanocomposite, a discernible diffraction peak emerged in the lower angle range, validating the intercalation of C93A’s silicate layers within the PHB matrix post-melt mixing. Conversely, the absence of integrally clay diffraction suggests the disorderly and stochastic dispersion of nanoclay platelets within the PHB matrix. Conversely, in the case of the PHB/C30B nanocomposite, a distinct diffraction peak emerged post-melt mixing with PHB at 4.16° (with an approximate d001-spacing of 2.12 nm). Furthermore, a transition from C93A to PHB/C93A led to the shifting of the diffraction peak to a lower angle, transitioning from 2θ = 3.8° (with an approximate d001-spacing of 2.32 nm) to 2θ = 2.61° (with an approximate d001-spacing of 3.38 nm). This observation implies an expansion of the layer spacing of C30B, while its ordered structure remained intact post-melt mixing. Through WAXD analysis, it can be inferred that PHB/C93A demonstrates enhanced characteristics concerning the compatibility and intercalation of nanoclays within the PHB matrix post-melt mixing. In contrast, PHB/C30B did not undergo significant exfoliation or intercalation post-mixing with PHB.
Additionally, it is worth noting that both nanocomposites showcased a secondary diffraction peak at an angle approximately around 9° (with an approximate d-spacing of 1 nm). This secondary peak could be ascribed to the intrinsic properties of the polymer matrix employed in this study, potentially indicating the presence of clay within the polymer matrix, in alignment with the findings from the WAXD examination of virgin PHB (depicted in Figure 5C) [42].
3.5 Morphological properties: FE-SEM analysis of organoclay/PHB composite
Field-emission scanning electron microscopy and scanning electron microscopy (SEM) were employed to examine various aspects of the composite morphology, including the clay particles, surface, and dispersion. The investigation aimed to assess the degree of polymer–filler adhesion and the allocation of filler nanoplates within the polymer matrix [43].
Figure 6A and B illustrate the FE-SEM images of the PHB composite with incorporated C30B clay at various magnifications. The images clearly depict the well-dispersed organoclay within the PHB matrix, indicating good intercalation [44]. Additionally, the layered structure of the clay layers can also be observed from the images. Additionally, the surface image (Figure 6B) reveal outstanding adhesion and a high extent of compatibility among the nanoclay particles and the polymer matrix, attributed to the proper melt mixing. The proper adhesion ensures the absence of void formation [45]. Moreover, the success of this adhesion can be attributed to the hydrophilic nature of both phases. Figure 6A and B also demonstrate the uniform dispersion of inorganic clay particles throughout the polymer matrix, especially at optimum loadings [46].
FE-SEM micrographs of (a) PHB/C30B and (b) PHB/C93A.
4 Conclusions
The research investigated the properties of PHB nanocomposites with nanoclays C93A and C30B prepared through melt blending. The nanocomposites exhibited superior mechanical properties, including increased Young’s modulus and tensile strength compared to V-PHB, attributed to the nucleation effect of nanoclays. The addition of clay improved crystallinity, with two melting peaks observed in the DSC thermograms, indicating a two-phase system. The Avrami exponent suggested three-dimensional crystal growth during isothermal crystallization. The presence of nanoclays enhanced thermal stability and viscoelastic properties leading to improved mechanical performance and higher storage and loss moduli. The morphological studies supported the above finding by displaying good dispersion over the matrix. Surprisingly, the nanocomposites degraded faster under composting conditions than V-PHB. Overall, the study highlights the potential of PHB nanocomposites as strong, thermally stable materials with promising applications in various industries, though further research is needed to understand their degradation behavior in specific environmental conditions.
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Research ethics: The research work is original of the authors.
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Author contributions: Aswini Kumar Mohapatra: Experiments, data curation, validation, compilation, writing and reviewing of original draft, supervision as first author and corresponding author; Aswathy N R: writing and reviewing of original draft as co author.
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Competing interests: Authors declare no conflict of interests.
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Research funding: No funding agency funded the current work.
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Data availability: Available on requst to the corresponding author after verification.
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