Kaolinite-reinforced bio-based polypropylene malonate nanocomposites incorporating itaconic acid with enhanced thermal properties

Abstract

This study investigates the effects of DMSO-intercalated kaolinite on the properties of poly(propylene malonate) (PPMI), synthesized from 1,3-propanediol and malonic acid, and cross-linked with itaconic acid (IA). The goal is to develop fully bio-based, saturated PPMI/kaolinite nanocomposites with varying kaolinite contents, focusing on how DMSO treatment enhances the dispersion of kaolinite and interfacial bonding within the polymer matrix. X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed improved dispersion and bonding. Thermogravimetric analysis (TGA) demonstrated an increase in thermal degradation temperature from 280 °C (neat PPMI) to 309 °C at 3 wt% kaolinite, indicating improved thermal stability due to enhanced polymer–clay interactions. Mechanical testing showed increased tensile strength (0.44 to 0.55 MPa) and Young's modulus (33.37 to 72.81 MPa) with rising kaolinite content, suggesting tunable flexibility and reinforcement. Although mechanical properties remain within the soft polymer range, these findings emphasize the role of DMSO-treated kaolinite in optimizing the thermal and mechanical properties of bio-based nanocomposites.

This study investigates the effects of DMSO-intercalated kaolinite on the properties of poly(propylene malonate) (PPMI), synthesized from 1,3-propanediol and malonic acid, and cross-linked with itaconic acid (IA).

Introduction

The growing environmental concerns surrounding petroleum consumption and its limited availability have led to a significant shift toward bio-based polymers, which are derived from renewable resources. These polymers present a promising alternative to petroleum-based materials due to their potential for sustainability and reduced environmental impact.¹ In recent years, there has been a surge in the demand for biomass-derived building blocks for polymer synthesis, marking a key trend in the transition towards a circular economy.^2,3 Bio-based polymers, such as poly(lactic acid) (PLA),⁴ poly(glycolic acid) (PGA),⁵ poly(butylene succinate) (PBS),⁶ poly(hydroxyalkanoates) (PHA),⁷ and poly(ε-caprolactone) (PCL), have garnered attention for their potential to replace conventional plastics in diverse industries, including packaging, biomedical applications, and agriculture.^2,8–10 Despite the progress, there remain challenges to overcome, including the cost of bio-based production and the performance of these polymers under practical conditions.^11,12 However, recent advances in bioprocessing and the expansion of global biorefineries have contributed to a decrease in production costs.^13,14 Polyesters derived from renewable sources, such as 1,3-propanediol (PDO), a bio-based monomer sourced from plants like corn, are another key area of focus.¹⁵ Within this growing field, aliphatic polyesters synthesized from a range of biobased acids, such as succinic acid, sebacic acid, adipic acid, malonic acid (MA), fumaric acid, and itaconic acid (IA), have shown great potential for producing both rigid plastics and elastomers. MA and IA, in particular, are of interest as key chemicals derived from sugars, identified by the U.S. Department of Energy as vital building blocks for future biorefineries.¹⁶ MA is a dicarboxylic acid with the chemical formula C₃H₄O₄, consisting of a three-carbon chain with carboxyl groups (–COOH) attached to both ends.¹⁷ It is a versatile compound used in organic synthesis, particularly for the preparation of various esters and as a precursor in the synthesis of pharmaceuticals and agrochemicals. MA is also known for its role in the malonate ester synthesis and its potential use in biodegradable polymers and copolymerization reactions.¹⁸ Moreover, IA-based polymers, which can be easily synthesized through polycondensation, offer excellent stability due to the C C bond in IA. Polypropylene malonate (PPM), developed in our earlier studies, belongs to the polyhydroxyalkanoates (PHA) family of polymers, known for their ability to degrade hydrolytically under environmental conditions.¹⁷ As observed in other PHAs, PPM also demonstrates a similar degradation tendency, making it a promising candidate for applications where biodegradability is important.¹⁹ PPM is a linear, amorphous polymer characterized by ester and ether bonds in its backbone, and it possesses a glass-transition temperature of −57 °C, making it suitable for use in specialty applications that require low-temperature flexibility such as in plasticizers or drug delivery systems, where the material's thermal properties can be tuned to match the intended use. However, the crosslinking of PPM with itaconic acid (IA) to form a gel-type product has never been studied before. This modification significantly alters the polymer's properties, including its flexibility, degradation rate, and potential for use in specialized applications such as drug delivery or plasticizers.

Incorporating natural clays, such as kaolinite, into polymer matrices has gained significant attention in recent years due to their ability to enhance material properties. Kaolinite, with the chemical formula Al₂O₃·2SiO₂·2H₂O,²⁰ is a clay mineral with a unique dioctahedral structure, containing alumino-silicate layers in a 1 : 1 ratio. When treated with organic molecules, kaolinite can be exfoliated, transforming its hydrophilic surface into an organophilic one, which allows for the intercalation of polymer chains within the kaolinite layers. This modification results in the formation of stable and homogeneous polymer/kaolinite nanocomposites.²¹ Kaolinite's fine particle size, inertness, and ability to improve mechanical and thermal properties make it an attractive filler for various industries, including paper, paint, and cosmetics.^22–26 Furthermore, kaolinite incorporation can enhance the mechanical strength, Young's modulus, heat resistance, and gas barrier properties of polymers, making it highly desirable for polymer/clay nanocomposites.^27,28

The advent of nanocomposite materials has sparked a revolution in materials science, leading to the development of high-performance materials with novel applications across many industries.^29,30 However, additional reinforcement of PPMI can enhance its mechanical properties for demanding applications, particularly in load-bearing conditions. Recent research has demonstrated that adding nanofillers such as carbon nanotubes, hydroxyapatite, and nanoclays can significantly enhance the strength, wear resistance, and thermal properties of composites.³¹ The use of kaolinite as a reinforcing agent in PPMI nanocomposites has not yet been explored. The incorporation of kaolinite into PPMI matrices could potentially improve the mechanical, thermal, and barrier properties of these composites, offering a more sustainable alternative to traditional materials.

Kaolinite is typically intercalated using polar organic solvents, such as dimethyl sulfoxide (DMSO) and N-methylformamide (NMF), which effectively break the hydrogen bonds between kaolinite layers, facilitating better dispersion within the polymer matrix.²² Among them, DMSO has been reported to produce larger basal spacing in kaolinite compared to NMF, which can further enhance its incorporation into the polymer matrix.³² In this study, the effects of DMSO-intercalated kaolinite on the properties of poly(propylene malonate) (PPMI) synthesized from 1,3-propanediol and malonic acid and cross-linked with IA were evaluated. The aim is to produce fully bio-based PPMI/kaolinite nanocomposites with varying filler contents, investigating how DMSO treatment improves kaolinite dispersion and enhances the mechanical and thermal stability of the resulting nanocomposites.

Experimental section

Materials

1,3-Propanediol (PDO, 98%), malonic acid (MA, 99%), itaconic acid (IA, 99+%), antimony(iii) oxide (Sb₂O₃, 99%), 4-methoxyphenol (HQ, 98%), dicumyl peroxide (DCP, 98%), dimethylsulfoxide (DMSO, ≥99.9%) and all solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA). All reagents and solvents were used as received without further purification.

Preparation of DMSO-intercalated kaolinite

The preparation of dimethylsulfoxide (DMSO)-intercalated kaolinite, denoted as KD, followed these steps: Initially, 180 mL of DMSO was thoroughly dispersed in distilled water at a volume ratio of 6 : 1. Subsequently, 30 g of kaolinite (Kao) was added to the solution, and the resulting suspension was stirred overnight at 60 °C followed by an additional 12 hours at room temperature. The suspension was then subjected to solid–liquid separation using a centrifuge at 4000 rpm for 20 min. The separated sample was washed three times with methanol to remove excess DMSO. The resulting product was dried at 60 °C for 48 hours and subsequently ground/sieved at 50 µm into a powder for further use.

Preparation of PPMI/kaolinite nanocomposites by in situ polymerization

The desired amount of 1, 3, 5 and 10 wt% of DMSO-intercalated kaolinite was added to PDO (0.11 mol) and dispersed using a magnetic stirrer at 500 rpm for 20 min at room temperature. Subsequently, MA (0.1 mol), IA (0.01 mol), Sb₂O₃ (0.1 mol% to acids), and HQ (0.5 wt% to all reactants) were added into the mixture. The experiments were run in a 100 mL round bottom flask equipped with a distillation setup and a nitrogen inlet to remove water and other by-products. All reactions were carried out with a diol/diacid molar ratio of 1.1 : 1. The reaction mixture was continuously stirred under nitrogen to limit oxidation and placed into an oil bath at 135 °C for 3 h to promote oligomer formation. After oligomerization, the reaction mixture was subsequently heated to 210 °C, and a vacuum of 7 mbar was applied for another 3 h to complete the polycondensation reaction. During this process, esterification occurred between the hydroxyl groups of PDO and the carboxylic acid groups of MA and IA, yielding unsaturated PPMI copolyester precursors containing pendant vinyl groups derived from IA. The final resin, except 10%, appeared as an opaque, viscous light grey liquid, and was used for the characterizations and further crosslinking step without any purification. The polymer sample containing 10% kaolinite was a rubber-like solid, which was cured directly without the addition of DCP (PPMI/K10X).

For the crosslinking step, the PPMI copolyesters were heated to 50 °C to decrease the viscosity and then mixed with DCP (0.5 wt%) as a free-radical initiator for 5 min. Then, the mixture was poured into Teflon mold coated with a silicone release agent and cured in an air oven at 160 °C for 24 h. During curing, thermal decomposition of DCP generated free radicals that initiated crosslinking through the pendant vinyl groups of the copolyester. After curing, the samples were carefully removed from the mold using tweezers. The diameter of mold was around 70 mm. The final products were named as PPMI/Kx, in which x was the weight percentage of KD in the prepared nanocomposites. A schematic illustration of the preparation of PPMI/kaolinite nanocomposites by in situ polymerization of PDO and MA/IA is presented in Fig. 1.

Fig. 1. Schematic illustration of the synthesis route of PPMI/kaolinite nanocomposites.

Characterization

The cross-linked polymer nanocomposites were characterized by Fourier transform infrared spectroscopy (FTIR) using a JASCO FTIR 6800 (JASCO International Co. Ltd, Hachioji, Tokyo, Japan) at 600–4000 cm⁻¹ wavelength range with 16 scans at a resolution of 4 cm⁻¹. Thermogravimetric analysis (TGA) was performed under nitrogen on a Mettler Toledo TGA/DSC 3+ (Mettler Toledo, Columbus, OH, USA). Samples were heated from 25 to 600 °C at 10 °C min⁻¹ heating rate. Differential scanning calorimetry (DSC) was performed using Mettler Toledo DSC 3 (Mettler Toledo, Columbus, OH, USA). The samples were first heated from room temperature to 160 °C at a rate of 10 °C min⁻¹, and held for 3 min to eliminate the thermal history. Then, they were cooled to −60 °C and heated again to 160 °C at a rate of 10 °C min⁻¹ under nitrogen flow. Wide-angle X-ray scattering were performed on powder samples using X-ray diffractometer (PANalytical, EMPYREAN XRD, Malvern Panalytical, Malvern, UK) with Cu Kα radiation. The scanning rate was 2° min⁻¹ from 5° to 60°. Morphological analysis of the powder samples was carried out using a FEI Quanta 650 Field Emission Scanning Electron Microscope with energy dispersive X-ray spectroscopy (SEM-EDS, FEI, Hillsboro, Oregon, USA) at 20 kV. Prior to imagining, the powder surfaces were coated with gold to reduce electrostatic charging. Tensile tests for five specimens of each nanocomposite with 50 × 5 × 1 mm were performed according to ASTM D882. The tests were performed on universal testing machine (WDW-50E, Jinan, China) at a crosshead speed of 5 mm min⁻¹ using a 50 kN load cell. Tensile properties such as tensile strength and Young's modulus were calculated from the stress–strain curves.

Results and discussion

X-ray diffraction (XRD)

The XRD analysis reveals changes in the diffraction peaks corresponding to the d-spacing of kaolinite layers after DMSO intercalation and the subsequent synthesis of the nanocomposites, indicating an expansion of the interlayer spacing. The absence of the (001) peak in the nanocomposites may suggest either exfoliation of kaolinite within the PPMI matrix or a low incorporation of kaolinite. Fig. 2 presents the XRD patterns of neat kaolinite, DMSO-intercalated kaolinite, and cross-linked PPMI/kaolinite nanocomposites. The characteristic peaks of kaolinite appear at diffraction angles of 12.27° (d = 0.72 nm) and 24.81° (d = 0.36 nm), corresponding to the (001) and (002) reflections, respectively. Following DMSO intercalation, the (001) peak shifts to a lower angle of 7.88° (d = 1.12 nm), which is consistent with previous literature.^21,33,34 Additionally, the intensity of the new peak increases, indicating more ordered kaolinite layers. The intercalation ratio (IR) of the DMSO-treated kaolinite is around 69.64%. In the XRD patterns of the PPMI/kaolinite nanocomposites (Fig. 3), a broad diffraction peak centered at approximately 20.72° is observed, which is attributed to the amorphous halo of the PPMI matrix. With increasing kaolinite content, a slight shift in this broad peak position and a reduction in intensity are observed, indicating changes in the short-range ordering and packing of the polymer chains induced by the incorporation of kaolinite.²⁵

Fig. 2. XRD patterns of neat kaolinite (Kao), DMSO-intercalated kaolinite (KD) and cross-linked PPMI/kaolinite nanocomposites.

Fig. 3. XRD patterns of cross-linked PPMI/kaolinite nanocomposites between 10° and 30°.

FTIR spectroscopy

The incorporation of organic modifiers into kaolinite led to interactions with the surface hydroxyl groups of the clay mineral. Four distinct bands were observed in the kaolinite, corresponding to these hydroxyl groups. The FTIR spectra of neat kaolinite and KD are shown in Fig. S1. After intercalating kaolinite with DMSO, two additional bands at 3536 and 3502 cm⁻¹, which are attributed to hydrogen bonding between DMSO and kaolinite, appeared. FTIR spectra of the synthesized PPMI polyesters and cross-linked PPMI/kaolinite nanocomposites, shown in Fig. 4, were recorded to assess their chemical composition. A broad O–H stretching peak around 3600 cm⁻¹ indicates the presence of –OH groups, likely from alcohols. Peaks around 2900 cm⁻¹ correspond to C–H stretching vibrations. A strong absorption band at approximately 1723 cm⁻¹ is attributed to C O stretching vibrations of the ester group. The absorptions at 1142 cm⁻¹ and 1034 cm⁻¹ are related to Si–O–Si stretching, while the peak at 910 cm⁻¹ corresponds to Al–OH–Al stretching in the kaolinite/PPMI nanocomposites, confirming that the kaolinite structure remained largely intact, spanning several layers. Additionally, the presence of crosslinking by itaconic acid is indicated by the absence or decrease in intensity of the characteristic C C stretching bands around 1642 cm⁻¹.

Fig. 4. FTIR spectra of neat PPMI and cross-linked PPMI/kaolinite nanocomposites in situ synthesized using Sb₂O₃ as catalyst.

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) and derivative thermal analysis (DTG) were performed on several synthesized nanocomposites, with the results summarized in Table 1. The TGA curves for neat PPMI and the nanocomposites are shown in Fig. 5. All samples exhibited thermal degradation in a single step. Neat PPMI decomposed around 280 °C, while the nanocomposites containing 3% kaolinite showed a higher decomposition temperature of approximately 309 °C. The addition of well-dispersed kaolinite into the polymer matrix enhanced the thermal stability by acting as a barrier, reducing the release of volatile degradation products. This was reflected in the T_5% values, which increased from 280 °C for neat PPMI to 295 °C with just 1% kaolinite. The maximum thermal degradation temperature (T_max) also increased to 309 °C with 3% kaolinite. However, when the kaolinite content increased to 5%, the degradation temperature slightly decreased, likely due to poor dispersion and weaker intermolecular interactions within the matrix.³⁵ Interestingly, the T_5% values for the uncured and cured 10% kaolinite nanocomposites showed a significant difference, indicating the impact of the curing process and DMSO content. The T_max values followed a similar trend. Overall, the incorporation of kaolinite delayed the thermal degradation of the neat polymer, improving the thermal stability of the composites. The residual weight loss was proportional to the kaolinite content but did not directly correspond, as it was also influenced by the loss of water and DMSO from the intercalated kaolinite. A mass loss of 15.75% was determined, corresponding to the DMSO content (Fig. S2). However, most of DMSO is expected to be removed after the polymerization at 210 °C.

Table 1. Thermal properties of PPMI/kaolinite nanocomposites^a.

Sample	T 5% (°C)	T max (°C)	Residue at 600 °C wt%	T g (°C)
PPMI	280	362	3.5	−29.2
PPMI/K1	295	380	9.0	−30.0
PPMI/K3	309	376	12.9	−28.0
PPMI/K5	307	378	15.3	−28.5
PPMI/K10	208	375	22.0	−37.6
PPMI/K10X	305	381	22.6	−23.3

^a T _g glass transition temperature, T_5% temperature at 5% weight loss, T_max temperature at 50% weight loss.

Fig. 5. TGA curves of neat PPMI and cross-linked PPMI/kaolinite nanocomposites with Sb₂O₃.

The derivative thermogravimetric (DTG) curves are shown in Fig. 6. Both neat PPMI and the nanocomposites exhibit double degradation behavior. The first degradation stage is likely due to the decomposition of low molar mass chains or volatile components, while the second stage corresponds to the main degradation process, indicating significant degradation related to the scission of hydrogen bonds in the polyester chains. As the kaolinite content in the nanocomposites increases, the degradation behavior is altered, with the maximum decomposition rate shifting to around 400 °C.

Fig. 6. DTG curves of neat PPMI and cross-linked PPMI/kaolinite nanocomposites with Sb₂O₃.

Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) was employed to investigate the effect of chemically functionalized kaolinite/PPMI nanoparticles on the glass transition temperature (T_g). The DSC curves of neat PPMI and cross-linked PPMI/kaolinite nanocomposites containing DMSO-intercalated kaolinite are shown in Fig. 7, and the corresponding values are summarized in Table 1. Neat PPMI exhibited a T_g of −29.2 °C. Upon the incorporation of low to moderate amounts of kaolinite (1–5 wt%) into the PPMI matrix, only minor changes in T_g were observed, suggesting that the filler had a limited effect on the segmental mobility of the polymer chains.³⁶ These small variations may be attributed to the interaction between the kaolinite platelets and the polymer chains that locally restrict chain motion. In contrast, the samples containing 10 wt% kaolinite exhibited markedly different behavior. The peroxide-cured sample (PPMI/K10) showed a lower T_g (−37.6 °C), whereas the non-peroxide-treated sample (PPMI/K10X) exhibited the highest T_g (−23.3 °C). This result suggests that, at high kaolinite loading, the thermal behavior is governed by a combination of filler-induced restrictions on chain mobility and structural changes associated with peroxide treatment. The higher T_g of PPMI/K10X may be attributed to stronger polymer–filler interactions and reduced chain mobility arising from the high kaolinite content, while peroxide-initiated reactions in PPMI/K10 may introduce competing effects that alter the polymer network structure. These results indicate that the influence of kaolinite on T_g becomes more pronounced at high filler loading and depends on the curing conditions.

Fig. 7. Second heating DSC curves of neat PPMI and cross-linked PPMI/kaolinite nanocomposites with Sb₂O₃.

Scanning electron microscopy (SEM)

Kaolinite dispersion within the polymer matrix was examined using SEM-EDS mapping. The EDS elemental analysis did not detect the expected 1 : 1 ratio of Al and Si signals, which might be due to surface effects or slight impurities.³⁷ However, the EDS elemental maps, shown in Fig. 8, demonstrated good dispersion of both Al and Si, indicating that kaolinite was effectively incorporated into the polymer matrix.

Fig. 8. Representative SEM-EDS elemental maps of 3 wt% cross-linked PPMI/kaolinite nanocomposites depicting composition of kaolinite filler.

SEM images of powdered kaolinite and DMSO-intercalated kaolinite are presented in Fig. 9. The images of kaolinite reveal fine-grain particles that are distinguishable as laminar hexagonal platelets, with some agglomerates visible (Fig. 9a and b). These agglomerates are likely formed through the aggregation of kaolinite particles due to strong intermolecular forces (cohesion) between them, which range in size from 0.25 to 1 µm in diameter and approximately 80 nm in thickness, consist of face-to-face stacking crystallites with sharply defined edges. Following DMSO intercalation, the kaolinite particles, shown in Fig. 9c and d, exhibit an increase in thickness, reaching over 100 nm, but their overall shape remains similar.

Fig. 9. SEM images of (a and b) kaolinite and (c and d) DMSO-intercalated kaolinite.

SEM images of the nanocomposites with varying kaolinite content, shown in Fig. 10a and d, reveal a uniform distribution of kaolinite within the polymer matrix, with no signs of agglomeration. This observation is consistent with the results from XRD and EDS mapping, indicating that the kaolinite is well-dispersed. The nanocomposites exhibit a clearly dispersed morphology and do not show inorganic domains, further suggesting that the kaolinite is uniformly incorporated into the polymer matrix. The kaolinite platelets are visible in the images. The major elements in the 5 wt% nanocomposites include carbon, oxygen, aluminum, and silicon, representing 48.84%, 47.38%, 0.66%, and 3.13%, respectively. While kaolinite should ideally have a 1 : 1 Al : Si ratio, the observed discrepancy could arise due to surface effects, small impurities, or slight variations in the crystal structure, which may influence the elemental distribution in the nanocomposite.

Fig. 10. SEM images of (a) 1 wt% kaolinite-PPMI, (b) 3 wt% kaolinite-PPMI, (c) 5 wt% kaolinite-PPMI, and (d) EDS result of 5 wt% kaolinite-PPMI cross-linked nanocomposites.

Tensile testing

Tensile testing of both the neat polymer and the 10% sample was not performed in this study due to the inherent brittleness of the neat polymer and the solid bulk form of the 10% sample, which made it challenging to prepare suitable test specimens. The mechanical properties of the bio-based cross-linked PPMI/kaolinite nanocomposites, presented in Fig. 11 and Table 2, showed a strong dependence on the kaolinite content. At 1 wt% kaolinite, the nanocomposite exhibited a tensile strength of 0.44 ± 0.17 MPa and a strain at break of 1.42 ± 0.33%. The Young's modulus was measured at 33.37 ± 7.98 MPa, and the toughness was 5.39 ± 2.53 kPa. The relatively low tensile strength and strain at break, combined with high toughness, indicate that the material retained some flexibility at this low clay concentration, with kaolinite particles potentially acting as a reinforcing filler without significantly hindering the polymer matrix's ability to deform. Upon increasing the kaolinite content to 3 wt%, a noticeable improvement in tensile strength was observed, reaching 0.55 ± 0.06 MPa, along with a marked increase in Young's modulus to 72.81 ± 9.35 MPa. This suggests that the addition of kaolinite particles facilitated stronger interfacial bonding between the filler and the polymer matrix, thus enhancing the overall stiffness of the nanocomposite. However, this increase in rigidity was accompanied by a decrease in strain at break (0.94 ± 0.18%) and toughness (3.56 ± 0.68 kPa). The reduction in strain at break and toughness indicates that, while kaolinite contributed to the strength and stiffness, it also made the material more brittle. This is a common behavior in nanocomposites, where the addition of filler particles strengthens the matrix but can reduce flexibility and impact resistance. At 5 wt% kaolinite, the nanocomposite showed a slight decrease in tensile strength (0.50 ± 0.23 MPa) compared to the 3 wt% sample, along with a further reduction in strain at break (0.74 ± 0.12%) and an increase in Young's modulus (60.57 ± 24.84 MPa). The increased modulus at this concentration suggests that the nanocomposite became stiffer, but the decrease in tensile strength and strain at break points to a possible overloading of the polymer matrix with kaolinite, leading to agglomeration or poor dispersion of the filler. These issues could have hindered the effective load transfer between the matrix and the filler, thus slightly diminishing the overall mechanical performance. Toughness increased slightly to 3.67 ± 1.29 kPa, which could be a result of better interaction between the matrix and filler at the higher clay content, though it remained lower than at 1 wt%. Overall, the mechanical properties of the nanocomposites indicated a complex relationship between the clay content and the material performance. While low kaolinite content (1 wt%) provided a balance of flexibility and strength, higher concentrations (3 wt% and 5 wt%) led to improved stiffness but also caused a reduction in flexibility and toughness, highlighting the challenge of optimizing filler content in nanocomposites. The results suggest that an optimal kaolinite content exists between 1 and 3 wt%, where the nanocomposites exhibit the best combination of mechanical strength and flexibility. Future studies could focus on improving the dispersion of kaolinite at higher concentrations to avoid agglomeration and enhance the performance of the nanocomposites.

Fig. 11. Stress–strain curves of cross-linked PPMI/kaolinite nanocomposites.

Table 2. Mechanical properties of cross-linked PPMI/kaolinite nanocomposites with kaolinite content.

Kaolinite (wt%)	Tensile strength (MPa)	Strain at break (%)	Young's modulus (MPa)	Toughness (kPa)
0	—	—	—	—
1	0.44 ± 0.17	1.42 ± 0.33	33.37 ± 7.98	5.39 ± 2.53
3	0.55 ± 0.06	0.94 ± 0.18	72.81 ± 9.35	3.56 ± 0.68
5	0.50 ± 0.23	0.74 ± 0.12	60.57 ± 24.84	3.67 ± 1.29

In comparison to commercially available biodegradable polymers such as polylactic acid (PLA), polybutylene succinate (PBS), and polyhydroxybutyrate (PHB), the cross-linked PPMI/kaolinite nanocomposites presented in this study exhibit a distinct mechanical profile. PLA typically has tensile strengths of 40–60 MPa and elongation at break around 5–10%, whereas the PPM/kaolinite composites show tensile strengths of 0.44–0.55 MPa and much lower elongation at break (0.74–1.42%). In terms of stiffness, the Young's modulus of the nanocomposites ranges from 33.37 MPa at 1 wt% kaolinite to 72.81 MPa at 3 wt%, which is much higher than that of PBS (typically 1–5 MPa) but similar to PHB's stiffness. Toughness is highest at 1 wt% kaolinite (5.39 kPa) and decreases with increasing filler content. Although the PPM/kaolinite composites are less flexible than PLA and PBS, their higher stiffness and thermal stability may make them suitable for applications requiring rigidity and bio-based content, despite the trade-off in mechanical performance.

Conclusions

Saturated bio-based nanocomposites were successfully synthesized from 1,3-propanediol and malonic acid via in situ melt-polycondensation and cross-linked with itaconic acid (IA), incorporating varying amounts of kaolinite clay. DMSO intercalation increased the basal spacing between kaolinite layers from 0.72 nm to 1.12 nm, confirming successful modification. This facilitated the effective dispersion of kaolinite within the polymer matrix, particularly up to 5 wt% kaolinite content. Structural analysis confirmed uniform dispersion, significantly enhancing the thermal and mechanical properties of the nanocomposites. For example, the cross-linked PPMI/kaolinite nanocomposites with 3% kaolinite exhibited a thermal degradation temperature of 309 °C, indicating improved thermal stability. Mechanical testing revealed that 1 wt% kaolinite enhanced tensile strength (0.44 ± 0.17 MPa), strain at break (1.42 ± 0.33%), and Young's modulus (33.37 ± 7.98 MPa). As kaolinite content increased, tensile strength and Young's modulus improved, with 3 wt% showing a significant rise to 0.55 ± 0.06 MPa and 72.81 ± 9.35 MPa, respectively. However, higher kaolinite content led to reduced strain at break and toughness, highlighting a trade-off between stiffness and flexibility. These results underscore the importance of optimizing kaolinite content, with 3 wt% being identified as the optimal concentration for balancing mechanical performance. Future work could focus on improving kaolinite dispersion at higher concentrations and evaluating the long-term stability of these nanocomposites for applications in packaging, automotive, and construction.

Conflicts of interest

No potential conflicts of interest was reported by the author(s).

Data availability

The data supporting this article are included within it and the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra04065a.