Eco-friendly starch-nanocellulose bio-nanocomposite films with improved structure–property interactions

Abstract

The development of sustainable and biodegradable substitutes has increased due to growing environmental concerns surrounding petroleum-based polymers. Herein, starch-nanocellulose bio-nanocomposite films were fabricated and thoroughly characterized in order to assess the impact of nanocellulose fiber loading on their physicochemical, structural, thermal, optical, and mechanical properties. Using a straightforward solution-casting technique, sulfuric acid hydrolysis was used to remove nanocellulose from a filter paper and incorporate it into a thermoplastic starch matrix plasticized with glycerol. Water uptake, moisture absorption, and solubility analyses, tensile testing, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and UV-vis spectroscopy were used to make and evaluate composite films containing 0, 5, 10, and 20 weight percent nanocellulose. Results demonstrated that increasing the nanocellulose content substantially decreased water uptake and moisture absorption because of the reduced free volume and increased hydrogen bonding within the matrix. The use of nanocellulose enhanced film continuity and decreased cracking, as demonstrated by morphological observations. TGA showed increased thermal stability at larger nanocellulose loadings, while XRD examination showed a steady rise in crystallinity from 32% for clean starch to 60% for the 20-weight percent nanocellulose composite. Although elongation at break reduced as a result of higher stiffness, mechanical testing revealed significant increases in tensile strength and Young's modulus with increasing nanocellulose concentration. These results show that the starch-nanocellulose bio-nanocomposites have enhanced strength, thermal resistance, and barrier qualities, underscoring their great promise as environmentally benign materials for biodegradable packaging and associated uses.

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1. Introduction

In recent years, synthetic materials have been widely used in many aspects of modern life such as in shopping bags and furniture.^1,2 Synthetic materials are more durable, versatile, uniform and environmentally sustainable, but most of the synthetic materials are expensive and have high processing costs. Besides, these synthetic materials are inert to microorganism attack, cause disposal problems, and create serious environmental pollution. At present, macromolecular substances are produced from petroleum. Most petroleum-derived polymers are not biodegradable and persist in the environment.^3,4 Considering these issues, natural polymeric composites need to be developed that are sustainable, suitable to use, and offers easy disposal, making them alternative sources to synthetic materials. For this, among different natural polymers, starch is of great interest. It is a linear polymer that is easily obtained from plant sources such as potato, corn wheat, maize etc. It is a semi-crystalline polymer and has two units: amylose and amylopectin. Amylose is a linear polymer chain, and amylopectin has a branched structure. Both of these units are attached through glycosidic bonds. Due to the presence of more hydroxyl groups, starch shows a hydrophilic nature and forms hydrogen bonds with ether and alcohol. It does not become a thermoplastic, like synthetic materials, in cold water, but in hot water, it breaks down its glycosidic bonds and turns into a thermoplastic.^5,6

Starch is considered a promising source for bio-composites owing to its biodegradability, renewability, and environmentally friendly nature.^7,8 Bio-composites are formed by combining heterogeneous materials to achieve properties superior to those of the individual components. Despite these advantages, starch-based polymeric materials exhibit inherent limitations, such as high moisture sensitivity and relatively low mechanical strength. These drawbacks can be effectively addressed by incorporating cellulose fibers as reinforcing fillers and glycerol as a plasticizer, which enhance stiffness, interfacial bonding, and overall durability.^9,10 Furthermore, compared to conventional natural cellulosic fiber-based systems, nanocellulose-based composites have emerged as advanced materials, offering remarkable improvements in mechanical performance at low fiber loadings while preserving transparency, processability, and sustainable characteristics.^11,12 Extracted from a variety of natural sources, nanocellulose has been utilized extensively to increase mechanical strength and decrease water sensitivity. In order to extract them, non-cellulosic components are usually removed by chemical or biological pretreatments.^13–15

Starch-nanocellulose bio-nanocomposites have gained significant attention as sustainable bio-composite materials due to the reinforcing potential of cellulose nanofibers (CNFs). CNFs act as effective nano-scale fillers within starch matrices, leading to notable improvements in mechanical strength, thermal stability, and controlled degradation behavior. Owing to the hydrophilic nature of cellulose systems, increased surface roughness and strong interfacial interactions between starch and nanocellulose play crucial roles in enhancing composite performance.^16–18 Various surface modification and processing strategies for nanocellulose have been explored to further tailor the structural and functional properties of these composites.^19–21 As a result of their small dimensions, high surface reactivity, and organized assembly, starch-nanocellulose bio-nanocomposites exhibit superior stiffness and strength at low filler loadings. These advantageous characteristics make them highly suitable for a wide range of applications, including packaging and construction and use in automotive components, furniture, and electronics, along with emerging uses in pharmaceutical, cosmetic, and biomedical fields.^22,23

Glycerol is added to increase a composite's flexibility, water resistivity, and sustainability. It increases water resistivity and sustainability of bio-composite films. Earlier, thermoplastic starch was mixed with plasticizers such as polyethylene glycol, polycaprolactone, polybutylene, and polyhydroxybutyrate for preparing bio-composites. However, these blends of thermoplastic starch with polycaprolactone, polybutylene, and polyethylene glycol results in brittleness, immiscibility and lower flexibility. To overcome these problems, glycerol is now added with thermoplastic starch, which reduces the brittleness and increases the flexibility and processability of composite films.^24,25 The combined development and methodical assessment of totally biodegradable starch-nanocellulose bio-nanocomposite films employing a straightforward solution-casting fabrication technique and a techno-economically feasible chemo-mechanical extraction process for nanocellulose constitute the novelty of this study. This work offers a thorough correlation between physicochemical, thermal, structural, optical, and mechanical properties through combined solubility, moisture and water absorption, and film integrity analyses, thermo gravimetric analysis (TGA), X-ray diffraction (XRD), UV-vis spectroscopy, and tensile analyses, in contrast to many earlier studies that concentrate on limited property assessments. In order to achieve a balance among mechanical strength, thermal stability, and biodegradability, the study specifically focuses on optimizing nanocellulose reinforcement within a thermoplastic starch matrix plasticized with glycerol. This work advances starch-based nanocomposites toward feasible, scalable, and commercially relevant biodegradable material alternatives by simultaneously addressing process simplicity, sustainability, and performance.

2. Materials and methods

2.1 Materials

Starch and cellulosic fiber were the raw materials for the preparation of polymeric composites. Starch was collected from a local market, and nanocellulose was extracted using a filter paper. Sulfuric acid (H₂SO₄; purity: 98%, manufacturer: BDH, England), glacial acetic acid (CH₃COOH; purity: 100%, manufacturer: BDH, England), methanol and ethanol (CH₃OH and C₂H₅OH, respectively; manufacturer: BDH, England), and di-methyl sulfo-oxide (manufacturer: E. Merck Ltd, Germany) were the other materials used in this study. Glycerol was collected from Dhaka, Bangladesh.

2.2 Methods

2.2.1 Preparation of nanocellulose from filter paper. Cellulose nano-fibres were prepared by the two-step acid hydrolysis of the filter paper. Hydrolysis was carried out with an H₂SO₄ solution (60 wt%) at 45 °C under continuous stirring. The fibre-to-acid ratio was 1 : 10 (10 mL of sulphuric acid for 1 g of cellulose). After 50 minutes, hydrolysis was stopped by adding 5-fold of excess distilled water (50 mL) to the reaction mixture. The resulting mixture was cooled down to room temperature and centrifuged. The solid fraction was washed off by distilled water until a neutral pH was obtained. Hydrolysis was carried out again following the same procedure. Finally, the nanocellulose suspension was obtained by stirring the solid fraction with required amount of water. The newly generated suspension was stored in refrigerator at 4 °C (Fig. 1).

Fig. 1 Block diagram of nanocellulose preparation.

2.2.2 Starch film preparation. 4 g of starch, 1 g of glycerol, and 0.21 g of acetic acid were taken in a beaker and heated with constant stirring. When the mixture was concentrated, it was poured on a Petri dish. After keeping it for 24 hours in open air, the mixture turned into a film. Then, the film was heated at 60 °C. This film was continuous and flexible. Finally, the dried film was collected and stored.

2.2.3 Composite film preparation. Though there were minor differences in film formulations during the initial stage of preparation, their impact on the final properties was considered negligible compared to the reinforcing effect of nanocellulose. Therefore, the compositions and corresponding results have been presented and interpreted based on this consideration.
2.2.3.1 Composite film containing 5 wt% nanocellulose. 3.5 g of starch, 1.5 g of glycerol, 0.18 g of acetic acid and 0.25 g of nanocellulose (5 wt% nanocellulose) with 50 mL of distilled water were taken in a beaker and heated with constant stirring. When the mixture was concentrated, it was poured onto a Petri dish to make the film. Then, the film was kept for 24 hours in open air and heated at 60 °C. Finally, the dried film was collected and stored.
2.2.3.2 Composite film containing 10 wt% nanocellulose. 3.5 g of starch and 50 mL of distilled water were mixed well. Then, 1.5 g of glycerol, 0.18 g of acetic acid and 0.5 g of nanocellulose (10 wt% nanocellulose) were mixed with the previous mixture, heated, and stirred until the mixture was concentrated. When the mixture concentrated, it was poured onto a Petri dish and kept for 24 hours. After 24 hours, the mixture turned into a continuous composite film. Then, the film was heated at 60 °C, and the dried film was collected and stored.
2.2.3.3 Composite film containing 20 wt% nanocellulose. 3.5 g of starch, 1.5 g of glycerol, 0.18 g of acetic acid, 50 mL of distilled water were mixed well. Then, 1 g of nanocellulose fiber was mixed with this mixture and heated with stirring until the mixture concentrated; this concentrated mixture was then onto a Petri dish and kept for 24 hours in open air. When the mixture turned into a composite film, it was heated at 60 °C, and the dried composite film was collected.

3. Analytical methods and analyses

A digital screw was used to measure film thickness. The thickness of the films was measured to be 0.25 mm. The films were heated to 60 °C for 4 hours to obtain a constant weight and kept in a desiccator to cool down to room temperature. The dried films were examined by naked eye, and the films were analyzed for assessing their various properties. For exact measurements, at least five measurements were done for each film.

3.1 Water absorption behavior of starch-nanocellulose bio-nanocomposite films

The composite film was dried at 60 °C for 4 hours to achieve a constant weight and was kept in a desiccator. The-dried film samples were sectioned into pieces of a uniform size (e.g., 20 mm × 20 mm) and an average thickness of ∼0.10 mm. These small pieces of composite films were kept in several test tubes with 50 mL of water. After three hours, the films were taken out and excess water was removed using tissue paper. After this, the films were weighed. Subtracting the initial weight from the observed weight, water absorption capacity was obtained.

3.2 Moisture absorption test

At first, the films were dried at 60 °C to achieve a constant weight. The film samples of identical dimensions were weighed, and the values were noted down. Then, a beaker with 50 mL of water was taken, and a metal wire mesh was kept on the beaker, with films placed on it. The dried films were kept on the wire mesh for 24 hours and weighed. Subtracting the previous weight and the final weight, the percent of moisture absorption was calculated. The formula for moisture absorption percentage was

3.3 Solubility test

The solubility of the prepared composite films was assessed using a qualitative method. For the experiment, film samples were cut to uniform sizes (about 20 mm × 20 mm) with a uniform thickness of ∼0.10 mm and placed in different test tubes containing 50 mL of different solvents such as methanol, ethanol, chloroform, distilled water, dimethyl sulfoxide (DMSO), and acetone. The samples were kept in different conditions, such as room temperature and high-temperature conditions, using a water bath (60 ± 2 °C) for 3 hours. After the desired time, the samples were checked visually for signs of dissolution and disintegration.

3.4 Thermogravimetric analysis

The thermogravimetric analysis of starch-based bio-nanocomposite films was conducted using a TGA/DTA made by PerkinElmer, and the weight loss of a sample was measured as a function of temperature. This was done by placing a sample onto a sample holder that hangs from a micro-gram balance during the entire experiment. The balance mechanism consisted of a sample pan holder suspended by a long hang-down wire. The hang-down wire was connected with the balance lever by a small quartz link to prevent any static build-up. The weight of the sample holder configuration was electrically balanced on the other side so that the balance could be zero before each run. TGA was run from room temperature to 600 degrees centigrade at a rate of 10 degree centigrade per minute under a nitrogen atmosphere. For high accuracy, every analysis was done two times for each sample.

3.5 Tensile test

Tensile tests were performed using an Instron 8821S testing system. The tensile specimens were cut in rectangular shapes with dimensions of 65 mm in length and 10 mm in width. The gauge length was fixed at 50 mm, and the cross-head speed was 5 mm min⁻¹ at room temperature. A basic mechanical assessment technique called tensile testing is used to find out how a material reacts to a tensile force. It is one of the easiest and most used tests for evaluating mechanical performance. Designers and quality managers can forecast how materials and products will behave under actual service circumstances by determining the force needed to stretch a specimen till failure. The findings guarantee that the materials, parts, and final goods are suitable for their intended uses by offering vital information on their integrity, dependability, and safety.

The stress–strain curve is a graphical representation of the relationship between the stress applied to a material and the corresponding amount of elongation that occurs when subjected to a tensile load, as shown in Fig. 2. The elastic portion will be a straight line, indicating that the material is in the elastic region before it reaches its yield strength, after which plastic deformation begins. After attaining yield strength, the material will continue in plastic deformation (strain hardening) until it reaches its ultimate tensile strength. After reaching its ultimate tensile strength, the material will undergo necking (reduction in cross-section), which ultimately leads to the failure (fracture) of the material.

Fig. 2 Illustration of elastic deformation, yield strength, strain hardening, ultimate tensile strength, necking, and various types of fracture by representative stress–strain curves, and how they would compare to actual specimen shape problems during tensile testing.²⁶

Stress is a measure of the internal force per unit cross sectional area. The formula for calculating stress is the same as the formula for calculating pressure, given as

σ = F/A (1)

where σ is the stress, F is the applied force per unit area, and A is the cross-sectional area of the sample.

Stresses cause strain. Applying pressure on an object causes it to stretch. Strain is a measure of how much an object is being stretched. The formula for strain is

where L₀ is the original length of a bar being stretched, and L is its length after it is stretched. (L − L₀), the difference between these two lengths, is the extension of the bar.

The (ultimate) tensile strength is the level of stress at which a material (rope, wire, or a structural beam) will fracture. Tensile strength is also known as fracture stress. The breaking load of a material is measured in Newton (N). Tensile stress and tensile strain were calculated using the following formula:

Young's Modulus is the measure of the stiffness of a material. It indicates how much a material will stretch as a result of a given amount of stress. The formula for calculating it is

E = σ/ϵ (5)

where σ is tensile stress and ϵ is tensile strength.

The elongation of the specimen expressed as a percentage of the gauge length is given as

3.6 UV-vis spectroscopy

UV-vis spectroscopy refers to absorption spectroscopy or reflectance spectroscopy in the ultra violet-visible light region. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence: fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state. Films were cut into 0.25 mm wide and 3 cm long samples and placed onto the sample holder, and UV-vis spectroscopy was conducted and results were noted down.

3.7 X-ray scattering

The X-ray defection analysis of pure polymers, fibres, blends and composites was performed at room temperature with a Siemens Kristalloflex 810 diffractometer, operating at 40 kV and 30 mA and using the CuKa radiation (λ = 0.1546 nm). The crystalline degree of the samples was determined as the ratio of the areas of crystalline reflections to the whole area (after subtraction of background) in the 2θ range.

4. Results and discussion

4.1 Morphology analysis

Fig. 3 (a–d) show how the films look when nanocellulose is added to them. Pure starch film exhibiting an uneven, less transparent surface with visible cracks and discontinuities. The film appears brittle and fragile, demonstrating limited structural stability without nanocellulose reinforcement. When 5–20 wt% of nanocellulose was added to starch films (Fig. 3b–d), the films got smoother, and they were easier to bend. This means that nanocellulose integrated well with the starch particles in the film, making the films stronger. We took these pictures with a camera and not a special microscope. Optical clarity, surface homogeneity, and mechanical reinforcement are all affected by the gradual addition of nanocellulose fibers, underscoring the significance of fiber concentration in adjusting the physicochemical characteristics of starch-based bio-nanocomposite films for possible biodegradable packaging applications.²⁷

Fig. 3 Photographs of (a) neat starch film and starch-nanocellulose bio-nanocomposite films containing (b) 5 wt%, (c) 10 wt%, and (d) 20 wt% nanocellulose fibers. SEM micrographs of (e) neat starch film and composites with (f) 5 wt%, (g) 10 wt%, and (h) 20 wt% nanocellulose fibers.

Fig. 3a–d help us understand the clarity and uniformity of the films. However, they do not show us what the surface of the film looks like in detail. The pictures at the bottom (Fig. 3e–h) are from a scanning electron microscope that helps us see the tiny details on the surface of a film, such as its roughness and fiber distribution. These SEM pictures show that when nanocellulose is added to the starch films, they get more compact as the particles are connected better. There are also empty spaces in the films, which help to strengthen them and enhance their functioning. Scanning electron microscopy was performed on the films because a scanning electron microscope can show us things that we cannot see with other tools. In order to confirm the successful integration of nanocellulose, a detailed characterization of the morphological properties of the films was carried out. From the SEM images (Fig. 3e–h), it can be seen that there are fibrillar structures and interconnected structures. This shows that nanoscale cellulose fibers are formed. The decrease in fiber size and improved distribution within the composite matrix show that acid hydrolysis and defibrillation were successful. Although the size of individual nanofibers cannot be precisely determined based on the resolution limit, the morphology shows good agreement with those reported for nanocellulose. Based on the pictures (Fig. 3a–d) and the SEM micrographs (Fig. 3e–h), we can understand how the structure of the film affects its properties of starch-nanocellulose bio-nanocomposite films.

4.2 Water uptake analysis

Water uptake test was done with different types of dried composite films, as shown in in Fig. 4. The water uptake capacity of starch and its composite films after the addition of nanocellulose fibers was evaluated. From Fig. 4, it is clear that the water uptake capacity was the highest for the pure starch film and the lowest for the composite film that contained 20 wt% nanocellulose. This is well known that glycerol, containing hydroxyl groups, and starch are highly hygroscopic in nature. For this reason, the highest water absorption capacity was observed for the pure starch film without nanocellulose fibers. On the other hand, a decrease in water content and free volume among the starch molecules were observed due to the addition of nanocellulose fibers in the composite films. That is why water absorption was reduced to the lowest point for the composite containing 20 wt% nanocellulose fibers, as shown in Fig. 4.

Fig. 4 Moisture absorption and water uptake capacities with increasing nanocellulose percentage.

4.3 Moisture absorption analysis

Starch is a highly hygroscopic material and is produced from natural sources. From Fig. 4, it is clear that with increased addition of nanocellulose content, the moisture absorption capacity of the pure films and composite films decreased. From some previous work on starch-based composites, it was noted that a slight diminution of water content was observed with the addition of nanocellulose content.^19,20 Besides this, glycerol contains hydroxyl groups. As a result, due to addition of glycerol in composite formulation, moisture absorption capacity was increased. The highest moisture absorption capacity was observed for the composite without nanocellulose fibers (1.15%), and the lowest moisture absorption capacity was observed for the composite containing 20 wt% nanocellulose content (0.88% to 0.81% decrease), as shown in Fig. 4.

4.4 Nanocellulose's function in preventing the cracking of starch-based films

Upon observing the films, it was noted that the film of 100 wt% starch was cracked and not continuous due to the breakdown of glycoside bond of starch, as shown in Fig. 3a. It was also found that microorganisms could be grown on the films under dark and cool conditions during the processing period. On the other hand, continuous and un-cracked films were obtained when 5 wt%, 10 wt%, & 20 wt% of nanocellulose were added to the composite. This was because of the reduced free volume of starch upon the binding of nanocellulose with starch.²⁸

4.5 Solubility test

Small pieces of starch-nanocellulose bio-nanocomposite films were immersed in different solvents taken in various test tubes under both normal (25 °C) and hot (60 °C) conditions. To analyze the solubility of the samples, distilled water, acetone, chloroform, methanol, ethanol, and dimethyl sulphoxide were selected as the solvents. Finally, it was found that starch-nanocellulose bio-nanocomposite films dissolved well in dimethyl sulphoxide.

4.6 X-ray diffraction (XRD) analysis

The structures of the pure starch film and starch-nanocellulose bio-nanocomposite films were evaluated using X-ray diffraction. The patterns showed peaks that are typical of starch and also had contributions from nanocellulose. When nanocellulose was added, it changed the intensity and sharpness of the peaks, which indicated that the structure of the composite was changed. The crystallinity index was calculated using a method called the Segal method. This method uses the intensity of the crystalline peak and the lowest intensity of the amorphous region. The formula is:
Here, I₀₀₂ is the maximum intensity of the crystalline phase, and I_am is amorphous scattering. This method gives an idea of how crystalline the composite is. The X-ray diffraction patterns originated from both starch and nanocellulose. Hence, the crystallinity values were of the composites and not just the individual parts. This indicates that the changes in crystallinity index should be seen as changes in the ordering of the structure upon the addition of nanocellulose, rather than as a direct comparison of how crystalline each part was. Table 1 summarizes the relevant crystallinity values. A large diffraction halo in the 2θ range of roughly 15–23° is seen in neat starch. This is typical of gelatinized or processed starch, where natural crystalline domains are destroyed during film production and is indicative of a primarily amorphous or semi-crystalline structure.²⁹ As a result, plain starch exhibited a comparatively low crystallinity of 32%. Table 1 displays the crystallinity percentages, which increase as the volume percentage of nanocellulose increases. Moreover, the increase in crystallinity, as determined from XRD results, shows that highly crystalline cellulose structures are formed, as observed for nanocellulose.

Table 1 Calculated crystallinity of starch films and its composite films with nanocellulose by XRD analysis

Sample	Compositions	Crystallinity%
Starch	100/0	32
Starch/NC 95/5	95/5	52
Starch/NC 90/10	90/10	53
Starch/NC 80/20	80/20	60

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The diffraction profiles clearly and systematically altered when nanocellulose was added. Increased peak intensity and more distinct diffraction characteristics were seen in the starch/NC composite films, especially around 2θ = 16–17° and 22–23°, which correlated to the distinctive reflections of cellulose I crystalline planes. A substantial sharpening of peaks was visible with a modest NC loading of 5% (Starch/NC 95/5), suggesting a minor improvement in molecular ordering and an increase in crystallinity to 52%. The diffraction peaks got sharper and more intense as the NC component was increased to 10% and 20%, indicating improved structural organization. The starch/NC (80/20) composite had the maximum crystallinity of 60%. The naturally high crystalline nature of nanocellulose and its efficient function as a nucleating and reinforcing agent within the starch matrix are responsible for the progressive improvement in crystallinity with increasing NC concentration. A better chain packing and the creation of more ordered domains are encouraged by strong intermolecular hydrogen bonds between the hydroxyl groups of starch and nanocellulose. Thus, the XRD data show that adding nanocellulose greatly enhances the crystalline structure of starch-based composites, which is anticipated to have a favorable impact on their mechanical strength and barrier qualities (Fig. 5).

Fig. 5 X-ray diffraction patterns of starch-nanocellulose bio-nanocomposites.

4.7 Optical properties

The optical properties of starch and starch-nanocellulose bio-nanocomposite films were evaluated using UV-vis spectroscopy. Results are shown in Fig. 6. When we look at the absorbance spectra in Fig. 6a, we can see that the absorbance goes up as the nanocellulose content increases. This means that light was being scattered more inside the matrix. On the other hand, the transmittance spectra in Fig. 6b show that light transmission decreases a lot as the nanocellulose loading increases from 0 to 20 wt%. To be specific, transmittance decreased from about 63% for starch to 42%, 19% and 10% for films that have 5, 10 and 20 wt% nanocellulose, respectively. This decrease in transparency is because of the increased crystallinity and formation of ordered structures, making it harder for light to pass through the films. Additionally, the increase in absorbance when nanocellulose was added suggested that the internal microstructure of the films changed and nanocellulose might have aggregated or not dispersed evenly at all loadings. Even though transmittance was reduced, all films were still somewhat transparent, which could be observed through naked eyes. There were no clumps visible, which was good. These results show that the amount of nanocellulose in the films is very important in determining their behavior with light and how they are structured. The starch-nanocellulose bio-nanocomposite films and their optical properties are really affected by the nanocellulose content and the way nanocellulose is organized inside the films.

Fig. 6 UV-vis absorbance and transmittance spectra of starch-nanocellulose bio-nanocomposites with different ratios of nanocellulose.

4.8 Thermal analysis

This analysis is carried out to measure the thermal stability of starch-nanocellulose bio-nanocomposite films. The TGA and DTG curves are shown in Fig. 7.

Fig. 7 TGA curves (a) and DTG curves (b) for starch-nanocellulose bio-nanocomposites containing different ratios of nanocellulose.

The thermal stability and degradation behavior of the clean starch and starch-nanocellulose bio-nanocomposite films were assessed using TGA; the associated TG and DTG curves are displayed in Fig. 7a and b, respectively. The TG curve for plain starch indicated the first weight loss that starts at about 31.9 °C. This is associated with the elimination of physically absorbed moisture that is responsible for roughly 10.1% of the mass loss. The breakage of the glycosidic bonds in starch molecules causes the primary degradation process at the onset temperatures of 149.3, 306.3, and 346.5 °C. This results in a total mass loss of roughly 67.9%. A tiny leftover mass remained after the degrading process finished at 346.5 °C. The initial weight loss for the pure starch film without nanocellulose began at about 29.4 °C, and the removal of moisture causes a 6.9% mass loss at about 111.4 °C. With an overall degradation of 54.8%, the primary degradation stage exhibits the maximum breakdown rates at the temperatures of 281.4, 315.6, and 348.9 °C. Up to 490 °C, there is an additional weight loss of roughly 21.9%, which is mostly related to ongoing volatilization and disintegration. The first moisture-related weight loss in the starch/NC composite films is around 10.2%. This is followed by a severe degradation stage, where the breakdown of amylose and amylopectin chains causes about 65.3% mass loss. The composite material shows stages of breaking down when it gets hot. This process starts at temperatures and the material breaks down the most, at even higher temperatures. The composite material has these stages of breaking down when it gets hot.

Another composite exhibits an initial moisture loss of 6.4%, with significant deterioration (56.2%) taking place at the maximum slopes and onset temperatures of 239, 300.2, and 364 °C, with degradation finishing at about 364 °C. The removal of moisture and volatile components accounts for an additional mass loss of 18.1%; the ash content of the composite film is responsible for the remaining residue. In contrast, composites with 20 wt% nanocellulose show a wider and shifted degradation range from about 239.1 to 364.1 °C, indicating improved thermal stability, while composites without nanocellulose show degradation onset at about 267 °C and completion at about 346.5 °C. Glycerol and starch break down at about 240 °C and 290 °C, respectively, while cellulose requires more energy to break down because of its strong glycosidic linkages and stiff glucose chains; it usually breaks down between 300 and 395 °C. The addition of nanocellulose improves the composites' overall thermal resistance; the film with 20 wt% nanocellulose showed the highest breakdown temperature. This behavior is explained by the build-up of glycerol at the amylopectin-nanocellulose interface, which encourages the development of crystalline areas surrounding nanocellulose fibers. The inorganic and carbonaceous components of starch-nanocellulose bio-nanocomposite films are represented by the stable solid residue that is left over after heating at temperatures over 440 °C.

4.9 Mechanical properties

The mechanical property of starch-nanocellulose bio-nanocomposite films was determined using a stress–strain curve (Fig. 8), and the obtained data are listed in Table 2 in the form of mean ± standard deviation, which ensures the reliability of the obtained data with the calculated values using the corrected cross-sectional area and gauge length.

Fig. 8 Stress vs. strain curve for starch-nanocellulose bio-nanocomposites containing different ratios of nanocellulose.

Table 2 Tensile strength, Young's modulus, and elongation at break of starch-nanocellulose bio-nanocomposite films with different NCF loadings

Materials	Tensile strength (MPa)	Young modulus (MPa)	Elongation at break (%)
0 wt% NCF	2.7 ± 0.15	100.0 ± 6.0	2.4 ± 0.17
5 wt% NCF	3.2 ± 0.10	143.0 ± 9.0	2.0 ± 0.10
10 wt% NCF	4.0 ± 0.19	192.1 ± 15.2	2.2 ± 0.11
20 wt% NCF	4.5 ± 0.26	250.5 ± 13.6	1.6 ± 0.10

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From calculations, it is found that there is a clear correlation between the data presented in Fig. 8 and Table 2. The neat starch film (0% NCF) shows the lowest tensile strength (2.7 ± 0.15 MPa), Young's modulus (100.0 ± 6.0 MPa) but the highest elongation at break (2.4 ± 0.17%) compared with the starch-nanocellulose bio-nanocomposite films. The starch-nanocellulose bio-nanocomposite films show improved tensile strength and Young's modulus with the addition of nanocellulose fibers. As the NCF loading increases to 5% NCF, tensile strength increases to 3.2 ± 0.10 MPa and Young's modulus increases to 143.0 ± 9.0 MPa. The elongation at break decreases slightly to 2.0% ± 0.10%. This indicates efficient stress transfer due to enhancements in interfacial interaction and dispersion of nanocellulose.

When the NCF content in the composite increases to 10%, tensile strength and Young's modulus increase to 4.0 ± 0.19 MPa and 192.1 ± 15.2 MPa, respectively. The elongation at break decreases slightly to 2.2% ± 0.11%. The trend observed in Fig. 8 and Table 2 indicates the enhancement of the mechanical properties of the composite without abnormal deviation. The maximum mechanical property is obtained for the 20 wt% NCF composite, where the tensile strength is 4.5 ± 0.26 MPa and Young's modulus is 250.5 ± 13.6 MPa. The elongation at break of the composite is reduced to 1.6% ± 0.10% with an increase in the nanocellulose content. Finally, it is evident from the corrected results that there is an increasing trend for tensile strength and stiffness with an increase in the nanocellulose content, while elongation at break is reduced with nanocellulose content. Results obtained by correlating the data (Table 2) and stress–strain curves (Fig. 8) show an agreement, which proves the accuracy of the mechanical analysis, thereby confirming the reinforcing effect of nanocellulose on starch-based bio-nanocomposites.

5. Conclusions

Starch-nanocellulose bio-nanocomposite films were successfully fabricated and systematically characterized to investigate the effect of nanocellulose fiber loading on the physicochemical, structural, thermal, optical, and mechanical properties of the films. Results clearly demonstrated that incorporating nanocellulose significantly enhanced the performance of starch-based films. Because starch is naturally hygroscopic and contains glycerol, clean starch films have the highest hydrophilicity, according to the tests of water uptake and moisture absorption. In contrast, increasing nanocellulose content significantly decreased both water uptake and moisture absorption. Strong hydrogen bonds between starch and nanocellulose are thought to be the cause of this decrease in free volume and limited molecular mobility inside the starch matrix. Additional solubility tests revealed that the composite films had strong chemical resistance and only dissolve in dimethyl sulphoxide, supporting their appropriateness for real-world applications. Pure starch films were brittle and broken, according to morphological studies, while films reinforced with nanocellulose were continuous and free of cracks, indicating better interfacial compatibility and film integrity. Morphological analysis confirmed that nanocellulose effectively improved film continuity and eliminated cracking, while solubility tests revealed good chemical resistance, with films remaining stable in most solvents except dimethyl sulfoxide. A gradual increase in crystallinity from 32% for pure starch films to 60% for films containing 20 wt% nanocellulose was confirmed by XRD analysis, highlighting the function of nanocellulose as an effective nucleating and reinforcing agent. Due to increased crystallinity and light dispersion, optical experiments revealed a progressive decrease in light transmittance with increasing nanocellulose loading while maintaining adequate transparency without obvious fiber aggregation. Larger degradation temperatures were noted at larger nanocellulose concentrations, especially at 20 weight percent nanocellulose loading, according to thermal analysis, which showed the increased thermal stability of the composites. Although elongation at break reduced due to increased stiffness, mechanical testing showed considerable gains in tensile strength and Young's modulus with increasing nanocellulose loading, indicating the strong reinforcing impact of nanocellulose. All these results demonstrate that nanocellulose is a useful reinforcement for starch matrices, allowing for the creation of biodegradable and sustainable films with enhanced mechanical, thermal, structural, and physicochemical properties for packaging applications. For a more thorough assessment, more research on comprehensive barrier characteristics such gas permeability and water vapor transmission is advised.

Conflicts of interest

The authors declare that they have no known financial or personal conflicts of interest that could have influenced the work reported in this paper.

Data availability

The data supporting the findings of this study can be obtained from the corresponding author upon request. Due to privacy concerns and other restrictions, the data are not publicly accessible.

Acknowledgements

The authors extend their sincere thanks to the Dept. of Applied Chemistry & Chemical Engineering, Islamic University, Bangladesh, for providing a supportive research environment and essential facilities. The authors are also thankful to the Chemistry Discipline, Khulna University, for providing supportive research facilities.

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