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. 2025 Feb 26;10(9):9018–9027. doi: 10.1021/acsomega.4c06854

Unveiling the Role of Ultrasonication Variables on Lignin-Containing Cellulose Nanocrystal Dispersion in Poly(ethylene oxide)-Based Suspension and Resulting Morphology and Mechanical Properties

Amirmohammad Raeisi †, Ismat Ara †, Greg Holt ‡, Dilpreet Bajwa †,*
PMCID: PMC11904664  PMID: 40092785

Abstract

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The biodegradability, abundant availability, and outstanding intrinsic properties of cellulose nanocrystals (CNCs) make them suitable candidates for functionalizing polymer materials. Lignin is another abundant material in nature and is a remarkable UV-blocking agent. Hence, their combination can produce a material with multifunctional properties. However, the self-assembling ability of CNCs can make it challenging to develop their well-dispersed suspension in polymer-based aqueous solutions. However, it is essential to identify the effective ultrasonication parameters to obtain the desired particle sizes and morphology. This study investigated the role of ultrasonication treatment in dispersing lignin-containing CNCs (L-CNCs) within the water-soluble poly(ethylene oxide) (PEO). The aqueous suspensions were prepared by dispersing L-CNC in 1 wt % of PEO solution, where varying ultrasonication times (3, 6, and 9 min) and different amplitudes (50 and 100%) were employed. The morphology, particle size, and dispersion of L-CNCs were analyzed by using zeta potential analysis, and scanning electron microscopy. Mechanical and physical properties were also assessed through dynamic mechanical analysis, differential scanning calorimetry and fourier transform infrared spectroscopy. The results indicated that an increase in sonication time and amplitude could significantly influence the dispersion of L-CNCs within the PEO polymer matrix, as evidenced by the increase in the zeta potential. Increased sonication time and amplitude improved dispersion and reduced the size and number of agglomerations. Ultrasonication at 100% amplitude for 9 min resulted in a 400% and more increase in the storage modulus of composite films. The comprehensive results obtained from this study aim to enhance our understanding of optimal ultrasonication parameters, contributing to an improved L-CNC dispersion and enhanced composite material performance.

1. Introduction

Biobased materials are increasingly used in a wide range of applications due to concerns over environmental sustainability. Biopolymers and biobased composites are being considered as appropriate substitutes for petroleum-based products and synthetic plastics due to their advantages, such as biodegradability and biocompatibility. The market size of biocomposites reached 25.1 billion USD in 2023 and is anticipated to attain 128.2 billion USD by 2030, with a compound annual growth rate (CAGR) of 16.01%.1 Cellulose is a biodegradable polymer found in diverse renewable resources, such as plants and bacteria. Consequently, making it the most naturally occurring polymer on the earth. Cellulose nanocrystals (CNCs) are the crystalline domain of cellulose, which can be obtained by various treatments, such as acid hydrolysis, enzymatic hydrolysis, and high-shear mechanical treatment of cellulose.2 CNCs have attracted attention due to their unique properties, such as low density (∼1.5 g/cm3), high surface area (150 m2/g), high aspect ratio (10–70), high strength (∼10 GPa = ∼Kevlar), and high stiffness (∼110–130 GPa = ∼Steel).2,3 Additionally, the CNCs have excellent stress-transfer and load-bearing properties along with chemically active surfaces, which allow CNC to be modified to enhance its properties and applications.

Many studies have focused on modifying and improving different polymer matrices’ physical and mechanical properties by incorporating CNCs. Incorporating CNCs into polylactic acid (PLA)-based composite filaments enhanced the tensile strength, elongation at break, and crystallinity of PLA.4 The CNCs were added to the PLA matrix in three different weight percentages (0.75, 1, and 2), and the highest tensile strength was achieved by adding 1 wt % with an 18% improvement compared to pure PLA.4 In another study, adding silane-treated CNCs to the poly(ethylene oxide) (PEO) matrix resulted in a significant improvement in the dynamic mechanical properties and crystallinity of nanocomposite thin films.5 The properties of bionanocomposites from polyhydroxybutyrate (PHB) and CNCs under environmental conditions were analyzed.6 It was observed that CNC acted as a nucleating agent, increasing the crystallinity of PHB. CNC enhanced the mechanical, thermal, optical, and barrier properties of PHB.6

However, CNCs tend to make agglomerations, which act as an area of stress concentration in composite materials. Generally, fillers often tend to aggregate, resulting from a stronger attraction force inside the fiber. In the case of CNC, the aggregation happens through an excessive hydrogen bonding present at the cellulose surface.7 According to reports, the dispersion of CNCs in liquid feeding is affected by the percentage of the liquid phase and the rate of liquid feeding.8 Scanning electron microscopy (SEM) analysis revealed that an increased CNC concentration led to substantial aggregation. Researchers suggested that this aggregation in PLA-CNCs composites may be due to insufficient shear force or shorter residence time in the twin-screw extruder.9 The cellulose’s inherent hydrophilicity causes a weak interaction, especially when incorporated with nonpolar or low-polar polymer matrices. This weak interaction results in cellulose aggregation, which leads to lower thermo-mechanical properties of final nanocomposites.10 Different approaches have been used to enhance CNC dispersion in polymer matrices, such as fiber surface modification treatments, sonication, and incorporation of compatibilizer materials, such as lignin.2,11

Lignin is an abundant amorphous aromatic macromolecule containing a variety of functional groups, such as hydroxyl, methoxyl, carboxyl, and carbonyl groups.11,12 These functional groups in the lignin structure greatly enhance the interaction between CNCs and hydrophobic matrix materials through hydrogen bonding and van der Waals interactions. Lignin-containing cellulose nanocrystals (L-CNCs) particles are biodegradable and have high strength resulting from the CNCs' crystallinity.13 Incorporating lignin reduces the reagglomeration of the CNCs without significantly impacting their initial dispersion.11 However, lignin offers limited enhancement, so additional mechanical or chemical methods are needed to disperse CNCs.11

Alternative techniques, such as ultrasonication, are primarily used to improve filler dispersion in polymer matrices. Ultrasonication is a noninvasive way to address the challenges of dispersing nanoparticles in polymer matrices using sound energy for acoustic cavitation. This process involves creating, growing, and breaking down bubbles in the liquid. This collapse of cavitation (bubble breaking down) and ultrasound causes the breakdown of the nanoparticle’s agglomeration and provides enhanced dispersion and wettability.14 A study on the impact of different sonication parameters on PVA-CNC nanocomposites reported that sonication time and amplitude significantly affect CNC morphology and dispersion.2 Two different amplitude levels, that is, 60 and 90 μm, and three different sonication times have been studied, where higher sonication amplitude for the same duration results in a lower size of particles and higher dispersion of CNCs in the polymer matrix.2 In another study, researchers have examined the effect of sonication duration on the dispersion and structural arrangement of nanoclay particles in an epoxy resin (diglycidyl ether bisphenol A (DGEBA)).15 The findings have indicated that increasing the sonication time up to 60 min improves nanoclay dispersion in the epoxy matrix. However, beyond 30 min, the impact of sonication on the dispersion becomes insignificant.15 Another research reported the impact of sonication for different durations, that is, 1, 2, 5, and 10 min on the properties of aqueous suspensions containing CNCs derived from acid hydrolysis of bleached cotton.16 The sonication results in improved nanocrystalline cellulose (NCC) dispersion in suspensions, facilitating the production of transparent and strong nanocellulose films with excellent oxygen barrier properties. The sonication breaks down the large NCC aggregates into individual nanowhiskers with average lengths of 171–118 nm and widths of 17–13 nm, depending on the sonication time. The 10 min sonication treatment led to a notable reduction in the optical haze of the CNC suspensions, decreasing from 98 to 52%. The films cast from sonicated suspensions exhibited lower haze and improved tensile strength by up to 57% with an increase in sonication time from 0 to 10 min. Similar to sonication duration, the amplitude has a crucial effect on CNCs, dimensions, morphology, and dispersion. As reported in the literature, increasing the sonication amplitude from 60 to 70% has decreased the diameter of CNCs from 20 to 16 nm at a constant sonication time of 20 min, indicating that the higher amplitude decreases particle size.2,17

Therefore, this paper aims to improve the dispersion of the lignin-containing CNCs in polymer matrices by manipulating the ultrasonication parameters. To this end, the research investigates the influence of different sonication parameters, such as time and amplitude, on the thermal and mechanical properties of a PEO-based composite. PEO is a biocompatible, water-soluble, and nontoxic polymer and is utilized in various fields from adhesive and coating to pharmaceuticals. This study underscores the significant role of sonication parameters in shaping the final performance of the PEO thin films. The extensive findings from this research are expected to deepen our understanding of the most effective ultrasonication parameters, thereby facilitating the improved dispersion of L-CNC and enhancing the overall performance of the biobased composite material, thus offering the potential for practical applications.

2. Materials and Methods

2.1. Materials

PEO powder with a molecular weight of 106 g/mol and a melting point of 70–80 °C was purchased from Sigma-Aldrich and was used as the polymer matrix. The L-CNCs were obtained from the USDA Forest Products Laboratory (Madison, WI, USA). They were extracted by hydrothermal treatment followed by 64% sulfuric acid hydrolysis from poplar wood.18 With a 30.6% lignin in the L-CNCs, the lignin was reported to be distributed as small nanoparticles among the CNCs where their average length was 82 ± 36 nm and average width was 8.2 ± 1.8 nm with an aspect ratio of 10.18 Deionized (DI) water, used as a solvent, was purchased from Millipore (St. Louis, MO, USA).

2.2. Nanocomposite Thin Films Preparation Method

The PEO/L-CNC nanocomposite thin films were prepared through the solvent casting method. Initially, 1 wt % of PEO was dissolved in 100 mL of DI water under stirring for 8 h at room temperature. A 1 wt % of L-CNC was added to the solution and homogenized using a homogenizer under a 5 min stirring. Each solution was ultrasonicated using an ultrasound device (Hielscher UIP1000hd, 20 kHz, Germany) with an effective power input of 1000 W and a 19 mm diameter probe. The probe was set at amplitudes of 50 and 100% and a constant frequency of 20 kHz, each for a duration of 3, 6, and 9 min. Throughout this process, the samples were kept in an ice bath to maintain a cool temperature and prevent any degradation of the matrix and fillers. The sonicated aqueous solutions were then poured into Petri dishes and allowed to form thin films at room temperature. Pure PEO thin films were also prepared as the control samples. Sample codes and ratios are provided in Table 1.

Table 1. Composite Thin Films Formulationsa.

sample code PEO (wt %) L-CNC (wt %) sonication time (min) sonication amplitude (%)
Pure PEO 1      
50A3MIN 1 1 3 50
50A6MIN 1 1 6 50
50A9MIN 1 1 9 50
100A3MIN 1 1 3 100
100A6MIN 1 1 6 100
100A9MIN 1 1 9 100
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a

Each sample code details the sonication parameters used in the process of integrating Lignin-containing CNC (L-CNC) into a poly(ethylene oxide) (PEO) matrix. The first number represents the amplitude percentage, and the second number shows the sonication duration. For example, 50A3MIN signifies samples sonicated at 50% amplitude for 3 min.

3. Experimental Methodology

3.1. Field Emission Scanning Electron Microscopy (FESEM)

The dispersion and microstructure of the L-CNC in the PEO-based film were investigated using a field emission scanning electron microscope (Supra 55VP, Zeiss, Thornwood, NY, USA) available in the image and chemical analysis laboratory (ICAL) at Montana State University. The samples were first spatter coated by iridium at 20 mA for 60 s to eliminate the charging effect of the nonconductive samples before being set under the electron microscope. The SEM images were taken at different magnifications at an accelerating voltage of 1 kV, a working distance of 5 mm, and an aperture of 30 μm.

3.2. Electrokinetic Analyzer (ζ-Potential)

The electrokinetic potentials (ζ-Potential) of the L-CNC in the PEO-based suspensions were measured using a Zeta-Meter system 4.0 available in the ICAL at Montana State University. The zeta potential was recorded for 15 charged particles’ movement for each sample at 22 °C and 300 V. The zeta meter automatically calculated the electrophoretic mobility for all systems and calculated the zeta potential for the aqueous systems. The zeta meter is designed to apply the Smoluchowski equation for zeta potential, and the simplified form of the equation is as follows:

3.2. 1

where ZP stands for zeta potential and it is in millivolts, Vt is for the viscosity of the suspending liquid in poises at temperature “t”, Dt is for dielectric constant, and EM is for electrophoretic mobility at actual temperature.

3.3. Fourier Transform Infrared (FTIR) Spectroscopy

The nanocomposite thin films were analyzed using FTIR spectroscopy using a Thermo-Scientific FTIR/ATR spectrometer, model Nicolet i550 (ThermoFisher Scientific, Madison, WI). An attenuated total reflection (ATR) mode ID7/ITX AR-coated diamond crystal was used for the purpose of the FTIR experiments. A total of 64 scans were performed at a resolution of 4 cm–1 and the wavenumber range of 400–4000 cm–1 to obtain the final spectrum for each formulation.

3.4. Dynamic Mechanical Analysis (DMA)

The dynamic storage and loss moduli of the samples were measured using a TA Instruments dynamic mechanical analyzer (Q800). The test was conducted in tensile mode with a constant frequency of 1 Hz, a temperature range of 30–90 °C, and a strain of 0.05. The sample dimensions were 12 × 6.5 × 0.1 mm, and the initial length of the specimen was determined using the dynamic mechanical analyzer after it was secured in the clamp. The storage and loss moduli were recorded as the temperature varied. Each formulation underwent examination with a minimum of 3 replicates, and the average values were reported.

3.5. Differential Scanning Calorimetry (DSC)

A TA Instruments DSC instrument (Q2500, New Castle, USA) under a nitrogen atmosphere at Montana State University was used to determine the melting peak and fusion enthalpy of the samples. Each sample, weighing around 7 mg, was scanned over a temperature range from −30 to 100 °C at a heating rate of 10 °C/min.

The crystallization behavior of composite thin films was studied using DSC. The crystallinity (Xc) was calculated using the following eq:

3.5. 2

where ΔHm is the enthalpy of melting, ΔH0m is the theoretical enthalpy of melting of 100% crystalline polyethylene Inline graphic, and x is the weight fraction of L-CNC.

3.6. Statistical Analysis

The dynamic mechanical properties are presented as mean ± standard deviation and were statistically analyzed using one-way analysis of variance (ANOVA) and Fisher’s least significant difference method to determine significant difference between values. Minitab Statistical Software 22 (Minitab Inc., State College, PA, USA) was used to evaluate the data. The analysis was performed with a “α” level of 0.05.

4. Results and Discussion

4.1. Dispersion and Agglomeration of L-CNC in PEO

The dispersion and agglomeration of the L-CNC-reinforced PEO films varied with the ultrasonification amplitude and time variation. Figure 1 shows the microstructure of the films at 50 and 100% ultrasonication amplitude at 3 and 9 min of sonication time. While 50A3MIN, 50A9MIN, and 100A3MIN samples had the largest extent of agglomeration, the 100A9MIN sample presented the lowest amount of agglomeration and visually most dispersion of the L-CNC crystals. The agglomerate sizes were measured using ImageJ software to obtain a quantitative assessment of agglomeration. The size of the agglomerates was 51.85 ± 10.29 μm for the 50A3MIN, 36.78 ± 1.98 μm for the 50A9MIN, 46.64 ± 4.31 μm for 100A3MIN, and 1.72 ± 0.19 μm for the 100A9MIN samples. Hence, the 100A9MIN sample, i.e., the sample prepared at the highest ultrasonication amplitude and time, had the smallest agglomerate size.

Figure 1.

Figure 1

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FESEM images of (a) 50A3MIN, (b) 50A9MIN, (c) 100A3MIN, and (d) 100A9MIN showing the L-CNC agglomeration and dispersion in the PEO matrix.

To understand the impact of the ultrasonication amplitude alone on the agglomeration and dispersion of L-CNC, the 50A9MIN and 100A9MIN samples have been compared, as shown in Figure 2. It is evident from the visual inspection of the FESEM images that the dispersion of the L-CNC improved with the increase in the amplitude. The size and distribution of the dispersed agglomerates were more uniform for the samples prepared at 100% amplitude, as shown in Figure 1. The size of the agglomerates decreased by 95% at 100% amplitude of ultrasonification compared to 50% amplitude when sonication time was constant at 9 min. However, the size of the agglomerates decreased by only 10% when the ultrasonication amplitude was increased, keeping the ultrasonication time fixed at 3 min, and there was no visible improvement in dispersion, as shown in Figure 1a,c. Hence, increasing the ultrasonication amplitude reduced the agglomeration size of the L-CNC in the PEO matrix. However, the reduction of the agglomeration size and dispersion with the amplitude increase was more prevalent at 9 min of sonication than at 3 min. Some other studies also reported that a higher amplitude of sonication decreases the particle size of CNCs, where the adjustment of amplitude from 60 to 70% leads to the reduction in CNC agglomeration size from 21 to 16 nm at a fixed sonication time of 20 min.19 As amplitude reflects the distance over which the sonication probe can fluctuate longitudinally, increasing sonication amplitude boosts the intensity of cavitation within the liquid.2 This implosive cavitation force induces extreme particle agitation that disintegrates the CNC agglomerates by enhancing the breakage of the fibrillary structure, reducing the size of the CNC particle.2,20

Figure 2.

Figure 2

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High magnification FESEM images of (a) 50A9MIN and (b) 100A9MIN samples showing the impact of amplitude on the agglomeration and dispersion of the L-CNC in the PEO matrix.

Similarly, a more homogeneous distribution of L-CNC agglomerates’ size and their even dispersion in the PEO matrix was observed when the sonication time was 9 min as opposed to 3 min of sonication with a constant amplitude of 100%, as evidenced by Figure 3. The agglomeration decreased by 96% in size at 9 min of ultrasonication compared to the samples with 3 min. When the amplitude was kept constant at 50%, the reduction of the agglomeration size was about 30% with no visible dispersions, as observed in Figure 3a,b. Hence, the impact of the ultrasonication time on the agglomeration size and dispersion was inevitable, and it became more prominent at a higher amplitude. The observed effect of the sonication time on dispersion and agglomerate size agrees with the literature.2,16 Shojaeiarani et al. reported that prolonged ultrasonication for 10 min leads to more even dispersion of CNC than sonication for a shorter time, that is, 7 min using the same input energy.2 Csiszar et al. observed that a gradual increase in the ultrasonication time, i.e., 2, 5, and 10 min of sonication, also gradually decreased the diameter of agglomerated particles from an initial size of 14.7 to 3.17, 2.71, and 2.23 μm, respectively.2 Similarly, Ni et al. identified that CNC particles’ size cuts down from 40.82 ± 2.12 to 34.18 ± 1.43 and 25.16 ± 1.55 nm when the sonication time is increased from 10 min to 30 and 60 min, respectively.21 The longer sonication time continues the process of agglomerate breakage for a more extended period and, hence, leads to more dispersion of the particles.

Figure 3.

Figure 3

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High magnification FESEM images of (a) 100A3MIN and (b) 100A9MIN samples showing the impact of amplitude on the agglomeration and dispersion of the L-CNC in the PEO matrix.

To get an idea of the extent to which the ultrasonication time impacted the agglomeration of the L-CNC in the PEO matrix, further FESEM was analyzed at the nanoscale. As shown in Figure 4a, the length of the smallest agglomerate detected in the samples formulated at 100% amplitude and 3 min of ultrasonication was 1.93 μm, and its maximum width was 0.93 μm, whereas the length of average agglomerates in the samples prepared with 100% amplitude and 9 min of ultrasonication was 1.62 μm and its maximum width was 0.18 μm, as shown in Figure 4b. Hence, even the smallest agglomerates in samples ultrasonicated for 3 min had larger sizes than the average size of agglomerates in 9 min ultrasonicated samples. Besides, none of the samples prepared with 3 min of ultrasonication provided any visible dispersion of L-CNC, as shown in Figure 1a,c.

Figure 4.

Figure 4

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High magnification FESEM images of individual agglomerates for (a) 100A3MIN and (b) 100A9MIN samples showing the comparative size of the agglomerates.

The size of the agglomerates measured from the FESEM analyses can be correlated to the zeta potential value of the samples, as shown in Figure 5. Figure 5a summarizes that the increase in the ultrasonication time and amplitude decreased the size of the agglomerates. Figure 5b presents the absolute zeta potential values for the L-CNC suspension at 1 wt % concentration in the PEO matrix at different ultrasonication amplitudes and times. The zeta potential of all of the samples was negative, which means that the L-CNC particles were negatively charged. The negative charge of the colloid intrigues the formation of an electrostatic double layer around it to neutralize the charged colloids and, consequently, develops an electrokinetic potential between the surface of the colloid and any point in the mass of the suspending liquid. Larger values in the zeta potential indicate that the adjacent particles repelled each other and prevented the agglomeration more than the ones with smaller values in the zeta potential. The zeta potential was increased by 41%, and the agglomeration size decreased by 29% when the ultrasonication time was raised from 3 to 9 min for L-CNC suspended liquid prepared at 50% amplitude. Similarly, for the sample at 100% amplitude, the zeta potential proliferated by 158%, and agglomeration size decreased by 96% when the ultrasonication time was increased from 3 to 9 min. The ultrasonication amplitude had the same impact on the agglomeration, as evidenced by a 26% rise in the zeta potential and a 95% reduction of agglomeration size when the amplitude was increased from 50 to 100%. Hence, it was further verified from the zeta potential estimation that increasing both ultrasonication time and amplitude resisted the agglomeration of the L-CNC particles, and the highest value of the zeta potential was recorded for the samples prepared at 100% amplitude and 9 min of ultrasonication.

Figure 5.

Figure 5

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Comparative analysis of (a) agglomerate size and (b) absolute zeta potential for different sonication amplitudes and time.

Hence, increasing the ultrasonication time at both amplitudes, i.e., 50 and 100% increased the zeta potential values for the suspended L-CNC particles. A similar conclusion has been reported by Guo 2023, where higher ultrasonication time leads to better dispersion of the CNCs in aqueous solution.22 However, the magnitude of the absolute zeta potential for all the samples was less than 10 mV, which indicates the high amount of agglomeration because agglomeration starts to happen below 15 mV of zeta potential. At a zeta potential value above 30 mV, the van der Waals force weakens, leading to electrostatic repulsion and hence improved dispersion stability.23,24 The effect of this low zeta potential in current samples can be evident in the SEM analyses as well, where the length and width of L-CNC observed in the best sample, i.e., the 100A9MIN, are much larger than the size of the original CNC, which is around 151–169 nm in length and 20 nm in width. This agglomerate size observation indicates that even at 9 min of ultrasonication, there was considerable agglomeration in L-CNC.

This low zeta potential and high agglomeration in the present L-CNC infused in the PEO film can be attributed to the presence of lignin. The high lignin containment in CNCs decreased the zeta potential of the colloidal suspension. The absolute zeta potential of 0.01% CNC in DI water was 43.75 ± 3.94 mV, which dropped to 27.92 ± 3.84 mV for 0.01% L-CNC in DI. A similar decrease in the zeta potential of CNC after lignin addition was reported in the literature.25 The CNC particles in the suspension have a negative charge because the particles are produced by sulfuric acid hydrolysis and, consequently, the presence of negative sulfate ester groups on their surface.18 The increase in the L-CNC concentration from 0.01 to 0.5% in DI water caused the decline in zeta potential from −27.92 ± 3.84 to −26.39 ± 4.44 mV, which are almost similar. However, an increase in the concentration of CNC on an aqueous solution was reported to decrease the zeta potential considerably in the literature because the dielectric constant increases with increasing CNC content due to the increased space charge (Maxwell–Wagner–Siller (MWS) polarization) and polarization contributions by the hydroxyl groups (−OH) present in the cellulose.22,26 Hence, the dispersion stability of L-CNC is not influenced by its concentration as much as the dispersion stability of the CNC and its concentration. The reason the L-CNC dispersion stability is less sensitive to its concentration can be the 2–2.5 times higher sulfur content in L-CNC than the lignin-free CNCs [18]. Therefore, the further decrease in the zeta potential of the L-CNC compared to that of lignin-free CNCs should be due to its interaction with the PEO solutions. When hydrophobic L-CNCs were mixed with the hydrophilic PEO solution, they experienced weak interfacial bonding that led to the aggregation of the L-CNC particles and, hence, poor dispersion. These hydrophobic L-CNCs will have better dispersion in a hydrophobic polymer instead of a hydrophilic one.27 Besides, when CNC is added to the PEO solutions, the adsorption of PEO happens on the CNC particles, and the zeta potential is decreased from −56 to −7 mV because of the weak adsorption.28

To improve the dispersion stability of the present L-CNC in aqueous PEO suspension, a higher ultrasonication time can be applied during the preparation of the solution so that the agglomerates can be broken up further. However, an aggressive and longer time of ultrasonication can decrease the crystallinity of the L-CNC particles by removing the crystalline and amorphous parts of the CNC structure because of the intense cavitation forces released by ultrasonication. Consequently, the mechanical properties of the CNC-based material are deteriorated.17 Shojaeiarani et al. reported that increasing the sonication time and amplitude leads to the decline of the crystallinity index of CNC by 12%.2 Similarly, Ni et al. 2021 observed that the crystallinity of the cellulose nanoparticles decreased by about 8, 12, and 19% after 10, 30, and 60 min of ultrasonication, respectively.21 Here, it is notable that the addition of lignin to the CNC does not degrade its crystallinity much because the crystallinity index of the L-CNC is in the range of 60–80% depending on the concentration of acid used during hydrolysis, which is the same as the crystallinity index of the CNCs.2,25,29 An et al. showed that the crystallinity index of the CNC slightly decreased from 89.8 to 88.5% after its modification by lignin because of a slight disorder in the surface of the CNCs.25

4.2. FTIR Spectroscopy Analysis of the Composite Thin Films

Figure 6 displays the FTIR spectra of pure PEO thin films and L-CNC composite films. In the spectrum of pure PEO, a peak between 2700 and 3000 cm–1 indicates methylene stretching. The primary peaks in the 750–1500 cm–1 range result from the presence of the methylene group and the stretching of ether groups.30 The peak observed at approximately 2880 cm–1 represents the C–H symmetrical stretching vibration of −CH2–. Another peak appears at 1116 cm–1, which is attributed to the −C–O–C– linkage.30 Incorporating L-CNC into the PEO matrix resulted in a higher overall intensity of FTIR absorption due to the presence of an increased number of molecules and a greater surface area that affects the strength of peaks seen in the IR spectra.31 Although the overall spectrum of composite films is similar to that of pure PEO, a new peak emerged in the hydroxyl stretching region around 3340 cm–1. This peak shows the presence of the stretching and vibrations of OH groups, suggesting the L-CNC integration into the PEO matrix.5 This O–H stretching at 3340 cm–1 is commonly observed for PEO/CNC composite films.32,33

Figure 6.

Figure 6

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FTIR spectra of (a) Pure PEO and (b) PEO/lignin-containing CNC composite thin films.

4.3. Dynamic Mechanical Properties Analysis of the Composite Thin Films

The DMA assessed the impact of different amplitudes and sonication durations on the dynamic-mechanical properties of composite films. Figure 7 illustrates the changes in the storage modulus (E′) and loss modulus (E″) as a function of the temperature of composite thin films. Table 2 represents the nanocomposite thin films’ storage and loss modulus values at different temperatures. As shown in Table 2, the storage modulus of a pure PEO thin film was observed to be 18.9 MPa. The addition of L-CNC to the PEO matrix significantly increased the storage modulus of composite thin films. This increase in the storage modulus indicates improved resistance to flow due to restricted polymer chain mobility and the reinforcing effect induced by L-CNCs.34,35 Across all formulations, the storage modulus consistently increased with higher sonication parameters. Generally, the longer sonication time and higher amplitudes resulted in higher storage modulus values, indicating better dispersion of L-CNC fillers within the PEO matrix.2,34 For instance, the storage modulus values at 30° C ranged from 250.3 MPa for the 50A3MIN samples to 1298.3 MPa for the 100A9MIN samples. Notably, it was observed that a higher amplitude had a greater effect on enhancing the storage modulus than a higher sonication time at a specific amplitude. While samples sonicated at 50% amplitude for different sonication durations (3, 6, and 9 min) did not exhibit a significant difference in storage modulus, higher amplitude (100%) had a greater effect on the final dynamic-mechanical properties of the nanocomposite thin films. Mainly, a significant increase in storage modulus of 195% was achieved with sonication parameters set at 9 min and an amplitude of 100% compared to 9 min of sonication at 50%. However, it is important to note that excessively high sonication amplitudes or prolonged sonication times could potentially degrade or damage the polymer matrix or nanofillers, adversely affecting the overall properties.37

Figure 7.

Figure 7

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Representative curves of (a) storage and (b) loss modulus as a function of temperature of composite thin films.

Table 2. Storage and Loss Modulus Values of the Composite Thin Films at Different Temperaturesa.

  storage modulus (MPa)
 loss modulus (MPa)
sample code 30 °C 70 °C 30 °C 70 °C
Pure PEO 18.9 ± 3.73a 0.7 ± 1.3a 1.17 ± 0.1a 0.1 ± 0.1a
50A3MIN 250.3 ± 47.7b 121.7 ± 17.6b 20.9 ± 2.5b 16.7 ± 2.0b
50A6MIN 331.4 ± 87.4b,c 102.0 ± 30.5b 36.5 ± 6.6b,c 19.1 ± 4.7b
50A9MIN 439.2 ± 169.6b,c 153.9 ± 62.4b 37.8 ± 9.2b,c 26.5 ± 6.9b
100A3MIN 529.4 ± 123.8c 149.8 ± 48.3b 41.6 ± 7.9c 29.5 ± 6.7b
100A6MIN 941.0 ± 268.0d 509.8 ± 164.6c 86.7 ± 23.0d 70.2 ± 16.8c
100A9MIN 1298.3 ± 84.1e 699.7 ± 106.6d 95.3 ± 7.0d 75.7 ± 7.0c
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a

For each column, values (means ± SD) with different letters are significantly different based on Fisher’s least significant difference test at α = 0.05.

The loss modulus values, representing the viscous or nonrecoverable component of the material’s response to deformation, also exhibit an interesting trend (Figure 7b and Table 2). At 30 °C, the loss modulus increased with the addition of L-CNC and longer sonication times and higher amplitudes, a pattern similar to that of the storage modulus. This suggests that while the L-CNC enhances the overall stiffness, it also contributes to increased energy dissipation and damping characteristics.37 A formulation, such as 100A9MIN, shows a higher loss modulus compared to others, potentially indicating better interfacial interactions.34,37 Overall, the sonication process caused a reduction in the size of the agglomerations, contributing to higher and more uniform dispersion and enhanced interfacial adhesion through the PEO matrix. This enhanced interfacial adhesion within the PEO matrix produced more homogeneous biocomposite films with improved mechanical properties. The SEM and zeta potential analyses from Section 4.1 confirmed that a higher sonication time and amplitude led to better dispersion and less agglomeration of L-CNC in the PEO matrix. The findings align with previous studies on sonication’s impacts on the nanoparticle’s dispersion and mechanical properties of biocomposites. A study examined the effects of sonication time and filler loading on the tensile and electrical properties of thin films of epoxy-containing graphene nanopowder (GNP) and multiwalled carbon nanotubes (MWCNTs).38 The results indicated that a 20 min sonication treatment had a higher effect on improving the tensile properties compared to a 10 min sonication process. Similarly, another study revealed that the nanoclay-reinforced starch subjected to the sonication process for more than 45 min exhibited superior mechanical properties.39 These samples showed higher elongation at break than those processed for shorter durations.

4.4. DSC Analysis

The crystallization behavior of composite thin films was studied by using DSC. The results are presented in Figure 8 and Table 3. The crystallinity of pure PEO was determined to be 84.6%. The glass transition temperature was not detected as the manufacturer reported it to be around −70 °C. According to Figure 8, despite pure PEO, the composite films containing L-CNC showed a lower crystallinity (Xc) and a lower crystallization temperature (Tc), suggesting the nucleation effect of L-CNC.5 While the onset of melting (thermodynamic melting temperature) is constant across all formulations, the decrease in the melting peak temperature is due to the significant reduction in the crystallinity of the composite films. Generally, a lower crystallinity results in a reduced melting peak temperature, though other factors like heating rate, molecular weight, and additives can also influence the melting peak temperature.40 The presence of nanofillers caused a heterogeneous nucleation process, leading to a decreased free energy barrier and a faster crystallization rate.5 The PEO polymer chains surrounded the crystal nucleus, creating a barrier that prevented the macromolecule chains from entering the lattice. This resulted in the disordered and metastable crystal structures formation.5,41 The lower crystallinity can also be attributed to the confinement effect of L-CNC and the sonication process used to disperse them. Although the presence of L-CNC leads to faster nucleation, it also confines the PEO chains, limiting their ability to form well-ordered, extensive crystalline regions. This confinement effect hinders large crystal growth, leading to overall lower crystallinity.5,34 The crystallinity of nanocomposite films decreased with increasing sonication time and amplitude, suggesting that more severe sonication treatment led to greater disruption of the crystalline domains. As evidenced by SEM results, higher ultrasonication parameters resulted in more uniform dispersion of nanoparticles within the polymer matrix by breaking down large filler aggregates. However, this improved distribution also created more polymer–filler interfaces, disrupting the regular polymer chain packing, thereby reducing the crystallinity of nanocomposite films.5,34

Figure 8.

Figure 8

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DSC thermograms of pure PEO and composite thin films.

Table 3. Crystallization Behavior of Pure PEO and Composite Thin Films.

samples code crystallization temp. (Tc) °C melting peak temp. (Tm) °C crystallinity Xc %
Pure PEO 49.5 65.1 84.6
50A3MIN 48.0 63.4 45.7
50A6MIN 45.7 63.3 42.5
50A9MIN 41.5 60.8 22.7
100A3MIN 46.2 63.5 45.1
100A6MIN 44.7 62.0 31.6
100A9MIN 42.2 60.7 22.5
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5. Conclusions

This study investigated the impact of different sonication parameters (amplitude and time) on the final thermomechanical properties, crystallinity behavior, and lignin-containing cellulose nanocrystal (L-CNC) dispersion in the PEO matrix. The L-CNC was successfully incorporated into the PEO matrix and was confirmed through FTIR spectroscopy, indicating a new hydroxyl peak. FESEM and zeta potential analysis revealed that increasing the ultrasonication amplitude from 50% to 100% and higher sonication time to 9 min significantly improved the dispersion of L-CNCs by reducing agglomeration size by 95–96% within the PEO matrix. DMA demonstrated that the enhanced dispersion resulting from a higher sonication time and amplitude led to a significant increase in the storage modulus of the nanocomposite thin films. Notably, a 418% increase was observed for the samples sonicated at 100% amplitude for 9 min compared to samples that underwent sonication treatment for 3 min at 50% amplitude. However, DSC showed a decrease in crystallinity with increasing sonication parameters due to the disruption of crystalline domains by intense ultrasound energy. Overall, the findings from this study highlight the importance of optimizing sonication parameters to achieve uniform dispersion of L-CNC within the polymer matrices, which leads to enhancement in the performance of the final nanocomposite. The comprehensive insights gained from this study contribute to the development of high-performance biobased nanocomposite materials with potential applications in various fields.

Acknowledgments

The authors are grateful to Montana State University, Bozeman, MT, USA for providing funding for this research project.

The authors declare no competing financial interest.

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