Cellulose nanofiber reinforced starch film with pH- responsive weakening and marine-degradability

doi:10.21203/rs.3.rs-2756600/v1

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Cellulose nanofiber reinforced starch film with pH- responsive weakening and marine-degradability

https://doi.org/10.21203/rs.3.rs-2756600/v1

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Conventional marine-degradable plastics can remain in seawater for several years without losing their strength and stability, creating major environmental problems and threatening marine life. In this study, we prepared a cellulose nanofiber-reinforced starch film with pH-responsive weakening in seawater and marine-microbial degradability. The TEMPO-oxidized cellulose nanofiber (TCNF) was blended with hydroxypropyl starch (HPS) and di-aldehyde starch to develop a TCNF/modified starch film. pH-responsive swelling and weakening were performed in freshwater and seawater. Moreover, marine microbial degradability was evaluated in an artificial marine environment. TCNF/modified starch film rapidly loses up to 80 % of its strength in seawater and has higher marine microbial growth and degradability than TCNF film. The TCNF/HPS film demonstrates pH-responsive weakening with adequate microbial degradability. Therefore, it is a potential next-generation packaging material that can help address the problem of floating debris.

cellulose nanofiber

modified starch

pH-responsive films

marine degradability

floating debris

Single-use petrochemical plastics have been overused and overproduced for decades owing to their low cost, high strength, versatility, and durability (Giacovelli et al., 2018). However, anthropogenic littering of single-use plastics has become a substantial source of pollutants in intertidal and subtidal benthos contamination (Blumenröder, Sechet, Kakkonen, & Hartl, 2017; Gao et al., 2022; Kumar, Singh, Singh, Pandey, & Bhargava, 2023; Markic et al., 2023; Weideman, Perold, Omardien, Smyth, & Ryan, 2020), and the amounts of floating plastic debris are continuously escalating and persisting in the environment (Coates & Getzler, 2020; Geyer, Jambeck, & Law, 2017).

One effort to alleviate this global issue has been blending biodegradable materials with polyolefins, creating semi-degradable blends that could partially undergo biological assimilation (Obasi & Egeolu, 2015; Panahi, Gholizadeh, & Hajimohammadi, 2020). As the marine environment is naturally alkaline, with an average pH of ~ 8.1(Harvey, 1955), and the intertidal zone has a variable pH of 7.5 to 9.5, several pH-responsive polymers, such as poly(vinyl pyrrolidone) (PVP), polyacrylic acid (PAA), polyurethane, and poly(ethyleneimine), have been developed and commercialized as pH-responsive marine degradable packaging. However, due to their hazardous synthesis processes, expensive large-scale production, inadequate mechanical properties, and fragmentation into microplastics, their application would only worsen the larger crisis of microplastics ahead. Thus, there is a huge demand for low-cost, mass-producible films that can have both pH-responsive strength weakening and microbial degradation in the marine environment.

Cellulose and starch to produce biomass-based films have shown suitable characteristics for marine degradable packaging, owing to their abundance in nature and their ease of enzymatic break down via cellulase and amylase (Carrasco et al., 2016; Trivedi, Reddy, & Lali, 2016). However, due to their reported opacity, poor mechanical properties, and freshwater durability, their uses in packaging are limited. One way to counter this issue was by introducing aldehyde moieties to polysaccharides, which creates hemiacetal crosslinking interactions to produce water-stable transparent films (Plappert et al., 2018; Rhim, Gennadios, Weller, Carole Cezeirat, & Hanna, 1998; Soni, Asoh, Hsu, Shimamura, & Uyama, 2020; Soni, Hsu, Asoh, & Uyama, 2021; Xu, Canisag, Mu, & Yang, 2015; Yu, Chang, & Ma, 2010). Although, aldehyde moieties were able to produce these stable films, the moieties’ introduction was also found to suppress marine microbial degradation (Soni, Asoh, Hsu, & Uyama, 2022). Another study by Soni et Al. demonstrated rapid marine disintegration of films owing to the presence of a poly-ion complex. This film was prepared by blending cationic starch with anionic cellulose nanofibers. However, the presence of ionic charge on the surface reduces the attachment of marine microbes, affecting the degradation of the films. In this study, cellulose nanofiber-reinforced starch films were prepared and tested for their freshwater durability and pH-responsive disintegration in marine environments. TEMPO-mediated oxidation was performed to prepare highly fibrillated cellulose nanofibers (TCNF). Hydroxypropyl starch (HPS) and di-aldehyde (di-aldS) were blended with the TCNF to develop TCNF/HPS and TCNF/di-aldS films. The films were kept in different pH conditions (4, 7, and 9) in fresh water and seawater to analyze their freshwater softening and pH-responsive weakening.

2.1. Materials

Commercially available cellulose powder was supplied by Nacalai Tesque, Japan. Sodium bromide, sodium borohydride, and 2,4-dinitrophenylhydrazine (DNPH) were purchased from Sigma Aldrich, Japan. (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) was supplied by TCI chemicals, Japan. Sodium hypochlorite was supplied by Nacalai Tesque, Japan. All reagents were laboratory grade. Two types of modified tapioca starch powder, hydroxypropyl starch (HPS, DS: 0.11) and dialdehyde starch (di-aldS, DS: 0.04), were supplied by Nihon Shokuhin Kako Co. Ltd., Japan, and used as received. Artificial seawater solution was made using Daigo’s Artificial Seawater SP for Marine Microalgae Medium from Shiotani MS Corporation, Japan. Milli-Q water was used throughout the experiment.

2.2. Preparation of cellulose nanofiber (TCNF)

Cellulose nanofiber was prepared by TEMPO-mediated oxidation as reported by Isogai (Isogai, Saito, & Fukuzumi, 2011). Briefly, cellulose powder (4.0 g), TEMPO (0.075 g), and NaBr (0.75 g) were added to 300 mL of Mili-Q water and dispersed using magnetic stirring for 1 h at a temperature range of 25 to 27°C. The obtained suspension was placed under an automatic titrator (Automatic Titrator COM A-19, Hiranuma, Japan) to maintain and confirm the reaction system at a pH value of 10.5 throughout. In total, 18 mL of NaClO was added. The obtained suspension was centrifuged and washed thoroughly. The cellulose suspension was mechanically treated for 15 min using a blender to disperse the cellulose into individual nanofibrils. The concentration of TEMPO-oxidized cellulose nanofibrils (TCNF) in suspension was calculated by weight measurement before and after lyophilization. The aqueous dispersion of TCNF with a concentration of 1.0% (W/V) was prepared by adding water and was stored in a refrigerator for further use. Before the experiment, the suspension was thoroughly stirred to ensure homogeneity.

2.3. Preparation of starch suspension

Starch powder (3.0 g) was added to 100 mL of Mili-Q water. The suspension was mixed using magnetic stirring at 90°C for 1 h to obtain a homogeneous solution. The prepared solution was stored at room temperature for further experiments.

2.4. Preparation of TCNF/starch composite film

The prepared TCNF dispersion (1.0%, W/V) and starch solution (3%, W/V) were blended together in a 1.5:1 weight ratio and homogenized via vortex tube mixer (VTX-3500, LMS, Japan). TCNF/starch dispersion was casted in a Teflon PFA Petri dish and kept in an oven at 45°C for 6 h. Dried membranes were cut into specimens for further experiments.

2.5. Marine microbial degradation test

The prepared TCNF, TCNF/HPS, and TCNF/di-aldS film were cut into 15 mm squares and used as degradation test samples. Films were placed on whalebone (containing marine microbes), which was kept at 14 to 15°C in a tank simulating the marine environment. The pH and salinity were 7.52 and 35 ppt, respectively. The films were kept in plastic cell strainers (Falcon, 40 µm nylon mesh) and covered with glass slides to prevent floating. The samples were incubated for four weeks. All films were stored in a biosafety cabinet and exposed to UV light before the test.

The whalebone used in this study was obtained from Kagoshima Bay, Japan, at a depth of 101 m by remote operated vehicular hyper dolphins. After collection, the bone was maintained under atmospheric pressure at a water temperature of 15°C in a 254-liter aquarium tank filled with artificial seawater. After the incubation period, the films were immediately fixed with a glutaraldehyde solution and lyophilized using a freeze-dryer. The dried samples were subjected to surface morphology analysis using scanning electron microscopy (SEM) imaging (Miyamoto, Shinozaki, & Fujiwara, 2013, 2014; Shinozaki et al., 2010).

2.6. Enzymatic degradation test

The fast biodegradation of the films was measured under laboratory conditions through enzymatic hydrolysis. Commercially available enzyme cellulase (activity of 6000 units/g) was dissolved in phosphate-buffered saline (PBS) (0.1 mol/L; pH 6). Sodium azide (0.1 g) was used as an antibacterial agent. The films were cut into small pieces (10 mm × 10 mm) and placed in a six-well plate. The required amount of enzyme mixture was added based on the dry weight of the films according to Eq. 2 (Leppänen, Vikman, Harlin, & Orelma, 2020).

Volume of enzyme mixture =\(\frac{50 \left(\frac{FPU}{g}\right) \times dry weight of film \left(g\right)}{Activity of prepared enzyme mixture \left(\frac{FPU}{ml}\right)}\)

Five days of incubation began with the addition of the enzyme mixture at 37°C. Each sample was measured three times. As a reference, films were kept in a PBS solution without the enzyme. After hydrolysis, the remaining film samples were gently removed using tweezers, washed with ethanol, and dried. The percentage weight loss was calculated by Eq. 3.

Percentage weight loss (%) = \(\frac{{W}_{BD}-{W}_{AD}}{{W}_{BD}}\times 100\) ,

where W_BD and W_AD are the weights of the dried films before and after degradation, respectively.

2.7. Characterization

A stress-controlled rheometer (Haake Rheo Stress 6000, Thermo Scientific, USA) was used to examine the rheological behavior of the obtained TCNF/starch suspensions in different pH conditions. All measurements were conducted with an aluminum cone-plate geometry (diameter, 40 mm; angle, 2°). The strain amplitude value was set as 0.01, which was in the linear viscoelastic region for all samples. Frequency sweeps were performed from 0.1 to 10 rad/s. A temperature controller (Haake UTM controller, Thermo Scientific, USA) was used to maintain the temperature at 25°C during the measurements. The TCNF, TCNF/HPS, and TCNF/di-aldS suspensions in different pH conditions were lyophilized and analyzed using Fourier transform infrared spectroscopy (FTIR) (Nicolet iS5, Thermo Scientific, USA) to confirm the formation of hemiacetal crosslinking, and the data were analyzed using OMNIC software.

Swelling tests were performed in freshwater and seawater with different pH values (4, 7, and 9). Dried film specimens (2 cm × 2 cm) were immersed in the water. After a fixed time period, swollen samples were gently taken out using tweezers and excess water was removed with tissue paper before samples were weighed. Swelling ratio (SR) was calculated as follows:

SR (%) = \(\frac{{W}_{ss}-{W}_{ds}}{{W}_{ds}}\times 100\) ,

where W_ds and W_ss are the weights of the dried and swollen samples, respectively.

The mechanical properties of the swollen films were evaluated by the universal testing machine (UTM) (EZ Graph, Shimadzu, Japan) with a 10-N load cell and 5.0 mm/min strain rate. Five specimens (20 mm × 5 mm) were cut from each film. The specimens were kept in freshwater and seawater with different pH values (4, 7, and 9) for 2 h. A micrometer was used to measure the thickness of swollen films before the tensile test. Measurements were taken at four different positions for each specimen, and the average value was used for the calculation. Wet strain energy density was calculated by taking the area under the stress-strain curves produced for each film and averaging the values.

3.1 pH-responsive weakening

pH-responsive TCNF/modified starch film was prepared by blending TCNF with HPS and di-aldehyde starch (di-aldS) in a 1.5:1 weight ratio. An illustrative representation of the preparation scheme is shown in Fig. 1. pH-responsive swelling and wet strain energy density were evaluated by immersing the film in different pH conditions in fresh water and seawater.

TCNF, TCNF/HPS, and TCNF/di-aldS film swelling ratio and wet strength data in freshwater with varying pH conditions are shown in Fig. 2a, 2b and summarized in Table 1.

After 2 h immersion in freshwater at pH 7, TCNF, TCNF/HPS, and TCNF/di-aldS films had swelling of 2623%, 1500%, and 811%, respectively, and strain energy density of 0.27, 1.10, and 1.49 kJ/m³, respectively. The swelling was reduced to 2432%, 1217%, and 545%, respectively, and strain energy density increased to 0.56, 1.73, and 2.53 kJ/m³ at pH 4 conditions. However, in freshwater at pH 9, swelling increased to 2948%, 1947%, and 1002%, respectively, and strain energy density reduced to 0.13, 0.87, and 1.21 kJ/m³.

Table 1

Swelling ratio, wet tensile strength, and strain energy density of TCNF, TCNF/HPS, and TCNF/di-aldS films in freshwater with different pH conditions.
pH	TCNF		TCNF/HPS		TCNF/Di-aldS
pH	Swelling (%)	Strain energy density (kJ/m³)	Swelling (%)	Strain energy density (kJ/m³)	Swelling (%)	Strain energy density (kJ/m³)
4	2432 ± 471	0.56 ± 0.02	1217 ± 265	1.73 ± 0.20	545 ± 123	2.53 ± 0.28
7	2623 ± 272	0.27 ± 0.06	1500 ± 357	1.10 ± 0.11	811 ± 110	1.49 ± 0.17
9	2948 ± 216	0.13 ± 0.02	1948 ± 85	0.87 ± 0.18	1002 ± 106	1.21 ± 0.04

During TEMPO oxidation, carboxylate and partial aldehyde moieties (~ 22 µmol/g) are introduced on the surface of cellulose nanofibers. TCNF film show swelling of 2623% owning to electrostatic repulsion of carboxylate. Partial aldehyde moieties form a network of hemiacetal bonding during TCNF film preparation. The blending of starch works as a flexible binder between TCNF and enhances hemiacetal crosslinking density (Soni, 2021; Xu et al., 2015; Yu et al., 2010). Therefore, TCNF/HPS and TCNF/Di-aldS films showed lesser swelling than TCNF film at pH 7.

The formation of hemiacetal bonding is reversible and depends on the pH conditions as the reaction proceeds in acidic conditions (Göpferich, 1996). Moreover, carboxylate is protonated in acidic conditions and converted to carboxylic, which forms ester bonding and reduces the electrostatic repulsion between fibers (Fig. S1). Therefore, TCNF film swelling decreased at pH 4. However, the hemiacetal bonding equilibrium is reversed in an alkaline environment. No carboxylic formation occurred in basic conditions (Fig. S1), so TCNF film swelling increased at pH 9.

A schematic representation of the crosslinking density of TCNF/modified starch film is shown in Fig. S2. Addition of starch helps to enhance hemiacetal and ester bonding density by efficiently providing carboxylic, hydroxyl, and aldehyde moieties for crosslinking (Xu et al., 2015). Therefore, TCNF/HPS and TCNF/Di-aldS show relatively lesser swelling in an acidic and basic environment compared to TCNF film. The minimum swollen film showed the highest wet strain energy density, and vice versa. Therefore, TCNF film had the lowest strain energy density of 0.56, 0.27, and 0.13 kJ/m³ at pH 4, 7, and 9, respectively. TCNF/Di-aldS film, on the other hand, had the highest wet strain energy density of 2.53, 1.49, and 1.21 kJ/m³, owing to a highly dense hemiacetal network.

Moreover, pH-responsive bonding in TCNF/modified starch suspension was confirmed through rheology analysis (Fig. 3), and the results are summarized in Table 2. Storage modulus (G’) and loss modulus (G’’) of TCNF and TCNF/modified starch suspension decreased with higher pH. These results confirm the role of hemiacetal and ester crosslinking in reducing the wet strength of TCNF/modified starch film in an alkaline environment.

Table 2

Storage moduli (G’) and loss moduli (G”) at frequency of 1 rad/s for TCNF, TCNF/HPS, and TCNF/Di-aldS suspensions prepared with different pH conditions.
pH	G’ (Pa)			G’’ (Pa)
pH	TCNF	TCNF/HPS	TCNF/Di-aldS	TCNF	TCNF/HPS	TCNF/Di-aldS
4	71.06	93.52	103.94	6.09	8.88	12.38
7	34.57	57.57	75.02	2.72	7.10	8.74
9	29.04	50.04	66	1.24	4.90	6.34

The films were kept in artificial seawater with different pHs of 4, 8, and 9 to evaluate pH-responsive weakening in the marine environment. Swelling and strain energy density data are shown in Fig. 2c, 2d, and summarized in Table 3. TCNF, TCNF/HPS, and TCNF/Di-aldS had swelling of 455%, 430%, and 395%, respectively, after 7 days of immersion in artificial seawater (pH 8). Swelling increased to 513%, 493%, and 443%, respectively, at pH 9, and swelling reduced in the case of pH 4. All films had similar swelling behaviors to the freshwater immersion conditions at different pHs but had lesser swelling than freshwater owing to counterion crosslinking of TCNF.

Table 3

Swelling ratio, wet tensile strength, and strain energy density of TCNF, TCNF/HPS, and TCNF/Di-aldS films in seawater with different pH conditions
pH	TCNF		TCNF/HPS		TCNF/Di-aldS
pH	Swelling (%)	Strain energy density (MJ/m³)	Swelling (%)	Strain energy density (MJ/m³)	Swelling (%)	Strain energy density (MJ/m³)
4	402 ± 11	1.71 ± 0.05	375 ± 14	0.85 ± 0.07	329 ± 11	0.45 ± 0.08
8	455 ± 15	1.55 ± 0.06	430 ± 17	0.71 ± 0.14	395 ± 5	0.33 ± 0.06
9	513 ± 12	1.32 ± 0.07	493 ± 16	0.42 ± 0.10	443 ± 26	0.14 ± 0.07

The TCNF film had a strain energy density of 1.55 MJ/m³, which increased to 1.71 MJ/m³ and reduced to 1.32 MJ/m³ at pH 4 and 9, respectively. However, TCNF/HPS and TCNF/Di-aldS film had a strain energy density of 0.71 and 0.33 MJ/m³, respectively, in normal seawater (pH 8), which reduced to 0.42 and 0.14 MJ/m³, respectively, at pH 9. These results suggest that the addition of starch made the TCNF/modified starch film weaker in lower pH environments, such as seawater.

TCNF film had strong packaging of fibers crosslinked with each other by counterions after immersion in seawater (Soni, Hsu, Asoh, & Uyama, 2022). Therefore, TCNF film had the highest wet strain energy density in seawater. However, the addition of starch disturbs the fiber packaging and weakens the counterion crosslinked matrix. Further, at higher pH, the hemiacetal network became weaker. The lowest strain energy density was observed in TCNF/Di-aldS film because of the depolymerized starch network owing to oxidation (Fonseca et al., 2015; Soni, Asoh, & Uyama, 2020). These results suggest that TCNF/modified starch film has pH-responsive weakening behavior that is effective in marine conditions.

3.2 Marine microbial degradation

TCNF, TCNF/HPS, and TCNF/di-aldS film microbial degradation tests were performed in an artificial marine environment. The films were kept on whalebone (containing marine microbes) to evaluate marine microbial degradation. Sample preparation for marine degradation was explained above, and an illustrative representation is shown in Fig. 4a. The surface morphology of the films after 28 days is shown in Fig. 4b.

TCNF/HPS film showed higher microbial growth and rapid degradation, while the neat TCNF film exhibited less microbial growth and slow degradation. The TCNF/di-aldS film had moderate degradation that was greater than that of the TCNF film. These results suggest that blending and modification of starch affect marine microbial degradation of TCNF/starch films. The addition of starch promoted microbial degradation of the films, as prior studies have found (Mohanan, Montazer, Sharma, & Levin, 2020). The introduction of aldehyde moieties reduced microbial growth on the surface, so TCNF/di-aldS film had lesser degradation than TCNF/HPS film. However, neat TCNF film had relatively poor microbial growth and degradation owing to anionic charges (Dang & Lovell, 2016). Moreover, higher chemical moieties substitution and the introduction of a longer hydrophobic chain (Table S1) on starch were found to reduce marine microbial degradation of TCNF/starch films (Fig. S3).

Further, the effect of film surface charge on enzymatic degradation was investigated by keeping the films in cellulase (onozuka R-10) under laboratory conditions. The percentage weight losses of the TCNF and TCNF/HPS film after immersion in cellulase for 28 days are shown in Fig. S4. The neat TCNF film showed ~ 25% weight loss, while the TCNF/HPS film showed ~ 60% weight loss in cellulase. These results can be explained by zeta potential analysis; cellulase enzyme had a zeta potential of -10 mV, and neat TCNF film had a zeta potential of -45 mV (Fig. S4). Therefore, electrostatic repulsion reduces the enzyme affinity to the TCNF film surface. TCNF/HPS film had a zeta potential of 12 mV, so the film surface had a higher affinity for the enzyme and higher degradation (Fig. S4). A similar phenomenon has been reported for lignin degradation. (Cai et al., 2017; Lou, Zhu, Lan, Lai, & Qiu, 2013; Yan et al., 2010)

Thus, the results of this study suggest that the pH-responsive weakening and marine degradability of TCNF/modified starch films can be optimized by adjusting the chemical moieties and their degree of substitution on the starch. The TCNF/HPS films have shown adequate pH-responsive strength weakening and microbial degradation. We believe that the TCNF/HPS film opens a research direction for next-generation renewable, pH-responsive, marine degradable, green packaging film.

Herein, we comprehensively studied the pH-responsive weakening and marine-microbial degradability of TCNF, TCNF/HPS, and TCNF/Di-aldS film in freshwater and seawater. Hemiacetal bonding and starch chemical structure are crucial in pH-responsive weakening in seawater. Modified starch blending with TCNF enhanced the marine-microbial degradability of TCNF/modified starch film. Higher chemical modification of starch reduced microbial degradability of the film. However, only TCNF film had lesser microbial growth and poor marine microbial degradability. Higher electrostatic charges and counterion crosslinking of TCNF film reduced the marine microbial growth on the film surface. The blending of starch helped to enhance microbial growth and degradability. TCNF/HPS film (DS 0.11) showed adequate pH-responsive weakening and marine microbial degradability. TCNF/HPS films will provide insight for further research on seawater weakening, marine-microbial degradable, and greener packaging films.

Acknowledgments

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants (No. 20H02797), and New Energy and Industrial Technology Development Organization (NEDO) (JPNP 20004). This work was also supported by JST Grant Number JPMJPF2218. This research was performed by the Environment Research and Technology Development Fund JPMEERF21S11900 of the Environmental Restoration and Conservation Agency of Japan. We would like to thank Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for performing marine microbial degradation test.

Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.

Ethics approval and consent to participate

Not applicable.

Consent for publication

All authors read, approved and consented to submit the final manuscript.

Authors contributions

All authors contributed to the study conceptualization and design. Material preparation, data collection and analysis were done by Izzah Durrati binti Haji Abdul Hamid and Raghav Soni. Yu-I Hsu did the review & editing, visualization and funding acquisition. Hiroshi Uyama did resources, funding acquisition. The first draft of the manuscript was written by Raghav Soni and all authors commented on previous versions of the manuscript.

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Cellulose nanofiber reinforced starch film with pH- responsive weakening and marine-degradability

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Abstract

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1. Introduction

2. Materials And Methods

2.1. Materials

2.2. Preparation of cellulose nanofiber (TCNF)

2.3. Preparation of starch suspension

2.4. Preparation of TCNF/starch composite film

2.5. Marine microbial degradation test

2.6. Enzymatic degradation test

2.7. Characterization

3. Results And Discussion

3.1 pH-responsive weakening

3.2 Marine microbial degradation

4. Conclusion

Declarations

Acknowledgments

Competing interests

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Authors contributions

References

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