Multifunctional Performance Lignin-Crosslinked-PVA Composite Film Based on a Dual Crosslinking Network

Abstract

The development of high-performance biocomposites based on poly vinyl alcohol (PVA) and lignin is often hindered by the limited interfacial compatibility. Herein, we reporte a synchronized crosslinking strategy to seamlessly integrate lignin and PVA into a uniform and robust composite film. The vinyl groups were introduced into both lignin and PVA molecular chains, which enable the formation of dense covalent bonds through reactions between these unsaturated carbon–carbon double bonds. This dual network structure combining covalent crosslinking with hydrogen bonding effectively strengthened the interfacial compatibility between lignin and PVA, which substantially enhanced film toughness, exhibiting an elongation at break of up to 4300%. Furthermore, the prepared composite film also demonstrated outstanding UV-blocking efficiency (>90%), strong antioxidant activity (82% DPPH scavenging), enhanced hydrophobicity (water contact angle of 97.9°), and improved thermal stability. The dramatic enhancements were attributed to the homogeneous dispersion of modified lignin within the covalently bonded network, which ensured efficient stress transfer and reduced the availability of hydrophilic groups. This synchronized crosslinking approach presents a versatile and effective route for fabricating high-value lignin-based composite materials.

1. Introduction

Escalating global environmental concerns and the drive towards a circular bioeconomy have intensified the demand for sustainable materials derived from renewable resources [1]. Among synthetic polymers, poly vinyl alcohol (PVA) stands out due to its excellent film-forming ability, high tensile strength, biocompatibility, biodegradability, and non-toxicity. These properties render PVA-based materials widely applicable in packaging, biomedical devices, and separation membranes [2]. However, the inherent strong hydrophilicity of PVA, stemming from abundant hydroxyl groups, compromises its dimensional stability and barrier properties in humid environments [3,4]. Furthermore, PVA offers limited resistance to ultraviolet (UV) radiation, which can accelerate its photodegradation and limit its lifespan in outdoor applications [5]. Therefore, developing strategies to enhance the moisture resistance and multifunctionality of PVA without sacrificing its biodegradability and safety is of paramount importance.

Lignin, the second most abundant natural polymer on earth, represents a vastly underutilized and sustainable source of aromatic structures [6]. It is a three-dimensional, amorphous macromolecule composed of methoxylated phenylpropane units, which contains valuable functional groups such as phenolic hydroxyl, aliphatic hydroxyl, and carboxyl groups [7,8,9]. These structural features endow lignin with several attractive properties, including inherent UV absorption, potent antioxidant activity, and rigidity [9,10,11]. The incorporation of lignin into a PVA matrix promises to create composite materials that are not only more sustainable but also possess enhanced functional properties, such as UV shielding and oxidative stability [12]. This approach aligns perfectly with the principles of green chemistry and biomass valorization.

The practical application of lignin/PVA composites has been persistently hampered by two interconnected challenges: poor interfacial compatibility and low lignin loading capacity [13,14]. In conventional blending processes, the interaction between lignin and PVA relies predominantly on hydrogen bonding [15]. When the lignin content becomes excessively high, the interfacial adhesion provided solely by hydrogen bonding is insufficient to prevent the aggregation of lignin molecules, consequently leading to macroscopic phase separation [16]. The resulting lignin aggregates act as stress concentration points, which markedly deteriorate the mechanical properties of the composite films, particularly the elongation at break and toughness. Moreover, the dark color and limited dispersion of lignin often lead to a drastic loss in transparency and an upper limit on its incorporation level (typically well below 30 wt%) [6]. Consequently, lignin has often been perceived as a low-value filler that compromises the mechanical integrity of the composite. The key to overcoming these limitations lies in transforming the interfacial interactions from relying solely on physical crosslinking (hydrogen bonding) to a dual-crosslinked network comprising both hydrogen bonds and chemical bonds (covalent crosslinking) [17,18,19]. Directly establishing covalent bridges between lignin and PVA chains could contribute to fundamentally resolving the issues of phase separation and inefficient stress transfer [20,21]. However, the inherent chemical incompatibility and low chemical reactivity of conventional lignin make this challenging.

In this work, we introduce a novel “synchronized modification and crosslinking” strategy to overcome these challenges. We extracted a lignin (TsPL) with exceptionally high phenolic hydroxyl content, providing abundant reactive sites [22]. The core of our approach is based on a one-pot process where both TsPL and PVA undergo chemical modification and subsequent crosslinking using acryloyl chloride (AC). AC reacts with hydroxyl groups on both TsPL and PVA, grafting vinyl groups onto their molecular chains [23]. Subsequently, the vinyl groups on lignin and PVA molecules undergo thermally initiated radical polymerization to form an extensive covalent network [24]. Together with hydrogen bonding from unreacted hydroxyl groups, this dual-network structure intimately connects lignin and PVA chains. The optimization and regulation of the lignin modification using acryloyl chloride was systematically studied. Comprehensive evaluation of the composite films formed via the synchronized crosslinking process was conducted. The morphology and mechanical, optical, thermal, surface, and antioxidant properties were extensively investigated [25]. The results compellingly demonstrated that this strategy successfully created homogeneous, ultra-tough, and multifunctional PVA/TsPL composite films, providing a new paradigm for the high-value utilization of lignin.

2. Materials and Methods

2.1. Materials

The pine wood powder used in this research was obtained by cutting pine boards from Finland into small pieces, then crushing them into powder using a micro pulverizer (FZ102, Keheng, Shanghai, China), and finally filtering through a sieve. p-Toluenesulfonic acid (p-TsOH, 104-15-4, 99%), phenol (108-95-2, ≥99.5%), polyvinyl alcohol (PVA, 104-15-4, 1788 type, 87–89 mol%), acryloyl chloride (AC, 814-68-6, 96%), and sodium bicarbonate (NaHCO₃, 144-55-8, ≥99.5%) were all purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). N,N-Dimethylformamide (DMF, 68-12-2, ≥99.5%) and 1,4-Dioxane (123-91-1, ≥99.5%) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). All chemicals were analytical grade or above and used without further purification.

2.2. Extraction and Modification of Lignin

A mixed solution of p-TsOH and phenol in a ratio of 6.5:3.5 was placed in a pressure resistant bottle. The pressure resistant bottle was then put into an ultrasonic hot-water bath (70 °C). After all the p-TsOH was dissolved, 5 g of pine wood powder (40 mesh) was added, and the reaction was carried out at 70 °C for 20 min. After the reaction, the obtained mixture was filtered, and the residue solid and extract solution were separated. The extract was added to 10 times its volume of deionized water to precipitate the lignin. After further centrifugation and freeze-drying, phenolated lignin (TsPL) was obtained.

The acryloylation of TsPL was performed to introduce carbon–carbon double bonds. Specifically, 0.3 g of TsPL was dissolved in 10 mL of anhydrous N,N-dimethylformamide (DMF) in a narrow-mouthed bottle equipped with a magnetic stirrer. The bottle was maintained in an ice-water bath to ensure low-temperature conditions. Sodium bicarbonate (NaHCO₃), selected as the optimal acid-binding reagent based on preliminary tests, was added to neutralize the generated HCl. A calculated amount of acryloyl chloride (AC) (e.g., 0.278 mL for an AC-to-OH molar ratio of 1:1) was then added dropwise. After adding ingredients, the reaction was maintained at a predetermined temperature and a constant stirring speed of 400 rpm for a specified time. After the reaction was completed within a certain time period, the reaction mixture was precipitated in a large volume of ice-cold deionized water (10 times volume). The precipitated solid was collected by centrifugation, thoroughly washed with deionized water to remove residual salts and solvent, and then finally freeze-dried to obtain the acryloylated lignin, designated as TsPL–AC. To optimize the reaction conditions, a series of controlled experiments were conducted, including varying the amount of AC (AC-to-OH molar ratios of 0.5:1, 1:1, and 2:1), reaction temperature (20, 25, 30, and 35 °C), and reaction time (2, 4, 8, and 12 h).

2.3. Preparation of TsPL-Based Composite Film

A total of 0.5 g of solid components (TsPL–AC and PVA), with lignin content varying from 5% to 40% relative to the total solid mass, was added to 15 mL of DMF. The mixture was first subjected to ultrasonication to disperse and dissolve the solid components, followed by stirring and heating at 90 °C for approximately 1 h until complete dissolution, yielding a homogeneous solution. The solution was then cooled to a lower temperature (20 °C, 25 °C, or 35 °C) in a water bath, and acryloyl chloride was added dropwise slowly. The reaction was allowed to proceed for 8 h with continuous stirring. Upon completion of the reaction, the mixture was cast onto a clean, level polytetrafluoroethylene (PTFE) mold. The mold was placed in a ventilated oven at 70 °C to initiate radical polymerization and promote the formation of the crosslinked network. After 24 h, the dried film was carefully peeled from the mold and immersed in deionized water to remove unreacted components and residual solvent. The washing process was repeated three times. The purified composite film was finally dried at 25 °C and 60% relative humidity for 48 h, obtaining the TsPL/PVA composite film.

2.4. Characterization of Lignin and Composite Films

Fourier transform infrared (FTIR) spectra of lignin samples were recorded on a spectrophotometer (Prestige-21, Shimadzu, Kyoto, Japan) at a frequency of 4000–500 cm⁻¹. The 1 mg dried sample was mixed with 100 mg dried KBr, ground in an agate mortar, thoroughly mixed, finely ground and compressed into slices for measurement. The measuring of FTIR spectra of the composite film samples was performed with an Alpha infrared spectrophotometer (Bruker, Berlin, Germany) with a scanning wave number range of 4000–500 cm⁻¹. ¹H NMR analyses of lignin samples were performed on AVANCE II 400 spectrometers (JEOL, JNM-ECZL400S, Bruker, Germany). A total of 60 mg of lignin was added to 0.5 mL DMSO-D6 to dissolve it completely, and Tetramethylsilane (TMS) was used as the internal standard and transferred into a 5 mm nuclear magnetic tube. The hydroxyl content of lignin samples was analyzed using ³¹P NMR spectroscopy.

The surface morphology of composite film was observed with a SEM Regulus 8220 (Hitachi, Tokyo, Japan) after the composite film was sprayed with gold with an ion sputtering instrument (MCIOOO, Hitachi, Tokyo, Japan). The UV shielding performance of the film with the size of 40 × 50 mm was determined using a UV-visible spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan), and the wavelength range was set to 800–200 nm. The mechanical tensile properties of the dried composite film (5 × 30 mm long strips) were measured with a texture analyzer (TA.XT plus C Stable Micro Systems, Surrey, UK). The initial spacing was set to 15 mm and the drawing speed was 0.2 mm/s. The water contact angle on the surface of the composite film was measured using the fully automatic optical contact angle measuring instrument OCA 50 (Data Physics, Filderstadt, Germany). The contact angle was recorded at 25 s with a drop volume of 5 μL per drop. The thermal stability of the film was measured using a thermogravimetric analyzer (Q50, TA Instruments, New Castle, DE, USA). The sample was heated from room temperature to 600 °C at a rate of 10 °C/min under a nitrogen flow of 10 mL/min, and the mass loss was recorded. Water uptake measurements of the composite films were conducted using the gravimetric method. The antioxidant properties of the film were measured using the DPPH free radical method.

3. Results and Discussion

3.1. Acryloyl Chloride-Modified Lignin and the Fabrication of Composite Films

The phenolic lignin (TsPL) was produced according to our previous research, which succeeded in endowing the extracted lignin with abundant hydroxyl groups [22]. The phenolic hydroxyl groups enable lignin to acquire more chemically active sites. In this work (Figure 1), the phenolic hydroxyl groups possessed in the lignin structure reacted with acryloyl chloride (AC), introducing unsaturated vinyl groups onto the lignin backbone and forming acrylated lignin (TsPL–AC). Unsaturated vinyl groups were also introduced into the structure of polyvinyl alcohol (PVA) in the reaction system. Simultaneously, the unsaturated carbon–carbon double bonds present in the lignin and PVA structures further reacted, thereby crosslinking the lignin and PVA together to form the precursor of the composite film.

The conversion of TsPL into its vinyl-grafted derivative, named TsPL–AC, was confirmed by FTIR and ¹H NMR spectroscopy according to previous research [17]. As shown in Figure 2a, the FTIR spectrum of TsPL showed a broad O-H stretching vibration at around 3438 cm⁻¹. After reaction with AC, this stretching vibration significantly declined, confirming the consumption of hydroxyl groups. Concurrently, two new significant peaks appeared at around 1738 cm⁻¹ (C=O stretching of the ester linkage) and 1607 cm⁻¹ (C=C stretching of the acryloyl group) [26]. The appearance of these peaks provides definitive evidence for the successful grafting. This transformation of lignin was further confirmed by ¹H NMR measurement (Figure 2b). The spectrum of unmodified TsPL showed virtually no signals in the 5.5–6.5 ppm region attributed to vinyl protons. In contrast, the spectrum of the AC-modified TsPL (TsPL–AC) displayed a clear set of signals at 6.13, 5.89, and 5.65 ppm, corresponding to the vinyl protons of the acrylate group [27]. In addition, the effect of acid binders on the overall reaction system was explored. By integrating the hydroxyl proton signals at 6.13, 5.89, and 5.65 ppm in the ¹H NMR spectra, the most pronounced decrease in hydroxyl content was observed in the control group which employed sodium bicarbonate (NaHCO₃) as the acid-binding reagent. This result demonstrated that the grafting rate of acryloyl chloride (AC) is highest under these conditions, thus confirming sodium bicarbonate as the optimal acid scavenger because it was far superior to sodium acetate in neutralizing HCl and preventing the degradation of the acryloyl groups, thereby maximizing the grafting efficiency, as evidenced by the stronger vinyl proton signals in FTIR spectra.

We further explored the effect of reaction conditions on the lignin modification using acryloyl chloride. It was found that the –OH stretching vibration peak of unmodified lignin at 3438 cm⁻¹ gradually decreased when increasing the addition amount of acryloyl chloride, reaction temperature and reaction time (Figure 3a₁–a₃). Concurrently, new peaks appeared at 1738 cm⁻¹ (ester C=O stretching) and 1607 cm⁻¹ (C=C stretching), directly confirming that acryloyl chloride underwent esterification with lignin hydroxyl groups, successfully introducing carbon–carbon double bonds into the lignin structure. A comparison between these lignin samples showed that their relative intensities of the fingerprint region were rather similar except for the differences in the typical characteristic peaks mentioned above, confirming that the modified lignin still retained typical lignin patterns. The residual phenolic hydroxyl content after lignin modification was determined using the Folin–Ciocalteu method, and the results are shown in Figure 3b₁–b₃. The results were consistent with the trend of the FTIR spectrum above. Furthermore, the quantitative calculation of the integration of the carbon–carbon double bond signals in ¹H NMR spectra revealed an increase in double bond content, which was consistent with the observed decrease in hydroxyl groups. Upon increasing the addition of acryloyl chloride, all the above characterizations indicated that the introduction of carbon–carbon double bonds increased while the hydroxyl content decreased. This is because acryloyl chloride, as a reactant, has a higher probability of contacting the lignin hydroxyl groups at higher doses, thus participating more in the reaction with hydroxyl groups. Furthermore, acryloyl chloride exhibited better reactivity at higher temperatures, as excessively high temperatures may lead to self-polymerization of double bonds or degradation of lignin aromatic side-chain ether bonds, thereby disrupting the formed ester and double bond structures. Therefore, the optimal reaction temperature was considered to be 35 °C. When the reaction time exceeded 8 h, the number of carbon–carbon double bonds introduced was almost negligible, which may be due to the lignin aggregation hindering the contact between hydroxyl groups and acryloyl chloride.

Based on the acryloyl chloride-modified lignin (TsPL–AC), a homogeneous composite film was constructed through synchronous modification and a crosslinking strategy. In the reaction system containing TsPL–AC, PVA and acryloyl chloride, PVA underwent an esterification reaction with acryloyl chloride, introducing carbon–carbon double bonds [28]. Simultaneously, the carbon–carbon double bonds present in both TsPL–AC and PVA underwent a crosslinking reaction, creating a unified three-dimensional network where TsPL was no longer a physically blended filler but an integral chemically bonded part of the polymer architecture. Furthermore, this network was reinforced by hydrogen bonding between the remaining hydroxyl groups on TsPL–AC and PVA. This synergistic dual-network structure not only substantially increases the allowable lignin loading but also resolves the poor interfacial adhesion between lignin and PVA, resulting in a uniform composite film.

3.2. Morphology of the Composite Films

The effectiveness of this strategy is clearly demonstrated by the film morphology. As shown in Figure 4a₁,a₂, the composite film without modification (TsPL-PVA) exhibited inhomogeneous color and dispersed surface particles. This can be attributed to the weak hydrogen bonding interactions between lignin and PVA, as lignin tended to self-aggregate due to their polarity differences. In contrast, the AC-modified composite film (TsPL–AC–PVA) was significantly smooth and uniform in color, maintaining considerable transparency even at a TsPL content of 40%, indicating a highly uniform dispersion of lignin molecules throughout during the preparation process of the composite films (Figure 4b). This is because AC established a covalent crosslinking network between TsPL and PVA. This robust covalent bonding fundamentally suppressed the strong π-π stacking and hydrophobic self-aggregation tendency of lignin molecules, thereby thermodynamically eliminating the primary driving force for phase separation. Scanning Electron Microscopy (SEM) revealed a sharply defined contrast between the crosslinked and non-crosslinked structures. The surface (Figure 4c) and cross-section (Figure 4e) of the composite film with 25% TsPL content were exceptionally smooth, continuous, and dense, without visible cracks, voids, or lignin aggregates. This homogeneous morphology is the direct physical manifestation of successful interfacial crosslinking. However, when the TsPL content was increased to 40% (Figure 4d,f), small particles reappeared on the surface, and the cross-section became less uniform. This could be attributed to the significant increase in steric hindrance caused by excessively high lignin content. The limitation of available reaction sites prevented the lignin from participating in the crosslinking, resulting in phase separation of the unbound lignin.

3.3. Multifunctional Performance of TsPL–AC–PVA Composite Films

As mentioned above, we selected three different reaction conditions to prepare the TsPL–AC–PVA composite films. We could achieve the regulation of the carbon–carbon double bonds content in modified lignin by controlling the degree of modification. This approach was strategically designed to regulate the balance between covalent crosslinking and hydrogen bonding in the composite film system, thereby enabling the optimization of its performance, especially in terms of mechanical properties. The profound impact of the covalent crosslinking network was reflected in the mechanical behavior of the prepared materials, thus achieving the transformation of brittle mixture into composite with ultra-high toughness. The stress–strain curves are shown in Figure 5a₁–a₃; the optimal TsPL–AC–PVA composite films (25% TsPL addition, 35 °C reaction temperature) achieved an astonishing elongation at break of 4300% which far exceeded that of pure PVA film and composite films without acryloyl chloride crosslinking. This remarkable enhancement in toughness did not originate from lignin acting as a conventional reinforcing filler, which typically improves strength at the expense of elongation. Instead, this is directly attributed to the unique deformation mechanism enabled by the covalent crosslinking network. In the composite films without acryloyl chloride crosslinking, the interface between TsPL and PVA was relatively weak, dominated primarily by reversible hydrogen bonds [29]. Under stress, lignin aggregates act as stress concentration points, which can rapidly induce and propagate microcracks, eventually leading to the fracture of the composite material [30]. For the synthesized TsPL–AC–PVA composite films, the formed covalent bonds at the interface serve as robust permanent anchors, which create a continuous network that efficiently transfers applied stress from the flexible PVA chains to the more rigid lignin domains [31]. Furthermore, the dual-network structure, where covalent crosslinks were supplemented by a dynamic hydrogen-bonding network, provided an additional energy dissipation pathway. The hydrogen bonds can break and re-form during deformation, further delaying fracture of the prepared materials. Under the optimal lignin addition of 25%, the composite films prepared at 35 °C demonstrated superior performance compared to those prepared at 20 °C and 25 °C (Figure 5a₁–a₃). This further clearly demonstrated that the higher crosslinking density obtained under optimized reaction conditions had a crucial influence on the material performance. Although higher TsPL–AC addition could enhance the film strength, it caused a dramatic reduction in toughness. The composite film with a TsPL–AC content of 25% to 30% achieves the best balance between toughness and strength.

As shown in Figure 5b, the UV–Vis spectra demonstrated that the TsPL–AC–PVA composite films provided over 90% UV-blocking efficiency while maintaining high visible-light transmittance. This functionality was intrinsically derived from the aromatic structure of lignin [32], which contains chromophores that absorb UV radiation effectively [33]. The homogeneous dispersion of TsPL enforced by the covalent network was also crucial for ensuring the optical transparency of the film material and the uniform absorption of UV light on the film without scattering by large aggregates [34]. The TsPL–AC–PVA composite films exhibited an 82% DPPH radical scavenging rate after 12 h, indicating strong antioxidant activity (Figure 5c) [35]. This result also demonstrated that a sufficient population of active phenolic hydroxyl groups still remained accessible within the crosslinked matrix to quench free radicals [36]. Therefore, the regulation of the carbon–carbon double bond content in modified lignin not only facilitates the formation of a stable covalent bond network but also appropriately preserves the inherent properties of lignin itself.

The surface properties of TsPL–AC–PVA composite films were also analyzed. As shown in Figure 6a₁–a₃, the water contact angle increased from 47.9° for pure PVA film to a maximum of 97.9° for the composite film. A clear trend can be observed from the results: a higher TsPL content resulted in the stronger hydrophobicity of the composite films. This shift toward hydrophobicity can be directly attributed to the chemical and physical consequences of the crosslinking process [37,38]. Chemically, the lignin modification reaction consumed most of the hydrophilic hydroxyl groups on both PVA and TsPL, replacing them with more hydrophobic ester groups [39]. Physically, the formation of a dense crosslinked network reduced chain mobility and free volume, thereby restricting the penetration and interaction of water molecules with the remaining polar groups. This mechanism is consistently corroborated by water uptake measurements (Figure 6b), which showed that the water absorption rate decreased significantly with the increase in TsPL content and crosslinking degree.

Thermogravimetric analysis revealed a marked enhancement in thermal stability for the TsPL–AC–PVA composite films (Figure 6c,d). In the medium temperature range (300–450 °C), the DTG peak of the composite film is lower than that of the PVA film. This is because the aromatic structure of lignin has a certain thermal stability and acts as a physical barrier [23]. The char residue at 600 °C increased from 7% (pure PVA film) to 18% (composite films), and the DTG peaks broadened and shifted to higher temperatures. This is because the AC modification enables the formation of chemical bonds (ester bonds) between TsPL and PVA, altering the interfacial structure [27]. TsPL with complex aromatic structure underwent multi-stage pyrolysis and formed a stable char [40]. More importantly, the covalently crosslinked network could restrict the segmental mobility of the PVA chains. This restrictive effect acted as a barrier, hindering the release of volatile decomposition products, thereby raising the apparent decomposition temperature and increasing the yield of carbonaceous residue.

4. Conclusions

In this study, acryloyl chloride-modified lignin and polyvinyl alcohol were successfully crosslinked together through their grafted carbon–carbon double bonds, synthesizing a multifunctional composite membrane material (TsPL–AC–PVA). Acryloyl chloride modification effectively strengthened the interfacial compatibility between lignin and PVA, forming a stable three-dimensional network combining covalent crosslinking with hydrogen bonding. This dual network structure substantially enhanced film toughness, exhibiting an elongation at break of up to 4300%. Furthermore, the composite film achieved over 90% UV-blocking efficiency while maintaining high visible light transmittance. Its DPPH radical scavenging rate reached 82% after 12 h, indicating excellent antioxidant activity. Hydrophilicity and thermal stability of the composite film were also significantly improved. The acryloyl chloride-crosslinked lignin–PVA composite films thus combine excellent optical properties, antioxidant capacity, thermal stability, and mechanical performance, demonstrating great potential for applications in active packaging, biomedical materials, and functional optical films.