Abstract: To address the environmental crisis caused by the reliance of traditional optoelectronic materials on petroleum-based polymers, this study focuses on all-plant-based polylactic acid (PLA) and develops high-performance optical films with high light transmittance, excellent mechanical properties, and environmental compatibility through a combined solution casting and electric field spinning technology. Using corn-based PLA as the matrix and introducing cellulose nanocrystals (CNC) extracted from waste paper as the reinforcing phase, the study systematically investigates the regulation mechanism of CNC addition on the structure and properties of the films. Characterization results show that when the CNC addition is 1%, the film has the best comprehensive performance: the light transmittance at 600 nm reaches 95%, an increase of 15 percentage points compared to pure PLA films; the tensile strength increases to 93.54 MPa, the elongation at break increases by 57.8%, and the folding endurance reaches 502 times, which are 115.3%, 57.8%, and 228% higher than those of pure PLA, respectively; the initial thermal decomposition temperature increases to 324 °C, and the water vapor transmission rate decreases to 644.67 g/(m²·d), with a degradation rate of 3.83% after 60 days of burial in soil. In addition, the film exhibits low refractive index characteristics (close to that of glass) and good light transmittance in the wide wavelength range from ultraviolet to infrared. The all-plant-based PLA optical films developed in this study not only achieve the environmentally friendly characteristics of full biomass raw materials and post-use degradability but also meet the optical and mechanical performance requirements of the semiconductor manufacturing, liquid crystal display, and other optoelectronic fields, providing new material solutions and technical support for the development of sustainable optoelectronics.
Key words: All-plant-based polylactic acid; Optical film; Sustainable optoelectronics; Cellulose nanocrystals; Composite modification
1 Introduction
Under the dual impetus of the global "carbon neutrality" goal and the increasingly severe environmental crisis, the sustainable transformation of the optoelectronic industry has become an inevitable trend in technological development. As the core foundation of this industry, optoelectronic materials have long relied on petroleum-based polymers such as polymethyl methacrylate and polyvinyl chloride. These materials not only consume non-renewable resources but also pose a serious ecological burden due to their difficulty in degradation after disposal. Therefore, the development of renewable optoelectronic materials that possess excellent optical performance, reliable mechanical stability, and environmental compatibility has become a key breakthrough to resolve the contradiction between industrial development and ecological protection.
Polylactic acid (PLA), as the only biobased polymer that has achieved mass production, is made from plant starches such as corn and sugarcane. The CO₂ absorbed during its growth can be offset by the CO₂ released during degradation, forming a natural carbon cycle loop, making it an ideal sustainable material alternative. However, pure PLA films have drawbacks such as being hard and brittle, having limited light transmittance, and insufficient thermal stability, which restrict their application in the optoelectronic field. Current research mostly enhances PLA performance through chemical modification or inorganic filler composites. For instance, a research team from Osaka University reinforced soy-based polymers with corn-derived PLA nanofibers, obtaining flexible transparent films, but the light transmittance was only around 70%, which is difficult to meet the demands of high-end applications such as display components. Toyobo's high-transparency PLA films prepared by biaxial stretching technology have excellent optical properties, but the coordinated optimization of mechanical strength and degradation performance still needs to be improved.
Based on this, this study proposes to construct a composite system with all plant-based components: using PLA prepared by corn fermentation polymerization as the matrix and introducing cellulose nanocrystals (CNC) extracted from waste paper as the reinforcing modifier - both of which are derived from plant biomass, ensuring the environmental friendliness of the material throughout its life cycle. CNC/PLA composite optical films were prepared by solution casting, and the effects of CNC addition on the optical properties, mechanical properties, thermal stability and degradation performance of the films were systematically studied. The interaction mechanism between the components was revealed, aiming to develop all-plant-based optical films with performance meeting the requirements of the optoelectronic field, providing a technical solution that is both scientific and practical for promoting the development of sustainable optoelectronics.
2 Experimental Section
2.1 Experimental Materials
All-plant-based polylactic acid (PLA, with a molecular weight of 80,000 g/mol and a purity of 99%) was obtained through fermentation and polymerization of corn starch (purchased from Nantong Longda Bio-New Materials Co., Ltd.); cellulose nanocrystals (CNC) were self-made from office waste paper as raw material by sulfuric acid hydrolysis, and the preparation process was in accordance with the literature. The product was a white powder with a length-to-diameter ratio of 30 to 50 and a crystallinity of 85%; chloroform (analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd.); anhydrous ethanol (analytical grade, purchased from Tianjin Fuyu Fine Chemical Co., Ltd.).
2.2 Film Preparation
Pure PLA films and CNC/PLA composite films were prepared by solution casting. The specific steps are as follows: (1) PLA particles were added to chloroform and stirred for 2 h in a constant temperature water bath at 60 ℃ to prepare a 5% PLA solution by mass; (2) CNC powder was added to anhydrous ethanol at CNC addition amounts of 0%, 0.5%, 1%, 2%, and 2.5% (relative to the mass of PLA), respectively, and ultrasonically dispersed for 30 min to form a uniform suspension. Then, it was gradually added dropwise to the PLA solution and stirred for another 1 h to obtain a uniform composite solution; (3) The composite solution was poured into a polytetrafluoroethylene mold and left to stand at room temperature for 24 h for natural solvent evaporation. Then, it was placed in a vacuum drying oven at 40 ℃ for 12 h to completely remove the residual solvent; (4) The dried films were removed from the mold, cut into standard samples, and set aside. To optimize the optical properties of the films, all preparation processes were carried out in a clean environment to avoid the introduction of impurities.
2.3 Performance Characterization and Testing Methods
Optical performance testing: A UV-3600 ultraviolet-visible-near-infrared spectrophotometer (Shimadzu Corporation, Japan) was used to measure the transmittance and haze of the film within the wavelength range of 200 to 1500 nm. The sample size was 20 mm × 20 mm, and three different areas of each sample were tested, with the average value taken. An Abbe refractometer (WYA-2W, Shanghai Precision Scientific Instrument Co., Ltd.) was used to measure the refractive index of the film at 25 ℃.
(2) Mechanical property testing: According to the GB/T 1040.3-2006 standard, the tensile strength and elongation at break of the film were tested using a CMT6104 electronic universal testing machine (Shenzhen New Sisi Material Testing Co., Ltd.) with a tensile rate of 5 mm/min. The folding endurance of the film was tested using a manual folding tester (Shanghai Hengyi Precision Instrument Co., Ltd., HY-5000 model) until the film broke.
(3) Thermal stability test: A TG-DSC simultaneous thermal analyzer (STA 449 F3, NETZSCH, Germany) was used. Under a nitrogen atmosphere, the temperature was raised from 30 ℃ to 600 ℃ at a rate of 10 ℃/min. The thermal decomposition curve of the film was recorded to determine the initial thermal decomposition temperature (T₅%) and the temperature of the maximum thermal decomposition rate (Tₘₐₓ).
(4) Degradation performance test: The soil burial degradation method was adopted, referring to the GB/T 19277.1-2011 standard. Film samples (10 mm × 10 mm, with initial mass m₀) were buried in farmland soil, with soil moisture controlled at 60% and temperature at 25 ℃. The samples were retrieved at 30 days and 60 days respectively, washed with distilled water, vacuum dried, and weighed (mₜ). The degradation rate was calculated as follows: Degradation rate = (m₀ - mₜ) / m₀ × 100%.
(5) Microstructure characterization: The surface and cross-sectional morphology of the films were observed using a SU8010 scanning electron microscope (Hitachi, Japan) with an acceleration voltage of 5 kV. The samples were gold-sputtered. The interaction between the components of the films was analyzed using a Nicolet iS50 Fourier transform infrared spectrometer (Thermo Fisher Scientific, USA) in the range of 4000 - 400 cm⁻¹. The crystal structure of the films was analyzed using a D8 Advance X-ray diffractometer (Bruker, Germany) with a 2θ scanning range of 10° - 60° and a scanning rate of 8°/min.
3 Results and Discussion
3.1 Microstructure and Component Interaction of Thin Films
Figure 1 shows the SEM morphologies of pure PLA film and 1% CNC/PLA composite film. The surface of the pure PLA film is relatively smooth but has a few tiny pores (Figure 1a), which are formed during the solvent evaporation process and may affect its optical and mechanical properties. In contrast, the surface of the composite film with 1% CNC is denser and more uniform, with a significant reduction in pore numbers (Figure 1b). The cross-section shows a continuous matrix structure, and CNC is uniformly dispersed in the PLA matrix without obvious agglomeration (Figure 1d), indicating good compatibility between CNC and the PLA matrix. This good dispersion is attributed to the hydrogen bond interaction between the hydroxyl groups on the CNC surface and the ester groups in the PLA molecular chains, a conclusion that can be verified by FTIR spectroscopy (Figure 2). In the FTIR spectrum, the ester stretching vibration peak of pure PLA film at 1750 cm⁻¹ shifts to a lower wavenumber by 3 cm⁻¹ in the composite film, and a hydroxyl stretching vibration peak appears at 3350 cm⁻¹, with the peak intensity increasing with the addition of CNC. This confirms the intermolecular hydrogen bond interaction between CNC and PLA, which helps enhance the binding force between components and optimize the film properties.
The XRD analysis results (Figure 3) show that the pure PLA film has characteristic diffraction peaks at 2θ = 16.5°, 18.8° and 22.5°, corresponding to the α crystal structure of PLA. After adding CNC, the positions of the characteristic diffraction peaks of the composite film did not shift significantly, but the diffraction peak intensity increased. When the CNC addition amount was 1%, the crystallinity index increased from 35% of pure PLA to 42%. This indicates that CNC plays a role of heterogeneous nucleation in the PLA matrix, promoting the crystalline arrangement of PLA molecular chains, reducing the defects in the amorphous region, and thereby enhancing the structural compactness and performance stability of the film.
3.2 Optical Performance Regulation and Optimization
Transmittance and haze are the core performance indicators of optical films, directly determining their application potential in the optoelectronic field. Table 1 lists the optical performance parameters of the films under different CNC addition amounts. The pure PLA film has a transmittance of 80% and a haze of 2.5% at a wavelength of 600 nm. As the CNC addition amount increases, the transmittance of the film first rises and then drops, while the haze first decreases and then increases. When the CNC addition amount is 1%, the transmittance reaches a maximum of 95%, and the haze drops to 1.2%, which is better than the plant-based transparent film developed by Osaka University (transmittance of 70%) and close to the level of the high-transmittance PLA film of Teijin. The increase in transmittance is mainly attributed to the uniform dispersion of CNC and its heterogeneous nucleation effect: the uniform dispersion of CNC avoids local aggregation-induced light scattering, and the increase in crystallinity reduces the light refraction loss in the amorphous region. When the CNC addition amount exceeds 1%, some CNC aggregates, leading to enhanced light scattering, a decrease in transmittance, and an increase in haze.
In addition, the composite films prepared in this study exhibit excellent light transmittance in the wavelength range of 200 to 1500 nm (Figure 4). Specifically, the light transmittance in the ultraviolet region (200 to 380 nm) is above 85%, and in the infrared region (760 to 1500 nm), it remains above 90%. Meanwhile, the refractive index is 1.52, close to that of low refractive index glass (1.50). This characteristic gives it a significant advantage in scenarios such as short-wavelength ultraviolet processing in semiconductor manufacturing and pixel defect detection in liquid crystal displays, effectively reducing light reflection and refraction losses and enhancing detection accuracy and processing efficiency.
3.3 Mechanism for Improving Mechanical Properties and Thermal Stability
The hard and brittle nature of pure PLA film limits its practical application. The addition of CNC significantly improves the mechanical properties of the film (Table 2). The tensile strength of pure PLA film is 43.44 MPa, the elongation at break is 1.8%, and the folding times are 153. When the addition amount of CNC increases, the mechanical properties of the film first improve and then decline. When the addition amount of CNC is 1%, the tensile strength increases to 93.54 MPa, the elongation at break increases to 2.9%, and the folding times reach 502, which are increased by 115.3%, 57.8%, and 228% respectively compared with pure PLA. This improvement effect is better than that of PLA films modified by traditional inorganic fillers, mainly due to the excellent mechanical properties and good interfacial adhesion of CNC: CNC has a high specific surface area and high aspect ratio, which can construct a three-dimensional network structure in the PLA matrix, effectively transfer stress, and at the same time, the intermolecular hydrogen bonding enhances the adhesion between components and reduces stress concentration. When the addition amount of CNC exceeds 1%, the agglomeration phenomenon leads to an increase in interface defects, and the mechanical properties decline instead.
The results of the thermal stability test (Figure 5) show that the initial thermal decomposition temperature (T₅%) of pure PLA film is 304 ℃, and the maximum thermal decomposition rate temperature (Tₘₐₓ) is 378 ℃. After adding CNC, the thermal stability of the composite film is significantly improved. When the CNC addition amount is 2.5%, T₅% increases to 324 ℃ and Tₘₐₓ increases to 395 ℃. This is because CNC has good thermal stability, and the three-dimensional network structure formed in the PLA matrix can hinder the thermal motion of molecular chains and delay the thermal decomposition process. At the same time, the hydrogen bond interaction between CNC and PLA enhances the rigidity of the molecular chains, further improving the thermal stability. The initial thermal decomposition temperature of 324 ℃ can meet the processing temperature requirements of most optoelectronic device manufacturing processes (usually below 300 ℃), providing a temperature guarantee for the practical application of the film.
3.4 Environmental Compatibility and Degradation Performance Evaluation
The all-plant source characteristics endow the composite film with excellent environmental compatibility. The results of the soil burial degradation test (Table 3) show that the degradation rate of pure PLA film after 60 days of soil burial is 1.09%; after adding CNC, the degradation rate of the film significantly increases. When the CNC addition amount is 1%, the degradation rate reaches 3.83%, which is 252% higher than that of pure PLA. This is because the hydroxyl groups on the surface of CNC can promote the adhesion and growth of microorganisms, and the addition of CNC increases the porosity of the film, facilitating the entry of microorganisms and enzymes into the film interior and accelerating the degradation process of PLA. In addition, the degradation products are carbon dioxide and water, which can be absorbed and utilized by plants through photosynthesis, forming a natural carbon cycle and not causing pollution to the environment. Notably, even after 60 days of degradation, the film can still maintain a certain structural integrity, indicating that it has reliable performance stability during its service life and achieves a harmonious unity of "use performance" and "environmental friendliness".
4 Conclusion and Prospect
This study successfully prepared a fully plant-based composite optical film using corn-derived PLA as the matrix and CNC extracted from waste paper as the reinforcing phase through solution casting. The regulatory mechanism of CNC addition on the structure and performance of the film was systematically investigated, and the following core conclusions were drawn: (1) When the CNC addition was 1%, the film had the best comprehensive performance, with a light transmittance of 95% at a wavelength of 600 nm, a haze of 1.2%, a tensile strength of 93.54 MPa, an initial thermal decomposition temperature of 324 ℃, and a degradation rate of 3.83% after 60 days of soil burial;
(2) The hydrogen bond interaction between CNC and PLA and the heterogeneous nucleation of CNC are the key mechanisms for enhancing the optical properties, mechanical properties and thermal stability of films. (3) The composite film has good light transmittance in a wide wavelength range from ultraviolet to infrared, and its refractive index is close to that of low refractive index glass, thus having potential for application in optoelectronic fields such as semiconductor manufacturing and liquid crystal display.
The all-plant-based PLA optical film developed in this research has achieved the unification of full biomimetic raw materials, high-end performance in use and minimized environmental impact, providing a new material solution for the development of sustainable optoelectronics. Future research can further optimize the preparation process, such as combining biaxial stretching technology to enhance the dimensional stability of the film. Explore the composite of multiple plant-based components, such as the introduction of soy-based polymers and plant essential oils, to endow the film with multi-functional properties like antibacterial and antioxidant properties; At the same time, carry out research on large-scale production and long-term service performance, promote its practical application in the optoelectronics industry, and assist in the green transformation of the industry and the realization of the "carbon neutrality" goal.
