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epoxy composites with cellulose nanocrystals
Journal Pre-proof Synergistic improvement for mechanical, thermal and optical properties of PVA-co-PE nanofiber/epoxy composites with cellulose nanocrystals Mufang Li, Xu Zhao, Yingying Li, Wen Wang, Weibing Zhong, Mengying Luo, Ying Lu, Ke Liu, Qiongzhen Liu, Yuedan Wang, Dong Wang PII:
S0266-3538(19)33080-5
DOI:
https://doi.org/10.1016/j.compscitech.2020.107990
Reference:
CSTE 107990
To appear in:
Composites Science and Technology
Received Date: 4 November 2019 Revised Date:
25 December 2019
Accepted Date: 1 January 2020
Please cite this article as: Li M, Zhao X, Li Y, Wang W, Zhong W, Luo M, Lu Y, Liu K, Liu Q, Wang Y, Wang D, Synergistic improvement for mechanical, thermal and optical properties of PVA-co-PE nanofiber/epoxy composites with cellulose nanocrystals, Composites Science and Technology (2020), doi: https://doi.org/10.1016/j.compscitech.2020.107990. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Synergistic improvement for mechanical, thermal and optical properties of PVA-co-PE nanofiber/epoxy composites with cellulose nanocrystals Mufang Li,a, # Xu Zhao,a, # Yingying Li,a Wen Wang,b Weibing Zhong,b Mengying Luo,a Ying Lu,a Ke Liu,a Qiongzhen Liu,a Yuedan Wang,a Dong Wanga, b, * a Hubei Key Laboratory of Advanced Textile Materials & Application, Hubei International Scientific and Technological Cooperation Base of Intelligent Textile Materials & Application, Wuhan Textile University, Wuhan 430200, China b College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, China * Corresponding author: #
Dong Wang, E-mail: [email protected]
These authors (Mufang Li and Xu Zhao) contribute equally to this work
Abstract: The rapid development of optoelectronic devices puts forward higher requirements for flexible transparent films with good mechanical and thermal properties. To further improve the mechanical, thermal and optical properties of the PVA-co-PE nanofiber reinforced epoxy composite film, cellulose nanocrystals (CNCs) was selected as modifier to enhance the interfacial adhesion between the reinforced nanofibers and matrix. Benefiting from excellent hydrophily, high strength, transparency and ultra-low thermal expansion coefficient (CTE) of CNCs, dramatic improvement
in
tensile
CNCs/PVA-co-PE/epoxy
strength, composites
transparency were
and
observed
thermal over
pure
stability
of
PVA-co-PE
nanofiber/epoxy film. Typically, when the CNCs concentration was 3%, the transparency of CNCs/PVA-co-PE/epoxy film increased from 83.4% to 90.4%, the tensile strength increased from 11.4 MPa to 16.4 MPa, the CTE dropped from 33 to 10 ppm/K. Furthermore, as the reinforcement for matrix and the carrier for functional materials, the CNCs/PVA-co-PE nanofiber membrane was used to prepare the patterned photochromic and thermochromic film encapsulated with epoxy, indicating the potential application of CNCs/PVA-co-PE/epoxy composites in the light emitting devices. Keywords: cellulose nanocrystals; transparent; mechanical property; thermal stability;
composites
1. Introduction
As the demands for flexible optoelectronic devices increasing, light weight, flexible and transparent polymers for flexible transparent electrodes, display and packaging have received significant attention, such as polyethylene terephthalate (PET), poly-dimethyl siloxane (PDMS), polyimides (PI), epoxy and so on. [1-4] To ensure the performance, lifetime and reliability of the optoelectronic devices, the transparent polymers must be able to fulfill the machinal and thermal demands for various processing and applications. [5, 6] However, most of the plastic films possess higher coefficient of thermal expansion (CTE) of 50-200 ppm/K, which may cause the “heat death” (thermal runaway or heat destruction) of the flexible electronic devices. [7, 8] Besides, when the optically transparent polymers are used as structural materials, mechanical properties become extremely important. [9] Therefore, the development of transparent flexible films with synergistically improved mechanical properties and thermal stability are highly desirable and important. Lots of efforts have been made to improve the mechanical properties and CTE of polymers. The most used method is to prepare composites by integrating strong or highly thermal conductive fillers into the polymers, such as various strong whiskers, fibers or graphite, carbon nanotubes, metal and ceramic particles. However, traditional composites are rarely transparent due to the light scattering by the fillers embedded within the polymers. [10-13] Therefore, to improve the mechanical and thermal properties without sacrificing the transparency is still the biggest challenge
for flexible transparent polymers. To solve this problem, two strategies were adopted to obtain optically transparent composites. The first one was matching the refractive indices (RI) of matrix and fillers (±0.001) and the other was reducing the filler size below the critical scattering length (less than 200 nm). [14-19] Typically, the nanosized polymer fibers, such as cellulose, nylon-6 (PA 6), polyacrylonitrile (PAN) and various cellulose nanofibers, could be used as the reinforcement to prepare optically transparent composites. [20-26] However, despite so many researches, flexible composite films with synergistically improved mechanical, thermal and optical properties were rarely reported. [27-30] In our previous study, the transparent PVA-co-PE nanofiber reinforced epoxy composites were prepared. [31] PVA-co-PE is a semi-crystalline random copolymer with good hydrophilic, excellent barrier property and low CTE (3.634 ppm/K). Moreover, the PVA-co-PE nanofiber membrane was prepared via a high throughput method of melting blending invented in our group. It was composed of numerous short nanofibers and the structure was denser than that prepared from electrospinning. However, the increase in nanofiber percentage affected the transparency of composites seriously. To solve this problem, cellulose nanocrystals (CNCs) were selected in this study to modify the PVA-co-PE nanofibers by enhancing the interfacial adhesion between the nanofibers and epoxy. CNCs are attractive as reinforcing fillers due to their advantages of high specific area, high strength, transparency, renewable and biodegradable abilities. The mechanical and thermal properties of CNCs are seriously dependent on the direction
relative to the cellulose crystalline structure. The average elastic modulus value of CNCs in the axial direction is around 130 GPa, while the CTE of CNCs in the axial direction is as small as 0.1 ppm/K. [32-34] Therefore, the mechanical, thermal and optical properties of CNCs/PVA-co-PE/epoxy composites were synergistically improved compared with the pure PVA-co-PE/epoxy composites by combining the CNCs with PVA-co-PE nanofibers as reinforcement. Meanwhile, the existence of flexible nanofibers may minish the drawback of low strain caused by the rigid CNCs. Moreover, the transparent CNCs/PVA-co-PE/epoxy composite films could be fabricated in a largescale, providing a basic material for the development of flexible optoelectronic devices. 2. Experimental 2.1 Materials Poly
(vinyl
alcohol-co-ethylene)
(PVA-co-PE)
was
purchased
from
Sigma-Aldrich (Milwaukee, WI, USA). Cellulose acetate butyrate (CAB) was purchased from Eastman Chemical Company. Cellulose nanocrystals (CNCs) was purchased from NingBo EneRol Nanotechnologies, Inc.. The bisphenol-A type epoxy resin was provided by Aladdin, China (E44, Hexion, USA). Polyether amine was used as the curing agent and was purchased from Aladdin, China (D-400, Hexion, USA). The thermochromic and photochromic powders were provided by the Shenzhen Qiansebian New Materials Technology Co., Ltd. The ethyl alcohol, acetone and isopropanol were purchased from Aladdin, China. 2.2 Preparation of CNC/PVA-co-PE composite nanofiber membrane
The PVA-co-PE nanofibers were prepared based on the melting blending method invented in our group. [31] To prepared the CNCs/PVA-co-PE composite nanofiber membrane (NM), different composition of CNCs were added into the PVA-co-PE suspension. Then the CNCs/PVA-co-PE blended suspension was coated on the substrate. After the drying and peeling processes, the CNCs/PVA-co-PE composite NM was obtained. For the pure PVA-co-PE/epoxy composite, the weight percentage of PVA-co-PE nanofibers was 35%. Based on the weight percentage of CNCs to nanofibers, the composite membranes were defined as 0%, 1%, 2%, 3%, 5%. The fabrication process was shown in Figure 1. 2.3 preparation of transparent CNCs/PVA-co-PE/epoxy composite film The prepared CNCs/PVA-co-PE NM was immersed into the epoxy resin solution containing the curing agent of polyamide (50 wt%). After the epoxy resin impregnated the CNCs/PVA-co-PE NM, the originally white membrane became transparent, as shown in Figure 1. Then the CNCs/PVA-co-PE/epoxy composite film was taken out from the solution and was cured at 90 °C for 4 h to obtain the transparent CNCs/PVA-co-PE/epoxy composite film. All the samples were cut to 4 cm*5 cm for further characterization.
Figure 1 Fabrication process of CNCs/PVA-co-PE/epoxy composite film 2.4 Preparation of the flexible transparent thermochromic and photochromic film The CNCs/PVA-co-PE NM was used mainly to reinforce the mechanical and thermal properties of epoxy in this study. Meanwhile it could be used as support layer for the thermochromic and photochromic materials as well. Firstly, the thermochromic and photochromic powders were dispersed in the epoxy resin solution containing the curing agent and then were coated onto the surface of CNCs/PVA-co-PE NM by the stencil printing process. After curing at room temperature for 12 h, the CNCs/PVA-co-PE NM containing the patterned thermochromic and photochromic materials were immersed into the epoxy resin solution containing the curing agent of polyamide (50 wt%). After the epoxy resin impregnated the CNCs/PVA-co-PE NM, the composite was taken out from the solution and was cured at 90 °C for 4 h to obtain the transparent thermochromic and photochromic film. 2.5 Characterization The
morphology
and
structure
of
the
CNCs/PVA-co-PE
NM
and
CNCs/PVA-co-PE/epoxy composite films were analyzed by the FESEM (7800F, JEOL, Japan). The hydrophilicity of CNCs/PVA-co-PE NMs were evaluated by the
contact angle goniometry (KRUSS DSA30S, KRUSS Co., Germany). The thermal properties of CNCs/PVA-co-PE/epoxy composites were analyzed by thermo gravimetric analyzer (TG 209 F3, Netzsch). The Instron universal testing machine (Instron 5970, Instron Co., America) was used to evaluate the mechanical properties of CNCs/PVA-co-PE/epoxy composite films. The light transmittance curves at wavelengths of 300-800 nm were measured with a UV-vis spectrometer (UV2700, Shimadzu, Japan). The thermal stability of the composite films was analyzed by keeping the samples in the ovens at different temperature for 30 mins. The CTE of samples with 24 mm long and 3 mm wide were evaluated by a thermomechanical analyzer (TMA Q400, TA Instruments, America). The measurements were carried out two times from 25 °C to 100 °C with a temperature rate of 10
/min. Typically, the
CTE was calculated by the following equation (1). α
∆ ∆
(1)
α is the coefficient of thermal expansion, L0 is the original length, ∆L is change in length due to thermal expansion, ∆T is the temperature gradient. 3. Results and discussion
Figure 2 FESEM images of CNCs/PVA-co-PE NM with the CNCs concentration of (a) 0% (b) 1% (c) 2% (d) 3% (5) 5% and their corresponding water contact angles The physical and chemical structures of reinforcements are the most important parameters that affect the interfacial adhesion and properties of the composites. In this study, the CNCs were used as the interfacial modifier to modify the physical and chemical structures of PVA-co-PE nanofibers. The water contact angle of 100% CNCs membrane is 21.6 °, as shown in Figure S1. The excellent hydrophily of CNCs membrane is mainly attributed to the abundant hydroxyl in the cellulose macromolecular. Due to the hydrogen bond between the hydroxy groups of CNCs and PVA-co-PE, the CNCs mainly adhered on the surface of PVA-co-PE nanofibers forming the CNCs/PVA-co-PE NM, as shown in Figure 1. The morphologies of CNCs/PVA-co-PE NMs with different CNCs concentrations and the corresponding water contact angles were given in Figure 2. The diameter of PVA-co-PE nanofibers nearly unchanged before and after the CNCs modification. The surface of CNCs/PVA-co-PE composite nanofibers seemed rougher than the pure PVA-co-PE as the CNCs adhesion. Correspondingly, the water contact angle also decreased with the increase in the CNCs concentration. Although the PVA-co-PE nanofiber membrane is hydrophilic with a water contact angle of 56.9° due to the existence of abundant hydroxyl in the vinyl alcohol segment, the hydrophily of CNCs is much better than PVA-co-PE. With the CNCs concentration increased from 1% to 2%, 3% and 5%, the water contact angles dropped gradually from 51.3° to 46.5°, 42.0° and 35.0°. Due to the contact angle could be affected by both the surface chemical composition and
morphology
of
nanofiber
membrane,
so
the
enhanced
hydrophily
of
CNCs/PVA-co-PE NM with the increase of CNCs concentration was mainly attributed to the increased concentration of CNCs and the larger roughness of CNCs/PVA-co-PE NM. [35, 36] Moreover, the better hydrophily is beneficial for enhancing the interfacial adhesion between the reinforcement and matrix.
Figure 3 FESEM images of cross section of CNCs/PVA-co-PE/Epoxy composite membrane with the CNCs concentration of (a) 0% (b) 1% (c) 3% (d) 5% To characterize the interfacial adhesion, the cross section images of CNCs/PVA-co-PE/epoxy composites were given in Figure 3. From the full view of cross section in Figure 3 (a1-d1), we could see that the CNCs/PVA-co-PE reinforcements distributed uniformly in the matrix, forming a complete coverage of nanofiber structures by the epoxy matrix. [37] Besides, an ultrathin pure epoxy superficial layer could be observed on both sides of the composite films, which were mainly caused by the adhesion of matrix after the casting process. The interface between the nanofibers and matrix could be investigated in the magnified Figure 3 (a2-d2). Due to the good hydrophily of CNCs/PVA-co-PE NM, all the
CNCs/PVA-co-PE/epoxy composites with different CNCs concentration exhibited better interface morphology, no obvious interface debonding occurred on the cross-sections, demonstrating that the adhesion between the nanofibers and matrix was very strong. Moreover, the interfacial boundary became more unsharp with the increase of CNCs concentration, indicating the introduce of CNCs on the surface of PVA-co-PE nanofibers enhanced the interfacial adhesion. It was mainly attributed to the increased hydrophily and surface areas after the CNCs adsorption.
Figure 4 (a) Stress-strain curves of CNCs/PVA-co-PE/epoxy composites with different CNCs concentrations, photographs of (b) transparence and (c) flexibility of CNCs/PVA-co-PE/epoxy composite films with 3 % CNCs. The mechanical properties of composites were significantly depended on the interactions between fillers and polymer matrix. In this study, CNCs were used as the interfacial modifier and the effect of CNCs concentration on the tensile stress and strain of CNCs/PVA-co-PE/epoxy composites was investigated. As shown in Figure 4 (a), compared with the pure PVA-co-PE reinforced epoxy composite, the tensile strength of all the CNCs/PVA-co-PE/epoxy composites increased. When the CNCs
concentration was 2%, the tensile strength of CNCs/PVA-co-PE/epoxy composite was highest, nearly 1.7 times of pure PVA-co-PE/epoxy composite. This was because the CNCs
adsorbed
on
the
surface
of
PVA-co-PE
nanofibers,
forming
the
nanofiber-CNCs-epoxy interaction in the micro-region of interface. The better hydrophily and larger surface areas of CNCs increased the efficiency of stress transfer between the reinforcement and matrix, thus leading to the high increase in mechanical properties. [38-41] However, the strain of CNCs/PVA-co-PE/epoxy composites decreased a lot with the increase of CNCs concentration, this was mainly attributed to the higher stiffness of CNCs. So the tensile strength of CNCs/PVA-co-PE/epoxy composites started to decline when the CNCs concentration was higher than 2%. Anyhow, the tensile strength of all the CNCs/PVA-co-PE/epoxy films were much higher than that of pure PVA-co-PE/epoxy film. The photograph in Figure 4 (b) and (c) demonstrated
further
the
excellent
transparence
and
flexibility
of
CNCs/PVA-co-PE/epoxy composites with 3% CNCs concentration. The chemical structures of PVA-co-PE, CNCs and epoxy and the mechanism of the enhanced interfacial adhesion were characterized and given in Figure 5. To CNCs, the peaks at 2900 cm-1 corresponded to the C–H stretching vibration. The strong peak at 1020 cm-1 was assigned to the C–O–C pyranose ring skeletal vibration of cellulose and the characteristic peaks at 892 cm-1 was attributed to the b-glycosidic linkages between the sugar units. [42] To epoxy, the absorption peak at 1606 and 1507 cm-1 represent to the stretching vibration stretching peak of carbon skeleton of benzene. The absorption peaks at 1294 and 1242 cm-1 were attributed to the C-N deformation
vibration peak and the aromatic ether deformation vibration peak. The split peaks at 1085 and 1032 cm-1 correspond to the deformation vibration peak of fatty ether. Notably, the unreacted epoxy groups appear at 923 cm-1, indicating the curing process of epoxy. [43] Besides, a strong absorption band at 3300 cm−1 could be found in all the three components of PVA-co-PE, CNCs and epoxy, representing the stretching vibration of OH groups. Therefore, the hydrogen bond generated from the interaction between the OH groups of PVA-co-PE and CNCs, CNCs and epoxy enhanced the interfacial adhesion between the reinforcement and matrix, as shown in Figure 5 (c).
Figure 5 (a) the chemical structures and (b) corresponding FTIR spectra of PVA-co-PE, CNCs and epoxy (c) scheme of structure of transparent CNCs/nanofiber/epoxy film and mechanism of the enhanced interfacial adhesion
Figure 6 (a) Transmittance and (b-f) photographs of CNCs/PVA-co-PE/Epoxy composite membranes with different CNCs concentration The category, diameter, orientation and composition of fibers are key parameters that affect the transmittance of fiber reinforced composites. [19, 26] Typically, nanofiber reinforcements possessed the most similar RI with matrix, smaller size, parallel fiber orientation and fewer composition are perceived as beneficial to the optical transmission due to fewer light scattering. In this study, the RI of the PVA-co-PE nanofibrous membrane and epoxy matrix are 1.62 and 1.53, respectively. Therefore, the high optical transparency of the CNCs/PVA-co-PE/epoxy composites is mainly attributed to the size effect of nanofibers. [27] As shown in Figure 6, the wavelength used to express the single transmittance for samples was 600 nm and the thicknesses for each sample measuring transmittance in Figure 6 (b-f) were 91, 90, 93, 90, 92 µm, respectively. In Figure 6 (a), the transmittance of pure PVA-co-PE/Epoxy composites reduced to 83.4% from 92.5% of pure epoxy, indicating that the addition of PVA-co-PE nanofiber affected the transparence of matrix seriously. After the
modification of PVA-co-PE nanofibers with CNCs, the transmittance of CNCs/PVA-co-PE/epoxy composites increased obviously and more CNCs percentage lead to larger transmittance. This was mainly ascribed to the enhanced interfacial adhesion and the better transparence of CNCs. When the CNCs concentration is 5%, the transmittance of CNCs/PVA-co-PE/epoxy is 91.2%, only 1.3% lower than that of pure epoxy. The optical photographs of CNCs/PVA-co-PE/epoxy films with CNCs concentration of 0, 1%, 2%, 3%, 5% were given in Figure 6 (b-f). All of the composite films are transparent and no obvious difference to the naked eye, demonstrating the perfect transparence of CNCs/PVA-co-PE/epoxy films.
Figure 7 TG and DTG curves of neat epoxy, PVA-co-PE, CNCs and CNCs/PVA-co-PE/epoxy composites with different CNCs concentration The
TG
and
DTG
curves
of
neat
PVA-co-PE,
epoxy,
CNCs
and
CNCs/PVA-co-PE/epoxy composites were given in Figure 7. For the neat matrix of epoxy, the peak value of mass loss rate appears in the range of 320-473 °C and the largest degradation temperatures is 354 °C, corresponding to the decomposition of epoxy. For the pure PVA-co-PE nanofiber, the onset temperature of degradation is
346 °C. Two peaks are observed on the DTG curve as PVA-co-PE is copolymer of PVA and PE. The first peak is in the range of 250-380 °C, corresponding to the degradation of vinyl alcohol segment in the copolymer. The second peak appears in the range of 390-500 °C, derived from the degradation of ethylene unit in the copolymer. The significant weight loss of CNCs appears in the range of 232-400 °C. [42] After the composition, both the degradation peaks of epoxy and PVA-co-PE could be observed in the CNCs/PVA-co-PE/epoxy composites, but the degradation peak of CNC disappeared. It is mainly attributed to the combination of CNCs with PVA-co-PE and epoxy by hydrogen bond, so the degradation peak of CNCs may be overlapped with that of epoxy and PVA-co-PE. Besides, the largest degradation temperature for the CNCs/PVA-co-PE/epoxy composites moves to higher value compared with the pure PVA-co-PE reinforced epoxy, and the tendency become more obvious with the increase of CNCs concentration, indicating the enhanced thermal stability of CNCs/PVA-co-PE/epoxy composites. This is also caused by the enhanced interaction between the nanofibers and matrix after the introduction of CNCs on the interface. [44]
Figure 8 (a) CTE of epoxy and CNCs/PVA-co-PE/epoxy composite films with
different CNCs concentration, (b) thermal stability test of CNCs/PVA-co-PE/epoxy composites with CNCs concentration of 3% at different temperature for 30 mins The CTE of transparent polymer film is extremely important for ensuring the safety and reliability of optical devices. Benefit from the excellent thermal stability of CNCs (with CTE as low as 0.1 ppm/K), the CNCs/PVA-co-PE/epoxy composite films exhibited distinctly lower CTE value compared with the pure PVA-co-PE/epoxy film. As shown in Figure 8, the CTE value for epoxy and pure PVA-co-PE/epoxy film are 62 and 33 ppm/K, respectively. The thermal stability is improved obviously after the addition of PVA-co-PE nanofibers, but it is not yet better enough for application in light emitting devices cause the required CTE level is less than 20 ppm/K. Although further increasing the nanofiber concentration may reduce the CTE value, but the transparency decreases simultaneously. This problem was solved by combination of CNCs with PVA-co-PE. The CTE value became smaller with the increase of CNCs concentration. When the CNCs concentrations are 1%, 2%, 3%, 5%, the CTE values are 26, 15, 10, 6 ppm/K respectively. This mainly ascribes to the ultra-low CTE value of CNCs and the enhanced interface adhesion between the matrix and reinforcement, realizing the efficient transfer of thermal stress from the matrix to reinforcements and restricting the thermal expansion of the matrix. [28, 45] Thermal stability testing further demonstrated the above conclusion, as shown in Figure 6 (b). The CNCs/PVA-co-PE/epoxy composite film was put into the oven at different temperature for 30 mins. It exhibited good thermal stability below 120 °C, which was much higher than the normally used polyethylene terephthalate (PET) substrate of
100 °C. [8]
Figure 9 The transparent (a) photochromic and (b) thermochromic membrane, (c, d) the corresponding response time and repeatability To develop the application of the reinforced transparent composite film in the light emitting devices, the photochromic and thermochromic membranes were prepared and given in Figure 9. In this study, the CNCs/PVA-co-PE NM is the reinforcement for epoxy matrix, and also it can be used as carrier for the luminescent materials. As shown in Figure 9 (a) and (b), we coated the photochromic and thermochromic materials on the surface of CNCs/PVA-co-PE NM respectively by the stencil printing method, then impregnated them into the epoxy resin solution containing the curing agent. After the curing process, the luminescent materials were fully encapsulated by the epoxy, preventing them from oxidation and abrasion. Meanwhile, the originally
opaque CNCs/PVA-co-PE NM substrate became transparent after the impregnation, making the luminescent pattern more distinguished and clearer. Under different light and temperature environment, the photochromic and thermochromic film changed color repeatedly, demonstrating their potential application as the luminescent devices. 4. Conclusion The CNCs/PVA-co-PE/epoxy composite membranes with different CNCs concentration were prepared in this study. The CNCs were used as modifier to enhance the interfacial adhesion between the PVA-co-PE nanofibers and epoxy matrix. Benefiting from the perfect hydrophily of CNCs, the hydrophily of CNCs/PVA-co-PE composite NM increased with the increase in the CNCs concentration. The FESEM demonstrated the enhanced interfacial adhesion between the CNCs/PVA-co-PE nanofibers and matrix. Due to the high strength, transparency and ultra-low CTE of CNCs and the enhanced interfacial adhesion, the tensile strength, thermal stability and transparence of CNCs/PVA-co-PE/epoxy composites increased obviously after the introduction of CNCs. When the CNCs concentration is 3%, the tensile strength of CNCs/PVA-co-PE/epoxy film is 16.4 MPa, the CTE is 10ppm/K, the transmittance is 90.4%, distinctly higher than that of pure PVA-co-PE/epoxy film whose tensile strength is 11.4 MPa, CTE is 33 × 10−6 K−1, transmittance is 83.4%. As the reinforcement for epoxy matrix and the carrier for the luminescent materials, the patterned photochromic and thermochromic films were prepared and demonstrated repeatable color change properties under different light and temperature environment, indicating their potential applications as optical devices.
Acknowledgements This work was supported by the Research Projects of Education Department of Hubei Province (Q20181712), National Nature Science Foundation (51873165, 51873166),
Program of Hubei Technology Innovation – International Collaboration
(2017AHB065) and Applied Fundamental Research Program of Wuhan Science and Technology Bureau (2017060201010165). We also thank “Wuhan Engineering Technology Research Center for Advanced Fibers” providing partial support for materials processing.
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Highlights: The transparent CNCs/PVA-co-PE/epoxy film with enhanced mechanical and thermal properties was prepared. The interfacial adhesion between the PVA-co-PE nanofibers and epoxy matrix was enhanced by introduction of CNCs. The CNCs/PVA-co-PE NM is not only the reinforcement for matrix but also the carrier for functional materials The photochromic and thermochromic films with repeatable color change properties were prepared
AUTHOR CONTRIBUTIONS Mufang Li performed writing – original draft, methodology, conceptualization, supervision and funding acquisition Xu Zhao performed writing– original draft, methodology, visualization and investigation Yingying Li performed methodology and visualization Wen Wang performed methodology and resources Weibing Zhong performed visualization Mengying Luo performed resources Ying Lu performed methodology Ke Liu performed investigation Qiongzhen Liu performed investigation Yuedan Wang performed visualization Dong Wang performed conceptualization, writing - review & editing, supervision and funding acquisition.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S0266-3538(19)33080-5
DOI:
https://doi.org/10.1016/j.compscitech.2020.107990
Reference:
CSTE 107990
To appear in:
Composites Science and Technology
Received Date: 4 November 2019 Revised Date:
25 December 2019
Accepted Date: 1 January 2020
Please cite this article as: Li M, Zhao X, Li Y, Wang W, Zhong W, Luo M, Lu Y, Liu K, Liu Q, Wang Y, Wang D, Synergistic improvement for mechanical, thermal and optical properties of PVA-co-PE nanofiber/epoxy composites with cellulose nanocrystals, Composites Science and Technology (2020), doi: https://doi.org/10.1016/j.compscitech.2020.107990. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Synergistic improvement for mechanical, thermal and optical properties of PVA-co-PE nanofiber/epoxy composites with cellulose nanocrystals Mufang Li,a, # Xu Zhao,a, # Yingying Li,a Wen Wang,b Weibing Zhong,b Mengying Luo,a Ying Lu,a Ke Liu,a Qiongzhen Liu,a Yuedan Wang,a Dong Wanga, b, * a Hubei Key Laboratory of Advanced Textile Materials & Application, Hubei International Scientific and Technological Cooperation Base of Intelligent Textile Materials & Application, Wuhan Textile University, Wuhan 430200, China b College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, China * Corresponding author: #
Dong Wang, E-mail: [email protected]
These authors (Mufang Li and Xu Zhao) contribute equally to this work
Abstract: The rapid development of optoelectronic devices puts forward higher requirements for flexible transparent films with good mechanical and thermal properties. To further improve the mechanical, thermal and optical properties of the PVA-co-PE nanofiber reinforced epoxy composite film, cellulose nanocrystals (CNCs) was selected as modifier to enhance the interfacial adhesion between the reinforced nanofibers and matrix. Benefiting from excellent hydrophily, high strength, transparency and ultra-low thermal expansion coefficient (CTE) of CNCs, dramatic improvement
in
tensile
CNCs/PVA-co-PE/epoxy
strength, composites
transparency were
and
observed
thermal over
pure
stability
of
PVA-co-PE
nanofiber/epoxy film. Typically, when the CNCs concentration was 3%, the transparency of CNCs/PVA-co-PE/epoxy film increased from 83.4% to 90.4%, the tensile strength increased from 11.4 MPa to 16.4 MPa, the CTE dropped from 33 to 10 ppm/K. Furthermore, as the reinforcement for matrix and the carrier for functional materials, the CNCs/PVA-co-PE nanofiber membrane was used to prepare the patterned photochromic and thermochromic film encapsulated with epoxy, indicating the potential application of CNCs/PVA-co-PE/epoxy composites in the light emitting devices. Keywords: cellulose nanocrystals; transparent; mechanical property; thermal stability;
composites
1. Introduction
As the demands for flexible optoelectronic devices increasing, light weight, flexible and transparent polymers for flexible transparent electrodes, display and packaging have received significant attention, such as polyethylene terephthalate (PET), poly-dimethyl siloxane (PDMS), polyimides (PI), epoxy and so on. [1-4] To ensure the performance, lifetime and reliability of the optoelectronic devices, the transparent polymers must be able to fulfill the machinal and thermal demands for various processing and applications. [5, 6] However, most of the plastic films possess higher coefficient of thermal expansion (CTE) of 50-200 ppm/K, which may cause the “heat death” (thermal runaway or heat destruction) of the flexible electronic devices. [7, 8] Besides, when the optically transparent polymers are used as structural materials, mechanical properties become extremely important. [9] Therefore, the development of transparent flexible films with synergistically improved mechanical properties and thermal stability are highly desirable and important. Lots of efforts have been made to improve the mechanical properties and CTE of polymers. The most used method is to prepare composites by integrating strong or highly thermal conductive fillers into the polymers, such as various strong whiskers, fibers or graphite, carbon nanotubes, metal and ceramic particles. However, traditional composites are rarely transparent due to the light scattering by the fillers embedded within the polymers. [10-13] Therefore, to improve the mechanical and thermal properties without sacrificing the transparency is still the biggest challenge
for flexible transparent polymers. To solve this problem, two strategies were adopted to obtain optically transparent composites. The first one was matching the refractive indices (RI) of matrix and fillers (±0.001) and the other was reducing the filler size below the critical scattering length (less than 200 nm). [14-19] Typically, the nanosized polymer fibers, such as cellulose, nylon-6 (PA 6), polyacrylonitrile (PAN) and various cellulose nanofibers, could be used as the reinforcement to prepare optically transparent composites. [20-26] However, despite so many researches, flexible composite films with synergistically improved mechanical, thermal and optical properties were rarely reported. [27-30] In our previous study, the transparent PVA-co-PE nanofiber reinforced epoxy composites were prepared. [31] PVA-co-PE is a semi-crystalline random copolymer with good hydrophilic, excellent barrier property and low CTE (3.634 ppm/K). Moreover, the PVA-co-PE nanofiber membrane was prepared via a high throughput method of melting blending invented in our group. It was composed of numerous short nanofibers and the structure was denser than that prepared from electrospinning. However, the increase in nanofiber percentage affected the transparency of composites seriously. To solve this problem, cellulose nanocrystals (CNCs) were selected in this study to modify the PVA-co-PE nanofibers by enhancing the interfacial adhesion between the nanofibers and epoxy. CNCs are attractive as reinforcing fillers due to their advantages of high specific area, high strength, transparency, renewable and biodegradable abilities. The mechanical and thermal properties of CNCs are seriously dependent on the direction
relative to the cellulose crystalline structure. The average elastic modulus value of CNCs in the axial direction is around 130 GPa, while the CTE of CNCs in the axial direction is as small as 0.1 ppm/K. [32-34] Therefore, the mechanical, thermal and optical properties of CNCs/PVA-co-PE/epoxy composites were synergistically improved compared with the pure PVA-co-PE/epoxy composites by combining the CNCs with PVA-co-PE nanofibers as reinforcement. Meanwhile, the existence of flexible nanofibers may minish the drawback of low strain caused by the rigid CNCs. Moreover, the transparent CNCs/PVA-co-PE/epoxy composite films could be fabricated in a largescale, providing a basic material for the development of flexible optoelectronic devices. 2. Experimental 2.1 Materials Poly
(vinyl
alcohol-co-ethylene)
(PVA-co-PE)
was
purchased
from
Sigma-Aldrich (Milwaukee, WI, USA). Cellulose acetate butyrate (CAB) was purchased from Eastman Chemical Company. Cellulose nanocrystals (CNCs) was purchased from NingBo EneRol Nanotechnologies, Inc.. The bisphenol-A type epoxy resin was provided by Aladdin, China (E44, Hexion, USA). Polyether amine was used as the curing agent and was purchased from Aladdin, China (D-400, Hexion, USA). The thermochromic and photochromic powders were provided by the Shenzhen Qiansebian New Materials Technology Co., Ltd. The ethyl alcohol, acetone and isopropanol were purchased from Aladdin, China. 2.2 Preparation of CNC/PVA-co-PE composite nanofiber membrane
The PVA-co-PE nanofibers were prepared based on the melting blending method invented in our group. [31] To prepared the CNCs/PVA-co-PE composite nanofiber membrane (NM), different composition of CNCs were added into the PVA-co-PE suspension. Then the CNCs/PVA-co-PE blended suspension was coated on the substrate. After the drying and peeling processes, the CNCs/PVA-co-PE composite NM was obtained. For the pure PVA-co-PE/epoxy composite, the weight percentage of PVA-co-PE nanofibers was 35%. Based on the weight percentage of CNCs to nanofibers, the composite membranes were defined as 0%, 1%, 2%, 3%, 5%. The fabrication process was shown in Figure 1. 2.3 preparation of transparent CNCs/PVA-co-PE/epoxy composite film The prepared CNCs/PVA-co-PE NM was immersed into the epoxy resin solution containing the curing agent of polyamide (50 wt%). After the epoxy resin impregnated the CNCs/PVA-co-PE NM, the originally white membrane became transparent, as shown in Figure 1. Then the CNCs/PVA-co-PE/epoxy composite film was taken out from the solution and was cured at 90 °C for 4 h to obtain the transparent CNCs/PVA-co-PE/epoxy composite film. All the samples were cut to 4 cm*5 cm for further characterization.
Figure 1 Fabrication process of CNCs/PVA-co-PE/epoxy composite film 2.4 Preparation of the flexible transparent thermochromic and photochromic film The CNCs/PVA-co-PE NM was used mainly to reinforce the mechanical and thermal properties of epoxy in this study. Meanwhile it could be used as support layer for the thermochromic and photochromic materials as well. Firstly, the thermochromic and photochromic powders were dispersed in the epoxy resin solution containing the curing agent and then were coated onto the surface of CNCs/PVA-co-PE NM by the stencil printing process. After curing at room temperature for 12 h, the CNCs/PVA-co-PE NM containing the patterned thermochromic and photochromic materials were immersed into the epoxy resin solution containing the curing agent of polyamide (50 wt%). After the epoxy resin impregnated the CNCs/PVA-co-PE NM, the composite was taken out from the solution and was cured at 90 °C for 4 h to obtain the transparent thermochromic and photochromic film. 2.5 Characterization The
morphology
and
structure
of
the
CNCs/PVA-co-PE
NM
and
CNCs/PVA-co-PE/epoxy composite films were analyzed by the FESEM (7800F, JEOL, Japan). The hydrophilicity of CNCs/PVA-co-PE NMs were evaluated by the
contact angle goniometry (KRUSS DSA30S, KRUSS Co., Germany). The thermal properties of CNCs/PVA-co-PE/epoxy composites were analyzed by thermo gravimetric analyzer (TG 209 F3, Netzsch). The Instron universal testing machine (Instron 5970, Instron Co., America) was used to evaluate the mechanical properties of CNCs/PVA-co-PE/epoxy composite films. The light transmittance curves at wavelengths of 300-800 nm were measured with a UV-vis spectrometer (UV2700, Shimadzu, Japan). The thermal stability of the composite films was analyzed by keeping the samples in the ovens at different temperature for 30 mins. The CTE of samples with 24 mm long and 3 mm wide were evaluated by a thermomechanical analyzer (TMA Q400, TA Instruments, America). The measurements were carried out two times from 25 °C to 100 °C with a temperature rate of 10
/min. Typically, the
CTE was calculated by the following equation (1). α
∆ ∆
(1)
α is the coefficient of thermal expansion, L0 is the original length, ∆L is change in length due to thermal expansion, ∆T is the temperature gradient. 3. Results and discussion
Figure 2 FESEM images of CNCs/PVA-co-PE NM with the CNCs concentration of (a) 0% (b) 1% (c) 2% (d) 3% (5) 5% and their corresponding water contact angles The physical and chemical structures of reinforcements are the most important parameters that affect the interfacial adhesion and properties of the composites. In this study, the CNCs were used as the interfacial modifier to modify the physical and chemical structures of PVA-co-PE nanofibers. The water contact angle of 100% CNCs membrane is 21.6 °, as shown in Figure S1. The excellent hydrophily of CNCs membrane is mainly attributed to the abundant hydroxyl in the cellulose macromolecular. Due to the hydrogen bond between the hydroxy groups of CNCs and PVA-co-PE, the CNCs mainly adhered on the surface of PVA-co-PE nanofibers forming the CNCs/PVA-co-PE NM, as shown in Figure 1. The morphologies of CNCs/PVA-co-PE NMs with different CNCs concentrations and the corresponding water contact angles were given in Figure 2. The diameter of PVA-co-PE nanofibers nearly unchanged before and after the CNCs modification. The surface of CNCs/PVA-co-PE composite nanofibers seemed rougher than the pure PVA-co-PE as the CNCs adhesion. Correspondingly, the water contact angle also decreased with the increase in the CNCs concentration. Although the PVA-co-PE nanofiber membrane is hydrophilic with a water contact angle of 56.9° due to the existence of abundant hydroxyl in the vinyl alcohol segment, the hydrophily of CNCs is much better than PVA-co-PE. With the CNCs concentration increased from 1% to 2%, 3% and 5%, the water contact angles dropped gradually from 51.3° to 46.5°, 42.0° and 35.0°. Due to the contact angle could be affected by both the surface chemical composition and
morphology
of
nanofiber
membrane,
so
the
enhanced
hydrophily
of
CNCs/PVA-co-PE NM with the increase of CNCs concentration was mainly attributed to the increased concentration of CNCs and the larger roughness of CNCs/PVA-co-PE NM. [35, 36] Moreover, the better hydrophily is beneficial for enhancing the interfacial adhesion between the reinforcement and matrix.
Figure 3 FESEM images of cross section of CNCs/PVA-co-PE/Epoxy composite membrane with the CNCs concentration of (a) 0% (b) 1% (c) 3% (d) 5% To characterize the interfacial adhesion, the cross section images of CNCs/PVA-co-PE/epoxy composites were given in Figure 3. From the full view of cross section in Figure 3 (a1-d1), we could see that the CNCs/PVA-co-PE reinforcements distributed uniformly in the matrix, forming a complete coverage of nanofiber structures by the epoxy matrix. [37] Besides, an ultrathin pure epoxy superficial layer could be observed on both sides of the composite films, which were mainly caused by the adhesion of matrix after the casting process. The interface between the nanofibers and matrix could be investigated in the magnified Figure 3 (a2-d2). Due to the good hydrophily of CNCs/PVA-co-PE NM, all the
CNCs/PVA-co-PE/epoxy composites with different CNCs concentration exhibited better interface morphology, no obvious interface debonding occurred on the cross-sections, demonstrating that the adhesion between the nanofibers and matrix was very strong. Moreover, the interfacial boundary became more unsharp with the increase of CNCs concentration, indicating the introduce of CNCs on the surface of PVA-co-PE nanofibers enhanced the interfacial adhesion. It was mainly attributed to the increased hydrophily and surface areas after the CNCs adsorption.
Figure 4 (a) Stress-strain curves of CNCs/PVA-co-PE/epoxy composites with different CNCs concentrations, photographs of (b) transparence and (c) flexibility of CNCs/PVA-co-PE/epoxy composite films with 3 % CNCs. The mechanical properties of composites were significantly depended on the interactions between fillers and polymer matrix. In this study, CNCs were used as the interfacial modifier and the effect of CNCs concentration on the tensile stress and strain of CNCs/PVA-co-PE/epoxy composites was investigated. As shown in Figure 4 (a), compared with the pure PVA-co-PE reinforced epoxy composite, the tensile strength of all the CNCs/PVA-co-PE/epoxy composites increased. When the CNCs
concentration was 2%, the tensile strength of CNCs/PVA-co-PE/epoxy composite was highest, nearly 1.7 times of pure PVA-co-PE/epoxy composite. This was because the CNCs
adsorbed
on
the
surface
of
PVA-co-PE
nanofibers,
forming
the
nanofiber-CNCs-epoxy interaction in the micro-region of interface. The better hydrophily and larger surface areas of CNCs increased the efficiency of stress transfer between the reinforcement and matrix, thus leading to the high increase in mechanical properties. [38-41] However, the strain of CNCs/PVA-co-PE/epoxy composites decreased a lot with the increase of CNCs concentration, this was mainly attributed to the higher stiffness of CNCs. So the tensile strength of CNCs/PVA-co-PE/epoxy composites started to decline when the CNCs concentration was higher than 2%. Anyhow, the tensile strength of all the CNCs/PVA-co-PE/epoxy films were much higher than that of pure PVA-co-PE/epoxy film. The photograph in Figure 4 (b) and (c) demonstrated
further
the
excellent
transparence
and
flexibility
of
CNCs/PVA-co-PE/epoxy composites with 3% CNCs concentration. The chemical structures of PVA-co-PE, CNCs and epoxy and the mechanism of the enhanced interfacial adhesion were characterized and given in Figure 5. To CNCs, the peaks at 2900 cm-1 corresponded to the C–H stretching vibration. The strong peak at 1020 cm-1 was assigned to the C–O–C pyranose ring skeletal vibration of cellulose and the characteristic peaks at 892 cm-1 was attributed to the b-glycosidic linkages between the sugar units. [42] To epoxy, the absorption peak at 1606 and 1507 cm-1 represent to the stretching vibration stretching peak of carbon skeleton of benzene. The absorption peaks at 1294 and 1242 cm-1 were attributed to the C-N deformation
vibration peak and the aromatic ether deformation vibration peak. The split peaks at 1085 and 1032 cm-1 correspond to the deformation vibration peak of fatty ether. Notably, the unreacted epoxy groups appear at 923 cm-1, indicating the curing process of epoxy. [43] Besides, a strong absorption band at 3300 cm−1 could be found in all the three components of PVA-co-PE, CNCs and epoxy, representing the stretching vibration of OH groups. Therefore, the hydrogen bond generated from the interaction between the OH groups of PVA-co-PE and CNCs, CNCs and epoxy enhanced the interfacial adhesion between the reinforcement and matrix, as shown in Figure 5 (c).
Figure 5 (a) the chemical structures and (b) corresponding FTIR spectra of PVA-co-PE, CNCs and epoxy (c) scheme of structure of transparent CNCs/nanofiber/epoxy film and mechanism of the enhanced interfacial adhesion
Figure 6 (a) Transmittance and (b-f) photographs of CNCs/PVA-co-PE/Epoxy composite membranes with different CNCs concentration The category, diameter, orientation and composition of fibers are key parameters that affect the transmittance of fiber reinforced composites. [19, 26] Typically, nanofiber reinforcements possessed the most similar RI with matrix, smaller size, parallel fiber orientation and fewer composition are perceived as beneficial to the optical transmission due to fewer light scattering. In this study, the RI of the PVA-co-PE nanofibrous membrane and epoxy matrix are 1.62 and 1.53, respectively. Therefore, the high optical transparency of the CNCs/PVA-co-PE/epoxy composites is mainly attributed to the size effect of nanofibers. [27] As shown in Figure 6, the wavelength used to express the single transmittance for samples was 600 nm and the thicknesses for each sample measuring transmittance in Figure 6 (b-f) were 91, 90, 93, 90, 92 µm, respectively. In Figure 6 (a), the transmittance of pure PVA-co-PE/Epoxy composites reduced to 83.4% from 92.5% of pure epoxy, indicating that the addition of PVA-co-PE nanofiber affected the transparence of matrix seriously. After the
modification of PVA-co-PE nanofibers with CNCs, the transmittance of CNCs/PVA-co-PE/epoxy composites increased obviously and more CNCs percentage lead to larger transmittance. This was mainly ascribed to the enhanced interfacial adhesion and the better transparence of CNCs. When the CNCs concentration is 5%, the transmittance of CNCs/PVA-co-PE/epoxy is 91.2%, only 1.3% lower than that of pure epoxy. The optical photographs of CNCs/PVA-co-PE/epoxy films with CNCs concentration of 0, 1%, 2%, 3%, 5% were given in Figure 6 (b-f). All of the composite films are transparent and no obvious difference to the naked eye, demonstrating the perfect transparence of CNCs/PVA-co-PE/epoxy films.
Figure 7 TG and DTG curves of neat epoxy, PVA-co-PE, CNCs and CNCs/PVA-co-PE/epoxy composites with different CNCs concentration The
TG
and
DTG
curves
of
neat
PVA-co-PE,
epoxy,
CNCs
and
CNCs/PVA-co-PE/epoxy composites were given in Figure 7. For the neat matrix of epoxy, the peak value of mass loss rate appears in the range of 320-473 °C and the largest degradation temperatures is 354 °C, corresponding to the decomposition of epoxy. For the pure PVA-co-PE nanofiber, the onset temperature of degradation is
346 °C. Two peaks are observed on the DTG curve as PVA-co-PE is copolymer of PVA and PE. The first peak is in the range of 250-380 °C, corresponding to the degradation of vinyl alcohol segment in the copolymer. The second peak appears in the range of 390-500 °C, derived from the degradation of ethylene unit in the copolymer. The significant weight loss of CNCs appears in the range of 232-400 °C. [42] After the composition, both the degradation peaks of epoxy and PVA-co-PE could be observed in the CNCs/PVA-co-PE/epoxy composites, but the degradation peak of CNC disappeared. It is mainly attributed to the combination of CNCs with PVA-co-PE and epoxy by hydrogen bond, so the degradation peak of CNCs may be overlapped with that of epoxy and PVA-co-PE. Besides, the largest degradation temperature for the CNCs/PVA-co-PE/epoxy composites moves to higher value compared with the pure PVA-co-PE reinforced epoxy, and the tendency become more obvious with the increase of CNCs concentration, indicating the enhanced thermal stability of CNCs/PVA-co-PE/epoxy composites. This is also caused by the enhanced interaction between the nanofibers and matrix after the introduction of CNCs on the interface. [44]
Figure 8 (a) CTE of epoxy and CNCs/PVA-co-PE/epoxy composite films with
different CNCs concentration, (b) thermal stability test of CNCs/PVA-co-PE/epoxy composites with CNCs concentration of 3% at different temperature for 30 mins The CTE of transparent polymer film is extremely important for ensuring the safety and reliability of optical devices. Benefit from the excellent thermal stability of CNCs (with CTE as low as 0.1 ppm/K), the CNCs/PVA-co-PE/epoxy composite films exhibited distinctly lower CTE value compared with the pure PVA-co-PE/epoxy film. As shown in Figure 8, the CTE value for epoxy and pure PVA-co-PE/epoxy film are 62 and 33 ppm/K, respectively. The thermal stability is improved obviously after the addition of PVA-co-PE nanofibers, but it is not yet better enough for application in light emitting devices cause the required CTE level is less than 20 ppm/K. Although further increasing the nanofiber concentration may reduce the CTE value, but the transparency decreases simultaneously. This problem was solved by combination of CNCs with PVA-co-PE. The CTE value became smaller with the increase of CNCs concentration. When the CNCs concentrations are 1%, 2%, 3%, 5%, the CTE values are 26, 15, 10, 6 ppm/K respectively. This mainly ascribes to the ultra-low CTE value of CNCs and the enhanced interface adhesion between the matrix and reinforcement, realizing the efficient transfer of thermal stress from the matrix to reinforcements and restricting the thermal expansion of the matrix. [28, 45] Thermal stability testing further demonstrated the above conclusion, as shown in Figure 6 (b). The CNCs/PVA-co-PE/epoxy composite film was put into the oven at different temperature for 30 mins. It exhibited good thermal stability below 120 °C, which was much higher than the normally used polyethylene terephthalate (PET) substrate of
100 °C. [8]
Figure 9 The transparent (a) photochromic and (b) thermochromic membrane, (c, d) the corresponding response time and repeatability To develop the application of the reinforced transparent composite film in the light emitting devices, the photochromic and thermochromic membranes were prepared and given in Figure 9. In this study, the CNCs/PVA-co-PE NM is the reinforcement for epoxy matrix, and also it can be used as carrier for the luminescent materials. As shown in Figure 9 (a) and (b), we coated the photochromic and thermochromic materials on the surface of CNCs/PVA-co-PE NM respectively by the stencil printing method, then impregnated them into the epoxy resin solution containing the curing agent. After the curing process, the luminescent materials were fully encapsulated by the epoxy, preventing them from oxidation and abrasion. Meanwhile, the originally
opaque CNCs/PVA-co-PE NM substrate became transparent after the impregnation, making the luminescent pattern more distinguished and clearer. Under different light and temperature environment, the photochromic and thermochromic film changed color repeatedly, demonstrating their potential application as the luminescent devices. 4. Conclusion The CNCs/PVA-co-PE/epoxy composite membranes with different CNCs concentration were prepared in this study. The CNCs were used as modifier to enhance the interfacial adhesion between the PVA-co-PE nanofibers and epoxy matrix. Benefiting from the perfect hydrophily of CNCs, the hydrophily of CNCs/PVA-co-PE composite NM increased with the increase in the CNCs concentration. The FESEM demonstrated the enhanced interfacial adhesion between the CNCs/PVA-co-PE nanofibers and matrix. Due to the high strength, transparency and ultra-low CTE of CNCs and the enhanced interfacial adhesion, the tensile strength, thermal stability and transparence of CNCs/PVA-co-PE/epoxy composites increased obviously after the introduction of CNCs. When the CNCs concentration is 3%, the tensile strength of CNCs/PVA-co-PE/epoxy film is 16.4 MPa, the CTE is 10ppm/K, the transmittance is 90.4%, distinctly higher than that of pure PVA-co-PE/epoxy film whose tensile strength is 11.4 MPa, CTE is 33 × 10−6 K−1, transmittance is 83.4%. As the reinforcement for epoxy matrix and the carrier for the luminescent materials, the patterned photochromic and thermochromic films were prepared and demonstrated repeatable color change properties under different light and temperature environment, indicating their potential applications as optical devices.
Acknowledgements This work was supported by the Research Projects of Education Department of Hubei Province (Q20181712), National Nature Science Foundation (51873165, 51873166),
Program of Hubei Technology Innovation – International Collaboration
(2017AHB065) and Applied Fundamental Research Program of Wuhan Science and Technology Bureau (2017060201010165). We also thank “Wuhan Engineering Technology Research Center for Advanced Fibers” providing partial support for materials processing.
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Highlights: The transparent CNCs/PVA-co-PE/epoxy film with enhanced mechanical and thermal properties was prepared. The interfacial adhesion between the PVA-co-PE nanofibers and epoxy matrix was enhanced by introduction of CNCs. The CNCs/PVA-co-PE NM is not only the reinforcement for matrix but also the carrier for functional materials The photochromic and thermochromic films with repeatable color change properties were prepared
AUTHOR CONTRIBUTIONS Mufang Li performed writing – original draft, methodology, conceptualization, supervision and funding acquisition Xu Zhao performed writing– original draft, methodology, visualization and investigation Yingying Li performed methodology and visualization Wen Wang performed methodology and resources Weibing Zhong performed visualization Mengying Luo performed resources Ying Lu performed methodology Ke Liu performed investigation Qiongzhen Liu performed investigation Yuedan Wang performed visualization Dong Wang performed conceptualization, writing - review & editing, supervision and funding acquisition.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.