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Cellulose nanofiber reinforced poly(vinyl alcohol) composite film with high visible light transmittance
Composites: Part A 39 (2008) 1638–1643
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Cellulose nanofiber reinforced poly(vinyl alcohol) composite film with high visible light transmittance Chunyi Tang a, Haiqing Liu a,b,* a b
College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China Key Laboratory of Polymer Materials of Fujian Province, Fuzhou 350007, China
a r t i c l e
i n f o
Article history: Received 2 December 2007 Received in revised form 26 June 2008 Accepted 13 July 2008
Keywords: A: Nano-structures A. Polymer fiber B: Mechanical properties E: Casting
a b s t r a c t In this paper, we presented the fabrication and characterization of poly(vinyl alcohol) (PVA) composite film reinforced with high volume of electrospun cellulose nanofibrous mat (CNM). Its visible light transmittance and mechanical properties were examined in relation to fiber content in the composite. Optimal CNM content in the composite was found to be 40 wt% in terms of its overall properties. This composite film exhibited visible light transmittance of 75%, and its mechanical strength and Young’s modulus were increased by 50% and 600%, respectively, as compared to neat PVA film. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced plastic (FRP) composites exhibit excellent flexural strength and elastic modulus [1]. FRPs with high transmittance of visible light are of great interest in the manufacture of transparent or translucent structural panel for applications as bendable displays, airplane canopies/windows, and light transmitting electromagnetic wave shielding material [2,3]. Factors such as fiber content, size, and refractive index (RI) of reinforcement fibers play significant roles in the light transmittance of composite materials. For example, lowering fiber content in composite may improve light transmittance; utilization of bigger fiber while fiber content is kept constant reduces the frequencies of reflection/refraction at fiber/resin interfaces, hence imparting composite good light transmittance; the close RI matching of fiber and polymer matrix to a third decimal place may prevent light scattering at the fiber/resin interfaces, resulting in high quality light transmittance of such composites [4]. However, this matching at certain temperature becomes mismatch because RI of resin matrix varies with temperature. As a result, the initial transparent composite turns into opaque as environmental temperature changes [5]. Additionally, it has been demonstrated that the mismatching of RI is enhanced with the increasing of fiber content in the composite [1]. Therefore, the manufacture of a transparent or
* Corresponding author. Address: College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China. Tel.: +86 591 83465225; fax: +86 591 83597537. E-mail address: [email protected] (H. Liu). 1359-835X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2008.07.005
translucent FRP with high content of reinforcement fiber has become a challenge. It has been reported that the application of nanofibers as reinforcement in the composite is highly effective in the preparation of optical transparent FRP. Bacterial cellulose nanofibers (10 nm thick and 50 nm wide) reinforced resin with fiber content as high as 70% are highly transparent with light transmittance of over 80% in the wavelength of 500–800 nm. Moreover, its light transmittance is insensitive to temperature variation. Additionally, its mechanical strength reaches up to 325 MPa, and Young’s modulus to 20–21 GPa, which is more than five times that of engineered plastics [6]. Thus this type of FRP is a very promising optically functional material. As far as we know, bacterial and plant cellulose is the only nature-made nanofiber applicable for FRPs [3,7]. Its manufacturing process is complex, time- and energy-consuming. Up to now, electrospinning is reported to be the only available simple but versatile technique in producing sufficient amount of nanofibers for application [8]. Several tens of various organic/inorganic polymer nanofibers have been obtained so far [9,10]. In electrospinning, the polymer jet is drawn up to 100,000 times in less than one-tenth of a second. This extremely high draw ratio can closely align polymer molecular chains along the fiber axis and make the electrospun nanofibers mechanically strong [11,12]. Although it has been found applications in various areas [9], electrospun nanofiber reinforced plastic with enhanced mechanical properties and good transparency has been rarely reported. Bergshoef et al. studied the fabrication of transparent epoxy resin reinforced with ultrathin electrospun nylon 4,6 fiber, whose content in the composite film was reported to be ca. 3.9% only [11].
C. Tang, H. Liu / Composites: Part A 39 (2008) 1638–1643
In view of the mechanical properties of transparent FRPs are highly correlated with the content of reinforcement fiber in the composites, in this study we try to make a FRP by impregnating as much as 60 wt% electrospun nanofiber into a transparent polymer matrix, and then to characterize its light transmittance and mechanical properties over a broad range of fiber content. For this purpose, cellulose nanofiber and poly(vinyl alcohol) (PVA) were used as reinforcement and polymer matrix, respectively. The large amount of hydroxyl groups on these two components assures satisfying fiber/resin interfacial interactions through hydrogen bonding, leading to desirable adhesion at the fiber/PVA interfaces. Consequently, the as-obtained composite may possible show high percent light transmittance even at high fiber content, and improved mechanical properties. 2. Experimental methods
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actual weight of composite film. Fiber content was calculated by dividing fiber mass by the mass of the final composite. The thickness of composite film obtained in this work was around 120– 210 lm. 2.4. Characterization FTIR spectra were collected on Thermo-Nicolet 5700 spectrometer in KBr form. The fiber morphology was examined by scanning electron microscope (SEM, JEOL, JSM-6380LV). All samples were sputter-coated with gold prior to SEM observation. The tensile strength of the film strips with size of 50 mm 10 mm was measured on Twin Column testing machine (LLOYD LR5K) with crosshead speed of 5 mm/min at 25 °C. Five replicates were conducted for each sample. Light transmittance of composite film was observed on UV–vis spectrometer (Lambda 850) in visible light wavelength range at 25 °C.
2.1. Materials 3. Results and discussion Cellulose acetate (CA) (Mw = 3.0 104, acetyl content 39.8 wt%) was purchased from Eastman. Poly(vinyl alcohol) (DP = 1750 ± 50, 99 + % hydrolyzed) was from commercial market in China. 2.2. Preparation of cellulose nanofibrous films 2:1 (v/v) acetone/N,N-dimethylacetamide(DMAc) mixture solvent was used as the spinning solvent for cellulose acetate as reported in our previous work [13]. The concentration of CA was 20 wt% in the solution. The CA spinning solution was placed in a syringe with a stainless needle of gauge 18. A negative electrode was clamped on the needle and connected to a power supply (DW-P303-IAC, Tianjin Dongwen High Voltage Plant, China). Grounded counter electrode was connected to collector aluminum foil. The electrospinning conditions of voltage of 8 kV and tip-tocollector distance of 15 cm were used. The feeding rate was 10 ll/min set by a syringe pump (TS2-60, Longer Precision Pump Co. Ltd., Baoding, China). Such obtained CA nanofibrous films on collector were dried under vacuum at 80 °C for 10 h to remove any residual solvents. They were subsequently hydrolyzed in 0.05 M NaOH/ethanol solution for 24 h to make cellulose nanofibrous mats (CNM). 2.3. Preparation of cellulose nanofibrous film/PVA composite film A swatch of CNM with size of 5 1 cm2 was placed flatly in a glass trough containing 5 wt% PVA aqueous solution, as illustrated in Scheme 1. After fully air-dried at ambient conditions, the composite film was vacuum dried at 50 °C for 24 h. CNMs with different thickness (15–160 lm) were applied in order to prepare composite films with varied cellulose nanofiber content up to 60 wt%. The neat PVA film outside the reinforcement cellulose nanofibrous film was trimmed off carefully in order to obtain the
Due to the high acetyl content of 39.8%, cellulose acetate nanofiber has characteristic hydrophobic surface [13]. This normally would lead to poor interfacial interaction with hydrophilic PVA matrix. In order to enhance the compatibility at the interface between the filler fiber and PVA matrix, acetyl groups of cellulose acetate nanofiber were hydrolyzed to regenerate hydroxyl groups and turn it into cellulose nanofiber. Fully hydrolysis was confirmed by the disappearance of absorption peak at 1751 cm1 caused by the vibrations of carbonyl groups on cellulose acetate [13,14]. CNM with fibers randomly stacked into non-woven film was obtained from electrospinning followed by hydrolysis in this research (Fig. 1A). Unlike conventional fibers with uniform diameter and good unidirectional alignment, fibers from electrospinning generally show broad size distribution from several tens nanometer to several hundreds nanometer [15], also they are randomly oriented on the collector to form nanofibrous film. The random orientation of fibers makes them be isotropic films ideal for the manufacturing of reinforced composite materials with isotropic properties such as similar mechanical strength in all directions. The highly porous cellulose nanofibrous film was composed of cylindrical nanofibers with diameters broadly ranging from 190 to 900 nm (Fig. 1B). Nearly 72% nanofibers are bigger than 400 nm. Actually this is a drawback for the manufacture of transparent fiber reinforced composite since the lower wavelength scale of visible light is 400 nm, as the light tends to reflect/refract on the surface of these big fibers. Solution parameters, such as the CA concentration and component composition ratio in the mixed solvent, were varied in an attempt to produce much finer cellulose nanofibers. Nanofibers with diameter of ca. 200 nm could be indeed generated, but they often showed bead-on-string morphology with beads size of several microns, as reported in our previous work [13,16]. These nanofibers are not ideal for manufacturing the targeted composite since large
Scheme 1. Preparation of CNM/PVA composite membrane.
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B
25 28% φ<400 nm
Frequency (%)
20
72% φ>400 nm
15 10 5 0 0
200
400
600
800
1000
Fiber diameter (nm) Fig. 1. (A) SEM of CNM and (B) diameter distribution of cellulose nanofiber in (A), the inset pie chart shows the percent of fibers with diameter less and larger than 400 nm.
beads would cause severe light reflectance and refraction and certainly lower the light transmittance of composites. Ultrafine cellulose nanofibers were also reported to be produced from cellulose/ LiCl-N,N0 -dimethylacetamide [17] and cellulose/N-methylmorpholine-N-oxide [18], respectively. However, in both cases, beads defect is presented along the cellulose nanofibers. Thus it is really hard to obtain cellulose nanofibers with diameter less than 200 nm, while with narrow size distribution. Actually this is a common phenomenon for most polymeric nanofibers generated from the electrospinning technique. The CNM shows very low visible light transmittance (ca. 7%) (Fig. 2A) as demonstrated by the fact that letters under the CNM cannot be seen (Fig. 2B). The CNM is a loosely packed porous film which has large amount of air/fiber interfaces. The incident light not only is reflected and refracted many times at these interfaces, but is absorbed within the CNM, resulting in little light being transmitted through the CNM. Bacterial cellulose nanofibrous sheet is white in color and also exhibits very low light transmittance [3]. PVA is transparent with nearly 92% light transmittance at wavelength of 500 nm (Fig. 2A). The composite film with 2 wt% CNM shows nearly same percent of light transmittance as that of neat PVA film, suggesting low content of the CNM in the composite causes no additional loss of light transmittance. Similar phenomenon has been found for nylon-4,6/epoxy composite with fiber volume of ca. 4 wt% [11]. The light transmittance rates of composite films containing 8.4 and 25 wt% CNM were reduced to 90% and 82%, respectively. With substantially increasing of CNMs in the composite film, light transmittance decreased to 75%, 60% and 50% for composite films containing 40, 50, and 60 wt% of CNMs, respectively. In view of the ‘mismatch’ of RI of PVA (RI = 1.52) and cellulose ½RI ¼ 1:54ð?Þ; 1:62ðkÞ [6], and the high content of CNMs as much as 40 wt%, light is supposed to scatter severely,
Fig. 2. (A) Light transmittance of CNM/PVA composite film in the visible light wavelength range. (B) Appearance of film. CNM/PVA composite film contains 40 wt% CNM.
causing significant loss of transmitted light. On the contrary, it is very interesting to find that the composite film still shows good light transmittance as letters under the film is clearly observed (Fig. 2B). Unlike the optical transparent bacterial cellulose nanofiber reinforced composites [3], the composite film prepared in this study is non-transparent. As illustrated in Scheme 2, visible light may theoretically pass nanofibers whose diameters are less than
φi
φi
Air Resin Nanofiber Microfiber
Air
φ t1
φ t2
φ t3
Scheme 2. Simplified model of light transmitted through fiber reinforced resin. RIs of resin and fiber do not match. /i is the incident light; /t is the transmitted light. Green arrows mean reflected light at the air/resin and fiber/resin interfaces; Red arrows mean refracted light at the interfaces. Assuming the reflected light does not go into the other end of composite, /t of transmitted light through microfiber is significantly less than /i. Light would pass through nanofiber without the occurrence of reflection/refraction at the fiber/resin interfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
C. Tang, H. Liu / Composites: Part A 39 (2008) 1638–1643
400 nm, without the occurrence of reflection/refraction at the fiber/resin interface, because light is a electromagnetic wave. However, light experiences reflection/refraction when it encounters nanofibers with diameters larger than 400 nm. Therefore, the transmitted light is reduced in the CNM/PVA composite film with 72% cellulose nanofibers bigger than 400 nm in diameter. Also the light scattering at the fiber/PVA interfaces results in the formation of opaque composite film. The other reason for the loss of transmitted light is possible associated with the imperfect contact of the nanofiber with PVA matrix at the interfaces, as will be discussed below. From the cross-section of CNM/PVA composite film which was stretched to break (Fig. 3), it is found that most fibers are intimately contacted with PVA resin, suggesting that most micropores within the CNM are filled with PVA resin. This indicates that the air/fiber interfaces in the CNM were replaced with PVA/fiber interfaces after the CNM was embedded in the PVA resin. This replacement is beneficial to the good light transmittance of the CNM/PVA composite film. It is well-known that the amount of light reflection at the interface is highly related to the refractive index of the two materials forming the interface. This relationship is described by the formula below.
r ¼ ½ðnR nF Þ=ðnR þ nF Þ2
ð1Þ
where r is the reflective coefficient, and nR and nF are RI of resin and fiber, respectively. Therefore, the more the difference between the RI of resin and fiber is, the higher the reflective coefficient is. As a result, more amount of light is reflected. The RI of air, PVA and cellulose nanofiber is 1, 1.49–1.52, 1.54–1.62, respectively. Thus the light reflection at the cellulose nanofiber/PVA interfaces is much less than that at the cellulose nanofiber/air interfaces, leading to much better light transmittance for the CNM/PVA composite film than that for the CNM as showed in Fig. 2. It was reported that part of the filler nylon 6 nanofibers could be peeled off from composite resin and some pores would be left behind [12], partly because the interfacial adhesion is not strong enough. In the case of CNM/PVA composite films, upon stretching to break, cellulose nanofibers were not pull out from PVA matrix because no cylindrical hole was created (Fig. 3), suggesting the adhesion at the interface is very strong. The features of ultrafine diameter, high surface areas and hydrophilicity of cellulose nanofibers substantially enhance the intermolecular interaction through forces such as hydrogen bonding at the interfaces of cellulose nanofiber/PVA matrix. A few cellulose nanofibers indicated by arrows are free from contacting with PVA resin (Fig. 3), indicating PVA resin did not penetrate into some capillaries to reach these
Fig. 3. Cross-section SEM of composite film containing 8.4 wt% CNM after stretched to break.
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‘big’ fibers whose diameter is ca. 0.8–1 lm in the CNM. Generally these fibers are big in diameter. Some capillaries formed around these fibers are large and the capillary forces to drive the penetration of PVA solution is much weaker as compared to that in the small capillaries formed among much finer nanofibers. The liquid movement in the capillary is expressed in the Lucas–Washburn equation:
dh ¼ rcLV cos h=4gh dt
ð2Þ
where t is time; h is the length of the capillary; r is the capillary radius; g is viscosity; c is the interfacial tension with LV meaning liquid and vapor, respectively; and h is the contact angle. In addition, the flow of PVA solution from the two ends of capillary would trap air within it. Therefore, some unfilled capillaries are presented in the composite film, as indicated by the circles (Fig. 3). When light transmits through the CNM/PVA composite film, light reflects/refracts not only on the interface of fiber/PVA, but also on air/PVA and air/fiber interface in the unfilled capillaries. This would certainly result in more light loss than that of the fully filled pore-free composite film. Hence, in order to prepare a cellulose nanofiber/ PVA composite film with high visible light transmittance, on the one hand, the diameter of filler cellulose nanofiber should be smaller than 400 nm; on the other hand, the pores formed among nanofibers should be fully filled with PVA resin to completely eliminate air/fiber and air/PVA interfaces within the composite. Typical stress–strain curves of CNM shows characteristic low stress of 2.8 MPa and Young’s modulus of 14.7 GPa, respectively (Fig. 4j). Due to the loose and random packing of short nanofibers, it is very common that nanofibrous film shows much lower stress than counterpart of conventional fiber or casting film [15,19]. It should be pointed out that the mechanical strength and modulus of a single cellulose nanofiber were not measured because the available final product in electrospinning is nanofibrous mats which are composed of randomly organized nanofibers as shown in Fig. 1A, and because few techniques have been developed to characterize the mechanical properties of a single nanofiber. The very few available data for the modulus of single electrospun nanofibers suggest that nanofibers possess better stiffness or modulus than bulk samples, in that nanofibers have a high degree of molecular chain orientation induced by electrospinning. For instance, polyacrylonitrile (PAN) nanofiber exhibits modulus up to 50 GPa, which is much higher than 1.2 GPa for the modulus of bulk PAN samples [20]. However, the high modulus of a single nanofiber
Fig. 4. Stress–strain curves of CNM/PVA composite films. Mass content of CNM in the composite films is (a) 0, (b) 4.7, (c) 5.5, (d) 8.4, (e) 23, (f) 34, (g) 40, (h) 50, (i) 60 and (j) 100.
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does not guarantee that nanofibrous mats are mechanically strong mainly because most nanofibers are shorter than 1 cm. Their length is much less than the cross-head distance (5–10 cm) for the mechanical test, therefore most nanofibers in the nanofibrous mats do not break while stretched, instead they shift and re-orient along the stretching direction [21]. The stress–strain behavior of CNM reinforced composites act like fiber instead of plastic (Fig. 4b–i). With even a small percent of fibers embedded in the PVA, the stress of composites increases almost linearly with strain, and the composites break at strain of less than 30% (Fig. 4). As less than 10 wt% of CNM was impregnated into the PVA resin, both mechanical strength and strain of composite film decreased (Fig. 5A). Since the stress of the CNM (r = 2.8 MPa) is much smaller than that of PVA matrix (r = 42 MPa), low content of the CNM is not expected to reinforce the composite film. Instead, it is highly possible to become stress concentration, and weak the overall mechanical properties of the composite film. Mechanical strength of soybean cellulose nanofiber/PVA composite also reduces with less than 10 wt% nanofiber in the PVA composite [22]. As CNM content was further increased to 23 wt%, the stress of composite film substantially increased to 51.8 ± 3.7 MPa. The maximum stress of 60.4 ± 5.7 MPa was reached for the CNM/PVA composite with CNM content of 40 wt%. This value is 1.5 times of that of neat PVA film, while its strain significantly decreased from 200% to 30%. Mechanical strength of composite films does not increase further as CNM content was raised to 50 and 60 wt% (Fig. 5A). Young’s moduli of composite films increase substantially with fiber content in the range studied in this work. The E of composite film with 40 wt% CNM is 3.9 GPa, which is 11
70
250
Stress Strain
Stress (MPa)
60
200 150
50 100 40
Strain (%)
A
50 30 20
0 0
20
40
16
E/E0
12
8
4
0
4. Conclusion The impregnation of non-transparent CNM into PVA film may manufacture composite film with high visible light transmittance of as much as 75% even the fiber content in the composite is 40 wt%, largely due to the intimate contact and strong interfacial adhesion between ultrafine dimension of cellulose nanofiber and PVA matrix. The CNM/PVA composite film is non-transparent because the transmitted light is scattered. High fiber content (P40 wt%) in the composite increased its mechanical strength by 50%, and its Young’s modulus by more than 600%. These findings strongly suggest that electrospun nanofibers have promising application as reinforcing fibers for composite materials with high fiber content and with high visible light transmittance as well. In full consideration of the mechanical properties and light transmittance of CNM/PVA composite films, the optimal fiber content in the composites is 40 wt%.
60
CNM content (wt.%)
B
times that of neat PVA film (Fig. 5B). The E of the CNM is 14.7 GPa (Fig. 4j). Consequently, the embedding of CNM in PVA could greatly improve the E of the composite. As already stated above, the large amount of hydroxyl groups on cellulose nanofiber surfaces would form strong hydrogen bonding with PVA matrix, resulting in good adhesion at the fiber/PVA interfaces, as confirmed by the PVA remnants on cellulose nanofibers (Fig. 3). This leads to stress transfer from PVA to cellulose nanofibers during stretching. After the composite was stretched to break, most of the embedded nanofibers were broken, while a few nanofibers which were nearly perpendicular to the stretching direction showed curved shape (Fig. 3). The occurrence of fiber bowing is originated from the stress transfer, and in turn it improves the mechanical strength of the composite. Although the mechanical strength of CNM/PVA composites increased with fiber content, this improvement is not as significant as expected, in view of the large content of CNM in composites and good adhesion at fiber/PVA interfaces. This is believed to be due to (1) the inherent low stress of CNMs (r = 2.8 MPa, Fig. 4j); (2) poor uniaxial alignment of nanofibers in the CNMs (Fig. 1); and (3) unfilled capillaries defects within the composite (Fig. 3), furthermore this type of defects grows with the fiber content or the thickness of the CNM. It should be pointed out that the substantial increasing of moduli is not in accordance with the moderate gain of stress (Fig. 5). This is because that the strain of the composite reduced largely even with a small fiber content in the composite, as found in the case that the strain of the composite with 20 wt% reinforcing fiber is about one-sixth of that of the neat PVA film (Fig. 5A). As a consequence, significant moduli increasing are achieved. A similar finding was also found by Bergshoef and Vancso [11]. He reported that the modulus of epoxy resin reinforced with electrospun nylon 4,6 nanofiber was ca. 36 times more than that of the control film, whereas the stress of the same composite was only three times that of the reference. Referring to FRPs reinforced with conventional fiber, highly aligned nanofiber yarns instead of randomly oriented CNMs would serve better in the manufacturing of nanofiber reinforced composites. New techniques need to be developed in order to decrease the unfilled pores/capillaries defects in the composite. By this way, the mechanical properties and light transmittance of composite can be further improved.
Acknowledgements 0
10
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CNM content (wt.%) Fig. 5. (A) Stress and strain of CNM/PVA composite membrane as a function of CNM content. (B) Young’s moduli of CNM/PVA composite membranes as a function of CNM content. E0 = 0.35 GPa for neat PVA film.
The authors thank the financial support from the Initiative Fund for the Returned Overseas Chinese Scholar administered by the State Education Ministry, and the Key Project of Natural Science Foundation of Fujian Province (Grant No. E0620001).
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Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Cellulose nanofiber reinforced poly(vinyl alcohol) composite film with high visible light transmittance Chunyi Tang a, Haiqing Liu a,b,* a b
College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China Key Laboratory of Polymer Materials of Fujian Province, Fuzhou 350007, China
a r t i c l e
i n f o
Article history: Received 2 December 2007 Received in revised form 26 June 2008 Accepted 13 July 2008
Keywords: A: Nano-structures A. Polymer fiber B: Mechanical properties E: Casting
a b s t r a c t In this paper, we presented the fabrication and characterization of poly(vinyl alcohol) (PVA) composite film reinforced with high volume of electrospun cellulose nanofibrous mat (CNM). Its visible light transmittance and mechanical properties were examined in relation to fiber content in the composite. Optimal CNM content in the composite was found to be 40 wt% in terms of its overall properties. This composite film exhibited visible light transmittance of 75%, and its mechanical strength and Young’s modulus were increased by 50% and 600%, respectively, as compared to neat PVA film. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced plastic (FRP) composites exhibit excellent flexural strength and elastic modulus [1]. FRPs with high transmittance of visible light are of great interest in the manufacture of transparent or translucent structural panel for applications as bendable displays, airplane canopies/windows, and light transmitting electromagnetic wave shielding material [2,3]. Factors such as fiber content, size, and refractive index (RI) of reinforcement fibers play significant roles in the light transmittance of composite materials. For example, lowering fiber content in composite may improve light transmittance; utilization of bigger fiber while fiber content is kept constant reduces the frequencies of reflection/refraction at fiber/resin interfaces, hence imparting composite good light transmittance; the close RI matching of fiber and polymer matrix to a third decimal place may prevent light scattering at the fiber/resin interfaces, resulting in high quality light transmittance of such composites [4]. However, this matching at certain temperature becomes mismatch because RI of resin matrix varies with temperature. As a result, the initial transparent composite turns into opaque as environmental temperature changes [5]. Additionally, it has been demonstrated that the mismatching of RI is enhanced with the increasing of fiber content in the composite [1]. Therefore, the manufacture of a transparent or
* Corresponding author. Address: College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China. Tel.: +86 591 83465225; fax: +86 591 83597537. E-mail address: [email protected] (H. Liu). 1359-835X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2008.07.005
translucent FRP with high content of reinforcement fiber has become a challenge. It has been reported that the application of nanofibers as reinforcement in the composite is highly effective in the preparation of optical transparent FRP. Bacterial cellulose nanofibers (10 nm thick and 50 nm wide) reinforced resin with fiber content as high as 70% are highly transparent with light transmittance of over 80% in the wavelength of 500–800 nm. Moreover, its light transmittance is insensitive to temperature variation. Additionally, its mechanical strength reaches up to 325 MPa, and Young’s modulus to 20–21 GPa, which is more than five times that of engineered plastics [6]. Thus this type of FRP is a very promising optically functional material. As far as we know, bacterial and plant cellulose is the only nature-made nanofiber applicable for FRPs [3,7]. Its manufacturing process is complex, time- and energy-consuming. Up to now, electrospinning is reported to be the only available simple but versatile technique in producing sufficient amount of nanofibers for application [8]. Several tens of various organic/inorganic polymer nanofibers have been obtained so far [9,10]. In electrospinning, the polymer jet is drawn up to 100,000 times in less than one-tenth of a second. This extremely high draw ratio can closely align polymer molecular chains along the fiber axis and make the electrospun nanofibers mechanically strong [11,12]. Although it has been found applications in various areas [9], electrospun nanofiber reinforced plastic with enhanced mechanical properties and good transparency has been rarely reported. Bergshoef et al. studied the fabrication of transparent epoxy resin reinforced with ultrathin electrospun nylon 4,6 fiber, whose content in the composite film was reported to be ca. 3.9% only [11].
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In view of the mechanical properties of transparent FRPs are highly correlated with the content of reinforcement fiber in the composites, in this study we try to make a FRP by impregnating as much as 60 wt% electrospun nanofiber into a transparent polymer matrix, and then to characterize its light transmittance and mechanical properties over a broad range of fiber content. For this purpose, cellulose nanofiber and poly(vinyl alcohol) (PVA) were used as reinforcement and polymer matrix, respectively. The large amount of hydroxyl groups on these two components assures satisfying fiber/resin interfacial interactions through hydrogen bonding, leading to desirable adhesion at the fiber/PVA interfaces. Consequently, the as-obtained composite may possible show high percent light transmittance even at high fiber content, and improved mechanical properties. 2. Experimental methods
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actual weight of composite film. Fiber content was calculated by dividing fiber mass by the mass of the final composite. The thickness of composite film obtained in this work was around 120– 210 lm. 2.4. Characterization FTIR spectra were collected on Thermo-Nicolet 5700 spectrometer in KBr form. The fiber morphology was examined by scanning electron microscope (SEM, JEOL, JSM-6380LV). All samples were sputter-coated with gold prior to SEM observation. The tensile strength of the film strips with size of 50 mm 10 mm was measured on Twin Column testing machine (LLOYD LR5K) with crosshead speed of 5 mm/min at 25 °C. Five replicates were conducted for each sample. Light transmittance of composite film was observed on UV–vis spectrometer (Lambda 850) in visible light wavelength range at 25 °C.
2.1. Materials 3. Results and discussion Cellulose acetate (CA) (Mw = 3.0 104, acetyl content 39.8 wt%) was purchased from Eastman. Poly(vinyl alcohol) (DP = 1750 ± 50, 99 + % hydrolyzed) was from commercial market in China. 2.2. Preparation of cellulose nanofibrous films 2:1 (v/v) acetone/N,N-dimethylacetamide(DMAc) mixture solvent was used as the spinning solvent for cellulose acetate as reported in our previous work [13]. The concentration of CA was 20 wt% in the solution. The CA spinning solution was placed in a syringe with a stainless needle of gauge 18. A negative electrode was clamped on the needle and connected to a power supply (DW-P303-IAC, Tianjin Dongwen High Voltage Plant, China). Grounded counter electrode was connected to collector aluminum foil. The electrospinning conditions of voltage of 8 kV and tip-tocollector distance of 15 cm were used. The feeding rate was 10 ll/min set by a syringe pump (TS2-60, Longer Precision Pump Co. Ltd., Baoding, China). Such obtained CA nanofibrous films on collector were dried under vacuum at 80 °C for 10 h to remove any residual solvents. They were subsequently hydrolyzed in 0.05 M NaOH/ethanol solution for 24 h to make cellulose nanofibrous mats (CNM). 2.3. Preparation of cellulose nanofibrous film/PVA composite film A swatch of CNM with size of 5 1 cm2 was placed flatly in a glass trough containing 5 wt% PVA aqueous solution, as illustrated in Scheme 1. After fully air-dried at ambient conditions, the composite film was vacuum dried at 50 °C for 24 h. CNMs with different thickness (15–160 lm) were applied in order to prepare composite films with varied cellulose nanofiber content up to 60 wt%. The neat PVA film outside the reinforcement cellulose nanofibrous film was trimmed off carefully in order to obtain the
Due to the high acetyl content of 39.8%, cellulose acetate nanofiber has characteristic hydrophobic surface [13]. This normally would lead to poor interfacial interaction with hydrophilic PVA matrix. In order to enhance the compatibility at the interface between the filler fiber and PVA matrix, acetyl groups of cellulose acetate nanofiber were hydrolyzed to regenerate hydroxyl groups and turn it into cellulose nanofiber. Fully hydrolysis was confirmed by the disappearance of absorption peak at 1751 cm1 caused by the vibrations of carbonyl groups on cellulose acetate [13,14]. CNM with fibers randomly stacked into non-woven film was obtained from electrospinning followed by hydrolysis in this research (Fig. 1A). Unlike conventional fibers with uniform diameter and good unidirectional alignment, fibers from electrospinning generally show broad size distribution from several tens nanometer to several hundreds nanometer [15], also they are randomly oriented on the collector to form nanofibrous film. The random orientation of fibers makes them be isotropic films ideal for the manufacturing of reinforced composite materials with isotropic properties such as similar mechanical strength in all directions. The highly porous cellulose nanofibrous film was composed of cylindrical nanofibers with diameters broadly ranging from 190 to 900 nm (Fig. 1B). Nearly 72% nanofibers are bigger than 400 nm. Actually this is a drawback for the manufacture of transparent fiber reinforced composite since the lower wavelength scale of visible light is 400 nm, as the light tends to reflect/refract on the surface of these big fibers. Solution parameters, such as the CA concentration and component composition ratio in the mixed solvent, were varied in an attempt to produce much finer cellulose nanofibers. Nanofibers with diameter of ca. 200 nm could be indeed generated, but they often showed bead-on-string morphology with beads size of several microns, as reported in our previous work [13,16]. These nanofibers are not ideal for manufacturing the targeted composite since large
Scheme 1. Preparation of CNM/PVA composite membrane.
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B
25 28% φ<400 nm
Frequency (%)
20
72% φ>400 nm
15 10 5 0 0
200
400
600
800
1000
Fiber diameter (nm) Fig. 1. (A) SEM of CNM and (B) diameter distribution of cellulose nanofiber in (A), the inset pie chart shows the percent of fibers with diameter less and larger than 400 nm.
beads would cause severe light reflectance and refraction and certainly lower the light transmittance of composites. Ultrafine cellulose nanofibers were also reported to be produced from cellulose/ LiCl-N,N0 -dimethylacetamide [17] and cellulose/N-methylmorpholine-N-oxide [18], respectively. However, in both cases, beads defect is presented along the cellulose nanofibers. Thus it is really hard to obtain cellulose nanofibers with diameter less than 200 nm, while with narrow size distribution. Actually this is a common phenomenon for most polymeric nanofibers generated from the electrospinning technique. The CNM shows very low visible light transmittance (ca. 7%) (Fig. 2A) as demonstrated by the fact that letters under the CNM cannot be seen (Fig. 2B). The CNM is a loosely packed porous film which has large amount of air/fiber interfaces. The incident light not only is reflected and refracted many times at these interfaces, but is absorbed within the CNM, resulting in little light being transmitted through the CNM. Bacterial cellulose nanofibrous sheet is white in color and also exhibits very low light transmittance [3]. PVA is transparent with nearly 92% light transmittance at wavelength of 500 nm (Fig. 2A). The composite film with 2 wt% CNM shows nearly same percent of light transmittance as that of neat PVA film, suggesting low content of the CNM in the composite causes no additional loss of light transmittance. Similar phenomenon has been found for nylon-4,6/epoxy composite with fiber volume of ca. 4 wt% [11]. The light transmittance rates of composite films containing 8.4 and 25 wt% CNM were reduced to 90% and 82%, respectively. With substantially increasing of CNMs in the composite film, light transmittance decreased to 75%, 60% and 50% for composite films containing 40, 50, and 60 wt% of CNMs, respectively. In view of the ‘mismatch’ of RI of PVA (RI = 1.52) and cellulose ½RI ¼ 1:54ð?Þ; 1:62ðkÞ [6], and the high content of CNMs as much as 40 wt%, light is supposed to scatter severely,
Fig. 2. (A) Light transmittance of CNM/PVA composite film in the visible light wavelength range. (B) Appearance of film. CNM/PVA composite film contains 40 wt% CNM.
causing significant loss of transmitted light. On the contrary, it is very interesting to find that the composite film still shows good light transmittance as letters under the film is clearly observed (Fig. 2B). Unlike the optical transparent bacterial cellulose nanofiber reinforced composites [3], the composite film prepared in this study is non-transparent. As illustrated in Scheme 2, visible light may theoretically pass nanofibers whose diameters are less than
φi
φi
Air Resin Nanofiber Microfiber
Air
φ t1
φ t2
φ t3
Scheme 2. Simplified model of light transmitted through fiber reinforced resin. RIs of resin and fiber do not match. /i is the incident light; /t is the transmitted light. Green arrows mean reflected light at the air/resin and fiber/resin interfaces; Red arrows mean refracted light at the interfaces. Assuming the reflected light does not go into the other end of composite, /t of transmitted light through microfiber is significantly less than /i. Light would pass through nanofiber without the occurrence of reflection/refraction at the fiber/resin interfaces. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
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400 nm, without the occurrence of reflection/refraction at the fiber/resin interface, because light is a electromagnetic wave. However, light experiences reflection/refraction when it encounters nanofibers with diameters larger than 400 nm. Therefore, the transmitted light is reduced in the CNM/PVA composite film with 72% cellulose nanofibers bigger than 400 nm in diameter. Also the light scattering at the fiber/PVA interfaces results in the formation of opaque composite film. The other reason for the loss of transmitted light is possible associated with the imperfect contact of the nanofiber with PVA matrix at the interfaces, as will be discussed below. From the cross-section of CNM/PVA composite film which was stretched to break (Fig. 3), it is found that most fibers are intimately contacted with PVA resin, suggesting that most micropores within the CNM are filled with PVA resin. This indicates that the air/fiber interfaces in the CNM were replaced with PVA/fiber interfaces after the CNM was embedded in the PVA resin. This replacement is beneficial to the good light transmittance of the CNM/PVA composite film. It is well-known that the amount of light reflection at the interface is highly related to the refractive index of the two materials forming the interface. This relationship is described by the formula below.
r ¼ ½ðnR nF Þ=ðnR þ nF Þ2
ð1Þ
where r is the reflective coefficient, and nR and nF are RI of resin and fiber, respectively. Therefore, the more the difference between the RI of resin and fiber is, the higher the reflective coefficient is. As a result, more amount of light is reflected. The RI of air, PVA and cellulose nanofiber is 1, 1.49–1.52, 1.54–1.62, respectively. Thus the light reflection at the cellulose nanofiber/PVA interfaces is much less than that at the cellulose nanofiber/air interfaces, leading to much better light transmittance for the CNM/PVA composite film than that for the CNM as showed in Fig. 2. It was reported that part of the filler nylon 6 nanofibers could be peeled off from composite resin and some pores would be left behind [12], partly because the interfacial adhesion is not strong enough. In the case of CNM/PVA composite films, upon stretching to break, cellulose nanofibers were not pull out from PVA matrix because no cylindrical hole was created (Fig. 3), suggesting the adhesion at the interface is very strong. The features of ultrafine diameter, high surface areas and hydrophilicity of cellulose nanofibers substantially enhance the intermolecular interaction through forces such as hydrogen bonding at the interfaces of cellulose nanofiber/PVA matrix. A few cellulose nanofibers indicated by arrows are free from contacting with PVA resin (Fig. 3), indicating PVA resin did not penetrate into some capillaries to reach these
Fig. 3. Cross-section SEM of composite film containing 8.4 wt% CNM after stretched to break.
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‘big’ fibers whose diameter is ca. 0.8–1 lm in the CNM. Generally these fibers are big in diameter. Some capillaries formed around these fibers are large and the capillary forces to drive the penetration of PVA solution is much weaker as compared to that in the small capillaries formed among much finer nanofibers. The liquid movement in the capillary is expressed in the Lucas–Washburn equation:
dh ¼ rcLV cos h=4gh dt
ð2Þ
where t is time; h is the length of the capillary; r is the capillary radius; g is viscosity; c is the interfacial tension with LV meaning liquid and vapor, respectively; and h is the contact angle. In addition, the flow of PVA solution from the two ends of capillary would trap air within it. Therefore, some unfilled capillaries are presented in the composite film, as indicated by the circles (Fig. 3). When light transmits through the CNM/PVA composite film, light reflects/refracts not only on the interface of fiber/PVA, but also on air/PVA and air/fiber interface in the unfilled capillaries. This would certainly result in more light loss than that of the fully filled pore-free composite film. Hence, in order to prepare a cellulose nanofiber/ PVA composite film with high visible light transmittance, on the one hand, the diameter of filler cellulose nanofiber should be smaller than 400 nm; on the other hand, the pores formed among nanofibers should be fully filled with PVA resin to completely eliminate air/fiber and air/PVA interfaces within the composite. Typical stress–strain curves of CNM shows characteristic low stress of 2.8 MPa and Young’s modulus of 14.7 GPa, respectively (Fig. 4j). Due to the loose and random packing of short nanofibers, it is very common that nanofibrous film shows much lower stress than counterpart of conventional fiber or casting film [15,19]. It should be pointed out that the mechanical strength and modulus of a single cellulose nanofiber were not measured because the available final product in electrospinning is nanofibrous mats which are composed of randomly organized nanofibers as shown in Fig. 1A, and because few techniques have been developed to characterize the mechanical properties of a single nanofiber. The very few available data for the modulus of single electrospun nanofibers suggest that nanofibers possess better stiffness or modulus than bulk samples, in that nanofibers have a high degree of molecular chain orientation induced by electrospinning. For instance, polyacrylonitrile (PAN) nanofiber exhibits modulus up to 50 GPa, which is much higher than 1.2 GPa for the modulus of bulk PAN samples [20]. However, the high modulus of a single nanofiber
Fig. 4. Stress–strain curves of CNM/PVA composite films. Mass content of CNM in the composite films is (a) 0, (b) 4.7, (c) 5.5, (d) 8.4, (e) 23, (f) 34, (g) 40, (h) 50, (i) 60 and (j) 100.
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does not guarantee that nanofibrous mats are mechanically strong mainly because most nanofibers are shorter than 1 cm. Their length is much less than the cross-head distance (5–10 cm) for the mechanical test, therefore most nanofibers in the nanofibrous mats do not break while stretched, instead they shift and re-orient along the stretching direction [21]. The stress–strain behavior of CNM reinforced composites act like fiber instead of plastic (Fig. 4b–i). With even a small percent of fibers embedded in the PVA, the stress of composites increases almost linearly with strain, and the composites break at strain of less than 30% (Fig. 4). As less than 10 wt% of CNM was impregnated into the PVA resin, both mechanical strength and strain of composite film decreased (Fig. 5A). Since the stress of the CNM (r = 2.8 MPa) is much smaller than that of PVA matrix (r = 42 MPa), low content of the CNM is not expected to reinforce the composite film. Instead, it is highly possible to become stress concentration, and weak the overall mechanical properties of the composite film. Mechanical strength of soybean cellulose nanofiber/PVA composite also reduces with less than 10 wt% nanofiber in the PVA composite [22]. As CNM content was further increased to 23 wt%, the stress of composite film substantially increased to 51.8 ± 3.7 MPa. The maximum stress of 60.4 ± 5.7 MPa was reached for the CNM/PVA composite with CNM content of 40 wt%. This value is 1.5 times of that of neat PVA film, while its strain significantly decreased from 200% to 30%. Mechanical strength of composite films does not increase further as CNM content was raised to 50 and 60 wt% (Fig. 5A). Young’s moduli of composite films increase substantially with fiber content in the range studied in this work. The E of composite film with 40 wt% CNM is 3.9 GPa, which is 11
70
250
Stress Strain
Stress (MPa)
60
200 150
50 100 40
Strain (%)
A
50 30 20
0 0
20
40
16
E/E0
12
8
4
0
4. Conclusion The impregnation of non-transparent CNM into PVA film may manufacture composite film with high visible light transmittance of as much as 75% even the fiber content in the composite is 40 wt%, largely due to the intimate contact and strong interfacial adhesion between ultrafine dimension of cellulose nanofiber and PVA matrix. The CNM/PVA composite film is non-transparent because the transmitted light is scattered. High fiber content (P40 wt%) in the composite increased its mechanical strength by 50%, and its Young’s modulus by more than 600%. These findings strongly suggest that electrospun nanofibers have promising application as reinforcing fibers for composite materials with high fiber content and with high visible light transmittance as well. In full consideration of the mechanical properties and light transmittance of CNM/PVA composite films, the optimal fiber content in the composites is 40 wt%.
60
CNM content (wt.%)
B
times that of neat PVA film (Fig. 5B). The E of the CNM is 14.7 GPa (Fig. 4j). Consequently, the embedding of CNM in PVA could greatly improve the E of the composite. As already stated above, the large amount of hydroxyl groups on cellulose nanofiber surfaces would form strong hydrogen bonding with PVA matrix, resulting in good adhesion at the fiber/PVA interfaces, as confirmed by the PVA remnants on cellulose nanofibers (Fig. 3). This leads to stress transfer from PVA to cellulose nanofibers during stretching. After the composite was stretched to break, most of the embedded nanofibers were broken, while a few nanofibers which were nearly perpendicular to the stretching direction showed curved shape (Fig. 3). The occurrence of fiber bowing is originated from the stress transfer, and in turn it improves the mechanical strength of the composite. Although the mechanical strength of CNM/PVA composites increased with fiber content, this improvement is not as significant as expected, in view of the large content of CNM in composites and good adhesion at fiber/PVA interfaces. This is believed to be due to (1) the inherent low stress of CNMs (r = 2.8 MPa, Fig. 4j); (2) poor uniaxial alignment of nanofibers in the CNMs (Fig. 1); and (3) unfilled capillaries defects within the composite (Fig. 3), furthermore this type of defects grows with the fiber content or the thickness of the CNM. It should be pointed out that the substantial increasing of moduli is not in accordance with the moderate gain of stress (Fig. 5). This is because that the strain of the composite reduced largely even with a small fiber content in the composite, as found in the case that the strain of the composite with 20 wt% reinforcing fiber is about one-sixth of that of the neat PVA film (Fig. 5A). As a consequence, significant moduli increasing are achieved. A similar finding was also found by Bergshoef and Vancso [11]. He reported that the modulus of epoxy resin reinforced with electrospun nylon 4,6 nanofiber was ca. 36 times more than that of the control film, whereas the stress of the same composite was only three times that of the reference. Referring to FRPs reinforced with conventional fiber, highly aligned nanofiber yarns instead of randomly oriented CNMs would serve better in the manufacturing of nanofiber reinforced composites. New techniques need to be developed in order to decrease the unfilled pores/capillaries defects in the composite. By this way, the mechanical properties and light transmittance of composite can be further improved.
Acknowledgements 0
10
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60
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CNM content (wt.%) Fig. 5. (A) Stress and strain of CNM/PVA composite membrane as a function of CNM content. (B) Young’s moduli of CNM/PVA composite membranes as a function of CNM content. E0 = 0.35 GPa for neat PVA film.
The authors thank the financial support from the Initiative Fund for the Returned Overseas Chinese Scholar administered by the State Education Ministry, and the Key Project of Natural Science Foundation of Fujian Province (Grant No. E0620001).
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