Cellulose Nanocrystals: Dispersion in Co-Solvent Systems and Effects on Electrospun Polyvinylpyrrolidone Fiber Mats

Transcription

1 Cellulose Nanocrystals: Dispersion in Co-Solvent Systems and Effects on Electrospun Polyvinylpyrrolidone Fiber Mats Ryan J. Going, Dan E. Sameoto, PhD, Cagri Ayranci, PhD University of Alberta, Edmonton, Alberta CANADA Corresondence to: Cagri Ayranci ABSTRACT This study reports a method to achieve dispersion of freeze-dried Cellulose nanocrystals (CNCs) with polyvinylpyrrolidone (PVP) in a water-methanol cosolvent system using purely mechanical means; in this case, magnetic stirring and sonication. During this study, no chemical modifiers or surfactants of any kind were added during the dispersion process as they increase the cost and duration of the manufacturing process for CNC reinforced composites and nanocomposites. The effect of CNC loading (0-20wt% of PVP) and preparation method on solution viscosity, dispersion, and mechanical properties of electrospun PVP/CNC nanocomposite fiber mats were examined. In particular, this study demonstrates that pre-dispersion of the hydrophilic CNCs in pure deionized water before the addition of methanol and PVP is critical to improving dispersion and achieving greater homogeneity of the system. All samples were examined for birefringence by polarized light microscopy, which was correlated to the level of CNC dispersion within the polymer matrix. The effect of CNC loading on the mechanical properties of the composite mats was investigated via tensile testing. Humidity was identified as an important factor affecting the PVP nanofiber morphology and strength, though its effects were not characterized in this study. Keywords: Electrospinning, nanofibers, composite materials, Cellulose nanocrystals (CNC), polyvinylpyrrolidone (PVP), polarized light microscopy. INTRODUCTION Electrospinning is a polymeric nanofiber manufacturing method to produce mats of nanofibers with diameters ranging from tens of nanometers to several microns [1]. There has recently been a great deal of research regarding the electrospinning of certain polymers such as polyvinyl alcohol (PVA) [2] and polyethylene oxide (PEO) [3]. These polymers are non-toxic and highly soluble in water; therefore, they are used in many biomedical applications, including tissue engineering and drug delivery systems [4]. However, polymeric fiber mats produced via electrospinning technique suffer from inherently low mechanical properties of polymers for many applications. Consequently, investigations into methods to electrospin polymer-based composite nanofibers using different nano-reinforcements are of great interest. Cellulose nanocrystals (CNCs) are nano fibers derived from wood pulp. CNCs present desirable mechanical properties and low density. They are also non-toxic which makes them a suitable material for many nanocomposites in biomedical applications [5]. Their hydrophilic nature [6] also makes them ideal for use as reinforcing particles in water-soluble polymers. A great deal of work has already been done to establish the dispensability of CNCs in water [7-9] as well as in certain organic solvents including N,N-dimethylformamide [10], but their dispensability in water-based co-solvents has not been fully investigated. CNCs effects on the mechanical and rheological properties of a number of electrospun polymers, such as PLA, PVA, and PEO, have also been well characterized [11-14]. The effects of CNCs on the properties of electrospun polyvinylpyrrolidone (PVP) fiber mats, another commonly used watersoluble polymer, however, have not been widely reported. In this study, composite nanofiber mats were produced via electrospinning from methanol-water suspensions of PVP with various levels of CNC loading. The objectives were twofold: first, to characterize the dispersion of CNCs in the watermethanol co-solvent system; and second, to study the influence of CNC concentration on the geometrical and mechanical properties of the electrospun composite fiber mats. To our knowledge this is the Journal of Engineered Fibers and Fabrics 155

2 first study to report the effect of CNCs on electrospun PVP nanofibers. EXPERIMENTAL SECTION Raw Materials PVP with a molecular weight of 1,300,000 g mol -1 and bulk density 1.2 g ml -1 as well as methanol was obtained from Fisher Scientific (Ottawa, ON). CNCs were obtained in freeze-dried powder form from Alberta Innovates Technology Futures (AITF) in Edmonton, AB. AITF reports that Whatman No. 1 filter paper was used as raw material to obtain cellulose nanocrystals. The filter paper was shredded to small pieces using a paper shredder before hydrolysis of the materials. The reaction mixture solid content was 10 wt%. First, the shredded filter paper was added into 65 wt% of sulfuric acid in a water bath with mechanical mixing at 45 ºC for 120 minutes. Following hydrolysis, the cellulose suspension was diluted with de-ionized (DI) water; DI water volume was approximately 10 times that of the acid solution. Dilution was done to stop the reaction and the mixture was centrifuged to remove the acid. Next, 2 wt% sodium carbonate solution was added to neutralize the residual acid and subsequently washed one more time. At the final stage, the suspension was purified by dialysis with DI water. Finally, the suspension was frozen with a freeze dryer to obtain dry CNCs. Using the SEM images provided by AITF, the average length and width of the CNCs manufactured using this method was measured as 72.2 nm (± 21.1 Standard Deviation) and 6.3 nm (±1.9 Standard Deviation), respectively. This corresponds to an average aspect ratio (length/diameter) of 11.9 (±3.1 Standard Deviation). The aqueous solutions/suspensions in all experiments conducted for this study were prepared using DI water. CNC Dispersions In order to obtain solutions that were suitable for the electrospinning process, a reliable method to produce dispersions of CNCs in a methanol-water co-solvent system with added PVP was found. Use of co-solvents is a common practice in electrospinning technique for various reasons, such as spinning ease and stabilization, and end-product fiber morphological properties. PVP was first dissolved in pure methanol at a ratio of 15:85 by weight by magnetically stirring overnight at 200rpm. Also, CNCs were dispersed in DI water in a mass ratio of 1:10. To improve dispersion, the samples were alternately magnetically stirred for 1 hour at 300rpm and sonicated for 20 minutes in an ultrasonic cleaner (Model FS220H, Fisher Scientific, Pittsburgh, PA). This cycle was repeated three times for each sample (except Sample 3 see below). The final dispersions in water were homogenous with no visible agglomerates. Later, varying amounts of the 10 wt% CNC-water solution were then added to 6 samples of the PVPmethanol mixture. During the process extra DI water or CNC was added as needed to obtain desired concentrations. The CNC amount in each sample varied from 0% to 20% of the total weight of PVP. Table I summarizes the 6 experimental samples and their constituents. TABLE I. Constituents of experimental samples, in grams. Sample Methanol Water PVP CNC Sample 3 varied from other dispersions in that the entire mass of CNCs were added directly to the methanol-pvp solution concurrently with DI water (ie, the CNCs were not pre-dispersed in water) followed by the same combination of magnetic stirring and ultrasonication as listed above. This was done as a comparison to Sample 2, and to test the effects of CNC pre-dispersion in water. Early experiments revealed qualitative differences in CNCs directly dispersed in methanol/water versus predispersion within water followed by mixing with pure methanol. The purpose of comparison between Sample 2 and Sample 3 was to demonstrate the importance of order of mixing on dispersion quality, but this pre-dispersing process was used for all higher concentrations of CNCs in PVP. Figure 1 shows the final as-prepared solutions. FIGURE 1. As-prepared solutions. From left to right: 0, 5, 10, 15, and 20wt% CNC. Journal of Engineered Fibers and Fabrics 156

3 Electrospinning Process The PVP/CNC solutions were loaded into 5 ml BD plastic syringes with an 18 gauge (inner diameter = mm) blunted stainless steel needle, which was then connected to a high-voltage DC power supply (Gamma High Voltage Research, Ormond Beach, FL). The flow rate was controlled by an NE-300 syringe pump (New Era Pump Systems Inc., The voltage supply was grounded to a steel collector plate, which was wrapped in aluminum foil to facilitate removal of the fibers. The plate was positioned 20cm (horizontally) from the needle tip. Figure 2 shows the electrospinning apparatus used for this work. The dispersion of CNCs in the solutions was then investigated by polarized light microscopy. In this method, optically resolvable agglomerates of CNCs can be detected by birefringence under crosspolarized light [16]. Microscope samples were prepared by placing 0.5 g of each solution on a plain 3 x2 glass microscope slide and drying under a fume hood for 24h. It is assumed that the same amount of methanol and water evaporated from all samples, so that the amount of CNCs in each sample may be correlated to the level of birefringence in the images. Images were taken using a model RM-4200 GE camera (JAI Inc., San Jose, CA) and samples were illuminated from behind using a plasma light source (ThorLabs Inc., Newton, NJ.). Electrospun CNC-PVP and PVP-only fiber mats were examined by a Tescan Vaga-3 Scanning Electron Microscope (SEM). Samples containing each level of CNC loading from 0-20wt% were randomly chosen, and 10 images were taken at 25000x magnification from each sample. Of these 10 images, 4 were chosen at random for further analysis. Diameter measurements for fibers were taken digitally at 3 locations on 10 fibers for each image, resulting in 120 distinct fiber diameter measurements for each level of CNC loading. Figure 3 shows a characteristic SEM image of the as-spun fibers. FIGURE 2. Electrospinning apparatus used to create fiber mats. During manufacturing a flow rate of 25 µl min -1, a needle tip-collector distance of 20cm, and an average voltage of 15 kv, (a ratio of 0.75 kv cm -1 ) was used. In the electrospinning process, the continuously discharged solution forms a short stable conical region at the needle tip that is known as the Taylor cone. Stability of this Taylor cone region is crucial to produce desirable fibers of relatively consistent dimension [15]; therefore, the voltage was periodically adjusted +/- 10% during the electrospinning process. Each sample was spun for 16 minutes (until 0.4 ml of solution was dispensed from the syringe). Ambient humidity was measured as ±2% using a digital hygrometer (Fisher Scientific, Ottawa, ON) during both the electrospinning process and tensile testing. Relative humidity measurements were taken at 30-minute intervals during manufacturing and testing. Measurement and Testing Viscosities of the 6 solution samples were determined at room temperature using a Brookfield DV-III programmable rheometer (CAN-AM Instruments, Oakville, ON). The shear rate was varied from 10 to 50 s -1. Mechanical properties of the mats were investigated using an MTS Synergie 400 tensile tester with a 500N load cell. At the time of testing this was the smallest load cell available to the authors for this application. The fiber mats were first carefully removed from the aluminum foil substrate. Removal of the mats from the substrate was completed with extreme care to ensure there was no residue left after extraction. The mats were then placed on a graph paper template (10mm x10mm spacing) and sheared using a pair of scissors to produce strips 10mm in width and approximately 50mm in length. It was typically not possible to produce more than 5 tensile test specimens from a single mat because of the small area of the as-spun mats (< 100 cm 2 ). Width measurements were taken at 5 locations for each strip using a MasterCraft digital caliper, with the average value being used in stress calculations. The samples were loaded in tension using a crosshead speed of 2mm/min until failure occurred. A total of 10 tensile tests were analyzed for each sample. The total number of tensile tests was higher than this, but the only specimens that were analyzed were those that exhibited necking during testing and failed due to crack propagation from the middle of the strip as in Figure 4. In some tests, cracks propagated from one Journal of Engineered Fibers and Fabrics 157

4 of the two tensile grips. These tests were not analyzed nor included in the average values presented in this study because the material failure is caused by an induced stress concentration. For all tests, gauge length was recorded based on the first point at which the load cell records a force. The load cell was zeroed before each test, so it is assumed that no load acts on the cell until all slack in the specimen has been eliminated. The target gauge length for tensile testing was 30mm; resultant gauge lengths were between mm and mm as measured by the MTS tester. Also, there is high variability in the area over which the solution is deposited during the electrospinning process. Consequently, it was necessary to normalize the results by accounting for differences in the amount of PVP and CNC present in the mats per unit area. FIGURE 4. Tensile test specimen showing failure in middle of strip. FIGURE 3. SEM image of electrospun fibers at 5000x magnification, scale bar 10 µm. 1) 0wt% CNC, 2) 5wt% predispersed CNC, 3) 5wt% non-predispersed CNC, 4) 10wt% predispersed CNC, 5) 15wt% predispersed CNC, 6) 20wt% predispersed CNC. A separate mat spun from each solution was measured for thickness in 10 locations using a Miyamoto Vernier micrometer in order to determine cross-sectional area. Similar to the width measurements, the average value was used for stress calculations. The fiber mats were also weighed to determine their density as they are highly porous and simply dividing the applied force by the cross-sectional area does not give a true indication of the stress within the fibers. Square pieces of mats with areas between 2500 and 4900 mm 2 were cut from each sample and weighed using a scale with precision of 0.1 mg (Sartorius AG, Goettingen, Germany). The density for each mat was calculated based on the measured mass, area, and thickness. Mat porosity was then calculated by (1) where = 1.2 g ml -1 as specified by the supplier. The thickness, density, and porosity of as-spun mats are summarized in Table II. TABLE II. Geometric Properties of as-spun mats. CNC loading 0wt% 5wt% 10wt% Thickness, t (μm) 58.4± ± ±24.1 Width, w (mm) 10.0± ± ±0.12 Mat Density, ρ mat (g ml -1 ) Porosity, ϕ mat (%) Journal of Engineered Fibers and Fabrics 158

5 RESULTS AND DISCUSSION Birefringence of Solutions Figure 5 shows the results of cross-polarized microscopy images, labeled by sample number corresponding to the numbers in Table I. The amount of light in the images due to birefringence of microscale CNC agglomerates increases with the amount of CNCs that have not been fully dispersed in the medium; this is the opposite of viscosity, which decreases when agglomerates form. A higher level of birefringence corresponds to a less homogenous suspension with more cellulose agglomerates; an image of a solution with complete (i.e., perfect) dispersion should therefore appear black. As predicted, the amount of birefringence in Sample 3, in which the CNCs were not predispersed in water, is significantly higher than its counterpart in Sample 2. It can be easily seen that the level of homogeneity of Sample 3 (5wt% CNC; not predispersed) is even lower than that of Sample 4 (10wt%; predispersed). This result shows the effectiveness of the predispersion technique developed in this study. It also presents a solution to one of the key problems in the production of CNC-reinforced composite materials, which is to obtain homogenous dispersions of CNCs in aqueous media without the use of chemical surfactants or additives. This allows any composite material produced from these suspensions to take full advantage of the mechanical and chemical properties of CNCs, and may also reduce associated manufacturing costs by decreasing the material requirements and duration of the process. This predispersion method in water is also applicable to the manufacture of composite materials that must be biocompatible, since many water-soluble polymers including PVP are non-toxic; additionally, many of these polymers easily dissolve in solvents which easily mix with water such as methanol. Naturally, a drawback of the method is that the hydrophilicity of CNCs and the polarity of water make further mixing with non-polar liquids and organic solvents extremely difficult, which may prohibit production of composite materials using certain acrylics and plastics. FIGURE 5. Cross-polarized light images of CNC-PVP solutions showing birefringence of CNC. 1) 0wt% CNC, 2) 5wt% predispersed CNC, 3) 5wt% non-predispersed CNC, 4) 10wt% predispersed CNC, 5) 15wt% predispersed CNC, 6) 20wt% predispersed CNC. Scale bar 100 µm. Rheology of Solutions Figure 6 shows the rheology measurements of Solutions 1-6 with shear rates from 10 to 50 s -1. As expected, the viscosity increases steadily with the amount of CNCs dispersed in the solution. All solutions also exhibited shear-thinning behavior as the shear rate was increased, with the relative rate of shear thinning increasing with the concentration of CNCs; this behavior is consistent with the findings in [11, 17-18]. Journal of Engineered Fibers and Fabrics 159

6 FIGURE 6. Shear rate vs. apparent viscosity of PVP-CNC solutions from 0-20wt% CNC loading. One important finding of the shear thinning experiments is the difference in viscosity between Solutions 2 and 3. Both contain 5% CNC by PVP weight and the same methanol/water content, but the viscosity of Solution 3 (in which the CNCs were not predispersed in water) is approximately 25% lower than the viscosity of Solution 2. This is most likely because the cellulose clumps into micro-scale agglomerates rather than fully dispersing in the cosolvent system, as is the case with the other solutions. Scanning Electron Microscopy of Electrospun Fiber Mats Electrospun fibers were observed to be free of beads, and the average fiber diameters were measured as 484, 372, 443, 384, and 385 nm for 0, 5, 10, 15, and 20 wt% CNC reinforcements, respectively. Figure 7 shows the minimum and maximum (whiskers), and first and third quartiles (boxes) of fiber diameter for each level of CNC loading. The change in fiber diameter and deviation does not seem to have a direct correlation with the amount of CNCs present in the mats. Stress-Strain Behavior of Mats Figure 8 shows characteristic stress-strain curves for fiber mats containing 0, 5, and 10 wt% CNC respectively. Tensile tests were also attempted for fiber mats containing 15 wt% and 20 wt% CNC, but many of these samples failed due to crack propagation from the tensile grips. The result is that the sample sets were not sufficient for purposes of comparison to the lower levels of CNC loading. FIGURE 7. CNC loading vs. diameter distribution of as-spun PVP fibers. The non-reinforced PVP mats exhibit a pseudo-yield point between 2 and 5% strain at very low stress values. These findings are consistent with tensile tests of other electrospun fibers produced from watersoluble polymers [19, 20]. This behavior may be as a result of a certain number of fibers being randomly aligned to the direction of elongation at the onset of the test. Journal of Engineered Fibers and Fabrics 160

7 FIGURE 8. Characteristic stress-strain curves of tensile samples. The addition of reinforcing CNC particles does not seem to change this behavior. The mats containing CNCs also yield at low strain; however, this yielding happens at a stress level that is much closer to the specimen s ultimate tensile strength as seen in Figure 8. The addition of reinforcing CNCs results in slightly higher E and significantly higher yield stress. To account for porosity in the mats, stress for all tests was calculated by F σ = (2) 1 ϕ ) A( mat where F is the measured force and A is the crosssectional area of the specimen. Unlike many classical engineering materials, nanofiber woven mats do not have a well-defined failure point. As the mats are elongated, failure occurs at individual fibers within the composite mat; therefore, once the ultimate tensile strength has been exceeded the calculated engineering stress in the mat slowly decreases with the number of remaining intact fibers that are being loaded. In similar studies a common method used is to record εb, strain at break, as the strain at which the stress is again reduced to zero or some other arbitrarily small value. On the other hand, for practical purposes, it is also useful to report the strain at which the Ultimate Tensile Strength is reached, εuts. In this study, εb was considered to be reached once the calculated stress fell below 1 MPa. Table III shows the elastic modulus, ultimate tensile strength (UTS), and % strain at maximum stress (εuts) and at break (εb) for 0, 5, and 10 wt% CNC reinforced PVP electrospun fiber mats. The average elastic modulus, E, of the samples nearly doubled with the addition of 5wt% CNC from 247.3MPa to MPa. However, the average E dropped to MPa at 10wt% CNC loading. This is similar to the findings reported in [11] that suggested that the decrease in stiffness might be due to decreased fiber size uniformity. Authors believe in the need and intend to further investigate in future studies the effect of CNC reinforcement on elastic modulus using smaller wt% increments rather than that of 5wt% as done in this study. Smaller increments will show a clear optimal wt% CNC loading for the elastic modulus change for the mats. TABLE III. Mechanical Properties of as-spun mats. CNC loading 0wt% 5wt% 10wt% E (MPa) 247.3± ± ±106.9 UTS (MPa) 25.2± ± ±6.8 ε UTS (%) 30.4± ± ±3.0 ε B (%) 41.2± ± ±9.0 Average UTS decreased by 23% (from 25.2 to 19.4 MPa) with the addition of 5wt% CNC. Likewise, the corresponding strain εuts decreased from 30.4% to 12.3% and εb dropped from 41.2% to 19.6%. This Journal of Engineered Fibers and Fabrics 161

8 may be explained by the fact that the incorporation of CNCs creates stress concentrations which allow cracks to originate and propagate faster than in the samples containing only PVP, leading to failure earlier in the test. There was very little change in UTS and εuts from 5wt% to 10wt% CNC loading. This may suggest, as discussed in the elastic modulus findings, that there may be some critical level of CNC loading below 5wt% at which a sudden decrease in ultimate tensile strength and % strain at break is observed. However, despite the similarity in UTS and εuts from 5wt % to 10wt % CNC loading, the average strain at break increases significantly from 19.6% to 24.0% strain. In this case, the latter average has been skewed upwards by two specimens which failed at 40.3% and 38.4% strain, which when combined with the small sample size yields an unrepresentative average. The median % strain at break for 5 wt% - and 10 wt% - CNC loading were 19.0% and 21.3% respectively. Humidity and Anisotropy of Mats Ambient humidity was a critical factor identified during testing. PVP is extremely hygroscopic; as a result, the structural integrity of the mats quickly degrade when exposed to high ambient humidity, and samples become sticky and lose most of their structural integrity within 36 hours at 30% RH. The effects of ambient humidity on PVP have been well characterized [21], as have the effects of ambient humidity on the microstructure of electrospun PVP fiber mats [22]. Humidity could not be controlled during manufacturing and testing, so all mats were sealed in airtight storage containers immediately after spinning and then tested within 72 hours. It is desirable in the future to study the effects of ambient humidity during the electrospinning process on the tensile behavior of as-spun mats. There is also potential for investigation into the isotropy of electrospun nanocomposite mats with regards to mechanical properties. Because of the inherent variability in the electrospinning process there is potential for anisotropy in the mats based on the alignment of randomly distributed fibers. Electrospun micro-fibers are much stronger in tension than in shear [23], consequently the plane along which tensile samples are extracted may have an effect on modulus or tensile strength of the mat. In this study, all tensile samples were cut such that the direction of elongation was kept constant with respect to the original orientation of the mat during the electrospinning process. CONCLUSION A method for dispersing cellulose nanocrystals (CNCs) in a water-methanol co-solvent system at varying concentrations was developed. Examination of solutions by polarized light microscopy and rheology testing show that pre-dispersion of CNCs in water is necessary to obtain greater dispersion of suspensions in the final co-solvent system. Prepared solutions were used to manufacture Polyvinylpyrrolidone (PVP) nano-fibrous mats via electrospinning. Varying levels of CNC with respect to PVP weight were incorporated into these mats for structural reinforcement. Investigation by scanning electron microscopy showed that as-spun fibers were sufficiently uniform, and furthermore that the level of CNC loading did not have a significant effect on fiber diameter. Mechanical properties of fiber mats were obtained via tensile testing. It was found that the incorporation of CNCs caused increase in stiffness while reducing the percent strain at break and ultimate tensile strength. The addition of CNCs to electrospun PVP fiber mats is desirable for applications requiring enhanced stiffness. In the future, the authors wish to investigate further the effects of 0-10% CNC loading using smaller increments in order to optimize the reinforcing material use. ACKNOWLEDGMENTS The CNC used in this research was provided by Alberta Innovates Technology Futures (AITF) in Edmonton, AB, Canada; therefore, the authors would like to thank AITF for the material provided. REFERENCES [1] Reneker, D.H.; Yarin, A.L.; Alexander, L. Electrospinning jets and polymer nanofibers, Polymer, vol. 49, 2008, pp [2] Deliu, R. et al. "Study of the Influence of Electrospinning Parameters on the Structure and Morphology of Polyvinyl Alcohol Nanofibers," Revista de Chimie, vol. 6, 2012, pp [3] Deitzel, J.M.; Kleinmeyer, J.D.; Hirvonen, J.K.; Beck Tan, N.C. Controlled deposition of electrospun poly(ethylene oxide) fibers, Polymer, vol. 42, 2001, pp [4] Huang, Z.M. et al. "A review on polymer nanofibers by electrospinning and their applications in nanocomposites," Composites Science and Technology, vol. 63, 2003, pp Journal of Engineered Fibers and Fabrics 162

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