外文翻译原文：光滑纳米纤维素（I）薄膜接触角的测量

Journal of Adhesion Science and Technology 25 (2011) 699708 brill.nl/jast Contact Angle Measurements on Smooth Nanocrystalline Cellulose (I) Thin Films Theresa A. Dankovich and Derek G. Gray McGill University, Department of Chemistry, Pulp and Paper Building, Montreal, QC, Canada H3A 2A7 Abstract Interactions of cellulose fi ber surfaces with water and other liquids depend on surface morphology as well as intrinsic material properties. Smooth nanocrystalline cellulose (I) fi lms can be used as models to study surface phenomena, where the effects of surface morphology and roughness are minimized. Contact angle measurements are particularly sensitive to surface roughness. In this work, we measured the advancing and receding contact angles for water on thin model cellulose (I) and regenerated cellulose (II) fi lms. The advancing and receding contact angles on model cellulose (I) surfaces were lower than on cellulose (II) surfaces,andthecontactanglehysteresiswasalsolowerforthesmoothmodelcellulose(I)surfacesprepared from nanocrystal suspensions. The surface free energy was evaluated for the various cellulose surfaces from contact angle measurements. Koninklijke Brill NV, Leiden, 2011 Keywords Nanocellulose, contact angle, cellulose, thin fi lms, model surfaces 1. Introduction Cellulose fi bers occur in plants as crystalline cellulose (I) regions interspersed with less ordered amorphous regions. The wettabillity and adhesion properties of cellu- losic surfaces are crucial for the utilization of wood, wood fi ber, pulp and cotton in paper, textiles and composites. In general, the surface morphology of natural cellu- lose from plant fi bers is complex, and the wetting and adhesion properties depend on both composition and morphology. It is often diffi cult to disentangle the contri- butions from each of these factors, especially in conventional wetting experiments. While it is possible to make smooth cellulose surfaces by dissolving cellulose or cellulose derivatives in suitable solvents and regenerating the cellulose from solvent-cast fi lms, this results in changes of the crystalline structure to cellulose (II) or other less common crystal forms 1. Wetting properties and surface free en- *To whom correspondence should be addressed. Tel.: (1-514) 398-6182; Fax: (1-514) 398-8254; e-mail: derek.graymcgill.ca Koninklijke Brill NV, Leiden, 2011DOI:10.1163/016942410X525885 700T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708 ergies have been measured on fi lms regenerated from viscose and cellulose acetate 25, and on thin fi lms prepared by spin coating or LangmuirBlodgett deposi- tionmethodsfromsolventsystemssuchasN-methylmorpholine-N-oxide (NMMO) with dimethyl sulfoxide (DMSO) or dimethylacetamide with lithium chloride 6 8. The cellulose in these smooth fi lms is in the cellulose (II) and/or amorphous form. However, highly crystalline cellulose (I) nanocrystals may be cast from aque- ous suspension and heat-stabilized to give model cellulose (I) fi lms; the nanocrystal dimensions are small enough to give surfaces with root mean square roughness val- ues in the range of a few nanometers, depending on substrate and cellulose source 9. Since macroscopic fi lms that are essentially pure cellulose (I) may thus be pre- pared, this facilitates surface studies on materials more closely resembling the form of cellulose found in natural cellulose fi bres, without the problems introduced by surface roughness and porosity. The difference in contact angles measured for droplets advancing and receding on the surface of a material is generally referred to as contact angle hysteresis, and may be due to surface roughness, surface heterogeneity, or sorption of the droplet liquid into the bulk of the material 10. For example, Toussaint and Luner observed a rapid decrease of the water contact angle with time for cellulose fi lms, which they attributed to water penetrating into the cellulose 4. This caused the cellulose to swell, lowering the interfacial energy and decreasing the contact angle. Contact angle hysteresis in the range of 2030for water on cellulose has been reported 2, 5, and it has been suggested that the degree of crystallinity plays a role in wetting hysteresis and adhesion 3, 11. In this study, the advancing and receding water contact angles were measured for a variety of cellulose surfaces and fi lm thicknesses, including surfaces of nanocrys- talline cellulose (I) and of regenerated cellulose (II) from cellulose acetate. Contact angles for diiodomethane and formamide were also determined. The results are dis- cussed in terms of contributions 12, 13 to the surface free energy of the cellulose. 2. Experimental 2.1. Materials Following a previously published method, we made colloidal suspensions of cellu- lose nanocrystals from cotton fi bers (Whatman No. 1 fi lter paper) 9. Cellulose acetate powder (Mn 30000, acetyl content 39.8%), sodium hydroxide, diiod- methane, and formamide were supplied by Sigma-Aldrich. 2.2. Preparation of Cellulose Films We prepared cellulose fi lms from nanocrystalline suspensions either on glass slides cleaned in a bath of 3:1 concentrated sulfuric acid/30% hydrogen peroxide mixture or on polystyrene (PS) Petri dishes. The cleaned glass slides showed a zero contact angle for water. To form a thin fi lm on glass, we cast approximately 200 l of 2.65% T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708701 (w/w) cellulose nanocrystal suspension onto a glass slide and dried it at 60C for an hour. To form a thick and free-standing fi lm, we evaporated 5 ml aliquots of cellulose nanocrystal suspension on a PS petri dish at 60C overnight. To form thin fi lms on the nanoscale, we used a spin coater to deposit thin layers of cellulose nanocrystal suspension on glass slides. The range of fi lm thickness varied from approximately 100 nm for the spin coated fi lms to 10 m for the fi lms cast on glass to 130 m for the free-standing fi lms. To stabilize the fi lms and prevent the re- dispersion of the nanocrystals, we heated them at 105C for 2030 min. The heat treatment removes sulfate groups from the surface of the cellulose nanocrystals 14. Regenerated cellulose (II) surfaces were prepared by sodium hydroxide hydroly- sis of cellulose acetate fi lms 15, 16. To facilitate adhesion of the cellulose acetate spin coated fi lms to the glass surfaces, the glass was rendered less hydrophilic by treatment with dichlorodimethylsilane 17. After spin-coating from acetone solution, the cellulose acetate surface was then hydrolysed with aqueous sodium hydroxide to form a regenerated cellulose (II) surface. 2.3. Contact Angle Measurement Contact angles were measured on sessile drops with a goniometer (contact angle system OCA20, Dataphysics, Germany) and a CCD camera at room temperature (23C) in air with a relative humidity of 1025%. A 3 l drop volume of the probe liquid was placed on the cellulose surface from a micro-syringe (Hamilton- Bonaduz). To determine the advancing contact angles, a, we added 2 l to the drop on the surface to gently advance the contact line. For receding contact angles, r, liquid was withdrawn with the syringe needle until the contact line re- tracted (Fig. 1). Measurements were made on three or more drops and averaged. Unless stated otherwise, each measurement was made on a new spot. Water, di- iodomethane, and formamide were used as probe liquids; their surface tension and surface tension component values are summarized in Table 1. Figure 1. Side view of a sessile liquid drop on a solid surface showing advancing and receding contact angles, aand r. (a) Liquid is added to the drop and the contact angle advances. (b) As liquid is withdrawn, the contact line remains pinned until the contact angle decreases to r, then the contact angle recedes 21. 702T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708 Table 1. Literature values for liquid surface tension and its components (mN/m) LiquidlLW,dp+ Water72.821.851.025.525.5 Diiodomethane50.850.80.00.00.0 Formamide58.039.029.039.62.28 LW: Lifshitzvan der Waals. Table 2. Advancing (a), receding (r) contact angles and hysteresis (ar) () with standard deviations for water on cellulose fi lm surfaces Surfacear(ar) Nanocrystalline cellulose (I) Spin-coated fi lm13.10.68.00.55.20.9 Cast fi lm17.81.110.92.66.93.2 Free-standing fi lm29.2 2.715.52.413.71.7 Regenerated cellulose (II) Spin-coated fi lm48.53.831.06.917.55.3 Free-standing fi lm37.65.218.94.318.64.0 The time dependence of the contact angle for water on the cellulose surfaces was measured in a separate set of experiments. The advancing angle was measured 30 s after the removal of the capillary tip (initial advancing angle), and again every 30 s for a period of around 8 min. 2.4. Water Absorption Following the ASTM D570-98 method 18, we measured the water absorption of the free-standing cellulose fi lms. Triplicate samples were dried at 105C for 20 30 min, cooled in a desiccator, and weighed. Due to the thinness and high rate of absorption of cellulose fi lms, the samples were submerged for only two hours in deionized water at 23C. The samples were then wiped dry, and reweighed imme- diately. 3. Results 3.1. Contact Angles of Water on Nanocrystalline Cellulose (NCC) and Regenerated Cellulose The advancing and receding contact angles on the surfaces of nanocrystalline cel- lulose and on the cellulose surfaces regenerated from cellulose acetate fi lms are summarized in Table 2. The spin coated nanocrystalline cellulose fi lms had the T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708703 Figure 2. Time dependence of the (initially advancing) water contact angle on cellulose surfaces. Error bars show standard deviations. lowest advancing water contact angle (13.1), the lowest receding contact angle (8) and the hence the lowest hysteresis value of 5 . The other fi lms of cellulose nanocrystals showed higher contact angles ( = 1828) and higher hysteresis (7 14 ) as the fi lm thickness increased from around 100 nm for the spin coated fi lm to 10 m for the cast fi lm on glass to 130 m for the free-standing fi lm. The fi lms with regenerated cellulose surfaces showed even higher values for contact angles and hysteresis. The decrease with time of the advancing contact angles on the cellulose surfaces is shown in Fig. 2. Again, the very thin cellulose nanocrystal layer spin-coated on glass reached the lowest contact angle (5) in about 5 min. This value is a little lower than the receding contact angle measured on the same surface. The decrease incontactanglewithtimeappearstobelevelingoffonlyforthethinnestnanocrystal sample. Measurements of contact angles for some specifi c liquids are often used to es- timate various contributions to the surface free energy of the substrate. Contact angles for water, diiodomethane and formamide on the cellulose surfaces are listed in Table 3, along with data for a spin coated cellulose triacetate fi lm (as used for surface hydrolysis to give the regenerated cellulose fi lm) and some data on a cotton cellulose sample (more typical of contact angles measured on fi brous sur- faces). 704T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708 Table 3. Contact angle measurements () for water (W), diiodomethane (D) and formamide (F) SurfaceWDF Nanocrystalline cellulose (I) Spin-coated fi lm13.1 0.625.9 2.65.2 0.7 Cast fi lm17.8 1.132.5 1.85.5 1.0 Free-standing fi lm29.2 2.735.0 2.45.6 1.7 Regenerated cellulose (II) Free-standing fi lm37.6 5.231.1 4.233.8 6.6 Cellulose acetate Spin-coated fi lm58.4 3.437.4 0.733.1 3.3 Cotton cellulose fabric Scoured (base-treated)41.0 3.434.6 5.821.0 4.5 *Contact angle data from Dankovich and Hsieh, 2007 20. 4. Discussion It was hoped that the high crystallinity and fl at surface of fi lms cast from cellulose (I) nanocrystal suspensions would avoid the complexities due to surface roughness and water absorption often observed for contact angle measurements on natural fi bres. Overall, the measured contact angles are in line with recent literature values. Yamane et al. 8 measured contact angles of around 12for water on regenerated cellulose and on cellulose (II) nanocrystals acid-hydrolysed from cuprammonium cellulose for a 500 m thick fi lm. Eriksson et al. 11 reported a somewhat higher value of 19.5for a 120 nm thick layer of cellulose (I) nanocrystals spin coated on a silicon wafer, and Aulin et al. measured a contact angle of 23.7for a very similar nanocrystal fi lm 19. However, there are some signifi cant trends in our results. (i) All the contact angles decreased with time. (ii) There is an apparent trend to higher contact angles and larger contact angle hysteresis with increasing thickness. Evidently, effects of roughness and water interactions must still be considered. Although the nanocrystal surfaces must have a topography of the same order of magnitude as the nanocrystal dimensions, these are of the order of tens of nanome- ters at maximum, and even for the upper surface of cast suspensions, AFM mea- surements suggest an rms roughness of 2.3 nm 9, 11. Compared to most fi brous cellulose sample, even the thickest nanocrystal fi lms are thus very smooth. Further- more, the simplest model for contact angles on rough surfaces suggests that for true contact angles less than 90, the apparent angle should be decreased by roughness, whereas the upper surface of the thick cast fi lm (potentially the roughest surface 9) showed the largest contact angles (Fig. 2). On the other hand, interaction with water is a key to most applications of cel- lulose. We had hoped that because the nanocrystals were very crystalline, and T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708705 crystalline cellulose does not swell in water, time-dependent effects on contact angle measurements due to swelling and water absorption would be minimized. However, there is strong evidence that in fact fi lms of nanocrystals do absorb sig- nifi cant amounts of water. After immersion in water for 2 h, we found that a sample of the free-standing fi lm absorbed about 16% water by weight, and Aulin et al. 19 recently determined a higher degree of swelling (26%) for thin (23.6 nm) fi lms of cellulose nanocrystals on quartz, by means of quartz crystal microbalance with dissipation (QCM-D). So, despite the highly crystalline nature of the nanocrystals, there is evidently suffi cient pore space between them for water sorption and fi lm swelling to occur. The hysteresis and the decrease in contact angles with time are thus most probably a consequence of the usual uptake of water by cellulosic materi- als, as also demonstrated by the results for the regenerated fi lms. The YoungDupr equation gives lvcos = o sv(t)slwhere the liquidvapor and solidvapor sur- face tensions are lvand o sv, respectively, and the solidliquid interfacial tension, (t)sl is time dependent, due to the diffusion of water into the nanoporous fi lm. As the pores between the nanoparticles imbibe water, this reduces the interfacial tension between the water and the cellulose, reaching a limit when the cellulose is saturated with water. Assuming that this is a diffusion process, the thin fi lms should reach saturation much faster than the free-standing fi lms, and so the lat- ter should have values for (t)slthat are initially close to that for the water/dry cellulose interface, dropping with time to that for the water/saturated cellulose in- terface. Since we assume that lvand o sv are time-independent, this suggests that the corresponding contact angles decrease with time to a value corresponding to the saturated nanocrystalline cellulose fi lm, and that this should occur much faster for the thinnest fi lm, in accord with the experimental observations. The results for diiodomethane and formamide advancing contact angles (Table 3) may be used to estimate the polar ( p s) and dispersion ( d s) components 22 of the cellulose surface free energies. Following Owens and Wendt 12, assuming that the interfacial free energy across the celluloseliquid interface is related to the geometric mean of the polar and dispersion surface free energies of the cellulose and the liquid, the surface free energy of the solid can be determined from: (1+cos)lv= 2? ? d l d s + ? p l p s ?, (1) where lvis the surface tension of the test liquid, subscripts l and s refers to the test liquid and cellulose surfaces, respectively, and superscripts d and p refer to the dispersion and polar contributions to the surface free energies, respectively. A si- miliar expresion has been given by van Oss, Good, and Chaudhury 13 to include Lewis acidbase components, where the term in polar interactions on the right in equation (1) is replaced by: ? + s l + ? s + l ?, where the superscripts + and refer to electron acceptor (acid) and electron donor (base) parameters, respectively. 706T. A. Dankovich, D. G. Gray / J. Adhes. Sci. Technol. 25 (2011) 699708 Table 4. Comparison of total and constituent surface free energies (mJ/m2) determined from dispersionpolar and Lifshitzvan der Waals (dispersion)/acidbase approaches Surface typeTotal sdpAB+ Nanocrystalline cellulose (I) (spin coated fi lm) Dispersionpolar78.945.833.1 Acidbase60.945.815.150.31.1 Nanocrystalline cellulose (I) (cast fi lm) Dispersionpolar7543.131.9 58.443.115.347.51.2 Nanocrystalline cellulose (I) (free-standing fi lm) Dispersionpolar69.542.027.5 57.842.015.837.71.6 Regenerated cellulose (II) (free-standing fi lm) Dispersionpolar64.843.721.1 Acidbase49.443.75.740.80.2 Cellulose acetate (spin coated fi lm) Dispersionpolar55.640.914.7 Acidbase49.440.98.514.51.2 Cotton cellulose fabric(base treated) Dispersionpolar63