Abstract
Silica nanoparticle (SiNP)-poly(vinyl alcohol) (PVOH) coating is an important material system in paper coating applications, where particle distribution critically affects coating performance. In the present study, the authors investigated a role of physicochemical interaction between SiNP surface and PVOH chain in SiNP distribution in the coating layer, with a comparison of the suspension at pH 3 (good interaction) and pH 10 (poor interaction) as PVOH concentration was varied. Rheological properties and sedimentation behavior of the suspensions showed the dispersion stability of SiNP at pH 3 was improved by the addition of PVOH, whereas it was independent of the PVOH concentration at pH 10. Scanning electron microscopy and small angle x-ray scattering intensity of dried coating layer showed the uniform and dense structure with homogeneous distribution of SiNPs at pH 3, where spatial arrangement of SiNPs depended on the addition of PVOH. However, non-uniform and porous structures with SiNP aggregates were observed at pH 10, where the spatial arrangement of SiNPs was independent to the addition of PVOH. The stress development during drying of the coating suggested that the mechanical property was related to the spatial arrangement of individual SiNPs at pH 3, whereas to the distribution of SiNPs aggregates at pH 10.
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Particle Dispersion in Silica-Poly(vinyl alcohol) Coatings: Role of Particle-Polymer Interaction
Sunhyung Kim,￡ Sang Hoon Sung, Sanghyuk Lim, and Kyung Hyun Ahn*
Silica nanoparticle (SiNP)-poly(vinyl alcohol) (PVOH) coating is an important material system in paper coating applications, where particle distribution critically affects coating performance. In the present study, the authors investigated a role of physicochemical interaction between SiNP surface and PVOH chain in SiNP distribution in the coating layer, with a comparison of the suspension at pH 3 (good interaction) and pH 10 (poor interaction) as PVOH concentration was varied. Rheological properties and sedimentation behavior of the suspensions showed the dispersion stability of SiNP at pH 3 was improved by the addition of PVOH, whereas it was independent of the PVOH concentration at pH 10. Scanning electron microscopy and small angle x-ray scattering intensity of dried coating layer showed the uniform and dense structure with homogeneous distribution of SiNPs at pH 3, where spatial arrangement of SiNPs depended on the addition of PVOH. However, non-uniform and porous structures with SiNP aggregates were observed at pH 10, where the spatial arrangement of SiNPs was independent to the addition of PVOH. The stress development during drying of the coating suggested that the mechanical property was related to the spatial arrangement of individual SiNPs at pH 3, whereas to the distribution of SiNPs aggregates at pH 10.
Keywords: Silica nanoparticle-poly(vinyl alcohol)l coating; Particle-polymer interaction; Particle distribution; Dispersion stability; Rheology; Stress development during drying
Contact information: School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul National University, Seoul 151-744, Republic of Korea; *Corresponding author: ahnnet@snu.ac.kr
￡Present address: Corporate R&D, LG chem. Ltd, Republic of Korea
INTRODUCTION
Hybrid organic/inorganic coatings combine the flexible and ductile nature of the organic polymer with the rigid and thermo-mechanically stable nature of the inorganic particles. Silica nanoparticles (SiNPs) exhibit many advantages as an inorganic candidate because of low cost, high thermal and chemical stability, high abrasion resistance, and relatively low refractive index. Therefore, SiNP-polymer coatings have received much attention in applications such as adhesives, protective coating, optical coating, and barrier film (Zou et al. 2008; Ribeiro et al. 2014).
Poly(vinyl alcohol) (PVOH) is one of the most commercialized plastics in use due to its water-soluble and biodegradable properties and excellent mechanical properties as a barrier film and adhesive. By mixing with inorganic particles (e.g., silica) or biomaterials (e.g., cellulose), PVOH has been used in various applications, including paper coating as an ink-absorbing media (Zhang et al. 2015; Chen et al. 2017), protective coatings (Chang et al. 2015), electrolyte membranes (Yang et al. 2014), pharmaceutical coatings (Koo et al. 2011) bio-composites (Su et al. 2013), hydrogels (Tummala et al. 2016), and organic-inorganic hybrid materials (Ching et al. 2015; Pingan et al. 2017).
Controlling the dispersion state of SiNPs in polymer matrix, as well as the resulting properties of the SiNP-polymer nanocomposite, has been intensively studied because the distribution of SiNP and its aggregates affect the material properties. One way to control SiNP dispersion is to adjust the physicochemical interaction between silica surface and polymer segment, for example, by selecting a suitable solvent (Bansal et al. 2005; Jouault et al. 2014) or varying the suspension pH (Kim et al. 2009). In aqueous SiNP/poly(vinyl alcohol) coatings, a physicochemical interaction between SiNP surface and polymer segment can be controlled by varying the suspension pH, which affects the hydrogen bonding between the silanol group of SiNP and the hydroxyl group in PVOH chain (Otsubo 1986). In a previous study on the SiNP-PVOH coating, the SiNP distribution was improved in SiNP-PVOH coating at low pH, resulting in increased PVOH adsorption on the SiNP surface (Kim et al. 2009).
Despite the importance of the particle-polymer interaction in particle distribution in SiNP-PVOH coatings, a fundamental understanding on the role of particle-polymer interaction in particle distribution, and resulting coating structure and property, is not yet fully understood. The role of particle-polymer interaction on structural evolution during drying and the resulting particle distribution in SiNP-PVOH coatings has been investigated previously (Kim et al. 2009, 2016; Lee et al. 2017). However, these studies were rather focused on the mechanism of drying behavior, and thus did not contribute much information on the property and structure of resulting dried coatings. Therefore, there has been a need to understand the structure and properties of the SiNP-PVOH coating resulted from varied particle-polymer interaction to obtain a controllable coating structure. It is particularly important in a paper coating application as an ink absorbing media, where porosity control is a key factor affecting product performance (Lamminmäki et al. 2012).
In the present study, the role of particle-polymer interaction in structure and property of a SiNP-PVOH coating was systemically investigated at a ‘good’ interaction and a ‘poor’ interaction by varying PVOH concentration in the suspension. First, the structure of the coating suspension was characterized by a sedimentation experiment and rheological measurement. Second, microstructure of the dried coating layer was probed at a particle and aggregate length scale by small angle x-ray scattering intensity and scanning electron microscopy (SEM) respectively. Finally, the mechanical property of coating layer was characterized by stress development measurement during drying. From the results, the dependency of structure and property of SiNP-PVOH coating was determined relative to PVOH and exhibited a marked difference between the ‘good’ and ‘poor’ particle-polymer interaction.
EXPERIMENTAL
Materials
Charge-stabilized aqueous SiNP suspension (Ludox HS-30, Aldrich, St. Louis, MO, USA) was used as received. The specific surface area of SiNP was 220 m2/g, and the density was 2.2 103 kg/m3according to the supplier. From the SAXS measurement, average diameter of SiNP d was determined to be 15 nm, with /d = 0.145, where is the width parameter of the Schulz distribution function (Kim et al. 2013). The PVOH of molecular weight of 31 103 g/mol to 50103 g/mol, degree of hydrolysis 87% to 88%, and density of 1.19 103 kg/m3, was supplied by Aldrich. A 15 wt.% PVOH solution was prepared as a stock solution by dissolving it in a deionized water at 80 °C for 3 h. The SiNP-PVOH suspensions were prepared to 10 wt.% of silica with a varied PVOH concentration(PVOH) from PVOH= 0.5 wt.% to 7.5 wt.%. Because pH of the suspension was initially pH 10, a 0.3 M HCl solution was gradually dropped into the suspension on the magnetic stirrer for the suspension pH 3. As time dependence was observed from the previous research (Kim et al. 2010), all measurements were performed for the suspensions of strictly 24 h stirring at room temperature.
Rheological Measurements
An ARES strain-controlled rotational rheometer (TA Instruments, New Castle, DE, USA) was used to investigate the rheological properties of the suspensions. Parallel plates geometry with 50 mm in diameter were used. The temperature was maintained at 25 °C during measurements by a Peltier system. A solvent trap was used to reduce evaporation during measurements. The steady shear viscosity of the suspensions was measured with 30 s of equilibration time and 30 s of measurement time. A frequency sweep measurement of the storage and loss moduli G′ and G″ was conducted at a strain of 0.5%, which was within the linear viscoelastic regime obtained from the amplitude sweep test at the oscillatory frequency = 1 rad/s.
Coating Microstructure after Drying
The microstructure of the SINP-PVOH coating after drying was probed by small angle x-ray scattering (SAXS) and SEM (JSM-840A, JEOL, Akishima, Japan). SAXS measurements were performed on a PLS-II 9A U-SAXS beam line at the Pohang Accelerator Laboratory (Pohang, Korea). The two-dimensional (2D) scattering image from the sample of 0.1 mm thickness was recorded on a CCD detector (SX165, Rayonix, Evanston, USA). The wavelength of the x-ray beam was 1.1159 Å, and the sample-to-detector distance was 4511 mm, covering a range of 0.06 nm-1 to1 nm-1.
Stress Development of Coating Layer During Drying
The cantilever deflection technique was utilized to measure stress development during drying of SiNP-PVOH coating (Payne et al. 1997). The cantilever made of a silicon wafer 70 mm long and 6 mm wide was clamped at one end with the other end free. After coating the suspension by blade coating applicator, the cantilever was brought to the drying chamber with a relative humidity of 10 3% and drying temperature of 252C for the measurement of cantilever deflection during drying. Deflection was measured by laser (LM-6501NAP, Lanics Co, Seoul, Korea) and position sensing detector (PSDM4, Thorlabs, USA) and recorded in situ during drying through a data acquisition system and program coded by LabVIEW (National Instruments, Austin, TX, USA). Deflection was transformed to stress using the Corcoran equation (Corcoran 1969). Details on the measurement equation can be found in Kim et al. (2009).
RESULTS AND DISCUSSION
SiNP-PVOH Interaction
The zeta potential () and adsorption amount of PVOH on the SiNP surface ( ) was evaluated by varying suspension pH, as shown in Fig.1 (Kim et al. 2016). A PVOH chain is adsorbed onto the SiNP surface by the hydrogen bonding of protonated silanol group (-SiOH) in SiNP and hydroxyl group (-OH) in PVOH chain. Protonation of the silanol group depends on suspension pH; e.g., SiO– is favorable in basic condition, whereas SiOH is favorable in acidic condition (Tadros 1978). Therefore, hydrogen bonding resulting in the increased amount of PVOH adsorption is promoted at the acidic condition. Therefore, the measurement of allows for the quantification of the interaction between SiNP and PVOH, which depends on pH. Low and the strongly negative were measured at pH 10, which implied a poor SiNP-PVOH interaction. Low was attributed to the ionized silanol group (-SiO–) on the SiNP surface at the basic condition. As pH decreased, increased by exhibiting a weakening of , which suggested the protonation of the silanol groups (i.e. SiO–+ H+ SiOH). As a result, pH 3 showed the largest in the range of pH in this study, suggesting the best SiNP-PVOH interaction in the tested range of pH.
Based on the measurement of , suspensions of pH 3 and pH 10 were selected as a representative ‘good’ interaction and ‘poor’ interaction, respectively, in this study. In the following section, the role of SiNP-PVOH interaction on dispersion stability, dried coating structure, and mechanical property was described by comparing pH 3 and pH 10.
Fig. 1. Zeta potential of SiNP () and the amount of PVOH adsorption () as a function of suspension pH (Kim et al. 2016)
Sedimentation Behavior
Sedimentation behavior, which illustrates the effect of particle-polymer interaction on the dispersion stability, was evaluated for the SiNP/PVOH suspensions at pH 3 and 10. As shown in Fig. 2a, at pH 3 the sedimentation was fastest at PVOH = 0.5 wt.%, but became slow as PVOH increased. In a previous study about the measurement as a function of PVOH and the dispersion stability at pH 3, the authors found the dispersion stability improved as increased, owing to steric stabilization as a result of adsorbed PVOH (Kim et al. 2015). The study showed that increases as PVOH increases, and the system approaches the plateau level at around PVOH = 3 wt.%. Therefore, the improvement of sedimentation with increasing PVOH in the range of 3.5 wt.% in Fig.2a can be understood by the steric stabilization because of increased . When PVOH increased above PVOH = 3.5 wt.%, sedimentation was not observed any more, which suggested that the adsorption of PVOH was fully saturated. However, it is notable that the suspensions at pH 3 showed a turbid image, which suggested that the SiNPs of 15 nm in diameter formed clustered structures in the suspension, where the length scale of the cluster was comparable to the wavelength of visible light; i.e. several hundred nm. In the previous study on the PVOH solution property at varied pH, the deterioration of PVOH chain was observed in a strongly acidic condition (Kim et al. 2009). However, neither a considerable turbidity change nor viscosity increase was observed as a signature of aggregation of the PVOH chain at a lower pH value. This suggests the deterioration of PVOH chain at least was not a dominant origin of the observed turbid image at pH 3. The cluster formation will be further discussed in terms of the rheological properties in the next section.
Sedimentation behavior at pH 10 was also evaluated in the range of PVOH from 3.5 wt.% to 7.5 wt.%. Unlike the turbid image at pH 3, the suspension at pH 10 did not sediment, as shown in Fig. 2b. Furthermore, the suspension at pH 10 was nearly transparent, which implied that SiNPs at pH 10 do not form a clustered structure in the length scale of wavelength of visible light. This result was attributed to the charged nature of SiNPs at pH 10, which led the electrostatic stabilization of SiNPs in the suspension. In addition, the image at pH 10 did not change with the increased PVOH, which implied that increased PVOH does not affect dispersion stability, probably owing to the poor interaction between SiNP and PVOH.
Fig. 2. Sedimentation image of 10 wt.% silica with a various PVOH at pH 3(a) and pH 10(b)
Rheological Property
The steady shear viscosity () and the linear viscoelastic property of the SiNP-PVOH suspensions were measured at pH 3 and pH 10, respectively, to further understand the structure in the range of PVOHbetween 3.5 wt.% and 7.5 wt.%.
At pH 3, strong shear thinning behavior was observed in the range of PVOH from 3.5 wt.% to 5 wt.%, as shown in Fig. 3a. The shear thinning behavior in particle-polymer mixtures indicates particle cluster formation, which breaks down into a smaller cluster by shear flow, resulting in the reduced viscosity with increasing shear rate (Nasser et al. 2016). Therefore, the shear thinning behavior observed at PVOH = 3.5 wt.% to 5 wt.% indicated that there was considerable formation of clusters. When PVOHincreased from 4.5 wt.% to 5 wt.%, viscosity decreased at the whole range of shear rate, and finally became a Newtonian fluid at PVOH = 7.5 wt.%. This indicated that the cluster size reduced when PVOHincreased above 4.5 wt.% (Kim et al. 2015). The clustered structure was further characterized by the frequency sweep test. At PVOH = 3.5 wt.% to 5wt.%, G‘ and G” exhibited a power law behavior with a nearly equal slope, as shown in Fig. 3b and 3c. The power law behavior with the same slope of G‘ and G” can be seen elsewhere (Aoki et al. 2003), suggesting the formation of fractal-like particle clusters. This confirmed that there were clusters of SiNPs in PVOH = 3.5wt.% to 5wt.%, as also supported by the turbid suspension images in Fig. 2a. When PVOH increased up to 5wt.%, both G‘ and G” decreased, which suggested a reduced size of clusters. Finally, the significant reduction of both G‘ and G” was observed at PVOH = 7.5 wt.%, which implied reduced cluster size. The cluster formation probed by rheological measurement could be further understood by the measurement of a small angle x-ray scattering and a small angle light scattering intensity for SiNP-PVOH suspension of pH 3 (Kim et al. 2015). The scattering intensity showed the formation of SiNP clusters were mainly dominated by depletion flocculation below PVOH = 4 wt.%, whereas it was dominated by depletion stabilization above PVOH = 4wt.%. On the basis of this study, the power law behavior of G‘ and G” at PVOH = 3.5 wt.% to 5wt.% was attributed to depletion clustering, and the serious reduction of G‘ and G” at PVOH= 7.5 wt.% was a signature of depletion stabilization.
Fig. 3. (a) Steady shear viscosity curve and (b, c) frequency sweep measurement results of G‘ and G” for the SiNP-PVOH suspension at pH 3 by varying PVOH. PVOH= 3.5(●, ○), 4.5 (▼▽), 5 (■, □), 7.5 (◆, ◇) (by wt.%)
The rheological property of the suspension at pH 10 is displayed in Fig. 4.
Fig. 4. (a) Steady shear viscosity curve and (b, c) frequency sweep measurement results of G‘ and G” for the SiNP-PVOH suspension at pH 10 by varying PVOH. PVOH = 3.5(●, ○), 4.5 (▼▽), 5 (■, □), 7.5 (◆, ◇) (by wt.%)
Unlike the shear thinning behavior at pH 3, which significantly depended on PVOH, the viscosity curves at pH 10 exhibited Newtonian behavior, which was independent of PVOH, as shown in Fig. 4a. The Newtonian behavior was observed in a stable suspension with a good dispersion, where there is no formation of particle clusters which breakdown by shear flow, even at high shear rates (Di Giuseppe et al. 2012). Therefore, the observed Newtonian behavior in Fig. 4a suggested that charged SiNPs do not form clusters at pH 10, as supported by transparent images in Fig. 2b. In the frequency sweep test, G‘ was negligibly small as shown in Fig. 4b, which indicated that there was no considerable aggregated structure. G” increased in the order of increased PVOH, which suggested the contribution of increased PVOH in the medium.
Microstructure of the Silica/PVOH Coatings after Drying
The structure of the dried SiNP-PVOH coating was investigated by means of SEM and SAXS. The authors previously studied the structure of dried silica-PVOH coating at a limited PVOH between 4 wt.% and 5 wt.% (Kim et al. 2016). To further understand the effect of PVOH on the coating structure, here the silica-PVOH coating was tested over a wider range of PVOH from 3.5 wt.% to 7.5 wt.%. SEM images of the dried coating layer at pH 3 and pH 10 are shown in Fig. 5. A uniform and dense structure with unobservable aggregates was clearly seen for pH 3 within the whole PVOH range under the investigation (Fig. 5a), whereas non-uniform and porous structure with large aggregates was observed at pH 10 (Fig.5b). The SEM image did not show a dependency of PVOH at pH 3, but showed a strong dependency on PVOH at pH 10; i.e. pores gradually reduced as PVOH increased, and finally disappeared at PVOH = 7.5 wt.%, with the aggregates which became considerably small.
Fig. 5. Scanning electron microscopy image of silica/PVOH coating surface after drying at (a) pH3 (b) pH 10 as a function of PVOH. The coatings were prepared from the aqueous 10 wt.% silica suspensions with varying PVOH concentration (wt.%, noted in the figure).
The SAXS technique was employed to characterize the structure of the coating film in Fig.6 at the particle length scale. This technique provided the information on the spatial organization of SiNPs in the film at the particle length scale, which clarified the relative position of the SiNP and PVOH in the dried coating film because of SiNP-PVOH interaction.
A broad peak at around q = 0.39 nm-1 was observed at pH 3, as can be seen in Fig. 6a.The peak position at q = 0.39 nm-1 allowed for calculation of the center-to-center distance between neighboring SiNPs h through h = 2/q, which yielded h = 16.1 nm. This was slightly larger than the diameter of SiNP, d=15nm (Kim et al. 2013), which suggested the presence of PVOH and the accompanying bound water (Hatakeyama et al. 1987) between SiNPs. The broad peak indicated that the particles in the film were loosely arranged, where PVOH was well-mixed with SiNPs. The broad peak became wider with the slight increase in the low q intensity as PVOH increased, finally disappeared at PVOH = 7.5 wt.%. This can be understood as a gradual disappearance of SiNP arrangement because SiNPs became far apart from each other with increased PVOH, as a result of a favorable mixing with PVOH in the solid film (Kim et al. 2016).
SAXS intensity at pH 10 showed a marked distinction from that at pH 3. Upturn at the lowest q and a shoulder at q = 0.43 nm-1was observed in SAXS intensity of pH 10, as shown in Fig.6b, which suggested that the particles formed disordered aggregates. The center-to-center distance between SiNPs h was calculated to 14.6 nm from the shoulder at q = 0.43 nm-1, nearly the same as the SiNP diameter. This implied that the aggregates shown in the SEM image (Fig.5b) were formed by the contact of bare SiNPs, in the absence of PVOHs inside the aggregates. Furthermore, the peak position at pH 10 did not shift with increasing PVOH, indicating that the increasing PVOH did not affect the SiNP structure in the aggregates. In conclusion, the location of PVOH observed in the SEM image (Fig.5b) and aggregate structure of SiNPs seen in SAXS intensity suggested that PVOH stayed outside of the SiNPs aggregates and did not play a role in improving SiNP distribution at pH 10, owing to the poor interaction between SiNP and PVOH (Kim et al. 2016).