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The effect of acrylamides copolymers on the stability and rheological properties of yellow iron oxide dispersion
Accepted Manuscript Title: The Effect of Acrylamides Copolymers on the Stability and Rheological Properties of Yellow Iron Oxide Dispersion Author: Caizhen Liang Bin Wang Jianjun Chen Yuewen Huang Tianyong Fang Yingying Wang Bing Liao PII: DOI: Reference:
S0927-7757(16)30873-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.10.020 COLSUA 21087
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-7-2016 25-9-2016 11-10-2016
Please cite this article as: Caizhen Liang, Bin Wang, Jianjun Chen, Yuewen Huang, Tianyong Fang, Yingying Wang, Bing Liao, The Effect of Acrylamides Copolymers on the Stability and Rheological Properties of Yellow Iron Oxide Dispersion, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The Effect of Acrylamides Copolymers on the Stability and Rheological Properties of Yellow Iron Oxide Dispersion
Caizhen Lianga,b,
a
Bin Wanga, Jianjun Chena,b, Yuewen Huanga, Tianyong Fanga,b, Yingying Wanga,b and Bing Liaoc,*
Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou
510650, PR China b
University of Chinese Academy of Sciences, Beijing 100039, PR China
c
Guangdong Academy of Sciences, Guangzhou 510650, PR China
Graphical abstract
* Corresponding author. Telephone: +86 13500023169 E-mail address: [email protected] (B. Liao)
Highlights
The acrylamides copolymers PMBAs synthesized by RAFT polymerization were designed as dispersants based on the amino groups as anchoring groups and alkyl groups as solvent chains
Yellow iron oxide dispersion achieved excellent colloidal stabilization and dispersity at PMBAs’s presence by H-bond interaction and steric stabilization.
Yellow iron oxide dispersion obtained high fluidity and low viscosity fluid with high solid content with a small amount of PMBAs.
PMBAs showed outstanding dispersing ability might be a prospect in material dispersing area and the information that yellow iron oxides can be dispersed in organic liquid might be favorable in colloidal material area in future.
Abstract It is still a challenge to disperse yellow iron oxides in organic media due to their susceptible aggregation, which limits their application in ceramic material area. In order to achieve colloidal stabilization in organic liquid, yellow iron oxides were dispersed in ethanol with p[methyl methacrylate-r-butyl methacrylate-r-N-(3-dimethylaminopropyl) methacrylamide] random copolymers (PMBAs) based on amino groups as anchoring groups and alkyl as solvent chains by reversible-addition fragmentation chain-transfer radical (RAFT) polymerization. The effect of PMBAs on viscosity, rheological properties and dispersion stability was investigated. In the presence of PMBAs, yellow iron oxide dispersion performed excellent colloidal stability and low viscosity with high solid loadings. Furthermore, it is suggested that PMBAs have outstanding dispersion ability, retarding yellow iron oxides aggregating and keeping it stable for more than 60 days at ambient temperature. This study reveals that PMBAs might find promising application in material dispersion area.
Keywords:
Yellow iron oxide; PMBAs; rheological property; stability; low viscosity; dispersion ability
1. Introduction With a composition of α-FeOOH and the structure of goethite, yellow iron oxide has long been used as coloring materials. Owing to its free toxicity, chemical stability and economy, yellow iron oxide has been found various application in paints, printing inks, building materials and cosmetics [1]. Also, its preparation and use in environment protection are extensively studied [2-4]. However, as the pigment, yellow iron oxide has high viscosity due to its shape of the particles in liquid media. Moreover, yellow iron oxide particles are apt to adhere and aggregate in organic media, which limit their applications in ceramic ink area. Therefore, the use of dispersants for stabilizing yellow iron oxides in organic media is urgent. It is generally accepted that adding polymers as dispersants is one of the main routes to regulate the viscosity, rheological properties and stability of particle dispersion, facilitating the processing of colloidal dispersion [5-7]. Nevertheless, the structure, molecular weight and concentration of polymeric dispersant have significant effects on the stability of the suspension. Thus, kinds of polymeric dispersants with different structures and molecular weights such as polyacrylates [810], polyacrylamides [11], polyethleneimine [12, 13] and other substances [14, 15] have been applied into various suspensions to obtain the final goal. Compared to homopolymers, copolymers with two or more monomers will be better for dispersing solid particles [16-18]. In general, the copolymeric dispersants have anchoring functional groups absorbing onto the surfaces of the solid particles and soluble segments stretching into the solvent freely imparting sufficient distance to counteract the inter-particle attractions, which bring about effective steric stabilization [19]. In non-aqueous dispersion media, steric stabilization will be a vital mechanism for achieving long-term colloidal stability [20, 21]. In non-aqueous colloidal dispersion, the phenomenon of agglomeration is severe due to the van der Waals force between particles [12], which results in bad stability and high viscosity. The key to minimize this phenomenon is to increase the distance between inter-particles and form steric stabilization. Therefore, adding polymeric dispersant will be an effective measure to provide steric hindrance to obtain colloidal dispersion with high stability and fluidity. There have been some reports about copolymers used as dispersants in ceramic dispersions [8, 22]. Herein, our attention was focused on the copolymer dispersant that has anchoring groups and soluble teeth. Also, the dispersion stability and rheological properties of yellow iron oxides in organic media have not been reported. Our strategy is to optimize the stability of yellow iron oxide particles in organic liquid as a choice for ceramic ink, p[methyl methacrylate-r-butyl methacrylate-r-N-(3-dimethylaminopropyl) methacrylamide] random copolymers were synthesized to disperse yellow iron oxide powders with hydrogen bond interaction and steric hindrance. In this study, the apparent viscosity,
rheological property, Dynamic light scattering (DLS), transmission electron microscope (TEM) and sedimentation measurements were explored, which can reflect the stability, dispensability and homogeneity of the colloidal dispersion. The correlation between viscosity and solid loading was also evaluated via the apparent viscosity and rheological property measurement.
2. Experimental section
2.1 Materials
Solvents and reagents were purchased from commercial sources and used without purification unless noted otherwise. Methyl methacrylate (MMA, Aldrich, 99 %), Butyl methacrylate (BMA, Aldrich, 99 %), and N-(3-(dimethylamino) propyl) methacrylamide (DMAPMA, Aldrich, 99 %) were passed through a basic oxide alumina column to remove the inhibitor prior to use. 2,2’- azobis(isobutyronitrile) (AIBN, Aldrich, 98 %) was recrystallized from ethanol twice and dried under vacuum at room temperature. THF was dried with calcium hydroxide and 5 Å molecular sieves by refluxing and distilled out before used. The chain transfer agent (CTA) 2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP) was synthesized according to the reported procedure [23]. Other chemicals were analytical grade and used without further purification. The yellow iron oxides (α-FeOOH), a pigment for ceramic ink, was measured by the Brunauner-Emmet-Teller (BET) method (3H-2000PS1) specific surface and pore size analyzer Beishide Instrument, China), and its value was 20.027 m 2/g.
2.2 Instruments
1
H NMR (400MHz) spectra were recorded on a Bruker DRX-400 Nucllear magnetic resonance spectrometer using CDCl3
or DMSO-d6 as solvent. All chemical shifts were reported in ppm (δ) and referenced to the chemical shifts of the residual solvent resonances. Mass spectrometry was measured with a Shimadzu GC-MS QP5050A in electron ionization mode. FTIR analysis was obtained from KBr disks on an FT-IR Nicolet 5100 spectrometer over the range 400 – 4000cm-1. Numberaverage molecular weight (Mn) and dispersity (Mw/Mn,PDI) were measured by gel permeation chromatography (GPC) (flow rate 0.5ml/min) using DMF as eluent. Monomer conversion was evaluated using 1H NMR spectroscopy (via end-group analysis). The viscosity was characterized by a low shear viscosimeter Brookfield (R/S plus). The particle size of the suspension with additive was determined using a zetasizer nano ZS and the suspension morphology was observed using a JEM-100CXII type miscroscopy (Japan).
2.3 Synthetic methods
2.3.1 Synthesis of 2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP)
2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP)
was synthesized as follows [23]. At room temperature, a
50 % NaOH solution (24.30 g, containing 12.00 g, 300 mmol of NaOH) was added to a stirred mixture of 2-methyl-1propanethiol (27.00 g, 300 mmol) and water (45 g). Acetone (15 mL) was then added, and the resulting clear, colorless solution was stirred for 30 min then treated with carbon disulfide (26.65 g, 337.5 mmol) to give a clear orange solution. This solution was stirred for 30 min then cooled in an ice bath to an internal temperature of <10 ℃. 2-Bromopropanoic acid (47.05 g, 307.5 mmol) was then added at such a rate that the temperature did not exceed 30 ℃ followed by 50 % NaOH (12.33 g, 300 mmol), also added at such a rate that the temperature did not exceed 30 ℃. When the exotherm had stopped, the ice bath was removed and water (45 mL) was added. The reaction was stirred at ambient temperature for 24 h then diluted with water (75 mL) then stirred and cooled in an ice bath while 10 M HCl (45 ml) was added at a rate which kept the temperature < 10 ℃. The yellow oil separated, and stirring of the mixture was continued at ice temperature until the oil solidified and the solution became colorless. The solid was collected by suction filtration, pressed and washed with cold water, and dried under reduced pressure to a state of semi-dryness. The lumps were crushed with a spatula; the nowgranular solid was resuspended in fresh cold water and stirred for 15 min then refiltered. The residue was washed with cold water and air-dried to afford a powdery yellow solid, 60.02 g, which was recrystallized from hexane three times with gentle stirring to give bright yellow microcrystals (55 g, 77 %). IR (KBr): 3465, 2960, 2867, 2713, 2590, 2490, 1706, 1450, 1415, 1317, 1205, 1087, 1064, 981, 910, 831, 648, 457 cm-1. 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.47 (br, 1H, CO2H), 4.87 (q, J = 7.2 Hz, 1H, SCH), 3.27 (t, J = 8.0 Hz, 2H, CH2S), 2.01 (heptet, J = 6.8 Hz, 1H, CHCH2S), 1.63(d, J = 7.2 Hz, 3H, SCHCH3), 1.02 (d, J = 6.4Hz, 6H, (CH3)2CH). MS (EI): m/z (%) = 238 [M+], 165 ([M-CH3CHCOOH]+), 106 ([M(CH3)2CCH2SC=S]+), 57 ([M-SC=SSCHCH3COOH]+).
2.3.2 RAFT copolymerization of PMBAs
A typical procedure for the copolymerization of PMBAs (Poly (methyl methacrylate-r-butyl methacrylate-r-N-(3(dimethylamino)propyl)methacrylamide) is as follows: MAA (4.00 g, 40.0 mmol), BMA (5.65 g, 40 mmol), DMAPMA (13.62 g, 80 mmol), IBCP (476.1 mg, 2.0 mmol), AIBN (65.7 mg, 0.4 mmol), and tetrahydrofuran (160 mL) were added into a 250 mL round-bottom septum-sealed flask equipped with a magnetic bar, where the mixture with an overall monomers
concentration controlled at 1 mol/L. The mixed solution in a sealed reaction vessel was degassed for 40 min with argon and subsequently placed in a preheated thermostated oil bath at 75 ℃ to carry out the RAFT polymerization. The reaction was stopped after 5 h by immersion of the flask in an iced bath and exposure to the air. The copolymer was precipitated three times in the petroleum ether and dried overnight in vacuo at 40 ℃. A bright yellow solid was obtained (10. 64 g, 46% yield).
Insert Scheme 1 Insert Table 1 2.4 Characterization
2.4.1 Viscometric measurements
The steady state viscosity curves of concentrated mixed yellow iron oxide, as a function of dispersant dosage, were performed by a digital rotational viscometer (DV-III ULTRA, BROOKFIELD) at 20 ℃ and 40 ℃. The suspensions prepared through adding 40 parts or 45 parts iron oxide yellow powder and 60 parts or 55 parts ethanol with different content of polymers, and then the mixture were ultrasonic for 15 min before testing. The total volume of the testing suspensions is 100 mL and using NO.1 rotor with the rate 150 r/s calculates the static viscosity of the system.
2.4.2 Rheological measurement
The rheological experiments were carried out in a coaxial cylinder rotational rheometer with controlled shear rate (Brookfield R/S + CC Rheometer, USA) at 20 ± 1 ℃. The yellow iron oxide suspensions of varying solid loading with various amounts of polymeric dispersants in ethanol, dispersed by magnetically stirring and ultrasonicating for 10 min (2 min in every 2 min), and then the mixture were allowed to stand for 24 h. The rheological test consisted of the following steps: (1) a linear increasing of shear rate from 0 s−1 to the maximum shear rate for 5 min (LS-HS); (2) sustaining the maximum shear rate for 5 min to get the steady viscosity;(3) linearly decreasing the shear rate from the maximum shear rate to 0 s−1 for 5 min (HSLS). The important influences of solid loading and dosage of polymeric dispersant were investigated during the measurements. The parameters about shear rate, shear stress and viscosity were recorded and analyzed on the computer.
2.4.3 Elemental analysis
The elemental analysis was test by energy dispersive spectrometer (EDS) (S4800). The yellow iron oxide dispersion with
PMBAs were stirred for 24h at ambient atmosphere. After that the dispersion was centrifuged at 12000 r/min for 5 min. The supernatant was discarded and the precipitation was washed with ethanol. Then the precipitation was dry in vacuo overnight and test by EDS.
2.4.4 Dynamic light scattering (DLS) measurement
Dynamic light scattering (DLS) measurements were performed using a Malvern Zetasizer Nano ZS (4 mW He−Ne 633 nm laser) on nanoparticle dispersions at 0.2 % (unless otherwise stated) and 25 ℃. Size measurements were obtained as an average of three individual measurements. Nanoparticle dispersions were measured directly without additional filtration or centrifugation. All suspensions were ultrasonic for 10 min before testing.
2.4.5 Sedimentation Measurements
The sedimentation measurement of yellow iron oxide suspensions was performed in a 25 mL graduated test tube with stopper. The suspensions were prepared through adding 30 parts yellow iron oxide yellow particles and 70 parts ethanol with different concentration of polymeric dispersants, then magnetically stirring for 30 min and ultrasonicating for 15 min. The obtained suspensions were put into 25 mL test tubes and set. The scenes of the sedimentation were recorded every week. The dispersing efficiency is calculated by the following formula [24]: F = (H-h) × 100 %
(1)
Where h is the settling height of colloidal dispersion, H is the whole height of colloidal dispersion, F is the dispersing efficiency. The sedimentation diagram of colloidal dispersion is showed in scheme 2.
Insert Scheme 2
3. Results and discussions Insert Table 2 Insert Scheme 3 3.1 Apparent viscosities
Fig. 1(a) shows the viscosities of yellow iron oxide dispersions containing 40 wt % yellow iron oxides with polymeric dispersants PMBA1, PMBA2, PMBA3 and PMBA4 (Table 2; Scheme 2) at 25 ℃ and 40 ℃. From fig. 1(a), we can see that
the static viscosities of the yellow iron oxide dispersions originally decreased with polymers amount increased, then started to increase slowly with continuing adding. It also displays that the dispersions with all tested copolymers have the minimum viscosities as the dosages increased to some extent. The viscosities of dispersions with PMBA1, PMBA3 and PMBA4 reached to the minimum at around 1.0 % adding amount, while the one with PMBA2 at 1.5 %. In the non-aqueous dispersion medium, generally, the dispersant with effective dispersion ability consists of the functional groups adsorbing on the surface of particles and the solvent chains which are long enough to overcome van der Waals attractive forces [25, 26], achieving low viscosities with high solid loadings. In this measurement, the dispersion with PMBA1 had lower viscosity compared with the ones with PMBA2, PMBA3 and PMBA4. It is probably that the PMBA1 has more proper mole ratio of anchoring groups and soluble teeth or molecular weights [19, 27] for restraining the inter-particles interaction. It is considered that the amino groups chains acted as anchoring groups interact with the yellow iron oxide particles [28], meanwhile the butyl and methyl chains dangled freely in the solution, which create effective steric barrier, preventing other particles from approaching too closely and stabilize the dispersions (Scheme 3). For the yellow iron oxide dispersion, the viscosity is closely related to the polymeric dispersants. Also, the temperature has an effect on the viscosity. The fig. 1(a) notes that the weakening of inter-particle and inter-molecular adhesion force, which is consistent with previous reports [29-31]. When the adding amounts of polymeric dispersants was less than 1 %, the viscosity had a strong dependence on dispersant concentration as well as temperature, decreasing obviously. While the amounts of polymeric dispersants added up to 1 %, the viscosity changes subtly.
Insert Fig. 1 To further study the viscosity we measured the effect of solid loading of yellow iron oxides and the result is shown in fig. 1(b). For the suspension with 45 wt % solid loadings, its viscosity was much higher than that with 40 wt % solid loadings. Furthermore, the variation of trend curves in viscosity with 45 wt % particles was more marked, which reveals that the increase in solid loadings limited the movement of the particles, exacerbating the collision between inter-particles, resulting in increase in viscosity. Besides, its viscosity at 25 ℃ was higher compared to that at 40 ℃.
3.2 Rheological properties
Previous papers indicated that the rheological behavior of the dispersion is not only a function of the shear rate, but also a function of the shear history, the concentrated suspension and the type or concentration of the polymeric dispersants [32-
36]. Herein, the effects of solid loadings, concentrated polymeric dispersants on the rheological behaviors of yellow iron oxide dispersions were investigated. The steady shear stress and viscosities of several yellow iron oxide dispersions as a function of shear rate from 0 to 150 s-1 are shown in fig. 2. Fig. 2A shows the variation of shear viscosities of yellow iron oxide dispersions with polymeric dispersant concentrations. Within the range of the shear rate from 0 to 150 s-1, an interesting phenomenon was found on the plot (Fig. 2A (i)). At 40 wt % yellow iron oxide particles, the dispersion with 0.5 % of PMBA1 exhibited a pronounced shear-thinning behavior. However, as the PMBA1 concentrations added up to 0.7 % and 1.0 %, the shear behavior transformed into gentle shearthickening, which exhibited no viscosity at low shear rates. At high shear rates (> 50 s-1), the flow curves of the dispersions were almost Newtonian with slightly shear-thickening behaviors. It demonstrates that the polymeric dispersants increase the distance between the inter-particles [32], preventing the inter-particles aggregating. Moreover, at high concentrated polymeric dispersants, the effect of concentration on the shear viscosity of dispersion is negligible. In fig. 2A(ii), the flow behaviors of the dispersions exhibited parallel process with shear-thickening, analogous to the Al2O3-water nanofluids researched by Fei duan group [37]. With the increase in solid loadings the viscosities of colloidal dispersions increased evidently and the shear-thickening behaviors became remarked at low shear rate. The dispersion with 30 wt % yellow iron oxide powder exhibited nearly Newtonian flow with PMBA1, while in the 50 wt % dispersion, the flow curve initially exhibited lucidly shear-thickening behavior then become slightly shear-thickening as shear rate increased. The system with 50 wt % solid loadings was more of consistency compared to that with 30 wt %, 40 wt % and 45 wt % solid loadings at corresponding shear rates. We confirmed that the flow behavior is uniform in the shear-thickening regimes. Meanwhile, the critical shear rate for exhibiting the initial viscosity of the dispersion decreased. It reveals that lower shear rate could break down the original flow behavior to obtain viscosity as the solid loadings increase [35]. Among 40 wt % solid loadings of the dispersions, the suspension with PMBA4 had higher viscosity relative to that with PMBA1, PMBA2 and PMBA3, displaying similar shear viscosities (Fig. 2A(iii)).
Insert Fig. 2 Insert Table 3 We also noted the shear stress of various dispersions as a function of shear rates as shown in fig. 2B. Undoubtedly, the dispersions exhibited non-Newtonian fluid mechanics though the flow curves exhibit nearly linearly in the range of shear
rates. Exceptionally, for the dispersion with 0.5 % PMBA1, it diverged from the flow behavior of shear-thickening. With respect to the concentrated yellow iron oxide dispersions, a Hershel-Bulkley model was applied to make a comparison on the flow behaviors determined by the final colloidal dispersion stability, which is expressed as follows: τ = τy + Kγn
Eq. (2)
Where τy is the yield stress determined from the Herschel-Bulkley model, K is a viscosity coefficient, and n is the shear rate exponent. Therein, K and n set the equilibrium rheology. As shown in table 3, the correlation coefficients R for the HerschelBulkley model are higher than 0.99 over the applied experimental conditions, implying that the Herschel-Bulkley model could be well fitted in our systems. It is obvious that the yield stress increased with the solid loadings elevated, appearing as the solid loading increased above 45 wt %. For the same solid loadings, the yield stress vanished when the polymeric dispersant dosage arrived at critical concentration manifesting zero yield stress. At 0.7 % concentration, the four polymers exhibited no yield stress. Furthermore, all these dispersions presented low K and n, indicating that these polymeric dispersants we synthesized have an excellent ability to diminish the viscosity and keep the dispersions more stable.
3.3 Elemental analysis From fig. 3, we can see that the PMBAs have absorbed onto the surfaces of yellow iron oxide particles. Before adding PMBAs, there were just three elements of C, O, Fe of yellow iron oxides (Fig. 3A). When we added PMBAs, two extra elements of N, S turned up. As seen in fig. 3, after added PMBAs, the weights of C, O, Fe changed and weights of N, S varied with the four copolymeric dispersants. Among the four dispersants, the PMBA1 absorbed onto yellow iron oxides was a little more than the others. However, the increase of weights of N, S are not desirable. It is probably that the H-bond interaction between PMBAs and yellow iron oxides is not strong enough. Meanwhile, it is easy to be broken up by the external force such as centrifugation. Nevertheless, PMBAs copolymers were successfully absorbed onto the surfaces of yellow iron oxide particles.
Insert Fig. 3 3.4 TEM images
The yellow iron oxide dispersions were further characterized with TEM. The results are shown in fig. 4. These images clearly show the goethite structure of yellow iron oxide. In the absence of dispersant, the tendency to flocculate is dominant for the yellow iron oxide particles. When the polymeric dispersants PMBA 1, PMBA2, PMBA3 and PMBA4 participated in the
systems, the phenomenon of aggregation lessened availably. It could be seen that PMBA1 is outstanding in dispersing the yellow iron oxide particles. It suggests that these copolymeric dispersants could stabilize the yellow iron oxide particles with good dispensability in ethanol owing to their proper structure and moderate amounts of anchoring groups.
Insert Fig. 4 3.5 Dynamic Particle size
For a better understanding of the dispersion efficiency of the different polymeric dispersants synthesized, the particle size of yellow iron oxide dispersions is determined by dynamic light scattering measurement in the presence of these dispersants. The result is shown in fig. 5. It illustrates that the variation trend in Z-average diameter were nearly identical for PMBA1, PMBA2, PMBA3 and PMBA4. As seen in the plot, the dispersant concentration seems to be an important parameter, affecting the particle size of the dispersion. Increase in dispersant concentrations, the Z-average diameters decreased apparently, then varied slowly, consistent with the viscosities analysis. Within the range of dispersant concentrations, PMBA 1 had the lowest Z-average diameter (273 ± 1 nm) followed by PMBA2 (290 ± 1 nm), PMBA3 (406 ± 1 nm) and PMBA4 (478 ± 1 nm). The result suggests that the dispersants we synthesized provide significant steric stability in ethanol [38-40]. With the increase of dispersant, more dispersants would adsorb onto the particle surfaces and their solvent segments produce steric hindrance between inter-particles, inhibiting the inter-particles tangling up efficiently [41]. When the dispersants on yellow iron oxide particle surfaces were at saturation level, the trend of aggregation was impeded and yellow iron oxide particles were stabilized in the dispersion. Hence, with dispersants’ participation, the Z-average diameter of yellow iron oxide dispersion decreases remarkably.
Insert Fig. 5 3.6 The stability and dispensability of yellow iron oxide dispersion
For long-term stability measurement, the dispersions of yellow iron oxide particles treated with PMBA1 in ethanol were detected by the sedimentation test, showing in fig. 6. The dispersion stability decreased as the sedimentation proceeded. Nevertheless, the dispersions stabilized by PMBA1 could be stable for at least 60 days, and the dispersion efficiency could achieve about 50 wt % with 1.0 % or more mass fraction of PMBA1. It means that the settling rate is significantly susceptible to polymeric dispersant concentrations. To be more precise, at high concentration, it was beneficial for PMBA1 to restrict the fierce movement of yellow iron oxide particles and construct the steric barrier so that the particles can suspend stably in
ethanol for days. On the contrary, the agglomerates were easy to form because of the van der Waals force. Nevertheless, the colloidal dispersions remained stable for a long time with PMBA1 though the sedimentation is in the process. It is confirmed that the dispersion system is bound up with the steric stabilization effect of polymeric dispersant. In this nonaqueous system, the copolymers PMBAs provide proper solvent chains and anchoring groups so that the yellow iron oxide particles can disperse stably in ethanol.
Insert Fig. 6
4. Conclusion In this paper, the copolymers PMBAs were fabricated as dispersants to investigate the viscosity, rheological properties and stability of yellow iron oxides in ethanol. The concentration-dependence of viscosity, particle size and sedimentation measurements were studied. It reveals that viscosity and stability of dispersions have a close relationship with the copolymer amount. In the apparent viscosity measurements, the smallest value was about 10 mPa.s or slightly higher as the copolymers dosage was 1.0 %, which indicates the feeble interaction between inter-particles. In the valid concentration ranging from 0.01 % to 0.2 % studied, the Z-average particle diameter decreased from micrometer to submicron, which is favorable for ceramic ink. For the rheological property, shear-thinning behavior switched to shear-thickening behavior as the copolymer dosage increased. The shear-thickening behavior became remarkable as the solid loadings added up to 50 wt %. The four copolymers displayed similar or almost identical flow behavior as the concentration was 0.7 %. Sedimentation measurement conveys the yellow iron oxide dispersions could be stabilized for more than 60 days despite the incessant aggregation, which shows brilliant dispensability stabilized by these copolymers. The prominent stabilization of yellow iron oxide dispersion is owing to the proper structure of copolymers with the major groups amino adsorbing on the particle surface via hydrogen-bond interaction. Meanwhile, the teeth of butyl cooperate with N-dimethylaminopropyl impart steric stabilization. From the application viewpoint, the copolymers PMBAs with excellent dispersion ability might be attractive in colloidal material area. With respect to yellow iron oxide dispersion, good colloidal stability and dispensability with low viscosity and high solid loadings are significant improvement for the ultimate goal of ceramic application.
Acknowledgements This work was supported by Key Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of Sciences
and Foshan Centre for Functional Polymer Materials and Fine Chemicals. The support is also acknowledged from Guangzhou Municipal Science and Technology Program (No. 201510010183)
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Figure captions: Fig. 1 (a) Apparent viscosities of yellow iron oxide colloidal dispersions with PMBAs as a function of concentration measured at 25 ℃ and 40 ℃; (b) Apparent viscosities of yellow iron oxide colloidal dispersions with 40 wt % and 45 wt % solid content and various concentrations of PMBA1 at 25 ℃ and 40 ℃
Fig. 2 The rheological properties of yellow iron oxide colloidal dispersions with PMBAs: A(i), B(i) Rheological properties of yellow iron oxide colloidal dispersions containing 40 wt % yellow iron oxides and various concentration of PMBA 1; A(ii), B(ii) Rheological properties of yellow iron oxide colloidal dispersions with different solid content of yellow iron oxides and 0.7 % PMBA1; A(iii), B(iii) Rheological properties of yellow iron oxide colloidal dispersions containing 40 wt % yellow iron oxides with 0.7 % PMBA1, PMBA2, PMBA3 and PMBA4.
Fig. 3 The elemental analysis of yellow iron oxides (A), yellow iron oxides with PMBA1 (B), yellow iron oxides with PMBA2 (C), yellow iron oxides with PMBA3 (D), yellow iron oxides with PMBA4 (E).
Fig. 4 TEM images of yellow iron oxide colloidal dispersion (A), colloidal dispersion with PMBA 1 (B), colloidal dispersion with PMBA2 (C), colloidal dispersion with PMBA3 (D), colloidal dispersion with PMBA4 (E).
Fig. 5 The Z-average dynamic particle sizes of yellow iron oxide with various amount of PMBA1, PMBA2, PMBA3 and PMBA4 in ethanol.
Fig. 6 The sedimentation test of yellow iron oxide colloidal dispersion with PMBA1.
Fig. 1
A
B
Fig. 2
Fig. 3
(A)
(B)
(C)
(D)
Fig. 4
(E)
Fig 5
.
Fig 6
Schemes
Scheme 1 The synthetic route of PMBAs.
Scheme 2 The sedimentation diagram of colloidal dispersion.
Scheme 3 Schematic illustration of the probable mechanism for the stabilization of yellow iron oxide particle dispersions with PMBAs.
Tables
Table 1 The synthesized conditions of PMBAs. Polymer
a
b
PMBA1
1:1:2
80:1:0.2
PMBA2
1:1:2
80:1:0.25
PMBA3
1:1:2
90:1:0.2
PMBA4
0:2.5:1.5
80:1:0.2
a
[M]: [B]: [A]
[M]0: [IBCP]: [AIBN]
monomers: M = methyl methacrylate, B = butyl methacrylate,
A = N-(3-(dimethylamino) propyl) methacrylamide; b
general polymerization conditions: T = 75 ℃, t = 5 h.
Table 2 Summary of monomer conversions and molecular weight of PMBAs. polymer
b
Monomer conversion (%)
c
Mn, theo
Mn, GPC
Mw, GPC
d
PDI
MMA
BMA
DMAPMA
PMBA1
93
66
22
5237
23101
28251
1.17
PMBA2
99
72
22
5765
26380
31346
1.19
PMBA3
91
64
24
6173
27910
33482
1.21
PMBA4
-
46
32
5143
26304
30262
1.15
a
Calculated by gravimetric analysis; b Determined by 1H NMR spectroscopy; c Mn, theo (theoretical Mn) = ([Monomer]0/[IBCP]0) ×
Conv. × Mmonomer (monomer molar mass) + MIBCP; d PDI denotes the polydispersity, measured by GPC, DMF as solvent.
Table 3 Rheological parameters of τy, K, N and values of steady state viscosities determined from Herschel-Bulkley Model Rheological parameters Samples (wt %) τy (Pa)
K (Pa.sn)
N
Viscosity (Pa.s)
R
0.5 PMBA1α
0.2863
0.0720
0.7810
0.0268
0.9982
0.7 PMBA1α
0
0.0024
1.2260
0.0071
0.9953
1.0PMBA1α
0
0.0012
1.3385
0.0065
0.9942
0.7 PMBA2α
0
0.0019
1.2592
0.0067
0.9910
0.7 PMBA3α
0
0.0015
1.3055
0.0069
0.9943
0.7 PMBA4α
0
0.0038
1.1708
0.0088
0.9948
0.7 PMBA1β
0
3.2594×10^-6
1.0495
0.0039
0.9911
0.7 PMBA1γ
0.1045
0.0061
1.1545
0.0141
0.9990
0.7 PMBA1δ
0.1486
0.0057
1.2568
0.0219
0.9952
yellow iron oxide particle solid content: α 40 wt %; β 30 wt %; γ 45 wt %; δ 50 wt %
S0927-7757(16)30873-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.10.020 COLSUA 21087
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-7-2016 25-9-2016 11-10-2016
Please cite this article as: Caizhen Liang, Bin Wang, Jianjun Chen, Yuewen Huang, Tianyong Fang, Yingying Wang, Bing Liao, The Effect of Acrylamides Copolymers on the Stability and Rheological Properties of Yellow Iron Oxide Dispersion, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.10.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The Effect of Acrylamides Copolymers on the Stability and Rheological Properties of Yellow Iron Oxide Dispersion
Caizhen Lianga,b,
a
Bin Wanga, Jianjun Chena,b, Yuewen Huanga, Tianyong Fanga,b, Yingying Wanga,b and Bing Liaoc,*
Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou
510650, PR China b
University of Chinese Academy of Sciences, Beijing 100039, PR China
c
Guangdong Academy of Sciences, Guangzhou 510650, PR China
Graphical abstract
* Corresponding author. Telephone: +86 13500023169 E-mail address: [email protected] (B. Liao)
Highlights
The acrylamides copolymers PMBAs synthesized by RAFT polymerization were designed as dispersants based on the amino groups as anchoring groups and alkyl groups as solvent chains
Yellow iron oxide dispersion achieved excellent colloidal stabilization and dispersity at PMBAs’s presence by H-bond interaction and steric stabilization.
Yellow iron oxide dispersion obtained high fluidity and low viscosity fluid with high solid content with a small amount of PMBAs.
PMBAs showed outstanding dispersing ability might be a prospect in material dispersing area and the information that yellow iron oxides can be dispersed in organic liquid might be favorable in colloidal material area in future.
Abstract It is still a challenge to disperse yellow iron oxides in organic media due to their susceptible aggregation, which limits their application in ceramic material area. In order to achieve colloidal stabilization in organic liquid, yellow iron oxides were dispersed in ethanol with p[methyl methacrylate-r-butyl methacrylate-r-N-(3-dimethylaminopropyl) methacrylamide] random copolymers (PMBAs) based on amino groups as anchoring groups and alkyl as solvent chains by reversible-addition fragmentation chain-transfer radical (RAFT) polymerization. The effect of PMBAs on viscosity, rheological properties and dispersion stability was investigated. In the presence of PMBAs, yellow iron oxide dispersion performed excellent colloidal stability and low viscosity with high solid loadings. Furthermore, it is suggested that PMBAs have outstanding dispersion ability, retarding yellow iron oxides aggregating and keeping it stable for more than 60 days at ambient temperature. This study reveals that PMBAs might find promising application in material dispersion area.
Keywords:
Yellow iron oxide; PMBAs; rheological property; stability; low viscosity; dispersion ability
1. Introduction With a composition of α-FeOOH and the structure of goethite, yellow iron oxide has long been used as coloring materials. Owing to its free toxicity, chemical stability and economy, yellow iron oxide has been found various application in paints, printing inks, building materials and cosmetics [1]. Also, its preparation and use in environment protection are extensively studied [2-4]. However, as the pigment, yellow iron oxide has high viscosity due to its shape of the particles in liquid media. Moreover, yellow iron oxide particles are apt to adhere and aggregate in organic media, which limit their applications in ceramic ink area. Therefore, the use of dispersants for stabilizing yellow iron oxides in organic media is urgent. It is generally accepted that adding polymers as dispersants is one of the main routes to regulate the viscosity, rheological properties and stability of particle dispersion, facilitating the processing of colloidal dispersion [5-7]. Nevertheless, the structure, molecular weight and concentration of polymeric dispersant have significant effects on the stability of the suspension. Thus, kinds of polymeric dispersants with different structures and molecular weights such as polyacrylates [810], polyacrylamides [11], polyethleneimine [12, 13] and other substances [14, 15] have been applied into various suspensions to obtain the final goal. Compared to homopolymers, copolymers with two or more monomers will be better for dispersing solid particles [16-18]. In general, the copolymeric dispersants have anchoring functional groups absorbing onto the surfaces of the solid particles and soluble segments stretching into the solvent freely imparting sufficient distance to counteract the inter-particle attractions, which bring about effective steric stabilization [19]. In non-aqueous dispersion media, steric stabilization will be a vital mechanism for achieving long-term colloidal stability [20, 21]. In non-aqueous colloidal dispersion, the phenomenon of agglomeration is severe due to the van der Waals force between particles [12], which results in bad stability and high viscosity. The key to minimize this phenomenon is to increase the distance between inter-particles and form steric stabilization. Therefore, adding polymeric dispersant will be an effective measure to provide steric hindrance to obtain colloidal dispersion with high stability and fluidity. There have been some reports about copolymers used as dispersants in ceramic dispersions [8, 22]. Herein, our attention was focused on the copolymer dispersant that has anchoring groups and soluble teeth. Also, the dispersion stability and rheological properties of yellow iron oxides in organic media have not been reported. Our strategy is to optimize the stability of yellow iron oxide particles in organic liquid as a choice for ceramic ink, p[methyl methacrylate-r-butyl methacrylate-r-N-(3-dimethylaminopropyl) methacrylamide] random copolymers were synthesized to disperse yellow iron oxide powders with hydrogen bond interaction and steric hindrance. In this study, the apparent viscosity,
rheological property, Dynamic light scattering (DLS), transmission electron microscope (TEM) and sedimentation measurements were explored, which can reflect the stability, dispensability and homogeneity of the colloidal dispersion. The correlation between viscosity and solid loading was also evaluated via the apparent viscosity and rheological property measurement.
2. Experimental section
2.1 Materials
Solvents and reagents were purchased from commercial sources and used without purification unless noted otherwise. Methyl methacrylate (MMA, Aldrich, 99 %), Butyl methacrylate (BMA, Aldrich, 99 %), and N-(3-(dimethylamino) propyl) methacrylamide (DMAPMA, Aldrich, 99 %) were passed through a basic oxide alumina column to remove the inhibitor prior to use. 2,2’- azobis(isobutyronitrile) (AIBN, Aldrich, 98 %) was recrystallized from ethanol twice and dried under vacuum at room temperature. THF was dried with calcium hydroxide and 5 Å molecular sieves by refluxing and distilled out before used. The chain transfer agent (CTA) 2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP) was synthesized according to the reported procedure [23]. Other chemicals were analytical grade and used without further purification. The yellow iron oxides (α-FeOOH), a pigment for ceramic ink, was measured by the Brunauner-Emmet-Teller (BET) method (3H-2000PS1) specific surface and pore size analyzer Beishide Instrument, China), and its value was 20.027 m 2/g.
2.2 Instruments
1
H NMR (400MHz) spectra were recorded on a Bruker DRX-400 Nucllear magnetic resonance spectrometer using CDCl3
or DMSO-d6 as solvent. All chemical shifts were reported in ppm (δ) and referenced to the chemical shifts of the residual solvent resonances. Mass spectrometry was measured with a Shimadzu GC-MS QP5050A in electron ionization mode. FTIR analysis was obtained from KBr disks on an FT-IR Nicolet 5100 spectrometer over the range 400 – 4000cm-1. Numberaverage molecular weight (Mn) and dispersity (Mw/Mn,PDI) were measured by gel permeation chromatography (GPC) (flow rate 0.5ml/min) using DMF as eluent. Monomer conversion was evaluated using 1H NMR spectroscopy (via end-group analysis). The viscosity was characterized by a low shear viscosimeter Brookfield (R/S plus). The particle size of the suspension with additive was determined using a zetasizer nano ZS and the suspension morphology was observed using a JEM-100CXII type miscroscopy (Japan).
2.3 Synthetic methods
2.3.1 Synthesis of 2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP)
2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid (IBCP)
was synthesized as follows [23]. At room temperature, a
50 % NaOH solution (24.30 g, containing 12.00 g, 300 mmol of NaOH) was added to a stirred mixture of 2-methyl-1propanethiol (27.00 g, 300 mmol) and water (45 g). Acetone (15 mL) was then added, and the resulting clear, colorless solution was stirred for 30 min then treated with carbon disulfide (26.65 g, 337.5 mmol) to give a clear orange solution. This solution was stirred for 30 min then cooled in an ice bath to an internal temperature of <10 ℃. 2-Bromopropanoic acid (47.05 g, 307.5 mmol) was then added at such a rate that the temperature did not exceed 30 ℃ followed by 50 % NaOH (12.33 g, 300 mmol), also added at such a rate that the temperature did not exceed 30 ℃. When the exotherm had stopped, the ice bath was removed and water (45 mL) was added. The reaction was stirred at ambient temperature for 24 h then diluted with water (75 mL) then stirred and cooled in an ice bath while 10 M HCl (45 ml) was added at a rate which kept the temperature < 10 ℃. The yellow oil separated, and stirring of the mixture was continued at ice temperature until the oil solidified and the solution became colorless. The solid was collected by suction filtration, pressed and washed with cold water, and dried under reduced pressure to a state of semi-dryness. The lumps were crushed with a spatula; the nowgranular solid was resuspended in fresh cold water and stirred for 15 min then refiltered. The residue was washed with cold water and air-dried to afford a powdery yellow solid, 60.02 g, which was recrystallized from hexane three times with gentle stirring to give bright yellow microcrystals (55 g, 77 %). IR (KBr): 3465, 2960, 2867, 2713, 2590, 2490, 1706, 1450, 1415, 1317, 1205, 1087, 1064, 981, 910, 831, 648, 457 cm-1. 1H NMR (400 MHz, CDCl3): δ (ppm) = 9.47 (br, 1H, CO2H), 4.87 (q, J = 7.2 Hz, 1H, SCH), 3.27 (t, J = 8.0 Hz, 2H, CH2S), 2.01 (heptet, J = 6.8 Hz, 1H, CHCH2S), 1.63(d, J = 7.2 Hz, 3H, SCHCH3), 1.02 (d, J = 6.4Hz, 6H, (CH3)2CH). MS (EI): m/z (%) = 238 [M+], 165 ([M-CH3CHCOOH]+), 106 ([M(CH3)2CCH2SC=S]+), 57 ([M-SC=SSCHCH3COOH]+).
2.3.2 RAFT copolymerization of PMBAs
A typical procedure for the copolymerization of PMBAs (Poly (methyl methacrylate-r-butyl methacrylate-r-N-(3(dimethylamino)propyl)methacrylamide) is as follows: MAA (4.00 g, 40.0 mmol), BMA (5.65 g, 40 mmol), DMAPMA (13.62 g, 80 mmol), IBCP (476.1 mg, 2.0 mmol), AIBN (65.7 mg, 0.4 mmol), and tetrahydrofuran (160 mL) were added into a 250 mL round-bottom septum-sealed flask equipped with a magnetic bar, where the mixture with an overall monomers
concentration controlled at 1 mol/L. The mixed solution in a sealed reaction vessel was degassed for 40 min with argon and subsequently placed in a preheated thermostated oil bath at 75 ℃ to carry out the RAFT polymerization. The reaction was stopped after 5 h by immersion of the flask in an iced bath and exposure to the air. The copolymer was precipitated three times in the petroleum ether and dried overnight in vacuo at 40 ℃. A bright yellow solid was obtained (10. 64 g, 46% yield).
Insert Scheme 1 Insert Table 1 2.4 Characterization
2.4.1 Viscometric measurements
The steady state viscosity curves of concentrated mixed yellow iron oxide, as a function of dispersant dosage, were performed by a digital rotational viscometer (DV-III ULTRA, BROOKFIELD) at 20 ℃ and 40 ℃. The suspensions prepared through adding 40 parts or 45 parts iron oxide yellow powder and 60 parts or 55 parts ethanol with different content of polymers, and then the mixture were ultrasonic for 15 min before testing. The total volume of the testing suspensions is 100 mL and using NO.1 rotor with the rate 150 r/s calculates the static viscosity of the system.
2.4.2 Rheological measurement
The rheological experiments were carried out in a coaxial cylinder rotational rheometer with controlled shear rate (Brookfield R/S + CC Rheometer, USA) at 20 ± 1 ℃. The yellow iron oxide suspensions of varying solid loading with various amounts of polymeric dispersants in ethanol, dispersed by magnetically stirring and ultrasonicating for 10 min (2 min in every 2 min), and then the mixture were allowed to stand for 24 h. The rheological test consisted of the following steps: (1) a linear increasing of shear rate from 0 s−1 to the maximum shear rate for 5 min (LS-HS); (2) sustaining the maximum shear rate for 5 min to get the steady viscosity;(3) linearly decreasing the shear rate from the maximum shear rate to 0 s−1 for 5 min (HSLS). The important influences of solid loading and dosage of polymeric dispersant were investigated during the measurements. The parameters about shear rate, shear stress and viscosity were recorded and analyzed on the computer.
2.4.3 Elemental analysis
The elemental analysis was test by energy dispersive spectrometer (EDS) (S4800). The yellow iron oxide dispersion with
PMBAs were stirred for 24h at ambient atmosphere. After that the dispersion was centrifuged at 12000 r/min for 5 min. The supernatant was discarded and the precipitation was washed with ethanol. Then the precipitation was dry in vacuo overnight and test by EDS.
2.4.4 Dynamic light scattering (DLS) measurement
Dynamic light scattering (DLS) measurements were performed using a Malvern Zetasizer Nano ZS (4 mW He−Ne 633 nm laser) on nanoparticle dispersions at 0.2 % (unless otherwise stated) and 25 ℃. Size measurements were obtained as an average of three individual measurements. Nanoparticle dispersions were measured directly without additional filtration or centrifugation. All suspensions were ultrasonic for 10 min before testing.
2.4.5 Sedimentation Measurements
The sedimentation measurement of yellow iron oxide suspensions was performed in a 25 mL graduated test tube with stopper. The suspensions were prepared through adding 30 parts yellow iron oxide yellow particles and 70 parts ethanol with different concentration of polymeric dispersants, then magnetically stirring for 30 min and ultrasonicating for 15 min. The obtained suspensions were put into 25 mL test tubes and set. The scenes of the sedimentation were recorded every week. The dispersing efficiency is calculated by the following formula [24]: F = (H-h) × 100 %
(1)
Where h is the settling height of colloidal dispersion, H is the whole height of colloidal dispersion, F is the dispersing efficiency. The sedimentation diagram of colloidal dispersion is showed in scheme 2.
Insert Scheme 2
3. Results and discussions Insert Table 2 Insert Scheme 3 3.1 Apparent viscosities
Fig. 1(a) shows the viscosities of yellow iron oxide dispersions containing 40 wt % yellow iron oxides with polymeric dispersants PMBA1, PMBA2, PMBA3 and PMBA4 (Table 2; Scheme 2) at 25 ℃ and 40 ℃. From fig. 1(a), we can see that
the static viscosities of the yellow iron oxide dispersions originally decreased with polymers amount increased, then started to increase slowly with continuing adding. It also displays that the dispersions with all tested copolymers have the minimum viscosities as the dosages increased to some extent. The viscosities of dispersions with PMBA1, PMBA3 and PMBA4 reached to the minimum at around 1.0 % adding amount, while the one with PMBA2 at 1.5 %. In the non-aqueous dispersion medium, generally, the dispersant with effective dispersion ability consists of the functional groups adsorbing on the surface of particles and the solvent chains which are long enough to overcome van der Waals attractive forces [25, 26], achieving low viscosities with high solid loadings. In this measurement, the dispersion with PMBA1 had lower viscosity compared with the ones with PMBA2, PMBA3 and PMBA4. It is probably that the PMBA1 has more proper mole ratio of anchoring groups and soluble teeth or molecular weights [19, 27] for restraining the inter-particles interaction. It is considered that the amino groups chains acted as anchoring groups interact with the yellow iron oxide particles [28], meanwhile the butyl and methyl chains dangled freely in the solution, which create effective steric barrier, preventing other particles from approaching too closely and stabilize the dispersions (Scheme 3). For the yellow iron oxide dispersion, the viscosity is closely related to the polymeric dispersants. Also, the temperature has an effect on the viscosity. The fig. 1(a) notes that the weakening of inter-particle and inter-molecular adhesion force, which is consistent with previous reports [29-31]. When the adding amounts of polymeric dispersants was less than 1 %, the viscosity had a strong dependence on dispersant concentration as well as temperature, decreasing obviously. While the amounts of polymeric dispersants added up to 1 %, the viscosity changes subtly.
Insert Fig. 1 To further study the viscosity we measured the effect of solid loading of yellow iron oxides and the result is shown in fig. 1(b). For the suspension with 45 wt % solid loadings, its viscosity was much higher than that with 40 wt % solid loadings. Furthermore, the variation of trend curves in viscosity with 45 wt % particles was more marked, which reveals that the increase in solid loadings limited the movement of the particles, exacerbating the collision between inter-particles, resulting in increase in viscosity. Besides, its viscosity at 25 ℃ was higher compared to that at 40 ℃.
3.2 Rheological properties
Previous papers indicated that the rheological behavior of the dispersion is not only a function of the shear rate, but also a function of the shear history, the concentrated suspension and the type or concentration of the polymeric dispersants [32-
36]. Herein, the effects of solid loadings, concentrated polymeric dispersants on the rheological behaviors of yellow iron oxide dispersions were investigated. The steady shear stress and viscosities of several yellow iron oxide dispersions as a function of shear rate from 0 to 150 s-1 are shown in fig. 2. Fig. 2A shows the variation of shear viscosities of yellow iron oxide dispersions with polymeric dispersant concentrations. Within the range of the shear rate from 0 to 150 s-1, an interesting phenomenon was found on the plot (Fig. 2A (i)). At 40 wt % yellow iron oxide particles, the dispersion with 0.5 % of PMBA1 exhibited a pronounced shear-thinning behavior. However, as the PMBA1 concentrations added up to 0.7 % and 1.0 %, the shear behavior transformed into gentle shearthickening, which exhibited no viscosity at low shear rates. At high shear rates (> 50 s-1), the flow curves of the dispersions were almost Newtonian with slightly shear-thickening behaviors. It demonstrates that the polymeric dispersants increase the distance between the inter-particles [32], preventing the inter-particles aggregating. Moreover, at high concentrated polymeric dispersants, the effect of concentration on the shear viscosity of dispersion is negligible. In fig. 2A(ii), the flow behaviors of the dispersions exhibited parallel process with shear-thickening, analogous to the Al2O3-water nanofluids researched by Fei duan group [37]. With the increase in solid loadings the viscosities of colloidal dispersions increased evidently and the shear-thickening behaviors became remarked at low shear rate. The dispersion with 30 wt % yellow iron oxide powder exhibited nearly Newtonian flow with PMBA1, while in the 50 wt % dispersion, the flow curve initially exhibited lucidly shear-thickening behavior then become slightly shear-thickening as shear rate increased. The system with 50 wt % solid loadings was more of consistency compared to that with 30 wt %, 40 wt % and 45 wt % solid loadings at corresponding shear rates. We confirmed that the flow behavior is uniform in the shear-thickening regimes. Meanwhile, the critical shear rate for exhibiting the initial viscosity of the dispersion decreased. It reveals that lower shear rate could break down the original flow behavior to obtain viscosity as the solid loadings increase [35]. Among 40 wt % solid loadings of the dispersions, the suspension with PMBA4 had higher viscosity relative to that with PMBA1, PMBA2 and PMBA3, displaying similar shear viscosities (Fig. 2A(iii)).
Insert Fig. 2 Insert Table 3 We also noted the shear stress of various dispersions as a function of shear rates as shown in fig. 2B. Undoubtedly, the dispersions exhibited non-Newtonian fluid mechanics though the flow curves exhibit nearly linearly in the range of shear
rates. Exceptionally, for the dispersion with 0.5 % PMBA1, it diverged from the flow behavior of shear-thickening. With respect to the concentrated yellow iron oxide dispersions, a Hershel-Bulkley model was applied to make a comparison on the flow behaviors determined by the final colloidal dispersion stability, which is expressed as follows: τ = τy + Kγn
Eq. (2)
Where τy is the yield stress determined from the Herschel-Bulkley model, K is a viscosity coefficient, and n is the shear rate exponent. Therein, K and n set the equilibrium rheology. As shown in table 3, the correlation coefficients R for the HerschelBulkley model are higher than 0.99 over the applied experimental conditions, implying that the Herschel-Bulkley model could be well fitted in our systems. It is obvious that the yield stress increased with the solid loadings elevated, appearing as the solid loading increased above 45 wt %. For the same solid loadings, the yield stress vanished when the polymeric dispersant dosage arrived at critical concentration manifesting zero yield stress. At 0.7 % concentration, the four polymers exhibited no yield stress. Furthermore, all these dispersions presented low K and n, indicating that these polymeric dispersants we synthesized have an excellent ability to diminish the viscosity and keep the dispersions more stable.
3.3 Elemental analysis From fig. 3, we can see that the PMBAs have absorbed onto the surfaces of yellow iron oxide particles. Before adding PMBAs, there were just three elements of C, O, Fe of yellow iron oxides (Fig. 3A). When we added PMBAs, two extra elements of N, S turned up. As seen in fig. 3, after added PMBAs, the weights of C, O, Fe changed and weights of N, S varied with the four copolymeric dispersants. Among the four dispersants, the PMBA1 absorbed onto yellow iron oxides was a little more than the others. However, the increase of weights of N, S are not desirable. It is probably that the H-bond interaction between PMBAs and yellow iron oxides is not strong enough. Meanwhile, it is easy to be broken up by the external force such as centrifugation. Nevertheless, PMBAs copolymers were successfully absorbed onto the surfaces of yellow iron oxide particles.
Insert Fig. 3 3.4 TEM images
The yellow iron oxide dispersions were further characterized with TEM. The results are shown in fig. 4. These images clearly show the goethite structure of yellow iron oxide. In the absence of dispersant, the tendency to flocculate is dominant for the yellow iron oxide particles. When the polymeric dispersants PMBA 1, PMBA2, PMBA3 and PMBA4 participated in the
systems, the phenomenon of aggregation lessened availably. It could be seen that PMBA1 is outstanding in dispersing the yellow iron oxide particles. It suggests that these copolymeric dispersants could stabilize the yellow iron oxide particles with good dispensability in ethanol owing to their proper structure and moderate amounts of anchoring groups.
Insert Fig. 4 3.5 Dynamic Particle size
For a better understanding of the dispersion efficiency of the different polymeric dispersants synthesized, the particle size of yellow iron oxide dispersions is determined by dynamic light scattering measurement in the presence of these dispersants. The result is shown in fig. 5. It illustrates that the variation trend in Z-average diameter were nearly identical for PMBA1, PMBA2, PMBA3 and PMBA4. As seen in the plot, the dispersant concentration seems to be an important parameter, affecting the particle size of the dispersion. Increase in dispersant concentrations, the Z-average diameters decreased apparently, then varied slowly, consistent with the viscosities analysis. Within the range of dispersant concentrations, PMBA 1 had the lowest Z-average diameter (273 ± 1 nm) followed by PMBA2 (290 ± 1 nm), PMBA3 (406 ± 1 nm) and PMBA4 (478 ± 1 nm). The result suggests that the dispersants we synthesized provide significant steric stability in ethanol [38-40]. With the increase of dispersant, more dispersants would adsorb onto the particle surfaces and their solvent segments produce steric hindrance between inter-particles, inhibiting the inter-particles tangling up efficiently [41]. When the dispersants on yellow iron oxide particle surfaces were at saturation level, the trend of aggregation was impeded and yellow iron oxide particles were stabilized in the dispersion. Hence, with dispersants’ participation, the Z-average diameter of yellow iron oxide dispersion decreases remarkably.
Insert Fig. 5 3.6 The stability and dispensability of yellow iron oxide dispersion
For long-term stability measurement, the dispersions of yellow iron oxide particles treated with PMBA1 in ethanol were detected by the sedimentation test, showing in fig. 6. The dispersion stability decreased as the sedimentation proceeded. Nevertheless, the dispersions stabilized by PMBA1 could be stable for at least 60 days, and the dispersion efficiency could achieve about 50 wt % with 1.0 % or more mass fraction of PMBA1. It means that the settling rate is significantly susceptible to polymeric dispersant concentrations. To be more precise, at high concentration, it was beneficial for PMBA1 to restrict the fierce movement of yellow iron oxide particles and construct the steric barrier so that the particles can suspend stably in
ethanol for days. On the contrary, the agglomerates were easy to form because of the van der Waals force. Nevertheless, the colloidal dispersions remained stable for a long time with PMBA1 though the sedimentation is in the process. It is confirmed that the dispersion system is bound up with the steric stabilization effect of polymeric dispersant. In this nonaqueous system, the copolymers PMBAs provide proper solvent chains and anchoring groups so that the yellow iron oxide particles can disperse stably in ethanol.
Insert Fig. 6
4. Conclusion In this paper, the copolymers PMBAs were fabricated as dispersants to investigate the viscosity, rheological properties and stability of yellow iron oxides in ethanol. The concentration-dependence of viscosity, particle size and sedimentation measurements were studied. It reveals that viscosity and stability of dispersions have a close relationship with the copolymer amount. In the apparent viscosity measurements, the smallest value was about 10 mPa.s or slightly higher as the copolymers dosage was 1.0 %, which indicates the feeble interaction between inter-particles. In the valid concentration ranging from 0.01 % to 0.2 % studied, the Z-average particle diameter decreased from micrometer to submicron, which is favorable for ceramic ink. For the rheological property, shear-thinning behavior switched to shear-thickening behavior as the copolymer dosage increased. The shear-thickening behavior became remarkable as the solid loadings added up to 50 wt %. The four copolymers displayed similar or almost identical flow behavior as the concentration was 0.7 %. Sedimentation measurement conveys the yellow iron oxide dispersions could be stabilized for more than 60 days despite the incessant aggregation, which shows brilliant dispensability stabilized by these copolymers. The prominent stabilization of yellow iron oxide dispersion is owing to the proper structure of copolymers with the major groups amino adsorbing on the particle surface via hydrogen-bond interaction. Meanwhile, the teeth of butyl cooperate with N-dimethylaminopropyl impart steric stabilization. From the application viewpoint, the copolymers PMBAs with excellent dispersion ability might be attractive in colloidal material area. With respect to yellow iron oxide dispersion, good colloidal stability and dispensability with low viscosity and high solid loadings are significant improvement for the ultimate goal of ceramic application.
Acknowledgements This work was supported by Key Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of Sciences
and Foshan Centre for Functional Polymer Materials and Fine Chemicals. The support is also acknowledged from Guangzhou Municipal Science and Technology Program (No. 201510010183)
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Figure captions: Fig. 1 (a) Apparent viscosities of yellow iron oxide colloidal dispersions with PMBAs as a function of concentration measured at 25 ℃ and 40 ℃; (b) Apparent viscosities of yellow iron oxide colloidal dispersions with 40 wt % and 45 wt % solid content and various concentrations of PMBA1 at 25 ℃ and 40 ℃
Fig. 2 The rheological properties of yellow iron oxide colloidal dispersions with PMBAs: A(i), B(i) Rheological properties of yellow iron oxide colloidal dispersions containing 40 wt % yellow iron oxides and various concentration of PMBA 1; A(ii), B(ii) Rheological properties of yellow iron oxide colloidal dispersions with different solid content of yellow iron oxides and 0.7 % PMBA1; A(iii), B(iii) Rheological properties of yellow iron oxide colloidal dispersions containing 40 wt % yellow iron oxides with 0.7 % PMBA1, PMBA2, PMBA3 and PMBA4.
Fig. 3 The elemental analysis of yellow iron oxides (A), yellow iron oxides with PMBA1 (B), yellow iron oxides with PMBA2 (C), yellow iron oxides with PMBA3 (D), yellow iron oxides with PMBA4 (E).
Fig. 4 TEM images of yellow iron oxide colloidal dispersion (A), colloidal dispersion with PMBA 1 (B), colloidal dispersion with PMBA2 (C), colloidal dispersion with PMBA3 (D), colloidal dispersion with PMBA4 (E).
Fig. 5 The Z-average dynamic particle sizes of yellow iron oxide with various amount of PMBA1, PMBA2, PMBA3 and PMBA4 in ethanol.
Fig. 6 The sedimentation test of yellow iron oxide colloidal dispersion with PMBA1.
Fig. 1
A
B
Fig. 2
Fig. 3
(A)
(B)
(C)
(D)
Fig. 4
(E)
Fig 5
.
Fig 6
Schemes
Scheme 1 The synthetic route of PMBAs.
Scheme 2 The sedimentation diagram of colloidal dispersion.
Scheme 3 Schematic illustration of the probable mechanism for the stabilization of yellow iron oxide particle dispersions with PMBAs.
Tables
Table 1 The synthesized conditions of PMBAs. Polymer
a
b
PMBA1
1:1:2
80:1:0.2
PMBA2
1:1:2
80:1:0.25
PMBA3
1:1:2
90:1:0.2
PMBA4
0:2.5:1.5
80:1:0.2
a
[M]: [B]: [A]
[M]0: [IBCP]: [AIBN]
monomers: M = methyl methacrylate, B = butyl methacrylate,
A = N-(3-(dimethylamino) propyl) methacrylamide; b
general polymerization conditions: T = 75 ℃, t = 5 h.
Table 2 Summary of monomer conversions and molecular weight of PMBAs. polymer
b
Monomer conversion (%)
c
Mn, theo
Mn, GPC
Mw, GPC
d
PDI
MMA
BMA
DMAPMA
PMBA1
93
66
22
5237
23101
28251
1.17
PMBA2
99
72
22
5765
26380
31346
1.19
PMBA3
91
64
24
6173
27910
33482
1.21
PMBA4
-
46
32
5143
26304
30262
1.15
a
Calculated by gravimetric analysis; b Determined by 1H NMR spectroscopy; c Mn, theo (theoretical Mn) = ([Monomer]0/[IBCP]0) ×
Conv. × Mmonomer (monomer molar mass) + MIBCP; d PDI denotes the polydispersity, measured by GPC, DMF as solvent.
Table 3 Rheological parameters of τy, K, N and values of steady state viscosities determined from Herschel-Bulkley Model Rheological parameters Samples (wt %) τy (Pa)
K (Pa.sn)
N
Viscosity (Pa.s)
R
0.5 PMBA1α
0.2863
0.0720
0.7810
0.0268
0.9982
0.7 PMBA1α
0
0.0024
1.2260
0.0071
0.9953
1.0PMBA1α
0
0.0012
1.3385
0.0065
0.9942
0.7 PMBA2α
0
0.0019
1.2592
0.0067
0.9910
0.7 PMBA3α
0
0.0015
1.3055
0.0069
0.9943
0.7 PMBA4α
0
0.0038
1.1708
0.0088
0.9948
0.7 PMBA1β
0
3.2594×10^-6
1.0495
0.0039
0.9911
0.7 PMBA1γ
0.1045
0.0061
1.1545
0.0141
0.9990
0.7 PMBA1δ
0.1486
0.0057
1.2568
0.0219
0.9952
yellow iron oxide particle solid content: α 40 wt %; β 30 wt %; γ 45 wt %; δ 50 wt %