Characterization of Heterocoagulation with Oppositely Charged

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Characterization of Heterocoagulation with Oppositely Charged Polymer Colloid Particles through On-line Tracking of Light Transmittance Hongyan Cao, Libin Zhang, Lili Wu, and Xiang Zheng Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08916 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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ACS Applied Materials & Interfaces

Characterization of Heterocoagulation with Oppositely Charged Polymer Colloid Particles through On-line Tracking of Light Transmittance Hongyan Cao, † Libin Zhang ,†,‡ Lili Wu,†,§ Xiang Zheng Kong*, † † College of Chemistry & Chemical Engineering, University of Jinan, Jinan 250022, China ‡ Department of Medical Technology, Zaozhuang Vocational College of Science & Technology, Tengzhou 277500, China. § Shandong Product Quality Inspection Research Institute, 31000 Jingshi East Road, Jinan, 250102, China

ABSTRACT: Heterocoagulation of colloid particles with opposite surface charge has been used for preparation of composite microspheres with specifically designed suprastructure, such as those with raspberry-like surface morphology and core-shell microspheres, which are difficult to achieve through other techniques. Here we report our investigation on the heterocoagulation of cationic polystyrene (PS) particles with anionic poly(methyl methacrylate) (PMMA) particles by a novel technique, i.e. by on-line following up the evolution of the light transmittance in the process with practically no disturbance of the dispersion and no any post-treatment for the samples. Different heterocoagulation was conducted with PS and PMMA latexes with different latex mixing regime and of different particle size for both latexes. Evolution of the light

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transmittance in all these processes, mainly with anionic latex addition to cationic latex, was followed on-line. Combined with TEM to detect the morphology of the composite microspheres formed, and with light scattering to follow the size evolution in the heterocoagulation, this work gives a clear picture for the heterocoagulation process. In addition, a set of mathematical equations are established in order to estimate the number ratio of the particles with opposite charge and the surface coverage percent of the large primary particle by the small ones. Results show that the particle size plays a key role in the process. The mechanism of the heterocoagulation process is discussed.

KEYWORDS: heterocoagulation, composite microspheres, light transmittance, particle surface coverage, self-assembly

1. INTRODUCTION Polymer composite particles with specially designed morphology and specific properties have received increasing attention in academic and industrial areas, owing to their various applications in many fields,1-6 including for instances biotechnology, medicine, chromatograph, controlled release and photonic crystals.7 These materials are featured by their characteristics, such as large surface areas, specific morphology and surface properties and high light-scattering ability. It is to note that, amongst possible protocols to achieve microspheres with specially designed morphology, orderly organization or self-assembly of primary particles have been the most common means, though the protocols are different one from another.1,3,5-14


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Heterocoagulation refers to a flocculation process of polymer dispersion with charged particle surface by blending with another polymer dispersion with the particles surface bearing opposite charge.15-24 Through the process, composite polymer microspheres with special suprastructure and morphology are obtained. The heterocoagulation of oppositely charged particles has been widely studied. Among the reported works, Vincent et al. 15-18 and Ottewill et al. 22,25 have done pioneering works. Using oppositely charged polystyrene (PS) particles of different sizes, their studies were focused first on the equilibrium adsorption-desorption behavior of small and positive PS particles onto large and negative ones in the presence of poly(viny1 alcohol) or nonionic surfactants like polyethoxides to adjust the inter-particle interactions.26 Depending on the ionic strength in the aqueous phase, the adsorption of the small particles was classified into highaffinity and low-affinity adsorption, the former was irreversible, and the latter was in general reversible. They developed also a freeze-fracture technique to observe the morphology of the resulting composite microspheres under SEM,27 and the process was also studied through rheological measurement to determine the extent of flocculation in heterocoagulation with concentrated dispersions.28 In the 1990’s, Okubo group conducted a series of studies on heterocoagulation between two groups of latex particles with opposite charges.19 The two latexes were blended without coagulation at pH 3 at room temperature, and heterocoagulation was induced by increasing pH of the mixed dispersion to 9. The size evolutions of the initial particles and the composite microspheres, large particles with the small ones adsorbed on the surface, were also determined by light scattering.20,21 Using the small polymer particles with a lower glass transition temperature (Tg) than that of the large particles at the core in the heterocoagulated composite microspheres, and by heating up the system up to a temperature above Tg of the small particles adsorbed on the large ones, core-shell composite microspheres


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were prepared.22,23,29 In addition, based on the interface energy of the large particles and the small particles towards water as well as that between the two polymers of the particles with opposite charges, the authors established equations to estimate the conditions required for the formation of a perfect core-shell structure, i.e. the interface energies required to achieve a complete spreading of the shell-forming polymer (of the small particles) over the surface of the core-forming material, the large particles.22,23,29 Up to date, besides the works conducted to fabricate composite polymer microspheres through heterocoagulation, all related studies have been focused either on the equilibrated adsorption of the small particles on the large ones with fixed ratio of the oppositely charged particles, or on the effects of the presence of adsorbed surfactants or non-ionic polymers and of ionic strength in the solvent used in the process. It is to point out that, in real lives where the heterocoagulation might be involved (such as adhesion of particles to a surface, formulation of pesticide and pharmaceutical products, soil conditioning, water purification, as well as in the aggregation and adhesion of biological cells, for examples), the process of heterocoagulation itself and its evolution in dispersions are more interesting, because coagulation or flocculation of the coexisting particles in pragmatic cases is time related. To understand how the flocculation is occurring in the process is useful to enable one to better control the processes. In addition, it is to note that, in the reported works the process was studied through different techniques, such as isotherm adsorption,17,18 electrophoresis,22 freeze-fracture at extreme low temperature,27 SEM observation of the particulates17 or other techniques etc. In all these methods, either particulates separation by centrifugation, or sample drying and heating, or all of them were involved. These post-treatments of the samples are accompanied with known and unknown side effects on the samples, because the equilibrium between the oppositely charged


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particles and that between the primary particles and the flocculated suprastructure (composite microspheres, in most cases) are often quite fragile. Clearly, a better method of the choice would be the one in which the system is disturbed as little as possible. Under this background, small PS particles with cationic surface charge (denoted as SC particles hereafter) and large-sized anionic polymer particles, of poly(methyl methacylate) (PMMA, denoted as LA particles hereafter), were prepared by emulsion polymerization. Heterocoagulation of the two groups of particles with opposite charge was carried out, using one given group of particles against a series of latexes with opposite charge and different particle size each at a time. And the process was closely followed by on-line detection of the light transmittance, without need for sampling and any post-treatment. The number ratios of the SC particles adsorbed on the LA particles in the entire process, from the very first adsorption of the SC particles on the LA particles to the saturation of this adsorption till the moment where the flocculation of the latex mixture started to occur, were determined. The effects of latex addition regime and particle size for both the SC and LA latexes on the heterocoagulation were investigated. An adsorption and desorption mechanism of the SC particles on the LA particles was proposed. Based on the process, a set of mathematical equations were established which enabled one to estimate the relative amounts of the two groups with oppositely charged particles at any moment up to the coagulation of the binary latex mixture. The surface coverage of the large particle by the small particles was also obtained, which was used to interpret the transmittance evolution in the heterocoagulation. 2. EXPERIMENTAL


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2.1 Materials. Styrene (S) and methyl methacrylate (MMA), of Tianjin Damao Chemicals, were purified by distillation under reduced pressure in a nitrogen atmosphere and stored in a refrigerator prior to use. 2,2′-azobis(2-methylpro-pionamide) dihydrochloride (AIBA, AR, Du Pont), potassium peroxydisulfate (KPS, AR) and sodium dodecyl sulfate (SDS, AR), both from Tianjin Fuchen Chemicals, were used as received. Dimethylaminoethyl methacrylate and nhexadecanebromide, both analytical grades from Sinopharma Chemical Reagents Co., were also used as received. Water used was always double-distilled and deionized in the laboratory. Cationic functional monomer, methacryloxy ethylhexadecyl dimethylammonium bromide (DMHB), was synthesized by reacting dimethylaminoethyl methacrylate with nhexadecanebromide as previously described.24 2.2 Preparation of Surface Charged Polymer Particles. A series of cationic polymer latexes of P(S-DMHB) was prepared through emulsion copolymerization of S with different amount of the cationic monomer DMHB using AIBA as a cationic initiator. For a typical run, 80 g of water was first added to a glass reactor of 250 mL equipped with reflux condenser and mechanical overhead stirrer, followed immediately by N2 purge under gentle stirring for 30 min. 0.5 g of cationic monomer DMHB was then added, followed by addition of 9.5 g of S. After 20 min of emulsification of the mixture under stirring at 500 r/min, the reactor was located to a water bath of 70 ºC. With the stirring reduced to 250 r/min, the emulsion polymerization was initiated by addition of 10 mL of aqueous AIBA solution of 0.25 wt%, and allowed to proceed for 4 h. By changing the amount of DMHB, cationic latexes with varying particle sizes were prepared. Latex solids varied from 8.5 to 11.0 wt% depending on the monomer conversion in different runs.


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Anionic latex of PMMA was prepared by emulsion polymerization by batch process. In a typical process, a given amount of anionic surfactant SDS was first added to a glass reactor of 500 mL, equipped with reflux condenser and mechanical overhead stirrer, followed by addition of 270 g of water. The content was stirred with N2 bubbling at the same time to make SDS full dissolved. 30 g of MMA was then added and the mixture was stirred at 500 r/min for 20 min to make a preemulsion. The reactor was located to a water bath of 70 ºC with the stirring reduced to 200 r/min. The emulsion polymerization was initiated by addition of 30 mL of aqueous KPS solution of 1.0 wt% and allowed to proceed for 4 h. By changing the amount of SDS, anionic latexes with varying particle sizes (75 nm to 196 nm) were prepared. Particle size, size distribution and surface morphology were determined using transmission electron microscope (TEM, JEM-100CXII, Japan) and dynamic light scattering (nano-Zetasizer, Malvern, UK). ζ potential of the latex particles was also determined using this Zetasizer instrument. In these tests, all samples were diluted to solids of 0.1 wt% or lower. 2.3 Process of Heterocoagulation and its Characterization. A typical protocol for the heterocoagulation using two latexes with their particles oppositely charged is as the following: both cationic and anionic latexes were diluted to solid content of 0.05 wt%. 100 mL of the diluted cationic latex was located into an Erlenmeyer flask held on the plate of a magnetic stirrer. Anionic latex was added into the flask at 5.0 mL/min under gentle stirring using a magnetic bar placed in the flask. During the process, the transmittance of the binary mixture of the latexes was followed using a photometer (Metrohm 662, Switzerland) at wavelength of 550 nm. Depending on the particle size and latex solids, the evolution of the light transmittance in the process of heterocoagulation was different. However, the end of the process, where flocculation or breaking-down of the latex mixture occurred, was always indicated by a sharp increase in light


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transmittance, because the flocculated latex particles started to settle down as sediment. The order of latex addition and the ratio of the two charged particles were changed in order to study the process. 3. Estimation of the Numbers of the Oppositely Charged Primary Particles in the Heterocoagulation. To accomplish the heterocoagulation, two groups of primary particles are required. There exist also two prerequisites for the primary particles: the first is that the two groups of primary particles ought to be surface charged, and in particular, with opposite charges; the second is that the two classes of primary particles have to be different in their size. It is easy to understand that a heterocoagulation, with two groups of oppositely charged particles of equal size, ought to be hard to proceed if not impossible owing to the nature of the process, in which the particles self-organize based on electrostatic interaction1,15-23 or hydrogen bonding3,30 between the two groups of the particles. In addition, from simple geometric viewpoint, the formation of the suprastructure by heterocoagulation between two groups of particles of equal size is difficult to achieve (as will be shown later in Section 3 by Figure 1), and it must be not stable or at very low degree of order if formed. This must be the reason why all reported studies on heterocoagulation have been carried out using two groups of particles with largely different size.1,3,15-30 That is to say, in order for the heterocoagulation to occur, one has to have two groups of primary particles: one group of the primary particles, of particle size D1 (denoted as their diameter) with anionic (or cationic) surface charges, and another group, of particle size denoted as D2, with their surface charges opposite to the first group. By appropriately adjusting the surface charge density and the relative numbers of the two groups of particles, the smaller particles ought to adhere onto the larger ones through electrostatic interaction, or static interaction by simplification. Obviously, the structure and the morphology of the finally


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assembled suprastructure (denoted as composite microspheres throughout this paper) are closely dependent on the size difference of the two group particles. Assuming that the primary particles with size D1 is negatively charged on their surface and those with size D2 positively charged, two different possibilities exist as schematized in Figure1: (1). One is D1/D2=1, which means that the two groups of primary particles are equal in size (Figure 1A). In this case, if one group of the particles closely packed onto the other group with opposite charge, an equilateral triangle, in which each particle at the center has six coordinated particles of opposite charge, is formed, leading to the common face-centered cubic packing.31-33 As a matter of fact, this form of ordered colloidal suprastructure, i.e. equilateral triangle, is possible to occur only on a planar substrate surface. When a surface with curvature is concerned as in the present case, where one group of spheres organizes on another group of spheres, defaults are necessarily present.32 (2). The second case is D1/D2>1 (Figure 1B), where the two groups of primary particles are different in size. In this case, the structure of the equilateral triangle between the two groups of primary particles will not form. Instead, the small particles will assembly on the large ones, and each large particle at centre will have more than six coordinated small particles. Larger is D1/D2, larger is the number of the coordinated particles. When focus exclusively on the close-packed small particles adhered on a large one, the problem turns to be the same as in the case of D1/D2=1, the only change is that it becomes the packing of the primary particles of equal size (Figure 1A) with the same surface charge. In this case, an equilateral triangle structure, in which each particle at the center has six coordinated particles, is formed, providing that the D1/D2>>1, i.e. the large particle is large enough that its surface can be considered as planar. Obviously, there will be defaults because the surface of the large particles is not truly planar. Larger is D1/D2, less there will be the defaults. (3). The third case is D1/D21 as depicted in Figure 1.

Figure 1. Possible packing in heterocoagulation using two groups of uniform polymer particles of different size (D1 and D2) and opposite surface charge (A, D2=D1; B, D1>D2; C, D1

Characterization of Heterocoagulation with Oppositely Charged

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