Influence of the functionalization pattern of ethyl cellulose on the interactions with polystyrene latex particles in aqueous mixtures Alexandra Wennerstrandª,#, Martin Olssonª, and Lars Järnström a Andreas Koschella b, Dominik Fenn b and Thomas Heinze b ª Department of Chemical Engineering, Karlstad University, SE-651 88 Karlstad, Sweden. b Kompetenzzentrum Polysaccharidforschung, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany. # Present address: Yara International ASA, Yara Teknologisenter Porsgrunn, N-3908 Porsgrunn, Norway. Present address: Borregaard AS, P.O. Box 162, N-1701 Sarpsborg, Norway. Abstract The interactions between polystyrene latex particles and of ethyl cellulose (EC) with different functionalization pattern have been investigated. 3-Mono-O-ethyl cellulose and EC with statistic functionalization pattern in the glucose units were studied in aqueous solutions and dispersions. EC belongs to a group of polymers that phase separate upon heating. The two types of EC showed large differences in phase separation temperature, which was explained as an effect of different interactions with water for the two polymers due to different functionalization pattern. Both types of EC did adsorb on polystyrene particles, which indicated a favorable interaction between EC and polystyrene latex particles. This favorable interaction influenced the properties of EC and polystyrene latex particle mixtures quite different depending on the structure of EC. The conventionally synthesized ethyl cellulose with statistic functionalization pattern formed much stronger networks with polystyrene latex particles than 3-mono-O-ethyl cellulose did. The lower phase separation temperature and the slightly higher molecular weight of the conventional synthetic ethyl cellulose gave it higher preference for interacting with polystyrene latex particles to form network. Introduction Of central importance for the use of cellulose derivates in aqueous-based applications are their physicochemical properties as well as their interactions with other colloidal sized materials like particles and surfactants. A group of cellulose derivates, mostly cellulose ethers, possess the unusual physicochemical property of decreased solubility in water upon heating, i.e., they phase separate at a certain temperature. The phase separation temperature is commonly referred to as the cloud point temperature, T cp. At temperatures above T cp, the solutions phase separate into one polymer rich phase and one phase depleted in polymer. Ethyl cellulose (EC), methylcellulose (MC) and ethyl(hydroxyethyl) cellulose (EHEC) are examples of cellulose ethers that phase separate upon heating. Only few studies have been performed concerning the interaction between cellulose derivates and colloidal sized particles. However, insights into
these interactions were gained from studies on adsorption behavior of cellulose derivates to macroscopic surfaces. In this study, we have investigated aqueous mixtures of ethyl cellulose (EC) and polystyrene latex (PSL) particles. Industrially, these kinds of mixtures are of importance in e. g. paint and paper coating formulations. For EC, the solubility in water is dependent on the degree of substituted ethyl groups (DS ethyl ) on the cellulose backbone (Bock 1937, Dönges 1990, Koschella et al. 2006). Between DS ethyl of 0.7-1.5, EC is water soluble, while at DS ethyl >1.5 EC is only soluble in organic media. The latter type of EC is widely used as coating material in the pharmaceutical industry (Porter 1989, Hjärtstam and Hjertberg 1998). In the present study, two types of water soluble EC were used. 3-Mono-O-EC has been synthesized according to Koschella et al. (2006) and the other type is a conventionally synthesized EC with a statistic functionalization pattern. The purpose of the study has been to investigate the interactions between PSL particles and EC, and to examine how these interactions are influenced for different types of EC. The interactions between EC and PSL particles were not investigated up to now. Neither the interaction nor the adsorption behavior of EC to macroscopic surfaces is known. In order to monitor the interactions between EC and PSL particles, we investigate the adsorption behavior of EC onto PSL particles. Further, information of how the interactions between EC and PSL particles influence the solution properties of these mixtures was gained through rheological measurements and AFM. The studied interactions are also dependent on the phase behavior of the two-component system of EC and water. Therefore, T cp of EC at various concentrations in water was determined spectrophotometrically. Experimental section Materials The two studied ethyl cellulose were 1) 3-mono-O-ethyl cellulose (3-EC) and 2) conventionally synthesized ethyl cellulose with statistic functionalization pattern (S-EC, see figure 1) Figure 1. The chemical structure for 3-EC (left figure) and S-EC (right figure). 3-EC was synthesized according to Koschella et al. (2006) by means of a protecting group technique. The degree of substitution of ethyl groups (DS ethyl ) was 1.05. A value of DS ethyl close to 1 for 3-EC implies that the functional group is uniformly distributed along the polymer chain. S-EC was a laboratory sample prepared by reacting ethyl chloride onto a cellulose backbone in the presence of sodium hydroxide. A DS ethyl of 1.3 was determined for
S-EC. Contradictory to 3-EC, S-EC may have a non-uniform functionalization pattern along the polymer chain. Two different batches of polystyrene latex (PSL) particles were used in this study. For the determination of the adsorbed amount of EC onto PSL particles well controlled PSL particles supplied from Sigma-Aldrich Production GmbH was used. These PSL particles were chosen since a minimal amount of surfactants has been used to stabilize the particle suspension and that the particle size was well controlled and rather monodisperse. The diameter of the particles was 590 nm. For the rheological measurements and the AFM studies PSL particles supplied from Dow Europe GmbH were used. These particles were chosen to obtain a higher solid content in the mixtures and for comparison to previous studies. The diameter of the particles was 260 nm. Methods Molecular weight distribution. For the molecular weight distribution of the EC, a size exclusion chromatograph (SEC) was used. A calibration curve was established by using pullulan standards of known molecular weight. Determination of cloud point temperature. The cloud point temperature, T cp, of aqueous EC solutions was determined by an UV/VIS spectrometer, where the turbidity of the sample was determined at a wavelength of 600 nm. The cloud point temperature, T cp, was determined as the temperature where a significant increase in turbidity happened. Adsorption measurements. PSL particles and EC mixtures were mixed and let to equilibrate. Thereafter, the samples were centrifuged at 6000 rpm for 2 3 h to obtain a clear aqueous (continuous) phase of the suspensions. The adsorbed amount of EC onto PSL particles was determined by analyzing the content of EC in the continuous phase using a color reaction of carbohydrates (Vasseur 1948). Rheological measurements. Rheological properties were examined using a controlled shear stress rheometer (MCR 300, Physica Messtechnik GmbH) equipped with a cone - plate geometry (50 mm diameter, 1 ). The material functions obtained in these measurements were storage modulus (G ), loss modulus (G ) and critical strain (γ c ). AFM measurements. The surface layer structure of coatings, prepared from mixtures of EC and PSL particles on nonporous polyester films, was investigated by Atomic Force Microscope. The topographic images of the coating surfaces that were collected with the AFM were subsequently processed by an image analysis program (ImagePro, Media Cybernetics Inc.). Results and Discussion The cloud point temperature (T cp ) was determined for aqueous solutions of 3-EC and S-EC at various polymer concentrations between 0.1 and 2.0 wt%. It was found at all investigated concentration that the 3-EC had T cp of ca. 30 C higher than for the S-EC. The slight difference in DS ethyl between the two investigated EC will give a difference in T cp, which has been shown e.g. for the more investigated EHEC by Thuresson et al. (1995). Thuresson et al. showed that increasing DS ethyl lead to a decreased T cp, where an increase in DS ethyl with 0.1 units decreased T cp by ca. 4 C. From these findings by Thuresson et al. for EHEC, the large difference in T cp in our study can not solely be explained by the difference in DS ethyl between
the two EC. We think that the additional effect is due to the different functionalization pattern within the repeating unit. Another possible explanation to the difference in T cp between the two EC is if the various substitution created an even distribution of ethyl groups along the cellulose backbone. If this is not the case, glucose units with low or no amount of substituted groups create hydrophobic parts on the cellulose derivate especially if several low substituted glucose units follow in a row. These hydrophobic parts influence not only T cp, but also the viscosity of aqueous solutions of cellulose derivatives. Karlson et al. (2000) give a procedure to determine the amount of hydrophobic parts in synthesized cellulose derivatives. The results obtained when performing this procedure showed that no large hydrophobic parts could be found in the chemical structure for either of the two EC. Therefore, possible explanations of the differences in T cp and in viscosity seen between the two EC are limited to be the slight variations in DS ethyl and molecular weight as well as the different functionalization pattern, i.e. the positions of the ethyl groups in the AGU. From the adsorption measurements, it was found that both EC absorbed to the PSL particles already at low concentrations of EC. The maximum adsorption for the two EC was found to be ca. 1 g/m 2, which is similar to values reported for the maximum adsorption of EHEC to polystyrene surfaces by Malmsten and Tiberg (1993). More information about the interactions between EC and PSL particles can be obtained from rheological measurements. Both frequency and strain (γ) sweeps were performed on EC/PSL particle mixture. The measurements showed that a strong and stable network was formed for the S-EC/PSL particle mixtures, while a considerable weaker network was formed for the 3- EC/PSL particle mixtures. There were quite remarkable differences of how the two EC interacted with PSL particles and thereby influenced the rheological properties of the EC/PSL particle mixtures. Once again, we think that these differences are a consequence of not only the molecular weight, but also differences in chemical structure between the two EC AFM images on coating layers based on pure PSL particles (with no cellulose derivate added) and mixtures of EC/PSL particles were established. High degree of order was seen for the pure PSL particle coating layers, while an addition of cellulose derivates decreased the degree of order of the coating layers. A difference in order between the EC could be detected, where the coating layers formed from the mixtures of 3-EC/PSL particles were the most regularly ordered. This behavior follows the same trend as the rheological measurements, where the strongest interactions between the components were seen for S-EC and PSL particle mixtures leading to a more solid-like behavior than in the mixtures of 3-EC and PSL particles. Measurements on various types of added polymers by Wallström and Järnström (2005) support the general conclusion that a more liquid-like behavior of the polymer and PSL particle mixtures promotes a more ordered surface structure. Conclusions In this study, it was found that the interactions between ethyl cellulose and polystyrene latex particles are largely dependent on the functionalization pattern, namely statistic (random) and regioselective functionalization, of the ethyl cellulose. This was found in rheology measurements and AFM analysis of coating layers, which revealed a different adsorption behavior of the two types of ethyl cellulose on polystyrene latex particles mixtures. The statistically modified ethyl cellulose formed much stronger network with polystyrene latex particles than the 3-O-ethyl cellulose.
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