Chinese Chemical Letters  2015, Vol.26 Issue (09): 1174-1178   PDF    
Adsorption of cationic copolymer nanoparticles onto bamboo fiber surfaces measured by conductometric titration
Xiu-Ming Liu, Dong-Qin He, Kuan-Jun Fang     
School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China
Abstract: Monosized nanoparticles of 57.3 nm were prepared by cationic emulsion polymerization using a polymerizable emulsifier DMHB. The adsorption of nanoparticles onto bamboo fibers was measured by conductometric titration. The results indicated that the adsorption capacity increased with increasing contact time until 120 min. The equilibrium data for nanoparticles adsorption onto bamboo fibers were well fitted to the Langmuir equation. Moreover, the monolayer adsorption capacity of nanoparticles in the concentration range (from 0.03 g/L to 0.6 g/L) studied, as calculated from Langmuir isotherm model at 25℃, was found to be 38.61 mg/g of fibers. The SEM images showed that the nanoparticles form a uniform monolayer on bamboo fiber surfaces.
Key words: Adsorption     Copolymer     Cationic nanoparticles     Bamboo fibers    
1. Introduction

Cationic latexes have emerged as the subject of vigorous research for applications in paints, coating industries, papermaking, textile and fiber treatments, functional materials, and so on [1, 2, 3, 4, 5, 6, 7], because cationic copolymer nanoparticles with positive charge have many attractive characteristics such as preferentially adding/adsorbing the negative charge on surfaces, eliminating electrostatic repulsion, and improving properties of materials and subsequent process [8, 9].

As a new regenerated cellulose fiber, bamboo fiber is hygroscopic and permeable with natural antibacterial and bacteriostatic properties. Further, it is soft and smooth, as well as opaque to ultraviolet light. Thus bamboo fibers have wide applications in different areas. In most cases bamboo fiber surfaces have negative charges, and cationic polymers nanoparticles are readily adsorbed onto bamboo fiber surfaces by electrostatic attraction. Therefore, the adsorption of positively charged colloidal nanoparticles on bamboo fibers is becoming a promising way to manufacture functional textiles and to enhance paper strength [10, 11, 12, 13]. Recently, much more attention has been given to the adsorption of cationic nanoparticles onto cotton fibers [14, 15, 16], however, the adsorption of cationic nanoparticles onto bamboo fibers has rarely been reported.

Spherical polymer nanoparticles with positive surface charges are often prepared by emulsion polymerization [17, 18]. Cationic nanoparticles obtained by common emulsifiers are sensitive to water used in processing [19]. Thus, more stable cationic nanoparticles were synthesized by using polymerizable emulsifiers [20].

The aim of the present work is to investigate the adsorption mechanism of cationic nanoparticles onto bamboo fibers by conductometric titration. We investigate the adsorption rate and adsorption mechanism of the nanoparticles onto bamboo fibers. In addition, the morphology of bamboo fiber surfaces deposited by the nanoparticles is also observed by a field emission scanning electron microscopy (FE-SEM).

2. Experimental 2.1. Method

Conductometric titration [21, 22, 23] was carried out by adding AgNO3 solution to the nanoparticle dispersion. To obtain highresolution data with a constant concentration (AgNO3 = 5.875 × 10-3 mol/L), separate titrations were carried out using a wide range of nanoparticle (from 0.03 g/L to 0.6 g/L) concentrations. In addition, all the measurements of the conductometric titration were carried out at room temperature and atmospheric pressure, and all the conductivity data were automatically compensated to 25 ℃ by the conductivity meter. The electrode was immersed in the dispersion for 1-2 min, and then the conductivity value was recorded. The same measurement was repeated several times to check the reliability of the data and the averaged values were given for the measurement.

2.2. Materials

The materials used in this investigation and their sources are as follows: Styrene (St) was purchased from Tianjin Guangcheng Chemical Co., Ltd., China, butyl acrylate (BA) was purchased from Shanghai Aibi Chemical Co., Ltd., China, and they were purified by washing with a 10% (w/w) sodium hydroxide aqueous solution and stored at -18 ℃. The cationic polymerizable emulsifier, methacryloxyethyl hexadecyl dimethylammoniumbromide (DMHB, purity >98.5%), was prepared according to the reference. The cationic initiator used was 2, 2'-azobis[2-methylpropionamidine] dihydrochloride (AIBA, purity 98%, Qiongdao Kexin Materials Technology Co., Ltd.). Silver nitrate (AgNO3) was purchased from Shanghai Chemical Reagent Factory. Deionized water was purified by standard procedures and used in all the experiments. Bamboo fibers, deionized and scoured were supplied by Huafang Co., Ltd., China. Other materials and solvents were used as received.

2.3. Semicontinuous polymerization of cationic copolymer nanoparticles

P(St-co-DMHB-co-BA)n+.Brn- was obtained by semicontinuous emulsion copolymerization of Styrene and DMHB with BA in different molar ratios. The steps are as follows: The polymerizations were carried out in a 250 mL round bottomed flask equipped with a stainless steel stirrer, nitrogen inlet tube, and reflux condenser. The nitrogen was injected into the flask after the device was prepared, and then part of DMHB and deionized water (80 g) were added. After 15 min, parts of St and BA were added at room temperature for 30 min to make the emulsifier evenly dispersed. After a part of initiator AIBA was added, the reaction system was heated to 80 ℃. Then residual mixed monomer (St and BA) were dropped into the reaction system within 1.5 h. After that, residual DMHB and AIBA were added into the emulsion, and the reaction system was kept at 80 ℃ for 3 h and cooled to room temperature. A dispersion of cationic nanoparticles, P(St-co-DMHB-co-BA)n+.Brn-, was obtained at the end of the reaction. The recipes used in the emulsion polymerization were given in Table 1.

Table 1
Recipes used in the emulsion polymerizations.
2.4. Adsorption of the nanoparticles

Square pieces of bamboo fabric (2 g) of about 1 cm2 were washed with deionized water several times. The washed samples were put into the aqueous suspension of cationic latexes (200 mL) and kept at 60 ℃ for 1 h. The adsorption of cationic nanoparticles onto bamboo fibers was conducted at 25 ℃ and a moderate shaking speed in an SHA-V shaker (Changzhou Guohua Electric Appliance Co., Ltd., China). After that, these samples were washed thoroughly with deionized water. The conductivity of the dispersion was measured several times to check the reliability of the data. Subsequently, the bamboo samples were dried under vacuum at 50 ℃ for 30 min for other tests.

2.5. Characterization 2.5.1. Size and distributions of the cationic nanoparticles

The size and distributions of the nanoparticles were measured by using a Nano-ZS90 instrument (Malvern, UK) at 25 ℃. All samples were diluted with deionized water before test.

2.5.2. Scanning electron microscopy (SEM)

SEM images were obtained using a Hitachi SU-8010 field emission scanning electron microscope. Prior to the observations, the samples were coated with Au.

3. Results and discussion 3.1. Properties of the cationic nanoparticles

P(St-co-DMHB-co-BA)n+.Brn- was prepared by a semicontinuous emulsion polymerization, and sizes and distributions of seeded nanoparticles were measured using the Nano ZS90 at 25 ℃. Scanning electron micrograph (SEM) of the nanoparticles was shown in Fig. 1. And it is also clear that most of the nanoparticles have a diameter less than 100 nm(see Fig. 2). The average diameter of the prepared nanoparticles in water is 57.3 nm, and the PDI is 0.122. The colloid titration analysis indicates that the charge density and density of the nanoparticles are 0.821 × 10-4 C/cm2 and 1.003 g/cm3, respectively. The positive charges on the nanoparticles surfaces came from the cationic groups of the cationic initiator and the cationic emulsifier.

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Fig. 1.SEM photograph of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles prepared by semicontinuous emulsion polymerization.

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Fig. 2.Particle sizes and distribution of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles prepared by semicontinuous polymerization.
3.2. Relationship between the concentrations of nanoparticles and bromide ions

Similar to the previously reported phenomena [21], the concentration of bromide ions measured by conductometric titration is the concentration of bromide ions at the diffusion layer of the nanoparticles. It is obvious that the relationship between the concentration of the P(St-co-DMHB-co-BA)n+.Brn- nanoparticles and the bromide ion is a good linear relationship with R2 = 0.9982 in Fig. 3, which can be written as:

Here, Cp is the concentration of particles and CBr- is the concentration of bromide ion.

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Fig. 3.The linear relationship between the concentration of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles and the bromide ion. The temperature was 298 K in the conductivity titration, CAgNO3 = 5.875 × 10-3 mol=L, the interval of stirring time is 2 min, dn = 57.3 nm.
3.3. Adsorption of the nanoparticles onto bamboo surfaces by conductivity titration 3.3.1. Adsorption rate

The adsorption rate was systematically studied to further investigate the adsorption process. Fig. 4 shows that the adsorption rate enhanced very fast within the first 50 min, this phenomenon confirmed that the strong electrostatic attractive force between the positively charged nanoparticles and the negatively charged bamboo fiber surfaces was the main driving force for adsorption. Then the amount of particles adsorbed onto the bamboo fiber increases gradually with increasing contact time until the maximum amount is reached after 120 min, which means that the adsorption equilibrium was reached. This can be explained as following: the quantity of negative charges on bamboo surface is constant; when partial nanoparticles contact with the bamboo surfaces, the double electrode layer repulsion exists between the adsorbed and adsorbing particles, which results in the decrease of particle adsorption rate.

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Fig. 4. The adsorption rate curve of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles onto the surface of bamboo fibers. dn = 57.3 nm, the fiber weight:liquor weight = 1:250, T = 298 K, Cp = 0.29 g/L, pH 6.53.
3.3.2. Adsorption isotherms

The adsorption of particles on negatively charged bamboo fiber surfaces was governed by electrostatic attractive force. The adsorption of particles onto bamboo interface mainly takes place through the mechanisms of ion exchange [24]. To further understand the adsorption mechanism of the nanoparticles onto bamboo fibers, the adsorption isotherms were measured at the Cp = 0.03-0.6 g/L according to Eq. (1). The obtained results were shown in Fig. 5, which revealed that the adsorbed amount increased with the nanoparticles concentration at adsorption equilibrium until the saturation value of adsorption was reached.

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Fig. 5.The fitting curve of adsorption isotherms of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles onto bamboo fibers. dn = 57.3 nm, T = 298 K, Cp = 0.03–0.6 g/L, pH 6.53, t = 120 min.

The measured equilibrium adsorption data were fitted according to Langmuir equation (see Fig. 6) [25]:

where Q0 is the saturated adsorption amount in mg/g, KL the equilibrium adsorption constant, qe the nanoparticles amount adsorbed on the bamboo surfaces at the equilibrium adsorption in mg/g, and Ce is the concentration of cationic nanoparticles in the dispersion at the equilibrium in g/L. According to the equation, the calculated parameters are shown in Table 2. It can be seen that the saturated amount of the nanoparticles on bamboo fibers is 38.61 mg/g, the equilibrium constant KL is 3.364. The coefficient of correlation R2 equals 0.9914, indicating that the adsorption of the nanoparticles onto bamboo fibers quite well conforms to the Langmuir model.

Table 2
The parameters of the adsorption isotherm of P(St-co-DMHB-co-BA)n+.Brn- onto bamboo fibers.

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Fig. 6.The fitting curve of Adsorption isotherm of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles onto bamboo fibers.
3.3.3. Adsorption model

As shown in Fig. 7, the SEM observation reveals that the bamboo fibers have a smooth surface compared to bamboo fibers (Fig. 7a0 and b0). Thus, after the adsorption, the nanoparticles densely arranged onto bamboo fiber surfaces at high magnification (Fig. 7b1) and partial nanoparticles aggregate onto bamboo fibers. This can be explained as follows: (1) Because bamboo fibers have a relatively smooth surface, the nanoparticles presented stronger interaction with themselves than fiber surfaces; (2) the particles are more inclined to attract each other, which decreases the stability of the copolymer suspension; thus it can be seen that aggregates of partial nanoparticles can be adsorbed onto bamboo fiber surfaces. In addition, the nanoparticles have spherical morphology and form a monolayer on the fiber surface, which further confirms that the adsorption of the cationic nanoparticles coincides with the Langmuir model, and chemical adsorption is predominant in the adsorption processes.

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Fig. 7.SEM images of adsorption of P(St-co-DMHB-co-BA)n+.Brn- nanoparticles onto bamboo fibers. a0 and b0 are the original fibers. a1 and b1 are the fibers adsorbed P(St-co-DMHB-co-BA)n+.Brn- nanoparticles. The fiber weight:liquor weight = 1:250, dn = 57.3 nm, T = 298 K, Cp = 0. 45 g/L, pH 6.84, t = 120 min.
4. Conclusion

In conclusion, we have successfully synthesized P(St-co-DMHBco-BA)n+.Brn- cationic nanoparticles with uniform average size ca. 57.3 nm by semicontinuous emulsion polymerization. The present study demonstrated the adsorption rate increased sharply within the first 50 min because of the strong electrostatic attractive force. The obtained adsorption experimental data are well fitted to the Langmuir adsorption model. The saturated amount of the nanoparticles on the bamboo fibers is 38.61 mg/g. In addition, the morphological investigation by SEM reveals that a uniform monolayer of nanoparticles is presented on bamboo fiber surfaces, which further confirms that the adsorption of the nanoparticles onto bamboo fibers coincides with the Langmuir model.

Acknowledgments

This work is supported by National Natural Science Foundation of China (No. 1173086), National Key Technology R&D Program (Nos. 2014A1302 and 2014AEOQO1) and Natural Science Fund of Tianjin, China (No. 14JCZDJC37200).

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