- Email: [email protected]
Preparation and characterization of cationic poly vinyl alcohol with a low degree of substitution
European Polymer Journal 47 (2011) 997–1004
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation and characterization of cationic poly vinyl alcohol with a low degree of substitution Pedram Fatehi a,⇑, Robin Singh a,b, Zainab Ziaee a, Huining Xiao a, Yonghao Ni a a b
Department of Chemical Engineering, Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, India
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 16 February 2011 Accepted 27 February 2011 Available online 3 March 2011 Keywords: Cationization Poly vinyl alcohol AFM NMR FTIR Polymer
a b s t r a c t The cationization of polymers has been regarded as an effective method to improve their performance for various applications. In this work, the cationization of poly vinyl alcohol (PVA) was investigated under different conditions, i.e., various glycidyl-trimethylammonium chloride (GTMAC) to PVA ratios, reaction temperatures, times, PVA and NaOH concentrations and solvent compositions. The results showed that the overall efficiency of the cationic modification was rather low, which was due to the hydrolysis of both GTMAC and cationic-modified PVA (CPVA) under the strong alkaline conditions employed. The results also showed that the optimum GTMAC/PVA ratio depended on the solvent composition. The cationization was confirmed by means of 1H NMR and FTIR analyses. The maximum efficiency in water was obtained under the conditions of 95 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration, while that in the ethanol/DMSO mixture (1.25 v/v) was obtained under the conditions of 70 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration. Additionally, the interaction of CPVAs with a silicon wafer (as a substrate) was determined by employing an atomic force microscope (AFM) in water and air. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Cationic polymers have been applied in various industries for different purposes. For example, they have been used as strength agents or retention aids in papermaking [1–4]. Copolymerization and grafting have been regarded as useful methods to induce tailored cationic polymers in the literature [1,5]. Although copolymerization is an efficient method to produce the cationic polymers, simultaneously controlling the molecular weight (MW) and charge density of the resulting cationic polymers may be a challenging task, as these properties are interrelated [5]. The copolymerization of vinyl acetate and diallyldimethyl ammonium chloride (DADMAC) was conducted in our previous work to produce
⇑ Corresponding author. Tel.: +1 506 452 6084; fax: +1 506 453 4767. E-mail address: [email protected] (P. Fatehi). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.02.021
cationic PVA (CPVA) [5]. The highest MW of the resulting CPVA was 80,000–90,000. On the other hand, the cationization of already produced polymers on the basis of grafting is a simpler process since the cationization degree can be controlled without any effect on the molecular weight of polymers. The modification of poly vinyl alcohol (PVA) using various chemical agents was reported in the literature [6–9]. In our previous work, the cationic-modified poly vinyl alcohol (CPVA) showed promising results in improving the pulp and paper strength [10,12–14], and the best results were obtained at employing PVA with the molecular weight of 142,000–186,000 [11]. Additionally, it was found that the optimum charge density of CPVA for various fiber systems may be different, varying between 0.3 and 0.72 meq/g, depending on the types of the fiber systems [10,12,13]. Therefore, it is important to understand the influence of parameters affecting the cationization of PVA with a tailored charge density.
998
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
In the literature, the cationization of starch and hemicelluloses were conducted under various conditions, including wet or semi-dried environments [15–18]. The cationization reaction was also conducted in various solvents, e.g., water, ethanol or dimethyl sulfoxide (DMSO) [19,20]. In the cationization reaction, glycidyl-trimethylammonium chloride (GTMAC) has been commonly used as a cationic reagent [15–20]. An atomic force microscopy (AFM) has been employed as a powerful device to investigate the interactions between various materials [21–23]. This device can be used to determine how the charge of polymers influences their attraction/repulsion force with soft or hard surfaces. In this research, we aimed at understanding the factors influencing the cationization of high MW PVA to induce CPVA with a low charge density (<1 meq/g) under various conditions. The parameters evaluated were: GTMAC/PVA ratio, temperature, time, PVA concentration, solvent composition and NaOH concentration. The cationization of PVA was confirmed qualitatively via FTIR and quantitatively via 1H NMR analyses. Based on these results, the conditions that could induce the CPVA with a desired charge density and the maximum cationization efficiency were determined. Moreover, the interaction of CPVA (having a low charge density of 0.2 meq/g or a high charge density of 1.3 meq/g) with a silicon wafer (as a substrate) was determined via employing an AFM in either water and air.
2. Experimental 2.1. Raw materials Poly vinyl alcohol (PVA) with a molecular weight (MW) of 142,000–186,000 (99% hydrolyzed), glycidyl-trimethylammonium chloride (GTMAC, 75% in water), potassium bromide (KBr, analytical grade), and dimethyl sulfoxide (99.7%, sterile-filtered), were all obtained from Sigma– Aldrich, and used as received. Anionic polyvinyl sulfate (PVSK) with a MW of 100,000–200,000 (97.7% esterified) was provided by Wako Pure Chem. Ltd. Japan. Ethanol (95%) was received from Fisher Scientific.
2.2. CPVA preparation In this research, 1 g of PVA was mixed with GTMAC under various conditions. The reaction parameters investigated were: GTMAC/PVA ratio (0.25–2 mol), PVA concentration (4–33% wt.), solvent type (water, ethanol, ethanol/water or DMSO/ethanol), reaction time (20 min– 240 min) and temperature (60 °C–95 °C) and NaOH concentration (1–15% mol based on PVA). All the reactions were conducted in a 250 ml 3-neck glass flask under a constant stirring. The cationization reaction was stopped via neutralizing the solution using HCl (1 M). Then, the solutions were cooled to room temperature, and unreacted GTMAC was separated by dialyses membranes having a molecular weight cutoff of 1000, while changing water every 2 h for the first 6 h and then once a day for 2 days.
2.3. CPVA characterization To determine the charge density of CPVAs, their solutions (1% wt.) were titrated against a PVSK standard solution (0.5 mM) by using a Particle Charge Detector Mütek PCD 03 (Herrsching, Germany). To quantify the degree of substitution of GTMAC with hydroxyl group on cationicmodified PVA (CPVA), CPVAs were dried at 105 °C overnight, and their 1H NMR spectrum was recorded in D2O at 25 °C using an Oxford 300 MHz spectrometer operating at 300.13 MHz [8,9]. The unmodified PVA and CPVA with the charge density of 1.08 meq/g, which was prepared under the conditions of 11% (wt.) PVA, 0.5 (mol) GTMAC/PVA ratio, 5% (mol) NaOH concentration(based on PVA), 90 °C, 1 h, were characterized by using a Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum 100 FT-IR Spectrometer, USA). The oven-dried PVA or CPVA were embedded in KBr pellets in a mixture of about 1% (wt.) KBr. The spectra were recorded in a transmittance mode in the range 800– 3800 cm 1. 2.4. AFM analysis In the literature, the AFM technique was employed to determine the interaction between CPVAs and a hard surface [21–23]. In fact, the silicon wafer with a 100 nm thermally oxidized layer was used as a model substrate for cellulose fibers [21–23]. In this work, similar silicon wafers were first immersed in 98% H2SO4 for 6–8 h and rinsed with deionised and distilled water prior to the use [21– 23]. To investigate the interaction of CPVA with the silicon wafers, an atomic force microscope (AFM) Nanoscope IIIa, Veeco Instruments Inc., Santa Barbara, CA, was employed. The force analysis was conducted by using silica nitrate probes (NP-S20) with a spring constant of 0.32 N/m, and a resonance frequency of 273 kHz. This silica probes were coated with CPVAs according to the following procedure: at first, the probes were immersed in CPVA solutions (2% wt.) overnight; then, the CPVA-coated probes were washed with deionised and distilled water to remove unadsorbed CPVAs from the probes. The adhesion/repulsion force developed between the CPVA-coated probes and silicon wafers was measured in Picoforce mode with a vendorsupplied fluid cell at the temperature of 25 °C and the pH of 7 in water or in air. Each of 50 different spots was scanned 5 times on an area of 1 1 lm2 at a constant tip velocity of 0.5 lm/s. The force-distance curves were obtained from cantilever deflection and displacement of the piezo. 3. Results and discussion 3.1. Preparation of cationic PVA 3.1.1. Effect of PVA concentration Fig. 1 shows the charge density of the CPVA as a function of PVA concentration. As can be seen, by increasing the PVA concentration from 4% to 33% (wt.), the charge density of resulting CPVA increased from 0.22 meq/g to
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
999
DMSO was mixed with 2.5 ml of ethanol and the reaction was conducted at 65 °C for 1 h (GTMAC/PVA molar ratio of 0.5). The resulting CPVA had a charge density of 0.61 meq/g. Thus, compared with the ethanol and ethanol/water solutions, the ethanol/DMSO solution was more effective in producing CPVA with a high charge density. Others have found that the ethanol/DMSO system was more effective than the ethanol and ethanol/water systems when cationizing starch [19,20]. Therefore, the remaining experiments were conducted under the semi-dried condition in the ethanol/DMSO system (1.25 v/v).
Fig. 1. Effect of PVA concentration on the charge density of CPVA (1 h, 80 °C, GTMAC/PVA molar ratio = 0.5, [NaOH] = 5% (mol)).
1 meq/g. In the literature, by increasing the molar ratio of water/GTMAC from 2.8 to 100 (increasing the water content of the system), the cationization efficiency of starch was reduced from 80% to 27% [16]. Also, it was reported that the cationization efficiency of hemicelluloses using GTMAC was generally low, which was attributed to the hydrolysis of both GTMAC and resulting cationic hemicelluloses under strong alkaline conditions [17,24–27]. It was reported that the hydrolysis of GTMAC under an alkaline condition mainly resulted in forming N-(2,3-dihydroxy)propyl-N,N,N-trimethylammonium chloride, and dimer and trimer of GTMAC [15]. We propose that similar hydrolyses probably occurred in the GTMAC/PVA reaction system, which impaired the cationization efficiency of PVA. The results in Fig. 1 imply that, at a low PVA concentration, the hydrolysis of GTMAC can be more pronounced, which is undesirable and reduces the efficiency of the cationization reaction. To improve the cationization efficiency, the reaction of PVA with GTMAC was conducted under semi-dried conditions. This method has been utilized for the cationization of hemicelluloses in the past [17]. In one set of experiments, 1 g PVA granule (rather than PVA solutions) was mixed with GTMAC with 2.5 ml of ethanol or with ethanol/water mixture (2.5 v/v) at 65 °C for 1 h (GTMAC/PVA molar ratio of 0.5). The charge density of the resulting CPVAs were 0.23, 0.32 meq/g, respectively. As is well known, the high MW PVA does not dissolve in pure ethanol under the conditions specified above. However, it can dissolve in the ethanol/water mixture. Thus, the former system was heterogeneous, while the latter was homogenous. It was reported that the efficiency of homogenous cationization reaction of starch (in water) was higher than the heterogeneous reaction (in ethanol) [20]. The higher solubility of starch in water, which favored the cationic reaction between starch and GTMAC, was the main reason for these results [20]. In another set of experiment, 1 g of PVA granule was dissolved in 2 ml DMSO at 65 °C, and then cooled down to room temperature. Subsequently, the PVA solution in
3.1.2. Effect of GTMAC/PVA molar ratio Fig. 2 shows the cationic modification of PVA as a function of GMTAC/PVA molar ratio in water or DMSO/ethanol. Clearly, the highest charge densities of 0.78 meq/g and 1.2 meq/g were obtained at GTMAC/PVA molar ratio of 0.5 in water and 1 in DMSO/ethanol solutions, respectively. In the cationic modification of starch, the GTMAC/PVA molar ratio of 3 was optimum, which was due to the fact that starch has three hydroxyl groups in its repeating unit, which can react with GTMAC [19]. By increasing the molar ratio of GTMAC to PVA, more GTMAC was available for the cationization reaction. However, its hydrolysis would also occur at the same time. These results showed that, by increasing the molar ratio of GTMAC/PVA more than 0.5 in water or 1 in the ethanol/DMSO mixture, more hydrolysis of GTMAC took place under the alkaline conditions studied, which reduced the efficiency of the cationization [17,24–26]. Furthermore, a higher charge density was obtained in the ethanol/DMSO mixture at 70 °C than in water at 80 °C, under otherwise the same conditions. 3.1.3. Effect of reaction temperature Fig. 3 shows the charge density of the cationic-modified PVA as a function of the reaction temperature in water or in the DMSO/ethanol solution. Interestingly, by increasing the reaction temperature from 60 °C to 95 °C, the charge density of resulting CPVA was significantly increased in water, which implied that a higher temperature favored the cationization of PVA. However, a maximum charge density of 1.2 meq/g was achieved at 70 °C for the cationization in the ethanol/DMSO solution. Further increasing the temperature resulted in a decrease in the charge density of CPVA in the ethanol/DMSO mixture. This behavior is probably due to the evaporation of ethanol from the ethanol/DMSO solution. In the literature, the cationization efficiency of starch was lower in a pure DMSO solution than that in the ethanol/DMSO solution [20]. 3.1.4. Effect of reaction time Fig. 4 shows the charge density of the CPVA as function of the reaction time. Clearly, the maximum charge density of 0.80 meq/g was obtained at 1 h in water. By extending the reaction time, the charge density of CPVA in water was reduced. A similar trend was reported for the cationization of hemicelluloses [17]. However, the maximum charge density of 0.9 meq/g was obtained in the ethanol/ DMSO solution after 1 h, and the further prolonging of the reaction had a marginal effect on the cationization effi-
1000
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
Fig. 2. Effect of GTMAC/PVA molar ratio on the charge density CPVA, [NaOH] = 5% (mol), 1 h, (in water: [PVA] = 11%, 80 °C; in ethanol/DMSO (1.25 (v/v)), 70 °C).
Fig. 3. Effect of reaction temperature on the charge density CPVA at [NaOH] = 5% (mol) for 1 h. (in water: [PVA] = 11%, GTMAC/PVA molar ratio = 0.5; in ethanol/DMSO (1.25 v/v): GTMAC/PVA molar ratio = 1).
Fig. 4. Effect of reaction time on the charge density of CPVA, GTMAC/PVA molar ratio = 0.5, and [NaOH] = 5% (mol) (in water: [PVA] = 11%, 80 °C; in ethanol/ DMSO (1.25 v/v), 70 °C).
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
ciency. It was reported that, by prolonging the reaction time, the hydrolysis of cationic hemicelluloses was enhanced, which reduced the efficiency of the reaction between GTMAC and hemicelluloses [17,23]. A similar phenomenon probably happened here by prolonging the reaction time for GTMAC and PVA system in water. However, the occurrence of side reactions was less in the ethanol/DMSO solution, as shown in Fig. 4. 3.1.5. Effect of NaOH Fig. 5 shows the charge density of CPVA as a function of the NaOH concentration. As can be seen, the charge density was the maximum via adding NaOH up to 5% (mol), regardless of the solution media. A similar result was reported by Wang and Xie on the cationization of corn starch [19]. Additionally, it was reported that NaOH functioned as a catalyst for the cationization of hemicelluloses and the hydrolysis of GTMAC [15,19]. Thus, the concentration of NaOH in solutions should be sufficient to promote the main cationization reaction. However, excessive amount of NaOH would result in more extensive GTMAC and CPVA hydrolyses. The results in Fig. 5 show that the maximum of 5% (mol) was sufficient to obtain the maximum cationization efficiency, regardless of the cationization medium. 3.2. Characterization of CPVA The reaction scheme of GTMAC with PVA was reported in our previous work [10]. Fig. 6 shows the 1H NMR spectrum of CPVA. The peak at 3.2 ppm belongs to protons attached to the quaternary ammonium group of the PVA backbone, which confirmed the cationic modification of PVA [7]. The peak at 4 ppm is assigned to the CH2 of the PVA backbone, while the peaks between 3.2 and 3.8 ppm are due to the CH2 of the attached cationic group on the PVA backbone. Also, the peaks between 1.5–2 ppm belong to CH of the PVA backbone. The degree of substitution can be quantified by considering the areas under the peaks at 3.2 and 4 ppm and the number of protons associated with
1001
each peak, which is 4.9% (mol) for the CPVA studied in Fig. 6. The charge density of this CPVA can be theoretically calculated by considering the degree of substitution via the 1 H NMR analysis and assuming that each quaternary mol of ammonium groups attached to PVA backbone has a charge density of 1 eq. The charge density of the CPVA via this 1H NMR analysis was 0.99 meq/g, which is in a good agreement with that of 1.08 meq/g obtained via the PCD analysis. The efficiency of the cationic modification can be determined based on the degree of substitution analysis of 1H NMR. In this respect, the efficiency is identified by considering the amount of GTMAC added to the system for the cationic reaction and the amount of GTMAC attached to the backbone of resulting CPVA. For the CPVA investigated in Fig. 6, the amount of GTMAC added was 0.5 mol (per mole of PVA) and the amount of GTMAC attached to PVA backbone was 0.049 mol (per mol of PVA), which implied that the efficiency was about 10%. A similar analysis was conducted on the other CPVAs produced, and the maximum efficiency obtained in water was about 12% under the conditions of 95 °C, 1 h, GTMAC/PVA molar ratio of 0.5 (mol), and the NaOH concentration of 5% (mol). The maximum efficiency in the ethanol/DMSO mixture was about 10% under the conditions of ethanol/DMSO (1.25 v/ v), 70 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and NaOH concentration of 5% (mol). The rather low efficiency is due to the occurrence of the side reactions, as discussed earlier. The cationic modification of PVA or hemicelluloses using other cationic agents was also reported to be less than 10% [7,24,27]. The results in Figs. 2 and 3 also shows that the maximum charge density of CPVA was approximately 1.2 meq/g, regardless of the solvent composition; while the reaction efficiency was higher in ethanol/DMSO system than in water system at any GTMAC/PVA molar ratio studied (Fig. 2). Fig. 7 shows the FTIR spectrum of unmodified PVA and the CPVA sample investigated in Fig. 6. A similar spectrum was reported for unmodified PVA in the litera-
Fig. 5. Effect of NaOH concentration (mol) on the charge density of CPVA, 1 h (in water: [PVA] = 11%, GTMAC/PVA molar ratio = 0.5, 80 °C; in ethanol/DMSO (1.25 v/v): GTMAC/PVA molar ratio = 1 at 70 °C).
1002
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
Fig. 6. 1H NMR spectrum of cationic PVA (CPVA) prepared under the following conditions: [NaOH] = 5% (mol), [PVA] = 11%, 90 °C, 1 h, GTMAC/PVA molar ratio = 0.5.
Fig. 7. FTIR spectra of original PVA and cationic PVA (CPVA) prepared under the following conditions: [NaOH] = 5% (mol), [PVA] = 11%, 90 °C, 1 h, GTMAC/ PVA molar ratio = 0.5.
ture [28,29]. The peak at 2850–3000 cm 1 is assigned to C–H broad alkyl stretching band, 3600–3650 cm 1 to – OH stretching band, and 3200–3570 cm 1 to hydrogen bonding of the PVA molecule. The peak at 1090– 1150 cm 1 is assigned to C–O band [30]. An increase in the intensity of the absorbance at 1044 cm 1 of CPVA
(C–O–C bond) provides the evidence of the ether linkages between GTMAC and PVA [24]. Additionally, an increase was observed on the intensity of the absorption at 1462 cm 1, which could be assigned to CH2 bending and methyl groups of the GTMAC, which is now a part of the resulting CPVA [24,31].
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
3.3. Adhesion/repulsion force Fig. 8 shows the adhesion force developed between the CPVA-coated AFM-probe and the silicon wafer in water. Interestingly, the adhesion force ( 1.5 mN/m) developed for the CPVA having a charge density of 1.3 meq/g, was higher than that ( 0.2 mN/m) for the CPVA having a charge density of 0.2 meq/g. A similar adhesion force ( 1 mN/m) has been reported between the AFM-probe, coated with cationic starch having a charge density of 0.5 meq/g, and a silicon wafer [32,33]. In contrast, a marginal interaction between the uncoated AFM-probe and the wafer was observed either in water or air. Fig. 9 shows the adhesion/repulsion force developed between the AFM-probes, coated with the same CPVAs, and the silicon wafer in air. The adhesion force for the CPVA
1003
with a charge density of 1.3 meq/g was strong ( 70 mN/ m), while the adhesion force for the CPVA having a charge density of 0.2 meq/g was weak ( 15 mN/m). From the results of Figs. 8 and 9, it can be inferred that the interaction between the CPVAs and oppositely charged surfaces (silicon wafers) is more significant in air than that in water, which may be due to the fact that the water molecules can screen some of the charges of CPVAs in aqueous solutions [34]. In the above AFM force analysis, the approaching phase of the CPVA-coated AFM probe to the silicon wafer resembles the approaching of CPVA segment to a hard/soft surface for adsorption. Therefore, the attraction/repulsion force simulates the interaction of the CPVA with the surfaces [12]. The results show that the CPVA with a high charge density (1.3 meq/g) has a higher affinity for the
Fig. 8. Adhesion/repulsion force developed between the AFM-probes, coated with CPVAs having different charge densities (meq/g), and silicon wafer in retrace mode in water.
Fig. 9. Adhesion/repulsion force developed between the AFM-probes, coated with CPVAs having different charge densities (meq/g), and silicon wafer in retrace mode in air.
1004
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
adsorption compared with the CPVA with a low charge density (0.2 meq/g). 4. Conclusions In this work, the PVA cationization efficiency was determined as functions of a number of process parameters. The results showed that the overall efficiency of the cationization was rather low, which was due to the hydrolysis of GTMAC and CPVA under the strong alkaline environment. The optimum GTMAC/PVA molar ratio was 0.5 in water and 1 in the ethanol/DMSO mixture (1.25 v/v). The maximum charge density was obtained after 1 h and a further extension of the reaction time reduced the charge density of CPVA. The maximum efficiency in water was obtained under the conditions of 95 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and 5% (mol) NaOH concentration; while that in the ethanol/DMSO mixture was obtained under the conditions of 70 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and 5% (mol) NaOH concentration. Additionally, the attraction force developed between a silicon wafer and the AFM-probe coated with CPVA having a charge density of 1.3 meq/g was larger than that developed between the silicon wafer and the AFM-probe coated with CPVA having a charge density of 0.2 meq/g. The attraction force was significantly higher in air than in water under otherwise the same conditions. Acknowledgments The financial support for this project is from an NSERC Strategic Net Work (ForValueNet) and Atlantic Innovation Fund (AIF). References [1] Gaiolas C, Costa AP, Santos Silva M. Influence of cationic starch addition on printing grade paper formation. Cell Chem Technol 2006;40(9–10):783–90. [2] Gaiolas C, Mendes P, Silva M. The role of cationic starch in carbonatefilled papers. Appita 2005;58(4):282–7. [3] Baba Y. New paper strength agent from grafted starch. Japan Tappi J 2001;55(9):11–6. [4] Xiao H, Pelton P, Hamielec A. Novel retention aids for mechanical pulps. Tappi J 1996;79(4):129–35. [5] Sang Y, Xiao H. Preparation and application of cationic cellulose fibers modified by in situ grafting of cationic PVA. Colloid Surf A: Physicochem Eng Aspects 2009;335(1–3):121–7. [6] Moritani T, Yamauchi J. Functional modification of poly (vinyl alcohol) by copolymerization: II. Modification with a sulfonate monomer. Polymer 1998;39(3):553–7. [7] Moritani T, Yamauchi J. Functional modification of poly (vinyl alcohol) by copolymerization: III. Modification with cationic monomers. Polymer 1998;39(3):559–72. [8] Moritani T, Kajitani K. Functional modification of poly (vinyl alcohol) by copolymerization: I. Modification with carboxylic monomers. Polymer 1997;38(12):2933–45. [9] Moritani T, Okaya T. Functional modification of poly (vinyl alcohol) by copolymerization: IV. Self-crosslinkable poly (vinyl alcohol)s. Polymer 1998;39(4):923–31. [10] Fatehi P, Xiao H. Adsorption characteristics of cationic-modified poly (vinyl alcohol) on cellulose fibers – A qualitative analysis. Colloid Surf A: Physicochem Eng Aspects 2008;327(1–3):127–33.
[11] Fatehi P, Xiao H. Effect of cationic PVA characteristics on fiber and paper properties at saturation level of polymer adsorption. Carbohyd polym 2010;79:423–8. [12] Fatehi P, Liu X, Ni Y, Xiao H. Interaction of cationic modified poly vinyl alcohol with high yield pulp. Cellulose 2010;17:1021–31. [13] Fatehi P, McArthur T, Xiao H, Ni Y. Improving the strength of old corrugated carton pulp (OCC) using a dry strength additive. Appita 2010;63(5):364–9. [14] Fatehi P, Ward JE, Ates S, Ni Y, Xiao H. Impact of cationic poly (vinyl alcohol) on the properties of papers made from two different pulps. Appita 2009;62(4):303–7. [15] Kavaliauskaite R, Klimaviciute R, Zemaitaitis A. Factors influencing production of cationic starches. Carbohyd Polym 2008;73:665–75. [16] Bendoraitiene J, Kavaliauskiate R, Klimaviciute R, Zemaitaitis A. Peculiarities of starch cationization with glycidy-methylammonium chloride. Starch 2006;58:623–31. [17] Z. Liu, Y. Ni, P. Fatehi, A. Saeed, Isolation and cationization of hemicelluloses from pre-hydrolysis liquor of the kraft-based dissolving pulp production process, Biomass Bioen. 2010, doi:10.1016/j.biombioe.2011.01.008. [18] Tuting W, Wegemann K, Mischnick P. Enzymatic degradation and electrospray tandem mass spectroscopy as tools for determining the structure of cationic starches prepared by wet and dry methods. Carbohyd Res 2004;339:637–48. [19] Wang Y, Xie W. Synthesis of cationic starch with a high degree of substitution in an ionic liquid. Carbohyd Polym 2010;80:1172–7. [20] Heinze T, Haack V, Rensing S. Starch derivatives of high degree of functionalization. 7. Preparation of cationic 2-hydroxypropyltrimethylammonium chloride starches. Starch 2004;56:288–96. [21] Paakko M, Vappavuori J, Silvennoinen R, Kosonen H, Ankerfors M, Lindstrom T, et al. Long and entangled native cellulose i nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Mat 2008;4:2492–9. [22] Ouajai S, Shanks RA. Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose. Cellulose 2006;13:31–44. [23] Wang J, Feldberg SW, Bard AJ. Measurement of double-layer forces at the polymer film/electrolyte interfaces using atomic force microscopy: concentration and potential-dependent interactions. J Phys Chem B 2002;106:10440–6. [24] Ren JL, Peng F, Sun RC, Liu CF, Cao ZN, Luo W, et al. Synthesis of cationic hemicellulosic derivatives with a low degree of substitution in dimethyl sulfoxide media. J Appl Polym Sci 2008;109:2711–7. [25] Bendoraitiene J, Kavaliauskaite R, Kilmaviciute R, Zemaitaitis A. Peculiarities of starch cationization with glycidyltrimethylammonium chloride. Starch 2006;58(12):623–31. [26] Ren JL, Peng F, Sun RC, Kennedy JF. Influence of hemicellulosic derivatives on the sulfate kraft pulp strength. Carbohy Polym 2009;75:338–42. [27] Xie W, Wang Y. Synthetic pf cationic starch with a high degree of substitution in an ionic liquid. Carbohyd Polym 2010;80:1172–7. [28] Hema M, Selvasekarapanadian S, Arunkumar D, Sakunthala A, Nithya H. FTIR, XRD and ac impedance spectroscopic study on PVA based polymer electrolyte doped with NH4X (X = Cl, Br, I). J Noncrystal Sol 2009;355:84–90. [29] Jia X, Li Y, Cheng Q, Zhang S, Zhang B. Preparation and properties of poly (vinyl alcohol)/silica nanocomposites derived from copolymerization of vinyl silica nanocomparticles and vinyl acetate. Euro Polym J 2007;43:1123–31. [30] Mansur HS, Orefice RL, Mansur AAP. Characterization of poly (vinyl alcohol)/poly (ethylene glycol) hydrogels and PVA-derived hybrids by small-angle X-ray scattering and FTIR spectroscopy. Polymer 2004;45:7193–202. [31] Kacurakova M, Ebringerova A, Hirsch J, Hromadkova Z. Infrared study of arabinoxylans. J Sci Food Agri 1994;66:423–7. [32] Guan Y, Qian L, Xiao H, Zheng A, He B. Synthesis of a novel antimicrobial-modified starch and its adsorption on cellulose fibers: part II – Adsorption behaviors of cationic starch on cellulose fibers. Cellulose 2008;15:619–29. [33] Abraham T, Christendat D, Xu Z, Masliyah J, Gohy JF, Jerome R. Role of polyelectrolyte charge density in tuning colloidal forces. AIChE J 2004;50(10):2613–26. [34] Leite FL, Riul AJR, Herrmann PSP J. Mapping of adhesion forces on soil minerals in air and water by atomic force spectroscopy (AFS). Adhesion Sci Technol 2003;17(16):2141–56.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Preparation and characterization of cationic poly vinyl alcohol with a low degree of substitution Pedram Fatehi a,⇑, Robin Singh a,b, Zainab Ziaee a, Huining Xiao a, Yonghao Ni a a b
Department of Chemical Engineering, Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, India
a r t i c l e
i n f o
Article history: Received 23 December 2010 Received in revised form 16 February 2011 Accepted 27 February 2011 Available online 3 March 2011 Keywords: Cationization Poly vinyl alcohol AFM NMR FTIR Polymer
a b s t r a c t The cationization of polymers has been regarded as an effective method to improve their performance for various applications. In this work, the cationization of poly vinyl alcohol (PVA) was investigated under different conditions, i.e., various glycidyl-trimethylammonium chloride (GTMAC) to PVA ratios, reaction temperatures, times, PVA and NaOH concentrations and solvent compositions. The results showed that the overall efficiency of the cationic modification was rather low, which was due to the hydrolysis of both GTMAC and cationic-modified PVA (CPVA) under the strong alkaline conditions employed. The results also showed that the optimum GTMAC/PVA ratio depended on the solvent composition. The cationization was confirmed by means of 1H NMR and FTIR analyses. The maximum efficiency in water was obtained under the conditions of 95 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration, while that in the ethanol/DMSO mixture (1.25 v/v) was obtained under the conditions of 70 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration. Additionally, the interaction of CPVAs with a silicon wafer (as a substrate) was determined by employing an atomic force microscope (AFM) in water and air. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Cationic polymers have been applied in various industries for different purposes. For example, they have been used as strength agents or retention aids in papermaking [1–4]. Copolymerization and grafting have been regarded as useful methods to induce tailored cationic polymers in the literature [1,5]. Although copolymerization is an efficient method to produce the cationic polymers, simultaneously controlling the molecular weight (MW) and charge density of the resulting cationic polymers may be a challenging task, as these properties are interrelated [5]. The copolymerization of vinyl acetate and diallyldimethyl ammonium chloride (DADMAC) was conducted in our previous work to produce
⇑ Corresponding author. Tel.: +1 506 452 6084; fax: +1 506 453 4767. E-mail address: [email protected] (P. Fatehi). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.02.021
cationic PVA (CPVA) [5]. The highest MW of the resulting CPVA was 80,000–90,000. On the other hand, the cationization of already produced polymers on the basis of grafting is a simpler process since the cationization degree can be controlled without any effect on the molecular weight of polymers. The modification of poly vinyl alcohol (PVA) using various chemical agents was reported in the literature [6–9]. In our previous work, the cationic-modified poly vinyl alcohol (CPVA) showed promising results in improving the pulp and paper strength [10,12–14], and the best results were obtained at employing PVA with the molecular weight of 142,000–186,000 [11]. Additionally, it was found that the optimum charge density of CPVA for various fiber systems may be different, varying between 0.3 and 0.72 meq/g, depending on the types of the fiber systems [10,12,13]. Therefore, it is important to understand the influence of parameters affecting the cationization of PVA with a tailored charge density.
998
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
In the literature, the cationization of starch and hemicelluloses were conducted under various conditions, including wet or semi-dried environments [15–18]. The cationization reaction was also conducted in various solvents, e.g., water, ethanol or dimethyl sulfoxide (DMSO) [19,20]. In the cationization reaction, glycidyl-trimethylammonium chloride (GTMAC) has been commonly used as a cationic reagent [15–20]. An atomic force microscopy (AFM) has been employed as a powerful device to investigate the interactions between various materials [21–23]. This device can be used to determine how the charge of polymers influences their attraction/repulsion force with soft or hard surfaces. In this research, we aimed at understanding the factors influencing the cationization of high MW PVA to induce CPVA with a low charge density (<1 meq/g) under various conditions. The parameters evaluated were: GTMAC/PVA ratio, temperature, time, PVA concentration, solvent composition and NaOH concentration. The cationization of PVA was confirmed qualitatively via FTIR and quantitatively via 1H NMR analyses. Based on these results, the conditions that could induce the CPVA with a desired charge density and the maximum cationization efficiency were determined. Moreover, the interaction of CPVA (having a low charge density of 0.2 meq/g or a high charge density of 1.3 meq/g) with a silicon wafer (as a substrate) was determined via employing an AFM in either water and air.
2. Experimental 2.1. Raw materials Poly vinyl alcohol (PVA) with a molecular weight (MW) of 142,000–186,000 (99% hydrolyzed), glycidyl-trimethylammonium chloride (GTMAC, 75% in water), potassium bromide (KBr, analytical grade), and dimethyl sulfoxide (99.7%, sterile-filtered), were all obtained from Sigma– Aldrich, and used as received. Anionic polyvinyl sulfate (PVSK) with a MW of 100,000–200,000 (97.7% esterified) was provided by Wako Pure Chem. Ltd. Japan. Ethanol (95%) was received from Fisher Scientific.
2.2. CPVA preparation In this research, 1 g of PVA was mixed with GTMAC under various conditions. The reaction parameters investigated were: GTMAC/PVA ratio (0.25–2 mol), PVA concentration (4–33% wt.), solvent type (water, ethanol, ethanol/water or DMSO/ethanol), reaction time (20 min– 240 min) and temperature (60 °C–95 °C) and NaOH concentration (1–15% mol based on PVA). All the reactions were conducted in a 250 ml 3-neck glass flask under a constant stirring. The cationization reaction was stopped via neutralizing the solution using HCl (1 M). Then, the solutions were cooled to room temperature, and unreacted GTMAC was separated by dialyses membranes having a molecular weight cutoff of 1000, while changing water every 2 h for the first 6 h and then once a day for 2 days.
2.3. CPVA characterization To determine the charge density of CPVAs, their solutions (1% wt.) were titrated against a PVSK standard solution (0.5 mM) by using a Particle Charge Detector Mütek PCD 03 (Herrsching, Germany). To quantify the degree of substitution of GTMAC with hydroxyl group on cationicmodified PVA (CPVA), CPVAs were dried at 105 °C overnight, and their 1H NMR spectrum was recorded in D2O at 25 °C using an Oxford 300 MHz spectrometer operating at 300.13 MHz [8,9]. The unmodified PVA and CPVA with the charge density of 1.08 meq/g, which was prepared under the conditions of 11% (wt.) PVA, 0.5 (mol) GTMAC/PVA ratio, 5% (mol) NaOH concentration(based on PVA), 90 °C, 1 h, were characterized by using a Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum 100 FT-IR Spectrometer, USA). The oven-dried PVA or CPVA were embedded in KBr pellets in a mixture of about 1% (wt.) KBr. The spectra were recorded in a transmittance mode in the range 800– 3800 cm 1. 2.4. AFM analysis In the literature, the AFM technique was employed to determine the interaction between CPVAs and a hard surface [21–23]. In fact, the silicon wafer with a 100 nm thermally oxidized layer was used as a model substrate for cellulose fibers [21–23]. In this work, similar silicon wafers were first immersed in 98% H2SO4 for 6–8 h and rinsed with deionised and distilled water prior to the use [21– 23]. To investigate the interaction of CPVA with the silicon wafers, an atomic force microscope (AFM) Nanoscope IIIa, Veeco Instruments Inc., Santa Barbara, CA, was employed. The force analysis was conducted by using silica nitrate probes (NP-S20) with a spring constant of 0.32 N/m, and a resonance frequency of 273 kHz. This silica probes were coated with CPVAs according to the following procedure: at first, the probes were immersed in CPVA solutions (2% wt.) overnight; then, the CPVA-coated probes were washed with deionised and distilled water to remove unadsorbed CPVAs from the probes. The adhesion/repulsion force developed between the CPVA-coated probes and silicon wafers was measured in Picoforce mode with a vendorsupplied fluid cell at the temperature of 25 °C and the pH of 7 in water or in air. Each of 50 different spots was scanned 5 times on an area of 1 1 lm2 at a constant tip velocity of 0.5 lm/s. The force-distance curves were obtained from cantilever deflection and displacement of the piezo. 3. Results and discussion 3.1. Preparation of cationic PVA 3.1.1. Effect of PVA concentration Fig. 1 shows the charge density of the CPVA as a function of PVA concentration. As can be seen, by increasing the PVA concentration from 4% to 33% (wt.), the charge density of resulting CPVA increased from 0.22 meq/g to
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
999
DMSO was mixed with 2.5 ml of ethanol and the reaction was conducted at 65 °C for 1 h (GTMAC/PVA molar ratio of 0.5). The resulting CPVA had a charge density of 0.61 meq/g. Thus, compared with the ethanol and ethanol/water solutions, the ethanol/DMSO solution was more effective in producing CPVA with a high charge density. Others have found that the ethanol/DMSO system was more effective than the ethanol and ethanol/water systems when cationizing starch [19,20]. Therefore, the remaining experiments were conducted under the semi-dried condition in the ethanol/DMSO system (1.25 v/v).
Fig. 1. Effect of PVA concentration on the charge density of CPVA (1 h, 80 °C, GTMAC/PVA molar ratio = 0.5, [NaOH] = 5% (mol)).
1 meq/g. In the literature, by increasing the molar ratio of water/GTMAC from 2.8 to 100 (increasing the water content of the system), the cationization efficiency of starch was reduced from 80% to 27% [16]. Also, it was reported that the cationization efficiency of hemicelluloses using GTMAC was generally low, which was attributed to the hydrolysis of both GTMAC and resulting cationic hemicelluloses under strong alkaline conditions [17,24–27]. It was reported that the hydrolysis of GTMAC under an alkaline condition mainly resulted in forming N-(2,3-dihydroxy)propyl-N,N,N-trimethylammonium chloride, and dimer and trimer of GTMAC [15]. We propose that similar hydrolyses probably occurred in the GTMAC/PVA reaction system, which impaired the cationization efficiency of PVA. The results in Fig. 1 imply that, at a low PVA concentration, the hydrolysis of GTMAC can be more pronounced, which is undesirable and reduces the efficiency of the cationization reaction. To improve the cationization efficiency, the reaction of PVA with GTMAC was conducted under semi-dried conditions. This method has been utilized for the cationization of hemicelluloses in the past [17]. In one set of experiments, 1 g PVA granule (rather than PVA solutions) was mixed with GTMAC with 2.5 ml of ethanol or with ethanol/water mixture (2.5 v/v) at 65 °C for 1 h (GTMAC/PVA molar ratio of 0.5). The charge density of the resulting CPVAs were 0.23, 0.32 meq/g, respectively. As is well known, the high MW PVA does not dissolve in pure ethanol under the conditions specified above. However, it can dissolve in the ethanol/water mixture. Thus, the former system was heterogeneous, while the latter was homogenous. It was reported that the efficiency of homogenous cationization reaction of starch (in water) was higher than the heterogeneous reaction (in ethanol) [20]. The higher solubility of starch in water, which favored the cationic reaction between starch and GTMAC, was the main reason for these results [20]. In another set of experiment, 1 g of PVA granule was dissolved in 2 ml DMSO at 65 °C, and then cooled down to room temperature. Subsequently, the PVA solution in
3.1.2. Effect of GTMAC/PVA molar ratio Fig. 2 shows the cationic modification of PVA as a function of GMTAC/PVA molar ratio in water or DMSO/ethanol. Clearly, the highest charge densities of 0.78 meq/g and 1.2 meq/g were obtained at GTMAC/PVA molar ratio of 0.5 in water and 1 in DMSO/ethanol solutions, respectively. In the cationic modification of starch, the GTMAC/PVA molar ratio of 3 was optimum, which was due to the fact that starch has three hydroxyl groups in its repeating unit, which can react with GTMAC [19]. By increasing the molar ratio of GTMAC to PVA, more GTMAC was available for the cationization reaction. However, its hydrolysis would also occur at the same time. These results showed that, by increasing the molar ratio of GTMAC/PVA more than 0.5 in water or 1 in the ethanol/DMSO mixture, more hydrolysis of GTMAC took place under the alkaline conditions studied, which reduced the efficiency of the cationization [17,24–26]. Furthermore, a higher charge density was obtained in the ethanol/DMSO mixture at 70 °C than in water at 80 °C, under otherwise the same conditions. 3.1.3. Effect of reaction temperature Fig. 3 shows the charge density of the cationic-modified PVA as a function of the reaction temperature in water or in the DMSO/ethanol solution. Interestingly, by increasing the reaction temperature from 60 °C to 95 °C, the charge density of resulting CPVA was significantly increased in water, which implied that a higher temperature favored the cationization of PVA. However, a maximum charge density of 1.2 meq/g was achieved at 70 °C for the cationization in the ethanol/DMSO solution. Further increasing the temperature resulted in a decrease in the charge density of CPVA in the ethanol/DMSO mixture. This behavior is probably due to the evaporation of ethanol from the ethanol/DMSO solution. In the literature, the cationization efficiency of starch was lower in a pure DMSO solution than that in the ethanol/DMSO solution [20]. 3.1.4. Effect of reaction time Fig. 4 shows the charge density of the CPVA as function of the reaction time. Clearly, the maximum charge density of 0.80 meq/g was obtained at 1 h in water. By extending the reaction time, the charge density of CPVA in water was reduced. A similar trend was reported for the cationization of hemicelluloses [17]. However, the maximum charge density of 0.9 meq/g was obtained in the ethanol/ DMSO solution after 1 h, and the further prolonging of the reaction had a marginal effect on the cationization effi-
1000
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
Fig. 2. Effect of GTMAC/PVA molar ratio on the charge density CPVA, [NaOH] = 5% (mol), 1 h, (in water: [PVA] = 11%, 80 °C; in ethanol/DMSO (1.25 (v/v)), 70 °C).
Fig. 3. Effect of reaction temperature on the charge density CPVA at [NaOH] = 5% (mol) for 1 h. (in water: [PVA] = 11%, GTMAC/PVA molar ratio = 0.5; in ethanol/DMSO (1.25 v/v): GTMAC/PVA molar ratio = 1).
Fig. 4. Effect of reaction time on the charge density of CPVA, GTMAC/PVA molar ratio = 0.5, and [NaOH] = 5% (mol) (in water: [PVA] = 11%, 80 °C; in ethanol/ DMSO (1.25 v/v), 70 °C).
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
ciency. It was reported that, by prolonging the reaction time, the hydrolysis of cationic hemicelluloses was enhanced, which reduced the efficiency of the reaction between GTMAC and hemicelluloses [17,23]. A similar phenomenon probably happened here by prolonging the reaction time for GTMAC and PVA system in water. However, the occurrence of side reactions was less in the ethanol/DMSO solution, as shown in Fig. 4. 3.1.5. Effect of NaOH Fig. 5 shows the charge density of CPVA as a function of the NaOH concentration. As can be seen, the charge density was the maximum via adding NaOH up to 5% (mol), regardless of the solution media. A similar result was reported by Wang and Xie on the cationization of corn starch [19]. Additionally, it was reported that NaOH functioned as a catalyst for the cationization of hemicelluloses and the hydrolysis of GTMAC [15,19]. Thus, the concentration of NaOH in solutions should be sufficient to promote the main cationization reaction. However, excessive amount of NaOH would result in more extensive GTMAC and CPVA hydrolyses. The results in Fig. 5 show that the maximum of 5% (mol) was sufficient to obtain the maximum cationization efficiency, regardless of the cationization medium. 3.2. Characterization of CPVA The reaction scheme of GTMAC with PVA was reported in our previous work [10]. Fig. 6 shows the 1H NMR spectrum of CPVA. The peak at 3.2 ppm belongs to protons attached to the quaternary ammonium group of the PVA backbone, which confirmed the cationic modification of PVA [7]. The peak at 4 ppm is assigned to the CH2 of the PVA backbone, while the peaks between 3.2 and 3.8 ppm are due to the CH2 of the attached cationic group on the PVA backbone. Also, the peaks between 1.5–2 ppm belong to CH of the PVA backbone. The degree of substitution can be quantified by considering the areas under the peaks at 3.2 and 4 ppm and the number of protons associated with
1001
each peak, which is 4.9% (mol) for the CPVA studied in Fig. 6. The charge density of this CPVA can be theoretically calculated by considering the degree of substitution via the 1 H NMR analysis and assuming that each quaternary mol of ammonium groups attached to PVA backbone has a charge density of 1 eq. The charge density of the CPVA via this 1H NMR analysis was 0.99 meq/g, which is in a good agreement with that of 1.08 meq/g obtained via the PCD analysis. The efficiency of the cationic modification can be determined based on the degree of substitution analysis of 1H NMR. In this respect, the efficiency is identified by considering the amount of GTMAC added to the system for the cationic reaction and the amount of GTMAC attached to the backbone of resulting CPVA. For the CPVA investigated in Fig. 6, the amount of GTMAC added was 0.5 mol (per mole of PVA) and the amount of GTMAC attached to PVA backbone was 0.049 mol (per mol of PVA), which implied that the efficiency was about 10%. A similar analysis was conducted on the other CPVAs produced, and the maximum efficiency obtained in water was about 12% under the conditions of 95 °C, 1 h, GTMAC/PVA molar ratio of 0.5 (mol), and the NaOH concentration of 5% (mol). The maximum efficiency in the ethanol/DMSO mixture was about 10% under the conditions of ethanol/DMSO (1.25 v/ v), 70 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and NaOH concentration of 5% (mol). The rather low efficiency is due to the occurrence of the side reactions, as discussed earlier. The cationic modification of PVA or hemicelluloses using other cationic agents was also reported to be less than 10% [7,24,27]. The results in Figs. 2 and 3 also shows that the maximum charge density of CPVA was approximately 1.2 meq/g, regardless of the solvent composition; while the reaction efficiency was higher in ethanol/DMSO system than in water system at any GTMAC/PVA molar ratio studied (Fig. 2). Fig. 7 shows the FTIR spectrum of unmodified PVA and the CPVA sample investigated in Fig. 6. A similar spectrum was reported for unmodified PVA in the litera-
Fig. 5. Effect of NaOH concentration (mol) on the charge density of CPVA, 1 h (in water: [PVA] = 11%, GTMAC/PVA molar ratio = 0.5, 80 °C; in ethanol/DMSO (1.25 v/v): GTMAC/PVA molar ratio = 1 at 70 °C).
1002
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
Fig. 6. 1H NMR spectrum of cationic PVA (CPVA) prepared under the following conditions: [NaOH] = 5% (mol), [PVA] = 11%, 90 °C, 1 h, GTMAC/PVA molar ratio = 0.5.
Fig. 7. FTIR spectra of original PVA and cationic PVA (CPVA) prepared under the following conditions: [NaOH] = 5% (mol), [PVA] = 11%, 90 °C, 1 h, GTMAC/ PVA molar ratio = 0.5.
ture [28,29]. The peak at 2850–3000 cm 1 is assigned to C–H broad alkyl stretching band, 3600–3650 cm 1 to – OH stretching band, and 3200–3570 cm 1 to hydrogen bonding of the PVA molecule. The peak at 1090– 1150 cm 1 is assigned to C–O band [30]. An increase in the intensity of the absorbance at 1044 cm 1 of CPVA
(C–O–C bond) provides the evidence of the ether linkages between GTMAC and PVA [24]. Additionally, an increase was observed on the intensity of the absorption at 1462 cm 1, which could be assigned to CH2 bending and methyl groups of the GTMAC, which is now a part of the resulting CPVA [24,31].
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
3.3. Adhesion/repulsion force Fig. 8 shows the adhesion force developed between the CPVA-coated AFM-probe and the silicon wafer in water. Interestingly, the adhesion force ( 1.5 mN/m) developed for the CPVA having a charge density of 1.3 meq/g, was higher than that ( 0.2 mN/m) for the CPVA having a charge density of 0.2 meq/g. A similar adhesion force ( 1 mN/m) has been reported between the AFM-probe, coated with cationic starch having a charge density of 0.5 meq/g, and a silicon wafer [32,33]. In contrast, a marginal interaction between the uncoated AFM-probe and the wafer was observed either in water or air. Fig. 9 shows the adhesion/repulsion force developed between the AFM-probes, coated with the same CPVAs, and the silicon wafer in air. The adhesion force for the CPVA
1003
with a charge density of 1.3 meq/g was strong ( 70 mN/ m), while the adhesion force for the CPVA having a charge density of 0.2 meq/g was weak ( 15 mN/m). From the results of Figs. 8 and 9, it can be inferred that the interaction between the CPVAs and oppositely charged surfaces (silicon wafers) is more significant in air than that in water, which may be due to the fact that the water molecules can screen some of the charges of CPVAs in aqueous solutions [34]. In the above AFM force analysis, the approaching phase of the CPVA-coated AFM probe to the silicon wafer resembles the approaching of CPVA segment to a hard/soft surface for adsorption. Therefore, the attraction/repulsion force simulates the interaction of the CPVA with the surfaces [12]. The results show that the CPVA with a high charge density (1.3 meq/g) has a higher affinity for the
Fig. 8. Adhesion/repulsion force developed between the AFM-probes, coated with CPVAs having different charge densities (meq/g), and silicon wafer in retrace mode in water.
Fig. 9. Adhesion/repulsion force developed between the AFM-probes, coated with CPVAs having different charge densities (meq/g), and silicon wafer in retrace mode in air.
1004
P. Fatehi et al. / European Polymer Journal 47 (2011) 997–1004
adsorption compared with the CPVA with a low charge density (0.2 meq/g). 4. Conclusions In this work, the PVA cationization efficiency was determined as functions of a number of process parameters. The results showed that the overall efficiency of the cationization was rather low, which was due to the hydrolysis of GTMAC and CPVA under the strong alkaline environment. The optimum GTMAC/PVA molar ratio was 0.5 in water and 1 in the ethanol/DMSO mixture (1.25 v/v). The maximum charge density was obtained after 1 h and a further extension of the reaction time reduced the charge density of CPVA. The maximum efficiency in water was obtained under the conditions of 95 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and 5% (mol) NaOH concentration; while that in the ethanol/DMSO mixture was obtained under the conditions of 70 °C, 1 h, GTMAC/PVA molar ratio of 0.5, and 5% (mol) NaOH concentration. Additionally, the attraction force developed between a silicon wafer and the AFM-probe coated with CPVA having a charge density of 1.3 meq/g was larger than that developed between the silicon wafer and the AFM-probe coated with CPVA having a charge density of 0.2 meq/g. The attraction force was significantly higher in air than in water under otherwise the same conditions. Acknowledgments The financial support for this project is from an NSERC Strategic Net Work (ForValueNet) and Atlantic Innovation Fund (AIF). References [1] Gaiolas C, Costa AP, Santos Silva M. Influence of cationic starch addition on printing grade paper formation. Cell Chem Technol 2006;40(9–10):783–90. [2] Gaiolas C, Mendes P, Silva M. The role of cationic starch in carbonatefilled papers. Appita 2005;58(4):282–7. [3] Baba Y. New paper strength agent from grafted starch. Japan Tappi J 2001;55(9):11–6. [4] Xiao H, Pelton P, Hamielec A. Novel retention aids for mechanical pulps. Tappi J 1996;79(4):129–35. [5] Sang Y, Xiao H. Preparation and application of cationic cellulose fibers modified by in situ grafting of cationic PVA. Colloid Surf A: Physicochem Eng Aspects 2009;335(1–3):121–7. [6] Moritani T, Yamauchi J. Functional modification of poly (vinyl alcohol) by copolymerization: II. Modification with a sulfonate monomer. Polymer 1998;39(3):553–7. [7] Moritani T, Yamauchi J. Functional modification of poly (vinyl alcohol) by copolymerization: III. Modification with cationic monomers. Polymer 1998;39(3):559–72. [8] Moritani T, Kajitani K. Functional modification of poly (vinyl alcohol) by copolymerization: I. Modification with carboxylic monomers. Polymer 1997;38(12):2933–45. [9] Moritani T, Okaya T. Functional modification of poly (vinyl alcohol) by copolymerization: IV. Self-crosslinkable poly (vinyl alcohol)s. Polymer 1998;39(4):923–31. [10] Fatehi P, Xiao H. Adsorption characteristics of cationic-modified poly (vinyl alcohol) on cellulose fibers – A qualitative analysis. Colloid Surf A: Physicochem Eng Aspects 2008;327(1–3):127–33.
[11] Fatehi P, Xiao H. Effect of cationic PVA characteristics on fiber and paper properties at saturation level of polymer adsorption. Carbohyd polym 2010;79:423–8. [12] Fatehi P, Liu X, Ni Y, Xiao H. Interaction of cationic modified poly vinyl alcohol with high yield pulp. Cellulose 2010;17:1021–31. [13] Fatehi P, McArthur T, Xiao H, Ni Y. Improving the strength of old corrugated carton pulp (OCC) using a dry strength additive. Appita 2010;63(5):364–9. [14] Fatehi P, Ward JE, Ates S, Ni Y, Xiao H. Impact of cationic poly (vinyl alcohol) on the properties of papers made from two different pulps. Appita 2009;62(4):303–7. [15] Kavaliauskaite R, Klimaviciute R, Zemaitaitis A. Factors influencing production of cationic starches. Carbohyd Polym 2008;73:665–75. [16] Bendoraitiene J, Kavaliauskiate R, Klimaviciute R, Zemaitaitis A. Peculiarities of starch cationization with glycidy-methylammonium chloride. Starch 2006;58:623–31. [17] Z. Liu, Y. Ni, P. Fatehi, A. Saeed, Isolation and cationization of hemicelluloses from pre-hydrolysis liquor of the kraft-based dissolving pulp production process, Biomass Bioen. 2010, doi:10.1016/j.biombioe.2011.01.008. [18] Tuting W, Wegemann K, Mischnick P. Enzymatic degradation and electrospray tandem mass spectroscopy as tools for determining the structure of cationic starches prepared by wet and dry methods. Carbohyd Res 2004;339:637–48. [19] Wang Y, Xie W. Synthesis of cationic starch with a high degree of substitution in an ionic liquid. Carbohyd Polym 2010;80:1172–7. [20] Heinze T, Haack V, Rensing S. Starch derivatives of high degree of functionalization. 7. Preparation of cationic 2-hydroxypropyltrimethylammonium chloride starches. Starch 2004;56:288–96. [21] Paakko M, Vappavuori J, Silvennoinen R, Kosonen H, Ankerfors M, Lindstrom T, et al. Long and entangled native cellulose i nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Mat 2008;4:2492–9. [22] Ouajai S, Shanks RA. Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose. Cellulose 2006;13:31–44. [23] Wang J, Feldberg SW, Bard AJ. Measurement of double-layer forces at the polymer film/electrolyte interfaces using atomic force microscopy: concentration and potential-dependent interactions. J Phys Chem B 2002;106:10440–6. [24] Ren JL, Peng F, Sun RC, Liu CF, Cao ZN, Luo W, et al. Synthesis of cationic hemicellulosic derivatives with a low degree of substitution in dimethyl sulfoxide media. J Appl Polym Sci 2008;109:2711–7. [25] Bendoraitiene J, Kavaliauskaite R, Kilmaviciute R, Zemaitaitis A. Peculiarities of starch cationization with glycidyltrimethylammonium chloride. Starch 2006;58(12):623–31. [26] Ren JL, Peng F, Sun RC, Kennedy JF. Influence of hemicellulosic derivatives on the sulfate kraft pulp strength. Carbohy Polym 2009;75:338–42. [27] Xie W, Wang Y. Synthetic pf cationic starch with a high degree of substitution in an ionic liquid. Carbohyd Polym 2010;80:1172–7. [28] Hema M, Selvasekarapanadian S, Arunkumar D, Sakunthala A, Nithya H. FTIR, XRD and ac impedance spectroscopic study on PVA based polymer electrolyte doped with NH4X (X = Cl, Br, I). J Noncrystal Sol 2009;355:84–90. [29] Jia X, Li Y, Cheng Q, Zhang S, Zhang B. Preparation and properties of poly (vinyl alcohol)/silica nanocomposites derived from copolymerization of vinyl silica nanocomparticles and vinyl acetate. Euro Polym J 2007;43:1123–31. [30] Mansur HS, Orefice RL, Mansur AAP. Characterization of poly (vinyl alcohol)/poly (ethylene glycol) hydrogels and PVA-derived hybrids by small-angle X-ray scattering and FTIR spectroscopy. Polymer 2004;45:7193–202. [31] Kacurakova M, Ebringerova A, Hirsch J, Hromadkova Z. Infrared study of arabinoxylans. J Sci Food Agri 1994;66:423–7. [32] Guan Y, Qian L, Xiao H, Zheng A, He B. Synthesis of a novel antimicrobial-modified starch and its adsorption on cellulose fibers: part II – Adsorption behaviors of cationic starch on cellulose fibers. Cellulose 2008;15:619–29. [33] Abraham T, Christendat D, Xu Z, Masliyah J, Gohy JF, Jerome R. Role of polyelectrolyte charge density in tuning colloidal forces. AIChE J 2004;50(10):2613–26. [34] Leite FL, Riul AJR, Herrmann PSP J. Mapping of adhesion forces on soil minerals in air and water by atomic force spectroscopy (AFS). Adhesion Sci Technol 2003;17(16):2141–56.