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Temperature-responsive character of acrylic acid and N-isopropylacrylamide binary monomers-grafted celluloses
European Polymer Journal 37 (2001) 807±813
Temperature-responsive character of acrylic acid and N-isopropylacrylamide binary monomers-grafted celluloses On Huan Wen, Shin-ichi Kuroda, Hitoshi Kubota * Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 30 September 1999; received in revised form 4 April 2000; accepted 21 August 2000
Abstract Grafted celluloses with grafted chains consisting of two types of monomer components, acrylic acid (AA) and Nisopropylacrylamide (NIPAAm), were prepared by two photografting methods using periodic acid oxidized cellulose. Dissolving pulp from softwoods was used as the cellulose sample. In the ®rst method (one-step procedure), AA and NIPAAm monomer mixtures were photografted at 50°C on the oxidized sample. With the second method (two-step procedure), AA was ®rst photografted on the oxidized sample and then NIPAAm was further photografted on the AAgrafted sample. The resulting grafted celluloses (one- and two-step samples) prepared by the one- and two-step procedures, respectively, were characterized by the monomer sequence distribution of the grafted chains; that is, the grafted chains of the one-step sample are composed of a random copolymer of AA and NIPAAm, while the monomer sequence of the grafted chains of the two-step sample is of block type with respect to each monomer component. The both grafted celluloses exhibited a temperature-responsive character, in which it swelled and shrank in water at 5°C and 50°C, respectively. The extent of the character was larger for the one-step sample compared to the two-step sample. The temperature-responsive character was considerably improved by treating the grafted celluloses with N ,N 0 -methylenebisacrylamide, by which a crosslinked structure was introduced into the grafted samples. Moreover, the grafted celluloses were found to have an ability to adsorb cupric ion, the extent of which depended on the preparation methods, such as the one- and two-step procedures. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photografting; N-Isopropylacrylamide; Acrylic acid; Binary monomers-grafted celluloses; Temperature-responsive character
1. Introduction It was found in our previous papers [1±6] that photografting is a useful means for introduction of various functions into cellulose substrate, such as catalytic activity [1], chelating function [2,3], highly waterabsorbing character [4,5] and temperature responsivecharacter [6]. This paper deals with preparation of
*
Corresponding author. Tel.: +81-277-30-1370; fax: +81277-30-1371. E-mail address: [email protected] (H. Kubota).
grafted celluloses having two dierent functions by means of photografting and its characteristics. The method to synthesize the grafted celluloses contains two kinds of photografting as shown schematically in Scheme 1. One is the photografting using a mixture of two monomer components M1 and M2 (one-step procedure), and the other is a two-step procedure, in which the monomer component M1 is ®rst photografted and then the component M2 is photografted on the M1 grafted cellulose. In the one-step procedure, the two monomer components are copolymerized to give a copolymer, where the monomer sequence depends on a monomer reactivity ratio of the component monomers. The two-step procedure could form a block-like copolymer of these monomer units. It is expected that
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 7 3 - 7
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O.H. Wen et al. / European Polymer Journal 37 (2001) 807±813
2. Experimental 2.1. Materials
Scheme 1. Photografting procedures.
Commercial dissolving pulp from softwoods was milled and classi®ed to remove ®bers less than 40 mesh and then used as the cellulose sample. The cellulose sample was treated with 20 mmol/l aqueous periodic acid solution (ratio of liquor to solids 100:1) at 45°C for 60 min to prepare the oxidized sample. It is wellknown that periodic acid oxidation leads to introduction of aldehyde groups into the cellulose sample [12,13]. The oxidation of cellulose with periodic acid [14] was also proven to result in a high activity to initiate photografting of various vinyl monomers. The carbonyl groups on the oxidized sample was determined to be 68.3 mmol per 100 g of cellulose sample according to the hydroxylamine method [15]. AA and NIPAAm were puri®ed by distillation under reduced pressure and recrystallization from a benzene/n-hexane mixture solvent, respectively. N ,N 0 -methylenebisacrylamide (MBAAm) was reagent grade and used without further puri®cation. 2.2. Photografting
distribution of the monomer sequences in the grafted chains is closely related with functions of the grafted celluloses. We selected acrylic acid (AA) and N-isopropylacrylamide (NIPAAm) as the two monomer components to be introduced. The AA-grafted chains act as reaction sites to introduce various functions through the carboxyl groups. It is well-known, on the other hand, that hydrogels [7±10] formed by poly(NIPAAm) swell and shrink in water below and above a lower critical solution temperature (LCST 32°C) of the linear polymer. Moreover, a copolymer [8,11] of AA and NIPAAm has been observed to undergo a discontinuous phase transition in response to a change in temperature. Accordingly, AA and NIPAAm binary monomersgrafted cellulose is expected to exhibit a temperatureresponsive character, in which it swells and shrinks in water below and above the LCST of the copolymer, depending on the composition of AA and NIPAAm. Based on the temperature-responsive character of the grafted celluloses, it is supposed that the functions introduced through the carboxyl groups of the AA component in the grafted chains may be in¯uenced in response to a change in temperature. In the present study, AA and NIPAAm binary monomers-grafted celluloses were synthesized by the two procedures described above, and temperatureresponsive character of the resulting grafted celluloses was compared. In order to understand reactivity of the grafted celluloses, moreover, adsorption of cupric ion with the grafted celluloses was also examined.
In the one-step procedure, photografting was carried out in a Pyrex glass tube containing 0.20 g oxidized sample and 20 ml water, in which given molar ratios of AA and NIPAAm binary monomers (total monomer concentration 0:44 mol/l) were dissolved, at 50°C under a nitrogen atmosphere. Irradiation with a highpressure mercury lamp (400 W) was carried out at 50°C using a Riko rotary photochemical reactor (RH40010W), around which the Pyrex glass tubes were rotated. The polymerized samples were extracted for 3 days with water to remove homopolymers. The percentage of grafting was taken as the percentage of weight increase of the original sample. The grafted samples thus obtained were denoted as a one-step sample. The NIPAAm content of the grafted chains was determined by nitrogen analysis. With the two-step procedure, AA (0.73 mol/l) was ®rst photografted on the oxidized sample according to the one-step procedure described above. The resulting AA-grafted sample was subjected to the following peroxidation; that is, the AA-grafted sample (0.20 g), 10 ml aqueous hydrogen peroxide solution (30%) and 2 ml methanesulfonic acid were placed in a ¯ask adjusted at 25°C for 30 min. After the reaction, the sample was washed with ice water. The peroxide content of the sample was determined iodometrically [16]. Finally, second-step photografting was performed in a Pyrex glass tube containing 0.20 g peroxidized AA-grafted sample and 20 ml water, in which a given quantity of NIPAAm (0.44 mol/l) was dissolved, at 50°C under a nitrogen atmosphere. The polymerized samples were
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extracted with water for 3 days to remove homopolymers. The percentage of grafting of NIPAAm was taken as the percentage of weight increase of the peroxidized AA-grafted sample. The grafted samples were denoted as a two-step sample. 2.3. Crosslinking treatment Crosslinking treatment was carried out in a Pyrex glass tube containing 0.30 g grafted sample and 30 ml aqueous hydrogen peroxide solution (5 mmol), in which known quantities of MBAAm were dissolved, under a nitrogen atmosphere. Irradiation was performed at 30°C for 30 min using the same reactor as that described in Section 2.2. 2.4. Measurement of swelling The grafted sample (Wo g), which was put into a teabag of nonwoven fabric, was immersed in water at 5°C and 50°C for 24 h, alternately. The treated bag was allowed to hang on a holder for 10 min to separate the swollen sample (Ws g) from the unabsorbed water. Degree of swelling was de®ned as follows: Swelling % 100 Ws ÿ Wo =Wo : 2.5. Adsorption of cupric ion The grafted sample (0.20 g) was added to 40 ml aqueous CuCl2 2H2 O (Cu2 ) solution (0.02 M), whose pH was adjusted by Clark±Lubs buer solution, and then adsorption reaction was carried out at 5°C or 50°C for 24 h. After the reaction, the reaction mixture was ®ltered o, and the concentration of Cu2 in the ®ltrate was determined by chelate titration [17] using EDTA standard solution and (2-pyridylazo)-2-naphthol indicator in order to calculate the amount of adsorbed Cu2 .
Fig. 1. Photografting of AA and NIPAAm binary monomers on cellulose oxidized by periodic acid. Irradiation, 50°C, 30 min, total monomer concentration 0:44 mol/l.
chains, which is also shown in Fig. 1, increased with the NIPAAm composition in the binary monomers. Monomer reactivity ratios of NIPAAm (r1 ) and AA (r2 ) were calculated for reference from data of Fig. 1 using the Fineman±Ross method. The values were 0:47 0:13 and 0:89 0:43, respectively, which were similar to AA (r1 ), 0:36±1:73, and acrylamide (r2 ), 0:03±1:38, depending on copolymerization conditions [18]. This suggests that grafted chains of the grafted cellulose prepared by the one-step procedure are composed of a random copolymer of AA and NIPAAm.
3. Results and discussion 3.1. Photografting Fig. 1 shows the results of photografting by the onestep procedure using binary monomers of AA and NIPAAm. The percentage of grafting increased with an increase in the NIPAAm composition in the binary monomers. It was found thus that the periodic acid oxidized sample has an activity to initiate the photografting of AA and NIPAAm binary monomers. The activity of the oxidized sample is conceivable to be originated in the oxidized states [14] such as aldehyde groups attached to cellulose substrate by periodic acid oxidation. The NIPAAm composition in the grafted
Fig. 2. Photografting of NIPAAm on AA-grafted cellulose. Irradiation temperature 50°C, NIPAAm 0:44 mol/l, percentage of grafting of AA 116:3%, peroxide 38:3 mmol/ 100 g. AA-grafted sample.
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Fig. 2 shows the photografting of NIPAAm on AAgrafted cellulose. With the two-step procedure, AAgrafted cellulose was ®rst peroxidized with hydrogen peroxide in the presence of acid to yield peroxides of peracid type on the AA-grafted chains (Eq. (1)). Then, the resulting polymer peracids were subjected to the second-step photografting of NIPAAm. It was observed that the second-step photografting of NIPAAm is successively performed by using the polymer peracids and the percentage of grafting of NIPAAm increases with the irradiation time. The polymer peracids [16,19,20] derived from AA-grafted cellulose and polyethylene are known to have an ability to initiate grafting of vinyl monomers. It is supposed that the polymer peracids are
used thermosensitivity [21], which was de®ned as the ratio of degrees of swelling of the grafted cellulose at 5°C and 50°C, and the results are shown in Fig. 4. The thermosensitivity increased with increasing the percentage of grafting, and the magnitude of the increase was larger for the one-step sample compared to the two-step sample. Moreover, the value was higher for the grafted cellulose with a higher NIPAAm content in the grafted chains. It is conceivable that the dierent thermosensitivity between the both grafted celluloses is ascribed to the monomer sequence distribution of grafted chains. As shown in Scheme 1, the monomer sequence distribution of the grafted chains in the two-step sample is of block type with respect to each monomer component. There-
1
2
3
photodecomposed according to Eqs. (2) and (3) and the resultant radicals on the AA-grafted chains initiate the grafting of NIPAAm monomer. Accordingly, the monomer sequence distribution of the grafted chains of the two-step sample is of block type with respect to each monomer component. 3.2. Temperature-responsive character Fig. 3 presents the degree of swelling of AA/NIPAAm-grafted cellulose prepared by the one-step procedure when immersed in water at 5°C and 50°C for 24 h, alternately. The grafted cellulose exhibited a reversible change in the degree of swelling between 5°C and 50°C. It was found thus that the grafted cellulose shows the temperature-responsive character, in which it swells and shrinks in water at 5°C and 50°C, respectively. In order to compare the extent of the temperature-responsive character between the one- and two-step samples, we
fore, only poly(NIPAAm) component of the grafted chains shrinks when immersed in water at a temperature higher than the LCST of poly(NIPAAm). This may cause a small contribution of the poly(NIPAAm) component to the shrinkage of the whole grafted chains including the poly(AA) component. With the one-step sample, on the other hand, the poly(NIPAAm) component is expected to participate directly in the shrinkage of grafted chains because the grafted chains are composed of a random copolymer of AA and NIPAAm monomers. It was observed in a previous paper [22] that the temperature-responsive character of NIPAAm-grafted cellulose is improved by treating it with MBAAm crosslinker. So, the crosslinking treatment was applied to AA/NIPAAm-grafted celluloses, and the results are shown in Fig. 5. The thermosensitivity increased with an increase in the MBAAm concentration, which was commonly observed for the one- and two-step samples.
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Fig. 5. Relationship between thermosensitivity and MBAAm concentration. Grafted samples (percentage of grafting 350± 370%, AA=NIPAAm 5=5) were treated with aqueous hydrogen peroxide solution (5 mM) at 30°C for 30 min under photoirradiation. ( ) one-step sample and (s) two-step sample. Fig. 3. Temperature-responsive character of AA/NIPAAmgrafted cellulose prepared by one-step procedure. Total percentage of grafting 200:7%.
that crosslinking treatment of AA/NIPAAm-grafted celluloses with MBAAm crosslinker is a useful means for improving the temperature-responsive character of the grafted samples. 3.3. Adsorption of Cu2
Fig. 4. Relationship between thermosensitivity and percentage of grafting. One-step sample is (h) AA=NIPAAm 5=5 and ( ) AA=NIPAAm 3=7 and two-step sample is (s) AA= NIPAAm 5=5 and (d) AA=NIPAAm 3=7.
It is conceivable that swelling and shrinking nature [22] of grafted cellulose is facilitated with the introduction of a crosslinked structure into the grafted chains, resulting in the increased thermosensitivity. It was con®rmed thus
Adsorption reaction of Cu2 with AA/NIPAAmgrafted celluloses was examined to understand its reactivity, and the results are shown in Fig. 6. The amount of adsorbed Cu2 in AA-grafted cellulose, which is also included in the ®gure, increased with an increase in the percentage of grafting, showing that carboxyl groups on the AA-grafted chains act as a reaction site for the chemical adsorption of Cu2 . The amount of adsorbed Cu2 in the one-step sample also increased with the percentage of grafting of AA component in the grafted chains. However, the amount was considerably lower compared to the AA-grafted cellulose, suggesting a steric hindrance due to a bulky side group of the NIPAAm component in the grafted chains. Fig. 7 shows the comparative examination in adsorption reaction of Cu2 with the one- and two-step samples. The amount of adsorbed Cu2 increased with increasing pH of the system, and the magnitude of the increase was smaller for the two-step sample compared to the one-step sample. It is inferred that poly(AA) component in the two-step sample is partly covered with poly(NIPAAm) component since poly(NIPAAm) is introduced into the AA-grafted cellulose by second-step
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the adsorption reaction of Cu2 because of the shrinkage of the grafted chains at 50°C. This leads to the lower amount of adsorbed Cu2 at 50°C. However, the difference in the amount of adsorbed Cu2 between the systems at 5°C and 50°C was not observed for the twostep sample since the temperature-responsive character of the two-step sample was smaller than the one-step sample as shown in Fig. 4.
4. Conclusions
Fig. 6. Adsorption of Cu2 with grafted cellulose. Cu2 0:02 M, pH 5:0, temperature 5°C, time 24 h, (d) AAgrafted cellulose and (h) one-step sample AA=NIPAAm 3=7.
It is concluded that grafted celluloses having grafted chains, which consist of two types of monomer component such as AA and NIPAAm, could be prepared by two photografting procedures using periodic acid oxidized cellulose sample; that is, one is the photograting of AA and NIPAAm binary monomers on the oxidized sample, and the other is the photografting of NIPAAm on the AA-grafted sample. The resulting grafted celluloses were characterized by the monomer sequence distribution of the grafted chains depending on the procedures, resulting in a dierent extent of properties, such as the temperature-responsive character and the reactivity toward adsorption of Cu2 , between the grafted celluloses prepared by the two procedures.
References
Fig. 7. Relationship between amount of adsorbed Cu2 and pH in AA/NIPAAm (3/7)-grafted celluloses (percentage of grafting 365±375%). Cu2 0:02 M, time 24 h, one-step sample is ( ) 5°C and (h) 50°C and two-step sample is (d) 5°C and (s) 50°C.
photografting, resulting in the lower amount of adsorbed Cu2 . It was observed, moreover, that the amount of adsorbed Cu2 in the one-step sample is lower for the system at 50°C than that at 5°C. It is conceivable that some parts of carboxyl groups on AA component in the grafted chains cannot participate in
[1] Kubota H, Ujita S. J Appl Polym Sci 1995;56:25. [2] Kubota H, Shigehisa Y. J Appl Polym Sci 1995;56: 147. [3] Kubota H, Suzuki S. Eur Polym J 1995;31:701. [4] Kuwabara S, Kubota H. J Appl Polym Sci 1996;60: 1965. [5] Kubota H, Kuwabara S. J Appl Polym Sci 1997;64: 2259. [6] Kubota H, Shiobara N. React Funct Polym 1998;37: 219. [7] Heskins M, Guillet JE. J Macromol Sci Chem 1968; A2:1441. [8] Hirotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987; 89:1392. [9] Hirokawa Y, Tanaka T. J Chem Phys 1984;81:6379. [10] Otake K, Inomata M, Konno M, Saito S. Macromolecules 1990;23:283. [11] Chen G, Homan AS. Nature 1995;373:49. [12] Ott E, Sparlin HM, Grain MW, editors. Cellulose and cellulose derivatives, part I. New York: Interscience, 1954. p. 148. [13] Rydholm SA. Pulping processes. New York: Wiley, 1965. p. 131. [14] Ogiwara Y, Hayase K, Kubota H. J Appl Polym Sci 1979;23:1. [15] Strole U. Makromol Chem 1956;20:19. [16] Kubota H, Ogiwara Y. J Appl Polym Sci 1979;22:241.
O.H. Wen et al. / European Polymer Journal 37 (2001) 807±813 [17] Ueno K. Chelatometry. Tokyo: Nankodo, 1989. p. 143. [18] Brandrup J, Immergut EH, editors. Polymer handbook. New York: Wiley-Interscience, 1975. p. II-115. [19] Hsiue GH, Perng TL, Yanf JM. J Appl Polym Sci 1991;42:1899.
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[20] Kubota H. React Polym 1992;17:263. [21] Nagaoka N, Safrang A, Yoshida M, Omichi H, Kubota H, Katakai R. Macromolecules 1993;26:7386. [22] Nakajima T, Kubota H, Katakai R, Yoshida M. J Macromol Sci-A, Pure Appl Chem 2000;A37:205.
Temperature-responsive character of acrylic acid and N-isopropylacrylamide binary monomers-grafted celluloses On Huan Wen, Shin-ichi Kuroda, Hitoshi Kubota * Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan Received 30 September 1999; received in revised form 4 April 2000; accepted 21 August 2000
Abstract Grafted celluloses with grafted chains consisting of two types of monomer components, acrylic acid (AA) and Nisopropylacrylamide (NIPAAm), were prepared by two photografting methods using periodic acid oxidized cellulose. Dissolving pulp from softwoods was used as the cellulose sample. In the ®rst method (one-step procedure), AA and NIPAAm monomer mixtures were photografted at 50°C on the oxidized sample. With the second method (two-step procedure), AA was ®rst photografted on the oxidized sample and then NIPAAm was further photografted on the AAgrafted sample. The resulting grafted celluloses (one- and two-step samples) prepared by the one- and two-step procedures, respectively, were characterized by the monomer sequence distribution of the grafted chains; that is, the grafted chains of the one-step sample are composed of a random copolymer of AA and NIPAAm, while the monomer sequence of the grafted chains of the two-step sample is of block type with respect to each monomer component. The both grafted celluloses exhibited a temperature-responsive character, in which it swelled and shrank in water at 5°C and 50°C, respectively. The extent of the character was larger for the one-step sample compared to the two-step sample. The temperature-responsive character was considerably improved by treating the grafted celluloses with N ,N 0 -methylenebisacrylamide, by which a crosslinked structure was introduced into the grafted samples. Moreover, the grafted celluloses were found to have an ability to adsorb cupric ion, the extent of which depended on the preparation methods, such as the one- and two-step procedures. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photografting; N-Isopropylacrylamide; Acrylic acid; Binary monomers-grafted celluloses; Temperature-responsive character
1. Introduction It was found in our previous papers [1±6] that photografting is a useful means for introduction of various functions into cellulose substrate, such as catalytic activity [1], chelating function [2,3], highly waterabsorbing character [4,5] and temperature responsivecharacter [6]. This paper deals with preparation of
*
Corresponding author. Tel.: +81-277-30-1370; fax: +81277-30-1371. E-mail address: [email protected] (H. Kubota).
grafted celluloses having two dierent functions by means of photografting and its characteristics. The method to synthesize the grafted celluloses contains two kinds of photografting as shown schematically in Scheme 1. One is the photografting using a mixture of two monomer components M1 and M2 (one-step procedure), and the other is a two-step procedure, in which the monomer component M1 is ®rst photografted and then the component M2 is photografted on the M1 grafted cellulose. In the one-step procedure, the two monomer components are copolymerized to give a copolymer, where the monomer sequence depends on a monomer reactivity ratio of the component monomers. The two-step procedure could form a block-like copolymer of these monomer units. It is expected that
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 7 3 - 7
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O.H. Wen et al. / European Polymer Journal 37 (2001) 807±813
2. Experimental 2.1. Materials
Scheme 1. Photografting procedures.
Commercial dissolving pulp from softwoods was milled and classi®ed to remove ®bers less than 40 mesh and then used as the cellulose sample. The cellulose sample was treated with 20 mmol/l aqueous periodic acid solution (ratio of liquor to solids 100:1) at 45°C for 60 min to prepare the oxidized sample. It is wellknown that periodic acid oxidation leads to introduction of aldehyde groups into the cellulose sample [12,13]. The oxidation of cellulose with periodic acid [14] was also proven to result in a high activity to initiate photografting of various vinyl monomers. The carbonyl groups on the oxidized sample was determined to be 68.3 mmol per 100 g of cellulose sample according to the hydroxylamine method [15]. AA and NIPAAm were puri®ed by distillation under reduced pressure and recrystallization from a benzene/n-hexane mixture solvent, respectively. N ,N 0 -methylenebisacrylamide (MBAAm) was reagent grade and used without further puri®cation. 2.2. Photografting
distribution of the monomer sequences in the grafted chains is closely related with functions of the grafted celluloses. We selected acrylic acid (AA) and N-isopropylacrylamide (NIPAAm) as the two monomer components to be introduced. The AA-grafted chains act as reaction sites to introduce various functions through the carboxyl groups. It is well-known, on the other hand, that hydrogels [7±10] formed by poly(NIPAAm) swell and shrink in water below and above a lower critical solution temperature (LCST 32°C) of the linear polymer. Moreover, a copolymer [8,11] of AA and NIPAAm has been observed to undergo a discontinuous phase transition in response to a change in temperature. Accordingly, AA and NIPAAm binary monomersgrafted cellulose is expected to exhibit a temperatureresponsive character, in which it swells and shrinks in water below and above the LCST of the copolymer, depending on the composition of AA and NIPAAm. Based on the temperature-responsive character of the grafted celluloses, it is supposed that the functions introduced through the carboxyl groups of the AA component in the grafted chains may be in¯uenced in response to a change in temperature. In the present study, AA and NIPAAm binary monomers-grafted celluloses were synthesized by the two procedures described above, and temperatureresponsive character of the resulting grafted celluloses was compared. In order to understand reactivity of the grafted celluloses, moreover, adsorption of cupric ion with the grafted celluloses was also examined.
In the one-step procedure, photografting was carried out in a Pyrex glass tube containing 0.20 g oxidized sample and 20 ml water, in which given molar ratios of AA and NIPAAm binary monomers (total monomer concentration 0:44 mol/l) were dissolved, at 50°C under a nitrogen atmosphere. Irradiation with a highpressure mercury lamp (400 W) was carried out at 50°C using a Riko rotary photochemical reactor (RH40010W), around which the Pyrex glass tubes were rotated. The polymerized samples were extracted for 3 days with water to remove homopolymers. The percentage of grafting was taken as the percentage of weight increase of the original sample. The grafted samples thus obtained were denoted as a one-step sample. The NIPAAm content of the grafted chains was determined by nitrogen analysis. With the two-step procedure, AA (0.73 mol/l) was ®rst photografted on the oxidized sample according to the one-step procedure described above. The resulting AA-grafted sample was subjected to the following peroxidation; that is, the AA-grafted sample (0.20 g), 10 ml aqueous hydrogen peroxide solution (30%) and 2 ml methanesulfonic acid were placed in a ¯ask adjusted at 25°C for 30 min. After the reaction, the sample was washed with ice water. The peroxide content of the sample was determined iodometrically [16]. Finally, second-step photografting was performed in a Pyrex glass tube containing 0.20 g peroxidized AA-grafted sample and 20 ml water, in which a given quantity of NIPAAm (0.44 mol/l) was dissolved, at 50°C under a nitrogen atmosphere. The polymerized samples were
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extracted with water for 3 days to remove homopolymers. The percentage of grafting of NIPAAm was taken as the percentage of weight increase of the peroxidized AA-grafted sample. The grafted samples were denoted as a two-step sample. 2.3. Crosslinking treatment Crosslinking treatment was carried out in a Pyrex glass tube containing 0.30 g grafted sample and 30 ml aqueous hydrogen peroxide solution (5 mmol), in which known quantities of MBAAm were dissolved, under a nitrogen atmosphere. Irradiation was performed at 30°C for 30 min using the same reactor as that described in Section 2.2. 2.4. Measurement of swelling The grafted sample (Wo g), which was put into a teabag of nonwoven fabric, was immersed in water at 5°C and 50°C for 24 h, alternately. The treated bag was allowed to hang on a holder for 10 min to separate the swollen sample (Ws g) from the unabsorbed water. Degree of swelling was de®ned as follows: Swelling % 100 Ws ÿ Wo =Wo : 2.5. Adsorption of cupric ion The grafted sample (0.20 g) was added to 40 ml aqueous CuCl2 2H2 O (Cu2 ) solution (0.02 M), whose pH was adjusted by Clark±Lubs buer solution, and then adsorption reaction was carried out at 5°C or 50°C for 24 h. After the reaction, the reaction mixture was ®ltered o, and the concentration of Cu2 in the ®ltrate was determined by chelate titration [17] using EDTA standard solution and (2-pyridylazo)-2-naphthol indicator in order to calculate the amount of adsorbed Cu2 .
Fig. 1. Photografting of AA and NIPAAm binary monomers on cellulose oxidized by periodic acid. Irradiation, 50°C, 30 min, total monomer concentration 0:44 mol/l.
chains, which is also shown in Fig. 1, increased with the NIPAAm composition in the binary monomers. Monomer reactivity ratios of NIPAAm (r1 ) and AA (r2 ) were calculated for reference from data of Fig. 1 using the Fineman±Ross method. The values were 0:47 0:13 and 0:89 0:43, respectively, which were similar to AA (r1 ), 0:36±1:73, and acrylamide (r2 ), 0:03±1:38, depending on copolymerization conditions [18]. This suggests that grafted chains of the grafted cellulose prepared by the one-step procedure are composed of a random copolymer of AA and NIPAAm.
3. Results and discussion 3.1. Photografting Fig. 1 shows the results of photografting by the onestep procedure using binary monomers of AA and NIPAAm. The percentage of grafting increased with an increase in the NIPAAm composition in the binary monomers. It was found thus that the periodic acid oxidized sample has an activity to initiate the photografting of AA and NIPAAm binary monomers. The activity of the oxidized sample is conceivable to be originated in the oxidized states [14] such as aldehyde groups attached to cellulose substrate by periodic acid oxidation. The NIPAAm composition in the grafted
Fig. 2. Photografting of NIPAAm on AA-grafted cellulose. Irradiation temperature 50°C, NIPAAm 0:44 mol/l, percentage of grafting of AA 116:3%, peroxide 38:3 mmol/ 100 g. AA-grafted sample.
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Fig. 2 shows the photografting of NIPAAm on AAgrafted cellulose. With the two-step procedure, AAgrafted cellulose was ®rst peroxidized with hydrogen peroxide in the presence of acid to yield peroxides of peracid type on the AA-grafted chains (Eq. (1)). Then, the resulting polymer peracids were subjected to the second-step photografting of NIPAAm. It was observed that the second-step photografting of NIPAAm is successively performed by using the polymer peracids and the percentage of grafting of NIPAAm increases with the irradiation time. The polymer peracids [16,19,20] derived from AA-grafted cellulose and polyethylene are known to have an ability to initiate grafting of vinyl monomers. It is supposed that the polymer peracids are
used thermosensitivity [21], which was de®ned as the ratio of degrees of swelling of the grafted cellulose at 5°C and 50°C, and the results are shown in Fig. 4. The thermosensitivity increased with increasing the percentage of grafting, and the magnitude of the increase was larger for the one-step sample compared to the two-step sample. Moreover, the value was higher for the grafted cellulose with a higher NIPAAm content in the grafted chains. It is conceivable that the dierent thermosensitivity between the both grafted celluloses is ascribed to the monomer sequence distribution of grafted chains. As shown in Scheme 1, the monomer sequence distribution of the grafted chains in the two-step sample is of block type with respect to each monomer component. There-
1
2
3
photodecomposed according to Eqs. (2) and (3) and the resultant radicals on the AA-grafted chains initiate the grafting of NIPAAm monomer. Accordingly, the monomer sequence distribution of the grafted chains of the two-step sample is of block type with respect to each monomer component. 3.2. Temperature-responsive character Fig. 3 presents the degree of swelling of AA/NIPAAm-grafted cellulose prepared by the one-step procedure when immersed in water at 5°C and 50°C for 24 h, alternately. The grafted cellulose exhibited a reversible change in the degree of swelling between 5°C and 50°C. It was found thus that the grafted cellulose shows the temperature-responsive character, in which it swells and shrinks in water at 5°C and 50°C, respectively. In order to compare the extent of the temperature-responsive character between the one- and two-step samples, we
fore, only poly(NIPAAm) component of the grafted chains shrinks when immersed in water at a temperature higher than the LCST of poly(NIPAAm). This may cause a small contribution of the poly(NIPAAm) component to the shrinkage of the whole grafted chains including the poly(AA) component. With the one-step sample, on the other hand, the poly(NIPAAm) component is expected to participate directly in the shrinkage of grafted chains because the grafted chains are composed of a random copolymer of AA and NIPAAm monomers. It was observed in a previous paper [22] that the temperature-responsive character of NIPAAm-grafted cellulose is improved by treating it with MBAAm crosslinker. So, the crosslinking treatment was applied to AA/NIPAAm-grafted celluloses, and the results are shown in Fig. 5. The thermosensitivity increased with an increase in the MBAAm concentration, which was commonly observed for the one- and two-step samples.
O.H. Wen et al. / European Polymer Journal 37 (2001) 807±813
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Fig. 5. Relationship between thermosensitivity and MBAAm concentration. Grafted samples (percentage of grafting 350± 370%, AA=NIPAAm 5=5) were treated with aqueous hydrogen peroxide solution (5 mM) at 30°C for 30 min under photoirradiation. ( ) one-step sample and (s) two-step sample. Fig. 3. Temperature-responsive character of AA/NIPAAmgrafted cellulose prepared by one-step procedure. Total percentage of grafting 200:7%.
that crosslinking treatment of AA/NIPAAm-grafted celluloses with MBAAm crosslinker is a useful means for improving the temperature-responsive character of the grafted samples. 3.3. Adsorption of Cu2
Fig. 4. Relationship between thermosensitivity and percentage of grafting. One-step sample is (h) AA=NIPAAm 5=5 and ( ) AA=NIPAAm 3=7 and two-step sample is (s) AA= NIPAAm 5=5 and (d) AA=NIPAAm 3=7.
It is conceivable that swelling and shrinking nature [22] of grafted cellulose is facilitated with the introduction of a crosslinked structure into the grafted chains, resulting in the increased thermosensitivity. It was con®rmed thus
Adsorption reaction of Cu2 with AA/NIPAAmgrafted celluloses was examined to understand its reactivity, and the results are shown in Fig. 6. The amount of adsorbed Cu2 in AA-grafted cellulose, which is also included in the ®gure, increased with an increase in the percentage of grafting, showing that carboxyl groups on the AA-grafted chains act as a reaction site for the chemical adsorption of Cu2 . The amount of adsorbed Cu2 in the one-step sample also increased with the percentage of grafting of AA component in the grafted chains. However, the amount was considerably lower compared to the AA-grafted cellulose, suggesting a steric hindrance due to a bulky side group of the NIPAAm component in the grafted chains. Fig. 7 shows the comparative examination in adsorption reaction of Cu2 with the one- and two-step samples. The amount of adsorbed Cu2 increased with increasing pH of the system, and the magnitude of the increase was smaller for the two-step sample compared to the one-step sample. It is inferred that poly(AA) component in the two-step sample is partly covered with poly(NIPAAm) component since poly(NIPAAm) is introduced into the AA-grafted cellulose by second-step
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the adsorption reaction of Cu2 because of the shrinkage of the grafted chains at 50°C. This leads to the lower amount of adsorbed Cu2 at 50°C. However, the difference in the amount of adsorbed Cu2 between the systems at 5°C and 50°C was not observed for the twostep sample since the temperature-responsive character of the two-step sample was smaller than the one-step sample as shown in Fig. 4.
4. Conclusions
Fig. 6. Adsorption of Cu2 with grafted cellulose. Cu2 0:02 M, pH 5:0, temperature 5°C, time 24 h, (d) AAgrafted cellulose and (h) one-step sample AA=NIPAAm 3=7.
It is concluded that grafted celluloses having grafted chains, which consist of two types of monomer component such as AA and NIPAAm, could be prepared by two photografting procedures using periodic acid oxidized cellulose sample; that is, one is the photograting of AA and NIPAAm binary monomers on the oxidized sample, and the other is the photografting of NIPAAm on the AA-grafted sample. The resulting grafted celluloses were characterized by the monomer sequence distribution of the grafted chains depending on the procedures, resulting in a dierent extent of properties, such as the temperature-responsive character and the reactivity toward adsorption of Cu2 , between the grafted celluloses prepared by the two procedures.
References
Fig. 7. Relationship between amount of adsorbed Cu2 and pH in AA/NIPAAm (3/7)-grafted celluloses (percentage of grafting 365±375%). Cu2 0:02 M, time 24 h, one-step sample is ( ) 5°C and (h) 50°C and two-step sample is (d) 5°C and (s) 50°C.
photografting, resulting in the lower amount of adsorbed Cu2 . It was observed, moreover, that the amount of adsorbed Cu2 in the one-step sample is lower for the system at 50°C than that at 5°C. It is conceivable that some parts of carboxyl groups on AA component in the grafted chains cannot participate in
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