Temperature-Responsive Cellulose by Ceric (IV) Ion-Initiated Graft

Biomacromolecules 2003, 4, 758-765

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Temperature-Responsive Cellulose by Ceric(IV) Ion-Initiated Graft Copolymerization of N-Isopropylacrylamide K. C. Gupta* and Keerti Khandekar Polymer Research Laboratory, Department of Chemistry, Indian Institute of Technology, Roorkee-247 667, UA, India Received December 12, 2002; Revised Manuscript Received February 18, 2003

Temperature-responsive cellulose has been obtained by graft copolymerization of N-isopropylacrylamide (NIPAAm) monomer using ceric ammonium nitrate (CAN) as initiator at 25.0 ( 0.1 °C in acidic medium. Kinetic and grafting parameters were evaluated at different concentrations of NIPAAm ranging from 1.25 × 10-3 to 12.5 × 10-3 mol dm-3 and varying concentrations of CAN from 1.5 × 10-3 to 9.0 × 10-3 mol dm-3 at constant concentration of nitric acid (2.5 × 10-2 mol dm-3). The graft copolymerization of NIPAAm onto cellulose has shown a significant increasing trend below lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAAm) and shown low energy of activation (18.0 kJ mol-1) for graft copolymerization within the temperature range of 10-35 °C as determined with Arrhenius plot. The PNIPAAm-grafted cellulose has shown improved thermal stability and shown temperature-dependent degree of swelling. Variation in degree of swelling of PNIPAAm-grafted cellulose as a function of temperature has been used to determine LCST of PNIPAAm-grafted cellulose. The contact angle (θ) has shown variation on increasing the graft yield and temperature. On the basis of experimental observations, the reaction steps for graft copolymerization have been proposed and a rate expression has been derived. Introduction Cellulose is a most-abundant naturally occurring biopolymer, and its derivatives have many important applications in fiber, paper, and paint industries. Cellulose consists of polydisperse glucose polymer chains, which form supramolecular structures through hydrogen bonding.1 The polymeric material with desired properties is a current need of society. To control properties, such as hydrophobicity, adhesivity,2 selectivity,3 drug delivery,4 wettability,5 and thermosensitivity,6 by graft copolymerization of suitable monomer is an indispensable technique for cellulose modification without any loss in its original properties. The graft copolymerization of varieties of monomers onto cellulose has been carried out by different techniques, such as irradiation with ultraviolet light, gamma rays, plasma ion beams, atom-transfer radical polymerization,7 and by ceric(IV) ion initiation methods.8 The ceric(IV) ion initiation offers great advantages of forming radicals at cellulose backbone through a singleelectron-transfer process9 to promote grafting of monomer onto cellulose. However, the ceric(IV) ion-initiated grafting depends on pH of the medium and the type of acid used for graft copolymerization.10 The poly(N-isopropylacrylamide) (PNIPAAm) has sharp phase-transition properties, which makes it a most-suitable material to develop temperatureresponsive smart surfaces. The temperature-responsive property of PNIPAAm has been utilized to develop columnpacking material for temperature-responsive high-performance * To whom correspondence should be addressed. E-mail: kcgptfcy@ iitr.ernet.in.

liquid chromatography (HPLC) with aqueous mobile phase.11 The PNIPAAm shows a thermally reversible solubleinsoluble change in response to temperature around its lower critical solution temperature (LCST) of 32 °C in aqueous solution. The PNIPAAm below its LCST forms wellhydrated expanded random coil structures and forms hydrophobic compact structures by rapid dehydration above LCST. The remarkable phase-transition behavior of PNIPAAm has been utilized for drug-delivery systems,12 cell culture substrate,13 and bioconjugates for bioreactors.14 The PNIPAAm has been used to modify the surface of silica and to develop cross-linked stimuli-responsive hydrogel, but no report is available in the literature for graft copolymerization of PNIPAAm onto cellulose; hence, in this paper, an attempt has been made to investigate the graft of copolymerization N-isopropylacrylamide (NIPAAm) onto cellulose in the presence of ceric ammonium nitrate (CAN) as initiator and grafting parameters have been evaluated as a function of experimental conditions. Experimental Section Chemicals Used. The cellulose powder (Loba Chemie, Mumbai, India) was purified by washing with methanol, acetone, and deionized water and finally dried in a vacuum desiccator on calcium chloride. The N-isopropylacrylamide (NIPAAm) was procured from Aldrich Chemical Company (Milwaukee, WI) and purified by recrystallization from hexane and dried at 25 °C in a vacuum desiccator. The ceric ammonium nitrate (CAN) and other chemicals were procured from E. Merck, India, and used without further purification.

10.1021/bm020135s CCC: $25.00 © 2003 American Chemical Society Published on Web 03/18/2003

Temperature-Responsive Cellulose

Graft Copolymerization. The solution graft copolymerization of NIPAAm onto cellulose was carried out in a threenecked round-bottom flask fitted with an electrically operated stirrer and maintained at 20.0 ( 0.1 °C. For graft copolymerization, 1.0 g of cellulose was added in a three-necked flask containing 100 mL of a solution of CAN in nitric acid. The solution was purged with nitrogen gas for about 30 min before adding NIPAAm monomer to initiate graft copolymerization. The reaction mixture was stirred at a constant rate15 to avoid the effect of stirring on the rate of graft copolymerization. The reaction was arrested by adding a 1.0% solution of hydroquinone at a fixed time of reaction. The reaction mixture was finally poured into a water-methanol mixture (1:1) and grafted cellulose crude was separated with sintered crucible. The separated grafted cellulose crude was washed repeatedly with water to remove unreacted monomer (NIPAAm) and its low molecular weight homopolymer (PNIPAAm) and other impurities coprecipitated along with grafted crude. Finally, the crude was dried under vacuum to constant weight at 60 °C because at this temperature the PNIPAAm has minimum water absorbency. Extraction of Ungrafted Poly(N-isopropylacrylamide). The ungrafted PNIPAAm formed during graft copolymerization was removed partially by washing the crude with water, but to ensure its complete removal, the crude was Soxhlet extracted for about 6-8 h in water, and the extract was precipitated in acetone, which was freeze-dried to constant weight at 60 °C. The weight of ungrafted PNIPAAm was used to calculate the percentage of homopolymer (% Hp) and grafted PNIPAAm (% Cg). Extraction of Ungrafted Cellulose. During graft copolymerization, a small amount of cellulose has remained ungrafted; hence, extraction of this ungrafted cellulose was carried out by keeping homopolymer-extracted crude in cuoxam solution. After 2 h, the cellulose extracted crude was separated by filtration and washed with distilled water and vacuum-dried to constant weight at 60 °C. The difference in weight of crude after extraction of cellulose was taken as the weight of ungrafted cellulose and used to calculate the true grafting (% GT) and percent cellulose conversion (% Cc). Extraction of Grafted Poly(N-isopropylacrylamide). To estimate the amount of grafted PNIPAAm and to characterize the grafted chains for molecular weight, the grafted PNIPAAm chains were separated from cellulose by hydrolyzing PNIPAAm-grafted cellulose in a 72% solution of sulfuric acid at room temperature. At the end of 6 h, the cellulose was removed, and the crude was washed repeatedly with distilled water and dried to constant weight at 60 °C. The hydrolyzed PNIPAAm chains were obtained from the filtrate by precipitating with methanol, which was freeze-dried to constant weight at 60 °C. The weight of PNIPAAm was used subsequently to calculate grafting parameters.8 During hydrolysis of PNIPAAm from the cellulose by sulfuric acid, the PNIPAAm chains remained unaffected, as confirmed by keeping PNIPAAm samples for 24 h in sulfuric acid and comparing their molecular weights with their initial weights.

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Characterization FTIR Spectra. The grafting of PNIPAAm onto cellulose was verified by comparing FTIR spectra of ungrafted cellulose, PNIPAAm, and PNIPAAm-g-cellulose recorded on KBr pallets using Perkin-Elmer 1600 Fourier-transform infrared spectrometer. Thermal Studies. The thermal stability of cellulose and PNIPAAm-grafted cellulose was estimated by recording thermogravimetric (TG) and differential thermogravimetric (DTG) curves using Perkin-Elmer 7.0 thermal analyzer system at a heating rate of 10 °C/min under nitrogen atmosphere. The energy of activation (Ea) for thermal decomposition of cellulose and grafted cellulose was determined by taking the fraction of sample (R) decomposed at a particular temperature (T) and using following equation.16 ln ln(1 - R)-1 )

Ea

100 θ+C T RTs f - Ti 2

(1)

where Ti and Tf are the initial and final decomposition temperatures taken as the point of 5% derivation from baseline in the DTG curve (Figure 2) and θ is the difference of decomposition temperature (T) and temperature of reference (Ts). Molecular Weight Measurements. The molecular weights of cellulose and PNIPAAm-grafted chains were estimated by a viscometric method using following equations: for cellulose,17 h V0.76 (in Cadoxen) (2) [η]25°C ) 38.50 × 103 cm3 g-1 M and for PNIPAAm,18 h V0.65 (in THF) [η]30°C ) 9.59 × 10-3 cm3 g-1 M

(3)

Temperature-Sensitive Degree of Swelling. To test the thermoresponsive nature of PNIPAAm-grafted cellulose, the degree of swelling of cellulose, PNIPAAm, and PNIPAAmgrafted cellulose was determined by immersing samples (2.0 g) into 100 mL of distilled water maintained at a specified temperature between 10 and 60 °C. After 24 h, the samples were separated and weighed (W1) after wiping surface water with tissue paper. The degree of swelling of the samples was determined by using following equation: Degree of swelling (%) )

W1 - W0 × 100 W0

(4)

where W0 is the initial dried weight of samples. Contact Angle (θ) Measurements. Static water contact angles (θ) of cuoxam-cast films of cellulose and PNIPAAmgrafted cellulose were measured at 25 °C and 50% humidity using a contact angle goniometer (NRL-100, Rame Hart). A drop of deionized water was placed on the film, and after 30 s, the contact angle (θ) was measured. The average of five measurements was used for each droplet. Results and Discussion The poly(N-isopropylacrylamide) alone and its interpenetrating polymer system undergo reversible phase transitions

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Figure 2. TG and DTG curves of (s) pure cellulose and (- - -) PNIPAAm-grafted cellulose at a heating rate of 10 °C/min.

Figure 1. FTIR spectra of (A) cellulose, (B) PNIPAAm, and (C) PNIPAAm-grafted cellulose.

in response to external stimuli,19 such as, temperature, pH, and nature of the medium; hence, graft copolymerization of NIPAAm makes a material surface smart and environmentally responsive. The cellulose is a naturally occurring biopolymer and is used in various applications, but PNIPAAmgrafted cellulose will have intelligence applications in medicine, biotechnology, industry, sensors,20 responsive membranes21 and drug-delivery vehicles.22 The distribution of PNIPAAm chains onto cellulose surface offers the ability to control important interfacial phenomena such as wetting,23 fluid flow,24 adhesion,25 and molecular recognition.26 The PNIPAAm-grafted silica has revealed interesting properties; hence, grafting of PNIPAAm onto cellulose has become a subject of great interest to develop cellulose with responsive character to external stimuli. Ceric(IV) ions are known to create active sites on the cellulose backbone; hence, NIPAAm has participated only in graft copolymerization and its participation in formation of homopolymer was insignificant. The grafting of PNIPAAm onto cellulose was confirmed by comparing the IR spectra of pure cellulose (Figure 1A), PNIPAAm (Figure 1B), and PNIPAAm-grafted cellulose (Figure 1C). The appearance of an absorption band at 1648 cm-1 in PNIPAAm-grafted cellulose corresponds to carbonylamide and N-H stretching of PNIPAAm (Figure 1B) and has provided evidence for the grafting of PNIPAAm onto cellulose (Figure 1C). The sharp bands at 1384 and 1365 cm-1 in grafted cellulose, which were initially absent in ungrafted cellulose (Figure 1A), are assigned to the isopropyl

group of PNIPAAm (Figure 1B). The grafted cellulose has shown improvement in thermal stability as clear from TG and DTG curves (Figure 2). The initial decomposition temperature of the cellulose on grafting was increased from 190 to 248 °C. The temperature corresponding to the peak height in the DTG curve has been taken as Tmax and has been found to be shifted to 456 °C in PNIPAAm-grafted cellulose (Figure 2), in comparison to original decomposition temperature of 390 °C (Figure 2) of pure cellulose. These observations have clearly indicated that grafting of PNIPAAm has improved the thermal stability of cellulose, and thermal stability was found to be dependent on degree of grafting onto cellulose. The energy of activation for the decomposition of cellulose and grafted cellulose estimated by taking the decomposed fraction of the cellulose or grafted cellulose (R) at different temperatures (T) has clearly indicated that stability of cellulose has increased on graft copolymerization of PNIPAAm onto cellulose (Figure 3). The energy of decomposition of grafted cellulose was higher (253 kJ mol-1) than that of pure cellulose (202 kJ mol-1). Thermosensitivity. The variation in degree of swelling of PNIPAAm-grafted cellulose as a function of temperature (Figure 4) has clearly indicated the thermoresponsive character of PNIPAAm-grafted cellulose. The degree of swelling of PNIPAAm-grafted cellulose (Figure 4C) and PNIPAAm (Figure 4B) has shown a marked decreasing trend around 32 °C, which corresponds to the LCST of PNIPAAm. This decreasing trend in degree of swelling above LCST (32 °C) was due to enhanced hydrophobic interactions of the isopropyl group of PNIPAAm and due to a significant decrease in hydrogen bonding between water molecules and grafted



Table 1. Contact Angle (θ) of PNIPAAm-Grafted Cellulosea θ

a

Figure 3. Determination of energy of activation of decomposition of (A) cellulose and (B) PNIPAAm-grafted cellulose using DTG data.

Figure 4. Variation of degree of swelling as a function of temperature for (A) cellulose, (B) PNIPAAm, and (C) PNIPAAm-grafted cellulose.

PNIPAAm. On increasing the temperature, a sudden change in volume of ungrafted PNIPAAm (Figure 4B) and grafted PNIPAAm chains onto cellulose (Figure 4C) has taken place, which has reduced the opportunities of water molecules to form hydrogen bonds with PNIPAAm chains. The change in degree of swelling above LCST (>32 °C) was almost negligible as no further change in structure and configuration of PNIPAAm chain takes place. However, pure cellulose has shown a slight increase in degree of swelling on increasing the temperature from 10 to 60 °C, which has been attributed to the breaking of hydrogen bonds between glucose units of the cellulose, which allowed water molecules to penetrate into the cellulose matrix. But in the case of PNIPAAm-grafted cellulose, the hydrophobic character of PNIPAAm was predominant, which prevented the diffusion of water molecules in the cellulose matrix; hence, the degree of swelling of PNIPAAm-grafted cellulose was low in comparison to pure cellulose at sample temperatures above LCST. But pure PNIPAAm chains are more hydrophobic above its LCST (>32 °C) in comparison to pure cellulose and PNIPAAm-grafted cellulose as confirmed from lowest degree of swelling above its LCST (>32 °C) (Figure 4B).

% graft yield

at 25 °C

at 35 °C

cellulose 12.9 36.0 69.0 85.0

35.0 ( 0.18 38.5 ( 0.16 52.0 ( 0.10 83.0 ( 0.12 105.0 ( 0.10

36.0 ( 0.20 43.0 ( 0.21 62.0 ( 0.18 101.0 ( 0.15 127.0 ( 0.16

Time ) 30 s; SD of five measurements.

Effect of Grafting on Contact Angle (θ). The hydrophilic and hydrophobic properties of the cellulose were confirmed by contact angle (θ) measurements. The contact angle has increased on increasing the extent of graft yield (% G) and on increasing the temperature from 25 to 35 °C (Table 1). The increase in contact angle on increasing the graft yield (% G) was due to the hydrophobic character of the isopropyl group of grafted PNIPAAm in comparison to the hydroxyl group of pure cellulose. The grafted PNIPAAm chains have decreased the wettability of cellulose in comparison to pure cellulose. The increase in contact angle (θ) on increasing the temperature from 25 to 35 °C is due to the coiling effect of grafted chains onto cellulose surface, which ultimately has decreased the extent of hydrogen bonding between water molecules and nitrogen atom of PNIPAAm; hence, the wettability of the PNIPAAm-grafted surface was decreased at higher temperature (>25 °C). Reaction Kinetics of Graft Copolymerization. In addition to variation in properties of cellulose on grafting of PNIPAAm, the effect of various experimental conditions has also been studied to optimize the conditions for maximum graft yield and to explain the mechanism involved in graft copolymerization of NIPAAm onto cellulose. Effect of NIPAAm Concentration. The graft copolymerization onto cellulose has been studied at different concentrations of NIPAAm ranging from 1.25 × 10-2 to 12.5 × 10-2 mol dm-3 at constant concentration of CAN (6.0 × 10-3 mol dm-3) and nitric acid (2.5 × 10-3 mol dm-3) at 25.0 ( 0.1 °C. The graft yield (% G) of NIPAAm onto cellulose has increased linearly up to 7.5 × 10-2 mol dm-3 concentration of NIPAAm, but deviation from linearity was observed on increasing its concentration further beyond 7.5 × 10-2 mol dm-3 (Figure 5). True grafting (% GT), graft conversion (% Cg), and efficiency of grafting (% GE) have shown similar trends as is clear from the data given in Table 2, section A. The cellulose conversion (% Cc) has maintained an increasing trend even at high concentration of NIPAAm (>7.5 × 10-2 mol dm-3) in the reaction mixture. The decreasing trend in efficiency of grafting (% GE), graft conversion (% Cg), and molecular weight (M h V) of grafted chains (Gp) and the increasing trend in cellulose conversion (% Cc), frequency of grafting (GF), and number of grafted chains (Ngp) beyond 7.5 × 10-2 mol dm-3 concentration of NIPAAm are clear indications for premature termination of grafted chains and inception of new chains onto cellulose matrix (Table 2, section B). These steps have justified the increasing trends in frequency of grafting (GF) and cellulose conversion (% Cc) and decreasing trends in molecular weight (M h V) of the grafted chains (Table 2, section B). At high

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Table 2 A. Effect of [NIPAAm] on Grafting Parameters at 25 °Ca [NIPAAm] × 102, mol dm-3

% GT

% CT

% Cg

% CHP

% Cc

% GE

1.25 2.50 5.00 7.50 10.00 12.50

190 197 202 213 208 174

46.10 65.00 80.00 94.12 92.92 85.11

31.91 46.07 63.16 81.18 75.22 65.25

14.18 15.71 16.84 12.94 17.70 19.86

36 38 45 54 65 70

69.23 74.57 78.95 86.25 80.95 76.66

B. Effect of [NIPAAm] on Grafting Parameters at 25 °Ca [NIPAAm] × 102, mol dm-3

GF

Ngp × 106

NHP × 106

1.25 2.50 5.00 7.50 10.00 12.50

3.41 7.71 14.22 16.33 20.10 21.52

3.00 7.17 15.65 21.56 30.36 36.80

6.67 11.00 16.00 15.71 29.41 41.79

M h v × 10-3 g mol-1 GP HP

%N

15 18 23 32 28 25

0.53 1.42 3.28 5.06 5.69 5.94

3.0 4.0 6.0 7.0 6.8 6.7

Figure 6. Double logarithmic plot of Rp vs [NIPAAm]: [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min; temp ) 25 °C.

a [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min.

Figure 7. Effect of concentration of CAN on graft yield (% G): [NIPAAm] ) 7.5 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min; temp ) 25 °C.

Figure 5. Effect of concentration of PNIPAAm on graft yield (% G): [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min; temp ) 25 °C.

concentration (>7.5 × 10-2 mol dm-3), the NIPAAm has played a role as a swelling agent for cellulose27 and as a solvent for cellulose-grafted PNIPAAm chains; hence, the graft yield (%G) has increased continuously on increasing the concentration of NIPAAm (Figure 5). However, the extent of grafting inside the cellulose matrix was comparatively low. The termination of cellulose-grafted PNIPAAm chains at high concentration of NIPAAm (>7.5 × 10-2mol dm-3) has facilitated the formation of homopolymer as is clear from the observed increasing amount of (Table 2, section A) homopolymer (Hp) at higher concentration of NIPAAm (>7.5 × 10-2 mol dm-3). The decreasing trend in molecular weight (M h V) of the grafted chains (Gp) and homopolymer (HP) at high concentration of NIPAAm (>7.5 × 10-2 mol dm-3) has been attributed to the increased rate of chain termination and transfer processes due to solvent effect of NIPAAm and its participation in an activity transfer process with growing chains at the cellulose. The rate of

grafting (Rp) has shown a linear increasing trend on increasing the concentration of NIPAAm from 1.25 × 10-2 to 7.5 × 10-2 mol dm-3 as clear from log-log plot (Figure 6) drawn between rate of graft copolymerization (Rp) and concentration of NIPAAm. The slope of this curve (Figure 6) was found to be 1.5, indicating a different mechanism of graft copolymerization in comparison to the mechanism reported for graft copolymerization with other vinyl monomers onto cellulose.28 Effect of Ceric(IV) Ion Concentration. The graft copolymerization of NIPAAm onto cellulose was also studied at different concentrations of ceric(IV) ions ranging from 1.5 × 10-3 to 9.0 × 10-3 mol dm-3 (Figure 7), keeping constant concentrations of nitric acid (2.5 × 10-2 mol dm-3) and NIPAAm (7.5 × 10-2 mol dm-3). The graft yield (% G) and other grafting parameters, such as true grafting (% GT), graft conversion (% Cg), and efficiency of grafting (% GE), have increased up to 6.0 × 10-3 mol dm-3 concentration of ceric(IV) ions (Figure 7 and Table 3, section A) but decreased on taking ceric(IV) ion concentration beyond 6.0 × 10-3 mol dm-3. The initial increasing trend in graft yield (% G) was due to the formation of sufficient number of active sites


Temperature-Responsive Cellulose Table 3 A. Effect of [CAN] on Grafting Parameters at 25 °Ca [CAN] × 102, mol dm-3

% GT

% CT

% Cg

% CHP

% Cc

% GE

1.5 3.0 4.5 6.0 7.5 9.0

200 204 211 213 205 203

50.59 67.76 81.18 94.12 92.94 87.06

41.18 57.65 69.41 81.18 78.82 74.12

9.41 10.59 11.76 12.94 17.64 18.82

45.0 49.0 51.0 54.0 53.7 53.4

81.39 84.38 85.51 86.25 81.01 79.49

B. Effect of [CAN] on Grafting Parameters at 25 °Ca [CAN] × 102, mol dm-3

GF

Ngp × 106

NHP × 106

1.5 3.0 4.5 6.0 7.5 9.0

8.15 11.34 13.99 16.33 16.28 16.17

8.97 13.61 17.35 21.56 21.33 21.02

6.96 8.18 12.50 15.71 22.73 25.81

M h v × 10-3 g mol-1 GP HP

%N

39.0 36.0 34.0 32.0 30.0 29.5

3.21 4.05 4.60 5.06 4.83 4.74

11.5 11.0 8.0 7.0 6.6 6.2

Figure 8. Double logarithmic plot of Rp vs [CAN]: [NIPAAm] ) 7.5 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min; temp ) 25 °C.

a [NIPAAm] ) 7.5 × 10-3 mol dm-3; [HNO ] ) 2.5 × 10-2 mol dm-3; 3 [cellulose] ) 1.0 g; time ) 50 min.

onto cellulose by ceric(IV) ions due to the presence of sufficient amount of nitric acid29 in the reaction mixture, but on increasing the concentration of ceric(IV) ions beyond 6.0 × 10-3 mol dm-3, the activity of ceric(IV) ions was decreased because of the hydration of ceric(IV) ions; hence, decreasing trends in graft yield (% G) and other grafting parameters was noticed (Figure 7 and Table 3). The role of ceric(IV) ions in the formation of active sites on cellulose is evident from the variation in frequency of grafting (GF) and number of grafted chains (Ngp) on the cellulose (Table 3, section B) upon varying the activity of the ceric(IV) ions in the reaction mixture. The values of these parameters (GF and Ngp) have decreased when the ceric(IV) ions became inactive because of disproportionate concentration of nitric acid in the reaction mixture. The variation in activity of ceric(IV) ions was also evident from the decreasing trend in cellulose conversion (% Cc) at higher concentration of the ceric(IV) ions (Table 3, section A). At high concentration of the ceric(IV) ions (>6.0 × 10-3 mol dm-3), the available nitric acid (2.5 × 10-2 mol dm-3) was not sufficient to prevent the hydration of the ceric(IV) ions; hence, ceric(IV) ions of the reaction mixture were hydrated, which makes them inactive for generating active sites on the cellulose for grafting of NIPAAm. These hydrated ceric(IV) ions were subsequently consumed in oxidative termination of growing chains and in formation of homopolymer. The participation of hydrated ions in the chain termination process is evident from the decreasing trends in molecular weights (M h V) of grafted PNIPAAm chains (Table 3, section B) and from the increasing trend in formation of homopolymer (% HP) at high concentration (>6.0 × 10-3 mol dm-3) of ceric(IV) ions. The increasing trend in number of homopolymer chains (NHP) at high concentration of ceric(IV) ions (>6.0 × 10-3 mol dm-3) is indicative of participation of hydrated ceric(IV) ions in inception of new homopolymer chains by interaction with NIPAAm molecules in the reaction mixture. These observations have clearly indicated that ceric(IV) ions are able to

Figure 9. Effect of temperature on graft yield (% G): [NIPAAm] ) 7.5 × 10-3 mol dm-3; [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min.

form active sites on cellulose, if an optimum amount of nitric acid is available in the reaction mixture to maintain the oxidation potential of ceric(IV) ions, otherwise the efficiency of ceric(IV) ions is changed significantly. The log-log plot (Figure 8) between rate of graft copolymerization (Rp) and concentration of ceric(IV) ions has given a slope value of 0.5, indicating a square-root dependence of grafting rate on concentration of ceric(IV) ions. Effect of Temperature. The grafting behavior of NIPAAm onto cellulose has been studied by varying reaction temperature from 10 to 45 °C at constant concentration of NIPAAm (7.5 × 10-2 mol dm-3), CAN (6.0 × 10-3 mol dm-3), and nitric acid (2.5 × 10-2 mol dm-3). The graft yield (% G) and other grafting parameters have increased up to 35 °C, and after that, a decreasing trend was noticed (Figure 9 and Table 4). The decreasing trend in graft yield (% G) at high temperature (>35 °C) was due to thermoresponsive reversible phase transition of grafted chains from a water-soluble hydrophilic state to a water-insoluble hydrophobic state, which ultimately has affected the penetration and diffusion of NIPAAm molecules to reach active sites on the cellulose. This is also indicative that LCST of PNIPAAm-grafted chains on cellulose is slightly different than that of ungrafted

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Table 4 A. Effect of Temperature on Grafting Parametersa temp, °C

% GT

% CT

% Cg

% CHP

% Cc

% GE

10 15 20 25 30 35 40 45

208 210 211 213 218 219 190 163

78.82 84.71 87.06 94.12 95.29 95.88 92.94 83.53

60.00 67.06 71.76 81.18 88.24 92.94 83.53 74.12

18.82 17.65 16.47 12.94 9.41 9.41 7.06 2.94

49.0 50.5 51.7 54.0 55.0 57.0 59.0 62.0

76.12 79.17 82.43 86.25 88.24 90.11 89.87 88.75

B. Effect of Temperature on Grafting Parametersa

M h v × 10-3 g mol-1 temp, °C

GF

Ngp × 106

NHP × 106

GP

HP

%N

10 15 20 25 30 35 40 45

11.83 12.87 14.67 16.33 19.30 22.73 20.54 18.97

13.42 15.83 18.48 21.56 25.86 31.6 29.58 28.64

18.82 18.75 18.42 15.71 9.23 3.97 13.33 13.79

38 36 33 32 29 25 24 22

8.5 8.0 7.6 7.0 6.5 6.3 6.0 5.8

4.18 4.5 4.69 5.06 5.31 5.47 5.14 4.79

Figure 10. Arrhenius plot of log kp vs 1/T: [NIPAAm] ) 7.5 × 10-3 mol dm-3; [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min.

a [NIPAAm] ) 7.5 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; time ) 50 min.

PNIPAAm and has also shown dependence on extent of grafting. This temperature dependence property of grafted chains has decreased the frequency (GF) and number of grafted chains (Ngp) onto cellulose (Table 4, section B) and also decreased the molecular weight (M h V) of the grafted chains (GP) at higher temperature (>35 °C). The initial decreasing trend in formation of homopolymer (% HP) up to 35 °C and increasing trend beyond 35 °C is due to the change in reaction activity from cellulose surface to the solution phase. But on further increasing the temperature beyond 40 °C, the formation of homopolymer (% HP) has become almost constant because of the change in property and structure of growing homopolymer (PNIPAAm) chains, which stopped the further growth of the homopolymer chains (Table 4, section A). The increasing trend in cellulose conversion (% Cc) beyond 35 °C is due to oxidation of cellulose by ceric(IV) ions. The energy of activation for graft copolymerization of NIPAAm onto cellulose was calculated with an Arrhenius plot (Figure 10) and has been found to be 18.0 kJ mol-1 within a temperature range of 10-35 °C. This has indicated that NIPAAm has affinity for grafting onto cellulose at low temperature, but a decreasing trend in affinity for grafting at high temperature (>35 °C) was due to the variation in property of grafted chains from hydrophilic to hydrophobic, which reduced the affinity of NIPAAm molecules for grafting. Effect of Additives. The graft copolymerization of NIPAAm onto cellulose was also studied in the presence of different additives (Figure 11). Sodium lauryl sulfate (NaLS) has retarded the graft yield because of the formation of a negatively charged layer on the cellulose surface by lauryl sulfate anions, which prevented NIPAAm molecules from reaching active sites on the cellulose, causing a reduction in graft yield of NIPAAm onto cellulose. The increasing trend

Figure 11. Effect of additives: control [NIPAAm] ) 7.5 × 10-3 mol dm-3; [CAN] ) 6.0 × 10-3 mol dm-3; [HNO3] ) 2.5 × 10-2 mol dm-3; [cellulose] ) 1.0 g; temp ) 25 °C; [NaLS] ) 7.5 × 10-3 mol dm-3; [CTAB] ) 2.5 × 10-3 mol dm-3; [ROH ] ) 5% (v/v).

in graft copolymerization of NIPAAm in the presence of cetyltrimethylammonium bromide (CTAB) has confirmed the retarding effect of lauryl sulfate anions because the increasing trend in graft copolymerization of PNIPAAm onto cellulose in the presence of CTAB has been assumed to be due to the formation of positive layer of preferentially adsorbed cations of CTAB, which increased the number of NIPAAm molecules on cellulose surface and ultimately has increased the rate of graft copolymerization. Thus the concentration of monomer molecules at the cellulose surface was controlled by time-average potential of the electrical double layer30 formed by potential-determining ions of added surfactants in the reaction mixture. The addition of water-miscible organic solvents (ROH), such as ethanol (EtOH) and methanol (MeOH), has shown a decreasing trend on graft copolymerization of NIPAAm, which has been attributed to the formation of inactive radicals and dehydration of cellulose, which has reduced the diffusion of NIPAAm molecules at active sites on cellulose. Thus on the basis of kinetic data of graft copolymerization of NIPAAm onto cellulose in the presence of CAN, the following reaction steps have been proposed:



(7)

the surface of cellulose and the formation of inactive radicals. In addition to kinetic parameters of graft copolymerization, the NIPAAm-grafted cellulose has also shown significant improvement in its properties such as thermal stability, water absorbency, and degree of swelling. The graft copolymerization of NIPAAm onto cellulose has introduced a thermoresponsive character, which has shown variation in degree of swelling and contact angle (θ) of the modified cellulose surface. The PNIPAAm-modified cellulose will have versatile applications in formulation of controlled-release and selective-separation devices.

cellulose-NIPAAm•- + NIPAAm 98 cellulose-NIPAAm2•- (8)

Acknowledgment. The financial assistance from UGC, New Delhi, is thankfully acknowledged. Authors are thankful to I. I. T. Roorkee for providing facilities to complete this research.

Reaction Mechanism Radical Formation K

cellulose + Ce(IV) 798 complex

(5)

kd

complex + NIPAAm 98 cellulose• + Ce(III) + NIPAAmH+ (6) Initiation ki

cellulose• + NIPAAm 98cellulose-NIPAAm•Propagation kp

cellulose-(NIPAAm)n-1-NIPAAm•kp

+ NIPAAm 98cellulose-(NIPAAm)n-NIPAAm•- (9) Termination ktc

cellulose-(NIPAAm)n•- + -•(NIPAAm)m-cellulose 98 grafted cellulose (10) Considering reaction steps 5-10 for graft copolymerization of NIPAAm onto cellulose, the following expression for overall rate of graft copolymerization (Rp) has been derived: R p ) kP

( ) kdK 2ktc

1/2

[NIPAAm]3/2[cellulose]1/2[CAN]1/2 (11)

where K and kd are the equilibrium constants for the formation of complex between ceric(IV) ions and cellulose. The kd is the rate constant for the decomposition of the complex in the presence of NIPAAm. The constants ki, kp, and ktc are the rate constants for initiation, propagation, and termination reactions involved in graft copolymerization of NIPAAm onto cellulose in the presence of CAN. The rate expression (eq 11) has shown total agreement to the experimental data for the dependence of the reaction rate of graft copolymerization of NIPAAm onto cellulose as a function of concentration of NIPAAm and CAN; hence, the proposed steps 5-10 are correct for graft copolymerization of NIPAAm onto cellulose. Conclusion The CAN has shown its capability for graft copolymerization of NIPAAm onto cellulose without forming a significant amount of homopolymer in the reaction mixture. The rate of graft copolymerization has shown dependence on concentration of NIPAAm and on relative concentration of ceric(IV) ions to nitric acid in the reaction mixture. The graft copolymerization is also affected in the presence of surfactants and alcohols because of the variation in force field at

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BM020135S

Temperature-Responsive Cellulose by Ceric (IV) Ion-Initiated Graft

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