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Preirradiation grafting of acrylonitrile onto polypropylene monofilament for biomedical applications: I. Influence of synthesis conditions
ARTICLE IN PRESS
Radiation Physics and Chemistry 75 (2006) 161–167 www.elsevier.com/locate/radphyschem
Preirradiation grafting of acrylonitrile onto polypropylene monofilament for biomedical applications: I. Influence of synthesis conditions Bhuvanesh Guptaa,, Rachna Jaina,b, Nishat Anjuma, Harpal Singhb a
Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Centre for Biomedical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
b
Received 21 July 2004; accepted 12 April 2005
Abstract Graft polymerization of acrylonitrile onto polypropylene (PP) monofilament was carried out by a preirradiation method using a 60Co gamma radiation source. The influence of synthesis conditions, such as preirradiation dose, reaction time, monomer concentration, reaction temperature and additives was determined. The grafting was considerably influenced by the instantaneous swelling of the monofilament in the reaction mixture during the course of the grafting process. The order of dependence of the rate of grafting on monomer concentration was found to be 1.04. The nature of the medium of the grafting and the additives had profound influence over the grafting reaction. The accelerative effects of solvent medium on the grafting were higher in methylethyl ketone (MEK) and dimethylformamide (DMF) as compared to methanol. At the same time, partial replacement of DMF with water led to acceleration in the grafting with peak maxima at 20% solvent composition. The addition of a small amount of sulfuric acid to the reaction mixture also resulted in a significant acceleration of the degree of grafting. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polypropylene; Acrylonitrile; Suture; Radiation; Graft polymerization
1. Introduction Polymers have generated considerable interest as biomaterials in medical science and biotechnology. The innovative application domains of biomaterials are surgical sutures, medical implants and bioreceptive interfaces/scaffolds for tissue engineering (Langer, 2000, 2001; Gupta et al., 2002; Hutmacher, 2000; Tessmar et al., 2000). With increasing demands for a
Corresponding author.
E-mail address: [email protected] (B. Gupta).
biomaterial with better acceptability and functionality to the biosystem, stress has been focussed on the development of newer materials. One of the ways to develop such materials is to modify existing polymers and design them keeping in view their specific application areas. Sutures are used in surgical operations and require optimum physico-chemical characteristics. However, medical reports show that local infections brought after surgery are very frequent. In these situations, surgical sutures with antibacterial properties provide a promising solution. Biocidal materials and antibacterial finishing of textiles have been reviewed by many authors (Kim and Sun, 2001; Chung et al., 1998; Nakashima et al.,
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.04.003
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B. Gupta et al. / Radiation Physics and Chemistry 75 (2006) 161–167
2001; Stevanato and Tedesco, 1998; Rahbaran, 1999; Lin et al., 2001). Polypropylene (PP) is one of the widely used biostable sutures due to its optimum tensile strength and low level of tissue reaction as compared to other sutures. However, the microbial infection on the suture site often has been observed to lead to the deterioration of the wound and related complications especially where post-surgical care is not well taken up. Therefore, the modification of PP sutures may be carried out in such a way that it acquires functional groups where a drug may be immobilized. This drug is released from the suture once in contact with the biosystem and provides antimicrobial action. The polymer modification by radiation-induced graft polymerization of monomers into polymers has attracted attention to induce desirable properties in a material (Choi and Nho, 1999; Gupta et al., 1994, 1996, 2000; Plessier et al., 1998; Yang et al., 2003; Dessouki and Taher, 1998; Izumi et al., 2001; Lope˘rgolo et al., 2000; Bucio and Burillo, 1996). The radiation-induced graft polymerization of monomers such as 2-hydroxyethylmethacrylate (HEMA), methacrylic acid, acrylamide and N-vinylpyrrolidone onto PP has been reported to introduce hydrogel characteristics in the polymer for biomedical applications (Singh and Tyagi, 1989; Park et al., 1998; Rao and Rao, 1987; Singh and Ray, 1994). In our previous work, we carried out the grafting of hydroxyethylmethacrylate onto PP monofilament to immobilize 8-hydroxyquinoline as the antimicrobial drug (Tyagi et al., 1990; Gupta et al., 1993). However, the grafting led to considerable homopolymer formation that remained occluded within the polymer matrix and its complete separation from the suture matrix could not be achieved due to the hydrogel nature of the poly-HEMA. Thereby the suture characteristics were greatly deteriorated. Moreover, the inherent incompatibility of the grafted ionic component with the nonionic PP matrix added a loss in tensile strength. In order to overcome this problem, we have carried out the grafting of nonionic monomer, acrylonitrile, onto the PP monofilament using preirradiation where homopolymerization is very little and its separation from the grafted PP matrix would be much easier as compared to hydrogel poly-HEMA. Moreover, polyacrylonitrile grafts, because of their nonionic nature, would provide better compatibility of polyacrylonitrile grafts with the PP base matrix. This may lead to better retention in tensile properties. The grafted PP monofilament was subsequently hydrolyzed to get carboxyl groups for subsequent antimicrobial drug immobilization. In the present study, the graft polymerization of acrylonitrile onto PP monofilament is carried out by a preirradiation method to develop sutures with different graft levels. The influence of the reaction conditions on the degree of grafting has been investigated.
2. Materials PP used for this study was manufactured by IPCL, India. The monofilament was prepared by melt spinning of PP at 230 1C under nitrogen atmosphere. The monofilament was collected on bobbins and was further drawn to a 1:5 ratio unless otherwise mentioned. The filament had a diameter of 0.32 mm and a denier of 760 gpd. Acrylonitrile monomer was received from GS Chemicals, India and was purified by distillation under vacuum. Sulfuric acid, dimethylformamide (DMF), methanol (MeOH) and methylethyl ketone (MEK) obtained from GS Chemical India Ltd. was used as received. Distilled water was used for all experiments.
3. Irradiation PP monofilaments were exposed to g-rays from a 60Co source (900 Curies) in the presence of air. The dose rate of radiation was 0.27 kGy/h. After the irradiation, samples were kept at 4 1C prior to the grafting reaction.
4. Grafting procedure Grafting was carried out in glass ampoules of 2 10 cm2 size with a B-24 socket. A weighed amount of preirradiated monofilament (appprox. 0.04–0.06 g) was placed in ampoules containing monomer and the solvent. Nitrogen was purged into the ampoule to remove air trapped inside the reaction mixture and the ampoule was subsequently placed in a water bath maintained at the constant temperature. After a desired period, the ampoule was removed and soxhlet extracted with DMF to remove any homopoylmer adhering to the sample surface. The samples were dried in an oven at 50 1C under vacuum and the degree of grafting was obtained from the following equation: Wg Wo 100, Wo where Wo and Wg are the weight of the original and grafted samples, respectively. Degree of grafting ð% ¼Þ
5. Swelling measurement The swellings of the PP filaments in various solvents were determined by placing a known amount of them in a specific medium for 6 h. Samples were subsequently taken out and the surface solution was wiped out using a Whatmann filter paper. The samples were weighed and swelling was obtained as percent increase in the weight of the samples.
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6. Results and discussion The radiation-induced graft polymerization of acrylonitrile onto PP monofilament was carried out to investigate the influence of synthesis conditions on the degree of grafting. The influence of additives, solvents, monomer concentration, preirradiation dose and reaction temperature on the degree of grafting was studied. The variation in degree of grafting with the monomer concentration in DMF solvent is presented in Fig. 1. The grafting increases with the increase in the monomer concentration up to 80% and then decreases abruptly. This trend is related to the swelling behavior of PP monofilament in the grafting reaction and subsequent availability of the monomer to the grafting sites. As long as DMF (a solvent for the polyacrylonitrile (PAN) grafts) is present in the grafting reaction, the polyacrylonitrile grafted PP layers swell in the medium and regulate the monomer diffusion through the swollen layer to the grafting sites (Plessier et al., 1998). As a result, the propagation of growing chains proceeds smoothly. In the absence of DMF, a lower degree of grafting was observed (as evident from 100% monomer concentration). This is because acrylonitrile is a nonsolvent for PAN chains, which diminishes the swelling of the grafted monofilament; hence, diffusion of the monomer through grafted zone is reduced and leads to low graft levels. It is, therefore, the instantaneous swelling of the monofilament that influences the grafting at a specific monomer–solvent composition. This is evident from the fact that the PP monofilament with 4.2% degree of grafting exhibits swellings of 5.9% and 3.5% at monomer concentrations of 20% and 80% acrylonitrile in DMF as solvent as compared to 1.6% in pure monomer indicating a more amenable matrix for the grafting in the presence of DMF as solvent.
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It is important to note in Fig. 1 that all solvents, DMF, methylethyl ketone and methanol, show identical trends in the grafting reaction and exhibit maxima at 20% solvent in a monomer–solvent mixture. However, methanol produces the lowest graft levels. The PP matrix has negligible swelling in methanol and hence the monomer diffusion within the monofilament matrix is restricted leading to low level grafting in spite of the accelerative effect of the methanol. It may be therefore stated that the solubility parameter plays a key role in swelling. PP has a solubility parameter of 9.4, which is close to that of MEK (9.3) as compared to both DMF (12.1) and methanol (14.5). This would account for the higher swelling of the PP matrix in MEK than other solvents and is refluxed in the higher degree of grafting. The partial replacement of DMF with water has been observed to show an accelerative effect on the degree of grafting. The influence of DMF–water composition on the grafting is presented in Fig. 2. The degree of grafting increases with the increase in the water content up to 20% water concentration and thereafter tends to decrease. The initial increase in the grafting may be probably due to coiling up of the growing PAN chains in the presence of water, which acts as nonsolvent for the polyacrylonitrile grafts. The coiling may lead to the decrease in the rate of termination of growing polyacrylonitrile chains and hence the propagation process continues smoothly. It seems that at higher water content, in spite of the autoaccelerative effect of water on grafting, it is the instantaneous swelling of the grafted monofilament that dominates the grafting reaction. Beyond 20% water content, the swelling of the PP matrix decreases to an extent that would reduce the monomer accessibility within the matrix for smooth propagation to continue, thereby lowering the graft levels. It is important to mention that the degree of grafting is low and the bar on all points in Fig. 2 10 DEGREE OF GRAFTING (%)
DEGREE OF GRAFTING (%)
6 MEK DMF METHANOL
4
2
0
8
6 4
Dose 45 kGy [t] 8h [T] 60 °C [M] 80%
2 0
0
20
40
60
80
100
MONOMER CONCENTRATION (%)
Fig. 1. Variation in the degree of grafting with monomer concentration in different solvents. Preirradiation dose, 45 kGy; temperature, 60 1C; time, 10 h.
0
20
40
60
80
100
WATER IN DMF-WATER MIXTURE (%)
Fig. 2. Variation in the degree of grafting with water content in DMF–water mixture. Preirradiation dose, 45 kGy; monomer concentration, 80%; temperature, 60 1C; time, 10 h.
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represents the low level of variation, in the range of 2–4% of the grafted component. The addition of sulfuric acid from 0.01 to 0.25 mol/l to the reaction medium leads to acceleration in the degree of grafting (Fig. 3) A sharp increase in the grafting with the increase in acid concentration was observed. This is evident from an increase in graft level by 140% for sulfuric acid concentration of 0.05 mol/l. Such a behavior has been observed earlier by different workers for the grafting of styrene and acrylic acid onto PE films (Gupta and Chapiro, 1989; Ang et al., 1983; Choi and Nho, 2000). It is important to mention that grafting reaction is free from homopolymerization up to 0.05 mol/l sulfuric acid concentration, beyond which homopolymerization originates in the medium (Table 1). The higher the sulfuric acid concentration, the higher the homopolymerization. We have observed that the homopolymerization is initiated early in the medium with the increasing sulfuric acid concentration. This is evident from the fact that homopolymerization at 0.14 mol/l sulfuric acid concentration starts at 4 h, while at 0.36 mol/l the homopolymerization is initiated within a minute. The quick leveling off in the grafting beyond
PERCENT INCREASE IN GRAFTING
200
150
Homopolymer
100
50
Dose [t] [T]
45 kGy 8h 60 °C
0.05 mol/l sulfuric acid concentration may be attributed to the extensive homopolymerization that follows in the reaction medium. This not only depletes the availability of the monomer to the grafting sites, but also enhances the viscosity of the medium to a level where the diffusion of monomer across the viscous medium diminishes fast and reduces the propagation step. These results certainly provide a window where high graft levels may be achieved without any homopolymerization proceeding during the reaction. Several authors have attributed accelerative effect to the partitioning of the monomer within the polymer backbone in the presence of acid, which enhances the monomer supply to the propagating chains. It may also be possible that the presence of acid favors decomposition of hydroperoxides and regulates the grafting. The extensive homopolymerization in the presence of acid further suggests the formation of a large amount of OH which is involved in homopolymerization. However, proper understanding of the possible reason for such an effect still needs a more elaborate investigation. The variation in the degree of grafting with the time for different monomer concentrations is presented in Fig. 4. The grafting increases with the reaction time and reaches equilibrium within 10 h. The higher the monomer concentration, the higher the degree of grafting. This is due to the higher availability of monomer at the grafting sites with increasing monomer concentration. Both the equilibrium degree of grafting (at 24 h) and the initial rates of grafting were observed to increase with the monomer concentration. The log–log plot of the rate of grafting vs. monomer concentration is presented in Fig. 5. The dependence of the rate of grafting on monomer concentration was found to be 1.04. The equation may therefore be represented as follows: Rg / ½M1:04 .
0 0
0.05
0.1
0.15
0.2
0.25
SULFURIC ACID CONCETRATION (mole/l)
Fig. 3. Variation in the degree of grafting with sulfuric acid concentration. Perirradiation dose, 45 kGy; monomer concentration, 80%; temperature 60 1C; time, 10 h; reaction medium, DMF–water.
The first-order dependence on monomer concentration indicates that the grafting proceeds without monomer wastage via side reactions. Since homopolymerization does not take place during the grafting, the medium does not undergo viscosity variation which facilitates the proper monomer accessibility to the growing chains. The
Table 1 Variation of homopolymer yield with the acid concentration Acid content (mol/l) 103
Percent increase in grafting
Homopolymer (%) (8 h)
Observation
7 14 29 50 140 220
73 93 106 140 153 173
— — — 0.2 4.5 5.7
No homopolymer No homopolymer No homopolymer Homopolymer formation in 2 h Homopolymer formation in 4 h Homopolymer formation in 4 h
Reaction conditions: dose, 45 kGy; time, 8 h; temperature, 60 1C; monomer concentration, 80%.
ARTICLE IN PRESS B. Gupta et al. / Radiation Physics and Chemistry 75 (2006) 161–167 10
10% 20% 40% 60% 80%
8
DEGREE OF GRAFTING (%)
DEGREE OF GRAFTING (%)
10
6
4 2
4
8
12
16
18
24
28
10 kGy 30 kGy 45 kGy
8 6
4 2 0
0 0
0
4
8
12
Fig. 4. Variation in the degree of grafting with time for different monomer concentrations. DMF solvent; preirradiation dose, 45 kGy; water in DMF–water mixture, 20%; temperature, 60 1C.
10
1.4
1.6
1.8
2.0
- 0.4 Rg ∝ 1.04
DEGREE OF GRAFTING (%)
Log Rate of Grafting [%/h]
0.4
1.2
20
24
28
Fig. 6. Variation in the degree of grafting with time for different preirradiation doses. DMF solvent; monomer concentration, 80%; water in DMF–water mixture, 20%; temperature, 60 1C.
Log [M]
1.0
16
TIME (h)
TIME (h)
0
165
80°C 70°C 60°C 50°C
8
4
2
- 0.8
0 0 - 1.2
Fig. 5. Log–log plot of rate of grafting vs. monomer concentration.
higher dependence of the order 1.7 and 1.86 of the rate of grafting on monomer concentration has been observed in the polypropylene-g-polyacrylamide systems (Gupta et al., 2000). This behavior of higher dependence was explained in terms of the faster termination of growing chains by transfer to impurities, solvent fragment and other constituents in the reaction medium. It seems that in the present system, the grafting of acrylonitrile on PP proceeds smoothly with efficient monomer consumption in the propagating step. The side reactions and chains’ transfer to different constituents in the reaction medium is very significant. The influence of preirradiation dose on the degree of grafting is presented in Fig. 6. For all doses, the grafting increases with time and reaches saturation in 10 h. Higher graft yields are obtained with the increasing preirradiation dose, due to the formation of higher amount of hydroperoxide groups within the polymer
4
8
12
16
18
24
28
TIME (h)
Fig. 7. Variation in the degree of grafting with time for different reaction temperatures. DMF solvent; preirradiation dose, 45 kGy; monomer concentration, 80%; water in DMF–water mixture, 20%.
matrix. These results are similar to an earlier study on grafting using pure DMF as the reaction medium (Lope˘rgolo et al., 2000). The reaction temperature has profound influence on the degree of grafting. As the grafting reaches saturation within 10 h, the equilibrium grafting at 24 h as a function of the temperature in the range of 50–80 1C is presented in Fig. 7. It can be seen from the results that both the degree of grafting and the initial rate of grafting increase with the increase in the reaction temperature. In our earlier investigation, similar behavior has been observed for grafting of acrylonitrile on PP fiber (Plessier et al., 1998). It seems that at higher temperature, the diffusion of monomer within the PP matrix is accelerated and is reflected in the better accessibility of monomer to the growing chains. These results are different from the
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the additives play an important role in graft regulation. While DMF and MEK produce higher graft levels, methanol led to the lower grafting. The addition of water in DMF further enhances the degree of grafting. A significant enhancement of degree of grafting is observed for the addition of sulfuric acid to the reaction medium. Therefore, both the water and sulfuric acid addition may be used favorably to achieve higher graft levels. An increase in preirradiation dose and temperature led to a higher rate of grafting and equilibrium graft levels.
DEGREE OF GRAFTING (%)
8
6
4 Dose [t] [T]
2
0
40
50
60
45 kGy 10 h 60° C
70
CRYSTALLINITY
Fig. 8. Variation in the degree of grafting with crystallinity; preirradiation dose, 45 kGy, monomer concentration, 80%; water in DMF–water mixture, 20%; time, 10 h; temperature, 60 1C.
grafting of monomers onto other hydrocarbon polymers such as polyethylene where the initial rate of grafting tends to decrease with the increase in the reaction temperature (Gupta et al., 2000). This was attributed to the enhanced deactivation of primary radicals within the polyethylene matrix and faster termination of growing chains in spite of the better diffusion of monomer within the polymer matrix. It seems that in the present system of acrylonitrile grafting onto PP monofilament, the growing chain termination is not influenced much and grafting proceeds smoothly with higher diffusion and more availability of monomer to the growing chains. Faster termination of primary radicals may still be there but this is overshadowed by the positive effect of the diffusivity of the monomer within the polymer matrix. The variation in degree of grafting with crystallinity is presented in Fig. 8. It has been observed that grafting increases with an increase in the crystallinity. It may be because more free radicals are available for the grafting reaction within the fiber. The radiation leads to the generation of free radicals both in crystalline and amorphous region. In case of less crystalline monofilament, the mobility of the chains is high which results in the deactivation of the radicals due to recombination during irradiation (Plessier et al., 1998).
7. Conclusion The present study was carried out with a view to modify PP monofilament by radiation-induced graft polymerization of acrylonitrile. The irradiation and reaction conditions influence the degree of grafting significantly. The increase in monomer concentration up to 80% increases the grafting beyond which the graft level decreases. The nature of the solvent medium and
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ARTICLE IN PRESS B. Gupta et al. / Radiation Physics and Chemistry 75 (2006) 161–167 Hutmacher, D.W., 2000. Scaffolds in tissue engineering bone and cartilage. Biomaterials 21, 2529–2543. Izumi, Y., Nagaike, H., Tabuse, S., Yoshida, Y., Tagawa, S., 2001. Radiation grafting of styrene onto polyethylene. Radiat. Phys. Chem. 62, 83–88. Kim, Y.H., Sun, G.J., 2001. Durable antimicrobial finishing of nylon fabrics with acid dyes and a quaternary ammonium salt. Textile Res. J. 71, 318–323. Langer, R., 2000. Biomaterials: status, challenges and perspectives. AIChE J. 46, 1286–1289. Langer, R., 2001. Drug delivery: drugs on target. Science 293, 58–59. Lin, J., Winnkekman, C., Worley, S.D., Broughton, R.M., Williams, J.F., 2001. Antimicrobial treatment of nylon. J. Appl. Polym. Sci. 81, 943–947. Lope˘rgolo, L.C., Catalani, L.H., Machado, L.D.B., Reta, P.R., Luga˜o, A.B., 2000. Development of reinforced hydrogels-I. Radiation-induced graft copolymerization of methylmethacrylate on nonwoven polypropylene fabric. Radiat. Phys. Chem. 57, 451–454. Nakashima, T., Sakagami, Y., Ito, H., Matsuo, M., 2001. Antimicrobial activity of cellulose fabrics modified with metallic salts. Textile Res. J. 71, 688–694. Park, J.S., Kim, H.J., Nho, Y.C., Kwon, O.H., 1998. Antibacterial activities of acrylic acid grafted polypropylene fabric and its metallic salts. J. Appl. Polym. Sci. 69, 2213–2220. Plessier, C., Gupta, B., Chapiro, A., 1998. Modification of polypropylene fiber by radiation-induced graft copolymer-
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Radiation Physics and Chemistry 75 (2006) 161–167 www.elsevier.com/locate/radphyschem
Preirradiation grafting of acrylonitrile onto polypropylene monofilament for biomedical applications: I. Influence of synthesis conditions Bhuvanesh Guptaa,, Rachna Jaina,b, Nishat Anjuma, Harpal Singhb a
Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Centre for Biomedical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
b
Received 21 July 2004; accepted 12 April 2005
Abstract Graft polymerization of acrylonitrile onto polypropylene (PP) monofilament was carried out by a preirradiation method using a 60Co gamma radiation source. The influence of synthesis conditions, such as preirradiation dose, reaction time, monomer concentration, reaction temperature and additives was determined. The grafting was considerably influenced by the instantaneous swelling of the monofilament in the reaction mixture during the course of the grafting process. The order of dependence of the rate of grafting on monomer concentration was found to be 1.04. The nature of the medium of the grafting and the additives had profound influence over the grafting reaction. The accelerative effects of solvent medium on the grafting were higher in methylethyl ketone (MEK) and dimethylformamide (DMF) as compared to methanol. At the same time, partial replacement of DMF with water led to acceleration in the grafting with peak maxima at 20% solvent composition. The addition of a small amount of sulfuric acid to the reaction mixture also resulted in a significant acceleration of the degree of grafting. r 2005 Elsevier Ltd. All rights reserved. Keywords: Polypropylene; Acrylonitrile; Suture; Radiation; Graft polymerization
1. Introduction Polymers have generated considerable interest as biomaterials in medical science and biotechnology. The innovative application domains of biomaterials are surgical sutures, medical implants and bioreceptive interfaces/scaffolds for tissue engineering (Langer, 2000, 2001; Gupta et al., 2002; Hutmacher, 2000; Tessmar et al., 2000). With increasing demands for a
Corresponding author.
E-mail address: [email protected] (B. Gupta).
biomaterial with better acceptability and functionality to the biosystem, stress has been focussed on the development of newer materials. One of the ways to develop such materials is to modify existing polymers and design them keeping in view their specific application areas. Sutures are used in surgical operations and require optimum physico-chemical characteristics. However, medical reports show that local infections brought after surgery are very frequent. In these situations, surgical sutures with antibacterial properties provide a promising solution. Biocidal materials and antibacterial finishing of textiles have been reviewed by many authors (Kim and Sun, 2001; Chung et al., 1998; Nakashima et al.,
0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.04.003
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2001; Stevanato and Tedesco, 1998; Rahbaran, 1999; Lin et al., 2001). Polypropylene (PP) is one of the widely used biostable sutures due to its optimum tensile strength and low level of tissue reaction as compared to other sutures. However, the microbial infection on the suture site often has been observed to lead to the deterioration of the wound and related complications especially where post-surgical care is not well taken up. Therefore, the modification of PP sutures may be carried out in such a way that it acquires functional groups where a drug may be immobilized. This drug is released from the suture once in contact with the biosystem and provides antimicrobial action. The polymer modification by radiation-induced graft polymerization of monomers into polymers has attracted attention to induce desirable properties in a material (Choi and Nho, 1999; Gupta et al., 1994, 1996, 2000; Plessier et al., 1998; Yang et al., 2003; Dessouki and Taher, 1998; Izumi et al., 2001; Lope˘rgolo et al., 2000; Bucio and Burillo, 1996). The radiation-induced graft polymerization of monomers such as 2-hydroxyethylmethacrylate (HEMA), methacrylic acid, acrylamide and N-vinylpyrrolidone onto PP has been reported to introduce hydrogel characteristics in the polymer for biomedical applications (Singh and Tyagi, 1989; Park et al., 1998; Rao and Rao, 1987; Singh and Ray, 1994). In our previous work, we carried out the grafting of hydroxyethylmethacrylate onto PP monofilament to immobilize 8-hydroxyquinoline as the antimicrobial drug (Tyagi et al., 1990; Gupta et al., 1993). However, the grafting led to considerable homopolymer formation that remained occluded within the polymer matrix and its complete separation from the suture matrix could not be achieved due to the hydrogel nature of the poly-HEMA. Thereby the suture characteristics were greatly deteriorated. Moreover, the inherent incompatibility of the grafted ionic component with the nonionic PP matrix added a loss in tensile strength. In order to overcome this problem, we have carried out the grafting of nonionic monomer, acrylonitrile, onto the PP monofilament using preirradiation where homopolymerization is very little and its separation from the grafted PP matrix would be much easier as compared to hydrogel poly-HEMA. Moreover, polyacrylonitrile grafts, because of their nonionic nature, would provide better compatibility of polyacrylonitrile grafts with the PP base matrix. This may lead to better retention in tensile properties. The grafted PP monofilament was subsequently hydrolyzed to get carboxyl groups for subsequent antimicrobial drug immobilization. In the present study, the graft polymerization of acrylonitrile onto PP monofilament is carried out by a preirradiation method to develop sutures with different graft levels. The influence of the reaction conditions on the degree of grafting has been investigated.
2. Materials PP used for this study was manufactured by IPCL, India. The monofilament was prepared by melt spinning of PP at 230 1C under nitrogen atmosphere. The monofilament was collected on bobbins and was further drawn to a 1:5 ratio unless otherwise mentioned. The filament had a diameter of 0.32 mm and a denier of 760 gpd. Acrylonitrile monomer was received from GS Chemicals, India and was purified by distillation under vacuum. Sulfuric acid, dimethylformamide (DMF), methanol (MeOH) and methylethyl ketone (MEK) obtained from GS Chemical India Ltd. was used as received. Distilled water was used for all experiments.
3. Irradiation PP monofilaments were exposed to g-rays from a 60Co source (900 Curies) in the presence of air. The dose rate of radiation was 0.27 kGy/h. After the irradiation, samples were kept at 4 1C prior to the grafting reaction.
4. Grafting procedure Grafting was carried out in glass ampoules of 2 10 cm2 size with a B-24 socket. A weighed amount of preirradiated monofilament (appprox. 0.04–0.06 g) was placed in ampoules containing monomer and the solvent. Nitrogen was purged into the ampoule to remove air trapped inside the reaction mixture and the ampoule was subsequently placed in a water bath maintained at the constant temperature. After a desired period, the ampoule was removed and soxhlet extracted with DMF to remove any homopoylmer adhering to the sample surface. The samples were dried in an oven at 50 1C under vacuum and the degree of grafting was obtained from the following equation: Wg Wo 100, Wo where Wo and Wg are the weight of the original and grafted samples, respectively. Degree of grafting ð% ¼Þ
5. Swelling measurement The swellings of the PP filaments in various solvents were determined by placing a known amount of them in a specific medium for 6 h. Samples were subsequently taken out and the surface solution was wiped out using a Whatmann filter paper. The samples were weighed and swelling was obtained as percent increase in the weight of the samples.
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6. Results and discussion The radiation-induced graft polymerization of acrylonitrile onto PP monofilament was carried out to investigate the influence of synthesis conditions on the degree of grafting. The influence of additives, solvents, monomer concentration, preirradiation dose and reaction temperature on the degree of grafting was studied. The variation in degree of grafting with the monomer concentration in DMF solvent is presented in Fig. 1. The grafting increases with the increase in the monomer concentration up to 80% and then decreases abruptly. This trend is related to the swelling behavior of PP monofilament in the grafting reaction and subsequent availability of the monomer to the grafting sites. As long as DMF (a solvent for the polyacrylonitrile (PAN) grafts) is present in the grafting reaction, the polyacrylonitrile grafted PP layers swell in the medium and regulate the monomer diffusion through the swollen layer to the grafting sites (Plessier et al., 1998). As a result, the propagation of growing chains proceeds smoothly. In the absence of DMF, a lower degree of grafting was observed (as evident from 100% monomer concentration). This is because acrylonitrile is a nonsolvent for PAN chains, which diminishes the swelling of the grafted monofilament; hence, diffusion of the monomer through grafted zone is reduced and leads to low graft levels. It is, therefore, the instantaneous swelling of the monofilament that influences the grafting at a specific monomer–solvent composition. This is evident from the fact that the PP monofilament with 4.2% degree of grafting exhibits swellings of 5.9% and 3.5% at monomer concentrations of 20% and 80% acrylonitrile in DMF as solvent as compared to 1.6% in pure monomer indicating a more amenable matrix for the grafting in the presence of DMF as solvent.
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It is important to note in Fig. 1 that all solvents, DMF, methylethyl ketone and methanol, show identical trends in the grafting reaction and exhibit maxima at 20% solvent in a monomer–solvent mixture. However, methanol produces the lowest graft levels. The PP matrix has negligible swelling in methanol and hence the monomer diffusion within the monofilament matrix is restricted leading to low level grafting in spite of the accelerative effect of the methanol. It may be therefore stated that the solubility parameter plays a key role in swelling. PP has a solubility parameter of 9.4, which is close to that of MEK (9.3) as compared to both DMF (12.1) and methanol (14.5). This would account for the higher swelling of the PP matrix in MEK than other solvents and is refluxed in the higher degree of grafting. The partial replacement of DMF with water has been observed to show an accelerative effect on the degree of grafting. The influence of DMF–water composition on the grafting is presented in Fig. 2. The degree of grafting increases with the increase in the water content up to 20% water concentration and thereafter tends to decrease. The initial increase in the grafting may be probably due to coiling up of the growing PAN chains in the presence of water, which acts as nonsolvent for the polyacrylonitrile grafts. The coiling may lead to the decrease in the rate of termination of growing polyacrylonitrile chains and hence the propagation process continues smoothly. It seems that at higher water content, in spite of the autoaccelerative effect of water on grafting, it is the instantaneous swelling of the grafted monofilament that dominates the grafting reaction. Beyond 20% water content, the swelling of the PP matrix decreases to an extent that would reduce the monomer accessibility within the matrix for smooth propagation to continue, thereby lowering the graft levels. It is important to mention that the degree of grafting is low and the bar on all points in Fig. 2 10 DEGREE OF GRAFTING (%)
DEGREE OF GRAFTING (%)
6 MEK DMF METHANOL
4
2
0
8
6 4
Dose 45 kGy [t] 8h [T] 60 °C [M] 80%
2 0
0
20
40
60
80
100
MONOMER CONCENTRATION (%)
Fig. 1. Variation in the degree of grafting with monomer concentration in different solvents. Preirradiation dose, 45 kGy; temperature, 60 1C; time, 10 h.
0
20
40
60
80
100
WATER IN DMF-WATER MIXTURE (%)
Fig. 2. Variation in the degree of grafting with water content in DMF–water mixture. Preirradiation dose, 45 kGy; monomer concentration, 80%; temperature, 60 1C; time, 10 h.
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represents the low level of variation, in the range of 2–4% of the grafted component. The addition of sulfuric acid from 0.01 to 0.25 mol/l to the reaction medium leads to acceleration in the degree of grafting (Fig. 3) A sharp increase in the grafting with the increase in acid concentration was observed. This is evident from an increase in graft level by 140% for sulfuric acid concentration of 0.05 mol/l. Such a behavior has been observed earlier by different workers for the grafting of styrene and acrylic acid onto PE films (Gupta and Chapiro, 1989; Ang et al., 1983; Choi and Nho, 2000). It is important to mention that grafting reaction is free from homopolymerization up to 0.05 mol/l sulfuric acid concentration, beyond which homopolymerization originates in the medium (Table 1). The higher the sulfuric acid concentration, the higher the homopolymerization. We have observed that the homopolymerization is initiated early in the medium with the increasing sulfuric acid concentration. This is evident from the fact that homopolymerization at 0.14 mol/l sulfuric acid concentration starts at 4 h, while at 0.36 mol/l the homopolymerization is initiated within a minute. The quick leveling off in the grafting beyond
PERCENT INCREASE IN GRAFTING
200
150
Homopolymer
100
50
Dose [t] [T]
45 kGy 8h 60 °C
0.05 mol/l sulfuric acid concentration may be attributed to the extensive homopolymerization that follows in the reaction medium. This not only depletes the availability of the monomer to the grafting sites, but also enhances the viscosity of the medium to a level where the diffusion of monomer across the viscous medium diminishes fast and reduces the propagation step. These results certainly provide a window where high graft levels may be achieved without any homopolymerization proceeding during the reaction. Several authors have attributed accelerative effect to the partitioning of the monomer within the polymer backbone in the presence of acid, which enhances the monomer supply to the propagating chains. It may also be possible that the presence of acid favors decomposition of hydroperoxides and regulates the grafting. The extensive homopolymerization in the presence of acid further suggests the formation of a large amount of OH which is involved in homopolymerization. However, proper understanding of the possible reason for such an effect still needs a more elaborate investigation. The variation in the degree of grafting with the time for different monomer concentrations is presented in Fig. 4. The grafting increases with the reaction time and reaches equilibrium within 10 h. The higher the monomer concentration, the higher the degree of grafting. This is due to the higher availability of monomer at the grafting sites with increasing monomer concentration. Both the equilibrium degree of grafting (at 24 h) and the initial rates of grafting were observed to increase with the monomer concentration. The log–log plot of the rate of grafting vs. monomer concentration is presented in Fig. 5. The dependence of the rate of grafting on monomer concentration was found to be 1.04. The equation may therefore be represented as follows: Rg / ½M1:04 .
0 0
0.05
0.1
0.15
0.2
0.25
SULFURIC ACID CONCETRATION (mole/l)
Fig. 3. Variation in the degree of grafting with sulfuric acid concentration. Perirradiation dose, 45 kGy; monomer concentration, 80%; temperature 60 1C; time, 10 h; reaction medium, DMF–water.
The first-order dependence on monomer concentration indicates that the grafting proceeds without monomer wastage via side reactions. Since homopolymerization does not take place during the grafting, the medium does not undergo viscosity variation which facilitates the proper monomer accessibility to the growing chains. The
Table 1 Variation of homopolymer yield with the acid concentration Acid content (mol/l) 103
Percent increase in grafting
Homopolymer (%) (8 h)
Observation
7 14 29 50 140 220
73 93 106 140 153 173
— — — 0.2 4.5 5.7
No homopolymer No homopolymer No homopolymer Homopolymer formation in 2 h Homopolymer formation in 4 h Homopolymer formation in 4 h
Reaction conditions: dose, 45 kGy; time, 8 h; temperature, 60 1C; monomer concentration, 80%.
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10% 20% 40% 60% 80%
8
DEGREE OF GRAFTING (%)
DEGREE OF GRAFTING (%)
10
6
4 2
4
8
12
16
18
24
28
10 kGy 30 kGy 45 kGy
8 6
4 2 0
0 0
0
4
8
12
Fig. 4. Variation in the degree of grafting with time for different monomer concentrations. DMF solvent; preirradiation dose, 45 kGy; water in DMF–water mixture, 20%; temperature, 60 1C.
10
1.4
1.6
1.8
2.0
- 0.4 Rg ∝ 1.04
DEGREE OF GRAFTING (%)
Log Rate of Grafting [%/h]
0.4
1.2
20
24
28
Fig. 6. Variation in the degree of grafting with time for different preirradiation doses. DMF solvent; monomer concentration, 80%; water in DMF–water mixture, 20%; temperature, 60 1C.
Log [M]
1.0
16
TIME (h)
TIME (h)
0
165
80°C 70°C 60°C 50°C
8
4
2
- 0.8
0 0 - 1.2
Fig. 5. Log–log plot of rate of grafting vs. monomer concentration.
higher dependence of the order 1.7 and 1.86 of the rate of grafting on monomer concentration has been observed in the polypropylene-g-polyacrylamide systems (Gupta et al., 2000). This behavior of higher dependence was explained in terms of the faster termination of growing chains by transfer to impurities, solvent fragment and other constituents in the reaction medium. It seems that in the present system, the grafting of acrylonitrile on PP proceeds smoothly with efficient monomer consumption in the propagating step. The side reactions and chains’ transfer to different constituents in the reaction medium is very significant. The influence of preirradiation dose on the degree of grafting is presented in Fig. 6. For all doses, the grafting increases with time and reaches saturation in 10 h. Higher graft yields are obtained with the increasing preirradiation dose, due to the formation of higher amount of hydroperoxide groups within the polymer
4
8
12
16
18
24
28
TIME (h)
Fig. 7. Variation in the degree of grafting with time for different reaction temperatures. DMF solvent; preirradiation dose, 45 kGy; monomer concentration, 80%; water in DMF–water mixture, 20%.
matrix. These results are similar to an earlier study on grafting using pure DMF as the reaction medium (Lope˘rgolo et al., 2000). The reaction temperature has profound influence on the degree of grafting. As the grafting reaches saturation within 10 h, the equilibrium grafting at 24 h as a function of the temperature in the range of 50–80 1C is presented in Fig. 7. It can be seen from the results that both the degree of grafting and the initial rate of grafting increase with the increase in the reaction temperature. In our earlier investigation, similar behavior has been observed for grafting of acrylonitrile on PP fiber (Plessier et al., 1998). It seems that at higher temperature, the diffusion of monomer within the PP matrix is accelerated and is reflected in the better accessibility of monomer to the growing chains. These results are different from the
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the additives play an important role in graft regulation. While DMF and MEK produce higher graft levels, methanol led to the lower grafting. The addition of water in DMF further enhances the degree of grafting. A significant enhancement of degree of grafting is observed for the addition of sulfuric acid to the reaction medium. Therefore, both the water and sulfuric acid addition may be used favorably to achieve higher graft levels. An increase in preirradiation dose and temperature led to a higher rate of grafting and equilibrium graft levels.
DEGREE OF GRAFTING (%)
8
6
4 Dose [t] [T]
2
0
40
50
60
45 kGy 10 h 60° C
70
CRYSTALLINITY
Fig. 8. Variation in the degree of grafting with crystallinity; preirradiation dose, 45 kGy, monomer concentration, 80%; water in DMF–water mixture, 20%; time, 10 h; temperature, 60 1C.
grafting of monomers onto other hydrocarbon polymers such as polyethylene where the initial rate of grafting tends to decrease with the increase in the reaction temperature (Gupta et al., 2000). This was attributed to the enhanced deactivation of primary radicals within the polyethylene matrix and faster termination of growing chains in spite of the better diffusion of monomer within the polymer matrix. It seems that in the present system of acrylonitrile grafting onto PP monofilament, the growing chain termination is not influenced much and grafting proceeds smoothly with higher diffusion and more availability of monomer to the growing chains. Faster termination of primary radicals may still be there but this is overshadowed by the positive effect of the diffusivity of the monomer within the polymer matrix. The variation in degree of grafting with crystallinity is presented in Fig. 8. It has been observed that grafting increases with an increase in the crystallinity. It may be because more free radicals are available for the grafting reaction within the fiber. The radiation leads to the generation of free radicals both in crystalline and amorphous region. In case of less crystalline monofilament, the mobility of the chains is high which results in the deactivation of the radicals due to recombination during irradiation (Plessier et al., 1998).
7. Conclusion The present study was carried out with a view to modify PP monofilament by radiation-induced graft polymerization of acrylonitrile. The irradiation and reaction conditions influence the degree of grafting significantly. The increase in monomer concentration up to 80% increases the grafting beyond which the graft level decreases. The nature of the solvent medium and
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