CLAY CATALYZED SYNTHESIS OF BIO-DEGRADABLE POLY(GLYCOLIC ACID)

Transcription

1 Chinese Journal of Polymer Science Vol. 26, No. 4, (2008), Chinese Journal of Polymer Science 2008 World Scientific CLAY CATALYZED SYNTHESIS OF BIO-DEGRADABLE POLY(GLYCOLIC ACID) K. Durai Murugan a, S. Radhika a, I. Baskaran a and R. Anbarasan b* a Department of Chemistry, S.N. College, Madurai , Tamil Nadu, India b Advanced Nanotechnology Research Laboratory, Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar , Tamil Nadu, India Abstract Glycolic acid was polymerized under vacuum in the presence and absence of nano sized clay. The added clay catalyzed the condensation polymerization which can be confirmed by recording FTIR spectroscopy and intrinsic viscosity (IV) values. The relative intensity of C=O/CH is increased while increasing the amount of clay. DSC showed the appearance of multiple endotherms of poly(glycolic acid). TGA showed the percentage weight residue remain above 750 C for polymer-nano composite system was 21% and hence proved the flame retardancy (char forming) nature. TEM confirmed the nano size of the clay used to catalyze the condensation reaction. The intrinsic viscosity value was increased with the increase of percentage weight of Hectorite type clay. Keywords: Synthesis; Poly(glycolic acid); Clay; FTIR; DSC; TGA; Intrinsic viscosity. INTRODUCTION Poly(glycolic acid) (PGA) is one of most simplest bio-degradable polymer and is widely used in medical and pharmaceutical sectors because of its excellent performance with very high bio-degradability. Such an environment friendly polymer has been synthesized normally by condensation polymerization with the help of esterification catalyst [1]. In order to achieve high molecular weight, high thermal and chemical stability with fantastic electrical conductivity PGA has been synthesized under different experimental conditions such as in the presence of ionic liquid [2]. The ionic liquids used here are acting as a reaction medium as well as co-catalyst. Regular honey comb porous structure and high humidity PGA can be obtained by copolymerization procedure, Lactic acid is a co-monomer. Improved degradation and bio-activity of glycolic acid copolymer has reported by Loher and research team [3]. Recently, the polymer chemists turned their attention towards the synthesis of polymer in the presence of nano sized materials resulted in polymer-nanocomposites [4 6]. Nano composites are materials in which more than 80 percent by weight or by volume of the dispersed phase should be in the dimension less than 100 nm. The uniform dispersion of nano sized materials on the polymer matrix is said to be polymer-nano composites. Such a polymer-nanocomposites have been reported with improved crystallization behavior [7] thermal stability [8], mechanical properties [9] and electrical conductivity [10]. Last year one article was published for the production of nano sized poly(acrylic acid) (PAA) through template method and the size of PAA is confirmed by TEM [11]. In the present communication, we are going to report about the synthesis of PGA in the presence of natural clay and characterized by FTIR, DSC and TGA techniques. Novelty of the present investigation is to study the effect of nano clay on the structure property relationship of PGA via FTIR approach which could be supported by intrinsic viscosity measurements. * Corresponding author: R. Anbarasan, anbu_may3@yahoo.co.in Received May 23, 2007 ; Revised August 2, 2007; Accepted August 20, 2007

2 394 K.D. Murugan et al. EXPERIMENTAL Materials and Characterizations Glycolic acid (Ottokemi, India) was used as such as received. Sb 2 O 3 (Ottokemi, India) was taken as a polycondensation catalyst. Hectorite type clay [Na 0.6 (Li 0.6 Mg 54 )Si 8 O 28 Cl 4 ], Na + is an interchangeable cation, collected from KCET campus and purified by alkali and acid wash and finally by distilled water. The presence of acid or alkali in the clay as excessive was checked by ph determination. The final washing should show the ph of 7.1 and was used as nano material. Grafting of PGA onto clay may not be a feasible reaction because of charge neutralization process. Clay plays a vital role during the condensation reaction through its surface catalytic activity. During the in situ condensation polymerization reaction, there is a chance for the formation of PGA-Clay nano composites. FTIR spectrum was obtained by using Shimadzu 8400 S FTIR spectrophotometer in the transmission mode from 400 to 4000 cm 1. 7 mg of the solid samples were pelletized by KBr pelletization method and FTIR measurements were carried out. The peak area (relative intensity) due to carbonyl stretching and C H in-plane bending vibration can be measured by FTIR software after proper baseline correction. Relative intensity (RI) = A 1745 /A The thermo gravimetric analysis of both the pristine polymer and nano composites samples were performed under air atmosphere using STA 409PC simultaneous TGA and DSC Analyzer, TA instrument. The weight losses of the samples are measured from 30 C to 800 C at the heating rate of 10 K/min. DSC analysis was performed under nitrogen atmosphere at the heating rate of 10 K/min in the range of 30 C to 300 C by using Universal V42E TA instrument. PGA samples were melted at 160 C and made into a thin film. Isothermal crystallization behavior of the same is under investigation. Nano size of the Clay was confirmed by recording TEM by using JEM 3010 instrument, a product of JEOL. The intrinsic viscosity of the samples was determined through viscosity method by dissolving the samples in DMSO solvent after filtration. Synthesis of Poly(glycolic acid) (PGA) and its Nano Composites PGA has been synthesized by simple condensation method. The two way necked 250 ml round bottom flask (RBF) was charged with 10 ml of glycolic acid, required quantity of clay, 50 ml of water and 30 mg of Sb 2 O 3, the condensation catalyst, and mixed well for half an hour by using a magnetic stirrer under nitrogen atmosphere at 70 C. The two necks were used as nitrogen inlet and outlet respectively. During this mixing time, glycolic acid may intercalate into the nano clay and leads to intercalation/delamination/exfoliation process. Then, the flask was fitted with a reflux condenser and the temperature of the bath was slowly raised to 110 C at the heating rate of 10 K/min. Water molecules liberated from the condensation reaction were collected in a measuring jar and measured for every 10 min. After half an hour the temperature was raised again to 120 C and maintained till the completion of the condensation reaction under the vacuum of Pa. When there was no liberation of water molecules from RBF, the flask was removed from the oil bath and the contents were carefully transferred to a Petri dish and placed in an air oven maintained at 110 C for 48 h for drying. After 48 h of drying, polymer and polymer nano composite samples were collected and stored in a zipper bag. The same procedure was followed for the synthesis of PGA in the absence of nano clay. RESULTS AND DISCUSSION Recently, the nano sized materials play a vital role in the polymer science and engineering field because of (i) increase of flame retardancy of the polymer, (ii) crystallinity of the polymer is increased through nucleation process, (iii) increase of mechanical property of the polymer, (iv) increase of electrical conductivity of the polymer and (v) catalytic activity of nano material. In the present investigation the catalytic activity of nano sized clay was tested and quantitatively proved by FTIR spectroscopy and viscosity measurements. FTIR Study PGA synthesized in the presence and absence of clay was characterized by FTIR spectroscopy (Fig. 1). The important peaks are mentioned below: The carbonyl stretching vibration of PGA appeared at 1745 cm 1. The chain end OH stretching vibration of PGA can be seen at 3421 cm 1 with the C H symmetric and anti-

3 Clay Catalyzed Synthesis of Bio-degradable Poly(glycolic acid) 395 symmetric stretching appearing at 2933 and 2962 cm 1 respectively. Peaks appear at 1427 and 1093 cm 1 are due to CH 2 bending vibration and C H in-plane deformation vibration cm 1 peak is responsible for C O C stretching vibration. For polymer nano composites small peaks appeared between 600 and 800 cm 1. This confirms the presence of metal-oxide stretching vibration in PGA nano composites (due to clay). Clay catalyzes the condensation reaction of GA. During condensation reaction of GA in the presence of clay, there is a chance for intercalation of GA into the basal spacing of clay through thermal driving force. Hence, the appearance of small peaks in the lower wave number confirms the presence of nano sized clay in the polymer nano composite. Moreover, their thermal and molecular weight also changed. Hence, we come to a conclusion that during the in situ polymerization of glycolic acid there will a formation of nano composite between the added clay and GA. For the sake of comparison, the FTIR spectrum of clay and clay catalyzed PGA is represented in Fig. 2(a) and 2(b) respectively. Peaks corresponding to clay in the PGA-clay system may be suppressed. Figure 2(a) infers that appearance of peaks between 400 cm 1 and 1100 cm 1 may be associated with the M OH stretching vibration of clay. A sharp peak appeared at 1650 cm 1 is due to the bending vibration of water molecules. A broad peak at 3400 cm 1 is predicted to the OH stretching and a small shoulder appear around 3600 cm 1 is due to the inter molecular hydrogen bonding. Fig. 1 FTIR spectrum of PGA and its nano composites Fig. 2 FTIR spectrum of (a) clay and (b) PGA-0.03 g clay End group analysis method is a conventional tool to estimate the functional group concentration with some tedious process. After analysis, the polymer solution should be discord and result in environmental pollution due to solvent. Now, we are in a position to select a method for the estimation of functional group without an environmental pollution (i.e) environmental friendly method. Recently, the FTIR spectroscopy is used for the quantitative calculation. In the present investigation our aim is to prove the catalytic activity of nano sized clay during the condensation reaction. In FTIR spectroscopy, the basic principle is absorbance area of the peak is directly proportional to the concentration of the substance. Based on that we have measured the peak area (relative intensity) of C=O (1745 cm 1 ) and C H (in plane) (1093 cm 1 ) stretching vibrations because the backbone of PGA is made up by C=O group and CH 2 group and their ratio (C=O/CH) was also determined. Interestingly it was found that the ratio of peak area (i.e) (RI C =O/C H) increased with the increase of amount of percentage weight of clay. This confirms that during the condensation polymerization more and more ester groups are chemically linked with each other. Figure 3 shows the plot of log(percentage weight of clay) versus log(ri C =O/C H), with a slope value of This authenticates the 0.50 order dependence of RI C =O/C H on percentage weight of clay. This supports the catalytic activity of clay which can be further supported by measuring the intrinsic viscosity (IV) value.

4 396 K.D. Murugan et al. Fig. 3 Effect of percentage weight of clay on relative intensity of PGA [GA] = 0.10 mol/l; Weight of Sb 2 O 3 = g; Time = 2 h; Temperature = 150 C Normally Sb 2 O 3 is used as a condensation catalyst. Sb 2 O 3 is not an environmentally green chemical. Inorder to avoid or to reduce the usage of such a hazardous chemical the condensation reaction is carried out with the help of 0.03 g clay in the absence of Sb 2 O 3. Here also we are getting PGA but its RI C =O/CH and IV values are lower. The RI C =O/CH value of Sb 2 O 3 catalyzed PGA is whereas the clay catalyzed PGA shows Hence, we decided to carryout the condensation of GA in the presence of mixture of catalysts (i.e) combination of both Sb 2 O 3 and clay. And we tested the effect of percentage weight of clay on RI C =O/CH and IV values of PGA. The molecular weight of PGA can be indirectly studied by measuring the RI C =O/CH while changing the percentage weight of clay. If the value of RI C =O/CH is high one can come to a conclusion that more amount of ester group is linked with each other resulting in high molecular weight PGA. TGA Profile Thermal stability of PGA and its nano composite was tested by TGA method. Figure 4(a) showed the TGA of PGA, the homopolymer. The thermogram showed the two step degradation process. Up to 150 C it showed slow weight loss due to the removal of moisture and physisorbed water molecules. The second step is the major weight loss step due to the degradation of PGA backbone. The maximum degradation occurred at 350 C and the degradation extended up to 750 C. This can be confirmed by seeing DTA of PGA (Fig. 4b). The percentage of weight residue remain above 750 C was approximately 11%. Fig. 4 Effect of percentage weight of clay on thermal property of PGA a) TGA of PGA; b) DTA of PGA TGA of PGA with 5 percentage weight of clay is shown in Fig. 5(a). Here also, one can observe the same weight loss steps. The major weight loss occurred at 350 C and the degradation still extended to 700 C. The

5 Clay Catalyzed Synthesis of Bio-degradable Poly(glycolic acid) 397 percentage of weight residue remained above 750 C was approximately 21%. The thermal stability of PGA-5% weight of clay system can be further evidenced by DTA (Fig. 5b). The percentage weight residue remained above 750 C confirmed the flame retarding nature of the possible polymer-nano composites. Hence we claimed the formation of PGA-natural clay nano composites during the in situ condensation of glycolic acid and natural clay. Fig. 5 Effect of percentage weight of clay on thermal property of PGA-nano composite a) TGA of PGA-5 % weight of clay; b) DTA of PGA-5% weight of clay Differential Scanning Calorimetry Figure 6 showed the DSC (heating scan) of PGA. DSC showed the multiple endotherms for the homopolymer, PGA. The first endothermic peak due to melting of PGA appeared at 145 C whereas the second appeared at C. Appearance of multiple endotherms for aliphatic & aromatic polyester was already discussed in the literature [12]. Appearance of multiple endothermic peaks was due to the following reasons: (i) difference in molecular weight, (ii) may be due to the existence of different stable form of PGA (α and β form), (iii) reorganization of micro structure of PGA during heating process. DSC of PGA-5% weight of clay system is shown in Fig. 7. Here also T m appeared in the same temperature as mentioned for PGA with multiple endotherms. Because of clear vision and discussion, we have separately presented the DSC thermogram of the samples. Fig. 6 DSC of PGA Fig. 7 DSC of PGA-5% weight of clay TEM Nano size of the clay can be confirmed by producing HR-TEM photograph (Fig. 8). Figure 8 showed the clay size of nm with layered structure. During the condensation polymerization there is a chance for the formation of PGA-natural clay nano composites through exfoliation or de-lamination or intercalation reactions. But photograph produced in Fig. 8 confirmed the nano size of natural clay alone. Further study regarding the PGA-natural clay nano composites are going on in our Advanced Nanotechnology Research Laboratory (ANTRL).

6 398 K.D. Murugan et al. Fig. 8 HR-TEM of pure clay Intrinsic Viscosity Table 1 showed the intrinsic viscosity value of PGA and its posssible nano composites. Homo PGA showed lower intrinsic viscosity value when compared with its possible nano composites. Another one interesting observation was noted. The intrinsic viscosity values were increased while increasing the amount of clay. The increase in intrinsic viscosity values of PGA-natural clay nano composites strongly supports the catalytic activity of clay in condensation polymerization of glycolic acid. Table 1. Intrinsic viscosity values Sample no. System Intrinsic viscosity (dl/g) PGA + Sb 2 O 3 PGA + Clay PGA-Sb 2 O 3-1% clay PGA-Sb 2 O 3-2% clay PGA-Sb 2 O 3-3% clay PGA-Sb 2 O 3-4% clay PGA-Sb 2 O 3-5% clay CONCLUSIONS Natural clay catalyzed the condensation polymerization of glycolic acid. The FTIR spectrum confirmed the increase in peak area ratio of carbonyl stretching and C H bending vibration of PGA. The catalytic activity of natural clay was further confirmed by measuring the intrinsic viscosity of polymer-nano composites. The percentage weight residue remain above 750 C evidenced the char forming nature of polymer-nano composites. Multiple endotherm appeared for the PGA, homo polymer and its nano composites. TEM confirmed the nano size of the natural hectorite type clay alone. REFERENCES 1 Dali, S., Lefebre, H. and Gharbi, R.E., J. Polym. Sci. Part A: Chem. Ed., 2006, 44: Tian, Y., Ding, H., Jiao, Q. and Wang, X., J. Appl. Polym. Sci., 2006, 100: Loher, S., Reboul, V., Brunner, T.J. and Stark, J., Nanotech., 2006, 17: Son, Y., Lee, J., Kim, J. and Lee, Y., Mol. Cryst. Liq. Cryst., 2006, 444: Harrane, S., Meghabar, R. and Belbachin, M., Design. Mono. Polym., 2005, 8: 11 6 Kickelbick, G., Prog. Polym. Sci., 2003, 28: 83 7 Zengin, H., Zhou, W. and Jin. J., Adv. Mater., 2002, 14: Park, S.J., Jun, J.R. and Suh, D.H., J. Korean Ind. Engg. Chem., 2003, 14: Chang, J.H., Kim, S.J. and Im, S.S., Polymer, 2004, 45: Manoratne, C.H. and Dissanayake, L., Int. Electrochem. Sci., 2006, 1: Wang, G., Cai, F., Si, L., Wang, Z. and Duan, X., Chem. Let., 2005, 34: Thakor, N., Trivedi, G. and Patel, K.C., Indian J. Biotech., 2006, 5: 137