J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/jmbbm

Research paper

Synthesis of polylactic acid–polyglycolic acid blends usingmicrowave radiation

Anurag Pandey, Girish C. Pandey, Pranesh B. Aswath∗

Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019, United States

A R T I C L E I N F O

Article history:

Received 21 April 2007

Received in revised form

5 December 2007

Accepted 9 December 2007

Published online 15 December 2007

Presented at the 2007 TMS Annual

Meeting Symposium on “Biological

Materials Science”, February

25–March 1 2007, Orlando, Florida.

Keywords:

Microwave

Biodegradable polymer blends

FTIR

NMR

PLLA

PLGA

A B S T R A C T

Degradation rates of a copolymeric PLGA can be controlled by varying the constituent

amount in the copolymer. In the present study we have made an attempt to utilize

microwave irradiation to blend PLLA and PGA in different concentrations. FTIR, NMR and

DSC measurements clearly show the blending and cross-linking between the constituents.c© 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Research in bioresorbable polymers has received increasedattention in recent years because of their wide applicationsin environmental and clinical medicine (e.g., drug delivery,dental, orthopedic applications) (Sodergard and Stolt, 2002;Amass et al., 1998; Vainiopaa et al., 1989; Zhang et al.,1994; Kost and Langer, 1992). The most popular and

∗ Corresponding address: Materials Science and Engineering DepartmArlington, TX 76019, United States. Tel.: +1 817 272 7108; fax: +1 817 2

E-mail address: aswath@uta.edu (P.B. Aswath).

1751-6161/$ - see front matter c© 2007 Elsevier Ltd. All rights reserveddoi:10.1016/j.jmbbm.2007.12.001

important biodegradable polymers are aliphatic polyesters,such as Poly-L-lactic acid (PLLA), polycaprolactone (PCL),and polyglycolic acid (PGA). Aliphatic polyesters are usedin tissue fixation (i.e., bone screws, bone plates, and pins),drug delivery systems (i.e., diffusion control), wound dressing(i.e., artificial skin), and wound closure (i.e., sutures andsurgical staples) (Shikinami and Okuno, 1999; Pitt et al.,1979; Vert et al., 1994). Bone screws, bone plates and pin

ent, 500 West First Street, Rm 325, University of Texas at Arlington,72 2538.

.

228 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3

Fig. 1 – (a) Esterification of L-lactic Acid to Poly-L-lacticAcid (b) Esterification of glycolic Acid to polyglycolic Acid.

structures made of polylactic acid (PLA) and Poly-DL-lactic-co-glycolic acid (PLAGA) are being used and are likely toreplacemetal implants in the near future (Agrawal et al., 1999;Lu et al., 2000). These bioabsorbable products have severaladvantages overmetal implants: (1) no stress-shielding effect,(2) no need for removal after surgery, and (3) no metalliccorrosion (Kawahara et al., 1980; Rice et al., 1978; Pilliar,2003; Harris and Tarr, 1978). However, each of these hassome shortcomings that restrict its applications. The majorlimitation of PGA is its hydrolytic instability resulting invery fast degradation. Many co-monomers have been usedin an attempt to increase the hydrolytic stability, but havealways produced materials with inferior properties comparedto the homopolymers (Gilding and Reed, 1979). PLLA besidesits long degradation time is usually hard and brittle, whichhinders its usage in medical applications like orthopedicand dental applications (Matsusue et al., 1997; Bergsmaet al., 1995). Some modifications, such as the additionof plasticizers or surfactants/compatibalizers, are usuallyrequired to improve its original properties. Poly-DL-lacticacids (PDLLA) can degrade quickly due to their amorphousstructure, thus shortening the degradation time but havepoor mechanical properties (Tsuji, 2002, 2000). Blendingtechniques are an extremely promising approach that canimprove the original properties of the polymers (Chen et al.,2003). Tsuji showed that in a well-stereo-complexed 1:1 blendand non-blended films prepared from PLLA and PDLA, thedegradation rate and tensile strength of the blends is muchhigher than the non-blended PDLA film (Tsuji, 2000). Biresawet al. in their recent study, blended a polystyrene with a seriesof biodegradable polymers to study the effect of interfacialenergy on the mechanical properties of the blend (Biresawand Carriere, 2004). Kadla et al. recently reported that theviscoelastic properties of lignin can be altered throughchemical modification or polymer blending (Kadla andKubo, 2004). However, to date there has been studies usingmicrowaves to synthesize blends of the PGA and PLLA, bytaking advantage of their microwave absorption capacity.This approach offers the possibility of developing clean, highpurity blends at rates much faster than traditional blendingmethods.

In the present study we have made an attempt tosynthesize blends of conventionally immiscible polymers,one having high mechanical strength and longer degradationtime with one that has poor mechanical strength and shorterdegradation time, in order to optimize the mechanical andhydrolytic stability of the blend which could be tailoredto a specific application. Microwave irradiation provides aneffective, selective, and fast synthetic method by heatingthe molecules directly through the interaction between themicrowave energy and molecular dipole moments of thematerial. This internal heating is believed to produce anefficient reaction because the reactive sites, which havestrong dipole moments, are the primary source of activationin the microwave electromagnetic field (Adachi et al., 2005;Pandey, 2003; Hayes, 2002). Microwave irradiation was usedto blend PLLA and PGA under different concentration ratiosin order to obtain the most compatible blend.

2. Materials and experimental procedure

2.1. Materials

The polyglycolic acid (PGA) (TLF 6267) with average molecularweight of 600 Daltons was obtained from Dupont. Poly-L-lactic acid (PLLA) (MW > 500 KD); Chloroform (99.95%) wasobtained from EM Science and was used as obtained withoutfurther purification or processing. The structures of PGA andPLLA are provided in Fig. 1(a,b).

2.2. Apparatus

The apparatus used for blending was a Panasonic domesticmicrowave oven (2450 MHz, 1300 W) with different powerlevels from ∼130 W to 1300 W without any modifications; allreactions were carried out in a hood with strong ventilation.

2.3. Measurements

PLLA and PGA and the blends obtained after the microwavetreatment were finely ground under liquid nitrogen andthen stored in a desiccator over night. Approximately 1 gof powdered sample was mixed with 10 g of KBr andmixed thoroughly and compacted into a solid 5 mm pellet.These pellets were then placed in a Bruker Vector 30 FTIRwith diffuse reflectance setup. Backgroundmeasurement wasmade using pure KBr pellet and then sample measurementwas made. Measurements were at 2 cm−1 resolution for 256scans.

Differential Scanning Calorimetry (DSC) data for polymerblends were conducted on Perkin Elmer DSC 7 in N2atmosphere at a rate of 5 ◦C/min. 1H-NMR was performedon a Bruker AM 400 MHz apparatus using tetramethylsilane(TMS) in CDCl3 at 25 ◦C as an internal standard.

2.4. Polymer blending

The PLLA/PGA blends were prepared by the followingprocedure. PLLA was dissolved in chloroform to obtain10% w/v solutions. Finely powdered PGA particles were

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3 229

Table 1 – Blend Composition % w/w

I 100% PLLAII 67% PLLA

33% PGAIII 50% PLLA

50% PGAIV 33% PLLA

67% PGAV 100% PGA

separately suspended in chloroform to obtain a 10% w/vsuspension. The two solutions were mixed together indifferent ratios to obtain a wide range of blend compositionas listed in Table 1. The solutions were subjected to ∼260 Wpower microwave irradiation for a predetermined amount oftime (25 min). Chloroform being a solvent that has a lowdielectric constant at 2450 MHz is not a good absorber ofmicrowave and hence provides a cooling effect by drainingthe heat from the microwave absorbing PLLA and PGA, and inthe process evaporated rapidly. Solid PLLA/PGA blends wereobtained having good structural stability, were cryogenicallyground in liquid N2 for DSC, NMR and FTIR measurements.

3. Results

3.1. IR spectra of pure polymers

The IR spectral features of both the pure polymers beforeand after microwave treatments did not show any significantdifference (Fig. 2(a) and (b)). The typical characteristic featuresobserved in IR spectra of both the polymers are reportedbelow; The IR spectra showed characteristic bands mainlydue to methylene and carboxylic >C=O bonds, in the caseof PGA (V), typically around 2960 cm−1 (γ-CH), 1430 cm−1 (δ-CH), 1182 cm−1, 1102 cm−1 (γ-C–O), and group of bands inthe region 1000–800 cm−1 possibly due to a mix of vibrationalmodes of [–C–C–]n repeat units and C–Htwist. The strong band(doublet) around 1727 cm−1 is a characteristic of [C=O] group.In the case of PLLA (I), on the other hand, due to methylsubstitution, the IR spectral features showed considerableshift in the corresponding frequency range at 2994 cm−1 (γ-CH) and 1465 cm−1 (characteristic of methyl group δ-CH),1229, 1147 cm−1 (characteristic of –C–O stretching esters). Inaddition a significant band shift in band due to C=O group isalso seen (1764 cm−1) as a result of methyl substitution in thepolymer backbone.

3.2. IR spectra of blends

Blends of the three polymer composition ratios [PLLA, PGA]were selected for the present study; 67 w/o PLLA (II), 50 w/oPLLA (III) and 33 w/o PLLA (IV). Fig. 3(a) and (b) shows theIR spectrum of pure PLLA (I), pure PGA (V) and the threeblends. The IR spectral analysis of all the compositionsas expected showed typical bands arising out of –C–H,(both methyl and methylene) >C=O, [C–O] ester bands. Thespectral features were analyzed in light of band shifts orappearance/disappearance of characteristic bands.

Fig. 2 – (a): FTIR spectrum of PLLA before and aftermicrowave treatment. A: FTIR spectrum of PLLA withoutmicrowave irradiation B: FTIR spectrum of PLLA aftermicrowave irradiation. (b): FTIR spectrum of PGA before andafter microwave treatment. A: FTIR spectrum of PGAwithout microwave irradiation B: FTIR spectrum of PGAafter microwave irradiation.

(a) γ-C–H region: As expected significant shifts/changes inthree compositions [(II), (III), (IV)] were observed. The doublets∼2994/2960 due to the γ-C–H vibration of methyl/methylenegroups can be observed. These doublets are either not presentor not resolved in the case of parent pure polymers.

(b) δ-C–H region: This region as expected did show shiftin the frequency 1465 cm−1 and 1430 cm−1 to around1458/1418 cm−1 in all the compositions indicating some sortof linkage between the two polymers as a result of blending.However a band ∼1583 cm−1 becomes significant in thecase of blend III (50:50 compositions). This band possiblymay be assigned to polyester linkage formed as a result ofesterification of the terminal hydroxyl of one of the polymersupon blending.

(c) C=O region: The spectral characteristics of the C=Obond indicate that the intensities of the doublet aroundI1780 > I1720 in the parent polymers tend to equalize uponblending. A significant positive shift of band near 1720 cm−1

to a higher frequency at 1740 cm−1 is also observed among allthe blends. This is also supported by the fact the carboxylicgroups of the repeat units of the two polymers may beforming linkages, which becomes significant in the cases ofblends II and III (67% and 50% of PLLA), but due to lowerconcentration of PLLA blend IV (33%) this band shift is not

230 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3

Fig. 3 – (a): FTIR spectrum of PLLA, PGA and PLLG/PGABlends from 2700 cm−1 to 3300 cm−1. I: FTIR spectrum of100% PLLA after microwave irradiation. II: FTIR spectrum of67% PLLA 33% PGA after microwave irradiation. III: FTIRspectrum of 50% PLLA 50% PGA after microwaveirradiation. IV: FTIR spectrum of 33% PLLA 67% PGA aftermicrowave irradiation. V: FTIR spectrum of 100% PGA aftermicrowave irradiation. (b): FTIR spectrum of PLLA, PGA andPLLG/PGA Blends from 700 cm−1 to 2000 cm−1. I: FTIRspectrum of 100% PLLA after microwave irradiation. II: FTIRspectrum of 67% PLLA 33% PGA after microwaveirradiation. III: FTIR spectrum of 50% PLLA 50% PGA aftermicrowave irradiation. IV: FTIR spectrum of 33% PLLA 67%PGA after microwave irradiation. V: FTIR spectrum of 100%PGA after microwave irradiation.

resolved i.e. they tend tomerge resulting in only one relativelybroad band.

The above observations indicate a strong possibilityof linkages similar to those expected from cross-linkingbetween the two polymers as a result of treatment withmicrowave radiations. The various spectral features of theblends indicate a good compatibility between the twopolymers of which 50:50 (III) composition ratios appear to bethe most compatible.

Fig. 4 – DSC of PLLA–PGA Blend I: DSC of 100% PLLA aftermicrowave irradiation. II: DSC of 67% PLLA 33% PGA aftermicrowave irradiation. III: DSC of 50% PLLA 50% PGA aftermicrowave irradiation. IV: DSC of 33% PLLA 67% PGA aftermicrowave irradiation. V: DSC of 100% PGA after microwaveirradiation.

Fig. 5 – 1H NMR spectrum of high molecular weight PLLA.

3.3. Differential scanning calorimetry (DSC)

For comparison purposes, the DSC results of all the blendsand those of the component polymers are shown in Fig. 4.Themelting peaks (Tm) of component polymer, PLLA and PGAare observed at 164 ◦C and 203 ◦C respectively. In the cases ofblends II and IV two melting peaks are observed at 156 ◦C and203 ◦C for blend II and 164 ◦C and 210 ◦C for blend IV that aresomewhat shifted from those of component polymers. In thecase of blend III (50:50) onemajor Tm is observed with aminorendotherm at 205 ◦C.

3.4. Proton nuclear magnetic resonance (1H NMR)

Proton NMR spectra of PLLA and the three blends wererecorded with a view to see if any copolymerization betweenthe two polymers was occurring or not. 1H NMR ChemicalShifts (δ, in ppm) of CH, CH2 and CH3 protons were usedas markers for identifying chemical interaction between thetwo polymer components. Presented in Figs. 5 and 6(a)–(c) arethe proton NMR spectra of PLLA and its 3 blends with PGAprepared (PLLA:PGA = 67:33, 50:50 and 33:67 by weight) after25 min of microwave treatment respectively.

1H NMR Chemical Shifts at δ = 5.2 ppm (peak E)correspond to methylene (CH) shifts in the polymer main

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3 231

Fig. 6 – (a): 1H NMR spectrum of 33:67 PLLA–PGA blendprepared by microwave treatment. (b): 1H NMR spectrum of50:50 PLLA–PGA blend prepared by microwave treatment.(c): 1H NMR spectrum of 67:33 PLLA–PGA blend prepared bymicrowave treatment.

chain. Chemical shift due to the methyl group present inPLLA backbone can be observed as a strong signal aroundδ = 1.52 ppm. This is a split signal arising due to CH3 grouppresent as backbone (peak C) and as terminal ones presentat the polymer end chain (peak A, B). Very low intensity ofpeak A and B compared to peak C suggests high molecularweight of PLLA. Comparing the spectrum of PLLA with thethree blends under study, a similar spectrum was obtainedexcept the presence of a peak at δ = 4.8 ppm (peak D). Thispeak can be attributed to the CH2 unit of PGA in the polymerblend/copolymer. In view of the negligible to no solubility ofPGA in deuterated chloroform (CDCl3), this signal could notbe attributed to the parent PGA polymer component. Thissignal at δ = 4.8 ppm (peak D) corresponding to CH2 canonly arise if the dissolved polymer blend has CH2 units inthe polymer main chain suggesting formation of PLGA typepolymer.

4. Discussion

Esters are very important organic molecules in both chemicaland pharmaceutical industries. Esterification of carboxylicacid, alkalization of carboxylate ion and trans-esterificationare three methods for ester synthesis. With conventionalheating, the conditions used are harsh e.g. use of catalysts,strong mineral acids etc. and can take anywhere from 2 hto 2 days, however in the case of microwave treatment thereaction times tend to reduce to 3–30 min. (Adachi et al., 2005;Pandey, 2003; Hayes, 2002). Extending this observation, we

decided to explore a novel approach for blending of polyesterbased polymers (PLLA and PGA) using microwave, which alsohave terminal –OH and a repeat polymer moiety containingC=O linkage, both known to be good absorbers of microwaveradiations (Hayes, 2002). As result, the two polymers maybe modified possibly via (a) esterification where H of theterminal O–H of one of the polymers is replaced by a secondpolymer, or (b) copolymerization or (c) cross-linking, leadingto a homogeneous composition of the new product in all thecases. Among the three options the last one (c) seems to themost probable one, however the role of trans-esterificationmechanism (a) cannot be excluded. This is clearly seen in thecase of the most compatible composition range (viz; 50:50).This observation is further supported from the IR/NMR/DSCdata detailed earlier. It may be mentioned that chloroform,(a low microwave absorber) was used as the common solventfor PLLA and PGA, which apart from acting as mixing media,also acted as the heat sink and helped to maintain a uniformreaction temperature.

FTIR data have been used to establish the compatibilityof the two component polymers and the concentrationratio of the most compatible blend, DSC and OpticalMicroscopy results confirm the results. FTIR spectroscopyhas been widely used by organic chemists to characterizesynthesized compounds (NOËL, 1994). In addition, it hasbecome a powerful tool for investigation of the three-dimensional structure of various bimolecular entities suchas proteins (Byler et al., 1984), and nucleic acids (Sclaviet al., 1994). With the help of technological advancementin hardware, FTIR can offer attractive applications in thefields of cell biology and medicine. Investigation of polymerstructure (Kang et al., 2001), coatings (Pandey et al., 1998) anddetermination of physical and physico-mechanical propertiesof polymers’ polymeric composites (Ignjatovic et al., 2001) andpolymeric blends using FTIR have been established in therecent past (Pandey et al., 2002a,b).

Both the polymers, PLLA and PGA are formed as a result ofpolyesterification leading to an ester group within the repeatunit, alcohol group (–OH) and a carboxylic group as a terminalend group as shown in Fig. 1(a, b). The main (structural)difference between the two (Fig. 1a and b) polymeric unitsis due to the substitution of one of the hydrogen (–H) ofmethylene (–CH2) group of glycolic acid (Fig. 1b) by a methylgroup (–CH3) in the case of L-lactic acid (Fig. 1a). Thisdifference is expected to be clearly visible in the IR spectrum,not only due to (–CH3/CH2) groups but on the neighboringcarboxylic group specially >C=O due to the inductive effectof –CH3 group. In order to establish that interaction of theparent molecules of PLLA and PGA with the microwaveradiation takes place, an initial study examining the roles ofmicrowaves in these polymers is shown in Fig. 2a and b andthe results clearly indicate that no structural changes occurin the parent polymers on microwave irradiation.

On blending the PLLA and PGA in various ratios usingchloroform (poor microwave absorber and acts as a coolingsystem) as a solvent and subjecting the mixtures tomicrowave irradiation, it is clear from the FTIR spectrashown in Fig. 3 that the δC–H, γC–H and C=O regions of theblends show significant variance after microwave irradiationcompared to the parent polymers. The results indicate a

232 J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3

direct linkage between the carboxylic acid groups of the PGAand PLLA in the blends indicating the formation of a chemicallinkage between the two molecules as opposed to just aphysical blend. The composition with 50:50 PGA and PLLAindicated the greatest extent of linkage.

The IR results are supported by the DSC data. Differentialscanning calorimetry of the parent polymers and the blendsindicate that the 50:50 blend of PGA and PLLA is themost compatible blend. While blends II and IV exhibit twosignificant peaks associated with individual melting of PGAand PLLA blend III, that is 50:50 (PGA:PLLA) has one majorpeak and one minor peak indicating that the bulk of thepolymer is melting at one temperature. This observationfurther supports the fact that blend III appears to be themost compatible composition. It may be mentioned herethat structurally the blends may be similar to the copolymeras suggested by Ajioka et al. (1998), and the Tm’s reportedfor the copolymer are 145 ◦C and 135 ◦C depending onthe polymerization process and hence the polymerizationsequence. On the other hand the observed Tm for the mostcompatible blend (III) is 161 ◦C. The increase in Tm couldpossibly be due to associated cross-linking type of linkagesupon microwave radiation.

Proton NMR studies of the parent polymers and theblends were also conducted after microwave irradiation. Thepresence of the CH2 peak (peak D in Fig. 6(a–c)) at d = 4.8 ppmindicates a linkage between the two polymers yielding acopolymer. Comparison of the spectra of the three blendsshows that the intensities of CH2 peak at δ = 4.8 ppm in allthe three blends are variable, the maximum being that for50:50 composition. This observation corresponds well withthose of DSC and FTIR spectroscopic data, and very clearlysupports the hypothesis that among the three blends themost compatible one is a 50:50 mixture of PGA and PLLA. Thisfurther indicates that the microwave assisted synthesis is anovel method for the synthesis of blends and copolymers ofPGA and PLLA.

5. Conclusions

The present method gives a new method of synthesisof copolymeric blends of PLLA and PGA using microwaveactive/absorbing reactants suspended in an inactive solvent.The copolymer thus prepared is believed to be a blockcopolymer and the block size is dependent on the molecularweight of the starting polymer systems. Microwave treatmentof polymers offers the possibility of tailoring the propertiesof the copolymer by cross-linking or esterification withoutsignificantly affecting the physical properties such ascrystallinity which sometimes are of great importance for invivo applications.

Acknowledgements

The authors graciously acknowledge the support from theMaterials Science and Engineering Department and theChemistry and Biochemistry Department at the University ofTexas at Arlington.

R E F E R E N C E S

Adachi, K., Iwamura, T., Chujo, Y., 2005. Microwave assistedsynthesis of organic-inorganic polymer hybrids. Polymer Bull.55 (5), 309–315.

Agrawal, C.M., Parr, J.E., Lin, S., Synthetic bioabsorbable polymersfor implants, ASTM Special Technical Publication, Vol 1396,Nov 16-17 1999, Kansas City, MO, United States, p. 175.

Ajioka, M., Suizu, H., Higuchi, C., Kashima, T., 1998. Aliphaticpolyesters and their copolymers synthesized through directcondensation polymerization. Polymer Degradation and Sta-bility 59, 137–143.

Amass, W., Amass, A., Tighe, B., 1998. Review of biodegrad-able polymers: Uses, current developments in the synthesisand characterization of biodegradable polyesters, blends ofbiodegradable polymers and recent advances in biodegrada-tion studies. Polym. Int. 47, 89.

Bergsma, J.E. (Univ Hospital Groningen), de Bruijn, W.C.,Rozema, F.R., Bos, R.R.M., Boering, G., 1995. Late degradationtissue response to poly(L-lactide) bone plates and screws.Biomaterials 16, 25–31.

Biresaw, G., Carriere, C.J., 2004. Compatibility and mechanicalproperties of blends of polystyrene with biodegradablepolyesters. Composites Part A: Appl. Sci. Manufacturing 35 (3),313–320.

Byler, M.D., Susi, H., Gerasimowicz,W.V., 1984. Vibrational spectra,assignments and valence force field for s-nitrocysteine andisotropic analogs. Appl. Spectroscopy 38 (2), 200–203.

Chen, C.-C., Chueh, J.-Y., Tseng, H,, Huang, H.-M., Lee, S.-Y.,2003. Preparation and characterization of biodegradable PLApolymeric blends. Biomaterials 24, 1167–1173.

Gilding, D.K., Reed, A.M., 1979. Biodegradable polymers for usein surgery polyglycolic/polylactic acid homo- and copolymer.Polymer 20, 1459–1464.

Harris, L.J., Tarr, R.R., 1978. Implant failures in orthopedic surgery.Biomaterials, Medical Devices, and Artificial Organs 7 (2),243–255.

Hayes, B.L., 2002. Microwave Synthesis; Chemistry at the speed oflight. CEM Publishing, NC, ISBN: 0972222901, 30.

Ignjatovic, N., Savic, V., Najman, S., Plavsic, M., Uskokovic, D.,2001. A study of HAp/PLLA composite as a substitute for bonepowder, using FT-IR spectroscopy. Biomaterials 22, 571.

Kadla, J.F., Kubo, S., 2004. Lignin-based polymer blends: Analysisof intermolecular interactions in lignin-synthetic polymerblends. Composites Part A: Appl. Sci. Manufacturing 35,395–400.

Kang, S.H., Shaw, L., Stidham, H.D., Smith, P.B., Yang, X.,Anne, L.M., 2001. A spectroscopic analysis of poly(lactic acid)structure. Macromolecules 34, 4542.

Kawahara, H., Hirabayashi, M., Shikita, T., 1980. Single crystalalumina for dental implants and bone screws. J. Biomed.Mater. Res. 15 (5), 597–605.

Kost, J., Langer, R., 1992. Responsive polymer systems forcontrolled delivery of therapeutics. Trends Biotechnol. 10 (4),127–131.

Lu, L., Peter, S.J., Lyman, M.D., Lai, H.-L., Leite, S.M., Tamada, J.A.,Vacanti, J.P., Langer, R., Mikos, A.G., 2000. In vitro degradationof porous poly(L-lactic acid) foams. Biomaterials 21 (15),1595–1605.

Matsusue, Y. (Kyoto Univ), Nakamura, T., Iida, H., Shimizu,K., 1997. Long-term clinical study on drawn poly-L-lactideimplants in orthopaedic surgery. J. Long-Term Effects Med.Implants 7, 119.

NOËL, P.G.R., 1994. A Guide to the Complete Interpretationof Infrared Spectra of Organic Structures. John Wiley, UK,ISBN: 0471939986.

Pandey, G.C., 2003 Application of Microwave for synthesis ofzeolites, Internal Report.

J O U R N A L O F T H E M E C H A N I C A L B E H AV I O R O F B I O M E D I C A L M A T E R I A L S 1 ( 2 0 0 8 ) 2 2 7 – 2 3 3 233

Pandey, G.C., Kumar, A., Boccara, A.C., Fournier, D., 1998.Nondestructive evaluation of polymer coatings bymirage-FTIRspectroscopy: A tool for surface characterization. J. Appl. Poly.Sci. 67, 1249–1252.

Pandey, G.C., Kumar, A., Garg, R.K., 2002a. Nondestructiveevaluation of physico-mechanical properties of polypropylenecopolymer by infra-red spectroscopy. European Poly. J. 38,745–750.

Pandey, G.C., Kumar, A., Garg, R.K., 2002b. A novel approachfor the determination of the physicomechanical properties ofpolymers by vibrational spectroscopy: The tensile strength ofpolybutadiene rubber’. J. Appl. Poly. Sci. 82, 2135–2139.

Pilliar, R.M., Metals and orthopaedic implants — Past successes,present limitations, future challenges, Medical Device Materi-als in: Proceedings of the Materials and Processes for MedicalDevices Conference 2003, Anaheim, CA, United States, ISBN:0871707985 ASM International, pp. 8–22.

Pitt, C.G., Jeffcoat, A.R., Zweidinger, R.A., Schindler, A., 1979.Sustained drug delivery systems: 1. The permeability ofPoly (epsilon-Caprolactone), poly(DL-lactic acid), and theircopolymers. J. Biomed. Mater. Res. 13 (3), 497–507.

Rice, R.M., Hegyeli, A.F., Gourlay, S.J., Wade, C.W.R., Dillon, J.G.,Jaffe, H., Kulkarni, R.K., 1978. Biocompatibility testing ofpolymers: In Vitro studies with in-vivo correlation. J. Biomed.Mater. Res. 12 (1), 43–54.

Sclavi, B., Peticolas, W.L., Powell, J.W., 1994. Fractal-like patternsin DNA films, B form at 0% relative humidity, andantiheteronomous DNA: An IR study. Biopolymers 34 (8),1105–1113.

Shikinami, Y., Okuno, M., 1999. Bioresorbable devices made offorged composites of hydroxyapatite (HA) particles and poly-L-lactide (PLLA): Part I. Basic characteristics. Biomaterials 20,859.

Sodergard, A., Stolt, M., 2002. Properties of lactic acid basedpolymers and their correlation with composition. Prog. Polym.Sci. 27, 1123.

Tsuji, H., 2000. In vitro hydrolysis of blends from enantiomericpoly(lactide)s. Part 1. Well-stereo-complexed blend and non-blended films. Polymer 41, 3621–3630.

Tsuji, H., 2002. Autocatalytic hydrolysis of amorphous-madepolylactides: Effects of L-lactide content, tacticity, andenantiomeric polymer blending. Polymer 43, 1789–1796.

Vainiopaa, S., Rokkanen, P., Tormala, P., 1989. Surgical applicationsof biodegradable polymers in human tissues. Prog. Polym. Sci.14, 679.

Vert, M., Mauduit, J., Li, S., 1994. Biodegradation of PLA/GA poly-mers: Increasing complexity. Biomaterials 15 (15), 1209–1213.

Zhang, X., Wyss, U.P., Pichora, D., Goosen, M.F.A., 1994.Investigation of poly(lactic acid) degradation. Bioact. Compos.Polym. 9, 80.