Effect of hydrolytic degradation and dehydration on the microstructure of 50:50 poly(glycolide-co-D,L-lactide)

Polymer International 45 (1998) 313È320

Effect of Hydrolytic Degradation andDehydration on the Microstructure of50 : 50 Poly(glycolide-co-D,L-lactide)

Elizabeth King & Ruth E. Cameron*¤

Department of Materials Science and Metallurgy, University of Cambridge, New Museums Site, Pembroke Street, Cambridge,CB2 3QZ, UK

(Received 27 August 1997 ; accepted 10 October 1997)

Abstract : The e†ect of hydrolytic degradation on the microstructure ofunoriented, random 50 : 50 poly(glycolic acid-co-D,L-lactic acid) was examinedusing simultaneous small and wide angle X-ray scattering (SAXS/WAXS) anddi†erential scanning calorimetry (DSC). Samples were degraded in phosphate-bu†ered saline solution at 37É5¡C and studied wet and after dehydration.

There was no evidence of crystalline material within the sample at any stage ofdegradation or dehydration from either X-ray scattering or thermal analysis.Thus, chain scission does not enable crystallization of the copolymer, and theglycolic acid and lactic acid fragments formed on degradation do not crystallize,even when the samples are dehydrated. Because such fragments are clearlyformed (Hakkarainen, M., Albertsson, A. C. & Karlsson, S., Polym. Deg. Stab., 52(1996) 283), and because they are crystalline in the dry state, it must be assumedeither that these species are not present in any quantity in the degrading sampleand that they di†use easily from the bulk into the surrounding medium, or thatthe bulk polymer prevents them from crystallizing.

SAXS gave evidence of small voids within the structure. Unlike dehydrateddegraded semi-crystalline samples, there is no evidence for voiding on a macro-scopic scale. The number and size of the small voids in the dehydrated samplesrises with degradation. The voids close as samples are heated above the glasstransition temperature and the amorphous chains gain mobility. The glass tran-sition, although clearly visible in the undegraded samples, becomes less visible byDSC on degradation. After 28 daysÏ degradation, there is some evidence that thestructure begins to close up, perhaps as a result of reduced viscosity arising fromthe increased fraction of low molecular weight material. 1998 SCI.(

Polym. Int. 45, 313È320 (1998)

Key words : hydrolytic degradation ; poly(glycolide-co-D,L-lactide) ; amorphous ;voids

INTRODUCTION

Poly(hydroxy acid)s are an important class of biode-gradable polymers for biomedical applications due totheir biocompatibility and their physiologically toler-able degradation products. Poly(glycolic acid), (PGA)

* To whom all correspondence should be addressed.¤ e-mail : rec11=cam.ac.ukContract/grant sponsor : PÐzer Central Research.Contract/grant sponsor : EPSRC.

and poly(lactic acid) (PLA) copolymers have beenmanufactured as absorbable sutures, dental and ortho-paedic implants and drug delivery devices.1

PGA and PLA are linear aliphatic polyesters. Thehydrophilic nature of the ester bond allows the poly-mers to be hydrolytically degraded by body Ñuids. PGAis a semi-crystalline thermoplastic with a high meltingpoint2 of 224È227¡C and a glass transitiontemperature3 of around 37¡C. In contrast, PLA can beeither amorphous or semi-crystalline, depending on thecomposition of D-LA and L-LA stereoisomers.4 PL-LA

3131998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

314 E. King, R. E. Cameron

has a melting point around 190¡C and a glass transitiontemperature1 around 57¡C. Their copolymers, termedgenerically PGLA in this paper, have varying degrees ofcrystallinity, depending on their composition. Therandom 50 : 50 copolymer of PGA with D,L-PLA usedin this study is intrinsically amorphous.1

The hydrolytic degradation of PGLA copolymersoccurs preferentially at the PGA sites, either because ofthe higher hydrophilicity of PGA, or due to easiercleavage of the GAÈGA ester bonds than LAÈLA orGAÈLA ester bonds.4 According to Hakkarainen et al.5the degradation of random copolymers of PGA andPLA occurs in three stages. In the Ðrst stage, the molec-ular weight of the polymer decreases rapidly, but fewsoluble oligomers are formed and there is little weightloss. In stage two, the molecular weight decreases at aslower rate, but the increased production of soluble oli-gomers means that weight loss occurs much morerapidly. The Ðnal stage is marked by total weight loss,when about 50% of the polymer is converted tomonomer. The timescale of these changes varies withboth the copolymer composition and the temperature ofdegradation. Hakkarainen et al.5 found that in pow-dered samples of random 50 : 50 copolymer of PGAwith D,L-PLA, degraded at 37¡C, the Ðrst stage wascomplete after 26 days. After this time, the molecularweight had fallen to 7% of its original value, but thefraction of material converted to soluble oligomers wassmall : the weight loss was only 18%, and only 12% ofthe polymer has been hydrolysed to monomer. Fullweight loss occurred after around 100 days.

Certain copolymers of PGA, PLLA and PDLA,although intrinsically amorphous, have semi-crystallineresidues after degradation. The presence of thesechanges and their extent is controlled by the copolymercomposition. A sample of random 50 : 50 copolymer ofPGA with D,L-PLA is likely to be amorphous through-out the degradation.1

Full characterization of hydrolytically degraded poly-mers usually requires sample dehydration in order toprevent further hydrolytic attack and to allow concisestructural analysis, such as thermal analysis and elec-tron microscopy. In an earlier study,6 we reported thee†ect of dehydration in hydrolytically degraded PGA. Itwas shown that dehydration causes additional structur-al damage, causing the formation of large voids visibleby scanning electron microscopy (SEM). The e†ect ofdehydration on a fully amorphous degraded structuremight be expected to di†er from a semi-crystalline one,where degradation occurs preferentially within theamorphous layers.7

We present a study of the microstructure ofunoriented plates of random 50 : 50 copolymer of PGAwith D,L-PLA during the Ðrst stage of degradation.During this stage, the molecular weight falls sharply,but the formation of monomer and the mass loss isfairly small. Simultaneous small and wide angle X-ray

scattering (SAXS/WAXS) and di†erential scanning ca-lorimetry (DSC) were used to monitor the morpho-logical changes associated with degradation anddehydration. These techniques allow any changes incrystallinity and other inhomogeneities such as voids tobe monitored. Such features are likely to control thebehaviour of the material in service as a controlledrelease matrix or a mechanical support.

EXPERIMENTAL

Materials

Powdered random 50 : 50 poly(glycolic acid-co-D,L-lactic acid) was obtained from Boehringer Ingelheim; itsinherent viscosity was 0É67 dl g~1. Plates 1 cm] 4 cmwere formed by melting 0É8 g copolymer at 230¡C in acopper mould with a PTFE-coated aluminium base,then holding at 160¡C for 5 min before cooling in air toroom temperature. Phosphate-bu†ered saline solutionpH 7É4, from Sigma-Aldrich Company was made upwith distilled water and 1% penicillinÈstreptomycinantibiotic solution, from Sigma-Aldrich Company, wasadded. All the experimental apparatus was autoclavedfor 30 min before use.

Plates were immersed in 50 ml bu†er solution at37¡C. After a predetermined time, the plates wereremoved from the bu†er solution and dried in a vacuumoven at room temperature, at 1 bar for 48 h, then storedin a vacuum desiccator Ðlled with anhydrous calciumsulphate at 4¡C to await analysis.

DSC

A Perkin Elmer DSC 7 was calibrated using sapphire.Approximately 5 mg of material was placed into an alu-minium pan with a crimped lid and heated at10¡Cmin~1 from 0 to 250¡C.

Time resolved SAXS /WAXS

Simultaneous SAXS/WAXS was performed on station8.2 at the SRS Laboratory at Daresbury, UK. The SRSlaboratory allows data to be obtained over short expo-sure times because of the high intensity of the synchro-tron radiation. Ryan et al.8 have described theexperimental technique of SAXS/WAXS used to deter-mine polymer structure.

The WAXS data were obtained using a curved knife-edge detector and the SAXS data were obtained using aquadrant detector located 3É5 m from the sample posi-tion. The SAXS detector was calibrated using wet rattail collagen and the WAXS using high densitypoly(ethylene). The SAXS data were divided by thedetector response found by uniform illumination of thedetector. Both WAXS and SAXS data were correctedfor background scattering by subtracting the scattering

POLYMER INTERNATIONAL VOL. 45, NO. 3, 1998

E†ect of hydrolytic degradation and dehydration 315

from the straight-through beam, and for sample thick-ness and transmission by dividing by the signal from anionization chamber placed directly behind the sample.A constant “liquid scatterÏ term was subtracted from theSAXS data. The “tacÏ hole, an electronic function of thedata collection, was removed.

Samples were each exposed in the beam for 30 s atroom temperature. Dynamic X-ray proÐles wereobtained by placing the samples into the beam in aDSC heating stage and heating at 10¡Cmin~1 from 0 to250¡C. Data were binned at 30 s intervals.

Analysis of SAXS data

The SAXS proÐles were analysed using Guinier analysisand invariant analysis to characterize the size andvolume fraction of scattering objects.

Guinier analysis is based on the assumption that thesample consists of dilute scattering entities.9 The radiusof gyration of three dimensional scattering objects Rgis obtained from the gradient of the plot of the nat-ural logarithm of intensity ln(I) against the square ofthe scattering vector q2 as q tends to zero, according toeqn (1).

I(q) \ I(0)expC[q2Rg2

3D

(1)

The extrapolated intensity at zero scattering angle I(0),obtained from the intercept of the Guinier plot, obeys10the following equation :

I(0)\ Nv2(o1[ o2)2 (2)

where N is the average number of scattering objects inthe irradiated volume, v is the average object volumeand and are the electron densities of the twoo1 o2phases.

The invariant of a SAXS proÐle10 Q can be calculatedusing the equation :

Q\ kP0

=I(q)q2 dq (3)

where k is a constant arising from the fact that theintensity is in arbitrary units. It is una†ected by theshape of the scattering entities, but dependent on thechange in electron density di†erences within the struc-ture.10 In a two-phase system the following applies :

Q\ 2n2Vt[/(1 [ /)](o1[ o2)2 (4)

where / is the volume fraction of one of the phases andis the total volume irradiated. At the low volumeVt

fractions required by the Guinier analysis, this approx-imates to :

Q\ 2n2Vt /(o1[ o2)2 (5)

and hence the invariant is directly proportional to thevolume fraction of scattering objects. Because mayVt/alternatively be written as the scattered intensity atNv ,

zero angle I(0), and the invariant Q may be combined toyield the average volume of a single scattering object v :

v\ 2n2 I(0)Q

(6)

The invariant was calculated by integrating between thelimits of data collection for all proÐles.

RESULTS

Preliminary observations

The samples retained structural integrity at all times.No cracks or voids were visible by eye in any of thesamples, although after 28 days of degradation, thesamples appeared slightly sticky. The surfaces of bothdegraded and undegraded dehydrated samplesappeared smooth at the length scales visible by SEM.ReÑection FTIR spectra of degraded and subsequentlydehydrated samples showed the formation of carboxylicacid groups, with degradation indicating that theexpected mechanisms of degradation were in operation.No IR peaks associated with crystalline material,11 suchas at 972, 901, 806, 627 and 590 cm~1 were observed inthe FTIR analysis.

WAXS

The WAXS proÐles indicate a fully amorphous struc-ture, within the accuracy of the data. On degradationfollowed by dehydration, the amorphous halo decreasesin intensity slightly, but the shape of the broad halodoes not change. Preliminary data from wet samplesbefore dehydration also showed no evidence of crys-talline forms.12

WAXS during heating

No signiÐcant changes occur to the starting amorphousmorphology on heating even in the highly degradedmaterial (Fig. 1). The scattering from the undegraded,dehydrated samples exhibits a drop in intensity atapproximately 200¡C. This is presumably due to thereduced viscosity of the sample on heating causing it tofall out of the X-ray beam. As degradation progresses,the temperature at which the sample fell out of thebeam decreases, presumably as a result of the fallingmolecular weight.

SAXS

The SAXS data obtained after various stages of degra-dation after dehydration are shown in Fig. 2. No SAXSlamellar peak is observed in any of the samples evenafter advanced hydrolytic attack and dehydration. Thescattering at low scattering angle increases sharply afterincreasing times of degradation before the dehydration



Fig. 1. The dynamic WAXS intensity proÐles for dehydratedrandom 50 : 50 copolymer of PGA with D,L-PLA degraded for(a) 0 days and (b) 28 days. The amorphous structure is unaf-

fected by the heating process.

step. Such di†use scattering is typical of the scatteringfrom irregularly sized voids. The size and volume frac-tion of the voids may be characterized by furtheranalysis of the proÐle.

The invariant is shown against degradation time inFig. 3. Because the irradiated volume and the density

Fig. 2. The SAXS intensity proÐles of the dehydrated random50 : 50 copolymer of PGA with D,L-PLA after various stagesof degradation. The curves have been o†set for clarity. Theintensity at low angles increases as degradation proceeds and

because of the e†ects of dehydration.

Fig. 3. The invariant Q from the SAXS intensity proÐle,plotted against degradation time for samples after dehydra-

tion.

di†erence between void and polymer are approximatelyconstant, this quantity provides a measure of the voidvolume fraction via eqn (5). This increases as degrada-tion progresses in dehydrated samples. Preliminary datafrom samples before dehydration also show an increasein Q, implying that the void volume fraction alsoincreases in the wet samples, before dehydration.12

The data shown in Fig. 2 give linear Guinier plots(Fig. 4). The corresponding radii of gyration of the voidsare shown in Fig. 5. The radius of gyration of voidsrises slightly as the amount of degradation before thedehydration step increases. Preliminary data fromsamples before dehydration gave values of a similarmagnitude, suggesting that the voids are of similar sizebefore and after dehydration.12

Figure 6 shows the average void volume calculatedfrom the invariant and the extrapolated intensity atzero angle. The voids increase in volume as the amountof degradation before dehydration increases.

Fig. 4. The Guinier plots, from the SAXS intensity proÐle forsamples after dehydration. Samples were degraded for 0 days

1 day 7 days 14 days 21 days (]) and 28(L), (K), ()), (|),days (]).



Fig. 5. The radius of gyration, determined from the Guinieranalysis, for samples after dehydration.

SAXS during heating

Figure 7 shows the dynamic SAXS data obtained fordehydrated samples degraded for 0, 7 and 28 days. Theintensities are in the same arbitrary units in each graph.Little change is observed in the SAXS proÐle of thedehydrated, undegraded morphology on heating to200¡C. Past this temperature, the intensity drops as themobility of the sample increases, causing it to fall out ofthe X-ray beam. As also seen in the WAXS data, asdegradation proceeds the temperature at which thepolymer falls out of the beam decreases.

After at least 7 daysÏ degradation, a drop in the SAXSintensity is observed above about 50¡C. The scatteringafter this transition is similar to that of the undegradedsample. No corresponding change is seen in the WAXSproÐles, indicating that this is not due to sample lossfrom the beam.

Figure 8 shows the invariant during the heating runs.It is largely una†ected by temperature in the unde-graded sample. However, in the dehydrated samplesdegraded for at least 7 days, it decreases on heating thepolymer past about 50¡C, indicating a fall in the volume

Fig. 6. The average void volume, determined from the invari-ant and Guinier analysis, for samples after dehydration.

Fig. 7. The dynamic SAXS intensity proÐle for dehydratedrandom 50 : 50 copolymer of PGA with D,L-PLA degraded for

(a) 0 days (b) 7 days and (c) 28 days.

fraction of voids to a value similar to that of the un-degraded sample.

Guinier plots are again linear (Fig. 9). As the morehighly degraded, dehydrated samples are heated, theradius of gyration (Fig. 10) and the average void volume(Fig. 11) both fall to a value similar to that seen in theundegraded sample.

DSC

Figure 12 shows the DSC results for dehydratedsamples degraded for 1 day and 28 days. The un-degraded material exhibits a glass transition temperature



Fig. 8. The invariant Q from the SAXS intensity proÐle,plotted against temperature for dehydrated samples degradedfor 0 days 1 day 7 days 14 days 21 days (])(L), (K), ()), (|),and 28 days (]). The value decreases on heating the samples

damaged by degradation, past 50¡C.

of approximately 50¡C. The peak shape of the glasstransition suggests that the heating rate of 10¡Cmin~1is considerably faster than the cooling rate around theglass transition during processing, a fair assumption

Fig. 9. The Guinier plots, from the SAXS intensity proÐles :(a) for undegraded samples after heating at 10¡Cmin~1 to25¡C 75¡C 125¡C and 175¡C (b) for dehy-(L), (K), ()) (|) ;drated samples degraded for 21 days after heating at10¡Cmin~1 to 25¡C 50¡C 75¡C 100¡C and(L), (K), ()), (|)

125¡C (]).

Fig. 10. The radius of gyration, determined from the Guinieranalysis, plotted against temperature for dehydrated samplesdegraded for 0 days 1 day 7 days 14 days(L), (K), ()), (|),

21 days (]) and 28 days (]).

Fig. 11. The average void volume plotted against temperaturefor dehydrated samples degraded for 0 days 1 day 7(L), (K),days 14 days 21 days (]) and 28 days (]). The value()), (|),decreases on heating the samples damaged by degradation,

past 50¡C.

Fig. 12. The DSC trace of dehydrated random 50 : 50 copoly-mer of PGA with D,L-PLA dehydrated samples degraded for 1day (ÈÈ) and 28 days (È È È). The samples were heated at10¡C/min from 0 to 250¡C. The traces have been o†set for

clarity.



since the samples were cooled in air.13 On further deg-radation the glass transition is undetectable. Neither ofthe samples show evidence of crystalline melting peaksfor either the polymer, oligomers or the degradationproducts glycolic acid, L-lactic acid and D-lactic acid.

DISCUSSION

Crystallinity , degradation and dehydration

As expected, the random copolymer used in this studywas found to be fully amorphous. After degradationthere was still no evidence for any crystalline phases,whether of polymer sequences, oligomers or monomers.WAXS, FTIR and DSC data all indicate completelyamorphous structures at all stages of degradation, andin both wet and dehydrated states (Figs 1 and 12). TheSAXS data are also consistent with a completelyamorphous structure in all samples (Fig. 2).

Since glycolic acid and lactic acid monomers are cer-tainly formed during the experiment, although still infairly small quantities,5 and since they would beexpected to be crystalline in the dehydrated state, itmust be assumed either that the bulk polymer preventsthem from crystallizing, or these species have suc-cessfully di†used from the bulk sample into the degrad-ing bu†er solution. Trace quantities may still be presentin the bulk, at a level not observable with these tech-niques. The lack of crystalline polymer or oligomer resi-dues is entirely consistent with the phase diagramproposed by Hakkarainen et al.,5 and is presumablydue to the random composition of the copolymer inwhich runs of like units do not occur.

Formation of voids on degradation

The samples in this experiment were all in the Ðrst stageof degradation as proposed by Hakkarainen et al.,5 inwhich large changes in molecular weight are seen, but inwhich mass loss and monomer formation are still fairlyslow. The linearity of all the Guinier plots (Figs 4 and 9)is consistent with a low void volume fraction in allsamples, which in turn in consistent with the low massloss expected in this regime of degradation.

Preliminary data from wet samples indicate that thevoid volume fraction increases with degradation, andthat the radius of gyration and average void volume areof the same order of magnitude as those in the dry state.This rough uniformity of size on dehydration suggeststhat, although it is quite likely that the removal of waterstresses the sample sufficiently to create new voids, theyare of similar magnitude to the pre-existing voids.Further, the size of existing voids is not dramaticallyincreased by dehydration. The lack of large scale

voiding visible either by eye, or by SEM, is consistentwith this interpretation.

The lack of dramatic voiding on dehydration in thesesamples is in contrast to semi-crystalline PGA, wheremacroscopic cracks and voids appear on dehydration.In the semi-crystalline material, the degradation ishighly inhomogeneous, occurring in the amorphousinterlamellar regions in preference to the crystallinelamellae. When such degraded material is dehydrated,internal stresses are created, some lamellae collapsetogether and large voids are opened up.6 In the fullyamorphous copolymer studied here, the degradation ispresumably much more homogeneous, occurring overall regions of the sample. When water is removed, thestructure is able to respond more uniformly and largecracks and voids are not opened up.

On degradation followed by dehydration, the radiusof gyration rises slightly with the degree of degradation(Fig. 5). The average void volume fraction apparentlyincreases by a larger percentage (Fig. 6). The averagevoid volume is slightly smaller than would be expectedfor perfectly spherical voids of the radius of gyrationquoted. This suggests that the voids may be a littleanisotropic, although still three dimensional. Theincreasing average void volume with respect to theradius of gyration may indicate a decreasing degree ofanisotropy with degradation.

The increased average volume of the voids cannotfully account for the change in volume fraction impliedby the change in invariant (Fig. 3). This means that thenumber of voids also increases with increasing levels ofdegradation before dehydration. To summarize, indehydrated samples, the voids increase in size andnumber, and apparently become more isotropic withincreasing levels of prior degradation.

The sample degraded for 28 days had a stickyappearance, probably linked to the onset of the secondstage of degradation when an increasing fraction of lowmolecular weight fragments is present. There is someevidence that the structure begins to close up, perhapsas a result of reduced viscosity arising from theincreased fraction of low molecular weight material.

Effect of heating on amorphous morphology andvoid content

On heating degraded, dehydrated samples above a tem-perature close to the glass transition temperature, thevalues of the radius of gyration, average void volumeand volume fraction all fall to values similar to that ofthe undegraded material (Figs 8, 10, 11). These changesindicate a closure of the voids formed on degradationand dehydration. As the material is heated above theglass transition, the mobility of the chains increases andthe void closure seen is presumably a consequence ofthe drive to reduce surface energy. It is interesting to



note that although the glass transition is not resolvableby DSC in the highly degraded dehydrated sample (Fig.12), it has apparently not moved signiÐcantly duringdegradation and is still controlling the behaviour of thesample.

CONCLUSIONS

Hydrolytic degradation of the random 50 : 50 copoly-mer of PGA with D,L-PLA causes signiÐcant micro-structural changes. These changes do not include anychange in the amorphous structure of the copolymerand the changes observed are very di†erent to thoseseen in the quenched amorphous PGA homopolymer,where crystallization occurs on degradation.12 Therandom 50 : 50 copolymer of PGA with D,L-PLA isincapable of crystallizing. Furthermore, the hydrolyticdegradation products of glycolic acid and lactic acid donot crystallize.

On degradation and subsequent dehydration, thevoids increase in size and number, and apparentlybecome more isotropic. After 28 days of degradation thepolymer architecture begins to collapse. Heating thestructures damaged by hydrolysis, past their glass tran-sition temperature causes the voids to collapse as theamorphous chains gain sufficient mobility for largescale rearrangements.

ACKNOWLEDGEMENTS

The authors are grateful to PÐzer Central Research andthe EPSRC for Ðnancial support. The X-ray experi-ments were performed at the CCLRC DaresburyLaboratory with the help and advice of Dr B. U.Komanschek. The DSC and FTIR experiments wereperformed at the Department of Materials Science &Metallurgy, University of Cambridge with the help ofMr C. SetterÐeld.

REFERENCES

1 Gilding, D. K. & Reed, A. M., Polymer, 20 (1979) 1459.2 Chu, C. C., J. Biomed. Mater. Res., 15 (1981) 19.3 Ginde, R. M. & Gupta, R. K., J. Appl. Polym. Sci., 33 (1987) 2411.4 Vert, M., Li, S. M. & Garreau, H., J. Biomater. Sci., Polym. Ed., 6

(1994) 639.5 Hakkarainen, M., Albertsson, A. C. & Karlsson, S., Polym. Deg.

Stab., 52 (1996) 283.6 King, E. & Cameron, R. E., J. Macromol. Phys. Chem. submitted

for publication.7 Chu, C. C. & Browning, A., J. Biomed. Mater. Res., 22 (1988) 699.8 Ryan, A. J., Naylor, S., Komanschek, B., Bras, W., Mant, G. R. &

Derbyshire, G. E., ACS Symp., 581 (1994) 162.9 Glatter, O. & Kratky, O., Small Angle X-ray Scattering, Academic

Press, London, 1982.10 Balta-Calleja, F. J. & Vonk, C. G., Polymer Science L ibrary 8 :

X-ray Scattering of Synthetic Polymers, Elsevier, Oxford, 1989.11 Chu, C. C., Zhang, L. & Coyne, L. D., J. Appl. Polym. Sci., 56

(1995) 1275.12 King, E., PhD Thesis, University of Cambridge, 1997.13 Hunt, B. J. & James, M. I., Polymer Characterisation, Blackie Aca-

demic & Professional, Glasgow, 1993.


Documents

Effect of hydrolytic degradation and dehydration on the microstructure of 50:50 poly(glycolide-co-D,L-lactide)