Poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethyleneglycol)-b-poly (D,L-lactic acid-co-glycolic acid) triblockcopolymer and thermoreversible phase transition in water

Myung Seob Shim,1 Hyung Tak Lee,1 Woo Sun Shim,1 Insun Park,3 Hyunjung Lee,3 Taihyun Chang,3

Sung Wan Kim,2 Doo Sung Lee1

1Center for Advanced Functional Polymers, Department of Polymer Science and Engineering, SungkyunkwanUniversity, Suwon, Kyungki 440-746, Korea2BPRB 205, CCCD, University of Utah, Salt Lake City, Utah 841123Department of Chemistry and Polymer Research Institute, Pohang University of Science and Technology, Pohang,790-784, Korea

Received 29 June 2001; revised 10 October 2001; accepted 27 November 2001

Abstract: Novel thermoreversible gelation behavior ofaqueous solutions of ABA-type triblock copolymers com-posed of the central polyethylene oxide (PEG) block and twopoly(D,L-lactic acid-co-glycolic acid) side blocks was found.Phase transition characteristics, such as critical gel concen-tration (CGC) and lower and upper critical gel temperature(CGT), are closely related to the molecular structure of thetriblock copolymers. The CGC and the lower CGT both in-creases with increasing PEG/PLGA molecular weight ratio.Increasing the GA content in PLGA block induces a some-what higher CGC. The copolymer forms micelles with aPLGA loop core and a PEG shell in water. Also groupedmicelles are identified seemingly due to the bridging of two

micelles sharing two PLGA blocks of a block copolymerchain. As the temperature increases the association of mi-celles increases, which results in gelation. The ABA-type co-polymers exhibit a relatively low CGC (<10%) and low sol-gel transition temperatures compared to BAB-type copoly-mers. As the temperature increases further gel-sol transitionis observed, which would result from the shrinkage of mi-celles with temperature increase. The hydrodynamic size ofthe micelles is monitored by dynamic laser scattering, and apossible gelation mechanism was suggested. © 2002 WileyPeriodicals, Inc. J Biomed Mater Res 61: 188–196, 2002

Key words: biodegradable; thermoreversible; phase transi-tion; triblock copolymer

INTRODUCTION

The stimuli-sensitive phenomena of polymer hydro-gels, especially the thermoreversible gels, have at-tracted extensive attention in the biodegradable mate-rials field in the past decades because a reversible sol-gel transition can take place upon heating or cooling.It can be developed for various applications as poly-meric drug carriers, implantation, and other medicaldevices.1–7 Pluronic (BASF) or Poloxamer products(ICI), composed of hydrophilic poly(ethylene oxide)

(PEG) and hydrophobic poly(propylene oxide) (PPO),are typical thermoreversible triblock (PEG-b-PPO-b-PEG) copolymers based on micellar gelation.6,10 Thegelation was induced with increasing temperature be-cause of the rapidly increasing micellar packing oraggregation. The diblock or triblock copolymers ofPEG and poly(butylene oxide) (PBO) show a similarphase transition behavior dependent on temperature,but they tend to associate more easily because of themore hydrophobic PBO compared with PPO.4,8,9

However, these copolymers were found to induce thetoxic enhancement of plasma cholesterol and triglyc-erol after intraperitional infection because they werenonbiodegradable and likely accumulated in thebody.11,12 Therefore, the temperature-sensitive biode-gradable polymers consisting of PEG and biodegrad-able polyester such as poly(L-lactic acid) (PLLA),poly(D,L-lactic acid) (PDLLA), poly(lactic acid-co-glycolic acid) (PLGA), and poly(DLLA-co-e-caprolactone) (PDLLA-co-PCL) have been investigatedextensively in recent years.13–21 Kim and his cowork-

Correspondence to: D. S. Lee; e-mail: dslee@skku.ac.krContract grant sponsor: Center for Advanced Functional

PolymersContract grant sponsor: KRF (BK21 program)Contract grant sponsor: KOSEF (Center for Integrated

Molecular Systems) (to T.C.)

© 2002 Wiley Periodicals, Inc.

ers reported the temperature-dependent phase transi-tion of a series of PEG-aliphatic ester diblock or BAB-type triblock copolymers.14–17 A typical BAB-typePEG-PLLA-PEG triblock copolymer (Mn, 5000–2040–5000) could be designed for injectable drug deliverysystem, owing to its sharp phase transition near thebody temperature. The lower molecular weight PEGblock (<5000) is required to be eliminated from thebody by kidney filtration. The ABA-type triblock co-polymers with a central PEG block could form highhydrophobic matrices and be considered suitable forthe safely releasing delivery of proteins because of thehigh biodegradability of polyester. Thus, the thermo-reversible phase transition in such ABA-type triblockcopolymer–water systems have gained much interestsrecently.

In the last communication,22 we reported that ABA-type PLGA-b-PEG-b-PLGA triblock copolymers dem-onstrated an interesting reversible sol-gel transition inwater. This PLGA-b-PEG-b-PLGA triblock copolymerforms micelles, and the grouped micelles bridged be-tween micelles because the hydrophobic PLGA endblocks can be located in different micelles. As a com-parison, the ABA-type PPO-PEG-PPO triblock copoly-mer generally formed a ball-shaped individual mi-celle, without any connection with other micelles.23,24

In this study, temperature-dependent transition be-havior for these triblock copolymers is investigated inmore detail. The micellar topology is monitored bydynamic laser scattering (DLS). A micellar gelationmechanism is proposed.

MATERIALS AND METHODS

Materials

Poly(ethylene oxide)s (PEGs) (Mn, 1000 and 1500, Mw/Mn:1.1 and 1.2, respectively) were acquired from Aldrich andpurified by dissolution in dried chloroform and then pre-cipitation in n-hexane before use. D,L-lactide (DLLA) andglycolide (GA) supplied from Polyscience and BoehringerIngellheim, were purified by recrystallization in dry ethylacetate. Stannous octoate [Sn(Oct)2, received from Sigma]was dried for 24 h under vacuum at ambient temperatureprior to use. Toluene was thoroughly purified by distillationover calcium hydride (CaH2). All other reagents were ofanalytical grade and used without further purification.

Synthesis

The synthesis of PLGA-b-PEG-b-PLGA triblock copoly-mers was performed through a ring-opening copolymeriza-tion with the PEG as an initiator. Feed ratios of PEG/GA and

DLLA/GA were used to adjust the composition and mo-lecular weight. The detailed synthesis process (taking 908/292-1000-292/908) can be described as below: To a driedthree-neck round-bottom flask equipped with a condenser,DLLA(3.78 g), GA(1.22 g), PEG(Mn = 1000) (2.00 g), stannousoctoate (0.03 g), and 80 mL toluene were added and refluxedunder a dry nitrogen atmosphere at 110°C for over 60 h. Thereaction was ended by precipitation in diethyl ether. Theresulting copolymers were further purified through dissolv-ing in chloroform and precipitation in diethyl ether. Thefinal products were dried under vacuum at room tempera-ture for over 48 h. Consequently, PLGA-b-PEG-b-PLGA(930/299-1000-299/930, 5 g, B-3) triblock copolymer was ob-tained. The overall yield of these triblock copolymers wasover 70% after drying.

Characterization

The number-average molecular weight (Mn) and molecu-lar weight distribution (MWD) of these triblock copolymerswere determined by gel permeation chromatography (GPC)on a Waters Model 410 apparatus equipped with four m-sty-ragel columns from 500 to 10 Å in series at a flow rate of1.2 mL/min (eluent: THF, 36°C, polystyrene as standard).IR spectra of these triblock copolymers were recorded byUNICAM Mattson 5000. 1H-NMR measurement was pre-formed on a Varian Unity Inova 500 instrument (500 MHz)to determine the molecular structure and compositions ofPEG, LA, and GA.15,17

The glass transition temperature (Tg) of the triblock co-polymers in bulk state was measured by a differential scan-ning calorimeter (DSC, DuPont TA2910) at a scanning rangeof −80–100°C and a heating rate of 10°C/min.25

Phase diagram measurement

The sol (flow)-gel (no flow) phase transition temperatureof triblock copolymer in aqueous solution was determinedusing inverting test method with a 4-mL vial test tube at atemperature interval of 1°C. Each sample at a given concen-tration was dissolved in water at 0°C for 2–4 days, and thenthe gel-sol transition temperature was determined by invert-ing the vial horizontally after keeping for 10 min at a con-stant temperature as described in our previous papers.15,17

Dynamic light scattering (DLS)

The average size and distribution of micelles of PLGA-b-PEG-b-PLGA triblock copolymer in water as a function oftemperatures were measured using Malvern PCS100 spec-trogoniometer and Brookhaven BI-9000AT digital autocor-relator at a wavelength of 488 nm of an Ar ion laser (Lexel95). The block copolymers were dissolved in deionized wa-ter to prepare two different concentration samples, 2 and 10

189TRIBLOCK COPOLYMERS AND THERMOREVERSIBLE PHASE TRANSITION IN WATER

wt %. Deionized water was prefiltered with 0.2 mm poremembrane filter. The solution was not clear, and it was fil-tered directly into a cleaned cell through a 1-mm pore mem-brane filter to avoid the removal of large sized aggregates.Measurements were then performed with increasing tem-perature toward CGT and the autocorrelation functionswere taken at three different scattering angles between 40and 120° at each temperature. The intensity autocorrelationfunctions were analyzed by either CONTIN method to ob-tain a distribution function or cumulant method to obtain anaverage particle size present in the samples.5,26 When thecumulant method was used the average hydrodynamic ra-dius (RH) was calculated from the angular dependence of thedecay constants, t of the autocorrelation functions:

RH =kT

6phD, t = Dq2, and q =

4pnl

sin~u/2!

where kT has the usual meaning, h is the viscosity of themedium, D is the diffusivity, n is the refractive index of themedium, l is the wavelength of the light, and u is the scat-tering angle.

RESULTS AND DISCUSSION

Molecular structure characterization

IR spectra of PLGA-b-PEG1000-b-PLGA obviouslydemonstrated the presence of both PEG and PLGAblocks. The GPC curves of various PLGA-b-PEG-b-PLGA triblock copolymers show a unimodal peakwith narrow molecular weight distribution of Mw/Mn

< 1.3 as listed in Table I.The molecular structure of PLGA-b-PEG-b-PLGA

triblock copolymer was further characterized with 1H-NMR. The characteristic signals appearing at 4.80ppm, 3.63 ppm, and 1.55 ppm are assigned to themethylene hydrogen of the GA units, the methylenehydrogen of the EO units, and the methyl hydrogen ofthe LA units, respectively. The compositions were ob-tained by calculation of the corresponding peak areasas described in the previous article.17

TABLE IPhysical Parameters of PLGA-b-PEG-b-PLGA Triblock Copolymers Synthesized

PLGA-b-PEG-b-PLGAMna

PEGMa

bPEG/PLGA

(wt/wt)aDLLA/GA(mol/mol)a Mw/Mn

c Tg (°C)d

1148/309-1000-309/1148(A-1) 1000 1/2.9 3.0 1.28 −8

1046/293-1000-293/1046(A-2) 1000 1/2.7 2.9 1.17 −10

969/256-1000-256/969(A-3) 1000 1/2.5 3.0 1.24 −13

809/205-1000-205/809(A-4) 1000 1/2.0 3.1 1.24 −18

1143/375-1000-375/1143(B-1) 1000 1/3.0 2.5 1.23

1051/317-1000-317/1051(B-2) 1000 1/2.7 2.6 1.21 −10

930/299-1000-299/930(B-3) 1000 1/2.5 2.5 1.28

800/230-1000-230/800(B-4) 1000 1/2.0 2.6 1.24

1052/399-1000-399/1052(C-1) 1000 1/2.9 2.1 1.31

922/396-1000-396/922(C-2) 1000 1/2.6 1.9 1.28 −11

731/276-1000-276/731(C-4) 1000 1/2.0 2.1 1.28

966/492-1000-492/966(D-1) 1000 1/2.9 1.5 1.24

902/468-1000-468/902(D-2) 1000 1/2.7 1.5 1.28 −11

777/459-1000-459/777(D-3) 1000 1/2.5 1.4 1.22

711/357-1000-357/711(D-4) 1000 1/2.1 1.6 1.31

1507/369-1500-369/1507(E-1) 1500 1/2.5 3.2 1.15aP(DLLA/GA)-b-PEG-b-P(GA/DLLA) number-average molecular weight calculated from 1H-NMR.bProvided by Aldrich.cMeasured by GPC (PS standard sample).dMeasured by DSC.

190 SHIM ET AL.

The calculated results are listed in Table I. The char-acterization results demonstrate that the structure-controlled ABA-type PLGA-b-PEG-b-PLGA triblockcopolymers with narrow molecular weight distribu-tion have been well synthesized.

Solubility of triblock copolymer in water

The phase transition tests were performed on thePLGA-b-PEG-b-PLGA triblock copolymers with differ-ent structures, and the results are listed in Table II. Inthe table, “I” denotes the copolymer insoluble in wa-ter, and “S” denotes the soluble copolymer, whichdoes not form a hydrogel. The samples showing sol-gel transition behavior are marked with sample codesfor later use. The triblock copolymers based on a cen-tral PEG block (Mn = 1000) with two PLGA end blockswith molecular weight over 1600 were all insoluble inwater. The triblock copolymers with PLGA blockswith molecular weight under 900 were soluble in wa-ter. Such a phenomenon indicates that the balance ofhydrophobic and hydrophilic block length is critical toexhibit a reversible phase transition in water. Onlythose triblock copolymers with PLGA blocks with themolecular weight within the range of 900 to 1600showed a thermoreversible sol-gel transition, as listedfrom A-1 to D-4, which are focused on in this article.

Phase diagrams

Phase diagrams of BAB-type PLGA-b-PEG1000-b-PLGA triblock copolymers with various PEG/PLGA

and DLLA/GA ratios are illustrated in Figures1–4. The phase diagrams show critical gel concentra-tion (CGC) and two critical gelation temperature(CGT) curves (a lower transition temperature curvefrom sol to gel and an upper transition temperaturecurve from gel to sol). In the phase diagrams, thelower CGT usually did not change as much as theupper CGT with varying polymer concentration. Tak-ing PLGA969/256–PEG1000–PLGA256/969 (A-3) as an ex-ample in Figure 1(a), the former changes only a few

TABLE IISol-Gel Phase Transition of PLGA-b-PEG-b-PLGA

Triblock Copolymers in Water

DLLA/GA(mol/mol) 3.4 3.0 2.5 2.0 1.5 1.3

PEG/PLGA(wt/wt)

1/3.2(1600-1000-1600) I I I I I I

1/3.0(1500-1000-1500) I A-1 B-1 C-1 D-1 —

1/2.7(1350-1000-1350) — A-2 B-2 C-2 D-2 —

1/2.5(1200-1000-1200) — A-3 B-3 C-3 D-3 —

1/2.0(1000-1000-1000) — A-4 B-4 C-4 D-4 S

1/1.8(900-1000-900) S S S S S S

Note: I, insoluble; S, soluble; —, not prepared

Figure 1. Phase diagrams of PLGA-b-PEG1000-b-PLGA tri-block copolymers with the comparabile DLLA/GA ratio andvarious PEG/PLGA ratios in water.

191TRIBLOCK COPOLYMERS AND THERMOREVERSIBLE PHASE TRANSITION IN WATER

degrees with concentration varying from 5 to 20%, butthe latter changes an order of magnitude more withinthe same concentration range. This indicates that thesol-to-gel phase transition of ABA-type triblock co-polymer in water is less dependent of the polymerconcentration compared with the gel-to-sol phasetransition, consistent with the BAB-type systems.15 Itis noted that the CGC for ABA type triblock copoly-mers is lower than BAB type, which is typically upper20% for BAB type.15–17 As can be expected from the

structure, two hydrophobic PLGA blocks can easilylocate into different micelles, leading to bridging con-nections. It facilitates the aggregation of micelles.

Figure 1 displays the phase diagrams of the ABA-type PLGA-b-PEG-b-PLGA triblock with varyingPEG/PLGA ratios and comparable DLLA/GA ratio.The CGC [taking (A-1)–(A-4) ( DLLA/GA ratio = 3.0/1) in Table II] sharply drops from 9 to 2% by varyingthe PEG/PLGA ratios from 1/2.0 to 1/2.9 [see Fig.1(a)]. As PLGA blocks gets longer, micelles are gener-ally formed with a less number of associated chains.16

The more bridging connections between micelles pro-duce the more grouped micelles, inducing sol-to-gelphase transition at lower temperatures, and gel-to-solphase transition at higher temperatures. In otherwords, the gel zone in the phase diagram becomeslarger with increasing PLGA/PEG ratio. The compa-rable tendency was also observed for the series withlower DLLA/GA ratio (1.5/1) as displayed in Figure1(b).

For the copolymers with the PEG/PLGA ratio (1/2.7), the CGC increases gradually from 3.5 to 5.5% byvarying DLLA/GA ratios from 2.9 to 1.5 [taking (A-2)–(D-2); see Fig. 2(a)]. When the molecular weight ofPLGA is kept constant, the gel zone decreases slightlywith decreasing the DLLA/GA ratio. A similar ten-dency was observed for the series with lower PEG/PLGA ratios (1/2.0) as shown in Figure 2(b). In thesesamples, the gel zone decreases dramatically. Thisabrupt change is due to the molecular weight changeof PLGA block from 1350 to 1000. If it decreases fur-ther down to 900, it does not form a gel at all. as shownin Table II, which indicates that the molecular weighteffect of PLGA is strong and more significant than theDLLA/GA ratio of PLGA on the gel-sol transition dia-gram in the composition ranges of Table II.

Thermal analysis of the triblock copolymers in bulkstate was performed [taking the two series with vary-ing PEG/PLGA or DLLA/GA as mentioned in Figs.1(a) and 2(a) above] by DSC. All the copolymers areamorphous, and show single glass transition as listedin Table I. The phase mixing between the PEG andPLGA blocks is good in the bulk state. The glass tran-sition temperature of the copolymers increases from−18 to −8°C when the PLGA block length was variedfrom 1000 to 1500, but nearly independent on theDLLA/GA ratio. Thus, the chain flexibility of PLGA inwater seems to be mainly determined by the PLGAblock length as shown in gel-sol phase diagram.

PLGA-b-PEG1500-b-PLGA (1507/369-1500-369/1507,E-1) with a long PEG block (Mn = 1500) displays asimilar CGT curve shape but shifted to a high tem-perature compared with PLGA-b-PEG1000-b-PLGA (A-3), as shown in Figure 3. The two copolymers have thesame PEG/PLGA and DLLA/GA ratios except for thetotal molecular weight. This may indicate that theshape of the gel-sol diagram does not change much as

Figure 2. Phase diagrams of PLGA-b-PEG1000-b-PLGA tri-block copolymers with the comparable PEG/PLGA ratioand various DLLA/GA ratios in water.

192 SHIM ET AL.

far as the composition of the copolymers unchanged,while the transition temperature strongly depends onthe total molecular weight.

Such an interesting vertical shift of the phase dia-gram was also observed when different additiveswere added to copolymer solutions such as PEG1000(Mn = 1000) and NaSCN as shown in Figure 4. Addinga small amount (2%) of NaSCN raised the phase tran-

sition curves to higher temperatures without influenc-ing CGC. This is because NaSCN as a typical salt-inconsolute is capable of solubilizing the hydrophobicPLGA blocks in water, inducing gelation at highertemperatures.10 While PEG1000 in aqueous solutionplayed a role of bridging connections due to the stronginteraction between the PEG1000 molecules and PEGmiddle block, and enhanced the bridging possibilitybetween micelles. In other words, the free PEG in so-lution can enhance the packing between micelles withless effect on micelle–micelle interaction, inducing ge-lation at low temperatures in accordance with obser-vation for BAB-type PEG—PLGA–PEG triblock co-polymer aqueous solution.16 It is expected that theaddition of more PEG shifts the phase transitioncurves to lower temperatures (see Fig. 4). These resultsindicate that the suitable additives do not influencethe CGC but strong modulation in phase transitiontemperature in accordance with BAB-type triblock co-polymer–water systems.15

Micellar topology and gelation mechanism

Micelle formation and micellar behavior of the co-polymer are dependent on the molecular structureand its hydrophobic/hydrophilic balance, whichplays an important role in the gelation of ABA andBAB-type copolymer in water.5,6,27–30 The micellar to-pology in aqueous solution of the ABA-type triblockcopolymer is characterized by the micellar size anddistribution as a function of temperature measured bydynamic laser scattering (DLS). Figure 5 displays thesize distribution of micelles of the 10 wt % aqueoussolution of B-3 at different temperatures, which wasobtained by the CONTIN analysis of the scattered in-tensity autocorrelation function measured at the scat-tering angle of 70°. The small peak below 5 nm seemsto be due to unimers. Below the lower CGT, two dis-tinct peaks are assigned to individual micelle of about10 nm and grouped micelle (>20 nm), bridged struc-tures of micelles, respectively. Individual micelles areconstructed with two PLGA blocks looped to form acore and a center PEG block to form a shell.22 As thetemperature is increased, the size of the individualmicelles does not change much because the associatednumber in a micelle is independent on temperaturebut determined by molecular structure.16 On the otherhand, the size of the grouped micelles increase gradu-ally from 24 to 41 nm by varying temperature from 3to 12°C. This indicates that the grouped micelle ex-pands through bridging individual micelles. Whentemperature is raised up to 14°C (near the low CGT of16°C ), the peak grows up dramatically to 95 nm, andbecomes wider and larger.

Furthermore, DLS measurement for E-1 at different

Figure 3. Phase diagrams of PLGA-b-PEG-b-PLGA triblockcopolymers (A-3, E-1) with different PEG molecular weightsin water.

Figure 4. Phase diagrams of ABA-type PLGA-b-PEG1000-b-PLGA triblock copolymers (B-3) with different additives inwater.

193TRIBLOCK COPOLYMERS AND THERMOREVERSIBLE PHASE TRANSITION IN WATER

concentrations (2 and 10 %) was also performed, andthe average sizes of micelles were measured by cumu-lant analysis method and results are displayed in Fig-ure 6. At a high polymer concentration (10%, above theCGC of 5%) the average size does not change much atlow temperature. Approaching the lower CGT (32°C),

however, the average micelle size increased abruptlydue to the bridging of individual micelles, whicheventually results in gelation [see Fig. 6(a)]. At a lowpolymer concentration [2%, Fig. 6(b)], the average sizeof micelles also increases rapidly near the gel tempera-ture, seemingly due to the bridging of individual mi-celles. In this case, however, gelation does not takeplace because the concentration is lower than CGC.These results further support the micellar-bridgingmechanism for the gelation of PLGA-b-PEG-b-PLGAtriblock copolymers in water. This is in a sharp con-trast with the micellar-packing mechanism proposedfor the gelation of BAB-type triblock copolymer byothers in the literature.3,4,17,23,24,27–29 BAB-type triblockcopolymers such as Pluronics form micelles in equi-librium with unimer in water. Each micelle has a hy-drophobic core and a hydrophilic shell, and can moverelatively freely without any bridging connection be-tween micelles. Sol-gel transition occurs when the to-tal volume fraction of micelles is larger than the maxi-mum packing fraction.

The temperature-dependent micellar bridging gela-tion mechanism is schematically illustrated in Figure7. At a temperature much lower that CGT [Fig. 7(A)],unimers, individual micelles, and grouped micellescoexist in the sol state. The fraction of unimer de-creases with temperature [Fig. 7(B)]. The size ofgrouped micelle grows rapidly resulting in sol-geltransition [see Fig. 7(C)]. The aggregation and packinginteraction between micelles increases to form moredense gel by raising temperature [Fig. 7(D)], while thegel changed from translucent to opaque (C and D inFig. 3). If the temperature was raised further, the hy-drophobic chains in the micelle core shrink tightly.Also, the hydrophilic PEG block underwent dehydra-tion (E in Fig. 3). The shrunk micelle groups are re-stored from the tight aggregation and strong packinginteraction, and tend to flow in water, for example,gel-sol transition begin [see Fig. 7(E)]. When the pointF in Figure 3 is reached, the overshrunk micellegroups precipitated in water, for example, the solutionbecame two separated parts of water and polymerphase as schematized in Figure 7(F).

CONCLUSION

PLGA-b-PEG-b-PLGA triblock copolymers of vary-ing PEG/PLGA, DLLA/GA ratios, and PEG lengthwere synthesized and their thermoreversible phasetransition behavior was investigated. A typical phasediagram exhibited a CGC and two CGT curves. TheCGC was mainly determined by the balance of hydro-phobic PLGA and hydrophilic PEG blocks. The lowerCGT showed a weaker dependence on the polymerconcentration than the upper CGT curve. The triblock

Figure 5. Micellar size and its distribution in aqueous so-lution of a typical triblock copolymer (B-3, 10%) as a func-tion of temperature. The Dynamic Laser Scattering (DLS)measurements were performed at a scatting angle of 70°,and the intensity autocorrection function was analysezed bythe CONTIN method.

Figure 6. The average sizes variation of micelles as a func-tion of temperature in aqueous solution of triblock copoly-mer (E-1) with different polymer concentrations: (a) 10%; (b)2%.

194 SHIM ET AL.

copolymer synthesized from longer PEG molecularchain (Mn = 1500) in water show the increased CGTwithout altering the CGC and curve shape. Also, theCGT curve shape could be adjusted by adding differ-ent additives, such as PEG1000 and NaSCN. The tri-block copolymer aqueous solution first formed micellewith a PLGA loop core and a PEG shell under thestrong hydrophobic chain interaction of PLGA, andthe bridged micelle groups due to PLGA locating intodifferent micelles. A micellar gelation was suggested.

The authors thank Dr. Feng-jun Hua for his comments.

References

1. Shargel L, Yu A. Applied biopharmaceutics and pharmaco ki-netics, 3rd ed. Norwalk, CT: Appleton & Lange; 1993. 111 p.

2. Chen G, Hoffman AS. Graft copolymers that exhibit tempera-ture-induced phase transitions over a wide range of pH. Na-ture 1995;373:49–52.

3. Kim SW, Bae YH, Okano T. Hydrogel: Swelling, drug loading,and release. Pharm Res 1992;9(3):283–290.

4. Park K, Shalaby W, Park H. Biodegradable hydrogels for drugdelivery, Ch. 4. Chemical gels. Lancaster, PA: Technomic PubCo Inc.; 1993. 67 p.

5. Yang YW, Yang Z, Zhou Z K, Attwood D, Booth C. Associationof triblock copolymers of ethylene oxide and butylene oxide inaqueous solution. A study of BnEmBn copolymers. Macromol-ecules 1996;29:670–680.

6. Berglund F. Renal excretion and volume of distribution ofpoly(ethylene glycol)(PEG) in the rat and the dog. Acta PhysiolScand 1968;73:20A–21A.

7. Marcotte N, Polk A, Goosen MFA. Kinetics of protein diffusionfrom a poly(D, L-lactide) reservoir system. J Parm Sci 1990;79:407–410.

8. Zhuo Y, Simon P, Deng NJ, Barlow RJ, Attwood D, Booth C.Effect of block structure on the micellization and gelation ofaqueous solution of copolymers of ethylene oxide and butyl-ene oxide. Macromolecules 1994;27:2371–2379.

9. Luo YZ, Nicholas CV, Attwood D, Collett JH, Price C, Booth C.Micellization and gelation of block copoly(oxyethylene/oxybutylene/oxyethylene), E58B17E58. Colloid Polym Sci 1992;270:1094–1105.

10. Malmsten M, Lindman B. Water self-diffusion in aqueousblock copolymer solution. Macromolecules 1992;25:5446–5450.

11. Wout Z, Pec EA, Maggiore JA, Williams RH, Palicharla P,Johnston TP. Poloxamer 407-mediated change in plasma cho-lesterol and tryglycerides following intraperitoneal injection torats. J Parenteral Sci Technol 1992;46(6):192–200.

12. Palmer WK, Emeson EE, Johnston TP. Poloxamer 407-inducedatherogenesis in the C57BL/6 mouse. Atherosclerosis 1998;136:115–123.

13. Suh H, Jeong BM, Rathi R, Kim SW. Regulation of smoothmuscle cell proliferation using paclitaxel-loaded poly(ethyleneoxide)-poly(lactide/glycolide) nanospheres. J Biomed MaterRes 1998;42(2):331–338.

14. Jeong BM, You HB, Lee DS, Kim SW. Biodegradable blockcopolymers as injectable drug-delivery system. Nature 1997;388:860–682.

15. Jeong BM, Lee DS, Shon Ji, Bae YH, Kim SW. Thermoreversiblegelation of poly(ethylene oxide) biodegradable polyester blockcopolymers. J Polym Sci Part A 1999;37:751–760.

16. Jeong BM, Bae YH, Kim SW. Thermoreversible gelation ofPEG-PLGA-PEG triblock copolymer aqueous solution. Macro-molecules 1999;32:7064–7069.

17. Choi SW, Choi SY, Jeong BM, Kim SW, Lee DS. Thermorevers-ible gelation of poly(ethylene oxide) biodegradable polyesterblock copolymers II. J Polym Sci Part A 1999;37(13):2207–2218.

18. Bae YH, Kang MH, Kim YG, Park KH. Biodegradable amphi-philic multiblock copolymers and their implications for bio-medical applications. J Controlled Res 2000;64:3–13.

19. Kang MH, Bae YH. Synthesis and characterization of poly(eth-ylene glycol)/poly(L-lactic acid) alternating multiblock co-polymers. Polymer 1999;40:6147–6155.

20. Li YX, Christian V, Kissel T. In-vitro degradable and bovineserum albumin release of the ABA triblock copolymers con-sisting of poly(L(+)lactic acid), or poly(L(+)lactic acid-co-glycolic acid) A-blocks attached to central polyoxyethylene B-blocks. J Controlled Rel 1994;32:121–128.

21. Sung YK, Kim SW. Advances in biodegradable polymers fordrug delivery systems. Kor Polym J 2000;8(5):199–208

22. Lee DS, Shim MS, Kim SW, Lee HJ, Chang T. Novel thermor-eversible gelation of biodegradable PLGA-b-PEO-b-PLGA tri-block copolymers in aqueous solution. Macromol Rapid Com-mun 2001;22:587–592.

Figure 7. A schematic diagram of the micellar gelationmechanism for ABA-type triblock copolymer in aqueous so-lution in the process of thermoreversible transition.

195TRIBLOCK COPOLYMERS AND THERMOREVERSIBLE PHASE TRANSITION IN WATER

23. Vaughn MN. Nonionic surfactants: Polyoxyalkylene block co-polymers. New York; Marcel Dekker, Inc,; 1996. 266 p.

24. Rassing J, Atwood D. Ultrasound velocity and light scatteringstudies on the polyoxyethylene–polyoxypropylene copolymerPluronic F127 in aqueous solution. Int J Pharm 1983; 13:47–55.

25. Cho JC, Khang G, Choi HS, Rhee JM, Lee HB. Preparation ofbiodegradable PLGA microspheres for sustained local anesthe-sia and their in vitro release behavior. Polymer (Korea)2000;24(5):728–735.

26. Koppel DE. Analysis of macromolecular polydispersity in in-tensity correlation spectroscopy: The method of cumulants. JChem Phys 1972;57:4814–4820.

27. Zhou Z, Chu B. Light scattering study on the association be-havior of triblock polymers of ethylene oxide and propylene

oxide in aqueous solution. J Colloid Interface Sci 1988;126:171–180.

28. Mortensen K, Perdersen S. Structure study on micelle forma-tion of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer in aqueous solution.Macromolecules 1993;26:805–812.

29. Alexandrisdis P, Holzwarth JF, Hatton TA. Micellization ofpoly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) tri-block copolymers in aqueous solutions. Thermodynamics ofcopolymer association. Macromolecules 1994;27:2414–2425.

30. Jorgensen EB, Hvidt S, Brown W, Schilen K. Effect of salts onthe micellization and gelation of a triblock copolymer studiedby rheology and light scattering. Macromolecules 1997;30:2355–2364.

196 SHIM ET AL.