Rheology of Aqueous Solutions of Polyglycidol-Based Analogues to Pluronic BlockCopolymers

Silvia Halacheva, Stanislav Rangelov,* and Christo Tsvetanov

Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. BoncheV 103-A, Sofia 1113, Bulgaria

ReceiVed: June 1, 2007; In Final Form: NoVember 28, 2007

The aqueous solution properties of a series of polyglycidol-poly(propylene oxide)-polyglycidol (PG-PPO-PG) block copolymers were investigated by means of rheology. The copolymers are considered as analoguesto the commercially available Pluronic, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO), block copolymers in which the flanking PEO blocks are substituted by blocks of structurallysimilar PG bearing a hydroxyl group in each repeating monomer unit. In the dilute regime, the samplesnormally behave as Newtonian fluids. Shear thinning was observed only for the solutions of LGP65 (thecopolymer of 50 wt % PG content) as well as at concentrations well above the critical micellizationconcentration for the rest of the copolymers. The zero shear viscosities exhibited pronounced maxima at PGcontent of 50 wt % and were found to decrease with increasing temperature. The concentrated solutions wereinvestigated using oscillatory measurements. Large hystereses were observed during the temperature sweeps15-70-15 °C. The evolutions of the loss and storage moduli with frequency, PG content, and temperaturedisplayed transitions from a non-elastic to elastic behavior of the solutions. A phase diagram showing areasof predominant elasticity or fluidity was constructed.

Introduction

The poly(ethylene oxide)-poly(propylene oxide)-poly-(ethylene oxide) (PEO-PPO-PEO) block copolymers, com-mercially available under the trade name Pluronic, havestimulated great interest,1-11 because of the unique self-assemblyin selective solvents. Particularly in water the Pluronic copoly-mers have proven to be versatile systems as far as the phasebehavior and microstructure are concerned. They form dispersedmicrophases with the hydrophobic, that is PPO, chains as-sembling into microdomains of a certain shape and the hydro-philic, that is PEO, chains orienting toward the aqueous phase.In the composition-concentration-temperature continuum, allliquid-crystalline phases have been observed.1-11 Pluroniccopolymers have been extensively investigated by a variety ofresearch techniques including rheology.12-16 Although it isdifficult to generalize the rheological behavior of these particularcopolymers because of their complex character and differentexperimental conditions, the following features can be out-lined: (i) Newtonian in the unimer and micellar phase, thesolutions exhibit rheothining behavior typically at concentrationsabove 10-12%; (ii) upon heating an increase in both storage(G′) and loss (G′′) moduli as well as the zero shear viscosity isobserved within a narrow temperature range; (iii) hysteresis isobserved on cooling; (iv) gel-like systems withG′ > G′′ areformed at concentrations typically above 20-22%, at whichthe initially disordered micellar dispersions transform into anordered lattice; (v) in the gel phase the moduli are comparablein magnitude with slightly predominant elasticity; (vi) the gelsformed can be classified as “soft” (G′ ≈ 103 Pa) and “hard”(G′ > 104 Pa).

Aiming to diversify the properties of the Pluronic copolymers,attempts have been made to substitute the blocks of PPO withpoly(butylene oxide) and to alter the chain architecture.17-26 Inrecent studies,27-29 we have reported the preparation andaqueous solution properties of a series of analogues to Pluroniccopolymers based on polyglycidol (PG) in which the hydrophilicPEO blocks have been substituted by blocks of PG. PG is ahydrophilic and biocompatible30 material, bearing a hydroxylgroup in each repeating unit (see Figure 1 for the formulas ofthe monomer units of PEO, PPO, and PG).

Table 1 gives the composition, molecular weights, andabbreviations of the novel copolymers. Considering theircomposition and particularly the PPO molecular weight, theytake a position between the Pluronic series L61-F68 and L72-F77. Similarly to Pluronic copolymers, the PG-based copolymers(hereinafter LGP copolymers) self-associate in water above acertain critical concentration (cmc), which depends on the PGcontent and temperature.27 However, the particles that areformed are considerably larger in size (63-1300 nm) andaggregation number (>100) than the familiar micelles of thePluronic copolymers.28 In the interior of the large particlesdiscrete PPO domains of slightly prolate spherical shape, asrevealed by small angle neutron scattering (SANS),29 aredistributed in a hydrogen-bonded continuous medium consistingof unassociated chains and water. At concentrations well above

* Corresponding author. Tel.:+ 359 2 9792293. Fax:+ 359 2 8700309.E-mail: rangelov@polymer.bas.bg.

Figure 1. Monomer units of (a) poly(ethylene oxide), (b) poly-(propylene oxide), and (c) linear polyglycidol.

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10.1021/jp0742463 CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 01/26/2008

the overlap concentration of the large particles, the PPO domainsarrange in a cubic lattice.29

With the present article, we aim to relate the molecularcomposition and aggregate structure with the rheologicalproperties of the aqueous solutions of the LGP copolymers. Twolimiting concentration ranges were selected to investigate theeffects of the PG content and temperature: dilute (0.5-5.0 wt%) and concentrated (33 wt %) solutions.

Experimental Section

Materials. The copolymers used in the present study weresynthesized as described elsewhere27 and characterized by gelpermeation chromatography and1H nuclear magnetic resonance(NMR). They are symmetric linear triblock copolymers of ageneral formula (G)n(PO)34(G)n, where G and PO denoteglycidol and propylene oxide units, respectively, andn variesfrom 3 to 70. The composition, polyglycidol content, totalmolecular weight, and the codes of the copolymers are givenin Table 1. Their aqueous solution properties have beeninvestigated by dye solubilization, turbidimetry, and NMR,27

light scattering and cryogenic transmission electron micros-copy,28 and SANS.29

Sample Preparation.Aqueous solutions in the concentrationrange from 0.5 to 33 wt % were prepared by dilution of stocksolutions. The latter were prepared gravimetrically by addingwater to a preweighed quantity of the copolymer and allowingit to mix overnight by shaking occasionally. An additionalamount of water was then added to obtain solutions of desiredconcentration.

Methods. Oscillatory shear and steady shear experimentswere carried out on a Thermo Haake 600 rheometer equippedwith cone/plate and plate/plate geometry, respectively. Thediameter and the gap width of the rotating inner bob were 60and 2 mm, respectively, whereas the cone sensor diameter was25 mm. Both series of experiments were carried out in acontrolled stress mode. Copolymer solutions were transferredto the instrument and carefully overlaid with a low-viscositysilicone oil to minimize water evaporation. Oscillatory experi-ments were performed for high-viscosity samples (concentratedsolutions). The storage modulus (G′) and the loss modulus (G′′)were measured over the frequency range 0.05-100 Hz. Thevalues of the stress amplitude were checked to ensure that allmeasurements were performed within the linear viscoelasticregion, where the dynamic moduli are independent of the appliedstress. For low-viscosity samples (dilute solutions), steady shearexperiments were performed in the range 0.5-1000 s-1. Themeasurements were performed at different temperatures ranging

from 15 to 70 °C. The temperature was controlled with anaccuracy of(0.1 °C.

Results and Discussion

Dilute Solutions.Steady shear viscosity measurements wereperformed in the dilute solution region (0.5-5 wt %). Theinvestigated concentration range is above the cmc’s,27 implyingthat we typically deal with aggregates. Despite the large particledimensions,28 viscosity measurements reveal that the solutionsnormally behave as Newtonian fluids; that is, viscosity isindependent of shear rate, as shown in Figure 2 for the 2 wt %solution of LGP68. Exceptions, however, are the solutions ofthe copolymer of PG content of 50 wt % (LGP65), which exhibita weak shear rate dependence of the viscosity (Figure 2). Thisfinding is in line with the light scattering results showingmaximum (several hundreds nanometers) in the dimensions ofthe particles formed by LGP65.28 At concentrations well abovethe cmc, at which the overlap concentration is presumablyexceeded, the solutions are no longer Newtonian and the shearthinning becomes more pronounced.

The zero shear viscosity values were obtained by extrapolat-ing the viscosity curves to zero shear rate. The viscosities atzero shear rate as a function of temperature and PG content atconcentration of 0.5 wt % are presented in Figure 3. Thefollowing features give a general picture of the behavior of thecopolymers in the dilute regime: (i) the viscosities havepronounced maxima at 50 wt % PG content; (ii) at PG contentse40 wt % andg70 wt % the zero shear viscosities are ofroughly the same order of magnitude; (iii) the effect ofincreasing concentration (not shown) is substantial for LGP65.For the rest of the copolymers, increasing concentration doesnot cause substantial changes in the zero shear viscosity in the

TABLE 1: Polyglycidol Content, Composition, CopolymerMolecular Weight, and Codes of the Copolymers Used inThis Study

PG content (wt %) compositiona molecular weight codeb

20 (G)3(PO)34(G)3 2400 LGP6230 (G)6(PO)34(G)6 2900 LGP6340 (G)8(PO)34(G)8 3200 LGP6450 (G)13(PO)34(G)13 3900 LGP6560 (G)17(PO)34(G)17 4500 LGP6670 (G)26(PO)34(G)26 5800 LGP6780 (G)51(PO)34(G)51 9500 LGP6884 (G)70(PO)34(G)70 12 400 LGP68+

a G and PO denote glycidol and propylene oxide monomer units,respectively.b The last digit multiplied by 10 gives the content of thehydrophilic polyglycidol in wt %; the only exception from this rule isthe copolymer LGP68+, which contains 84 wt % polyglycidol. Figure 2. Viscosity versus shear rate for 2 wt % aqueous solutions of

LGP68 (2) and LGP65 (9). Temperature 40°C.

Figure 3. Zero shear viscosity as a function of PG content andtemperature for aqueous solutions of PG-PPO-PG block copolymersat a concentration of 0.5 wt %.

1900 J. Phys. Chem. B, Vol. 112, No. 7, 2008 Halacheva et al.

investigated concentration interval; (iv) the zero shear viscositygenerally decreases with increasing temperature. The effect oftemperature is most pronounced in the interval 25-40°C, whichimparts the different shapes of the curves at 25°C. From 40 to50 °C, the viscosities decrease is marginal, whereas practicallyno changes were observed with a temperature increase from 50to 60 °C.

A remarkable aspect disclosed by the rheological measure-ments is that no systematic dependence of zero shear viscosityon the PG content was observed. The Newtonian rheologyobserved for the samples of PG contentse40 wt % andg60wt % for all concentrations and temperatures is consistent withwell-separated and isotropic particles.31,32 In that context, thenon-Newtonian behavior and the abrupt increase of the zeroshear viscosity of LGP65 could be attributed to the formationof large aggregates or highly anisotropic, often referred to asthreadlike, particles. However, the light scattering experiments28

do not evidence structural evolution and particle shape transitionwith temperature and copolymer composition. In fact, thevariations of the zero shear viscosity copy exactly the variationsof the hydrodynamic radii of the aggregates, which were foundto go through a maximum centered at 50 wt % of PG content.28

Apparently, the zero shear viscosity maxima can be related tothe large (hundreds of nanometers) particles formed by LGP65.Later in the text, we speculate on another possible reason.

Furthermore, the aggregates of the LGP copolymers werefound to decrease in size and aggregation number with risingtemperature,28 which is manifested by a noticeable decrease ofthe zero shear viscosity (cf., the curves at 25 andg40 °C inFigure 3). The most dramatic changes occurred in the temper-ature interval 25-40 °C, because with a further temperatureincrease the macroscopic response of the systems is considerablyless pronounced.

Concentrated Solutions: Moduli as a Function of Fre-quency.To characterize the mechanical properties, frequency-dependent oscillatory experiments were carried out. The mea-surements were performed on all copolymers at temperatures15 and 60°C, concentration of 33 wt %, and constant shearstress. At this concentration, the solute volume fraction changesonly slightly (in the 0.30-0.32 range) with copolymer molecularweight. Figure 4 gives the mechanical spectra of LGP64,LGP65, and LGP67 that typify the behaviors of the copolymersof PG contentse40, 50, andg60 wt %, respectively. The firstimpression from Figure 4 is that the two moduli for allcopolymer solutions and temperatures studied show frequencydependence, which is indicative for the presence of lyotropicmesophases.33 The finding is in line with the SANS results29

showing arrangement of the PPO domains in a cubic lattice.At lower PG contents (Figure 4a), the samples typically have

very low moduli values. In the frequency range studied and atboth temperatures the solutions behave as viscous fluids with aloss modulus,G′′, larger than the storage modulus,G′. As thePG content is increased tog60 wt % (Figure 4b), the systemsshow both fluid-like and gel-like behaviors. At 15°C thesolution of LGP67 shows non-elastic responses at smalldeformation, that is,G′′ > G′, whereas at high frequencies thestructures do not have time to relax from the applied deformationand store energy elastically, which results inG′ rising aboveG′′. At 60 °C G′ ≈ G′′ for nearly the whole frequency range;only at high frequencies do the two moduli tend to diverge,giving rise to an appearance of a flow zone. In good agreementwith the results obtained in the dilute limit, the behavior ofLGP65 is distinctly different from that of the rest of thecopolymers. Figure 4c presents the rheological properties of the

33 wt % solution of this copolymer. At 15°C and lowerfrequencies, the solution is more elastic withG′ ≈ G′′ than athigher frequencies where the response is clearly non-elastic. At60 °C the solution is entirely elastic at all frequencies. This isthe main difference from the solutions of the rest of thecopolymers, which at these conditions are either fluids (G′′ >G′) or viscoelastic fluids withG′ ≈ G′′. Another feature is themagnitudes of the moduli, which were found to be considerablygreater than the corresponding moduli of the solutions of therest of the copolymers. This is better illustrated in Figure 5,which shows the storage and loss moduli measured at afrequency of 0.1 Hz and 60°C as a function of PG content. Aswith the steady shear experiments, pronounced maxima of bothmoduli at a PG content of 50 wt % are observable. However,in contrast to the zero shear viscosity in the dilute limit, bothmoduli show steady rise with the PG content (cf., PG contentintervals 20-40 and 60-84 wt % in Figures 3 and 5).

The evolution of the moduli with the PG content is notablebecause it shows very nicely the transitions from a non-elastic

Figure 4. Storage,G′ (triangles), and loss,G′′ (squares), moduli as afunction of frequency at 15°C (open symbols) and 60°C (closedsymbols) for (a) LGP64, (b) LGP67, and (c) LGP65. Copolymerconcentration 33 wt %.

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to elastic behavior of the solutions. At PG contents up to 40 wt%, the responses are clearly non-elastic asG′′ > G′. Both moduliabruptly increase andG′ exceedsG′′ at a PG content of 50 wt%, indicating that the system is predominantly elastic. Followinga fall in the magnitudes of the moduli at PG contents of 60 wt% and more, the samples exhibit both fluid-like and gel-likebehavior with comparable storage and loss moduli. In thatcomposition range, theG′/G′′ ratio increases slightly withincreasing PG content, implying slight enhancement of theelastic character of the samples. Probably, the most intriguingfinding is the large moduli values and the clear elastic responseof the solution of LGP65.

Concentrated Solutions: Moduli on Heating. In thissection, the rheological properties of aqueous solutions of thecopolymers are studied as a function of temperature. Theviscoelastic measurements were performed at a frequency of0.04 Hz, 33 wt % solutions, and a heating rate of 1°C/ min.The results are presented in Figure 6.

Figure 6a shows the variations of the loss moduli withtemperature for all copolymers studied. The curve patterns arequite similar to those previously observed not only for PEO-PPO-PEO copolymers but also for polymers of rather differentnature.9,12-16,34-38 The fully developed evolution of the modulus,which applies for the storage modulus as well, displays at leastthree distinct regions: (i) a low-temperature region in whichthe moduli do not or hardly change; (ii) a narrow interval of asharp increase; and (iii) a high-temperature region where themoduli reach plateau values. All three regions for the storageand loss moduli were observed for LGP65 (Figure 6a and b).Even an additional high-temperature region of a slight drop ofboth moduli is visible. For LGP62, LGP67, LGP68, andLGP68+, the plateau values ofG′′ are either not (the first two

copolymers) or nearly (the latter two) reached (Figure 6a). Threeregions are observable for LGP64 and LGP66; however, onlya several-fold increase ofG′′ was registered as compared to anincrease of typically orders of magnitude for the rest of thecopolymers (Figure 6a).

The variations of the storage moduli with temperature arepresented in Figure 6b. At temperatures below ca. 40°C, storagemoduli were not detected, indicating that solutions behaved asliquids because presumablyG′′ . G′. The only exception wasLGP65 displaying a detectable storage modulus in the wholetemperature region. At temperatures above 40°C the storagemoduli appeared and were found to sharply increase with afurther temperature increase. GenerallyG′ increases faster thanG′′, indicating that the systems become increasingly elastic athigher temperatures. However, in the investigated temperaturewindow, crossovers, at which the elasticity becomes predomi-nant, were observed only for LGP65, LGP68, and LGP68+ (seebelow Figure 7, where the storage and loss moduli are plottedtogether). Even in those cases,G′ only slightly exceedsG′′.Noteworthy, the crossover temperatures are above 60°C forthe latter two copolymers, whereas for LGP65 it is 35°C.

Concentrated Solutions: Moduli on Heating-CoolingCycles.The viscoelastic properties of the aqueous solutions ofPG-PPO-PG copolymers were investigated during temperaturesweeps 15-70-15 °C (Figure 7). In general, large hystereseswere observed for all copolymers and, at the end of the cycles,the systems did not return to their initial state. Althoughexhibiting a general similarity, the curve patterns are somewhatdifferent, indicating the effect of the copolymer composition.The thermal cycles also exhibited similarity to those previouslyobserved for Pluronic F108.13 According to these authors, thelarge hystereses are due to kinetic effects deriving from theintermicellar molecular entanglements; the relaxation of suchentangled structures is additionally hindered due to the highviscosity of the systems at elevated temperatures. In addition,as shown elsewhere,27 the solubility of PG decreases withdecreasing temperature, which may also contribute to theappearance of the hystereses. It is noteworthy that samples werekept at the end of the cycle, that is, at 15°C for hours until theinitial values of the moduli were reached, as demonstrated inFigure 8 for LGP64.

Another aspect of the systems to be considered is thecrossover temperatures on heating and cooling and the onsettemperatures of moduli increase. The first two are indicativefor the gelling and melting points, respectively, whereas thelatter are transition temperatures for possible structural changesin the solution. Unfortunately, in the investigated temperaturewindow some of the crossover temperatures were not observed.

Figure 5. Storage,G′ (2), and loss,G′′ (9), moduli as a function ofPG content at a frequency of 0.1 Hz and temperature of 60°C.Copolymer concentration 33 wt %.

Figure 6. Variations of loss (a) and storage (b) moduli with temperature for 33 wt % aqueous solutions of LGP62 (b), LGP64 (0), LGP65 (2),LGP66 (]), LGP67 ([), LGP68 (4), and LGP68+ (9) at a frequency of 0.04 Hz and a heating rate of 1°C/min.

1902 J. Phys. Chem. B, Vol. 112, No. 7, 2008 Halacheva et al.

Figure 9 summarizes the results obtained from the temperaturesweeps. Because the method for determination of the gelling/melting and transition temperatures is known to be dependenton the frequency and heating/cooling rate used in the experi-ments, the results in Figures 9 describe the kinetic behaviorrather than the behavior in an equilibrium state.

The upper and lower curves in Figure 9 represent the gellingupon heating and melting upon cooling boundaries, respectively.Thus, above the upper curve elastic gels with slightly predomi-nantG′ are formed. Below the melting boundary (lower curve)the systems are clearly viscous liquids. In the region betweenthe two curves, which indicates the width of the hystereses, thesystems exhibit either a liquid-like (on heating) or a gel-like(on cooling) behavior. The dashed curve in this region corre-sponds to the boundary at which the moduli start to sharplyincrease on heating, which could be associated with possiblestructural changes leading to gelation on further heating. Upon

cooling, the structural changes are retarded or occur graduallyrather than sharply as on heating (see Figure 7).

Composition-Structure-Properties Relationships.It hasbeen previously shown that seemingly small chemical modifica-tion of the hydrophilic block as compared to the Pluronic blockcopolymers (Figure 1) is manifested in large differences in theaggregation behavior.27-29 Although the fundamental mechanismof the aggregation (an entropy-driven process) is not alteredand the cmc values are from hardly to slightly changed uponthe substitution of the PEO with PG,27 the aggregates of theLGP copolymers appeared quite large in size and aggregationnumber. Two equally involved attractive processes, that is,hydrophobic interactions of the PPO blocks and multiple intra-and interchain hydrogen bonding in the PG moieties, result inthe formation of large compound particles.28 The most intriguingfinding is that the average density of the polymer material withinthe particles is low and comparable to the density of flexiblepolymer coils in a good solvent.28 Furthermore, in line withthis finding are the results from the investigations of the internalstructure by SANS revealing coexistence of slightly prolate PPOdomains and individual copolymer chains in the interior of themulticore particles.29 Despite the large particle dimensions mostof the copolymers display Newtonian rheology in the diluteregime. Obviously, the specific particle structure, that is, manyPPO domains that are spaced by individual copolymer chains,which results in low overall density of the polymer materialand noncompact interior, determines this behavior. The onlyexception is the behavior of LGP65 exhibiting shear thinning,

Figure 7. Storage modulus,G′ (triangles), and loss modulus,G′′(squares), as a function of temperature for 33 wt % aqueous solutionsof (a) LGP68, (b) LGP65, and (c) LGP64 during heating-coolingcycles. Closed symbols: heating. Open symbols: cooling.

Figure 8. Storage,G′ (2), and loss,G′′ (0), moduli of a 33 wt %solution of LGP64 as a function of time at 15°C. The sample wasfirst heated to 70°C and then cooled to 15°C. Heating/cooling rate 1°C/min, frequency 0.1 Hz.

Figure 9. Variations of the onsets of the moduli increase (2 and dashedcurve), and crossover temperatures on heating (9 and upper curve)and cooling (b and lower curve) as a function of PG content. Frequency0.1 Hz, heating/cooling rate 1°C/min, concentration 33 wt %. See textfor more information.

Rheology of Analogues to Pluronic Block Copolymers J. Phys. Chem. B, Vol. 112, No. 7, 20081903

maximum in zero shear viscosity, and a substantial effect ofincreasing concentration (Figures 2 and 3). To explain this wecan speculate on the existence of a critical particle dimensionbeyond which the size, rather than the specificities of the interior,has a decisive role or, alternatively, that the LGP65 aggregatesare of different (higher) density and compactness. It must benoted that at this stage of investigations little is known aboutthe density of the polymer material in the aggregates of LGP65.However, preliminary results on copolymers having the same(50 wt %) content of PG but different PPO block lengths indicatedeviations in the size and structural variations with the PGcontent, implying that this particular PG content is critical.

The size and molar mass of the large compound particleslinearly decrease with increasing temperature.28 The firstimpression is that this finding does not correspond to the inzero shear viscosity variations with temperature, more pro-nounced in the 25-40 °C range, marginal between 40 and 50°C, and practically no changes were observed in the 50-60 °Cinterval (Figure 3). However, it should be taken into accountthat upon the molar mass decrease the number of the particleseffectively increases, which probably compensates the effectof the size decrease on zero shear viscosity. The combined effectof these rearrangements is reflected by the in zero shear viscosityvariations with temperature documented in this study.

In the regime of high concentrations, the large compoundparticles are no longer well-separated; they fuse, the PPOdomains get in close contact because their number increaseswith increasing concentration, and arrange into a cubic lattice.29

The formation of such a liquid-crystalline mesophase isresponsible for the frequency dependence of the moduli (Figure4). The behavior of the systems is thus largely determined bythe PPO domains. The latter interact via PG chains of varyinglength grafted on their surface. Unassociated copolymer chains,detected even at high concentrations and temperatures,29 mediatethe interdomain interactions. The longer are the PG chains, thestronger are the interactions (entanglements), which is consistentwith the non-elastic response of the copolymers of short PGchains and the increasing elastic character at PG contents above60 wt % (Figure 5). In that context, the clear elastic responseand maxima in the moduli values of LGP65 (Figure 5) issomewhat conflicting but can be rationalized in terms offormation of particles of supposedly different structure repeat-edly manifesting itself in deviating behaviors in the concentratedas well as dilute regimes (see Figures 3, 5, and 9).

Both constituent blocks of the LGP copolymers are thermo-sensitive, but in contrast to those of the Pluronic blockcopolymers their solubility in water changes in oppositemanners,27 which imparts a number of differences in theaggregation behavior and properties of the particles. The mostdrastic are the changes in the domains;29 we have found that attemperatures as low as 15°C small size domains (pre-domains)exist and upon increasing temperature they serve as nucleationcenters for the formation of the PPO domains. With a furthertemperature increase the surface of the latter becomes sharperand more distinct and the number of the PPO chains in thedomains increases. These processes are opposed by the increas-ing solubility of the PG moieties, which has been found togradually decrease the particle dimensions and molar massesin the dilute regime28 and even at sufficiently high temperaturesand/or PG contents cause disintegration.28 However, the tem-perature variations of the moduli (Figure 6), the hystereses inthe heating-cooling cycles (Figure 7), and the dependencesshown in Figure 9 are qualitatively similar to those observedfor the Pluronic copolymers,8,13 implying that the systems are

apparently not sensitive to the variations in the solubility ofPG. This is not surprising because the gel-like behavior of theconcentrated solutions of both Pluronic and LGP copolymersis related to the close packing of the same structural units. Theeffect of the opposite solubility of PG is seen in the retardedrecovery of the initial state in the temperature cycles (Figure8).

Conclusions

Rheological measurements were used to study the propertiesof aqueous solutions of a series of polyglycidol-poly(propyleneoxide)-polyglycidol block copolymers. Typically Newtonianin the vicinity of the cmc, the solutions exhibit rheothiningbehavior at concentrations well above the cmc. However,rheothining is observed at considerably lower (up to 5 wt %)concentrations than those of the Pluronic copolymers (typically10-12 wt %). The dilute solutions of LGP65 (50 wt % PGcontent) behaved as non-Newtonian fluids in the whole dilutelimit investigated, which imparts pronounced maxima of thezero shear viscosity versus PG content plots. These findingsare consistent with the presence of large non-micellar and oflow density aggregates formed by the LGP copolymers.28 Incontrast to Pluronic copolymers, the zero shear viscosity wasfound to decrease with increasing temperature due to the particlesize reduction.28 At higher concentrations (33 wt %), themechanical properties were studied using frequency-dependentoscillatory experiments. BothG′ and G′′ for all copolymersolutions and temperatures studied show frequency dependence.The evolution of the moduli with PG content at 60°C exhibitstransitions from a non-elastic to elastic behavior with a clearelastic response at a PG content of 50 wt %. The measurementsmade as a function of temperature reveal a number of similaritiesbetween the rheological properties of LGP and Pluroniccopolymers, for example, a sharp increase of both moduli withina narrow temperature interval, enhancement of elasticity athigher temperatures, comparable in magnitudesG′ andG′′ withslightly predominant elasticity, and large hystereses during theheating-cooling cycles. With regard to the magnitudes ofG′the gels formed by this particular series of LGP copolymerscan be considered as soft. A phase diagram that defines areasof PG contents and temperatures in which the systems pre-dominantly exhibit either fluidity or elasticity is constructed.

Acknowledgment. The donations of Alexander von Hum-boldt Stiftung and the Bulgarian Academy of Sciences topurchase the rheometer Thermo Haake 600 are gratefullyacknowledged. Dr. V. Samichkov is thanked for the criticalreading and helpful discussions.

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