Contribution to the explanation of the association

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Oct 1, 2016 - a computer permitting the control of parameters via the software. Rheocalc (V2. 4), (Brookfield Engineering Laboratories Inc., USA) [3,. 29].
Journal of Molecular Liquids 224 (2016) 279–283

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Contribution to the explanation of the association process of two triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers and their mixtures in an aqueous solution N. Ghaouar a,b,⁎, M. Baroudi a, T. Othman a a b

Université de Tunis El Manar, Faculté des Sciences de Tunis, Laboratoire de Physique de la Matière Molle et de la Modélisation Electromagnétique, 2092 Tunis, Tunisie Université de Carthage, Institut National des Sciences Appliquées et de Technologie, Centre Urbain Nord, BP. 676, Tunis, Tunisie

a r t i c l e

i n f o

Article history: Received 20 August 2016 Accepted 23 September 2016 Available online 01 October 2016 Keywords: Pluronics F68 and L64 Micelles Mixtures Gel phase

a b s t r a c t An explanation of various transitions occurring in aqueous solutions of two triblock polyethylene oxide (PEO)/ polypropylene oxide (PPO)/polyethylene oxide (PEO) copolymers (Pluronics L64 and F68) and their volumetric mixtures is given. We first characterized the conformational changes of the individual polymer under shear versus temperature. The L64 did not form micellar conformations or a gel phase. A schematic model is proposed explaining the transitions for L64 in the range of temperatures varying from 20 to 80 °C. For the mixture with the volumetric ratio of F68:L64 = 4:1, a shift in the compaction temperature of the micelles from 30 to 45 °C was observed and the gel phase becomes large. For mixture with the volumetric ratio of F68:L64 = 1:4, a shift was observed in the compaction temperature but the gel phase did not occur. A schematic model is proposed explaining the various transitions occurring for a mixture of F68:L64 = 1:4. © 2016 Elsevier B.V. All rights reserved.

1. Introduction For some triblock copolymers based on poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in aqueous solutions, the existence or non-existence of free individual triblock molecules, micelles and/or elongated structures are controlled by the critical micellar temperature (CMT) at fixed concentrations [1–6]. For temperatures lower than the CMT, the PEO and the PPO blocks are both water-soluble and the copolymers remain as individual soluble molecules of PEO-PPO-PEO. With increasing temperatures, the PPO blocks become insoluble as the hydrophobic interactions increase, inducing an enhancement of the number of aggregates and the formation of spherical micelles around the CMT [1–6]. The micelles formed are constituted by hydrophobic PPO cores and hydrated PEO blocks coronas. Thus, several structural and conformational transitions can occur; the micelles become structured and organized to form, probably, after successive conformational transitions can occur leading to a simultaneous increase in the viscosity due to the intensification of the inter-micellar attractive interactions, with a physical gel being developed in certain situations [7–11]. The micelles become structured and organized to form behind successive conformational transitions lyotropic liquid crystalline phase. The transitions are described as follows: (i) individual micelles organize to form a body⁎ Corresponding author at: Université de Tunis El Manar, Faculté des Sciences de Tunis, Laboratoire de Physique de la Matière Molle et de la Modélisation Electromagnétique, 2092 Tunis, Tunisie. E-mail address: [email protected] (N. Ghaouar).

http://dx.doi.org/10.1016/j.molliq.2016.09.123 0167-7322/© 2016 Elsevier B.V. All rights reserved.

centered cubic phase of spherical micelles; (ii) body-centered cubic phase to hexagonal phase; (iii) hexagonal phase to lamellar phase [3, 12–14]. These properties have resulted in their use in many applications, especially in the pharmaceutical field [15–19]. For example, the hydrophobic micelle core with PPO blocks is able to load lipophilic drugs, but is surrounded by the hydrophilic corona constituted with PEO blocks providing micellar stability [20]. However, the formation of spherical micelles as well as their crystallization in various packings or the formation of gel phases has not been evident for some L64 solutions [21,22]. Some of them do not present the same transitions and the observed increase in the viscosity with temperatures does not signify the formation of a viscous gel phase [21,22]. Despite the numerous studies reporting, in detail, the micellization process and various conformational transitions of Pluronics in solution, information on the mixtures of two Pluronics in aqueous solution are still missing. It is known that triblock copolymer mixtures are expected to form mixed micelles since they have equivalent hydrophobicity [23,24], whereas for the mixtures of two Pluronic copolymers with different hydrophobic block lengths, there exist different claims on the nature of the micelles [23–27]. In this work, we first studied the rheoviscosimetric properties of the individual Pluronics L64 and F68 polymers in aqueous solution at fixed concentration and various temperatures. The Pluronics L64 and F68 have the same hydrophobic block lengths and dissimilar hydrophilic block lengths. Secondly, based on the obtained results for the individual Pluronics and the several interpretations given in the literature [21,22], we investigated the rheoviscosimetric properties and the various transitions that occurred in binary mixtures of Pluronics

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L64 and F68 in water. Schematic models explaining various conformational changes of Pluronics L64 and F68 and their mixtures are given. 2. Materials and methods The PEO-PPO-PEO block copolymers, Pluronics L64 and F68, were donated by BASF Corp. (Germany) and were used without further purification. The Pluronic L64 copolymer has a nominal molecular weight of 2900 and is represented by the formula (PEO)13(PPO)30(PEO)13 whereas the F68 copolymer has a molecular weight of 8600 and is represented by the formula (PEO)78(PPO)30(PEO)78. The deionized water used was treated with a Millipore-Q water purification system. The sample preparation was based on the “Cold Method” technique [28]. This method consists in the dissolving of the copolymer triblocks in cold water where the temperature does not exceed 4 °C. After agitation, the solution was equilibrated for 24 h at 4 °C [28]. The samples were kept at room temperature for 30 min before use. Rheoviscosimetric measurements in shear were performed using a Brookfield DV-II + Pro Viscometer cone-plate geometry (Brookfield Engineering Laboratories Inc., USA). The temperature was controlled by a thermostatic controlled water bath, type TC-502D-230, (Brookfield Engineering Laboratories Inc., USA), with an accuracy of 0.1 °C. The system was connected with a computer permitting the control of parameters via the software Rheocalc (V2. 4), (Brookfield Engineering Laboratories Inc., USA) [3, 29]. All curves were treated using the software OriginPro (OriginLab/ OriginPro 8, USA). All values were determined by performing five replicates. 3. Results and discussion 3.1. Rheoviscosimetric properties of pluronics F68 and L64 Fig. 1 shows the variation of the relative viscosity of an aqueous solution of Pluronic F68 as a function of temperature for a concentration of 30 mg/ml under shear. The viscosity varied following three regions according to the temperature where various transitions occurred. In the first region, in which the temperature varied from 20 to 30 °C, the relative viscosity decreased due to the diminution of the PPO blocks solubility and the increase of the hydrophobic interactions in solution that facilitated the association process between the individual PEO-PPOPEO triblocks to form micelle conformations. When the micelles were formed a compaction process occurred by the shrinkage of the coronas, explaining the minimal value of the viscosity around 30 °C [1,3]. In the second region, in which the temperature varied between 30 and

F68/water

2,7

60 °C, the viscosity increased indicating the onset of a viscous liquid phase. We suggest that between 30 and 50 °C, several conformational changes occurred. An association process between the compact micelles took place to ensure the formation of cubic and hexagonal network structures [1,3]. Above 50 °C, the viscosity increased rapidly indicating the presence of a series of radical changes in the structures formed. In fact, the structures formed interact with each other through the PEO blocks to form large and non-homogeneous arrangements as elliptical or prolate structures [21]. It was reported, using small-angle X-ray scattering, smallangle neutron scattering and indexation of the Bragg diffraction peaks, that during the thermal gelation a hard sphere crystallization of the micelles in a cubic phase usually occurred [30–32]. For example, Pluronics F88 and P85 [33] are able to form a body-centered cubic phase. Other works [34,35] suggested that Pluronic F127 forms a face-centered cubic structure for lower concentrations whereas, at a higher concentration, a body-centered cubic packing of micelles was observed [21]. Around 60 °C, packed elongated structures are formed which led to the formation of a lamellar phase [36,37]. This phase is constituted with an inner layer of poly(propylene oxide) protected by a layer formed with poly(ethylene oxide) blocks in contact with the solvent. In the third region, in which the temperature varied between 60 and 75 °C, the decrease in the viscosity indicates the end of the gel phase. In this range of temperatures, the PEO-PPO-PEO blocks are highly dehydrated which facilitate their disintegration from the lamellar structure leading to a macrophase separation between the solvent and the solute with various irregular conformations. This macrophase separation indicates the presence of a gel-precipitation transition. The solution is then divided into two phases; the first rich in dehydrated PEOPPOPEO blocks and the second rich in water. The macrophase separation leads to a sol-precipitation transition and precipitation-coalescence transition of Pluronic F68 at still higher temperatures as shown in the figures. Hun et al. [7], using rheological measurements, reported from the analysis of the phase diagram of PLGA-PEG-PLGA triblock copolymer in water at lower temperatures, the existence of the three regions (sol, gel, and precipitation) divided by three transition boundaries: a sol-gel transition, a gel-precipitation transition and a sol-precipitation transition. For that reason, in the range of temperatures lying between 20 and 75 °C, we consider the existence of four transitions for our Pluronic F68 aqueous solution, namely a sol-gel transition, a gel-precipitation transition, a sol-precipitation transition and a precipitation-coalescence transition. Fig. 2 shows the variation of the relative viscosity under shear of an aqueous solution of Pluronic L64 as a function of temperature for a

gel-precipitation

L64/water

2,6

20

Relative viscosity

2,4 2,3

sol-gel precipitation -coalescence

2,2 2,1 2,0

Relative Viscosity

sol-precipitation

2,5

15

10

5

1,9 1,8

0

1,7 20

25

30

35

40

45

50

55

60

65

70

75

Temperature (°C) Fig. 1. Variation of the relative viscosity of Pluronic F68 for a concentration of 30 mg/ml as a function of temperature.

20

30

40

50

60

70

80

Temperature (°C) Fig. 2. Variation of the relative viscosity of Pluronic L64 for a concentration of 30 mg/ml as a function of temperature.

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concentration of 30 mg/ml. The relative viscosity varied following three regions; it appears practically constant between 20 and 50 °C, there was an abrupt increase between 50 and 60 °C and then it slightly decreased for temperatures superior to 60 °C. In the first region, in which the temperature varied from 20 to 50 °C, the relative viscosity remained constant. This result is in discordance with those given in several studies [1–6] where it was shown that the viscosity of Pluronic solutions decreases in the same range of temperatures. Those results were generally interpreted according to the Arrhenius law which considers that, at low temperatures, the viscosity decreases for increasing temperatures due to the increasing molecular mobility [37] or by using the Israelachvili model [38,33]. A question thus arises: why the viscosity was constant for this range of temperatures and what is the model that we can adapt to explain the various transitions and conformational changes? To give an explanation for the constancy of the relative viscosity, Fig. 2, we consider two different possible assumptions: (i): we suggest, firstly, that in this range of temperatures the presence of shear prevents the onset of the association process between the PPO blocks of Pluronic L64 to form spherical micelles because the individual triblocks preserve their initial conformation and do not undergo any deformation under shear. This suggestion was rapidly eliminated because the association processes and the formation of micellar structures would not be prevented under shear for Pluronic F68 aqueous solution. (ii): we considered that the formation of spherical micelles did not occur for Pluronic L64. For Pluronic L64 the length of the hydrophilic PEO blocks was smaller than the length of hydrophobic PPO block. With increasing temperatures, the solubility of the PEO blocks is reduced and the hydrophobic attractive interactions between PPO blocks are developed. As the viscosity remained constant, we suggest that the process of dehydration and aggregation occurred without influencing the hydrodynamic volume occupied by the blocks. In fact, the association process was probably effectuated along the horizontal shear axis to form “braid” conformations. The size of the micelles was then approximately equal to the length of the PEO-PPOPEO blocks to form “elliptical packed” conformations in which the number of PEO-PPO-PEO blocks is limited. The saturation of the braid conformations induces a change in the balance of interactions in solution. The interactions become repulsive and there is no association between the braids formed. In view of that, the viscosity remained constant during this range of temperatures. A recent theoretical study using molecular simulations showed that the Pluronic L64 micelles are formed in ellipsoidal elongated structures and the interactions between them are repulsive [21]. This result was obtained by determining the difference in the interaction energy between the ellipsoidal conformations of elongated micelles along their horizontal axis and the spherical structures using the Israelachvili model [33,38]. In the second region, in which the temperature varies from 50 to 60 °C, we believe the small increase in the viscosity between 50 and 55 °C can be attributed to the association process of braids along their horizontal axes through the PEO blocks and to a certain disorder-order transition ensuring the gradual homogenization of the novel structures formed. Between 55 and 60 °C, the drastic increase in the viscosity shows the existence of a highly viscous liquid phase and a novel disorder in the homogeneity of the solution was established. This viscous phase is attributed to the manifestation of strong attractive interactions inter-braids through the PEO blocks inducing the formation of non-homogenous aggregates. As we have considered that the interactions between elongated braids are repulsive, the possibilities of the formation of large aggregates or the existence of a gel phase responsible for the enhancement of the viscosity is impossible. In this context, measurements were

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conducted by SANS for Pluronic L64 solutions for various concentrations. They showed that the percolation parameter was very low, in the vicinity of the cloud point, compared to the critical value relative to Pluronics association estimated at pc = 10.3 [33]. This fact eliminates the possibility of the formation of massive aggregates responsible for the increase in the viscosity around the cloud point. Therefore, the drastic increase was probably due to the establishing of a certain disorder between the dehydrated braids where they act as “thin sticks” randomly organized inducing an increase in the relative viscosity. We recall that the higher value of the relative viscosity in this range of temperature is in concordance with the results obtained for Pluronic L64 using numerical models [19]. In the third phase, where the temperatures varied from 60 °C to 80 °C, the viscosity decreased slightly with increasing temperatures. The individual PPO-PEO-PPO triblocks became highly dehydrated leading to their abrupt disintegration from the structure formed. The blocks undergo then a chaotic movement in solution where they lose their energy and a precipitation- coalescence transition starts. Fig. 3, represents a schematic model explaining the various transitions of Pluronic L64 in the range of temperatures varying from 20 to 80 °C. 3.2. Rheoviscosimetric properties of F68:L64 mixtures To a solution with a concentration of L64 of 30 mg/ml were added various amounts of F68 solution of concentration of 30 mg/ml. The mixtures were prepared according to the cold method in a volume of 4 ml. The mixtures F68:L64 have better solubility at lower temperatures than that at room temperature. The volumetric mixtures chosen are F68:L64 = 4:1 and F68:L64 = 1:4 respectively. Fig. 4 illustrates the variation of the relative viscosity of the mixtures for various temperatures under shear. For the mixture of F68:L64 = 4:1, the relative viscosity decreased more slowly between 20 and 45 °C compared with the variation observed for the solution of F68 (Fig. 1). In this range of temperature, we consider that with increasing temperatures the attractive interactions between PEO-PPO-PEO blocks led to the mixed association through the hydrophobic PPO blocks of F68 and L64. The solution behaved probably as a non-homogeneous mixture formed by coronas of PEO blocks with different lengths (mixed micelle) and others micelles formed by the same homogeneous blocks F68 or L64. Liu et al. [34] suggested that for a mixture of two block copolymers, one would obtain two types of micelles: mixed micelles containing both copolymer species and two separate families of micelles each incorporating only one copolymer species [20,23,36]. Consequently, if we consider that the compaction temperature of the micelles corresponds to the minimal value of the relative viscosity [3,38], we suggest that the majority of the micelles in the mixture are constituted with PEO blocks having dissimilar lengths because for a mixture of F68:L64 = 4:1 we remark that the compaction temperature was around 45 °C whereas for individual F68 in aqueous solution is around 30 °C. Above 45 °C, the relative viscosity increased slowly up to 65 °C and then rapidly to reach its maximum around 75 °C, contrary to individual F68 solution where the maximum was around 60 °C. In this range of temperatures, the hydrophobic interactions increase and the mixed micelles interact with their neighbours through PPO and PEO blocks to form a highly viscous liquid micelle phase. The shift in the maximum to 75 °C and the widening of the gel phase can be probably attributed to the formation of various crystalline networks with defects due to the dissimilarity of the PEO blocks constituting the mixed micelles. For temperatures superior to 75 °C, the viscosity decreased indicating the end of the gel phase. This decrease is followed by a macrophase separation between the solvent and the highly dehydrated crystalline arrangements leading to their precipitation and coalescence toward higher temperature. For the mixture of F68:L64 = 1:4, the PEO-PPO-PEO blocks relative to L64 were in the majority compared with those of F68. We suggest

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20°C

50°C

PEO PPO

Alignment of PEO-PP

PEO blocks along their horizontal axis

Formation of braid conformations

Thin stick randomly organized

Precipitation and

Disintegration of PEO-PPO-PEO blocks

Coalescence

80°C

60°C

Fig. 3. A schematic model explaining the various transitions of Pluronic L64 in the range of temperatures varying from 20 to 80 °C.

that the dissimilarity between the length of the PEO blocks relative to L64 and F68 in the mixture favoured the association of PEO-PPOPEO along their horizontal axes to form “braid” conformations with lateral hydrophilic blocks having dissimilar lengths. This explains the lower values of the viscosity compared with the mixture F68:L64 = 4:1 in

3,4

F68:L64 = 1:4 F68:L64 = 4:1

3,2 3,0

Relative Viscosity

2,8 2,6 2,4 2,2 2,0

the range of temperatures varying between 20 and 30 °C. Beyond 30 °C, the viscosity decreases slightly to reach its minimal value around 40 °C. This decrease probably originated to the contraction of the braid due to the reduction of the solubility of the PEO blocks. For temperatures superior to 45 °C, the viscosity of the mixture increased slowly to reach its maximum around 60 °C. We notice that the maximum of viscosity reached was lower than these obtained for individual L64 and F68 solutions around 60 °C which excluded the presence of the gel phase. To describe this, we have considered that the association process between braids occurred through the hydrophilic PEO blocks to form “ring” conformations as we have considered that the inter-braids interactions are repulsive through the hydrophobic PPO blocks. Indeed, the lower values of the viscosity are then due to the fact that the ring conformations facilitate the flow of the solvent. Above 60 °C, the viscosity of the solution decreases indicating that the highly dehydrated rings are disintegrated and a coalescence process was started. In Fig. 5, we illustrated a schematic model explaining the various transitions occurred in the mixture of F68:L64 = 1:4 in the range of temperatures varying from 20 to 80 °C.

1,8

4. Conclusion

1,6 1,4 1,2 15

20

25

30

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40

45

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55

60

65

70

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80

Temperature (°C) Fig. 4. Variation of the relative viscosity for mixtures of F68:L64 = 4:1 and F68:L64 = 1:4 as a function of temperature.

We recall that during this work, we have given an explanation of the various transitions of PEO-PPO-PEO copolymers triblocks in an aqueous solution. We have chosen Pluronics L64 and F68 having the same hydrophobic blocks length and dissimilar hydrophilic blocks length. We have studied firstly the various conformationel changes of Pluronic F68 under shear and identified the various transitions versus temperature. Secondly, we are interested to the identification of the various

N. Ghaouar et al. / Journal of Molecular Liquids 224 (2016) 279–283

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20°C F68

L64

Mixedbraids

Alignment of PEO-PPO-PEO blocks L64 and

containing both copolymer species

F68 Diminution of the solubility of PEO of F68 and L64

Repulsive

interactions Dehydratation Disintegration of

Formation of ring conformations

andcoalescence ring conformations

75°C

60°C

40°C

Fig. 5. A schematic model explaining the various transitions occurred in the mixture of F68:L64 = 1:4 in the range of temperatures varying from 20 to 80 °C.

conformationel changes of Pluronic L64. We have demonstrated that the L64 does not form micelle conformations and gel phase. We have proposed a schematic model explaining the various transitions of Pluronic L64 in the range of temperatures varying from 20 to 80 °C. The volumitric mixtures between F68 and L64 in aqueous solution are also studied. We have observed a shift in the compaction temperature of micelles for a mixture of F68:L64 = 4:1 and the gel phase become large. We have considered that the dissimilarity of PEO blocks induces the formation of crystalline networks with defects responsible to the shift of the compaction and the growth of the gel phase. For a mixture of F68:L64 = 1:4, we have showed that even the shift observed in the temperature of the compaction, the gel phase is missed. We have proposed a model explaining the various transitions occurred for a mixture of F68:L64 = 1:4. Acknowledgement The authors wish to acknowledge the Ministry of Superior Education and Scientific Research and Technology of Tunisia, which has facilitated the carried work. References [1] N. Ghaouar, M. Ben Henda, A. Aschi, A. Gharbi, J. Macromol. Sci.: Phys 50 (2011) 2150. [2] N. Ghaouar, M.M. Jebari, A. Aschi, A. Gharbi, E-Polymers (2005) 5621. [3] (a) M. Ben Henda, N. Ghaouar, A. Gharbi, J. Polym. (2013) 1–7; (b) M.M. Jebari, N. Ghaouar, A. Aschi, A. Gharbi, Polym. Int. (2006) 55176. [4] R. Gérard, Prog. Poly. Sci. A. 28 (2003) 1107. [5] M. Almgren, P. Bahadur, M. Jansson, P. Li, W. Brown, J. Colloid Interface Sci. 151 (1992) 157. [6] C. Perreur, J.P. Habas, J. François, J. Peyrelae, A. Lapp, Phys. Rev. E 65 (2002) 041802.

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