Effect of additives on the micellization of PEO/PPO/PEO block copolymer F127 in aqueous solution

Abstract

The micellization of an ethylene oxide-propylene oxide (PEO-PPO-PEO) symmetrical triblock copolymer (Pluronic®) F127 (EO₉₉PO₆₅EO₉₉, mol. wt. PPO=1750, %PEO=70) in aqueous solution in the presence of various additives (i.e. sodium chloride, urea and sodium dodecyl sulfate (SDS)) is examined by cloud point, surface tension, dye spectral change, sound velocity, viscosity and dynamic light scattering measurements over the temperature range 25–50°C. The critical micelle concentration (CMC) of copolymer altered significantly in the presence of additives. While the addition of sodium chloride lowered the CMC, the addition of urea showed the reverse trend. The presence of added sodium chloride develops hydrophobicity in the PPO moiety and reduces hydrophilicity of PEO moieties, favoring micellization of the block copolymer at relatively lower concentrations than in water at ambient temperature. The increase in CMC of copolymer in presence of urea is interpreted in terms of enhanced solubility of semipolar PPO moiety and also PEO moiety. The critical micelle temperatures (CMTs) show a marked decrease in the presence of added sodium chloride. CMCs obtained by different methods are in good agreement. The addition of SDS to aqueous copolymer solutions leads to the formation of copolymer-SDS complex (or mixed micelle) showing polyelectrolyte nature. Surface tension/dye spectral change measurements reveal aggregation of SDS taking place at concentration much below its CMC, indicating clearly SDS-copolymer interaction. The addition of SDS suppresses the micellization of copolymer and beyond a particular SDS concentration only, SDS micelles with one or two copolymer molecules are present predominantly.

Introduction

Polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO/PPO/PEO) are commercially available non-ionic polymeric surfactants due to two dissimilar moieties, i.e. hydrophilic PEO block and hydrophobic PPO block, within the same molecule and their micellization in aqueous solution and adsorption onto interfaces resemble those of non-ionic surfactants [1]. These triblock copolymers possess symmetrical structure (EO)_x(PO)_y(EO)_x where x and y denote the number of ethylene oxide and propylene oxide units per block and are available in a range of x and y values under trade names such as Pluronic® (BASF) and Synperonics (ICI), poloxamers or generic names such as EPE polyols as flakes, liquids and pastes [2], [3], [4], [5]. These block copolymers due to their unique surface activity are extensively used for a variety of applications, most notably in cosmetics, pharmaceuticals and textile industries [6].

In analogy with conventional low molecular weight surfactants [7], block copolymers form different kinds of aggregates, depending on the molecular weight, the block sizes, the solvent composition and temperature. Strongly concentration and temperature dependent micellization has been observed for a variety of copolymers differing in molecular characteristics [8], [9], [10] and is driven by hydrophobic PPO block [10], [11], [12] with micelle core consisting of PPO blocks and a corona of PEO blocks [13]. Their critical micelle concentrations (CMC) decrease very rapidly upon increasing temperature, by two orders of magnitude for a temperature increase of 20°C around ambient temperature [3], [12]. While large discrepancies in CMCs of several copolymers existed and led to several publications in this field, it is now well established that variable CMCs were due to batch variations, the presence of homopolymer and diblock copolymer impurities and polydispersity in these commercial samples [14], [15], [16], [17], [18], [19], [20].

The strong dependence of the CMC on temperature has led to an extensive use of the concept of critical micellization temperature (CMT) [2], [12], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], the temperature at which micelles appear in a PEO/PPO/PEO solution at a given concentration. Most reported studies were performed as a function of temperature and the measured properties showed sharp changes at CMT. However, like CMC, the CMT also is not a single temperature but more or less a wide temperature range due to copolymer polydispersity in composition. Besides, a good amount of literature exists on micellar transitions, phase diagrams and thermorheological behavior in aqueous solutions of block copolymers as influenced by concentration and temperature [9], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. It has been shown that micelles grow in size on increasing temperature and undergo sphere-to-rod transitions at elevated temperatures (15–20°C below the cloud point). Liu et al. [37] and Jain et al. [29] have recently examined micellar structures and micelle interactions in aqueous solutions of pluronics P84 and P104 over a broad concentration and temperature range using small-angle neutron scattering (SANS), and observed micellar growth and transitions as already stated in earlier references.

The presence of additives (i.e. electrolytes [8], [26], [28], [29], [39], [40], [41], [42], [43], [44], [45], [46], non-electrolytes [44], [47], [48], organic solvents [47], homopolymers [49] and ionic surfactants [50], [51], [52], [53]) has been found to exert a remarkable influence on the aggregation and gelation of PEO/PPO block copolymers. However, data on the effect of these additives influencing micellar characteristics are rather scarce although the presence of such additives has been shown to alter significantly the solvent properties [13], [28] as well as surface/adsorption characteristics of copolymers in solution [26], [45]. Micellization, rheology and clouding behavior of block copolymers in aqueous solution in the presence of added salts have been extensively studied by Bahadur and coworkers [26], [29], [42], [43] and other workers in the past [8], [39], [40], [41]. From small-angle neutron scattering (SANS) study on a very highly hydrophilic block copolymer F88 in aqueous salt solutions, Jain et al. [29] have shown that micelles in aqueous salt solutions do not undergo any kind of transition even at temperatures close to cloud point. The effect of added inorganic salt on the micellization of PEO/PPO/PEO triblock copolymers was analogous to that of temperature. For applications where temperature cannot be altered appreciably, the addition of salt provides an easy method for inducing micellization needed to optimize a surfactant's performance. Besides, a recent study by Jorgensen et al. [40] on micellization and gelation of pluronic P85 in presence of salt potassium fluoride, has shown that spherical micelles transform into rod-like ∼38°C in 1 M KF, which is ∼36°C below the transition temperature in salt free system. Moreover, temperatures for sphere-to-rod transitions, gel formation and cloud points are shifted to lower values with salt addition. Moreover, the effect of sodium chloride upon micellization and phase separation transitions in aqueous solutions of various triblock copolymers using a high sensitivity differential scanning calorimetry has been recently documented by Armstrong et al. [41]. They also concluded that addition of salts to these systems lowers critical micellization temperature (CMT) and cloud point (CP) in agreement with previously reported findings. Micellar growth and change of micelles from spheres to prolate ellipsoids near to CP were also noted.

While sufficient literature exists on micellization of block copolymers in aqueous solution in the presence of added salts, studies on the presence of organic solvents and non-electrolytes are rather scarce [44], [47], [48]. Armstrong et al. [47] studied the effect of various cosolutes and cosolvents upon the micellization of the PEO/PPO/PEO block copolymer F87 and noted that methanol, ethanol and urea incorporation in the aqueous surfactant solution raised the CMT. Similar results for P105 in aqueous urea solutions have also been reported by Alexandridis et al. [48] whereas butanol tended to reduce the CMT. The results are explained assuming that methanol and ethanol are incorporated into the hydration sphere of the PO block, thereby increasing CMT and temperature range over which micellization occurs. Here too, the alteration of solution composition through the use of cosolvents and cosolutes makes accessible to the surfactant technologist a method for producing a solution with fairly precise properties. Besides, few reports demonstrating the interaction of EO/PO block copolymers with conventional surfactants have been documented [50], [51], [52], [53], [54]. It has been observed that interaction was more predominant with anionic surfactants compared to that with cationic and zweterionic. The micellization of copolymer is suppressed completely by addition of surfactant and the CP of copolymer showed a marked increase although CP increasing effect was more distinct for anionic surfactants.

In this paper we present a comprehensive account of the effect of different kinds of additives, viz. electrolyte (sodium chloride), non-electrolyte (urea) and anionic surfactant (sodium dodecyl suphate) on the micellization of an extensively studied block copolymer F127 (EO₉₉/PO₆₅/ EO₉₉, %PEO=70, mol. wt.=12 500) in aqueous solution by different physico-chemical methods in continuation of our ongoing studies on PEO/PPO/PEO block copolymer in aqueous solution in presence of additives.