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Tuning the visco-elastic properties of bis-urea based supramolecular polymer solutions by adding co-solutes Kelly Roberta Francisco, Cecile Ayako Dreiss, Laurent Bouteiller, and Edvaldo Sabadini Langmuir, Just Accepted Manuscript • Publication Date (Web): 11 Sep 2012 Downloaded from http://pubs.acs.org on September 13, 2012
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Tuning the visco-elastic properties of bis-urea based supramolecular polymer solutions by adding co-solutes
Kelly Roberta Franciscoa, Cécile A. Dreissb*, Laurent Bouteillerc, Edvaldo Sabadinia.
a
Department of Physical-Chemistry, Institute of Chemistry, P.O. Box 6154, 13084-862, University of Campinas, Campinas-SP, Brazil;
b
Institute of Pharmaceutical Science, King’s College London, 150 Stamford Street, SE1 9NH London, U.K.;
c
UPMC Univ Paris 06, UMR 7610, Chimie des Polymères, F-75005, Paris, France, and CNRS, UMR 7610, Chimie des Polymères, F-75005, Paris, France.
* Corresponding author; e-mail:
[email protected] (C.A. Dreiss).
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ABSTRACT
Polymers formed by the self-assembly of a bis-urea-based polymer, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in organic solvents such as octane are promising systems with remarkable rheological properties. This is the first self-assembled polymer recently reported as a hydrodynamic drag reducer for hydrocarbons. The rheology of diluted and semi-diluted EHUT solutions can be tuned by specific interactions between the chains, modulated by the nature of the solvent and the presence of additives. In this manuscript, rheological, thermal and SANS measurements were performed in order to investigate the competition between EHUT self-assembly and its interaction with specific molecules (benzene, benzyl alcohol and ethanol) that can interact with EHUT unimers via hydrogen bonds and π-π interactions. No substantial rheological, thermal or structural effect is observed when benzene is added to the systems. However, ethanol and benzyl alcohol interact with EHUT unimers through hydrogen bonds, drastically decreasing the viscoelasticity of the solutions. In addition, benzyl alcohol can interact with EHUT polymers by π-stacking interactions, playing an important role to tune rheological properties of the systems.
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INTRODUCTION Molecules are traditionally based on covalent bonds. Weak interactions however, such as metal-ligand complexation, hydrogen bonding and π-π interactions, have received increasing interest in recent years to synthesize supramolecular polymers.1-3 These supramolecular polymer chains reproduce many of the characteristics associated with covalent polymers; for instance, they can form viscous solutions or gels, depending on the concentration.4-6 However, the reversibility of the bonds leads to significant differences between traditional and supramolecular polymers, since in a supramolecular polymer the average number of monomers in a chain is not fixed, but depends on the physico-chemical conditions, such as temperature, solvent polarity, monomer and co-solutes concentrations7-10, imparting the polymer novel functionalities, notably, stimuli-responsiveness and self-healing. Supramolecular polymers can break and reform on the experimental time scale, similarly to wormlike micelles systems, in which the dynamic process of breaking and reforming of chains has a notable effect on the rheological response, as it adds an extra mechanism for the relaxation of stress in these systems. In others words, as the chain length can be adjusted in situ by varying the conditions, the resulting solutions show tunable viscoelasticity by modification of the self-assembly structures.11-13 One of these interesting supramolecular polymers formed by multiple hydrogen bonding between the monomers is 2,4-bis(2-ethylhexylureido)toluene (EHUT).14 This compound self-assembles in several apolar solvents into two types of supramolecular polymeric structures, depending on the temperature or the solvent: at room temperature, EHUT forms viscoelastic solutions, due to the cooperative formation of four hydrogen bonds per monomer, leading to the formation of long and semi-flexible tubes (T) with three molecules in the cross section, while at high temperatures, a sharp decrease of the viscosity is observed due to the conversion of tube chains into filaments (F), with a single molecule in the cross section.15 The degree of polymerization and the length of the tube and filament forms have been estimated using the association model and their parameters have been empirically determined by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC).16,17 For example, at ACS Paragon Plus Environment
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an EHUT concentration of 0.1 mmol L-1 in toluene at 20ºC, the tube structure is the form which is predominant, and its average curvilinear length was estimated as ca. 400 nm (Mn ≅ 106 g mol-1), whereas at 40ºC the filament form is the main self-assembly structure present, and its average curvilinear length is ca. 6 nm (Mn ≅ 6 × 103 g mol-1).16 Recently, an interesting application for EHUT has been reported, based on the changes in the average curvilinear length of the supramolecular polymers as a function of the temperature. Systems formed predominantly by tubes have a good potential to be used as drag reducers in low-polarity solvents as compared to filament assemblies, and temperature can be used as a switch to tune this property.18 Bouteiller and co-workers have demonstrated another way to tune the viscoelasticity of EHUT solutions by adding carefully designed organic compounds and ionic species as co-solutes, such as bis-urea, bisthiourea and tetrabutylammonium salts (fluoride, chloride, bromide, iodide, dihydrogen phosphate, hydrogensulfate and hexafluorophospate), demonstrating their ability to interact with EHUT monomers and break down the polymer chains.19,20 In this case, they considered the hydrogen bonds as the driving force for the interaction between the co-solutes and the monomers, which has to be at least as strong as the interactions between the monomers to promote a decrease in the average curvilinear length or transitions on polymeric chains. Surprisingly, there have been no rheological and structural studies on simple organic molecules interacting through hydrogen bonds and π-π interactions with EHUT. In this contribution, we report the effect of adding to EHUT solutions minute amounts of ethanol, benzyl alcohol and benzene (up to 0.2 wt%) as a way to tune the rheological properties of these systems, where co-solutes can interact with EHUT monomers mainly through hydrogen bonds (ethanol), π-π interactions (benzene) and both hydrogen bonds and π-π interactions (benzyl alcohol). Microstructural characterization was performed by SANS measurements, in order to detect any potential structural changes of EHUT chains underlying the rheology and thermal properties.
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EXPERIMENTAL SECTION Materials. The synthesis of EHUT has been described previously.14 Octane, ethanol, benzyl alcohol and benzene were acquired from Sigma-Aldrich. For hydrodynamic drag reduction experiments chemicals were purchased from Merck. Deuterated octane from Qmx laboratories was used to optimise the contrast with the solutes in the SANS measurements. All reagents were used without further treatment. Sample preparation. EHUT solutions were prepared by dilution of stock solutions of EHUT to the following concentrations: 0.4, 5.0 and 10.0 mmol L-1 EHUT. Stock solutions at 15.0 mmol L-1 were prepared by weighing adequate quantities of EHUT as required and adding octane under vigorous stirring at 50ºC for 48 hours. Co-solutes were added to EHUT solutions, and systems containing 5.0 and 20.0 mmol L-1 co-solutes were kept under stirring at 50ºC for 24 hours. All solutions were left to rest at least 3 days at room temperature and were equilibrated at 25ºC (in a water-bath) for 12 hours before any measurement.
Rheology Experiments. Shear oscillatory experiments were conducted on a controlled-strain rheometer (ARES, TA Instruments) equipped with a Peltier Temperature Control system, in the plate-plate configuration (50 mm diameter, 0.8 mm gap). All oscillatory experiments were conducted within the viscoelastic linear range as determined by strain sweep measurements (1 - 3%). Measurements were conducted under a solvent-trap in order to minimize solvent loss by evaporation. Experiments were carried out at 25.0°C and measurements were carried out in duplicates, with very good reproducibility (an error of ±3% is estimated on the rheological curves). In order to measure the capability of hydrodynamic drag reduction (HDR) of the EHUT systems, rheological experiments were conducted on a Haake RheoStress I rheometer equipped with a double-gap cell. The internal and external cup diameters were 17.75 and 21.70 mm, respectively, and the internal ACS Paragon Plus Environment
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and external rotor diameters were 18.35 and 20.99 mm, respectively, with a rotor length of 55.00 mm. The temperature sweep experiments were performed over a temperature range of 25 to 75ºC at a fixed angular velocity of 900 rpm, and the temperature was controlled by an external water-bath system to a precision better than 0.1ºC. Data treatment. A theoretical model, referred to as the “living polymer”, has been proposed by Cates and Candau to describe the linear and non-linear rheological behaviour of systems formed by supramolecular polymers.21 According to this model, previously developed for wormlike micelles, a competition between reptation and breaking dynamic processes is established, which is associated with specific relaxation times, τrep and τbreak, respectively. For processes in which τrep
