J. Phys. Chem. B 2007, 111, 371-378
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Rheological Properties of a Telechelic Associative Polymer in the Presence of r- and Methylated β-Cyclodextrins Dongsheng Liao, Sheng Dai, and Kam Chiu Tam* School of Mechanical and Aerospace Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798 ReceiVed: October 3, 2006; In Final Form: NoVember 2, 2006
The viscosity of hydrophobic ethoxylated urethane (HEUR) solution decreased in the presence of R-CD or m-β-CD; however their interactions were quite different. When the R-CD/hydrophobe molar ratio exceeded 5.0, the viscosity was close to that of a PEO solution of similar molecular weight. Oscillatory shear indicated that the mechanically active chains in HEUR solution decreased with the addition of R-CD. This agreed with the hypothesis that R-CD formed an inclusion complex with the hydrophobic moiety of the HEUR polymer, thereby destroying the transient hydrophobic associative network. The viscosity/temperature relationship of the R-CD/HEUR system (for HEUR with 70% of the PEO chains capped at both ends) did not obey the Arrhenius relationship for R-CD/hydrophobe molar ratio in the range 0.8-5.0. The low shear viscosity increased with increasing temperature at molar ratio of 1.0, and this was attributed to the competitive complexation of the R-CD/hydrophobe and the R-CD/PEO chain. Increasing temperature favored R-CD/PEO complexation. Comparison between the behavior of R-CD/HEUR and m-β-CD/HEUR resulting from the different binding characteristics was discussed.
Introduction In the past decade, research on the aggregation behaviors of telechelic associative polymers such as hydrophobic ethoxylated urethane (HEUR) has been conducted using various techniques such as rheology,1-3 laser light scattering,4 fluorescence spectroscopy5,6 and pulse-gradient NMR.7,8 For HEUR with C16H33 hydrophobic end-caps and molecular weight greater than 10 000 g/mol, the polymer chains self-associate into discrete micelles consisting of a hydrophobic core surrounded by a corona of looping poly(ethylene oxide) (PEO) chains. At high concentrations, these micelles are connected to each other through bridging HEUR chains, which form a network that exhibits interesting rheological behavior.9 More recently, the concept of controlling the self-assembly behavior of associative polymers like hydrophobically modified alkali-soluble emulsion (HASE) and HEUR by the addition of a third component has attracted increasing attention. Organic cosolvents, such as propylene glycol, can be used to suppress the viscosity of aqueous solution containing associative thickeners needed to facilitate the preparation of coating formulations.10 While these organic cosolvents perform their intended function, they possess potential environmental, safety, and health disadvantages. Despite the importance of hydrophobic interactions in enhancing the viscosity of the HEUR system, there is also a need to adjust the viscosity-shear rate profile to accommodate different industrial requirements. To achieve this, the addition of surfactant or cyclodextrin (CD) is needed to partially or completely remove hydrophobic interactions. Viscosity suppression may be accomplished by the use of surfactants.11 While this presents no specific health/environmental hazard, it does degrade formulation performance. The cyclodextrin compound suppresses the viscosity of associative thickener solutions and * To whom correspondence should be addressed. Fax: (65) 67911859. E-mail:
[email protected].
eliminates the usage of organic cosolvents. In addition, the viscosity suppressing function of cyclodextrin compounds is readily reversed by the addition of a compound having greater affinity for cyclodextrin (e.g., surfactants with longer hydrophobic groups), thereby providing an effective mean of utilizing associative thickener in the preparation of aqueous formulation.12 This concept is based on the idea of using cyclodextrin to encapsulate hydrophobic segments so that the molecular properties of single chain and the solution viscosity of associative polymers can be controlled.13,14 Karlson15,16 proposed a simple Langmuir adsorption model for describing the complexation mechanism between cyclodextrin and hydrophobically modified PEO. They concluded that cyclodextrin molecules could complex with hydrophobic end groups of associative polymers resulting in the disruption of the three-dimensional associating network. However, only few studies have been conducted on CD/polymer systems to investigate the dynamics of their interactions, such as the effect of cyclodextrin on the temperature dependence of viscosity. The focus of this paper is to examine the rheological properties of aqueous HEUR solution in the presence of R-CD and m-β-CD. The effect of temperature on the interaction of unmodified R-CD/PEO and hydrophobe/RCD systems was investigated and compared to those of m-βCD/PEO and hydrophobe/m-β-CD. Experimental Section Materials. HEUR was synthesized by Dow Chemicals (formerly Union Carbide) with a Mn of 51 000 Da and Mw/Mn of 1.7 and designated as HEUR-C16-51K. The degree of end functionalization was determined from NMR studies to be 1.7;17 i.e., 30% of the HEUR chains are only capped with linear C16H33 hydrophobic segment at one end. The chemical structure is C16H33O-(DI-PEO)6-DI-OC16H33, where DI is an isophorone diisocyanate group and PEO is a poly(ethylene oxide) segment of nominal molecular weight of 8200 Da. R-Cyclodextrin (R-
10.1021/jp066490k CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
372 J. Phys. Chem. B, Vol. 111, No. 2, 2007
Liao et al.
SCHEME 1: Synthesis of Fully Encapped Hydrophobically Modified PEO with Narrow MW Distributions
Figure 1. 1H NMR of synthesized encapped hydrophobically modified PEO.
CD) and methylated β-cyclodextrin (m-β-CD) were purchased from Cyclodextrin Technologies Development Inc (High Springs, FL). The deionized water was from Alpha-Q Millipore water purifying system. Preparation of Fully End-Capped PEO with Narrow Molecular Weight Distribution. Isophorone diisocyanate (IPDI) was purchased from Sigma-Aldrich and distilled under vacuum prior to use. PEO with a Mn of 20 000 Da was purchased from Fluka. Toluene, tetrahydrofuran, and petroleum ether were stirred for 24 h over 40 mesh calcium hydride and distilled under argon. Dibutyltin diacetate and dibutyltin dilaurate were obtained from Alpha. The synthesis of fully endcapped hydrophobically modified PEO with narrow MW distribution is shown in Scheme 1 in accordance with Lundberg et al.18 The prepolymer of PEO was prepared on a large scale by stoichiometric reaction of PEO, with excess of IPDI (1:200 equiv ratio) and subsequently dividing the product into smaller portions for reaction with alcohols containing different hydrophobes (Scheme 1). In a typical experiment, the prepolymer was prepared by adding 20 g of PEO to a 100 mL, four-necked, break-away reaction flask, equipped with a Dean Stark water trap and condenser, argon inlet tube, and mechanical stirrer. The PEO was dried by azeotropic distillation in toluene. About 30 mL of the toluene was removed through the Dean Stark trap, and 40 g of dry, distilled tetrahydrofuran and 40 g of IPDI were then added. A small portion of the solution was withdrawn from the mixture after 36 h at 45 °C and placed in methanol for size exclusion chromatographic analyses. The remainder of the solution was precipitated into petroleum ether, filtered, and dried under vacuum. The isocyanate content was determined by dibutylamine titration, and the prepolymer was stored under argon at dry ice temperature until it was ready for reaction with alcohols containing different alkyl chain lengths. The following description of the hydrophobic modification of telechelic prepolymer with hexadecanol is a representative of all end-capping reactions with hydrophobic, active-hydrogencontaining compounds. In a 100 mL three-necked, round-bottom flask equipped with a magnetic stirrer, argon inlet tube, and thermometer, 5 g of the telechelic prepolymer (0.29 mmol of -NdCdO) was dissolved in 15 mL of dry, distilled THF. Dry hexadecanol (1.9 g, 8.5 mmol of -OH; 25 mol excess of -OH to -NdCdO) was dissolved in a small amount of THF and added to the flask, and the contents were heated to 40 °C. A drop of dibutyltin diacetate was added to catalyze the addition reaction, and the mixture was stirred for 12 h. After the reaction was completed, it was filtered to remove triethylamine hydrobromide. The solution was concentrated and precipitated in
n-hexane, filtered, and dried under vacuum. The procedure was repeated twice to ensure the complete coupling of end groups. Finally the crude polymer was dissolved in water at pH of 7-8 and extracted from methylene chloride. After the sample was dried over magnesium chloride for 12 h, the solvent was removed under vacuum and dried to obtain the purified polymer. The final polymer is designated as PEO-C16-20K. Characterization of Polymers. The 1H NMR spectra of PEO-C16-20K were recorded on a Bruker DRX-400 NMR spectrometer at 400 MHz at room temperature. The 1H NMR measurements were carried out with an acquisition time of 3.2 s, a pulse repetition time of 2.0 s, a 30° pulse width, 5208Hz spectral width, and 32K data points. Chemical shifts were referred to the solvent peaks (δ ) 7.30 and 2.50 ppm for CDCl3 and DMSO-d6, respectively). The details of the 1H NMR spectrum are shown in Figure 1. 1H NMR of the PEO-C1620K revealed two peaks at 4.18 ppm (c) and at 4.36 ppm (a), which corresponds to ester -CH2 of cetyl group and ester -CH2 PEO, respectively. The integration of theses two peaks (1:1) confirmed >95% substitution of IPDI to PEO at both ends as well as the complete esterification of cetyl alcohol to IPDI. Rheology. The steady and dynamic rheological behaviors of the solutions were measured using a controlled stress Carri-Med CSL500 and a controlled-rate Contraves LS40 rheometer. Rheological measurements of less viscous samples were conducted using the Contraves LS40 rheometer, which was fitted with a concentric cylinder of geometry, with bob and cup radii of 5.5 and 6 mm, respectively. Measurements involving more viscous solutions were performed on the CSL500 rheometer with a coneand-plate geometry with a diameter of 40 mm and a cone angle of 2°. All the rheological measurements were carried out at a temperature of 25 °C controlled by a PolyScience water bath unless otherwise stated. For steady flow measurements of semidilute solutions, shear rates ranging 0.001-100 s-1 for LS40 and shear stresses ranging 0.2-1000 Pa for CSL500 were used. The frequency-dependent moduli were measured over an angular frequency range 0.03-200 rad/s within the linear viscoelastic region. Results and Discussion Effect of R-CD on Viscosity of HEUR Solution. The steady shear viscosities of 2.0 wt % HEUR-C16-51K aqueous solution in the presence of different amounts of R-CD were determined over a broad deformation regime (Figure 2). The HEUR-C1651K solution without cyclodextrin exhibits Newtonian behavior at low shear rates. It shear thickened at moderate shear rates and then shear-thinned at high shear rates. With the addition of
Rheological Properties of a Telechelic Polymer
Figure 2. Dependence of shear viscosity on shear rate for 2.0 wt % HEUR in different concentrations of R-CD. (The numbers represents the molar ratio of R-CD to hydrophobic end-group.)
Figure 3. Effect of R-CD on the low shear viscosity of 2.0 wt % HEUR aqueous solution at 25 °C. The open circle is the referenced m-β-CD/HEUR system as measured under the same conditions.
cyclodextrin, the low shear viscosity decreased sharply, and the characteristic shear-thickening and shear-thinning became less significant with increasing R-CD concentration. It was also clear that the onset of shear-thickening and shear-thinning shifted to higher shear rate as R-CD concentration was increased. When the molar ratio of R-CD/hydrophobe exceeded 1.0, the mixed solutions exhibited predominantly Newtonian behavior, confirming that the network associating structure had been destroyed by R-CD molecules. When the R-CD/hydrophobe molar ratio was greater than 5.0, negligible change in viscosity was observed. Similar rheological behavior was observed for the m-β-CD/HEUR system.19 The dependence of low shear viscosity, η0, on R-CD concentration is shown in Figure 3. Both R-CD/HEUR and m-βCD/HEUR systems showed a similar trend. The interaction between R-CD and HEUR can be divided into 3 regimes. At very low R-CD concentration (R-CD/hydrophobe molar ratio 5), excess amounts of R-CD are present to encapsulate all the hydrophobes, thereby deactivating most of the bridging chains yielding a solution viscosity that is similar to that of PEO of identical molecular weight. Note Added after ASAP Publication. There was an error in eq 1 in the version published ASAP December 16, 2006; the corrected version was published ASAP December 18, 2006. References and Notes
Figure 12. Schematic representation of the complexation mechanism between R-CD and HEUR. The coarse line represents PEO with one end-capped hydrophobe, and the fine line represents fully end-capped PEO.
temperature dependence characteristics between HEUR and PEO-C16-20K strongly suggested that the unique viscositytemperature profiles of R-CD/HEUR solution were attributed to the competitive complexation between the 30% uncapped PEO chain ends and the 70% C16H33 alkyl chain ends. The mechanism of R-CD and HEUR interaction is presented schematically in Figure 12. Conclusions The interaction of R-CD/HEUR is completely different from m-β-CD/HEUR interaction, where the interaction can be divided into 3 regimes. In region 1 (R-CD/hydrophobe molar ratio
