Synthesis of Novel Amphiphilic Dendron–Coil Macromonomer and

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Synthesis of Novel Amphiphilic Dendron−Coil Macromonomer and Rheological Behavior of Its Acrylate Copolymer Emulsion Huijun Ye,† Biye Ren,*,†,‡ Rui Liu,† Jun Peng,† and Zhen Tong† †

Research Institute of Materials Science and ‡The Key Laboratory of Polymer Processing Engineering, Ministry of Education, South China University of Technology, Guangzhou 510641, China S Supporting Information *

ABSTRACT: A novel amphiphilic dendron−coil macromonomer, 3,5-di(n-hexadecyloxy)benzoyloxy polyoxyethylene methacrylate [(3,5)16G1-CO2PEG1000MA, 6], was synthesized by esterification of polyoxythylene (molecular weight M = 1000) with 3,5-di(n-hexadecyloxy)benzoyl chloride followed by methacrylate chloride. The compound was then characterized by means of NMR and FT-IR. Moreover, a new kind of dendron hydrophobically modified alkali-soluble emulsion (DHASE) polymer was synthesized by emulsion copolymerization of macromonomer 6 with mathacrylic acid and ethyl acrylate. The rheological behavior of a series of DHASE was tested. It has been found that, during alkalization, while the value of pH reached to 6.5, all the samples of DHASE had a significant increase in terms of viscosity and obvious shear-thinning behavior. Besides, the solution viscosity increased with increasing the macromonomer 6 content and the polymer concentration. Moreover, the mechanism of the association process was discussed by examining the changes of G′ and G″ in the angular frequency range of 10−2−102 rad/s. The results illustrate the potential of this novel amphiphilic dendron−coil macromonomer in hydrophobically modified alkali-soluble emulsion polymers and associative thickeners for the first time.

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INTRODUCTION Hydrophobically modified alkali-soluble emulsion (HASE) polymers are a particular class of associative polymers. They are long chain acrylic polymers interspersed with carboxylate groups, having hydrophobic end-capped macromonomers distributed along the chain.1 Under basic conditions, the HASE solution viscosity significantly increases, because the expansion of the polymer backbone due to the carboxylate repulsion of anion backbone results in an increase of hydrodynamic volume and hydrophobic groups associate dynamically and form a network of reversible hydrophobic junctions. Therefore, these HASE polymers are widely used as effective thickeners for improving the rheological properties for a variety of applications, such as waterborne coating, cosmetics, dyestuff, and medicines. Several studies had focused on the aggregation behavior of the HASE polymers.2 It is well-known that the factors influencing the thickening effect of HASE polymers are as follows: association of hydrophobic groups, chain expansion of high molecular weight polyelectrolyte backbone, hydrophobe size, acid content, molecular weight, HASE polymer concentration, the type of functional monomers with hydrophobic groups, pH level, the addition of surfactants and salt, etc.3 Jenkins et al.3a found that solution viscosity of linear HASE polymers dramatically increased within the pH range of 6.0− 7.5. For a higher pH, the viscosity remained approximately constant. English et al.4 studied the influence of linear HASE polymer concentration on solution viscosity. Their results indicated the existence of a thermodynamically favored transition from intramolecular to predominantly intermolecular associations, as the polymer concentration was increased from the dilute to the nondilute regime. Seng et al.5 studied the effects of surfactant on the solution rheological behavior of © 2013 American Chemical Society

linear HASE. Under the critical association concentration, c*, the addition of surfactant resulted in an increase of the viscosity to a maximum value caused by the increase in the number of mechanically active intermolecular hydrophobic junctions and the lifetime of the average junctions. Further addition of surfactants caused the viscosity to decrease, due to the decrease in the number of intermolecular junctions and the strength of the network structure. These observations are attributed to the disruption of the associating network and the electrostatic shielding. Islam et al.6 determined the molecular weight of single-chain HASE polymer by using methyl-β-cyclodextrin to shield the hydrophobes from associating. The molecular weights of single-chain HASE polymers with C8, C16, and C20 hydrophobes were found to be ∼2.0 × 105 g/mol. Wu et al.7 controlled the molecular weight of linear HASE polymer by adding chain transfer agent (CTA) during polymerization. When the CTA level was below 0.1 wt %, the polymer solutions displayed shear-thinning behavior. However, a small increase in CTA concentration beyond 0.1 wt % caused a dramatic improvement on the solution viscosity. Tam et al.8 studied viscoelastic properties of linear HASE in salt solution. They found that the presence of a simple salt had profound influence on the rheological behavior of linear HASE solution. The solution viscosity declined progressively with the increase of salt concentration. At the same time, the viscoelastic properties of the hydrophobe-containing polymer solutions changed from predominantly elastic to viscous behavior. Received: Revised: Accepted: Published: 11858

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transform infrared (FTIR) spectra of the complexes were recorded on a Bruker Vector 33 FTIR spectrometer using KBr pellets at room temperature (25 °C). The rheological properties of DHASE polymers were measured by an ARG-2 controlled stress rheometer (TA Instrument Inc., America). All of the tests were carried out at room temperature (25 °C). Syntheses. Synthese of Methyl 3,5-Di(n-cetyl-1-yoxy)benzoate (2). A 5.0 g (29.75 mmol) portion of methyl 3,5dihydroxybenzoate, 13.57 g (98.18 mmol) of K2CO3 in 70 mL of DMF, and 18.15 g (59.45 mmol) of 1-bromohexadecane were added to a 150-mL, three-necked, round-bottom flask reactor equipped with a magnetic stirrer. The temperature was increased to 80 °C and the mixture was stirred under the Ar atmosphere. Twenty-four hours later, we used TLC analysis to determine the end point of the reaction. After the completion of reaction, the mixture was cooled down to 50−60 °C, and the DMF filtrate was removed by means of reduced pressure distillation. Then 300 mL of deionized water was added into the crude product, the temperature rose to 65 °C, and the product stirred for another 1 h. Then the mixture was cooled to room temperature (25 °C) and filtered. The filter cake was dried and purified by silica gel column chromatography (eluent petroleum ether/ethyl acetate = 10:1, v:v) to yield 16.09 g (92.5 wt %) of a white solid (2). 1H NMR (CDCl3, TMS, δ, ppm): 0.88 (a, 6H, CH3), 1.29 (c, 16H, −(CH2)7−), 1.47 (b, 4H, CH2CH2CH2OAr), 1.79 (c, 4H, CH2CH2OAr), 3.89 (g, 3H, CO 2CH 3), 4.01 (d, 4H, CH2 OAr), 7.16 (f, 2H, ArHCO2CH3), 6.63 (e, 1H, OArH). Rf = 0.68 (eluent petroleum ether/ethyl acetate = 10:1, v:v). The 1H NMR spectrum of 2 is given in the Supporting Information (Figure S1). Synthesis of Methyl 3,5-Di(n-cetyl-1-yoxy)benzoic Acid [(3,5)2C16−CO2H, 3]. A 4.80 g (7.79 mmol) portion of 2, 40 mL of 80% EtOH, and 4.37 g (77.9 mmol) of KOH were placed into a 150-mL, three-necked, round-bottom flask reactor equipped with a magnetic stirrer. The mixture was heated to 80 °C and stirred for about 10 h with the temperature kept constant. The progress of the hydrolysis was tracked by TLC analysis. After the completion of hydrolysis, the mixture was cooled to 60 °C and then 30 mL of THF was added into the flask. The solution was acidified with dilute HCl until the pH was 1, and the mixture was reacted for 2 h, cooled to room temperature, and poured it into 80 mL of Et2O. The organic phase was washed twice with 50 mL of NaCl solution, and the organic layer was separated and dried by MgSO4. After filtration, the crude product was recrystallized twice from ethanol, resulting in 4.50 g (95.8%) of white crystals of 3. 1H NMR (CDCl3, TMS, δ, ppm): 0.88 (a, 6H, CH3), 1.26 (c, 48H, −CH2−), 1.44 (b, 4H, CH2CH2CH2OAr), 1.77 (c, 4H, CH2CH2OAr), 3.65 (i,j, 80H, O(OCH2CH2)20), 3.96 (d, 4H, CH2OAr), 4.36 (g, 2H, CH2CO2), 7.16 (f, 2H, ArHCO2CH3), 6.63 (e, 1H, OArH). Rf = 0.6 (eluent dichloromethane/ methanol = 10:1). The 1H NMR spectrum of 3 is given in the Supporting Information (Figure S2). Synthesis of 3,5-Di(n-dodecan-1-yloxy)benzoyl Chloride [(3,5)2C16−COCl, 4]. A 100-mL, three-necked, round-bottom flask reactor equipped with a magnetic stirrer was charged with 2.65 g (4.4 mmol) of 3, 20 mL of CH2Cl2, and 0.1 mL of DMF. The flask was flushed with Ar and cooled in an ice bath. SOCl2 (0.54 mL, 6.6 mmol) was added dropwise to the chilled mixture. After the removal of the ice bath, the mixture was stirred for 3 h. The solvent was evaporated, and the resulting

The HASE polymers are usually prepared by emulsion polymerization with methacrylic acid (MAA), ethyl acrylate (EA), and functional monomers containing hydrophobic groups. For hydrophobic functional monomers, the alkyl tail length of hydrophobic groups of monomers usually ranges from C12 to C22, while C20 is the most popular choice. Tirtaatmadja et al.1,9 studied the relationship between the HASE solution viscosity and its hydrophobic chain length. They found out that the hydrophobic association increased in strength with increasing hydrophobic chain length. Thus, the solution viscosity rose. The number of moles of EO (spacer length) between polymer backbone and hydrophobic group normally varies from 30 to 40, while 35 is preferred. However, nearly all the studies of HASE emphasize hydrophobes of a single alkyl tail chain or a single substituted tail chain linked to the PEO spacer. Alkyl-substituted benzoid acids and their derivatives are a class of dendrons that are used for self-assembled and nanostructured materials.10 Percec et al.10c synthesized the monoesters with 3,4,5-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzoic acid and mono-, di-, tri-, tetraethylene glycol and then composed them with methacryloyl chloride to form a series of polymizable dendrons. After studying the monomers and their self-assembly behavior, they found that the length of the alkyl chain and the degree of polymerization had an impact on the self-assembly behavior. These polymizable dendrons have three hydrophobic chains. Supposing that the polymizable dendrons can be introduced to the HASE polymers, the hydrophobic interaction between alkyl groups could be greatly improved. Thus, it may form a more complete hydrophobic association network, which could have a significant thickening and shear-thinning effect. However, the combination of synthetic thickeners with a linear polyelectrolyte block and dendritic moiety has seldom been seen. Little was known about the rheological effect of dendritic amphiphiles on the HASE as well. In this work, a novel amphiphilic dendron−coil macromonomer, 3,5-di(n-hexadecryloxy)benzoyloxy polyoxyethylene methacrylate [(3,5)16G1-CO2PEG1000MA, 6], was prepared and characterized with FTIR and 1H NMR. Then a new kind of dendron hydrophobically modified alkali-soluble emulsion (DHASE) copolymer was synthesized by emulsion copolymerization of macromonomer 6 with methacrylic acid (MA) and ethyl acrylate (EA). The objective of this study is to obtain detailed insights on the rheological behavior of DHASE and demonstrate the protential of this novel amphiphilic dendron− coil macromonomer in hydrophobically modified alkali-soluble emulsion (HASE) polymers and associative thickeners in the future.

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EXPERIMENTAL SECTION Materials. Methyl 4-hydroxybenzoate, methyl 3,5-dihydroxybenzoate (both from Aldrich, 98% pure), methyl acryloylchloride (Aladdin, 95% pure), and polyethylene glycol (PEG1000, Shanghai Pudong Gaonan Chemical Plant, Mw = 1000 g/mol) were used as received. 1-Bromohexadecane (Yancheng Longsheng Fine Chemical Factory, 98% pure) was distilled under vacuum. Other reagents, such as methacrylic acid (MAA), ethyl acrylate (EA), SOCl2, Et2O, tetrahydrofuran (THF), CH2Cl2, and N,N′-dimethylformamide (DMF), were all analytical reagents and freshly dried and distilled before use. Measurements. 1H NMR spectra were obtained on a Bruker Avance Digital 400 MHz spectrometer. Fourier 11859

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Figure 1. Synthesis scheme of amphiphilic dendron−coil macromonomer 6.

Figure 2. 1H NMR spectrum of amphiphilic dendron−coil macromonomer 6.

analysis. After evaporating the solvent, the resulting white solid was put into 100 mL of Et2O and stirred for 2 h. The mixture was filtered and the solvent was evaporated. The crude product was purified by silica gel column chromatography (eluent dichloromethane/methanol = 10:1, v:v) to yield 4.59 g (68.5 wt %) of a white solid (5). 1H NMR (CDCl3, TMS, δ, ppm): 0.88 (a, 6H, CH3), 1.26 (c, 48H, −CH2−), 1.44 (b, 4H, CH2CH2CH2OAr), 1.77 (c, 4H, CH2CH2OAr), 3.65 (i,j, 80H, O(OCH2CH2)20), 3.96 (d, 4H, CH2OAr), 4.36 (g, 2H, −CH2−CO2), 7.16 (f, 2H, ArHCO2CH3), 6.63 (e, 1H, OArH). Rf = 0.6 (eluent dichloromethane/methanol = 10:1). The 1H

2.59 g (95%) of white solid was dried under vacuum. The crude product was put into the next reactor directly. Synthesis of 3,5-Di(n-dodecan-1-yloxy)benzoxy polyoxyethylene [(3,5)2C16−COOPEG1000, 5]. A 41.8 g (41.8 mmol) portion of PEG1000 and 80 mL of toluene were put into a 150mL, four-necked, round-bottom flask reactor equipped with a magnetic stirrer, a reflux condenser, and a thermometer. The mixture reacted in an Ar atmosphere at 112 °C for 2 h. After the reaction, the flask was cooled down to 80 °C. Then 0.64 g (6.27 mmol) of Et3N and 2.59 g (4.18 mmol) of 4 were added dropwise to the mixture. The mixture was stirred for another 5 h at 80 °C. The progress of the reaction was tracked by TLC 11860

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NMR spectrum of 5 is given in the Supporting Information (Figure S3). Synthesis of 3,5-Di(n-hexadecyloxy)benzoxy Polyoxyethylene Methacrylate [(3,5)16G1-CO2PEG1000MA] (6). As shown in Figure 1, the functional monomer 6 was synthesized by esterification of methacryloyl chloride and compound 5. A 3.604 g (3.0 mmol) portion of 5 and 2.195 g (21.0 mmol) of methacryloyl chloride in 20 mL of CH2Cl2 were placed into a 100-mL, round-bottom flask. Then the mixture was cooled to 0−5 °C with an ice−water bath. Dry Et3N (2.125 g, 21.0 mmol) was added dropwise and the mixture was stirred with an ice bath for 1h and at room temperature for 3 h. Then the mixture was poured into saturated NaCl solution and extracted by CH2Cl2. The organic phase was separated and dried with anhydrous MgSO4. Then the organic phase was removed with a rotary evaporator to yield the crude product, which was then purified by silica gel column chromatography (eluent CH2Cl2/ CH3OH = 15:1, v:v) to give 6. The 1H NMR spectrum of 6 is shown in Figure 2. As seen from Figure 2, the measured area ratio of peaks l, m, and a is 1:1.01:6.012, while the theoretical value is 1:1:6. 1H NMR (CDCl3, TMS, δ, ppm): 0.88 (t, 6H, CH3), 1.26 (c, 48H, −CH2−), 1.44 (b, 4H, CH2CH2CH2OAr), 1.77 (c, 4H, CH2CH2OAr), 1.95 (k, 3H, CCCH3), 3.65 (i,j, 80H, O(OCH2CH2)20), 3.96 (d, 4H, CH2OAr), 4.36 (g, 2H, CH2CO2), 7.16 (f, 2H, ArHCO2CH3), 6.63 (e, 1H, OArH), 5.58 and 6.13 (l, m, 2H, CCH2). The FTIR spectrum of 6 is given in the Supporting Information (Figure S4). The peak at 1039−1122 cm−1 is the characteristic absorption bands of −CH2OCH2− from PEG, the absorption peak at 1638 cm−1 is the stretching vibration of −CC− from acrylic acid groups, the peak at 951 cm−1 represents the out-of-plane bending vibration of −CHCH2, the peak at 1715 cm−1 indicates the stretching vibration of −CO− from the ester group of aromatic acids, and the peak at 1260 cm−1 is also the stretching vibration of −C−O− from the ester group of aromatic acids. Emulsion Polymerization. The modified alkali-soluble associative polymer latex was synthesized by semicontinuous, seed emulsion, and pre-emulsification polymerization technology. The recipe of a single batch is given in Table 1. A detail

Figure 3. Synthesis scheme of dendron hydrophobically modified alkali-soluble emulsion (DHASE) copolymer.

drying the product in an oven at 100 °C and then weighing at room temperature. Sample Preparation. The polymer latexes obtained from the emulsion polymerization process were dialyzed in regenerated cellulose membrane tubes for approximately 30 days, with frequent changing of water. The solid content of each polymer in the series was again determined by drying in an oven at 100 °C and weighing. Stock solutions were dried to powder in an oven at 100 °C, from which the final solutions of different concentration were prepared. Samples for viscosity and rheological measurements were prepared by adding triethanolamine (TEOA) to the originally powder materials to reach a required pH value. The DHASE solutions were left to stand for 1 day before testing.

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RESULTS AND DISCUSSION Rheological Behavior in the Course of Alkalinization. In order to test the thickening effect of the DHASE polymers, we first tested the steady shear viscosity profile of 1.5 wt % latex MED-10.5 at different pH values, as shown in Figure 4. The zero shear viscosity η0 (0.01 1/s) of the solutions jumps at pH 6.2, reaches the maximum at pH 6.8 (5 orders of magnitude higher than the viscosity at pH 5.8) and stays almost constant at higher pH values. As mentioned above, the structure of the associative polymers in solution at high pH is a network of temporary hydrophobic junctions, which dissociates and

Table 1. Raw Ratios in the Preparation of DHASE Polymers sample

MAA/EA/6 mole ratio

MAA/EA/6 mass ratio

MED-0 MED-3.5 MED-5.17 MED-10.5

47.23/52.76/0.00 47.16/52.63/0.21 47.07/52.61/0.32 46.90/52.47/0.63

43.50/56.50/0.00 42.00/54.50/3.50 41.24/53.59/5.17 38.90/50.60/10.50

emulsion polymerization process is described below in Figure 3. A certain amount of methacrylic acid, ethyl acrylate, functional macromonomer 6, TR-70 surfactant, and distilled deionized water were added to a flask and then dispersed by vigorous agitation. Thus, the pre-emulsion was prepared. The initiator solution was prepared by mixing some ammonium persulfate and distilled deionized water together. In N2 atmosphere, the pre-emulsion was fed in a single shot into a 150-mL glass reactor that was heated to 80 °C. The initiator solution was fed dropwise to the reactor over a 2.5 h period, while the whole mixture was continuously stirred at 80 °C. Then the reactor was heated to 85 °C for another 1 h for residual monomers to react. The obtained DHASE polymers are listed in Table 1. The final solid content of the polymers was approximately 10% by weight. These results were subsequently confirmed again by

Figure 4. Steady shear viscosity profile of 1.5 wt % latex MED-10.5 solutions at the indicated pH values. 11861

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reassociates dynamically. At higher pHs, beyond 6.8, there seems to be a slight decline in the viscosity of the solutions, which is believed to be the cause of partial shielding of negative charges along the polymer backbone in the existence of excess positive ions. Furthermore, the amount of intermolecular hydrophobic associations decreases as a result of the shielding of negative charges; thus, the polymer chains contract slightly and the viscosity of higher pH values stabilize at the same level. We find from Figure 4 that the solution viscosity of MED10.5 sample is essentially Newtonian at low pH values. While the pH values of polymer solutions are higher than 6.2, the solutions show large low-shear viscosities and a highly shearthinning behavior, just like a typical characteristic of associative thickeners. The drop in the viscosity with increasing shear rate is a result of the disruption of the network junctions. The association of hydrophobic groups can be both intramolecular from the same polymer chain and intermolecular from the neighboring polymer chain, and the latter plays a dominant role. Hence the predominant intermolecular associations of hydrophobes from neighboring chains form a 3-dimensional network of associating junctions that dramatically increase the hydrodynamic radius and solution viscosity. As the association strength is weak, the network can be destroyed by the shear stress. The rate of junction disruption surpasses the rate at which hydrophobic associations can be re-formed; thus, the solution shows a rapid shear-thinning behavior.1 The results indicate that the introduction of amphiphilic dendron−coil macromonomer 6 can greatly improve the thickening effect of the emulsion due to strong hydrophobic association between long chain alkyl groups, as mentioned in the Introduction. However, compared with some well-known commercial thickeners, like the Sterocoll FD and Sterocoll D series, at the same pH level and polymer concentration (1.5 wt %, pH 8) the viscosity of the Sterocoll FD (pH 8, 0.7−5 wt %), as reported by Kheirandish etc.,11 is about 3 orders of magnitudes lower than that of MED-10.5 (pH 8, 1.5 wt %), indicating the potential of the dialkylsubstituted benzoic acid dendritic amphiphile in hydrophobically modified alkali-soluble emulsion thickeners. In order to illustrate the influence of pH value on the storage (G′) and loss (G″) modulus of 1.5 wt % MED-10.5 sample, a typical set of results for the emulsion are shown in Figure 5. Some consequences are multiplied by 10a to distinguish the curves clearly, and the values of a are listed in the figure. All dynamic measurements in the figure are performed in the linear viscoelastic region. As we can see from Figure 5, the sample MED-10.5 behaves more like the viscous liquid at low pH. At pH 5.8, for ω > 1 rad/s, G′ > G″, while for 0.1 < ω < 1 rad/s, G′ < G″. This indicates that at this pH, the hydrophobic groups began to form networks with associative junctions. As the backbone chains still coil, the hydrophobic groups of the polymer branches prefer intramolecular rather than intermolecular associative junctions, and hence, the networks are incomplete. With the increase of pH value, carboxylic acid groups from the polymer chains dissociate more; thus, the electrostatic repulsive forces cause the polymer chain to expand and then the intrachain associative junctions become interchain associative junctions. So when pH 6.2, G′ > G″ at a larger range in the emulsion system.12 With the addition of alkali, the hydrophobic groups associate interchain as the backbones of the chain expand entirely. Both G′ and G″ increase a lot because the interchain associative junctions take the lead and the network is becoming complete.

Figure 5. Frequency dependence properties of 1.5 wt % Latex MED10.5 solutions at the indicated pH values.

More hydrophobic junctions and greater associative forces drive higher G′ and G″ values. Besides, the value of G′ grows faster than the value of G″. As a result, at a certain condition, curves G′ and G″ will intersect at one point, and then G′ > G″. This phenomenon suggests the formation of a physical associative network.9,13 At a higher pH level, the storage modulus is independent of the testing frequency and remains a constant value, suggesting that the emulsion should behave like a highly viscoelastic gel. Tam et al.14 attributed this behavior to the large number of mechanically active junctions. Those active junctions formed by association of the hydrophobe in the polymers solution systems are temporary. The lifetime of those junctions is very short, which is expected to be on the order of 0.1 s. Since the junctions in the network are dynamic, they are broken and re-formed all the time by thermal energy. So the capacity to rupture and re-form the existing junctions allows these gels to relax applied stress of strains. Effect of Polymer Concentration on Rheological Behavior. As previously mentioned, upon neutralization, the electrostatic repulsive force of the negative charges along the backbone drive the polymer chains to expand as the carboxylic acid groups are ionized. At the same time, a network consisting of both intra- and intermolecular associations of the hydrophobic groups is formed. Furthermore, the ionic strength (i.e., the range of electrostatic interaction) decreases with increasing polymer concentration. Therefore, when the polymer concentration is lower than the cac (critical association concentraion), as each polymer chain is separated in the solution and the hydrophobes are far away from each other, the intramolecular associations play a dominant role. Thus, a complete physical network could not develop to give a high viscosity. However, at a higher polymer concentration approaching the cac, intermolecular interactions become more probable, and these associative junctions link hydrophobes from different polymer chains to build a complete 3-D network. In general, it presents pronounced thickening effects. Evidence can be found from Figure 6, that shows the steady shear viscosity profile of pH 6.8 11862

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than G′. However, both G′ and G″ increase as the angular frequency rises. This phenomenon is attributed to the high proportion of intrachain association, which induces the physical network to be established. The result is in accord with the consequence from Figure 7. When the polymer concentration is above 1.0 wt %, at the whole range of angular frequencies, the value of G′ is higher than the value of G″. Moreover, G′ is independent of the angular frequency. This phenomenon indicates that the polymer concentration is above the cac and the predominant associations of interchains occur. Thus, a complete network can be formed in the solution. With the increase of the association junctions, both G′ and G″ increase and G′ grows faster than G″. To the deformation of low frequency, G′ does not show its dependency on the frequency. We can believe that 1.0 wt % polymer concentration is higher than the polymer’s cac. Effect of Functional Monomer Content on Rheological Behavior. Figure 8 illustrates the influence of varying

Figure 6. Steady shear viscosity profile of Latex MED-10.5 solution at indicated solid contents at pH 6.8.

latex MED-10.5 at different polymer concentration. At 0.5 wt % concentration, the relatively long distance between neighboring chains of DHASE polymer cause more intramolecular hydrophobic associations; thus, DHASE does not show an effective thickening effect. At higher concentrations like 1.0 and 1.5 wt %, the network junctions formed by the intermolecular hydrophobic associations lead to the improvement of viscosity, which is increased by about 2 orders of magnitude compared to 0.5 wt %. The more evident shear-thinning behavior from higher polymer concentrations also reflects the influence of polymer concentration. The similar results can be found from Tirtaatmadja’s research about associative polymers.9 In order to demonstrates the influence of polymer concentration on the inner structure of the solution, storage modulus (G′) and loss modulus (G″) of 1.5 wt % MED-10.5 sample are shown in Figure 7. Some consequences are multiplied by 10a to distinguish the curves clearly, and the values of a are listed in the figure. When the polymer concentration was 0.5 wt % and the angular frequency (ω) 30 1/s), the viscosity of the solutions rises as the proportion of 6 content rises, but when the content of 6 reaches 10.5 wt %, the viscosity is a bit smaller than that of 5.17 wt %. Under high shear stress, the associative junctions between neighboring chains are ruptured, so the interchain association becomes intrachain association. If the content of 6 surpasses a certain limit, more hydrophobic groups along the backbone chain cause intramolecular associations. Hence, the coil of the polymer chain and the fall of hydrodynamic volume lead to the drop of viscosity. Figure 9 displays the dynamic shear modulus against angular frequency (ω) for 1.0 wt % DHASE with varying 6 content.

Figure 7. Frequency dependence properties of latex MED-10.5 solutions at indicated solid contents at pH 6.8. 11863

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ASSOCIATED CONTENT

S Supporting Information *

H NMR spectra of (3,5)2C16−CO2CH3 (2) (Figure S1), (3,5)2C 16−COOH (3) (Figure S2), and (3,5) 2C16 − COOPEG1000 (5) (Figure S3) and the FTIR spectrum of (3,5) 2C16−COOPEG1000MA (6) (Figure S4).This material is available free of charge via the Internet at http://pubs.acs.org. 1

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-20-87112708. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the NSFC (21274047) and the Specialized Research Fund for the Doctoral Program of the Education Ministry (20120172110005) is gratefully acknowledged.

Figure 9. Frequency dependence properties of 1.5 wt % latex MED10.5 solutions at indicated macromonomer 6 contents at pH 6.8.

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REFERENCES

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The data are vertically shifted by a factor of 10a to avoid the curves overlapping, and the a values are listed in Figure 9. According to Figure 9, when polymer contains no functional monomers, G′ is always lower than G″ in the whole ω range. This means that, without functional monomers, polymer has no hydrophobic side groups. The polymer chains just entangle together instead of building an associative network, so G′ < G″ and the solution presents obvious viscous behavior. After introducing 6, the backbone chain has hydrophobic branches that can help associate the hydrophobes and establish the network. Even when the amount of the 6 is low, like the 3.5 wt % sample, a complete network can be established. Thus, when the content of 6 is higher than 3.5 wt %, as shown in Figure 5, the storage modulus (G′) is higher than the loss modulus (G″) and G′ is independent of ω, meaning that these DHASE samples exhibit viscoelastic behavior.

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CONCLUSIONS A novel amphiphilic dendron−coil macromonomer, 3,5-di(nhexadecyloxy)benzoyloxy polyoxyethylene methacrylate [(3,5)16G1-CO2PEG1000MA, 6], was synthesized by the grafting-onto approach. After emulsion copolymerizing with methacrylic acid and ethyl acrylate, a new catalog of dendron hydrophobically modified alkali-soluble emulsion (DHASE) thickners were synthesized. It has been found that, during alkalization, when the value of pH reached 6.5, all the samples of DHASE had an obvious improvement in terms of viscosity and rapid shear-thinning behavior. Besides, the solution viscosity rose upon increasing the macromonomer 6 content and the polymer concentration. Moreover, the mechanism of the association process was discussed by examining the changes of G′ and G″ in the angular frequency range of 10−2−102 rad/s. As expected, these DHASE had a high initial viscosity and rapid shear-thinning behavior under basic conditions due to strong hydrophobic association between long alkyl chains. In general, this study indicates the potential application of the dendritic amphiphile in hydrophobically modified alkali-soluble emulsion for the first time and opens a new perspective for more efficient thickeners. 11864

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Industrial & Engineering Chemistry Research

Article

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