A novel cationic surfmer, methacryloxyethyl-dimethyl cetyl ammonium chloride (DMDCC), is synthesized. The micellar properties, including critical micelle concentration and aggregation number, of DMDCC-SDS mixed micelle system are studied using conductivity measurement and a steady-state fluorescence technique. A series of water-soluble associative copolymers with acrylamide and DMDCC are prepared using the mixed micellar polymerization. Compared to conventional micellar polymerization, this new method could not only reasonably adjust the length of the hydrophobic microblock, that is,
The incorporation of a few hydrophobic units into a hydrophilic backbone results in hydrophobically associative water-soluble polymers. In aqueous solution, above a certain polymer concentration, the hydrophobic moieties associate and build a transitory three-dimensional cross-links, leading to unique rheological properties such as dramatic viscosity enhancement [
There have been substantial efforts with regard to the synthesis and properties of hydrophobically modified water-soluble polyacrylamides. Following Strauss et al. [
To overcome these drawbacks, the conventional micellar copolymerization method is modified by using polymerizable surfactants, that is, surfmers, to substitute the traditional ones. Surfmers are a family of unique surfactants which not only have amphiphilic structures, that is, cationic or anionic long-chain alkyl structure, but also contain polymerizable vinyl double bonds [
It is thought that only the surfmer-surfactant mixed micellar system corresponds to the same charge, either positive or negative, the microblock of length could be adjusted. Can the cationic surfmer-anionic surfactant binary system share the same capacity in the preparation of hydrophobically associative polyacrylamide with good performance? This paper just addressed this issue. Firstly, a long-chain alkyl cationic surfmer of methacryloxy-ethyl-dimethyl cetyl ammonium chloride (DMDCC) was synthesized, and the properties of DMDCC-SDS (sodium dodecyl sulfate) mixed micelles are determined by conductivity measurement and a steady-state fluorescence technique. We discuss the synthesis and characterization of cationic associating copolymers prepared by DMDCC-SDS mixed micellar copolymerization with DMDCC as cationic hydrophobic comonomer. Particularly, we focus on the effect of the length and number of the hydrophobic microstructures on the rheological properties. In addition, we look at the effect of salt and surfactant on the apparent viscosity of copolymers.
Acrylamide (Chengdu KeLong Chemical Reagent Co., Ltd.) was recrystallized twice from acetone and vacuum dried at room temperature prior to use. The azo-initiator V-50 (2,2′-azobis(2-amidinopropane)dihydrochloride) (UA, Alfa Aesar 99%) was used without further purification. N,N-Dimethyl-ethylamine methacrylate and cetyl chloride, purchased from Aldrich chemicals (purity, 99%), were used as received. Water was double deionized with a Millipore Milli-Q system. Other reagents were analytically pure and used as received.
Scheme
Synthesis of the cationic surfmer (DMDCC).
CMC values of DMDCC and mixture of DMDCC-SDS were determined by conductivity on a digit conductivity meter. The conductivity,
Using pyrene as a probe, the aggregation numbers (
A 250 mL three-necked round-bottom flask is equipped with a mechanical stirrer, nitrogen inlet, and a thermometer. Acrylamide (AM) and methacryloxyethyl-dimethyl cetyl ammonium chloride (DMDCC) and sodium dodecyl sulfate (SDS) in the desired ratio are dissolved in 100 mL of deionized water and then placed in the flask. The total concentration of the monomers is kept constant at 1.0 M. The flask is purged with a small N2 stream for half an hour prior to being heated to 50°C. Polymerization is then initiated by addition of 0.35 mL of V-50 (1 wt% preferentially dissolved in the deionized water) via a pipette scaled to 1 mL under slow stirring. In this case, instead of potassim persulfate, v-50 was used as initiator to avoid the redox side reaction between mercaptan and persulfate. Polymerization is conducted continuously for 8 h at 50°C. The reaction mixture is diluted with five volumes of distilled water, and two volumes of acetone are then added with stirring to precipitate the polymers. The precipitated polymers are further washed twice with acetone and extracted with ethanol to remove all traces of water, surfactant, residual monomers, and initiator. The polymers are recovered by freeze-drying after vacuum-drying at 50°C for three days. For reference, PAM is prepared under identical experimental conditions and purification method mentioned above.
IR was carried out using Shimadzu-1800S spectrometer on KBr pellets in the range of 4000–400 cm−1. The peak intensities are characterized as follows: vs = very strong, s = strong, w = weak,
The intrinsic viscosities [
Polymer solutions were prepared by dissolution of a known amount of the polymer powder in water and NaCl solution. The apparent viscosity of samples solution at low concentrations was determined by a Brookfield DV-III rheometer equipped with different sizes of spindles (different diameter depending on solution viscosity) at 25°C. Rheological experiments were carried out with a RS100 controlled-stress rheometer equipped with a cone plate geometry (angle 1°, diameter 35 mm). We measured flow curves by increasing the shear stress in regular steps and waiting at each step until equilibrium was attained. The shear rate
The microstructure of polymers in both aqueous and salt solutions was observed by environment scanning electron microscope (ESEM XL 30). The sample solutions were maintained at −3.5°C and the pressure was controlled below 5 Torr to keep samples solution state during the whole observation.
Fluorescence spectra were measured on a Lengguang 970CRT fluorescence spectrometer. All measurements were conducted at ambient temperature. The slit width of excitation and emission was kept at 5 nm during experiments. The excitation wavelength (
SDS was chosen to study the micellar behavior of DMDCC-SDS binary mixed system, because it bears the same hydrophobic long chain as DMDCC and the most common surfactant used in radical micellar copolymerization, and, most importantly, mixture of cationic (DMDCC) and anionic (SDS) surfactants has a net interaction (
Figure
Variation of critical micelle concentration of DMDCC-SDS binary mixture system as a function of DMDCC mole fraction (
The CMC of mixed surfactants strongly depends on parameter values
Variation of critical micelle concentration of DTAB-SDS binary mixture system as a function of DTAB mole fraction [
The much lower CMC values of SDS are obtained by adding cationic surfactant DTAB. We note that DMDCC bears almost the same structure as DTAB. Therefore, we can draw a conclusion that the CMC behavior of DMDCC-SDS mixed system, as shown in Figure
Schematic representation of the mixed DMDCC-SDS micelles compared to pure DMDCC micelles and pure SDS micelles.
In the DMDCC/SDS mixed system,
The copolymers poly(AM/DMDCC) with low amounts of cationic hydrophobe were obtained in water using a DMDCC-SDS binary mixed micellar polymerization (Scheme
Structural parameters of the copolymers.
Samplea | Polymer composition mol % |
|
Polymer characterization | ||||||
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AM | DMDCC | SDS |
|
|
[ |
|
Solubility in watere | ||
PAM | 100 | 0 | 0 | 0 | 0 | 3.50 | 610 | 755 |
|
|
|||||||||
0.5DM4.5 | 99.5 | 0.5 | 1.75 | 4.5 | 25 | 3.15 | 586 | 697 |
|
0.5DM9.3 | 0.8 | 9.3 | 12.4 | 3.29 | 540 | 720 | + | ||
0.5DM12.7 | 0.5 | 12.7 | 9 | 3.25 | 520 | 714 | + | ||
0.5DM92 | 0 | 92 | 0.6 | 1.64 | 299 | 428 |
| ||
|
|||||||||
0.75DM4.6 | 99.25 | 0.75 | 5.5 | 4.6 | 25 | 2.22 | 335 | 536 |
|
0.75DM9.3 | 2.1 | 9.3 | 17 | 3.06 | 470 | 683 | + | ||
0.75DM12.8 | 1.35 | 12.8 | 13 | 3.35 | 418 | 730 | + | ||
0.75DM92 | 0 | 92 | 0 | / | / | / | − |
Copolymerization of AM and DMDCC.
IR spectra of the DMDCC and copolymer 0.75DM12.8.
1HNMR spectra of copolymer 0.75DM12.8.
The effect of the hydrophobic microblock length, that is,
Apparent viscosity as a function of polymer concentration for copolymers 0.75DM12.8, 0.5DM12.7, and 0.5DM9.3 samples and for a PAM sample (
All the samples are well measured at a constant shear rate of 7.34 s−1. In extremely dilute solution regime (
Apparent viscosity as a function of shear rate is conducted for three copolymers modified by 0.5 mol% DMDCC and for PAM. The viscosity/shear rate curves of the copolymers and PAM at concentration of 1.0 g dL−1 and 2.0 g dL−1 are shown in Figures
Variation of apparent viscosity as a function of shear rate for PAM, 0.5DM12.7, and 0.5DM4.5 (
Variation of apparent viscosity as a function of shear rate for PAM, 0.5DM12.7, and 0.5DM4.5 (
PAM exhibits the classic rheological behavior, that is, a slight shear thinning effect after Newtonian plateau. All the copolymers present the same type of behavior. For poly(AM/DMDCC), at 1.0 g dL−1 concentration, a shear thickening domain reaches first followed by shear thinning. The shear thickening is interpreted in terms of the balance between intra- and intermolecular associations. At a certain shear rate, the shear forces are strong enough to extend the polymer coils and disrupt the intramolecular hydrophobic interactions. The intramolecular hydrophobic microdomain is released and favors forming intermolecular hydrophobic interactions, resulting in an increase in viscosity. Further increasing shear rate, these interactions are gradually broken and shear thinning behavior is observed. This behavior was general agreement with the nonionic hydrophobically modified polyacrylamides [
To determine the role of the
Rheological values of
Polymer |
|
| ||||
---|---|---|---|---|---|---|
|
|
|
|
|
|
|
PAM | 0.93 | 10.3 | 90 | 3.6 | 10 | 357 |
0.5DM4.5 | 1.09 | 1.66 | 658 | 8.2 | 0.56 | 14650 |
0.5DM9.3 | 1.55 | 1.23 | 1264 | 9.78 | 0.36 | 27160 |
0.5DM12.7 | 2.07 | 0.76 | 2723 | 13.15 | 0.25 | 62600 |
In case of PAM, a higher
The stress threshold values
We should figure out that even at a higher concentration (2.0 g dL−1), shear thickening domain is also observed, and both copolymers show classical shear thickening subsequent shear thinning behavior within the range of shear rate. This owning to the fact that beside molecular weight and polymer concentration the nature of hydrophobic monomer also influences shear thickening efficiency. The apparent viscosity/shear rate curves at the characteristic values at the Newtonian plateau are also given in Table
Figures
Storage (
Storage (
However, for copolymers modified by hydrophobic groups, at low frequencies, the behavior of the shear modulus is Maxwellian, indicating the variations of
Values of the slopes of the storage modulus
Polymer | Slope |
|
|
|
---|---|---|---|---|
|
|
|||
PAM | 2.2 | 0.98 | / | <0.01 |
0.5DM4.5 | 1.6 | 1.17 | 1.51 | 0.32 |
0.5DM9.3 | 1.35 | 1.05 | 2.27 | 0.86 |
0.5DM12.7 | 1.27 | 0.86 | 2.45 | 2.85 |
0.75DM4.6 | 1.46 | 1.15 | 1.79 | 0.82 |
0.75DM9.3 | 1.25 | 0.93 | 2.78 | 1.92 |
0.75DM12.8 | 1.32 | 0.96 | 3.56 | 6.25 |
The presence of the hydrophobic group into macromolecular chain increased both moduli and the terminal zone is shifted toward low frequencies due to the transitional network junctions by hydrophobic associations. 0.5 DM4.5 exhibits a slight increase in the modulus at Hz compared to that of PAM, while copolymer 0.5 DM9.3 shows a more marked enhancement with
On the other hand, time
The apparent viscosity as a function of NaCl concentration for 0.75DM12.8 and PAM at polymer concentration of 0.15 g dL−1 and 0.75 g dL−1 was represented in Figure
Relative variation of the viscosity with NaCl concentration for copolymer 0.75DM12.8 and PAM at two different concentrations (shear rate 7.34 s−1,
The behavior of PAM exhibits almost the same trend at two different concentrations in the whole salt concentration range. At low salt concentration, the viscosity decreases smoothly upon the increase in salt content, and above the certain salt concentration, relative constant apparent viscosity is exhibited. For PAM, increasing the concentration slightly shifts the constant apparent viscosity towards the lower salt concentration. At the highest salt level, the viscosity of PAM at 0.75 g dL−1 is about 85% of its initial value in pure water. This can be explained that the polarity of solvent is changed in the presence of salt, resulting in the oppression of macromolecular chain accompanied by slight decrease of the apparent viscosity. However, for copolymer 0.75DM12.8 in salt solution, the variation of the viscosity depends significantly on the polymer concentration. For copolymer 0.75DM12.8 dilute solution (0.15 g dL−1), slightly higher than critical concentration (0.11 g dL−1), apparent viscosity increases with increasing of NaCl concentration. At NaCl concentration of 0.80 g dL−1, apparent viscosity reaches the maximum value (3 times that value in salt free solution), subsequently decreases steeply, and it finally becomes close to that of PAM. Solution polarity induced by the presence of NaCl enhances intermolecular interaction between hydrophobic groups of the chains and the good salt-thickening behavior is observed. On the other hand, with further increasing the salt concentration, the hydrophobic microstructures turn compact and isolated macromolecules resulting in the disappearance of intermolecular interaction and the decrease of the apparent viscosity. This behavior supports the conclusion that, below the chain overlap regime, the thickening efficiency arises from the intermolecular interaction between the hydrophobic segments of the chains. Once the transient network is disrupted into small clusters, the viscosity is significantly lowered. Conversely, over the chain overlap regime (0.75 g dL−1), apparent viscosity of copolymer 0.75DM12.7 almost keeps constant after attaining the maximum value (3.5 times that value in salt-free solution). This can be ascribed to the high degree of chain overlap and even entanglement at this concentration prevents the network from disruption. For copolymer 0.75DM12.7, when the salt concentration is above 10 g dL−1, either clouding or separation is observed.
Figure
Variation of apparent viscosity of as a function of SDS concentration for copolymers 0.75DM12.8 and 0.75DM9.3 at 0.5 g dL−1 (shear rate 7.34 s−1,
It is noteworthy that the increasing of
The intensity ratio
Fluorescence spectra of pyrene in 0.25 g dL−1 0.75DM12.8 solutions at different conditions.
The microscopic structures of poly(AM/DMDCC) at different experimental conditions are observed through ESEM at room temperature and the morphological images are shown in Figure
ESEM images of poly(AM/DMDCC) at different experimental conditions; (a) 0.5DM12.7 with 0.075 g dL−1; (b) 0.5DM12.7 with 0.15 g dL−1; (c) 0.75DM12.8 with 0.15 g dL−1 in 2 g dL−1 salt solution; (d) 0.75DM12.8 with 0.75 g dL−1 in 2 g dL−1 salt solution; (e) 0.5DM12.7 with 0.5 g dL−1 in 8 mM SDS solution; (f) 0.75DM12.8 with 0.5 g dL−1 in 8 mM SDS solution.
A novel cationic surfmer, methacryloxyethyl-dimethyl cetyl ammonium chloride (DMDCC), was synthesized in the experiment. For DMDCC-SDS binary mixed surfactants system, conductivity measurement and a steady-state fluorescence technique indicated that synergetic mixed micelles were obtained due to strong net attraction between cationic (DMDCC) and anionic (SDS) surfactants, which dramatically reduce the CMC and keep the Nagg almost stable. Therefore, this new method could not only reasonably adjust the length of the hydrophobic microblock (
The incorporation of hydrophobic segments into polymer chains induces a strong enhancement of apparent viscosity. Rheological measurements including steady flow and linear viscoelasticity experiments are used to study the association process in the semidilute regime of copolymers. In the whole shear rate range of steady flow experiments, a shear thickening behavior of copolymers is exhibited and followed by shear thinning. Increasing hydrophobic microblock enhances the degree of association and results in shear thickening behavior towards lower share rate. The presence of the hydrophobic group into macromolecular chain increased both storage (
In the vicinity of the critical associative concentration, the presence of salt influences the dynamics transformation between intramolecular and intermolecular junctions resulting in changing the solution viscosity. However, at higher copolymer concentration, governed by chains entanglement and intermolecular associations, viscosity enhancement is observed in the whole salt concentration range. Apparent viscosity measurement performed on the copolymer indicates that both ionic attraction and intermolecular interactions play a great role. At the CMC of SDS, the formation of mixed micelles between the copolymer chain and surfactant which serve as junction bridges for transitional network remarkably enhances the viscosity. What is more, the microscopic structures for copolymers at different experimental conditions conducted by ESEM confirm the results discussed above.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to the Major Special Project of China (Grant no. 20082X05049-05-03) for financial support of this work.