Synthesis and Aqueous Solution Properties ofHydrophobically Modified Graft Copolymer of SodiumCarboxymethylcellulose with Acrylamide andDimethyloctyl(2-methacryloxyethyl)ammonium Bromide

JIAN ZHANG,1,2 LI-MING ZHANG,1 ZHUO-MEI LI1

1 Institute of Polymer Science, Zhongshan University, Guangzhou, 510275, People’s Republic of China

2 State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation, Sichuan, 637001, People’s Republic of China

Received 14 September 1999; accepted 17 December 1999

ABSTRACT: The hydrophobically modified water-soluble graft copolymer of sodiumcarboxymethylcellulose with acrylamide and dimethyloctyl(2-methacryloxyethyl)am-monium bromide has been synthesized using potassium persulfate and dimethylami-noethyl methacrylate as initiators in aqueous solution. The structure and compositionof the graft copolymer were characterized by infrared spectrum and elementary anal-ysis. Molecular weight was determined by gel permeation chromatography. The solu-bility and viscosity of the graft copolymers in aqueous solution were investigated, as afunction of copolymer concentration, added salt, temperature, shear rate, and surfac-tant. In addition, the intermolecular hydrophobic association in aqueous solutions inthe presence of added salt and surfactant was demonstrated through the retention timefrom gel permeation chromatography measurement. © 2000 John Wiley & Sons, Inc. J ApplPolym Sci 78: 537–542, 2000

Key words: carboxymethylcellulose; graft copolymer; hydrophobic association; solu-bility; viscosity; retention time

INTRODUCTION

Water-soluble polymers modified with a smallamount of hydrophobic groups (1–5 mol %) havebecome of great interest in recent years.1 In par-ticular, the hydrophobically modified polyacryl-amide (HMPAM) has attained extensive researchbecause of its important commercial applications

in enhanced oil recovery, frictional drag reduc-tion, flocculation, superabsorbency, personal careformulation, controlled drug release, papermak-ing, and coatings.2–4 In its aqueous solution, in-termolecular hydrophobic interaction occursabove a certain polymer concentration, leading toenhancing the viscosity prominently. But somedisadvantages have also been found for HMPAMin practice, such as poor solubility in aqueoussolution, sharp decrease of viscosity at high shearrate,5 and environmental damage of the effluent.

The previous studies in our laboratory showthat sodium carboxymethylcellulose (NaCMC) ex-hibits excellent viscosification at relatively highshear rate, better solubility in water, and no en-vironmental pollution owing to its biodegradabil-ity.6 What’s more, the cellulose resources are very

Correspondence to: L.-M. Zhang.Contract grant sponsor: Natural Science Foundations of

Guangdong Province and China.Contract grant sponsors: State Key Laboratory of Polymer

Materials Engineering, State Key Laboratory of Oil/Gas Res-ervoir Geology and Exploitation, Laboratory of Cellulose andLignocellulosic Chemistry, Academia Sinica.Journal of Applied Polymer Science, Vol. 78, 537–542 (2000)© 2000 John Wiley & Sons, Inc.

537

abundant and the production cost is low. In thisreport, we try to incorporate the advantages ofNaCMC and HMPAM to prepare a new hydropho-bically modified water-soluble graft copolymerthat may effectively increase the viscosity ofaqueous solution at certain range of temperature,shear rate, and surfactant concentration in thepresence of added salt.

EXPERIMENTAL

Materials

NaCMC (commercial grade) was purified bywashing with 80% alcohol, with a degree of sub-stitution of 0.68 determined using conductometrictitration7 and an average molecular weight of 1.43 105 measured by gel permeation chromatogra-phy (GPC). Dimethyloctyl(2-methacryloxyethyl-)ammonium bromide (DMOAAB) was synthe-sized as described in Scheme 1 according to theliterature.8 Acrylamide (AM) was recrystallizedtwice from acetone prior to use. Ammonium per-sulfate (APS) (analytical grade) was purified byrecrystallization from water, and dimethylamin-oethyl methacrylate (DMAEMA) (commercialgrade) was purified by distillation under reducedpressure. 1-Bromooctylane and sodium dodecylsulfonate (SDS), purchased from Aldrich Chemi-cal Co., were used without further purification.

Synthesis of CAO

In a 500-mL four-necked round-bottom flaskequipped with a mechanical stirrer, a thermome-ter, a reflux condenser, inert-gas nitrogen (N2)inlet, and thermostated water bath, 150 mLNaCMC aqueous solution was placed, and purgedwith N2 for 30 min at 40°C. Solutions of APS andDMAEMA were introduced, and the pH was ad-justed to 6 or so with HCl. After 20 min, the mixedsolution of AM and DMOAAB was added drop-wise through a dropping funnel within 30 min.Then the solution was continuously stirred at40°C for 4–6 h under a continuous flow of N2. Theresulting products were washed extensively inacetone three times, and then dried under vac-uum for 24 h. Further purification of the driedproducts was conducted by extraction with 10 mLmixed solvent (acetone/methanol/dimethylform-amide/water 5 60:20:10:10 by volume) per grampolymer twice to remove homopolymers and co-polymers so as to obtain the pure graft copoly-mers CAO listed in Table I. For comparison, thegraft copolymer (CA) of NaCMC and AM withoutDMOAAB was also prepared at the same condi-tions as CAO.

Analysis and Measurements

1. The infrared (IR) spectra of graft copoly-mers were recorded with a NICOLET FT-IR 20SX spectrophotometer, KBr pellet.

2. The analysis of the composition for graftcopolymers was determined by HERAEUSCHN-O-Rapid element meter.

3. The molecular weight (MW) of graft copoly-mers was obtained using Waters-244GPC:

Scheme 1 Synthesis of DMOAAB monomer.

Table I Parameters of Graft Copolymers Obtained by EA and GPC

Sample

Feed Ratio

N (%)Br(%)

Compositions ofCopolymers

Yield(%)

Mw

(3106)E/g F/g G/g E/% F/% G/%

CA 3 10 0 14.517 0 26.38 73.62 0 76.92 1.31CAO-1 3 10 1.00 14.697 1.119 21.56 73.54 4.90 71.43 0.63CAO-2 3 10 0.51 15.060 0.349 22.40 76.06 1.54 74.02 0.87CAO-3 4 10 0.78 13.063 0.744 31.16 65.59 3.25 67.66 0.71

E, F, and G denote NaCMC, AM, and DMOAAB, respectively.Reaction conditions: [APS] 5 3 3 1023M; [DMAEMA] 5 3.5 3 1023M; pH 5 6; 4 ; 6 h; 40°C.

Yield ~%! 5Mass of the graft copolymers

Mass of the raw materials 3 100.

538 ZHANG, ZHANG, AND LI

Ultrahydrogel 1000 column; standard spec-imen, pullulan; eluant, mixture of waterand methanol containing 0.1M NaNO3;temperature, 30°C. The polymer concen-trations were obtained using ChromatixKMX-16 differential refractometer.

4. The viscosity measurements of aqueous so-lution for graft copolymers were performedon a Contraves LS 30 low-shear rheometerat low shear rate, on a Haake Rheocon-troller equipped with CV20C RV20 PK45-4.0 rotasystem at high shear rate.

5. Hydrophobic associations were identifiedin the light of retention time gained byGPC.

RESULTS AND DISCUSSION

Structure Characteristics of Graft Copolymer

IR Analysis

The FTIR spectra of graft copolymers CAOmanifest the peaks of NaCMC (cycloether, 1068cm21; —COO2, 1592 cm21, and 1412 cm21), AM(3200 cm21, 1650 cm21, 1330cm21), andDMAOAB [—(CH2)nCH3 n . 4, 723 cm21;—COO—, 1180 cm21, weak peaks], confirmingthat CAO consists of the components of NaCMC,AM, and DMOAAB.

Composition Analysis

According to the data of elemental analysis (EA)for nitrogen (N/%) and bromide (Br/%), the con-tents of NaCMC (A/%), AM (B/%), and DMOAAB(C/%) in 100 g CAO can be calculated as follows:

A 5 100 2 B 2 C

B 5 S N14 2

C349.9D 3 71

C 5Br

79.9 3 349.9

where 14, 79.9, and 71, 349.9 represent theatomic mass of nitrogen and bromide, the molec-ular weight of AM and DMOAAB, respectively.The results are listed in Table I.

Aqueous Solution Properties

Solubility

The graft copolymers with various contents ofDMOAAB exhibit the difference of solubility indeionized water according to the transmittance ofthe solution determined by a spectrophotometer(Table II). As usual, the larger transmittance in-dicates the better homophase formed betweenpolymer and water, i.e., good solubility.9 It hasbeen found that the solubility of CAO with hydro-phobe is less than that of CA without hydrophobe,and the solubility of CAO reduces with the in-crease of hydrophobe. But because of CAO con-taining hydrophilic groups of quaternary ammo-nium cation, —CONH2 and —COO2, CAO is stillwater soluble under certain condition. For in-stance, CAO-1 with high content of hydrophobecan dissolve completely in water within a day onagitating at 30°C.

Viscosity Behaviors

Effect of polymer concentration. The viscosity ofaqueous solutions for graft copolymers increaseswith increasing polymer concentration (Cp) (Fig.1), which is in agreement with the general regu-

Figure 1 Plot of the viscosity of aqueous solution forgraft copolymers as a function of polymer concentra-tion: 30°C; 10 s21.

Table II Dependence of Solubility on theContent of Hydrophobe in Graft Copolymers

Sample CA CAO-1 CAO-2 CAO-3

Transmittance (%) 98 83 94 87

Determined conditions: polymer concentration, 0.4%; sol-vent, deionized water; 30°C.

HYDROPHOBICALLY MODIFIED GRAFT COPOLYMER 539

larity. But it is interesting to find that for CAO-1with lowest molecular weight (see Table I), theincrement of h is the largest and its viscosity ismuch larger than CAO-2 and CA. This is attrib-uted to the least ratio of NaCMC and the highestcontent of hydrophobe in CAO-1. The lowest pro-portion of NaCMC bearing semi-rigid backbone isfavorable to hydrophobic association and lesspolyelectrolyte effect. The highest content of hy-drophobe (DMOAAB) causes the nearest space forhydrophobic pendant groups of adjacent chains,therefore interactions between hydrophobicgroups are strengthened, forming plenty ofbridged crosslinking complexes containing hydro-phobe, hydrophile, and solvent.10 It is noteworthythat the graft copolymer with very low level ofDMOAAB displays interesting solution proper-ties. For example, the solution viscosity of CAO-2with 1.54% DMOAAB is much larger than that ofCA with the highest molecular weight of 1.313 106 at the Cp more than 0.2%, but it is lowerthan the viscosity of CA solution at the Cp lessthan 0.2%. This is because the intermolecularhydrophobic associations exceed greatly intramo-lecular associations at high Cp over 0.2% and theintramolecular associations become the majorones at Cp below 0.2%.

Effect of added salt. It can be seen from Figure 2at NaCl concentration from 0 to 0.05M, the solu-tion viscosity of graft copolymers decreases withthe increase of NaCl content, as is analogous withliterature.2 However, at NaCl concentration over0.05M, the viscosity of CAO and CA solution ex-hibits two opposite tendencies with increasingNaCl content: viscosity of CAO is enhanced

nearly linearly, but that of CA drastically fallsdown. It is well known that the addition of NaClto the polyelectrolyte solution causes electrostaticscreening of the charged groups and a strikingreduction in hydrodynamic volume leading to thedecrease of viscosity; CA is consistent with thisrule. As for CAO, it is necessary to take the spe-cialty of hydrophobe into account. The polarity ofsolvent promoted by NaCl motivates intermolec-ular hydrophobic association, which results in in-crease of viscosity.11 The electrostatic shield ofCl2 to cation of quaternary ammonium ofDMOAAB will weaken the repulsion between pos-itive charges of DMOAAB, leading to easier hy-drophobic association. In addition, the ionic spe-cies may enhance ordering of the hydrophobicmicrodomains in favor of viscosity enhancement.2

It can be inferred that for CAO solution in thepresence of added salt, the more the hydrophobecontent, the higher the viscosity. As shown inFigure 2, in 1M NaCl solution, the viscosity of0.5% CAO-1 solution is high up to 158 mPa.s, thatof CAO-3 solution is only 120 mPa.s.

Effect of temperature. Figure 3 shows the depen-dence of solution viscosity on temperature. Theviscosity of CA solution reduces gradually withincreasing temperature due to weakening solva-tion and disruption of intermolecular hydrogenbond of CA. But for CAO solution, the behavior isquite different: the viscosity increases with in-creasing temperature within certain range tomaximum and then drops down remarkably. Ob-viously there exist two opposite effects of temper-ature on viscosity of CAO solution. Because thehydrophobic interaction is an endothermic pro-

Figure 3 Plot of the viscosity of aqueous solution forgraft copolymers as a function of temperature. Cp,0.8%; 10 s21.

Figure 2 Plot of the viscosity of aqueous solution forgraft copolymers as a function of salt concentration: Cp,0.5%; 25°C; 10 s21.

540 ZHANG, ZHANG, AND LI

cess driven by entropy,2 rising temperature isfavorable for intermolecular hydrophobic associa-tion resulting in enhancement of the solution vis-cosity. But at higher temperature, the activemovement of molecules would destroy the hydro-phobic association and the dehydration of the hy-drophilic groups would give birth to the coilingloose structure of molecular chains. These lattertwo effects would result in decreasing viscosity ofCAO solution. As shown in Figure 3, the solutionviscosity of CAO-1 with more hydrophobe ishigher than CAO-2 and hmax of CAO-1 solutionappears at higher temperature than CAO-2 solu-tion. This infers that CAO with higher content ofhydrophobe can keep its higher solution viscosityup to higher temperature. It is possible to varythe content of the hydrophobe in CAO to meetdifferent requirements for viscosity of aqueoussolution in practice.

Effect of shear rate. Figure 4 shows the viscosity-shear relationships of graft copolymers. The vis-cosity of CA falls down with increasing shear rate.But increasing the shear rate from 10 to 110 s21

results in a large increase in viscosity of CAO-1followed by a decrease at higher shear rate. Sam-ple CAO-2 shows moderate viscosity increment. Itis reported2 that this peculiar viscosity behaviorof CAO solution may be a result of an increase inthe order of the hydrophobic domains induced bythe application of shear, thus the hydrophobicdomains are enhanced by groups that had notparticipated in the intermolecular association be-fore the utilization of shear. Further increment inshear rate (110 s21) leads to the breakage of in-termolecular associations and the copolymers ex-

hibit shear-thinning behavior on account of thephysical reversibility of the association frame.12

Effect of SDS. Figure 5 illustrates the viscosityfeature of graft copolymers in the presence ofsurfactant SDS. When SDS is added to the aque-ous solution of CA, a continuous and steady de-crease of the viscosity is observed. This is equiv-alent to the augment of the ionic strength of thesolution. However, the solutions of CAO-1 andCAO-2 display completely different viscosity be-haviors on addition of SDS. The viscosity risesnoticeably at SDS concentration less than its crit-ical micelle concentration (cmc, >8 3 1023mol/L),13 up to a maximum and then down rapidlyabove cmc. Despite the unfavorable electrostaticrepulsion between CAO and SDS, hydrophobicinteraction becomes the driving force of associa-tion. The hydrophobic alkyl groups of CAO andSDS form mixed hydrophobic clusters containingtwo or more distinct polymer chains13 resulting inthe increase of hydrodynamic volume of CAO andthe increase of viscosity of CAO solution. At ap-proximate cmc of SDS, maximum of crosslinkingof the polymer chains is realized through mixedhydrophobic clusters. Accordingly, the viscosity ofCAO reaches maximum. Well above cmc, SDSmicelles are in large excess, the polymer-alkylgroups are distributed in a maximum number ofSDS micelles rather than involved in a smallamount of clusters for mixed micelles containingtwo or more chains. Thus, effective crosslinking isprevented and the network structure is brokendown, ands eventually results in the descent ofviscosity.

Figure 5 Plot of the viscosity of aqueous solution forgraft copolymers as a function of SDS concentration.Cp, 0.4%; 25 °C; 10 s21.

Figure 4 Plot of the viscosity of aqueous solution forgraft copolymers as a function of shear rate. Cp, 0.6%;25°C (from Haake rheometer).

HYDROPHOBICALLY MODIFIED GRAFT COPOLYMER 541

Evidence of Hydrophobic Associations

GPC was used to study the intermolecular hydro-phobic association in CAO solution. In general,the shorter the retention time (Rt), the largerhydrodynamic volume of the polymer due to theintermolecular aggregation. Table III describesRt of CAO-1 solution containing various contentsof NaCl and SDS. When the concentration ofNaCl is raised from 0.1M to 0.5M, Rt decreasesfrom 12.8 to 12.4 min. But there is the quitepronounced difference of the influence of SDS onRt. For example, at 0.1M NaCl, Rt varies from12.6 to 11.7 min as SDS concentration increasesfrom 0.002 to 0.006M. However, Rt rises from11.7 to 12.0 min with increasing SDS from 0.006to 0.01M. The above results are consistent withthe h–NaCl plot in Figure 2 and h–SDS plot inFigure 5. It has been demonstrated, once more,that the intermolecular hydrophobic associationis the main effect on the peculiar viscosity behav-ior of CAO aqueous solution.

CONCLUSIONS

The graft copolymers are of good solubility inwater, even if the content of hydrophobe(DMOAAB) in CAO-1 is up to 4.9%. The depen-dence of solution viscosity (h) for CAO on Cpincreases greatly with the increment of hydro-phobe. The upward slope of the plots of h versusCp above 0.2% is much larger than that below0.2%. Solution viscosity increases drastically withthe addition of NaCl, but nonmodified CA withoutDMOAAB exhibits opposite tendency. Tempera-ture in the range of 20 to 60°C is favorable tointermolecular hydrophobic association. Solutionviscosity rises with the increment of shear rateless than 110 s21. An increase in the solutionviscosity of CAO is observed on the addition of

SDS below cmc and the continuous addition ofSDS above cmc leads to dropping viscosity. Butthe solution viscosity of CA without DMOAAB isdiminished with increasing SDS. Retention timeattained by GPC is capable of demonstrating theinterchain associations of hydrophobic groups ofCAO.

The authors appreciate support from the State KeyLaboratory of Polymer Materials Engineering, theState Key Laboratory of Oil/Gas Reservoir Geology andExploitation and the Laboratory of Cellulose andLignocellulosic Chemistry, Academia Sinica.

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Table III Retention Time of CAO-1 as a Function of Concentrations ofNaCl and SDS by GPC

Polymeric Water Solution Eluant Rt (min)

0.5% CAO-1 1 0.1M NaCl 0.1M NaCl 12.80.5% CAO-1 1 0.5M NaCl 0.5M NaCl 12.40.5% CAO-1 1 0.1M NaCl 1 0.002M SDS 0.1M NaCl 1 0.002M SDS 12.60.5% CAO-1 1 0.1M NaCl 1 0.006M SDS 0.1M NaCl 1 0.006M SDS 11.70.5% CAO-1 1 0.1M NaCl 1 0.01M SDS 0.1M NaCl 1 0.01M SDS 12.0

Determined conditions: driving speed, 0.6 mL/min; temperature, 30°C; polymer concentrationsare inspected by Chromatix KMX-16 differential refractometer.

542 ZHANG, ZHANG, AND LI