(Hydroxyethyl)cellulose - American Chemical Society

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18 Synthesis and Solution Properties of Hydrophobically Modified (Hydroxyethyl)cellulose Arjun C . Sau and Leo M . Landoll 1

2

Aqualon Company, Research and Development Center, Little Falls Centre One, 2711 Centerville Road, P.O. Box 15417, Wilmington, DE 19850-5417 Hercules, Inc., Research Center, Wilmington, DE 19894

1

2

Hydrophobically modified water-soluble polymers (HMWSPs) exhibit enhanced solution viscosity and unique rheological properties. These properties can be explained in terms of intermolecular associations via hydrophobes. This chapter describes the synthesis and solution properties of HMWSPs. Particularly discussed are the solution properties of hydrophobically modified (hydroxyethyl)cellulose (HMHEC) in aqueous and surfactant systems. HMHECs interact with surfactants and thus modify solution viscosities. The structure and the concentration of the surfactant dictate the solution behavior of HMHEC. The unique solution properties of HMHEC can be exploited to meet industrial demands for specific formulations or applications.

ATERS -OLUBLE POLYMERS

(WSPs) are an important class of industrial polymers. They have many applications in solution and in the solid state. In solution, they are widely used as thickeners to control the rheology of various water-based formulations, such as latex paints, drilling muds, foods, cosmetics, and building materials. Chemically modified natural polysaccharides such as starch, cellulose, and guar are a large class of commercial water-soluble polymers. The appropriate chemical modification of these polysaccharides can lead to the modified solution properties needed for specific applications.

0065-2393/89/0223-0343$06.50/0 © 1989 American Chemical Society

In Polymers in Aqueous Media; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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POLYMERS IN A Q U E O U S M E D I A


In many applications and formulations, WSPs with surface-active properties are desirable. These properties are imparted to a polymer by chemical modification of the hydrophilic W S P with appropriate hydrophobic substituents. Examples of commercially available surface-active WSPs based on cellulose include its methyl, hydroxypropyl, and methylhydroxypropyl derivatives. Recently, a new class of surface-active synthetic WSPs called "synthetic associative thickeners" was introduced to the latex coatings industry. These compounds have generated interest because of their unique rheological characteristics in aqueous systems (J). These synthetic macromolecules are composed of hydrophilic and hydrophobic components. A few years ago, Landoll (2-4) reported that grafting a small amount of long-chain alkyl hydrophobes onto a nonionic water-soluble polymer leads to associative thickening behavior (i.e., enhanced viscosity, surface activity, and unusual rheological properties). This chapter deals with the general methods of preparation and solution properties of hydrophobically modified nonionic WSPs. Particularly described are the solution properties of hydrophobically modified (hydroxyethyl)cellulose ( H M H E C ) in aqueous and surfactant systems.

Experimental Details The H M H E C s discussed in this chapter were prepared according to LandoH's method (2) (i.e., by the reaction of H E C with the appropriate 1,2-epoxyalkane or the alkyl halide (^ C ) in an alkaline slurry process). The hydrophobe content of the H M H E C was determined by exhaustive cleavage of the polymer with 57% aqueous hydroiodic acid for 2 h at 185-195 °C followed by analysis of the iodoalkane formed by gas chromatography on a column packed with 10% OV-17 (phenylmethylsilicone) on 80-110-mesh Chromosorb W-HP (diatomaceous earth, Alltech Associates) in a Perkin-Elmer 3920B gas chromatograph. All samples were run in triplicate. Aqueous viscosity was measured at room temperature with a Brookfield and Ubbelohde viscometer. Intrinsic viscosities were measured by a five-point dilution method; no shear-rate corrections were made for the data. Interfacial tension was measured with a DuNouy ring tensiometer against toluene at various polymer concentrations. The formulations used to evaluate the H M H E C s for latex paints have been described elsewhere (5). 8

Results and Discussion Methods of Preparation of Hydrophobically Modified WSPs (HMWSPs). Incorporation of Hydrophobes into WSPs. Water-soluble cellulose derivatives ((hydroxyethyl)cellulose, (hydroxypropyl)cellulose, methylcellulose, etc.) or synthetic polymers containing hydroxyl groups (e.g., polyvinyl alcohol)) can be reacted with a long-chain alkyl halide (2), acyl halide (2), acid anhydride (6), isocyanate (2), or epoxide (2, 3) under appropriate conditions to form an H M W S P . These reactions are shown in Scheme I. These postmodifications can be done in solution or in hetero-


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i)

R—OH

Hydrophobically Modified (Hydroxyethyl)cellulose

+ Χ-

R—Ο

Ο

Ο

II

II ii)

R—OH +

R — Ο — Ο

Χ — Ο

Ο iii)

R—OH

+ Ο

345

ο

II \ / \ / II

Ν


R—Ο—C—C—C—C—OH Ο Ο iv) v)

R—OH R—OH

+ 0 = C = N +

^-7

> »

R—Ο—C—NH~

R — O — C H

2

— C H —

I

OH

R: polymeric backbone containing hydroxyl group alkyl hydrophobe X: halogen atom Scheme I geneous slurry processes. However, on an industrial scale, slurry processes are preferred because of viscosity build-up during modification in solution. Synthetic H M W S P s can be prepared according to

Copolymerization.

this approach by the copolymerization of a vinyl or epoxide monomer (4) with a small amount of a specific hydrophobic monomer that is copolymerizable with the primary monomer as shown in Scheme II. The hydrophobe content of the polymers can be tailored by controlling the amount of the hydrophobic comonomer used in the polymerization process. The nature of the group (i.e., ether, ester, carbamate, etc.) connecting the "long-chain

i)

CH =CH

+ CH =CH

2

polymerization^

2

OAC

0~~ HQ ^ 2

HCH —CH) 2

M

-(CH —CH)„ 2

OH iî)

Q H 2

ι

S^^2

Ο

+

O H 2 ^CH

ο

Ο —

polymerization

* -£-(CH —CH —0) —(CH —CH—0)„-J 2

2

m

2

Scheme 11


346


alkyl group" to the W S P backbone has no noticeable effect on the solution properties of the resulting H M W S P . Thickening Mechanism of H M W S P s . H M W S P s exhibit enhanced solution viscosity as compared to their unmodified counterparts. Figure 1 compares the Brookfield viscosity of a C hydrophobically modified H E C 1 6

( C H M H E C ) with that of a similar molecular weight H E C . The enhanced solution viscosity of H M W S P s is the result of intermolecular associations via the hydrophobic groups (Figure 2). These hydrophobic associations can be viewed as pseudo-cross-links. A three-dimensional network results because of the tendency of the hydrophobes to cluster and thereby minimize the disruption of water structure. This network leads to an apparent increase in the hydrodynamic volume, which is reflected in enhanced solution viscos ities.


1 6

Effect of Hydrophobe Level on Solution Behavior of HMHEC The general solution viscosity behavior of a C H M H E C is shown in Figure 3. This behavior is common to all nonionic H M H E C s that contain hydro phobes of different chain lengths ( C - C ) . The peak viscosity and solubility of a given type of H M H E C depend on the amount and chain length of the hydrophobe (3). At a given hydrophobe level, the longer the chain length, the higher the viscosity. Also, the peak viscosity at a given hydrophobe level 1 2

8

2 4

100,000

Ί

ι

0

1

1 2

Γ 3

Solution Concentration, % Figure 1. Brookfield viscosity (at 30 rpm) vs. concentration of C wHMHEC and HEC of equivalent molecular weight.


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347


Polymer Backbone

Figure 2. Structure of HMHEC in solution.

500 h

Wt.

%

C-

Figure 3. Brookfield viscosity of a C HMHEC as a function of polymer concentration. (Reproduced with permission from ref. 10. Copynght 1987 TAPPI Press.) l2

American Chemical Society Library

1155 16th SI, HW. In Polymers in Aqueous Media; Glass, J.; Advances in Chemistry; Washington. American Chemical Society: DC, 1989. 0.C 20O3Washington, B

348


is much higher when the alkyl chain is longer. These relationships are shown in Figure 4, which compares the aqueous viscosities of H M H E C s that contain C , C , and C hydrophobes.


8

1 2

1 6

As the hydrophobe content of the H M H E C increases, the viscosity first increases, reaches a maximum, and then decreases rapidly. The viscosity finally approaches that of the solvent as the polymer becomes insoluble. A minimum hydrophobe level (discussed in detail later) is necessary to achieve an observable increase in viscosity as compared to that of the unmodified H E C . Above this threshold, viscosity increases rapidly with increase in the hydrophobe content. After reaching the peak viscosity at an appropriate hydrophobe level, the viscosity decreases upon further increase in hydrophobe level. This decrease is not caused by a diminution in the degree of hydrophobic association, but by the incomplete solubility of the sample. That is, a given H M H E C sample has a distribution of polymer chains with different hydrophobe levels. The proportion of insoluble species increases as the overall hydrophobe content of the sample increases. The hydrophobe

Alkyl Content (wt. %)

•

Figure 4. Brookfield viscosity (at 30 rpm) of HMHEC solution vs. alkyl group content of hydrophobe. (Reproduced with permission from ref 3. Copyright 1982 Wiley).


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349

substitution level, when analytically measured, reflects the average degree of substitution for all H M H E C species.

Correhtion of Hydrocarbon Chain Length and Level with Solution Properties Landoll (3) proposed a scheme to quantify structural effects of hydrophobes on solution properties. Two particular hydrophobic modification levels, designated H


0

and H

ly

were used. H

0

is the hydrophobe level of H M H E C at

which solution viscosity begins to increase markedly; H is the level at which t

the H M H E C becomes water-insoluble. O n the basis of experimental data, it has been found that H is logarithmically related to the alkyl hydrophobe l

chain length. If H is defined in terms of hydrophobes per chain, then the l

polymer backbone molecular weight does not significantly influence H

h

a general curve, as shown in Figure 5, is obtained.


and

350


In Figure 5, the number of hydrophobes necessary to render the H M H E C insoluble is between 1 and 40 depending on the carbon chain length (C25 to C ) . This number is in sharp contrast with an earlier report 1 0

(7) that hydrophobically modified polyelectrolytes with 100 or more hydrophobes are water-soluble. These polyelectrolytes were made by the reaction of n-dodecyl bromide with poly(4-vinylpyridines) with varying amounts of n-bromododecane and bromoethane. Because of their enhanced hydrophilicity, polyelectrolyte backbones can effectively drag more hydroDownloaded by UNIV OF SOUTHERN CALIFORNIA on July 14, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch018

phobes into solution. Strauss and Gershfeld (8) have reported the intrinsic behavior of these systems. Figure 6 shows Strauss and GershfekTs results for polysoaps, which are polymers to whose chain structure soap molecules are chemically attached. Their data suggest a contraction of the flexible polymer coil (Figure 6) in solution. As a result, these polyelectrolytes have reduced hydrodynamic volumes and do not exhibit enhanced viscosity.

7.5 5.0 -

Figure 6. Effect of C content on reduced viscosity of polyelectrolytes. Key: 1, poly(vinyl-N-ethylpyridinium bromide); 2, 6.75% polysoap; 3, 13.6% polysoap; 4, 28.5% polysoap; 5, 37.9% polysoap. (Reproduced from ref. 8. Copyright 1954 American Chemical Society.) i 2


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This solution behavior might be explained in light of intramolecular hydrophobic association of molecules. The abundance of hydrophobes on the polyelectrolyte's backbone favors intramolecular hydrophobic associa tion. The nonionic cellulose ethers containing fewer hydrophobes, on the other hand, do not have this choice and tend to undergo strong intermolecular hydrophobic association. This association results in the formation of large aggregates that are manifested by higher viscosity. The argument just made hinges on the concept that aggregates of hy Downloaded by UNIV OF SOUTHERN CALIFORNIA on July 14, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch018

drophobes behave very much like surfactant micelles, in that they effectively form only above a certain critical concentration, H , which can be estimated 0

by measuring several other solution properties, such as intrinsic viscosity and interfacial surface tension. The results of these measurements are shown in Figures 7 and 8. If these phenomena are used to find H

0

hydrophobe chain length (C

as a function of

is the number of carbons in the hydrophobe),

N

the result is the logarithmic relationship shown in Figure 9. The observed linear relationship logH

0

= -0.078C* -

0.53

(1)

is quite similar to that normally found for the critical micelle concentration ( C M C ) of nonionic surfactants (9) log ( C M C ) =

-kC

N

+ X

(2)

HMHECs (MW 80,000) that contain 0.7 (O) and 0.65 (Δ) wt % hydrophobe. (Reproduced with permission from refi 3. Copyright 1982 Wiley.)



352

POLYMERS IN A Q U E O U S

In fact, the value of Η

σ

MEDIA

(ca. 10" g/dL) is exactly that found for a wide 3

range of nonionic surfactants (9). Thus, the same thermodynamic forces that cause micellization of surfactants or phase separation of hydrophobic solvents appear to govern the solution properties of H M H E C s

(9).

Dilute Solution Properties of HMHEC Figure 10 shows a plot of intrinsic viscosity versus hydrophobe level for two H M H E C samples that have different hydrophobes. As the hydrophobe con tent at fixed molecular weight increases, the intrinsic viscosity decreases. This behavior is even more striking for C H M H E C . In dilute solutions, the H M H E C molecules are separate and untangled, and no intermolecular association occurs. However, to minimize the disruption of water structure, hydrophobes on the same chain tend to cluster. This clustering results in a much reduced hydrodynamic volume, and a lower viscosity is observed. Therefore, the dilute solution properties of H M H E C are consistent with intramolecular hydrophobic association (10). Because of intramolecular as sociation and incomplete solubility, the intrinsic viscosity of H M H E C cannot be appropriately correlated with its molecular weight. Hence, to obtain meaningful intrinsic viscosity data, intramolecular association must be com1 6


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353


18.

Figure 10. Aqueous intrinsic viscosity of C HMHEC (O) andC HMHEC (•) vs. their hydrophobe contents. (Reproduced with permission from ref. 10. Copyright 1987 TAPPI Press.) 8

16


354

POLYMERS IN A Q U E O U S

MEDIA

pletely disrupted. Gelman and Barth (II) found that this disruption can be done by a number of solvents that contain methanol or surfactants, and they reported the Mark-Houwink constants for H M H E C in 0.1% sodium oleate solution.


Rheological Properties of HMHEC Solutions Solutions of H M H E C with specific ranges of hydrophobic modification exhibit non-Newtonian behavior at low shear rates. This behavior is interesting in that solutions of H E C under similar conditions are Newtonian. Because the enhanced viscosity arises from the intermolecular association of polymers, the application of shear results in a disruption of the aggregates, and viscosity drops quickly. Figure 11 compares the viscosity profiles of H M H E C and H E C of equivalent molecular weight under different shear rates. Because of their tendency to aggregate, H M H E C solutions exhibit increased elasticity or higher normal forces (Figure 12). A high molecular weight H E C is included for comparison. As can be seen from Figure 12, an H M H E C with molecular weight equivalent to that of H E C exhibits higher normal forces at 0.8% than the low-viscosity H E C does at 4.4% concentration. However, under shearing conditions, the association is completely disrupted and elasticity is destroyed. This unique behavior of an H M H E C can be exploited in applied-flow systems, such as latex paints. High molecular weight H E C s tend to retain their elasticity even at high shear because of the entropie recoiling forces

0

125

250

500 Shear Rate, s e c

1000 - 1

Figure 11. Apparent viscosity vs. shear rate for HEC and C HMHEC equivalent molecular weight. m


of


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ll 1


ι

ι

10

100

355

ι

(sec," ) Figure 12. First normal force difference vs. shear rate for an HMHEC and an HEC of equivalent molecular weight and an HEC of high molecular weight (HEC-H). The percentages indicate the respective polymer concentration. 1

that result in paint spattering (12). By contrast, latex paints thickened with low molecular weight H M H E C exhibit excellent spatter resistance (5).

Interaction of HMHEC with Surfactants H M H E C s can be viewed as polymeric surfactants. They tend to interact with low molecular weight surfactants and thus modify solution viscosity. The viscosity changes, however, depend on the nature (nonionic or ionic) and concentration of the surfactant. The molecular behavior of H M H E C in the presence of various types of surfactants has been described by Gelman (10) and Steiner (13). Nonionic Surfactants. Gelman (JO) studied the effect of a nonionic surfactant (Triton X-100, an ethoxylated octylphenol surfactant containing 67.5 wt % ethylene oxide) on the viscosity of two C H M H E C s . The results are shown in Figure 13. The Brookfield viscosity of these polymers along with that of the unmodified H E C is included for comparison. In the presence of 0.5% surfactant, there was a dramatic drop in viscosity. In fact, at this surfactant level, the H M H E C viscosity profile was very similar to that of the control H E C . The following explanation has been offered to account for these findings. 1 6


356



Because of their chemical similarity, the polymer-bound hydrophobes have a tendency to interact with the hydrophobic part of the surfactant molecule. If the surfactant concentration in the system is high enough, micelles are formed. If there are enough micelles in the system, then all the hydrophobes will get bound to micelles (Figure 14). As a result, there will be no intermolecular hydrophobic association (Figure 2) and no viscosity

POLYMER CONCENTRATION

(%)

Figure 13. Brookfield viscosity as a function of HMHEC concentration with and without a surfactant. (Reproduced with permission from refi 10. Copyright 1987 TAPPI Press.)

Figure 14. Schematic of the interactions between HMHEC surfactant molecules above the critical micelle concentration. (Reproduced with permission from ref. 10. Copyright 1987 TAPPI Press.)


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enhancement. In fact, this was the situation in the experiments in which the concentration of surfactant used was such that there were more micelles than the number of hydrophobes.

Ionic Surfactants.

The change in H M H E C viscosity with the change

in surfactant concentration is interesting (JO). The chemical nature of the surfactant is an important factor in dictating its effect on the association of the polymer. Figure 15 shows the effect of sodium oleate concentration on Downloaded by UNIV OF SOUTHERN CALIFORNIA on July 14, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch018

the viscosity of two H M H E C samples. At very low surfactant concentrations (0.01-0.1 wt %), the viscosity first increases, reaches a maximum, and then drops sharply to the original value. The magnitude of the peak viscosity varies with the sample's hydrophobe content. However, the peak viscosity occurs at a particular sodium oleate concentration, regardless of the sample's hydrophobe content. For C

1 6

H M H E C samples, the peak viscosity occurred

at a surfactant concentration of about 0.05% (w/v). These results imply that the dramatic changes observed in viscosity were not due to specific interactions between polymer and surfactant, but were related to the concentration at which micelles form. We suggest that at a surfactant concentration just below the critical micelle concentration, poly-

100 to ο ο

HMHEC

0.4% C

1 6

·

HMHEC

0 7% C

] 6

x

CO

50 ο ο

0J

0.2

0.3

SODIUM OLEATE CONCENTRATION (%) Figure 15. Brookfield viscosities of HMHECs as a function of sodium oleate concentration. (Reproduced with permission from ref. 10. Copyright 1987 TAPPI Press.)


358



mer-bound hydrophobes are present in sufficient quantity to permit micelle formation. If more than one hydrophobe from different polymer chains is incorporated into these micelles, the polymer chains are effectively cross-linked (Figure 16) and occasion a large viscosity enhancement. As the surfactant concentration is increased, the number of micelles per polymerbound hydrophobe increases, and the micelles can no longer act as crosslinkers. Consequently, the viscosity decreases. As mentioned earlier, the magnitude of the peak viscosity increases with the increase in the polymer concentration in the presence of oleate (Figure 17). At low polymer concentrations (

(Hydroxyethyl)cellulose - American Chemical Society

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