Hydrophobically and cationically modified hydroxyethyl cellulose and their interactions with surfactants

Colloids and Surfaces A : Physicochemical and Engineering Aspects, 82 (1994) 279-297 0927-7757/94/$07.00 G 1994-Elsevier Science B .N . All rights reserved .

279

Hydrophobically and cationically modified hydroxyethyl cellulose and their interactions with surfactants U . Kastnera •* , H. Hoffmanna, R . Dongesb, R . Ehrlerb aUnipersitar Bayreuth, Physikalische Chemie I, D-95440 Bayreuth, Germany 'Hoechst AG, Werk Kalle-Albert, Rheingaustr . 190, D-65174 Wiesbaden, Germany (Received 19 July 1993; accepted 10 August 1993) Abstract We investigated the solution behavior and the macroscopic properties of samples of hydroxyethyl cellulose (HEC) which arc modified by perfiuoroalkyl chains (F-HMHEC) and by cationic groups (cat-HEC) . The systems were characterized by theological, electric birefringence and scanning electron microscopic measurements. A solution of 1% HEC (molecular weight, 5 x 10' g mol - ') has a viscosity of about 100 mPa s . The chemical modification causes an increase in viscosity up to 1000 mPa s . The solutions of the modified systems show shear thinning behavior . On the addition of surfactant the viscosity of the F-HMHEC and cat-HEC solutions increases even more. Such solutions show viscoelastic behavior and a yield stress value . The theological properties were characterized by oscillatory measurements . The results are interpreted in terms of associations of the side chains of the polymer with the surfactant molecules . The surfactants and the modified HEC form a three-dimensional network structure . It is shown for the first time that the three-dimensional network structure can be made visible by freeze-fracture scanning electron microscopy, The influence of the cooling rate on the microscopic structures was shown by using melting nitrogen and liquid propane as cryogens . Key words : Hydroxyethyl cellulose : Surfactants

Introduction Water-soluble cellulose derivatives are largescale commercial products which are used in many industrial applications . Of particular interest in this investigation is hydroxyethyl cellulose (HEC) and the cationically and hydrophobically modified HEC, cat-HEC and HMHEC respectively . HEC is a water-soluble compound and because of its "compatibility", it is used as a thickener, a protective colloid or water-retarding agent in cosmetic products, pharmaceutical preparations, polymerization processes, emulsion paints and in many other industrial applications . Commercial products *Corresponding author . SSDI 0927-7757(93)02629-S

are available with molecular weights ranging from 25000 to 800 000 g tool -' . An aqueous solution of 1% HEC (molecular weight, 5 x 10 5 g mol - t) possesses a viscosity of about 100 mPa s . The chemical modification causes an increasing viscosity up to 1000 mPa s . During the last few years it has been shown by several groups [1-9] that the efficiency of HEC as a thickener can considerably be improved by hydrophobic modification of the HEC . Usually alkyl groups of intermediate chain length are used to modify the HEC systems . The degree of substitution has to be kept low, otherwise the polymers become insoluble in water . Typically one alkyl group per 100 cellulose units is sufficient to increase the thickener capacity . In this respect the situation

280

U. Kastner et al./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994) 279 297

is very similar to that of hydrophobically modified polyelectrolytes where substitution of a few per cent of the ionic groups by alkyl groups is sufficient to change the macroscopic properties drastically [3] . The thickening action in both systems is due to intermolecular and intramolecular cross-linking of the polymers through hydrophobic interactions of the alkyl groups . Tanaka et al, [4,5] have shown that the unmodified HEC is present as individual molecules which interact through hydrodynamic forces while the modified systems form transient networks. The macroscopic properties of such systems can be further improved and optimized by small concentrations of surfactants . It is believed that the surfactant molecules bind on the alkyl groups of the polymers and strengthen the cross-links . Solutions with similar properties to those described can be obtained with cat-HEC in combination with anionic surfactants . Such systems have been investigated systematically during recent years by Goddard and co-workers [8-10] . It can be concluded from these studies that the anionic surfactants bind to the cationic charges of the polymer and the tails of the surfactant act again as cross-links between different polymer coils . If enough surfactant is added to compensate the charge completely the system phase separates . With excess anionic surfactant a single-phase solution is obtained . Very recently Goddard and Leung [10] have shown that systems with even stronger elastic properties can be obtained with HEC when the polymer is substituted with cationic groups which also carry a hydrophobic alkyl group . With an increasing concentration of anionic surfactant such systems become viscoelastic, then phase separate and finally become single phase again and behave as very strong stiff hydrogels. Rheological measurements on such systems have shown that the storage moduli (C') are much higher than the loss moduli (G") over a frequency range of several decades and that the absolute values of the moduli are more than ten times larger than for those systems which

were only hydrophobically or cationically modified . In the present work we also studied modified H EC . The object of the investigation was to study the influence of perfluoroalkyl side-chains on the HEC (F-HMHEC) and also the influence of the degree of cationic substitution (cat-HEC) on the macroscopic properties . For both types of system the influence of surfactants on the properties of the system was studied . Materials The HEC samples were commercial products (hydroxyethyl cellulose : Tylose" H (Hoechst AG .)) . Modified samples were prepared according to the literature . HEC and F-HMHEC [111 Four samples of a neutral perfluoroalkyl hydroxyethyl cellulose (F-HMHEC) and the basic HEC were used for our measurements . The molecular weight of all samples is 5 x 10 5 g mol' (from light-scattering experiments) . The molar substitution (MS) of hydroxyethyl (HE) is 2 .3-2 .6 per anhydroglucose unit; for the degree of hydrophobic substitution and the approximate number of perfluoroalkyl side-chains (z 5) see Table 1 . The structure of the hydrophobic substituent is -CH,-CHOH-CHz O-CH(CH2 CI)-CH iO(CH 2 ) 2 - ( CF 2 )7 1 1-CF 3 resulting from the corresponding glycidyl ether used in the synthesis .

cat-HEC [12] Four samples of a cationically modified hydroxyethyl cellulose (cat-HEC) were used . The cationic substituent is a glycidyltrimethylammonium chloride : -CH,-CHOH-CH,N(CH 3 ) 3 CI

U. Kastner et al./Colloids Surfaces A : Physieoehem . Eng. Aspects 82 (1994) 279-297

Table 1 Molecular weights (M w ) determined by light-scattering experiments, the degree of molar substitution of hydroxyethyl (MS,,) and perfluoroalkyl or cationic (MS S) chains and the approximate number of side chains (z s) for HEC, the four F-HMHEC and the four cat-HEC samples Sample

Mw (g mol - ' I

MS„,

NEC

500000

2 .3-2 .6

F-HMHEC 1 F-HMHEC 2 F-HMHEC 3 F-HMHEC 4

500000 500000 500000 5000(

2 .3-2 .6 2 .3-2 .6 2 .3-2 .6 23-2 .6

0.0007 0.0027 0.0031 0.0043

cat-HEC 1 cat-HEC 2 cat-HEC 3 cat-HEC 4

33000 43000 120000

1 .90 1 .38 2 .09 2 .36

0.27 0.21 0.10 0.11

150"

MS5

1 4 6 8 37 41 48 63

The samples are of different molecular weight and degree of substitution (see Table 1) . The following surfactants were used : SDS (sodium dodecyl sulfate) C12H25SO4Na from Fluka ; dodecyltrimethylammonium bromide C 12 H25 N(CH 3 ),Br from Aldrich ; sodium tetradecyl sulfate C 1gH 29 SO 4Na from Aldrich ; lithium perfluorononanoate 2 Li, acid from Aldrich C8 F 17 CO ; n-perfluorooctyl butyltrimethylammonium iodide C,F 17 C 4 H 8N (CH 3 )3 1 from Hoechst . Methods All polymer solutions were continuously stirred for at least 18 h prior to use . The appropriate amount of surfactant was added after the complete dissolution of the polymers. The static light-scattering experiments were carried out on a light-scattering photometer KMX 6 (Chromatix) by using a small angle of 6-7 = . The differential refractive index was measured on a differential refractometer KMX 16 (Chromatix) . The light source was an He-Ne laser (632.8 nm) . The dynamic light-scattering data were obtained on a Brookhaven laser light-scattering goniometer (wavelength ,1=632.8 nm) connected to a Brookhaven digital correlator (BI 8000 AT) . For the electric birefringence measurements we

281

placed the solution in a cell between crossed polarizer and analyzer . On shining an He-Ne laser through this system we measured the light intensity transmitted when an electric field was applied to the solution . Details of this method are summarized elsewhere (see, for example, Refs . 13 and 14) . The viscosities of the aqueous solutions of the polymers over the concentration range 0.001-0.5% were determined using an oscillating capillary rheometer Paar OCR-D. All oscillatory rheological measurements were recorded using a Bohlin CS rheometer . For solutions with a lower viscosity we used the double-gap measuring geometry and for the gels we used the cone/plate measuring geometry . The temperature of all measurements was constant at 25 ° C. Visualization of the expected network structure was performed using a scanning electron microscope JEM 840-A (Jeol) . Two different preparation methods were used : (a) the preparation of a thin film, in which small droplets of the samples were placed on glass, dried for at least 24 h and then coated with a fine film of gold about 10-20 nm thick by a sputtering technique ; and (b) the cryo-jet preparation freezefracture scanning electron microscopy (FFSEM) in which the samples were prepared as sandwiches between two copper rivets . These sandwiches were cooled by jet freezing (using nitrogen slush or liquid propane) and were then transferred in liquid nitrogen to the evacuated cold stage of the preparation chamber . After freeze fracturing, the specimens were warmed up to 190 K for about 10-12 min, for surface etching by water sublimation . The etching time included a heating time of 9 min and a variable time at a constant temperature (190 K) . After etching, the samples were cooled again and coated with a gold layer approximately 10-20 nm thick . The required sputter time was divided into six parts, each of 30 s, with pauses to provide a change in the sample temperature . The prepared samples were then transferred under vacuum to the microscope stage, which was held at 100 K by use of cold nitrogen gas .



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U. Kastner et al./Colloids Surfaces A: PhVsicochem . Eng. Aspects 82 (1994) 279-297

Results The main emphasis in this investigation was on the rheological results and their relationship to scanning electron microscopy . Some static and dynamic light-scattering and electric birefringence measurements were also carried out in order to characterize the polymers . Phase behavior All four investigated polymer samples which are modified by a perfluoroalkyl chain are derived from HEC having the same molecular weight . Aqueous solutions of HEC and of the samples which carry the lowest number of perfluoroalkyl groups are completely water soluble at room temperature up to a 2% polymer concentration . The two F-HMHEC samples with the highest number of perfluoroalkyl chains are water soluble up to polymer concentrations of 0 .3% and 0.1% respectively (see Fig . 1) . At higher concentrations they are soluble at 45-60°C, but the aqueous solutions prepared at this temperature separated into two macroscopic phases at room temperature. Phase separation at room temperature does not occur when small amounts of ionic surfactants are added to the polymer solution . Phase separation

phase separation

dpolymer) [%] Fig . 1 . The solubility of the two samples of F-HMHEC with the highest numbers of perfluoroalkyl groups as a function of temperature .

occurs, however, for the F-HMHEC systems in the presence of larger concentrations of surfactant . In this case, obviously the polymer is charged by the surfactant and it is not clear why phase separation occurs . It is conceivable that the phase separation which occurs with excess anionic surfactants is due to a salt effect . The excess surfactants form micelles and screen the charges on the surfactant-polymer complex . Attractive forces then become dominant again and the aggregates contract. It is also conceivable that two-phase formation in the F-HMHEC-surfactant system is a special case of the phenomenon that is generally known under the name of depletion flocculation [15] . The flocculation of latex dispersions by polyelectrolytes or other charged colloidal particles is usually explained on the basis of this mechanism . The flocculation is then the result of the large osmotic pressure of the charged particles and not so much the result of the attractive forces between the flocculating particles . In our case the charged particles would be micelles formed when excess surfactant is added beyond the amount which is necessary to saturate the hydrophobic chains on the polymer . The process of flocculation is shown in Fig. 2 for a sample of F-HMHEC 4 solution with an increasing concentration of anionic perfluoroalkyl surfactant . The flocculation starts at a surfactant concentration of 18 mmol. The portion of the flocculated phase increases up to 42 mmol and reaches a constant value. Figure 3 shows a plot of the viscosity data and the ratio between the liquid and the flocculated phase of this sample as a function of the lithium perfluorononanoate concentration . (For rheological behavior on the addition of surfactants see the Rheological Results section below .) The cationically modified samples are all water soluble at low and high temperatures up to polymer concentrations of 2% . Phase separation occurs with increasing concentration of oppositely charged surfactant. The phase separation starts at a surfactant concentration large enough to com-



U. Kastner et al./Collards Surfaces A: Physicochem . Eng. Aspects 82 (1994) 279-297

283

Fig. 2 . A photograph of the 1% F-HMHEC 4 solutions with different concentrations of added lithium perfluorononanoate : (from the left) 4 mmol (the maximum viscosity) ; 18 mmol (the beginning of flocculation) ; 24 mmol; 30 mmol ; 34 mmol ; 38 mmol ; 42 mmol ; 50 mmol .

pensate the charge of the polymers . The two phases separate macroscopically and a sharp phase boundary is observed between the two phases . Owing to this fact the surfactants are not likely to bind in a cooperative way on the polymer . This is often the case with classic polyclectrolytes . In these situations, one observes already signs of two phase separations as soon as the smallest amounts of

naccwzled M e

:

37

35

s7

..l

5i ~5 1SC

te2„eo, rut-moa

Fig. 3 . The viscosity and the ratio of the liquid and the flocculated phase of a 1% solution of F-HMHEC 4 as a function of concentration of added lithium perfluorononanoate . The given viscosity is measured in the liquid and in the homogeneous phase .

surfactant are added to the charged polyelectrolytes. For the present system, we can attain 90% charge neutralization without observing phase separation . The two-phase systems change again to singlephase solutions when more surfactant is added than that which is necessary for charge compensation . It is likely that the excess surfactants bind to the surfactant-polymer compounds and reverse the charges. This phase behavior of the cat-HEC with added SDS (as shown in Fig . 4) is consistent with that found from observations on other simple systems and is also consistent with the general behavior of charged gels. Figure 5 shows the viscosity data and the ratio between the liquid and the flocculated phase of the same sample of cat-HEC 4 as a function of the SDS concentration . (For rheological behavior on the addition of surfactants see the Rheological Results section below .) Light scattering Static light-scattering measurements are shown in Fig. 6 where the forward and backward scatter-



U. Kdstner et al ./Colloids Surfaces A : Physicoehem. Eng . Aspects 82 (1994) 279-297

284

Fig. 4 . A photograph of the 1% cat-HEC 4 solution with different concentrations of added SDS : (from the left) 4 mmol (the maximum viscosity); 6 mmol (the beginning of flocculation) ; 8 mmol; 10 mmol; 12 mmol; 18 mmol; 26 mmol; and 30 mmol (single phase again) .

sro S eox n

w

a

roa 91

o% o %

4 .05 0

gei

/ /~~ . ^ / / ! (localated /,

gel

Q

a .or OWS-

ewt `"

V . I

.~.~ ;

10% ° 1l

ox

• HEC (tocwacdl • HEC Ibacu: a cot-HEC 1 • 6 moot NeCI caPHEC 2 +6 mmol NeCI c cat-HEC 3 • 6 mmol NeCI c H EC 4_6 moot lad -

a(SG9 emon

Fig. 5 . The viscosity and the ratio of the liquid and the flocculated phase of a 1% solution of cat-HEC 4 as a function of added SDS concentration. The given viscosity is measured in both the liquid and in the gel phases .

Fig .. 6. The Rayleigh factor determined from static lightscattering experiments as a function of the polymer concentration of HEC and the four cat-HEC samples .

ing of HEC and the forward scattering of the four cat-HEC samples are given. The F-HMHEC samples are not measured because of the intermolecular and intramolecular associations of the hydrophobic side-chains in aqueous media. Both forward and backward scattering for the HEC sample are different and allow the determination of a radius of gyration of 52 .8 nm. This radius can be compared with the hydrodynamic radius of

51 .4 nm which is determined from dynamic light scattering data of HEC . Both radii are indications that the polymer exists as a coil . The cat-HEC samples show no difference between the forward and backward light scattering . The molecular weights that have been evaluated are listed in Table 1 . The degree of molar substitution and the approximate number of side chains (zs) are included in Table 1 .



./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994) 279-297 U. Kastner et al

Electric birefringence

Signals that are obtained by the transient and dynamic electric birefringence methods are given in Fig . 7 . The normal d .c . signals appear for the HEC solution up to polymer concentrations of 0.4% and for the modified F-HMHEC samples up to concentrations of 0.15% (Figs . 7(a)-7(c)) . For higher polymer concentrations, no constant plateau is reached even with the a .c . method (Fig . 7(d)) . In this case we assume that the forma-

f7~ 2 ms

f

r

.4

2 ms

3s

2 ms d)

c)

4 ms

t s

e) Fig. 7 . Typical signals from electric birefringence measurements : (a) d.c. signal of a sample of HEC at a concentration of 0 .1% and at 700 V ; (b) the same sample at a concentration of 0.4% and at 800 V : (c) d .c . signal of a sample of F-HMHEC 2 at a concentration of 0 .1% and at 1100 V ; (d) ac. signal of the same sample at a concentration of 0 .2% and at 300 V and 3 kHz ; (e) d .c. signal of a sample of cat-HEC 4 at a concentration of 0 .1% and at 500 V; (f) the same sample at a concentration of 0 .3% and at 700 V .

285

tion of a transient network by association of the hydrophobic side-chains prevents the orientation of the polymers in the electric field . The cationically modified samples show normal d .c . signals up to a concentration of 0.2% (Fig . 7(e)) and no constant plateau for higher concentrations (Fig, 7(f)) . Some formation of polymeric domains . In comparison with other may be the reason studies on polyelectrolytes [16-18] it is interesting to note that no electric birefringence anomaly is observed at the cross-over concentration from the dilute to the semidilute range . It is conceivable that the lack of this anomaly has something to do with the low charge density of the investigated compounds . A persistence length of a charged polymer has a contribution from the backbone (intrinsic persistence length) and a contribution from the electrostatic repulsion between the charges . For classical polyelectrolytes, the intrinsic persistence length is usually very short and of the order of a few bond lengths . With the cationically modified HEC derivatives, this seems to be different . It is also interesting to note that the d .c . signal is not symmetric . The process of orientation takes more time than the process of relaxation . For low fields the birefringence increases with E2 and a Kerr constant is determined (Fig. 8(a)) . The specific Kerr constants are plotted in Fig . 8(b) against the concentration for several compounds . The Kerr constants of HEC and the F-HMHEC samples are smaller than the Kerr constants of the cat-HEC samples . The Kerr constants are relatively independent of the concentration . The time constants obtained from the birefringence decay were used to determine a dimension for the molecules . For the HEC sample, this length agreed with the diameter measured by light scattering (table 2 shows all results determined from electric birefringence measurements) . Thus it seems that the birefringence monitors the rotation of the hydrodynamic coil . For a statistical Gaussian coil with a molecular weight of 500000 g mol - ' and complete freedom of orientation for each monomer unit, we would expect an end-to-end distance or a dimension for the coil of ((roo>)'/2=(n x h)'/2 : (1900 x



./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994) 279-297 U. Kdstner et al

286

at-HEC

Table 2 Data from electric birefringence measurements on the HEC, two F-HMHEC and four cat-HEC samples

^

.2n

._1

,i

as

HEC

22

1200

82

>0 .2

It

F-HMHEC 1 F-HMHEC 2

22-45 22-50

1200 1200

82 82

0 .1 0 .1

to

t~

it a

A

22-64 64-119 174-393 216-504

72 102 260 300

80 115 160 185

T.

mm)

at-HEC 2

//

c

c *d (%)

L,,° mm)

Itr)`

(µs)

Sample

F1-MHEC 2

- -~ ~

F-HMHEC 1

s

(a)

cat-11EC cat-HEC cat-HEC cat-HEC

1 2 3 4

0 .15 0.15 0 .17 0 .18

f W >

-t -~---. cal-HEC 4

rr --~- ~

-*-

--~----cat-HEC1

--° F--IMHEC2

HEC as - -

(b)

s=s

~

a1

0 . 15 a r= ctpolymarl[%i

Fig. 8. (a) Electric birefringence as a function of the electric field strength E for some samples of HEC, F-HMHEC and catHEC at a polymer concentration of 0 .1%, which means just before interactions between the single molecules occur . (h) The specific Kerr constants vs. the polymer concentration of HEC, F-IIMHEC 2, and all cat-HEC samples .

(0.63nm)2)t'"- x30 nut (where n is the number of monomer units per polymer backbone and I is the length of one monomer unit) . The dimensions of the coils, however, are three times larger, which shows that the density of the polymer inside the coil is 3 3 times smaller than that of a statistical coil . This shows that the persistence length of HEC is of the order of 100 nm. In spite of their low charge density of only one cationic group per 4-10 anhydroglucose units, the cat-HEC compounds show a behavior that is typical for polyelectrolytes . This can he concluded from a comparison of the contour length L,, for the molecules, which is based on the molecular weight and a length of 0 .63 nm for a monomer unit . For the cat-HEC samples 1 and 2 the dimension that is determined from the birefringence

.r gives the relaxation times for the investigated polymer concentrations below c* . h Le is the contour length; t-k= (Mw/ .M„) x 0 .63 nm . where M e, represents the molecular weight of one monomer unit and 0.63 nm is the length of one monomer unit . °1(r) is the calculated length from the relaxation times before reaching c* . dc* is the critical concentration from where the Kerr constants are no longer independent of the polymer concentration.

decay 1(r) is about the same as Lk . This shows that the molecules in solution prefer a more or less stretched conformation . For the two samples with the highest molecular weight the contour length is somewhat larger than the length determined from the birefringence data . The difference is, however, less than a factor of 2, which shows that even those molecules that have an even lower charge density arc in a stretched conformation and have a persistence length which is longer than 100 nm. We measured the orientation time in the dilute range (0 .1% of cat-HEC) when surfactant ions of opposite charge were added . We observed indeed a decrease in the orientation times, as shown in Fig. 9 . The decrease was, however, small . This result shows that even the uncharged molecules are in a stretched conformation and have a persistence length of around 100 nm . Rheological results [19 22] Pure polymer solutions

Figure 10 shows a log log plot of the zero shear viscosity

(qo=hmlq*(w)l ~ w+0

as a function of the



U . Kastner et al /Colloids Surfaces A: Ahysicocherre Eng . Aspects 82 (1994) 279-297

i- ce--HEC 2 1,1%

nt

o5

os

a

d5D8)[mmoil

Fig . 9 . Relaxation times for 0 .1% solutions of cat-HEC 2 (A) and cat-HEC 4 (∎) as a function of added SDS concentration .

Fig. 10. The zero shear viscosity (qo) as a function of the polymer concentration of HEC (s), F-HMHEC 2 (A) and catHEC 4 (/) .

polymer concentration for the HEC, one sample of F-HMHEC and one sample of cat-HEC . At higher polymer concentrations the F-HMHEC solutions show thixotropic behavior and a zero shear viscosity cannot be determined . The viscosity data from 0.5% to 1% are the measured viscosities at the lowest frequency (0.001 Hz) . The HEC and F-HMHEC solutions display the viscosity of water up to polymer concentrations of 0 .02% . At concentrations above 0.4%, the viscosity of the HEC solution increases . This concentration c* marks the point where the flexible coils of HEC are just in contact with each other . For the F-HMHEC sample the viscosity increases at a concentration c* above 0.1% . This concentration characterizes the beginning of the intermolecular

287

association between the perfluoroalkyl chains . At 0.4%, the F-HMHEC concentration reaches the coil overlap concentration of the HEC sample . The slope of the plots corresponds approximately to the scaling law (c/c*) 5 5 characteristic for polymers . Comparing the viscosity data at HEC and F-HMHEC concentrations of 1% it is likely that the higher values for the modified sample are caused by the formation of transient networks . At very low concentrations the slope of the increasing viscosity for the cat-HEC sample is higher than that for the HEC and F-HMHEC solutions while for high concentrations it is the other way around . The viscosity increases from 0,005% slowly, as for typical polyelectrolytes where the viscosity follows a scaling law of (c/c*)r/ O . At a concentration of approximately 0.2% (c*) the viscosity increases faster and the slope of approximately three characterizes again the polymeric behavior. Above the critical concentration, all polymer solutions show shear thinning behavior. Figure 11 shows this behavior for a sample of cat-HEC and a sample of F-HMHEC . The phenomenon of thixotropy is also shown : the plots of viscosity against an increasing and a decreasing shear rate are not identical and no zero shear viscosity occurs .

Fig . 11 . The shear rate dependence of the viscosity of the sample 1% cat-HEC 4 (∎) and a sample of 1% F-HMHEC 2 (A .C,) . The closed symbols characterize measurement with increasing shear rate and the open symbols with decreasing shear rate.



U. K6stner et at/Colloids Surfaces A: Physicochem.

288

Eng. Aspects 82 (1994) 279-297

Influence of added surfactant The influence of added surfactants was investigated for 1% polymer solutions . The solubility of the F-HMHEC samples changes on the addition of surfactants : the more highly modified F-HMHEC sample number 3 becomes soluble with added alkyl surfactants ; the F-HMHEC samples 3 and 4 become soluble with added perfluoroalkyl surfactants . This result points to direct interactions between the surfactant molecules and the polymeric side-chains . The insertion of surfactants in the network structure of the polymers causes an increase in the viscosity . Figure 12 shows, for increasing SDS concentration, the viscosity at a constant shear rate for an HEC solution, two samples of the cat-HEC and two samples of the F-HMHFC solutions . Figure 13 shows the same procedure for the system F-HMHEC-perfluoroalkyl surfactant. The neutrality of the F-HMHEC molecules permits the use of anionic and cationic surfactants . The viscosity of the cat-HEC solutions increases with increasing molecular weight and number of cationic sidechains of these compounds . The viscosity of the F-HMHEC solutions increases with growing molar substitution of the perfluoroalkyl side-chains of the polymer while the HEC solution is not influenced by the addition of surfactants . The viscosity passes over a maximum for both polymer

HEC

a

4

z

s rs~,~atnmi t, .,dt

a

s

e

HEC 10

IwdecIenU fmmop

Fig . 13 . The viscosity at a constant shear rate of 0.5 s - ` as a function of the cationic and anionic perfluoroalkyl surfactants for the four 1% solutions of F-HMHEC and the 1% solution of unmodified HEC . The numbers of the F-HMHEC solutions characterize the increasing hydrophobic substitution of the polymers at a constant molecular weight of 5 x 10' g mol - '. types . We found the maxima generally at lower concentrations of surfactant than the CMC . This means, that the increasing viscosity is not a process of pure micellization of the surfactant in the solution . The results are interpreted in terms of the formation of "premicelles" between the surfactant molecules and the side chains of the modified HEC . The solutions become viscoelastic as shown for an F-HMHEC example in Fig . 14. The "premicellization" causes an increasing number of association points between the polymer

ia . so -

I s d5DS) tmmon

Fig. 12. The viscosity at a constant shear rate of 0.5 s - ' as a function of the SDS concentration for I% solutions of HEC (k), two samples of F-HMHEC (O, F-HMHEC 1; A, F-HMHEC 2) and two samples of cat-HEC (A, cat-HEC 2 ; ∎, cat-HEC 4) .

Fig . 14 . The storage modulus G' (closed symbols) and the loss modulus C" (open symbols) as a function of the frequency for a 1% solution of F-HMHEC 2 (A,4) and for a 1% solution of F-HM HEC: 2 with 4 mmol added lithium pcrtfuorononanoate ( •, fl) .



U. K6stner et al./Colloids Surfaces A . Physicochem . Eng, Aspects 82 (1994) 279-297

backbones. A parameter that describes these numbers is the storage modulus G' . Figures 15 and 16 show the plots of the storage modulus G' at a frequency of 10 Hz (constant value of G') vs . the surfactant concentration for two samples of catHEC and two samples of F-HMHEC solution with added SDS (Fig . 15) and the four F-HMHEC solutions with added anionic and cationic perfluoroalkyl surfactants (Fig. 16). For all polymer-sur-

Fig. 15 . The storage modulus G at a frequency of IO Hz as a function of the SDS concentration for 1% solutions of two samples of F-HMHEC (0, F-HMHEC 1 ; A, F-HMHEC 2) and two samples of cat-HEC (A, cat-HEC 2 ; ∎, cat-HEC 4). The arrows characterize the point of maximum viscosity for each polymer-SDS system.

C(„C,H,N(CH,)

CF CO,Li 100

to

a 4 6 2 daurfaclant) Immoll

2

4

6

dwrfadant) [mmoll

8

Fig. 16 . The storage modulus G' at a frequency of 10 Hz as a function of the concentration of an anionic and a cationic perfluoroalkyl surfactant for the four 1% solutions of F-HMHEC . The numbers of the F-HMHEC solutions characterize the increasing hydrophobic substitution of the polymers at a constant molecular weight of 5 x 10' g mot - ' .

289

factant systems, G' passes over a maximum . The maxima of G' for the cat-HEC solutions with SDS and the four F-HMHEC solutions with lithium perfluorononanoate occur at the same concentration of anionic surfactant as the maxima of the viscosity of these samples . Both maxima characterize the point at which the formation of strong hydrogels is optimized . In Table 3 we compare micellar densities of contact (u,) determined from G' (10 Hz) with the densities of the hydrophobic or cationic chains (an approximation from G°= AV,kT, where A denotes a numerical factor on the order of 0.5 < A < 1, T is the temperature, k is the Boltzmann constant and v, indicates the number of elastically effective chains per unit volume of the transient network) . In the table, n is the ratio between v, and the number of dissolved macromolecules per unit volume (cx =N,, x (c(polymer)/M w ) and describes the number of junction points which build up the network structure. Each of the polymer chains has z s substituents. The maximum number of contacts between different macromolecules is, hence, of the order of zs . The results determined from the theological measurements indicate that only a few of these potential junction points are used to form the network . The F-HMHEC samples use approximately one quarter of the hydrophobic chains to form the transient network while the other chains form intramolecular contact points. For the catHEC samples the number of junction points increases from 0 .5% to 4% with their molecular weight and so does the strength of the observed gels. The majority of the cationic chains covered by anionic surfactant molecules seem to interact by intramolecular association . This indicates that the polymers are now in a more coiled conformation . Plots of the F-HMHEC samples with added cationic and anionic surfactants are not symmetrical (see Figs . 13 and 16), It is known that the influence of cationic surfactants is often lower in comparison with equal anionic surfactants [23-25] . These results seem to indicate that the



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Table 3 Data from rheological measurements of F-HMHEC with added lithium perfluorononanoate and cat-IIEC with added SDS Sample 1%

Surfactant concentration (mmol)

Added C 8 F, 7 CO 2Li F-HMHEC 1 F-HMHEC 2 F-HMHEC 3 t'-HMHEC 4

3 .5 4 .0 5 .0 4 .0

Added SDS cat-HEC 1 cat-HEC 2 cat-HEC 3 cat-HEC 4

6 5 4 4

(1022 m ')

1 .2 1 .2 1 .2 1 .2 18 14 4

or b (1022 m - ')

0.4 1 .3 1 .5 2.4

0 .3 1 1 .25 2

3 .8 4.7 7.1 10.0

0 .2 0 .3 1 .4 2 .5

'Number of dissolved macromolecules in a 1 % solution determined from the molecular weight . 6 Number of elastically effective chains building up the network determined from G' (10 Hz) . `Ratio between above parameters.

F-HMHEC samples have a weak cationic nature ; this helps in binding the anionic surfactants and suppresses the binding of the cationic surfactant . The question then arises of where the differences in behavior are coming from . It is conceivable that these differences are due to the influences of the counterions of the surfactants . The situation seems to be similar for the interaction of ionic surfactants with non-ionic surfactants . Usually a small synergism is found between anionic and non-ionic surfactants . It is also observed in many experiments that non-ionic surfactants have a weak cationic character . It is conceivable that these observations have their origin in the weak complexation of metal ions by ethylene oxide (EO) groups . The F-HMHEC samples also have EO groups which could bind a few sodium ions and thus support the binding of the anionic but not the cationic surfactants . The maximum increase in viscosity is found for the perfluoroalkyl surfactant . This behavior describes the best miscibility of the perfluoroalkyl surfactant molecules with the perfluoroalkyl sidechains of the polymer and characterizes a direct association between both perfluoroalkyl parts . Mixtures of hydrocarbons and perfluoroalkanes have a miscibility gap which is due to their mutual

phobicity . On the basis of this phobicity it is actually surprising that strong interaction between the F-HMHEC and the hydrocarbon surfactants is observed . The lower miscibility of alkyl with perfluoroalkyl groups is indicated out in the displacement between the maxima of the viscosity and the storage modulus G' as shown in Fig. 15 for the two samples of F-HMHEC with added SDS . We found the strongest hydrogels for the system F-HMHEC 4 with 4 mmol of added lithium perfluorononanoate and for the system cat-HEC 4 with 4 mmol of SDS . Both polymer solutions are (again) 1% by weight . While the F-HMHEC-perfluoroalkyl surfactant system shows viscoelastic behavior with a characteristic cross-over point of the storage modulus G' and the loss modulus G" and a zero shear viscosity, the cat-HEC-SDS system behaves differently . Figure 17 shows an oscillatory experiment for the sample cat-HEC 4) with 4 mmol of SDS . The complex viscosity shows no constant value at any frequency . The storage modulus is always higher than the loss modulus and there is no cross-over point . We found a yield stress value of approximately 10.8 Pa for this system . The slope of G' is 0.25 . Figure 18 shows the stress relaxation measurement on this sample .



U. Kastner ei al .%Colloids Surfaces A: Physicodunn. Eng . Aspects 82 (1994) 279-297

Fig . 17. Result of the oscillatory measurement of a sample of 1% cat-HEC 4 with 4 mmol of added SDS .

Fig. 18 . Result of the stress relaxation experiment on a 1% solution of eat-HEC 4) with 4 mmol of added SDS (stretched exponential fit a=a` exp[-(tit)'] ; t=3 .3s ; a=0-27) : z, measured data ; -, fit .

We found that we can fit the stress relaxation measurement with a stretched exponential equation :

a=ao exp[ - ( tlr)Q I Therefore r is the measured relaxation time and a is the exponent of decay . For the cat-HEC sample, a is of the order of the slope of G' determined from the oscillatory measurements . Influence of added surfactant in excess

After having passed the maximum the viscosity and the storage modulus decrease continuously . The number of associations decreases as the number of surfactant molecules increases . This

291

process is completed when each side-chain of the HEC backbone is approximately covered with surfactant molecules. Thereby the network structure is finally destroyed . For even higher surfactant concentrations the solutions separate into two phases : a liquid phase of water and some surfactant molecules or micelles with a viscosity near that of water and a phase which contains the polymer-surfactant aggregates . When all the cationic sites are occupied by the surfactants and the charge on the cat-HEC is completely neutralized we observe phase separation. The polymer-surfactant aggregates contract to a new dense phase . When more surfactant is added in excess, more surfactant molecules bind on the polymer-surfactant complex and reverse the charge . As a consequence the osmotic pressure inside the dense phase increases and leads to a swelling of the phase and finally to a single-phase situation . Somewhat unexpected, however, is the observed phase separation of the F-HMHEC systems with excess anionic surfactant . The viscosity of the liquid phase is still higher than the viscosity of water . This might be caused by some polymer molecules stabilized in the very dilute solution . Furthermore, there is not such a sharp interface as there is for the cat-HEC-SDS system . Some dynamic process of exchange between both phases may be the reason for this .

Scanning electron microscopy Influence of different preparation methods

Each manipulation during the preparation of the samples may influence the structure inside an aqueous solution [26-28] . We investigated various types of preparations in order to minimize undesired artifacts . First, we prepared a thin dried film of a sample of F-HMHEC 2 with 4mmol of added lithium perfluorononanoate on glass . Figure 19(a) shows a photograph of the whole droplet with some dendrites . At higher magnification the surface of a

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Fig . 19 . (a) and (b) A sample of a l , solution of F-HMHEC 2 and 4 camel of added lithium perfluorononanoate prepared as a thin dried film on glass (scale bar in (a) represents 100 gm ; (b) scale bar in (b) represents I gm) . (c) and (d) The same sample prepared as a sandwich in melting nitrogen (FFSEM) (scale bar in (c) represents 10 gm; scale bar in (d) represents 1 gm .

dendrite is studied but no polymeric structure can be seen (see Fig . 19(b)) . Figure 19(c) shows the same sample prepared by the freeze-fracture method in melting nitrogen . It shows a network structure with large holes between the polymeric network strands . The network structure is directed toward the middle of the sandwich . Figure 19(d) shows the same part at higher magnification . The strands are built up from connected spherical aggregates . Figures 20(a) and 20(b) show the same polymer sample with added surfactant prepared in liquid propane . The same spherical aggregates are visible, but they are much smaller than the globules in

Fig. 19(d) . The network structure is much finer and of higher density than that shown in Fig . 19(c) . The preparation of polymers in aqueous solutions as a dried thin film is not suitable for the visualization of network structures . With the freeze-fracture method a network structure occurs . For the two cooling liquids, melting nitrogen and liquid propane, we observed a large difference . The cooling rate of melting nitrogen is much lower than the rate for liquid propane [28,29] . This lower rate allows a slow growth of ice crystals inside the solution . The polymeric network is pushed onto the border of these ice crystals. The direction of the network structure in Fig . 19(c)

./Colloids Surfaces A : Physicochem. Eng . Aspects 82 (1994) 279-297 U. Kastner et al

293

Fig. 20. FFSEM of a 1°% solution of F-HMHEC 2 and 4 mmol added of lithium perfluorononanoate prepared as a sandwich in liquid propane: (a) scale bar represents 10 gm; (b) scale bar represents 1 µm.

points to the middle of the sample and shows the direction of freezing when the cooling rate is too slow . The differences in the size of the spherical aggregates (see Figs . 19(d) and 20(b)) are also caused by different cooling rates . During the freeze process in melting nitrogen, ice crystals grow inside the water-swollen polymer particles . For further preparations we used liquid propane exclusively to characterize the polymeric structure .

Characterization of the polymeric structure To characterize the polymeric structure we used a sample of F-HMHEC 2 (because of the water solubility) and a sample of cat-HEC 4 (because of the strongest measured hydrogels) . We chose polymer concentrations below and above the critical point (0 .05% and 1%) and at 1% with an added surfactant concentration at the maximum viscosity . The expected size of the particles that were free of interactions should be comparable to the calculated length from electric birefringence measurements . For the F-HMHEC solutions below the critical concentration c* of side-chain association we found a constant diameter of 100 nm . Figure 21 (a) shows some spherical molecules with this latter size . The particles are spherical because of the intramolecular associations of the hydrophobic perfluoroalkyl side-chains . These associa-

tions are inside the globules to minimize their contact with water . At a concentration above c*, the expected network structure appears . For the 1% F-HMHEC sample (see Fig. 21(b)) a fine network structure is observed, which is built up of the same spherical aggregates as those seen in Fig . 21(a) . The network is embeded in the water surface and some broken ends are visible . The network structure seems to arise from intermolecular associations of some of the hydrophobic side-chains while the majority of the perfluoroalkyl chains are connected to each other by intramolecular associations and form the globules . For the cat-HEC solutions below and above the concentration c* we observed no structure in the FFSEM photographs . The conformation of the cat-HEC samples determined from electric birefringence measurements indicates almost stretched chains with a length comparable to their contour length . The diameter of these rod-like particles is about 0 .6 nm. In this case the particles would break in the center and we would not be able to see these small diameters in the broken surface . Finally we are interested in the network structure of the polymer-surfactant systems at the points of highest viscosity . Figure 22(a) shows an FFSEM photograph of the system of 1% F-HMHEC 2

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U. Kastner et al./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994) 279-297

Fig. 21 . (a) FFSEM of a 0 .05% solution of F-HMHEC 2 . (b) FFSEM of a 1% solution of F-HMHEC 2 . Scale bar represent I

µm .

Fig . 22 . (a) FFSEM of a 1% solution of F-HMHEC 2 with 4 mmol of added lithium perfluorononanoate . I b) FFSEM of a 1% solution of cat-HEC 4 with 4 mmol of added SDS . Scale bars represent I µm .

with 4 mmol of lithium perfluorononanoate and Fig . 22(b) shows a photograph of 1% cat-HEC 4 with 4 mmol of SDS . Both network structures are built up of connected spherical aggregates . These globules are formed by intramolecular associations of substituent-surfactant micelles. The observed structure is consistent with the rheological results, from which it was concluded that the majority of substituents are not used to build up the transient network structure. Only a few chains act as crosslinks and connect the spherical particles to the observed network structure . The networks show no direction and the mesh density of the cat-

HEC-SDS system is higher than in Fig . 22(a) (F-HMHEC) . Discussion The two polymer types show a different behavior of their pure aqueous solutions above c* . The F-HMHEC samples interact through their hydrophobic side-chains and form spherical aggregates . The cat-HEC samples behave like polyelectrolytes . The electrostatic repulsion leads to almost stretched rod-like chains . Above c* these chains form interacting polymeric domains .

U. Kastner et al ./Colloids SurJaees A : Physicochem. Eng . Aspects 82 (1994) 279-297

On the addition of surfactants the differences disappear . For both polymer types we found a maximum in the viscosity and in the storage modulus with increasing surfactant concentration . The polymeric structure occurs as spherical aggregates connected to a three-dimensional network . A model of the structure of the HMHEC in the presence of surfactant molecules is interpreted as an association between the hydrophobic tails of the surfactant and the hydrophobic side-chains of the polymer . In one of the first investigations concerning the associations between HMHEC and surfactant molecules, Gelman [1] observed that the maximum value of the viscosity is associated with a surfactant concentration above their CMC . He suggested that the increase in viscosity is based on the dynamic interaction of surfactant micelles and the polymeric association points of hydrophobic side-chains . In later investigations [2-7] on such systems the maximum viscosity occurred in general at lower surfactant concentrations than their CMCs . According to our investigations it is likely that the amount of surfactant molecules corresponds to the density of the hydrophobic sidechains . The "premicellar" interaction between the surfactant molecules and the hydrophobic sidechains protects the hydrophobic parts of the polymer from the surrounding water . The ideal model of such interactions corresponds to the situation in which two hydrophobic side-chains of different polymeric backbones are connected and stabilized by the surfactant molecules . However, this model is still a matter of speculation . Counting the junction points which form the elastic network we found that only a few per cent of the hydrophobic side-chains build up the network structure . Tanaka et al . [5] discovered with use of spin resonance spectroscopy measurements employing spinlabeled polymer molecules that the number of junction points in the polymer solutions with and without added surfactant did not differ very much . It is likely that the main part of the hydrophobic side-chains forms intramolecular associations,

295

forming the globules seen in the FFSEM photographs . The model for cationically modified hydroxyethyl cellulose with added anionic surfactant is interpreted in interactions between the charged head groups of the surfactants and the cationic substituents of the polymer . The hydrophobic tails of the surfactant molecules form thejunction points of the network structure . Goddard et al . [8] found that the strong increase in viscosity just before phase separation occurs is due to the stiffness of the polymeric backbone . Using polycationic cellulose polymers with high molecular weights they found that the storage modulus G' dominates the loss modulus over the entire frequency range . We found this yield stress behavior for the cat-HEC sample with the highest molecular weight also . Goddard et al . concluded that the formation of the transient network has a strong component of entanglements of the stiff polymeric backbones and a component of the bound surfactant ions on the cationic substituents. We determined the junction points of the network at the maximum of the storage modulus of each cat-HEC-SDS system and we found that less than 5% of the cationic substituents are used to build up the network structure . Furthermore, we found a charge compensation of 90% at the maxima of the viscosity and the storage modulus . In this case the polymer takes on a strong hydrophobic character and we suggested that the hydrophobic parts (the tails of the surfactants) interact by forming intramolecular associations . This would cause a more coiled conformation of the polymeric backbones, as seen in the FFSEM photographs . Conclusions The HEC polymer investigated has a molecular weight of 500000g mol' . The polymer forms large coils with a diameter of about 100 run . Four modified compounds (F-HMHEC) of the same molecular weight and with 1-8 perfluoroalkyl chains attached to each cellulose backbone were also investigated. Large changes in the behavior of

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the compounds were observed even for this small degree of hydrophobic substitution . HEC and F-HMHEC with one and four perfluoroalkyl chains are completely water soluble at concentrations up to 1%, while the compounds with six and eight perfluoroalkyl chains form two-phase systems at room temperature . At very low polymer concentrations, the solutions show a water-like viscosity . The coils of the polymers can be visualized by FFSEM experiments . These coils are formed by intramolecular associations of the hydrophobic side-chains to minimize their contact with water . The modification results in the aggregation of the coils and in lower cross-over concentrations (c*) from the dilute to the semidilute range as shown by the increase in viscosity . Above this concentration c* of approximately 0 .1% the F-HMHEC forms a fine network structure of connected spherical aggregates . The cross-over concentration of the unmodified HEC sample (where the particles just contact each other) is observed at a concentration of 0 .4% . The viscosity increases even more by the addition of surfactants to the F-HMHEC samples . The viscosity and the storage modulus G' pass with increasing surfactant concentration over a maximum . The peaks of the viscosity and G' rise with the degree of molar substitution of the polymer . The type of the surfactant is only important with respect to the level of both parameters . The systems of F-HMHEC with added perfluoroalkyl surfactants result in stronger gels than the systems containing hydrocarbon surfactants . The maxima of both parameters depend also on the ionic charge of the surfactants . We found that the maxima for cationic surfactants were generally lower than for anionic surfactants added to the F-HMHEC solutions . The four cationically modified compounds (catHEC) used have a smaller molecular weight, which varies between 34000 and 150000 g mol' . The cationic degree of substitution per anhydroglucose unit ranges from 0 .1 to 0 .2 . Solutions in the dilute range show a higher viscosity than

the F-HMHEC compounds at the same weight concentration . We observed a cross-over concentration c* at 0 .2% . The length of the molecules with the lower molecular weight determined from electric birefringence measurements is comparable to their contour length . The cat-HEC samples with the highest molecular weights show a difference between both parameters of less than a factor of 2 . This indicates that the molecules are almost in a stretched conformation . In the case of these rod-like molecules, we could not see a structure in the FFSEM photographs . The phase behavior of the cat-HEC samples is strongly dependent on the amount of added anionic surfactant. We found an increase in the viscosity up to 90% of the charge neutralization . Soon after, the maximum phase separation occurs and the viscosity of the liquid phase decreases abruptly . After the complete recharging of the polymer at a very high surfactant concentration the solution becomes single phase and the viscosity rises again . Both polymers form transient networks above c* . On the addition of surfactants the viscosity and the storage modulus G' pass over a maximum for both polymer types with increasing surfactant concentration . The number of elastically effective junction points increases less than suspected . Only a few per cent of the substituents are used to build up the network structure . We concluded that the strength of the hydrogels is not a matter of the number of network points . The main emphasis is on the degree of hydrophobicity of the connected side-chains/surfactant parts and their stabilization through substituent-surfactant interactions.

Acknowledgment Clarissa Drummer and Werner Reichstein are thanked for skillful technical assistance and help with the FFSEM preparations . The FFSEM was financially supported by the Deutsche Forschungsgemeinschaft (SFB 213) .



U. Kdstner et al./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994 ) 279-297

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4 5 6 7 8 9 10 11 12 13

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