Polymers in Aqueous Media - American Chemical Society

19 The Rheological Properties of a Hydrophobically Modified Cellulose Downloaded by UNIV OF LEEDS on August 7, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch019

1

1

1

2

J . W . Goodwin , R . W . Hughes , C . K . Lam , J. A . Miles , and B . C . H. Warren 2

1

2

Department of Physical Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1 T S , England Chemical Defence Establishment, Porton D o w n , Wiltshire, England

Dilute solution behavior of a water-soluble cellulose derivative that had been modified by grafting approximately three hexadecyl hydrocarbon chains per cellulose molecule onto the backbone was studied by capillary viscometry and refractometry. Both techniques indicated that aggregation occurred at concentrations over 0.075 g dL . At concentrations over 0.1 g dL , phase separation was observed. The number-average weight of the hydrophobically modified cellulose was 1 x 10 daltons. The viscoelastic properties were investigated at concentrations up to 2 g dL . Shear stress-shear rate measurements were performed, and power-law behavior was observed. Forced-oscillation measurements were carried out over a frequency range of 0.001-20 Hz, and shear-wave rigidity measurements were carried out at 200 Hz to give the wave-rigidity modulus. -1

-1

5

-1

THE USE OF HYDROPHOBIC INTERACTIONS

to p r o d u c e associative t h i c k eners has increased m a r k e d l y o v e r the past 10 years i n s u c h diverse areas as surface coatings a n d e n h a n c e d o i l recovery. T h e d e s i r e d t h i c k e n i n g p r o p erties are p r o d u c e d b y r e l a t i v e l y l o w m o l e c u l a r w e i g h t p o l y m e r s that are r e v e r s i b l y c r o s s - l i n k e d b y p e n d a n t h y d r o p h o b i c moieties to give a t h r e e d i m e n s i o n a l n e t w o r k . T o m a i n t a i n solubility, the n u m b e r o f h y d r o p h o b e s p e r soluble m o l e c u l e is l o w , a n d the c h a i n l e n g t h is t y p i c a l o f that u s e d i n surfactants (i.e., C - C ) . I n solution, the h y d r o p h o b e s appear to associate i n an analogous fashion to m i c e l l i z a t i o n , i n that, i n the absence of surfactants, 8

1 6

0065-2393/89/0223-0365$06.00/0 © 1989 American Chemical Society

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

366

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

a critical concentration appears to be required. Added surfactants can aid the association at low concentrations by contributing to the molecular ag gregates, whereas at concentrations in excess of their critical micelle con centration, surfactants may prevent association by forming micelles around the pendant groups.

Downloaded by UNIV OF LEEDS on August 7, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch019

This chapter is concerned with just one of this class of thickeners, namely a hydrophobically modified (hydroxyethyl)cellulose ( H M H E C ) , Hercules W S P D-47. Gelman and Barth (J) reported on the viscosity of such H M H E C s . This sample contained up to four hexadecyl chains grafted to the cellulose backbone. The preparative route was that described by Landoll (2) . The dilute solution properties were characterized by capillary viscometry, whereas the more concentrated solutions were characterized by con tinuous-shear viscometry, forced oscillation measurements, and shear-wave propagation. In addition, the adsorption onto polymer latex particles was investigated.

Experimental Details Materials. The H M H E C was used as supplied. All the water was doubledistilled. The NaCl was B D H (British Drug Houses) Analar grade. The polymer was dissolved in water containing 5 Χ 10" mol L* NaCl; the solutions were stored at 4 °C for at least 48 h before use to ensure dissolution. The polystyrene latex was prepared by the emulsion polymerization of styrene (3) by using Alcopol OS (Allied Colloids pic) as the emulsifier and potassium persulfate as the initiator. After preparation, the latex was extensively dialyzed against distilled water to remove emulsifiers. The particle size was determined from electron mi croscopy and the latex was found to be monodisperse with a number-average diameter of 140 mm and a coefficient of variation of less than 2%. 3

1

Capillary Viscometry. The viscosity of dilute solutions of the polymer was determined with a Canon-Fenske capillary viscometer set in a thermostat bath at 25 ± 0.05 °C. A stock solution of 0.15 g d L was prepared in distilled water, stored at 4 °C for 2 days, and then filtered through a No. 1 sinter glass funnel to remove fibers and detritus from the preparation of the cellulose ether. Nine other solutions were prepared from this stock by dilution with filtered distilled water. All these solutions were then stored at 4 °C for a further 24 h to ensure that all the solutions had the same temperature history, as this factor has been shown to be important in producing reproducible solutions of cellulose ethers (4, 5). The concentration range prepared was 0.015-0.15 g d L , and theflowtimes ranged from 294 to 1609 s. 1

1

Adsorption Isotherm. A stock latex suspension was prepared at a volume frac tion, φ, of 0.005 in 5 x 10 mol L NaCl solution. Four grams of this latex was used in each case. Increasing quantities of a 1% H M H E C stock solution in NaCl were added, and the systems were made up to 11 g with NaCl solution. The sus pensions were mixed by slowly turning them end-over-end for 4 days at 25 °C to allow adsorption equilibrium to be attained. Centrifugation at 15,000 rpm was used -4

l


19.

GOODWIN E T AL.

Λ Hydrophobically Modified Cellulose

367

to separate the particles from the continuous phase, which was then analyzed for polymer in solution. The assay was carried out by using a Hilger-Watts interfer ometer, and by comparing the refractive indexes of the supernatant solutions with a calibration graph covering the range 0-0.25% polymer.

Shear Wave Propagation. A pulse shearometer (Rank Bros.) was used to meas ure the propagation velocity of a shear wave through the weak gels formed by the solutions of H M H E C in dilute NaCl. The polymer concentration range studied was 0.5-2.0%. With this apparatus, the frequency of the shear wave is approximately 1200 rad s , and the strain is < 1 0 . At this strain, mpst systems behave in a linear viscoelastic fashion, and the wave-rigidity modulus, G is _1

-4


G =

P

y

(D

where ν is the measured wave velocity and p is the density of the solution. When the wave attenuation is small (i.e, when the storage modulus is much greater than the loss modulus), s

G -

G(oo)

(2)

G(») is the high-frequency limit of the storage modulus and is a useful probe of the structure in the gel (6).

Rotational Rheometry. Two instruments were used for these measurements. An Instron model 3250 rheometer was used for measuring rotational viscometry and forced oscillation as a function offrequencywith strains in the region of 0.5. A Bohlin VOR rheometer was used for dynamic measurements at smaller strains (i.e., « 0 . 1 ) . The Bohlin VOR was also used for stress relaxation experiments. Experiments were carried out over afrequencyrange from 0.01 to 60 rad s" . Strain sweeps were carried out on both instruments at 6 rad s , and the viscoelastic response was linear over a range of strain of 0.25-0.65 on the Instron instrument, whereas the Bohlin VOR also showed a linear response over the range used (i.e., 0.05-0.20). 1

_1

Results and Discussion Capillary Viscometry.

The viscosity data are plotted in Figure 1 as

the reduced viscosity (η ι) as a function of concentration. The curve shows Γβ(

two linear plots that intersect at a polymer concentration, c, of 0.075 g d L " . 1

If the Huggins equation (5)

TW = [η] +

(3)

is applied to the lowest concentration data, an intrinsic viscosity, [ η ] , of 4.7


368

POLYMERS IN A Q U E O U S

MEDIA

d L g " is obtained with a Huggins coefficient, fc', of 2.25. These values are close to those obtained by Gelman and Barth (I) for a similar H M H E C , with 0.9% by weight of hexadecyl chains, dissolved in water. 1

If the Mark-Houwink equation (5) is used with [η] = 4.7 d L g" , constant = 4.7 χ ί ο , and power law index a = 0.8 (4) in 1

K

1

- 4

[η] = K M x

(4)

a

n

where M is the number-average mol wt, then M = 1 Χ 10 , which is similar to the values quoted by Gelman and Barth (J) for the molecular weight of their H M H E C . However, those authors also pointed out that the high value of the Huggins coefficient indicated that molecular aggregation occurred in water. We agree with this interpretation but wish to make an additional distinction. At low concentrations (i.e., at c < 0.075 g d L ) , a limited aggregation of the polymer molecules leads to effectively higher molecular weight units. At c > 0.075 g d L " , a much more extensive ag gregation leads to a three-dimensional network or gelation of the polymer solution. This behavior is analogous to the micellization of short-chain sur factants and can be referred to as the critical aggregation concentration


n

n

5

1

1

Figure 1. Reduced viscosity as a function of concentration for HMHEC polymer D-47.


19.

GOODWIN ET AL.

A Hydrophobically Modified Cellulose

369

(CAC) for the associative thickener. Although it is tempting to call this the "critical gelation concentration", this name would be misleading as systems a little above this concentration were observed to phase-separate into a lower gel phase and an upper lower viscosity phase if left to stand for a time in excess of 2 weeks. The increased slope of the t)

vs. c plot for c > 0.075

Ted

g d L " is a clear indication of very extensive aggregation. 1

Adsorption Studies.

The calibration curve for the Hilger and Watts

interferometer is shown in Figure 2. Again, two straight-line portions were observed. In this case, the break occurred at the slightly higher concentration


of

0.10 g dL" . 1

Figure 3 shows the adsorption isotherm for the H M H E C D-47 onto a cleaned polystyrene latex with a particle diameter of 140 nm. The curve shows a relatively slow approach to a plateau. At concentrations in excess of 0.08 g d L " , a step occurs in the isotherm. The area occupied per molecule on the plateau before the step was 26.1 n m mol" , whereas after the step, the area was reduced to 17.1 n m m o l , although a true plateau was not reached even at equilibrium concentrations as high as 0.18 g d L " . The data in Figures 2 and 3 confirm the polymer aggregation process at c > 0.075 g 1

2

2

1

1

1

1.20

S

1.10 h

B

LOO

h

i Χ!

13 β

£

0.90 h

0.80 k

0.70h

0.05

0.10

0.15

0.20

0.25

Concentration of D-47 (wt % Figure 2. Interferometer calibration curve with refractive index in arbitrary units.



370

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

0

0.15 0.05 0.10 Equilibrium Concentration of D-47 (wt %)

Figure 3. Adsorption isotherm for HMHEC on cleaned polystyrene latex of particle diameter 140 nm. d L " with steps in the plots at concentrations just above this value. The adsorption density is 4 to 5 times that found for an ethyl H E C ( E H E C ) of similar molecular weight (6). This much higher value of adsorption on the low-concentration side of the step indicates that some molecular aggregation may be occurring in addition to an increase in adsorption due to the pendant alkyl chains. Because the extensive aggregation occurring at c > 0.075 g d L " results in only a 25% increase in adsorption, the preaggregation process probably does not account for more than this. The tentative conclusion is that the pendant alkyl chains cause a marked enhancement of adsorption of the polymer by direct interaction with the surface. 1

1

Wave Propagation Experiments. These experiments were carried out at much higher polymer concentrations, the range covered from 0.5 to 2.0 g d L " . A monotonie increase in the wave-rigidity modulus was observed throughout this range, and the experimental data are shown in Figure 4. The size of the experimental points indicates the uncertainty in each measurement. The values are similar to those observed for E H E C (6). 1

At these polymer concentrations, the solutions form continuous viscoelastic gels with moduli in the region 0.02-0.3 kPa. Such swollen networks can be treated as heavily swollen elastomers (6) by applying the theory of rubber elasticity (5, 7-9) to them. For a continuous network that is subjected to a small strain and at a frequency that is sufficiently high for there to be no stress relaxation, the wave rigidity modulus can be equated to the highfrequency limit of the storage modulus. The rigidity is then proportional to the cross-link density. G (oo) = 2ANkT


(5)

19.

GOODWIN ET AL.

A Hydrophobically Modified Cellulose


Nj=4

371

N-, =3

Concentration of D-47 (wt %) Figure 4. Wave-rigidity modulus as a function of concentration. The points are experimental data, and the solid line is calculated for Ni = 4 with the broken line for Ni = 3. where kT is the thermal energy (the Boltzmann constant and the absolute temperature), Ν is the number of cross-links in unit volume of gel, and the front factor A (Vz

Polymers in Aqueous Media - American Chemical Society

Recommend Documents