Structure and Properties of Hydrophobically Associating Polymers

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Downloaded by UCSF LIB CKM RSCS MGMT on December 3, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0223.ch022

Structure and Properties of Hydrophobically Associating Polymers 1

Jan Bock, D . B . Siano, P. L. Valint, Jr. , and S. J . Pace Exxon Research and Engineering Company, Clinton Township, Route 22 East, Annandale, N J 08801

Hydrophobically associating polymers are synthetically derived water-soluble polymers containing a small number of oil-soluble or hydrophobic groups. In aqueous solution, hydrophobic associations can dominate polymer conformation and, in turn, solution rheological properties. We have studied a series of copolymers of acrylamide and N-substituted alkylacrylamides and terpolymers that contain anionically charged carboxyl groups to define the relationships between polymer structure and solution properties. Intrinsic viscosity and Huggins interaction coefficients provided information on the conformation and intramolecular aggregation behavior of these polymers in dilute solution. Viscoelastic properties above the polymer overlap concentration provided a measure of intermolecular interactions. These rheological properties were studied as functions of polymer composition, molecular weight, and solvent quality to observe the structure-property relationships in hydrophobically associating polymers.

THE RHEOLOGICAL PROPERTIES OF WATERB -ASED FLUIDS

can b e c o n t r o l l e d w i t h h y d r o p h o b i c a l l y associating p o l y m e r s . Hydrophobically associating polymers are synthetically d e r i v e d , water-soluble p o l y m e r s that c o n t a i n a small n u m b e r o f oil-soluble o r h y d r o p h o b i c groups. W h e n these p o l y m e r s are d i s s o l v e d i n aqueous s o l u t i o n , the h y d r o p h o b i c groups aggregate to m i n i m i z e t h e i r exposure to water, i n a fashion analogous to that o f surfactants C u r r e n t address: Bausch & Lomb, 1400 North Goodman Street, Rochester, N Y 14692

0065-2393/89/0223-0411$06.00/0 © 1989 American Chemical Society

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

412

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

above the critical micelle concentration. The influence of this association on the rheological properties of water-based fluids is studied in this chapter. The nature of hydrophobic interactions and their effects on the structure and properties of water have been extensively studied, particularly for small molecules (1-3). In contrast, the introduction of hydrophobic associations into synthetic water-soluble polymers to control solution rheology has re ceived rather limited and recent study (4-7). To better understand the relationships between polymer structure and solution properties, we have


synthesized and characterized a series of copolymers of acrylamide and Nsubstituted alkylacrylamides and terpolymers containing anionically charged carboxyl groups. Solution properties of these systems have been obtained in both the dilute and semidilute concentration regime, to probe the influ ence of intra- and intermolecular interactions. In addition, the influence of the shear field and solvent quality on the associations was studied.

Experimental Details Polymer Synthesis. Copolymers of alkylacrylamide (R) and acrylamide (AM), which we called RAM, were prepared with a micellar polymerization technique (4). A micellar surfactant solution was used to disperse the hydrophobic alkylacrylamide monomer into an aqueous phase that contained acrylamide. The monomers were polymerized with a standard free-radical initiator (e.g., potassium persulfate) or a redox initiator to yield the desired random copolymer. Varied temperature and initiator concentrations were used to provide polymers of different molecular weights. Polymerizations were taken to essentially complete conversion. Compositions, in terms of hydrophobe level reported in this chapter, were based on amounts charged to the reactor. Further details on the synthesis and structure of these RAM polymers

n-fllkyl

η

= 3 - 11

Dialkyl

R

=

i-C H 3

7

-

Bilinear

n-C H 8

1

7

m + η = 5,

fllkylphenyl

6,

9

R

=

Chart I. Hydrophobe monomer structures.


CgH , 5

C H 4

9

22.

B O C K ET AL.

Hydrophobically Associating Polymers: Structure

413


can be found elsewhere (5). A series of different hydrophobic monomers (see Chart I) were synthesized to explore the effect of monomer structure on solution properties. To impart ionic charge onto the polymer molecule, a controlled level of hydrolysis (H) was achieved under base (sodium hydroxide)-catalyzed conditions at elevated temperature (e.g., 50 BC). The amount of base, temperature, and reaction time were used to regulate the level of carboxyl groups in the resulting terpolymers, which we called HRAM.

Characterization. Polymer composition was determined by a variety of classical analytical techniques that included elemental analysis and NMR. The carboxyl content of the polymers was determined by potentiometric titration following conversion to the acid form with an ion-exchange column. Analysis of the sodium content in the polymers gave carboxyl values within a few percent of those found by the titration technique. The number of hydrophobic groups in the polymers in this study was too low to allow quantification by conventional analytical techniques. The levels cited in this chapter refer to amounts added to the reactor and complete incorporation into the polymer was assumed. A recent study (8) using a UV spectroscopic technique on model hydrophobic monomers indicated that this was a fairly good assumption. The molecular weights of many of the polymers used in this study were determined by either a classical laser light-scattering (LLS) technique or by a sedimentation- LLS technique (8, 9) that also provided information about the molecular weight distribution. A multiangle LLS apparatus (Dawn model B , Wyatt Technology) was used to determine the scattered light intensity at 15 angles simultaneously. Molecular weights were determined by performing a Zimm analysis of the data with the software supplied by the manufacturer.

Solution Rheology. Polymers were hydrated in distilled, filtered water and were agitated gently until dissolution was complete. To prepare polymer solutions containing salt, concentrated sodium chloride solutions were added to polymer previously dissolved in distilled water. An alternative procedure was used to evaluate the effect of salinity on solution rheology. Solid sodium chloride was slowly added to various concentrations of polymer in solution. To ensure complete dissolution, the solutions were allowed to equilibrate for approximately 24 h before viscometric measurements were obtained. Turbidity measurements were made with a turbidimeter (Hach) on 1500-ppm solutions in 3% NaCl and 0.3% CaCl brine, which we called 3.3% brine. Low-shear-rate solution viscosity was measured on a Couette-type rheometer (Contraves LS 30) with a No. 1 bob and cup. The viscosity-shear rate profile was determined from 10~ to 10 s" at 25 BC. The system was allowed to reach steady state at each shear rate before the measured viscosity was recorded. Dilute solution viscosities used for determining the intrinsic viscosity of the polymer systems in 2.0 wt % NaCl were obtained in a capillary viscometer (Ubbelohde) by using standard methods (JO, 11). Some of the measurements were obtained from an automatic capillary viscometer (Schott AVS/G). A conventional Huggins relationship (11) in which reduced viscosity is a linear function of polymer concentration was used to fit the data. A regression analysis was used to yield the intrinsic viscosity (the intercept) and the Huggins interaction coefficient, Kh, (the slope divided by the square of the intrinsic viscosity). 2

2

2

1


414

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


It was assumed that the solutions were Newtonian at the shear rates in the capillary. This assumption was assessed with Couette viscometer measurements of these dilute solutions over a range of shear rates and was reasonable. Deviations were found for solutions at the higher concentrations, as indicated by negative de parture from linearity of the reduced-viscosity-concentration plots; these values were not used for intrinsic viscosity and Huggins constant determination. Another issue was the applicability of this approach to characterize the dilutesolution properties of associating polymers. Reduced viscosity is a linear function of the polymer concentration at low concentrations for the systems in this study, so the use of a Huggins-type relationship is justified.

Results and Discussion Dilute Solution Properties. The rheology of dilute polymer solu tions has been used extensively to gain insight into the structure and con formation of polymers in solution (II). The intrinsic viscosity provides a measure of the molecular weight of a polymer through a relationship such as the Mark-Houwink-Sakurada equation. Earlier studies of polyacrylamide (PAM) systems and details of the complexity of the characterization of highmolecular-weight water-soluble systems can be found in references 9, 13, and 14. To explore the influence of hydrophobe structure and content, a ref erence P A M was prepared that had a weight-average molecular weight of 3 Χ 10 g/mol and an intrinsic viscosity of 7.3 d L / g (Table I). These values agreed with the Mark-Houwink parameters found for PAMs synthesized by other techniques (13). With data obtained from a large variety of P A M samples studied in water at 25 ° C , Kulicke et al. (13) proposed the following relationship: 6

[η] = 1.0 Χ Ι Ο " X M W 0.755 4

(1)

where [ η ] is the intrinsic viscosity in d L / g and M W is the viscosity average molecular weight. The molecular weight and intrinsic viscosity for our P A M sample was also consistent with the other Mark-Houwink relationships for P A M in water containing various levels of salt (13). Thus, the presence of salt in the water did not appear to affect the measured intrinsic viscosity or molecular weight (determined by light scattering) of P A M polymers. This observation is rea sonable because, in solution, P A M behaves as an uncharged random-coil polymer.


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Hydrophobically Associating Polymers: Structure

415

Table I. Polymer Systems and Solution Properties


Polymer Hydrophobe Class Type PAM ΗΡΑΜ ΗΡΑΜ ΗΡΑΜ RAM HRAM HRAM HRAM HRAM

MW Hydrophobe Charge Level (mol %) (mol %) (X JO ) a

6

0.4 24 20

n-C

6

n-C n-C n-C n-C

8

8

8

8

1.0 1.0 1.0 1.0 1.25

18 0.3 18 18 18 15

2.9

d

0.49 3.0 5.8 3.0

Structure and Properties of Hydrophobically Associating Polymers

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