Preparation, Characterization, and Rheological Behavior of Water

Chapter 4

Preparation, Characterization, and Rheological Behavior of Water-Swellable Polymer Networks

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch004

H. Nottelmann and W.-M. Kulicke

1

Institut für Technische und Makromolekulare Chemie, Universität Hamburg, Bundesstrasse 45, 2000 Hamburg 13, Germany

The swelling and rheological behavior has been investigated on highly absorbent water-swellable polymers prepared from anionic and cationic monomers with tetra- and hexafunctional cross-linking agents. Because of their technical application, the absorption capa city of gel blocks and their reversibility of swelling in salt solutions with multivalent cations have been studied. A method based on very few experimental data was introduced, which allows to deter mine the parameters necessary for predicting the degree of swelling at any desired salt concentration. In addition, rheological investiga tions on gel blocks and particle solutions provide information about regions where inhomogeneities occur and data for determin ing the absorption capacities and their elasticity values.

Water-swellable polymer networks, referred to generically as hydrogels, represent highly elastic, solvent-containing bodies with very specific properties and may be either synthetic or biopolymeric in nature. As has been stated in diverse publi cations (2—5), there is an increasing number of applications for covalently cross-linked hydrogels in the field of engineering and medicine. Their range of properties spans from a slight degree of swelling, such as that necessary in soft contact lenses, to the cyclically reversible absorption of more than 1000 times the weight of their solid content, which enables them to be used as additives for incontinence aids or as moisture regulators in areas with prolonged dry periods. They exhibit a high retention capacity under load, as well as resistance to biological and chemical attack over various time periods. These properties, which may in part seem contradictory, can be influenced by varying a number of factors, namely, the chemical nature of the polymer Corresponding author

0097-6156/91/0462-0062$07.50/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


4. N O T T E L M A N N & K U L I C K E

Water-Swellable Polymer Networks

63

components, the proportion of ionic and nonionic groups, and the distribution of sequence length and mesh width, that is, network density. The result of this is that a particular hydrogel with specified properties can be allocated to the appropriate field of application. To determine or predict the property profile, it is necessary to have an exact knowledge of the formation and structure of the hydrogels (9-12). The condi tions required for this are laid down at the time of synthesis. As far as the published data on gel systems are concerned, the degree of comparability often has shortcomings. There would thus appear to be a need for the introduction of a reference state for the synthesis. The synthesis of uniform gel blocks was therefore carried out according to a preparation concept to ensure simple, direct, and partially quantitative comparability between differing gel systems. As an extension of previously published results for synthetic, semisynthetic, and biopolymer gels based on polyacrylamide and starch gels (13, 14), synthetic ionic hydrogels based on 2-acrylamido-2-methylpropanesulfonic acid and N,N~ dimethylaminoethylacrylate with triallylamine or bisacrylamide as cross-linking agents were synthesized according to the abovementioned concept. Because the industrial use of hydrogels largely calls for gels in the form of dried particles, or solutions and suspensions of these particles, whose macroscopic material properties depend on their surface configuration and size, some dry gel particles of different particle sizes were synthesized on the basis of 2-acrylamido2-methylpropanesulfonic acid and triallylamine. The investigations deal with technically relevant, phenomenological aspects, such as the stability in salt solutions of different concentrations with charges ranging from 1 to 3, the equilibrium degrees of swelling within these solutions, the reversibility of swelling, and the rheological characterization with respect to the synthesis parameters. The results are to be compared with the property profiles of gels from analogous syntheses, which have been obtained with starch as a basis. Correlat ing the recorded data with molecular parameters should enable qualitative and quantitative relationships to be derived, which can then be compared with net work theories, such as the Flory-Rehner relationship, as modified by the Donnan theory (25). Experimental Details The total mass of components in each preparation batch, including solvent, was set at 10 g. The total substrate concentration of the network components is given in percentage by weight and represents a dimension that is technically very easy to manipulate. Because the mesh width in a network system is dependent upon the ratio of cross-linker concentration to the concentration of monomer or, where a polymer is used, to the number of repeating units, the proportion of cross-linking agent is given in mole percentage. The gels will henceforth be identified by two numbers separated by an oblique stroke (% w/w/mol-%). The first number deals with the total substrate content, and the second with the associated cross-linking agent concentration. All gels were synthesized by a radical-initiated, cross-linking copolymerization in aqueous solution. The redox initiator system ammonium peroxodisulfate/TEMED was used for the radical copolymerization. This has often been used in already published studies (16—21). The initiator content, irrespective of the total monomer content, was the same for each preparation batch: 30 mg of ammonium peroxodisulfate and 50 μ ι (38.8 mg) of TEMED per 10 g. The


64

POLYMERS AS R H E O L O G Y MODIFIERS

following gel systems have been prepared as gel blocks according to the above concept.


Anionic Gels. PAAm-AAc-BisAAm

Poly-(acrylamide-co-acrylate-co-bisaciylamide)

PAAm-AAc-TriAA

Poly-(acrylamide-co-acrylate-co-triallylamine)

PAMPS-BisAAm

Poly-(2-acrylamido-2-methylpropane sulfonic acid-cobisacrylamide)

PAMPS-TriAA

Poly-(2-acrylamido-2-methylpropane sulfonic acid-cotriallylamine)

Production of Dried PAMPS-TriAA Gel Particles. After synthesis, PAMPS-TriAA gel blocks were placed in a plastic bag and pounded to a uniform mass and, after 24 h, dried in an oven at 50 *C. The dried gel particles were pulverized in a mortar and sized through sieves of differing widths. The fractions 100-300 μπι, 300-500 μπι, and 500-720 μπι were used for rheological investigations. Cationk Gels. PDAEA-TriAA

Poly-(^^-dimethylaminoethylacrylate-co-triallylamine)

Swelling Measurements on Gel Blocks. A mould (Figure 1) was fabricated for the swelling measurements that enables the gels to be produced as uniform cylinders of a defined geometry and removed without being destroyed. Equili brium swelling measurements were carried out in NaCl, MgCl , and Al(NO~) solutions with concentrations ranging from 1 χ 10" to 1.0 mol/L and in dis tilled water. In addition, the gel cylinders obtained were weighed and swollen in 250 ml of their respective solution for at least a week. Hie gels that had attained equilibrium were then removed from the solution with as little damage as possible and swabbed dry with absorbent paper before being weighed. 2

3

4

Rheological Investigations on Gel Blocks. Gels may be regarded as entropy-elastic bodies and can be reversibly deformed over a wide range. At sufficiently small deformation, the Hookian Law applies so that the quotient from the resulting stress (σ„) and the imposed deformation (7) is constant. The constant in this so-called linear viscoelastic region is given by the shear modulus (G) (22). The investigations on the gel blocks were carried out by nondestructive mechanical oscillation measurements. The complex shear modulus (G*), calculated from the resulting torque, is composed of the storage or elasticity modulus (G') and the loss modulus (G"), although the latter was negligibly small in every case. As can be seen from Figure 2, gelation takes several hours to reach comple tion (constant value of the storage modulus). In the radical-initiated copolymerization of synthetic hydrogels, gelation takes approximately 2-3 h, whereas the base-induced cross-linking of starch hydrogels takes 10-12 h because of the slower ionic mechanism. After completion,frequency-dependentmeasurements were then carried out in the linear viscoelastic region to determine the storage modulus in the plateau region (G' ). This value can be linked to the number of elastically effective chains (i/ ) via the theory of rubber elasticity (25). e



Water-SweUable Polymer Networks


height : 14.S mm

65

radius : 7.5 mm

perspex PTFE

1

1

Figure 1. Mould for the preparation of cylindrical get blocks.

25000 (17.5/ S) S t - S T M P 20000 -triallylamine)

PAMPS-BisAAm

Poly-(2-acrylamido-2-methylpropane sulfonic acid-cobisacrylamide)

PAMPS-TriAA

Poly-(2-acrylamido-2-methylpropane sulfonic acid-co triallylamine)

PDAEA-TriAA

Poly-(N,N-dimethylaminoethylacrylate-co-triallylamine)

PMAAc-DVB

Poly-(methacrylic acid-co-divinylbenzene)

St-STMP

Starch—sodium trimetaphosphate

St-ECH

Starch—epichlorohydrin

TEMED

Ν,Ν,Ν',Ν'-tetramethylethylenediamine

a., b.

Individual constants

a

H»

a

s

Limit of the gradients in pure water and in concentrated salt solution, respectively



c , c* s

Water-Swellable Polymer Networks

85

Molar salt concentration within a gel and in the

s

surrounding medium, respectively f

Functionality of cross-linking unit

G*, G', G "

Complex shear modulus, storage modulus, and loss


modulus, respectively G'p

Storage modulus in the plateau region

I*

Ionic strength of the counter ions in the external solution Concentration of fixed charges referred to the unswollen network Mean average molecular weight of the monomer or repeating units

(i/v ) u

M

Q

M

c

n

Mean molecular weight per chain of a perfect net work Total number of monomer or repeating units

tQt

q

Degree of swelling according to Flory (~ l / t ^ )

Q

Degree of swelling (V/V ) referred to the prepara tion volume

Vj

Molar volume of solvent

Q

V, V X

Volume of the gel in the equilibrium state of swel ling and the preparation state, respectively

0

Mole fraction of cross-linking agent for a perfect network

c

X*

Minimum critical mole fraction of cross-linking

c

agent for a perfect network z , z_

Valencies of cation and anion, respectively

AF

el

Free energy change for elastic deformation

AF

i o n

Free energy change from osmotic pressure of the

+

ions AF^

Free energy change on mixing

u

Number of chains of a perfect network

Q

i/

e

^(ch)' ^(ph)

Xj

Total number of elastically effective chains in a real network Number of elastically effective chains generated by chemical and physical links, respectively. Volume fraction of polymer in swollen equilibrium with pure solvent Flory—Huggins polymer/solvent interaction parameter divided by kT.



86

POLYMERS AS R H E O L O G Y MODIFIERS

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Water-Swellable Polymer Network

29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

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Preparation, Characterization, and Rheological Behavior of Water

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