polyelectrolyte–surfactant complexes in the solid state: facile building blocks for...

ADVANCED MATERIALS

Polyelectrolyte-Surfactant Complexes in the Solid State: Facile Building Blocks for S elf-0 rganizing Materials ** By Christopher K. Ober* and Gerhard Wegner

Even though mixtures of polyelectrolytes and surfactants are used in a variety of technologies, little is known about the solid- state properties of complexes formed by the two components. Recently reported methods for preparing polyelectrolyte- surfactant complexes and their solid-state structure will be described in the context of the self-assembly behavior of the source surfactant molecules. This facile process offers the opportunity of producing a variety of new materials with applications that may range from switchable, permselective biological membranes to fluorinated materials with non-wetting properties. In the form of a solid-state complex, remarkably diverse mechanical properties ranging from elastomers to crystalline solids can easily be achieved. This review discusses the growing quantity of research in this new field, and in particular, the fab- rication of such complexes in the form of processable self-doped conducting polymers is described.

1. Introduction

Efforts to produce ordered polymer structures are wide- spread and have largely focused on the use of liquid crystalline behavior in main chain or side-group polymers, the de- velopment of complex morphologies in block copolymers via tailored molecular weight and composition ratios, or the use of supramolecular assembly. Ordered structures have a host of possible applications ranging from improved mechanical behavior to unusual optical, electrical, and even biological properties. Despite the interest in develop- ing new methods for the construction of ordered materials, a largely overlooked tool for producing organized solid- state structures has been the relatively facile combination of surfactants with oppositely charged polyelectrolytes.

This combination occurs by happenstance in a variety of technological processes and formulations including the production of photographic film, cosmetics, paper manufac- ture, and food science, among many well-known technologies.''] The resulting complexes, produced during the

[*I Prof. C . K. Ober Department of Materials Science and Engineermg Cornell University Ithaca, N Y 14850 (USA) Prof. G. Wegner Max-Planck-Institut fur Polymerforschung Ackermannweg 10, D-55128 Mainz (Germany)

[**I C.K.O. thanks the A.V. Humboldt Foundation for financial support during his sabbatical leave at the MPI fur Polymerforschung, Mainz, Germany. C.K.O. also thanks Dr. Ralph Colby (Penn State) for his collaboration and many helpful discussions on this topic. C.W. thanks Cornell University and the Chemistry Department for their hospitality during his stay as Baker Lecturer in Ithaca, NY, USA. In particular, discussions with Dr. J. Wang and Dr. A. Kameyama were essential in the preparation of this review.

combined use of polyelectrolytes and surfactants in the same process, may be either soluble or insoluble depending on the precise stoichiometry of the reactants. Their structure depends greatly on the competition between the rate of complex formation and the rate by which a thermodyna- mically equilibrated complex phase is eventually formed.

It is the remarkable range of intricate, molecular scale structures and the ease of complex creation that provides the potential advantages of using such materials. A schematic of a simple neutralization process for complex formation and examples of the vast array of building blocks possible are given in Figure 1. Complexes with natural polymers have been long studied and are usually based on charged poly(peptides), polysaccharides, and DNA. Many of the natural polyelectrolytes are quite stiff macromolecules and can form mesophases leading to highly ordered structures when mixed with surfactants. The interplay of natural polymers with surfactants has been briefly consid- ered in a recent report by Ciferri.['I More extended discussion of complex formation between surfactants and oppositely charged polysaccharides has been provided by Thalberg and Lindman.[31 They point out that the precipitation of surfactants with cellulosics of opposite charge has been exploited for the purification of polysaccharides. En- zymes have in some cases been shown to resist denatura- tion when mixed with surfactants, suggesting their stabiliza- tion by the creation of a particular mi~ro-environment.[~] More frequently surfactants can be used to denature proteins by disrupting or replacing their tertiary structures.r51 These differences are related to the ability of the protein to bind with the surfactant.

While many complexes can persist into the solid-state and strongly influence the final properties of the resulting material, only recently have their solid-state properties be- gun to be examined in some detail. The purpose of this re-

Ad". Maier. 1991, 9, No. 1 0 VCH Verlagsgesellschaft mbH, D-69469 Weinheim, 1997 0935-9648/97/0101-0017 $10.00~.25/0 17

ADVA MATE

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State 4CED ZIALS

Building Blocks of Poly(electro1yte)-Surfactant Complexes

sulf onate -so; 0

.-.---o-b-o phosphonate

--w.w-Q' '0

b diphosphonate

-P032. phosphate

-8-0 carboxylate 0

Poly(anions)

-(A)% Synthetic fc%-cpy -

Counterions (X+)

X Li+, Na+, K+, R4N+, R3S+, etc.

X Ca2+, Mg2+,

X Al3+, Fe3+

Natural

poly(styrene sulfonate) 101

swallow tailed - F H 3

d t C H , quaternary ammonium salt

0 CH3

Poly(cations)

-@+I-, b - c r t ' poly(N-alkyl vinyl pyrldlne)

OH-, etc.

poly(dlallyl dialkyl ammonium)

Hyaluronlc acid anion

Fig. 1. Schematic showing the simple formation of a polyelectrolyte-surfactant complex by combination of a polyelectrolyte and an oppositely charged surfactant. Examples of the building hlocks used in the construction of surfactant-polyelectrolyte complexes. Surfactants are typically mono- or divalent and have one or two long aliphatic tails.

18 0 VCII Verlugsgrsel1,chujt mhH, D-69469 Weinhelm, 1997 0915-964~/97/0101-001~ $10 OO+ 2570 A d v Marer 1997, 9, N o 1

view is to describe the current work in the area of polyelectrolyte-surfactant solid-state complexes and to suggest some additional areas of possible interest. The review is by necessity limited in scope and will only discuss coulombic interactions. No detailed treatment of complexes between polymers and non-ionic surfactants will be given.

Before continuing, it is worth summarizing some work that has been carried out in related areas. A number of research groups have been exploring the same interactions that produce polyelectrolyte-surfactant complexes to man- ufacture ordered thin film structures using layer-by-layer assembly.['] These groups make use of polyelectrolytes of opposite charge to produce layered structures in which different functions may be introduced in each layer. Con- ducting films and electroluminescent devices have been produced in such a fashion and long-term stability is claimed for these structures. Charged nanoparticles such as viruses can also be incorporated into layers using this technique.l7] However, the size scale and order within the layers is quite different from the structures produced by the self- organizing interactions found in the polyelectrolyte-surfactant complexes.

Surfactants are also being used as templates to produce zeolites of very large, uniform pore size distribution.'" In this work, an inorganic polymer (silica produced from tet- raethyl orthosilicate) is built up in the presence of cationic surfactants at a controlled pH. The growing silica chain be- haves as a polyelectrolyte in this system. Charge neutralization occurs and an insoluble complex is produced which has a hexagonal structure and which can be fired to produce a zeolite. The pore size and hexagonal structure of the zeolite corresponds strongly to the dimensions and arrangement of the insoluble ordered complex. In more recent work, precise concentrations were found to be unne- cessary, since at the correct pH the complex self-organizes into the appropriate ~tructure.'~'

One method of incorporating and/or stabilizing the organization of surfactants in a polymeric form is to create and polymerize surfactants with reactive functions["] In this manner, a phase produced by a surfactant-water mixture may be trapped in the solid-state by use of a polymer network. Alternatively, it is possible to create a variety of phase structures by using polymeric surfactants (e.g. block copolymers) and subsequently crosslinking the surfactants via reactive groups.["] This solid-state organization may also be produced by a superficially very simple polymer- based approach, one that involves a direct combination of polyelectrolytes and surfactants. Upon careful study, the resulting complexes are very intricate systems and it is this latter approach for the formation of solid-state structures that is the subject of this review. The production of such complexes has the advantage that they can be easily formed from readily available materials to produce polymer-based materials with a wide variety of structures and properties ranging from elastomers to thermoplastics and thermosets.

ADVANCED MATERIALS

C. K. Ober, G. WegneriPolyelectrolyte-Surfactant Complexes in the Solid State

2. Ordered Structures of Simple Surfactants and Their Mixtures

It is important to briefly describe the nature of surfactants in order to appreciate their role as structure-forming elements in solids. Generally, ionic surfactants are constructed of a short hydrocarbon chain, 8 to 18 carbon atoms in length, with a charged head group. This structure has a practical effect on complex formation in determining factors such as the volume ratio of the components; the choice also depends on the commercial availability of some materials.

Some typical structures of the building blocks of the complexes (the surfactants and polyelectrolytes) are listed in Figure 1. For example, anionic surfactants are typically composed of salts of aliphatic sulfates, sulfonates, carboxylates, and phosphates. Implicit in our figures of surfactants and polyelectrolytes are counterions which are shown later (with selected polycations and polyanions). The solubility and surfactant behavior of the carboxylates and phosphates are in particular dependent on pH. A number of naturally occurring surfactants are also well known and include the phosphoglycerides. This latter category has two aliphatic tails that strongly effect the molecular geometry of the micellar and mesomorphic structures formed by the surfactant. Cationic surfactants are generally produced from alkyl amines and are used in primary, secondary, tertiary, and quaternary ion form. The pH-insensitive quaternary ammonium salts are the most common examples and are listed along with some other varieties. They are generally used as linear aliphatic materials, but surfactants based on pyridinium and piperidinium groups are also of importance in the pharmaceutical industry. Other examples of anionic and cationic surfactants may be found in a variety of reviews on the topic of surface active

Surface active agents exhibit a number of ordered phases which are the basis of the structure-forming abilities of the polyelectrolyte-surfactant complexes. As single component systems, that is neat surfactants, compounds such as fatty acid alkali soaps may have several different phases with increasing temperature. They are essentially all lamellar and, as described in the literature,['] the lateral extent of each layer varies so that they may exist as sheets, ribbons, or disks. The ribbon structure is reportedly the most common form observed. By altering the single-tailed surfactant to a swallow-tailed structure such as di-2-ethylhexylsulfo- succinate (Aerosol OT), a stable liquid crystalline hexagonal phase is formed even at 20 "C as a result of the geo- metric and volume filling constraints imposed by the 2 alkyl segments attached to the same head groups.

When a second component (e.g. water) is added to the surfactant, the volume ratio of the ionic and hydrocarbon components is changed and more complex liquid crystalline behavior is observed. Among the most widely used examples of anionic and cationic surfactants are sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium

Adv. Mater. 1991, 9, No. I 0 VCH Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-9648/97/0101-0019 $10.00+.25/0 19

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State ADVANCED- MATERIALS

20

bromide (CTAB), respectively. To illustrate this sort of behavior, binary mixtures of either CTAB or SDS with water and ternary mixtures with other compounds are described briefly. Phase diagrams of the surfactants CTAB[13’ and SDS1’41 are shown in Figures 2 and 3, respectively. These

micellar + Cr

I I I I

-

I I I I

0 25 50 75 101 CTAB ( W t - Y o k

Fie. 2. Phase diagram of cetvl trimethyl ammonium bromide (CT B). This

for a wide range of surfactant types. These phases are the most common and follow a general progression of phases with decreasing water content from micellar (I) ++ hexagonal ( H a ) ++ cubic (Q,) +* lamellar (La). The hexagonal phase is identified by the appearance of the 1, J3, J4, 47, and 4 9 ratios of the diffraction ring radii, but may prove difficult to identify optically. The binary phase diagram for CTAB (Fig. 2) shows two lamellar phases in which a change in spacing is attributed to the onset of interdigita- tion of the aliphatic tails in the La (11) phase whereas the L, (1) phase has only end-to-end contact of the tails. While nearly identical in terms of organization, this small effect can create a major difference in rheological properties. Schematic diagrams showing the structures of several of these mesophases are given in Figure 4.

Changes in mesophase structure with both concentration and temperature are a function of the differences caused in

a) La /----?

c) Pn 3m (Q224)

phasc diagram covers the entire composition range and shows the more common mesophascs posiblc. Based on data from [13].

e) Ia 3d (Q230)

Fig. 3. Phase diagram of sodium dodecyl sulfate (SDS). This diagram covers the rangc of 50% or more surfactant showing the variety of phases possible. Based on data from [14].

phase diagrams illustrate the complexity and richness of the self-organized structures that can be created using surfactants.

Phase diagrams are constructed using a combination of techniques, but mainly optical microscopy and X-ray diffraction.[”’ Phases with different types of organization including lamellar, hexagonal, and cubic phases are known

d ) Im 3m(Q229)

Fig. 4. Organization of various mesophases possible in mixtures constructed from water and surfactants. These same phase structures should be observa- ble in polyelectrolyte-surfactant complexes. This figure is adapted from in- formation provided in the review by J. M. Seddon. Biochimica et Biophysica Acfa 1990, 1031, 1.

20 0 V C H Verlagsgesellschaft mbtf, 0-69469 Wecnherm, 1997 09.~5-9648/97/0101-~~20 $10 00+ 25/0 Adv Muter 1997, 9, No. 1

ADVANCED MATERIALS

C. K. Oher, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State

cross-sectional areas of the paraffinic chains and the hy- drated polar head groups, and conformation of the alkyl tails and volume fractions of the two components in the molecule. These differences are strongly affected by the nature of the other components in the surfactant mixtures and are the basis for the very complex self-organized structures that can be produced by polyelectrolyte-surfactant mixtures. Similar spatial arrangements of two components are of course observed in block copolymers, a major difference being that in the surfactant systems, the polar and non-polar components form a single phase in terms of the thermodynamics, despite the current tendency to call such mixtures microphase separated. That means that the re- spective phases differing in composition exhibit a global symmetry, but segments of different polarity are segregated on the length scale of the chemical entities.

Similar phase behavior is seen in SDS-water mixtures as shown in the SDS-water binary phase diagram shown in Figure 3.[14] This second phase diagram focuses on a smaller composition and temperature range, and a greater variety of mesophases are identified. Again, the nature of the phases is largely a function of water content leading to nearly vertical phase boundaries. Changing temperature has only a small affect on the observed phase. In addition to the phases found in CTAB-water mixtures, there are the monoclinic (Ma), rhombohedra1 (Ra), and tetragonal (T,) phases, as well as a number of biphasic regions.

Nearly identical observations have been made of binary systems of either fluorinated or swallow-tailed surfactants with water, provided account is taken of the altered geometrical factors caused by the enlarged aliphatic components.["] The geometrical factors, as well as solubility lim- itations, mean that these latter two types of surfactants usually have shorter non-polar tails than the corresponding hydrocarbon surfactants.

Ternary mixtures of surfactant, water, and a third component depend very much on the nature of the latter ingredi- ent. The third component may be hydrophobic, amphiphi- lic, or ionic and therefore contribute to the mesophase in a variety of ways. It is possible to produce inverted ordered structures in these ternary systems in which the hydrocarbon phase is the continuous structure. An example of the phases present in such mixtures is shown in the phase diagram (Fig. 5) for the CTAB-water-l-hexanol mixture at 25 OC.['*] In this simple system, only normal phases are observed. On decreasing the water content or increasing the content of the organic component, the phases change from micellar (I) ++ hexagonal (H) tf lamellar (L) ++ pure phase along with several biphasic regions. The progression from spheres to cylinders to layered structures is often observed with increased surfactant content. It may also be noted that in this system, the number of phases present is less than in the binary mixture described above.

This example is merely meant to be illustrative, for just as it is almost impossible to predict all the phases produced by a ternary mixture with a surfactant, at this point far too little

11 +

Hexanol

Fig. 5. Ternary phase diagram of CTAB-water-1-hexanol mixture at 25 "C. adapted from 1121.

is known about polyelectrolyte-surfactant complexes to be able to predict the expected phase structures. Threading a polyelectrolyte chain through these mesophases will ob- viously create different geometrical constraints from a small molecule, but it presently appears that observed phases are essentially confined to those known for surfactants.

3. Polyelectrolyte-Surfactant Complexes

Returning to our discussion of solvent-surfactant mixtures, if the third component is a polymer instead. many of the same sorts of phase behavior can be observed. How- ever, if the polymer is a polyelectrolyte, then the additional effect of charge neutralization can occur when oppositely charged surfactants are used. Such interactions are quite strong and micelle-like clusters will form at concentrations much lower than the critical micelle concentration (cmc) of the surfactant.'15] The concentration where binding of the surfactant with the polyelectrolyte begins is called the critical aggregation concentration (cac) and is several orders of magnitude lower than the cmc. Both electrostatic interactions between charged components as well as hydrophobic interactions between the polymer backbone and the surfactant's alkyl tail are important in stabilizing the complex.[l6I An additional feature of these mixtures is that phase separation can take place and that these two phases consist of a concentrated phase containing polyelectrolyte and surfactant in equilibrium with a dilute solution of the excess component.['71 It is this process that can be used for the direct synthesis of polyelectrolyte-surfactant complexes.

Models for this aggregation behavior can be generally described by the schematic shown in Figure 6. There are strong similarities in the interaction of charged surfactants with oppositely charged polyelectrolytes and the binding of surfactants to oppositely charged metal oxides."'] At low

Adv. Muter. 1997,9, No. 1 0 VCH Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-9648/97/0101-0021$10.00+.25/0 21

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State ADVANCED- MATERIALS

---_ .- 6

PRECIPITATION WITHOUT~OLYMER , ZONE ,

CONCENTRATIONSURFACTANT-

Fig. 6. Model for possible structures produced by the interaction of polyelectrolytes and surfactants at varying relative concentrations starting from soluble components. (Rased on model proposed by E. D. Goddard, [15].)

surfactant concentrations, some charge neutralization occurs and complex formation takes place at the cac. At roughly stoichiometric concentrations, where the surfactant concentration equals the concentration of charged sites on the polyelectrolyte, a precipitate is formed, since no charge-induced solubility is present. It is the formation and solid-state structure of the insoluble precipitate that had rarely been explored until recently.

With excess surfactant, resolubilization can occur and a resulting "string-of-beads'' or "string-of-pearls'' structure is formed. The beads are micelles that are associated with the polymer chain either as bound or adsorbed species. Re- cently, researchers have also explored the interaction of surfactants with ionomers in solution (N.B. ionomers are ion-containing copolymers with less than 10" mol-% ionic repeat).[Iy1 A cac is also observed in these materials and the string of beads model has also been proposed to explain it. The interesting aspects of these materials are the hydrophobic nature of the complex and the important effect of kinetics on complex formation. Non-stoichiometric phases appear in processes used in a variety of industries and products (e.g. the photographic industry, paper products, cosmetics, printing inks, food industry). While they are important technologically, they have been discussed else- where."]

4. General Principles for Creation of Complexes

As described above, the addition of a surfactant to a polyelectrolyte is in principle quite simple, but there exist several routes to complex formation. Most researchers add aqueous solutions of polyelectrolyte to surfactant to form the complex using metathesis (or ion exchange). For example, if the salt of poly(acry1ic acid) and a quaternary ammonium salt are mixed, a gel will form when the polyelectrolyte and surfactant are added in stoichiometric quantities. The gel is then extracted until no more soluble surfactant can be removed. The resulting purified gel is dried to form a solid with properties varying from elastomeric to brittle

and from crystalline to non-crystalline depending on the components of the complex.

This simple process is subject to many variations depending on the nature of the polyelectrolyte, the surfactant and the counterions. Examples of synthetic complexes are dominated by poly(anions) based on poly(acry1ic acid) de- rivatives and salts of poly(styrene sulfonic acid), several examples of which are listed in Figure 1 along with possible counterions The use of multivalent counterions compli- cates the complex-forming process and will be described later. Poly(cations) have also been used as the polyelectrolyte in complex formation and are also listed in Figure 1 along with possible counterions. There are generally more examples of complexes based on poly(cations) described in the literature. Of particular interest are the conjugated polycations which can be simultaneously doped during complex formation. Such complexes have been used to produce processable conducting polymers and blends, as described below.

In addition to metathesis, several other processes are conceivable. A selection of the most important of these is given in Figure 7 and includes neutralization, chemical change, redox chemistry, and quaternization reactions. In this figure, the poly(anion) or its source polymer is represented by -(AQ)- while the poly(cation) is given by -(BO)-. Counterions are shown as X9 and Yo and the surfactant is represented by RF*, where F is some function that deter- mines the positive or negative charge on the surfactant fragment. The use of multivalent counterions enables the formation of polyelectrolyte-surfactant complexes with both polyelectrolyte and surfactants of like charge.

Several of these processes are not necessarily solution reactions, and may be carried out in the polymer melt. This can be an advantage when processing new materials. Sev- eral factors in complex formation should be commented on at this time. In the studies of polyelectrolyte-surfactant complexes of conducting polymers described below it has been observed that conductivity suddenly jumps when the mixture is heated above a certain temperature. After cooling, the enhanced conductivity remains as an indication of an organizational change in the complex. This effect is associated with a kinetically-controlled change in complex structure and a competition between kinetic and equilibrium effects is therefore present in these and other types of complexes. It is therefore important that care be taken in the analysis and assessment of complex structures in polyelectrolyte-surfactant mixtures as they may not be in equilibrium. Despite the many possible synthetic strategies, the majority of complexes are produced using the ion exchange or metathesis reaction as seen below.

5. Studies of Solid-State Complexes

The following section summarizes the recent work on the formation and structure of polyelectrolyte-surfactant com-

22 8 VCH Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-V648/Y7/0101-0022 $ 10.00+.2.5/0 Adv. Mater. 1997, 9, No. I

ADVANCED MATERIALS

C. K. Ober, C. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State

Metathesis

Neutralization I .

Chemical Change

tsTn + n F - Ti;$, e'g' T+ .;)- -T

Me

MA S03Me SO; MeN+-

Redox e.g. poty(pyrroie), poly(ani1ine)

e.g. mn + FeC13 +n RF- +n Na'

- wn + FeCI2 +n CI-

RF.

Quaternization +B'+n + Alkylating Agent

(Acid Doping) + f N O ! + q

R F-

Miscellaneous (charge reversal via poly(electro1yte)

e.g. M*+ = Zn, RF = AOT

Fig. 7. General techniques for and some examples of the production of polyelectrolyte-surfactant complexes, including metathesis, neutralization, chemical change, redox reactions, and quaternization.

plexes in the solid-state. In perhaps the first reported study, Harada and Nozakura[2"1 described transmission electron microscopy (TEM) studies of solid polyelectrolyte-surfactant complexes made from poly(viny1 sulfate), PVS, with CTAB. By mixing dilute aqueous solutions of the potas- sium salt of PVS with CTAB, precipitates were formed

which showed maximum turbidity when a 1:l mixture was prepared. The precipitates possessed complex lamellar structures which were measured by X-ray diffraction to be of thickness 50-60 A, the length of two surfactant molecules. Similar layered structures were reported for the complex made from ionene-3,4 and SDS.

It is known in the biological community that if an amphiphile with two alkyl chains is dissolved in water and then dried, bilayer structures are often formed in the solid-state. This is a result of the amphiphile shape contour, alkyl chain length, and the size of the charged group. In order to stabilize such structures, Okahata et a1.1211 carried out research, reported in a series of papers, in which polystyrene sulfonate was neutralized with a viologen to produce a bilayer immobilized film containing redox sites. These complexes in film form were shown to undergo electrochemical changes in their transport behavior that depended on the mesophase present in the films, as shown in Figure 8. In general, such complexes may make excellent barrier or permselective materials.

below red 20°C Jc'

Tc 1 t24'C

red - - 25- 35OC ox

red above A 7

40°C ox

Oxidized Reduced

Fig. 8. Schematic of permeation mechanism which depends on the temperature and electrochemical redox reactions of a poly(styrene sulfonate)-viologen complex thin film. Below 25 "C, the film is crystalline and above 40°C it is isotropic. Only in the mesophase is selectivity possible [21].

Building on this concept, Taguchi and co-workersPZ1 formed bilayer structures from cationic ammonium surfactants (e.g. dimethyldioctadecyl-ammonium bromide) and poly(styrene sulfonate). Again, dilute aqueous solutions of the surfactant and polyelectrolyte were mixed to produce the complex. The complexes were purified by repeated precipitation from chloroform into ethanol. Stable films were prepared which were analyzed using dynamic mechanical analysis with 14,16, and 18 carbon long alkyl chain surfactants. Glass transition temperatures were different

A& Mater. 1997, 9, No. 1 0 V C H Verlagsgesellschaft mbH, 0-69469 Wemheim, I997 0935-9648/97/0101-0023 $10.00+.25/0 23

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State ADVANCED MATERIALS

for each complex and followed the expected trends corresponding to values observed by differential scanning calori- metry (DSC). Electron spin resonance (ESR) studies using 12-doxy1 stearic acid displayed line shapes indicating the fast motion limit above 50°C and the slow motion limit below 0 “C, corresponding to the transition temperatures observed by thermal and mechanical measurements.

Also reported were companion studies in which cholesterol (CH) was added to the surfactant-polymer com- p l e ~ . [ ~ ” ’ ~ ~ Both the previous complexes, as well as those made from sodium poly[2-(acrylamido)-2-methyl-l- propane-sulfonate] with dimethyldihexadecyl ammonium bromide, were studied. These complexes were prepared in thin films by casting them onto silanized glass surfaces. Cholesterol (CH) was added to these complexes to determine if the effects seen on natural bilayers would also be observed in these artificial analogs. Addition of cholesterol altered the complex’s structure to produce a non-bilayer film. The presence of CH was seen to broaden and weaken the X-ray peak of the bilayer form and with 40 YO CH to al- together eliminate the layered structure. A variety of mechanical and solvent treatments were used to create films in which the surfactant chains could be oriented either parallel or perpendicular to the drawing direction. The ability to control orientation in such thin films is a potentially un- ique feature of these materials that has not been exploited.

Layered structures appear to be the predominant arrangement so far reported for polyelectrolyte-surfactant complexes, the equivalent of the smectic phase in many side group LC polymers. A major reason for this may be the use of aliphatic groups in the surfactant and the relative volume fractions of the two components. A schematic of the lamellar structure is given in Figure 9. As in most cases, the side groups appear to be placed head-to-head rather than in an interdigitated fashion. This observation may be due to the fact that while many surfactants exist, only a few common examples have been investigated. Only recently have Anto- nietti and coworkers12‘l started to uncover the more intricate phases possible by examining a broad range of complexes.

Several groups have examined the use of poly(e1ectro- lytes) with surlactants to stabilize surfactant films to act as model cell membranes. For example, when ionene-6,6 was complexed with dipalmitoylphosphatidyl glycerol it was found that while the bilayer was stabilized, the melting temperature increased slightly and the layer thickness was reduced compared to the surfactant itself.[*’] By combining lecithin with poly(diallyldimethylammonium chloride), model cell membranes were produced which mimic a number of cell membrane physical properties[261 In contrast to most other complexes, this system shows a thermal transition that is associated with surfactant melting. An undulat- ing bilayer structure was proposed to describe the rather complex small angle X-ray scattering (SAXS) results observed for films of these materials. The excellent mechanical properties of these films may provide for a number of possible uses, including the target model lipid bilayers.

Fig. 9. Bilayer structure typically observed in polyelectrolyte~surfactant complexes. Both single-tailed and double-tailed surfactants tend to adopt this arrangement.

Antonietti and Conrad[271 have also prepared complexes from poly(acry1ic acid) (PAA) neutralized with sodium hydroxide, which was subsequently reacted with dodecyl trimethyl ammonium chloride (CTAC1). The complex was purified by dissolving in 2-butanol and adding water until a gel phase was formed. The water phase was removed and this procedure was repeated until no chloride was detected with AgNO?. The product gel was dried in vacuum to produce a highly elastomeric and deformable material which using optical microscopy showed large, highly birefringent domains. Both SAXS and wide angle X-ray diffraction (WAXS) was carried out. WAXS revealed an amorphous material with reportedly no order on the side group length scale. However, SAXS studies uncovered a high degree of organization in the complex at lamellar dimensions.

The SAXS data showed signs of both FCC and cylindrical structures; a possible explanation for this is the presence of a periodically oscillating cylinder where the thick portions of the cylinder are arranged in an FCC lattice. The relative phase composition of the ionic versus alkyl phase matrix is in the range of 0.3 to 0.7, where one might expect a cylindrical morphology as found in block copolymers. Such periodic oscillations are known in nature, in surfactant films, and in ordered block copolymer structures.[**] The rheological behavior of these complexes has also been examined by our research group (see Fig. 10) and the PAA-CTAC1 complex exhibits the sort of thermoplastic behavior expected of a side group smectic LC polymer.[291 While there is a steady decrease of G’ and G” with increasing temperature, there is no evidence of a mesomorphic transition over the temperature range examined which is

24 0 VCH Verlngrgaellschaff mhH, 0-69469 Weinhelm, 1997 0935-9648/97/0101-0024 3 10 00+ 2510 Adv Muter 1997, 9, No I

ADVANCED MATERIALS

C. K. Ober, G. WegneriPolyelectrolyte-Surfactant Complexes in the Solid State

Temperature ("C)

Fig. 10. Plot of G' and G" versus temperature for a I:1 complex made from poly(acry1ic acid) and cetyl trimethyl ammonium chloride.

consistent with X-ray and thermal data. This sample con- tained an unknown amount of water, since drying occurred during the course of measurement. Completely dry samples have been observed to be intractable.

Also in our group, poly(methacry1ic acid) (PMA) segments prepared by group transfer polymerization of t-butyl methacrylate followed by hydrolysis were also observed to form a lamellar structure when complexed with the same CTACl surfactant. Tacticity therefore has little effect on structure, since both atactic and syndiotactic materials produced the same complex. The X-ray pattern of the room temperature complex is identical to that shown in Figure 11

17.66 8.838 5.901 4.436 3.559 2.976

2800

5 I0 1; 2'0 2'5 30

Fig. 11. Plot of intensity versus 28 for a 1:l complex made from poly- (methacrylic acid) and cetyl trimethyl ammonium chloride.

and is representative of those reported in the literature for many layered polyelectrolyte-surfactant complexes. This complex also shows the typical head-to-head arrangement seen for most complexes reported above. Another interesting observation is the large water uptake observed in these materials, as much as 20 % by elemental analysis. The water has little effect on the observation of a layered structure, but has a very large effect on the Tg of the complex. No sys- tematic studies have yet been reported on the role that water plays on structure formation, stability of the complexes and the solid-state properties in general.

As an example of the versatility of this technique, the Antonietti research group has prepared complexes with fluorinated surf act ant^.[^^] Using PMA and PAA as back- bones, several commercial fluorinated surfactants, as well as C6F13C000 Na@ were examined. Again, layered structures are produced which were found to have the rather large correlation length of -550 A. Surface energies below that of PTFE but above that expected for a true -CF3 surface have been measured. While the use and stability of these films in aqueous environments as non-wetting coatings was never discussed, such materials are interesting from the perspective of self-lubricating coatings. In view of the high solubility of oxygen in fluorocarbons, gas permeation applications may also be of interest.

In other work, Antonietti et al.'311 described complexes made from poly(styrene sodium sulfonate) (PSS) with greater than 90 % sulfonation of a monodisperse polystyrene. The surfactant was again CTACl as well as its higher homologues. It was impossible to accurately characterize the molecular weight of the complex and it was concluded that some dissociation of the complex must be occurring in solution. The resulting complexes were, in the solid-state, flexible, transparent films with no transition detectable by DSC. The films were mechanically stable up to the decom- position temperature of around 200 "C.

The complexes were not crystalline and as before there was no side group crystallization. Peaks observed at small angles were like those of a smectic phase with a spacing of around 30 A, which is close to the dimensions of closely- packed alkane layers. Layered structures were observed, but they were not isomorphous. Analogous behavior has been observed in both block copolymer and surfactant phases in which increase in either length or volume fraction leads to interfacial curvature, destabilizing layered morphology and producing new phase structures. As expected, however, an increase in the surfactant carbon number did cause an increase in the long period observed in the layers of these complexes. This result is shown in Figure 12. For a C14 surfactant, assuming standard densities, the ionic layer was calculated to be 14 A, whereas the alkane layer was es- timated to be 18 A. Finally, difunctional surfactants were

0 I - I

- j A 2 10 12 14 16 18 20

Number of corbons

Fig. 12. Increase in long period of the mesomorphic layered structures as a function of carbon number in the surfactant in complexes made from poly- (styrene sulfonate) and trimethyl alkyl ammonium halides. Schematic shows the volume ratio of the layers for the C14 surfactant [31].

Adv. Mater. 1991, 9, No. 1 0 VCH Verlagsge.sellschu,ft rnbli, D-69469 Weinheim, 1997 093S-9648/97/0101-0025 $10.00+.2S/0 25

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State L D V A N C E D MATERIALS

investigated in these systems as a type of crosslinking group. Small effects were observed, but the general picture remained the same. The reported observation of interfacial curvature highlights the similarity of the self-organized structures which can form in surfactants, block copolymers, and polyelectrolyte-surfactant complexes.

We have also prepared PSS-CATCI complexes for rheological studies and they were observed to form lamellar phases of the same dimensions reported by Antonietti and coworkers.[”1 We also observed that while they can be readily processed from solution, their melt behavior is quite different than the PMA-CTACI complexes. Instead, when dried they are extremely intractable and decompose before undergoing melt flow even with no crosslinking units.‘”’ Thus, at present, the long-term thermal and, in particular, environmental stability of polyelectrolyte- surfactant complexes is only poorly understood and needs much more study.

Antonietti has recently coupled w-telechelic block copolymers to a polyele~trolyte.[~~~ The poly(styrene-b-ethylene oxide) with RO-S03NH4 end group was complexed with poly(diallyldimethy1ammonium chloride). A multilayered structure with styrene, ethylene oxide and complex layers was formed. The complex itself exhibited highly viscoelas- tic, rubber-like behavior. As noted by the authors, the block copolymer was easily removed from the complex, however.

Complexes can also be formed from surfactants and ionomers, that is, ion-containing polymers with relatively low charge density. Complexes of such polymers, while self-assembling into a two phase structure, are nevertheless not as well organized as polyelectrolyte-surfactant complexes. Study of the behavior of ionomer-surfactant complexes can be instructive concerning the mobility and phase separation of the components in the polyelectrolyte-surfactant mixtures.

E i ~ e n b e r g ~ ~ ~ I has studied the ionic cluster phase plasticization of ionomers using surfactant molecules. Using ionomers with less that 15 mol-% ionic groups, properties of poly(styrene-co-styrene sodium sulfonate) were changed by adding sodium dodecylbenzene sulfonate (SDBS). The concentration of sodium sulfonate groups in the polymer ranged from 5 to 10 mol-%. It is well-known that groups attached to ionic species have reduced mobility and that the mobility is localized roughly in regions of the size of the persistence length of the polymer (-10 A for PS). This leads to two glass transition temperatures (Tg), one for the ionic region (low mobility and higher Tg) and one for the bulk of the polymer.

The surfactant, SDBS, was added to the ionomer in quantities of 10, SO, and 100 mol-% per sulfonate unit. The mass of the surfactant is large and for example, with 10 % sulfonated PS and 100 mol-% SDBS, we have 33 wt.-% surfactant in the blend. It was expected that the ionic groups would still aggregate with the surfactant and since the tail of the SDBS was only 14 A, the surfactant could not

extFnd far from the restricted mobility ionic region. It was observed that the ionic cluster could be preferentially plasticized and that the Tg of the cluster decreased as the SDBS content increased, indicating that the surfactant was indeed localized. This cluster model is reminiscent of the string-of-beads picture of polyelectrolyte-surfactant complexes described by Goddard and others.”] An important similarity with ionomers is that the beads form and saturate an ionic site with many surfactant molecules. This leads to solubility in non-polar solvents since the ionic group is now masked. However, there are fewer sites along the ionomer backbone than along the polyelectrolyte chain. This model is shown below in Figure 13.

...

( b ) Fig. 13. “String-of-pearls” structure of ionomer-surfactant complexes. This structure appears to be present in both solid-state and solution, since the number of sites for surfactant complexation is small compared to a polyelectrolyte (351.

In another variation of the stoichiometric polyelectrolyte-surfactant complexes described in this review, Bakeev and coworkers have prepared a series of ionomers based on non-stoichiometric complexes.~3s1 These were created by interaction of poly(N-ethyl-4-vinyl pyridinium bromide) and SDS in non-polar organic solvents. The complex was isolated by vacuum evaporation of the solvent. Study of stoichiometric and non-stoichiometric complexes by ultra- centrifugation showed that the complexes behaved as single species.

These same researchers have also studied ionomer-surfactant complexes formed from divalent cations.[761 This combination causes charge reversal at the complex binding site and permits the charge neutralization of ionomers and surfactants of similar type, for example, only anionic materials. Sulfonated polystyrene ionomers were combined in organic solvents with aerosol OT which had been prepared in inverse micelle form. The authors surmise that micelles of aerosol OT are distributed along the ionomer backbone, similar to the string-of-beads model of the water soluble polyelectrolyte-surfactant complexes and similar to the complex for ionomer plasticization. While only the solution properties were discussed, the work is relevant as another technique for the preparation of polyelectrolyte-surfactant complexes from components of similar charge.

26 0 VCH Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-9648/97/0101-0026 3 10.00+.25/0 Adv. Mater. 1991, 9, No. I

ADVANCED MATERIALS

C. K. Ober, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State

Poly(electro1yte)

Fig. 14. Complexes formed from a polymer and ionic or hydrogen-bonding mesogenic groups: a) Mesomorphic complexes created by the addition of mesogenic groups to polyelectrolytes: b) Possible orientation of mesogens in contact with surfaces coated with polyelectrolyte-mesogenic group complex; c) Illustration of the creation of mesogenic groups through hydrogen bonding.

H-Bond Acceptor

H-Bond Donor

Formation of mesogenic group by complexation

A series of thermotropic complexes were prepared by Ujiie and Iimura from PVS and several mesogenic azobenzene-containing ammonium compounds.[371 The purpose of this study was to investigate the effect of ionic groups on LC behavior, and the ordering observed in such materials. The resulting stoichiometric complexes, prepared by metathesis techniques, were glassy with a smectic mesophase which seemed to be stabilized by the complex. The glass transition temperature was evidently established by the polyanion structure since no Tg was observed in the azobenzene ammonium iodide. Perhaps the most interesting feature of this complex was its strong interaction with surfaces, which manifested itself in the formation of homeotropic textures without the need for surface treatment of the glass. The observation that surface interactions can establish the orientation of a complex, shown schematically in Figure 14, has not been tested systematically in other systems and may provide a very interesting application for these materials in general. In this figure, the general structures from Ujiie and I i m ~ r a , [ ~ ~ ] as well as those of Kato and Bazuin, described from the literature given below, are shown.

While the bulk of the review has concentrated on complexes formed from charge neutralization, complexes may also be produced via hydrogen bonding. These polymer- based complexes will also be briefly described here. In part of a growing family of H-bonded LC materials, Kato and

associates[381 have prepared thermotropic polymers which are composed of complexes formed between carboxylic acids and amine bases. These lock and key complexes are not based on charge neutralization, rather hydrogen bonding, but are similar in concept and will be mentioned here. Complexes were prepared from solution by adding poly(4- (hydroxyhexyloxy benzoic acid) methacrylate) (P60BA) to a mesogenic alkoxy stilbazole (nOSz) and dried. Spec- troscopically, only one type of H-bond was observed due to the fact that the benzoic acid group gives a strong H bond where the proton remains associated with the acid. The presence of H bonding leads not only to uniform LC phases, but to good miscibility between the polymeric and LC components, a feature shared by the surfactant-polyelectrolyte complexes. A large number of ingenious structures have been developed by these researchers, including new mesomorphic network complexes based on double hydrogen bonds with bis(acylamino)pyridine~.~~~]

In work that combines aspects of both that of Kato and Ujiie, Bazuin et al.14'] prepared LC materials using non- covalent interactions between ionic or ionizable groups. A polymer backbone of poly(4-vinyl pyridine) was combined with aliphatic acid mono- and difunctional mesogens based on biphenyl. A second family was prepared by interaction of poly(acry1ic acid) with a tertiary amine functional mesogenic group also based on biphenyl. It was observed that

Adv. Muter. 1991, 9, No. 1 0 VCH Verlagsgesellschuft mbH, 0-69469 Weinheim, 1997 0935-9648/97/0101-0027 $10.00+.25/0 27


complex formation does not assure the existence of a mesophase, particularly when non-stoichiometric quantities are added. When mesophases were noted, they tended to be smectic. Similar to the work of Bazuin, Tal'roze et aLi4'] complexed PAA and PMA with mesogenic tertiary amines. In these weakly crystalline materials, the characteristic complex corresponds to two acid groups for each amine group. That the polymer backbone has an important effect on the organization of these materials is shown by the fact that while the layer spacings increase dramatically with temperature in the case of the PAA-complexes, it is nearly constant in the PMA materials. This appears to be in contrast to the polyelectrolyte-surfactant complexes produced with either PMA or PAA in which little difference in behavior was observed. Surprisingly little amine (< 10%) is needed to induce a layered structure in these complexes.

Recently, complexes formed from poly(4-vinyl pyridine), P4VP, and surfactant-like molecules have been exam- i~~ed. '~ '] Fredricksoni4" has theorized that a bottle-brush macromolecular structure formed by complexing a alkyl tail surfactant with a polymer will form a mesophase in the appropriate solvent. This theory proposes that if a critical amount of surfactant per repeat is exceeded, then due to steric crowding and electrostatic effects the persistence length of the chain will be sufficiently high to cause mesomorphic behavior, as shown in Figure 15. In order to test

( a )

( b ) Fig. 15. Polymer-surfactant complexes a) in a coiled configuration where surfactant i\ in a low coverage limit and b) in the bottle-brush arrangement whcre the complex exhibits high coverage. In the latter case if a critical coverage is exceeded. then the complex acts in a rod-like manner [43].

this idea and examine the resulting solid-state structures of these materials, a series of stoichiometric and off-stoichiometry complexes were produced from P4VP and dodecylbenzene sulfonic acid. Layered structures were formed in the solid-state and samples with up to 50 % toluene also retained this smectic ordering.

While these polymers are ionized due to protonation of the pyridyl group by the sulfonic acid, an alternative ap-

proach to prevent ionization has been examined.i441 In a second study, these researchers looked at coordination complexes in which a Zn-neutralized surfactant was coordi- nated to the pyridyl group via the nitrogen lone pair. A long term goal of this study was to determine whether the polymer backbone could be stretched out by proper complexation. These materials were demonstrated to be mesomorphic both from optical and X-ray studies. Birefrin- gence, taken as an indication of liquid crystallinity. was observed even in non-stoichiometric complexes with as little as a single Zn-neutralized surfactant for every second 4VP repeat. At lower concentrations, indications of mesomorphic behavior were lost. In another study, the same group examined the formation of complexes using hydrogen bonding in order to eliminate any bound charges.i4s1 These new complexes were made using P4VP and pentade- cylphenol (PDP). In the melt, there was a continuous change in the thickness of layered structures, with values of -50 A at low concentrations of PDP to values of -35 A for neat PDP, such that the lamellar thickness scales as (mole fraction surfactant)-'. The imposition of lamellar structures at even low concentrations of PDP (-5 YO) is surprising, but consistent with the work of Tal'roze. Despite a logical analysis using arguments derived from block copolymer theory, no clear explanation could be given.

While the polyelectrolyte-surfactant complexes have been largely confined to analysis of their mesomorphic properties, examination of their potential as optical materials has also been undertaken. Ionenes have been complexed with the salts of sulfonic acid dyes to make glassy, polable non-linear optic-active (NLO) filrns.l"] Non-stoichiometric structures could be successfully produced with high refractive indices (> 1.6) when doped with inorganic counterions. This work demonstrates the ease with which different colors can be realized in such complexes. Pro- spects for using these materials in optical applications therefore seem good provided possible environmental sen- sitivity is accounted for.

6. Complexes with Conjugated Polymers

Probably the first example of a polyelectrolyte-surfactant complex based on a conducting polymer involved poly(pyrr~le).["~ These complexes, prepared and studied by Wegner and coworkers, could be prepared as self-sup- porting films by electrochemical polymerization of pyrrole in water in the presence of an anionic surfactant, for example, SDS. It was also possible to produce a layered structure in the presence of n-alkyl sulfate, sulphonate, and phosphate. The result of this synthetic approach was a stable, conducting polymer with tailorable conductivity. In addition, environmentally-questionable counterions like hexa- fluoroarsenate could be replaced. Data listing the conductivity of these polymers is given in Table t . Of particular note is the high conductivity of the complex made with

28 0 V C I I Verlag~gewllschuft mbH, 0-69469 Wernherm, 1997 0975-9648/97/0101-00228 $ I0 00+ 25/0 Ad" Mater 1991, 9, NO 1

ADVANCED MATERIALS

C. K. Ober, G. Wegner/Polyelectrolyte-Surfactant Complexes in the Solid State

Table 1. Complexes formed from poly(pyrro1e) and alkylsulfonates, -sulfates and phosphonates. Data listed iiicludes room temperature conductivity cRT, d-spacing of observed Bragg peak and composition of complex.

Surfactant 6.d S-cm-' d-spacing Stoichiometry Anion Wt-

(A) fraction (%) H( CH2) 1 ~ - 0 S 0 3 - HO 35.4 3.41 51.5 H(CHZ),~-OSO~- 5 3.8-1 56.5 H(CH>),l-S03 5-10 19.4 3 5:l 37.5

H(CH2)n-SOo 160 26.9 3:l 49.7 H(CHz)h-S03~ 70 23.4 3.5 1 42.1

H ( C H ~ ) I ~ - S O ~ - 50 32.2 3 1 53.1 H(CHz) M-S% 41.4 3.9.1 54.6

SOi(CH2) io-SO3- 25 19.6 8 1 36.5 H(CH&OP03H~ 12 24.6 4 1 43.1 kUC1 bhn-OI'O~H- 20 30.7 4 1 47.2

100400 S/cm when doped with DBSA or camphor sulfonic acid (CSA) are possible. In contrast to the HC1-doped PANi, there is a weaker temperature dependence of conductivity. It seems that the PANi-CSA complex is partially crystalline and analysis of the electrical properties by the authors showed that the material is close to the metal/insu- lator (M/I) boundary. Conducting blends have been made of PANi-CSA complexes in polymers such as poly(ethy1- ene, PMMA, PVC, and others. With the PANi-CSA complex at approximately 20 wt.-% in the blend, conductivity between 0.1 and 10 S/cm is possible. This increase in conductivity is seen in Figure 17.

H(CH2)8-S0<. Other data showed that maximum conductivity was effectively independent of surfactant length up until alkyl side groups of -12 carbons. After this length, conductivity decreased as the conducting polymer became diluted. The X-ray studies showed a rather short correlation length despite the layered structure and these materials were macroscopically disordered since there was no or- ienting field used in the production of these films. An interesting feature of these materials is the confinement of the conducting polymers to planar regions, as determined by X-ray diffraction and shown in Figure 16. Like many

3000 4000[

'"""I OO

///

28

Fig. 16. Plot of intensity versus 29 for two complexes with poly(pyrro1e). Dc- spite the relative chain stiffness, lamellar structures are formed, but the small angle peak is broader than that of the complex shown in Figure 11, indicating a smaller correlation length [47].

complexes, the increase in layer thickness is proportional to the number of carbons in the surfactant.

The solution and melt processing of poly(aniline), PANi, has recently been carried out using functionalized sulfonic acids. These acids are typically the acid form of surfactants such as dodecyl benzene sulfonic acid (DBSA), which act to acid-dope the polymer, thereby making it conductive, induce melt processability, and produce a polymer capable of blending with others. Long chain protonic acids were used by Heeger and coworkers to dope PANi to the acid form and leave it soluble in a variety of solvents such as toluene, decalin, xylene, et~.[~*I Conductivities in the range

k I

t I 0 - 5 5

0 0.05 0.10 0.15 0.20 PANI-surfactant content in blend (w/w)

Fig 17 Plot of electrical conductivity versus weight fraction of a PANi-camphor sulfonic acid complex blended in PMMA [48]

Blends of the PANi-DBSA complex in ultra-high molecular weight polyethylene (UHMWPE) were spun into oriented films and fibers. The resulting strong orientation leads to parallel alignment in the fiber direction, and for some optical absorptions associated with the phenyl rings there is orientation perpendicular to the alignment direction. The resulting solid-state structures depend very much on the interactions between the selected counterions, solvent, and co-~olvent.[~~]

In principle, an increase in the number of functional arms on the surfactant counterion should lead to better processing. The additional arms might also produce more efficient blending. Phosphoric acid esters are available with two aliphatic arms and were used as doping agents with PANi.["I General structures of such materials are shown in the list of surfactants (Fig. 1) and in this study, the aliphatic tails used were the n-octyl, ethylhexyl, and isobutyl groups. An increase in conductivity up to -5 Slcm was observed in the PANi-diethylhexyl complex with the surfactant concentration less than 23 wt.-%. Above this value, a plasticized blend was produced. If the blend was dissolved in an organic solvent, a soluble and insoluble fraction were formed, with the soluble fraction having a PANi-surfactant molar ratio of 1:l. The plasticized film can be processed as a thermoplastic. When blended with PVC (15 wt.-% PANi, 50 wt.-% PVC, and 35 wt.-% surfactant), it is possible to form a mixture with conductivities as high as 0.01 Slcm.

Adv. Mater. 1997, 9, No. I 0 VCH Verlagsgesellschaft mbH, 0-69469 Weinhem, 1997 0935-9648/97/0101-0029 $10.00+.25/0 29


Until much higher PANi-surfactant levels were reached, little increase in conductivity was observed. Uses for such materials might include electrostatic dissipation, static dis- charge, and electromagnetic interference shielding (EMI).‘”] Some advantages of blends of conducting polymers include control of conductivity, improved mechanical properties, cost, transparency, color adjustment, and processing behavior. Many of these properties are shared with polyelectrolyte-surfactant complexes in general.

It is possible to form these complexes simply by combining the surfactant with the correct form of PANi followed by heating of the mixture. Above about 450 K, the true complex forms, and high conductivity appears which is retained on cooling. If such complexes are introduced to a block copolymer such as a styrene-hydrogenated buta- diene-styrene triblock, the elastomer-complex blend is conductive. Upon stretching, the conductivity initially increases and then at higher extension drops off. This sug- gests that the conductive component is part of the continuous phase, but eventually disconnects. The authors of this study would not speculate on the causes of this effect.

The role of solvent and solubility on complex formation has also been studied.[521 It was found that PANi-DBSA complexes of ratio -1:l were needed for solubility in non- polar solvents, but that an excess of DBSA was needed to form non-gelling solutions. After thermal processing it was possible to extract the excess DBSA with acetone. As an alternative to additional DBSA for processability, a co-solvent such as dodecyl phenol was used. This material was shown to enhance the conductivity of the complex, but the reason for this was not clear. Large excesses of surfactant were also investigated.[”] Annealing of the initially poly- morphic PANi and DBSA mixtures above 190°C was shown to promote single phase formation and to produce enhanced conductivity. Diffraction studies showed that the mixture produced a layered structure, the layer spacing of which corresponded to that of the surfactant length. Their studies suggested a random placement of the main chain and possible disordering of the alkyl side groups in the solid-state. Maximum conductivity of 10 S/cm was observed when the PANilDBSA ratio was 1:3 and with more DBSA the conductivity dropped to 1-3 Slcm.

The polyelectrolyte-surfactant complexes can be cast from organic solvents, therefore it is possible to make fibers of these complexes. There have been earlier attempts to obtain the crystal structure of PANi and to calculate the local geometry via torsion angles, but the experimental data has been largely limited to X-ray powder patterns. Re- cently, making use of the complexes it was possible to study several oriented fibrils made from the complexes by electron It was shown that the unit cell consists of two PANi chains and one DBSA as abbreviated by (4NH4NH) @ (DBSA)@. A schematic of the unit cell is shown in Figure 18.

The lattice parameters for the cell were a = 1.18, b = 1.79, and c = 0.72 nm. Assuming an orthorhombic space

( a ) - lab 3f4

Fig. 18. Schematic drawing ot the main chain and interchain packing of the PANi-DBSA complex determined from electron diffraction of drawn fibrils ~541.

group, these values give a density of 1.12 gcm-3, which is close to that measured by other methods. For the fibrils, a Hermann’s orientation parameter of -0.96 was calculated and from the Debye-Scherrer formula, a column length of -300 A was determined. The c-axis is also the polymer chain axis, which is consistent with what would be expected for the extended chain polymer. The spacing between the chains is controlled by the surfactant and this observation is in line with observations made on the other polyelectrolyte-surfactant complexes described in this review.

This view of the complex is useful in that it provides a slightly different picture than that usually drawn. The surfactant DBSA molecules are extended perpendicular to the PANi chains. The PANi chains are also more or less fully extended. While this is a layered structure, the orien- tations of the DBSA molecules are. however, alternating and tilted. Some aspects of this organization must be due to the relative stiffness of the PANi chain versus the more flexible poly(e1ectrolytes) usually studied.

7. Summary and Comments

The simplicity of producing new melt-processable, organic soluble materials from simple, readily available polyelectrolytes and surfactant molecules is truly exciting. New structures have appeared in the last 5 years with properties that range from thermoplastic to elastomer and thermoset. A variety of molecular organizations have been observed in these complexes including lamellar, cylindrical, and un- dulating layered structures. Applications in biology and materials science including electrically switchable membranes, NLO materials, and extremely low surface energy materials have been proposed. New, self-doped complex

30 0 VCH Verlug~gesellschaft mhH, D-69469 Weinheim, 1997 0935-9648/97/0101-0030 $10.00+.25/0 Adv. Muter. 1997, 9, No. I

ADVANCED MATERIALS

C. K. Oher, G. WegnerlPolyelectrolyte-Surfactant Complexes in the Solid State

films and blends with conducting polymers have been produced with near metallic conductivity. One can even speculate that barrier or transport materials with controlled permselectivity to gases can be produced. Such complexes might be useful for the creation of ordered materials via templating reactions.

Despite these accomplishments, a number of critical is- sues need to be addressed. Repeatedly, layered structures are produced with a variety of components. This is clearly the preferred arrangement, even with little added surfactant. Control over this molecular arrangement needs to be addressed before these complexes achieve their full potential. Little is known of the processing of these complexes in the melt and our preliminary work indicates it will be a challenge, in part due to the presence of water in the complex. Effects of the molecular weight of the polyelectrolyte backbone have not been addressed. Finally, the role of water in these complexes needs to be examined in detail. Water appears to be present in many of these structures and may be overlooked in a number of applications, espe- cially those focused on biological uses. However, in a number of materials science activities, variable water content may prove to be a challenge. Examples of ways to address this are already appearing in the work on water-repellent fluorinated materials. The prospects of this emerging field therefore appear bright and one can expect much more work in this area to appear in the near future.

Received December 18,1995 Final version August 28, 1996

[l] Interactions of Surfactants with Polymers and Proteins (Eds: E. D. Goddard, K. P. Ananthapadmanabham), CRC Press, Boca Raton, FL 1993.

[2] A. Ciferri, Macromol. Chem. Phys. 1994,195,457. [3] K. Thalberg, B. Lindman, in Surfactants in Solution, Vol.11 (Eds:

K. L. Mittal, D. Shah), Plenum Press, New York 1991. [4] H. Morawetz, personal communication. [5] K. P. Ananthapadmanabham, in Interactions of Surfactants with Poly-

mers and Proteins (Eds: E. D. Goddard, K. P. Ananthapadmanab- ham). CRC Press, Boca Raton, FL 1993.

[6] G. Decher, J. D. Hong, Makromol. Chem., Macromol. Symp. 1991,46, 321; J. H. Cheung, A. C. Fou, M. F. Rubner, Thin Solid Films 1994, 244, 985: M. Ferreira, J. H. Cheung, M. F. Rubner, Thin Solid Films 1994,244,806.

[7] Y. Lvov. H. Haas, G. Decher, H. Mohwald, A. Mikhailov, B. Mtched- lishvily, E. Morgunova, B. Vainshtein, Langmuir 1994,10,42324236.

[8] J. S. Beck, J. C. Vartuli, W. J. Roth, M.E. Leonwicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenkar, J. Am. Chem. Soc. 1992,114, 10 834.

[9] A. Monnier, E Schuth, Q. Huo, D. Jumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurthy, P. Petroff, A. Firouzi. M. Janicke. B. F. Chmelka, Science 1993,261, 1299.

[lo] H. Meier, I. Sprenger, M. Barmann, E. Sackmann, Macromolecules 1944.27,7581.

1111 J. Wang, G. Wegner, Macromolecules 1992,25,1786. [I21 P. Ekwall, in Advances in Liquid Crystals, Vol. 1 (Ed G. H. Brown),

Academic Press, New York 1975; H. Hoffmann, Adv. Mater. 1994, 6, 116 M. J. Rosen, Surfactant.s and Interfacial Phenomena, Wiley, New York 1989: E. Kissa, Fluorinated Surfactants, Marcel Dekker, New York 1994.

[13] X. Auvray, C. Petipas, R. Anthore, I. Rico, A. Lattes, J . Phys Chem. 1989,93,7458.

[14] P. KBkicheff, J. Colloid Interface Sci. 1989,131,133. [lS] E. D. Goddard, Colloids SurJ 1986.19, 301. [I61 B. Lindman; K. Thalberg, in Interactions of Surfactants with Polymers

and Proteins (Eds: E. D. Goddard, K. P. Ananthapadmanabham), CRC Press, Boca Raton, FL 1993.

[17] B. Magny, I. Iliopoulos, R. Zana, R. Audebert, Langmuir 1994,10,3180. 1181 E. D. Goddard, in Surfactants in Solution, Val. 11 (Eds: K. L. Mittal,

D. Shah), Plenum Press, New York 1991. 1191 K. N. Bakeev, S.A Chugnov, T.A. Larina, W. J. MacKnight, A. B.

Zezin, V. A. Kabanov, Polym. Sci. 1994,36,200. [20] A. Harada, S. Nozakura, Polym. Bull. 1984,II, 175. [21] Y. Okahata, G. Enna, K. Tagucbi, T. Seki, J. A m . Chem. SOC. 1985,107,

5300. [22] K. Taguchi, S. Yano, K. Hiratani, N. Minoura, Y. Okahata,

Macromolecules 1988,21,3336. 1231 K. Ydguchi, S. Yano, K. Hiratani, N. Minoura, H. Umehara,

Macromolecules 1989,22,4140. [24] K. Taguchi, S. Yano, K. Hiratani, N. Minoura, Macromolecules 1991,

24,5192. [25] D.A. Tirrell, A. B. Truek, D.A. Wilkinson, T. J. McIntosh,

Macromolecules 1985,18,1512. [26] M. Antonietti, A. Kaul, A. Thunemann, Langmuir 1995, 11, 2633: M.

Antonietti, A. Wenzel, A. Thunemann, Langmuir 1996,12,2111. [27] M. Antonietti, J. Conrad, Angew. Chem., 1nt. Ed. Engl. 1994,33,1869;

M. Antonietti, C. Burger, J. Effing, Adv. Mater. 1995,7,751. [28] K. Almdal, K.A. Koppi, F. Bates, K. Mortenson, Macromolecules

1992, 25, 1743; I. Hamley, K.A. Koppi, J. Rosendale, F. Bates, K. Almdal, K. Mortenson, Macromolecules 1993,26,5959.

1291 C. K. Ober, A. Kameyama, R. H. Colby, unpublished results. [30] M. Antonietti, S. Henke, A. Thunemann, Adv. Mater. 1996,X 41. [31] M. Antonietti, J. Conrad, A. Thunemann, Macromolecules 1994, 27,

6007. [32] R. H. Colby, C. K. Ober, A. Kameyama, unpublished results. 1331 M. Antonietti, M. Maskos, Macromol. Rapid Commun. 1995,16,763. [34] J. S. Kim, S. B. Roberts. A. Eisenberg, R. B. Moore, Macromolecules

1993,26,5256. [35] K. Bakeev, Y. Shu, W. J. MacKnight, A. Zezin, V.A. Kabanov,

Macromolecules 1994,27,300. [36] K. Bakeev, S.A. Chugunov, I . Terakoa, W. J. MacKnight, A. Zezin,

V. A. Kabanov, Macromolecules 1994,27,3926. [37] S. Ujiie, K. Iimura, Macromolecules 1992,25,3174. [38] T. Kato, H. Kihara, T. Uryu, A. Fujishima, J. M. J. FrCchet,

Macromolecules 1992,25, 6836 T. Kato, H. Kihara, U. Kumar, T. Ur- yu, J. M. J. FrBchet, Angew. Chem., In/. Ed. Engl. 1994,33,1644.

1391 T. Kato, M. Nakano, T. Moteki, T. Uryu, S. Ujiie, Macromolecules 1995,28,8875.

1401 C. G. Bazuin, F.A. Brandys, T. M. Eve, M. Plante, Macromol. Symp. 1994,84,183; C. G. Bazuin, A. Tork, Macromolecules 199.5,28,8877.

[41] R. Tal’roze, S. Kuptsov, T. Sycheva, V. Bezborodov, N. Plate, Macromolecules 1995,28,8689.

[42] 0. Ikkala, J. Ruokalainen, G. ten Brinke, M. Torkkeli, R. Serimaa, Macromolecules 199.5,28,7088.

[43] G. H. Fredrickson, Macromolecules 1993,26,2825. [44] J. Ruokalainen, J. Tanner, G. ten Brinke, 0. Ikkala, M. Torkkeli, R.

Serimaa, Macromolecules 1995,28,7779. [45] J. Ruokalainen, G. ten Brinke, 0. Ikkala, M. Torkkeli, R. Serimaa,

Macromolecules 1996,29,3409. (461 W. H. Meyer, J. Pecherz, A. Mathy, C. Wegner, Adv. Mater. 1991,3,151. 1471 G. Wegner, J. Ruhe, Faraday Discuss. Chem. Soc. 1989,88,333. 1481 A. S. Heeger, Synth. Met. 1993, 55-57, 3471; Y. Cao, P. Smith, A. J.

Heeger, Synth. Met. 1993,55-57,3514. [49] Y. Cao, P. Smith, Synrh. Met. 1995,69, 191. [50] A. Pron, J. Lostra, J.-E. Osterholm, P. J. Smith, Polymer 1993, 34,4235:

A. Pron, .I.-E. Osterbolm, P. Smith, A. J. Heeger, J. Laska, M. Zagorska, Synth. Met. 1993,55-57,3520.

[51] 0. T. Ikkala, J. Lackso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. Jarviene, T. Taka, P. Passiniemi, J.-E. Osterholm, Y. Cao, A. Andreat- ta, I? Smith, A. J. Heeger, Synth. Met. 1995,69, 97.

[52] Y. Cao. J. Qin, I? Smith, Synth. Met. 1995,69,187. [53] W. Y. Zheng, R. H. Wong, K. Levon, Z. Y. Rong, T. Taken, unpub-

lished results. [54] C. Y. Yang, P. Smith, A. J. Heeger, Y. Cao, J. E. Osterholm, Polymer

1993,34,5178.

Adv. Mater. 1997, 9, No. I 0 V C I i Verlagsgesellschaft mbH, 0-69469 Weinheim, 1997 0935-964X/97/0101-0031$10.00+.25/0 31

polyelectrolyte–surfactant complexes in the solid state: facile building blocks for...

Documents