A molecular complex is formed by the association of two or more molecules or ions. More specifically, a polyelectrolyte complex (PEC) is made up of differing macromolecules bound together by noncovalent bonds, which include primarily coulombic interactions.1 PECs often are referred to in the patent and technical literature as polymer-polymer complexes, interpolyelectrolyte complexes, or complex coacervates; these terms all are based on the interaction of large macromolecules. Regarding the formation and application of PECs, the authors propose they instead be characterized by the term complex system, as defined by complexity theory.
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A molecular complex is formed by the association of two or more molecules or ions. More specifically, a polyelectrolyte complex (PEC) is made up of differing macromolecules bound together by noncovalent bonds, which include primarily coulombic interactions.1 PECs often are referred to in the patent and technical literature as polymer-polymer complexes, interpolyelectrolyte complexes, or complex coacervates; these terms all are based on the interaction of large macromolecules. Regarding the formation and application of PECs, the authors propose they instead be characterized by the term complex system, as defined by complexity theory.
In this paper, PECs will be described in some detail, including their basic chemistry and formation process. Also, a mechanism will be proposed to explain how a microgel cross-linking structure formed from a PEC consisting of the two polymers PVM/MA copolymer and polyquaternium-28 repairs damaged hair. The potential benefit of this PEC for hair treatments illustrates how this complex system can produce results beyond what could be predicted from the characteristics of its individual polymers. As defined by complexity theory, a complex system contains multiple parts that have local relationships to each other. From the interaction of these parts, a new property of behavior emerges that could not have been predicted from a study of its individual parts alone.2
Although some may view complexes as insoluble species that form when something goes awry during formulating or processing a formula, they do have a functionality that has been demonstrated in various industries. Some examples include microencapsulation, separation membranes, controlled and sustained release of drugs, flocculation of colloidal dispersions, bioadhesion and, less frequently, personal care.
The Chemistry Behind
The Complex
Figure 1 shows the basic reaction in which mixing an anionic polyelectrolyte with a cationic polyelectrolyte produces a PEC. Magnifying one section shows the localized ionic interaction of the two oppositely charged macromolecules.3 Complexes also can be based on hydrogen bonding and/or hydrophobic bonding through Van Der Waals forces acting between the hydrophobic grafted side chains on the molecule. Usually all three types of bonds are involved in the complex; however, the predominant bond is electrostatic. A typical PEC is based on a polyacrylic acid and a cationic polyelectrolyte consisting of multiple quaternary groups (see
Figure 2).4 Besides synthetic polymers, PECs also can be made from naturally derived polymers.5
The degree of interaction depends on the charge density of each polymer. For the cationic polyelectrolyte, this would be the level of cationic substitution; for the anionic polymer, the level of substitution with carboxylate groups and pH. Increasing the pH ionizes the anionic polymer so that a greater interaction with the cationic polymer is obtained.
Sometimes creating PECs can prove challenging; for example, if an incorrect order of addition is used to make a simple clear hair gel, a complex will form if a fixative resin like PVP is added to an unneutralized anionic polyacrylate gellant such as carbomer. In its acidic form, the polyacrylate donates a proton to the tertiary amine of the PVP. This will cause a differential charge in the two polymers, which is responsible for forming the complex. Therefore, to produce a clear, uniform system, it is recommended to add the neutralizer either first to the gellant, or to the fixative resin before adding the fixative resin to the gellant.
The Complex Nature of PECs
There are several variables to consider when designing PECs, many of which can be controlled by the chemist, but there also are uncontrollable variables such as those found in typical synthesis reactions involving covalent bonds. There are three types of controllable independent variables: intrinsic polymer variables, formulation variables and processing variables. Intrinsic variables include polymer molecular weights, charge densities and distribution, and functional groups that provide a specific chemistry to the molecule. Formulation variables include solvent type, electrolyte content, weight ratios of the polymers, total polymer solids, and pH. Processing variables include order of addition, viscosity/concentration of pre-phases, rate of addition, mixing time/speed/type, and temperature. To determine the effect of these variables would take a multitude of experiments.
Consideration of this overabundance of variables leads to the realization that a world of opportunity exists not only for the number of PECs that can be made, but also for potential applications as yet undiscovered. Finding these correct polymer ratios through the production of phase diagrams would be tedious; fortunately, other experimental strategies can be utilized to help the discovery process.
Design of experiment: One strategy is Design of Experiment, commonly termed DoE. Here, a computer generates design points in the factor space. Regression analysis of the measured response variables produces a response surface. Phase regions can be identified and the region of maximum complexation can be predicted, which may dictate the optimal weight ratio of polymers to use.
Stoichiometry calculations: Another technique to reduce the number of experiments required is calculating the stoichiometry of charge neutralization. Maximum complexation usually occurs when anionic and cationic charges are equal on a molar basis. The weight ratio of the polymers can then be determined based on these stoichiometric calculations.
Combinatorial high-throughput methods: Although not employed here, a third possibility to increase the efficiency of experimentation is combinatorial methods for high-throughput techniques utilizing automated workstations. This type of equipment is currently being used to explore polymer-surfactant complexes, the study of which is providing an increased understanding of polymer molecular weight, charge density, electrolyte content and order of addition on coacervate formation.6, 7
Uncontrollable Variables
Having a number of interacting variables does not make PECs complex from a complexity theory point of view. Their complexity lies in their uncontrollable variables. Small molecules tend to react in a random fashion. When the orientation and the threshold in the energy of activation are correct, new covalent bonds are formed. When polymers associate through electrostatic bonds, the process is also random. Figure 3 illustrates the process.8
The reacting species in polymers, however, are tied to each other since they are part of the polymer chain. Due to the conformation of the polymer in solution, there is an incomplete reaction between polymers of unlike charge. Some parts of each chain remain unassociated. Dautzenberg designated this as the scrambled egg structure, as illustrated in Figure 4.9 The complex formed between the two polymers, although it may have a net zero charge on a stoichiometric basis, may still have a residual charge on the surface based on the dangling parts of each of the unassociated chains.
In the formation of polymer complexes, there is no way to hook one monomer with a cationic charge precisely with a specific monomer of another polymer with an anionic charge; interaction is completely governed by local interactions of the two types of polymers in a random fashion. This self-association illustrates one of the uncontrollable factors that make the system complex.
A Synergistic Effect
Another factor leading to the complexity of PECs is synergism. Granted, there are mixtures that are synergistic in behavior without forming a complex—for example, two discrete ingredients intimately mixed with a solvent to form a mixed solution. This can be represented as:
Mixture: A + B = [A + B] Eq. 1
If a dependent response variable such as viscosity is higher than the additive effects of the two ingredients by
themselves, then synergism has occurred. However, when two polymers join together to form a complex, the result is not just a mixture of two polymers, but the formation of a new species, which can be represented as:
Synergy: A + B = C Eq. 2
R. B. Fuller defined this new species (C) as synergistic because its properties could not be predicted from its component parts. To illustrate synergy, Fuller cited the use of alloys to increase the tensile strength of metals. Combining chrome, nickel and steel alloys achieves a tensile strength of 350,000 pounds per square inch (psi). Yet the tensile strength of the individual components ranges from 60,000 to 80,000 psi. This significant disparity can be explained by the spatial arrangement of each of the individual types of atoms in space. This increased packing due to the arrangement of atoms provides for greater interatomic attractions and results in an increase in tensile strength. In this case, the behavior of the whole is unpredictable by the behavior of its parts.10
PVM/MA Copolymer
and Polyquaternium-28
The factors necessary to make a system complex, namely self-association and synergism as defined by Fuller, can be exhibited by combining anionic and cationic polymers together. A complex is formed when the copolymer of methyl vinyl ether-maleic anhydride (INCI: PVM/MA Copolymer), the anionic portion of the complex, is combined at certain ratios with different types of cationic polyelectrolytes. Examples of the cationic polyelectrolytes explored include polyquaternium-7, -10, -28 and -55. Also, complexes have been made using the hydrophobically modified anionic polymer VP/acrylates/lauryl methacrylate copolymer, in order to interact with the polymers containing the cationic quaternary group.11
The complexation of many other polymers can be envisioned based on the disparity of charge between two or more polymers. The complex described in the present paper for hair mending benefits is formed between PVM/MA copolymer and polyquaternium-28. Their structures are represented in Figure 5. PVM/MA copolymer acts as the anionic component containing the ionizable carboxylate groups, and polyquaternium-28 is the cationic component containing multiple quaternized nitrogens.
Figure 6 shows a phase diagram constructed to determine the region of maximum complexationa. The phase diagram was constructed by combining various weight ratios of the two polymers together keeping various formulation variables constant. The response surface was built by quantifying the macroscopic character of the combination of the two polymers on a weight basis; see Figure 6 legend for a description of each of the phase regions. The area designated as “1” represents the weight ratios where the resultant products exhibited a milky appearance. This was initially judged from a macroscopic point of view as the region of maximum complexation.
The controllable variables in making each of the formulations for this phase diagram included such things as polymer-weight ratios, order of addition, temperature, solvent type, and pH of the PVM/MA copolymer, which controls the level of anionic charge of this polymer. However, it is when the uncontrollable variables are considered that complexity is revealed. These uncontrollable variables are responsible for the self-association of the two polymers into polyelectrolyte complexes. The nature of these self-associating species is shown upon closer analysis. This was accomplished through optical microscopy and measuring the viscosity of the mixtures of the two polymers. These techniques proved to be more revealing of the nature of the PEC system compared to simply observing the macroscopic character of their solutions.
Viscosities were measured on a series of combinations of polyquaternium-28 and PVM/MA copolymer, keeping the PVM/MA copolymer constant at 0.20%. Viscosity was then plotted against the molar ratio of the reacting species of each polymer. As Figure 7 shows, there was a trend found in the unit mole ratio of the two polymers with viscosity, as portrayed by the concave response curve. Stoichiometric calculations further revealed that this minimal level was the result of a one-to-one charge neutralization ratio of each of the polymers identifying the point of maximum complexation. On either side of this ratio, an excess of uncomplexed polyanion or polycation was responsible for increasing solution viscosity. This property has been reported in other systems as typical of PECs.12 The 1:1 charge neutralization line is drawn in Figure 6 to show other polymer concentrations, which theoretically shows the points of minimum viscosity and maximum complexation.
The systems produced to build the phase diagram of Figure 6, when observed microscopically, except for the clear systems, contained small dispersed particles, or microgels. The morphology of these microgels, illustrated in
Figure 8, is characterized as translucent, odd-shaped, free-flowing particles with an average measured particle size of
5 to 10 microns. The microgel structure explains why systems that have a 1:1 charge neutralization produce a low-viscosity response: this is the condition for maximum complex formation. These microgel particles are not produced by any variable controlled by the researcher, but result from the way that the polyelectrolytes self-assemble. The microgels produced from PVM/MA copolymer and polyquaternium-28 result in properties that exemplify their synergistic behavior, again that cannot be fully explained by considering its component parts.
From Synergy to Split Ends
Concurrent to exploring the nature of PECs, research efforts were under way to design a test method to identify an ingredient or composition to repair damaged hair. These efforts tested the efficacy of a leave-in treatment product to mend the split ends that form during aggressive hair styling processes such as brushing and blow drying.13 The method consisted of evaluating split-end fibers under a stereomicroscope to assess the degree of mending, then confirming via scanning electron microscopy (SEM).
The procedure included tagging the split-end fibers at their root end in a tress, which allows their assessment during the different steps of the experimental protocol. Durability of the mend can be assessed in a realistic fashion since the treated fibers are part of the tress. In this case, durability is tested by shear forces induced by combing. The mending as well as cuticular smoothing is also observed at higher resolution by SEM, as shown in Figure 9.
If a complex were to be considered as efficacious for hair mending, it was conjectured that it would be in the region of maximum complexation, as judged from the character of the composition. This was shown by testing a complex in the region designated as “1” in Figure 6 and achieving positive results. The exact composition tested is shown as a blue dot.
This result initiated a flurry of activity to investigate the nature of these microgels and their ability to mend damaged hair. The series of experiments revealed that the highest efficacy was found when the polymers were complexed such that their charge neutralization was at a 1:1 ratio of anionic to cationic charge. Complexes formed at charge neutralization ratios deviating from this, as well as testing the polymers alone, had lower efficacy; the 1:1 charge neutralization line is depicted in Figure 6.
These microgels represent a new species of material even though they are not formed through the breaking and making of covalent bonds; that is, without the use of chemical transformation. The proposed mechanism consists of their ability to form cross-linking structures that bind and mend the broken subassemblies of the fibers.
The first step begins with the interaction of the microgels with the damaged components of the hair fiber through adhesive ionic interactions. During application, the microgels infiltrate the subassemblies of the broken fiber and form cross-linking structures to bind these components together. During drying, the microgels contract, pulling the subassemblies together as well as sealing the cuticle, thus helping to increase mending durability. The mechanism is illustrated in Figure 10. The mend based on PVM/MA copolymer and polyquaternium-28 is semi-permanent in nature in that reapplication is necessary after shampooing.
Besides hair mending, there are many examples of complexes formed between two polymers having properties that are not apparent simply by considering the mere blending of their component parts. The interest in PECs is evident from the extent of the references available in patent and technical literature. Although applications are not prevalent in the personal care industry, examples exist, such as rheological effects for suspensions applied in surfactant-free formulations,14, 15 moisture delivery,16 and facial-wrinkle masking.17
Conclusion
Complexity theory teaches that viewing systems as a whole and not just as simple mixtures of their component parts will provide researchers with a new paradigm for scientific inquiry.18, 19 Considering all the factors in the formation of PECs, and the fact that their resultant novel application properties cannot be predicted by the properties of their component parts alone, the case can be made that PECs are indeed complex systems. A good example of an emergent property of this system is the complex formed between PVM/MA copolymer and polyquaternium-28, which can be used in a treatment product for mending damaged hair. Numerous examples of PECs containing synergistic properties abound in many industries, which hopefully will encourage chemists to consider them as another avenue in the discovery of new compounds to achieve new performance benefits.
References
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