Field Responsive Polymers - ACS Publications - American Chemical

Chapter 20

Synthetic Design of "Responsive" Surfaces David E. Bergbreiter

Downloaded by UNIV OF LIVERPOOL on June 16, 2017 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch020

Department of Chemistry, Texas A&M University, College Station, TX 77843

Responsive polymer-modified surfaces whose properties and character changes in response to external stimuli such as temperature, pH or solvent can be prepared by a number of procedures. Such surfaces can be monolayers of functional groups or multilayers of groups. Using synthetic organic polymer chemistry, it is possible to design such interfaces such that predictable changes in reactivity, wettability or permeability can be effected. Examples of such chemistry using surface oxidation or surface graft polymerization are described.

Chemistry to modify surfaces to change the properties of a material or to engender new properties for a material is of interest in many disciplines. " In this paper, I describe and review some of the work our group has done to modify surfaces that can then respond to their environment. This work has implications in polymer chemistry, in the design of biocompatible surfaces, in catalysis chemistry, corrosion passivation and sensor chemistry. This review highlights the general ideas underlying this work. Readers interested in specific applications and in specific materials should consult the original papers cited where we and our collaborators detail our original ideas. To begin with, it is important to define what exactly is meant by the term 'responsive surface'. There are two parts to this term, both of which require some clarification. In general, 'responsive' surfaces are surfaces of organic polymers or inorganic materials that have been modified in some way so that their properties change in response to an external stimulus like pH or temperature. In this context, responsive surfaces are much like other so-called intelligent materials. The second part of this term refers to a substance's 'surface'. This term too needs to be clarified. In the context of our work, we have construed the term 'surface' to include more than the top few A of material in contact with a gas. We generally use the term surface to loosely define the interfacial portion of a surface functionalized polymer or solid in which functional groups readily react 1

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© 1999 American Chemical Society Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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302 with soluble reagents that would not normally penetrate the underlying polymer or solid. Thus, functional groups in a self-assembled monolayer, functional groups in a spun-cast polymer or functional groups at polymer-solution interfaces all would be construed as functional groups at a 'surface . Modification of existing polymers surfaces to make so-called 'smart' surfaces has precedent both in our work and in that of others. " One example of this effect would be changes in wettability in surface functionalized polymers on thermal treatment in vacuum or water. For example, in our studies on sulfonated polyethylene surfaces, we were able to show that reversible changes in hydrophobicity could be seen both for PE-[C02H] and PE-[SC>3H] (surfaces prepared by C1O3/H2SO4 etching and SO3/H2SO4 etching, respectively). As shown in Figure 1, the P E 4 C O 2 H ] surface becomes more hydrophobic on heating in vacuum and more hydrophilic on heating in water. In contrast, the PE[SO3H] surface becomes more hydrophilic on heating in vacuum and more hydrophobic on heating in water. However, these changes in hydrophobicity are modest and the changes are relatively slow (vacuum heating requires ca. 1 h). Moreover, prolonged treatment leads to an irreversible change to a more hydrophobic surface in all cases - the normal reconstruction seen with many functionalized polymer surfaces. Similar work has also been described by Ferguson. In this latter work, the changes were more reversible, more rapid and more substantial. 1

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9

610


6

9

Initial

Vacuum 1

Water 1

Vacuum 2

Water 2

Figure 1. Changes in wettability of PE-[C0 H] (O) or PE-[S0 H] ( « ) o n heating in vacuum or water in comparison to initial oxidized surface wettability (Initial). 2

3

Reversibly responsive polymer surfaces can also be prepared by other strategies. One strategy our group has favored is a process we call entrapment functionalization. This process has the advantage of allowing us to prepare sophisticated, labeled polyethylene oligomers that we have characterized by solution state analyses (principally N M R spectroscopy). The resulting oligomers can then be mixed with virgin high density polyethylene polyethylene that is free of additives. Such mixtures typically have 1% or less of 11

Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

303 the oligomer by weight. Codissolution of the virgin polymer and the functionalized oligomer then produces a homogeneous solution that can be used to solvent cast a functionalized polyethylene film. Our experience shows that the oligomer's functional groups are located at the "surface" of the film derived from this casting process. Two different examples illustrate how we can use this process to prepare a responsive surface. In the first example, we used the chemistry shown in equation 1 to prepare a pyrenylphenylmethyl-terminated polyethylene oligomer. This oligomer in turn was 12

13

HC


2

CH

2

B

u

L

J

(D

mixed with virgin polyethylene (1:100, w:w) and dissolved in o-dichlorobenzene at 120 °C. The resulting solution was then poured into a flat dish and the solvent was evaporated in an explosion-proof oven to form the film 1 shown in equation 2.

(2)

1, PE/PE -CH(Ph)(pyrene) 01ig

^ [CH C(C0 C(CH ) )] H 2

2

3

3

N

R = -C(CH0, 2, PE-g-poly(terf-butylacrylate)

3, PE-g-poly(acrylic acid)

The pyrene-labeled polyethylene film 1 contains a weakly acidic C - H bond (highlighted H) that can be converted into a lithiated initiator on treatment with a


304 strong base. Thus, this film was then deprotonated with w-BuLi and allowed to react withterf-butylacrylate to form a poly(ter/-butyl acrylate graft) that was in turn hydrolyzed with acid to form a poly(acrylic acid) graft. The surfacefunctionalized polyethylene so-formed contains a pyrenefluorescentlabel at the base of an amphoteric poly(acrylic acid) graft. Exposure of this grafted surface to a series of buffers shows that the buffer has no effect on the fluorescence intensity of the pyrene in this film. However, if 1 M Nal were present in the buffer solution, the amount of pyrenes quenched by the iodide significantly increased in the pH region of 6.2-6.5. This pH-induced change in the amount of surface-bound pyrene quenched by the soluble iodide quencher is reversible. We have interpreted this change to suggest that the surface structure changes with the polyelectrolyte graft becoming completely deprotonated and solvent swollen in the indicated pH range, thus exposing the underlying pyrene groups. When similar studies were carried out with an amine quencher (Et3N, Figure 2), the amount of pyrene quenched was again sensitive to pH. However, in this instance the significant quenching was only seen above the pKa of EtaNrT (i.e., above a pH of 9 where the Et3N concentration becomes significant).


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Figure 2. Effect of pH on PE/PEoiig-C(Ph)(pyrene)-g-(CH CH(C02H)) quenching based on changes in the intensity of the I3 peak in fluores-cence spectra of; • , 1 M Nal in buffered aqueous solutions; • , buffer solutions without added quencher; A, 1 M Et3N in a buffer solution. 2

n


305 Responsive surfaces like the modified polyethylene film 3 suggest that it may be possible to change the character of a surface significantly by changing pH. Two recent papers further illustrate this idea. Thefirstof these papers is work by Ikada's group in which poly(acrylic acid) grafted surfaces are used as responsive membranes. In this work, the porosity of a poly(acrylic acid)-grafted membrane is significantly changed by virtue of changes in the size of the poly(acrylic acid) graft as a function of pH. In this example, the presumption of changes in size of the pores as a function of ionization of the graft was supported by AFM studies. The second example of a responsive surface with a poly(acrylic acid) graft is workfromthe Crooks/Bergbreiter groups. In this instance, we prepared hyperbranched graft surfaces using a forgiving synthetic approach to design a highly functionalized reactive surface. This approach to synthesis of a functionalized responsive does not rely on a grafting from the surface strategy. Instead, we use soluble polymers (NH2-teiminated poly(terf-butyl acrylate)) to graft a poly(acrylic acid) precursor onto the surface. In order to compensate for synthetic vagaries associated with even simple reactions at surfaces, our two groups designed a new strategy for grafting that used repetitions of this simple step to create dendritic, hyperbranched poly(acrylic acid grafts). Unlike our earlier polyethylene work, this chemistry is considerably simpler and much more versatile. The grafts prepared in this chemistry can be as thick as desired with thicknesses of up to 1000 A being accessible in six or seven synthetic stages. The resulting grafted surfaces illustrate responsiveness much like that seen by Ikada. In this case, we find that a so-called 3 PAA surface (a surface grafted through three stages of activation (C1C0 R), coupling (H N-PTBA-NH ) and hydrolysis) has a thickness of 300-400 A depending on the molecular weight of the diamino poly(terf-butylacrylate) (H N-PTBA-NH ) polymer used in the functionalization step. This thickness, which was measured after immersion of 14


15, 6

2

2

2

2

3

2

4

5

6

Cycle Number Figure 3 Changes in hydrophilicity of a hyperbranched 3-PAA graft with acid (0.1 N HC1, O) or base (0.1 N NaOH, ) treatment.


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306 the surface in dilute acid and drying with an ethanol rinse and N stream, reversibly expands to 40-50% after immersion in aqueous base. The reversibility of these thickness changes is correlated with a similar reversible change in surface hydrophilicity as is illustrated by the data shown in Figure 3. While polymer surfaces sometimes experience some hysteresis with surface changes attenuating as the number of cycles increases or with temporary changes becoming permanent after long term treatment, these surfaces seem more robust. It is also worth noting that both the dry thickness of the acid form and the dry thickness of the basic form of these 3-PAA surfaces are likely differentfromthe thickness of these same surfaces in situ. Indeed, recent work by our joint groups has indicated that these surfaces thicknesses expand to over 1000 A in their basic form when then are completely solvated by an aqueous basic buffer. Recent work by the Crooks group has shown the potential for surface modification with dendrimers alone. Our group in conjunction with the Crooks group has recently extended this chemistry by coupling the chemistry of dendrimers to the chemistry of surface chemisorbed reactive polymer brushes. ' The result has been that we are able to prepare surfaces with thicknesses between 100 - 500 A whose permeability changes significantly and reversibly with pH. In this instance, we significantly extend the idea of using a single polyelectrolyte bonded to a surface by covalently assembling a matrix of a polybase in a polyacid. The polyacid in this case is a poly(maleic anhydride)-cpoly(methyl vinyl ether) copolymer commercially available as Gantrez. After reaction with amines or after reaction with an alcohol, this polymer forms half acid-half amide (amic acid) groups or half acid-half ester groups and is thus a polyacid. The polybase in our procedure is a fourth generation PAMAM dendrimer assembled by sequential reactions of ethylene diamine with an acrylate moiety. This dendrimer contains tertiary amine groups in its interior and 64 NH groups on its periphery. When an appropriately functionalized surface (an amine-functionalized SAM on a gold-coated Si wafer) is modified by covalent deposition of Gantrez, the resultant surface contains a reactive anhydride brush that can react with a methanolic solution of this PAMAM dendrimer. The result is a covalent multilayer assembly of partially functionalized dendrimers in a random coil carboxylic acid-containing polymer matrix. This mixture of groups is precludedfromphase separating by the amic acid crosslinks. Moreover, the initial Gantrez layer (Gzl) which is then modified by a dendrimer (Dl) can be further modified because the dendrimer treatment leaves the surface as an aminerich surface - a surface that resembles functionally the starting surface. Thus, repetition of the Gantrez and dendrimer treatments leads to increasingly thicker films. 2

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19 20

2

The supported nanocomposite membrane formed by two such Gantrez/dendrimer treatment cycles is termed a D2 surface. ' When this surface is examined electrochemically in the presence of anionic or cationic electroactive species, interesting pH-dependent permselective behavior is seen because of how the surface structure changes with pH. These changes in surface structure are illustrated by the schematic structures shown in Figure 4 below. At pH 3, the amine groups of the dendrimer are protonated and the carboxylic acid groups are neutral. The result is a cationic surface-supported membrane as illustrated in 19 20


307 Figure 4. Such a membrane is permeable to Fe(CN>6 . However, a cationic redox species, Ru(NH3>6 , cannot permeate through this cationic supported membrane and this species is electrochemically silent. In contrast at pH 11, the carboxylic acid groups are carboxylate groups and anionic and the amine groups are neutral. The result is reversed activity toward the charged redox species with oxidation/reduction being facile for Ru(NH3)6 and Fe(CN)6 " being electrochemically silent. At intermediate pH values, oxidation/reduction occurs for both species. These electrochemical experiments carried out in 3 M NaCl show that responsive surfaces can affect the reactivity of their underlying support toward soluble reagents. 3+


3+

pH3

3

pH11

Figure 4. Reversible changes in a D2 Gantrez/PAMAM nanocomposite membrane with changes in pH. One example of how we have prepared materials that usefully change their reactivity in response to temperature changes is our use of poly(Nisopropylacrylamide) as a support for reactants in catalysis and as a support for catalysts. " The changes in reactivity of a nitroarene bound to p o l y ^ isopropylacrylamide) via an amide bond toward a heterogeneous hydrogenation catalyst (Pt/C) exemplify the concept of using a soluble polymer to imbue into a reaction an anti-Arrhenius response to temperature. While reactions normally run increasingly faster with temperature, this substrate on this polymer in water has reactivity that changes little with temperature in the 0 - 33 °C range. More notably, above 3 9 °C, this nitroarene is unreactive because above this temperature the polymer (and thus the substrate) is insoluble. Temperature-responsive surface chemistry like that seen for 'smart' substrates can also be seen for appropriate surface-functionalized polyethylene samples. In other work, we had noted that polyethylene surfaces can be functionalized with PEG oligomers or with PEG oligomers that are tagged with a pyrene fluorophore. " When a pyrene fluorophore was used, we successfully showed that the resulting polymer/water interface's environment changed in response to changes in temperature. These studies specifically showed that 21

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308 normal and inverse solvation of these interfaces occur with increasing temperature. Our studies also showed that functional groups at thermally responsive interfaces could have reactivity that has an unusual temperature dependence like that of the 'smart' substrates described above. Since the pyrene ester groups and PEG chains collapse at higher temperature in water suspension in a manner qualitatively illustrated in eq. 3, the reactivity of the end groups should be affected. Kinetic studies of ester hydrolysis at these surfaces show that hydrolysis rate of the surface bound pyrene esters decreases slightly at higher temperatures. However, the reaction does not stop as it did in the case of soluble polymer-supported substrates. We speculate that the polyethylene film/solvent interface in this instance changes enough to suppress the hydrolysis reaction rate but that the less hydrophilic interface is still 7


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reactive at higher temperature. It is possible that other thicker functional surfaces could exhibit a more pronounced change in solvation and reactivity. Regardless, the changes in reactivity are notable in that they imply that there could be "surface" reactions that would best be run at low temperature because they could be kinetically slow at high temperature. Conclusion Surfaces that respond to temperature can be designed to serve several roles. Recent work both from our group and others shows that surface hydrophilicity and surface hydration can change with temperature in a manner that resembles the better known changes of soluble polymer solvation with temperature. Moreover, such changes in surface solvation can have an effect on surface reactivity - an effect that can be used to molecularly engineer useful responsive reactivity into a substrate. Likewise pH can significantly affect surface solvation and reactivity. In the case of a simple polyelectrolyte, reversible changes in hydrophilicity and reactivity of the surface induced by pH can be seen. In the case of membranes, changes in permeability as a result of either surface size changes or as a result of surface charge changes can be seen. Since the range of what can be done synthetically at surfaces is now developing rapidly, it seems likely even more sorts of responsive surfaces will be developed. Such materials should find useful roles in areas like adhesion, permeability, biocompatibility, sensor chemistry and corrosion passivation.


309 Acknowledgment Generous support of our work in surface chemistry and catalysis from the National Science Foundation and the Robert A. Welch Foundation is gratefully acknowledged.


References

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21. Bergbreiter, D. E.; Zhang, L.; Mariagnanam, V. M. J. Am. Chem. Soc., 1993, 115, 9295-6. Bergbreiter, D. E.; Mariagnanam, V. M.; Zhang, L. Advanced Materials, 1995, 7, 69-71 22. Bergbreiter, D. E.; Liu, Y.-S. Tetrahedron Lett., 1997, 38, 7843-6. 23. Bergbreiter, D. E.; Case, B. Tetrahedron Lett., submitted for publication. 24. Bergbreiter, D. E.; Caraway, J. W. J. Am. Chem. Soc., 1996, 118, 6092-3. 25. Bergbreiter, D. E.; Srinivas, B. Macromolecules, 1992, 25, 6360-5. 26. Bergbreiter, D. E.; Aguilar, G. J. Polym. Sci., Polym. Chem. Ed., 1995, 33, 1209-1217.


Field Responsive Polymers - ACS Publications - American Chemical

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