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Hybrid Nanomaterials Research: Is It Really Interdisciplinary?

Hybrid Nanomaterials Research: Is It Really Interdisciplinary?

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650



Chapter 22 Emerging Concepts in Interfacial Chemistry of Hybrid Materials



of controlling the properties of substrate coatings and improves performance. Upon

laser light illumination, the absorption centers locally disrupt the shells of the

nanocontainers. The loaded inhibitor is then released, covering and healing the

corrosion area.

To make the containers sensitive to IR laser light, preformed silver nanoparticles

were directly incorporated into the polyelectrolyte shell. For this purpose, AgNPs were

added into solutions of polyelectrolytes for LbL deposition and hence fixed between

the polyelectrolyte layers. The AFM images of the resultant nanocontainerimpregnated film showed a uniform distribution of the containers over the coating,

with the concentration of the silica containers equalling $107 containers per meter

squared.

Opening of the polyelectrolyte – silver SiO2 containers loaded with benzotriazole

was performed in two modes. First, a home-made laser setup was used in which a

collimated pulsed laser (time duration 700 ps, and power 3 mJ, 820 nm) was focused

onto the sample through a microscope objective (100 Â magnification). Images were

recorded by a charge-coupled device camera connected to a computer in transmission

mode using a 150 W light source. In the second mode, an unfocused CW frequencydoubled Nd : YAG laser (operating at 532 nm) with an average incident power up to

150 mW illuminated the sample directly for $30 seconds.

The effects observed during these studies can be attributed to the photothermal

change of the conformation of the polyelectrolyte shell as a consequence of particle

heating induced by laser light absorption, resulting in a switching of the shell to the

open state. The rupture of the containers demonstrates that laser –nanoparticle interaction through thermal processes22 is responsible for container activation. Other

processes, for instance transport of protons or electron redistribution around the nanoparticles, do not activate the containers, because the polyelectrolyte multilayers were

shown to be permeable for protons23 and the local redistribution of electrons cannot

cause the rupture of entire containers. When illumination is conducted in the NIR,

the concentration of nanoparticles and their aggregation state in particular21 play an

important role. Since only NP aggregates show NIR absorption, closely located nanoparticles have to interact to open this wavelength range for use in self-healing

applications. Therefore, higher concentrations of aggregated nanoparticles are

needed for NIR-responsive nanocontainers. The local temperature rise on nanoparticles can be rather high, more than 100 degrees, but it can be controlled and adjusted

(e.g., to just several degrees) simply by tuning the incident light intensity, allowing

induction of truly controlled release of the encapsulated material. Most importantly,

the presence of absorption centers in the shell of a nanocontainer assures that the

temperature rise occurs only locally in the vicinity of the nanoparticle, because the

temperature decreases rapidly on the nanometer scale with increasing distance from

the particle surface,24 assuring that the surrounding environment is unaffected.

Figure 22.5 shows the current map above the surface of the sol – gel film loaded

with NIR-sensitive containers recorded after 24 h of immersion before and after

irradiation with an IR-laser. The corrosion disappears immediately after irradiation

of the local defect area. Thus, the healing of the corrosion defect is induced by

remote IR opening of the inhibitor-loaded containers near the defects. Moreover,

these films are characterized by high anticorrosion ability during aging.



Acknowledgments



651



Figure 22.5 Scanning vibrating electrode (SVET) measurements of the ionic currents above the surface

of a SiOx : ZrOx sol– gel film with inhibitor-loaded containers sensitive to IR irradiation upon 24 hours of

immersion in 0.1 M NaCl before (left) and after (right) local IR-laser irradiation of the corrosion area.



22.4 CONCLUSIONS

Nanocontainers possessing the ability to release encapsulated active materials in a controlled way can be employed to develop a new family of feedback active, in particular

self-healing, anticorrosion coating. Several approaches to fabricate such self-healing

coatings by impregnation of SiOx : ZrOx sol– gel films, with inhibitor-loaded mesoporous SiO2 or TiO2 containers, halloysites or dense SiO2 nanoparticles on aluminum

alloy substrates were demonstrated. All containers were modified with layer-by-layer

assembled polyelectrolyte or polyelectrolyte/nanoparticle shells in order to provide

them permeability properties controlled by changes of pH in a local area or by external

electromagnetic irradiation. Nanomaterials are most useful in this respect because of

their high specific surface area that is available for loading, little mechanical disturbance, and advantageous optical properties, qualifying them especially for applications

that utilize remote light sources. The release of the anticorrosion materials occurs only

when triggered by the corrosion processes themselves, which prevents leakage of

the active component out of the intact coating. Achievements for better control of

the encapsulation/release parameters have been brought about by the progress in the

fabrication of the functional nanocontainers. Moreover, the concept presented in this

chapter can also be transferred to active coatings with several active functionalities

(e.g., antibacterial, anticorrosion, and antistatic) when several types of nanocontainers

loaded with the corresponding active agents are incorporated simultaneously into a

coating matrix. This will surely be a matter of future intense research, which, as a

result, may lead to highly sophisticated hybrid structures.



ACKNOWLEDGMENTS

The authors would like to thank Dr. A. Skirtach and D. Fix for carrying out experimental work presented in this chapter. The research was supported by the

NanoFutur program of the German Ministry for Education and Research (BMBF)

and the Volkswagen Foundation.



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Chapter 22 Emerging Concepts in Interfacial Chemistry of Hybrid Materials



REFERENCES

1. S. R. WHITE, N. R. SOTTOS, P. H. GEUBELLE, J. S. MOORE, M. R. KESSLER, S. R. SRIRAM, E. N. BROWN,

S. VISWANATHAN, Nature 2001, 409, 794–797.

2. W. FENG, S. H. PATEL, M-Y. YOUNG, J. L. ZUNINO, M. XANTHOS, Adv. Polym. Technol. 2007, 26, 1–13.

3. W. LI, L. M. CALLE, Smart Coating for Corrosion Sensing and Protection, Proceedings of the US Army

Corrosion Summit 2006, Clearwater Beach, FL, Feb. 14 16, 2006.

4. S. LEE, M. MUăLLER, R. HEEB, S. ZUăRCHER, S. TOSATTI, M. HEINRICH, F. AMSTAD, S. PECHMANN,

N. D. SPENCER, Tribol. Lett. 2006, 24, 217– 223.

5. B. WESSLING, Adv. Mater. 1994, 6, 226– 228.

6. D. G. SHCHUKIN, M. ZHELUDKEVICH, K. YASAKAU, S. LAMAKA, M. FERREIRA, H. MOăHWALD, Adv. Mater.

2006, 18, 1672–1678.

7. A. N. KHRAMOVA, N. N. VOEVODIN, V. N. BALBYSHEV, M. S. DONLEY, Thin Solid Films 2004, 447–448,

549–557.

8. Y. CHEN, L. JIN, Y. XIE, J. Sol-Gel. Sci. Technol. 1998, 13, 735– 738.

9. M. L. ZHELUDKEVICH, R. SERRA, M. F. MONTEMOR, K. A. YASAKAU, I. M. M. SALVADO, M. G. S. FERREIRA,

Electrochim. Acta 2005, 51, 208– 217.

10. C. SCHMIDT, Anti-Corrosive Coating Including a Filler with a Hollow Cellular Structure, U.S. Patent

6383271, May 7, 2002.

11. R. G. BUCHHEIT, S. B. MAMIDIPALLY, P. SCHMUTZ, H. GUAN, Corrosion 2002, 58, 3 –14.

12. R. B. LEGGAT, W. ZHANG, R. G. BUCHHEIT, S. R. TAYLOR, Corrosion 2002, 58, 322– 328.

13. G. SCHNEIDER, G. DECHER, Nano Lett. 2004, 4, 1833–1839.

14. S. S. SHIRATORI, M. F. RUBNER, Macromolecules 2000, 33, 4213– 4219.

15. S. T. DUBAS, J. B. SCHLENOFF, Langmuir 2001, 17, 7725– 7727.

16. D. M. DELONGCHAMP, P. T. HAMMOND, Chem. Mater. 2003, 15, 1165–1173.

17. D. V. ANDREEVA, D. FIX, D. G. SHCHUKIN, H. MOăHWALD, J. Mater. Chem. 2008, 18, 1738– 1740.

18. S. R. LEVIS, P. B. DEASY, Int. J. Pharm. 2002, 243, 125– 134.

19. R. R. PRICE, B. P. GABER, Y. LVOV, J. Microencapsul. 2001, 18, 713–722.

20. X. LUO, C. ZHA, B. LUTHER-DAVIES, J. Sol-Gel Sci. Technol. 2004, 32, 297– 301.

21. A. G. SKIRTACH, A. MUN˜OZ JAVIER, O. KREFT, K. KOăHLER, A. PIERA ALBEROLA, H. MOăHWALD, W. J. PARAK,

G. B. SUKHORUKOV, Angew. Chem. Int. Ed. 2006, 45, 4612–4617.

22. R. E. HOLMIN, R. F. ISMAGILOV, R. HAAG, V. MUJICA, M. A. RATNER, M. A. RAMPI, G. M. WHITESIDES,

Angew. Chem. Int. Ed. 2001, 40, 2316– 2320.

23. R. VON KLITZING, H. MOăHWALD, Langmuir 1995, 11, 35543559.

24. A. G. SKIRTACH, C. DE´JUGNAT, D. BRAUN, A. S. SUSHA, A. L. ROGACH, G. B. SUKHORUKOV, J. Phys. Chem.

C 2007, 111, 555–564.



Chapter



23



Molecular Schizophrenics:

Switchable Materials with

Multiple Functions

ROBERT BYRNE AND DERMOT DIAMOND

23.1 INTRODUCTION

23.1.1 CHEMICAL SENSING PARADOX



653

655



23.2 FLUID CONTROL USING STIMULI-RESPONSIVE MATERIALS

23.2.1 ELECTRORHEOLOGICAL FLUIDS

23.2.2 CONDUCTING POLYMER ACTUATORS

23.2.3 PHOTORHEOLOGICAL MATERIALS

23.2.4 CHEMICALLY CONTROLLED FLOW SYSTEMS BASED

ON RESPONSIVE HYDROGELS



656

656

657

660



23.3 MOLECULAR RECOGNITION/TRANSDUCTION

23.3.1 ELECTROCHEMICAL SENSORS

23.3.2 OPTICAL SENSORS



665

665

667



23.4 CONCLUSIONS



670



ACKNOWLEDGMENTS



670



REFERENCES



671



663



23.1 INTRODUCTION

Digital communication networks are at the heart of modern society. The digitization

of communications, development of the Internet, and the availability of relatively

inexpensive but powerful mobile computing technologies have established a global

communications network capable of linking billions of people, places, and objects.



The Supramolecular Chemistry of Organic–Inorganic Hybrid Materials. Edited by Knut Rurack and

Ramo´n Martı´nez-Ma´n˜ez

Copyright # 2010 John Wiley & Sons, Inc.



653



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



The move from traditional analog landline to digital mobile phones has been an

important part of this revolution. Inexpensive mobile phones and other wireless technologies, coupled with palmtop PCs and personal digital assistants (PDAs), provide

individuals with communications capabilities unimaginable a decade ago. The

exchange of files containing text, graphics, and media to multiple remote locations

and websites provides a platform for instantaneous notification and dissemination of

information globally. This technology is now pervasive, and those in research and

business have multiple interactions with this digital world every day. However, this

technology might simply be the foundation for the next wave of development that

will provide a seamless interface between the real and digital worlds.

Sensornets are large-scale distributed sensing networks made up of many small

sensing devices equipped with memory, processors, and short-range wireless communications capabilities.1 These devices, known as Motes can gather and share

sensor data from multiple locations through in-built wireless communications capabilities. The vision of incorporating chemical and biological sensing dimensions

into these platforms is very appealing, and the potential applications in areas critical

to society are truly revolutionary.2 For example,





Healthcare: personalized access for individuals, relatives, care givers, and other

specialists to real-time or historical information generated by wearable sensors,

implantable devices, or home-based diagnostics units will facilitate the movement towards home- or community-based healthcare rather than the current,

unsustainable, hospital-centric model in the developed world. In addition,

access to low cost communications and diagnostics will also provide a means

to rapidly improve the delivery of healthcare in less well-developed regions.







Environment: Sensors monitoring air and water quality will be able to provide

early warning of pollution events arising at industrial plants, landfill sites,

reservoirs, and water distribution systems at remote locations. The “environmental nervous system” concept likens the rapid access and response capabilities of widely distributed sensor networks to the human nervous system; that is,

it is able to detect and categorize events as they happen, and organize an appropriate response.

Emergency/disaster and threat detection: Given the increased concern over

terrorist incidents involving chemical, biological, or radiological threats, this

is a major driving force for the development of sensornets, so that such events

can be quickly identified and appropriate action taken to minimize the impact.

However, at the moment, chemical and biological measurements are overwhelmingly post-event, and related to gathering remedial and forensic information.3







The crucial missing part in this scenario is the gateway through which these

worlds will communicate; how can the digital world sense and respond to changes

in the real world? Unfortunately, it would appear from the lack of field deployable

devices in commercial production that attempts to integrate molecular sensor science

into portable devices have failed to bear the fruits promised; this problem is what we

call the chemo-/biosensing paradox.4



23.1 Introduction



23.1.1



655



Chemical Sensing Paradox



Chemo/biosensors must have an active surface incorporating sites that are predesigned to bind with specific target species in order to generate the chemically or

biologically inspired signal; at the same time, these surfaces must be passive to

changes that cause signal drift and loss of sensitivity.

The interactions involved in these binding events can be very subtle, and even

slight changes in the surface or bulk characteristics through processes like leaching,

fouling, or decomposition can have a significant effect on the output signal, and the

overall performance of the device. This is in contrast to physical transducers like

thermistors that can be completely enclosed in a tough protective coating without

inhibiting their ability to function. Hence chemo-/biosensors suffer from baseline

drift and variations (usually reduction) in sensitivity, as well as cross-response to

interferents that may be present in the sample.

Consequently, chemo-/biosensors and analytical instruments must be regularly

calibrated, meaning that the sensing surface is periodically removed from the

sample and exposed to standards, the response characteristics checked, and any baseline drift or change in sensitivity compensated. Therefore autonomous analytical

devices typically incorporate liquid handling for sampling, reagents, and waste,

which requires pumps, valves, and liquid storage. This drives up the complexity,

price, and power requirements, which makes the realization of small, autonomous,

reliable, chemical sensing/biosensing devices impractical at present.

Therefore, the concept of the chemo-/biosensor as a device with an active membrane attached to a pen-like probe is outdated, and needs to be completely rethought.

The key to progress will require breakthroughs arising from new concepts in fundamental materials science, such as the development of adaptive materials that have

externally or locally controllable characteristics. In particular, the issue of how to

predict and control surface characteristics at the interface between the device and

the real world needs fresh thinking, as this is where the molecular interactions that

generate the observed sensor signal happen. In a sense, these materials could be

regarded as having the capability to switch between several completely different

“personalities”—schizophrenics at the molecular level!

This means that analytical and material scientists, working together, can realize

one of the most exciting challenges facing science today, the bridging of the digital

and molecular worlds through the generation of new types of chemo/biosensing platforms. The fundamental requirement of all chemo/biosensors is the need to generate a

selective response to a particular analyte, for example, by means of a selective binding

event as happens in host– guest complexation, enzyme – substrate reactions, antibody– antigen interactions, or other forms of molecular recognition. Much research

focuses on developing a fuller understanding of the basis for molecular recognition,

which may ultimately lead to more selective devices that will be required in futuristic

applications. Coupled with this attention to selectivity is the need to provide a transduction mechanism, so that the binding event can be observed via an electronic or

optical signal. Researchers typically look to electrochemistry (potentiometry, voltammetry, and amperometry) or spectroscopy (visible absorbance and fluorescence) for



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



this signal, which is generated by appropriate redox-active sites or chromophores or

fluorophores, either as part of the molecular sensor itself or as part of a sensing cocktail. Success depends on the molecular binding event triggering transduction of the

signaling moiety without adversely affecting the overall selectivity of the binding

process. In parallel with sensor research, tremendous advances have been made in

the development of compact, portable analytical instruments. For example, labon-a-chip devices enable complex bench processes (sampling, reagent addition, temperature control, and analysis of reaction products) to be incorporated into a compact,

low-power format to provide reliable analytical information within controlled internal

environments.

In this chapter, we shall discuss how sensors and sensing systems are likely to

develop in the coming years, with a particular focus on the critical importance of

new concepts in fundamental materials science to the realization of these futuristic

chemo-/biosensing systems. Due to space limitations, we will focus on a small

number of fundamental challenges, such as the ability to control the characteristics

and behavior of solids (polymers) and fluids, and processes occurring at solid –

liquid interfaces. In particular, we will highlight the key role that stimuli-responsive

materials can play in producing new “adaptive” materials capable of exhibiting

dramatic changes in properties by external stimuli, such as temperature,5,6 an electric

or magnetic field,7–10 photon irradiation,11,12 or specific chemicals (pH, ionic

strength).13,14 For more details, the reader should consult sources like the excellent

recent special issue of Langmuir on Stimuli-Responsive Materials.15



23.2 FLUID CONTROL USING STIMULI-RESPONSIVE

MATERIALS

Conventional solid-state actuators and valves such as peristaltic pumps and solenoid

valves require external power and complex fabrication schemes which limit their

use in many microfluidic applications. Of particular interest at the moment is the

development of low-cost, efficient, polymer-based actuators and valves for sample

handling in microfluidics systems.



23.2.1



Electrorheological Fluids



Electrorheological (ER) fluids are materials whose rheological properties (viscosity,

yield stress, shear modulus, etc.) can be readily controlled using an external electric

field. For example, in some cases, they can switch from a liquid-like material to a

solid-like material within a millisecond with the aid of an electric field, by means

of the so-called ER effect.16,17 The unique feature of the ER effect is that ER fluids

can reversibly and continuously change from a liquid state to a solid state. ER fluid

research is focused mainly on the automotive and robotics industry as electrical and

mechanical interfaces for applications such as clutches, brakes, damping devices,

fuel injection, and hydraulic valves. However, more recently, there is growing



23.2 Fluid Control Using Stimuli-Responsive Materials



657



realization of the potential impact of ER fluids in analytical science, and in particular,

the area of microfluidics.18,19

Three types of ER effects have been observed so far:





The positive ER effect—occurs when the rheological properties of a fluid

increase with applied electric field;







The negative ER effect—occurs when the rheological properties decrease with

applied electric field;







The photo-electrorheological (PER) effect—both the positive and negative ER

effect can be enhanced by UV illumination in some ER systems.



Much effort has been focused on developing high-performance positive ER

materials. A good positive ER fluid should have the following features: (1) a high

yield stress preferably equal to or larger than 5 kPa under an electric field of 2 kV

mm21; (2) a low current density passing through the ER fluid, preferably less than

20 mA cm22; (3) a strong ER effect within a wide temperature range from 2308C

to 1208C; (4) a short response time, usually less than 1023 s; (5) a high stability

and no particle sedimentation or material degradation problems.17,20

Amorphous silicate ceramics are very important ER materials and constitute a

large number of ER fluids.21,22 Aluminosilicates are popularly used to make hydrous

and anhydrous ER fluids with a strong ER effect, the yield stress of such an ER

material can easily reach 10 kPa at 2.5 kV mm21 and a particle loading of 45 wt%.

Recently, Wen et al.23 developed electrorheological suspensions of urea-coated

barium titanyl oxalate nanoparticles that show electrically reversible liquid – solid transitions. The solid state can reach yield strengths in the region of 130 kPa, breaking the

theoretical upper boundary of conventional ER static yield stress.23 For more details

on the structure and mechanisms of ER fluids, readers should consult the recent excellent review by Wen et al.24

Organic and polymeric semiconductive materials also show a strong ER effect.

They are generally electronic conductive materials with a p-conjugated bond structure. It is believed that they have better dispersing ability compared to inorganic

materials. However, the ER effect of organic and polymeric ER fluids is relatively

weak compared to that of aluminosilicate materials.



23.2.2



Conducting Polymer Actuators



Conjugated polymer (CPs) actuators are based on conducting polymers that undergo a

volume change on oxidation or reduction of the material. Multiple mechanisms are

involved in the physical swelling of CPs. The macroscopic volume change of the polymer is due mainly to the combined effect of the insertion of ions and their solvent/

hydration shell, which occupies free space within the polymer matrix. For small and

mobile counteranions, polymer oxidation predominantly results in the penetration

of negatively charged anions from the surrounding electrolyte into the positively

charged polymer, causing the polymer to swell. On reduction, charge on the polymer

backbone is neutralized, electrostatic forces between the polymer – solvent and



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



polymer – counterions become negligible, and polymer – polymer interactions become

strong. As a result, counteranions can diffuse freely across the polymer – electrolyte

interface to electrolyte phase, resulting in polymer shrinkage. At the opposite extreme,

if the counteranion is bulky and immobile, on polymer reduction, these counterions

would be entangled within the polymer network and remain inside the neutralized

polymer. Consequently, charge compensation is dominated by mobile cations from

the electrolyte to balance the entrapped negatively charged counterions, resulting in

polymer expansion. On oxidation, the polymer backbone again becomes positively

charged, promoting the expulsion of these mobile cations from the polymer and resulting in polymer shrinkage. Moreover, there is also an inflow of solvent, governed by

osmosis, which acts to balance the altered ionic concentration inside the polymeric

matrix. Third, conformational changes and coulombic repulsion acts on the existing

polymer backbones to provide an additional contribution to the structural changes.

The degree of volume expansion and its speed is thus dependent on a number of factors. These include ion types and sizes, ionic concentration, the solvent, the thickness

of the CP film, and the external voltage applied. Polyanilines (PANI) and polypyrroles

(PPY), Figure 23.1, and their derivatives, are popular CP actuators, as they can be

prepared easily using simple procedures. These organic polymers are soft and nonabrasive, and have attracted a great deal of attention over the past few years, resulting

in a large number of publications.25,26

Actuators based on low-voltage electrochemical systems, such as CPs, are convenient and safe, and power inputs are potentially low (55 mW).27 One deficiency

of conducting polymers compared to skeletal muscle is their low actuation strain,

less than 15% in the case of CPs. It has become obvious that force generation is limited

by the breaking strength of the actuator material. It has been reported that the maximum stress that can be generated by an actuator is typically 50% of the breaking

stress, so that for PANI fibers, stresses on the order of 190 MPa should be achievable.

However, in practice the breaking strain of CP fibers when immersed in electrolyte

and operated electromechanically are significantly lower than their dry-state strengths.



Figure 23.1 Molecular structure of polyaniline (top) and polypyrrole (bottom).



23.2 Fluid Control Using Stimuli-Responsive Materials



659



The highest reported stress that can be sustained by conducting polymers during actuator work cycles is in the range of 20 to 34 MPa for PPY films. Therefore, devices that

require low strain actuation would be a suitable place to employ these actuators. With

this in mind, we have developed a low powered pump suitable for operating fluid handling within a microfluidic chip. Figure 23.2 shows a fully functioning diaphragm pump

based on a hybrid material of PPY and poly(dimethylsiloxane) (PDMS) (PPYPDMS).28 PPY actuation is due to electrochemical oxidation and reduction of the

membrane that causes it to swell or contract. In this PPY-PDMS diaphragm design,

a PPY membrane was grown on each side of poly(vinylidene fluoride) (PVDF) membrane. The two membranes on each side were connected to a potentiostat such that one

worked as the working electrode and the other as the counter electrode. When electrical voltage is applied, one side swells while the other side contracts to produce maximum mechanical force during actuation. This micropump produced a maximum flow

rate of 52 mL min21 and a nominal minimum flow rate of 18 mL min21 at +1.5 V

supply voltage.29

It has been argued that the low actuating strain can be mechanically amplified

(levers, bellows, hinges, etc.) to produce useful movements, but higher forces are

needed to operate these amplifiers. To improve the mechanical performance of CPs,

single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) have been

added to various CP matrices, and this has produced significant improvements in

strength and stiffness. For example, it has been shown that the modulus of PANI

can be increased by up to four times through the incorporation of relatively small

amounts (,2%) of nanotubes.

Even with their impressive physical changes, electrorheological materials have

not solved the microactuator problem. This is due to the complex fabrication schemes

required to incorporate these materials into microfluidic manifolds. Therefore,



Figure 23.2 Schematic of PPY-PDMS diaphragm pump (above) and photograph of fabricated

microfluidic pump (below). (See color insert.)



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



controlling physical properties with temperature,1,2 photon irradiation,11,12 or specific

chemicals (pH, ionic strength)13,14 would be of great benefit for rapid prototyping.



23.2.3



Photorheological Materials



Chemistry is fundamentally important for the emergence of the new molecular designs

and synthetic schemes that will underpin the development of molecules that are better

able to transduce external stimuli into a reversible change in viscosity, even to the

point of gelation. Some of the most successful approaches in this field employ the

use of photochemistry.

Wolff and Muller30 were among the first to report the ability to switch between sol

and gel states by the conformational changes that can be brought about by irradiation

with light. Their studies concerned the selective production of the unstable 9-methylanthracene by preorganization in micellar solutions of cetyltrimethylammonium

bromide and subsequent irradiation with light (Fig. 23.3).

As a side effect the rheological properties of the solutions also changed when irradiated, which led the authors to investigate further. The addition of 9-methylanthracene

led to a strong enhancement of viscosity, upon photodimerization (l . 300 nm),

viscosity was reduced to approximately half the value. Subsequent irradiation with

light of shorter wavelength (l , 300 nm) led to an increase in the viscosity.

The cis-trans isomerization in azo compounds and stilbene has been used for

switching viscosity.31,32 Shinkai et al. developed a cholesterol-based gelator containing azobenzene; irradiation of the system at 330 to 380 nm led to isomerization of

the trans azo linkage to its cis conformation and thereby disrupted gelation.33

Irradiation of the sol at wavelengths longer than 460 nm resulted in reversal of the

isomerization and re-formation of the gel. Pozzo and coworkers34 employed the



Figure 23.3 Photodimerization of 9-methyl-anthracene leading to head-to-head (top) and head-to-tail

(bottom) dimers.



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