Meeting Report

Getting smarter

As engineers discover applications for ever smaller gadgets, they are seeking ways to pack greater functionality into tinier spaces. One way to achieve this goal is through smart materials—polymers and other materials that respond to external stimuli. For some materials and applications, promoting "intelligence" is still in the basic research stage—understanding how materials behave in certain physical and chemical environments. In other cases, smart materials are already on the market.

An April 23, 2009, meeting of the Academy's Soft Materials Discussion Group, organized by Suresh Rajaraman of Air Products and Chemicals, focused on some basic research that is helping to develop smart materials and on several applications that have already reached consumers. Eric Dufresne of Yale University described research exploring electrostatic interactions between molecules in different environments. Steven P. Bitler of Landec Corporation described uses of thermally responsive polymers to solve problems in areas such as food packaging, adhesives, drug delivery, and seed coatings.

Understanding long-range electrostatics

Electrostatic interactions are important throughout the biological world. For example, they shuttle information between neurons in the brain, and electrostatic attractions and repulsions support both the structure and function of biological molecules. Such interactions are also important within colloids.

Colloidal materials are mixtures containing undissolved but evenly dispersed particles. For the last century, such mixtures have been used in consumer products such as pigments, personal care products, and medical diagnostics. To design new colloidal materials with smart properties, researchers need to understand both how interactions between the particles in the mixtures change in different environments and how to control those interactions.

Particles within colloids can aggregate or fall out of suspension, but researchers have discovered that controlling charged interactions is one way to stabilize these mixtures and build new applications. One recent smart technology involving electrostatics has been E-ink, the technology now used in the Amazon Kindle. The screen pixels are made with white and black particles with oppositely signed charges. Applying an electric field makes a pixel either black or white and with a page "turn" another electric field can flip the sign.

Building new materials based on electrostatic interactions first means understanding the details of these forces, Dufresne says. Studying those interactions requires both creativity in figuring out how to measure the forces at work between particles and then careful analysis of what those results mean in a microscopic system.

Measuring tiny forces

Electrostatic interactions between particles decrease as the distance between them increases. In a vacuum, that fall-off is slow. But in everyday systems such as water solutions, or physiological conditions containing water and salts, other charges within a liquid environment can have significant impact, screening the effect of an individual charge of interest. Therefore the distance over which an electrostatic force can act—the Debye screening length—can be as little as 1 nm (the size of a small molecule) under physiological conditions; in typical deionized water the distance is probably only 100 times greater.

In addition to occurring at such small distances, the forces between these particles are miniscule, measured in femtoNewtons (fN). All mechanical measurements of force require careful calibrations, and even on a macroscale, measuring a force through Hooke's law—the tug of an object on a spring—means that the force of the spring has to be taken into account. Looking at small scales, surface forces can be measured in microNewtons and atomic forces in nanoNewtons, while optical tweezers can reach picoNewtons.

At the femtoNewton scale, thermal fluctuations (Brownian motion) can hide these forces. But by setting up rapidly blinking optical tweezers (20–40 times per second) and acquiring large amounts of data (100–1000 times per second), Dufresne and his colleagues can produce Gaussian-shaped probability curves that allow them to measure both the velocity and the diffusion coefficient and then calculate the force between the particles.

The strange world of long-range forces

To gain new insights about how the electrostatic effects operate at longer ranges, Dufresne, graduate student Jason Merrill, and former postdoctoral researcher Sunil Sainis have been examining interactions between pairs and small groups of particles in oil-based solvents. As a model system they use plastic spheres made from polymethylmethacrylate (1.2 µm diameter) in hexadecane. To vary the electrostatic environment, they vary concentrations of a surfactant, aerosol-OT.

When they compared mixtures with two different surfactant concentrations (1 mM AOT compared to 10 mM AOT), they found that the mixture with higher surfactant concentration behaved as they expected, forming a regular lattice type structure. However, in a system with a lower surfactant concentration, where the electrostatic repulsions between particles should have been stronger and had a longer range, the particles did not form a lattice. Instead they aggregated, forming highly irregular structures. This behavior suggested the interactions between the particles were not pairwise additive and charge did not accumulate simply based on the number of particles in the system.

The Three Body System: Plastic spheres act as batteries and modulate their charge to maintain a constant potential.

Dufresne and his colleagues examined pairs and increasingly larger groupings of particles to understand the electrostatic interactions at work in this system. They first looked at pairs of particles under these two different sets of conditions: lower and higher surfactant concentrations with longer and shorter Debye screening lengths. They then added a third particle to the system to test whether the forces added up pairwise. The system with the longer Debye screening length—several times larger than the radius of the particle—had electrostatic forces that were significantly lower than those predicted for a pair wise interaction. In other words, in this system, charges of one plus one do not add up to equal two.

The anomalies aren't due to nonlinearity or a breakdown in the mean field theory, Dufresne says. Their observations can be explained, however, if they look at the plastic particles as tiny conducting spheres that act like batteries. As those charges get close together, the spheres modulate their charge to keep a constant electrostatic potential. Therefore, adding up the charges overestimates the amount of force experienced by each sphere. The effects are greater for 3 particles compared to 2 particles, and Dufresne and his colleagues are examining what their results mean for bulk systems. That understanding could lead to new principles for building smart materials.

Thermal polymers

Researchers must study many interactions in materials systems before bringing a product to market. Steven Bitler, his colleagues at Landec Corporation, and their partners have had several successes in integrating temperature-responsive polymers into everyday products.

The polymers at the core of Landec's technology are based on designs for side chain crystalline polymers developed as academic curiosities by E.F. Jordan, Jr. of the U.S. Department of Agriculture, and Shibaev and Platé in the former Soviet Union in the 1950s and 1960s, Bitler said. Unlike typical polymers whose melting temperature gradually increases with molecular weight, these materials show a sharp melting transition that is not based on molecular weight. Side chain crystalline polymers are typically constructed with an acrylic backbone with long hydrophobic side chains (such as a steryl group with 18 carbon atoms). The liquid crystalline properties of these polymers result from the structure of these long hydrophobic side chains, which line up like a comb. By inserting a variety of co-monomers, researchers can modulate the polymer properties. Landec has been able to tune the polymer construction so that the melting temperature can be anywhere between 0° C and 70° C.

Lengthening food shelf life

One application of Landec's work has come in breathable membranes for vegetable packaging. Maximizing the freshness of cut vegetables is often a matter of maintaining an optimal balance of gases such as oxygen and carbon dioxide within plastic packaging. However, cut vegetables continue to respire, so simply sealing them in plastic isn't the best solution. A breathable membrane in the packaging that allows air within the package to exchange with air in the environment can maintain that balance.

The optimal oxygen and carbon dioxide balance for maintaining freshness varies among fruits and vegetables.

For example, the optimal balance for maintaining the freshness of broccoli florets is an environment with 3% oxygen and 8% carbon dioxide, according to research from the University of California, Davis. Broccoli respires more slowly at lower temperatures, requiring lower membrane permeability, and increases rapidly at higher temperatures, requiring greater gas exchange between the interior and exterior of the package. Landec used thermal polymers create a smart membrane that covers a hole in the plastic bag. The simple membrane is then coated with a thermally responsive polymer designed to switch at 10° C, the temperature at which the respiration rate of cut broccoli changes.

Working with Chiquita, Landec has also produced packaging that minimizes the browning of bananas. Their smart wrap on individual bananas allows the company to sell them individually as healthy snacks and make more money per banana.

Thermosets and adhesives

Mixing is critical to having the best adhesion with thermoset polymers, materials that irreversibly cure based on temperature. But premixing these materials often means that they have to be stored cold to prevent the catalytic reaction from occurring early.

Working with Air Products, Landec has produced thermally responsive polymers, such as epoxy, with good adhesive properties and longer shelf life. A thermally responsive polymer called Intelimer 6050 traps a cobalt catalyst within a crystalline polymer so that it's not activated until the material is warmed to a specific curing temperature.

Polymer encapsulates the catalyst, but then releases it when heated to a specific temperature.

Construction crews use these smart thermoset polymers to repair breaks in underground water pipe without digging them up, Bitler says. The cracked pipe can be lined with the polymer mix, and then they cure it by filling the pipe with steam to repair the breach.Landec has also exploited the sharp temperature transition of side chain crystalline polymers to develop a heat-sensitive adhesive that's tacky at one temperature but allows materials to be peeled off as it cools. Landec and Nitta Corporation in Japan have developed one application of these quick release adhesives for polishing the silicon wafers and ceramic electronic components of cell phones, which are the size of a grain of salt.

Controlling germination and drug delivery

One of the most important decisions a farmer makes each year is when to plant a crop. If the seeds are in the ground too early, they might rot before they germinate. If he waits and plants later, crop yields are often lower. A thermally responsive polymer called Intelicoat (developed with Monsanto) extends the time window for planting. Seeds covered in the polymer are protected in the ground, but at the germination temperature of 18° C, the coating melts. Other similar applications coat male and female seed types with different polymers to help farmers balance cross pollination.

Landec is also developing polymers for drug delivery that have both a temperature and pH trigger. Such polymers can help delay the activity of drugs until they reach their target and eliminate burst—the fast release of active molecules on the surface of a pill or device. The pH trigger helps protect drugs against stomach acid, and slow release polymers could allow patients to take necessary medications less frequently.

Understanding the chemical and physical interactions of polymeric materials is increasingly important for the design of new devices and for improving existing products. Whether it's sorting out electrostatics or tuning polymer melting temperatures, smart materials research promises many new discoveries and applications.

Open Questions

What are the implications of the non-pairwise additivity of long-range electrostatic interactions in bulk materials?

What new applications could be based on materials with these unusual electrostatic properties?

How versatile are side chain crystalline polymers? Is it possible to create polymers with many different environmental switches?

How much can these polymers optimize drug delivery? How widely applicable is the sustained release technology for drugs?