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It is well established that materials, and particularly polymeric materials, may serve many functions. These functions range from life-saving medical devices to paints protecting cars from rusting, or plastic utensils that rapidly fill up landfill sites, to name just a few. Regardless of their function, there is a global need for functional materials, devices or complex systems to be either fully sustainable or entirely degradable. This chapter outlines fundamental principles governing the role of physical states and dimensional changes in chemical and physical stimuli-responsive processes. While the complexity of sequential and concurrent processes that occur during vision are used as an example, energetic requirements for solutions, surfaces and interfaces, gels and solids set the stage for the remaining chapters.

It is well established that materials, and particularly polymeric materials, may serve many functions. These functions range from life-saving medical devices to paints that protect cars from rusting, or plastic utensils that rapidly fill up our landfill sites, to name just a few. Regardless of their function, there is a global need for functional materials, devices or complex systems to be either fully sustainable or entirely degradable. Making an object sustainable implies that all initial functions will be retained during its lifetime, but at some point it will lose its functions. While one term does not exclude the other, making materials degradable requires that at the end of its useful life, a given object will be turned into a new product of equal if not greater value. This concept, known as the cradle-to-cradle approach, seeks to create functional materials, devices or systems in such a way that they are not only efficient, but also waste-free. While it is environmentally advantageous to design all materials using the cradle-to-cradle approach, it is even more desirable to incorporate into materials design active functions that will be manifested by their property changes. For example, on a sunny hot day, it would be desirable to have a house with a white roof in order to minimize absorption of the sun's rays so the house would stay cool. But in winter we want the same white roof to change color to black in order to absorb sunlight to keep the house warmer. Thus, there is a stimulus, sun radiation, and there is a response, the change of color from white to black. If this process can be repeated many times, we have a roof that is stimuli-responsive to the external source of energy. Can internal sources lead to stimuli? If there are internally built mechanical stresses or concentration gradients within a given material, molecular relaxations may lead to responses manifested by crack formation, which typically results from the non-equilibrium state of matter.

Although Mother Nature provides multiple inspirations for the design and development of new materials, creating synthetic systems capable of responding to stimuli in a controllable and predictable fashion presents significant but fascinating challenges. Particular challenges lie in mimicking biological systems where structural and compositional gradients at various length scales are necessary for orchestrated and orderly responsive behaviors. To tackle these challenges several stimuli-responsive systems have been developed, with the majority of studies dealing with polymeric solutions, gels, surfaces and interfaces, and to some extent, polymeric solids. These states of matter impose different degrees of restriction on the mobility of polymeric segments or chains, thus making dimensional responsiveness easily attainable for systems with a higher solvent content and minimal energy inputs. Significantly greater challenges exist when designing chemically or physically crosslinked gels and solid polymeric networks that require maintenance of their mechanical integrity. Restricted mobility within the network results from significant spatial limitations, thus imposing limits on obtaining stimuli responsiveness. The challenge in designing these stimuli-responsive polymeric systems is to create networks capable of inducing minute molecular yet orchestrated changes that lead to significant physicochemical responses upon external or internal stimuli.

To illustrate spatial restrictions on mobility in the x-, y- and z-directions in solutions, at surfaces and interfaces, in gels, and in solids, Figure 1.1 is a schematic diagram of the four states and relative dimensional restrictions within each state.1  When going from a solution phase to surfaces and interfaces, or gels to solid state phases, segmental mobilities of polymer chains decrease due to significant spatial restrictions manifested by smaller displacement vectors in the x-, y- and z-directions. Consequently, the energetic requirements for responses to temperature, mechanical stimuli, electromagnetic irradiation, or electrochemical stimuli, pH, ionic strength, or bioactive species will be different for each physical state. Examples of responses are depicted in Figure 1.1 and are classified into chemical and physical categories, where multiple stimuli may result in one or more responses, or one stimulus may result in more than one response. Because spatial restrictions also dictate energetic requirements, the next section will discuss these relationships.

Figure 1.1

Physical state and dimensional changes in chemical and physical stimuli-responsive processes. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

Figure 1.1

Physical state and dimensional changes in chemical and physical stimuli-responsive processes. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

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How do our eyes work, and why we are able to see? Why do flowers bloom every spring? Why does self-healing of wounds occur in mammals and plants? How do we age? Why do living organisms metabolize and materials do not? Why are grizzly bears able to hibernate in the winter and humans cannot? Some of these questions might be obvious, but for the most part they are not. Why? The majority of these processes are only partially understood and the complexity arises from the lack of correlations between chemical reactions and physical processes responsible for the overall outcome. Another way of looking at it is that using chemically the same material to make different objects will require different physical designs. Making a chair for a child and an adult from the same material will require different designs. On the contrary, using the same design for a child and an adult may require different materials. To take a closer look at stimuli responsiveness let us consider the processes involved in vision. To see we need an object, visible light and a detector. This detector is a human eye, which can only sense reflected rays when an object is illuminated by light – in darkness we cannot see. Some may consider the eye as a camera; but is it really that simple? As depicted in Figure 1.2 the human eye has many components, which act in a synchronized manner. This rather remarkable device is capable of adjusting to various distances and illumination conditions, converting light signals to impulses and transmitting them to the brain where an image is created.

Figure 1.2

The complexity of a human eye structure. (Eyeball cross-section image © 1989–2001 by Lippincott Williams & Wilkins, Baltimore, MD.)

Figure 1.2

The complexity of a human eye structure. (Eyeball cross-section image © 1989–2001 by Lippincott Williams & Wilkins, Baltimore, MD.)

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To realize the interplay of chemical and physical processes let us now consider selected chemical processes responsible for the basic physical eye function, vision.

Figure 1.2 illustrates the complex structure of a human eye. It is covered by a white, tough wall called the episclera, a fibrous layer between the conjunctiva and sclera. The eye muscles are connected to the conjunctiva. The cavity in the front part of the eye, between the lens and cornea, is the anterior chamber, which is filled with aqueous fluid that is recirculated every 100 minutes; this is produced by the ciliary body and drains back into the blood circulation through channels in the chamber angle. The structure behind the iris (mostly invisible), which produces the fluids filling the front part of the eye and maintains the eye pressure, is the ciliary body. Another important function of this part of the eye is to facilitate focusing. The iris acts like the diaphragm of a camera (and is responsible for the color of the eyes), allowing only a certain amount of light to enter the eye by dilating and constricting the pupil. The pupil is the dark opening in the center of the colored iris controlling how much light enters the eye. Immediately behind the iris is the lens, which is responsible for focusing light rays onto the retina. The white part of the eye, which is a thin lining over the sclera and inside the eyelids, is the conjunctiva. The cells of the conjunctiva produce mucous, which lubricates the eye. The primary focusing element of the eye is the cornea, also known as the epithelium. It is made of transparent cells capable of rapid regeneration. Its inner layer is transparent, allowing light to pass through. The narrow channel that runs from the optic disc to the back surface of the lens is called the hyaloid canal, and the body of the eye is filled with a jelly-like clear substance called vitreous humor. The retina is a layer of membrane lining the back of the eye. It contains photoreceptor cells that react to light intensity by sending impulses to the brain via the optic nerve. The light passes through the cornea and lens, creating an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. The macula is the most sensitive part of the retina and is responsible for the central (or reading) vision. Being near the optic nerve, directly at the back of the eye, it is also responsible for color vision. The optic disc is located in the back of the eye where the nerve, along with arteries and veins, enters the eye. The optic nerve consists of a bundle of a million nerves and is responsible for transferring information from the retina as electrical signals and delivering it to the brain, where this information is processed as a visual image.

There are two types of photoreceptor cell within the retina layer: rods and cones. As the light hits the photoreceptor cells, the first step is for chromophore 11-cis-retinal to isomerize to all-trans-retinal. The protein that is covalently bonded to 11-cis-retinal is opsin, and the complex 11-cis-retinal–opsin is known as rhodopsin. Opsin has seven hydrophobic α-helical regions that pass through the lipid membrane of the pigment-containing discs (Figure 1.3), forming a hydrophobic pocket where 11-cis-retinal resides. In the dark, 11-cis-retinal binds the opsin as an inverse agonist and holds it in an inactive conformation. When light strikes the visual pigment, the isomerization of 11-cis-retinal to all-trans-retinal in the binding pocket pushes the opsin into an active conformation and initiates phototransduction, which ultimately leads to the generation of nerve impulses. The maximum absorbance radiation for rhodopsin is ∼500 nm. Phototransduction is the most impressive signal transduction cascade leading to the generation of a nerve impulse to the brain. The changes in rhodopsin activate transducin, which, in turn, activates another enzyme, phosphodiesterase. The latter catalyzes the hydrolysis of cyclic GMP, as shown in Figure 1.4. Hydrolysis of cyclic GMP will subsequently close the Na+ channels. In the dark, the Na+ channels are open. The influx of Na+ is compensated for by an outflux of potassium ions (K+) through the K+ channel, resulting in depolarization of photoreceptors in the dark. The depolarized state of the membrane will trigger continual transmitter release from the synaptic terminals of the photoreceptor cells. Under light, Na+ channels are closed so that a large charge difference across the rod's outer membrane can build up, caused by the outflow of K+, a process known as membrane hyperpolarization, which decreases the release of neurotransmitters. Meanwhile, after all-trans-retinal is formed, it is released from opsin within nanoseconds (10−9 s). Because opsin itself cannot absorb light, 11-cis-retinal needs to be regenerated and bind to opsin again. This is accomplished in the retinal pigment epithelium (RPE) via the visual cycle (Figure 1.4). RPE is a single layer of cells located between the retina and sclera. Following isomerization and release from the opsin protein, all-trans-retinal is reduced to all-trans-retinol and travels back to the RPE to be ‘recharged’. It is first esterified by lecithin retinol acyltransferase (LRAT) and then converted to 11-cis-retinol by the isomerohydrolase RPE65. The isomerase activity of RPE65 has been shown; it is still uncertain whether it also acts as a hydrolase. Finally, it is oxidized to 11-cis-retinal before traveling back to the rod outer segment where it is again conjugated to an opsin to form a new, functional visual pigment (rhodopsin).

Figure 1.3

Schematic representation of the activation of rod phototransduction. Upon photon absorption, activated rhodopsin (R*) activates heterotrimeric G protein, and catalyzes the exchange of GDP for GTP, thus producing active Gα*-GTP. Two Gα*-GTP bind to the two inhibitory subunits of PDE, thereby releasing the inhibition on the catalytic α and β subunits, and forms PDE*. This, in turn, catalyzes the hydrolysis of cGMP. The consequent decrease in the cytoplasmic free cGMP concentration leads to the closure of the cGMP-gated channels on the plasma membrane and blockage of the influx of cations into the outer segments, which results in reduction of the circulating dark current. Reproduced from ref. 2. © 2015 Webvision.

Figure 1.3

Schematic representation of the activation of rod phototransduction. Upon photon absorption, activated rhodopsin (R*) activates heterotrimeric G protein, and catalyzes the exchange of GDP for GTP, thus producing active Gα*-GTP. Two Gα*-GTP bind to the two inhibitory subunits of PDE, thereby releasing the inhibition on the catalytic α and β subunits, and forms PDE*. This, in turn, catalyzes the hydrolysis of cGMP. The consequent decrease in the cytoplasmic free cGMP concentration leads to the closure of the cGMP-gated channels on the plasma membrane and blockage of the influx of cations into the outer segments, which results in reduction of the circulating dark current. Reproduced from ref. 2. © 2015 Webvision.

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Figure 1.4

Anticipated chemical reactions during the visual cycle of a human eye.

Figure 1.4

Anticipated chemical reactions during the visual cycle of a human eye.

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As shown in Figure 1.5, when a photon of a visible portion of electromagnetic radiation is absorbed, 11-cis-retinal undergoes all-trans-retinal configuration. As a result, the size and the shape of this molecule is altered. This rather sketchy physical description of an eye and the complexity of the chemical reactions leading to vision clearly illustrate how illumination of light leads to the responses of this remarkable device that allow us to see. It is quite apparent from Figure 1.1 that physical processes are driven by minute chemical changes that must occur in an orchestrated and sequential manner. One also needs to keep in mind that the event connectivity is time-dependent. Because ‘cause’ and ‘effect’ are typically connected or separated in time by intermediate processes, in designing stimuli-responsive materials one needs to identify what causes and links exist in a given time frame. If an event A is the cause and an event B is the effect, ‘cause A’ and the ‘effect B’ may be related by transient overlap or entirely separated, but connected in a hierarchical manner. There are a few fundamental classes of causes, which can be applicable to chemical processes or physical events in materials (Figures 1.6 and 1.7).

Figure 1.5

Upon absorption of electromagnetic radiation, 11-cis-retinal undergoes all-trans-retinal configuration.

Figure 1.5

Upon absorption of electromagnetic radiation, 11-cis-retinal undergoes all-trans-retinal configuration.

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Figure 1.6

Reaction of 11-cis-retinal to opsin.

Figure 1.6

Reaction of 11-cis-retinal to opsin.

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Figure 1.7

The addition of a water molecule (red) to GMP and breaking the bond between the phosphate group (blue) and carbon using a phosphor-diesterase catalyzed process leads to hydrolysis of cyclic GMP.

Figure 1.7

The addition of a water molecule (red) to GMP and breaking the bond between the phosphate group (blue) and carbon using a phosphor-diesterase catalyzed process leads to hydrolysis of cyclic GMP.

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To illustrate energy relationships in the context of stimuli responsiveness for a given state, Figure 1.8 depicts a series of equilibrium energy diagrams as well as the relative stimuli energy input as a function of response for each state. The main features of each diagram are the magnitude of the energy required to undergo transitions from one state to another, while maintaining chemical/physical integrity and functionality of a given state represented by the equilibrium energy (ΔEeq). For materials to exhibit stimuli responsiveness it is essential to maintain the equilibrium state in order to preserve the functionality and, at the same time, create a physical and/or chemical environment that will require significantly lower amounts of energy (ΔESR) for a system to undergo stimuli-responsive transitions. The latter is represented by two usually smaller metastable energy minima. Transitions between these minima will represent the energy required for a system to go from one responsive state to another, while maintaining the ‘functional’ equilibrium. Examples of such stimuli-responsive transitions are molecular cistrans rearrangements or other conformational changes, hydrogen bonding-induced rearrangements, aggregation–dissociation, penetration–separation, order–disorder transitions, or protonation–deprotonation. These lower energy transitions may or may not be reversible, and their energy requirements will depend on initial physical and chemical states. The main challenge, however, is to obtain responsiveness at longer length scales while maintaining mechanical integrity of the network.

Figure 1.8

Equilibria energy for stimuli responsiveness in solutions. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

Figure 1.8

Equilibria energy for stimuli responsiveness in solutions. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

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Stimuli-responsive behavior is easily obtained in polymeric solutions because the Brownian motion of solvent molecules requires relatively low energies for macromolecular segments to displace solvent molecules. For an ideal system, the kinetic energy required for the Brownian motion at room temperature is 1.5 kBT (where kB is the Boltzmann constant (1.38×10−23 J K−1), and T is the temperature).3,4  To illustrate stimuli-responsive transitions in solutions, Figure 1.8 depicts two energy minima, A and B, where in order for a given polymeric solution to go from energy state A to B, a certain amount of energy ΔESR(solution) is required. One example of these transitions in polymeric solutions is the lower critical solution temperature (LCST),5  which is the lowest temperature of phase separation on the concentration–temperature phase diagrams. Below the LCST, polymer chains and solvent molecules are in one homogenous mixing phase, and exhibit favorable free energy (ΔG<0), which is believed to be facilitated by hydrogen bonding interactions between the two phases. Above the LCST, a phase separation occurs as enthalpic (ΔH) energy overcomes the entropic (ΔS) contributions resulting in unfavorable free energy (ΔG>0) of the entire system. For the majority of polymeric solutions,6–11  temperature-induced LCST transitions result in a particle or aggregate size decrease above the LCST, and the reported size changes are substantial, typically in the range from 250–3000 nm below the LCST, to 100–1000 nm above. For polymeric solutions that exhibit LCST behavior, the relationship between the solution equilibrium energy (z-axis), the stimulus energy input (x-axis) and physical/chemical response (y-axis) is depicted in Figure 1.8. Initially, the system is in the equilibrium energy state A, and as a result of the stimulus energy input, such as temperature A′→B′, the system undergoes transition to the equilibrium state B. As a consequence, the system will go from the physical state A″ below the LCST to a new state B″ above the LCST, manifested by a physical response in the form of collapse of the particles or aggregate size changes.

As illustrated in Figure 1.1, the mobility of responsive chains on surfaces is restricted and depends on many factors. Due to the anchoring of one end of a polymeric segment to the surface, restricted freedom of movement is ‘transmitted’ along the chain. If we define the stimuli-responsive surface as a point on the anchored chain, the energy required for the segments further away from the anchoring point to respond is a function of the distance from the surface. Figure 1.9 illustrates the relationship between the surface layer equilibrium energy (z-axis), stimulus energy input (x-axis) and a physical/chemical response to a stimulus (y-axis). For segments that are close to the anchoring points of the surface, relatively higher energy (ΔESR(surface)) input will be required to undergo A to B transitions, because more space and free volume are available further away from the anchored point, thus providing energetically and spatially favorable rearrangement conditions. This is illustrated in Figure 1.9, which depicts the magnitude changes between the energy minima A and B for segments close to the surfaces (anchored ends) and those further away (free ends). This will also be reflected in the Tg changes as a function of distance from the anchoring point at the surface. The first experimental evidence manifesting the importance of the Tg changes as a function of distance from the surface showed that the Tg values for 1000 nm thick poly(methyl methacrylate) (PMMA) films vary by as much as 20 K for the 25 nm thick layer at the free surface compared to the same layer in the bulk.12  Generally, stimuli responsiveness at polymeric surfaces is an entropy (ΔS) driven process,13  in which the disorder (mobility fluctuation) of anchored chains (ΔS) has greater contributions to the ΔG values than the conformational changes resulting from the enthalpic (ΔH) component of the free energy. Examples of this behavior are switching of surface wettability,14,15  which involves transformation from one equilibrium state to another (A→B) in response to the external energy input (A′ to B′), resulting in physical/chemical state changes from A″ to B″.

Figure 1.9

Equilibria energy for stimuli-responsiveness at surfaces and interfaces. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

Figure 1.9

Equilibria energy for stimuli-responsiveness at surfaces and interfaces. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

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The energy diagram for polymer interfaces is illustrated in Figure 1.9, which in principle depicts similar energetic behavior, but the presence of two anchor points forming an interface will reduce chain mobility and consequently stimuli responsiveness. The energy input required for the stimuli-responsive A→B transitions will be significantly greater than for the surfaces, and will be a function of molecular distance between the two surface anchoring points. This is schematically depicted in Figure 1.9. One of the challenges will be to design stimuli-responsive interfaces with various molecular structures at the interfacial regions that exhibit stimuli responsiveness. While each chain of the interface must contain stimuli-responsive components, it will also be necessary to control the interfacial chain lengths, molecular weight of chains, and chain stiffness. Thus, incorporating well-defined responsive copolymers with desired polydispersity index (PDI) into the interfacial regions using precision polymerizations will be essential (Chapter 2). As shown in Figure 1.9, molecular weight of the interfacial chains will have a significant effect on responsiveness. Another scenario will be to control interfacial entities with higher PDI where the shorter chains are responsible for mechanical integrity of the interface whereas the longer chains will serve as stimuli-responsive components.

Stimuli-responsive gels are typically formed by physical and/or chemical crosslinking, or by supramolecular associations of molecular chains dispersed in solvents. The forces driving the molecular makeup of gels are covalent bonds16  and non-covalent interactions, such as hydrogen bonding, hydrophobic or van der Waals interactions, and π–π stacking.17  As a polymer matrix is tied by crosslinked points or entanglements, relatively stable hydrogels and/or organogels with maintained bulk structures are achieved. However, because gels are usually porous or solvent-containing networks, their integrity may be destroyed when responsive dimensional changes (deformation) occur. Typically, dimensional changes within stimuli-responsive gels are at micro-scale levels because responses occur along the network components, which was observed in PNIPAAm gels by utilizing laser scanning confocal microscopy (LSCM).18  In contrast, for dispersed polymer chains in solutions, dimensional changes are at the nano-range levels. The relationship between the gel equilibrium energy (z-axis), stimuli energy input (x-axis) and physical/chemical response (y-axis) is illustrated in Figure 1.10. If we consider again that the equilibrium energy (ΔEeq(gel)) is responsible for gel network stability, which exhibits the higher energy difference between the minimum E and the higher energy states, two metastable energy minima A and B will provide stimuli responsiveness. This is controlled by the entropic component (ΔS), as ΔH is smaller and contributes to a lesser degree to overall ΔG values. One example is the expansion and shrinkage of hydrogel networks which result in the changes of physical and/or chemical state (A″→B″) when the stimuli energy is delivered, resulting in A′ to B′ transitions.

Figure 1.10

Equilibria energy for stimuli-responsiveness in gels. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

Figure 1.10

Equilibria energy for stimuli-responsiveness in gels. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

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Due to spatial limitations resulting from a limited access to free volume, creating stimuli-responsive solid polymeric networks represents a great challenge. Similarly to solutions, surfaces and gels, Figure 1.11 illustrates the relationship between the equilibrium energy (z-axis), stimuli energy input (x-axis) and chemical/physical response (y-axis). Similar to polymeric gels, the minimum energy at equilibrium (E) provides stability of the network, whereas metastable energy states A and B facilitate stimuli responsiveness. In polymeric solids, the ΔEeq(solid) at equilibrium is significantly greater compared to solutions, surfaces and gels, thus resulting in a greater network integrity due to tightly entangled or crosslinked polymeric chains. As a consequence, the Tg is also relatively higher compared to other states. However, in solid polymeric networks, the entropic term (ΔS) does not contribute significantly to stimuli responsiveness because spatial mobility in these tighter networks is restricted.

Figure 1.11

Equilibria energy for stimuli-responsiveness in solids. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

Figure 1.11

Equilibria energy for stimuli-responsiveness in solids. Reprinted from ref. 1, Copyright (2010), with permission from Elsevier.

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