Chapter 1. Introduction
1.3 Stimuli-responsive polymers with micro/nanostructures
Among many smart polymers, such as temperature, pH, light, electro-responsive polymers, thermoresponsive polymers are the most widely studied type (Figure 1.7).49 First, the thermoresponsive polymers have LCST or UCST, and the miscibility of polymer chains and solvent molecules change around the transition temperature due to the thermodynamic principles.14 For example, PNIPAM has the LCST of ~32 °C; it changes the chain structure between the random coil and the compact globule reversibly as temperature changes around the LCST.15 Above LCST, the enthalpic energy overcomes the entropic energy contributions, which makes a phase separation.
Therefore, the thermoresponsive polymers can be used as temperature sensors and thermoresponsive actuators. In addition, the transition temperature can be increased or decreased by adding more hydrophilic or hydrophobic comonomers.50
Second, pH-responsive polymers have ionizable functional groups for donation or acceptance of protons according to the surrounding pH condition.16 The pH-responsive polymers can be divided into polyacids and polybases based on their characteristics. Polyacids, for example, PAAc and PMAAc, release protons at high pH. In contrast, polybases, such as PEI, poly(vinyl pyridine) (PVP), and poly(N,N’-dimethyl aminoethyl methacrylate) (PDMAEMA)), accept protons at low pH conditions.
The solvation states of polymer chains change around the pK values, so pH sensors use them to measure the pH value through the electrical signal.
Light-responsive polymers have a noticeable advantage compared to other responsive polymers, which is a remote-controllable property.49 One example of light-responsive polymer is azobenzene- incorporated polymers, they show trans-cis isomerization and even volume change under UV light.17 Another example is spiropyran; it is ionized when exposed to UV light, which induces electrostatic repulsion between the ionic molecules.18 Its structural change leads to color changes between colorless and colored states. In the same way, a leuco dye also exhibits color changes by the electromagnetic radiation.51 The other type of light-responsive polymer is cinnamate, which is dimerized upon UV light irradiation.52 By controlling the light frequency, light-responsive actuators can exhibit fast change between on and off states.53
Electro-responsive polymers, such as conducting polymers and dielectric elastomers, change their physical or/and chemical properties under the electric field. Conducting polymers including PANI, PPy, and poly(3,4-ethylenedioxythiphene) (PEDOT) have conjugated double bonds in the polymer backbone, and the polymers become conductive after removing or adding an electron.54 They can be used as electrochemical actuators because they expand and contract by reduction and oxidation, respectively.19 Additionally, a dielectric elastomer film sandwiched between two compliant electrodes
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has an electroactive property.20 As applying a voltage, the electrostatic force induces the expansion of elastomer layer by squeezing it. Ecoflex is a representative dielectric elastomer that is widely used as the electro-responsive actuators due to its high flexibility.
Multistimuli-responsive polymers have responsive properties upon various stimuli. For example, spiropyran and leuco dye exhibit color change by two or more stimuli.18, 51 The spiropyran changes in response to UV light, and it recovers the original color by visible light or heat. It also responds to the mechanical force, therefore, it can be used as the color-changing strain sensors. The leuco dye shows color changes between colorless and colored form by heat, pH, or UV light. PDMAEMA also shows temperature- and pH-responsive properties because it has LCST and tertiary amine groups that show pH-dependent charged states.55 In addition, the multistimuli-responsive polymers can be designed by copolymerization of monomers with different types, such as temperature- and pH-responsive poly (N- isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) made by copolymerization of N- isopropylacrylamide and acrylic acid monomers.56
Figure 1.7. Stimuli-responsive polymers with various stimuli.
PEDOT
Spiropyran
Leuco dye UV Vis / Heat
UV Heat
PNIPAM PAAc Azobenzene
OH- H+
350 nm 450 nm / Heat
Temperature pH Light
Electro Multi-stimuli
28 1.3.2 Micro/nanostructured smart polymers
To further improve the responsive performance of smart polymers, micro/nanostructured responsive polymers can be a good choice in terms of the key factors, sensitivity and response time, attributed to the increase in surface area. According to the purpose of sensors and actuators, responsive polymers can be fabricated as 2D films and 3D particulates (Figure 1.8).57-63 First, 2D films include graft polymer brushes and layer-by-layer (LbL) films which have responsive surfaces. Reconstruction of the polymer chains induces the responsive behavior. The responsive 2D surfaces can be used in applications such as cell harvesting, bioapplications, micro/nanopatterning, tissue engineering.64
Homopolymer and block-copolymer brushes switch the chain conformation from the stretched to the collapsed conformation in response to the stimuli.65 The grafted polymer chains reversibly change the conformation on the substrate. Especially, block copolymer brushes switch the chain conformation due to the phase segregation of different blocks. Therefore, responsive polymer brushes lead the change in surface characteristics such as morphology, mechanical strength, wettability, or adhesive properties.
Compared to the responsive polymer brushes, the film thickness of LbL films can be simply controlled by changing the number of layers. The LbL film is assembled based on the ion pairing of the oppositely charged molecules or the hydrogen bonding interactions.66 The resulting LbL films are used to control surface chemical properties or porous structures in response to the stimuli. The responsive porous structure can also be made by membrane structures. Nano/microporous membrane can easily control the pore size and the permeation of chemicals under the proper stimulus.67
Furthermore, responsive 3D particulates, such as microgels, nanogels, polymer micelles, and core- shell particles, have a unique characteristic that the particles can be dispersed in solvents or other polymers and exhibit high stability. When applying the appropriate stimulus, the responsive polymer particles exhibit reversible aggregation dispersion68.
Microgels are the crosslinked polymer particles with the size of tens of nanometers to a few micrometers. In case of thermoresponsive PNIPAM microgels, they swell and collapse upon temperature changes.69 The PNIPAM microgels exhibit the change between hydrophilic and hydrophobic properties. The response time of responsive microgels is much faster than the bulk responsive polymers due to the large surface area. The aggregation can also lead to cloudy microgel dispersion, which induces the change in the optical transparency. Nanogels have much smaller diameter in a nanometer range and larger surface area compared to microgels. Especially, unsaturated nanogels can be used as crosslinkers of the bulk polymer to improve stimuli-responsive performance.70
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Among the responsive polymer particles, polymer micelles are appropriate to be used in the targeted drug delivery.71 The responsive polymer micelles assemble and disassemble according to stimuli such as temperature or pH. After injecting the drug containing micelles into the body, the micelles can be disassembled on the targeted spot by applying the stimulus and they release the drug at the exact location. Moreover, responsive polymer chains can be grafted on the core nanoparticles to fabricate core-shell nanoparticles, which give responsive properties to the desired nanoparticles.72
Figure 1.8. Stimuli-responsive polymers with micro/nanostructures. Homopolymer brush (T.
Zhou et al. Nat. Commun. 2016, 7, 11119). Block copolymer brush (C. Cummins et al. Nanoscale 2015, 7, 6712). LbL film (S. C. Mun et al. Carbon 2017, 125, 492). Membrane (N. D. Koromilas et al.
Polymers 2019, 11, 59). Microgels (Y. Kim et al. Sens. Actuators B 2015, 207, 623). Core-shell particles (M. Ballauff et al. Polymer 2007, 48, 1815).
Microgels
Membrane LbL film
Homopolymer brush
Core-shell particles
1µ m
100nm
Block copolymer brush
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