Pavlovian polymers

\u3cp\u3eThe development of smart functions and advanced processing leads to a paradigm shift in polymer applications. One example is polymer-based soft robots that sense changes in environment and (re)act by undergoing shape deformations. The Priimägi group added an additional level of intelligence by introducing a training step that leads to a conditioned response, similar to the classic Pavlov's dog experiment.\u3c/p\u3

Densely cross-linked liquid crystal networks by controlled photopolymerization of ordered liquid crystal monomers:properties and applications

Polymers with a controlledmolecular organization in all three dimensions are of interest because of their unusual, but very accurately adjustable and addressable optical, electrical, and mechanical properties. An established method to produce 3-D ordered polymers is the photoinitiated polymerization of liquid crystal (LC) monomers [1-4]. The variety in possible LC phases of low-molar-mass reactive mesogens provides diversity in the choice of the type of the molecular order, all being accessible to be fixed by the polymerization process [5-8]. Known techniques to establish monolithic molecular order in LCs, such as rubbed surfaces, surfactant-treated surfaces, external electric, or magnetic fields or flow, can be applied or even can be combined with each other to create films of even more complex molecular architectures [9]. The molecular structure of LC monomers can be tailored, for example, to optimize on the mechanical and optical properties of the films. Furthermore, blends of monomers can be made to adjust the properties in the monomeric state, such as the LC transition temperatures and the flow viscosity, and in the polymeric state, such as the elastic modulus, the glass transition (Tg) temperature, and the refractive indices

The use of cholesterically-ordered polymer networks in practical applications

Cholesteric polymer films produced by the photo-initiated polymerization of liquid-crystalline diacrylates containing a small amount of chiral dopant are demonstrated to be applicable as foils to compensate for the wavelength dependence of the transmission of supertwisted nematic liquid-crystal displays and to possess the appropriate thermal behaviour for use as a coating or encapsulation material for low-stress packaging of electronic devices

Switching surface topographies based on liquid crystal network coatings

The surface topography of materials is the local deviations of a surface from a perfectly flat plane. This chapter proposes the use of liquid crystal (LC) networks to induce the formation of surface topographies. The driving principle is the light‐triggered trans‐to‐cis isomerization of a copolymerized azobenzene molecule added in a relatively low concentration to the network. An additional advantage of the use of photopolymerized LC networks is that one can chose between a variety of alignment strategies, which leads to the formation of preprogrammed surface structures. The principle of photoactivated deformation of LC network films has many parallels with the deformation induced by heating of structured LC network films. In this case, a copolymerized azobenzene molecule disturbs the order when it transforms from its trans state to the cis state. The formation of LC polymeric networks by in situ polymerization of LC monomers provides the possibility to create films or coatings with director patterns

Light controlled friction at a liquid crystal polymer coating with switchable patterning

We describe a new methodology that enables dynamically control of motion through modulating friction at coating surfaces by exposing with UV light. The principle is based on reversibly switching the surface topographies of the coating by light. The coating surface transfers from flat in the dark to corrugated in the presence of UV by forming regular ridge-like line gratings. Both the static and the kinetic friction coefficients are investigated in a dynamic manner by switching between the off (flat surface) and the activated (with ridges) state. By dynamically changing the friction, we are able to bring the sample from a static state into motion via UV exposure. When in motion, the friction coefficient can be altered further by modulating the light conditions. E.g. a smooth sliding can transfer into an interlocking state, or vice versa. Moreover, we can dynamically reduce the contact area in the interface and thus lowering friction forces

Liquid crystal polymer networks : switchable surface topographies

Coating of liquid crystal (LC) networks modified with azobenzene moieties as crosslinks was reviewed to create surface topographies in the micrometer range by exposure with UV light. Various configurations were worked out. Homogeneous cholesteric networks and homeotropic nematic networks could form well-defined protrusions by local exposure. The protrusions can be dynamic, i.e. disappear as soon as light is switched off, or be permanent keeping stable topological structures even after the exposure stops. The formation of the protrusions was proven to be induced mainly by a decrease in density when the order in the LC network is reduced under the trans to–cis isomerization of the azobenzene units. Next to the photochemical effect also heating by absorption played a role. When the polymer network coating was build up from alternating cholesteric and homeotropic areas a uniform exposure led to the surface topography. When exposed, the cholesteric areas are protruding and the homeotropic areas are indenting. In this the deformation is enhanced by the orchestrated linear geometric shear deformations in the neighboring areas where the homeotropic areas tend to expand in the lateral direction and the cholesteric areas in the direction perpendicular to the coating

New insights into photoactivated volume generation boost surface morphing in liquid crystal coatings

Photoactivated generation of disorder in a liquid crystal network produces free volume that leads to the controlled formation of dynamic corrugations at its surface. The liquid crystal order amplifies the deformation of copolymerized azobenzene, which takes place on molecular length scales, to a micrometre-sized macroscopic phenomenon based on changes in density. We postulate a new mechanism in which continuous oscillating dynamics of the trans-to-cis isomerization of the azobenzene overrules the net conversion, which is currently considered as the origin. This is supported by a significant local density decrease when both the trans and cis isomers are triggered simultaneously, either by dual-wavelength excitation or by the addition of a fluorescent agent converting part of the light to the cis-actuating wavelengths. This new insight provides a general guideline to boost free volume generation leading not only to larger macroscopic deformations but also to controllable and faster non-equilibrium dynamics

Dynamic tribology in liquid crystal coatings

This chapter demonstrates that under UV illumination dynamic surface topographies can be used to modulate the surface friction. The modulation of friction is based on a reversible switch between a flat and a pre‐designed surface texture. The change in friction, both and dynamic, is employed by utilizing some materials. The dynamic friction behavior of the system is quantified by the kinetic friction coefficient (μk) and is measured by two coatings that are sliding against each other at a constant speed. A home‐built measuring system is designed to measure the dynamic surface friction. The static friction coefficient (μs) is measured by placing two coatings against each other and tilting them from the horizontal position to a deflection angle β. The chapter also demonstrates a method to bring a sample into motion by decreasing the static friction at two surfaces through UV light