Jelle E. Stumpel

Photoswitchable ratchet surface topographies based on self protonating spiropyran-NIPAAM hydrogels

In this work, self-protonating spiropyran-based poly(N-isopropylacrylamide) polymer networks are prepared. These photoresponsive hydrogel coatings can change their surface topography upon exposure with visible light in a neutral environment. Photoresponsive surface-constrained films have been fabricated for which the swelling behavior can be controlled in a reversible manner. In a first step, symmetrical switchable surface topologies with varying cross-link density are obtained by polymerization-induced diffusion. Under light exposure, the areas with low cross-link density swell more than the areas with high cross-link density, thus forming a corrugated surface. Asymmetric ratchet-like photoresponsive surfaces have been prepared on prestructured asymmetric substrates. As a result of thickness variation of the surface-confined hydrogel layer, an asymmetric swelling behavior is obtained. Depending on the cross-link density of the hydrogel, it is possible to switch between a ratchet and flat surface topography or even an inverse ratchet surface by light.

The mechanism of the oxidation of benzyl alcohol by iron(III)nitrate: Conventional versus microwave heating

The mechanism of the oxidation of benzyl alcohol with iron(III)nitrate nonahydrate under conventional and under microwave heating conditions has been investigated and the reaction conditions have been optimized. A series of redox reactions leads to the formation of benzaldehyde and other products. Direct comparison between conventional and microwave heating revealed identical conversions profiles. Mastering the microwave induced heat, absence of a real microwave effect and byproduct formation are the major factors to advise a traditional batch-wise way of process development to a larger scale.

Photoswitchable Hydrogel Surface Topographies by Polymerisation‐Induced Diffusion

Herein, we describe the preparation of patterned photoresponsive hydrogels by using a facile method. This polymer-network hydrogel coating consists of N-isopropylacrylamide (NIPAAM), cross-linking agent tripropylene glycol diacrylate (TPGDA), and a new photochromic spiropyran monoacrylate. In a pre-study, a linear NIPAAM copolymer (without TPGDA) that contained the spiropyran dye was synthesised, which showed relatively fast photoswitching behaviour. Subsequently, the photopolymerisation of a similar monomer mixture that included TPGDA afforded freestanding hydrogel polymer networks. The light-induced isomerisation of protonated merocyanine into neutral spiropyran under slightly acidic conditions resulted in macroscopic changes in the hydrophilicity of the entire polymer film, that is, shrinkage of the hydrogel. The degree of shrinkage could be controlled by changing the chemical composition of the acrylate mixture. After these pre-studies, a hydrogel film with spatially modulated cross-link density was fabricated through polymerisation-induced diffusion, by using a patterned photomask. The resulting smooth patterned hydrogel coating swelled in slightly acidic media and the swelling was higher in the regions with lower cross-linking densities, thus yielding a corrugated surface. Upon exposure to visible light, the surface topography flattened again, thus showing that a hydrogel coating could be created, the topography of which could be controlled by light irradiation.

Water-responsive dual-coloured photonic polymer coatings based on cholesteric liquid crystals

This work describes a straightforward method to prepare patterned photonic coatings which alter their colour when exposed to water. Various kinds of dual-coloured patterns were made, which become visible or fade away when placed in water. These effects are reversible and can be repeated many times.

Optical and topographic changes in water-responsive patterned cholesteric liquid crystalline polymer coatings

In this work, we present patterned water-responsive coatings, which alter both their topological and optical properties. The polymer coatings are based on a hydrogen-bonded cholesteric liquid crystalline polymer network. A two-step photopolymerization procedure leads to a patterned coating with repeating liquid crystalline and isotropic areas. The cholesteric liquid crystalline areas reflect green light, whilst the isotropic areas are transparent for visible light. Treatment with alkaline solution results in a hygroscopic polymer salt coating. When placed in demineralized water, the polymer films swells, leading to an enhancement of the surface topography structure in which the liquid crystalline areas swell more. Moreover, the pitch of the helical organization in the cholesteric areas increases due to this swelling leading to a color change from green to red.

Hot pen and laser writable photonic polymer films

An orange-reflecting photonic polymer film has been fabricated based on a hydrogen-bonded cholesteric liquid crystalline (CLC) polymer consisting of non-reactive (R)-(+)-3-methyladipic acid as the chiral dopant. This polymer film can be patterned easily by evaporating the chiral dopant at specific locations with a hot pen or a laser beam. Removal of chiral dopant leads to a decrease in the helical pitch at the heat treated areas leading to a change in color from orange to green revealing a high contrast pattern. The photonic patterns are irreversible and stable at ambient conditions. This makes such a CLC polymer film interesting as writable photonic paper.

Responsive Polymer Photonics

Awarding Institute: Eindhoven University of Technology, Eindhoven (The Netherlands) Date Awarded: November 6, 2014 Supervisors: Prof. Dr. Albertus P. H. J. Schenning, Eindhoven University of Technology, Eindhoven (The Netherlands); Prof. Dr. Dirk J. Broer, Eindhoven University of Technology, Eindhoven (The Netherlands) Polymer coatings are of major importance in everyday life. Most of the surfaces that we use daily have been modified with a protective or aesthetic coating. However, the majority of these coatings are static. In other words, they are not responsive to external influences such as light or humidity. It is expected that responsive coatings will play an important role in meeting social challenges in the fields of sustainable energy, health care, and food safety. These functional properties are often determined by the surface topography and/or wettability. Recently, the development of stimulus-responsive polymer coatings that have dynamic rather than static properties has been the focus of considerable attention. For these so-called smart coatings, the properties change in response to an external stimulus. The functional properties can be adjusted autonomously depending on user needs or upon environmental changes. Such coatings hold great promise in a variety of fields including optical sensors, reflecting devices, as well as easy-to-clean materials and robotics (responsive topographies). The research described in this thesis focuses on the development of responsive materials that operate in a reversible way. Photoresponsive and stimulus-responsive photonic coatings were prepared. The first type of coating undergoes photoinduced topographical alterations; coatings in the second category change colour upon exposure to a specific stimulus. Both types are based on liquid crystal or hydrogel-based materials and, in both cases, the response is a dimensional alteration that leads to changes in the initial surface topography and/or colour. The first chapter gives an overview of the stimuli-responsive photonic coatings that have been produced in the past. The following two chapters describe the use of light-induced topographical changes in a crosslinked poly(N-isopropylacrylamide)-spiropyran hydrogel coating. When this coating is placed in an acidic environment, the colourless spiropyran derivative (Sp) spontaneously changes into its protonated strongly coloured merocyanine isomer (McH+), as depicted in Figure 1. Exposure to visible light then results in back-isomerisation into the Sp form. The protonated McH+ species is hydrophilic and its Sp isomer is hydrophobic. Spiropyran-based polyNIPAAM (N-isopropylacrylamide) hydrogels therefore undergo a light-induced hydrophilicity change, resulting in an alteration in the dimensions of the hydrogel, which can be visually observed (Figure 1). The degree of swelling and shrinkage in the hydrogel depends on the crosslink density of the material. Polymerisation-induced diffusion was used to control the spatial crosslink density in a hydrogel coating. This resulted in a corrugated structure after the material was swelled in a slightly acidic aqueous solution. Photo-exposure resulted in a reversible reduction of the height of this surface topography. Figure 1 Isomerisation behaviour of protonated merocyanine (McH+) into spiropyran (Sp) upon visible light exposure at pH 2.7. The left photograph displays a fully swollen hydrogel; the right photograph is the same hydrogel after photo-exposure (the scale ... Further development of this material resulted in photoresponsive hydrogels that operate at neutral pH. In these hydrogels, acrylic acid acts as an internal proton source, resulting in a hydrogel coating that is photoresponsive in a neutral environment. After being placed in demineralised water, the slightly off-colour hydrogel film changed to a more intense orange colour, which indicates that the protonated merocyanine isomer formed spontaneously in water because of the carboxylic acid that is incorporated in the polymer backbone. Polymerisation-induced diffusion was used to obtain responsive surface topographies of the kind described above. Moreover, asymmetric ratchet-like structures were prepared using a prestructured substrate (Figure 2). In this system, the asymmetric change of the surface topography is a result of variation in the thickness of the hydrogel. The extent of the photoinduced alteration of the coating can be controlled by adjusting the crosslink density of the hydrogel. Figure 2 3 D height profiles of a hydrogel ratchet before (a) and after (b) light exposure and c,d) corresponding cross-sections of the surface topography, as observed by interferometry (dimensions of the surface are 505×946 μm ... The succeeding chapters report on responsive photonic coatings based on cholesteric liquid crystals (CLCs). In this liquid crystalline phase, an alternating refractive index change through the material results in the partial reflection of the incident light. CLC coatings which alter their colour upon water uptake have been prepared. Furthermore, water-responsive surface topographies were prepared in which the hydrogen-bonded supramolecular polymer coating had alternating regions with a CLC or an isotropic alignment. Treatment with an alkaline solution led to the disruption of the hydrogen bonds and the formation of carboxylates. Due to the hygroscopic nature of these moieties, the materials swelled when placed in water. Interestingly, the isotropic areas swelled uniformly, while the CLC areas mostly swelled unidirectionally. The structure of the surface topography was therefore accentuated after placement in water. A change in the helical pitch of the CLC region also led to a different reflected colour, while the isotropic areas are colourless in both the dry and the swollen state. Additionally, coatings were prepared that underwent dual-coloured patterned changes when placed in water. A novel approach was used to produce the humidity-responsive optical sensors with a broad operational range. An interpenetrating polymer network (IPN) which combines CLC-based materials with hydrogels was used for this coating. A cholesteric polymer network was swollen with an acrylic acid-based mixture which was subsequently photopolymerised. The activation of this coating with an alkaline solution led to the formation of a hygroscopic potassium polyacrylate network in the CLC scaffold. Humidity-dependent volumetric changes in this IPN network led to a relative large colour change. Moreover, an IPN with a responsive surface topography was prepared. This coating consisted of regions containing both responsive potassium polyacrylate areas and static polystyrene areas. Depending on the pH or relative humidity, the surface became flat or structured (Figure 3). Figure 3 a) Schematic representation of the working mechanism of the patterned polystyrene poly(acrylic acid)-responsive IPN polymer film. b) Optical micrographs of a patterned IPN coating at a relative humidity (RH) of 15 % and 85 %. ... Lastly, the fabrication of a hydrogen-bonded CLC coating which responds to gaseous trimethylamine (TMA), a substance produced by decay in fish is presented. Like the alkaline treatment, TMA deprotonates the carboxylic acids in the material and carboxylate salts are formed. Under anhydrous conditions, this salt formation causes a decrease in the molecular order that results in a colour loss. However, the hygroscopic coating swells under humid conditions, resulting in a clearly visible colour shift from green to red. This type of colour-changing CLC coatings can be used as optical sensors for the real-time detection of various analytes. In a proof-of-principle experiment, it was shown that these CLC films can be used as optical sensors to detect volatile amines that are produced by decaying fish (Figure 4). Figure 4 Photographs of the green-reflecting CLC film before and after exposure to spoiling cod. Reproduced with permission from Ref. 5. Copyright 2014, John Wiley and Sons. Overall, this thesis shows that responsive photonic coatings that are of interest for a range of applications can be produced. The technology assessment discusses some of these applications in greater detail. The development of photoresponsive coatings has considerable potential for applications ranging from microfluidic devices to cell culturing. Turning to the use of responsive photonic coatings as sensors, preliminary results are presented which are expected to improve the selectivity of the sensor. Finally, the photochromic behaviour of spiropyran is discussed as a way of preparing smart windows and writable optical materials.