Fast, multi-responsive microgels based on photo-crosslinkable poly(2-(dimethylamino)ethyl methacrylate)

Polymer 45 (2004) 6771–6778 www.elsevier.com/locate/polymer

Fast, multi-responsive microgels based on photo-crosslinkable poly(2-(dimethylamino)ethyl methacrylate) Jan M.D. Heijl, Filip E. Du Prez* Department of Organic Chemistry, Polymer Chemistry Research Group, Faculty of Science, Ghent University, Krijgslaan 281 S4bis, B-9000 Ghent, Belgium Received 25 March 2004; received in revised form 9 August 2004; accepted 10 August 2004

Abstract Multi-responsive microgels based on poly(2-(N,N-dimethylamino)ethyl methacrylate) were developed and their properties were investigated. The primary goal of this research was to speed up the stimulus-response time of the hydrogels to a level usable for actuator applications, by reducing the diffusion distance of water. The gels were prepared by a UV induced photodimerization of a copolymer of 2(dimethylamino)ethyl methacrylate and 4-cinnamoyl-phenyl methacrylate. Patterning studies showed that these materials can be used as photo-resist materials with high resolution at short exposure times. They showed lower critical solution temperature behavior in water, as well as pH dependent solubility and swelling ratios. While 1 mm thick gels showed response times to temperature and pH-changes of several hours, Si-supported microgels of 300 nm thickness had response times in the range of only a few seconds. The copolymer was prepared by free radical copolymerization, and the reactivity ratios were determined with the extended Kelen Tudos method. Spin-coating of this copolymer on Si supports and subsequent UV-irradiation yielded microgels of variable thickness (200 nm–15 mm), which was determined by confocal scanning laser microscopy. Surface plasmon resonance spectroscopy measurements demonstrated the fast, stimuli-responsive swelling behavior, while differential scanning calorimetry gave insight into the morphology of the networks. q 2004 Elsevier Ltd. All rights reserved. Keywords: Multi-responsive hydrogels; UV-crosslinked microgels; Poly(2-(dimethylamino)ethyl methacrylate)

1. Introduction Crosslinked polymer networks swollen in water, socalled hydrogels, have been shown to be able to respond to a variety of stimuli, such as pH, temperature, magnetic and electrical fields, and have been proposed for their use in various applications. These include mainly biological and biomedical applications such as drug delivery [1–3], artificial muscles [4] and surfaces in biomaterials [5], but also for membrane separations [6,7], sensors and actuators [8]. One of the problems in applying these gel materials, is that the response of the gel to the stimulus is a slow, diffusion-controlled process. Thermo-responsive or pHresponsive gels will swell or deswell as a result of respectively their lower critical solution temperature (LCST) behavior and the protonation or deprotonation of

* Corresponding author. Tel.: C32-926-449-32; fax: C32-926-449-72 E-mail address: [email protected] (F.E. Du Prez). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.08.013

ionizable groups. In this process, water is diffused either from the surface towards the centre of the gel, or vice versa. Because the swelling and deswelling rate is inversely proportional to the square of the distance that the water molecules have to traverse, it is possible to reduce the response time by decreasing the gel dimensions [9]. To reach acceptable response times, it is necessary to decrease the gel size dramatically, to the mm range or smaller [10]. The most suitable method for preparing such microgel films is by a photo-crosslinking process. It is a well known method from photolithographic literature, and often used in microsystem technology [11], in which pendant chromophoric groups are attached to the polymer as crosslinkers. However, there are relatively few reports of gels being prepared via photo-crosslinking of water-soluble polymers [9,12,13]. In these reports, water-soluble polymers are used as water-developable photo-resists [13], as absorbents for nonionic surfactants with temperature adjustable adsorption properties [12], and for the preparation of LCST polymer

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systems [8,14–16]. Kuckling et al. prepared poly(Nisopropylacrylamide) (PNIPAAm) microgels based on a [2C2]-photocyclo-addition [9]. These PNIPAAm gels did not show the desired thermo-responsive properties, because the demixing temperature of the homopolymer (LCSTPNIPAAmZ32 8C) is lowered below room temperature by the incorporation of hydrophobic chromophores. The use of polymers with high LCST values, often of little interest because this temperature is out of the bio-applicable range, can solve this problem. Poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMAEMA) is a thermo-responsive polymer with an LCST value of around 50 8C [17,18]. Due to the presence of amine groups, the polymer also shows pH sensitivity, giving it a wider range of applications [19]. Lowering the LCST by the incorporation of hydrophobic chromophores, brings the transition temperature into the applicable range around body temperature. Another advantage of the use of PDMAEMA is that, because of its low Tg (19 8C) and thus higher mobility in comparison with PNIPAAm (TgZ130 8C) [20], the use of solvents during synthesis and crosslinking can be restricted, and the kinetics and the conversion of the crosslinking process can be improved. The chromophore used in this research is 4-cinnamoylphenyl methacrylate (CPMA), a photo-dimerizable methacrylate monomer known from literature [21,22]. This chromophore was synthesized, the copolymerization with DMAEMA was studied, and the resulting polymer was spincoated and crosslinked on a silicon or quartz surface. The stimuli-responsive behavior of the copolymer, the coatings and the nanometer-thick microgels were investigated by several methods, such as surface plasmon resonance (SPR) spectroscopy, confocal scanning laser microscopy (CSLM) and differential scanning calorimetry (DSC). These methods respectively allowed to investigate the swelling properties, the layer thickness and the morphology of the copolymer networks.

was prepared from benzaldehyde and p-hydroxy-acetophenone by an aldol condensation, followed by the dehydration of the b-hydroxyketone and esterification with methacryloylchloride (yield: 67%), as reported by Subramanian [21, 22]. After recrystallization in MeOH, the resulting shiny, yellow–green flakes were stored in the dark and refrigerated. The photoreactive double bond is the a–b unsaturation of the ketone in Scheme 1. 2.3. Copolymer synthesis The poly(DMAEMA-co-CPMA) copolymers were prepared by a free radical copolymerization initiated by AIBN in MEK. A typical procedure is described below, and the reaction is illustrated schematically in Scheme 2. For a copolymer with 1 mol% of chromophore, 10 ml of DMAEMA (5.93!10K2 mol) and 173 mg of CPMA (5.93!10K4 mol) were dissolved in 40 ml of MEK, and oxygen was removed by bubbling nitrogen through the solution for 20 min. After addition of AIBN (19 mg, 0.2 wt% versus monomers), the solution was refluxed for 10 h. The solution was concentrated, and the copolymer was precipitated in 400 ml of pentane (more than tenfold excess to MEK). 1 H NMR data: broadened overlapping peaks at 7.2– 8.1 ppm (aromatic systemCconjugated double bond); 4–4.2 ppm (–O–CH2–CH2–N–); 2.5–2.7 ppm (–O–CH2– CH2–N–); 2.1–2.4 ppm (–CH2–N–(CH3)2); 1–1.7 ppm (polymer backbone: –C–CH2–C– and –C–CH3). IR data: alkene C–H and aromatic C–H from CPMA at 3100 cmK1, aliphatic C–H at 2870–2960 cmK1 from polymer backbone and amino-ethyl side chains, region of low intensity peaks at G2000 cmK1 from aromatic system, CZO stretch from unsaturated ketone in CPMA at 1618 cmK1 and from DMAEMA and CPMA esters at 1638 cmK1, C–N at 1180–1210 cmK1. UV data: absorption bands with maxima at 200 and 330 nm.

2. Experimental

2.4. UV-crosslinking procedure

2.1. Materials 2-(N,N-Dimethylamino)ethyl methacrylate (DMAEMA, Acros, 99%) and methacryloylchloride (Acros, 98%) were distilled under vacuum and stored under argon in a refrigerated environment prior to use. Methyl ethyl ketone (MEK, Acros, 2-butanone, 99C%), azo-bis-isobutyronitrile (AIBN, Aldrich, 98%) and Michler’s ketone (Acros, 4,4 0 bis(dimethylamino)benzophenone, 98%) were used as received from Acros Organics and Aldrich. All other chemicals were of analytical grade and used as received.

Thin films of the photopolymer were prepared by casting and spin-coating (at 5000 rpm and maximum torque) of solutions of the copolymer in MEK, with 10 wt% of Michler’s ketone added as sensitizer, onto either quartz slides or silicon wafers. The concentration (2–20 wt%) of these solutions has been used to vary the layer thickness. The solvent was evaporated and the films were placed under a 900 W VL 400-L UV lamp at 10 cm distance (16 mW/cm2), with an emission maximum at 365 nm, for various irradiation times. The unreacted chains were washed away with acetone and water.

2.2. Chromophore synthesis

2.5. Characterization

4-Cinnamoyl-phenyl methacrylate (CPMA) (Scheme 1)

The molecular structures and copolymer compositions

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Scheme 1. CPMA synthesis.

were determined by 1H NMR spectroscopy, on a Bru¨ker Avance 500 or 300 MHz NMR spectrometer. IR spectra were taken on a Perkin–Elmer 1600 FTIR, and UV spectra on an Uvikon XL (BIO-TEK instruments). Elemental analysis was performed at the CNRS (De´partement d’analyse e´le´mentaire) in Vernaison, France. DSC measurements were performed on a Perkin–Elmer DSC 7 with TAC 7/DX thermal analysis controller and Pyris software, to determine glass transition temperatures (Tg) and demixing temperatures (Tdem.), at a heating rate of 5 8C/min, in a temperature range of 0–150 8C for Tg and 5–85 8C for Tdem.. For the determination of the Tdem. 10 wt% aqueous solutions were used, as the solubility in water is limited to about 10–20 wt% and lower concentrations showed no clear endothermic signal. Tdem. is defined as the onset of the endothermic signal. Molecular weights of the copolymers were determined with a Waters gel permeation chromatography system (with Viscotek software and a PLgel 5 mm Mixed C column) on tetrahydrofuran (THF) as eluent, doped with 5% Et3N and calibrated with polystyrene standards at 22 8C and 1 ml/min flow rate. CSLM was performed on a BioRad Radiance 2000 MP instrument, to measure the thickness of the coated films. These films were prepared from MEK solutions of the copolymer with varying concentrations. When several layers were placed on top of each other, each layer was irradiated for 30 min and dried in vacuum before the next was applied. The patterning pictures of the copolymer were taken with an Olympus BH2 optical microscope (800!enlarged) and a Minolta digital camera. For the study of the behavior of the (sub)micrometer-thick films, SPR spectroscopy was applied. This technique was previously applied by Kuckling et al. [10,23] on thin films containing poly(N-isopropylacrylamide). To use this technique, a crosslinked PDMAEMA coating is made on a 45 nm gold layer supported on a quartz slide. A prism of special LaSFN9

glass is set on top of the free quartz side to catch the incoming laser light, and a special flow-cell was developed to bring the other side (with the polymer coating) into an aqueous environment in order to study the swollen polymer coating.

3. Results and discussion 3.1. Synthesis of the photoreactive copolymer In general, the incorporation of hydrophobic, photoreactive CPMA groups in a thermo-responsive polymer results in a weakening of the thermo-sensitivity, and in a decrease of the Tdem. [9,14]. For example, when the wellknown polyNIPAAm is used as thermo-responsive polymer, the demixing temperature is no longer above room temperature in the photo-crosslinkable copolymers, and the polymer structure is no longer utile for thermoresponsive applications. In the case of PDMAEMA, it was expected that more of the chromophore could be used without loss of thermo-responsiveness, because of the higher LCST of the homopolymer, which is about 50 8C [17]. The chromophore was synthesized as reported by Subramanian [21], and shown in Scheme 1. The copolymerization with DMAEMA was studied with the extended Kelen Tudos method for the determination of the copolymerization parameters, as the polymerizations could not be stopped at low conversions [24,25]. The results of this study are shown in Table 1. From these results, it was calculated that the r1 value for the copolymerization of DMAEMA with CPMA is 0.9 while r2 (for CPMA with DMAEMA) is 6.5. Although the reactivity of CPMA is much higher than that of DMAEMA, no problems were encountered during the crosslinking

Scheme 2. Copolymerization process for the preparation of poly(DMAEMA-co-CPMA) photopolymer.

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Table 1 Extended Kelen Tudos study of the copolymerization of DMAEMA and CPMA M1 (%)

M2 (%)

dM1a (%)

dM2a (%)

hb

xb

Conversion (%)a

90 80 70 60 50 40 30 20 10

10 20 30 40 50 60 70 80 90

85.0 72.0 56.0 39.1 25.4 16.2 9.0 3.9 1.8

15.0 28.0 44.0 60.9 74.6 83.8 91.0 96.1 98.2

0.43 0.26 0.07 K0.13 K0.32 K0.52 K0.79 K1.26 K1.57

0.82 0.67 0.58 0.53 0.49 0.43 0.37 0.33 0.18

12 11 13 11 14 11 15 14 16

a b

dM1, dM2 and conversion were determined with 1H NMR spectroscopy. h and x are parameters calculated from M1, M2, dM1 and dM2 to create a linear relationship that allows the determination of r1 and r2 [24,25].

reactions, as only small amounts of CPMA are incorporated in the copolymer (1–2.5 mol%). The composition of the copolymers will be shown as in the following example: poly(DMAEMA97.5-co-CPMA2.5), meaning a copolymer with 97.5 mol% DMAEMA units and 2.5 mol% CPMA units. 3.2. Polymer characterization The average molecular weight (Mn) and polydispersity of the copolymers was respectively 18,500 g/mol and 3.1 as measured against polystyrene standards with a conversion factor of 0.5 [26]. Although for the different batches used in this work, the molecular weights varied from about 17,000– 20,000 g/mol, significant changes in the final properties of the microgels are not observed. The copolymer is soluble in several organic solvents (acetone, MEK, toluene, etc.) but also in water, as required for the eventual applications. The copolymers exhibited demixing temperatures ranging from 31 to 39 8C, as determined by the endothermic signal in the DSC analysis of copolymer solutions (Fig. 1). The Tdem.’s for 10 wt% polymer solutions in water decrease with increasing CPMA content, as expected from its hydrophobic character [27,28]. The signal in the DSC curve, which is a clear peak for poly(DMAEMA99-co-

CPMA 1) and poly(DMAEMA 97.5-co-CPMA 2.5), also becomes weaker and broader with increasing CPMA content, until no distinguishable maximum is observed for poly(DMAEMA90-co-CPMA10). In the dry copolymer samples, DSC analysis unexpectedly showed three Tg’ (G 19 8C, from PDMAEMA, G50 8C, from the mixed phase, and G138 8C, from PCPMA), of which the exact position and the relative size was influenced by the chromophore concentration (Table 2). These data suggest that the chromophores partially aggregate in the polymer bulk, in spite of their quite low weight fractions. Possible evidence for this is found in the properties of the polymer. Below 19 8C, both homopolymers and thus also the copolymer are in the glassy state. Between 19 and 50 8C, the copolymer is a hard and tough material, which bends and stretches elastically before breaking. Above 50 8C the material becomes less tough, but holds the rubbery properties. Above 138 8C, the copolymer becomes soft and viscous. This seems to indicate that at temperatures below 138 8C (the Tg of PCPMA [21]), some degree of physical crosslinking is present, attributed to the aggregation of the bulky CPMA units. This aggregation is facilitated by the composition drift in the copolymer, originating from the large difference in copolymerization parameters. 3.3. Photogelation and patterning

Fig. 1. Tdem. of 10 wt% polymer solutions in water as determined by DSC analysis of the copolymers with various chromophore concentrations. From top to bottom: poly(DMAEMA99-co-CPMA1), poly(DMAEMA97.5-coCPMA2.5 ), poly(DMAEMA95 -co-CPMA5), poly(DMAEMA92.5 -coCPMA7.5) and poly(DMAEMA90-co-CPMA10).

The photo-crosslinking reaction took place in dry films of the copolymer, either cast or spincoated on a quartz or silicon surface. As casting resulted in films with irreproducible thickness and rough surfaces, spincoating was used throughout the experiments to prepare the polymer films. After coating the copolymer onto the surface from a MEK solution, it was crosslinked by UV-irradiation. As the absorption maximum of the copolymer is at 330 nm, a sensitizer (Michler’s ketone, lmaxZ360 nm) was added to the solution before coating, to improve the quantum efficiency [11]. The kinetics of the crosslinking were studied by on-line UV absorption measurements, as shown in Fig. 2.

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Table 2 Tg values of copolymers with various chromophore concentrations, as observed from DSC Poly(DMAEMAx-co-CPMAy) x

y

99 97.5 95 92.5 90

1 2.5 5 7.5 10

Tg,1 (8C)

Tg,2 (8C)

Tg,3 (8C)

19 20 21 23 25

48 50 52 52 54

132 134 134 135 137

The figure represents the UV absorption spectra of a coating from a 2 wt% solution of a poly(DMAEMA-coCPMA) copolymer in MEK, during the crosslinking reaction. After 10 s, the main part of the maximum at 320–330 nm, i.e. the unreacted copolymer, has already disappeared, and a new maximum appears at 270 nm, due to the formation of dimerized CPMA groups, so-called truxillic derivatives [29]. The measurement of the UV absorption curve is a relatively slow process, i.e. 10 s is the shortest period of irradiation that could be measured. For a better understanding of the kinetics of the curing reaction, patterning resolutions of the coated films were investigated as a function of exposure time. Only complete crosslinking of the film should give clear patterning and good resolution. The resolution values are given in Table 3 for different exposure times. A picture of such a patterned coating is shown in Fig. 3. From this experiment, we can conclude that the sensitizer speeds up the reaction by a factor of 6, and that the crosslinking reaction is overall a quite fast process (in the order of seconds). The latter can be interpreted as another indication of a phase separated morphology of the coated films, as was mentioned earlier based on DSC analysis. If this aggregation is indeed present, the chromophoric groups are in each others vicinity, and can react more easily and effectively upon irradiation than would be expected of a copolymer with only 1 mol% of chromophore. Normally, in

such low concentrations, the chromophore should behave like in a very dilute solution, and should not dimerize, but should isomerize instead from the trans- to the cis-isomer of CPMA [30]. CSLM was used to determine the thickness of the dry coatings. This technique allows the detection of the polymer layer in confocal planes. However, for the determination of the thickness, it also gives the possibility to study a crosssection of the copolymer layer and the support material. From Table 4, it can be observed that the thickness of the polymer film was varied from 300 nm up to about 8 mm by variation of the concentration of the coated solution and of the number of layers applied. 3.4. Multi-responsive properties of PDMAEMA microgels: classical methods The usual methods for determination of the thermoresponsive properties of polymer materials are designed for

Fig. 3. Photograph of positive and negative lines (4 mm width) in the patterned thermo-responsive coating on a Si surface, after 5 s of irradiation with Michler’s ketone as sensitizer, and washing with acetone. Table 3 Patterning resolution study of a coating obtained from a 2 wt% MEK solution of poly(DMAEMA99-co-CPMA1)

Fig. 2. Irradiation kinetics of a coating of the poly(DMAEMA99-coCPMA1) copolymer: (1) before irradiation; (2) 10s; (3) 20s; (4) 40s; (5) 50s; (6) 60s; (7) 90s; (8) 105s.

Sensitizera (wt%)

Exposure time (s)

Resolution (mm)

0 0 0 0 0 10 10

1 5 10 20 30 1 5

20 15 12 6 4 6 4

a

Sensitizer concentration is mentioned relative to the copolymer.

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Table 4 CSLM measurements of thickness of coatings prepared from poly(DMAEMA95-co-CPMA5) copolymer in a MEK solution. Each layer was irradiated and dried in vacuum before the next was applied # Layers

Concentration (wt%)

Thickness (mm)

1 1 1 2 3 4 5

20 4 2 4 4 4 4

7.3 1.4 0.3 3.2 4.2 5.0 6.2

materials with macroscopic sizes. DSC measurements on swollen films of 1 mm thickness showed no clear endothermic signal, and gravimetric measurements were impossible to perform. As a first indicator to the properties of the thin films, thicker films (1 mm) were prepared and investigated with the methods previously mentioned. Demixing temperatures of 30–50 8C were found, depending on the chromophore content (high Tdem. for 1 mol%, low Tdem. for 7.5 mol%), as explained previously for the results in Fig. 1 from the DSC measurements. The pH-responsivity of these materials was also investigated with the use of buffer solutions, and the influence of pH on the Tdem. was determined. The results are shown in Figs. 4 and 5. From these figures, it becomes clear that these 1 mm thick films do not give large changes in the swelling degree when plotted versus temperature. The position of the Tdem. is not significantly changed with pH, but changes in the pH do give rise to a large difference in the swelling degree. In all thick films, the reaction to the stimuli was slow. The first swelling of the 1 mm thick samples took at least 24 h, and

Fig. 4. Thermo-responsive swelling behavior of a 1 mm thick film of poly(DMAEMA95-co-CPMA5) at pH 2 (:) and pH 12 (6).

Fig. 5. Swelling degree of a poly(DMAEMA95-co-CPMA5) film of 1 mm thickness versus pH of buffer solutions at 22 8C.

4–6 h were needed to reach equilibrium after a temperature or pH change. 3.5. Multi-responsive properties of PDMAEMA microgels: surface plasmon resonance The SPR-technique measures the intensity of laser light reflected on a coated surface with varying angles of incidence and detection (incidence and detection vary in a q/2q ratio). In the resulting curves of the water-swollen PDMAEMA coatings, two phenomena can be observed. First of all, there is a broad minimum in reflected intensity (R) at relatively high angles (approximately 758). The exact position and shape of this minimum is mostly dependent on the refractive index (RI) of the surface, but also to some extent on the thickness. Next, there is a sharp minimum (called ‘wave guide mode’) in reflected intensity at lower angles (approximately 458), the position and shape of which are primarily related to the thickness of the layer, but also to a lesser extent to the RI. By making use of curve fitting techniques, both RI and layer thickness can be determined independently, and the swelling degrees can be calculated. A typical series of SPR spectra is shown in Fig. 6. In order to use this technique and to find both a wave guide mode and a minimum in the SPR spectrum, coatings of less than 500 nm had to be made on the gold surface.

Fig. 6. SPR curves for poly(DMAEMA99-co-CPMA1) at various temperatures, with a dry thickness of G300 nm. The curves, from left to right at the broad minimum, or right to left at the sharp minimum represent the measurements at respectively: 33.2; 38.7; 44.2; 53.6; 64.5; 69.6 8C.

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With a 2 wt% solution in MEK, coatings of about 300 nm were made, which could be tested on their swelling behavior in the flow-cell that was specially designed for this purpose. The coatings with 1 and 2.5 mol% of chromophore in the copolymer proved to have the best thermo-responsive properties (largest change in film thickness). Their swelling curves and changes in RI are shown in Fig. 7. The shift in the SPR curves starts at about 35 8C and ends at approximately 65 8C (Fig. 6). This confirms the continuous demixing, which is also seen in the gravimetric curves for the 1 mm thick films. The changes in film thickness seem to be relatively small, but it has to be pointed out that the change measured here is a one-dimensional change only, and would, for unsupported films, be increased to the third power. While the speed of swelling and deswelling was in the range of several hours to a day for the thick films, it decreased drastically in these microgel films. The first swelling of the sample was complete in 20 min, and the response of the coating to the temperature change was faster than the response time of the thermostat (!1 min). To check this apparently instantaneous response, a kinetic test was performed on the film with 1 mol% chromophore. For that reason, the reflected intensity of the SPR laser beam is measured at a detector angle of 678, which is the inflection point of the curve at room temperature. Under these conditions, a relatively large change in reflected intensity is measured for a small change in the refractive index. Fig. 8 clearly indicates that the goal of a low response time has been reached, as the film thickness reaches a constant value at the same time as the temperature inside the flow cell. Moreover, it shows the reversibility of this process. The film with 1 mol% chromophore was also subjected to a pH-responsive test in the SPR flow cell (Fig. 9). With decreasing pH, the swelling degree of the microgel starts to increase at a pH of 7, which is in accordance with the start of the buffering zone of the polymer (pKa,DMAEMA z8.5;

Fig. 7. Film thickness (full line, :,6) and refractive index (broken line, %,>) of the poly(DMAEMA99 -co-CPMA1 ) (:,%) and poly(DMAEMA97.5-co-CPMA2.5) (6,>) coatings plotted versus the water temperature in the flow-cell.

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Fig. 8. Fast, reversible swelling and deswelling behavior of a poly(DMAEMA99-co-CPMA1) microgel (dry thickness: 300 nm).

pKa,PDMAEMA,networkz5.2; buffering zone: pHZpKaG2) [31,32]. At a pH below 5, the gel was swollen to such an extent that it was released from the gold substrate, and no further measurements could be performed.

4. Conclusion It can be concluded that the objective of creating a fast, multi-responsive system has been reached, as the nano- or micrometer thin polymer coatings consisting of photocrosslinked PDMAEMA, react within a few seconds both to temperature and pH changes with a change in swelling degree. First, the thermo-responsive behavior has been confirmed with DSC measurements of copolymer solutions. Then, the fast, reversible temperature and pH-response of the microgels has been evidenced in SPR measurements in a custom designed flow cell. Moreover, the photodimerization reaction, which was followed by UV spectrometry, occurs so fast and efficient that the copolymer is a well-patternable photo-resist material, with patterning resolutions in the micrometer range as evidenced in optical microscopy. This opens good prospects for a wide range of applications, for example in patterned ‘lab-on-a-chip’ experiments for adsorption of biomaterials [33], but also in textile applications as an alternative for thermal or plasma coatings.

Fig. 9. pH responsive measurement of the swelling of a poly(DMAEMA99co-CPMA1) microgel in the SPR flow cell.

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Acknowledgements The authors thank Dr Dirk Kuckling and Katja Kretschmer of the research group of Prof. Adler at the Institut fu¨r Makromolekulare und Textilchemie of the Technische Universita¨t Dresden, for their help with the SPR measurements. Jan Vanden Bulcke from the Faculty of Agricultural and Applied Biological Science of Ghent University is acknowledged for the CSLM measurements. The Institute for the Promotion of Innovation through Science and Technology in Flanders (IWTVlaanderen) and the ESF-programme SUPERNET are acknowledged for financial support.

References [1] Langer R. Nature 1998;392:5–10. [2] Dinarvand R, D’Emanuele A. J Controlled Release 1995;36:221–7. [3] Kuptsova S, Markvicheva E, Kochetkov K, Belokon Y, Rumsh L, Zubov V. Biocatal Biotransform 2000;18(2):133–49. [4] Kato N, Yamanobe S, Takahashi F. Mater Sci Engng C 1997;5:141–7. [5] Okano T, Yamada N, Okuhara M, Sakai H, Sakurai Y. Biomaterials 1995;16:297–303. [6] Huang RYM. Membrane science and technology series, 1. Pervaporation membrane separation processes. Amsterdam: Elsevier; 1991. [7] Lequieu W, Du Prez FE. Polymer 2004;45:749–57. [8] Arndt KF, Kuckling D, Richter A. Polym Adv Technol 2000;11: 496–505. [9] Kuckling D, Adler HJ, Arndt KF, Hoffmann J, Plo¨tner M, Wolff T. Polym Adv Technol 1999;10:345–52. [10] Kuckling D, Harmon ME, Frank CW. Macromolecules 2002;35: 6377–83. [11] Reiser A. Photoreactive polymers—the science and technology of resists. Chichester, UK: Wiley; 1989. [12] Yamagiwa K, Komi T, Yoshida M, Ohkawa A, Iida T. J Chem Engng Jpn 2001;34(9):1171–6.

[13] Chae KH, Sun GJ, Kang JK, Kim TK. J Appl Polym Sci 2002;86(5): 1172–80. [14] Vo CD, Kuckling D, Adler HJP, Schohoff M. Colloid Polym Sci 2002; 280(5):400–9. [15] Kuckling D, Adler HJP, Ling L, Habicher WD, Arndt KF. Polym Bull 2000;44(3):269–76. [16] Laschewsky A, Rekai ED, Wischerhoff E. Macromol Chem Phys 2001;202(2):276–86. [17] Chen Y, Yi M. Radiat Phys Chem 2001;61:65–8. [18] Cho SH, Jhon MS, Yuk SH, Lee HB. J Polym Sci Part B—Polym Phys 1997;35(4):595–8. [19] Webber GB, Wanless EJ, Butun V, Armes SP, Biggs S. Nano Lett 2002;2(11):1307–13. [20] Brandrup J, Immergut EH, Grulke EA, editors. Polymer handbook, 4th ed. New York: Wiley; 1999. [21] Subramanian K, Rami Reddy AV, Krishnaswamy V. Makromol Chem Rapid 1991;12:211–4. [22] Subramanian K, Nanjundan S. J Macromol Sci—Pure Appl Chem 2000;A37(10):1211–25. [23] Harmon ME, Jakob TAM, Knoll W, Frank CW. Macromolecules 2002;35(15):5999–6004. [24] Kelen T, Tudos F. J Macromol Sci Chem 1975;A9(1):1–27. [25] Tudos F, Kelen T, Fo¨ldes-Beresznich T, Turcsanyi B. J Macromol Sci Chem 1976;A10(8):1513–40. [26] Huan K, Bes L, Haddleton DM, Koshdel E. J Polym Sci, Polym Chem 2001;39(11):1833–42. [27] Verbrugghe S, Bernaerts K. Du Prez FE. Macromol Chem Phys 2003; 204(9):1217–25. [28] Loos W, Verbrugghe S, Goethals EJ, Du Prez FE, Bakeeva IV, Zubov VP. Macromol Chem Phys 2003;204(1):98–103. [29] Atkinson SDM, Almond MJ, Hollins P, Jenkins SL. Spectrochim Acta, Part A—Mol Biomol Spectrosc 2003;59(3):629–35. [30] Chae B, Lee SW, Ree M, Jung YM, Kim SB. Langmuir 2003;19(3): 687–95. [31] Simmons MR, Patrickios CS. Macromolecules 1998;31(25): 9075–7. [32] Simmons MR, Yamasaki EN, Patrickios CS. Macromolecules 2000; 33(8):3176–9. [33] Min JH, Baeumner AJ. Ind Engng Chem 2003;9(1):1–8.