Photocleavable epoxy based materials

Accepted Manuscript Photocleavable epoxy based materials Simone Radl, Manuel Kreimer, Jakob Manhart, Thomas Griesser, Andreas Moser, Gerald Pinter, Gerhard Kalinka, Wolfgang Kern, Sandra Schlögl PII:

S0032-3861(15)30019-7

DOI:

10.1016/j.polymer.2015.05.055

Reference:

JPOL 17893

To appear in:

Polymer

Received Date: 21 March 2015 Revised Date:

28 May 2015

Accepted Date: 30 May 2015

Please cite this article as: Radl S, Kreimer M, Manhart J, Griesser T, Moser A, Pinter G, Kalinka G, Kern W, Schlögl S, Photocleavable epoxy based materials, Polymer (2015), doi: 10.1016/ j.polymer.2015.05.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

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hv

NO2

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CHCl3

hv O

R1

400

O

NO O

+

HO

250 200 150 100

H

hv

50 0 0

20

40

60

80

100

displacement / µm

120

R1 O

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300

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force / mN

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450

R2

ACCEPTED MANUSCRIPT

Photocleavable epoxy based materials Simone Radla, Manuel Kreimera, Jakob Manharta, Thomas Griesserb,c, Andreas Moserd, Gerald Pinterd, Gerhard Kalinkae, Wolfgang Kerna,b, Sandra Schlögla,*

Polymer Competence Center Leoben GmbH, Roseggerstraße 12, A-8700 Leoben, Austria

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a

Tel : (+43) 3842 402 2354; Fax: (+43) 3842 402 2352; E-mail: [email protected] b

Chair of Chemistry of Polymeric Materials, University of Leoben, Otto Glöckel-Straße 2, A-8700

Leoben, Austria

Christian Doppler Laboratory for Functional and Polymer based Ink-Jet Inks, Otto Glöckel-

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c

Straße 2, A-8700 Leoben, Austria d

Chair of Materials Science and Testing of Plastics, University of Leoben, Otto Glöckel-Straße 2,

e

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A-8700 Leoben, Austria

BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205

Berlin, Germany

Abstract

The present study aims at the development of photodegradable epoxy based

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materials comprising o-nitrobenzyl ester links that undergo well defined bond cleavage in response to UV irradiation. New bi-functional epoxy based monomers bearing onitrobenzyl ester groups are synthesized and thermally cured with an anhydride hardener to

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yield photosensitive polymers and duromers. The UV induced changes in solubility are exploited for the preparation of positive-type photoresists. Thin patterned films are

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obtained by photolithographic processes and characterized by microscopic techniques. The results evidence that sensitive resist materials with good resolution and high contrast behavior can be accomplished. Along with resist technology, the applicability of onitrobenzyl chemistry in the design of recyclable polymer materials with thicknesses in the millimeter range is evaluated. By monitoring the thermo-mechanical properties upon UV illumination, a distinctive depletion of storage modulus and glass transition temperature is observed with increasing exposure dose. Additionally, single fiber pull-out tests are carried out revealing a significant decrease of the interfacial adhesion at the fiber-matrix interface due to the phototriggered cleavage reaction. Keywords: epoxy based network; photocleavage; o-nitrobenzyl ester 1

ACCEPTED MANUSCRIPT 1. Introduction Whilst the photoisomerization of o-nitrobenzyl (o-NB) alcohol derivatives was first described in 1966, the pioneering work of Woodward et al. on the employment of o-NB as photolabile protecting group in organic chemistry has become the starting point for the extensive use of this photocleavage reaction in the architecture of polymer materials.[1–3]

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Recently, Wirz et al. have studied the phototriggered isomerization reaction of o-NB links in detail. Upon UV irradiation with wavelengths between 300 and 365 nm the photoisomerization of o-NB esters yields the corresponding o-nitrosobenzaldehyde under the release of a carboxylic acid (see Figure 1).[4] It was found, that the formation of

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photodimerized byproducts can be avoided by attaching selected substituents onto the

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aromatic ring or on the benzyl position of the o-NB linker.[5]

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Figure 1 – Photoisomerization of o-NBE derivatives Along with the use as protecting groups in organic synthesis, the controlled

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cleavage of covalent links simply by light exposure has made the photoisomerization of oNB a popular reaction to create photocleavable block copolymers and bioconjugates.[6–8] The photochemistry of o-NB alcohol derivatives has also gained increased attention in the side functionalization of copolymers and in the preparation of photo-responsive selfassembled monolayers.[9,10] In an effort to combine photodegradable properties with good biocompatibility and low cytotoxicity the development of o-NB-based hydrogels comprising photocleavable crosslinks presents a unique opportunity.[11,12] In particular, biocompatible polymer networks based on poly(ethylene glycol), dextran, agarose or hyaluronan bearing o-NB-linkers have been developed which are promising materials for tissue engineering and controlled drug delivery.[13–16] One of the salient features of 2

ACCEPTED MANUSCRIPT photocleavable hydrogels is the spatially controlled degradation of the network in response to UV exposure. Recent studies describe the preparation of photopatternable o-NB based hydrogels, which represents an innovative strategy towards a well-defined immobilization of proteins and the preparation of materials for biomedical applications.[17,18] Going a step beyond one-photon UV exposure, Kasko et al. used two-photon photolithographic

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techniques to accomplish defined 3D channels in hydrogels for a controlled migration of cells.[19] In addition, Li and coworkers demonstrated the synthesis of new photodegradable polymers bearing o-NBE links that can be further modified by “click” reactions such as UV induced thiol–ene chemistry or copper catalyzed azide–alkyne

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cycloaddition for the preparation of reactive micropatterns.[20]

Besides the patterning of hydrogels, the photochemistry of o-NB has become

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widely used in the design of photoresists that are based on UV induced changes in polarity and solubility.[3] It represents a versatile route as it allows the photogeneration of organic acids (e.g. from o-NB carboxylates and sulfonates) as well as organic bases (e.g. from oNB carbamates).[21–23] In particular, positive-type photoresists were developed by incorporating o-NB ethers in the polymer backbone of poly acrylates and polyethylene glycols or by the synthesis of polyimide precursors with pedant o-NB groups.[24,25] The

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successful implementation of o-NB-based photoresists in organic electronics was demonstrated by Zojer et al. who applied photoreactive layers containing o-NB groups for organic thin-film transistors.[26] Another study from Wilkins and coworker describes the synthesis of an o-NB ester of cholic acid which was used as photosensitive solution

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inhibitor for copolymers of methyl methacrylate and methyl acrylic acid. Upon illumination with deep UV the insoluble o-NB ester based inhibitor was cleaved to

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products which were highly soluble in alkaline solutions promoting positive-type photoresists with resolutions in the range of 0.5 µm.[27] Taking advantage of the photocleavage of o-NB linkers, the present work focuses

on the design of photodegradable epoxy based thermosetting resins. Bi-functional epoxy based monomers bearing photolabile o-NB ester (o-NBE) moieties are synthesized and photodegradable polymers and duromers are then achieved by a thermal curing step using an anhydride based hardener and a tertiary amine acting as accelerator. Upon UV irradiation (λ = 300 - 350 nm) photocleavage of the o-NBE groups and efficient decomposition of the highly crosslinked network and polymer chains is accomplished.

3

ACCEPTED MANUSCRIPT Due to the incorporation of photolabile links within highly crosslinked epoxy based networks, this strategy offers a new approach towards duromer materials with improved recycling properties. Following the visions of a low-carbon-economy, the development of new chemical recycling techniques for highly crosslinked materials such as cured thermosetting resins has become a key issue in current materials research.[28,29] As

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crosslinked polymer materials, in particularly thermosets, cannot be remelted or reshaped such as thermoplastics, recycling at the end of their life cycle poses a demanding challenge.[30] Industrially established recycling techniques for thermosets still rely on thermal and mechanical comminution processes whilst the role of chemical recycling is

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negligible. However, in recent years numerous routes have been pursued to introduce sensitive crosslinks into the duromer network which can be cleaved more easily in

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response to an external stimulus such as heat, UV exposure or change in pH.[31] One prominent concept involves the application of reversible crosslinks, which can be controllably cleaved by heat or UV exposure.[32,33] Particularly, the reversible cleavage of disulfide links, the reversible acid-catalyzed ring opening polymerization of spiroorthoester and the reversible Diels-Alder cycloaddition reaction between bismaleimide and furan groups have been successfully employed to generate recyclable

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polymer based networks.[34–37]

Going a step beyond the application in thin films, the present work also highlights the employment of o-NBE chemistry for the preparation of recyclable crosslinked polymers and glass fiber reinforced composites. The efficient degradation of the

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photosensitive polymer and duromer materials with a thickness in the millimeter range is clearly demonstrated by thermo-mechanical analysis and gel permeation chromatography.

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In addition, single fiber pull-out tests evidence that the cleavage of the photolabile matrix influences the adhesion strength of the fiber-matrix interface which significantly decreases upon UV exposure.

2. Experimental Part 2.1 Materials All chemicals were purchased from Sigma-Aldrich (United States) unless otherwise stated and were used without further purification. (2-Nitro-1,4-phenylene) bis(methylene) bis(2-(oxiran-2-yl)acetate) was synthesized as previously reported.[38,39] 4

ACCEPTED MANUSCRIPT Caution: For preparative work, hazardous chemicals and solvents were employed. Reactions must be carried out in a fume hood and protective clothes and goggles must be used. 2.2 Synthesis of (2-nitro-1,4-phenylene) bis(methylene) bis(2-(oxiran-2-yl)acetate) (epoxyNBE)

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3-Butenoic acid (8.1 mL; 95 mmol) was added to a stirred solution of (2-nitro-1,4phenylene) dimethanol (6.7 g; 36.6 mmol) in 40 mL dichloromethane. The reaction mixture was cooled to 0°C and 4-(N,N-dimethylamino) pyridine (DMAP) (0.8 g; 6.5 mmol) was added. A solution of N,N'-dicyclohexylcarbodiimide (DCC) (20 g;

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96.9 mmol) in 50 mL dichloromethane was placed drop-wise to the reaction mixture that was subsequently stirred at room temperature for 2 h. The yellowish precipitate was

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collected by filtration and washed with 50 mL dimethylformamide. The yellowish solid was then dried under vacuum at 60°C. Chromatography on silica gel afforded the product which was analysed by 1H NMR spectroscopy (δ, 400 MHz, 20°C, CDCl3) 8.08 (d, 1H, ph3); 7.63-7.56 (m, 2H ph5,6); 5.97-5.87 (m, 2H, CH4a,4b); 5.51 (s, 2H, CH21b); 5.23-5.1 (m, 2H, CH5a’’,5b’’); 5.19 (s, 2H, CH21a); 5.16-5.65 (m, 2H, CH5a’,5b’); 3.29-3.15 (m, 4H, CH23a,3b) and 13C NMR spectroscopy (δ, 125 MHz, 20°C, CDCl3) 170.94 (s, 1C, C=O2b);

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170.66 (s, 1C, C=O2a); 149.24 (s, 1C, ph2); 147.49 (s, 1C, ph4); 137.37 (s, 1C, ph5); 132.87 (s, 1C, ph1); 131.77 (s, 1C, ph6); 129.56 (s, 1C, CH4b); 129.51(s, 1C, CH4a); 124.28 (s, 1C, ph3); 121.9 (s, 1C, CH25b); 119.05 (s, 1C, CH25a); 64.47 (s, 1C, CH21b); 62.08 (s,

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1C, CH21a); 38.85 (s, 1C, CH23b); 38.82 (s, 1C, CH23a). An excess of 3-chloroperbenzoic acid 77% (m-CPBA) (36.0 g; 162 mmol) was drop-wise

at

room

temperature

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added

phenylene)bis(methylene)bis(but-3-enoate)

to

(10.8 g;

a

solution 33.8 mmol)

of

(2-nitro-1,4in

54 mL

dichloromethane. The reaction mixture was stirred at room temperature for 2 days and 3chloroperbenzoic acid 77% (5.0 g; 22 mmol) was then added drop-wise to the solution again. This procedure was repeated and after a reaction time of 6 days a yellowish solution with a white-yellowish precipitate was obtained. The product mixture was solved with dichloromethane and the organic layer was extracted with saturated sodium bicarbonate solution and deionized water. Finally, the solution was dried over sodium sulfate, filtered and the solvent was removed by rotary evaporation at 30°C. Chromatography on silica gel (cyclohexane:ethyl acetate = 20:1) afforded the product as a yellowish liquid (50% of the 5

ACCEPTED MANUSCRIPT theoretical yield) which was analysed by 1H NMR spectroscopy, (δ, 400 MHz, 20°C, CDCl3) 8.11 (s, 1H, ph3); 7.67-7.61 (m, 2H, ph5,6); 5.56 (s, 2H, CH21b); 5.23 (s, 2H, anCH21a); 3.34-3.27 (m, 2H, CH4a,4b); 2.87-2.85 (m, 2H, CH5a‘,5b‘); 2.74-2.57 (m, 6H, CH23a, CH23b, CH5a,5b),

13

C NMR spectroscopy (δ, 125 MHz, 20°C, CDCl3) 131.63 (s, 1C, ph1);

147.56 (s, 1C, ph2); 124.41 (s, 1C, ph3); 137.27 (s, 1C, ph4); 133.02 (s, 1C, ph5); 129.49 (s,

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1C, ph6); 63.10 (s, 1C, CH21a); 169.62 (s, 1C, O=C2a-O); 37.86 (s, 1C, CH23a); 46.62 (s, 1C, CH4a); 47.74 (s, 1C, CH25a); 64.71 (s, 1C, CH21b); 169.88 (s, 1C, O=C2b-O); 37.91 (s, 1C, CH23b); 46.67 (s, 1C, CH4b); 47.78 (s, 1C, CH25b) and FT-IR spectroscopy (3003 cm-1, 1735 cm-1, 1532 cm-1, 1348 cm-1, 1324 cm-1, 1165 cm-1, 1001 cm-1, 905 cm-1, 840 cm-1, 13

C NMR spectra of epoxy-NBE are provided in the Supporting

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754 cm-1). The 1H and

Information (see Figure S1 and Figure S2).

1

H NMR and

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2.3 Physico-chemical characterization of the photolabile monomer

C NMR spectra were recorded with a Varian (United States) 400-

NMR spectrometer operating at 399.66 MHz and 100.5 MHz, respectively. The spectra were referenced to Si(CH3)4 and a relaxation delay of 10s and 45° pulse were employed for the acquisition of 1H NMR spectra.

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UV-Vis measurements were performed with a Varian (United States) Cary 50 UVVis spectrophotometer in absorbance mode. For irradiation experiments, epoxy-NBE was dissolved in acetonitrile (0.1 mg/mL), placed in a quartz cuvette and UV-Vis spectra were taken prior to and after UV exposure. UV irradiation was carried out with a medium

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pressure Hg lamp (Omnicure S1000, Lumen Dynamics, Canada). 2.4 Preparation and thermal curing of photodegradable resin formulations

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Resin formulations were prepared by dissolving different amounts of the

photolabile monomer epoxy-NBE in the low viscous glycidyl 2-methylphenyl ether (epoxy-2). Hexahydro-4-methylphthalic anhydride (1.0 epoxide equiv.) and N,Ndimethylbenzylamine (0.1 wt.% related to total reaction mixture) were then added to the epoxy based monomers and the resin mixtures were stirred at room temperature for 60 min. The composition of the photodegradable resin formulations is summarized in Table 1. Thermal curing was carried out at 70°C for 12 h yielding solid samples.

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ACCEPTED MANUSCRIPT Table 1 – Composition of photocleavable epoxy based resin formulations

sample

bi-functional

mono-functional

monomer

monomer a

hardener /

molar

accelerator

mol%b

ratioc

/ wt.%d

a

epoxy-2 / mol%

resin-1

100

0

100

1

0.1

resin-2

20

80

60

0.6

0.1

resin-3

10

90

55

0.55

0.1

resin-4

5

95

52.5

0.525

0.1

a

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epoxy-NBE / mol%

Molar concentration of the epoxy based monomers in the base resin Molar concentration of the anhydride crosslinker in the base resin c Molar ratio of anhydride crosslinker to epoxy monomers in the resin formulation d Weight percentage related to the total weight of the resin formulation

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b

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2.5 Characterization of the curing and cleavage kinetics

Both the thermally induced ring-opening reaction between epoxy monomers and anhydride based hardener as well as the photoinduced cleavage of the o-NBE links were characterized with FT-IR spectroscopy using a Vertex 70 spectrometer (Bruker, United States). 16 scans were accumulated with a resolution of 4 cm-1 and the absorption peak

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areas were calculated with OPUS software. All spectra were recorded in transmission mode. For sample preparation 10 µL of each resin formulation (see Table 1) were dissolved in 1 mL tetrahydrofuran and were then spin cast on CaF2 discs. The kinetics of the curing reaction was monitored at 70°C over 12 h by recording FT-IR spectra every

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hour. The cured thin films were then irradiated with a medium pressure Hg lamp (Omnicure S1000, Lumen Dynamics, Canada) and the cleavage reaction was monitored

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upon 20 min UV exposure. The light intensity (power density P; in mW cm-2) into the sample plane was determined with an integrating radiometer (Powerpuck II, EIT Instrument Markets, United States) while qualitative spectra were measured with a spectroradiometer (Solascope 2000TM, Solatell, United Kingdom). The power density was 20.75 mW cm-2 between 250 and 470 nm. Gel permeation chromatography (GPC) was performed on a Merck Hitachi system (Germany) with a combined refractive index – viscosity detector from Viscotec (Viscotec 200, Germany). Chloroform was used as eluent with separation columns of Polymer Standards Service, 8 x 300 mm STV 5 µm grade size (106, 104 and 103 Å). Polystyrene standards supplied by Polymer Standard Service were used for calibration. 7

ACCEPTED MANUSCRIPT 2.6 Preparation and characterization of patterned films Thin films were obtained by spin casting a tetrahydrofuran solution of resin-1 (10 wt.%) on Si wafer. The samples were thermally cured at 70°C for 12h. Photolithographic patterning was carried out with a mask aligner (MJB4, SUSS, Germany) equipped with a 500 W HgXe lamp. No filter was used and the exposure time amounted to 100 s. The

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illuminated films were developed by dipping the samples in chloroform. The patterned films were examined by confocal microscopy (MicroProf, Fries Research and Technology, Germany). The measuring frequency was 2,000 Hz and the measuring speed amounted to 607 µm s-1. The resolution in height was 10 nm and a lateral

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resolution < 2.5 µm was accomplished. In addition, the resolution of the patterned film was evaluated by scanning electron microscopy (Auriga 60, Zeiss, Germany) operating at

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20 kV after gold sputtering using an Agar Sputter Coater (Agar Scientific, United Kingdom).

Sensitivity and contrast were investigated by irradiating the spin cast Si-wafer with a medium pressure Hg lamp (Omnicure S1000, Lumen Dynamics, Canada) over different periods of time. After subsequent development in chloroform the insoluble gel fraction was determined by quantitative FT-IR spectroscopy using a Vertex 70 spectrometer

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(Bruker, United States) with a resolution of 4 cm-1 accumulating 16 scans. 2.7 Characterization of the thermo-mechanical properties The storage modulus (G0) was determined at 25°C with a Perkin Elmer Dynamic

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Mechanical Analyzer 8000 (United States) using a frequency of 1 Hz and amplitude of 20 µm. For sample preparation 1 mL of each resin formulation was placed in metal forms

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(40x10x1 mm), degassed in vacuum, cured at 70°C for 12 h and then were finally removed from the forms. By using a continuous inline UV irradiation with a medium pressure Hg lamp (Omnicure S1000, Lumen Dynamics, Canada) the change of the storage modulus upon prolonged UV exposure was determined. The power density was 96.4 mW cm-2 between 250 and 470 nm. The equipment was cooled to 25°C with a nitrogen flow to avoid heating of the sample during UV irradiation. Five samples were measured and the values were averaged. Differential scanning calorimetry measurements were performed with a MettlerToledo DSC 821e (United States) using a nitrogen flow of 20 mL min-1. The cured resins were heated from -30 to 90°C with a heating rate of 20°C min-1. The glass transition 8

ACCEPTED MANUSCRIPT temperature (Tg) was determined from the second heating run and was read as the midpoint in heat capacity. The maximum pull-out force Fmax was determined by single fiber pull-out tests as measure of the adhesion between glass fibers and the photodegradable epoxy based matrix prior to and after UV exposure. For sample preparation, commercially available glass

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bundles were treated with Caro’s acid (15 min) to remove the sizing and were then rinsed with deionized water. A single fiber was selected from the bundle, cut to a length of 10 mm with a scalpel and fixed to a steel fibre holder with cyan acrylate glue. The resin formulation was placed in a small hole (d = 1 mm) in an aluminum sample carrier with a

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syringe. The aligned fiber was then embedded with a length of approx. 250 µm in the resin droplet by using a homemade apparatus.[40,41] The resin was thermally cured at 70°C for 12 h after heating the aluminum sample carrier from room temperature to 70°C with a

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heating rate of 10°C min-1. To initiate cleavage reaction, the cured resin was treated with UV light with a medium pressure Hg lamp (Omnicure S1000, Lumen Dynamics, Canada) using a power density of 20.7 mW cm-2. The embedded fiber was then cut from the steel fiber holder. The free part of the fiber was glued onto a screw platform which was attached to a force transducer and the small aluminum block holding the polymer droplet with the

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embedded part of the fiber was attached to a piezo actuator. The embedded fiber was loaded at a ramp speed of 1.0 µm s-1 while time, force and displacement were recorded. The resolution of the used force transducer was better than 1mN. After each test, the pullout zone and the fiber surface were studied by a 3D laser scanning microscope (VK-X100,

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Keyence, Japan) as only the tests where the sample broke at the interface between fiber and matrix were employed for further data processing. Ten measurements were performed

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of each sample and averaged.

3. Results and discussion 3.1 Synthesis and design of photodegradable epoxy based materials (2-Nitro-1,4-phenylene) bis(methylene) bis(2-(oxiran-2-yl)acetate) (epoxy-NBE) as photodegradable bi-functional epoxy based monomer was obtained by two-step synthesis in 50% of the theoretical yield (see Figure 2). A Steglich esterification of (2-nitro-1,4phenylene) dimethanol (1) with 3-butenoic acid (2) afforded the product (3). The terminal olefin groups of (3) were epoxidized by a subsequent Prilezhaev epoxidation reaction 9

ACCEPTED MANUSCRIPT using an excess of 3-chloroperbenzoic acid. Epoxy-NBE (4) was purified by column chromatography and 1H NMR,

13

C NMR and FT-IR spectra were in accordance to the

proposed structure. All details to the synthesis procedure and to the characterization are

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given in the experimental part.

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Figure 2 – Synthesis of (2-nitro-1,4-phenylene) bis(methylene) bis(2-(oxiran-2-yl)acetate)

When it comes to the design of epoxy based materials bearing photocleavable o-

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NBE links, several factors have to be taken into account. Curing of the photolabile epoxy monomer with primary or secondary amines is not feasible as the ester group of the o-NBE is highly susceptible to hydrolysis in the presence of alkaline compounds. For thermal curing of the epoxy monomer an alternative route was pursued involving the employment of hexahydro-4-methylphthalic anhydride as hardener. A tertiary amine was selected as accelerator that increases the reaction rate and reduces the reaction temperature from 140

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to 70°C but does not hydrolyze the ester group of the o-NBE link (see Figure 3).

Figure 3 – Design of epoxy based photocleavable materials In terms of the photocleavage reaction, it has to be considered that o-NBE derivatives are well known for their high optical density.[3] Upon UV exposure the 10

ACCEPTED MANUSCRIPT characteristic absorption maxima of the o-NBE at 272 and 278 nm slightly decrease. At the same time the absorption bands between 240 and 250 nm increases whilst a new signal

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arises between 325 and 380 nm (see Figure 4).

Figure 4 - UV-Vis spectra of cured resin-1 (1) prior to and (2) after UV exposure with 70 J cm-2

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Along with thin films, the UV induced absorbance changes of the photocleavable monomer were determined in acetonitrile. In solution the decrease of the o-NBE absorption maxima and the formation of the photoproducts are more pronounced and the o-NBE absorption band disappears upon UV exposure with 28.9 J/cm2 (see Figure 5). The

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results correlate well with the absorption spectra published by Wirz et al. who have monitored the photoisomerization reaction of 2-nitrobenzyl methyl ether upon UV

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exposure with 254 nm and demonstrated that the increase of the signals at 240-250 nm and 325-380 nm can be related to the formation of the o-nitrosobenzaldehyde and azoderivatives.[42] Since both, o-NBE moieties as well as the cleavage products, reduce the light transmissivity of the material and thus the penetration depth of the incident light, the applicability of the new photodegradable materials is limited to thin films. Along with the high optical density, undesired crack formation that can be related to the high brittleness of the photocleavable duromers is observed in cured resins with a thickness in the millimeter range. The brittleness of materials can be governed by several factors including molecular structure, use of additives or crosslink density.[43–45] In the

11

ACCEPTED MANUSCRIPT present work, glycidyl 2-methylphenyl ether (epoxy-2) as mono-functional epoxy

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monomer was added (see Figure 3) to reduce the number of crosslink sites.

Figure 5 – Monitoring the UV-Vis absorbance of epoxy-NBE (0.1 mg/mL in acetonitrile) upon UV exposure

3.2 Characterization of the thermal curing and photoinduced cleavage in thin films

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The curing kinetics of selected resin formulations (see Table 1) with varying amount of epoxy-NBE was monitored by FT-IR spectroscopy. Thin films cast from the resin formulations on CaF2 substrates were used for the characterization of the crosslink

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reaction. The catalyzed ring-opening reaction between epoxy and anhydride moieties yielding ester bonds represents a state-of-the-art curing technique for epoxy based thermosetting resins and has been studied in detail in previous work.[46] In Figure 6 the

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FT-IR spectra of resin-1 are depicted prior to and after the thermal curing at 70°C. A depletion of the absorption peaks at 1860 and 1778 cm-1 is observed during the thermal curing at 70°C which can be assigned to the typical C=O absorption bands of the anhydride hardener. This decrease corresponds to the appearance of the absorption peak at 1740 cm-1 which can be attributed to the C=O signal of the ester group confirming the ring-opening reaction between epoxy monomers and anhydride hardener.[47] The curing kinetics was determined by following the disappearance of the IR signal related to the epoxy group at 917 cm-1.

12

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ACCEPTED MANUSCRIPT

12h

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Figure 6 – FT-IR spectra of resin-1 (1) prior to and (2) after thermal curing at 70°C for

Figure 7 illustrates the normalized peak area of the epoxy absorption band versus reaction time at a curing temperature of 70°C. The results give evidence that quantitative conversion is accomplished in each sample after 12 h whilst the reaction rate is not significantly influenced by the amount of the bi-functional epoxy-NBE that ranged

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between 5 and 100 mol%.

Figure 7 – Following the thermal curing of photolabile epoxy based resins by means of real time FT-IR spectroscopy: Depletion of the normalized epoxy band at 917 cm-1 in ) 10 mol%, ( ) 20 mol% and ( ) 100 mol% epoxyresins containing (

) 5 mol%, ( NBE. 13

ACCEPTED MANUSCRIPT The subsequent photocleavage reaction was characterized by following the conversion of the nitro group to the nitroso group. Figure 8 provides the FT-IR spectra of cured resin-2 prior to and after UV exposure. It is evident, that the characteristic absorption of the NO2 stretching band at 1537 cm-1 disappears upon UV exposure which

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gives rise to the photoisomerization of the o-NBE links.

Figure 8 – FT-IR spectra of resin-2 (1) prior to and (2) after UV exposure with 250 J cm-2

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By monitoring the depletion of the normalized NO2 signal, the rates of the cleavage reaction were determined in thin films (see Figure 9). From the results it can be concluded that the rates of cleavage are influenced by the amount of epoxy-NBE present in the cured resins. At low epoxy-NBE concentrations (5 mol%) a quantitative consumption of the

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nitro groups is accomplished with 60 J cm-2 exposure dose, whilst at higher concentrations (20 mol%) an exposure dose of 250 J cm-2 is required. In terms of resin-1 comprising 100 2

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mol% epoxy-NBE, the conversion yield is 92% after prolonged UV exposure (1260 J cm). The correlation between epoxy-NBE concentration and rates of cleavage might be

related to differences in both optical density as well as chain mobility. With a lower amount of o-NBE links the optical density decreases and thus the exposure dose required for complete cleavage is reduced. However, along with the light transmissivity, the ratio between mono- and bi-functional epoxy monomers also influences glass transition temperature as well as crosslink density of the materials and thus the mobility of the polymer chains. Although the cleavage rates obtained from FT-IR studies give a good indication of the corresponding network degradation, it has to be considered that undesired

14

ACCEPTED MANUSCRIPT side reactions such as the formation of photodimerized byproducts can lead to a

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reformation of covalent links again.[23]

Figure 9 – Following the photocleavage reaction of o-NBE links by real time FT-IR spectroscopy upon prolonged UV exposure: Depletion of the normalized intensity of the
) 5 mol%, ( ) 10 mol%, ( ) 20 nitro band at 1537 cm-1 in cured resins containing (
mol% and ( ) 100 mol% epoxy-NBE.

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Thermal curing of resin-3 and resin-4 comprising 5 and 10 mol% epoxy-NBE, respectively, yields non-crosslinked thermoplastic polymers which are highly soluble in chloroform. GPC measurements reveal that the molecular weight of these photosensitive

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polymers ranges from 2,790 g mol-1 for resin-4 to 3,170 g mol-1 for resin-3 (see Table 2).

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Table 2 – Mw and PD of the soluble fraction of cured photodegradable resins prior to and after UV exposure with 96.4 mW cm-2

Mw of soluble fraction / g mol-1

a

PD / -

sample

non illuminated

illuminated

non illuminated

illuminated

resin-1

-a

120,650

-a

4.60

resin-2

-a

4,000

-a

2.17

resin-3

3,170

2,210

2.04

1.73

resin-4

2,790

2,040

1.83

1.60

soluble fraction was not determined from crosslinked samples 15

ACCEPTED MANUSCRIPT When the amount of bi-functional epoxy-NBE exceeds 20 mol% in the resin formulation, crosslinked and non soluble epoxy based networks are achieved. The change in the molecular weight distribution and the formation of crosslinks is further reflected by the Tg of the cured materials (see Table 3).

96.4mW cm-2

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Table 3 – Tg of cured photodegradable resins prior to and after UV exposure with

sample

66

resin-2

60

resin-3

57

resin-4

54

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resin-1

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glass transition temperature / °C non illuminated illuminated 59 56 53 49

As expected, the Tg increases with both molecular weight of the photocleavabe polymers and crosslink density of the corresponding network. Correlating these results

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with the kinetics of the cleavage reaction obtained by FT-IR spectroscopy it can be clearly seen that the mobility of the polymer chains governs the cleavage rate. However, even in highly crosslinked epoxy based networks the photorearrangement reaction of the o-NBE

illumination.

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links proceeds reasonably efficient and promotes high cleavage yields upon UV

In a further step, the photoinduced change in the solubility of epoxy based

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networks was evaluated by sol-gel analysis. For this study, thin films of the cured resin-1 were illuminated for selected periods of time, developed with chloroform as a solvent and the insoluble fraction was investigated by quantitative FT-IR spectroscopy. Figure 10 depicts the gel fraction versus exposure dose. It is evident, that the gel fraction decreases upon prolonged UV exposure demonstrating the formation of soluble cleavage products and complete solubility is accomplished at an exposure dose of 55.5 J cm-2.

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Figure 10 – (1) Normalized film thickness versus exposure dose of the positive-type resist prepared from resin-1 after the development in chloroform and (2) determination of the contrast (γ) by calculating the slope of the linear area of the curve. The optical micrograph

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presents an inscribed pattern in detail.

3.3 Characterization of thin patterned films

The efficient conversion of insoluble epoxy based networks into soluble species was further exploited to prepare positive-toned photoresists. For these studies, thin films

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were spin cast from resin-1 on Si wafer. After curing at 70°C, the thin films were photopatterned by mask aligner equipment using a quartz-chromium mask. The inscribed

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patterns were characterized by microscopic techniques after the development step involving a dipping of the thin films in chloroform for a few seconds. The SEM and optical micrographs, provided in Figure 11a and 11c, demonstrate that the photodegradable epoxy based networks enable rapid development on the exposed areas whilst no significant swelling is observed in the crosslinked areas, which have not been illuminated. These results are confirmed by confocal microscopy (see Figure 11b) which gives a 3D visualization of the inscribed patterns. Without any optimization, a resolution of 8 µm was accomplished. The contrast, which provides a measure of the solubility change of the resist upon UV exposure, was further determined from the results of the solgel measurements. The normalized thickness of the film is plotted as a log function of 17

ACCEPTED MANUSCRIPT exposure dose and by taking the slope of the linear portion of the curve, a contrast of 4.8 is obtained (see Figure 10). The dose for clearance was further determined by extrapolating

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the linear portion of the curve to a gel content of 100% and amounted to 1.9 J cm-2.

Figure 11 – (a) Optical, (b) confocal and (c) SEM micrographs of photo patterned positive-type resists based on resin-1 after the development in chloroform

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3.4 Characterization of the thermo-mechanical properties of photodegradable epoxy based networks in macroscopic films

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Going a step beyond the application of photocleavable epoxy based materials in

thin films, the cleavage reaction was characterized in samples with a thickness of 1 mm. GPC measurements were performed to determine the efficiency of the photocleavage reaction in the macroscopic samples. In Table 2 the molecular weight of the photosensitive polymers with varying amount of epoxy-NBE and crosslink density, respectively, are summarized prior to and after UV exposure. In terms of the non crosslinked thermoplastic polymers prepared from resin-3 and resin-4, it is obvious that UV illumination leads to a decrease of both the molecular weight and the polydispersity (PD). Furthermore, it is evident, that even in highly crosslinked samples comprising only epoxy-NBE as base resin, the cleavage reaction proceeds efficient enough for the formation of soluble 18

ACCEPTED MANUSCRIPT fragments. Upon UV exposure with 96.4 mW cm-2, the crosslinked samples became completely soluble in chloroform and the soluble cleavage products yielded a molecular weight of 120,650 g mol-1 with a corresponding PD of 4.6. Although the results evidence the formation of soluble photocleavage products in both crosslinked and non crosslinked samples, the formation of photodimerized byproducts, which lead to a regeneration of

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covalent links again, cannot be excluded. To correlate the cleavage of the o-NBE links in the polymer backbone and the 3D network with thermo-mechanical performance, DSC and DMA experiments were carried out. In the first step, the change in Tg was determined prior to and after UV irradiation. As

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discussed in the previous section, the results clearly reveal that the Tg of the cured resin samples rises from 54 to 66°C with increasing amount of epoxy-NBE (5 to 100 mol%).

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On the one hand, this can be related to the increase in molecular weight of the photocleavable thermoplastic polymers obtained from resin-3 and resin-4. On the other hand, the higher Tg is also influenced by the formation of the 3D network and the rising crosslink density of cured resin-1 and resin-2. Upon UV exposure a significant decrease of the Tg in the range of 4 to 7°C is observed in each sample (see Table 3) which gives an indication of the UV induced bond cleavage in the epoxy based materials.

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In addition, the change in the storage modulus at room temperature was continuously monitored upon in-line UV illumination. It should be noted that it was not possible to characterize the performance of cured resin-1 as a larger sample geometry was required for these measurements and the brittle samples could not be taken out of the

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molds without cracking. Figure 12 provides the storage modulus versus illumination time for cured resin samples with varying concentrations of epoxy-NBE. In particular, the non

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crosslinked thermoplastic polymer prepared from resin-4 promotes a distinctive decrease of the storage modulus from 3,200 to 1,900 MPa in conjunction with a high depletion rate. The decrease of the storage modulus is less pronounced in the crosslinked sample obtained from resin-2 and does not exceed 300 MPa. It is obvious, that the results of the DMA measurements exhibit a good correlation with the photocleavage kinetics in thin films achieved by FT-IR spectroscopy. They confirm that the efficiency of the photocleavage reaction is influenced by the chain mobility. Consequently, a high molecular weight, a high glass transition temperature and a high crosslink density lower the efficiency of the photocleavage reaction. Furthermore, the formation of photodimerized byproducts might

19

ACCEPTED MANUSCRIPT be favored in a restrained and rigid matrix, which would also explain the low depletion of

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the storage modulus in crosslinked resin-2.

Figure 12 – Storage modulus of photocleavable epoxy based materials determined at room

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temperature versus illumination time (power density amounted to 96.4 mW cm-2); ( ) resin-2, ( ) resin-3 and (

) resin-4.

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3.5 Single fiber pull-out tests in photocleavable epoxy based networks Single fiber pull-out tests were finally performed to study the influence of the

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photoinduced matrix cleavage on the adhesion strength of the interface between non modified glass fibers and the photosensitive matrix. Prior to the experiments the sizing of the commercially available glass fibers was removed by a wet chemical etching with Caro’s acid to avoid any influences of the organic layer. The quantitative removal of the sizing from the glass surface was confirmed by photoelectron spectroscopy and zetapotential measurements. In Figure 13 the force-displacement curves for glass fibers embedded with a comparable embedding length in cured resin-2 prior to and after UV exposure are summarized. After each experiment, the surface of the glass fibers was characterized by laser scanning microscopy to evaluate the failure mode. 20

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Figure 13 – Force-displacement curves for single glass fibers in cured resin-2 with a comparable embedding length (1) prior to and (2) after UV exposure with 174 J cm-2. (a) The optical micrograph provides a glass fiber after the pull-out experiment where

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complete fiber-matrix debonding was accomplished.

With respect to non illuminated samples, the micrographs show relatively smooth and clean glass fibers with a small resin droplet at the end of the embedding length which

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indicates an adhesive failure at the interface between fiber and matrix (see optical micrograph in Figure 13). However, in the majority of the irradiated samples the matrix

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failed cohesively rather than the fiber-matrix interface and a part of the matrix remained on the glass fiber after the pull-out test. Although the change in the failure mode gives an indication of UV induced matrix cleavage, a quantitative determination of the adhesion strength is only possible if complete fiber-matrix debonding occurs. The apparent shear strength (τ) can be then estimated by τ = Fmax/A where Fmax corresponds to the maximum pull-out force and A to the embedded fiber surface. A can be easily calculated as follows A= dfiber*π*l where dfiber is the fiber diameter and l is the embedding length which were both measured by laser scanning microscopy.[48,40] To determine the apparent shear strength in illuminated samples more than 20 pullout tests were carried out to obtain ten force-displacement curves where fiber-matrix 21

ACCEPTED MANUSCRIPT debonding and thus adhesive failure mode was observed. Comparing the results a distinctive decrease of the apparent shear strength from 16.8 ± 3.8 MPa to 10.5 ± 3.6 MPa is accomplished upon UV exposure with 174 J cm-2. It is confirmed that the photoinduced cleavage reaction does not only influence the properties of the matrix but also the interface between inorganic filler and the photosensitive matrix. Although the efficiency of the

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cleavage reaction is limited by the sample thickness, the versatility of o-NBE photochemistry makes it an attractive route for the development of recyclable polymer materials and fiber-reinforced composites with film thicknesses in the millimeter range.

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4. Conclusions

Epoxy based polymers and 3D networks bearing o-NBE links promoting efficient

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photocleavage upon UV exposure have been prepared. In thin films, the photoinduced conversion of the nitro group to the nitroso group is strongly influenced by the mobility of the polymer chains. FT-IR measurements reveal that an increase in molecular weight and crosslink density corresponds to a decrease of the cleavage rate. However, sol-gel analysis of highly crosslinked networks evidences that the formation of soluble cleavage products

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is reasonably efficient with conversion yields above 90% at prolonged UV exposure. The conversion of insoluble epoxy based networks into soluble species was used to prepare positive-toned photoresists with a resolution of 8 µm and a contrast of 4.8. Along with thin films, the applicability of the o-NBE chemistry in samples with film thicknesses in the

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millimeter range was evaluated. A mono-functional epoxy monomer was added to the resin formulation in order to overcome the brittleness of the highly crosslinked samples

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and to avoid crack formation. The successful photocleavage of o-NBE links in polymers and even in highly crosslinked networks was demonstrated by GPC measurements. A distinctive decrease of both molecular weight and PD was observed. These results correspond to the change of the thermo-mechanical properties of the photolabile epoxy based materials which were measured prior to and after UV exposure. The DSC and DMA curves clearly show that both storage modulus and glass transition temperature decrease upon prolonged UV exposure evidencing the photocleavage of the o-NBE links. Along with the matrix properties, single fiber pull-out tests reveal that the cleavage reaction also influences the adhesion strength of the interface between glass fibers and the epoxy based materials. A decrease of the apparent shear strength at the fiber-matrix 22

ACCEPTED MANUSCRIPT interface is observed after UV illumination, which demonstrates the applicability of the photocleavable materials for thin composite materials with improved recyclability.

Acknowledgements The research work was performed within the K-Project “PolyComp” at the

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Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry for Economy, Family and Youth. Funding is provided by the Austrian Government and the State Government of Styria. The authors thank Melanie Brasch

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(PCCL) for the GPC measurements. T. Griesser thanks the Christian Doppler research

financial support. References

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association and the Austrian Ministry for Economy, Family and Youth (BMWFJ) for

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ACCEPTED MANUSCRIPT Highlights • Synthesis of photocleavable epoxy based monomers with o-nitrobenzyl ester links. • UV induced cleavage reaction is studied in epoxy based polymers and 3D networks. • Conversion of nitro groups is influenced by the mobility of the polymer chains. • Changes in solubility are exploited for preparation of positive-type photoresists.

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• Applicability for thin composite materials with improved recyclability is confirmed.

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Supporting Information

Photocleavable epoxy based materials Simone Radla, Manuel Kreimera, Jakob Manharta, Thomas Griesserb,c, Andreas Moserd,

a

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Gerald Pinterd, Gerhard Kalinkae, Wolfgang Kerna,b, Sandra Schlögla,*

Polymer Competence Center Leoben GmbH, Roseggerstraße 12, A-8700 Leoben, Austria

Tel : (+43) 3842 402 2354; Fax: (+43) 3842 402 2352; E-mail: [email protected]

Chair of Chemistry of Polymeric Materials, University of Leoben, Otto Glöckel-Straße 2, A-8700

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b

Leoben, Austria c

Christian Doppler Laboratory for Functional and Polymer based Ink-Jet Inks, Otto Glöckel-Straße 2,

d

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A-8700 Leoben, Austria

Chair of Materials Science and Testing of Plastics, University of Leoben, Otto Glöckel-Straße 2, A-

8700 Leoben, Austria e

BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin,

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Germany

Figure S1 – 1H NMR spectrum of epoxy-NBE

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Figure S2 – 13C NMR spectrum of epoxy-NBE