Photopolymerization of the coumarin-containing reversible photo

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Materials and Interfaces

Photopolymerization of the coumarin-containing reversible photo-responsive materials based on the wavelength selectivity Qiang Chen, Qian Yang, Pei Gao, Baihong Chi, Jun Nie, and Yong He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05164 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Industrial & Engineering Chemistry Research

Photopolymerization of the coumarin-containing reversible photo-responsive materials based on the wavelength selectivity Qiang Chen,a Qian Yang, a Pei Gao,a Baihong Chi,b Jun Nie a and Yong He a* a

College of Materials Science and Engineering, Beijing University of Chemical Technology,

Beijing, 100029, P. R. China b

Beijing Institute of Satellite Information Engineering, Beijing, 100086, P. R. China

ABSTRACT

Photo-response

of

the

coumarin

derivative

based

on

the

reversible

photodimerization and photocleavage has been studied for long time. This paper reports a novel approach of preparing photo-responsive polymer materials containing coumarin structure by photopolymerization based on the wavelength selectivity. A photo-polymerizable and photoresponsive monomer 7-(hydroxyethoxy)-4-methyl-coumarin (AECM) was synthesized and performed photo polymerization upon UV light irradiation (λ=405 nm). After polymerization the polymer with coumarin pendant groups can achieve reversible photoinduced [2+2] dimerization under UV light (λ>300 nm) and the photo-cleavage of crosslink to recovery back to the original structure through further irradiation with UV light (λ=254 nm). What’s more, this monomer could be used to produce high-resolution pattern and photo responsive pressure sensitive adhesive through photopolymerization. The demonstrated approach opens new perspectives for the design of photo-response materials. KEYWORDS: photo-response, coumarin, photopolymerization, wavelength selectivity

1.

Introduction

Stimulus response systems based on photo-reversible reaction have been widely investigated in recent years as smart functional materials such as shape memory polymer,1 self-healing materials 2-5

and self-folding.6 Photo-reversible reactions have an attractive interest because light is non-

destructive and allows remote activation and delivery of energy to a system resulting in a photo-

ACS Paragon Plus Environment

1

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response. What is more, photo-reversible reactions are considered as green synthetic pathways due to no residue left, and ability to be conducted at ambient temperature and often in the solid-state. At the same time, light-induced responses represent one of the most desirable methods for economical, easy, rapid, and efficient control of the material properties by tuning light parameters such as wavelength, power, and time of irradiation.7 Typically, photo-responsive materials include polymers bearing photosensitive groups in the main chain or pendant to the main chain.8 These materials are able to change reversibly between two specific situations, e.g. by bond forming and cleavage or by conformational changes, depending on the irradiation conditions. Photosensitive groups such as cinnamate,

9,10

anthracene,

3,11-12

thymine, 7 or coumarin derivatives13-16 are able to form covalent bonds reversibly, under controlled light irradiation. In the last few years, exciting advancements have been made in the development of various photo-responsive “reversible” polymeric architectures employing coumarin and its derivatives due to the excellent photo-stability and an extended spectrum range for excitation.1, 1719

Coumarin derivatives can undergo a [2+2] photodimerization upon irradiation with light

wavelengths above 300 nm by forming a cyclobutane ring, and photocleavage while irradiated with light of wavelength below 254 nm.20 In our knowledge that the reported photo-responsive materials are all prepared by thermal polymerization method.7,21-22 However, the thermal polymerization method presents several drawbacks such as the long time for polymerization, the complex process of post treatment, most important, the inability to prepare high resolution pattern. All of these drawbacks could overcome by photopolymerization, in which the only hinder is the undesirable photodimerization of photoresponsive group upon irradiation for photopolymerization. In our mind, this hinder can be perfectly solved by wavelength selectivity, which means that the light inducing the photopolymerization does not overlap with the wavelength range of photodimerization and photocleavage (Scheme 1). For photo-responsive groups, their sensitive wavelength could not be changed except changing the structure. However, the wavelength range for photopolymerization could be modulated through variety of methods, such as employing different initiators or sensitizers.23-27 Hence, the strategy to produce the photo-responsive materials through photopolymerization in this work is that firstly accurately define the sensitive wavelength of photodimerization and photocleavage, secondly, select the special initiator system not overlapped with these wavelength ranges, and lastly realize the photopolymerization of photo-responsive


2

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polymer materials and confirm their photo-response ability. In this way, the photo-responsive and high resolution patterned smart materials could be obtained, which could find grant potential in photo-responsive control released bio-materials, smart device by 3D printing.

Scheme 1. The structure of compounds used and illustration of the approach reported

2.

Experimental Section

Materials 7-Hydroxyl-4-methylcoumarin (HCM, 98%, Aladdin, China), ethanol (99.7%, Beijing Chemical Works, China), triethylamine (TEA, 99%, Sinopharm Chemical Reagent, China), acryloyl chloride (99.5%, Beijing Chemical Works, China), ethylene chloride (DCM, 99.5%, Beijing Chemical Works, China), acetonitrile (99.7%, Beijing Chemical Works China), dimethyl formamide (DMF, 99.5%, Beijing Chemical Works, China), tetrahydrofurfuryl acrylate (SR285, Sartomer, China.) ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPOL,98%, Tianjin Jiuri Chemical) and ethylene carbonate (98%, Aladdin, China)were all used without further purification. The linear polyacrylic polymer (LP32, prepared by random radical solution polymerization with butyl acrylate, octadecyl acrylate and 2-ethyl hexyl acrylate as monomers, Mw 320 k, PDI 2.8, Tg -55 o

C)

Instrument Real-time infrared spectra (RTIR) were recorded on a Nicolet 5700 instrument (Thermo, USA) and used to calculate the conversion of double bonds. The formula composed of monomers and photoinitiator were applied between two KBr crystals and irradiated with UV LED 405 nm spot light source (Lamplic, China). UV-Vis spectra were measured on a UV–vis spectrophotometer UV-3010 (Hitachi, Japan) from 200 to 500 nm with 300 nm/min scan speed. Morphology were


3


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investigated by optical microscope (Leica Microsystems Wetzlar GmbH, Germany). Peel strength was measured using a BLD-200S instrument (Labthink, China) at room temperature. The 180° peel tests were conducted between a glass substrate and a PET film. The test speed was 5 mm/s. The ZF-5 UV light source (254 nm, Qinke, China) and OmniCure S1000 (Mercury arc, 320-500 nm, Exfo, Canada) were used for investigation of photoresponse. Synthesis of the 7-(2-acryloyloxyethoxy)-4-methylcoumarin (AECM) The synthesis route of AECM was showed in Scheme 2. First, HCM (0.05 mol) and ethylene carbonate (0.05 mol) were dissolved in 40 mL of DMF. After adding of potassium carbonate (0.1 mol), the mixture was stirred for 10 h at 100 °C in nitrogen atmosphere. The product was precipitated in cold water and recrystallized twice from ethyl acetate. As a result, pure 7(hydroxyethoxy)-4-methylcoumarin (0.043 mol, HECM) was obtained. Second, HECM (0.01 mol) and TEA (0.02 mol) were dissolved in DCM (20 mL) and cooled in ice bath. A solution of acryloyl chloride (0.02 mol) in DCM (10 mL) was added dropwise into the mixture for 0.5 h at 05 ℃. The mixture was further stirred at room temperature for 12 h, and the precipitate was removed by filtration. The obtained solution was concentrated by rotary evaporation. Finally, the residue was recrystallized in ethyl acetate twice to obtain the 7-(2-methacryloyloxyethoxy)-4methylcoumarin (AECM). The characterization were included in Supporting Information (Fig. S1 and Fig. S2). O HO

O

Cl

O +

O

O

K2CO3

HO

O

O

O

O O

O

O

O

O

TEA ice bath

HCM

HECM

AECM

Scheme 2. Synthesis route of AECM Photopolymeriztion and photoresponse of monomer mixture Drops of the mixture of AECM (8 wt%)/TPO-l (2 wt%) and SR285 (90 wt%) were placed on a quartz slide and covered another quartz slide to control the thickness of the liquid film about 2030 um. After UV irradiation with UV LED 405 nm light source (10 mW/cm2) for 60 s and put it in darkness for 2 h. The obtained sample films are used to check photo-response by UV–Vis spectrophotometer. To further understanding the reversibility of photoresponse of material made through photopolymerization, it was alternatively exposed to >300 nm and 254 nm UV light and the


4

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conversion were measured by UV-Vis spectrometry and plotted as a function of UV irradiation time. The dimerization degree (PDD) and photocleavage degree (PCD) can calculate by the following equations. 4-5 PDD = (1 −

𝐴𝑡 ) × 100% 𝐴0

(1)

where A0 and At represent area of absorbance peak around 320 nm before and after time t exposure (>300 nm UV light) PCD =

𝐴𝑡2 −𝐴𝑡1 × 100% 𝐴0 − 𝐴𝑡1

(2)

where At1 represent the minimum area of absorbance peak around 320 nm after photodimerization and At2 represent the area of absorbance peak around 320 nm after time t exposure of 254 nm UV light, and A0 has the same meaning as that in Equation (1). Photo-patterning method The mixture of AECM (8 wt%)/TPO-l (2 wt%)/SR285 (90 wt%) were coated on glass and covered by a mask. After UV irradiation with UV LED 405 nm light source (10 mW/cm2) for 10 s and removing the mask, ethanol was used to remove the unpolymerized monomers, then the pattern was obtained (Scheme 3).

Scheme 3. Schematic illustration of photo-patterned Preparation of pressure-sensitive adhesive UV-curable PSA formulas were prepared by blending of the polymer LP32 (45.5 wt%) with photoinitiator TPO-l (1 wt%), SR285 (45.5 wt%) and AECM (8 wt%). They were coated onto PET using a bar-coater (100 μm thickness) then covered with a quartz slide, and irradiated upon UV LED 405 nm light source (10 mW/cm2) for 5 min. The reversible photoreactions were performed by irradiation with different wavelength light from the quartz side.

3. Results and Discussion


5


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Firstly, the wavelength selectivity of the synthesized coumarin-containing acrylate monomer (AECM) was investigated in acetonitrile solutions. As shown in Figure 1a, there is no meaningful absorbance in the range of longer than 360 nm, so 405 nm LED device could be tried as the non photodimerization sensitive light source, which is a kind of widely commercial used UV curing light source and completely spectroscopy different with AECM. Upon irradiation of 405 nm LED light (10 mW/cm2) for 60 min, there is no discernible change found in the absorption spectrum of AECM, which means that 405 nm irradiation will not lead to its photodimerization (Fig. 1a). However, exposure of AECM solutions to mercury lamp with Pyrex glass filter (only >300 nm light can effectively transmit, which UV absorption and transmission spectrum was shown in Fig. S3) resulted in the obvious decline of the absorbance peak at 320 nm (Fig. 1b), which indicates photodimerization effectively occurred. AECM can undergo photodimerization with >300 nm UV (30 mW/cm2) irradiation, while the 405 nm UV light will not trigger the reaction of photodimerization, So the 405 nm UV can be used as light source for the photopolymerization of photo-responsive AECM without leading to the photodimerization. Next, the photopolymerization kinetic of the AECM was investigated. Upon 405 nm UV irradiation at room temperature, as shown in Figure 1c, the acrylate double bond conversion of the AECM with 2 wt% TPO-l as photoinitiator in acetonitrile with the 40 wt% concentration arrived about 60 % at 60 s and 70 % after 350 s, and the max conversion rate appeared at 20 s. We also checked the photo-copolymerization kinetic of the mixture of AECM and SR285 (one commercial available acrylate monomer) with the ratio of AECM (9 wt%), SR285 (89 wt%) and TPO-l (2 wt%). The reason to select SR 285 as co-monomer is that AECM is solid and can be dissolved in liquid SR 285, which can realize solvent free liquid formula and lead to better film performance. Also, the best comprehensive property of SR285 in good solubility to AECM, low volatile rate and odor, and proper filming ability among the test monomers (isobornyl acrylate, 2-phenoxyethyl acrylate, butyl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate). In addition, the photopolymerization reactivity of SR285 should be much higher than AECM because of its higher double bond density, less stereo-hindrance and low viscosity. The double bond conversion of monomer mixture can achieve about 80 % at 25 s and 90 % after 100 s and the max conversion rate was observed at 10 s (Fig. 1d), which is fast enough to prepare a polymer film. So, this mixture system was adopted to further evaluation of the photo-response behaviour of the polymer produced through photopolymerization.


6

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Figure 1. UV-Vis adsorption spectra of AECM solution (4×10-5 mol/L in acetonitrile) upon irradiation of (a) 405 nm (10 mW/cm2) and (b) λ>300 nm UV light （30 mW/cm2）and double bond conversion and rate of AECM (c) and AECM+SR285 (d) as function of irradiation time with 2 wt% TPO-l as photoinitiator It can be seen from Fig. 2a that upon >300 nm UV light, absorption peak around 320 nm of P(AECM+SR285), obtained via photopolymerization, progressively decreased with the irradiation time increased, indicating that coumarin groups undergo photodimerization effectively. The equilibrium state achieved after approximately 120 s. And the subsequent exposure to 254 nm UV light resulted in the recovery of this absorbance peak (Fig. 2b), which demonstrated photocleavage of the dimerized coumarin fragments. The equilibrium achieved after 500 s. The adsorption spectra of P(AECM/SR285) and P(AECM) in acetonitrile solution alternate exposed to the λ>300 nm and 254 nm UV light were showed in Fig. S4 and Fig. S5. Their reversible change of the absorbance spectra upon irradiation with different wavelengths UV light indicates the reversibility of the photoreaction of AECM in both solution and solid state, which provides a possibility for fabrication of reversibly photoresponse. It can be seen from Fig. 2c and Fig. 2d that the photodimerization approaches to 90% of the equilibrium value at 120 s, which means that a maximum of 90% of coumarin moieties in the copolymer of P(AECM/SR285) can dimerize under the circumstances.4-5 However, the photocleavage showed lower reaction rate, just 45% of the equilibrium value reached at 700 s. It


7


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is also found that the maximum absorbance at 320 nm declines after each cycle of photodimerization and photocleavage, which exhibits the decay of the photo-reversibility. It should attribute to asymmetric fission of coumarin dimer during photocleavage under 254 nm light, which results in cleft products different from original coumarin moieties.28-32 We also calculate the photo-response kinetics of P(AECM) in acetonitrile solution (Fig. S6), which showed lower photodimerization and photocleavage degrees than P(AECM/SR285) due to the relative low AECM concentration.

Figure 2. Photodimerization (a) and photocleavage (b) of P(AECM/SR285) produced through photopolymerization and recyclability of photodimerization (c) and photocleavage (d) photodimerization: irradiation of λ >300 nm (30 mW/cm2) for 120s, photocleavage: irradiation of λ =254 nm (3 mW/cm2) for 500 s.

This kind of photopolymerizable and photo-responsive system could certainly be used in the preparation of fine resolution pattern photo-responsive materials. After mask exposure to 405 nm light with initiator, the different patterns could produce easily. As showed in Fig.3a–c, the circle, strip and grid were obtained for AECM/SR285 system with less than 0.002 mm resolution, which enable it very suitable to produce scaffold to load cells or photo switch. Also, this photoinduced dimerization and cleavage could lead to the volume change and consequently interface property modulation reversibly, which could be adopted in some on demand adherent materials. As an example, a series of photo-curing pressure sensitive adhesive


8

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(PSA) formulas (Fig. 3e) were prepared and the photo-response adhere ability measured. As shown in Fig. 3d, the non-photo-responsive PSA (A0) and 5 wt% AECM containing PSA (A1) showed 0.33 kN/m and 0.27 kN/m of 180°peel strength, in which the difference came from the rigid structure of AECM after photopolymerization. But after >300 nm irradiation, the 180° peel strength declined significantly to 0.19 kN/m (A2), then came back to 0.23 kN/m (A3) after 254 nm irradiation. The second reversible process can lead to 0.17 kN/m (A4), then came back to 0.22 kN/m (A5) peel strength, which means that the photo-response PSA achieved the stable state. This drop and back rise must obviously be attributed to the volume change resulted from photodimerization, because this dimerization and back reaction could lead to crosslinking or decrosslinking then change the interface morphology. The crosslink could increase the volume shrinkage and produce the deformation of the interface,33 then decrease the contact area of PSA and substrate, which could consequently decrease the adhere strength. In these reversible processes, the bulk glass transmission temperature (Tg) could not surpass the operation temperature, because the added linear polyacrylic polymer possesses extremely low Tg (-55 oC).

Figure 3 ． Optical images of the different P(AECM/SR285) pattern produced through photopolymerization （a, b and c）and peel strength of different press sensitive adhesive (d) and the formulas(e) of press sensitive adhesive

4. Conclusions


9


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In this work, we developed a novel photopolymerization method for preparing reversibly photoresponsive material containing coumarin structure based on the wavelength selectivity. In the approaches, the photopolymerization could achieve with very high double bond conversion in short time and the obtained materials showed quick and good recycled reversible photo-response. In addition, this kind of photo-responsive materials could be patterned to different grains on demand with high resolution and the photo modulated adhere ability change was realized. This strategy could be applied for smart photo-responsive biomaterials, switch or easy to peel off and reuse adhere materials, which is very important in microelectronic industry. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1

H NMR and FTIR spectra of AECM; UV absorption and transmission spectra of Pyrex glass;

UV−vis adsorption spectra of P(AECM+SR285) and PAECM after irradiation with different wavelength UV light; Photodimerization conversion and photocleavage conversion of PAECM. AUTHOR INFORMATION Corresponding Author *Phone: +86-10-64421310. E-mail: [email protected],cn. Yong He: 0000-0002-4689-966X Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors thank the National Key Research and Development Program of China (2017YFB0307800) and National Natural Science Foundation of China (51573011 and 51803010) for their financial support. REFERENCES (1) Rochette, J. M.; Ashby, V. S., Photoresponsive Polyesters for Tailorable Shape Memory Biomaterials. Macromolecules 2013, 46, 2134-2140. (2) Chan-Moon Chung, Young-Suk Roh,Sung-Youl Cho, and Joong-Gon Kim, Crack Healing in Polymeric Materials via Photochemical [2+2] Cycloaddition. Chem. Mater. 2004, 16, 3982-3984. (3) Froimowicz, P.; Frey, H.; Landfester, K., Towards the generation of self-healing materials by means of a reversible photo-induced approach. Macromol. Rapid. Commun. 2011, 32, 468-73.


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GRAPHICAL ABSTRACT Coumarin-containing acrylate can photo polymerize while remaining the good photodimerization activity, based on the wavelength selectivity. The obtained polymer can further perform reversible photoresponse upon different

wavelength irradiation.

The effective wavelength

for

photopolymerization is 405 nm, and that for photodimerization and photocleavage are >300 nm and 254 nm separately. GRAPHICAL ABSTRACT FIGURE


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Photopolymerization of the coumarin-containing reversible photo

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