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Reversible CO2-responsive and photopolymerizable prepolymers for stepwise regulation on demand Pengcheng Zhu, Zhixin Liu, Jun Nie, and Yong He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04383 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Industrial & Engineering Chemistry Research

Reversible CO2-responsive and photopolymerizable prepolymers for stepwise regulation on demand Pengcheng Zhu,a,b,c Zhixin Liu, c Jun Nie, a,c Yong He b,c,* a

State Key Laboratory of Chemical Resource Engineering, Ministry of Education, Beijing

University of Chemical Technology, Beijing 100029, PR China b

Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical

Technology), Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, PR China c

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

Beijing, 100029, PR China KEYWORDS: reversible CO2 response; photopolymerization; prepolymer; stepwise regulation

ABSTRACT: CO2-responsive and photopolymerizable prepolymers are designed and synthesized. Their good photopolymerization kinetics and reversible CO2 and N2 responses to a bulk state and in a solution are reported. Stepwise modulation is verified in template imprinting. Results reveal that stable and high-resolution patterns can be produced by imprinting liquid prepolymers in CO2 atmosphere, erased by warm heating, or retained permanently by chemical cross-linking through photopolymerization with the aid of a photoinitiator. By contrast, only blurry patterns are obtained in air. INTRODUCTION Stimulus-responsive polymers have been widely explored because of their potential for various applications in functional materials,1 optoelectronics,2 diagnostics,3 bio-imaging,4 sensing,5,6 medicine,7 and nanotechnology.7 Various stimuli, including gas, pH, temperature, light, redox

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agents, enzymes, ions, mechanical force, and electric and magnetic fields9-15, have been investigated in detail. Unlike other chemical stimuli that may accumulate byproducts, which cause contamination due to the repeated addition of chemical agents,16 CO2 can react with polymers containing certain responsive groups, including primary or secondary amine and amidine, on side chains to convert them into hydrophilic species, such as carbamate and bicarbonate.17 This process is readily reversible with an inert gas, such as nitrogen and argon, passing through the system or mildly heating to remove CO2. This CO2 switch on/off cycle can be used as a basis for versatile, reversible, and repeatable CO2-responsive processes without accumulating contaminants.18 CO2 as a continuous gas flow passing through polymers can be tuned precisely and can be modulated by the gas flow rate as a result of the corresponding stimulating strength. CO2 as a gas emitted by the massive combustion of fossil fuels has caused challenging global warming problems,19 and approaches that can help utilize CO2 will contribute to addressing environmental concerns.20 Many CO2-responsive polymers, including particles,21 hydrogels,22 micelles,23 fluids,24 surfactants,25 and emulsions,26 vesicles,27 membrane,28 have been studied. In 2009, Weiss et a.29 reported the reversible cross-linking of amino-polysiloxanes upon stimulation by CO2. In this process, the viscosity of polymers significantly increases from a flowing state to a gel state when CO2 is bubbled through polymers. Their initial state can also be recovered upon N2 aeration with heating. In 2011, Nagai et al.30 developed a hydrogel by using CO2 as a gallant that reacts with polyallylamine to form urea cross-linking points that change a polyallylamine aqueous solution into a non-flowing hydrogel. Zhao et al.31 discovered that a CO2-responsive microgel suspension can be transformed from a liquid into a solid-like material as CO2 is bubbled into it, and it returns to a liquid after CO2 is blown away with N2.


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CO2-responsive systems have yet to be used as functional materials because of their low mechanical strength and stability. Cross-linking can also be diminished during storage by air exposure or continuous heating. In our strategy, chemical cross-linking on demand will be further applied into these systems, in which photopolymerization is a good candidate because it can be performed mildly and rapidly with time and space controllability. This technique is also widely applied in fine manufacturing, 3D fabrication, photoresist, and biomaterial production. When prepolymers react with CO2 to form a physical cross-linking structure, their viscosity increases remarkably from a flowing state to a non-flowing state. They can easily retain the nonflowing state, return to their original state on demand, or be further chemically cross-linked upon UV light irradiation with the aid of a photoinitiator. With this characteristic, prepolymers can be applied to manufacture liquid materials stepwise. Such liquid prepolymers can also be coated on substrates and be converted into a non-flowing state after they are exposed to a CO2 environment. These semi-solid materials can be stored for subsequent photopolymerization and can thus form a permanent chemical cross-linking system with better adhesion than that of traditional systems from solid materials because this CO2 cross-linking system can be returned to their liquid forms by heat from photopolymerization. Patterns can be produced from liquid prepolymers through micro- and nano-imprinting technique combined with CO2 responsiveness, can be erased easily by mild heating, or can be chemically cross-linked through photopolymerization.

EXPERIMENTAL Materials.


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Vinylbenzyl chloride (VBC,99 wt%), propylamine (>99 wt%) were purchased from Tianjin Guangfu Fine Chemical Research Institute and were used without further purification, 2ethylhexyl acrylate (EHA, 99 wt%) was supplied by Aladdin Industrial Corporation and used as received,2-isocyanatoethylmethacrylate (IEM, 98wt%) was obtained from J&K Chemical and used without further purification, triethylamine (≥98.0 wt%) was obtained from Tianjin Fuchen Chemical Reagents Factory and used as received.2-Hydroxy-2-methyl-2-phenyl acetone (Irgacure 1173) and TPGDA (Tripropylene Glycol Diacrylate) was donated by Runtec Chemical (China) Co. Azobisisobutyronitrile (AIBN) were purchased from Aladdin Agents (China) and were purified by recrystallization in ethanol prior to use. 1-Dodecanethiol (99 wt%) was purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. All of the organic solvent (AR grade pure) was provided by Beijing Chemical Works and used as received.

Synthesis of the amino-containing methacrylate prepolymers Synthesis of amino-containing methacrylate oligomer consists of three steps which was described in Scheme. 1. Taking sample B in Table 1 as an example, the detailed processes are described as follows: Firstly, 5.28 g of VBC, 3.68 g of 2-EHA and 1.5 wt% n-dodecanethiol (to monomers) were dissolved in 50 mL of toluene, then the solution was poured into the reactor. A solution of 1.3 mol% (to monomer) AIBN in toluene (10 mL) was added dropwise to the reaction system at 79 o

C with magnetic stirring in nitrogen atmosphere. After 3 h, another 10 mL of 1.3 mol% (to

monomer) AIBN toluene solution was again added to the reactor dropwise and keep reaction for further 3 h. Then the product was evaporated under reduced pressure to remove the most solvent and then the condensed mixture was reprecipitated in n-hexane to remove the unreacted


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molecules. Afterwards the obtained syrup was dried under vacuum at 60 oC overnight. The oligomer was obtained as yellow syrup. The 1H NMR spectrum was provided in Fig. S1. In the second step, 7.98 g last step product was dissolved in 50 mL of DMF and poured into the dropping funnel, then 9.97 g propylamine and 3.03 g triethylamine were added to the three neck flask. With magnetic stirring, the DMF solution was dropped slowly into the reactor at room temperature and keep reaction for 8 h. Then the reactant was mixed with 50 mL dichloromethane to get a transparent faint yellow solution. The solution was washed with 1 mol/L hydrochloric acid in a separating funnel to get rid of the excessive propylamine, when the upper layer of colorless transparent aqueous solution reached pH of 7, then the lower layer of product solution was separated out and washed by 1mol/L NaOH solution and deionized water to make sure that the excess HCl was fully removed. After removing solvent by rotary evaporator and dry under vacuum at room temperature overnight, a faint yellow transparent viscous liquid polymer was obtained. In the third step, 5.18 g of the second step product was dissolved in 30 mL ethyl acetate and was added into a three neck flask, 0.98 g of IEM was then dropped into the flask under magnetic stirring in room temperature and keep reaction for 2 hours, then the solvent was removed with rotary evaporator and faint yellow viscous liquid polymer was obtained after dry under vacuum at room temperature overnight. The 1H NMR spectrum was showed in Fig. S1


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Scheme 1. Synthesis route of the prepolymers Table 1. Composition and molecular weight of the synthesized prepolymers

A

VBC/EHA mol/mol 6:4

AC mol% 40

DBC mol% 20

Mn/103 g/mol 7.3

B

6:4

40

20

3.2

C

6:4

60

20

7.0

20

2.9

Prepolymers

D 6:4 60 AC: Amino content, DBC: Double bond content. Instrumentation.

Infrared spectra (IR) were recorded on a Nicolet iS5 FT-IR (Thermo Fisher Scientific, USA) Spectrometer (32 scans at 4 cm-1). 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 prepolymers and 4 wt% Irgacure 1173 as photoinitiator was applied between two KBr crystals and irradiated with UV spot light source (30 mW/cm2, Rolence-100 UV, Taiwan, China). Because the decreasing of the absorption peak area was directly proportional to the number of polymerized double bond, the degree of conversion (DC) of the double bond could be expressed as follows:


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DCሺ%ሻ =

‫ܣ‬଴ − ‫ܣ‬௧ × 100 ‫ܣ‬଴

where A0 is the absorption peak area around 815 cm−1 before irradiation and At is the absorption peak area at irradiation time t. Differential scanning calorimetry (DSC) measurements were performed on a Q2000 calorimeter (TA Instruments, DE, USA) interfaced to a TA Thermal Analyst 3100 controller that was connected to an RCS90 cooling system under a N2 flow rate of 20 mL/min with a heating and cooling rate of 10 °C /min. Thermal gravimetric analysis (TGA) measurements were conducted on a TGA Q50 thermogravimetric analyzer (TA Instruments, DE, USA)under a N2 flow rate of 60 mL/min at a heating rate of 10 °C/min from room temperature to 600 °C. Transmittance spectra were measured on UV–Vis spectrophotometer Hitachi UV-3010 (Hitachi, Japan).The gel permeation chromatography (GPC) was performed with Waters 1515 GPC (Waters, USA) equipped with four chromatographic columns of Styragel HT2\HT3\HT4\HT6E, which was calibrated with standard polystyrene using THF as eluent. Rheological measurements were performed with an Anton Paar Physica MCR 301 rheometer (Anton Paar GmbH, Graz, Austria), equipped with a Peltier temperature-controller and parallel plates (25 mm diameter) at 25 °C. The data were collected using Rheoplus/32 Service V3.10 software. Before data were recorded, each sample was placed directly onto the steel base plate, and the upper parallel steel plate was moved into the 1 mm initial gap and contact with the upper sample surface, after which any excess substrate sample was removed. Then, the samples were kept undisturbed at 25 °C for 5 min to ensure thermal equilibration. Duplicate measurements were performed with new samples. Sample Preparations.


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CO2 response of prepolymers were performed by coating 1 mm depth liquid film on PTFE plate and then the plate was placed into an oxygen bomb purged with 2 MPa of CO2 for 5 min. The stripe patterns were formed through imprinting by PDMS stripe pattern template and observed by a Leica DM2500P polarization microscope. First a 10 µm resolution stripe pattern of tripropylene glycol diacrylate (TPGDA) on glass substrate was made through photolithography. Then poly(dimethylsiloxane) (PDMS) mold was produced by covering curable liquid PDMS on TPGDA mold and staying it overnight at 50 oC to let it cured to solid.32 Last, prepolymer C with 4 wt% of Irg 1173 as photoinitiator was dissolved in acetone to form 80 mg/mL solution and dipcoated on a glass substrate with 20 µm depth. Covered with PDMS mold, they were put into an oxygen bomb purged with 2 MPa CO2 and stayed for 5 min to make the prepolymer reacted with CO2, and the pattern could be well transferred from PDMS to physical crosslinking synthesized prepolymers.

RESULTS AND DISCUSSION FTIR was used to verify the structure of the obtained products, as illustrated in Fig. 1. The presence of peaks at 678 cm-1 for C-Cl group and 1726 cm-1 for C=O and absence of peak at 809 cm-1 for acrylate double bond in Fig. 1(a) confirmed the first step product. And the existence of peaks at 1637 cm-1 and 815 cm-1 for methacrylate double bond and 1726 cm-1 for C=O, 3322 cm1

for N-H, and disappearance of peaks at 2277 cm-1 for NCO in Fig. 1(c) proved the successful

synthesis of target polymer containing double bond and secondary amine group in the side chain of polyacrylic backbone.


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Figure 1. Infrared spectra of the first step (a), the second step (b) and the third step (c) products of prepolymer D

The CO2 response of prepolymer solutions was investigated. The prepolymer D was dissolved in acetone to form a transparent solution with a concentration of 50 g/mL, when bubbling CO2 through the solution with a flow rate of 30 mL/min, no evident change was observed within the initial 40 s (Fig. 2a). However, the transparency of the solution gradually decreased and became thoroughly opaque at 100 s possibly because of the formation of ammonium carbamate from a secondary amine and CO2 that could cross-link the prepolymer and considerably decrease its solubility. The aeration gas was changed to N2, and the opaque solution gradually returned to a transparent one within 600 s (Fig. 2c). N2 recovered ammonium carbamate back to a secondary amine and CO2 and released CO2, thereby causing the prepolymer solution to return to its original state. These results confirmed the good and reversible CO2/N2 responsive property of such prepolymers in an acetone solution.17 The reversible CO2/N2 response of prepolymer D solution was characterized by using a UV– Vis spectrophotometer. The real-time change in the transmittance of this solution (30 mg/mL)


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with a CO2 and N2 purging flow rate of 30 mL/min was determined at 800 nm. In terms of CO2 responsiveness (Fig. 2b), after an induction period of 10 s, the transmittance rapidly decreased from 88% to 45% at 24 s and to 0 at 60 s with the continuous CO2 bubbling in the solution. The transmittance then remained constant, indicating that the solution became completely turbid, and the response was accomplished. Likewise, we conducted a reversible N2 responsiveness test at the same wavelength and flow rate (Fig. 2d). In the first 50 s of aeration, the transmittance remained 0 because the bubbled N2 only expelled CO2 dissolved in the solution instead of desorbing CO2 that reacted with the amino-containing prepolymer in the previous aeration period. The obvious increase in transmittance after 50 s implied that N2 completely released CO2 dissolved in the solution, started to induce the decomposition of formed ammonium carbamate to amine and CO2, and pushed the gas out. The N2 response rate increased after 50 s and reached the highest content at 99 s with 40% transmittance. The solution returned totally to the original transparent state at 200 s. The reversible N2 response was generally slower than the CO2 response. The N2 response needed 200 s to recover to the initial state, whereas the total CO2 response process could be achieved within 60 s. Secondary amines could react rapidly with CO2 to form carbamate and participate from solutions, whereas such amines hardly directly reacted with N2 in the recovery process. N2 should initially replace the dissolved CO2 in the solution first and subsequently stimulate the reversible reaction to the back direction to regenerate soluble prepolymer. Hence, N2 response was slower than CO2 response. This CO2/N2 response showed a good cycle property. The transmittance remained at approximately 90% of the original value after 10 cycles of reversible processes, and each process involved 300 s CO2 response and 300 s N2 response (Fig. 2e).


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Figure 2. Photographs and transmittance change curve versus time of prepolymer D acetone solution in CO2 response (a, b), reversible N2 response (c, d), and recyclability (e). The response behavior of pure prepolymers is more important than that of prepolymer solutions because the former can disclose the state transition process and the degree to which it can occur upon CO2 stimulation. For example, the graphs of prepolymer D (Fig. 3a) show that the prepolymer changed from a flowing state to a non-flowing state after 5 min of CO2 aeration. N2 combination with mild heating in a 50 °C bath was purged to overcome the slow diffusion rate in high-viscosity systems, and the prepolymer returned to the initial flowing state.


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Figure 3. Prepolymer D before (left) and after (right) bubbling through CO2 (a), viscosity before (black) and after (gray) bubbling through CO2, and CO2 absorbed (blue) by four prepolymers (b). The process was also verified through differential scanning calorimetry (Fig. S2) 29,. The DSC thermogram of prepolymer D after CO2 response presents a clear endothermic peak at 60 °C in the first heating process, and this curve corresponded to the decomposition of ammonium carbamate and the release of CO2. In the second run, no peak was discernible, indicating that no carbamate remained. The first run curve exhibited a glass transition temperature (Tg ) at −27.42 °C, which was higher than that in the second run (−45.68 °C). Therefore, the CO2 response process could increase Tg of prepolymer D by 18 °C. This observation also explained why the prepolymer was transformed from a flowing state to a non-flowing state. The temperature range of the endothermic peak in the first run demonstrated that CO2 could be easily removed from the prepolymer through moderate heating. Static rheological investigations performed on all of the prepared prepolymers before and after CO2 response showed that the steady-shear viscosities were independent of the shear rate over the explored range. The viscosity before the response was low between 10 and 200 Pa.s. The


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viscosities of A and B were much lower than those of C and D because of the lower phenyl content in the former than in the latter (Fig. 3b, black and gray). When their amino contents were the same, the initial viscosities of high-molecular-weight A and C were high. After the CO2 response was achieved, the viscosity of the prepolymers increased by two to three orders of magnitude. The amino content, as well as molecular weight and phenyl content, might influence viscosity because numerous amino groups corresponded to considerable cross-linking points for CO2 to “lock” the polymer chain and subsequently increase the viscosity of the resulting crosslinked polymer network. Thus, the highest viscosity before (190 Pas) and after (1.0×105 Pas) CO2 response should belong to prepolymer C, which has high molecular weight, amino content, and phenyl content. The thermogravimetric analysis results (Fig. S3) also demonstrated that the prepolymers with high amino content and low molecular weight could absorb substantial amounts of CO2 (Fig. 3b, blue).

Figure 4. Stripe patterns of prepolymer C imprinted on a PDMS template, imprinted pattern in air (a); imprinted pattern in CO2: just imprinted (b); after storing with CO2 for 7 days (c); after


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the photopolymerization in air (d); after heating at 50 °C (e); and photopolymerization kinetics of different prepolymers with 4 wt% Irgacure 1173 as a photoinitiator at 30 mW/cm2 irradiation intensity (f). Prepolymer C, which presented the largest increase in viscosity, was used to conduct an imprinting experiment with a PDMS template to verify its application feasibility. In Fig. 4b, after the PDMS template was removed, the pattern was observed with a Leika microscope for the system in the CO2 environment. However, the pattern of the system in air was blurred (Fig. 4a) because the prepolymer easily flowed around, and the stripe pattern was destroyed. Consequently, the pattern could not be clearly copied without the aid of CO2. The stability of the imprinted pattern (Fig. 4c) in CO2 atmosphere was satisfactory. As a result, the pattern remained in good shape and did not lose any detail. After the prepolymer was heated at 50 °C for 2 min, the stripes completely disappeared (Fig. 4e), indicating that the produced pattern could be erased upon heating, while it is believed useless thereafter or should be permanently deleted. Afterward, it is ready for reuse. Upon UV irradiation, the imprinted pattern containing some photoinitiators could photopolymerize to form a cross-link network and be saved permanently (Fig. 4d). Mild heating from the initial photopolymerization could induce the occurrence of the subsequent photopolymerization in a low viscosity state by returning the system to its original state before CO2 response, which could not affect the pattern resolution due to the very short photopolymerization time. As a consequence, its rate and adhesion increased. Therefore, the system could be used to transfer the pattern on substrates on demand. The photopolymerization kinetic results tested by real-time Fourier transform infrared spectroscopy (Fig. 4f) confirmed that the four synthesized prepolymers showed good photopolymerization activities, in which over 90% double bond conversion could be reached within 300 s with different rates.


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CONCLUSIONS CO2-responsive photopolymerizable prepolymers showed a good CO2/N2-induced transparentturbid transition in an acetone solution with good recyclability evidenced by vision and UV–vis spectroscopy. The prepolymers could undergo transition from a flowing state to a non-flowing state due to a remarkably change in viscosity upon CO2 and N2 stimulation, which could be more than two orders to three orders of magnitude. A high amino content led to a large viscosity variation. The synthesized prepolymers also showed good photopolymerization ability. Thus, they could be used as a step-wise physical–chemical cross-link to achieve various effects. The pattern with 10 µm resolution could be more stable when such prepolymers were imprinted under a PDMS mold in CO2 atmosphere than when they were imprinted under a PDMS mold in air through a CO2 physical cross-link, which could be erased completely by simply heating or be permanently fixed through chemical cross-linking by photopolymerization with a photoinitiator on demand. This strategy of designing multifunction prepolymers would enable scientists and technologists to develop additional applications in wide fields of UV curing and CO2-responsive technologies, such as information or pattern recording, erasing, or transferring on demand. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 1

H NMR spectra of different steps products of prepolymers B; the DSC curves of prepolymer D

after response to CO2; and the TGA curves of different prepolymers before and after the response

to CO2. AUTHOR INFORMATION Corresponding Author


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*Phone: +86-10-64421310. E-mail: [email protected],cn. ORCID Yong He: 0000-0002-4689-966X Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors thank National Key Program of China (2017YFB0307800) and National Natural Science Foundation of China (51373015 and 51573011) for their financial support. REFERENCE 1. Katsuno, C.; Konda, A.; Urayama, K.; Takigawa, T.; Kidowaki, M.; Ito, K., PressureResponsive Polymer Membranes of Slide-Ring Gels with Movable Cross-Links. Adv. mater. 2013, 25 (33), 4636-4640. 2. Tee, C. K.; Wang, C.; Allen, R.; Bao, Z., An electrically and mechanically self-healing composite with pressure- and flexion-sensitive properties for electronic skin applications. Nat. Nanotechnol. 2012, 7 (12), 825-832. 3. Lee, S. M.; Ahn, R. W.; Chen, F.; Fought, A. J.; O'Halloran, T. V.; Cryns, V. L.; Nguyen, S. T., Biological Evaluation of pH-Responsive Polymer-Caged Nanobins for Breast Cancer Therapy. ACS Nano. 2010, 4 (9), 4971-4978. 4. Shim, M. S.; Kwon, Y. J., Stimuli-responsive polymers and nanomaterials for gene delivery and imaging applications. Adv. Drug Delivery Rev. 2012, 64 (11), 1046-1059. 5. Paek, K.; Yang, H.; Lee, J.; Park, J.; Kim, B. J., Efficient colorimetric pH sensor based on responsive polymer-quantum dot integrated graphene oxide. ACS Nano. 2014, 8(3), 28482856. 6. Hamer, M.; Lázaro-Martínez, J. M.; Rezzano, I. N., Fluorescent responsive chlorophyllide-hydrogel for carbon dioxide detection. Sensors and Actuators B: Chemical 2016, 237, 905-911. 7. Mura, S.; Nicolas, J.; Couvreur, P., Stimuli-responsive nanocarriers for drug delivery. Nat. mater. 2013, 12 (11), 991-1003. 8. Park, J. S.; Cho, M. K.; Lee, E. J.; Ahn, K. Y.; Lee, K. E.; Jung, J. H.; Cho, Y.; Han, S. S.; Kim, Y. K.; Lee, J., A highly sensitive and selective diagnostic assay based on virus nanoparticles. Nat. Nanotechnol. 2009, 4 (4), 259-264. 9. Dai, S.; Ravi, P.; Tam, K. C., pH-Responsive polymers: synthesis, properties and applications. Soft Matter 2008, 4 (3), 435-449. 10. Chilkoti, A.; Dreher MRMeyer, D. E.; Raucher, D., Targeted drug delivery by thermally responsive polymers. Adv. Drug Delivery Rev. 2002, 54 (5), 613-630. 11. Gohy, J. F.; Zhao, Y., Photo-responsive block copolymer micelles: design and behavior. Chem. Soc. Rev. 2013, 42 (17), 7117-7129. 12. Huo, M.; Yuan, J.; Tao, L.; Wei, Y., Redox-responsive polymers for drug delivery: from molecular design to applications. Polym. Chem. 2014, 5 (5), 1519-1528.


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GRAPHICAL ABSTRACT Pengcheng Zhu, Zhixin Liu, Jun Nie, Yong He* Reversible CO2-responsive and photopolymerizable prepolymers for stepwise regulation on demand CO2-responsive and photopolymerizable prepolymers are designed. Reversible CO2 and N2 responses for bulk and solution are described, and their photopolymerization kinetics are reported. Stepwise modulation is obtained through template imprinting to produce or erase highresolution patterns on demand by combining reversible CO2 response and photopolymerization or mild heating. GRAPHICAL ABSTRACT FIGURE

In Air

In CO2

Heating

Coating

Substrate

UV Curing

PDMS template


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Reversible CO2-responsive and ... - ACS Publications

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