Influence of Supramolecular Interaction Type on Photoresponsive

Jun 24, 2016 - aThe effect of three types of interactions on surface relief grating (SRG) formation is compared: (a) pure hydrogen bonding, (b) proton...
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Influence of Supramolecular Interaction Type on Photoresponsive Azopolymer Complexes: A Surface Relief Grating Formation Study Xin Wang,†,‡ Jaana Vapaavuori,‡ Xiaoxiao Wang,‡ Ribal Georges Sabat,§ Christian Pellerin,‡ and C. Geraldine Bazuin*,‡ †

School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, China 610500 Département de chimie, Centre de recherche sur les matériaux auto-assemblés (CRMAA/CSACS), Université de Montréal, C.P. 6128, succursale Centre-Ville, Montréal, QC, Canada H3C 3J7 § Department of Physics, Royal Military College of Canada, Kingston, ON, Canada K7K 7B4 ‡

S Supporting Information *

ABSTRACT: The influence of the supramolecular interaction typehydrogen bonding, proton transfer, and pure ionic bondingon photoresponsive behavior, particularly surface relief grating (SRG) inscription, has been investigated using a homopolymer with an ambient temperature glass transition (Tg). To this end, poly(dimethylaminoethyl methacrylate) (PDMAEMA or PDM) of relatively high molecular weight (42K) was complexed at various azo/DM molar ratios with dimethylaminoazobenzene derivatives functionalized by hydroxyl (azoOH) or carboxylic acid (p-methyl red or azoCOOH) groups, and quaternized PDM (PDMQ) was complexed at equimolar ratio with a sulfonated analogue (methyl orange or azoSO3), and the structural and thermal properties of these complexes were determined. SRG inscription on spin-coated films was found to be possible because complexation increases the Tg above ambient. The efficiency of SRG inscription on spin-coated films, which increases with azo content, is highest for the purely ionic azoSO3/PDMQ complex despite its very high Tg, while the proton transfer azoCOOH/ PDM complexes, involving ionic and H-bonding with acid-salt structures, show somewhat more efficient SRG inscription than do the purely hydrogen-bonded azoOH/PDM complexes, and this despite some azoCOOH crystallization at the highest molar ratios. These findings clarify the comparative effectiveness of different supramolecular bond types on SRG inscription and provide a useful guide for the design of supramolecular photoresponsive polymers for SRG applications.

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it are still not well understood.2,6,8,9 To reach a better experimental understanding as well as to design easily tailorable new materials, several groups in recent years have turned to a supramolecular strategy, where appropriately functionalized azo-containing small molecules are noncovalently bonded to passive polymers possessing complementary groups.10−27 This approach greatly facilitates the preparation of series of photoresponsive materials where specific material parameters can be varied systematically. Among the parameters that are crucial in designing supramolecular materials is the type of interaction which, for SRG studies, has included hydrogen,10−16 ionic,17−22 and halogen23−25 bonding. A number of these have been shown to lead to efficient SRG formation, but almost no direct comparisons between the different interaction types or strengths on SRG formation have been made, excepting analogous hydrogen- and halogen-bonding materials.23−25 For these, it was found that halogen bonding leads to more efficient SRG formation compared to hydrogen bonding,23−25 which

INTRODUCTION Azobenzene motifs, which undergo trans−cis photoisomerization under irradiation of light in the UV−vis region, are widely used in photoresponsive materials.1,2 These materials have a variety of potential applications, some of them based on the formation of surface relief gratings or SRGs. Photoinduced SRGs are inscribed in photoresponsive films by exposure to an interference pattern of light whose wavelength lies in the absorption band of the azo group, causing directional polymer chain motion on a macroscopic scale,1,3−5 a phenomenon that has given rise to an alternative term, directional photofluidization lithography,6 to describe SRG inscription. Because the periodicity, amplitude, and form of the gratings can be controlled by the optical setup, SRGs can be exploited in areas such as photonics, either by using SRGs directly or as microand nanopatterning templates for diffraction gratings, antireflection coatings, light-polarization converters, and plasmonic sensors.7 Although much research has been conducted on SRG formation with many different materials, the mechanism underlying it and the various material parameters that influence © XXXX American Chemical Society

Received: May 13, 2016

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DOI: 10.1021/acs.macromol.6b01009 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules was attributed to greater directionality and hence molecular linearity23,24 and to the absence of plasticization effects25 in the halogen-bonded systems. Many of the supramolecular systems used in SRG investigations to date are based on polymers having relatively high glass transition temperatures (Tg), especially poly(4-vinylpyridine) (P4VP)10,13,14,20,23,26 and sometimes polyphenol.12,27 For the former, complexes were typically made by simple mixing of the polymer with OH- or COOH-functionalized azo molecules, whereas for the latter pyridine-functionalized azo molecules were appropriate. These all lead to hydrogen-bonded complexes, albeit with different strengths. In the above context, SRG formation in supramolecular complexes based on poly(dimethylaminoethyl methacrylate) (PDMAEMA, abbreviated here to PDM) is of interest. Because of the more basic character of the tertiary amine in PDM compared to pyridine in P4VP, the nature of PDM bonding with OH- and COOH-functionalized azo molecules is fundamentally different, notably pure hydrogen bonding for the former and a form of ionic bonding through proton transfer (but also involving H-bonding and acid-salt structures, as explained later) for the latter, allowing a direct comparison of these two bonding types. It will be shown that although the Tg of PDM itself is low (near ambient), which can negatively affect the temporal stability of the SRGs, complexation increases the Tg to thereby enable SRG inscription. On the other hand, it has generally been considered that very high Tg’s inhibit efficient SRG inscription, although this is not supported by our previous results for ionic complexes based mainly on quaternized P4VP ion-exchanged with sulfonated azo molecules where the influence of the molecular flexibility on photoinduced birefringence and SRG formation was determined.20 Since PDM can also be quaternized and ion-exchanged to obtain a high-Tg complex characterized by pure ionic bonding,20 it is a simple matter to directly compare SRG inscription using this ionic complex with the hydrogen-bonded and proton-transfer complexes. Thus, the aim of the present work is to characterize the three types of complexes shown in Scheme 1, where the azo molecules in each case are identical except for the functional group forming the complex with PDM, and to compare the SRG inscription on films of these complexes in relation to their properties. The complexes are referred to hereafter as azoCOOH/PDM(y), azoOH/PDM(y), and azoSO3/PDMQ(1.0), where y refers to the molar ratio of the azo molecule to polymer repeat unit (DM or DMQ), which is varied for the azoOH and azoCOOH complexes and equimolar only for the azoSO3 complex. They will sometimes be referred to collectively as azoSM (SM for small molecule).

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Scheme 1. Targeted Azo-Containing Complexes SelfAssembled from (a) azoOH, (b) azoCOOH, or (c) azoSO3 and the Polymer, Poly(dimethylaminoethyl methacrylate) (PDM; PDMQ When Quaternized);a (d) Simple Schematic of SRG Inscription

a The effect of three types of interactions on surface relief grating (SRG) formation is compared: (a) pure hydrogen bonding, (b) proton transfer leading to mixed ionic and hydrogen bonding, and (c) pure ionic bonding via ion exchange. AzoSM/DM molar ratios vary between 0.1 and 1.0 for (a) and (b) and are 1.0 only for (c).

or DM, 98%) was passed through a neutral Al2O3 column just before polymerization to eliminate the inhibitor. Copper(I) bromide (CuBr, 98%) was washed with boiling acetic acid before use to remove any soluble oxidized species. Deionized water was obtained from a Millipore Gradient A10 Milli-Q system. Instrumentation. The molecular weight and polydispersity of PDM were measured at room temperature by size exclusion chromatography coupled with quasi-elastic light scattering (SEC-LS), using a Wyatt QELS system equipped with a Waters 600E pump, two PLgel 300 × 7.5 mm columns with a particle size of 5 μm and pore sizes of 103 and 105 Å, respectively, a refractive index detector, and a photodiode array detector. THF with 2 vol % triethylamine (TEA) was used as the eluent, the flow rate was 1 mL/min, and the solution concentration was about 3 mg/mL. Freeze-drying was effected using a FTS Systems FD-3-85A-MP freeze-dryer working at 1−3 mT with the condenser at −90 °C. Spin-coating was done at 3000 rpm for 30 s at room temperature using a Headway Research EC101D instrument. The thicknesses of the spin-coated films on glass slides were measured using a Bruker DektakXT profilometer with Vision64 software across a scratch made close to the inscribed SRG using a bamboo toothpick dipped in DMF. 1 H NMR spectra were recorded on a Bruker Avance spectrometer (400 MHz). Energy dispersive spectrometric (EDS) analysis was done using a FEI Quanta 200 FEG environmental scanning electron microscope equipped with an energy dispersive spectrometer. Fourier transform infrared (IR) spectra in transmission mode were recorded using a Thermo Scientific Nicolet spectrometer. Those in ATR (attenuated total reflection) mode were recorded using a Tensor 27 spectrometer (Bruker Optics) equipped with a HgCdTe detector and a MIRacle (Pike Technologies) ATR accessory with a silicon element. The spectra were obtained with a resolution of 4 cm−1 by averaging 200 scans. UV−vis spectra were recorded using an Agilent Cary Series UV−vis−NIR spectrophotometer. Surface relief grating inscription was achieved by guiding a circularly polarized beam of an Ar+ laser, operating at a wavelength of 488 nm and an irradiance of 50 or 190 mW/cm2 (referred to hereafter as lower and higher irradiance, respectively), to a Lloyd’s mirror interferometer for producing an interference pattern incident to the thin film of the sample, leading to SRG periodicities of 750 and 790 nm for inscription at lower and higher irradiance, respectively. SRG inscription was monitored by measuring the first-order diffraction of a 633 nm HeNe laser recorded by a photodiode. The gratings were maintained in

EXPERIMENTAL SECTION

Materials. All materials were obtained from Sigma-Aldrich unless otherwise specified. 4-Hydroxy-4′-dimethylaminoazobenzene (azoOH, >98.0%, Tokyo Chemical Industry), 4-(dimethylamino)azobenzene4′-carboxylic acid (azoCOOH, also called p-methyl red, >97.0%, Tokyo Chemical Industry), sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate (azoSO3Na, also called methyl orange), aluminum oxide (Al2O3, activated neutral), ethylene bis(2-bromoisobutyrate), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), anhydrous anisole (99.7%), anhydrous DMF (99.8%), DMSO (ACS grade, American Chemical), THF (HPLC grade, Fisher Scientific), hexane (ACS grade, Fisher Scientific), nitromethane (ACS grade, J.T. Baker), iodomethane (99.5%), and triethylamine (≥99.0%) were all used as received. N,N′-Dimethylaminoethyl methacrylate (DMAEMA B

DOI: 10.1021/acs.macromol.6b01009 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. UV−vis absorption spectra of spin-coated films of (A) azoOH/PDM, (B) azoCOOH/PDM, and (C) azoSO3/PDMQ complexes at the molar ratios indicated, compared to the spectra of the pure azo molecules in dilute DMF solution (0.01, 0.01, and 0.04 mg/mL, respectively). The absorbance maxima were normalized to 1. The arrows identify the wavelength at which the SRGs were inscribed. ambient conditions for 1 and 2 days for inscription at lower and higher irradiance, respectively, before being stored in a vacuum oven at room temperature. They were imaged by tapping mode atomic force microscopy (AFM) under ambient atmosphere using a Digital Instruments (Bruker) Multimode microscope, a Nanoscope V controller, Nanoscope V7.30 software, and Nanoworld tips (Arrows NC model, spring constant 42 N/m, oscillation frequency 285 kHz, tip radius azoOH. The slopes of the linear portions of the DE curves are plotted in Figure 7 as a function of complexation ratio, y. For SRG inscription at lower irradiance, these values follow the same tendency as the grating depths, notably in their increase as a function of azo content for the azoOH and azoCOOH complexes and in the higher value at each complexation ratio for azoCOOH compared to azoOH. Furthermore, while the increase for the azoOH complexes appears linear from y = 0.25 to 1.0, the values for the azoCOOH complexes tend to flatten out for complexation values greater than about 0.5. The latter can be attributed to significant azoCOOH crystallization (consistent with the GIWAXD and POM data), where the crystallized portion cannot participate in the SRG writing process and may even interfere with the process. This crystallization is clearly observed in the AFM image of the SRG for y = 1.0 only (Figure 5), probably indicating that any crystallites for y = 0.75 are very small and the degree of crystallization too low to be observable by AFM. In contrast, with no or insignificant crystallization in the azoOH films according to the GIWAXD and POM data (infrared suggests I

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Macromolecules when it is well above the writing temperature,53 which does not seem to be the case here or in the (high-Tg) ionic complexes in ref 20. Possibly, the liquid crystal order in the azoSO3/PDMQ complex plays a significant role in the high DE despite the high Tg; for example, irradiation possibly causes isotropization of the LC phase due to disruption by the cis isomers54 and simultaneously decreases the Tg. The overall greater DE for the azoCOOH complexes compared to the azoOH complexes, even in the presence of azoCOOH phase separation/ crystallization at the highest molar ratios studied, may also be related to the partially ionic character of the proton transfer complex involved, although simultaneous H-bonding and acidsalt structures obscure drawing a clear correlation. Importantly, this work also shows that SRGs with relatively good to excellent temporal stability can be written on supramolecular complexes of functionalized azo molecules with PDM despite the intrinsically low Tg of pure PDM. This is possible because complexation increases the Tg of the material to above ambient, which is critical since SRGs are generally not temporally stable in amorphous materials with Tg’s near or below ambient temperature45,52,55 Finally, it is noteworthy that the molecular weight of the PDM used for the complexes is relatively high, although previous literature concerning all-covalent azo polymers as well as H-bonded supramolecular complexes, including several based on P4VP, indicated that SRG formation efficiency tends to decrease with increase in molecular weight beyond an optimal (oligomeric) range.2,12,56 A few other examples from the literature similarly indicate efficient SRG formation with high molecular weight systems,57 including the complex of azoSO3 with quaternized P4VP.19,20 The role of molecular weight, which also influences the Tg, is clearly another aspect that requires further investigation in order to draw firmer conclusions regarding structure−property relations for SRG inscription. In future work, to further understand the effect of supramolecular bond type on SRG formation, it will be of interest to compare the influence of the Tg, by variation of the polymer molecular weight, in a purely ionic complex in parallel with a purely H-bonded complex, complemented by dynamic infrared investigations. Analogous azoSMs that give a liquid crystal structure and an amorphous (i.e., isotropic) structure for a given complex type would help determine how this structure may also play a significant role in SRG formation efficiency.

critical factor for the azoOH and azoCOOH systems, in that azoSM crystallization can occur relatively easily if the solvent evaporation time is too slow (as in solvent-cast films) or if the drying temperature is too high (near or above the Tg), particularly for azoCOOH due to the contribution of acid dimerization to the driving force for crystallization. This work adds to the increasing body of knowledge and understanding concerning the optical properties of azo-containing materials. It also emphasizes the ease with which different molecular characteristics can be readily modified and compared using the supramolecular strategy, providing a convenient means to deepen understanding of photoresponsive materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01009. Supplementary NMR, IR, XRD, POM, DSC, and AFM data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.G.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada (R.G.S., C.P., C.G.B.), le Fonds de recherche du Québec−Nature et technologies (FRQ-NT) (C.P., C.G.B.), and the Canadian Defence Academy Research Program (CDARP) (R.G.S.). X.W. acknowledges a four-year scholarship through the StateSponsored Study Abroad Programs (SSSAP) granted by the China Scholarship Council (CSC) as well as start-up funding from Southwest Petroleum University. J.V. acknowledges a FRQ-NT postdoctoral fellowship and funding from the Emil Aaltonen Foundation and Finnish Cultural Foundation. Francine Bélanger and Sylvain Essiembre are specially thanked for their help with the XRD instruments.

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REFERENCES

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CONCLUSIONS A series of analogous azo-containing supramolecular polymer complexes of various azo/polymer molar ratios were prepared with the aim of comparing the nature of the supramolecular bond on SRG formation, notably pure hydrogen bonding using azoOH and PDM, mixed ionic and hydrogen bonding via proton transfer and involving acid-salt structures using azoCOOH and PDM, and pure ionic bonding via ion exchange using azoSO3 and quaternized PDM (this last at equimolar composition only). It was found that pure ionic bonding is clearly superior in SRG inscription efficiency to the others, while the mixed proton transfer system is somewhat superior to the purely hydrogen-bonded system despite partial azoSM phase separation and crystallization at higher azoSM content for the former. SRG inscription is possible in these systems in spite of the ambient temperature Tg of PDM because complexation increases the Tg, which is particularly high for azoSO3/PDMQ. Sample preparation was also found to be a J

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DOI: 10.1021/acs.macromol.6b01009 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01009 Macromolecules XXXX, XXX, XXX−XXX