Using Light To Tune Thermo-Responsive Behavior and Host–Guest

Article pubs.acs.org/Macromolecules

Using Light To Tune Thermo-Responsive Behavior and Host−Guest Interactions in Tegylated Poly(azocalix[4]arene)s Szymon Wiktorowicz, Heikki Tenhu, and Vladimir Aseyev* Department of Chemistry, Laboratory of Polymer Chemistry, University of Helsinki, PB 55 (A.I.Virtasen aukio 1), FIN-00014 HY, Finland S Supporting Information *

ABSTRACT: Polymers consisting of azocalix[4]arenes in the main chain and tetraethylene glycol monomethyl ether chains in the lower rim of the calix[4]arene units have been prepared. The polymers undergo reversible photoisomerization between the trans and the cis forms, the extent of which depends on the solvent. A lower critical solution temperature (LCST) type behavior is observed for aqueous solutions of the polymers, which is strongly affected by the molar mass and concentration. More importantly, the same polymers exhibit an upper critical solution temperature (UCST) type transition in alcohols. It is shown that the temperature of the phase transition in alcohols decreases proportionally to the decrease in the trans content of the samples thus offering a unique possibility to reversibly tune the UCST behavior by adjusting the irradiation exposure time. An exciting photoassisted writing on solutions of the polymer in alcohols is demonstrated. Furthermore, the host−guest complex formation with a low molar mass guest is influenced by the photostationary state of the polymers.

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INTRODUCTION Numerous polymeric systems have been reported in which a dramatic response is triggered as a result of applying a single stimulus (pH,1 temperature,2 irradiation,3 etc.). The applicability of such “smart” materials spans over various fields and remains a subject of strong interest.4 However the preparation of nonionic multistimuli responsive systems,5 in which stimuliderived effects co-operate to result in unique control of physicochemical properties is not a feasible task and requires an interdisciplinary approach. In a recent review, Theato et al.6 describe the overall progress in the preparation of temperature and light responsive smart polymers indicating the importance of further development of new systems and applications. Photoassisted tuning of a lower critical solution temperature (LCST) transition has been observed in the past,6,7 but using light to tune an upper critical solution temperature (UCST) type behavior of polymers has been very limited. Lodge and Watanabe8 reported on influencing the UCST of an azobenzene containing random copolymers of poly(N-isopropylacrylamide), PNiPAM in an ionic liquid. Calix[4]arenes are well established cavitands in supramolecular chemistry.9 Their conformational adaptability toward hosting low molar mass guests along with easy derivatization of the lower rim (hydroxyl groups) and upper rim (para positions to the hydroxyl groups), as well as the possibility of locking a desired conformation (cone, partial cone, 1,2-alternate, 1,3alternate) under specific reaction conditions, makes them interesting candidates for the preparation of stimuli-responsive systems. However, only a few examples exist in which © XXXX American Chemical Society

calix[4]arenes are introduced into polymers, either as cores for star polymers10 or side groups11 or into the main chain.12 We have recently developed a new type of photoswitchable polymers, the poly(azocalix[4]arene)s.13 In these, the calix[4]arene units are locked in the cone conformation with aliphatic substituents. The polymer backbone consists of calix[4]arenes joined with azo-bridges to induce photoisomerization between the trans and cis forms of the azo-moiety. The polymers were investigated for the influence of irradiation on the degree of intermolecular complex formation with a low molar mass pyridinium-based compound.13 Interestingly, the polymers could reversibly experience enhanced or suppressed host− guest interaction, depending on their photostationary state. In the current study we report on the preparation of a new tegylated (substituted with tetraethylene glycol monomenthyl ether chains) poly(azocalix[4]arene) system (Figure 1) with exceptional multistimuli-responsive properties. Apart from undergoing reversible photoisomerization between the trans and the cis form of the azo-bridge, the polymers also exhibit LCST-type behavior in water and UCST transition in alcohols. Tuning the photoisomerization state of the polymers leads to changes in the UCST demixing temperature. Moreover, localized irradiation of the phase separated polymer solutions in alcohols results in areas of translucency corresponding to lower demixing temperature and thus offers an interesting photoassisted solution writing/erasing application of the Received: June 3, 2013 Revised: July 8, 2013

A

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model compound for irradiation studies, 4,4′-bis(3,6,9,12-tetraoxatridec-1-yloxy)azobenzene, MCTEGOMe is presented in Supporting Information. 1H and 13C NMR spectra were recorded on a 500 MHz Bruker Avance III spectrometer using deuterated chloroform (CDCl3), dimethyl sulfoxide (DMSO) or ethanol as solvents with the chemical shifts being presented in ppm from internal TMS standards. Samples of the reaction mixtures and polymer fractions in THF were analyzed for determining molar mass and polydispersity by means of size exclusion chromatography, SEC, which was performed using a Waters instrument equipped with Waters Styragel HR6, HR4 and HR2 columns (7.8 × 300 mm each), monitored with Waters 2487 UV (set to 230 or 365 nm) and Waters 2410 RI detectors at a flow rate of 0.8 mL/min and referenced against PS standards (Scientific Polymer Products Inc.). Light Scattering. Measurements were conducted using a Brookhaven Instruments goniometer BIC-200SM, a BIC-TurboCorr digital auto/crosscorrelator, and a BIC-CrossCorr detector combining two BIC-DS1 detectors. The light source was a BIC Mini-L30 diode laser operating at the wavelength of 637 nm and the power of 30 mW. Alcohol solutions were passed through a hydrophilic Millipore MillexHV 0.45 μm pore size and 13 mm in diameter filters prior to measurements, to remove dust particles. The temperature of the samples (c = 2.5 g/L) was controlled by means of a Lauda RC 6C thermostat and the measurements were done at a 90° scattering angle. Irradiation of Samples. This was done using an internal xenon lamp of a Fluoromax-4 Spectrofluorometer from HoribaJobin Yvon (operational wavelengths included 360, 365, and 450 nm; exit slit = 3 nm, detection mode blocked). For other experiments requiring lower photostationary state values, a High Power fiber-coupled LED, BlackLED-365, from Prizmatix was used. All trans-samples were kept at elevated temperatures prior to irradiation. UV−Vis. Spectra were recorded on a Shimadzu UV-1601 PC spectrometer. Relative trans content is calculated from dividing the observed maximum absorption value Amax(t) at a given time by the maximum absorption observed prior to irradiation, Amax(0) and expressed as percent [%]. The preirradiation state indicates a relaxed “predominantly-trans” state of the polymer which was heated overnight to induce highest possible trans content (referred to as 100% trans) and/or irradiated with 450 nm). Photoassisted Writing Applications. Samples of the polymers were prepared in ethanol or 2-propanol (c = 2.5 g/L) and kept overnight at 50 °C prior to the experiment. The solutions were transferred to a thin 1 mm quarz cuvette and irradiated using a BlackLED-365 (44 mW power output) from Prizmatix. Turbidity. These measurements (600 nm, 1 cm path length) were performed on a JASCO J-815 CD spectrometer, with varying heating and cooling rates of 1, 1.5, and 2 °C/min in the temperature range between 0 and 40 °C (in the case of lower demixing temperature values the experiments were done between −10 and 40 °C). Cloud point is presented as the temperature at which 50% transmittance is observed. MALDI ToF. The mass spectra of the synthesized compounds were obtained on a Bruker Microflex, equipped with 337 nm N2 laser in the reflector mode using 2,5-dihydroxybenzoic acid (DHB) in THF as the matrix and sodium trifluoroacetate, NaTFA as the cationizing agent. Reductive Coupling of 25,26,27,28-Tetrakis(3,6,9,12-tetraoxatridec-1-yloxy)-5,17-di(nitro)butylcalix[4]arene. Synthesis of Polymers. Calix[4]arene C5 (1 equiv) was dissolved in toluene (2 mL/mmol) and added dropwise, under argon, to a solution of sodium bis(2-methoxyethoxy)aluminum hydride (Red-Al) maintained at 0 °C. The resulting suspension was thawed to room temperature and stirred for 3 days. The reaction was quenched by slow and careful dropwise addition of methanol (Caution! exothermic reaction). This was done until no gas evolution could be observed. Subsequently, the residue was then taken up with 10% aqueous HCl and extracted with chloroform. The organic layer was separated, washed with water, and dried over MgSO4. After filtration, the solvent was evaporated to give the crude reaction mixture, which was further analyzed with SEC.

Figure 1. Structure of poly(azocalix[4]arene) with tetraethylene glycol monomethyl ether side chains, TEGOMe. Illustration of tegylated poly(azocalix[4]arene in the trans (a) and the cis (b) form.

system. Depending on the experimental conditions (temperature, concentration, degree of polymerization of the polymers, alcohol, irradiation exposure time) the stability of the produced images can be controlled. The trans-to-cis photoisomerization can also be used to tune the degree of host−guest interaction of the polymers in chloroform.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were of reagent grade quality as obtained from suppliers. Solvents were dried prior to use by means of molecular sieves or distillation. 5,17-Di(tert)butylcalix[4]arene, C1 (See Supporting Information, Scheme S1), was obtained in a three stage reaction procedure starting from tert-butylcalix[4]arene as described previously.12 Potassium carbonate was dried in a vacuum oven prior to use. Tosylation of triethylene glycol monomethyl ether14 and preparation of N-methylpyridinium iodide, NMPI17c was done according to known literature procedures. The synthesis and characterization of the tegylated 5,17-dinitrocalix[4]arene monomers, B

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Figure 2. Comparison of change in UV−vis absorbance maxima ratios (Amax(t)/Amax(0) with time upon trans-to-cis isomerization in (a) THF and (b) chloroform of the model compound MCTEGOMe (■), and polymers AZTEGOMe10 (•) and AZTEGOMe20 (▲). Fractionation of the Polymers. Hexane was added dropwise to concentrated solutions of the crude products in THF (25 g/L) to a point where phase separation was evident. The suspensions were left stirring on a vortex mixer for 2 h, after which a clear yellow film formed at the bottom of the flask, which constituted for one fraction. The residual solution was decanted off, and further amount of hexane was added to induce phase separation. The procedure was repeated several times until SEC eluograms indicated satisfactory polydispersity values of the fractions.

tegylated polymer adopt a pinched cone (C2v symmetry) conformation. This is supported by the upfield shift of diametrically positioned aromatic signal (Supporting Information, Figure S1, parts b* and c*) alongside the shifts observed in the 3.8−4.8 ppm region, which corresponds to the methylene bridge and TEGOMe protons closest to the calix[4]arene cavity. This distortion may be due to steric hindrance present as a result of bridging multiple calix[4]arenes. Photoisomerization and Thermal Relaxation. Dilute solutions of the selected polymer fractions AZTEGOMe10 and AZTEGOMe20 (c = 0.1 g/L) were prepared in THF and chloroform and subjected to irradiation with 365 nm (trans-tocis, Supporting Information, Figure S2) and 450 nm (cis-totrans, Supporting Information, Figure S3) wavelengths. The same experiments were done for the model compound, MCTEGOMe (Supporting Information, Figure S4). When analyzing the results obtained for polymer samples in THF (Figure 2a), and chloroform (Figure 2b) we can clearly see that under the same experimental conditions, the photostationary state, PSS, observed for the trans-to-cis transition of the polymers in THF has a lower value in THF (25% trans), than in chloroform (36% trans). This indicates that the isomerization is more restricted in chloroform and can be attributed to lower solubility. The model compound, MCTEGOMe follows the same pattern and the PSS values are 0.1 or 10% trans in chloroform and 0.05 or 5% trans in THF. Upon irradiation with 450 nm wavelength the polymers undergo cis-to-trans isomerization (Supporting Information, Figure S5), reaching a PSS of 88% (chloroform) and 92% (THF) in the first 40 min of the experiment, and completely regain the state prior to irradiation after 24 h. In a subsequent experiment, samples in THF were irradiated to the cis-rich PSS and thermal relaxation at 20 and 50 °C was recorded as a function of time (Supporting Information, Figure S6). As expected, at both temperatures the samples undergo thermal relaxation, the extent of which is more pronounced at 50 °C. The relaxation rate of the model compound is higher than that observed for the polymers, which is not surprising considering steric restrictions present in the poly(azocalix[4]arene)s. 1H NMR spectra obtained for the relaxation of the polymers in chloroform at 20 °C (Supporting Information, Figure S7) clearly suggest that multiple units remain in the trans form (Supporting Information, Figure S7, H1) at PSS. Furthermore, a change in the intensity of signals corresponding to the methylene bridge protons and ethylene glycol units closest to

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RESULTS AND DISCUSSION Synthesis and Background. The main factors in the design of the monomer for the preparation of thermo-sensitive poly(azocalix[4]arene)s were the diametric placement of the functional groups in the upper rim (5, 17 positions) and persistence of the cone conformation of the calix[4]arene.13 Recently, Roth and Theato et al.15 presented a study in which poly[oligo(ethylene glycol) methyl ether methacrylate], POEGMA was shown to undergo a phase transition of the UCST-type in various alcohol solutions. In order to induce a thermal response of the polymer, as well as to ensure that the functional groups survive the harsh reductive conditions during the polymerization, we selected tetraethylene glycol monomethyl ether as the substituent for the lower rim of the calix[4]arene. Since direct tegylation results in a mixture of conformers,16 an additional synthetic step was taken, to promote the cone conformer as the major product (Supporting Information, Scheme S1). The introduction of the spacer, followed by its reduction, tregylation (derivatization with triethylene glycol monomethyl ether chains) and ipso-nitration led to the preparation of the monomer, tegylated 5,17-dinitrocalix[4]arene, with a good yield. The polymerization of the monomers was done using Red-Al in concentrated toluene solutions over a period of 3−5 days. The crude reaction mixtures were analyzed using SEC. Because of the step-growth character of the reductive coupling, the mixtures were very polydisperse and fractionation was needed. This was achieved by careful precipitation of the polymers from THF solutions using hexane. The same protocol was repeated until satisfactory PDI values were achieved. For further analysis and characterization, two fractions were selected from the available species (Supporting Information, Table S1), AZTEGOMe20 (M w = 28000 g/mol, PDI = 1.55) and AZTEGOMe10 (Mw = 15000 g/mol, PDI = 1.5) corresponding to 20 and 10 units in length, respectively. As was observed for the previously described aliphatic poly(azocalix[4]arene)s,13 the calix[4]arene units in the C

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polymerization. In recent years, poly[oligo(ethylene glycol) methyl ether methacrylate], POEGMA,18 has been demonstrated to be a versatile LCST-type polymer, the phase transition temperature of which is dependent on the length of the OEG side chain.19 In order to achieve the desired thermal sensitivity of the polymers, the lower rim of the calix[4]arene was derivatized with tetraethylene glycol monomethyl ether chains. The fact that the calix[4]arene units have been locked in the cone conformation, translates to a high density of substituents oriented in the same direction with respect to the cavity. This, along with the length and nature of the substituent could potentially lead to solubilization in polar solvents. While the monomer could not be dissolved in water at any temperature, preparation of aqueous solutions of the tegylated poly(azocalix[4]arene)s was dependent on the degree of polymerization, DP of the polymer used, temperature and concentration (Figure 4) and the polymers showed lower

the cavity suggests a distortion in the conformation of the calix[4]arene, which regains the preirradiation state after 24 h. Complex Formation. Calix[4]arenes are known cavitands and thus titration of the polymers with low molar mass guests can lead to formation of dynamic complexes between the two species in solution.17 1H NMR is a very informative tool for probing this discrete interaction. As in the case of previously reported interaction of aliphatic poly(azocalix[4]arene)s with hexadecylpyridinium chloride,13 C16-Py, phototunable host− guest interaction was probed for the tegylated counterparts. The extent of the interaction was decreased when referenced to the n-butoxy poly(azocalix[4]arene)s. For the purpose of the current study, N-methylpyridinium iodide, NMPI, was used as the guest to ensure that steric hindrance from a long hexadecyl chain of the C16-Py guest does not decrease the interaction. When in chloroform solution, the proton signals appear at 9.32 (Hα), 8.53 (Hγ) and 8.15 (Hβ) ppm (Supporting Information, Figure S8). Addition of NMPI to a solution of the trans-rich poly(azocalix[4]arene) host results in an upfield shift of the aromatic proton signals of the pyridinium aromatic ring, the extent of which informs about the strength of the interaction. Subsequent additions (Supporting Information, Figure S8, parts a−i) result in decreasing upfield shift values when referenced to the free guest solution. The fact that only an average chemical shift value and no two distinct populations of the pyridinium moieties (free and bound) can be observed may be assigned to the short lifetime of the complex compared to the time scale of the NMR measurement. The guest molecules “sense” the presence of the polymers, but no permanent supramolecular structure is attained. Obvious differences in the strength of the complex formation can be deduced when the same experiment is performed on cisrich polymer samples (Figure 3). In this case, the interaction is

Figure 4. Transmittance plots as a function of temperature for aqueous solutions of AZTEGOMe10 at c = 0.5 g/L (−), c = 1.5 g/L (red −) and AZTEGOMe 7 at c = 1.5 g/L (blue −) measured with a heating rate of 1 °C/min in the range 7−35 °C. Photographic inserts represent the solution of AZTEGOMe7 before (left) and after (right) the LCST phase transition.

critical solution temperature (LCST) type transition behavior. The transmittance values of the samples obtained from turbidity measurements suggest, that the lower the DP, the better its solubility. Oligomers (DP < 7) exhibited transmittance values of 90%. At the same concentration, polymers with higher DP, AZTEGOMe10 were turbid (68% transmittance) and despite elongated dissolution times, no significant changes in transmittance could be observed. Diluting the sample led to higher transmittance (87%), but made the transition very broad. This dependence on DP translates directly to the observed phase transition temperature values, leading to a difference for the samples of approximately ΔT = 5.1 °C. It is worth mentioning, that while the phase separation is instantaneous, resolubilisation of the samples requires longer times but is completely reversible. Furthermore, lack of solubilization of the monomer indicates that the induced aqueous solubility in the case of the tegylated poly(azocalix[4]arene)s and its temperature dependence are results of the specific architecture of the polymers. Irradiation of a dilute (c = 0.1 g/L) aqueous solution of AZTEGOMe10 with 365 nm wavelength (trans-to-cis isomerization) resulted in only a slight 25% decrease in UV−vis absorbance maximum (Supporting Information, Figure S10) which may indicate that aggregation of the species in solution

Figure 3. Comparison of the chemical shifts monitored for Hα of the pyridinium guest in free solution (■) and in the presence of trans-rich (•) and cis-rich (▲) host AZTEGOMe20 (c = 4.3 mmol/L as calcd per calix[4]arene unit).

less effective, which may be a result of the aforementioned distortion of the conformation of the calixarene units in the cisrich state. This offers the possibility to control the degree of complex formation by means of irradiation with different wavelengths of light (Supporting Information, Figure S9). LCST in Water. Introducing poly(ethylene glycol), PEG blocks into the structure of a (co)polymer may result in inducing water solubility of a system. Furthermore, by incorporating the oligo(ethylene glycol), OEG units as side chains onto methacrylates, polymers exhibiting LCST type phase transition can be prepared via controlled radical D

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occurs and thus the photoisomerization of the azo-bridges is hindered. In the future, a possible way to overcome aggregation present in the system, while retaining thermal sensitivity will be to use longer (n > 4) oligoethylene glycol substituents. UCST in Alcohols. Solutions of the tegylated poly(azocalix[4]arene) fractions, AZTEGOMe10 and AZTEGOMe20 in methanol, ethanol, n-propanol and 2-propanol were prepared at varying concentrations (0.5, 1.5, 1.8, and 2.5 g/L). All samples were kept in an oven at 40 °C to ensure complete dissolution, after which transmittance was measured as a function of temperature in the range between 0 and 40 °C. In alcohols, the polymers show UCST type behavior. At the lower concentration of 0.5 g/L (Supporting Information, Figure S11), only the samples in 2-propanol exhibited phase separation, however, the transition was very broad and in the measured temperature range did not reach 0% transmittance. This result indicates that more branched alcohols will increase the UCST value. At the concentration of 1.5 g/L the phase transition becomes apparent in ethanol and n-propanol (Supporting Information, Figure S12). Clouding of the methanol solution could only be observed at −20 °C (*not within the scanning range of the instrumental set up). The UCST values obtained for 2-propanol (Supporting Information, Figure S13a) were too high with respect to future photoisomerization studies (competing thermal relaxation of samples at elevated temperatures) and in the case of n-propanol (Supporting Information, Figure S13b) the experimental data were similar to those in ethanol (Figure 5a). When analyzing the turbidimetry results (Table 1), it becomes evident that the UCST behavior of the polymer solutions is strongly dependent on the degree of polymerization and concentration. With increase in the molecular weight and/ or the sample concentration an increase in the transition temperature value can be expected. This, along with proper choice of solvent allows for control over UCST. Light Scattering, LS, measurements (Figure 5b) were conducted on the trans-AZTEGOMe20 in ethanol (c = 2.5 g/L) to determine how the hydrodynamic diameters of the poly(azocalix[4]arene)s change with temperature. For each temperature step, the solution was allowed to reach equilibrium. Constant intensity of the light scattered at 90° angle was used as a criterion of the solution equilibrium. At higher temperatures (28−35 °C) most of the polymers were dissolved on a molecular level (dh = 8−9 nm) with only few aggregates present in solution. Interestingly, upon reaching the phase transition temperature, with increase in the scattered light intensity, the hydrodynamic diameter of the particles left in solution decreases. This indicates that larger polymers undergo phase separation prior to the smaller species, resulting in a thermally driven fractionation. The oligomers, which remain in the system, form stable suspensions under the applied experimental conditions. Microcalorimetry measurements on samples of the polymers in alcohols showed no changes in enthalpy and thus did not yield any definite evidence as to the character of the UCSTtype transition. To see whether photoisomerization was possible, diluted samples of the model compound (c = 0.06 g/L, Supporting Information, Figure S14) and polymers (c = 0.1 g/L, same concentration of azo-groups as for model compound) in ethanol (Supporting Information, Figure S15) and n-propanol (Supporting Information, Figure S16) were irradiated with 365

Figure 5. (a) Transmittance vs temperature plots for samples AZTEGOMe10 and AZTEGOMe20 at different concentration in ethanol done with a cooling rate of 1 °C/min. (b) Intensity of scattered light at 90° angle (left axis) and the hydrodynamic diameter (right axis) as a function of temperature for solution of trans- rich AZTEGOMe20 in ethanol (c = 2.5 g/L).

Table 1. Summary of UCST Cloud Point Characteristics of Polymer Samples AZTEGOME10 and AZTEGOMe20 in Alcohol Solutions

a

sample

DP

solvent

concentration (g/L)

cloud point (°C)

AZTEGOMe10 AZTEGOMe10 AZTEGOMe10 AZTEGOMe20 AZTEGOMe20 AZTEGOMe20 AZTEGOMe20 AZTEGOMe20 AZTEGOMe20

10 10 10 20 20 20 20 20 20

EtOH nPrOH iPrOH MeOH EtOH EtOH EtOH nPrOH iPrOH

1.5 1.5 1.5 1.5 1.5 1.8 2.5 1.5 1.5

−5 −5 27 −20a 7 11 23 7 38

Discerned for refrigerated sample.

(trans-to-cis) and 450 (cis-to-trans) nm wavelength and UV−vis spectra were recorded at varying time intervals at 25 °C. When comparing the trans-to-cis plots of change in absorbance maxima ratios (Amax(t)/Amax (0) with time for ethanol (Supporting Information, Figure S17) and n-propanol (Supporting Information, Figure S18) it becomes evident, that in both cases the PSS has a high value of approximately 50% trans content under the same irradiation conditions. To induce lower PSS and thus decrease the trans content of the samples a new irradiation source with a higher power output was used (see below). E

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Figure 6. (a) Transmittance vs temperature plots for AZTEGOMe20 in ethanol (c = 2.25 g/L) upon irradiation to different photostationary states (trans content). Cooling rate = 1 °C/min. (b) Plot of the cloud point as a function of photoinduced trans content from measurements done with cooling rate of 1 °C/min.

and 100% by means of elevated temperature (40 °C for 30 min). The following LS experiment was performed to showcase the relaxation process. AZTEGOMe20 in ethanol (c = 2.5 g/L, cloud point 23 °C) was irradiated to PSS of 75% trans and the sample was kept at 20 °C. Intensity of scattered light was recorded as a function of time (Figure 7). We can discern that

Photoassisted Tuning of UCST in Ethanol. Next, we wanted to evaluate the influence of the photostationary state of the polymers on the UCST transition. Ethanol was chosen as the solvent for the sample AZTEGOMe20 and the concentration set to 2.25 g/L, which would result in a cloud point value of 21 °C. Prior to each turbidity measurement, the PSS was determined from UV−vis spectra (Supporting Information, Figure S19). The samples were irradiated with 365 nm wavelength (trans-to-cis isomerization) at 25 °C, which is above the phase separation temperature for this specific concentration and degree of polymerization. This was done to ensure complete solubilization of the polymers. If higher temperature conditions were to be used, competing thermal relaxation could diminish the trans-to-cis photoisomerization, hence resulting in a higher PSS value. Interestingly, irradiation of the trans-rich polymer solution results in a shift of the UCST transition (Figure 6a). To be more specific, the change in the cloud point temperature decreases proportionally to the decrease in the trans content of the sample (Table 2). At 92% trans content the

Figure 7. Intensity of scattered light from solution of AZTEGOMe20 in ethanol (c = 2.5 g/L) irradiated to 75% trans content as a function of time. Temperature of measurement depicted as regions on timeline.

Table 2. Cloud Point Values Obtained from Transmittance Measurements of AZTEGOMe20 in Ethanol (c = 2.25 g/L) at Varying PSSa

a

trans content (%)

cloud point (°C)

100 92 85 78 72 68 63, 58, 52, 46, 40

21 14 11 7 3 0 undetectable

the inflection point for the increase in scattered light intensity after 4h denotes the occurrence of phase separation. The intensity of scattered light continuously increases with time as more polymers relax back to the trans form. Increasing the temperature of the measurement to 25 °C leads to a decrease in intensity, indicating the redissolution of the sample. It is noteworthy that similarly to the report by Theato et al.15 minimal addition of water (0.5% v/v) to the alcohol solution of our tegylated polymers results in a dramatic decrease in the cloud point temperature (ΔTcp = 10 °C). However, the reversible phototuning of the UCST is still possible in the system (Supporting Information, Figure S20). When the irradiation is done with a high power UV-LED we can induce the photoisomerization below Tcp, i.e., where the polymers are in the phase separated state. As a result, the poly(azocalix[4]arene)s redissolve in the alcohol solution due to decrease in trans content. Furthermore, we could also observe that given enough power of the irradiation source, the trans-to-cis photoisomerization below UCST is a quick process. This is best visualized when irradiation of the solution is done in a thin 1 mm quartz cuvette

Cooling rate = 1°C/min.

cloud point temperature, Tcp shifts to 14 °C and decreases upon further irradiation. At 68% the cloud point is close to 0 °C, and becomes undetectable below 63% (limitations of instrumental setup). Upon plotting these Tcp values against trans content, we can see that the trend has a linear character (Figure 6b). It is worth mentioning, that thermal relaxation can also be used as the driving force in the change of Tcp of irradiated samples. Same results could be obtained upon irradiating the sample to 72% trans content and inducing the relaxation to 85% F

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Irradiation of samples in THF and chloroform demonstrated that photoisomerization was possible, the extent of which was dependent on the choice of a better solvent. Formation of dynamic complexes with a low molar mass pyridinium guest was evaluated as a function of photostationary state, PSS in deuterated chloroform. It was shown that the trans-rich polymers exhibited stronger interaction with the guest than the cis species, thus offering the potential to tune the degree of interaction with the guest through irradiation and/or thermal relaxation. The explanation of the light-induced difference in host−guest interaction was attributed to slight change in the conformation of the calix[4]arene unit upon photoisomerization. Thermo-responsive properties were investigated in different solvents. The polymers showed LCST type phase transitions in water which could only be studied for lower degree of polymerization samples, as the solubility of the samples decreased proportionally to the increase in molecular weight. UCST-type phase transition was demonstrated for poly(azocalix[4]arene) samples in alcohols. The transition is dependent on degree of polymerization, concentration and choice of alcohol, providing a pathway to selectively adjust the UCST value of the system through experimental conditions. In addition, the thermo-responsive behavior of the polymers in alcohols could be manipulated by means of irradiation. The photoinduced isomerization from the trans to the cis form of the units along the polymer backbone resulted in a significant decrease in the phase transition temperature, which was evidenced by a linear dependence of the cloud point on the trans content of the polymers. This phototuning of UCST is completely reversible and the cloud point values prior to irradiation can be regained through thermal relaxation or irradiation with 450 nm wavelength characteristic to the cis-totrans isomerization. The origin of this light-derived control of UCST may be assigned to the difference in inter/intramolecular interaction between the polymers in alcohol solution when in the trans and the cis form. Finally, irradiation of polymer samples in the phase separated state, below UCST results in trans-to-cis photoisomerization which in turn reduces the value of UCST, rendering the polymers soluble at a given temperature and offers a possibility to write on the polymer solutions using light when the irradiation is done locally. This process can be completely erased through thermal relaxation. The described multistimuli responsiveness of the tegylated poly(azocalix[4]arene)s can result in exciting new control of properties and makes them an interesting candidate in the future development of other “smart” phototunable materials.

(Figure 8). Before irradiation the solution is homogeneously turbid. Irradiation of specific areas of the sample results in

Figure 8. Photographs of AZTEGOMe20 in ethanol (c = 2.5 g/L): (a) at 20 °C before irradiation; (b) after photoassisted writing at 20 °C; (c) at 40 °C and (d) at 20 °C after thermal relaxation/erasing.

immediate appearance of localized regions with translucent character closest to the irradiation source. This allows us to write on the phase-separated polymer solution using wavelength of light typical for the trans-to-cis photoisomerization. Owing to molecular motion and the slow relaxation from the cis to the trans form, the image slowly vanishes, in the present case during two to 3 h at 20 °C (Supporting Information, Figure S21). This depends on the concentration, alcohol and irradiation exposure time. Also leaving the sample at elevated temperature and subsequent cooling, results in complete erasing of the photowritten transparent regions (Figure 8c,d). If the image was erased by shaking the dispersion (high trans content remaining), one could rewrite again with the light beam (365 nm). A possible explanation of the phototuning of UCST may be attributed to the trans-orientation of the chains along the rigid polymer backbone. This particular arrangement could promote intermolecular interaction between the polymers, as opposed to the intramolecular interaction in the cis-rich form. As a result, the larger the trans content of the samples, the higher the temperature of the UCST transition. Effectively, in this novel system, the irradiation of the polymer leads to photoisomerization, which in turn influences the outcome of thermo-response in alcohols demonstrating co-operation between both stimuli-responses. Such tuning of properties may result in further exciting applications and is subject of ongoing investigations.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of tegylated monomers, model compound, molecular weight characteristics of polymer samples, UV−vis studies in different solvents, 1H NMR spectra for photoisomerization and complex formation studies, and turbidity measurements in alcohols. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS Poly(azocalix[4]arene)s with tetraethylene glycol monomethyl ether chains in the lower rim of the locked cone calix[4]arene unit have been successfully prepared in a reductive coupling procedure from their 5,17-dinitrocalix[4]arene counterparts. Careful precipitation of the polymers led to isolation of fractions with moderate polydispersity values, which were then used for characterization of physicochemical properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (V.A.) Vladimir.Aseyev@helsinki.fi. G

dx.doi.org/10.1021/ma4011457 | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Academy of Finland (project numbers 127329 and 260486) is gratefully acknowledged. REFERENCES

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dx.doi.org/10.1021/ma4011457 | Macromolecules XXXX, XXX, XXX−XXX