Hierarchical Structure of Supramolecular Polymers Formed by N,N

Article pubs.acs.org/JPCB

Hierarchical Structure of Supramolecular Polymers Formed by N,N′‑Di(2-ethylhexyl)urea in Solutions ,† ́ Jolanta Swiergiel,* Laurent Bouteiller,‡ and Jan Jadzẏ n† †

Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, PL-60-179 Poznań, Poland CNRS, Institut Parisien de Chimie Moléculaire, Equipe Chimie des Polymères, Sorbonne Universités, UPMC Univ Paris 06, 4 Place Jussieu, F-75005 Paris, France

‡

ABSTRACT: Supramolecular chain polymers formed by N,N′-di(2ethylhexyl)urea (EHU) dissolved at low concentrations (up to 0.1 mole fraction) in heptane were investigated with the use of the dielectric spectroscopy. The experimental data show an exceptional ability of the chains for the antiparallel self-aggregation due to dipole−dipole interactions, leading to an anomalous dependence of the static permittivity of EHU + heptane solutions on temperature and concentration of the urea. The primary molecular assembly into polymeric chains is therefore followed by a secondary bundling of the chains which facilitates a longitudinal translation of the chains. That peculiarity and an asymmetry of the alkyl substituent in the EHU molecule making the system a mixture of diastereoisomers of unfavorable packing of the side group, are the most probable molecular mechanisms which prevent the crystallization of EHUthe only known liquid urea derivative.

1. INTRODUCTION Supramolecular polymerization is a result of the spontaneous formation of hydrogen bonds between molecules revealing simultaneously acidic and basic properties. That self-assembling process is unusually widespread in the nature where it offers an amazing efficiency in formation of the multimolecular entities of a special structure and functionality, as for example, polypeptides and nucleic acidsthe supramolecular polymers of fundamental biological significance.1−7 The peculiarities in the physical properties of hydrogenbonded supramolecular polymers, when compared to conventional polymers, result mostly from a relatively low energy linking the basic elements of the polymers, i.e., the monomers. The energy of a hydrogen bond typically amounts to about a few tens of kJ/mol, while the energy of a covalent bond, which links the monomers in conventional polymers, is higher by 1 order of magnitude (≈500 kJ/mol). That circumstance makes the hydrogen-bonded structures much more sensitive to the thermal energy of the system and manifests itself in the incessant interruptions and reconnections of the hydrogen bonds (in normal conditions, the mean value of the thermal energy per mole of molecules, RT, is about 3 kJ/mol, R is the gas constant, and T is the absolute temperature). It is just the thermal reversibility of the supramolecular polymers which essentially differentiates them from the conventional polymers and imposes to the formers a specific physicochemical behavior. One of the most important features of hydrogen-bonded systems concerns their exceptional sensitivity to the changes in the molecular environment of the hydrogen bonding sticker. The experiment shows that even a relatively small modification of the steric hindrance of the sticker can lead to quite unexpected changes in the extension of the self-association © XXXX American Chemical Society

process as well as in the structure of the entities formed. It especially concerns the vicinity of the hydrogen atom.8 Apparently, that circumstance gives a specific way to control the polymerization process, but unfortunately, actually one is not able to predict, in general, the results of a hindrance modification. However, the problem is extremely fascinating, and its significance for strongly developing molecular engineering is difficult to overestimate.9 The compounds from the family of sym-dialkyl-substituted urea (R·NH·CO·NH·R) are well-known for their strong ability to form supramolecular polymers of a linear shape, where the monomers are linked to each other by two CO···H−N hydrogen bonds, as sketched in Figure 1.10−14 The trans−trans configuration of the ureide group, which is required for formation of such bifurcated hydrogen bonds, has been proved by infrared spectroscopy for most of

Figure 1. Polymerization of N,N′-dialkylureas with trans−trans configuration of the ureide group. The arrows represent the dipole moments of monomers (μ1). Received: July 31, 2015 Revised: September 14, 2015

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DOI: 10.1021/acs.jpcb.5b07406 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B N,N′-dialkylureas synthesized up to now.11,15 At room temperature these compounds are solid, starting from the simplest derivative with R = CH3 and ending with rather complex substituent R = CH2−CH[(CH2)4−CH3]3.11 The melting points of the compounds are rather high (about 100 °C or higher), and mostly they are soluble in some organic solvents, although in a limited range. Among known urea-based compounds, there is, however, one amazing exception:11,15 N,N′-di(2-ethylhexyl)urea (EHU), R = CH2C*H(C4H9)C2H5 (C* is a chiral carbon):

more concentrated solutions (xEHU > 0.3), where the reorientation of the strongly developed supramolecular chains immersed in a medium of high viscosity is practically improbable, one records the dynamics of some parts of polymers resulting from the thermally stimulated braking of the hydrogen bonds within the chains. We have estimated that the mean relaxing part of the polymer chains is composed of about five hydrogen-bonded monomers.16 It seems that such a pentameric entities are rigid and compact enough for reorientation as a whole in a viscous medium. The experiments have shown that the dielectric response of such very complex system is the simplest possible: the dielectric relaxation spectra are of the Debye type. Besides, the thermal activation energy of the reorientation of the parts of the urea chains is quite close to the energy required for breaking two CO···H−N hydrogen bonds. Of course, such a phenomenon cannot be observed with conventional polymers. Unfortunately, the experimental results obtained for concentrated EHU solutions cannot be compared to those obtained for other (solid) sym-disubstituted ureas which are rather poor soluble in nonpolar solvents. Still, the presented conclusions on the dynamic behavior of the supramolecular chains of EHU immersed in a medium of high viscosity seem to be quite natural and, finally, not very surprising for the reversible polymers. So, the studies of EHU in its concentrated solutions seem not to be the best way for searching some peculiar molecular behavior which can be related to unexpected state of neat EHU. In the present paper we analyze the static and dynamic dielectric properties of EHU in dilute solutions where a rational molecular explanation of the macroscopic properties of EHU will be searched. Unlike the concentrated solutions, the experimental data on the EHU dilute solution can be compared to the data obtained for other homologues of the disubstituted ureas. In this paper the dielectric data obtained previously for dilute solutions of (solid in neat state) MPHU will be taken as the reference ones.

The neat compound is a liquid of high viscosity (η ≈ 5 Pa·s at room temperature; i.e., the consistency of fresh honey) and exhibits a transition to the glassy state at about −90 °C. Like for nearly all dialkyl-substituted ureas, the infrared spectra recorded for dilute solutions of EHU in carbon tetrachloride, at various concentrations and temperatures, have shown the trans−trans configuration of ureide group.11 So, the self-association of EHU molecules certainly leads to linear polymeric entities, as sketched in Figure 1. The quantitative analysis of the IR spectra, performed in the frame of a self-association model with two equilibrium constantsdimerization (K2) and multimerization (K) revealed an unusual ability of EHU molecules to form the supramolecular polymers.11 The ability is rather similar to that of the urea derivative with a very small substituent (R = C2H5). The equilibrium constant of EHU multimerization (K = 350 dm3/mol, at 25 °C) is twice as large as that obtained for N,N′-di(2-methyl-2-pentylheptyl)urea, (MPHU), R = CH2C(C5H11)2CH3, a derivative with substituent R larger only by a few CH2 groups in comparison to R of EHU:

As shown in ref 11, the strong supramolecular association of EHU manifests itself as a large increase of the viscosity recorded for increasing EHU concentration in heptane: for EHU mole fraction x ≈ 0.025; the viscosity of the solution is 6 times higher than the viscosity of neat heptane. That increase of the viscosity markedly exceeds that observed for MPHU solutions. In our recent paper,16 we have analyzed the dielectric results obtained mainly for concentrated solutions of EHU in nonpolar medium. Because of a relatively high viscosity of the solutions, the measurements were performed at somewhat higher temperatures than it is in this paper, and a suitable nonpolar solvent of a high boiling point, 4-n-propylcyclohexyl-4′-npentylphenyl, was used. It was found that the dielectric dynamic behavior of the EHU solutions is quite different in a low concentration range (xEHU < 0.3) and in a high concentration range. Namely, in dilute solutions, where the self-association process is gradually developing as the urea concentration increases and the viscosity of solutions is not too high, one observes the reorientations of the whole multimers. Because of the distribution of the degree of polymerization, the Brownian dynamics of the chains of distributed length (and polarity) reveals in the dielectric spectrum as the band of the Davidson− Cole type17 which in the complex plane (the Cole−Cole plot) presents a skewed semicircle. Such type of dielectric spectrum is recorded also in some rigid conventional polymers.18−20 In

2. EXPERIMENTAL SECTION 2.1. Materials. The supramolecular polymer under investigation is N,N′-di(2-ethylhexyl)urea (EHU) dissolved in heptane. Synthesis and purification of EHU were described previously.11,15 Heptane of spectrophotometric grade was purchased from Sigma-Aldrich with stated purity 99.3% and was stored over 4 Å molecular sieves (Sigma-Aldrich) for several weeks before measurements. 2.2. Methods. The dielectric spectra were recorded with the use of an HP 4194A impedance/gain phase analyzer in the frequency range from 500 Hz to 5 MHz. Electrical heating of high performance with the use of a “Scientific Instruments” temperature controller, model 9700, assured very good temperature stabilization. Such equipment allows one to determine the impedance with an uncertainty less than 1%. The details of the used experimental setup can be found in a recent paper.21 The measurements were performed for EHU concentrations in heptane up to about 10% (in mole fraction) and in the temperature range from 5 to 50 °C. 3. RESULTS AND DISCUSSION Figure 2 presents the real (a) and imaginary (b) parts of the dielectric relaxation spectra recorded for a solution of EHU + heptane with a urea mole fraction x = 0.110 at different B


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The Journal of Physical Chemistry B

permittivity of free space. The numerical analysis of the experimental dielectric spectra with the use of eq 1 yielded the values of σDC, εs, ε∞, β, and τD as a function of the EHU concentration and the temperature. The corresponding dependences are presented and discussed below. First, we present in Figure 3 the σDC conductivity dependence on EHU mole fraction in heptane at different

Figure 3. Direct current conductivity of the solutions of EHU in heptane, as results from the best fit of eq 1 to the imaginary part of the dielectric spectra recorded in experiment.

temperatures, obtained from the best fit of eq 1 to the imaginary part (ε″) of the dielectric spectra of the solutions. The data allow one to remove from the experimental spectra that part of the losses which are due to the electrical conductivity. The data presented in Figure 3 show the conductivity dependence on the urea concentration as well as on the temperature. The conductivity results are not discussed in this paper, but it seems to be important to mention here that the recent studies 23 on the electrical conductivity in concentrated solutions of EHU in nonpolar medium exhibited some peculiar behavior of the conductivity. Namely, the thermal activation energy of the conductivity is independent of the medium viscosity in a large range of EHU mole fraction (0.3 < x ≤ 1), in full analogy to the behavior of the activation energy of the dipolar relaxation time of the entities resulting from the thermally stimulating breaking of the hydrogen bonds chains. Besides, in that urea concentration range one observes the correlation between the relaxation times of the charge curriers translation and the dipolar entities rotation. The data point out for a very probable participation of the protons, releasing in the acts of the hydrogen bonds breaking, in the electrical conductivity of the studied supramolecular system. Figure 4 presents the dielectric spectra from Figure 2, after removing the contribution of the electrical conductivity from the dielectric losses. The solid lines in the figure represent the best fit of the Davidson−Cole equation without the conductivity term. The effect of the double-layers formation, seen in part a of Figure 4 as the dotted lines, will be omitted in our further analysis and discussion of the experimental results. The dielectric relaxation spectra depicted in Figure 5 refer to the EHU in heptane solutions of different mole fractions of the urea at constant temperature (25 °C). The spectra, when presented in the complex (ε″, ε′) plane (Figure 6), take the form of skewed semicircles, the skewing of which increases with increasing the difference between the exponent β and 1. As it was mentioned above, for β = 1 the dielectric spectrum has the form of an ideal semicircle (the Debye’s shape of the spectrum). Figure 7 presents the concentration dependence of the exponent β for solutions of EHU + heptane. The data are

Figure 2. Real (a) and imaginary (b) parts of the dielectric relaxation spectra recorded at different temperatures for solution of EHU in heptane with a urea mole fraction x = 0.110.

temperatures. The presented spectra are typical for all solutions studied in this work. As seen in the figure, both parts of the dielectric spectra reveal a relatively strong contribution of the electric charge carriers present in the studied solutions. In a low frequency range of the real part of the spectra one observes an effect caused by formation of the ionic double layers near the blocking electrodes of the measuring cell. That effect leads to a large increase of the capacity (and apparently the permittivity) of the system. In the imaginary part of the spectra, the losses due to the electrical conductivity strongly mask the dielectric losses due to dipolar reorientations of supramolecular polymers existing in the studied solution. A quantitative analysis of the spectra were performed with the use of the Davidson−Cole dielectric relaxation approach,17 taking into consideration the contribution of the electrical conductivity to the dielectric losses: εs − ε∞ σ ε*(ω) = ε′(ω) − jε″(ω) = ε∞ + + DC β jε0ω (1 + jωτD) (1)

Equation 1 gives the most adequate theoretical description of the experimental dielectric spectra recorded for studied solutions of EHU in heptane. Here, εs and ε∞ denote respectively the static and the high frequency limits of the permittivity, τD is the dipolar relaxation time, ω = 2πf is the angular frequency of the probing electric field, f is the frequency of the field, and j = (−1)1/2. The Davidson−Cole exponent β is a measure of deviation of the experimental dielectric spectrum from the Debye’s shape,22 for which β = 1. In the conductivity term of the dielectric losses, σDC denotes a direct current conductivity of the studied solution and ε0 = 8.85 pF/m is the C


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Figure 6. Dielectric relaxation spectra of EHU solutions in heptane with different mole fractions of the urea in the complex plane at 5 °C. The solid lines have the same meaning as in Figure 4.

Figure 7. Dependence of the Davidson−Cole exponent β on the mole fraction of EHU in heptane depicted at two temperatures: 5 °C (▲) and 50 °C (△).

Figure 4. Dielectric relaxation spectra of EHU + heptane solution (x = 0.110), at different temperatures, after removing from the losses spectra (b) the contribution of the electrical conductivity. The solid lines represent the best fit of the dielectric part of the Davidson−Cole eq 1 to the experimental data (points). An increase of the permittivity, presented in (a) as dotted lines, refers to the double-layers formation near the blocking electrodes of the measuring cell.

presented at two extreme temperatures used in our experiment: 5 and 50 °C. As shown by Böttcher and Bordewijk,24 the dielectric spectra of the Davidson−Cole type are related to the distribution of the relaxation times, and the exponent β is a measure of that distribution. So, the two following conclusions result from the data presented in Figure 7. First, the distributions of the dielectric relaxation time strongly evolve from a quite large effect in very dilute solutions (β ≈ 0.4) to a relatively narrow distribution (β ≈ 0.9), noted for the concentration of about 10% (mole fraction) of EHU in heptane. Second, the β(x) dependence is practically independent of the temperature in the range used in our studies. However, that quite strong dependence of the exponent β on EHU concentration has, unexpectedly, little correspondence in the concentration behavior of the dielectric relaxation time. As seen in Figure 8, with increasing EHU concentration, the relaxation time is practically constant with rather symbolic increasing in the higher temperatures of the measurements. The problem seems to arise, however, from the physical

Figure 5. Dielectric relaxation spectra of solutions of different mole fractions (x) of EHU in heptane at 5 °C. The solid lines have the same meaning as in Figure 4. The dashed line in part b shows the shift of the dielectric losses maximum following the change of the urea concentration, and the dotted lines represent the frequencies corresponding to Davidson−Cole relaxation time, τD = (2πf D−C)−1.

Figure 8. Davidson−Cole dielectric relaxation time determined for solutions of EHU + heptane as a function of the urea mole fraction at different temperatures. The temperature difference between the neighboring curves is equal to 10 °C. D


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as obtained from the best fit of eq 1 to the experimental dielectric spectra. As expected for a compound exhibiting selfassociation into chains, the permittivity increases as the concentration of EHU increases. However, the comparison of the data obtained for EHU solutions to the εs(x) dependence obtained previously 25 for MPHU solutions in carbon tetrachloride, shown in Figure 9b, is really amazing. The significantly less self-associated MPHU exhibits a much stronger εs(x) dependence than EHU, which, as it results from IR spectroscopic studies, is one of the most self-associated disubstituted ureas synthesized up to now.11,15 Besides, as it was mentioned above, the spectroscopic studies undoubtedly confirmed the trans−trans configuration of the ureide group NH.CO.NHin both EHU and MPHU molecules, which means that hydrogen-bonded chains are present in solutions of the both urea derivatives. The next peculiarity, seen in Figure 4 or 9a, and explicitly presented in Figure 10a, concerns the anomalous temperature

meaning of the relaxation time determined with the use of Davidson−Cole formalism. The formalism concerns the system where the relaxation times are distributed and the determined time constant is some mean value of the relaxation times. As seen in Figure 5b, the frequencies related to the Davidson− Cole relaxation time (the dotted line) are shifted from the loss maxima (the dashed line) and that shift depends on the β value. That effect may result in an apparent independence of the Davidson−Cole relaxation time on EHU concentration. In the Debye-shaped spectra the two lines shown in Figure 5b, dotted and dashed, are identical. In the studied system the distribution of the dielectric relaxation time reflects the distribution of the length of the supramolecular chains at the given EHU concentration and, as results from β behavior, that length distribution is practically temperature independent. It means that the τD dependence on the temperature, clearly seen in Figure 8, results from the temperature dependence of the viscosity of the medium. Above, we presented the concentration and temperature dependences of the quantities which concern the dynamic behavior of two basic elements of the studied supramolecular system, namely, the translation of the charge carriers (σDC) and the rotation of the dipolar multimers (τD and β). Although the dynamic dielectric response of EHU + heptane mixtures shows some deviations from the typical response of a polar or selfassociated liquid, one could have intuitively expected a much stronger dependence of τD on the urea concentration or a distinct temperature dependence of the exponent β. However, there are no particular dynamic property which decisively differentiates EHU from other self-associated compounds. Still, we will see in the next part of the paper that EHU manifests its really unusual behavior when one analyzes the static dielectric properties of the solutions. These static properties reflect the dipolar interations between the hydrogen-bonded entities formed by EHU molecules in nonpolar medium. Figure 9a presents the static permittivity (εs) dependence on the mole fraction of EHU in heptane, at different temperatures,

Figure 10. (a) Temperature behavior of the static permittivity of solutions of EHU + heptane for different mole fractions of the urea. (b) Comparison of the permittivity dependence on temperature of two disubstituted ureas, EHU and MPHU, dissolved in nonpolar media (x = 0.110) at 25 °C.25

behavior of the static permittivity of EHU solutions. Namely, at constant concentration of EHU, an increase of the temperature is followed by an increase of the permittivity. It is an inverse temperature behavior than usually observed for the permittivity of polar liquids, including liquids self-associated by hydrogen bonds, as MPHU solution presented in Figure 10b. There are two known exceptions from the normal temperature behavior of the static permittivity of liquids. The first one concerns the hydrogen-bonded liquids composed of molecules exhibiting a cis-configuration of the center forming the hydrogen bonds, such as carboxylic acids26 or lactams.27−29 Cyclic dimers with a significantly compensated dipole moment are formed in equilibrium with open (polar) dimers. An increase of the temperature shifts the equilibrium toward the open structures, causing an increase of the permittivity of the system. However, this is not the case for the studied EHU molecules with trans−trans configured of the ureide group. The second case of an anomalous temperature dependence of the permittivity concerns the nematogenic liquids composed

Figure 9. (a) Dependence of the static permittivity of EHU + heptane solutions on mole fraction of the urea at different temperatures (the temperature difference between the neighboring curves is equal to 10 °C). (b) Comparison of the experimental εs (x) dependences for EHU and (less self-associated) MPHU in nonpolar solvents at 25 °C.25 E


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The Journal of Physical Chemistry B of the highly polar molecules from the family of n-alkylcyanobiphenyls.30,31 With decreasing temperature of the isotropic liquid cyanobiphenyls, several degrees before the phase transition to the nematic phase, the slope of the εs(T) dependence changes from negative (normal permittivity behavior) to positive. So, in the vicinity of the phase transition to the nematic phase, the permittivity of liquid cyanobiphenyls decreases with decreasing temperature, similarly to the temperature behavior of the permittivity of studied EHU + heptane solutions. It was shown32 that the anomalous permittivity behavior in the vicinity of the phase transition from the isotropic liquid to the nematic liquid crystalline phase is caused by a dipolar aggregation process which is strongly enhanced when the phase transition approaches. The pseudonematic domains with antiparallel arrangement of the molecular dipoles, i.e., with considerably reduced polarity, are formed in the isotropic liquid as the precursors of the approaching nematic phase. As the size of the domains increases as the temperature decreases, the reason for the anomalous permittivity behavior in nematogenic isotropic liquids is obvious. The supramolecular chains composed of hydrogen-bonded of N,N′-disubstituted ureas can exhibit a quite large dipole moment even in a relatively dilute solution. For example, it was estimated with the use of IR spectroscopy11,15 that EHU dissolved in nonpolar solvent at the mole fraction of x ≈ 1.5 × 10−3 forms chains composed on average of 10 molecules, i.e., the entities of polarity of about 50 D (see Figure 1). Besides, a broad distribution of the degree of EHU polymerization shows also quite considerable number of multimers composed of about 30 monomers (polarity of about 150 D) or even longer. The electrostatic forces of the dipole−dipole interactions certainly can be enormous here, but due to a very strong dependence of these forces on the distance between the dipoles (Fdip ∼ r−4), their efficiency strongly depends on the structure of molecular dipoles. In the case of the above-mentioned mesogenic n-alkylcyanobiphenyls, a relatively simple molecular structure allows the molecules to approach each other, which results in a spontaneous formation of antiparallel dipolar aggregates. The situation is much more complex in the case of N,N′-disubstituted ureas, where substituents R of different size and nature can be introduced. However, the experimental data show that the substituent R = CH2CH(C4H9)C2H5 in EHU molecules, despite its relative complexity, distinguishes itself very much from other substituents. Namely, it results from IR spectroscopy,11,15 which in the presence of this substituent the supramolecular polymerization process is more favored than in the case of other substituents. Next, as results from the static dielectric data presented in this paper, the hydrogen-bonded chains formed with EHU molecules exhibit an exclusive ability to antiparallel dipolar aggregation. The primary polymerization process leads to an increase of the polarity of the system, and the secondary bundling process causes a decrease of the polarity. In experiment one observes the resulting effect which can be expressed as the Kirkwood correlation factor (gK). The factor relates the apparent dipole moment per one molecule (μapp), obtained for a given EHU + heptane solution with the use of the Onsager formula:24 2 εs(ε∞ + 2)2 N0μapp εs − ε∞ = 2εs + ε∞ 9ε0kT

to the dipole moment (μ1) of single molecules (monomers):

gK = μapp2 /μ12

(3)

In eq 2, N0 denotes the number of dipoles in the unit volume and T is the absolute temperature. The factor g K reflects the prevailing type of the intermolecular interactions in the liquid under investigation. Only in the case of liquids where the dipole−dipole correlations are absent, μapp is equal to the dipole moment of a single molecule, μ1, and the Kirkwood factor, gK = 1. The case of gK < 1 corresponds to the antiparallel dipolar correlation which leads to the reduction of the dipole moment per molecule, and gK > 1 corresponds to the parallel dipoles correlation with enhancement of the molecular apparent dipole moment. It is obvious that for increasing temperature of the system gK → 1, independently of the type of the dipolar interaction. The data presented in Figure 11a show that the ability for the antiparallel aggregation of the highly polar chains composed of

Figure 11. (a) Temperature dependence of the Kirkwood correlation factor determined for solutions of EHU + heptane at different mole fractions of the urea. (b) Comparison of temperature dependences of gK determined for solutions of EHU and MPHU in nonpolar medium when the ureas mole fraction is equal to 0.110.

EHU molecules is really unusual. The values of the Kirkwood correlation factor are so much below unity that even though these values are only estimated roughly, the effect of the antiparallel dipole−dipole aggregation of the polymeric chains is here certainly very strong. Such a behavior clearly differentiates EHU from the other disubstituted ureas which are represented by MPHU in Figure 11b.

4. CONCLUSION In the paper we presented the experimental data that prove a peculiar behavior of the dialkyl-substituted urea R·NH·CO·NH·R, R = CH 2 CH(C 4 H 9 )C 2 H 5 , (acronym EHU), which reveals two-step hierarchical aggregation in nonpolar medium: (i) the primary step, where monomers selfassociate via the bifurcated CO···H−N hydrogen bonds forming the polymeric chains of extremely high polarity, and (ii) the secondary step, where the chains self-organize via the dipole−dipole interactions leading to formation of the antiparallel structures of considerably compensated dipole

(2) F


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(18) Pissis, P.; Fragiadakis, D.; Kanapitsas, A.; Delides, K. Broadband Dielectric Relaxation Spectroscopy in Polymer Nanocomposites. Macromol. Symp. 2008, 265, 12−20. (19) Kremer, F.; Schoenhals, A. Broadband Dielectric Spectroscopy; Springer: Berlin, 2003. (20) Runt, J. P.; Fitzgerald, J. J. Dielectric Spectroscopy of Polymeric Materials; American Chemical Society: Washington, DC, 1997. (21) Świergiel, J.; Jadżyn, J. Static Dielectric Permittivity of Homologous series of Liquid Cycling Ethers, 3n-Crown-n, n = 4 to 6. J. Chem. Eng. Data 2012, 57, 2271−2274. (22) Debye, P. Polar Molecules; Chemical Catalog Co.: New York, 1929. ́ (23) Swiergiel, J.; Bouteiller, L.; Jadżyn, J. Electrical conductivity studies for hydrogen-bonded supramolecular polymer formed by dialkylurea in non-polar solvent. Electrochim. Acta 2015, 170, 321− 327. (24) Böttcher, C. J. F.; Bordewijk, P. Theory of Electric Polarization: Dielectric in Time-Dependent Fields; Elsevier: Amsterdam, 1992; Vol. II. ́ (25) Swiergiel, J.; Jadżyn, J.; Bouteiller, L. Molecular Dynamics and Entropy Effects in hydrogen-Bonded Supramolecular Polymer N,N′Di(2-methyl-2pentylheptyl)urea Dissolved in Nonpolar Medium. J. Phys. Chem. B 2010, 114, 737−741. (26) Riniker, S.; Horta, B. A.; Thijssen, B.; Gupta, S.; van Gunsteren, W. F.; Hünenberger, P. H. Temperature Dependence of the Dielectric Permittivity of Acetic Acid, Propionic Acid and Their Methyl Esters: A Molecular Dynamics Simulation Study. ChemPhysChem 2012, 13, 1182−1190. (27) Jadżyn, J.; Małecki, J.; Jadżyn, C. Dielectric Polarization of 2Pyrrolidinone-Benzene Solutions. J. Phys. Chem. 1978, 82, 2128−2130. (28) Walmsley, J. A.; Jacob, E. J.; Thompson, H. B. An investigation of the self-association of 2-pyrrolidinone in cyclohexane and carbon tetrachloride by means of spectroscopic and dielectric polarization measurements. J. Phys. Chem. 1976, 80, 2745−2753. (29) Hopmann, R. F. W. Chemical Relaxation as a Mechanistic Probe of Hydrogen Bonding. Thermodynamics and Kinetics of Lactam Isoassociation in Nonpolar Solvents. J. Phys. Chem. 1974, 78, 2341− 2348. (30) Drozd-Rzoska, A.; Rzoska, S. J.; Zioło, J.; Jadżyn, J. Quasicritical behavior of the low-frequency dielectric permittivity in the isotropic phase of liquid crystalline materials. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2001, 63, 052701. (31) Jadżyn, J.; Czechowski, G.; Legrand, C.; Douali, R. Static and dynamic dielectric properties of strongly polar liquids in the vicinity of first order and weakly first order phase transitions. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2003, 67, 041705. (32) Jadżyn, J.; Dejardin, J. L.; Czechowski, G. Singular pretransitional behavior of the electric field-dependent part of the thermodynamic quantities of strongly polar mesogenic liquids in the isotropic phase. Acta Phys. Pol., A 2007, 111, 877−884.

moment. It seems that the secondary structure of the dipolar chains of EHU provides the system with the way of relatively easy translations of the chains along their axis which can effectively prevent crystallization of that urea derivative. However, most probably the inherent liquid nature of EHU is due to its unsymmetrical alkyl substituent making the system a mixture of diastereoisomers of unfavorable packing of the side group. The rather flexible nature of the resultant polymers should endow them with further self-assemble abilities under thermodynamic control, thus leading to the creation of antiparallel assemblies.

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AUTHOR INFORMATION

Corresponding Author

*Tel +48 61 86 95 162; e-mail [email protected] ́ (J.S.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Hierarchical Structure of Supramolecular Polymers Formed by N,N

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