Study of Structure Formation in Side-Chain Liquid Crystal Copolymers

Article pubs.acs.org/IECR

Study of Structure Formation in Side-Chain Liquid Crystal Copolymers by Variable Temperature Fourier Transform Infrared Spectroscopy Alfonso Martínez-Felipe,*,† Corrie T. Imrie,‡ and Amparo Ribes-Greus† †

Institute of Materials Technology, Universitat Politècnica de València, Camino de Vera S/N, 46022 Valencia, Spain Department of Chemistry, School of Natural and Computing Sciences, Meston Building, University of Aberdeen, Aberdeen AB24 3UE Aberdeen, Scotland, U.K.

‡

ABSTRACT: The formation of smectic phases in side-chain liquid crystal copolymers, SCLCPs, containing sulfonic acid-based nonmesogenic units, has been investigated using variable temperature FT-IR microscopy. Two copolymers have been characterized, namely, the poly[10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate]−copoly[2-acrylamido-2-methyl-1propanesulfonic acid]s, the X-MeOAzB/AMPS copolymers, containing X = 0.71 and 0.56 mol fraction of mesogenic sidechains, respectively. For comparative purposes the corresponding side chain liquid crystal homopolymer, poly[10-(4-methoxy-4′oxy-azobenzene) decyl methacrylate], MeOAzB, has also been characterized. The 0.56-MeOAzB/AMPS copolymer exhibits a bilayer smectic A phase, in which the mesogenic side chains constitute one layer with a SmA1 packing arrangement and the sulfonic acid groups another; whereas in the smectic A phase shown by the 0.71-MeOAzB/AMPS copolymer, the acid groups are located within the smectic layers giving a partially interdigated SmAd phase and reducing side chain packing efficiency. Smectic stabilization is attributed to a combination of stronger interactions involving the ester groups, as reflected in changes to the C O stretching band at ν ∼ 1730 cm−1, and hydrogen bonding between the amide groups within the acid-based layers, as inferred by changes to the NH stretching band at ν ∼ 3320 cm−1. The temperature response observed for groups with different chemical environments permits the mapping of the short-range interactions between the various structural components in SCLCPs with a view to controlling the functionality of the materials.

1. INTRODUCTION Side-chain liquid crystal polymers, SCLCPs, have attracted considerable scientific and technological interest over the last 30 years due to their combination of macromolecular and liquid crystalline behavior;1 they exhibit the viscoelastic behavior of conventional polymers and the electro-optic characteristics of low molar mass liquid crystals, albeit on a much slower time scale. This gives rise to their well-established application potential in a diverse range of electro-optic technologies including, for example, optical information storage and nonlinear optics.1 This unique duality of properties stems from the molecular architecture of SCLCPs which consists of three structural components: a mesogenic group, a conventional polymer backbone, and, connecting these, a flexible alkyl spacer.1 The spacer plays a critical role in determining phase behavior since it decouples, at least to some extent, the opposing tendencies of the polymer backbone to adopt random coil conformations from those of the mesogenic units to self-assemble into a liquid crystal phase.2−7 Multifunctional SCLCPs may be obtained by the introduction of nonmesogenic units into the polymer structure via copolymerisation. Following this approach, several SCLCPs have been designed as potential components in electrolytes for advanced electrochemical technologies, by the incorporation of metal ions, such as alkali metal ions, or ionogens, such as carboxylic and sulfonic acid groups.8−10 Different types of materials containing liquid crystals have been reported recently © 2013 American Chemical Society

with a view to controlling their transport properties through the formation of anisotropic morphologies.11−19 Here we analyze the role of the interactions between the different structural components in SCLCPs and their influence on structure formation. The Fourier transfrom infrared (FT-IR) temperature-dependent spectra of the liquid crystal homopolymer, poly[10-(4-methoxyazobenzene-4′-oxy)decyl methacrylate], MeOAzB, and of the poly[10-(4-methoxy-4′-oxyazobenzene) decyl methacrylate]−copoly[2-acrylamido-2methyl-1-propanesulfonic acid]s, the X-MeOAzB/AMPS copolymers where X denotes the mol fraction of mesogenic side chains, see Scheme 1, were obtained in the glassy, smectic, and isotropic phases. The use of temperature-dependent FT-IR spectroscopy to map the short-range interactions between the various structural components in polymers is an effective methodology to investigate and optimize the molecular architecture of SCLCPs for application such as electrolyte membranes, since the number, distribution, and location of the charged or polar groups will, to a large extent, determine the physicochemical and conductivity properties of these materials. Special Issue: Giulio Sarti Festschrift Received: Revised: Accepted: Published: 8714

November 14, 2012 February 8, 2013 March 26, 2013 March 26, 2013 dx.doi.org/10.1021/ie303130e | Ind. Eng. Chem. Res. 2013, 52, 8714−8721

Industrial & Engineering Chemistry Research

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below the glass transition temperature to the isotropic phase. The sample was allowed to equilibrate at each temperature prior to the spectrum being recorded. The spectra were collected over 32 scans with a resolution of 4 cm−1, meaning that changes of 2 wavenumbers are significant, in the 4000−400 cm−1 range, and the samples were run in duplicate; the average was used.

Scheme 1. MeOAzB/AMPS Copolymers

3. RESULTS AND DISCUSSION 3.1. FT-IR Spectra Recorded below Tg. The different regions of the FT-IR spectra of the MeOAzB and AMPS

2. EXPERIMENTAL SECTION The X-MeOAzB/AMPS copolymers were obtained by radical polymerization of the commercial monomer 2-acrylamido-2methyl-1-propanesulfonic and 10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate, which was synthesized according to a method described previously.20−23 The appropriate amounts of the monomers were dissolved in dimethyl formamide (10%, weight %) and 1,1′-azobis(cyclohexane carbonitrile) added as the initiator (∼3% mole equiv). The reaction mixtures were flushed with nitrogen for 45 min and then heated at 80 °C in the absence of oxygen to initiate the polymerizations, which were terminated after 24 h by precipitation into diethyl ether. The polymers were then purified by several reprecipitations from dichloromethane into diethyl ether. The composition of the copolymers was determined using 1 H NMR spectroscopy (in CDCl3), and specifically by the ratios of the signals at δ = 6.9 ppm (arising exclusively from the liquid crystal side chain) and the signal at δ = 2.7 ppm (assigned to the methylene protons adjacent to the sulfonic group). Two copolymers were obtained with mol fractions of 0.71 and 0.56 in mesogenic side chains units, designated as 0.71-MeOAzB/AMPS and 0.56-MeOAzB/AMPS, respectively. The liquid crystalline behavior of the polymers has previously been characterized using differential scanning calorimetry, polarized light microscopy, and X-ray diffraction.24,25 The MeOAzB homopolymer shows a glass transition at Tg = 71 °C, and a smectic phase extending to the clearing point at TSmA‑I = 135 °C. A nematic phase emerges in a very short temperature range and is detected by DSC as a shoulder to the phase transition envelope (TSmA‑N = 127).24 On the other hand, the AMPS homopolymer, poly[2-acrylamido-2-methyl-1-propanesulfonic acid], is amorphous with Tg = 124 °C. The introduction of nonmesogenic units promotes a slight depression of the clearing point (TSmA‑I = 123 and 125 °C for 0.71-MeOAzB/AMPS and 0.56-MeOAzB/AMPS, respectively), and the glass transition remains essentially unaltered (Tg ∼ 71 °C).24,25 The FT-IR spectra of the polymers as a function of temperature were obtained using a Nicolet Nexus FT-IR bench attached to a Continuum FT-IR microscope equipped with a Linkam FTIR 600 heating stage and a TMS 93 control unit. The sample was sandwiched between a gold-coated microscope slide and a 3 mm KBr disc. A thin film of the sample was obtained by first heating it into the isotropic phase and cooling to room temperature. The spectra were recorded on subsequent reheating in a trans-reflectance mode, i.e., the IR beam passed through the sample was reflected by the gold surface back through the sample and then detected. Spectra were recorded approximately every 3 °C from a temperature

Figure 1. FT-IR spectra in the 3600−2600 cm−1 region of MeOAzB (red), 0.71-MeOAzB/AMPS (blue), and 0.56-MeOAzB/AMPS (green) measured at 60 °C in the glassy smectic A phase. The solid black line corresponds to the IR curve of AMPS.

homopolymers and the copolymers below their glass transition temperatures, measured at 60 °C, are shown in Figures 1, 2, and 3. The frequencies of the characteristic IR absorption bands seen in these spectra along with their assignments are listed in Table 1. The 3600−2600 cm−1 region, Figure 1, contains distinct contributions arising from the structural components of the MeOAzB homopolymer, and more specifically, the region comprises C−H stretching (st) bands, including those associated with aromatic (ν > 3000 cm−1) and the alkyl (ν < 3000 cm−1) groups. The AMPS homopolymer also shows several signals associated with stretching of CH2 and CH3 groups, together with a broad signal appearing in the high frequency range, ν ∼ 3600−3200 cm−1. At least two contributions are distinguishable in that region. A prominent band is observed with a maximum at ca. 3450 cm−1, which can be assigned to hydrogen bonded OH groups of the sulfonic acid groups. At lower frequencies, a second contribution in the ν ∼ 3325−3315 cm−1 range is also observed, associated with stretching of the NH groups. As expected, the C−H stretching bands also appear for the copolymers in Figure 1, while the prominent OH signal cannot be seen. In the same region a new shoulder with low intensity can now be detected at high frequencies (ν ∼ 3460 cm−1) which may be related to free NH groups in the copolymers. Figure 2 shows the 1800−1350 cm−1 IR region, involving vibrations associated with several groups from the main and side chains of the polymers under study. The ester group vibration band (st CO, ν ∼ 1727 cm−1) is visible in this region for the MeOAzB and MeOAzB/AMPS copolymers, together with three prominent bands at 1601, 1582, and 1501 8715

dx.doi.org/10.1021/ie303130e | Ind. Eng. Chem. Res. 2013, 52, 8714−8721


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can be observed for the bending vibration band of the aliphatic CH groups (ν ∼ 1480−1420 cm−1). These changes are accompanied with shifts of the bands related to the ester group, and more specifically, 0.71-MeOAzB/ AMPS shows the stretching IR bands at higher frequencies (CO, 1731 cm−1 and CO, 1265 cm−1), than 0.56-MeOAzB/AMPS (CO, 1725 cm−1 and CO 1251 cm−1). • The copolymers display the amide I and II peaks at lower intensities than AMPS, appearing at 1672 and 1541 cm−1 in Figure 2. It is noteworthy that the CO amide groups in the copolymers appear displaced to higher frequencies, with respect to the AMPS homopolymer, as previously reported by other authors.30−36 The low frequency range of the IR spectra is displayed in Figure 3. Several strong bands associated with CO vibrations from the MeOAzB groups are visible, with maxima at ν = 1251, 1179, and 1032 cm−1. Other bands observed in this region may be assigned to structural vibrations of the polymer backbone (CC st) and in-plane and out-of-plane bending of aromatic CH bonds (rocking, ρ, and torsion, τ). On the other hand, a prominent peak is observed in the AMPS IR curve in the vicinity of 1043 cm−1, related to stretching of the SO3 group. The FT-IR spectra of the 0.71-MeOAzB/AMPS and 0.56MeOAzB/AMPS copolymers contain signals from the two homopolymers in this region, as expected, but the band associated with st SO3 (ν = 1043 cm−1) is overlapped with strong bands arising from st C−O−C; even semiquantitative analyses are unwise. These spectra reflect changes in the chemical environment of some of the functional groups contained in the mesogenic units, the alkyl spacers, and the polymer backbone of the copolymers in the glass state.37−40 More specifically, the variations seen for the IR stretching bands of the carbonyl bonds, corresponding to the ester and amide groups (st CO, ν ∼ 1727 cm−1 and ν ∼ 1672 cm−1, respectively), and also for the NH stretching region (ν ∼ 3320 cm−1) may be interpreted in terms of changes to the local environments. Thus, two effects can be discriminated. On the one hand, the shifts in the CO bands are indicative of modification in the interactions of the groups near the backbone in the copolymers, with respect to the MeOAzB homopolymer. On the other


cm−1 associated with CC stretching in the aromatic rings of the mesogenic cores (st ar(CC)). At lower frequencies, several bands may be distinguished between 1480 and 1350 cm−1 and attributed to contributions from CH2 and CH3 bending (in plane, scissoring, δ) and aliphatic CC stretching modes, respectively. The IR signals associated with vibrations of the cis and trans forms of the NN bond are also normally seen in this region (ν ∼ 1550−1400 cm−1). The IR spectrum of the AMPS homopolymer in this region is dominated by two broad and intense bands at 1672 and 1541 cm−1, assigned to vibrations of the amide group (amide I, C O stretching, and amide II, NH bending, respectively). Several contributions are also visible in the frequency range where the structural CH bending and CC stretching IR vibrations (ν ∼ 1500−1100 cm−1) are expected to appear. In particular, contributions from the C(CH3)2 group are expected at 1365 cm−1 (bending, δ) and 1228 cm−1 (rocking, ρ). It is possible to observe several variations in the IR signals of this region by the introduction of AMPS units in the MeOAzB homopolymer: • The CO stretching band broadens for the copolymers but becomes narrower at sufficiently high AMPS concentrations; see inset in Figure 2. A similar effect 8716


8717

a

ar(CH) stas CH2 sts CH3

2928 2853

3320 3450 3000 cm−1) and the shift of the ν ∼ 1672 cm−1 band suggest that hydrogen bonding is disrupted by the presence of mesogenic groups when compared to that seen in AMPS; see Figure 2 (ν ∼ 1650 cm−1).10,25,30−33 3.2. Temperature Dependence of the FT-IR Spectra. The evaluation of the IR temperature response of the MeOAzB/AMPS copolymers is now shown in order to study with more detail the intermolecular interactions between the mesogenic units in the liquid crystalline chain after addition of acid units, in terms of local chemical environment, packing

Figure 5. Contour surfaces of IR regions of the temperature-dependent FT-IR spectra of MeOAzB (a−c), 0.71-MeOAzB/AMPS (d−f), and 0.56MeOAzB/AMPS (g−i). On each contour surface, the lower dashed line indicates the glass transition temperature, and the higher line indicates the smectic A to isotropic transition temperature. Increasing intensity from yellow < orange < red < violet. 8718



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Figure 8. Schematic representations of the smectic templates in the MeOAzB homopolymer (a), 0.71-MeOAzB/AMPS (b), and 0.56MeOAzB/AMPS (c).

some compositional effects. In general, while the spectra of the homopolymer and the 0.56-MeOAzB/AMPS copolymer undergo significant variations with temperature (Figures 5a−c and g−i), those of 0.71-MeOAzB/AMPS exhibit little temperature dependence in the glass and smectic phases. The IR bands associated with the mesogenic monomeric units are studied in further detail in the 1750−1450 cm−1 region, by calculating the peak positions of the main bands as a function of temperature; see Figure 6. A different temperature dependence of the aromatic bands and ester groups, already observed for the MeOAzB homopolymer in Figure 4, is also seen in the copolymers The bands associated with the aromatic C−C vibrations, at 1600 and 1500 cm−1, shift to lower frequencies on increasing temperature, while the band associated with the CO stretching vibration, ca. 1727 cm−1, to higher frequencies. In the spectra of both the homopolymer and 0.56-MeOAzB/AMPS, the linear shifts in these peak positions on varying temperature exhibit an increase of gradient at the glass transition temperature, suggesting a higher capability of the groups and units to rearrange and change the intermolecular interactions, and this is not observed for 0.71-MeOAzB/AMPS. The shift to higher frequencies of the CO band may be understood as a decrease in the interactions between the ester groups arising from an increase in the free volume associated with the polymer backbone regions. The differences in the glass phase between the homopolymer and 0.56-MeOAzB/AMPS, see Figure 6, can be

Figure 6. Temperature dependence of the peak positions of selected bands in the 1750−1450 cm−1 region of the FT-IR spectra for the homopolymer (◇), 0.71-MeOAzB/AMPS (△), and 0.56-MeOAzB/ AMPS (□).

7, respectively. Figure 5 shows the different temperature dependence of some IR regions within the samples, revealing

Figure 7. Temperature dependence of the 3600−3100 cm−1 region of the infrared spectra showing the band associated with the stretching of the N− H bond for (a) 0.71-MeOAzB/AMPS and (b) 0.56-MeOAzB/AMPS copolymers. The spectra have been recorded on heating from the glass phase. 8719



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related to some reminiscences of a more ordered isotropic phase in the former, as is normally observed by a reduction on the entropy change of the isotropic to smectic transition in liquid crystal ionomers.10,43 The coincidence of the peak at higher temperature values is then indicative of similar liquid crystalline environments in these samples, promoted by the activation of segmental motions above the Tg. The effect of temperature on the chemical environment of the AMPS groups was investigated through the NH stretching band at about 3300 cm−1 in Figure 7. A reduction in intensity and shift to higher frequency of this band is observed for the two copolymers on increasing temperature. These results indicate a reduction in the extent of hydrogen bonding as would be expected, being more pronounced for 0.56-MeOAzB/ AMPS. The absence of a strong change in either the position or intensity of this band in the vicinity of the glass transition of the copolymers strongly suggests that vitrification is not driven by a sudden change in the extent of hydrogen bonding. Instead, this result could indicate that the intermolecular interactions between the AMPS groups occur exclusively in acid-rich regions of the copolymers. The distinct IR temperature dependences of the different molecular groups are indicative of microphase separation typical of smectic phases in SCLCPs,2−7,24,44 by confining the mesogenic groups between the backbone layers. In the case of the MeOAzB homopolymer this leads to fully interleaved smectic A1 templates with optimized packing efficiency, as illustrated in Figure 8a. On the other hand, the IR results of 0.71-MeOAzB/AMPS denote higher heterogeneity in the chemical environment of the polymer, caused by the inclusion of the acid groups into the smectic templates. This results in poorer packing efficiency, with coexisting SmAd and SmA1 regions, and ultimately destabilization of the smectic range25,45−47 (Figure 8b). At higher AMPS concentrations, the similarities in the IR response of 0.56-MeOAzB/AMPS and MeOAzB can be then explained by the formation of similar smectic-rich domains, from which the acid groups are excluded. More concretely, a bilayer microstructure is proposed for 0.56MeOAzB/AMPS, see Figure 8c, which is also consistent with previous structural findings in similar and other liquid crystalline ionomers.8,10,25 Therefore, the liquid crystalline character in the MeOAzB/AMPS copolymers is not affected to a great extent by hydrogen bonding, which must be confined in the acid layers within this scheme, and instead is ruled by maximizing the packing efficiency of the side chains.

In both MeOAzB and 0.56-MeOAzB/AMPS, temperature promotes an increase in free volume of the backbone and better mesogenic alignment. By contrast, 0.71-MeOAzB/AMPS shows weaker interactions between the backbones and the mesogenic units which may be attributed to the disruptive effect of the nonmesogenic units in the copolymer, resulting in a lower packing efficiency and the formation of incommensurate SmA phases. The evaluation of the CO interactions and hydrogen bonding will be important when designing SCLCPs copolymers with new functionalities, as a way to anticipate structure forming factors between the mesogenic and nonmesogenic units.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is meant to be a tribute and recognition to the research activity of Professor Giulio Sarti. The authors also thank the Spanish Ministry of Science and Innovation, through the Research Projects ENE2007-67584-C03, UPOVCE-3E-013, IT2009-0074, ENE2011-28735-C02-01 and two FPI and FPU predoctoral grants, and the financial support of the Generalitat Valenciana, through the Grisolia (GRISOLIAP/2010/057, GRISOLIAP/2012/056, GRISOLIAP/2013/A/036) and Forteza programs and the ACOMP/2011/189 program. The UPV is also thanked for additional support through the PAID 05-094331 and PAID-06-11 program.

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REFERENCES

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4. CONCLUSIONS Variable temperature FT-IR microscopy has revealed a number of differences in the chemical environments of the functional groups of two poly[10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate]−copoly[2-acrylamido-2-methyl-1-propanesulfonic acid]s copolymers and the corresponding liquid crystal homopolymer, MeOAzB, in the glass, smectic, and isotropic phases. The copolymer with highest nonmesogenic unit content, 0.56-MeOAzB/AMPS, shows similar behavior to the liquid crystal homopolymer, and this is consistent with the presence of phase separated bilayers of mesogenic and acid groups. In this sample, the smectic phase formation appears to be driven by a combination of stronger interactions between the ester groups and hydrogen bonding between the AMPS-based units driving the microphase separation of the acid-rich sublayers. 8720



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Study of Structure Formation in Side-Chain Liquid Crystal Copolymers

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