Side-Chain Liquid Crystalline Polyacetylenes with Increasing Length

Jan 22, 2013 - The adjacent SmA blocks slide halfway to each other, leading to a highly ordered smectic phase with frustrated molecular packing at low...
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Side-Chain Liquid Crystalline Polyacetylenes with Increasing Length of Alkyl Tails: From Highly Ordered Smectic to Smectic C Phase Zhen-Qiang Yu,*,†,‡ Jacky W. Y. Lam,§ Cai-Zhen Zhu,‡ Er-Qiang Chen,*,† and Ben Zhong Tang*,§ †

Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering and the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ School of Chemistry and Chemical Engineering, Shenzhen Key Laboratory of Functional Polymers, Shenzhen University, Shenzhen 518060, China § Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China ABSTRACT: Phase structures and transitions of a series of side-chain liquid crystalline polyacetylene (SCLCPA) with a short spacer of three methylene units and different lengths of alkyl tails, namely, poly(5-{[(4′-alkyl-4-biphenylyl)carbonyl]oxy}-1-pentyne) (P-3,m, m is the number of the carbon atoms in the alkyl tail, m = 5, 7, 9, 11), were investigated using differential scanning calorimeter, polarized light microscopy, and one- and two-dimensional wide-angle X-ray diffraction. With the short spacer, P-3,m possesses the mesogenic groups on the side chain and polyacetylene backbone coupled together and thus renders sheetlike shape with the width nearly twice of the extended side-chain length. Experimental results reveal that the liquid crystalline (LC) structure of P-3,m is strongly dependent on the side-chain tail length, different from that of other SCLCPAs. For m < 11, several layers of the sheetlike P-3,m molecules stack together to form a smectic A (SmA) block, and the number of molecular layers increases with increasing m. The adjacent SmA blocks slide halfway to each other, leading to a highly ordered smectic phase with frustrated molecular packing at low temperatures. The enantiotropic phase transition sequence of P-3,m (m < 11) follows: highly ordered smectic with additional ordering on the subnanometer scale ↔ highly ordered smectic ↔ semctic C (SmC) ↔ isotropic. However, when m is increased to 11, the packing of sheetlike P-3,11 gives the SmC phase, with the transition sequence of SmC with additional ordering on the subnanometer scale ↔ SmC ↔ isotropic. The phase structures of P-3,m were studied by computer modeling. The phase changing from the highly ordered one to SmC with increasing m may be ascribed to that the P3,11 molecules intend to maximize the interaction between the biphenyl moieties in neighboring chains. We also investigated the orientation behavior of the highly ordered smectic phase under electric field, wherein a unique striplike texture was well developed, with the side chain and main chain parallel and perpendicular to the electric field, respectively.

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INTRODUCTION

structures and transitions of thermotropic SCLCPA are important topics. The molecular design of SCLCPA usually follows the concept of “flexible spacer” proposed by Finkelmann and Ringsdorf.17,18 Insertion of flexible spacers with reasonable length between the backbone and the mesogenic group can effectively decouple the dynamics of both the components. As a result, with the improved mobility of mesogen, LC phases such as nematic and smectic, which are largely dependent on the LC properties of the side chains, can be readily obtained in sidechain LC polymers. Many researches of SCLCPA have been focused on the flexible spacer length effect on the LC behavior. Similar to that observed in other side-chain LC polymers with

From the viewpoint of chemical structure, incorporating pendant groups into polyacetylene (PA) may result in new functional materials combining the π-electron delocalization along backbone and other functionalities offered by the properly selected substituents.1−7 Meanwhile, substituted PAs can possess the improved stability, good solubility, and excellent film forming capability, etc., which facilitate the fabrication of substituted PA in different application fields.8−14 Among the substituted PAs, side-chain liquid crystalline polyacetylene (SCLCPA) has been widely and deeply investigated in the past years.5−7 Shirakawa and Akagi first suggested that taking the advantage of easy alignment of mesogenic side chain in the liquid crystalline (LC) state, the orientation and effective conjugation of PA backbone of SCLCPA could be improved.15,16 In this regards, phase © 2013 American Chemical Society

Received: December 10, 2012 Revised: January 12, 2013 Published: January 22, 2013 588

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flexible backbones, the existence of flexible spacer is crucial with respect to the LC phase formation of SCLCPAs.19−41 For example, Hsu et al. have demonstrated that without spacers the SCLCPA carrying the mesogen of 4-alkanyloxyphenyl-trans-4alkylcyclohecanoate cannot display liquid crystallinity, but those with three or four methylene units as the spacer can form both smectic A (SmA) and smectic C (SmC) with an interdigitated bilayer structure.19 It is interesting to note that the spacers used in SCLCPAs can be rather short. Costa et al. have studied a series of SCLCPAs bearing biphenyl or benzoyloxy benzoate mesogens with short spacers.20,21 The SCLCPAs they synthesized with the spacer of two methylene units show enantiotropic smectic phase, although the corresponding monomers did not have any LC property. Even when the spacer length is as short as only one methylene unit, their polymers carrying biphenyl mesogenic moieties with ester or ether linkage can form bilayer SmC or interdigitated SmA phase. This observation is different from that recently reported by Chen et al., which shows a poly(1-alkyne) with the side group of cyanoterphenyl moiety directly linked to the backbone with ether linkage through one methylene unit is amorphous.22 The comparison between the works by Costa20,21 and Chen22 indicates that in addition to the spacer length the chemical structure of bridge group is also important, of which the effect has been discussed by Tang et al.23 When dealing with SCLCPAs, one should keep in mind that the backbone is (semi)rigid with considerably high steroregularity. Moreover, the PA backbone with trans structure or extended cis-transoidal conformation is planar. In this context, the main chain in SCLCPA may also join in the LC phase formation as an important and constructive component,24,25 particularly when the dynamics of main chain and side chain cannot be completely decoupled by the short spacers applied. In our previous research, we have found that a SCLCPA with a short spacer of three methylene units, poly(5-{[(4′-heptoxy-4biphenylyl)carbonyl]oxy}-1-pentyne) (denoted as P-3,7), can form a highly order smectic phase with frustrated molecular packing.25 The molecule of P-3,7 with the trans-cisoidal conformation of backbone is sheetlike as a whole.25,42 On both sides of the backbone, the mesogenic side chains are parallel to each other but extended to opposite directions, and therefore, the width of the molecular sheet is almost twice of the side-chain length. When the LC phase is formed, five layers of the sheetlike P-3,7 molecules stack together, forming a SmA block. However, the adjacent SmA blocks need to glide half the width of the molecular sheet along the side-chain direction, resulting in the frustrated molecular packing. As a result, we demonstrate that once the SCLCPA molecule takes an overall sheetlike shape, not only the mesogenic side chain but also the PA backbone will participate together in the LC structure. In the present work, we intend to elucidate if the length of side-chain tail will affect the LC structure of the sheetlike SCLCPA. Compared to the flexible spacer, side-chain tails have gained less attention in the research of SCLCPAs. In many cases, it is found no significant tail length effect on the phase structures of SCLCPAs.43−45 However, for a SCLCPA molecule with a persistent sheetlike shape, varying the sidechain tail length can cause different sheet width, which may also lead to different molecular packing behaviors. Here, using mainly one- and two-dimensional (1D and 2D) wide-angle Xray diffraction (WAXD), we study the phase structures and transitions of a series of poly(5-{[(4′-alkyl-4-biphenylyl)carbonyl]oxy}-1-pentyne) (Chart 1, denoted as P-3,m, where

Chart 1. Chemical Structure of P-3,m (m = 5, 7, 9, and 11)

m is the number of carbon atoms of the tail, m = 5, 7, 9, and 11). Similar to P-3,7 we studied before,25 P-3,5 and P-3,9 form a highly ordered smectic phase with frustrated molecular packing at low temperatures. However, to maximize the intermolecular interaction, P-3,11 with the longest side-chain tail abandons the frustrated molecular packing, just exhibiting a SmC structure. We also investigated the orientation of the highly ordered smectic phase after the samples were subjected to an external electric field. The result shows that the side chain and main chain are well aligned parallel and perpendicular to the electric field, respectively, and large domains with striplike texture can be obtained.

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EXPERIMENTAL SECTION

Materials. The samples of P-3,m (m = 5, 7, 9, 11) with different length of alkyl tails were synthesized by solution polymerization using WCl6−Ph4Sn as catalyst. The detailed synthetic procedures and structural characterizations of the polymers have been reported previously.23 Number-average molecular weight and polydispersity of the polymers, which are (3.0−5.0) × 104 g/mol and ∼2.0, respectively, were measured by gel permeation chromotography (Waters 150C) calibrated using polystyrene standard. 1H NMR analysis indicated that P-3,m was of good stereoregularity with the trans content of >80%. The decomposition temperature of P-3,m determined by thermogravimetric analysis at a heating rate of 10 °C min−1 was higher than 300 °C in a dry nitrogen atmosphere. Instruments and Methods. Differential scanning calorimetry (DSC) experiments were carried out using a PerkinElmer Pyris I with a mechanical refrigerator. The temperatures and heat flows were calibrated using benzoic acid and indium as standard materials. 1D WAXD experiments were performed on a Philips X’Pert Pro diffractometer equipped with an X’celerator detector in the reflection mode. 2D WAXD experiments were conducted on a Bruker D8Discover diffractometer with a 2D detector of GADDS in the transmission mode. In the thermal WAXD experiments, the sample temperature was controlled within ±1 °C, and the samples were protected by dry nitrogen gas. The X-ray sources (Cu Kα, λ = 0.154 nm) were provided by 3 kW ceramic tubes, and the peak positions were calibrated with silicon powder (2θ > 15°) and silver behenate (2θ < 10°). For both the 1D and 2D diffractions, the background scattering was recorded and subtracted from the sample patterns. The oriented samples used in 2D WAXD were prepared by mild mechanical shearing from the LC phase when applicable, and the point-focused X-ray beam was aligned perpendicular to the shear direction. To observe the LC textures, polarized light microscopy (PLM) experiments were carried out on a Leitz Laborlux 12 microscope with a Leitz 350 hot stage. Molecular dynamics (MD) simulation was also performed to study the single molecules and their packing models. A molecule of P-3,m (m = 5, 7, 9, 11) containing 10 repeating units with a perfect trans-cisoidal conformation was constructed by Materials Studio (Accerlrys), and the spacers and alkyl tails on side chains were assumed to be all-trans. The energy of this molecule was minimized using Smart Minimizer incorporated in the Discover Molecular Simulation Program. For P3,m (m < 11), the highly ordered smectic structures were constructed according to the rectangular lattice with the parameters of a and b deduced from the WAXD experiments at room temperature. The backbone axis (c-axis) is assumed to be perpendicular to the ab-plane, and the sheetlike molecules are parallel to the ac-plane. For P-3,11, the model of SmC structure was also based on the WAXD result. After construction of the LC structure, the energy was minimized using steepest descent, Fletcher−Reeves conjugate gradient, and BFGS 589

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Figure 1. DSC traces of P-3,m (m = 5, 7, 9, 11) recorded during cooling and subsequent heating at a rate of 10 °C/min. Newton with the convergence levels of 1000, 10, and 0.1 kcal/(mol Å), respectively. Compass force field was used during the minimization. The cutoff length during the energy calculation was 0.95 nm, and a van der Waals long-range energy correction after 0.95 nm was applied. The simulated 1D WAXD patterns were obtained by fitting the discrete WAXD lines of the simulated molecular packing using Reflex incorporated in Materials Studio.

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RESULTS AND DISCUSSION Phase Transition Behaviors. The phase transition behaviors of P-3,m were first studied by means of DSC. Figure 1 shows the DSC cooling and subsequent heating curves of the four samples at a rate of 10 °C/min. Upon using various cooling/heating rates, we found that for all the samples the transition temperatures detected by DSC were just slightly ratedependent. This implies that the transitions are close to thermodynamic equilibrium, in most cases associated with the LC phase transition. As shown in Figure 1, the samples of P3,m with m = 5, 7, 9 exhibit multiple transitions. P-3,7 possesses three exotherms peaked at 166, 157, and 94 °C, respectively. On the cooling curve, P-3,5 also has clearly three transitions. For P-3,9, in addition to the weak transition located around 90 °C, the other two high temperature transitions are largely overlapped but still can be identified from the DSC cooling curve. It is found that the isotropic temperature of P-3,m with m ≤ 9 decreases significantly with the increase of the alkyl tail length. But for P-3,11, only two broad transitions can be clearly observed, with the isotropic temperature almost identical to that of P-3,9. The phase transitions of P-3,m detected by DSC could be confirmed by thermal experiments of 1D WAXD. Figure 2 presents the 1D WAXD powder patterns of P-3,7 recorded during heating. In the high-angle region (Figure 2b), the scattering halo with the maximum at 2θ of ∼20° (d-spacing of ∼0.44 nm) is relatively sharp at low temperatures, of which the full width at half-height (fwhh) is ∼2.2°. However, the fwhh of the halo is ∼4.0° at 120 °C. The abrupt broadening of the halo starting at around 100 °C corresponds to the low-temperature transition observed by DSC. In the low-angle region (Figure 2a), two strong diffraction peaks (marked as 1 and 2 in Figure 2a) are observed, with a ratio of scattering vector q (q = 4π sin θ/λ, with λ the X-ray wavelength and 2θ the scattering angle) of 1:1.3. Both of the diffractions decrease in intensity when the temperature exceeds 130 °C, indicating that the sample enters the middle transition range as that shown in the DSC heating curve. At above 165 °C, while some residual of peak 1 can be still detected, peak 2 completely disappears, which may be associated with the finish of the middle transition. At 175 °C,

Figure 2. Sets of 1D WAXD powder patterns of P-3,7 recorded upon heating: (a) low-angle region; (b) high-angle region.

two typical amorphous halos can be observed in both the lowand high-angle region, indicating an isotropic phase of P-3,7. For P-3,5 and P-3,9, the thermal 1D WAXD experiments showed their structural evolution similar to that of P-3,7. When the carbon number of alkyl tail is increased to 11, the phase transition behavior of P-3,11 became completely different from that of other three samples. Figure 3 describes the 1D WAXD patterns of P-3,11 recorded upon heating. Compared with that shown in Figure 2b of P-3,7, the scattering halo of P3,11 in the high-angle region is relatively boarder with a fwhh of

Figure 3. Sets of 1D WAXD powder patterns of P-3,11 recorded upon heating: (a) low-angle region; (b) high-angle region. 590

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∼4.4° (Figure 3b), suggesting that on a subnanometer scale the molecular packing is less ordered. Heating the sample can lead to a further increase of fwhh. A discontinuous jump of the fwhh happens when the temperature is increased from 110 to 130 °C, in accordance with the low-temperature endotherm observed in the DSC heating scan of P-3,11. In Figure 3a, there are also two low-angle diffraction peaks, and the first one at the lower angle is much stronger than the second one. Note that the q-ratio of these two peaks is identical to 1:2, different from that of other P-3,m samples studied. This indicates that P3,11 forms a typical smectic phase. At above 120 °C, the two diffraction peaks slightly move toward higher angle of 2θ with increasing temperature, which should be mainly due to that the long alkyl tails adopt more and more gauche conformations. Meanwhile, the diffractions gradually become weaker, and they vanish simultaneously when the sample becomes isotropic. Phase Structure Identification. For comparison, the 1D WAXD patters of the four P-3,m samples obtained at room temperature are depicted in Figure 4. As mentioned, the

Figure 5. 2D WAXD patterns of oriented P-3,5 (a) and P-3,9 (b) at room temperature. The sample orientation was obtained by mild mechanical shearing. The shear direction is along the meridian and perpendicular to the incident X-ray beam.

P-3,5 and P-3,9, respectively. On the other hand, we can also observe the diffraction appearing in quadrants (marked as 1), which is away from the equator with an angle of ∼45° for P-3,5 and ∼38° for P-3,9. According to the analysis of P-3,7, we consider that P-3,5 and P-3,9 should form a highly ordered smectic phase with frustrated molecular packing.46−49 When a mild mechanical shear force was applied to orient the sample on a solid surface, the LC domains got aligned along the shear direction and the chain backbone is perpendicular to the substrate. In this case, the diffractions of 1 and 2 shown in Figure 5 can be assigned to be (11) and (20) of a 2D rectangular lattice. On the basis of the diffraction results, the lattice parameters of a and b can be determined. For P-3,5, a = 4.35 nm and b = 3.55 nm; for P-3,9, a = 4.9 nm and b = 5.30 nm. On the other hand, the PA backbone is perpendicular to the ab-plane of rectangular lattice. We also monitored the evolution of 2D WAXD patterns when the phase transitions took place upon heating. Figure 6 describes a set of 2D WAXD patterns of an oriented P-3,5 sample obtained at various temperatures. Compared with that shown in Figure 5a, the high-angle scattering becomes more diffusive when the temperature passes the low-temperature transition at around 130 °C, as evidenced by Figure 6a of 145 °C. This indicates that the ordering on the subnanometer scale of P-3,5 is lost. Meanwhile, the low-angle diffractions look almost unchanged, and therefore, the basic structure of the highly ordered smectic phase remains. Figure 6b shows that at 175 °C, which is close to the end of the middle transition, diffraction 2 on the equator almost vanishes. However, diffraction 1 in the quadrants is still very clear, giving the 2D WAXD pattern for a SmC phase. Further increasing the temperature to 190 °C, two amorphous rings are observed in both the low- and high-angle region (Figure 6c), indicating the isotropic state. Combining the results of DSC, 1D WAXD, and 2D WAXD, we suggest that the phase transition of P-3,5 follows the sequence of highly ordered smectic with additional ordering on the subnanometer scale ↔ highly ordered smectic ↔ SmC ↔ isotropic. For P-3,7 and P-3,9, the same phase transition sequence can be justified. According to the DSC and 1D WAXD results, we find that P3,11 should have two transitions. Upon heating, the lowtemperature transition at around 128 °C is related to the loss of ordering on the subnanometer scale within the smectic phase. On the other hand, the smectic structure remains until the isotropization. Figure 7a depicts the 2D WAXD pattern of a oriented P-3,11 sample at room temperature. In the low-angle region, the diffraction arcs with the q-ratio of 1:2 locate on the equator, indicating that the layer normal of smectic phase is

Figure 4. 1D WAXD powder patterns of P-3,m (m = 5, 7, 9, 11) recorded at room temperature.

diffraction behavior of P-3,11 is quite different from that of the samples with shorter alkyl tails. The low-angle diffractions with the q-ratio = 1:2 indicates that P-3,11 forms a smectic phase. However, for P-3,m with m = 5, 7, 9, two diffractions (marked as 1 and 2 in Figure 4) with the q-ratio ≠ 1:2 can be observed, suggesting that these three samples may share a same molecular packing behavior in the LC phase which must be different from that of P-3,11. To identify the phase structure clearly, we further performed the experiments of 2D WAXD using oriented samples. Figure 5 presents the 2D WAXD patterns of P-3,5 and P-3,9 recorded at room temperature. To obtain the chain orientation, the sample was mechanically sheared with a mild external force at a proper temperature below the isotropic temperature and was then quenched to room temperature. For P-3,5 and P-3,9, the shear temperature was 160 and 120 °C, respectively. In Figure 5, the shear direction is along the meridian and perpendicular to the incident X-ray beam. Obviously, P-3,5 and P-3,9 share the same diffraction feature, which is in fact the same as that of P-3,7 reported before.25 In the high-angle region, the scattering with a d-spacing of ∼0.44 nm is concentrated on the meridian. In the low-angle region, the diffraction on the equator (marked as 2) is located at 2θ of 4.1° (d-spacing of 2.15 nm) and of 3.6° (d-spacing of 2.45 nm) for 591

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Figure 6. Sets of 2D WAXD patterns of an oriented P-3,5 sample recorded at various temperatures upon heating: (a) 145, (b) 175, and (c) 190 °C. The sample orientation was obtained by mechanical shearing. The shear direction is along the meridian.

Figure 7. (a) 2D WAXD pattern of an orientated P-3,11 recorded at room temperature. (b) The azimuthal scan data of the high-angle scattering shown in (a).

the all-trans conformation. This dimension in fact determines the a parameter of P-3,7. A similar relationship is observed for P-3,5 and P-3,9 (see Table 1). With the backbone perpendicular to the ab-plane, the sheetlike molecules of P3,m (m = 5, 7, 9) are parallel to the a-axis. On the other hand, the increase of b parameter should reflect that the number of molecular sheets involved in the 2D rectangular lattice is increased with increasing m. The d-spacing of ∼0.44 nm detected from the halo maximum in the high-angle region of WAXD gives the average intermolecular distance between the parallel stacked molecules. Interestingly, we find that the value of b of P-3,m (m = 5, 7, 9) is nearly an integral multiple of the dimension of 0.44 nm. Therefore, the number of molecular layers can be estimated as ∼b/0.44, which is 8, 10, and 12 for P3,m with m = 5, 7, and 9, respectively. Note that each rectangular lattice involves two SmA block. Therefore, the SmA block of P-3,5, P-3,7, and P-3,9 contains 4, 5, and 6 layers of molecules, respectively. We performed molecular simulation of the highly ordered smectic structure of P-3,m based on the lattice dimensions determined by WAXD. Using P-3,5 as an example, the computer simulation result of molecular packing is shown in Figure 8a. In the model, the molecular “sheet” is slightly bended due to the rotation of ester bridge (OCO group) on both sides of the backbone. Four layers of molecules form a SmA block. Moreover, the upper and lower blocks glide halfway from each other along the a-axis, giving the frustrated molecular packing. Such a frustration structure is caused by the coupling of density and dipole moment modulation, which can maximize the entropy of LC phase. Here, it is worth noting that after the glide of SmA blocks the biphenyl groups from different layers are still well stacked together. Therefore, in the highly ordered smectic phase the intermolecular interaction of P-3,5 is also largely satisfied. The corresponding simulation of 1D WAXD

perpendicular to the shear direction which is along the meridian. It is worthy to note that the high-angle scattering halo with the d-spacing of ∼0.44 nm splits into four appearing in the quadrants. Figure 7b gives that the corresponding azimuthal integrity of the high-angle scattering halo, showing that the four arcs tilt 35° away from the meridian direction. As a result, a SmC phase can be determined for P-3,11. We propose that the phase transition sequence of P-3,11 is SmC with additional ordering on the subnanometer scale ↔ SmC ↔ isotropic. Alkyl Tail Length Dependence of LC Phase of P-3,m. The LC phase structures and lattice dimensions of P-3,m at the low temperature are summarized in Table 1. We propose that Table 1. Side-Chain Length, Lattice Dimensions of LC Structure of P-3,m (m = 5, 7, 9, 11) highly ordered smectic

P-3,5 P-3,7 P-3,9 P-3,11

smectic C

lSCa (nm)

ab (nm)

bb (nm)

layer period (nm)

tilt angle (deg)

2.15 2.40 2.65 2.90

4.35 4.80 4.90

3.55 4.40 5.30

3.02 3.40 3.93 4.40

45 45 48 55

a

Side-chain length estimated with the assumption that the spacer and tail adopt an all-trans conformation. bParameter of the 2D rectangular lattice for the highly ordered smectic structure.

the P-3,m with m = 5 and 9 form the LC structure same to that of P-3,7, which is highly ordered smectic. For these three P-3,m, both the a and b parameter of rectangular lattice increase with increasing m. As mentioned, the width of P-3,7 molecular “sheet” is nearly the double of the side-chain length (lSC) estimated under an assumption that the spacer and tail adopt 592

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Figure 8. Computer simulation of the LC structure of P-3,5 (a) and P-3,11 (c). The simulated 1D WAXD patterns based on the (a) and (c) are shown in (b) and (d), respectively.

Figure 9. Schematics of the frustrated molecular packing of P-3,m (m = 5, 7, 9, 11) viewing along the PA backbone. The molecule is assumed to be perfectly sheetlike. The lengths of “main chain + spacer (including the ester bridge)”, “mesogen”, and “alkyl tail” are drawn based on their real sizes.

pattern agrees well with the experimental data (Figure 8b), providing a support to the chain packing model proposed. P-3,11 adopts a SmC structure rather than the highly ordered smectic one. The sheetlike molecular shape of P-3,m is mainly due to that the short spacer used to connect the PA backbone, and the biphenyl mesogenic group can couple these two parts together. We first presumed that the P-3,11 molecule remained the basic feature of sheetlike shape, and the SmC structure was formed by parallel stacking of the molecules. In this case, the P3,11 backbones form a sublayer separated by the two adjacent sublayers of side chains, and the side chain is titled nearly 55° with respect to the normal of the backbone sublayer. Figure 8c gives the computer simulation result of the SmC structure of P3,11 with the layer periodicity of 4.4 nm. Compared with that

shown in Figure 8a of P-3,5, the shape of P-3,11 molecule is more bended. Nevertheless, in the SmC structure, the bended “sheets” well stack together, forming clearly the sublayers of backbones and biphenyls. Figure 8d describes the simulated 1D WAXD profile based on the chain packing model shown in Figure 8c, which is in accordance with the experimental results. Thermodynamically, the phase structure of P-3,m changing from the highly ordered smectic to the SmC with increasing m should arise from the competitive balance between enthalpic and entropic contributions. We find that the interaction between biphenyl groups may be crucial for the molecular packing behavior selected by the P-3,m molecules. Figure 9 shows a simple geometrical argument of the molecular stacking. As an approximation, the cross section of the sheetlike P-3,m is 593

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simplified to be a rectangle with the thickness of ∼0.44 nm. Further assuming that the side chain is extended and perpendicular to the conjugated backbone, the width of the rectangle is the sum of the lengths of different moieties in the side chain. For the highly ordered smectic phase, the two neighboring SmA blocks need to glide halfway to each other. Here, we just pay attention to the two molecules coming from the top and bottom layer of the two adjacent SmA blocks. As shown in Figure 9, the biphenyl groups of the two adjacent P3,5 molecules can almost fully contact with each other after the molecular gliding (also see Figure 8a). Increasing the alkyl tail length will result in less contact between the biphenyls. For P3,11, the extended alkyl tail is of ∼1.45 nm, nearly identical to the combined length from the backbone to the end of biphenyl. If the P-3,11 molecules packed with the frustration scheme, the biphenyls would no longer contact to each other. Therefore, to seek more intermolecular interaction between the biphenyls, a SmC phase becomes favorable for P-3,11. This implies that when the entropy gain due to the frustrated molecular packing cannot compensate for the loss of molecular interaction, P-3,m will select the phase of SmC. Unique LC Texture of the Highly Ordered Smectic Phase. The P-3,m samples with highly ordered smectic phase could exhibit some unique LC texture under PLM. After the samples were cooled from the isotropic state to below the isotropic temperature, a sanded texture was observed, which should correspond to the formation of SmC phase at high temperatures. When a relatively fast cooling rate was applied, the sanded texture largely retained even after the highly ordered smectic phase was completely formed at room temperature. However, when the sample was subjected to a slow cooling (e.g., at a rate of 0.1 °C/min), some faint striplike texture emerged from the sanded background. Isothermal annealing of the samples at the temperatures below the peak temperature of the middle transition of P-3,m (m ≤ 9) could result in clear striplike texture. Figures 10a and 10b illustrate the PLM images

molecular alignment of LC polymers can be achieved by applying magnetic field,50 external electric field,51−53 and mechanical shearing in the LC phase temperature region. To orientate the P-3,m samples using electric field (E), thin film sandwiched between two ITO glasses was subjected to an E strength of ∼107 V/m. Under the applied E, the sample slowly cooled from isotropic state was isothermally annealed for ∼10 h at 150 °C, followed by cooling to room temperature. Figure 11a

Figure 11. (a) LC texture of P-3,7 recorded at 150 °C under an electric field with the strength of ∼107 V/m. Image size: 125 × 125 μm2. (b) 2D WAXD pattern of the E-oriented P-3,7 with the direction of electric field along the equator.

shows the resulted texture of P-3,7 thin film. Compared with that shown in Figure 10b, the PLM image of Figure 11a indicates that striplike domains are prevailing, and there is no sanded texture detected. Considering that the biphenyl-based mesogen possesses relatively large dipole moment and can response to E more easily than the PA backbone, we presume that the side chains of P-3,m will be oriented along the E direction. Figure 11b presents the 2D WAXD pattern of the Eorientated P-3,7 film, wherein the X-ray incident beam is perpendicular to the E direction which is along the equator. It is interesting to note that the 2D WAXD looks similar to that shown in Figure 5 and also that in the previous report of P3,7.25 The diffraction on the equator (i.e., the E direction) can again be assigned to be (20), indicating the side chains are indeed aligned along the E direction. In this case, the PA backbones lie down in the film.

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CONCLUSION In summary, we have investigated the phase structures and transitions of a series of SCLCPA (P-3,m) with different sidechain tail lengths. Different from that of many other SCLCPAs, the LC structure of P-3,m is strongly dependent on the tail length. Since the spacer of three methylene units used in P-3,m is short, the mesogenic groups on the side chain and PA backbone of P-3,m are largely coupled together, resulting in the sheetlike molecule with the width nearly a twice of the extended side-chain length. Consequently, both the main chain and side chain participate in the LC phase formation. On the basis of the 1D and 2D WAXD results, we determine that P3,m with m = 5, 7, and 9 can form the highly ordered smectic phase with frustrated molecular packing at low temperatures. Namely, several layers of the sheetlike molecules of P-3,m stack together to form a SmA block, and the adjacent blocks need to glide halfway from each other along the a-axis of the 2D rectangular lattice (i.e., nearly the side-chain direction) of the LC structure. Increasing the tail length leads to the increase of the a dimension of the 2D rectangular lattice. Moreover, we find that the b parameter of the 2D lattice is also increase with m. The number of molecular layers involved in the SmA block

Figure 10. LC textures recorded after the P-3,9 cooled from isotropic state to 150 °C followed by annealing at that temperature for (a) 10 h and (b) 20 h. Image size: 125 × 125 μm2.

of P-3,9 taken at 150 °C with annealing time of 10 and 20 h, respectively. When monitoring the development of the striplike domains under PLM, we found that they were probably initiated by merging some small LC domains together, and their number increased with increasing the annealing time. Compared to that shown in Figure 10a, the striplike domains in Figure 10b are much brighter, implying that the chain packing within the domains becomes more perfect after prolonged annealing. The “strips” can be nearly millimeter long. However, the reason for anisotropic growth is not known at this moment. We found that external electric field could facilitate the development of striplike domains. It is well-known that 594

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is 4, 5, and 6 for P-3,5, P-3,7, and P-3,9, respectively. P-3,m (m = 5, 7, 9) displays three enantiotropic transitions following the sequence of highly ordered smectic with additional ordering on the subnanometer scale ↔ highly ordered smectic ↔ SmC ↔ isotropic. In terms of the tail length dependences of phase structure of P-3,m, the most important one is that increasing the tail length will eventually result in a SmC phase, as evidenced by P-3,11. The reason may be that for P-3,11 the frustrated molecular packing will sacrifice too much the intermolecular interaction, and thus the SmC is favorable. The transition sequence of P-3,11 is SmC with additional ordering on the subnanometer scale ↔ SmC ↔ isotropic. We also investigated the orientation of the highly ordered smectic structure induced by external electric field. The resulted sample can show large domains with striplike texture, and the PA backbones are well aligned perpendicular to the electric field.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-Q.Y.); [email protected] (E.Q.C.); [email protected] (B.Z.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NNSFC Grants 21074073, 20990232, and 51273002) and Shenzhen Science and Technology Bureau (ZYC201105130112A and JC201105130384A). We thank Dr. G. Q. Xu for the simulation of LC structures.

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