He-lou Xie*a,
Bin Nia,
Quan Liua,
Jun Wangb,
Shuang Yangb,
Hai-liang Zhang*a and
Er-qiang Chen*b
aKey Laboratory of Special Functional Polymer Materials of Hunan Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges and Universities of Hunan Province, Key Lab of Environment-friendly Chemistry and Application in Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China. E-mail: xhl20040731@163.com; hailiangzhang@xtu.edu.cn
bBeijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering and the Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Center for Soft Matter Science and Engineering, Peking University, Beijing 100871, P. R. China. E-mail: eqchen@pku.edu.cn
First published on 4th November 2015
A series of combined main-chain/side-chain liquid crystalline polymers (MCSCLCP) based on “jacketing” effect, poly{(2,5-bis[n-(4-butoxy-4′-oxybiphenyl)n-alkyl]oxycarbonyl}styrene) with different lengths of alkyl spacers (denoted as Pn, n represents the number of carbon atoms in the alkyl spacers, n = 2–10) have been successfully synthesized via atom transfer radical polymerization (ATRP). The chemical structures of Pns and the corresponding monomers were characterized using combined techniques with satisfactory analysis data. The phase structures and transitions of Pn were investigated using differential scanning calorimetry (DSC), polarized optical microscope (POM), and one- and two-dimensional wide-angle X-ray diffraction (1D and 2D WAXD). It has been identified that P2 and P4 with short alkyl spacers form typical smectic phase. For n ≥ 6, Pns exhibit similar hierarchical ordered structure at low temperatures, bearing double orderings on the nanometer and sub-nanometer scales. In the hierarchical structure, the main-chains based on “jacketing” effect form a 2D centered rectangular scaffold, and the side-chain biphenyl mesogens within the scaffold pack into a smectic E-like structure. The a dimension of rectangular lattice enlarges with n. When the temperature is increased, different from P6, P8 and P10 present the same phase behavior, forming smectic B-like packing of side chains and maintaining their main-chain scaffold until isotropization.
Combined main-chain/side-chain LC polymer (MCSCLCP)26–34 is a hybrid one combining the chemical features of MCLCP and SCLCP. MCSCLCP is usually prepared by condensation polymerization method. From the chemical viewpoint, the main chain of MCSCLCP can either be rigid rod or possess an alternative structure of rod-like mesogenic groups with flexible spacers; and its side chain is usually composed of mesogenic group linked to the main-chain repeating unit through a flexible spacer. Compared with MCLCP and SCLCP, the interplay between main- and side-chain of MCSCLCP becomes more complicated, which leads to versatility of the LC properties and supramolecular structures. Recently, we have proposed that MCSCLCP can be synthesized based on chain polymerization.35–37 The first example is poly(2,5-bis{[6-(4-butoxy-4′-oxybiphenyl)hexyl]oxycarbonyl}styrene) (PBBHCS). Experimental results reveal that PBBHCS with sufficiently high molecular weight (Mw) can self-assemble into a hierarchical structure containing double orderings on the nanometer and sub-nanometer scales.36 Taking the advantage of “jacketing” effect, the rodlike main chain of PBBHCS is generated by the central rigid portion of the side chains laterally attached to every second carbon atom along the polyethylene backbone. At low temperatures, the main chains can form a two-dimensional (2D) scaffold with centered rectangular arrangement on the nanometer scale, and the biphenyl-containing side chains fill in the space between the main chains, forming a smectic E (SmE)-like structure with the side-chain axis perpendicular to that of main chain. It is further found that incorporating different functional components into the side chain, such as azobenzene mesogen, triphenylene (Tp) mesogen and polyhedral oligomeric silsesquioxane (POSS) can also offer interesting property.37–39
It is well known that the length of flexible spacer of LCP plays an important role in determining the LC behaviours of LCP.40–46, The decoupling effect proposed by Finkelmann et al. Elucidates that the flexible spacer with sufficiently long length can efficiently decrease the dynamics interaction between main chain and mesogenic side groups, beneficial to the LC formation in SCLCPs.47,48 It should be worth noting that for the MCSCLCP with the main chain based on “jacketing” effect, the “flexible spacer” takes part in two roles. On one hand, the flexible spacer can facilitate the LC phase formation of mesogenic side chains. On the other hand, it may also affect the packing of main chains. In order to further excavate the effect of spacer, we synthesized a series of MCSCLCP bearing the spacers with different lengths, poly(2,5-bis{[n-(4-butoxy-4′-oxybiphenyl)n-alkyl]oxycarbonyl}styrene) (denoted as Pn, n represents the number of carbon atoms in the alkyl spacers, n = 2–10), of which the chemical structure is shown in Scheme 1. The phase structures and transitions of P6 (i.e., PBBHCS mentioned above) has been well studied.36 Here, using P6 as the reference, we attempt to elucidate how the variation of spacer length will influence the molecular packing behavior. We find that with short spacers P2 and P4 just form smectic structure. For P8 and P10, the hierarchical structure containing double orderings on the nanometer and sub-nanometer scales, which is similar to that of P6, is demonstrated at low temperatures. Moreover, different from P6, P8 and P10 allows the biphenyl mesogen packing in a smectic B (SmB)-like structure after the melt of their SmE-like structure, and the main-chain scaffolds remain until isotropic temperature.
LC texture of the polymers were recorded under polarized optical microscopy (POM, Leica DM-LM-P) coupled with a Mettler-Toledo hot stage (FP82HT). For POM experiments, the film samples with the thickness of ∼10 μm were obtained via cast-coating followed by slow drying at room temperature. The thermal transitions of the polymers were detected by differential scanning calorimetry (DSC, TA-Q10). Samples were encapsulated in sealed aluminum pans with a typical mass of 3–10 mg. The temperature and heat flow were calibrated using standard materials (indium and zinc) at cooling and heating rates 10 °C min−1.
One-dimensional (1D) wide-angle X-ray diffraction (WAXD) experiments were carried out on a Bruker D8 advance diffraction with a 3 kW ceramic tube as the X-ray source (Cu Kα) and lynxeye detector as the detector. The reflection peak positions were calibrated with silicon powder (2θ > 15°) and silver behenate (2θ < 10°). Background scattering was subtracted from the sample patterns. A temperature control unit (Paar Physica TCU 100) connected with the diffractometer was exploited to study the structure evolutions as a function of temperature. 2D-WAXD shear patterns were obtained using a Bruker D8 Discover diffraction with a Vantec-500 detector. The positions of diffraction patterns were calibrated using the same method mentioned above. The samples were put on the sample stage, and the point-focused X-ray beam was aligned perpendicular or parallel to the mechanical shearing or fiber direction. The 2D diffraction patterns were recorded in a transmission mode at different temperatures upon heating and cooling.
Sample | Yielda (%) | Mn,GPCb (×104) | Mw/Mnb | Phase transition (°C)c | Td,N2d (°C) | Td,aire (°C) |
---|---|---|---|---|---|---|
a Yield calculated according to previous paper.39b Obtained from GPC with linear PS as standards.c Phase transitions and corresponding transition temperature evaluated by DSC at a rate of 10 °C min−1 under heating.d The temperature at which 5% weight loss of the sample was reached from TGA under nitrogen atmosphere and air atmosphere, respectively.e The temperature at which 5% weight loss of the sample was reached from TGA under nitrogen atmosphere and air atmosphere, respectively.f Orthogonal SmE-like structure of side-chain mesogens fills in 2D centered rectangular scaffold of main chain.g Smectic side-chain mesogens fills in 2D centered rectangular scaffold of main chain.h Hexagonal SmB-like structure of side-chain mesogens fills in 2D centered rectangular scaffold of main chain. | ||||||
P2 | 70 | 1.90 | 1.10 | SmA(192 °C) isotropic | 371 | 308 |
P4 | 65 | 1.51 | 1.15 | SmA(187 °C) isotropic | 362 | 301 |
P6 | 65 | 1.55 | 1.06 | LC1f(136 °C)LC2g(161 °C)SmA(172 °C) isotropic | 377 | 294 |
P8 | 68 | 2.02 | 1.13 | LC1f(130 °C)LC3h(136 °C) | 381 | 303 |
LC2g(171 °C) isotropic | ||||||
P10 | 65 | 2.21 | 1.15 | LC1f(116 °C)LC3h(132 °C) | 387 | 299 |
LC2g(186 °C) isotropic |
Fig. 2 DSC thermograms of P2 (a) and P10 (b) at a rate 10 °C min−1 during cooling and subsequent heating process. |
POM results also implied the phase variation tendency of Pns similar to that revealed by DSC. Below the highest transition temperature, all the samples presented strong birefringence under POM, indicating the existence of LC phases. At different temperatures, the five Pns may exhibit different textures. The distinct change of textural color of P6 has been observed when the sample went through different temperature areas.36 P8 and P10 displayed the typical the fan-shaped texture at room temperature. Upon heating, the texture became significantly different when the samples crossed over the low- and middle-temperature transitions. The representative POM images of P10 at different temperatures are shown in Fig. 3a–c. However, P2 and P4 showed grainy textures until up to the isotropic temperature, and the texture did not change during the heating or cooling process. Fig. 3d is a POM image of P2 at 170 °C. The POM results are good consistent with the DSC results.
Fig. 3 Representative textures of P10 at 30 °C (a), 130 °C (b), 170 °C (c) and P2 at 170 °C (d), (200× magnification). |
1D WAXD thermal experiment was performed to elucidate the phase behavior. Similar to P6,36 P8 and P10 presented two sets of diffractions in both the low- and high-angle areas, respectively. The 1D WAXD profiles of P10 recorded at various temperatures are shown in Fig. 4a. At low temperatures, four distinct diffraction peaks have been observed in the low-angle region, of which the q-ratio (q = 4πsinθ/λ) is 1:1.38:1.92:2 at room temperature, indicating a LC phase structure other than lamellar and hexagonal. On the other hand, the diffractions in the high-angle region at low temperatures are highly reminiscent of that of biphenyl containing SCLCPs forming a SmE phase.50–52 From left to right, the three peaks can be assigned as (11), (20), and (21) diffractions of SmE. When the temperature is located between 30 and 110 °C, there are hardly change of the diffraction peaks for both low- and high-angle region. From 120 to 130 °C, the low-angle diffractions slightly shift to left due to thermal expansion. Meanwhile, the two peaks at 2θ of 20.81° and 26.66° disappear, only the diffraction peak of 19.58° is still kept. This outcome suggests that the SmE ordered structure of the side chain transforms into another one, which can be smectic B (SmB).53,54 With further increasing temperature, the first low-angle diffraction gradually reduces its intensity, but others peaks always remains. In the meantime, the peak at 19.58° becomes very diffuse, indicating that the SmB structure of biphenyl mesogens is destroyed. When the temperature exceeds 200 °C, only scattering holes appear in the high- and low-angle region, respectively, indicating that the sample enters isotropic state. In Fig. 4b, the d-spacing and the full-width of half-height (FWHH) of the first peak in the high-angle region is plotted as functions of temperature. Three dramatic jumps of the d-spacing and FWHH have been observed, indicating the three transitions which well agree with the results observed in DSC (see the dashed vertical lines). Similar experimental results of P8 are shown in Fig. S4.†
For P2 and P4 with short spacer, they presented different diffractive behaviour from P6, P8 and P10. As show in Fig. 5, P2 exhibits three peaks in the low angle region with the q-ratio of 1:2:3, indicating that P2 forms a lamellar structure (smectic phase). In the high angle region, a scattering halo appears implying the smectic structure is low ordered. With increasing temperature, the high-angle halo becomes broader and continuously shifts to left side. On the other hand, the three peaks with the q-ratio of 1:2:3 in the low-angle region remain during heating. They disappear until the temperature is higher than 180 °C, indicating the transition from smectic to isotropic. The 1D WAXD result of P4 is shown in Fig. S3,† similar to that shown in Fig. 5.
Fig. 6 2D WAXD patterns of P10 recorded with X-ray beam parallel to the X-direction at 30 °C (a); 125 °C (b); 170 °C (c). |
The schematic draw of the P8 and P10 chain packing at low temperatures is shown in Fig. 7a and b. In order to understand more the molecular packing, the relative electron density map of P10 was reconstructed based on the 1D WAXD data, of which the result is shown in Fig. 7c (for P8, see ESI†). According to chemical structure, the red circular and green area in Fig. 7c with the relatively high electron density should be the column constructed by the main chain. It can be calculated that the area (i.e., red + green) is about 1.3 nm2, in agreement with the main-chain dimension estimated in our previous work.36 The location nearby the main-chain column (blue area) has the lowest electron density, which could be assigned to the flexible spacer. The blue area has the size of ∼1 nm, which is close to the length of 1.2 nm for the flexible spacer with all trans conformation. The green colored zig-zag area should correspond to the parallel packing of the biphenyl groups.
With the decrease of spacer length, P2 and P4 presented totally different diffraction patterns from P6, P8 and P10. As shown in Fig. 8a, three pairs of sharp diffraction arcs of P2 in the low-angle region show on the equator with the q-ratio of 1:2:3, confirming the lamellar structure. In addition, a pair of diffuse halo at the high angle appears on the meridian, indicating that the side chains are largely parallel to the normal of smectic layer. Therefore, P2 forms a SmA phase. Assuming the spacer and tail of side chain takes all trans conformation, the calculated side-chain length of P2 is 5.37 nm. The experimental measured d-spacing of 3.65 nm is smaller than the calculated value but larger than a half of it. Thus, P2 should adopt partially interdigitated packing of smectic phase (SmAd). The reconstruction of relative electron density map of P2 is shown in Fig. 8b. We assume that the zone with high electron density (red colored) is ascribed to the biphenyl mesogens which are parallel packed to each other. The zone thickness of ∼1.4 nm is larger than the size of biphenyl (∼1 nm), implying the interdigitated packing. The blue colored area should be occupied by the alkyl spacers, and the area between two adjacent blue zones belongs to the main chains. During the heating process, this diffraction pattern hardly changed until isotropization, meaning P2 forms a stable SmAd phase during the whole area below the isotropic temperature. The 2D WAXD results and reconstructed relative electron density map of P4 are similar to that of P2 (see ESI†).
Fig. 8 (a) 2D WAXD pattern of P2 recorded with X-ray beam perpendicular to the shear direction at room temperature. (b) Relative electron density map of the smectic phase of P2. |
Combined DSC and 1D/2D WAXD results, the phase transition sequences of Pns and the LC structure parameters at low temperatures are summarized in Tables 1 and 2, respectively. Obviously, the spacer length plays an important role in the formation of supramolecular structures of this type of MCSCLCP. It has proved that the decoupling effect of spacer length can affect the molecular shape, resulting in the formation of the different supramolecular structure. Herein, the short spacer length with ethylidene or butylidene cannot provide effectively the decoupling effect. The side chains can either extend or fold in the smectic phase. While the former one can result in the two biphenyl groups towards two opposite directions, the later one makes a hairpin shape of the side chain. In both cases, the side chain packing will be influenced by the polyethylene backbone, and the detailed molecular arrangement of the resultant SmAd is not clear at this moment. For n ≥ 6, the microphase separation from the side chain and main chain is able to occur and meanwhile, the alkyl spacer may partly involve in the construction of rod-like main chain. Thus, the whole molecular shape of Pn dramatically changes. The synergistic effect of the main and the side chain facilitates the formation of hierarchically ordered structure. Careful analysis can find the dimension of the ordered structures is dependent on the spacer length. Fig. 9a describes the d-spacings measured from WAXD as functions of the spacer length n, wherein the data for n ≤ 4 correspond to the first order diffraction (i.e., smectic layer spacing) and that for n ≥ 6 correspond to the second order diffraction (i.e., d20 of the rectangular lattice). As mentioned above, Pns with n ≤ 4 form SmAd. Thus a half of the slope (i.e., 0.07 nm/methylene unit) represents the increment caused by adding one methylene unit. This value is smaller than that of 0.125 nm/methylene unit for the spacer with all-trans conformation, implying that the spacers adopt some gauche conformation. For Pn with n ≥ 6 formed 2D centered rectangular scaffold of main chain (see Fig. 7a and b), the value of d20 in fact represents a half of the side-chain length. It increases with n with a slope of 0.093, also smaller than 0.125, implying the existence of the gauche conformations in the spacer. On the other hand, the short axis b of 2D centered rectangular lattice determined based on WAXD results hardly change with n. It means that for n ≥ 6 Pns always have four layers of biphenyl parallel stack in the rectangular lattice.36
Sample | The value of dhka (nm) | Lattice | Lattice parametersb (nm) |
---|---|---|---|
a Indices are based on the unit cells proposed.b Obtained via Bragg's formula and the orthorhombic equation.c Obtained from the ref. 36. | |||
P2 | d01 = 3.65, d02 = 1.82, d03 = 1.29 | Lamellar | Layer spacing = 3.65 |
P4 | d01 = 3.93, d02 = 1.96, d03 = 1.33 | Lamellar | Layer spacing = 3.93 |
P6c | d20 = 2.76, d11 = 1.96, d31 = 1.43, d40 = 1.38 | Rectangular | a = 5.52, b = 2.15 |
P8 | d20 = 2.98, d11 = 2.05, d31 = 1.55, d40 = 1.52 | Rectangular | a = 5.96, b = 2.18 |
P10 | d20 = 3.13, d11 = 2.08, d31 = 1.62, d40 = 1.57 | Rectangular | a = 6.26, b = 2.20 |
It is worth mentioning that the side-chain mesogens of P8 and P10 with longer spacers can form SmB-like structure, which is not observed in P6. This outcome suggests that longer spacers promote the ordering of biphenyl groups. We noted that the dimensions of the rectangular scaffold fabricated by the main-chain changed with the temperature. Fig. 9b illustrates the temperature dependence of the calculated value of a and b of the rectangular lattice for P10. The similar result of P8 is shown in ESI.† In Fig. 9b, it is observed that slight increases of a and b occur at 120 °C, correlating to the transition of biphenyl side chain from SmE-like to SmB-like packing; at 150 °C, the significant expansion of a infers the transition of biphenyl side chain from SmB to SmA. The main-chain scaffold of P8 and P10 is always kept until the isotropic transition, which is different from the results of P6. Therefore, P8 and P10 with lengthened spacers can fabricate more stable scaffold of the main chain compared to P6.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21257j |
This journal is © The Royal Society of Chemistry 2015 |