Influence of the gelator backbone architecture

If the block copolymer architecture is changed from a linear ABA structure to a three arm star architecture, this change is expected to result in higher number of physical crosslinking points per gelator molecule due to the introduction of a chemical crosslinking connecting the arms (see chapter 5.6). This increase in the number of network point should increase the network density, thus creating a more elastic gel structure. Additionally, by the introduction of the chemical crosslink the probability for the formation of back loops might be reduced.

In Figure 5.33 the molecules are schematically shown. The linear ABA’ triblock copolymer gelator 19d is compared to the star-shaped block copolymer gelator 25a. Each arm in the star consists of a ABA’ triblock copolymer 19d. 25a is comprised of a mixture of three arm stars, dimers and unimers.

Figure 5.33: Schematic representation and chemical structure of the linear ABA triblock copolymer gelator 19d (left) and the mixture containing the three arm star-shaped block copolymer 25a (right). Schematical representation is shown for y = 3.

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Samples containing 5CB with 5 wt% of 19d or 25a were analyzed in the same way as discussed above. In the frequency-dependent measurement, shown in Figure 5.34, both samples show a temperature dependency. The frequency dependency is much more pronounced for the sample with 19d, although the sample with 25a exhibits a crossing in G’(ω) and G’(ω) at ω ≥ 31 rad/s and, thus, shows liquid-like behavior above that copolymer 19d (left) and the gelator mixture containing the star-shaped block copolymer 25a (right). Measurements were performed at different temperatures.

The temperature dependence of the linear dynamic viscoelastic behavior for both sample is given in Figure 5.35. The behavior of the sample containing 19d was already described above in detail. Apparently, no gel is formed that corresponds to the gel definition in chapter 5.1. For the sample containing the star-shaped block copolymer gelator 25a a gel like behavior is observed. In the gel state a high storage modulus (G’ = 111 Pa) is determined that sharply decreases around the crossover point of G’ and G’’. This temperature at which the gel network disassembles is at 31.4 °C and thus significantly shifted to lower temperature compared to the clearing transition. The second heating DSC trace exhibits a broadened clearing transition with two peak maxima of equal heights at 35.2 ° and 35.6 °C.

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25 30 35 40 45

10^-2 10^-1 10⁰ 10¹ 10²

25 30 35 40 45

G', G'' /Pa

T/°C G' G''

5CB + 5wt% 19d 5CB + 5wt% 25a

heatflow

T/°C

heatflow (endo up)

Figure 5.35: Temperature dependence of linear dynamic viscoelastic behavior of 5CB gel samples with 5 wt% of linear ABA triblock copolymer gelator 19d (left) and the star-shaped block copolymer gelator 25a (right) on heating with a heating rate of 1 K/min. Overlaid are respective the second heating DSC traces at a heating rate of 1 K/min (arrows indicates direction of sweep).

The change of the block copolymer architecture from linear to a mixture of three arm star-shaped block copolymers, dimers of ABA triblock copolymers and unimers results in a significant increase in the quality of network formation. At the same gelator mass concentration where the linear triblock copolymer gelator 19d failed to produce a gellating network the corresponding star-shaped block copolymer gelator 25a yielded a sample filling network. The storage modulus for the resulting gel was the highest value at 25 °C determined in the series of gelators that were investigated. The high temperature dependency as well as the shift in gelation temperature is attributed to the short A-blocks, although the increase in crosslinking points, resulting from the partial star architecture, leads to a network formation. For gelator based on star-shaped block copolymers with longer A-blocks a significantly improved performance might be expected.

189 5.8 Electrooptical investigation of LC gels ¹

In cooperation with Dr. Maxim Khazimullin and Prof. Ingo Rehberg of Experimental Physics V electro-optical methods have been employed to characterize the viscoelastic properties of binary mixtures of the block copolymer gelators as well as the cyanobiphenyl-containing homopolymers in 5CB. All measurements and data analysis in this chapter have been performed by Dr. Maxim Khazimullin. For the different experiments a wide range of samples containing different concentration of homopolymer or gelators have been prepared by myself as well as Dr. Klaus Kreger using the sample preparation method described in chapter 5.7.

In this chapter exemplary investigations are presented to illustrate the usage of the discussed materials for fundamental studies. Results obtained for the gelator 19c and the respective cyanobiphenyl-containing homopolymer IX were selected that were also submitted as a publication.^[230]

The transition from the diluted regime to the gel state of solutions of triblock copolymer 19c in 5CB by varying the polymer concentration was investigated. To this end, the viscoelastic properties of a binary mixture of the gelator 19c in 5CB and the homopolymer IX in 5CB were characterized over a mass concentration (c) range from 0.2 % up to 2.5 %. . Infiltration of the mixtures with IX in 5Cb and 19c in 5CB into liquid crystal cells coated with rubbed polyamide produced well aligned samples that behaved like usual nematics up to mass concentrations of c = 2.5 % as shown in Figure 5.36

1 Parts of this chapter were already published: M. Khazimullin, T. Müller, S. Messlinger, I. Rehberg, W.

Schöpf, A. Krekhov, R. Pettau, K. Kreger, H.-W. Schmidt, „Gel formation in a mixture of a block copolymer and a nematic liquid crystal “, Phys. Rev. E 2011, 84, 2171.

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Figure 5.36: Microscopic images taken between crossed polarizers of cells filled with samples of different concentrations c of the block copolymer 20c. a) c = 1.1 %; b) c = 3.0 %; c) c = 5.0 %. Note the different scales.

For the characterization of the viscoelstic properties the dynamic Fréedericksz transition technique was used.^[231] This electro-optical technique is based on the optical detection of the reorientation of the liquid crystal in an electric field thus, the dependence of the splay elastic constant and the rotational viscosity on the polymer concentration could be obtained. The anisotropy of the dielectric permittivity, as well as the splay elastic constant, of the mixtures did not exhibit any pronounced dependence on concentration within the accuracy of the measurements. In contrast, the dynamic properties, namely the rotational viscosity ( ), display a more pronounced influence of the polymer concentration.

The self-assembly of the A-blocks of the gelator was investigated by comparing the behavior of samples containing homopolymer IX to samples of the same concentration of gelator 19c. The increase of rotational viscosity of the samples depending of the mass concentration of homopolymer IX and triblock copolymer 19c is shown in Figure 5.37.

For small concentrations c < 1 %, the rotational viscosities of both mixtures increase with c and show similar values. With increasing concentrations c > 1 %, the rotational viscosity of the mixture with 20c sharply increases and tends to diverge while the rotational viscosityfor the mixtures with IX exhibits an almost linear dependence on c.

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Figure 5.37: Increase of the rotational viscosity for the homopolymer (δγ^h1 ) and for the block copolymer (δγ^c1) solutions. The solid line is a linear fit with δγ^h1 = 0.156 c, while the dashed line is a guide to the eye.

By the comparison of the homopolymer to the ABA triblock copolymer the influence of the A-blocks can be isolated as shown in Figure 5.38. The increase of the ratio / with concentration yields a measure for the effective size of the attached block copolymer chains in units of a single chain size. These measurements of the dynamic behavior reveal that above a mass concentration of 1 % self-assembling of the block copolymer chain segments in clusters occurred resulting in a gel state at higher concentrations. The effective cluster size was estimated as a function of the gelator concentration.

Until the critical gelator concentration / should obey a scaling law. The experimental values were fitted as shown in Figure 5.38. Using this fitting procedure the critical gelator concentration could be calculated to a mass concentration of 2.7 %. As discussed in chapter 5.7.2, the critical concentration determined by rheological measurements resulted in a mass concentration of c ≈ 3 %.The value obtained from the rotational viscosity data is very close to the value extrapolated from rheological measurements. Thus, the dynamic Freedericksz transition technique can serve as an alternative approach for the determination of the critical concentration and has apparent advantages in comparison with rheological and dynamic light scattering measurements.

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Figure 5.38: Dependence of (δγ^c1 /δγ^h1)^1/2 on the block copolymer concentration c. The solid line is obtained by a fitting procedure for the data above c = 1 %.

Achievements

In this chapter functional ABA triblock copolymers were synthesized and characterized that were designed as block copolymer gelators for the low molecular weight liquid crystal 5CB. Two different synthetic routes were employed for the anionic polymerization of the ABA and ABA’ block copolymer backbone with varying block lengths and compositions. Cyanobiphenyl-containing homopolymers and block copolymers were prepared in a polymer analogous reaction and structure-property relations regarding the required solubility of the functionalized B-block in the liquid crystal 5CB were established using the respective functionalized homopolymers. Combination of oscillating rheology measurements and DSC measurements were performed to study gels containing 5 wt% of the gelators. The influence of the gelator backbone on the gel properties was investigated. In cooperation with the Department of Experimental Physics V the gel formation was investigated by electro-optical techniques on selected examples.