Organic ER Materials Polymeric semiconducting material

4 Design and Preparation of ER Materials

4.1 ER Materials Based on Molecular and Crystal Structure Design

4.1.2 Organic ER Materials Polymeric semiconducting material

Since Block reported the first polymeric semiconductive materials, polyaniline (PANI), show a strong ER effect. The polymeric semiconductive materials including polypyrroles (PPy), poly(p-phenylene) (PPP), polythiophenes, poly(naphthalene quinine radicals) (PNQR), poly(acene quinine radicals) (PANQ), poly(phenylenediamine), oxidized polyacrylonitrile and their derivatives have been developed as ER active materials [9,53,54]. Figure 17 gives the molecular structure of various available polymeric semiconductive ER materials. The generally characteristic of the polymeric semiconductive materials are electronic conductive materials with a

S

-conjugated bond structure. The interfacial polarization, induced by electronic movement in particles, is believed to be attributed to ER effect. Therefore, the adjustment of electronic concentration or conductivity is key to ER effect of polymeric semiconducting materials.

(a) (b)

(c) (d)

Figure 17 molecular structure of various available polymeric semiconductive ER materials (a) PANI, (b) PNQR, (c) PPy, and (d) PPP

Polyanilines and its derivatives such as polymethylaninline, polyethylaniline, polymethoxylaniline, and polyethoxyaniline are popularly used ER materials because they can be easily prepared by oxidization polymerization and their conductivity is easily controlled by protonation and charge transfer doping using conventional chemical and electrochemicalmethods [54]. The protonation/deprotonation equilibrium occurs for two of the oxidation states, depending on the pH value. Figure 18 shows oxidized state, reduced state, and mixed state of PANI. Due to the condcuctivity requirement of ER materials, the PANI with mixed state is most used in ER fluid.

Figure 18 shows oxidized state, reduced state, and mixed state of PANI.

The polymeric semiconductiing materials are believed to have better dispersing ability and mechanical properties compared to inorganic materials due to lower apparent density and soft and non-abrasive to ER devices. These advantages have attracted much attention to investigate ER effect of polymeric semiconductive materials in the past few years. However, some problems, such as high current density and relative weak ER effect compared with inorganic ER materials have not well overcome up to now. Various approaches have been proposed to improve the performance, for example, coating conducting polyaniline surfaces with non-conductive polymer layer, synthesizing N-substituted copolyaniline and so on [55,56].

(2) Polymer with polar groups

The second type of organic ER material is polymer containing polar group such as amine (–NH₂), hydroxyl (–OH), amino-cyano (–NHCN), and so on [57]. The high ER performance

of these materials is closely related to their molecular structure, in particular branched polar group, and it is believed that the dipole orientation polarization of polar groups is dominated to the ER effect. Besides the specially developed ER materials such as sulfonic polystyrene [58], some organic polymers that contain polysaccharide or consist of glucose units, such as starch, chitosan, and cellulose, have been adopted as ER dispersed phase. In the early studies, polysaccharide polar polymer such as starch, cellulose and chitosan often need the presence of water and impurity ions to promote ER effect. This limited its temperature stability and result in large current density. Recently, those wet-base polysaccharide materials including cellulose and starch have been converted into the dry-base materials that exhibit optimal ER performance by the modified structures. Choi et al [59] prepared the phosphate cellulose, phosphate starch and found ER effect in dry state. Figure 19 shows the chemical structure of linear reacted amylase group in potato starch. Poato tuber starch is characterized by high content of phosphate in which the phosphate groups are located as monoester at the C-6 (~70%) and the C-3 (~30%) positions of the glucose residues. This phosphate starch was found to show good ER effect when dry. Unlike polymeric semiconductive materials and sulfonic polystyrene, one of the advantages of these polysaccharide materials is no toxicity, low cost, and bio-consistent. Therefore, more attention indeed needs to be paid on polysaccharide based ER materials. Of course, the disadvantages from natural structure of polysaccharide result in thermal instability.

Figure 19 Chemical structure of potato starch: (a) 30% and (b) 70% (Reproduced fro Ref [59], Copyright American Chemical Society 2005)

Recently, Zhao et al [60-62] further proposed the cyclodextrin based ER materials due to the high thermal stability of cyclodextrin and its special structure (see Figure 20) that could be modified by supramolecular assembly for optimal dielectric properties. According to the point that the host structure and the properties can be easily modified by the formation of host–guest complex, the supramolecular complexes of

E

-cyclodextrin cross-linking polymer/1-(2-pyridlazo)-2-naphthol (

E

-CDP-PAN) particles were synthesized. Figure 21 shows the comparison of yield stress of the typical

E

-CDP,

E

-CDP-PAN ER fluids under different electric fields. It was found that the yield stress of the typical

E

-CDP-PAN ER fluid was 6.16 KPa in 5 kV/mm, which is much higher than that of pure

E

-cyclodextrin polymer (

E

-CDP), that of pure 1-(2-pyridlazo)-2-naphthol (PAN) as well as that of the mixture of the host with the guest(

E

-CDP-PAN). As expected, the improvement of dielectric and conductivity properties of b-CDP resulted in good ER effect of

E

^-CDP-PAN.

Furthermore, it was found that the cross-linking degree (CLD) of the polymer strongly influences the ER behavior of

E

-CDP-PAN and

E

-CDP. When CLD remains in the range of 4-6,

E

-CDP-PAN exhibits much stronger ER effect, and for

E

-CDP, its suitable range is 5–8. The significant preponderance of the host–guest complex formation is that the host structure can be controlled easily by adding different guests. Thus, Zhao et al further synthesized six supramolecular complexes of ȕ-cyclodextrin cross-linking polymer with salicylicacid (ȕ-CDP-1), 5-chlorosalicylic acid (ȕ-CDP-2), 3,5-dichlorosalicylic acid (ȕ-CDP- 3), 5-nitrosalicylic acid (ȕ-CDP-4), 3,5-dinitrosalicylicacid (ȕ-CDP-5), or 3-hydroxy-2- naphthoic acid (ȕ-CDP-6) particles (see structure in Figure 22). It was found that the yield stress of the typical ȕ-CDP-1 ER fluid was 5.6 kPa in 4 kV/mm, which is much higher than that of pure ȕ-cyclodextrin polymer (ȕ-CDP), that of pure salicylic acid as well as that of the mixture of the host with the guest. It is clearly indicated that the formation of supramolecular complexes between ȕ-CDP and salicylic acid can enhance the ER properties of the host. The similar results for other supramolecular complexes with different guests have also been obtained under the same DC electric fields. The yield stress of supramolecular complexes is strongly affected by the structure of guests. Among the six investigated guests, 3-hydroxy-2- naphthoic acid gave the highest ER property having a yield stress of 9.8 kPa under 4 kV/mm DC while cross-linked with ȕ-CDP to form ȕ-CDP-6. The yield stress of ȕ-CDP-6 was significantly increased by 72%in comparison with that of the pure ȕ-CDP. However, the yield stress of ȕ-CDP-1–5 slightly increased by 34–41% as compared with that of the pure ȕ-CDP.

The achieved results indicate that the ER effect of host–guest complexes can be greatly affected by the changes of the tremendous guest structure, whereas the slight guest structural transposition, such as altering different groups of a guest, can only obtain the adjacent electrorheological behavior. The dielectric properties of these host–guest complexes also proved that the ER effect can be affected by the properties of guest.

1.37nm 0.57

1.53nm 0.78

1.69nm 0.95

0.78nm

Figure 20 Chemical structure and corresponding molecular size

Figure 21 The yield stress of ȕ-CDP-PAN ȕ-CDP PAN, simple mixture of ȕ-CDP and PAN ER fluids as a function of dc electric fields (T= 25^oC, particle volume fraction = 31%)

Figure 22 Schematic structure of the preparation of ȕ-CDP and supramolecular composite ȕ-CDP-1-6