Antiferroelectric polymer

In fact, two new representatives of polar achiral systems have been discovered quite recently antiferroelectric polymer-monomer mixtures [286] and ferroelectric biaxial smectic A phases composed of ba-nana-like molecules [287]. 

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

Fig. 13.27 Achiral antiferroelectric. Voltage dependence of pyroelectric coefficient describing the double hysteresis loop (a) and dependence of the field-induced polarization on the content of a monomer in the polymer-monomer mixtures (b)...

As shown by the X-ray diffraction, polymer-monomer mixture consists of SmC bilayers. A bilayer is the principal unit cell having either non-polar C2h or polar C2v (b) S5mimetry. The former is incompatible with both ferroelectricity or antiferroelectricity, because such a structure has an inversion centre. On the contrary, in sketch (b) each bilayer is polar with Pq vector located in the tilt plane along the y-axis. In a stack of such layers the direction of Pq alternates and the stmcture (b) is antiferroelectiic in its ground state. Only strong electric field Ey causes the transition to the ferroelectric structure shown in sketch (c) as observed in experiment. Note that both the Pq and P = X Pq vectors are always lying in the tilt plane. 

Link, D.R., Clark, N.A., Ostrovskii, B.I., Soto Bustamante, E.A. Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture. Phys. Rev. E 61, R37-R39 (2000)... 

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

Table A..5-1 The 72 families of ferroelectric materials. The number assigned to each family corresponds to the number used in LB III/36. The numbers in parentheses (A sub>. f+a ) after the family name serve the purpose of conveying some information about the size and importance of the family. The numbers indicate the following A sub the number of pure substances (ferroelectric, antiferroelectric, and related substances) which are treated as members of this family in LB III/36 A f+A the number of ferroelectric and antiferroelectric substances which are treated as members of this family in LB III/36 n, the number of representative substances from this family whose properties are surveyed in Sect. 4.5.4. For some of these families, additional remarks are needed for instance, because the perovskite-type oxide family has many members and consists of several subfamilies because the liquid crystal and polymer families have very specific properties compared with crystalline ferroelectrics and because the traditional names of some families are apt to lead to misconceptions about their members. Such families are marked by letters a-m following the parentheses, and remarks on these families are given under the corresponding letter in the text in Sect. 4.5.3.1...

Soto Bustamante et al. [103,104] have reported antiferroelectricity in the smectic A phase of polymer-monomer mixtures. Mixtures of the achiral polyacrylate... 

Boemelburg, J., Heppke, G., and Hollidt, J., Evidence for an antiferroelectric smectic phase in a chiral side-chain polymer, Makromol. Chem. Rapid Commun., 12, 483-488 (1991). 

Polymer 21 is a typical ferroelectric liquid crystal poly(acrylate) with a rather simple chiral terminal chain commonly seen in chiral liquid crystals. A range of chiral liquid crystal phases are exhibited by polymer 21, complicated by the reported presence of two phases. The recent interest in the antiferroelectric phase (see above) has been... 

Polymer 22 exhibits what is thought to be an antiferroelectric S phase below a conventional ferroelectric S phase. This is perhaps to be expected because of the... 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

The antiferroelectric SmC structure (see Fig. 17) can also occur in racemates [94] or in nonchiral compounds such as symmetric dimers [136, 137], nonsymmetric dimers [133], and main chain liquid crystal polymers [138], where its formation is driven by steric and/or conformational effects. Antiferroelectric ordering has been shown to increase the smectic order parameters in ferroelectric liquid crystals [94, 95]. 

Concepts like piezoelectric, pyroelectric, ferroelectric, ferrielectric, antiferroelectric, paraelectric, electrostrictive, and several more, relate to distinct phenomena and are themselves interrelated. They are bound to appear in the description of liquid crystals and liquid crystal polymers, as they do in normal polymers and crystalline solids. Presently, great confusion is created by the uncritical use of these terms. For example, in the latest edition of the Encyclopedia Britannica [4] it is stated that pyroelectric-... 

In polymer 76 (Fig. 90), which contains a chiral oxiranc ring in the spacer, a three-state switching was observ in the Sc phase [59,118]. In SSFLC ceUs thb polymer shows a characteristic stripe texture. The electrooptical response exhibits typical antiferroelectric properties, ix., two current response peaks, an optical and electrical double... 

A proof of the antiferroelectric nature of the chiral smectic phase of a polymer was offered by Nishiyanu and Goodby (121). They performed a misdbility test between the... 

Investigadon of tbe Beld dependence of the apparent till angle in the antiferroelectric phase of polymer 79 revealed that the tilt angle gradually increases with the field before tbe transition from tbe antiferroelectric state to the electrically induced ferroelectric stale occurs (Fig. 97). The hysteresis behavior observed for low-molar-mass antiferoelectric LCs was not observed for tbe polymer. 

PMMA/PZT Composites (Polar-Polymer/Antiferroelectric Ceramic Systems)... 

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