Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 2. Polycondensation of mesogenic diols and diphosgene

A series of new side-chain liquid crystalline polymers was prepared with a polycarbonate backbone, bearing (E)-4′-nitrostilbene mesogenic groups, connected to the backbone by spacers of different length. The polymers were synthesized in 1,4-dioxane by polycondensation of diphosgene and diol monomers with the general structure 2-[ω-(4′-nitrostilben-4-ylcxy)alkyloxy]-1,3-propanediol in the presence of pyridine as a proton trap and catalyst. The mesomorphic properties of all monomers, polymers and intermediates were studied by polarized optical microscopy and differential scanning calorimetry (DSC). The polymers were also studied by X-ray diffraction of non-oriented and oriented samples (fibres). All monomers were liquid crystalline with nematic and/or smectic phases. For the polymers a clear relation was found between spacer length and glass transition temperature (T_g), which decreases with increasing spacer length and between spacer length and clearing temperature, which increases with increasing spacer length. X-ray diffraction measurements indicated the presence of a smectic A double layer structure (S_Ad) for all polymers at room temperature, with an antiparallel of overlapping mesogenic groups. A distinct difference in the orientation of the mesogens was found when fibres were drawn from the smectic phase or from the nematic or isotropic phase. In the first case the mesogenic groups were oriented perpendicular to the fibre axis and in the second case the mesogenic groups were oriented parallel to the fibre axis. By gel-permeation chromatography (GPC) measurements of heated samples and by thermogravimetric analysis (TGA) and DSC the polymers were found to be thermally stable up to temperatures well above their clearing point.     

Synthesis of novel sulfonyl-containing liquid-crystalline side-chain poly(vinyl ethers)

The synthesis of some new liquid-crystalline polymers with sulfonyl-containing mesogenic groups is described. 4-[(S)-(-)-2-MethylbutyIsulfonyl]-4′-[(11-vinyloxy)undecyloxy]biphenyl ( 10 -11), 4-[(S)-(-)-2-methylbutylsulfonyl]-4′-[8-(vinyloxy)octyloxy]biphenyl ( 10 -8), 4-[(S)-(-)-2- methylbutylsulfonyl]biphenyl 4-[11-(vinyloxy)undecyloxy]benzoate ( 12 -11), 4-[(S)-(-)-2-methylbutylsulfonyl]biphenyl 4-[8-(vinyloxy)octyloxy]benzoate ( 12 -8), 4-[(S)-(-)-2-methylbutyloxy]-4′- [11-(vinyloxy)undecylsulfonyl]biphenyl ( 18 -11) and 2-[11-(vinyloxy)undecyloxy]-6-{4-[(S)-(-)-2- methylbutylsulfonyl]phenyl}naphthalene ( 23 -11) were all synthesized, and polymerized with the initiating system CF₃SO₃H/S(CH₃)₂ in CH₃Cl₃ at 0°C. Monomers 10 -11, 10 -8, 18 -11 and 23 -11 are crystalline, while both 12 -11 and 12 -8 show an enantiotropic smectic A phase. All polymers exhibit the same thermotropic behaviour as their corresponding monomers, except poly( 23 -11) which exhibits an enantiotropic smectic A (s_A) and a monotropic chiral smectic C phase (S*_C).     

Synthesis and characterization of a homologous series of chiral smectic C side-chain polymers

The synthesis and characterization of a homologous series of liquid-crystalline side-chain polymers with a chiral smectic C phase are described. The polymer backbone consists of methacrylate monomeric units which link the mesogenic moieties of the chiral cinnamic acid 2S- (?)-4-(2-chloro-4-methylpentanoyloxy)phenyl ester via a flexible spacer of six to eleven CH₂ groups. The influence of this variation on this variation on transition temperatures, phase structure and range of the l. c. phases is studied. In contrast to the monomers, the chiral smectic C phase of the polymers is significantly broadened with respect to the temperature regime and can be vitrified in the glassy state.     

Synthesis of ferroelectric liquid-crystalline polymethacrylates containing 1,2-diphenylethane based mesogens

The synthesis of side-chain liquid-crystalline polymethacrylates containing (2S)-2-chloro-4-methylvaleric acid end groups and 1,2-diphenylethane based mesogenic units is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction were used to determine the thermal transitions and to analyze the anisotropic phases. Among the methacrylate monomers prepared in this study, those without lateral chloro substituent show only a smetic A phase while those with a lateral chloro substituent display a cholesteric phase. All polymers exhibit smectic mesomorphism and do not undergo side-chain crystallization. All four polymethacrylates without lateral chloro substituents reveal enantiotropic smectic A and smectic B phases while the other four polymethacrylates with lateral chloro substituents reveal enantiotropic smectic A and chiral smectic C phases. The results seem to demonstrate that incorporating a lateral chloro substituent into the mesogenic core enhances the formation of a tilted chiral smectic C phase.     

The effects of variation of a core ring system in the mesogenic side chain of liquid-crystal polysiloxanes

Fourteen novel side-chain polysiloxanes 2 and their alkene precursors 1 containing methylene-cyclohexane, cyclohexane, benzene, bicyclooctane, 3 -oxobicyclooctane, bicyclohexane and biphenyl moieties in the mesogenic core, and with (CH₂)_n spacer lengths of n = 3, 6 and 11 are reported. The order of efficiency of these cyclic moieties in promoting liquid-crystal properties in the polymers is the same as that observed for the precursor alkenes, indicating that similar qualitative structural rules govern mesophase type and thermal stability in both monomeric and macromolecular systems. The apparent occurrence of smectic C properties for two of the polymers is particularly interesting.     

Thermally stimulated current and DSC studies of the dual glass transitions in side-chain liquid crystalline copolysiloxanes containing 4-[(S)-2-methylbutoxy]phenyl 3-chloro-4-alkenyloxybenzoate side groups

The synthesis of side-chain liquid crystalline copolysiloxanes containing ω-[4-[4-[(S)-2-methylbutoxy]phenoxycarbonyl]-2-chlorophenoxy]alkyl side groups is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffractometry reveal smectic mesomorphism for most of the obtained polymers. The copolysiloxane with three methylene units in the spacer is the only one showing no mesomorphic property. The other four copolysiloxanes, containing four, five, six or eleven methylene units in the spacer, display a smectic A phase. All of the obtained polymers present dual glass transition behavior by both DSC and thermally stimulated current (TSC) techniques. The first glass transition (T_g1) at lower temperature is due to the segmental motions of the polysiloxane backbone, while the second glass transition (T_g2) is due to the cooperative relaxation motions of spacers and mesogenic units.     

Transitional properties of liquid-crystalline side-chain polymers derived from poly(p-hydroxystyrene)

A series of liquid-crystalline polystyrene derivatives with ω-(4-(4-fluorophenylazo)phenoxy)-alkoxy pendant groups have been synthesized and characterized. Homologues with four or more methylene units in the spacer chain exhibit smectic liquid-crystalline phases, while the homologue with three methylene units forms an amorphous glass. The smectic A-isotropic (S_A-I) transition temperatures show a marked odd-even effect as a function of the parity of the spacer chain. This odd-even behaviour is also found in the variation of ΔS/R at the S_A-I transition across the series. A marked hysteresis between the transition temperatures determined on heating and the transition temperatures determined on cooling is observed. A molecular interpretation of these phenomena is proposed.     

Phase transitions of hydroxy-telechelic side-chain liquid crystalline polyethers and polyurethane networks derived thereof

Hydroxy-telechelic polymers with side-chain mesogenic groups have been prepared from epoxy monomers using a BF₃/1,6-hexanediol initiating system. Homopolymers showed either nematic or smectic A and C phases, depending on the length of the alkoxy substituent on the aromatic mesogenic group. In copolymers both nematic and smectic phases could be observed, depending on composition. Liquid crystallinity was found to be retained after crosslinking with a triisocyanate at temperatures below the clearing temperature of the mixture. On the other hand, the mesophase was found to disappear on rapid cooling after a certain degree of crosslinking above the clearing temperature, only to reappear after a period of heating above the glass transition temperature.     

Fluorescence study on intermolecular complex formation between mesogenic 4,4′-biphenyldicarboxylate moieties of thermotropic liquid-crystalline polyesters with 5 and 6 methylene units

The change in intermolecular complex formation between mesogenic 4,4′-biphenyldicarboxylate moieties of thermotropic liquid-crystalline (LC) polyesters containing 5 and 6 methylene units (BB-n, n = 5, 6) was studied by fluorescence technique. BB-5 shows a crystalline, an S_CA, and an isotropic phase during heating. BB-6 shows a crystalline mixture of β- and γ-phase, a pure γ-phase, an S_A, and an isotropic phase during heating. The fluorescence of BB-n is confirmed to be due to the formation of an intermolecular ground-state complex between two mesogenic 4,4′-biphenyldicarboxylate moieties, and shows a wavelength shift of the fluorescence peak to lower wavelengths upon heating. This is because the increase in thermal fluctuation with increasing temperature induces a decrease in interaction between two 4,4′-biphenyldicarboxylate moieties. Because of the constancy of the fluorescence peak wavelength in the LC phase of BB-n, LC phases for S_CA and S_A type molecular arrangements of the LC polyester with mesogenic biphenyl moieties were proved to show fluorescence around 376 and 360 nm, respectively. However, the shift of the fluorescence peak wavelength to lower wavelengths upon heating has been observed for the first time in the present study. The change in molecular interaction between the mesogenic moieties of the LCPs during heating is shown to result in various fluorescence patterns.     

Liquid crystalline polyacrylates containing biphenyl and chiral terminal groups in the side chain, 2. Influence of the nature of the chiral group

Acrylate monomers with a chiral group have been synthesized bearing a propyl, isopropyl or benzyl substituent on the asymmetric carbon. All of the monomers are crystalline. Above their melting point, they give a clear, isotropic melt. The corresponding polymers have been prepared by free radical polymerization. They were examined by differential scanning calorimetry, optical microscopy and X-ray diffraction. They were found to show a reproducible liquid-crystal behavior. On cooling from the isotropic state, S_A, S (or S) and crystal-like (probably S_H or S_H′) phases become evident. Our results do not preclude the existence of a S phase between the S_A and S phases. It is noteworthy that polyacrylates of the same series but with side chains terminated by a chiral group bearing a methyl substituent exhibit S_A, S_B and crystal-like (probably S_E) phases.     

Layer Morphology of a Poly(ester imide) LCP in Different Solid States

A poly(ester imide) (PEI) in the smectic crystalline (S_E) state exhibits both a medium‐angle X‐ray scattering (MAXS) peak and discrete small‐angle X‐ray scattering (SAXS). Based on quantitative analysis of absolute scattering data the observed SAXS is attributed to the formation of a sandwich structure in which a central, well‐ordered layer‐shaped zone is enclosed in disordered buffer zones. Additionally the MAXS of material in the frozen liquid‐crystalline S_A and S_B states is analysed. The results show that the S_A state is characterised by long ranging correlation in the direction of the chain, whereas the lateral correlation range is short. Thus the smectic layer is considerably warped. The lateral range of order is wider in the S_B state and approaches infinity in the S_E state. This increase of lateral order and layer flatness goes along with considerable loss of longitudinal order. Nevertheless the range of longitudinal order remains high enough to assure correlation among the mesogens from the ordered zone with those from the disordered zones. Thus chain‐folding cannot be the primary reason for the formation of ordered and disordered zones. Results indicate that asymmetry, orientation and internal twist of mesogenic groups in a liquid‐crystalline main‐chain polymer are important parameters controlling structure formation of the smectic morphology.     

Phase structures,transition behavior and surface alignment in polymers containing rigid rod-like backbones with flexible side chains, 3. The even-odd effect on phase transitions in combined liquid crystal polyesters

A series of "combined" liquid crystal (LC) polyesters consisting of aromatic main-chain backbones and flexible aliphatic side chains with 4-cyanobiphenyl end groups was synthesized based on the polycondensation of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldicarbonyl chloride with 2,2′-bis{ω-[4-(4-cyanophenyl)phenoxy]-n-alkoxycarbonyl]}-4,4′-biphenyldiol (PEFBP). In a recent study of PEFBP(n = 11) containing eleven methylene units in the side chains, four different phases were identified in addition to the isotropic melt (I). They are as follows: an orthorhombic crystalline (K_O) phase, two triclinic crystalline (K_T1 and K_T2) phases in the high temperature region, and a nematic (N) phase. In this report, we focus on the even-odd effect of the methylene units in the side chains on the phase structure and transition behavior of PEFBP(n = 8, 9, 10 and 11)s. Two even-numbered PEFBPs with either eight or ten methylene units in the side chains [PEFBP(n = 10 and 8)s] were studied. An odd-numbered PEFBP containing nine methylene units, PEFBP(n = 9), was also investigated in addition to PEFBP(n = 11) which was reported on previously. It was found that both of the PEFBPs with even-numbered methylene units in their side chains [PEFBP(n = 10 and 8)s] exhibit only a nematic and a smectic A (S_A) phase. Their phase transition sequence during heating and cooling is T_g (S_A LC glass) ↔ S_A ↔ N ↔ I Since only LC phases exist, this is a thermodynamically reversible transition sequence independent upon thermal history. For PEFBP(n = 9 and 11)s, crystalline phases with three-dimensional order are observed above the NLC glass transition temperature in addition to the N phase. In PEFBP(n = 9), the N and K_O phases are similar to those seen in the case of PEFBP(n = 11). However, this polymer presents only one triclinic crystalline phase, while two triclinic crystal phases are observed in PEFBP(n = 11). The phase transition sequence in these odd-numbered PEFBPs during heating between 2.5°C/min –10°C/min for a sample cooled from the isotropic melt at the same rate is T_g (N LC glass) → K_O → N → K_T(s) → N → I Nevertheless, this sequence does not represent a case of thermodynamic equilibrium. The K_O and K_T(s) phases in PEFBP(n = 9 and 11)s and the S_A phase in PEBFP(n = 8 and 10)s are characteristics of the even-odd effect observed in this series of combined LC polyesters. The number of methylene units in the side chains thus plays a critical role in the formation of phase order and transition behavior in a way quite different from the common even-odd effect seen in main-chain LC polymers. Furthermore, this effect also indicates that the 4-cyanobiphenyl groups in the side chains are incorporated with the main-chain backbones to form the liquid crystal and crystal phases.     

LC Ionomers with Phosphonate Groups and Their Multilayer Build‐Up

This paper describes the synthesis of new liquid‐crystalline (LC) ionomers (semiflexible LC main‐chain polymers with ionic groups as side groups) by polycondensation of substituted malonates with mesogenic diols. Phosphonic acids were used as negative groups, and phosphonium and ammonium ions as positive groups. The presence of these ionic groups leads to gelation (physical crosslinking) of the polymers, which display a rubbery consistency. As expected from the reference polymers without an ionic group, all polymers show smectic phases. These phases are destabilized by bulky side groups (e. g., phosphonium ions or phosphonic acid esters). They are, however, stabilized by phosphonic acid groups and the corresponding salts. Together with X‐ray measurements this supports the assumption that different smectic sublayers are formed for the mesogens and the ionic groups. Dielectric relaxation measurements show four different relaxation processes for these polymers. They are, with rising temperature, the χ‐ and the β‐relaxation (local relaxations, active below T_g) and an α‐ and a δ‐relaxation above T_g. The α‐relaxation corresponds to the dynamic glass process and the δ‐relaxation is due to a reorientation of the mesogenic groups around their short axis. Despite the fact that these LC ionomers have only one ionic group per repeating unit (one group per 3.3 nm), they could be successfully used for a multilayer built‐up by electrostatic deposition. The resulting multilayers are rather hydrophobic, if the LC ionomer is the outermost layer.     

Synthesis and thermotropic properties of novel side‐chain dimer liquid crystalline polymers

A new class of side‐chain liquid crystalline polymers with polymethacrylate or polyether backbones and two different mesogenic moieties in one side‐chain, i.e. a biphenyl group and a cholesterol group, have been synthesized. The thermotropic behavior of both, the monomeric precursors and the polymers, has been investigated. The monomers exhibit smectic A and chiral nematic phases and the polymers exhibit smectic A mesophases. For both, monomers and polymers, the isotropization temperatures and the corresponding dimensionless entropy changes ΔS/R depend on the parity of the spacer between the two mesogens. Replacement of the biphenyl group in the polymethacrylates by a phenyl group lowers the isotropization temperatures, but the smectic A phase is retained.     

Liquid crystalline polyfumarates, 2 Polymerization of fumarates with two mesogenic groups per unit

The paper describes synthesis and characterization of some new side-chain liquid crystalline polymers (SCLCP) obtained by homo- and copolymerization of fumarates with two mesogenic groups per unit. The homopolymerization of spacerless bis[4-(4-butoxy-benzoyloxy)phenyl] fumarate ( 1 ) did not take place due to steric hindrance of the propagation reaction by the bulky phenyl benzoate groups. The radically induced polymerization of the fumarates with spacer between fumaric unit and the mesogenic groups was performed in bulk. At the polymerization temperature the monomers were isotropic. During polymerization the mixtures became turbid due to phase separation of the isotropic monomer and the resulting polymerization products which formed a liquid crystalline (lc) phase. The molecular weights of the obtained homopolymers were relatively low and depended on spacer length. Transfer reactions to the monomer occurred to a considerable extent. The homopolymers showed an enantiotropic S_A phase. This was confirmed by X-ray measurements. A model for the arrangement of the polymer backbones in the smectic phase is given. Copolymerization of bis{6-[4-(4-butoxybenzoyloxy)benzoyloxy]hexyl} fumarate (4) with styrene yielded copolymers 6 with different percentages of styrene. All copolymers formed an enantiotropic S_A phase. With increasing percentage of styrene the transition to the isotropic state was shifted toward lower temperatures.     

Side-chain liquid-crystalline polymers containing a siloxane spacer, 1. Synthesis of side-chain liquid-crystalline polymers containing siloxane bond in the spacer unit

Side-chain liquid-crystalline polymers containing siloxane bond in the spacer units were prepared. For the synthesis of the desired monomers, novel silanol compounds carrying a mesogenic group were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, p- or p′-cyano-substituted biphenyl group, phenyl and biphenylyl benzoates were chosen, and the structural effects on the thermal properties were investigated. Introduction of a disiloxane group in the side chain resulted in a decrease of T_g which was more marked for longer siloxane units. Furthermore, only when a terphenylene-type mesogen was introduced, a stable smectic mesophase was observed with wide range of phase stability.     

Side-Chain liquid-crystalline polymers containing a siloxane spacer, 2. Effects of mesogenic groups on the thermal properties of side-chain liquid-crystalline polymers containing a disiloxane unit in the spacer

A series of side-chain Iiquid-crystalline polymers was prepared containing disiloxane units in the spacer. The polymers consist of a polymethacrylate backbone and several kinds of mesogenic side groups. For the synthesis of the desired monomers, silanol compounds carrying a mesogenic groups were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, linear-type p- or p′-substituted phenyl and biphenylyl benzoates, p- or m-substituted benzoyloxybiphenyl groups and laterally attached biphenylyl benzoate-type mesogens were chosen, and the structural effects on the thermal properties of the polymers were investigated. In the case of linear-type mesogens, only when triple-core mesogens were introduced a stable smectic mesophase was observed with a wide range of phase stability, while on mesophase was observed for the linear double-core and the laterally attached mesogens. On the other hand, a highly ordered smectic phase was observed for polymers having linear-type triple-core mesogens with p-alkoxy and m-cyano end groups. Furthermore, it was revealed that the introduction of a disiloxane unit in the spacer lowered the temperature range of the mesophase, in comparison with polymers having an alkylene spacer with the same backbone component and mesogens.     

Smectic behaviour of linear random copolymers containing the stilbene-4,4′-dicarboxy group

Copolymeric systems of the general formula: P₁ = {((? OCH₂CH₂)_a? O? R? )_m ((? OCH₂CH₂)_b? O? R? )_n}_x or P₂ = {((? OCH₂CH₂)_a? O? R? )_m (? O(CH₂)_b? O? R? )_n}_x with R = ? OC? C₆H₄? CH?HC? C₆H₄? CO? were prepared and their mesophasic properties studied by polarizing microscopy, differential scanning calorimetry and X-ray diffraction analysis. Enantiotropic smectic mesomorphism was directly observed in most cases. For all systems but P₂(a = 2, b = 12) a single smectic phase was observed for all compositions, with a continuous variation of the smectic structural periodicity, isotropization temperatures and molar entropic changes close to the weighted average. This indicates a complete compatibility of the different monomer units in the formation of the smectic layers. On the contrary, system P₂ (a = 2, b = 12) exhibits two smectic phases each within a definite range of composition. The two phases are sharply distinct as far as the structural periodicity is concerned. Both phases are present in the mesophasic state of copolymer with m = 75.     

Synthesis and characterization of novel crosslinkable side-chain liquid crystalline polyesters

Crosslinkable side-chain liquid crystalline polyesters PCn from N-[n-(4-(4-nitrophenylazo)phenyloxy)alkyl]diethanolamine (Cn, n = 3, 5, 6, 10) as mesogenic monomers and maleic anhydride were synthesized and characterized. The thermal properties of PCn's were studied by means of DSC, polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD), and the results showed that all the polymers studied exhibit enantiotropic liquid crystallinity. In the molar mass independent region, the relatively high content of cis —CH=CH— groups in the polymer backbone of PC3 causes an increase of the melting temperature (T_m) and a decrease of T_g and isotropisation temperature (T_i). The crosslinking of PCn in the radical polymerization with styrene was confirmed by FTIR spectroscopy. The absorption band at 1 300 cm^–1 attributed to the in-plane C—H-bending vibration of trans —CH=CH— in the polymer backbone disappeared after crosslinking, indicating that the trans —CH=CH— functions are consumed in the crosslinking polymerization of styrene.     

Preparation of ordered and crosslinked films from liquid crystalline p-vinylphenoxy-based monomers

The synthesis of the following monofunctional and bifunctional liquid crystalline p-vinylphenoxy-based monomers is described: 4-methoxyphenyl 4-[11-(4-vinylphenoxy)-undecyloxy]benzoate ( 1 ), 4-cyanophenyl 4-[11-(4-vinylphenoxy)undecyloxy]benzoate ( 2 ) and 3-(4-vinylphenoxy)propyl 4-{4-[11-(4-vinylphenoxy)undecyloxy]benzoyloxy}benzoate ( 3 ). Both free radical and cationic polymerization of the monofunctional monomers 1 and 2 yielded side-chain liquid crystalline polymers exhibiting smectic A mesomorphism. The polymers exhibited high molar masses (M _n = 40000 – 100000 g/mol) and relatively narrow molar mass distributions (M _w/M _n between 1.5 and 3). Ordered thin films were prepared by in-situ photopolymerization of monomers 1 , 2 and 3 oriented in their nematic mesophases. Thin films of a thermally stabilized ordered side-chain liquid crystalline polymer were prepared by copolymerization of the monofunctional monomer 1 and the bifunctional monomer 3 , the latter present in low concentration. The films regained orientation when cooled down from temperatures above the isotropization point (137°C) as evidenced by polarized FT-Raman measurements.