Linear and hyperbranched liquid crystalline polysiloxanes

A series of liquid crystalline (LC) polymers having biphenyl (-C₆H₄C₆H₄R; R=H, OC₁₁H₂₃, OC(O)CH(Cl)CH₂CH₂COOCH₂CH₃) 4-methoxyphenyl benzoate (-C₆H₄C(O)OC₆H₄OCH₃) and cholesteryl type mesogenic moieties were synthesized. They were made from respective -Si(CH₃)₂H terminated mesogens and vinyl functionalized linear and star shape branched polysiloxanes of comb-like and dendritic topologies and were analyzed using WAXS and SAXS techniques. Contrary to the well defined typical dendrimers, in which mesogenic groups are present in an outer sphere, the dendritic systems described here contain such groups also inside the dendritic core. It was found that star shape and dendritic LC structures exhibited various calamitic mesophases (SmA, N* and SmC*) depending on the type of mesogenic groups. On the contrary, the comb-like structures give rise to formation of hexagonal phase, even though they contain typical rod-like mesogenic moieties. For the series of 4-methoxyphenyl benzoate substituted polymers the thorough studies of the relationship between their liquid crystalline properties and topology of siloxane skeleton was determined. Mechanical properties of the LC materials were also studied.     

Synthesis and properties of liquid crystalline polysiloxanes

Most studies on the structure-property relationship of liquid crystalline polysiloxanes are focused on the side-chain-type polymers bearing calamitic or discotic side groups decoupled from the backbone via a flexible spacer. However, main-chain liquid crystalline polymers have been obtained by combining flexible siloxane segments with rigid rod-type mesogens. The synthesis and properties of these two types of liquid crystalline polysiloxanes are examined. One of the most widely used methods of preparation is the hydrosilylation of unsaturated mesogens. However, this reaction is not always as clean and clear-cut as would be necessary for the obtention of polymers with reproducible characteristics. This point is discussed and specific examples of side reactions are given. Some data concerning liquid crystalline polysiloxanes with no mesogenic groups as well as liquid crystalline elastomers are presented. Potential applications of these polymers and future developments are discussed.This review is from the Second International Topical Workshop, Advances in Silicon-Based Polymer Science.     

Synthesis and characterization of side-chain liquid crystalline polysiloxanes

The synthesis and characterization are described for a series of side-chain liquid crystal polysiloxanes using polyhydrosilylation reaction between a poly(hydrogen methyl-co-dimethylsiloxanes),-(OSiHMe)_x,-(OSiMe2)_y-, where x/y was 13/87, 30/70, 55/45, 73/27 and 98/2, and [4-(allyloxy)benzoyll biphenyl mesogenic group. The side-chain liquid crystal polysiloxanes were characterized by¹H NMR,¹³C NMR, IR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and optical polarizing microscopy. The dimethylsiloxane segment factors governing thermal transition temperatures and activation energy (E_a) of the nematic-to-isotropic phase transition are discussed.     

Synthesis,structure, and phase behavior of new chiral liquid‐crystalline polysiloxanes based on mesogenic menthyl monomers

Six new chiral monomers ( M ₁ – M ₆ ), and their corresponding side chain polymers ( P ₁ – P ₆ ) containing menthyl groups were synthesized. The chemical structures of M ₁ – M ₆ were characterized with FTIR and ¹H NMR. The structure–property relationships of the monomers and polymers obtained are discussed. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction measurements. Some compounds containing menthyl groups formed mesophase when a flexible spacer was inserted between the mesogenic core and the menthyl groups by reducing the steric hindered effect. The monomers M ₁ , M ₄ , and M ₅ did not reveal mesomorphic properties because of the weaker rigid core or the longer flexible terminal groups, whereas M ₂ , M ₃ , and M ₆ all revealed cholesteric phase. Except P ₄ and P ₅ , the homopolymers P ₁ – P ₃ and P ₆ showed a smectic A phase. In addition, melting, glass transition temperature, or clearing temperature increased, and the mesophase temperature range widened with increasing the rigidity of mesogenic core or decreasing the length of the flexible spacer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and characterization of chiral smectic side‐chain liquid crystalline polysiloxanes and ionomers containing sulfonic acid groups

The synthesis of new chiral smectic A (S_A) side‐chain liquid crystalline polysiloxanes (LCPs) and ionomers (LCIs) containing 4‐allyloxy‐benzoyl‐4‐(S‐2‐ethylhexanoyl) p‐benzenediol bisate (ABB) as mesogenic units and 4‐[[4‐(2‐propenyloxy)phenyl]azo]benzenesulfonic acid (AABS) as nonmesogenic units is presented. The chemical structures of the monomers and polymers are confirmed by FTIR spectroscopy or ¹H–NMR. Differential scanning calorimetry (DSC), optical polarizing microscopy, and X‐ray diffraction measurements reveal that all the polymers P_I–P_IV and ionomers P_V–P_VI exhibit S_A texture. The results seem to demonstrate that the tendency toward the S_A‐phase region increases with increasing sulfonic acid concentration, and the thermal stability of the S_A phase is determined by the flexibility of the polymer backbones and the interactions of sulfonic acid groups. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2335–2340, 2001     

Synthesis and phase behavior of chiral side‐chain liquid‐crystalline polysiloxanes containing two mesogenic groups

The synthesis of chiral side‐chain liquid‐crystalline polysiloxanes containing both cholesteryl undecylenate (M_I) and 4‐allyloxy‐benzoyl‐4‐(S‐2‐ethylhexanoyl) p‐benzenediol bisate (M_II) mesogenic side groups was examined. The chemical structures of the obtained monomers and polymers were confirmed with Fourier transform infrared spectroscopy or ¹H‐NMR techniques. The mesomorphic properties and phase behavior of the synthesized monomers and polymers were investigated with polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis (TGA). Copolymers IIP–IVP revealed a smectic‐A phase, and VP and VIP revealed a smectic‐A phase and a cholesteric phase. The experimental results demonstrated that the glass‐transition temperature, the clearing‐point temperature, and the mesomorphic temperature range of IIP–VIP increased with an increase in the concentration of mesogenic M_I units. TGA showed that the temperatures at which 5% mass losses occurred were greater than 300°C for all the polymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2670–2676, 2002     

Effect of the different rigidity of the chiral crosslinker on phase behaviors of side‐chain chiral liquid crystalline elastomers

In the present work, the phase behaviors of two series of side‐chain liquid crystalline elastomers ( PI and PII series) derived from the same nematic liquid crystalline monomer and the different rigidity of chiral bisolefinic crosslinking units have been compared and studied extensively, and the effect of the different rigidity of crosslinker on the phase behavior of elastomers has been discussed. The chemical structure of the monomers and polymers obtained were confirmed by FTIR and ¹H NMR spectroscopy. The phase behaviors were investigated by differential scanning calorimetry, polarizing optical microscopy measurement, thermogravimetric analyses, and X‐ray diffraction measurement. The two series of elastomers showed smectic or cholesteric phases. When the amount of different crosslinking units was less than 15 mol %, both of the elastomers displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. It is shown that the isotropization temperature values of PII series are higher than those of PI series, and the glass transition temperature values of PII series varied smoothly and that of PI series changed smoothly first and then abruptly with increasing the contents of crosslinkers with different rigidity. In addition, PI series showed an interesting change in LC texture near clearing point, but PII series did not. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Phase behaviors of comb-like liquid crystalline polysiloxanes containing fluorinated mesogenic units

Several liquid crystalline polysiloxanes (Pa-Pf) bearing fluorinated mesogenic units were synthesized using poly(methylhydrogeno)siloxane, 4′-(4-undec-10-enoyloxy-benzoyloxy)-biphenyl-4-yl 4-fluoro-benzoate and 4′-propionyloxy-biphenyl-4-yl 4-allyloxy-benzoate, and effect of fluorinated mesogenic units on phase behaviors of the fluorinated LC polysiloxanes was studied as well. The samples Pa and Pb showed single nematic mesophase when they were heated and cooled, but Pc, Pd, Pe and Pf exhibited both smectic and nematic phases. The glass transition temperature and smectic A-nematic mesophase transition temperature of polymers increased slightly with increase of fluorinated units in the polymer systems, but mesophase-isotropic phase transition temperature decreased slightly. In XRD curves, the intensity of sharp reflections at low angle increased with increase of fluorinated mesogenic units in the polymers' systems, indicating that the longer spacer and the fluorophobic effect of fluorinated mesogenic units could lead to a significant stabilization and even to modifications of smectic mesophases.     

Smectic liquid crystalline polyoxetane with novel mesogenic group

Summary Polyoxetane was found to be a good main chain for well-developed smectic liquid crystalline state, when cyano substituted biphenyls were used as mesogenic groups attached through four methylene spacer. With this main chain and spacer, fluorine substituted biphenyl was found to be a novel and good mesogenic group for liquid crystalline state.     

Mesomorphic properties of cholesteric side‐chain liquid crystalline polysiloxanes and elastomers

A series of crosslinked liquid crystalline polymers and corresponding uncrosslinked liquid crystalline polymers were prepared by graft copolymerization. Their liquid crystalline properties were characterized by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The results showed that the crosslinking obtained in the isotropic state and the introduction of nonmesogenic crosslinking units into a polymeric structure could cause additional reduction of the clearing point (T_i) of the crosslinked polymers, compared with the corresponding uncrosslinked polymers. The crosslinked polymers (P‐2–P‐4) with a low crosslinking density exhibited cholesteric phases as did the uncrosslinked polymers. In contrast, a high crosslinking density made the crosslinked polymer P‐5 lose its thermotropic liquid crystalline property. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 773–778, 2004     

The influence of lateral substituents on the phase behavior of side-chain liquid crystalline polysiloxanes containing trans-2,5-disubstituted-1,3-dioxane based mesogenic side groups

The synthesis and characterization of the polysiloxanes containingtrans-2,5-disubstituted-1,3-dioxane-based mesogenic side groups were described in this present study. Among three polysiloxanes containing three methylene units in their spacers, one without the lateral substituent exhibited two enantiotropic smectic phases while the other two polymers with a lateral methyl- or methoxy-substituent displayed an enantiotropic nematic phase. For polymers containing a longer spacer length (n 4), those without lateral substituent presented two enantiotropic smectic phases while the others containing a lateral substituent showed only an enantiotropic smectic A phase. The lateral substitution exerted a strong effect on the mesomorphic properties of the synthesized polymers.     

Synthesis of a side chain liquid crystalline polycarbonate with a chiral backbone

We report the experimental demonstration of a novel and environmentally benign supercritical carbon dioxide (ScCO₂) technique that yields an optically active, side chain liquid crystalline polycarbonate in a single‐step reaction. The obtained polymer is worthwhile, since it is highly stereoregular and can find applications in an enlarged mesomorphic temperature range compared to its acrylic analogues. The synthesized materials were characterized by IR, ¹H‐NMR, and ¹³C‐NMR, while the thermal properties were measured by DSC. Polarized optical micrograph and wide angle X‐ray diffraction were used for the mesogenic property characterization of the copolymer. The transfer chirality from the backbone to the mesophase is demonstrated on the optical microscopy textures. Being consistent with the ¹³C‐NMR, X‐ray implies an ordered polymeric structure. The DSC analysis of the copolymer indicates that the T_i (the clearing point temperature) value does not change dramatically, whereas a pronounced decrease in T_g (the glass transition temperature) value is observed from that of its acrylic analogues. Hence, the obtained polymer exhibits another practical benefit by widening the mesomorphic temperature range. This study is the insightful combination of material processing and chemical design that elucidates the advantages of ScCO₂ application, in terms of liquid crystallinity and the tacticity of the obtained polymer. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1915–1921, 2006     

A novel side chain liquid crystalline polyurethane

Summary Side chain liquid crystalline polyurethanes (SCLP) without flexible spacer were synthesized by a two step block copolymerization reaction. The polyurethanes were based on azobenzene-type mesogenic diol chain extender (DR-19), a poly(tetramethylene oxide) (PTMO) soft segment, and different diisocyanates, including 4,4-diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI). The polyurethane samples obtained from DR-19 or DR-19 and PTMO with HDI had mesomorphic phases as determined by DSC and polarizing microscopy. Received: 19 March 1998/Revised version: 27 May 1998/Accepted: 17 June 1998     

Synthesis and characterization of biphasic liquid crystalline polysiloxanes containing 4-undecanyloxy-4′-cyanobiphenyl side-groups

Summary The synthesis and characterization of liquid crystalline polysiloxanes and copolysiloxanes containing 4-undecanyloxy-4-cyanobiphenyl side-groups is presented. The polysiloxane presents a single glass transition temperature followed by a S_c and a S_A mesophase. Copolysiloxanes presenting around 50% weight fraction of side-groups exhibit two glass transition temperatures i.e., one due to the independent motion of the main chain and the other due to the cooperative but independent motion of the side groups, and in addition to the S_c and S_A phases exhibited by the homopolymer, present also side-chain crystallization.     

Selective oxidation of alcohols using novel crystalline Mo-V-O oxide as heterogeneous catalyst in liquid phase with molecular oxygen

Catalytic selective oxidation of benzyl alcohol with molecular oxygen under mild conditions was carried out over novel crystalline Mo-V-O oxide. The present research is focused on investigation of recycling, reusability and stability of the crystalline oxide in the liquid-phase reaction. The Mo-V-O oxide catalyst was used at least four times with comparable activities to that of fresh catalyst. The separation of the catalyst from reaction medium can stop the conversion of benzyl alcohol, and the addition of the catalyst to the reaction medium can trigger the reaction immediately. The catalytic oxidation of 2,3,6-trimethylphenol as a reference reaction suggested that there were no leached active species in the reaction mixture. The results of the ICP–MS analysis, XRD, and SEM characterization confirmed that the structure and composition of the catalyst were stable. Besides, the Mo-V-O oxide can catalyze the oxidation of a series of alcohols with high selectivities for corresponding carbonyl compounds.     

Infrared spectroscopic studies of a nematic liquid crystalline poly(ester amide) in the solid phase

Infrared spectroscopy has been used to study the hydrogen bonding conditions in a nematic liquid crystalline poly(ester amide). The results showed that the material formed interchain hydrogen bonding as a result of the inclusion of amide groups in the polymer chains. The hydrogen bonding was found to be stable in the temperature range studied (up to 150°C). However, the studies suggested that the hydrogen bond strength was weaker than in polyamide and did not appear to affect the mechanical properties of the material significantly.     

Synthesis of chiral side-chain liquid crystalline polyacetylenes bearing succinic acid spacer

A chiral acetylene monomer having a cholesteryl group (PSCh) and novel three types of chiral acetylene monomers having a o-substituted tyrosine methyl ester (PSMY-Rs: –COC₆H₅=PSMY-Bz; –COC₆H₄CN=PSMY-BzCN; –COC₁₂H₉=PSMY-PhBz) as a pendant group were synthesized from 4-oxo-4-(prop-2-ynyloxy)butanoic acid (PS), and polymerized with the rhodium-catalyzed system. Structures and properties of the monomers and the resulting polymers were characterized and evaluated by NMR, IR, GPC, and DSC. In addition, these optical properties were investigated by polarimetric detector and circular dichroism (CD) analyses.     

Tailor made liquid crystalline networks exhibiting a chiral smectic C (S C * ) mesophase via living cationic copolymerization

Summary Living cationic copolymerization of (2S, 3S)-(+)-2-chloro-3-methylpentyl 4-(8-vinyloxyoctyloxy)biphenyl-4-carboxylate (15-8) with 2-vinyloxyethyloxy methacrylate (16-2) leads to reactive copolymers poly[(15-8)-co-(16-2)]X/Y (where X/Y is the mol ratio between the two monomers in the copolymer) containing methacryloyl side groups. The thermally crosslinked copolymers with X/Y=5/5 to 9/1 display an enantiotropic chiral smectic C (S _C ^* ) mesophase.This paper is part 28 in the series: Molecular engineering of liquid-crystalline polymers by living polymerization. Part 27: V. Percec, Q. Zheng: Polym. Bull., previous paper in the issue     

Neutron diffraction studies of liquid and crystalline trilaurin

Neutron diffraction has been employed to investigate the structure of trilaurin in both liquid and crystalline states. The combination of long-wavelength (4.5 Å) neutrons and wide angular ranges of detection (～6<2θ[^o]<～100) per mitted a large range of momentum transfer to be acceessed (～0.15?1]<～2) in a single scan of the detector. The chemical technique of selective deuteration of chosen parts of the triglyceride molecules was used to enhance specific aspects of the diffraction pattern and elucidate structures formed. The well established layer structure in the solid phases was confirmed in the diffraction patterns. It was also established that long-range ordering in the solid resulting from chain-chain registry disappeared when the solid melts. However, we found no evidence for long-range ordering in the form of persistence of a layer-layer spacing correlation in the liquid state. We postulate from the diffraction patterns a molecular arrangement resembling the arrangement found in the nematic phase of liquid crystals, where the layer structure is relaxed from that smectic-like arrangement for the crystalline phase. The data also reveal that this lack of order is not dependent on temperature in the liquid phase and persists into the super-cooled region below the normal β melting point.