Wim J. Mijs

Liquid Crystalline Epoxide Thermosets Copolymerization with aromatic diamines

Abstract Highly ordered networks were produced by copolymerization of a low molecular weight liquid crystalline (LC) diepoxide with an aromatic diamine. A series of aromatic diamines was synthesized and their Polymerization reaction with the LC-diepoxide was studied by means of differential scanning calorimetry. By adjusting the melting point of the crosslinking agents an eutectic LC mixture was found on which all the physical measurements were carried out. The degree of order of a certain dye guest in the polymer networks as determined by UV-dichroism was 0.76. Birefringence measurements have confirmed that about the same level of ordering can be maintained up to the thermal degradation point (i.e., 300°C). Microscopy and X-ray studies have revealed that the epoxy-amine mixture is transformed from a nematic to a smectic phase, as a result of the polymerization reaction at 120°C.

Specific interactions in discotic liquid crystals

A series of novel mesogens have been prepared by a five-fold Sonogashira reaction of terminal acetylenes with a functionalized pentabromophenol derivative. The corresponding side-chain substituted polymers were prepared by an analogous polymer substitution reaction. The mesogens differ in the nature of the substituents, i.e. CH2, O, S, SO2 and CONH groups, linking five hexyl tails to the core. A wide range of mesophases and corresponding transition temperatures have been detected, varying from low melting nematic phases to highly stable columnar phases. The wide spread in phase behaviour is described in terms of specific intermolecular interactions. The addition of planar electron deficient molecules resulted in the formation of charge transfer complexes. The observed stabilisation or destabilisation of the mesophases is explained by considering the complexation strength of the complex as well as steric factors.

Dynamic mechanical properties of some liquid crystalline polyacrylates

SummaryThe dynamic mechanical properties of some LC (nematic and smectic) polyacrylates with biphenyl mesogens with different spacers and tails are described. From plots of G′ and G″ against frequency and temperature the phase transitions could be clearly detected, in agreement with DSC analysis. Furthermore, the dynamic viscosities of the LC polyacrylates compared with an isotropic model polymer of similar stucture showed a strong shear rate and temperature dependence. Surprisingly it was found that the viscosity of the LC polymers was higher in the nematic phase than in the isotropic phase.

The Nematic Discotic Phase in Materials Containing a Siloxane Core

The synthesis of a series of novel multipodes based on disc-shaped mesogens with a siloxane core is described. Their phase behaviour is characterised and compared to the liquid crystalline properties of the discotic molecules without the siloxane moieties. We observed that incorporation of the siloxane group results in a strong decrease of all transition temperatures, resulting for one series in (close to) room temperature nematic mesophases. In a second series all liquid crystalline properties were suppressed. Introduction of nematic phases was accomplished via the formation of charge transfer complexes.

Network formation in polyurethanes based on triisocyanate and diethanolamine derivatives

Abstract The effect of the side chain length and the presence of charge in diols on gelation of polyurethane networks from diols based on diethanolamine derivatives and tris(4-isocyanatophenyl) thiophosphate was studied. The samples having different initial ratios of the reactive hydroxy [OH] and isocyanate [NCO] groups ( r H =[OH]/[NCO] ranging from 1.1 to 3) were investigated. The number- and weight-average molecular weights in the pregel stage ( M n , M w ) together with the critical ratio of the reactive groups, r H c , at which gelation occurs, were determined. The experimental M n and M w values are lower than the theoretical ones calculated for the ring-free case suggesting that cyclization takes place. The fraction of bonds lost in cycles, S , is 0.02–0.04 in the bulk state, increasing with increasing dilution and decreasing r H . A pronounced decrease in r H c with dilution was found; while the length of the side chain of the diol has only a minor effect on the decrease, the introduction of the charge (quaternization of the nitrogen) into the diol leads to a less pronounced decrease (lower extent of cyclization). The S values calculated from M w and r H c were found to be always higher than S values calculated from M n . A possible supramolecular structure is proposed for networks prepared from the uncharged diethanolamine-based diols when prepared in the presence of diluent.

Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 4. Synthesis of side-chain liquid crystalline polycarbonates with mesogenic groups having tails of different lengths

Side-chain liquid crystalline polycarbonates with alkoxyphenylbenzoate side groups, having a short spacer and tails ranging from 1 to 8 C-atoms, were synthesized. The polymers were prepared by an organo-zinc catalysed copolymerization of carbon dioxide and mesogenic 4-alkoxyphenyl 4-(2,3-epoxypropoxy)benzoates. Model polycarbonates were prepared by copolymerization of glycidyl phenyl ether (GPE) and carbon dioxide and by terpolymerization of GPE, propylene oxide and CO2. The copolymerizations of LC monomers were carried out at 70°C in an autoclave at 60 atm CO2 pressure in dioxane as the solvent. Catalysts based on diethylzinc and water or diethylzinc and resorcinol were used. High catalyst concentrations (up to 20 mol-% on the basis of diethylzinc with respect to epoxide) and long reaction times (72 h) were found to be necessary for acceptable yields and reasonable molar masses. The resulting polymers had a high polydispersity and the maximum yield was about 30%. A major side product, formed directly from CO2 and epoxide, as well as indirectly by depolymerization, was the corresponding five-membered cyclic carbonate. The polymers on average contained about 30 mol-% of ether units and 70 mol-% of carbonate units due to a weak tendency of the epoxides to undergo homopolymerization. 1H and 13C NMR analysis revealed detailed information about the overall ether content and the distribution of the ether and carbonate groups in the chain, as well as the tacticity of the polymers. Surprisingly, the monomer distribution and the tacticity were molecular weight dependent. Some possible explanations for these phenomena are suggested.

Substituent effects in discotic liquid crystals

A series of novel mesogens have been prepared by a five-fold Sonogashira reaction of terminal acetylenes with a functionalized pentabromophenol. The corresponding side chain polymers were prepared by a polymer analogous substitution reaction. The mesogens differ in the nature of the substituents, linking five hexyl tails to the aromatic core, i.e. CH2, O, S, SO2 and CONH groups. A wide range of mesophases and corresponding transition temperatures has been detected, ranging from low melting nematic phases to highly stable columnar phases. The widely variable phase behavior is described in terms of specific intermolecular interactions.

A Series of Novel Liquid Crystalline Polymers Showing a Nematic Discotic and/or a Nematic Columnar Phase

Abstract A series of novel liquid crystalline side chain polyacrylates was prepared, bearing a disk-shaped pentakis(4-methylphenylethynyl)benzene mesogen via an undecanoxy spacer. By using the concept of the polymer analogous reaction, a variety of copolymers with different alkyl acrylate “co-monomers” and increasing mesogen contents were synthesized. The thermal properties of the polymers were investigated by optical polarization microscopy and differential scanning calorimetry. The results indicated that the liquid crystalline characteristics of the polymers are not determined by nature of the co-monomers, but mainly by the weight fraction of rigid mesogen present in the material. All polymers exhibit a nematic columnar phase at lower temperatures. Polymers with a high mesogen content (>61%-wt) exhibit an additional nematic discotic phase at high temperatures, which is unique for polymeric materials.

Synthesis and characterization of novel side‐chain liquid crystalline polycarbonates, 5. Mesophase characterization of side‐chain liquid crystalline polycarbonates with tails of different lengths

The mesomorphic properties and thermal stability of side-chain LC polycarbonates with alkoxyphenyl benzoate side groups having a short spacer and alkoxy tails ranging from 1 to 8 carbon atoms were studied by DSC, X-ray diffraction and polarized light optical microscopy. All polymers have a smectic A structure. Mesogens having short tails organize preferably in a monolayer structure, and mesogens having long tails in a double layer structure. A sharp increase in the clearing temperatures and enthalpies as a function of tail length was observed. No clear relation between the tail length and the glass transition temperature was found. Both the Tg and the clearing temperature show a strong dependence on the molecular weight. Up to about Mw = 20000 a rapid increase in phase transition temperatures with increasing molecular weight was observed, eventually levelling off to a constant maximum value. The LC polycarbonates were found to be thermally stable up to about 200°C. At higher temperatures, random chain scission, resulting in a rapid decrease in molecular weight, and unzipping, resulting in the formation of a five-membered cyclic carbonate, are the main initial degradation mechanisms. The degraded polymers have lower transition temperatures than the original ones.

Synthesis and Characterization of Novel Side-Chain Liquid Crystalline Polycarbonates

Abstract Side-chain liquid crystalline (SCLC) polycarbonates were prepared by copolymerization of carbon dioxide with epoxides, carrying a nitrostilbene mesogenic group and spacers of different lengths. The polymerization, carried out in an autoclave at high carbon dioxide pressure, was catalyzed by an organozinc catalyst. The number average molecular weight of the polymers, as determined by GPC, was in the range of 4,300 to 38,000. The polydispersity of the polymers ranged from 5.9 to 80. 1H NMR and 13C NMR studies revealed that the obtained polymers contain carbonate linkages (67-94%) as well as ether linkages in their backbone. Liquid-crystalline properties of the polymers were investigated by DSC, X-Ray diffraction and polarized optical microscopy.