Dipolar relaxations in the glass transition region and in the liquid crystalline phase of two side-chain liquid crystalline polysiloxanes

The molecular relaxation mechanisms in the glass transition region and in the liquid crystalline phase exhibited by two side-chain liquid crystalline polysiloxanes have been studied by Thermally Stimulated Discharge Currents. These results were compared with those previously obtained by dielectric relaxation spectroscopy. It was observed that two relaxation mechanisms were present in the liquid crystalline phase, and we suggest that these might correspond to the motions of the mesogenic moieties in the liquid crystalline phase. © 1996 John Wiley & Sons, Inc.     

Dielectric relaxation in a mixture of a side chain liquid-crystalline polymer and a low molecular liquid crystal

Dielectric measurements on two mixtures of a low and high molecular liquid crystal and the pure components have been carried out in the frequency range from 1 Hz to 100 kHz. In the S_A and supercooled S_A phase two absorption ranges were observed. The low frequency absorption belongs to the low molecular weight liquid crystal. The shift in the relaxation frequency of both compounds indicates complete miscibility. The strong difference of the absorption intensity of the polymer on heating and cooling can be associated with the formation of mixed crystals at smaller concentrations of the low molecular weight compound.     

DSC investigation of vitreous selenium in the glass transition region

The present paper describes the experimentally observed changes in the glass transition temperatureT _g and the corresponding endothermic effect of a glassy Se with annealing at 10°. The changes were observed by the DSC method. During the annealing process the values ofT _g andH increase by 10° and 3 J/g, respectively, in comparison to the values of the samples as prepared. BothH andT _g depend upon the scanning speed in the DSC measurements. The glass transition temperature as a function of heating ratea fits well the relation 1T _g=C ₁–C ₂ Ina; on the other hand, the value ofH with decreasing heating rate approaches zero (disappears).

---

Zusammenfassung Es werden die auf dem Versuchswege ermittelten Änderungen der Glas-ÜbergangstemperaturT _g und der entsprechende endotherme Effekt eines glasförmigen Se mit Mattätzung bei einer Temperatur von 10° beschrieben. Die Änderungen wurden mittels der DSC-Methode beobachtet. Während des Mattätzungsvorganges steigen die Werte vonT _g undH um 10° bzw. 3 J/g im Vergleich zu den Werten der ursprünglichen Proben. SowohlH wieT _g hängen von der Abtastgeschwindigkeit bei der DSC-Messung ab. Die Glas-Übergangstemperatur als Funktion der Aufheizgeschwindigkeita kann mit der Gleichung 1/T _g=C ₁–C ₂ Ina gut beschrieben werden; andererseits nähert sichH mit abnehmenden Aufheizgeschwindigkeiten dem Nullwert (verschwindet).

---

Résumé L'article décrit les variations observées expérimentalement par analyse calorimétrique différentielle (DSC) pour la températureT _g de la transition vitreuse d'un Se vitreux recuit à la température de 10°, et pour l'effet endothermique correspondant. Pendant le recuit les valeurs deT _g et de H augmentent respectivement de 10° et de 3 J.g^–1 par rapport aux valeurs des échantillons bruts de préparation. Les valeurs deH et deT _g dépendent toutes deux de la vitesse de chauffage. La température de la transition vitreuse peut être représentée en fonction de la vitesse de chauffagea par l'équation 1/T _g=C ₁–C ₂ Ina; d'autre part, la valeur deH tend vers zéro lorsque la vitesse de chauffage diminue.

---

T _g T=10°. (). T _g 10° 3 / . _g . , , 1/T _g=C ₁–C ₂, ln. , , ( ).

---

---

The authors would like to express their gratitude to Miss Vyovská for her assistance with the DSC measurements.     

Dipolar motions and phase transitions in a side‐chain polysiloxane liquid crystal. A study by thermally stimulated depolarization currents

The relaxation mechanisms present in a side‐chain liquid crystalline polymer have been studied by Thermally Stimulated Depolarization Currents (t.s.d.c.), in a wide temperature range covering the glassy state, the glass transition region, and the liquid crystalline phase. The thermal sampling procedure was used to decompose the complex relaxations into its narrowly distributed components. Three relaxation mechanisms were observed in this polymer: a relaxation below the glass transition temperature that is broad and extends from −150°C up to −110°C, the glass transition relaxation whose maximum intensity appears at ∼20°C, and a relaxation above the glass transition temperature, in the liquid crystalline phase. The attribution of these relaxations at the molecular level is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 227–235, 1999     

A deuteron NMR study of axial motion and side chain conformation in the mesophase of discotic liquid crystal main-chain polymers

Pulsed deuteron NMR spectroscopy has been used to examine the axial motion and the side-chain conformation in both oriented and unoriented mesophases of discotic liquid crystal main-chain polymers based on pentyloxy or heptyloxy substituted triphenylenes. Lineshape simulations show that the rotational motion of the triphenylene rings about the column axes can be described by an inhomogeneous distribution of reorientation angles around 45°. However, only about 60% of the discs are involved in such large amplitude motions; the remaining 40% have reorientation angles below 10°. This illustrates the severe restrictions imposed on the rotation of the discs by the interlinkage of the columns via the alkylene spacers. Furthermore, the simulations demonstrate that, at the -carbon, the side chains show very little fast internal motion, but have a relatively complex conformation involving a disorder which does not change on the microsecond timescale. Since such a disorder is not present in the corresponding monomeric samples it is ascribed to the presence of the spacers. These results are also consistent with the presence of large sterical hindrances between the first side chains segments of adjacent discs, and they indicate a correlated reorientation of the discotic units within a column.     

New hydrocarbon side chain liquid crystalline polysiloxane polymers: Synthesis,characterization, and thermotropic behavior

A series of new and high-purity hydrocarbon liquid crystal monomers were synthesized through the acylation reaction, deoxygenation reaction, and Grignard reaction. ¹H-NMR spectra and elemental analyses were used to examine their purity. The liquid crystalline polysiloxane polymers were obtained by grafting the monomers onto poly(methylhydrosiloxane). The thermal transition temperature, mesomorphic properties, and mesophase textures of the monomers and the polymers were determined by differential scanning calorimetry (dsc), polarized optical microscopy, and X-ray diffraction analysis. Moreover, we observed the even–odd effect of the smectic/isotropic transition temperature with the length variation of the substituents. In this study, we found by X-ray diffraction that the liquid crystalline polysiloxane polymers undergo a transition from smectic B to smectic E mesophase. However, dsc has difficulty detecting the phase transition process. By considering the spin–lattice relaxation time (T₁), we can systematically explain the relation between the flexibility of the substituent with the smectic/isotropic transition temperature. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2849–2863, 1998     

Interpretation of the observed influence of mesophase structures on the β-relaxation in side chain liquid crystal polymers

A strong dependence of the rotational dynamics of the mesogenic units (β-relaxation) on the order of the mesophase was found in sidechain liquid crystal polymers. The preexponential frequency factor, $ f_{\beta \infty }^* $ and the activation energy $ {\rm E}_{\alpha \beta }^{\rm *} $ of the β-relaxation rate both increase significantly (i.e., obeying a compensation law) with increasing order of the mesophase which is accompanied by a decrease of the mean lateral mesogenic distance. In this work, we show how these experimental results can be interpreted in a quantitative manner by using the general results of the coupling model for cooperative motions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1927–1934, 1998     

Synthesis, micro structure, and thermal stability of side chain liquid crystalline polysiloxane polymers with an oligo (ethylene oxide) unit in the side chain

A series of new alkene monomers [MS3BDBEn, n=1-3] containing 4-oligo (ethylene oxide) monomethyl ether 4-biphenyl ether carboxyl benzoate as terminal groups were synthesized. These polymers were prepared by grafting these monomers onto the poly (methylhydrosilox-ane) (PMHS) backbone. The transition temperatures, liquid crystalline textures, and thermal stability of the polysiloxane polymers have been determined by thermal data, by optical texture, and by X-ray diffraction patterns. Polymers PS3BDBEn showed smectic or smectic and nematic phases which were not analogous to their precursor nematic monomers. The terminal length of the polymers affects not only the mesophase transition temperatures but also the layer-spacing length (d₁) and the side-chain distance (d₂). The long- and short-range orders can remain to some extent above the isotropization temperature and below the melting point. The polymer PS3BDBE3 decomposed in air 20°C above the isotropization temperature and lost its short range orders as detected by the X-ray diffraction analysis.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

Static and dynamic properties of smectic liquid crystal side chain polymers

On the basis of models for the conformation of liquid crystal side chain polymers in the smectic phase, some dynamic aspects of such systems are considered using the reptation approach. The overall diffusion constants of the polymer along and perpendicular to the director of the side chains are derived.     

2H NMR study of phase transitions and orientational order in a side chain liquid crystal polymer: Evidence of a nematic-nematic transition

We have investigated the mesomorphic behaviour of a partially deuteriated side chain LC polymethacrylate, poly[4-[6-methacryloyloxyhexyl-oxy]-4'-methoxyazobenzene], by means of ²H NMR, DSC and optical microscopy. Evidence for a phase transition occurring in the middle of the nematic phase has been found.     

Molecular dynamics and the glass transition in a columnar liquid crystal formed by a chiral discotic mesogen

A new type of chiral discotic liquid crystal is described. It is based on a triphenylene core but carries only one asymmetric side chain per mesogen. This system displays a columnar liquid-crystalline mesophase with a helical superstructure and a pitch of 3·0 nm over a temperature range of 227 K. Upon cooling it forms a glassy state. By broadband dielectric and ²H NMR spectroscopy two motional processes are detected. The axial rotation of the discs around the column axis exhibits non-Arrhenius behaviour and is directly related to the glass transition. The second process is ascribed to localized side chain motions involving the ester linkages.     

Direct observation of smectic layers in side chain liquid crystal polymer films

By applying stringent conditions to the imaging process it has been possible to produce electron micrographs of the smectic planes in a highly beam sensitive polymethcrylate side chain liquid crystal polymer. These planes show a high degree of internal perfection, while, at the same time, showing clear evidence of curvature. The director orientation was determined over a fairly large region.     

Order parameter of side groups of a polysiloxane in liquid crystalline uniaxial phases

The orientational order of a liquid crystalline side chain polysiloxane has been investigated by means of polarized light spectroscopy and dielectric relaxation measurements. The order parameters (P₂) and (P₄) have been determined as a function of the temperature for the smectic and nematic phases. The orienting properties of the polysiloxane investigated have been compared with those of a polysiloxane with the same mesogenic groups, but a shorter spacer.     

Enthalpic relaxation and the glass transition in polymer blends

The kinetics of enthalpic relaxation are reviewed and applied to the ageing of a range of blends made from polyether imide and polyether ether ketone. DSC has been used to follow the development of enthalpic relaxation and a Williams-Watt stretched exponential equation relating the extent of relaxation, ϕ(t), to the ageing time t and an average relaxation time, t́_a', has been used to quantify the ageing process. where β' is inversely related to the breadth of the relaxation spectrum such that 0<β>1.0. The relationship was modified to incorporate non-linearity in the relaxation behaviour. ϕ(t) was measured directly from the enthalpy change observed in the endotherms on heating aged specimens through the glass transition in the DSC. The PEI/PEEK blends were compatible over the full composition range in that they exhibited a single glass transition with a temperature that varied almost linearly with composition between those of the homopolymers. Enthalpic relaxation was found to be a useful technique for probing the molecular relaxations of polymer blends and confirming the degree of compatibility of the system. The β' values changed systematically with the blend composition between those of the homopolymers suggesting that the breadth of the relaxation spectra were similar in the blends to that in the homopolymers. Physical ageing was observed to embrittle the blends, and there was a close correlation between the extent of enthalpic ageing and the change in mechanical and impact behaviour. The yield stress increased and the elongation to break decreased progressively with ϕ(t) in addition to a reduction in impact strength. The model of enthalpic relaxation and the kinetic relationships, outlined above, have been used to determine the onset of the glass transition temperature and subsequent progress of enthalpic relaxation at fixed ageing temperatures, for direct comparison with the change in specific heat observed in DSC experiments. Good agreement was observed between experiment and calculated glass transitions and the effect of variables, such as activation enthalpies, pre-exponential factors, non-linear factors such as X and β' and fictive temperature on the observed glass transition temperatures and the temperature range over which the glass transition occurred determined. Modifications to the model for the enthalpic relaxation have been suggested.     

Investigation of side chain liquid crystal polymers bearing cholesterol and bile acid derivatives

Cholic acid (or 3a,7a,12a-trihydoxyl-5a-cholan-24-oic acid) and lithocholic acid (or 3a-hydroxyl-5a-cholanic-24-oic acid) are commonly occurring bile acids synthesized from cholesterol in the liver in mammals. They all possess a steroid skeleton containing four rings, three with six carbons and one with five carbons. The transformation of cholesterol to cholic acid results in two major structural changes that affect the steroid skeleton. The first is the hydrogenation of the double bond between C5 and C6 and the second is a conformational flip of ring A from the 5a-position to the 5a-position. In addition, one or more hydroxyl groups are added to the steroid skeleton. Outside of the ring system, C24 is converted from a saturated alkyl to a carboxylic acid group.Side chain polymers based on cholesterol moiety have been made as reported in the literature.Since bile acids and cholesterol are all in the family of steroid molecules, it is of interest to investigate whether bile acids may also act as mesogenic groups.Therefore, flexible spacer groups with 10 carbons are introduced between bile acid skeleton and the poymerizable double bonds. The monomers and polymers are compared with cholesterol and dihydrocholesterol monomers and polymers with the same spacers. Dihydrocholesterol is chosen to investigate the influence of the double bond in the formation of LC, given that both cholesterol and dihydrocholesterol have a planar structure but there is no double bond in the latter. These monomers and their corresponding polymers were characterized for their liquid crystalline (LC) properties by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction.It was found that only the compounds bearing the planar cholesterol moieties possess LC phases. It is concluded that the 5a-configuration between the first and second cycles on the steroid skeleton of bile acids does not favor proper alignment of the rigid part of the bile acid moieties. It is interesting to see the effect of a small variation in structure on the properties of the otherwise structurally-similar compounds and the materials made from them.