Molecular dynamics in liquid-crystalline side chain polymers studied by dielectric relaxation spectroscopy: A comparative study

A series of liquid-crystalline side chain copolymers with different main chains have been studied by the dielectric method in a maximum frequency range of 9 decades. Oriented samples were used throughout. The data were analysed in terms of the Havriliak-Negami and Fuoss-Kirkwood formulae for the relaxation functions. Two well separated dispersion regions with their strengths depending strongly on the macroscopic orientation were found. The low frequency or δ-relaxation shows a marked change in its curve form and width with different main chain structure, its strength being determined by the longitudinal dipole moment of the mesogenic unit. The high frequency relaxation shows a more complicated dependence of its characteristic parameters on the molecular structure. In some cases a decomposition into two underlying relaxations was successfully attempted. We discuss the models for molecular motions developed for low molecular weight liquid crystals and for amorphous polymers, in order to explain the behaviour of the different dispersions found.     

Molecular dynamics and ordering of side chain liquid crystal polymers as studied by paramagnetic resonance

Abstract

Molecular dynamics of side chain liquid crystalline polymers (LCP) and their components were studied using the technique of paramagnetic resonance. A cigar shape spin probe (COL) and a nearly spherical spin probe (TPL) were used to study the motions and order of the LCPs. Computer simulations of the observed spectra were performed. Both rotational correlation times and order parameters were extracted from these simulations. We found that LCPs containing 30 per cent and 50 per cent of mesogenic side chains had about the same viscosity as indicated by nearly equal tumbling times at the same temperature. In addition, the LCPs motion is considerably slower than that of the monomeric liquid crystal indicating that the spacer couples the motions of the side chains to those of the main chain. Rotations about axes perpendicular to the side chain are slowed more than rotations about an axis parallel to the side chain. DSC measurements were employed to study the phase transitions. The 30 and 50 per cent LCPs displayed first order NS_A transitions, but the 50 per cent LCPs transition was much weaker, in agreement with McMillan's theory which predicts a first order transition for T _NS/T _NI>0.87 (observed ratios are 0.98, 0.90 and 0.86 for 30, 50 and 100 per cent LCPs, respectively). The 30 per cent LCP has a very short nematic range so that the nematic order, which is not saturated at the NS transition, can couple with the smectic order. This was indicated by a sharp change in slope of the order parameter versus temperature plot as the smectic is entered. The LCPs studied formed a highly ordered glass when cooled in a 1 T field. If one could find a LCP with similar ordering properties whose glass temperature is well above room temperature, then one would have a useful binder for the manufacture of haze-free polymer dispersed liquid crystal displays.     

Broadband dielectric spectroscopy on ferroelectric liquid-crystalline side group polymers

Broadband dielectric spectroscopy (10^-1 to 10⁷Hz, 100 to 450 K) has been applied, for the first time, to investigate the molecular dynamics of recently synthesized fast switching ferroelectric side group polymers. The softmode could be studied in detail in the S_c* phase and in the S₁* phase. At the transition a pronounced increase of the relaxational strength was found whereas its relaxational frequency remains constant The softmode is the molecular basis for the electroclinic effect which has high application potential for ferroelectric liquid crystal polymers.     

Molecular dynamics in fluorinated side-chain maleimide copolymers as studied by broadband dielectric spectroscopy

A series of alternating maleimide (MI) copolymers with fluorinated side chains have been investigated using broadband dielectric spectroscopy. The side chains consist of fluoroalkane (–C _x F_2x+1, x=1, 7, 9) end groups connected to the main chain via methylene spacers. The experiments were carried out in a frequency range of 0.1 Hz to 10 MHz and at temperatures between 120 K and 500 K. The fluorinated MI copolymers show a fast sub-T _g (β) relaxation characterized by an Arrhenius-type temperature dependence with activation energy in the range of 30–37 kJ/mol. Two more processes (α and δ-like) are observed, corresponding to independent relaxations of the main chain and the fluoroalkane domains respectively. For shorter side chains, the δ-like process is not observed but instead another relaxation process, α _S , occurs at temperatures higher than either the α and δ-like processes. When compared with unfluorinated MI copolymers, the fluorinated MI copolymers show the δ-like process and a slower β-relaxation unlike their unfluorinated counterparts. A model to explain the molecular origin of the four processes is proposed, supplemented by differential scanning calorimetry and published WAXS/SAXS data.     

Discotic liquid-crystalline side chain polymers with inositol derivatives as mesogens

The synthesis of monofunctionalized myo- and scyllo-inositol derivatives and their attachment as side groups on a polysiloxane backbone are described. All polymers with scyllo-inositol mesogens show liquid-crystalline behaviour. In contrast to this the myo-inosital mesogens containing polymers exhibit no liquid-crystalline phases.     

Thermotropic liquid-crystalline polymers. XXVII. Reentrant nematic phase of side chain liquid-crystalline polymers and their optical properties

The phase diagrams of side chain liquid-crystalline acrylic copolymers with cyanobiphenyl mesogenic groups are described. These copolymers are shown to form a reentrant nematic phase. The main regularities of the reentrant behaviour of polymer systems are studied. Certain peculiarities of the electric field induced orientation phenomena are discussed.     

Molecular addressing? Studies on light-induced reorientation in liquid-crystalline side chain polymers

Polarized light leads to an effective reorientation of the optic axis in the glassy state of liquid-crystalline side chain polymers containing azobenzene mesogenic groups, via a trans-cis and cis-trans isomerization. Using a combination of U V and IR dichroitic studies it is shown for copolymers consisting of chromophores (azobenzene) and non-chromophores (phenylbenzoate) that only the chromophores are reoriented by light as far as the glassy state is concerned. Individual chromophores are thus addressed by photoselection. Photoselection in the fluid nematic state, on the other hand, leads also to a reorientation of the non-chromophores.     

Photochromic liquid-crystalline polymers. Main chain and side chain polymers containing azobenzene mesogens

Abstract

Two classes of thermotropic polymers were synthesized containing the trans-azobenzene unit as both a mesogenic and a photochromic group. In the former class (I) the azobenzene unit is incorporated into the main chain of substituted polymalonates, while in the latter class (II) it is appended as a side chain substituent to a polyacrylate backbone. The liquid-crystalline properties of the polymers were studied as a function of the chemical structure. All of the prepared polymers I have smectic phases. Polymers II are nematic and/or smectic, or cholesteric when including a chiral residue R'. Polymers I and II when radiated at 348 nm in chloroform solution undergo trans-to-cis isomerization of the azobenzene moiety. The calculated rate constants are comparable with those of low molar mass model compounds, and indicate that the macromolecular structure does not significantly affect the photoisomerization rate.     

Relation between solvent and protein dynamics as studied by dielectric spectroscopy

We present results obtained by dielectric spectroscopy in wide frequency (10(-2)-10(9) Hz) and temperature ranges on human hemoglobin in the three different solvents water, glycerol, and methanol, at a solvent level of 0.8 g of solvent/g of protein. In this broad frequency region, there are motions on several time-scales in the measured temperature range (110-370 K for water, 170-410 K for glycerol, and 110-310 K for methanol). For all samples, the dielectric data shows at least four relaxation processes, with frequency dependences that are well described by the Havriliak-Negami or Cole-Cole functions. The fastest and most pronounced process in the dielectric spectra of hemoglobin in glycerol and methanol solutions is similar to the alpha-relaxation of the corresponding bulk solvent (but shifted to slower dynamics due to surface interactions). For water solutions, however, this process corresponds to earlier results obtained for water confined in various systems and it is most likely due to a local beta-relaxation. The slowing down of the glycerol and methanol relaxations and the good agreement with earlier results on confined water show that this process is affected by the interaction with the protein surface. The second fastest process is attributed to motions of polar side groups on the protein, with a possible contribution from tightly bound solvent molecules. This process is shifted to slower dynamics with increasing solvent viscosity, and it shows a crossover in its temperature dependence from Arrhenius behavior at low temperatures to non-Arrhenius behavior at higher temperatures where there seems to be an onset of cooperativity effects. The origins of the two slowest relaxation processes (visible at high temperatures and low frequencies), which show saddlelike temperature dependences for the solvents water and methanol, are most likely due to motions of the polypeptide backbone and an even more global motion in the protein molecule.     

Molecular dynamics of a biodegradable biomimetic ionomer studied by broadband dielectric spectroscopy

Broadband dielectric spectroscopy was used to investigate the bulk molecular dynamics of a recently developed biodegradable biomimetic ionomer potentially useful for biomedical applications. Isothermal dielectric spectra were gathered for a phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) (PTMC) ionomer and unfunctionalized PTMC at temperatures ranging from 2 to 60 degrees C over a broad frequency range of 10(-3) to 10(6) Hz. Four relaxations were clearly identified, two of which were shown to stem from the PTMC polymer backbone. A detailed analysis showed that the formation of zwitterionic aggregates was responsible for the material's bulk functionality and that bulk conduction processes may provide useful information for assessing the PC ionomer as a candidate for drug delivery applications. Finally, it was concluded that absorbed water concentrates around the aggregates, resulting in an increased mobility of the PC end-groups.     

Molecular dynamics of amorphous/crystalline polymer blends studied by broadband dielectric spectroscopy

The molecular dynamics of poly(vinyl acetate), PVAc, and poly(hydroxy butyrate), PHB, as an amorphous/crystalline polymer blend has been investigated using broadband dielectric spectroscopy over wide ranges of frequency (10⁻² to 10⁵ Hz), temperature, and blend composition. Two dielectric relaxation processes were detected for pure PHB at high and low frequency ranges at a given constant temperature above the T_g. These two relaxation peaks are related to the α and α′ of the amorphous and rigid amorphous regions in the sample, respectively. The α′-relaxation process was found to be temperature and composition dependent and related to the constrained amorphous region located between adjacent lamellae inside the lamellar stacks. In addition, the α′-relaxation process behaves as a typical glass relaxation process, i.e., originated from the micro-Brownian cooperative reorientation of highly constraints polymeric segments. The α-relaxation process is related to the amorphous regions located between the lamellar crystals stacks. In the PHB/PVAc blends, only one α-relaxation process has been observed for all measured blends located in the temperature ranges between the T_g’s of the pure components. This last finding suggested that the relaxation processes of the two components are coupled together due to the small difference in the T_g’s (ΔT_g = 35 °C) and the favorable thermodynamics interaction between the two polymer components and consequently less dynamic heterogeneity in the blends. The T_g’s of the blends measured by DSC were followed a linear behavior with composition indicating that the two components are miscible over the entire range of composition. The α′-relaxation process was also observed in the blends of rich PHB content up to 30 wt% PHB. The molecular dynamics of α and α′-relaxation processes were found to be greatly influenced by blending, i.e., the dielectric strength, the peak broadness, and the dielectric loss peak maximum were found to be composition dependent. The dielectric measurements also confirmed the slowing down of the crystallization process of PHB in the blends.     

Molecular motions in amorphous ibuprofen as studied by broadband dielectric spectroscopy

The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as alpha, beta, gamma, and D, were detected and characterized, and a complete relaxation map was given for the first time. The gamma-process has activation energy E a = 31 kJ.mol (-1), typical for local mobility. The weak beta-relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the alpha-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, T B = 265 K. From the low temperature VFTH regime, a T g (diel) (tau =100 s) = 226 K was estimated, and a fragility or steepness index m = 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the alpha-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.     

The molecular dynamics of thermoreversible networks as studied by broadband dielectric spectroscopy

Polybutadienes modified by a small number of 4-phenyl-1,2,4-triazoline-3,5-dione form thermoreversible networks via hydrogen bonding between the polar stickers. The molecular dynamics of systems with different contents of polar stickers are investigated by broadband dielectric spectroscopy in the frequency regime of 10^–1–10⁹ Hz. Unmodified polybutadiene shows two relaxation processes, the -relaxation which is correlated to the dynamic glass transition of the polybutadiene, and a -relaxation corresponding to a local relaxation of polybutadiene segments. In the polar functionalized systems, besides these two relaxations, an additional relaxation process (called ^*) is observed, which occurs at lower frequencies than the -process. While the -relaxation remains unaffected by the functionalization the cooperativity of the -relaxation increases by the formation of reversible junctions and slows down considerably. This indicates a decreased mobility of the polymer matrix. At the same time the dipole moment of relaxing units contributing to the -relaxation is increased by free phenyl urazole units. The ^* is assigned to the local complex dynamics resulting from the dissociation and formation of dimeric contacts. Hence, for this dynamic process, the absolute value of the dipole moment fluctuates with time and causes a dielectric absorption. This interpretation is in agreement with the hindered reptation model of Leibler, Rubinstein and Colby and simultaneous measurements of infrared dichroism and birefringence.     

Molecular alignment of ammonia studied by electron-ion-ion coincidence spectroscopy

Electron-ion-ion coincidence measurements carried out at discrete resonances near the N 1s threshold in ammonia are reported. The measured coincidence spectra show clear alignment of the molecule upon resonant core-electron excitation. The coincidence data are analyzed to extract information about the molecule in the excited state by simulating the alignment and the dissociation processes. Dynamic changes in molecular geometry are found as the photon energy is scanned through the N 1s-->4a(1) resonance, whereas for the N 1s-->2e state the geometry and kinetic energy released upon dissociation remain unchanged. The alignment of the core-excited molecules is found to be preserved even in two-step dissociation processes.     

Surface molecular relaxation in smectic C* phase as studied by dielectric spectroscopy

The molecular relaxation process of a ferroelectric liquid crystal with a high tilt angle and a high spontaneous polarization in a homeotropically aligned cell has been studied by the dielectric relaxation method in the frequency range 10 Hz to 10 MHz. The measurements have been done using thin (3.5μm) cells with gold coated electrodes and samples aligned by a magnetic field. It has been observed that the molecular relaxation around the short axis of the molecule is detected in the chiral nematic and smectic C* phases. The surface molecular process is observed in the S*_c phase down to nearly 6 to 7 K below the transition temperature of the N* to the S*_c phase. The experimental results of the surface molecular process are analysed by theoretical calculations. The experimental results agree with the theoretical predictions.     

Mesomorphic and luminescent properties of side chain nematic liquid-crystalline polymers containing Ir(III)

Iridium-containing liquid-crystalline polymers were obtained by graft copolymerisation using poly(methylhydrogeno)siloxane, 1-methyl-4-(4-(4-vinylcyclohexyl)cyclohexyl)benzene (M₁) and an iridium complexes monomer (Ir-M₂). The series of polymers contained different molecular fractions of Ir-M₂ from 0% to 1.2%. All of these polymers showed mesomorphic behaviours. The introduction of small amount of iridium ions endowed liquid-crystalline polymers with luminescent properties. The chemical structures were characterised by IR and ¹H NMR. The mesomorphic properties and phase behaviour were investigated by differential scanning calorimetry, thermal gravimetric analysis, polarising optical microscopy and X-ray diffraction. With an increase of iridium complexes units in the polymers, the glass transition temperature (T_g) did not change significantly; the isotropic temperature (T_i) decreased. All polymers showed typical nematic marble textures, which was confirmed by X-ray diffraction. The temperatures at which 5% weight loss occurred (T_d) were greater than 300°C for the polymers. The introduction of iridium complexes units did not change the liquid-crystalline state of polymer systems. With Ir³⁺ ion contents ranging between 0.6 and 2.4 mol%, luminescent intensity of polymers gradually increased.     

Molecular dynamics simulations of side chain liquid crystal polymer molecules in isotropic and liquid-crystalline melts

A detailed molecular dynamics simulation study is described for a polysiloxane side chain liquid crystal polymer (SCLCP). The simulations use a coarse-grained model composed of a combination of isotropic and anisotropic interaction sites. On cooling from a fully isotropic polymer melt, we see spontaneous microphase separation into polymer-rich and mesogen-rich regions. Upon application of a small aligning potential during cooling, the structures that form on microphase separation anneal to produce a smectic-A phase in which the polymer backbone is largely confined between the smectic layers. Several independent quenches from the melt are described that vary in the strength of the aligning potential and the degree of cooling. In each quench, defects were found where the backbone chains hop from one backbone-rich region to the next by tunneling through the mesogenic layers. As expected, the number of such defects is found to depend strongly on the rate of cooling. In the vicinity of such a defect, the smectic-A structure of the mesogen-rich layers is disrupted to give nematiclike ordering. Additionally, several extensive annealing runs of approximately 40 ns duration have been carried out at the point of microphase separation. During annealing the polymer backbone is seen to be slowly excluded from the mesogenic layers and lie perpendicular to the smectic-A director. These observations agree with previous assumptions about the structure of a SCLCP and with interpretations of x-ray diffraction and small angle neutron scattering data. The flexible alkyl spacers, which link the backbone to the mesogens, are found to form sublayers around the backbone layer.     

Recent development in dielectric relaxation spectroscopy of polymers

An evaluation strategy for dielectric measurements in the frequency and/or time domain is presented which provides complete information about a relaxation mechanism (intensity, position, and shape of the relaxation function) independent of overlapping with neighboring mechanisms. As an example results on poly-(ethyleneterephthalate) are given.