Analysis of orientation fluctuation in fluids by small angle X-ray scattering

The small angle X-ray scattering of molecular fluids contains information on particular aspects of their orientational order. Examples are given for the case of the isotropic, nematic and cholesteric phases of mesogenic molecules. It is shown that the distribution of the molecular centers relative to the direction defined by the molecular long axes can be analysed by means of small angle X-ray scattering. An approximate expression for the circulation correlation function is given.     

Structure analysis of side chain liquid crystal polymer films by means of electron microscopy

Using the combined techniques of electron diffraction, bright and dark field electron microscopy as well as light microscopy, it has been possible to obtain detailed structural information about the arrangement of the smectic layers in a polymethacrylate side chain liquid crystal polymer with a biphenylester as the mesogenic group.     

Linear viscoelasticity of side chain liquid crystal polymer

Small amplitude oscillatory shear has been used to study thermotropic liquid-crystalline polymers that have mesogenic groups pendant to flexible backbones. The polymers studied form nematic and smectic glasses, enabling viscoelastic response to be studied over a wide range of frequencies using time-temperature superposition. In contrast to main chain liquid-crystalline polymers, the nematic side chain polymers exhibit linear viscoelastic response over a wide range of strain amplitudes that is independent of thermal and shear histories. Viscoelastic response is very sensitive to smectic-nematic and smectic-isotropic transitions, but insensitive to the nematic-isotropic transition, as time-temperature superposition applies across this transition. We compare viscoelastic data with diffusion data by calculating the time τ that it takes a polymer to diffuse a distance equal to its coil size R (τ=R₂/D). At frequencies lower than 1/τ side chain polymers in their nematic show the terminal response characteristic of viscoelastic liquids. In their smectic, they are still strongly viscoelastic at frequencies lower than 1/τ and approach the terminal response of a viscoelastic solid at the lowest frequencies. Implications of such behaviour are discussed.     

Structure of maltodextrin gels — a small angle X-ray scattering study

The structure of maltodextrin gels was investigated by small angle X-ray scattering. The inhomogeneities in the maltodextrin gels are molecular aggregates with maximum dimensions of almost 300 nm. Their shape can be approximated by oblate ellipsoids of revolution with an axial ratio of 0.1. The radius of gyration of the aggregates amounts to about 90 nm. For this reason in the nomenclature of Papkov the gels are polymer gels of the 2^nd type. The melting of these aggregates is measured by SAXS with a position sensitive detector in the range near 56 °C.     

A small angle neutron scattering study of the conformation of a side chain liquid crystal poly(methacrylate) in the smectic C phase

The conformation of the backbone in the side chain liquid crystal polymer poly\[ omega (4-methoxybiphenyl-4-yloxy)butyl methacrylate] has been studied in the smectic C and nematic phases. Small angle neutron scattering experiments were performed on mixtures of molecules with perdeuteriated backbones and unlabelled molecules. The polymer is found to adopt an oblate conformation in the smectic C phase. The components of the radius of gyration parallel and perpendicular to the director are determined as a function of temperature from Guinier plots of the SANS data. The radii of gyration do not vary across the smectic phase and are found to be Rg,||=(27+/-1)A, Rg, =(42+/-1)A. These results are compared with recent SANS results for other side chain liquid crystal polymers.     

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.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2?wt?%) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4'-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

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.     

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.     

X-ray small angle scattering of bulk polyethylene

Summary A computor method for slit-height correction is described, in which the experimental X-ray small angle scattering curve is represented by aFourier series. It is particulary applicable to curves showing broad maxima or shoulders, like those of solid polymers.

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Zusammenfassung Es wird eine Methode zur elektronischen Berechnung der Entschmierung beschrieben, bei der die Röntgen-Kleinwinkelstreuungskurve durch eineFouriersche Reihe dargestellt wird. Sie eignet sich insbesondere für Kurven mit breiten Maximalbereichen oder Schultern, wie sie bei festen Polymeren erhalten werden.

     

Crystallization and chain conformation of semicrystalline and amorphous polymer blends studied by wide‐angle and small‐angle scattering

We examine the crystallization and chain conformation behavior of semicrystalline poly(ethylene oxide) (PEO) and amorphous poly(vinyl acetate) (PVAc) mixtures with wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering (SAXS), and small‐angle neutron scattering (SANS) experiments. For blends with PEO weight fractions (wt_PEO) greater than or equal to 0.3, below the melting point of PEO, the WAXD patterns reveal that crystalline PEO belongs to the monoclinic system. The unit‐cell parameters are independent of wt_PEO. However, the bulk crystallinity determined from WAXD decreases as wt_PEO decreases. The scattered intensities from SAXS experiments show that the systems form an ordered crystalline/amorphous lamellar structure. In a combination of WAXD and SAXS analysis, the related morphological parameters are assigned correctly. With the addition of amorphous PVAc, both the average amorphous layer thickness and long spacing increase, whereas the average crystalline layer thickness decreases. We find that a two‐phase analysis of the correlation function from SAXS, in which the scattering invariant is linearly proportional to the volume fraction of lamellar stacks, describes quantitatively the crystallization behavior of PEO in the presence of PVAc. When wt_PEO is close to 1, the samples are fully spaced‐filled with lamellar stacks. As wt_PEO decreases from 1.0 to 0.3, more PVAc chains are excluded from the interlamellar region into the interfibrillar region. The fraction outside the lamellar stacks, which is completely occupied with PVAc chains, increases from 0 to 58%. Because the radius of gyration of PVAc with a random‐coil configuration determined from SANS is smaller than the average amorphous layer thickness from SAXS, we believe that the amorphous PVAc chains still persist with a random‐coil configuration even when the blends form an ordered structure. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2705–2715, 2001     

Analysis of spherical polyelectrolyte brushes by small angle X-ray scattering

Spherical polyelectrolyte brushes （SPBs） with PS core and poly（acrylic acid） （PAA） brushes were prepared and analyzed by SAXS in this article. A radial electron density profile of SPB was brought up, which fits well with the SAXS result and shows a core-shell structure. The effect of pH on SPB form was represented by SAXS and it proves that the chains of SPB will stretch in response to increased pH owning to the increased electrostatic repulsion. SPBs immobilized with magnetic nanoparticles or bovine serum albumin （BSA） were prepared and analyzed by SAXS as well. SAXS could characterize the changes of electron density inside brushes of SPBs due to the immobilization of magnetic nanoparticles or BSA. This provides significant supports for further application of immobilized metal nanoparticles or proteins.     

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.     

Conformation and orientation of tetraalanine in a lyotropic liquid crystal studied by nuclear magnetic resonance

The (1)H NMR spectra of two isotopomers of tetraalanine deuterated on the two external methyl groups and on the two internal ones, respectively, were recorded in the lyotropic solvent cesium pentadecafluorooctanoate (CsPFO)/water. Eight dipolar couplings could be estimated from the spectra. The set of dipolar couplings was fitted assuming that one rigid conformer is present. Of the four major conformers considered, selected on the basis of theoretical calculations, the one characterized by the two couples of internal dihedral angles in the Ramachandran region of PPII resulted to be the only one to fit the set of couplings within experimental error. The data indicate that the molecule is oriented with the long molecular axis tilted with respect to the surface of the micelles formed by CsPFO.     

Analogy between liquid crystal side chain polysiloxanes and liquid crystal twins with siloxane spacers

Abstract

The goal of this present work was to compare the properties of dimers with a siloxane spacer to those of side chain polysiloxanes including the same mesogenic groups and the same dilution with dimethylsiloxane segments. A systematic study of the influence of the dilution parameter on the mesomorphic and structural properties of dimers and diluted side chain polysiloxanes, as well as on the behaviour of their solutions with low molar mass liquid crystals is reported.     

Analogy between liquid crystal side chain polysiloxanes and liquid crystal twins with siloxane spacers

The goal of this present work was to compare the properties of dimers with a siloxane spacer to those of side chain polysiloxanes including the same mesogenic groups and the same dilution with dimethylsiloxane segments. A systematic study of the influence of the dilution parameter on the mesomorphic and structural properties of dimers and diluted side chain polysiloxanes, as well as on the behaviour of their solutions with low molar mass liquid crystals is reported.