Chain segment ordering in uniaxially strained networks: A deuterium NMR investigation

The deuterium NMR (²H-NMR) is used for probing the chain segment orientation in polymer networks under uniaxial stress. The method is based on the observation of an incomplete time averaging of quadrupolar interactions affixed to deuterated segments. The samples are end-linked polydimethylsiloxane networks. The ²H-NMR experiments are performed either on labelled network chains or an labelled probe polymer chains dissolved in the network. The basic results are the following: — The induced uniaxial order is related to a uniaxial dynamics of chain segments around the direction of the applied constraint. — A permanent orientation is observed on free polymer chains dissolved in the deformed networks. — The mean degrees of orientational order induced along short and long chains in bimodal networks are the same. These experimental facts appear as evidences for cooperative orientational couplings between chain segments in the deformed networks.     

Segmental orientation and chain relaxation of polymers by fourier transform infrared dichroism

Fourier transform infrared dichroism has been used to investigate molecular orientation in polymeric materials. It is first applied to characterize network behavior in some elastomeric systems such as model networks of poly(dimethylsiloxane). The strain dependence of segmental orientation is analyzed through networks of known degree of cross-linking and experimental results are compared with calculation predictions based on the rotational isomeric state formalism. Infrared dichroism spectroscopy has also been used to analyze orientational relaxation in binary blends of long and short polystyrene chains. The effect of short deuterated chains (M_w = 3000 to 72000) on the orientational relaxation of long entangled chains (Mw = 2 000 000) is examined in the bidisperse melts uniaxially deformed above the glass transition temperature. While the long chain relaxation is found to be dependent on the short-chain concentration, the local orientational order of the latter is molecular weight dependent in agreement with the classical relaxation theories.     

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete 2H and 13C NMR Study

A liquid crystal ( HZL 7/* ) containing an (S)‐2‐methylbutyl‐(S)‐lactate unit in the chiral chain, is investigated by means of ²H and ¹³C NMR spectroscopy in order to obtain information on its orientational order, its molecular structure and the effect of external magnetic fields on the supramolecular structure of its phases. This mesogen presents very peculiar mesomorphic properties and exhibits frustrated TGBA* and TGBC* phases in a wide temperature range up to 60 °C, as well as an additional phase transition from TGBC₁* to TGBC₂*. ²H NMR measurements show, for the first time, a peculiar magnetic field effect in unwinding the supramolecular structure of both the TGBA* and TGBC* phases. This effect is particularly evident at higher magnetic fields, while different behaviour is observed at lower magnetic fields. This indicates that the supramolecular structure is very sensitive to magnetic fields of the order of 1 Tesla. Moreover, the analysis of the ²H and ¹³C NMR spectra of HZL 7/* allow us to obtain several structural properties, such as the tilt angle of the TGBC* phases and the local orientational order parameters referred to the phenyl and biphenyl fragments. This is the first structural characterization of the frustrated phases of these complexes by means of NMR.     

Dynamic uniaxiality in strained networks: A deuterium NMR investigation

The orientational order generated in a polymer network by a uniaxial stress is probed using deuterium NMR. The experiments are performed on end-linked polydimethylsiloxane networks. When a uniaxial force is applied, the observed NMR spectra show that the chain segments reorient uniaxially around the external force direction. This experimental fact appears as new evidence for cooperative orientational interactions between chain segments in the deformed networks.     

Microscopic theory of orientational order, structure and thermodynamics in strained polymer liquids and networks

A microscopic integral equation theory of the segmental orientational order parameter, structural correlations and thermodynamics of strained polymer solutions, melts and networks has been developed. The nonclassical problem of the consequences of intermolecular excluded volume repulsions and chain connectivity is addressed. The theory makes several novel predictions, including effective power law dependences of the orientational order parameter on monomer concentration and chain degree of polymerization, and strain hardening of the bulk modulus. The predictions of a nearly classical strain dependence, and supralinear scaling with segment concentration, of the strain-induced nematic order parameter is in agreement with nuclear magnetic resonance experiments. The absolute magnitudes of the a priori calculated orientational order parameter agree with simulations and experiments to within a factor of 2. The possible complicating influence of "trapped entanglements" in crosslinked networks is discussed. Extensions of the theory are possible to treat the mechanical response of flexible polymer liquids and rubbers, and the structure, thermodynamics, and mechanical properties of strained liquid crystal forming polymers.     

Solid-state 13C NMR characterization of molecular orientation of hot drawn nylon 6

Two-dimensional cross polarization (CP), magic angle spinning (MAS) rotor synchronization NMR spectroscopy has been used to determine quantitatively the molecular orientational distribution function on hot-drawn Nylon 6. Both films and fibers are studied that had been thermally deformed at temperature above T_g, from 60 to 175°C at draw ratios in the range of 1-5.5. In the two-dimensional NMR spectrum, the sidebands that intrinsically originate from the chemical shift anisotropy reveal the degree of molecular orientational order. No preferential orientational order is detected for the sample without thermal deformation, and the highest degree of order is observed for samples which have been hot drawn above T_g at ratios ca. 5. Based on the aggregate model the maximum achievable order parameters are determined. © 1994 John Wiley & Sons, Inc.     

Atomistic Simulation of Bulk Mechanics and Local Dynamics of Amorphous Polymers

Summary: In order to have better insight into the polymer specifics of the dynamic glass transition molecular dynamics (MD) computer simulations of three glass-formers have been carried out: low-molecular-weight isopropylbenzene (iPB), brittle atactic polystyrene (PS) and tough bisphenol A polycarbonate (PC). Simulation of the uniaxial deformation of these mechanically different types of amorphous polymers shows that the mechanical experimental data could be realistically reproduced. Now the objective is to study the local orientational mobility in the non-deformed isotropic state and to find the possible connection of the segmental dynamics with the different bulk mechanical properties. Local orientational mobility has been studied via Legendre polynomials of the second order and CONTIN analysis. Insight into local orientational dynamics on a range of length- and time scales is acquired. The fast transient ballistic process describing the very initial part of the relaxation has been observed for all temperatures. For all three simulated materials the slowing down of cage escape (α-relaxation) follows mode-coupling theory above T_g, with non-universal, material-specific exponents. Below T_g universal activated segmental motion has been found. At high temperature the α process is merged with the β process. The β process which corresponds to the motions within cage continues below T_g and can be described by an activation law.     

Studies of solution dynamics of poly(N-vinyl pyrrolidone) and its iodine adduct

Carbon-13 NMR spin-lattice relaxation times T₁ of poly(N-vinyl pyrrolidone) (PVP) and PVP-iodine have been studied in several solvents and at different temperatures. Three kinds of motion can be identified from the T₁ data: segmental motion, ring rotation, and ring puckering. The effective correlation time for segmental motion is calculated to be 1 × 10^?9s, in good agreement with published proton NMR data. Another solvent, 1,1,2,2-tetrachloroethane, behaves like D₂O, the segmental correlation time being 3 × 10^?9s. In benzene, however, the linewidths are very broad and tend to narrow with increasing temperature, but the T₁s are not very different from those of PVP in D₂O. The results suggest association of pyrrolidone rings in benzene that reduces chain dimensions and also restricts chain mobility. As for PVP-iodine in water, again broad resonances are observed which sharpen considerably at higher temperatures. The result agrees with previous suggestions of specific interactions between the pyrrolidone group and iodine.     

Ferroelectric liquid crystals derived from isoleucine II. Orientational ordering by carbon-13 separated local field spectroscopy

The orientational ordering of a series of ferroelectric liquid crystals 4'-[(2S, 3S)-3-methyl- 2-halopentanoyloxy]-4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates have been studied by two-dimensional carbon-13 separated local field spectroscopy combined with off-magic-angle spinning. All carbons are well-resolved in the carbon-13 NMR spectrum so that carbon-proton dipolar coupling constants for all carbons in the liquid crystal molecule can be determined, giving detailed segmental orienta- tional ordering information. (The order parameters of the biphenyl core segments and each carbon-proton bond in the aliphatic chains were measured as a function of temperature.) The results show that the substitution of different halogens on the chiral chain affects the carbon- proton bond orientational ordering of the entire chiral chain, while the orientational ordering of the rest of the liquid crystal molecule (core and achiral chain) remains unchanged. For the fluoro-substituted liquid crystals, carbon-fluorine dipolar couplings are also observed. At the SmA-to-SmC* transition, there is a discontinuous change in the magnitudes of all C-H bond order parameters. However, the aliphatic ordering relative to the core ordering is unchanged, suggesting that there is a negligible change in the mesogen conformation and the environment of the aliphatic chain at this transition, i.e. there is no evidence for substantial 'chiral interactions' in the SmC* phase.     

N.M.R. study of segmental molecular interactions in liquid crystals

Abstract

A simple statistical model of interacting non-rigid molecules, based on a perturbation expansion of the pair correlation function and the additivity of segmental interactions, is applied to the study of orientational order as measured by N.M.R. in the nematic and S_A phases of 4-n-alkyl-4′-cyanobiphenyls (N-CB, N = 5 to 8), the nematic and S_C phases of 4-n-alkyloxybenzoic acids (N-OBA, N = 7, 8) and the D _h0 columnar discotic phase of hexa-alkyloxytriphenylenes (N-THE, N = 5 to 8). The order parameters of each homologous series are correctly described in terms of two isotropic and two anisotropic segmental coupling constants. The model predicts certain relations among coupling constants pertaining to different homogous series. These predictions are supported by the results obtained for the three types of compounds studied.     

The optical anisotropy and orientational order parameter of two mesogens having slightly different flexible side chain – a comparative study

Variation of the phase transition temperatures and existence of different number of mesophases, due to the changes of -CH₂ groups at the oxygen containing flexible side chain between two Schiff’s base liquid crystals compounds p-ethoxybenzylidine p-heptylaniline (EBHA) and p-butoxybenzylidine p-heptylaniline (BBHA) have been studied by using POM and DSC. Refractive indices as a function of temperature of these two compounds have also been measured by the well-known thin prism technique. To get the best accuracy of refractive indices data a comparison between the experimental data and theoretical values which are obtained from modified four-parameter model based on Vuks’ hypothesis have been done. A comparative study of some important parameters such as, optical anisotropy, orientational order parameters of these two liquid crystal compounds have also been conducted.     

Determination of the orientational ordering of 4′-cyanophenyl-4-alkylbenzoates by 13C NMR

Abstract

The orientational ordering of three 4′-cyanophenyl-4-alkylbenzoates (with number of carbons in the alkyl chain, n = 6,7 and 8; hereafter abbreviated as n-CPBs) has been investigated by ¹³C NMR. The order parameters of different molecular segments in the nematic phase of the n-CPBs were determined by the two-dimensional technique of separated local field (SLF) spectroscopy combined with off-magic-axis, variable-angle spinning (VAS) of the sample. The carbon-13 chemical shifts for each carbon nucleus in these compounds were determined by slowly spinning the sample parallel to the applied magnetic field. The order parameters obtained from SLF/VAS studies are linearly related to the corresponding anisotropic carbon-13 chemical shifts. These results provide a convenient way to obtain the order parameters for other homologous members of this liquid crystal series by direct measurement of only their carbon-13 chemical shifts in conjunction with the observed linear relationship between order parameters and chemical shifts.     

Induced orientation of free chains in a strained rubber: A deuterium magnetic resonance study

Linear free polydimethylsiloxane (PDMS) chains are dissolved in a PDMS model network. When the system is uniaxially stretched, the free chains, as well as the network chains themselves, acquire a uniaxial orientational order, which is monitored by ²H NMR. The induced orders measured on both kinds of chains are comparable. The order is then investigated as functions of the free chain length and concentration.     

Thermodynamics, orientational order and elasticity of strained liquid crystalline melts and elastomers

A microscopic polymer liquid-state theory has been developed for the structure, thermodynamics and mechanical properties of strained liquid crystalline elastomers. The theory captures the experimentally observed phenomenon of spontaneous distortion and establishes a direct correlation between it and the nematic order parameter. Strain induced softening of the elastic modulus is predicted to emerge due to coupling of the induced orientational order and anisotropic interchain excluded volume interactions. Comparison of our results with limited experiments shows good qualitative and sometimes quantitative agreement. The theory predicts that deformation in the liquid crystalline state results in an increase of the amplitude of density fluctuations (compressibility) which becomes more pronounced as chain degree of polymerization and/or segmental density are decreased.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

The 1H NMR spectrum of pyrazole in a nematic phase

The experimental ¹H nuclear magnetic resonance (NMR) spectrum of 1H‐pyrazole was recorded in thermotropic nematic liquid crystal N‐(p‐ethoxybenzylidene)‐p‐butylaniline (EBBA) within the temperature range of 299–308 K. Two of three observable dipolar D_HH‐couplings appeared to be equal at each temperature because of fast prototropic tautomerism. Analysis of the Saupe orientational order parameters using fixed geometry determined by computations and experimental dipolar couplings results in a situation in which the molecular orientation relative to the magnetic field (and the liquid crystal director) can be described exceptionally by a single parameter. Copyright © 2016 John Wiley & Sons, Ltd.     

Magnesium and calcium surfactants Ternary phase diagrams of magnesium and calcium dodecylsulphate with decanol and water

The isothermal ternary phase diagrams for the systems magnesium dodecylsulphate-decanol-water at 40 °C and calcium dodecylsulphate-decanol-water at 50 °C are determined by water deuteron NMR and polarizing microscopic studies. In the magnesium system, three liquid crystalline phases (lamellar and normal and reverse hexagonal) and two isotropic (normal and reverse) solution phases are characterized and their ranges of existence are obtained. The calcium system yields the same liquid crystalline phases, but only the lamellar liquid crystalline phase is investigated in detail. The important observations made are: (i) The lamellar liquid crystalline phase for the magnesium and calcium systems can incorporate, respectively, a maximum of 22.5 and 14.3 mole water per mole surfactant ion against 139 mole water for the corresponding sodium system. (ii) The reverse hexagonal liquid crystalline phase is formed for both the magnesium and calcium systems while no such liquid crystalline phase exists for the corresponding sodium system. (iii) The²H NMR quadrupole splittings obtained in the liquid crystalline phases for C₈SO ₄ ^– and C₁₂SO ₄ ^– surfactant systems with different counterions (Ca²⁺,Mg²⁺,Be²⁺,Na⁺) reveal that surfactant hydration is almost independent of alkyl chain length and counterions.     

The nature and origin of structural defects in polymeric sulfur nitride

Defect structures in (SN)_x crystals obtained by solid-state polymerization are so numerous and varied that they are likely to conceal inherent property aspects of the defect-free polymer. In addition to frequent macroscopic twinning, three different kinds of defect sites are observed. These sites are related to the disruption of molecular orientational order about the chain axis direction and the sequencing of chain types in the periodic phase. The origin of these defects is explained by the nonuniqueness of solid-state polymerization and recrystallization processes, which transform the initial dimer phase to the final polymer phase. The orientational relationship between the dimer and polymer phase is predicted from the observed twinning mode and the structural relationship between these phases. The change from dimer phase (at zero and at intermediate conversions involving solid solution formation) to polymer phase can be described by a shear transformation on the (001) dimer plane. This observation suggests an analogy with known martensitic reactions and a method for improving the perfection of (SN)_x.     

Correlation between the mechanical properties and the homogeneity of some photopolymerized acrylic/methacrylic networks1

Proton T_1ρ data obtained by CP/MAS solid-state ¹³C NMR at 100.56 MHz for different polyacrylate networks were correlated with their mechanical properties in order to determine the best mixture in terms of stress-at-break (σ), Young's modulus of elasticity (Y), and homogeneity of the monomers that are forming the solid network. The mixtures were formed by the photopolymerization of 40% trimethylolpropane triacrylate (TMPTA), 40% dipentaerythritol pentaacrylate (DPHPA), and 20% of a monomer from a series of poly(ethylene glycol) and poly(ethylene) dimethacrylate. Proton T_1ρ's in networks were not quite averaged to a single value by spin diffusion as in polymer blends, indicating extensive intermixing of TMPTA and DPHPA in forming the network, but with phase separation between the dimethacrylate homopolymer and the rest of the network that is dependent on the laser power. The stress-at-break and the Young's modulus of elasticity were determined for each of the mixtures at different values of the laser power used for the polymerization. © 1996 John Wiley & Sons, Inc.