Relaxation spectroscopy of LC main chain polymers: Experimental and theoretical investigations

A series of thermotropic main chain polymers with extended dimethylsiloxane segments was investigated. The study of dielectric properties of polymers revealed the relationship between their ability to form a mesophase and their molecular mobility. The peculiar behaviour of dielectric α-relaxation parameters during the transition from mesomorphic to isotropic members of this polymeric series was shown. The theoretical approaches were developed for understanding macrochain dynamics of liquid crystalline polymers. The theoretical and experimental results are in good agreement.     

Proton and carbon-13 high resolution NMR spectroscopy of high polymers

Using a super-conducting magnet, the examination of high polymers by ¹H- and ¹³C-NMR provides complementary and precise information about their microstructures. The configuration of the polymer skeleton and chemical structures which are present in low concentrations are easily observed. Relationships between polymerization mechanisms, microstructure and properties thus become clearer.     

Relaxation response of polymers containing highly flexible side groups monitored by broadband dielectric spectroscopy

The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10(-1)-10(9) Hz and 123-473 K, respectively. Above the glass transition temperature T(g) the dielectric loss in the frequency domain exhibits a prominent alpha absorption, followed in increasing order of frequencies by two secondary absorptions called beta and gamma. At temperatures slightly higher than T(g), the a relaxation is well separated from the beta, but as temperature increases overlapping between both relaxations augments forming an alphabeta absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the y relaxation. The strength of the a relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the alphabeta absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the beta relaxation also describes that of the alphabeta absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the a relaxation feeds on the beta absorption until its complete disappearance. The gamma relaxation, in turn, seems to increase at the expense of the alphabeta process at high temperature.     

Orientation of amorphous polymers

Orientation of amorphous polymers stretched at a temperature above their glass-transition temperature, is involved in thermoforming processing. The molecular processes controlling the orientation and chain relaxation of polymers have been investigated by infrared dichroism in a large series of materials: polystyrene, polymethylmethacrylate of various tacticity and its copolymers with styrene and acrylonitrile. Polystyrene with hydrogenated and deuterated blocks leads to information on the behavior of each block (central part, chain ends) and allows a quantitative comparison with the Doi-Edwards model for chain relaxation. In order to analyse the effect of polydispersity, blends of hydrogenated and deuerated polystyrene chains with various molecular weights have been studied. Short chains with molecular weights smaller than the molecular weight between entanglements, enhance the relaxation of long chains. Furthermore an anisotropic orientational coupling effect exists between a chain segment and its oriented surrounding. By comparing the orientation of polymers with different chemical structures, it results that they behave differently under temperature conditions where T - T_g = const, but they undergo identical relaxations when the experiments are performed at temperatures chosen in such a way that the monomer friction coefficients are identical. In copolymers of styrene and methylmethacrylate, the two monomer units have different orientations due to local conformational constraints. This effect also accounts for the difference observed between an alternated and a random copolymer.     

Relaxation of entangled polymers in melts

The stress relaxation function of a monodisperse or polydisperse melt, and the corresponding viscosity, have been calculated in the entanglement domain by applying the Doi-Edwards theory which relies on the reptation concept introduced by P. G. de Gennes. Though the theory has been considered as successful, the agreement with precise experiments is only qualitative, and strong anomalies remained to be explained. A new theory is presented here: it is obtained by introducing simple approximations derived from elementary but novel considerations. This theory is shown to be in good agreement with experiments on monodisperse and polydisperse melts. In particular, it explains the well-known fact that the viscosity of a monodisperse polymer melt of molecular mass M seems to increase proportionally to M^3,4 when M is large.     

Meander model of amorphous polymers

Assuming bundles (of shortrange ordered macromolecules, folding back and forth statistically), their equilibrium superstructure and diameter are described on the basis of cluster-entropy-hypothesis (CEH). As primary blocks in the bulk polymer and in thin films coupled meander cubes are most probable, which are linked via their cube diagonals serving as axis of statistical rotation and aggregate to coarse grains. Magnetic birefringence, SANS and elctronmicroscopy are used as further methods to determine the cube side length. Applying the same concept to myosin-, collagen-, and elastin-aggregates, these can be interpreted as equilibrium meander fibrils, additionally stabilized by specific interactions and by the length of the molecules.     

Depolarization thermocurrents in amorphous polymers

The depolarization thermocurrent (DTC) method gives the dependence of the dielectric relaxation time on temperature. It has been used for investigations of relaxations obeying an Arrhenius-like law in crystalline polymers. The analysis of this method shows it is possible to study mechanisms described by the Williams-Landel-Ferry (WLF) equation. The critical temperature appearing in the free-volume theory of Cohen and Turnbull and also in the statistical thermodynamic theory of Adam and Gibbs can then be measured with good accuracy. The thermal coefficient of expansion of the free-volume and the WLF coefficient for any reference temperature can also be obtained. Since analysis of the experimental DTC spectrum is particularly simple, this method seems to be a very useful tool for examination of relaxation transitions in amorphous polymers. As an example, results obtained for poly(methyl methacrylate) are presented; they are consistent with published data.     

Structural relaxation in amorphous polymers

A theoretical calculation based on the general solution of a multi-ordering-parameter model is found to be in good agreement with the measured volume relaxation of poly(vinyl acetate). This suggests that a limiting equilibrium state is eventually reached, which may resolve the disagreement between the behavior of the model and experiments discussed in the literature. In addition, the asymmetric character of the isothermal response and memory effect is satisfactorily calculated from the same basic equation. The distribution function and the temperature-structure dependence of relaxation times are discussed.     

Miscible blends of amorphous polymers

Thirty-five polymethacrylate/chlorinated polymer blends were investigated by differential scanning calorimetry. Poly(ethyl), poly(n-propyl), poly(n-butyl), and poly(n-amyl methacrylate)s were found to be miscible with poly(vinyl chloride) (PVC), chlorinated PVC, and Saran, but immiscible with a chlorinated polyethylene containing 48% chlorine. Poly(methyl) (PMMA), poly(n-hexyl) (PHMA), and poly(n-lauryl methacrylate)s were found to be immiscible with the same chlorinated polymers, except the PMMA/PVC, PMMA/Saran, and PHMA/Saran blends, which were miscible. A high chlorine content of the chlorinated polymer and an optimum CH₂/COO ratio of the polymethacrylate are required to obtain miscibility. However, poly(methyl), poly(ethyl), poly(n-butyl), and poly(n-octadecyl acrylate)s were found to be immiscible with the same chlorinated polymers, except with Saran, indicating a much greater miscibility of the polymethacrylates with the chlorinated polymers as compared with the polyacrylates.     

Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50–300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles (‘holes’). The value of the critical matrix ligament thickness (ID_c) uniquely depends on the molecular structure: at an increasing network density, ID_c increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency.     

Submolecular dynamics of amorphous polymers probed by two-dimensional infrared (2D IR) spectroscopy

Two-dimensional infrared (2D IR) spectroscopy was used to probe the submolecular dynamics of atactic polystyrene. 2D IR is a powerful analytical technique especially suited for the elucidation of localized motions of polymer segments. In 2D IR, a polymer sample is excited by a small-amplitude oscillatory strain at a frequency in the acoustic range. The fluctuation of IR dichroism signals resulting from the strain-induced reorientation of electric dipole-transition moments is monitored with a time-resolved spectrometer. Spectra defined by two independent wavenumber axes are constructed by applying a correlation analysis to such signals. The 2D spectra provide detailed information about the local dynamics of submolecular constituents of the system. From the sign of cross peaks in the synchronous 2D IR spectrum of glassy polystyrene, it is shown that the main-chain backbone of polystyrene reorients in the direction of applied strain. Cross peaks in an asynchronous 2D IR spectrum reveal highly localized reorientational motions of phenyl side groups occurring more or less independently of the main-chain realignment. In the glassy state, the phenyl ring tends to fold back along the main-chain, indicating that there exists a highly constrained local distortion of side groups during deformation.     

Free volume microstructure of amorphous polymers at glass transition temperatures from positron annihilation spectroscopy data

The analysis of annihilation characteristics of ortho-positronium at conventional calorimetric glass transition temperatures for a series of amorphous polymers reveals empirical correlations of average lifetime of o-Ps , and of its product with a relative intensityI _3g with appropriateT _g ^DSC values. These trends in terms of free volume mean that both the average size of free volume hole entityv _hg and the fractional free volume grow with increasingT _g ^DSC . The results are discussed considering the chemical microstructure as well as possible mechanisms acting in glass transition. A relation is indicated between geometric and flexibility characteristics of chains and thev _hg andf _g parameters of free volume microstructure on the one side and potential motional processes responsible for solidification of the amorphous system on the other side.     

Relaxation processes in oriented liquid crystalline polymers

A review is presented of research on oriented thermotropic liquid crystalline polymers undertaken at Leeds University. A range of main-chain polyesters has been studied based on 1,4-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid as the primary monomers. By using a combination of dynamic mechanical, dielectric and NMR measurements insight has been gained into the molecular motions associated with the three principal relaxation processes. The behaviour in tension, for stresses parallel and perpendicular to the principal orientation direction, has been related to that in shear by use of mechanical models.     

Solvent diffusion in amorphous glassy polymers

In this article, a mathematical model is proposed for predicting solvent self‐diffusion coefficients in amorphous glassy polymers based on free volume theory. The basis of this new model involves consideration of the plasticization effects induced by small molecular solvents to correctly estimate the hole‐free volume variation above and below the glass‐transition temperature. Solvent mutual‐diffusion coefficients are calculated using free volume parameters determined as in the original theory. Only one parameter, which can be predicted by thermodynamic theory, is introduced to express the plasticization effect. Thus, this model permits the prediction of diffusion coefficients without adjustable parameters. Comparison of the values calculated by this new model with the present experimental data, including benzene, toluene, ethyl benzene, methyl acetate, and methyl ethyl ketone (MEK) in polystyrene (PS) and poly(methyl methacrylate) (PMMA), has been performed, and the results show good agreement between the predicted and measured values. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 846–856, 2000     

Photoresponsive liquid crystalline and amorphous polymers

Isomerization processes of azobenzene dyes dissolved in a glassy polymeric matrix or attached in glassy amorphous or liquid crystalline polymers to the backbone as side groups are induced by light. The isomerization process, in turn, causes the dye to reorient provided that polarized light is used: the long axis of the dye is oriented perpendicular to the polarization direction in the stationary case. Such a reorientation gives rise to strong modifications of the optical properties. This contribution is concerned with the analysis of the correlation between the nature of the azobenzene dyes, the isomerization, reorientation and modulations discussed above and with possible applications in the optical holographic storage. Considered are, in particular, dye/matrix combinations giving rise to nonlinear holographic responses, two photon holography, transient holographic modes applicable for holographic displays and the optical switching of other than optical properties.     

Magnetic orientation of diamagnetic amorphous polymers

Molecular order in an amorphous polymer with anisotropic magnetic susceptibility is altered by applying external magnetic fields. Disks of atactic polystyrene (a‐PS) are prepared by solvent casting outside or inside a magnet. The effect of the magnetic field on the polymer samples is investigated by magnetic levitation and solid state NMR spectroscopy. Magnetic levitation of the a‐PS disks shows that the orientation of disk symmetry axis with respect to the magnetic field gradient depends on the magnitude and direction of the applied field during casting. Similarly, carbon‐13 two‐dimensional cross‐polarization/magic angle spinning rotor‐synchronized NMR measurements in these samples show modulation patterns of the spinning side bands only on disks prepared in the presence of a magnetic field. These findings suggest that macromolecular order could be induced in a fluid or fluid–solid phase transition with cooperative segmental motions reorienting the diamagnetic susceptibility tensor to minimize the magnetic contribution to free energy of the sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1009–1015, 2010     

Near-infrared spectroscopy of polymers

Although near–infrared (NIR) spectroscopy has been used over many decades for the – primarily quantitative – analysis of polymers containing OH–, NH– and CH–functionalities (e.g. determination of OH–number, water content and residual carbon–carbon double–bonds), it has never been established as a wide–spread analytical and physical tool comparable to other spectroscopic techniques. In the late seventies, however, two new developments have initiated a renaissance of NIR spectroscopy in analytical chemistry. On the one hand, chemo–metric data evaluation techniques have – in combination with diffuse reflection measurements – opened up the possibility of non–destructive, rational multicomponent analysis and identity control of solid polymers with varying morphologies. On the other hand, the introduction of optical light fibres has contributed to an enormous expansion of conventional NIR spectroscopy in terms of remote control. Thus, specific fibre–optic probes allow a separation of the spectrometer and the location of sample measurement over several hundred meters and tremendously alleviate the analysis of toxic and hazardous materials including process and reaction control. Recently, further progress is made by the development of new, rapid-scan NIR monochromator systems without mechanically moved parts, such as acousto–optic tunable filters. Last, but not least, it should be mentioned, that – although not treated here – the new approach of Fourier–Transform Raman spectroscopy with Nd–YAG laser excitation at 1064 nm is principally also a near–infrared technique.     

Hole-burning spectroscopy on organic molecules in amorphous silica

Amorphous silica samples of high optical quality, doped with organic molecules, were prepared at room temperature by the sol-gel method. Spectral hole-burning experiments were performed in the S₁←S₀ absorption bands of chlorin (2,3-dihydroporphyrin) and of oxazine-4 perchlorate. Homogeneous linewidths of 380 and 750 MHz were obtained at 1.7 K. The temperature dependence between 1.7 and 5.7 K was T^145±0.07 for chlorin and T^127±0.05for oxazine-4.