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.     

Methyl rotational barriers in amides and thioamides

The methyl rotational barriers for a series of N-methyl-substituted amides and thioamides have been calculated at the MP2/6-311+G** level. A comparison of the N-methylformamide and methyl formate barriers indicates that the H [bond] C(Me) [bond] N [bond] H eclipsed torsional arrangement destabilizes an amide by about 0.8 kcal/mol. A comparison of thioamides and amides showed the importance of steric repulsion between the sulfur and a methyl hydrogen in the Z-forms of the thioamides. The C [bond] N bond rotation transition states of the N,N-dimethyl amides have much larger methyl rotational barriers than found in the ground states. They can be attributed to the smaller CH(3)(-)N [bond] CH(3) bond angles in the transition states.     

Conformational populations and rotational barriers in esters

The $\text{trans}$ conformations of methyl, ethyl and isopropyl formate were shown to be present in equilibrium in a polar solvent with the $\text{cis}$ conformations to significant (> 1% 230 K) but much lower extents than for t-butyl formate; rotational barriers for the former compounds are greaters.     

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.     

Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.     

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.     

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.     

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     

Modelling of solid polymers in chemical detail - gases in amorphous polymers

The diffusion of gases in dense polymers, above and below the glass-transition temperature, is described with a new Transition State Theory model that is based on the concept that the dynamics of small molecules dissolved in dense polymers is separated from the structural relaxation of the dense polymers. The model is used to study the dynamics of rare gases dissolved in atomistic micro-structures of four polymers at 300 K: poly(dimethylsiloxane), poly(isobutylene), atactic poly(vinylchloride) and the polycarbonate of 4,4′-isopropylidenediphenol (bisphenol-A). Short-time-scale MD runs (5 ps) are used to characterize the elastic thermal motion of the host matrix; this information on mobility is then used for a stochastic simulation of solute dynamics up to ca. 1ms. All dissolved molecules show similar behavior by displaying three time regimes: a short-time, high-mobility domain, an intermediate time domain of anomalous diffusion, and a diffusive regime at long times. From the long-time data diffusion coefficients are estimated; comparison with experimental data results in good agreement.     

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     

The nature of the rotational barriers in simple carbonyl compounds

The rotational barriers around the CO and CC bonds are investigated in formic acid, ethanedial and glycolaldedyde molecules on the basis of DFT-B3LYP/aug-cc-pVDZ calculations. Natural bond orbitals analysis is applied to enhance physical understanding of rotational barriers. In the case of attractive barriers in formic acid and Gc-glycolaldehyde, the barrier originates from the loss of hyperconjugation that determines the equilibrium structures while for the repulsive barriers in ethanedial and Go-glycolaldehyde, both Lewis and hyperconjugation terms contribute.     

Density fluctuations in amorphous and semicrystalline polymers

Summary The small-angle scattering of amorphous and semicrystalline polymers contains an intensity component due to density fluctuations within the crystalline and amorphous domains.For amorphous polymers, the density fluctuations aboveT _g correspond to the theoretical value for a fluid system in thermodynamic equilibrium. BelowT _g , a temperature dependence proportional to T is observed over a range of about 50°. At lower temperatures, a linear relationship with a smaller slope has been found which extrapolates to a non-zero value at 0 °K. This value corresponds to the frozen-in disorder, the slope at low temperatures is related to thermal vibrations and can be evaluated in terms of photon-phonon scattering.Semicrystalline polymers show a temperature dependence of the density fluctuation similar to that of the amorphous polymers. At constant temperature the density fluctuations vary linearly with crystallinity.Natural rubber shows an increase of the density fluctuations with increasing cross-linking densities from which information on the density changes in the vicinity of a cross-link and on the statistics of the distribution of cross-linking can be obtained.

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Zusammenfassung Die Kleinwinkelstreuung amorpher und teilkristalliner Polymere besitzt eine Intensitätskomponente, die von Dichtefluktuationen innerhalb der kristallinen und amorphen Bezirke herrührt. Für amorphe Polymere entspricht die Dichtefluktuation oberhalb vonT _g dem theoretischen Wert für ein fluides System im thermodynamischen Gleichgewicht. UnterhalbT _g wird eine Temperaturabhängigkeit proportional zuT über einen Bereich von etwa 50° beobachtet. Bei tieferen Temperaturen wird eine lineare Beziehung mit einer geringeren Steigung gefunden, welche zu einem endlichen Wert bei 0 °K extrapoliert werden kann. Dieser Wert bezieht sich auf die eingefrorene Fehlordnung, die Steigung bei tiefen Temperaturen ist auf thermische Schwingungen zurückzuführen und kann als Photon-Phonon-Streuung ausgewertet werden.Teilkristalline Polymere zeigen eine Temperaturabhängigkeit der Dichtefluktuation, die der von amorphen Polymeren ähnlich ist. Bei konstanter Temperatur ändert sich die Dichtefluktuation linear mit der Kristallinität.Naturkautschuk zeigt eine mit der Vernetzungsdichte ansteigende Dichtefluktuation, aus der man Information über die Dichteänderung in der Umgebung eines Netzpunktes und die Statistik der Netzpunktverteilung erhalten kann.

     

Diffusion of large penetrant molecules in amorphous polymers

A new version of the free-volume theory of diffusion is modified to describe self-diffusion phenomena in polymer + solvent systems for solvents which do not move as single units. The free-volume equations are used to interpret data which appear to illustrate that some large solvent molecules do indeed move in a segmentwise manner. The dependence of the effective activation energy for diffusion on solvent size is examined, and the importance of energy effects on the diffusion process is considered.     

Visualization of deformation-induced structural rearrangements in amorphous polymers

A technique is proposed for decorating amorphous polymers: Before the deformation (shrinkage) of an amorphous polymer, its surface is decorated with a thin metal coating. The subsequent deformation is accompanied by surface structure formation, which makes the processes that occur in the polymer visible. The proposed technique makes it possible to visualize and describe the mechanism of transfer of the polymer from the surface into the bulk and vice versa and to obtain direct information about the direction of the actual local stress. The technique makes it possible to obtain information about the topological heterogeneity of rubber networks, to reveal the features of structural rearrangements that occur during the cold rolling of amorphous polymers, and to describe the phenomenon of self-elongation during annealing of the oriented PET. These microscopic data explain the following features of the structural and mechanical behavior of glassy polymers from a unified viewpoint: stress relaxation in a polymer in the elastic (Hookean) region of the stress-strain curve, an increase in stress in a deformed glassy polymer during its isometric annealing below T _g, the low-temperature shrinkage of a deformed polymer glass in the strain range below its yield point, the storage of internal energy in a deformed glassy polymer in the strain range below the yield point, some anomalies of thermophysical properties, and some other features.     

Surface segregation in blends of miscible amorphous polymers

The isothermal luminescence decay kinetics in near-surface nanolayers of plasma-activated bulk samples of amorphous polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and their miscible blends with weight ratios PS/PPO of 75/25 and 50/50 has been studied at 77 K. The intensities of isothermal luminescence (I) of homopolymer and blend surfaces have been compared. It has been found that the ratio between the luminescence intensities for PS and PPO (I(PS)/I(PPO)) may be as high as 50, while the luminescence intensities for the PS–PPO blends are close to I(PPO). The results obtained indicate that the PPO concentration in the surface layers of the blends is higher than that in the bulk.     

The rate of solvent diffusion in amorphous polymers

The present work is a contribution to our understanding of one aspect of the diffusion process, the diffusion rate. It attempts to show that all diffusion theories must satisfy the following: 1) the rate of diffusion at the initial stages of the process must be finite 2) the rate of diffusion must have square root time dependence at longer diffusion distances. Deviations of experimental data from these rules usually result from experimental inaccuracies. Whereas Fickian approach satisfies the second but not the first rule and Case II sorption the opposite, their combination satisfies neither. Two alternative explanations, which provide a very good correlation with the experimental data, are suggested: Limited diffusion rate and sorption kinetics.