NMR studies of molecular mobility in uniaxially stretched oriented polymers

The effect of elasticlike uniaxial tension on molecular mobility in polymers have been studied over a wide temperature range using the broad-line NMR technique. The studies were carried out on oriented semicrystalline samples of nylon 6, poly(ethylene terephthlate), polypropylene, polyethylene, polyoxymethylene, poly(vinyl alcohol), and polytetrafluorethylene. Above the low-temperature transition the NMR spectra are reversibly transformed under tension. Increases in the second moments of the spectra are attributed to weaker molecular motion in stressed polymers. The only exception is polypropylene, in which the reverse, i.e., enhancement of molecular mobility, can be observed in a certain temperature range. In the spectra of polymers stretched above the glass transition temperature the narrow component decreases, thus indicating inhibition of micro-Brownian motion, a phenomenon we call “mechanical vitrification.” Such mechanical vitrification is proved to result from reduction in the number of possible tie-chain conformations in the non-crystalline regions and not from closer packing of chains. In discussing the results we use the experimental data on the reduction of the number of gauche isomers under tension (on average, one transition of a gauche link to the trans state causes at least five methylene groups in the main chain to become immobile). The results of studies of molecular mobility in stretched polymers are used for more accurate definition of the mechanisms of molecular motion at different temperatures. A method for evaluation of the energy of intermolecular interactions which hinder small-scale motion at low temperature is suggested.     

Transport properties of uniaxially oriented aliphatic polyketone

The oxygen, carbon dioxide, and water‐transport properties of a uniaxially oriented aliphatic polyketone were determined. The polyketone was drawn to 5–10 times its original length. The transport properties were related to changes in crystallinity estimated by differential scanning calorimetry and density measurements and by changes in the molecular and crystal orientation assessed by, respectively, infrared and X‐ray spectroscopy. The film structures were characterized by confocal scanning laser microscopy and scanning electron microscopy. Stress‐strain tests on the drawn specimens enabled the impacts of orientation on the transport and mechanical properties to be compared. A draw‐induced increase in crystallinity and molecular orientation yielded permeabilities at a draw ratio of 10 that were 30–40% of the original value, and the percentage decrease was basically independent of the type of gas/vapor molecule. Also, the diffusivities of oxygen and carbon dioxide decreased by an order of magnitude. The fact that the amorphous permeability was peaking at a draw ratio of about 5 was a consequence of a peak in amorphous solubility, which was very high for oxygen and absent for water. It was suggested that the peak in solubility was mainly caused by the destruction of the polymer hydrogen‐bond network during drawing and crystal reorientation. The impact of structural reorganization within the polymer and presence of surface valleys seemed to have less impact on the mechanical properties than on the transport properties. This suggested that transport data are more sensitive than mechanical data in probing material defects and changes in molecular packing and morphology. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 947–955, 2004     

Laser Raman scattering in uniaxially oriented atactic polystyrene

Raman polarization measurements of atactic polystyrene (APS) films were recorded after uniaxial stretching. Twelve different polarization directions were observed by varying the direction and polarization of the laser and scattered radiation. The polarization behavior of several vibration modes were shown clearly in the Raman spectra. On the basis of these polarization properties and previous vibrational studies of model compounds of polystyrene as well as of monosubstituted benzene derivatives, a reassignment of some of the vibrational modes of APS is made and discussed.     

Dynamic rheo-optical characterization of uniaxially oriented polyamide 11

Dynamic mechanical analysis, coupled with polarized step-scan FTIR transmission spectroscopy, has been used to monitor the submolecular motional behavior of uniaxially oriented polyamide 11. The dynamic in-phase spectra depend upon the morphology of the samples as well as on the polarization direction of the infrared radiation. The lineshape features of the dynamic in-phase spectra and their relationship to sample deformation are analyzed on the basis of changes of the internal coordinates, the reorientation movement of several functional groups, and the thickness change of the film during the stretching cycle. Dynamic infrared spectra are helpful for deconvolution of overlapping bands on the basis of their different responses to the external perturbation, which sometimes cannot be resolved well by derivative spectroscopy or curve-fitting analysis. The lineshape features have been used to follow microstructural changes after isothermal heat treatment. Near the N H stretching frequency, two bands at 3270 cm⁻¹ and 3200 cm⁻¹ are resolved and analyzed in terms of Fermi resonance between the N H stretching fundamental mode and the overtone and combination modes of the amide I and II vibrations. The dynamic response of the N H stretching mode correlates with the modulation of hydrogen bond strength in uniaxially oriented PA-11. After thermal treatment at the highest temperature (190°C), the dynamic response in this region is mainly caused by the modulation of crystals. In amide I region, three bands at 1680 cm⁻¹, 1648 cm⁻¹, and 1638 cm⁻¹ are separated and assigned to hydrogen bond-free, hydrogen-bonded amorphous, and hydrogen-bonded crystalline regions, respectively. The dynamic responses of the hydrogen-bonded regions are more sensitive to external perturbation. Two components are found in the amide II region, and the band at 3080 cm⁻¹ is assigned to the overtone resonance of the component with perpendicular polarization. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2895–2904, 1998     

NMR study of the tie-chain length distribution in oriented polymers

Wide-line NMR has been used in an investigation of noncrystalline (amorphous) regions in oriented semicrystalline polymers. Nylon 6 was chosen as a model material. The tie-chain length distribution function, the fraction of tie chains in the total number of chains in the crystallite cross section, and the relative number of taut tie chains have been determined. The data on the tie-chain length distribution are used in discussing specific features of vitrification of the amorphous regions in oriented polymers and in prediction macroscopic mechanical properties.     

Morphology of uniaxially oriented poly(ethylene terephthalate)

The morphological character of uniaxially oriented poly(ethylene terephthalate) (PET) films was investigated as a function of draw ratio. Dynamic mechanical, infrared, and crystallite-size measurements were made on the samples. In addition, selective degradation experiments and molecular weight determinations were employed. The dynamic mechanical measurements indicated a sharp decrease in irregular folds for draw ratios of 3.0 and higher, which also coincided with the essentially complete disappearence of regular folds (from the 988 cm^?1 band in the infrared spectra) in unannealed samples. Infrared studies of drawn samples annealed under different conditions gave evidence in support of a structure in which the chains are stretched out. Apparent crystallite-size measurements showed a sudden increase in length of the crystals in the direction of the draw beyond a draw ratio of 3.0. Molecular weight measurements showed a large increase in average chain length in the residue after selective degradation of amorphous material and folds; undrawn and slightly drawn samples gave a much lower M _n. Based on these observations, it is postulated that for higher draw ratios and present drawing conditions, the crystals are of the straight chain type, somewhat similar to the fringed-micelle crystal concept.     

Single-molecule spectroscopy of uniaxially oriented terrylene in polyethylene.

Single terrylene molecules doped into linear low-density polyethylene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confocal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 W cm(-2) is consistent with an uniaxial orientation of terrylene in the sample plane.     

Longitudinal sound velocity in uniaxially stretched rubberlike polymers

We present a new method to interpret data obtained by Brillouin-scattering from oriented networks. Because in rubberlike samples the transverse sound waves are normally not observable, it is not possible to extract the complete set of elastic constants. We will show that by combining a model of the orientation process with the lattice-model from Kondo and Igarashi, force constants can be determined from the sound velocities. These force constants can be discussed in terms of molecular conformations and interactions. The anisotropic hypersound velocity was measured in polyurethane- and polysiloxane networks. With the help of the combined orientation-lattice-model evaluation, different kinds of forces can be distinguished. By comparing the force constants with Raman- or infrared spectroscopic data, information about the conformation of the polymer chains can be determined. The effective force constants of the segments are an order of magnitude lower than that of the polymer main chains and the ratio of force constants along and between the segments lies between 1 : 1 to 5 : 1. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1661–1676, 1997     

Fourier transform infrared study of uniaxially oriented atactic polystyrene

Infrared measurements of the dichroic ratio of atactic polystyrene absorption bands provide a valuable method of determination of the overall orientation of chains as well as the particular orientation of the trans conformational segments. The orientation process produces the alignment of the chains as well as an increase in the amount of trans conformational segments. A linear relationship is observed between birefringence and dichroic ratio for the absorption bands characteristic of overall orientation. A value of 35° ± 3° is obtained for the angle between the normal to the plane of the benzene ring and the chain axis from both infrared and birefringence determinations.     

Polarization of fluorescent light from a uniaxially oriented rubber

The polarization of fluorescence from a polymer characterizes two segment orientation functions, f_s⁽²⁾ and f_s⁽⁴⁾. These may be calculated as a function of elongation using the kinetic theory of rubber elasticity. Three cases are considered: (a) the transition moment direction lies parallel to the segment axis and does not change its orientation during the lifetime of the excited state, (b) the transition moment direction lies at an angle γ to the segment axis, and (c) the transition moment changes its orientation during the lifetime of the excited state.     

On the CO2 permeation of uniaxially drawn polymers

The CO₂ permeation coefficient and the difficient were measured using the permeation time-lag method for films of atactic polystyrene and high-density polyethylene, each as a function of uniaxial draw ratio. The reduction of permeability with draw ratio is observed for polystyrene and for polyethylene. In the latter it is associated with an increase in crystallinity. In both cases the premeability decreases and the solubility constant remains unchanged. The reduction of permeability is thus caused only by the reduction in diffusion of CO₂ in the drawn polymers. The mechainism is different for the two polymers, as is confirmed by measurements of birefringence, glass transition temperature, and crystallinity.     

On the sub-Tg annealing of uniaxially oriented and unoriented atactic polystyrene

DSC analysis was performed on uniaxially oriented and unoriented atactic polystyrene samples prior to and after annealing at 80 °C. With increasing annealing times, an endothermic peak appeared, whose area increased with the duration of the annealing period. No difference was found between the endotherm areas for the oriented and the unoriented polymer.The DSC curve of the unannealed oriented polystyrene exhibited a relaxation exotherm. DSC and relaxation studies indicated that this relaxation exotherm was independent of the main chain orientation; it may rather be due to sample densification.

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Zusammenfassung Uniaxial orientierte und unorientierte ataktische Polystyrenproben wurden vor und nach dem Tempern bei 80° mittels DSC untersucht. Mit länger werdender Temperungszeit erscheint in endothermer Peak, dessen Fläche mit der Dauer der Temperungsperiode ansteigt. Die Flächen der endothermen Peaks von orientierten und unorientierten Polymeren unterscheiden sich nicht. Die DSC-Kurve von ungetempertem orientiertem Polystyren zeigt einen exothermen Relaxationseffekt. DSC und Relaxationsuntersuchungen ergaben, daß dieser exotherme Relaxationseffekt unabhängig von der Orientierung der Hauptkette ist und eher einer Probeverdichtung zuzuschreiben ist.

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We are grateful to Prof. B. Wunderlich of the Rensselaer Polytechnic Institute, New York, for useful discussions.

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The financial support of C.N.R. (progetto finalizzato chimica fine e secondaria) is gratefully acknowledged.     

Dielectric dispersion of oriented polymers

Summary Expressions have been derived for the tensor of the complex dielectric permitivity of an oriented polymer containing polar groups in the main chain. The relevant intramolecular and intermolecular interactions have been assumed to be independent of the degree of orientation; the anisotropy of dielectric permitivity is expected to be given only by the orientation of local equilibrium positions of the dipoles. For the low-temperature dispersion of a polymer oriented by cold-drawing, both the spectra of relaxation times and the magnitudes of the components of the dielelectric permitivity tensor have been calculated. Relation between the calculated values and the dipole-dipole interaction parameters, diffusion coefficients and average dipole-chain angle are given.

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Zusammenfassung Es wurden Ausdrücke für den komplexen Tensor der Dielektrizitätskonstantante an orientierten Polymeren, die polare Gruppen in der Hauptkette enthalten, abgeleitet. Die maßgebenden intra- und intermolekularen Wechselwirkungen wurden als unabhängig vom Orientierungsgrad angenommen; die Anisotropie der Dielektrizitätskonstante dürfte nur durch die Orientierung lokaler Gleichgewichtspositionen der Dipole gegeben sein. Für die Tieftemperaturdispersion eines kalt verstreckten Polymeren wurden das Spektrum der Relaxationszeiten und die Relaxationsstärken des Tensors berechnet. Beziehungen zwischen berechneten Werten und den Dipol-Dipol-Wechselwirkungsparametern, Diffusionskoeffizienten und den mittleren Winkel zwischen Dipol und Kette sind angegeben.

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With 5 figures     

Orientation and relaxation in uniaxially stretched styrene-co-methyl methacrylate random copolymers

Orientation and relaxation behavior in uniaxially stretched styrene-co-methyl methacrylate random copolymers was investigated. When compared at a reference temperature T = T_g + constant, orientation of methyl methacrylate units (MMA) decreases while styrene units orientation increases with a decrease in the styrene percentage. This behavior can be related to intermolecular interactions between MMA units and to the stiffness of styrene-MMA units, which do not undergo conformational changes upon stretching. Both monomer units relax the same in a given copolymer and chain relaxation increases when the styrene percentage increases. Orientation relaxation of styrene and MMA units can be reduced to two general relaxation master curves whatever the blend composition, when the results are compared at same monomeric friction coefficient. © 1994 John Wiley & Sons, Inc.     

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.     

Mechanical properties of epitaxially crystallized 1,4-polybutadiene on uniaxially oriented isotactic polypropylene

The structure and mechanical properties of polypropylene (PP)-trans-1,4-polybutadiene (PBD) blends were investigated. It was found that PBD can epitaxially crystallize on PP. An increase in mechanical properties (Young's modulus, fracture stress) is observed, despite the fact that pure PBD has much worse mechanical properties compared to PP. The epitaxial morphology is responsible for the synergetic effects in the mechanical properties.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

Broad line NMR study of molecular motion in uniaxially stretched polymers

Capability of NMR technique to study fracture and elementary destruction acts in polymers is discussed. Tensile stress influences macromolecular mobility As a rule mobility of macromolecules decreases under deformation. So far deformation remains reversible, this effect is reversible, too. As a result of chain micro-Brownian motion hindering under stress the amorphous layers can pass from high-elastic to solid state even at temperatures rather higher than T_g measured under initial conditions. It is mechanical vitrification (not structure changes) that is regarded as the general causes of draw process interruption (destruction of polymer). Deformation-induced electron emission was detected. It is caused by ionization of stressed chains and tunnel transitions of electrons into deep traps. Influence of molecular mobility on the traps and their destroying under stress are considered.     

Micro-Raman study of the transition front in uniaxially stretched semicrystalline polymers

The transition front of the neck between the isotropic and oriented region in uniaxially stretched polyvinylidene fluoride (PVDF) and polypropylene (PP) is analyzed with a high spatial resolution by micro-Raman spectroscopy. The variation of the microstructure, i.e. change in the degree of crystalline modification in the case of PVDF and in the orientation of the PP chains in function of the strain rate, is correlated with other parameters associated with the drawing response of these polymers and the temperature rise during deformation. The results here described can be understood through the clear increase of temperature that was detected in the neck due to the heat generated by the deformation work of the drawing process.