Changes In Molecular Mobility Research Articles

Specifics of bahaviour under load of oriented high density polyethylene samples of various molecular mass

The orientation capacity of high density PE samples in the studied broad range of λ is shown to depend on MM. A characteristic feature of all weakly oriented samples (with the exception of the samples with M v = 7 × 10 6) is the increase of molecular mobility on reversible stretching, and a decrease of these changes with increasing orientation of the amorphous regions. The mean rigidity K of an individual chain holding load under strain is estimated. It is shown that the extent of change of molecular mobility in the studied polymers is mainly determined by the value of K.

PIEZOELECTRICITY AND ENTHALPY RELAXATION IN THE COPOLYMER OF VINYLIDENE CYANIDE AND VINYL ACETATE

In amorphous copolymer of vinylidene cyanide and vinyl acetate showing high piezoelectricity, a considerable enthalpy relaxation was observed at the glass transition temperature (Tg) by annealing below Tg. The relationships between the excess thermodynamic properties and the piezoelectric properties were elucidated by investigating the effect of isothermal annealing on the depolarization process by the thermal stimulated current (TSC) measurements. From the comparison of activation energies between the enthalpy relaxation and the depolarizing process, the molecular motion associated with the energy absorption in DSC endothermic peak is considered to be the conformational rearrangement of C-CN bond induced by dipole rotations in the glassy matrix. Also corresponding changes in molecular mobility have an influence upon the stability of dipole orientation in depolarizing process. Consequently, annealing below Tg increases the stability of the remanent polarization (Pr) and its corresponding piezoelectricity.

Effect of strain on dielectric relaxation in filled rubber

The effect of tensile strain on molecular mobility in the surface layers of rubber filled with Aerosil has been studied. The results of the investigation show that tensile strain significantly influences molecular mobility both in volume and in the surface layers. This change in molecular mobility results from increase in the rigidity of the macrochains and change in the conformational set of the macromolecules.

The variation in molecular mobility during crosslinking of oligoester acrylates

The kinetics of three dimensional polymerization of α,ω- bis-(methacrylyloxyethylenoxycarbonyloxy)-1,3-propylene monomer and the nature of the change in molecular mobility in the course of formation of highly crosslinked polymer, have been investigated by the broad line and pulsed NMR methods. It is shown that when formation of a three dimensional network begins during polymerization of this monomer, a broad component of the NMR line appears, corresponding to protons of low mobility, belonging to the polymethacrylate chain being formed. Quantitative characteristics of the monomer-polymer system have been obtained by separation of the two component, resonance absorption spectra into the narrow and broad components. In study of the polymerization kinetics of the above monomer, all other conditions being the same, the results of the two NMR methods are in agreement.

The effect of chemical modification of the surface of a filler on the behaviour of a polymer in boundary layers of different thickness

Polymeric compositions on the surface of Aerosil modified with organic compounds, at different concentrations of the filler in the polymer systems, i.e. with interlayers of different thickness, have been investigated by the dielectric relaxation method. It was found that the variation in the properties of the surface layer of polymer as the thickness of the layer is changed, is not uniform. Modification of the surface does not affect the nature of the change in molecular mobility in the boundary layer.

The effect of the conditions of solid phase polymerization of trioxan on the confirmation of the polyoxymethylene chains

It is shown that by varying the method and conditions of solid phase polymerization of trioxan, polyoxymethylene chains differing considerably in conformation are produced. A metastable chain conformation with vibrational frequencies at 850, 920, 970, 1030, 1150 and 1165 cm −1 was discovered by infrared spectroscopy. It is produced under certain polymerization conditions, and when the polymer is treated ultrasonically or is recrystallized it is transformed to the well known gauche-gauche conformation. Compression of single crystals of trioxan by application of a shock wave has a considerable effect on the conformation of the polyoxymethylene chains formed. This effect is evidently a result of specific changes in molecular mobility and to deformation of the substance under the action of the shock wave and shows that polymerization occurs mainly during the time of compression.

Relaxation Behaviour of Poly(methacrylic acid) and Its Copolymers with 2-Hydroxyethyl Methacrylate

A free-vibrating torsional pendulum was used to determine the temperature dependence of the storage and loss shear moduli of poly(methacrylic acid) (PMAAc), poly(2-hydroxyethyl methacrylate) (PHEMA) and their copolymers within the temperature range from −196 to 120°C. A decrease in the number of hydroxyethyl side chains per unit volume of polymer leads to a reduction of the low-temperature (γ) dispersion, which is simultaneously shifted toward lower temperatures. The intensity of the secondary (β1) dispersion increases with concentration of the carboxyl groups, while its temperature falls from 28°C for PHEMA to −20°C for PMAAc. Water causes a continuous decrease in temperature and an increase in the intensity of the secondary dispersion of PMAAc. The observed changes in molecular mobility in the glassy state are interpreted qualitatively as a consequence of changes in both steric hindrance and intermolecular interaction.

Spin-lattice relaxation in the system epoxy resin-thermoplastic polymer

In our earlier papers [1–4] we reported our findings regarding change in the molecular mobility of oligomer molecules on solid surfaces based on dielectric relaxation and NMR investigations. We found that for linear oligomers there was a single relaxation process associated with the motion of molecular segments and of the molecular chains themselves. Moreover it was found that on solid surfaces interaction between the latter and oligomer molecules (adsorption) leads to displacement of the minimum spin-lattice relaxation time T 1 towards higher temperatures. This was attributed to adsorption and to limitation of the number of possible molecular conformations near the surface with consequent stiffening of the oligomer molecules. In all the previous investigations we dealt with processes taking place at the interface with a solid substance, and only the effect of the latter on the behaviour of the boundary layer of polymer were considered. In view of the growing interest being shown in mixtures of polymers it appeared necessary to investigate the behaviour of the boundary layers in polymer-polymer or oligomer-polymer systems, the assumption being that the properties of the substrate (the latter being the more rigid polymer) may also vary as a result of interaction with the boundary layer of the other more flexible polymer or oligomer. In this investigation our aim was to clarify changes in molecular mobility in the system oligomer-thermoplastic polymer the latter having the role of filler or substrate. A study was made of relaxation processes (based on change in T 1), in the surface leyers ofED-5 and ED-6 epoxy resins coated on polymeric substrates consisting of the stren (St+-methyl methacrylate (MMA) copolymer investigated by us in reference [5]. The behaviour of a mixture of both the epoxy resins was also investigated.

LA MOBILITÉ DES CONFIGURATIONS DANS LES LIQUIDES SURFONDUS ET LES VERRES

The phenomenological aspects of the glass transition are analysed on the basis of the (( free volume )) concept. Attention is called to the fact that the variations of molecular mobility during the slow isothermal and isobaric evolution of glasses is directly related to the changes of the primary therrnodynamic parameters such as volume or enthalpy, which depend on the thermal history of the sample. Thus coupled thermodynamic and viscoelastic investigations should provide a convenient means for the separation of the contribution of temperature and volume to changes of molecular mobility. Preliminary experiments show that the contribution of free volume to the configurational mobility is predominant. However, the quantitative interpretation of the data is made delicate by the fact the mechanical relaxation spectrum is getting troncated in the glass transition range in its high frequency tail. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971555

Magnetic Relaxation in Ordered Systems

NUCLEAR magnetic resonance (NMR) spectroscopy is one of the most promising techniques to be recently applied to the study of lipid, protein and water interactions in biological and other ordered and semi-ordered systems. NMR studies of the water in many hydrated biological systems have been interpreted in terms of the existence of “structured”, “ice-like” or “bound” water1–6. Likewise, line broadening in ordered liquid crystalline and biological systems has often been taken as an indication of restriction of molecular motion7–13. We should like to point out some possible pitfalls in interpreting the results of limited relaxation or line width data in terms of changes in molecular mobility arising from specific interactions, in systems which are not isotropic liquids and are not always homogeneous. We have carried out a spin-echo NMR study of the dimethyldodecylamine oxide–deuterium oxide system—a system in which anisotropic ordered structures exist and one for which considerable X-ray and continuous wave NMR data are available9,14. This system is an ideal one for the study of the effects of molecular ordering and anisotropy on the measured NMR parameters because the structure can be changed from isotropic to anisotropic simply by a slight change in composition or temperature.

Creep of linear unoriented polymers in tension, compression, and torsion

The creep of polymethyl methacrylate, polystyrene, and polyethylene in tension, compression, and torsion has been investigated over a broad interval of temperature below the glass transition point. It is shown that the creep criterion changes depending on the temperature. This is associated with changes in molecular mobility.

THE COLD-DRAWING OF NYLON 6.6

An investigation has been made into the variation in shape of the loadelongation curve of undrawn polyhexamethylene adipamide (nylon 6.6) filaments with change in ambient conditions, isotropic refractive index of the undrawn filament and speed of extension at low speeds of extension (0.008cm/sec–0.43cm/sec). The interaction of these variables and their resultant effect on the load-elongation curve are traced to a change in molecular mobility within the amorphous regions of the filament. The internal fracture which occurs when a nylon filament is drawn at a “neck” under particular conditions is discussed, and the experimental results are shown to be consistent with the formation of a negative hydrostatic pressure at the cusp of the neck. Drawn filaments which have fractured internally have, compared with normal nylon filaments, approximately the same strength, 20% lower density and a much higher opacity. A general molecular mechanism for the drawing process is put forward which combines ideas set out previously by Bunn and Alcock, Jackl, and Marshall and Thompson. A suggestion is made that drawing at a neck is equivalent to melting at reduced pressure at the centre of the neck, and an attempt has been made to express this idea quantitatively. The theory produced inter-relates the latent heat of fusion of the undrawn filament, the maximum natural draw ratio and the work of drawing.