S.G. Karpova

On the molecular mobility of polypropylene chains in amorphous regions of the stressed polymer

Abstract The segmental mobility in amorphous regions of isotactic PP subjected to mechanical loading has been investigated by ESR, using the stable nitroxyl radical (probe) 2,2,6,6-tetramethylpiperidine-1-oxyl. A complex relation between the correlation time τ for the probe and the degree of stress has been found. The value of τ decreases during the initial stage of loading and thereafter increases gradually, reaching a maximum under strains of ∼11%. It is surmised that the experimentally observed dependence is determined by an additional orientation of the specimen and by increasing segmental mobility of PP macromolecules accompanying increased stress.

Oxidative degradation of polymers under load. Ozone-oxygen action on oriented polyethylene

Abstract A study has been made of the influence of external loads on the rate of ozone oxidation of oriented high- and low-density polyethylene specimens, under regimes of constant stress and strain. It is shown that tensile stress has an accelerating effect on the kinetics of H atom abstraction. The higher sensitivity to loading of low density PE specimens compared with high density specimens is noted, and a relationship between loading sensitivity and MWD is observed for PE.

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 Mv = 7 × 106) 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.

Effect of tensile stress on the molecular mobility of oriented amorphous-crystalline polymers☆

Abstract ESR spectroscopy was used to study the effect of dilatational strain on the molecular mobility of uniaxially oriented PP and PE of high density and varying width of MD, which are oriented to varying degrees in elongation. For slightly oriented PP and PE samples it is typical to observe increased mobility in the elastic zone, the effect decreasing with an increase in orientation. For highly oriented samples segmental mobility decreases in the whole section of elongation. The dependences observed are explained on the basis of ideas concentraning the volumetric variation of the amorphous phase during elongation.

Effect of oxidation on the dynamic and structural parameters of oriented polyethylene in the deformed state

ESR spectroscopy using a radical probe and X-ray analysis have been employed to study the behaviour under a load of oriented HDPE samples with narrow and wide MD first oxidized with an ozone-oxygen mixture. Loosening of the amorphous phase on reversible loading of the polymer films increase with rise in the degree of oxidation. The elasticity modulus E in the initial stage rises but falls for deeper degrees of oxidation. For oxidation of short duration no appreciable changes were found in the orientation; for longer oxidation times the orientation considerably diminishes. Oxidation leads to a sharp increase in the correlation time of the radical-probe.

Effect of anisotropy of high-density polyethylene on special features of its behaviour on uniaxial elongation

Abstract ESR spectroscopy with a radical probe, deformation calorimetry and X-ray analysis were used to study the behaviour of high density oriented polyethylene under stress. A relation was derived between segmental mobility, the heat effect, density of samples and deformation. The decisive role of anisotropy in the behaviour of the polymer in the range of elastic deformations was shown. Elongation of slightly oriented films is accompanied by an increase in segmental mobility of macro-chains, heat absorption and a reduction in the density of the amorphous phase. The opposite changes were observed for highly oriented samples.

Special features of elastic deformation of uniaxially oriented high-density polyethylene

ESR spectroscopy and a nitroxyl radical-probe was used to study the change in segmental mobility of macrochains of high-density polyethylene in the amorphous phase by the action of tensile.stress. In the elastic zone of stressing a link was observed between changes of kinetic parameters of the radical-probe and thermodynamic parameters of the polymer sample during deformation, which may be expressed by the equation. δδS≠ = αδS/1−κ where δδS≠ is the experimental variation of activation entropy of rotary motion of the radical-probe, δS is the entropy variation of the polymer sample, α-coefficient of proportionality, κ—proportion of the crystalline phase. It was shown that in the elastic zone the type of variation of molecular mobility is determined by the degree of orientation of the sample.

High-temperature relaxation transitions in polyolefins

Abstract High-temperature relaxation transitions in oriented HDPE and PP samples have been investigated by ESR spectrometry, using a radical-probe method. Jump-like changes in mechanical properties and in molecular mobility parameters occurred at 40–50° for the PE and at 60–70° for PP. A mechanism proposed for the α-relaxation transition is based on thawing amorphous phase microregions with denser packing of extended chains.

Influence of external effects on the structure and molecular dynamics of oriented copolymers and blends on the basis of polypropylene and polyethylene

Abstract In polyethylene (PE)-polypropyle (PP) blends, the greatest changes in dynamic and structural parameters occur on the addition of 1–10% of PE to PP. In this case the orientation of PE chains in amorphous domains increases appreciably, the rigidity of the amorphous domains grows and the elasticity modulus is enhanced. Reversible stretching of oriented blends is accompanied by a growth of molecular mobility which is independent of the degree of orientational drawing and blend composition. Stability with respect to ozone changes slightly with blend composition, whereas stability towards UV irradiation drops sharply in blends of PP with small amounts of PE. The behaviour of copolymers on stretching changes sharply on addition of 5% PE units to PP. Reversible strain of oriented copolymers containing

Influence of external agents on structure and molecular dynamics in oriented polyolefins review

Abstract Oriented PE and PP samples have been studied on exposure to mechanical, temperature and oxidative factors. The complex nature of the change in molecular mobility on static loading of oriented polyolefins has been established. Deformation, degree of stretching, the MMD and the MM were found to influence the value and direction of thi change. At the initial stage of oxidation there is considerable improvement in mechanical properties. For high degrees of oxidation the character of the dependence of the molecular mobility on reversible strain for highly oriented samples with mean MM and narrow MMD changes to the opposite. In the region 40°C in PE and 60°C in PP a relaxational transition was found. After oxidation by ozone at >95°C and also oxidation in the loaded state and subsequent annealing (at the temperature of the relaxational transition) this transition was not found.