Yu. M. Boiko

Tensile, stress relaxation and dynamic mechanical behaviour of polyethylene crystallized from highly deformed melts

High-density polyethylene (HDPT) specimens were obtained by standard extrusion and also by a procedure of solidification from a highly deformed melt (Sodem) in wide ranges of temperature T, time t, and draw ratio 2 from 1.0 to 12.2. Tensile tests were conducted isothermally between 20 and 120 C and stress relaxation at constant tensile strain studied as a function of time also isothermally at several temperatures ill the range from -50 to + 100C. Dynamic mechanical testing was similarly conducted in the range from - 150 to + 120 C. The time-temperature equivalence principle, an equation for the temperature shift factor al as a function of the reduced volume ? and the Hartmann equation of state were applied to the properties so established, including the stress relaxation and the mechanical loss tangent. The earlier shift factor equation has been generalized so that it now includes the draw ratio in two ways: In ar = 1/[a + c).] + B/I?- 1]; ,, c and the Doolittle constant B are characteristic for a given material but independent of the degree of orientation and of temperature. The reduced volume ? depends on temperature T via equation (7) and on 2 via equation (8). Drawing causes a decrease in the number of available chain conformations, which is reflected in the first term; it also changes intersegmental interactions, as reflected in the second term through equation (8). The Sodem procedure improves mechanical properties of HDPE. Specimens with the highest draw ratio 2= 12.2 exhibit the highest elastic modulus and the highest tensile strength as well as high relaxation rates during long-term testing.

Nanoporous structure of ultrahigh-molecular-weight polyethylene reactor powder

The influence of the catalyst system and synthesis conditions on the morphology and molecular dynamics of reactor (nascent) powders of ultrahigh-molecular-weight PE synthesized over supported Ziegler-Natta catalysts in laboratory reactors was studied by means of electron microscopy and 1H broadline NMR spectroscopy. For comparison, commercial reactor powders were studied as well. The type of the catalyst system and the temperature of slurry polymerization have a substantial effect on the supermolecular structure of the nascent polymer. The proton NMR spectra of the reactor powders synthesized at low temperatures display a narrow component. An analysis of its behavior at low temperatures and different humidities led to the conclusion that the signal is due to water localized in nanopores of 2–4 nm in size in the nascent polymer. The role of nanopores in the sintering of reactor particles is discussed.

Relationship between the Contact Time of Polymer Surfaces and Their Bond Strength

Films of amorphous polystyrene (PS) (mono- and polydisperse polymers) and poly(phenylene oxide) (PPO) were brought into contact at a low pressure (0.8 MPa) under temperatures Th from 62 to 156°C. The contact time t was varied from 2 min to 96 h. Partially “healed” PS-PS, PPO-PPO, and PS-PPO contacts were broken at room temperature and the shear strength in the contact zone was measured. The experimental σ-t curves were analyzed in double logarithmic coordinates. The value of for the three interfaces investigated was within 0.07-0.32 at Th < Tg, where is the glass transition temperature. In the same way, the data on strength development taken from the literature for a number of polymer-polymer contacts formed both above and below Tg were analyzed, and the range of variations in α proved to be roughly the same (0.05-0.35). The results obtained are discussed within the framework of theories of molecular dynamics for the polymer bulk and the interface.

Evolution of the elastic modulus at the interfaces of amorphous glassy polymers

The method of shear deformation of adhesive compounds based on high-molecular-mass glassy polymers is used for the pioneering analysis of changes in the elastic modulus as a function of contact time and temperature at the symmetric and asymmetric polymer-polymer interfaces. The kinetics of changes in the elastic modulus and strength of adhesive joints during shearing at a contact temperature below the actual glass-transition temperature of the polymer bulk is analyzed. The elastic modulus can be generally described in terms of the classical principle of the temperature-time analogy with respect to the conditions of the formation of adhesive joints. Molecular and thermoactivation mechanisms of the evolution of adhesive viscoelastic and strength characteristics are discussed.

Self-adhesion of amorphous polymers and their miscible blends

The amorphous film surfaces of polystyrene (PS), poly(2,6-dimethyl 1,4-phenylene oxide) (PPO), and their miscible blends are brought into overlap contact below the glass transition temperature Tg for 10 min and 24 h in order to obtain PS—PS, PPO—PPO, and blend—blend self-adhesive joints. It is shown that after the contact of the blend surfaces, i.e., when the molecules of both PS and PPO are present at the interface, it is possible to attain higher values of shear strength as compared with those at PS—PS and PPO—PPO interfaces. This points to the contribution of a specific interaction between the segments of PS and PPO to the strength development at the interface.

Influence of chemical structure and chain length on conducting properties of dielectric polymers in metal/polymer/metal structures

The influence of weight-average molecular mass Mw on the maximal width of a conducting layer hmax of polysterene, polymethylmethacrylate, and polyamidine films was investigated. It was shown that hmax increases in proportion to MwS for all investigated polymers. It points to the fact that the hmax value is defined by the macromolecular coil size.

Rupture of molecules upon fracture of adhesive joint between two polymer samples

The surfaces formed after fracture of the joint of two polystyrene (PS) samples have been studied by attenuated total reflection Fourier-transform infrared spectroscopy. The adhesive joint between samples was created by pressing them one against another and holding at a pressure of 0.8 MPa and a temperature of 80°C, which is ∼23°C lower than the glass transition temperature of PS. It has been found that, after the joint fracture, the concentration of molecule ends formed after the rupture of carbon-carbon bonds in the back-bone of the PS molecule increases.

A study of ultra-strength polymer fibers via calorimetry

Xerogel reactor powders and supramolecular polyethylene fibers with various degrees of hood have been studied via differential scanning calorimetry. A higher strength of laboratory fibers in comparison with industrial ones is found to be achieved due to a multistage band high-temperature hood that causes the thermodynamic parameters of supramolecular polymer structure.

Specific features of the melting of poly(ethylene terephthalate) after its thermal and mechanical treatment

Specific features of the melting of PET samples with different molecular masses after their thermal and mechanical treatment (orientational drawing) are studied by the DSC method. Orientational drawing of the samples leads to the development of a supermolecular structure, which depends on the molecular mass and thermal prehistory of the initial samples.

X-ray structural study of poly(ethylene terephthalate) after solid-state postpolycondensation

The structure of PET samples exposed to multistep annealing below the melting temperature T m in high vacuum (the so-called solid-state postpolycondensation) has been studied by wide-angle X-ray diffraction. The sizes of crystallites have been calculated through the analysis of half-widths of corresponding reflections via the Rietveld, Scherrer, and Hosemann methods. As the molecular mass (M η) of PET is increased from 4.5 × 104 to 3 × 105, the sizes of crystallites increase in three crystallographic directions (100, 010, and 001). An increase in the thickness of the crystal core of a lamella (the fold length in direction