V. A. Marikhin

ON THE MEMORY EFFECT IN UHMWPE NASCENT POWDERS

Morphology and molecular structure of three nascent ultra high molecular weight polyethylene (UHMWPE) powders synthesized on unsupported and supported Ziegler-Natta heterogeneous catalysts were studied with the help of electron microscopy and infrared (IR) spectroscopy. A synthesis was carried out in a slurry process in n-heptane at a temperature of about 70°C, which resulted in production of UHMWPE of one and the same molecular weight, approaching 106g/mol. The morphology of the nascent particles dramatically differed, however, which is evidence of the important role of the catalytic system used. Nascent particles synthesized on unsupported TiCl4 catalyst with EtAl as the cocatalyst had a globular structure, while those synthesized on supported TiCl4/Al2O3·SiO2 and TiCl3·0.3 AlCl3/MgCl2 Ziegler-Natta catalysts with the same cocatalyst demonstrated fibrillar (cobweb) and spiral (wormlike) structures, respectively. As revealed by IR spectroscopy, there was a significant difference in the nature of the crystalline regions formed during the synthesis/crystallization process; in the amount of short trans sequences not included in crystallites (taut tie molecules); in the content of irregular conformers of various types, including typical point defects 2G1 kinks; and so on. The melt-crystallized films produced from the investigated reactor powders were markedly distinguished by their drawability and by the existence and the amount of different molecular conformations despite a sufficiently high recrystallization temperature and prolonged preheating. The difference in molecular structure of the drawn films was retained in a whole range of draw ratios. It is concluded that there is a well-pronounced “memory effect” and the morphology of the original nascent particles depending on the catalyst/synthesis conditions clearly play a role in the properties of the end product. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Electrical properties of the conducting polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine

The electrical properties of the conducting polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine (poly-THD) are investigated for the first time. The stability of the conductivity with time, the current-voltage characteristics over a broad range of applied voltages, the pressure dependence of the conductivity, and the low-frequency conductivity fluctuations in the range of analysis frequencies 20 Hz–10 kHz are studied. Appreciable anisotropy of the conductivity is discovered under strong uniaxial compression. Hooge’s parameter α is evaluated (α∼20 at 300 K).

Specific features of phase transitions in molecular crystals of diols

A comparative analysis of the phase transitions in molecular crystals of diols with different chain lengths [1,15-pentadecanediol (CH2)14(OH)2 and 1,16-hexadecanediol (CH2)15(OH)2] has been performed using differential scanning calorimetry. A quantitative analysis of the temperature dependence of the heat capacity within the theory of diffuse (Λ-shaped) first-order phase transitions and the use of the temperature hysteresis have revealed a number of new effects associated with the specific features of the phase transitions in molecular crystals of diols.

On fine structure of nascent UHMWPE reactor powders

Reactor powder of a number of commercial ultra-high molecular weight polyethylenes synthesized on heterogeneous Ziegler–Natta catalyst in a conventional slurry process have been investigated with the help of scanning electron microscopy (SEM), wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. The SEM study reveals a complicated structure of nascent particles consisting of a small (0.5–1.0 μm) spongy-like fibrillar spheres. It is suggested that the elementary morphological units are fibrils formed by the small defective crystallites with the dominating crystalline distortions caused by microstrains. The comparison of longitudinal crystallite sizes derived from WAXS data with the straight chain segment length calculated from Raman shift frequency and the length of extended segments computed from the true melting interval measured by DSC allows to conclude that there are a large number of extended tie chains bridging the neighboring crystallites, which is not in agreement with a lamellar model. The extended chains passing through the noncrystalline regions stabilize a structure and provides its thermal stability. A possible tilting of molecules in the crystals, the location of defects and the distribution of crystallite sizes are discussed.

Features of the amorphous-crystalline structure of UHMWPE

To reveal the features of the structure of UHMWPE that depend on the catalytic system used for synthesis, a comparative study of the sets of laboratory and commercial powders of UHMWPE synthesized on different catalysts in the slurry process under different conditions is conducted. This study includes the use of various modern physical methods, such as transmission and scanning electron microscopy, X-ray analysis, Raman scattering spectroscopy, differential scanning calorimetry, nuclear magnetic resonance, and thermoluminescence. All nascent particles are shown to have a complex hierarchical structure. In all reactor powders, the elementary structural unit is crystalline lamellas, whose dimensions and mutual orientation are dependent on the type of catalytic system. In the synthesis on the supported catalysts, the character of the formed structure depends on the characteristics of the substrate. During the breakdown of the substrate in the course of the synthesis, fibrils form. The colloidal dimensions of the catalyst particles are responsible for a more uniform lamellar structure of the reactor powders. The conformational composition of the segments of molecules in the interlamellar regions of the reactor powders is characterized.

Phase transitions in molecular crystals of dicarboxylic acids

A comparative analysis of phase transitions in molecular crystals of saturated dicarboxylic acids with {COOH(CH2)nCOOH} chains of different lengths has been performed by differential scanning calorimetry, which revealed a number of new effects related with specific features of the phase transitions. A quantitative analysis of temperature dependence of the heat capacity has been carried out in the context of the theory of smeared (Λ-shaped) first-order phase transitions.

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH3(CH2) n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.

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.

Phase transitions in molecular crystals of n-alkane alcohols

A comparative analysis of the phase transitions in molecular crystals of n-alkane alcohols with different lengths of the (CnH2n + 2OH) chain was performed using differential scanning calorimetry (DSC). Investigation of the temperature hysteresis made it possible to reveal a number of new effects associated with the specific features of the first-order phase transitions. A quantitative analysis of the temperature dependence of the heat capacity was carried out in the framework of the theory of diffuse (Λ-shaped) first-order phase transitions.

Specifics of first-order phase transitions in 1,22-docosanediol and 1,44-tetratetracontanediol

A comparative analysis of phase transitions in diols with different chain lengths, (CH2)44(OH)2 and (CH2)22(OH)2, was performed using differential scanning calorimetry. The use of temperature hysteresis made it possible to reveal a number of new effects associated with the specifics of first-order phase transitions. The parameters of transitions were quantitatively analyzed in terms of the self-consistent field theory for diffuse (A-shaped) first-order transitions.