Nano- and Micromechanics of Crystalline Polymers

It is currently thought that crystalline polymers consists of lamellar crystals which are separated from each other by a layer of amorphous polymer and are held together by tie molecules through the amorphous phase [e.g. 1]. The lamellae are formed from mostly folded chains. The thickness of lamellae is determined by the parameters such as interfacial energies, glass transition temperature and melting temperature, undercooling, segmental diffusivity, etc. The thickness reported lies usually in a narrow range between 3 and 20 nm as obtained from observations in various types of microscopes or calculated from the degree of crystallinity and long period. It has been recognized that chain folding is not so regular as it was thought and molecular packing in lamellae is subject to considerable and irregularly distributed disorder depending on undercooling- regimes of crystallization. It has been demonstrated in various ways that the planar growth front will always break up into fibrous or cellular growth. Also crystallization of polymers leads to interface instability. More sophisticated treatment of the instabilities involve perturbation analyses of planar interfaces, correlating diffusion, temperature gradients along the interface, and interfacial energy with the size of the growing crystals

Crystallization kinetics of polymer fibrous nanocomposites

Through applying both a probabilistic approach and a combination of probabilistic and the Avrami ‘extended volume’ approaches we have derived a theory of overall crystallization kinetics of polymers reinforced with nanofibers. The theory describes the crystallization kinetics in the presence of straight or curved nanofibers, with different nucleation ability and orientation, and allows to account for their variable length. The analytic results are supported by computer simulations of spherulitic structures. The derived mathematical formulas are in exponential forms suggesting the use of the Avrami logarithmic coordinates for detailed analysis of experimental data. Experimental data on crystallization of several nanocomposites, including polypropylene reinforced with poly(tetrafluoroethylene) nanofibers and polyamide 12 with carbon nanotubes, are in a good agreement with the theoretical predictions

The influence of chemical composition of aliphatic-aromatic copolyesters on their properties

The chain microstructure and properties of a series of aliphatic-aromatic copolyesters in a range of compositions from 10 to 100% of aromatic components were studied by examining melting and crystallization behaviors, dynamic mechanical response, morphology, wide- (WAXS) and small-angle X-ray scattering (SAXS), and tensile deformation. Chain microstructure was analyzed by $^{1}H$ NMR. The results indicate that most of copolyesters used in this study have essentially random distribution of comonomers. Copolyesters with more than 30 mol% of aromatic part crystallize with a crystal structure characteristic for homopolymer poly(butylene terephthalate) (PBT). However, some of the reflections from crystal planes are shifted towards lower diffraction angles as compared to butylene terephthalate homoplymer. The phase transition temperatures decrease with increasing aliphatic content. By means of polarized light microscopy (PLM), small-angle light scattering (SALS) and SAXS, crystallization behavior of a selected aliphatic-aromatic copolyester was further explored. Selected copolyester crystallizes in the form of thin fibrous crystals, few nanometers thick, which is the main factor influencing the depression of its melting temperature

Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 2: crystallinity and supra molecular structure

Test methods including OM, SEM, TEM, DSC, SAXS, WAXS, and IR were used to characterise supra-molecular structure in three batches of polyethylene (PE), which had weight-average relative molar masses ¯¯¯¯ M w of approximately 0.6 × 106, 5 × 106, and 9 × 106. They were applied to compression mouldings made by the polymer manufacturer. Electron microscopy showed that powders formed in the polymerization reactor consisted of irregularly shaped grains between 50 and 250 μm in diameter. Higher magnification revealed that each grain was an aggregate, composed of particles between 0.4 and 0.8 μm in diameter, which were connected by long, thin fibrils. In compression mouldings, lamellar thicknesses ranged from 7 to 23 nm. Crystallinity varied between 70 and 75 % in reactor powder, but was lower in compression mouldings. Melting peak temperatures ranged from 138 to 145 °C, depending on processing history. DMTA showed that the glass transition temperature θg was −120 °C for all three grades of polyethylene. IR spectroscopy found negligibly small levels of oxidation and thermal degradation in mouldings. Optical microscopy revealed the presence of visible fusion defects at grain boundaries. It is concluded that relatively weak defects can be characterized using optical microscopy, but there is a need for improved methods that can detect less obvious fusion defects

Kompozyty w pełni polimerowe – nowe podejście z użyciem rozplątanych częściowo krystalicznych polimerów. Cz.II. Otrzymywanie kompozytów zawierających częściowo rozplątany polilaktyd

The mechanical properties of two uniaxially stretched polylactides have been studied: the polylactide with reduced density of entanglements of macromolecules and with equilibrium density of entanglements. It has been shown that plastic deformation is easier in a partially disentangled polymer. Thus, this polymer was used to form an all-polymer composite by blending it with a second molten polymer: poly(butylene succinate) or polystyrene. During mixing, polylactide fibers were produced that strengthened the prepared composite.Oceniano właściwości mechaniczne podczas jednoosiowego rozciągania dwóch rodzajów polilaktydu: ze zmniejszoną gęstością splątań makrocząsteczek i z równowagową gęstością splątań. Wykazano, że częściowo splątany polimer łatwiej ulega deformacji plastycznej. Z tego względu użyto go do wytworzenia kompozytu całkowicie polimerowego w wyniku zmieszania z drugim stopionym polimerem – poli(bursztynianem butylenu) lub polistyrenem. Podczas mieszania polimerów powstawały włókna polilaktydowe wzmacniające wytworzony kompozyt

Crystalline Lamellae Fragmentation during Drawing of Polypropylene

Filling free volume pores of the amorphous phase with the molecules of low molecular weight modifier leads to a complete elimination of the cavitation during tensile drawing. Such way of modification of a solidified material makes the polypropylene/modifier system a model system that enables the analysis of the influence of cavitation on thermomechanical properties and the mechanisms activated during its deformation. In this paper we have presented the influence of cavitation on the intensity of the lamellae fragmentation. In the case of cavitating material, on the basis of X-ray measurements and scanning electron microscopy, we have observed substantial decrease of the undisturbed crystallites lengths during its deformation up to 50–55% of their initial value. The deformation of noncavitating material proceeded with smaller decrease of average crystallites lengths by only 15–20% of their initial value. The changes of the SAXS’s long period of noncavitating polypropylene indicated that only a small fraction of lamellae stacks that are oriented parallel to the tensile direction undergo fragmentation. This type of fragmentation is connected with excessive lamellae thinning and interfacial instabilities but by no means by cavitation

Kompozyty w pełni polimerowe – nowe podejście z użyciem rozplątanych częściowo krystalicznych polimerów. Cz. I. Rozplątywanie i właściwości rozplątanego polilaktydu

This contribution discusses the preparation of partially disentangled polylactide from solution. The rheological method determined how the density of macromolecular entanglement depends on the concentration of the prepared solution. From a 0.1 wt % solution, the polymer having 20% of the original entanglements was obtained. The crystallization studies in isothermal conditions showed that the growth of crystals can be 10% faster if the polylactide is partially disentangled. For the first time, it was shown that in the partially disentangled polylactide there are shifts in temperatures of crystallization regimes.W artykule omówiono otrzymywanie z roztworu częściowo rozplątanego poliaktydu. Metodą reologiczną określono gęstość splątań makrocząsteczek w funkcji stężenia sporządzonego roztworu. Z roztworu o stężeniu 0,1% mas. otrzymano polimer zawierający ok. 20% wyjściowych splątań. Na podstawie wyników badań procesu krystalizacji w warunkach izotermicznych stwierdzono, że wzrost kryształów może być o 10% szybszy w przypadku tylko częściowego splątania polilaktydu. Po raz pierwszy wykazano, że w częściowo rozplątanym polilaktydzie występuje przesunięcie wartości temperatury, w których następuje zmiana reżimu krystalizacji

Plasticization of Polylactide after Solidification: An Effectiveness and Utilization for Correct Interpretation of Thermal Properties

Polylactide/triethyl citrate (PLA/TEC) systems were prepared in two ways by introducing TEC to solidified polymer matrix (SS) and by blending in a molten state (MS) to investigate the effectiveness of the plasticization process after solidification of polylactide. The plasticization processes, independent of the way of introducing the TEC into PLA matrix, leads to systems characterized by similar stability, morphology, and properties. Some differences in mechanical properties between MS and SS systems result primarily from the difference in the degree of crystallinity/crystal thickness of the PLA matrix itself. Based on the presented results, it was concluded that the plasticization process after solidification of polylactide is an alternative to the conventional method of modification-blending in a molten state. Then, this new approach to plasticization process was utilized for the interpretation of thermal properties of PLA and PLA/TEC systems. It turned out that double melting peak observed at differential scanning calorimetry (DSC) thermograms does not result from the melting of a double population of crystals with different lamellar thickness, or the melting of both the α′ and α crystalline phase (commonly used explanations in literature), but is associated with the improvement of perfection of crystalline structure of PLA during heating process