Ewa Piorkowska

Mechanical and Thermal Properties of Green Polylactide Composites with Natural Fillers

Green composites of PLA with micropowders derived from agricultural by-products such as oat husks, cocoa shells, and apple solids that remain after pressing have been prepared by melt mixing. The thermal and mechanical properties of the composites, including the effect of matrix crystallization and plasticization with poly(propylene glycol), have been studied. All fillers nucleated PLA crystallization and decreased the cold-crystallization temperature. They also affected the mechanical properties of the compositions, increasing the modulus of elasticity but decreasing the elongation at break and tensile impact strength although with few exceptions. Plasticization of the PLA matrix improved the ductility of the composites.

Modeling of crystallization kinetics in fiber reinforced composites

The kinetics of spherulitic crystallization in fiber reinforced thermoplastics was determined by means of the theoretical model based on the statistical approach and by and the computer simulation of spherulitic crystallization. The computer simulation of the spherulites nucleation and growth was performed to verify the derived expressions and to visualize the polymer morphology. The obtained results indicate that the classic Avrami equation is inappropriate for the description of systems with fibers serving as additional nucleation sites. The kinetics of crystallization is governed by the nucleation inside a polymer matrix and at fiber surfaces as well as by fibers content and fiber radius. The developed model permits to predict the half-time of crystallization in fiber reinforced systems.

Effect of negative pressure on melting behavior of spherulites in thin films of several crystalline polymers

The melting behavior of spherulites in thin films of isotactic polypropylene, poly(ethylene oxide), poly(methylene oxide), and poly(ethylene adipate) crystallized isothermally at various temperatures has been studied by polarized light microscopy. The local increase of melting temperature in regions surrounding cavities and multiple boundary points, dependent on the crystallization temperature, was observed in all studied polymers. In pockets of occluded melt an arising negative pressure lowers an equilibrium melting temperature; hence, decreases an undercooling, which results in the increase of lamellae thickness and their melting temperature. The elevation of melting temperature and the negative pressure buildup depend on the polymer and the crystallization temperature.

Statistical approach to the description of spherulite patterns. Two-and three-dimensional cases

A new statistical approach was applied for the description of spherulite patterns for athermal and thermal type of primary nucleation. The two-and three-dimensional cases were treated in the same way.The analytical expressions were developed for spherulite boundary and inner point distances distributions. It was shown that in the case of the thermal nucleation the nuclei distance are correlated although they were nucleated in a random way.The results were confirmed by the data from computer simulation. The analysis of the theoretical formulas showed that the spherulite pattern remains unchanged if the primary nucleation density is constant (the growth rate can vary) in the case of the athermal nucleation. In the case of the thermal nucleation if the ratio of the nucleation rate and growth rate does not change (both can vary but the ratio must remain constant) the spherulite pattern is not modified.

Strain hardening of molten thermoplastic polymers reinforced with poly(tetrafluoroethylene) nanofibers

The influence of poly(tetrafluoroethylene) (PTFE) nanofibers on the extensional viscosity of various molten thermoplastic polymers, including isotactic polypropylene (iPP), high density polyethylene (HDPE), low density polyethylene (LDPE), and atactic polystyrene (PS), has been investigated. It has been shown that PTFE nanofibers, generated in situ by shearing of crystalline PTFE inclusions during compounding with another molten polymer, formed an entangled network, which in turn drastically changed the rheological behavior of polymers studied. The entangled network of PTFE nanofibers induced the strain hardening effect in the nanocomposites based on iPPs, HDPE, and PS, which do not show the strain hardening themselves. Moreover, the strain hardening in the nanocomposite with LDPE was enhanced in comparison to neat LDPE. The higher the content of PTFE nanofibers and the larger the strain rates applied, the more pronounced the strain hardening occurred. Additionally, the presence of PTFE nanofibers signifi...

Thermal effects due to polymer crystallization

The problem of dissipation of latent heat of fusion by thermal conductivity in polymer crystallizing in bulk is discussed. It is shown that the liberation of latent heat of fusion causes the increase of the temperature at the crystallization front. The increase of thermal conductivity and diffusivity of a polymer due to phase transition decreases while the presence of another crystallization front increases the temperature elevation. The temperature buildup is limited by the decrease of growth rate with temperature, directly related to the release of latent heat of fusion. The thermal effects due to crystallization of numerous spherulites in a plate are also estimated. It is shown that the crystallization causes a significant increase of the temperature inside the material as the crystallization proceeds, which slows the rate of conversion of melt into spherulites. The temperature increase in the interior of material depends on the initial temperature and on the sample sizes. The predictions were confirmed by the measurements of the temperature in bulk samples of isotactic polypropylene during crystallization at isothermal ambient conditions.

Influence of the liberation of heat of fusion on the temperature near the crystallization front in polymers

Abstract The solution of the heat conduction equation with appropriate boundary and initial conditions makes it possible to determine the influence of the latent heat of fusion evolving during polymer crystallization on the temperature distribution near the crystallization front. It is shown that the increase in temperature at the crystallization front does not exceed a fraction of a degree.

Heat conduction anisotropy of drawn high density polyethylene samples

In the paper the influence of the morphology of cold drawn samples on heat conduction anisotropy is discussed. It was shown that the important role is played by the existence of air gaps between fibrils of oriented polymer and the perpendicular microcracks appearing with the increase of the draw rate of the specimens.Rolled drawn samples have a significantly reduced air gap contents thus the heat conduction anisotropy measurements on these rolled samples provide real data on polymeric material properties.

Nucleation of Polypropylene with Gold Nanoparticles. Part 1: Introduction of Sandwich Method for Evaluation of Very Weak Nucleation Activity

Although gold particles are known to nucleate isotactic polypropylene (PP), the nucleating effect of chemically pure 5 nm Au, prepared in vacuum sputter coater, was found to be hardly observable. In order to detect such a weak effect, we deposited a homogeneous layer of Au nanoparticles between thin PP films and evaluated the nucleation activity by a combination of three independent methods: polarized light microscopy (PLM), differential scanning calorimetry (DSC), and 2D wide-angle X-ray scattering (2D-WAXS). This new technique, which was called sandwich method, allowed us to demonstrate that gold nanoparticles were able to nucleate PP crystallization, although the effect was much weaker than that produced by commercial α-nucleant [1,2,3,4-bis(3,4-dimethylbenzylidene)sorbitol] and β-nucleant (N,N-Dicyclohexyl-2,6-naphthalene dicarboxamide). The sandwich method appeared to be quite universal and applicable for any micro-sized nucleants or nanonucleants.

Nucleation of Polypropylene Crystallization with Gold Nanoparticles. Part 2: Relation between Particle Morphology and Nucleation Activity

ABSTRACT The influence of gold nanoparticle morphology on nucleation of isotactic polypropylene (PP) crystallization was investigated. Previous experiments indicated certain nucleation activity of gold nanoparticles, varying with their size. In this work, eight types of gold micro/nanoparticles were used: vacuum-sputtered nanostructures (nanoparticles, nanoislands, and nanolayers), chemically prepared isometric gold nanocrystals (5, 20, and 100 nm diameters), and two types of gold microcrystals with well-developed crystal facets [with (100) and (111) facets, respectively]. To minimize the effect of particle agglomeration, we used our recently introduced sandwich method, in which the nucleating agent was deposited between thin PP films and the nucleation was evaluated by polarized light microscopy (PLM), X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The nucleation activity of Au particles in PP was lower than it might be expected from the previous studies and depended on their morphology. The nucleation activity of Au microcrystals with well-developed facets was higher than the activity of non-faceted Au nanocrystals.