Zlatan Denchev

Molecular dynamics and microstructure development during cold crystallization in poly(ether-ether-ketone) as revealed by real time dielectric and x-ray methods

The isothermal crystallization process of poly(ether-ether-ketone) from the glass has been studied in real time by dielectric spectroscopy and x-ray scattering experiments. The combination of these two techniques revealed a complete picture of the crystallization processes from the point of view of both amorphous and crystalline phases. Analysis of results shows that the sample morphology consists of lamellar stacks, separated by rather broad amorphous regions. The lamellar stacks are highly crystalline (∼70%), as obtained from both dielectric and x-ray scattering measurements, and the amorphous phase within the stacks is constrained up to a level where no segmental relaxation is possible. The remaining amorphous phase, after completion of the primary crystallization process, still has a certain mobility, but it is significantly slower than the initial amorphous mobility. Dielectric data and x-ray results are found to be highly congruent.

On the Origin of the Multiple Melting Behavior in Poly(ethylene naphthalene-2,6-dicarboxylate): Microstructural Study as Revealed by Differential Scanning Calorimetry and X-ray Scattering

In this article a study on the melting behavior and microstructure of semicrystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) prepared by crystallization from the glass under different annealing conditions is presented. The influence of the annealing temperature (Ta), annealing time (ta), and the heating rate (Rh) at which Ta is reached on the endothermic behavior of the samples was investigated by means of differential scanning calorimetry (DSC). A dual melting behavior appeared for low Rh values (2 deg · min−1) within the range of 145 °C < Ta < 250 °C and 1 min ≤ ta. ≤ 16 h. Samples subjected to fast heating rates (Rh = 200 deg · min−1) to reach a Ta ≥ 230 °C showed DSC traces in which a transition is observed from three peaks to a single melting peak when ta increases in the 30–240 min range. On the basis of the DSC results, PEN samples were prepared displaying single or dual endothermic behavior. The microstructure of these samples was studied by wide (WAXS) and small-angle X-Ray scattering (SAXS) techniques. The SAXS data were analyzed using the correlation function and interface distribution function formalisms, respectively. In samples with a single melting behavior, microstructural parameters such as the long spacing, the amorphous and the crystalline phase thicknesses are consistent with a lamellar stacking model in which the thickness distributions of both phases are almost the same. For samples exhibiting two melting endotherms, a dual lamellar model, which is in agreement with the experimental results is proposed.

Transcrystallization with reorientation in drawn PET/PA12 blend as revealed by WAXS from synchrotron radiation

Studying the isotropisation kinetics during melting of a drawn poly(ethylene terephthalate) (PET)/polyamide 12 (PA12) blend (60/40 wt %) at TmPA12 < Ta < TmPET by means of wide-angle X-ray scattering (WAXS) from synchrotron radiation a transcrystallization behaviour of PA12 was observed. It was also found that the nonisothermal transcrystallization behaviour of PA12 taking place during cooling down of the molten PA12 is characterized by reorientation of PA12 chain axis by 90° with respect of PET chains axis and of the starting chains axis direction as concluded from the WAXS patterns. A conclusion is drawn that PET microfibrils are not only effective nuclei for transcrystallization of the matrix but they cause a drastic reorientation of the matrix chains placing them perpendicular to the initial common for both PET and PA12 orientation direction.

Cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) by simultaneous measurements of X-ray scattering and dielectric spectroscopy

The isothermal cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) was investigated by simultaneous small and wide angle X-ray scattering and dielectric spectroscopy (DS). By this experimental approach, simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization, namely about the chain ordering through wide angle X-ray scattering, about the lamellar crystals arrangement by means of small angle X-ray scattering, and about the amorphous phase evolution by means of DS. The results indicate that average mobility of the amorphous phase suffers a discontinuous decrease upon passing from the primary to the secondary crystallization regime. We interpret these results assuming that the restriction to the mobility of the amorphous phase occurs mainly in the amorphous regions between the lamellar stacks and not in the amorphous regions within the lamellar stacks.

Sequential Reordering in Condensation Copolymers, 6. Average Block Lengths in Poly(ethylene terephthalate) – Polyamide 6 Copolymers As Revealed by NMR Spectroscopy

An equimolar physical blend comprising poly(ethylene terephthalate (PET), polyamide 6 (PA6) and catalytic amounts of p-toluene sulfonic acid (pTSA) was studied by differential scanning calorimetry (DSC) 13 C and C NMR spectroscopy before and after reactive blending for various times at 280°C that enables interchange reactions. With the extension of the reaction time, DSC traces showed an abrupt decrease in ability to crystallize, and after two jours of blending the system did not reveal any crystallization or melting behavior. By means of 13 C CMR spectroscopy, a clear distinction was made between the PET/PA6 physical blend and the copolymers with different degrees of interchange reactions. The 13 C NMR sequence length determination in various PET-PA6 based copolyesteramides showed PET block lengths of 4-10 repeat units which was in good agreement with the crystallizability of these systems. A combination of selective degradation of the PET blocks and 1H NMR allowed, for the first time, the characterization of both PET and PA6 sequence lengths. This method was shown to be more sensitive, as compared to 13C NMR spectroscopy without selective degradation, for sequence analysis in copolymers with close to random distributions of PET and PA6 sequence lengths.

Simultaneous measurements of small angle x-ray scattering, wide angle x-ray scattering, and dielectric spectroscopy during crystallization of polymers

A novel experimental setup is described which allows one to obtain detailed information on structural and dynamical changes in polymers during crystallization. This technique includes simultaneous measurements of small angle-wide angle x-ray scattering and dielectric spectroscopy (SWD). The capabilities of the technique have been probed by following in real time the crystallization process of a model crystallizable polymer: poly(ethylene terephthalate). By performing these experiments, simultaneous information from both, the amorphous and the crystalline phase is obtained providing a complete description of changes occurring during a crystallization process. The SWD technique opens up new promising perspectives for the experimental study of the relation between structure and dynamics in materials science.

Induction time for cold crystallization in semi-rigid polymers: PEN and PEEK

The appearance of long range density fluctuations in two isotropic semi-rigid polymers (PEN and PEEK), during the induction period prior to crystallization, when heating above the glass transition temperature, Tg, has been demonstrated by simultaneous wide- and small-angle X-ray scattering techniques. The parallel use of dielectric spectroscopy, to characterize the segmental dynamics of the amorphous polymers polymer above Tg, and of X-ray scattering, to estimate the induction period for cold crystallization, reveals that segmental mobility of the supercooled melt is a key factor in controlling the development of the pre-crystalline nanostructures.

Characterization of composite materials based on PP-Cork blends

This article studies the possibility of making a cork fiber to reinforce plastics (polypropylene, PP). The thermal and structural properties of natural fibers are determined. The hydrophilic character of cork and the hydrophobic character of PP eliminate all chemical adhesion between the fiber and the matrix. This article gives a method of surface modification of cork which ameliorates the fiber-matrix adhesion (their results effects on the mechanical properties of composites). The method consists of hot water treatment. First, the influence of water treatment on the mechanical, thermal, and structural properties of the cork used to reinforce the composites is studied. This study is used to prepare a composite material with good mechanical and thermal properties without destruction of the cellular structure. The specimens of cork are treated with boiling water at ambient pressure for 1, 2 and 3 h and then washed with cold water. The specimens are dried in a dry oven at 70 C for 3 h. The results obtained show that density decreases as a function of water treatment time. This study also describes the influence of water treatment on the thermal behavior of the cork. An increase in thermal stability with water treatment time, in a nitrogen atmosphere, is noticed. In an isothermal study, a loss of mass of 5% after 30 min at 180 Cis observed. The specimens of treated and untreated cork are used to reinforce the PP matrix. The tensile test shows the importance of the surface modification of cork used in composites, particularly after treatment for 1 h. The adhesion properties of PP-cork are described with scanning electron microscopy (SEM).

Sequential reordering in condensation copolymers, 4. Crystallization-induced sequential reordering in poly(ethylene terephthalate)/polycarbonate copolymers as revealed by the behavior of the amorphous phases†

An equimolar blend of poly(ethylene terephthalate) (PET) c and bisphenol-A-polycarbonate (PC) d is studied by dynamic-mechanical thermal analysis (DMTA) and X-ray scattering after thermal treatment that enables transesterification. As demonstrated by wide-angle X-ray scattering (WAXS) measurements, prolonged thermal treatment at 280°C gives rise to a copolymer that no longer reveals melting or crystallization. In accordance with previous reports, this effect is attributed to the formation of a random copolymer. Additional annealing of such samples below the melting temperature of PET results in restoration of the crystallization ability. This effect is explained by crystallization-induced sequential reordering from random to block copolymer by means of transreactions which closes the cycle of transformations from two homopolymers via block- and random copolymer back to a block copolymer. The behavior of the amorphous phases is studied by means of DMTA demonstrating that their glass transition temperatures T g s vary in accordance with the crystallinity changes. The random copolymer is characterized by a more or less homogeneous amorphous phase. In contrast to this, the mechanical mixture and the two block copolymers (the initial and that with the restored blocky structure) show DMTA peaks of two amorphous phases, clearly separated and with distinct individual Tgs. Viscosity measurements also demonstrate that the random copolymer significantly differs in its viscosity as compared to all other samples. These results represent a further evidence for the effect of block restoration via crystallization-induced sequential reordering.

CRYSTALLIZATION IN PARTIALLY MOLTEN ORIENTED BLENDS OF POLYCONDENSATES AS REVEALED BY X-RAY STUDIES*

Oriented fibers or films of binary polymer blends from polycondensates were investigated by two-dimensional (2D) wide-angle X-ray scattering (WAXS) during the finishing process of microfibrillar reinforced composite (MFC) preparation, that is, heating to a temperature between the melting temperatures of the two components, isothermal annealing, and subsequent cooling. It is shown that the crystallization behavior in such MFC from polycondensates depends not only on the blend composition, but also on thermal treatment conditions. Poly(ethylene terephthalate)/polyamide 12 (PET/PA12), poly(butylene terephthalate)/poly(ether ester) (PBT/PEE), and PET/PA6 (polyamide 6) composites were prepared in various compositions from the components. Materials were investigated using rotating anode and synchrotron X-ray source facilities. The effect of the annealing time on the expected isotropization of the lower melting component was studied in the PET/PA6 blend. It was found that PA6 isotropization took place after 2 h; shorter (up to 30 min) and longer (up to 8 h) melt annealing results in oriented crystallization due to different reasons. In PET/PA12 composites, the effect of PA12 transcrystallization with reorientation was confirmed for various blend compositions. The relative strength of the effect decreases with progressing bulk crystallization. Earlier presumed coexistence of isotropic and highly oriented crystallites of the same kind with drawn PBT/PEE blend was confirmed by WAXS from a synchrotron source. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.