Alfréd Menyhárd

Crystallization, melting and structure of polypropylene/poly(vinylidene-fluoride) blends

Blends were prepared from isotactic polypropylene (iPP) along with its b-nucleated form and poly(vinylidene-fluoride) (PVDF). Melting, and crystallization characteristics as well as structure of the blends were studied by polarized light microscopy (PLM) and differential scanning calorimetry. According to PLM studies, the phase structure of these blends is heterogeneous in the molten state. The temperature range of crystallization of PVDF during cooling is higher than that of iPP. PVDF has a strong α-nucleating effect on iPP. The crystallization of iPP starts on the surface of dispersed PVDF droplets and an α-transcrystalline layer forms on the surface of the crystalline PVDF phase. The iPP matrix crystallizes predominantly in a-form in spite of the presence of a highly active b-nucleating agent.

Kinetics of competitive crystallization of β- and α-modifications in β-nucleated iPP studied by isothermal stepwise crystallization technique

Crystallization kinetics of β-nucleated isotactic polypropylene (β-iPP) under isothermal conditions were investigated by differential scanning calorimetry. iPP was nucleated by a trisamide derivative, namely tris-2,3-dimethyl-hexylamide of trimesic acid (TATA). In the presence of TATA possessing dual nucleating ability, the formation of the α- and β-form occurs simultaneously. An isothermal stepwise crystallization method is suggested in this study, which can separate the crystallization process of β- and α-iPP and consequently their crystallization kinetics can be evaluated separately. The results indicated that the mechanism of crystallization changes in temperature especially in the vicinity of the upper critical temperature of the formation of the β-phase. In addition, it was found that the ratio of the growth rates of β- and α-modification determines the characteristics of crystallization and influences the apparent rate constant of crystallization of both polymorphs.

Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

The chain regularity of isotactic polypropylene (iPP) homopolymer and random copolymers was characterized by different thermal fractionation methods in this study. Different stepwise temperature programs were applied in a calorimeter (DSC), in order to develop a method which is significantly faster than stepwise isothermal segregation technique (SIST) and provides reliable information about the chain regularity of iPP. Our studies prove that self-seeding accelerates the crystallization process during annealing in SSA–DSC experiments (successive self-nucleation and annealing). Consequently, the time of isothermal steps can be shortened significantly in the SSA–DSC method. On the other hand, we found that step time should not be too short if the goal of the measurement is the determination of average chain regularity. Our results clearly indicate that both the experimental conditions and the evaluation technique influence the obtained results. A standard experimental procedure is proposed for reliably determining the average chain regularity of iPP. The length of the SSA–DSC temperature program developed in this study is much shorter compared to that of the conventional SIST measurements used recently for such experiments. The proposed SSA–DSC program makes the reliable characterization of a large number of samples on an acceptable timescale possible.

The role of solubility and critical temperatures for the efficiency of sorbitol clarifiers in polypropylene

The optical properties of polypropylene (PP) were modified by nine different sorbitol type clarifiers available commercially or synthesized in the study. The solubility of the clarifiers in PP was estimated by thermodynamic model calculations. The results showed that the solubility of these additives in PP is small, a few 1000 ppm at most. Solubility is determined by the chemical structure of the sorbitol, and the heat of fusion of the latter changes solubility by at least one order of magnitude. Solubility can be estimated reasonably by the Flory–Huggins lattice theory. The morphology of most sorbitols transforms at a temperature much below their melting point upon heating. This transformation, which is accompanied by crystal perfection, seems to influence melting and solubility. A fibrillar structure forms upon the cooling of molten sorbitols, but the diameter of the fibrils is much larger than those forming in the polymer melt. The nucleating effect of the clarifier depends on solubility, but also on processing conditions. Nucleus density is related to the amount of dissolved clarifier. A close correlation was found between the Flory–Huggins interaction parameter of sorbitols and the smallest achievable haze, which can be explained by the effect of solubility and nucleus density.

Crystallization, melting, supermolecular structure and properties of isotactic polypropylene nucleated with dicyclohexyl-terephthalamide

Dicyclohexyl-terephthalamide (DCHT) is an efficient and partially soluble nucleating agent for isotactic polypropylene. Its properties, nucleating efficiency and selectivity were characterized using infrared spectroscopy, calorimetry and polarized light microscopy techniques with special attention to the dissolution and recrystallization of DCHT in the iPP melt. The mechanical properties of the nucleated samples were studied using standard tensile tests. It was found that DCHT possesses dual (α and β) nucleating ability. DCHT is soluble in polypropylene melt and recrystallized from the melt during the cooling. It was observed that a special supermolecular structure is formed in its presence, where micron-sized α-iPP is dispersed finely in the β-iPP matrix. This unique structure promoted good stiffness and toughness simultaneously.

Separation of simultaneously developing polymorphic modifications by stepwise crystallization technique in non-isothermal calorimetric experiments

Crystallization kinetics of simultaneously developing polymorphic modifications was investigated in isotactic polypropylene (iPP) under non-isothermal conditions. iPP was nucleated by a trisamide derivative (TATA), which is a non-selective β-nucleating agent, in order to obtain mixed polymorphic structure. The crystallization process was followed using differential scanning calorimetry under constant cooling rates and a stepwise experimental technique is presented, which can separate the development of α- and β-modifications. Ozawa method was applied to evaluate the crystallization kinetics of the two modifications separately. The results indicate clearly that the kinetics of the modification is different. Accordingly, any methods, which evaluate the crystallization process of samples containing mixed polymorphic structure without separation the modifications, are incorrect, and this is the reason why these complicated processes are difficult to linearize using the conventional kinetic evaluation techniques.

Some historical aspects of thermal analysis on the mid-European territory

History of thermal science is shortly described accentuating the impact of Czech, Slovak and Hungarian thermoanalysts, books and journals, personalities and showing the gradual development of associated conferences. It is documented by some unfamiliar photographs of the historic range.

Prediction of tensile modulus of semicrystalline polymers from a single melting curve recorded by calorimetry

This work introduces an easy calculation method, which results in the elastic modulus of semicrystalline polymers from data obtained from a single melting curve recorded by calorimetry. It is well known that the mechanical properties of semicrystalline polymers depend on the crystalline structure; however, the number of direct correlations between crystalline structure and mechanical properties is limited. The melting and crystallization of the polymers studied in this work were measured using calorimetry; the mechanical properties were estimated by tensile tests on the standard shape specimens. Good agreement was found between the calculated and measured modulus values, indicating that the correlation is reliable. In addition, the method was tested for polypropylene and polyamide-6 which proves clearly that the correlation reported here is generally valid for semicrystalline polymers.

Best reviewer award 2011: Professor Dr. Li-Xian Sun

This is the second time when the Best Reviewer Award of Journal of Thermal Analysis and Calorimetry is delivered to one of our outstanding reviewers in order to express our gratitude for his valuable work. The award ceremony was held at the 15th ICTAC conference in Osaka, where the award was delivered to Prof. Dr. Li-Xian Sun. He is an excellent reviewer, who has completed 26 assignments since 2009 within very short time. In addition, he is the Regional Editor of Journal of Thermal Analysis and Calorimetry. His activity contributes significantly to the steep increase of high-quality scientific research in China, which makes him an essential member of Chinese Scientific Community.