Separation and kinetic analysis of the thermo-oxidative reactions of polyacrylonitrile upon heat treatment

A polyacrylonitrile (PAN) homopolymer and a copolymer with methyl acrylate were analyzed by differential scanning calorimetry (DSC) in order to establish a method for the kinetic analysis of the stabilization reaction steps during heat treatment. The application of a program rarely used for the study of the chemical transformation of PAN and its copolymers resulted in five separate steps, which can be assigned with large probability to various cyclization and oxidation reactions. A more exact identification of the reactions needs further measurements and considerations. Overlapping peaks appearing in the DSC traces were deconvoluted and then quantitatively analyzed to obtain kinetic parameters. The kinetics of the processes were described by the Kissinger model. The model could be fitted well to four of the five processes that may indicate that these belong to individual reactions of the first order. The parameters obtained agree well with the few values of similar investigations published in the literature up to now. The results have large practical relevance, since the kinetic parameters obtained can be used in the preparation of intermediates and for the optimization of the stabilization process.     

Surface characteristics of layered silicates: Influence on the properties of clay/polymer nanocomposites

A sodium montmorillonite and six organophilic montmorillonites coated with different surfactants were characterized in various ways in the study. Particle and surface characteristics were determined by nitrogen adsorption and inverse gas chromatography, respectively. The gallery structure of organophilic montmorillonite, the orientation of surfactants in the galleries, and surface coverage were estimated by X-ray diffraction measurements and model calculations. The effect of organophilization on the properties of polypropylene/clay composites was determined by the measurement of tensile properties. The results showed that the surface energy of uncoated layered silicates is large; thus, the forces keeping the layers together are very strong. The long chain surfactants used for the coating of montmorillonite orientate more or less parallel to the surface and usually cover the platelets in a single layer in commercial silicates. Surplus surfactant is not located in the galleries, but among the particles, and might influence the properties of composites negatively. Organophilization leads to the drastic decrease of surface free energy. Surface tension of all coated clays is practically the same, irrespective of the type of the surfactant used for treatment. Low surface energy leads to weaker forces between the layers, which might facilitate exfoliation. This effect can be further enhanced by the use of surfactants with two long aliphatic chains, one of which orientates vertically to the surface, leading to larger gallery distance. Polymer/silicate interaction also decreases as an effect of decreasing surface tension proved by the decrease of tensile yield stress of polypropylene/montmorillonite composites. Besides surface tension, the exfoliation of layered silicates is influenced by several other factors as well, like gallery distance, mutual solubility of the components, competitive adsorption, or possible chemical reactions.     

Quantitative analysis of functional groups in HDPE powder by DRIFT spectroscopy

FT-IR spectra of ethylene homopolymers and ethylene/1-hexene copolymers polymerized under different conditions were studied by transmission and diffuse reflection (DRIFT) spectroscopy. The absorbance spectra of film samples were compared with the DRIFT spectra of powders ground from the films. For determining the concentration of the methyl and unsaturated (vinyl, vinylidene and trans-vinylene) groups of polyethylene powders the DRIFT spectra were calibrated by comparing the IR intensities of the corresponding bands measured by the two methods. The results proved that the effect of differences in scattering of the polymer powder originating from the irregularity of the top surface, as well as the size and shape of the particles can be eliminated by the use of a proper internal standard. Linear correlation was established between the logarithms of the normalized intensities measured in absorbance and Kubelka-Munk units. In the case of polyethylene the selection of the internal reference band affects significantly the accuracy of the calibration due to the difference in the refractive indices of the crystalline and amorphous phases.     

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.     

Critical review of analytical methods for determination of inorganic mercury and methylmercury compounds

This review describes determinations of mercury compounds under three categories: total mercury; separate determinations of inorganic mercury(II) and organomercury compounds by selective reduction; and speciation of inorganic mercury(II), monomethylmercury cation, and dimethylmercury. Topics described for each category include sample treatment, separation, detection, and limit of detection. Finally, we note that most methods would not detect dimethylmercury if it were present.     

Thermal analysis of cellulose acetate modified with caprolactone

Cellulose acetate (CA) was modified with caprolactone (CL) under various reaction conditions in an internal mixer. The thermal behavior and relaxation transitions of the samples were determined by dynamic mechanical analysis and differential scanning calorimetry. Various relaxation transitions were detected in externally and internally modified cellulose acetate by DMTA. These were assigned to the glass transition of the main chain, to the movement of single glucose units and to hydroxymethyl groups. The β′ transition must belong to structural units larger than a single glucose ring and their formation must depend on sample preparation conditions. No transition could be assigned to grafted polycaprolactone (PCL) chains by DMTA. Contrary to other groups, we could not detect even the transitions of modified CA by DSC. Only the crystallization of oligomeric PCL homopolymer was observed mostly when it diffused to the surface of the sample.     

Structure and surface coverage of water-based stearate coatings on calcium carbonate nanoparticles

In a preceding paper it was found that, during coating with solutions of a stearin salt in water, whatever the concentration used, a considerable part of the PCC surface remains free, indicating the development of an incomplete monolayer. This was explained by assuming a micelle adsorption mechanism as the dominating process in water, resulting in the formation of a multilayer structure composed of an inner incomplete chemisorbed monolayer and one or more physically adsorbed layers. This model predicted a physisorbed layer in which polar groups are oriented outwards of the particles, resulting in a hydrophilic surface, and contrary to experimental evidence. In this paper we propose that during the drying stage the physisorbed calcium stearate layers undergo a complex rearrangement leading to a hydrophobic coating with the aliphatic tails oriented outwards of the particles. The results of XRD measurements proved that the physisorbed stearate layer is crystalline, while DSC model experiments indicated that the layer goes through phase transitions during heat treatment. The proposed model matched with IGC measurements, showing a clear dependence of the specific component of surface energy on the amount of absorbed stearin. The agreement with values obtained for solvent and dry-coated particles support the proposed rearrangement of alkanoate molecules in the coating.     

Quantitative Characterization of the Structure of PP/Layered Silicate Nanocomposites at Various Length Scales

The structure and rheological properties of a large number of layered silicate polypropylene nanocomposites were studied with widely varying compositions. Unlike general practice, morphology was characterized at different length scales covering several decades by SEM, TEM, and XRD. Rheological measurements supplied additional information about structure. The results showed that these materials possess a very complex structural architecture. The introduction of a functionalized polymer decreases the size of the original clay particles. However, relatively large silicate particles were found also in composite samples yielding XRD traces without silicate reflection. We found that not only two (individual layers, intercalated stacks), but four morphological entities, i.e. also particles and a silicate network, may be present simultaneously in the composites. An attempt was made to characterize morphology quantitatively, wherever possible. SEM and TEM micrographs were evaluated by image analysis, XRD traces by curve fitting, while a model was developed to determine relaxation time from rheological measurements, but only XRD and SEM results are reported here.     

Thermoanalytical methods in the study of structure/properties relationships of modified and crosslinked linear polyethylene

Linear polyethylene is a typical semicrystalline polymer. Its physical and mechanical properties are strongly dependent on the crystallinity (crystalline fraction), and on the size and structure of the crystallites.Modification of linear polyethylene by the grafting of alkoxysilane groups and by subsequent network formation influences the crystalline properties of the polymer. DSC is eminently suitable to follow these changes. For crosslinked polyethylenes above the crystalline melting point, the modulus of elasticity and the thermal expansion coefficient determined by TMA give useful information on the structure. Comparison of the mechanical properties of grafted and crosslinked samples as a function of the crystallinity allows the separation of structural effects. A close correlation has been found between the structure and properties of modified and subsequently crosslinked polyethylenes.

---

Zusammenfassung Lineares Polyethylen ist ein charakteristisch semikristallines Polymer. Seine physikalischen und mechanischen Eigenschaften hÄngen sehr stark von der KristallinitÄt (kristallinen Fraktion) und von der Grö\e und Struktur der Krsitallite ab. Eine Modifizierung linearen Polyethylenes durch Propfung von Alkoxysilangruppen und anschlie\ende Vernetzung beeinflu\t die Kristalleigenschaften des Polymers. Zu einer Untersuchung dieser VerÄnderungen ist DSC Äu\erst gut geeignet. Der mittels TMA ermittelte ElastizitÄtsmodul und WÄrmeausdehnungskoeffizient liefern bei vernetzten Polyethylenen über dem kristallinen Schmelzpunkt nützliche Informationen zur Struktur. Ein Vergleich der mechanischen Eigenschaften modifizierter und vernetzter Proben als Punktion der KristallinitÄt erlaubt die Herausstellung struktureller Einflüsse. Zwischen Eigenschaften und der Struktur von modifizierten und anschlie\end vernetzten Poly-ethylenen konnte eine enge Korrelation festgestellt werden.