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.

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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.

     

Structure and impact resistance of short carbon fiber reinforced polyamide 6 composites

Short carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by homogenization of the components in a twin-screw extruder and by injection molding. Fiber content was varied between 0 and 16 vol%, while specimens were injection molded at rates between 2.0 and 22.6 cm/s. Average fiber length and orientation were measured to characterize structure. Average fiber length decreased with increasing fiber content and processing rate. The observed structure is contradictory to those reported in the literature for short glass fiber reinforced composites. Fibers were oriented randomly relative to the mold fill direction in the skin layer, while they were oriented parallel to this direction in the middle of the specimen. The thickness of the skin decreased with increasing injection rate and decreasing fiber content. Although instrumented impact testing indicated brittle failure at all combinations of the variables, the strain energy release rate could not be determined by the usual technique using varying notch depths because of the different properties of the skin and the core. Also, the mechanism of failure seems to be different in the two layers. A minimum appears in the fracture toughness and impact resistance at low fiber contents, indicating that fibers might promote fracture initiation at such compositions. Fiber length changed in a narrow range in the studied composites; thus, properties are determined mainly by orientation. As a consequence, both increased fiber content and injection rate lead to an increase of stiffness and toughness.     

Network Formation in PP/Layered Silicate Nanocomposites: Modeling and Analysis of Rheological Properties

PP/organophilic montmorillonite composites were prepared from PP homopolymer, maleinated polypropylene and a clay organophilized with octadecyl amine. Various approaches were used to analyze the rheological properties of the composites. Cole-Cole plots proved to be useful, since they detect the network very sensitively. The frequency dependence of the real part of complex viscosity was described with a series of Maxwel bodies coupled parallel to each other. The relaxation time of the melt changed in the range of 0.0005 and 200 sec. Under certain conditions long relaxation times appear in the spectrum, which indicate the formation of new structural units, probably a silicate network. A certain number of silicate layers are needed to create a house-of-cards structure. A threshold concentration of MAPP exists for network formation, which depends on silicate content.     

On chemical kinetics of silicon deposition from silane (III). LPCVD poly silicon formation in the temperature range 900–950 K

LPCVD poly Silicon deposition form silane has been investigated for limited conditions regarding temperature, silane input and pumping speed. It has been found that layer growth is controlled by a chemical reaction of 0.5^th-order in consequence of which growth rate linearly decays along the axis of an open isothermal reactor tube. The slope of that decay is determined not only by the reaction rate constant but also by linear gas velocity within the tube and that part of total substrate surface area that is effectively exposed to silane at each wafer position. In conseqence growth rate decay is the steeper not only the higher temperature will be chosen but also the slower gas velocity is adjusted and the smaller wafers are separated to each other. The kind of how axial layer growth rate distribution is effected by changing wafer spacing is a proof for the heterogeneous reaction mechanism. The silicon forming reaction is characterised by an activation energy of about 52 kcal/mole.     

Thermal analysis of the structure of segmented polyurethane elastomers

Polyurethanes were prepared from 4,4′-methylenebis (phenyl isocyanate) (MDI), 1,4-butanediol (BD), and poly(tetrahydrofurane) polyether polyol (PTHF) by melt polymerization. The –OH functional group ratio of polyol/total diol was kept constant at 0.4, while the ratio of the isocyanate and hydroxyl groups (NCO/OH) changed between 0.940 and 1.150. The thermal analysis of the polymers by DSC and DMTA measurements indicated several transitions. The three glass transition temperatures observed were assigned to the relaxation of the aliphatic –CH₂– groups of the polyol, and to that of the soft and hard segments, respectively. The glass transition temperature of the soft and hard phase changed with the NCO/OH ratio indicating changes in phase structure and composition confirmed also by the maximum in the number of relaxing soft segments. Changes in the relatively small number of end-groups result in considerable modification of mechanical properties. Strength is determined by molecular mass and interactions, while stiffness depends mainly on phase structure. Surprisingly enough, –OH excess yields stiffer polymers, since the interaction of the –OH groups results in a decrease in the amount of the soft phase. A unique correlation was found between tensile modulus and the number of relaxing soft segments.     

In-situ doping of LPCVD poly silicon (II). Temperature influence

Poly silicon deposition by pyrolysis of silane under low pressure conditions has been investigated with respect to the influence of temperature when simultaneously in-situ doping of the deposited layer takes place. The growth rate of poly silicon is retarded in the presence of phosphine provided that a certain lower PH₃/SiH₄-ratio has been exceeded. It has been shown how that lower ratio depends on temperature. Increasing PH₃/SiH₄-ratio not only slows down layer growth rate but also the apparent activation energy of the layer forming reaction. An empirical equation describing the temperature dependence of that activation energy has been derived. Phosphine adsorption has been discussed as a cause of both layer growth rate and activation energy reduction. Additionally, incorporation of phosphorus during layer growth has been investigated with respect to the total amount and the electrically active concentration, the latter measured after a postdeposition anneal at 1000 °C.     

Surface tension and mechanical properties in polyolefin composites

Polyolefin composites were prepared with CaCO₃ fillers of different specific surface area. The fillers were surface treated with stearic acid between 0 and 100% surface coverage. As an effect of the treatment, surface tension of the fillers and also polymer/filler interaction decreased. The relation between interfacial interaction and mechanical properties of the composites was analysed by the equation developed earlier to describe the composition dependence of the tensile yield stress. The characteristics of the interphase were calculated, its yield stress decreases and thickness increases with increasing surface coverage. Reversible work of adhesion can be successfully related to the tensile yield stress, but a more complicated correlation exists between the thickness of the interphase and the strength of the interaction than assumed earlier. Other mechanical properties also change with the surface treatment; modulus and strength decrease and extensibility increases with decreasing polymer/filler interaction.     

Synthesis and Applications of Cinchona Squaramide-Modified Poly(Glycidyl Methacrylate) Microspheres as Recyclable Polymer-Grafted Enantioselective Organocatalysts

This work presents the immobilization of cinchona squaramide organocatalysts on poly(glycidyl methacrylate) solid supports. Preparation of the well-defined monodisperse polymer microspheres was facilitated by comprehensive parameter optimization. By exploiting the reactive epoxy groups of the polymer support, three amino-functionalized cinchona derivatives were immobilized on this carrier. To explore the effect of the amino linker, these structurally varied precatalysts were synthesized by modifying the cinchona skeleton at different positions. The catalytic activities of the immobilized organocatalysts were tested in the Michael addition of pentane-2,4-dione and trans-β-nitrostyrene with excellent yields (up to 98 %) and enantioselectivities (up to 96 % ee). Finally, the catalysts were easily recovered five times by centrifugation without loss of activity.     

Two polymorphs of aqua[N,N′-ethyl­enebis(salicylideneaminato-N,O)]oxovanadium(V) nitrate

The title compound, aqua­[bis­(salicyl­idene)­ethyl­enediamin­ato-O,N,N′,O′]­oxovanadium(V) nitrate, [VO(C₁₆H₁₄N₂O₂)(H₂O)]NO₃, crystallizes as two polymorphs in the triclinic and monoclinic crystal systems. In both, the V atom has a distorted octahedral coordination geometry with a long V—O_water bond trans to V=O. The coordinated water mol­ecules are hydrogen bonded to the nitrate ions so that pairs of cations are linked to give neutral centrosymmetric dimers. The V=O and V—O_water distances are 1.598 (2) and 2.257 (2) Å, respectively, in the triclinic form, and 1.588 (3) and 2.230 (3) Å, respectively, in the monoclinic form. In the triclinic form, the dimers pack so that the salen [bis(salicyl­idene)ethylenediaminate] ligands are parallel to each other, whereas in the monoclinic form, which is the denser, there is a herring-bone arrangement.