Orientation mechanisms during biaxial drawing of polymer films

Summary Second drawing of monoaxially drawn PP and PET films perpendicular to the first drawing direction has been investigated. At low draw ratios structural systems having an angle of 45° with regard to the second draw direction are formed. By increasing the draw ratio the structures rotate into the new preferred direction. The results lead to the impression that the structural transformations do not turn out molecule by molecule, but domain by domain.

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Zusammenfassung Die Zweitverstreckung monoaxial verstreckter PP- und PET-Folien senkrecht zur Erstverstreckrichtung wird untersucht. Bei kleinen Verstreckgraden bilden sich Struktursysteme unter 45° zur Zweitverstreckrichtung; bei weiterer Verdehnung drehen sich die Strukturen in die Vorzugsrichtung. Die Ergebnisse vermitteln den Eindruck, daß die Strukturänderung nicht Molekül für Molekül, sondern Bereich für Bereich abläuft.

With 10 figures and 2 tables     

ON MORPHOLOGY AND MULTIPLE MELTING IN POLYPROPYLENE

A series of polypropylenes, including two modern high crystallinity materials (HCPP) were subjected to a stepwise crystallization procedure as a guide to their properties. The results were complicated by the development of double melting endotherms at the highest crystallization temperature. Both HCPP, when grown at the temperature of 145°C, give a double melting peak, but the proportions of the two peaks varied according to the density of nucleation. By partially melting a specimen between the two peaks, it was possible to assign the higher peak to radial dominant lamellae and the lower peak to mostly tangential subsidiary lamellae filling the space in between. Uniformly cross-hatched lamellae at the center of spherulites melt along with the lower melting population in the outer regions of the spherulites. Even if differences in crystallization temperature are eliminated, the properties of nucleated as opposed to nonnucleated PP may therefore be influenced by differences between the center and periphery of spherulites, with “central” properties much more in evidence in nucleated material. The development of these double endotherms is related to morphological constraint, rather than molecular fractionation. Their form is strongly influenced by cross-hatching, but the phenomenon is still found at 160°C where cross-hatching does not form.     

Rheological and Mechanical Characterization of Multi-Walled Carbon Nanotubes/Polypropylene Nanocomposites

The rheology and morphology of multi-walled carbon nanotube (MWNT)/polypropylene (PP) nanocomposites prepared via melt blending was investigated. The minor phase content of MWNT varied between 0.25 and 8 wt%. From morphological studies using a scanning electron microscopy technique a good dispersion of carbon nanotubes in the PP matrix was observed. The rheological studies were performed by a capillary rheometer, and mechanical properties of the nanocomposites were studied using a tensile and flexural tester. Both PP and its nanocomposites showed non-Newtonian behavior. At low shear rates the addition of MWNT content causes an increase in viscosity; however, viscosity is less sensitive to addition of MWNT content at higher shear rates. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. From this calculation it was concluded that the temperature sensitivity of nanocomposites was increased by increasing of nanotube content. An increase in tensile and flexural moduli and Izod impact strength was also observed by increasing the MWNT content. From rheological and mechanical tests it was concluded that the mechanical and rheological percolation threshold is at 1.5 wt%.     

NUCLEATING EFFECT OF MONTMORILLONITE NANOPARTICLES IN POLYPROPYLENE

The nucleation effect of two layered montmorillonite silicates of different origin was studied in polypropylene (PP). Composites were prepared as a function of composition in an internal mixer or by homogenization in a twin-screw compounder. Melting and crystallization characteristics were determined by differential scanning calorimetry. The gallery distance of the silicates and the structure of the composites were studied by WAXS. The results prove that montmorillonite may considerably influence the crystallization of PP. The effect depends very much on the origin and treatment of the filler. Both treatment and composite preparation lead to considerable changes in the separation distance of the silicate layers. Even though the composites contain fillers with more than one population of layer distances, the nucleating effect is related to the completely collapsed galleries of 1 nm distance. Besides gallery distance, organophillization also modifies the surface tension of the filler, but this apparently does not influence its nucleating efficiency. The efficient gallery distance is twice as large as the characteristic matching lattice dimension determined by Lotz et al. Although a few observations could not be explained and the tentative explanation given earlier may need further verification, the results prove that nucleation does not occur at the flat surface of the filler but between its galleries, where polymer molecules may adsorb preferentially.     

Rheological Behaviors of Polypropylene/Poly(1-butene) Blends

The rheological behaviors of polypropylene (PP)/poly(1-butene) (PB) blends with homo-polypropylene (PP₁) or impact polypropylene (PP₂), a poly(propylene-co-ethylene) as the PP component were studied. With increasing of PB resin content for both PP/PB blends, the blends showed higher G'(ω), G''(ω) and η*(ω) at low frequencies but lower values at high frequencies which implied that the processability was improved. A two-phase morphology was observed through the various rheological responses, including G'(ω)-ω terminal region curves, Cole-Cole plots and the weighted relaxation spectra with the PB contents up to 40?wt%. With the same PB content, the rheological parameters of the PP₂/PB blends were quite different from those of the PP₁/PB, which can be attributed to the stronger interaction between PB chains and the ethylene-co-propylene copolymer in PP₂. The impact strength of the PP₂/PB blends was improved dramatically over that of the PP₁/PB. The more significant toughening effect for the PP₂/PB blends can be attributed to the special responses of its rheological behaviors.     

Preparation and characterization of polypropylene/waste tyre dust blends with addition of DCP and HVA-2 (PP/WTDP-HVA2)

Preparation of polypropylene (PP)/waste tyre dust (WTD) blends with addition of dicumyl peroxide (DCP) and N,N′-m-phenylenebismaleimide (HVA-2) (PP/WTD_P-HVA2) was described. The blends were characterized based on their tensile properties, swelling resistance and morphology, as well as thermal properties. The results reveal that the presence of both chemicals in PP/WTD_P-HVA2 blends brought about noticeable improvements in tensile properties, swelling resistance and interfacial adhesion. The thermal stability was also improved, indicating existence of some chemical interaction between WTD and the PP matrix in the presence of both DCP and HVA-2 during processing. The results imply that both chemicals had great influences on WTD destabilization, leading to the enlargement and intensification of the copolymer formation and, hence, improved the interfacial adhesion between WTD and the PP matrix. A competing reaction mechanism that may contribute to the property enhancement of PP/WTD_P-HVA2 blends was proposed.     

Increasing the Hydrophobicity of a PP Film Using a Helium/CF<Subscript>4</Subscript> DBD Treatment at Atmospheric Pressure

Plasma surface modification is widely used to tailor the surface properties of polymeric materials. Most treatments are performed using low pressure plasma systems, but recently, atmospheric dielectric barrier discharges (DBDs) have appeared as interesting alternatives. Therefore, in this paper, an atmospheric He + CF₄ DBD is used to increase the hydrophobicity of a polypropylene (PP) film. The surface characterization of the PP film is performed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results show that the hydrophobic properties of the polymer films are greatly enhanced after plasma treatment as evidenced by an increased contact angle. The incorporation of fluorine on the surface is significant (45 at%), demonstrating the ability of the used DBD set-up to generate fluorine-containing functional groups on the PP surface.     

Antistatic ability of 1-n-tetradecyl-3-methylimidazolium bromide and its effects on the structure and properties of polypropylene

In this study, 1-n-tetradecyl-3-methylimidazolium bromide ([C₁₄mim]Br), one kind of imidazolium ionic liquid (imi-IL), was incorporated into polypropylene (PP) via melt blending. The measurement of surface resistance (R_s) and volume resistance (R_v) of neat PP and PP/[C₁₄mim]Br blends indicated that [C₁₄mim]Br had excellent antistatic properties. The PP/[C₁₄mim]Br blend had the best antistatic ability, when the weight ratio of [C₁₄mim]Br to PP reached 3/100. The surface resistance of PP/[C₁₄mim]Br decreased from the 7.67 × 10¹³ to 1.40 × 10⁷ Ω whereas the volume resistance of PP decreased from 2.67 × 10¹⁴ to 2.60 × 10⁷ Ω. Semicrystalline morphology and crystal structure were investigated by polarized optical microscopy (POM) and X-ray diffraction (XRD). The spherulites of PP became smaller with the addition of [C₁₄mim]Br, implying that [C₁₄mim]Br had a nucleating effect in the PP matrix. The XRD study indicates the crystallization process of PP was affected by [C₁₄mim]Br, and the crystallinity of PP was thus decreased. Scanning electron microscopy (SEM) studies reveal that [C₁₄mim]Br had good dispersion in PP; thermogravimetric analysis (TGA) shows the addition of [C₁₄mim]Br remarkably increased the thermal stability of PP even though it is a small molecule.