J. Petermann

Transcrystallization in fiber‐reinforced isotactic polypropylene composites in a temperature gradient

Transcrystallization of isotactic polypropylene (iPP) on different fibers (carbon fiber, glass fiber, and aramid fiber) was conducted in a temperature gradient. The Ultra-High-Module carbon fiber (UHMCF), the High-Module carbon fiber, and the aramid fiber (Twaron) showed sufficient nucleation ability to form transcrystallization of iPP in certain temperature ranges. Among them, the UHMCF showed the best nucleation ability. On the contrary, the Intermediate-Module carbon fiber, the High-Tenacity carbon fiber, and the E-glass fiber showed too low nucleation ability to form transcrystallization of iPP. One efficient way to induce transcrystallization on these fibers was proved by pulling the fibers in supercooled iPP melts. The interface shear between fiber and supercooled matrix melt on crystallization and the interface temperature gradient between fiber and supercooled matrix melt on crystallization are considered to be two very important factors for the formation of transcrystallization.

DSC and TEM investigations on multiple melting phenomena in isotactic polystyrene

The multiple melting behavior and morphologies of isotactic polystyrene (iPS) isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The combination of thermal analysis and morphological results indicates that two lamellar populations are responsible for the so-called double melting behavior in iPS. The low-temperature melting peak is attributed to the melting of less perfect (thinner or defect containing) subsidiary lamellae formed in the framework of the dominant (thicker or more perfect crystalline) lamellae upon isothermal crystallization. The high-temperature one is mainly due to the melting of the dominant thicker lamellae, and to some less extent, the melting of a recrystallized population coming from the melted defect lamellae during the heating process in DSC.

Observation of shish crystal growth into nondeformed melts

Thin films of isotactic polystyrene (iPS) were deformed on a carbon supported TEM grid at temperatures between 210 °C and 240 °C. The elongation of the supercooled polymer melt films is localized in a deformation zone due to the formation of cracks within the brittle carbon support film, whereas the adjoining areas stay in a relaxed state. Since the nucleation of fibrous (shish) crystals needs an orientation mechanism to align the molecular chains, their origin is located in the deformation zone. It is shown that the subsequent growth of the shish crystal can propagate into the relaxed melt although it has to surmount the adhesion to the carbon substrate. From these results the conclusion is drawn that the shish crystal growth can be an autocatalytic process, which induces a self-orientation of the molecules in the growth front of the crystal tip and does not necessarily need an external flow field.

Controlling factors for the occurrence of heteroepitaxy of polyethylene on highly oriented isotactic polypropylene

The controlling factors for the epitaxial crystallization of high-density polyethylene (HDPE) on highly oriented isotactic polypropylene (iPP) substrates have been studied in detail by means of transmission electron microscopy and electron diffraction. The results obtained in this work indicate that the crystallization process must be considered in the investigation of epitaxial growth of polymers on polymeric substrates, because of the unique morphological and crystallization characteristics of polymers. Crystallization rate has an important effect on the epitaxial crystallization of polymers. Higher rates result in the formation of thicker epitaxial layers. Isothermal crystallization temperature is another factor affecting epitaxial growth of polymers. Lower temperatures are favorable to epitaxial crystallization of polymers. There exists a critical epitaxial temperature at given experimental conditions, above which no epitaxial growth occurs at all. The influence of crystal dimensions of both the substrates and the deposited polymers on epitaxial growth confirms that secondary nucleation is an important controlling factor for the occurrence of epitaxial crystallization in polymers. The requirement satisfying the secondary nucleation criterion is that the substrate crystal dimension in the matching direction must be greater than the crystal thickness of the deposited polymer. Once the requirement of the secondary nucleation is satisfied, subsequent epitaxial growth is based on the lamellar growth habit of the deposited polymer itself.

Shish-Kebab Crystals in Polyethylene Investigated by Scanning Force Microscopy

Abstract Ultra-thin, highly oriented polyethylene (PE) films drawn from the melt have been investigated using scanning force microscopy (SFM) to study their surface morphology. The results obtained from SFM measurements exhibit the lamellar morphology and evidence for extended chain crystals of the films. Morphological defects, such as structural thickness modulations of lamellae and steps on lamellar crystals of ⩽1 nm height, were obtained. The fact that the lamellar crystals protrude out of the film surface is explained by a combination of film processing and crystal growth effects. On a molecular scale, high resolution SFM investigations of the crystalline parts of the film show extended PE chains with an intermolecular distance repeat of 0.56 ± 0.1 nm.

EPITAXIAL CRYSTALLIZATION OF SYNDIOTACTIC POLYPROPYLENE ON UNIAXIALLY ORIENTED POLYETHYLENE

The epitaxial crystallization of syndiotactic polypropylene (sPP) on uniaxially oriented polyethylene (PE) has been investigated by electron microscopy and electron diffraction. It is found that the sPP lamellae grow epitaxially on the PE substrate film with the preference in sPP for the b axis as the fast growth direction. Instead of 50-degrees as in the system of isotactic polypropylene with PE, the molecular chains of the sPP crystals are approximately +/- 37-degrees inclined to the chain direction of PE.

Morphologies and mechanical properties of syndiotactic polypropylene (sPP)/polyethylene (PE) blends

The tensile properties of blends based on syndiotactic polypropylene (sPP) and high-density polyethylene (HDPE) have been studied. In order to understand the unexpected decrease in ductility, the crystallization behavior of these blends was characterized by transmission electron microscopy and in-situ Raman spectroscopy. For all investigated blends demixing occurs, and results for low HDPE concentrations in an island-like morphology of HDPE domains embedded in a continuous sPP matrix. For all investigated sPP/HDPE blends the start temperature of the sPP crystallization significantly increases with increasing HDPE content.

Morphological studies on syndiotactic polypropylene prepared by ansa- metallocenes

Abstract Propylene was homopolymerized with the ansa -metallocene Ph 2 C(fluorenyl)(cyclopentadienyl)ZrCl 2 / methylaluminoxane catalyst to a highly syndiotactic polymer using a constant set of parameters. The properties of the pure syndiotactic polypropylene (sPP) such as molecular weight, chain microstructure and melting point were investigated, and the morphologies of uniaxially oriented films were studied by transmission electron microscopy. The oriented sPP films have a lamellar structure and a single-crystal-like texture from typical preparation conditions. In comparison with the X-ray diffraction pattern reported by De Rosa and Corradini, the electron diffraction pattern shows more detail but leads to the same structure (cell III structure).

Epitaxial crystallization in the sPP/nylon-12 semicrystalline polymer system

The epitaxial crystallization behaviour of syndiotactic polypropylene (sPP) on highly oriented nylon-12 substrates has been investigated by means of transmission electron microscopy. The results obtained from bright field electron microscopy and electron diffraction indicate that sPP crystals grow epitaxially on the oriented nylon-12 substrate with their c-axes +/- 37 degrees apart from the chain axis of the nylon-12 substrate. The contact planes of the sPP crystals are the (100) lattice planes. Moreover, the epitaxial crystallization of nylon-12 on highly oriented sPP substrates from a dilute solution in cyclohexanone has also been studied using optical microscopy. The results show that the nylon-12 crystals grow epitaxially on the oriented sPP substrate with the oriented nylon-12 lamellae forming large, anisotropic domains. Copyright (C) 1996 Elsevier Science Ltd.

Morphological studies of PE crystallized between iPP and PTFE oriented films

The crystallization behavior of polyethylene (PE) between oriented isotactic polypropylene (iPP) and poly(tetrafluoroethylene) (PTFE) substrates in sandwich triple layered samples at various crystallization conditions was studied by means of transmission electron microscopy and electron diffraction. It is well known that both iPP and PTFE oriented substrates are active nucleation agents for PE, but result in different crystallization manners of PE. While the iPP substrate leads to an epitaxial crystallization of PE in a way, with the molecular chain directions of both polymers ±50° apart, an oriented overgrowth of PE on the PTFE substrate with parallel chains of both polymers is identified. By using sandwich samples with the PE in between the two different substrates, i.e. iPP and PTFE, the nucleation efficiency of both polymers for PE can be compared. The results indicate that at high undercoolings, the iPP and PTFE oriented crystals have about the same nucleation ability onto PE, but at lower undercoolings, the PE crystallizes all on the PTFE side. This indicates unambiguously that the nucleation of PE on the PTFE substrate starts at a higher temperature than that on the iPP oriented substrate.