Sven Henning

Chevron morphology in deformed semicrystalline polymers

Chevron morphology was observed using transmission electron microscopy in various semicrystalline polymers deformed in tensile experiments. The morphological and mechanical prerequisites for chevron structure formation in semicrystalline polymers were revealed. It was demonstrated that chevron folding is a common deformation mode which can appear in real, i.e. globally unoriented or partially oriented samples, in areas where the lamellar stacks are oriented perpendicular to the deformation direction. Similarities with the behaviour of other layered systems were found. The mechanism of chevron formation is discussed in the light of the fundamental statements of the folding theories and is related to the specific microstructure of the polymers. The effect of boundary conditions, deformation temperature and macroscopic strain on the characteristics of the chevron structure is described.

Nanocristalline Hydroxyapatite for Bone Repair

The substitution of bone defects is of the main problems in contemporary orthopaedic surgery. The progress of skeletal surgery has opened new horizons of joint replacement, trauma management and skeletal repair in bone tumors. But loosening of joint protheses, tumor resection and reconstruction of traumatized bone needs to handle large size defects in the skeleton. Because bone grafts are limited for a number of reasons such as availability, risk of different viral infections, problems of storage or cost-intensive procedure of manufacturing, still remained a lack of adequate substitute materials to fill the bone defects. Bioactive ceramics became interesting and widely used as substitute materials for bone defects. An optimal substitute should replace the lacking tissue for a limited period, should be replaced step by step by new bone and should disappear without any remnants after doing its task. It has to be biocompatible, sterilizable, non mutagenic and non cancerogenic. However, the compact bioceramic implants used today mostly outlast in the bone. Their degradation behaviour in vivo is discussed controversially. It depends on a variety of characteristics such as sintering parameters, chemical composition, crystallinity and phase purity. Nanocrystalline hydroxyapatite had been described in the literature as interesting alternative to common compact or spongeous ceramic materials [1,2]. The present work describes the results of testing nanocrystalline hydroxyapatite in vivo in a rabbit animal model.

Morphology and stress-relaxation of biaxially oriented cross-linked polyethylene films

In the present work, the morphology and stress-relaxation of cross-linked polyethylene films were investigated. The stress-relaxation of cross-linked films was studied as a function of temperature and as a function of the irradiation dose applied. The relaxation time spectra of cross-linked films were observed to display additional relaxation times. Irradiation dose and temperature were shown to influence the relaxation time spectrum of cross-linked and non-cross-linked films. The crystallization of cross-linked and non-cross-linked polyethylene was compared. It was shown that the lower mobility of the cross-linked molecules affects the final crystallinity.

Influence of γ‐Irradiation on the Deformation Behaviour of Lamellar SBS Triblock Copolymers

The micromechanical deformation behaviour of lamellar styrene/butadiene block copolymers crosslinked by γ-irradiation was studied by transmission electron microscopy and scanning electron microscopy. Irrespective of the irradiation dosage, the micromechanical mechanism in these samples is predominantly necking and drawing of glassy lamellae accompanied by shearing in the rubbery phase. However, the samples behave in a brittle fashion at higher irradiation dosages, which is associated with the surface damage caused by γ-irradiation.

Morphology, Mechanical Properties and Stress-relaxation of Biaxially Oriented Polyethylene Films Irradiated with β-radiation

The present study describes an influence of β-radiation on morphology, mechanical properties and stress-relaxation of biaxially oriented polyethylene films. The mechanical properties and stress-relaxation of irradiated crosslinked films were studied as a function of the irradiation dose applied. The relaxation time spectra of crosslinked films were observed to display additional relaxation times. Irradiation dose of 4 megarads (MR) was found to be optimal from the viewpoint of mechanical properties. The crystallization of crosslinked and noncrosslinked polyethylene was compared. It was shown that irradiation affects the final crystallinity of polyethylene. Lower crystallinity was observed due to lower mobility of crosslinked molecules.

Comparison of structure formation upon drawing of gel-cast and melt-crystallised UHMWPE films

Abstract Structure and mechanical properties of ultra-high molecular weight polyethylene (UHMWPE) films of different draw ratios produced from gel-cast and meltcrystallised samples by multi-stage zone drawing have been studied in order to investigate the structural development and to clarify the role of interfaces between morphological units in the mechanical behaviour. The complex hierarchy of fibrillar structures generating upon neck formation is revealed. It is shown that the rearrangement of initial chain-folded crystallites into a microfibrillar structure occurs inside the volumes of supermolecular structures of larger sizes (spherulites in the melt-crystallised films and stacks of lamellae in the gel-crystallised films), the boundaries between the latter not having disappeared. Both the initial spherulites and stacks of lamellae with internally rearranged microfibrillar structure elongate upon drawing and comprise spindle-like units - macrofibrils - which are well resolved in scanning electron microscopy. Moreover, superfibrils have been revealed in the melt-crystallised films for the first time. They are, as suggested, the remnants of deformed and rearranged nascent polymer particles. Thus, the volume of the drawn films is considered to be full of interfaces. It is suggested that the majority of conformational defects, like chain ends, loops, tie molecules, entanglements, etc., are expelled from fibrillar crystallites in both intra- and interfibrillar regions. In addition to that, the interfibrillar spaces are crossed by tie molecules and micro- and macrofibrils. All of them hinder interfibrillar slip. The differences in creep and recovery of the investigated films are discussed in terms of density of packing and degree of connectivity of various scale fibrillar structural units. The lateral boundaries and the tips of fibrils are considered to be the weakest structural sites from which kink band formation and subsequent fracture of the loaded material begin.

Scanning Electron Microscopy, ESEM, and X-ray Microanalysis

Abstract The chapter gives a brief summary of the principles, advantages, and applications of scanning electron microscopy (SEM) and electron probe-based X-ray microanalysis (EDX). The focus is put on practical aspects, as there are issues of sample preparation and mechanisms of signal detection and contrast generation that should enable users to define the problems and interpret the results adequately. Further on, the advantages of environmental (or variable pressure) SEM are discussed. The general part is completed with a number of examples from different groups of materials. Finally, an overview of recent developments toward ultrahigh-resolution SEM and new mechanisms of contrast enhancement is given, covering interactions and specific advantages obtained by low-voltage techniques in combination with a new generation of signal detectors, especially the so-called inlens detectors.