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Über dieses Buch

The first edition of this book had been written with the special aim to provide the necessary information for an understanding of the deformation and scission of chain molecules and its role in polymer fracture. In this field there had been an intense ac­ tivity in the sixties and early seventies. The new results from spectroscopical (ESR, IR) and fracture mechanics methods reported in the first edition had complemented in a very successful way the conventional interpretations of fracture behavior. The extremely friendly reception of this book by the polymer community has shown that the subject was timely chosen and that the treatment had satisfied a need. In view of the importance of a molecular interpretation of fracture phenomena and of the continued demand for this book which still is the only one of its kind, a second edition has become necessary. The aims of the second edition will be similar to those of the first: it will be at­ tempted to reference and evaluate completely the literature on stress-induced chain scission, now up to 1985/86. References on other subjects such as morphology, vis­ coelasticity, plastiC deformation and fracture mechanics, where the treatment was never meant to be exhaustive, have remained selective, but they have been updated.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Deformation and Fracture of High Polymers, Definition and Scope of Treatment

Abstract
The importance of a thorough understanding of the deformation behavior and the strength of polymeric engineering materials need not be emphasized. It is obvious to anyone who wants to use polymers as load bearing, weather-resisting, or deformable components or who wants to grind or degrade them. “Strength” and “fracture” of a sample are the positive and negative aspects of one and the same phenomenon, namely that of stress-biased material disintegration. The final step of such disintegration manifests itself as macroscopic failure of the component under use, be it a water pipe, a glass fiber reinforced oil tank, or a plastic grocery bag. The preceding intermediate steps — nonlinear deformation, environmental attack, and crack initiation and growth — are often less obvious, although they cause and/or constitute the damage developed within a loaded sample.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 2. Structure and Deformation

Abstract
The basic structural elements of high polymer solids are the chain molecules. The large variety of their chemical structures and their flexibility permit widely different modes of organization and mechanical interaction, thus giving rise to a variety of deformation mechanisms and fracture patterns. At this point the characteristic elements of structure and superstructure of amorphous and semicrystalline polymers will be introduced. The interrelations between chain parameters (structure and regularity), crystal or superstructural parameters (degree of crystallinity, lattice structure, nucleation, growth kinetics, and defects of crystals), and external parameters are extensively discussed in the literature and are not the principal object of this book [see Ref. 1–3, 107, for an introduction and for further references].
Hans-Henning Kausch-Blecken von Schmeling

Chapter 3. Statistical, Continuum Mechanical, and Rate Process Theories of Fracture

Abstract
Test specimens loaded under laboratory conditions, but also regular engineering components fracturing in service, provide many different data which lend themselves to evaluation of the fracture process. These data are, for instance, the time to onset and completion of fracture, details of the fracture pattern (ductile or brittle fracture, appearance of the breaking specimen and of the fracture surface), crack dynamics, and change in physical or chemical properties. Naturally the most straightforward evaluation of a test or a set of data is the direct correlation of the property of interest (e.g., time under stress) to the environmental parameter(s) of interest (e.g., stress and temperature). In Figure 1.4 a set of data with just these variables has been plotted (PVC pipes under internal pressure). If one uses such a plot a number of questions arise:
  • — what is the statistical significance of an individual data point
  • — what are the failure conditions and how do they depend on external parameters and material properties
  • — what is the likely cause of failure
  • — what conclusions can be drawn with respect to extrapolating the set of curves into hitherto non-accessible regions of time, pressure, or temperature?
Hans-Henning Kausch-Blecken von Schmeling

Chapter 4. Strength of Primary Bonds

Abstract
In the last chapter, in the section on molecular theories of fracture, almost throughout an Arrhenius equation has been used to describe the activation of element breakage. The energy of activation, U0, frequently turned out to be equal to (or was assumed to be equal to) the dissociation energy of the weakest main chain bond. Before further analyzing the kinetics of element — and possibly chain — breakage a definition of the mechanical strength of a bond and of a chain have to be given. In order to do this basic results of quantum chemistry [1,2] are recalled in this chapter which concern the “strengths” of intramolecular bonds and factors influencing the binding potential such as electronic excitation or ionization.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 5. Mechanical Excitation and Scission of a Chain

Abstract
A chain molecule as part of a thermoplastic body is in thermal contact with other chains and constantly in thermal motion. The atoms vibrate and take part in the more or less hindered rotations of groups and even of chain segments. With no external forces acting all molecular entities try to approach — and fluctuate around — the most probable conformation attainable to them. The action of external forces causes — or maintains — displacements of the chain from those positions and evokes retractive forces. Let us consider a chain or a bundle of chains in thermal contact with the surrounding and at constant volume. The condition of thermodynamic stability of such a system is that the free energy
$$ F = U - TS $$
assume a minimum. The differential changes of the free energy of the system are given by the changes of its internal energy U and its entropy S. Under isothermal conditions has:
$$ dF = dU - TdS $$
.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 6. Identification of ESR Spectra of Mechanically Formed Free Radicals

Abstract
The action of axial tensile forces on a molecular bond R1–R2 results in a decrease of the apparent binding energy of the bond and thus in an increase of the probability for bond scission. If the reduction of the apparent binding energy is sizeable the mechanical action may be considered as the main cause of chain destruction. In as much as the scission of a chain molecule into organic radicals and the resulting appearance of unpaired free electrons is governed by mechanical forces a study of radical formation and of radical reactions will reveal information on the forces acting on a chain at a molecular level. The method of investigating free radicals by paramagnetic resonance techniques has been highly developed during the last thirty years [1,2]. Since then it was successfully applied to elucidate the mechanism of the formation of free radicals in chemical reactions and under irradiation of visible and ultraviolet light, of x- and 7-rays, and of particles [1.3]. Also the value of the spectroscopic splitting factor g, the magnetic surrounding of the unpaired free electron spin, and the structure of the free radical have been studied. In all these cases the free electron spin acts as a probe, which, at least temporarily, is attached to a certain molecule, takes part in the motion of the molecule, and interacts with the surrounding magnetic field.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 7. Phenomenology of Free Radical Formation and of Relevant Radical Reactions (Dependence on Strain, Time, and Sample Treatment)

Abstract
Historically, the first ESR experiments with mechanically produced free radicals have been made in the Ioffe-Institute in Leningrad in 1959 [1] with ground or milled polymers, the samples being investigated after completion of the fracture process. To elucidate the effect of structural or environmental parameters on the kinetics of stress-induced free radical formation it is necessary to study — by ESR — highly stressed chains during the loading process. As outlined in Chapter 5 a notable energy elastic straining of a chain can only be achieved if the chain cannot relieve stresses internally by a change of conformation or externally by slippage against the field exerting the uniaxial forces. Vice versa the mechanical scission of a chain must be taken as an indication that at the time of chain rupture axial stresses ψ equal to the chain strength ψ c had not only been reached but also maintained during the average lifetime period T c of the chain segment.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 8. The Role of Chain Scission in Homogeneous Deformation and Fracture

Abstract
Section II B of Chapter 2 gave a description of the uniaxial deformation behavior of an unoriented thermoplastic polymer. It was indicated that — depending on experimental and material parameters — failure could occur at any of the different stages of a tensile loading process:
  • — as brittle fracture due to crack initiation and propagation during anelastic deformation
  • — as a consequence of plastic flow after necking or homogeneous yielding
  • — as ultimate failure following plastic deformation with strain hardening.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 9. Molecular Chains in Heterogeneous Fracture

Abstract
In Chapter 8 principally the contribution of spatially homogeneously distributed molecular rearrangements to polymer fracture was studied. The term spatially homogeneous referred to the absence of flaws, inclusions, cracks, or notches of a size sufficient to act as a stress concentrator. Under those conditions during an initial phase of external loading damage development or growth is homogeneously distributed on a macroscopic scale. Heterogeneous fracture now is defined as the converse of homogeneous fracture or briefly as fracture through crack propagation. In this case cracks, notches, inclusions, or accumulated crack nuclei act as macroscopic stress concentrations and essentially confine further damage development to the close proximity of the then existing defect(s). The phenomenon of crazing has been included in this chapter because of the well observable structural irregularities and despite the fact that with increasing stress new crazes can be formed at arbitrary nucleation sites.
Hans-Henning Kausch-Blecken von Schmeling

Chapter 10. Fracture Mechanics Studies of Crack Healing

Abstract
The formation of cracks, i.e. fracture, and the healing of cracks in polymer solids are two complementary processes. Some of the molecular mechanisms described in the previous chapters on fracture also apply — by simply changing direction — to crack healing, such as relaxation of chains towards an equilibrium conformation, establishment of network isotropy, closure of voids and formation of entanglements. The role of these mechanisms in fracture has become clearer through the study of crack healing. It is for this reason that a chapter on this subject is included in a book on fracture.
Hans-Henning Kausch-Blecken von Schmeling

Backmatter

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