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HIERARCHICAL STRUCTURE AND MECHANICAL PROPERTIES
IN POLYMERIC MATERIALS

ERIC BAER, ANNE HILTNER

Center for Applied Polymer Science and Department of Macromolecular Science,

Case Western Reserve University, Cleveland, Ohio 44106, USA

In order to aid the understanding of complex macromolecular systems, especially mechanical properties, the authors have suggested ”three rules of complex assemblies”. The first rule states that the structure is organized into discrete levels of scale: molecular, nanoscopic, microscopic, and macroscopic. The second rule states that the levels are held together by specific interactions between the components. Without the integrity supplied by the interactions on all scales, the system would not function as designed. The third rule states that these highly interacting levels are organized into a hierarchical architecture that is designed to meet a complex spectrum of functional requirements. The mechanical properties of both biological and synthetic polymers are explained with this hierarchical approach.

The mechanical properties of biological systems are used as examples of the relationship of hierarchical structure to mechanical properties. Collagen, a main component in soft connective tissues, is shown to be organized into different hierarchical structures in the form on tendons or intervertebral discs as examples. The hierarchical structure of tendon is designed to accommodate a uniaxial tensile force. The stress-strain behavior of tendon has four distinct regions and each region is explained by the hierarchical structure of tendon. Collagen in intervertebral discs undergoes axial compression and shear forces and therefore has a different, yet still hierarchical structure. Finally, the excellent mechanical properties of wood are explained by the hierarchical structures.

The rules of complex assemblies is also applied to synthetic polymers for a clearer understanding of the mechanical properties. Classical examples of the phenomenon of crystalline structure and craze formation are described according to the technique above. The impact modification of polycarbonate is more easily understood when the system is explained in a hierarchical manner. Finally, the mechanical properties of a nano-composite of ATBN and MMT is explained by the hierarchical approach.

It also now possible to create or force hierarchical structures in synthetic polymers to achieve specific mechanical properties by microlayering technology. Brittle polymers such as PMMA or SAN can be microlayered with PC to create tough materials due to the scale, interaction and architecture of the microlayered composite. The interfacial adhesion mechanism of PC/SAN microlayered composite are also explained with the above approach. Furthermore, bulk properties such as barrier or electrical conduction of microlayers are explained with the rules of complex assembly.

ML2

STRUCTURAL FEATURES AND FRACTURE PHENOMENA IN POLYETHYLENE

E.Q.CLUTTON, L.J. ROSE and G. CAPACCIO

BP Chemicals Ltd., Applied Technology, P.O. Box 21, Grangemouth FK3 9XH,
United Kingdom

ML3

DEFORMATION BEHAVIOUR AND MECHANICAL PROPERTIES OF HARD ELASTIC AND POROUS FILMS OF POLYETHYLENE.

G.K. ELYASHEVICH

Institute of Macromolecular Compounds, Russian Academy of Sciences,
Bolshoy pr. 31, RU-199004 St. Petersburg, Russia

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GELATION/CRYSTALLIZATION MECHANISM OF CRYSTALLINE POLYMER SOLUTIONS AND THE MORPHOLOGY AND DRAWABILITY OF THE RESULTANT FILMS

MASARU MATSUO

Faculty of Human Life and Environment, Nara Women`s University, Nara 630-8263, Japan

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FEEDBACK IN MECHANICAL BEHAVIOUR OF POLYMERS

MIROSLAV RAAB

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic

A hierarchical structure of polymers [1] includes several levels starting from the molecular architecture to the supermolecular morphology and up to the macroscopic geometry of a structural part. Under mechanical stress, a corresponding hierarchy of the deformations and fracture processes can be followed. The hierarchical levels of the mechanical behaviour, however, do not develop independently; the processes in individual structural levels control each other and these mutual interrelations can frequently be described in terms of feedback processes [2]. Several examples of such behaviour are described and discussed in the lecture:

• Crack propagation in ductile polymers [3].
• Equivalence between the degradation and the crack.
• Interrelations between the macroscopic and microscopic plasticity.
• Cracks and necks as mechanical amplifiers or oscillators.
• Long-range interrelations between the crack and plasticity.
• Interactions between the primary and secondary cracks [4].
• In summary, the concept of feedback is a useful tool for the structural understanding of the complex processes of macroscopic mechanical behaviour of polymeric materials.

References

1. E. Baer, A. Hiltner: Hierarchical Structure and Mechanical Properties of Polymeric Materials. This Conference, Lecture ML 1.
2. M. Raab, F. La Mantia, J. Pospíšil: Competition between Degradation and Orientation in Semicrystalline Polymers. Angew. Makromol. Chem. 176/177, 93 (1990).
3. M. Raab, Schulz E., Pelzbauer Z.: Eine Anomalie des Festigkeitsverhaltens von Polymeren. Plaste Kautsch. 33, 341 (1986).
4. F. Lednický, Z. Pelzbauer: Determination of Changes in Fracture Propagation from Fracture Curves. Intern. J. Polymeric Mater. 3, 301 (1975).

Support from the Grant Agency of the Czech Republic (Projects 106/98/0718, 106/96/1372) and from the Grant Agency of the Academy of Sciences of the Czech Republic (Project A2050601) is gratefully acknowledged.

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NEW TOUGHENING MECHANISMS OF POLYMERS

G. H. MICHLER

Institute of Materials Science, Martin Luther University Halle-Wittenberg,
D-06099 Halle, Germany

A profound understanding of the relationships between the morphology and deformation properties of polymers is important for the development of polymer systems with improved mechanical properties. A very direct way of studying such structure-property-relations is opened up a better knowledge of micromechanical processes of deformation and fracture. For many applications of polymers toughness is a decisive property. To improve toughness of a polymeric material, various modifier particles with different physical properties (e.g. rubber particles, core-shell particles, heterogeneous particles) can be added to the polymeric matrix. In such a modified polymer the modifier particles act as stress concentrators of the applied stress and initiate at and between particles plastic deformation processes in the matrix, either in form of crazes or shear yielding.

Direct investigation of the deformation processes in polymers is rendered possible by techniques of electron microscopy, including scanning (SEM), transmission (TEM), and high voltage electron microscopy (HVEM) as well as methods of scanning force microscopy SFM) [1].

Using these direct microscopic techniques, micromechanical properties of different polymer blends on the basis of amorphous and semicrystalline polymers are discussed:

• SAN blends reveal the different efficiency of core-shell (PBA) particles and homogeneous rubber (PB) particles to initiate multiple matrix yielding and to prevent premature cracking. Additionally, a process of plastic deformation of the cores inside the modifier particles has been found as a new toughening mechanism [2].
• PP blends have been deformed at lower temperatures up to –100 °C to study low temperature toughness. A surprising result has been found in PP/EPR blends:

  The EP shell is cavitated with formation of coarse fibrils and the adjacent matrix shows fibrillated crazes. This is an effect far below the glass transition temperature T_g of the rubber phase and it demonstrates a new toughening mechanism [3].

References