Main lectures

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ML 01

CERTIFICATION OF COMPOSITE STRENGTH RELIABILITY USING PROBABILITY MODELING

E.M. WU¹, J.L. KARDOS²

This paper focuses on the probability modeling of fiber composite strength, wherein the failure modes are dominated by fiber tensile failures. The probability model is the Tri-modal Local Load Sharing Model which is the Phoenix-Harlow Local Load Sharing Model with the filament failure model extended from one mode to three modes. This model results in increased efficiency in the determination of fiber statistical parameters and in lower cost when applied to a) quality control in materials (fiber) manufacturing, b) materials (fiber) selection and comparison, c) accounting for the effect of size scaling in design, and d) qualification and certification of critical composite structures that are too large and expensive to test statistically. In addition, possible extensions to proof testing and time-dependent life prediction are discussed and preliminary data are presented.

ML 02

HOW WILL ADDITIVES SHAPE THE FUTURE OF PLASTICS?

R. PFAENDNER

Additives are essential components of plastic formulations providing maintenance and/or modification of properties, long-term use, and performance. The extension of polymer properties by additives has been playing a substantial role in the growth of plastics. Processing and use of plastic materials in outdoor applications would not be feasible at all without efficient additives. Polymers such as polypropylene, PVC and elastomers cannot even be transformed thermally without efficient stabilizers.

In the past, additives were mainly used as materials to maintain polymer properties and to help plastics to survive heat treatment during transforming (heat stabilizers, antioxidants, processing aids, lubricants), to extend their service life (light stabilizers, biocides), to provide protection (flame retardants) or to modify mechanical and physical properties (fillers, glass fibers, impact modifiers, antistats, nucleating agents). These well-established additives cover the requirements of standard plastics and today´s mass applications.

The more recent developments of additives address more stringent or new requirements, more severe processing and use conditions and/or environmental concerns. However, despite of an improved performance these additives still do not exceed the target of maintaining the initial properties of plastics. Examples include grafted antioxidants with non-staining performance, HALS light stabilizers which perform in the presence of greenhouse pesticides, additives for recyclates which address specifically the predegraded structures of post used plastics and stabilizers targeting the specific stability requirements of nanocomposites.

What will be the future trend for plastic additives? Additives will introduce more and more functionalities in plastic applications. They will not only maintain but more and more influence and modify the basic properties of polymers, broadening the potential use of plastics and driving the substitution process of the different plastic materials where e.g. commodity plastics are used in the traditional field of engineering plastics. Furthermore, additives will not only modify the polymer itself and add new properties, but can, incorporated in the plastic, beneficially influence outside properties. Several emerging examples will illustrate the hypothesis e.g. reactive additives to tailor polymer properties during processing resulting in the modification of polymer rheology with custom designed performance such as melt strength enhancement of polyesters.

Surface modifiers target hydrophilic or hydrophobic behavior of plastics and provide water absorption or water repellency. Smart additives protect the content of packaged goods, or shift UV light to visible light range. (For example, they act as growth promoters of plants if incorporated in greenhouse films.)

The presentation shows the role of additives used in plastics from past to present and outlines future trends.

ML 03

THE FUTURE TREND OF CLAY/POLYMER NANOCOMPOSITES

F. GAO

Clay/polymer nanocomposites are one of those few limited technologies in nano-field, which can be applied immediately for commercial application. Because of this reason together with those promising improvement of a wide range of physical and engineering properties made in the early stage of the development, this subject of science has received great attention by academics, industries and politicians. Expectations were high in the early stage of the development whilst some technical barriers encounted in the recent development casted doubt on the future applications of this technological approach. This presentation will give a rational analysis of the potential of using clay/polymer nanocomposites as reinforcing and functional materials regardless of the current technical barriers.

ML 04

POLYOLEFIN-CLAY NANOCOMPOSITES: TAILORING MORPHOLOGIES FROM THE INTERFACIAL PHYSICO-CHEMISTRY AND SHEARING CONDITIONS

S. BOUCARD^a,b, J. DUCHET-RUMEAU^a, P. PRELE^b, J.F. GERARD^a

Polyolefin nanocomposites are a new class of materials with promising properties for many usual applications. These new filled materials represent a new strategy in polymer reinforcement in terms of routes for processing via a molten process nanomaterials combining hydrophilic nanoparticles such as clays and an hydrophobic polymer. With the high specific surface of nanofillers, numerous interactions can be developed at the interface with the polymer chains and a small volume fraction of nano-objects ( less than 10 wt. %) is enough to increase mechanical, thermal, and gas barrier properties keeping a rheological behaviour suitable for further processing.

The paper deals with the strategy of processing nanocomposites based on an isotactic polypropylene matrix and organophilic nanoclays, such as montmorillonites. To develop a maximum of interfacial interactions, the most efficient dispersion of the nanoclays is required, i.e. exfoliation as individual nanoplatelets or the intercalation of polymer chains in the intergallery regions. The combination between the physico-chemistry developed at the interface between the organic medium and the exchanged inorganic surfaces and the shear effects induced via the extrusion process is discussed.

• The interfacial physico-chemistry is managed using various coupling agents, from reactive solvent to maleic anhydride grafted polymers having different microstructures, i.e. grafting ratio and molar masses which govern the miscibility and further entanglements/co-crystallization with the matrix chains and kinetics of diffusion in the PP matrix to the clay surface
• The shearing process is also design by varying the parameters of extrusion process (screw profile, viscosity of the medium defined from temperature and rheological behaviour of the components, etc.)

Different morphologies can be designed and characterized at different scales (from the d-spacing scale to few tens of micrometers using WAXS and TEM) by tailoring this combination between interfacial phenomena and shearing conditions. Rheological behaviours and mechanical properties of nanocomposites are discussed in order to characterize these morphologies compatibilizer used. From this work, we are able by coupling chemistry with processing to design various morphologies of nanocomposites relevant to desired macroscopic properties.

ML 05

MICROMECHANICAL EFFECTS OF BALANCING STRENGTH AND TOUGHNESS OF POLYMERS

G.H. MICHLER

Usually, polymers with a high strength posses only a limited toughness and elongation at break, whereas tough polymers are not very stiff and strong. In accordance to this, the different methods and techniques of toughness enhancement (e.g. toughening due to rubber particles, network or inclusion yielding, filler particles) yield to modified polymers with reduced stiffness and strength. However, recent developments in polymer research have shown that a significant variation and improvement of mechanical properties can be achieved by structural modification not only on the µm-scale but also on the nm-scale. Modification on the nm-scale can cause novel nanomechanical effects and mechanisms, which can be used for defined materials improvement, particularly for a combination of the contradictory properties stiffness, strength and toughness.

A very direct way to study the structural hierarchy and the µm- and nm-scale is investigation of loaded and deformed samples using the different techniques of electron and scanning force microscopy. There are special micro-tensile devices available, enabling deformed samples to study directly in the microscopes.

Three groups of nanostructured polymers are discussed in detail:

• In block copolymers unusual periodic structures can be realized due to variations of the molecular architecture (asymmetry, interfaces, …). A layered, lamellar morphology was created in PS/PB block copolymers with a high content of PS above 80 % and large elongations at break of about 200-300 %.
• By coextrusion procedures polymer blends with alternating continous layers with thicknesses ranging from some microns to several 10 nm can be produced. Only by variation of the layer thicknesses transitions are possible from brittle to tough behaviour.
• Using the electrospinning process nanofibres with diameters of some 10 nm containing inorganic particles with an improved spacial distribution have been prepared, yielding stiff and tough composites.

ML 06

PLASTIC DEFORMATION AND ORIENTATION OF SEMICRYSTALLINE POLYMERS BY PLANE-STRAIN COMPRESSION AND ROLLING

Z. BARTCZAK

Plastic deformation behavior of several semicrystalline polymers including polyethylene and isotactic propylene, was investigated. The micromechanisms involved as well as the resulting orientation were studied. In order to avoid any unwanted side-effects a cavity-free compression in the plane-strain conditions was chosen as a deformation mode. This deformation mode was realized either by static compression in a channel-die or by a continuous method of rolling with side constraints.

It was found that both plane-strain compression and rolling result in homogeneous deformation without any instabilities or premature fracture up to high strains, frequently exceeding compression ratio 10 at room temperature. The strain is accommodated by deformation mechanisms of crystallographic nature, operating in a crystalline component, supplemented by shear of the amorphous layers.

The primary deformation mechanisms active in crystals are the crystallographic slip systems, especially those operating along the chain direction. They are supplemented occasionally by twinning or martensitic transformations. Deformation of amorphous layers by shear substitutes lacking slip systems in the directions crossing the chain direction in crystals, which allows for strain accommodation of the entire semicrystalline material. An advanced cooperative action of crystallographic mechanisms and amorphous shear leads frequently to partial or even complete destruction of the initial lamellar structure and eventual restructurization into a new highly oriented fibrillar structure.

The studies of the deformation in plane-strain compression and rolling allowed to describe the entire deformation sequence and the role of particular deformation mechanisms (operating in either crystalline or amorphous component) in the process, as well as to find out the key parameters responsible for the deformation behavior of a given polymer.

ML 07

MOLECULAR DYNAMICS SIMULATIONS OF MECHANICAL AND TRIBOLOGICAL BEHAVIOR OF POLYMERIC MATERIALS

W. BROSTOW

We use computer simulations to study the mesoscale behavior of polymeric materials. Time-dependent deformation mechanisms leading eventually to fracture have been followed for our viscoelastic materials as fuction of the material structure for 1- and 2-phase polymers. The latter include polymer liquid crystals (molecular composites) with relatively rigid LC-rich islands.

The mechanical properties of computer generated materials under an external tensile force have been determined under a variety of loading conditions. Computer simulations allow us to monitor the true stress level in the material and compare it with the engineering stress. Defects in the form of vacancies are seen to influence the mechanical behavior. The influence of the presence and spatial distribution of a rigid second phase in the flexible matrix has also been investigated.

Scratch resistance and recovery was simulated by applying a force perpendicular to a surface of the material and moving it along the surface. The local structure was found to greatly influence the tribological properties in terms of scratching depth and surface healing. The distribution of the second phase also plays an important role.

Our simulations provide us with information which is not available from experiments. They also allow us to independently study the effect of individual process variables and morphological features. The graphical visualization methods developed for analysis of simulation results have a broad application scope, from education to terrain mapping.

1. S. Blonski, W. Brostow and J. Kubát, Phys. Rev. B 1994, 49, 6494.

2. W. Brostow, M. Donahue III, C.E. Karashin and R. Simões, Mater. Res. Innovat. 2001, 4, 75.

3. W. Brostow and R. Simões, Rev. Plasticos Modernos 2002, 83, 177.

4. W. Brostow, A.M. Cunha, J. Quintanilla and R. Simões, Macromol. Theory & Simul. 2002, 11, 308.

5. W. Brostow, A.M. Cunha and R. Simões, Mater. Res. Innovat. 2003, 7, 19.

6. W. Brostow, J.A. Hinze and R. Simoes, J. Mater. Res. 2004, 19, 851.

ML 08

CHARACTERISITICS OF CARBON FILMS PREPARED FROM POLYMER COMPOSITES CONTAINING CARBON NANOTUBES AND METAL PARTICLES

M. MATSUO, Y. BIN, A. KOGANEMARU, Q. CHEN

This paper deals with characteristics of carbon films prepared from polymer composites by heat treatment. The discussions were done for two kinds of composite, the polyimide-nickel powder composites and polyacrylonitrile (PAN)-multiwall carbon nanotubes (MWNTs) composites.

The catalytic effect of nickel was investigated for carbonization of polyimide films prepared film mixing polyamic acid with nickel particles. The three-layered polyimide film was prepared to obtain graphite films with thickness beyond 100. The middle layer composed of polyimide with nickel particles. The thickness of each layer was 50 and the film thickness became 150. The carbonization was done at ca. 1600°C for 5h. The morphology of the carbonized films was observed by scanning electron microscopy. The graphitization degree was investigated on the basis of X-ray diffraction intensity distribution from the (002) plane. The analysis was done in terms of the comparison between the experimental and theoretical diffraction intensity curves.

Secondly, composite films of PAN-MWNTs were fabricated by gelation/crystallization from solution. The composites films were elongated 2-fold and heat-treated at 2800°C to form oriented carbon films. The measurements were done for the orientation of resultant carbonized PAN-MWNTs films as well as morphology. The degree of graphitization was enhanced by the orientations of MWNTs as well as molecular chains of the matrix PAN. Interestingly, the graphitization of PAN by the heat-treatment was promoted on the surface of each MWNT oriented along the stretching direction. Furthermore, the relationships between the morphology and the electrochemical properties for the undrawn and drawn composite films were investigated by the electrochemical lithium insertion and extraction of the carbonized films as active working electrode materials. The results indicated that the reversible capacity and the coulomb efficiency of the drawn films were higher than those of the undrawn films suggesting that the reversible capacity can be improved by the structural orientation of the composite.

ML 09

New functionalities and property enhancement through non-conventional INJECTION moulding techniques

C.A. Silva, J.C. Viana, Z.A. Denchev, A.M. Cunha

Non-conventional injection moulding techniques opens new possibilities for functional enhancement of polymer-based devices. Fluid assisted moulding, micromoulding and multi-material have interesting examples as controlled drug release or combined mechanical performance and surface activity.

The use a controlled shear and elongation actions allows for constraining the polymer microstructural development, namely in semicrystalline thermoplastics. In reinforced materials the resulting distribution and orientation of the fillers can also be enhanced and controlled. As a result, mechanical performance and dimensional stability can be improved, but other effects associated to very particular arrangements of fibres matrix can also be achieved.

This communication will revise some report results obtained with a special mould (RCEM) was used to superimpose external rotation and axial actions to the pressure driven advancing flow front, during the moulding filling stage. The results evidence the microstructural and morphological manipulation possibilities in both reinforced and non-reinforced propylene homopolymers. Among other effects, novel bimodal oriented structures produced, as presented in figure 1.

a) conventional moulding	b) moulding obtained under rotation at 300 rpm

Fig. 1 - Bimodal semicrystalline structure produced with RCEM mould (SAXS pattern obateined in the DESY Synchrotron Facility).

ML 10

MULTI-SCALE APPROACH TO RESEARCH, EDUCATION AND PROMOTION OF POLYMERIC MATERIALS

M. RAAB

Finding complex relations between structure and end-use properties of polymeric materials is an important task of polymer physics. The structure, however, could be studied at various levels: starting from molecular, supermolecular and crystalline levels in the case of semi‑crystalline polymers, through phase and interphase morphology of polymer blends up to macroscopic structural gradients and structural defects of real injection-molded parts^1,2. The lecture will give a critical outline of the many physical and physical-chemical methods currently available to characterize the individual structural features. From the combination of their results a picture of the whole structural hierarchy can be compiled. Between individual structural levels feedback relations can also be recognized. Their understanding is important for optimizing technological processes, but also for students of polymer physics and engineer-ing. The structure-property relations of polyethylenes and isotactic polypropylene can serve here as instructive examples^3,4. Finally, the producers of polymeric materials should be able to explain to their customers how the specific molecular structure of their products affects the higher structural levels of the materials and how they, in turn, decide about the properties of the final structural parts.

1. E. Baer, A. Hiltner, D. Jarus:
Relationship of Hierarchical Structure to Mechanical Properties. Macromol. Symp. 147, 37-61 (1999).

2. Schmidt P., Baldrian J., Ščudla J., Dybal J., Raab M., Eichhorn K.-J.:
Structural transformation of polyethylene phase in oriented polyethylene/polypropylene blends: A hierarchical structure approach. Polymer 42, 5321-5326 (2001).

3. Kotek J., Raab M., Baldrian J., Grellmann W.:
The effect of specific β-nucleation on morphology and mechanical behavior of isotactic polypropylene. J. Appl. Polym. Sci. 85, 1174-1184 (2002).

4. Ščudla J., Raab M., Eichhorn K.-J., Strachota A.:
Formation and transformation of hierarchical structure of β-nucleated polypropylene characterized by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. Polymer, 44, 4655-4664 (2003).

The support of the Academy of Sciences of the Czech Republic (project AVOZ 40500505) is gratefully acknowledged.