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Key-Words: Polymers, plastics, phase transitions, crystallization, mesophases, glass relaxation, Grüneisen parameter, hydrostatic pressure, volume, compressibility, mechanical properties, dynamic shear compliance, resonators, dielectric relaxation, Pockels-effect, non-linear optics, polarization microscope, Cotton-Mouton-effect, Langmuir trough, molecular modelling, self-consistent harmonic approximation, semiempirical quantum mechanical PCILO method, molecular parameters
We are working towards a more thorough understanding of the relations between structure and properties of amorphous, semi-crystalline and liquid crystalline polymers. This implies the development of models of mole- cular structure and superstructure for melts, networks, solutions and gels. The following problems in polymer physics are investigated:
Atomistic calculations with semiempirical potentials (molecular mechanics, molecular modelling) yield important molecular parameters (conformations, lattice parameters, defect energies and -volumes of dis- locations and interfaces) for a quantitative description of molecular structure and superstructure of polymers and oligomeric model systems. Calculations of conformations and packing of trans-polybutadiene chains in the low and high temperature phase have been performed. The combination of the calculated molecular data with statistical thermodynamics resulted in pressure-dependent phase transition data which are in good agreement with experiments (Chwalek, Hägele, Bux).
The self-consistent harmonic approximation (SCHA) has been extended to molecular crystals. Starting from well-known low temperature potentials the anisotropy of the thermal expansion of polyethylene up to 300 K was calculated in good agreement with experimental data. Now other temperature dependent properties (phonon frequencies, elastic moduli, e.g.) are investigated. The limits of the SCHA have been discussed in detail and in comparison with other methods (Stobbe, Hägele, Matuschek).
The semiempirical PCILO method in quantum mechanics is useful to calculate the conformations of rather large chain molecules with a moderate amount of computer time. The contributions to the perturbation ex- pansion have been used to visualize the interaction energies. In cooperation with the FH Ulm a graphical user interface and improved minimalization algorithms have been added to an existing computer program. An implementation on a workstation was done. Another extension of the program was developed to calculate lattice data of crystals. Further investigation will include calculations on polymers where no reliable atomic potentials are available up to now (Mallon, Hägele, Hänisch, Doser).
Phase transitions in polymers and oligomeric model systems are characterized by cooperative changes of the intra- and intermolecular order, from the crystal to the melt. The experimental and theoretical study of phase transitions on the basis of atomistic parameters leads to a deeper understanding of molecular organisation and the properties of the different phases.
Condis phases (in polyethylene and trans- 1,4-polybutadiene), nematic phases (in poly- methacrylates and polysiloxanes with mesogenic side groups) as well as lamellar/columnar mesophases (in polydialkyl-siloxanes, cyclolinear polysiloxanes and polydialkoxyphosphazenes) were investigated. The experimental methods are polarization microscopy, calorimetry in cooperation with the section for calorimetry (Dr. G. Höhne), dilatometry and dynamic mechanical spectroscopy.
In addition, turbidity measurements were performed for blends of PS with TMPC up to 100 MPa in order to investigate the phase separation. These are in good agreement with calculations on the basis of the meander model, which predicts either homogeneous or nano-structural blends and phase separated ones. Moreover, this model can account for the melting behaviour and the kinetics of crystallization of polyethylene. Progress was made in the theoretical treatment of the most important phase transitions (melting, evaporation, order/disorder-transitions) (Hägele, Pechhold, Sautter, Theobald).
Thermal properties of polymeric crystals, glasses and melts are investigated in a wide range of temperature and pressure by calorimetry (in cooperation with Dr. G. Höhne) and dilatometry. The growing use of computers in polymer processing needs more and more precise volume data for polymeric materials as a function of temperature and pressure. Moreover, these data are needed for the test of a new analytical equation on the basis of solid state physics, which proved successful already for the glassy state of polycarbonate and polystyrene. Polymer glasses, which are deformed or prepared under high pressure, show an instability (changes of elastic moduli, peak in cp-curves) during the first heating run; we are trying to describe the corresponding molecular motions. Experimental data for the differences cp, and between glass and melt as well as for the melting temperature, entropy and volume change (Tm, Sm, Vm) are measured and used for the study of model theories (Dollhopf, Pechhold, Stoll, Mayr).
Models of the superstructure are based on the intra- and intermolecular interaction energies which are highly anisotropic. They may predict some important properties of polymers. The size and the structure of the supermolecular meander cubes have been determined for liquid crystalline polymers starting from their relaxation kinetics and relaxation strengths. This model is able to explain the anisotropy of the radius of gyration in magnetic field which was determined from small angle neutron scattering as well as the anisotropy of the shear compliance observed in a monodomain sample.
Moreover, the radius of gyration of polymer chains in solution can be calculated for different values of the -Parameter by minimization of the Gibbs free energy (for chain folding, superfolding and mixing with the solvent inside of topological "tubes"). The gelation of atactic polystyrene in CS2 exhibits a volume change in the order of 0.001, but, on the other hand, a very strong orientational correlation of polymer segments, which exceeds that of the solution by more than a factor of 1000 in the case of low Mw. (Brackenhofer, Groß, Pechhold, Sautter).
The molecular motion in polymers is influenced by the chemical structure and the intramolecular potentials on one hand, by the molecular packing, by defects and superstructure on the other. Our research is focused on the glass relaxation, which is investigated by means of dieletric and mechanical spectroscopy in a wide range of frequency, temperature and pressure. The analysis of these data in the frame of the meander model yields information about the size of the superstructural units and the energy of quasi-dislocations. The latter are responsible for the kinetics of cooperative molecular motions. These data can be used to calculate the specific heat capacity and the volume during cooling and heating in the vicinity of the glass temperature. This model is able - without further assumptions - to predict fluctuations of the superstructure which lead to a broadening of the glass relaxation; this is in agreement with experimental data.
Polymers with integrated chromophores are recently investigated for application in non-linear optics and in optical waveguides for telecommunication. The linear electro-optical effect, which is characterized by the Pockels coefficient, should be high and remain constant for a long time. Measurements of the Pockels-coefficient were performed in our department above and below Tg. They show a strong correlation of the reorientation of the chromophores with the segmental motion of the macromolecular chains as predicted by the meander model. The incorporation of "label molecules" (with a strong dipole moment) in polymer systems could reveal the rotation of the superstructural units (Pechhold, von Soden, Stoll, Theobald, Wedekind).
Mechanical relaxation and rubber elasticity of polymer melts, blends and polymer networks are studied by the analysis of master-curves of the dynamic shear compliance. Deformation and flow are assigned to molecular motions within and between meander blocks and it was possible to calculate the relaxation parameters for glass process, shear band process and flow relaxation as a function of molecular weight, density of crosslink, swelling ratio, filler content and particle size. The stress-strain behaviour at high elongations and the fracture of crosslinked rubbers are also investigated as well as the amplitude dependence of the dynamic properties (Eckert, Pechhold, von Soden).
Publications:
Doctorate Degrees:
1993 P. Schwarzenberger:
Beschreibung und Anwendung der Quarzresonatormethode zur Bestimmung
des komplexen Schermoduls von viskoelastischen Stoffen auch unter
hydrostatischem Druck"
1993 M. Gnoth:
"Dielektrische Untersuchungen im Frequenzbereich von 10-2
Hz bis 108 Hz, auch unter hydrostatischem Druck, an Mischungen
von Polymeren und niedermolekularen Flüssigkristallen und
ihre Interpretation im Rahmen eines molekularen Modells"
1994 F. Soergel:
Biomechanische Charakterisierung der menschlichen Augenhornhaut
mit dynamisch-mechanischer Spektroskopie
1994 J. Siegel:
Dielektrische Relaxation von NLO-Farabstoffen in Polymeren
1994 W. Diegritz:
Konstitution von Kautschukwerken unterschiedlicher Vernetzungsart
aus dynamischer Scherkomplianz und Grenzeigenschaften
1995 M. Schmid:
Mechanische Spektroskopie mit ausgewählten Polymernetzwerken,
auch amplitudenabhängig
1995 W.E. Mehr:
Entwicklung und Anwendung einer Schwingungssonde zur Messung der
komplexen Scherkomplianz auch unter Vorlast und mit großen
Amplituden im Frequenzbereich 10-3 Hz bis 102 Hz sowie mechanisch-dynamische
Untersuchungen an ausgesuchten Netzwerksystemen