Instrument-independent dynamic mechanical analysis of polymeric systems (original) (raw)
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Ultrasonic dynamic mechanical analysis of polymers
2005
The propagation of ultrasonic waves in polymers depends on their viscoelastic behaviour and density, resulting significantly affected by phase transitions occurring with changing temperature and pressure or during chemical reactions. Therefore, the application of low intensity ultrasound, acting as a high frequency dynamic mechanical deformation applied to a polymer, can monitor the changes of viscoelastic properties associated with the glass transition, the crystallization, the physical or chemical gelation, the crosslinking. Thanks to the non-destructive character (due to the very small deformation amplitude), low intensity ultrasound can be successfully used for polymer characterization. Moreover, this technique has a big potential as a sensor for on-line and in-situ monitoring of production processes for polymers or polymer matrix composites. Recently, in the laboratory of Polymeric Materials of Lecce University a custom made ultrasonic set-up for the characterization of polymeric material, even at high temperatures, has been developed. The ultrasonic equipment is coupled with a rotational rheometer. Ultrasonic waves and shear oscillations at low frequency can be applied simultaneously on the sample, getting information on its viscoelastic behaviour over a wide frequency range. The aim of this paper is to present the potential and reliability of the ultrasonic equipment for the ultrasonic dynamic mechanical analysis (UDMA) of both thermosetting and thermoplastic polymers. Three applications of UDMA to different polymeric systems will be reviewed, concerning the cross-linking of a thermosetting resin, the crystallisation from melt of a semicrystalline polymer and the water sorption in a dry hydrogel film. From the ultrasonic velocity and attenuation measurements, the viscoelastic properties of the tested polymers are evaluated in terms of complex longitudinal modulus and compared with the results of conventional dynamic mechanical analysis, carried out at low frequency.
Mechanical performance of polymer systems: The relation between structure and properties
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A direct relation between molecular characteristics and macroscopic mechanical properties of polymeric materials was subject of a vast number academic and industrial research studies in the past. Motivation was that an answer to this question could, in the end, result in guidelines how to construct tailored materials, either on the molecular level or, in heterogeneous materials, on the micro-scale, that could serve our needs of improved materials without the need of extensive trial and error work. Despite all attempts, no real universally applicable success was reported, and it was only after the introduction of the concept of the polymer's intrinsic deformation behavior that some remarkable progress could be recognized. Thus, it is first important to understand where intrinsic deformation behavior of polymeric materials stands for. Second, it is interesting to understand why this intermediate step is relevant and how it relates to the molecular structure of polymers. Third and, in the end, the most computational-modeling-based question to be answered is how intrinsic behavior relates to the macroscopic response of polymeric materials. This is a multi-scale problem like encountered in many of our present research problems. q
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Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformation of Polyethylene one can quote shear crystallites, fragmentation of crystalline lamellae, cavitation or martensitic transformation. Distinctly, the determination of the initiation of plasticity and damage within this kind of polymers remains questionable. The aim of this study is to characterize the plasticity and damage of several Polyethylene (PE) during tensile tests using the ultrasonic (US) monitoring technique. The proposed methodology uses both guided and longitudinal waves in the ultrasonic frequency range and enables to separate the geometrical effects from those of the material. It is shown that the US attenuation increases when the degree of crystallinity decreases. Besides, the US attenuation appears to be higher in the amorphous phase than in the crystal. During a tensile test, a strong decrease of the transmitted energy is observed once the yield point is reached, due to the formation of the fibrillar microstructure, which breaks the crystalline percolation. Finally, the results evidence that the chain alignment during a tensile test favors the wave propagation; in addition, cavitation induces a significant attenuation, which is strongly anisotropic as the voids are aligned with the fibrils.
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This paper reviews the literature on the response of polymers to high strain rate deformation. The main focus is on the experimental techniques used to characterize this response. The paper includes a small number of examples as well as references to experimental data over a wide range of rates, which illustrate the key features of rate dependence in these materials; however this is by no means an exhaustive list. The aim of the paper is to give the reader unfamiliar with the subject an overview of the techniques available with sufficient references from which further information can be obtained. In addition to the 'well established' techniques of the Hopkinson bar, Taylor Impact and Transverse impact, a discussion of the use of time-temperature superposition in interpreting and experimentally replicating high rate response is given, as is a description of new techniques in which mechanical parameters are derived by directly measuring wave propagation in specimens; these are particularly appropriate for polymers with low wave speeds. The vast topic of constitutive modelling is deliberately excluded from this review.
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Quantitative characterization of deformation in polypropylene fibers
Journal of Polymer Science Part A-2: Polymer Physics, 1968
The submicroscopic morphology of uniaxially deformed isotactic polypropylene films has been examined by small-angle light scattering (SALS), electron microscopy, optical microscopy, small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction, birefringence, sonic modulus, and density methods. Several new interpretations and extensions of existing theories are developed and verified experimentally as follows. (I) The V , SALS pattern is shown to be a new tool for the identification of the sign of the birefringence of spherulites too small to be seen in the optical microscope. The theoretical dependence of the V , SALS pattern is developed and verified experimentally with patterns from isotactic polypropylene, polyethylene, Penton, nylon 6,6, poly(ethy1ene terephthalate), and nylon 6,lO. (2) Intraspherulitic lamellar behavior during deformation can be identified from the SAXS pattern. This includes quantitative evaluation of the long spacing between lamellae and their average orientation. (3) The two-phase sonic modulus theory is valid over the wide range of deformations, crystallinities, processing temperatures, and molecular weights used in this study. The deformation of isotactic polypropylene films drawn a t 110 and 135°C. has been characterized quantitatively in terms of an integrated picture of m&s movement on all morphological levels: the molecular, the interlamellar, and the spherulitic. At both temperatures, the spherulites deform affinely with extension, whereas the deformation mechanisms within the spherulite depend on the location of the radii with respect to the applied load. During spherulite deformation, lamellar orientation and sep@ration processes predominate, whereas a t high extensions, fibrillation occurs and crystal cleavage processes predominate. The noncrystalline region orients throughout the draw region. At 135'C. non-orienting relaxation processes appear in the noncrystalline region which retard the rate of molecular orientation with extension. QUANTITATIVE CHARACTERIZATION OF DEFORMATION 1103 nylon 6,6 (Du Pont Zytel 101) produced positive spherulitic film by a treatment similar to that applied to poly(ethy1ene terephthalate). Negative spherulitic nylon 6,G film wsls prepared from dried polymer by fusion in a press at 500°F. for 1 min. at atmospheric pressure, then 1 min. at 900 psi, slow cooling under pressure for 10 min. to 4G0°F., and subsequent cold water quenching. Positive spherulites of nylon 6,lO (Du Pont Zytel 31) were prepared in a press by fusion for 2 min. at 440°F. with subsequent slow cooling from the melt under 1000 psi pressure for 10 min. Negative spherulites of nylon G,10 were prepared by fusion for 2 min. at 440°F. and subsequent room-temperature quenching. Solution Grown Spherulites. Three-dimensional spherulites were grown from Pro-fax 6601 flake. Negative spherulites, 2-5 p in diameter, were obtained by heating a 1% solution of the Pro-fax in p-xylene for 10 min. at 130°C. and then quenching the solution in a bath of Dry Ice and methylene chloride. Positive spherulites, 12-15 p in diameter, were obtained by heating a 15% solution of Pro-fax in a 70:30 mixture (by volume) of xylene and butyl Cellosolve at 134°C. arid then allowing the solution to cool Density slowly. Densities were measured in a 3A alcohol-water density-gradient column at 23.2 I 0.1"C. Wide-Angle X-Ray Diffraction These measurements were made with a Phillips diff ractometer equipped with a copper target, nickel filter, and a scintillation counter detector system. The measured intensities were corrected for polarization, absorption, background, and incoherent scattering. Calculations were made on an SDS 920 computer. Small-Angle X-Ray Scattering (SAXS) Small-angle x-ray scattering patterns of films were obtained with a W. H. Warhus vacuum camera equipped with 0.025411. pinholes. Nickel-filtered copper radiation was used, and the diffraction pattern was obtained at a sample-to-film distance of 29 cm. Birefringence Film retardation was measured in a Zeiss polarizing microscope at a wavelength (A) of 546 mp. Both quartz and calcite Ehringhaus compensators were used to measure the retardation. Thickness measurements were made with a Pratt and Whitney Electro-Limit Gage. Sonic Modulus An H. M. Morgan Company, Inc., pulse propagation meter, PPM-5 was used to determine the sonic modulus from film strips 1 111111. wide and 15-25
Journal of The Mechanics and Physics of Solids, 2008
This paper details a methodology to test the mechanical response of soft, pressure sensitive materials, over a wide range of strain rates. A hybrid experimental-numerical procedure is used to assess the constitutive parameters. The experimental phase involves axial compression of a cylindrical specimen which is confined by a tightly-fit sleeve that is allowed to yield plastically, thus applying a constant confining pressure. The usually neglected frictional effects between the specimen and the sleeve are fully accounted for and characterized in detail. With commercial polycarbonate as a typical example, it is shown that pressure-sensitivity and rate-sensitivity are not coupled, thus reducing the number of tests needed to characterize a material. The results of numerical simulations indicate that the pressure sensitivity index (angle β in the Drucker-Prager material model) has little influence on the hydrostatic and confining pressures, whereas the equivalent stress sustained by the specimen increases with β, which for commercial polycarbonate is found to be 0 15 β = .