Polypropylene viscoelastic properties

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

More recently, Lin and Masuda [47] measured the viscoelastic properties of polypropylene melts filled with small (0.15 pm) and larger (4.0 pm) calcium carbonate particles. The dynamic modulus and viscosity were found to rise with filler loading especially at low frequencies. With highly filled compositions (at... 

The viscoelastic properties of polypropylene melts containing magnesium hydroxide fire retardant fillers have been studied using parallel plate dynamic rheology [36]. In this work the filler variants differed in particle size, surface area and morphology, ranging from approximately spherical particles formed... 

Homopolymers are sometimes modified by a mechanical admixture of another homopolymer. As only about 5 % of pairs of all known polymers are mutually miscible, compatibility may be a problem in mixtures (blends). Copolymerization is technically applied to overcome, for example, the brittleness of polystyrene, polypropylene and PVC. It is also applied for improving the curing properties and modifying the viscoelastic properties of rubbers. By copolymerization, the relation between the hydrophobic and hydrophilic properties of macromolecules can also be modified. Their resistance to solvents may be enhanced. 

Rahim N. A. A. (2010). Flow behaviour and viscoelastic properties of polypropylene-kaolin composites. MSc Thesis, Universiti Sains Malaysia. 

Fejes-Kozma, Zs. Karger-Kocsis, J. (1994). Fracture Mechanical Characterization of a Glass Fiber Mat-reinforced Polypropylene by Instrumented Impact Bending. Journal of Reinforced Plastics and Composites, Vol.l3, No.9, pp. 822-834 ISSN 0731-6844 Ferry, J. D. (1980). Viscoelastic Properties of Polymers, 3rd Edition, Wiley Press, ISBN 978-0471048947, New York... 

Nishitani, Y. Kitano, T. Nagatsuka, Y. Nakamura, K. Sekiguchi, I. (1998). Influence of Surface Treatment on the Mechanical Properties of Short Glass Fiber Reinforced Polypropylenes and their Viscoelastic Properties in Molten State.. Reserch Report of Kogakuin University (in Japanese), Vol.84, pp. 11-20 ISSN 0368-5098... 

Nishitani, Y. Sekiguchi, I. Hausnerova, B. Nagatsuka Y. Kitano, T. (2001). Dynamic Viscoelastic Properties of Long Organic Fibre Reinforced Polypropylene in Molten State. Polymers Polymer Composites, Vol.9, No.3, pp. 199-211 ISSN 0967-3911... 

Scholz, P., Froelich, D., and Muller, R. (1989) Viscoelastic properties and morphology of two-phase polypropylene/ polyamide 6 blends in the melt. Interpretation of results with an emulsion model. J. Rheol, 33 (3), 481-499. 

A special case of interest is reinforced polypropylene with various fibers. Often transcrystallinity in polypropylene occurs which is due to dense heterogeneous nucleation by a substrate. The occurrence of transcrystallinity depends on the type of fiber and the temperature. In contrast to transcrystallinity in quiescent crystallization, the application of stress at the interface between a fiber and a PP melt results in the crystallization of polypropylene on a row-nuclei around a fiber. This effect is caused by strain-induced nucleation via some self-nucleation mechanism and is independent of the type of fiber and less dependent on the temperature of crystallization [5,6]. Axial stress arises also during cooling of two materials with a large difference in thermal expansion coefficients. As such, the stress-induced nucleation in reinforced PP depends also on the cooling rate, fiber length, position along the fiber and viscoelastic properties of the PP melt [5]. 

And although thermoforming is basically a rubbery solid deformation process, the viscoelastic character of the polymer may need to be understood, particularly for the plug-assisted forming process. Computer-aided design programs also may need polymer viscoelastic properties. This may be particularly true for crystalline polymers such as polyethylene and polypropylene when formed above their melt temperatures. This is discussed below. 

Yang J-J, Fang H-G, Zhang Y-Q, Shi W-T, Chen P, Wang Z-G (2013) Studies on influences of viscoelastic properties on melt extrusion of carbon nanombes-filled isotactic polypropylene composites. Acta Polym Sin 10 1325—1333... 

Molecular dynamics (MD) is an invaluable tool to study structural and dynamical details of polymer processes at the atomic or molecular level and to link these observations to experimentally accessible macroscopic properties of polymeric materials. For example, in their pioneering studies of MD simulations of polymers, Rigby and Roe in 1987 introduced detailed atomistic modeling of polymers and developed a fundamental understanding of the relationship between macroscopic mechanical properties and molecular dynamic events [183-186]. Over the past 15 years, molecular dynamics have been applied to a number of different polymers to study behavior and mechanical properties [187-193], polymer crystallization [194-196], diffusion of a small-molecule penetrant in an amorphous polymer [197-199], viscoelastic properties [200], blend [201,202] and polymer surface analysis[203-210]. In this article, we discuss MD studies on polyethylene (PE) with up to 120,000 atoms, polyethylproplyene (PEP), atactic polypropylene (aPP) and polyisobutylene (PIB) with up to 12,000 backbone atoms. The purpose of our work has been to interpret the structure and properties of a fine polymer particle stage distinguished from the bulk solid phase by the size and surface to volume ratio. 

The dynamic viscoelastic properties are sensitive to the morphology of polymers. In the case of highly crystalline polymers, such as polyethylene and polypropylene, sometimes a relaxation appears above the glass transition temperature. Figure 24 shows the temperature dependence of E and E" for polyethylene (Alkathene) at 100 Hz determined by Takayanagi et al. [23]. A relaxation (a -relaxation) appears at about 70°C along with a relaxation at — 20°C (a -relaxation) when the specimen crystallizes at 100°C but for the... 

The studies of nonlinear viscoelastic behavior have been performed not only for rubber matrices, but also reported for polyolefin matrices, for example polypropylene-reinforced Ti02 nanoparticles. Work by Bahloul et al.—concerning preparation of PPA i02 nanocomposites based on the sol-gel method—reported strain dependence of the viscoelastic properties. The authors observed a change in the storage modulus (G ) versus the strain amplitude and a characteristic decrease. 

Habas and co-workers [43] carried out an investigation into the viscoelastic properties of aqueous solutions of a four-branched polyethylene oxide - polypropylene oxide - polyethylene oxide triblock copolymer in the unimer range using TMA, DMTA and small-angle neutron scattering. The aqueous solutions were characterised in the crystalline phase and a modified version of Eyring s theory was used to describe the stress-time dependence of ordered solutions. 

The relaxation behavior of partially crystalline systems is complex and different from amorphous polymers. Observations give the general impression that, in comparison to amorphous systems, partially crystalline samples are much less uniform in behavior. Many of the systems exhibit peculiarities and these can dominate the viscoelastic properties. This is not the place to explore this large field in the necessary depth, which would mean we would have to discuss separately the mechanical behavior of polyethylene, poly(ethylene terephtha-late), polypropylene, it-polystyrene, poly(tetrafluoroethylene) etc. What can be done for illustration is to pick out one instructive example and we select polyethylene. 

Mechanical testing (strain-stress, tensile strength, elongation at break, elastic modulus, melt flow, viscoelastic properties, etc), have frequently been used in the study of the photodegradation of polyethylene [711, 1656, 1704, 1750, 1957, 2124, 2128], polypropylene [1750, 1899, 1903], poly(styrene) [748], poly(styrene-co-carbon monoxide) [1429], poly(styrene-co-acrylonitrile) [747], EPDM [896], poly(vinyl chloride) [806,1137,1138,1232,1748,1938], impact modified poly(vinyl chloride) [761, 764,1232], nylon 6 [672, 726, 727, 1395,1396,2300,2305], polyethylene blends with nylon 6 [506], and polyurethanes and its blends with poly(vinyl chloride), poly(vinyl alcohol), poly(vinyl acetate) and poly(vinyl chloride-co-vinyl acetate) [652]. 

Nazockdast E, Nazockdast H, Goharpey F. Linear and nonlinear melt-state viscoelastic properties of polypropylene/organoclay nanocomposites. Polym Eng Sci 2008 48 1240-1249. 

Eckstein, A., Friedrich, C., Lobbrecht, A., Spitz, R., Miilhaupt, R. Comparison of the viscoelastic properties of syndio-and isotactic polypropylenes. Acta Polymerica (1997) 48, pp. 41-46... 

Controlled-rheology polypropylene, peroxide, nitroxyl radical generator, nrolecular weight distribution, viscoelastic properties, rheological properties... 

MEASUREMENT OF THE HIGH FREQUENCY VISCOELASTIC PROPERTIES OF POLYPROPYLENE USING A SLIDING PLATE RHEOMETER... 

The measurement of viscoelastic properties on commercial rotational rheometers is hmited to frequencies < 30 Hz as inertial and edge effects are known to become significant at high frequencies [1-3]. Extension of the frequency range to higher frequencies (> 50 Hz) by means of time-temperature superposition (TTS) is not always possible, especially for semi-ciystalline polymers such as polypropylene, as the highest frequency attainable in TTS is limited by the melting point of the polymer [1,4]. 

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