Mechanical response of polymeric

The nonlinear optical and dielectric properties of polymers find increasing use in devices, such as cladding and coatings for optical fibres, piezoelectric and optical fibre sensors, frequency doublers, and thin films for integrated optics applications. It is therefore important to understand the dielectric, optical and mechanical response of polymeric materials to optimize their usage. The parameters that are important to evaluate these properties of polymers are their dipole moment polarizability a, hyperpolarizabilities 0... 

Polymers differ from other substances by the size of their molecules which, appropriately enough, are referred to as macromolecules, since they consist of thousands or tens of thousands of atoms (molecular weight up to 106 or more) and have a macroscopic rectilinear length (up to 10 4 cm). The atoms of a macromolecule are firmly held together by valence bonds, forming a single entity. In polymeric substances, the weaker van der Waals forces have an effect on the components of the macromolecules which form the system. The structure of polymeric systems is more complicated than that of low-molecular solids or liquids, but there are some common features the atoms within a given macromolecule are ordered, but the centres of mass of the individual macromolecules and parts of them are distributed randomly. Remarkably, the mechanical response of polymeric systems combines the elasticity of a solid with the fluidity of a liquid. Indeed, their behaviour is described as viscoelastic, which is closely connected with slow (relaxation time to 1 sec or more) relaxation processes in systems. 

Rybicky, E.F., and Kanninen, M.F., The Effect of Different Behavior in Tension and Compression on the Mechanical Response of Polymeric Materials . Deformation and Fracture of High Polymers, (H. Kausch, et al, Ed. s.) Plenum Press, NY, 1973, p. 417-427. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Although the mechanical response of macromolecular solids is complex, it is possible to gain an understanding of the broad principles that govern this behavior. Polymeric articles can be designed rationally, and polymers can be synthesized for... 

The viscoelastic response of polymeric materials is a subject which has undergone extensive development over the past twenty years and still accounts for a major portion of the research effort expended. It is not difficult to understand the reason for this emphasis in view of the vast quantities of polymeric substances which find applications as engineering plastics and the still greater volume which are utilized as elastomers. The central importance of the time and temperature dependence of the mechanical properties of polymers lies in the large magnitudes of these dependencies when compared to other structural materials such as metals. Thus an understanding of viscoelastic behavior is fundamental for the proper utilization of polymers. 

Rich and reliable viscoelastic data can be obtained from measuring the dynamic mechanical response of the polymeric liquid to a rate-of-strain X t) which is harmonically oscillatory ... 

The investigations mentioned above, are focused principally on the processing and mechanical response of polymers reinforced with natural fibers, without considering that their mechanical properties decrease after exposition to alkaline environmental of cement materials or weather. Natural fibers reduce their mechanical properties after exposition to alkaline environment of the cement matrix, nevertheless the use of polymeric matrix as a binder aroimd the natural fibers provides protection for them. However, if the interface of composites is not good, and/or matrix is not alkaline resistant, hydration products like calcium hydroxide will migrate to interface, and polymer composite will deteriorate. Several studies have demonstrated that the mechanical properties of natural fibers decrease after exposure to alkaline environment of the cement matrix due to three different mechanisms [30-39] ... 

The term rheology dates back to 1929 (Tanner and Walters 1998) and is used to describe the mechanical response of materials. Polymeric materials generally show a more complex response than classical Newtonian fluids or linear viscoelastic bodies. Nevertheless, the kinematics and the conservation laws are the same for all bodies. The presentation here is condensed one may consult other books for amplification (Bird et al. 1987a Huilgol and Phan-Thien 1997 Tanner 2000). We begin with kinematics. 

While elastic modulus is an important characteristic of a polymeric material, it mainly describes material behavior at small deformations only. This could be important for some applications (note, for example, that scratch resistance or hardness often can be directly linked to the modulus) however, in general, one requires the knowledge of mechanical response of the material over a broad range of deformations. For polyurethane elastomers and TPU s, as well as for... 

Considered individually, these interactions are not stronger than those observed in a system composed of simple molecules. However, in polymeric systems, the multiplicity of interactive groups and the forces resulting from their repetition along the same macromolecular chain lead to considerable cohesion energies that are in turn responsible for the peculiar mechanical properties of polymeric materials. 

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