Elastic modulus various polymers

Table 14.2 Effect of various treatments on the elastic modulus of polymer matrix...

The importance of polymer composites arises largely from the fact that such low density materials can have unusually high elastic modulus and tensile strength. Polymers have extensive applications in various fields of industry and agriculture. They are used as constructional materials or protective coatings. Exploitation of polymers is of special importance for products that may be exposed to the radiation or temperature, since the use of polymers make it possible to decrease the consumption of expensive (and, sometimes, deficient) metals and alloys, and to extent the lifetime of the whole product. 

The mechanical properties can be studied by stretching a polymer specimen at constant rate and monitoring the stress produced. The Young (elastic) modulus is determined from the initial linear portion of the stress-strain curve, and other mechanical parameters of interest include the yield and break stresses and the corresponding strain (draw ratio) values. Some of these parameters will be reported in the following paragraphs, referred to as results on thermotropic polybibenzoates with different spacers. The stress-strain plots were obtained at various drawing temperatures and rates. 

This is a theoretical study on the entanglement architecture and mechanical properties of an ideal two-component interpenetrating polymer network (IPN) composed of flexible chains (Fig. la). In this system molecular interaction between different polymer species is accomplished by the simultaneous or sequential polymerization of the polymeric precursors [1 ]. Chains which are thermodynamically incompatible are permanently interlocked in a composite network due to the presence of chemical crosslinks. The network structure is thus reinforced by chain entanglements trapped between permanent junctions [2,3]. It is evident that, entanglements between identical chains lie further apart in an IPN than in a one-component network (Fig. lb) and entanglements associating heterogeneous polymers are formed in between homopolymer junctions. In the present study the density of the various interchain associations in the composite network is evaluated as a function of the properties of the pure network components. This information is used to estimate the equilibrium rubber elasticity modulus of the IPN. 

Fig. 2.22. Dependence of the elastic modulus E and the mechanical loss factor 6 on temperature for various polymers. Curves 1 elastomer (statistical copolymer of ethylene and propylene) curves 2 isotactic polypropylene (semicrystalline)...

After an introductory chapter we review in Chap. 2 the classical definition of stress, strain and modulus and summarize the commonly used solutions of the equations of elasticity. In Chap. 3 we show how these classical solutions are applied to various test methods and comment on the problems imposed by specimen size, shape and alignment and also by the methods by which loads are applied. In Chap. 4 we discuss non-homogeneous materials and die theories relating to them, pressing die analogies with composites and the value of the concept of the representative volume element (RVE). Chapter 5 is devoted to a discussion of the RVE for crystalline and non-crystalline polymers and scale effects in testing. In Chap. 6 we discuss the methods so far available for calculating the elastic properties of polymers and the relevance of scale effects in this context. 

Until the 1970s there was a substantial gap between the theoretical modulus of polymer chains and the practical stiffness achieved in the existing processes. Since then the fibres made by extended chain crystallisation have bridged this gap. Solution spun fibres of high molecular linear polyethylene have been prepared with a Young modulus (at low temperature) of 90% of the theoretical value. Tables 19.9 and 19.10 illustrate the whole extent of elastic moduli in the various materials. 

The answers to these questions can be gleaned from Table 13-2, which compares approximate values of the tensile modulus for various polymers. Rubbers or elastomer are also amorphous, of course, but they respond to a stress in an entirely different manner to all other types of materials. Because they have low Ts, at ordinary temperatures, they respond to a load by changing their distribution of chain conformations, the chains becoming more extended as the material is stretched. A rubber has to be extended many limes its original dimensions before the covalent bonds take the load. We will consider rubber elasticity as a separate topic later. 

The objectives of this test pattern is to analytically resolve these problems into three manageable segments. The first task will be to define the viscoelastic kinetic properties of a material as a function of various reaction temperatures. These properties (viscosity, viscous modulus, elastic modulus, tan delta) define the rate of change in the polymers overall reaction "character" as it will relate to article flow consolidation, phase separation particle distribution, bond line thickness and gas-liquid transport mechanics. These are the properties primarily responsible for consistent production behavior and structural properties. This test is also utilized as a quality assurance technique for incoming materials. The reaction rates are an excellent screening criteria to ensure the polymer system is "behaviorally" identical to its predecessor. The second objective is to allow modeling for effects of process variables. This will allow the material to undergo environmental... 

If a polymer sample is subjected to a mechanical stress or to an electric field the structure responds, i.e. relaxes, in such a way as to reach an equilibrium under the stress or the field. This response generally involves some rearrangement of the structural units of the polymer, i.e. changes in the proportions of the various conformational or orientational states of its molecules, and it is the nature of these changes and the extent to which they take place that determine the elastic modulus or dielectric constant of the polymer. In the rest of this section and in section 5.7.3 it is assumed that the equilibrium ratio of the numbers of molecules in a particular pair of states between which jumps take place is actually influenced by the stress or electric field. This is so for many such pairs of states, but not for all pairs (see sections 7.6.1 and 9.2.5). 

Polymers such as EVA, are used as admixture because it modify the elastic modulus, toughness, permeability and bond strength to various substrates in cement and mortars [10]. The polymer forms a film that creates a network inside the cement matrix, partially covering hydrated and anhydrous cement particles, sealing pores and bridging microcracks. Besides, this addition also changes the hydration rate. Silva et al [11] compare the effects of two polymers a water soluble polymer (HPMC — hydroxypropylmethylcellulose) and a latex [EVA-poly(ethylene-co-vinyl acetate)] on... 

PLA films with various concentrations of a-TOC and resveratrol were prepared by compression moulding. The addition of these antioxidant agents resulted in deterioration of optical properties and reduction in Tg and Tm, but a significant improvement of thermostability. Mechanical properties were also improved since this plasticizing effect was accompanied by an increase in the elastic modulus, which is interpreted as a beneficial interaction between polymer and agents. No data about agents activity or release were reported. 

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