The relatively high levels of elongation that most polymers exhibit without breaking are due in large part to chain entanglement. However, this entanglement also restricts the freedom required at a molecular level to organize into crystals. Consequently, no polymer under normal processing conditions is fully crystalline, and some polymers do not crystallize to any significant degree. This lack of a predictable and repeatable structure gives rise to a situation where changes in temperature always influence the mechanical properties of these materials.
This phenomenon can be captured by performing mechanical tests at various temperatures on any given material. One of the simplest ways to capture the load-bearing properties of a material over a wide range of temperatures involves a method known as dynamic mechanical analysis DMA.
This technique can measure so many aspects of polymer behavior that whole books have been devoted to the topic. However, for our purposes we can focus on one small aspect: the ability to measure the elastic modulus of a material as a function of temperature. Both of the materials tested are unreinforced. Nylon 6 is a semi-crystalline polymer, while PC is amorphous, and the results shown here represent typical behavior for these two classes of materials.
But while most data sheets provide little or no information on the effect of temperature on properties, the plots in Fig. Amorphous PC exhibits only one transition temperature, known as the glass-transition temperature Tg. This represents the temperature at which the individual polymer chains become sufficiently mobile at a molecular level to move independently despite the fact that they remain entangled.
Structurally, this event can be likened to a softening temperature, and for engineering purposes the material loses all load-bearing properties as it passes through this transition. However, between room temperature and the DTUL, most data sheets provide little guidance on the effects of temperature on load-bearing capabilities. The behavior of the semi-crystalline nylon 6 is somewhat different than that of the amorphous PC.
Nylon 6 is referred to as semi-crystalline because, like all polymers in this class, it consists of a structural mix of crystalline and amorphous regions. As the temperature increases, the amorphous regions become mobile and this mobility is again signaled by the glass transition.
But it does not fall to zero like PC does because of the presence of the crystalline structure. All amorphous polymers follow a temperature response that is similar to that of PC. Nylon 6 serves as a model for the temperature-dependent behavior of all semi-crystalline polymers. The distinguishing features among semi-crystalline polymers include the glass-transition temperature, the melting point, and the degree of decline in modulus associated with the glass transition.
It is important to emphasize that the Tg and the melting point Tm are fundamental properties for each polymer. We can reduce the effect that the Tg has on the elastic modulus of a semi-crystalline polymer by adding fillers and reinforcements. Despite the very short span compared to the thickness, the shear effect is disregarded.
Polymer mortar is considered to be an isotropic material, and the theory of plane cross-section is used. Damage in the flexure is assumed to be caused primarily by the axial tension and compression stresses. Cylinder polymer mortar specimens were tested in compression at the loading rate of 1.
Both the flexural and compressive test set-ups are presented in Figure 2. Flexural test results of the epoxy and unsaturated polymer mortars under different temperatures are presented in Table 2. Figure 3 represents the loss of flexural strength in the epoxy and unsaturated polyester mortars over the entire range of test temperatures. Upon increasing the test temperature, a significant loss in flexural strength is observed.
Epoxy polymer mortars present a strong and marked temperature dependency. The decrease in the flexural strength is consistent, and a In the analysis of the unsaturated polyester specimens, a lower temperature dependency was observed than that of the epoxy specimens; in addition, a smaller decrease is reported. Upon increasing the test temperature, a gradual decrease in flexural strength occurs; in fact, a decrease of The relationships between the test temperature and flexural strength of both formulations of polymer mortar are presented in Figures 4 and 5.
The epoxy mortar becomes less brittle, but no significant ultimate flexural strength change is reported. The HDT is the temperature at which a polymer sample deforms under a specified load. The HDT corresponds to the initial softening point and relates closely to the glass transition temperature of the polymer. Evaluating Figure 5 , the same behavior reported for the epoxy mortars specimens is observed for the unsaturated polyester ones.
As shown in Figures 4 and 5 , for both formulations, the loss of load bearing capacity increases with temperature. This behavior is associated with a progressive loss of material stiffness, turning the brittle characteristics of the polymer mortar into ductility. The change in the polymer mortars behavior is related to the HDT values of the polymers used as binders. Table 3 presents the test results of the epoxy and unsaturated polyester mortars under compression at different temperatures.
Analysis of the results of the compressive tests reveals that the polymer mortars behavior is similar to that in the flexural tests. A decrease in the compressive strength for both the epoxy and unsaturated polyester mortars is reported. The loss computed in the epoxy specimens is, again, higher than the loss reported for the unsaturated polyester mortars.
A lower compressive strength decrease is observed for the unsaturated polyester mortars. A decrease of The behavior of the compressive strength over the whole temperature range is shown in Figure 6.
As the test temperature increased, a trend in the increase of the ductile behavior can be seen. Again, the epoxy polymer mortars present a strong temperature dependency, and the rate of decrease in the unsaturated polyester mortars is lower than in the epoxy binder specimens.
Figures 7 and 8 compare the computed values for the epoxy and unsaturated polyester polymer mortars, respectively, under different test temperatures. They allow us to evaluate the compressive stress-strain behavior and to follow the phenomena that occur during temperature increase. As the test temperature increases, the compressive elasticity modulus decreases, and failure becomes less brittle.
This is very similar to the results of the flexural tests. As reported for flexure, the unsaturated polyester mortars exhibit ductile behavior as the test temperature increases.
As presented in Figures 7 and 8 , for both formulations, a loss of brittleness capacity with an increase in temperature is observed.
The change in the polymer mortars behavior within the temperature increment that is associated with the HDT values of the polymers used as binders occurs at lower steps. These values are lower than those observed in the flexural tests. Parrish and N.
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You can also search for this author in PubMed Google Scholar. Translated from Mekhanika Polimerov, No. Reprints and Permissions. Sogolova, T. Temperature dependence of the mechanical properties of polymers of different chemical structure in the temperature range from 4. Polymer Mechanics 13, — Download citation.
Received : 04 August
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