Investigation of temperature dependent mechanical properties of polymers measured by dynamic mechanical analysis
urrently, polymers are among the most widely used materials in industry, construction, medicine. When used as structural materials, depending on operating conditions, they can be subjected to heating, cooling, exposure to high humidity and other external factors. Investigation of the mechanical properties of materials under conditions close to operational ones is a paramount task facing materials scientists.
This work is devoted to the study of the change in the mechanical properties (hardness, modulus of elasticity and loss modulus) of polymeric plastics – polycarbonate and ultrahigh molecular weight polyethylene (UHMWPE) in the temperature range from 2°C to 60°C. In , indentation at high temperatures of polymers, biomaterials and solid composites was carried out, which revealed the effectiveness of this technique for determining the temperature-dependent mechanical characteristics. Temperatures from the above range can be achieved with the help of Peltier elements. Work  covers issues related to the use of Peltier elements for cooling polymer samples (using atactic polypropylene as an example) tested by instrumental indentation.
Polycarbonate is a widely known thermoplastic with high mechanical and optical qualities, due to which it is used as a material in the manufacture of lenses, CDs, headlights, computers, glasses and lighting products. In this work, polycarbonate was used as a reference sample to test the methodology.
The second sample is made of ultrahigh molecular weight polyethylene, which is a kind of synthetic polymers with long chains of molecular bonds that is of interest to many researchers in connection with the possibility of its use as polymer matrices in the construction of structural materials. UHMWPE, characterized by an ultrahigh mass of molecules and a crystallinity of up to 85%, also has a relatively high modulus of elasticity and holds shock loads well. However, its applications are limited due to the low melting point, although the thermal stability of UHMWPE is sufficient, for example, for use in bearings with water lubrication, operating at temperatures from 40 to 60°C . Also among the properties of this polymer can be noted a high degree of corrosion resistance, chemical and biological inertness, the retention of radiation and ultraviolet rays. The change in the properties of ultrahigh molecular weight polyethylene as a function of the dose of UV irradiation was investigated in .
UHMWPE is used in the production of bullet-proof vests, protective helmets, prostheses, machine and instrument parts with low coefficient of friction, ropes, ship sheathing and of sports protection as one of the components of heavy-duty fabrics. Particularly it is possible to note the biomedical field of application of UHMWPE. A group of scientists from MISIS  proposed a method for the production of a porous material based on UHMWPE, capable of replacing bone tissue lost by the patient as a result of trauma. Since polyethylene does not decompose in the human body, this will make "eternal" dentures. From the literature, the results of tests of polymer materials are known, showing that during operation at high temperatures, UHMWPE is characterized by increased wear . Therefore, studies of its mechanical properties in the temperature range from 2 to 60°C are extremely urgent.
In this paper, mechanical properties studies in this temperature range were carried out using the NanoScan-4D nano-hardness tester equipped with a special stage. This device (Fig.1.) allows to realize about 30 different testing methods for metal, ceramic, composite materials, solid plastics both in the form of bulk samples and in the form of thin films .
One of the effective techniques implemented in NanoScan-4D is the method of dynamic mechanical analysis (DMA) . Obtaining an almost continuous curve of the dependence of mechanical properties on depth is one of the key advantages of DMA over other methods of studying mechanical properties. As in the standard nanoindentation method, non-destructive testing is performed, and only thin, near-surface layers of the material are subjected to deformation. However, this method is especially interesting as a tool that allows investigating the viscoelastic properties of a number of polymer materials. The DMA method is based on the process of continuous indentation into the material, carried out simultaneously with the harmonic oscillations of the tip. When the oscillating indenter is brought into contact with a sample having viscous properties, the phase of the oscillations changes, which in turn generates an imaginary component of the modulus of elasticity: the loss modulus: E = E' + iE". The loss tangent is the ratio of the imaginary to the real component of the elastic modulus.
Since this work consisted in controlling mechanical properties under special temperature conditions, a simple universal temperature module based on the Peltier element was developed . The basic scheme of the temperature stage is shown in Fig.2. This module allows you to change the temperature of the test sample by 30°C both in the large and lower side of the room. Temperature control was carried out using a thermocouple attached to the surface of the sample in the immediate vicinity of the location of the mechanical tests. The heated area was isolated from the surrounding area by means of a protective casing. To increase the efficiency of the system while cooling the sample, it was necessary to ensure heat removal from the side of the Peltier element opposite to where the sample was fixed. To do this, the whole measuring cell was installed on the radiator, cooled by laminar airflow. During tests of the developed design it turned out that this solution allows to achieve a minimum temperature drift of the device, since the spatial domain with a significant difference in temperature from room temperature is in practice limited only by the indenter and the sample. The temperature change on the body of the device did not exceed 1°C.
Since the method of dynamic mechanical analysis does not impose specific requirements on the shape of the samples (in contrast to the tensile and compressive tests ), they were cut out in the form of plane-parallel plates 10 Ч 10 mm in size, the surface of which was sufficiently smooth and did not require additional polishing. The thickness of the samples was about 1 mm, so the difference between the temperature of the Peltier element and the sample surface did not exceed 1°C.
The tests were carried out in the loading mode with the control of linear scan of the displacement of the indenter. During the experiments, the following test parameters were selected: maximum deepening – 1 µm, oscillation frequency of indentor – 20 Hz, amplitude of oscillations – 200 nm, loading time – 60 seconds, time of maintaining constant load – 20 seconds. An important condition is that the speed of the oscillatory motion of the indenter must exceed the rate of introduction of the indenter into the sample by at least an order of magnitude. Otherwise, the error in determining the loss modulus becomes significant in comparison with the value of the real component of the elastic modulus .
DISCUSSION OF RESULTS
The obtained hardness values of polycarbonate and UHMWPE samples as a function of temperature are shown in Fig.3a. As can be seen from the presented dependences, the hardness of both polycarbonate and UHMWPE increases with decreasing temperature. The change in hardness with heating/cooling at 20°C relative to room temperature is about 15%. Plastics are in some sense water-like materials, so when cooled, the transition of amorphous regions to semi-crystalline ones takes place: from the "rubber" state at room temperature to the "glassy" state when approaching the glass transition temperature. The obtained data on the hardness of UHMWPE coincide within the error with the data of .
The dependences of the modulus of elasticity of both samples on temperature were also obtained (Fig.3b). The modulus of elasticity of polycarbonate depends on temperature in the same way as hardness (i.e., increases with decreasing temperature) and varies within 10% of the initial value. For the UHMWPE sample, the elastic modulus decreases with decreasing temperature, which indicates an increase in the brittleness of the material.
The dependencies of the loss modulus and loss tangent of the UHMWPE sample are shown in Fig.4. The loss modulus, and accordingly, the loss tangent for polycarbonate are close to zero. The growth of the loss modulus of ultrahigh-molecular polyethylene with an increase in temperature is explained by the increase in the appearance of viscous properties (increased "stickiness" of the polymer). The loss tangent, found as the ratio of the imaginary to the actual components of the elastic modulus, changes insignificantly for the UHMWPE sample, since the loss and elastic moduli vary in the same way.
The use of polymeric plastics as structural materials, as well as for some specific purposes such as biomedicine, requires temperature stability, so it is necessary to monitor their mechanical characteristics. Polymers with sufficient hardness can be investigated by the method of dynamic mechanical analysis in combination with the use of technical solutions that allow creating temperature conditions close to operational ones.
In this work, on the basis of the NanoScan-4D device, a compact and easy-to-use module was created that allows the sample to be heated or cooled to a temperature different from room temperature by 30°C. Using this module, the hardness, modulus of elasticity and loss of polymeric materials at a temperature between + 2°C and + 60°C were investigated. Polycarbonate, as one of the most common structural plastics, and ultrahigh molecular weight polyethylene, as a polymer of interest for the creation of high-strength composite materials on its basis, were chosen as the samples to be studied. The high mechanical properties of UHMWPE are caused by the superlong linear chains of polyethylene with weak intermolecular bonds.
In the indicated temperature range, a noticeable decrease in the hardness value of the samples (by about 15%) was observed with increasing temperature, since upon cooling of the sample the degree of its crystallinity increases as a result of approaching the glass transition point. The polycarbonate sample also exhibited a hardening effect with decreasing temperature. At the same time, if the modulus of elasticity of UHMWPE falls with decreasing temperature, then the modulus of elasticity of polycarbonate grows. The loss modulus and, accordingly, the loss tangent for polycarbonate is close to zero, and the loss modulus of UHMWPE changes in a similar way to the modulus of elasticity of this material. ■