Commercially available shape memory polymer (SMP) Estane (designation: ETE75DT3 NAT022) is investigated by means of dynamic mechanical thermal analysis (DMTA) technique in torsion mode using the Modular Compact Rheometer MCR-301 (Anton Paar GmbH). Amplitude sweep tests have been run below and above the glass transition temperature to establish the linear viscoelastic range (LVR) in glassy and rubbery phase of this SMP for the correct physical interpretation of DMTA data. Temperature sweep tests were performed at various frequencies to study the influence of this parameter on values of the storage and loss moduli and the storage and loss compliances as well as the viscosities. These tests have been carried out in heating mode with different rates and at different strain amplitudes. The short- and long-term behavior of SMP Estane have been studied by frequency sweep tests performed at different temperatures and data have been transformed into time-domain properties by applying time-temperature superposition principles. All these DMTA data provide the experimental basis for the study of relaxation processes, property-structure relationships, and the shape memory effect in this little-known SMP.
Thermoresponsive shape memory polymers (SMPs) have the ability to recover a permanent shape from a programmed temporary shape upon heating. The characterization and modeling of this phenomenon, known as the shape memory effect (SME), require comprehensive experimental studies of mechanical, thermal, and functional properties of this class of smart materials [
The viscoelastic behavior of polymers including SMPs may be studied in several experimental methods like steady-state deformation, stress relaxation, creep, or oscillatory dynamic deformation. The results of tests are quantified using material functions such as steady viscosity, relaxation modulus, creep compliance, and storage and loss modulus. Additional tests are needed to study the coupling between viscoelastic and thermal properties of polymers. The standard quasi-static tensile tests together with creep and stress relaxation tests are often carried out at different ambient temperatures to determine the temperature-dependent mechanical response of the particular polymer under monotonic and steady-state loading [
Dynamic mechanical thermal analysis (DMTA) is a very efficient alternative technique for the study of time-, frequency-, and temperature-dependent mechanical properties of polymeric materials [
In this work, we present results of an extensive DMTA study of the commercially available thermoplastic polyurethane (TPU) based SMP Estane (purchased from Lubrizol, Oevel Westerlo, Belgium) performed in torsion deformation mode using the Modular Compact Rheometer MCR-301 equipped with temperature chamber. The DMTA investigations include the following specific experiments: strain amplitude sweep tests at different temperatures to determine the linear viscoelastic range (LVR) for the tested polymer, temperature sweep tests at different frequencies with the aim to study the coupling between temperature and time-dependent properties and to evaluate relaxation processes, temperature sweep tests in heating mode with different rates to evaluate the influence of this parameter on viscoelastic properties, temperature sweep tests on samples cut out from plates in two perpendicular directions to identify a possible anisotropy in material structure or processing of the tested polymer, frequency sweep tests at different isothermal temperatures to determine the short- and long-time response of this SMP.
These special tests serve to characterize various aspects and relative contributions of viscous and elastic responses of the Estane. In particular, the frequency of oscillation defines the timescale of tests and it follows that by observing polymer response as a function of frequency, the material can be probed at different timescales. These measurements are important in SMPs characterization because the overall response of these materials is due to contribution from several mechanisms at the molecular and microscopic levels. These mechanisms can be identified by observing material response at different frequencies.
Conventional DMTA equipments such as the one used in this study provide data for a tested SMP in a limited temperature and frequency range. However, when combined with theoretical concepts generally known as superposition principles and related concepts of the so-called master curves, these data may be used to determine the viscoelastic behavior of the same material over a wider frequency (or time) range. The application of these theoretical concepts to SMP Estane and the representative results are presented in the second part of the paper. Finally, the experimental and analytical (theoretical) results for the tested polymer obtained in this study are shortly discussed in reference to other thermoresponsive SMPs extensively investigated in the literature.
The SMP Estane has not yet drawn much attention on its thermoviscoelastic properties. The only known study is [
The thermoplastic polyurethane-based shape memory polymer Estane (designation: ETE75DT3 NAT022) was purchased from Lubrizol (Oevel Westerlo, Belgium) in the form of plates of dimensions
Rectangular specimens cut out of the SMP Estane plate.
The same SMP has been previously studied in [
Dynamic mechanical thermal analysis (DMTA) tests in torsion deformation mode were performed in the temperature range from −5°C to 150°C using the Modular Compact Rheometer MCR-301 (Anton Paar GmbH) [
Modular Compact Rheometer MCR-301 equipped with temperature chamber (a) and details of specimen clamping (b).
In DMTA torsion mode tests, a small axial force (around −0.5 N) is applied to the sample in order to maintain it under net tension. On this state of sample, the harmonic twist angle (rotation) with prescribed amplitude and frequency is superimposed and the resulting harmonic torque as well as the phase lag or loss angle
Besides the shear modulus and the shear compliance, there is another quantity called the viscosity to characterize the rheological behavior of polymeric materials. The complex viscosity is defined as the ratio of the stress and strain rate and may be computed from the complex shear modulus
The rheometer used in this study can perform a wide range of DMTA experiments including temperature ramp, frequency, and amplitude sweep tests in both stress- and strain-controlled modes. From such tests, the determined shear moduli and the loss factor are obtained as functions of test parameters in the specified range.
In order to use DMTA technique to accurately determine thermorheological properties and to develop morphological relationships of materials, a tested polymer must be deformed at amplitudes that remain within the linear viscoelastic region (LVR). Within LVR, the viscoelastic response of the polymer is independent of the magnitude of deformation. As a general rule, this region must be determined for every type of polymer by DMTA amplitude sweep tests, in which a frequency is fixed and the strain amplitude is incrementally increased.
From the plot of the storage and loss moduli against the strain amplitude for the SMP Estane shown in Figure
Amplitude sweep test at different temperatures: variation of the storage and loss shear moduli with strain amplitude.
The temperature-sweep test involves measurements of the storage and loss moduli and the loss factor over a specified temperature range at constant strain (or stress) amplitude and constant frequency. Temperature sweeps can be carried out in ramp or stepwise fashion.
Figures
Temperature scan at different frequencies: variation of the storage and loss shear moduli (a) and loss factor (b) with temperature.
Temperature scan at different frequencies: variation of the storage and loss compliances with temperature.
Temperature scan at different frequencies: variation of the dynamic and out-of-phase viscosities with temperature.
In Figure
Typically, the DMA temperature sweep tests of a polymer sample scanned at different frequencies show that at higher frequencies the storage modulus demonstrates higher values and the glass transition temperature shifts to a higher temperature. Figure
DMTA measurements over a range of temperatures provide valuable insight into the structure, the morphology, and the viscoelastic behavior of SMPs. In particular, these measurements are an important part of the technique for establishing relaxation transitions. For example, during temperature sweep, the temperature at crossover modulus
A further characterization of the tested SMP is obtained by plotting the storage and loss compliances as well as the dynamic and out-of-phase viscosities as functions of temperature for different frequencies (Figures
Figure
Temperature scan at different heating rates: variation of the storage and loss shear moduli (a) and the loss factor (b) with temperature.
The loss factor results are shown in Figure
Only limited research has focused on possible nonisotropic effects in polymeric materials. Such effects could be due to material anisotropy, processing anisotropy, or deformation. Each type of anisotropy greatly complicates the interpretation of DMTA data. The analysis of the structural anisotropy in oriented semicrystalline polymers presented in [
Both the material and processing-induced anisotropy in polymers may be detected by DMTA technique. In [
Variation of the storage and loss moduli with temperature (a) measured on samples cut out from the Estane plate in two orthogonal directions (b).
The frequency sweep is probably the most efficient DMTA test in characterizing the viscoelastic behavior of polymeric materials including SMPs. Such a test performed in torsion mode at fixed strain amplitude and temperature provides the storage and loss shear moduli as well as the loss factor as functions of frequency. The corresponding shear compliance and viscosity of a material may then be computed using formulae (
A typical dynamic mechanical analyzer such as that used in this study can provide data only over a limited range of frequency or time and this is inadequate to track the long-term viscoelastic behavior of a tested material. The time-temperature superposition principle not only offers the opportunity to obtain the long-term behavior of polymeric materials from the standard DMTA tests but also provides data that are difficult to measure directly [
In order to verify the applicability of the time-temperature (equivalently the frequency-temperature) superposition principle for the tested polymer Estane, the frequency sweeps were conducted at different isothermal temperatures ranging from 10°C to 75°C and stepping every 5°C for each sweep step. In all these tests, the same range of frequencies from
Plot of the storage and loss moduli versus frequency (log-log) for different temperatures.
From these data, the master curve has been constructed by shifting some of these curves along the logarithmic frequency axis to the left (to lower frequencies) and others to the right (to higher frequencies) relative to the reference curve at the temperature
The storage and loss moduli versus frequency master curves.
The materials for which the time-temperature or equivalent principle applies are referred to as thermorheologically simple materials and this may be verified in a number of ways depending on the material parameter used for the study [
In terms of the dynamic moduli, the time-temperature superposition principle underlying the construction of master curves reads [
Dozens of formulas have been proposed in the literature to link the shift factor of master curve to the chosen reference temperature. The most recognized empirical formula is known as the Williams-Landel-Ferry (WLF) equation [
The second widely considered theoretical equation for the shift factor is known as the Arrhenius model [
A comparison with experimental values of the shift factor shows that the WLF equation fits data reasonably well except at the lowest and highest temperature values (Figure
Experimental shifted data points versus temperature and comparison with WLF and Arrhenius models.
A smooth master curve for the storage or loss modulus of the tested polymer may be constructed within the more general concept of thermorheologically complex (TRC) materials [
This work is complementary to the parallel study by Mogharebi et al. [ The linear viscoelastic range (LVR) determined by amplitude sweep tests run at different temperatures proves that this polymer exhibits the linear behavior in both glassy and rubbery phase in a far wider range than it is usually suggested for the DMTA measurements. The temperature dependency of the storage and loss moduli determined from temperature sweep tests shows the characteristic behavior typical for thermoplastics. The commonly used Williams-Landel-Ferry equation and Arrhenius model to describe the temperature- and time-dependent behavior of polymers are not strictly applicable for the SMP Estane. The master curves built up by means of a procedure based on the time-temperature superposition principle show that the tested polymer may be considered as rheological simple only in limited time range.
The primary aim of the related work [ in [ DMTA data presented here and in [
When comparing the experimental results obtained in this work for the Estane with partial data published in the literature for other types of SMPs, the following aspects may be noted. The shift in tan The storage and loss moduli as well as the loss factor of the Estane measured at isochronal conditions (
It may be briefly stated that the results of this work provide the experimental basis for the study of structure-property relationships and shape memory properties of the Estane.
The authors declare that there is no conflict of interests regarding the publication of this paper.