Comparative analysis of new-generation thermoplastic polymers: Cyclic olefin copolymer and polycarbonate urethane elastomers

Abstract

In this study, the thermal, rheological, mechanical, and viscoelastic properties of two new-generation thermoplastic polymers, namely cyclic olefin copolymer (COC) and polycarbonate urethane (PCU) elastomers, were compared to those of a conventional thermoplastic elastomer, thermoplastic polyurethane (TPU). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used for thermal examinations, while rheological, tensile, and solid-state creep tests were used for viscoelastic and mechanical analyses. The DSC results revealed that all elastomers had two different T_g values, which were −23 and 7.5°C for PCU, −40 and 95°C for TPU, and 4 and 55°C for COC. Moreover, PCU had an amorphous and more compatible structure than PCU and TPU. In the DMA, it was also observed that COC melted at approximately 90°C, while PCU and TPU melted at about 155°C. In the tensile tests, it was observed that COC showed higher strength at 30°C, but it lost its strength more effectively than the other polymers with increasing temperature and exhibited similar performance to all specimens at 50°C. Finally, in the solid-state creep tests, COC exhibited the highest creep resistance at 30°C, while its creep strain increased with temperature more effectively than those of the other elastomers.

Keywords

Thermoplastic elastomer viscoelastic properties cyclo-olefin copolymer thermoplastic polyurethane polycarbonate urethane

Introduction

The term thermoplastic elastomer (TPE) is used for a specific class of polymers that combine the melt-processability of thermoplastics and the elastomeric properties of rubbers.^1,2 Due to recyclability and melt processability, TPEs have gained popularity in applications such as automotive,³ footwear,⁴ medical devices,⁵ and sports equipment instead of chemically crosslinked elastomers.⁶ In the automotive industry, they are widely used in interior trims, sealing systems, and flexible gaskets, while in the medical field, flexible catheters and drug delivery devices benefit from their unique properties. Additionally, TPEs are preferred in many sports equipment, such as shock-absorbing shoe soles and protective gear. The unique properties exhibited by elastomers depending on their chemical structure significantly determine their application areas. For example, PCUs are particularly preferred in biomedical applications requiring high fatigue resistance, such as cardiovascular implants and artificial cartilage tissues, due to their biocompatibility and durability while TPUs are widely used in industrial applications where cost-effectiveness and mechanical strength are critical, including conveyor belts, protective coatings, and automotive components such as interior trims, sealing systems, and flexible gaskets.

The presence of different structures called hard and soft segments in TPE plays a crucial role in exhibiting this unique behavior.⁷ Isocyanates, norbornene, and polyamides are examples of hard segments, while polyethylene and polyol derivatives are the main examples of soft segments.^8–10 In a TPE structure, hard segments form clusters that act as physical crosslinks between soft blocks to improve and impart high stiffness,¹¹ mechanical strength,¹² thermal resistance,¹³ and shape memory behavior,¹⁴ while soft segments provide flexibility and molecular mobility at low temperatures.^15,16 Thus, elastomeric behavior at a wider temperature range is obtained. However, the main disadvantages of TPEs are the upper service temperature limitation, lower creep and fatigue resistance, and lower mechanical strength for high-performance applications compared to chemically crosslinked elastomers.^17–19 In general, most of these disadvantages arise from the softening and melting of the hard phase at lower temperatures and the lower rate of interaction of physically crosslinked structures compared to chemically crosslinked structures. In this context, the synthesis of different block-structured or random copolymer-structured TPEs by different catalysts, monomers, and polymerization techniques has been investigated widely in scientific and industrial research.

TPUs were commercially produced for the first time in the late 1950s, marking the period when elastomeric properties were combined with thermoplastic characteristics.⁸ In general, diisocyanate compounds form the rigid block of polyurethane, while diol compounds form the flexible block. Additionally, a chain extender as a third component is also used to obtain TPU by condensation polymerization. The most popular isocyanates are 4,4-diphenylmethane diisocyanate (MDI),²⁰ 2,4-toluene diisocyanate or 2,6-toluene diisocyanate (TDI),²¹ 1,4-phenylene diisocyanate (PDI),²² 1,6-hexamethylene diisocyanate (HDI),²³ isophorone diisocyanate (IPDI),²⁴ and 4,40-methylene bis (cyclohexyl isocyanate) (HMDI).²⁵ Polyols that are commonly used in the synthesis of TPU can be classified as ether- and ester-based polyols.²⁶ Polyurethane obtained using ester-based polyols possess good mechanical properties and thermal stability, but they are hydrolytically unstable compared to ether-based polyols.²⁶ Commonly used chain extenders are diols, for example, 1,2-ethanediol,²⁷ 1,4-butanediol,²⁸ 1,6-hexanediol,²⁹ or 1,10-decanediol³⁰ and diamines,³¹ for example, 1,4-butanediamine,³² 4,40–methylene-bis-benzeneamine, and ethylene diamine.³³ The properties of TPUs can vary depending on the type of diol and diisocyanate used.

Through the development of various formulations, TPUs have become versatile materials that can exhibit a wide range of hardness values, high strength, and resistance to different chemicals, making them a preferred choice across many industries. Furthermore, hard and soft segments are thermodynamically incompatible, and this incompatibility causes them to aggregate into separated phase domains which affect their mechanical properties. Thanks to technical specifications that can vary over a wide range and their widespread use in many application areas, TPUs have become a reference material for TPE-class polymers. Therefore, TPUs have been taken as a reference for the comparison of new-generation TPEs both in this study and many others.

PCUs, a unique class of TPUs and TPEs, show great promise for orthopedic implants.^34,35 PCUs have become especially popular due to their relative biocompatibility, as they are more resistant to hydrolytic and oxidative degradation than polyester and polyether urethanes.³⁶ Oligo(carbonate) segments constitute the soft segments of PCUs, while isocyanates are the rigid phase of PCU. Similarly, with TPU, PCUs with different properties can be prepared according to the types and ratios of their monomers. Therefore, in recent years, PCU applications in biomaterial engineering increased significantly for medical devices, implants, and artificial cartilage tissue applications.^37–39

COC is a newer member of TPEs, and it is a random copolymer in which the soft segment is made of polyethylene, and a norbornene structure forms the hard segment.⁴⁰ Depending on their monomer composition, COCs have variable thermal and mechanical properties, such as glass transition temperature (T_g) ranging from −6 to 178°C and elastomeric to highly glassy physical properties.^41,42 COC grades containing a higher ratio of norbornene have an amorphous structure and high T_g values, while those containing a lower ratio of norbornene have a semi-crystalline structure and lower T_g values.^43,44 With this structure, elastomeric COC appears to be a high-transparency polymer with high chemical resistance to ordinary solvents, moisture impermeability, and higher T_g than TPU. COCs are widely used in the as high-toughness films, medical tubing, over-molding for soft-touch components packaging and other electrical, optical, and biomedical applications.^45–49 The review of the relevant literature on semi-crystalline elastomeric COCs shows that a limited number of studies focused on its crystallization behavior, medical device applications, and the preparation and characterization of blends with ABS for various applications.⁵⁰

This study represents the first comparative analysis of the new-generation elastomers, COC and PCU, with the conventional elastomer, TPU. Unlike previous studies that are limited to basic mechanical tests, this work provides a comprehensive evaluation by examining both the solid and melt-state viscoelastic properties of these materials. Furthermore, the analysis includes assessments of their short- and long-term performance, as well as deformation behavior under cyclic loading conditions. These detailed investigations aim to offer a deeper understanding of the strengths and limitations of these elastomers, providing insights into their suitability for specific applications.

Material and method

Materials

In this study, three types of commercially available TPE, which were TPU, PCU, and COC, were used. An ester-based TPU with Shore A values of 80 provided by Ravago Petrochemical with the code Ravathane^®130-A80 was used in the study, as well as an aromatic polycarbonate-based PCU with Shore A values of 80 which was supplied by AdvanSource Biomaterials with the trade name ChronoFlex^® C-80A. An elastomer and semi-crystalline grade COC with Shore A values of 89 were kindly provided by TOPAS Advanced Polymers with the commercial name E-140.

Methods

Test sample preparation

The granule form of the specimens was used for rheological, DSC, and melt state creep analyses, while thin film specimens at about 150–300 µm of thickness were used for the DMA and solid-state creep tests. All granules were dried under vacuum at 70°C for 4 h before processing and characterization. The film specimens were prepared using the compression molding technique. Compression molding was performed using the Gülnar (Turkey) programmable hot and cold press. In compression molding, a temperature of 220°C was applied to the specimens. Here, the specimens were first left between the hot plates in the press for 3 min before applying pressure to soften the specimen. The pressure was then increased to 8–10 psi, and the specimens were left for three more minutes. Finally, the pressure was increased to 120 psi, and the specimens were kept there for three more minutes. After hot compression molding, the specimens were removed from the hot plates, placed into the cold press cooled by cold water at a temperature of 12°C and cooled to ambient temperature under the same pressure (120 psi). Finally, under the same conditions, plates with the dimensions of 15 × 15 × 2 mm were prepared, and dog-bone shaped specimens were cut from these plates according to the ASTM D638 standard (Type IV). These specimens were used for tensile and solid-state tensile creep tests.

Characterizations

The thermal properties of the elastomers were investigated using a DSC testing device (Seiko, DSC 7020) under a nitrogen atmosphere. A total of 15 mg of the samples were weighted and heated from −80°C to 230°C at a heating rate of 10°C min⁻¹ and kept there for 5 min to eliminate thermal history, then cooled down to −80°C at 10°C min⁻¹ and finally heated to 280°C again at 10°C min⁻¹. The thermomechanical properties of the film specimens were characterized by DMA. DMAs were performed in the temperature range of 30–200°C with a heating rate of 3°C/min. and a frequency of 1.0 Hz. The rheological, tensile, melt-state, and solid-state creep tests and DMA results of the elastomers were investigated with a hybrid dynamic rheometer equipped with a solid tensile test and parallel plate test geometry (Discovery Hybrid Rheometer-1, DHR-1, TA). Strain and frequency sweep tests were carried out for each specimen. Strain sweep tests were used in the linear viscoelastic regions (LVR) of the specimens, while frequency sweep tests were carried out in LVR were used to characterize their melt-state viscoelastic structures. The frequency sweep tests were carried out at 200°C in the angular frequency range of 600–0.1 rad/s and the strain value of 1%. In the tensile tests, the specimens were tested at three different temperature values (30, 40, 50°C) up to a maximum strain of 175% considering the limit values of the device. During the tensile tests, the test speed was adjusted to 10 mm/min. The tensile tests were carried out in compliance with the ASTM D638 standard. In the cyclic tensile tests, a 10 mm/min tensile speed was applied, and repeated strain and recovery steps were performed 100 times at 23, 30, 40, and 50°C up to a 75% elongation value. In the solid-state tensile creep tests carried out under a pressure of 1.5 MPa, the test specimens were thermally conditioned for 5 min at the test temperature (30–40–50°C). The creep and recovery times for the test were selected as 30 min. The melt-state creep tests were conducted at 220°C at a stress value of 3 Pa. The stress and recovery times for the test were selected as 3 and 6 min, respectively.

Result and discussion

Differential scanning calorimeter (DSC)

Before the mechanical, viscoelastic, and rheological characterization steps, all specimens were subjected to DSC analysis to investigate their thermal and morphological properties, as well as phase transition temperatures (T_g and T_m). The DSC thermograms of the specimens are presented in Figure 1. Figure 1(b) shows that PCU had two different T_g (T_g1, T_g2) values, which were −23 and 7.5°C, respectively. T_g1 was considered the T_g of soft polycarbonate segments in the structure, while T_g2 was considered the T_g of hard urethane structures.^51,52 T_g values of the pure component of PCUs were reported as −60 and (−40)°C for aliphatic macro diols and 110°C for hard urethane structures.⁵¹ Considering the T_g values determined using the DSC method in this study, it can be concluded that the two phases showed a relatively homogeneous distribution within each other to display a more compatible single phase compared to many TPU grades. In Figure 1(b), a small endothermic change is observed at around 200°C, which was attributed to the melting of the hard phase.⁵³ The phase transitions during the cooling of PCU indicated a delayed small exothermic increase compared to heating at 160°C in Figure 1(a). However, it was speculated that this increase was far from crystallization, and therefore, the structure was assumed to be amorphous.

As seen in Figure 1(b), the T_g and T_m values of the soft segment of TPU were determined as −20 and 5°C, while those of the rigid phase were determined as 95 and 175°C, respectively. A homopolymer polyurethane composed entirely of hard parts was reported in the literature to have a T_g of 108°C,⁵⁴ but it was considered that this value decreased to 7.5°C for PCU and to 95°C for TPU due to the better distribution of the hard isocyanate parts in the polycarbonate structure and the formation of a compatible structure. Furthermore, a significant crystalline phase formation was not observed in TPU during cooling. The cooling curves of COC in Figure 1(a) show a crystallization peak that started at 63°C and reached a maximum at 60°C, unlike the other elastomers. The semi-crystalline nature of elastomeric COC can be associated with the fact that it is insufficient to hinder the arrangement of ethylene chains via the norbornene units due to its low norbornene content, unlike its amorphous grades with high T_g values. In the examinations of the T_g values of COC, two separate T_g values of the soft ethylene and hard norbornene phases were observed at 4°C and 55°C, respectively, while a melting peak was detected at around 88°C. The high T_g of the ethylene phase compared to the homopolymeric oligomeric ethylene is associated with the hindrance of the chain mobility of the rigid norbornene structure, while the low T_m is associated with the melting of the oligomeric ethylene units. An interpretation of compatibility based on changes in the homopolymer and copolymer structures was not made for COC because it was synthesized by the radical addition polymerization method, unlike the other elastomers, TPU and PCU.^55–57

The T_g value of the soft segment in TPEs generally determines their minimum service temperature. In this context, the T_g of COC, being around 4°C, indicates that its elastomeric behavior will be largely lost below this temperature, which is a disadvantage for applications operating in cold environments. On the other hand, the T_g values of PCU and TPU are sufficiently low to meet the requirements of many applications. On the other hand, T_g of the hard segment influences the many mechanical properties (stiffness, elastic modulus, creep strength etc.) however it cannot be solely attributed as the decisive parameter due to the interplay of chemical structure and interphase interactions. Therefore, the T_g of the hard segment was not directly correlated with the mechanical properties. Lastly, the T_m value, as expected, determines the maximum service temperature. In this regard, COC exhibits a lower maximum service temperature compared to the other elastomers.

Dynamic mechanic analyzes (DMA)

The temperature-dependent changes in tensile storage modulus (E′), tensile loss modulus (E″), and damping factor (tanδ) values obtained by DMA are presented in Figure 2. It is observed that both modulus values (E′ and E″) of all three elastomers decreased with an increase in temperature, as a result of high chain mobility caused by the low T_g values of the soft segments. It is also seen that the decreasing trend of the E′ and E″ values of COC was more efficient with temperature, while that of TPU showed the least sensitivity to temperature. This low-temperature dependence of TPU was attributed to the high T_g values of the rigid isocyanate (95°C) segment in the TPU, while the highest degree of temperature sensitivity, which was found in COC, was attributed to its low T_m value (88°C). Considering the E′ values around room temperature in Figure 2(a), it can be observed that the E′ values in COC are the highest. This is likely due to the restricted chain mobility caused by the cyclic norbornene units and the crystalline segments present in the structure of COC. PCU and TPU also showed similar E' values up to 70–80°C. A sharp decrease in E′ values for COC was also observed at about 90°C, while a similarly sharp decrease was observed for PCU and TPU at about 160°C. These results were attributed to the melting of the polymers. It is seen that the T_m values of PCU and TPU determined by DMA were significantly lower than those determined by DSC. During DMA, an oscillation force is applied to the specimens to create a certain deformation, which was 40 µm for this study. In this respect, depending on the applied force, lower transition temperature values can be determined by DMA compared to those determined by DSC. It was determined that when the T_g values of both the soft and rigid segments of PCU and TPU exceeded at 95°C, both elastomers lost their mechanical strength at lower temperatures compared to the melting temperatures determined in DSC. Based on these results, it was concluded that the upper temperature limit for COC in practical use conditions was 65°C, even at the deformation limits in the linear field, while this temperature was between 130 and 140°C for PCU and TPU.

Figure 2(b) shows the temperature-dependent changes in E″ values representing energy losses during the deformation of polymer chains due to friction and other factors. It is seen that there was a similar order (COC>PCU>TPU) in E″ values to that in E′ values at lower temperatures, but much higher E″ values of PCU compared to TPU draw attention. The remarkably higher E″ values of PCU compared to those of TPU, while they had similar E′ values at the same temperatures, were associated with a higher friction force between the hard and soft phases of PCU due to higher compatibility and the interaction, as previously revealed by the DSC analysis results. The highest E″ values, which were found in COC at room conditions, were also attributed to restricted chain mobility due to the bulky cyclic norbornene structure and its semi-crystalline structure.

Finally, tanδ values, calculated using the ratio E″/E′, are presented in Figure 2(c). In some papers, linear relationships between the area under the tanδ-temperature curve and the impact strength of some polymers have been revealed.^58,59 The energy absorption values of TPEs cannot be determined in many impact tests because the breaking of the specimens is not observed due to high flexibility.^60,61 In this concept, the evaluation of tanδ values can be considered an important indicator to compare the energy damping performance results of the elastomer specimens. In Figure 2(c), it is revealed that TPU showed the lowest damping performance, while PCU showed the highest performance in temperatures up to 60°C, which corresponds to typical usage conditions, due to its higher E″ values.

Rheological and melt-state viscoelastic analyzes

Variations in the rheological parameters (G′, G″, tanδ, η^*) of the specimens in melt conditions were investigated depending on the angular frequency (ω), and the results are given in Figure 3. In Figure 3(a), which shows the variation of G′ values depending on the angular frequency, it is seen that all elastomers had the same G′ values at a high frequency. However, they had different values and behaviors in the lower-frequency regions. The variation of G′ values with the angular frequency in the low (or terminal)-frequency region indicated that TPU and COC showed a similar character that is known as classical thermoplastic polymer behavior (G′∼ω⁻²). On the other hand, in the same frequency region, it is seen that PCU exhibited the frequency-independent modulus behavior called solid-like behavior.^62,63 The frequency-independent modulus values in the terminal region in the G′-ω plot can also be explained by a solid or solid-like material-filled polymer structure along with a percolated structure. The solid-like behavior of PCU was attributed to the incompletely melted and dispersed solid crystal segments of PCU (200°C) compared to TPU (175°C) and COC (95°C), which was previously determined in the DSC analyses.

As shown in Figure 3(b), the analysis of G″ values concerning frequency also shows a trend similar to that determined for G′. However, since G′ values are more sensitive to viscoelastic changes, it is seen that the degree of difference in G″ values was lower. In Figure 3(c), the phase angle (tanδ) values, which are also determined as the ratio of modulus values (G″/G′), are presented given. The tanδ value in the low-frequency region is used to analyze structural parameters such as structural incompatibility, polymer/polymer interactions, and filler/polymer interactions. The tanδ curve of COC is shown in Figure 3(c), which shows a classical single-phase thermoplastic behavior. Despite this, it is seen that while the tanδ curve of TPU became flatter in the low-frequency region, that of PCU first becomes flatter at higher frequencies and then enters a decreasing trend. These results revealed that the structural compatibility of COC was better than that of TPU. Although this can be considered structural incompatibility for PCU when evaluated in this sense, it was thought that the main reason for the reduced character behavior in the low-frequency region of PCU was that the hard phase was not completely dispersed at the test temperatures.

Finally, regarding the variations in η^* values with the angular frequency, Figure 3(d) shows that in the low-angular-frequency region (corresponding to low shear values in flow tests), COC and TPU exhibited a Newtonian character, while PCU exhibited a non-Newtonian character due to the incompletely melted hard phase of PCU. The comparison of the viscosity values of the specimens in the low-shear region showed the order of the viscosity value as PCU>COC>TPU. Since the influence of parameters such as molecular weight, branching structure, and the polydispersity index (PDI) is not known, the differences in viscosity could not be discussed in terms of a cause-and-effect relationship.

Tensile test

The stress-strain curves obtained as a result of the tensile tests at different temperatures (30, 40, and 50°C) applied to the elastomer film specimens are shown in Figure 4, and the parameters obtained by the analysis of these curves are shown in Table 1. In, Figure 4 the stress-strain behaviors of the specimens indicate that all specimens showed a classical thermoplastic elastomer behavior, which can be defined as a linear deformation area up to the yield point and strain-hardening properties at higher strain values than the yield point. It is also seen that all specimens showed a yield point of about 25% strain regardless of temperature. In addition, it is noted that strain-hardening occurred more effectively at lower temperatures in comparison to higher ones, while the rate of increase in stress values decreased with increasing temperatures.

Table 1.

Tensile test parameters of the samples.

Temp.	Sample	E	σ_max	σ_@%100	Area
(⁰C)	Sample	(MPa)	(MPa)	(MPa)	Ƴ = %5	Ƴ = %25	Ƴ = %50	Ƴ = %100	Ƴ = %150
30⁰C	COC	34	6,45	5,21	4,89	59,35	154,27	392,27	673,05
	TPU	26	5,53	4,34	3,24	51,38	192,06	345,39	581,29
	PCU	19	6,12	4,62	2,27	42,41	127,94	342,27	593,66
40⁰C	COC	27	5,10	4,13	3,11	45,23	120,47	309,68	531,27
	TPU	25	4,78	3,98	2,89	47,85	131,20	320,48	444,11
	PCU	17	4,60	3,67	1,92	36,02	107,65	280,41	476,77
50⁰C	COC	22	4,20	3,41	2,10	36,01	98,30	254,35	438,70
	TPU	19	3.90	3,31	2,07	38,28	107,01	264,65	440,43
	PCU	14	3,73	3031	1,63	29,72	88,45	230,36	392,13

The strain-hardening behavior is generally explained by the orientation of the polymer chains, the crystallization of the polymer chain, and the stretching of the chemically or physically crosslinked structures in the literature, and an increase in the viscous movement of the polymer chain decreases its strain-hardening properties.^64,65 In this respect, the decreasing trend of strain-hardening properties with temperature was an expected result. On the other hand, it is seen that the strain-hardening performance of the specimens was in the order of COC>PCU>TPU. The same ordering in the E″ values that shows the resistance of the polymer chains against viscous deformation in Figure 2(b) obtained by DMA also supports the explanation of the higher strain-hardening properties of COC by hindered viscous movement originating from the crystal segment.

Table 1 shows that the Elastic modulus (E) values of the specimens were ordered as COC>TPU>PCU at all temperatures. The morphologies and T_g values of specimens are the main parameters determining their E values. In this respect, the highest E values, which were found in COC, can be attributed to its semi-crystalline structure and moderate T_g value (55°C), while the lowest E values, which were found in PCU, can be explained by the lowest T_g values among the specimens found in PCU and its amorphous structure. The higher modulus of COC at low temperatures, while limiting its elastomeric behavior, offers advantages in applications requiring structural rigidity and stability under cold conditions. For example, this property can be beneficial in automotive components that demand dimensional stability and high modulus, such as brackets or housings, and in outdoor equipment exposed to sub-zero environments. Three additional key parameters evaluated in the tensile test include the maximum stress (σ_max, up to 175% elongation), toughness and the stress at 100% elongation (σ_@%100). As seen in Table 1, the order for both σ_max and σ_@%100values was COC>TPU>PCU at all test temperatures, similar to the E values. Moreover, it was noted that both stress values decreased as expected with increasing temperature, and very close values were obtained for all three elastomers at 50°C. In this sense, it was revealed that COC lost its mechanical strength more effectively by increasing temperature than the other specimens, as verified by DMA previously. Another significant property of elastomer materials is their energy absorption capability which is calculated by the area under the stress-strain curve of the specimen.⁶⁶ Since elastomer materials can work under different deformation rates in practical operating conditions, the area values of the specimens in this study for different elongation rates were calculated. The values show that COC had the highest toughness value at 30°C for all strain values, while COC and TPU had similar area values as temperature increased. Additionally, it was observed that the area increased more for COC compared to the other two polymers at increasing strain values due to its more effective strain-hardening behavior at low temperatures.

Cyclic tensile test

One of the most valuable features of elastomers is that they show low degrees of permanent deformation and exhibit a reproducible stress-strain behavior, unlike conventional thermoplastics, even when subjected to high deformation.^67–69 Crosslinked elastomers do not show considerable permanent deformation even at strain ratios up to 100%, while higher permanent deformation occurs in thermoplastic elastomers due to their nature. Therefore, in the development of thermoplastic polymers, less permanent deformation at cyclic tensile tests has always been an objective for scientific and industrial research.⁷⁰ In this respect, within the scope of this study, cyclic tensile tests of PCU, COC and TPU were carried out, and the changes in their structures were investigated.

In the scope of the test, all specimens were strained and relaxed 100 times at the four different temperatures (23, 30, 40, 50°C) up to a certain strain value (75%), and the stress values at this strain value for each step were recorded. Theoretically, the same stress value must be obtained for a specimen with a fully elastomeric structure at a definite strain value in each step of the cyclic tensile test. On the other hand, a certain permanent deformation is expected in each deformation cycle, in which a strain value of 75% is applied for a thermoplastic elastomer. This permanent deformation also causes a lower stress value in each cycle. Figure 5 shows the time-dependent stress values recorded during the test for different temperatures (23, 30, 40, and 50°C). It was aimed to demonstrate stress reduction more comprehensibly by drawing a line from the first point to the last point of these curves (from the first cycle to the two hundredth cycle) in the plot. In all three specimens, the slope decreased with increasing temperature. An almost horizontal line was also obtained, especially in the tests performed at 40 and 50°C for COC. The slopes are presented quantitively in Figure 6(a)–(c), and the variations of the slope depending on the temperature in all three polymers are summarized in Figure 6(d). In Figure 6(d), it is seen that the lowest slope belonged to COC, and the slope values gradually decreased with increasing temperature and reached 1 at 40°C and above. Furthermore, it is seen that the highest slope was obtained for PCU at all temperature values.

It was considered that obtaining the same stress values at higher temperatures for COC could have been caused by two reasons. First, a certain amount of permanent deformation may have occurred on the specimen during the first few deformation periods of the cyclic tensile test. This means that the length of the specimen for each cycle may have increased compared to the previous cycle. This is because the constant strain value that is calculated according to the initial length of the specimen at the beginning of the test as 75% is applied to the specimen during the cyclic tensile test. For example, a specimen that undergoes 25% permanent deformation during the first few deformation periods will only deform by 50% when subjected to the next 75% strain (referring to the initial strain value at the beginning of the test). In case the origin of variations in stress values in the cyclic tensile test is not fully understood, there is a possibility that the specimen with the highest performance, on the contrary, might be the specimen with the greatest permanent deformation, and this could be misleading. For this case, the changes in the amount of permanent deformation on the specimens every 50 cycles were calculated, and the results are given in Figure 7. The total amount of permanent deformation that occurred in each cycle during the test was determined by taking the point where the axial force value measured in the upper jaw of the test machine as 0 in the relaxation section of each cycle. Figure 7 shows that the highest permanent deformation belonged to COC at all test temperatures. It is seen that the permanent deformation amount of COC reached 25% at 23°C in the 100th cycle, while it reached 34% at 50°C, and these values did not increase significantly above this cycle. In Figure 7, it is also seen that PCU showed a lower degree of permanent deformation at 23°C than TPU, while both showed similar performance at higher temperatures.

As a result, it was seen that PCU showed the minimum structural deformation at 23°C while its permanent deformation got closer to that of TPU with increasing temperature. In contrast, a higher degree of permanent deformation was observed for COC at all temperatures. The material properties for different applications suggest that more reproducible results can be achieved with COC in stress-sensitive applications subjected to cyclic deformation and operating at temperatures above 30°C. However, the potential sagging in the specimen due to permanent deformation should be considered. Accordingly, there will be no stress-strain linearity in long-term use due to permanent deformation. On the other hand, it was observed that PCU was structurally less deformed compared to the other elastomers, and therefore, it could considered to have higher fatigue resistance. The higher permanent deformation of COC was associated with the crystalline segments in its structure, and the low permanent deformation of PCU was associated with its low T_g values and the high compatibility between its hard and soft segments. This property is particularly critical for elastomers that need to maintain performance under repetitive loading conditions. Among the elastomers studied, the superior resistance of PCU to permanent deformation provides enhanced service life and reliability in biomedical applications, such as flexible implants, cardiovascular devices, and prosthetic components, as well as in industrial applications, including vibration damping systems.

Creep test

The creep behaviors of polymers are directly related to the mobility of polymer chains, similar to many other mechanical and rheological properties. Chain mobility is also determined by structural parameters such as molecular weight, chemical structure, chain branching, and solid organic/inorganic fillers or crystal segments in the polymer structure that behave as a solid below the melting temperature.⁷¹

In the specimens examined within the scope of this study, one of the most important parameters that would affect chain mobility, except the chemical structure of the polymers, was the influence of the crystalline segments and molecular architecture due to all specimens being unfilled. However, the effects of these two parameters cannot be clearly determined separately by the solid-state creep test because they are effective on the solid-state creep resistance of the specimens simultaneously. To determine these two effects separately, first, melt-state creep tests were performed. Thus, it was aimed to examine only the effect of the molecular structure since the crystalline phases were dispersed in the molten state. Then, by performing solid-state creep tests, the effects of parameters such as crystallization or physical crosslink formation were also investigated. Figure 8 shows the time-dependent creep strain values found at 220°C in the melt state by using a rotational rheometer. In Figure 8, it is seen that the creep strain values of the specimens varied in the order of TPU>COC>PCU. In the Figure 8 in which all specimens show similar creep strain-time behaviors, it is seen that the order of slope values in the last part of the curves was TPU>COC>PCU. The last parts of the creep-time curves have been attributed to the creep rate in connection with the Burgers model in various studies. They are associated with completely viscous chain movements in the polymer structure. In this sense, it was thought that the maximum chain entanglement value, which depends on parameters such as molecular weight and chain branching, was found in PCU, while the minimum chain entanglement value was found in TPU.

In Figure 9, the solid-state creep performances of the specimens at three different temperatures (30, 40, and 50°C) are shown. Figure 9(a) demonstrates that COC showed the lowest creep strain, while PCU showed the highest one at all test temperatures, contrary to the results of the melt-state creep tests. The comparison of these results to the melt-state creep test results shows that the crystal structures formed in COC significantly reduced chain mobility, as expected. Additionally, it was revealed that because the hard parts of TPU had a higher melting temperature compared to those of PCU, it led to the restriction of chain mobility in these structures, although not as much as the crystalline parts in COC. It is seen in Figure 9(b) and (c) that the creep values of all three specimens increased with temperature, again, as expected.

Interestingly, however, the creep values of COC increased much more with temperature compared to the other two polymers. Such that, while COC showed the lowest creep strain value at 30°C, it showed a similar creep performance to TPU at 40°C and exhibited creep strain values between those of TPU and PCU at 50°C. The higher degree of increase in the creep values of COC than the other polymers can be explained by approaching T_g of the hard (norbornene) parts of the structure, which was determined as 55°C in the DSC analysis or approaching the α transition temperature of the soft ethylene parts (approximately 60°C in the literature). To reveal solid-state viscoelastic properties and the effects of the crystalline and amorphous segments in the polymer structure on the creep properties of the polymers, experimental creep strain-time curves were modelled by the four-parameter Burgers model that combines the Maxwell and Kelvin-Voight methods in series.^72,73 A schematic representation of the Burgers model is given in Figure 10.

According to the Burgers model, the total creep strain of a viscoelastic solid is equal to the total strain of the Maxwell spring (ɛ_M1), Kelvin unit (ɛ_K), and Maxwell damper system (ɛ_M2) results, as given in equation (1).

ε (t) = ε_{M 1} + ε_{K} + ε_{M 2}

(1)

The basic representation of the Burger model, equation (2), is obtained as:

ε (t) = \frac{σ}{E_{M}} + \frac{σ}{E_{K}} [1 - \exp (\frac{- E_{K} t}{η_{K}})] + \frac{σ t}{η_{M}}

(2)

In the equations, E_M and η_M represent the Young’s modulus of the spring in the Maxwell element and the viscosity of the dashpot, and E_K and η_K represent the Young’s modulus of the spring and the viscosity of the dashpot in the Kelvin unit in the model, respectively. Finally, σ and t represent the applied stress and time, respectively. The Burgers model-fitted creep strain-time plots are given in Figure 11, and the Burgers model parameters are summarized in Table 2.

Table 2.

Burger model parameters for the samples at different temperatures.

Sample code	30°C				40°C				50°C
Sample code	E_m (MPa)	E_k (MPa)	η_m (MPa.s)	η_k (MPa.s)	E_m (MPa)	E_k (MPa)	η_m (MPa.s)	η_k (MPa.s)	E_m (MPa)	E_k (MPa)	η_m (MPa.s)	η_k (MPa.s)
COC	0.33	0.77	1433	80.2	0.32	0.22	635	13.65	0.33	0.10	345	7.71
TPU	0.30	0.29	959	13.34	0.26	0.22	615	13.65	0.21	0.16	475	11.73
PCU	0.18	0.29	613	21.01	0.16	0.22	427	15.35	0.16	0.09	259	10.94

In Figure 11, it is also seen that the model described creep behaviors with sufficient consistency. In general, the E_m value represents the purely elastic character of the viscoelastic material, and it is related to the solid content in the specimen, such as an inorganic filler in polymer composites and crystalline segments in semi-crystalline polymers. In this respect, the E_M values that were found were only attributed to the crystalline segment of the polymers due to the fact that none of the specimens contained any type of filler. The Table shows that the E_M values of the specimens were ordered as COC>TPU>PCU at all test temperatures. It is also seen that the E_M values of all specimens decreased with temperature except for COC. The decrease in the E_M values of PCU with temperature may have been caused by low T_g values, while that in the values of TPU can be explained by higher chain mobility due to the melting of the soft segments.

The E_k parameter of the Burgers model represents the elastic character in the viscoelastic structure, and it is associated with rigid components (such as norbornene and isocyanate) in the structure. The Table shows that the E_K values of all specimens generally decreased with temperature, but interestingly, this decrease was much more pronounced for COC in comparison to the other specimens, unlike the decreasing trend in the E_M values. Such that, the order of the E_K values for 30°C was COC>TPU = PCU, it was COC = TPU = PCU for 40°C, and it was COC<PCU<TPU for 50°C. This significant decrease in the E_K values of COC may be explained by the convergence of the test temperature to the T_g values of the rigid isocyanate segments of PCU, which were determined as 55°C in the DSC analysis.

The η_K values, which are indicative of the viscous character in the viscoelastic structure, also showed a similar change and ordering to the E_k parameter. In general, the η_K of TPEs is associated with soft segments that provide mobility by being localized among the rigid components in the structure. In the test temperatures reaching the alpha transition temperatures of ethylene which are about 55–60°C, the soft part of COC lost its viscosity, η_K decreased directly, and E_K decreased indirectly much more effectively than those of the other specimens.

Finally, the fourth parameter of the Burgers model is η_m, which is an indicator of resistance to completely irreversible deformation in the structure and is associated with the permanent deformation of the amorphous polymer chains. In Table 2, it is seen that the η_m values were ordered as COC>TPU>PCU for 30°C, COC≈TPU>PCU for 40°C, and TPU>COC>PCU for 50°C. The Burgers model was applied to analyze changes in the viscoelastic structures of the three elastomers that behaved differently at different temperatures. The results showed that the main reason for the increase in the creep elongation rates of COC with increasing temperature may be the increased mobility of the ethylene moieties in the structure caused by approaching the α-transition temperature. Thus, it was thought that with this increased mobility, the sliding of the hard parts became easier, and as a result, COC showed higher strain with increasing temperature compared to the other specimens.

One of the important results obtained from the creep tests applied at different temperatures was the prediction of creep strain values for a longer period. Although there are different methods in the literature for predicting long-term creep performance, one of the most common and accepted methods is the time-temperature superposition (TTS) principle.^71,74 In the TTS principle, it is basically assumed that temperature and time have similar effects on the final properties of a viscoelastic material at a particular ratio. The methodology applied in determining the long-term creep performance in creep tests was explained in detail in our previous study.⁷⁴ The longer-term time-dependent creep strain values for 30°C obtained using the TTS principle are presented in Figure 12.

In the Figure, it is seen that PCU showed very high creep strain compared to the other two specimens at 30°C, while COC showed the lowest strain. According to the TTS predictions, at the end of the first day, COC was elongated by about 13%, TPU was elongated by 20%, and PCU was elongated by about 33%. The creep elongation trends of the specimens also shows that PCU displayed a higher rate of elongation than the other two polymers. Accordingly, it was considered that COC was more suitable for applications that required working under stress for a long time, while PCU would have a significant disadvantage.

Conclusion

In this study, a comparative analysis of two new-generation thermoplastic elastomers (COC and PCU) and a conventional thermoplastic elastomer (TPU) was performed to evaluate their thermal, mechanical, and viscoelastic properties. The thermal analysis revealed that all three elastomers exhibited two distinct Tg values, corresponding to the soft and hard segments, with COC showing the lowest melting temperature (Tm) at approximately 88°C. This lower Tm limits its performance at elevated temperatures but makes it suitable for applications requiring moderate thermal resistance. In contrast, TPU and PCU displayed higher Tm values, indicating better thermal stability up to 155°C. The tensile test results demonstrated that COC achieved the highest Young’s modulus, especially at low temperatures, indicating superior rigidity. However, its stiffness decreased more significantly with rising temperature compared to TPU and PCU, which maintained a more balanced mechanical response. PCU, on the other hand, showed excellent resistance to permanent deformation, making it highly suitable for cyclic loading applications, such as biomedical devices and vibration damping systems. TPU provided a balance between flexibility and mechanical strength, making it versatile for industrial applications, including automotive components and protective coatings. The solid-state creep tests combined with Burgers model analysis highlighted that COC exhibited the highest creep resistance at lower temperatures due to its crystalline structure. However, its performance declined with increasing temperature, whereas TPU and PCU showed more stable viscoelastic behavior across varying thermal conditions. These findings suggest that COC is ideal for low-temperature, load-bearing applications, while PCU’s superior fatigue resistance positions it as the best candidate for long-term cyclic applications. Future studies could explore strategies to improve the thermal stability and creep resistance of COC, such as incorporating reinforcing fillers or optimizing its crystalline structure. For PCU, enhancing its high-temperature performance through the addition of thermally stable segments or heat-resistant nanofillers could further expand its application potential, particularly in demanding environments.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Alper KASGOZ

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