Journal of Alloys and Compounds 586 (2014) S214–S217
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Comparison of shape memory effect in UHMWPE for bulk and fiber state Aleksey Maksimkin ⇑, Sergey Kaloshkin, Mikhail Zadorozhnyy, Victor Tcherdyntsev National University of Science and Technology ‘‘MISIS’’, Moscow 119049, Russia
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Article history: Available online 13 December 2012 Keywords: Shape memory effect UHMWPE Recovery stress
a b s t r a c t The shape memory effect in pure ultra-high molecular weight polyethylene (UHMWPE) and UHMWPE fibers has been studied using dynamic mechanical analysis. The temperature dependences of properties that define functional characteristics of shape memory polymers (SMP), such as recovery stress, recovery strain, and activation temperature of transition, were determined. The isothermal recovery stress in UHMWPE deformed by 200% and UHMWPE fibers achieved rather high values, up to 6 MPa and 22 MPa, respectively. The shape memory effect in UHMWPE is the result of a very high entanglement coupling chains. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The shape memory effect consists in the controlled change in the geometrical dimensions of material under the action of an external stimulus. Commonly, the external stimulus is a change in temperature; however, it could be electricity, light, magnetism, and even moisture [1]. These materials are widely applied in bioengineering, protection systems, heat-shrinkable tubes, smart fabrics, electrical industries, and aerospace [2,3]. Materials with shape memory effect can be metals, ceramics, and polymers. The shape memory polymers (SMP) have a much lower recovery stress compared to metals, 1–10 MPa for SMP against 400 MPa and more for shape memory alloys (SMA). However, the polymers have one significant advantage, which consists in the ability to larger attainable strains. SMPs can lengthen by hundreds percent, while SMAs and ceramics can be deformed only by about 10% and 1%, respectively [3]. This property makes promising further research and development of polymer materials with shape memory. At present, researches of many scientists aim at increasing recovery stress in SMPs. Cross-linked polyethylene is a widely used shape-memory material in the industrially developed countries. It is produced by using various grades of polyethylenes (LDPE or HDPE), which is cross-linked by one of the following methods: radiation, peroxide or silane. As a result of cross-linking, C–C cross bonds, which impart shape memory behavior to PE, form between polymer chains. Cross-linked polyethylene can be deformed up to 300% of the permanent shape dimensions above the melting temperature and should then be fixed firmly during cooling [4]. Upon reheating above Ttrans, polyethylene returns to its permanent shape, developing recovery stress of up to 3 MPa [3]. ⇑ Corresponding author. E-mail address:
[email protected] (A. Maksimkin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.014
UHMWPE is a relatively new polymer, and has not yet received its distribution as a material with shape memory. Due to very long molecular chains, the polymer has many unique properties and is fundamentally different from other grades of polyethylene. The distinctive properties are a low friction coefficient, high wear resistance, and higher mechanical properties. Especially high strength is reached for the UHMWPE fibers due to orientation of its very long polymeric chains. Therefore, it is of particular interest to study an effect of long polymeric chains in UHMWPE on its shape memory behavior. At present, no data on UHMWPE as the shape-memory material are available from the literature. The effect was only mentioned. For instance, in [5,6], the ability of deformed UHMWPE to recover its permanent shape in heating was used to remove plastic deformation from the polymer surface after the tribological tests. In connection with this, here, we study UHMWPE for bulk and fiber state and characterize is as the shape-memory material.
2. Experimental details The shape memory effect was studied for Ticona GUR 4120 UHMWPE powder with an average molecular weight of 3–5 106 g/mol and highly oriented UHMWPE fibers. UHMWPE GUR 4120 powder was thermo pressed into samples with dimensions of 78 10 2 mm at a temperature of 180 °C and a pressure of 60 MPa. Then the monolithic samples were elongated at room temperature by 200% using Instron tearing machine at a speed of 10 mm/min. Thermal analysis of the initial thermo pressed samples and the deformed samples was performed using differential scanning calorimetry (DSC) on NETZSCH DSC 204 F1 calorimeter in argon. The measurements were taken in controlled heating and cooling regime at 10 °C/min. Shape memory properties were studied using a DMA Q800 dynamic mechanical analyzer. For the DMA testing, the deformed samples were cut into rods with square cross-section of 1 mm2, total length of 36 mm, and working length of 16 mm. The UHMWPE fibers were taken in the form of bundles with a cross section of approximately 0.2 mm2. Several test series were conducted to determine the main shape-memory properties of UHMWPE.
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To study the recovery strain, the samples were heated from room temperature up to 155 °C at 2 °C/min with one end of a sample fixed in a still clamp and the other end fixed in a freely moving clamp with a minimal preload force of 0.001 N. To study the isothermal recovery stress, the samples were thermostated at the temperatures from 100 to 160 °C and kept for 5 h. Both ends of a sample were fixed in still clamps so that distance between the clamps did not change during an experiment. The samples were heated to a temperature of thermostating at 10 °C/min. The generation of recovery stress with time was recorded.
3. Results and discussion It was observed that the deformed UHMWPE can recover its permanent shape in heating. Fig. 1 shows the staged process of shape recovery of the pre-deformed UHMWPE sample as it is heated on a hot surface. Upon melting of crystalline phase, UHMWPE becomes translucent, which allows registering the polymer melting visually. When heated below melting (Fig. 1b), the sample partially recovers its permanent shape. Then UHMWPE begins to melt starting from the surface that is in contact with the hot surface (Fig. 1c). As the heat is conducted to the center, the sample continues to recover the shape (Figs. 1d and e). After the crystalline phase has completely melted, the full shape recovery is achieved (Fig. 1f). Fig. 2a shows the recovery strain of deformed UHMWPE. The polymer completely recovers its permanent shape at 150 °C. When heated to 153 °C, the samples continued to contact by further 6% of its permanent shape. This shrinkage compared to the original shape can be a related to the technology of sample preparation. After thermo pressing, the samples retain residual tensions that would lead to sample contraction during subsequent annealing. Fig. 2b shows the reduction of UHMWPE fibers during the heating. During the heating, the fibers are gradually reduced and at a temperature of 153 °C, this process becomes dramatic. The UHMWPE fibers were reduced by 315%, which is the limit of fiber reduction in the heating. With further heating the fiber length remained constant. Fig. 3a and b gives the isothermal recovery stress curves for the deformed UHMWPE and UHMWPE fibers, at different thermostating temperatures. When deformed UHMWPE was thermostated at 100 °C, a recovery stress of 5.2 MPa was observed in the polymer (Fig. 3a). The recovery stress remained constant for 5 h of thermostating. With increasing thermostating temperature of deformed UHMWPE, the recovery stress increased. The highest isothermal recovery stress was 6 MPa. However, the higher the termostating temperature, the more abrupt was a decrease of recovery stress with time. At the thermostating temperatures of 110, 120, and 130 °C, the recovery stress decreased by 14%, 20% and 41% respectively. Fig. 3b shows a similar behavior of recovery stress for the UHMWPE fibers. Thermostating of the fibers at the temperatures of 130 and 140 °C led to the recovery stresses of 9 MPa and 9.9 MPa, which remained constant throughout the experiment. Heating of the fibers to 150 °C led to an increase in the recovery stress to 10.5 MPa; however, after 130 min, the stress decreased.
Fig. 2. Heat-induced (a) UHMWPE recovery strain for the sample pre-deformed by 200% state and (b) UHMWPE reduction strain for the fibers state.
By the end of the experiment, the recovery stress decreased by 33%. The development of the recovery stress in the fibers during heating passed through a local minimum, which became most pronounced, when fibers were heated to 150 °C. The heating of the fibers to 160 °C raised the recovery stress to the highest value of 22 MPa; then, the stress steeply decreased and, in 24 min, the fibers were destroyed. The local minimum stress is not observed; however, when a recovery stress of 16 MPa was reached, the curve’s slope changed. UHMWPE in the fiber state has a more oriented polymer chains than UHMWPE in the deformed bulk state. The polymer chains behave like a spring that stores internal energy by elongation under an external force. After removal of the external force, the system tries to get back to the original state by decreasing the stored energy. At room temperature the polymer
Fig. 1. UHMWPE shape recovery induced by heating.
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Fig. 3. Heat-induced recovery stress curves for (a) UHMWPE deformed state, (b) UHMWPE fibers state.
chains are ‘‘frozen’’, their oscillations are very small, so the return to the initial state is impossible. As the temperature increases, the molecular mobility of polymer chains grows and the system begins going back. Therefore, comparing the values of recovery stress in the UHMWPE deformed and fibers state, it can be concluded that an increase in elongation of the chains raises the recovery stress. The recorded recovery stresses in the deformed UHMWPE and fibers are very high for SMP, which typically demonstrate recovery stress in the range of 1–10 MPa [7]. The recovery stress for crosslinked low density polyethylene does not exceed 3 MPa [3]. A decrease of recovery stress with time for samples of UHMWPE in the deformed bulk and fiber state can be explained by using the DSC curves of these samples. Fig. 4a and b shows the DSC curves of UHMWPE (first heating) for deformed bulk and fiber state. In the case of deformed UHMWPE (Fig. 4a), the heating to 100 °C does not lead to melting of the crystalline phase of UHMWPE, and the recovery stress also does not change with time, Fig. 3a. Heating to 110 °C and 120 °C falls in the range of melting point of the crystalline phase and the recovery stress gradually decreases with time. Heating to 130 °C melts even larger fraction of the crystalline phase and leads to a steeper decrease in the recovery stress. A similar dependence of the isothermal recovery stress on the molten fraction of the crystalline phase is observed for UHMWPE fibers (Fig. 4b). It can be concluded that the crystalline phase acts as a hard lock of the polymer chains (physical cross-linking) and prevents them from spreading, when their molecular mobility increases during heating. After melting of the crystalline phase, the polymer chains start to slide relative to each other leading to a decrease in the recovery stress.
Fig. 4. DSC curves for (a) UHMWPE deformed state and (b) UHMWPE fibers state.
The local minimum stress, which is observed at force generation in the fibers (Fig. 3b), can be caused by the presence of two crystalline phases in the fibers. The presence of two crystalline phases in UHMWPE fiber is confirmed by double melting peak of the polymer in the DSC curve (Fig. 4b). The melting point of the second high-temperature crystalline phase also has a wide range of fusion, which is difficult to determine due to a superposition of two melting peaks. The molten part of high-temperature phase can again recrystallize into the low-temperature phase. Therefore, in the period of time, when the recrystallization proceeds, the stress decreases, because a fraction of the crystalline phase melts, and the sliding of the polymer chains increases. After the completion of recrystallization, a fraction of crystalline phase increases, and a possibility of polymer chains slipping decreases. The closer the temperature of heating to the melting point of the second peak, the more complete recrystallization occurs, and the steeper is the temporary decrease in the local stress. The mechanism of the shape memory effect in the polymers was described in several reviews [8,9]. Fig. 5 shows the model that describes the shape memory effect in pure UHMWPE. In the permanent shape, amorphous-crystalline polyethylene is isotropic and energetically stable. A load applied to the polymer leads to a deformation of the polymer chains. At room temperature the polymer chains are ‘‘frozen’’, their oscillations are very small, so the return to the initial state is impossible. As the temperature increases, the molecular mobility of polymer chains grows, and the system begins going back. Heating PE below its melting point T < Tm results in partially return to the permanent shape, which can be seen in Fig. 1b. The molecular mobility of polymer chains at a given temperature does not allow the complete recover of its
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Fig. 5. Molecular mechanism of thermally induced SME in UHMWPE.
original shape. At this stage, the crystalline phase of UHMWPE acts as physical cross-linking chains, preventing them to slide relative to each other. When the polymer is heated above its melting temperature, the crystalline phase is melted, and the molecular mobility of polymer increases to a level that allows the polymer to return to its permanent shape. One of the mechanisms of polymer chains interaction is entanglement coupling, which results in viscoelastic behavior of polymer melt [10]. Chain entanglements can form a semi-continuous network inside the polymer material and act as physical cross-links [11]. In the case of PE with low molecular weight, the polymer chains slip relative to each other, the polymer spreads out and loses its shape (cross-linked polyethylene chains hamper the process). Therefore, PE has a shot-term memory. In the case of UHMWPE, the polymer chains are longer and have a stronger entanglement coupling. Slippage of polymer chains is extended in time, so UHMWPE has the long-term memory. Entanglement coupling plays the role of physical cross-links in the melted UHMWPE. 4. Conclusions It is found that UHMWPE exhibits the shape memory behavior. Eminent shape memory properties of UHMWPE are due to its very
long polymer chains that form a network of entanglements acting as physical cross-links. Melting of crystalline phase leads to the gradual disappearance of SME due to the polymer chains sliding. The value of recovery stress depends on a degree of polymer chain elongation, because the polymer chains behave like a spring, that stores internal energy by elongation under external force. Therefore, the recovery stress of UHMWPE in the fiber state. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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