Preparation of shape memory polyurethane gels containing iron

There are many interesting materials in the world, and every day a new one is produced according to the needs with engineering applications. One of the most important of these materials are able to maintain the original shape against changing physical and chemical stimuli. The types of stimuli include; heat, voltage, pressure, electric current, magnetic field, change of pH, solvent or moisture level, light and other factors. These type of materials are known as stimuli-responsive materials. Stimuli-responsive materials are used in many areas due to protect their original shape against these stimulus. Stimulus-responsive polymers and composites in recent years, drug release, and in issues such as gene transfer has become a very important materials. Showing heat sensitivity of shape memory polymers are composed of crystalline phase and amorphous phase. Melting point and glass transition temperature, is a key information nematic phase transition from the isotropic phase. Physical and chemical crosslinking is an important factor for ability to return to the original shape. Developed most important shape memory materials are shape memory alloys and shape memory polymers. Shape memory polymers based on polynorborne is manufactured as first generation shape memory polymers in 1984 by Nippon Zeon Company in Japan. Later trans-izopoliprem and styrene-butadiene based shape memory polymers have been developed. Thermoplastic shape memory polyurethane was invented in 1990 by Dr. S. Hayashi. The main advantages of shape memory polymers are; low density and price, easy production of high quality, high tension strength, shape recovery in wide temperature range, high damping ratio in transition region, low-cost recycling, excellent chemical stability, biocompatibility, high biodegradability, suitability to use in the human body and flexibility. Shape memory polymers are important materials that have dual-shape capability. Shape memory effect only relies on the molecular architecture. Therefore, material properties can be adjusted with monomer type or comonomer ratio. Shape memory polymers, stimuli sensitive switches is equipped with an elastic polymer networks. Polymer network comes from molecular switches and netpoints. Physical or chemical crosslinking determines the shape of permanent polymer network. Physical crosslinking of the polymers is observed in morphology occurring in areas that have at least two segregated domains. High thermal transition temperature of these areas serves as netpoints (a hard segment). Second high thermal transition temperature transition zones acting as molecular switches. The shape memory polymer network are composed of crosslinks or covalent bonds. Physical crosslinking is carried out by crystallization or vitrification of domains at transition temperature. Shape memory polymers can be easily deformed above the glass transition temperature. A small force is sufficient for the preservation of the temporary shape. Tg is glass transition temperature that is activated that shape memory effect in shape memory polymer. Shape memory polymer below Tg is rigid, glassy state and hard to deform. In this case, soft segment of the shape memory polymer is frozen state. Significant segmental motion can not be observed in this region. In glassy state, the rigid material is hard to deform. When the shape memory polyurethane heated, material enters the glass transition region. Within this range, the soft segment moves and the heat capacity the shape memory polymer increases. Polyurethanes, is smart material, gradually increasing attention in recent years due to their shape-memory property. They started to be used widely in the biomedical field such as stent, implant and drug delivery because of its mechanical, thermal, magnetic and biocompatibility properties. Nowadays, especially composite materials, is prepared by using material having magnetic properties, have great interest for use in the medical field. Through these composites, polymeric materials are being developed that show the shape-memory property under a magnetic field. The aim of this study, to be used in the medical field, synthesizing biocompatible polyurethane (PU) gels showing shape-memory property under magnetic field .In the study; polymeric composite materials formed by polyurethane and magnetite (Fe3O4) will be synthesized. In the synthesis of these composite, polyethylene glycol 300 (PEG300), polyethylene glycol 3000 (PEG3000), and castor oil (CO) as polyol, 1,4-butanediol (BDO) as chain extender, hexamethylene diisocyanate as the isocyanate source (HDI) were used. Castor oil is also crosslinking agent in the synthesis. Any solvent or catalyst are used in the synthesis. Micro magnetite (Fe3O4) was added into synthesized polyurethane films in order to show shape-memory feature under magnetic field except the effect of heat. The magnetite ranges from 0% to 10% amount by weight in the polyurethane. Thus, the percentage of magnetite, under different magnetic field and its effect at various temperature on shape-memory property of the composite materials were investigated. Only the performance of magnetite percentage effect on shape-memory feature was investigated keeping CO / PEG ratio constant in the study. In determining of shape-memory performance of PU containing different percentage of magnetite, bending test, which is relatively simple test, was utilized. With this test, PU films' shape fixity and shape recovery ratio were determined. CO / PEG ratio of PU films in the synthesis is not changed so that contents of rigid area is the same in all the films. Hence, contents of rigid area is encountered as an invariant parameter in shape-memory performance and its impact is not examined. In the bending test, the effect of the percentage of magnetite on polyurethane's shape-memory properties were investigated. Due to the biocompatibility of PU films, to investigate the availability of possible biomedical applications, the shape fixity at 40 ° C and shapes recall rates of films were analyzed. Furthermore, bending test results, applied under 40° C and 65° C temperature and magnetic field were compared with each other. Fourier transform infrared (FT-IR) spectroscopy for structural characterization, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) for thermal characterizationand dynamic mechanical analysis to determine the viscoelastic properties (DMA) devices are used for determining of synthesized magnetite particles PU films' properties. The swelling behavior of the synthesized PU films in pure water and the percent gel content is determined with the Soxhlet extractor. Average molecular weight of crosslinking density and between the two crosslinks of PU composite films are calculated starting from the DMA data. Contact angle measurement of the synthesized PU films in order to determine the effect of the hydrophilic property of the polyurethane with magnetite additive was performed. Contents of hard segments, crystallinity, transition temperature, crosslinking density and the additive in the film affects shape-memory property of the polyurethane. In this study, the contents of hard segments kept constant, impacts of magnetite additive on the shape-memory properties was investigated. Varying percentage of magnetite in PU films affects crystallinity the transition temperature, and crosslinking density value. Therefore, shape-memory performance varies the percentage of additive. According to the results obtained in the study; increasing magnetite percentages in PU film, caused to decrease shape fixity ratio (Rf) due to hardening of material. On the other hand, shape recovery (Rr) increased by increasing magnetite percentage in the films. In this study, shape recovery step applied at two different temperature, 40 °C and 65 °C, and under magnetic field. The first step of the bending test, the heating step, was at 40 °C or 65 °C and the second step, is cooling step, was performed at 4 °C. According to the results obtained, Rf and Rr were increased a little with increasing temperature. In order to determine reusability of a sample, its shape memory behavior was determined in ten consecutive measurements by bendin test. Glass transition temperature of all the synthesized PU films are quite lower than the room temperature. As expected, with increasing percentage of magnetite, the density of PU composites showed also an increase. With increasing percentage of additive in the synthesized PU films, the water retention of films became difficult and swelling ratio was decreased. Magnetite additive caused the decrease of the surface hydrophilic properties of the polyurethane films. The overall result of the study is that magnetite added polyurethane films have a great potential for biomedical applications as shape memory material.