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Title: Microstructure and water vapor transport properties of temperature sensitive polyurethanes
Authors: Ding, Xuemei
Degree: Ph.D.
Issue Date: 2004
Abstract: Temperature sensitive polyurethane (TS-PU) is one novel type of smart polymers. The water vapor permeability (WVP) of its membrane could undergo a significant increase as temperature increases within a predetermined temperature range. Such smart property enables this material to have a broad range of applications to textile industry, medicine, environmental fields and so on. However, based on the literature review, contradicting results were found on some TS-PUs (as claimed) regarding Tg as transition temperature. The predetermined temperature range of other TS-PU products, regarding T1 as transition temperature, is so high as to restrict their applications to many areas, particularly in textile industry and medicine area. Moreover, they all did not provide a clear theoretical explanation of this smart permeable phenomenon. The aims of this project are to synthesize TS-PU with Tm in the broader temperature range including ambient temperature range, and then investigate systematically the relationships between microstructure and water vapor transport properties of TS-PU. Such researches enable to identify the key parameters governing the water vapor permeability of TS-PU membrane and open the way for new tailor-made polymeric materials with specified properties as well as fashion innovation and so on accordingly. For this purpose, in this project, a series of polyurethanes (PU) were synthesized using five different crystalline polyols with approximately similar molecule weight and three different hydrophilic contents, and dense membranes were prepared accordingly. Some of these PUs were designed as semi-crystalline polymers with different degree of crystallinity in the broader temperature range, while others were fully amorphous or contain very low crystallinity for comparison, The microstructure and properties of these PUs were investigated using Differential scanning calorimetry (DSC), Wide angle X-ray diffraction (WAXD), Dynamic mechanical analysis (DMA), Fourier transform infra red spectroscopy (FTIR), Polarizing Microscopy (POM), Transmission electron microscopy (TEM), Instron tensile instrument and Positron annihilation lifetime techniques (PALS). Their equilibrium water sorption and water vapor permeability were measured accordingly. Results show that crystal melting of these resulting semi-crystalline PUs take place in the temperature range from -10-60 C as desired. These polyols form crystallites in the segmented PUs, and the crystalline properties of the PUs, including degree of crystallinity and crystal melting temperature (Tm), depend on chemical structure of the polyols and hard segment concentration (HSC). Storage modulus (E') drops down quickly in the temperature range of crystal melting, suggesting a great transition in the predetermined temperature range. Crystal melting triggers the motion of molecule chain of soft segment. The presence of hard segments hinders the formation of crystallinity in polyols. The decreased HSC as well as regular chemical structure of polyols results in the larger spherulites and higher melting end temperature, and the higher crystallinity induces the more obvious incompatibility of soft segment and hard segment in the PUs. These PUs are proved to have good enough elastic about 10 times in strain at max-load and strength about 25MPa in stress at max-load for applications. The mean free volume size and fractional free volume increase more significantly in the temperature range of crystal melting than in other temperature intervals, Different characterizing methods have proved that the microstructure of PUs is just as designed. Finally, as expected, the WVP of semi-crystalline PU membranes increases significantly in the temperature range of crystal melting. Equilibrium water sorption keeps approximately constant, suggesting that the significant increase in WVP mainly results from the significant increase in diffusion coefficient. By contrast, those PUs, which are fully amorphous PUs or contain less crystallites, do not exhibit this smart property. The relationships between microstructure and WVP of the PUs as a function of temperature are observed. Results indicate that the high degree of crystallinity results in the small WVP in rough. The water sorption content (G) reveals a quasi-exponential dependence on the hydrophilic PEO content. However, hydrophilic PEO content has no obvious influence on WVP. Chemical structure of soft segment and variation of free volume with temperature present a profound influence on WVP. The significant increase in WVP of semi-crystalline PU is obviously correlated with the sharp increase in the free volume in the predetermined temperature range. Such relationship proves that the permeation process of water vapor through TS-PU membranes within the temperature range of crystal melting obeys Fijita's free volume model. In this project, a series of temperature-sensitive polyurethane (TS-PU) with the predetermination of smart transition temperature close to room temperature range through molecule design have been synthesized, which have wide applications to textile industry, medicine area and so on. Such smart waterproof and breathable fabrics can be developed through coating or laminating fabrics with the smart membrane. In the meantime, the microstructure and morphology of TS-PU have been investigated in details The variation of free volume in the temperature range of soft-segment crystal melting is first observed using PASL. This project also studies in-depth on the relationship between microstructure and water vapor transport property of the TS-PU in the specified transition temperature range. Particularly, water vapor permeability of TS PU membrane is correlated with fractional free volume. Such relationships lead to a scientific approach to prepare temperature-sensitive polyurethane in order to meet different requirements for various innovation applications. Potential applications of this smart material to textile industry, medical area, food packaging, gas separation as well as chemical industry are presented, and future study is suggested in the end of this thesis.
Subjects: Hong Kong Polytechnic University -- Dissertations
Smart materials
Pages: xxv, 254 leaves : ill. ; 30 cm
Appears in Collections:Thesis

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