|Title:||Studies of structure and water vapor transport properties of shape memory segmented polyurethanes for breathable textiles|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Shape memory effect
|Department:||Institute of Textiles and Clothing|
|Pages:||xxvii, 273 leaves : ill. ; 30 cm.|
|Abstract:||Shape memory polyurethanes (SMPU) have segmented structure which is capable of changing their shape upon application of heat. Large change of thermomechinical properties would occur across the glass transition temperature (Tg) or soft segment crystal melting temperature (Tms) of SMPU. In addition to the change of thermomechanical properties of SMPU, it also has large change in moisture vapor permeability above and below the Tg/Tms. From the literature review it was found that the transition temperature for water vapor permeability is above 40℃, which would not be suitable for breathable textile applications. Therefore the key objective of this study is to investigate the role of segmental structure and soft segment crystal melting on water vapor permeability of shape memory segmented polyurethane (SPU) for breathable textiles. In this study, six unique factors on structure, shape memory and water vapor transport properties of segmented polyurethanes (SPU) have been investigated. These factors are hard segment, hydrophilic block length, hydrophilic segment content, hydrophilic and/or carboxylic unit content, mixed polyol block and multi wall carbon nano tube (MWNT). For this purpose, three different kinds of polyols such as polycaprolactone diol (PCL), polytetramethylene glycol (PTMG) and polypropylene glycol (PPG) with different molecular weight were used as soft segment, on the other hand 4,4'-methhylene (bisphenyl) diisocyanate (MDI) and 1,4-butane diol (1,4-BDO) were used as hard segment. SPUs were modified by hydrophilic segment such as polyethylene glycol (PEG) with different molecular weight, and/or carboxylic group containing unit such as dimethyl amino propionic acid (DMPA). In addition SPUs were reinforced with different quantity of functionalized multi wall carbon nano tube. Polymers were synthesized by two or three steps polymerization techniques. SPUs were characterized by Fourier transform infra red (FTIR) spectroscopy, Raman spectroscopy, wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic mechanical thermal analysis (DMTA), positron annihilation lifetime spectrometry (PALS), thermogravimetry analysis (TGA), Instron, and shape memory behavior which was measured by Instron having temperature control chamber. Water vapor transport properties were measured by equilibrium sorption, dynamic sorption and water vapor permeability measurements (ASTM 96 E).|
Experimental results revealed significant variations in macromolecular structure, shape memory effect and water vapor transport properties of SPUs depending on segmental architecture. There is evidence that the microcrystalline structure formation in SPU would depend on the selection of soft segment. SPUs are completely amorphous, when the polyol is non-crystalline at room temperature. Furthermore the introduction of little amount of MWNT slightly increases the soft segment crystallinity. These micro crystal melting point temperatures are in the room temperature range (15 - 23℃) which enhances the water vapor permeability (WVP) above room temperature through the nonporous membrane structure. The membranes with completely amorphous structure have no such abrupt changes of WVP in the experimental temperature range. The behavior of water vapor transport of SPU membranes were not only influenced by soft segment crystal melting point but also interpreted on the basis of chemical nature of soft segment in the SPU backbone, hydrophilicity, hard/soft segment content and free volume. TEM images show two phase micro-phase separated structure which was depended on the type of polyol used. Experimental result shows that the shape recovery effect was improved by increasing the percent crystallinity or physical cross-linking between the polymer chains. Selected SPUs were applied to the cotton fabrics by coating method. Water pressure resistance value of 24.5 mbar was achieved for the coated fabric with PTMG (Mn 2900) based SPU containing 15 wt% of PEG (Mn 3400). Coated fabrics also maintained good water vapor permeability, therefore confirmed the physiological comfort to the wearer. MWNT reinforced SPU coated fabrics show excellent UV blocking properties. With only 1 wt% of MWNT in the SPU, the UV protection factor (UPF) was about 125 (rated as excellent, >+50) achieved as compared to the uncoated fabric having UPF of 5.6 which is non rate able. This study stated development of temperature stimulating shape memory segmented polyurethanes membrane with improved water vapor permeability and soft segment crystal melting temperature in the room temperature range. Such nonporous membrane would be applicable to develop smart breathable textiles by laminating solid nonporous membrane instead of microporous membrane on a suitable base fabric which would have lower water vapors permeability at low temperature and significant increase of water vapor permeability with increasing temperature. This "flexible barrier function" would enable the garment to intelligently adjust its insulating properties in response to temperature changes which will assure optimum comfort regardless of temperature. Systematic study on structure and water vapor transport properties presented in this thesis throws light to the polymer chemist to develop smart polyurethane membrane through molecular design from wide range of raw materials available for polyurethane synthesis.
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