Preparation And Application Of Polyurethane Complex Films

To improve the bad permeability and the responsiveness to environment of synthetic leather and reduce the large amount of organic solvents used in polyurethane leather manufacturing techniques, the incorporation of amphiphilic macromolecular polymers into polyurethane was adopted and it was synthesized by radical polymerization and cast by wet method to get the porous semi-interpenetrating polymer networks as a base layer of polyurethane leather. By controlling the polymerization conditions of amphiphilic monomer and its molecular weight and distribution, an even porous structure of the semi-IPNs with good permeability of water vapor was obtained. The function of the resulted porous film was similar to the polyurethane base of synthetic leather. This invention opened a new world of the application of IPNs and semi-IPNs in leather industry and made it possible for semi-IPNs putting into practice. Moreover, this environmental friendly application lowered the amount of organic solvents, which provides a new method of leather industry in theoretical and practical fields.To make clear of the existence and properties of poly-N-isopropylacrylamide (PNIPAM) in the composite polymer materials and the effects of wet-casting conditions, pure PNIPAM hydrogels were synthesized with N,N’-methylene-bis-acrylamide (BIS) as the crosslinker in two solutions, one—mixed solvents of dimethylformamide (DMF) and water with different ratios, the other—mixed solvents of DMF and tetrahydrofuran (THF). For the later one, the products were mixed with water and tested at different temperatures for turbidity tests. The results of differential scanning calorimeter (DSC) and turbidity tests indicated that the water existence in PNIPAM and its lower phase transition temperature (LCST) were greatly affected by the ratio of solvents. On the basis of PNIPAM hydrogels, PU/PNIPAM semi-IPNs were prepared using DMF and THF as solvents with a ratio of7:3(v/v) under nitrogen atmosphere. The synthetic parameters, such as the ratios of PU and NIPAM, the amount of crosslinker, the reaction time, were investigated to study their effects on material’s structure and properties. Later, the materials were casted in different filming mediums selected from water, ethanol and isopropanol to study the mechanism of PU/PNIPAM semi-IPNs filming theory. By comparing the water existence in pure PNIPAM with that in PU/PNIPAM semi-IPNs and studying the solvent effects on PNIPAM hydrogels and PU/PNIPAM semi-IPNs films, the interaction between PU and PNIPAM molecular chains can be deduced. The materials obtained were characterized by Fourier transform infrared spectroscopy(FT-IR), scanning electron microscope (SEM), differential scanning calorimeter (DSC), dynamic mechanical analysis(DMA), Thermal Gravity(TG), X-ray diffraction analysis(XRD) and Scanning Probe Microscopy(SPM). The porosity, mechanical properties, swelling ratios and water vapor permeability of porous films cast by wet method were measured and made comparison with dense membranes.PU/PNIPAM semi-IPNs can be formed, which was proved by FT-IR spectra and DMA measurements. FT-IR spectra confirmed the existence of strong hydrogen bond interaction between PU and molecular chains. DSC and turbidity tests showed that:the ratios of water and solvents had great effect on the swelling behavior of PNIPAM and the existence of water in semi-IPNs; the phase transition of PNIPAM was influenced by the interaction of solvents and water, with more significant phase transition behavior in water-dominated solution; the water in PU/PNIPAM semi-IPNs showed exact opposite trends from the water in pure PNIPAM hydrogels. That was due to that most of the polar groups in PNIPAM formed hydrogen bonds with PU, leading to its weak linkages with water. Swelling ratio tests also showed that the phase transition temperature scope of PNIPAM in PU/PNIPAM semi-IPNs are between35℃and45℃.The results of dynamic mechanical analysis (DMA) showed that with the increase of reaction time, the interpenetrating degree of PU and PNIPAM improved, making the glass transition temperature (Tg) of materials elevated. The PNIPAM molecular chains are very short when the BIS amount is2%. At that condition, when the ratio of PU and PNIPAM was4:1, the Tg of materials was elevated as PNIPAM promotes the crystallization of molecular chains, but with the content of PNIPAM increasing, it destroyed the crystalline regions of PU, causing the drop of Tg When BIS amount was5%, PNIPAM destroyed the interaction of PU chains, making the Tg lower; and with more PNIPAM incorporated, the phase separation of PU and PNIPAM alleviated and the Tg of materials boosted. When the amount of BIS was8%, with the ratios of NIPAM increasing, the compatibility of PU and PNIPAM improved; the crystalline regions in hard segments of PU were ruined, leading to lower Tg. XRD tests indicated that the introduction of PNIPAM promotes the crystallization of materials; when BIS amount was5%, the crystallization of materials was the highest and the distance between crystalline layers was the longest; with the increasing amount of NIPAM, the destruction of crystalline regions aggregated, and more amorphous regions were formed; the increasing amount of BIS made the crystallization of materials went up first and then down with the maximum at5%BIS.SEM showed that the forming of PU/PNIPAM semi-IPNs films and the size of pores are affected by the solubility parameters of solvents. Binodal phase separation and nucleation were happened when water and ethanol were used as coagulation solution, while spinodal phase separation was happened when isopropanol as the medium. The larger gaps of solubility parameters between the filming medium and mixed solvents, the bigger pore size of the films is. Therefore, under the same conditions, the pore size of films formed in water is much larger than that in ethanol and isopropanol. On the other hand, the existence of PNIPAM makes the exchange rate of solvents and nonsolvents slow, leading to the finger-like pores turning to round-like ones.The measurement of porosity and physical properties implied that the microporous films exhibits better swelling and water vapor permeability properties, but lower tensile strength compared with dense films. The porosity also changed the tensile strength regulation of materials. To the dense membrane, the initial modulus properties increased and the tensile strength elevated, yet the elongation at breaking points went up and down with the increasing ration of PNIPAM in materials. For microporous films, their porosity was lowest and physical properties were best at5%BIS.The test results of dynamic mechanical analysis (DMA), Thermal Gravity (TG), X-ray diffraction analysis (XRD) and the mechanical properties of materials implied that the introduction of PNIPAM elevates the crystallization of PU, making it more strong and stable to heat.To make clear of the reasons for the change of contact angle of materials, Wenzel and Cassie model were adopted. Combined with the results of SPM, it can be found that contact angle was affected by the properties of surface polymer and the roughness of the surface. The hydrophilicity of PU/PNIPAM semi-IPNs was improved after heating and its thickness became smaller with the semi-IPNs reaction time longer, leading to the decrease of surface roughness. The strong interaction of polar moleculars between molecular chains of PU and PNIPAM (weight ratio3:1, 5%BIS) makes the phase change of PNIPAM difficult, thus leading to little change of contact angle. Those phenomena can also be proved by water vapor permeability tests and the Ep values of PU/PNIPAM semi-IPNs. The introduction of PNIPAM greatly improved the water adsorption ability of PU materials, and made the materials more hydrophilic. For PU, the surface contact angle was82.5°, while for PU/PNIPAM semi-IPNs it can reach at55°. The contact angle disparity was16°between room temperature and60℃.Swelling ratios tests implied that the swelling properties are related with the volume phase transition temperature of PNIPAM, but have little relationships with BIS amount and coagulation solution; the longer reaction time of semi-IPNs, the lower swelling ratios of semi-IPN films. To dense membrane, the thermosensitivity of membranes with PU/PNIPAM ratio at2:1and BIS8%are the most significant ones, but turn to less distinct with longer reaction time. The swelling curves at different temperature with the time showed that the swelling of films changes like waves at the initial time from0to240mins and then become stable after300mins. The wave-like change of swelling ratio was due to the fact that the shrinkage of PNIPAM molecular chain impels the water inside but leaves room for free water to come in later. According to the experimental results of water retention(WR) with time, the equation of fitting curves of ln(WR) with t is:ln(WR)=-kt+b, among which k is the rate of water retention. It can be seen from k values that the dehydration speed turns faster with the BIS amount; when put into40℃water, the dehydration of films becomes slower with the reaction time of semi-IPNs, yet turns opposite side when put into60℃water; the higher the temperature of swelling water solution, the higher the dehydration rate of films. The swelling ratios dropped from46.77to8.95within a narrow temperature gap, from which it can be seen that PNIPAM imparts the materials thermosensitivity,According to water permeability formulae and the Arrhenius equation, the permeability active energy (Ep) can be calculated. The relationships of water permability and time can be described as ln(WVP)=K*(1/T)+B, and Ep=8.314K. The Ep values decreased with BIS amount for films with6hours reaction time, and visa versa for films with9hours reaction time; the effect of reaction time on Ep values was the lest when BIS amount was5%and the water vapor permeability for films with PU:NIPAM at2:1were the highest.In all, PU/PNIPAM semi-IPNs have good water vapor permability and thermosensitivity, and exhibits good mechanical properties at the same time, showing great potential for application.