Gas Permeation Of Polymer Studied By Positrons

In this dissertation, a series of polymers (including polyurethane and polymer nanocomposites) are systematically studied by Positron Annihilation Lifetime Spectroscopy (PALS) and the other experimental methods. Effects of the temperature and chemical composite on the free volume, molecular motion, gas permeability and barrier properties are discussed. The main results are as follows:1. Polyurethane polymersa. Polyolefine based polyurethanesPALS measurements were performed on a series of polyolefine based polyurethanes including HTPB-PU, HTBS-PU and HTBN-PU. It is observed that as the content of hard segments increases, o-Ps lifetime, the free volume hole radius and fractional free volume decrease in these polyurethanes. Because the hard segments congregation can concur in glassy state through hydrogen bond and the stronger interaction between hard and soft segments restrains the segmental motion. The free volume hole size and fraction have a decreasing order of HTPB-PU>HTBS-PU>HTBN-PU under the same amount of hard segment content. The effect of different soft segment on free volume is mainly attributed to the degree of phase separation and polymer chain mobility. The results of program MELT show that two peaks are found in the o-Ps lifetime distribution, due to the phase separation. According to the free volume theory, the correlations between free volume and gas permeation and diffusion follow the Fujita' model. The gas permeability in these polyurethanes is mainly controlled by the free volume.b. Polyether based polyurethanesPALS were utilized to study the free volume properties for PECH and PPG based PU. The free volume hole radius and fraction in PECH-PU are smaller than those in PPG-PU, attributed to chloric groups in soft segments which decrease the flexibility of softsegments and hinder the formation of hydrogen bond between soft segments and hard segments. According to the results of free volume distribution studied by program MELT, the FWHM of o-Ps distribution in PECH-PU is smaller than that in PPG-PU. Gas permeation coefficients in PECH-PU are much smaller than that in PPG-PU, as a result of smaller free volume.c. Interpenetrating polymer networks (IPN)PALS were applied to investigate the free volume properties of PU/EP IPN composed of polyurethane and different resin. The free volume hole radius and free volume fraction in IPN are smaller than those in pure polyurethane and resin, which shows the closer packing of polymer chains in IPN. It is found that IPN with Bisphenol epoxy resin has the bigger free volume than that with Phenolic epoxy resin as the latter has higher density of cross-linking. The relationship between the free volume and gas transport shows that IPN with Bisphenol epoxy resin has the higher gas permeation coefficient than that with Phenolic epoxy resin. The correlations between free volume and gas permeability in pure PU and IPN can be explained by Fujita model based on the theory of free volume.2. Hydrophilic polyurethanesPALS measurements were performed on polyester and polyether based polyurethanes with different content of hydrophilic group of PEG as a function of temperature. At a temperature range of 253-323K, the increase of o-Ps lifetime and free volume with temperature have been observed attributed to the thermal expansion of the free volume. Near melting point, the varying rate of o-Ps lifetime has a marked increase, due to the melting of crystalline region in these materials. These phenomenona are not found in PU without crystalline region which is in agreement with the WAXD and DSC results. The analysis by MELT indicates the distribution of free volume hole becomes broader when the temperature increased above the melting temperature. Water vapor permeation (WVP) coefficients increase with temperature for polyester based PU and decrease with temperature for polyether based PU, respectively. The activation energy of permeation (Ep) is affected by the activation energy of diffusion (Ed) and the heat of sorption (Hs), whichcan be express as Ep=Ed+Hs. PU with much higher free volume exhibits lower Ed resulting in the negative Ep when Hs is negative. Effect of the PEG content on water vapor permeability and the free volume in PBA-based PUs has been studied. More PEG groups added exert no significant effects on the water vapor permeability, because there are two opposite effects of PEG groups including the increase of hydrophilicity and the decrease of free volume on water vapor permeation. In hydrophilic PU, water vapor permeability is influenced by not only the free volume, but also the hydrophilicity and crystalline region.3. Polymer nanocompositesThe free volume properties of rectorite/SBR and MMT/SBR nanocomposites and N326(carbon black)/SBR composites were studied by PALS. As the rectorite and MMT content increases, the o-Ps lifetime is nearly a constant, which indicates that the free volume of SBR is not filled with the clay. With the increasing volume fraction of filler, a rapid decrease of o-Ps intensity I3 and the increase of I2 have been observed, which shows stronger interaction between the clay and SBR matrix. A simple model has been developed by Neilson to depict the effect of size on the barrier performance of nanocomposites containing platelet particles. The rectorite/SBR and MMT/SBR have much higher gas barrier coefficients than N326/SBR, owing to recrtorite and MMT plateletlike morphology increasing the small molecules diffusing path. The relationships between free volume and gas permeation in nanocomposites do not follow the Fujita free volume model indicating that the reduction in permeability and diffusion is due to more than the decrease of fractional free volume. The gas barrier of nanocomposites is controlled by not only free volume, but also tortuous diffusional path effects.