Molecular dynamics simulations have been performed for hydrogel models of poly(vinyl alcohol) (PVA), poly(vinyl methyl ether) (PVME), and poly(N-isopropylacrylamide) (PNiPAM). The spatial distributions and stability of hydrogen bonds in the vicinity of polymer segments are analyzed to investigate the properties of water which is highly cooperative with the surrounding polymer chains. Water-water hydrogen bonds are enhanced around hydrophobic groups especially for PVME and PNiPAM by the hydrophobic hydration. Hydrogen bonds are also stabilized around hydrophilic groups for PVME and PNiPAM. The stabilization is accounted for by a severe constraint of the mutual orientation between water and polar group.
Molecular dynamics simulations have been carried out in order to examine the mechanism of diffusion of small penetrants in amorphous polymer membranes. Diffusion processes of methane, water, and ethanol in poly(dimethylsiloxane) (PDMS) and in polyethylene (PE) were investigated. Pure liquid water and ethanol were also simulated. The insertion probabilities P(R) of hard-sphere atoms of radius R into the polymers and the liquids were calculated. The free volume fraction, P(0), of PDMS is large and the insertion probability of a finite size atom into PDMS is widely distributed compared with the other polymers and liquids. Simulations of 5 ns were performed for PDMS and in PE with a penetrant species, methane. The diffusion of methane in the polymer matrix exhibits anomalous (non-Einstein) behavior for time scales of 1 and 0.3 ns in PDMS and PE, respectively. Aggregates of water and ethanol are found to be formed in PDMS. Diffusion coefficients of water and ethanol in PDMS are reduced by more than 1 order of magnitude due to the aggregation. The calculated diffusion coefficients of the nonaggregated penetrants in PDMS and in the pure liquids agree well with the experimental values.
Molecular dynamics simulations have been performed for hydrogel
models of poly(vinyl
alcohol) (PVA), poly(vinyl methyl ether) (PVME), and
poly(N-isopropylacrylamide) (PNiPAM).
The
dynamics of hydrogen bonds and the translational and rotational motions
of water molecules in the vicinity
of the polymer segments are analyzed to investigate the properties of
water molecules which are highly
cooperative with the surrounding polymer chains. The characters of
the hydrophilic groups which are
reflected in the lifetime of hydrogen bonds, the spectral density,
etc., are examined. The mobility of
water molecules is significantly lowered around polymer chains for both
translational and rotational
motions. This is partly because of the hydrogen bonds between
water and polymers around the hydrophilic
groups and partly because of the structuralization of water around the
hydrophobic groups.
The mean-square radius of gyration (S2) was determined by small-angle X-ray scattering and light scattering for 20 samples of atactic oligo-and poly(methyl methacrylate)s (a-PMMA), each with the fraction of racemic diads f, = 0.79, in the range of weight-average molecular weight M" from 4.02 x 102 to 2.83 X 106 in acetonitrile at 44.0 °C (0). The ratio (S2)/xw as a function of the weight-average degree of polymerization, xw, exhibits unusual behavior; it passes through a maximum at xw ca 50 before reaching its asymptotic value for large xw. First, a comparison is made of the experimental data with the theoretical values on the basis of three types of the rotational isomeric state model, and it is shown that none of them can explain the observed maximum. Then, it is shown that the helical wormlike (HW) chain theory may well explain the data with the parameter values X"*k0 = 4.
The excess chemical potentials of methane, water, and ethanol in poly(dimethylsiloxane) (PDMS) and polyethylene (PE) were calculated by the Widom method. The excess chemical potentials of water and ethanol in aqueous ethanol solutions (0, 50, 100 wt %) were also calculated by the Shing-Gubbins method. The excluded volume map sampling (EVMS) method and the continuum configurational bias (CCB) method were used to increase the efficiency of sampling. In spite of the polarity and the internal degrees of freedom of the molecules, the excess chemical potentials could be calculated with a small statistical error. The solubilities of methane, water, and ethanol in the polymers were calculated from excess chemical potentials. Permeation rates calculated from diffusion coefficients and solubilities were in reasonable agreement with experimental data. The free volume cluster of each system was analyzed and was related to the permeation of small penetrants in the membranes.
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