Effect of surface swelling on diffusion on polymers. II. Delamination

Swelling in chloroform resulted in delamination of surface-hydroxylated styrene-butadiene-styrene block copolymers, with the formation of three fractions: crosslinked gel, reacted sol, and unreacted chloroform-soluble material. From the relative weights of these fractions, the depth of penetration of the reaction and the shape of the reaction front were determined. The weight of the gel fraction gave directly an estimate of the depth of penetration, λ_x which decreased with increasing film thickness. This was attributed to the increased amount of reacted polymer being noncrosslinked material (sol) in the interior compared to the 100% crosslinked nature of the surface region and to a reduction in the penetration of the reaction front resulting from the effect of stress transfer in lowering the permeation rate of peracetic acid. Comparison of the depth of penetration estimates λ_x and λ_s determined from the weight of the unreacted polymer demonstrated that, for the conditions used, the reaction front is quite broad. (For example, at 40°C, in 71% acetic acid for 80 min, the fully crosslinked portion is 15 μm thick and the partially reacted region extends for another 8.5 μm.) The combined influences of the increase in penetration of the reactants in the polymer and the relief of stress secondary to this increase in penetration were reflected in the time dependence of the depths of penetration λ_x and λ_s.     

Infrared spectroscopy of electroluminescent conjugated polymers

The infrared spectra of the light-emitting diodes and the metal-insulator-semiconductor devices based on a poly(p-phenylenevinylene) derivative MEH-PPV have been measured in situ with a reflection configuration. The voltage-induced infrared spectra of these devices have been measured by the FT-IR difference-spectrum method. The observed bands have been attributed to the carriers injected into the polymer layers. The observation of positive carriers in the polymer light-emitting diode is probably related to the predominance of injected positive carriers, which is one of the factors in the low efficiency of the polymer light-emitting diode. In situ infrared reflective absorption measurements provide the information about injected carriers, which play a central role in the properties and the functions of polymer electronic devices.     

Investigation of swelling and diffusion in polymers by NMR imaging

The kinetics of diffusion in polymers ranges from simple Fickian diffusion to higher order diffusion, such as Case II diffusion^1-2). The conventional method for determining the characteristics of solvents into polymer matrices is by measuring the mass uptake of the polymer as the solvent penetrates the matrix. However, since such measurements perform observations at a macroscopic level, little information has been obtained relating to the nature of the solvent in the polymer matrix and the mechanisms of the processes that control the diffusion. Nuclear magnetic resonance (NMR) imaging (‘MRI’) has been used to observe the penetration of solvents into solid systems in realtime. The method provides a one- or more-dimensional image of the density and the mobility of the solvent in a material or of the network changes of the material itself due to the softening influence of the solvent. The first (imaging of the solvent) can be used for a quantitative measurement of the diffusion whereas the observation of the network gives information about the changing of the network (mobility, de-crystallization…) during the swelling process. For example the diffusion of organic solvents in some polymeric materials (natural rubber, water gels (PNIPAAm), and nematic diblock-copolymers) are investigated.     

Low surface energy polymers and surface active block polymers. I. t-butylphenyl containing polymers

Two new monomers in the 2-oxazoline series were synthesized and polymerized. These were 2-[4-(t-butyl)phenyl]-2-oxazoline ( I ) and 2-[(3-(3,5-di-t-butyl)phenoxy)propyl]-2-oxazoline ( II ). The polymer from I crystallized readily during bulk polymerization and showed T_m at 592 K (319°C). After annealing, the polymer showed a critical surface tension of 23.2 dyn/cm. Polymer from II was amorphous; hence, annealing showed little effect on contact angles. Block polymers were made with I (Xn = 10) and ethyl oxazoline (Xn = 6,20,60). Very sharp molecular weight distributions were obtained. All samples crystallized when annealed. The block polymer was an effective emulsifier for emulsion polymerization of butyl acrylate at 0.1%.     

Solute and penetrant diffusion in swellable polymers. I. Mathematical modeling

A mathematical model was developed to describe diffusion of a penetrant and a solute in a swellable polymer slab. The model was applied to the case of a hydrophilic polymer loaded with a soluble bioactive agent, in which the penetrant (water) is sorbed and solute is desorbed. The model allows the incorporation of any appropriate form of the diffusion coefficients. A Fujita-type exponential dependence on penetrant concentration was chosen and shown to be adequate for prediction of a range of transport behavior. Dimensional changes in the sample were predicted by allowing each spatial increment to expand according to the amount of penetrant sorbed. During the initial period of release, the swelling was restricted to one dimension by the glassy core of the sample. At a later point in the process, the center of the sample had sorbed enough penetrant to plasticize it, and the sample relaxed to an isotropically swollen state; thereafter swelling was three-dimensional.     

Linear polymers in nonhomogeneous flow fields. I. Translational diffusion coefficient

The translational diffusion coefficient D, needed in treatments of flow-induced polymer migration phenomena, is studied. Explicit results are reported for bead–spring model macromolecules with two, three, and four beads, respectively. If the number of beads is large, an integral formulation is used. The results obtained show that D depends upon the hydrodynamic interaction parameters h^* and h^*N^1/2. Nondraining behavior of D is predicted irrespective of h if h^* is close to 0.2357. Approximations for D are obtained and the error involved is estimated.     

Statistical mechanical model of diffusion of complex penetrants in polymers. I. Theory

A previously developed model of simple penetrant diffusion is extended to encompass complex penetrants of idealized molecular shape, characterized by dimensions of length, width, and thickness. Expressions are obtained for D(0,T), the diffusion coefficient at zero penetrant concentration (c), and the fractional increase in D(0,T) as a function of c and temperature (T). The model predicts that D(0,T) will exhibit Arrhenius behavior at temperatures well above T_g and gives the limiting activation energy as a function of penetrant thickness and the polymer energy/distance constants used previously. For T_g < T ? T_g + 150 K the model requires two new disposable parameters, in addition to the jump-length parameter of the simple penetrant theory. These parameters, however, have precise physical meanings (all are lengths) and together with the penetrant dimensions and polymer constants determine the absolute magnitude of the diffusion coefficient as well as its relative dependence on c and T. For T ? T_g + 40 the relative concentration dependence may be calculated in terms of the penetrant dimensions and polymer constants only.     

Statistical mechanical model for diffusion of simple penetrants in polymers. I. Theory

Following Di Benedetto it is proposed that noncrystalline polymer regions possess an approximate semicrystalline order with chain bundles that are locally parallel along distances of several nanometers. Packing with on-average four nearest neighbors is assumed. A spherical molecule may move through such a substrate in two distinct ways: (a) along the axis of a “tube” formed by locally parallel chains or (b) perpendicular to this axis by two polymer chains separating sufficiently to permit passage of the molecule. The first process is relatively fast, generally requires little activation energy, and determines the effective jump length in diffusion. The second is responsible for the activation energy of diffusion, which is taken as the minimum energy necessary to produce a symmetrical chain separation which allows transfer of a molecule. This is calculated as a function of the penetrant diameter d and parameters Γ and β which characterize the interchain cohesion and chain stiffness, respectively. Γ is estimated from the polymer density and cohesive energy density by suitably linearizing a relation given by Di Benedetto for the potential between two polymer chains approximated as infinite strings of Lennard-Jones force centers. β is shown to be approximately obtainable from the polymer chain backbone geometry and bond rotation potentials. An expression for the diffusion coefficient D is developed which contains only one disposable parameter, the effective jump length.     

Effect of doping on thermoluminescence in polymers. I. 9,10-phenanthrenequinone-doped polydiancarbonate

The thermoluminescence (TL) and thermally stimulated depolarization currents (TSDC) observed in polycarbonate doped with 9,10-phenanthrenequinone (PhQ) have been compared in order to test to what extent the TL phenomenon can be correlated with intrinsic relaxation properties of the macromolecular chains. Good qualitative agreement has been found between the two types of curves, showing that the two TL peaks appearing at about 140 and 185 K are attributable to untrapping of electrons induced by the onset of local motions. However, the values and distribution of the activation energies determined by the two techniques differ markedly. The apparent activation energy deduced from TL data presumably reflects the energy level of traps decreased by molecular motions rather than the thermal energy of these relaxations. The primary process responsible for light emission is related to a transition of the n-π type occurring in PhQ at 420 nm, while the trapping sites are probably formed by carbonate groups of the polymeric matrix. The same trapping mechanism has been postulated for the two TL peaks on the basis of their parallel evolutions as a function of dose and dopant concentration.     

Infrared spectroscopy of methanol-hexane liquid mixtures. I. Free OH present in minute quantities

Methanol and hexane mixtures covering the whole solubility range are studied by Fourier transform infrared attenuated total reflectance spectroscopy in order to evaluate OH groups that are H-bond-free. The mixtures from 0 to 0.25 and from 0.75 to 1.00 mole fractions form homogeneous solutions, whereas those from 0.25 to 0.75 mole fractions are inhomogeneous, forming two phases. Factor analysis (FA) was used to find out if free OH groups were present. These were found in minute quantities at the lowest mole fraction by evaluating the OH stretch absorption. The bulk of the absorption is due to the greater than 99.9% of hydrogen-bonded methanol molecules, with a band maximum situated at 3340 cm(-1). The stretch band of the free OH groups absorbs at 3654 cm(-1), with a full width at half maximum of 35 cm(-1). The concentration is very weak but constant at less than 5 mM in the mole fraction between 0.252 and 0.067. Below this range, OH concentrations are even smaller. This represents less than 1% of the amount of methanol at the mole fraction of 0.067 (0.543M). Above 0.25 mole fraction, free methanol OH groups are not observed. Since the free OH band is very weak, almost at the noise level, we verified its presence with mixtures of hexanol in hexane. There, we found a similar free OH band with almost the same band characteristics, but with almost three times the concentrations found with methanol, which we attribute to the difference in the hydrocarbon chain length. The present study indicates clearly that solutions of methanol in hexane contain free OH groups but in minute quantities and only in the low methanol concentrations. This situation is much different from that observed in solutions of methanol in CCl(4), where free OH groups are clearly observed at all concentrations except at the concentration limits. Whereas in CCl(4), methanol is believed to form H-bonded chains, the situation is different in n-hexane: methanol in the low concentration region would form reverse micelles with the OH groups in the core and the CH(3) groups mixed with n-hexane molecules.     

Gas sorption and diffusion in hydrogen-bonded polymers. I. Vinyl alcohol/vinyl butyral copolymers

A series of vinyl alcohol/vinyl butyral copolymers was examined to assess the effect of internal hydrogen bonding on gas sorption and diffusion. Sorption and permeation measurements for carbon dioxide and methane were performed on four vinyl alcohol/vinyl butyral copolymers. Upon comparing the various data, it was found that hydrogen-bonded copolymers exhibit a much wider variation in diffusion coefficient than non-hydrogen-bonded copolymers. The fractional free volumes of the studied copolymers were considerably lower than expected based on values of the diffusion coefficient. This may be due to the fact that predicted occupied volumes are too large and that the effect of internal hydrogen bonding is not accounted for properly. Using a relationship between infrared spectral shift and hydrogen-bond length, fractional free volumes in the hydrogen bonded copolymers were correlated with the interatomic spacing associated with the hydrogen bond. This implies that the average length of a hydrogen bond can be used as a measure of chain packing in hydrogen-bonded polymers. © 1993 John Wiley & Sons, Inc.     

Gas diffusion in partially crystalline polymers part I: Concentration dependence

In this work, a phenomenological model for the gas diffusion in partially crystalline polymers using differential effective medium theory is presented. By making an analogy with the power law known as Archie's law which relates the d.c. conductivity of a brine saturated porous rock to its porosity; we show that gas diffusion through semicrystalline polymers can be described in a similar way. It is assumed that the diffusion coefficient in the crystalline region is zero, while in the amorphous region it is given by a free volume model, and an effective diffusion coefficient D^eff, is obtained using the mentioned analogy. The variation of D^eff upon concentration is analyzed through its free volume dependence. The crystallinity dependence is considered through an average chain immobilization factor 〈β〉 which is explicitely derived. Finally, the results of this model are compared with experimental data given by Kreituss and Frisch, obtaining a good agreement. © 1996 John Wiley & Sons, Inc.     

Effect of microheterogeneities on the diffusion of gases through glassy polymers

The diffusion of gases through glassy polymers is studied and the effective diffusion coefficient D^eff is represented as the result of the superposition of two fundamental mechanisms, namely slipping and hopping. D^eff is calculated by a two-point correlation method. Comparisons are made with experimental data of Meares for diffusion coefficients of Kr, O₂, He, and A in poly(vinyl acetate) in the glassy state. Good fits are obtained and yield significant parameters.     

Effect of end‐tethered polymers on surface adhesion of glassy polymers

The adhesion between a glassy polymer melt and substrate is studied in the presence of end‐grafted chains chemically attached to the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of the areal density ∑ of tethered chains and tensile pull velocity v on the adhesive failure mechanisms. The initial configurations are generated using a double‐bridging algorithm in which new bonds are formed across a pair of monomers equidistant from their respective free ends. This generates new chain configurations that are substantially different than the original two chains such that the systems can be equilibrated in a reasonable amount of cpu time. At the slowest tensile pull velocity studied, a crossover from chain scission to crazing is observed as the coverage increases, while for very large pull velocity, only chain scission is observed. As the coverage increases, the sections of the tethered chains pulled out from the interface form the fibrils of a craze that are strong enough to suppress chain scission, resulting in cohesive rather than adhesive failure. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 199–208, 2004     

Vibrational spectroscopy of water in hydrated lipid multi-bilayers. I. Infrared spectra and ultrafast pump-probe observables

The vibrational spectroscopy of hydration water in dilauroylphosphatidylcholine lipid multi-bilayers is investigated using molecular dynamics simulations and a mixed quantum/classical model for the OD stretch spectroscopy of dilute HDO in H(2)O. FTIR absorption spectra, and isotropic and anisotropic pump-probe decay curves have been measured experimentally as a function of the hydration level of the lipid multi-bilayer, and our goal is to make connection with these experiments. To this end, we use third-order response functions, which allow us to include non-Gaussian frequency fluctuations, non-Condon effects, molecular rotations, and a fluctuating vibrational lifetime, all of which we believe are important for this system. We calculate the response functions using existing transition frequency and dipole maps. From the experiments it appears that there are two distinct vibrational lifetimes corresponding to HDO molecules in different molecular environments. In order to obtain these lifetimes, we consider a simple two-population model for hydration water hydrogen bonds. Assuming a different lifetime for each population, we then calculate the isotropic pump-probe decay, fitting to experiment to obtain the two lifetimes for each hydration level. With these lifetimes in hand, we then calculate FTIR spectra and pump-probe anisotropy decay as a function of hydration. This approach, therefore, permits a consistent calculation of all observables within a unified computational scheme. Our theoretical results are all in qualitative agreement with experiment. The vibrational lifetime of lipid-associated OD groups is found to be systematically shorter than that of the water-associated population, and the lifetimes of each population increase with decreasing hydration, in agreement with previous analysis. Our theoretical FTIR absorption spectra successfully reproduce the experimentally observed red-shift with decreasing lipid hydration, and we confirm a previous interpretation that this shift results from the hydrogen bonding of water to the lipid phosphate group. From the pump-probe anisotropy decay, we confirm that the reorientational motions of water molecules slow significantly as hydration decreases, with water bound in the lipid carbonyl region undergoing the slowest rotations.     

Adhesion of high polymers. I. Influence of diffusion,adsorption, and physical state on polymer adhesion

It is possible to identify three distinct types of polymer adhesion on the basis of the physical state of adhesive and adherend: (1) rubbery polymer–rubbery polymer (R–R adhesion); (2) rubbery polymer–glassy polymer (R–G adhesion); (3) rubbery polymer–nonpolymer (R–S adhesion). Limitations of the diffusion and adsorption theories and their conflicting results are discussed within the framework of the proposed classification. By defining the physical state of the polymer as an adhesive or as an adherend, it is possible to eliminate many of the discrepancies commonly noted in attempted application of the diffusion and adsorption theories. As predicted by the Bueche-Cashin-Debye equation, the diffusion of a polymer into another should be greatly reduced as it changes from the rubbery to the glassy state. For this reason, diffusion, which depends to a great extent on the physical state of the polymer, is actually a limited, selective process. Assuming a 10¹³ poise bulk viscosity at glass temperature, self-diffusion constants of forty polymers were calculated to be 10^?21cm.²/sec. or 10^?5A.²/sec. This slow rate of diffusion is unmeasurable and insignificant. Adsorption, which is less dependent on the physical state of the polymer, is more frequently encountered.     

Mechanical relaxations in some ionene polymers. I. Effect of Ion concentration

Dynamic mechanical properties of m,n-ionenes, the structure of which are shown in Figure 1, were examined by torsional braid analysis. Three relaxations designated as α,β and γ were found. The α relaxation, ascribed to the primary relaxation due to an amorphous phase, was observed at 70–130°C, the temperature increasing with an increase of the ion concentration along the polymer chains. The β relaxation at around 0°C was related to the ionic portions of the polymers. The γ relaxation at around–120°C was a so-called local mode relaxation. The γ relaxation peak was split into two peaks in the very slowly cooled 12,10-ionene sample and the formation of an inhomogeneous structure in the amorphous phase is proposed.     

Stabilization of high nitrile polymers. I. Effect of dienophilic compounds

The effect of certain stabilizers on the discoloration of high nitrile polymers is discussed in terms of an in situ Diels-Alder reaction. Dienophiles react with cis azadienes formed during thermal degradation of the nitrile polymer to interrupt conjugated sequences prior to oxidative chromophore formation. Several nondienophilic additives decompose upon heating to release a strong dienophile that stabilizes the polymer.     

Impedance spectroscopy of reactive polymers. 1

Dielectric measurements were utilized to follow the advancement of cure in an epoxy/amine formulation. In contrast to earlier studies, complex impedance was measured during cure and used to calculate ionic resistivity. By using complex impedance were able to separate according to their frequency dependence the contributions to overall polarization from electrode blocking layers, migrating charges, and dipole relaxations. At any stage of cure, there is a unique frequency at which ionic resistivity can be singularly measured. Our approach does not involve trial-and-error frequency search and is conducive to the development of phenomenological models based on equivalent circuits. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. © 1994 John Wiley & Sons, Inc.