Hydrogen bonding in polyamides

Figure 2. The dipole moment of the absorbed water molecules varies from approximately 1.8 to 0.9 for the polyimides and from 1.1 to 0.7 for the polyamide-imides corresponding to fractional polarizabilities of l.O/i -0.4/x. The low values of p (<0.5p ) as seen in all the amide-imide po ymers and several of the ° polyimides, indicate restricted mobility of the water molecules. In the amide-imide polymers, we believe this is due to increased water-polymer interactions such as hydrogen bonding. Other evidence of hydrogen bonding in polyamide-imides is the water-induced plasticization and Tg lowering frequently observed.

It should be mentioned that similar approach has been used by Kotek and coworkers [28-30] for suppression of the intermolecular hydrogen bonding in polyamides in order to achieve better molecular orientation making it possible to prepare Nylon filaments with superior mechanical properties. 

A hydrocarbon chain is in a constant thermal motion, and without external force field, the chains fluctuate around the most stable position given by the distribution of possible conformations at the temperature. The action of external forces at the ends of a molecule causes displacements of chains from their equilibrium conformations and evokes retractive forces. For a hydrocarbon chain of M = 14,000, extended length 125.5 nm, and the end-to-end distance r = 1 mn, the maximum exerted force is 10 MPa. The level of forces exerted by the random coil macromolecules are much lower than the theoretical strength of the primary bonds. The presence of strong intermolecular interactions, such as hydrogen bonds in polyamides, affects the retractive force substantially, causing a restriction of the number of possible chain conformations. In addition, the transitions... 

Finally, reference is made to the dynamic dichroic absorption difference, which is related to an absorption change induced by an external time-dependent perturbation. Dynamic absorption spectroscopy is performed with linearly polarized IR light and serves for example to detect intermolecular hydrogen bonds in polyamides of the structure... 

The incorporation of the 2 2 -disubstituted biphenyl moiety into the polymer backbone of wholly-aromatic polyamides, polyesters, and polyesteramides significantly affects their optical, morphological, and solution properties. Experimental evidence strongly suggests that intermolecular attractive forces, such as hydrogen bonding in polyamides, are significantly weakened in these materials. 

We can be reasonable confident that the actual torsion angles in the solid state structure will be close to the predicted values, except for Xj and X2> which define the phenyl-urethane orientations. The model diphenyl-diurethane structures (6-8) have X angles in the range 10-37°. The deviations from X=90° could be due to electron delocalization between the phenyl and urethane groups, which would favor the sterically disallowed X=0° conformation. Such electron delocalization has been considered elsewhere for aromatic polyamides and polyesters. However, a simpler explanation may be that significant space occurs between the molecules if they are hydrogen bonded in the X= 90° conformations, and that this is eliminated when X is varied i.e. the urethanes are twisted away from a position... 

Improvement of Membrane Selectivity (UTC-70R) Tb improve membrane selectivity, polyamide chains in the membrane are prefened to be packed tightly. We found that a certain posttreatment aimed to cleavage hydrogen bond of polyamide and reorder polymer chains effectively improved membrane selectivity of UTC-70 (Figure 8). This type of membrane are commercialized as "UTC-70R", and membrane performance is shown Figure 7. 

Hydrogen bonding between polyamide chains plays an important role in determining the properties of a nylon such as nylon 6,6 (Table 12.5). Draw the structural formulas for two adjacent chains of nylon 6,6 and show where hydrogenbonding interactions could occur between them. 

Coleman and Painter [4-6] consider hydrogen bonding as the central strong interaction in polymers that cause the observed phase behavior and miscibility. They have used Fourier transform infrared (FTIR) spectroscopy to study the hydrogen bonding in polymer blends in systems such as polyamides and polyurethanes. A large class... 

X-ray analysis reveals that poly(ethylene terephthalate) (Dacron ) belongs to the triclinic system (25). The cell dimensions are a = 4.56, b = 5.94, c = 10.75 A, with the angles being a= 98.5°, J5 = 118°, y= 112°. Both the polyamides and the aromatic polyesters are high melting polymers because of hydrogen bonding in the former case and chain stiffness in the latter case (see Table 6.1). As is well known, both of these polymers make excellent fibers and plastics. 

M. F. Roberts and S. A. Jenekhe, Sitc-spectfle reversible scission id hydrogen bonds in polymers. An investigation of polyamides and their Lewis acid-base complexes by infrared apectroacopy, Macramoleatl 24-J142 (1991V... 

Figure 3.17 Schematic of hydrogen bonding in a pair of polyamide polymer chains [4],...

Here, the H-bonds enforce an increase in the length of adjacent bonds, which leads to lower stretching frequencies. When H-bonds are weakened or destroyed, however, the position of IR absorption bands related to the stretching vibrations of N—H and C=0 are blue-shifted. This technique has been employed to detect hydrogen bonds in polyester urethanes and polyamide-6 that had been exposed to sinusoidally varying strain at a stretching frequency of... 

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