Stretching, molecular

When we compared the viscosities of solutions of natural rubber and of guttapercha and of other elastomers and later of polyethylene vs.(poly)cis-butadiene, with such bulk properties as moduli, densities, X-ray structures, and adhesiveness, we were greatly helped in understanding these behavioral differences by the studies of Wood (6) on the temperature and stress dependent, melting and freezing,hysteresis of natural rubber, and by the work of Treloar (7) and of Flory (8) on the elasticity and crystallinity of elastomers on stretching. Molecular symmetry and stiffness among closely similar chemical structures, as they affect the enthalpy, the entropy, and phase transitions (perhaps best expressed by AHm and by Clapeyron s... 

Eq. (13.37) shows that the modulus of a rubber increases with temperature this is in contrast with the behaviour of polymers that are not cross-linked. The reason of this behaviour is that rubber elasticity is an entropy elasticity in contrast with the energy elasticity in "normal" solids the modulus increases with temperature because of the increased thermal or Brownian motion, which causes the stretched molecular segments to tug at their "anchor points" and try to assume a more probable coiled-up shape. 

When polymer melts are accelerated during the manufacturing processes such as injection molding and afterwards cooled down quickly, the stretched molecular chains cannot relax, which leads to orientations in the direction of melt flow. These oriented polymer chains act differently toward ESC influences as compared to nonoriented molecules. Orientation perpendicular to the crack propagation direction may result in a higher ESCR, as will be shown in the following discussion. 

Two parameters seem to govern the temperature behavior of the maximiun craze width the relationship between the length of stretched molecular chain and... 

We now consider models of higher dimensionality. The first is still fully quantum mechanical but a special form for the PES is used (Karikorpi et al. 1987 Halstead and Holloway 1988 Holloway 1989) that is most appropriate for the H2/Ni and Hj/Cu systems. The basic assumptions are that a barrier to chemisorption exists in the entrance channel, much like point A in Fig. 17 this barrier is nearly independent of the molecular orientation and vibrational bond length and all molecules that pass over this barrier dissociate (i.e. there is a sink in the PES). The justification for the orientation independence is the free rotational motion in the physisorbed species. The justification for the independence of bond length is the absence of a significantly stretched molecular bond at the position of the activation barrier A. The irreversible dissociative behavior after surmounting a point like A in Fig. 17 assumes that either no barrier or at most an extremely small barrier exists in the exit channel, unlike point D in that figure. 

Morita and Hynes S have attempted an analysis of the sum frequency generation spectrum at the surface of water from ab initio calculations of normal mode shifts and hyperpolarizability in the frequency range of the OH stretch. Molecular dynamics has been used to generate a sample of structures. Agreement between the simulated and experimental spectra is good. The spectrum appears to depend on signals from molecules in a few standard orientations in the top two layers at the surface. 

As described in Section 12.2.2, the stretched molecular chains of the chemical coupling adduct are bonded at one end to the glass fiber surface with the free end oriented in the normal direction. Milner (30) defined this unique arrangement of long-chain molecules as bristles of a polymer brush. 

A totally different approach to rubber elasticity has been developed by Stepto and co-workers [15, 16], which also accounts for the Mooney-Rivlin softening. Their approach is not phenomenological, but is based on structural considerations that give an accurate description of the moleeular eonformational states of the units in the polymer chains as the network is stretched. They have proposed a method for calculating the free energy of a stretched molecular network based on the rotational isomeric state of the network chains, with conformational energies determined from observations of conformational properties. 

The H H distance in stretched molecular iridium dihydrogen complexes may be very sensitive to the presence of weak intra- and intermolecular interactions. In this regard, Gusev etal. reported, on the basis of DFT calculations and experimental spectroscopic measurements, that the complex [IrH(H2)Cl2(PPp3)2] 998, which contains a stretched dihydrogen ligand in the solid state, " is a trihydride in solution. ... 

---