Geometric Hindrance Research Articles

The Flow Properties of γ-Polyglutamic Acid Solutions in Capillary

Abstract Dependence of the viscosity of the solution of γ-polyglutamic acid in eater at various pH’s on the bore-size of the capillary viscometer has been studied at 25°C at various concentrations (0.01–0.05 g/100 ml). Measurements were carried out with a Maron-Belner type low shear capillary viscometer with various capillary radii (1.9×10−2–7.6×10−2cm) within the shear stress range 0.2–20 dyn/cm2 at the capillary wall. The polymer solutions showed a non-Newtonian flow behavior and a capillary bore-size dependence and both of these properties were diminished with the decreasing of pH. This capillary bore-size dependence may be attributed to the geometric hindrance of the capillary wall to the thermal motion of the polymer molecules near the wall. The dissolved state of the polymer molecules in solutions of various pH’s has been studied by means of optical rotatory dispersions, flow birefringence and intrinsic viscosity. The dissolved state of the polymer solution as well as the capillary bore-size dependence were sensitive to pH. The polymer molecule in neutral and alkaline media may be in the form of a highly expanded chain and in acid medium in a contracted form having a some ordered chain conformation.

Protracted pressor response to angiotensin after bilateral nephrectomy in rats.

The pressor response of nephrectomized rats to an injection of renin is marked by a prolonged stable plateau. A similar, prolonged response to angiotensin has been demonstrated and the following aspects of this pressor effect were evaluated. 1. The protracted response was shown not to be due to a persistence of angiotensin in the blood of nephrectomized animals. There are several indications from unrelated studies that angiotensin is destroyed rapidly. Cross-circulation experiments indicate that during the protracted pressor phase blood levels of constrictor materials have fallen to undetectably low levels. 2. No evidence could be obtained to indicate that the neurogenic buffer system is involved in the development of the protracted response. Ganglionic blockade was ineffective in preventing its development. 3. The hemodynamic basis for the response must be a persistent increase in total peripheral resistance (TPR) since there is no increase in cardiac output (CO) in response to the angiotensin infusion. Before infusion, the CO of nephrectomized animals is higher than that of sham-operated control animals. 4. The increase in TPR must be due to an increase in geometric hindrance for no changes in hematocrit or in viscosity result from angiotensin infusion. Blood viscosity is lower in nephrectomized than in sham-operated animals, but this can account for only part of the lower pre-injection control pressure seen in this group. It appears that in animals depleted of endogenous renin, a large infusion of angiotensin increases vascular resistance by some non-neurogenic, non-humoral mechanism which may represent another type of vascular tone.

Molecular structure and rubber-like elasticity - III. Molecular movements in rubber-like polymers

The evidence of X -ray diffraction photographs shows that in gutta-perch a and in polychloroprene at the ‘melting ’-point, major changes of molecular configuration due to rotation round the single bonds occur spontaneously. Melting is ascribed to the onset of chain-bond oscillation between alternative, geometrically equivalent positions (‘molecular wriggling’), and the low melting-points of these substances (and also, by analogy, of rubber) as compared with polyethylene are ascribed to the greater freedom of rotation round the single bonds in the former substances (and also, by analogy, of rubber) as com pared with polyethylene are ascribed to the greater freedom of rotation round the single bonds in the former substances. It is postulated that the energy required for rotation round the single bonds is made up of two factors— the bond-orientation energy, and the interaction of the atoms or groups held by the bonds. The comparative freedom of rotation in rubber and polychloroprene is due to two causes: first, the presence of double bonds in the chains, which has the effect of making rotation round adjacent single bonds m ore free than in comparable saturated molecules; an d secondly, the small hindrance to rotation offered by the side substituents. In gutta-perch a the bond-orientation energy is presumably the same as in rubber an d polychloroprene, but geometrical hindrance is greater, an d it is suggested that this is the reason for its rather higher melting-point. The structure of amorphous rubber is considered. Starting from the known structures of crystalline polymers, which consist of crystals tied together by molecules each of which passes through a number of crystals, the minimum change on melting is assumed, and that change— the wriggling an d consequent disarrangement of the molecules— is well founded on experimental evidence; it is not necessary to assume that the molecules have a completely random configuration, an d there are reasons why this is unlikely. The mechanical properties above and below the melting-point are correlated on this basis. The statistical effects to which Mark has drawn attention operate through the ‘tying ’ portions of the molecules.