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James and Guth [3] have used the inverse Langevin function-based expression of tension in a polymer chain to build the so-called 3-chain model of uncompressible rubber elasticity. It is an analytical model, but it is not isotropic. If one considers an isotropic superposition of an infinite number of chains in all the directions, one obtains an isotropic model, but it is no more tractable analytically. Following Cohen [12], we propose to replace the inverse Langevin function by its Pade approximant, the two functions being very close. The isotropic model is then analytically integrable, and yields large strain strain-stress relationship.

The invention relates to a rotatable stopcock inserted into a male coupling (2) having a 90 DEG offset connecting piece (3) for connecting to fluid lines. A contact hole (6) for receiving a female pin (8), which is connected to the stopcock (1) and which is provided with a continuous hole (9) having a 90 DEG angle curve for connecting the coupling housing (5) to the connecting piece (3), is situated along the straight extension of the male coupling (2), and the end of the said female pin is connected to a turning knob (7) while forming one piece. In order to be able to securely fix the stopcock (1) to the coupling housing (5) after insertion into the contact hole (6), the housing wall (10) comprises a peripheral flange ring (11) on the contact hole (6). For the same purpose, two ring segments (13) located opposite one another are shaped onto the front wall (12) of the turning knob (7). Said ring segments are concentric to the housing wall (10) and are provided with detent catches (15) which, after elastic expansion of the ring segments (13), engage behind the flange ring (11) with positive engagement when inserting the female pin (8) into the contact hole (6). A separate detent limb (22) is also provided on the front wall (12) of the turning knob (7) and is shaped onto the front wall (12) of the turning knob (7) on one side and is provided with two inwardly pointing detent teeth (24) on the other side. Said detent teeth engage inside a detent notch (25) projecting from the housing wall (10) in order to lock the turning knob (7) in the open position of the stopcock (1).