Bond bending and stretching

A deformation due to bond bending and stretching which is effectively instantaneous and independent of temperature (ordinary elastic deformation, oe)-... 

Fig. 3 Idealized temperature scan of a polymer starting at low temperature, the modulus decreases as the molecules gain more free volume resulting in more molecular motion. This shows main curve as divided into six regions, which corresponds to local motions (6), bond bending and stretching (5), movements in the side chain or adjacent atoms in the main chain (4), the region of the Tg (3), coordinated movements in the amorphous portion of the chain (2), and the melting region (1). Transitions are marked as described in the text. (From Ref. l) (View this art in color at www.dekker.com.)...

Further improvements have also enabled the careful measurement of combination bands involving van der Waals or hydrogen-bond bending and stretching modes, depending on the composition of the complex. 

The molecular mechanics method (Section 16.6) is not a quantum-mechanical method and does not use a molecular Hamiltonian operator or wave function. Instead, it views the molecule as a collection of atoms held together by bonds and expresses the molecular energy in terms of force constants for bond bending and stretching and other parameters. 

From the melt it is now well established that sPS crystallizes primarily in an all-trans (T4) form (u-form) with a planar zigzag backbone structure (11, 12) (see Fig. 2). A second (jS-form) structure (T2G2) is also present, but imder normal crystallization conditions the level of this structure is quite low (13-15). Calculations of the ultimate elastic modulus of the a-form sPS crystal are as high as 83 GPa (16). Experimentally, using x-ray diffraction, this has been reported at 86 GPa (12.5 x 10 psi) (17). The stiffness of the crystalline sPS is due primarily to intrachain interactions involving bond bending and stretching. The applied stress... 

This freedom is not exercised in tension because it would be energetically less favorable than bond bending and stretching. The axial compression modulus of a zig-zag chain is essentially determined by the hindered rotation mode of deformation, it corresponds to the extension modules of a highly kinked chain (n = n/4) and has been taken to be 5 GN/m. ... 

The ideal rubber should respond to an external stress only by uncoiling, that is, dE/dL = 0. Experiments in which f, L, and T are varied confirm that for many amorphous, cross-linked materials above T, dE/dL is very small (Figure 9.17). Below we have said that polymer segments cannot deform in the timescale in which was measured, so dS/dL = 0. In a glass we expect external stresses to be countered by bond bending and stretching. 

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