Energetics of strain-induced conformational transitions in polymethylene chains

Abstract

Deformation potentials have been calculated for highly extended polymethylene chains containing simple torsional defects. The calculations were performed by using molecular mechanics techniques. The deformation potentials show abrupt discontinuities, which have been identified as gauche-trans transitions of chains containing strained defects into chains in the fully extended zigzag form. The reverse transgauche transition was observed during compression of all-trans chains. The observed discontinuity in the conformational transition is a direct consequence of the input of mechanical energy into the torsional angles and also into the bond lengths and bond angles in the chain. The transition point imposes a limit on the amount of energy stored in chains containing defects; the energy is fully recovered at the transition. It is suggested that abrupt conformational transitions may occur upon drawing crystalline and glassy polymers as a mechanism of avoiding large translational movements of atoms under the conditions of strong constraints from neighboring chains. The Hookean behavior of single chains containing a conformational defect was tested and the effect of this defect on the chain modulus was estimated.

Energetics of Stretching of Conformational Defects in Extended Poly(methylene) Chains

“…Theoretical studies on the ultimate Young's modulus of a PM chain using molecular mechanics, semiempirical and ab initio methods resulted in a wide variety of values covering the approximate range 170-400 GPa. [1,5,7,14,18] The above value of E T predicted by the MM+ method is too low in comparison to the quantumchemical estimates of over 300 GPa. [18] Apparently, the previous observations [5,7] that force-field methods tend to provide low values of the E T modulus are endorsed.…”

“…[1,5,7,14,18] The above value of E T predicted by the MM+ method is too low in comparison to the quantumchemical estimates of over 300 GPa. [18] Apparently, the previous observations [5,7] that force-field methods tend to provide low values of the E T modulus are endorsed. Since we are interested mainly in the influence of the defects on the chain modulus, a relative measure, the ratio E/E T (Table 1) is utilised in further discussion instead of the absolute values.…”

“…[5] Aside from localised conformational defects, some molecular mechanical calculations were reported on the crystallographic defects. [7,19] These defects may be involved in diffusion, translation and rotation of molecules inside the PM crystals.…”

“…[3 -7] Calculations have shown that the transition of a defect into a more extended conformation proceeds by an abrupt change of the deformation potential. [3][4][5][6] A flip-flop interconversion of the torsional angles within a defect brings about the change from the shorter (kink) into the longer (all-trans) state. For chains containing the single-kink or jog defects the stored elastic energy was calculated.…”

Fracture toughness of interface of polyethylene modified in bulk

“…The same factors should be relevant for fracture toughness of semicrystalline polymers, but in addition, influence of the crystalline micro-and macrostructure in the bulk and especially near the interface have to be considered. [7][8][9] The role of crystallites in adhesion becomes even more pronounced in the case of semicrystalline polymers containing additives. An additive, depending on its nature, may behave as a nucleating agent or as a plasticizer.…”