The Deformation of Rigid Gels of One-Step Polyamide Networks and Their Constituent Stiff Segments
Titel:
The Deformation of Rigid Gels of One-Step Polyamide Networks and Their Constituent Stiff Segments
Auteur:
Aharoni, Shaul M.
Verschenen in:
International journal of polymeric materials
Paginering:
Jaargang 17 (1992) nr. 1-2 pagina's 35-49
Jaar:
1992-04-01
Inhoud:
When a gel of rigid polyamide network is prepared in solution by a single step polymerization, some of its stiff segments are straight and others are bent. The motional freedom of the majority of the segments is greatly constrained by having fully or mostly reacted multifunctional rigid branchpoints at both their ends. A minority of the segments are less constrained because they are attached to the network at one end only. We believe that a most efficient mechanism for bending of constrained straight stiff polyamide segments is by a pair of syn-anti interconversions occurring in the same segment. For polyamides these are the least energy consuming changes in which 180° rotations around the ring-to-carbonyl or ring-to-nitrogen bonds take place at an activation energy as low as 4 kcal/mol. In this way, significant translations of one branchpoint relative to the other take place without any torsions of the moving branchpoint and with the bent portions of the stiff segment defining no or almost no “cone of rotation” in space. Conversely, the most efficient straightening mechanism for constrained bent stiff segments is the reversal of the above process, i.e., two placement interconversions occurring per segment. Stiff segments attached to the network at only one end can easily deform by a single inter-conversion, of course. During macrodeformation the gels maintain constant volume. In this deformed state, the size of the populations of straight and bent stiff segments changes from the relaxed state. In the directions where the stressed sample increases its dimensions, the population of straight segments grows at the expense of the bent segments. In the perpendicular directions where the dimensions of the gel decrease, the population of bent segments increases at the expense of the straight ones. The inability of straight stiff segments to stretch results in increases in modulus with deformation. After the stress is removed the populations of straight and bent segments revert back to their original size. From the above it is obvious that the deformation of rigid gels is not affine. Similarities exist between deformation and swelling, except for the fact that in the swollen gel the population of straight segments increases isotropically throughout the sample which is not the case in the deformed gel. The strong effects of network imperfections are discussed in the context of rigid network swelling.
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