4 Three-Network Concept
In a filled rubber, agglomeration of the particles produces a filler network, in
addition to the network of covalently-bonded polymer chains. In fact, Reichert
et al. [30] modeled the deformation of single network of filled rubber as a double
network, adopting an approach similar to that used to analyze unfilled double
networks [35–37]. This implies that double-network rubber reinforced with filler
can be viewed as a composite of three distinct networks.
5 Double Network in Metal Salts of Unsaturated
Carboxylic Acids Reinforced Rubbers [38–59]
Another type of double network is that the constituted networks are formed by
different chemical bonds, namely covalent bond and ionic bond. The typical
representative of this kind of double network is the metal salts of unsaturated
carboxylic acids reinforced rubbers. This kind of metal salts can be polymerized
during the vulcanization of rubber matrix in the present of free radicals. The ionic
crosslinks are formed by metal salts graft-polymerized onto the rubber chains
because of large numbers of ion pairs in polymerized metal salts molecules and
the strong electrostatic interaction between ion pairs, while the polymerized metal
salts repulse from the rubber matrix, forming nano-size particles. Thus, the
crosslink network contains covalent crosslinks and ionic crosslinks [38–52]. Chen
and Xu [53–59] have commenced a series of studies on the crosslink network of
zinc dimethacrylate (ZDMA) and magnesium dimethacrylate (MDMA) reinforced
rubbers. They found that a developed ionic crosslink network can be formed in the
ZDMA or MDMA reinforced natural rubber (NR) at high filler contents. The ionic
crosslink network exerts a strong influence on the non-linear viscoelasticity of NR.
5.1 Viscoelasticity of Uncured NR/ZDMA Compounds
In the uncured NR/ZDMA compounds, the interaction between ZDMA-ZDMA is
very weak since the ZDMA particles are micrometer grade. They may agglomerate
and even form a filler-filler network at high loading. This filler-filler network is
somewhat similar to the CB network (in fact, the CB rigid network is much stronger
than the ZDMA network) but so weak that it can be disrupted at low shear strain
amplitudes. Figure 7 shows the G
0 and G
00 versus the strain for the uncured
NR/ZDMA compounds. The compounds having higher content of ZDMA exhibit
strong dependence of G
0 on shear strain, showing the severe disruption of filler-filler
interaction. More agglomeration or filler-filler network structure are formed at high
loading of ZDMA, thus more energy dissipated during the rupture of the filler
Effect of Double Networking on Non-Linear Viscoelasticity of Elastomers
175
In a filled rubber, agglomeration of the particles produces a filler network, in
addition to the network of covalently-bonded polymer chains. In fact, Reichert
et al. [30] modeled the deformation of single network of filled rubber as a double
network, adopting an approach similar to that used to analyze unfilled double
networks [35–37]. This implies that double-network rubber reinforced with filler
can be viewed as a composite of three distinct networks.
5 Double Network in Metal Salts of Unsaturated
Carboxylic Acids Reinforced Rubbers [38–59]
Another type of double network is that the constituted networks are formed by
different chemical bonds, namely covalent bond and ionic bond. The typical
representative of this kind of double network is the metal salts of unsaturated
carboxylic acids reinforced rubbers. This kind of metal salts can be polymerized
during the vulcanization of rubber matrix in the present of free radicals. The ionic
crosslinks are formed by metal salts graft-polymerized onto the rubber chains
because of large numbers of ion pairs in polymerized metal salts molecules and
the strong electrostatic interaction between ion pairs, while the polymerized metal
salts repulse from the rubber matrix, forming nano-size particles. Thus, the
crosslink network contains covalent crosslinks and ionic crosslinks [38–52]. Chen
and Xu [53–59] have commenced a series of studies on the crosslink network of
zinc dimethacrylate (ZDMA) and magnesium dimethacrylate (MDMA) reinforced
rubbers. They found that a developed ionic crosslink network can be formed in the
ZDMA or MDMA reinforced natural rubber (NR) at high filler contents. The ionic
crosslink network exerts a strong influence on the non-linear viscoelasticity of NR.
5.1 Viscoelasticity of Uncured NR/ZDMA Compounds
In the uncured NR/ZDMA compounds, the interaction between ZDMA-ZDMA is
very weak since the ZDMA particles are micrometer grade. They may agglomerate
and even form a filler-filler network at high loading. This filler-filler network is
somewhat similar to the CB network (in fact, the CB rigid network is much stronger
than the ZDMA network) but so weak that it can be disrupted at low shear strain
amplitudes. Figure 7 shows the G
0 and G
00 versus the strain for the uncured
NR/ZDMA compounds. The compounds having higher content of ZDMA exhibit
strong dependence of G
0 on shear strain, showing the severe disruption of filler-filler
interaction. More agglomeration or filler-filler network structure are formed at high
loading of ZDMA, thus more energy dissipated during the rupture of the filler
Effect of Double Networking on Non-Linear Viscoelasticity of Elastomers
175