analogous to the movement of a spring according to Hooke’s Law. The parallel
spring and dashpot model explains viscoelasticity as non-linear, time dependent
mechanical response. An ideal elastomer only exhibits an elastic response whereas
in the real elastomer case both viscoelastic and elastic responses are seen especially
at higher strains. Depending on the mechanical response the chemical structure and
molecular architecture of elastomers change considerably. The application of high
strain to the elastomers uncoils the random molecular coils and aligns them into
more linear conformations. When molecules are in fully extended conformations
uncoiling of chain segments occurs and this limits the elastic response. The
elastomer macromolecules have flexible chains and so it responds immediately
and reversibly. This explanation is valid at ambient conditions of temperature as the
elastomer properties vary at temperatures below the glass transition. The viscous
flow occurring in elastomers at high extensions and under strain for longer times is
called creep. All these behavior of elastomer chains is observed if the polymer is
uncrosslinked. But chemical cross-linking prevents the molecular movement and
the unstrained shape of an elastomer cannot be altered and the elastomer cannot be
reprocessed or recycled once crosslinked. This fact is considered to be a disadvantage for rubber applications [8–15].
1.3 Rubber Nanocomposites
Rubber however has some limitations such as swelling in oils, ageing, ozone attack,
and attack by flame, although most of them can be overcome to a great extent by
compounding techniques and with the use of specialty synthetic rubbers. Therefore
various kinds of fillers such as metal oxides, carbon black, silica particles, graphitic
particles and carbon nanotubes are usually employed for their effective reinforcement. Upon the incorporation of stiff fillers into elastomers, the stiffness enhances
while retaining the important attributes of large strain resilient behavior and large
strain-to break. Fillers improve the processability, increase the toughness, fracture
resistance and stress transfer in elastomers. Filler reinforcement provides fabrication of stronger, lighter or less expensive composites than their traditional neat
counterparts. Due to very small size and large surface area and hence surface
properties, nanoparticles play an important role as fillers. In nanocomposites, a
stronger filler/matrix interaction at the interface exists which leads to a more
immobilized rubber shell compared with filler particles of micro dimensions. In
compounds containing filler with identical surface area and chemical nature but
different shape the storage modulus increases with increase in anisometry. Additive
effects of the physical and chemical crosslinks of the chains play an important role
at the beginning of the deformation and give higher modulus than pure elastomer
[5, 6]. Though rubbers are thermal and electrical insulators, incorporation of
conductive fillers could produce conducting composite materials as well. Thus
the study of viscoelasticity in case of rubber composites is of much importance
and this is indeed the aim of this book.
4
D. Ponnamma and S. Thomas
spring and dashpot model explains viscoelasticity as non-linear, time dependent
mechanical response. An ideal elastomer only exhibits an elastic response whereas
in the real elastomer case both viscoelastic and elastic responses are seen especially
at higher strains. Depending on the mechanical response the chemical structure and
molecular architecture of elastomers change considerably. The application of high
strain to the elastomers uncoils the random molecular coils and aligns them into
more linear conformations. When molecules are in fully extended conformations
uncoiling of chain segments occurs and this limits the elastic response. The
elastomer macromolecules have flexible chains and so it responds immediately
and reversibly. This explanation is valid at ambient conditions of temperature as the
elastomer properties vary at temperatures below the glass transition. The viscous
flow occurring in elastomers at high extensions and under strain for longer times is
called creep. All these behavior of elastomer chains is observed if the polymer is
uncrosslinked. But chemical cross-linking prevents the molecular movement and
the unstrained shape of an elastomer cannot be altered and the elastomer cannot be
reprocessed or recycled once crosslinked. This fact is considered to be a disadvantage for rubber applications [8–15].
1.3 Rubber Nanocomposites
Rubber however has some limitations such as swelling in oils, ageing, ozone attack,
and attack by flame, although most of them can be overcome to a great extent by
compounding techniques and with the use of specialty synthetic rubbers. Therefore
various kinds of fillers such as metal oxides, carbon black, silica particles, graphitic
particles and carbon nanotubes are usually employed for their effective reinforcement. Upon the incorporation of stiff fillers into elastomers, the stiffness enhances
while retaining the important attributes of large strain resilient behavior and large
strain-to break. Fillers improve the processability, increase the toughness, fracture
resistance and stress transfer in elastomers. Filler reinforcement provides fabrication of stronger, lighter or less expensive composites than their traditional neat
counterparts. Due to very small size and large surface area and hence surface
properties, nanoparticles play an important role as fillers. In nanocomposites, a
stronger filler/matrix interaction at the interface exists which leads to a more
immobilized rubber shell compared with filler particles of micro dimensions. In
compounds containing filler with identical surface area and chemical nature but
different shape the storage modulus increases with increase in anisometry. Additive
effects of the physical and chemical crosslinks of the chains play an important role
at the beginning of the deformation and give higher modulus than pure elastomer
[5, 6]. Though rubbers are thermal and electrical insulators, incorporation of
conductive fillers could produce conducting composite materials as well. Thus
the study of viscoelasticity in case of rubber composites is of much importance
and this is indeed the aim of this book.
4
D. Ponnamma and S. Thomas