DISPERSION NUCLEATING--EFFECTS OF POLYMER NANOCOPMPOSITES

1. AMRUTHA K S
2ND M.Sc BPS
CBPST, KOCHI

2. Introduction
 While large surface-to-volume ratio of nanoparticles makes
them superior reinforcements over conventional fillers, it
also brings difficulties in dispersing the nanoparticles due to
their strong interactions among themselves. Due to these
inter nanoparticle interactions, it is extremely difficult to
disperse nanoparticles uniformly, specially at higher particle
loadings.
 Several processing methods including shear mixing,
mechanical mixing, in-situ polymerization and sonication
have been used for dispersing nanoparticles in polymers.

3. Effect of Dispersion at High Particle
Loadings
 In shear mixing and sonication to disperse montmorillonite
clay nanoparticle into vinyl ester resin. The clay nanoparticle
loading was 3-8% by weight. The compressive results of
clay modified resin showed that the resin with 5% nanoclay
exhibited a reasonable improvement in compressive strength
and a considerable (20%) improvement in modulus.
 However the mechanical properties, especially strength,
tended to go down when nanoclay was loaded beyond 5
wt%.

4.  16% improvement in tensile strength and 90% improvement
in tensile modulus were reported at 5wt% loading of fillers
3wt% 5wt% 8wt%

5.  Epoxy matrix modified with modified with silica nanoparticles
(up to 14 vol%) via sol-gel method enhanced the mechanical
properties of polymer nanocomposites such as modulus,
microhardness, and fracture toughness. It is therefore
reasonable to say that the sol-gel method can be used to produce
well dispersed high nanoparticle loading nanocomposites with
much improved mechanical properties.

6. Dispersion of Particles of Different
Sizes
 While particle dispersion plays the foremost role in
enhancing polymer’s properties.
 it is also greatly influenced by the size of particles. Smaller
particles have higher surface area (at same particle volume
fraction) and,
 Hence, greater surface interactions among the particles. This
is what makes the dispersion of nanoparticles harder.
 Nanocomposites show increase in modulus with the
decrease in particle size that leads to dispersion more
difficult.

8. Nucleation
 In polymer technology, especially for injection molding the
desired high crystallization rates often require the addition of
nucleating agents.
 A nucleating agent is substance that forms nuclei for the growth
of crystals in super cooled polymer melt. virtually ay solid body
with a high energy surface may act as a nucleating agent closely
matches the crystallization surface of polymer.
 The nucleating agent may preferably induce a crystallographic
form of the polymer. for e.g., addition to isotactic PP of either
1,2,34-bis(3,4-dimethyl-benzylidene sorbitol) or N,N’-
dicyclohexyl-2,6-naphthalate dicarboxamide, preferably generate
a-iPP or b-iPP, respectively

9.  The nucleating efficiency depends on several independent
variables, such as temperature,pressure,stress,part thickness
as well as the presence of the other processing additives
 An efficient nucleating agent must have a high-energy
surface-the large the specific surface ,the more efficient it is
expected to be.
 Nucleation involves initial adsorption of macromolecules on
the surface. the process is particularly efficient if the foreign
body is able to provide a energetic matrix for the formation
of thermodynamically favorable crystalline forms.

10.  Alternatively, the crystalline cell type ad size of the nucleating
agent may induce a transitory crystalline polymer forms that
upon annealing transforms into a stable form of higher packing
density
 The nucleating efficiency may be expressed in terms of the
energy ratio required to generate a nucleus in a heterogeneous
nucleation over that in a homogenous one.
 The nucleation is the rate determine step, while the extend of
crystallinity is constant. Thus the nucleation rate ,r , depends on
the degree of super cooling ,

11. Where,
 Em is the energy required to form a nucleus ,
 is a geological factor ,
 is the specific surface energy ,
 Vm is the molar volume of the crystalline substance.
 is the melting entropy,
 n is the Avrami exponent and
 kb is the Boltzmann constant.

12. Nanofillers such as clay may have high
nucleating effect. The wide variety of
intercallatants,intercalating methods and
compatibilisers may form a barrier between
high energy clay surface and semicrystaline
polymer matrix.