Biomedical Engineering Reference
In-Depth Information
interface or interphase plays a key role in defining the overall properties of a
composite.
42
For simplification, the term interface will be used throughout
this chapter to describe both terminologies.
The following discussion involves the interfaces within (e.g., filler-matrix)
and around the composite (composite-surrounding environment). These
interfaces significantly influence the composite mechanics (e.g., strength,
modulus and toughness) and behavior (its interaction with surrounding
tissues, circulating cells, macromolecules and biologics) respectively. At
these interfaces, dynamic processes such as ion diffusion/precipitation and
cell attachment are usually occurring. Appreciation of the dynamics of these
processes helps in tailoring material interfaces with desired properties.
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7.2.1 Filler-Matrix
The filler can be bonded primarily through ionic or covalent bonds or sec-
ondarily through van der Waals bonds to the resin matrix. The level of filler-
matrix interaction affects fillers loading/dispersibility, stress-transfer across
the filler-matrix interface and hence the composite performance.
43
With low
filler-matrix interaction, macroscopic phase separation,
43
stress build-up,
cracks, delamination or debonding can occur at the interface.
44,45
This
condition would become even worse in composites continually subjected to
fluctuating thermal and mechanical stresses. Improving interfacial prop-
erties is therefore important for enhancing the durability of these compos-
ites. This could be achieved by tuning the morphology of the filler,
46
e.g.,
with mesoporous fillers, the polymer chains act as a necklace passing
through the nano-sized channels in the filler.
47
Functionalization
48-50
and
nano-structuring
51
of the interface has also been attempted to improve the
interfacial properties.
Functionalization of the interface involves surface treatment of fillers to
achieve covalent interactions with the matrix.
48-50
It improves the distribution
of filler particles within the matrix
52
and subsequently immobilization of the
matrix in the vicinity of the filler.
43
Surface treatments of fillers include sila-
nization using coupling agents
53-55
[e.g., 3-aminopropyltriethoxysilane
(AMPTES), methyltriethoxysilane (MTES) and 3-mercaptopropyltrimethoxy-
silane (MPTMS),
56
g-methacryloyloxypropyltrimethoxy-silane (g-MPS)
53
], elec-
trolytic or gas phase modification
41
or grafting polymer chains.
43,57,58
The
coupling agents have both hydrolysable (e.g., trialkyloxysilane) and non-
hydrolysable organofunctional (e.g., amino, methacrylate or vinyl) groups.
These groups form strong interactions with filler particles (by forming co-
valent and hydrogen bonds with silanols or similar groups on the filler sur-
face)
59
and polymer matrix,
60
respectively. For effective grafting of polymer
chains on the surface of fillers, and hence a strong filler-matrix interaction,
appropriate selection of the side chains of the grafted polymer is a key
factor.
61
Nano-structuring the interface is another option to improve the interfacial
interactions between the filler and resin matrix. In nano-composites, the
.
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