Biomedical Engineering Reference
In-Depth Information
polyurethane [4, 5] to lower the friction coefficient. An optimal amount
of added silicone (1.5 to 2 wt %) increased wear-resistance by up to 25%.
Moreover, the ultimate tensile strength and elongation at break were also
both enhanced. The thermoplastic poly(urethanes) (TPU) involved in the
system (Fig. 1) were composed of both soft segments (poly(tetramethylene
oxide): PTMO) and hard segments (4,4
-diphenylmethanediisocyanate: MDI,
extended with 1,4-butanediol). The phase separation between the two kinds
of segment is responsible for the good mechanical properties of the TPU.
However, it was found that the incorporation of PDMS had no effect. The
PDMS is incorporated in the soft domains because it is more miscible with
PTMO than with TPU hard segments. However, the authors showed that the
soft segment volume did not increase whereas the density did. This is a clear
demonstration of the chain packing effect of the soft segments by the PDMS
and explains the enhanced Young's modulus and tensile strength. Last but not
least, the hydrophobicity was also increased.
Fig. 1
TPU used by Hill et al. [4]
It should be noted that this type of blend (PU/PDMS) presents an im-
proved chemical resistance [6] that makes them more base-resistant than
acid-resistant. In addition, the relative viscosity
η
r
(or
λ
) which is defined as
the ratio of the dispersed phase viscosity (
η
m
)
plays an important role in the quality of the blend morphology formation.
An empirical relationship [7] links the capillary number (Ca) to the viscosity
ratio (Eq. 1).
η
d
) to the matrix viscosity (
η
d
/η
m
)
k
k
=0.84for
λ
≥
1and
k
=- 0.84 for
λ
≤
Ca = 4(
1.
(1)
Since the capillary number is related to the drop deformation under shear
stress during mixing and drop break up occurs for a maximum critical value
of Ca (Ca
cr
), a
η
r
value close to unity is required in order to optimize the
structure of the blend. To the best of our knowledge, no such study involving
polydimethylsiloxane discretely dispersed in a matrix is dedicated to the vis-
cosity ratio influence in binary blends. Nonetheless, Maric and Macosko [8],
in a work on copolymer effect that will be discussed later, measured the evo-
lution of the morphology with the viscosity ratio (
)ofaPS/PDMS
blend (Fig. 2). Their results fall in agreement with theory but, as expected,
these blends were not thermodynamically stable and coarsening was ob-
served after annealing.
D
vs.
λ
