Because water keeps the CNF and polymer dispersed in their
mixture, cellulose agglomeration and particle sedimentation can
happen during water removal. In one experiment, the mixture
was quickly frozen in liquid nitrogen and freeze-dried into afine
powder. After compression molding, large aggregates of CNFs
were found in the clear PDLLA matrix (data not shown). When
water is frozen into ice, the spontaneous nucleation of ice
crystals drives the dissolved or suspended substances out,
concentrating them in the shrinking water phase. This caused
the cellulose to agglomerate even before the sublimation
process took place. Water removal by membranefiltration was
found to be like a sol−gel process except that the gelation was
not caused by chemical cross-linking but by the concentrated
CNFs. Beforefiltration, the mixture was a free-flowing liquid
sol of suspended CNFs and microparticles. With water being
drawn away by vacuum, the mixture became a gel-like material
when the CNFs were concentrated to form a continuous
network, trapping the microparticles. Only a fraction of all of
the particles in the mixture (from 34 to 63%) could be retained
by the gel, while the rest precipitated out from the network,
resulting in a two-layered wet cake formed on the membrane.
The phenomenon could have been caused by the difference in
the stabilities of CNF and PDLLA in water and the nonuniform
size distribution of the microparticles. Reducing particle sizes
with higher surfactant loadings and increasing thefiltration
speed (to about 15 s per batch) by feeding smaller sample
volumes or by using large-pore-size membranefilters did not
prevent it from happening (data not shown)
Because water keeps the CNF and polymer dispersed in their
mixture, cellulose agglomeration and particle sedimentation can
happen during water removal. In one experiment, the mixture
was quickly frozen in liquid nitrogen and freeze-dried into afine
powder. After compression molding, large aggregates of CNFs
were found in the clear PDLLA matrix (data not shown). When
water is frozen into ice, the spontaneous nucleation of ice
crystals drives the dissolved or suspended substances out,
concentrating them in the shrinking water phase. This caused
the cellulose to agglomerate even before the sublimation
process took place. Water removal by membranefiltration was
found to be like a sol−gel process except that the gelation was
not caused by chemical cross-linking but by the concentrated
CNFs. Beforefiltration, the mixture was a free-flowing liquid
sol of suspended CNFs and microparticles. With water being
drawn away by vacuum, the mixture became a gel-like material
when the CNFs were concentrated to form a continuous
network, trapping the microparticles. Only a fraction of all of
the particles in the mixture (from 34 to 63%) could be retained
by the gel, while the rest precipitated out from the network,
resulting in a two-layered wet cake formed on the membrane.
The phenomenon could have been caused by the difference in
the stabilities of CNF and PDLLA in water and the nonuniform
size distribution of the microparticles. Reducing particle sizes
with higher surfactant loadings and increasing thefiltration
speed (to about 15 s per batch) by feeding smaller sample
volumes or by using large-pore-size membranefilters did not
prevent it from happening (data not shown)
การแปล กรุณารอสักครู่..
