Formation of Porous Spherulites of Poly(L-lactide) Grown from Solutions
Polymer Journal
Formation of Porous Spherulites of Poly(L-lactide) Grown from Solutions
By Takashi SASAKI
Ryuya ASAKAWA
Kensuke SAKURAI
PLLA / Spherulites / Solutions / Crystallization /
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Porous soft materials possessing high surface to volume
ratio have been attracted much attention because of their
potential applications such as functional filters, catalyst, and
biomedical materials. Various techniques to fabricate materials
with high porosity have been developed up to the present,
which utilize the layer-by-layer deposition,1 freeze-drying,2
electro-spinning,3 etc. In addition, crystallization of polymers
from suitable solutions is another potential technique to obtain
porous materials as small particles. Very porous particles of
polyamide and poly(ethylene oxide) (PEO) have been obtained
via spherulitic growth in solutions.4,5 Such characteristic
feature of morphology seems to be closely related to
complicated mechanisms specific to solution crystallization.
We have studied the mechanism of solution crystallization
especially in viscous solvents, and have revealed an unusual
diffusion aspect for isotactic polystyrene/viscous solvent
systems.6 Another important feature of solution crystallization
is that polymer-solvent interaction plays an important role,
which is responsible for the resulting crystalline morphology.
For example, crystallization of PEO in a very viscous solvent
results in highly swollen spherulites with solvent, while in
dimethyl sulfoxide crystallites with outer polygonal contours
are formed suggesting that liquid-liquid phase separation
occurs during the crystallization.7
For many biomedical applications of the porous materials,
biocompatibility and biodegradability is usually required.
In this respect, poly(L-lacide) (PLLA) is one of the most
promising materials for the fabrication of porous particles by
the solution crystallization method. It has been shown that
single crystals of PLLA are formed from very dilute solutions,
and their structure has been well characterized.8,9 On the other
hand, spherulites of PLLA are generally formed from more
concentrated solutions and from melt.8,10 Porous PLLA fibres
can be obtained by precipitation in a non-solvent under
stirring.11
In this paper, we investigate the morphology of very porous
PLLA particles which are obtained by solution crystallization.
We employed four solvents with different viscosities, i.e.,
diethyl phthalate (DEP), glycerol tri-n-propionate (TP),
N,Ndimethylformamide (DMF), and dimethyl sulfoxide (DMSO).
Porous spherulites exhibiting remarkable morphology of a
solvent
viscosity (cP)
flower-like appearance (assembled petals) are successfully
obtained especially from viscous solvents (DEP and TP).
EXPERIMENTAL
PLLA (Mw ? 210 kg mol 1) was supplied from Mitsui
Chemicals Co., which contained 98% L units. The four
solvents (DEP, TP, DMF, and DMSO) were distilled under
reduced pressure before use. Table I shows viscosity of the
solvents at 25 C. PLLA was added to each solvent in a glass
tube and heated at 150 C for 15?30 min to make an apparently
homogeneous solution of which the PLLA content was
5.0 wt %. Then, the sample tube was rapidly immersed in a
water bath, where the temperature was controlled at 25, 30, and
35 C, and the temperature was kept constant within 0:1 K for
74 h to allow isothermal crystallization. The obtained
crystallites were separated from the solution by filtration using a
Millipore filter (1.0 or 0.2 mm pore size), and they were washed
with methanol several times. Finally, the crystallites were dried
under vacuum at 30 C for 24 h. To investigate further the
crystalline morphology by removing amorphous portion in
the obtained PLLA crystallites, etching treatment (hydrolysis)
was performed as follows.8,10 The PLLA crystallites were
immersed in a 0.025 mol L 1 NaOH solution of H2O/methanol
(1:2 by weight), and the mixture was stirred for 6 h at
60 C.
Morphology of the obtained PLLA crystallites was
investigated by scanning electron microscopy (SEM) by using a
Hitachi S-2600 electron microscope. Polarized optical
microscopy was also performed by using an Olympus BH-2
microscope. Rough estimation of dissolution temperature of
PLLA in the four solvents was done by differential scanning
calorimetry (DSC). PLLA was first crystallized from the melt
at 80 C for 1 h (isothermal crystallization), and the obtained
crystalline PLLA was encapsulated in a hermetically sealed
pan together with the solvent (PLLA content was ca. 10 wt %).
We estimated the dissolution temperature as the observed
endothermic peak due to dissolution that was observed in a
DSC heating scan at a rate of 10 C min 1. Also, degree of
crystallinity of the solution-grown PLLA samples was
estimated as the peak area of melting endotherm observed during
the DSC heating scan. We here employed a reported value
90.9 J g 1 K 1 for the enthalpy of fusion of complete
crystalline PLLA.12 All the DSC measurements were performed in a
nitrogen atmosphe (...truncated)