Formation of Porous Spherulites of Poly(L-lactide) Grown from Solutions

Polymer Journal, Jul 2009

Takashi Sasaki, Ryuya Asakawa, Kensuke Sakurai

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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 / - 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)


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Takashi Sasaki, Ryuya Asakawa, Kensuke Sakurai. Formation of Porous Spherulites of Poly(L-lactide) Grown from Solutions, Polymer Journal, 2009, pp. 787-791, Issue: 41, DOI: 10.1295/polymj.PJ2009078