Intriguing morphology transformation due to the macromolecular rearrangement of poly(l-lactide)-block-poly(oxyethylene): from core–shell nanoparticles to band structures via fragments of unimolecular size
Introduction
Various types of microphase-separated structures have been observed in the solid state of block copolymers, where two or more polymer chains are covalently linked at the molecular level [1]. Most of the copolymers extensively studied are made of non-crystalline segments, and the driving force for their phase separation is mainly ascribed to the repulsive force between the block segments. Another class of block copolymers, consisting of semi-crystalline segments, can also form a microphase-separated structure. The morphological behavior in the latter case is very different from that of the amorphous block copolymers because of segment crystallization in addition to micro phase separation [2], [3], [4], [5]. In spite of the importance of crystalline block copolymers, few analyses have been done on the structure of the latter or its formation process because of its complexity. Recently, much attention has been paid to the block copolymers involving poly-l-lactide (PLLA) and poly(oxyethylene) (PEG) as hydrophobic and hydrophilic blocks, respectively, because of their wide applications in the biomedical field. For example, versatile drug-delivery systems [6], [7] and temperature-dependent sol–gel systems [8] have been developed with these PLLA-PEG copolymers having different block lengths. In addition to their importance in applications, the PLLA-PEG copolymers inherent dual nature as crystalline and amphiphilic copolymers provide a great opportunity to investigate the structural organization of semicrystalline block copolymers.
In the present study, a typical A-B diblock copolymer comprising PLLA (A) and PEG (B) was prepared, and its core–shell nanoparticles were made in an aqueous medium. The resultant particle dispersion was then developed on a germanium substrate to follow the particle aggregation on surface by atomic force microscopy (AFM) and fourier-transform infrared spectroscopy (FT-IR). Additionally, the particles were coagulated into a bulk material in order to analyze the solid state structure by small- and wide-angle X-ray scattering techniques. These analyses revealed the dynamic process of macromolecular dissociation, aggregation and ordering, promoting the understanding of the mechanism of anomalous structural organization from the core–shell nanoparticles reported previously [9] and in the present paper.
Section snippets
Materials
l-Lactide was supplied by Purac Biochem (Netherlands) and purified by recrystallization from ethyl acetate. Monomethoxy-terminated poly(oxyethylene) (PEGMe) with a number average molecular weight (Mn) of 5000 Da and a polydispersity of Mw/Mn (weight/number average molecular weight)=1.05 was purchased from Aldrich Chemical Co., Inc., and lyophilized from benzene before use. Stannous 4-ethylhexanoate (Sn(Oct)2) was supplied by Nacalai Tesque Co. (Japan) and distilled under high vacuum. It was
Characterization of nanoparticles
The suspension of the PLLA-PEG diblock copolymer was prepared by mixing a THF solution of the copolymer into an aqueous medium with ultrasonic wave applied. Although the method employed in the present study is different from that reported by other researchers [6], [7], [8], the particle formation of the copolymer has been established. Fig. 1b shows the 1H NMR spectrum of a similar suspension prepared in D2O. Different from Fig. 1a, only the PEGMe signals are shown because the aggregation of the
Conclusions
Core–shell nanoparticles of PLLA-PEG were prepared in an aqueous medium. When the particles were cast and heat-treated on a germanium substrate, the particles burst into small fragments, which then aggregated into the band structures. Since the crystallinity of the PLLA blocks significantly increased with the band formation (from 0.11 to 0.24), the band structures were considered to be guided by the formation of crystalline lamellae. Any ordered structure was not observed when the same PLLA-PEG
Acknowledgements
We acknowledge the support from a Grant-in-aid for Scientific Research on Priority Area, “Sustainable Biodegradable Plastics”, No. 11217210 from the Ministry of Education, Science, Sports and Culture. The SAXS experiments were performed in the Photon Factory with an approved number of 99G232.
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