We study the effect of the cyclization of a linear poly(styrene-b-isoprene) PS−PI copolymer on the morphology of the micelles formed in heptane (a selective solvent for PI). The linear and the cyclic copolymers have exactly the same degree of polymerization, i.e. 290 for PS and 110 for PI, i.e., volume fraction ΦPS = 0.78. We use dynamic light scattering (DLS) and atomic force microscopy (AFM) techniques to characterize the micellar morphology made from copolymer molecules, focusing on the effect of cyclization. We also report the concentration effect on the micelle size and shape made from the linear and cyclic copolymers. Our results show that the micelles made from linear PS−PI adopt a “classical” spherical shape and keep the same morphology parameters over the range of investigated concentrations. In contrast, the micelles arising from cyclic copolymer chains adopt a giant wormlike shape. In this later case, both the size and the shape evolve when the concentration is increased, starting from small in...

Micellar Morphological Changes Promoted by Cyclization of PS-b-PI Copolymer: DLS and AFM Experiments

Self-assembled blends of PS-b-P4VP(PDP) supramolecules, obtained by hydrogen bonding of pentadecylphenol (PDP) side chains to poly(4-vinylpyridine), and poly(2,6-dimethyl-1,4-diphenyl oxide) (PPE) were investigated by thermal analysis and small-angle X-ray scattering (SAXS) and compared with blends of PS-b-P4VP and PPE. Differential scanning calorimetry (DSC) measurements showed a single composition dependent Tg of the PPE/PS layers for both systems, demonstrating that PPE is distributed throughout the PS layers. Furthermore, DSC showed that for the PPE/PS-b-P4VP(PDP) blends the presence of PDP is not restricted to the P4VP layers. Its partial presence in the PS-containing domains was confirmed by nuclear magnetic resonance (NMR) spectroscopy on PS−P4VP core−corona nanorods prepared from hexagonally self-assembled PS-b-P4VP(PDP) supramolecules. The results of the SAXS study on the dependence of the lamellar period of PPE/PS-b-P4VP blends on the amount of PPE were in excellent agreement with a theoretical ...

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Incorporation of PPE in Lamellar Self-Assembled PS-b-P4VP(PDP) Supramolecules and PS-b-P4VP Diblock Copolymers

We have studied the morphology of self-assembled micelles made of linear and cyclic poly(styrene-b-isoprene) PS-b-PI block copolymers dispersed in selective solvents of the PI block (n-heptane, n-decane). Up to a copolymer concentration of 5 mg mL−1, the micelles made from linear block copolymer chains adopt a spherical shape. Those arising from cyclic copolymer chains having exactly the same molar mass and volume fraction self-assemble into (i) planar sunflower-shaped particles at low concentration (c < 0.1 mg mL−1), (ii) giant wormlike micelles at intermediate concentration (0.1 mg mL−1 < c < 2 mg mL−1) and (iii) vesicles at higher concentration (2 mg mL−1 < c < 5 mg mL−1). Those results were obtained using dynamic light scattering and in situ freeze-drying cryo-transmission electron microscopy. In this contribution, we discuss the effects of concentration and temperature on the morphology of the self-assembled particles made from both linear and cyclic PS–PI copolymers, and highlight the surprising vesicle formation in cyclic block copolymer solutions.

Vesicles made of PS-PI cyclic diblock copolymers: In situ freeze-drying cryo-TEM and dynamic light scattering experiments

In this article, we studied the effect of hyaluronic acid (HA) on thermogelation of poly(caprolactone-b-ethylene glycol-b-caprolactone) (PCL-PEG-PCL) aqueous solution designed as an injectable system for prevention of postsurgical tissue adhesion. The PCL-PEG-PCL triblock copolymers were simply synthesized by ring-opening polymerization of e-caprolactone (CL) in the presence of PEG as a polymeric initiator. The synthesized copolymers were confirmed by proton nuclear magnetic resonance (1H-NMR) spectroscopy. Possible interactions between HA and PCL-PEG-PCL triblock copolymers in the blend were evaluated by Fourier-transform infrared spectroscopy (FTIR). The effect of HA on the micellization of PCL-PEG-PCL aqueous solution was investigated by dye solubilization method and electrophoretic lighting scattering (ELS) spectrophotometer. Also, the thermogelling behaviors of the PCL-PEG-PCL triblock copolymers in the presence of HA and their mechanism were investigated by test tube inverting method, 13C-NMR, 1H-NMR, Advanced Rheometic Expansion System (ARES), and differential scanning calorimetry (DSC). The PCL-PEG-PCL/HA blend aqueous solutions undergo the sol-gel-sol transition in response to an increase in temperature (10–60 °C) and the gelation of the PCL-PEG-PCL was rather accelerated by HA. Presumably, this accelerated gelation seems to arise from the attractive interactions between them and the effect of chain confinement in the micelle corona region. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3629–3637, 2008

Thermogelling behaviors of poly(caprolactone‐b‐ethylene glycol‐b‐caprolactone) triblock copolymer in the presence of hyaluronic acid

Diblock copolymers self-assemble into a variety of microstructures such as lamellae (L), hexagonally ordered cylinders (HEX), and body-centered-cubic (bcc) spheres.1 In recent studies, a number of so-called complex phases, such as gyroid (G) with Ia3hd symmetry and hexagonally perforated phase (HPL), have also been observed in a narrow region of composition between L and HEX phases.2-5 When a homopolymer is homogeneously mixed with a diblock copolymer, various structures similar to those observed with pure block copolymer can be formed as the volume fraction of one block.6-9 In recent experimental studies, Yang et al.8 observed HPL phase in blends of styrene-isoprene graft block copolymer and polystyrene (PS) homopolymer in a narrow volume fraction region between L and HEX. Lee et al.9 also found a morphology similar to HPL phase during the thermally induced transition from L to HEX in blends of asymmetric styrene-isoprene-styrene triblock copolymer (S-I-S)/PS homopolymer. Recent self-consistent mean-field theory by Matsen suggested that the HPL phase is stabilized by the addition of homopolymer to a diblock copolymer at certain compositions in contrast to the case of pure block copolymer.10,11 However, the relative stability of the HPL phase in diblock copolymer/homopolymer blend has not been known exactly until now. In this paper, we investigate the effect of homopolymer molecular weight on the formation of HPL phase in the intermediate transition from L to G phase. Styrene-isoprene diblock copolymer (PSI67) was synthesized by living anionic polymerization using highvacuum techniques. The polyisoprenes (PI) were obtained from Polysicences Co. and Daelim Industry Ltd. The molecular characteristics of diblock copolymer and homopolymers are listed in Table 1. Blends were prepared by solution-casting in toluene (∼10 wt % solid) in the presence of an antioxidant (Irganox 1010, CibaGeigy Group), and then the solvents were slowly evaporated. After complete removal of the solvent, the samples were annealed at 120 °C for 10 h. To observe morphological transitions, the weight fraction of homopolymer PI in the blend was designed to be 2.5 wt % (the total volume fraction of isoprene in blends ) 0.35). The characteristics of microdomain structure for neat block copolymer and its binary blends are given in Table 2. Neat PSI67 copolymer has a gyroid morphology, while its binary blends has a lamellar morphology at room temperature, which the lamellar microdomain spacings of blends increase with increasing PI molecular weight. SAXS measurements were performed on the 4C1 beamline using synchrotron X-ray radiation sources at the Pohang Accelerator Laboratory, Korea. The X-ray source wavelength was 1.608 Å. The experimental details were presented elsewhere.12 Figure 1a shows the SAXS patterns for unsheared PSI67/PI1K blend at each temperature during the heating process. Below 100 °C, the specimen has a lamellar (L) microstructure having peaks at positions of q*, 2q*, and 3q*. At 120 °C, the scattering profile shows the splitting of the first-order peak as indicated by open triangle and arrow, which implies that the mixture begins to transform from L to a new morphology. After subsequent heating to 140 °C, the SAXS pattern clearly exhibits three strong peaks at 0.92q*, q*, and 1.08q*, which is consistent with the scattering patterns of HPL shown in previous work,5 indicating the completion of morphological transition. Therefore, it means that L and HPL morphologies coexist between 120 and 140 °C because a morphological transition from L to HPL occurs at this temperature range. At temperatures between 170 and 175 °C, the first-order peak at the position of 0.92q* disappears, and the position of the third peak jumps from 1.08q* to 1.15q*, indicating a morphological transition from HPL to G at this temperature range. It also means that the two-phase region of HPL and G exists in a narrow range of temperatures. Therefore, it is concluded that the blend transforms from HPL to G phase between 170 and 175 °C. Finally, at 215 °C, the SAXS profiles shows a single broad peak which indicates a disordered state. Thus, the SAXS profiles for PSI/PI1K indicate the OOT from L through HPL to G and the ODT from G to the disordered phase. To confirm the thermodynamic stability of HPL phase, the blend was annealed at 150 °C for 48 h. After * To whom correspondence should be addressed: e-mail wczin@postech.ac.kr; Tel 82-54-279-2136; Fax 82-54-279-2399. Table 1. Characteristics of Polymers Used

Effect of Polyisoprene Molecular Weight on Morphological Transition in Binary Blends of Styrene-Isoprene Diblock Copolymer and Polyisoprene

The characteristics of micelle formed in blends of styrene-isoprene diblock copolymer (SI) and poly(vinyl methyl ether) (PVME) were investigated by using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and light scattering techniques. The results of SAXS and TEM experiments indicate that the blends form spherical micelles with isoprene-rich cores and styrene-corona mixed with PVME in a matrix of PVME. As temperature increased, the blends underwent macrophase separation into a PVME-rich phase and a SI-rich phase which was also microphase-separated to ordered microdomains. Upon further heating, the ordered structure of the SI-rich region transforms to a disordered phase in a manner resembling the order-disorder transition in pure block copolymer melts.

Si Tae Noh

Papers

Micelle formation in mixtures of styrene-isoprene diblock copolymer and poly(vinyl methyl ether)