We have investigated systematically the morphology of thin films spin-coated from solutions of a semicrystalline diblock copolymer, poly(L-lactic acid)-block-polystyrene (PLLA-b-PS), in solvents with varying selectivity. In neutral solvents (chloroform and tetrahydrofuran (THF)), a spinodal-like pattern was obtained and the pattern boundary was sharpened by diluting the solution. Meanwhile, loose spherical associates, together with larger aggregates composed of these associates by unimer bridges, formed partly due to crystallization of the PLLA blocks in relatively concentrated solutions. In slightly PS-selective solvent (e.g., benzene), both loose and compact spherical micelles were obtained, depending on the polymer concentration, coexisting with unimers. When enhancing the selectivity with mixed solvents, for example, mixing the neutral solvent and the slightly selective solvent with a highly PS-selective solvent, CS 2, loose assemblies (nanorods in CS2/THF mixtures and polydisperse aggregates in CS2/benzene mixtures) and well-developed lamellar micelles were obtained. For the lamellar micelles observed, a collapsed corona model is proposed to describe the geometry of dry platelets spin-coated on the substrates instead of the swollen coils in solutions.

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Summary: A novel reversible addition‐fragmentation transfer (RAFT) agent, 10‐carboxylic acid‐10‐dithiobenzoate‐decyltrimethylammonium bromide (CDDA), was synthesized and intercalated into montmorillonite (MMT). Successively, the CDDA‐intercalated MMT was used as RAFT agent in the in situ RAFT polymerization for preparation of the polystyrene/MMT nanocomposites. After separation of MMT, the polymers obtained have predictable molecular weight and narrow polydispersity. XRD spectra and TEM images of the nanocomposites demonstrated exfoliated structure. Thermal stability of the composites has been noticeably improved.image

A well‐defined branched copolymer with PLLA‐b‐PS2 branches was prepared by combination of reversible addition‐fragmentation transfer (RAFT) polymerization, ring‐opening polymerization (ROP), and atom transfer radical polymerization (ATRP). The RAFT copolymerization of methyl acrylate (MA) and hydroxyethyl acrylate (HEA) yielded poly(MA‐co‐HEA), which was used as macro initiator in the successive ROP polymerization of LLA. After divergent reaction of poly(MA‐co‐HEA)‐g‐PLLAOH with divergent agent, the macro initiator, poly(MA‐co‐HEA)‐g‐PLLABr2 was formed in high conversion. The following ATRP of styrene (St) produced the target polymer, poly(MA‐co‐HEA)‐g‐(PLLA‐b‐PS2). The structures, molecular weight, and molecular weight distribution of the intermediates and the target polymers obtained from every step were confirmed by their 1H NMR and GPC measurements. DSC results show one T = 3 °C for the poly(MA‐co‐HEA), T = −5 °C, T= 122 °C, and T = 157 °C for the branched copolymers (poly(MA‐co‐HEA)‐g‐PLLA), and T = 51 °C, T = 116 °C, and T = 162 °C for poly(MA‐co‐HEA)‐g‐(PLLA‐b‐PS2). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 549–560, 2006

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We have investigated the hole nucleation and growth induced by crystallization of thin crystalline-coil diblock copolymer films. Semicrystalline rodlike assemblies from neutral/selective binary solvent are used as seeds to nucleate crystallization at temperatures above the glass transition temperature (T g) but below melting point (Tm). The crystallization of nanorods drives neighboring copolymer chains to diffuse into the growing nanorods. Depletion of copolymer chains yields hole nucleation and growth at the edge of the nanorods. Simultaneously, the polymer chains unassociated into the nanorods were oriented by induction from the free surface and the substrate, leading to limitation of the hole depth to the lamellar spacing, ∼20 nm. The holes, as well as the nanorods, grow as t R , where t is the annealing time and a crossover in the exponent R is found. The orientation and stretching of the copolymer chains by the surface and interface are believed to accelerate the crystallization, and in turn, the latter accelerates the growth rate of the holes. At T > Tm, the grains melt and the copolymer chains relax and flow into the first layer of the film.

The interplay of microphase separation and crystallization has been investigated at the early stage of annealing the amorphous thin films of the crystalline−coil poly(l-lactic acid)-block-polystyrene (PLLA-b-PS) diblock copolymer. The homogeneous and heterogeneous films were annealed at the temperatures between the glass transition temperature of PS (T g PS) and the melting point of PLLA (T m PLLA) so that the microphase separation and crystallization were simultaneously possible. (1) The homogeneous films formed spinodal-like pattern, with the amplitude amplified to the lamellar spacing (L 0), indicating the microphase separation and perpendicular orientation of the copolymer chains with respect to the surface and substrate. Afterward, abnormal relief structures (domain II, ∼30 nm in height) started and grew laterally within the original relief patterns (domain I). The surface wavevector q decayed with the time t as q ∼ t -1/3, while the growth of domain II could be described by the Avrami equation with the exponent of 1.0. It was hypothesized that the PLLA may crystallize within the mesophase, increasing the lamellar spacing. (2) Using THF/CS2 mixtures as solvent, semicrystalline nuclei were integrated to the spin-coated PLLA-b-PS films. These nuclei initiated the crystallization of the thin films. The crystallization kinetics follows the Avrami equation with the exponent of 2.6. The crystallization induced cracks in the films, which nucleated holes at the surface. The coarsening kinetics of the holes is an Ostwald ripening type. Comparison of the crystallization halftimes of the homogeneous and heterogeneous films suggests that the crystallization of these films may be a relaxation-controlled process.

Polystyrene-block-poly(4-vinylpyridine) (PSb-P4VP) was synthesized by two steps of reversible addition-fragmentation transfer (RAFT) polymerization of styrene (St) and 4-vinylpyridine (4VP) successively. After P4VP block was quaternized with CH 3 I, PS-b-quaternized P4VP/ montmorillonite (PS-b-QP4VP/MMT) nanocomposites were prepared by cationic exchange reactions of quaternary ammonium ion in the PS-b-QP4VP with ions in MMT. The results obtained from X-ray diffraction (XRD) and transmission electron microscopy (TEM) images demonstrate that the block copolymer/MMT nanocomposites are of intercalated and exfoliated structures, and also a small amount of silicates' layers remained in the original structure; differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) results show that the nanocomposites displayed higher glass transition temperature (T g ) and higher thermal stability than that of the corresponding copolymers. The blending of PS-b-QP4VP/MMT with commercial PS makes MMT to be further separated, and the MMT was homogeneously dispersed in the polymer matrix. The enhancement of thermal stability of PS/PS-b-QP4VP/MMT is about 208C in comparison with commercial PS.