P4VP Block Research Articles

Self-assembly of an A–B diblock copolymer blended with a C homopolymer and a C–D diblock copolymer through hydrogen bonding interaction

In this study, we investigated the miscibility, phase behavior, and self-assembled nanostructures formed from the immiscible crystalline-amorphous diblock copolymer poly(ɛ-caprolactone-b-4-vinyl pyridine) (PCL-b-P4VP, A–B) when blended with the homopolymer poly(vinyl phenol) (PVPh, C) and the diblock copolymer poly(vinyl phenol-b-styrene) (PVPh-b-PS, C–D). Long-range-ordered microphase separation was difficult to achieve in the PCL-b-P4VP/PVPh (A–B/C) blend system because PVPh interacted with both the P4VP and PCL blocks simultaneously through hydrogen bonding interactions. In contrast, we observed sharp, multiple orders of diffraction in the SAXS profiles of the PCL-b-P4VP/PVPh-b-PS (A–B/C–D) blend system, indicating that perfect microphase separation occurred because the incorporation of the PS block induced the PVPh block to hydrogen bond preferentially with the P4VP block. This simple A–B/C–D (PCL-b-P4VP/PVPh-b-PS) diblock copolymer mixture exhibited self-assembly behavior (a three-lamella phase) similar to that of a corresponding ABC triblock copolymer.

Transition Behavior of Hydrogen-Bonding-Mediated Block Copolymer Mixtures

We have investigated transition behavior for block copolymer (BCP) mixtures composed of a lamella-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and phenylacetamide derivatives, where the hydroxyl functionality at phenylacetamide unit is available only for 4-hydroxyphenylacetamide (HPA) and not for phenylacetamide (PA). Unlike a similar transition temperature for BCP mixtures with PA, measured by in situ small-angle X-ray scattering (SAXS) and depolarized light scattering (DPLS), an annealing temperature dependence for BCP mixtures with HPA indicates that only the complex by H-bonding mediation between the nitrogen units of P2VP block and hydroxyl group in the HPA enhances nonfavorable segmental interactions between two block components, leading to a significant increase in d-spacing for BCP mixtures with HPA. This result illustrates the importance of the availability for H-bonding mediation to control over transition behavior for BCP mixtures with the functional molecules.

Control of Optical Hysteresis in Block Copolymer Photonic Gels: A Step Towards Wet Photonic Memory Films

AbstractPolystyrene‐block‐poly(2‐vinyl pyridine) (PS‐b‐P2VP) block copolymer photonic gels are fabricated that exhibit controllable optical hysteresis in response to a cyclic pH sweep. The optical hysteresis is tuned by controlling the ion‐pairing affinity between various anions and the protonated pyridinium ions on the P2VP block, which is highly dependent on the hydration energy of the ions, the dielectric constant of the solvent, and the ionic strength of the medium. The pH coercivity defining the magnitude of hysteresis of the photonic gels could be varied from 0.26 to 7.4. Photonic gel films with strong optical hysteresis can serve as wet photonic memory films where information can be cyclically recorded and erased at least 15 times and maintained for at least 96 h. The memory colors can be further tuned by selection of the copolymer molecular weight.

A simple method for creating nanoporous block-copolymer thin films

We introduce a simple method to create block copolymer films with controlled porosity. We show that the pore structure can be varied over a broad range of length scales not obtainable in homopolymer blend films. The morphology is a random two phase kinetically trapped structure that is not limited by the equilibrium block copolymer structure. The morphology is obtained through blending homopolymer poly(4-vinylpyridine) (P4VP) with block copolymer polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and then removing the homopolymer P4VP by washing with ethanol. The structure obtained prior to washing (which templates the nanoporous structure) is stabilized in the kinetically trapped morphology during spincoating and is not obtainable from either homopolymer blends or the pure block copolymer. When PS/P4VP blend solutions in tetrahydrofuran were spincoated at 25% relative humidity, continuous films with raised P4VP nanodomains were formed due to a preferential affinity of the spinning solvent for polystyrene. In a similar manner, when PS-b-P4VP/P4VP block copolymer/homopolymer solutions were spincoated, the P4VP homopolymer was solubilized in the P4VP block domains during spincoating, suppressing macro-phase separation. The film morphology is generated at the air surface and then propagates through the film, resulting in P4VP nanodomains oriented vertically to the substrate. In the resulting films, the size of P4VP nanodomains were varied by increasing the amount of P4VP homopolymer. The subsequent extraction of P4VP homopolymer from the PS-b-P4VP/P4VP blend films in ethanol resulted in nanopores with a distribution of length scales. The morphology of these materials makes the films potentially suitable for a range of applications such as anti-reflective coatings, nanoporous membranes and low-k materials. An illustrative example of an anti-reflective coating will be presented.

Association Strength of Metal Ions with Poly(4-vinylpyridine) in Inorganic/Poly(4-vinylpyridine)-b-poly(ε-caprolactone) Hybrids

A series of inorganic/poly(4-vinylpyridine)-b-poly(ε-caprolactone) (P4VP−PCL) hybrids were prepared to examine the phase behavior of hybridization. Various metal ions including Au3+, Cu2+, Cu+, and Ag+ ions were used for the hybridization. As demonstrated by transmission electron microscopy, the phase transformation of self-assembled nanostructures can be easily induced by adding small amount of metal ions due to the association of metal ions with P4VP block that profoundly creates the extra volume in P4VP microdomain (i.e., gives rise to the significant increase of effective excluded volume) beside the Ag+ ions. The variation in the effective excluded volume (relevant to the degree of domain swelling, as evidenced by small-angle X-ray scattering) is strongly dependent upon the association strength of the metal ions with the P4VP block, as determined by Fourier transform infrared spectroscopy. The association strength for the formation of the hybrids follows the order of Au3+ > Cu2+ and Cu+ > Ag+. Accordingly, the degree of domain swelling in the hybrids increases with the enhancement of association strength following the order of Au3+ > Cu2+ and Cu+ > Ag+. Furthermore, as demonstrated in the hybrids of Au nanoparticles and P4VP−PCL, a dramatic decrease of the association strength can be found in the hybrids after reduction. Consistent with the theoretical prediction, the reduction of metal ions causes the alleviation of the association strength. The association effect is also dependent upon the particle size; the larger the particle size is, the weaker the association will be. Consequently, the accommodation of the metal nanoparticles within the P4VP microdomain is justified by the size of the metal nanoparticles.

Self-Assembly of Supramolecular Triblock Copolymer Complexes

Four different poly(tert-butoxystyrene)-b-polystyrene-b-poly(4-vinylpyridine) (PtBOS-b-PS-b-P4VP) linear triblock copolymers, with the P4VP weight fraction varying from 0.08 to 0.39, were synthesized via sequential anionic polymerization. The values of the unknown interaction parameters between styrene and tert-butoxystyrene and between tert-butoxystyrene and 4-vinylpyridine were determined from random copolymer blend miscibility studies and found to satisfy 0.031 < χS,tBOS < 0.034 and 0.39 < χ4VP,tBOS <0.43, the latter being slightly larger than the known 0.30 < χS,4VP ≤ 0.35 value range. All triblock copolymers synthesized adopted a P4VP/PS core/shell cylindrical self-assembled morphology. From these four triblock copolymers supramolecular complexes were prepared by hydrogen bonding a stoichiometric amount of pentadecylphenol (PDP) to the P4VP blocks. Three of these complexes formed a triple lamellar ordered state with additional short length scale ordering inside the P4VP(PDP) layers. The self-assembled state of the supramolecular complex based on the triblock copolymer with the largest fraction of P4VP consisted of alternating layers of PtBOS and P4VP(PDP) layers with PS cylinders inside the latter layers. The difference in morphology between the triblock copolymers and the supramolecular complexes is due to two effects: (i) a change in effective composition and, (ii) a reduction in interfacial tension between the PS and P4VP containing domains. The small angle X-ray scattering patterns of the supramolecules systems are very temperature sensitive. A striking feature is the disappearance of the first order scattering peak of the triple lamellar state in certain temperature intervals, while the higher order peaks (including the third order) remain. This is argued to be due to the thermal sensitivity of the hydrogen bonding and thus directly related to the very nature of these systems.

Composition-Dependent Morphological Transitions and Pathways in Switching of Fine Structure in Thin Films of Block Copolymer Supramolecular Assemblies

The phase behavior of supramolecular assemblies (SMA) formed by polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with 2-(4′-hydroxybenzeneazo)benzoic acid (HABA) was investigated with respect to the molar ratio (X) between HABA and 4VP monomer unit in bulk as well as in thin films coated on a silicon substrate. In bulk, the SMA showed bcc packed spherical or hexagonally packed cylindrical microdomains composed of P4VP(HABA) blocks in PS matrix depending on the composition which varied with the HABA weight fraction. However, at a molar ratio of 2 where HABA was in excess, the cylindrical microdomains of P4VP(HABA) blocks were found to pack in a tetragonal lattice. The SMA in thin film also showed spherical or cylindrical morphology depending on the composition. Interestingly, the orientation of cylindrical microdomains of P4VP(HABA) depended on the selectivity of the solvent as well as on the degree of swelling. Hence, in a nonselective solvent like chloroform, the cylinders were found to lie parallel t...

Janus Micelle Formation Induced by Protonation/Deprotonation of Poly(2‐vinylpyridine)‐block‐Poly(ethylene oxide) Diblock Copolymers

AbstractA novel approach to amphiphilic polymeric Janus micelles based on the protonation/deprotonation process of poly(2‐vinylpyridine)‐block‐poly(ethylene oxide) (P2VP‐b‐PEO) diblock copolymers in THF is presented. It is found that addition of HCl to the micelles solution of P2VP‐b‐PEO copolymers leads to the formation of vesicles. Subsequently mixing a small amount of hydrazine monohydrate with the vesicle solution can induce the dissociation and reorganization of the vesicles into Janus micelles. When HCl is replaced by HAuCl4 precursors, composite Janus particles containing gold in P2VP blocks are obtained.magnified image

Exfoliation of organoclay nanocomposites based on polystyrene-block-polyisoprene-block-poly(2-vinylpyridine) copolymer: Solution blending versus melt blending

Exfoliated nanocomposites based on polystyrene-block-polyisoprene-block-poly(2-vinylpyridine) (SI2VP triblock) copolymer were prepared by solution blending and melt blending. Their dispersion characteristics were investigated using transmission electron microscopy, X-ray diffraction, and small-angle X-ray scattering (SAXS). For the study, SI2VP triblock copolymers with varying amounts of poly(2-vinylpyridine) (P2VP) block (3, 5, and 13 wt%) and different molecular weights were synthesized by sequential anionic polymerization. In the preparation of nanocomposites, four different commercial organoclays, treated with a surfactant having quaternary ammonium salt, were employed. It was found from SAXS that the microdomain structure of an SI2VP triblock copolymer having 13 wt% P2VP block (SI2VP-13) transformed from core-shell cylinders into lamellae when it was mixed with an organoclay. It was found further that the solution-blended nanocomposites based on a homogeneous SI2VP triblock copolymer having 5 wt% P2VP block (SI2VP-5) gave rise to an exfoliated morphology, irrespective of the differences in chemical structure of the surfactant residing at the surface of the organoclays, which is attributable to the presence of ion–dipole interactions between the positively charged N+ ion in the surfactant residing at the surface of the organoclay and the pyridine rings in the P2VP block of SI2VP-5 and SI2VP-13, respectively. Both solution- and melt-blended nanocomposites based on microphase-separated SI2VP-13 having an order–disorder transition temperature (TODT) of approximately 210 °C also gave rise to exfoliated morphology. However, melt-blended nanocomposite based on a high-molecular-weight SI2VP triblock copolymer having a very high TODT (estimated to be about 360 °C), which was much higher than the melt blending temperature employed (200 °C), gave rise to very poor dispersion of the aggregates of organoclay. It is concluded that the TODT of a block copolymer plays a significant role in determining the dispersion characteristics of organoclay nanocomposites prepared by melt blending.

Complex aggregation of TPPS and PEG-b-P4VP in confined space

To explore the structure and property features of complex aggregates assembled by polymer and organic dye molecules in microcosmic soft confined space, a three-dimensional microconfinement is constructed by W/O inverse emulsion as the site for the complex aggregation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP). It is found that porphyrin molecules exist in emulsion droplets mainly as the deprotonated monomer form, even though the pH of the water phase is much lower than the pK(a) of TPPS in bulk (pK(a)≈ 4.7). In spatially confined circumstances, upon blending of the single-component emulsions, complex aggregates are formed due to the thermodynamic collision of emulsion droplets and the subsequent electrostatic interaction between TPPS and polymer. The resultant novel complexes possess a relatively precise structure and homogenous core consisting of a porphyrin and P4VP block. Upon emulsion-breaking, the metastable complex kinetic intermediates generated in original acidic emulsion droplets display a remarkable trend of reaggregation within a certain period of time followed by a disassociation and redispersion. Complex aggregates with some novel morphologies are observed. The final product has a core-shell structure with the electrostatic complex of deprotonized porphyrin molecules homogenously dispersing in the P4VP clew as the core and the soluble PEG as the shell. However, those complexes produced in basic emulsion droplets exhibit a relatively higher stability against further aggregation after breakage of emulsion.

Highly ordered arrays of magnetic nanoparticles prepared via block copolymer assembly

In the present study we report a simple and reproducible method to prepare highly ordered arrays of Fe3O4 superparamagnetic nanoparticles (MNPs) via block copolymer (BCP) self assembly. Pre-synthesized MNPs with a mean diameter of 6.1 nm are selectively segregated within the lamellae or hexagonally packed cylinders composed of PVP blocks in poly(styrene-b-vinylpyridine) (PS-b-PVP) block copolymers without any additional surface modification. The density of the stabilizing shell in the MNPs as well as the position of pyridine nitrogen in the PVP block of the BCPs are found to be crucial for selective incorporation of MNPs into the PVP domains. The obtained results suggest a key importance of a mutual affinity between active blocks and the nanoparticles which should be maximized in order to attain high nanoparticles loadings and long-range structural orders.

Crystallization and Physical Ageing of Poly (2‐vinyl pyridine)‐b‐poly(ethylene oxide) Diblock Copolymers

AbstractSummary: A set of melt miscible Poly(2‐vinyl pyridine)‐b‐Poly(ethylene oxide) (P2VP‐b‐PEO) block copolymers of different compositions were studied. Transmission electron microscopy shows phase separation in the materials during the crystallization process of the PEO block as crystalline lamellae are observed for all compositions evaluated. The isothermal crystallization kinetics of PEO is progressively retarded as the P2VP content in the copolymer increases, since P2VP hinders molecular mobility in the miscible amorphous phase. Polarized light optical microscopy demonstrated that the glassy P2VP block has a negative effect on the secondary nucleation of the PEO. Finally, physical ageing experiments performed in the glassy state of the amorphous mixed phase, at different ageing times, demonstrated that a nucleating effect can be induced in the glassy state as a consequence of the reorganization of the amorphous regions. This nucleating effect significantly alters the cold crystallization rate upon subsequent heating above the glass transition temperature.

Polymeric core-shell stars with a novel fluorescent, cross-linked and swollen core: Their efficient one-step preparation, further self-assembly into superparticles and application as a chemosensor

Here we report that block copolymer core-shell stars with a novel fluorescent, cross-linked and swollen core can be efficiently prepared by simply cross-linking the poly(4-vinylpyridine) (P4VP) block of polystyrene (PS)-block-P4VP diblock copolymer in N,N-dimethylformamide (DMF, the common solvent of the copolymer) using propargyl bromide (PB) as the cross-linker. Propargyl bromide first quaternized pyridine groups in the repeat units of the P4VP block chains, grafting propargyl groups onto the P4VP block chains. After the quaternization, the acetylene groups in the grafted propargyl groups were activated and thus could polymerize with each other, which resulted in the cross-linking of the P4VP block chains. The cross-linking reaction forced the aggregation of the P4VP block chains. Meanwhile, with the cross-linking reaction taking place, the PS block chains which were still soluble, gathered around the P4VP aggregates and localized the cross-linking reaction. Therefore, block copolymer core-shell stars with the P4VP/PB cross-linked network as the core and the PS block chains as the shell formed. The average aggregation number of the core-shell stars was determined by static light scattering to be 26, which is much less than that of micelles formed by the same block copolymer with similar structure parameters in a selective solvent for PS. The polymerization of the acetylene groups in the grafted propargyl groups also led to the formation of a fluorescent conjugated polymer polyacetylene in the core. Therefore, the as-prepared core-shell stars have a fluorescent core (the fluorescence of the product of PB and pyridine (or products of PB analogues with pyridine derivatives) has never been reported). We further confirmed that the core of the as-prepared core-shell stars had a loosely aggregated structure and, to a certain extent, swelled in DMF, which made penetration of small molecular species into the core convenient. Therefore, the chromophores within the core could respond promptly to various metallic ions and basic species. It was also due to the loosely aggregated structure that the primary core-cross-linked core-shell stars could further self-assemble into dispersible superparticles in water. Although originally existing as the core, the P4VP/PB cross-linked network acted as the shell of and thus stabilized the superparticles. This simple, yet efficient method for preparing polymeric core-shell stars with a novel fluorescent and loosely aggregated core that can respond promptly to ionic species and further self-assemble into superparticles should show promise to address related problems in the field of materials chemistry.

The Self‐Assembled Structure of the Diblock Copolymer PCL‐b‐P4VP Transforms Upon Competitive Interactions with Octaphenol Polyhedral Oligomeric Silsesquioxane

This paper describes the miscibility and self-assembly, mediated by hydrogen-bonding interactions, of new block copolymer/nanoparticle blends. The morphologies adopted by the immiscible poly[(ε-caprolactone)-block-(4-vinyl pyridine)] (PCL-b-P4VP) diblock copolymer changes upon increasing the number of competitive hydrogen-bonding interactions after adding increasing amounts of octaphenol polyhedral oligomeric silsesquioxane (OP-POSS). Transmission electron microscopy reveals morphologies that exhibit high degrees of long-range order, such as cylindrical and spherical structures, at relatively low OP-POSS contents, and short-range order or disordered structures at higher OP-POSS contents. Analyses performed using differential scanning calorimetry, wide-angle X-ray diffraction, and FT-IR spectroscopy provide positive evidence that the pyridyl units of the P4VP block are significantly stronger hydrogen-bond acceptors toward the OH group of OP-POSS than are the CO groups of the PCL block, thereby resulting in excluded and confined PCL phases.

Controlling Orientation and Functionalization in Thin Films of Block Copolymers

Functionalizing and controlling nanostructures resulting from block copolymer self-assembly are key factors in defining their application. In this work, a simple but quite general route to achieve both goals simultaneously is discussed. In thin films of polystyrene-block-poly(vinyl pyridine) (PS-b-PVP) with small concentrations of a gold salt, the salt is found to complex with the PVP block which leads to an orientation of the microdomains normal to the surface after solvent annealing together with functionalization. By increasing the amount of gold salt, on the other hand, micelles are found to form in solutions leading to a range of different morphologies in the thin films.

Shell–core–corona aggregates formed from poly(styrene)-poly(4-vinylpyridine) block copolymer induced by added homopolymer via interpolymer hydrogen-bonding

The solution behavior of poly(styrene)-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer with added poly(4,4′-oxydiphenylenepyromellitamic acid) (POAA) homopolymer in DMF is studied by dynamic light scattering (DLS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM). It is found that coaggregation takes place when mixing PS-b-P4VP block copolymer and POAA homopolymer in DMF due to the strong interpolymer hydrogen-bonding between POAA chains and P4VP blocks. The coaggregation is a diffusion-controlled process and the complexation-induced aggregates are very stable. NMR measurements demonstrate that the resultant aggregates are much more swollen compared with typical amphiphilic block copolymer core–shell micelles. DLS measurements with Eu3+ as a probe combined with TEM observation, are employed to study the structure of the aggregates. Results reveal that the formed aggregates are core–shell spheres with the P4VP/POAA complexes as core and the PS blocks as shell when the weight ratio of POAA to PS-b-P4VP is lower. However, a core–shell–corona structure forms with a thin layer of POAA chains adsorbed on the initial core–shell aggregates with increasing weight content of POAA to 60%. Finally, possible mechanisms of the structural transitions are proposed.

Arrays of Inorganic Nanodots and Nanowires Using Nanotemplates Based on Switchable Block Copolymer Supramolecular Assemblies

AbstractHere, a novel and simple route to fabricate highly dense arrays of palladium nanodots and nanowires with sub‐30 nm periodicity using nanoporous templates fabricated from supramolecular assemblies of a block copolymer, polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) and a low molecular weight additive, 2‐(4′‐hydroxybenzeneazo) benzoic acid (HABA) is demonstrated. The palladium nanoparticles, which are directly deposited in the nanoporous templates from an aqueous solution, selectively migrate in the pores mainly due to their preferential attraction to the P4VP block covering the pore wall. The polymer template is then removed by oxygen plasma etching or pyrolysis in air resulting in palladium nanostructures whose large scale morphology mirrors that of the original template. The method adopted in this work is general and versatile so that it could easily be extended for patterning a variety of metallic materials into dot and wire arrays.

Responsive Hybrid Nanotubes Composed of Block Copolymer and Gold Nanoparticles

We demonstrate the facile fabrication of high aspect ratio nanotubes from responsive block copolymer, polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), and in situ grown gold nanoparticles. PS-b-P2VP nanotubes are fabricated by template-assisted assembly by utilizing cylindrical pores of porous alumina membranes followed by its dissolution. Solvent annealing was employed to facilitate ordered phase separation of the block copolymer with in the nanotube structure, followed by in situ formation of gold nanoparticles in the P2VP domains occupying the inner and outer shell of the nanotubes. Owing to the pH-sensitive nature of P2VP block, which exhibits a coil to globule transition above pH 3.6, the PS-b-P2VP−gold nanoparticles hybrid nanotubes show reversible changes in topology, size (up to 25%), and collective plasmon resonance appearance. We suggest that PS-b-P2VP−gold nanoparticles hybrid nanotubes with their pH-responsive properties, large surface-to-volume ratio, and through open pore can find intriguing applications in sensing and microfluidic systems.

Structures of the cylindrical and vesicular micelles of an P4VP-longer asymmetric PS-b-P4VP

The control of the self-assembled micellar morphology of amphiphilic block copolymers in a selective solvent has engrossed substantial interests in the theoretical and applied research fields. The micellar morphology of the block copolymers can be controlled by solvent selectivity and block copolymer composition. There are three main factors for controlling these morphologies, i.e., the stretching of core blocks, the repulsive interaction among corona chains, and the surface tension of the core/corona interface at the onset of micellization. The block copolymer can selfassemble into spherical, cylindrical, and vesicular micelles in a given solution. Among those micelles, the vesicle has fascinated scientists because of its diverse applications such as drug delivery diagnostics, electronics (chip nanopatterning), and catalysis (enzyme entrapment). Vesicles (generally kinetically trapped and non-equilibrium) are usually formed by low molecular materials such as surfactants or lipids. Wittmann et al. for the first time reported the polymeric vesicles of poly(styrene-b-isoprene) (PS-b-PI) and poly(styrene-b-ethylene oxide) (PS-b-PEO) in a mixture of ethyl benzene (a selective solvent for both blocks) and n-decane (a PI and PEO selective solvent). Schrage et al. investigated vesicular morphologies of quaternized poly(1-methyl-4vinyl pyridine-b-styrene) and poly(1,2-butadiene-b-(cesium methacrylate)) which were self-assembled in THF (nonselective for PS and poly(1,2-butadiene) but selective for charged blocks). Kesselman et al. also studied the change in the vesicular morphologies of PS-b-PI by controlling solvent selectivity in diethyl phthalate (less PS-selective) and dimethyl phthalate (highly PS-selective) mixtures. Putaux et al. investigated the vesicular morphologies of PS-b-PI in n-heptane and n-decane, both of which are selective solvents for PI. The PS-b-P4VP vesicles have been observed when the P4VP blocks were complexed with other small molecules. The vesicles of PS-b-P4VP in chloroform were observed when perfluorooctanoic acid (PFOA) was added to be complexed with P4VP. Peng et al. also reported another interesting research on the vesicular morphology of PS-b-P4VP in a complex form with low molecular-weight aliphatic acids (stearic acid, decanoate, acetic acid, and formic acid) in chloroform (non-selective for PS-b-P4VP and aliphatic acids but selective in a complex form). But, PS-bP4VP vesicles without complexation were rarely observed. The cylindrical morphology of an asymmetric PS(19600)b-P4VP(5100) (PS longer) in P4VP selective solvents was reported. However, the vesicles with asymmetric PS-b-P4VPs (P4VP longer) in PS selective solvents have not been reported. The block length of P4VP of the PS-b-P4VP is of prime importance because a number of valuable metals or small molecules could be complexed with the P4VP core of micelles (in the PS selective solvents) which is utilized as a nanoreactor. The screening of the solvent selectivity is necessary to check the possible micellar morphologies of the given block copolymer. Recently the micellar structures and their ordered structures of PS(12K)-b-P4VP(11.8K) and PS(19.6K)-b-P4VP (5.1 K) in the toluene/ethanol solvent system were reported. However, we believe that solvent selection for PS-b-P4VP is important in this study. We used the toluene/ethanol solvent system where toluene and ethanol are selective solvents for PS and P4VP, respectively. The toluene/ethanol system covers from PS-selective, to neutral, and to P4VPselective solvents although our previous THF/toluene system covers only from neutral to PS-selective solvents. Thus, we expected that a wider range of the micellar morphologies can be formed in the toluene/ethanol solvent system. The solvent selectivity was narrowly screened by changing the mixing ratio of the two solvents. But the vesicular structure was not found probably due to the less-asymmetric composition of the block copolymer and/or the less selectivity of the used solvent mixtures. The research in this article used a more-asymmetric block copolymer of PS (3.3K)-b-P4VP(18.7 K) (P4VP longer) in the toluene/ethanol mixtures to screen the solvent selectivity.

PVPylated Poly(alanine) as a New Thermogelling Polymer

In order to investigate poly(N-vinyl pyrrolidone) as an alternative to poly(ethylene glycol) in preparing a biomedical polymer, we synthesized a series of reverse thermogelling poly(N-vinyl pyrrolidone-b-alanine) (PVP-PA). The amphiphilic polymers consisting of the hydrophilic PVP block and the hydrophobic PA block formed micelles in water and the micelles aggregated as the temperature increased. FTIR spectroscopy, circular dichrosim spectroscopy, and 13C NMR spectroscopy showed that the aggregation behavior accompanied a change in PA conformation as well as a decrease in the molecular motion of PVP-PA. The sol-to-gel transition temperature decreased as the PA block length increased, PVP block length decreased, and l-alanine/dl-alanine ratio of PA increased. This paper suggests that PVP can be a promising alternative to poly(ethylene glycol) in designing a reverse thermogelling biomaterial.