Functionalization of amphiphilic coil-rod-coil triblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) with florescence moiety and C60

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride in the presence of pyridine. This led to the introduction of an active methylene group that is a site for further functionalization in the rod block. The Michael addition reaction of the triblock copolymer with 7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rod block. The fluorescent labeled copolymers were isolated in ∼94% yields. Similarly C₆₀ pendent was introduced to the rod block by the Bingel reaction. The yields of C₆₀ functionalized copolymers were ∼54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphologies, such as micelles and vesicles by simply changing the solvent. For the C₆₀ functionalized block copolymer, structural constraints in micelles and vesicles prevented C₆₀ pendents to aggregate.     

A novel biodegradable poly(p-dioxanone)-grafted poly(vinyl alcohol) copolymer with a controllable in vitro degradation

A novel biodegradable copolymer was synthesized from poly(vinyl alcohol) and poly(p-dioxanone) by ring-opening polymerization. The molecular structure of the copolymer was characterized by one- and two-dimensional NMR spectroscopy. The results of differential scanning calorimetry (DSC) show that the amphiphilic and comb grafted structure of the copolymer make its crystalline behavior different from that of the poly(p-dioxanone) homopolymer (PPDO). The in vitro degradation rate of the copolymers can be controlled via adjusting the number and length of PPDO segments of PVA-g-PPDO copolymers. The copolymer has a potential application in biomedical materials or in the controlled release of drug.     

Synthesis and micelle behavior of (PNIPAm-PtBA-PNIPAm)m amphiphilic multiblock copolymer

A linear amphiphilic multiblock copolymer (PNIPAm-PtBA-PNIPAm)_m was successfully synthesized by a two-step reversible addition-fragmentation transfer (RAFT) polymerization in the presence of a cyclic trithiocarbonate as RAFT agent. The micelle behavior of (PNIPAm-PtBA-PNIPAm)_m multiblock copolymer in aqueous solution was then investigated by means of normal TEM, cryo-TEM, static and dynamic light scattering. The morphology, size, and size distribution of (PNIPAm-PtBA-PNIPAm)_m micelles were found to be dependent on the initial concentration of multiblock copolymer in THF. Spherical micelles, associated aggregates of spherical micelles, cage-like micelles, layered structures, and vesicular micelles were experimentally observed, which were in good agreement with the prediction of theory and simulations on linear amphiphilic multiblock copolymer in selective solvent. The (PNIPAm-PtBA-PNIPAm)_m micelles also exhibit thermo-sensitive behavior in aqueous solution because of the PNIPAm blocks.     

Poly(methyl methacrylate-co-acrylonitrile)

Stable macroradicals of methyl methacrylate were prepared by the azobisisobutyronitrile-initiated polymerization of methyl methacrylate in hexane whose solubility parameter value (δ) differed from that of the macroradical by more than 1.8 hildebrand units and in 1-propanol at temperatures below its theta temperature (84.5°C). The rates of heterogeneous polymerization in hexane and 1-propanol were much faster than that of the homogeneous polymerization in benzene. Stable macroradicals were not obtained in benzene which was a good solvent nor at temperatures above the glass transition temperature (T_t) of the macroradicals. Thus, stable macroradicals of butyl methacrylate (T_g20°C) and and methyl acrylate (T_g3°C) were not obtained at a polymerization temperature of 50°C. Good yields of block copolymers of methyl methacrylate and acrylonitrile were obtained by the addition of acrylonitrile to the methyl methacrylate macroradical in methanol, ethanol, 1-propanol and hexane at 50°C. The rate of formation of the block copolymer decreased in these poor solvents as the differences between the solubility parameter of the solvent and macroradical increased.The block copolymer samples prepared at temperatures of 50°C and above were dissolved in benzene which is a non-solvent for acrylonitrile homopolymer, but is a good solvent for poly(methyl methacrylate) and the block copolymer. The presence of acrylonitrile and methyl methacrylate in the benzene-soluble macromolecule was demonstrated by pyrolysis gas chromatography, infra-red spectroscopy and differential thermal analysis.     

Long-tailed spherical aggregates formed from ABA triblock copolymer by changing the properties of selective solvent

A convenient method of tuning aggregate morphologies from amphiphilic block copolymer by adding second selective solvent is introduced in this paper. Some novel aggregate morphologies, i.e. hierarchical vesicles (and compound spherical micelles) with one or more tails, were formed by introducing a second selective solvent for core-forming blocks into the poly(4-vinyl pyridine)-b-polystyrene-b-poly(4-vinyl pyridine) ABA amphiphilic block copolymer/co-solvent/water systems. Addition of selective solvent (toluene) for core-forming blocks (PS blocks) has significant effect on the aggregate morphologies from the amphiphilic triblock copolymer. The aggregate morphologies changed from spheres to rods, long tailed solid large compound spheres, and to long tailed hierarchical vesicles by adding 0.5, 10 and 30 wt% of toluene to the organic solvent, respectively. There exists an aggregate morphological transition of the long tailed hierarchical vesicles to long tailed solid spheres by decreasing the content of toluene in the organic solvent mixture. The tails disappeared, and irregular vesicular and spherical structures were formed when the toluene content was 20 wt%. The toluene addition is expected to increase the stretching of the core-forming blocks (PS), and to modify the interfacial tension of core-corona interface, which are the main reasons for the aggregate morphology transition. To the best of our knowledge, these tailed vesicles and spherical morphologies have not been found in block copolymer aggregates system up to now.     

Photo-responsive block copolymer containing azobenzene group: Synthesis by reversible addition-fragmentation chain transfer polymerization and characterization

The design and synthesis of a new azobenzene-based methacrylate monomer (Azo-IEM) was demonstrated, and its polymerization behavior during reversible addition-fragmentation chain transfer (RAFT) polymerization was investigated. Well-defined homopolymer and amphiphilic block copolymer containing Azo-IEM monomeric units were successfully prepared as evidenced by NMR and GPC analysis. Moreover, the photo-triggered reversible isomerization of polymer products in selected solvents was investigated. Finally, TEM analysis showed that there were significant differences in the nanoparticle morphologies when the block copolymer samples were irradiated with different wavelengths of light (i.e., UV and visible). The size and shapes of the p(HEMA-b-Azo-IEM) polymer capable of transitions upon changes in Vis/UV light exposure which prepared from MeOH/CHCl₃ mixture solvent. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47870.     

Dissipative particle dynamics simulation study on complex structure transitions of vesicles formed by comb-like block copolymers

Vesicles are membrane-enclosed capsules that can store or transport substances. Their structures and the corresponding structural transitions are important to fulfill specific functions. Using dissipative particle dynamics method, we study the complex structure transitions of vesicles that are spontaneously formed by A₆(B₂)₃ type comb-like block copolymers. In the simulations, the interaction parameters between different components are tuned to mimic the variations of amphiphilicity of the block copolymers and the selectivity of the solvent which are experimentally tractable by, for example, a temperature quench. Complex vesicle structures are found in this research; their transitions, such as fission and reversal, are studied in detail with this dynamic simulation method. We find that the line tension plays a decisive role on the vesicle fission pathways. Moreover, the tube-like vesicles tend to transform to a special layered micelle structure, whereas the onion-shape vesicles tend to transform to reverse onion-shape vesicles when vesicle reversal takes place due to the variation of solvent selectivity.     

Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(?-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid)

Nanostructured poly(?-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP)/poly(acrylic acid) (PAA) interpolyelectrolyte complexes (IPECs) were prepared by casting from THF/ethanol solution. The morphological behaviour of this amphiphilic block copolymer/polyelectrolyte complexes with respect to the composition was investigated in a solvent mixture. The phase behaviour, specific interactions and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelle formation occurred due to the aggregation of hydrogen bonded P2VP block and polyelectrolyte (PAA) from non-interacted PCL blocks. It was observed that the hydrodynamic diameter (D_h) of the micelles in solution decreased with increasing PAA content up to 40 wt%. After 50 wt% PAA content, D_h again increased. The micelle formation in PCL-b-P2VP/PAA IPECs was due to the strong intermolecular hydrogen bonding between PAA homopolymer units and P2VP blocks of the block copolymer. The penetration of PAA homopolymers into the shell of the PCL-b-P2VP block copolymer micelles resulted in the folding of the P2VP chains, which in turn reduced the hydrodynamic size of the micelles. After the saturation of the shell with PAA homopolymers, the size of the micelles increased due to the absorption of added PAA onto the surface of the micelles.     

A novel amphiphilic AB2 star copolymer synthesized by the combination of ring-opening metathesis polymerization and atom transfer radical polymerization

A new amphiphilic AB₂ star copolymer was synthesized by the combination of ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP). Two different routes (methods A and B) were employed firstly to prepare the poly(oxanorbornene)-based monotelechelic polymers as the hydrophobic arm bearing dibromo-ended group via ROMP in the presence of two different terminating agents catalyzed by first generation Grubbs catalyst. The values of capping efficiency (CE) of the polymers were determined by NMR, which were 94% and 67% for methods A and B, respectively. Then, the dibromo-ended ROMP polymers were used as the macroinitiators for ATRP of 2-(dimethylamino)ethyl methacrylate (DMAEMA) to produce two hydrophilic arms. The prepared amphiphilic AB₂ star copolymers poly(7-oxanorborn-5-ene-exo,exo-2,3-dicarboxylic acid dimethyl ester)-block-bis[poly(2-(dimethylamino)ethyl methacrylate)] (PONBDM_n-b-(PDMAEMA_m)₂) with a fixed chain length of hydrophobic PONBDM and various hydrophilic PDMAEMA chain lengths can self-assemble spontaneously in water to form polymeric micelles, which were characterized by dynamic light scattering, atom force microscopy, and transmission electron microscopy measurements.     

Self-assembly of pH-responsive and fluorescent comb-like amphiphilic copolymers in aqueous media

“Comb-like” graft copolymers, consisting of a poly((N-vinylcarbazole)-co-(4-vinylbenzyl chloride)) (P(NVK-co-VBC)) copolymer backbone from free radical polymerization and poly(((2-dimethylamino)ethyl methacrylate)-co-(tert-butyl acrylate)) (P(DMAEMA-co-tBA)) side chains from atom transfer radical polymerization (ATRP), were hydrolyzed to produce the acrylic acid (AAc)-containing “comb-like” graft copolymers of P(NVK-co-VBC)-comb-P(DMAEMA-co-AAc). The amphiphilic copolymers possess a fluorescent hydrophobic P(NVK-co-VBC) backbone and pH-sensitive hydrophilic P(DMAEMA-co-AAc) side chains. Arising from acid-base interaction of the hydrophilic side chains, the copolymers can self-assemble into pH-responsive fluorescent and multi-walled hollow vesicles of well-defined morphology in aqueous media. The size and layered wall thickness of the vesicles are also dependent on the length of the copolymer side chains, while the number of wall layers is dependent on the concentration of the vesicles in the aqueous media. In comparison, a N-isopropylacrylamide (NIPAAm)-containing comb-like amphiphilic copolymer (P(NVK-co-VBC)-comb-P(NIPAAm-co-DMAEMA)) of similar structure, albeit with non-interacting hydropholic side chains, self-assembles only into temperature and pH-responsive single-shelled hollow nanoparticles in aqueous media.     

Self-assembly microstructures of amphiphilic polyborate in aqueous solutions

A new type of amphiphilic homo-polyborate was synthesized with the surface tension (γ_CMC) in deionized water 31.2 mN m⁻¹ (pH = 7, 25 °C), which is similar to low-molecule surfactants. Self-assembly behaviors of the polyborate in aqueous solutions were investigated by TEM. Self-aggregation morphologies of the polyborate can be controlled by the concentration of NaCl. With the increase of the concentration of NaCl, micromorphologies from the amphiphilic polyborate transformed from spherical polymeric vesicles to tree-like or even vertebra-like morphologies. It was also found that, the polymeric vesicles show a strong stability against ethanol addition, even when the concentration of ethanol increases to a volume ratio of 33%, they still maintain good micromorphologies. A possible mechanism of vesicle formation was proposed based on packing parameter theory.     

Synthesis and characterization of well-defined hydrophilic block copolymer brushes by aqueous ATRP

Homopolymer brushes of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA) and poly(N-isopropylacrylamide)(PNIPAM) grown on atom transfer radical polymerization (ATRP) initiator functionalized latex particles were used as macroinitiators for the synthesis of PDMA-b-PNIPAM/PMEA, PMEA-b-PDMA/PNIPAM and PNIPAM-b-PDMA block copolymer brushes by surface initiated aqueous ATRP. The grafted homopolymer and block copolymer brushes were analyzed for molecular weight, molecular weight distribution, chain grafting density, composition and hydrodynamic thickness (HT) using gel permeation chromatography-multi-angle laser light scattering, ¹H NMR, particle size analysis and atomic force microscopy (AFM) techniques. The measured graft molecular weight increased following the second ATRP reaction in all cases, indicating the second block had been added. Chain growth depended on the nature of the monomer used for block copolymerization and its concentration. Unimodal distribution of polymer chains in GPC with non-overlap of molar mass-elution volume curves implied an efficient block copolymerization. This was supported by the increase in HT measured by particle size analysis, equilibrium thickness observed by AFM and the composition of the block copolymer layer by ¹H NMR analysis, both in situ and on cleaved chains in solution. ¹H NMR analysis of the grafted latex and cleaved polymers from the surface demonstrated that accurate determination of the copolymer composition by this method is possible without detaching polymer chains from surface. Block copolymer brushes obey the same power law dependence of HT on molecular weight as homopolymer brushes in good solvent conditions. The NIPAM-containing block copolymer brushes were sensitive to changes in the environment as shown by a decrease in HT with increase in the temperature of the medium.     

Amphiphilic cylindrical brushes with poly(acrylic acid) core and poly(n-butyl acrylate) shell and narrow length distribution

Core-shell cylindrical polymer brushes with poly(t-butyl acrylate)-b-poly(n-butyl acrylate) (PtBA-b-PnBA) diblock copolymer side chains were synthesized via ‘grafting from’ technique using atom transfer radical polymerization (ATRP). The formation of well-defined brushes was confirmed by GPC and ¹H NMR. Multi-angle light scattering (MALS) measurements on brushes with 240 arms show that the radius of gyration scales with the degree of polymerization of the side chains with an exponent of 0.57±0.05. The hydrolysis of the PtBA block of the side chains resulted amphiphilic cylindrical core-shell nanoparticles. In order to obtain a narrow length distribution of the brushes, the backbone, poly(2-hydroxyethyl methacrylate), was synthesized by anionic polymerization in addition to ATRP. The characteristic core-shell cylindrical structure of the brush was directly visualized on mica by scanning force microscopy (SFM). Brushes with 1500 block copolymer side chains and a length distribution of l_w/l_n=1.04 at a total length l_n=179 nm were obtained. By choosing the proper solvent in the dip-coating process on mica, the core and the shell can be visualized independently by SFM.     

Multiple morphologies of self-assembled star aggregates of amphiphilic PEO-b-PNPMA diblock copolymers in solution, synthesis and micellization

Novel amphiphilic block copolymers, poly(ethylene oxide)-b-poly(p-nitrophenyl methacrylate) (PEO-b-PNPMA) with controlled molecular weights and narrow molecular weight distribution were successively synthesized by ATRP of NPMA using PEO-Br as initiator. Self-assembling of the diblock copolymer PEO₁₁₃-b-PNPMA₂₈ in the different solvent mixtures yielded various morphologies of star micelle-like aggregates, such as spheres, vesicles, cauliflower-like aggregates and rod-like aggregates, which are determined by the nature of the common solvents and the selective solvents. Thus the critical selective solvent contents and the solvent contents in PNPMA-rich phase were measured, and they have the following order: ethanol > methanol > water, and THF > CH₃NO₂ > DMSO. The probable self-assembling mechanism is discussed. This method is convenient for preparation of multiple morphological star micelle-like aggregates in solution, especially from the amphiphilic block copolymers with relatively longer block shell.     

Well-defined skeletal macroporous polymer monoliths fabricated with a novel type of amphiphilic diblock copolymer as a phase separator

A novel type of amphiphilic diblock copolymer consisting of butyl methacrylate (BMA) block and glycidyl methacrylate (GMA) block (BG copolymer) was successfully synthesized via atom transfer radical polymerization (ATRP) and then utilized as a phase separator to control the porous structure of poly(butyl methacrylate-co-ethylene dimethacrylate) (poly(BMA-co-EDMA)) monoliths. It has been found that the addition of the BG copolymer had a great impact on the polymerization of the monoliths. When the amount of the BG copolymer added into the synthesizing solution was changed, the porous structure could be varied from aggregated microglobular structure to well-defined three-dimensional (3D) skeletal structure. The porous structure was characterized by scanning electron microscope, mercury intrusion porosimetry and nitrogen adsorption measurement. Finally, the separation of proteins demonstrated its potential applications in proteome research.     

Effects of neutral solvent addition on the body-centered cubic spheres of block copolymers

We employ self-consistent mean-field (SCMF) theory in studying the body-centered cubic (bcc) spheres of block copolymers in the presence of a neutral solvent. First we examine the accuracy of the dilution approximation then analyze the dependence of the bcc structural sizes with copolymer volume fraction ?, the interaction parameter χ_AB, and degree of copolymerization N. Our results reveal that both distribution of each component and the micro-structural length scales are greatly influenced by each parameter ?, χ_AB, and N. As expected, with decreasing ?, more solvent distributes non-uniformally in the segregated domains, therefore deviation from the dilution approximation increases. This also suggests that when the effective segregation parameter ?χ_ABN is fixed, a larger deviation is expected as χ_ABN increases (i.e. ? decreases). Although when both χ_ABN and ? are fixed, decreasing N (i.e. increasing χ_AB) enlarges the deviation from the dilution approximation. Furthermore, this solvent non-uniformity behavior is so significant that it even affects the dependence of the domain spacing L^* and the matrix length Λ^* with respect to (χ_AB)_effN=?χ_ABN near the ODT. When the systems are in molten state and/or in the concentrated regime, both L^* and Λ^* exhibit a sharp increase behavior as ODT is approached, due to many of the minority blocks being pulled from the spherical domains and swelling the matrix. With increasing solvent amount and/or χ_ABN, we observe that the increase of the degree for the minority blocks pulled from the spheres into the matrix near the ODT is not as significant as that in the melt. As such, the sharp increase behavior in L^* as well as Λ^* near the ODT smoothens and even disappears.     

Morphology and mechanical properties of nanostructured blends of epoxy resin with poly(?-caprolactone)-block-poly(butadiene-co-acrylonitrile)-block-poly(?-caprolactone) triblock copolymer

Poly(?-caprolactone)-block-poly(butadiene-co-acrylonitrile)-block-poly(?-caprolactone) triblock copolymer was synthesized via the ring-opening polymerization of ?-caprolactone with dihydroxyl-terminated butadiene-co-acrylonitrile random copolymer. The amphiphilic block copolymer was used to toughen epoxy thermosets via the formation of nanostructures. The morphology of the thermosets was investigated by means of atomic force microscopy, transmission electronic microscopy and small-angle X-ray scattering. It was judged that the formation of the nanostructures in the thermosets follows the mechanism of reaction-induced microphase separation. The thermal and mechanical properties of the nanostructured thermosets were compared to those of the ternary blends composed of epoxy, poly(butadiene-co-acrylonitrile) and poly(?-caprolactone) with the identical content of the modifiers. It is noted that at the same composition the nanostructured thermosets displayed higher glass transition temperatures (T_gs) than the ternary blends, which was evidenced by dynamic mechanical analysis. The fracture toughness of the thermosets was evaluated in terms of the measurement of critical stress field intensity factor (K_1C). It is noted that at the identical composition the nanostructured blends significantly displayed higher fracture toughness than the ternary blends. In addition, the K_1C of the nanostructured thermosets attained the maximum with the content of the modifier less than their counterpart of ternary blending.     

Interdiffusion and phase behavior at homopolymer/random copolymer interfaces

Interdiffusion in polymer bilayers of polystyrene (PS) and the statistically random copolymer poly(styrene-r-4-bromostyrene) (PBS), (C₈H_(8−x)Br_x)_N, where x is the mole fraction of brominated repeat units in the copolymer and N the degree of polymerization, was studied using Rutherford backscattering spectroscopy (RBS). PS/PBS bilayers with 0.04<x<0.63 and the ratio N_PS/N_PBS varied from 0.06<N_PS/N_PBS<18.1 were examined. PBS volume fraction versus depth profiles were obtained from the evolution of the bromine peak in the RBS spectra. It is shown that as the phase boundary is approached, interdiffusion occurs until layer compositions indicative of binodal conditions are reached. These observations are in very good agreement with phase diagrams obtained using Flory-Huggins theory and a PS/PBS interaction parameter measured using small angle X-ray scattering. For N_PS/N_PBS≠1, the mobility is dictated by the faster diffusing (lower N) component, resulting in an interface which moves toward the faster diffusing component. This result is consistent with fast mode theory; equilibrium conditions correspond to the asymmetry of the phase diagrams. Mutual diffusion coefficients were determined by comparison of the RBS data to a mean-field interdiffusion model using the fast mode expression for mobility. The mutual diffusion coefficient is shown to decrease with increasing N and x and increase with temperature. The implications of this miscibility dependence of the interdiffusion behavior, based on both composition of the copolymer and degree of polymerization, are discussed in the context of strengthening homopolymer/random copolymer interfaces.     

Co-aggregation process of poly(ethylene oxide)-b-polybutadiene/poly(acrylic acid) based on evolution of interpolymer hydrogen bonding in solutions

The co-aggregation process of a diblock copolymer poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) and a homopolymer poly(acrylic acid) (PAA) in solutions was studied. The number-average molecular weights of both the PEO and PB blocks are 5100 g/mol; the weight-average molecular weight of PAA is ∼2000 g/mol. The co-aggregation was induced by adding a PB selective solvent (i.e., alkane or cycloalkane) into the THF solution of the two polymers, with the processes characterized by turbidity, ¹H NMR, dynamic light scattering, and microscopy experiments. During the selective solvent titration, the solution underwent a macro-phase separation that was mainly related to PAA, followed by a micro-phase separation that corresponded to the formation of vesicles with the shell of PB block and the core of PAA/PEO complex. The experimental results indicated that the evolution of interpolymer hydrogen bonding complexation between the PAA and PEO blocks determined the co-aggregation process. The loose and soluble interpolymer complex could be formed at rather low selective solvent content (f). The complexation was enhanced with increasing f, resulting in “redissolving” the PAA-rich domains in the blend solutions. Afterwards, the more compact PAA/PEO complex chemically linked with a soluble PB block acted as the building blocks to form the vesicles at higher f.     

Size controlled polymersomes by continuous self-assembly in micromixers

The formation of vesicles based on the self-assembly of amphiphilic poly(butadiene)-b-poly(ethylene oxide) (PB₁₃₀-b-PEO₆₆) block copolymer in water has been studied using THF as co-solvent. To obtain a highly controlled mixing process for the polymer/THF- and the water-phase, we employed micro mixers with different mixing geometries. The high impact of this preparation method on the self-assembling process was verified by TEM and DLS characterization of the obtained structures. Spherical micelles, vesicles and worm-like micelles were found depending on the parameters of mixing. By additional parameter adjustment in the vesicle regime, the size of the assembled vesicles was controlled between 45 and 100 nm. This demonstrates the continuous preparation of narrowly distributed vesicle structures with controlled sizes.