Low Molecular Weight Block Copolymers Research Articles

Formation of giant polymer vesicles by simple double emulsification using block copolymers as the sole surfactant

Polymer vesicles that mimic the function of cell membranes can be obtained through the self-assembly of amphiphilic block copolymers. The cell-like characteristics of polymer vesicles, such as the core-shell structure, semi-permeability and tunable surface chemistry make them excellent building blocks for artificial cells. However, the standard preparation methods for polymer vesicles can be time consuming, require special equipment, or have low encapsulation efficiency for large components, such as nanomaterials and proteins. Here, we introduce a new encapsulation strategy based on a simple double emulsification (SDE) approach which allows giant polymer vesicles to be formed in a short time and with basic laboratory equipment. The SDE method requires a single low molecular weight block copolymer that has the dual role of macromolecular surfactant and membrane building block. Giant polymer vesicles with diameters between 20-50 μm were produced, which allowed proteins and nanoparticles to be encapsulated. To demonstrate its practical application, we used the SDE method to assemble a simple artificial cell that mimics a two-step enzymatic cascade reaction. The SDE method described here introduces a new tool for simple and rapid fabrication of synthetic compartments.

Enhanced microphase separation of thin films of low molecular weight block copolymer by the addition of an ionic liquid.

We report on the use of a selective, non-volatile ionic liquid (IL) to enhance the self-assembly via solvent annealing of a low molecular weight block copolymer (BCP) of styrene and 2-vinylpyridine (2VP) suitable for generating sub-10 nm features. Diblock and triblock copolymers of different molecular weights of styrene and 2VP are individually blended with the IL and then solvent annealed in acetone, a non-preferential solvent for the BCPs. Differential scanning calorimetry indicates that the IL selectively resides in the 2VP block of the BCP, resulting in a decrease of the block's Tg and an increase of the effective Flory-Huggins parameter (χeff) of the BCP. The influence of the IL on the non-preferential window of a random copolymer brush used to treat the substrate for self-assembly of the BCPs is also analyzed. Well-defined lamellar patterns form when the optimal weight ratio of IL (∼1%) is added to the BCPs. A detailed analysis of the orientational correlation length and pitch size of the BCPs quantitatively shows that the addition of the IL enhanced the microphase separation of the low molecular weight version of the BCP. Subsequent treatment of the self-assembled BCP with sequential infiltration synthesis yields sub-10 nm AlOx lines.

Isothermal crystallization kinetics of biodegradable poly(lactic acid)/poly(ε‐caprolactone) blends compatibilized with low‐molecular weight block copolymers

The isothermal crystallization kinetics of biodegradable blends made of poly(lactic acid) (PLA) and poly(ε‐caprolactone) (PCL) compatibilized with two different low molecular weight block copolymers, that is, ε‐caprolactone/tetramethylene ether glycol and ε‐caprolactone/aliphatic polycarbonate (CB), was done. Blends were prepared by melt mixing in an extruder, while isothermal crystallization kinetics and morphologies were investigated by thermal (differential scanning calorimetry) and thermo‐optical (quantitative polarized light optical microscopy [qPLOM]) quantitative methods. Data were analyzed using the Avrami equation, revealing 2D and 3D growth and simultaneous heterogeneous nucleation. The presence of low molecular weight compatibilizers, that is, 2,000 g mol−1, accelerated the PLA crystallization rate by two to threefold when compared with neat PLA, with high degrees of crystallinity (40–43%) as confirmed by PLOM images. The activation energy (Ea) showed that PCL inhibits PLA crystallization; however, the addition of block copolymers used as compatibilizers of the blends reduced Ea values, increasing the chain mobility of PLA and thus increasing the crystallization rate. POLYM. ENG. SCI., 59:E161–E169, 2019. © 2018 Society of Plastics Engineers

Constructing a molecular theory of self-assembly: Interplay of ideas from surfactants and block copolymers

Low molecular weight surfactants and high molecular weight block copolymers display analogous self-assembly behavior in solutions and at interfaces, generating nanoscale structures of different shapes. Understanding the link between the molecular structure of these amphiphiles and their self-assembly behavior has been the goal of theoretical studies. Despite the analogies between surfactants and block copolymers, models predicting their self-assembly behavior have evolved independent of one another, each overlooking the molecular feature considered critical to the other. In this review, we focus on the interplay of ideas pertaining to surfactants and block copolymers in three areas of self-assembly. First, we show how improved free energy models have evolved by applying ideas from surfactants to block copolymers and vice versa, giving rise to a unitary theoretical framework and better predictive capabilities for both classes of amphiphiles. Second we show that even though molecular packing arguments are often used to explain aggregate shape transitions resulting from self-assembly, the molecular packing considerations are more relevant in the case of surfactants whereas free energy criteria are relevant for block copolymers. Third, we show that even though the surfactant and block copolymer aggregates are small nanostructures, the size differences between them is significant enough to make the interfacial effects control the solubilization of molecules in surfactant micelles while the bulk interactions control the solubilization in block copolymer micelles. Finally, we conclude by identifying recent theoretical progress in adapting the micelle model to a wide variety of self-assembly phenomena and the challenges to modeling posed by emerging novel classes of amphiphiles with complex biological, inorganic or nanoparticle moieties.

Macromolecular Coupling in Seconds of Triazolinedione End-Functionalized Polymers Prepared by RAFT Polymerization.

The ultrafast and additive-free triazolinedione-click reaction with electron rich (di)enes is a powerful method for the ultrafast ligation of polymer segments. A versatile method is described for the introduction of clickable TAD end groups in various polymer segments, using reversible addition-fragmentation chain transfer polymerization. These triazolinedione-functionalized prepolymers were subsequently used for macromolecular functionalization with a low molecular weight diene and block copolymer synthesis of different types within seconds, at ambient conditions, through the coupling with diene-functionalized polymers such as poly(ethylene glycol) and poly(isobornyl acrylate).

Self-organization of homopolymer brush- and mixed homopolymer brush-grafted silica nanoparticles in block copolymers and polymer blends

In this report, hierarchical self-organization of poly(tert-butyl acrylate) (PtBA) brush- and mixed PtBA/polystyrene (PS) brush-grafted silica nanoparticles (NPs) in PtBA-b-PS block copolymer and PtBA/PS polymer blend matrices having different molecular weights was investigated using transmission electron microscopy (TEM). Driven by the entropic interactions between the brushes and the polymer matrices (i.e., wet- and dry-brush effects), various morphologies were observed. First, PtBA brush-grafted 67 nm silica NPs could induce an onion-like structure in a low molecular weight block copolymer matrix because of the wet-brush effect, low particle curvature, and low bending rigidity of the block copolymer lamellae. On the contrary, PtBA brush-grafted 23 nm silica NPs could not induce the onion-like structure, primarily owing to the high particle curvature and high bending penalty of the block copolymer lamellae. Second, the attempt to use ordered block copolymer lamellae to direct microphase separation of mixed homopolymer brushes with a goal of achieving ordered nanopatterns was unsuccessful, likely because the mixed PtBA/PS brushes on NPs underwent microphase separation prior to the ordering of block copolymers during the solution-casting process. Third, in polymer blends, the wet- and dry-brush entropic effects played an important role. In the wet-brush regime, the hairy NPs were uniformly distributed in the preferred polymer phase. In the dry-brush regime, the hairy NPs were driven to the interfaces of immiscible PtBA/PS domains. The knowledge gained on hierarchical self-organization of homopolymer brush- and mixed homopolymer brush-grafted NPs in block copolymer and polymer blend matrices will help us design better polymer nanocomposites with precisely controlled locations of NPs.

Synthesis of Reactive Polymers for Acrolein Capture Using AGET ATRP.

Acrolein is a toxic metabolite of the anticancer agent cyclophosphamide (CP). Current strategies to mitigate acrolein toxicity are insufficient, and in this brief article, we report the synthesis of well-defined low molecular weight block copolymers using activators generated by electron transfer atom transfer radical polymerization (AGET ATRP) capable of reacting with the cytotoxic small molecule acrolein. Acrolein reactivity was introduced into the block copolymers via incorporation of either (a) aminooxy or (b) sulfhydryl groups. The cytoprotective effect of the polymers was compared to sodium 2-sulfanylethanesulfonate (mesna) the current gold standard for protection from CP urotoxicity, and we found that the polymers bearing sulfhydryl moieties demonstrated superior cytoprotective activity.

Macrophase Separation of Blends of Diblock Copolymers in Thin Films

Symmetric poly(styrene-b-methyl methacrylate) block copolymers (BCP) with total molecular weights of 129–412 kg/mol were blended with a symmetric 44 kg/mol BCP. This set of polymers covers the parameter space of molecular weight ratios of the large vs small BCP in the pair, R = Mn,L/Mn,S from 2.9 to 9.4. The blends macrophase separate into a nearly pure small period phase and a blended large period phase when R is greater than 5. The influence of film confinement on the phase behavior of these blends was analyzed by comparing the morphology within films on nonpreferential substrates and bulk samples using scanning electron microscopy and X-ray scatterning techniques. In the thin film, the large period phase has approximately equal volume fraction of the high and low molecular weight BCP, but in the bulk, the large period phase is more concentrated with the high molecular weight BCP. This difference in the saturation composition not only narrows the composition window for macrophase separation but also res...

Effect of Crystallization on Proton Transport in Model Polymer Electrolyte Membranes

Polymer electrolyte membranes with bicontinuous microphases comprising soft hydrated domains and mechanically robust hydrophobic domains are used in a wide range of electrochemical devices including fuel cells and electrolyzers. The self-assembly, water uptake, and proton conductivity of model block copolymer electrolytes with semicrystalline hydrophobic blocks were investigated. A series of sulfonated polystyrene-block-polyethylene (PSS–PE) copolymers were synthesized to probe the interplay between crystallization, morphology, hydration, and proton transport. In block copolymer systems with amorphous hydrophobic blocks, it has been shown that higher water update and proton conductivity are obtained in low molecular weight systems. However, crystallization is known to disrupt the self-assembly of low molecular weight block copolymers. We found that this disruption results in lower water uptake and proton conductivity. Increasing molecular weight results in less morphological disruption and improvement in performance.

Ionic Conductivity of Low Molecular Weight Block Copolymer Electrolytes

The ionic conductivity and glass transition temperatures of nanostructured block copolymer electrolytes composed of polystyrene-b-poly(ethylene oxide) (SEO) doped with lithium bis(trifluoromethanesulfone)imide (LiTFSI) were studied in the small molecular weight limit (between 2.7 and 13.7 kg mol–1). In this range, the annealed conductivity exhibits a nonmonotonic dependence on molecular weight, decreasing with increasing molecular weight in the small molecular weight limit before increasing when molecular weight exceeds about 10 kg mol–1. We show that annealed electrolyte conductivity is affected by two competing factors: the glass transition temperature of the insulating polystyrene (PS) block and the width of the conducting poly(ethylene oxide) (PEO) channel. In the low molecular weight limit, all ions are in contact with both PS and PEO segments. The intermixing between PS and PEO segments is restricted to an interfacial zone of width, λ. Our experiments suggest that λ is about 5 nm. The fraction of io...

Architectural layer-by-layer assembly of drug nanocapsules with PEGylated polyelectrolytes.

150-200 nm diameter capsules containing 60-70 wt % of poorly soluble drugs, paclitaxel and camptothecin, were produced by layer-by-layer (LbL) assembly on drug nanocores in a solution containing uncharged stabilizers. Optimization of capsule shell architecture and thickness allowed for concentrated (3-5 mg/mL) colloids that are stable in isotonic salt buffers. Nanoparticle aggregation during the washless LbL-assembly was prevented by using low molecular weight block-copolymers of poly(amino acids) (poly-L-lysine and poly-L-glutamic acid) with polyethylene glycol (PEG) in combination with heparin and bovine serum albumin at every bilayer building step. Minimal amounts of the polyelectrolytes were used to recharge the surface of nanoparticles in this non-washing LbL process. Such PEGylated shells resulted in drug nanocapsules with high colloidal stability in PBS buffer and increased protein adhesion resistance. The washless LbL polyelectrolyte nanocapsule assembly process, colloidal stability and nanoparticle morphology were monitored by dynamic light scattering and electrophoretic mobility measurements, UV-vis spectroscopy, TEM, SEM and laser confocal microscopy imaging.

Plasticization of poly(lactide) with blends of tributyl citrate and low molecular weight poly( d, l-lactide)-b-poly(ethylene glycol) copolymers

Polylactide (PLA) is a potential candidate for the partial replacement of petrochemical polymers because it is biodegradable and produced from annually renewable resources. Characterized by its high tensile strength, unfortunately the brittleness and rigidity limit its applicability. For a great number of applications such as packaging, fibers, films, etc., it is of high interest to formulate new PLA grades with improved flexibility and better impact properties. In order to develop PLA-based biodegradable packaging, the physico-mechanical properties of commercially available PLA should be modified using biodegradable plasticizers. To this end, PLA was melt-mixed with blends of tributyl citrate and more thermally stable low molecular weight block copolymers based on poly( d, l-lactide) and poly(ethylene glycol) of different molecular weights and topologies. The copolymers have been synthesized using a potassium based catalyst which is expected to be non toxic and were characterized by utilization of TGA, GPC and NMR techniques. The effect of the addition of up to 25 wt% plasticizer on the thermo-mechanical properties of PLA was investigated and the results were correlated with particular attention to the relationship between properties and applications. To confirm the safety of the catalyst used for the preparation of the copolymers, in vitro cytotoxicity tests have been carried out using MTS assay and the results show their biocompatibility in the presence of the fibroblast cells. Compost biodegradation experiments carried out using neat and plasticized PLA have shown that the presence of plasticizers accelerates the degradation of the PLA matrix.

Fabrication of Thin Oriented Organic Layers and Micropatterns by Zone Casting

AbstractZone casting—a special solution processing method for obtaining highly ordered continuous layers of soluble organic materials—can also be applied for fabricating patterned layers. In this method, solution is continuously supplied onto a substrate that is withdrawn at a controlled rate, and the solvent evaporates from the surface of a meniscus formed between a flat nozzle and the substrate. Due to the gradient of a solute concentration, its solidification proceeds within the narrow zone localized in the meniscus. In this way solute molecules can be self‐assembled at surfaces into different anisotropic nanostructures (columns, rods, lamellae, or microcrystals) uniaxially aligned parallel to the substrate. Under some conditions various phenomena like dewetting, fingering instability, stick‐slip motion, etc., come into play, leading to various periodic perturbations in the film deposition. Examples of continuous and patterned layers of different materials such as low molecular weight (semi)conductors, chromophores, discotic molecules, and block copolymers, which can successfully be processed using this technique, are shown. Typical layers obtained through this method are shown to have good properties as active layers in field effect transistors.

Low molecular weight block copolymers as plasticizers for polystyrene

AbstractPolystyrene‐b‐alkyl, polystyrene‐b‐polybutadiene‐b‐polystyrene, and polystyrene‐b‐poly(propylene glycol)monotridecyl ether were synthesized using macro initiators and atom transfer radical polymerization or by esterifications of homopolymers. The aim was a maximum molecular weight of 4 kg/mol and minimum polystyrene content of 50 w/w %, which by us is predicted as the limits for solubility of polystyrene‐b‐alkyl in polystyrene. DSC showed polystyrene was plasticized, as seen by a reduction in glass transition temperature, by block copolymers consisting of a polystyrene block with molecular weight of approximately 1 kg/mol and an alkyl block with a molecular weight of approximately of 0.3 kg/mol. The efficiency of the block copolymers as plasticizers increases with decreasing molecular weight and polystyrene content. In addition, polystyrene‐b‐alkyl is found to be an efficient plasticizer also for polystyrene‐b‐polyisoprene‐b‐polystyrene (SIS) block copolymers. The end use properties of SIS plasticized with polystyrene‐b‐alkyl, measured as tensile strength, is higher than for SIS plasticized with dioctyl adipate. The polystyrene‐b‐polybutadiene‐b‐polystyrene and polystyrene‐b‐poly(propylene glycol)monotridecyl ether series were only partially soluble in polystyrene and insoluble in the polystyrene phase of SIS. For the lowest molecular weight samples, this leads to measurable plasticization of polystyrene but no plasticization of SIS. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 981–991, 2005

The Influence of Block Copolymers on Silica-Filled Polyisoprene

Abstract In the present work, precipitated silica, liquid polyisoprene, and PDMS-PI block copolymer are mixed in different formulations. The dynamic rheological properties of the resulting uncured materials are measured and are used to assess filler-filler networking. In small-amplitude tests, composites containing PDMS-PI exhibit lower values of the dynamic storage modulus (G′) than those observed in control samples prepared without copolymer additives. The control samples remain stable during several months of storage, while samples containing low molecular weight block copolymers show evidence of an aging process. The rheological manifestations of this aging include a pronounced decrease in small-amplitude G′ with time, accompanied by weakening amplitude-dependence of both G′ and G″. These suggest a progressive reduction of filler-filler interactions, presumably caused by the development of a block copolymer layer at the filler-polymer interface. The make-up of that interfacial region is investigated by means of magic-angle spinning solid-state 1H NMR experiments carried out on bound polymer fractions that survive extraction in a good solvent. Spin-spin relaxation times and the intensities of the relaxation modes associated with these characteristic times are used to identify the proportion of both PDMS and polyisoprene among the segments strongly immobilized by close contact with the silica surface and among the rest of the bound (un-extracted) interfacial layer. The changes in mechanical properties are correlated not with the overall composition of the bound polymer fraction, but with the increase in PDMS segments in close contact with the filler surface.

Characterization of poly(ethylene oxide)-b-poly(L-lactide) block copolymer by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A poly(ethylene oxide)-b-poly(L-lactide) diblock copolymer (PEO-b-PLLA) is characterized by matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and a block length distribution map is constructed. Although the MALDI-TOF mass spectrum of PEO-b-PLLA is very complicated, most of the polymer species were identified by isolating the overlapped isotope patterns and by fitting the overlapped peaks to the Schulz-Zimm distribution function. Reconstructed MALDI-TOF MS spectrum was nearly identical to the measured spectrum and this method shows its potential to be developed as an easy and fast analysis method of low molecular weight block copolymers.

Dynamics of Poly(styrene-b-2-vinylpyridine) in Toluene

Low molecular weight specifically labeled block copolymer poly(β-deuteriostyrene-b-2-vinylpyridine) (β-DSVP) and poly(α-deuteriostyrene) (α-DS) (7600 and 10000 g/mol with polydispersities of 1.15 and 1.7, respectively) have been prepared by anionic polymerization. Deuterium NMR relaxation times and self-diffusion coefficients of DSVP and DS were measured for the polymers in solution as a function of concentration. The relaxation times were interpreted through the use of the Hall−Helfand model for chain dynamics. In toluene, the dynamics of the styrene segments in block copolymers were different than in homopolystyrene. The effect of concentration on segmental motions was more pronounced for the low molecular weight block copolymers than it was for the homopolymer above 15 wt %. The results indicated that the short-range motions were similar for the homo- and copolymer while the long-range motions were different. Self-diffusion coefficients for the block copolymer also showed a sharp decrease at concentrat...

A Block Copolymer from Polystyrene and Columnar Liquid-Crystalline Poly(diethylsiloxane)

A block copolymer from polystyrene (PS) and poly(diethylsiloxane) (PDES) has been synthesized by sequential anionic polymerization. By DSC, the PDES block was shown to exist in the columnar mesophase at ambient temperatures. By transmission electron microscopy and scanning force microscopy on thin films, the block copolymer was shown to build regular structures, showing a large periodicity, which is unusual for relatively low molecular weight block copolymers and must be ascribed to the fact that the PDES block exists in the mesophase.

Microphase vs. macrophase formation in homopolymer/block copolymer blends with exothermic interaction

The formation of a microphase and a macrophase in solvent-cast homopolymer/block copolymer blends, where the homopolymer is exothermically miscible with one of the block copolymer segments, is demonstrated to depend on the molecular weights of the homopolymer and the incompatible segment of the block copolymer, in spite of the exothermic mixing. Binary blends of poly(vinyl chloride) (PVC) with poly( n-butyl-methacrylate- b-styrene) (PnBMA- b-PS) block copolymer, solvent cast from methyl ethyl ketone, were investigated. By varying the molecular weight, we vary the thermodynamic product N χPVC/PS which controls the phase morphology. Here N = N PVC N PS /N PVC 1 2 + N PS 1 2 ) 2 , where N PVC and N PS are the degrees of polymerization of PVC and PS block copolymer segment, respectively, and χ PVC/PS is the Flory—Huggins parameter which characterizes the repulsive interaction between PVC and PS. Microphase separation is observed when N χPVC/PS is small; and when N χPVC/PS is large, macrophase or macrophase-induced microphase separation occurs. The routes of phase formation are discussed using equilateral triangular phase diagrams, and a morphology diagram is constructed for homopolymer/block copolymer blends. Microphase formation is observed even when the molecular weight of the homopolymer is much larger than that of the miscible block copolymer segment, and when the homopolymer is polydisperse. Indeed, since macrophase formation occurs when N is large, it is clear that low molecular weight block copolymer is necessary to form the microphase when a homopolymer of high molecular weight is present.

Properties of low molecular weight block copolymers. 3. Mixtures of styrene-dimethylsiloxane diblock copolymers with polystyrene

Properties of low molecular weight block copolymers. 3. Mixtures of styrene-dimethylsiloxane diblock copolymers with polystyrene