Behavior Of Diblock Copolymers Research Articles

Synthesis, Thermoresponsive Behavior, and Catalytic Properties of Amphiphilic Diblock Copolymers of N-Vinylcaprolactam and N-Vinylimidazole

Thermoresponsive diblock copolymers (DCs) were prepared by two-stage reversible addition-fragmentation chain transfer/macromolecular design by interchange of xanthate (RAFT/MADIX) polymerization of N-vinylcaprolactam and N-vinylimidazole (VI). The poly(N-vinylcaprolactam) (PVCL) blocks were first synthesized and used as macro-chain transfer agent in VI polymerization. The temperature behavior of PVCL and DCs in aqueous media has been studied by static and dynamic light scattering. It has been shown that the phase separation temperature of both PVCLs and DCs depends on the length of the PVCL chain and the composition of aqueous solvent. The temperature range above the PVCL θ temperature and below the cloud point is characterized by the conformational rearrangements leading to the formation of mesoglobules. The study of catalytic activity of DCs in the hydrolysis reaction of p-nitrophenyl propionate has shown that their activity substantially increases in this transitional temperature region owing to the formation of highly developed hydrophilic–hydrophobic interfaces inside the mesoglobules.

Influence of pH-Responsive Monomer Content on the Behavior of Di-Block Copolymers in Solution and as Stabilizers of Pickering Latex Particle Emulsifiers

In this study, diblock copolymers poly(methyl methacrylate)—block—poly (2-dimethylaminoethyl methacrylate) (pMMA-b-pDMAEMA) are investigated for the steric stabilization of latex particles and the subsequent use of these latex particles as Pickering emulsifiers. Solution properties of the diblock copolymers highlight that the pDMAEMA block length influences the critical micelle concentration (CMC) and micelle hydrodynamic diameter in response to changes in pH and the pKa. The block length can also be used as a way to control the particle size of sterically stabilized polystyrene latex particles prepared via emulsion polymerization. The suspension properties of these latex particles are also presented. Emulsion studies using these latex particles as emulsifiers show that both continuous phase pH and electrolyte concentration affect emulsion stability to coalescence. At high pH, stable emulsions are formed due to the affinity of the particles to the interface. At low pH, protonation of the amine groups reduces the affinity and thus droplet coalescence is observed. Increasing the electrolyte concentration improves emulsion stability, but causes an increase in droplet size due to adsorption of flocculated/aggregated particles. Finally, it is shown that these latex particles can be used in conjunction with membrane emulsification techniques to produce emulsions with low polydispersity.

Micellisation of polystyrene-b-polyglycidol copolymers in water solution

The preparation of a set of amphiphilic polystyrene-b-polyglycidol (PS-b-PGL) diblock copolymers and their characteristics in aqueous media are described. The copolymers were prepared via anionic polymerisation starting from the synthesis of a polystyrene macroinitiator terminated by ethylene oxide, followed by anionic polymerisation of glycidol 1-ethoxyethyl ether to obtain a poly(1-ethoxyethyl glycidyl ether) block. The ethoxyethyl blocking groups of the hydroxyls were subsequently removed by hydrolysis. The copolymers differed in polymerisation degree of the polyglycidol block with DP values equal to 13, 43, 68 and 125, whereas the DP of polystyrene was 29. Two different organic media, N,N-dimethylformamide and dioxane, were used to prepare copolymer solutions dialysed afterward against water. The behaviour of diblock copolymers in water, their self-assembly and the size parameters of their aggregated structures were determined using dynamic light scattering. The critical micelle concentration (CMC) did not depend on the initial solvent and linearly increased with increasing content of hydrophilic glycidol units in the copolymer chain. However, above the CMC, the dispersions obtained from DMF contained a significant fraction of large aggregates. In the case of dioxane after dialysis, only one population of particles of the sizes, which can be related to typical core–shell micelles, was observed. Cryogenic transmission electron microscopy images revealed the presence of spherical objects and confirmed their sizes. PS-b-PGL micelles were highly stable and did not dissociate even after dissolution of their dispersions to concentrations below the CMC.

Morphology Transition of Diblock Copolymer with Nanoparticles Confined in Cylindrical Nanopores

Coarse-grained molecular dynamics simulations on self-assembly behavior of diblock copolymers (DCP) with nanoparticles confined in cylindrical nanopores are perfomed to study the morphology transition of DCP affected by the confined ratio of the cylindrical confinement diameter to the block copolymer domain spacing, the wall-polymer interactions, the particle-polymer interaction potential and the component concentration. The simulation results show that nanoparticles have a pronounced effect on the morphology transition of DCP and can therefore be considered as an important aspect in controlling the confined self-assembly in cylindrical confinement. Besides, the phase diagram indicates the process of morphology transition.

Amphiphilic diblock copolymers containing poly(N-hexylisocyanate): Monolayer behavior at the air-water interface

The behavior of amphiphilic diblock copolymers containing 80–89% of poly(N-hexylisocyanate) (PHIC) with different hydrophobic segments spread at the air–water interface has been studied. Surface pressure-area isotherms (π-A) at the air–water interface were determined. It was found that these diblock copolymers form stable monolayers and the isotherms present a pseudoplateau region at low surface pressure, irrespective of the nature of the partner block: poly(styrene) (PS) or poly(isoprene). Surface pressure variation at the semidilute region of the monolayer was expressed in terms of the scaling laws as power function of the surface concentration. The critical exponents of the excluded volume ν obtained for copolymers with PHIC and PS blocks are 0.58 for the copolymer with 85% of PHIC and 15% of PS, and 0.63 for the copolymer with 89% of PHIC and 11% of PS. The hydrophobicity degree of the diblock copolymers was estimated from the determination of the surface energy values by wettability measurements. The morphology of the monolayers was determined by means of Brewster angle microscopy. Molecular dynamic simulation was performed to explain the experimental behavior of diblock copolymers at the air–water interface. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Orientational Correlations and the Dynamical Behavior of Diblock Copolymers

Strong orientational effects in the static and dynamic structure factors of diblock copolymer melts have been shown to originate from the connected structure and mutual repulsion between the blocks. A collective variable based on the chain bond vectors is introduced and shown to have a dynamics comparable to that of the concentration fluctuations. Coupled equations of motion are derived which illustrate how the concentration fluctuations drive the orientational correlations in diblock chains. The coupling gives rise to two relaxation modes, one of which shows a critical slowing down and is approximately wavenumber independent on scales larger than the chain size. The amplitude of this mode increases as the microphase separation is approached in a manner similar to the static concentration fluctuations. These findings are in qualitative agreement with recent light-scattering experiments.

Change of phase behaviour of diblock copolymers upon application of pressure

Poly(styrene-b-paramethylstyrene) block copolymer samples have been prepared in a mechanical press, cooled under pressure below the glass transition temperature and investigated by small angle neutron scattering. We observe a microphase separation transition from an orderd to a disorderd state with ascending pressure. The observed effect is not expected solely due to pressure, but the situation in the press is more complicated: (i) Ascending pressure leads to an increase of the glass transition temperature which slows down relaxations and hinders the evolution of thermodynamic equilibrium, (ii) the uniaxial movement of the piston leads to an uniaxial shear field and samples get oriented. An adiabatic heating during the pressing procedure has been found to be of minor influence. The situation is discussed with respect to common sample preparation conditions and processing.