Liquid-crystalline symmetric triblock copolymers

Symmetric fully liquid-crystalline triblock copolymers of various structures containing optically active mesogenic groups are for the first time synthesized via pseudoliving radical reversible addition-fragmentation chain-transfer polymerization. Their phase behavior and physicochemical and optical properties are studied. It is shown that, depending on composition, at low temperatures block copolymers can form at temperatures phase-separated structures caused by microsegregation of blocks of different chemical natures and that, with an increase in temperature, these structures can mix to form a cholesteric mesophase characterized by a helical supramolecular structure. A model illustrating the molecular packing of block copolymers with a phase-separated lamellar structure is advanced. The effect of the molecular structure of the block copolymers on their optical properties is discussed.     

DNA multiblock copolymers

Single stranded (ss) DNA block copolymers were applied to synthesize DNA multiblock architectures by hybridization; these polymeric bioorganic hybrids were characterized by gel electrophoresis and MALDI-TOF mass spectrometry.     

Self-assembly of linear rod-coil multiblock copolymers

The self-assembly of the linear rod-coil multiblock copolymers is studied by applying self-consistent-field lattice techniques in a three-dimensional (3D) space. Compared to the copolymer with one rod, the copolymer with more rods (mrod≥2) exhibits rich order-order phase transitions with increasing temperature, where the ordered morphology changes from strips to perforated lamellae and finally to lamellae. In addition, taking the copolymer with mrod = 2 as a representative, we further study the effects of the volume fractions of the rods, the spacer coils and the end coils on the phase behaviors respectively, by which the detailed self-assembled mechanism of the linear rod-coil multiblock copolymers is revealed. Our results are expected to provide guidance for the design of the rod-coil materials.     

Anion conducting multiblock copolymers with different tethered cations

A multiblock copoly(arylene ether) polymer was used to quantitatively compare the ion conducting channels formed by three different, tethered cation head‐groups. The synthesis allowed for the formation of an exact number of tethers on each repeat unit. Three head‐groups, quaternary trimethylammonium (TMA), quinuclidium (ABCO), and tris(2,4,6‐trimethoxyphenyl)phosphonium (TTMPP) cation head‐groups were compared in terms of size of the conducting channels, ionic conductivity of the mobile hydroxide ion, mechanical properties, quantity of productive and unproductive water, and chemical stability of the membrane in base. The interdomain spacing showed that multiblock copolymers with larger cations formed larger ion conduction channels in the membrane. Larger cations resulted lower ion exchange capacity (IEC) even though the polymer backbone and tether arrangements were identical. TMA was the most stable cation after exposure to 1 M NaOH at 60 °C for 20 days. ABCO had a lower number of bound water molecules and a 22% loss in ion conductivity after treatment in 1 M NaOH at 60 °C for 20 days due to the higher hydroxide ion concentration in the ion conductive blocks. Membranes with TMA head‐groups also had the best mechanical properties. Two membrane preparation methods were compared. The presence of the cation head‐groups assists in phase segregation. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1395–1403     

A novel strategy for synthesis of multiblock copolymers

Multi-block copolymers with well-controlled numbers of blocks and block chain length are synthesized for the first time using a 'polyinitiator'.     

Lateral ordering during self-organization of statistical multiblock copolymers

The self-organization of statistical multiblock and Bernoulli AB copolymers is studied. The initial ensemble is generated via the polymer-analogous reaction A→B that proceeds with the accelerating effect of neighboring B units. In a two-dimensional model, the reaction is performed in a rectangle composed of stretched chains. Then, the rectangle is closed into a cylinder, so that ring chains are located on its side surface. The self-organization of the ensemble is simulated via the successive rotation of each upper ring over the lower ring until arrangement with the maximum (in modulus) energy of attraction between chains is attained. Self-organization by energy is accompanied by lateral ordering: the sizes of clusters—accumulations of the one-type units—and mean heights H _A (H _B) of stems—columns consisting of A or B units perpendicular to chains—increase. The ratio between the values of H _A, as well as H _B, for ordered and initial ensembles is independent of the average composition of the system and as a rule increases as the length of blocks increases and the length of chains decreases. Features of generation of the ensemble of short chains and their ordering are revealed. It is shown that, during ordering of multiblock copolymers, the probabilistic properties (the stochastic behavior) of the ensemble are disturbed. The self-organization of statistical multiblock copolymers in a three-dimensional model is investigated via rotation of rings in the torus of the rectangular cross-section. The effects of various factors on self-organization by energy and local ordering in 2D and 3D models are similar; however, the efficiency of ordering in the three-dimensional system is always lower because, in this case, arrangements with the maximum energy of attraction simultaneously to two neighboring chains, rather than to one, are implemented for the majority of chains.     

Supramolecular self-assembly of multiblock copolymers in aqueous solution

A unique pH-dependent phase behavior from a copolymer micellar solution to a collapsed hydrogel with micelles ordered in a hexagonal phase was observed. Small-angle neutron scattering (SANS) was used to follow the pH-dependent structural evolution of micelles formed in a solution of a pentablock copolymer consisting of poly((diethylaminoethyl methacrylate)-b-(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)-b-(diethylaminoethyl methacrylate)) (PDEAEM25-b-PEO100-b-PPO65-b-PEO100-b-PDEAEM25). Between pH 3.0 and pH 7.4, we observed the presence of charged spherical micelles. Increasing the pH of the micelle solution above pH 7.4 resulted in increasing the size of the micelles due to the increasing hydrophobicity of the PDEAEM blocks above their pKa of 7.6. The increase in size of the spherical micelles resulted in a transition to a cylindrical micelle morphology in the pH range 8.1-10.5, and at pH >11, the copolymer solution undergoes macroscopic phase separation. Indeed, the phase separated copolymer sediments and coalesces into a hydrogel structure that consists of 25-35 wt % water. Small-angle X-ray scattering (SAXS) clearly indicated that the hydrogel has a hexagonal ordered phase. Interestingly, the process is reversible, as lowering of the pH below 7.0 leads to rapid dissolution of the solid into homogeneous solution. We believe that the hexagonal structure in the hydrogel is a result of the organization of the cylindrical micelles due to the increased hydrophobic interactions between the micelles at 70 degrees C and pH 11. Thus we have developed a pH-/temperature-dependent, reversible hierarchically self-assembling block copolymer system with structures spanning nano- to microscale dimensions.     

Synthesis of electroactive and biodegradable multiblock copolymers based on poly(ester amide) and aniline pentamer

A new family of multiblock copolymers (PEA‐b‐AP) based on poly(ester amide) (PEA) and aniline pentamer (AP) with the unique properties of being both electroactive and biodegradable was synthesized via a two‐stage active solution polycondensation. The new synthesis approach proceeded smoothly, and avoided the complicated purification steps for separating the intermediate products. The molecular weight of PEA blocks was regulated by varying the nucleophilic/electrophilic monomers feed ratios. The chemical structures of the copolymers were confirmed by both IR and NMR spectra. UV‐Vis spectroscopy indicated that the copolymers possessed of the intrinsic electroactivity of AP blocks, and showed three reversible oxidation states. The copolymers had lower degradation rates than the PEA homopolymers with similar molecular weight, and their degradation rates were greatly affected by the proportion of AP blocks. In vitro cell culture studies of the PEA‐b‐APs revealed that they facilitated the proliferation of RSC96 Schwann cells and displayed a good biocompatibility. These biodegradable copolymers with electroactive function may have great potential for use as nerve repair and regeneration scaffold materials in tissue engineering. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4722–4731     

Liquid-crystalline polyesters based on branched propylene spacers

A new promising concept of spacer modification in main chain liquid crystalline polymers is the use of branched alkylene spacers. In this article we give as a first example the polyesters of p, p′-terphenyl-4,4′-dicarboxylic acid as mesogenic unit and disubstituted propylene spacers. The steric restrictions of the spacers lower the melting points compared with the polyesters with linear spacer but they do not flexibilize the main chains as linear alkylspacers do. This leads to a residual stability range of the liquid crystalline state in the homologues of polyesters with disubstituted spacer. The melting and clearing points are lowered to the same extent. Because of the solubility of the higher homologues in common solvents it is possible for the first time to determine molecular masses of this polymer class based on p, p′-terphenyl-4,4′-dicarboxylic acid and aliphatic diols more exactly. X-ray scattering indicates that long side chains are arranged parallel to the main chain and have no longer influence on transition temperatures and intermolecular distances. © 1996 John Wiley & Sons, Inc.     

Phase behavior of compressible melts of multiblock polydisperse copolymers

In terms of the previously proposed model, specific features of the phase behavior of Markovian polydisperse copolymers with allowance for their compressibility have been investigated via bifurcation analysis followed by continuation with respect to a parameter that characterizes the deviation of the temperature of the system from its value on the spinodal. These features above all include competition between microphase separation and macrophase separation under conditions when the local instability of the homogeneous state appearing at the spinodal corresponds to the macrophase separation only. Nevertheless, it was shown that depending on the structural parameters, the global instability characterized by a cloud-point hypersurface can result in either macrophase or microphase separation, with the microphase separation occurring in the vicinity of the critical point. In this case, the results are consistent with the conclusions of the Landau theory of phase transitions, whose applicability limits with respect to deviation from the critical point have been evaluated in this study. Outside the range of applicability of this theory, cloud-point curves that correspond to macrophase separation and microphase separation are very similar. These conclusions remain valid over a wide range of compressibility whose influence has been assessed for the first time. It has been found that the type of copolymers under consideration has a characteristic feature that was not noticed previously: Namely, the distribution of density in the nucleus of a new phase in this case will look like a spatially localized solitonlike profile.     

Elastic moduli of multiblock copolymers in the lamellar phase

We study the linear elastic response of multiblock copolymer melts in the lamellar phase, where the molecules are composed of tethered symmetric AB diblock copolymers. We use a self-consistent field theory method, and introduce a real space approach to calculate the tensile and shear moduli as a function of block number. The former is found to be in qualitative agreement with experiment. We find that the increase in bridging fraction with block number, that follows the increase in modulus, is not responsible for the increase in modulus. It is demonstrated that the change in modulus is due to an increase in mixing of repulsive A and B monomers. Under extension, this increase originates from a widening of the interface, and more molecules pulled free of the interface. Under compression, only the second of these two processes acts to increase the modulus.     

Synthesis and properties of multiblock copolymers based on polydimethylsiloxane and polyamides from dicarboxylic acid and diisocyanate terminated functionalities

Polydimethylsiloxane (PDMS)–polyamide multiblock copolymers were successfully synthesized via diisocyanate route by two different procedures, i.e., the one-step and two-step methods, In the two-step method, α, ω-diisocyanate-terminated polyamide oligomers, which were prepared in situ from a mixture of isophthalic acid (IPA) and azelaic acid (AZA) with 4,4′-methylenedi (phenyl isocyanate) (MDI) in 1,3-dimethyl-2-imidazolidone (DMI) in the presence of 3-methyl-1-phenyl-2-phosopholene 1-oxide catalyst, were reacted with α, ω-bis (10-carboxydecyl) polydimethylsiloxane (PDMS-diacid) leading to the formation of multiblock copolymers. In the one-step method, the reaction components, MDI, IPA, AZA, and PDMS-diacid were reacted all together in DMI in the presence of the catalyst. These polymerizations gave multiblock copolymers having inherent viscosities in the range of 0.36–1.12 dL/g in N,N-dimethylacetamide (DMAc). These multiblock copolymers were soluble in amide-type solvents, and transparent (or translucent) and ductile films could be cast from the solutions in a mixture of DMAc and bis(2-ethoxyethyl) ether. The multiblock copolymers prepared by the two-step method had better-defined, microphase-separated morphology than those obtained by the one-step method. The mechanical properties of PDMS–polyamide multiblock copolymer films were found to be highly dependent on the PDMS content; the tensile strength and modulus of the films decreased with increasing the PDMS content.     

Recognition of multiblock copolymers on nanopatterned surfaces: insight from molecular simulations

The recognition of multiblock copolymers on nanopatterned surfaces has been investigated by molecular simulations. All the copolymers (AnB12-n)5 are composed of 60 square-well segments, but with various architectures by changing n. Segment density profiles, radii of gyration, pattern transfer parameters, and three adsorption conformations (tail, loop, and train) are examined quantitatively. It is found that the copolymer can recognize the adsorbing stripes on surface and the surface vicinity. The recognition affinity becomes stronger with increasing the stripe width, the adsorption strength, and the number of adsorbing segments in copolymer chain. From surface to bulk phase, the shape of copolymer changes from elongated to elliptical, and finally to globular. Among the three adsorption conformations, tail has the greatest average size while train has the smallest. With the increased number of nonadsorbing segments, the average size shows an increase in tail but a decrease in train.     

The soliton mechanism of phase separation of polydisperse multiblock copolymers

The equations of the thermodynamics of compressible polydisperse multiblock copolymers have been analytically and numerically studied. The possible density distributions of copolymer units over a wide temperature range including the cloud point and the spinodal temperature have been determined. Along with periodic structures, spatially localized soliton-type distributions have been identified in order to estimate the energy barrier that must be overcome for the occurrence of the initial stage of the phase transition.     

Synthesis of PLA-b-PEG multiblock copolymers for stealth drug carrier preparation

An efficient method of preparing biodegradable and biocompatible multiblock copolymers from lactic acid and polyethylene glycol is proposed.     

Macro-and microphase separation in nonaqueous solutions of binary multiblock copolymers

In terms of the random phase approximation, a global analysis of the thermodynamic stability of solutions of multiblock copolymers (A_nB_m)_k and (A_nB_m)_kA_n was performed. Phase diagrams for various parameters of the copolymer structure including the macrophase and microphase separation regions were obtained. Critical lines near which this approximation still holds and the microphase separation corresponds to the weak segregation mode were also constructed. The formation of ordered structures with a period of the order of a wavelength of visible light becomes possible under certain conditions. It was shown that the homogeneous state of solutions of multiblock copolymers in certain narrow ranges of values of the parameters breaks down through a simultaneous growth of fluctuations with substantially different wavelengths, a phenomenon which must lead to the appearance of structures ordered at two length scales. The width of such regions increases with an increase in the number of blocks k and a decrease in the degree of polymerization n + m of a block. The potential use of these multiblock-copolymer solutions as photonic crystals is discussed.     

Liquid-crystalline comb copolymers with lipoamino acid side chains Synthesis and structure study

In order to obtain liquid-crystalline polymers without using classical mesogenic groups, comb copolymers consisting of a polyacrylamide main chain and lipoamino acid side chains have been synthesized. These copolymers were obtained by the polymerization of lipoamino acid macromonomers. These macromonomers were obtained from, α,ω-aliphatic amino acids by linking a polymerizable group at the amino end and an α-amino acid at the carboxyl end. The macromonomers were then transformed into comb copolymers by free-radical polymerization. These comb copolymers exhibit mesophases both in aqueous solution and in the anhydrous state. The range of stability and the structures of the mesophases were determined by X-ray diffraction. Two types of structures were found, corresponding to the lyotropic lamellar and hexagonal mesophases. The influences of the nature of the amino acid and the water concentration on the domain of stability and the geometrical parameters of the mesomorphic structures were investigated.