Morphologies of star-block copolymers in dilute solutions

The morphologies of star-block copolymer (AB)n and (BA)n in a selective solvent for A-block are investigated by using dissipative particle dynamics. For a star-block copolymer of (BA)n type with a large enough arm number n, since the solvophobic B-blocks are situated in the inner part of the star, it behaves as a unimolecular micelle with the B-block core and A-block hairy corona. These types of star copolymers repel each other, thus it is quite difficult to form multimolecular micelles. On the other hand, for a star-block copolymer of (AB)n type, a few aggregative domains develop on the outer rim of the molecule. As the length of B-blocks or the repulsive interaction between B-blocks and solvents is increased, the tendency of B-blocks to associate within the star increases and thus the average number of aggregative domains declines. Owing to the exposure of B-domains, (AB)n type star-blocks tend to form micelles with morphology different from typical micelles. Upon performing simulations for solutions with multiple stars, we have shown that the single molecular conformation may greatly affect the resulting morphology of the supramolecular structure, such as connected-star aggregate, multicore micelle, segmented worm, and core-lump micelle.     

Investigation of the thermal transformations of acrylonitrile copolymers with methylthiirane in dilute solution in dimethylformamide

The effect of introduction of the comonomer methylthiirane (MT) into the main chain of polyacrylonitrile (PAN) on the thermal transformations of the macromolecules in dilute DMF solution was studied. For this purpose, statistical copolymers of acrylonitrile and MT were synthetized and their pyrolysis in solution was investigated. It was found that introduction of the thioether group into the main chain of PAN results in essential changes in the thermal behaviour of the copolymers as compared to the homopolymer. The results of rheological, thermal and spectral studies on the solutions suggest that the direction of the thermochemical reaction changes on introduction of the thioether group into the PAN molecule. In such copolymers the reaction will preferably proceed by the -C≡N groups forming conjugated cyclic structures.     

Pressure-induced reentrant micellization of amphiphilic block copolymers in dilute aqueous solutions

The pressure-induced structural changes of a block copolymer, poly(2-ethoxyethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (pEOEOVE-b-pHOVE) in aqueous solutions, were studied by means of small-angle neutron scattering (SANS) and dynamic light scattering (DLS) from atmospheric pressure up to 400 MPa. pEOEOVE-b-pHOVE formed a spherical micellar structure above 40 degrees C due to poor solubility of pEOEOVE. Micellization phase diagram was determined by DLS, and a covex-upward pressure-temperature (P-T) phase diagram was obtained having a peak around (P,T)=(150 MPa,48 degrees C). The SANS curves at 50 degrees C were analyzed as a function of P. The micellar core size decreased by pressurizing at low P's (P相似文献   

Structural and polymer-analogous transformations in cellulose on esterification with polyfunctional acids

Structural transformations of cellulose in ethylenediamine and in liquid ammonia and on subsequent esterification with polyfunctional organic and inorganic acids have been studied.The authors thank Prof. G. A. Petropavlovskii for valuable advice and consultation.Tashkent Institute of Chemical Technology, Tashkent, 700029, ul. T. Shevchenko, 1. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 160–164, January-February, 1995. Original article submitted October 11, 1994.     

Multiblock copolymers of styrene and isoprene. II. Microstructure and dilute solution properties

Dilute solution properties of linear (SI)₃ six-block copolymers of styrene and isoprene are compared to those of random, two-block, and three-block copolymers of the same system. All the copolymers were prepared with sec-butyllithium in benzene. The microstructure of the polyisoprene blocks is close to that of homopolyisoprene prepared under the same conditions. In contrast, the random copolymer shows a larger amount of trans-1,4 isoprene units. The intrinsic viscosities of the copolymers in methylisobutyl ketone, a poor solvent for both polystyrene and polyisoprene, and in toluene, a good solvent for both homopolymers, are examined on the basis of the Fox–Flory relation for homopolymers. All the copolymers behave similarly in each solvent. In methylisobutyl ketone, the viscosity results indicate a random coil conformation with a small expansion owing to the extra repulsive interactions between the dissimilar units. In all cases, the heterocontact repulsive interactions are small and can be characterized by an interaction parameter χ_ab close to 0.025. In toluene, the perturbation caused by the heterocontacts becomes negligible and the expansion factor α_η can be predicted from a weighted average of those of the parent homopolymers of the same molecular weight as the copolymer.     

Geometric properties of micelles formed by triblock copolymers and solubilizates in dilute solutions

In this work we study the geometric properties of the triblock copolymer micelles with solubilized low-mass molecules in a selective solvent using the Monte Carlo technique on a cubic lattice. The triblock copolymers are of the ABA type, with the two insoluble blocks at the ends. The size of the micelles is characterized by the squared radius of gyration of the micellar core, R_g², while the shape is treated by the asphericity b and the acylindricity c, which are defined in terms of the principal moments of the radius of gyration tensor. The parameters varied are the amount of solubilizate molecules, the polymer concentration, the interaction parameters between A and B, A and solvent, solute and solvent, solute and B block, and the A and B block length. The micelle size, characterized by R_g², grows with increasing concentration of the solubilizates and/or the polymer, and stronger interactions between the incompatible species. The A block length is found not to modify R_g² monotonously, while an increase in B block length results in a decrease in R_g² at high concentrations. As the size expands, the micellar shape becomes less spherical but retains its cylindricity. In addition to an increase in the averaged R_g², the distribution of R_g² becomes broader and the system less homogeneous.     

Study of polystyrene-poly(tert-butyl acrylate) heteroarm star copolymers in dilute solutions

Heteroarm star copolymers with polystyrene and poly(tert-butyl acrylate) arms are studied in dilute solutions by means of viscometry and static light scattering. In a weakly selective solvent a segregated-nonsegregated conformational transition is observed induced by temperature change. In a strongly selective solvent association phenomena take place and the heteroarm star molecules form multimolecular micelles.     

“Recognition” of copolymers and stereoisomers in macromolecule complexation reactions in dilute solutions

Theoretical relationships have been obtained describing the dependence of selectivity (or molecular “recognition”) on the length of reacting polymer chains in reactions between complementary macromolecules. The possibility of highly efficient separation of mixtures of macromolecules, differing little in chemical structure, was predicted as well as of fractionation of compositionally inhomogeneous copolymers by means of polymer-polymer interactions. Experimentally it has been found that polyethylene glycol mixed with polymethacrylic acid stereoisomers is selectively bound in a polycomplex with macromolecules enriched with iso-triads; polyvinylpyrrolidone is selectively bound to those poor in iso-triads. Polyvinylpyrrolidone selectively binds polymethacrylic acid in mixtures with copolymers of acrylic and methacrylic acids. The degree of “recognition” of a macromolecule of given chemical structure by an oligomer increases with increasing oligomer chain length and is in a quantitative agreement with the theoretical equation. It has been shown that, in aqueous solution for interaction of polyacrylic acid with compositionally inhomogeneous copolymer of 4-vinylpyridine and N-ethyl-4-vinylpyridinium bromide (partially quaternized polyvinylpyridine), the polyacrylic acid binds selectively the fractions rich in non-alkylated units. This result can be explained quantitatively in terms of the theory.     

Cloud point suppression in dilute solutions of model gradient copolymers with prespecified composition profiles

Gradient copolymers of 2‐hydroxyethyl methacrylate (HEMA) and 2‐(dimethylamino)ethyl methacrylate (DMAEMA) having prescribed linear, parabolic, and hyperbolic composition profiles were synthesized with the guidance of a numerical model that determines the instantaneous comonomer feed rate. These materials exhibit low polydispersity indices (<1.1); the evolution of the overall DMAEMA content and the absolute molecular weight of the copolymers are all in good agreement with the quantities predicted by our model. Compared to random copolymers of HEMA and DMAEMA, the cloud points of dilute buffered aqueous solutions of gradient copolymers decrease with increasing gradient strength; where the gradient strength is defined as the largest difference in the instantaneous composition along the copolymer. The temperature range over which the solutions transition from transparent to turbid also broadens significantly with increasing gradient strength. Both observations suggest the onset of transition to be dictated by the least soluble ends of the polymer chains. These correlations point to the importance of monomer sequence distribution in determining the macroscopic physical properties of copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Sphere‐to‐cylinder transition in dilute solutions of diblock copolymers

AB diblock copolymers dissolved in a selective solvent for block B spontaneously self‐assemble into spherical or cylindrical micelles with an A core surrounded by a corona of the B block. Using a Monte Carlo lattice model, we find that the sphere‐to‐cylinder transition can be fully described in terms of a packing parameter that depends on the relative cross sections of the two blocks. The latter can be varied either by a block length being changed or by the B block being allowed to adopt an ordered conformation (e.g., helical). The model results are in good agreement with experimental observation. They also shed some light onto the molecular origins of the ease of processing and exceptional mechanical properties of spider dragline. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 890–895, 2002     

Temperature dependence of the fluorescence quantum yields of diphenylsiloxane‐based copolymers in dilute solutions

The monomer and excimer fluorescence quantum yields of well‐defined poly(dimethylsiloxane‐co‐diphenylsiloxane)s with different diphenylsiloxane (Ph₂SiO) contents have been determined, along with those of 1,1,3,3‐tetraphenyl‐1,3‐dimethyldisiloxane and 1,1,3,3,5,5‐hexaphenyltrisiloxane‐1,5‐diol used as model compounds, in a dilute organic solvent at different temperatures. The measured fluorescence quantum yields of the copolymers are correlated with the fraction of the ? (CH₃)₂SiO? (Ph₂SiO)_n? (CH₃)₂SiO? structures. The monomer fluorescence yield for copolymers with low Ph₂SiO contents is dominated mainly by the isolated ? (CH₃)₂SiO? (Ph₂SiO)? (CH₃)₂SiO? unit, and the apparent mean binding energy of the excimer does not increase significantly with increasing Ph₂SiO content. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 854–861, 2002     

Effect of structure of copolymers on the extent of selective sorption in dilute solutions in mixed solvents

The selective sorption of 2,2,3,3-tetrafluoropropanol (TFP) on to the macromolecules of polystyrene, poly(methyl methacrylate) and mixtures of these homopolymers, and also on to the macromolecules of the corresponding random and alternating copolymers has been measured. Dilute polymer solutions in a binary mixed solvent (benzene with 20 vol.-% TFP) have been studied by equilibrium dialysis and differential refractometry. The selective sorption of TFP in this system increases (at a constant copolymer composition) with increasing number of bonds between nonidentical monomer units; it takes up its highest value for the alternating copolymer and its lowest value for the mixture of the homopolymers.     

Properties of polystyrene-poly(methylmethacrylate) random and diblock copolymers in dilute and semidilute solutions

Solution properties for random and diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been measured by dynamic and total intensity light scattering in solvents of differing quality. The results are compared with the corresponding properties for PS and PMMA homopolymers of similar molecular weight, in order to determine if interactions between unlike monomers are significant. The hydrodynamic radius (R_h) and diffusion second virial coefficient (k_d) for the random copolymer are found to be larger than the corresponding values for the homopolymers in a solvent which is near-theta for the two homopolymers, whereas no such effect is observed for the block copolymer. This suggests that most intrachain interactions occur a relatively short distance along the chain backbone. In a mutual good solvent R_h and k_d of the random copolymer are comparable to the average of the values for the homopolymers, indicating that in a good solvent monomer/solvent interactions dominate over monomer/monomer interactions. For an isolated diblock copolymer in a mutual good solvent, there is no evidence that interactions between unlike monomers lead to additional expansion of the entire molecule, as measured by R_h, nor expansion of the individual blocks as probed by light scattering with one block optically masked. However, at low but finite concentration there is evidence (the coefficients of the binary interaction terms in the viscosity and the mutual diffusion coefficient, and the second and third virial coefficients) that a weak ordering effect may exist in block copolymer solutions, far from the conditions where microphase separation occurs. Finally, measurements of ternary polymer-polymer-solvent solutions show no dependence on monomer composition or monomer distribution for the tracer diffusion of probe PS-PMMA copolymers in a PMMA/toluene matrix. This indicate that the frictional interaction is largely unaffected by interactions between unlike monomers. However, there is evidence that the thermodynamic interaction is more unfavorable between a random copolymer and the homopolymer matrix than between a diblock and the matrix. © 1994 John Wiley & Sons, Inc.     

Solution properties of styrene-p-chlorostyrene diblock copolymers—I. Conformational behaviour in dilute solutions

The conformational behaviour of styrene-p-chlorostyrene diblock copolymers in dilute solutions was studied and compared with that of the corresponding triblock copolymers. Eight styrene-p-chlorostyrene diblock copolymers, of almost equimolar composition but with different molecular weights, were prepared using an anionic polymerization technique. The intrinsic viscosities of the copolymers were measured in non-selective solvents, such as toluene and 2-butanone, and in a selective solvent, cumene. The osmotic second virial coefficients of the diblock copolymers were measured in toluene. The data were analysed on the basis of two parameter theories. The unperturbed dimensions for the diblock copolymers can be expressed as a composition average of those for the parent homopolymers and the long-range interaction parameters of the diblock copolymers in toluene, 2-butanone and cumene are smaller than those of the triblock copolymers of the same composition. It means that the diblock copolymer chains in these 3 solvents had a more compact conformation than the triblock copolymers of the same composition and molecular weight.     

Polystyrene–polyisoprene block copolymers in dilute solutions: Characterization of the conformation by small-angle neutron scattering

The variable-contrast method in small-angle neutron scattering has been applied to a study of the conformation of polystyrene–polyisoprene block copolymers in dilute solution. The experimental results reveal no intramolecular segregation effect in dilute solutions in toluene and cyclohexane.     

Block copolymers with Zwitterionic groups at specific sites: Synthesis and aggregation behavior in dilute solutions

Well‐defined linear diblock and triblock copolymers of styrene and isoprene, nearly symmetric in composition, with one or two sulfobetaine associogenic groups at the ends of the polymer chain or the junction points between blocks, were synthesized by anionic polymerization high‐vacuum techniques. The synthetic strategy used the combined initiation of polymerization with 3‐dimethylaminopropyllithium, living end‐capping with 1‐(4‐dimethylaminophenyl)‐1‐phenylethylene, and postpolymerization reaction with cyclopropanesultone. The association behavior of these macromolecules in dilute solutions in the nonpolar solvent CCl₄, good for both blocks, was studied by a number of methods, including static and dynamic light scattering and viscometry. The number and position of the associogenic groups dramatically influenced the association behavior of these model block copolymers. The end‐functionalized samples formed larger aggregates than the junction‐point‐functionalized ones. The difference was attributed to stronger excluded volume effects when the zwitterion group was located within the chain than when the group was at the chain end(s). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3791–3801, 2000     

Micellization of styrene/butadiene/styrene block copolymers in mixed solvents

Dilute solutions of three block copolymers, styrene/butadiene/styrene in the mixed selective solvents dioxane/different alcohols is studied at different temperatures. The molecular weight, second virial coefficient and radius of gyration in all selective solvents are also determined. From these measurements it is concluded that a maximum in the dissymmetry of the system for all the selective solvents used is observed. This maximum is observed at higher temperatures for concentrated solutions. Similarly, this maximum is also shifted to lower temperature by using strong precipitants in selective solvents. The milky opalescence is observed for all the selective solvents either by lowering the temperature of the system or by increasing the ratio of the precipitant in the selective solvent.