Two-block Copolymers Research Articles

The Longitudinal Superdiffusive Motion of Block Copolymer in a Tight Nanopore.

The structure and dynamic properties of polymer chains in a confined environment were studied by means of the Monte Carlo method. The studied chains were represented by coarse-grained models and embedded into a simple 3D cubic lattice. The chains stood for two-block linear copolymers of different energy of bead–bead interactions. Their behavior was studied in a nanotube formed by four impenetrable surfaces. The long-time unidirectional motion of the chain in the tight nanopore was found to be correlated with the orientation of both parts of the copolymer along the length of the nanopore. A possible mechanism of the anomalous diffusion was proposed on the basis of thermodynamics of the system, more precisely on the free energy barrier of the swapping of positions of both parts of the chain and the impulse of temporary forces induced by variation of the chain conformation. The mean bead and the mass center autocorrelation functions were examined. While the former function behaves classically, the latter indicates the period of time of superdiffusive motion similar to the ballistic motion with the autocorrelation function scaling with the exponent t5/3. A distribution of periods of time of chain diffusion between swapping events was found and discussed. The influence of the nanotube width and the chain length on the polymer diffusivity was studied.

Controlled free-radical polymerization of vinyl chloride

Owing to the importance of poly(vinyl chloride) (PVC) as the second-largest plastic in volume after the polyolefins and above styrene polymers, the control of the free-radical polymerization of vinyl chloride (VC) is of high industrial and academic interest. But still the term “controlled” polymerization is not yet clearly defined. Often it is used for quasi-living polymerizations with equilibrium reactions in the initiation and/or termination step or for the control of the molecular weight distribution (MWD), but it can also be applied to several structural aspects such as stereochemistry, branching, or special technical properties. In the present article, the control of chain growth and chain transfer is discussed. It has been well known for many years that the propagation step in the VC polymerization is terminated to a large degree by the rather frequent and temperature-dependent chain transfer of the growing macromolecules to the monomer. Therefore, the degree of polymerization is strongly governed by the polymerization temperature. However, this transfer step does not result in a controlled or a narrow MWD. By means of free-radical nitroxide-mediated polymerization of VC in suspension, PVC with a narrower MWD can be obtained also at higher polymerization temperatures. The resulting PVC with nitroxide end groups can act as a macro-initiator for various monomers, resulting in two-block copolymers, which are, e.g., interesting compatibilizers in blends with PVC. J. VINYL ADDIT. TECHNOL., 11:86–90, 2005. © 2005 Society of Plastics Engineers

Theory of liquid adsorption chromatography of macromolecules applied to polyethylene glycol and fatty alcohol ethoxylates

Liquid adsorption chromatography retention modes of poly(ethylene glycol) (PEG), its mono- and dimethyl ethers (MMEs and DMEs) and also of fatty alcohol ethoxylates (FAEs) are studied both experimentally and theoretically. The experimental system under investigation was a Spherisorb S5W 80 Å column and isopropanol–water as the mobile phase. At various compositions of the mobile phase (in the range of 75 to 87% of isopropanol) chromatograms exhibiting good peak resolution were obtained and the dependencies of the elution volume on the number of repeating EO units for both PEG and FAE samples evaluated on the basis of a closer inspection of the chromatographic data. The experiments did not reveal any differences between the chromatographic behavior of PEGs, MMEs and DMEs, while the FAEs gave substantially smaller values of elution volume at all mobile phase compositions. These data were interpreted by using a molecular-statistic theory of homopolymers (to describe both PEG, MME and DME) and two-block copolymers (for FAEs) based on a continuum Gaussian chain model of macromolecules and a slit-like model of pores of stationary phase, wide pore approximation and the adsorption chromatography mode for PEG molecules were assumed in the development of this theory. This theory described very well the experimental data obtained, and two thermodynamic parameters characterizing interactions of EO units of PEG and both EO and (CH 2) n chains of FAE molecules with the adsorbent pore walls have been determined from the comparison of the theory and the experiments. Although the mean thickness of adsorbed oxyethylene chain, H, was estimated as being equal to about 3.5–4.5 Å, H proved to be slightly decreasing with increasing isopropanol content in mobile phase. Chromatograms visualizing the adsorption of PEG and FAE molecules are presented, and the correspondence between the theoretical approach and the experimental situation under investigation is discussed.

Novel materials through stereocomplexation

Both (−)- and (+)-poly(lactide) (PLA) crystallize into a stereocomplex with a melting point that is 50 °C higher than that of crystals of the same-handed enantiomers. According to Boyer et al. [Polym. Prepr., 36 (1995) 87] and Jiang et al. [J. Am. Chem. Soc., 117 (1995) 7037], alternating isotactic propylene-CO-copolymers (P(P-alt-CO)) also form a stereocomplex with a higher melting point (60 °C). Force-Field-simulated structures for both polymer systems were found to agree well with X-ray data, irrespective of whether they had a chiral or racemic packing. The almost similar results for both stereocomplexes indicate that they might form a mixed stereocomplex of (−)-PLA and (+)-P(P-alt-CO). In acetonitrile, both enantiomers of an AB block copolymer derivative, poly(lactide)-poly(ethyleneglycol) (PLA-PEG); were found to crystallize exclusively into a racemic lattice. The influence of racemic packing on self-assembly of the two-block copolymer was analyzed by atomic-force microscopy. An equimolar mixture of (−)- and (+)-PLA-PEG formed spherical particles through stereocomplexation, in contrast to chiral block copolymers that formed large crystal needles and long rods.

Synthesis of a new microphase‐separated polymer system by “counterion coupling” and its X‐ray characterization

AbstractA new type of highly ordered, microphase‐separated polymer material is synthesized by simple ionic coupling of a commercial styrene/ethylene oxide twoblock copolymer with a charged end group to an oppositely charged polyelectrolyte by common precipitation from aqueous solution. The characterization of the polymer/polymer complex by small‐angle X‐ray scattering and polarization microscopy reveals that the comb‐like addition complexes formed by ionic grafting build up a mesophase with lamellar morphology and a periodicity of d = 7.1 nm which is easily oriented in mechanic fields.

Block and graft copolymers of 2-oxazolines

The cationic ring-opening polymerization of 2-oxazolines is known to be initiated by alkyl halides, Lewis acids and esters of strong acids. The polymerization proceeds by a living mechanism. Numerous block and graft copolymers have already been described. Recently it was found that chloroformates (R-O-CO-CI) are also useful initiators. The mechanism of the initiation and propagation is discussed. This type of initiators allows the synthesis of different novel two-block and three-block copolymers, star-shape polymers, and a graft copolymer with a polyvinylacetate backbone.

Polylactones 27. Anionic polymerization of L‐lactide. Variation of endgroups and synthesis of block copolymers with poly(ethylene oxide)

AbstractL‐Lactide was polymerized in toluene with various alkoxide initiators. These initiators were prepared in situ from potassium t.‐butoxide and primary or secondary alcohols such as tetradecanol, diethylene glycol butyl ether, menthol or testosteron. All these alcohols were incorporated as ester endgroups into the polylactide chain. However 1H‐NMR spectroscopy proves the existence of more OH‐endgroup than ester endgroups. This finding and 10–20% racemization observed for all anionic polymerizations suggest that chain‐transfer reactions with the monomer via deprotonation take place. When poly(ethylene glycol) monomethyl ether in combination with KOtBu was used as initiator, twoblock copolymers were obtained. With poly(ethylene glycol) A‐B‐A‐triblock copolymers could be synthesized. The quantitative reaction of the poly(ethylene glycol)s with L‐lactide could be proven by both 1H NMR spectroscopy and gelpermeation chromatography. DSC‐measurements show that depending on their lengths either the polylactide or the poly(ethylene oxide)blocks can crystallize. Due to partial racemization the melting temperatures (Tm) of the poly(L‐lactide) blocks did not exceed 155°C.

Microphase and macrophase separation in blends with a two-block copolymer

The miscibility behaviour of blends A αβ of a homopolymer A and a symmetric two-block copolymer αβ, where the chains A and the blocks α are chemically the same and immiscible with the blocks β, is analysed theoretically and experimentally. The blends are compared with corresponding blends A αβ′ , where the copolymer αβ′ is random. The styrene (S)/methyl methacrylate (MMA) model systems investigated consist of a copolymer P(S- b-MMA) (two-block) or P(S- co-MMA) (random) and a homopolymer PMMA of varying chain length. The process of phase separation during film casting at different temperatures was analysed by mean-field calculations, and the morphologies of the cast films were studied by transmission electron microscopy. The block copolymer blends exhibit both microphase and macrophase separation, which develop in two steps in the order microphase-macrophase or macrophase-microphase. Which sequence is preferred can be predicted from the homopolymer-copolymer chain-length ratio.

Experimental verification of the concept of chromatographic “invisibility” in critical chromatography of block copolymers

The concept of chromatographic “invisibility” of block copolymers in chromatography carried out in critical conditions has been verified experimentally. Block copolymers of styrene with tert-butyl methacrylate of type A-B were studied. The critical chromatographic conditions for the poly( tert-butyl methacrylate) block were selected by varying the composition of a binary eluent. The obtained results confirm the validity of the concept of chromatographic “invisibility” for two-block copolymers consisting of flexible-chain blocks.

Theory of supermolecular structures in polydisperse block copolymers: 2. Lamellar superstructure consisting of two-block copolymers

The lamellar superstructure formed in polydisperse block copolymer melts has been considered. For the narrow interphase approximation, a mean-field analytical theory has been developed. This theory relates the equilibrium parameters of the superstructure to the characteristics of the copolymer molecules: the average molecular weight, the type of polydispersity and the surface tension. It is shown that, in a binary mixture of long block copolymers (with different molecular weights and compositions), the formation of a single lamella with common period is under certain conditions thermodynamically more advantageous than the segregation of the system into lamellae formed by the mixture components. The structure of the lamellar layers is investigated in detail. Theoretical and experimental data are compared.

Scaling theory of supermolecular structures in block copolymer-solvent systems: 2. Supercrystalline structures

The supermolecular structures formed in a block copolymer-solvent system were considered by using the scaling method. The equilibrium characteristics of the superstructures of different morphologies formed by two-block copolymers with non-crystallizable blocks were determined as a function of the molecular weight and composition of copolymer and the strength and concentration of the solvent in the system. The composition-concentration diagram of the system was obtained. The lamellar superstructure formed by a block copolymer with one crystallizable block was analysed. The results of the theory were compared with the experimental data.

Achievements and uses of critical conditions in the chromatography of polymers

A method for the analysis and separation of macromolecules is proposed, based on the effect of the chromatographic “invisibility” of some chain fragments, namely blocks of a certain type in block copolymers, grafted chains in graft copolymers and linear elements of macromolecules with rings. When certain critical conditions for such fragments of polymer molecules are created neither the length nor the molecules weight distribution of these molecules influences the retention volume, and this effect is treated as a chromatographic “invisibility” of the elements involved. Results of the modern theory of chromatography and the direct simulation of the equilibrium behaviour of macromolecules in a chromatographic column have beent aken into account and conclusion is draw that this method of “invisibles” provides an efficient separation of macrocycles and linear polymers and the separation of macromolecules according to the number of functional groups. The method of “invisibles” also makes it possible to separate two-block copolymers according to the size of the “visible” block only, and to separate grafted polymers according to the backbone length. The scope of the application of the method to chromatographic analyses of copolymers is discussed.

Scaling theory of supermolecular structures in block copolymer-solvent systems: 1. Model of micellar structures

The micellar structures formed by two-block copolymers in a block copolymer-solvent system were considered by using the scaling method. The equilibrium parameters of micelles were determined depending on the composition of the copolymer and the strength and concentration of the solvent in the system. It was shown that a ‘quasiglobular’ regime exists in a micellar solution at concentrations exceeding that of overlapping of individual micelles. The results of the theory are compared with the experimental results.

To the theory of superstructures in concentrated solutions of block copolymers

A concentrated solution of a two-block copolymer under conditions of phase separation is discussed. Besides chemical segregation (formation of domains of the minor component in the major component matrix), the segregation of matrix component blocks from various domains also occurs in such solution. A mean field theory of the equilibrium parameters of lamellar mesophases in the solution is developed, the dependence on the molecular mass of the copolymer, on its composition, and on the quality and quantity of solvent in the system. The segmental density profile and the distribution of free ends of component blocks in lamellar layers were determined.

Interpretation of small angle neutron scattering by copolymer solutions as a function of concentration

Using a generalization of the Random Phase Approximation (RPA) for a three-component system, the intensity scattered by a two-block copolymer has been calculated in the two special cases : a) the coherent scattering length of the solvent is identical to the coherent scattering length of one of the blocks, b) the difference between the average scattering length of the whole molecule and the solvent is zero. Expressions are explicitly given and discussed as a function of the concentration and the momentum transfert q assuming the solvent to be good for both sequences. Experimental results obtained by small angle neutron scattering on a two block copolymer PSH-PSD (hydrogenated and deuterated Polystyrene) in a mixture of hydrogenated and deuterated benzene show a good agreement between the theory and the experiments.

Methods of investigation theory of chromatographic separation of two-block copolymers according to composition

Chromatographic separation of two-block copolymers according to composition has been treated theoretically. The copolymer was modelled by a Gaussian chain consisting of two blocks which differed in the energy of interaction with the surface of a slit-like pore. The partition coefficients have been derived in dependence on the energy of interaction between the polymer and the adsorbent and on pore width for copolymers of different length and composition. Critical conditions have been found for copolymers in narrow-pore sorbents and values of the partition coefficient have been derived.

Temperature Effect on the Micelle Behavior of Styrene-Isoprene Block Copolymers in Selective Solvents

Abstract Micelle formation by several styrene-isoprene two-block copolymers in selective solvents, viz., n-heptane, n-dodecane, N,N′-dimethylformamide, and N,N′ -dimethylacetamide, was studied by viscometry and photon correlation spectroscopy (PCS). Aggregation number and micellar weight were deduced by combining viscosity and PCS data. The temperature effect on the viscosity behavior of micellar solutions of block copolymers in different selective solvents is examined and explained. 1H-NMR spectra of the copolymer in n-heptane were recorded at different temperatures to explain the characteristic features of micellar solutions.

Micellar behaviour of styrene—isoprene block copolymers in selective solvents

The micellar phenomenon of block copolymers in selective solvents has been observed analogous to the classical surfactants. Spherical polymolecular micelles of block copolymers consisting of a core of insoluble block surrounded by the fringes of the soluble block have been analysed. The micellar behaviour of several styrene—isoprene two-block copolymers in two kinds of solvents, (i) selectively good solvent for polystyrene, e.g. dimethylformamide (DMF), dimethylacetamide (DMA), methyl ethyl ketone (MEK) and (ii) selectively good solvent for polyisoprene, e.g. n-alkanes (heptane, octane, decane and dodecane), has been examined by photon correlation spectroscopy (PCS), viscosity data and NMR. Polymolecular micelles with a narrow size distribution and having a size of several nanometers were detected from PCS data. The size and aggregation number of the micelles largely depended upon the molecular characteristics (total molecular weight and the block composition) of the copolymer. The formation of micelles in different solvents has been interpreted in terms of solubility parameters. The influence of temperature and the formation of mixed micelles have also been examined.

Precipitation fractionation of block copolymers

Possibilities offered by classical precipitation fractionation in the preparative separation of polymer admixtures from block copolymers and in an estimate of the polydispersity in molar mass and of the heterogeneity in chemical composition of block copolymers have been examined. A mixture of a two-block copolymer, polystyrene-block-polyisoprene, with polystyrene was separated in the systems cyclohexane/1-propanol and 1,4-dioxan/1-propanol, and a mixture of three two-block styrene-isoprene copolymers having different chemical composition was fractionated in the system benzene/methanol. In the cyclohexane/1-propanol system, two commercial samples of three-block copolymers polystyrene-block-poly(ethene-co-butene)-block-polystyrene and their mixture were fractionated. The results showed that the polydispersity and chemical heterogeneity thus estimated are much lower than the real ones, especially due to the colloid properties of block copolymers.

The concentration dependence of the angular dissymmetry of light scattered by micellar solutions

A light scattering investigation was made of micelles formed from a polystyrene-poly(ethylene/propylene) two-block copolymer in n-nexane. n-Hexane is a selectively bad solvent for polystyrene and so polystyrene blocks formed the cores of the micelles. The light scattering measurements were made at 25°C for concentrations up to 0.015 g cm −3. The dissymetry ratio ( I 45° I 135° ) was found to decrease with increase in concentration and it was below unity for measurements at c > 4.8 × 10 −3 g cm −3. The effect of concentration on the dissymmetry ratio was predicted quite well by assuming the micelle packing could be described by radial distribution functions for hard spheres.