Dissipative particle dynamics study on the interfaces in incompatible A/B homopolymer blends and with their block copolymers

Dissipative particle dynamics, a simulation technique appropriate at mesoscopic scales, has been applied to investigate the interfaces in immiscible binary A/B homopolymer blends and in the ternary systems with their block copolymers. For the binary blends, the interfacial tension increases and the interface thickness decreases with increasing Flory-Huggins interaction parameter chi while the homopolymer chain length is fixed. However, when the chi parameter and one of the homopolymer chain length is fixed, increasing another homopolymer chain length will induce only a small increase on interfacial tension and slight decrease on interface thickness. For the ternary blends, adding the A-b-B block copolymer will reduce the interfacial tension. When the mole number of the block copolymer is fixed, longer block chains have higher efficiency on reducing the interfacial tension than the shorter ones. But for the block copolymers with fixed volume fraction, shorter chains will be more efficient than the longer ones on reducing the interfacial tension. Increasing the block copolymer concentration reduces interfacial tension. This effect is more prominent for shorter block copolymer chains.     

Temperature stability of the interfacial structure between a sulfonated crystalline alkyl side-chain polymer and a soft adhesive

The fracture toughness of interfaces between a sulfonated alkyl side-chain graft copolymer and a soft acrylic random copolymer containing acrylic acid monomers was investigated with a probe test method. Interfaces between a thin (100 nm) layer of the block copolymer and a thick (100 microm) layer of the acrylic copolymer were prepared at room temperature and subsequently annealed for 7 h at different temperatures. After the annealing step, the interface was quenched to room temperature and tested, a strategy that provides the advantage of keeping constant the mechanical properties of the materials on both sides of the interface so that any major difference in adhesive behavior can only be attributed to a change in the interfacial structure. For annealing temperatures below the crystalline to liquid crystalline transition temperature (86 degrees C), the adhesion energy remained very low and failure occurred by interfacial crack propagation. However when the interface was annealed above that temperature, a much higher adhesion energy was observed at room temperature because of the formation of a fibrillar structure upon debonding. The results indicate that the crystalline order at low temperature is very stable presumably because of the strong interactions between the sulfone groups in the side chains. However, when these interactions weaken and the side chains become liquid crystalline, the surface reconstruction mechanism cannot be prevented and strong interactions formed between the polar parts of the copolymer and the acrylic acid. These strong interactions remain during the cooling step, and a mechanism of surface reconstruction is proposed.     

分子量多分散性对AB二嵌段共聚物相行为的影响

采用Monte Carlo方法研究了分子量多分散性对AB型嵌段共聚物相行为的影响. 通过调整嵌段共聚物中组分含量, 考察了整体多分散性和单嵌段多分散性对嵌段共聚物共混物的有序-无序转变(Order-disorder transition, ODT)、 形貌及链尺寸的影响. 研究结果表明, 多分散度的增大使无序相向较大χN区域略微移动, 形成的片层结构厚度增加. 在形成微观有序形貌后, 较大分散度时各亚组分的链会得到更大的伸展, 表明分子链堆积受挫的程度减小, 因此, 涨落作用受到的抑制作用减小, 无序相区向更低温度区域移动.     

Effect of Endfunctionality of Reactive Polymers on the Reaction Kinetics at Immiscible Polymer Interfaces: A Monte Carlo Study

Reactions at the interface of two immiscible polymers containing different reactive groups at either one end or both ends are studied with Monte Carlo (MC) simulations. The MC simulation shows that the copolymer concentration at the interface is shown to dramatically increase during the early stage of reaction and then levels off at a constant value. The effect of endfunctionality, i. e., the effect of the number of endfunctional groups, is also investigated. While the saturation value of interfacial coverage is proportional to the initial reactive polymer density for the case of mono‐endfunctional polymer, the simulation results with di‐endfunctional polymers show that the saturation copolymer coverage is not exactly proportional to the initial reactive polymer density in the case of high concentrations of the initial reactive polymer. This is believed to be caused by the change of conformation of block copolymers formed at the interface due to reaction: the fraction of loop conformation decreases while the tail fraction increases with a large amount of initial reactive di‐endfunctional polymer. Also, the experimentally determined time‐dependent interfacial fracture toughness, which is, in turn, related to the copolymer coverage at the interface, is in good qualitative agreement with the simulation results.     

Self-assembly of polystyrene-block-poly(ethylene oxide) copolymers at the air-water interface: is dewetting the genesis of surface aggregate formation?

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.     

Dependence of the interfacial reaction and morphology development on the functionality of the reactive precursors in reactive blending

PMMA containing 50 wt% of anthracene-labeled PMMA chains end-capped by a phthalic anhydride group (anth-PMMA-anh) has been melt blended at 180°C with PS containing 33 wt% of chains end-capped by an aliphatic primary amine (PS-NH₂) and PS bearing 3.5 pendant amine groups (as an average) along the chains (PS-co-PSNH₂), respectively. The reactive chains have been synthesized by atom transfer radical polymerization. Conversion of anth-PMMA-anh into PS-b-PMMA and PS-g-PMMA copolymers has been monitored by SEC with a UV detector. The interfacial reaction mainly occurs in the initial melting and softening stage (<1.0 min.), although at a rate which strongly depends on the number of reactive groups attached to PS chains, the higher conversion being observed for the PS-co-PSNH₂ containing blends. The phase morphology depends on the architecture of the in-situ formed copolymer. Indeed, a coarser phase dispersion is observed in case of the graft copolymer compared to the diblock.     

Influence of the efficacy of interfacial modification on the coarsening of cocontinuous PS/HDPE blends during quiescent annealing

This article studies the quiescent annealing of three different cocontinuous polystyrene/high‐density polyethylene blends modified with two types of interfacial agents of widely different efficacies. Quantitative analysis of phase growth was obtained using mercury porosimetry. In a previous work, it was shown that one of these modifiers, a symmetrical diblock copolymer, has a high affinity for the interface and demonstrates virtually no micelle formation prior to saturation of the blend interface. The other modifier, a hydrogenated SEBS of 70/30 composition, forms micelles at elevated concentrations of modifier. In this study, it is shown that the cocontinuous phase size grows linearly without modifier, whereas the addition of both interfacial modifiers significantly suppresses the PE/PS phase coarsening and results in nonlinear phase growth behavior. The effect of the diblock copolymer on suppressing coarsening, however, is much more effective than that for the triblock case clearly supporting the tendency toward micelle formation for that latter modifier. In the case of unmodified PE/PS, the driving force for capillary pressure effects is so high that it is the capillary instability phenomena that dominate the coarsening and hence result in a linear growth of pore size with annealing time. When interfacial modifiers are added, the influence of reduced interfacial tension and lower pore size polydispersity significantly diminishes both capillary pressure effects and capillary instability phenomena. In that case, capillary pressure becomes the main rate determining step resulting in a nonlinear dependence of pore size with annealing time. It is shown that both the viscosity of the phases and the temperature of annealing can strongly influence coarsening behavior at low levels of interfacial modifier. Under all those conditions, however, nonlinear phase growth for the partially compatibilized system was maintained. These results clearly show that careful quantitative coarsening experiments using mercury porosimetry can be used as a tool to analyze the efficacy of interfacial modifiers for highly continuous or cocontinuous systems. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 711–721, 2006     

Formation of interfacial microstructures of Mo‐coating modified SiCf/Mo/Ti‐6Al‐4V composites

This paper addresses the role of Mo coating to modify the interface of SiC fiber reinforced Ti‐6Al‐4V composite (SiC_f/Mo/Ti‐6Al‐4V). The formation of microstructure as well as the diffusion of elements in the interface of as‐prepared and heat‐treated SiC_f/Mo/Ti‐6Al‐4V composites was investigated. The results show that the phases formed at the interfacial zone are: Mo coating∣TiC∣Mo + β‐Ti∣β‐Ti∣β‐Ti + α‐Ti_strip, ordering from fiber to matrix. Mo coating can effectively hinder the diffusion of elements between the matrix and fiber to some extent, thus it can inhibit fiber/matrix interfacial reaction and protect the fiber from damage. It is believed that the β‐Ti layer formed around the interface can improve the formability of composites. Furthermore, it indicates that Mo coating exhibits excellent thermal stability bellow 700 °C according to the heat treatment of the composites at 700 °C for up to 200 h. Copyright © 2012 John Wiley & Sons, Ltd.     

The effect of synthesis conditions on the microheterogeneity of the butadiene-isoprene copolymer

The dependence of the microheterogeneity of butadiene-isoprene copolymers synthesized in the presence of a titanium-based catalytic system on the hydrodynamic regime used at the initial moment of formation of the reaction mixture of copolymerization is studied. When copolymers are synthesized with the direct addition of the catalyst to the solution of the comonomers, copolymers featuring a statistical distribution of units are formed, regardless of the composition of the initial comonomer mixture. In this case, the growth of copolymer chains obeys the Bernoulli statistics. Depending of the composition of the initial comonomer mixture, hydrodynamic factors promote different extents of deviation from the statistical growth of copolymer chains.     

PAmXD,6/PP-g-MA blends. III. Microstructure,blend melt viscosity,and copolymer concentration relationship

This work deals with the relationship between microstructure, melt viscosity, and copolymer concentration of PAmXD,6/PP-g-MA blends [poly(m-xylylene adipamide)/maleic anhydride functionalized polypropylene]. The blends were processed in a Brabender plastograph at a temperature of 265 ± 5°C and at 45 rpm. The characterization of the microstructure was carried out through SEM analysis after microtome leveling and chemical etching. The melt viscosity of the components and of the blends was measured by the Brabender torque. It was found that the copolymers concentration controls the dimension of the dispersed phase. The composition of the blend (dispersed phase weight percent) has a more limited influence. Variations of the components viscosity ratio during the mixing time have little, if any influence on the dimension of the dispersed phase. A linear relation between the Brabender torque and the specific interfacial area was found. The determination of the copolymer weight fraction leads to the establishment of a close relation between the copolymer concentration and the specific interfacial area. For blends containing from 0 to 7.5 wt % of copolymer, this relation is linear and consequently the concentration of copolymer at the interface is constant at about one copolymer macromolecule per 16 nm². © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1313–1327, 1997     

Maillard Reaction between Leucine and Glucose in O/W Microemulsion Media in Comparison to Aqueous Solution

The Maillard reaction is controlled by temperature, pH, reactant nature (sugars and amino acids), and water activity. We carried out reactions between glucose and leucine in U‐type nonionic microemulsions to obtain regioselectivity and control reaction rates. Reactants were oriented at the interface and water activity was adjusted using blends of surfactant and propylene glycol (PG). U‐type microemulsions, previously studied by us, served as microreactors for the Maillard reaction. The reactions in the microemulsion media were slower than those carried out in aqueous solution and formed unique aroma compounds. Reaction rates increased when using systems richer in water, as the water activity was enhanced. The surfactant plays a key role in determining water activity and reagent reactivity in all the microemulsions. The presence of PG slows the reaction, mainly when it resides at the interface, facilitating the formation of a bicontinuous structure. Phase transitions within the U‐type microemulsions were determined by viscosity and SD‐NMR and were correlated to the interfacial presence of the reactants and their reactivity.     

Block copolymer-mediated synthesis of size-tunable gold nanospheres and nanoplates

We have successfully controlled the size and shape of isotropic and anisotropic gold nanocrystals through a one-step reaction by using amphiphilic polyethylene oxide-polystyrene oxide block copolymers as both reductant and stabilizing agents in water solution. Spherical or quasispherical nanoparticles were obtained at room temperature with tunable mean sizes and polydispersities depending on reaction conditions, that is, on copolymer block length, and copolymer and gold salt concentrations. By moderate increases of reaction temperature up to 65 degrees C, progressive formation of single-crystalline gold nanoplates in good yields takes place (up to 70%) without the necessity of additional reactants or growing solutions. These nanoplates are characterized by lateral mean sizes between 0.1-1.2 microm depending on copolymer concentration and reaction temperature, with mainly truncated or rounded triangular shapes with {111} planes as two basal surfaces. This allows us to tune the surface plasmon band of the nanoplates from ca. 850 nm to more than 1100 nm, well inside the near-infrared region (NIR), which enables the use of these type of nanostructures as a very promsing materials in applications such as optical coatings, SERS, and cancer cell hyperthermia. We proposed that the growth of these nanostructures can stem from a decrease in the reaction rate as temperature increases due to an enhanced copolymer hydrophobicity, which gives rise to a structure of interacting micelles formed from the fluid via a percolation transition (known as "soft gel") at elevated temperatures. In this way, reduction becomes slow enough to allow kinetic control of the reaction, and preferential adsorption of the copolymer molecules/micelles on certain crystallographic planes can favor the growth of certain nanocrystal facets to give the final structure. This alternative water-based system provides a more convenient and environmentally benign route to the synthesis of shape-controlled noble-metal nanocrystals in high yield because it does not involve toxic organic solvents or reagents and serves as a bridge between two frontline discipline: the block copolymeric science and anisotropic nanoparticles.     

Composition and solution properties of fluorinated block copolymers and their surface structures in the solid state

A series of diblock copolymers composed of methyl methacrylate and 2-perfluorooctylethyl methacrylate (PMMA₁₄₄-b-PFMA _n ) with various PFMA block lengths were prepared by atom transfer radical polymerization (ATRP). The surface structures and properties of these polymers in the solid state and in solution were investigated using contact angle measurement, X-ray photoelectron spectroscopy (XPS), sum frequency generation (SFG) vibrational spectroscopy, surface tension and dynamic laser light scattering (DLS). It was found that with increasing PFMA block length, water and oil repellency decreased, the ratio of F/C increased with increasing film depth, and the degree of ordered packing of the perfluoroalkyl side chains at the surface decreased. When the number of PFMA block units reached 10, PMMA segments were detected at the copolymer surface, which was attributed to the PFMA block length affecting molecular aggregation structure of the copolymer in the solution and the interfacial structure at the air/liquid interface, which in turn affects surface structure formation during solution solidification. The results suggest that copolymer solution properties play an important role in structure formation on the solid surface. Supported by the National Natural Science Foundation of China (Grant Nos. 50573069 and 20704038) and Program for Changjiang Scholars and Innovative Research Team in University (Grant No.IRT 0654)     

Micellar induced regioselectivity in the two-step consecutive reaction of SO3(2-) with Br-(CH2CH2)n-Br (n=2-5)

High field (800 MHz) (1)H NMR was used to monitor the two-step consecutive reaction of excess SO(3)(2-) with symmetrical bifunctional alpha,omega-dibromoalkanes with butane (DBB), hexane (DBH), octane (DBO), and decane (DBD) chains in CTAB micelles at 25 degrees C. The first-order rate constant for the first substitution step for DBB and DBH is about 5 times faster than for the second, but the kinetics for DBO and DBD were not cleanly first-order. After 40 min, the solution contained about 80% of the intermediate bromoalkanesulfonate from DBB and DBH and the remainder is alkanedisulfonate and unreacted starting material. The same reactions were carried out in homogeneous MeOH/D(2)O solutions at 50 degrees C. The rate constants for all four alpha,omega-dibromoalkanes were first-order throughout the time course of the reaction and the same within +/-10%. However, because micellar solutions are organized on the nanoscale and bring together lipophilic and hydrophilic reactants into a small reaction volume at the micellar interface, they speed this substitution reaction considerably compared to reaction in MeOH/D(2)O. The CTAB micelles also induce a significant regioselectivity in product formation by speeding the first step of the consecutive reaction more than the second. The results are consistent with the bromoalkanesulfonate intermediates having a radial orientation within the micelles with the -CH(2)SO(3)(-) group in the interfacial region and the -CH(2)Br group directed into the micellar core such that the concentration of -CH(2)Br groups in the reactive zone, i.e., the micellar interface, is significantly reduced. These results provide the first example of self-assembled surfactant system altering the relative rates of the reaction steps of a consecutive reaction and, in doing so, enhancing monosubstitution of a symmetrically disubstituted species.     

Stabilization of polymer blend structure by diblock copolymers

The stabilizing (emulsifying) effect of a symmetric diblock copolymer in the mixture of two immiscible homopolymers is considered. The equilibrium value of the interfacial area per copolymer chain is calculated via minimization of the free energy of the mixture for a fixed number of copolymer chains adsorbed to the interface. The size and concentration of droplets of the minor component are determined for the equilibrium state. The particles' radius is shown to be inversely proportional to the copolymer concentration, the coefficient of proportionality being dependent on the Flory-Huggins parameter and chain length. The penetration of homopolymer segments into the copolymer layer on the interface is taken into account and proved to be important for stabilization of the droplets by symmetric copolymers. The conditions of the validity of the presented approach are discussed in detail.     

Interfaces in polymer blends

We investigate the structure and thermodynamics of interfaces in dense polymer blends using Monte Carlo (MC) simulations and self‐consistent field (SCF) calculations. For structurally symmetric blends we find quantitative agreement between the MC simulations and the SCF calculations for excess quantities of the interface (e.g., interfacial tension or enrichment of copolymers at the interface). However, a quantitative comparison between profiles across the interface in the MC simulations and the SCF calculations has to take due account of capillary waves. While the profiles in the SCF calculations correspond to intrinsic profiles of a perfectly flat interface the local interfacial position fluctuates in the MC simulations. We test this concept by extensive Monte Carlo simulations and study the cross‐over between “intrinsic” fluctuations which build up the local profile and capillary waves on long (lateral) length scales. Properties of structurally asymmetric blends are exemplified by investigating polymers of different stiffness. At high incompatibilities the interfacial width is not much larger than the persistence length of the stiffer component. In this limit we find deviations from the predictions of the Gaussian chain model: while the Gaussian chain model yields an increase of the interfacial width upon increasing the persistence length, no such increase is found in the MC simulations. Using a partial enumeration technique, however, we can account for the details of the chain architecture on all length scales in the SCF calculations and achieve good agreement with the MC simulations. In blends containing diblock copolymers we investigate the enrichment of copolymers at the interface and the concomitant reduction of the interfacial tension. At weak segregation the addition of copolymers leads to compatibilization. At high incompatibilities, the homopolymer‐rich phase can accommodate only a small fraction of copolymer before the copolymer forms a lamellar phase. The analysis of interfacial fluctuations yields an estimate for the bending rigidity of the interface. The latter quantity is important for the formation of a polymeric microemulsion at intermediate segregation and the consequences for the phase diagram are discussed.     

Reactive blending: inhomogeneous interface grafting in melts of maleinated polystyrene and polyamides

To be competitive, most blends need compatibilizers, usually copolymers with a blocky architecture, the chains of which cover the interfaces between the blend phases, refining the phase morphology and improving the interface strength. When the blend components are suitably functionalized, such copolymers can be conveniently generated in situ, in processes of reactive blending. Normally, graft copolymers are created. The polymer–polymer coupling proceeds exclusively in the interfaces. This interface grafting is (i) pivotal in the design of modern blend systems and (ii) an interesting route towards novel copolymers. The complex kinetics of interface grafting in blend melts have so far attracted little attention. In a model study, amino terminated polyamide 12 (PA) was grafted in the melt onto heavily maleinated polystyrene (SMA; S: styrene and MA: maleic anhydride). Anhydride and amino functions react at high temperatures fast and irreversibly by imide condensation. A series of SMA/PA blends differing in composition and PA chain lengths was investigated, with the aim of driving the grafting to high conversions so a pure graft copolymer SMAgPA would result, instead of an SMA/PA/SMAgPA blend. However, a pure copolymer was never obtained. The grafting remained incomplete, except with very short-chained PA and only at equal weight fractions of SMA and PA. More importantly, the SMA chains were never grafted evenly. Instead, “overgrafted” and “undergrafted” chains SMAgPA coexisted in one and the same product. It appears that the SMAgPA chains form an auto-inhibitory barrier in the interfaces that prevents random grafting. Grafting proceeds to high conversion only in SMA/PA blends with a co-continuous phase morphology where the interfaces are constantly torn apart and renewed, during melt blending, so the reaction is constantly reactivated. © 1998 John Wiley & Sons, Ltd.     

Adsorption of hydrophobically modified polyelectrolytes at the n-octane/water interface

The interfacial activity of polyelectrolytes carrying alkyl side chains of different length has been studied. Potassium salts of poly(maleic acid-co-1-olefins), PA-n K2 with n=12 , 14, 16, 18, were synthesized, and the interfacial tension at the aqueous solution/n -octane interface was measured as a function of the length of the alkyl side chain. The results show that the interfacial tension lowering, the limiting excess concentration Gamma (m), and the efficiency of adsorption pC (20) depend on the number of methylene groups in the alkyl side chain. According to Rosen the last two parameters define two different contributions to the standard free energy of adsorption: one arises from the distribution of the polymer between the bulk of the solution and the interface Delta G (dist )(0), and another comes from the configuration adopted at the interface Delta G (int )(0). These free energies were plotted as a function of the number of carbon atoms in the alkyl side chain and a linear relation was found for both of them. From these plots contributions of 0.83 and -0.58 per methylene group were determined for Delta G (0)(dist ) and Delta G (0)(int ), respectively. The positive value for the incremental free energy of distribution is attributed to the formation of a polymer micelle which is stabilized by longer alkyl side chains. On the other hand, the negative value for Delta G (0)(int ) indicates that at the interface the polymer adopts a configuration where the hydrocarbon tail is interacting with the octane molecules.