The structure and phase transitions in polymer blends,diblock copolymers and liquid crystalline polymers: The Landau-Ginzburg approach

The polymer systems are discussed in the framework of the Landau-Ginzburg model. The model is derived from the mesoscopic Edwards Hamiltonian via the conditional partition function. We discuss flexible, semiflexible and rigid polymers. The following systems are studied: polymer blends, flexible diblock and multi-block copolymer melts, random copolymer melts, ring polymers, rigid-flexible diblock copolymer melts, mixtures of copolymers and homopolymers and mixtures of liquid crystalline polymers. Three methods are used to study the systems: mean-field model, self consistent one-loop approximation and self consistent field theory. The following problems are studied and discussed: the phase diagrams, scattering intensities and correlation functions, single chain statistics and behavior of single chains close to critical points, fluctuations induced shift of phase boundaries. In particular we shall discuss shrinking of the polymer chains close to the critical point in polymer blends, size of the Ginzburg region in polymer blends and shift of the critical temperature. In the rigid-flexible diblock copolymers we shall discuss the density nematic order parameter correlation function. The correlation functions in this system are found to oscillate with the characteristic period equal to the length of the rigid part of the diblock copolymer. The density and nematic order parameter measured along the given direction are anticorrelated. In the flexible diblock copolymer system we shall discuss various phases including the double diamond and gyroid structures. The single chain statistics in the disordered phase of a flexible diblock copolymer system is shown to deviate from the Gaussian statistics due to fluctuations. In the one loop approximation one shows that the diblock copolymer chain is stretched in the point where two incompatible blocks meet but also that each block shrinks close to the microphase separation transition. The stretching outweights shrinking and the net result is the increase of the radius of gyration above the Gaussian value. Certain properties of homopolymer/copolymer systems are discussed. Diblock copolymers solubilize two incompatible homopolymers by forming a monolayer interface between them. The interface has a positive saddle splay modulus which means that the interfaces in the disordered phase should be characterized by a negative Gaussian curvature. We also show that in such a mixture the Lifshitz tricritical point is encountered. The properties of this unusual point are presented. The Lifshitz, equimaxima and disorder lines are shown to provide a useful tool for studying local ordering in polymer mixtures. In the liquid crystalline mixtures the isotropic nematic phase transition is discussed. We concentrate on static, equilibrium properties of the polymer systems.     

Theory of crystallizable block copolymers

We present an equilibrium theory of diblock copolymers in which one of the blocks is crystallizable and the other is amorphous. The material is assumed to order in a lamellar structure of alternating semi-crystalline and amorphous layers with the chemical bonds which connect the copolymer blocks lying in the interfacial regions between the layers. The amorphous blocks are modelled as flexible chains, each with one end (the joint) anchored in an interface. Their contribution to the free energy is calculated via the self-consistent solution of the modified diffusion equations. The crystalline regions are modelled as folded chains, also with one end in an interfacial region (bonded to the corresponding end of an amorphous block). We find that the calculated amorphous block-free energies can be expressed as a single universal function depending on the total degree of polymerization of the amorphous block, its stretching, and a parameter proportional to the thickness of the interface. We have fitted an analytical form to this function, which can be used for any amorphous block, and we have combined it with our model of the crystallizable block to obtain scaling laws describing the equilibrium morphology.     

聚烯烃／极性聚合物界面的分子状态

为了克服高分子共混物界面不易表征的缺点，提出用溶剂选择性溶解方法使界面暴露．结合Ｘ－射线光电子能谱（ＸＰＳ）表征手段，研究了官能化聚合物，接枝型共聚物及带有反应性基团的聚合物作为共混物增容剂时在界面区域的分子状态．实验结果表明，作增容剂时，官能化聚合物在界面区内采取最有利的分子构象，充分发挥增容作用；接枝型共聚物主链、侧链向相应本体聚合物内扩散；而带有反应性基团的聚合物与某个本体聚合物发生反应之前存在反应基团在界面富集的过程     

A novel method to characterize the compatibilizer effect in polymer blend

Both components of a polymer blend were labeled slightly with donor (naphthalene) and acceptor (anthracene), respectively and were mixed with equal amounts of various amounts of diblock copolymer. Adding diblock copolymer as a compatibilizer would improve blend compatibility and the degree of mutual segmental penetration between both components was also increased. In contrast, adding random copolymer has very little effect. Making use of nonradiative energy transfer, we can evaluate the efficiency of a compatibilizer delicately and determine the optimum concentration of compatibilizer for a blend. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3215–3219, 2007     

模拟研究石墨烯/生物基尼龙复合材料的界面热阻

生物基尼龙(PA56)源于天然产物,具有优良的环保性能和广阔应用前景,有望替代传统的石油基尼龙材料.为了开发基于PA56的导热材料,利用分子动力学模拟研究方法探索了石墨烯/PA56复合材料界面热阻的影响因素.首先,利用实验测试商用PA56样品的玻璃化转变温度(Tg)和导热系数(Tc),验证了PA56模型的模拟参数.接着,通过设计和比较不同表面改性状态对石墨烯/PA56复合材料的界面热阻的影响规律,最后,为了降低界面改性的难度,设计了一种新型的二嵌段共聚物作为石墨烯/PA56复合体系的界面改性剂,研究了界面改性剂的结构对界面热阻的影响规律.研究结果对于实验研究制备生物基尼龙导热复合材料具有重要的参考价值.     

Tem- and neutron reflection studies on surfaces and interfaces with block copolymers

The behavior of block copolymers at various interfaces is studied by transmission electron microscopy and neutron reflection. A thin film of a symmetric diblock copolymer of styrene and methyl methacrylate forms layer structures when in contact with air and a random copolymer of styrene and acrylonitrile containing 35 wt% acrylonitrile. When the random copolymer has an acrylonitrile content of 25 wt%, a competition between layer formation and diffusion of disordered micelles takes place. Driving force for these processes are different interfacial tensions and a changing miscibility behavior as a function of acrylonitrile contents of the random copolymers. The ordering behavior of a symmetric diblock copolymer of deuterated styrene and isoprene in contact with poly(3,5-dimethyl phenylene ether) is studied by neutron reflection. Polystyrene-block-poly(ethene-co-but-1-ene)-block-polystyrene with cylindrical PS microdomains shows an interfacial phase transition to lamellae near to the interface with different polymers. The morphological studies are in agreement with adhesion data obtained by peel tests on different bilayer specimens.     

Surface segregation of polypeptide-based block copolymer micelles: An approach to engineer nanostructured and stimuli responsive surfaces

We describe the surface segregation of polypeptide-based block copolymer micelles to produce stimuli-responsive nanostructures at the polymer blend/air interface. Such structures were obtained by simultaneous surface migration and self assembly at the surface of diblock copolymer/homopolymer blends. We employed blends composed of homopolymer (PS) and an amphiphilic block copolymer polystyrene-b-poly(l-glutamic acid) (PS-b-PGA). The surface was functionalized based on the preferential segregation to the polymer blend/air interface of the hydrophilic PGA block of the diblock copolymer upon annealing to water vapor. The surface migration of the diblock copolymer to the interface was demonstrated both by XPS and contact angle measurements. As a consequence, the PGA interfacial attraction leads to a large surface excess on diblock copolymer which in turn, through macrophase and microphase separation, produced separated domains at the surface with regions composed either of homo or block copolymer. Herein we demonstrate that the use of asymmetric diblock copolymers with a higher content in PS lead to spherical micellar assemblies randomly distributed at the surface. As observed by AFM imaging the blend composition, i.e. the amount of block copolymer within the blend influences the density of micelles at the surface. Finally, when exposed to water, the pH affects the surface morphology. The PGA segments are collapsed at low pH values and extended at pH values above 4.8, thus inducing variations on the topography of the films at the nanometer scale.     

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‐Forming Interlocking Interfaces on the Immiscible Polymer Bilayers via Gelation‐Mediated Phase Separation

Gelation‐mediated phase separation is applied to prepare immiscible polymer bilayer films with an interlocking interface structure. Polymer systems consisting of copolymer of urea and polydimethylsiloxane and epoxy are selected to demonstrate the feasibility. When the epoxy fraction exceeds 25 wt%, well‐defined bilayer structures self‐form by a one‐pot casting method in which the phase separation state is fixed by an evaporation‐induced gelation. Microscopy studies of the resulting bilayers clearly reveal that interlocking structures form during the bilayer films construct. The interlocking structures lead to an enhanced interfacial adhesion and higher fracture energy. The current strategy might offer a facile way to in situ create an interlocking interface between immiscible polymer systems.     

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.     

Interfacial properties of diblock copolymers at penetrable interfaces: Density profile,stretching, and interfacial tension

The interfacial properties of diblock (AB) copolymers near an interface between two solvents are studied by using the exact Green's function of a Gaussian copolymer chain at an attractive penetrable interface. We have studied the mean‐square end‐to‐end distance of the copolymer, 〈R²(z)〉, as a function of the distance of the joint point of the copolymer to the interface, z, the segment density distribution ρ(z), and the reduction of the interfacial tension Δγ_c due to the presence of the diblock copolymer. The density profile and the stretching of the copolymer chain are in agreement with both experimental results and simulations. The reduction in the interfacial tension is found to decrease with the increase in the degree of polymerization of the copolymer chain.     

On the compatibilization of polymer blends

By means of statistical thermodynamics we consider the effect of a diblock copolymer on the interface of a demixed homopolymer blend. In contrast to the usual case where the blocks are identical with the components of the homopolymers, we investigate the use of arbitrary blocks XY respective to the A-B blend. As examples, we calculate interfacial properties such as the interfacial tension and the width of the interface as well as the concentration profiles of the blocks. We expect a strong compatibilization effect if the blocks are sufficiently long and have a preferential repulsive interaction with one of the two homopolymers.     

Interfacial tension of evaporating emulsion droplets containing amphiphilic block copolymers: effects of solvent and polymer composition

Evaporating droplets of volatile organic solvent containing amphiphilic block copolymers may undergo hydrodynamic instabilities that lead to dispersal of copolymer micelles into the surrounding aqueous phase. As for related phenomena in reactive polymer blends and oil/water/surfactant systems, this process has been ascribed to a nearly vanishing or transiently negative interfacial tension between the water and solvent phases induced by adsorption of copolymer to the interface. In this report, we investigate the influence of the choice of organic solvent and polymer composition for a series of polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock copolymers, by in situ micropipette tensiometry on evaporating emulsion drops. These measurements suggest that the sensitivity to the organic solvent chosen reflects both differences in the bare solvent/water interfacial tension as well as the propensity of the copolymer to aggregate within the organic phase. While instabilities coincident with an approach of the interfacial tension nearly to zero were observed only for copolymers with PEO content greater than 15 wt.%, beyond this point the interfacial behavior and critical concentration needed to trigger surface instability were found to depend only weakly on copolymer composition.     

Relationship between macroscopic and microscopic models of surfactant adsorption dynamics at fluid interfaces

In a companion preceding paper, we presented an experimental investigation into the adsorption dynamics of a diblock copolymer surfactant to a polymer/polymer interface and found them to be well-described by a microscopic model of diffusion in a potential generated using self-consistent field theory. We compare the predictions of the microscopic approach with a macroscopic (adsorption-diffusion) model and demonstrate the equivalence of the two models when the free-energy well underlying surfactant adsorption is flanked by barriers that are significantly larger than thermal energy (kT). However, when the energy barriers are nonexistent, as is the case for the experimental system of interest, a finite interfacial width must be introduced into the classical model to obtain physically meaningful results (i.e., nonnegative desorption rates). Surprisingly, we find that the predictions of the macroscopic finite interfacial width model with no adjustable parameters are in excellent agreement with experimental data presented in the companion paper even though the latter was obtained with molecular resolution. This agreement provides insight into aspects of the free-energy landscape that determine surfactant transport.     

稳态和振荡剪切流动下聚合物共混反应体系演化的格子玻尔兹曼模拟

采用A/B/S三元共混体系为模型体系,其中S由A和B通过可逆的化学反应生成并充当增容剂.我们采用自由能形式的格子玻尔兹曼模拟方法,考察了在稳态及振荡剪切流动下化学反应速率、剪切速率和振荡频率对体系形态结构演化的影响.模拟给出了增容剂平均密度和空间分布随时间的演化,结果表明增容剂S的生成能有效地降低分散相的尺寸,并且通过控制化学反应速率、剪切速率和振荡频率能够有效地调控增容剂在体系中的分布,从而为控制反应共混体系的形态结构提供帮助.     

用于聚合物共混体系的共聚物增容剂

在不相容多相聚合物体系中，共聚物增容剂的存在将对共混物的形态结构及力学性能产生极大的影响，它们的作用是增强相间粘合力，减小相表面张力。本文系统论述了共聚物增容剂的分子设计及其分子特性对增容效果的影响，并分别讨论了反应型及非反应型共聚物增容剂的近期发展。     

Interface Orientation and Chain Conformation in Simulated Symmetric Diblock Copolymer Lamellar Systems

Summary: We describe the results of Monte Carlo simulations, based on the cooperative motion algorithm, of the lamellar structure generated at finite temperature by a symmetric diblock copolymer. The (70 × 70 × 70) simulation box in which the polymer chains were embedded for each simulation was rotated, based on the interface orientation, to bring the interfacial planes of the simulated structure into parallel. We found that the interface thickness, as defined by the distribution of the junction points, became narrower at lower temperature, and that the interface plane was characterized by a waviness with a maximum peak‐to‐valley distance of 20–30 lattice bonds. Compared with the isotropic state (T/N = ∞), chains at lower temperatures were stretched in the direction perpendicular to the interface; but only modestly compressed in the direction parallel to the interface. Individual block chains within the lamellar domains still behave like random coils. The block copolymer molecules exhibit only a modest tendency to orient themselves with their end‐to‐end vector perpendicular to the plane of the lamellar interface. Considered as an ensemble average, the results we obtained are similar to those reported from small angle neutron scattering measurements for the mean conformation of the PSd blocks of symmetrical PSd‐PVP diblock copolymers.

2‐D projections onto the X‐Z plane of the end beads for the A‐ and B‐chains (gray) and the junction points J (black) at T/N = 0.2. The interface plane is oriented parallel to the Y‐Z plane by rotating the simulation box. The distribution profiles of junction points and the end beads across the system in the direction of interface normal are shown in the lower part of the figure.     

纳米CaCO_3/相容剂/PP中的界面相互作用研究

采用不同相容剂(PP-g-MAH、POE-g-MAH和EVA-g-MAH)制备了不同界面相互作用的纳米CaCO3(CC)/相容剂/PP体系,研究了相容剂/PP和相容剂/CC界面相互作用对PP/CC的结晶形态、结晶行为、熔融特性和力学性能的影响.观察到PP/CC界面相互作用提高PP结晶温度和PP/CC的模量和冲击强度,但降低了屈服强度.相容剂/CC界面相互作用进一步提高了PP/CC的结晶温度.PP/相容剂界面相互作用取决于PP与相容剂相容性.PP/PP-g-MAH相容性高有利于提高PP/CC的异相成核作用和PP/CC屈服强度和模量,但降低冲击强度.PP/POE-g-MAH部分相容对相容剂/CC界面的异相成核作用、PP/CC屈服强度和模量影响不大,可明显提高冲击强度.但PP/EVA-g-MAH不相容导致PP/CC冲击强度明显降低.