Phase separation induced ordered patterns in thin polymer blend films

Strategies for the utilization of phase separation to generate ordered pattern in polymer thin film are reviewed. First, the fundamental theory and factors influencing phase separation in polymer thin film are discussed. That is followed by a discussion of the formation of ordered patterns induced by phase separation in polymer thin films under the influence of a chemical heterogeneous substrate, convection or breath figures. The mechanisms and the conditions for well-ordered structures generated by phase separation are then discussed to show that multi-scaled/multi-component patterns, stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments more complex structures. Finally, applications of fabricated patterns in pattern generation and reproduction, antireflecting coating, catalysis, bio-chips and optoelectronics are also discussed.     

Composition effect on dewetting of ultrathin films of miscible polymer blend

Two kinds of dewetting and their transition induced by composition fluctuation due to different composition in blend [poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN)] films on SiO_x substrate at 145 °C have been studied by in-situ atomic force microscopy (AFM). The results showed that morphology and pathway of dewetting depended crucially on the composition. Possible reason is the variation in intensity of composition fluctuation resulted from the change of components in polymer blend. Based on the discussion of this fluctuation due to the composition gradient, parameter of U_q0/E, which describes the initial amplitude of the surface undulation and original thickness of film respectively, has been employed to distinguish the morphologies of spontaneous dewetting including bicontinuous structures and holes. Prior to the investigation of dewetting, it is confirmed that this blend is miscible at 145 °C using grazing incidence ultra small-angle X-ray scattering (GIUSAX).     

Studies on blends of binary crystalline polymers: Miscibility and crystallization behavior in PBT/PAr(I27-T73)

The miscibility and crystallization behavior of binary crystalline blends of poly(butylene terephthalate) [PBT] and polyarylate based on Bisphenol A and a 27/73 mole ratio of isophthalic and terephthalic acids [PAr(I27-T73)] have been investigated by differential scanning calorimetry (DSC). This blend system exhibits a single composition-dependent glass transition temperature over the entire composition range. The equilibrium melting point depression of PBT was observed, and Flory interaction parameter χ₁₂ = −0.96 was obtained. These indicate that the blends are thermodynamically miscible in the melt. The crystallization rate of PBT decreased as the amount of PAr(I27-T73) increased, and a contrary trend was found when PAr(I27-T73) crystallized with the increase of the amount of PBT. The addition of high-T_g PAr(I27-T73) would suppress the segmental mobility of PBT, while low-T_g PBT would have promotional effect on PAr(I27-T73). The crystallization rate and melting point of PBT were significantly influenced when the PAr(I27-T73) crystallites are previously formed. It is because not only does the amorphous phase composition shift to a richer PBT content after the crystallization of PAr(I27-T73), but also the PAr(I27-T73) crystal phase would constrain the crystallization of PBT. Thus, effects of the glass transition temperature, interaction between components, and previously formed crystallites of one component on the crystallization behavior of the other component were discussed and compared with blends of PBT and PAr(I-100) based on Bisphenol A and isophthalic acid.     

Influence of nanoparticle-polymer interactions on the apparent migration behaviour of carbon nanotubes in an immiscible polymer blend

We investigate the influence of nanoparticle-polymer interactions on the apparent migration behavior of multiwall carbon nanotubes (CNTs) in an immiscible polymer blend of ethylene-acrylate copolymer (EA) and polyamide 12 (PA). The polymer-CNTs interaction is tuned by using different surface modification strategies, comprising grafting and coating. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) are chosen as surface modifiers. The nanocomposite materials are prepared by melt-blending polymer-modified-CNTs in EA and PA. Polymer-grafted-CNTs tend to concentrate at the PA/EA interface, even if predispersed in PA, as opposed to pristine CNTs, which stay inside PA under the same circumstances. This new behavior is consistent with the morphology of PA/EA/(PMMA or PS) ternary blends and suggest a dominance of interfacial thermodynamics on CNTs localization. If we use polymer-coated-CNTs instead, the behavior depends on molar mass of the coating polymer. For low molar mass, it is similar to that of pristine CNTs and indicates desorption of the coating, owing to the weak interaction with the CNTs surface. Interestingly, we observe that long PS chains do not desorb and can drive the CNTs to the interface of the PA/EA blend. Moreover, the influence of kinetics is clearly observed through the dependence of CNTs interfacial confinement on dispersed droplet size.     

Study on miscibility of PEO and PCL in blends with PHB by solution viscometry

Viscometric studies were carried out for polymer-blend solutions in chloroform, i.e. for solutions of two polymers in a common solvent. Polymer blends comprised poly(3-hydroxy butyrate) (PHB) in combination with poly(ethylene oxide) (PEO) and poly(ε-caprolactone) (PCL), respectively. Experimental results obey the Huggins equation to an excellent approximation in the concentration range studied. Intrinsic viscosities vary linearly with blend composition to a good approximation. Huggins coefficients display generally nonlinear dependencies on blend composition. Positive deviations from perfect behaviour are attributed to attractions between different chain molecules or miscibility of the constituents. The viscometric study demonstrates that PEO is miscible with PHB, whereas PCL is immiscible.     

Effect of hydrodynamics on dynamics of phase separation in polystyrene/poly(vinyl methyl ether) blend

Polystyrene/poly(vinyl methyl ether) (PS/PVME) phase diagram was assessed by rheological tools and by on-line microscopy observations both under quiescent and shear flow conditions. Shear flow was found to induce both mixing and demixing of the mixture depending on the amplitude of the imposed shear rate. Viscoelastic properties of PS/PVME blends were also measured under steady shear flow near the phase separation temperature. At lower shear rate, flow enhances concentration fluctuation and induces phase segregation. At high shear rate, flow suppresses fluctuations and the polymer mixture keeps its miscible state. Several rheological signatures of phase transition were found. In steady shear flow, a secondary plateau in viscosity was observed when the temperature was close to T_s whereas, at the start-up shear flow, transient shear stress showed a second overshoot after a few minutes of shearing.     

Dendritic crystallization in thin films of PEO/PMMA blends: A comparison to crystallization in small molecule liquids

Dendritic crystallization of poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) thin films is reported. The film thickness is kept constant while the PMMA molar mass and blend composition are varied. Some basic features of dendritic growth, such as the diffusion length and tip curvature are discussed. The diffusion coefficient is tuned by varying the molar mass of the non-crystallizable PMMA and the blend composition. The observed dendrite tip radius is on the order of 50 nm and the shape of the growth envelope varies from square to needle-like as the PMMA molar mass or PMMA content is increased. The sidebranch spacing increases with the distance from the dendrite trunk with a power-law relationship that is also dependent on the PMMA molar mass and PMMA content. This coarsening process is similar to that reported for other classes of materials. These similarities (the curved dendrite tip, power-law relationship of the sidebranches, and the sidebranch coarsening processes) indicate that the large scale crystallization morphologies of the polymeric materials we study are similar to those found in crystallization of small molecules and metals.     

Nanocomposite polymer electrolyte comprising PEO/LiClO4 and solid super acid: effect of sulphated-zirconia on the crystallization kinetics of PEO

A novel PEO-based nanocomposite polymer electrolyte is prepared by using solid super acid sulphated-zirconia (, SZ) as the filler. Polarized optical microscopy (POM) and differential scanning calorimeter (DSC) results show that part of SZ particles may act as the nucleus of PEO spherulites and thus increase the amount of PEO spherulites. On the other hand, other SZ particles, which do not act as the nucleus, can restrain the recrystallization tendency of PEO chains through Lewis acid-base interaction and hence decrease the growth speed of PEO spherulites. As a result, the PEO component in PEO-LiClO₄-SZ can maintain a high amorphous state for a long time. The room temperature ionic conductivity of PEO-LiClO₄-SZ is relative high and stable compared with pristine PEO-LiClO₄, indicating that it is promising for all solid-state rechargeable lithium ion batteries.     

Surface energy inducing asymmetric phase distribution in films of a bynary polymeric blend

The phase morphology of blends of low density polyethylene (PE) with low molecular weight copolyamide (CPA) was investigated in films having 50-100 μm thickness. Films were prepared by compression moulding between two surfaces with different polarity, namely teflon and aluminium sheets, in a parallel plate heating press. The film surface characterization and surface energy deduction were performed by FT-IR/ATR spectroscopy and contact angle measurements, respectively. Moreover, the morphology and phase distribution were investigated by scanning electron microscopy both on the surfaces and on the cryogenic section of the films.The copolyamide resulted to be the dispersed phase in all compositions (90/10, 95/5, 97.5/2.5 and 99/1 PE/CPA) and the shape, dimension and distribution of the domains depending on temperature, pressure, time and nature of the surfaces used during the compression stage.The experimental evidences were discussed with respect to the different surface energies of the type of moulding material during the film preparation and related compatibility of the components.     

Solid-state NMR studies on phase behavior and motional mobility in binary blends of polystyrene and poly(cyclohexyl methacrylate)

The phase behavior and motional mobility in binary blends of polystyrene (PS) and poly(cyclohexyl methacrylate) (PCHMA) have been investigated by solid state ¹³C NMR techniques. The blend miscibility has been studied by examining the ¹H spin-relaxation times in the laboratory frame (T₁^H) and in the rotating frame (T_1ρ^H) for the PCHMA/PS blends with various compositions and pure components. The T_1ρ^H results show that PCHMA and PS are intimately mixed at the molecular level within the blends at all compositions. In addition, according to the results of carbon T_1ρ relaxation time measurements, we conclude that mixing is intimate enough to cause a reduction in local chain mobility for PS, but an increase in side chain mobility for PCHMA.     

Compatibilisation of heterogeneous acrylonitrile-butadiene rubber/polystyrene blends by the addition of styrene-acrylonitrile copolymer: effect on morphology and mechanical properties

Compatibility of polystyrene (PS) and acrylonitrile-butadiene rubber (NBR) blend is poor, hence technological compatibilisation was sought by the addition of styrene-acrylonitrile copolymer (SAN). The interfacial activity of SAN was studied as a function of compatibiliser concentration by following the morphology of three different blend series, viz. PS/NBR 30/70, 50/50 and 70/30. Incorporation of SAN into PS/NBR blends improved tensile, tear, hardness and impact properties. Addition of SAN beyond the saturation level (critical micelle concentration) adversely affected the ultimate properties. Attempts were made to understand the conformation of the compatibiliser at the interface. The protocol of mixing was varied, and, its effect on the mechanical properties was investigated. The experimental results were compared with the theoretical predictions of Noolandi and Hong.     

Density effect on mixing and segregation processes in a vibrated binary granular mixture

The mixing and segregation processes of binary granular mixture with identical sizes but different densities particles subjected to vertical oscillatory excitation are investigated in this study. The spatial distributions of vibrated binary steel-glass beads mixture are visualized by simulation and the results are similar to the experimental data. The time evolution of pattern formations show that the heavy particles first move toward the center of the bed and then concentrate near the centers of the two convection cells of the vibrated system. The mechanism causing the mixing and segregation is strongly dependent on the momentum exchange of each species which is related to the granular temperature gradients of mixture components. The influences of solid fraction and granular temperature profiles on the mixing of the mixture are examined under different operating conditions. The simulation results show that the granular temperatures of heavy particles are higher than those of light particles, indicating that the granular temperatures do not equilibrate for the mixture system. The convection motion also plays an important role in determining the mixing of the system. For understanding the extent of granular mixing, the segregation intensity was determined to quantify the mixing rate of binary mixtures. The segregation intensity shows that the mixing rate increases with the vibration strength, but decreases with the initial heights of mixture.     

Investigation of different crystal habits without chemical additives in a three-phase reactor

A lot of powders with fine particle sizes and specific crystal habits are produced by precipitation processes, in which growth and morphology of the particles are controlled by a wide range of chemical agents. This study investigates alternative ways to limit crystal growth and influence crystal morphology. Experiments are carried out in an ultrasound levitator, where a single droplet can be suspended against gravity by an acoustic levitation force. The ultrasound leviator is a three-phase reactor, which allows the investigation of precipitation with applying specific shear forces at growing crystals within the droplet. The investigated system is calcium carbonate. By variation of physical reaction parameters as temperature and level of applied shear forces it is possible to obtain different crystal habits and morphologies. The morphology of calcium carbonate produced in the leviator can vary between prismatic, shell-like and spherical shape. Also the particle size distribution of the precipitated product is influenced. Increasing mechanical stress leads to a shift of the particle size distribution to smaller sizes.     

Molecular weight effect on swelling of polymer gels in homopolymer solutions: a fluorescence study

A novel technique based on in situ steady state fluorescence measurements is introduced for studying swelling processes of gels formed by free radical crosslinking copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM) in homopolymer solutions. Gels were prepared at 55±2 °C for various EGDM contents. After drying these gels, swelling experiments were performed in chloroform solution of anthracene labeled poly(methyl methacrylate) (An-PMMA) in various molecular weights at room temperature by real time monitoring of anthracene fluorescence intensity. Anthracene labeled PMMA chains having various molecular weights were prepared by atom transfer radical polymerization at 90 °C. During the swelling experiments, it was observed that anthracene emission intensities increased due to trapping of An-PMMA chains into the gel as the swelling time is increased. The trapping of An-PMMA chains in swollen gel, increase by obeying parabolic law in time. Penetration time constant, τ of PMMA chains were measured and found to be increased as the crosslinker density of gel is increased. It is observed that τ values are much higher for high molecular weight An-PMMA chains than low molecular weight chains in all gel samples.     

Compatibilization mechanisms of nanoclays with different surface modifiers in UCST blends: Opposing effects on phase miscibility

The effects of organically modified and pristine nanoclays on the kinetics of thermodynamic equilibrium state attainment for semicrystalline binary blends of polyethylene (PE)/ethylene-vinyl acetate copolymer (EVA) have been investigated. Due to the non-equilibrium compatibilization mechanism, intercalated organoclay results in a slower rate of phase miscibility change at lower annealing temperatures, thereby worsening the PE/EVA compatibility state. In contrast to poorly dispersed pristine nanoclay, the homogeneous state is obtained at higher or equal rates by adding organoclay at higher annealing temperatures because of the dominant role of nanofiller equilibrium compatibilization mechanism. Phase diagrams of these UCST blends determined by a dynamic method shifts to higher temperatures by the incorporation of nanofillers and the unexpected reduction in miscibility window area is much more noticeable for nanocomposites having highly restricted molecular movements. This can verify that dynamic methods lose their efficiency for measuring the equilibrium phase diagram of polymer blends containing nanoparticles.     

Epoxidation of surface-bound polynorbornene films on silicon: Preventing degradation via solvent effect

We describe a strategy to prevent the detachment of polymer chain during the epoxidation of covalently grafted polynorbornene (PNb) films. Surface-bound PNb was prepared by surface-initiated ring-opening metathesis polymerization (SIROMP) from Si/SiO₂ substrates, and then epoxidized with meta-chloroperoxybenzoic acid by taking advantage of the double bonds along the polymer chains. The polymer-solvent interactions of three model solvents (dichloromethane, ether and methanol) with grafted PNb and grafted epoxidized PNb were studied by measuring the intrinsic viscosity of the free counterparts in solutions with a capillary viscometer. Oxidative degradation of the films was evaluated by ellipsometric thicknesses. The change in surface morphology was characterized with atomic force microscopy (AFM). Reactions of degradation and epoxidation of grafted PNb in three model solvents were studied. Kinetics study revealed that the surface-bound PNb is epoxidized gradiently from the surface toward the silicon substrate in ether. Through the solvent control, epoxidation of surface-bound PNb film without degradation was achieved under relatively mild condition.     

Microphase separation in ABA triblock copolymer-based model conetworks in the bulk: Effect of loop formation

This work estimates the free energy and domain size and predicts the phase diagram of microphase-separated model polymer conetworks, consisting of end-linked ABA triblock copolymers with cross-link points of relatively low functionality, in the strong segregation limit (highly incompatible A and B). Considering the case when these conetworks were prepared in the disordered state, their microphase separation is accompanied by extensive loop formation, with the fraction of loops depending on the morphology. The free energy of these conetworks is approximated using the expressions for linear block copolymers, modified to take into account the presence of loops whose free energy is estimated in two different approaches. The domain sizes in the conetwork system are found to be larger than those of the same morphologies in the linear system because of the reduction of the retractive force due to loop formation in the former system. The conetwork phase diagram is very sensitive to the expression used to describe the energy for loop formation.     

Phase dynamics and wetting layer formation mechanisms of pattern-directed phase separation in binary polymer mixture films with asymmetry compositions

Focusing on the binary polymer mixture films under the off-critical condition, the phase dynamics and wetting layer formation mechanisms of pattern-directed phase separation are numerically investigated. The simulated results demonstrate that, for different compositions, the polymer mixtures on the strip patterned surface can exhibit various phase morphologies in the strips of the bulk, which can be used to tailor the microscopic structures of films. The evolutions of these phase structures in the strips of the bulk obey almost the same power law with an exponent of 1/3, i.e., the Lifshitz-Slyozov growth law for the films with various off-critical degrees. It is found that the wetting layer thickness near the patterned surface grows logarithmically at the initial stages, just like the wetting layer formation mechanism of the polymer mixture near the surface with an isotropic potential. This revels that only patterning the surface potential may not change the growth law of the wetting layer. The simulated results also indicate that the diffusion of the component in the direction parallel to the surface originates from the edge of the strips.     

Semi-batch reactive crystallization of sodium perborate tetrahydrate: effect of mixing parameters on crystal size

Semi-batch reactive precipitation of sodium perborate tetrahydrate was studied in a stirred reactor. Effect of hydrodynamic parameters and reactant concentrations on the average crystal size was examined and it was found that secondary nucleation plays a dominant role in deciding the final crystal size. The data of average crystal size was correlated with power consumption per unit mass and solid loading.     

The effect of the polymerization route on the amount of interphase in structured latex particles and their corresponding films

Three series of hard/soft styrene-acrylic latex based systems with equivalent compositions were prepared either by blending of homopolymer latexes or by preparing structured latex particles having core shell (CS) or inverted core shell (ICS) morphologies. Transmission electron microscopy (TEM) was used to investigate the particle morphologies, which were correlated to the calculated fractional radical penetration for the propagating species during the reactions. The thermo-mechanical properties as well as the morphology of the resulting latex films were analyzed by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and TEM. The viscoelastic properties of the interphase between the first and second-stage polymers formed in the structured hard/soft latex films, as well as its qualitative amount and also the film morphologies were found to depend on the interplay between thermodynamic and kinetic parameters during the synthesis of the samples.