Session K22 - Phase Behavior, Morphology and Thermodynamics of Polymer Thin Films.
MIXED session, Wednesday morning, March 22
208C, MCC

[K22.001] Suppression of lateral phase separation in thin polyolefin blend films

The effectiveness of a compatibilizer suppressing lateral phase separation in thin films is investigated as a function of film thickness and temperature. Neutron and x-ray reflectivity measurements were made on spun cast thin blend films of partially deuterated and hydrogenated polyolefin blends with and without diblock compatibilizer as a blend component. For ultrathin films (\leq 30 nm), finite size effects and the surface enrichment of the hydrogenated blend component suppress the development of lateral (undulation) features of phase separated structures. In thicker films (\approx 100 nm), the development of undulation features is suppressed only in the presence of the block copolymer compatibilizer. This stabilization effect can be attributed to the reduction of interfacial tension and presence of diblock copolymer in both phases and at the interphases. In general, temperature has a weak effect on the profile development in compatibilized blend films. Thinner films (L\approx 30 nm) are inherently more stable than thicker (L \approx 100nm) films.

[K22.002] Evolution of Phase Morphology and Capillary Fluctuations in Thin Film Polymer Blends

We have investigated wetting and phase separation in thin film (200 nm) blends of deuterated poly(methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) by forward recoil spectrometry and atomic force microscopy (AFM). Upon quenching a critical blend into the unstable regime, the depth profile evolves by symmetric wetting of the dPMMA-rich phase (denoted as A) at the air and substrate (trilayer structure), apparent remixing of A, and the formation of a thick A wetting layer encapsulating SAN-rich (B) droplets. Upon removing A and analyzing by AFM, a bicontinuous morphology is first observed, followed by uniform disks of A spanning the wetting layer, and finally an interconnecting B network which evolves into droplets. During the intermediate stage, FFT analysis reveals maximum wave vectors, qm1 and qm2, which decrease with time. The longer features (qm1) display three regimes and are explained in terms of the B layer capillary fluctuations. The shorter features (qm2) reflect the internal phase evolution and decay as t -0.33 during the intermediate stage. The evolution of both length scales provides new insight into the interplay between phase separation and capillary fluctuations in thin film polymer blends.

[K22.003] Phase Behavior in Thin Film Blends of Polystyrene and Poly (bromo-styrene)

The phase behavior of thin film blends of Polystyrene (PS) and Poly (bromo-styrene) (PBS) is studied using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). The extent of bromination in the PBS samples is varied from 4.5 to 22 weight percent. These studies are complimentary to interdiffusion studies of PS/PBS interfaces using Rutherford Backscattering Spectroscopy (RBS) to understand the structure and morphology of the growing interface. From RBS, a composition profile of the interface is determined. Selected compositions are fabricated as thin film blends by co-dissolving PS and PBS in toluene and spin cast onto a silicon wafer. Phase behavior is studied as a function of PS molecular weight, molecular weight distribution, degree of miscibility (controlled via extent of bromination), and annealing conditions. The blends are annealed at temperatures above the Tg of each polymer and annealing times ranging from 15 minutes to 72 hours are studied. We observe that the PS/PBS system exhibits UCST behavior. The mechanism of phase separation changes from nucleation to spinodal decomposition as the annealing temperature is increased. In addition, the domain sizes increase with annealing temperature and these have been characterized using SAXS studies. Molecular simulations have been used to theoretically predict phase diagrams for all the systems studied. Implications of these studies for the interdiffusion measurements are discussed.

[K22.004] Phase segregation of PS and PMMA blend under confinement

We have studied the phase segregation of a PS (Mw=62k) and PMMA (Mw=25k) blend under confinement suing AFM and STXM. The results show that phase segregation is severely hindered by confinement. Uniform cylindrical structures with a very small wavelength form rapidly and appear to be stable even after long annealing times. These cylinders are oriented perpendicular to the Si substrate and appear to originate from a thin adsorbed uniform layer of PMMA which coats the Si interfaces. Addition of 10% diblock copolymer decreases the wavelength even further, but otherwise does not change the structure of dynamics. Detailed studies of partially confined transition regions indicate that the confined films may be under large elastic tension leading to the observed morphologies.

[K22.005] Phase-Separating Thin Film Polymer Blends: The Effect of Film Thickness on Roughening

>From depth profiling and morphology studies, thin film blends of deuterated poly(methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) on a substrate are found to evolve by symmetric wetting of the dPMMA-rich phase (A) , phase coarsening of A within the SAN-rich (B) interphase layer, and dewetting of B into droplets encapsulated by A. For films 100 nm to 500 nm thick (h), the surface roughness displays three distinct regimes of roughening. Roughness increases rapidly at first, reaches a constant value, increases moderately, and then approaches a final value. Initially, the rate of roughening is independent of h because phase coarsening, which is not yet influenced by confinement, dominates the roughness. Thus, the duration of this stage increases linearly with h. During the second stage, the roughness stops increasing because dPMMA begins to flow from the surface wetting layer into the domains. Here, the roughness value increases from 2 nm to 10 nm as h increases. After this plateau, the roughness increases at a moderate rate but with a much larger lateral length scale than before. Eventually, the films rupture between 600 min and 3500 min, as h increases. The roughness reaches a final value which increases as h increases. Although the roughening characteristics are found to scale with h, the rate of roughening before rupture is much faster then a prediction based on capillary fluctuations in an A/B/A structure.

[K22.006] Influence of Lateral Confinement on Phase Separation in Thin Film Polymer Blends

Thin films of polymer blends (50/50) of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN), are molded into strips for investigating the influence of lateral confinement on phase separation. AFM and confocal microscopy are used to identify and quantify the stages of phase evolution by measuring the wetting layer thickness, the number density of PMMA-rich (A) domains, and the domain size. After symmetric wetting of the A phase, the wetting layer thickness along the center of the strip increases dramatically during the early stage, decreases slowly during the intermediate stage, and approaches a constant value during the late stage. The A domains inside the SAN-rich (B) matrix also grow rapidly during the early stage. These domains continue to grow by diffusion and coarsening during the intermediate stage to a maximum value, and then heal during the late stage. The final morphology is an elliptical B core along the strip encapsulated by the A phase. These studies demonstrate that confinement directs the formation of a self-assembled core/shell morphology, which has potential applications for creating micro-wires.

[K22.007] Morphology of Annealed Metastable Bilayer and Trilayer Polymer Films

By using a combination of spincoating and water transfer techniques, we have fabricated PS/PMMA/Si(001) bilayer and PMMA/PS/PMMA/Si(001) trilayer films, where PS is polystyrene and PMMA is poly (methyl methacrylate). When the bilayer films are heated, PS dewets on PMMA. When the trilayer films are heated, the buried PS molecules segregate to the top surface of the film to minimize the interfacial area between the two polymers and to maximize the area of the PS/air interface. We have characterized the changes in morphology obtained after annealing the trilayer films at high temperatures for a long time for different PMMA capping layer thicknesses. We have also compared the annealed morphology of the trilayer films with that of the bilayer films.

[K22.008] Dispersion-Driven Morphology and Hole Formation in Freely-Standing Trilayer Polymer Films

Upon annealing of freely-standing trilayer polymer films, a periodic lateral morphology can self-assemble, driven by the attractive dispersion force which acts across the film(K. Dalnoki-Veress, B.G. Nickel and J.R. Dutcher, Phys. Rev. Lett. 82), 1486 (1999).. We have studied the dependence of the morphology of PS/PI/PS freely-standing trilayer films on the thermal history of the films (PS is polystyrene and PI is polyisoprene). In particular, we have observed distinct differences in the morphology by changing the heating rate and maximum annealing temperature and time. By heating the trilayer films to higher temperatures, we have also observed the formation and growth of holes in the films.

[K22.009] Compatibilizing Effect of Block Copolymers Added to the Polymer / Polymer Interface: Effects of Additive MW, Architecture, and Composition

The effect of block copolymer addition on the reduction of the interfacial tension between two immiscible homopolymers is investigated as a function of the additive concentration, molecular weight, macromolecular architecture, and composition for polystyrene / polyisoprene blends in the presence of poly(styrene - isoprene) block and graft copolymers using the pendant drop method. The reduction of the interfacial tension is a non-monotonic function of the copolymer additive molecular weight at constant copolymer concentration. At the same time, the reduction is a strong function of the additive architecture with graft copolymers leading to a larger reduction; for graft copolymers, the maximum efficiency observed was for a non-symmetric copolymer composition. The results strongly indicate the importance of micelle formation on the efficiency of block copolymers as emulsifying agents.

[K22.010] The Segregation of Alternating Copolymers to the Biphasic Interface of an Immiscible Polymer Blend

The use of copolymers is a practical method to reinforce the interface of immiscible polymer blends. However, to be useful in industrial applications, the copolymer must diffuse to the biphasic interface within a blend during processing. Thus, the process of getting to the interface can be a limiting step in the utilization of copolymers as interfacial modifiers. Moreover, the role of copolymer sequence distribution on its ability to compatibilize blends is of current interest. Pursuant to this, neutron reflectivity experiments have been completed to determine the rate at which an alternating copolymer of polystyrene (PS) and poly(methyl methacrylate) (PMMA) diffuses to a biphasic interface. The effect of the molecular weight of the copolymer has also been examined. Qualitatively, the results show that the diffusion of an alternating copolymer at 1050/50 random copolymer to the bilayer interface.

[K22.011] Compatibilization of Polymer Blends with POSS

Uniform thin films of poly(methylmethacrylate)- polyhedral oligomeric silesquioxane (PMMA- POSS) and blends of other polymers and PMMA -POSS could be produced by dissolving the polymers in a suitable solvents and spin casting onto native oxide covered silicon substrates. The amount of POSS on the PMMA backbone varied from 0 to 15%. The films were annealed (various times and temperatures) and analyzed with transmission x-ray and electron microscopy as well as scanning force microscopy and dynamic secondary ion mass spectroscopy (DSIMS). The results show that the POSS is uniformly distributed in the PMMA matrix and no crystalline POSS domains are observed. The interfacial width between PMMA-POSS and deuterated PMMA homopolymer was measured with DSIMS to determine the miscibility as a function of POSS concentration. The results show that the interfacial width is less than 10 nanometers, or the DSIMS resolution even after annealing for 4 days at 170 C. No dependence on POSS concentration for x>=5% was observed. In contrast to the PMMA/ PMMA-POSS blends, the addition of POSS to the backbone seemed to act as a compatibilizer when the materials were blended with monodisperse polystyrene. The interfacial tension, as deduced from contact angle measurements, was found to decrease by an order of magnitude with the addition of 10% POSS. The decrease was nearly exponential with POSS concentration. These results will be compared with SCF calculations of the compatibilization effectiveness of inorganic additives in blends.

[K22.012] Effect of Compressibility in an Athermal Blend Near a Surface

We present a pure blend (no free-volume with lattice completely filled with polymer segments) and compare its behavior with compressible blends with different amount of free-volume in the system. We compare their segment density profile, surface free energy and surface entropy for different ratios of average degree of polymerization and different blend compositions. In general, we find that presence of free-volume reduces the size of surface region, dampens the non-monotonic nature of segment density profile and that the surface entropy is lower for the incompressible blends compared to that for compressible blends. We do our caculations on a tree structure which reduces the computation time requirement and results are thermodynamically consistent.

[K22.013] Vapor-Liquid Phase Equilibria in Alkane Monolayers Physisorbed on a Metal Surface

Grand canonical histogram-reweighting Monte Carlo simulations are used to determine the vapor-liquid coexistence densities for alkane monolayers physisorbed on a flat gold substrate. Results are reported for normal alkanes ranging from methane to hexane, and for iso-butane (2-methylpropane). Comparisions are made to experimental data and to simulations of the corresponding bulk phases. Mixed-field finite-size scaling is used to locate the critical points of these systems. The ratio of 2-D to 3-D critical temperatures are found to depend only weakly on chain length and branching.

Part K of program listing