Reptation dynamics of a polymer melt near an attractive solid interface

Cluster secondary ion mass spectrometry of polymers and related materials

Cluster secondary ion mass spectrometry (cluster SIMS) has played a critical role in the characterization of polymeric materials over the last decade, allowing for the ability to obtain spatially resolved surface and in-depth molecular information from many polymer systems. With the advent of new molecular sources such as C(60)(+), Au(3)(+), SF(5)(+), and Bi(3)(+), there are considerable increases in secondary ion signal as compared to more conventional atomic beams (Ar(+), Cs(+), or Ga(+)). In addition, compositional depth profiling in organic and polymeric systems is now feasible, without the rapid signal decay that is typically observed under atomic bombardment. The premise behind the success of cluster SIMS is that compared to atomic beams, polyatomic beams tend to cause surface-localized damage with rapid sputter removal rates, resulting in a system at equilibrium, where the damage created is rapidly removed before it can accumulate. Though this may be partly true, there are actually much more complex chemistries occurring under polyatomic bombardment of organic and polymeric materials, which need to be considered and discussed to better understand and define the important parameters for successful depth profiling. The following presents a review of the current literature on polymer analysis using cluster beams. This review will focus on the surface and in-depth characterization of polymer samples with cluster sources, but will also discuss the characterization of other relevant organic materials, and basic polymer radiation chemistry.

Dendritic superstructures and structure transitions of asymmetric poly(L-lactide-b-ethylene oxide) diblock copolymer thin films

The evolution of morphologies of isothermally crystallized thin films with different thicknesses of poly(L-lactide-b-ethylene oxide) diblock copolymer was observed by optical microscopy (OM) and atomic force microscopy (AFM). Dendritic superstructures stacked with lamellae were investigated in thin films with approximately 200 nm to approximately 400 nm thickness. The lamellar structure was a lozenge- or truncated-lozenge-shaped single crystal of PLLA confirmed by AFM observations. The contour of the dendritic superstructures is hexagonal, and two types of sectors, [110] and [100], can be classified in terms of the chain-folding and crystal growth directions. These phenomena are due to the interplay of the crystallization of the PLLA block, the microphase separation of the block copolymer, and the effect of the film thickness. The growth process of the superstructure can be classified into three steps: the growth of the main branches, the growth of the secondary side branches along the main branch, and the tertiary side branches. PLLA growth rates decrease in copolymer films thinner than 1 microm. Layer-layer phase structure of the copolymer driven by the crystallization of PLLA and the microphase separation of the copolymer appears to be a key factor explaining the crystallization and morphological behavior of this system.

Chain dynamics of a weakly adsorbing polymer in thin films

Thin films of weakly adsorbing poly(dimethyl siloxane) (PDMS) on porous alumina are examined with NMR fast field cycling (FFC) relaxometry and NMR transverse relaxometry. The longitudinal relaxation dispersion of polymer amounts corresponding to approximate monolayer coverage shows substantial deviation from the bulk and is characterized by a particularly weak temperature dependence. Thicker films, however, show relaxation behavior and temperature dependence more similar to the bulk polymer. Transverse relaxation times were found to cover a range of several orders of magnitudes for any sample investigated; their dependence on temperature is a function of the total amount of adsorbed polymer. While thick films see an overall increase of molecular mobility at higher temperatures, monolayer films are best characterized by the decreasing fraction of a short, i.e. relatively rigid, component. These effects are consistent with the concept of two regions, one in which chain dynamics deviate from bulk and another where chain dynamics are reduced but bulk-like, although chains inside each region may also experience motional heterogeneity.

Surfactant mobility in nanoporous glass films

Polymer molecules when physically confined at nanometer length scales diffuse nonclassically and very differently depending on their molecular weight and the nature of the confinement. Long polymers that exhibit "snakelike" reptation based mobility in melts may diffuse faster in confined nanometer sized cylinders with pore diameter d approximately 15 nm, and short polymers subject to Rouse dynamics have shown signatures of reptation and slower diffusion when confined in nanoporous glass with d approximately 4 nm. However, the mobility of short polymers with radii of gyration similar to a smaller pore diameter (d < or = 2.1 nm) but with extended lengths well larger than the pore diameter has not as yet been studied. In this work, we demonstrate that those short molecules including nonionic surfactants can readily diffuse in strongly hydrophobic nanoporous glasses film with d < or = 2.1 nm. The diffusivity was found sensitive to molecular weight, hydrophilic-lipophilic balance, and molecular structure of surfactants. Remarkably, analysis of the measured diffusion coefficients reveals that short-chain surfactants exhibit signature of reptation based diffusion in the nanoscopic pore confinements. Such reptation mobility in agreement with theoretical predictions is not even observed in reptating polymer melts due to fluctuations of the entanglement pathway. The fixed pathways in the interconnected nanoporous films provide ideal nanoscale environments to explore mobility of confined molecules, and the results have implications for a number of technologies where nanoporous materials are in contact with surfactant molecules.

Density functional approach for modeling CO2 pressurized polymer thin films in equilibrium

We have used polymer density functional theory to analyze the equilibrium density profiles and interfacial properties of thin films of polymer in the presence of CO(2). Surface tension, surface excess adsorption of CO(2) on polymer surface, and width of the interface are discussed. We have shown the changes in these properties in the presence of CO(2) and with increasing film thickness and their inverse linear relationship with increasing chain length. One of our important findings is the evidence of segregation of end segments toward the interface. We have introduced a new method of representing this phenomenon by means of Delta profiles that show increase in segregation owing to the presence of CO(2) and with increasing chain length. We also make predictions for the octacosane-CO(2) binary system near the critical point of CO(2). Our results indicate qualitative trends that are comparable to the similar experimental and simulation studies.

Dielectric relaxation of ultrathin films of supported polysulfone

The dynamic glass transition (alpha relaxation, structural relaxation) of ultrathin polysulfone films prepared between aluminum electrodes is investigated by dielectric relaxation spectroscopy. As a main result, it is found that the glass transition temperature Tg does not depend on the thickness of the polymeric layer down to a thickness of 10 nm. For thicknesses lower than 10 nm, an increase of Tg is observed. A more detailed analysis of the temperature dependence of the relaxation rates reveals that the Vogel temperature increases and the fragility decreases systematically with decreasing film thickness d. Further, the dielectric strength Deltaepsilon decreases with decreasing d. This is discussed by the formation of a surface layer of adsorbed polysulfone segments having a reduced molecular mobility with regard to the time scale characteristic of the glassy dynamics of bulk polysulfone. Plotted versus inverse film thickness Deltaepsilon decreases linearly with 1/d and becomes zero for an extrapolated length scale of 10 nm. From that it is concluded that the thickness of the adsorbed boundary layer is about 5 nm. Contact-angle measurements were carried out to confirm the strong interaction between aluminum and polysulfone. It is also shown that preparation details like annealing conditions strongly influence the glass transition of supported ultrathin films.

Enhanced mobility of confined polymers

Non-classical behaviour, brought about by a confinement that imposes spatial constraints on molecules, is opening avenues to novel applications. For example, carbon nanotubes, which show rapid and selective transport of small molecules across the nanotubes, have significant potential as biological or chemical separation materials for organic solvents or gaseous molecules. With polymers, when the dimensions of a confining volume are much less than the radius of gyration, a quantitative understanding of perturbations to chain dynamics due to geometric constraints remains a challenge and, with the development of nanofabrication processes, the dynamics of confined polymers have significant technological implications. Here, we describe a weak molecular-weight-dependent mobility of polymers confined within nanoscopic cylindrical pores having diameters smaller than the dimension of the chains in the bulk. On the basis of the chain configuration along the pore axis, the measured mobility of polymers in the confined geometry is much higher than the mobility of the unconfined chain. With the emergence of nanofabrication processes based on polymer flow, the unexpected enhancement in flow and reduction in intermolecular entanglements are of significant importance in the design and execution of processing strategies.

Molecular Motion at Soft and Hard Interfaces: From Phospholipid Bilayers to Polymers and Lubricants

Spatially resolved and time-resolved understanding of complex fluid situations compose a new frontier in physical chemistry. Here we draw attention to the significance of spatially resolving systems whose ensemble average differs fundamentally from the spatially resolved individual elements. We take examples from the field of fluid phospholipid bilayers, to which macromolecules adsorb; the field of polymer physics, when flexible chains adsorb to the solid-liquid interface; and from the field of lubrication, when two solids are squeezed close together with confined fluid retained between them.

Molar mass dependent growth of poly(epsilon-caprolactone) crystals in Langmuir films

Poly(epsilon-caprolactone) (PCL) samples with number average molar masses (Mn) ranging from 3.5 to 36 kg.mol-1 exhibit molar mass dependent nucleation and growth of crystals, crystal morphologies, and melting properties at a temperature of 22.5 degrees C in Langmuir films at the air/water (A/W) interface. At surface area per monomer, A, greater than approximately 0.37 nm2.monomer-1, surface pressure, Pi, and surface elasticity exhibit molar mass independent behavior that is consistent with a semidilute PCL monolayer. In this regime, the scaling exponent indicates that the A/W interface is a good solvent for the liquid-expanded PCL monolayers. Pi-A isotherms show molar mass dependent behavior in the vicinity of the collapse transition, i.e., the supersaturated monolayer state, corresponding to the onset of the nucleation of crystals. Molar mass dependent morphological features for PCL crystals and their subsequent crystal melting are studied by in situ Brewster angle microscopy during hysteresis experiments. The competition between lower segmental mobility and a greater degree of undercooling with increasing molar mass produces a maximum average growth rate at intermediate molar mass. This behavior is analogous to spherulitic growth in bulk PCL melts. The plateau regions in the expansion isotherms represent the melting process, where the polymer chains continuously return to the monolayer state. The magnitude of Pi for the plateau during expansion decreases with increasing molar mass, indicating that the melting process is strongly molar mass dependent.

Blends of poly(epsilon-caprolactone) and intermediate molar mass polystyrene as Langmuir films at the air/water interface

Poly(epsilon-caprolactone)/polystyrene (PCL/PS) blends, where nonamphiphilic PS is glassy in the bulk state at the experimental temperature of 22.5 degrees C, are immiscible as Langmuir films at the air/water (A/W) interface. Surface pressure-area per monomer isotherm analyses indicate that the surface concentration of amphiphilic PCL is the only factor influencing the surface pressure below the collapse transition. For PS-rich blends, Brewster angle microscopy (BAM) studies at the A/W interface and atomic force microscopy studies on Langmuir-Schaefer films reveal that PS nanoparticle aggregates formed at very low surface pressures can form networks upon further compression. The morphologies seen in PS-rich blends (networklike rings) are consistent with a recent study of a nonamphiphilic polyhedral oligomeric silsesquioxane (POSS), octaisobutyl-POSS, blended with amphiphilic poly(dimethylsiloxane), suggesting that the nonamphiphilic PS aggregates at the A/W interface produce domains with dipole densities that differ from that of pure PCL. In all composition regimes, the amphiphilic PCL phase tends to spread and form a continuous surface layer at the A/W interface, while simultaneously improving the dispersion of nonamphiphilic PS domains. During film expansion, BAM images show a gradual change in the surface morphology from highly continuous networklike structures (PS-rich blends) to broken ringlike structures (intermediate composition) to small discontinuous aggregates (PCL-rich blends). This study provides valuable information on the morphological evolution of semicrystalline PCL-based polymer blends confined in a "two-dimensional" geometry at the A/W interface and fundamental insight into the influence of microstructure (domain size, phase-separated structures, crystalline morphology, etc.) on the interfacial properties of blends as Langmuir films.

Segmental dynamics in poly(methyl acrylate) on silica: Molecular-mass effects

The effect of molecular mass on the segmental dynamics of poly(methyl acrylate) (PMA) adsorbed on silica was studied using deuterium quadrupole-echo nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry. Samples adsorbed on silica (all about 1.5 mg PMA/m2 silica) were shown to have more restricted segmental mobility, and higher Tg's, than the corresponding bulk PMA samples. Around the glass-transition region, adsorbed samples exhibited segmental mobility, which could be classified as heterogeneous due to a superposition of more-mobile and less-mobile components present in the deuterium NMR spectra. This heterogeneity was consistent with a motional gradient with more-mobile segments near the polymer-air interface and the less-mobile species near the polymer-silica interface. The mobility of the adsorbed 77 kDa PMA sample was the lowest among the four different molecular-mass samples studied. Samples studied with masses both larger and smaller than 77 kDa had larger mobile-component fractions in the adsorbed polymer. The additional mobility was attributed to the presence of either longer tail and loop conformations in the higher molecular-mass samples or the inherent mobility of the tails in the lower molecular-mass samples on the surface.

Brewster angle microscopy study of poly(epsilon-caprolactone) crystal growth in Langmuir films at the air/water interface

Surface pressure-induced crystallization of poly(epsilon-caprolactone) (PCL) from a metastable region of the surface pressure-area per monomer (Pi-A) isotherm in Langmuir monolayers at the air/water (A/W) interface has been captured in real time by Brewster angle microscopy (BAM). Morphological features of PCL crystals grown in Langmuir films during the compression process exhibit four fully developed faces and two distorted faces. During expansion of the crystallized film, polymer chains slowly detach from the crystalline domains and diffuse back into the monolayer as the crystals "melt". Typical diffusion-controlled morphologies are revealed by BAM during the melting process as the secondary dendrites melt away faster, that is, at a higher surface pressure than the principal axes. Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the polymer chain axes perpendicular to the substrate surface, while atomic force microscopy reveals a crystal thickness of approximately 7.6 nm.

Carbon-13 Labeled Polymers: An Alternative Tracer for Depth Profiling of Polymer Films and Multilayers Using Secondary Ion Mass Spectrometry

13C labeling is introduced as a tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry (SIMS). Deuterium substitution has traditionally been used in depth profiling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulk-phase behavior and surface and interfacial interactions. SIMS can provide contrast by examining various functional groups, chemical moieties, or isotopic labels. 13C-Labeled PS (13C-PS) and unlabeled PS (12C-PS) and PMMA were synthesized using atom-transfer radical polymerization and assembled in several model thin-film systems. Depth profiles were recorded using a Cameca IMS-6f magnetic sector mass spectrometer using both 6.0-keV impact energy Cs+ and 5.5-keV impact energy O2+ primary ion bombardment with detection of negative and positive secondary ions, respectively. Although complete separation of 12C1H from 13C is achieved using both primary ion species, 6.0-keV Cs+ clearly shows improved detection sensitivity and signal-to-noise ratio for detection of 12C, 12C1H, and 13C secondary ions. The use of Cs+ primary ion bombardment results in somewhat anomalous, nonmonotonic changes in the 12C, 12C1H, and 13C secondary ion yields through the PS/PMMA interface; however, it is shown that this behavior is not due to sample charging. Through normalization of the 13C secondary ion yield to the total C (12C + 13C) ion yield, the observed effects through the PS/PMMA interface can be greatly minimized, thereby significantly improving analysis of polymer films and multilayers using SIMS. Mass spectra of 13C-PS and 12C-PS were also analyzed using a PHI TRIFT I time-of-flight mass spectrometer, with 15-keV Ga+ primary ion bombardment and detection of positive secondary ions. The (12)C7(1)H7 ion fragment and its 13C-enriched analogues have significant secondary ion yields with negligible mass interferences, providing an early indication of the potential for future use of this technique for cluster probe depth profiling of high molecular weight 13C-labeled fragments.

Modeling diffusion in miscible polymer blend films

Recent experiments designed to probe polymer transport in the bulk and in the vicinity of surfaces have examined the interdiffusion of multilayer sandwiches of isotopically labeled polymers. The measured time dependent concentration profiles normal to the surface are typically fit to Fick's law, with a single fitting parameter, the mutual binary diffusion coefficient (MBDC). The resulting MBDCs are found to vary over a broad range of film thicknesses and time, with the time dependence being viewed as a unique signature of the reptation mechanism of long chain motion, and the thickness dependence being attributed to the slowing down of chain dynamics near surfaces. Since the experiments are conducted at finite concentration, the MBDC, which is a product of the bare mobility and the concentration derivative of the chemical potential, could be dominated by the time and thickness dependence of this second term (which is ignored in Fick's law). To quantify this conjecture we consider the more rigorous Cahn formulation of the diffusion problem in terms of chemical potential gradients. We use square gradient theory to evaluate chemical potentials, and fit the resulting time dependent concentration profiles to the analytical solution of Fick's law. By thus mimicking the experimental analysis we find that the apparent MBDCs vary with time as t(-1/2) at short times, in good agreement with existing experiments. We show that this time dependence reflects the system's desire to minimize concentration gradients, a fact ignored in Fick's law. Since these arguments make no reference to the mechanism of chain motion, we argue that the time dependence of MBDC derived from interdiffusion experiments does not provide unequivocal support for the reptation mechanism of long chain transport. The MBDC values, which also vary with the degree of confinement, are predicted to increase with decreasing thickness for model parameters corresponding to experimental systems. In contrast, since the experimental fits yield an opposite trend, we suggest that the bare mobility of the chains decreases strongly with decreasing thickness. These findings strongly support the idea that the chains are "pinned" irreversibly to the surfaces, in good agreement with other, independent experiments.

Viscosity of entangled polystyrene thin film melts: Film thickness dependence

We determined the low-shear effective viscosity of entangled polystyrene thin film melts, in the thickness range of 27<h<100 nm, on SiO(x)/Si substrates. This was accomplished using a method based on the notion that thin liquid films can become unstable and rupture due to defects or to destabilizing, long-range van der Waals interactions (dewetting). The holes that are created in the film subsequently grow at a rate determined by a balance between the capillary driving forces and the viscous resistive forces. Based on the velocity of growth of holes on the substrate, we show that the viscosity decreases appreciably with decreasing thickness for 25<h<50 nm. These results are consistent with studies which suggest that the glass transition of entangled polystyrene thin film melts on SiO(x)/Si substrates exhibit an apparent decrease with decreasing film thickness over the same range of h.