Probing Polymer Material Properties on the Nanometer Scale (original) (raw)
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Micrometer and Sub-Micrometer Structure Formation of Phase Separating Polymer Films
2000
This thesis describes the structure formation on the micrometer and submicrometer scale in thin films of polymer blends. Dissolving two incompatible polymers in a common solvent and spin-coating the solution leads to phase separation during the evaporation of the solvent. Spinodal decomposition, a process which is well understood in bulk systems is the underlying process. In the thin film geometry, surface directed spinodal decomposition but also wetting properties of the different phases play an important role during the structure formation process. Thus, the morphology of the two coexisting phases can be organized normal or parallel to the film surface depending on the surface energy of the substrate. In this thesis, we make use of the substrate sensitivity of the demixing process. By using chemically heterogeneous pre-structured substrates made by micro-contact printing (µCP), an arbitrary substrate pattern can be used to induce a lateral composition variation in the polymer film. While µCP leaves only a molecularly thick patterned organic layer on the substrate, the laterally structured polymer film has a thickness in the order of 100 nm and it forms during the spin-coating process within a few seconds. Thus, by controlling the phase morphology, one can create surfaces with new properties, e.g. lithography masks , optical devices, or biological sensors. For instance this novel structuring technique can be applied to ion conducting organic-inorganic hybrid materials and polymer metal precursors for high T c superconductors. More complex morphologies are found after spin-casting ternary polymer blends on to homogeneous hydrophobic substrates. If one of the polymer-polymer interaction parameters exceeds the sum of the other two, the morphologies can be considered as emulsions. Simulation results performed by Nauman et al. [1], closely resemble the morphologies observed in our systematic study, which also can be controlled by using a ordered prepatterned substrate. But also laterally isotropic phase separated polymer films feature new physical properties. For example if one of the two polymers of a phase morphology with a lateral length scale below 200 nm is removed, the resulting nanoporous film can be used as a high-performance anti-reflection coating. The refractive index of these films can be adjusted in a range from 1.6 down to 1.05 by tuning the composition of the polymer solution. This allows the build up of multi-layer coatings with a broad spectral transmission.
Deuterated Polymers for Probing Phase Separation Using Infrared Microspectroscopy
Biomacromolecules, 2014
Infrared (IR) microspectroscopy has the capacity to determine the extent of phase separation in polymer blends. However, a major limitation in the use of this technique has been its reliance on overlapping peaks in the IR spectra to differentiate between polymers of similar chemical compositions in blends. The objective of this study was to evaluate the suitability of deuteration of one mixture component to separate infrared (IR) absorption bands and provide image contrast in phase separated materials. Deuteration of poly(3-hydroxyoctanoate) (PHO) was achieved via microbial biosynthesis using deuterated substrates, and the characteristic C−D stretching vibrations provided distinct signals completely separated from the C−H signals of protonated poly(3-hydroxybutyrate) (PHB). Phase separation was observed in 50:50 (% w/w) blends as domains up to 100 μm through the film cross sections, consistent with earlier reports of phase separation observed by scanning electron microscopy (SEM) of freeze-fractured protonated polymer blends. The presence of deuterated phases throughout the film suggests there is some miscibility at smaller length scales, which increased with increasing PHB content. These investigations indicate that biodeuteration combined with IR microspectroscopy represents a useful tool for mapping the phase behavior of polymer blends.
Mapping Polymer Heterogeneity Using Atomic Force Microscopy Phase Imaging and Nanoscale Indentation
Macromolecules, 2000
Polymer coatings often contain degradation-susceptible regions, and corrosion of the metallic substrate can occur directly underneath these regions. In this paper, the microstructure of model coating materials is investigated using atomic force microscopy (AFM). Specifically, AFM is used to study heterogeneity in thin film blends of polystyrene (PS) and polybutadiene (PB) as a function of annealing time at 80°C. PS/PB blend films with thicknesses of approximately 250 nm are prepared by spin casting from solutions onto silicon substrates. Both topographic and phase imaging in tapping mode AFM are performed on these films under ambient conditions and at different force levels using a silicon tip. For certain force levels, phase imaging provides good contrast between the phase-separated PS and PB regions, primarily because of the large compliance difference between the two materials. This contrast decreases with increasing annealing time because thermal oxidation causes cross-linking in PB, and thus, the compliance of the PB region increases toward that of PS. Nanoscale indentation measurements are then made on the observed phase-separated regions to identify these regions as PS-and PB-rich and to better understand the influence of relative surface stiffness on the phase images. Cast and free-standing films of pure PS and pure PB are also studied as a function of annealing time using AFM, contact angle measurements, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Results from studies of the individual PS and PB films are related to the AFM results for the blend films. The use of phase imaging for cure monitoring of polymers and for studies of chemically heterogeneous polymer systems is also discussed.
Investigation of morphologies and nanostructures of polymer blends by tapping mode phase imaging
Applied Physics A: Materials Science & Processing, 1998
Tapping mode atomic force microscopy (TMAFM) measurements were carried out for blends of a triblock copolymer, poly(styrene)-block-poly(ethene-co-but-1-ene)block-poly(styrene) (SEBS), with isotactic polypropylene (i-PP). Our TMAFM work on SEBS/i-PP blends show that phase imaging is an important and competitive tool for studying the microphase separation of polymers, and that the use of the film-glass interface is relevant in learning about the bulk morphology of polyolefinic samples by TMAFM.
Applied Spectroscopy, 2011
Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing sub-micrometer spatial resolution IR spectra and absorption images. This new capability enables the spectroscopic characterization of microdomain-forming polymers at levels not previously possible. Films of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) were solution cast on ZnSe prisms, followed by melting and annealing to generate crystalline microdomains of different sizes. A tunable IR laser generating pulses of the order of 10 ns was used for excitation of the sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Dramatic differences in the room-temperature IR spectra are observed in the 1200-1300 cm À1 range as a function of position on a spatial scale of less than one micrometer. This spectral region is particularly sensitive to the polymer backbone conformation. Such dramatic spectral differences have also been observed previously in bulk IR measurements, but only by comparing room-temperature spectra with ones collected at higher temperatures. Less dramatic, but significant, AFM-IR spectral differences are observed in the carbonyl stretching region around 1720 cm À1 as a function of location on the sample. Two overlapping, but relatively sharp, carbonyl bands are observed near 1720 cm À1 in more crystalline regions of the polymer, while a broader carbonyl stretching band appears centered at 1740 cm À1 in the more amorphous regions. Using this spectral region, it is possible to monitor the development of polymer crystalline structures at varying distances from a nucleation site, where the site was generated by bringing a heated AFM tip close to a specific location to locally anneal the sample.
Microphase separation at the surface of block copolymers, as studied with atomic force microscopy
Colloids and Surfaces B: Biointerfaces, 2000
Atomic force microscopy (AFM) is used to study the phase separation process occurring in block copolymers in the solid state. The simultaneous measurement of the amplitude and the phase of the oscillating cantilever in the tapping mode operation provides the surface topography along with the cartography of the microdomains of different mechanical properties. This technique thus allows to characterize the size and shape of those microdomains and their organization at the surface (e.g. cubic lattice spheres, hexagonal lattice of cylinders, or lamellae). In this study, a series of symmetric triblock copolymers made of a inner elastomeric sequence (poly(butadiene) or poly(alkylacrylate)) and two outer thermoplastic sequences (poly(methylmethacrylate)) is analyzed by AFM in the tapping mode. The microphase separation and their morphology are essential factors for the potential of these materials as a new class of thermoplastic elastomers. Special attention is paid to the control of the surface morphology, as observed by AFM, by the molecular structure of the copolymers (volume ratio of the sequences, molecular weight, length of the alkyl side group) and the experimental conditions used for the sample preparation. The molecular structure of the chains is completely controlled by the synthesis, which relies on the sequential living anionic polymerization of the comonomers. The copolymers are analyzed as solvent-cast films, whose characteristics depend on the solvent used and the annealing conditions. The surface arrangement of the phase-separated elastomeric and thermoplastic microdomains observed on the AFM phase images is discussed on the basis of quantitative information provided by the statistical analysis by Fourier transform and grain size distribution calculations.
Morphology mapping of phase-separated polymer films using nanothermal analysis
2010
Polymers films are attractive, in part, because their physical properties can be tuned by blending polymer with complementary characteristics. However, blending is typically challenging because most polymers will undergo phase separation, resulting in unpredictable behavior. Here, we introduce band excitation nanothermal analysis (BE-NanoTA) as a nondestructive AFM-based technique for mapping the near surface, thermal properties of polymeric coatings. BE-NanoTA was used to investigate phase separation and domain growth in poly(styrene-ran-acrylonitrile):poly(methyl methacrylate) SAN: PMMA films. The size and shape of PMMA-rich domains are consistent with prior measurements on the same system using a destructive method, namely UV-ozone etching of PMMA followed by topography mapping using standard AFM. Moreover, new insights into the mechanism of phase separation were uncovered including the observation of SAN-and PMMA-rich channels near the surface at early times as well as small SAN-rich domains trapped within large PMMA domains during intermediate times. Because it is nondestructive, BE-NanoTA can be used to explore in situ phase evolution in soft matter systems (e.g., polymer nanocomposites) which do not lend themselves to the UV-ozone etching method.
Progress Report on Phase Separation in Polymer Solutions
Advanced Materials, 2019
Polymeric porous media (PPM) are widely used as advanced materials, such as sound dampening foams, lithium-ion batteries, stretchable sensors, and biofilters. The functionality, reliability, and durability of these materials have a strong dependence on the microstructural patterns of PPM. One underlying mechanism for the formation of porosity in PPM is phase separation, which engenders polymer-rich and polymer-poor (pore) phases. Herein, the phase separation in polymer solutions is discussed from two different aspects: diffusion and hydrodynamic effects. For phase separation governed by diffusion, two novel morphological transitions are reviewed: "cluster-topercolation" and "percolation-to-droplets," which are attributed to an effect that the polymer-rich and the solvent-rich phases reach the equilibrium states asynchronously. In the case dictated by hydrodynamics, a deterministic nature for the microstructural evolution during phase separation is scrutinized. The deterministic nature is caused by an interfacial-tensiongradient (solutal Marangoni force), which can lead to directional movement of droplets as well as hydrodynamic instabilities during phase separation.