Surface structure analysis of thin dewetted polymer blend films

The surface structure of thin polymer blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) after annealing above the glass transition temperature was investigated. With scanning force microscopy (SFM) the surface topography originated by a dewetting process is detected. The sample surface is covered with small droplets consisting of several polymer molecules. Utilizing grazing incidence small angle neutron scattering (GISANS) the topographical information as well as the in‐plane composition is probed. For thin confined blend films a substructure of the droplets resulting from an additional phase separation process at different length scales is detected.     

Film thickness dependence of the domain size in weakly incompatible thin polymer blend films

The surface morphology of thin polymer blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) is investigated with scanning force microscopy (SFM) and optical microscopy. From a statistical analysis of the data the most prominent in-plane length picturing the domain size as a function of the blend film thickness is determined. In ultra-thin films surface patterns directly after preparation are absent, whereas for thicker films a linear dependence is observed. After a relaxation towards equilibrium, resulting from annealing or storage under toluene vapor, the power law observed changes for ultra-thin films and remains unchanged for thicker films. Received: 27 July 2000 Accepted: 30 October 2000     

Structure evolution in layers of polymer blend nanoparticles

The early stages of phase evolution, not available for nanometer polymer blend films spin-cast from solutions of incompatible mixtures, have been examined for films prepared from nanoparticles of deuterated polystyrene/ poly(methyl methacrylate) blends (1:1 mass fraction of dPS/PMMA) with PS-PMMA diblock copolymer additives. The initial phase arrangement, confined to the size of nanoparticles, has provided the homogeneity of the initial film composition. The early stages of structure formation, promoted by annealing and traced with atomic and lateral force microscopy (AFM, LFM) as well as secondary ion mass spectroscopy (SIMS), resulted in bilayers, observed commonly for as-prepared solvent-cast blends. The initiated capillary instability of the upper dPS-rich layer depended on copolymer additives, which enhanced the lateral structures pinning the dewetting process.     

Comparative analysis of nanostructured diblock copolymer films

Nanostructured polymer films of poly(styrene-block-paramethylstyrene) diblock copolymers P(Sd-b-pMS) on silicon substrates with a native oxide layer are investigated. Resulting from a storage under toluene vapor, a surface structure is installed. The early stages, characterized by the creation of a host structure out of an initially continuous film, are addressed. Grazing incidence small-angle X-ray scattering (GISAXS) experiments were performed as a function of exposure time. Results are compared to modelling of the scattering pattern and other experimental techniques, such as grazing incidence small-angle neutron scattering (GISANS) and atomic force microscopy (AFM) data. Possibilities and limits of the techniques are discussed.     

Surface versus confinement induced morphology transition in triblock copolymer films: a grazing incidence small angle neutron scattering investigation

The internal nanostructure resulting from microphase separation in triblock copolymer films of polyparamethylstyrene-block-polystyrene-block-polyparamethylstyrene, P(pMS-b-Sd8-b-pMS), has been investigated with grazing incidence small angle neutron scattering (GISANS). X-ray reflectivity, grazing incidence small-angle X-ray scattering (GISAXS), optical microscopy and atomic force microscopy (AFM) complement the investigation. The influence of two limiting interfaces present in confinement is compared to the presence of only one surface. GISANS allows for the detection of structures in the very limited sample volume of confined films as well as for a depth sensitivity to probe the near free surface part of bulk films. With respect to the surface a perpendicular oriented lamella is observed. In contrast to the shrinkage of the characteristic lamellar spacing in confinement at the free surface, a slight increase is determined.     

Solvent vapor mediated polymer adsorption in thin films

The effectiveness of a "solvent annealing" process was investigated for thin (approximately 150 nm) polystyrene films, in which the diffusion and reorganization of polymer chains were mediated by the controlled absorption of cyclohexane vapor. Results were compared with conventional "thermal annealing" of films under vacuum above the glass transition temperature. Elastic recoil detection analysis (ERDA) was used to determine the surface excesses of fluorocarbon end-capped polystyrene (hPSF) and poly(styrene-b-dimethylsiloxane) (hPS-PDMS) in deuterated polystyrene (dPS) films. Both annealing methods enabled diffusion of the surface-active polymers; however, only thermal annealing gave rise to a surface excess in hPSF/dPS films. The inhibition ofhPSF adsorption under solvent annealing was due to the low surface tension of cyclohexane. In contrast, hPS-PDMS, having a larger surface-active group than that of hPSF, was found in excess at the air surface under solvent annealing, and surface excesses were consistent with the formation of saturated monolayers in blended films. The mixing of hPS-PDMS with dPS was inhibited by the unfavorable interaction between the PDMS block of the copolymer and the homopolymer. The slow interdiffusion of hPS-PDMS in dPS is consistent with the formation of micelles, and the formation of an excess layer at the air surface may be kinetically inhibited by the rate of dissociation of hPS-PDMS micelles.     

Flat-on lamellae in spin-coated, stable films of poly(propylene azelate)

Thin films (5-500 nm thick) of the linear aliphatic polyester (3,7) poly(propylene azelate) (PPAz) were prepared by spin-coating of CHCl(3) polymer solutions with different polymer concentrations. The morphology and structure of the spin-coated thin films were investigated by atomic force microscopy (AFM) and by grazing incidence wide-angle X-ray scattering (GIWAXS) techniques. AFM revealed the continuous nature of the flat, spherulitic films which are stable against dewetting even for polymer coatings as thin as 15 nm. GIWAXS patterns revealed a high crystal orientation of the films. A sharp reflection on the meridian whose spacing is related to the polymer chain unit length (c-axis) supports the presence of flat-on lamellae morphology in the whole range of film thicknesses investigated. The flat-on lamellae morphology is also supported by AFM images. A triclinic unit cell with the c*-axis perpendicular to the substrate is proposed for PPAz. The repulsion of the long aliphatic spacer by the Si-substrate is invoked as the main reason for the flat-on morphology observed.     

Surface and interfacial segregation in blends of polystyrene with functional end groups and deuterated polystyrene

The effect of chain-end chemistry on surface and interfacial segregation in symmetric blends of polystyrene (hPS)/deuterated polystyrene (dPS) has been investigated by X-ray photoelectron and secondary ion mass spectroscopy in conjunction with neutron reflectivity measurements. Alpha,omega-fluoroalkyl- and alpha,omega-carboxy-terminated polystyrenes (alpha,omega-hPS(Rf)2 and alpha,omega-hPS(COOH)2) were used as end-functionalized polymers; the former possesses chain ends with lower surface energies, and the latter possesses higher surface energies compared with that of the main chain. In the case of an alpha,omega-hPS(Rf)2/dPS blend film, alpha,omega-hPS(Rf)2 was enriched at the surface owing to the surface localization of the Rf groups, although the surface energy of the hPS segments was slightly higher than that of the dPS ones. On the contrary, in the case of an alpha,omega-hPS(COOH)2/dPS blend film, dPS was preferentially segregated at the surface. This may be due to a surface depletion of COOH ends and an apparent molecular weight increase of alpha,omega-hPS(COOH)2 produced by a hydrogen-bonded intermolecular association of COOH ends in addition to the surface energy difference between hPS and dPS segments. Interestingly, both Rf and COOH chain ends were partitioned to the substrate interface for the alpha,omega-hPS(Rf)2/dPS and alpha,omega-hPS(COOH)2/dPS blend films, resulting in the segregation of the hPS component at the substrate interface for both blends. The results presented imply that surface and interfacial segregation in polymer blends could be regulated by incorporating functional groups into the end portions of one component.     

Morphology and film formation of poly(butyl methacrylate)–polypyrrole core–shell latex particles

 Core–shell latex particles made of a poly(butyl methacrylate) (PBMA) core and a thin polypyrrole (PPy) shell were synthesized by two-stage polymerization. In the first stage, PBMA latex particles were synthesized in a semicontinuous process by free-radical polymerization. PBMA latex particles were labeled either with an energy donor or with an energy acceptor, in two different syntheses. These particles were used in a second stage as seeds for the synthesis of the core–shell particles. The PPy shell was polymerized around the PBMA core latex in an oxidative chemical in situ polymerization. Proofs for the success of the core–shell synthesis were obtained using nonradiative energy transfer (NRET) and atomic force microscopy (AFM). NRET gives access to the rate of polymer chain migration between adjacent particles in a film annealed at a temperature above the glass-transition temperature T _g of the particles. Slower chain migration of the PBMA polymer chains was obtained with the PBMA–PPy core–shell particles compared to rate of the PBMA polymer chain migration found with the pure, uncoated PBMA particles. This result is due to the coating of PBMA by PPy, which hinders the migration of the PBMA polymer chains between adjacent particles in the film. This observation has been confirmed by AFM measurements showing that the flattening of the latex film surface is much slower for the core–shell particles than for the pure PBMA particles. This result can again be explained by the presence of a rigid PPy shell around the PBMA core. Thus, these two complementary methods have given evidence that real core–shell particles were synthesized and that the shell seriously hinders film formation of the particles in spite of the fact that it is very thin (thickness close to 1 nm) compared to the size (750 and 780 nm in diameter) of the PBMA core. Transparency measurements confirm the results obtained by NRET and AFM. When the films are placed at a temperature higher than the T _g of PBMA, the increase in transparency is faster for films made with the uncoated PBMA particles than for films made with the coated PBMA particles. This result indicates again that the presence of the rigid PPy layer around the PBMA core reduces considerably the speed at which the structure of the film is modified when heated above the T _g of PBMA. Received: 02 September 1999 Accepted: 21 December 1999     

Changes in thermodynamic interactions at highly immiscible polymer/polymer interfaces due to deuterium labeling

Deuterium labeling has been shown previously to affect thermodynamic interactions at polymer surfaces, polymer/polymer heterogeneous interfaces, and in bulk (away from a surface or interface). However, the changes in polymer-polymer interactions due to deuterium labeling have not been thoroughly investigated for highly immiscible systems. It is shown here that deuterium labeling can influence polymer-polymer interactions at heterogeneous interfaces with highly immiscible systems, namely, polystyrene/poly(2-vinylpyridine) (PS/P2VP), polystyrene/poly(4-vinylpyridine) (PS/P4VP), and polystyrene/poly(methyl methacrylate) (PS/PMMA). Using secondary ion mass spectrometry, segregation of deuterium labeled polystyrene (dPS) in a dPS + unlabeled PS (dPS:hPS) blend layer was observed at the dPS:hPS/hP2VP, dPS:hPS/hP4VP, and dPS:hPS/hPMMA heterogeneous interfaces. However, a reference system involving PS on a PS brush shows no segregation of dPS to the interface.     

Investigating polymer blend miscibility with forward recoil spectrometry

We tested forward recoil spectrometry (FRES) as a method to determine miscibility by measuring coexistence compositions in binary polymer blends. In this study, equilibrium phase compositions were determined for a compositionally symmetric poly(styrene‐ran‐methyl methacrylate) random copolymer (S_0.49‐r‐MMA) and two homopolymers, deuterated polystyrene (dPS) and deuterated poly(methyl methacrylate) (dPMMA). Sample preparation, film dewetting, and beam damage were addressed, and the results for these polymer blends were in good agreement with those obtained through other experimental techniques. Deuteration had a strong effect on the miscibility of the dPS/S_0.49‐r‐MMA and dPMMA/S_0.49‐r‐MMA blends, to the extent that the asymmetric miscibility observed separately for the PS/S_0.49‐r‐MMA and PMMA/S_0.49‐r‐MMA blends was not found. Although this deuteration effect may limit the applicability of FRES for some polymer systems, the accuracy with which phase compositions can be determined with FRES makes it an attractive alternative to other less quantitative methods for investigating blend miscibility. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1547–1552, 2000     

In situ measurement of the thermal expansion behavior of benzocyclobutene films

In situ measurement techniques suitable for determination of the coefficient of thermal expansion (CTE) in thin, spin‐cast polymer films in both the in‐plane and through‐plane directions are presented. An examination of the thermal expansion behavior of cyclotene thin films has been performed. In particular, the effect of film thickness on the in‐plane and through‐plane CTE and in‐plane Young's modulus of spin‐coated cyclotene films was examined. It is shown that the mechanical response of in situ cyclotene films can be adequately described by isotropic film properties. It was also demonstrated that there is no thickness dependence on the free‐standing mechanical properties or on the resulting through‐plane thermal strain in an in situ film. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 311–321, 1999     

Use of in‐situ atomic force microscopy to monitor the biodegradation of polyhydroxyalkanoates (PHAs)

Thin films of a co‐polymer mixture of poly 3‐hydroxybutyrate and poly 3‐hydroxyvalerate P(3HB‐3HV) were spun‐cast onto glass slides resulting in 35 nm thick layers with a spherulitic microstructure. An untyped strain of Streptomyces sp. bacteria was isolated from soil samples, and it's PHA depolymerase was used to degrade the P(3HB‐3HV) thin films. Both ex‐situ and in‐situ atomic force microscopy (AFM) biodegradation studies were performed to determine the kinetics of the biodegradation over the course of three hours at room temperature. Ex‐situ AFM was performed in Tapping Mode and in‐situ AFM was performed in the PHA depolymerase using contact mode AFM in the liquid cell, allowing for the real‐time analysis of P(3HB‐3HV) biodegradation. Biodegradation is observed uniformly throughout the surface, and can be observed within 30 min. of depolymerase exposure. In‐situ AFM analysis yields a linear degradation rate as a function of time, while the ex‐situ study suggests a more complex kinetics.     

Unusual Multilayered Structures in Poly(ethylene oxide)/Laponite Nanocomposite Films

Summary: The unusual structure of poly(ethylene oxide) (PEO) and Laponite clay in transparent nanocomposite films was investigated using scanning electron, atomic force, and optical microscopy, and X‐ray scattering. Each method is sensitive to different aspects of structural features and together they measure the resulting morphology and shear‐induced orientation. On nanometer length scales, clay platelets were found to orient in bundles while polymer crystallinity was suppressed. Microscopy led to the observation of unexpected and highly oriented multilayers on the micron length scale.

Scanning electron microscopy image of the freeze‐fractured surface of a poly(ethylene oxide)–Laponite film: the view on top of the x–y plane.     

Photoactive Nanostructures of Polypyrrole

The creation of nanostructures of the photoactive polymer polypyrrole (PPy) on glass substrates with the spin‐coating method is described. No additional post‐production treatment is necessary to obtain uniformly distributed photoactive nanostructures on macroscopically scaled substrates. Based on X‐ray reflectivity measurements, the critical solution concentration of PPy below which these nanostructures develop is determined. The PPy nanostructures are displayed with atomic force microscopy (AFM) measurements, which prove that the nanostructures form directly on the substrate. With UV/Vis spectroscopy the absorption behavior of the nanostructures is probed in comparison to PPy films and PPy solutions. A linear dependence of the absorption of the nanostructure on the surface coverage measured with AFM is detected. The influence of confinement on the conjugation length results in a modified absorption behavior of the nanostructures.     

Aggregation, adsorption, and surface properties of multiply end-functionalized polystyrenes

The properties of polystyrene blends containing deuteriopolystyrene, multiply end-functionalized with C8F17 fluorocarbon groups, are strikingly analogous to those of surfactants in solution. These materials, denoted FxdPSy, where x is the number of fluorocarbon groups and y is the molecular weight of the dPS chain in kg/mol, were blended with unfunctionalized polystyrene, hPS. Nuclear reaction analysis experiments show that FxdPSy polymers adsorb spontaneously to solution and blend surfaces, resulting in a reduction in surface energy inferred from contact angle analysis. Aggregation of functionalized polymers in the bulk was found to be sensitive to FxdPSy structure and closely related to surface properties. At low concentrations, the functionalized polymers are freely dispersed in the hPS matrix, and in this range, the surface excess concentration grows sharply with increasing bulk concentration. At higher concentrations, surface excess concentrations and contact angles reach a plateau, small-angle neutron scattering data indicate small micellar aggregates of six to seven F2dPS10 polymer chains and much larger aggregates of F4dPS10. Whereas F2dPS10 aggregates are miscible with the hPS matrix, F4dPS10 forms a separate phase of multilamellar vesicles. Using neutron reflectometry (NR), we found that the extent of the adsorbed layer was approximately half the lamellar spacing of the multilamellar vesicles. NR data were fitted using an error function profile to describe the concentration profile of the adsorbed layer, and reasonable agreement was found with concentration profiles predicted by the SCFT model. The thermodynamic sticking energy of the fluorocarbon-functionalized polymer chains to the blend surface increases from 5.3kBT for x = 2 to 6.6kBT for x = 4 but appears to be somewhat dependent upon the blend concentration.     

The molecular structure of poly(biphenyl dianhydride-p-phenylenediamine) polyimide thin films by infrared spectroscopy: Thickness dependence of structure in the nano- to micrometer range

The molecular structure of poly[biphenyl dianhydride-p-phenylenediamine] (BPDA–PDA) polyimide in ultrathin (3–300 nm) films on silicon has been characterized by polarized infrared spectroscopy in conjunction with ellipsometry and X-ray reflectivity measurements. In spite of the high degree of crystalline packing of the polymer chains, the results show that an unexpected and significant content of imide rings exhibit local structural perturbations, including out-of-plane twisting. Further, the fraction of perturbed rings increases with increasing film thickness while, in contrast, the high degree of in-plane uniaxial film symmetry and planar stacking of the chains remain constant with thickness. These results reveal a new structural aspect of localized ring disorder that arises within the otherwise well-ordered, chain-stacked structure of BPDA–PDA polyimide films. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1247–1260, 1998     

Amphiphilic and hydrophobic surface patterns generated from hyperbranched fluoropolymer/linear polymer networks: Minimally adhesive coatings via the crosslinking of hyperbranched fluoropolymers

Hyperbranched fluoropolymers (HBFPs), based on benzyl ether linkages and having a large number of pentafluorophenyl chain ends, were crosslinked by a reaction with diamino-terminated poly(ethylene glycol) (PEG) or diamino-terminated poly(dimethyl siloxane) (PDMS) to form hyperbranched–linear copolymer networks of different compositions, structures, and properties. The crosslinking reactions involved the nucleophilic aromatic substitution of the pentafluorophenyl para-fluorines of HBFP by the amine functionalities of the respective telechelic linear segments. The contact angles, differential scanning calorimetry, thermogravimetric analysis, tensile measurements, and atomic force microscopy (AFM) were used to characterize the resulting network film samples. The surface wettability of the crosslinked materials was affected by the nature and amount of the linear polymer crosslinking agent employed. Amphiphilic polymer networks were formed by the incorporation of diamino-terminated PEG as a crosslinker, whereas diamino-terminated PDMS produced polymer networks of a hydrophobic character. The mechanical properties improved upon crosslinking, as measured by tensile testing. The mechanical integrity of the films was also found to improve upon crosslinking, as measured by AFM machining protocols. The AFM images revealed topographical morphologies that appeared to be the result of phase segregation of HBFP from PEG or PDMS; the dimensions of the phase-segregated domains were dependent on the stoichiometry of HBFP to the linear polymer and the thickness of the coating. As the content of PEG increased, fouling by human fibrinogen, used as a model protein, decreased. Further studies are in progress to determine the effects of the surface composition, morphology, and topography on the biofouling characteristics. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3531–3540, 2003     

Influence of the electropolymerisation mode on PEDOTh films morphology and redox behaviour—an AFM investigation

The morphologies of PEDOTh films deposited on platinum electrodes, using different electrochemical modes but conditions selected to obtain layers with similar electroactivities, have been analysed by atomic force microscopy (AFM). Ex situ AFM showed a fibber-like topography for the potentiodynamically synthesised films, contrasting to the granular character of those formed at constant potential. In situ contact mode AFM, used to examine the initial stages of the polymer growth and the surface morphology evolution, disclosed the development of long segments with the number of potential cycles, and the formation of identical globular nodules, which diameter increases with the growth charge. The morphological changes imparted by the switching process revealed to be more marked in films grown at constant potential. Thickness measurements performed on oxidised and reduced films unveiled the swelling/shrinking of the polymers during the ingress/egress of the perchlorate anions, with film volume changes of ca. 20% upon oxidation.     

AFM phase imaging of thin films of electronically conducting polymer polybithiophene prepared by electrochemical potentiodynamic deposition

Nanoscale properties of thin films of conducting polymer polybithiophene (PBT) deposited under potentiostatic and potentiodynamic conditions were compared using ex-situ atomic force microscopy (AFM) and its extension called phase imaging (PI-AFM). While the morphologies of the films prepared using the two techniques were quite similar, the phase contrast measurements revealed a profound difference in the mechanisms of potentiostatic and potentiodynamic electropolymerization, as well as in the nanoscale crystallinity and grain structure of the resulting polymer films. The overall crystallinity and degree of order were always higher for films deposited at constant potential. The differences were especially pronounced at the early deposition stages (film thicknesses of ca. 10 nm).