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A simple yet versatile method was developed to prepare a low-density polymerization initiator gradient, which was combined with surface-initiated atom transfer radical polymerization (ATRP) to produce a well-defined poly(2-hydroxyethyl methacrylate) (HEMA) gradient substrate. A smooth variation in film thickness was measured across the gradient, ranging from 20 Å to over 80 Å, but we observed a nonmonotonic variation in water contact angle. Fits of X-ray reflectivity profiles suggested that at the low graft density end, the polymer chain structure was in a “mushroom” regime, while the polymer chains at high graft density were in a “brush” regime. It was found that the “mushroom” region of the gradient could be made adhesive to cells by adsorbing adhesion proteins, and cell adhesion could be tuned by controlling the density of the polymer grafts. Fibroblasts were seeded on gradients precoated with fibronectin to test cellular responses to this novel substrate, but it was found that cell adhesion did not follow the expected trend; instead, saturated cell adhesion and spreading was found at the low grafting density region.

A fast combinatorial approach to access information about the immobilization behavior and kinetics of enzymes on a variation of surfaces is presented. As a test system, Candida Antarctica Lipase B was immobilized on a self-assembled monolayer bearing a gradient of surface energy. The respective immobilization behavior was monitored by Fourier transform infrared microspectroscopy. In addition, the activity of the immobilized enzyme was monitored over the entire film in real time with a specially developed fluorescence activity assay embedded into a siloxane gel. It was found that the highest amount of active protein was immobilized on the hydrophilic end of the gradient surface. This effect is associated with a higher surface roughness of this area resulting in hydrophobic microenviroments in which the enzyme gets immobilized.

Rapid prototyping photolithography of a thiolene-based resin was used to fabricate microfluidic devices stable to aliphatic and aromatic organic solvents. The swelling of the cross-linked polymer matrix in various organic solvents was quantified, and the solvent resistance properties of these microfluidic devices are described. Discrete droplets of hexanes and toluene of uniform size were generated in microfluidic devices inside a water matrix containing SDS surfactant (SDS = sodium dodecyl sulfate). Variation of water and organic flow rates in the fluidic channels was used to control droplet size and separation. Droplet composition could be controlled by varying flow rates of two joined organic streams. Organic-phase synthetic reactions within the droplets were demonstrated with the bromination of alkenes inside benzene droplets.

The influence of nanometer thick, highly anisotropic organically modified layered silicate (montmorillonite) on the phase behavior of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) is investigated by a combination of small-angle neutron scattering (SANS) and a two-dimensional combinatorial method based on light scattering and corroborated by single-point static cloud-point light scattering. The presence of layered silicates up to a volume fraction of 0.04 is found to leave the phase diagram essentially unchanged, with the values of the Flory−Huggins χ parameter at high temperatures being nearly independent of added silicate for blends with layered silicates up to a volume fraction of 0.008. These surprising results, in light of the significantly higher polarity of PVME in comparison to PS, allows us to investigate the influence of such layered silicates on the kinetics and morphology of phase separation in polymer blends as detailed in a previous paper.

The influence of the disk diameter of nanometer thick anisotropic layered silicates on the phase-separated morphology of a near-critical polystyrene (PS)−poly(vinyl methyl ether) (PVME) blend was examined using atomic force microscopy. Films with comparable amounts of thermodynamically equivalent nanoparticles varying only in lateral disk diameters were examined using a temperature gradient method and showed dramatic differences in late-stage morphology. The blends with small disk diameter (30 nm and 0.5 μm) nanoparticles exhibit a pinning of domain sizes and demonstrate an increase in the number of domains with a higher fraction of near circular structures. On the other hand, for layered silicates with large disk diameters (10 μm), the nanoparticles do not affect the morphology of the phase-separated structure and only accelerate the phase separation kinetics. The extent of domain pinning increases with increasing silicate content and results in smaller domains at higher concentrations of silicate.

Combinatorial methods are utilized to study the influence of film thickness (13 nm < h < 120 nm) and crystallization temperature (131 °C < Tx < 193 °C) on the crystallization of isotactic polystyrene. Films with continuous gradients in film thickness were prepared by flow coating, crystallized on an orthogonal temperature gradient stage, and observed under an optical microscope. Spherulite growth rates were measured as a function of film thickness and temperature, as determined from an optical micrograph library of numerous subregions within the gradient films. We observed that the growth rate had a maximum near 180 °C and varied inversely to film thickness, consistent with the results of Sawamura et al. A transition from hexagonal plate to circular growth was observed with increasing undercooling in the thickness range between 23 and 80 nm. Both hexagonal and disordered branched crystalline morphologies formed in ultrathin regions where the film thickness is less than the radius of gyration.

We utilize chemically patterned substrates with arrays of progressively narrower stripes (1−15 μm) to investigate the influence of pattern size on the morphology of ultrathin dewetting polystyrene films. The scale and orientation of the spinodal-like height fluctuations of the dewetting patterns are coupled to the imposed substrate chemical frequency, providing a powerful means of morphological control. Dewetting patterns are correlated to the substrate pattern period leading to the formation of droplet arrays. The measurements confirm recent numerical simulations by Kargupta and Sharma of the existence of upper and lower cutoff scales for pattern recognition of a dewetting fluid. For pattern dimensions less than the characteristic scale on nonpatterned substrates, the droplets become anisotropic as they coarsen to a scale comparable to the stripe width, and then undergo a morphological transition to circular droplets that cross multiple stripes. This leads to quantization of droplet size and contact angles, as indicated by theory.

The phase diagram of coexisting liquid−liquid phase separation (LLPS) and crystallization in nearly isorefractive blends of statistical ethylene/hexane (PEH) and ethylene/butene (PEB) copolymers has been investigated. A variety of techniques that exploit crystallization-induced contrast, such as diffuse light scattering, small-angle light scattering, light transmission, atomic force microscopy, and differential scanning calorimetry, are used to identify the LLPS boundary, which is found to exhibit an upper critical solution temperature of 146 °C. The composition dependence of the LLPS boundary follows the prediction of Flory−Huggins theory for binary polymer mixtures. The equilibrium melting temperature of the blends decreases with increasing PEB concentration in the miscible phase, whereas it remains relatively constant at 127 °C within the LLPS coexistence region. Measurements of the LLPS boundary can be complicated by heterogeneity in the polymer microstructure.

Poly(propyleneimine) dendrimers with stearyl end groups are investigated as inverse micelles in toluene. While the initial dendrimers have a spherical structure with a collapsed core, solubilization of metal salt hydrate leads to the formation of cylindrical multidendrimer structures with swollen, metal-salt-filled dendrimer cores. The cylindrical structure can be deduced from the small-angle neutron (SANS) as well as from the X-ray scattering (SAXS) pair distance distribution function. While the neutron scattering leads to a two-step cross-sectional density profile, the X-ray scattering is sensitive only to the metal-salt-stained core. The gold salt inside the dendrimers was reduced to form colloidal particles, and upon reduction the cylindrical structure breaks up and spherical colloids are formed. The sizes are larger than would be expected if the gold salt loading of one dendrimer formed one particle, indicating that the ions from several dendrimers are combined. These dendrimer-stabilized metal colloids represent hybrid structures that have potential importance as selective catalysts and in the formation of composite materials with special optical properties.

The progression of intramolecular organizations within a series of dilute poly(amido amine) (PAMAM) dendrimer/methanol solutions is examined by use of small-angle X-ray scattering (SAXS) combined with comparisons to electron density models and radial density distributions extracted by the indirect transformation for the calculation of P(r) (ITP) method. The SAXS from generation 3 (G3) dendrimers are shown to possess scattering features similar to those of star molecules. This contrasts with the SAXS of the much larger G9 and G10 dendrimers. The G10 SAXS exhibits at least five resolvable secondary maxima. These features are successfully reproduced for a model consisting of constant-density spheres with a small amount of polydispersity in molecular size. Scattering from the intermediately sized G4 through G8 dendrimers reflects a consistent evolution of internal structure progressing from “starlike” to “hard-sphere-like” organizations.

Higher generation poly(amidoamine) (PAMAM) dendrimers have the unique ability to act as templates for the formation of inorganic nanoclusters. Here, we use dendrimers dispersed in a polymer matrix to create a new type of polymer−inorganic composite material. Hydrophilic polymer networks (poly(2-hydroxyethyl methacrylate)) that contain poly(amidoamine) dendrimers were swollen in aqueous solution, and metal ions were attached to the dendrimers. Chemical reduction on these precursor ions results in nanoparticles that are located inside the dendrimers, which are dispersed in the polymer matrix. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to characterize gold, platinum, and copper nanoclusters within the polymer networks. These new organic−inorganic hybrid materials may be important for a combination of optical or catalytic properties of the colloids with the mechanical properties provided by the polymer network.

Combinatorial methods involving data collection in multiparameter space allow a rapid identification of measured property trends as a function of system parameters. The technique has been applied with success to pharmaceutical, inorganic and organic materials synthesis, but not significantly to measurements of polymeric films and coatings. We demonstrate the use of 2-D combinatorial libraries to investigate thin-film dewetting. We have prepared libraries of thin films of polystyrene on silicon substrates containing orthogonal, continuous variations of thickness (h), and temperature (T) that represent about 1200 practical state points per library. The libraries were screened for dewetting behavior using automated optical microscopy. Dewetting trends were visibly apparent on the libraries, and a comprehensive map of the T, h, and time (t) dependence was generated in a few hours. The combinatorial libraries, spanning a large T, h, and t range, not only reproduced known dewetting structures and phenomena but also enabled a novel T, h superposition of the heterogeneous nucleated hole dewetting kinetics. We observed three hole nucleation regimes as a function of thickness:  heterogeneously nucleated holes (h > 55 nm), a crossover regime where both heterogeneous and capillary instability nucleation compete (33 nm < h < 55 nm), and a regime of holes nucleated by capillary instability (16 nm < h < 33 nm).

Charged poly(amidoamine) (PAMAM) dendrimers are used to create organic−inorganic hybrid colloids in aqueous solution. The formation of gold colloids upon reduction of a gold salt precursor serves as a model reaction to study the influence of reaction conditions and dendrimer generation on the resulting nanostructures. Characterization by transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) show that the gold particles are formed inside the dendrimer and located offset from the center. Although lower generation dendrimers aggregate when stabilizing the metal particles formed, dendrimers of generation 6−9 can template one gold colloid per dendrimer molecule, the size of which is well-controlled by the number of gold atoms added per dendrimer. For generation 10, multiple smaller gold particles per dendrimer are observed. The effectiveness of PAMAM dendrimers as templates in the host−guest nanoscale synthesis is confirmed for different chemical reactions.

To quantify the swelling response of polyelectrolyte dendrimers in aqueous solutions as affected by changes in solution conditions, we have studied eighth generation, G8, poly(amidoamine) (PAMAM) dendrimers in D2O using small-angle neutron scattering. The charge density is manipulated by varying the pH of the solution in order to protonate and deprotonate the primary and tertiary amines. The coherent scattered intensity at high q values yields size information from the form factor of individual dendrimers. The distance distribution functions of the dendrimers in a variety of experimental situations were determined by indirect Fourier transform. For all the conditions studied, we find that the size of G8 dendrimers is essentially insensitive to variations of the pH (4.7 < pH < 10.1) and of ionic strength (up to 3 M of NaCl). In all cases, the scattering curves yield density profiles that correspond well to a uniform sphere with a radius of gyration RG = 4.0 ± 0.15 nm. We conclude that the dendrimer size is essentially independent of the charge density or ionic strength of the solvent, contrary to the predictions of current models and simulations.

The perturbing influence of nanosize filler particles on the dewetting of spun-cast polymer films is investigated. Previous studies have shown that spun-cast films of unentangled polystyrene (PS) and polybutadiene (PB) dewet acid-cleaned silicon wafers when the films are annealed above their bulk glass transition temperature Tg. It is also known that “impurity” particles tend to promote dewetting in thin polymer films. Here we demonstrate that the addition of a small amount of C60 fullerene nanoparticles to the spin-casting polymer solution leads to an inhibition of dewetting in thin (L < 100 nm) PS and PB films cast on both acid-cleaned and carbon-coated silicon wafer substrates. Neutron reflection measurements indicate that this effect is associated with the formation of a diffuse fullerene layer near the solid substrate. Evidently, the immobilized fullerene particles form an enrichment layer at the solid boundary and “pin” the contact line of the growing dewetted (“dry”) regions so that hole growth in the filled films becomes arrested at a scale which diminishes with increasing filler concentration. Above a relative filler−polymer mass fraction of 1%, we no longer observe hole formation by atomic force microscopy on the time scales of our measurements (typically on the order of hours), suggesting the existence of a critical filler surface coverage for the suppression of dewetting. The roughness of the fullerene layer probably also contributes to the modification of the surface wetting properties through a change in the surface energy of the substrate, as suggested by recent measurements on substrates “roughened” by other means such as irradiation of polymer film substrates, grafting of polymer chains, etc.

Molecular structure and rheological properties of ethylene−propylene (EP) random copolymer gels formed in toluene and methylcyclohexane, respectively, have been investigated by small-angle neutron scattering (SANS) and oscillatory shear. In both solvents, SANS results are analyzed by considering a fiberlike structure with a radius cross-section distribution function expressed by w(r) ~ r^-λ. It is shown that a small change in the EP copolymer ethylene content has a large effect on the density of network connecting fibers but a much smaller effect on the diameter of the fibers. Oscillatory shear experiments reveal that the relaxation rate of the elastic modulus increases with decreasing EP copolymer ethylene content. This behavior indicates a decrease in the density of connections between fibers with decreasing ethylene content. The relationship between the elastic modulus and polymer concentration has also been examined. The results suggest, in light of the theory developed by Jones and Marques for rigid networks, that the rigidity of fibers formed in the gel is achieved only locally.

Solutions of the poly(propylene imine) dendrimers DAB-dendr-(PA)32 and DAB-dendr-(PA)64 in methanol are investigated with small angle neutron scattering over the range of dendrimer mass fraction 0.01 ≤ x ≤ 0.80. The single particle scattering function, P(q), is used to calculate the structure factor, S(q), of the dendrimer solutions and to evaluate the radius of gyration of the dendrimers in dilute solution giving %@mt;sys@%%@ital@%R%@rsf@%%@sx@%g%@be@%+%@sxx@%%@mx@% (DAB-dendr-(PA)32) = 12.4 ± 0.2 Å and %@mt;sys@%%@ital@%R%@rsf@%%@sx@%g%@be@%+%@sxx@%%@mx@% (DAB-dendr-(PA)64) = (15.6 ± 0.2) Å. In each case the segment density in dilute solution is 0.35 ± 0.03 g cm^-3, leaving a large fraction of the dendrimer volume accessible to solvent. We define the “dilute solution regime” for dendrimers to extend to a concentration where the swollen dendrimer volume fraction first equals the value 0.64, which comes at a dendrimer weight fraction of x = 0.25. At this concentration, the spatial arrangement of the dendrimers can be described as a random close packing. For higher concentrations, the experimental scattering functions appear to be self-similar up to a mass fraction of x ≈ 0.60. These observations are consistent with a model of concentrated dendrimer solutions which assumes that individual dendrimers collapse to maintain a volume fraction φ ≈ 0.64, which is the volume fraction of random close packing of hard spheres. Preliminary small X-ray scattering data give further evidence of the dendrimer collapse.

The average dimensions of poly(amido amine) (PAMAM) dendrimers of generations G5 and G8 are studied by small-angle neutron scattering (SANS) as a function of solvents of differing quality, the composition of mixtures of a good solvent and a nonsolvent, and temperature. The radius of gyration, Rg, of the G8 dendrimer decreases for the series of solvents D(CD2)mOD (with m = 0, 1, 2, 4) by approximately 10% from m = 0 to m = 4 with decreasing solvent quality. The average segment density in solution is calculated for G5 and G8. The size of PAMAM dendrimers, as reflected by Rg, is not influenced by the composition of mixtures of methyl alcohol/acetone in the full range of solubility of the dendrimers. In the temperature range −10 °C ≤ T ≤ 50 °C, the dimensions of the G8 and G5 dendrimer are constant, within ≈5% of each other.

The spatial distribution of the terminal groups of poly(amido amine) dendrimers have been determined experimentally by small-angle neutron scattering with deuterium labeling and scattering contrast variation. The radius of gyration of deuterated terminal units of generation 7 dendrimers is 39.3 ± 1.0 Å. This is significantly larger than the radius of gyration of the whole dendrimer, which is 34.4 ± 0.2 Å. These data indicate that dendrimers have terminal groups that are concentrated near the periphery. These results are inconsistent with many computer simulations and some molecular models.

Recent simulations suggest that the presence of filler particles in a phase separating blend can induce the development of composition waves having the symmetry of the filler particles. We investigate these predictions through atomic force microscopy (AFM) measurements on ultrathin (L ≈ 100 nm) polystyrene and poly(vinyl methyl ether) blend films containing a low concentration of model filler particles (silica particles having a nominal diameter ≈100 nm). The filled blend films were spun-cast on acid-cleaned silica wafers, and phase separation was induced by a temperature jump into the two-phase region (T ≈145 °C) of the bulk polymer blend. By rinsing off the polymer film with solvent, we show that the silica particles are associated with the substrate so that the filler particles represent a quenched disorder perturbation of the film phase separation. The presence of the filler particles leads to the development of circular composition waves (“target patterns”) about the filler particles during the intermediate stage of phase separation. These target patterns disintegrate as the background spinodal phase separation pattern becomes much larger than the filler particles. Our observations are consistent with idealized two-dimensional Cahn−Hilliard−Cook simulations of the phase separation of polymer blends having a small concentration of filler particles.

We present results of small-angle neutron scattering (SANS) investigations into the characteristics of charged poly(amidoamine) (PAMAM) dendrimer solutions in deuterium oxide. The study shows that ionic strength, surface charge density, and concentration are the crucial parameters controlling the degree of structural organization of the solution. Upon the addition of acid, HCl, we observe a peak in the scattering intensity. The results indicate that increasing ionization of terminal amines results in interparticle interactions giving rise to local, liquidlike ordering. These interactions can be screened by adding monovalent salt (NaCl) in large quantities. The short-range interactions were studied by systematically varying dendrimer and salt concentration (up to 2 M). Contrary to most weak polyelectrolyte systems, we did not observe a negative second virial coefficient, even for the highest salt concentrations.

Small-angle neutron scattering (SANS) has been applied to extract the single-chain form factors of the fatty acid (FA) and the dendrimer components in bulk and dilute solution of fatty acid modified poly(propyleneimine) dendrimers (Astramol). The fatty acid chains were covalently or noncovalently linked to the end groups of two generations, G3 and G5. Bulk samples were prepared under contrast match conditions of mixtures of dendrimer-h/FAH and dendrimer-h/FAD to obtain the form factor of the fatty acid components. The solutions were prepared with deuterated fatty acid modified dendrimers in decalin solvent using the contrast match technique. Mixtures of decalin-h and decalin-d were used to match either the dendrimer or FA components. The dendrimer core of both G3 and G5 appears collapsed, without the incorporation of a significant amount of solvent. The G3 FA portion has a size closer to that of the bulk than that of a collapsed FA shell. The G5 dendrimer has the opposite trend.

A steep upturn at low angles dominates the small-angle neutron scattering profile of low ionic strength polyelectrolyte solutions. This behavior is demonstrated for poly(N-methyl-2-vinylpyridinium chloride) in D2O by measurements that are extended to low angles not typical of most studies. This behavior is consistent with static and dynamic light scattering measurements performed on the same solutions. Specifically, the upturn at low q and the slow diffusive mode in dynamic light scattering indicate the presence of supramolecular structures, which we interpret as large domains of increased polymer concentration. In these same solutions, a shorter wavelength correlation is evident from the presence of a maximum at finite q in the small-angle scattering experiment. Fluctuations on a short wavelength are also apparent from the existence of the fast mode in the dynamic light scattering. The maximum is interpreted as reflecting a characteristic dimension defined by order within domains, while the fast mode results from electrodynamic coupling of fast moving counterions and portions of chains.

Individual dendrimer molecules of poly(amidoamine) (PAMAM) from generation 10 (G10) to G5 were imaged by conventional transmission electron microscopy (TEM) after staining with aqueous sodium phosphotungstate. The dendrimers were resolved as separate, beam-stable entities, and their sizes and distribution of sizes were statistically analyzed and compared with data from small-angle X-ray scattering in solution. Aqueous G10 dendrimers were also characterized by examination in the frozen, hydrated state after quench-freezing in liquid ethane (cryo-TEM) and the data compared with the results of the staining experiments. To a first approximation, the dendrimers appeared circular in projection and the diameters conformed to a Gaussian distribution which broadened somewhat with increasing generation number. Cryo-TEM, in general, confirmed the staining experiments but suggested that, in the native state, there is more variability in the shapes of the dendrimers, with polyhedral shapes occurring quite frequently.

Recent theoretical and experimental studies of polyelectrolyte solutions have suggested that hydrophobic interactions between the chain backbone and the solvent lead to some of the essential observations of polyelectrolyte solution scattering. In the current paper we present small angle neutron scattering data on a polyelectrolyte, poly(N-methyl-2-vinylpyridinium chloride) dissolved in ethylene glycol where the hydrophobic effect is not present. This system shows the same general pattern, a peak at finite wavevector and steep upturn at low wavevector, as for those having hydrophobic interactions so that the influences of poor backbone solvation appear to be unessential for explaining the characteristics of polyelectrolyte scattering data. We demonstrate that in the scattering from polyelectrolytes after the addition of salt, the unusual peak and steep upturn are suppressed and the scattering parallels scattering from the neutral chains.