Observation of polymer chain structures in two-dimensional films by atomic force microscopy

In Situ Real-Time Atomic Force Microscopy Observation of the Surface Mobility on Each Domain of a Polystyrene-b-poly(methyl methacrylate) Film at High Temperatures

The surface chain movements within the microdomains of a polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) and corresponding homopolymer films were observed via in situ real-time atomic force microscopy (AFM) at high temperatures and analyzed quantitatively using particle image velocimetry (PIV). At low temperatures, mobility within the PS microdomains resembled that within the PS homopolymer film, but movements in the PMMA microdomains were notably accelerated compared to the PMMA homopolymer. Conversely, at high temperatures, mobility within both PS and PMMA microdomains was considerably suppressed compared to their respective homopolymer films, likely owing to the fixed linkage of the block chains at the microdomain interface. This combination of real-time AFM observation and PIV analysis is an effective method for quantitatively evaluating surface chain mobility in real space.

Self-brushing for nanopatterning: achieving perpendicular domain orientation in block copolymer thin films

The self-assembly of thin films of block copolymers (BCPs) with perpendicular domain orientation offers a promising approach for nanopatterning on a variety of substrates, which is required by advanced applications such as ultrasmall transistors in integrated circuits, nanopatterned materials for tissue engineering, and electrocatalysts for fuel cell applications. In this study, we created BCPs with an A-b-(B-r-C) architecture that have blocks with equal surface energy (γair) and that can bind to the substrate, effectively creating a non-preferential substrate coating via self-brushing that enables the formation of through-film perpendicular domains in thin films of BCPs. We employed a thiol-epoxy click reaction to functionalize polystyrene-block-poly(glycidyl methacrylate) with a pair of thiols to generate an A-b-(B-r-C) BCP and tune γair of the B-r-C block. The secondary hydroxyl and thiol ether functionality generated by the click reaction was utilized to bind the BCP to the substrates. Scanning electron microscopy revealed that perpendicular orientation was achieved by simply annealing a thin film of the BCP on the bare substrate without the usual extra step of coating a random copolymer brush on the substrate. The self-brushing capability of the BCP was also examined on gold, platinum, titanium, aluminum nitride, and silicon nitride surfaces. These results demonstrate that self-brushing is a promising approach for achieving perpendicular domain orientation in thin films of BCP for nanopatterning on a variety of useful surfaces.

Insulin Extended Release from PLA-PEG Stereocomplex Nanoparticles

This report addresses the challenges of controlled drug delivery for peptide and protein therapeutics by introducing a novel approach of nano formulation fabricated in aqueous media applying stereo-interaction mechanism with poly(D-lactide)-polyethylene glycol (D-PLA-PEG). To overcome the inherent poor stability of peptide and protein therapeutics, stereocomplexation of the peptide, insulin, is applied, onto D-PLA-PEG in aqueous media. Nanoparticles of ≈400 nm are spontaneously formed when water-soluble D configured PLA-PEG diblock copolymer and L- configured insulin interlock into a stereocomplex, owing to their concave convex fitness. In vitro release of insulin from stereocomplex in phosphate buffer solution (PBS) pH 7.4 solution shows sustained release for 14 weeks. The therapeutic efficacy of the PLA-insulin stereocomplex nanoparticles are evaluated in diabetic Akita mice. Blood glucose levels and body weight are closely monitored for a period of 17 weeks, revealing a significant reduction in glucose levels of the Akita mice treated with insulin stereocomplex, as well as normal body weight gain. These findings suggest that the stereocomplex nanoparticles of insulin-D-PLA-PEG present a promising and effective sustained and extended release platform for insulin. Notably, the use of water-soluble D-PLA-PEG for stereocomplexation in water expands the applicability of this approach to fabricate controlled delivery systems for peptide and protein therapeutics.

In Situ Real-Time Atomic Force Microscopy Observations of Chain Mobility at Polymer/Water Interfaces of Poly(methyl methacrylate), Poly(2-hydroxyethyl methacrylate), and Poly(2-methoxyethyl methacrylate) Films in Water

Polymer materials are widely used in water or in contact with an aqueous environment. However, evaluating the chain mobility, a crucial parameter, at a polymer-water interface is challenging. In this study, we, for the first time, observed poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(2-methoxyethyl methacrylate) (PMEMA) film surfaces in water via in situ real-time atomic force microscopy (AFM) in tapping mode and quantified the chain mobility. The average displacement between adjacent images (nm/8.75 min) was evaluated using particle image velocimetry. The displacement of PMMA, which has a high bulk glass-transition temperature (Tg) (108 °C) and exhibits limited water absorption, was low both in air (0.54 nm/8.75 min) and water (0.86), while PHEMA, which has a high bulk Tg (99 °C) and exhibits high water absorption, exhibited low mobility in air (0.40) but two orders of magnitude higher mobility in water (60). PMEMA, which has a low bulk Tg (14 °C) and exhibits limited water absorption, already started to move in air (4.5), and its mobility moderately increased in water (20). These behaviors were reasonable, considering the bulk Tg and water absorption characteristics of the polymers. Further, the chain mobility in water was compared with that of dried samples at high temperatures in air. The mobility of PMMA, PHEMA, and PMEMA in water corresponded to that of the dried samples observed in air below the surface Tg (97 °C) for PMMA, at ∼125 °C for PHEMA, and at ∼35 °C for PMEMA. In situ real-time AFM analysis of polymer materials in water is an effective method for evaluating the chain mobility at the polymer/water interface.

Universal scaling of the osmotic pressure for dense, quasi-two-dimensionally confined polymer melts reveals transitions between fractal dimensions

A scaling law for the osmotic pressure of quasi-two-dimensional polymer melts as a function of concentration is obtained, which shows fractal characteristics. Structural properties such as the chains' contour length and their inner-monomer pair distribution function display fractal scaling properties as well. These predictions are confirmed with mesoscale numerical simulations. The chains are swollen and highly entangled, yet Flory's exponent is always ν = 1/2. The melt can be considered a fluid of "blobs" whose size becomes renormalized in terms of the contour's length while the fractal dimension df increases monotonically between 5/4 and 2, as the monomer concentration is increased. The semidilute scaling of the pressure is recovered when df = 1. Our results agree with recent experiments and with numerical reports on quasi-2d melts. This work provides a new paradigm to study and interpret thermodynamic and structural data in low-dimensional polymer melts, namely as fractal macromolecular objects.

Structure of Strongly Adsorbed Polymer Systems: A Computer Simulation Study

The structure of very thin polymer films formed by strongly adsorbed macromolecules was studied by computer simulation. A coarse-grained model of strictly two-dimensional polymer systems was built, and its properties determined by an efficient Monte Carlo simulation algorithm. Properties of the model system were determined by means of Monte Carlo simulations with a sampling algorithm that combines Verdier-Stockmayer, pivot and reputation moves. The effects of temperature, chain length and polymer concentration on the macromolecular structure were investigated. It was shown that at low temperatures, the chain size increases with the concentration, that is, inversely with high temperatures. This behavior should be explained by the influence of inter-chain interactions.

Screw sense excess and reversals of helical polymers in solution

The helix reversal is a structural motif found in helical polymers in the solid state, but whose existence is elusive in solution. Herein, we have shown how the photochemical electrocyclization (PEC) of poly(phenylacetylene)s (PPAs) can be used to determine not only the presence of helix reversals in polymer solution, but also to estimate the screw sense excess. To perform these studies, we used a library of well folded PPAs and different copolymers series made by enantiomeric comonomers that show chiral conflict effect. The results obtained indicate that the PEC of a PPA will depend on the helical scaffold adopted by the PPA backbone and on its folding degree. Then, from these studies it is possible to determine the screw sense excess of a PPA, highly important in applications such as chiral stationary phases in HPLC or asymmetric synthesis.

Chain Movements at the Topmost Surface of Poly(methyl methacrylate) and Polystyrene Films Directly Evaluated by In Situ High-Temperature Atomic Force Microscopy

The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (Tg) is significantly lower than that in the bulk material. However, the mobility of the chains at the top of the surface has never been directly evaluated. In this study, the movements of the topmost chains of poly(methyl methacrylate) (PMMA) and polystyrene (PS) bulk films were observed in situ at high temperatures with atomic force microscopy in tapping mode. PMMA and PS chains started moving at ∼97 and ∼50 °C, respectively, which were slightly and significantly below the values of their bulk Tg (PMMA, 108 °C; PS, 104 °C), respectively. The activation energies of the apparent diffusion constants of PMMA and PS, derived by particle image velocimetry analysis, were 193 and 151 kJ mol-1, respectively, and reasonable for the glass transition. Movements of isolated PMMA chains deposited on a PMMA film by the Langmuir-Blodgett technique were also observed and confirmed to be essentially the same as those on the PMMA film surface.

Full Control of the Chiral Overpass Effect in Helical Polymers: P/M Screw Sense Induction by Remote Chiral Centers After Bypassing the First Chiral Residue

In helical polymers, helical sense induction is usually commanded by teleinduction mechanism, where the largest substituent of the chiral residue directly attached to the main chain is the one that commands the helical sense. In this work, different helical structures with different helical senses are induced in a helical polymer [poly-(phenylacetylene)] when the conformational composition of two different dihedral angles of a pendant group with more than two chiral residues is tamed. Thus, while the dihedral angle at chiral residue 1 [(R)- or (S)-alanine], attached to the backbone, produces an extended or bent conformation in the pendant resulting in two scaffolds with different stretching degree, the second dihedral angle at chiral residue 2 [(R)- or (S)-methoxyphenylacetamide] places the substituents of this chiral center in a different spatial orientation, originating opposite helical senses at the polymer that are induced through a total control of the "chiral overpass effect".

Submolecular Resolution Imaging of P3HT:PCBM Nanostructured Films by Atomic Force Microscopy: Implications for Organic Solar Cells

The efficiency of organic bulk-heterojunction (BHJ) solar cells depends greatly on both the bulk and surface structure of the nanostructured bicontinuous interpenetrating network of materials, known as the active layer. The morphology of the top layer of a coated film is often resolved at the scale of a few nanometers, but fine details of the domains and the order within them are more difficult to identify. Here, we report a high-resolution atomic force microscopy (AFM) investigation of various stoichiometries of the well-studied poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) active layer mixture. Images of the surface were obtained using AC-mode AFM exciting higher-order resonance frequencies of a standard silicon probe, a promising technique for acquiring real-space images of organic-based thin films with nanoscale and even submolecular resolution. We provide firm evidence of the nanoscale organization of the P3HT polymer and of the P3HT:PCBM stoichiometric mixtures at the surface-air interface of the BHJ architecture. Our study shows the characteristic periodicity of the regioregular P3HT identified in the nanoscale domain areas with submolecular resolution. Such areas are then distorted in place when adding different quantities of PCBM forming stoichiometric mixtures. When the samples were exposed to ambient light, the morphologies were very different, and submolecular resolution was not achieved. This approach is shown to provide a precise view of the active layer's nanostructure and will be useful for studies of other materials as a function of various parameters, with particular attention to the role of the acceptor in tuning morphology for understanding optimum performance in organic photovoltaic devices.

Intrinsic structure and dynamics of monolayer ring polymer melts

We present the general structural and dynamical characteristics of flexible ring polymers in narrowly confined two-dimensional (2D) melt systems using atomistic molecular dynamics simulations. The results are further analyzed via direct comparison with the 2D linear analogue as well as the three-dimensional (3D) ring and linear melt systems. It is observed that dimensional restriction in 2D confined systems results in an increase in the intrinsic chain stiffness of the ring polymer. Fundamentally, this arises from an entropic penalty on polymer chains along with a reduction in the available chain configuration states in phase space and spatial choices for individual segmental walks. This feature in combination with the intermolecular interactions between neighboring ring chains leads to an overall extended interpenetrated chain configuration for the 2D ring melt. In contrast to the generally large differences in structural and dynamical properties between ring and linear polymers in 3D melt systems, relatively similar local-to-global chain structures and dynamics are observed for the 2D ring and linear melts. This is attributed to the general structural similarity (i.e., extended double-stranded chain conformations), the less effective role of the chain ends, and the absence of complex topological constraints between chains (i.e., interchain entanglement and mutual ring threading) in the 2D confined systems compared with the corresponding 3D bulk systems.

Molecular Combing of Various Poly(n-Alkyl Acrylate) Chains on Mica by the Dipping Method

If polymer chains could be deposited on a substrate as a fully extended chain, a procedure known as "molecular combing," the chain structure could be characterized by atomic force microscopy in more detail than has been possible with the measurements available today. We show here, for the first time, that flexible polymers can be molecularly combed to fully extended chains by the dipping method. We studied the molecular combing of a series of poly(n-alkyl acrylate)s on mica from a chloroform solution by the dipping method and found that poly(n-alkyl acrylate)s with an alkyl group longer than n-octyl can be molecularly combed into straight chains under optimized conditions. With increasing alkyl lengths, the number of chains deposited decreases by four orders of magnitude, and chains become molecularly combed under a wider range of conditions. The length of the molecularly combed chains is ∼80% for poly(n-octyl acrylate) but ∼100% of the all-trans conformation for poly(n-alkyl acrylate)s with an alkyl length longer than n-nonyl.

Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of a polyene backbone. This method not only allows classification of PPAs in cis-cisoidal (ω₁ <90°) or cis-transoidal structures (ω₁ >90°), but also approximating ω₁ . A PPA solution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPA helix by measuring the ECD spectra at different times. PPAs with a cis-cisoidal structure show a reduction of the ECD signal of at least 50 % before 30 min of irradiation, while cis-transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis-cisoidal and cis-transoidal helices require different times to decrease their ECD signal by 50 % (t_1/2 ), depending on the degree of compression or stretching of the helix, establishing a relationship between the secondary structure adopted by PPA (ω₁ ) and the time required to lose the ECD vinylic signal by light irradiation.

In situ AFM Observation of the Movements of Isolated Isotactic Poly(methyl methacrylate) Chains in a Precursor Film of an Oligo(methyl methacrylate) Droplet Spreading on Mica

Atomic force microscopy (AFM) is a powerful tool to observe polymer chains at the molecular level. In this study, we show that the movements of isolated linear polymer chains in a precursor film of a droplet of an oligomer spreading on a substrate could be visualized in situ at the molecular level by AFM for the first time. The system was an isotactic poly(methyl methacrylate) (it-PMMA) solubilized in an oligo(MMA) matrix (it-PMMA/oligo(MMA) = 1/10,000 w/w) spreading on mica under high humidity. Because of the limited resolution of the AFM instrument, condensed linear polymer chains could not be visualized, but a small amount of it-PMMA chains that were solubilized as isolated chains in the oligo(MMA) matrix could be visualized in the precursor film, the contrast of which came from a large difference in glass transition temperature (T_g) of it-PMMA and oligo(MMA). The it-PMMA chains in the precursor film spread in the radial direction of the droplet with vigorously changing chain conformations. The spreading rate of it-PMMA chains under 72% relative humidity was ∼1/30 of the spreading rate of the oligo(MMA) matrix, which was estimated based on the decrease in the volume of the macroscopic droplet. The spreading of the it-PMMA chains and droplet strongly depended on humidity and was suppressed with the decrease in humidity, most likely because of the increase in friction with the substrate. The difference in the spreading rate of it-PMMA and oligo(MMA) further increased under low humidity. The dynamic molecular information of a precursor film by AFM should help to elucidate the wetting dynamics on a substrate.

Crystallization of Poly(ethylene)s with Regular Phosphoester Defects Studied at the Air-Water Interface

Poly(ethylene) (PE) is a commonly used semi-crystalline polymer which, due to the lack of polar groups in the repeating unit, is not able to form Langmuir or Langmuir-Blodgett (LB) films. This problem can be solved using PEs with hydrophilic groups arranged at regular distances within the polymer backbone. With acyclic diene metathesis (ADMET) polymerization, a tool for precise addition of polar groups after a certain interval of methylene sequence is available. In this study, we demonstrate the formation of Langmuir/LB films from two different PEs with regular phosphoester groups, acting as crystallization defects in the main chain. After spreading the polymers from chloroform solution on the water surface of a Langmuir trough and solvent evaporation, the surface pressure is recorded during compression under isothermal condition. These π-A isotherms, surface pressure π vs. mean area per repeat unit A, show a plateau zone at surface pressures of ~ (6 to 8) mN/m, attributed to the formation of crystalline domains of the PEs as confirmed by Brewster angle and epifluorescence microscopy. PE with ethoxy phosphoester defects (Ethoxy-PPE) forms circular shape domains, whereas Methyl-PPE-co-decadiene with methyl phosphoester defects and two different methylene sequences between the defects exhibits a film-like morphology. The domains/films are examined by atomic force microscopy after transferring them to a solid support. The thickness of the domains/films is found in the range from ~ (2.4 to 3.2) nm depending on the transfer pressure. A necessity of chain tilt in the crystalline domains is also confirmed. Grazing incidence X-ray scattering measurements in LB films show a single Bragg reflection at a scattering vector qxy position of ~ 15.1 nm-1 known from crystalline PE samples.

Preparation of a Si(111) Atomically Flat Substrate via Wet Etching and Evaluation as an AFM Substrate for Observations of Isolated Chains, Crystals, and Crystallization of Isotactic Poly(methyl methacrylate) at the Molecular Level

To observe a polymer chain deposited on a substrate by atomic force microscopy (AFM) at the molecular level, the substrate should be atomically flat and stable under laboratory conditions and adsorb polymer chains firmly. Therefore, substrates used under laboratory conditions are practically limited to mica, highly ordered pyrolytic graphite, and atomically stepped sapphire, and polymers observed by AFM at the molecular level are also limited. A silicon wafer is frequently used as a substrate for AFM observation for somewhat macroscopic observations, but the surface of the silicon wafer is too rough to observe polymer chains deposited on it at the molecular level. In this study, we prepared an atomically stepped Si(111) substrate via wet etching in NH₄F and evaluated it as an AFM substrate. The Si(111) substrate was stable as an AFM substrate, and isolated poly(methyl methacrylate) (it-PMMA) chains and a crystalline monolayer deposited on the substrate were observed by AFM at the molecular level. An it-PMMA amorphous monolayer deposited on mica crystallized under high humidity, but that on the Si(111) substrate did not because of the difference in the surface nature and the crystal structure of the substrates. The Si(111) substrate was hydrophobic, and the it-PMMA monolayers could be deposited as a multilayer, which could not be formed on hydrophilic mica. The crystallization behavior of an it-PMMA amorphous multilayer and an amorphous/crystalline mixed multilayer on the Si(111) substrate was also evaluated.

Self-Assembly of Linear and Cyclic Polylactide Stereoblock Copolymers with a Parallel and Antiparallel Chain Arrangement Distinguishing Their Directions on a Water Surface

The self-assembly of molecules into a well-ordered structure is one of the most important processes in fabricating sophisticated materials. Here, we show that polymer chains can be self-assembled, distinguishing their direction (parallel or antiparallel), and could be a new useful scaffold for self-assembly in a controlled direction. The system that was used was a stereocomplex (SC) formation of linear and cyclic polylactide (PLA) stereoblock copolymers with a parallel and antiparallel chain arrangement in a Langmuir monolayer. The linear and cyclic stereoblock copolymers with a parallel arrangement formed a well-ordered lamellar SC in the first and second layers upon compression, but the linear and cyclic stereoblock copolymers with an antiparallel arrangement did not form a first-layer lamella and instead formed only the second-layer lamella. These results were only rationally explained by assuming that the enantiomeric PLA chains selectively assembled in a parallel direction, not in an antiparallel direction, in the SC. A simple polymer chain could be self-assembled, distinguishing the direction without a specific interaction group in it.

Morphology and microstructural analysis of bioactive-loaded micro/nanocarriers via microscopy techniques; CLSM/SEM/TEM/AFM

Efficient characterization of the physicochemical attributes of bioactive-loaded micro/nano-vehicles is crucial for the successful product development. The introduction of outstanding science-based strategies and techniques makes it possible to realize how the characteristics of the formulation ingredients affect the structural and (bio)functional properties of the final bioactive-loaded carriers. The important points to be solved, at a microscopic level, are investigating how the features of the formulation ingredients affect the morphology, surface, size, dispersity, as well as the particulate interactions within bioactive-comprising nano/micro-delivery systems. This review presents a detailed description concerning the application of advanced microscopy techniques, i.e., confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) in characterizing the attributes of nano/microcarriers for the efficient delivery of bioactive compounds. Furthermore, the fundamental principles of these approaches, instrumentation, specific applications, and the strategy to choose the most proper technique for different carriers has been discussed.

Quantifying the Chain Folding in Polymer Single Crystals by Single-Molecule Force Spectroscopy

Chain folding is a motif of polymer crystallization, which is essential for determining the crystallization kinetics. However, the experimental quantification of the chain folding remains a challenge because of limited instrumental resolution. Here, we quantify chain folding in solution-grown single crystals by using atomic force microscopy (AFM)-based single-molecule force spectroscopy. The fingerprint spectrum of force-induced chain motion allows us to decipher the adjacent and nonadjacent re-entry folding with spatial resolution of subnanometers. The average fractions of adjacent re-entry folds ⟨f⟩ are in the range 91-95% for polycaprolactone, poly-l-lactic acid, and polyamide 66, which is higher than the values determined by other classical technologies. The established single-molecule method is applicable to a broad range of crystalline polymer systems with different chain conformations or compositions.

Ultra-high resolution imaging of thin films and single strands of polythiophene using atomic force microscopy

Real-space images of polymers with sub-molecular resolution could provide valuable insights into the relationship between morphology and functionality of polymer optoelectronic devices, but their acquisition is problematic due to perceived limitations in atomic force microscopy (AFM). We show that individual thiophene units and the lattice of semicrystalline spin-coated films of polythiophenes (PTs) may be resolved using AFM under ambient conditions through the low-amplitude (≤ 1 nm) excitation of higher eigenmodes of a cantilever. PT strands are adsorbed on hexagonal boron nitride near-parallel to the surface in islands with lateral dimensions ~10 nm. On the surface of a spin-coated PT thin film, in which the thiophene groups are perpendicular to the interface, we resolve terminal CH3-groups in a square arrangement with a lattice constant 0.55 nm from which we can identify abrupt boundaries and also regions with more slowly varying disorder, which allow comparison with proposed models of PT domains.

Molecular Combing of a Flexible Polymer Chain by Simple Spin-Casting

If polymer chains could be fixed on a substrate as a fully elongated chain, a procedure known as "molecular combing", the chain structure could be analyzed more precisely than has been possible with the characterization techniques available today. Although the molecular combing of a rigid biomolecule, DNA, has been attained for the mapping of genetic information, that of flexible chains has never been achieved as yet. We show here that poly(n-nonyl acrylate) (PNA) can be molecularly combed on mica by a simple spin-casting method, and that the chain lengths were in good agreement with that of the all-trans conformation. One of the key factors for successful molecular combing was found to be the weak adsorption of PNA on mica, indicating that flexible polymers may be molecularly combed by adjusting their affinity to the substrate. The molecular combing of polymer chains may open a new way not only to characterize the chain structures more precisely but also to fabricate new nanomaterials based on polymers.

Poly(styrene-co-butadiene) random copolymer thin films and nanostructures on a mica surface: morphology and contact angles of nanodroplets

The self-assembly of poly(styrene-co-butadiene) random copolymers on mica surfaces was studied by varying solution concentrations and polymer molecular weights. Toluene solutions of the poly(styrene-co-butadiene) samples were spin coated onto a mica surface and the resulting polymer morphology was investigated by atomic force microscopy. At higher concentrations, thin films formed with varying thicknesses; some dewetting was observed which depended on the molecular weight. Total dewetting did not occur despite the polymer's low glass transition temperature. Instead, partial dewetting was observed suggesting that the polymer was in a metastable equilibrium state. At lower concentrations, spherical cap shaped nanodroplets formed with varying sizes from single polymer chains to aggregates containing millions of chains. As the molecular weight was increased, fewer aggregates were observed on the surface, albeit with larger sizes resulting from increased solution viscosities and more chain entanglements at higher molecular weights. The contact angles of the nanodroplets were shown to be size dependent. A minimum contact angle occurs for droplets with radii of 100-250 nm at each molecular weight. Droplets smaller than 100 nm showed a sharp increase in contact angle; attributed to an increase in the elastic modulus of the droplets, in addition, to a positive line tension value. Droplets larger than 250 nm also showed an increased contact angle due to surface heterogeneities which cannot be avoided for larger droplets. This increase in contact angle plateaus as the droplet size reaches the macroscopic scale.