Visualizing polymer crystallization in ultrathin layers using a single-macromolecule tracking method

Revealing the Existence of Long-Range Liquid-Liquid Interfacial Potential in Phase-Transfer Processes

By employing fluorescence wide-field microscopy and a nanoparticle-based phase transfer catalyst (PTC), consisting of a fluorescent silica nanoparticle functionalized with trioctylpropylammonium bromide, we demonstrate that in the presence of NaOH, single nanoparticles display subdiffusive motion along the axis normal to an aqueous liquid-organic liquid interface. This is because of an extended interfacial potential with a shallow well (∼1 kBT) that stretches a few μm into the organic phase, in contrast to previous molecular dynamics studies that reported narrow interfaces on the order of ∼1 nm. Spontaneous interfacial emulsification induced by NaOH results in the propagation of water-in-oil nanoemulsions into the organic solvent that creates an equilibrium hybrid-solvent composition that solvates the PTC. A greater mobility and longer residence time of the PTC at the potential well enhance the interfacial phase transfer process and catalytic efficiency.

Unusual Morphologies of Poly(vinyl alcohol) Thin Films Adsorbed on Poly(dimethylsiloxane) Substrates

Adsorption of poly(vinyl alcohol) (PVOH), 99% and 88% hydrolyzed poly(vinyl acetate), to poly(dimethylsiloxane) (PDMS) substrates was studied. The substrates were prepared by covalently attaching linear PDMS polymers of 2, 9, 17, 49, and 116 kDa onto silicon wafers. As the PDMS molecular weight/thickness increases, the adsorbed PVOH thin films progressively transition from continuous to discontinuous morphologies, including honeycomb and fractal/droplet. The structures are the result of thin film dewetting that occurs upon exposure to air. The PVOH film thickness does not vary significantly on these PDMS substrates, implicating the PDMS thickness as the cause for the morphology differences. The adsorbed PVOH thin films are less stable and have a stronger tendency to dewet on thicker, more liquid-like PDMS layers. When PVOH(99%) and PVOH(88%) thin films are compared, fractal and droplet morphologies are observed on high molecular weight PDMS substrates, respectively. The formation of the unique fractal features in the PVOH(99%) thin films as well as other crystalline and semicrystalline thin films is most likely driven by crystallization during the dehydration process in a diffusion-limited aggregation fashion. The only significant enhancement in hydrophilicity via PVOH adsorption was obtained on PDMS(2k), which is completely covered with a PVOH thin film. To mimic the lower receding contact angle and less liquid-like character of the PDMS(2k) substrate, light plasma treatment of the higher molecular weight PDMS substrates was carried out. On the treated PDMS substrates, the adsorbed PVOH thin films are in the more continuous honeycomb morphology, giving rise to significantly enhanced wettability. Furthermore, hydrophobic recovery of the hydrophilized PDMS substrates was not observed during a 1 week period. Thus, light plasma oxidation and subsequent PVOH adsorption can be utilized as a means to effectively hydrophilize conventional PDMS substrates. This study illustrates that the stability and morphology of adsorbed polymer thin films depend on polymer crystallinity as well as substrate physical properties.

Correlating polymer crystals via self-induced nucleation

Crystallizable polymers often form multiple stacks of uniquely oriented lamellae, which have good registry despite being separated by amorphous fold surfaces. These correlations require multiple synchronized, yet unidentified, nucleation events. Here, we demonstrate that in thin films of isotactic polystyrene, the probability of generating correlated lamellae is controlled by the branched morphology of a single primary lamella. The nucleation density n(s) of secondary lamellae is found to be dependent on the width w of the branches of the primary lamella such that n(s) ∼ w(-2). This relation is independent of molecular weight, crystallization temperature, and film thickness. We propose a nucleation mechanism based on the insertion of polymers into a branched primary lamellar crystal.

Single-molecule observation of long jumps in polymer adsorption

Single-molecule fluorescence imaging of adsorption onto initially bare surfaces shows that polymer chains need not localize immediately after arrival. In a system optimized to present limited adsorption sites (quartz surface to which polyethylene glycol (PEG) chains adsorb from aqueous solution at pH 8.2), we find that some chains diffuse back into bulk solution and readsorb at some distance away, sometimes multiple times before they either localize at a stable position or diffuse away into bulk solution. This mechanism of surface diffusion is considerably more rapid than the classical model in which adsorbed polymers crawl on surfaces while the entire molecule remains adsorbed, suggesting the conceptual generality of a recent report ( Phys. Rev. Lett. 2013 , 110 , 256101 ) but in a new experimental system and with comparison of different chain lengths. We find the trajectories with jumps to follow a truncated Lévy distribution of step size with limiting slope -2.5, consistent with a well-defined, rapid surface diffusion coefficient over the times we observe. The broad waiting time distribution appears to reflect that polymer chains possess a broad distribution of bound fraction: the smaller the bound fraction of a given chain, the shorter the surface residence time before executing the next surface jump.

Unconventional multiple ring structure formation from evaporation-induced self-assembly of polymers

The formation of multiring deposits of poly(2-vinylpyridine) (P2VP) from the evaporation of a P2VP-(2,6-lutidine + water) drop on a glass substrate does not conform to the conventional pinning-depinning mechanism. Instead, ringlike deposits are formed when the droplet undergoes several cycles of spreading and receding where, for each spreading event, a P2VP ridge is formed at the contact line when the polymer flows toward the outward advancing edge. The complex interplay between an outward solutal-Marangoni flow due to a higher concentration of the polymer at the contact line and an inward solvent-Marangoni flow arising from the differences in volatilities and surface tensions of the pure solvent components plays an important role in enhancing the droplet spreading rate. The newly discovered surface patterning mechanism has important implications in the development of novel techniques for inducing self-assembly of functional materials from evaporating drops.

Domain coalescence-induced nucleation and anomalous growth of holes in thin polymer blend film

The phase evolution of a thin polymer blend film of polystyrene (PS) and poly(2-vinyl pyridine) (P2VP) triggered by solvent annealing is examined at both the bulk and single-(macro)molecule levels using wide-field microscopy (WFM). The transitions between different evolutionary stages in the nucleation and growth process are clearly visualized in real time and without intermittent breaks. The nucleation of PS holes arises from the coalescence and growth of P2VP domains and the holes expand in a complex manner involving the dewetting of PS and the absorption of P2VP domains into the holes.

Single chain diffusion of poly(ethylene oxide) in its monolayers before and after crystallization

Lateral diffusion of single chain related to the crystallization process of poly(ethylene oxide) (PEO) in its monolayers on silica surfaces is studied by single molecule fluorescence microscopy and single molecule tracking techniques. Diffusion of PEO chains is observed in the supercooled state before crystallization as well as in the noncrystalline regions after crystallization. In the postcrystallization monolayers, the diffusion coefficient of PEO chains is an order of magnitude lower than that in the supercooled state before crystallization. The origin is attributed to the change of polymer surface concentration due to the consumption of polymer molecules in the crystal formation. This is supported by the results showing a monotonous decrease of diffusion coefficient with the thickness decrease of the monolayer in its supercooled state. The PEO chains take a more flattened conformation under lower surface concentration and stick stronger to the surface. As a consequence, the diffusion rate is reduced. The results clearly demonstrate a strong mutual effect between the crystallization process and the mass transportation for the polymer crystallization under surface confinement.