Crystal growth pattern changes in low molecular weight poly(ethylene oxide) ultrathin films

Confined Crystallization of Polymers within Nanopores

Crystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called "fractionated crystallization", i.e., multiple crystallization peaks during cooling.Because the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material.Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the "chain perpendicular to" to the "chain parallel to" the AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth direction, nucleation, and crystal growth rate. An orientation diagram has been established for the PEO/AAO system, considering the cooling condition and pore size.The interfacial polymer layer has different physical properties as compared to the bulk. In poly(l-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.

Morphological Variation of an LB Film of Giant Amphiphiles Composed of Poly(ethylene oxide) and Hydrophobically Modified POSS

Hydrophobically modified polyhedral oligomeric silsesquioxanes (XPOSS) are linked to one end of water-soluble poly(ethylene oxide) (PEO) to synthesize giant amphiphiles (XPOSS-PEO). XPOSS-PEO exhibit an interesting surface activation capacity owing to the synergy of the soft PEO segment and hydrophobic XPOSS when they are spread on the water surface and compressed by the barrier. The monolayers of XPOSS-PEO at the air-water interface are transferred onto the silicon substrate at different surface pressures using the Langmuir-Blodgett (LB) film deposition protocol. The modification of the POSS head significantly changes the crystallinity of the PEO tail, which affects the LB film morphologies of the giant amphiphiles. When the POSS are modified with fluorinated agents, the assembled LB films show a fractal growth pattern, but when the POSS are decorated with a pure alkane chain, the fractal growth pattern does not present in the resulting LB film.

Janus Polymer Single Crystal Nanosheet via Evaporative Crystallization

We show that liquid/liquid interface can guide polymer chain folding during crystallization. Evaporation-induced crystallization of telechelic dicarboxyl end-functionalized poly(ε-caprolactone) (COOH-PCL-COOH) at a water/pentyl acetate interface produced millimeter-scale, uniform polymer single crystal (PSC) films. Due to the asymmetric nature at the interface, the PSC nanosheets exhibited a Janus structure: the two surfaces of the crystal showed distinct water contact angle, which are quantitatively confirmed by in situ nanocondensation using environmental scanning electron microscopy (ESEM).

Structure Formation of Ultrathin PEO Films at Solid Interfaces—Complex Pattern Formation by Dewetting and Crystallization

The direct contact of ultrathin polymer films with a solid substrate may result in thin film rupture caused by dewetting. With crystallisable polymers such as polyethyleneoxide (PEO), molecular self-assembly into partial ordered lamella structures is studied as an additional source of pattern formation. Morphological features in ultrathin PEO films (thickness < 10 nm) result from an interplay between dewetting patterns and diffusion limited growth pattern of ordered lamella growing within the dewetting areas. Besides structure formation of hydrophilic PEO molecules, n-alkylterminated (hydrophobic) PEO oligomers are investigated with respect to self-organization in ultrathin films. Morphological features characteristic for pure PEO are not changed by the presence of the n-alkylgroups.

Morphology Diagram of Single-Layer Crystal Patterns in Supercooled Poly(ethylene oxide) Ultrathin Films: Understanding Macromolecular Effect of Crystal Pattern Formation and Selection

A series of single-layer crystal patterns were observed in ultrathin films of 10 poly(ethylene oxide) fractions of molecular weights ranging from 2.02k to 932.0k g/mol. Morphology transitions between these different crystal patterns were quantitatively identified, and a morphology diagram with respect to supercooling and molecular weight dependencies was constructed. This will foster understanding of the macromolecular effects on the crystal pattern formation and selection critically associated with the parameters of molecular diffusion length and growth anisotropy.