RESUMO
Upon cooling, semicrystalline polymers experience crystallization and form alternatively stacked layers consisting of thin crystal lamellae and amorphous ones. The unique morphology, crystallinity, and crystallization kinetics highly depend on the molecular weight. Therefore, it is deduced that entanglement impacts crystallization kinetics, as well as hierarchically crystalline structures. However, the impact of entanglement on folded crystalline chains has not been well understood due to experimental difficulties. In this work, chain-folding structures for seven 13C CH3 labeled poly(l-lactic acid)s with various molecular weights (Mws) were investigated by 13C-13C double quantum NMR spectroscopy. As a result, chain-folding events were categorized into three different Mw regimes: (i) The lowest Mw sample (2K g/mol) adopts an extended chain conformation (folding number, n = 0) (regime I); (ii) Intermediate Mw ones possess mixtures of non- and once-folded structures, and the once-folded fraction suddenly increases above the entanglement length (Me), up to Mw = 45K g/mol (regime II); (iii) The high Mw ones (Mw > 45K g/mol) adopt the highest chance for an adjacent re-entry structure with n = 1.0 in the well-developed entangled network (regime III). It was suggested that entanglement induces folding of the semicrystalline polymer.
RESUMO
An understanding of surfactant adsorption at solid-liquid interfaces is important for solving many technological problems. This work evaluates surfactant adsorption abilities of high surface area (200-600 m2/g), high porosity (>90%), hierarchically structured open pore polymer gels. Specifically, the interactions of a nonionic block copolymer surfactant, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with three polymer gels, namely, syndiotactic polystyrene (sPS), polyimide (PI), and polyurea (PUA) offering different surface energy values, are evaluated at surfactant concentrations below and well above the critical micelle concentration (CMC). Two distinct surfactant adsorption behaviors are identified from the surface tension and nuclear magnetic resonance data. At concentrations below CMC, the surfactant molecules adsorb as a monolayer on polymer strands, inferred from the Langmuir-type adsorption isotherm, with the adsorbed amount increasing with the specific surface area of the polymer gel. The study reports for the first time that the gels show a strong surfactant adsorption above CMC, with the effective surfactant concentration in the gel reaching several folds of the CMC values. The effective surfactant concentration in the gel is analyzed using surfactant micelle size, polymer surface energy, and pore size of the gel. The findings of this study may have strong implications in liquid-liquid separation problems and in the removal of small dye molecules, heavy metal ions, and living organisms from aqueous streams.
RESUMO
Semicrystalline polymers are categorized as either mobile or fixed crystals, depending on chain mobility in the crystalline region. In this work, we investigate molecular dynamics and phase structure in the cocrystal consisting of fixed and mobile polymer crystals by solid-state (ss) nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that (i) the mobile component begins large amplitude motions associated with crystal-crystal transition, while fixed ones keep their rigidity in the cocrystal, and (ii) asymmetric molecular dynamics leads to nanosegregations into mobile- and fixed-rich domains in the cocrystal below the melting temperature (Tm). The observed phase separation induced by asymmetric molecular dynamics is similar to the phase separation of the miscible amorphous polymer blend; however, it is limited to two dimensions due to the parallel packing of the stems inside the cocrystal, as well as chain connectivity at the crystalline-amorphous boundary.