Ultrathin Films of Poly(ethylene oxides) on Oxidized Silicon. 2. In Situ Study of Crystallization and Melting by Hot Stage AFM

Abstract

We report on the isothermal crystallization behavior of thin (film thickness d < 500 nm) and ultrathin (d < 100 nm) films of poly(ethylene oxide) (PEO), as well as pyrene end-labeled PEO, on native silicon studied by in situ hot stage atomic force microscopy (AFM). Individual lamellae were imaged during crystallization and melting. Using AFM, we have directly measured lamellar growth rates, lamellar thicknesses, and melting ranges as a function of film thickness (ca. 15->500 nm), crystallization temperature (40-62 °C), and molar mass (11-100 kg/mol). On the basis of the Hoffman-Weeks extrapolation, the Gibbs-Thomson equation, and the Hoffman-Lauritzen theory, we show that the crystallization of PEO in thin and ultrathin films can be described with the same laws as the bulk crystallization. In addition, we find that the equilibrium melting points and surface free energies of the fold surfaces agree quantitatively with literature data for bulk crystallization and hence are not altered due to confinement in ultrathin films. However, there is a monotonic decrease of lamellar growth rates with decreasing film thickness for films thinner than ca. 250 nm. The growth rates decrease to below 1% of their bulk value in the thinnest films; this is attributed to an increase in glass transition temperature of up to 30 °C for the confined PEO and the concomitant reduction of molecular mobility.

Paclitaxel distribution in poly(ethylene glycol)/poly(lactide-co-glycolic acid) blends and its release visualized by coherent anti-Stokes Raman scattering microscopy

“…Crystallization of polyethylene oxide in thin films has been reported [22]. Herein, the dependence of the CARS signal on excitation polarization was analyzed to determine the existence of PEG crystallization in the blend film.…”

SAXS study on the crystallization of PET under physical confinement in PET/PC multilayered films

“…In particular, for polymer systems it has been found that confinement can change the glass transition temperature [1][2][3][4], molecular mobility [5,6], behaviour of the phases and morphology [7,8], molecular orientation [9] and crystallization behaviour [10][11][12][13]. Polymeric materials can be subjected either to chemical or to physical confinement.…”

“…The mechanisms of nucleation are affected due to the reduced number of chain conformations and interacting molecules in comparison with the bulk [11]. Lamellar growth rate and the final degree of crystallinity are diminished if the chains in thin polymer films are subjected to a preferred orientation [10,15]. Confined crystallization of polypropylene layers in between amorphous layers of polystyrene results in discoidal morphologies which transform into long stacks of very short lamellae arranged in fan-like arrays, when decreasing layer thickness from the microscale to the nanoscale [16].…”

A Tutorial on the Mechanisms, Dynamics, and Control of Atomic Force Microscopes

“…The first applications were found in materials science and this field continues to make use of AFM today. Recent studies include experiments to understand the nanoscalephenomena underlying improved photovoltaic cells [19], surface forces [20], thin films [21]- [23], crystallization [24], [25], and semiconductor properties [26]- [28].…”