The molecular organization in ultrathin polymer films (thicknesses less than 1000 angstroms) and thin polymer films (thicknesses between 1000 and 10,000 angstroms) may differ substantially from that of bulk polymers, which can lead to important differences in resulting thermophysical properties. Such constrained geometry films have been fabricated from amorphous poly(3-methyl-4-hydroxy styrene) (PMHS) and semicrystalline poly(di-n-hexyl silane) (PD6S) by means of spin-casting. The residual solvent content is substantially greater in ultrathin PMHS films, which suggests a higher glass transition temperature that results from a stronger hydrogen-bonded network as compared with that in thicker films. Crystallization of PD6S is substantially hindered in ultrathin films, in which a critical thickness of 150 angstroms is needed for crystalline morphology to exist and in which the rate of crystallization is initially slow but increases rapidly as the film approaches 500 angstroms in thickness.

The effect of film thickness on the organization kinetics of ultrathin (50-1000 Å thick) spin-cast polymer films was studied using poly(di-n-hexylsilane) and UV absorption spectroscopy. We found an extensive reduction in the crystallinity as well as a reduction in the rate of crystallization for film thicknesses below 500 Å resulting from the constrained geometry. Modeling using polymer crystallization theories elucidated the surface-induced phenomena. We found that the dimensionality of the growth depended both on the film thickness and on the crystallization temperature. At low crystallization temperatures (below 0°C) and for films thicker than 220 Å, the nucleation is bulk and the growth is three-dimensional. However, at higher crystallization temperatures (above 3°C) and for low film thickness (below 150 Å), the growth is one-dimensional and heterogeneous nucleation becomes important. For 500 Å thick films, the transition between the two nucleation regimes occurs abruptly around 3°C. Experimental SectionSubstrate Preparation. The poly(di-n-hexylsilane) used in this study was synthesized as reported previously. 17 The weight-average molecular weight was 2.6 × 10 6 and the polydispersity 2.4. Solutions of the polymer were made in isooctane (2,2,4-trimethylpentane, 99+%, Aldrich Chemical Co.) in concentrations ranging from 0.1 to 2 wt %. The isooctane was used as received.

The effect of confinement on the crystallinity and chain orientation of ultrathin poly(di‐n‐hexylsilane) films has been investigated using UV absorption, fluorescence and IR spectroscopies. UV absorption measurements in a series of poly(di‐n‐hexylsilane) films having thicknesses between 50 and 3500 Å have shown that, for thicknesses less than 500 Å, the polymer backbone disorders and extensive crystallization of the films is hindered irrespective of molecular weight or surface hydrophobicity. Fluorescence studies showed that rapid energy transfer occurs from the disordered chain segments to the crystalline ones. The orientation of both the polymer backbone and side chains was probed with IR absorption and grazing incidence reflection measurements. The side chains are extended, although not completely in the all‐trans conformation, with their carbon plane mostly perpendicular to the substrate. The backbone lies extended, with the polymer axis parallel to the plane of the film. The hexyl side‐chains disorder in films less than 2000 Å thick and this disordering occurs through the introduction of gauche bonds. Our findings suggest the possibility of using thickness to control the chain organization and morphology of a polymer thin film. © 1996 John Wiley & Sons, Inc.

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