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.

32) Watanabe, M.; Togo, M.; Sanui, K.; Ogata, N.; Kobayoshi, T.; (33) The numbers in parentheses represent standard deviations (34) Armand, M. B.; Duclot, M. J.; Rigaud, Ph. Solid State Zonics (29) Hartman, J. R.; Wolf, R. E.; Foxman, B. M.; Cooper, S. R. J. Am. Chem. SOC. 1983,105, 131-132. (30) Ashworth, R. S.; Prout, C. K.; Domenicano, A.; Vaciago, A.; J. Chem. SOC. A 1968, 93-104. (31) Domenicano, A.; Scaramuzza, L.; Vaciago, A.; Ashworth, R. S.; Ohtaki, Z. Macromolecules 1984,17, 2908-2912. determined from multiple experiments. Prout, C. K. J. Chem. SOC. A 1968, 866-869.ABSTRACT A series of poly(di-n-allcylsilanes) having hexyl, heptyl, and octyl side chains have been investigated by spectroscopic methods. Whereas the UV spectrum provided information about the electronic structure of the polymer backbone, IR and Raman methods have been used to characterize the molecular conformation of both the main and side chains. These structural studies were correlated with results from thermal analyses and X-ray diffraction measurements. A mechanism involving the melting of the n-alkyl side chains with a subsequent disordering of the polymer backbone is proposed.

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Structural studies of asymmetrically substituted poly(di-n-alkylsilanes) as a function of pressure have shown that side-chain architecture is important for the observation of piezochromism. UV-vis, Raman, and FTIR measurements of poly(di-n-alkylsilanes) with linear side chains of two different lengths indicate that a piezochromic shift of the UV-vis absorption maximum at 315 nm is observed. However, if instead one or both of the side chains are branched, the Si backbone is prevented from adopting a planar zigzag conformation and hence no large changes in the UV-vis spectrum are observed.

The first bioactive glass, 45S5, discovered by Hench in 1969, remains as the "Gold standard" in the field of bioactive glasses due to its excellent bioactivity. 1,2 It has been used for numerous applications including hard and soft tissue regeneration, 3-6 oral care and cosmetic products. 7,8 Particulates of 45S5 glass have been commercialized in several successful medical products including Perioglas ® (NovaBone Products LLC), Novabone ® (NovaBone Products LLC), and NovaMin ® (Glaxo-Smith-Kline). 9 However, due to its limited ability of densification associated with crystallization during sintering, 10,11 the shaping of 45S5 glass into complex structure at elevated temperatures remains challenging. So far, there has been no commercial 3D-structured products made from 45S5 glass using a high-temperature processing method. Extensive studies have been conducted to investigate the crystallization mechanism and kinetics in 45S5 glasses to

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