Tacticity and surface chemistry effects on the glass transition temperature of thin supported PMMA films.
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Tacticity and surface chemistry effects on the glass transition temperature of thin supported PMMA films. Y.Grohens , L.Hamon, P.Carriere, Y.Holl, J.Schultz Institut de Chimie des Surfaces et Interfaces-CNRS 15, rue J.Starcky, BP 2488 68057 Mulhouse Cedex, France ABSTRACT The glass transition temperature (Tg) of thin supported films of stereoregular poly(methyl methacrylate) on silicon has been investigated by ellipsometry in temperature. The Tg of the spin coated layers of isotactic PMMA increases for thickness lower than 100 nm whereas a depression of the Tg is observed for the syndiotactic PMMA. This opposite trend in the Tg variation in thin layer between the two isomers evidenced a fascinating effect of the chain organization at an interacting interface. Hydrofluric acid surface etching of the silicon wafer was shown to decrease the threshold thickness at which the change in Tg occurs for both PMMA isomers. The influence of the interfacial interactions along with the tacticity dependent characteristics of the polymer will be discussed. INTRODUCTION The properties of macromolecules in an interfacial region is of technological importance in many industrial fields. It is known that the conformation [1,2] and the mobility [3,4] of the polymer chains will be strongly influenced by a surface. The glass transition temperature of thin layers of polymers has been investigated by many groups in the last years [5-8]. One of the main conclusion that can be drawn from these works is that Tg of thin films decreases for free standing films and for weekly interacting systems. Polymers at free surfaces or in contact with repulsive surfaces are dominated by entropic effects such as disentanglements, confinement effects or chain end segregation which lead to a Tg depression [9-10]. In contrast, the increase of Tg for polymer thin layers in strong interaction with a substrate is often ascribed to particular chain organization (conformation, orientation) or chain packing (local density fluctuation) [8,12]. In the present study, the thermal expansion of thin supported layers of stereoregular PMMA was investigated by ellipsometry. Hydrofluoric acid, UVozone and H2O plasma surface treatments were done to vary the chemistry of the Si wafer substrate. The interfacial interactions were studied by FT-IR in ATR mode. The importance of the static and dynamic stiffness of the polymer chain in the structuration of the interfacial zone was discussed.
EXPERIMENTAL DETAILS The PMMA used here were purchased from Polymer Source Inc. Their characteristics are given in table 1. PMMA
Mw/Mn
Tacticity (%)
Mn
i:h:s
(10-3g/mol)
FF1.7.1
Tg (°C)
i-PMMA
97:03:0
37
1,21
61
s-PMMA
0:20:80
33
1,05
131
Table 1 : Characteristics of the PMMA used. i, h and s represents the isotactic, heterotactic and syndiotactic triads, respectively. They were synthesized by anionic polymerization to ensure well defined tacticity, polydispersity and molecular weight. These polymers were spin cast from chloroform solutions on (111) silicon surfaces. The substr
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