Ultrathin polymer films, glass transitions

Recently, Vaia et al. [8] reported a new process for direct polymer intercalation based on a predominantly enthalpic mechanism. By maximization of the number of polymer host interactions, the unfavorable loss of conformational entropy associated with intercalation of the polymer can be overcome leading to new intercalated nanostructures. They also reported that this type of intercalated polymer chain adopted a collapsed, two-dimensional conformation and did not reveal the characteristic bulk glass transition. This behavior was qualitatively different from that exhibited by the bulk polymer and was attributed to the confinement of the polymer chains between the host s layers. These types of materials have important implications not only in the synthesis and property areas, where ultrathin polymer films confined between adsorbed surfaces are involved. These include polymer filler interactions in polymer composites, polymer adhesives, lubricants, and interfacial agents between immiscible phases. 

Prucker, S. Christian, H. Bock, J. Ruehe, C.W. Frank, and W. KnoU, On the glass transition in ultrathin polymer films of different molecular architecture, Macromol. Chem. Phys. 199, 1435 (1998) J.N.D Amour, C.W. Frank, and U. Okoroanyanwu, Influence of substrate chemistry on the properties of ultra thin polymer films, Microelectron, Eng. 73 74, 209 217 (2004). 

DeMaggio, W.E. Frieze, D.W. Gidley, M. Zhu, H.A. Hristov, and A.F. Yee, Interface and surface effects on the glass transition in thin polystyrene films, Phys. Rev. Lett. 78, 1524 (1997). " D.B. Hall, J.C. Hooker, and J.M. Torkelson, Ultrathin polymer films near the glass transition Effect on the distribution of a relaxation times as measured by second harmonic generation, Macro molecules 30, 667 (1997). 

Here, we would like to show our recent results on the glass transition dynamics of stacked thin polymer films and the aging dynamics of ultrathin polymer films, as investigated by DSC and DRS. For both cases, heterogeneous dynamics such as the... 

Efremov MYu, Olson EA, Zhang M, Zhang Z, Allen LH (2003) Glass transition in ultrathin polymer films calorimetric study. Phys Rev Lett 91 085703... 

Hall DB, Hooker JC, Torkelson JM (1997) Ultrathin polymer films near the glass transition effect on the distribution of alpha-relaxation times as measured by second harmonic generation. Macromolecules 30 667-669... 

Ellison CJ, Kim SD, Hall DB, Torkelson JM (2002) Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films novel fluorescence measurements. Eur Phys J E 8 155-166... 

Priestley RD, Broadbelt LJ, Torkelson JM (2005) Physical aging of ultrathin polymer films above and below the bulk glass transition temperature effects of attractive vs neutral polymersubstrate interactions measured by fluorescence. Macromolecules 38 654—657... 

Fryer, D. S., Nealey, R F., and de Pablo, J. J., Thermal probe measurements of the glass transition temperature for ultrathin polymer films as a function of thickness, Macromolecules, 33, 6439-6447 (2000). 

In this paper, we will address the complex issue of the mechanical determination of Tg at surfaces of thick bulk-like, as well as interfacially confined ultrathin polymer films. First, we will show that the pressure exerted by the tip does not affect the glass transition. Then, we will introduce a novel contact mechanical technique to determine the Tg of thin films, shear modulation SPM. Finally we will present Tg results on films where interfacial interactions, remaining stress and conformational changes, formed during film preparation, alter the bulk Tg. 

Tremendous interest has arisen in recent years regarding die issues of chain mobility and glass transition at polymer surfaces (1-4). Keddie et al used ellipsometric measurements of thermal expansivity to study Tg of polystyrene on silicon as a function of film thickness (1). The observed Tg asymptotically approached the bulk value of 100 C as thickness was increased, but for thinner films lO s of nm), Tg was significantly depressed. Clearly, such effects are of critical importance in the application of ultrathin polymer films as lubricants and protective coatings in magnetic data storage devices and micromechanical systems (5). Until recently, however, direct mechanical measurements of polymer behavior at the nanoscopic scale (which is necessary for mechanical analysis of ultrathin films) and at elevated temperatures has not been possible. 

At the end of this article, one should be able to briefly describe the anomalous transitions in near-surface layers, nanocomposites, and ultrathin films. Glass Transition on the Polymer Surface and in the Polymer Thin... 

Serghei, A., Tress, M., Kremer, F. Confinement effects on the relaxation time distribution of the dynamic glass transition in ultrathin polymer films. Macromolecules 39, 9385 (2006)... 

Singh, L., Ludovice, P.J., Hendaxon, C.L. Influence of molecular weight and film thickness on the glass transition temperature and coefficient of thermal expansion of supported ultrathin polymer films. Thin Solid Films 449(12), 231-241 (2004)... 

Addition of a thin-film heater to the back of a metal sample holder allowed studies of thin polymer films by GRAS at up to 200 °C (121). In a study of an ultrathin film of polymethylmethacrylate, the two doublets near 1240/1270 and 1150/1190 cm"1 exhibited changes in relative intensity above the glass transition temperature of 100 °C, indicating that the polymer glass structure was maintained even in such thin films. 

Koh YP, McKeima GB, Simon SL (2006) Calorimetric glass transition temperature and absolute heat capacity of polystyrene ultrathin films. J Polym Sci, Part B Polym Phys... 

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