Polystyrene films, confinement effects

Even if some polymers might be hydrophobic in the bulk, concerns about possible water adsorption effects in thin films are justified, because of the preponderant role of the interface in confinement. For example, negligible water adsorption is reported in the handbook of polymers for polystyrene in the bulk 0.05% at 23 °C and 50% relative humidity. In spite of this, strong water adsorption-desorption effects were observed in thin polystyrene films the dielectric loss decreased with more than one decade when a thin film was measured in a dry nitrogen atmosphere and after 2 horns of annealing at 135 °C (Fig. 9, example for a film thickness of 223 nm). A pronounced decrease of s ( 20% at 1 Hz) was detected as well. 

The fascinating properties exhibited by nanoparticles, such as blue shift of the absorption spectrum, size-dependent luminescence, etc., are various manifestations of the so-called quantum confinement effect. These unique properties make ZnO a promising candidate for applications in optical and optoelectronic devices [35-38]. Polymer-based nanocomposites are the subject of considerable research due to the possibility of combining the advantages of both polymers and nanoparticles. There are several applications of polymeric nanocomposites based on their optical, electrical and mechanical properties. Further, nanocrystals dispersed in suitable solid hosts can be stabilized for long periods of time. Polystyrene (PS)— an amorphous, optically clear thermoplastic material, which is flexible in thin-film form—is often chosen as a host matrix because of its ideal properties for investigating optical properties. It is one of the most extensively used plastic materials, e.g., in disposable cutlery, plastic models, CD and DVD cases, and smoke-detector housings. 

The glass transition temperatures were found to decrease with both decreasing film thickness and decreasing molecular weights of the polystyrene. For example, one 20 nm thick film of polystyrene had a Tg value reduced by 70 K below the bulk value of 373 K. The strong molecular weight dependence observed in these studies revealed the importance of chain confinement effects. 

Rathfon, LM., Cohn, R.W., Crosby, A.J., Rothstein, J.P., Tew, G.N. Confinement effects on chain entanglement in liee-standing polystyrene ultrathin films. Maciomolecules 44(13), 5436-5442 (2011)... 

Tg s of polymer thin films with thickness near the Rc of the polymer chains can be altered from the bulk values, largely due to confinement effects and interfacial attraction to substrates.In all of these examples, attraction of a thin film of a homopolymer to a substrate causes an inaease in Tg in thin films, while the opposite occurs when polymer chains are repulsed by the surface. Tsui et al. used surface-grafted PS-r-PMMA with varying composition to modify the chain interactions of a thin layer of polystyrene toward the substrate, and found that the Tg of polystyrene could be effectively controlled. ... 

Mundra, M. K., Ellison, C. J., Behhng, R. E., and Torkelson, J. M., Confinement, composition and spin coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene containing random copolymers sensing by intrinsic fluorescence. Polymer, 47, 7747-7759 (2006). 

Experimental results obtained with the techniques, listed in table 1, on similar systems (such as polystyrene with conq>arable molecular weights) have been found to be inconsistent. Although there is agreement that near the substrate interface, the value of Tg is shifted from the bulk due to confinement and size effects, there is evident contradiction in the direction of the shift. Problems such as local density variations normal to the interface, interfacial roughness, and film inq)erfections due to partial dewetting are listed as potential causes for these contradicting interpretations (21). 

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