Isothermal Relaxation of Rubbed Polystyrene Thin Films Probed With Optical Birefringence Measurements

In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

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Effects of confinement on material behaviour at the nanometre size scale

Alcoutlabi

McKenna²

2005

J. Phys.: Condens. Matter

1,208

1,246

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Relaxation of a rubbed polystyrene surface

Schwab

Dhinojwala

2003

Phys. Rev. E

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The relaxation dynamics of a rubbed polystyrene (PS) surface have been characterized using infrared-visible sum frequency generation spectroscopy (SFG). The SFG results were compared with previous relaxation of retardation measurements, and the results show that the rubbed PS surface has the same T(g) as the bulk where T(g) is defined as tau(T(g))=5 s, however, the surface has a lower activation energy (deltaE) and a larger stretching exponent (beta(KWW)) than bulk PS. This indicates that the surface region relaxes more quickly than the bulk. The thickness of this region of lower deltaE and larger beta(KWW) is estimated to be roughly 12 nm.

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The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

2005

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We have conducted a systematic study on the effects of post rubbing annealing on the relaxation of rubbing-induced birefringence of polystyrene. It is found that annealing at T(0) only affects the relaxation up to T(0)+T(Lag), where T(Lag) is proportional to the logarithm of the annealing time t(A). A theoretical model based on the distribution of relaxation times due to the individual birefringence elements is proposed. To remove its contribution to the net birefringence each element must overcome an energy barrier E=(317+1.17xi)x10(3) J/mol, and therefore must have a characteristic relaxation time tau which depends on temperature T and a barrier height which ranges from 340.4 kJ/mol to 445.7 kJ/mol. The relaxation of birefringence is expressed by the equation NB(T, t)=[Formula: see text]N(xi)e(-t/tau(T,xi))dxi, in which both the relaxation time tau(T,xi) and the distribution function N(xi) can be extracted from experimental data. The predictions of the model agree well with all the experimental results presented in this work. The differences and similarities of the relaxation of birefringence with respect to the physical aging of quenched PS are discussed. In particular, similarities in terms of the general temperature lag phenomena are noted.

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Isothermal Relaxation of Rubbed Polystyrene Thin Films Probed With Optical Birefringence Measurements

Cited by 4 publications

References 15 publications

Effects of confinement on material behaviour at the nanometre size scale

Effects of confinement on material behaviour at the nanometre size scale

Relaxation of a rubbed polystyrene surface

The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

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