Effect of nanoscale confinement on the glass transition temperature of free‐standing polymer films: Novel, self‐referencing fluorescence method

Abstract

The effect of nanoscale confinement on the glass transition temperature, T g , of freely standing polystyrene (PS) films was determined using the temperature dependence of a fluorescence intensity ratio associated with pyrene dye labeled to the polymer. The ratio of the intensity of the third fluorescence peak to that of the first fluorescence peak in 1-pyrenylmethyl methacrylate-labeled PS (MApyrene-labeled PS) decreased with decreasing temperature, and the intersection of the linear temperature dependences in the rubbery and glassy states yielded the measurement of T g . The sensitivity of this method to T g was also shown in bulk, supported PS and poly(isobutyl methacrylate) films. With free-standing PS films, a strong effect of confinement on T g was evident at thicknesses less than 80-90 nm. For MApyrene-labeled PS with M n ¼ 701 kg mol À1 , a 41-nm-thick film exhibited a 47 K reduction in T g relative to bulk PS. A strong molecular weight dependence of the T g -confinement effect was also observed, with a 65-nm-thick free-standing film exhibiting a reduction in T g relative to bulk PS of 19 K with M n ¼ 701 kg mol À1 and 31 K with M n ¼ 1460 kg mol À1 . The data are in reasonable agreement with results of Forrest, DalnokiVeress, and Dutcher who performed the seminal studies on T g -confinement effects in free-standing PS films. The utility of self-referencing fluorescence for novel studies of confinement effects in free-standing films is discussed.

Critical micelle concentrations of block and gradient copolymers in homopolymer: Effects of sequence distribution, composition, and molecular weight

“…(By using an intrinsic excimer to monomer intensity ratio rather than an intensity as our measurable, our technique is self-referencing, that is, independent of the thicknesses of the various films used in our study. Other self-referencing measurables that are useful in film studies include fluorescence lifetimes, 62,63 wavelength-shifting dye fluorescence, [64][65][66] and the ratio of excimer to monomer fluorescence from extrinsic dyes.…”

Perspective: Supercooled liquids and glasses

“…Even bigger decreases in T g (up to 70 K) have been reported for freestanding polymer films in the nanometer regime. 231,243,244 A nano-bubble inflation technique (as shown in Figure 6) has been used to directly study the deformation properties of freestanding films down to 3 nm in thickness. 245 T g shifts as large as 100 K have been reported with this technique, 246 along with equally striking qualitative changes in the response of larger length scale polymer motions.…”

“…245 T g shifts as large as 100 K have been reported with this technique, 246 along with equally striking qualitative changes in the response of larger length scale polymer motions. T g shifts in freestanding films have been reported to depend upon molecular weight for very long chains, 231,243,244 which is surprising since the glass transition in polymers has been understood to reflect structural relaxation on the scale of a few segments. Very recent measurements on freestanding polystyrene films indicate that such systems may show two thickness-vs.-temperature transitions (interpreted as two glass transitions), with the dominant transition being molecular weight independent and within 20 K of the bulk T g .…”

Temperature-ramping measurement of dye reorientation to probe molecular motion in polymer glasses

“…23 Among confined systems, polymer thin films have been studied extensively due to the simplicity of the geometry and the ease of sample preparation. For polystyrene thin films, many measurements show that the glass transition temperature becomes lower as the film becomes thinner and that the amount of deviation depends on whether the film is freestanding [24][25][26][27][28][29] or supported. 1,2 It is interesting to note that many of the techniques that show comparable results can be categorized as "dilatometric" methods.…”