Thickness dependence of structural relaxation in spin‐cast polynorbornene films with high glass transition temperatures (>613 K)

Abstract

The isothermal structural relaxation (densification) of a family of glassy polynorbornene films with high glass transition temperatures (T g > 613 K) is assessed via spectroscopic ellipsometry. Three polymers were examined: poly(butylnorbornene) (BuNB), poly(hydroxyhexafluoroisopropyl norbornene) (HFANB), and their random copolymer, BuNB-r-HFANB. The effective aging rate, b(T), of thick ($1.2 lm) spun cast films of BuNB-r-HFANB is approximately 10 23 over a wide temperature window (0.49 < T/T g < 0.68). At higher temperatures, these polymers undergo reactions that more dramatically decrease the film thickness, which prohibits erasing the process history by annealing above T g . The aging rate for thick BuNB-r-HFANB films is independent of the casting solvent, which infers that rapid aging is not associated with residual solvent. b (at 373 K) decreases for films thinner than $500 nm. However, the isothermal structural relaxation of thin films of BuNB-r-HFANB exhibits nonmonotonic temporal evolution in thickness for films thinner than 115 nm film. The thickness after 18 h of aging at 373 K can be greater than the initial thickness. The rapid aging of these polynorbornene films is attributed to the unusual rapid local dynamics of this class of polymers and demonstrates the potential for unexpected structural relaxations in membranes and thin films of high-T g polymers that could impact their performance.

Effect of adjacent hydrophilic polymer thin films on physical aging and residual stress in thin films of poly(butylnorbornene‐ran‐hydroxyhexafluoroisopropyl norbornene)

“…In this case, the film thickness surprisingly starts to increase after around 80 min. This increasing film thickness during aging has been reported previously for thin spin cast films that were not annealed above T g prior to aging . This increase is attributed to the highly nonequilibrium chain conformations induced during rapid vitrification associated with spin coating.…”

“…Recently, we had reported the unexpected rapid aging of similar polymers at temperature >200 °C below T g when supported on silicon wafers . For the thinnest films, the films expanded during isothermal aging . A similar isothermal expansion in thin films was reported for polycarbonates (PCs), which similar to this polynorbornene contain rigid cyclic rings in the polymer backbone.…”

“…Woollam Co. (Lincoln, NE); M‐2000UI). Significant thermal degradation of the BuNB‐ r ‐HFANB films (Fig. S9) occurred prior to reaching T g (≈200 °C), which prevents the thermal history of the films from being erased by annealing above T g prior to aging studies as is common .…”

“…High T g polymers should decrease the propensity for physical aging at the operating conditions for such membranes, so polynorbornene membranes with high selectivity for butanol and having T g s in excess of 300 °C are promising candidates for biobutanol production . Recently, we had reported the unexpected rapid aging of similar polymers at temperature >200 °C below T g when supported on silicon wafers . For the thinnest films, the films expanded during isothermal aging .…”

Temperature-Dependent Spectroscopic Ellipsometry of Thin Polymer Films

“…We will now discuss cases in which such analysis provides equivocal results. The problem is the accuracy of determining the temperature of a given thermal transition, which can be affected by several factors, such as those related to the quality of the sample, i.e., surface roughness, and thickness unevenness, , as well as the presence of residual solvent in the samples, − error measurement, noise, a large dispersion of data, and a small range of linear dependence of data values. Problems related to sample imperfections can and should be identified by measuring the degree of depolarization.…”