Glassy polymer films have been extensively examined over the past 2 decades to try to provide insights into the glass formation process and the fundamental underpinnings of the glass transition. The physical aging of glasses represents the approach of the glass towards equilibrium and aging can lead to dramatic changes in the physical and transport properties of the glass. For thin films, recent works have proposed a streamlined process for aging measurements using ellipsometry. This streamlined method has been developed to minimize the experimentation time, where it has been shown that 6 h is sufficient to obtain accurate aging rates for polystyrene (PS) thin films. In our investigation, we offer caveats to the streamlined procedures based on our investigation of different branched architectures of PS: Comb (cmbPS), 4-arm star (SPS), and Centipede (CPS). These branched polymers age significantly slower than their linear counterparts, especially for films less than 40 nm. Extrapolating the bulk-like aging rate for the cmbPS predicts that a 0.15 nm change in thickness for a 20 nm film would require several weeks of aging. For LPS, 6 h is a sufficient amount of time to observe at least 0.1 nm change in film thickness for films as thin as 11 nanometers. For sub-40 nm films of cmbPS, CPS, and SPS, the subtle changes in thickness due to the diminished aging rates render the thickness statistically invariant during the 6 h aging experiment. The small change in thickness necessitates examination of the accuracy of ellipsometric measurements for thickness changes to ensure that the change can be used to calculate the aging rate. We find that thickness change &lt; 0.1nm leads to wide variation in the calculated aging rates on repeated aging experiments for the thinnest films. Thus, an additional consideration of the total thickness change during aging is required for assessment of aging rates in thin polymer films.

Effect of chain architecture on the aging rate in thin polystyrene films

256th National Meeting (2018)

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