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SUMMARY:β-NMR studies of the temperature\, depth and molecular weight dep
 endence of dynamics in normal and ultrastable polystyrene glasses
DTSTART;VALUE=DATE-TIME:20220829T164000Z
DTEND;VALUE=DATE-TIME:20220829T170000Z
DTSTAMP;VALUE=DATE-TIME:20220826T110720Z
UID:indico-contribution-3717@indico.stfc.ac.uk
DESCRIPTION:Speakers: Iain McKenzie (TRIUMF)\nThe observation that the gla
 ss transition temperature of polystyrene (PS) thin films decreases with de
 creasing film thickness led to the suggestion that there is a very thin la
 yer near the free surface where the polymer motion is faster than in the b
 ulk. Direct confirmation of enhanced dynamics near the free surface has pr
 oved elusive as few techniques can measure how the dynamics varies with di
 stance from the free surface on the nm length scale and those that can eit
 her use bulky probes or multilayer films that modify the dynamic gradients
 . Implanted $^8$Li$^+$ is a much less perturbing probe and with $\\beta$-d
 etected nuclear magnetic resonance ($\\beta$-NMR) we can measure the depth
  dependence of the $\\gamma$ relaxation of PS\, which involves motion of t
 he phenyl side groups.$^{1\,2\,3}$ The $\\gamma$ relaxation has a double e
 xponential depth dependence from the free surface\, with a characteristic 
 length of ~6 nm. We have produced ultrastable glass (USG) films of low-mol
 ecular weight PS by physical vapor deposition that exhibit properties simi
 lar to those of a normal glass (NG) that has been aged for several years. 
 Our β-NMR measurements indicate the bulk $\\gamma$ relaxation is slower i
 n the USG compared with the NG while the opposite is true near the free su
 rface where the $\\gamma$ relaxation is faster in the USG compared with th
 e NG. These trends are more significant for samples with a larger apparent
  age.  \n1. I. McKenzie et al.\, *Soft Matter* **2015**\, *11*\, 175\n2. I
 . McKenzie et al.\, *Soft Matter* **2018**\, *14*\, 7324\n3. I. McKenzie e
 t al.\, *J. Chem. Phys.* **2022**\, *156*\, 084903\n\nhttps://indico.stfc.
 ac.uk/event/53/contributions/3717/
LOCATION:Science and Technology Campus\, University of Parma
URL:https://indico.stfc.ac.uk/event/53/contributions/3717/
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