An official website of the United States government
In the present work, we use ellipsometry to extract the physical aging response of thin glassy polystyrene (PS) layers from rubbery–glassy bilayer films of poly(n-butyl methacrylate) (PnBMA) atop PS. How the soft interface between rubbery and glassy polymer domains can impact the physical aging response of glassy domains is unclear. Measurements in the literature have shown that the local glass transition temperature Tg of PS is strongly reduced near a PnBMA/PS interface with a magnitude twice as large compared to that imparted by a free surface. As the free surface is known to reduce physical aging, we anticipated large changes in the physical aging response of PS within PnBMA/PS bilayer films. However, surprisingly the aging response remained equivalent to bulk down to 75 nm PS layer thicknesses that were the thinnest we found could be accurately measured given the optical limits of dispersion. With complementary fluorescence measurements, we show that the average Tg(hPS) of such PS layers within 150 nm PnBMA/75 nm PS bilayer films are also still bulk. These findings demonstrate that films with finite domain sizes have interfacial dynamical gradients that are significantly altered from those previously measured in systems with semi-infinite domain sizes.
more »
« less
Unexpected Molecular Weight Dependence to the Physical Aging of Thin Polystyrene Films Present at Ultra-High Molecular Weights
The physical aging behavior, time‐dependent densification, of thin polystyrene (PS) films supported on silicon are investigated using ellipsometry for a large range of molecular weights (MWs) from Mw = 97 to 10,100 kg mol−1. We report an unexpected MW dependence to the physical aging rate of h < 80‐nm thick films not present in bulk films, where samples made from ultra‐high MWs ≥ 6500 kg mol−1 exhibit on average a 45% faster aging response at an aging temperature of 40 °C compared with equivalent films made from (merely) high MWs ≤ 3500 kg mol−1. This MW‐dependent difference in physical aging response indicates that the breadth of the gradient in dynamics originating from the free surface in these thin films is diminished for films of ultra‐high MW PS. In contrast, measures of the film‐average glass transition temperature T g(h) and effective average film density (molecular packing) show no corresponding change for the same range of film thicknesses, suggesting physical aging may be more sensitive to differences in dynamical gradients. These results contribute to growing literature reports signaling that chain connectivity and entropy play a subtle, but important role in how glassy dynamics are propagated from interfaces.
more »
« less
- Award ID(s):
- 1709132
- PAR ID:
- 10097070
- Date Published:
- Journal Name:
- Journal of Polymer Science Part B: Polymer Physics
- Volume:
- 57
- ISSN:
- 0887-6266
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between the residual adsorbed layer thickness h ads ( t ) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse. This procedure, frequently called “Guiselin's experiment”, refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers h ads ( t ) from solvent washing melt films. We find h ads ( t ) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind h ads of comparable thickness. This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption. Preformed adsorbed layers of different thicknesses h ads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.more » « less
-
We report the use of suboxide molecular-beam epitaxy ( S-MBE) to grow β-Ga 2 O 3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 10 16 to 10 19 cm −3 . In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga 2 O, i.e., gallium suboxide, is supplied. Directly supplying Ga 2 O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga 2 O 3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature ( T sub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO 2 ) producing an SiO suboxide molecular beam are used to dope the β-Ga 2 O 3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 10 17 cm −3 reveal a room-temperature mobility of 124 cm 2 V −1 s −1 that increases to 627 cm 2 V −1 s −1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga 2 O 3 films grown by S-MBE at growth rates of ∼1 µm/h.more » « less
-
null (Ed.)CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2–) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.more » « less
-
Bismuth telluride-based thin films have been investigated as the active material in flexible and micro thermoelectric generators (TEGs) for near room-temperature energy harvesting applications. The latter is a class of compact printed circuit board compatible devices conceptualized for operation at low-temperature gradients to generate power for wireless sensor nodes (WSNs), the fundamental units of the Internet-of-Things (IoT). CMOS and MEMS compatible micro-TEGs require thin films that can be integrated into the fabrication flow without compromising their thermoelectric properties. We present results on the thermoelectric properties of (Bi,Sb)2(Se,Te)3 thin films deposited via thermal evaporation of ternary compound pellets on four-inch SiO2 substrates at room temperature. Thin-film compositions and post-deposition annealing parameters are optimized to achieve power factors of 2.75 mW m−1 K−2 and 0.59 mW m−1 K−2 for p-type and n-type thin films. The measurement setup is optimized to characterize the thin-film properties accurately. Thin-film adhesion is further tested and optimized on several substrates. Successful lift-off of p-type and n-type thin films is completed on the same wafer to create thermocouple patterns as per the target device design proving compatibility with the standard MEMS fabrication process.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/polb.24797
