• PDF / 545,430 Bytes
• 8 Pages / 612 x 792 pts (letter) Page_size
• 83 Downloads / 240 Views

DOWNLOAD

REPORT

---

Research Letter

Dynamics of interacting interphases in polymer bilayer thin films David D. Hsu, Department of Physics & Engineering, Wheaton College, 501 College Avenue, Wheaton, IL 60187, USA Wenjie Xia, Materials Science and Engineering Division, National Institute of Standards and Technology, 100 Bureau Drive, M/S 8550, Gaithersburg, MD 20899, USA; Center for Hierarchical Materials Design, Northwestern University, Evanston, IL 60208, USA; Department of Civil & Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA Jake Song, Department of Materials Science & Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA; Department of Materials Science & Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA Sinan Keten, Department of Civil & Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA; Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3109, USA Address all correspondence to Sinan Keten at [email protected] (Received 14 July 2017; accepted 28 September 2017)

Abstract We investigate how the local glass-transition temperature (Tg) depends on film thickness in monolayer and bilayer thin films with a polystyrene (PS) upper-layer and a poly(methyl methacrylate) (PMMA) lower-layer using coarse-grained simulations. Interactions between overlapping interphases demonstrate a superposition principle for describing their glass-transition behaviors. For supported bilayer films, the free surface effect on a PS film upper-layer is effectively eliminated due to an enhanced local Tg near the PS–PMMA interface, which cancels out depressed Tg near the free surface. However, at very low PMMA lower-layer thicknesses, the PMMA-substrate effect can penetrate through the polymer– polymer interface, leading to enhanced Tg in the PS upper-layer.

Introduction In nanoscale films, regions near interfaces exhibit properties which are strongly perturbed from their respective bulk identities, such as modulus, glass-transition temperature (Tg), density and relaxation dynamics, known as the nanoconfinement effect. As the characteristic size of the material decreases, these “interphase” regions constitute a larger fraction of the material by volume, and can greatly influence the macroscopic properties.[1–4] Significant efforts have been carried out to quantify the extent to which polymer–air[1,3–6] and polymer–substrate[7–9] interfaces cause deviation from bulk properties for nanoscopically thin films. However, the behavior of polymer–polymer “soft” interfaces[10–16] occurring at the junction of immiscible polymers is far less understood. Interphases formed near these discontinuities approach each other as the characteristic feature dimensions become small. Our understanding of the interactions of these interphases, specifically whether they have additive or subtractive effects in thin films remains to be clarified. Previous studies have show