Elsevier

Journal of Membrane Science

Volume 525, 1 March 2017, Pages 399-408
Journal of Membrane Science

Physical aging of glassy perfluoropolymers in thin film composite membranes. Part II. Glass transition temperature and the free volume model

https://doi.org/10.1016/j.memsci.2016.08.033Get rights and content

Highlights

  • Nano-thermal analysis determines Tg of selective layers in TFC membranes in situ.

  • A free volume model describes the Tg increase and permeance decrease during physical aging.

  • Polymers with stable permeability should have high free volume and low physical aging rate.

Abstract

Thin film composite (TFC) membranes for gas separation often comprise a thin selective layer of a glassy polymer, which, however, suffers from physical aging, i.e., gas permeance decreases with time. This study aims to provide a mechanistic understanding of the effect of physical aging on permeance reduction in TFC membranes. The Part I study reports gas permeances in two-layer TFC membranes comprising perfluoropolymers of Teflon® AF or Hyflon® AD with thicknesses of 50–400 nm. In this Part II study, apparent glass transition temperature (Tg) of thin selective layers was determined in situ over time using a nano-thermal analysis (nano-TA). Physical aging decreases gas permeances and increases apparent Tg, and the rate of changes is more significant for thinner selective layers. For example, N2 permeance decreases from 1000 gpu to 550 gpu while apparent Tg increases from 160 °C to 172 °C after aging for 2000 h in a membrane with 100-nm-thick Teflon AF1600. The measured Tg values are used to derive polymer fractional free volume and physical aging rate. A simplified free volume model is used to successfully correlate the gas permeance reduction with Tg increase during physical aging. Polymers with good stability of permeability should have low physical aging rate and high fractional free volume.

Section snippets

Introduction and Background

Glassy polymers have been widely explored for membrane gas separation, due to their rigid structure and strong size sieving ability [1], [2]. However, when these polymers are made into thin film composite (TFC) membranes, their gas permeability often decreases with time due to physical aging, which limits their potential for industrial applications [3], [4], [5], [6]. For example, Part I of this study investigates TFC membranes with selective layers of glassy perfluoropolymers such as Teflon®

Materials and preparation of TFC membranes

The materials and the preparation of TFC membranes were described in detail in the Part I of this study [7]. Briefly, TFC membranes were prepared by coating of perfluoropolymer solutions on top of polyethersulfone (PES) porous supports (PES-2 from Ultura™ High Recovery Membrane Technology, Long Beach, CA) using an automatic draw machine (DP-8301, The Paul N. Gardner Company, Pompano Beach, FL). Pure-gas permeance in the TFC membranes was determined using a constant pressure/variable volume

Tg of perfluoropolymers determined using nano-TA

Fig. 2 shows exampled deflection curves of nano-TA for Teflon AF1600 in TFC membranes after being aged for about 744 h. The deflection approaches a maximum at the apparent Tg of the thin film polymer. More specifically, Fig. 2a shows the deflection curves at five different positions of a 50-nm-thick Teflon AF1600, which demonstrate the reproducibility of nano-TA. Fig. 2b shows deflection curves of Teflon AF1600 with different film thicknesses in TFC membranes after aging for 744 h. The deflection

Conclusion

Thin film glassy polymers are subjective to physical aging, which decreases gas permeability over time. There is a critical need to understand the structure and property relationship to design advanced membrane materials with good stability over time, together with high permeability and selectivity. The Part I of this study reports gas permeance in two-layer TFC membranes with perfluoropolymers as the selective layers. In this Part II study, a new nano-thermal analysis was used to determine the

Acknowledgments

MY and HL gratefully acknowledge the financial support of this work by Korean Carbon Capture and Sequestration R&D Center (KCRC), and the partial support from the U.S. National Science Foundation (NSF) under the CAREER Award no. 1554236. YD and SM acknowledge the financial support from NSF under the Award no. CBET-1264276 and IIP-1432952.

References (54)

  • H. Lin et al.

    Effect of copolymer composition, temperature and carbon dioxide fugacity on pure- and mixed-gas permeability in poly(ethylene glycol)-based materials: free volume interpretation

    J. Membr. Sci.

    (2007)
  • H. Lin et al.

    Upper bound of polymeric membranes for mixed-gas CO2/CH4 separations

    J. Membr. Sci.

    (2015)
  • S.H. Maruf et al.

    Glass transition behaviors of interfacially polymerized polyamide barrier layers on thin film composite membranes via nano-thermal analysis

    Polymer

    (2011)
  • S.H. Maruf et al.

    Correlation between barrier layer T-g and a thin-film composite polyamide membrane's performance: Effect of chlorine treatment

    J. Membr. Sci.

    (2012)
  • T.M. Murphy et al.

    Physical aging of polystyrene films tracked by gas permeability

    Polymer

    (2013)
  • R.R. Tiwari et al.

    Gas permeation in thin films of "high free-volume" glassy perfluoropolymers: Part II. CO2 plasticization and sorption

    Polymer

    (2015)
  • R.R. Tiwari et al.

    Gas permeation in thin films of “high free-volume” glassy perfluoropolymers: Part I. Physical aging

    Polymer

    (2014)
  • S.J. Rigby et al.

    Low-temperature dielectric-relaxation in polymers containing an aromatic group in main chain

    Polymer

    (1974)
  • S. Kalakkunnath et al.

    Molecular relaxation in cross-linked poly(ethylene glycol) and poly(propylene glycol) diacrylate networks by dielectric spectroscopy

    Polymer

    (2007)
  • R.W. Baker et al.

    Gas separation membrane materials: a perspective

    Macromolecules

    (2014)
  • M. Yavari, T. Le, H. Lin, Physical aging of glassy perfluoropolymers in thin film composite membranes. Part I. Gas...
  • L. Zhu, M. Yavari, W. Jia, E. P. Furlani, H. Lin, Effect of porous Supports on the Permeance of Thin Film Composite...
  • C.H. Lau et al.

    Gas-separation membranes loaded with porous aromatic frameworks that improve with age

    Angew. Chem. Int. Ed.

    (2015)
  • C.H. Lau et al.

    Ending aging in super glassy polymer membranes

    Angew. Chem. Int. Ed.

    (2014)
  • L.C.E. Struik

    Physical aging in plastics and other glassy materials

    Polym. Eng. Sci.

    (1977)
  • D. Cangialosi

    Dynamics and thermodynamics of polymer glasses

    J. Phys.-Condens. Mater.

    (2014)
  • D.W. Van Krevelen

    Properties of Polymers: Their Correlation with Chemical Structure: Their Numerical Estimation and Prediction from Additive Group Contributions

    (1990)
  • Cited by (35)

    • Ultrathin-film composite (uTFC) membranes based on amorphous perfluoropolymers for liquid separations

      2022, Journal of Membrane Science
      Citation Excerpt :

      To demonstrate the versatility of the PFPs-based membranes, amorphous glassy polymers of Teflon AF1600 and Hyflon AD40 were also fabricated into uTFC membranes using 0.1 mass% solutions and PES10k and evaluated for desalination. Teflon AF2400, Teflon AF1600, and Hyflon AD40 show fractional free volume (FFV) of 0.33, 0.29 and 0.22, respectively [41,46]. The FFV is an important parameter influencing gas diffusion coefficient, i.e., increasing FFV in the polymer often increases gas diffusivity and decreases size-sieving ability (cf. Table S1).

    • Gas sorption and diffusion in perfluoro(butenyl vinyl ether) based perfluoropolymeric membranes

      2022, Journal of Membrane Science
      Citation Excerpt :

      They also exhibit better resistance toward physical aging and plasticization by both CO2 and organic vapors compared to conventional hydrocarbon polymers [7,8,12–16]. In Robeson's plots [6,17], perfluorinated polymers define the upper bound for several gas pairs, including He/H2, He/CH4 and N2/CH4 due to the low affinity of perfluorinated materials to hydrogen containing molecules (i.e. H2 and CH4) [7,8,15]. Thus, perfluorinated polymers are of particular interest in gas separation and organic pervaporation.

    View all citing articles on Scopus
    View full text