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Since the initial discovery that polystyrene (PS) films supported on silica exhibited a reduction in T_g with decreasing thickness [1], whereas poly(methyl methacrylate) (PMMA) films exhibited a thickness dependent T_g that was impacted by the type of supporting substrate [2], the T_g-confinement effect has been attributed to the presence of interfacial effects perturbing glass transition dynamics.  Nanoscale confinement and interfacial effects also impact, physical aging, i.e., glassy-state structural relaxation [3,4].  We have shown that the dispersion of nanofiller into a polymer matrix, in which there exists an attraction between the nanofiller and polymer, can lead to suppressed or nearly no physical aging of polymer nanocomposite films [5,6].

A factor affecting the stability or long-term throughput of glassy polymeric membranes is physical aging [7,8]. Physical aging leads to a decrease in free volume [9].  For glassy membranes the permeability can decrease greater than forty percent over a modest lifetime due to physical aging.  In this proposal, we investigate the stability of thin glassy polymeric membranes by Matrix Assisted Pulsed Laser Evaporation (MAPLE) and explore their use as stable ultra-barrier membranes.

We have completed an investigation of the glass transition and stability of supported PMMA films deposited by MAPLE.  A main result of this work, as illustrated in Figure 1A, is that, via MAPLE, we have been able to prepare films of PMMA that exhibit a remarkable enhancement in thermal stability (i.e., ~ 50 K increase in the glass transition temperature (T_g)) in comparison to ordinary PMMA.  We have determined the importance of substrate temperature in the formation of stable, high T_g PMMA films when deposited at a constant deposition rate of ~ 5 nm/sec; films not deposited at ~ 0.85T_g exhibited large endothermic peaks during heating, an indication of glassy-state structural relaxation, i.e., instability.  Furthermore, we have observed a dependence of deposition rate on the enhancement of thermal stability, i.e., a slower rate of deposition leads to a higher T_g. The results demonstrate, for the first time, the formation of a thermally stable polymer film by MAPLE.  A second significant finding of this work, as illustrated in Figure 1B, it that when isothermally annealed at T_g (T_g of the corresponding film), the time required for the glass to liquid transformation is ~ 2-orders-of-magnitude greater for MAPLE-deposited PMMA than ordinary PMMA.  In addition, MAPLE-deposited films that exhibit large endothermic peaks also exhibit minima in the C_p,rev vs. time curves.

Figure 1: A) Heat capacity of ordinary and MAPLE-Deposited PMMA. B) Reversing heat capacity of ordinary and MAPLE-Deposited PMMA as a function of time at T_g.

We will continue to investigate the impact of substrate temperature and deposition rate on the glass transition, stability and energy of MAPLE-deposited PMMA films and other polymers.  We are also interested in understanding the molecular structure of MAPLE-deposited PMMA films, and therefore plan to use x-ray scattering to probe the structure.  Lastly, we will undertake barrier studies on MAPLE-deposited films.

Career and educational impact.  Funding of this proposal has allowed our group to initially explore the impact of nanoscale confinement and interfaces on T_g and physical aging of confined polymer, and to investigate the stability of glassy polymeric films deposited by MAPLE.  Our work is the first to show that MAPLE-deposited glassy polymeric films exhibit a significant enhancement in thermal and kinetic stability.  A postdoctoral fellow and a graduate student have carried out this work.  Funding of this proposal has allowed them to develop a fundamental understanding of confinement effects on the properties of polymers and develop the ability to process films by MAPLE.

References:

1.  Keddie, J.L.; Jones, R.A.L.; Cory, R.A. Size-dependent depression of the glass transition            

     temperature in polymer films. Europhys. Lett. 1994 27, 59-64

2.  Keddie, J.L.; Jones, R.A.L.; Cory, R.A. Interface and surface effects on the glass-transition 

     temperature in thin polymer films. Faraday Discuss. 1994, 98, 219-230

3.  Huang, Y.; Paul, D.R. Effect of Temperature on Physical Aging of Thin Glassy Polymer Films. Macromolecules 2005, 38, 10148-10154

4.  Huang, Y.; Paul, D.R. Physical Aging of Thin Glassy Polymer Films Monitored by Optical Properties. Macromolecules 2006, 39, 1554-1559

5.  Priestley, R.D.; Rittigstein, P.; Broadbelt, L.J.; Fukao, K.; Torkelson, J.M.  Evidence for the Molecular-scale Origin of the Suppression of Physical Aging in Confined Polymer: Fluorescence   and Dielectric Spectroscopy Studies of Polymer-Silica Nanocomposites. J. Phys.: Condens.           Matter 2007, 19, 205120-1-205120-12

6.  Rittigstein, P.; Priestley, R.D.; Broadbelt, L.J.; Torkelson, J.M. Model Polymer Nanocomposites Provide an Understanding of Confinement Effects in Real Nanocomposites. Nature Materials 2007, 6, 278-282

7.  Pfromm, P.H; Koros, W.J.  Accelerated physical aging of thin glassy polymer films: evidence from gas transport measurements. Polymer 1995, 36, 2379-         2387

8.  Huang, Y.; Paul, D.R. Physical Aging in Thin Glassy Polymer Films Monitored by Gas Permeability. Polymer 2004, 45, 8377-8393

9.  Struik, L.C.E. Physical Aging of Amorphous Polymers and Other Materials. 1978, Elesvier, Amsterdam