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Processing glassy polymeric membranes with enhanced thermal and kinetic stability

Rodney D. Priestley, Princeton University, ACS PRF 49903-DNI10

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 1 (left), is that, via MAPLE, we have been able to prepare films of PMMA that exhibit a remarkable enhancement in thermal stability (i.e., ~ 33 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 ~ 0.25 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 1(right), 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. 

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

We have discovered that the enhancement in T_g of MAPLE-deposited glassy films is accompanied with a significantly reduced density compared to the standard glass.  From refractive index (RI) measurements at room temperature, the densities of MAPLE-deposited and standard PMMA were determined to be 0.681 ± 0.008 g/cm³ and 1.184 ± 0.002 g/cm³, respectively.  Hence, MAPLE-deposited PMMA can have an ~ 42 % reduced density, as estimated from RI measurements.  This finding, i.e., the dramatically reduced density of MAPLE-deposited PMMA, was corroborated by X-ray reflectivity measurements that estimated an ~ 38 % reduction of density.

We believe the concurrent enhancement in thermal/kinetic stability and reduction in density is due to the film morphology.  Figure 2 shows the surface topology of a MAPLE glass with T_substrate = 336 K, as revealed by AFM.  Noticeably, the surface structure of the MAPLE-deposited film contains spherical-like globules.  The globules range in size from ~ 20 – 500 nm and would each contain a few to tens to thousands of polymer chains.  The surface structure of the film suggests that glass formation proceeded by the assembly of polymer globules to form a supramolecular nanostructured glass.  The cross-sectional SEM image of Figure 4 supports this claim, as sub-surface nanoglobules are clearly visible.   

Figure 2: AFM amplitude image of MAPLE-deposited PMMA film with T_substrate = 336 K and cross-sectional SEM image of the film.

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.