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43616-AC7
Tracer Diffusion in Random and Block Copolymer Ionomers

Russell J. Composto, University of Pennsylvania

Introduction:  Ionomers are utilized as membranes, adhesives and coatings for applications ranging from sports materials to fuel cells.  In spite of their wide utility, the relaxation mechanisms underlying the dynamic properties of ionomers are not completely understood, preventing engineers from optimizing processing and transport related properties.  In the original proposal, we planned to systematically measure tracer diffusion coefficient, D*, in matrices of random and block copolymer ionomers.  This report presents completed studies of deuterated polystyrene (dPS) diffusing into lightly sulfonated polystyrene (SPS).  Also proposed, for year 3 (but not funded), were diffusion studies into block copolymers.  These studies were modified because self-assembly of the highly ordered ionomers proved difficult.  Thus, as reported in the 51th Annual Report of the ACS/PRF Fund, we investigated the self-assembly of amphiphilic block copolymers.  Such copolymers, upon neutralization, can be considered ionomers and future studies will investigate the diffusion into ordered copolymer ionomers. 

Materials. Two different molecular weight deuterated polystyrene (d-PS) samples were purchased from Polymer Source (M_w = 65,900 g/mol, M_w = 150,000 g/mol) and polystyrene (PS) from Pressure Chemical (M_w = 65,000 g/mol). Polystyrene was sulfonated to produce random copolymers P(S-SS_x), where x refers to mole percent of styrene sulfonate (x=0.2%, 0.6%, 0.7%, 0.9%, and 1.0%). 

Sample Preparation. Diffusion measurements were made on bilayer films. A thick bottom layer (~ 2 µm) of PS or P(S-SS_x) was spun-coat onto a silicon wafer. A thin top film (~ 20 nm) was produced by spinning a 0.5 wt% solution of d-PS onto a glass slide. The top film was floated off the slide onto the surface of a water bath and then picked up by the substrate. This diffusion couple was annealed at 150 °C under vacuum.

Forward Recoil Spectrometry. Forward recoil spectrometry was used to determine the volume fraction vs. depth profile of d-PS that has diffused into PS or P(S-SS_x) matrix. This depth profile was used to determine the tracer diffusion coefficient of d-PS.

Results.  Figure 1 (left) shows that the tracer diffusion coefficients, D*, for d-PS65.9 (triangles) and d-PS150 (squares) decrease monotonically as the mole percent sulfonation of the P(S-SS_x) matrix increases. Note that a three-fold decrease in D* is observed for only a 1% increase in sulfonation!  This slowing-down of the dynamics of dPS chains in a sulfonated polystyrene matrix could be due to either an increase in the glass transition temperature T_g of the matrix, or an increase in the monomeric friction coefficient z.  Because the highest sulfonation level is only 1%, T_g is taken as a constant value for the P(S-SS_x) matrices and the thus the decrease in D is attributed to z.

Discussion.  The tracer diffusion coefficient for d-PS of molecular weight M_d-PS diffusing in P(S-SS_x) can be related to  the effective monomeric friction coefficient encountered by the deuterated styrene monomer in a matrix of styrene-sulfonated styrene monomers, and  the friction on an effective styrene-sulfonated styrene monomer in a styrene-sulfonated styrene matrix. Using this relationship and the measured D*'s the two monomeric friction coefficients can be determined.

The friction coefficients (squares) and (triangles) are plotted in Figure 2 (below) as a function of volume fraction of styrene sulfonate. The friction coefficients from rheology are also given (open symbols). Both monomeric friction coefficients increase monotonically with styrene sulfonate content. Relative to the PS matrix (f=0), and  increase by ~ 2 and 6 times in P(S-SS_1.01) where . This increase in friction coefficient upon adding a small amount of styrene sulfonate to random copolymer explains the significant decrease in the tracer diffusion coefficients shown in Figure 1. Based on the miscibility behavior of PS: P(S-SS_x) blends, we can conclude that a styrene segment repels the styrene sulfonate, SS, segment. Diffusion of d-PS in P(S-SS_x) involves “dragging” a d-PS chain through surroundings containing SS segments.  We propose that these unfavorable interactions cause an increase in free energy proportional to c which provides a resistance to diffusive motion

Conclusion: The mobility of dPS in a P(S-SS_x) matrix decreases as the number of “repulsive” SS groups increases.  By analyzing contributions due to reptation and constraint release, the slowing down of dPS diffusion is attributed to an increase in effective monomeric friction coefficient.  Because microscopic models fail to capture experimental results, new models are clearly needed to describe diffusion in ionomers.

Participants:  The research was carried out by a PhD student with assistance from two female undergraduate engineering students from the University of Pennsylvania and Columbia.  One of these students is now a PhD candidate in Chemical Engineering.

Impact on PI's Career:  This ACS/PRF funding is the only support for the PI's research on ionomers.  Significantly, this study has lead to new discoveries of amphiphilic block copolymer organization/structure as well as their response to temperature and liquids (e.g., swelling, pH).

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