SIZE-EXCLUSION CHROMATOGRAPHY OF PERFLUOROSULFONATED IONOMERS

Nan ZhaoGeorge Geevarghese

PART – I : INTRO

Nan Zhao

INTRODUCTION

The authors want to measure the molar mass distributions , Polymer conformation and unperturbed dimensions of the Polyfluorosulfonated ionomers which are copolymers of tetrafluorethylene and sulfonic acid-functionalized (SO3H) perfluorinated vinyl ethers with SEC(with LS, DRI, DV)

SEC

LS

DRI, DV

EXPERIMENTAL

Testing Equipments:SEC: Agilent 1100 seriesisocratic pump, autosampler and two-wavelength spectrophotometric detector.LS: Agilent PD2020 two-angle LS detectorDV: Malvern Model 270 DV detectorDRI: Waters Corporation Model 410 DRI detector3 Columns: Agilent Olexis 7.5 mm×300 mm columns at 35.0 ◦C were used with N,N-dimethylformamide (DMF) containing 0.1 M LiNO3.

Materials: the authors used 3 classes of the Perfluorosulfonated ionomers.

The eluent was pre-filtered using 0.22 m Millipore GS filters. The flow rate was 1.0 mL/min 0.2% acetone added to the samples as a flow marker The columns were calibrated with 15 PMMA narrow standards

from Agilent with molar masses between 580 and 1,400,000.

EXPERIMENTAL

Sample dissolution: He prepares a dilute ionomer dispersion by using 80/20 n-propanol/water. The dilute dispersion is then taken in the acid digestion bomb.

Then place the bomb in an oven heated from 30 to 230 ◦C at 5 ◦C per min, and then held for 6 h. After that several hours were required to cool the vessels down to room temperature.

Then the autoclaved solutions were diluted 1:1 with SEC sample solvent to give the SEC sample injection concentration of ∼0.5 mg/mL.

RESULTS AND DISCUSSION

Five strategies the authors used SEC optimization

Ionomer disolution increasing SEC detector signals (Calculation of molar mass distributions) (Polymer conformation and unperturbed dimensions)

SEC OPTIMIZATION

Because the sample has two solubility parameters which δ1=9.68 (cal/cm3)1/2 and δ2=16.71 (cal/cm3)1/2, the authors need to find a proper solvent.

Previously, Dr. Lousenberg and Hommura et al. used δ values of the solvent closer to δ1 and closer to δ2 respectively.

Due to some “unknows reason”, he choose the DMF(δ= 12.1),with as the solvent.

Then again, none of the perfluorosulfonatedionomers eluted from the SEC columns using DMF(pure) he added 0.1M LiNO3 to improve the elution.

And because the non-autoclaved samples showed less linearity in the log M calibration curve, he used autoclaved Nafion.

SEC OPTIMIZATION

PART II – 4 STRATEGIES + CONCLUSION

George Geevarghese

Ionomer dissolution

• Martin et al. - autoclave idea!– 50/50 (v/v) n-propanol/water at 250 deg for 1hr (1wt%

incomer concentration)– Author repeats (at 260 deg for 1hr & same conc., but fails– But it worked (almost) at 230 deg for 6hrs & same conc., but

phase separated.

• Author finally used 0.1 M LiNO3 with DMF and added to 80/20 n-propanol/water – That worked. BUT only for Nafion. C2 and C4 still showed

prepeaks.

• Then again, he realized at low wt % of ionomer (~0.2wt%) prepeaks were less significant

• BUT, LS observed prepeaks at even small wt %. But, DRI doesnt (even at higher conc)

• So he found an optimized conc @ 0.1wt%

Increasing SEC detector signals

• The “optimized” “autoclaved” solution (0.1 wt%) was diluted 1:1 (w/w) with SEC samples– But, results in large solvent peaks and detector signal

disruptions in the system peak region.– This also reduced the injection concentrations of polymer ( to

0.5 mg/mL)• DRI and LS are weak at this conc. because of small specific

refractive index increments (between −0.02 and −0.04 mL/g)• Viscometry detector signals are weak because of the low

intrinsic viscosities

• So, he increased sample conc. by Solvent Exchange to 2 mg/mL∼SEC of this sample showed no aggregation(cos no prepeaks in

DRI)

• Can also improve DV signal by inc. the flow rate.

• Changing the solvent to NMP and DMSO will increase the specific refractive index increment and improve DRI and LS detector signals accordingly (complicated! high temp!)

1 g of SEC sample solvent is added to 3 g of 0.1 wt % 80/20 n-propanol/water autoclaved solution. The mixed solvent is then partially evaporated with a nitrogen stream to approximately 1.5 g. The lower boiling n-propanol and water are preferentially removed while the sample concentration is increased.

Calculation of molar mass distributions

• Variation in dn/dc across the molecular size distribution caused by comonomer compositional drift.

• Corrected by:• Accounting for minor variation in dn/dc across the chromatogram (OR)• Mark–Houwink calculation method

Polymer conformation & unperturbed dimensions

• Conformation plots of three ionomer classes are plotted. All PFSA ionomers were observed to have lower viscosity at equivalent molar mass than PMMA

• The unperturbed dimensions was calculated from SEC viscosity–molar mass data

• It suggested suspicious free rotation of the perfluoroalkane units (can be attributed to the unusual structure and behavior of these materials)

Our Conclusions

• Advantages:– SEC system with triple detection can• give high accuracy for perfluorosulfonated ionomers.• Most info can be obtained

– Solvent Exchange process can• improve all detector signals• Also reduce the water and n-propanol peaks observed

in the system peak region.– The autoclaving procedure • provides molecular solutions without re-aggregation

(better peaks)

Our Conclusions

• Disadvantages:– Dissolution temp is high compared to other

techniques– Solvent exchange is somewhat tedious and

requires careful weighing of solutions before and after evaporation to obtain accurate injected sample concentrations

– Even though small, Variation in dn/dc causes error in molar mass and viscosity from its original value

Reference• Size-exclusion chromatography of perfluorosulfonated ionomers by T.H.

Moureya, L.A. Slater a, R.C. Galipoa, R.J. Koestnerb : Journal of Chromatography A, 1218 (2011) 5801– 5809