surface relief, phase and surface potential investigations of

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Surface relief Phase and Surface PotentialInvestigations of Composite Polymer MembranesUsing AFMTo cite this article Julija Hodakovska and Janis Kleperis 2011 IOP Conf Ser Mater Sci Eng 23012017

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Surface relief Phase and Surface Potential Investigations of Composite Polymer Membranes Using AFM

Julija Hodakovska and Janis Kleperis

Institute of Solid State Physics University of Latvia Kengaraga Street 8 Riga LV-1063 Latvia E-mail julia_h_lvyahoocouk Abstract Composite membrane consisting from two different polymers ndash SPEKK and PVDF was used as two-phase sample for investigations with atomic force microscope (AMF) Three AFM modes were applied to analysis ndash surface relief phase and surface potential scanning Theoretical advantage of all three methods used for same sample is possibility to provide more detailed information on different phases in material AFM results of surface investigation for pure SPEKK showed small fluctuation in height and phase (only difference in some nm and about 5ordm) what means that material is homogeneous We found that for composite polymer membrane sample the changes in both modes are much more significant for height it reaches 600 nm but for phase ndash 20ordm If surface potential of composite polymer sample clearly identifies some areas as SPEKK polymer than in topography and phase images these places are identified as PVDF polymer only This can be explained with the fact that polymers dissipate charge not only from surface but also little in depth Even with such small potential contrast the surface potential mode of AFM can be used to detect distinct polymer structures not only on surface but also in a volume under the surface

1 Introduction Interest for alternative energy is causing development of variety of sciences methods to get energy from source itself and possibility to use their combinations (eg electricity and thermal energy from Sun) Also researchers focus on energy transfer and storage with minimal losses Hydrogen is one of the possible solution for renewable energy sources ndash it could be used for energy storage for energy transfer for energy production and usage as well as in transport as in power generation sectors and even in portable devices [1] History of hydrogen usage has started even before oil was made one of the main energy source for humankind eg first combustion engine was built up on hydrogen [2] Main question of hydrogen usage is how to get energy from gas It can be done in several ways and one of the possibilities is to use gas battery or fuel cell ndash the device which transforms chemical energy in electrical energy [3] Polymer electrolyte membrane fuel cell (PEMFC) has several advantages compared with fuel combustion technologies short recharge time high energy density good efficiency and no harmful emissions which makes it suitable for application such as transport and portable devices [4] To improve quality and decrease costs for PEMFC systems a lot of research is made in the area of membrane-electrode assembly (MEA) which makes more then half of price of PEMFC (for example see [5] and references therein) Different polymer materials are synthesized and investigated with different methods to find compliance with parameters important for PEMFC water

Annual Conference on Functional Materials and Nanotechnologies ndash FMampNT 2011 IOP PublishingIOP Conf Series Materials Science and Engineering 23 (2011) 012017 doi1010881757-899X231012017

Published under licence by IOP Publishing Ltd 1

absorption chemical stability conductivity gas permeability thermal stability mechanical stability [6] Information about polymer morphology and structure can help to explain parameters mentioned above therefore microscope and structural inspections are quite important Most common technique is scanning electron microscope (SEM) providing possibilities to explore both surface relief and cross-section only problem is complicate sample preparation method and expensive large equipment Atomic force microscope (AFM) nowadays is easy available and can work in ambient conditions without isolating sample in any kind of camera [7] In this work the complex research of AFM and SEM results for composite polymer membranersquos surface potential are analyzed along with topography and phase images to obtain more information on morphology and subsurface layers This investigation is a part of complex research of potential membrane material it which also connection between some parameters mentioned above and microscopy results is discussed [8]

2 Experimental

21 Materials Following chemicals without any additional purification or modification were used to prepare composite polymer membranes

bull Sulfonated poly(ether-ketone-ketone) (SPEKK) bull Polyvinylidene fluoride (PVDF) bull Dimethyl sulfoxide (DMSO)

Last two were purchased at Aldrich SPEKK with sulfonation degree 63 was synthesized previously in GKSS Polymer Institute [8] Two samples were prepared for measurements pure SPEKK and SPEKK with addition of 20 w PVDF Samples were prepared from solution in DMSO by casting on glass at 90 degC After solvent evaporation membrane was obtained

22 Methodology Two microscopy methods were used scanning electron microscope (SEM) and atomic force microscope (AFM) With LEO Gemini 1550 VP SEM samplesrsquo surface and cross-section was observed for cross-section measurements the sample was broken in liquid nitrogen For AFM measurements tapping mode and itrsquos derivatives were used as most appropriate for polymer samples Veecorsquos MultiMode AFM was used to investigate surface relief phase and surface potential sample was not isolated from room atmosphere

3 Results and discussion Polymer membranes for fuel cells are common subject of investigation and a lot of research was made for several types of membranes Sulfonated poly(aryl-ether-keton)s are one of the most frequently used materials known for itrsquos good chemical stability and proton conductivity Pure SPEKK membrane was used to compare surface relief taken by SEM and AFM methods The surface potential was not determined for that sample since pure SPEKK membrane is homogenous and surface potential difference between adjacent areas areas was minimal Both SEM and AFM topography diagrams show similar structure for pure SPEKK membrane small grains with the diameter of about 001 μm or smaller (Figure 1) AFM results of surface investigation for pure SPEKK showed small fluctuation in heigth and phase eg on phase images registered difference was only about 5ordm which means that material is homogeneous


2

Figure 1 Results of SEM (left) AFM (topography in middle) and AFM (phase right) microscopy for SPEKK samples (1 μm x 1 μm surface area is shown for AFM result)

Next sample is composite ndash made from SPEKK polymer with addition of 20 w PVDF polymer In SEM picture the SPEKK keeps producing small grains with the diameter about 001 μm while PVDF is noticed as larger grains reaching 25 μm in diameter (Figure 2 left) AFM phase diagram shows some difference between interfaces of two polymers in one case it is smooth change from one value to another in other darker ldquoringrdquo is observed around PVDF grain (Figure 2 right) This indicates significant change in properties straight beneath surface allowing making conclusion about week mixing between polymers in composite material which resulted in cracks (Figure 2 right)

Figure 2 Results of SEM (left) AFM (topography in middle) and AFM (phase right) microscopy for

SPEKK with 20 w PVDF samples (5 μm x 5 μm surface area is shown for AFM result)

Conclusion about cracks was confirmed when cross-section images were prepared and inspected by SEM (Figure 3 middle) Pure SPEKK membranes had the same structure inside and on surface consisting from small grains with no significant defects (Figure 3 left) In composite sample with 20 w PVDF the SPEKK polymer has the homogenous structure but PVDF forms specific inclusions with distinct appearance it looks solid compared to SPEKK with no inner structure The form of PVDF inclusions are representing processes during the formation of composite membrane (Figure 3 right) caused by gravitation force Based on this observations we can conclude that phase diagram can help in finding defects such as cracks near surface providing with information on inner structure of the sample Of cause there are limitation for AFM method since it would not be possible to get any information on structures in subsurface layers


3

Figure 3 Results of SEM for pure polymer SPEKK (left) and composite polymer SPEKK with 20

w PVDF samples (middle right) at different magnifications

Accordingly to AFM microscopy phase images around PVDF inclusions lighter sphere is observed (Figure 2 right) which has neither SPEKK nor PVDF accordingly topography data but something in between This corresponds well with PVDF ldquonailsrdquo outer part of inclusionrsquos ldquoheadrdquo is thin and this results in different properties on phase diagram with properties between 2 pure polymers Using AFM in surface potential mode to investigate composite polymer sample from SPEKK with 20 w of PVDF it is found that on surface potential image some areas are identified like SPEKK polymer but on surface relief and phase images these places are identified as PVDF polymer (Figure 4) But on surface potential image the specific PVDF area has more complicated form even compared to phase image This could be due to more complicated 3D structure of the ldquonailrdquo which is not clearly seen on SEM cross-section image which is 2D and in phase image (Figure 2 right) some smaller areas of PVDF are found around larger ones

Figure 4 AFM results for SPEKK with 20 w PVDF samples topography (left) phase (middle) and

surface (right) potential diagrams

Main problem with measuring surface potential is that resulting image of the composite polymer has comparably low contrast typically reaching 01 ndash 02 V This can be explained with the fact that polymers dissipate charge due to interaction between polymers But even with such a contrast the surface potential could be used to determine polymer structure in a layer near surface in case of lack of other methods

4 Conclusions Two microscopy methods are compared for analysing polymer membrane morphology scanning electron microscopy and atomic force microscopy SEM is common method for such investigations and allows observing the surface relief and sampling cross-section Nowadays the AFM microscopy is not commonly used for measurements of polymer membranes and has three possibilities ndash to operate


4

in topography phase and surface potential modes From our investigations of pure and composite polymer membranes it can be concluded that AFM can supplement the cross-section inspections by SEM till some extension providing some information on subsurface layers their composition

5 Acknowledgements Author (JH) acknowledges European Social Fund for stipend both authors acknowledge National Research Program in Energy for financial support

6 References 1 Bockris JO`M and Veziroglu TN 2007 Int J Hydrogen Energy 32 pp 1605-1610 2 Eckermann E 2001 World history of the automobile (Warrendale Society of Automotive

Engineers Inc) pp 18-19 3 Hogers G 2003 Fuel Cell Technology Handbook (Boca Raton CRT Press LLC) p 14 4 Wee JH 2007Applications of proton exchange membrane fuel cell systems Renewable amp

Sustainable Energy Reviews 11 1720-1738 5 Wright P 2010 Nanotechnology Market Review Fuel Cells Technologies (NY Tradition

Equities Equity Research amp Trading) p 65 6 Javaid Zaidi SM and Matsuura T 2008 Polymer Membranes for Fuel Cells (Springer) p 550 7 Batteas JD 2005 Applications of Scanned Probe Microscopy to Polymers CS Symp Series

(American Chemical Society vol 897) p 265 8 Hodakovska J 2011 Research of Materials for Membrane and Membrane-Electrode Assembly

for Application in Fuel Cells (Riga University of Latvia)


5

Surface relief Phase and Surface Potential Investigations of Composite Polymer Membranes Using AFM

Julija Hodakovska and Janis Kleperis

Institute of Solid State Physics University of Latvia Kengaraga Street 8 Riga LV-1063 Latvia E-mail julia_h_lvyahoocouk Abstract Composite membrane consisting from two different polymers ndash SPEKK and PVDF was used as two-phase sample for investigations with atomic force microscope (AMF) Three AFM modes were applied to analysis ndash surface relief phase and surface potential scanning Theoretical advantage of all three methods used for same sample is possibility to provide more detailed information on different phases in material AFM results of surface investigation for pure SPEKK showed small fluctuation in height and phase (only difference in some nm and about 5ordm) what means that material is homogeneous We found that for composite polymer membrane sample the changes in both modes are much more significant for height it reaches 600 nm but for phase ndash 20ordm If surface potential of composite polymer sample clearly identifies some areas as SPEKK polymer than in topography and phase images these places are identified as PVDF polymer only This can be explained with the fact that polymers dissipate charge not only from surface but also little in depth Even with such small potential contrast the surface potential mode of AFM can be used to detect distinct polymer structures not only on surface but also in a volume under the surface

1 Introduction Interest for alternative energy is causing development of variety of sciences methods to get energy from source itself and possibility to use their combinations (eg electricity and thermal energy from Sun) Also researchers focus on energy transfer and storage with minimal losses Hydrogen is one of the possible solution for renewable energy sources ndash it could be used for energy storage for energy transfer for energy production and usage as well as in transport as in power generation sectors and even in portable devices [1] History of hydrogen usage has started even before oil was made one of the main energy source for humankind eg first combustion engine was built up on hydrogen [2] Main question of hydrogen usage is how to get energy from gas It can be done in several ways and one of the possibilities is to use gas battery or fuel cell ndash the device which transforms chemical energy in electrical energy [3] Polymer electrolyte membrane fuel cell (PEMFC) has several advantages compared with fuel combustion technologies short recharge time high energy density good efficiency and no harmful emissions which makes it suitable for application such as transport and portable devices [4] To improve quality and decrease costs for PEMFC systems a lot of research is made in the area of membrane-electrode assembly (MEA) which makes more then half of price of PEMFC (for example see [5] and references therein) Different polymer materials are synthesized and investigated with different methods to find compliance with parameters important for PEMFC water


Published under licence by IOP Publishing Ltd 1


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surface relief, phase and surface potential investigations of

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