Study On Anisotropic Expansion PEDOT/PSS-PAM Films

Poly（3.4-ethylenedioxythiophene）/poly（4-styrenesulfonic acid）（PEDOT/PSS）has been widely studied and applied as the most successful conductive polymer due to its good electrical conductivity,transparency and processability.Polyacrylamide（PAM）is a water-soluble linear polymer which cannot keep in a film-shape in water.After the conductive PEDOT/PSS dispersion was mixed with linear PAM solution,we prepared PEDOT/PSS-PAM films by drop-casting the mixed solution and found that The films showed anisotropic expansion in water.This thesis mainly reported on the anisotropic expansion phenomenon of the PEDOT/PSS-PAM films and possible mechanism,as well as aggregated structures of PEDOT colloidal particles due to the expansion.1)During the PEDOT/PSS-PAM films were immersed in water,we found that the area of the PEDOT/PSS films increased and the thickness decreased.When the films were immersed for long time（to 25 h）,eventually the film area slightly retracted and the film thickness slightly increased.The expansion process of PEDOT/PSS-PAM films showed that the expansion has an induction period where the film thickness increases rapidly while the area is almost unchanged.After the induction period,the film area gradually increases while the film thickness decreases.The changes of the film area and the thickness during the expansion process showed that the expansion of the PEDOT/PSS-PAM films is evidently anisotropic.2)We investigated on the mechanism of anisotropic expansion by changing the temperature,adjusting the p H,ionic strength,percentage of PAM and adding water.Based on the above experimental results,we considered that expansion is related to osmic pressure induced by chemical potential of PAM in the film and water.Accordingly we proposed a Hydrophilic-Lipophilic-Competing（HLC）model to explain the anisotropic expansion of the film in water.When the expanding force of the hydrophilic groups in the film is greater than the hydrophobic contraction force of the hydrophobic groups the film area increases.When the two forces are balanced,the film area attains to maximum.When the immersion time is further prolonged,the film area contracted slightly because hydrophobic force duo to PEDOT domains is dominant.The time scale of change from dominant hydrophilic force to dominant hydrophobic force is much different between film thickness direction and film area direction,resulting in the anisotropic expansion phenomenon of the PEDOT/PSS-PAM films.3)Finally,we performed AFM-current imaging characterization on the film with different expansion times,and found that number of highly conductive spot decreased with increasing in the expansion time,i.e.,1,17,36,58,90,110 times.It is considered that the overlap or aggregate of the PEDOT colloidal particles on film thickness direction is gradually reduced during the film expansion,which induced high electrical resistance,therefore,the highly conductive spots were reduced on the AFM-current image.On the other hand,the conductivity of the films on the film area direction maintained in about 10^-4 S/cm.The above results indicated that the overlap or aggregate of the PEDOT colloidal particles on film thickness direction is reduced due to the expansion,while the PEDOT colloidal particles maintained in electrical contact on the film area direction.