Phase change of polyaniline on wetting with NMP

G. Delmas a, R. Marigot b, and B. Wessling, c

a Chemistry Dept, UQAM, CP8888, Succ. Centre ville, Montreal H3C3P8, Canada.
b Chemistry Dept., Faculté des sciences et techniques de Saint-Jerome, 13394 Marseille Cedex 20, France.
c Ormecon, Ammersbek, 22949, Germany.

Abstract:
The investigation of the interaction between Pani and NMP is pursued by calorimetry in a T-ramp. A phase-change is observed for the salt and the base between 75 and 100oC with a large exothermic heat (-2000J/g) in well-stirred mixtures. The origin of this phase change is discussed.

Introduction:
Mixing Polyaniline (Pani) with polar solvents such as alcohols and 1-methyl-2-pyrrolidone (NMP) 1, generates a strong interaction as indicated by the immediate development of color on polymer-solvent contact. Heats of immersion confirm the existence of a favorable interaction since the DH are exothermic2. In the present work, the heats are measured in a T-ramp.

*Apparatus: The calorimeter is a Setaram (Lyon, France) differential calorimeter. It gives stable signal even in a very slow T-ramp (0.1 K/min). The slow T-ramp used in previous work on polyolefins was chosen here since it reveals phase-changes not observed by the usual DSC operating at faster T-ramp (10 K/min). The experiment lasts about 17 hours and not including the time for equilibrium. *Solvent: The Aldrich NMP was kept on molecular seeves and flushed with N2 before use (1h).

*Polymer: Pani comes either from our synthesis or from Aldrich or from Ormecon (Ammersbeck, Germany). Results are given here for the Ormecon (the salt and the base). It is dried on P2O5 before use. *Sample preparation:The three following strategies were used in succession. Results are given only for two of them. In the first strategy, stirring was achieved manually during a few minutes, in the second a magnetic stirrer mixes the system during 25 min .In the third strategy a sonicator (Sonics & Materials, Vibro cell at medium power) helped to homogenize the mixture. The way of adding the polymer to the NMP was also controlled: In strategy two and three, small amounts of polymer were added at a time over a 10 min period. The calorimetric traces were used to choose the strategy. Heats were not reproducible in strategy one, so it was abandoned. It was found that in strategy three the gradual addition of polymer was not necessary. The mixture is place in glass tubing sealed under N2 and inside the calorimeter for equilibrium. Results: Tables 1, 2 give for 13 different runs the characteristics of the calorimetric traces namely after the run number, sample tested, the nominal concentration, the values of DH, the width of the peak and the temperature of phase change for two different nominal concentrations c nominal (1.3 and 0.3 mg /cm3). The value in parenthesis is the interval of integration of the exotherm. *Concentration = 1.3 mg/cm3 i. The values of DH and T phase-change are reproducible in the same strategy with sonication whatever the way of adding the polymer (No1 and 2) ii. A longer contact time between solvent and polymer after sonication lead to a diminution (about 25%) of DH and of T phase-change. (2K). It also lead to a higher kinetics of the phase changes (No 3) iii. The values of DH for the base have the same magnitude (-2000J/g) as for the salt while their T phase-change increase by about 5K (No 4 and 5 vs No 1, 2 and 3). iv. The values of DH for the salt for the preparations according to the strategy two (magnetic stirring instead of sonication) are similar to those of preparation with the strategy three but the kinetics of phase-change is slower since T phase-change has risen by 15 K (No 6 and 7 Vs No 4 and 5). v. The values of DH for the base are diminished by the preparation using a magnetic stirrer (No 8 and 9 Vs No 4 and 5). As for the salt, the kinetics of phase change are increased by sonication (about 20K). *Concentration = 0.3 mg/cm3

In Table 2, the characteristics of the exothermic peaks are given for the salt and the base at a very low nominal concentration .The values of H are multiplied by 10 for the salt and by 5 for the base. The values of T phase-change are unchanged. However, the peaks are wider.

(mg/cm3) a DH

(J/g) b,c FWMHe

(K) Tphase-change

(°C)

Table 1: Characteristics of phase-change of Pani/NMP mixtures at nominal =1.3 mg/cm3.

No Sample Conc.

(g/cm3) a DH

(J/g) b,c FWMHe

(K) Tphase-change

(°C)

10 Salt 0.3 -21746 (50-100oC) 13.7 77.7

79.1

Table 2: Characteristics of phase-change of Pani/NMP mixtures at 0.3mg/cm3.(a)=Nominal concentration (sample is not soluble). (b)=Using the nominal concentration. (c)=After strategy three if not indicated other wise. (d)=Residence time after sonication of three days. (e)=Full Width at Mid Height. (f)=After strategy two. Discussion: The similarity of the phase changes in the Pani chains, in the salt and the base suggests that it is not related to the conformation of the backbone chain of the polymer or to the organization of chains . We assume that the origin of the exothermic effect is the replacement of the PAni-water interface (which is the starting situation) by a PAni-NMP interface. The desorption / adsorption phenomenon takes place only at higher temperature. The high increase of DH at lower concentration is indicative of the fact, that one gets dramatically smaller particle size (= higher surface area) at lower concentration, therefore more water to be replaced by NMP on the particle surfaces 3-5 .These interfaces are the locus of the phase change observed by calorimetry. The widening of the peak in NMP (No 11 and 13) are consistent with a partial reassociation of Pani particles. Calorimetry with a variety of solvents and other techniques are necessary to give of finer description of the origin of the phase-change observed in the slow T-ramp used in the present experiments.

References.

1. Rannou, P., Gwalicka, A., Berner, D., Pron, A., Nechtschein,
M. and Djurado, D., Macromolecules, vol. 31, no. 9, 1998, p.3007.

2. K.Strobach, S.Amrani, N.Luce, H.P. Nguyen, P. Bernazzani and G.Delmas, Proceedings of the North American Thermal Analysis Society, "Heats of immersion of polyaniline in NMP ", Savannah,1999.

3. B.Wessling, Dissipative Structure Formation in Colloidal Systems, Advanced Materials ,5, 300 (1993 ).

4. B.Wessling Dispersion as the link between basic research and commercial applications of conductive polymers (polyaniline) .Synth. Met., 93, 143 ,(1998)

5.B. Wessling, in: E. Nalwa (ed.) Handbook of Nanostructured Materials and Nanotechnology, Vol 5.

Home