Research - (2021) Volume 10, Issue 3
Received: 03-Mar-2021 Published: 24-Mar-2021 , DOI: 10.37421/2169-0022.2021.10.565
Many studies were devoted in our Laboratory to the determination of physico-chemical and thermodynamic properties of polymers and/or oxides by using the inverse gas chromatography (IGC) at infinite dilution. More particularly, we studied the interactions of solid substrates with some model organic molecules and their acid-base properties, in Lewis terms, by determining the acidic and basic constants. We proposed in this paper to study the surface thermodynamic energetics, transition phenomena, specific interactions and acid-base properties of both the conducting polyaniline (PANI-HEBSA) and the non-conducting form (PANI-EB) on the light of the new progresses of IGC methods. This technique was used to obtain the net retention volume Vn and then the dispersive free enthalpy of n-alkanes adsorbed on PANI. The curves of the dispersive component of the surface energy of n-alkanes adsorbed on PANI, as a function of the temperature highlighted the presence of two transition temperatures on 383K and 430K respectively for PANI-HEBSA and PANI-EB. There results were confirmed by the curves of RTlnVn =f(1/T) of n-alkanes. The determination of the specific free enthalpy of polar molecules adsorbed on PANI proved a shift of 4K in the value of the glass transition of PANI-EB. From the variation of as a function of the temperature, one deduced the values of the specific enthalpy of the various polar molecules and determined the acidic constant KA and basic constant KD, the two constants characterizing the solid substrate. It was showed that PANI is highly more basic than acidic (about 2.6 times more basic) and an increase of the acid-base character was highlighted near the glass transition for PANI-EB.
Reagent stability, Plasma, Alzheimer’s disease, Immunomagnetic reduction
Polyaniline (PANI) has been widely studied due to its various useful properties as well as to its environmental stability [1]. Polyaniline is finding widespread use in novel organic electronic applications such as, light emitting diodes (LED), electroluminescence, metallic corrosion resistance, organic rechargeable batteries, biological and environmental sensors, composite structures, bioelectronics medical devices, and a variety of other applications where tunable conductivity in an organic polymer is desirable [2]. However, very few of the studies deal with the thermal behavior of the emeraldine salt form of PANI both as powder or film cast from solution [1]. Many researches were conducted on the thermal behavior of PANIEB [1,3-5]. Many papers studied the thermal and mechanical properties of polyaniline doped or non-doped, in powder form or in films, by means of dynamic mechanical analysis, differential scanning calorimetry and thermogravimetric analysis [6-11].
Inverse gas chromatography was also used to characterize both doped, the undoped polyaniline (PANI), and its blend with nylon-6 using many solutes. The change in the morphology of these polymers was detected in the temperature interval (80°C-180°C) including the study of the crystallinity [12].
Al-Saigh et al. [11] studied some surface thermodynamic characteristics, such as surface energy and other interaction properties of polyaniline in conducting and non-conducting forms by using inverse gas chromatography. They evaluated the interaction and dispersive force of polyaniline with alkyl acetates and alcohols. They observed for acetates and alcohols a maximum of around 145.8°C traducing a phase change from a semicrystalline to amorphous phase.
Data about the glass transition temperature (Tg) of EB are also scarce and rather controversial. By means of differential mechanical thermal analysis (DMTA) some authors have registered relaxations transitions in the chemically synthetized PANI-EB film cast from NMP solution [1]. Depending on the amount of the residual solvent in the film, Tg has been determined in the region 105°C -220°C. DMTA studies by Chen et al. [5] showed three relaxation transitions in PANI-EB films. According to them, the Tg of such films varied between 99 and 158 oC depending on the NMP content.
Polyaniline doped by camphor sulphonic acid (PANI-CSA) exhibits a high electronic conductivity [13]. All the authors agree to invoke in various extents the role of disorder in the evolution of the transport properties as a function of temperature. The rigidity of the PANI chains was confirmed, in both a conducting and a partially doped sample. All the observable quasielastic scattering occurs from the CSA dynamics.
Ding et al. [14] highlighted the presence of two glass transitions Tg: 70°C and 250°C, in PANI-EB. Powder was observed for the first time using MDSC during the process of heat treatment. Later, Han et al. [15] studied the effect of solvent and dopant on the thermal transition of polyaniline (PANI) by using dynamic mechanical analysis (DMA) and found for the solvent N-rnethyl-2-pyrrolidinone (NMP) containing film three thermal transitions temperatures (142, 198, and 272°C). Rannou et al. [16] showed that diesters doped PANI exhibits two glass transition temperatures Tg1=275 K and Tg2=304 K corresponding to the freezing of the movement of the dopant substituents and of the movements of polymer-dopant anion association, whereas, with PANI-CSA, they proved a glass transition between 403 and 425 K.
In 2012, Farbod and Khajehpour Tadavani [17] proved that the glass transition temperature of the pure PANI was measured Tg=378 K using electrical resistivity measurement. The Tg increased up to 394 K when the PANI is doped up to 16 wt.% of functionalized multiwall carbon nanotubes (MWCNTs). The Tg of pure PANI was observed to be dependent on the pelletizing pressure and increased from 373 K to 387 K when the pelletizing pressure is increased from 96 to 1150 MPa.
The restricted molecular motions in rigid crystalline polymers like PANI are the essential cause of the extreme difficulty to the detection of its glass transition Tg. As a result, the reported Tg of PANI varies from −12°C to +250°C [18-21]. The reported literatures also show that the Tg of PANI depends remarkably on the processing temperature, heat treatment time, measurement techniques, type and concentration of dopant, solvent content, presence of chain ordering material, the molecular weight of the polymer, presence of plasticizers fillers and on the experimental techniques and conditions of measurement [18-20].
We were interested in this paper to the determination of dispersive surface energy interactions and acid-base properties of both doped polyaniline and the non-conducting form by using inverse gas chromatography technique at infinite dilution. Different IGC methods were used and compared each other to quantify the dispersive component of the surface energy of PANI as well as the specific interactions and the acid base constants in Lewi terms. Organic molecules such as n-alkanes molecules and polar organic were used in this study. The n-alkanes were used to determine the dispersive energy of polymers and the polar molecules served to determine the specific interactions between polyaniline and these probes. The retention time obtained by IGC technique was revealed an excellent experimental parameter to characterize the surface properties of the polymer s.
Theory and Methods
Since 1982, many scientists used the inverse gas chromatography to determine surface phenomena, glass transitions and acid-base properties of solid materials [22-41]. The changes of the thermodynamic variables determined by IGC technique, as a function of temperature, of the surface properties of solid materials or nanomaterials, polymers, oxides or polymers adsorbed on oxides are considered now as powerful parameters to detect any evolution in the surface properties of materials. Model organic molecules of known properties are injected in the column containing the solid. One of the most important parameters is the retention time of injected molecules, measured at infinite dilution, allowing the determination of the interactions between the probes and the solid surfaces. The net retention volume was obtained from the experimental values of the retention time:
Vn=j Dc (tR-t0) (1)
Where tR is the retention time of the probe, t0 the zero retention reference time measured with a non-adsorbing probe such as methane, Dc the flow rate and j a correction factor taking into account the compression of the gas [42].
The free energy of adsorption ΔG0 of n-alkanes on the solid surface was calculated from the following equation:
ΔG0=RTlnVn+C (2)
where R is the ideal gas constant, T the absolute temperature and C a constant depending on the reference state of adsorption. The free energy of adsorption ΔG0 contains the two contributions relative to the dispersive and specific interactions. In the case of n-alkanes, ΔG0 is equal to the free energy of adsorption corresponding to the dispersive interactions ΔGd only. To calculate the specific interactions between the solid substrates and polar probes, several methods were used in the literature [43-48].
The dispersive component of the surface energy of PANI was calculated from Fowkes approach by using different models of the surface areas of the various molecules.
The Fowkes relation used the geometric mean of the respective dispersive components of the surface energy of the probe γl d and the solid γs d:
Where Wa is the energy of adhesion, N is Avogadro’s number and a is the surface area of one adsorbed molecule of the probe.
For polar molecules, the specific interactions are added to the dispersive interactions:
ΔG0=2 Na (γl d γs d)1/2+ΔGsp (4)
By plotting RTlnVn as a function of 2Na (γl d)1/2 of n-alkanes, it is possible to deduce, from the slope of the straight line, the value of dispersive component γs d of the surface energy of the solid. If γl d, γs d and a cross section of an adsorbed molecule, are known, it is possible to calculate the contribution to the free energy of adsorption of the Lewis acid–base surface interactions ΔGsp by using equation (4) [49].
In previous studies, Hamieh et al. [50,51] proved the effect of the temperature on the surface area of n-alkanes and polar molecules and concluded to the non-validity of Fowkes method to be used for the determination of the dispersive surface energy and the specific free enthalpy of adsorption. They proposed various theoretical models giving the molecular areas of n-alkanes: geometrical model, cylindrical molecular model, liquid density model, BET method, Kiselev results and the twodimensional Van der Waals and Redlich-Kwong models. These different models were used in the experimental section to compare between the obtained values of γs d of the polymer.
On the other hand, the limitations of Fowkes method due, in part, to the fact that the molecular area a is not exactly known and varies both with the nature of the solid, and the temperature and surface coverage, led us to consider the method proposed Papier et al. [52,53].
Papirer et al. method [43,44] obtained a straight line when plotting RTlnVn against the logarithm of the vapor pressure P0 on-alkanes. For a homologous series of n-alkanes:
RT lnVn=A lnP0+B (5)
Where A and B are constants depending of the nature of the solid substrate.
When polar molecules are injected into the column, specific interactions are established between these probes and the solid surface and ΔG0 is now given by:
ΔG0=ΔGd+ΔGsp (6)
Where ΔGsp refers to specific interactions of a polar molecule adsorbed on solid substrate.
The Papirer method was used to determine the specific free energy of adsorption of polar molecules on PANI surfaces.
From the values of ΔGsp of the polar molecules for different temperatures, the specific enthalpy
(ΔHsp) and entropy ΔSsp were deduced by using:
ΔGsp=ΔHsp-T ΔSsp (7)
Knowing ΔHsp of the various polar molecules, the acidic constant KA and basic constant KD, the two constants characterizing the solid substrate, are determined by using the following classical relationship:
(-ΔHsp)=(KA.DN+KD.AN) (8)
Where DN and AN are the donor and acceptor numbers of electrons of the polar molecules.
This relation was corrected by Hamieh et al. [45,46,54,55] and proposed a new relationship by adding a third parameter K reflecting the amphoteric character of the oxide according to:
(-ΔHsp)=KA.DN+KD.AN-K.AN.DN (9)
Experimental part
Materials and solvents: Polyaniline doped ethylbenzene sulfonic acid (PANI-EB) and the conducting form (PANI-HEBSA) was both obtained from Merck with a molecular mass of 50,000 in hexafluoroisopropanol (HFIP) to form a solution.
Organic molecules such as n-alkanes from C5 to C10 and polar molecules (alkyl acetates and alcohols) were used in this study. They are characterized by their donor and acceptor numbers [54-56]. On, we gave the donor and acceptor numbers of polar solvents.
GC conditions: The PANI powder was used with particle diameters ranging from 100 to 250 m. Particles of the correct size were introduced into a stainless-steel column, which was 30 cm long and had an internal diameter of 2 mm. A mass of 300 g of polymer was used to fill the chromatographic column. The column filled with the sample was conditioned at 120°C for 12 h to remove any impurities. The measurements of retention time were carried out with a DELSI GC 121 FB Chromatograph equipped with a flame ionization detector of high sensitivity. Helium was used as carrier gas; its flowrate was equal to about 20 mL min 1. IGC measurements at infinite dilution were done by varying the temperature from 40°C to 180°C. The IGC system has been used to make infinite dilution (ID) pulse experiments, probes were injected manually with a 1 μL Hamilton syringe. The injection volume for each probe was 0.1 μL, in order to approach linear condition gas chromatography. At least three injections were made for each probe and the average retention time, tR, was used for the calculations. The standard deviation was less than 1% in all measurements.
In this study, we determined the dispersive component of the surface energy, the specific interactions and the acid base constants of polyaniline in both conducting and non-conducting forms. Two transition temperatures were highlighted, the first one probably due to the modification of the surface of PANI-HEBSA at a temperature Tg1=383 K and the second one relative to the glass transition of PANI-EB given by Tg2=430 K. The evolution of the dispersive energy of polymers showed these two temperatures for n-alkanes used in this study (C5 to C10). The evolution of the specific free enthalpy of interaction between the polymer and the polar molecules as a function of the temperature confirmed the shift of the glass transition for the adsorbed polar solvents on PANI-EB. Our results giving the values of the acidic and basic constants of different polymers, showed an important basic character of PANI. However, an increase of the acid-base force was highlighted for PANI-EB near the glass transition.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
There is no conflict of interests.
Journal of Material Sciences & Engineering received 3677 citations as per Google Scholar report
