In the field of smart materials for photovoltaic (PV) industry and Organic Electronic (OE), polymeric films that exhibit modular wetting properties and conductive features are considered very promising for several applications. Self-cleaning films for the covering of photovoltaic cells and conductive polymer films are two important examples.
The present work is related to the use of Sulfonated Polyarylethersulfone (SPES) for two different applications:
i) for PV industry, as flexible and transparent polymeric film having both hydrophilic and hydrophobic features;
ii) in the research area of OE, as doping agent for both electrical conductive and polymeric films, and screen printer inks.
Polymers based on Polyarylethersulfones (PESs) are favorable materials for preparing membranes for several applications due to their excellent thermal and chemical stability, ion exchange properties, oxidation resistance as well as good mechanical behavior. PES-based membranes have been widely used in advanced separation technologies, including low-cost alternatives to expensive fluorinated polymers in fuel cells, in biomedical fields (as medical devices for blood purification and dialysis) and in food industry as membranes for wine, water and fruit-juices purification.
In order to diversify membrane properties and therefore widen possible application fields, chemical modifications of PES matrices can be studied; such modifications can be achieved by supplying different functional groups on the polymer matrix, i.e. sulfonic (-SO3-), hydroxyl (-OH), carboxyl (-COOH) and amino (-NH2) moieties.
SPES has received great attention in the last decade due to the possibility to improve the wetting properties of PES membranes thanks to the incorporation of sulfonic groups. This allowed the development of SPES membranes as advanced materials for a variety of separation processes, such as ion exchange, reverse osmosis and electro dialysis process; in all these fields, materials with highly hydrophilic behavior are requested.
SPESs can be prepared via two polymerization routes, either via post-sulfonation reaction with different sulfonating agents and solvents of the pre-formed PES polymer -heterogeneous synthesis-, or starting from pre-sulfonated monomers -homogeneous synthesis-. Although homogeneous synthesis allows an easy control of the degree of sulfonation (DS) of the resulting polymer and to avoid side and degradations reactions, heterogeneous synthesis is widely used, both in the industrial and academic area, due to its simplicity and low cost.
i) SPES films characterized by both hydrophobic and hydrophilic behavior.
In this work, a series of SPESs with different DS were synthesized via homogeneous synthesis starting from 4,4\u2019-difluorodiphenylsulfone, 4,4\u2019\u2013dihydroxydiphenyl and a sulfonated comonomer, 2,5-dihydroxybenzene-3-sulfonate potassium salt (sulfonated hydroquinone in potassium salt). The macromolecular structure was determined via 1H NMR and 1H-1H COSY spectroscopy and the presence of sulfonic groups was confirmed by Fourier Transform Infrared (FT-IR) spectra; the real DS values of SPESs were determined by 1H NMR calculations and confirmed by potentiometric tritation data. Intrinsic viscosities were measured using dimethylacetamide (DMAc) as solvent with and without lithium bromide (LiBr) solution in order to investigate polyelectrolyte effect on SPES samples. The effects of DS increasing on the thermal properties of SPES membranes obtained via solvent casting deposition in DMAc were studied by Thermogravimetric (TGA) analyses and Differential Scanning Calorimetry (DSC); wetting features were investigated by static water contact angle (SWCA).
Wettability measurements are commonly used to characterize the relative hydrophilicity or hydrophobicity of a polymer surface; here, in order to investigate the wetting properties of SPES membranes, SWCA measures for all the samples, obtained from solution casting in DMAc, were performed.
The samples were solution casted onto a PTFE mold; the wetting features of the membranes were measured both at the air-side and at the PTFE-side surface. Table I shows the results obtained.
Sample DS
(meq SO3-*g-1) (1H NMR) \u3b8w
(air-side) \u3b8w
(PTFE-side)
SPES_0.5 0.48 (65) \u305\ub12 (89) \u305\ub11
SPES_0.75 0.70 50\ub12 (85) \u305\ub11
SPES_1 0.98 (43) \u305\ub11 (81) \u305\ub12
Table I. Static water contact angles of samples synthesized.
The results reported indicate that the hydrophilicity of SPES membranes improves as -SO3-K+ amount in the SPES membranes increases, due to the high polarity of the -SO3- groups. It is important to underline that, thanks to a direct sulfonation reaction of, the hydrophilic properties of SPES samples are greatly enhanced using low amounts of sulfonic groups.
The increase of \u3b8w observed on the PTFE-side for SPES membranes with respect to the air-side is due to the different organization of the SO3-K+ groups of the polymeric chains occurring during the evaporation of the solvent, as confirmed by FT-IR spectra performed on both air-side and PTFE one of the polymeric films.
Standard materials used as photovoltaic cells cover are characterized by an internal hydrophilic side, optimal for the deposition on solar cells and by an external hydrophobic or hyper-hydrophobic side, designed ad hoc as protective self-cleaning layer from pollution.
To improve the hydrophobic properties, the use of SPESs can be very advantageous in fields where hydrophilic/hydrophobic properties can be modulated; moreover, the use of Ionic Liquids (I.Ls.) combined with SPES can be a way to create tailor-made hydrophobic materials for solar cells covering.
I.Ls., a class of molten salts, have excellent thermal stability and their physical-chemical properties can be modulated changing the nature of the cation or anion. Modulating cationic apolar groups can dramatically influence the tendency of I.L. towards efficient ion packing and, in turn, its hydrophobic features, e.g. the longer the alkyl chains, the more hydrophobic the salt.
The wetting properties of SPES could therefore be modulated by introducing different cationic apolar groups through a novel ionic exchange reaction between the K+ cation of the sulfonic moiety of SPES and the cation of an I.Ls.. The structure of I.Ls. used are reported in Figure I.
Figure I. Ionic Liquids used for the cation exchange reactions with SPES.
The hydrophobic properties of SPES treated with I.Ls. were found to improve with the DS of sulfonation of SPES, i.e. with the number of K+ ions available for substitution by I.Ls. cations, obtaining contact angles up to (131) \u305\ub0 (Table II) leading to self-cleaning surfaces (Figure II).
Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analyses for SPESs and SPES_I.Ls. samples were performed in order to clarify the influence of I.Ls. on the surface properties of the polymeric membranes prepared. Figure III presents SEM morphologies of representative membranes from both air-side and PTFE-side of SPESs and SPES_I.Ls. samples.
In the case of SPES_1, no surface differences are detectable between the air-side (IIIa) and the mold-side of the polymeric membrane (IIIb). When I.Ls. are added, it is possible to observe that the surface at the air-side remains smooth (IIIc), while the surface at the mold-side of the membrane changes from smooth to rough (IIId). The roughness of PTFE-side of SPES_I.Ls. membranes increases as the DS of SPESs increases, i.e. with the number of hydrophobic I.Ls. cations exchanged, as it is possible to observe comparing SPES_MEIM_0.5 (IIId) with SPES_MEIM_0.75 (IIIe) and SPES_MEIM_1 (IIIf), and it enhances as the length of the imidazolinium alkyl chains enhances, as shown in Figure IIIf for SPES_MEIM_1, in Figure IIIg for SPES_BBIM_1, in Figure IIIh for SPES_MOIM_1 and in Figure IIIi for SPES_BOIM_1.
Samples \u3b8w (air-side) \u3b8w (PTFE-side)
SPES_MEIM_0.5 (86) \u305\ub12 (108) \u305\ub11
SPES_MEIM_0.75 (85) \u305\ub11 (116) \u305\ub11
SPES_MEIM_1 (89) \u305\ub11 (121) \u305\ub11
SPES_BBIM_0.5 (88) \u305\ub11 (123) \u305\ub11
SPES_BBIM_0.75 (84) \u305\ub12 (124) \u305\ub12
SPES_BBIM_1 (81) \u305\ub12 (126) \u305\ub11
SPES_MOIM_0.5 (77) \u305\ub11 (124) \u305\ub11
SPES_MOIM_0.75 (80) \u305\ub11 (125) \u305\ub12
SPES_MOIM_1 (81) \u305\ub11 (128) \u305\ub11
SPES_BOIM_0.5 (85) \u305\ub11 (130) \u305\ub11
SPES_BOIM_0.75 (80) \u305\ub11 (131) \u305\ub11
SPES_BOIM_1 (81) \u305\ub11 (131) \u305\ub11
Table II. Static water contact angles of samples synthesized.
.
Figure II. Self-cleaning SPES films.
Comparing AFM topographies of the mold-side of pristine SPES (Figure IVa) and SPES_I.Ls. samples (Figure IVb, c, d, f, g and h), it is clear that the surfaces of SPES_I.Ls. are much rougher than the surface of SPES sample without I.Ls. When I.Ls. are present, it is possible to observe that the surface at the air-side of the membrane remains smooth (IVe). Conversely, the surface at PTFE-side changes from smooth to rough (IVd); Root-mean-square (RMS) roughness data range from 66.65 nm for the air-side to 185.15 nm for its PTFE- mold side.
The roughness of PTFE-side of SPES_I.Ls. membranes increases as the DS of SPESs increases, as it is possible to observe comparing SPES_MEIM_0.5 (IVb) with SPES_MEIM_0.75 (IVc) and SPES_MEIM_1 (IVd). This behavior was confirmed by RMS roughness values that for SPES_MEIM_0.5, SPES_MEIM_0.75 and SPES_MEIM_1 are 101.01 nm, 163.87 nm and 185.15 nm, respectively.
The influence of I.Ls. characterized by difference length of the imidazolinium alkyl chains was also investigated and the results obtained suggest that the roughness of the mold-side of SPES_I.Ls. membranes increases as the length of the imidazolinium alkyl chains increases, as shown in Figure IVe for SPES_MEIM_1 -RMS of 185.15 nm-, in Figure IVf for SPES_BBIM_1 -RMS of 192.78 nm-, in Figure IVg for SPES_MOIM_1 -RMS of 206.34 nm- and in Figure IVh for SPES_BOIM_1 -RMS of 236.85 nm-.
All AFM images and RMS roughness values obtained agree well with both SWCA data -i.e. the higher the contact angles, the rougher the surface- and SEM analyses, as previously shown in Figure III, confirming that the hydrophobic imidazolinium alkyl chains, orienting themselves during the evaporation of the solvent towards the mold surface, change the membrane surface in correspondence of PTFE-side from smooth to rough.
Figure III. (a) SPES_1 air-side; (b) SPES_1 PTFE-side; (c) SPES_MEIM_0.5 air-side; (d) SPES_MEIM_0.5 PTFE-side; (e) SPES_MEIM_0.75 PTFE-side; (f) SPES_MEIM_1 PTFE-side; (g) SPES_BBIM_1 PTFE-side; (h) SPES_MOIM_1 PTFE-side and (i) SPES_BOIM_1 PTFE-side. Figure IV. (a) SPES_1 PTFE-side; (b) SPES_MEIM_0.5 PTFE-side; (c) SPES_MEIM_0.75 PTFE-side; (d) SPES_MEIM_1 PTFE-side; (e) SPES_MEIM_1 air-side; (f) SPES_BBIM_1 PTFE-side; (g) SPES_MOIM_1 PTFE-side and (h) SPES_BOIM_1 PTFE-side.
ii) SPES as dopant agent for both electrical conductive and polymeric films and screen printer inks.
OE based on polymers is a research field that is gaining more and more interest, both from an academic and an industrial point of view. Organic conductive materials could open several possibilities in the production of new advanced electronic devices: in fact, these materials can conjugate useful features such as flexibility, transparency, durability and lightness typical of polymers, with conductive properties, conferred by highly conjugated organic systems.
Polycarbonate (PC) is a polymer characterized by high durability and mechanical resistance, coupled with good thermal properties. One of the most valuable features of PC relies on its extremely good optical behavior: PC is often used as a substitute of glass because the refractive index of the two materials is very similar (1.5237 for glass and 1.5856 for PC at \u3bb = 580 nm) but PC has higher mechanical properties, is lighter and is not fragile. Within this context also Polymethyl Methacrylate (PMMA) is an optimal material for transparent layers (PMMA = 1.4910 at \u3bb = 580 nm) and its monomers can be easily functionalized.
The goal of this project was to develop an innovative material bearing at the same time carbonate moieties, useful for optical features, and acrylate groups, functionalized with conductive molecules, in order to obtain transparent conductive films. At first Polyallyl carbonate homopolymers were successfully synthesized through free radical polymerization (NMR, Gel Permeation Chromatography (GPC) and thermal studies were performed). As result of homopolymerization kinetics, it seems that the radical of the propagating species is a stable \u201cpseudo-living\u201d radical, able to initiate the growth of another polymeric chain characterized by conductive functionalities, in order to obtain block copolymers with high molecular weights and conductive continuity along the chain.
Acrylate monomers based on conductive molecules were synthesized; the organic molecules used are based on highly aromatic structure, Fluorene Bisphenol, known as \u201ccardo structure\u201d. The high level of electronic delocalization given by the aromatic groups guarantees not only the conductivity, but also high thermal resistance. Polyallyl carbonate and functionalized PMMA block copolymers were indeed successfully synthesized (Figure V) and fully characterized by NMR, GPC and thermal characterizations.
Starting from this new material, a transparent polymer film was obtained through solvent casting deposition. In order to improve the conductive features of this material, functional end-capped conducting 3,4-ethylene dioxy thiophene (EDOT) oligomers were synthesized. In particular, EDOT is the monomer adopted for the polymerization of Poly(3,4-ethylene dioxy thiophene) (PEDOT) oligomers, a famous commercial conductive polymer -known as Baytron\uae- usually doped with Sulphonated Polystyrene (PSS), that it is insoluble in the common organic solvent and therefore difficult to process.
In the present work a synthetic route able to enhance PEDOT solubility and conductivity was developed. Methacrylate-terminated PEDOTs were successfully synthesized via oxidative polymerization of EDOT and cross-linkable methacrylate end-capped EDOT, with ferric sulfate as oxidant (Figure VI). EDOT end-capping monomer was prepared through Friedel Crafts acylation starting from EDOT and methacryloyl chloride. The chemical structure and the degree of polymerization of the end-capped PEDOTs were determined by 1H NMR spectroscopy.
Figure V. Block copolymers of Polyallyl carbonate and functionalized methacrylic polymers.
End-capped PEDOTs have excellent solubility in several organic and chlorinated solvents. Furthermore, the use of cross-linkable end-caps makes EDOT-based oligomers soluble in organic and chlorinated solvents; the cross-linking of PEDOT film is then possible after UV exposure.
In order to improve PEDOT conductive properties, the dopant agents based on sulfonic groups commonly used with PEDOT are 2-Naphthalenesulfonic acid, paratoluene sulfonic acid or others. Besides these molecules, also SPES, obtained as reported previously, can be used as dopant agent thanks to both the charge separation deriving from the use of the pre-sulfonated comonomer and the possibility to modulate the moieties of sulfonic groups in the polymeric chains.
Figure VI. Synthetic route for methacrylate end-capped PEDOT. Figure VII. SPES as dopant agent for methacrylate end-capped PEDOT.
In this work, the oxidative polymerization of EDOT and functional end-capped EDOT monomers with ferric sulfate as oxidant and SPES as dopant was performed (Figure VII); it was found that PEDOT conductive features increase as SPES DS increases reaching 210 S/cm, a value 50 S/cm far higher than the one of commercial PEDOT (160 S/cm). The cross-linking of end-capped PEDOT with the block copolymers of Polyallyl carbonate and functionalized methacrylic polymers, through the vinyl functionalities, in then possible after UV exposure
