Extrusion and Ionic Liquids: A Promising Combination To Develop

Aug 9, 2018 - *E-mails: [email protected] (L.C.L.); [email protected] (S.L.). DOI:10.1021/bk-2018-1304.ch010. Publication Date (Web):Augus...
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Chapter 10

Extrusion and Ionic Liquids: A Promising Combination To Develop High Performance Polymer Materials Luanda C. Lins*,1 and Sébastien Livi*,2 1Institute

of Chemistry, University of Campinas (UNICAMP), P.O. Box 6154, 13083-970 Campinas, Brazil 2Univ Lyon, INSA Lyon, CNRS, IMP UMR 5223, F-69621, Villeurbanne, France *E-mails: [email protected] (L.C.L.); [email protected] (S.L.).

In the last thirty years, the scientific community have put a lot of effort in order to develop advanced polymer materials with various functionalities such as gas or water barrier properties, fire retardancy, mechanical reinforcement, ionic conductivity, anti-bacterial or fouling properties. One of the best ways to achieve this objective and industrially preferred is the melt mixing, especially extrusion. In fact, the advantage of this route is that the use of solvent is not required. However, the use of compatibilizing agents such as ionomers or block copolymers are commonly required to improve the affinity between two polymers or the polymer matrix and an inorganic or organic nanoparticles. Recently, ionic liquids (ILs) which are small organic molecules composed of cations and anions with melting temperatures below 100 °C have shown a great potential when they are used as multifunctional additives of polymer matrices such as surfactant agents of nanoparticles, as processing aids of wood plastic composites and as compatibilizing agents of polymer blends. This book chapter aims to provide a unique place to discuss the state of the art approaches in the field of polymers and ionic liquids processed by extrusion in order to design advanced polymer materials.

© 2018 American Chemical Society Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Due to their unique physico-chemical properties combining an excellent thermal stability (up to 300 °C), a good chemical stability, a low saturated vapor pressure and a good ionic conductivity, ionic liquids have recently undergone a real boom in the academic research (1). Moreover, as they are considered also as environmental friendly components and “green solvents” depending of their cation/anion combination and due to their recyclability and their ability to be reused several times in various types of polymerization, they have commonly used in polymer science (2–4). Thus, they have been used as a polymerization medium to prepare functional polymers (5–7), as excellent dissolution agents or solvents for low solubility biopolymers such as lignin, cellulose, polysaccharides and pupunha (8, 9). For example, different authors have demonstrated that cellulose or lignin and ionic liquids combined with counter anions such as chloride, acetate, phosphate or phosphite presented a very strong affinity. They have explained that this affinity is due to the presence of six hydroxyl groups for each unit of cellulose or lignin inducing inter- or intra-molecular hydrogen bonds. Recently, Ohno et al but also Swatloski et al have highlighted the dissolution of cellulose in an ionic liquid medium thus opening new perspectives in the field of wood-based composites or biorefinery (10, 11). Another area of research where ionic liquids have played a key role is that as surfactant agents of nanoparticles, especially layered silicates such as montmorillonite (MMT) and layered double hydroxide (LDH) in order to prepare polymer nanocomposites with enhanced final properties (12–17). The majority of this research have focused on the use of ammonium, imidazolium and phosphonium ILs as modifiers agents of cationic and anionic clays and their incorporation in different thermoplastics by extrusion process. Thus, polymer nanocomposites based on layered silicates with good mechanical performances, an excellent thermal stability, gas and water barrier properties have been prepared. In recent years, ionic liquids have also shown a great potential in the field of polymer blends. For example, Lins et al have demonstrated that the use of only 1 wt% of phosphonium ILs combined with different counter anions led to a phase separated morphologies combined with a significant increase of the mechanical performances opening up prospects in the field of compostable films for agricultural lands as well as food packaging applications (18). This book chapter aims to highlight the potential of ionic liquids in the field of polymeric materials processed by melt mixing such as extrusion. Thus, we will describe in this chapter the use of ionic liquids as multifunctional agents of polymer matrices through four parts: A) Origin and Structure of ILs, B) Ionic Liquids as surfactant agents of layered silicates in order to develop polymer nanocomposites, C) Ionic Liquids as new additives of Wood Plastic Composites (WPC) and D) Ionic Liquids as compatibilizing agents of polymer blends.

190 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

A – Origin and Structure of Ionic Liquids (ILs) Ionic liquids are composed of positively and negatively charged cations and anions, respectively. Moreover, the ionic liquids are well-known to have a melting temperature below 100 ° C and can be liquid at ambient temperature which explains their designation of Room Temperature Ionic Liquids (RTILs) (19, 20). This advantage opens many perspectives in the processing of polymeric materials by extrusion. Although ionic liquids were recently used in polymer science, the first ionic liquid reported and discovered by P. Wadden in the literature was an ammonium salt denoted ethylammonium nitrate (EtNH3+NO3-). But it is mainly in the late 80’s, 90’s that organic chemists have focused on ILs. As previously discussed, ILs are mainly composed of an organic cation combined with an organic or inorganic anion and depending of this wedding between these two components, different applications can be considered. The majority of ionic liquids are composed of organic cations such as imidazolium, ammonium, thiazolium, phosphonium or pyridinium and can be functionalized by multiple groups: acid, amine, acrylate, thiol, ester etc… Regarding the anions, two types are widely reported in the literature: fluorinated anions such as hexafluorophosphate (PF6-), tetrafluoroborate (BF4-), trifluoromethanesulfonate (CF3SO3-), and the conventional anions i.e. bromide (Br-), chloride (Cl-) or iodide (I-). The Figure 1 represents the most commonly encountered cations and anions.

Figure 1. Different chemical structures of commonly encountered cations and anions. The choice of cation or anion will have great consequences on its melting temperature, viscosity, density, solubility, toxicity as well as biodegradability. For example, imidazolium IL denoted 1-methyl-2-butylimidazolium tetrafluoroborate is soluble in water but when one changes its anion by an anion named PF6-, the resulting ionic liquid becomes totally immiscible in water (21). The alkyl chain length and consequently the symmetry of the molecule will also have a major impact on the melting temperature of the ionic liquid as well as its viscosity. In other works, an increase in the viscosity of the ionic liquid has 191 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

been observed when the anion is hygroscopic leading also to a decrease of its thermal stability (22, 23). The acidic or basic nature of an ionic liquid can also influence the solubility of organic compounds and its reactivity. The Kamlet-Taft parameters allow to take into account the solute-solvent interactions thanks to the polarity indices π*, α, and β the acidity (H-bond donor) and the basicity (H-bond acceptor) of the ionic liquid, respectively (24, 25). For these reasons, ILs have been commonly used in organic chemistry, especially in homogeneous and heterogeneous catalysis and as solvent replacement for the synthesis of various compounds. Due to their ability to dissolve a wide variety of organic substances, they have thus substituted conventional organic solvents. They were used in the coupling reactions of Suzuki-Heck reactions, in the alkylation reactions of Friedal-Craft and Diels-Alder (26–28). In summary, the infinite cation / anion combinations proposed by ILs open up promising prospects in the field of polymers.

B − Ionic Liquids as Surfactant Agents of Layered Silicates in Order To Develop Polymer Nanocomposites In the world of polymer/layered silicates (PLS) nanocomposites, significant R&D resources have been devoted to the dispersion of a small amount (1-10 wt %) of organic-inorganic hybrid materials in order to design polymer nanocomposites with improved final properties (29–32). In fact, the low cost of layered silicates such as montmorillonite (MMT) or layered double hydroxide (LDH) is their main advantage. However, a surface treatment of the layered silicates is required in order to improve the compatibility/affinity between the polymer matrix and the nanoparticles. Thus, many works have been widely reported on the use of ammonium-modified layered silicates and more recently on the use of thermostable ILs based on phosphonium or imidazolium cations as modifiers agents of clays (33–38). In this part, we will discussed the influence of the chemical nature of thermostable ILs on the morphologies as well as on the mechanical performances, the thermal and barrier properties of the PLS nanocomposites processed by extrusion. As previously discussed, the surface modification of cationic clays is required and the most commonly used surface treatments are cationic exchange and anionic exchange. According to the literature, Giesekind and Hendricks have demonstrated for the first time the intercalation of organic cations instead of inorganic exchangeable cations such as sodium (Na+), potassium (K+), calcium (Ca2+) in an aqueous medium (39, 40). These clays are well-known to have a specific property denoted CEC for cationic exchange capacity which corresponds to the substitution of inorganic compensating cations initially present between clay layers by organic cation for 100 grams of clay and expressed in milliequivalent per gram (meq/g). These last years, many ILs have been used as surfactant agents of layered silicates leading to a larger interlayer spacing as well as a reduction of their surface energy making them more compatible with the polymer matrix which allows a better dispersion and distribution of the fillers in the polymer matrix (41–43). 192 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

In the case of LDHs containing water molecules and anions between the clay layers, only few studies have been reported on the preparation of organic-inorganic hybrids based on ionic liquids. As cationic clays, LDH have a specific property denoted AEC (Anionic Exchange Capacity) included between 200 to 450 meq per 100g. To modify LDH and to intercalate organic molecules between clay layers, different routes have been explored: i) direct synthesis, ii) anion exchange reaction, iii) reconstruction and iv) restacking (44–46). However, anionic exchange requiring two steps based on an initial calcination following by the counter anion intercalation in solvent medium is a promising route to design ILs-modified LDHs. According to the literature, the use of ILs as surfactant agents of layered silicates have been widely reported in various thermoplastic matrices, especially thanks to their excellent thermal stability (up to 300 °C) allowing their use at higher temperatures. Thus, different polymer nanocomposites based on polyolefins, poly(vinylidene fluoride) (PVDF) and polyesters have been processed by extrusion and investigated in terms of mechanical, thermal, gas and water barrier properties as well as in terms of morphologies. a – Polyolefins/IL Treated Montmorillonites Polymeric matrices such as polyethylene (PE) or polypropylene (PP) have been extensively studied. According to the literature, the majority of the authors have focused on the quality of dispersion and distribution of the ILs-modified clays by using transmission electronic microscopy and by studying the rheological properties of the resulting nanocomposites where an enhancement of the filler-matrix interaction is characterized by an increase of the viscosity (47–49). However, some works have highlighted the influence of the MMT-ILs on the final properties of the polypropylene matrix. Thus, different authors have demonstrated that the incorporation of MMT-ILs in PP matrix exhibited an increase of the maximum degradation temperature (+ 56 °C) when 5 wt % of imidazolium IL-treated MMT was used (50). In other studies, PP nanocomposites have displayed an improvement of the Young Modulus (+ 35%) combined with a decrease of a relative yield strain. In addition, the introduction of 4 vol % of MMT-ILs induced a significant improvement of the relative oxygen permeation as well as a better thermal behavior (51, 52). In 2010 and 2011, Livi et al have synthesized different imidazolium and phosphonium ILs functionalized by various alkyl chains lengths (C18, C22) and counter anions such as iodide (I-), bromide (Br-), hexafluorophosphate (PF6-) or tetrafluroborate (BF4-) (53–55). Then, they have used these “homemade” ILs as modifiers agents of MMT and they have incorporated them in high density polyethylene (HDPE) matrix by twin screw extrusion. In this work, they have demonstrated that the physically adsorbed species present on the clay surface play a key role on the dispersion of the clay layers into HDPE matrix. In fact, ILs act as compatibilizing agent generating an increase of the stiffness without reducing the fracture behavior coupled with a better thermal stability (+ 20 °C). More recently, germinal benzimidazolium ILs have been used as surfactants and polyethylene nanocomposites have been prepared via melt-compounding method 193 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

using a minilab twin-screw mixer at 190°C during 10 minutes (56). El achaby et al have highlighted increases of 66% of the Young’s Modulus, 30 % of the tensile strength as well as an improvement of 23 °C of the thermal behavior of HDPE nanocomposites.

b – Polyesters/IL Treated Montmorillonites Many polymer/layered silicates nanocomposites based on polyethylene terephthalate (PET), polycaprolactone (PCL), poly(butylene adipate-coterephtalate) (PBAT) and polylactide (PLA) have been processed by extrusion and investigated in the literature. The preparation of PET/layered silicates nanocomposites requires higher temperatures at around 280 °C. Thus, thermally stable organically modified clays based on imidazolium and phosphonium ILs have been used as reinforcing agents of PET matrix (57, 58). However, a color change without modification of the final properties of PET have been obtained. Others authors such as Ghasemi et al have prepared PET nanocomposites including imidazolium- and phosphonium-modified MMTs and they have shown that the use of phosphonium IL as surfactant agent of layered silicates led to a better thermal stability combined with an intercalated morphology (59). Other biodegradable polyester matrices derived from petroleum (PBAT) or from fermentation process and starch distillation (PLA) have been also studied. Recently, PBAT nanocomposites containing 2 and 5 wt % of IL-treated montmorillonites have been prepared using a twin screw extruder and the impact of these fillers on the mechanical, gas, water properties as well as the structuration of PBAT matrix have been investigated. In this work, two ILs based on phosphonium denoted tributyltetradecylphosphonium dodecylbenzenesulfonate and imidazolium denoted N-octadecyl-N′-octadecyl imidazolium iodide have been selected for the surface treatment of the MMT. In both cases, a good level of dispersion is observed leading to a significant improvement of the mechanical performances characterized by increases in the Young Modulus (15-25 %) without reducing the fracture behavior. Concerning the water barrier properties, the incorporation of MMT-ILs induced a reduction of water sorption but also the water permeability included between 60 to 90%. The authors have attributed this phenomenon to the hydrophobic nature of ILs as well as to the creation of a tortuous path due to a good distribution of MMT-ILs in PBAT matrix which delays the progress of the water molecules in PBAT matrix (60). Very recently, similar results were obtained when 2 and 5 wt % of IL-modified-LDH are incorporated into PBAT matrix. In fact, they have obtained good mechanical performances with an increase of 20-50% of the stiffness combined with an improvement of the water barrier properties included between 30 to 50% depending of the chemical nature of the phosphonium ILs used (61). Different authors such as Ha et al and Livi et al have developed PLA nanocomposites containing LDH-ILs and they have highlighted a slight plasticizing effect of ILs on the mechanical behavior of PLA and thanks to the potential antibacterial properties of phosphonium ILs, Ha et al proposed that LDH-ILs can be used as reservoir for the controlled release of phosphorous compounds (62, 63). 194 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

c – Poly(fluorure vinylidene fluoride) (PVDF)/IL Treated Montmorillonites Due to its polymorphism through its α, β, γ and δ forms as well as its dielectric and pyroelectric properties, PVDF is a polymer commonly used for energy applications (64–68). One of the best ways to generate the β form is the incorporation of organically modified clays (69). Thus, different ILs based on ammonium, pyridinium, imidazolium and phosphonium cations have been reported as surfactant agents of montmorillonite. Indeed, various authors have demonstrated that the introduction of a small amount (1-5 wt %) of IL-treated MMT into PVDF matrix prepared by extrusion induced partially exfoliated, exfoliated or intercalated morphologies depending of the chemical nature of ILs. As consequence, these morphologies have an impact on the mechanical properties of the resulting nanocomposites. Patro et al have demonstrated that the use of pyridinium- and ammonium-treated MMTs generated an increase of the strain at break of + 175% and + 200%, respectively. They have shown that the incorporation of phosphonium-modified MMT acts as an efficient nucleating agent and have the ability to induce β-phase (70). More recently, Livi et al have investigated the effect of “homemade” ILs as interfacial agents of MMT on the mechanical behavior of PVDF nanocomposites. Thus, they have synthesized imidazolium ionic liquid functionalized with fluorinated ligand to obtain a higher compatibility between MMT and PVDF matrix. Finally, the incorporation of only 1 wt % of imidazolium-treated MMT in PVDF matrix led to a plasticizing effect characterized with an increase of + 630 % of the elongation at break combined with a decrease of 15 % of the Young’s Modulus promoting also the formation of β-phase opening new applications in the field of energy (71). In summary, imidazolium and phosphonium ionic liquids (Figure 2) represent a new alternative for the preparation of high performance polymer nanocomposites. Although few studies have been reported on these systems, the incorporation of ILs-treated clays such as MMT or LDH leads to an enhancement of the thermal stability, mechanical behavior and barrier properties. These promising results also highlighting the role as compatibilizing agent as well as dispersant aids of ILs open a new way in the preparation of nanocomposites resulting from the biomass. Thus, new polymeric materials having a biobased matrix or mixed with renewable resources can be envisaged.

195 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 2. Various common ionic liquids described in literature used as surfactant agents of layered silicates. (A) N-octadecyl-N′-octadecylimidazolium iodide (53–55, 57, 60, 71); (B) octadecyltriphenylphosphonium iodide, octadecyltriphenylphosphonium bromide, and octadecyltriphenylphosphonium hexafluorophosphate (69, 70, 72); (C) N-octadecyl-N′-docosyl imidazolium iodide (73); (D) N-docosyl-N′-docosylimidazolium bromide (54); (E) 1-hexadecyl-3-methylimidazolium chloride (50); (F) 1-decyl-2-methyl-3-octadecylimidazolium bromide (51); (G) Tetraphenylphosphonium chloride and hexadecyltriphenylphosphonium chloride (52); (H) 2-(1-hydroxyethyl)-1,3-dihexadecyl-benzimidazolium bromide (56); (I) Tributyl hexadecyl phosphonium bromide (57), (J) hexadecylquinolinium bromide (58); (K) Tributyl tetradecyl phosphonium bromide and chloride (59), 196 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

(L) Dihexadecyl imidazolium chloride (32); (M) tributyltetradecylphosphonium dodecylbenzenesulfonate (60, 62); (N) Trihexyl(tetradecyl)phosphonium cation with bis(2,4,4-trimethylpentyl)phosphinate anion and bis(2-ethylhexyl)phosphate anion (63, 74); (O) Trihexyltetradecylphosphonium decanoate and Trihexyltetradecylphosphonium dodecylsulfonate (62, 67); and (P) N-methyl-N′-perfluorododecylimidazolium (71).

C − Ionic Liquids as New Additives of Wood Plastic Composites (WPC) Compared to conventional solvents commonly used for the processing of biopolymers such as ethylene diamine/salts, N,N-Dimethylacetamide with lithium chloride or N-Methylmorpholine N-oxide, ionic liquids thanks to their non-volatility and their high recyclability have a great potential to dissolve cellulose, hemicellulose, lignin or wood. Based on their ability, wood plastic composites (WPCs) can be considered (72, 73, 75–77). According to the literature, WPCs are emerging and have the advantage of being able to be processed by extrusion. However, the incorporation of wood content in polymer materials led to an increase of the viscosity requiring high torque to mix the blends. Moreover, these composites also required high energy consumption and sometimes the use of higher temperatures that can degrade the wood flour (78, 79). Thus, some studies postponing the use of ionic liquids to treat wood and manufacture composites have been reported (80). Imidazolium and phosphonium ionic liquids have been reported for the preparation of wood plastic composites based on high density polyethylene (HDPE), polystyrene (PS), and polypropylene (PP). Indeed, Xie et al have studied the use of imidazolium IL denoted 3-allyl-1-methylimidazolium chloride to dissolve and prepare lauroylated and benzoylated spruce (81). Then, they have introduced these modified wood flours into PS and PP matrices by using a co-rotating twin screw extruder (ThermoHaake) at 221 °C with a rotation speed of 120-150 rpm. Thus, the use of highly substituted wood based lignocellulosic materials led to an increase of the compatibility between wood and polymer matrices characterized by an improvement of the thermal stability while retaining the same mechanical strength. In 2009, Zhang et al have developed HDPE/wood composites studying on the influence of screw configuration, the incorporation of lubricants or the optimization of screw speed (82, 83). In this work, they have demonstrated that the incorporation of lubricants induced an enhancement of the wood flour flow due to the reduction of friction between the two components, i.e. HDPE and wood flour a well as a good uniformity of the composites. Recently, different authors have worked on the “in situ” thermoplasticization of poplar wood with the help of various ILs denoted 1-ethyl-3-methylimidazolium chloride ([Emim]Cl-), 1-ethyl-2,3-dimethylimidazolium chloride ([Edmim]Cl ), 1-benzyl-3-methylimidazolium ([Bzmim]Cl-), 1-(2-hydroxyethyl)-3-methylimidazolium and ([Hemim]Cl-). In this study, nonisothermal compression tests have been preferred to extrusion (84). However, they have demonstrated that ILs have the ability to introduce a permanent strain 197 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

in wood but also to generate a disintegration of intermolecular and intramolecular hydrogen bonds between cell wall polymers. The same year, Ou et al have treated wood flour by an ionic liquid named [Hemim]Cl- and they have processed HDPE/wood flour composites using a Haake microcompounder equipped with co-rotating twin screw extruder (L/D= 40, diameter=18) at a temperature from 150 °C to 175 °C. They have observed a decrease of the crystallinity and an increase in the thermoplasticity of the wood flour with the presence of imidazolium IL (85). Moreover, an increase of the IL content led to a significant increase of the viscosity as well as the melt torque for low processing speeds. In the opposite, a reduction of these parameters is obtained for high processing speeds. Thus, they have highlighted that the use of IL plays a key role on the filler-filler interaction resulting to an enhancement of the plasticity of rigid wood cell walls. Very recently, Croitoru et al have prepared HDPE/wood composites by compression molding at 140 °C. In this work, they have used ammonium and phosphonium ILs denoted methyltrioctylammonium bistriflimide and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate as interfacial adhesion agents in wood plastic composites (86). Due to the unique properties of ILs, the interfacial adhesion between polymer matrix and wood have been significantly improved leading to a better resistance to water action and good mechanical performances, especially for impact resistance as well as the tensile strength. In addition, they have demonstrated in presence of ILs a higher resistance of WPC to accelerated photo-oxidation and antifungal properties against brown rot inhibiting the growth of Postia Placenta fungal strain. In conclusion, these various studies have demonstrated that ionic liquids can be used as plasticizers, lubricants or interfacial agents in order to increase the compatibility but also the adhesion between wood and polymers improving the final properties of the WPC. Ionic liquids open new perspectives of the biomass valorization in the field of polymer materials. Thus, others lignocellulose agricultural residues can be explored such as peach palm trees (Bactris gasipaes kunth), also well-known as “pupunha”, kenaf, sisal, and munguba fibers (87–94).

D − Ionic Liquids as Compatibilizing Agents of Polymer Blends In recent years, the development of polymer blends has attracted the attention of academic and industrial research and represents an important economic challenge (95–97). In fact, binary and ternary mixtures are a promising way to produce high performance polymers at low cost. In the field of recycled polymers composed of polyolefins such as polypropylene (PP) or polyethylene (PE) and engineering plastics as polyamide (PA), many authors have shown the advantage of combining the properties of the corresponding neat polymers. For example, for polymeric blends composed of PP/PA, widely reported in the literature, the incorporation of PP as a minor phase leads to an increase of the water barrier properties (98–104). In the opposite, the incorporation of polyamide in a PP phase rich induces chemical and heat resistance. 198 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

However, it is well known that the majority of polymers are immiscible between them from thermodynamical point of view leading to unstable morphologies and poor interfacial adhesion. As a consequence, polymer materials processed by extrusion have poor mechanical performances. For these reasons, different routes have been investigated for forty years: the first is the use of block copolymers or ionomers which have the same chemical affinity with one of the two phases while the second path is the introduction of nanoparticles such as silica, carbon nanotubes, layered silicates (montmorillonite, mica, layered double hydroxide) in order to stabilize the morphologies of the polymer blends (105–114). Nevertheless, the major drawbacks of these two pathways are a significant increase of the viscosity of the polymer blends and require higher amounts (5 wt% to 20 wt%) of nanoparticles or block copolymers. Due to the unique properties of ionic liquids but also to their potential as “green” solvents compared to traditional organic solvents, many ILs have been reported for the dissolution of carbohydrates and other polysaccharides such as cellulose, lignin, starch, chitin and chitosan (115–119). In 2010, Sankri et al have studied the processing of thermoplastic starch (TPS) plasticized with imidazolium IL denoted 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) by a Clextral BC21 corotating twin screw extruder (120). In this work, the temperature in the feeding zone was 90°C while the temperature at the die exit was 120°C. Thus, the authors have finally shown that [BMIM]Cl is an excellent plasticizing agent compared to glycerol. Following these results, Leroy et al have used the same IL as plasticizer of starch, zein and their blends and processed at 130 °C the different samples with twin screw microextruder equipped with conical screws of 12 cm3 during 5 minutes with a screw speed of 50 rpm (121). The authors have prepared starch/zein blends with different ratio: 100/0; 90/10; 70/30; 50/50; 30/70; 10/90 and 0/100 by using 23 wt% of imidazolium ionic liquid. Then, they have demonstrated that the use of IL led to a significant decrease of the starch/zein viscosity combined with the formation of smaller zein aggregates observed by laser confocal scanning microscopy. Concerning the mechanical behavior of starch/zein (90/10 wt%) representative to the composition of corn flour and plasticized with [BMIM]Cl, they have demonstrated a significant increase of the strain at break combined with a drop of Young’s modulus by a factor of 10 compared to glycerol plasticized blends. Thus, they have suggested a compatibilization of the polymer blends. More later, the same authors have investigated the impact of IL and salts with the ability to form Deep Eutectic Solvents (DES) on the morphologies and mechanical performances of starch/zein blends and they have assumed that IL and DES (30 wt%) act as surfactant agents and that a part was at the starch/zein interface (122). These results open news perspectives in the processing of bioplastics by extrusion but require large amounts of ionic liquids. Although ionic liquids are commercially available, their price is very high. Recently, Livi et al have reported a new way of compatibilization of thermoplastics blends by using ILs based on tetraalkylphosphonium salts combined with phosphinate (IL-TMP) and trifluoromethylsulfonylimide (TFSI) counter anions (123). In this work, they have incorporated 1 to 20 wt% of ILs in PP/PA6 (80/20 wt%) blends processed by twin screw extrusion. They have 199 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

shown for the first time that a small amount (1 wt%) of phosphonium ILs led to a significant decrease of the minor phase (PA6) from 30 µm to 3 µm. They have explained this phenomenon by an accumulation of ILs at the interface of PP/PA6 blends which prevents the coalescence of polyamide phases. In addition, the incorporation of only 1 wt % of IL-TMP induced a significant increase of the strain at break (+ 1400 %) without reducing the Young’s Modulus while the incorporation of IL-TFSI generated a drop of 25% of the stiffness. In addition, an increase of the thermal stability of the polymer blends was obtained. In order to understand the effect of the ionic liquid on the affinity of IL-TMP and IL-TFSI with PP and PA, they have investigated the influence of the amount of ILs on the morphologies of polymer blends by transmission electronic microscopy (TEM) and they have shown a decrease of the minor phase to 1.8-1.9 µm for 10 wt% and an increase to 3-4 µm for 20 wt% highlighting that a high amount of ILs act as emulsifiers promoting coalescence of PA6 through ionic bonds. From the study of morphologies, mechanical properties and surface energy measurements of PP/PA6 blends, they have demonstrated that phosphonium ILs are located at the interphase of PA6 phases and have presented a schematic representation of the distribution of ILs in polymer blends in function of their amount and their chemical nature (Figure 3).

Figure 3. Schematic representation of the effect of the ILs on the morphology of polymer blends. Reproduced with permission from ref. (123). Copyright 2014 Elsevier. Following these promising results, Lins et al have investigated the effect of the phosphonium ionic liquids having different counter anions on biopolymer blends composed of a flexible aliphatic-aromatic copolyester named poly(butylene-adipate-co-terephtalate) (PBAT) and poly(lactid acid) (PLA) (18). Thus, phosphonium ILs combined with chloride (IL-Cl), phosphinate (IL-TMP), bistriflimide (IL-TFSI), hexanoate (IL-EHT) and phosphate (IL-EHP) have been introduced in PBAT/PLA blends by using a 15g-capacity DSM micro-extruder with co-rotating screws (L/D ratio of 18) at 160 °C during 5 minutes under 100 rpm speed. In this study, different morphologies have been highlighted by transmission electronic microscopy (Figure 4). 200 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 4. TEM micrographs of PBAT/PLA without ILs (left) and PBAT/PLA with ILs (right).

In all cases, the use of ILs led to a phase separated morphology characterized by the presence of fiber-like shape or droplets compared to large and non-uniform PLA particles for blends without ILs. The authors have attributed the distribution of PLA in PBAT matrix to the Marangoni effect which generates a local perturbation from ionic interactions leading to a reduction of interfacial tension. In order to locate of ionic liquids in polymer blends, Lins et al have determined by the Good-Girifalco equation the values of the different interfacial energies and they have found a wetting coefficient ω between -1 and 1 confirming that ILs are located at the interface of the two polymers. These results have been proven in 2016 by Megevand et al by using Atomic Force Microscope (AFM) with the PeakForce Quantitative NanoMechanics (QNM) mode in order to study the interfaces between PBAT and PLA polymers (124). The authors have demonstrated that IL-Cl led to the formation of a thick interphase generating a good local miscibility of PBAT and PLA phases while IL-TMP induced the formation of an interface layer ensuring interactions with the two polymers. They have suggested a schematic representation of the interactions as well as the interface structure (Figure 5) between ILs and PBAT, PLA polymers. Concerning the mechanical behavior of these polymer blends, Lins et al have shown that the use of only 1 wt% of phosphonium ionic liquid induced an excellent compromise between stiffness and stretchability where increases of the Young’s modulus and strain at break have been observed for IL-TMP and IL-EHT.

201 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 5. On the left, AFM topography and images with corresponding modulus profiles for PBAT/PLA (A–B); PBAT/PLA/il-Cl (C–D); and PBAT/PLA/il-TMP (E–F). On the right, the interface structure proposed for IL-Cl (H) and IL-TMP (I). Reproduced with permission from ref. (124). Copyright 2016 Royal Society of Chemistry. They have also demonstrated a dual role of phosphonium ILs as new catalyst of the transesterification reaction. In fact, size exclusion chromatography (SEC) have been performed in order to highlight the impact of ILs in PBAT/PLA blends and the authors have shown that ionic liquids having alkoxides groups such as IL-TMP, IL-EHT and IL-EHP generated stronger interactions with PBAT and PLA inducing the transesterification reaction. Then, other authors have prepared melted ternary blends composed of PBAT, PLA and lignin in one pot by using only 1 wt% of phosphonium ILs and they have explored the mechanical performances, the water vapor permeability as well as the morphologies of these blends (125). In this paper, they have highlighted that the use of ILs led to a finer dispersion as well as a significant reduction of the lignin particles around of 30-220 nm. They have explained these results by the strong interaction between phosphate and chloride counter anions with OH groups of lignin. Indeed, the presence of ILs resulted in partial dissolution of lignin particles under melt mixing promoting strong hydrogen bonds. Concerning the mechanical properties, IL-TMP and ILTFSI induced significant increases of the Young’s modulus of 60% and 30% as well as increases of the strain at break, respectively. In addition, the water barrier properties have been also investigated and the use of hydrophobic phosphonium salts combined with a good dispersion of PLA and lignin particles into PBAT 202 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

matrix generated the formation of a tortuous path delaying the diffusion of small water molecules. As a consequence, a significant decrease of the water sorption has been obtained. In conclusion, ionic liquids represent a promising route in the field of polymer blends and opens news perspectives in the preparation and development of bioplastics from biomass and natural polymer. Thus, ionic liquids can play a key role as multifunctional additives of polymer blends. In fact, they can be used as plasticizers, compatibilizing agents, dissolution agents and catalysts of the transesterification. Even though the price of ILs is high, we have demonstrated that the use of a small amount (1 wt%) of ionic liquids can lead to a significant improvement in the final properties of the polymers facilitating the extrusion process by reducing the viscosity. Moreover, several studies are required, especially on the use of biodegradable ionic liquids. The main objective of this chapter has been to highlight the great potential of ionic liquids as promising multifunctional agents of polymeric materials prepared by the extrusion process. Thus, we have demonstrated that depending on the chemical nature of the ionic liquid but also the polymer matrix, it becomes possible to produce polymer materials with improved properties. Nevertheless, academic research must continue this research on the polymer-ionic liquids field in order to develop industrial applications leading to a decrease of their cost.

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