applied sciences

Review Application of Quaternary Ammonium Compounds as Compatibilizers for Blends and Polymer Composites—A Concise Review

Ahmad Adlie Shamsuri 1,* and Siti Nurul Ain Md. Jamil 2,3,*

1 Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, University Putra Malaysia, Serdang 43400, Selangor, Malaysia 2 Department of Chemistry, Faculty of Science, University Putra Malaysia, Serdang 43400, Selangor, Malaysia 3 Centre of Foundation Studies for Agricultural Science, University Putra Malaysia, Serdang 43400, Selangor, Malaysia * Correspondence: [email protected] (A.A.S.); [email protected] (S.N.A.M.J.)

Abstract: A wide variety of quaternary ammonium compounds (QACs) have escalated the attraction of researchers to explore the application of QACs. The compounds have frequently been synthesized through alkylation or quaternization of tertiary amines with alkyl halides. Recently, QACs have been applied to compatibilize polymer blends and polymer composites in improving their thermo- mechanical properties. This concise review concentrates on the application of two types of QACs as compatibilizers for polymer blends and polymer composites. The types of QACs that were effectively applied in the blends and composites are quaternary ammonium surfactants (QASs) and quaternary   ammonium ionic liquids (QAILs). They have been chosen for the discussion because of their unique chemical structure which can interact with the polymer blend and composite components. The Citation: Shamsuri, A.A.; Jamil, influence of QASs and QAILs on the thermo-mechanical properties of the polymer blends and S.N.A.M. Application of Quaternary polymer composites is also described. This review could be helpful for the polymer blend and Ammonium Compounds as polymer composite researchers and induce more novel ideas in this research area. Compatibilizers for Polymer Blends and Polymer Composites—A Concise Keywords: surfactant; ionic liquid; polymer blend; polymer composite; compatibilizer Review. Appl. Sci. 2021, 11, 3167. https://doi.org/10.3390/app11073167

Academic Editor: María Emma Borges 1. Introduction Lately, the application of quaternary ammonium compounds (QACs) in the polymer Received: 9 January 2021 research has drastically developed because of the effectiveness of these ionic compounds Accepted: 15 February 2021 in forming the intermolecular interactions with synthetic and natural . QACs Published: 2 April 2021 can be synthesized through alkylation of amines by quaternization of tertiary amines with alkylating agents such as alkyl halides [1,2]. Figure1 demonstrates the schematic of quater- Publisher’s Note: MDPI stays neutral nization of N-methyldiethanolamine (tertiary amine) with benzyl chloride (alkyl halide) to with regard to jurisdictional claims in produce benzylbis(2-hydroxyethyl)methylammonium chloride (QAC). The quaternization published maps and institutional affil- of tertiary amines is commonly carried out in polar solvents such as methanol [3]. The iations. other alkylating agents, for example, benzylic, allylic, and α-carbonylated alkyl halides can also be employed in the synthesis of QACs. On top of that, the quaternization of tertiary amines has been used to produce a wide variety of QACs. The two most important QACs that were employed in various applications are surfactants and ionic liquids. In addition, Copyright: © 2021 by the authors. these compounds are able to utilize for processing polymeric materials, as well as for Licensee MDPI, Basel, Switzerland. enhancing the physicochemical properties of the materials [4]. QACs can also be applied This article is an open access article as compatibilizers for polymer blends and polymer composites due to their capability to distributed under the terms and improve the compatibility of the materials. conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Appl. Sci. 2021, 11, 3167. https://doi.org/10.3390/app11073167 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3167 2 of 16 Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 16

Cl Cl N + N HO OH HO OH

Figure 1. FigureSchematic 1. Schematic of the quaternization of the quaternization of N-methyldiethanolamine of N‐methyldiethanolamine with benzyl with benzyl chloride chloride to produce to pro‐ benzylbis(2- hydroxyethyl)methylammoniumduce benzylbis(2‐hydroxyethyl)methylammonium chloride. chloride.

In general, polymerIn general, blends polymer are combinations blends are of combinations two or more of polymers, two or more whereas polymers, pol‐ whereas poly- ymer compositesmer are composites compound are made compound up of polymer made up matrix of polymer and filler matrix components. and filler The components. The preparation of preparationpolymer blends of polymer and polymer blends composites and polymer is typically composites intended is typically to obtain intended to obtain materials that havematerials a combination that have of a combination excellent properties of excellent of the properties different of materials. the different This materials. This approach is commonlyapproach inexpensive is commonly and inexpensive less time‐consuming and less time-consuming than the development than the of development of new polymericnew materials. polymeric They materials. are classified They as are compatible classified and as compatible incompatible and materials. incompatible materials. Compatible polymerCompatible blends polymer and polymer blends composites and polymer have composites superior thermo have superior‐mechanical thermo-mechanical properties (e.g.,properties degradation (e.g., temperature, degradation glass temperature, transition glasstemperature, transition melting temperature, tempera melting‐ tempera- ture, tensile strength,ture, tensile flexural strength, strength, flexural impact strength, strength, impact etc.,), while strength, incompatible etc.), while matters incompatible matters commonly havecommonly poor thermo have‐mechanical poor thermo-mechanical properties. The properties. compatibilization The compatibilization is usually is usually done on the incompatibledone on the polymer incompatible blends polymer and polymer blends composites and polymer because composites they have because a they have a difference in polaritydifference that inaffected polarity their that thermo affected‐mechanical their thermo-mechanical properties. Moreover, properties. the use Moreover, the use of compatibilizersof compatibilizers in polymer blends in polymer and polymer blends composites and polymer is compositesto improve istheir to improve interfa‐ their interfacial cial adhesion [5,6]adhesion by creating [5,6] by the creating intermolecular the intermolecular interactions interactions between the between blend and the blendcom‐ and composite posite componentscomponents [7,8]. Table [7,8 ].1 Tableshows1 showsthe examples the examples of polymer of polymer blends blends compatibilized compatibilized by QACs. by QACs. Table 1. Examples of polymer blends compatibilized by quaternary ammonium compounds (QACs). Table 1. Examples of polymer blends compatibilized by quaternary ammonium compounds (QACs). Polymer Blend Abbreviation References -6/liquid Nylon-6/LNR [9] PolymerNatural Blend wool/cellulose Abbreviation NW/Cellulose References [10] NylonPolyethylene‐6/liquid natural terephtalate/polyethylene rubber Nylon‐6/LNR PET/PE [9] [11] NaturalPolybutylene wool/cellulose succinate/rice starchNW/Cellulose PBS/RS [10] [12] Poly(ethylene oxide)/chitosan PEO/Chitosan [13] Polyethylene terephtalate/polyethylene PET/PE [11] Agar/rice starch Agar/RS [14] PolybutylenePoly(3-hydroxybutyrate-co-3- succinate/rice starch PBS/RS [12] PHBV/Cellulose [15] Poly(ethylenehydroxyvalerate)/cellulose oxide)/chitosan PEO/Chitosan [13] Agar/rice starch Agar/RS [14] Poly(3‐hydroxybutyrateTable2‐ coindicates‐3‐hydroxyvalerate)/cellulose the examples of polymer PHBV/Cellulose composites compatibilized [15] by QACs. The selection of the polymer blends and polymer composites in this concise review is not Table 2 indicatesrandomly the basedexamples on the of polymer materials composites compatibilized compatibilized by QACs, butby QACs. it is based The on the thermo- selection of the mechanicalpolymer blends properties and polymer that were composites comprehensively in this concise studied review in the is previous not ran‐ research works. domly based on theIn materials the past tencompatibilized years, many by compatibilization QACs, but it is approachesbased on the have thermo been‐ recommended mechanical propertiesfor the intentionthat were ofcomprehensively enhancing the thermo-mechanicalstudied in the previous properties research of works. polymer blends and polymer composites. The application of QACs as compatibilizers could provide an advan- tage because of their unique chemical structure that owns both polar ionic and non-polar lipophilic functional groups [26]. These groups are able to interact with polar hydrophilic and non-polar hydrophobic polymers or materials. The intermolecular interactions could contribute to the compatibilization effect on polymer blends and polymer composites, and consequently improve the interfacial adhesion between their components. Yet, to the au- thors’ knowledge, there is no concise review prepared concentrating on the application of QACs as compatibilizers for polymer blends and polymer composites. That is the objective of making a categorized review in this paper, which consists of related research works.

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Table 2. Examples of polymer composites compatibilized by quaternary ammonium com- pounds (QACs).

Polymer Composite Abbreviation References High-/low-density polyethylene/cellulose HDPE/LDPE/Cellulose [16] Carboxylated styrene butadiene rubber/rice bran carbon XSBR/RBC [17] Poly(Bisphenol-A-co-epichlorohydrin)/boron nitride PBE/BN [18] Agarose/talc Agarose/Tc [19] Styrene-butadiene rubber/bentonite SBR/Bentonite [20] Diglycidyl ether of bisphenol A epoxy/chitin DGEBA epoxy/Chitin [21] High-density polyethylene/agar HDPE/Agar [22] Polyurethane/silica PU/SiO2 [23] Nylon-6/LNR/montmorillonite Nylon-6/LNR/MMT [24] Regenerated cellulose/montmorillonite RC/MMT [25]

2. Polymer Blend and Polymer Composite Materials 2.1. Polymer Blend Materials The polymer blend materials can be prepared either in the melt, solution, or condition. The blending process involves at least two different polymers. The melt blending process must be carried out at elevated temperatures, precisely at the fusing temperature of the polymer components [27]. The solution blending process can be conducted in appropriate solvents that are capable to dissolve polymers completely [14]. The emulsion blending process involves dispersion of emulsion of at least two different polymers [28]. The compatible polymer blends have remarkable thermo-mechanical properties based on the complementary behavior of their individual components [29]. The preparation of polymer blend materials is straightforward when conventional blending machines are used [30]. The polymer blends can be made either from synthetic polymers or from natural biodegradable polymers by blending between the synthetic/synthetic polymers, natural/natural polymers, or synthetic/natural polymers. Table3 shows the examples of polymer blends, blend types, blending conditions, and blending machines. Synthetic thermoplastic polymer blends can be prepared via melt, solution and emulsion blending processes [31]. The chemical structure of the main chains of thermoplastic polymers has allowed them to be processed in such conditions, including the nature of the intermolecular interaction of the polymers. Nevertheless, for natural biodegradable polymer (biopolymer) blends, they are usually prepared through solution blending process; this is because most of them do not melt but decompose at elevated temperatures [14]. The chemical nature of biopolymers has also allowed them to be processed in such condition because they were bonded with hydrogen bonding. Figure2 exhibits the chemical structures of PET, PBS, PEO, and PHBV. The melt blending process can be done by using compounder or extruder. In contrast, solution and emulsion blending processes can be performed by means of mechanical stirrer, magnetic stirrer, or KPG stirrer.

Table 3. Examples of polymer blends, blend types, blending conditions, and blending machines.

Polymer Blend Blend Type Blending Condition Blending Machine References PET/PE Synthetic/synthetic Melt Twin-screw compounder [11] Agar/RS Natural/natural Solution Magnetic stirrer [14] Chitin/Cellulose Natural/natural Solution KPG stirrer [32] PBS/RS Synthetic/natural Melt Twin-screw extruder [12] PEO/Chitosan Synthetic/natural Solution Mechanical stirrer [13] PHBV/Cellulose Synthetic/natural Solution Magnetic stirrer [15] Nylon-6/LNR Synthetic/natural Emulsion Mechanical stirrer [9] Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 16

Table 3. Examples of polymer blends, blend types, blending conditions, and blending machines.

Blending Condi‐ Refer‐ Polymer Blend Blend Type Blending Machine tion ences Synthetic/syn‐ Twin‐screw com‐ PET/PE Melt [11] thetic pounder Agar/RS Natural/natural Solution Magnetic stirrer [14] Chitin/Cellu‐ Natural/natural Solution KPG stirrer [32] lose PBS/RS Synthetic/natural Melt Twin‐screw extruder [12] PEO/Chitosan Synthetic/natural Solution Mechanical stirrer [13] PHBV/Cellu‐ Appl. Sci. 2021, 11, 3167 Synthetic/natural Solution Magnetic stirrer [15]4 of 16 lose Nylon‐6/LNR Synthetic/natural Emulsion Mechanical stirrer [9]

PET OH

O O

H O O n PBS O

O PEO O O H OH O n n PHBV O O

O O m n FigureFigure 2. 2. ChemicalChemical structures structures ofof thermoplasticthermoplastic polymers polymers and and the the abbreviations abbreviations are are explained explained in Table in 1. Table 1. 2.2. Polymer Composite Materials 2.2. PolymerThe polymer Composite composite Materials materials are typically prepared by incorporating a filler into the polymerThe polymer matrix. composite The process materials could are be done typically either prepared in the melt, by incorporating solution, or emulsion a filler into con- thedition polymer [24,33 ,matrix.34]. The The polymer process composites could be regularlydone either possess in the improved melt, solution, thermo-mechanical or emulsion conditionproperties [24,33,34]. compared The to their polymer polymer composites matrix [ 35regularly]. The polymer possess compositeimproved materialsthermo‐me are‐ chanicaleasy to prepare, properties and compared they also to need their processing polymer matrix machines [35]. such The aspolymer polymer composite blends. Table mate4‐ rialsdisplays are easy the examplesto prepare, of and polymer they also matrices, need matrixprocessing types, machines fillers, processing such as polymer conditions, blends. and Tableprocessing 4 displays machines. the examples Thermoplastic of polymer polymer matrices, composites matrix cantypes, be preparedfillers, processing through melt,con‐ ditions,solution, and and processing emulsion processing.machines. Thermoplastic However, for thermosetting polymer composites polymer can composites, be prepared they throughare generally melt, preparedsolution, throughand emulsion solution processing. processing However, before curing for thermosetting because they cannotpolymer be composites,melted, emulsified, they are and generally reshaped prepared after curing through [36]. solution The chemical processing nature before of thermosetting curing be‐ causepolymers they hascannot allowed be melted, them to emulsified, be processed and in reshaped such condition after curing because [36]. they The are chemical bonded naturewith irreversible of thermosetting covalent polymers bonds. Figure has allowed3 displays them the chemicalto be processed structures in ofsuch PBE, condition DGEBA becauseepoxy, andthey PU. are Onbonded the other with irreversible hand, biopolymer covalent composites bonds. Figure are ordinarily3 displays the prepared chemical via structuressolution processing of PBE, DGEBA [37]. Figure epoxy,4 demonstratesand PU. On the the other chemical hand, structures biopolymer of cellulose, composites chitin, are ordinarilychitosan, andprepared agarose. via The solution melt processing processing can [37]. be madeFigure by 4 meansdemonstrates of internal the mixer, chemical two- structuresroll mill, or of extruder. cellulose, In chitin, contrast, chitosan, the solution and agarose. and emulsion The melt processing processing can can be be operated made by by meansusing mechanicalof internal mixer, stirrer, two overhead‐roll mill, stirrer, or extruder. sonicator, In or contrast, magnetic the stirrer. solution and emulsion

Table 4. Examples of polymer matrices, matrix types, fillers, processing conditions, and process- ing machines.

Polymer Matrix Matrix Type Filler Processing Condition Processing Machine References HDPE/LDPE Thermoplastic Cellulose Melt Internal mixer [16] PE Thermoplastic Sawdust Melt Single-screw extruder [38] PBE Thermosetting BN Solution Mechanical stirrer [18] DGEBA epoxy Thermosetting Chitin Solution Mechanical stirrer [21] PU Thermosetting SiO2 Solution Overhead stirrer [23] Cellulose Biopolymer Chitosan Solution Sonicator [39] RC Biopolymer MMT Solution Magnetic stirrer [25] Agarose Biopolymer Tc Solution Magnetic stirrer [19] SBR Bentonite Melt Two-roll mill [20] Nylon-6/LNR Blend MMT Emulsion Mechanical stirrer [24] Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 16

processing can be operated by using mechanical stirrer, overhead stirrer, sonicator, or magnetic stirrer.

Table 4. Examples of polymer matrices, matrix types, fillers, processing conditions, and pro‐ cessing machines.

Polymer Matrix Matrix Type Filler Processing Condition Processing Machine References HDPE/LDPE Thermoplastic Cellulose Melt Internal mixer [16] PE Thermoplastic Sawdust Melt Single‐screw extruder [38] PBE Thermosetting BN Solution Mechanical stirrer [18] DGEBA epoxy Thermosetting Chitin Solution Mechanical stirrer [21] PU Thermosetting SiO2 Solution Overhead stirrer [23] Cellulose Biopolymer Chitosan Solution Sonicator [39] RC Biopolymer MMT Solution Magnetic stirrer [25] Agarose Biopolymer Tc Solution Magnetic stirrer [19] Appl. Sci. 2021, 11, 3167 SBR Elastomer Bentonite Melt Two‐roll mill [20]5 of 16 Nylon‐6/LNR Blend MMT Emulsion Mechanical stirrer [24]

Figure 3. Chemical structures of thermosetting polymers and the abbreviations are explained in Appl. Sci. 2021, 11, x FOR PEER REVIEWFigure 3. Chemical structures of thermosetting polymers and the abbreviations are explained6 of 16 in Table 2. Table2.

Chitin COCH3 Cellulose CH OH OH 2 CH2OH NH H O HO H O HO HO O O HO O HO O OH O O OH CH OH 2 NH CH2OH n n COCH3 (a) (b) Chitosan Agarose OH CH OH O CH2OH H3COCHN 2 O HO H O HO O O HO O O OH O H O HO NH CH OH OH 3 m 2 n n X (c) (d) FigureFigure 4. 4. ChemicalChemical structures structures of of ( (aa)) cellulose, cellulose, ( (bb)) chitin, chitin, ( (cc)) chitosan, chitosan, and and ( (dd)) agarose. agarose.

3.3. Types Types of of QACs QACs for for Compatibilization Compatibilization of of Polymer Polymer Blends Blends and and Polymer Polymer Composites Composites 3.1.3.1. Quaternary Quaternary Ammonium Ammonium Surfactants Surfactants (QASs) (QASs) InIn recent recent times, times, QACs QACs have have been been categorized categorized into into two two types, types, specifically specifically quaternary quaternary ammoniumammonium surfactants surfactants (QASs) (QASs) and and quaternary quaternary ammonium ammonium ionic ionic liquids liquids (QAILs). (QAILs). The cat The‐ egorycategory is based is based on onthe the state state of ofthe the QACs; QACs; for for instance, instance, QASs QASs are are usually usually present present in in solid solid-‐ statestate and and QAILs QAILs commonly commonly exist exist in in a a liquid liquid form. form. QASs QASs possess possess an an amphiphilic amphiphilic character, character, whichwhich contains contains both both a polar a polar functional functional group group and and a non a‐ non-polarpolar functional functional group group [40]. Fur [40‐]. thermore,Furthermore, QASs QASs could could reduce reduce the surface the surface tension tension between between the polar the and polar non and‐polar non-polar mate‐ rialsmaterials [41], including [41], including components components of the of polymer the polymer blends blends and polymer and polymer composites. composites. On top On oftop that, of that,QASs QASs have havebeen beenshown shown to function to function in many in many vital physical vital physical processes, processes, serving serving as an emulsifyingas an emulsifying agent, agent,wetting wetting agent, agent,dispersing dispersing agent, lubricating agent, lubricating agent, softening agent, softening agent, foaming agent, an anti‐foaming agent, and an antimicrobial agent [42–44]. Table 5 shows the examples of QASs applied for compatibilization of polymer blends and polymer com‐ posites. Figure 5 demonstrates the chemical structures of QASs collected in Table 5.

Table 5. Examples of quaternary ammonium surfactants (QASs) applied for compatibilization of polymer blends and polymer composites.

QASs Abbreviation Compatibilization References Dodecyltrimethylammonium bromide DTAB Blend/Composite [13,18,45] N‐isopropyl‐N, N‐dimethyldodecan‐1‐aminium IDAB Composite [20] bromide Benzylbis(2‐hydroxyethyl)dodecylammonium BDAC Blend [11] chloride Tetradecyltrimethylammonium bromide TTAB Composite [46] [9,22,24,47,48 Hexadecyltrimethylammonium bromide HTAB Blend/Composite ] N‐isopropyl‐N, N‐dimethylhexadecan‐1‐ IHAB Composite [20] aminium bromide Benzyldimethylhexadecylammonium chloride BHAC Blend [11] Hexadecylpyridinium chloride HPC Blend [49] Octadecyltrimethyl ammonium bromide OTAB Composite [18] Stearyltrimethylammonium chloride STAC Composite [50]

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agent, foaming agent, an anti-foaming agent, and an antimicrobial agent [42–44]. Table5 shows the examples of QASs applied for compatibilization of polymer blends and polymer composites. Figure5 demonstrates the chemical structures of QASs collected in Table5.

Table 5. Examples of quaternary ammonium surfactants (QASs) applied for compatibilization of polymer blends and polymer composites.

QASs Abbreviation Compatibilization References Dodecyltrimethylammonium bromide DTAB Blend/Composite [13,18,45] N-isopropyl-N, N-dimethyldodecan-1-aminium bromide IDAB Composite [20] Benzylbis(2-hydroxyethyl)dodecylammonium chloride BDAC Blend [11] Tetradecyltrimethylammonium bromide TTAB Composite [46] Hexadecyltrimethylammonium bromide HTAB Blend/Composite [9,22,24,47,48] N-isopropyl-N, N-dimethylhexadecan-1-aminium bromide IHAB Composite [20] Benzyldimethylhexadecylammonium chloride BHAC Blend [11] Hexadecylpyridinium chloride HPC Blend [49] Octadecyltrimethyl ammonium bromide OTAB Composite [18] Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 16 Stearyltrimethylammonium chloride STAC Composite [50]

BDAC

Cl‐ DTAB IDAB TTAB HTAB N+ N+ Br‐ N+ Br‐ C12H25 N+ Br‐ N+ Br‐ OH

C12H25 C12H25 HO C14H29 C16H33 BHAC

IHAB HPC OTAB STAC

‐ + ‐ Cl + N Br‐ N+ Cl N+ Br‐ N Cl‐ N+

C16H33 C16H33 C16H33 C18H37 C18H37 Figure 5. Chemical structures of QASs and the abbreviations are explained inin TableTable5 5..

3.2. Quaternary Ammonium Ionic Liquids (QAILs) Unlike QASs, QASs, QAILs QAILs have have a alow low melting melting temperature temperature (<100 (<100 °C).◦ QAILsC). QAILs are also are non also‐ volatile,non-volatile, non‐flammable, non-flammable, exhibit exhibit good good thermal thermal stability, stability, and andable ableto dissolve to dissolve organic organic and inorganicand inorganic materials materials [51]. In [51 the]. In past the decade, past decade, QAILs QAILs have been have utilized been utilized in adsorption/de in adsorp-‐ sorptiontion/desorption studies [52], studies catalysis [52], catalysis systems, systems, electrochemical electrochemical studies, metal studies, nanostructures, metal nanostruc- an‐ alyticaltures, analytical chemistry chemistry including including sensors, bio sensors,‐analytical bio-analytical chemistry, chemistry,electrochemical electrochemical biosensors [53], and CO2 capture systems [54]. The exceptional properties of QAILs, such as good biosensors [53], and CO2 capture systems [54]. The exceptional properties of QAILs, such interactionas good interaction with organic with organicand inorganic and inorganic materials, materials, make them make suitable them suitable chemical chemical com‐ compoundspounds for applying for applying in the in polymer the polymer blends blends and polymer and polymer composites composites [55]. Table [55]. Table 6 exhibits6 ex- thehibits examples the examples of QAILs of QAILs applied applied for compatibilization for compatibilization of polymer of polymer blends blends and polymer and polymer com‐ posites.composites. Figure Figure 6 demonstrates6 demonstrates the thechemical chemical structures structures of QAILs of QAILs collected collected in Table in Table 6. 6.

Table 6. Examples of quaternary ammonium ionic liquids (QAILs) applied for compatibilization of polymer blends and polymer composites.

QAILs Abbreviation Compatibilization References 1‐Ethyl‐3‐methylimidazolium acetate EmimAc Blend/Composite [21,56–58] 1‐Ethyl‐3‐methylimidazolium propionate EmimPr Blend [32] 1‐Ethyl‐3‐methylimidazolium trifluoro‐ EmimOTf Composite [59] methanesulfonate 1‐Allyl‐3‐methylimidazolium bromide AmimBr Composite [60] 1‐Allyl‐3‐methylimidazolium chloride AmimCl Blend [61] 1‐Butyl‐3‐methylimidazolium acetate BmimAc Composite [62] [14,19,25,39,63 1‐Butyl‐3‐methylimidazolium chloride BmimCl Blend/Composite ] 1‐Butyl‐3‐methylimidazolium hexafluorophos‐ BmimPF6 Composite [23] phate 1‐Hexyl‐3‐methylimidazolium hexafluoro‐ HmimPF6 Composite [17] phosphate 1‐Dodecyl‐3‐methylimidazolium bis(trifluoro‐ DmimNTf2 Blend [12] methylsulfonyl)imide

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Table 6. Examples of quaternary ammonium ionic liquids (QAILs) applied for compatibilization of polymer blends and polymer composites.

QAILs Abbreviation Compatibilization References 1-Ethyl-3-methylimidazolium acetate EmimAc Blend/Composite [21,56–58] 1-Ethyl-3-methylimidazolium propionate EmimPr Blend [32] 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate EmimOTf Composite [59] 1-Allyl-3-methylimidazolium bromide AmimBr Composite [60] 1-Allyl-3-methylimidazolium chloride AmimCl Blend [61] 1-Butyl-3-methylimidazolium acetate BmimAc Composite [62] 1-Butyl-3-methylimidazolium chloride BmimCl Blend/Composite [14,19,25,39,63] 1-Butyl-3-methylimidazolium hexafluorophosphate BmimPF6 Composite [23] 1-Hexyl-3-methylimidazolium hexafluorophosphate HmimPF6 Composite [17] 1-Dodecyl-3-methylimidazolium Appl. Sci. 2021, 11, x FOR PEER REVIEW DmimNTf2 Blend [128 ] of 16 bis(trifluoromethylsulfonyl)imide

EmimAc EmimPr O O

N+ N+ ‐ O‐ O N N

EmimOTf F O AmimBr AmimCl ‐ + ‐ Cl N ‐ Br F S O N+ N+ N N N F O BmimPF BmimAc BmimCl 6 F O ‐ Cl F ‐ F O ‐ + + N+ P N N F N N N F F HmimPF 6 F F F N+ P‐ N F F F DmimNTf2 O O N F F S S N+ N O F O F F F Figure 6. Chemical structures of QAILs and the abbreviations are explained in Table 6. Figure 6. Chemical structures of QAILs and the abbreviations are explained in Table6. 4. Influence of QACs on Thermo‐Mechanical Properties of Polymer Blends and Poly‐ 4.mer Influence Composites of QACs on Thermo-Mechanical Properties of Polymer Blends and Polymer Composites 4.1.4.1. InfluenceInfluence of QASs and QAILs on on Polymer Polymer Blends Blends Table7 7 indicatesindicates the thermo thermo-mechanical‐mechanical properties properties of of polymer polymer blends blends influenced influenced by byQASs QASs and and QAILs. QAILs. QAS like QAS HTAB like HTAB(chemical (chemical structure structure is shown is in shown Figure in 5) Figurehas been5) ap has‐ beenplied applied for compatibilization for compatibilization of Nylon of‐6/LNR Nylon-6/LNR blends [9]. blends The [application9]. The application of HTAB of influ HTAB‐ influencedenced the degradation the degradation temperature, temperature, glass transition temperature, temperature, tensile strength, tensile strength, tensile tensilemodulus, modulus, tensile extension, tensile extension, and impact and strength impact strengthof the blends. of the The blends. degradation The degradation tempera‐ temperatureture of the blends of the decreased blends decreased because LNR because possessed LNR possessed lower initial lower degradation initial degradation tempera‐ temperatureture than nylon than‐6, nylon-6, which deteriorated which deteriorated the degradation the degradation temperature temperature of the overall of the blends. overall blends.The result The also result shows also a showsdecreasing a decreasing trend with trend the withblends the containing blends containing high LNR high content, LNR content,which is which also due is also to the due low to degradation the low degradation temperature temperature of LNR compared of LNR compared to nylon to‐6. nylon- Fur‐ 6.thermore, Furthermore, the glass the transition glass transition temperature temperature of the blends of the also blends decreased also decreasedbecause of the because im‐ ofprovement the improvement of the compatibility of the compatibility between nylon between‐6 and nylon-6LNR. This and is caused LNR. Thisby HTAB, is caused which by HTAB,consisted which of polar consisted and non of‐ polarpolar andgroups non-polar that acted groups as a compatibilizer that acted as a by compatibilizer creating inter by‐ creatingactions between interactions them between [9]. Moreover, them [the9]. mechanical Moreover, properties, the mechanical such properties,as tensile strength such as tensileand tensile strength modulus and tensileof the blends modulus decreased of the blends because decreased of the increase because of of their the segmental increase of movement. It is considered that LNR acted as a stress dilutor, which is responsible for lowering the tensile strength and the tensile modulus. This is induced by the presence of HTAB that is responsible for better homogeneous distribution of LNR in the blends. How‐ ever, the tensile extension and impact strength of the blends increased by up to 100% and 35%, respectively, compared to the neat nylon‐6. This is attributed to the HTAB‐compati‐ bilized Nylon‐6/LNR blends, which eventually increased the elongation and toughness of the blends [9]. It can be seen in Table 7 that fewer studies have focused on the influence of the QASs on the thermo‐mechanical properties of polymer blends. QAS like BDAC (chemical struc‐ ture is shown in Figure 5) is applied for compatibilization of PET/PE blends [11]. The ap‐ plication of BDAC influenced the degradation temperature, tensile strength, tensile mod‐ ulus, and tensile extension of the blends. The degradation temperature of the blends de‐ creased because of the presence of the benzyl group and aromatic structure in BDAC that

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the compatibility between the blend components; this can take place when they have in- their segmental movement.termolecular It is considered interactions. that Nonetheless, LNR acted as the a stress mechanical dilutor, properties, which is for instance, tensile responsiblethe compatibility for loweringstrength, between the tensile tensilethe strengthblend modulus, components and theand tensile tensile; this can modulus. extension take place This of thewhen is inducedblends they increasedhave by in- by up to 429%, the presencetermolecular of HTAB interactions.190%, that is and responsible Nonetheless, 44%, respectively, for better the mechanical homogeneousin comparison properties, distribution with the for PHBV. instance, of LNR This intensile is because of the pres- the blends.strength, However, tensile themodulus,ence tensile of interaction extensionand tensile between and extension impact PHBV strengthof andthe cellulosblends of the increasede, blends which increased improved by up to by the429%, compatibility of the up to190%, 100% and 35%,44%, respectively,respectively,blends [15]. comparedin comparison to the with neat the nylon-6. PHBV. This This is is attributed because of to the pres- HTAB-compatibilizedence of interaction Nylon-6/LNR between PHBV blends, and cellulos whiche, eventually which improved increased the thecompatibility elongation of the and toughnessblends [15]. of the blendsTable 7. [9 Thermo-mechanical]. properties of polymer blends influenced by QASs and QAILs.

Thermo-Mechanical Properties * TableTable 7. Thermo-mechanical 7. Thermo-mechanicalPolymer properties properties Blend of polymer of polymer QASs/QAILs blends blends influenced influenced by QASs by andQASs QAILs. and QAILs. References Td Tg Tm TS TM TE FS FE IS Nylon-6/LNR Thermo-MechanicalHTABThermo-Mechanical ⇩ Properties ⇩ Properties - * ⇩ * ⇩ ⇧ - - ⇧ [9] Polymer Blend QASs/QAILs References Polymer Blend QASs/QAILs T T T TS TM TE FS FE IS References PET/PE Td TgBDACg Tmm TS TM⇩ - TE - FS ⇧ FE ⇧IS ⇩ - - - [11] Nylon-6/LNRNylon-6/LNR NW/Cellulose HTABHTAB ⇩ ⇩BmimCl - - ⇩ ⇧ ⇩ ⇧ ⇧ --- - ⇧ - ⇧ ⇧ ⇧[9 ] [9] - - - [10] PET/PEPET/PE BDACPBS/RSBDAC ⇩ DmimNTf --- - 2 ⇧ ⇧ ⇧ - ⇩ ⇩---[ - ⇩ - ⇩ - ⇧11 ] [11] ⇩ ⇧ - [12] NW/CelluloseNW/Cellulose PHBV/Cellulose BmimClBmimCl ⇧ ⇧BmimCl - - ⇧ ⇧ ⇧ ⇧ ⇧ ⇩---[ - ⇧ - ⇧ - ⇧10 ] [10] - - - [15]

PBS/RSPBS/RS Td = degradation DmimNTfDmimNTf2 2temperature, ⇧ -- ⇩T g = glass ⇩ transition ⇩ ⇧ temperature, ⇩ ⇧ -[ - Tm12 =] melting[12] temperature, TS = PHBV/CellulosePHBV/Cellulosetensile BmimCl strength,BmimCl TM = ⇧tensile ⇧ modulus, ⇩ ⇧ TE ⇧ = tensile ⇧ ---[ extension, - - -FS =15 flexural] [15] strength, FE = flexural Td = degradation temperature,extension, and Tg =IS glass = impact transition strength. temperature, * The symbol Tm = “melting⇧” corresponds temperature, to an TS incr =ease in the properties Td = degradation temperature, Tg = glass transition temperature, Tm = melting temperature, TS = tensile strength, TM = tensiletensile modulus, strength,TE TM=and tensile = tensile “⇩” extension, a decreasemodulus,FS in= TE flexuralthe = propertiestensile strength, extension, whileFE = flexural“-” FS describes = flexural extension, not strength, andavailable.IS = FE impact = flexural extension, and IS = impact strength. * The symbol “⇧” corresponds to an increase in the properties strength. * The symbol “ ”4.2. corresponds Influence to of an QASs increase on in Polymer the properties Composites and “ ” a decrease in the properties while “-”and describes “⇩” a decrease not available. in the properties while “-” describes not available. Table 8 shows the thermo-mechanical properties of polymer composites influenced 4.2. Influence of QASs on Polymer Composites It can be seen inby Table QASs.7 that QAS fewer like studiesHTAB has have been focused applied on for the compatibilization influence of the of HDPE/LDPE/Cellu- QASs on theTable thermo-mechanical 8 showslose the composites thermo-mechanical properties [16]. ofThe polymer applicationproper blends.ties ofof polymer QASHTAB like has composites BDAC influenced (chemical influenced the degradation tempera- structureby QASs. is shown QAS in liketure, Figure HTAB melting5) ishas applied temperature,been applied for compatibilization tensile for compatibilization strength, of tensile PET/PE of modulus, HDPE/LDPE/Cellu- blends tensile [ 11]. extension, and impact The applicationlose composites of BDAC [16].strength influenced The ofapplication the the composites. degradation of HT ABThe temperature,has degradation influencedtensile temperature the degradation strength, of the tensile tempera- composites decreased be- modulus,ture, andmelting tensile temperature, extensioncause of the oftensile thelower blends. strength, initial The degradation tensile degradation modulus, temper temperature tensileature extension,of HTAB of the blendsthan and otherimpact composite compo- decreasedstrength because of the of composites. thenents, presence which The ofaffected degradation the benzyl the degradationgroup temperature and aromatic temper of the aturecomposites structure of the in decreasedoverall BDAC composites. be- Neverthe- that loweredcause of the lower thermalless, initial stabilitythe degradation melting of the temperature blends.temperature Therefore, of the of HTAB composit BDAC thanes acted otherremains composite as aunchanged thermal compo- because of the pres- degradationnents, which promoter affectedence for the ofthe crystal blends, degradation lattice which oftemper can HTAB catalyzeature in the of the compthe polymer overallosites degradation composites.[16]. Moreover, withNeverthe- the mechanical proper- a thermalless, the dependence. melting ties,temperature This such decreasing as tensileof the trendcomposit strength also esand occurred remains tensile inunchanged mo thedulus PP/Nylon-6 of because the composites blendsof the pres- increased by up to compatibilizedence of crystal by PP-g-MA lattice25% ofand [HTAB64 25%,]. However, inrespectively, the comp theosites mechanicalcompared [16]. Moreover, to properties, the neat the HDPE/LDPE/Cellulose such mechanical as tensile proper- composite. This strengthties, and such tensile as tensile modulusis attributed strength of the andto blends the tensile existence increased modulus of byHTAB, upof the to whic 14%compositesh and enhanced 24%, increased respectively, the interfacial by up to adhesion between compared25% and to the 25%, neat respectively, PET/PEHDPE/LDPE blend. compared matrix This is attributedandto the cellulose neat to HDPE/LDPE/Cellulose the filler, vital as role a ofresult, the BDAC, improved composite. which the This stress transfer from actedis as attributed a compatibilization to theHDPE/LDPE existence agent of betweento HTAB, cellulose. whic PET However,h and enhanced PE [the11 ].the tensile Nevertheless, interfacial extension adhesion the and tensile impact between strength of the com- extensionHDPE/LDPE of the blends matrixposites decreased and decreased cellulose because causedfiller, of their as by a lower theresult, excess ductility improved of the compared crystal the stress lattice to the transfer blendof HTAB from in the composites, withoutHDPE/LDPE BDAC [11 ].to Moreover,cellulose.which increased However, the polymer their the brittleness chainstensile ofextension PET/PE [16]. This and blends is impact related containing strength to the increase BDAC of the com- of the stiffness prop- are stifferposites in comparisondecreasederty caused with of the the by composites. neat the excess blend that ofIt isthe reduced a crystalwell-known the lattice ductility fact of HTABthat and the deformabilityin crystal the composites, lattice with higher content as well.which This increased effectively theirin the decreased brittleness matrix can the [16]. decrease tensile This is extensionthe related tensile toof extensio the the increase BDAC-compatibilizedn and of impact the stiffness strength prop- of the composites as PET/PEerty blends. of the composites.a rule ofIt ismixture. a well-known fact that the crystal lattice with higher content Onin the the matrix other can hand, decrease QAILQAS the like tensile OTAB BmimCl extensio (chemical (chemicaln and structure structure impact is strength shown is shown in of Figure the in Figurecomposites 5) has6) been as applied for com- has beena rule applied of mixture. for compatibilizationpatibilization of PBE/BN of NW/Cellulose composites blends [18]. [The10]. Theapplication application of OTAB of has influenced the BmimCl influencedQAS like OTAB thethermal degradation (chemical conductivity, structure temperature, flexural is shownglass strength, in transition Figure flexural 5) temperature,has modulus, been applied and tensile impact for com- strength of the com- strength,patibilization tensile modulus, of positesPBE/BN and (Table tensilecomposites 8). extension The [18]. thermal ofThe the applicationconductivity blends (Table of 7 ofOTAB). the The composites thermalhas influenced prop- increased the by up to 79%, erties,thermal such as conductivity, degradationcompared flexural temperature to strength,the neat and PBE/BNflexural glass transition modulus,composite. temperature and This impact is attributed strength of the blendsto of the the improvementcom- of inter- increasedposites by up(Table to 25% 8).facial andThe 33%,adhesionthermal respectively, conductivity between compared PBE of matrix the to composites theand neat BN fi NW.ller, increased This which is attributedinducedby up to heat 79%, transfer across the to thecompared miscibility to between the interfaceneat NW PBE/BN and efficiently cellulose composite. [18]. as wellMoreThis asisover, theattributed formationthe mechanical to the of strongimprovement properties, interaction suchof inter- as flexural strength, betweenfacial these adhesion biopolymers betweenflexural generated PBE modulus, matrix by and and BmimCl impactBN filler, [10 strength]. which Moreover, ofinduced the the composites heat mechanical transfer increased prop- across bythe up to 38%, 8.5%, erties,interface for instance, efficiently tensileand [18]. 11%, strength, More respectively,over, tensile the modulus, inmechanical comparison and properties, tensile with the extension neatsuch composite.as of flexural the blends Thisstrength, is because of the long increased by up to 66%, 100%, and 56%, respectively, in comparison with the NW. This is flexural modulus,alkyl and chainimpact length strength of OTAB, of the which composites effectively increased adsorbed by up on to the 38%, surface 8.5%, of the composites due to the increase of the cellulose content in the blends and also good interfacial adhesion and 11%, respectively,[18]. Therefore, in comparison the compatibilization with the neat composite. by applying This isOTAB because not of only the increasedlong the thermal between them [10]. alkyl chain lengthconductivity of OTAB, which of the effectively composites adsorbed but also onimprov the surfaceed their of flexural the composites and impact properties as QAIL like DmimNTf (chemical structure is shown in Figure6) has been applied for [18]. Therefore, thewell. compatibilization2 by applying OTAB not only increased the thermal compatibilizationconductivity of of PBS/RS the composites blends [ 12but]. Thealso applicationimproved their of DmimNTf flexural 2andhas impact influenced properties the as degradation temperature, melting temperature, tensile strength, tensile modulus, tensile well.

Appl. Sci. 2021, 11, 3167 9 of 16

extension, flexural strength, and flexural extension of the blends (Table7). The degradation temperature of the blends increased by up to 2.3% compared to the neat PBS/RS blend. This is attributed to the high degradation temperature of DmimNTf2 and the existence of intermolecular interactions between the blend components generated by DmimNTf2. However, the melting temperature of the blends decreased because of the presence of DmimNTf2, which improved the interfacial adhesion between PBS and RS [12]. In the compatible polymer blends, they commonly have the melting temperature lower than their individual components [14]. Thus, it can be considered that the PBS/RS blends containing DmimNTf2 have good compatibility in comparison with the neat blend. Furthermore, the mechanical properties, such as tensile strength, tensile modulus, and flexural strength of the blends decreased since fewer loads are required for withstanding the exerted force. This is, of course, not unusual, the compatible polymer blends are not necessarily linked to the increase of the tensile and flexural strengths. Nevertheless, the tensile extension and flexural extension of the blends increased by up to 233% and 17%, respectively, in comparison with the blend without DmimNTf2. This is caused by the improvement of the compatibility between PBS and RS since DmimNTf2 acted as a compatibilizer [12]. BmimCl has also been applied for compatibilization of PHBV/Cellulose blends [15]. The application of BmimCl influenced the degradation temperature, glass transition tem- perature, melting temperature, tensile strength, tensile modulus, and tensile extension of the blends (Table7). The thermal properties, such as degradation temperature and glass transition temperature of the blends increased by up to 20% and 700%, respectively, compared to the neat PHBV. This is attributed to the partial miscibility between PHBV and cellulose in their regenerated form. However, the melting temperature of the blends decreased because of the formation of hydrogen bonding interaction between the blend components regenerated from BmimCl [15]. The lower melting temperature of the poly- mer blends than their individual components can also be considered as an indicator to show the compatibility between the blend components; this can take place when they have intermolecular interactions. Nonetheless, the mechanical properties, for instance, tensile strength, tensile modulus, and tensile extension of the blends increased by up to 429%, 190%, and 44%, respectively, in comparison with the PHBV. This is because of the presence of interaction between PHBV and cellulose, which improved the compatibility of the blends [15].

4.2. Influence of QASs on Polymer Composites Table8 shows the thermo-mechanical properties of polymer composites influenced by QASs. QAS like HTAB has been applied for compatibilization of HDPE/LDPE/Cellulose composites [16]. The application of HTAB has influenced the degradation temperature, melting temperature, tensile strength, tensile modulus, tensile extension, and impact strength of the composites. The degradation temperature of the composites decreased because of the lower initial degradation temperature of HTAB than other composite compo- nents, which affected the degradation temperature of the overall composites. Nevertheless, the melting temperature of the composites remains unchanged because of the presence of crystal lattice of HTAB in the composites [16]. Moreover, the mechanical properties, such as tensile strength and tensile modulus of the composites increased by up to 25% and 25%, respectively, compared to the neat HDPE/LDPE/Cellulose composite. This is attributed to the existence of HTAB, which enhanced the interfacial adhesion between HDPE/LDPE matrix and cellulose filler, as a result, improved the stress transfer from HDPE/LDPE to cellulose. However, the tensile extension and impact strength of the composites decreased caused by the excess of the crystal lattice of HTAB in the composites, which increased their brittleness [16]. This is related to the increase of the stiffness property of the composites. It is a well-known fact that the crystal lattice with higher content in the matrix can decrease the tensile extension and impact strength of the composites as a rule of mixture. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16

the compatibility between the blend components; this can take place when they have in- termolecular interactions. Nonetheless, the mechanical properties, for instance, tensile the compatibilitystrength, between tensile the blendmodulus, components and tensile; this extension can take ofplace the when blends they increased have in- by up to 429%, termolecular interactions.190%, and 44%, Nonetheless, respectively, the inmechanical comparison properties, with the PHBV.for instance, This is tensile because of the pres- strength, tensileence modulus, of interaction and tensile between extension PHBV andof the cellulos blendse, whichincreased improved by up theto 429%, compatibility of the Appl. Sci. 2021, 11, 3167 190%, and 44%,blends respectively, [15]. in comparison with the PHBV. This is because of the pres-10 of 16 Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 16 ence of interaction between PHBV and cellulose, which improved the compatibility of the blends [15]. Table 7. Thermo-mechanical properties of polymer blends influenced by QASs and QAILs.

Thermo-Mechanical Properties * TableTable Table7. Thermo-mechanical8. Thermo-mechanical 8. Thermo-mechanicalPolymer Blend properties properties properties QASs/QAILsof polymerof pol of polymerymer blends composites composites influenced influenced influenced by QASs by and byQASs. QASs.QAILs. References Td Tg Tm TS TM TE FS FE IS Nylon-6/LNR HTABThermo-MechanicalThermo-Mechanical Thermo-Mechanical⇩ ⇩ -Properties Properties Properties ⇩ ⇩ * * * ⇧ - - ⇧ [9] Polymer Composite QASs References PolymerPolymer Blend Composite QASs/QAILs QASs T T T k TS TM TE FS FMReferencesReferences IS PET/PE TdT BDACdT gTg g T mT m TSk ⇩TS TM - TM - TE TE ⇧FS FS FE ⇧FM IS ⇩ IS - - - [11] Nylon-6/LNRHDPE/LDPE/CelluloseHDPE/LDPE/Cellulose NW/CelluloseHTAB HTAB HTAB ⇩BmimCl ⇩ ⇩- - - ⇳ ⇩- - ⇧ ⇧ ⇩ ⇧ ⇧ - ⇧ ⇩ ⇧ - - ⇧ - -- - ⇧ ⇧ ⇩ - [9][16] - -[ 16] [10] PET/PEPBE/BNPBE/BN PBS/RSBDAC OTAB OTAB DmimNTf⇩ ------2 ⇧ ⇧ - ⇧ ---- - ⇩ ⇩ - ⇩ - ⇧ ⇩ - ⇧ - ⇧ ⇧ ⇩ [11][18] ⇧ -[ 18] [12] NW/CelluloseHDPE/AgarHDPE/Agar PHBV/Cellulose BmimCl HTABHTAB ⇧BmimCl ⇩ ⇧- - - ⇩ ⇧- - ⇧ ⇩ ⇧ ⇧ ⇩ ⇩ ⇧ ⇧ ⇧ - - ⇧ - -- - - ⇧ ⇧ - [10][22] - -[ 22] [15] PBS/RSSBR/BentoniteSBR/Bentonite Td = degradationDmimNTf IDABtemperature,IDAB2 ⇧ ⇩ - ---T - g = ⇩ glass - ⇩- transition ⇧ ⇩ ⇧ ⇧temperature, ⇩ ⇩ - ⇧ ---[ - T -m = - melting[12][20] temperature, 20] TS = PHBV/CelluloseNylon-6/LNR/MMTNylon-6/LNR/MMTtensile strength,BmimCl HTABTMHTAB = tensile ⇧ ⇧ ⇧modulus, ⇧ ⇩ --- ⇧- TE ⇧ = ⇧tensile ⇧ ⇧ extension, ⇩ - - - FS-- - -= flexural ⇧ [15] strength,[24] [24] FE = flexural Td = ddegradationextension, temperature, and TISg == g glassimpact transition strength. temperature, * The symbol T m“ ⇧=m” melting corresponds temperature, to an incr TSease = in the properties T =T degradationd = degradation temperature, temperature, T T =g glass= glass transition transition temperature, temperature, TTm == melting melting temperature, temperature, k = thermal tensilethermalconductivity, strength, conductivity,and TMTS “ =⇩ =tensile” tensile aTS decrease = modulus,tensile strength, in strength, the TMTE properties == tensiletensile TM = extension, while modulus,tensile “-” modulus, TE describesFS == tensileflexural TE not = extension, tensile strength,available. extension,FS FE = = flexural flexural FS = strength, extension,flexural andstrength, IS = impact FM = flexural strength. modulus, * The symbol and IS “ ⇧=” impact corresponds strength. to an* The incr symbolease in the“⇧” propertiescorresponds FM = flexural4.2. modulus Influence, and ofIS QASs= impact on strength.Polymer * TheComposites symbol “ ” corresponds to an increase in the properties ⇩ ⇩ ⇳ andto “ anand” increasea “decrease” a decrease in inthe the properties in properties the properties and while “ while” a“-” decrease “-” describes and “ in” describethenot propertiesavailable. not available while and “-” unchanged,and “ ” describe respectively. not available and unchanged,Table 8 respectively. shows the thermo-mechanical properties of polymer composites influenced 4.2. Influence ofby QASs QASs. on PolymerQAS like Composites HTAB has been applied for compatibilization of HDPE/LDPE/Cellu- 4.3. Influence of QAILs on Polymer Composites TableQAS 8 showslose like composites OTABthe thermo-mechanical (chemical [16]. The structure application proper is shownties of inofHT FigurepolymerAB has5) hasinfluencedcomposites been applied the influenced degradation for compat- tempera- ibilization of PBE/BN composites [18]. The application of OTAB has influenced the thermal by QASs.Table QAS 9ture, displays like meltingHTAB the has thermo-mechanicaltemperature, been applied tensile for prop strength,compatibilizationerties tensileof polymer modulus, of compositesHDPE/LDPE/Cellu- tensile influenced extension, and impact conductivity, flexural strength, flexural modulus, and impact strength of the composites loseby composites QAILs. QAILstrength [16]. like The ofHmimPF theapplication composites.6 (chemical of HT The structureAB degradation has influenced is shown temperature inthe Figure degradation of 6) thehas composites been tempera- applied decreased be- (Table8). The thermal conductivity of the composites increased by up to 79%, compared ture,for melting compatibilization temperature,cause of the of lowertensileXSBR/RBC initial strength, compositesdegradation tensile modulus,[17]. temper Theature tensileapplication of extension,HTAB of than HmimPF and other impact 6composite influ- compo- to the neat PBE/BN composite. This is attributed to the improvement of interfacial adhe- strengthenced ofthe the degradationnents, composites. which temperature, Theaffected degradation the glass degradation temperaturetransition temper temperature, of theature composites of tensile the overall decreasedstrength, composites. tensile be- Neverthe- sion between PBE matrix and BN filler, which induced heat transfer across the interface causemodulus, of the lowerandless, tensile initial the melting extensiondegradation temperature of temperthe comp ofature theosites. ofcomposit HTAB The degradation thanes remains other composite unchangedtemperature compo- because of the of the pres- efficiently [18]. Moreover, the mechanical properties, such as flexural strength, flexural nents,composites which affected decreasedence of crystalthe because degradation lattice of theof HTAB temperhydrolyzation inature the compof ofthe HmimPFosites overall [16]. 6composites. induced Moreover, by Neverthe-water the mechanical that is proper- modulus, and impact strength of the composites increased by up to 38%, 8.5%, and 11%, less,released the melting fromties, RBCtemperature such at aas temperature tensile of the strength composit above and 100es tensileremains°C. This mo unchangedis dulusdue to of HmimPF the because composites6 being of the sensitive pres-increased by up to respectively, in comparison with the neat composite. This is because of the long alkyl chain enceto hydrolyzation,of crystal25% lattice and especially of 25%, HTAB respectively, atin elevatedthe comp comparedtemperature.osites [16]. to Moreover, the Therefore, neat HDPE/LDPE/Cellulose the the mechanical polymer composites proper- composite. This length of OTAB, which effectively adsorbed on the surface of the composites [18]. Therefore, ties,heated such aboveas tensileis 100attributed strength°C released to andthe water existencetensile fr ommo of RBC,dulus HTAB, and of whicthe it hydrolyzed compositesh enhanced HmimPF increased the interfacial6. Asby aup adhesionresult, to between the compatibilization by applying OTAB not only increased the thermal conductivity of 25%it decreasedand 25%, HDPE/LDPE respectively,the degradation comparedmatrix temperature and to cellulosethe ofneat the HDPE/LDPE/Cellulosefiller, polymer as a composites. result, improved However, composite. the thestress This glass transfer from the composites but also improved their flexural and impact properties as well. is attributedtransition temperatureto the existence of theof HTAB, composites whic hincreased enhanced by the up interfacial to 3.4% compared adhesion betweento the neat HTABHDPE/LDPE has also been to cellulose. applied forHowever, compatibilization the tensile extension of HDPE/Agar and impact composites strength [22 of]. the com- HDPE/LDPEXSBR/RBC matrixcomposite. and Thiscellulose is attributed filler, as to a theresult, mobility improved of XSBR the macromolecularstress transfer from chains The applicationposites ofdecreased HTAB has caused influenced by the the excess degradation of the temperature,crystal lattice melting of HTAB temperature, in the composites, HDPE/LDPEwhich was torestricted cellulose. with However, the presence the tensile of HmimPF extension6 [17]. and Moreover, impact strength the mechanical of the com- prop- tensile strength,which increased tensile modulus, their brittleness tensile extension, [16]. This and is related impact to strength the increase of the of composites the stiffness prop- positeserties, decreased such as tensile caused strength, by the excess tensile of modulus, the crystal and lattice tensile of extensionHTAB in theof the composites, composites (Table8).erty The of thermal the composites. properties, It suchis a well-known as degradation fact temperature that the crystal and lattice melting with temper- higher content whichincreased increased by up their to 31%, brittleness 13%, and [16]. 7.0%, This respectively, is related to inthe comparison increase of with the stiffness the neat prop-compo- ature of thein the composites matrix can decreased decreasecompared the tensile to extensio the neatn and HDPE/Agar impact strength composite. of the This composites is as site. This is due to the formation of interactions between XSBR matrix and RBC filler gen- erty ofdue the to composites. thea rule presence of mixture.It is of a interactionswell-known betweenfact that HDPEthe crystal matrix lattice and with agar higher filler generatedcontent by erated by HmimPF6 [17]. in theHTAB matrix [ 22can]. Moreover,decreaseQAS like the theOTAB tensile mechanical (chemical extensio properties, structuren and impact foris shown instance, strength in tensileFigure of the strength5)composites has been and appliedas tensile for com- QAIL like BmimCl has been applied for compatibilization of Agarose/Tc composites a rulemodulus of mixture.patibilization of the composites of PBE/BN decreased composites because of[18]. their The ductile application behavior of withOTAB the has presence influenced the [19], it is blended with urea to increase its efficacy. The application of BmimCl has influ- QASof HTAB. like thermalOTAB The existence (chemical conductivity, of structure ductile flexural behavior is shown strength, leads in Figure flexural to the 5) decrease modulus, has been of andapplied brittleness impact for ofstrengthcom- the com- of the com- patibilizationencedposites, the degradation whichofposites PBE/BN also (Table reducedtemperature,composites 8). The the thermal stiffness[18]. glass The transition conductivity property application temperature, of theofof the composites.OTAB composites tensile has influenced strength, This increased is induced tensile the by byup to 79%, thermalmodulus,HTAB conductivity, thatandcompared improvedtensile flexural extension to thethe strength, interfacialneat of PBE/BNthe flexural composites adhesion composite. modulus, between (Table This and 9). HDPE isimpactThe attributed degradation and strength agar. to Nevertheless,the of temperature theimprovement com- the of inter- positesof thetensile (Table composites extension facial8). The adhesion decreased andthermal impact between conductivitybecause strength PBE of the ofmatrix of the lothewer composites and composites thermal BN filler, increasedresistance increased which induced by of by upurea up to heat 72%thanto 79%, transfer andother 24%, across the comparedcompositerespectively, to thecomponents,interface neat in comparison PBE/BN efficiently which composite. with affected[18]. the More neat Thistheover, composite.de isgradation attributed the mechanical This temperature to isthe attributed properties,improvement of tothe thesuch overall compatibiliza-of asinter- flexuralcom- strength, facialposites. tionadhesion effectNonetheless,flexural between of HTAB, modulus, thePBE which glass matrix improved andtransition and impact BN the te fistrength compatibilitymperatureller, which of ofthinduced ebetweenthe composites composites heat the transfer composite increased increased across components, by by theup up to 38%, 8.5%, interfaceto 23%as a efficiently result,in comparisonand increased 11%, [18]. respectively, Morewith the ductilityover,the neat the in ofAgarose/Tccomparisonmechanical the HDPE/Agar composite.withproperties, the composites neat Thissuch composite. is as attributed [ 22flexural]. This strength, to is thebecause for- of the long flexuralmation modulus, ofBesides, strongalkyl and QAS interactionschain impact like length IDAB strength between of (chemical OTAB, of agarose thwhich structuree composites matrix effectively is shownand increased theadsorbed in talc Figure fillerby on 5up) with hasthe to beensurface38%, the presence applied8.5%, of the for composites andof 11%, BmimClcompatibilization respectively, [19].[18]. Furthermore,Therefore, in of comparison SBR/Bentonite the the compatibilization mechanical with compositesthe neat properties, composite. by [ 20applying]. such The This application asOTAB tensileis because not strength ofonly of IDAB theincreased and long has ten- influ- the thermal alkylsile encedchain modulus length theconductivity degradationof theof OTAB, composites of temperature, which the compositesincreased effectively tensile by but adsorbed up strength, also to 26% improv on tensile and theed 62%, surface modulus, their respectively, flexural of andthe composites tensileand compared impact extension properties as [18].to theTherefore,of theneat composites composite.well. the compatibilization (TableThis is8 ).because The degradationby of applying the composites temperatureOTAB havingnot only of strong theincreased composites and stiff the properties,thermal is expected conductivityas wellto be as lower BmimClof the compared composites acted as to a thebut coupling neatalso improv SBR/Bentonite agent edor theircompatibilizer flexural composite. and for Thisimpact the iscomposites. becauseproperties of How- theas low well.ever, degradation the tensile temperatureextension of the of IDAB composites compared decreased to other because components the mo intion the of composites agarose mo- [20]. lecularHowever, chains the was mechanical restricted by properties, the talc filler such [19]. as tensile The result strength also andconfirmed tensile that modulus the stiff- of the nesscomposites property of increased the composites by up toincreased 20% and with 34%, the respectively, presence of incompatibilizer, comparisonwith which the en- neat hancedcomposite. the rigidity This isof attributed the composites, to the increase thus moving of the the cross-linking tensile extension density to of lower the SBR, values. which is causedOn the by other the presence hand, QAIL of interactions like EmimAc between (chemi SBRcal structure matrix and is shown bentonite in Figure filler, as 6) a has result, beenimproving applied for the compatibilization compatibility between of DGEBA them epoxy/Chitin [20]. Nonetheless, composites the tensile [21]. extensionThe applica- of the tioncomposites of EmimAc decreased influenced because the degradation of the increase temperature, of their stiffness glass propertytransition with temperature, the presence tensileof IDAB. strength, The tensile result modulus, indicated tensile that the extension, compatibilization and impact of strength the composites of the composites by IDAB has (Tableallowed 9). The bentonite degradation to restrict temperature the movement of the composites of SBR chains. remains Consequently, unchanged it because increased of the

Appl. Sci. 2021, 11, 3167 11 of 16

rigidity property, which reduced the strain behavior of the composites and led to the lower tensile extension. The compatibilization by applying QASs is not only limited to conventional polymer composites, but they can also be applied in polymer nanocomposites [43]. HTAB is ap- plied for compatibilization of Nylon-6/LNR/MMT nanocomposites [24]. The application of HTAB influenced the degradation temperature, glass transition temperature, tensile strength, tensile modulus, tensile extension, and impact strength of the nanocomposites (Table8). The thermal properties, such as degradation temperature and glass transition tem- perature of the nanocomposites increased by up to 19% and 81%, respectively, compared to the neat Nylon-6/LNR matrix. This is attributed to the formation of intermolecular interactions between Nylon-6/LNR matrix and MMT filler generated by HTAB [24]. More- over, the mechanical properties, for instance, tensile strength, tensile modulus, and impact strength of the nanocomposites increased by up to 48%, 149%, and 6.5%, respectively, in comparison with the neat matrix. This is due to the compatibilization by HTAB that has effectively provided a reinforcing effect on the nanocomposites [24]. Nevertheless, the tensile extension of the nanocomposites decreased because of the increase of their stiff- ness property. This typical phenomenon can occur not only in the conventional polymer composites but also in the polymer nanocomposites due to the compatibilization effect of HTAB.

4.3. Influence of QAILs on Polymer Composites Table9 displays the thermo-mechanical properties of polymer composites influenced by QAILs. QAIL like HmimPF6 (chemical structure is shown in Figure6) has been applied for compatibilization of XSBR/RBC composites [17]. The application of HmimPF6 influ- enced the degradation temperature, glass transition temperature, tensile strength, tensile modulus, and tensile extension of the composites. The degradation temperature of the composites decreased because of the hydrolyzation of HmimPF6 induced by water that is Appl. Sci. 2021, 11, x FOR PEER REVIEW ◦ 10 of 16 released from RBC at a temperature above 100 C. This is due to HmimPF6 being sensitive to hydrolyzation, especially at elevated temperature. Therefore, the polymer composites Appl. Sci. 2021, 11, x FOR PEER REVIEW ◦ 10 of 16 heated above 100 C released water from RBC, and it hydrolyzed HmimPF6. As a result, it decreasedthe compatibility the degradation between temperature the blend of components the polymer; this composites. can take place However, when the they glass have in- transitiontermolecular temperature interactions. of the composites Nonetheless, increased the mechanical by up to 3.4% properties, compared for toinstance, the neat tensile XSBR/RBCthestrength, compatibility composite. tensile modulus,between This is attributed the and blend tensile to components theextension mobility ; ofthis of theXSBR can blends take macromolecular placeincreased when by they chainsup haveto 429%, in- whichtermolecular190%, was restricted and 44%, interactions. with respectively, the presence Nonetheless, in comparison of HmimPF the mechanicalwith6 [17]. the Moreover, PHBV. properties, This the mechanicalis forbecause instance, of prop- the tensile pres- erties,strength,ence such of as interaction tensiletensile strength,modulus, between tensile and PHBV tensile modulus, and extensioncellulos ande, tensile whichof the extensionimprovedblends increased ofthe the compatibility composites by up to 429%, of the increased190%,blends by and up[15]. to44%, 31%, respectively, 13%, and 7.0%, in comparison respectively, with in comparison the PHBV. with This the is neatbecause composite. of the pres- This isence due of to interaction the formation between of interactions PHBV and between cellulos XSBRe, which matrix improved and RBC the filler compatibility generated of the by HmimPFblendsTable 7. 6[15]. Thermo-mechanical[17]. properties of polymer blends influenced by QASs and QAILs. Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 16

Thermo-Mechanical Properties * TableTable 9. Thermo-mechanicalPolymer 7. Thermo-mechanical Blend properties QASs/QAILs properties of polymer of polymer composites blends influenced influenced by by QAILs. QASs and QAILs.References Td Tg Tm TS TM TE FS FE IS Nylon-6/LNR HTAB ⇩ Thermo-Mechanical ⇩ Thermo-Mechanical - ⇩ ⇩ Properties Properties ⇧ *- -* ⇧ [9] Polymer Composite QAILs References Table 8. Thermo-mechanicalPolymer Blend properties QASs/QAILs of pol ymerT compositesT k TS influenced TM TE by FS QASs. FE IS References PET/PE BDAC T⇩d T -gg Tm - TS ⇧ TM ⇧ TE ⇩ FS - FE - IS - [11]

XSBR/RBCNylon-6/LNRNW/Cellulose HmimPFBmimClHTAB6 Thermo-Mechanical⇩⇧ ⇩ ⇧ - -- ⇩ ⇧ Properties ⇩ ⇧ ⇧ ⇧ ---[ * - - - - ⇧ - 17] [10][9] Polymer Composite QASs References Agarose/TcPET/PEPBS/RS BmimClDmimNTfBDACT d 2 Tg T⇩⇧m k - - TS -⇩ - TM ⇧ ⇩ TE ⇧ ⇩ FS ⇩ ⇧ FM ---[ -⇩ IS -⇧ - - 19] [11][12] DGEBAHDPE/LDPE/CellulosePHBV/Cellulose epoxy/ChitinNW/Cellulose EmimAcHTABBmimClBmimCl ⇩ - ⇧ ⇳⇧ -⇧ ⇧ ⇧ - ⇩ - ⇧ ⇧ ⇧ ⇩ ⇧ ⇧ - ⇧ ⇧ -- - - - ⇩ - - - -[16] [21 ] [10][15]

TPU/SiOd =PBE/BN degradationPBS/RS2 temperature,BmimPFOTABDmimNTf 6 Tg -= 2glass - ⇧transition - ⇧ - - ⇩ temperature, - ⇩ --- - ⇩ ⇧ T m ⇧ = ⇧melting ⇩ ⇧ ⇧ temperature,-[ - [18]23 ] [12] TS = tensileRC/MMTHDPE/AgarPHBV/Cellulose strength, TM = tensileHTAB BmimClBmimCl modulus, ⇩ - TE⇧ ⇩ = - ⇧tensile -- ⇩ ⇩ extension, ⇩ ⇧ ⇧ ⇧ FS - = ⇧ flexural---[ - - ⇧ strength, - - [22] FE25 ] =[15] flexural Textension,d = degradation and IS temperature, = impact strength. Tg =⇩ glass * - The transition - symbol - ⇧ “temperature,⇧” ⇧ corresponds ⇩ -T m =to melting - an incr - easetemperature, in the properties TS = Td = degradationSBR/Bentonite temperature, Tg =IDAB glass transition temperature, k = thermal conductivity, TS = tensile[20] strength, TM =Nylon-6/LNR/MMT tensiletensileand “ modulus,⇩ strength,” a decreaseTE TM= tensilein = thetensileHTAB properties extension, modulus, ⇧ FS while ⇧ = TE flexural - =“-” tensile - describes strength, ⇧ extension, ⇧ FEnot= ⇩ available. flexural FS - = flexural extension, - ⇧ strength, and IS[24]= FE impact = flexural extension, and IS = impact strength. * The symbol “⇧” corresponds to an increase in the properties Tstrength.d = degradation4.2. * TheInfluence symbol temperature, of “ QASs” corresponds Tong = Polymer glass to antransition increaseComposites temperature, in the properties Tm and = melting “ ” a decrease temperature, in theproperties k = ⇩ thermalwhile “-”and conductivity, and “ “” a” decrease describe TS = notin tensile the available properties strength, and unchanged, whileTM = “-”tensile respectively. describes modulus, not available.TE = tensile extension, FS = Table 8 shows the thermo-mechanical properties of polymer composites⇧ influenced flexural4.2. strength, Influence FM of= flexuralQASs on modulus, Polymer and Composites IS = impact strength. * The symbol “ ” corresponds to an increaseby QASs. in the QAS properties like HTAB and “ ⇩has” a decreasebeen applied in the propertiesfor compatibilization while “-” and of “⇳ HDPE/LDPE/Cellu-” describe not availablelose and Tablecomposites unchanged, 8 shows [16].respectively. the The thermo-mechanical application of HT properAB hasties influenced of polymer the composites degradation influenced tempera- byture, QASs. melting QAS temperature, like HTAB hastensile been strength, applied tensile for compatibilization modulus, tensile of extension, HDPE/LDPE/Cellu- and impact 4.3. Influence of QAILs on Polymer Composites losestrength composites of the composites. [16]. The application The degradation of HTAB temperature has influenced of the the composites degradation decreased tempera- be- Tableture,cause 9 melting displaysof the lower temperature, the thermo-mechanicalinitial degradation tensile strength, temperprop ertiestensileature of modulus, of polymer HTAB tensilethancomposites other extension, composite influenced and compo-impact by QAILs.strengthnents, QAIL which of like the affected HmimPFcomposites. the6 (chemical degradation The degradation structure temper temperatureis atureshown of in the Figure of overall the 6)composites has composites. been applieddecreased Neverthe- be- for compatibilizationcauseless, the of themelting lower of temperature initialXSBR/RBC degradation compositesof the composit temper [17].aturees The remains of application HTAB unchanged than of other HmimPF because composite6 influ- of the compo- pres- enced nents,theence degradation of which crystal affected lattice temperature, ofthe HTAB degradation glass in the transition comp temperosites temperature,ature [16]. of theMoreover, overall tensile the composites.strength, mechanical tensile Neverthe- proper- modulus,less,ties, and thesuch meltingtensile as tensile extension temperature strength of the andof thecomp tensile compositosites. mo dulusesThe remains degradation of the unchanged composites temperature because increased of of the theby pres-up to compositesence25% ofanddecreased crystal 25%, respectively,lattice because of HTABof the compared hydrolyzationin the comp to theosites neatof HmimPF [16].HDPE/LDPE/Cellulose Moreover,6 induced the by mechanical water composite. that isproper- This releasedties,is fromattributed such RBC as attensileto a the temperature existencestrength ofandabove HTAB, tensile 100 whic°C. mo Thishdulus enhanced is dueof the to the HmimPFcomposites interfacial6 being increased adhesion sensitive by between up to to hydrolyzation,25%HDPE/LDPE and 25%, especially matrixrespectively, andat elevated cellulose compared temperature. filler, to the as neata result,Therefore, HDPE/LDPE/Cellulose improved the polymer the stress composites composite. transfer fromThis heatedis HDPE/LDPEabove attributed 100 °C to to releasedthe cellulose. existence water However, of fr HTAB,om RBC, the whic tensile andh itenhanced extensionhydrolyzed the and interfacialHmimPF impact strength6. adhesionAs a result, of betweenthe com- it decreasedHDPE/LDPEposites the decreased degradation matrix caused and temperature celluloseby the excess filler,of the of aspolymer the a result,crystal composites. improvedlattice of However,HTAB the stress in the transfer composites,glass from transitionHDPE/LDPEwhich temperature increased to cellulose. oftheir the brittleness composites However, [16]. increasedthe This tensile is relatedby extension up to to 3.4%the and increase comparedimpact ofstrength theto thestiffness ofneat the prop-com- XSBR/RBCpositeserty ofcomposite. decreasedthe composites. This caused is Itattributed byis a the well-known excess to the of mobility thefact crystalthat of the XSBR lattice crystal macromolecular of latticeHTAB with in the higher chains composites, content which whichwasin the restricted matrixincreased can with their decrease the brittleness presence the tensile of[16]. HmimPF extensioThis is6 related[17].n and Moreover, impactto the increase strength the mechanical of of the the stiffness composites prop- prop- as erties, ertysucha rule of as ofthe tensile mixture. composites. strength, It tensileis a well-known modulus, factand thattensile the extensioncrystal lattice of the with composites higher content increasedin the byQAS matrixup tolike 31%, can OTAB decrease13%, (chemical and the 7.0%, tensile structure respectively, extensio is shownn in and comparison in impact Figure strength 5) with has the been of neat the applied compositescompo- for com- as site. Thisapatibilization rule is due of mixture. to the of formation PBE/BN ofcomposites interactions [18]. between The application XSBR matrix of OTABand RBC has filler influenced gen- the erated thermalby HmimPFQAS conductivity, like6 [17].OTAB (chemical flexural strength, structure flexural is shown modulus, in Figure and 5) impacthas been strength applied of forthe com- com- QAILpatibilizationposites like (Table BmimCl of 8). PBE/BN has The been thermal composites applied conductivity for [18]. compatibilization The of applicationthe composites of Agarose/Tcof OTAB increased has composites influencedby up to 79%, the [19], itthermal comparedis blended conductivity, towith the ureaneat flexuraltoPBE/BN increase strength, composite. its efficacy. flexural This The modulus,is applicationattributed and to ofimpact the BmimCl improvement strength has influ- of the of inter-com- enced positesthefacial degradation adhesion (Table 8). between temperature, The thermal PBE matrix glass conductivity transition and BN fiof temperature,ller, the which composites induced tensile increased heatstrength, transfer by tensile up across to 79%, the modulus,comparedinterface and tensile efficiently to the extension neat [18]. PBE/BN ofMore the composite.over, composites the mechanical This (Table is attributed 9). properties, The degradation to the such improvement as temperature flexural strength,of inter- of the facialcompositesflexural adhesion modulus, decreased between and becauseimpact PBE matrix strengthof the and lo werofBN th fithermaleller, composites which resistance induced increased of heat urea by transfer thanup to other 38%,across 8.5%, the compositeinterfaceand components,11%, efficiently respectively, which [18]. in Moreaffected comparisonover, the the de with mechanicalgradation the neat temperature composite.properties, Thisofsuch the is as overallbecause flexural com- of strength, the long posites.flexuralalkyl Nonetheless, chain modulus, length the andofglass OTAB, impact transition which strength te effectivelymperature of the compositesadsorbed of the composites on increased the surface increased by ofup the to by composites38%, up 8.5%, to 23%and[18]. in comparison11%, Therefore, respectively, thewith compatibilization the in comparisonneat Agarose/Tc with by applying composite.the neat composite.OTAB This not is attributed onlyThis increasedis because to the theof for- the thermal long mationalkylconductivity of strong chain interactions length of the of composites OTAB, between which butagarose effectivelyalso improvmatrix adsorbed anded their the talc flexuralon fillerthe surface andwith impact the of presencethe properties composites as of BmimCl[18].well. Therefore,[19]. Furthermore, the compatibilization the mechanical by properties, applying suchOTAB as not tensile only strength increased and the ten- thermal sile modulusconductivity of the ofcomposites the composites increased but alsoby up improv to 26%ed and their 62%, flexural respectively, and impact compared properties as to the neatwell. composite. This is because of the composites having strong and stiff properties, as well as BmimCl acted as a coupling agent or compatibilizer for the composites. How- ever, the tensile extension of the composites decreased because the motion of agarose mo- lecular chains was restricted by the talc filler [19]. The result also confirmed that the stiff- ness property of the composites increased with the presence of compatibilizer, which en- hanced the rigidity of the composites, thus moving the tensile extension to lower values. On the other hand, QAIL like EmimAc (chemical structure is shown in Figure 6) has been applied for compatibilization of DGEBA epoxy/Chitin composites [21]. The applica- tion of EmimAc influenced the degradation temperature, glass transition temperature, tensile strength, tensile modulus, tensile extension, and impact strength of the composites (Table 9). The degradation temperature of the composites remains unchanged because of

Appl. Sci. 2021, 11, 3167 12 of 16

QAIL like BmimCl has been applied for compatibilization of Agarose/Tc compos- ites [19], it is blended with urea to increase its efficacy. The application of BmimCl has influenced the degradation temperature, glass transition temperature, tensile strength, tensile modulus, and tensile extension of the composites (Table9). The degradation tem- perature of the composites decreased because of the lower thermal resistance of urea than other composite components, which affected the degradation temperature of the overall composites. Nonetheless, the glass transition temperature of the composites increased by up to 23% in comparison with the neat Agarose/Tc composite. This is attributed to the for- mation of strong interactions between agarose matrix and the talc filler with the presence of BmimCl [19]. Furthermore, the mechanical properties, such as tensile strength and tensile modulus of the composites increased by up to 26% and 62%, respectively, compared to the neat composite. This is because of the composites having strong and stiff properties, as well as BmimCl acted as a coupling agent or compatibilizer for the composites. However, the tensile extension of the composites decreased because the motion of agarose molecular chains was restricted by the talc filler [19]. The result also confirmed that the stiffness property of the composites increased with the presence of compatibilizer, which enhanced the rigidity of the composites, thus moving the tensile extension to lower values. On the other hand, QAIL like EmimAc (chemical structure is shown in Figure6) has been applied for compatibilization of DGEBA epoxy/Chitin composites [21]. The appli- cation of EmimAc influenced the degradation temperature, glass transition temperature, tensile strength, tensile modulus, tensile extension, and impact strength of the composites (Table9). The degradation temperature of the composites remains unchanged because of the small content of chitin in the composites; thus, the overall thermal degradation behavior is similar to the neat DGEBA epoxy matrix. Nonetheless, the glass transition temperature of the composites increased by up to 9.0% compared to the neat matrix. This is attributed to the presence of chitin, which restricted the mobility of DGEBA epoxy chains prompted by intermolecular hydrogen bonding [21]. Moreover, the mechanical properties, such as tensile strength, tensile modulus, tensile extension, and impact strength of the composites increased by up to 34%, 8.4%, 80%, and 91%, respectively, in comparison with the neat matrix. This is because of the better dispersion of chitin filler within the DGEBA epoxy matrix with the presence of EmimAc [21]. Besides, QAIL like BmimPF6 (chemical structure is shown in Figure6) has been applied for compatibilization of PU/SiO2 composites [23]. The application of BmimPF6 influenced the degradation temperature, glass transition temperature, thermal conductivity, flexural strength, and flexural extension of the composites (Table9). The thermal properties, such as degradation temperature and glass transition temperature of the composites increased by up to 10% and 13%, respectively, compared to the neat PU/SiO2 composite. This is attributed to the presence of BmimPF6 that has significantly enhanced the thermal stability of the composites. Nevertheless, the thermal conductivity of the composites decreased because of the low thermal conductivity of SiO2 filler, which is well dispersed in the cell walls of the PU matrix [23]. In addition, the BmimPF6-compatibilized composites improved the interfacial adhesion between PU and SiO2, which reduced the heat transfer on the matrix, thus decreasing the thermal conductivity of the composites. The flexural strength of the composites increased by up to 15% in comparison with the neat composite. This is because of the reinforcing effect of SiO2 and compatibilization effect of BmimPF6. However, the flexural extension of the composites decreased because of the existence of SiO2 aggregates that acted as defects within the PU matrix [23]. The aggregates normally exist at high content of SiO2 filler in the matrix. Hence, the formation of aggregates can be avoided or minimized during the preparation of the composites. The compatibilization by applying QAILs is not only limited to conventional polymer composites, but they can also be applied in polymer nanocomposites as QASs. BmimCl is applied for compatibilization of RC/MMT nanocomposites [25]. The application of BmimCl influenced the degradation temperature, tensile strength, tensile modulus, and tensile extension of the nanocomposites (Table9). The degradation temperature of the Appl. Sci. 2021, 11, 3167 13 of 16

nanocomposites increased by up to 154% compared to the neat RC matrix. This is attributed to the good dispersion and exfoliation of MMT filler within the RC matrix regenerated from BmimCl [25]. Furthermore, the mechanical properties, such as tensile strength and tensile modulus of the nanocomposites increased by up to 12% and 47%, respectively, in comparison with the neat matrix. This is caused by the presence of MMT that provided high rigidity property to the nanocomposites. Nonetheless, the tensile extension of the nanocomposites decreased because the slippage movement of RC chains was restrained by MMT filler [25]. This increased the stiffness property of the composites, which reduced the strain behavior and led to the lower tensile extension.

5. Conclusions Polymer blends and polymer composites compatibilized by quaternary ammonium compounds (QACs) were concisely reviewed in this paper. The focal thermo-mechanical properties, for instance, degradation temperature, glass transition temperature, melting temperature, tensile strength, flexural strength, and impact strength of the polymer blends and polymer composites were also ascertained in this concise review. QACs can be ap- plied for compatibilization of polymer blends and polymer composites because they own multipurpose features. QACs applied for different types of polymer blends and polymer composites are mainly classified into two types, namely quaternary ammonium surfactants (QASs) and quaternary ammonium ionic liquids (QAILs). The appropriate compatibi- lizations of polymer blends by QASs and QAILs could effectually create intermolecular interactions between the blend components. Furthermore, the polymer composites com- patibilized by QASs and QAILs could also effectively form intermolecular interactions between the polymer matrices and fillers. The presence of the interactions might improve the interfacial adhesion and compatibility between the components of polymer blends and polymer composites. The improved compatibility has consequently enhanced the thermo-mechanical properties of the polymer blends and polymer composites.

Author Contributions: Conceptualization, A.A.S. and S.N.A.M.J.; methodology, A.A.S.; validation, A.A.S. and S.N.A.M.J.; formal analysis, S.N.A.M.J.; investigation, A.A.S.; resources, S.N.A.M.J.; data curation, A.A.S.; writing—original draft preparation, A.A.S.; writing—review and editing, S.N.A.M.J.; project administration, A.A.S.; funding acquisition, S.N.A.M.J. All authors have read and agreed to the published version of the manuscript. Funding: This concise review was funded by the Universiti Putra Malaysia (vote number: 9001103). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank Jacek Nowaczyk from the Nicolaus Copernicus University in Toru´nfor inspiring the authors to write this concise review. Conflicts of Interest: The authors declare no conflict of interest. The funder had no role in the design of the review; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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