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Article Enhancement of the Surface Properties on Film Using Side-Chain Crystalline Block

Sho Hirai * , Patchiya Phanthong , Hikaru Okubo and Shigeru Yao

Research Institute for the Creation of Functional and Structural Materials, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan; [email protected] (P.P.); [email protected] (H.O.); [email protected] (S.Y.) * Correspondence: [email protected]; Tel.: +81-92-871-6631

 Received: 25 October 2020; Accepted: 16 November 2020; Published: 18 November 2020 

Abstract: The consumption of polypropylene (PP) has significantly increased over that of other materials because of its light weight, easy molding, and high mechanical strength. However, the applications of PP are limited, owing to the lack of surface properties, especially with respect to adhesive properties and hydrophilicity. In this study, we developed a surface modification method for enhancing the adhesive properties and hydrophilicity on the PP surface using a side-chain crystalline block (SCCBC). This method was simple and involved the dipping of a PP film in a diluted SCCBC solution. The optimized modification conditions for enhancing the adhesive properties of PP were investigated. The results revealed that the adhesion strength of PP modified with the SCCBC of behenyl acrylate and 2-(tert-butylamino)ethyl methacrylate was enhanced to 2.00 N/mm (T-peel test) and 1.05 N/mm2 (tensile shear test). In addition, the hydrophilicity of PP modified with the SCCBC of behenyl acrylate and di(ethylene glycol)ethyl acrylate was enhanced to a water contact angle of 69 4 . Surface analysis ± ◦ was also performed to elucidate a plausible mechanism for PP modification by the SCCBCs. This surface modification method is facile and enhances desirable properties for the wide application of PP.

Keywords: polypropylene; surface modification; adhesive properties; hydrophilicity; block copolymer

1. Introduction is one of the most used materials in daily life, owing to its light weight, low cost, and high mechanical and chemical properties. Since the mass production of plastic began in the 1950s, the world plastic production has significantly increased from 2 million tons in 1950 to 380 million tons in 2015, with an 8.4% increase in the production rate [1]. Many types of with a variety of desirable properties have been developed and produced to date. Among them, polypropylene (PP) has been widely used in consumer products such as packaging materials [2], pipes [3], rope [4], and automobiles [5,6]. This is because, compared to other plastics, PP has exhibited superior properties including high chemical and heat resistances, easy molding, and high mechanical tensile and compression strengths [7]. Therefore, PP had been used in various fields spanning from the industrial scale to daily life and accounts for 17% of the total plastic production [1]. Recently, the application of PP as a separator for lithium-ion batteries and as a biomaterial for various applications has been explored to expand its usage [8–10]. However, because PP solely consists of alkane chains, it exhibits a low surface free energy. As a result, PP shows low hydrophilicity, poor adhesion properties, and low dyeability, which limits its applications. Various modification methods have been developed to enhance the surface properties of PP. Indeed, PP polyfunctionalization was achieved by fabricating PP as a composite material using

Polymers 2020, 12, 2736; doi:10.3390/polym12112736 www.mdpi.com/journal/polymers Polymers 2020, 12, 2736 2 of 12 different techniques. For example, , which was stacked by some amount of thin films, showed the different properties with the fabrication by only one film [11–14]. The other technique was blending which was directly mixed with PP after adding some materials such as polyolefin elastomer [15], polyphenol [16], 6 [17], glass fiber [18], calcium carbonate [19], and montmorillonite [20]. A method in which PP was bonded with antioxidants by chemical reaction was also reported [21]. Although these methods have successfully improved the hydrophilicity, adhesive properties, and mechanical strength of PP, some drawbacks have also been encountered. Particularly, PP showed low compatibility with other materials, so it was difficult for the additives to be homogeneously dispersed. Therefore, not only did the modified products show insufficient desirable properties, but the natural properties of PP such as mechanical strength also decreased as the result of the slight difference in the modification conditions (i.e., added amount, component type, kneading time, and film thickness). As a result, these methods required a high level of experimental techniques and long processing times until the composite materials exhibited satisfactory properties. On the other hand, surface modification of PP is attractive because of its simplicity and good repeatability. In this technique, it is not necessary to change the properties of the whole compound. Instead, the products are tailored according to the market requirement by modification at the PP surface to achieve the required properties. Surface modification of PP has been mainly achieved by chemical and physical treatments. In the former, a functional group, such as a hydroxy and carboxy group, was generated by using a nitric acid or ozone [22,23]. However, these chemical treatments require hazardous reagents to oxidize the highly chemical stabile PP surface. Therefore, the surface modified by chemical treatment is difficult to apply in a thin film or as a biomaterial. Hence, most surface modifications have been performed by physical treatments such as plasma [24,25], corona [26,27], and flame treatment [28,29]. These methods could successfully impart hydrophilicity and adhesion properties by scission of the PP alkane chain at the material surface to generate functional groups. However, these methods also had some disadvantages, including the requirement of special equipment and short-lived modification stability. Thin film modification by physical treatment is also difficult because the high energy of plasma irradiation is necessary. As a result, the mechanical strength of PP is significantly decreased owing to the extensive scission of the PP alkane chains. Moreover, these methods are limited by the type and shape of the substrates such as the porous membranes and the large size of the materials because the plasma cannot penetrate inside the porous membranes. From this viewpoint, some methods have been demonstrated to be mild and sufficient procedures by using a combination of chemical and physical treatments are required. These modifications were achieved in chemical reactions such as graft and oxidization on the PP surface using chemical reagents accompanied by UV irradiation [30–32]. These methods successfully imparted various properties to the PP, with long-term modification effects; however, they required complex techniques and special apparatus. Therefore, the development of facile modification techniques that enhance the adhesive properties of the PP surface is still challenging. In our previous studies, the surface modification of (PE) was successfully achieved by using a side-chain crystalline block copolymer (SCCBC). This comprised a block copolymer with a long alkyl chain and a functional group (amino or hydroxy group) as the respective side-chain crystalline and functional units. The result revealed that the side-chain crystalline unit was adsorbed on the PE surface because of the similar structures of the SCCBC side-chain crystalline unit and PE chain. In addition, the functional unit covered the PE surface, which resulted in the specific characteristics from the functional unit that were exhibited on the PE surface. This method could enhance the dispersibility, hydrophilicity, and adhesive properties of PE [33–36]. However, it was only applied to the PE surface because the side-chain crystalline units could only be adsorbed by PE, owing to their similar structures. In this current study, surface modification was developed on the PP surface by using two types of SCCBCs to enhance the adhesive properties and hydrophilicity of PP.The surface modification conditions, namely the concentration of SCCBC solution, solvent types, processing temperature, and dipping time, were varied to investigate the optimal conditions for improvement of the adhesive properties of the PP film. The adhesion Polymers 2020, 12, 2736 3 of 12 strength was evaluated by the T-peel and tensile shear tests. In addition, surface analysis by atomic force microscopy (AFM), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy of the non-modified PP and PP modified with SCCBC was performed to elucidate the surface modification mechanism. Furthermore, the hydrophilicity enhancement of the PP modified with SCCBC was investigated. This study aimed at enhancing the surface properties of PP by using a facile method, to expand the usage of PP in a wide range of applications.

2. Materials and Methods

2.1. Material All PP pellets: homopolymers (h-PP; Prime PolyproTM J137G), random copolymers (r-PP; Prime PolyproTM B241), and block copolymers (b-PP; Prime PolyproTM B-150M), were purchased from Prime Polymer Co., Ltd., Tokyo, Japan. High-density polyethylene pellets (HDPE; FX201A) were purchased from Keiyo Polyethylene Co., Ltd., Tokyo, Japan. These PP and HDPE pellets were compressed at 210 ◦C (PP) and 180 ◦C (HDPE) for 2 min under 25 MPa by a hot compression machine IMC-180C (Imoto machinery Co., Ltd., Kyoto, Japan) to fabricate thin films of 0.5 mm thickness. Next, the thin films were cut into the dimensions 100.0 12.5 0.5 mm × × (length width thickness) for the T-peel tests and 100.0 25.0 0.5 mm for the tensile shear tests. × × × × (PVC) sheets (thickness: 0.5 mm) were purchased from Meiwa Gravure Co., Ltd., Osaka, Japan, while aluminum and copper metal sheets (thickness: 0.5 mm) were obtained from HIKARI Co., Ltd., Osaka, Japan. These substrates were cut to the same dimensions of the PP thin film. Aron Alpha® 201, which comprised α-cyanoacrylate as its main component, was obtained from Toagosei Co., Ltd. (Tokyo, Japan) and was used as an instant glue. The stabilizer like a monomethyl ether hydroquinone contained in the of behenyl acrylate (BHA, NOF Corp., Tokyo, Japan), 2-(tert-butylamino)ethyl methacrylate (TBAEMA, Sigma-Aldrich Co. LLC., St. Louis, MO, USA), and di(ethylene glycol)ethyl ether acrylate (DEEA, Sigma-Aldrich Co. LLC., St. Louis, MO, USA) were removed by inhibitor removers (Sigma-Aldrich Co. LLC., St. Louis, MO, USA) before use. 2-Methyl-2-[N-tert-butyl-N-(1-diethoxyphosphoryl-2,2-dimethylpropyl)aminoxy]propionic acid (BlocBuilder® MA) was supplied by Alkema Inc., Colombes, France, and used as received. Octane, xylene, butyl acetate, decalin, benzonitrile, isopropanol, N,N-dimethylformamide (DMF), and methanol were purchased from FUJIFILM Wako Pure Chemical Corp., Osaka, Japan and used without further purification.

2.2. Synthesis of the SCCBCs Two SCCBCs (BHA-TBAEMA and BHA-DEEA) were synthesized by nitroxide living according to our previously reported method [22]. BHA (5.0 g, 13.1 mmol), BlocBuilder® MA (0.4 g, 1.0 mmol) and butyl acetate (5.7 mL) were added to a separable flask. The mixture was then deoxygenated by stirring under nitrogen atmosphere for 20 min and subsequently heated at 110 ◦C for 24 h. Next, a solution of butyl acetate (5.7 mL) and TBAEMA (5.0 g, 27.0 mmol) was added to the mixture and stirred at 110 ◦C for 24 h, after which the mixture was exposed to air and cooled down to room temperature prior to reprecipitation by methanol. The afforded product was then collected by filtration prior to drying under reduced pressure at room temperature for 24 h to obtain BHA-TBAEMA as a pale yellow solid. The weight-average molecular weight (Mw) and polydispersity index (PDI) of BHA-TBAEMA, which were determined by gel permeation chromatography (GPC, HLC-8320GPC; Tosoh Corp. Tokyo, Japan) and calculated using as a standard, were 6.6 103 g/mol × and 1.2, respectively. BHA-DEEA was also prepared using a synthetic procedure similar to that of BHA-TBAEMA but with the functional group DEEA (5 g, 26.6 mmol) instead of TBAEMA. The afforded white BHA-DEEA solid of was obtained with 11.9 103 g/mol of Mw and 1.4 of PDI. The chemical × structure of SCCBCs are shown in Figure1. Polymers 2020, 12, 2736 4 of 12 Polymers 2020, 12, x FOR PEER REVIEW 9 of 13

Figure 1. Chemical structure of SCCBC SCCBCss ( a) BHA BHA-TBAEMA;-TBAEMA; (b) BHA-DEEA.BHA-DEEA.

2.3. Surface Modification of the PP Film Using SCCBCs 2.3. Surface Modification of the PP Film Using SCCBCs The PP film surface was cleaned by washing with acetone and drying at room temperature. The PP film surface was cleaned by washing with acetone and drying at room temperature. The The diluted SCCBC solution (0.1 wt%) was prepared by the adding SCCBC solid (0.4 g) into diluted SCCBC solution (0.1 wt%) was prepared by the adding SCCBC solid (0.4 g) into xylene (400 xylene (400 g). Next, the PP film was dipped in the diluted SCCBC solution at 80 C for 5 min. g). Next, the PP film was dipped in the diluted SCCBC solution at 80 °C for 5 min. Subsequently◦ , the Subsequently, the modified PP film was dried at room temperature for one day in order to remove the modified PP film was dried at room temperature for one day in order to remove the excess of solvent. excess of solvent. To investigate the optimal conditions to improve the PP film adhesive properties, the To investigate the optimal conditions to improve the PP film adhesive properties, the modification modification conditions were varied with respect to concentration of SCCBC solution (0.01, 0.05, 0.10, conditions were varied with respect to concentration of SCCBC solution (0.01, 0.05, 0.10, 0.50 and 0.50 and 1.00 wt%), solvent types of the diluted SCCBC solution (octane, xylene, butyl acetate, 1.00 wt%), solvent types of the diluted SCCBC solution (octane, xylene, butyl acetate, decalin, benzonitrile, decalin, benzonitrile, isopropanol, and DMF), dipping temperature (20, 40, 60, 80 and 100 °C), and isopropanol, and DMF), dipping temperature (20, 40, 60, 80 and 100 C), and dipping time (0.17, 1, 5, 10 dipping time (0.17, 1, 5, 10 and 30 min). ◦ and 30 min). 2.4. Characterization 2.4. Characterization The adhesive strengths of the non-modified PP film and PP films modified with the SCCBCs The adhesive strengths of the non-modified PP film and PP films modified with the SCCBCs were evaluated by the T-peel test and tensile shear tests by using an EZ-LX compact table-top tester were evaluated by the T-peel test and tensile shear tests by using an EZ-LX compact table-top tester (Shimadzu Corp., Kyoto, Japan). Two pieces of the modified PP film were glued with an instant glue (Shimadzu Corp., Kyoto, Japan). Two pieces of the modified PP film were glued with an instant glue and and attached with a bonded area of 12.5 × 75.0 mm (width × length) for the T-peel test and 12.5 × 25.0 attached with a bonded area of 12.5 75.0 mm (width length) for the T-peel test and 12.5 25.0 mm mm (width × length) for the tensile× shear test. The schematic× s of the test specimens are× shown in (width length) for the tensile shear test. The schematics of the test specimens are shown in Figure2. Figure 2.× The bonded PP films were cured at room temperature for 24 h and the averaged thickness The bonded PP films were cured at room temperature for 24 h and the averaged thickness of the of the bonded layer was 0.1 mm. Adhesive strength measurements were performed using a peeling bonded layer was 0.1 mm. Adhesive strength measurements were performed using a peeling rate of rate of 1 mm/min for the T-peel test and 10 mm/min for the tensile shear test. All sample data were 1 mm/min for the T-peel test and 10 mm/min for the tensile shear test. All sample data were averaged averaged from five tests. The surface topography was evaluated by atomic force microscopy (AFM) from five tests. The surface topography was evaluated by atomic force microscopy (AFM) using using a the JPK NanoWizard® Sense AFM (Bruker Nano GmbH., Berlin, Germany). The PP films were a the JPK NanoWizard® Sense AFM (Bruker Nano GmbH., Berlin, Germany). The PP films were observed in tapping mode by using a PPP-NCHAuD (NANOSENSORS™, Neuchâtel, Switzerland), observed in tapping mode by using a PPP-NCHAuD (NANOSENSORS™, Neuchâtel, Switzerland), which comprised a pyramidal cantilever with silicon coated by a metallic layer. Transmission electron which comprised a pyramidal cantilever with silicon coated by a metallic layer. Transmission electron microscopy (TEM) was performed by using a JEM-2100F (JEOL Ltd., Tokyo, Japan). The PP films microscopy (TEM) was performed by using a JEM-2100F (JEOL Ltd., Tokyo, Japan). The PP films were were coated with an epoxy resin and cut into small pieces with a thickness of 100 nm using an coated with an epoxy resin and cut into small pieces with a thickness of 100 nm using an ultramicrotome ultramicrotome with a diamond knife at −120 °C. The small PP pieces were then stained with RuO4 with a diamond knife at 120 C. The small PP pieces were then stained with RuO for 15 min and for 15 min and the TEM− measurement◦ s were performed with an accelerating voltage4 of 200 kV. the TEM measurements were performed with an accelerating voltage of 200 kV. Fourier transform Fourier transform infrared (FTIR) spectra were recorded by a Spectrum Two (PerkinElmer Inc., infrared (FTIR) spectra were recorded by a Spectrum Two (PerkinElmer Inc., Waltham, MA, USA). Waltham, MA, USA). The hydrophilicity was evaluated with a DropMaster DM-301 (Kyowa Interface The hydrophilicity was evaluated with a DropMaster DM-301 (Kyowa Interface Science Co., Ltd., Science Co., Ltd., Saitama, Japan) and the water contact angle was averaged from five testing samples. Saitama, Japan) and the water contact angle was averaged from five testing samples.

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FigureFigure 2. 2. SchematicSchematicss of of the the test test specimen specimen:: ( (aa)) T T-peel-peel test test;; ( (bb)) tensile tensile shear shear test test.. 3. Result and Discussion 3. Result and Discussion 3.1. Effect of the Surface Modification Conditions on the Adhesive Properties of PP Modified with BHA-TBAEMA 3.1. Effect of the Surface Modification Conditions on the Adhesive Properties of PP Modified with BHA- TBAEMAThe e ffects of the surface modification conditions were investigated in order to enhance the adhesive properties of PP. The surface modification factors were varied with respect to the dipping temperature, The effects of the surface modification conditions were investigated in order to enhance the dipping time, concentration, and types of solvent in the SCCBC solution. Figures3a and S1a showed the adhesive properties of PP. The surface modification factors were varied with respect to the dipping adhesion strengths of the non-modified PP and PP modified with BHA-TBAEMA with different dipping temperature, dipping time, concentration, and types of solvent in the SCCBC solution. Figure 3a and temperatures (20, 40, 60, 80 and 100 ◦C) evaluated by the tensile shear and T-peel tests, respectively. Figure S1a showed the adhesion strengths of the non-modified PP and PP modified with BHA- In the case of the non-modified PP, the adhesion strength was 0.03 N/mm for the T-peel test and TBAEMA with different dipping temperatures (20, 40, 60, 80 and 100 °C) evaluated by the tensile 0.18 N/mm2 for the tensile shear test, which implied that the neat PP surface lacked adhesion strength. shear and T-peel tests, respectively. In the case of the non-modified PP, the adhesion strength was The modified PP dipped in 0.1 wt% of BHA-TBAEMA in xylene for 10 min at 20 ◦C was also easily 0.03 N/mm for the T-peel test and 0.18 N/mm2 for the tensile shear test, which implied that the neat peeled off with an adhesive strength of 0.07 N/mm for the T-peel test and 0.46 N/mm2 for the tensile PP surface lacked adhesion strength. The modified PP dipped in 0.1 wt% of BHA-TBAEMA in xylene shear test. This adhesion strength was slightly enhanced when the dipping temperature of the PP for 10 min at 20 °C was also easily peeled off with an adhesive strength of 0.07 N/mm for the T-peel modified with BHA-TBAEMA was increased to 40 ◦C. Notably, when the dipping temperature was test and 0.46 N/mm2 for the tensile shear test. This adhesion strength was slightly enhanced when the further increased to 60 ◦C, the modified PP exhibited a significant increase in adhesion strength dipping temperature of the PP modified with BHA-TBAEMA was increased to 40 °C. Notably, when (1.97 N/mm for the T-peel test and 1.03 N/mm2 for the tensile shear test). This high adhesion strength the dipping temperature was further increased to 60 °C, the modified PP exhibited a significant was also observed when the PP was modified with the BHA-TBAEMA solution at dipping temperature increase in adhesion strength (1.97 N/mm for the T-peel test and 1.03 N/mm2 for the tensile shear of 80 and 100 ◦C. These results implied that the side-chain crystalline unit of SCCBC were strongly test). This high adhesion strength was also observed when the PP was modified with the BHA- adsorbed onto the PP surface because side-chain crystalline unit could penetrate until the inner layer TBAEMA solution at dipping temperature of 80 and 100 °C . These results implied that the side-chain of PP and form cocrystals at high temperature. Based on an above investigation, we concluded that crystalline unit of SCCBC were strongly adsorbed onto the PP surface because side-chain crystalline the optimized dipping temperature of the PP modified with BHA-TBAEMA solution was 80 ◦C, unit could penetrate until the inner layer of PP and form cocrystals at high temperature. Based on an which afforded the highest adhesion strengths of 2.00 N/mm for the T-peel test and 1.04 N/mm2 for the above investigation, we concluded that the optimized dipping temperature of the PP modified with tensile shear test. BHA-TBAEMA solution was 80 °C, which afforded the highest adhesion strengths of 2.00 N/mm for The adhesion strength of PP modified with BHA-TBAEMA at different dipping times (0.17, 1, 5, the T-peel test and 1.04 N/mm2 for the tensile shear test. 10 and 30 min) was next evaluated by tensile shear and T-peel testing (Figures3b and S1b, respectively). When the dipping time was in the range 0.17–30 min, it did not affect the adhesion strength of the modified PP, and the adhesion strength averaged at approximately 2.00 N/mm for the T-peel test and 1.03 N/mm2 for the tensile shear test. Notably, all the test pieces were torn at the area of neat PP prior to peel-off. Therefore, this surface modification method enhanced the adhesive properties of PP in a short processing time. Indeed, the optimized dipping time of the PP in the BHA-TBAEMA solution was 5 min, which afforded the highest adhesion strength of 2.00 N/mm for the T-peel test and 1.05 N/mm2 for the tensile shear test. The effect of the concentration of BHA-TBAEMA in xylene solution on the adhesion strength of PP is exhibited in Figure3c and Figure S1c. The concentration was varied at 0.01, 0.05, 0.10, 0.50 and 1.00 wt% of BHA-TBAEMA in xylene. At 0.01 wt%, the adhesion strength of the modified PP was slightly higher than that of the non-modified PP. These results considered that the adhesive properties on the PP surface were an insufficient enhancement owing to the lack of BHA-TBAEMA to modify the whole PP surface. A significant increase in adhesion strength was detected in the concentration range 0.05–0.10 wt%, reaching values of 2.00 N/mm for the T-peel test and 1.05 N/mm2 for the tensile shear at 0.10 wt%. However, the adhesion strength decreased with a further increase in concentration to

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0.50 wt% and decreased sharply when the concentration exceeded 0.50 wt%. This result implied that the functional unit did not work well because excess BHA-TBAEMA adsorbed onto the PP surface to form the multilayer of copolymer. Thus, we concluded that the optimal concentration of BHA-TBAEMA in Polymersxylene 20 was20, 1 0.102, x FOR wt%. PEER REVIEW 9 of 13

FigureFigure 3. 3. AAdhesiondhesion strength strength of of the the non non-modified-modified PPPP andand PPPP modified modified with with the the SCCBC SCCBC evaluated evaluated by by the thetensile tensile shear shear tests: test (a)s: 0.1 (a wt%) 0.1 ofwt% BHA-TBAEMA of BHA-TBAEMA in xylene in for xylene 10 min for at di 10ff erent min at dipping different temperatures; dipping temperature(b) 0.1 wt%s; of (b BHA-TBAEMA) 0.1 wt% of BHA in-TBAEMA xylene at in 80 xylene◦C with at 80 di °Cfferent with dippingdifferent times; dipping (c) time BHA-TBAEMAs; (c) BHA- TBAEMAat 80 ◦C forat 80 5 min°C for at 5 di minfferent at different concentrations concentration of BHA-TBAEMAs of BHA-TBAEMA in xylene in solution;xylene solution; (d) 0.1 ( wt%d) 0.1 of wt%BHA-TBAEMA of BHA-TBAEMA in different in different solvent solvent types attypes 80 ◦ Cat for80 5°C min. for 5 min.

TheThe adhesion adhesion strength strengths of of PP the modified non-modified with BHA PP and-TBAEMA PP modified at different with BHA-TBAEMA dipping times in(0.17, different 1, 5, 10types and of30 solventmin) was were next also evaluated evaluated, by as tensile shown shear in Figure and 3Td-peel and test Figureing S1d.(Figure The 3b adhesion and Figure strength S1b, respectivelywas significantly). When enhanced the dipping when time the was PP wasin the modified range 0.17 with–30 BHA-TBAEMA min, it did not inaffect octane the and adhesion xylene strengthas the solvents, of the modified while butyl PP, and acetate the adhesion and decalin strength only affordedaveraged a at slight approximately increase in 2.00 adhesion N/mm strengthfor the 2 T(approximately-peel test and 1.03 1.00 N/mm N/mm2 for the T-peeltensiletest shear and test 0.85. Notably N/mm , forall the tensile test shear pieces test). were On torn the at other the area hand, ofthe neat use ofPP benzonitrile, prior to peel isopropanol,-off. Therefore, and DMF this solvents surface did modification not affect the method adhesion enhance strength.d the This adh is becauseesive propertiesBHA-TBAEMA of PP in is morea short soluble processing in nonpolar time. Indeed, solvents, the such optimized as octane dipping and xylene, time thanof the the PP polar in the solvents, BHA- TBAEMAsuch as benzonitrile, solution was isopropanol, 5 min, which and DMF.afford Thus,ed the surface highest modification adhesion strength with BHA-TBAEMA of 2.00 N/mm in for a nonpolar the T- peelsolvent test wasand found1.05 N/mm to be2 suitable for the tensile to enhance shear the test. adhesion strength of PP. TheFrom effect the of investigation the concentration of the of surface BHA-TBAEMA modification in xylene conditions solution for on the the enhancement adhesion strength of the of PP PPadhesive is exhibited properties, in Figure the 3c optimized and Figure conditions S1c. The concentration were 0.1 wt% was BHA-TBAEMA varied at 0.01, in 0.05, xylene 0.10, solution 0.50 and at 1.0080 ◦wt%C for of 5 min,BHA- wherebyTBAEMA the in PPxylene. adhesion At 0.01 strength wt%, the was adhesion enhanced strength to 2.00 Nof/ mmthe modified for the T-peel PP was test 2 slightlyand 1.05 higher N/mm thanfor the that tensile of the shear non test.-modified These PP.optimized These conditions results considered were used that in thethe subsequent adhesive propertiescharacterization on the experiment.PP surface were an insufficient enhancement owing to the lack of BHA-TBAEMA to modifFigurey the4 shows whole the PP photos surface of. A the significant test pieces increas after testinge in adhesion by T-peel strength (Figure 4 wasa) and detected tensile in shear the concentrationtesting (Figure range4b). 0.05 The– non-modified0.10 wt%, reaching PP films values were of 2.00 easily N/mm peeled for o theff at T- thepeel adhesion test and 1.05 area. N/mm On the2 forother the hand,tensile the shear test at pieces 0.10 wt% of PP. However, modified the with adhesion BHA-TBAEMA strength weredecreased torn atwith an areaa further other increase than the inbonded concentration area. This to implied0.50 wt% that and the decreased adhesive sharply properties when of the the PP concentration surface were exceeded enhanced 0.50 by modification wt%. This resultwith BHA-TBAEMA.implied that the As functional a result, theunit PP did test not pieces work were well broken because at theexcess neat BHA PP area-TBAEMA prior to adsorbed peel-off at ontothe bonded the PP area. surface to form the multilayer of copolymer. Thus, we concluded that the optimal concentration of BHA-TBAEMA in xylene was 0.10 wt%. The adhesion strengths of the non-modified PP and PP modified with BHA-TBAEMA in different types of solvent were also evaluated, as shown in Figure 3d and Figure S1d. The adhesion strength was significantly enhanced when the PP was modified with BHA-TBAEMA in octane and xylene as the solvents, while butyl acetate and decalin only afforded a slight increase in adhesion strength (approximately 1.00 N/mm for the T-peel test and 0.85 N/mm2 for tensile shear test). On the other hand, the use of benzonitrile, isopropanol, and DMF solvents did not affect the adhesion

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strength. This is because BHA-TBAEMA is more soluble in nonpolar solvents, such as octane and xylene, than the polar solvents, such as benzonitrile, isopropanol, and DMF. Thus, surface modification with BHA-TBAEMA in a nonpolar solvent was found to be suitable to enhance the adhesion strength of PP. From the investigation of the surface modification conditions for the enhancement of the PP adhesive properties, the optimized conditions were 0.1 wt% BHA-TBAEMA in xylene solution at 80 °C for 5 min, whereby the PP adhesion strength was enhanced to 2.00 N/mm for the T-peel test and 1.05 N/mm2 for the tensile shear test. These optimized conditions were used in the subsequent characterization experiment. Figure 4 shows the photos of the test pieces after testing by T-peel (Figure 4a) and tensile shear testing (Figure 4b). The non-modified PP films were easily peeled off at the adhesion area. On the other hand, the test pieces of PP modified with BHA-TBAEMA were torn at an area other than the bonded area. This implied that the adhesive properties of the PP surface were enhanced by Polymersmodification2020, 12 ,with 2736 BHA-TBAEMA. As a result, the PP test pieces were broken at the neat PP7 area of 12 prior to peel-off at the bonded area.

Figure 4. PhotosPhotos of of the the test test pieces pieces after after evaluation evaluation by by adhesion adhesion measurement: measurement: ( a)) T T-peel-peel test; ( b) tens tensileile shear test.

TToo evaluate evaluate the the adhesion adhesion strength strength of ofPP PPwith with other other types types of substrate, of substrate, h-PP h-PPmodified modified with BHA with- TBAEMABHA-TBAEMA was attached was attached to another to another test piece test of piece h-PP, of r h-PP,-PP, b r-PP,-PP, or b-PP, HDPE or HDPE,, also modified also modified with BHA with- TBAEMABHA-TBAEMA underunder the same the samemodification modification condition conditions.s. Both the Both T-peel the T-peel test and test tensile and tensile shear sheartests were tests werethen performed then performed on these on thesesamples. samples. The results The results were exhibited were exhibited in Figure in Figures 5 and 5Figure and S2. S2. ItIt cancan bebe found that the adhesion strength was enhanced not only for the test piece comprising h h-PP-PP attached with h h-PP,-PP, but but also also for for h- h-PPPP attached attached with with r-PP, r-PP, b-PP, b-PP, and and HDPE. HDPE. In addition, In addition, all the all test the pieces test pieces were werebroken broken at the at neat the area neat prior area priorto peel to-off peel-o at theff at bonded the bonded area. area.The adhesive The adhesive strength strength of the of h the-PP h-PPfilm modifiedfilm modified with withBHA BHA-TBAEMA-TBAEMA was further was further studied studied with withother othertypestypes of substrates of substrates which which were werenon- modifiednon-modified aluminum, aluminum, copper, copper, and and polyvinyl polyvinyl chloride chloride (PVC) (PVC) film film.. H Highigh adhesion adhesion strength strength was 2 detected in the modifiedmodified h-PPh-PP attached with the metal substrates such as aluminum (1.03 NN/mm/mm2)) 2 and copper (1.05 N/N/mm2),), where where the the modified modified h h-PP-PP film film was was broken broken at the neat area out of the 2 adhesives zone.zone. The adhesion adhesion strength of the modified modified h h-PP-PP and non non-modified-modified PVC was 0.26 N/mm N/mm2,, where thethe PVCPVC filmfilm was was elongated elongated prior prior to to peel-o peelff-offfrom from the the bonded bonded area. area. These These results results indicated indicated that surface modification with BHA-TBAEMA successfully enhances the adhesive properties of PP with Polymersthat surface 2020, 12 , modificationx FOR PEER REVIEW with BHA-TBAEMA successfully enhances the adhesive properties9 of 13 PP withother other plastic plastic and metal and metal substrates, substrates which, which widens wide thens range the range of applications of applications for this for material. this material.

Figure 5. Adhesion strength of the modified h-PP attached with other substrates (h-PP, r-PP, Figureb-PP, 5. and Adhesion HDPE), strength which were of the also modified modified h-PP with attached BHA-TBAEMA with other under substrates the optimized (h-PP, r-PP, conditions, b-PP, andand HDPE) non-modified, which were aluminum, also modified copper, with and BHA PVC-TBAEMA film. These under results the optimized were evaluated condition bys, and the tensilenon- modifiedshear tests. aluminum, copper, and PVC film. These results were evaluated by the tensile shear tests.

3.2. Surface Analysis of the Non-Modified PP and PP Modified with BHA-TBAEMA The surface morphology and roughness average (Sa) of the non-modified PP and PP modified with BHA-TBAEMA were determined by AFM and the results are shown in Figure 6. The non- modified PP (Figure 6a) showed a surface with the Sa value of 1.8 nm, while the surface of the modified PP (Figure 6b) was rougher with the Sa value of 3.5 nm. This confirmed that surface modification by BHA-TBAEMA resulted in a higher roughness and changes in the PP surface, which led to an enhancement of the adhesive properties.

Figure 6. AFM topography images and surface roughness average parameters: (a) non-modified PP; (b) PP modified with BHA-TBAEMA.

Figure 7 shows the TEM cross-sectional image of PP modified with BHA-TBAEMA. It can be found that the thin layer of BHA-TBAEMA was formed on the surface of PP with a thickness approximating 20 nm. Interestingly, some parts of the BHA-TBAEMA penetrated the PP layer. This implies that the long alkane chain crystalline unit of BHA-TBAEMA was not only adsorbed on the PP surface but also formed cocrystals and penetrated the inner PP layer.

Polymers 2020, 12, x FOR PEER REVIEW 9 of 13

Figure 5. Adhesion strength of the modified h-PP attached with other substrates (h-PP, r-PP, b-PP, and HDPE), which were also modified with BHA-TBAEMA under the optimized conditions, and non- modified aluminum, copper, and PVC film. These results were evaluated by the tensile shear tests. Polymers 2020, 12, 2736 8 of 12 3.2. Surface Analysis of the Non-Modified PP and PP Modified with BHA-TBAEMA

3.2. SurfaceThe surface Analysis morphology of the Non-Modified and roughness PP and average PP Modified (Sa)with of the BHA-TBAEMA non-modified PP and PP modified with BHA-TBAEMA were determined by AFM and the results are shown in Figure 6. The non- The surface morphology and roughness average (Sa) of the non-modified PP and PP modified with modified PP (Figure 6a) showed a surface with the Sa value of 1.8 nm, while the surface of the BHA-TBAEMA were determined by AFM and the results are shown in Figure6. The non-modified PP modified PP (Figure 6b) was rougher with the Sa value of 3.5 nm. This confirmed that surface (Figure6a) showed a surface with the Sa value of 1.8 nm, while the surface of the modified PP (Figure6b) modification by BHA-TBAEMA resulted in a higher roughness and changes in the PP surface, which was rougher with the Sa value of 3.5 nm. This confirmed that surface modification by BHA-TBAEMA led to an enhancement of the adhesive properties. resulted in a higher roughness and changes in the PP surface, which led to an enhancement of the adhesive properties.

Figure 6. AFM topography images and surface roughness average parameters: (a) non-modified PP; (b) PP modified with BHA-TBAEMA. Figure 6. AFM topography images and surface roughness average parameters: (a) non-modified PP; Figure(b) PP modified7 shows thewith TEM BHA cross-sectional-TBAEMA. image of PP modified with BHA-TBAEMA. It can be found that the thin layer of BHA-TBAEMA was formed on the surface of PP with a thickness approximating 20 nm.Figure Interestingly, 7 shows somethe TEM parts cross of the-sectional BHA-TBAEMA image of penetratedPP modified the with PP layer. BHA This-TBAEMA implies. It that can the be longfound alkane that chain the thin crystalline layer of unit BHA of- BHA-TBAEMATBAEMA was was formed not only on the adsorbed surface on of the PP PP with surface a thickness but also formedapproximating cocrystals 20 andnm. penetratedInterestingly, the some inner part PP layer.s of the BHA-TBAEMA penetrated the PP layer. This Polymersimplies 20 that20, 1 2the, x FOR long PEER alkane REVIEW chain crystalline unit of BHA-TBAEMA was not only adsorbed on9 of the 13 PP surface but also formed cocrystals and penetrated the inner PP layer.

Figure 7. TEM c cross-sectionalross-sectional imageimage of PP modifiedmodified with BHA-TBAEMA.BHA-TBAEMA.

The surface chemical composition of the non-modified PP, BHA-TBAEMA, and PP modified with The surface chemical composition of the non-modified PP, BHA-TBAEMA, and PP modified BHA-TBAEMA were analyzed and FTIR spectra were displayed in Figure8. The characteristic peaks with BHA-TBAEMA were analyzed and FTIR spectra were displayed in Figure 8. The characteristic of the non-modified PP were visible at 2951, 2915, 2837, 1455 and 1375 cm 1, which are typical alkane peaks of the non-modified PP were visible at 2951, 2915, 2837, 1455 and 1375− cm−1, which are typical (C–H) peaks [37]. On the other hand, the modified PP showed new peaks that were assigned to the alkane (C–H) peaks [37]. On the other hand, the modified PP showed new peaks that were assigned C–H at 2915 and 2850 cm 1,C=O ester group at 1733 cm 1, and C–O–C asymmetric stretching peak at to the C–H at 2915 and 2850− cm−1, C=O ester group at 1733− cm−1, and C–O–C asymmetric stretching 1160 cm 1. These new peaks corresponded to the characteristic peaks of BHA-TBAEMA. As a result, peak at 1160− cm−1. These new peaks corresponded to the characteristic peaks of BHA-TBAEMA. As a the surface of PP was successfully modified with BHA-TBAEMA via this surface modification method. result, the surface of PP was successfully modified with BHA-TBAEMA via this surface modification method.

Figure 8. FTIR spectra: (a) non-modified PP; (b) BHA-TBAEMA; (c) PP modified with BHA-TBAEMA.

3.3. Plausible Mechanism for PP Modification with SCCBC The schematic diagram of the plausible mechanism for the modification of PP with SCCBC is proposed in Figure 9. After dipping of the non-modified PP film into the SCCBC solution, the side- chain crystalline unit of the SCCBC was adsorbed on the PP film by van der Waals forces. Thus, cocrystals were formed between the side-chain crystalline unit of SCCBC and the PP alkane molecule. In addition, the functional unit of SCCBC covered the PP surface, thereby modifying the properties on the PP surface. As a result, the PP modified with BHA-TBAEMA exhibited the enhancement of adhesive properties, derived from the cocrystal of BHA on PP and the functional unit of TBAEMA on the PP surface. The conditions of the surface modification method affected the mechanism. As shown in Figure 9, the difference in dipping temperature affected the adhesive properties of the PP modified with SCCBC. At low dipping temperatures, a weak adhesion strength was detected because of the small number of cocrystals between the side-chain crystalline unit of SCCBC and PP molecules. Most of the side-chain crystalline unit was adsorbed on the surface, and was easy to separate from the PP. On the

Polymers 2020, 12, x FOR PEER REVIEW 9 of 13

Figure 7. TEM cross-sectional image of PP modified with BHA-TBAEMA.

The surface chemical composition of the non-modified PP, BHA-TBAEMA, and PP modified with BHA-TBAEMA were analyzed and FTIR spectra were displayed in Figure 8. The characteristic peaks of the non-modified PP were visible at 2951, 2915, 2837, 1455 and 1375 cm−1, which are typical alkane (C–H) peaks [37]. On the other hand, the modified PP showed new peaks that were assigned to the C–H at 2915 and 2850 cm−1, C=O ester group at 1733 cm−1, and C–O–C asymmetric stretching peak at 1160 cm−1. These new peaks corresponded to the characteristic peaks of BHA-TBAEMA. As a result, the surface of PP was successfully modified with BHA-TBAEMA via this surface modification Polymers 2020, 12, 2736 9 of 12 method.

Figure 8. FTIR spectra: (a) non-modified PP; (b) BHA-TBAEMA; (c) PP modified with BHA-TBAEMA. Figure 8. FTIR spectra: (a) non-modified PP; (b) BHA-TBAEMA; (c) PP modified with BHA-TBAEMA. 3.3. Plausible Mechanism for PP Modification with SCCBC 3.3. Plausible Mechanism for PP Modification with SCCBC The schematic diagram of the plausible mechanism for the modification of PP with SCCBC is proposedPolymersThe 20sch20 in,ematic Figure12, x FOR 9diagram. PEER After REVIEW dipping of the plausible of the non-modified mechanism PP for film the into modification the SCCBC solution,of PP with the SCCBC side-chain9 of is 13 proposedcrystalline in unit Figure of the 9. SCCBCAfter dipping was adsorbed of the non on the-modified PP film byPP van film der into Waals the forces.SCCBC Thus, solution cocrystals, the side were- chainformedother crystalline hand, between at high unit the dipping of side-chain the SCCBC temperatures crystalline was adsorbed, the unit PP of surface SCCBCon the moleculesPP and film the by PP could van alkane dermove molecule. Waals easily, forces. affect In addition, Thusing the, cocrystalsthepenetration functional were of unitformed the of side SCCBC between-chain covered crystalline the side the-chain PP unit surface, crystalline of SCCBC thereby unit in modifyingto of the SCCBC inner the and layer properties the of PP PP alkane on and the moleculeform PP surface.ing as. InAscocrystal addition, a result,s. Asthe the afunctional PPresult, modified enhance unit with ofd adhesiveSCCBC BHA-TBAEMA covered properties the exhibited wPPere surface exhibited the enhancement, thereby on the modif PP of surfaceying adhesive the modified properties properties, with onderivedSCCBC the PP in from surface. the thetypes cocrystalAs of a BHAresult, of-TBAEMA. BHAthe PP on modified PP and the with functional BHA-TBAEMA unit of TBAEMA exhibited on the the enhancement PP surface. of adhesive properties, derived from the cocrystal of BHA on PP and the functional unit of TBAEMA on the PP surface. The conditions of the surface modification method affected the mechanism. As shown in Figure 9, the difference in dipping temperature affected the adhesive properties of the PP modified with SCCBC. At low dipping temperatures, a weak adhesion strength was detected because of the small number of cocrystals between the side-chain crystalline unit of SCCBC and PP molecules. Most of the side-chain crystalline unit was adsorbed on the surface, and was easy to separate from the PP. On the

Figure 9. Schematic diagram of the plausible mechanism of PP modified with the SCCBC. Figure 9. Schematic diagram of the plausible mechanism of PP modified with the SCCBC. The conditions of the surface modification method affected the mechanism. As shown in Figure9, 3.4. Enhancement of the Hydrophilicity on the PP Surface by Modification with BHA-DEEA the difference in dipping temperature affected the adhesive properties of the PP modified with SCCBC. At lowEnhancement dipping temperatures, of the hydrophilicity a weak adhesion on the strength PP surface was detected is attractive because for of membrane the small number and other of cocrystalsapplications. between By using the side-chain the surface crystalline modification unit of SCCBC in this and study, PP molecules. SCCBC Most also of improve the side-chaind the crystallinehydrophilicity unit of was PP. adsorbed Figure 1 on0 show the surface,s the contact and was angle easy of to a separatewater droplet from theon PP.non On-modified the other PP hand, and atPP high modified dipping with temperatures, BHA-DEEA. the The PP hydrophobic surface molecules surface could of the move former easily, was adetectedffecting theat a penetration water contact of theangle side-chain of 101 ± crystalline1°. However, unit the of SCCBCwater contact into the angle inner decreased layer of PP to and 69 forming ± 4° on asthe cocrystals. surface of As the a result,latter, enhancedimplying that adhesive the hydrophilic properties were properties exhibited of PP on were the PP enhanced surface. modified With the with suitable SCCBC selection in the of types the offunctional BHA-TBAEMA. group and modification conditions, SCCBC was able to enhance any desirable properties for the wide application of the PP surface. 3.4. Enhancement of the Hydrophilicity on the PP Surface by Modification with BHA-DEEA Enhancement of the hydrophilicity on the PP surface is attractive for membrane and other applications. By using the surface modification in this study, SCCBC also improved the hydrophilicity of PP. Figure 10 shows the contact angle of a water droplet on non-modified PP and PP modified with BHA-DEEA. The hydrophobic surface of the former was detected at a water contact angle of 101 1 . ± ◦

Figure 10. Water droplet images and the surface contact angle measurement: (a) non-modified PP; (b) PP modified with BHA-DEEA.

4. Conclusions The facile surface modification method for the enhancement of adhesive properties and hydrophilicity on the PP surface was developed. SCCBC comprising a side-chain crystalline unit and functional unit plays a key role in improving the surface properties of PP. The simple surface modification method comprised dipping of the PP film in a diluted solution of SCCBC. The optimized conditions were found to be 0.1 wt% BHA-TBAEMA in xylene solution at 80 °C for 5 min. As a result, the PP adhesion strength was enhanced to 2.00 N/mm for the T-peel test and 1.05 N/mm2 for the tensile shear test, in which the test pieces were broken at the neat area prior to peel-off at the bonded area. In addition, the adhesion strength was enhanced by h-PP attached to h-PP, other plastics, and metal substrates. From the surface characterization of PP modified with BHA-TBAEMA, it can be confirmed that BHA-TBAEMA adsorbed on the surface of PP. Moreover, it was deduced that the

Polymers 2020, 12, x FOR PEER REVIEW 9 of 13 other hand, at high dipping temperatures, the PP surface molecules could move easily, affecting the penetration of the side-chain crystalline unit of SCCBC into the inner layer of PP and forming as cocrystals. As a result, enhanced adhesive properties were exhibited on the PP surface modified with SCCBC in the types of BHA-TBAEMA.

Figure 9. Schematic diagram of the plausible mechanism of PP modified with the SCCBC.

3.4. Enhancement of the Hydrophilicity on the PP Surface by Modification with BHA-DEEA Enhancement of the hydrophilicity on the PP surface is attractive for membrane and other applications. By using the surface modification in this study, SCCBC also improved the Polymers 2020, 12, 2736 10 of 12 hydrophilicity of PP. Figure 10 shows the contact angle of a water droplet on non-modified PP and PP modified with BHA-DEEA. The hydrophobic surface of the former was detected at a water contact However,angle of 101 the ± water1°. However, contact anglethe water decreased contact to angle 69 decreased4 on the surface to 69 ± of4° theon the latter, surface implying of the that latter, the ± ◦ hydrophilicimplying that properties the hydrophilic of PP were properties enhanced. of PPWith were the suitableenhanced selection. With the of the suitable functional selection group of and the modificationfunctional group conditions, and modification SCCBC was condition able to enhances, SCCBC any was desirable able to properties enhance any for thedesirable wide applicationproperties offor the the PP wide surface. application of the PP surface.

Figure 10. WaterWater droplet droplet images images and and the the surface surface contact contact angle angle measurement measurement:: (a) (naon) non-modified-modified PP; PP;(b) (PPb) modified PP modified with with BHA BHA-DEEA.-DEEA.

4. Conclusions The facilefacile surface surface modification modification method method for the for enhancement the enhancement of adhesive of properties adhesive and properties hydrophilicity and onhydrophilicity the PP surface on wasthe PP developed. surface was SCCBC developed. comprising SCCBC a side-chain comprising crystalline a side-chain unit crystalline and functional unit unitand playsfunctional a key unit role plays in improving a key role the surfacein improv propertiesing the of surface PP. The properties simple surface of PP.modification The simple methodsurface comprisedmodification dipping method of thecomprised PP film indipping a diluted of solutionthe PP film of SCCBC.in a diluted The optimizedsolution of conditions SCCBC. The were optimized found to becondition 0.1 wt%s BHA-TBAEMAwere found to be in 0.1 xylene wt% solution BHA-TBAEMA at 80 ◦C for in 5xylene min. As solution a result, at the 80 PP°C adhesionfor 5 min. strength As a result, was 2 enhancedthe PP adhesion to 2.00 N strength/mm for thewas T-peel enhanced test and to 1.052.00 N N/mm/mm for for the the tensile T-peel shear test test, and in 1.05 which N/mm the test2 for pieces the weretensile broken shear attest the, in neat which area the prior test to pieces peel-off were at the broken bonded at the area. neat In area addition, prior the to peel adhesion-off at strength the bonded was enhancedarea. In addition, by h-PP attachedthe adhesion to h-PP, strength otherplastics, was enhanced and metal by substrates.h-PP attached From to the h- surfacePP, other characterization plastics, and ofmetal PP modifiedsubstrates. with From BHA-TBAEMA, the surface characterization it can be confirmed of PP that modified BHA-TBAEMA with BHA adsorbed-TBAEMA on the, it surfacecan be ofconfirmed PP. Moreover, that BHA it was-TBAEMA deduced thatadsorbed the BHA on side-chainthe surface crystalline of PP. Moreover unit penetrated, it was thededuced inner PPthat layer the to form cocrystals, while the functional TBAEMA unit covered the PP surface, thereby enhancing the adhesive properties of the material. In addition, PP modified with BHA-DEEA exhibited the increase of hydrophilicity, with a water contact angle of 69 4 . With the suitable selection of the functional group ± ◦ and modification conditions, SCCBC enhances any desirable properties to broaden the applications of the PP surface. In addition, this surface modification method was easily scaled up to an industrial scale because of its simple manipulation and its mild and wide range of modification conditions.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/11/2736/s1, Figure S1: Adhesion strength of the non-modified PP and PP modified with the SCCBC evaluated by the T-peel tests: (a) 0.1 wt% of BHA-TBAEMA in xylene for 10 min at different dipping temperatures; (b) 0.1 wt% of BHA-TBAEMA in xylene at 80 ◦C with different dipping times; (c) BHA-TBAEMA at 80 ◦C for 5 min at different concentrations of BHA-TBAEMA in xylene solution; (d) 0.1 wt% of BHA-TBAEMA in different solvent types at 80 ◦C for 5 min. Figure S2: Adhesion strength of the modified h-PP attached with other substrates, which were also modified with BHA-TBAEMA under the optimized condition. These results were evaluated by the T-peel tests. Author Contributions: Conceptualization, S.H. and S.Y.; methodology, S.H.; validation, P.P.; investigation, S.H. and H.O.; data curation, S.H. and H.O.; writing—original draft preparation, S.H.; writing—review and editing, P.P.; supervision, S.Y.; project administration, S.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by JSPS KAKENHI, grant number JP18K14010. Conflicts of Interest: The authors declare no conflict of interest. Polymers 2020, 12, 2736 11 of 12

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