Reactive Compatibilization of Polyamide 6/Olefin Block

materials

Article Reactive Compatibilization of Polyamide 6/Oleﬁn Block Copolymer Blends: Phase Morphology, Rheological Behavior, Thermal Behavior, and Mechanical Properties

Xintu Lin 1, Yuejun Liu 1,*, Xi Chen 2, Yincai Wu 2, Lingna Cui 1, Long Mao 3, Wei Zheng 2 and Minghao Lin 3

1 Key Laboratory of Advanced Packaging Materials and Technology of Hunan Province, School of Packaging and Materials Engineering, Hunan University of Technology, Zhuzhou 412007, China; [email protected] (X.L.); [email protected] (L.C.) 2 Xiamen Changsu Industrial Company, Limited, Xiamen 361026, China; [email protected] (X.C.); [email protected] (Y.W.); [email protected] (W.Z.) 3 Key Laboratory of Polymer Processing Principle and Application, Xiamen University of Technology, Xiamen 361024, China; [email protected] (L.M.); [email protected] (M.L.) * Correspondence: [email protected]

Received: 16 December 2019; Accepted: 2 March 2020; Published: 5 March 2020

Abstract: In this study, the morphology, rheological behavior, thermal behavior, and mechanical properties of a polyamide 6 (PA6) and olefin block copolymer (OBC) blend compatibilized with maleic anhydride-grafted polyethylene-octene copolymer (POE-g-MAH) were investigated. The morphological observations showed that the addition of POE-g-MAH enhanced the OBC particle dispersion in the PA6 matrix, suggesting a better interfacial compatibility between the pure PA6 and OBC. The results of the Fourier transform infrared (FTIR) spectroscopy analysis and the Molau test confirmed the compatibilization reactions between POE-g-MAH and PA6. The rheological test revealed that the melt viscosity, storage modulus (G’), and loss modulus (G”) of the compatibilized PA6/OBC blends at low frequency were increased with the increasing POE-g-MAH content. The thermal analysis indicated that the addition of OBC had little effect on the crystallization behavior of PA6, while the incorporation of POE-g-MAH at high content (7 wt%) in the PA6/OBC blend restricted the crystallization of PA6. In addition, the compatibilized blends exhibited a significant enhancement in impact strength compared to the uncompatibilized PA6/OBC blend, in which the highest value of impact strength obtained at a POE-g-MAH content of 7 wt% was about 194% higher than that of pure PA6 under our experimental conditions.

Keywords: compatibilization; melt blending; oleﬁn block copolymer; polyamide 6; rheology

1. Introduction Polymer blending modification is a widely used method to obtain materials with improved properties [1–3]. Many polymers, including poly(ethylene terephthalate) (PET) [4], polypropylene (PP) [5], poly(butylene terephthalate) (PBT) [6], polyamide [7], and poly(lactic acid) (PLA) [8,9], have been modified by blending modification. Polyamide 6 (PA6), a typical semi-crystalline polymer with excellent chemical and physical properties, has been widely used in automotive industries, electronics, chemical industries, and films, etc. However, the notched impact strength of PA6 is low, especially at low temperatures, which limits its wide applications. As a result, many efforts have been devoted to improving the toughness

Materials 2020, 13, 1146; doi:10.3390/ma13051146 www.mdpi.com/journal/materials Materials 2020, 13, 1146 2 of 16 of PA6 by melt blending with flexible polymers and elastomers, such as low density polyethylene (LDPE) [10], high density polyethylene (HDPE) [11,12], acrylonitrile-butadiene-styrene (ABS) [13,14], poly(ethylene-octene) (POE) [15,16], ethylene-propylene-diene rubber (EPDM) [17], ethylene vinyl acetate (EVA) [18], maleated styrene-ethylene-butylene-styrene (SEBS-g-MAH) [19], and maleated ethylene-octene copolymer (EOC-g-MAH) [20]. However, PA6 and flexible polymers and elastomers are often immiscible due to the difference of polarities, leading to a phase-separated morphology and poor compatibility between the PA6 matrix and the dispersed phase, which will result in unsatisfactory mechanical properties. Therefore, improving the compatibility between PA6 and flexible polymers and elastomers is critically important to achieve a high fracture toughness of the blends. Generally, blending PA6 with functionalized flexible polymers or functionalized elastomers or adding a third component, like a copolymer or compatibilizer, is a widely used method to improve the compatibility between PA6 and flexible polymers and elastomers. The functional group of the modified flexible polymer and compatibilizer, which is usually maleic anhydride or an epoxy group, can react with the amine groups of PA6 to form a graft copolymer, which can improve the compatibility between the PA6 matrix and the dispersed phase and then obtain the desired properties of the blends [21–24]. Xie et al. [15] prepared a series of POE-g-MAH as the compatibilizer for a PA6/POE blend, and they found that the increase of the grafting degree of maleic anhydride in the POE-g-MAH increased the notched Izod impact strength of the compatibilized PA6/POE blend. Das et al. [25] revealed that the mechanical properties of PA6/linear low-density polyethylene (PA6/LLDPE) blends were increased by using maleated LLDPE (LLDPE-g-MAH) as the reactive compatibilizer. Huang et al. [26] found that the impact strength of a PA6/ABS blend containing 20 wt% ethylene-acrylate-glycidyl methacrylate copolymer (EAGMA) was about 323% higher than that of pure PA6. Recently, olefin block copolymer (OBC), a novel thermoplastic polyolefin elastomer, was synthesized by the Dow Chemical Company through the method of chain shuttling polymerization [27]. Compared with random ethylene-octene copolymers, OBC exhibits a higher melting temperature, higher heat resistance and lower glass transition temperature, while maintaining excellent elastomeric properties at high temperatures [28]. The high toughness of OBC makes it a excellent toughening agent for plastics, and many studies have demonstrated that OBC can be used as an effective toughening agent [29–31]. Ren et al. [29] found that the addition of OBC effectively increased the Izod impact strength of polypropylene random copolymers (PPR). To the best of our knowledge, relatively few studies have been performed on the use of OBC as an impact modifier for PA6 based blends. Due to polarity difference between PA6 and OBC, it is necessary to improve the compatibility between the OBC dispersed phase and PA6 matrix to obtain the desired high mechanical performance. Previous researchers have reported that maleic anhydride-grafted POE (POE-g-MAH) can be used as an effective compatibilizing agent for PA6 based blends [15,32]. Thus, it was expected that the compatibility of PA6/OBC blends could be enhanced by using POE-g-MAH as a compatibilizer whose compatibilizing effects on this blend are not yet systematically investigated and reported in the literature. In this work, the influence of a POE-g-MAH compatibilizer on the morphology, rheological behavior, thermal behavior, and mechanical properties of PA6/OBC blends was determined by using scanning electron microscopy (SEM), rheometry, differential scanning calorimetry (DSC) and tensile, flexural, and notched Charpy impact tests, respectively. Furthermore, Fourier transform infrared (FTIR) spectroscopy analysis and the Molau test were also carried out to explore the possible chemical interactions between the PA6 matrix and POE-g-MAH. Materials 2020, 13, 1146 3 of 16

2. Experimental

2.1. Materials Polyamide 6 (PA6) (Novamid® 1010C2) with a density of 1.13 g/cm3 was purchased from the DSM Engineering Plastics, the Netherlands. Oleﬁn block copolymer (OBC) (INFUSE ™ 9107) was supplied by the Dow Chemical Company, Michigan, USA. The characteristics of the OBC are as follows: a density 3 of 0.866 g/cm , a melt index of 1.0 g/10 min (190 ◦C, 2.16 Kg), ultimate tensile strength of 5.1 Mpa, and ultimate tensile elongation of 600%. Maleic anhydride-grafted poly(ethylene-octene) (POE-g-MAH, CMG5805-L) with 0.8 wt% maleic anhydride was obtained from Fine-blend Compatilizer Jiangsu Co., Ltd., Nantong, China. Formic acid (AR, 98%) was supplied by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China.

2.2. Preparation of the Blends

PA6 pellets were dried at 80 ◦C for 12 h in a hot air oven. OBC and POE-g-MAH were dried using a vacuum oven at 50 ◦C for 12 h. Then, all specimens were prepared in a co-rotating twin-screw extruder (CHT35/600-18.5-40, L/D = 40, Nanjing Ruiya Polymer Processing Equipment Company, Nanjing, China). The screw speed was set at 120 rpm and the temperature proﬁle from the feed zone to die zone was set at 215, 220, 225, 230, 230, 230, 235, 240, 240, and 235 ◦C, respectively. The extrudates were then chopped into small pellets. Finally, the obtained pellets were dried at 85 ◦C for 12 h and injection-molded into standard specimens using an injection molding machine (PL860-260, Haitian plastics machinery Co., Ltd., Wuxi, Jiangsu, China). The barrel temperatures were 210–240 ◦C, and the mold temperature was kept at 25 ◦C. The compositions of the PA6/OBC/POE-g-MAH blends under investigation are given in Table1.

Table 1. The compositions of the studied blends.

Maleic Anhydride-Grafted Polyamide 6 Oleﬁn Block Copolymer Sample Code Polyethylene-Octene Copolymer (PA6) (wt%) (OBC) (wt%) (POE-g-MAH) (wt%) Pure PA6 100 0 0 PA6/OBC 90 10 0 PA6/OBC/1 90 10 1 PA6/OBC/3 90 10 3 PA6/OBC/5 90 10 5 PA6/OBC/7 90 10 7

2.3. Characterization Fourier transform infrared (FTIR) spectrometry (Nicolet iS5, Thermo Scientiﬁc, Madison, WI, USA) was used to study the possible chemical interactions between PA6 and POE-g-MAH. The Molau test was performed by mixing about 103 mg of the samples in 20 mL of 98% formic acid in diﬀerent test tubes. The mixture was stored at room temperature for 22 h. The morphology of the blends was investigated using scanning electron microscopy (SEM) (MIRA3 LMU, TESCAN, Brno, Czech). The specimens were fractured in liquid nitrogen and then coated with gold. The number average diameter (Dn), weight average diameter (Dw), and polydispersity index (PDI) of the OBC particles were calculated using the following equations [33]: P NiDi Dn = P (1) Ni

P 2 NiDi Dw = P (2) NiDi Materials 2020, 13, 1146 4 of 16

D PDI = w (3) Dn where Di is the diameter of OBC particles with the number Ni. Rheological behaviors of the samples were measured using a rheometer (Discovery HR-2, TA Instruments, New Castle, DE, USA) with a plate diameter of 25 mm at 230 ◦C. The frequency sweep test was performed from 0.1 and 628 rad/s, and the strain was 0.5% under linear viscoelastic conditions. Diﬀerential scanning calorimetry (DSC) (DSC1/500, Mettler Toledo, Zurich, Switzerland) was used to investigate the thermal properties of the samples. The specimens were initially heated from 30 ◦C to 260 ◦C at 20 ◦C /min and held at 260 ◦C for 5 min to erase the prior thermal history, and then cooled to 30 ◦C at 20 ◦C /min to determine the crystallization temperature (Tc), and reheated again to 260 ◦C with the same heating rate to determine the melting temperature (Tm). The degree of crystallinity (Xc) of PA6 and OBC was calculated from the melting enthalpy values using the following Equation (4):

∆H X (%) = m 100 (4) c 0 wf∆Hm ∗ where ∆Hm is the melting enthalpy of PA6 or OBC in the samples, wf is the weight percent of PA6 or 0 OBC in the samples, and ∆Hm is the melting enthalpy of 100% crystalline for PA6 (230 J/g) [34] and OBC (290 J/g) [35]). Notched Charpy impact tests were performed on an impact tester (PIT501J, Shenzhen Wance Testing Machine Co., Ltd., Shenzhen, China) at room temperature according to Chinese Standard GB/T1043-1993, and the specimens (80 mm in length, 10 mm in width, and 4 mm in thickness) were used with a 45◦ V-shaped notch and a notch depth of 0.8 mm (Figure1a). For each condition, more than five samples were tested to determine the average values. Tensile and flexural tests were carried out using a micro-controlled electronic universal testing machine (ETM502B-Ex, Shenzhen Wance Testing Machine Co., Ltd., Shenzhen, China) at room temperature according to Chinese Standard GB/T1040-2006 and GB/T9341-2008, and crosshead speeds of 50 mm/min and 20 mm/min were used, respectively. The dimensions of the specimens for the flexural tests were 80 mm 10 mm 4 mm (length width thickness) (Figure1b). The specimens × × × × for the tensile tests were dumbbell-shaped with dimensions of 150 mm 10 mm 4 mm (length × × × width narrow parallel portion thickness) (Figure1c). For each condition, five samples were tested, × and the results were averaged arithmetically. Materials 2020, 13, x FOR PEER REVIEW 5 of 17

FigureFigure 1. (a )1. Notched (a) Notched Charpy Charpy impact impact test specimen,test specimen, (b) flexural (b) flexural test specimen,test specimen and, (andc) tensile (c) tensile test specimen. test specimen.

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Materials3. Results2020, 13and, 1146 Discussion 5 of 16

3.1. Fourier Transform Infrared (FTIR) Spectroscopy Analysis 3. Results and Discussion Figure 2 depicts the FTIR spectra of pure PA6, pure OBC, and the PA6/OBC blends with and 3.1.wit Fourierhout POE Transform-g-MAH. Infrared As shown (FTIR) in Spectroscopy Figure 2, pure Analysis PA6 exhibited absorption characteristic bands at 1538 cm−1 (the combination of bending vibration of the N–H group and stretching vibration of the C– Figure2 depicts the FTIR spectra of pure PA6, pure OBC, and the PA6 /OBC blends with and N group (Amide II, N–H bending + C–N stretching)), at 1633 cm−1 (stretching vibration of the C=O without POE-g-MAH. As shown in Figure2, pure PA6 exhibited absorption characteristic bands at group (Amide I, C=O stretching)), at 2932 cm−1 (stretching vibration of the C–H group (C–H 1538 cm 1 (the combination of bending vibration of the N–H group and stretching vibration of the C–N stretching)),− as well as bands at 3294 cm−1 (stretching vibration of the N–H group (N–H stretching)). group (Amide II, N–H bending + C–N stretching)), at 1633 cm 1 (stretching vibration of the C=O group After addition of OBC and POE-g-MAH to PA6, the − spectra of the resulting PA6/OBC and (Amide I, C=O stretching)), at 2932 cm 1 (stretching vibration of the C–H group (C–H stretching)), PA6/OBC/POE-g-MAH blends showed− similar characteristic bands as those of PA6, and there was as well as bands at 3294 cm 1 (stretching vibration of the N–H group (N–H stretching)). After addition no appearance of new absorption− bands. of OBC and POE-g-MAH to PA6, the spectra of the resulting PA6/OBC and PA6/OBC/POE-g-MAH blendsHowever, showed similarwe noted characteristic that the N–- bandsH and asC=O those characteristic of PA6, and bands there of wasPA6 noidentified appearance on the of spectra new absorptionof the compatibilized bands. PA6/OBC blend were slightly stronger than those of pristine PA6 and the umcompatibilizedHowever, we noted samples. that the In N–Haddition, and C the=O absorption characteristic characteristic bands of PA6 bands identiﬁed corresponding on the spectra to the ofmaleic the compatibilized anhydride groups PA6 /ofOBC POE blend-g-MAH were disa slightlyppeared stronger in the thancompatibilized those of pristine PA6/OBC PA6 blend. and theThis umcompatibilizedobservation indicates samples. the In occurrence addition, theof aabsorption reaction characteristic between the bands PA6 and corresponding the POE-g to-MAH the maleiccompatibilizer. anhydride As groups shown of in POE-g-MAH Figure 3, when disappeared POE-g-MAH in was the compatibilizedincorporated into PA6 the/OBC uncompatibilized blend. This observationPA6/OBC blend, indicates the the amine occurrence groups ofof aPA6 reaction reacted between with the maleic PA6 and anhydride the POE-g-MAH groups compatibilizer.of POE-g-MAH Asto shownform the in functional Figure3, when groups POE-g-MAH of N–-H and was C=O, incorporated which are overlapped into the uncompatibilized with the original PA6 absorption/OBC blend,characteristic the amine bands groups of of PA6. PA6 reacted A similar with observation the maleic anhydride was reported groups by of Marco POE-g-MAH et al. to [36] form for the the functionalpolypropylene groups of functionalized N–H and C= O, with which maleic are overlapped anhydride with (PP the- originalg-MA) absorption compatibilized characteristic isotactic bandspolypropylene of PA6. A(iPP)/nylon similar observation 6 (PA6) blends. was This reported finding by is Marco in correspondence et al. [36] for with the the polypropylene result derived functionalizedfrom the Molau with test maleic in the anhydride next section. (PP-g-MA) compatibilized isotactic polypropylene (iPP)/nylon 6 (PA6) blends. This ﬁnding is in correspondence with the result derived from the Molau test in the next section.

Figure 2. Fourier transform infrared (FTIR) spectra of (a) pure OBC, (b) pure POE-g-MAH, (c) pure PA6, Figure 2. Fourier transform infrared (FTIR) spectra of (a) pure OBC, (b) pure POE-g-MAH, (c) pure (d) the uncompatibilized PA6/OBC blend, and (e) the 7 wt% POE-g-MAH compatibilized PA6/OBC PA6, (d) the uncompatibilized PA6/OBC blend, and (e) the 7 wt% POE-g-MAH compatibilized blend (PA6/OBC/7). PA6/OBC blend (PA6/OBC/7).

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Figure 3. Possible chemical reaction between PA6 and POE-g-MAH. (a) the reaction between MAH

groups of POE-g-MAH and –NH2 groups of PA6; (b) the reaction between MAH groups of POE-g-MAH andFigure –NH– 3. Possible groups ofchemical PA6. reaction between PA6 and POE-g-MAH. (a) the reaction between MAH groups of POE-g-MAH and –NH2 groups of PA6; (b) the reaction between MAH groups of POE-g- 3.2. MolauMAH and Test –NH– groups of PA6. The Molau test was used to conﬁrm the formation of a grafted copolymer in the compatibilized PA63.2. Molau/OBC Test blend, and the results of Molau test of pure PA6, pure OBC, pure POE-g-MAH, the uncompatibilizedThe Molau test was PA6 used/OBC to blend,confirm and the the formation 7 wt% POE-g-MAH of a grafted copolymer compatibilized in the PA6 compa/OBCtibilized blend (PA6PA6/OBC/OBC /7) blend are shown, and the in Figureresults4. ofIt was Molau observed test of that pure pure PA6, PA6 pure was completely OBC, pure dissolved POE-g-MAH, in formic the aciduncompatibilized and exhibited PA6/OBC a transparent blend, solutionand the (Figure 7 wt%4c), POE while-g-MAH pure OBCcompatibilized and pure POE-g-MAH PA6/OBC blend was insoluble(PA6/OBC/7) (Figure are4 showna,b). As in shown Figure in 4. Figure It was4d, observed for the uncompatibilized that pure PA6 was PA6 completely/OBC blend, dissolved PA6 phase in wasformic dissolved acid and in exhibited formic acid a transparent whereas the solution OBC phase (Figure ﬂoated 4c), on while the toppure of OBC the solution, and pure indicating POE-g-MAH poor interfacialwas insoluble adhesion (Figure between 4a,b). As the shown PA6 matrix in Figure and 4d, the for OBC the dispersed uncompatibilized phase. However, PA6/OBC a milky blend, white PA6 solutionphase was was dissolved observed afterin formic the addition acid whereas of 7 wt% the POE-g-MAH OBC phase to floated the PA6 / onOBC the blend top (Figure of the 4 solution,e). This phenomenonindicating poor indicates interfacial that adhesion a grafted between copolymer the wasPA6formed matrix and between the OBC the PA6dispersed and OBC phase. phase However, during thea milky melt white blending, solution which was can observed act as an after emulsifying the addition agent, of and7 wt% enhance POE-g the-MAH compatibility to the PA6/OBC between blend the two(Figu phases.re 4e). This phenomenon indicates that a grafted copolymer was formed between the PA6 and OBC phase during the melt blending, which can act as an emulsifying agent, and enhance the compatibility between the two phases.

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Figure 4. 4.The The Molau Molau tests tests of (a ) of pure (a OBC,) pure (b ) OBC, pure POE-g-MAH, (b) pure POE (c)- pureg-MAH, PA6, (dc) thepure uncompatibilized PA6, (d) the PA6uncompatibilized/OBC blend, and PA6/OBC (e) the 7blend, wt% POE-g-MAH and (e) the 7 compatibilized wt% POE-g-MAH PA6/ OBCcompatibilized blend (PA6 PA6/OBC/OBC/7). blend (PA6/OBC/7). 3.3. Morphological Analyses 3.3. MorphologicalThe phase morphologies Analyses of the uncompatibilized and compatibilized PA6/OBC blends were investigatedThe phase by morphologies SEM. Figure of5 delineates the uncompatibilized the cryo-fractured and compatibilized surfaces of PA6/OBC PA6 /OBC blends and were the PA6investigated/OBC/POE-g-MAH by SEM. blends. Figure 5For delineates the uncompatibilized the cryo-fractured PA6/OBC surfaces blend of (Figure PA6/OBC5a),the and large the voidsPA6/OBC/POE with clear-g contours-MAH blends. representing For the uncompatibilized the OBC dispersed PA6/OBC particles blend pulled (Figure out from 5a), the the PA6 large matrix, voids arewith readily clear contours observable representing on the fractured the OBC surfaces. dispersed The OBC particles particles pulled were out poorly from dispersedthe PA6 matrix, in the PA6are µ matrixreadily withobservable a larger on size the of fractured about 8.2 surfaces.m. This The revealed OBC particles the poor compatibilitywere poorly dispersed and weak in interfacial the PA6 interactionsmatrix with betweena larger thesize PA6 of about and the 8.2 OBC μm. phases.This revealed In general, the poor the final compatibility morphologies and ofweak the immiscibleinterfacial blendsinteractions are believed between to thbee aff PA6ected and by the the droplet OBC phases. break-up In and general, coalescence the final [37 , morphologies38]. According of to the workimmiscible of Sundararaj blends et are al. [believed39], for uncompatibilized to be affected by blends, the droplet higher concentrations break-up and of coalescence the dispersed [37,38]. phase causedAccording larger to particlethe work sizes of Sundararaj due to increased et al. [39], coalescence. for uncompatibilized With the addition blends, of OBC, higher the concentr coalescenceations of OBCof the particles dispersed became phase relatively caused larger obvious particle due to sizes the highdue to interfacial increased tension coalescence. between With the PA6the addition matrix and of theOBC, OBC the dispersedcoalescence phase, of OBC resulting particles thus became in coarsened relatively morphology. obvious due The to poor the high compatibility interfacial between tension thebetween PA6 and the OBCPA6 phasematrix was and also the responsibleOBC dispersed for the phase, limited resulting improvement thus in coarsened in the mechanical morphology. properties. The Therefore,poor compatibility a suitable compatibilizer between the wasPA6 needed and OBC to promote phase the was dispersion also responsible of the OBC for particles the limited in the PA6improvement matrix. in the mechanical properties. Therefore, a suitable compatibilizer was needed to promoteWhen the the dispersion POE-g-MAH of the was OBC added particles to thein the PA6 PA6/OBC matrix. blend, the scale and the number of voids corresponding to the pulled out OBC particles decreased, and the particle size of the OBC phase decreasedWhen significantlythe POE-g-MAH with was increasing added to POE-g-MAH the PA6/OBC content, blend, asthe can scale be and observed the number in Figure of voids5b,c. Thecorresponding average particle to the sizepulled of OBCout OBC in the particles uncompatibilized decreased, PA6and/ OBCthe particle blend wassize 8.2of µthem OBC (Figure phase5a), whereasdecreased it significantly decreased from with 8.2 increasing to 5.7 µm POE with-g the-MAH increase content, of POE-g-MAH as can be observed contents in fromFigure 0 5b,c. to 3 wt%The (Figureaverage5 a,c).particle In addition, size of OBC the in PDI the decreased uncompatibilized from 1.29 PA6/OBC for uncompatibilized blend was 8.2 PA6 μm/OBC (Figure blend 5a), (Figure whereas5a) toit decreased 1.15 for PA6 from/OBC 8.2 to blends 5.7 μm containing with the increase 3 wt% POE-g-MAH of POE-g-MAH (Figure contents5c). fromHemelrijck 0 to 3 wt% et al. (Figure [40,41] suggested5a,c). In addition, that the the presence PDI decreased of compatibilizers from 1.29 couldfor uncompatibilized improve the compatibility PA6/OBC blend between (Figure the phases, 5a) to and1.15 resultedfor PA6/OBC thus inblends a finer containing morphology 3 wt% by POE reducing-g-MAH the (Figure interfacial 5c). tension Hemelrijck and et suppressing al. [40,41] dropletsuggested coalescence. that the presence of compatibilizers could improve the compatibility between the phases, and resultedThe decrease thus in in a particle finer morphology size distribution by reducing and the the average interfacial particle tension size and of thesuppressing OBC phase droplet was probablycoalescence. due to the inhibition of the coalescence of the dispersed phase and the decrease of interfacial tension caused by the compatibilizing effect of POE-g-MAH. For the compatibilized PA6/OBC blends The decrease in particle size distribution and the average particle size of the OBC phase was containing high concentrations of POE-g-MAH (>3 wt%), it was found that the OBC particles were probably due to the inhibition of the coalescence of the dispersed phase and the decrease of interfacial almost invisible and the phase interface between the PA6 matrix and OBC phase became blurred tension caused by the compatibilizing effect of POE-g-MAH. For the compatibilized PA6/OBC blends (Figure5d,e). This observation was indicative of the suppression of droplet coalescence and the containing high concentrations of POE-g-MAH (>3 wt%), it was found that the OBC particles were improved compatibility and interface combination between the PA6 matrix and OBC phase. Therefore, almost invisible and the phase interface between the PA6 matrix and OBC phase became blurred the compatibilized PA6/OBC blends exhibited finer morphology of the dispersed phase, which (Figure 5d,e). This observation was indicative of the suppression of droplet coalescence and the was responsible of the increased mechanical properties of the PA6/OBC blends as discussed in the improved compatibility and interface combination between the PA6 matrix and OBC phase. corresponding section. The results obtained from the morphological analyses are also consistent with Therefore, the compatibilized PA6/OBC blends exhibited finer morphology of the dispersed phase, those obtained from the rheological analyses discussed in the next section. which was responsible of the increased mechanical properties of the PA6/OBC blends as discussed in the corresponding section. The results obtained from the morphological analyses are also consistent with those obtained from the rheological analyses discussed in the next section.

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Figure 5. Scanning electron electron microscopy microscopy (SEM) (SEM) images images of of the the cryo cryo-fractured-fractured surfaces surfaces of of ( (aa)) PA PA66/OBC,/OBC, (b) PA6PA6/OBC/1,/OBC/1, ((cc)) PA6PA6/OBC/3,/OBC/3, ( d(d)) PA6 PA6/OBC/5/OBC/5 and and (e ()e PA6) PA6/OBC/7./OBC/7.

3.4. Rheological Behavior Figure 66 illustratesillustrates thethe complexcomplex viscosityviscosity ((η*)η*) of of pure pure PA6, PA6, pure OBC, and the PA6/OBC PA6/OBC and PA6PA6/OBC/POE/OBC/POE-g-MAH-g-MAH blends with with different different contents contents of of POE POE-g-MAH-g-MAH as as a function a function of offrequency. frequency. It Itwas was observed observed that that PA6 PA6 exhibited exhibited a aNewtonian Newtonian behavior behavior in in the the lower lower frequency frequency region region followed followed by a shear-thinningshear-thinning behavior in the higher frequency region, while OBC showed a typical shear shear-thinning-thinning behavior over the entire studied ra range.nge. This This may may be be due due to to the the much much more more rigid rigid structure of PA6 compared with with OBC. OBC. It Itwas was also also noted noted that thethat addition the addition of OBC of enhanced OBC enhanced the shear-thinning the shear- behaviorthinning ofbehavior pure PA6 of andpure caused PA6 and the Newtoniancaused the plateauNewtonian to shift plateau toward to shift lower toward frequencies. lower Thisfrequencies. behavior This was morebehavior visible was in more the PA6visible/OBC in/ thePOE-g-MAH PA6/OBC/POE blends,-g-MAH which blends indicated, which that indicat compatibilizered that compatibilizer POE-g-MAH withPOE- flexibleg-MAH chains with contributedflexible chains a lot contributed to the observed a lot shear-thinning to the observed behavior. shear In-thinning addition, behavior. the complex In viscosityaddition, ofthe pure complex PA6 increased viscosity appreciablyof pure PA6 with increased the addition appreciably of OBC. with This the was addi probablytion of OBC. due to This the inherentwas probably larger due melt-viscosity to the inherent of OBC larger and melt more-viscosity entanglement of OBC points and more formed entanglement in the melt points caused formed by the additionin the melt of OBC,caused resulting by the addition in the larger of OBC, viscosity resulting of the in uncompatibilized the larger viscosity PA6 of/OBC the uncompatibilized blend. PA6/OBCWith blend. the incorporation of POE-g-MAH, it can be seen that the complex viscosity of PA6/OBC/POE-g-MAH blends increased gradually with increasing POE-g-MAH contents, especially in theWith low frequency the incorporation range. To get of a POE straightforward-g-MAH, it comparison, can be seen the that complex the viscosity complex values viscosity of pure of PA6,PA6/OBC/POE pure OBC,-g PA6-MAH/OBC blends and PA6increased/OBC/POE-g-MAH gradually with blends increasing with di POEfferent-g-MAH amounts contents, of POE-g-MAH especially in the low frequency range.1 To get a straightforward comparison, the complex viscosity values of at the frequency of 10− rad/s were summarized in Table2. As can be seen from this table, the complex viscositypure PA6, of pure all samples OBC, PA6/OBC at low frequency and PA6/OBC/POE increased in-g- theMAH order blends of: PA6with< differentPA6/OBC amounts< PA6/OBC of POE/1 <- PA6g-MAH/OBC at/3 the< PA6 frequency/OBC/5 of< 10PA6−1 rad/s/OBC /were7 < OBC. summarized The results in Table of increase 2. As can in the be complexseen from viscosity this table, of the compatibilizedcomplex viscosity PA6 of/OBC all samples blends were at low attributed frequency to the increased chemical in reactions the order of of: the PA6 maleic < PA6/OBC anhydride < groupsPA6/OBC/1 of POE-g-MAH < PA6/OBC/3 with < thePA6/OBC/5 amine groups < PA6/OBC/7 of PA6 (Figure < OBC.3). The As shown results in of Figure increase3, the in formation the complex of theviscosity POE-g-PA6 of the copolymer compatibilized at the interface PA6/OBC of blends the blends were caused attributed the enhancement to the chemical of the reactionmacromoleculars of the entanglementsmaleic anhydride and group inhibiteds of POE the flow-g-MAH of the with melt, the resulting amine groups in an increased of PA6 (Figure complex 3). viscosity As shown of the in blends.Figure 3 Similar, the formation behavior ofwas the observed POE-g-PA6 in other copolymer compatibilized at the interface PA6 based of blends the blends [15]. In caused addition, the atenhancement a higher frequency, of the macromolecular the disentanglement entanglements of the molecular and inhibited entanglements the flow of decreased the melt, theresulting complex in viscosityan increased of the complex blends. viscosity of the blends. Similar behavior was observed in other compatibilized PA6 based blends [15]. In addition, at a higher frequency, the disentanglement of the molecular entanglements decreased the complex viscosity of the blends.

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Figure 6. The complex viscosity of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH Figure 6. The complex viscosity of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g- blends at various POE-g-MAH contents. MAH blends at various POE-g-MAH contents. Table 2. The complex viscosity (η*), storage modulus (G’), and loss modulus (G”) of pure PA6, pure OBC,Table and 2. The the PA6complex/OBC viscosity and PA6 (/OBC *), /storagePOE-g-MAH modulus blends (G’), atand various loss modulus POE-g-MAH (G”) of contents pure PA6, and pure a 1 frequencyOBC, and of the 10− PA6/OBCrad/s. and PA6/OBC/POE-g-MAH blends at various POE-g-MAH contents and a frequency of 10−1 rad/s. Sample Complex Viscosity (η*) (Pa s) Storage Modulus (G’) (Pa) Loss Modulus (G”) (Pa) · PureSample PA6 Complex Viscosity 330.0 ( *) (Pa·s) Storage Modulus 0.6 (G’) (Pa) Loss Modulus 31.1 (G”) (Pa) Pure OBC 10472.9 110.6 1018.3 PurePA6/ OBCPA6 461.0330.0 1.90.6 43.331.1 PA6Pure/OBC OBC/1 485.510472.9 2.2110.6 46.61018.3 PA6PA6/OBC/OBC/3 602.6461.0 4.71.9 60.043.3 PA6/OBC/1PA6/OBC/5 758.4485.5 6.92.2 75.646.6 PA6/OBC/3PA6/OBC/7 806.6602.6 10.54.7 80.060.0 PA6/OBC/5 758.4 6.9 75.6 PA6/OBC/7 806.6 10.5 80.0 Figure7 illustrates the storage modulus (G’) and the loss modulus (G”) of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH blends with diﬀerent contents of POE-g-MAH. As depicted Figure 7 illustrates the storage modulus (G’) and the loss modulus (G”) of pure PA6, pure OBC, in Figure7a, the G’ of the samples in the entire frequency interval exhibited a similar trend to that and the PA6/OBC and PA6/OBC/POE-g-MAH blends with different contents of POE-g-MAH. As of the η*. It was clear that the addition of OBC increased the G’ of pure PA6, and the addition of Materialsdepicted 2020 in, 13 Figure, x FOR 7a,PEER the REVIEW G’ of the samples in the entire frequency interval exhibited a similar11 trendof 17 POE-g-MAH further increased the G’ of the blends. For instance, as shown in Table2, at the frequency to that of the η*. It was clear that the addition of OBC increased the G’ of pure PA6, and the addition ofPA6/ABS 10 1 rad blends./s, the valuesThis study of the indicated storage modulusthat this behavior for PA6/OBC was/ POE-g-MAHdue to the fact blends that all increased the samples from were 1.9 of POE− -g-MAH further increased the G’ of the blends. For instance, as shown in Table 2, at the topredominately 10.5 Pa with increasingmade of PA6 POE-g-MAH phase and ratiohad good from processability. 0 to 7 wt%, which It is were interesting all much to highernote that than the that result of frequency of 10−1 rad/s, the values of the storage modulus for PA6/OBC/POE-g-MAH blends pureabove PA6 is in (0.6 correspondence Pa). with the result derived from the morphological analyses above. increased from 1.9 to 10.5 Pa with increasing POE-g-MAH ratio from 0 to 7 wt%, which were all much higher than that of pure PA6 (0.6 Pa).

The higher storage modulus values of the compatibilized PA6/OBC blends point out that the addition of the compatibilizer to the blends generated larger elasticities, indicating improved compatibility of the PA6/OBC blends. Figure 7b shows a trend of G” similar to that of G’. In addition, it was observed that G” was higher than G’ over the entire frequency range of testing indicating thus FigureFigure 7. 7.( a()a) Storage Storage modulus modulus (G’) (G’) and and (b) loss (b) modulus loss modulus (G”) of(G”) pure of PA6, pure pure PA6, OBC, pure and OBC, the PA6 and/OBC the that all samples exhibited a liquid-like behavior, and that the compatibilized blends are viscoelastic andPA6/OBC PA6/OBC and/POE-g-MAH PA6/OBC/POE blends-g-MAH at various blends POE-g-MAHat various POE contents.-g-MAH contents. materials [43]. Arsad et al. [44] also reported liquid-like behavior in the study of compatibilized 3.5. Thermal Behavior of The Materials The DSC cooling curves and the second heating curves of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH blends with different contents of POE-g-MAH are given in Figure 8. The results of Tm, ΔHm, Tc, and Xc are summarized in Table 3. The data revealed that the addition of OBC and POE-g-MAH had little effect on the Tm of PA6 in the uncompatibilized and compatibilized blends. However, the Tc of PA6 in all the mixtures was only influenced by the addition of the POE-g-MAH compatibilizer. Indeed, the Tc of PA6 gradually shifted to lower temperatures with the addition of POE-g-MAH to PA6/OBC blend. When the dosage of POE-g-MAH increased from 0 to 7 wt%, the Tc of PA6 in the compatibilized blends decreased from 189.05 to 187.67 °C, which suggested that the addition of POE-g-MAH had a negative effect on the crystallization of PA6. The reason for reducing the crystallization temperature of PA6 might be due to the formation of the POE- g-PA6 copolymer resulting from the reaction between PA6 and POE-g-MAH, which hindered the crystallization of PA6.

In addition, from Figure 8b, it can be seen that, compared with the pure PA6, the PA6/OBC and the PA6/OBC/POE-g-MAH blends exhibited two melting peaks in the heating process. Here, the major endothermic peak located at around 220 °C was attributed to the melting of PA6, and the small peak located at around 120 °C was attributed to the melting of OBC [45]. This was because the OBC consisted of crystallizable ethylene-octene blocks [35]. In addition, the Xc of the pure PA6, the PA6/OBC and the PA6/OBC/POE-g-MAH blends with a lower dosages (1–5 wt%) of POE-g-MAH obtained from the DSC heating thermograms was around 31%, pointing out that the crystallinity of PA6 was not significantly affected by the OBC or POE-g-MAH at the low contents. However, at a 7 wt% addition of POE-g-MAH, the Xc of PA6 decreased to 26.92%, 4.74% lower than that of pure PA6. When the addition of POE-g-MAH exceeded a certain amount, the crystallization ability of PA6 decreased because of the chemical interactions between the POE-g-MAH and PA6 matrix, which limited the molecular chain mobility of the PA6 that is necessary for crystallization, resulting thus in a decrease in PA6 crystallinity.

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It has been reported that the storage modulus relates to the elastic behavior of the materials and represents the amount of stored energy [42]. Therefore, for the uncompatibilized PA6/OBC blend, the ﬂexible macromolecules of OBC were able to store further elastic energy, and were thus responsible for the increased storage modulus of the PA6. For the compatibilized PA6/OBC blends, the increase in G’ might have resulted from the compatibilizing eﬃciency of POE-g-MAH. After the addition of POE-g-MAH to the PA6/OBC blend, the PA6 matrix had a stronger interaction with OBC particles through the interface by reducing the interfacial tension. The higher storage modulus values of the compatibilized PA6/OBC blends point out that the addition of the compatibilizer to the blends generated larger elasticities, indicating improved compatibility of the PA6/OBC blends. Figure7b shows a trend of G” similar to that of G’. In addition, it was observed that G” was higher than G’ over the entire frequency range of testing indicating thus that all samples exhibited a liquid-like behavior, and that the compatibilized blends are viscoelastic materials [43]. Arsad et al. [44] also reported liquid-like behavior in the study of compatibilized PA6/ABS blends. This study indicated that this behavior was due to the fact that all the samples were predominately made of PA6 phase and had good processability. It is interesting to note that the result above is in correspondence with the result derived from the morphological analyses above.

3.5. Thermal Behavior of The Materials The DSC cooling curves and the second heating curves of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH blends with different contents of POE-g-MAH are given in Figure8. The results of Tm, ∆Hm,Tc, and Xc are summarized in Table3. The data revealed that the addition of OBC and POE-g-MAH had little effect on the Tm of PA6 in the uncompatibilized and compatibilized blends. However, the Tc of PA6 in all the mixtures was only influenced by the addition of the POE-g-MAH compatibilizer. Indeed, the Tc of PA6 gradually shifted to lower temperatures with the addition of POE-g-MAH to PA6/OBC blend. When the dosage of POE-g-MAH increased from 0 to 7 wt%, the Tc of PA6 in the compatibilized blends decreased from 189.05 to 187.67 ◦C, which suggested that the addition of POE-g-MAH had a negative effect on the crystallization of PA6. The reason for reducing the crystallization temperature of PA6 might be due to the formation of the POE-g-PA6 copolymer resulting from the reaction between PA6 and POE-g-MAH, which hindered the crystallization of PA6. In addition, from Figure8b, it can be seen that, compared with the pure PA6, the PA6 /OBC and the PA6/OBC/POE-g-MAH blends exhibited two melting peaks in the heating process. Here, the major endothermic peak located at around 220 ◦C was attributed to the melting of PA6, and the small peak located at around 120 ◦C was attributed to the melting of OBC [45]. This was because the OBC consisted of crystallizable ethylene-octene blocks [35]. In addition, the Xc of the pure PA6, the PA6/OBC and the PA6/OBC/POE-g-MAH blends with a lower dosages (1–5 wt%) of POE-g-MAH obtained from the DSC heating thermograms was around 31%, pointing out that the crystallinity of PA6 was not significantly affected by the OBC or POE-g-MAH at the low contents. However, at a 7 wt% addition of POE-g-MAH, the Xc of PA6 decreased to 26.92%, 4.74% lower than that of pure PA6. When the addition of POE-g-MAH exceeded a certain amount, the crystallization ability of PA6 decreased because of the chemical interactions between the POE-g-MAH and PA6 matrix, which limited the molecular chain mobility of the PA6 that is necessary for crystallization, resulting thus in a decrease in PA6 crystallinity. Materials 2020, 13, 1146 11 of 16 Materials 2020, 13, x FOR PEER REVIEW 12 of 17

FigureFigure 8. The 8. The differential diﬀerential scanning scanning calorimetry calorimetry (DSC) (DSC) plots plots of of(a) ( athe) the cooling cooling curves curves and and (b) ( bthe) the second second heatingheating curves curves of of pure pure PA6, PA6, pure pure OBC, OBC, and and the the PA6 PA6/OBC/OBC and and PA6 PA6/OBC/POE/OBC/POE-g-MAH-g-MAH blends blends at various at variousPOE-g-MAH POE-g-MAH contents. contents.

Table 3. The DSC results of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH blends Table 3. The DSC results of pure PA6, pure OBC, and the PA6/OBC and PA6/OBC/POE-g-MAH at various POE-g-MAH contents. blends at various POE-g-MAH contents.

Sample Tm (◦C) Tc (◦C) ∆Hm (J/g) Xc (%) Sample Tm (°C) Tc (°C) ∆Hm (J/g) Xc (%) Pure PA6 Pure PA6220.78 220.78189. 189.5555 72.8272.82 31.6631.66 Pure OBC Pure OBC121.06 121.0678.34 78.34 18.0 6.216.21 PA6/OBC 220.86 189.05 65.85 31.81 PA6/OBCPA6 /OBC/220.186 220.87189. 188.7705 63.1565.85 30.8131.81 PA6/OBC/1PA6 /OBC/220.387 220.67188. 188.6277 63.9863.15 31.8430.81 PA6/OBC/3PA6 /OBC/220.567 221.17188.6 187.782 61.5763.98 31.2331.84 PA6/OBC/5PA6 /OBC/221.717 221.46187. 187.6778 52.0861.57 26.9231.23 PA6/OBC/7 221.46 187.67 52.08 26.92 3.6. Mechanical Properties 3.6. Mechanical Properties Values of the notched Charpy impact strength (IS) of pure PA6, PA6/OBC and the PA6/OBC/POE-g-MAH blendsValues with of various the notched contents Charpy of POE-g-MAH impact strengthare shown (IS) in Figure of pure9. As PA6, can bePA6/OBC seen, very and limited the PA6/OBC/POEincrements in-g IS-MAH were obtainedblends with in the various case of contents uncompatibilized of POE-g-MAH PA6/OBC are shown blend as in compared Figure 9. As to thatcan of bepure seen, PA6, very which limited might increments be ascribed in IS to were the poorobtained compatibility in the case between of uncompatibilized the PA6 matrix PA and6/OBC OBC blend phases as ascompared demonstrated to that byof pure the SEM PA6, micrographswhich might inbe Figureascribed5a. to However, the poor compatibility when POE-g-MAH between was the added PA6 matrixto the and PA6 OBC/OBC phases blend, as thedemonstrated IS of the compatibilized by the SEM micrographs blends increased in Figure significantly 5a. However, with when increasing POE- g-MAHPOE-g-MAH was added contents. to the InPA6/OBC particular, blend, the PA6the /ISOBC of the/7 sample compatibilized exhibited blends the highest increased IS (19 significantly kJ/m2), which withwas increasing about 194% POE higher-g-MAH than contents. that of pure In particular, PA6 under the our PA6/OBC/7 experimental sample conditions. exhibited the highest IS (19 kJ/mThis2), which successful was tougheningabout 194%improvement higher than that of PA6 of pure could PA6 be due under to the our chemical experimental reaction conditions. of the maleic anhydride groups of POE-g-MAH with the amine groups of PA6 to form the POE-g-PA6 copolymer, This successful toughening improvement of PA6 could be due to the chemical reaction of the which could act as bridges between PA6 matrix and OBC phase to enhance the compatibility of the maleic anhydride groups of POE-g-MAH with the amine groups of PA6 to form the POE-g-PA6 PA6/OBC blends. In addition, considering the corresponding SEM images, it is believed that the copolymer, which could act as bridges between PA6 matrix and OBC phase to enhance the increase of IS might have resulted from the inhibition of the coalescence of the dispersed phase and the compatibility of the PA6/OBC blends. In addition, considering the corresponding SEM images, it is promoted dispersion of OBC particles in the PA6 matrix, as well as the improved interfacial adhesion believed that the increase of IS might have resulted from the inhibition of the coalescence of the and molecular chain entanglements between the two phases, which allowed for efficient stress transfer dispersed phase and the promoted dispersion of OBC particles in the PA6 matrix, as well as the between the phases and caused a large yield deformation, which dissipated large amounts of energy improved interfacial adhesion and molecular chain entanglements between the two phases, which responsible for the improved IS of the blends [46,47]. The toughening effect of maleic anhydride allowed for efficient stress transfer between the phases and caused a large yield deformation, which functionalized elastomer particles has also been reported by other researchers [48,49]. The obtained dissipated large amounts of energy responsible for the improved IS of the blends [46,47]. The results indicate that the compatibility of the PA6/OBC blend was greatly improved by the incorporation toughening effect of maleic anhydride functionalized elastomer particles has also been reported by of POE-g-MAH. other researchers [48,49]. The obtained results indicate that the compatibility of the PA6/OBC blend was greatly improved by the incorporation of POE-g-MAH.

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Figure 9. Notched Charpy impact strength of pure PA6, and the PA6/OBC and PA6/OBC/POE -g- MAH blends at various POE-g-MAH contents. FigureFigure 9. 9.Notched Notched Charpy Charpy impact impact strength strength of pure of pure PA6, PA6, and theand PA6 the/OBC PA6/OBC and PA6 and/OBC PA6/OBC/POE/POE-g-MAH-g- blendsMAH atblends various at various POE-g-MAH POE-g contents.-MAH contents. Figure 10 depicts the tensile and flexural properties of pure PA6, and the PA6/OBC and PA6/OBC/POEFigure 10 -gdepicts-MAH the blends tensile with and different flexural properties contents of pure POE- PA6,g-MAH. and As the PA6expected,/OBC and the Figure 10 depicts the tensile and flexural properties of pure PA6, and the PA6/OBC and PA6incorporation/OBC/POE-g-MAH of OBC increased blends with the dielongationfferent contents at break of POE-g-MAH.of pure PA6. This As expected, might bethe attributed incorporation to the PA6/OBC/POE-g-MAH blends with different contents of POE-g-MAH. As expected, the ofelastomeric OBC increased nature the of elongation OBC. In addition, at break the of pure elongat PA6.ion This at the might break be of attributed the compatibilized to the elastomeric blends incorporation of OBC increased the elongation at break of pure PA6. This might be attributed to the naturegradually of OBC. increased In addition, with the the increase elongation of POE at the-g break-MAH of contents, the compatibilized and reached blends the graduallymaximum increased value at elastomeric nature of OBC. In addition, the elongation at the break of the compatibilized blends withthe concentration the increase ofof POE-g-MAH7 wt% POE-g contents,-MAH, which and reached was about the maximum3.8 times higher value than at the that concentration of pure PA6. of gradually increased with the increase of POE-g-MAH contents, and reached the maximum value at 7This wt% could POE-g-MAH, be ascribed which to the was improved about 3.8 interfacial times higher adhesion than that between of pure the PA6. PA6 This matrix could and be OBC ascribed phase to the concentration of 7 wt% POE-g-MAH, which was about 3.8 times higher than that of pure PA6. theresulting improved from interfacial the formation adhesion of the between POE-g-PA6 the PA6 copolymer matrix and at the OBC interface, phase resulting which increased from the the formation plastic This could be ascribed to the improved interfacial adhesion between the PA6 matrix and OBC phase ofdeformation the POE-g-PA6 or ductility copolymer of the at blend. the interface, However, which owing increased to the elastomeric the plastic deformationnature of OBC or and ductility POE- ofg- resulting from the formation of the POE-g-PA6 copolymer at the interface, which increased the plastic theMAH, blend. the However,addition of owing OBC toand the POE elastomeric-g-MAH naturedecreased of OBC the andtensile POE-g-MAH, strength, flexural the addition strength, of OBCand deformation or ductility of the blend. However, owing to the elastomeric nature of OBC and POE-g- andflexural POE-g-MAH modulus of decreased the PA6. the tensile strength, flexural strength, and flexural modulus of the PA6. MAH, the addition of OBC and POE-g-MAH decreased the tensile strength, flexural strength, and flexural modulus of the PA6.

Figure 10. (a) The elongation at the break, (b) the tensile strength, (c) the ﬂexural strength, and Figure 10. (a) The elongation at the break, (b) the tensile strength, (c) the flexural strength, and (d) the (flexd) theural ﬂexural modulus modulus of pure of PA6, pure and PA6, the and PA6/OBC the PA6 and/OBC PA6/OBC/POE and PA6/OBC-g/POE-g-MAH-MAH blends blends at various at various POE- Figure 10. (a) The elongation at the break, (b) the tensile strength, (c) the flexural strength, and (d) the POE-g-MAHg-MAH contents. contents. flexural modulus of pure PA6, and the PA6/OBC and PA6/OBC/POE-g-MAH blends at various POE- g-MAH contents.

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4. Conclusions In this work, olefin block copolymer (OBC) was used as an impact modifier for PA6, along with a commercialized POE-g-MAH as a reactive compatibilizer for the PA6/OBC blend. Blends of PA6/OBC (PA6/OBC = 90/10 w/w) were prepared by melt blending in the presence of 0–7 wt% POE-g-MAH contents. The effects of POE-g-MAH on the morphology, rheological behavior, thermal behavior, and mechanical properties of the blends were investigated. The morphological analysis showed that pure PA6 and OBC were immiscible with sea-island type morphologies. SEM investigation indicated that the addition of 7 wt% POE-g-MAH to the PA6/OBC blend enhanced the dispersion of the OBC particles in the PA6 matrix with a blurred phase interface, suggesting a better interfacial compatibility between the pure PA6 and the OBC, which was consistent with the results of the rheological analysis. The rheological test showed that the addition of POE-g-MAH increased the melt viscosity, storage modulus (G’), and loss modulus (G”) of the compatibilized blends at low frequencies, mainly due to the reaction between the maleic anhydride groups of POE-g-MAH and the amine groups of PA6, which was confirmed by Fourier transform infrared (FTIR) spectroscopy analysis and the Molau test. Moreover, DSC analysis demonstrated that the addition of OBC had little effect on the crystallization behavior of pure PA6, while the incorporation of POE-g-MAH at high concentrations (7 wt%) in PA6/OBC blend hindered the crystallization of PA6. In addition, the notched Charpy impact strength and the elongation at break of the PA6/OBC blend increased with increasing POE-g-MAH compatibilizer content, and a maximum impact strength of 19 kJ/m2 was reached for the blend PA6/OBC/7, which was approximately 194% higher than that of pure PA6 under our experimental conditions. However, owing to the elastomeric nature of OBC and POE-g-MAH, the addition of OBC and POE-g-MAH decreased the tensile strength, flexural strength, and flexural modulus of the PA6.

Author Contributions: Conceptualization, X.L. and X.C.; methodology, X.L., Y.L. and Y.W.; validation, Y.L., L.M. and W.Z.; formal analysis, X.L.; investigation, X.L. and M.L.; resources, L.M.; data curation, X.L., L.C., L.M. and M.L.; writing—original draft preparation, X.L.; writing—review and editing, X.L., Y.L. and W.Z.; visualization, X.L.; supervision, X.L.; project administration, X.L. and X.C.; funding acquisition, Y.L. and L.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (No. 11872179), the Natural Science Foundation of Hunan Province (No. 2018JJ4072), High-Level Talents Support Plan of Xiamen University of Technology (No. YKJ19008R), the Scientific Research Project of Education Department of Hunan Province (No.19A138), the Scientific Research Project of Education Department of Hunan Province (No.19B152), Natural Science Foundation of Hunan Province, China (No. 2019JJ50132), the Science and Technology Plan Projects of Fujian Province (No. 2018H6024), and the open fund for innovation platform in Hunan province (No. 18k079). Conflicts of Interest: The authors declare no conflict of interest.

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