Structural Evolution of Two-Phase Blends of Polycarbonate and PMMA by Simultaneous Biaxial Stretching

Article Structural Evolution of Two-Phase Blends of Polycarbonate and PMMA by Simultaneous Biaxial Stretching

Takumi Kobayashi and Hiromu Saito *

Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, Koganei-shi, Tokyo 184-8588, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-42-388-7294

Received: 30 July 2018; Accepted: 23 August 2018; Published: 27 August 2018

Abstract: We investigated the structural evolution of the two-phase blends of polycarbonate (PC) and poly(methyl methacrylate) (PMMA) at various blend compositions by simultaneous biaxial stretching, using optical microscopy and SEM observation. The spherical PMMA domains and PC matrix of 30/70 PC/PMMA were enlarged uniformly at the all in-plane direction, while the anisotropic-shaped co-continuous structure in 50/50 PC/PMMA was deformed to a crosshatched structure by the in-plane bimodal orientation. In 70/30 PC/PMMA, the phase inversion was found to occur by simultaneous biaxial stretching; that is, the spherical PMMA domains were changed to a crosshatched matrix by the in-plane bimodal orientation due to coalescence of the PMMA domains during the stretching. Owing to the phase inversion, the surface hardness estimated by the pencil hardness test became harder, from 2B to 2H, increasing the strain from 1.0 to 2.0.

Keywords: biaxial stretching; blend; polycarbonate; PMMA; phase inversion; surface hardness

1. Introduction Biaxially stretched polymer films are often used as packaging materials for food and industrial products, because the mechanical and gas barrier properties can be improved using biaxial stretching in the manufacturing process [1]. Many researchers have investigated how the structures of polymer films obtained by biaxial stretching vary in a number of aspects [2,3] for crystalline polymers [4–27], polymer blends [28–34] and composites [35–37]. The dimensional stability is enhanced due to the increase in the degree of the stress-induced crystallinity [4], surface morphology is changed [5,6], hardness is enhanced [7], and high toughness polylactide film can be obtained by development of highly-oriented small crystallites [8] in the crystalline polymers. The mechanical property is improved in the uncompatibilized polymer blends by addition of the interfacial modifier due to suppression of the interfacial voiding [28], oxygen barrier property is improved in the multi-layered polypropylene (PP)/polyethylene oxide system [29], and microporous structure is developed in the PP/nylon 6 blends [30]. The silica nanofillers are elongated parallel or perpendicular to the deformation directions depending on the type of nanofiller [35] and biaxial stretching of a PP/clay nanocomposite results in delamination and orientation of clay stacks [36,37]. Orientation by biaxial stretching is characterized by wide-angle x-ray diffraction [4,9–17,28,31,32], birefringence [9,13–20], infrared spectroscopy [13,20–23], fluorescence anisotropy [24], stress-strain behavior [14,16,17,25–27,32,33]. Isotropic plane film can be produced by simultaneous biaxial stretching, in which films are stretched in the X and Y directions at the same time, so that the properties of the stretched specimen thus produced are isotropic in the in-plane direction [14,16,32]. This indicates that a uniform orientation at all in-plane direction is caused by simultaneous biaxial stretching; that is, the polymer chains and crystalline lamellae are uniformly

Polymers 2018, 10, 950; doi:10.3390/polym10090950 www.mdpi.com/journal/polymers Polymers 2018, 10, 950 2 of 14 oriented at all in-plane direction. The in-situ polarized infrared spectroscopic study by Nitta et al. suggests that the molecular chain is elongated uniformly at the all in-plane direction by simultaneous biaxial stretching of PP [20,23]. Though the uniform orientation without the preferred orientation is considered in the simultaneous biaxial stretching, the bimodal orientation to the stretching directions of X and Y is also suggested by a wide angle x-ray diffraction study in crystalline polymers such as polyethylene terephthalate (PET) and polyethylene naphthalate [16,31]. However, it is difficult to clarify the bimodal orientation of the structure in neat polymers because of the difficulty in observing the stretched structure. Bisphenol-A polycarbonate (PC) is a ductile material. Because of its excellent mechanical properties, PC is used for various goods such as window glass, bottles, bulletproof glass, and more. Although it has high impact-resistance, it exhibits low scratch-resistance due to low surface hardness; its pencil hardness is 4B. To improve the scratch-resistance, hard coatings are applied on the PC surface by physical or chemical vapor deposition [38] and sol-gel methods [39,40]. On the other hand, poly(methyl methacrylate) (PMMA) has poor toughness, but exhibits high scratch-resistance due to high surface hardness; its pencil hardness is 4H. The tensile property of glassy PMMA changes from brittle to ductile with increasing temperature [41], while that of glassy PC changes from ductile to brittle when the molecular weight is low [42,43]. Fracture toughness of PMMA and PC abruptly increases at very high loading rates [44]. Yamaguchi et al. found that the surface hardness of PC could be enhanced by blending PMMA, due to the surface localization of PMMA by segregation during the injection-molding. The durometer D hardness increased from 80 to 83 by blending 5% of PMMA [45]. The segregation of PMMA might be attributed to the orientation-induced phase separation resulting from variations in viscosity, due to composition fluctuations [46,47]. A PC/PMMA blend is partially miscible and is suggested to exhibit a lower critical solution temperature (LCST) type phase diagram, in which liquid–liquid phase separation occurs at a temperature above the LCST [48,49]. When a blend has a two-phase structure, the spherical domain dispersed in the matrix deforms to an ellipsoidal one by uniaxial elongational flow [50–52], and the deformed structure recovers to a spherical one by relaxation after the flow stops [52]. A fibrillar shaped two-phase structure is also obtained by the extrusion process [53–57]. Owing to the structural color induced by the fibrillar shaped two-phase structure, iridescent luster is generated by the uniaxial elongation of 70/30 PC/PMMA. Our interest in this study is the structural evolution of two-phase blends of PC and PMMA by simultaneous biaxial stretching, and the change of the surface hardness achieved by this stretching. To observe the stretched structure obtained by simultaneous biaxial stretching, we investigated the evolution of the two-phase structure of the PC/PMMA blends by using optical microscopy and SEM observation. When the two-phase structure obtained after simultaneous stretching is isotropic, the domain structure is enlarged uniformly at the all in-plane direction, for instance. We chose blends of PC and PMMA because various two-phase structures with a size in a micrometer scale can be obtained at different blend compositions. For example, the spherical domain structure is obtained at a 30/70 composition, while a co-continuous structure is obtained at a 50/50 composition. Hence, various structural evolutions by simultaneous biaxial stretching can be observed by microscopic observation. The result of the surface hardness estimated by the pencil hardness test is also presented, in order to discuss the structure change achieved by stretching. The control of the surface hardness is significant for the application of the packaging materials.

2. Materials and Methods The PC and PMMA specimens used in this study were commercial polymers. The PC was bisphenol-A polycarbonate supplied by the Mitsubishi Gas Chemical Company, Inc. (Tokyo, Japan); 4 −1 Iupilon S-2000 N, Mw = 2.4 × 10 g mol . The PMMA was supplied by Kurarey Co., Ltd. 5 −1 (Tokyo, Japan); Parapet G, Mw = 1.0 × 10 g mol . The glass transition temperatures (Tgs) of PC and PMMA were 150 ◦C and 100 ◦C, respectively. Polymers 2018, 10, 950 3 of 14

The PC and PMMA were melt-mixed at a temperature of 250 ◦C and at a rotor speed of 90 rpm forPolymers 5 min 2018 in, 9, ax FOR mixing PEER chamber REVIEW of a miniature mixing machine (Imoto IMC-18D7, Kyoto, Japan).3 of 14 The melt mixed blend was extruded and chopped into pellets. The pellets were then melt pressed inmelt a vacuummixed blend hot-press was extruded machine (Imotoand chopped IMC-11FD, into pe Kyoto,llets. The Japan) pellets at 250 were◦C andthen 10melt MPa pressed for 5 min,in a tovacuum obtain hot-press the unstretched machine film (Imoto with IMC-11FD, a thickness Kyoto, of about Japan) 100 at 250µm, °C which and 10 was MPa then for cooled5 min, to to obtain room temperature.the unstretched The film PC with and a thethickness blends of used about in 100 this µm, study which did was not showthen cooled crystallization-and-melting to room temperature. behaviorThe PC and in the these blends experimental used in this conditions. study did Tonot erase show the crystallization-and-melting effect of local ordering of behavior PC [58], in the these PC ◦ blendsexperimental were melt-pressedconditions. To at erase 250 C,the which effect wasof lo toocal highordering compared of PC [58], with the the PCTg ofblends PC and were PMMA. melt- Thepressed unstretched at 250 °C, filmwhich was was cut too into high square-shaped compared with specimens the Tg of PC of 65and mm PMMA.× 65 The mm unstretched to undergo film the biaxialwas cut stretching. into square-shaped specimens of 65 mm × 65 mm to undergo the biaxial stretching. The biaxialbiaxial stretching stretching was was performed performed by aby biaxial a bi stretcheraxial stretcher equipped equipped with a temperature with a temperature controller (IMC-1A94,controller (IMC-1A94, Imoto Machinery Imoto Machinery Co., Ltd., Kyoto, Co., Ltd., Japan). Kyoto, The Japan). film specimen The film was specimen gripped was on eachgripped side on by twoeach pairsside by of fivetwo clampspairs of (Figure five clamps1), and (Figure the initial 1), an gauged the area initial before gauge stretching area before was stretching50 mm × 50was mm 50. Themm specimen× 50 mm. wasThe specimen heated and was then heated biaxially and stretchedthen biaxially at 140 stretched◦C for 30/70at 140 and°C for 50/50 30/70 PC/PMMA, and 50/50 160PC/PMMA,◦C for 70/30 160 °C PC/PMMA, for 70/30 PC/PMMA, and 180 ◦C and for 180 neat °C PC for after neat maintaining PC after maintaining the temperature the temperature for 5 min. Thesefor 5 min. temperatures These temperatures were the most weresuitable the most for suitable stretching for stretching uniformly uniformly at the all in-planeat the alldirection. in-plane Thedirection. biaxial The stretching biaxial stretching was carried was out carried at a draw out at speed a draw of speed 100 mm of 100 min mm−1 with min− various1 with various drawratios, draw byratios, moving by moving the clamps the clamps simultaneously simultaneously in two in stretching two stretching directions directions (the X and(the YX directions,and Y directions, as shown as byshown arrows by inarrows Figure in1 )Figure at the same1) at the speed same and speed to the and same to drawthe same ratio. draw Then, ratio. the stretchedThen, the specimenstretched wasspecimen cooled was to roomcooled temperature to room temperature for the observation. for the observation.

Figure 1. Photograph of clamps in a biaxial stretcher. Figure 1. Photograph of clamps in a biaxial stretcher.

The phase structure of the blends was observed both under an unpolarized optical microscope and aThe polarized phase structure optical microscope of the blends (Olympus was observed BX53 both-P, Tokyo, under anJapan), unpolarized each equipped optical microscope with a charge- and acoupled polarized device optical (CCD) microscope camera (Olympus (Olympus BX53-P, DP73, Tokyo,Tokyo, Japan), Japan). each The equipped structure with under a charge-coupled the polarized deviceoptical microscope (CCD) camera was (Olympus observed by DP73, the optical Tokyo, micros Japan).cope The equipped structure with under a sensitive the polarized tint plate, optical with microscopean optical path was difference observed of by 530 the nm optical under microscope crossed polarizers equipped P withand A. a sensitiveHere, the tintoptical plate, axis with of the an opticalpolarizer path was difference rotated at of45° 530 to the nm stretching under crossed directio polarizersns of X and P andY, and A. that Here, of thethe opticalsensitive axis tint of plate the ◦ polarizer(ST). was rotated at 45 to the stretching directions of X and Y, and that of the sensitive tint plateThe (ST). phase structure of the blend was also observed under a scanning electron microscope (SEM) (HitachiThe phaseS2100A, structure Tokyo, of Japan). the blend In wasorder also to observedobserve the under surface a scanning and cross electron section microscope of the (SEM)blend (Hitachispecimen, S2100A, the PMMA Tokyo, component Japan). In orderwas etched to observe by 2-butanone the surface for and 3 crossh at 20 section °C, and of thethe blendPC component specimen, ◦ thewas PMMA etched componentby sodium washydroxide etched (NaOH) by 2-butanone solution for (30 3 h wt at 20% NaOH)C, and thefor PC48 componenth at 20 °C. The was etched ◦ byspecimen sodium was hydroxide sputter coated (NaOH) with solution platinum (30 using wt % a NaOH) sputter for system 48 h at(JEOL 20 JFC-1300,C. The etched Tokyo, specimen Japan). wasAfter sputter the conductive coated with coating, platinum SEM was using operated a sputter with system an accelerating (JEOL JFC-1300, voltage Tokyo, of 20 kV Japan). at 20 After°C under the ◦ conductivevacuum. coating, SEM was operated with an accelerating voltage of 20 kV at 20 C under vacuum. The surface hardness of the blend specimen was estimated by the pencil hardness test according to ISO 15184. A graphite pencil was used to draw on the film specimen with uniform pressure, maintaining the pencil at a constant angle of 45°. The surface hardness was determined using pencils of various degrees of hardness by judging whether an indentation was caused or not; the specimen was judged as harder than the pencil hardness when no indentation was seen.

Polymers 2018, 10, 950 4 of 14

The surface hardness of the blend specimen was estimated by the pencil hardness test according to ISO 15184. A graphite pencil was used to draw on the ﬁlm specimen with uniform pressure, maintaining the pencil at a constant angle of 45◦. The surface hardness was determined using pencils of various degrees of hardness by judging whether an indentation was caused or not; the specimen Polymerswas judged 2018, 9 as, x harderFOR PEER than REVIEW the pencil hardness when no indentation was seen. 4 of 14

3. Results and and Discussion Figure 2 2 showsshows unpolarizedunpolarized andand polarizedpolarized optical optical micrographs micrographs of of 30/70 30/70 polycarbonatepolycarbonate (PC)/poly(methyl methacrylate) methacrylate) (PMMA) (PMMA) obtained obtained by bysimultaneous simultaneous biaxial biaxial stretching stretching at a draw at a drawratio λratio of 1.5λ of (λ 1.5 = 1.5), (λ = and 1.5), the and unstretched the unstretched blend blend (λ = 1.0). (λ = Here, 1.0). Here, the specimen the specimen was stretched was stretched at 140 at °C. 140 The◦C. blendThe blend specimen specimen in this in this composition composition was was fractured fractured at ata draw a draw ratio ratio above above 1.5. 1.5. Spherical Spherical PC PC domains domains with diameters diameters of of several several micrometers micrometers we werere dispersed dispersed in inthe the PMMA PMMA matrix matrix at λ at = 1.0λ = (Figure 1.0 (Figure 2a). 2Thea). sphericalThe spherical domain domain became became larger larger without without a change a change of ofthe the shape, shape, and and the the domain domain distance distance became longer by simultaneous biaxial biaxial stretching stretching (Figure (Figure 22b),b), indicatingindicating thatthat thethe two-phasetwo-phase structurestructure ofof thethe spherical domain and matrix is enlarged uniformly at the all in-plane direction. PC can be elongated ◦ ◦ to a high high strain [59], [59], though the stretching temperature ofof 140140 °CC was below the Tg ofof the the PC PC (150 (150 °C);C); meaning that that the the crazing crazing structure structure and and debondi debondingng at at the the interphase interphase were were not not observed observed at λ at =λ 1.5.= 1.5. In spiteIn spite of ofthe the enlargement enlargement of ofthe the two-phase two-phase structur structure,e, the the change change of of the the interference interference color color due due to stretching was small and the structure was unclear under polarized optical microscopy microscopy (Figure (Figure 22d),d), indicating that the deformed structure was optically isotropic. If the deformation is truly biaxial, it is expected to be optically isotropic in the plane of th thee film. ﬁlm. Thus, Thus, the the results results su suggestggest that that the the two-phase two-phase structure of the spherical domain and matrix is is el elongatedongated uniformly uniformly at at the the all all in-plane in-plane direction, direction, without the preferred orientation, by simultaneous biaxial stretching. These results are consistent with those demonstrated in neat polymers such as polypropylene, polypropylene, poly(lactic acid) and poly(ethylene poly(ethylene terephthalate), in in which the stretched specimen is optically isotropic due to uniform deformation without the preferred in-plane orientation byby simultaneoussimultaneous biaxialbiaxial stretchingstretching [[14,16,32].14,16,32].

Figure 2.2. Unpolarized and polarizedpolarized opticaloptical micrographsmicrographs ofof 30/7030/70 polycarbonate/poly(methylpolycarbonate/poly(methyl methacrylate) (PC/PMMA)(PC/PMMA) obtained byby simultaneoussimultaneous biaxial biaxial stretching stretching at at draw draw ratios ratiosλ ofλ 1.0of 1.0 and and 1.5. 1.5.(a) λ (a=) 1.0λ = (unpolarized);1.0 (unpolarized); (b) λ(b=) 1.5λ = (unpolarized);1.5 (unpolarized); (c) λ (c=) 1.0λ = (polarized);1.0 (polarized); (d) λ(d=) 1.5λ = (polarized).1.5 (polarized).

Polymers 2018, 10, 950 5 of 14

A co-continuous two-phase structure was obtained in 50/50 PC/PMMA. The change of the co-continuous two-phase structure by simultaneous biaxial stretching is shown in Figure3. The blend specimen in this composition was fractured at a draw ratio above 1.5. The size of the co-continuous structure became larger by stretching (Figure3b). The crazing structure and debonding at the interphase were not observed at λ = 1.5. The interesting result here is that blue and yellow interference colors appeared, and a crosshatched pattern was seen in the stretched blend under the polarized opticalPolymers 2018 microscopy, 9, x FOR PEER (Figure REVIEW3d), indicating that the component phase is optically anisotropic and5 of the 14 anisotropic-shaped phase is formed along the crosshatched structure. The change and enlargement of the phase phase structure structure might might be be attributed attributed to to the the ev evolutionolution of ofthe the liquid–liquid liquid–liquid phase phase separation, separation, as asdiscussed discussed later later in the in results the results of 70/30 of PC/PMMA. 70/30 PC/PMMA. Blue and Blue yellow and interference yellow interference colors were colors seen along were seenthe crosshatched along the crosshatched structure, which structure, was elongated which was to elongatedthe X and toY stretching the X and directions. Y stretching These directions. results Thesesuggest results that the suggest anisotropic-shaped that the anisotropic-shaped domain is defor domainmed to the is deformed X and Y stretching to the X anddirections Y stretching by an directionsin-plane bimodal by an in-plane orientation bimodal to yield orientation the crosshatched to yield the structure. crosshatched structure.

Figure 3.3. UnpolarizedUnpolarized and and polarized polarized optical micrographs ofof 50/5050/50 PC/PMMA obtained by simultaneous biaxial stre stretchingtching at at draw ratios λ of 1.0 and 1.5. ( aa)) λ = 1.0 (unpolarized); ((bb)) λ = 1.5 (unpolarized); (c) λ == 1.0 (polarized); (d) λ = 1.5 (polarized).

Figure 44 showsshows thethe schematicschematic illustration for for the the deformation deformation of of the the two-phase two-phase structure structure of ofPC/PMMA PC/PMMA blends blends by simultaneous by simultaneous biaxial biaxial stretchi stretching.ng. The symmetric The symmetric spherical sphericaldomain is domaindeformed is deformeduniformly at uniformly the all in-plane at the direction, all in-plane without direction, the preferred without orientation the preferred by orientationstretching (Figure by stretching 4a). On (Figurethe other4a). hand,Onthe the other co-continuous hand, the co-continuoustwo-phase struct two-phaseure consists structure of an consists unsymmetrical of an unsymmetrical anisotropic- anisotropic-shapedshaped domain, and domain, the long and axis the longof the axis anisot of theropic anisotropic shaped shaped domain domain is rotated is rotated to the to theX and X and Y Ystretching stretching directions, directions, with with the the domains domains deformed deformed by by the the in-plane in-plane bimodal bimodal orientation orientation (Figure (Figure 4b).4b). In 70/30 PC/PMMA, PC/PMMA, the the characteristic characteristic change change of of the the two-phase two-phase structure structure occurred occurred as as shown in Figure5 5.. Here,Here, thethe specimenspecimen waswas stretchedstretched atat 160160 °C.◦C. The The two-phase two-phase structure of the PMMA spherical domains dispersed in the PC matrix was seen in th thee unstretched blend (Figure5 5a).a). ThoughThough thethe phasephase structure with spherical spherical domains domains was was similar similar to to th thatat observed observed in inFigure Figure 2 2for for 30/70 30/70 PC/PMMA, PC/PMMA, the thestructural structural evolution evolution by stretching by stretching was wasquite quite different different in 70/30 in 70/30 PC/PMMA. PC/PMMA. The spherical The spherical shape shapeof the ofPMMA the PMMA domains domains changed changed to the to theanisotropic-shap anisotropic-shapeded domain domain (Figure (Figure 5b,c).5b,c). Blue Blue and yellowyellow interference colors appeared, and and a a crosshatched pattern was was seen along the X and Y stretching directions under polarized optical microscopy (Figure 5e,f). The results indicate that the component phase is optically anisotropic, and the anisotropic-shaped phase is formed along the crosshatched structure by the in-plane bimodal orientation, though the deformed structure was optically isotropic and no change of the spherical shape was seen in 30/70 PC/PMMA by stretching at 140 °C, which was below the Tg of PC (150 °C).

Polymers 2018, 10, 950 6 of 14 directions under polarized optical microscopy (Figure5e,f). The results indicate that the component phase is optically anisotropic, and the anisotropic-shaped phase is formed along the crosshatched structure by the in-plane bimodal orientation, though the deformed structure was optically isotropic Polymers 2018, 9, x FOR PEER REVIEW 6 of 14 and no change of the spherical shape was seen in 30/70 PC/PMMA by stretching at 140 ◦C, which was ◦ belowPolymers the 2018T,g 9,of x FOR PC (150PEER REVIEWC). 6 of 14

Figure 4. Schematic illustration for the deformation direction of the two-phase structure by Figure 4. Schematic illustration for the deformation direction of the two-phase structure by simultaneous biaxial stretching. (a) symmetric spherical domain; (b) anisotropic-shaped domain. simultaneousFigure 4. Schematic biaxial stretching. illustration (a )for symmetric the deformatio sphericaln domain; direction (b )of anisotropic-shaped the two-phase structure domain. by simultaneous biaxial stretching. (a) symmetric spherical domain; (b) anisotropic-shaped domain.

Figure 5.5. UnpolarizedUnpolarized and and polarized optical micrographsmicrographs ofof 70/3070/30 PC/PMMAPC/PMMA obtained by simultaneoussimultaneous biaxial biaxial stretching stretching at various at various draw draw ratios ratiosλ. The upperλ. The pictures upper are picturesoptical micrographs, are optical Figure 5. Unpolarized and polarized optical micrographs of 70/30 PC/PMMA obtained by micrographs,and the lower and pictures the lower are polarized pictures are optical polarized micrographs. optical micrographs. (a) λ = 1.0 (unpolarized); (a) λ = 1.0 (unpolarized); (b) λ = 1.5 simultaneous biaxial stretching at various draw ratios λ. The upper pictures are optical micrographs, ((unpolarized);b) λ = 1.5 (unpolarized); (c) λ = 2.0 ((unpolarized);c) λ = 2.0 (unpolarized); (d) λ = 1.0 (d(polarized);) λ = 1.0 (polarized); (e) λ = 1.5 ((polarized);e) λ = 1.5 (polarized); (f) λ = 2.0 and the lower pictures are polarized optical micrographs. (a) λ = 1.0 (unpolarized); (b) λ = 1.5 ((polarized).f) λ = 2.0 (polarized). (unpolarized); (c) λ = 2.0 (unpolarized); (d) λ = 1.0 (polarized); (e) λ = 1.5 (polarized); (f) λ = 2.0 (polarized). To understandunderstand thethe evolutionevolution ofof thethe phasephase structurestructure duringduring thethe simultaneoussimultaneous biaxialbiaxial stretchingstretching of 70/30To understandPC/PMMA, PC/PMMA, thethe the evolution magnified magniﬁed of images the images phase for for structurthe the unpolarized unpolarizede during theoptical optical simultaneous microscopies microscopies biaxial are are stretchingshown shown in Figureof 70/30 6. PC/PMMA,The PMMA thespherical magnified domains images shown for inthe Figure unpolarized 5a were optical coalesced microscopies and aggregated are shown to yield in largeFigure anisotropic-shaped 6. The PMMA spherical distorted domains domains, shown and in the Figure PC matrix 5a were was coalesced deformed and to aggregateda fibrillar structure to yield inlarge the anisotropic-shaped X and Y stretching distorted directions domains, by theand in-planethe PC matrix bimodal was deformedorientation to (Figurea fibrillar 6a,b). structure The deformationin the X and of theY stretchingPC matrix bydirections the in-plane by thebimodal in-plane orientation bimodal might orientation be caused (Figure by the rotation6a,b). The of thedeformation anisotropic-shaped of the PC matrix PMMA by domains the in-plane to the bimodal X and orientationY stretching might directions. be caused The by phase the rotation structure of becamethe anisotropic-shaped larger with increasing PMMA λ ,domains and a continuous to the X PMMAand Y stretching phase was directions. seen (Figure The 6c,d). phase The structure change became larger with increasing λ, and a continuous PMMA phase was seen (Figure 6c,d). The change

Polymers 2018, 10, 950 7 of 14 in Figure6. The PMMA spherical domains shown in Figure5a were coalesced and aggregated to yield large anisotropic-shaped distorted domains, and the PC matrix was deformed to a ﬁbrillar structure in the X and Y stretching directions by the in-plane bimodal orientation (Figure6a,b). The deformation of the PC matrix by the in-plane bimodal orientation might be caused by the rotation ofPolymers the anisotropic-shaped 2018, 9, x FOR PEER REVIEW PMMA domains to the X and Y stretching directions. The phase structure7 of 14 became larger with increasing λ, and a continuous PMMA phase was seen (Figure6c,d). The change andand enlargementenlargement ofof thethe phase phase structure structure observed observed in in 70/30 70/30 PC/PMMAPC/PMMA mightmight bebe attributedattributed toto thethe ◦ evolutionevolution ofof thethe liquid–liquidliquid–liquid phase separation by by stretching stretching at at 160 160 °C,C, which which was was above above the the TgTsg ofs ofboth both component component polymers. polymers. Since Since PC/PMMA PC/PMMA blends blends are are partially partially miscible miscible and and are suggestedsuggested toto exhibitexhibit anan LCSTLCST typetype phasephase diagramdiagram [48[48,49],,49], thethe evolutionevolution ofof thethe liquid–liquidliquid–liquid phasephase ofof separationseparation mightmight be be caused caused by by shifting shifting the the LCST LCST phase phase diagram diagra tom ato lower a lower temperature temperature by stretchingby stretching [60] [60] and and the orientation-inducedthe orientation-induced phase phase separation separation [46,47 [46,47,61].,61].

FigureFigure 6.6.Unpolarized Unpolarized opticaloptical micrographs micrographs of of 70/30 70/30 PC/PMMAPC/PMMA obtainedobtained byby simultaneoussimultaneous biaxialbiaxial stretchingstretching at at various various draw draw ratios ratiosλ λ.(. a(a))λ λ= = 1.2; 1.2; ( b(b))λ λ= =1.5; 1.5;( c(c))λ λ= =1.8; 1.8;( (dd))λ λ= = 2.0.2.0.

FigureFigure7 7shows shows SEM SEM micrographs micrographs of of the the surface surface of of 70/30 70/30 PC/PMMA PC/PMMA obtained obtained by by simultaneous simultaneous biaxialbiaxial stretching,stretching, which which correspond correspond to to the the optical optical micrographs micrographs shown shown in in Figures Figures5 5and and6. 6. Since Since SEM SEM observationobservation waswas carried out out after after extraction extraction of of the the PMMA PMMA phase phase by byetching etching with with 2-butanone, 2-butanone, the theremaining remaining material material was was PC. PC. Thus, Thus, spherical spherical holes holes with with a diameter a diameter of ofseveral several micrometers micrometers (seen (seen in inFigure Figure 7a)7a) are are assigned assigned to tothe the PMMA PMMA domains, domains, while while the white the white matrix matrix is assigned is assigned to the toPC the matrix. PC matrix.This indicates This indicates that the that PMMA the PMMA domain domain is dispersed is dispersed in the in PC the matrix PC matrix in the in unstretched the unstretched blend. blend. The Thespherical spherical domain domain of PMMA of PMMA changed changed to toellipsoidal ellipsoidal domain, domain, and and the the aspect aspect ratio ratio increased in thethe XX andand Y Y direction direction with with an increasing an increasing draw ratiodraw (Figure ratio7 b).(Figure This result7b). supportsThis result the resultsupports demonstrated the result indemonstrated Figures5 and 6in; thatFigures spherical 5 and PMMA 6; that domains spherica arel PMMA elongated domains to the are X and elongated Y stretching to the directions X and Y bystretching the bimodal directions in-plane by the orientation. bimodal in-plane At λ = 2.0, orientation. a crosshatched At λ = network 2.0, a crosshatched structure was network formed structure in the PMMAwas formed phase in (Figure the PMMA7c). That phase is, phase (Figure inversion 7c). That occurred is, phase by simultaneousinversion occurred biaxial by stretching—the simultaneous PMMAbiaxial sphericalstretching—the domain PMMA was inverted spherical to thedomain crosshatched was inverted network to the matrix. crosshatched To our knowledge, network matrix. this is theTo ﬁrstour knowledge, study to observe this is the the phase first study inversion to obse ofrve two-phase the phase polymer inversion blends of two-phase by simultaneous polymer biaxial blends stretching.by simultaneous The phase biaxial inversion stretching. might The bephase caused inversion by the might coalescence be caused and aggregationby the coalescence of PMMA and domainsaggregation owing of toPMMA Ostwald domains ripening owing [62], to due Ostwald to the evolutionripening of[62], the due liquid–liquid to the evolution phase separationof the liquid– by stretching,liquid phase as separation demonstrated by stretching, in the following as demonstrated SEM micrographs. in the following SEM micrographs.

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Figure 7. SEM micrographs of the surface of 70/30 PC/PMMA obtained by simultaneous biaxial Figure 7. SEM micrographs of the surface of 70/30 PC/PMMA obtained by simultaneous biaxial stretching at various draw ratios λ.(a) λ = 1.0; (b) λ = 1.5; (c) λ = 2.0. stretching at various draw ratios λ. (a) λ = 1.0; (b) λ = 1.5; (c) λ = 2.0.

FigureFigure8 8shows shows SEM SEM micrographs micrographs of of the the cross cross section section of of 70/30 70/30 PC/PMMA. PC/PMMA. Since Since the the SEM SEM observationobservation waswas carriedcarried outout afterafter extractionextraction ofof thethe PCPC phasephase byby etchingetching withwith NaOH,NaOH, thethe remainingremaining materialmaterial waswas PMMA.PMMA. TheThe sphericalspherical domaindomain ofof PMMAPMMA changedchanged toto anan ellipsoidalellipsoidal oror fibrillarfibrillar domain,domain, inin whichwhich thethe majormajor axisaxis ofof thethe domaindomain isis parallelparallel toto thethe planeplane ofof thethe filmsfilms byby stretchingstretching (Figure(Figure8 8a).a). ThisThis isis consistentconsistent withwith thethe in-planein-plane orientationorientation andand stretch-thinningstretch-thinning inin thethe normalnormal thicknessthickness directiondirection suggestedsuggested inin thethe simultaneouslysimultaneously biaxialbiaxial stretchedstretched filmsfilms [[14,16,32].14,16,32]. TheThe PMMAPMMA domainsdomains werewere thenthen aggregatedaggregated inin thethe thicknessthickness direction,direction, andand thethe elongatedelongated domainsdomains becamebecame thickerthicker inin thethe thicknessthickness directiondirection by by the the biaxial biaxial stretching stretching (Figure (Figure8b). 8b). These These results results suggest suggest that that coalescence coalescence and and aggregation aggregation of PMMAof PMMA domains domains occurred occurred by simultaneous by simultaneous biaxial biaxial stretching. stretching. Thus, phaseThus, inversionphase inversion from the from PMMA the domainPMMA todomain the PMMA to the matrix PMMA by simultaneousmatrix by simultaneous biaxial stretching, biaxial demonstratedstretching, demonstrated in Figure6, is in attributed Figure 6, tois theattributed coalescence to the and coalescence aggregation and of theaggregatio PMMA domains.n of the PMMA Such coalescence domains. andSuch aggregation coalescence of and the PMMAaggregation domains of the might PMMA be caused domains by might Ostwald be ripeningcaused by [62 Ostwald], due to ripening the evolution [62], due of the to liquid–liquidthe evolution phaseof the separationliquid–liquid by stretching.phase separation by stretching. FigureFigure9 9shows shows the the schematic schematic illustration illustration for for the th structurale structural evolution evolution of of 70/30 70/30 PC/PMMA PC/PMMA during during simultaneouslysimultaneously biaxialbiaxial stretching.stretching. The spherical PMMA domains dispersed inin thethe PCPC matrixmatrix areare coalescedcoalesced andand aggregatedaggregated byby OstwaldOstwald ripeningripening duedue toto thethe evolutionevolution ofof thethe liquid–liquidliquid–liquid phasephase separationseparation by stretching, and and the the anisotropic-shap anisotropic-shapeded aggregated aggregated domains domains are are formed. formed. The Thelong long axis axisof the of anisotropic-shaped the anisotropic-shaped PMMA PMMA domain domain is rotated is rotated to the to theX and X and Y stretching Y stretching directions directions by bythe the inin-planeplane bimodal bimodal orientation, orientation, as as suggested suggested in in Figure Figure 4b4b ( λλ == 1.5: 1.5: Figure Figure 99a).a). OwingOwing toto thetheexistence existence of of thethe anisotropic-shapedanisotropic-shaped PMMA PMMA domain, domain, the PCthe matrix PC matrix is deformed is deformed by the in-plane by the bimodal in-plane orientation. bimodal Dueorientation. to subsequent Due coalescenceto subsequent and coalescence deformation an ofd the deformation anisotropic-shaped of the PMMAanisotropic-shaped domains, the PMMAPMMA domainsdomains, arethe invertedPMMA domains to a crosshatched are inverted matrix to a crosshatched (λ = 2.0: Figure matrix9a). (λ During = 2.0: Figure the coalescence 9a). During and the deformationcoalescence inand the deformation X and Y stretching in the X directions, and Y stre thetching anisotropic-shaped directions, the PMMA anisotropic-shaped domain is rotated PMMA in thedomain in-plane is rotated projection in the and in-plane is squashed projection in the thicknessand is squashed direction in bythe stretch-thinning thickness direction (Figure by9 b).stretch- thinning (Figure 9b).

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FigureFigure 8.8. SEMSEM micrographs micrographs ofof thethe crosscrosscross section sectionsection of ofof 70/30 70/3070/30 PC/PMMA PC/PMMA obtainedobtained byby simultaneoussimultaneous biaxial biaxial stretchingstretching atat drawdraw ratiosratios λλ ofofof 1.51.51.5 andandand 2.0.2.0.2.0. ((aa))) λλ = == 1.5;1.5; ((bb)) λλ == 2.0.2.0.

FigureFigure 9.9. SchematicSchematic illustrationillustration forforfor thethethe structure structurestructure change chanchangege of ofof 70/30 70/3070/30 PC/PMMAPC/PMMA byby simultaneous simultaneous biaxial biaxial stretching.stretching. ( (aa)) surface; surface; ((bb)) crosscross section.section.

FigureFigure 1010 showsshows thethe surfacesurface hardnesshardness ofof thethe unstretchedunstretched PC/PMMAPC/PMMA filmsfilms atat variousvarious blendblend Figure 10 shows the surface hardness of the unstretched PC/PMMA ﬁlms at various blend compositions.compositions. Here,Here, thethe surfacesurface hardnesshardness waswas estiestimatedmated byby thethe pencilpencil hardnesshardness test.test. TheThe surfacesurface compositions. Here, the surface hardness was estimated by the pencil hardness test. The surface hardnesshardness ofof neatneat PCPC andand neatneat PMMAPMMA waswas 4B4B andand 4H,4H, respectively.respectively. TheThe surfacesurface hardnesshardness becomesbecomes hardness of neat PC and neat PMMA was 4B and 4H, respectively. The surface hardness becomes harder,harder, fromfrom 4B4B toto 4H4H withwith increasingincreasing thethe PMMAPMMA composition.composition. AsAs shownshown inin FiguresFigures 22 andand 5,5, thethe harder, from 4B to 4H with increasing the PMMA composition. As shown in Figures2 and5, the matrix matrixmatrix inin 30/7030/70 PC/PMMAPC/PMMA andand 70/3070/30 PC/PMMAPC/PMMA waswas PPMMAMMA andand PC,PC, respectively.respectively. Hence,Hence, thethe surfacesurface in 30/70 PC/PMMA and 70/30 PC/PMMA was PMMA and PC, respectively. Hence, the surface hardnesshardness ofof thethe blendsblends becamebecame harderharder byby increasingincreasing thethe amountamount ofof PMMAPMMA atat thethe surface,surface, withwith thethe hardness of the blends became harder by increasing the amount of PMMA at the surface, with the increaseincrease ofof thethe compositioncomposition ofof PMMA.PMMA. TheThe changechange ofof thethe pencilpencil hardnesshardness waswas largelarge atat aroundaround aa increase of the composition of PMMA. The change of the pencil hardness was large at around a PMMA PMMAPMMA compositioncomposition ofof 5050 wtwt %,%, inin whichwhich aa co-continuousco-continuous structurestructure waswas formed,formed, suggestingsuggesting thatthat thethe composition of 50 wt %, in which a co-continuous structure was formed, suggesting that the critical criticalcritical compositioncomposition forfor thethe changechange ofof thethe phasephase isis estimatedestimated toto bebe 5050 wtwt %.%. OnOn thethe otherother hand,hand, thethe composition for the change of the phase is estimated to be 50 wt %. On the other hand, the change of changechange ofof thethe pencilpencil hardnesshardness waswas smallsmall atat thethe PMMAPMMA compositioncomposition aboveabove 70%,70%, suggestingsuggesting thethe the pencil hardness was small at the PMMA composition above 70%, suggesting the existence of large existenceexistence ofof largelarge amountamount ofof PMMAPMMA atat thethe surfacsurfacee duedue toto thethe surfacesurface localizationlocalization atat thethe PMMAPMMA amount of PMMA at the surface due to the surface localization at the PMMA composition above 70%. compositioncomposition aboveabove 70%.70%.

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2B Pencil hardness

0 20 40 60 80 100 PMMA (wt%)

FigureFigure 10.10. SurfaceSurface hardnesshardness ofof unstretchedunstretched PC/PMMAPC/PMMA blends at various compositions.

Figure 1111 shows the surface hardnesshardness ofof PC/PMMAPC/PMMA film ﬁlm obtained obtained at various blend compositions by simultaneous biaxial biaxial stretching stretching at at various various draw draw ratios. ratios. The Thesurface surface hardness hardness of the of neat the PC neat (100/0 PC (100/0PC/PMMA) PC/PMMA) did not didchange not changeby stretc byhing. stretching. Note that Note there that is there no report is no reporton the onenhancement the enhancement of the ofsurface the surface hardness hardness by stretching by stretching of neat ofamorphous neat amorphous polymers polymers such as PC, such though as PC, the though surface the hardness surface hardnessof the crystallized of the crystallized PET is enhanced PET is by enhanced the biaxial by stre thetching biaxial due stretching to the increase due toof the crystallinity increase of the[7], crystallinityand the hardness [7], and appears the hardness to relate appears to the toyield relate stress to the [63] yield and stressthe yield [63] andstress the can yield be increased stress can beby increasedstretching. by No stretching. change of No the change surface of hardness the surface by hardness stretching by was stretching observed was in observed the blends in theof 30/70 blends and of 30/7050/50 PC/PMMA and 50/50 PC/PMMAeither; the pencil either; hardness the pencil of hardness30/70 PC/PMMA of 30/70 was PC/PMMA 3H at λ = was 1.0 3Hand at 1.5,λ = and 1.0 andthat 1.5,of 50/50 and thatPC/PMMA of 50/50 was PC/PMMA H at λ = was1.0 and H at 1.5.λ = As 1.0 shown and 1.5. in As Figures shown 2 in and Figures 3, molecular2 and3, molecular chains are chainsoriented are by oriented stretching, by stretching,but phase butinversion phase did inversion not occur did in not 30/70 occur and in 30/7050/50 PC/PMMA. and 50/50 PC/PMMA. The results Thesuggest results that suggest the surface that the hardness surface cannot hardness be cannotenhanc beed enhancedby only the by molecular only the molecular orientation orientation of PC and of PCPMMA. and PMMA. The most The interesting most interesting result is result that the is that surface the surface hardness hardness of 70/30 of PC/PMMA 70/30 PC/PMMA became became harder harderwith an with increasing an increasing draw ratio; draw the ratio; pencil the hardness pencil hardness was 2B at was λ = 2B1.0, at HBλ = at 1.0, λ =HB 1.5,at andλ = 2H 1.5, atand λ = 2.0. 2H atByλ combining= 2.0. By combining the results theshown results in Figure shown 10, in the Figure surface 10, thehardness surface of hardness the blend of at the λ = blend 1.5 and at λλ == 1.52.0 andwas λclose= 2.0 to was that close of 50/50 to that PC/PMMA of 50/50 PC/PMMAand 30/70 PC/PMMA, and 30/70 PC/PMMA,respectively. respectively. Note that the Note surface that thehardness surface of hardnessPC was enhanced of PC was by enhanced the surface by theloca surfacelization localization of PMMA due of PMMA to segregation due to segregation during the duringinjection-molding; the injection-molding; the durometer the D durometer hardness Dincreased hardness from increased 80 to 83 from by blending 80 to 83 by5%blending of PMMA 5% [45]. of PMMAThe change [45]. Theof the change surface of thehardness surface in hardness 70/30 PC/PMMA in 70/30 PC/PMMA by biaxial by stretching biaxial stretching is attributed is attributed to the toincrease the increase of the of amount the amount of PMMA of PMMA at the at thesurfac surfacee by bystructure structure change change from from the the PMMA domaindomain dispersed inin thethe PCPC matrixmatrix to to the the PC PC domain domain dispersed dispersed in in the the PMMA PMMA matrix matrix during during stretching, stretching, due due to theto the phase phase inversion. inversion. This This result result conﬁrms confirms the the phase phase inversion inversion of of the the blend blend byby simultaneoussimultaneous biaxialbiaxial stretching, demonstrateddemonstrated inin FiguresFigures5 5–9.–9. SinceSince thethe criticalcritical compositioncomposition estimatedestimated fromfrom FigureFigure 10 10 was was 50 wt %,%, thethe criticalcritical strainstrain for thethe phasephase inversioninversion was estimated to be λ == 1.8, in whichwhich thethe pencilpencil hardness waswas samesame asas thatthat ofof 50/5050/50 PC/PMMA. PC/PMMA.

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PC/PMMA 3H 30/70 2H

H 50/50 HB

2B 70/30

Pencil hardness Pencil 3B

4B 100/0

1.0 1.2 1.4 1.6 1.8 2.0

FigureFigure 11. 11.Surface Surface hardness hardness of of PC/PMMA PC/PMMA blendsblends obtainedobtained byby simultaneous biaxial stretching at atvarious various draw draw ratios ratios λλ. .◯: 100/0: 100/0 PC/PMMA; PC/PMMA; ●: 70/30: 70/30 PC/PMMA; PC/PMMA; △: 50/504: 50/50PC/PMMA; PC/PMMA; □: 30/70 :PC/PMMA. 30/70 PC/PMMA. #

4.4. Conclusions Conclusions WeWe found found the the characteristic characteristic structural structural evolution evolution in in the the two-phase two-phase blends blends of of polycarbonate polycarbonate (PC) (PC) andand poly(methyl poly(methyl methacrylate) methacrylate) (PMMA) (PMMA) byby simultaneoussimultaneous biaxial stretching. stretching. The The phase phase structure structure was wasenlarged enlarged in inall all directions directions without without the preferred deformationdeformation whenwhen the the symmetric symmetric spherical spherical PC PC domainsdomains were were dispersed dispersed in in the the PMMA PMMA matrix matrix in in 30/70 30/70 PC/PMMA, while while an an anisotropic-shaped anisotropic-shaped co- co-continuouscontinuous structure structure was deformeddeformed toto yield yield a a crosshatched crosshatched structure structure in in 50/50 50/50 PC/PMMA. PC/PMMA. On On the the otherother hand, hand, in in 70/30 70/30 PC/PMMA PC/PMMA thethe phasephase inversioninversion occurredoccurred from the spherical PMMA domains to tothe the crosshatched PMMAPMMA networknetwork matrixmatrix byby in-plane in-plane bimodal bimodal orientation, orientation, due due to to coalescence coalescence and and aggregationaggregation of of the the PMMA PMMA domains domains during during thethe biaxialbiaxial stretching.stretching. Owing Owing to to the the phase phase inversion, inversion, the thepencil pencil hardness hardness became became harder, harder, from from 2B to 2H,2H, duedue toto thethe increase increase of of the the amount amount of of PMMA PMMA at at the the ﬁlmfilm surface surface by by the the simultaneous simultaneous biaxial biaxial stretching. stretching Hence,. Hence, biaxial biaxial stretching stretching of of two-phase two-phase polymer polymer blendsblends is expectedis expected to enhance to enhance the surface the surface hardness hardne of packagingss of packaging ﬁlms when films the when phase the inversion phase inversion occurs, foroccurs, instance. for instance.

AuthorAuthor Contributions: Contributions:T.K. T.K. and and H.S. H.S. conceived conceived and and designed designed the the experiments; experiments; T.K. T.K. performed performed the the experiments; experiments; T.K.T.K. analyzed analyzed the the data; data; T.K. T.K. and and H.S. H.S. wrote wrote the the paper. paper.

Funding:Funding:This This research research was was partially partially supported supported by by the the Japan Japan Society Society for for the the Promotion Promotion of Scienceof Science (Grant-in-Aid (Grant-in-Aid forfor Scientific Scientific Research Research (C), (C), No. No. 18K05231). 18K05231). Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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