Phase Structure and Properties of Ternary Polylactide/Poly(Methyl Methacrylate)/Polysilsesquioxane Blends

Article Phase Structure and Properties of Ternary Polylactide/ Poly(methyl methacrylate)/Polysilsesquioxane Blends

Anna Kowalewska 1,* , Agata S. Herc 1 , Joanna Bojda 1, Maria Nowacka 1 , Mariia Svyntkivska 1, Ewa Piorkowska 1, Witold Kaczorowski 2 and Witold Szyma ´nski 2

1 Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland; [email protected] (A.S.H.); [email protected] (J.B.); [email protected] (M.N.); [email protected] (M.S.); [email protected] (E.P.) 2 Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; [email protected] (W.K.); [email protected] (W.S.) * Correspondence: [email protected]

Abstract: Ternary blends of polylactide (PLA, 90 wt.%) and poly(methyl methacrylate) (PMMA, 10 wt.%) with functionalized polysilsesquioxanes (LPSQ-R) were obtained by solution blending. R groups in LPSQ containing hydroxyethyl (LPSQ-OH), methylglycolic (LPSQ-COOMe) and pentaflu- orophenyl (LPSQ-F5) moieties of different chemical properties were designed to modify PLA blends with PMMA. The effect of the type of LPSQ-R and their content, 1–3 wt.%, on the structure of the blends was studied with scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (SEM-EDS), dynamic mechanical thermal analysis (DMTA) and Raman spectroscopy. Citation: Kowalewska, A.; Differential scanning calorimetry (DSC) and tensile tests also showed various effects of LPSQ-R on Herc, A.S.; Bojda, J.; Nowacka, M.; the thermal and mechanical properties of the blends. Depth-sensing indentation was used to resolve Svyntkivska, M.; Piorkowska, E.; Kaczorowski, W.; Szymański,W. spatially the micro- and nano-scale mechanical properties (hardness and elastic behaviour) of the Phase Structure and Properties of blends. The results showed clearly that LPSQ-R modulate the structure and properties of the blends. Ternary Polylactide/Poly(methyl methacrylate)/Polysilsesquioxane Keywords: PLA/PMMA blends; ternary polymer blends; polysilsesquioxanes Blends. Polymers 2021, 13, 1033. https://doi.org/10.3390/ polym13071033 1. Introduction Academic Editor: Aziz Biodegradable poly(lactic acid) (PLA) exhibits high strength and stiffness. Brittleness, Mansurovich Muzafarov low toughness, low melt strength, slow crystallization rate and low heat deflection temperature largely limit its wide use [1–3]. PLA may be toughened using various methods Received: 28 February 2021 (plasticization, copolymerization, filling and melt blending with other polymers) and the Accepted: 19 March 2021 relationships between PLA-based polymer blends composition, their morphology, mechan- Published: 26 March 2021 ical properties and toughening mechanisms behind the observed phenomena have been thoroughly studied [4–7]. The interfacial compatibility on a nanoscale level and the phase Publisher’s Note: MDPI stays neutral behaviour have a decisive influence on polymer blends. Miscibility may be favoured when with regard to jurisdictional claims in published maps and institutional affil- some special interactions, stronger than van der Waals forces (ionic interactions, dipole- iations. dipole, donor-acceptor and weak hydrogen bonding) exist between the blended materials. The intrinsic properties of the components, as well as the characteristic morphology of the blend determine the improvement of the final properties, e.g., mechanical, thermal and barrier properties. Interesting results were obtained on formation of miscible/immiscible binary blends Copyright: © 2021 by the authors. containing poly(methyl methacrylate) (PMMA). PMMA is known for intrinsic high mechan- Licensee MDPI, Basel, Switzerland. ical properties, UV resistance and long-term durability. Full miscibility of PLA and PMMA This article is an open access article was achieved through solution precipitation [8,9] and melt blending [10] whereas solution distributed under the terms and conditions of the Creative Commons casting resulted in two separate phases (co-continuous or separated inclusions) [8,10,11]. Attribution (CC BY) license (https:// The immiscibility–miscibility transition behaviour of PLA/PMMA blends changes with creativecommons.org/licenses/by/ composition [12,13] and temperature [14]. Rheological, thermomechanical, and mechanical 4.0/). properties of PLA/PMMA miscible blends with different compositions were extensively

Polymers 2021, 13, 1033. https://doi.org/10.3390/polym13071033 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 19

changes with composition [12,13] and temperature [14]. Rheological, thermomechanical, and mechanical properties of PLA/PMMA miscible blends with different compositions were extensively examined. The role of entanglement network in rheological and thermomechanical properties was explained [15–20]. The small size of the dispersed inclusions Polymers 2021, 13, 1033 as well as the strong interfacial interactions in PLA/PMMA blends is responsible2 of 19 for increased tensile strength. PMMA was also shown to provide increased hydrolytic and structural stability to the PLA/PMMA blends [21–23]. However, the improvement of im- examined.pact toughness The role in of binary entanglement blends network requires in addi rheologicaltion of and quite thermomechanical large amount prop-(>20 wt.%) of ertiesPMMA, was which explained has [15an– 20adverse]. The smalleffect sizeon the of the compostability dispersed inclusions of PLA. as In well addition, as the the im- strongprovement interfacial of impact interactions toughness in PLA/PMMA may be often blends accompanied is responsible by for a increased significant tensile reduction of strength.the tensile PMMA strength. was also shown to provide increased hydrolytic and structural stability to theTernary PLA/PMMA blends blends are [much21–23 ].more However, complex the improvementsystems with of a impactwide range toughness of possible in mi- binarycrostructures. blends requires Many additionvariables of that quite influenc large amounte the morphology, (>20 wt.%) of and PMMA, properties which has include the an adverse effect on the compostability of PLA. In addition, the improvement of impact volume fraction, inherent properties of the components and their miscibility. In the im- toughness may be often accompanied by a significant reduction of the tensile strength. miscibleTernary blends blends the are interfacial much more tension complex between systems withcomponents a wide range can ofbe possibleefficiently mi- reduced crostructures.through compatibilization, Many variables thatwhich influence results the in morphology,a better dispersion and properties of a minor include component the and volumeimproved fraction, interfacial inherent adhesion. properties That of the may components prevent and premature their miscibility. fracture In resulting the immis- from for- ciblemation blends of thelarge interfacial cavities tension inside between a minor components component can inclusions be efficiently or due reduced to separation through of the compatibilization,components. Ternary which resultsPLA/PMMA in a better blends dispersion with of aother minor thermoplastic component and improvedpolymers can be interfacialpromising adhesion. materials. That The may compatibilizers prevent premature should fracture be miscible resulting with from at formationleast one component of largeof the cavities blend. inside Addition a minor of componentcopolymers inclusions containi orng due functionalized to separation ofmethacrylate the components. blocks is an Ternary PLA/PMMA blends with other thermoplastic polymers can be promising materials. effective way to achieve good compatibility and toughening [24–34]. Nanofillers, e.g., na- The compatibilizers should be miscible with at least one component of the blend. Addition ofnosilica copolymers [17,35] containing or other functionalized minerals [36,37], methacrylate also blockshave isa anpositive effective effect way toon achieve properties of goodPLA/PMMA compatibility blends. and It toughening is worth mentioning [24–34]. Nanofillers, that PMMA e.g., nanosilica was also [applied17,35] or as other an effective mineralscompatibilizer [36,37], also that have provided a positive a toughening effect on properties effect ofin PLA/PMMA blends of PLA/polyvinylidene blends. It is worth fluo- mentioningride [38], PLA/polybutadiene-g-poly(styrene-c that PMMA was also applied as an effectiveo-acrylonitrile) compatibilizer [39,40], that PLA/poly(butylene provided a tougheningadipate-co-butylene effect in blends terephthalate) of PLA/polyvinylidene [41], PLA/polystyrene fluoride [38], [42] PLA/polybutadiene-g- or PLA/poly(epichlorohy- poly(styrene-co-acrylonitrile)drin-co-ethylene oxide) [43]. [39 ,40], PLA/poly(butylene adipate-co-butylene terephthalate) [41We], PLA/polystyrenehave recently [found42] or PLA/poly(epichlorohydrin-co-ethylene that linear, ladder-like polysilsesquioxanes oxide) [43]. (LPSQ-R) We have recently found that linear, ladder-like polysilsesquioxanes (LPSQ-R) (Scheme1 ) (Scheme 1) may be applied as interesting modifiers of PLA [44–47]. LPSQ-R are amor- may be applied as interesting modifiers of PLA [44–47]. LPSQ-R are amorphous substances ofphous low glass substances transition of temperatures low glass transition that are partially temperatures miscible that with are PLA partially [44–46]. miscible Their net with PLA effect[44–46]. may Their be attributed net effect to plasticization may be attributed and dispersion to plasticization in PLA matrix and and dispersion supramolecular in PLA matrix interactionsand supramolecular that involve interactions side functional that groups involve R [6, 44side–46 ].functional In this report, groups we focus R [6,44–46]. on the In this influencereport, we of selectedfocus on LPSQ-R the influence on PLA/PMMA of select (90:10)ed LPSQ-R blend. Theon PLA/PMMA composition allowed (90:10) for blend. The studyingcomposition dispersion allowed of PMMA for studying particles within dispersion PLA matrix. of PMMA Moreover, particles this blend within exhibited PLA matrix. goodMoreover, ductility. this blend exhibited good ductility.

SchemeScheme 1. 1.Chemical Chemical structure structure of (poly[2-(hydroxyethylthio)-ethylsilsesquioxane] of (poly[2-(hydroxyethylthio)-ethylsilsesqu (LPSQ-OH),ioxane] (LPSQ-OH), poly[2- (methylthioglycolate)-ethylsilsesquioxane]poly[2-(methylthioglycolate)-ethylsilsesquioxa (LPSQ-COOMe)ne] (LPSQ-COOMe) and poly[2-(pentaﬂuorophenylthio)- and poly[2-(pentafluoro- ethylsilsesquioxane]phenylthio)-ethylsilsesquioxane] (LPSQ-F5). (LPSQ-F5).

Phase structure and dispersion of PMMA in PLA matrix was studied with scanning electron microscopy (SEM) and Raman mapping. The observed differences were correlated with the bulk thermal and tensile properties of the blends as well as surface nanomechanical characteristics and wettability. The obtained results show that small amounts of functionalized polysilsesquioxanes may improve the dispersion of PMMA in PLA and

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modulate the properties of such ternary blends. Although polysilsesquioxanes are not biodegradable, they are non-toxic and biocompatible as other silsesquioxanes [48].

2. Materials and Methods 2.1. Materials Ladder-like polysilsesquioxanes LPSQ-R (Scheme1) were prepared as previously described [44,45]. The precursor used for their synthesis was poly(vinylsilsesquioxane) of −1 Mn = 1 kg mol , Mw/Mn = 1.4. PLA used in the studies was a commercially available −1 NW4032D grade from NatureWorks LLC, Minnetonka, MN, USA (Mw = 130 kg mol , Mw/Mn = 1.9) containing 1.2 mol% of D-lactide units. Poly(methyl methacrylate) was −1 Altuglas BS 572 Clear (Resinex, Warsaw, Poland, Mn = 179 kg mol , Mw/Mn = 2.4, ◦ Tg = 119.5 C). Solvents used for the synthesis and preparation of blends (THF, CH2Cl2, Krakchemia S.A., Kraków, Poland) were purified following literature procedures [49]. Blends of PLA and PMMA containing LPSQ-R modifiers (10, 20 or 30 wt.% with respect to the amount of PMMA) were prepared by solution blending (Table1). The blends are referred through this paper as, for example B-R-10, where R indicates the type of functional group, and the number stands for the LPSQ-R weight content with respect to PMMA. For comparison, PLA blend with 10 wt.% of PMMA was also prepared. In general, the PLA/PMMA mixture (9.0 g and 1.0 g, respectively) was dissolved in CH2Cl2 (68 mL), to obtain 10 wt.% solution. After 24 h, a specific volume of LPSQ-R solutions in CH2Cl2 (3 wt.% LPSQ-COOMe or LPSQ-F5; corresponding to 0.1 g in 2.4 mL of the solvent for B-R-10) or THF (4 wt.% LPSQ-OH; corresponding to 0.1 g in 2.7 mL of the solvent for B-R-10) was added dropwise. The mixtures were stirred magnetically for 2 h at room temperature (RT). The mixtures containing LPSQ-OH became cloudier in a short time after the addition of the polysilsesquioxanes and remained turbid on prolonged stirring. The effect was not observed for LPSQ-COOMe and LPSQ-F5. No increase of viscosity of the mixtures was observed. The prepared compositions were then poured into Petri dishes and left for free solvent evaporation. The obtained samples were not tacky. Solid products were dried under high vacuum (0.01 Torr) for 24 h at 80 ◦C, then for 24 h at 100 ◦C, and studied for their thermal stability with thermogravimetric analysis. For further studies, 0.5 and 1 mm thick films were compression moulded at 190 ◦C and rapidly quenched between metal blocks held at 0 ◦C to hinder crystallization of PLA matrix. Differential scanning calorimetry (DSC) measurements evidenced that all the prepared films were amorphous.

Table 1. Component contents in the prepared blends.

Component Content Sample Code PLA (wt.%) PMMA (wt.%) LPSQ-R (wt.%) B-0 90.0 10.0 - B-R-10 89.1 9.9 1.0 B-R-20 88.2 9.8 2.0 B-R-30 87.4 9.7 2.9 R = OH, COOMe, F5.

2.2. Analytic Methods Morphological investigations were carried out for amorphous ﬁlms of neat PLA, PMMA, PLA/PMMA (B-0) blend and ternary PLA/PMMA/LPSQ-R blends that were cryo-fractured, sputtered with gold using a Jeol Fine Coater 1200 (JEOL, Tokyo, Japan) and analysed with scanning electron microscopy (SEM) using SEM Jeol 6010LA (JEOL, Tokyo, Japan) operating in the high vacuum mode at an accelerating voltage of 10 kV. The distribution of silicon atoms of LPSQ-R in the studied samples was examined with SEM with energy dispersive spectroscopy (SEM-EDS). Prior to the examination, the surfaces were sputtered with carbon using a coater Q150R ES (Quorum Technologies, Lewes, UK). To have a better insight into the structure, ﬁlms of selected materials were enzymatically etched according to [50,51]. The solution of Proteinase K (4 mg), Trizma base (61 mg) and Polymers 2021, 13, 1033 4 of 19

sodium azide (2 mg), all from Sigma Aldrich (Munich, Germany), in distilled water (5 mL), was prepared. The samples were etched in the solution at 37 ◦C for 1–2 h, washed with a distilled water, dried, sputtered with gold and examined by SEM. Dynamic mechanical thermal analysis (DMTA) was carried out on 1 mm thick rectan- gular specimens, 17.5 mm × 12 mm, in a single cantilever bending mode, using a DMTA TA Q-800 Thermal Analyser (TA Instruments, New Castle, DE, USA) at a frequency of 1 Hz and a heating rate of 2 ◦C min−1 from −100 to 140 ◦C. Raman signal (spectra and maps) were collected with an InVia Raman microscope (Renishaw plc, Gloucestershire UK). The polymer surfaces were analysed using a 532-nm laser with 20× objective lens (Carl-Zeiss Jena, Germany), providing approximately 25 mW of laser power at the sample. Single spectra were collected with an exposure time of 20 s in the spectral collection range 700–1800 cm−1. 2-D Raman maps were collected from 200 µm × 100 µm 2 areas with 10 µm spatial resolution. All operations on the data (baseline correction, normalization) and direct classical least squares (DCLS) component analysis were performed using Wire 5.1 application. For better visualization, the maps appearance was interpolated. Phase transitions in the studied samples were detected using a DSC 2920 (TA Instru- ments, New Castle, DE, USA). Thermograms were recorded for samples of blend cast ◦ −1 ◦ ﬁlms heated in N2 atmosphere at a rate of 10 C min from 20 to 190 C. After the ﬁrst heating, the samples were kept at 190 ◦C for 5 min and then cooled down to ensure the same thermal history. The phase transitions were studied during the second heating, at ◦ −1 5 C min . The degree of crystallinity (Xc) was estimated from the enthalpies of cold crystallization exotherms and melting endotherms using Equation (1):

∆Hm·100% Xc = (1) ∆H0

where ∆Hm and ∆H0 are the enthalpy of fusion of PLA in the blends and the crystalline phase of PLA (93 J/g, enthalpy of melting of α-crystals [52]), respectively. Thermogravimetric analyses of the studied polymeric materials were carried out with a Hi-Res TGA 2950 Thermogravimetric Analyzer (TA Instruments, New Castle, DE, USA) ◦ −1 in N2 atmosphere (heating rate of 10 C min , resolution 3, sensitivity 3). Tensile properties were measured using Linkam Tensile Stress Testing System TST 350 (Linkam Scientific Instruments Ltd., Waterfield, UK). At least five 0.5 mm thick oar- shaped specimens, with 3.81 mm gauge length and gauge width of 1.59 mm, were drawn to fracture at a rate of 5% min−1, at 25 ◦C. The average values of mechanical parameters were calculated. Selected specimens were analysed after fracture with polarized light microscopy (PLM) using microscope Nikon Eclipse 80i (Nikon, Tokyo, Japan). Nanomechanical properties on the surface and at the cross-section of the compression moulded 0.5 mm thick polymer films were determined with a G-200 Nano Indenter (MTS Nano Instruments/KLA-Tencor Corporation; Milpitas, CA, USA) equipped with TestWorksPro 4 software. The impressions were made with a Berkovich type indenter with a half angle equal to 65.3◦ and a radius of roundness. The data analysis with the Analyst program was based on Oliver and Pharr’s approach [53]. The studies were carried out using two methods: CSM (Continuous Stiffness Measurement) and BASIC [54]. The CSM method is based on continuous measurement of the stiffness (S) during the material penetration to estimate hardness (H) and modulus of elasticity (E) of the material as a function of penetration depth (h). The test was used to measure the mechanical properties of the specimens from the front with 9 prints per sample (penetration depth 1500 nm). BASIC method was used to map the mechanical properties from the front at the maximum load point. Maps of E and H were obtained with 100 indents in a 10 × 10 matrix with a step of 20 µm. Surface free energy was estimated for the compression moulded 0.5 mm thick films by contact-angle measurements (sessile drop technique) at the film–air interface, as described earlier [46], using deionized water and glycerol (Chempur, Piekary Slaskie, Poland; pure Polymers 2021, 13, 1033 5 of 19

p.a., anhydrous) as the reference liquids. Static and advancing contact angles of sessile droplets of reference liquids at the ﬁlm–air interface were measured right after the deposition at RT with a Ramé-Hart NRL contact-angle goniometer (Ramé-Hart Instrument Co., Succasunna, NJ, USA) equipped with a video camera JVC KYF 70B (Yokohama, Japan) and drop-shape analysis software (“Drop”, version 2.1, written and authored by Dr. hab. Stanisław Sosnowski at Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences). The surface free energy (including the polar and dispersive components) was estimated by the Owens–Wendt method [55]. The values of contact angles are averages of at least four measurements taken on different areas of the same sample and calculated with their standard deviations. The contact angle for each drop was calculated as the arithmetic mean of the left and right angles.

3. Results and Discussion 3.1. Phase Distribution in the PLA/PMMA Mixture and Ternary Blends Containing LPSQ-R The application of linear double strand polysilsesquioxanes as modifiers in PLA/PMMA blends is related to our previous works on PLA/LPSQ-R systems [44–46]. Linear polysilsesquioxanes have a specific double strand backbone, which makes them much less flexible than linear polysiloxanes and prevents chain coiling. The presence of functional groups at each of the silicon atoms increases effectiveness of interactions, e.g., in supramolecular systems. The functionalized LPSQ-R of similar molecular weight ◦ selected for this study have low glass transition temperature (Tg) (LPSQ-COOMe: −41 C; LPSQ-OH: −46 ◦C; LPSQ-F5: −10 ◦C) [45,46]. We have previously showed that LPSQ- COOMe [45] and LPSQ-F5 [46] are well dispersed and partially miscible with PLA at c ≤ 5 wt.%. LPSQ-OH formed separated inclusions with sizes up to a few micrometers in the polyester matrix for lower contents of the additive, yet at 5 wt.% the maximum inclusion size increased to about 10 µm [45]. The structural differences may be correlated with the strength of interactions between the functional groups and PLA matrix. LPSQ-COOMe may form only rather weak hydrogen bonds with carbonyl groups (-CH3···O=C-). This phenomenon may be of importance as similar mechanisms are known to operate during the formation of PLA stereocomplex structures [56]. The contacts between LPSQ-F5 and PLA were explained by the formation of complexes similar to n→π* attractive interactions. Both LPSQ-COOMe and LPSQ-F5 could act as effective modifiers of PLA drawability and tensile toughness without a significant reduction of Tg of PLA [45,46]. Moreover, enhancement of low temperature nucleation, which intensified cold crystallization, was observed. In the present study, PLA/PMMA blends modified with LPSQ-R were prepared by solution blending. It is known that miscible PLA/PMMA blends are most readily formed by melt blending while solution blending results in phase separation [12]. Nevertheless, this obsta- cle helped to demonstrate clearly the effect of LSPQ-R. The low weight ratio of PMMA to PLA was also chosen to better illustrate the changes in the components’ distribution. It should be stressed that the effective content of LPSQ-R in the blends is very low (≤3 wt.%). Figure1 shows the DMTA results, the storage modulus ( E’) and loss modulus (E”) of PLA, PMMA, PLA/PMMA blend (B-0) and the blends with the 3 wt.% of LPSQ-R. The E” peaks of PLA and PMMA, with maxima at 62 ◦C and at 124 ◦C, correspond to the glass transition in these polymers. In addition, E” plot of PMMA exhibits a broad β-peak with a maximum at 20 ◦C, correlated with the mobility of ester side groups with respect to the main chain and overlapping the α-peak corresponding to the glass transition. E’ of PMMA and E’ of PLA decreased with increasing temperature and dropped above 100 ◦C and above 50 ◦C, respectively, in the glass transition regions of these polymers. The pronounced E” peak of PLA/PMMA blend at 61 ◦C corresponded to the glass transition in PLA. In the glassy region, E” of the blend was slightly higher that of PLA because of PMMA β-relaxation contribution. The result suggested that the PLA/PMMA blend was immiscible. The glass transition of PMMA in the blend did not show up because of the small PMMA content and a strong decrease of stiffness of the blend above PLA glass transition that made correct measurement impossible. E” plots of B-OH-30, B-F5-30 and Polymers 2021, 13, x FOR PEER REVIEW 6 of 19

a maximum at 20 °C, correlated with the mobility of ester side groups with respect to the main chain and overlapping the α-peak corresponding to the glass transition. E′ of PMMA and E′ of PLA decreased with increasing temperature and dropped above 100 °C and above 50 °C, respectively, in the glass transition regions of these polymers. The pronounced E″ peak of PLA/PMMA blend at 61 °C corresponded to the glass transition in PLA. In the glassy region, E″ of the blend was slightly higher that of PLA because of PMMA β-relaxation contribution. The result suggested that the PLA/PMMA blend was immiscible. The glass transition of PMMA in the blend did not show up because of the Polymers 2021, 13, 1033 small PMMA content and a strong decrease of stiffness of the blend above PLA6 of 19glass transition that made correct measurement impossible. E″ plots of B-OH-30, B-F5-30 and B- COOMe-30 were similar to that of PLA/PMMA blend, with E’ peak temperature of 60, 61 B-COOMe-30and 59 °C, respectively. were similar The to that small of PLA/PMMAdecrease of E blend,″ peak with temperatureE’ peak temperature resulted from of disper- 60,sion 61 of and LPSQ-R 59 ◦C, respectively. fraction in ThePLA small on a decrease molecular of E” level,peak as temperature it was observed resulted previously from for dispersionPLA blends of LPSQ-Rwith LPSQ-R fraction [45,46]. in PLA This on a molecularconclusion level, was as corroborated it was observed by previouslythe results of SEM forEDS PLA analysis. blends with LPSQ-R [45,46]. This conclusion was corroborated by the results of SEM EDS analysis. E′ plot of PLA/PMMA blend resembled that of PLA. E’ of the blend was slightly E’ plot of PLA/PMMA blend resembled that of PLA. E’ of the blend was slightly higherhigher than thanE0 Eof′ of PLA PLA in thein the low low temperature temperature range range and slightly and slightly lower above lower 0 above◦C, and 0 it °C, and it ﬁnallyfinally dropped dropped in in the the glass glass transition transition region. region.E’ temperature E’ temperature dependencies dependencies of B-OH-30, of B-OH-30, B-F5-30B-F5-30 and and B-COOMe-30 B-COOMe-30 resembled resembled that of that PLA/PMMA of PLA/PMMA blend, except blend, for except lower E’forvalues lower E′ val- inues the in glassy the glassy region region caused caused by the presence by the presence of LPSQ-R. of LPSQ-R.

FigureFigure 1. 1.Storage Storage modulus modulus (E’) and(E′) loss and modulus loss modulus (E”) of neat (E″ polylactide) of neat polylactide (PLA), poly(methyl (PLA), methacry-poly(methyl latemethacrylate (PMMA), PLA/PMMA (PMMA), PLA/PMMA blend (B-0) and blend selected (B-0) LPSQ-R and selected modified LPSQ-R blends vs. modified temperature. blends vs. temperature. The blend structure was studied by SEM. The results of SEM analysis, summarized brieﬂy in Table S1, evidenced phase separation in the blends. SEM micrographs (Figure2 ) of the cryo-fracture surface of PLA/PMMA (B-0) blend reveal the presence of oval areas, with sizes of several micrometers, where fracture propagated differently than in the surrounding matrix, which is suggestive of phase separated inclusions. Figure3 shows the etched surface of this blend ﬁlm. Proteinase K is known to degrade L-lactide units, hence the etching should expose PMMA inclusions. Indeed, on the etched surface particles of various sizes are seen, some laying on the surface and some still embedded in the matrix. The sizes Polymers 2021, 13, x FOR PEER REVIEW 7 of 19

Polymers 2021, 13, x FOR PEER REVIEW 7 of 19

The blend structure was studied by SEM. The results of SEM analysis, summarized briefly in Table S1, evidenced phase separation in the blends. SEM micrographs (Figure 2) of theThe cryo-fracture blend structure surface was studied of PLA/PMMA by SEM. (BThe-0) results blend revealof SEM the analysis, presence summarized of oval areas, withbriefly sizes in Table of several S1, evidenced micrometers, phase where separation fracture in the propagated blends. SEM differently micrographs than (Figurein the sur- rounding2) of the cryo-fracture matrix, which surface is suggestive of PLA/PMMA of phase (B-0) separatedblend reveal inclusions. the presence Figure of oval 3 shows areas, the etchedwith sizes surface of several of this micrometers, blend film. Proteinasewhere fracture K is propagated known to degrade differently L-lactide than in units, the sur- hence rounding matrix, which is suggestive of phase separated inclusions. Figure 3 shows the Polymers 2021, 13, 1033 the etching should expose PMMA inclusions. Indeed, on the etched surface7 of particles 19 of variousetched surface sizes are of thisseen, blend some film. laying Proteinase on the suK rfaceis known and tosome degrade still embeddedL-lactide units, in the hence matrix. the etching should expose PMMA inclusions. Indeed, on the etched surface particles of The sizes of the large particles correspond to the sizes of the oval areas visible on the frac- various sizes are seen, some laying on the surface and some still embedded in the matrix. ture surface in Figure 2 that confirms that the latter are PMMA inclusions. It is worth ofThe the sizes large of particles the large correspond particles tocorrespond the sizes of to the th ovale sizes areas of visiblethe oval on areas the fracture visible surfaceon the frac- noting that after longer etching, the sizes of the visible particles somewhat diminished, inture Figure surface2 that in conﬁrms Figure that2 that the confirms latter are PMMAthat the inclusions. latter are ItPMMA is worth inclusions. noting that It after is worth except for those still embedded in matrix. This suggest that the PMMA particles could longernoting etching, that after the sizeslonger of etching, the visible the particles sizes of somewhat the visible diminished, particles exceptsomewhat for those diminished, still embeddedcontainexcept for a fraction inthose matrix. still of ThisPLA,embedded suggest despite in that thematrix. the lack PMMA Thisof DMTA suggest particles evidence. that could the contain PMMA a fractionparticles of could PLA,contain despite a fraction the lack of ofPLA, DMTA despite evidence. the lack of DMTA evidence.

Figure 2. Scanning electron microscopy (SEM) micrograph of cryo-fracture surface of PLA/PMMA Figure 2. Scanning electron microscopy (SEM) micrograph of cryo-fracture surface of PLA/PMMA Figureblend. 2. Scanning electron microscopy (SEM) micrograph of cryo-fracture surface of PLA/PMMA blend. blend.

FigureFigureFigure 3. 3.SEM SEM micrographs micrographsmicrographs of of PLA/PMMA of PLA/PMMA PLA/PMMA blend blend blend ﬁlm film etchedfilm etched etched for 1for h. for 1 h. 1 h.

SEMSEM micrographs micrographsmicrographs of of surfacesof surfaces surfaces of cryo-fracturedof of cryo-fractured cryo-fractured ternary ternary ternary blends blends blends with with 1 andwith 1 3and wt.%1 and 3 wt.% of 3 wt.% of of LPSQ-RLPSQ-RLPSQ-R are are shown shownshown in in Figurein Figure Figure4. On4. 4. On theOn the surfaces the surfaces surfaces oval oval areas oval areas areareas visible,are are visible, visible, where where the where fracture the thefracture fracture propagated differently than in the surrounding polymer, as in PLA/PMMA blend in propagatedpropagated differentlydifferently than than in in the the surrounding surrounding polymer, polymer, as asin PLA/PMMAin PLA/PMMA blend blend in Fig- in Fig- Figure2 , and identiﬁed as PMMA inclusions. However, in the micrographs, also other ureure 2, and identifiedidentified as as PMMA PMMA inclusions. inclusions. However, However, in inthe the micrographs, micrographs, also also other other phase phase phaseseparated separated particles particles are present, are present, with withsizes sizesdependent dependent on the on additive the additive type type and andcontent. separated particles are present, with sizes dependent on the additive type and content. content.They were They accompanied were accompanied with holes with of holes similar of similar sizes that sizes were that inhabited were inhabited by the by inclusions the They were accompanied with holes of similar sizes that were inhabited by the inclusions inclusionsbefore the before fracture. the fracture.However, However, Si mapping Si mapping of the blends of the blendswith 3 withwt.% 3 of wt.% LPSQ-R, of LPSQ-R, presented presentedbefore the in fracture. Figure5, However, conﬁrms that Si mapping LPSQ-R formed of the blends inclusions with in 3 the wt.% blends of LPSQ-R, but were presented in Figure 5, confirms that LPSQ-R formed inclusions in the blends but were also dispersed alsoin Figure dispersed 5, confirms in the surrounding that LPSQ-R polymers. formed Previously, inclusions we in havethe blends found thatbut were PLAblends also dispersed with LPSQ-R were phase separated but partially miscible [44–46]. In the present study, the phase separated LPSQ-R rich inclusions did not show up in DMTA results, most possibly because of their small content. SEM analysis of cryo-fracture surfaces of the blends with LPSQ-OH also demonstrated the presence of large LPSQ-OH-rich inclusions of sizes of several micrometers, irrespectively of the additive content. Although they were dispersed Polymers 2021, 13, x FOR PEER REVIEW 8 of 19

in the surrounding polymers. Previously, we have found that PLA blends with LPSQ-R were phase separated but partially miscible [44–46]. In the present study, the phase separated LPSQ-R rich inclusions did not show up in DMTA results, most possibly because of Polymers 2021, 13, 1033 8 of 19 their small content. SEM analysis of cryo-fracture surfaces of the blends with LPSQ-OH also demonstrated the presence of large LPSQ-OH-rich inclusions of sizes of several micrometers, irrespectively of the additive content. Although they were dispersed in the inPLA the rich PLA matrix, rich matrix, many of many them of were them embedded, were embedded, in whole in or whole in part, or in PMMA part, in particles. PMMA particles.In the ternary In the blends ternary with blends LPSQ-F5 with and LPSQ-F5 LPSQ-COOMe, and LPSQ-COOMe, the largest the additive largest rich additive inclu- richsions inclusions were markedly were markedly smaller smallerthan in thanthe ma interials the materials with LPSQ-OH. with LPSQ-OH. Although Although the largest the largestinclusion inclusion size increased size increased with increasing with increasing additive additive content, content, it did itnot did exceed not exceed 3 μm 3inµ mB-F5- in B-F5-30.30. In the In blends the blends with with LPSQ-COOMe, LPSQ-COOMe, the sizes the sizes of the of thelargest largest inclusions inclusions were were similar. similar. As Asshown shown in Figure in Figure 4, in4, inboth both types types of ternary of ternary blends, blends, numerous numerous additive additive rich rich inclusions inclusions were werelocated located in the in PMMA the PMMA particles. particles. SEM SEM EDS EDS Si mapping Si mapping did did not not show show any any depletion depletion of of Si Siaround around LPSQ-R LPSQ-R rich rich inclusions; inclusions; hence, hence, LPSQ-R LPSQ-R were were not only disperseddispersed onon aa molecularmolecular levellevel in in PLA PLA but but also also in in PMMA. PMMA.

Polymers 2021, 13, x FOR PEER REVIEW 9 of 19

FigureFigure 4. 4.SEM SEM micrographs micrographs of of cryo-fracture cryo-fracture surfaces surfaces of of PLA/PMMA PLA/PMMA blends blends modiﬁed modified with with LPSQ-R. LPSQ- R.

FigureFigure 5.5.SEM SEM EDSEDS SiSi mappingmapping ofof PLA/PMMAPLA/PMMA blends modiﬁedmodified withwith LPSQ-R.LPSQ-R.

Figure 6 shows SEM micrographs of the blends with 3 wt.% of LPSQ-R after 2 h etching. Similarly to PLA/PMMA blend, numerous particles are visible, located on the surface or still embedded in the polymer. The small particles in B-OH-30 were accompanied by large ones, with diameters exceeding 10 μm. In contrast, sizes of particles in B-F5-30 and B-COOMe-30 were more uniform, up to several micrometers. Judging from their size, the particles might be PMMA or LPSQ-R rich particles. However, as seen in Figure 5, numerous LPSQ-R particles were located inside PMMA. Moreover, LPSQ-R are amorphous substances with low Tg, unable to retain their shape at RT. Therefore, the particles shown in Figure 5 are rather PMMA-rich particles.

Figure 6. SEM micrographs of etched PLA/PMMA blends modified with 3 wt.% of LPSQ-R.

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Figure 5. SEM EDS Si mapping of PLA/PMMA blends modified with LPSQ-R.

FigureFigure6 6 shows shows SEMSEM micrographs of of the the blends blends with with 3 wt.% 3 wt.% of ofLPSQ-R LPSQ-R after after 2 h etch- 2 h etching.ing. Similarly Similarly to PLA/PMMA to PLA/PMMA blend, blend, numerous numerous particles particles are visible, are visible, located located on the onsurface the surfaceor still orembedded still embedded in the inpolymer. the polymer. The small The small particles particles in B-OH-30 in B-OH-30 were were accompanied accompanied by bylarge large ones, ones, with with diameters diameters exceeding exceeding 10 μ 10m.µ m.In contrast, In contrast, sizes sizes of particles of particles in B-F5-30 in B-F5-30 and andB-COOMe-30 B-COOMe-30 were were more more uniform, uniform, up to up severa to severall micrometers. micrometers. Judging Judging from from their their size, size, the theparticles particles might might be PMMA be PMMA or LPSQ-R or LPSQ-R rich particles. rich particles. However, However, as seen as in seen Figure in 5, Figure numer-5, numerousous LPSQ-R LPSQ-R particles particles were located were located inside inside PMMA. PMMA. Moreover, Moreover, LPSQ-R LPSQ-R are amorphous are amorphous sub- substances with low T , unable to retain their shape at RT. Therefore, the particles shown stances with low Tg, unableg to retain their shape at RT. Therefore, the particles shown in inFigure Figure 5 5are are rather rather PMMA-rich PMMA-rich particles. particles.

FigureFigure 6.6.SEM SEM micrographsmicrographs ofof etchedetched PLA/PMMAPLA/PMMA blends modiﬁedmodified withwith 33 wt.%wt.% ofof LPSQ-R.LPSQ-R.

Raman spectroscopy mapping was used to detect changes of concentration of PMMA in the studied samples (Figure7 and Figures S1–S4, Table S2). Very clear data were obtained, and evaluation of the main components’ dispersion was possible. Raman spectra of PLA and PMMA differ in the position of ν(C=O) mode (1767 cm−1 for PLA and 1726 cm−1 for −1 −1 PMMA) and the presence of ν(C-COO) (873 cm , PLA), ν(CH2) (853 cm , PLA) and −1 O–CH3 rock (990 cm , PMMA) vibrations. Polymers 2021, 13, x FOR PEER REVIEW 10 of 19

Raman spectroscopy mapping was used to detect changes of concentration of PMMA in the studied samples (Figures 7 and S1–S4, Table S2). Very clear data were obtained, and evaluation of the main components’ dispersion was possible. Raman spectra of PLA and PMMA differ in the position of ν(C=O) mode (1767 cm−1 for PLA and 1726 cm−1 for PMMA) Polymersand2021, 13the, 1033 presence of ν(C-COO) (873 cm−1, PLA), ν(CH2) (853 cm−1, PLA) and O–CH3 rock10 of 19 (990 cm−1, PMMA) vibrations.

FigureFigure 7. Raman7. Raman maps maps of PLA/PMMA of PLA/PMMA and ternary and blends ternary with blends 2 wt.% contentwith 2 ofwt.% LPSQ-R content illustrating of LPSQ-R distribution illustrat- of PMMAing distribution in the blends (withof PMMA corresponding in the opticalblends micrographs (with corresponding in the background) optical and spectramicrographs of selected in regions. the background) and spectra of selectedComparative regions. analysis of the relative intensity of those bands can be indicative of the blending effectiveness. Raman maps evidenced phase separation in the blends; PMMA Comparative analysisformed inclusions, of the relative as already intensit shown byy of SEM, those although bands it may can be be also indicative partly dispersed of the in blending effectiveness.PLA. TheRaman size of maps inclusions eviden seemsced to decreasephase separation on increasing in concentration the blends; of LPSQ-OH.PMMA Better dispersion was observed for LPSQ-COOMe. PMMA was well dispersed within PLA formed inclusions, evenas already at a very shown low content by SEM, of this additivealthough (B-COOMe-10). it may be also LPSQ-F5 partly is an dispersed intermediate— in PLA. The size of inclusionsrather good seems dispersion to decrease was noted on for increasing B-F5-20. The concentration maps illustrating of distributionLPSQ-OH. of Better dispersion wasPMMA observed corroborate for the LPSQ-COOMe. structure of blends PMMA shown in was optical well micrographs dispersed (Figure within7 and Figures S1–S4) and microstructures shown in Figure6. Samples containing LPSQ-COOMe PLA even at a veryare low the content most uniform of this among addi thosetive materials. (B-COOMe-10). Unfortunately, LPSQ-F5 the intensity is an of intermedi- characteristic ate—rather good dispersionRaman vibrations was ofnoted LPSQ-R for was B-F5-2 below0. the The detection maps limit. illustrating distribution of PMMA corroborate the structure of blends shown in optical micrographs (Figures 7 and S1–S4) and microstructures shown in Figure 6. Samples containing LPSQ-COOMe are the most uniform among those materials. Unfortunately, the intensity of characteristic Raman vibrations of LPSQ-R was below the detection limit.

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The structural differences of the studied PLA/PMMA/LPSQ-R blends may be explained in terms of supramolecular interactions characteristic of R groups. Side groups of LPSQ-OH act as donors of hydrogen bonds to oxygen atoms of carbonyl groups in both PLA and PMMA. That is why it appears to be worse dispersed in the blend polymers. Methylglycolic moieties of LPSQ-COOMe are rather inert, although they can act as accep- tors/donors of weak -CH3···O=C- hydrogen bonds. LPSQ-F5 is a speciﬁc case—as it was mentioned, pentaﬂuorophenyl species may form weak complexes with carbonyl groups, which may bring about attractive interactions between LPSQ-F5 and both PLA and PMMA.

3.2. Thermal and Mechanical Properties of the Hybrid Blends The thermal properties of the blends were examined with DSC. DSC first heating thermograms of cast films collected in Figure S5 show melting endotherms with peak ◦ temperatures (Tm) at 166–167 C and melting enthalpies (∆Hm) ranging from 40 to 50 J/g. The lack of cold crystallization exotherms evidenced that crystallization in the films was accomplished during preparation. During subsequent cooling from 190 ◦C no crystallization was observed, as shown in Figure S6; thus, the cooled films were amorphous and able to crystallize during second heating. Preliminary experiments demonstrated that during the second heating at 10 ◦C min−1 only weak cold crystallization occurred, followed by melting, and thus the phase transitions in the blends was studied at a decreased heating rate of 5 ◦C min−1. The second heating thermograms of the neat polymers and the blends are collected in Figure8. The PMMA thermograms shows only glass transition with Tg ◦ ◦ of 120 C. In turn, for neat PLA, above the glass transition with Tg at 60 C, the cold ◦ crystallization exotherm was visible, with the peak temperature Tcc at 131 C and cold crystallization enthalpy (∆Hcc) of 29 J/g, corresponding to 31 wt.% crystallinity. The cold ◦ crystallization was followed by melting with Tm at 166 C and ∆Hm equal to ∆Hcc. The , , thermal behaviour of PLA/PMMA blend was similar with nearly the same Tcc Tm and ∆Hm of PLA component. Due to overlapping of cold crystallization exotherm of PLA and glass transition of PMMA, only ∆Hm allows to estimate correctly the crystallinity of PLA in the blends. It is worth noting that crystallization of PLA was restricted in the presence of PMMA in their miscible blends [20,57]. PMMA retarded also the crystallization of PLLA plasticized by PEO by reducing the chain mobility in the blends [58,59]. In the present study, the phase separated structure of the PLA/PMMA blend resulted in the lack of significant influence of PMMA on PLA crystallization in the blend. Similarly to PLA/PMMA, ther- ◦ mograms of the ternary blends exhibited the glass transition, with Tg of 59–60 C, and the cold crystallization exotherms followed by melting endotherms. The slight enhancement of cold crystallization due to the addition of LPSQ-R was observed only in the blends with ◦ 2 and 3 wt.% of LPSQ-COOMe, reflected in a decrease of Tcc to 125–126 C and an increase of ∆Hm to 31–32.5 J/g PLA, which corresponds to 33.5–35 wt.% crystallinity. The significant enhancement of the cold crystallization of PLA in the blends with LPSQ-COOMe was observed by us previously [44,45]. However, in the LPSQR-COOMe modified PLA/PMMA blends, the dispersion of the additive was much worse, and its content in PLA was smaller due to location of inclusions in PMMA rich domains. Despite the differences in crystallinity developed during cold crystallization, the melting behaviour of all the blends was similar ◦ to that of PLA, and their Tm values were practically the same, at 166–167 C. It is worth noting that the cooling rate of 10 ◦C min−1 was sufficient to cool the blends to the glassy state without crystallization. Thus, one can expect that the compression moulded films of the blends quenched between metal blocks were also amorphous. The thermal stability of the blends was studied with TGA. The prepared PLA/LPSQ-R were thermally stable, and the additives did not change the structure of PLA backbone (Figure S7). The main decomposition of PLA was noted at the temperature of peak weight ◦ ◦ −1 loss derivative Td = 354.2 C at a rate of weight loss of 9.5%·min C , and leaving char residue of 1.2 wt.% at 600 ◦C. The B-0 and blends with LPSQ-R were more thermally stable ◦ (Td = 360–365 C, irrespectively of the kind of R and the content of LPSQ-R). The rate of decomposition was the same as that of neat PLA, and the char residue was negligible. Polymers 2021, 13, 1033 12 of 19

The only exception was the char left after decomposition of B-OH blends that increased to about 2 wt.%, which corroborates earlier reported observations [44]. The hydroxyethyl Polymers 2021, 13, x FOR PEER REVIEW 12 of 19 groups may take part in thermally induced process of redistribution of ester bonds, which would result in incorporation of LPSQ-OH in PLA matrix.

FigureFigure 8. 8.DSC DSC thermograms thermograms of neat of PLA,neat PMMA,PLA, PMMA, PLA/PMMA PLA/PMMA and PLA/PMMA and PLA/PMMA blends modiﬁed blends modified withwith LPSQ-R LPSQ-R recorded recorded during during the second the second heating heating at 5 ◦C min at −5 1°C. min−1.

MechanicalThe thermal properties stability of of the the obtained blends blends was st wereudied studied with byTGA. tensile The testing. prepared Ex- PLA/LPSQ- emplary engineering stress-engineering strain dependencies of the materials examined areR were shown thermally in Figure9 ,stable, whereas and average the additives values of mechanicaldid not change parameters the structure are collected of PLA in backbone Table(Figure S3. PLAS7). exhibitedThe main the decomposition yield stress (σy) closeof PLA to 44 wa MPas noted and fractured at the temperature at the elongation of peak weight −1 (lossεb) and derivative stress (σb) T ofd 27%= 354.2 and 42°C MPa, at a respectively. rate of weight PLA/PMMA loss of blend9.5%·min exhibited °C slightly, and leaving char higherresidueσy ofof 1.2 46 MPa.wt.% Beyondat 600 °C. the The yield, B-0 the and stress blends decreased with toLPSQ-R about 28 were MPa, more and εthermallyb of stable 95% was reached, at the average σ of 28.5 MPa, indicating an improved drawability as (Td = 360–365 °C, irrespectivelyb of the kind of R and the content of LPSQ-R). The rate of compareddecomposition to neat PLA.was Thethe stress–strainsame as that dependencies of neat PLA, of alland ternary the char blends residue also showed was negligible. a The decrease of stress beyond the yield, similarly to PLA/PMMA blend. Because of that, the only exception was the char left after decomposition of B-OH blends that increased to yield strength of all blends was determined by σy. However, only B-OH-10 and B-COOMe- 10about exhibited 2 wt.%, signiﬁcantly which improvedcorroborates drawability earlier in repo comparisonrted observations to PLA/PMMA. [44]. Their The σhydroxyethyly values,groups 45 may MPa take and 48part MPa, in respectively,thermally induced were close proc to thatess ofof PLA/PMMA redistribution blend, of ester but the bonds, which averagewould εresultb of about in incorporation 160 and 190, respectively, of LPSQ-OH was in achieved PLA matrix. at σb close to 28 MPa. With increasingMechanical LPSQ-OH properties content εb of of the the blends obtained worsened blends dramatically, were studied to below by 15%, tensile despite testing. Exem- a decrease of σ to 39–40 MPa. The increase of LPSQ-COOMe content to 2 wt.% resulted plary engineeringy stress-engineering strain dependencies of the materials examined are in a decrease of εb to about 110% and to 50% at the 3 wt.% content. εb of the blends with LPSQ-F5shown in also Figure worsened 9, whereas with increasing average additive values content, of mechanical but even atparameters 1–2 wt.% content, are collected it in Ta- wasble onlyS3. PLA about exhibited 70% and dropped the yield to 11%stress at 3(σ wt.%y) close content. to 44 MPa and fractured at the elongation (εb) and stress (σb) of 27% and 42 MPa, respectively. PLA/PMMA blend exhibited slightly higher σy of 46 MPa. Beyond the yield, the stress decreased to about 28 MPa, and εb of 95% was reached, at the average σb of 28.5 MPa, indicating an improved drawability as com- pared to neat PLA. The stress–strain dependencies of all ternary blends also showed a decrease of stress beyond the yield, similarly to PLA/PMMA blend. Because of that, the yield strength of all blends was determined by σy. However, only B-OH-10 and B- COOMe-10 exhibited significantly improved drawability in comparison to PLA/PMMA. Their σy values, 45 MPa and 48 MPa, respectively, were close to that of PLA/PMMA blend, but the average εb of about 160 and 190, respectively, was achieved at σb close to 28 MPa. With increasing LPSQ-OH content εb of the blends worsened dramatically, to below 15%, despite a decrease of σy to 39–40 MPa. The increase of LPSQ-COOMe content to 2 wt.% resulted in a decrease of εb to about 110% and to 50% at the 3 wt.% content. εb of the blends

Polymers 2021, 13, x FOR PEER REVIEW 13 of 19

Polymers 2021, 13, 1033 13 of 19 with LPSQ-F5 also worsened with increasing additive content, but even at 1–2 wt.% content, it was only about 70% and dropped to 11% at 3 wt.% content.

Figure 9. 9.Engineering Engineering stress-engineering stress-engineering strain strain dependencies dependencies of PLA, of PLA/PMMA PLA, PLA/PMMA and PLA/PMMA and blendPLA/PMMA modiﬁed blend wit 1 wt.%modified of LPSQR, wit 1 recordedwt.% of duringLPSQR, drawing recorded at 5%/min.during drawing at 5%/min.

DuringDuring drawing drawing of of all all the the blends, blends, the neckingthe neck wasing observedwas observed indicating indicating the predomi- the predomi- nant deformation mechanism associated with shear bands. The neck region of PLA/PMMA nant deformation mechanism associated with shear bands. The neck region of blend did not exhibit a significant loss of transparency. PLM micrograph of this region (FigurePLA/PMMA S8) showed blend crazes did not but ratherexhibit short a significant and distorted loss by of later transparency. formation of PLM shear micrograph bands. of Wethis hypothesize region (Figure that theS8) propagationshowed crazes of crazes, but rather initiated short on specimenand distorted surfaces, by waslater termi- formation of natedshear onbands. PMMA We inclusions, hypothesize which that prevented the propagat for earlyion fracture of crazes, and enabled initiated the on formation specimen sur- offaces, shear was bands. terminated In the neck on regions PMMA of inclusions, the ternary blendswhich exhibitingprevented enhanced for early drawability, fracture and ena- alsobled short the formation and distorted of crazesshear bands. were seen, In the as shown neck regions in Figure of S9. the However, ternary theblends intense exhibiting whiteningenhanced wasdrawability, observed inalso the short neck regionsand distorte of thesed crazes blends, were indicating seen, the as occurrenceshown in ofFigure S9. cavitation.However, Cavitationthe intense in whitening PLA based blends,was observed inside the in inclusionsthe neck ofregions minor of component these blends, or indi- at the interface, was observed also by others [4,6,60]. The shear yielding of PLA matrix cating the occurrence of cavitation. Cavitation in PLA based blends, inside the inclusions triggered by the cavitation was identified as the main toughening mechanism [4,6]. Thus, cavitationof minor component related to LPSQ-R or at the inclusions interface, could was improve observed the also drawability by others of the [4,6,60]. LPSQ-R The shear modifiedyielding blendsof PLA in matrix comparison triggered to PLA/PMMA, by the cavita althoughtion was the identified increase of as LPSQ-R the main content toughening causingmechanism an increase [4,6]. Thus, of inclusion cavitation size and related number to LPSQ-R could result inclusions in a premature could fracture.improve the drawability of the LPSQ-R modified blends in comparison to PLA/PMMA, although the in- 3.3.crease Mechanical of LPSQ-R Properties content in Micro-Scale causing an and increase Surface Characteristics of inclusion size and number could result in a prematureThe extracted fracture. data allowed to evaluate mechanical properties of the blends along their surfaces and cross sections (Figures 10 and 11; Figures S10 and S11). Nanoindentation experiments3.3. Mechanical showed Properties that the in hardness Micro-Scale of PLA, and PMMA,Surface B-0Characteristics and the blends with LPSQ-OH and LPSQ-F5 is similar while the average value of H for B-COOMe decreased by about 10% irrespectivelyThe extracted of thedata amount allowed of polysilsesquioxane.to evaluate mechanical The elastic properties modulus ofE theremained blends along unchangedtheir surfaces at the and level cross characteristic sections (Figures of PLA in10 theand less 11; homogenous Figures S10 blendsand S11). B-0, Nanoindenta- B-OH andtion B-F5.experimentsE of PMMA showed is slightly that smaller, the hardness and it was of reflectedPLA, PMMA, in the B-COOMe B-0 and ofthe rather blends with uniformLPSQ-OH concentration and LPSQ-F5 of the is components.similar while the average value of H for B-COOMe decreased by about 10% irrespectively of the amount of polysilsesquioxane. The elastic modulus E remained unchanged at the level characteristic of PLA in the less homogenous blends B- 0, B-OH and B-F5. E of PMMA is slightly smaller, and it was reflected in the B-COOMe of rather uniform concentration of the components.

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Figure 10. Average mechanical properties (hardness H and elastic modulus E) in the studied samples Figure 10. Average mechanical properties (hardness H and elastic modulus E) in the studied samples(A: along(A: along the surface the surface B: cross-section B: cross-section at the edge; at the C: cross-section,edge; C: cross-section, middle area). middle area). Hardness and modulus mapping using the BASIC method allowed for their quantitative determinationsHardness and ofmodulus the studied mapping samples. using All mapsthe BASIC are prepared method based allowed on 200 for indents their quanti- tativewith spacing determinations 10 µm (single of the map studied includes samples. 200 indents All coveringmaps are the prepared 190 × 90 basedµm area). on 200 The indents withspacing spacing of the 10 indents μm (single is selected map taking includes into 200 account indents the rulescovering of minimum the 190 × indentation 90 μm area). The spacingspacing, of which the indents ensure that is selected no deformations/stresses taking into account of one the indent rules affect of minimum the neighbour indentation spacing,indent. The which minimum ensure safety that distance no deformations/s is assumed totresses be 20 times of one the indent indentation affect depth the [61neighbour]. indent.The depth The of minimum the indents safety for the distance tested samples is assume wasd aboutto be 50020 times nm, i.e., the the indentation minimum depth dis- [61]. tance between the indents should be 10 µm. Maps of nanomechanical properties (Figure 11 The depth of the indents for the tested samples was about 500 nm, i.e., the minimum dis- and ESI-10-11) were inﬂuenced by the mixture composition and correspond to the content tanceof PMMA between showed the indents in Raman should spectra be (Figure 10 μm.7 andMaps Figures of nanomechanic S2–S4). B-COOMeal properties exhibit (Figure 11uniform and ESI-10-11) distribution were of H influencedand E. by the mixture composition and correspond to the content ofThe PMMA decrease showed of elastic in Raman modulus spectra and hardness (Figures seems 7 and to beS2–S4). related B-COOMe both to the exhibit intrinsic uniform distributionproperties of of PMMA H and and E. the character of the additive (viscous liquids) at RT. Samples containing LPSQ-OH and LPSQ-F5 were not softened, but contrary to the features showed in B-0 maps, the structure of samples is similar on the surface and at the cross-section. B-OH contain areas of increased E and H, which may be the result of local physical crosslinking by hydrogen bonds donated by hydroxyethyl groups of LPSQ-OH. We have recently showed that nanomechanical properties of PLA blends containing low molecular weight cyclotetrasiloxane analogues of LPSQ-OH were distributed rather uniformly while more pronounced local variations of E and H were observed (in accordance with SEM EDS) for samples containing cyclosiloxanes with carboxylic groups [62]. The changes of surface hardness and elasticity observed for B-OH indicate that the accumulated number of functional groups grafted to the polysiloxane backbone hinder blending of the components, which results in local change of properties.

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Figure 11. MapsMaps of of nanomechanical nanomechanical properties properties (hardness (hardness HH andand elastic elastic modulus modulus EE)) in in compression compression moulded samples samples of of the the neat neat components components and and select selecteded blends blends (A: (A: surface surface area; area; C: cross-section, C: cross-section, middle area).

TheWe havedecrease also foundof elastic that modulus addition and of functionalized hardness seems LPSQ-R to be (R related = OH, both F5) to to PLA/PMMA the intrinsic propertiesblend resulted of PMMA in a slight and decrease the character of surface of th freee additive energy (viscous (γs) with liquids) respect toat theRT. neatSamples PLA containingand PLA/PMMA LPSQ-OH (B-0) and (Figure LPSQ-F5 12 ).were The not decrease softened, of γbuts does contrary not seemto the tofeatures depend showed much inon B-0 the maps, amount theof structure the additive. of samples ForB-F5, is simila ther moston the pronounced surface and effect at the may cross-section. be typically B- OHrelated contain to the areas presence of increased of ﬂuorine E and atoms. H, Thewhich explanation may be the of theresult increase of local of hydrophobicityphysical cross- linkingin B-OH by is hydrogen not intuitive. bonds The donated inhomogeneity by hydroxyethyl of surface groups mechanical of LPSQ-OH. properties We (Figure have re-11, centlyFigures showed S10 and that S11 nanomechanical) might correspond properties to a slightly of PLA higher blends surface containing roughness low molecular and thus weightan increase cyclotetrasiloxane of hydrophobicity analogues (e.g., of [ 63LPSQ-OH]), but full were clariﬁcation distributed requires rather uniformly more detailed while morestudies. pronounced The observed local increase variations of the of polarE and component H were observed may be (in related accordance to the presence with SEM of EDS)silsesquioxane for samples structures containing on the cyclosiloxanes surface. In the with case carboxylic of B-COOMe, groups the free[62]. surface The changes energy of is surfacecomparable hardness to that and of elasticity neat components observed and for PLA/PMMA,B-OH indicate which that the is anotheraccumulated indicator number of a ofvery functional good blending groups ofgrafted components. to the polysiloxane backbone hinder blending of the components, which results in local change of properties.

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We have also found that addition of functionalized LPSQ-R (R = OH, F5) to PLA/PMMA blend resulted in a slight decrease of surface free energy (γs) with respect to the neat PLA and PLA/PMMA (B-0) (Figure 12). The decrease of γs does not seem to depend much on the amount of the additive. For B-F5, the most pronounced effect may be typically related to the presence of fluorine atoms. The explanation of the increase of hydrophobicity in B-OH is not intuitive. The inhomogeneity of surface mechanical properties (Figures 11, S10 and S11) might correspond to a slightly higher surface roughness and thus an increase of hydrophobicity (e.g., [63]), but full clarification requires more detailed studies. The observed increase of the polar component may be related to the presence of Polymers 2021, 13, 1033 silsesquioxane structures on the surface. In the case of B-COOMe, the free surface energy16 of 19 is comparable to that of neat components and PLA/PMMA, which is another indicator of a very good blending of components.

FigureFigure 12.12. ChangesChanges ofof surfacesurface energyenergy measuredmeasured forfor nativenative PLA,PLA, PMMA,PMMA, theirtheir blendblend B-0B-0 andand blends withblends LPSQ-R. with LPSQ-R.

4.4. ConclusionsConclusions TheThe PLA/PMMA blends blends modified modiﬁed with with 1–3 1–3 wt.% wt.% of functionalized of functionalized linear linear pol- polysilsesquioxanesysilsesquioxanes (LPSQ-R) (LPSQ-R) were were partially partially misc miscibleible and and phase phase separated. separated. The The obtained obtained re- resultssults show show that that small small contents contents of LPSQ-R of LPSQ-R may may improve improve the thedispersion dispersion of PMMA of PMMA in PLA in PLAand modulate and modulate the properties the properties of such of suchternary ternary blends. blends. PMMA PMMA in the inblends the blends formed formed inclu- inclusions,sions, as revealed as revealed by SEM. by SEM. SEM SEMand SEM-EDS and SEM-EDS demonstrated demonstrated that LPSQR-R that LPSQR-R were wereaccu- accumulatedmulated in inclusions in inclusions but but were were also also dispersed dispersed in in blend blend polymers polymers on on a a molecular level,level, which was accompanied by a slight decrease of T of PLA rich phase. Interestingly, many which was accompanied by a slight decrease of Tgg of PLA rich phase. Interestingly, many LPSQ-RLPSQ-R containingcontaining inclusionsinclusions werewere locatedlocated insideinside PMMAPMMA richrich domains,domains, especiallyespecially inin thethe blendsblends withwith LPSQ-COOMeLPSQ-COOMe andand LPSQ-F5.LPSQ-F5. RamanRaman analysisanalysis evidencedevidenced thatthat thethe additionaddition ofof LPSQ-F5LPSQ-F5 andand LPSQ-COOMeLPSQ-COOMe resultedresulted inin thethe improvedimproved dispersiondispersion ofof PMMAPMMA inin the blends. TheThe tensiletensile testtest ofof thethe blendsblends demonstrateddemonstrated thethe improved improved drawability drawability of of PLA/PMMA PLA/PMMA blendblend in comparison to to neat neat PLA. PLA. Further Further increase increase of of the the drawability drawability was was achieved achieved for forthe the blends with the small contents of LPSQ-OH and LPSQ-COOMe, resulting in nearly blends with the small contents of LPSQ-OH and LPSQ-COOMe, resulting in nearly two- two-fold increase of elongation at break without a decrease of the yield strength. fold increase of elongation at break without a decrease of the yield strength.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: 10.3390/polym13071033/s1, Table S1: Summary of the results of SEM analysis. Figure S1: Raman Summary of the results of SEM analysis. Figure S1: Raman spectra of− 1PLA and PMMA. Table S2: spectracharacteristic of PLA Raman and PMMA. shifts (cm Table−1) S2:in PLA characteristic and LPSQ-R Raman and shifts their (cm assignments.) in PLA Figure and LPSQ-R S2: Raman and theirmaps assignments. illustrating distribution Figure S2: Raman of PMMA maps and illustrating selected distributionspectra of B-0 of and PMMA B-OH and blends. selected Figure spectra S3: ofRaman B-0 and maps B-OH illustrating blends. distribution Figure S3: Raman of PMMA maps and illustrating selected spectra distribution of B-0 ofand PMMA B-F5 blends. and selected Figure spectraS4: Raman of B-0 maps and illustrating B-F5 blends. distribution Figure S4: of Raman PMMA maps and illustratingselected spectra distribution of B-0 ofand PMMA B-COOMe and selected spectra of B-0 and B-COOMe blends. Figure S5: DSC thermograms of neat PLA, PMMA and PLA/PMMA blends modified with LPSQ-R recorded during the first heating at 10 ◦C min−1. Figure S6: DSC thermograms of neat PLA, PMMA and PLA/PMMA blends modified with 3 wt.% of LPSQ-R recorded during the first cooling at 10 ◦C min−1. Figure S7: exemplary TGA traces of B-OH ◦ and B-COOMe blends in N2 at heating rate 10 C/min. Table S3: yield stress, stress and elongation at break of PLA, PLA/PMMA and LPSQ-R modified PLA/PMMA blends. Figure S8: PLM micrograph of the neck formed in PLA/PMMA specimen during drawing taken after fracture at elongation of approx. 110% (drawing direction horizontal). Figure S9: PLM micrograph of the neck formed in B-COOMe-10 specimen during drawing taken after fracture at elongation of approx. 200% (drawing direction horizontal). Figure S10: maps of hardness (H) in compression moulded films of the neat components and the blends (A: surface area; C: cross-section, middle area). Figure S11: maps of elastic modulus (E) in compression moulded films of the neat components and the blends (A: surface area; C: cross-section, middle area). Polymers 2021, 13, 1033 17 of 19

Author Contributions: Investigation and formal analysis, A.S.H., J.B., M.N., M.S., W.K. and W.S.; supervision: A.K. and E.P.; conceptualization: A.K.; writing—original draft, review and editing, A.K. and E.P. All authors have read and agreed to the published version of the manuscript. Funding: The authors thank Polish National Centre of Sciences (NCN) for the financial support (including APC) within grant No. 2016/21/B/ST5/03070). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Additional data are available on request. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Wu, S. Chain structure, phase morphology, and toughness relationships in polymers and blends. Polym. Eng. Sci. 1990, 30, 753–761. [CrossRef] 2. Grijpma, D.W.; Pennings, A. (Co)polymers of L-lactide, 2. Mechanical properties. J. Macromol. Chem. Phys. 1994, 195, 1649–1663. [CrossRef] 3. Ito, M.; Abe, S.; Ishikawa, M. The fracture mechanism of polylactic acid resin and the improving mechanism of its toughness by addition of acrylic modifier. J. Appl. Polym. Sci. 2010, 115, 1454–1460. [CrossRef] 4. Liu, H.; Zhang, J. Research Progress in Toughening Modification of Poly(lactic acid). J. Polym. Sci. Part B: Polym. Phys. 2011, 49, 1051–1083. [CrossRef] 5. Nofar, M.; Sacligil, D.; Carreau, P.J.; Kamal, M.R.; Marie-Claude Heuzey, M.-C. Poly (lactic acid) blends: Processing, properties and applications. Int. J. Biol. Macromol. 2019, 125, 307–360. [CrossRef][PubMed] 6. Zhao, X.; Hu, H.; Wang, X.; Yu, X.; Zhou, W.; Peng, S. Super tough poly(lactic acid) blends: A comprehensive review. RSC Adv. 2020, 10, 13316–13368. [CrossRef] 7. Kowalewska, A.; Nowacka, M. Supramolecular Interactions in Hybrid Polylactide Blends—The Structures, Mechanisms and Properties. Molecules 2020, 25, 3351. [CrossRef] 8. Zhang, G.; Zhang, J.; Wang, S.; Shen, D. Miscibility and phase structure of binary blends of polylactide and poly(methyl methacrylate). J. Polym. Sci. B Polym. Phys. 2003, 41, 23–30. [CrossRef] 9. Cossement, D.; Gouttebaron, R.; Cornet, V.; Viville, P.; Hecq, M.; Lazzaroni, R. PLAPMMA blends: A study by XPS and ToF-SIMS. Appl. Surf. Sci. 2006, 252, 6636–6639. [CrossRef] 10. Samuel, C.; Raquez, J.-M.; Dubois, P. PLLA/PMMA blends: A shear-induced miscibility with tunable morphologies and properties? Polymer 2013, 54, 3931–3939. [CrossRef] 11. Eckelt, J.; Enders, S.; do Carmo Gonçalves, M.; Queiroz, D.P.; Wolf, B.A. Polydispersity effects on the phase diagram of the system chloroform/poly-L-lactic acid/polymethyl methacrylate. And morphology of PLA/PMMA films. Fluid Phase Equilibria 2000, 171, 219–232. [CrossRef] 12. Li, S.-H.; Woo, E.M. Immiscibility-miscibility phase transitions in blends of poly(L-lactide) with poly(methyl methacrylate). Polym. Int. 2008, 57, 1242–1251. [CrossRef] 13. Canetti, M.; Cacciamani, A.; Bertini, F. Miscible Blends of Polylactide and Poly(methyl methacrylate): Morphology, Structure, and Thermal Behavior. J. Polym. Sci. Part B Polym. Phys. 2014, 52, 1168–1177. [CrossRef] 14. Li, S.H.; Woo, E.M. Effects of chain configuration on UCST behavior in blends of poly(L-lactic acid) with tactic poly(methyl methacrylate)s. J. Polym. Sci. Part B Polym. Phys. 2008, 46, 2355–2369. [CrossRef] 15. Imre, B.; Renner, K.; Pukanszky, B. Interactions, structure and properties in poly(lactic acid)/thermoplastic polymer blends. Express Polym. Lett 2014, 8, 2–14. [CrossRef] 16. Hao, X.; Kaschta, J.; Liu, X.; Pan, Y.; Schubert, D.W. Entanglement network formed in miscible PLA/PMMA blends and its role in rheological and thermo-mechanical properties of the blends. Polymer 2015, 80, 38–45. [CrossRef] 17. Wu, J.-H.; Kuo, M.C.; Chen, C.-W. Physical properties and crystallization behavior of poly(lactide)/poly(methyl methacrylate)/silica composites. J. Appl. Polym. Sci. 2015, 132, 42378. [CrossRef] 18. Anakabe, J.; Zaldua Huici, A.M.; Eceiza, A.; Arbelaiz, A. Melt blending of polylactide and poly(methyl methacrylate): Thermal and mechanical properties and phase morphology characterization. J. Appl. Polym. Sci. 2015, 132, 42677. [CrossRef] 19. Teoh, E.L.; Chow, W.S. Transparency, ultraviolet transmittance, and miscibility of poly(lactic acid)/poly(methyl methacrylate) blends. J. Elastom. Plast. 2017, 50, 596–610. [CrossRef] 20. Gonzalez-Garzon, M.; Shahbikian, S.; Huneault, M.A. Properties and phase structure of melt-processed PLA/PMMA blend. J. Polym. Res. 2018, 25, 58. [CrossRef] 21. Rodriguez, E.; Shahbikian, S.; Marcos, B.; Huneault, M.A. Hydrolytic stability of polylactide and poly(methyl methacrylate) blends. J. Appl. Polym. Sci. 2018, 135, 45991. [CrossRef] 22. Mangin, R.; Vahabi, H.; Sonnier, R.; Chivas-Joly, C.; Lopez-Cuesta, J.-M.; Cochez, M. Improving the resistance to hydrothermal ageing of flame-retarded PLA by incorporating miscible PMMA. Polym. Degr. Stab. 2018, 155, 52–66. [CrossRef] Polymers 2021, 13, 1033 18 of 19

23. Boudaoud, N.; Benali, S.; Mincheva, R.; Satha, H.; Raquez, J.-M.; Dubois, P. Hydrolytic degradation of poly(L-lactic acid)/poly(methyl methacrylate) blends. Polym. Int. 2018, 67, 1393–1400. [CrossRef] 24. Choochottiros, C.; Chin, I.-J. Potential transparent PLA impact modifiers based on PMMA copolymers. Eur. Polym. J. 2013, 49, 957–966. [CrossRef] 25. Zhang, K.; Nagarajan, V.; Misra, M.; Mohanty, A.K. Supertoughened Renewable PLA Reactive Multiphase Blends System: Phase Morphology and Performance. ACS Appl. Mater. Interfaces 2014, 6, 12436–12448. [CrossRef] 26. Yuryev, Y.; Mohanty, A.K.; Misra, M. Novel super-toughened bio-based blend from polycarbonate and poly(lactic acid) for durable applications. RSC Adv. 2016, 6, 105094–105104. [CrossRef] 27. Anakabe, J.; Zaldua Huici, A.M.; Eceiza, A.; Arbelaiz, A. The effect of the addition of poly(styrene-co-glycidyl methacrylate) copolymer on the properties of polylactide/poly(methyl methacrylate) blend. J. Appl. Polym. Sci. 2016, 133, 43935. [CrossRef] 28. Petchwattana, N.; Naknaen, P.; Jakrabutr, W.; Sanetuntikul, J.; Narupai, B. Modification of poly(lactic acid) by blending with poly(methylmethacrylate-co-ethyl acrylate) for extrusion blow molding application). J. Eng. Sci. Technol. 2017, 12, 2766–2777. 29. Bouzouita, A.; Notta-Cuvier, D.; Delille, R.; Lauro, F.; Raquez, J.-M.; Dubois, P. Design of toughened PLA based material for application in structures subjected to severe loading conditions. Part 2. Quasi-static tensile tests and dynamic mechanical analysis at ambient and moderately high temperature. Polym. Test. 2017, 57, 235–244. [CrossRef] 30. Chen, Y.; Wang, W.; Yuan, D.; Xu, C.; Cao, L.; Liang, X. Bio-Based PLA/NR-PMMA/NR Ternary Thermoplastic Vulcanizates with Balanced Stiffness and Toughness: “Soft−Hard” Core−Shell Continuous Rubber Phase, In Situ Compatibilization, and Properties. ACS Sustain. Chem. Eng. 2018, 6, 6488–6496. [CrossRef] 31. Anakabe, J.; Orue, A.; Zaldua Huici, A.M.; Eceiza, A.; Arbelaiz, A. Properties of PLA/PMMA blends with high polylactide content prepared by reactive mixing in presence of poly(styrene-co-glycidyl methacrylate) copolymer. J. Appl. Polym. Sci. 2018, 135, 46825. [CrossRef] 32. Xue, B.; He, H.; Zhu, Z.; Li, J.; Huang, Z.; Wang, G.; Chen, M.; Zhan, Z. A Facile Fabrication of High Toughness Poly(lactic Acid) via Reactive Extrusion with Poly(butylene Succinate) and Ethylene-Methyl Acrylate-Glycidyl Methacrylate. Polymers 2018, 10, 1401. [CrossRef][PubMed] 33. Qu, Z.; Bu, J.; Pan, X.; Hu, X. Probing the nanomechanical properties of PLA/PC blends compatibilized with compatibilizer and nucleation agent by AFM. J. Polym. Res. 2018, 25, 138. [CrossRef] 34. Jiang, Y.; Li, Z.; Song, S.; Sun, S.; Li, Q. Highly-modified polylactide transparent blends with better heat-resistance, melt strength, toughness and stiffness balance due to the compatibilization and chain extender effects of methacrylate-co-glycidyl methacrylate copolymer. J. Appl. Polym. Sci. 2020, e50124. [CrossRef] 35. Hao, X.; Kaschta, J.; Pan, X.; Liu, X.; Schubert, D.W. Intermolecular cooperativity and entanglement network in a miscible PLA/PMMA blend in the presence of nanosilica. Polymer 2016, 82, 57–65. [CrossRef] 36. Geng, Z.; Zhen, W.; Song, Z.; Wang, X. Synthesis, characterization of layered double hydroxidepoly(methylmethacrylate) graft copolymers via activators regenerated by electron transfer for atom transfer radical polymerization and its effect on the performance of poly(lactic acid). Polym. Adv. Technol. 2018, 29, 1765–1778. [CrossRef] 37. Gonzalez-Garzon, M.; Shahbikian, S.; Huneault, M.A. Properties of mineral filled poly(lactic acid)/poly(methyl methacrylate) blend. J. Appl. Polym. Sci. 2019, 136, 46927. [CrossRef] 38. Yang, J.-H.; Feng, C.-X.; Chen, H.-M.; Zhang, N.; Huang, T.; Wang, Y. Toughening effect of poly(methyl methacrylate) on an immiscible poly(vinylidene fluoride)/polylactide blend. Polym. Int. 2016, 65, 675–682. [CrossRef] 39. Mazidi, M.M.; Berahman, R.; Edalat, A. Phase morphology, fracture toughness and failure mechanisms in supertoughened PLA/PB-g-SAN/PMMA ternary blends: A quantitative analysis of crack resistance. Polym. Test. 2018, 67, 380–391. [CrossRef] 40. Mazidi, M.M.; Edalat, A.; Berahman, R.; Hosseini, F.S. Highly-Toughened Polylactide- (PLA-) Based Ternary Blends with Significantly Enhanced Glass Transition and Melt Strength: Tailoring the Interfacial Interactions, Phase Morphology, and Performance. Macromolecules 2018, 51, 4298–4314. [CrossRef] 41. Guo, Y.; Zuo, X.; Xue, Y.; Zhou, Y.; Yang, Z.; Chuang, Y.-C.; Chang, C.-C.; Yuan, G.; Satija, S.K.; Gersappe, D.; et al. Enhancing Impact Resistance of Polymer Blends via Self-Assembled Nanoscale Interfacial Structures. Macromolecules 2018, 51, 3897–3910. [CrossRef] 42. Zuo, X.; Xue, Y.; Zhou, Y.; Yin, Y.; Li, T.-D.; Wang, L.; Chuang, Y.-C.; Chang, C.-C.; Rafailovich, M.H.; Guo, Y. The use of low cost, abundant, homopolymers for engineering degradable polymer blends: Compatibilization of poly(lactic acid)/styrenics using poly(methyl methacrylate). Polymer 2020, 186, 122010. [CrossRef] 43. Jiang, H.; Ding, Y.; Liu, J.; Alagarsamy, A.; Pan, L.; Song, D.; Zhang, K.; Li, Y. Supertough Poly(lactic acid) and Sustainable Elastomer Blends Compatibilized by PLLA-b-PMMA Block Copolymers as Effective A-b-C-Type Compatibilizers. Ind. Eng. Chem. Res. 2020, 59, 13956–13968. [CrossRef] 44. Herc, A.S.; Lewiński,P.; Ka´zmierski,S.; Bojda, J.; Kowalewska, A. Hybrid SC-polylactide/poly(silsesquioxane) blends of improved thermal stability. Thermochim. Acta 2020, 687, 178592. [CrossRef] 45. Herc, A.S.; Bojda, J.; Nowacka, M.; Lewiński,P.; Maniukiewicz, W.; Piorkowska, E.; Kowalewska, A. Crystallization, structure and properties of polylactide/ladder poly (silsesquioxane) blends. Polymer 2020, 201, 122563. [CrossRef] 46. Kowalewska, A.; Herc, A.S.; Bojda, J.; Palusiak, M.; Markiewicz, E.; Ławniczak, P.; Nowacka, M.; Sołtysiak, J.; Rózański,A.;˙ Piorkowska, E. Supramolecular interactions involving fluoroaryl groups in hybrid blends of laddder polysilsesquioxanes and polylactide. Polym. Test. 2021, 94, 107033. [CrossRef] Polymers 2021, 13, 1033 19 of 19

47. Svyntkivska, M.; Makowski, T.; Piorkowska, E.; Brzezinski, M.; Herc, A.; Kowalewska, A. Modification of Polylactide Nonwovens with Carbon Nanotubes and Ladder Poly(silsesquioxane). Molecules 2021, 26, 1353. [CrossRef] 48. Janaszewska, A.; Gradzinska, K.; Marcinkowska, M.; Klajnert-Maculewicz, B.; Stanczyk, W.A. In Vitro Studies of Polyhedral Oligo Silsesquioxanes: Evidence for Their Low Cytotoxicity. Materials 2015, 8, 6062–6070. [CrossRef] 49. Armarego, W.L.F.; Chai, C.L.L. Purification of Laboratory Chemicals, 5th ed.; Elsevier Science: Oxford, UK, 2003. 50. He, Y.; Wu, T.; Wei, J.; Fan, Z.; Li, S. Morphological investigation on melt crystallized polylactide homo- and stereocopolymers by enzymatic degradation with proteinase K. J. Polym. Sci. Part B Polym. Phys. 2008, 46, 959–970. [CrossRef] 51. Bojda, J.; Piorkowska, E. Shear-induced nonisothermal crystallization of two grades of PLA. Polym. Test. 2014, 50, 172–181. [CrossRef] 52. Fischer, E.W.; Sterzel, H.J.; Wegner, G. Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reaction. Kolloid-Zu Z-Polym. 1973, 251, 980–990. [CrossRef] 53. Oliver, W.C.; Pharr, M.G. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 1992, 7, 1564–1583. [CrossRef] 54. Li, X.; Bhushan, B. A review of nanoindentation continuous stiffness measurement technique and its applications. Mater. Charact. 2002, 48, 11–36. [CrossRef] 55. Owens, D.K.; Wendt, R.C. Estimation of the surface free energy of polymers. J. Appl. Polym. Sci. 1969, 13, 1741–1747. [CrossRef] 56. Tsui, H. Poly(lactide) stereocomplexes: Formation, structure, properties, degradation, and applications. Macromol. Biosci. 2005, 5, 569–597. [CrossRef] 57. Eguiburu, J.L.; Iruin, J.J.; Fernandez-Berridia, M.J.; San Román, J. Blends of amorphous and crystalline polylactides with poly(methyl methacrylate) and poly(methyl acrylate): A miscibility study. Polymer 1998, 39, 6891–6897. [CrossRef] 58. Auliawan, A.; Woo, E.M. Nanocomposites based on vermiculite clay and ternary blend of poly(L-lactic acid), poly(methyl methacrylate), and poly(ethylene oxide). Polym. Compos. 2011, 32, 1916–1926. [CrossRef] 59. Auliawan, A.; Woo, E.M. Crystallization kinetics and degradation of nanocomposites based on ternary blend of poly(L-lactic acid), poly(methyl methacrylate), and poly(ethylene oxide) with two different organoclays. J. Appl. Polym. Sci. 2012, 125, E444–E458. [CrossRef] 60. Kowalczyk, M.; Piorkowska, E.; Dutkiewicz, S.; Sowinski, P. Toughening of polylactide by blending with a novel random aliphatic-aromatic copolyester. Eur. Polym. J. 2014, 59, 59–68. [CrossRef] 61. Sudharshan Phani, P.; Oliver, W.C. A critical assessment of the effect of indentation spacing on themeasurement of hardness and modulus using instrumentedindentation testing. Mater. Des. 2019, 164, 107563. [CrossRef] 62. Herc, A.S.; Włodarska, M.; Nowacka, M.; Bojda, J.; Szymański, W.; Kowalewska, A. Supramolecular interactions between polylactide and model cyclosiloxanes with hydrogen bonding-capable functional groups. eXPRESS Polym. Lett. 2020, 14, 134–153. [CrossRef] 63. Yang, H.; Ji, F.; Li, Z.; Tao, S. Preparation of Hydrophobic Surface on PLA and ABS by Fused Deposition Modeling. Polymers 2020, 12, 1539. [CrossRef][PubMed]