G G Oseke
email: ggoseke@abu.edu.ng

E E Peter

A Y Atta

B Mukhtar

B El-Yakubu

B Aderemi

• B Mukhtar

• B J El-Yakubu

• B O Aderemi

Improved aromatic yield and toluene selectivity in propane aromatization over Zn-Co/ZSM-5: effect of metal composition and process conditions

Keywords: Propane conversion, Aromatic yield, Toluene selectivity, Zn-Co/ZSM-5

In this report, a catalytic enhanced-conventional process production background was employed to determine the most cost-effective and environmentally friendly techniques to improve the catalytic production of toluene and other aromatic compounds from propane aromatization. 2 wt% of zinc was co-impregnated with 1-3 wt% of cobalt on HZSM-5. Characterizations and analysis showed that catalysts are crystalline and microporous. Propane conversion was carried out at 540 °C, 1200 ml/g-h gas hourly space velocity and atmospheric pressure over Zn-Co/ZSM-5 bimetallic catalysts. Toluene selectivity in the aromatic products was greatly improved and sustained significantly together with other aromatic products. Catalytic conversion of propane and aromatic yield over Zn-Co/ZSM-5 was improved and stabilized due to metallic collaboration on HZSM-5. Aromatic yield averaged 46, 32, and 36%, respectively, for 1-3 wt% Co in Zn-Co/ ZSM-5 bimetallic catalyst. Average toluene selectivity in the aromatic products for 12 h time on stream from 60, 50 and 51% for 1-3 wt% Co loading. The threshold loading of cobalt with zinc was 2% above which the general aromatic selectivity declined. A decrease in conversion from 73 to 15% was observed for flowrate increase from 6 to 35 ml min -1 and an increase in aromatic selectivity from 80 to 87%. An increase in temperature of 500-560 °C increased catalytic performance, 32-47% for propane conver-sion, and 79-86% aromatic selectivity.

Introduction

Aromatic compounds which comprise benzene, toluene, and xylenes (m, o, and p,) and ethylbenzene serves as feedstocks for a good number of intermediates that are used for the manufacture of synthetic rubber dyes, explosives, pesticides, detergents, resins. etc. [1,2] Styrene, linear alkyl benzene, and cumene are serves as the foremost consumer of benzene. These are largely in demand, making aromatic compound production to have and still attracting attention both in academic and industrial research [3][4][5][6].

Quite a lot of conventional industrial processes for the manufacture of aromatic compounds include; Carbonization of coal, naphtha steam cracking, catalytic reforming of low octane naphtha, cyclar technology of BP-UOP, naphtha dearomatization, toluene hydro-dealkylation, and toluene disproportionation, KTI Pyroforming, Cheveron's Aromax process, Mitsubishi's Zforming process etc [7][8][START_REF] Mall | Petrochemical Process Technology[END_REF][START_REF] Frey | Reaction Kinetics[END_REF][11][12][13][START_REF] Csicsery | Shape-selective Catalyst in Zeolite[END_REF].

Aromatization is the catalytic process that converts light hydrocarbons to usable aromatic compounds. ZSM-5 among many other catalysts is known to be an exceptional averagesized pore zeolite making it fit for lower alkanes aromatization [15,16]. Cracking, oligomerization, and cyclization are major reaction steps in aromatization [17,18]. Toluene is a non-polar aromatic hydrocarbon produced mostly from petroleum which is majorly used as solvent toluene, the production of benzoic acid, chloro-toluene derivatives, trinitro-toluene (explosives), toluene sulphonic acid, toluene sulphonamide, benzaldehyde, polyurethane, toluene diisocyanate, recreational inhalant among others. Toluene is an important non-polar aromatic feedstock to petrochemical industries commercially produced alongside other aromatic compounds by carbonization of coal, steam cracking of naphtha, and catalytic reforming of low octane naphtha.

Aromatic compound production from light alkanes from previous research employed modified catalysts with impregnated mono or bimetallic as dehydrogenating agents and promoters to improve aromatic yield and selectivity as HZSM-5 has a low aromatic yield. Zn, Ga, and Pt among many metals have emerged as outstanding at dehydrogenation. The economy of procurement is of advantage with the use of Zn with no health concern but unstable at reaction condition. HZSM-5 and other metal-modified catalysts (mono and bi) have been used in aromatization in previous research. Cr was impregnated with Ga/ZSM-5 which increased the dehydrogenation step of the aromatization in which Cr inhibited dispersion which could no longer promote propane conversion and overall aromatic yield [START_REF] Xu | Propane aromatization tuned by tailoring Cr Modified Ga/ZSM-5 catalysts[END_REF]. The promotional Effect of CO 2 on propane aromatization over Zn/ZSM-5 was studied alongside the addition of second metal to eradicate carbon deposit on the catalyst thus enhancing catalyst stability [START_REF] Fan | Mechanistic Insight into the Promotional Effect of CO2 on Propane Aromatization over Zn/ ZSM[END_REF]. Zn-Ni/ZSM-5 was used for catalytic aromatization of olefin/paraffin which showed improved aromatic selectivity [START_REF] Pan | Synthesis and consequence of Zn modified ZSM-5 zeolite supported Ni catalyst for catalytic aromatization of olefin/paraffin[END_REF]. Propane was converted to liquid hydrocarbons using Pt-Zn/SiO2 + ZSM-5. Catalyst performed better at high temperatures, improved aromatic selectivity, and alteration of reaction pathways due to changes in temperature [START_REF] Chang | Catalytic process development strategies for conversion of propane to liquid hydrocarbons[END_REF]. Lanthanum/Gallium-Modified Zn/ZSM-5 Zeolite was used to investigate the isomerization/aromatization of FCC Light Gasoline which revealed that La and Ga aided the dispersion of Zn thus enhancing the isomerization/aromatization activities and prolonging the service life of the catalysts [START_REF] Song | Lanthanum/ gallium-modified Zn/ZSM-5 zeolite for efficient isomerization/ aromatization of FCC light gasoline[END_REF]. Ga and Zn/ ZSM-5 were used for the conversion of LPG to Aromatics in which Ga showed better performance according to the industrial feed [START_REF] Hajimirzaee | Modified ZSM-5 zeolite for conversion of LPG to aromatics[END_REF].

General improved selectivity had been reported but not tailored selectively towards toluene production [25][26][27][28][29]. Due to the increasing demand and use of toluene in petrochemical industries as feedstocks for benzene and other petrochemical products, there is a need to develop stable bimetallic catalysts highly selective towards toluene production owing. Through this investigation, we report efficient stable and toluene-selective catalysts having an unaltered reaction pathway for propane aromatization and overall improved aromatic selectivity.

Experimental procedures

Preparation of catalysts

Zn-Co/ZSM-5 catalyst was prepared using NH 4 -ZSM-5, hydrates of Zn(NO 3 ) 2 and (Co(NO 3 ) 2 by the wet co-impregnation method employed in our previous studies [26][27][28].

Characterization of catalysts

FTIR, FTIR-pyridine, XRD, BET, and textural properties, SEM, TEM, hydrogen-TPR, and XPS were employed in catalyst characterization as reported in our earlier studies procedures [26][27][28].

Catalytic activity test

Aromatization propane was carried out using already established procedures in our previous research on stainless-steel fixed bed reactor with propane (C 3 ) conversion and product selectivity calculated using the same equations [26][27][28].

Results and discussions

FTIR spectra in Fig. 1 show the presence of peak spectra of characteristic functional groups of Zn-Co/ZSM-5. 450 and 550 cm -1 peaks that appeared depict T-O bending and crystalline double-5 ring of ZSM-5. 1100 and 1225 cm -1 peaks match to inner and outer asymmetric stretch respectively [26][27][28][29]. Broader and wider were observed with an increase in metal loading showing metallic bond stretching. 3400 and 1700 cm -1 match the stretch and vibrational bend of the OH¯ group of absorbed water on the surface of H-ZSM-5 [29][30][31].

XRD of Zn-Co/ZSM-5 catalysts are presented in Fig. 2 which demonstrated their lattice structures. From our previous studies, there were slight decreases in the intensities of characteristic diffraction peaks as metal dosage on ZSM-5 increased because of metal deposition on zeolite pores and surface [32][33][34][35]. The surface metal modification did not affect catalysts' crystallinity as there was no collapse or alteration of structure [8,[32][33][34][35]. There was no distinct characteristic peak and mismatch in the diffractogram or new phase formations on bimetallic Zn-Co/ZSM-5 because of low metal dosage [36,37]. The observed typical peaks of HZSM-5 appeared for all modified catalysts at 2θ angle = 7.89°, 8.73°, 14.82°, 23.04°, 23.86°, and 24.26° [38][39][40][41].

Figure 3 show the pyridine-IR of Zn-Co/ZSM-5 catalysts. Three distinguishing peaks about 1450,1550 and 1510 cm -1 were detected in the catalysts though there were little shifts which characterize the amount of Lewis, Bronsted and additive Bronsted and Lewis acid sites [26,27]. Lewis acid sites improve dehydrogenation and aromatization process. There was an increase in peak 1450 cm -1 corresponding to Lewis showing the effect of metal to increase Lewis acid sites and reduced Bronsted acidity [39,42,43].

Figure 4 show the surface morphology of zeolite catalysts from parent HZSM-5 (A) to metal impregnated catalysts Zn/ZSM-5 (B) and Zn-Co/ZSM-5 respectively of the same magnifications. Figure 4. Slight surface morphological changes were observed on modified catalysts due to impregnation of heavy metal zinc and cobalt on HZSM-5 [26][27][28][29]44].

The transmission emission micrograms (TEM) are presented in Fig. 5 for parent HZSM-5, monometallic Zn/ ZSM-5 and bimetallic Zn-Co/ZSM-5 (2 wt% each for the metals) at same resolutions. Catalyst surface voids were filled with impregnated metals when related to parent protonated ZSM-5 as the surface became finer [26][27][28][29]44].

Table 1 presents the textural features of ZSM-5 catalysts. They all displayed Type-I N 2 -isotherms deprived of distinct hysteresis loops owing to their uniform microporosity despite surface metal impregnation as shown in Fig. 6. Zn on HZSM-5 had an effect surface area reduction in relation to HZSM-5, displaying reduced microporosity and mesoporosity as a result of ZnO particles deposits on the surface by pore blockages on HZSM-5 openings as reported in our previous studies [26][27][28][29]. On co-impregnation of Zn with Co on the HZSM-5 catalyst, there was a reduction in the areas of micro and mesopore on account of blockage. This could be accounted for by metal particles' penetration inside the micropores, while others were available on the external surface which led to a decrease in both micropores and external surface areas. An increase in metal loading decreased total pore volume [30,35,36,45]. The XRD peaks reduction of either from the normal single metal thereby promoting the formation of stable zinc species and good electronic intermetallic interactions [26,27,46,47]. The charge relocation moves the electrons from zinc and co-impregnated cobalt [48][49][50].

The XPS survey spectra in Fig. 8 show that the deposited metals were on the surface of ZSM-5 and the metals did not affect the entire survey spectra except for the characteristic peaks of zinc and zinc-cobalt denoting their presence on HZSM-5. XPS showed that Zn and Co both possessed the catalysts as metals were impregnated in Fig. 2 supported these surface properties [46,47].

Zinc-modified zeolite and hydrogen-ZSM-5 have the same TPR plot trend as presented in our previous research in Fig. 7 under the experimental conditions showing that Zn was not reduced by hydrogen on ZSM-5 [31]. The Peak reduction for cobalt on ZSM-5 as reported by Dao et al., 2015 is 335 °C. Zn-Co/ZSM-5 showed a little shift in hydrogen consumption at around 340, 400, and 600 °C. It was reported that metal-metal interactions cause peak shifts oxidation state of 2+. Increase in metal binding energies showed zinc-cobalt interaction which had enhanced stability of zinc. Reduction binding energy of Zn 2p 3/2 showed dispersion of ZnO clusters over zeolite surface while that of Zn 2p 1/2 was credited to presence of ZnOH + species [31,38]. Co loading (Cop 3/2 788 eV) improved the distribution of ZnO and the concentration of ZnOH + which promoted alkane aromatization and suppressed the formation of light olefins [46]. The shift in the binding energies of Zn and Co (780 and 796 eV) lower and higher energy level from their oxides on zeolite are attributed to inter-metallic bonding and electron balancing on ZSM-5 [27][28][29]. Figures 9 and10 show propane conversion and aromatic yield on ZSM-5 catalysts. Dehydrogenation as an important reaction step in aromatization occurs in; carbonium ion to carbenium ion and finally to small alkenes, higher olefins to di-olefins; cyclization of di-olefins to cyclic olefins and dehydrogenation of cyclic olefins to cyclic di-olefins and finally to aromatics. HZSM-5 catalyst had low selectivity owing high cracking due to beta-scission as reported in our previous studies [25][26][27][28][29]37]. This occurred from reaction of surface hydrogen from hydride transfer step reacting back with alkene to form light alkanes and cyclohexane. The presence of Co with Zn on HZSM-5 helped in improving propane conversion and catalytic activity as presented in Fig. 9. Propane conversion was sustained for twelve hours of time on stream through Zn-Co interaction synergy and electron mobility on zeolite. The higher activity in propane conversion after the co-impregnation of Zn and Co 1 and 3 wt% was attributed to enhanced dehydrogenation activity of Zn by Co on HZSM-5 channels and improved stability of Zn at reaction temperature. This was clearly shown in XPS and H 2 -TPR by increased binding energy and reduction temperature respectively as a result of interaction between the Zn and Co species. However, 2 wt% equal metal loading of Zn and Co had lower conversion, this was observed in our previous studies for other bimetallic catalyst.

Aromatic yield of bimetallic catalyst with varying cobalt loading co-impregnated with zinc on ZSM-5 catalysts are shown in Fig. 10. The synergy of co-impregnated metal species improved aromatic yield when compared with parent and Zn/ZSM-5 in the previous studies resulting from the recombination of surface generated hydrogen during hydride transfer process [25]. This suppressed cracking, minimized hydrogenation backward reaction and enhanced oligomerization and cyclization for twelve hours' time on stream [25][26][27][28][29][51][52][53]. Zn-Co/ZSM-5 bimetallic catalyst with Co as second metal had lower conversion compared with other bimetallic catalyst. Same observations were recorded for equal metal loading bimetallic catalysts of equal 2 wt% loading of Fe, Cu and Ni as second metal with zinc on zeolite. Performance test over ZSM-5 catalysts as earlier reported had high percentage of light gases because of surface hydrogen from hydride transfer reaction with alkene to form alkanes thereby leading to cracking and formation of cyclohexane and heptane (C 6 -C 7 ) [32,[52][53][54][55]. Zn high dehydrogenating ability was reported as there was a boost in the aromatic compound selectivity as a result of minimized cracking and side reactions. Lighter alkanes, propene and C 9+ components increased showing loss of Zn active site due to its instability at reaction temperature [26][27][28][29]. Further co-impregnation as experimented in this studies of Co with Zn helped to stabilize zinc thus improving and sustaining aromatic selectivity with toluene being the highest as compared to Zn/ZSM-5. Toluene was the most stable among the aromatic compound because of the methyl-group as electron donor to the benzene ring. Presence of cobalt aided alkylation of benzene and demethylation of xylene to aromatic product distribution. This would have resulted from the coverage active sites for dehydrogenation thus cracking set in as observed in Fig. 11. Figure 14 shows the toluene selectivity in the aromatic products for twelve hours' time on stream for 1-3 wt% cobalt co-loading with 2 wt% zinc on ZSM-5 catalyst. It was observed that 1 wt% cobalt had sustained toluene selectivity of 65% while 2 wt% of cobalt was on the increase from 35 to 70%. 3 wt% had light gases and hexanes which are not desired.

Effect of temperature and feed flow rate on propane aromatization over Zn-Co/ZSM-5

Process condition performance test were carried out for 2 wt% Zn with 2 wt% Co on ZSM-5 being the best performed catalyst with respect to aromatic selectivity and increased toluene content in the aromatic product.

Figure 15 shows the effect of temperature on catalytic performance. Aromatization is an endothermic reaction and the energy-demanding process makes temperature an important process condition. Temperature from 500 to 580 °C favour catalytic performance as it enhances dehydrogenation, oligomerization and cyclization with both increase in propane conversion and aromatic selectivity [30,48,56]. It can be seen that C 9+ cracked to lower aromatics which were clearly seen in toluene, benzene, o and m-xylene increase in Fig. 16. Demethylation and methylation processes were observed to be happening concurrently with variation increase in one aromatic compound leading to a decrease in another.

Figures 17 and18 show the effect of increase in propane flow rate on catalytic performance and product distribution respectively. Propane shorter time of contact with the surface of the catalyst reduced led to a reduction in the conversion with an increase from 6 to 35 ml min -1 ( 29). An increase in feed rate improved aromatic selectivity as the C 9+ build-up time of higher aromatics was limited [25]. Figure 18 showed the product distribution as C 1 -C 3 gases were reduced because more of lighter hydrocarbons were consumed during benzene alkylation to toluene and other xylenes. Benzene was fairly steady among the aromatic compounds while toluene selectivity was on the increase. The effect of flowrate increase was observed in the toluene increase resulting from decrease in C 9+ because of limited build-up time for higher aromatics formation, thus an overall increase in aromatic selectivity.

Conclusion

Propane was aromatized catalytically over Zn-Co/ZSM-5 bimetallic catalysts. The performance test showed that the catalyst is stable and promotes aromatic compound formation and of high selectivity toward toluene. The characterization techniques employed showed crystallinity and structure zeolite retained despite impregnation of metals. Synergy of cobalt with zinc improved and sustained catalyst activity and over all aromatic selectivity especially toluene among other aromatic compounds.

Fig. 1 Fig. 2 X

 12 Fig. 1 FTIR-spectra of Zn-Co/ ZSM-5 catalysts

Fig. 3 Fig. 4 Fig. 5

 345 Fig. 3 Pyridine-FTIR of Zn-Co/ZSM-5 catalysts

Fig. 6

 6 Fig. 6 N 2 -adsorption of Zn-Co/ ZSM-5 catalysts

Fig. 8 Fig. 9

 89 Fig. 8 XPS spectra of Zn-Co/ ZSM-5 catalyst

Fig. 10 5 Fig. 11

 10511 Fig. 10 Aromatic yield over bimetallic Zn-Co/ZSM-5 catalysts

Fig. 12

 12 Fig.12 Product distribution and aromatic selectivity over Zn-2Co/ZSM-5

Fig. 14

 14 Fig. 14 Toluene selectivity in the aromatic products over Zn-Co/ZSM-5 catalysts

Fig. 16 Fig. 17

 1617 Fig.16 Effect temperature on product distribution

Table 1

 1 Textural properties of Zn-Co/ZSM-5 catalysts

	Catalyst(s)	S BET a (m 2 /g)	S micro	c (m 2 /g)	S mesoc (m 2 /g)	V total b (cm 3 /g)	V micro c (cm 3 /g)	Pore width (nm)
	Zn-1Co/ZSM-5	358.34	324.71	34.64	0.25	0.15	2.77
	Zn-2Co/ZSM-5	359.19	324.45	34.73	0.25	0.15	2.74
	Zn-3Co/ZSM5	362.02	324.72	37.30	0.25	0.15	2.72

Acknowledgements A deep appreciation from the authors goes to the Petroleum Technology Development Fund (PTDF), Abuja, Nigeria for providing support and funding for this research.

Funding This research was funded by Petroleum Technology Development Fund, PTDF, Ministry of Petroleum Resources, Abuja-Nigeria.

Data availability All data generated or analysed during this study are included in this published article and are provided in the supplementary information files.

Author contributions All authors were involved in the manuscript proofreading but below are the unique contributions of an individual with the corresponding author, Dr. Oseke Godwin Gbenga carried out the research in the laboratory and prepared the manuscript. Professor AAY was deeply involved in catalyst synthesis and effective operations of the fixed bed reactor. Professor MB and Professor EYBJ contributed to catalyst performance tests in relation to characterization. Professor ABO contributed to catalyst characterization, understanding the reaction mechanism in relation to performance tests, and general organization of the research. EEP contributed to understanding the reaction mechanism, temperature effect on product distribution, and metal catalyst interactions.

Declarations

Conflict of interest The authors of this paper declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


E Nigito

F Diemer

S Husson

S F Ou

M H Tsai

F Rézaï-Aria

Microstructure of NiTi superelastic alloy manufactured by selective laser melting

A C T

Nitinol is used in dental tools for its superelasticity, shape memory and biocompatibility. Selective laser melting (SLM) on a powder bed is a potential process for manufacturing such as dental tools. Microstructural studies are conducted on NiTi-SLM using a parallel strip strategy and 90 • laser beam rotation. The multi-scale (nano-, microand mesoscopic) microstructure is revealed. Similar "cast" type microstructure features are found. The SLM melt is casted in a "micro-mold". Fine micro-grains with random crystallographic orientation are formed near the mold walls and served as seats for the rapid growth of columnar dendritic grains along the 〈0 0 1〉 direction with epitaxial crystallographic relationships. The growth of the columnar grains curves towards the center of the "micro-mold", coinciding with the center of the laser beam. It is proposed that SLM processing resembles "Metallic Self Micro-Mold Casting" (MSµMC) and/or "Self Micro-Molding" (S2M).

Introduction

NiTi (Nitinol) alloys exhibit superelastic behavior and shape memory capabilities [START_REF] Walia | An initial investigation of the bending and torsional properties of nitinol root canal files[END_REF]. Due to good corrosion resistance, biocompatibility and low modulus of elasticity, they are candidates for dental tooling [START_REF] Walia | An initial investigation of the bending and torsional properties of nitinol root canal files[END_REF]. However, their high elastic spring-back, stress-induced phase transformation and the formation of burrs during machining and forming make their manufacturing very difficult [START_REF] Elahinia | Manufacturing and processing of NiTi implants: A review[END_REF]. Powder bed selective laser melting (SLM) has great potential to manufacture complex parts and therefore, is a promising process to manufacture NiTi tools. The effects of main SLM parameters (power, laser scanning speed, hatching, etc.) on microstructure, mechanical properties or phase transformation are already reported [START_REF] Saedi | The influence of heat treatment on the thermomechanical response of Ni-rich NiTi alloys manufactured by selective laser melting[END_REF][START_REF] Speirs | On the transformation behavior of NiTi shape-memory alloy produced by SLM[END_REF], but limited number of papers deals with microstructure/solidification resulting from extremely rapid "heating/ melting" and "solidification/cooling" rates [START_REF] Qian | Influence of position and laser power on thermal history and microstructure of direct laser fabricated Ti-6Al-4V samples[END_REF]. Present contribution deals with NiTi SLM microstructures according to solidification perspective.

Material and experimental procedures

A Ni 50.63 Ti 49.37 powder (Plasma Atomisation TM ) with a very spherical shape, few satellites and a size distribution between 22 and 48 µm (±1µm) was purchased from AP&C Company (Canada). SEM examinations (FEI Nova Nano SEM 450 FEG) of the powder revealed a microcellular and dendritic microstructure. These features are specific to powder metallurgy roots, due to a rapid solidification.

SLM-125 HL machine with infrared laser (400 W, Nd-YAG fiber, λ = 1075 nm, d≈85 µm) and a silicone coater is used. 5 mm side cubes (supports of 5 mm high) are produced similarly to [START_REF] Elahinia | Fabrication of NiTi through additive manufacturing: A review[END_REF] with a parallel stripe strategy (laser power 250 W, scan speed 1250 mm.s -1 , layer thickness 30 µm, hatching 120 µm, laser rotation θ = 90 • and heated TA6V plate (200 • C)), Fig. 1a. The cubes make an angle α = 30 • with the coater axis (Fig. 1b).

The "as-fabricated" cubes are sectioned, polished and etched (10-60 s) with a solution of 15 mL HNO3, 5 mL HF and 35 mL HCl. EBSD investigations are conducted in a JEOL JSM-7100TTLS LV field emission SEM equipped with an Aztec Advanced Nordlys Nano EBSD detector.

Results & discussions

SEM observations (Fig. 2) along the XY (a-b-c) and XZ (d-e-f-g-h) plans show that NiTi exhibits a complex multi-scale microstructure with "square patch features" (sides≈120 µm, Fig. 2.a) due to the fabrication strategy (h = 120 µm; θ = 90 • ). Two features are observed in the XY plan. First, "circular" grains (diameters≈100 µm) within patches oriented mainly along the 〈1 1 1〉 and 〈1 0 1〉 crystallographic directions. Secondly, elongated grains (width≈5µm; length≈100 µm) oriented mainly along 〈0 0 1〉 crystallographic direction and surrounding the patches. At higher magnification, Fig. 2.b, long micro-lamellae (length≈20 µm; width≈500 nm) are visible within the patches (white color). Examinations of the inter-lamellar regions (Fig. 2.c) reveal square-like nano-cellular microstructures of about few hundred nanometers sides. The micro-lamellae can be considered as primary dendritic arms and the nano-cells as secondary dendritic arms.

Fig. 2 (d-f) shows XZ plan feature of the last solidified layer/microbath. It has a depth of 200 µm (about 7 layers of powder) and a diameter at the surface of about twice the laser spot diameter. Each layer is thus rapidly heated, melted and then rapidly cooled for about 7 times. The EBSD color images show the formation of 3 kinds of grains. Firstly, long vertical columnar grains in the center of the micro-bath/micromold, with the main axis following 〈0 0 1〉 and 〈1 0 1〉 orientations, almost parallel to the laser beam. The orientations 〈0 0 1〉 and 〈1 0 1〉 in the XZ plan coincide with the 〈0 0 1〉 direction in the XY plan. Secondly, preferentially 〈0 0 1〉 oriented grains are formed on either side of the columnar grains, represented here by alternating purple and orange grains. Like a "mirror", the grains grow on either side of the laser spot during the forward and backward movement of the laser track. Thirdly, small grains, never seen before, are solidified on the upper surface of the micro-bath. Strong convection/radiation heat exchanges at the free surface of the melt bath with the "inert gas" flow in the fabrication chamber, enhanced by the generated plasma, could form these small grains which are re-melted during the next laser exposure and are therefore only visible on the last fabricated layer.

The GROD analysis, Fig. 2 (e-f), shows very small distortions inside grains developed from the melt/mold interface, "solid wall", towards the center of the mold and bend towards the Z-axis (upper surface of the melt pool). These distortions can be seen as evidence of the formation of the aforementioned micro-lamellae. It should be noted that the microlamellae are not visible on the EBSD IPF map along the XY and XZ plans. It can be assumed that the micro-lamellae are sub-grains with very small misorientations (≈1 • ) and therefore not identifiable by conventional IPF mapping.

The cross-section along the XZ plan, Fig. 2 (g-h), shows the interface between two successive melt pools. As conventional foundry, different granular microstructures can be observed in micro-mold. Solidification essentially starts from the "melt pool walls" (1st initial region), develops over a certain distance (2nd intermediate region) and ends in the center of the mold/micro-melt pool (3rd final region).

In SLM, the previously melted layers serve as walls of the micro-mold for the melt and act as heat extractors from the liquid. Near the wall, the heat flow is locally almost perpendicular to the surface. However, due to thermally activated reactions between the liquid and the solid (wall) which is heated below the melting temperature, the dissolution and "washing" of the micro-mold wall surface disrupts the liquid/solid interface as in conventional casting [START_REF] Salem | Effect of aluminizing and oxidation on the thermal fatigue damage of hot work tool steels for high pressure die casting applications[END_REF]. As a result, the micro-mold surface becomes self-roughening. It is reported that the heat "contact resistance" between the liquid and the solid depends on the roughness of the mold surface [START_REF] Hamasaiid | A predictive model for the evolution of the thermal conductance at the casting-die interfaces in high pressure die casting[END_REF]. Fine grains or equiaxed cells (Fig. 2.h) are formed near the mold wall and constitute the "child cast" zone. They are randomly oriented in all crystallographic direction, probably due to the self-roughening of the wall surface, which randomly directs the heat extraction vectors.

Assuming that the micro-molds have a parabolic shape (2D crosssection, XZ plan), the columnar grains (Fig. 2.g) grow from micromold wall along the 〈0 0 1〉 direction. However, due to upward thermal gradients and Marangoni effects, the direction of heat flows converges progressively from the mold wall (lower temperature) towards the center and top of the micro-mold (melt free surface). Solidification follows these heat directions and generates columnar curved grains. This "necessary grain curvature" deforms the atomic planes and cells and generate internal stresses and strains.

In NiTi, as in many alloys with a body-centered cubic lattice, the grains solidify preferentially along 〈0 0 1〉 direction, which is commonly referred to as the "natural direction" of solidification. As can be seen in Fig. 2 (g-h), in the bulk of the sample, columnar grains near the center of • SLM processing is considered as "Metallic Self Micro-Mold Casting" (MSµMC) and/or "Self Micro-Molding" (S2M). • SLM NiTi exhibits complex multi-scale microstructures with patches, micro-lamellae and nano-cells. • Solidification front basically starts from "micro-mold wall" towards center of melt pool along the heat extraction directions. • EBSD reveals crystallographic texturing of grains respectively along 〈0 0 1〉 direction at the center of laser spot and along 〈1 0 1〉 and 〈1 1 1〉 directions between the wall and the center of micro-mold. • Columnar grains grow along the 〈0 0 1〉 crystallographic direction (natural direction of solidification) and preserved an epitaxial relationship with the grains at the surface of the micro-mold. the melt solidify from the previous layers. These columnar grains are separated by very fine cellular grains, which maintain the same natural direction of solidification. At the solidification front, mushy zones form and lead to local microsegregation by rejection and/or diffusion of Ni and Ti elements on a microscopic scale (micro-lamellae), as in conventional casting. Active exchange of elements can disrupt the solidification front. Wetting phenomenon at grains boundaries [START_REF] Straumal | Grain Boundary Wetting by a Second Solid Phase in the Zr-Nb Alloys[END_REF] could explain theses phenomenon of micro-segregation, but results are not enough detailed to conclude it. Observations at higher magnification (microstructure, phases), localized at grains boundaries could allow to have better understanding of solidification and wetting phenomenon on theses specifics microstructural sites.

SLM is similar to "micro-casting" in "metallic micro-mold". The micro-volume of alloy consists in several solidified layers and the last layer of spread powder, which is rapidly heated and melted for a short period of time, then rapidly cooled and solidified. In micro-casting, solidification is governed by similar physical phenomena (nucleation and "micro-grain" growth mechanisms) as in conventional die-casting. SLM processing can be considered as "Metallic Self Micro-Mold Casting" (MSµMC) and/or "Self Micro-Molding" (S2M).

Conclusions

Fig. 1 .

 1 Fig. 1. Schematic of manufacturing with "stripe" strategy and 90 • rotation between two consecutive layers (a), sample position on fabrication plate (b).

Fig. 2 .

 2 Fig. 2. SEM images of NiTi multi-scale microstructure. Progressive magnification of XY and XZ plans are respectively presented in a-b-c and d-e-f-g-h.

•

  Thermal convection and radiation play important role in solidification during SLM manufacturing.

	review & editing, Visualization. F. Diemer: Conceptualization, Re-
	sources, Writing -review & editing, Supervision. S. Husson: Concep-
	tualization, Resources. S.F. Ou: Conceptualization, Resources. M.H.
	Tsai: Conceptualization, Resources. F. Rézaï-Aria: Conceptualization,
	Resources, Data curation, Writing -review & editing, Supervision,
	Project administration, Funding acquisition.
	CRediT authorship contribution statement
	E. Nigito: Conceptualization, Investigation, Data curation, Writing -

Acknowledgments

Joint financial support from French Occitanie Region and IMT Mines Albi are acknowledged. Micro-analysis Raymond Castaing Center, Toulouse is acknowledged for EBDS investigations.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Seif Eddine Fenni

Mariangela Spigno

Wei Wang

Andrea Costanzo

Alejandro J Müller
email: alejandrojesus.muller@ehu.es

Dario Cavallo
email: dario.cavallo@unige.it

How nucleating particles migration affects the fractionated crystallization of isotactic polypropylene/polystyrene immiscible blends

We investigate the inter-phase migration of nucleating particles and their effect on the fractionated crystallization of polypropylene droplets dispersed in an immiscible polystyrene matrix by preparing melt-mixed 90/10 wt% PS/PP blends with selected nucleating agents. Different concentrations of nucleating agents (NAs): Phtalocianene Blue, NA11, Talc, and sodium benzoate, were initially incorporated either in neat PP or in neat PS. Then the different PS/PP/NAs blends were prepared. Contact angles were determined for the neat polymers and the NAs, and the results indicated that, from a thermodynamics point of view, all NAs should remain or migrate to the PS phase. According to SEM observations, a sea-island morphology was obtained that did not vary with NA addition. Hence the results should be independent of differences in the morphology.

PP droplets crystallized in neat blends in three well-differentiated crystallization peaks (i.e., fractionated crystallization). The NAs induce a concentration-dependent increase in the high-temperature crystallization peak at the expense of reducing the low-temperature peaks, as droplets become in contact with or contain the NAs. When compared at a constant concentration in one of the phases, the nucleating agents have efficiencies that decrease in the order: Pht Blue ≈NA11>Talc>Sod Benz. Surprisingly, for the three most effective NAs, it was found that adding them initially to the PS phase led to a much higher nucleation efficiency (evaluated in terms of increases in peak crystallization temperature) than adding them to PP. This indicates that these NAs, initially present in the PS phase, migrate to the interface or to the bulk of the PP phase (against thermodynamics). Sod Benz was the only exception, as it shows a higher thermodynamic affinity to the PS phase. We explain the migration of most particles from the PS phase to the phases' boundary or to the bulk of the PP droplets by considering that kinetic factors dominate the behavior, as, from a rheological perspective, the minor component (PP) always displays a lower viscosity than the matrix component (PS) under the chosen mixing conditions.

Introduction

Polymer nucleation often initiates at the interface with impurities or heterogeneities present in the bulk sample, which act as primary heterogeneous nuclei. The primary nucleation step can be largely affected if the semicrystalline polymer is mixed with a second immiscible polymer.

In particular, the phenomenon of fractionated crystallization might be found when the semicrystalline component is the minor phase in an immiscible blend [START_REF] Frensch | Fractionated crystallization in incompatible polymer blends[END_REF][START_REF] Arnal | Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix[END_REF][START_REF] Sangroniz | Fractionated crystallization in semicrystalline polymers[END_REF]. In fact, the number of the microdomains of the dispersed phase can be of the same order of magnitude or exceed that of the nucleating impurities. In such a case, some of the microdomains (commonly polymer droplets) will be free of any heterogeneity. A typical DSC cooling trace of this immiscible blend will display several crystallization exotherms attributed, in order of increasing undercooling, to very active nucleating heterogeneities, less efficient impurities, nucleation at the interface with the matrix, and homogeneous nucleation.

During the mixing process of two immiscible polymers, the

ω 12 = γ P-2 -γ P-1 γ 1-2 (1)
where γ P-i is the interfacial tension between the particle and the polymer i, and γ 1-2 is the interfacial tension between the two polymers. When ω 12 > 1the particle is located only in polymer 1, for values of ω 12 <-1 the particle is exclusively found in polymer 2, while for other values, the particle concentrates at the interface between the two polymers. It must be underlined that particle migration cannot be achieved under static conditions, but it only occurs under the action of shear forces as a result of particle-polymer droplets collisions [START_REF] Fenouillot | Uneven distribution of nanoparticles in immiscible fluids: morphology development in polymer blends[END_REF][START_REF] Elias | Migration of nanosilica particles in polymer blends[END_REF].

Besides the above-mentioned equilibrium considerations, it should be reminded that mixing is a dynamic process, and several kinetics effects can arise and dominate over the thermodynamics, depending, for instance, on the mixing procedure (whether the particle is first mixed with the most or least favourable phase), the viscosity ratio and mixing time [START_REF] Fenouillot | Uneven distribution of nanoparticles in immiscible fluids: morphology development in polymer blends[END_REF]. For example, carbon nanotubes, which, accordingly to wettability, should be selectively located in the polylactide (PLA) phase of a polylactide/polycaprolactone (PCL) blend, are found in the PCL phase because of its lower viscosity [START_REF] Wu | Selective localization of nanofillers: effect on morphology and crystallization of PLA/PCL blends[END_REF]. Similar findings are reported for carbon black particles in different blends [START_REF] Clarke | Compatibilising effect of carbon black on morphology of NR-NBR blends[END_REF][START_REF] Zhou | Morphology and electrical properties of carbon black filled LLDPE/EMA composites[END_REF]. By tuning the mixing time, Gubbels et al. could confine carbon black particles in a HDPE/PS blend in either of the polymers or at the interface between them [START_REF] Gubbels | Selective localization of carbon black in immiscible polymer blends: a useful tool to design electrical conductive composites[END_REF].

In the present work, we want to investigate the effect of migration of purposely added nucleating particles on the fractionated crystallization of the dispersed PP droplet phase in PS/PP blends. We have recently shown that heterogeneous particles can efficiently nucleate PP droplets [START_REF] Wang | Heterogeneous nucleation and self-nucleation of isotactic polypropylene microdroplets in immiscible blends: from nucleation to growth-dominated crystallization[END_REF][START_REF] Carmeli | Competing crystallization of αand β-phase induced by β-nucleating agents in microdroplets of isotactic polypropylene[END_REF]. In the previous works, the different additives were mixed directly with the semicrystalline phase, before final blending with the immiscible matrix. In this study, instead, we aim to check the possibility of impurity transfer between the two components. Several nucleating particles, chosen by their different affinity for the two blend components, are selected. The role of additive concentration in both phases on the fractionated crystallization of the droplet ensembles is evaluated via differential scanning calorimetry.

Materials and methods

An isotactic polypropylene, provided by Borealis Polyolefine GmbH (Austria), with weight-average molecular weight (M W ) and polydispersity index (M w /M n ) of 365 kg/mol and 5.4, respectively, was employed as the crystallizing minor blend component. An atactic polystyrene from Sigma-Aldrich with M W and M W /M n of 350 kg/mol and 2.1, respectively, was used as the matrix polymer.

Four different known nucleating agents (NAs) for PP were used in this study. Phthalocyanine blue pigment (Pht Blue), (sodium 2,2 ′methylene bis-(4,6-di-tert-butylphenyl) phosphate) (NA11), and talc were kindly provided by Borealis Polyolefine GmbH, in the form of fine powders. Sodium benzoate (Sod Benz) was purchased from Sigma-Aldrich. All the materials were used as received.

All the blends were prepared in a Brabender Plastograph W 50 internal mixer at 200 • C, using a rotor speed of 100 rpm for 10 min. The mixing strategy was the following: masterbatches of the two polymers containing different concentrations of the various nucleating agents were first prepared, then each of the nucleated polymers were mixed with the neat second component at a constant weight concentration (90 wt% PS and 10 wt% PP). A blend with neat PS and PP was also prepared for comparison under the same mixing conditions. Table 1 reports all the prepared compositions, in which the weight concentration of the polymers in the additivated samples represents the content of polymer masterbatches. For the sake of shortening the blends nomenclature, since all blends have the 90/10 composition, the 90/10 code will not be reported in the following.

The additive concentrations were chosen according to the capabilities for accurate weighting. The blends in which the NAs were added to PP covers the lower range of overall additive concentration in the

Table 1

Composition of the prepared blends expressed in wt%. NA refers to each of the employed nucleating agents (i.e., Pht Blue, NA11, Sod Benz, or Talc), whereas the asterisk (*) refers to the polymer in which the nucleating agent is added. nucleating heterogeneities of the droplets (minor phase) could be transferred to the matrix phase. The opposite situation, i.e., migration of heterogeneities from the matrix to the dispersed phase, is also possible, in principle. Bartczak et al. have investigated in detail the possibility of impurity transfer during melt blending of isotactic polypropylene (PP) with several different components, namely high-density polyethylene (HDPE) [START_REF] Bartczak | Spherulite nucleation in blends of isotactic polypropylene with high-density polyethylene[END_REF], low-density polyethylene (LDPE) [START_REF] Gałeski | Spherulite nucleation in polypropylene blends with low density polyethylene[END_REF], ethylene-propylene random copolymer (EP) [START_REF] Bartczak | Primary nucleation behaviour in isotactic polypropylene/ethylene-propylene random copolymer blends[END_REF] and atactic polystyrene (PS) [START_REF] Bartczak | Primary nucleation and spherulite growth rate in isotactic polypropylene-polystyrene blends[END_REF]. The nucleation density of PP was measured by quantifying the overall crystallization rate via DSC or by polarized optical microscopy. It was found that the number of PP nuclei per unit volume either decreases [START_REF] Bartczak | Spherulite nucleation in blends of isotactic polypropylene with high-density polyethylene[END_REF][START_REF] Gałeski | Spherulite nucleation in polypropylene blends with low density polyethylene[END_REF] or increases, [START_REF] Bartczak | Primary nucleation behaviour in isotactic polypropylene/ethylene-propylene random copolymer blends[END_REF][START_REF] Bartczak | Primary nucleation and spherulite growth rate in isotactic polypropylene-polystyrene blends[END_REF] depending on the specific blend system, increasing the amount of the second component in the blend and the mixing time. This result clearly indicates a transfer of nucleating heterogeneities between the two polymers. The concept was tested by purposely adding nucleating agents to either of the polymers. It was shown that an equilibrium PP nucleation density is achieved after sufficient mixing time, independently from the polymer that was hosting the nucleating agents at the beginning of the blending stage. It was then concluded that the driving force for the migration is of thermodynamic origin and can be quantified by the difference between the interfacial free energies of the impurities with respect to both blend components.

The topic of migration of (nano)particles in immiscible polymer blends has been intensively explored more recently, with the main aim of localizing the nanofiller at the interface between components to obtain compatibilization and improved properties [START_REF] Fenouillot | Uneven distribution of nanoparticles in immiscible fluids: morphology development in polymer blends[END_REF][START_REF] Taguet | Structuration, selective dispersion and compatibilizing effect of (nano)fillers in polymer blends[END_REF][START_REF] Nofar | Influence of nanoparticles and their selective localization on the structure and properties of polylactide-based blend nanocomposites[END_REF]. Examples of the studied particles include carbon black [START_REF] Gubbels | Kinetic and thermodynamic control of the selective localization of carbon black at the interface of immiscible polymer blends[END_REF], nanosilica [START_REF] Elias | Morphology and rheology of immiscible polymer blends filled with silica nanoparticles[END_REF][START_REF] Elias | Migration of nanosilica particles in polymer blends[END_REF][START_REF] Sangroniz | The outstanding ability of nanosilica to stabilize dispersions of nylon 6 droplets in a polypropylene matrix[END_REF], carbon nanotubes [START_REF] Wu | Selective localization of multiwalled carbon nanotubes in poly(ε-caprolactone)/polylactide blend[END_REF][START_REF] Wu | Selective localization of nanofillers: effect on morphology and crystallization of PLA/PCL blends[END_REF] and nanoclays [START_REF] Filippone | The role of organoclay in promoting co-continuous morphology in high-density poly(ethylene)/poly (amide) 6 blends[END_REF][START_REF] Filippone | Selective localization of organoclay and effects on the morphology and mechanical properties of LDPE/ PA11 blends with distributed and co-continuous morphology[END_REF].

At the equilibrium, the localization of the particle is dictated by thermodynamics. The possibly uneven distribution of particles among the blended polymers can be predicted knowing the surface tensions of the three components, according to the wetting parameter, ω 12 :

Results and discussion

The surface tensions of the various solid substrates, polymers, and NAs were determined by measuring the contact angles with two test liquids, water, and diiodomethane. The average values of the contact angles are reported in the Supporting Information, Table S1. From these values, the interfacial free energy of the solid substrate can be evaluated according to: [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF]

γ l (1 + cosθ) 2 ̅̅̅̅ ̅ γ d l √ = ̅̅̅̅ ̅ γ d s √ + ̅̅̅̅ γ p s √ ( ̅̅̅̅ γ p l √ ̅̅̅̅ ̅ γ d l √ ) (2)
where γ l is the surface tension of the liquid, θ is the contact angle, γ s is the surface tension of the solid substrate, and the superscripts d and p are related to the dispersive and polar component of the interfacial free energies, respectively. By using two probe liquids, a set of two equations can be written, from which the values of the two unknown variables (γ s d ,

γ s p ) are found.
The values of the interfacial free energies for the various solids are reported in Table 2. The values found for PP and PS are in excellent and good agreement with the literature, respectively. [START_REF] Elias | Morphology and rheology of immiscible polymer blends filled with silica nanoparticles[END_REF] The solid additives have larger surface energies, which increase in the order: Pht Blue <NA11<Talc<Sod Benz.

Once the polar and dispersive components of the surface tensions are known for all the blend constituents, the polymer/polymer and particle/ polymer interfacial tensions can be calculated with the use of the Owens-Wendt equation:

γ 12 = γ 1 + γ 2 -2 ̅̅̅̅̅̅̅̅ ̅ γ d 1 γ d 2 √ -2 ̅̅̅̅̅̅̅̅ γ p 1 γ p 2 √ (3)
where the subscripts 1 and 2 refer to the considered constituents (e.g., particle/polymer or polymer 1/polymer 2).

From the values of the interfacial tensions of the different blend components, the wettability parameter of the various NAs can be evaluated using equation 1. Table 3 shows the interfacial tensions of the different particles with the two polymers. The wettability parameters of the same particles are calculated using equation 1, with polymer 2 being PS and polymer 1 being PP.

The obtained values demonstrate that for all the particles, the surface tensions with respect to PS are lower than those with respect to PP. This suggests that migration of the particles towards the PS matrix during blending will be favoured. In fact, all the wettability parameters are clearly lower than -1, indicating the preferential location of the NAs in the PS phase, from the thermodynamics standpoint. In particular, the affinity for the polystyrene matrix is maximum for Sod Benz and minimum for Pht Blue, with intermediate values found for Talc and NA11 (Talc<NA11). It should be noted that the wettability parameters are calculated at room temperature since the temperature coefficient of the surface tensions of the various additives is unknown. As such, they might differ from the actual ones which are established at the mixing temperature of 200 • C. Therefore, the obtained values should be considered only as an approximation of the real ones, notwithstanding they are useful for further data interpretation and discussion.

After the initial thermodynamic characterization of the systems, the morphological aspects of the prepared blends are considered. Selected SEM micrographs of additivated blends are compared in Fig. 1. Samples containing 1 wt% of NAs added either in PP or in PS are chosen for comparison, together with the neat PS/PP blend. As expected, all blends exhibit the typical droplet-in-matrix morphology, which is characteristic of a 90/10 PS/PP composition [START_REF] Arnal | Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix[END_REF][START_REF] Wang | Heterogeneous nucleation and self-nucleation of isotactic polypropylene microdroplets in immiscible blends: from nucleation to growth-dominated crystallization[END_REF][START_REF] Carmeli | Competing crystallization of αand β-phase induced by β-nucleating agents in microdroplets of isotactic polypropylene[END_REF]. No apparent differences in the PP droplet size are found when the NA is added, either in the dispersed or in the matrix phase, for all the investigated particles. Moreover, the droplet diameter also seems unaffected by the type of added particle.

The PP droplet size distribution was quantitatively determined by an image analysis software. The results for the selected concentration of NAs, added either in PP or PS, are shown in Figs. 2a and2b, respectively. The bar plots highlight the following facts: i) the type of particle does not affect the obtained average droplet size; ii) increasing the concentration of the NAs does not meaningfully vary the PP droplet size; iii) mixing the additive in PP or PS does not significantly change the droplet size; iv) considering the relative standard deviation of the droplet size distribution, the blends containing the additives are comparable to the neat blends. It can thus be concluded that the addition of NAs practically does not influence the morphology of the blends. Therefore, the crystallization data presented in this contribution can be safely considered to be the result of the sole effect of NA incorporation and their possible blends, while those where the NAs were added in PS attain a maximum concentration of about 1 wt%. A partial overlap exists between the two series, in the range of overall concentrations 0.06-0.3 wt%. Scanning Electron Microscopy (SEM) analysis of the internal surfaces of the blends was performed. At first, the samples were cryo-fractured under liquid Nitrogen, and then they were sputter-coated with thin carbon layers (Polaron E5100). The morphology of the different NAs was also observed via SEM. A field emission instrument (Supra 40 VP model, Zeiss, Germany) was used.

The crystallization kinetics of the various blends was investigated via differential scanning calorimetry (DSC). Samples of about 3-5 mg were inserted in aluminum pans and analyzed with a DSC1 STARe System (Mettler-Toledo, Switzerland), under a constant nitrogen flow of 20 mL/ min. The DSC was calibrated with indium. All the samples were first heated to 210 ºC, kept there for 3 min to erase any previous thermomechanical treatment, then cooled to -10 ºC and subsequently heated again to 210 ºC with scan rates of 10 K/min.

The surface tension of the different components was determined by measuring the contact angles with two probe liquids (water and diiodomethane) with a contact angle meter One Attension Theta. PP and PS thin films were prepared for the determination of the contact angle by manual compression molding on a hot plate. Contact angles of solid powder substrates with a test liquid is an established method to obtain the wettability of particles [START_REF] Shang | Comparison of different methods to measure contact angles of soil colloids[END_REF][START_REF] Alghunaim | Techniques for determining contact angle and wettability of powders[END_REF]. For measurements of the various NA particles, tablets were prepared by compressing a small amount of powder in a Specac Vertex 70 press, under a pressure of 10 ton for a few minutes.

Rheological analysis was performed using an HR 10 rheometer (TA Instruments, USA). The polymers were prepared in the form of approximately 1 mm-thick plates with a compression molding press and subsequently subjected to a frequency sweep test at 200 • C, consistent with the adopted blending temperature. Specifically, the rheological measurements were carried out in a range of frequencies from 0.01 to 100 rad/s, using an oscillation amplitude of 3% (within the linear viscoelastic range). In addition to that of the blends, the morphology of the nucleating particles has been checked as well. Representative SEM images and average sizes of the various NAs are reported in Fig. S1 and Table S2 of the Supporting Information, respectively. Although some of the particles exhibit larger sizes with respect to PP droplets, we note that the actual morphology after blending could be largely different due to possible not further investigated.

The results related to Pht Blue and Talc are reported in Fig. S2 of the Supporting Information. Phtalocyanine blue pigment has a very efficient nucleating behavior, promoting the formation of a high temperature crystallization peak already at very low concentrations and independently of the initial distribution (PP vs. PS phase). Talc presents instead a nucleating efficiency intermediate between Sod Benz and NA11, it gives rise to fractionated crystallization peaks at high temperatures but of relatively low intensity. Both nucleants are effective when added to PP or PS phases, indicating the possible kinetic localization of some particles at the PS/PP interface or in the PP droplet phase.

To directly compare the nucleating efficiency of the different NAs particles, the DSC cooling curves of the blends containing the same concentration of nucleating particles dispersed firstly in either PP or PS are shown in Fig. 4.

By comparing the exotherms obtained after adding a constant weight percent of the different NAs in the dispersed PP phase (Fig. 4a), it can be deduced that Pht Blue is the most efficient nucleating agent, as the high temperature crystallization peak is the sharpest and with the largest area. The second in the rank is NA11, for which the heterogeneously nucleated PP droplets crystallize at a temperature comparable to that of Pht Blue, but with substantially smaller enthalpy. Also, a part of the droplet ensemble is still clean of any impurity, as judged by the area of the homogeneous nucleation peak at 45 • C. Besides the possible difference in intrinsic nucleating efficiency between the two particles, this suggests that a larger number of droplets is nucleated in the case of Pht Blue with respect to NA11, possibly due to an easier migration of the particles to the interface or to the PP phase, in agreement with the lower affinity with the PS matrix deduced by the wettability parameter (Table 3).

The enthalpy of crystallization of the heterogeneously nucleated droplet fraction for the systems containing Sod Benz and Talc is much lower than that of the other two nucleants indicating a mild or low nucleating efficiency (Fig. 4). The addition of Talc gives rise to multiple exotherms in the high-temperature region, possibly because of different droplet sizes or Talc particle specific surface. Considering the same concentration of nucleants added to the PS matrix leads to the same ranking of NAs efficiency. The high-temperature peak area is typically larger (and, correspondingly, the homogeneous nucleation peak smaller) with respect to the case of NA addition in the dispersed PP phase, due to the higher overall concentration of particles in the blend (see Table 1). Again it is evidenced how, for any of the nucleating agents, the particles are not exclusively confined to the PS phase, as the thermodynamics would prescribe, but interact with PP droplets (by migration at the interface or to the bulk), being able to enhance their crystallization kinetics.

In order to exclude possible differences between intrinsic nucleating efficiency of the various NAs when added to bulk or blended PP, Fig. S3 reports the crystallization curves of the various additivated PPs, together with that of neat PP. Crystallization temperature increases of around 15 • C, 16 • C, 4 • C, and 8.5 • C for Pht Blue, NA11, Sod Benz, and Talc, respectively, were measured independently of the nucleant concentration. The observed nucleation effects of the various NAs in neat PP are thus consistent with those reported for the sample blended with PS (Fig. 4).

To better understand the ability of particle's inter-phase migration, the crystallization of blends containing the same overall concentration of NAs, but incorporated first either in the PP or in the PS phases are compared in Figs. 5a-d.

The curves of Fig. 5 are grouped and coloured according to the overall concentration of NA in the blend. Given the chosen 90/10 PS/PP blend composition, a concentration of 0.07 wt% in PS and of 0.75 wt% in PP corresponds to an approximate total NA concentration of 0.07 wt%, an overall concentration on average of 0.12 wt% is obtained with 0.15 wt% in PS or 1 wt% in PP. Finally, a global concentration of about 0.26 wt% is given by adding 0.25 wt% NA in PS and 3 wt% in the dispersed exothermic peaks located at about 110, 75 and 45 • C. These events can be related, from high to low temperatures, to naturally occurring nucleating impurities in PP, PS interface, and homogeneous nucleation of the droplet population with smallest sizes, respectively [START_REF] Arnal | Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix[END_REF][START_REF] Wang | Heterogeneous nucleation and self-nucleation of isotactic polypropylene microdroplets in immiscible blends: from nucleation to growth-dominated crystallization[END_REF][START_REF] Santana | Homogeneous nucleation of the dispersed crystallisable component of immiscible polymer blends[END_REF]. This is consistent with the low concentration of PP in the blend and the small average droplet size with a relatively wide dispersion.

Considering the blends containing NA11 added to PP (Fig. 3a), it can be seen that with increasing particles concentration in the dispersed phase, a crystallization event at lower undercoolings (around 120 • C) develops and increases in intensity at the expense of the peak at 75 • C first, and more gradually of the peak at 45 • C. The high crystallization temperature peak increases its temperature slightly with increasing concentration, but its peak area increases with concentration significantly. This indicates a growing population of nucleated droplets, while the parallel decrease of the homogeneous nucleation peak (at 45 • C) suggests that also the droplets with the smallest sizes get gradually heterogeneously nucleated. Interestingly, the low undercooling peak tends to disappear for higher nucleating agent concentration. This effect might be due to agglomeration of the particles, thus decreasing their nucleating ability, or to a more efficient transfer to the thermodynamically favoured PS phase during mixing, because of the increased number of collisions with the higher particle concentration [START_REF] Elias | Migration of nanosilica particles in polymer blends[END_REF].

These results are consistent with those of Baer et al., who also studied droplets of PP in a PS matrix, but obtained via breakup of PP nanolayers and thus of smaller size. The droplets were nucleated with different soluble or solid heterogeneous nucleating agents [START_REF] Jin | Effect of a sorbitol nucleating agent on fractionated crystallization of polypropylene droplets[END_REF][START_REF] Jin | Effect of an organic dicarboxylic acid salt on fractionated crystallization of polypropylene droplets[END_REF][START_REF] Langhe | Fractionated crystallization of αand β-nucleated polypropylene droplets[END_REF]. Similarly to our case, it was seen that the low-temperature exotherm, characteristic of the homogeneous nucleation process, disappears in favor of a high-temperature one with increasing nucleant's concentration, provided that the size of the PP droplet was commensurate to the size of the added particle.

A similar situation occurs when NA11 is added to the PS matrix (see Fig. 3b). In particular, a peak at even lower undercoolings develops (around 125 • C) and grows with concentration. The almost complete disappearance of the homogeneous nucleation peak suggests an even higher nucleating efficiency in this case, compared to the addition of NA11 to the dispersed phase, possibly due to the higher overall particle concentration with this mixing procedure. Remarkably, despite the particle wettability values significantly lower than -1, which would predict the location of the nucleating particles in the PS phase, nucleation of the PP droplet can still occur when the NA is added to the PS matrix. This result shows that a certain number of nucleating NA11 particles is either located at the polymer/polymer interface or has been transferred to the PP droplets during mixing, indicating a predominant kinetic effect against the thermodynamics considerations.

The effect of adding Sod Benz on PP droplets fractionated crystallization can be evaluated from Figs. 3c and3d. It is possible to note that the nucleating effect of these particles is far less marked than that of NA11. This is evident because the crystallization peak at lower undercooling is found at lower temperatures and is by far less intense. Moreover, the droplet population that nucleate homogeneously is always present, also in samples with Sod Benz concentration of 1 wt% in the dispersed phase. Adding the nucleant to the PS phase does not significantly improve the nucleating efficiency, despite the higher overall concentration, the heterogeneous nucleation peak has a small intensity; it is broad and located at relatively low temperatures. To explain the behavior of Sod Benz, with respect to that of NA11, two factors can be considered. The first is the intrinsically lower nucleating efficiency of the former nucleant compared to the latter [START_REF] Wang | Heterogeneous nucleation and self-nucleation of isotactic polypropylene microdroplets in immiscible blends: from nucleation to growth-dominated crystallization[END_REF]. The second consideration is the higher affinity of Sod Benz to the PS phase, as demonstrated by the higher absolute value of the wettability parameter (Table 3). The thermodynamics thus implies a stronger driving force for the migration or selective localization of Sod Benz particles in the PS matrix, with respect to PP droplets. It should be mentioned that the recorded thermal behavior of the PS*(+0.07 Sod Benz)/PP blend (see Fig. 3d) is peculiar and out of the trend and the reason behind this was PP phase.

The data relating to phthalocyanine blue are reported in Fig. 5a. For all three overall particle concentrations, the position of the heterogeneous nucleation peak (i.e., crystallization temperature) and the enthalpy of crystallization are remarkably similar, notwithstanding the initial phase of additive incorporation. This result confirms the easiness of additive migration from PS to the PP (or to the interface), against interfacial tensions equilibrium but in agreement with the lowest possible affinity to the PS phase (i.e., lowest wettability absolute value). Interestingly the behavior is independent of nucleant concentration, evidencing that the migration can occur already at low particle loadings. NA11 particles, instead, display meaningful differences depending on the phase to which they are initially added (Fig. 5b). Considering the lowest overall concentration (0.07 wt%), when the nucleant is added to the PP dispersed phase, the heterogeneous nucleation peak around 120 • C is prominent, and the homogeneous nucleation peak is small. On the contrary, when NA11 is incorporated first in the PS matrix, the major fractionated crystallization peak is that of homogeneous nucleation at 45 • C, and the peak at low undercooling is weak. Similar behavior is found for an overall concentration of 0.12 wt%. The fact that, despite its relatively high intrinsic nucleation efficiency, NA11 does not give a meaningful contribution to PP droplet nucleation when added to the PS matrix, suggests its low tendency to migrate from PS to the interface or bulk PP phase. Or, equivalently, the NA11 has a preference to be located in the PS matrix, in agreement with the high negative value of the wettability parameter, ranking second among all the nucleants. An anomalous behavior is instead found for the highest overall concentration, where the nucleating effect when adding NA11 to PS is larger than when it is added to the PP dispersed phase. However, as previously discussed, the reason might be the too large initial concentration in the PP phase, which can cause particles agglomeration phenomena or more favourable migration of the nucleant to the PS phase.

Sod Benz nucleant (Fig. 5c) has a low intrinsic nucleating efficiency, as already discussed. Dispersing it first in PS or PP phase provokes differences in the fractionated crystallization behavior. In particular, a weak heterogeneous nucleation peak is only discernible when Sod Benz is initially added to the PP phase. The observation leads to the hypothesis that this nucleant locates preferentially in the PS matrix, which is supported by the highest thermodynamics affinity for that phase (see wettability parameters in Table 3). For what concerns Talc (Fig. 5d), the situation is similar to Sod Benz, although a small nucleating effect is also present when the particles are added initially to the PS matrix (see weak peaks above 100 • C). This might indicate that the possibility of migration between the phases exists, although it is less relevant than what was observed for Pht Blue (Fig. 5a). In fact, according to the wettability parameter, the affinity of Talc to PS is lower than that displayed by Sod Benz and NA11.

The crystallization temperatures of the heterogeneous nucleation crystallization peak (i.e., highest crystallization temperature peak) and the related enthalpies for the various blends are collected in Fig. 6 and Fig. S4, respectively. The data are represented by filled symbols when the nucleant is added first to the PP dispersed phase, or by empty symbols, for blends prepared by dispersing the particles initially in the PS matrix. As deduced from Table 1, the series of samples with nucleants in PP covers the low range of overall concentrations, while the other samples extend the range to about 1 wt%. The overlapping range of concentration spans from 0.07 to 0.26 wt%, as shown in the DSC curves of Fig. 5.

The data related to Pht Blue, NA11, and Talc (Figs. 6a, b andd) display a similar behavior: the crystallization temperature increases with the overall concentration of nucleant until an asymptotic value is reached. The highest value at saturation is reported for NA11, closely followed by Pht Blue, while Talc is about 4-6 • C lower. For the blends with nucleants initially incorporated in PS, the increase in crystallization temperature with increasing particles concentration must be related to a more efficient transfer to the droplets (or to the interface), as a result of the augmented number of particle-droplet collisions during mixing [START_REF] Elias | Migration of nanosilica particles in polymer blends[END_REF]. Notably, for all these NAs, the blend with the highest concentration incorporated initially in the PP phase deviates from the overall trend, showing a markedly reduced crystallization temperature. As previously suggested, the reason for this can be found in a possible aggregation of the nucleating particle, which decreases the effective surface area available for nucleation. Alternatively, an increased migration towards the thermodynamically favoured PS phase at high nucleant loading is also possible. Considering the region of overlapping concentrations between the nucleants added to PP and PS, it can be seen that the crystallization temperature is always higher when the NA particles are initially incorporated in the PS matrix, rather than in the crystallizing PP phase. This result is rather counterintuitive and indicates that the effective concentration in the droplet (or at the interface between the droplet and matrix) is higher in the former case, i.e., when the NAs are firstly dispersed in the matrix. The crystallization temperature gap between the two blending strategies is particularly low for Pht Blue and Talc, while it is larger in the case of NA11. We hypothesize that this difference is correlated to the easiness of inter-phase particle migration, being substantial for Pht Blue and Talc (the NAs with the lowest affinity with the PS phase) and lower for NA11 (larger thermodynamic affinity towards the PS matrix).

A completely different situation holds for Sod Benz particles (Fig. 6c). Here the maximum crystallization temperature value achieved is the lowest among the various NAs, in agreement with its lowest intrinsic nucleation efficiency. Moreover, the increase of crystallization 7. The two overall concentrations of 0.07 and 0.12 wt% are considered only, to avoid issues with possible particle aggregation phenomena for the higher concentration (3 wt% in PP). The crystallization temperature recorded for the nucleant initially dispersed in PS has been subtracted from the one detected when the nucleant is firstly located in PP. An average ΔT C for the two above mentioned overall concentrations is calculated.

For all the NAs, excluding Sod Benz, the ΔT C are negative, meaning that it is more efficient from the nucleation point of view to add the nucleant's particles to the PS matrix than to the crystallizing PP phase, for the same overall NA concentration. As previously mentioned, the opposite behavior of Sod Benz must be related to its higher affinity for the PS phase, which prevents a substantial migration of the additive towards the droplet-matrix interface or to the bulk of the PP droplet phase. Interestingly, the two nucleating agents displaying the lowest affinity to the PS matrix (Pht Blue and Talc), show the lowest values of ΔT C . This means that their migration towards PP, driven by the mechanical forces during mixing rather than by thermodynamics, is somewhat easier in these NAs. NA11, instead, presents a large temperature gap, although opposite with respect to the one of Sod Benz. This can be the result either of a certain migration from PP droplets to PS phase (assisted by thermodynamics) or of a better migration in the opposite direction (against thermodynamics), resulting in higher crystallization temperatures when the nucleant is in PS with respect to when it is in PP. The non-monotonic trend of ΔT C as a function of the wettability parameter underlines that the migration of additives able to affect crystallization is not a sole function of thermodynamics and interfacial tensions, but other kinetic factors must come into play. These include the shear forces acting during mixing, and are thus related, for instance, to the viscosity ratio between the phases and the particle size/shape, which affects the particles' dispersion state.

To investigate these kinetics effects, and most importantly, those that lead to the remarkable migration of the particles from the PS matrix to PP droplets or the interface, opposite to the thermodynamic prescription, a rheological analysis of the two polymers under melt blending conditions was conducted. Fig. 8 reports the viscosity of PS and PP as a function of the applied shear rate at a constant temperature of 200 • C. The shear rate (γ) representative of the mixing conditions inside the mixer chamber is indicated in the plot, and has been evaluated according to: [START_REF] Kim | The effect of PS-GMA as an in situ compatibilizer on the morphology and rheological properties of the immiscible PBT/PS blend[END_REF] Fig. 7. Average difference in crystallization temperatures (for the overall concentrations of 0.07 and 0.12 wt%) considering the nucleant initially added to the PP dispersed phase with respect to blends where it is first mixed with the PS matrix, as a function of the wettability parameter of the various NAs. rate with increasing nucleant overall concentration is only found for samples where the particles are originally added to the PP dispersed phase (with the exception of the 3 wt% concentration, as discussed above). On the other hand, for Sod Benz added to the PS matrix at first, the trend of the data is practically flat. The most striking difference between Sod Benz and the behavior of the other NAs is, however, that for this additive the crystallization temperatures obtained when it is added to the crystallizing phase are higher than those recorded when incorporating it in PS. This is exactly contrary to the finding for Pht Blue, NA11 and Talc, reported above. It is thus deduced that, as a consequence of the higher affinity of Sod Benz for the PS matrix, when the particles are added to this latter phase, no meaningful migration to the interface or to the bulk of the PP droplets takes place, resulting in a practical absence of nucleating effect.

For what concerns the associated crystallization enthalpies, reported in Fig. S4, it can be stated that a general increasing trend is found with increasing the overall NAs concentration, indicating an increase in the number of heterogeneously nucleated droplets. The highest enthalpies are found for Pht Blue and NA11, while lower values are recorded for Sod Benz and Talc. Regarding the overlapping concentration range between the two nucleant addition strategies, it can be observed that Pht Blue presents comparable values, perhaps as a consequence of the already underlined easiness of particle migration between the phases. Instead, for the rest of the nucleants, higher enthalpies are recorded when the particles are initially located in the crystallizing PP phase, in agreement with the higher thermodynamic driving force for the localization of the particles in the PS phase, but in contrast with the larger crystallization temperatures determined for the nucleants dispersed in the PS phase (Fig. 6a,b,d). It must be deduced that when these NAs are added to the PS matrix, the nucleation is more efficient (higher crystallization temperature), but a lower number of droplets is heterogeneously nucleated (lower crystallization enthalpy).

To explore the hypothesis of correlation between the crystallization temperature gap for the same overall concentrations (with incorporation of the additive either in PP or PS) and the thermodynamic driving force for NAs migration, the average value of crystallization temperature difference (ΔT C ) is plotted against the measured wettability parameter in

γ = 2πRN 60h (4)
where R is the radius of the rotor blade (which changes with position), N is the rotor speed during mixing (in rpm), and h is the gap between the mixer chamber wall and the rotor blade. The average shear rate between the maximum and minimum possible values is calculated, using the following parameters, measured directly from the mixing chamber and rotors: R max = 18.5 mm, h min = 1 mm and R min = 10.5 mm, h max = 9 mm. Notably, the viscosity of PS is always higher than that of PP, in all the investigated range of shear rates. In particular, the viscosity ratio PS/PP goes from about 8 in the Newtonian region of low shear rates to approximately 2 at the shear rate characteristic of the mixing conditions (about 30 rad/s, assuming the validity of the Cox-Merz rule [START_REF] Kim | The effect of PS-GMA as an in situ compatibilizer on the morphology and rheological properties of the immiscible PBT/PS blend[END_REF]). As such, the PP dispersed phase is the one presenting the highest fluidity during mixing, a fact that can affect the selective localization of the NA particles. It has been shown in the literature that particle distribution among the phases opposite to those predicted by thermodynamics considerations alone could be achieved when one of the phases is characterized by a lower viscosity [START_REF] Wu | Selective localization of nanofillers: effect on morphology and crystallization of PLA/PCL blends[END_REF][START_REF] Clarke | Compatibilising effect of carbon black on morphology of NR-NBR blends[END_REF][START_REF] Zhou | Morphology and electrical properties of carbon black filled LLDPE/EMA composites[END_REF]. In fact, independently from the phase in which the particles were initially incorporated, CNT and carbon black have been shown to migrate towards the interface and into the lower viscosity phase. The results of fractionated crystallization of our systems, especially those in which the nucleants are initially incorporated in PS, are thus consistent with such migration from the thermodynamically favoured PS phase towards the interface or the bulk of the dispersed PP phase, possessing a lower viscosity.

Conclusions

The effect of nucleating particle migration on fractionated crystallization of polypropylene droplets in an immiscible PS matrix was investigated by adopting a dual blending strategy. Selected PP nucleating agents were initially incorporated either in the PP crystallizing phase or in the PS matrix at different concentrations.

Contact angle measurements on the polymers and particles reveal that, from the thermodynamics point of view, all the nucleating agents should be preferentially located in the PS matrix phase, because of the lower interfacial tension with this polymer with respect to PP.

The droplet-in-matrix morphology of the blend is practically unaffected by the addition of the NAs in either of the phases, thus allowing a meaningful comparison of the sole effect of NAs incorporation and interphase migration on the fractionated crystallization of PP droplets.

The fractionated crystallization behavior of PP droplets is instead largely affected by the NAs incorporation, as judged by the emergence of a crystallization peak due to heterogeneously nucleated droplets at higher temperatures, at the expense of the homogeneously and interfacially nucleated droplets crystallization at larger undercoolings. When compared at an identical concentration in one of the phases, the nucleating agents demonstrate a different efficiency, decreasing in the order Pht Blue ≈NA11>Talc>Sod Benz.

The comparison of the crystallization curves for equivalent overall concentrations, but with NAs particles dispersed first either in PP or PS, reveals similar results for Pht Blue, while for the rest of the nucleants it is more efficient (in terms of crystallization enthalpy) to initially add the nucleant to the PP phase. Despite this, it is remarkable that a meaningful nucleating effect is recorded for the nucleant original addition to the PS matrix, implying a migration of the particles to the interface or to the bulk of the PP phase (against thermodynamics).

Although the calculated wettability parameters of the various particles are of general help for understanding the observed behavior, they do not explain all the effects, as kinetic factors affecting migration probably arise during mixing. For instance, the surprising migration of most particles from the PS phase to the phases' boundary or to the bulk of the PP droplets can be tentatively explained from a rheological perspective, as the minor component displays a lower viscosity with respect to the matrix under the chosen mixing conditions.

This work provides a systematic evaluation of the effect of nucleating particle migration on crystallization in immiscible blends and can therefore help further understanding of the fractionated crystallization phenomenon.

Fig. 1 .

 1 Fig. 1. SEM micrographs of the neat blends and blends with 1 wt% of the NAs added either in the PP or in the PS phase.

Fig. 2 .

 2 Fig. 2. Average sizes (and related standard deviations) of the PP microdomains in the 90/10 PS/PP blends at the indicated overall concentrations of additive: (a) corresponds to blends where NAs are mixed with PP first, whereas (b) refers to blends where NAs are added to the PS phase before mixing with PP. The shaded area represents the size interval (standard deviation around the average value) of the PP droplets in the neat PS/PP blend.

Fig. 4 .

 4 Fig. 4. DSC curves during cooling from the melt for the 90/10 PS/PP blends with 0.5 wt% of the different NAs added either in the PP (a) or PS (b) phase.

Fig. 5 .

 5 Fig. 5. DSC curves during cooling from the melt for the 90/10 PS/PP blends with a similar overall concentration of the different NAs: Pht Blue (a); NA11 (b); Sod Benz (c); Talc (d). The curves are grouped in pairs according to the overall concentration of particles in the blend, which corresponds approximately to 0.07, 0.12, and 0.26 wt%, from bottom to top.

Fig. 6 .

 6 Fig. 6. Crystallization temperatures of the heterogeneous nucleation crystallization peak of the various blends, as a function of the overall NA concentration for Pht Blue(a); NA11 (b); Sod Benz (c); Talc (d). The filled symbols refer to blends where the nucleant is initially added to the PP dispersed phase, the empty ones to blends where it is first mixed with the PS matrix.

Fig.

  Fig.7. The two overall concentrations of 0.07 and 0.12 wt% are considered only, to avoid issues with possible particle aggregation phenomena for the higher concentration (3 wt% in PP). The crystallization temperature recorded for the nucleant initially dispersed in PS has been subtracted from the one detected when the nucleant is firstly located in PP. An average ΔT C for the two above mentioned overall concentrations is calculated.For all the NAs, excluding Sod Benz, the ΔT C are negative, meaning that it is more efficient from the nucleation point of view to add the nucleant's particles to the PS matrix than to the crystallizing PP phase, for the same overall NA concentration. As previously mentioned, the opposite behavior of Sod Benz must be related to its higher affinity for the PS phase, which prevents a substantial migration of the additive towards the droplet-matrix interface or to the bulk of the PP droplet phase. Interestingly, the two nucleating agents displaying the lowest affinity to the PS matrix (Pht Blue and Talc), show the lowest values of ΔT C . This means that their migration towards PP, driven by the mechanical forces during mixing rather than by thermodynamics, is somewhat easier in these NAs. NA11, instead, presents a large temperature gap, although opposite with respect to the one of Sod Benz. This can be the result either of a certain migration from PP droplets to PS phase (assisted by thermodynamics) or of a better migration in the opposite direction (against thermodynamics), resulting in higher crystallization temperatures when the nucleant is in PS with respect to when it is in PP. The non-monotonic trend of ΔT C as a function of the wettability parameter underlines that the migration of additives able to affect crystallization is not a sole function of thermodynamics and interfacial tensions, but other kinetic factors must come into play. These include the shear forces acting during mixing, and are thus related, for instance, to the viscosity ratio between the phases and the particle size/shape, which affects the particles' dispersion state.To investigate these kinetics effects, and most importantly, those that lead to the remarkable migration of the particles from the PS matrix to PP droplets or the interface, opposite to the thermodynamic prescription, a rheological analysis of the two polymers under melt blending conditions was conducted. Fig.8reports the viscosity of PS and PP as a function of the applied shear rate at a constant temperature of 200 • C. The shear rate (γ) representative of the mixing conditions inside the mixer chamber is indicated in the plot, and has been evaluated according to:[START_REF] Kim | The effect of PS-GMA as an in situ compatibilizer on the morphology and rheological properties of the immiscible PBT/PS blend[END_REF]

Fig. 8 .

 8 Fig. 8. Complex viscosity as a function of angular frequency for rheological measurement carried out on PS and PP at a temperature of 200 • C. The vertical dashed line indicates the average shear rate value holding during mixing in the Brabender Plastograph.

Table 2

 2 Surface tensions of the employed polymers and nucleating agents.

	PP	30.9	0.0	30.9
	PS	36.6	1.4	35.2
	Pht Blue	46.2	6.8	39.4
	NA11	50.4	27.5	22.9
	Talc	54.2	16.7	37.5
	SodBen	71.5	44.0	27.5

Materials

γ

[mN/m] γ p [mN/m] γ d [mN/m]

Table 3

 3 Values

	ω 12

of the interfacial free energies (with respect to PP and PS) and of the wettability parameters of the used nucleating agents. Materials γ PP [mN/m] γ PS [mN/m]

Acknowledgments

AJM acknowledges funding from the Basque Government through grant IT1503-22. The authors would like to thank Prof. B. Wunderlich, whose works have been a constant source of inspiration for our research in polymer crystallization.

Data availability

Data will be made available on request.

Author statement

Seif Eddine Fenni, Mariangela Spigno, Wei Wang and Andrea Costanzo: experiments, data analysis. Seif Eddine Fenni: writing first draft. Alejandro J. Müller and Dario Cavallo: Experimental design and draft manuscript revision. Dario Cavallo: conceptualization and supervision.

Declaration of Competing Interest

The authors declare no conflict of interest.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.tca.2022.179407.