Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites

Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites

Journal Pre-proof Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites Shu-Gui Yang, Hui-Jie ...

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Journal Pre-proof Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites Shu-Gui Yang, Hui-Jie Xie, Hina Saba, Liliana Cseh, Goran Ungar PII:

S0032-3861(20)30088-4

DOI:

https://doi.org/10.1016/j.polymer.2020.122246

Reference:

JPOL 122246

To appear in:

Polymer

Received Date: 10 September 2019 Revised Date:

31 December 2019

Accepted Date: 31 January 2020

Please cite this article as: Yang S-G, Xie H-J, Saba H, Cseh L, Ungar G, Fluorescence microscopy tracking of dyes, nanoparticles and quantum dots during growth of polymer spherulites, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2020.122246. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit Author Statement

Shu-Gui Yang: Conceptualization; Data curation; Formal analysis; Investigation; Roles/Writing - original draft; Writing - review & editing.Methodology; Hui-Jie Xie:

Data curation; Formal analysis;

Hina Saba:

Data curation; Formal analysis; Investigation;

Liliana Cseh:

Conceptualization; Data curation; Investigation; Methodology;

Goran Ungar:

Conceptualization; Funding acquisition; Investigation; Methodology;

Project administration; Supervision; Roles/Writing - original draft; Writing - review & editing.

Fluorescence Microscopy Tracking of Dyes, Nanoparticles and Quantum Dots During Growth of Polymer Spherulites Shu-Gui Yanga,b,c, Hui-Jie Xied, Hina Sabad, Liliana Csehe, and Goran Ungara,c,d,* a

State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Center for Soft Matter, Xi’an Jiaotong University, Xi’an, 710049, P.R. China. b

College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, P.R. China.

c

Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K.

d

Department of Physics, Zhejiang Sci-Tech University, 310018, Hangzhou, P.R. China. e

Romanian Academy, ”Coriolan Dragulescu” Institute of Chemistry, Timisoara 300223, Romania. E-mail: [email protected]

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Fluorescence Microscopy Tracking of Dyes, Nanoparticles and Quantum Dots During Growth of Polymer Spherulites

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Shu-Gui Yanga,b,c, Hui-Jie Xied, Hina Sabad, Liliana Csehe, and Goran Ungara,c,d,*

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a

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b

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c

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d

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e

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E-mail: [email protected]

State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Center for Soft Matter, Xi’an Jiaotong University, Xi’an, 710049, P.R. China. College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, P.R. China. Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K. Department of Physics, Zhejiang Sci-Tech University, 310018, Hangzhou, P.R. China. Romanian Academy, ”Coriolan Dragulescu” Institute of Chemistry, Timisoara 300223, Romania.

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Abstract: The distribution of additives and nanoparticles in a semicrystalline polymer is largely dependent on the complex hierarchical polymer morphology and the kinetics of its development. Here we show by in-situ fluorescence microscopy (FM), coupled with polarized optical and scanning electron microscopy, that a substantial fraction of the additive (Nile Red, NR) and even NR-labelled silica nanoparticles (NP) and quantum dots (QD) was pushed ahead of the growing spherulites during melt-crystallization. For the 35 and 200 nm NPs this was unexpected because their diffusion rate based on Stokes-Einstein equation should have been 2-3 orders of magnitude slower than the rate at which spherulites were growing. Another surprising finding was that much of the initially rejected NR and some QDs subsequently re-entered the spherulites through back-diffusion, posing the question why they were then rejected in the first place. The excessive initial rejection of NR and QDs and the unexpectedly rapid migration of NPs are both explained by the additive preferentially filling and being carried along by the polymer depletion zone in the melt ahead of the growing spherulite, and the high negative pressure in these zones. The effectiveness of FM in detecting minute cracks and cavities is also demonstrated. The results also show that the most severe clustering of additives occurs where spherulites did not nucleate, a problem preventable by the addition of nucleating agent.

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Keywords: nile red, silica nanoparticles, polymer nanocomposites, cavitation, isotactic polypropylene, poly(lactic acid)

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1. Introduction

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Commercial semicrystalline polymers contain additives such as antioxidants and UV absorbers, while nanocomposites also contain nanoparticles (NPs) to improve their mechanical, electrical and other properties.1-10 NPs can also be added to a polymer to control its crystallinity, since high NP loading restricts crystallization through confinement.11 Thus e.g. to make poly(ethylene oxide) amorphous at room temperature and make it usable as polymer electrolyte, beside chemical modification,12 it can be loaded with ceramic NPs, with a loading of ca. 75 vol% suppressing crystallinity completely.11 Considering the complex hierarchical 1

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inhomogeneous morphology of semicrystalline polymers, for most purposes it is important to know how such additives are distributed. Keith and Padden already realized that melt-miscible but noncrystallizeable polymeric impurities and the low molecular weight fraction partially segregate into “inter-fibrillar” and inter-spherulitic regions.13 Runt et al.14 reached a similar conclusion, based mainly on evidence from SAXS about the extent of intercrystalline layer expansion with increasing guest content. The guest fractions in those studies were substantial, as required by the limited sensitivity of the technique. Also, it was difficult to establish whether the excess guest, unaccounted for by SAXS, resided between lamellar stacks (“interfibrillar” zones) within the spherulites, or at spherulite boundaries.

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A number of studies have been devoted to the distribution of nanoparticles in semicrystalline polymers.4,5,7,8,9 These studies build on more general experimental and theoretical background work on the problem of how colloidal particles, initially dispersed in a liquid, are being moved by a solid-liquid interface sweeping through the liquid. 15,16,17,18 Much of that background work was prompted by the technology of “freeze-casting”, i.e. creating microporous solids by nucleated ice crystallization in aquous suspension of particles. Assuming that the particles are initially uniformly spread through the liquid, and that they are excluded from the crystalline phase, for low-molecular liquids it was shown that their spatial distribution in solidified composite is governed mainly by the relationship between crystal growth rate G and the particle diffusion coefficient D. The latter is given by Stokes-Einstein diffusion law defining the viscous drag, D = kT/(fπηr), where k is Boltzmann constant, T the temperature, η the viscosity of the medium, r the particle radius and f a constant between 4 and 6.19 The viscous friction force is balanced against the “disjoining force” pushing the particles away from the advancing solid. In the general case of a moving solid-liquid interface there is a “critical velocity” Gc of the crystal front below which the particles are pushed ahead of the crystal and above which the particles are engulfed by it.15,16,17,18 In the case of polymers, according to the currently prevailing opinion20,21,22,23 the diffusion rate of NPs that are either larger or smaller than the critical entanglement mesh size (reptation tube diameter) dT of the matrix follow the Stokes-Einstein law. Some controversy exists about intermediate sized NPs whose diameter is comparable to dT.20,21,22

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A number of past works have given “post mortem” information on NPs on a local scale using different electron and scanning probe microscopies and small-angle X-ray scattering. 4,5,8,9,11,24,25 Recently a combined SAXS and transmission electron microscopy (TEM) study of poly(ethylene oxide) loaded with PMMA-grafted silica NPs showed that while in quenched nanocomposite the NPs were randomly dispersed, in blends crystallized very slowly over several days, they were confined to intercrystalline layers, thus forming 1-D arrays of silica-rich platelets.8

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In order to study the spatial distribution of smaller amounts of polymer additives and to visualize it directly on the spherulitic and multi-spherulitic scale of tens and hundreds of µm, Calvert et al.26,27,28 used UV and fluorescence microscopy (FM) to view the distribution of an UV absorber in i-polypropylene (iPP) and confirmed its preferential aggregation at spherulite boundaries. It would be of great value to apply such methods to the study of nanocomposites, using fluorescent NPs or fluorescent dye-labelled NPs in order to determine their spatial distribution and, using a real-time method, establish the pathway and kinetics by which the final distribution is achieved. To our knowledge such optical studies have not been performed. This should complement the above-cited “post-mortem” studies of nanocomposites which, in most cases, gave only local (≤1 µm) information, thus often missing the bigger picture, i.e. “not seeing the wood for the trees”.

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Here we present the results of direct in-situ visualization studies of a dye additive (nile red, NR), NR-labelled silica NPs and perovskite quantum dots (QD) in isotactic polypropylene (iPP) and poly(lactic acid) (PLA) during crystallization and subsequent thermal annealing, We show that much of the dye is initially rejected from the growing spherulites, but a substantial proportion of it is subsequently re-admitted. As to the nanoparticles, yet more surprisingly, we find that a considerable fraction of them is also being “pushed” ahead of the spherulite growth front even when it moves much faster than the calculated critical velocity Gc. While the results are a challenge to the current theories, the clear evidence, highlighted by FM, of cavitation and areas of high negative pressure developing during crystallization, seems to provide a clue, as discussed below. We also conclude that fluorescence microscopy is an undervalued technique in the study of synthetic polymers that has considerable untapped potential.

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2. Results and Discussion

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In Section 2.1. we describe the behavior of iPP and PLA containing fluorescent dye NR during isothermal spherulite growth and subsequent annealing. NR was chosen because of its hydrophobicity and thus compatibility with the polymers. Spatial distribution and redistribution of NR-grafted silica NPs and QDs dispersed in iPP and PLA during and after spherulite growth is described in Section 2.2. In addition to optical and fluorescence microscopy, scanning electron microscopy (SEM) results are shown, confirming the findings of the in-situ FM experiments. In Section 2.3. we propose a mechanism primarily responsible for the non-uniform distribution of the additives in the final semicrystalline polymer matrix. Finally in Section 2.4 the additional advantage of the FM method in visualizing fine cracks in the polymer is illustrated. The results are summarized in Section 3, and the experimental details are given in Section 4. Additional fluorescence micrographs, further details on the synthesis of NR-labelled silica NPs and on the linearization of the response of the digital camera used are given in Supporting Information (SI).

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2.1. Polymer – Nile Red blends

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Figure 1 shows micrographs of spherulites of iPP containing 0.05 wt% NR growing from the melt at 130 °C recorded alternatively by crossed polarized optical microscopy (POM) and FM. As can be seen in Figure 1d-f, darker circles grow within the initially uniformly bright melt, coincident with the birefringent α-spherulites seen in (a-c). Somewhat brighter rim can be seen in FM around the growing spherulites, indicating an increased concentration of the dye that is being “pushed” ahead of the spherulite growth front. Bright lines are left at boundaries between the spherulites upon their collision. A larger and still brighter uncrystallized region is seen in the middle of the image, which is furthest away from the initial spherulite nuclei. This area fully crystallizes only on subsequent cooling, indicating that it contains the highest concentration of impurities, NR being only one of them. As Figure 1 shows, and as already noted by Calvert,27 partial exclusion of the additive from the spherulites leaves it distributed rather non-uniformly in the solidified polymer. It is also notable that the heterogeneity is exacerbated by the non-uniform coverage by the nucleation sites, a problem that can be mitigated to some extent by the addition of well dispersed nucleating agent.

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Figure 1. (a-c) Transmission crossed polarizer and (d-f) reflection fluorescence micrographs of iPP containing 0.05 wt% NR recorded during crystallization at Tc = 130 °C. Times counted from reaching Tc are indicated.

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Figure 2. Line profiles of fluorescence intensity across two spherulites and

their boundary in a iPP-NR blend crystallized at 130 °C for 10, 30 and 60 min. As seen in Figure 1, spherulite growth is complete within the first 10 min. Insets show the FM images at t = 10 and 60 min and the line along which the profiles were measured. The recorded light levels in this and subsequent figures have been corrected for camera non-linearity to obtain ordinate values proportional to actual fluorescence photon count (for details of the correction see SI).

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Following the completion of crystallization, interestingly, it was observed that upon further isothermal annealing at the crystallization temperature Tc, the 4

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segregation of the dye into the interspherulitic regions is partially reversed. This is illustrated in Figure 2 where linear luminescence intensity scans, taken along the dashed line in the image on the left, are shown for different times indicated, counting from the moment of reaching Tc = 130°C. As can be seen, the dye concentration at the boundary is gradually decreasing on annealing, while inside the spherulites, particularly close to their centres, it is increasing. This phenomenon will be discussed further below.

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Somewhat different behaviour is observed with NR-doped PLA. Figure 3 shows FM images of crystallization at 130 °C (a1-a3). While the spherulites are again darker than the melt, and the brighter aura around the growing spherulites again indicates the build-up of dye ahead of the crystallization front, the sharp bright boundaries between impinged spherulites are absent. Instead, diffuse slightly brighter areas are left between the darker spherulite cores. With further time at Tc the differences in brightness gradually even out. These changes are clearer from the line scans through three adjacent growing spherulites in Figure 3b.

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The line profiles in Figure 3b that start flat in the melt develop dips where the spherulites nucleate. The dips widen as spherulites grow, with brightness increasing between the spherulites, until they collide, at which point the peaks in brightness collapse abruptly. Broad maxima and minima still remain, but with time they even out. Calvert et al.27 attributed the darker spherulite centres in iPP to their apparently higher crystallinity, but they did not observe the isothermal levelling as in Figures 2 and 3b. Whatever the cause of the initial difference between the core and the peripheral region of the spherulites, it is clear that there is a process of back diffusion of the dye from the periphery toward spherulite centre. Back diffusion through a newly formed spherulite also seems to occur simultaneously with the forward diffusion of the dye through the melt ahead of the spherulite front. This two-way process could account for the absence of bright inter-spherulitic boundaries in PLA.

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It should be mentioned that, while partial rejection of the dye from the growing spherulites accounts for part of the contrast between spherulites and the melt in Figures 1-3, we still need to explain the overall drop in fluorescence after full crystallization, illustrated in line scans in Figures 2 and 3b. Within the spherulites the dye is trapped in the amorphous layers, where it is considerably more concentrated than in the bulk melt. Planar aromatic molecules like NR are known to be prone to stacking, which may result in fluorescence quenching. This could be responsible for part of the loss of fluorescence in the spherulites. In order to avoid such aggregation-induced quenching and thus obtain a more quantitative measure of inhomogeneous dye distribution, in future we will look for non-planar fluorophores.29

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For reference, the spherulite growth rates as a function of temperature for iPP-NR and PLA-NR blends are plotted in Figure 8.

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Figure 3. Fluorescence images of PLA containing 0.05 wt % NR recorded during isothermal crystallization at 130 °C (a1-a3). Note the large area between spherulites in a3 delineated by a Newton interference ring indicating delamination of the matrix from the glass surface due to negative pressure; the centre of the cavity is rich in segregated NR (bright). In this and subsequent FM figures the green colour has been converted to 256 grey levels to improve visual intensity resolution. (b) Line scans along the dashed white line in a1 recorded at successive intervals between 0 and 100 minutes during crystallization at 130 °C.

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2.2. Polymer – nanoparticle blends 13 14 15 16 17 18 19

Figure 4. Left: TEM images of SiO2 NPs of 35 and 200 nm diameter, used in this work. Right: Siloxane‐modified NR molecule chemically bonded to a SiO2 NP.

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We have also monitored the distribution of NPs in nanocomposites with the two crystallizable polymers, iPP and PLA, during their spherulite growth. Three kinds of highly monodisperse NPs were used: SiO2 of 35 nm diameter, SiO2 of 200 nm diameter, and perovskite QDs of 8 nm diameter passivized with a surface layer of oleic acid. While the QDs were highly fluorescent, the SiO2 NPs had to be made visible by grafting NR molecules on them. For this purpose NR had to be synthesized that contained a siloxane head group that could then be chemically bound to the NPs (Figure 4, right). For synthetic details see Experimental section and Supporting Information. TEM images of the silica NPs are shown in Figure 4, left.

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Figure 5. Selected FM images of spherulites of iPP blend with 35 nm NR‐labelled SiO2 NPs (1.3 wt%) growing at Tc = 130°C (a1‐a3), PLA blends with 200 nm NPs (1.3 wt%) at Tc = 130°C (b1‐b3) and at Tc = 120°C (c1-c3), and PLA with 35 nm NPs (also 1.3 wt%) at Tc = 130°C (d1‐d3). All images are contrast enhanced. Small aggregates of 35 nm NPs are seen in the initial iPP and PLA melt, persisting throughout the crystallization.

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Selected FM images of iPP with 1.3 wt% of labelled 35 nm SiO2 NPs, recorded during crystallization at 130 °C, are shown in Figure 5(a1-a3). Again, the growing spherulites are darker than the surrounding melt. However, as the contrast is now significantly lower it has been digitally enhanced for clarity. A comparison between Figures 1 and 5 (a1-a3) confirms the already reported finding that NPs increase the nucleation density. 4,5,9,30 In the iPP-NP 200 nm sample (5 wt%) the contrast is reduced even further – Figure S1 in SI). This presumably reflects the fact that spherulite growth rate exceeds the diffusion rate of the larger NPs in the melt even more than in the case of the 35 nm NPs. The slow diffusion of the NP results in many of them being trapped in the spherulites close to their original location.8 However, there is no doubt that significant spatial redistribution of the NPs takes place during spherulite growth. This is shown quantitatively by the fluorescence intensity scans in Figure S1e, where the brightness, hence NP concentration, in the boundary area between spherulites increases by about 27% from the uniform level in the original melt.

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For comparison, in Figure 5 we also present FM images of growing spherulites in PLA containing 35 and 200 nm NR-labelled SiO2 NPs (b1-b3, c1-c3), (d1-d3). Compared to iPP nanocomposites, in PLA ones there is an even higher degree of rejection of SiO2 NPs from the spherulites, as indicated by the brighter melt in their immediate surrounding (Figures 5b2,c1,c2,d2). On spherulite collision, the 7

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boundaries are diffuse and only marginally brighter than the spherulite centres (Figure 5b3,c3,d3), similar to the case of neat NR-doped PLA in Figure 3. However, near the completion of primary crystallization, in both 35 and 200 nm composites there are also many highly fluorescent pockets of concentrated NPs at locations furthest away from the initial spherulite nuclei (Figure 5b3,c3,d3). This indicates that a significant fraction of NPs have been swept ahead of the growing spherulites to end up forming such clusters.

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Figure 6. Line profiles of (a) PLA-NP 200 nm blend and (b) PLA-NP 35 nm

blend crystallized at 130 °C for different times as indicated. The insets show where the line profiles were scanned.

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In Figure 6 we show time evolution of fluorescence across growing spherulites and the interposed melt in a PLA-NP 200 nm blend (a) and a PLA-NP 35 nm blend (b). As before, minima and maxima correspond to spherulite centres and interspherulitic regions, respectively. While an increase in NP concentration between the spherulites is clearly seen, it is not on the scale of the increase in neat PLA (Figure 3). However, the scans in Figure 6 do not include the bright clusters of very high fluorescence. As in neat NR-doped PLA the fluctuations in fluorescence intensity even out toward the end of the crystallization process, with the interspherulitic maxima almost disappearing. However, unlike in neat PLA-NR, the minima at spherulite centres do 8

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not fill in. It therefore remains unclear whether any NPs diffuse back into the spherulites as does the free NR dye.

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Regarding the NR-labelled SiO2 NPs, there is still a global drop in fluorescence as crystallization proceeds, but the decrease is only about half that in the polymers without NPs. To explain this we propose that fluorescence quenching may still occur as the NPs cluster within the spherulites allowing a degree of π-stacking of the relatively loosely tethered fluorophores (Figure 4). The situation is different with QDs (see below), where the above quenching mechanism does not apply.

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Figure 7a,b shows FM images of spherulite growth in PLA containing quantum dots. The QDs used were cube-shaped CsPbBr3 perovskite NPs 8 nm in size and coated with oleic acid.31 The FM images recorded during spherulite growth present a picture rather different from that for silica nanocomposites. Here a clear bright, somewhat diffuse rim is seen around all growing spherulites. On their impingement these rims remain, slightly intensified, as straight inter-spherulite boundaries. However, unlike in the case of NR, the brightness inside and outside the spherulites remains more-or-less equal. This is clearly seen in the intensity profiles in Figure 7c, e.g. the blue curve recorded at 40 min. Note the cusp-like peaks marking the growth fronts, moving toward the eventual spherulite boundary in the middle of the scans. This signifies the fact that the QD concentration increases in the melt as the spherulite front approaches and then decreases again with time as the particles left behind redisperse within the spherulite. We consider the evidence of this transiently heightened particle concentration around the growth font highly significant, and will return to it in Section 2.4. After the collision of the spherulites the boundary stays richer in QDs than the surrounding, but on further annealing the particles gradualy dissipate and the intensity cusp diminishes. Thus it can be said that limited back diffusion of QDs takes place on a local scale. This seems plausible in view of QDs’ intermediate size between that of the NR dye molecules that do diffuse back and the silica NPs that do not.

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An additional feature of QDs is that they, unlike silica NPs, seem not to have any effect on nucleation of the spherulites. Nucleation density is low, as in pure iPP, illustrated by the fact that in Figure 7a (Tc = 130°C) there is only one nucleus in the field of view.

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Figure 7. Fluorescence images of PLA blend with 8 nm quantum dots (0.5 wt%) recorded during crystallization at (a) 130°C and (b) 125°C at times indicated. (c) Intensity line profiles

of PLA-QD crystallized at 125 °C for different times as indicated, scanned along the line in (b4). Coloured arrows point at cusps marking the moving spherulite growth front. Temperature dependence of the spherulite growth rate is shown in Figure 8 for all samples. While GiPP increases exponentially with supercooling, GPLA(T) passes through its maximum in the temperature range of the current experiments. While considerably facilitating primary nucleation, as is well known,32 the nanoparticles do not seem to have an effect on crystal growth at the relatively low concentrations used in this work. This is consistent with some similar studies,30 but is at some variance with others.5,32

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Figure 8. Temperature dependence of spherulite growth rate for iPP and PLA with and without nanoparticles, as indicated. The dashed curves are there to guide the eye.

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2.3. Nanoparticle-facilitated cavitation

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Comparing the appearance of FM images of silica NP-doped polymers, such as Figure 5 (right hand column), with those of polymers without NPs (Figures 1-3), it is clear that cracks between spherulites that form near the end stage of primary crystallization, appear much more readily in the former than in the latter. They are most common in places furthest away from spherulite centres, where the negative pressure on the residual melt due to crystallization-induced contraction was the greatest. 33 , 34 In PLA-NP blends the cracks are often surrounded by Newton interference rings, particularly clear in Figure 5d3. These form around areas of detachment of the polymer from the glass substrate, caused by the highly negative pressure. The role of the NPs here is believed to be to reduce long-distance chain migration and thus hinder melt flow to the negative pressure spots. This conclusion is supported by the statistical data in Figure 9 where the fraction of cavitation area at the end of crystallization is plotted against crystallization temperature for PLA-NP 35 nm and PLA-NP 200 nm. As can be seen, the cavity area increases with lowering Tc, i.e. with decreasing chain mobility. Also the 35 nm NPs are seen to be more effective in inducing crack than the 200 nm NPs at the same weight fraction. However, raising the temperature makes that distinction less pronounced. The fact that no Newton rings, hence no surface detachment, is seen in NP-doped iPP is consistent with the much higher chain mobility in iPP compared to that in PLA in the studied temperature range. Suspension of larger particles in polymeric liquids is known to produce an increase in viscosity, 35 even when clay–polymer nanocomposites are involved. Increased viscosity of a liquid by suspended particles is at least qualitatively consistent with Einsteins expression for viscosity η = η0(1 + 2.5φ) where φ is the volume fraction of the particles. 36 The two observations, (a) that NPs at concentrations used here have no effect on growth rate and yet (b) strongly affect cavitation, are not contradictory. While cavitation is affected by large scale movement of polymer molecules involving the (long) critical relaxation time τ0, crystallization can proceed with hardly any change to the radius of gyration.37

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We note that an alternative explanation has been proposed for NPs role in inducing cracks, i.e. that the NPs interpose themselves between different areas of polymer matrix or between the matrix and the substrate and thus reduce the surface energy.38 We accept that there is merit in that argument, as is indeed indicated by 11

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our SEM observations described below. However the large increase in cavity area with the modest decrease in Tc from 130 to 120°C seen in Figure 10 suggests that the reduction in polymer self-diffusion rate plays a more important role in increasing cavitation than does the reduction in surface energy. At 120°C PLA crystal growth rate G(T) is just below its maximum Gmax (see Figure 8), meaning that viscosity had increased substantially on cooling from 130 to 120°C, while surface enrgy is unlikely to have changed much. Thus the addition of NPs, especially the 35 nm ones, has a similar effect as reducing Tc toward glass transition, i.e. to reduce chain mobility and increase the negative pressure.

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Figure 9. Relative area of cracks and Newton rings vs. crystallization

temperature. The insets are selected FM images as marked by a, b, c.

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Interestingly, we did not observe cavitation in PLA doped with QDs (e.g. Figure 7). One may argue that this is due to their lower concentration than that of silica NPs. But we see cavitation even in pure PLA (Figure 3a3). Here we point to several studies that showed that smaller NPs which are of the order of the entanglement mesh size dT , i.e. are ≤10 nm in diameter, actually decrease rather than increase the viscosity.39,40 Since our QDs fall in that size category, the lack of cavitation in the PLA-QD blend may be explained by this intriguing viscosity-reducing properties of small NPs.

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We also investigated the PLA-NP composites by SEM of freeze-fracture surfaces. Figure 10a shows a spherulite that had been partly pulled out from the bulk in a 35 nm NP-doped PLA sample whose isothermal crystallization at 120 °C was interrupted by quenching. The blown-up area in Figure 10b shows the spherulite surface covered with aggregated NPs, proving that a proportion of them had indeed been “pushed” ahead of the spherulite front, consistent with FM images. Their aggregation appears to have occurred during spherulite growth. The weakening of the matrix by NPs in close proximity to each other is likely to be helped by the formation of the entropically induced polymer depletion layer between such particles.41,42

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Figure 10c shows a crack between two spherulites having the crack surfaces lined with a uniform layer of individual 35 nm NPs. These images illustrate graphically the deleterious effect of NPs aggregating at spherulite boundaries on mechanical integrity of the nanocomposite.

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Figure 10. SEM images of a fracture surface of a 35 nm NP loaded sample of PLA crystallized at 120 °C for 40 minutes, then quenched at room temperature. (a) A spherulite partially pulled out from the bulk. (b) Higher magnification detail of the spherulite surface covered with aggregated NPs. (c) A crack between two spherulites decorated with individual NPs.

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2.4. Mechanism of spatial redistribution of the dye and nanoparticles by growing spherulites

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The results of this study show quite conclusively that to a certain degree both the dye molecules and the NPs migrate ahead of the growing spherulites. To summerize the evidence: the bright rim around growing spherulites (Figures 1, 2, 3, 5 [middle column], and Figures 6 and 7), the NPs densely covering the surface of the spherulite in Figure 10a,b, the NPs seen to line the crack surface in Figure 10c, and the bright patches at the 3- or 4-spherulite meeting points in Figures 5b3,c3,d3 and 7b4. The second intriguing observation is the return of the dye into the spherulites after it had been expelled during spherulite growth (Figures 2 and 3b); this “back diffusion” is to a certain degree also occurring in the blend of PLA with QD (Figure 7), where it takes place simultaneously with spherulite growth.

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According to the currently prevailing view, the 35 nm and 200 nm silica NPs used in this work fall in the “large” NP category, since they are significantly larger than the entanglement mesh size of PLA, estimated as dT ≈ 5 nm. The estimation is done as follows. According to rubber elasticity theory, a relationship between the characteristic value of the plateau modulus, GN0, and the entanglement molecular weight, Me, is given by:43

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Me =

ρ RT GN0

where ρ is the density and R the gas constant. According to Dorgan et al44 GN0 is near 1.0 MPa for PLA, and ρ = 1.152 g/cm3 at 140 °C. Hence, Me of PLA is ca. 4000 g/mol. In estimating dT of PLA melt we used the ratio of the mean square unperturbed chain dimensions (end-to-end distance) to the molecular weight 0/Me = 0.681 Å2mol/g.45 Thus dT = 0½ = (0.681 Me)½ = 5.2 nm. We can therefore say that the size of our QDs is comparable to that of the entanglement mesh in PLA melt, while the silica NPs of 35 and 200 nm diameter are significantly larger. Hence, if not the QDs, at least the silica NPs are expected to obey the Stokes-Einstein diffusion law. To check whether they do, the critical crystal growth rate of PLA was calculated by using8,23

Gc =

kT 6πη rd

where d is the crystal lattice spacing. Here we took for η the zero shear viscosity for PLA (4032D) of η0 = 1.35 × 105 Pa•s at 130 °C.46 For d we take the value a/2 where a = 10.7 Å is the crystal lattice parameter along the assumed growth direction 13

1 2 3 4 5 6 7 8 9 10

for the α-form of PLA.47 Combining the above information, at 130 °C the critical crystal growth rates Gc of PLA with the loading of 8, 35, and 200 nm NPs are calculated as 1.0 × 10−3 µm/s, 2.3 × 10−4 µm/s, and 4.1 × 10−5 µm/s, respectively. These values should be compared with the measured spherulite growth rate of NP-doped PLA at 130°C of (4 ± 1) × 10-2 µm/s (Figure 8). Thus for our 8, 35, and 200 nm NP composites the spherulites were growing 40, 170 and 1,000 times faster than the calculated Gc. On that basis the NPs would be expected to be engulfed by the spherulites. Yet, as we have seen, a sizeable portion of them have been pushed ahead of the growth front, ending up at spherulite boundaries or in discrete pockets at the meeting points of several spherulites.

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

There have been suggestions that under certain circumstances NPs could move at rates different than predicted by the SE equation. E.g. Rubinstein et al.48 suggested a hopping motion of NPs through the entanglement mesh, although this would not apply to larger particles. Yamamoto and Schweizer49 predicted positive deviations from purely hydrodynamic SE-type NP mobility as a consequence of the non-diffusive nature of collective density fluctuation relaxation, but again, this affects NPs of size ≤ dT, the reptation tube diameter. Experimentally, Tuteja, Mackay et al.50 measured the diffusion rate of CdSe QDs of core diameter 2.2 nm in polystyrene melt by X-ray photocorrelation spectroscopy and found it to be 200 times faster than predicted by the continuum SE theory. Similarly, by following 3rd harmonic luminescence from even smaller (1-2 nm) gold NPs in poly(hexyl methacrylate), Grabowski et al.51 found the diffusion coefficient in entangled melt to be 250 times faster then predicted by the SE relation. These results are at least qualitatively consistent with the large fall in the NP drag coefficient, predicted by Brochard-Wyart and deGennes20 when the particle diameter drops below or dT.

26 27 28 29 30 31 32 33 34

We can thus say that the observed behavior of our PDA-QD blends is in qualitative agreement with these latter studies. Even though the spherulite growth front travels 40 times faster than Gc calculated on the basis of SE hydrodynamics, the QDs can follow the growth front by “slipping” through the entanglement mesh. We also note that the symmetrical cusp shape of the peaks in the luminescence profiles in Figure 7c suggests an increased QD concentration at the spherulite front, which decays gradually both ahead and behind the front. The latter gradual decay can only be explaine by back-diffusion of QDs into the spherulite. We will return to this issue below.

35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Now we arrive at the most unexpected result of our experiments, i.e. that a significant portion of silica NPs are being “pushed” ahead of the growing spherulites, even though their size is way above that of the entanglement mesh or the reptation tube diameter. As spherulite front travelled up to 1,000 times faster than they were expected to diffuse according to SE hydrodynamics, all the NPs should have been engulfed by the spherulites. Related to this anomalous behavior is also the unusual back diffusion phenomenon, where molecules of the NR dye and apparently also QDs, that had initially been pushed out of the spherulites subsequently diffuse back into them. This observation begs the question why then had they been rejected in the first place. In our view the two phenomena are related and both point to a specific situation at the spherulite growth front. It is well known that a density depletion layer precedes the growth front of polymer crystals and spherulites, as the polymer chains are reeled in by chain-folded crystallization and the volume contracts upon crystallization. 52 , 53 , 54 , 55 In this work evidence of the resulting strong negative pressure is seen in the formation of cracks and Newton rings.

50 51

According to Pawlak and Galeski 56 a negative pressure outside spherulite boundaries can build up to

52

P = -B (∆V/V0) 14

1 2 3 4 5 6 7 8 9

where ∆V/V0 is the relative change in specific volume on crystallization and B is the bulk modulus of the melt. To estimate P we used literature data for iPP and PLA on specific volumes of crystalline state (both α-form) and melt (at 130°C), as well as the bulk modulus of the melt – see Table 1. The very high negative pressures of 161 and 254 MPa for iPP and PLA are the upper limits, those that may develop locally in front of individual lamellae, but a more relevant values are perhaps the P values multiplied by crystallinity X of the spherulites, i.e. PX. Thus for X = 0.5 the corresponding negative pressures would still have impressive values of 80 and 127 MPa, respectively.

10 11 12 13

Table 1. Maximum negative pressure P developing in the melt during crystallization at 130°C due to volume contraction, including specific volumes of crystal and melt and the bulk modulus of the melt (B) Polymer

Vcryst (cm3/g)

Vmelt130C (cm3/g)

B (MPa)

P (MPa)

iPP

1.07257

1.26658

97058

-161

PLA

0.79659

0.93260

1186cal

-254

culated from data in 61

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

The above P or PX values exceed by far the “disjoining pressure” normally associated with a solid-liquid interface. A high-pressure driven diffusion is likely to obey kinetics different from that based on unbiased Brownian motion. While polymer chain diffusion is inhibited by entanglements, additives such as NR or NPs could be quicker to take over the depletion zone (Figure 11). Thus an additive-rich polymer-defficient layer is believed to precede the spherulite front, as most clearly seen in QDs-doped PLA (Figure 7). The excessive non-equilibrium concentration of the additive at the growth front, resulting from the imbalance in diffusion of the host and the guest, apparently starts to return to a more equilibrium distribution, as the transient depletion zone fills up behind the spherulite growth front. Note the nearly

symmetrical shape of the moving cusps in Figure 7c, marking the traveling NP-rich spherulite front, with the concentration maximum dissipating back toward the spherulite interior once the growth front had passed. The back-diffusion presumably occurs through the amorphous interlamellar layers or through voids between lamellar stacks (“interfibrillar zones”). In contrast, no clear evidence of back-diffusion is seen with the larger SiO2 NPs. A higher negative pressure seems to build up in the latter blends, due to more effective immobilization of polymer chains by the largher NPs. Thus particularly in these blends, once spherulites collide, cavities may open trapping in them the excess additive content of the travelling depletion zones.

36

15

1 2 3

Figure 11. Schematic showing the NPs filling the melt depletion zone ahead of a growing spherulite.

4 5 6 7 8 9 10 11 12 13 14

It is interesting that, in their recent SAXS study of commercial linear polyethylene doped with 14 nm diameter spherical n-octadecyl-capped silica NPs, Kumar et al.23 found NP aggregation induced by isothermal crystallization. It is likely that these aggregates are equivalent to those seen in our micrographs at spherulite boundaries and in pockets and cavities at 3-spherulite junctions. As there was no sign of increasing long period, the authors concluded that the NPs did not reside in interlcrystalline layers. Since the spherulites in our iPP and PLA nanocomposites retail much of the fluorescence of the initial melt, we can tentatively conclude, by implication, that most NPs engulfed by the spherulites reside in spaces between discrete lamellar stacks on a submicron length scale, unresolved by FM.

15 16

2.5. Advantage of FM in visualizing microcracks

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Finally, we draw attention to an additional benefit of using fluorescence microscopy in studies of polymer morphology. Four different optical microscopy modes are compared in Figure 12 showing a few α-spherulites in a thin NR-doped iPP film: FM (a), POM (b), bright-field (c) and POM with λ-plate (d). Cracks are formed in the areas of the film solidified by quenching after incomplete isothermal crystallization. Although they can be seen faintly in in other three panels, the microcracks at the upper left and right corners are by far the clearest in the FM image in panel a as bright lines on dark background. These should also be compared with the bright lines around cracks in Figures 5a3, b3, d3. In fact in the low-contrast FM images of polymer-NP blends, before digital contrast enhancement, bright spots are seen to suddenly light up when crystallization is nearly complete, indicating opening of micron or submicron cracks at three-spherulite meeting points. The origin of the black crack lines in bright field is the wave guiding along a vertical crack, which removes the light from the image plane of the microscope. However, the intense evanescent wave of incident radiation flowing along the crack surface generates strong fluorescence which is not guided and hence does contribute to the image.62 A similar effect is also responsible for the high FM contrast delineating the radial lamellar bundles within the spherulites. Such “artefacts” can be very useful, but must be understood when interpreting FM images of polymers.

16

1 2 3 4 5 6

Figure 12. Comparison of different optical microscopy imaging modes: (a) FM, (b) POM, (c) bright field, (d) POM with a full-wave (λ) plate. The images show an α-spherulite in a thin film of iPP after growing at 130 °C for 180 minutes and then quenching at room temperature. The larger indicatrix in the inset in (d) is that of the retarder and the small ones are those of the spherulite in the different sectors.

7 8

3. Conclusions

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

By combined fluorescence and polarized microscopy, supported by SEM, it is shown how fluorescent additives and nanoparticles can be traced in situ during melt-crystallization of polymers and how further post-crystallization rearrangements can be monitored. It was found that not only dye molecules, but also a sizeable fraction of 8, 35 and 200 nm nanoparticles were able to move ahead of growing spherulites respectively 40, 170 and 1,000 times faster than predicted by Stokes-Einstein hydrodynamics. This is particularly surprising for the larger NPs, as previous works suggested that they should obey it. FM images indicate that NPs and low-molar additives coalesce in the depletion zone ahead of the spherulite growth front, assisted by the buildup of high negative pressure. The cavitation that this causes is particularly pronounced in blends with larger NPs that inhibit polymer diffusion to the depletion zones. This mechanism whereby additives are carried ahead of the spherulite by the depletion zone also helps explain why they may in certain cases be “over-rejected”, some of it subsequently returning to the spherulite through back-diffusion.

24 25 26 27 28 29

Furthermore, the work shows that the worst clustering of additives occurs where spherulites had not nucleated, highlighting the benefits of adding well-dispersed nucleating agent to ensure more uniform additive distribution. It is also shown how FM can be effective in detecting cavities, even minute microcracks. A further advantage of FM is its ability to allow quantitative in-situ measurement of otherwise unavailable concentration profiles, and thus direct determination of diffusion rates.

30 31

4. Experimental section

32

4.1. Materials

33 34 35 36 37

iPP was produced by Sinopec Yanshan Petrochemical Company (Beijing, China), Mn = 48 kDa, Mw = 287 kDa, MFI = 12 g/10 min. PLA (4032D) was obtained from NatureWorks (Minnetonka, USA) containing around 2% D-lactide, Mn = 106 kDa, Mw = 223 kDa. Perovskite QDs (CsPbBr3) with a diameter of 8 nm, covered with grafted oleic acid, were provided by Nanjing MKNANO Tech. Co., Ltd (Nanjing, China). NR 17

1 2

(C20H18N2O2, Sigma-Aldrich), anhydrous p-xylene 99% (Energy Chemicals), and anhydrous methanol 99.9% (Energy Chemicals) were used as received.

3

4.2 Synthesis of dye-labelled silica NPs

4 5 6 7

The syntheses of 9-diethylamino-2-(triethoxysilyl-3-propyloxy)-5H-benzo[α]phenoxazin-5-one (L) and SiO2 NPs of 35 nm and 200 nm were carried out according to 63 and 64 , respectively. The hybridization was achieved as described in SI.

8

4.3. Preparation of dye- and NP-doped polymers

9 10 11 12 13 14 15 16

To obtain a uniform mixture of NR and the polymers, freeze-drying of mixed solution was applied. For example a iPP-NR sample was obtained by first preparing 2000:1 wt ratio solution in p-xylene (130 °C, mild stirring). The solution was frozen by quenching in liquid nitrogen, after which the solvent was sublimed off under vacuum at 0 °C. For preparation of PLA-NR blends the solvent was dioxane and the solution mixing temperature was 50 °C. The last traces of solvent were removed by vacuum drying at room temperature. The final concentration of NR in both iPP and PLA was 0.05 wt%.

17 18 19 20 21 22

To prepare polymer-NP blends, NPs were first dispersed in solvent (sonication, 60 min), then the NPs suspension was mixed with the solvent/polymers solution followed by freeze-drying. The solvents and dissolution temperatures were the same as above. The concentrations of 35 and 200 nm functionalized SiO2 NPs in iPP were 1.3 and 5 wt%, respectively. The concentration of both 35 and 200 nm SiO2 NPs in PLA was 1.5 wt%, and the concentration of QDs in PLA was 0.5 wt%.

23

4.4. Microscopy methods

24 25 26 27

For TEM of the NPs, the NPs powders were dispersed in water under sonication for 15 min, and then a drop of the dispersion was spread onto a formvar-coated copper grid. TEM observations were taken on a Hitachi H-8100 IV electron microscope under 200 kV operating voltage.

28 29 30 31 32

SEM was performed on a field-emission scanning electron microscope (Inspect F, FEI, USA), operating at 5 kV. Small pieces of a 200 µm thick PLA-NP 35 nm sample melt-crystallized at 120 °C for 40 min were immersed in liquid nitrogen for 1 h and then cryogenically fractured. The smooth fracture surface was sputter-coated with gold.

33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

For the combined optical and fluorescence microscopy an Olympus BX-53 microscope was used with a Mettler HS82 hot-stage. Polarized (crossed polarizers) and bright field microscopy was done in transmission. A 530 nm full wave retarder (λ) plate was used with the polarizers where necessary. Alternating with transmission images, fluorescence micrographs were recorded in reflection using a LED source (PE300-White from CoolLED) and a dichroic mirror/filter combination optimized for 510 nm maximum absorption. The polymer blends with NR and NPs were placed between the glass slide and a cover slip, annealed at 210 °C for 10 min to remove memory of previous treatments. Film thickness was typically 20−30 µm. Afterwards, the sample was cooled to Tc at the maximum cooling rate allowed by the hot stage (ca 30 ºC/min) and maintained at Tc for a period. Once the temperature reached Tc, transmission (bright field, polarized and polarized with λ-plate) and fluorescence microscopy images were captured in sequence, tracing the growth of spherulites. Flipping between the transmission and fluorecsence modes took about 4 seconds. The coloured fluorescence microscopy images were converted to a brightness scale with 256 grey levels to improve visual intensity resolution. Here 255 means maximum number of photons per pixel and 0 means no photons above the dark current and read-out noise of the CCD chip (cf. Fig 2b). As shown in Figure S2, the camera had a 18

1 2

non-linear response; a correction function was constructed by using neutral density filters (see Supporting Information).

3 4

Acknowledgements

5 6

Financial support from NSFC (grant 21674099) is acknowledged. S.G.Y. thanks China Scholarship Council for scholarship.

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Table of Contents Entry

2 3 4

Fluorescence Microscopy Tracking of Dyes, Nanoparticles and Quantum Dots During Growth of Polymer Spherulites

5 6

Shu-Gui Yanga,b,c, Hui-Jie Xied, Hina Sabad, Liliana Csehe, and Goran Ungara,c,d,*

7 8

a

9 10

b

11 12

c

13 14

d

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e

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E-mail: [email protected]

State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Center for Soft Matter, Xi’an Jiaotong University, Xi’an, 710049, P.R. China. College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, P.R. China. Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K. Department of Physics, Zhejiang Sci-Tech University, 310018, Hangzhou, P.R. China. Romanian Academy, ”Coriolan Dragulescu” Institute of Chemistry, Timisoara 300223, Romania.

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Highlights • • • • •

A dye, nanoparticles (NP) and quantum dots (QD) were followed during spherulite growth All were partly excluded from spherulites but a fraction of dye and QDs diffused back in Diffusion of excluded 35, 200 nm NPs much faster than expected from Stokes-Einstein Additives carried along by high negative pressure depletion zone ahead of spherulite Fluorescence microscopy also very useful for showing cavities and microcracks

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: