Systematic Control of Self-Seeding Crystallization Patterns of Poly

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Systematic Control of Self-Seeding Crystallization Patterns of Poly(ethylene oxide) in Thin Films Binghua Wang,† Shaohua Tang,† Yan Wang,† Changyu Shen,† Renate Reiter,‡ Günter Reiter,‡ Jingbo Chen,*,† and Bin Zhang*,† †

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s Republic of China Institute of Physics, University of Freiburg, 79104 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: Using optical microscopy and atomic force microscopy, we studied systematically crystallization patterns in thin films of a low molecular weight poly(ethylene oxide) (PEO) resulting from a kinetically controlled self-seeding approach. In particular, the influence of seeding temperature (Ts) and heating rate (Vh) on the various resulting crystallization patterns was investigated. Crystallization at 49 °C resulted in dendritic PEO crystals consisting of almost exclusively twice-folded chains. Upon heating these crystals, we observed crystal thickening due to a reduction in the average number of chain folds. On the basis of the detected morphology, we deduced that the density of seeded PEO crystals decreased when increasing Ts from 54 to 57 °C. At the highest Vh (i.e., 100 °C/min), only a few well-separated faceted single crystals of PEO were grown from individual seeds. In contrast to such random distribution of crystals, because of a faster reduction of chain folds at the edges of PEO lamellae, an almost continuous sequence of seeded crystals was formed at the periphery of the original crystals at significantly lower Vh (i.e., 10 °C/min). Interestingly, reflecting the different metastable states within the initial crystal resulting from seeding at Ts = 54 °C, the seeding probability for crystals at the diagonals was higher than for the major side branches. In addition, we estimated activation energies (213−376 kJ/mol) for thickening of PEO lamellar crystal from an Arrhenius-type behavior of the lateral spreading rates as a function of Vh. Our findings suggest that the interplay between thickening and melting of metastable states within the initial crystals is considered as responsible for the resulting nucleation density and crystal morphology induced by self-seeding. nucleation, which is entitled as “self-nucleation” or “selfseeding” because crystallization starts from not molten, chemically identical crystalline parts (nuclei or seeds).23−25 The process of self-nucleation was identified nearly six decades ago by Keller and Sharples.26−28 In recent reviews by Müller and co-workers, the principles and applications of successive self-nucleation and annealing (SSA) have been introduced.29,30 The influence of self-nucleation on crystallization, the resulting morphology, and the overall behavior of polymers has been widely studied.31−36 The following aspects of self-nucleation have been described: The overall crystallization kinetics of polymer is enhanced as the nucleation density can significantly increase through the presence of selfnuclei. 37−39 The nucleation density can be controlled predictably by choosing the appropriate conditions of melting or dissolution of polymer crystals.23 The nucleation density is strongly influenced by metastable states present in the starting crystal.24 Self-nucleation enables the generation of orientationcorrelated polymer crystals of uniform shape and size having their orientation inherited from the initial single crystal.40 Self-

1. INTRODUCTION Nucleation and crystal growth, first intensively investigated in metals and for small crystallizable molecules, have attracted considerable interest in polymer science.1−6 Polymer crystallization is a first-order transition, which requires to transform a material consisting of random coiled polymeric chains into a nearly perfectly ordered state.7−10 In contrast to the melting behavior of crystals of small molecules, long chain polymer crystals may possess a strong melt memory of the previously molten crystalline structures, having for example a dramatic influence on the dynamics of subsequent crystallization processes and the resulting crystal morphology.11−14 For most semicrystalline polymers, crystallization usually initiated either by homogeneous nucleation or via heterogeneous nucleation.15−18 All memory of previous crystalline structures is expected to relax and disappear if these crystals are melted above the equilibrium melting point for a sufficiently long time.19,20 Melting or annealing a polymer crystal below the equilibrium melting point will allow for a number of surviving “small crystals” or “bundles of partially ordered chain segments”. This number depends drastically on the temperature used for melting and annealing. Upon subsequent crystallization, these remnants of the initial crystalline structures can act as predetermined nucleation sites.21,22 This initiation process can be considered as a special type of heterogeneous © XXXX American Chemical Society

Received: November 20, 2017 Revised: January 15, 2018

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DOI: 10.1021/acs.macromol.7b02445 Macromolecules XXXX, XXX, XXX−XXX

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experiments on the influence of seeding temperature (Ts), heating rate (Vh), and metastable states within the initial crystal on the resulting crystallization patterns. Our here presented observations indicate that the number and location of thickened domains depend on the seeding temperature, heating rate, and the initial crystal structures. For higher Ts or Vh, the onset of thickening was retarded, often accompanied by a lower density and random location of nuclei. At lower Ts or Vh, dominant thickening occurred at the periphery of the starting crystal where thus seeds were preferentially formed. With increasing Ts, the nucleation density at the periphery decreased. We found that the lateral spreading rates of lamellar thickening decreased with temperature, obeying the Arrhenius law with activation energies EA ≈ 213−376 kJ/mol for heating rates ranging from 5 to 100 °C/ min. We conclude that the competition between lamellar thickening and melting is responsible for the variations observed in the crystallization patterns for different Ts and Vh.

nucleation can be applied for crystallizing polymers from solutions, thin films, and bulk samples.41−46 Low molecular weight poly(ethylene oxide) (LMW PEO) is an important model system to study theoretical and experimental points of polymer crystallization.4,47−50 A large and still-expanding body of literature on crystallization of bulk samples51,52 and the formation of single crystals53,54 of LMW PEO is available. Pioneering work by Kovacs et al.55 has studied isothermal growth, lamellar thickening, melting temperature, single crystal morphology, and integral folded chains (IF) of LMW PEO. By systematically varying molecular weight and molecular structure (i.e., molecular shape, defects in molecules, and end-groups), Cheng and Chen et al.47,51,56 found that noninteger folded chain (NIF) crystals are thermodynamically less stable than IF crystals, despite their growth is more rapid. The lamellar thickening process of PEO, in particular for lowmolecular-weight fractions with monodisperse or narrow mass distribution, has been widely studied by small-angle X-ray scattering (SAXS), polarized light microscopy (PLM), DSC, Raman spectrometry, and atomic force microscopy (AFM).57 Wang et al.58,59 studied the morphological changes of PEO crystals during heating by performing in situ measurements using an AFM equipped with a hot stage. They revealed a change of the crystal thickness, which was strongly dependent on annealing time and temperature. Hu et al.60,61 reported that chain-sliding diffusion of polymers during annealing is responsible for the thickening process of monolamellar crystal by using Monte Carlo simulations. In our previous studies, we found that thickening mainly occurs at the edges of isolated lamellae, leading to the formation of rims whose width increased with time and temperature.62,63 The thickening behavior of LMW PEO has been extensively studied in the past decades, yielding a large amount of insight into the central processes of reorganization within polymer lamellar crystals. The previous studies mostly involved the thickening process and changes of fold length in lamellae. However, until now, very few experimental studies were concerned with the mechanism responsible for the formation of self-nuclei, their density and distribution in space, and their influence on the final crystallization patterns. In the case of selfseeding, when a polymer crystal is heated to a temperature below the equilibrium melting point, a competition of lamellar thickening and melting will take place. Annealing may induce thickened crystalline domains, which exhibit a higher melting temperature and therefore are potentially capable of surviving the heating process as nuclei. Therefore, an in-depth understanding of lamellar thickening will be an essential in controlling polymer crystallization patterns by self-seeding. The objective of the present study is to explore how annealing conditions affect lamellar thickening and subsequent crystal structures and morphologies induced by self-seeding in LMW PEO thin films. The ultimate goal is to control and predict the resulting crystallization patterns quantitatively by enabling appropriate site-directed self-nucleation. Here, three interesting questions arise: Which mechanism is responsible for the formation of such self-seeded crystallization patterns? What roles do thickening and melting play during the seeding process? Can such patterns be controlled quantitatively by changes of the annealing condition or the spatial distribution of metastable states of different degrees of order within the initial crystal? In order to answer these questions and to shed some light on the variations of crystallization patterns in thin film induced by a self-seeding process, we carried out a series of

2. EXPERIMENTAL SECTION Poly(ethylene oxide) (Mn = 4600 g/mol, PDI = 1.14) used in this work was purchased from Sigma-Aldrich. PEO was dissolved in acetonitrile at concentrations of 0.4 wt % by heating the solution for 60 min to 65 °C, which is well above the nominal dissolution temperature of ca. 40 °C.64 Thin films were prepared by spin coating the solution onto a silicon substrate (P111 type, UV treated for 145 min before being placed on the spin-coater) with a KW-4A spin-coater (Institute of Microelectronics, Chinese Academy of Sciences, China). The spin speed and time were 4000 rpm and 30 s, respectively. The temperature protocol adopted for self-nucleation experiments is schematized in Figure 1 and consisted of the following steps: (1)

Figure 1. Experimental protocol employed for self-seeding of PEO crystals in thin films. Ts, self-seeding temperature; Tc1, precrystallization temperature of the dendritic crystals; Tc2, isothermal crystallization temperature after seeding; and Tm, the equilibrium melting temperature.51,65

heating the films from room temperature to 70 °C at a rate of 30 °C/ min; (2) holding at 70 °C for 3 min; (3) cooling at 10 °C/min down to Tc1 and holding at Tc1 for tc1; (4) heating from Tc1 to Ts = 54−57 °C with heating rate (Vh) ranging between 5 and 100 °C/min, followed by isothermal holding at Ts for 180 s; (5) cooling to the crystallization temperature (Tc2); (6) after quenching to room temperature, samples were examined by AFM. B


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which can be calculated as L/(n + 1),59,68 are thus 14.5 and 9.6 nm, respectively. Figure 2d shows the mean lamellar thickness recorded by AFM after crystallizing films at 49−54 °C. At lower crystallization temperatures, e.g., Tc = 49 °C, a typical dendritic crystal can be observed; its thickness is about 9.7 nm, which corresponds to the average twice-folded chain length of the IF(n = 2) crystal. When increasing Tc to 52 °C, a faceted PEO single crystal is formed (see Figure S2); the measured thickness of 14.6 nm agrees very well with theoretical thickness of IF(n = 1) crystal. The thicknesses of crystals grown at other Tc do not correspond to either the IF(n = 1) or IF(n = 2) crystals, but rather to NIF crystals. From Figure S1, it becomes apparent that the growth rate decreases with increasing crystallization temperature and inverse of film thickness. 3.2. Self-Seeding from a Dendritic Crystal. In Figure 3, we show the crystallization patterns of PEO crystals obtained in

For morphological observation, an Olympus BX-51 optical microscope (Olympus, Tokyo, Japan) equipped with a Linkam THMS 600 hot stage (Linkam Scientific Instruments, Tadworth, UK) was used. The crystallization patterns resulting from such a selfseeding procedure were characterized by atomic force microscopy (Dimension Icon, Bruker, USA). Tapping mode was applied throughout this study using Bruker probes (model number SCANASYST-AIR; tip radius, 12 nm; force, 0.4 N/m; frequency, 70 kHz).

3. RESULTS AND DISCUSSION 3.1. Typical Micrographs of a PEO Dendritic Crystal. In many past studies, morphologies of crystals of poly(ethylene oxide) and its block copolymer have been well investigated in thin films.50,54,66 Here, as a model system, a LMW PEO was used to study the effect of self-seeding conditions on resulting crystallization patterns. In Figure 2, we present typical AFM

Figure 3. Optical micrographs showing the small faceted PEO single crystals grown within the region occupied by the starting large compact dendritic crystal after applying the self-seeding procedure described in the text. (a) The starting crystal, crystallized for 10 min at Tc1 = 49 °C. The same crystal, after subsequent annealing for (b) 15 s and (c) 180 s at 56 °C, and (d) after recrystallization at Tc2 = 52 °C for 120 s. The size of the scale bar is 50 μm.

Figure 2. (a, b) AFM height images showing a typical dendritic PEO crystal grown in a 7.5 nm thick film at 49 °C. (c) Height histograms obtained from the AFM height image in (b). (d) Lamellar thickness (dc) as a function of crystallization temperature (Tc) obtained from crystals grown in 7.5 nm thick films.

height micrographs of a PEO dendritic crystal grown at 49 °C in a ca. 7.5 nm thick film, indicating that the pattern resulted from a diffusion limited-aggregation (DLA) process.67 As shown in Figure 2a, one can clearly identify a monolamellar crystal with four orthogonal main branches and many side branches lying flat on the silicon substrate. The height histogram of the region indicated by the dotted box in Figure 2a is shown in Figure 2c. Three peaks can be identified which correspond to the substrate (peak 1), the thickness of thin crystals formed during quenching (peak 2), and the thickness of the dendritic crystal formed at 49 °C (peak 3) with a mean value of about 9.7 nm. As a remarkable feature of lowmolecular-weight fractions of PEO, crystallization may yield lamellae containing integer folded (IF) or noninteger folded (NIF) chains with the chain axis normal to the substrate.68 The maximum length (L) of the PEO molecule (Mn = 4600 g/mol) in the fully extended state is L = luN = 28.9 nm with lu = 0.2783 nm58 (lu is the average length of one monomer in the crystalline lattice) and N = 104 (N is the average number of monomers). The theoretical thicknesses of IF(n = 1) and IF(n = 2) crystals,

the course of a typical self-seeding procedure, starting from a preformed dendritic crystal IF(n = 2) obtained by crystallization at 49 °C for 10 min. Self-seeding is based on the residual memory and thus the previous thermal history of polymer crystals which controls the resulting crystallization patterns as well as number density and location of “baby” crystals. As shown in Figure 3b, when annealing this crystal at about 56 °C for 15 s, dendritic crystalline features became rather diffuse. The dendritic pattern was lost completely after holding the sample at 56 °C for 3 min (see Figure 3c). Interestingly, it can be observed that the tiny crystal (as denoted by white arrows in Figure 3) at the nucleation site was stable enough to survive and continued growing at this temperature. When lowering the temperature to recrystallization temperature (Tc2 = 52 °C), tens of small faceted single crystals appear in the range of the initial crystal. All these reappearing crystals have the almost uniform size and shape, and their orientation is the same as the unit cell of the initial dendritic crystal. This crystallization patterns indicate that most C


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Figure 4. Effect of seeding temperature and heating rate on crystallization patterns of the PEO crystals induced via a self-seeding procedure. AFM height images of PEO crystals obtained after annealing for 3 min at different temperatures (a) 57, (b) 56, and (c) 55 °C for different heating rates from Tc1 to Ts (ranging from 5 to 100 °C/min), followed by isothermal recrystallization at Tc2 = 52 °C for 30 s. The size of the scale bar is 10 μm.

limited by polymer diffusion. Therefore, thickening (in our case conversion from IF(n = 2) to IF(n = 1) crystal) occurs most rapidly at the edge of crystals. In particular, there are two key aspects to understand the mechanism of crystal thickening at the periphery: First, a lamellar crystal is less stable at the periphery, and thus thickening is more pronounced there. The large lateral surface free energy and less perfectly ordered and more often folded chains attached at the crystal growth front favor thickening. Second, if the thickening process takes place at the periphery of crystals (growth front), transportation of polymer chains toward the “thickening site” is less difficult, and the time needed for attachment and lateral packing of these chains into the crystal lattice is short compared to times required for rearrangement of chains inside a crystalline domain. In other words, thickening proceeds fastest at position closest to the reservoir of molten polymer chains. For high seeding temperatures and fast heating rates, the attachment probability for molten chains is low. Conversely, for low heating rates and a low seeding temperatures, sufficient time for growth and reorganization (for example, unfolding from IF(n = 2) to IF(n = 1)) will be available. The results obtained at lower seeding temperatures (55 and 56 °C), shown in Figures 4b and 4c, clearly demonstrate that the number of faceted PEO crystals, that is, the nucleation or seed density, increased with decreasing seeding temperature. Compared to results obtained for higher Ts = 55−57 °C, a notable difference in morphology was observed when a PEO dendritic crystal was annealed (“seeded”) at 54 °C. Except for the thick crystalline periphery, additional nucleation sites were found on locations of the main branches and some side branches of the initial dendritic crystal. The patterns obtained at Ts less than ca. 55 °C suggest that the thermodynamically most stable parts of the initial dendritic crystal were mainly located on the main branches. Interestingly, as shown in Figure 5a for Ts = 54 °C, at low Ts we did not observe the formation of a thick crystalline rim at the periphery of the initial dendritic

crystalline structures from the initial dendritic crystal were erased at the seeding temperature. However, a few randomly distributed “seeds” survived, which acted as the nuclei for subsequent crystallization. Such self-seeding phenomenon was also observed for many other polymers, such as poly(ferrocenyldimethylsilane) and poly(2-vinylpyridine-block-poly(ethylene oxide)).23,24,69 It has been explained as the difference in thermal stability of metastable states within chain-folded lamellar polymer crystals.43,70 Regions of higher degrees of order within crystalline structures resist melting. At the seeding temperature, these regions can be considered as thermodynamically “stable regions” (they will not disappear even when the waiting time at the seeding temperature is increased) which only can be erased by increasing the seeding temperature above their nominal melting temperature. In the case of self-seeding, the density of seeds (nuclei) has been reported to depend on seeding temperature Ts but not on the time spent at Ts.24 In present study, the crystalline patterns induced by self-seeding in PEO thin films exhibit both a dependence on Ts and the heating rate Vh. Figure 4 shows a series of patterns that were induced by self-seeding in precrystallized samples which were heated from 49 °C to Ts (55, 56, and 57 °C) at various Vh (5−100 °C/min). Significant differences in nucleation density accompanied by changes in locations of the reappearing crystals were observed, for example, when approaching Ts = 57 °C at different Vh (Figure 4a). There, for a slow heating rate (e.g., 5 °C/min), crystal growth was dominantly occurring at the periphery of the initial crystal, accompanied by a few isolated faceted crystals inside this region. Clearly, when increasing Vh from 5 to 10 °C/min, the crystalline periphery first became discontinuous and smaller in width and even disappeared completely for Vh higher than 20 °C/min. In addition, we found that the number of isolated crystals also decreased with increasing Vh (from 20 to 100 °C/ min). Similar to crystal growth in thin films, the process of crystal thickening during annealing is often controlled and D


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exception of the formation of a thickened rim at the crystal. At the same Ts and Vh, the width of this rim was larger for crystals in thicker films, as can be seen in Figure S4 for a PEO crystal in a 9.2 nm thick film. On the basis of the above AFM results, one can demonstrate that the seeding conditions together with the properties of the metastable states within the initial dendritic crystal play an important role in the formation of the resulting crystallization patterns. Thus, these patterns should also depend on the initial lamellar thickness, which varies with crystallization temperature. Accordingly, we need to explore if we can predictably produce crystallization patterns of diverse morphologies by choosing well-defined metastable states of the initial crystal, which we exploit by apply appropriate seeding protocols. In order to investigate this question, we employ the isochronous decoration method, which has often been used as an approach to obtain lamellae with controlled but varying thicknesses within an individual lamella. A typical AFM image and a corresponding cross section profile of an isochronously decorated dendritic crystal are shown in Figure 6a. There, we have varied the crystallization temperature between 49 and 53 °C, yielding lamellar thicknesses dc ranging from dc ≈ 9.6 nm and dc ≈ 18 nm, respectively. During the second and fourth growth steps at relatively high temperature, the thickness of the thin crystal increased from 9.6 nm IF(n = 2) to 14.5 nm IF(n = 1), and an even thicker lamellar thickness was obtained at 53 °C (dc ≈ 18 nm) at the periphery. This thickest region at the periphery enabled us to produce self-seeding only at the periphery of these crystals, as shown in Figure 6b. When the initial crystal, generated through a four-step growth process, was annealed at Ts = 57 °C, the seeding temperature was sufficiently high to completely melt all thin parts of the crystal interior (dc = 9.6 nm IF(n = 2) and 14.5 nm IF(n = 1)) but

Figure 5. AFM height images measured at room temperature for PEO crystals obtained after annealing for 3 min at Ts = 54 °C, which has been reached at different heating rates (a) 100, (b) 50, (c) 20, and (d) 10 °C/min from Tc1 to Ts, followed by isothermal recrystallization at Tc2 = 52 °C for 30 s. The size of the scale bar is 10 μm.

crystal. The regrown crystalline morphology has some similarities with the original morphology of the starting crystal shown in Figure 2a. With decreasing heating rate, the morphology changed from a coarse dendritic pattern (for Vh = 100 °C/min) toward a finer dendritic pattern surrounded by a discontinuous crystalline rim (e.g., for Vh = 50 °C/min). At relatively low Vh, AFM measurements revealed that the original morphology of the starting dendritic crystals was essentially unchanged by partial melting during the seeding process, with

Figure 6. (a) AFM height images of a PEO crystal that was grown according to the following sequence of steps varying in crystallization temperature (Tc) and crystallization time (tc) (isochronous decoration method). Step 1: [(Tc, tc) = (49 °C, 80 s)]; step 2: [(Tc, tc) = (53 °C, 120 s)]; step 3: [(Tc, tc) = (49 °C, 80 s)]; step 4: [(Tc, tc) = (53 °C, 120 s)]. (b) AFM measurements were performed at room temperature, after annealing another isochronously decorated PEO crystal at Ts = 57 °C for 3 min, which was then recrystallized at Tc2 = 52 °C for 30 s. Cross-section profiles along the directions indicated with the dotted black lines in the AFM images. The size of the scale bar is 10 μm. E


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Figure 7. Seeding temperature dependence of the height and width of the thickened rim. AFM height images of representative regions close to a corner of PEO crystals. AFM height images were measured after annealing the PEO crystals (Tc1 = 49 °C, tc1 = 2.5 min) at (a) 57, (b) 56, (c) 55, and (d) 54 °C for 3 min with heating rate of 5 °C/min, followed by isothermal recrystallization at Tc2 = 52 °C for 30 s. The size of the scale bar is 1 μm. Cross-section profiles along the directions indicated with the same colors of the dotted lines in (a), (c), and (d) are given in (e), (f), and (g), respectively.

Figure 8. Heating rate dependence of the height and width of the thickened rim. AFM height images of representative regions close to the growth tips of PEO crystals. Measurements were performed at room temperature after annealing the PEO crystals (Tc1 = 49 °C, tc1 = 2.5 min) at 56 °C for 3 min for different heating rates [(a) 50, (b) 20, (c) 10, and (d) 5 °C/min], followed by isothermal recrystallization at Tc2 = 52 °C for 30 s. The size of the scale bar is 1 μm. Cross-section profiles along the directions indicated with the same colors of the dotted lines in (a), (b), and (d) are given in (e), (f), and (g), respectively.

vary the degree of metastability and thus the local thickening and subsequent melting behavior of a dendritic crystal by changing the crystallization temperature during growth processes.

insufficient to remove crystalline structures at the thicker periphery (high temperature growth part, Tc = 53 °C, dc = 18 nm). Thus, this self-seeding procedure led to the formation of small faceted crystals. Our result suggests that we can discretely F


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Figure 9. (a) Plots of the width (w) of thickened rim versus heating rate, Vh. (b) Arrhenius plot of the lateral spreading rate of lamellar thickening, ρ (from the thickened area divided by the heating period), versus inverse absolute temperature.

To have a quantitative description of lamellar thickening during the self-seeding process, the characteristic width of the thickened rim, w, and the temperature dependence of the lateral spreading rate of lamellar thickening, ρ (calculated from the thickened area, including both nonisothermal (TP I) and isothermal (TP II) thickening part, divided by the heating period), at different Vh, were determined. As indicated in Figure 9a, at higher Ts, e.g., 57 °C, the width of thickened rim w exhibits no measurable changes with heating rate Vh. However, with an increase in Vh, w becomes smaller at lower Ts. The change of w can be described by an exponential function, b + aekVh where k = −7.5 × 10−2 at Ts = 54 °C. Thus, the exponential decrease of w with increasing Vh could be attributed to an exponentially decreasing probability to produce thicker crystalline regions within the seeding crystal for increasing Vh. Chain-sliding motion and a mechanism of melting−recrystallization have been invoked to explain thickening of lamellar crystals. Basically, the mechanism of chain-sliding motion suggests that chains in thinner parts of the lamellae slide along the c-axis which leads to the generation of less-folded chains; melting−recrystallization involves melting of thinner parts of crystalline lamellae and their subsequent recrystallizing into thicker lamellae.72 Wang et al.59,73 have obtained the annealing time dependence of the height, area, and volume of a thickened part within the LMW PEO crystals during annealing. Furthermore, within the temperature range from 50 to 58 °C, spontaneous thickening and induced thickening took place during the entire annealing process. The corresponding temperature dependence obeyed an Arrheniustype behavior. Chen et al.74 also proposed that thickening of PEO crystal was controlled by an “on-site sliding diffusion” process, which was related to a nucleation-controlled mechanism resulting in an Arrhenius type temperature dependence. Figure 8b shows that the height of the isothermally thickened part (TP II) is almost uniform (thickness of ≈18.8 nm, i.e., IF(n = 0.5)) over the whole lamella. However, the lamellar thickness depends not only on annealing temperature but also on heating rate, as can be anticipated from the incompletely thickened part of the interior region (TP I). Furthermore, space between neighboring branches in a dendrite can be filled during a slow heating process (i.e., low Vh). Such significant morphological changes length scales much larger than the size of the molecules, as represented by a doubling of the

The above results also indicate that thickening of PEO lamellar crystals of short chains is sensitively dependent on seeding temperature (controlling the allowed temperature window for thickening) and on heating rate (controlling the time provided for the process of reorganization). For higher molecular weight polymers, probably due to the comparatively slow relaxation processes, this influence is less pronounced.71 The thicker rims at the edges of the initial dendritic lamellar crystals have a nearly round shape. In order to quantitatively explore the effect of thickening conditions on the resulting crystallization patterns in LMW PEO thin films induced via kinetically controlled self-seeding conditions, the width and height of resulting crystals after self-seeding have been measured from the cross section profiles obtained at the periphery part. First, thickening “walls” (the highest regions in AFM image) of different heights can be obseved in Figure 7. Within the region enveloped by these “walls”, the interior exhibits crystals which are few nanometers lower in height. As depicted in Figure 7e, according to their heights, the resulting crystals can be divided into three subsections: thickened part I (TP I, dc ≈ 14.5−16.5 nm), thickened part II (TP II, dc ≈ 18.2−18.8 nm), and the isothermal recrystallization part (IP, Tc2 = 52 °C yielding dc ≈ 14.5 nm). The thickening process has been suggested as follows: The nonisothermal thickening part TP I in Figure 7e was formed during heating from Tc1 to Ts. It corresponds to thickening from IF(n = 2) to IF(n = 1) or NIF. We notice that the region of TP I outgrowth around TP II (IF(n = 0.5)) is about 400 nm in width (see TP II in Figure 7e). Lamellae within the IP part have a lower thickness of 14.2 nm than the TP I and TP II regions. A thickness of 14.2 nm corresponds to crystals consisting of once-folded chains which grew at Tc2 = 52 °C epitaxially from the TP II region. From Figure 7, it can be easily noticed that with decreasing Ts the width of TP I region increased but the height of crystals in the TP I region decreased. The influence of changing heating rates on crystal thickening can be seen in Figure 8. It is clearly shown that the width of the TP I region decreased while the correponding height increased with increasing Vh. We asumme that the TP II region arose from isothermal annealing at Ts, since the width of this region was almost independent of the heating rate. At high seeding temperatures, in particular when combined with high Vh, only few isolated crystals were observed. G


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Macromolecules lamellar thickness or the filling of space between initial branches, suggest that thickening is related to a melting and recrystallization process.75 Such a mechanism may also involve sliding motion of molecular segments along the c-axis combined with melting recrystallization. Such processes occur for any Vh in the investigated range. The chain-sliding motion may dominate the thickening process at low annealing temperatures. Melting and recrystallization may require higher energies (i.e., higher annealing temperatures) as many steps are involved like melting of thin parts of the initial crystals, diffusion of free molecules to the remaining thick crystals, and their attachment and integration into these thicker crystals. In order to obtain more information about the thickening processes of LWM PEO, we measured the spreading rate (ρ) at which the thickened area increased laterally (see Figure 9b). For lamellar thickening from IF(n = 2) to IF(n = 1, 0.5) or NIF crystals, ρ decreases with temperature, following an Arrheniustype (ln ρ ∝ T−1) behavior in the temperature range from 54 to 57 °C. Furthermore, such an Arrhenius-type thickening kinetics of lateral spreading rates (ρ) agrees well with Hu and coworkers’60 simulation results in the similar situation. Assuming the validity of such behavior, we can further estimate an activation energy which is needed for thickening of PEO crystals for different heating rates (ranging from 5 to 100 °C/ min), increasing with increasing Vh from EA ≈ 213 kJ/mol to EA ≈ 376 kJ/mol. On the basis of the temperature and heating rate dependence of the thickening patterns presented in Figures 7 and 8, we conclude for the mechanisms of lamellar thickening and the formation of crystallization patterns induced by self-seeding as the following stages. The nonisothermal thickening of lamellar crystals from IF(n = 2) to IF(n = 1, 0.5) or NIF may resulted from a mechanism which involve chain-sliding motion combined with melting recrystallization. However, the isothermal thickening process depends largely upon the thermodynamic driving force and cooperative molecular motion within the solid crystal along the crystallographic caxis. Hikosaka et al.76 have emphasized that at high temperatures lamellar thickening proceeded more likely by chain sliding diffusion, due to the more frequent occurrence of conformational disordered segments. As a result, comparing thickening rates for different heating rates provides an informative approach for the investigation of the influence of chain sliding on lamellar thickening behavior.72 If lateral spreading rate of lamellar thickening is comparable to or faster than the melting rate, then we expect to obtain more thickened lamellae. The final thickening patterns may basically be attributed to the interplay of thickening the more stable crystalline parts and melting of thinner crystalline parts. In addition, the activation energy for the thickening process is very much related to the competition between thickening (induced e.g. by chain sliding in the crystalline lattice), yielding thicker and more stable lamellar crystals, and melting (detachment of chains from the crystalline lattice), which increases the fraction of amorphous (molten) polymers. The increase of the activation energy with heating rate can be explained as follows: Usually, upon heating, a lamellar crystal is thermodynamically stable if its thickness exceeds a certain value via thickening. Thus, the time required for such thickening needs to be shorter than the duration of the heating process. This is not always fulfilled for the higher Vh.77 Therefore, the required lamellar thickness may be reached in (few) regions consisting of highly mobile chains.

4. CONCLUSION In our study, seeding temperature (Ts) and heating rate (Vh) were found to have a tremendous impact on crystallization patterns resulting from kinetically controlled self-seeding process. Morphologically, at the highest Ts and Vh, only isolated nuclei or a discontinuous thickened rim at the crystal periphery can be obtained. However, at lower Vh or Ts, thickening dominantly occurred at the periphery of the starting crystals. With increasing Ts, the self-seeding nucleation density at the periphery decreased. Structurally, for different Ts and Vh, both IF(n = 2) to IF(n = 1, 0.5) crystals and IF(n = 2) to noninteger folded (NIF) transitions were observed. The lateral spreading rate of lamellar thickening (ρ) studied here obeyed an Arrhenius-type temperature dependence, yielding activation energies ranging from ca. 213 to 376 kJ/mol at different Vh with the increase in heating rate. The well-controlled thickening procedure allowed us to tune not only the seeding density but also the locations of these seeds. Furthermore, our results suggest that the interplay of thickening and melting of metastable states of the initial crystals represents the key factor, which is responsible for the morphology of the final crystallization patterns induced by self-seeding. Thus, we can predictively design crystallization patterns by choosing appropriate site-directed nucleation processes by adjusting self-seeding conditions (e.g., seeding temperature, and heating rate) and initial crystal structures.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02445. Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.C.). *E-mail: [email protected] (B.Z.). ORCID

Renate Reiter: 0000-0003-2294-1445 Günter Reiter: 0000-0003-4578-8316 Bin Zhang: 0000-0002-8293-1321 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Wenbing Hu and Prof. Murugappan Muthukumar for inspiring and very helpful discussions. The authors are grateful to the National Science Foundation of China (No. 11372284 and 51773182), Outstanding Young Talent Research Fund of Zhengzhou University (1521320004), China Postdoctoral Science Foundation (2016M592302), and Startup Research Fund of Zhengzhou University (1512320001).

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