In Situ Real-Time Observation of Polymer Folded-Chain Crystallization

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In Situ Real-Time Observation of Polymer Folded-Chain Crystallization by Atomic Force Microscopy at the Molecular Level Yuki Ono and Jiro Kumaki* Department of Organic Materials Science, Graduate School of Organic Materials Science, Yamagata University, Yonezawa, Yamagata 992-8510, Japan

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S Supporting Information *

ABSTRACT: The crystallization process of a folded-chain crystal (FCC), the most common crystalline structure of polymer chains, was clearly visualized at the molecular level in situ and in real time for the first time. The sample was an isotactic poly(methyl methacrylate) monolayer deposited on mica in an amorphous state, the crystallization of which occurred under high humidity and was followed by atomic force microscopy. Detailed crystallization behaviors, especially the stepwise growth of the FCC with blocks shorter than the chain, cooperative chain slipping in the FCC, the formation of an anisotropic nucleus and its growth, and the formation of a small nucleus with a short lifetime, were clearly visualized at the molecular level. The stepwise growth of the FCC differed from that expected by the classical Lauritzen−Hoffman theory and was consistent with recent reports that indicated the formation of some ordered state in the amorphous phase. We believe this molecular information will improve our understanding of the molecular mechanism of polymer crystallization.

1. INTRODUCTION The folded-chain crystal is the most common crystalline structure of polymer chains and has been extensively studied as one of the main issues in polymer science. 1−6 The crystallization of the folded-chain crystal from long flexible polymer chains that strongly entangle each other is a complex process and still retains unclear, especially at the molecular level. If one could directly observe the crystallization process at the molecular level, our understanding of the crystallization would significantly improve. The invention of scanning probe microscopy, especially atomic force microscopy (AFM), enables us to observe materials at atomic and molecular levels, and various polymer structures have been observed at the molecular level;7−17 however, the crystallization process of folded-chain crystals has never been observed in situ at the molecular level. In a classical way, crystallization processes have been studied in situ by polarized optical microscopy, and the nucleation and the growth of spherulites were evaluated.1−3 Around 2000, the invention of AFM enabled the visualization of not only the overall growth of spherulites but also the growth of individual lamellae inside the spherulites,18−22 showing that the lamella did not grow at a constant rate but grew at various rates changing them with time.21 Depending on the crystallization temperature of the polymer, the observations were done in situ at room temperature or at high temperatures by AFM instruments equipped with a hot stage for heating samples. However, the limited resolution of the AFM instruments prevented visualization of the chain packing inside the growing lamella crystals. © XXXX American Chemical Society

AFM is a type of surface probe microscopy that visualizes materials by scanning their surface with a probe; therefore, the maximum resolution is expected to be attained by the use of atomically or molecularly flat samples. Kumaki and co-workers showed that by using two-dimensional (2D) samples, especially Langmuir−Blodgett (LB) films, various polymer structures could be visualized at the molecular level by tappingmode AFM,14,17 for example, single chains23,24 and their movements,25 crystals26 and their melting process,27 singlechain crystals,28 multistranded helices of a stereocomplex,29,30 and polymer blends31 at the molecular level. They visualized folded-chain crystals of isotactic poly(methyl methacrylate) (itPMMA),26 where a monolayer spread on a water surface was compressed to crystallize, deposited on mica, and then observed by AFM. Chain packing in lamellae, as well as folded-chain structures at their edge and loose and tight tie chains that combine the lamellae, was clearly observed at the molecular level.17,26 Therefore, in principle, if an amorphous itPMMA monolayer is deposited on mica at a low surface pressure and observed by AFM at a high temperature to accelerate crystallization, the crystallization process of the folded-chain crystal could be visualized at the molecular level. However, a large thermal drift caused by heating the sample was unavoidable and completely disturbed the observation under high magnification at the molecular level. To crystallize molecules frozen in an amorphous state, the mobility of the Received: July 5, 2018 Revised: September 7, 2018

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Figure 1. (a) High-resolution time-lapse AFM phase images of a monolayer of it-PMMA(290K) deposited on mica at a surface pressure of 5 mN/ m, subsequently precrystallized under 85% RH for 30 min, and then observed under 80.2 ± 1% RH at 0 and 130 min. (b) Magnified image of crystal F in (a). (c) Schematic representation of a crystal that is composed of a single it-PMMA(290K) chain, where the length of a doublestranded helix composed of an it-PMMA chain (290K) (chain length) was estimated to be 304 nm from the molecular weight, and if it folded with the average lamella thickness (14 nm) derived from (a), the double-stranded helix folded 21.7 times on average. (d) Length of chains contained in each crystal as a function of time. The red dotted lines indicate a crystal containing one, two, and three chains. The movie of (a) (Movie S1 (Figure 1)) is available in the Supporting Information.

torsional-tapping-mode 32,33 and bimodal-tapping-mode AFM;34 however, the crystallization has not been observed in situ at the molecular level.

molecules needs to be accelerated by heating or adding plasticizer. Recently, we found that an amorphous it-PMMA monolayer could be crystallized under high humidity at room temperature in a controlled manner, where a small amount of water absorbed in the monolayer from the high humidity accelerated the mobility of the amorphous chains to be crystallized. Thus, the crystallization could be followed at high magnification without thermal drift. In this paper, we report the crystallization process of a folded-chain crystal of it-PMMA observed under high humidity at the molecular level. Lamella growth, a chain slipping inside the lamella, nucleation and growth, and a small nucleus with a short lifetime were clearly observed by AFM at the molecular level. To the best of our knowledge, this is the first observation of the crystallization of a folded-chain crystal at the molecular level. The high-resolution molecular images of polyethylene crystals formed in melt oriented thin films on substrates were recently visualized by modified tapping-mode AFM, such as

2. RESULTS AND DISCUSSION 2.1. In Situ Real-Time AFM of Crystallization. The surface pressure−area (π−A) isotherm of the it-PMMA (a number-average molecular weight (Mn) of 2.9 × 105, a polydispersity index (Mw/Mn) of 1.13, and a mm content of 98%) showed a plateau corresponding to its crystallization at 8 mN/m (Figure S1 in the Supporting Information). A single amorphous monolayer was deposited on mica at a π from 3 to 5 mN/m in the amorphous region, and the crystallization was observed by AFM under a relative humidity (RH) from 85 to 90% RH (see Figure S2). Both the nucleation and crystal growth rates were increased with increasing deposition π and RH during the observation. At higher π, the monolayer was closer to the crystallization transition, thus thermodynamically B

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chain crystal might be affected by the two-dimensionality of the present sample. 2.2. Typical Crystallization Processes. 2.2.1. Stepwise Growth of Lamella. Some typical crystallization processes seen in Movie S1 (Figure 1) are summarized below. Figure 2

closer to crystallization. Under higher RH during observation, absorbed water plasticized the monolayer to a greater extent, accelerating the mobility of the chains for crystallization. The crystallization was also followed for monolayers precrystallized under high humidity before observation. The π and the RH dependence of crystallization rates are shown in Figure S3. In the precrystallized monolayers, the nucleation rates during the subsequent observation were rather slow, but both the nucleation and crystallization rates showed a similar dependence on the π and the RH (see details in Figure S3). Figure 1 shows in situ AFM phase images of the it-PMMA monolayer deposited at 5 mN/m in the amorphous region, precrystallized at 85% RH for 30 min, and then observed under 80 ± 1% RH for 130 min. Using preformed crystals, we first optimized the scanning conditions for high-resolution imaging at the molecular level and then started the in situ AFM observation of the crystallization process. Figure 1a shows AFM images at 0 and 130 min; a movie constructed from 46 sequential images is available in the Supporting Information (Movie S1 (Figure 1)). The growth of crystals A to D and F and the nucleation and growth of a new crystal E were apparently observed. As shown in a magnified image of crystal F in Figure 1b, the chain packing inside the lamella crystal was clearly observed. Kusanagi, Tadokoro, and Chatani reported that it-PMMA formed a double-stranded-helix crystal in which two 101 helices of it-PMMA chains intertwined to form a double-stranded helix and packed into an orthorhombic unit cell with a = 2.098, b = 1.217, and c (fiber axis) = 1.05 nm.35,36 The helical pitch was 1.05 nm, and the distances between the double-stranded helices were 1.213 and 1.217 in the (110) and (100) planes, respectively, which agreed fairly with the observed values of ca. 1.11 and 1.29 nm in the AFM images, respectively. Based on the crystal model, the length of a double-stranded helix composed of a single intertwined itPMMA(290k) chain is 304 nm (chain length). If we assume the chain folds with the average lamella width in the images (14 nm), then it should fold 21.7 times on average, and the lateral length of the single-chain lamella is 25 nm, as shown in Figure 1c. The scale of the schematic representation of the single-chain crystal is equal to Figure 1a. Interestingly, the sizes of the crystals were comparable to or few times larger than that of the single-chain crystal. As shown in the magnified image presented in Figure 1b, the total chain length contained in the crystal could be evaluated. Figure 1d shows the chain length that was included in each crystal as a function of time. The red dotted lines indicate a crystal composed of one, two, and three chains. The chain length in the crystals did not grow at a constant rate but grew in a stepwise manner, which was apparent for smaller crystals. For a larger crystal A, which corresponded to approximately three chains, the total chain length appeared to grow continuously, but the individual small crystals in crystal A appeared to grow in a stepwise manner (see Movie S1). The chain length of the smaller crystals, especially crystal E, appeared as if after one chain finished crystallization and then another chain added on to it; however, because of the molecular weight distribution of the it-PMMA, we could not derive a conclusive result from the present experiment. In two-dimensional states, chain entanglements are known to be suppressed, and the chains are expected to form rather condensed conformations in comparison with those in three-dimensional states.37 The stepwise growth of the crystals with a roughly comparable size to that of the single-

Figure 2. Magnified time-lapse AFM phase images of crystal B shown in Figure 1. The position of the growing front at a given time is indicated by blue and pink arrows. Crystallization growth stopped at the position indicated by the time period. The crystal did not grow at a constant rate but grew stepwise. The movie of Figure 2 (Movie S2) is available in the Supporting Information.

shows magnified time-lapse AFM images of crystal B in Figure 1a. A movie of Figure 2 is available in the Supporting Information (Movie S2). The positions of the growing fronts of the crystal at a given time are indicated in Figure 2 by blue and pink arrows. As shown, the crystal did not grow at a constant rate but grew in a stepwise manner, with intervals indicated in the images. For example, the crystal grew to the right side but stopped growing during the periods of 0−30, 46−54, 66−75, 83−95, and 101−130 min at the positions indicated by the blue arrows. Similarly, crystal growth stopped C

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water surface to crystallize only the it-PMMA polymer.28 We found that the chain direction in each single-chain crystal was identical throughout the crystal, as indicated even on a water surface (Figure 12 in ref 28); thus, without the substrate effect, the chains crystallized as single crystals. Therefore, some substrate effect may have been present in our observations; however, we may derive key features of the polymer crystallization from our observation. 2.2.2. Chain Slipping in Lamella. Figure 3 shows magnified time-lapse AFM images of crystal C in Figure 1. The size of the

at the left side at 8−17 and 26−130 min (see Movie S2). As shown in Figure 1d, the overall growth of crystal B exhibited a step, where the first crystallization rate slowed down at ∼60 min, and then the second crystallization started. The stepwise growth shown in Figure 2 was smaller than the overall stepwise growth shown in Figure 1d, which corresponded to the single entire chain, and the stepwise growth in Figure 2 indicated that the parts of the chain that were much shorter than the entire chain attached as small blocks. We note that this crystallization behavior attaching parts of the chain as small blocks is apparently different from that expected by the classical Lauritzen−Hoffman (LH) theory, in which crystallization proceeds by attaching an amorphous chain to the surface of the crystal and the amorphous chain changes conformations to an ordered state at the surface.38 This observation indicated that the chains in the amorphous region might form a somewhat ordered structure and attach to the crystal. Miyoshi and coworkers performed a solid-state 13C−13C double-quantum NMR study and revealed the formation of an ordered domain composed of regularly folded chains in the early stage of crystallization, and the folded-chain cluster was deposited on the growing front of the crystal.39,40 Our result is also consistent with the bundle model proposed by Allegra and Meille, where a precrystalline structure composed of a bundle of chains attaches to the crystalline surface.41 Our observation indicates that the crystallization mechanism may require reconsideration. We also note that the shape of the precrystallized crystal at 0 min remained nearly unchanged, and additional crystals seemingly added to it. This indicates that chain slipping inside the crystal did not occur, and the crystallization proceeded from the preformed crystal. During the crystallization, an abrupt shift in the growth of the lamella along the chain direction occurred, resulting in the formation of a clear step in the lamella. This behavior was frequently observed; for example, such a shift occurred between 78 and 83 min in Figure 2. As will be mentioned later, these abrupt shifts were not caused by the tip scanning effect, which was negligible in our experiments (see section 2.3). Although, the crystal had some steps, the chain direction was nearly identical throughout the crystal, indicating the crystal had a single-crystal structure. Crystal A in Figure 1 had a more complicated shape, but it had an identical chain direction throughout the crystal and was recognized as a single crystal. These crystals were formed on mica and therefore might be affected by the substrate. We recognized a tendency for the it-PMMA to crystallize epitaxially with the crystal plane of the mica. An AFM image of a well-crystallized monolayer deposited at 5 mN/m and crystallized under 90% RH for 69 min is shown in Figure S3. The mica surface has a pseudohexagonal two-dimensional lattice;42,43 therefore, itPMMA may epitaxially crystallize along the 3-fold symmetrical axes of the lattice. The crystals presented some tendency to align in the 3-fold symmetrical directions; however, a close examination indicates that the chain directions in the crystals did not precisely align along the 3-folded symmetrical directions, but the directions varied from crystal to crystal. This indicated that the identical chain directions in each crystal were not fully controlled by the interaction between mica and the chains but were dominantly controlled by the chain−chain interaction within each crystal. Previously, we studied the crystallization of single it-PMMA chains, where isolated itPMMA chains were solubilized in a monolayer of an it-PMMA oligomer, and the mixed monolayer was compressed on a

Figure 3. Magnified time-lapse AFM phase images of crystal C shown in Figure 1. The preformed crystal at 49 min (pink dotted line) reduced in size and flew into a new crystal, indicating a cooperative chain slipping occurred in the crystal. The movie of Figure 3 (Movie S3) is available in the Supporting Information.

preformed crystal at 0 min grew at 49 min (red dotted line) and then reduced its size, and the chain appeared to flow into a new crystal underneath at approximately 75−90 min (see Movie S3 (Figure 3) in the Supporting Information). Because the chains were significantly folded in the crystal, the reduction of the crystal size and the flow of the chains were only possible with a significant cooperative slipping of the chains in the crystal, indicating that the chains in the folded-chain crystal were mobile and could slip significantly along their chain length in this crystal. This is in contrast to crystal B in Figure 2, which did not change in shape, and the growth of the new crystal underneath also looked rather smooth in comparison with crystal B. 2.2.3. Nucleation and Growth. Nucleation and growth were observed in crystal E, as shown in Figure 4 (see Movie S4 in the Supporting Information). First, an anisotropic nucleus with a length of ∼13 nm and a width of ∼5 nm appeared at 62 min, and it immediately grew to an alignment of four strands at D

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crystallization. The multistep mechanism leads to a lower crystal Gibbs free energy than that in the classical nucleation theory. The nucleation and growth observed in this study were still fast, and we cannot discuss the details of the primary nucleation process and are not sure whether the disordered structure of the present nucleus observed at 62 min corresponds to the amorphous prenucleus or not. Detailed experiments by slowing down the nucleation process by reducing the humidity may reveal the primary nucleation process. The gradual change of the crystal to become more perfect observed from 70 to 110 min indicates that the crystallization proceeded via a multistep process with a reduction in Gibbs free energy. Strobl proposed a multistep crystallization mechanism where the crystallization proceeds via a mesomorphic phase.47 Polymers first attach to the growth front of the lamella, similar to the mesomorphic phase, in which chains are disordered but somewhat stretched in the direction of the stems of the lamella. Upon proceeding with crystallization, the chains in the mesomorphic phase elongate and align in an ordered state to increase the width and finally form a well-ordered lamella. From this mechanism, a wedge shape of the growing front of the crystal is generally expected. In our study, we did not observe an apparent wedge shape at the front of a growing lamella. Currently, we are not sure whether this indicates that the stabilization from a mesomorphic phase to a crystal in our system is too fast to be observed or not. We just note here that the gradual change of the crystalline shapes indicated a multistep mechanism of the crystallization in our experiments. The size of crystal E was comparable to that of a single-chain crystal, though there is no direct evidence of this. However, if this is a single-chain crystal, then a single chain intertwined to form a double-stranded helix, a part of which folded to form an anisotropic nucleus, grew into a folded-chain crystal, and the chain inside the crystal slipped to form a well ordered foldedchain crystal. A single it-PMMA chain does not need to form a double-stranded helix for the entire chain, but if it can form, then significant chain slipping inside the double-stranded helix also has to occur, as well as the slipping of the double-stranded helix itself. Currently, we have no concrete evidence whether the crystal was composed of a single chain or a self-assembly of parts of several chains, but we feel that the crystal is composed of a single chain, or at most, very few chains, from the crystallization behavior. We are planning to study the molecular-weight dependence of similar crystallization processes to clarify this point. Though we successfully observed the nucleation and growth at the molecular level, the crystallization was still too fast to follow the details of nucleation process and chain folding and the chain slipping inside the crystal; we are trying to observe more details of the crystallization process by reducing the crystallization rate by lowering the humidity and by trying to improve the resolution of AFM. 2.2.4. Small Nucleus with a Short Lifetime. In Movie S1 (Figure 1), we also recognized many small objects with an indefinite shape and a short lifetime repeatedly appearing and disappearing within the amorphous region between the crystals. Typical examples are shown in Figure 5. Figure 5a shows that a small anisotropic nucleus with a width of ca. 4 nm and a length of ca. 8 nm appeared at 87 min upward of crystal F and then immediately disappeared. The contrast of the nucleus was similar to, but the size was smaller than, the nucleus of crystal E (Figure 4). Most likely, this crystalline

Figure 4. Magnified time-lapse AFM phase images of crystal E shown in Figure 1. An anisotropic nucleus formed at 62 min and grew to a 4 times folded chain (64 min) and then a 9−20 times folded chain. The crystal formed at 17 min (yellow dotted line) and then changed its shape to be a more perfect crystal (pink, violet, and green dotted lines). The movie of Figure 4 (Movie S4) is available in the Supporting Information.

64 min and 9 strands at 67 min. With time, it grew into an alignment of 17 strands, and the shape of the crystal gradually changed to be a well-ordered crystal, further slowly increasing the number of strands in it to 20 at 110 min. The anisotropic nucleus that appeared at 62 min did not show a clear crystalline order. Recently, reinvestigations of primary nucleation in various systems (e.g., proteins44 and inorganic systems45,46) have been very popular. These results suggest a multistep mechanism of primary nucleation: the formation of a dense amorphous cluster followed by its E

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Figure 5. (a, b) Magnified time-lapse AFM phase images of the unstable nucleus with a short lifetime (yellow arrows) observed in Figure 1.

nucleus was too small and unstable to grow into a crystal. Figure 5b shows an example of the rather vague objects with a short lifetime, and similar vague objects were frequently seen in Movie S1 (Figure 1). The contrast and the shape were rather vague compared with those of the nucleus in Figure 5a and might be an embryo of a nucleus or a small aggregation of chains. These small objects with a short lifetime shown in the amorphous region indicated that a homogeneous nucleation process occurred in this experiment. 2.3. Negligible Tip Effect on Crystallization. The crystallization was observed using tapping-mode AFM; thus, the sample was always tapped by a tip during the observation. We must consider the possible effect of the tip on the crystallization; however, we think the effect was negligible in our experiment. Figure 6 shows lower magnification images observed before and after the long-time scanning. The yellow squares correspond to the scanning area, and the movie was constructed from the area indicated by the pink square. There was no recognizable difference between crystals formed inside and outside the scanning areathe shapes, density, and sizes of the crystals were nearly sameindicating the scanning effect was negligible in our observation, and the crystallization process observed here was a true crystallization process of the sample.

Figure 6. Lower magnification AFM phase images observed before and after the long-time scanning for Figure 1. The areas for the longtime scanning (yellow square) and from which the movie was constructed (pink square) are shown. There was no recognizable difference between crystals formed inside and outside the scanning area, indicating the scanning effect on the crystallization was negligible.

growth process, and the small nucleus with a short lifetime were observed. The stepwise growth of the crystals differed from that expected by the classical Lauritzen−Hoffman theory, indicating that reconsideration of the molecular mechanism of crystallization is necessary. We believe that the accumulation of experimental evidence by direct observations of the crystallization process at the molecular level will improve our understanding of the molecular mechanism of the crystallization process.

4. EXPERIMENTAL SECTION 4.1. Materials. An it-PMMA with a Mn of 2.9 × 105, a Mw/Mn of 1.13, and an mm of 98% (it-PMMA(290K)) was prepared by the isotactic-specific anionic living polymerization of MMA in toluene at −78 °C with tert-butylmagnesium bromide as an initiator.48 The Mn and Mw/Mn values were measured by size exclusion chromatography (SEC) in chloroform using PMMA standards (Shodex, Tokyo, Japan) for the calibration. The tacticity was determined from the 1H NMR signals of the α-methyl protons. Highly purified chloroform (Infinity Pure, Wako Chemicals, Osaka, Japan) was used as the solvent for the

3. CONCLUDING REMARKS We successfully observed a crystallization process of long, flexible polymer chains at the molecular level using a twodimensional film suitable for observation by AFM. Various crystallization behaviors such as the stepwise growth of the crystals, chain slipping inside the crystal, the nucleation and F

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18H02025). We also appreciate the financial support from Toray Research Center, Inc.

spreading solutions without further purification. Water was purified by a Milli-Q system and used as the subphase for the LB investigations. Mica was purchased from Okenshoji Co., Ltd. (Tokyo, Japan). 4.2. π−A Isotherm Measurement and LB Film Preparation for AFM. The π−A isotherms were measured as follows. An itPMMA solution in benzene having a concentration of ca. 4.0 × 10−4 was spread on a water surface at 23 °C in a commercial LB trough with an area of 60 × 15 cm2 and an effective moving barrier length of 15 cm (FSD-300AS, USI, Fukuoka, Japan). The surface pressure was measured using filter paper as the Wilhelmy plate. The π−A isotherms were measured at a constant compression rate with a moving barrier speed of 0.5 mm/s. For AFM observations, an it-PMMA monolayer was compressed at a moving barrier speed of 0.5 mm/s and then deposited on a piece of freshly cleaved mica by pulling it out of the water at a rate of 4.2 mm/min while compressing the monolayer at the prescribed pressure (the vertical dipping method). 4.3. In Situ AFM Observations of Crystallization under High Humidity. The crystallization of the monolayers deposited on a mica substrate was observed by a commercial AFM (NanoScope IIIa or IIId/MutiMode AFM unit, Bruker AXS, Santa Barbara, CA) with standard silicon cantilevers (NCH, Bruker AXS) at 28 ± 1 °C in tapping mode with the AFM unit being enclosed in an acrylic desiccator, the humidity in which was controlled by a humidity/ temperature-controlled air supplier (PAP01B-KJ, Orion Machinery Co., Ltd., Nagano, Japan). The typical settings of the AFM observations were as follows: an amplitude of 1.0−1.5 V, a set point of 0.68−0.92 V, and a scan rate from 2.0 to 3.1 Hz. The AFM images obtained are presented without any image processing except flattening. The movies were constructed from sequential AFM images canceling the drifts of the images manually by ImageJ, a public domain software from the National Institutes of Health, and further registered using the StackReg plug-in of ImageJ by Philippe Thévenaz to eliminate the small residual drift effect.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01428. π−A isotherm; crystallization behavior of monolayers deposited at various surface pressures and observed under various humidity values; list of movies (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI) Movie S4 (AVI) Movie S5 (AVI) Movie S6 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.K.). ORCID

Jiro Kumaki: 0000-0001-9552-3303 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate Associate Professor Takehiro Kawauchi, Ryukoku University, for the polymerization of the it-PMMA. We also thank an anonymous reviewer of the manuscript who provided valuable comments and suggestions for the crystallization mechanism. This work was supported by JSPS KAKENHI (Grants 25107706, 15H03861, 17K19147, and G

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Macromolecules

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DOI: 10.1021/acs.macromol.8b01428 Macromolecules XXXX, XXX, XXX−XXX