Single-Molecule Force Spectroscopy Study on Force-Induced Melting

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Single-Molecule Force Spectroscopy Study on Force-Induced Melting in Polymer Single Crystals: The Chain Conformation Matters Yu Song,†,‡ Ziwen Ma,† Peng Yang,† Xiaoye Zhang,† Xiujuan Lyu,† Ke Jiang,† and Wenke Zhang*,† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, and ‡Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China

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S Supporting Information *

ABSTRACT: Understanding the conformation effect on forceinduced melting is important for developing advanced semicrystalline polymer materials. Here, two types of polymer single crystals, polycaprolactone (zigzag conformation) and poly(Llactic acid) (helical conformation), have been selected to study the conformation effect on force-induced melting by using the atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS). We find that the zigzag chains facilitate the stick−slip motion, and the single helical chain takes smooth motion during force-induced melting from the single crystals. Furthermore, we illustrate that the conformation acts on the force-induced melting by defining the interaction in between the adjacent stems. This SMFS study deepens our understanding on the relationship between the chain conformation and nanomechanical properties of polymer crystals.

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and folding,34−37 have been investigated by SMFS based on optical tweezers, magnetic tweezers, or atomic force microscopy (AFM). The AFM-based SMFS can work in a variety of environments, such as aqueous solution,38−42 organic solvent,43−46 and even in air47 and vacuum,48 making it a powerful technique for investigating the molecular behaviors of synthetic polymer systems. Furthermore, our previously established method for nanomechanical manipulation on polymer single crystals can be applied to study the force-induced melting behaviors experimentally.49,50 In this study, to test the effect of chain conformation on the nanomechanical properties of a polymer single crystal, we used the AFM-based SMFS to stretch the single chains from polycaprolactone (PCL) and poly(L-lactic acid) (PLLA) single crystals (Figure S1). As shown in Figure 1, these polymer chains can form nonplanar zigzag (PCL) and helical (PLLA) conformations within the single crystals. We demonstrate the conformation effect on force-induced melting through comparing the chain mobility in these two kinds of single crystals. Moreover, the molecular mechanisms of the conformation effect are revealed by analyzing the interaction in between the adjacent stems. Our results show that the zigzag chains facilitate the stick−slip motion, and the helical chain takes smooth helical motion during force-induced melting from the single crystals. This SMFS study deepens our understanding of the relationship between chain conformation and nanomechanical properties of polymer crystals.

INTRODUCTION The force-induced melting is critical for the drawability and toughness of semicrystalline polymer materials.1−5 The forced melting process occurs at the large deformation that is governed by the mobility of the crystalline chain.6−9 Usually, in the crystalline region of polymer materials, the closely packed polymer chains form a zigzag or helical conformation.10−12 Understanding the conformation effect on force-induced melting is therefore essential.13 The relevant knowledge is important for developing advanced polymer materials, such as polymer fiber with tunable mechanical properties and highly stretchable semiconductor films in wearable electronics.14−17 However, to establish the direct relationship between chain conformation and nanomechanical properties is quite challenging because it is difficult to distinguish the force-induced melting from other deformation behaviors including crystalline lamellae slip and recrystallization in bulk materials.5,18−21 This issue can be resolved by stretching individual polymer chains of different conformations from the single crystal. Although theoretical simulations have been used to predict the effect of chain conformation on the nanomechanical properties of polymer single crystals,22 there is no direct experimental evidence to prove the simulations due to the limitation of traditional detection method and difficulty in sample preparation. Single-molecule force spectroscopy offers a possibility to stretch a single polymer chain and measure its mechanics.23−28 The single-molecule mechanics can provide the time evolution of molecular structures while measuring the length of the unfolded chain with subnanometer precision.29−32 Many molecular behaviors, such as single polymer growth,33 mechanical task of molecular motor,24 and protein unfolding © XXXX American Chemical Society

Received: December 19, 2018 Revised: January 17, 2019

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DOI: 10.1021/acs.macromol.8b02702 Macromolecules XXXX, XXX, XXX−XXX

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nonplanar zigzag main chain stabilize the crystal structure by favorable electrostatic dipolar interaction in the adjacent stems (Figure 1A).51 We prepared the PCL single crystals from dilute solution and immobilized them onto the amino-modified silicon substrate. Then, the AFM tip was forced to contact with the single crystal surface, which can produce strong molecule attachment via physical adsorption.52−54 Eventually, the PCL molecule could be stretched out of the crystal after tip−sample separation (Figure 2A). Stretching PCL from single crystal results in the force−extension (F−E) curves shown in Figure 2B. The F−E curves are characterized by two types of sawtooth peaks (the big and small ones), and the small sawtooth peaks reside on each of the big peaks. Fitting the wormlike chain (WLC) model55 of polymer elasticity to consecutive big peaks shows contour length increments ΔLc of 18.3 ± 2.2 nm (Figure 2C). This value is in good agreement with the contour length of one complete fold containing two stems in the 9.1 nm thick single crystal, thus corroborating that each big peak corresponds to unfolding of one fold from single crystal. Closer inspection of the small sawtooth peaks that reside on the big one uncovers sequential multiple unfolding (Figure 3A). The statistical analysis of the distances Δlc between the small sawtooth peaks shows two populations located at approximately 0.86 ± 0.18 and 1.73 ± 0.16 nm (Figure 3B). These two values are close to the length of one and two repeat units of PCL, respectively. Therefore, each small peak results from the slipping of one or two repeat units of PCL chain from single crystal, whereas the last small peak corresponds to the catastrophic fracture of the remaining chain in the fold. Proper identification of chain mobility in the crystalline region during these sequential slipping events is essential to decipher the molecular mechanisms of the force-induced melting. Figure 3C shows the interpretation in one slipping event corresponding to stages 1−3 in Figure 3A. The progressive stretching of the free chain (i.e., unfolded chain) leads to the force accumulation, giving a characteristic parabolic curve (stage 1 in Figure 3A). When the force exceeds the strength of the electrostatic dipolar interactions in between adjacent stems, these interactions are broken (stage 2). The electrostatic dipolar interactions will reform either by rotating and forming an opposite side-chain orientation by slipping a repeat unit of PCL chain (from a to b1 in Figure 3D) or by preserving the side-chain orientation and slipping two repeat units of PCL chain (from a to b2 in Figure 3D). After slipping, the length of free chain increases, leading to the force release (stage 3 in Figure 3A). During the sequential

Figure 1. AFM images and crystal unit structure of (A) PCL and (B) PLLA. The middle row has been viewed from the X−Y plane, and the bottom row is the projection viewed along the y-axis. The polymer chains exist as two types of conformations, including nonplanar zigzag (PCL) and helical (PLLA) conformations. Color key: aqua, carbon atoms; red, oxygen atoms.

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RESULTS AND DISCUSSION Force-Induced Melting of a Single PCL Chain from the Single Crystal. The PCL single crystal was first selected to study the force-induced melting. In the PCL single crystal, the polar groups (CO) that are arranged on the outside of the

Figure 2. SMFS experiments on PCL single crystals. (A) Schematic illustration of the force-induced unfolding of single PCL chain from the single crystal. (B) Typical F−E curves obtained from PCL single crystals. For the sake of clarity, all curves were separated by 0.75 nN along the y-axis. Wormlike chain (WLC) fit curves (red dashed lines) allow determining the contour length increments ΔLc of the big peaks. (C) Histogram and Gaussian fit (black curve) of ΔLc. B

DOI: 10.1021/acs.macromol.8b02702 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Stick−slip motion in PCL single crystal. (A) The enlarged region at the F−E curve surrounded by dashed circle in Figure 2B. WLC fits (red dashed lines) allow determining the distances Δlc in-between the small sawtooth peaks. (B) Histogram and Gaussian fits (black curve) of Δlc. (C) Schematic of the unfolding event corresponding to stages 1−3 in (A): (1) progressive stretching of the free chain; (2) the breakage of electrostatic dipolar interactions in between adjacent stems; (3) after the breakage, one or two repeating units of PCL chain are slipping out of the crystalline region, leading to the length increase and relaxation of the free chain. (D) Schematic of the movement of a PCL repeat unit (bright chain) in the crystalline region. Each dim chain with three repeat units shows the structure of PCL chain before slipping. From a to b1, the PCL chain slips a repeat unit by rotating and forming an opposite side-chain orientation. From a to b2, the PCL chain slips two repeat units by preserving the side-chain orientation.

Figure 4. Effect of free-chain length (in between the AFM tip and the crystal surface) on the distance, ΔLs, of stick−slip motion. (A) Measurement of ΔLs on a big peak by WLC fit. (B) Plot of ΔLs versus the sequence number of big peaks. The inset shows the definition of sequence number of big peaks.

unfolding, the force changes and chain motion in the crystalline region will repeat in the same fashion. Consequently, the small sawtooth peaks reveal the stick−slip motion of the PCL chain in the force-induced melting process. This result is in good agreement with the molecular dynamics (MD) simulations by Sinan et al. on the silk,14 where they found the stick−slip motion in pulling out the polypeptide chain from β-sheet nanocrystals. At the same time, the peak forces decrease with the increase of the extension as the sequential slipping shortens the PCL stems (Figures 2B and 3C and Figure S2). This phenomenon further suggests the mechanical strength of the crystalline PCL chain is dominated by the electrostatic dipolar interaction in between the adjacent stems. The existence of the stick−slip motion in the force-induced melting is a result of the strong interaction in between the nonplanar zigzag stems.

Because of the decrease of mechanical stability within PCL stems during chain slipping, the stick−slip motion fails catastrophically once the residual force (i.e., the force after chain slip) in the external components (including the free chain and cantilever) exceeds the mechanical strength of the stems. Because the length of the free chain influences force releasing by defining its elasticity,56,57 it is thus expected to have significant effect on the failure of stick−slip motion. To experimentally test this hypothesis, we analyzed the distance of stick−slip motion, ΔLs, on each big peak (Figure 4A), in which the free chain length increases while the free chain stiffness decreases with the increase of the extension length. Statistical analysis shows that the distance of stick−slip motion decreases with the increase of the sequence number of big peak (Figure 4B). This analysis proves that longer/softer free chain leads to the failure of stick− C

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Figure 5. SMFS experiments on PLLA single crystal. (A) Typical curves obtained from PLLA single crystal. ΔLc is the distance between the adjacent regular plateau-like peaks. (B) Histogram and Gaussian fit (black curve) of ΔLc. (C) Schematic of the helical movement of seven PLLA repeat units (bright chain). Each dim chain with 11 repeat units shows the structure of PLLA chain before slipping. (D) The enlarged region at the F−E curve surrounded by dashed circle in (A). ΔLs is the distance of smooth motion. (E) Plot of ΔLs versus the sequence number of plateau-like peaks.

relatively small, meaning that the PLLA chain moves smoothly within the crystalline region (Figure 5C). In the PLLA single crystal, the polar groups (CO) are arranged inside the PLLA helix and the nonpolar groups (−CH3) are arranged outside the PLLA helix (Figure 1B). These structural features result in the weak interaction in between the adjacent helix stems, which explains the smooth motion of PLLA chain in the force-induced melting. In addition, the distance ΔLs of this smooth motion are similar in all plateau-like peaks (Figure 5D,E). This phenomenon proves that the distance of smooth motion, as well as the total dissipated energy during the force-induced melting, is independent of the elasticity of external component such as the free chain. Conformation Effect on the Force-Induced Melting. The series of SMFS experiments enables us to reveal the conformation effect on the force-induced melting in polymer single crystals. The nonplanar zigzag PCL chain facilitates the stick−slip motion during the force-induced melting (green curve in Figure 6) and the distance of stick−slip motion is enhanced by the stiffer external component (e.g., shorter free chain; see Figure 4 and Figure S4). Moreover, our recent experiments demonstrated that the PA 66 and PA 6 chains with planar zigzag conformation take the stick−slip motion during force-induced melting from the single crystals.60 The hydrogen bonds break and re-form by slipping one repeat unit. The overall trend of chain mobility obtained from the PCL system is similar to that in PA66 and PA 6 system (red and blue curves in Figure 6). Therefore, polymer chains with the zigzag conformations including both nonplanar zigzag and planar zigzag conformations perform the stick−slip chain motion during the forceinduced melting. In contrast, for the helical PLLA chain, the force fluctuation is relatively small and smooth motion dominates the force-induced melting (pink curve in Figure 6). The distance of smooth motion is independent of the elasticity of the external component (Figure 5E). This mobility feature is

slip motion at much shorter distance because of the weaker force-release capacity, which significantly decreases the total dissipated energy and increases the residual force during the force-induced melting. Force-Induced Melting of a Single PLLA Chain from the Single Crystal. From the results above, we prove stick−slip motion of the nonplanar zigzag chain in force-induced melting. What will happen on a polymer chain with helical conformation? To address this issue, we performed SMFS experiments on the PLLA single crystal, in which the polymer chain adopts a 107 helical conformation.58,59 Figure 5A shows the typical F−E curves of stretching single PLLA chains from the single crystals. Comparing with the F−E curves of PCL, the unfolding of PLLA chain from single crystals results in the plateau-like peaks. The distance between adjacent major peaks, ΔLc, is around 20.5 ± 2.8 nm (Figure 5B), corresponding to the complete unfolding of one fold from the 9.7 nm thick single crystals. In the F−E curves, the short-range force fluctuation can be observed abundantly in the first two force plateaus but rarely in the rest of force plateaus (Figure 5A and Figure S3). In the case of the first two force plateaus, the shorter free chain provides higher detection sensitivity to the force change (Figure S4), which leads to the abundant force fluctuations on the plateau. In contrast, the longer free chain that emerges at rest of force plateaus provides lower detection sensitivity to the melting force. Most of force-fluctuation events are buried in the thermal noise, which leads to the rare force fluctuation on the plateau. We attribute such fluctuations to the change of free energy, which originate from the relative movement of helical PLLA chain in the crystalline region. The previous steered molecular dynamics (SMD) simulations on the poly(ethylene oxide) (PEO) single crystal revealed that rotating movement of helical segment (including the PEO loop) within the crystalline region during stretching results in the small force fluctuation.22 Indeed, the short-range force fluctuation in all PLLA force plateaus is D

DOI: 10.1021/acs.macromol.8b02702 Macromolecules XXXX, XXX, XXX−XXX

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manipulation can directly characterize molecular behavior that are otherwise masked in ensemble measurements. Therefore, our method is in principle applicable to a broad range of singlemolecule mechanical behaviors in polymer crystals.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials and Instruments. Poly(caprolactone) (Mn = 80000) was purchased from Polymer Source Inc. Poly(L-lactide) (Mw = 85000−160000), 1,4-butanediol, and amyl acetate were purchased from Aldrich. 3-Aminopropyldimethylmethoxysilane was purchased from Fluorochem, UK. All materials were used as received. All AFM and AFM-based SMFS experiments were performed on a NanoWizardII BioAFM (JPK instrument AG, Berlin, Germany). Preparation of Solution-Grown Crystals. PCL and PLLA single crystals were prepared from solution by the self-seeding method. The polymer (0.8 mg for PCL; 0.6 mg for PLLA) was put in 10 g of amyl acetate at the dissolving temperature (60 °C for PCL; 130 °C for PLLA) for 10 min. After dissolution, the hot solution was quickly transferred to another bath at 4 °C (for PCL) or 40 °C (for PLLA) for 10 h. Then the cold solution was heated to the seeding temperature (29.5 °C for PCL; 125 °C for PLLA) with a heating rate of 10 °C/h and kept for at least 10 min to form stabilized nucleus. Subsequently, the solution was brought to crystallization temperature (23 °C for PCL; 75 °C for PLLA). Crystallization was conducted in the bath for 10 h (PCL) or 6 h (PLLA), respectively. Substrate Modification and Sample Fixation. Silicon wafers were treated by freshly prepared piranha solution (H2SO4/H2O2, 7:3) for 3 h and then were rinsed thoroughly with deionized water. After drying in an oven (around 80 °C), the silicon wafers were put into a desiccator containing 50 μL of 3-aminopropyldimethylmethoxysilane. The desiccator was sealed and placed 2 h at room temperature. Then the wafers were cleaned by methyl alcohol and dried in an oven (around 115 °C) for 10 min. After cooling sufficiently, several drops of the crystal suspensions were deposited onto the silanized silicon wafers (PCL and PLLA) for 30 min. Next the samples were dried under a gentle flow of nitrogen gas. To remove the noncrystallized molecules, corresponding solvents were used to rinse the silicon wafers. AFM Imaging and Single-Molecule Force Spectroscopy. For air-phase AFM imaging, silicon cantilevers from BRUKER (RTESPA300, 300 kHz, 40 N/m) were used. For AFM imaging and SMFS experiments in liquid, bare Si3N4 AFM tips (MLCT, BRUKER) were used. The spring constants of the cantilevers were measured using the thermal noise method.61 For random picking and stretching experiment, single crystals fixed on the wafer were imaged by intermittent contact mode in liquid at first. Then a specific single crystal was selected and zoomed in. Next the working mode was switched to contact mode to perform force spectroscopy measurements. During the experiment, a grid was set on the surface of the single crystal, and the tip was allowed to do “approach−contact−retract” movement at each point of the grid using the constant speed mode. The pulling speeds were set as 0.5 μm/s for PLLA, PCL, PA 66, and PA 6 and as 0.02 μm/s for PEO. To avoid the influence of nonspecific tip−sample adhesion and the end of polymer chain on the result, the first (at short extension, ≤10 nm) major peaks were excluded for force and length analysis, and the last major peaks were excluded for force analysis. The probability for successful grabbing a single polymer chain from the single crystal was around 0.4%.

Figure 6. Comparison of unfolding patterns obtained from PA 66 (red), PA 6 (blue), PCL (green), PLLA (pink), and PEO (sky blue) single crystals. The inset shows the unfolding pattern of PEO with enlarged view along the Y-axis.

similar to that in PEO single crystals, in which the polymer chains also take the helical conformation (sky blue curve in Figure 6). These results prove that it is the chain conformation in the polymer single crystal that dominates the mobility difference of a polymer chain during the force-induced melting. The big difference in the absolute melting force between PA 66 (PA 6) and PCL comes from the different chemical composition. For PA 66 and PA 6, the multiple hydrogen bonds (formed between the N−H and CO groups) between the adjacent stems contributed to the higher melting force. While for PCL, there is no very strong hydrogen bond but some dipolar interactions between adjacent stems. Although the absolute melting forces are different, the interactions between adjacent stems produce the similar chain motion and force pattern for PA and PCL. Whereas, for helical polymer chains (such as PEO or PLLA), the structures of the polar groups (C O and −O−) are arranged inside the helix and the nonpolar groups (−CH3 and −CH2−) are arranged outside the helix, resulting in the weak interaction in between the adjacent stems (Figure 1B). Thus, the zigzag and helical conformations act on force-induced melting in different ways. The conformation effect revealed here presents a general law of chain mobility during the force-induced melting in polymer single crystal.

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CONCLUSION In summary, we have presented an efficient single-molecule method for investigating chain behaviors such as stick−slip motion and smooth motion during force-induced melting. Using this method, we have found distinct effect of polymer conformation on the chain motion and nanomechanical properties of polymer single crystals. The zigzag chains facilitate the stick−slip motion producing larger force fluctuations, while the helical chain takes smooth motion with smaller force fluctuations during force-induced melting from the single crystals. The conformation effect on chain mobility revealed here provides deep insights into molecular mechanism in forceinduced melting. Our findings are of fundamental importance to understanding the mechanical properties of a broader class of helical-chain-rich or zigzag-chain-rich semicrystalline polymer material and provide new concepts in the design of crystalline polymer materials. In addition, the single crystal provides welldefined chain arrangement and loading direction, and the SMFS

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02702. Figure S1: AFM images, height profiles, and histograms of (A) PCL and (B) PLLA single crystals; Figure S2: the relationship between small peak force and its sequence; Figure S3: the comparison of force fluctuation in plateauE

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like peaks; Figure S4: effect of free chain length on detection sensitivity (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Z.). ORCID

Wenke Zhang: 0000-0002-4569-6035 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by National Natural Science Foundation of China (21525418, 21474041, 91127031, and 21504030), the National Basic Research Program (2013CB834503), and the Program for New Century Excellent Talents in University (NCET-11-0205).

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DOI: 10.1021/acs.macromol.8b02702 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02702 Macromolecules XXXX, XXX, XXX−XXX