Microstructure and Microtribology of Polymer Surfaces - American

Chapter 20

Scanning Force Microscopic Study of Polyethylene Single Crystals Prepared by a Self-Seeding Method

Downloaded via UNIV OF TEXAS AT EL PASO on November 1, 2018 at 08:03:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Tomokazu Fujii, Atsushi Takahara, and Tisato Kajiyama Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

The surface structures of high-density polyethylene (HDPE) single crystals with weight average molecular weight, Mw=32k, 120k and 520k prepared by a self-seeding method were investigated by atomic force microscopy (AFM) and lateral force microscopy (LFM). A characteristic lozenge shape lamellar crystal of ca. 12 nm thick was observed with AFM. The contrast of lateral force of the HDPE single crystal in LFM images was observed between adjacent sectors, which means that the chain folding directions were different. The magnitude of lateral force in LFM measurement increased with scanning angle, Θ, which was defined as the angle between scanning direction of the cantilever and {110}, that is the growth face of the HDPE single crystal. These results indicate that the surfaces of the HDPE single crystals prepared by a self-seeding method exhibit regular sharp chain folding along {110} at the surface. Furthermore, it was suggested that the folding structure of the HDPE single crystal surface might depend on the crystallization methods, such as isothermal crystallization or self-seeding one.

Since polyethylene (PE) single crystal prepared by isothermal crystallization method from a dilute xylene solution was discovered by Keller et al. (i), several chain folding models on the PE single crystal surface have been proposed (2-4). As the thickness of lamellar crystal is much shorter than the length of a PE chain, a PE molecular chain should fold at the lamellar surface with an appropriate conformation. However, since direct experimental evidence on the chain folding structure have not been found out yet, various possibilities on the chain folding structure have been proposed for the past three decades (3,4). It is reasonable to consider that the chain folding structure of the single crystal strongly depends on the crystallization temperature, the cooling rate, the molecular weight and its distribution (4). 1

Corresponding author.

336

© 2000 American Chemical Society

Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

337 Scanning force microscopy (SFM) is one of the new scanning probe microscopic (SPM) techniques for investigating the surface topology or the surface properties of solid materials with spatial resolution on a molecular level. SFM can image the surface morphology on the basis of the various types of forces acting between sharp microcantilever tip and sample surface, such as van der Waals, electrostatic, frictional, and magnetic forces. Moreover, it has been proved that the molecular resolution image of lipid monolayer can be observed by atomic force microscopy (AFM) (5) and lateral force microscopy (LFM) (6). The molecular resolution image can be observed because the stick-slip motions of the cantilever and tip occur on an molecular scale in LFM measurement. The direct observation of chain folding on the PE single crystal surface was attempted by using AFM (7). However, since the molecular motion of the chain folding part might be active at room temperature (293 K) that is higher than the glass transition temperature of the PE, the direct observation of the chain folding part at room temperature is very difficult by using AFM or LFM. Anisotropic LFM measurement can reveal the properties of sample (8). It has been indicated that the LFM can distinguish the different molecular tilt angle of the thiolipid monolayer based on friction anisotropic and asymmetric measurements using frictional loop (9). Anisotropic lateral forces on the single crystal surface of polyoxymethylene and PE have been revealed by LFM (10-12). In our previous report (12), the LFM measurement of the isothermally crystallized fractionated highdensity polyethylene (HDPE) single crystals with weight average molecular weight, Mw=10k and 45k revealed that the lateral force on the single crystal surface was dependent on the scanning direction against the {110} growth face. On the other hand, the lateral force was independent of the scanning direction for the HDPE single crystal surface of unfractionated HDPE with Mw=520k. These LFM results indicate the sharp regular fold for the fractionated HDPE single crystals with Mw=10k and 45k, whereas the loose loop with randomly reentry chain folding for the unfractionated HDPE single crystal with Mw=520k. In this study, the AFM observation and the LFM measurement were carried out for the HDPE single crystals prepared by a self-seeding method (13) in order to discuss the effect of crystallization conditions on the chain folding structure of the HDPE single crystals. Experimental Method. Sample Preparation. HDPEs used were unfractionated HDPE (Marlex 9, Mw=520k) with a broad molecular weight distribution and fractionated HDPEs (Mw=32k and 120k) with a narrow molecular weight distribution. The HDPE single crystals were prepared by a self-seeding technique (13). Table I shows the crystallization conditions used in this study. The HDPEs (Mw=32k, 120k and 520k) were dissolved in p-xylene at T , Κ to obtain a 0.01 wt% solution. The solutions were kept at the first crystallization temperature, T Κ for 30 min. Then, the solution was heated again to T^ Κ and the solutions were kept again for crystallization at the second crystallization temperature, Τ^ Κ for 48 hrs. Finally, the solution was cooled to 293 K. A small amount of solution with the suspended HDPE single crystals was dropped onto a cleaned silicon wafer substrate and allowed to air-dry at 293 Κ for the AFM observation and the LFM measurement. d

cl


338 AFM observation and LFM measurement. The AFM and the LFM images of the HDPE single crystals were obtained with SPA300 (Seiko Instruments Industries, Co. Ltd., Japan). The cantilever used in this study was a rectangular-shaped one with a quadrangular pyramid of silicon nitride (Si N ) microtip (Olympus, Co. Ltd., Japan). The bending spring constant of the cantilever was 0.09 N m" . In the AFM observations and the LFM measurements, the distance between tip and sample surface was kept constant by a feedback loop, moving the sample surface (the sample position) up and down with the piezoelectric scanner (topography mode). The images were taken with 20 μπι χ 20 μπι scanner. The AFM observations and the LFM measurements were carried out under a reference force of -0.022 nN (repulsive force) at 298 Κ in air. Figure 1 shows the definition of the scanning angle, Θ, in the LFM measurement. In the LFM measurement, the magnitude of anisotropic lateral force was measured by changing the scanning direction of cantilever from 0° (the scanning direction is parallel to the {110} growth face) to 90° (the scanning direction is perpendicular to the {110} growth face) against the {110} growth face of the HDPE single crystal. Since the scanning direction was perpendicular to the long axis of the cantilever, the degree of cantilever torsion reflected the magnitude of lateral force between microtip and sample surface. The magnitude of lateral force was determined based on the measurement under bidirectional scanning. 3

4

1

Results and Discussion. AFM observation and LFM measurement. Figures 2, 3 and 4 show the AFM topographical (a), the LFM (b) images for the Mw=32k, 120k and 520k HDPE single crystal prepared by a self-seeding method and the schematic representation of sharp chain folding on the HDPE single crystal surface and the scanning direction of cantilever in LFM measurement. In the cases of AFM observation and the LFM measurement shown in Figures 2, 3 and 4, the scanning direction of the cantilever was from left to right and the images were captured during scanning without any filter treatments. Characteristic lozenge shaped lamellar crystals with ca. 12 nm thick were observed for any molecular weight used in this study. The thickness and dimension of a single crystal agreed well with those obtained from small-angle X-ray scattering measurement and transmission electron microscopic observation (4), respectively. The LFM images showed a different contrast corresponding to the different lateral force between adjacent sectors, when the scanning direction was parallel to the {110} growth face. This means that the fold domain boundaries are clearly distinguishable on the basis of LFM measurement. Since a disordered fold surface composed of random irregular chain folds or physically adsorbed polymer chains in a random coil state on single crystal surface would not exhibit such a lateral force anisotropy, it seems reasonable to consider that the regular chain folding is formed on the single crystal surface, even in the case that the unfractionated HDPE single crystal with high Mw of 520k was prepared by the self-seeding method. This conclusion is quite different from that the switchboard type chain folding was formed for the single crystal of the same HDPE prepared by an isothermal crystallization (72). Since the self-seeding technique means that higher molecular weight components crystallize in a solution and lower ones dissolve in solvent, the fractionation based on molecular weights proceeds during a crystallization process. Therefore, it seems reasonable to consider that thefractionationof molecular weights in the self-seeding process affects the surface structure of the HDPE single crystal. Also, it should be noticed that in the case that the diagonal axis of the single crystal Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

339

Table I.

Crystallization conditions of HDPE single crystals in self-seeding method. Mw

Mw/Mn

T /K

T /K

T /K

T /K

32k 120k 520k

Lll 1.19 large

411 411 411

339 338 338

368 370 374

343 343 343

DL

CL

D2

C2

Figure 1. Schematic representation of the definition of the scanning angle, Θ, in the lateral force measurement.



Figure 2. A F M (a), L F M (b) images and relationship between scanning direction of cantilever and chain folding direction (c) of single crystals of the fractionated HDPE (Mw=32k) single crystals.


Figure 3. A F M (a), L F M (b) images and relationship between scanning direction of cantilever and chain folding direction (c) of single crystals of thefiactionatedHDPE (Mw=120k) single crystals.


Figure 4. A F M (a), LFM (b) images and relationship between scanning direction of cantilever and chain folding direction (c) of single crystals of the unfractionated HDPE (Mw=520k) single crystal.

Ni

343 was parallel to the scanning direction, the HDPE single crystal did not show any difference in the magnitude of lateral force between adjacent domains as shown by the upper single crystal in Figure 4 (b). This indicates that even though the chain folding direction, that is, the crystal growth surface, is different between adjacent sectors, the angle between cantilever and direction of chain folding is the same. Then, these LFM results apparently indicate the sharp regular fold along the {110} growth face is formed on the HDPE single crystal surface prepared by the selfseeding method as shown in Figures 2 (c), 3 (c) and 4 (c). Scanning angle dependence of lateral force. The scanning angle, Θ, dependence of lateral force was measured in order to investigate the regularity of chain folding on the HDPE single crystal surface. If the chain folding was formed predominantly along the {110} growth face of the single crystal, the θ dependence of lateral force should be observed due to the existence of directionally ordered or arrayed "loops" as shown in Figures 2 (c), 3 (c) and 4 (c). When the scanning direction of the tip is along the sharp fold loops, in other words, is parallel to the growing edge of the crystal, the LFM tip might experience lower lateral force. On the other hand, the tip might experience higher lateral force if the tip bumps into the fold loops. Figure 5 shows the schematic representation of lateral force curve upon bidirectional scanning in lateral force measurement. The bidirectional lateral force measurement can distinguish the torsion of cantilever due to lateral force of the HDPE single crystal surface from that due to topographical origin. The quantitative evaluation can be performed based on the average of magnitudes of lateral force in both scanning directions. Figure 6 shows the scanning angle dependence of lateral force for the single crystal of unfractionated HDPE of Mw=520k and fractionated HDPEs of Mw=120k and 32k. The magnitude of lateral force increased gradually with the scanning angle, Θ, from 0° to 90° for all cases, in other words, the minimum and maximum lateral forces were observed at 9=0° and 0=90°, respectively. This apparently indicates that the sharp chain folds were formed parallel to the {110} growth face as described above. Figure 7 shows the adjacent reentry and regular sharp fold model (a) and the random reentry and loose loop fold model (switchboard model) with {110} (b). It is obvious that in the case that the chains on the HDPE single crystal surface form the adjacent reentry and regular sharp fold (Figure 7 (a)), the tip of cantilever should experience the different magnitude of lateral force depending on the θ values. On the other hand, when the surface structure of the HDPE single crystal corresponds to the switchboard like fold (Figure 7 (b)), the tip of cantilever might feel the same lateral force without any θ dependence. Therefore, it can be concluded from Figure 6 that the surface structure of the HDPE single crystal prepared by a self-seeding method is composed of the adjacent reentry and regular sharp fold. We have reported that in the case of the single crystal of unfractionated HDPE of Mw=520k prepared by an isothermal crystallization technique, the magnitude of lateral force was independent of θ (12). Therefore, it was concluded that the regular direction of fold loops was not present on the single crystal surface of unfractionated HDPE with Mw=520k prepared by an isothermal crystallization method but the switchboard like fold was formed as shown in schematically Figure 7 (b). It seems reasonable to consider that the adjacent reentry and regular sharp fold structure of the HDPE single crystal is formed due to the fractionation of molecular weights based on Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

344 from left to right .sliding

8

Lateral force

Î

2°

.3

Reverse torsion

Lateral force

Traveling distance of cantilever Figure 5. Schematic representation of lateral force curve with bidirectional scannings.

l.oMw=520k 2.oMw=120k 3.Δ Mw=32k

30

90

60

Scanning angle, θ / degree Figure 6. Scanning angle dependence of lateral force for the HDPE single crystal with Mw of 520k, 120k and 32k prepared by self-seeding method.

(a)

(b)

fa {110}

{110}

Figure 7. Adjacent reentry and regular sharp fold model (a) and random reentry fold model (switchboard model) (b) at the surface of HDPE single crystal.


345 their different clouding points during a self-seeding process. These results suggest that surface structures of the HDPE single crystal depend on the crystallization method. The origin of lateral force has not been completely understood yet because the lateral force reflects many properties of sample surface. The results of LFM measurement do not always have intuitive explanation, the counterintuitive results in the asymmetric LFM measurement have been obtained in the system of thiolipid monolayer (8). In the case of PE single crystal, the molecular tilt direction was equivalent in four sectors of one HDPE lamellar single crystal (4). However, the folding direction of HDPE chain on the surface in adjacent sectors of HDPE single crystal seems to be different. Therefore, it is reasonable to consider that the anisotropy of the lateral force in our experiment was due to the difference in folding direction of HDPE chain. Conclusion. The anisotropic lateral force measurements with varying the scanning angle, Θ, against the {110} growth face of the HDPE single crystals prepared by the selfseeding method indicates that the adjacent reentry and regular sharp fold is formed parallel to the {110} growth face on the single crystal surface, even in the case that the unfractionated HDPE single crystal with Mw=520k. In addition, it was suggested that the state of chain folding on the HDPE single crystal surface strongly depends on the crystallization methods. Acknowledgement This study was partially supported by Grant-in-Aid for COE research and Scientific Research on Priority Areas, "Near-field Nano Optics" (No. 286-09241222), from Ministry of Education, Science, Sports and Culture of Japan. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Keller, A. Phil. Mag. 1957, 2, 1171. Reneker, D. H.; Geil, P. H. J. Appl. Phys. 1960, 31, 1916. Faraday Disc. Chem. Ser. 1979, 68, 7. Keller, A. Kept. Prog. Phys. 1968, 31, 623. Meyer, E.; Howald, L.; Overney, R. M . ; Heinzelmann, H.; Frommer, J.; Güntherodt, H. J.; Wagner, T.; Schier, H.; Roth, S. Nature 1991, 349, 398. Overney, R. M.; Takano, H.; Fujihira, M. Phys. Rev. Lett. 1994, 72, 3564. Reneker, D. H.; Chun, I. In Scanning Probe Microscopy of Polymers; ACS Symp. Ser. 694; American Chemical Society: Washington, DC, 1996; Chapter 2, pp 32-52. Schwarz, U. D.; Bluhm, H.; Hölscher, H.; Allers, W.; Wiesendanger, R. In Physics of Sliding Friction; Persson, B. N . P.; Tosatti, E., Ed.; Kluwer Academic Publishers: Dordrecht, Netherlands, 1996; pp 369-402. Liley, M.; Gourdon, D.; Stamou, D.; Meseth, U.; Fisher, T. M.; Lautz, C.; Stahlberg, H.; Vogel, H.; Burnham, Ν. Α.; Duschl, C. Science 1998, 280, 273. Nisman, R.; Smith, P.; Vancso, G. J. Langmuir 1994, 10, 1667. Smith, P.; Nisman, R.; Ng, C.; Vancso, G. J. Polym. Bull. 1994, 33, 459. Kajiyama, T.; Ohki, I.; Takahara, A. Macromolecules 1995, 28, 4768. Blundell, D. J.; Keller, A. J. Macromol. Sci.-Phys. 1968, B2, 301. Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

Microstructure and Microtribology of Polymer Surfaces - American

Recommend Documents