Article pubs.acs.org/Langmuir
Force Estimation on the Contact of Poly(L,L‑lactide) and Poly(D,D‑lactide) Surfaces Regarding Stereocomplex Formation Hiroharu Ajiro,†,‡,§ Shun Takahama,∥ Masashi Mizukami,⊥ Kai Kan,†,‡ Mitsuru Akashi,*,# and Kazue Kurihara*,⊥,∇ †
Institute for Research Initiatives and ‡Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma, Nara 630-0192, Japan § JST PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ∥ Department of Applied Chemistry and #Graduate School of Frontier Biosciences, Osaka University, 2-1 Yamada-oka, Suita, 565-0871, Japan ⊥ Institute of Multidisciplinary Research for Advanced Materials and ∇WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: The stereocomplex formation of poly(L,Llactide) (PLLA) and poly(D,D-lactide) (PDLA) was selected in order to investigate the interaction of the two surfaces including hydrogen bonding and van der Waals interaction. Adhesion force measurement using surface force apparatus (SFA) equipped with an optical microscope was conducted on the PLLA and PDLA spin-coated films. The adhesion forces, Fad, phenomenologically followed the linear relation with the applied normal load, L. For the force Fad between PLLA and PDLA films with low molecular weights (PLLA, Mn = 2800; PDLA, : Mn = 2100), the slope of linear fitting of Fad vs L was significantly larger for the heterointerface (PLLA/PLDA) compared with that for the homointerface (PLLA/PLLA and PDLA/ PDLA). However, when polymers with higher molecular weights (PLLA, Mn = 8500; PDLA, Mn = 8300) were measured, the slopes of linear fitting lines were almost the same for hetero- and homointerfaces. This indicated that the mobility of the lower molecular weight PLLA/PDLA films promoted the selective interaction of PLLA and PDLA under the applied normal loads. The adhesion between the outermost PLLA layer and PDLA layer prepared by layer-by-layer (LbL) assembly was also measured. It is interesting that the adhesion force was very weak in this case. This weak adhesion could be explained by the much less mobility of the polymer chain due to the stereocomplex formation within the LbL layers. This study demonstrated that the adhesion force due to the selective interaction of PLLA and PDLA between PLLA/PDLA films could be directly measured, and depended on the mobility of the outermost polymer chains, which reflected the different structures of polymer chains in the organized complex films.
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INTRODUCTION The stereocomplex of poly(L,L-lactide) (PLLA) and poly(D,Dlactide) (PDLA) has been widely investigated since the first report in 1987.1 The stereocomplex formed with PLLA and PDLA can be used as renewable material derived from natural products. It serves as polymer material substituted for those supplied by the oil industry, because of its improved thermal properties and mechanical strength. In order to elucidate the properties and the possible application of the stereocomplex, the thermal characteristics and mechanical strength of the stereocomplex have been well investigated for decades.2,3 It is possible to introduce additional properties by chain end modification4−7 and block copolymers with poly(ethylene glycol) (PEG),8−10 because the stereocomplex forms by the interaction of the polymer main chain sequence between PLLA and PDLA with the degree of polymerization of at least 7.11 It is © 2016 American Chemical Society
important to clarify the interaction of the stereocomplex in detail at the polymer chain level, in order to utilize the stereocomplex formation as a building block moiety for further functional materials. There are several data available regarding the interaction of PLLA and PDLA at the polymer chain level. For example, the stereocomplex formation of PLLA and PDLA at the air−water interface was investigated using IR spectroscopy.12 A more relevant study of interactions on the substrate was carried out by IR spectroscopy and it found that PDLA was selectively adsorbed on the PLLA grafted surface due to stereocomplex formation.13 Recently, PLLA and PDLA were grafted onto Received: July 15, 2016 Revised: August 28, 2016 Published: August 30, 2016 9501
DOI: 10.1021/acs.langmuir.6b02623 Langmuir 2016, 32, 9501−9506
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polyhedral oligomeric silsesquioxane to observe aggregation by stereocomplex formation on the particle surfaces.14,15 However, the interaction between PLLA and PDLA on the interface is still poorly understood. A commonly used method to evaluate the polymer−polymer interaction is the lap and shear test, which measures the shear force between two surfaces (polymer film) after keeping them in contact. This method has been employed to study rather strong forces which involved electrostatic and hydrogenbonding interactions. For example, adhesion between two planar substrates coated with layer-by-layer (LbL) assembled films of cationic and anionic polymers has been evaluated.16 It is noteworthy that selective and reversible adhesion was achieved using oppositely charged polyelectrolyte brushes.17 Hydrogen bonding interaction18 and host−guest interaction19 were also found to be involved in the adhesion of two surfaces. However, to evaluate the weak interaction forces involved in stereocomplex formation, the methods with higher sensitivity are necessary. One of the most effective methods for quantitatively studying the polymer interaction with high sensitivity is the direct forces measurement using atomic force microscopy (AFM) and/or surface forces apparatus (SFA). AFM has been employed to study single polymer chain interactions.20,21 For example, AFM has been used to measure adhesion forces between poly(acrylic) acid chains fixed to an AFM tip and solid surfaces such as mica,22,23 CH3/COOH terminated self-assembled monolayer surfaces,24 and single crystal ZnO(0001)−Zn surface.25 AFM was also applied to measure the force for extracting a single poly(ethylene oxide) chain form a single crystal.26 Reduction of attractive forces between polylactide surfaces by grafting hydrophilic polymer chains was observed using colloidal probe AFM.27 SFA has been used for studying the adhesion force between polymer films of micrometer level thickness such as poly(ethylene terephthalate),28 poly(butyl methacrylate),29 polystyrene, and poly(dimethylsiloxane).30 We have used SFA for studying the adhesion forces between polyimide film and Ni/Cr alloy surfaces.31,32 SFA has advantages for studying adhesion forces. The contacting area such as shape and roughness changes can be directly monitored using an optical microscope.31,32 The contact area vs load relation can be analyzed to provide the adhesion and surface energies, and their changes during the contact using a theory of adhesion such as JKR (Johnson−Kendall−Roberts) or DMT (Derjaguin−Muller− Toporov).33,34 Therefore, SFA can be a quite useful tool for studying the interaction forces involved in the selective interaction of PLLA and PDLA. In this study, we study the interaction between the surfaces of PLLA and PDLA using SFA, in order to clarify the weak polymer−polymer interaction and its selectivity, which are related to stereocomplex formation. We measured adhesive interactions between PLLA/PDLA, PLLA/PLLA, and PDLA/ PDLA films spin-coated on mica surfaces, and the adhesion forces were measured at various applied loads. The spin-coated films of PLLA and PDLA were prepared with two different molecular weight polymers. The adhesion between PLLA/ PDLA interfaces was also measured using the outermost layer of films prepared by layer-by-layer assembly. Based on these measurements, the effect of polymer chain mobility on the adhesion forces was evaluated.
Article
EXPERIMENTAL SECTION
Materials. L,L-Lactide and D,D-lactide (Musashino Chemical Laboratory, Ltd., Japan) were recrystallized from ethyl acetate and dried in vacuo at room temperature. Benzyl alcohol (Tokyo Chemical Industry, Ltd., Japan) was distilled with calcium hydride. PLLA (Mn = 8500, polydispersity index (PDI) = 1.2, or Mn = 2800, PDI = 1.4) and PDLA (Mn = 8300, PDI = 1.2, or Mn = 2100, PDI = 1.5) in this study were synthesized according to the literature.35 Preparation of Polymer Films. Spin-coated films were prepared with chloroform solutions of PLLA (20 mg/mL) or PDLA (20 mg/ mL) by spin-coating (1500 rpm, 60 s) using a spin-coater (1H-D7, Mikasa Co., Ltd., Japan), and dried under vacuum for 1 h at room temperature. The polymer films were prepared on mica sheets glued on half-cylindrical silica disks (curvature radius, R = 20 mm) for adhesion force measurement using SFA. Adhesion Force Measurement Using SFA. Adhesion forces between polymer films deposited on mica substrates were directly measured using SFA equipped with an optical microscope as previously reported (Figure 1).31,32 After the spin-coating of polymer,
Figure 1. Schematic illustration of SFA equipped with optical microscope for observing the surfaces in contact. a cylindrical disk was fixed to an upper lens holder, and the other disk was fixed to a lower lens holder which was connected to a pulse motor stage (M-111.12S, PI Japan) via a horizontal spring (spring constant, k = 241 N/m). The surface separation distance and the normal load were controlled by the pulse motor drive. Typically the lower surface was driven by the pulse motor at a velocity of 1 μm/s. The duration of contact was within the range 5−60 min in this study. Monochromatic light from a sodium lamp (Na/10, IRIE Corp.) was irradiated to the surfaces from the bottom. Newton’s ring generated by the interference between upper and lower surfaces was monitored using an optical microscope placed above the chamber. Newton’s ring was recorded as a movie during one cycle of adhesion force measurement, that is, approaching, contact, loading, retraction, and separation (see Figure S1 in Supporting Information). The deflection of the cantilever needed to separate surfaces (Δd) was determined as the difference in the pulse motor positions where the surfaces reached contact and the surfaces jumped apart on retraction. The adhesion force (Fad) was obtained using the following equation: Fad = kΔd. The contact position was changed after several times of adhesion forces measurement to see the average tendencies of the adhesion.
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RESULTS AND DISCUSSION Adhesion Forces between Spin-Coated Films of Low Molecular Weight Polymer PLLA (Mn = 2800, PDI = 1.4) and PDLA (Mn = 2100, PDI = 1.5). The adhesion forces (Fad) for a set of low molecular weight samples of PLLA (Mn = 2800, PDI = 1.4) and PDLA (Mn = 2100, PDI = 1.5) measured at various applied loads are shown in Figure 2 with those for 9502
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significantly with the increasing applied load. On the other hand, the values of F0 in the eq 1, which corresponds to the adhesion forces without the applying load (Table 1, runs 1−3), showed no significant difference. This indicated that the loading was necessary for the selective interaction to form the stereocomplex. To see the difference in the adhesion forces at high normal load (L), the average adhesion forces (Fad) were calculated at the applied load (L) of 8000 mN/m using eq 1. The Fad values at L = 8000 mN/m were 152.1 mN/m for PLLA/PLLA (n = 39) (Table 1, run 1), 134.8 mN/m for PDLA/PDLA (n = 18) (Table 1, run 2), and 343.8 mN/m for PLLA/PDLA (n = 14) (Table 1, run 3). Thus, it is clear that the average adhesion force (Fad) for PLLA/PDLA, which is the polymer pair for selective interaction of PLLA and PDLA, was significantly larger than those for PLLA/PLLA and PDLA/ PDLA under high applied normal loads (L). The results indicated that stereocomplexes were formed between the solid surfaces coated with the low molecular weight (Mn ∼ 3000) PLLA and PDLA by applying high load (pressure). Differences in the contacting areas have to be considered because of the possibility that the contact areas increased under the high-pressure conditions and thus could increase the adhesion. Therefore, the dependence of the adhesion force on the contact area was studied, which did not show any dependence (Supporting Information, Figures S2−S4). Thus, it was clear that adhesive forces were influenced not by the contact area, but by the applied load (pressure). Adhesion Forces between Spin-Coated Films of Higher Molecular Weight Polymer PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2). The high molecular weight samples were next examined, considering the effect of molecular weight, which can be correlated with the facile dynamic rearrangement of the polymer on the surface. The obtained adhesion forces of PLLA/PLLA (Mn = 8500, PDI = 1.2) (circles), PDLA/PDLA (Mn = 8300, PDI = 1.2) (squares), and PLLA/PDLA (triangles) are plotted against the applied load in Figure 3. The distributions of adhesion forces are shown in Figure 3, and no clear difference between three combination of surfaces. For each combination of PLA films, all the data of the adhesion force vs the applied load were fitted with eq 1. The fitting lines are shown in the Figure 3 as PLLA/ PLLA (dotted line) (a = 7.3) (Table 1, run 4), PDLA/PDLA (dashed line) (a = 4.9) (Table 1, run 5), and PLLA/PDLA (solid line) (a = 8.2) (Table 1, run 6). The adhesion forces increased with the increasing applied load on average for all combinations of PLA films; however, the slopes (a) of the linear fitting showed no significant difference between three combinations of surfaces in the case of higher molecular weight (Mn) samples. This result was quite different from the case of low molecular weight polymers (see Figure 2). The Fad values at L = 8000 mN/m calculated using eq 1 were 58.7 mN/m for
Figure 2. Adhesion force between PLA surfaces measured in ambient air with PLLA (Mn = 2800, PDI = 1.4) and PDLA (Mn = 2100, PDI = 1.5).
PLLA/PLLA (circles), PDLA/PDLA (squares), and PLLA/ PDLA (triangles). As can be seen in the distributions of adhesion forces shown in Figure 2, the adhesion forces for PLLA/PDLA were larger than those for PLLA/PLLA and PDLA/PDLA. The adhesion forces for PLLA/PDLA increased almost proportional to the increase in the applied load. On the other hand, the adhesion forces for PLLA/PLLA and PDLA/ PDLA showed large scattering. For example, two Fad values of PLLA/PLLA in Figure 2 for L above 7000 mN/m are higher than those of PLLA/PDLA, and some Fad values of PLLA/ PLLA and PDLA/PDLA were around zero for the entire L range. The large scattering indicated that homointerfaces could exhibit adhesion; however, it depended on the contacting position, and its probability was much less compared with that for PLLA/PDLA. This implied that the adhesion forces of homointerfaces scattered due to the local properties of films, e.g., roughness and orientation of polymer chains. In this study, to discuss the overall tendencies of the adhesion forces including all of the observation, the adhesion forces vs applied load (L) data were fitted with a linear function of eq 1 for each case.
Fad = aL + F0
(1)
where a is the slope, L (mN/m) is the applied load on the contacting surfaces, and F0 (mN/m) is the intercept. The fitting lines are also shown in Figure 2, for PLLA/PLLA by a dotted line, PDLA/PDLA by a dashed line, and PLLA/PDLA by a solid line. The values of a and F0 obtained from fitting are summarized in Table 1. Interestingly, the slope (a) of eq 1 was significantly larger for PLLA/PDLA (a = 37.4) (Table 1, run 3) compared with those for the homogeneous pairs of PLLA/PLLA (a = 13.8) (Table 1, run 1) and PDLA/PDLA (a = 14.9) (Table 1, run 2). This means that the adhesion force for PLLA/PDLA increased more
Table 1. Parameters Obtained from Adhesion Force vs Applied Load Plots Using eq 1 run
substrate 1
substrate 2
a (×10−3)
F0 (mN/m)
Fad at L = 8000 mN/m (mN/m)
1 2 3 4 5 6 7
PLLA (Mn = 2800) PDLA (Mn = 2100) PLLA (Mn = 2800) PLLA (Mn = 8500) PDLA (Mn = 8300) PLLA (Mn = 8500) PLA−LbL
PLLA (Mn = 2800) PDLA (Mn = 2100) PDLA (Mn = 2100) PLLA (Mn = 8500) PDLA (Mn = 8300) PDLA (Mn = 8300) PLA−LbL
13.8 14.9 37.4 7.3 4.9 8.2 −0.5
41.7 15.6 44.6 0.3 23.2 4.3 9.9
152.1 134.8 343.8 58.7 62.4 69.9 5.9
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Figure 3. Adhesion force between PLA surfaces measured in ambient air with PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2).
Figure 4. Adhesion force of spin-coated PLLA/PDLA surfaces and PLA−LbL surfaces measured in ambient air with PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2).
PLLA/PLLA (n = 24), 62.4 mN/m for PDLA/PDLA (n = 22), and 69.9 mN/m for PLLA/PDLA (n = 36) (Table 1, runs 4− 6), showing no significant difference. This indicated that stereocomplex was not significantly formed for the PLLA/ PDLA films prepared by higher Mn samples even at high applied loads up to ca. 10 000 mN/m, which corresponded to the pressure of ca. 3.8 MPa calculated using the contact area of ca. 0.056 mm2. This should be related to the lower mobilities of PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2), and the stereocomplex could not be formed within the duration of contact. On the other hand, the polymer chains in the films prepared by the lower Mn samples (Figure 2) possess mobility high enough to form selective interaction of PLLA and PDLA on the contacting area of two surfaces. Adhesion Forces between PLLA/PDLA Interfaces of LbL Films with Higher Molecular Weight PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2). Finally, the adhesion forces between the PLLA/PDLA interfaces of outermost layers prepared by LbL assembly were studied. It has been reported that PLLA/PDLA formed stereocomplexes in the LbL films. According to the literature,36 the acetonitrile solutions at 10 mg/mL were prepared for PLLA (Mn = 8500, PDI = 1.2) and PDLA (Mn = 8300, PDI = 1.2). Then, mica was used as the substrate for LbL procedures. After 20 steps of LbL assembly, two LbL assembled films on mica substrates were prepared: one with a PLLA layer on the outermost surface and another with a PDLA layer on the outermost surface. The stereocomplex structure in the LbL films was confirmed by Fourier transform infrared spectroscopic (FT-IR) and AFM measurements (Supporting Information, Figures S5 and S6). Then, the two surfaces were employed to study the adhesion forces by SFA (Figure 4). The adhesion forces for the LbL films (Figure 4, diamonds) were significantly weak even at the applied loads above 10 000 mN/m compared with the results for spin-coated films of PLLA/PDLA (Figure 4, triangles, and those in Figure 3). This was also clearly seen in the distribution of the adhesion forces shown in Figure 4. The data of adhesion force vs applied load were fitted with eq 1. The obtained slope value a was nearly zero, and no effect of loading force was observed. The average adhesion force at L = 8000 mN/m was calculated to be 5.9 mN/min (n = 21) (Table 1, run 7). This was significantly smaller than that obtained for PLLA/PDLA films prepared by spin-coating (69.9 mN/m) (n = 36). The results indicated that it was difficult for the crystallized stereocomplex moieties of polymer chains36 to rearrange themselves in the contact area to
form selective interaction of PLLA and PDLA. The spin-coated films of high molecular weight polymers exhibited greater adhesion force than the LbL films, indicating that the even nonspecific chain−chain interactions can be much restricted in case of the LbL films. The surface roughnesses of spin-coated film and LbL film were estimated by AFM as 4.4 and 15 nm, respectively (Supporting Information, Figure S6). The larger roughness of LbL film could contribute to smaller adhesion compared with those of spin-coated films; however, low mobility of the main chain in LbL film due to the stereocomplex formation on each substrate could also contribute to smaller adhesion properties at the interface. The mobility of polymer chains in the contacting area could vary from sample to sample. This should be the reason for rather scattering adhesion measured for spin-coated films.
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CONCLUSION In this study, we measured the interaction forces between the surfaces of PLLA and PDLA using SFA, in order to clarify the weak polymer−polymer interaction and its selectivity, which are related to stereocomplex formation. The spin-coated films of PLLA and PDLA were prepared with two different molecular weight polymers (Mn ∼ 3000 and Mn ∼ 8000). The adhesion between PLLA/PDLA interfaces was also measured using the outermost layer of films prepared by LbL assembly. For smaller molecular weight polymers (Mn ∼ 3000), the adhesion force increased with increasing applied load, and the PLLA/PDLA interfaces exhibited significantly larger adhesion force (343.8 mN/m for applied load of 8000 mN/m) compared with those for PLLA/PLLA and PDLA/PDLA interfaces (152.1 and 134.8 mN/m, respectively, for applied load of 8000 mN/ m). For higher molecular weight polymers (Mn ∼ 8000), the increase in the adhesion force by applying higher applied load became less significant, and no clear difference in the adhesion force was observed between PLLA/PDLA, PLLA/PLLA, and PDLA/PDLA interfaces (69.9, 58.7, and 62.4 mN/m, respectively, for applied load of 8000 mN/m). These results indicated that the mobility of the lower molecular weight PLLA/PDLA films promoted the selective interaction of PLLA and PDLA under the applied normal loads, while it was difficult to form a stereocomplex for the PLLA/PDLA films prepared using higher Mn polymers. The adhesion force measured between the outermost PLLA layer and PDLA layer prepared by LbL assembly showed no increase in the adhesion force with increasing applied load (5.9 mN/m for applied load of 8000 mN/m). This result indicated 9504
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(9) Fujiwara, T.; Mukose, T.; Yamaoka, T.; Yamane, H.; Sakurai, S.; Kimura, Y. Novel Thermo-Responsive Formation of a Hydrogel by Stereo-Complexation between PLLA-PEG-PLLA and PDLA-PEGPDLA Block Copolymers. Macromol. Biosci. 2001, 1, 204−208. (10) Yang, L.; Wu, X.; Liu, F.; Duan, Y.; Li, S. Novel Biodegradable Polylactide/poly(ethylene glycol) Micelles Prepared by Direct Dissolution Method for Controlled Deliverly of Anticancer Drugs. Pharm. Res. 2009, 26, 2332−2342. (11) deJong, S. J.; van Dijk-Wolthuis, W. N. E.; Kettenses-van den Bosch, J. J.; Schuyl, P. J. W.; Hennink, W. E. Macromolecules 1998, 31, 6397−6402. (12) Bourque, H.; Laurin, I.; Pézolet, M.; Klass, J. M.; Lennox, R. B.; Brown, R. B. Investigation of the Poly(L-lactide)/Poly(D-lactide) Stereocomplex at the Air-Water Interface by Polarization Modulation Infrared Reflection Absorption Spectroscopy. Langmuir 2001, 17, 5842−5849. (13) Tretinnikov, O. N.; Kato, K.; Iwata, H. Adsorption of Enantiomeric Poly(lactide)s on Surface-Grafted Poly(L-lactide). Langmuir 2004, 20, 6748−6753. (14) Tan, B. H.; Hussain, H.; Lin, T. T.; Chua, Y. C.; Leong, Y. W.; Tjiu, W. W.; Wong, P. K.; He, C. B. Stable Dispersions of Hybrid Nanoparticles Induced by Stereocomplexation between Enantiomeric Poly(lactide) Star Polymers. Langmuir 2011, 27, 10538−10547. (15) Tan, B. H.; Hussain, H.; Leong, Y. W.; Lin, T. T.; Tjiu, W. W.; He, C. Tuning Self-assembly of Hybrid PLA-P(MA-POSS) Block Copolymers in Solution via Stereocomplexation. Polym. Chem. 2013, 4, 1250−1259. (16) Matsukuma, D.; Aoyagi, T.; Serizawa, T. Adhesion of Two Physically Contacting Planar Substrates Coated with Layer-by-Layer Assembled Films. Langmuir 2009, 25, 9824−9830. (17) Kobayashi, M.; Terada, M.; Takahara, A. Reversible Adhesivefree Nanoscale Adhesion Utilizing Oppositely Charged Polyelectrolyte Brushes. Soft Matter 2011, 7, 5717−5722. (18) Faghihnejad, A.; Feldman, K. E.; Yu, J.; Tirrell, M. V.; Israelachvili, J. N.; Hawker, C. J.; Kramer, E. J.; Zeng, H. Adhesion and Surface Interactoins of a Self-Healing Polymer with Multiple Hydrogen-Bonding Groups. Adv. Funct. Mater. 2014, 24, 2322−2333. (19) Ahn, Y.; Jang, Y.; Selvapalam, N.; Yun, G.; Kim, K. Supramolecular Velcro for Reversible Underwater Adhesion. Angew. Chem., Int. Ed. 2013, 52, 3140−3144. (20) Binnig, G.; Quate, C. F.; Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 1986, 56, 930−933. (21) Alessandrini, A.; Facci, P. AFM: A Versatile Tool in Biophysics. Meas. Sci. Technol. 2005, 16, R65−R92. (22) Clausen-Schaumann, H.; Seitz, M.; Krautbauer, R.; Gaub, H. E. Force Spectroscopy with Single Bio-molecules. Curr. Opin. Chem. Biol. 2000, 4, 524−530. (23) Seitz, M.; Friedsam, C.; Jöstl, W.; Hugel, T.; Gaub, H. E. Probing Solid Surfaces with Single Polymers. ChemPhysChem 2003, 4, 986−990. (24) Friedsam, C.; Seitz, M.; Gaub, H. E. Investigation of polyelectrolyte desorption by single molecule force spectroscopy. J. Phys.: Condens. Matter 2004, 16, S2369−S2382. (25) Valtiner, M.; Grundmeier, G. Single Molecules as Sensors for Local Molecular Adhesion Studies. Langmuir 2010, 26, 815−820. (26) Liu, K.; Song, Y.; Feng, W.; Liu, N.; Zhang, W.; Zhang, X. Extracting a Single Polyethyelen Oxide Chain from a Single Crystal by a Combination of Atomic Force Microscopy Imaging and SingleMolecule Force Spectroscopy: Toward the Investigation of Molecular Interactions in Their Condensed States. J. Am. Chem. Soc. 2011, 133, 3226−3229. (27) Nugroho, R. W. N.; Pettersson, T.; Odelius, K.; Höglund, A.; Albertsson, A. C. Force Interactions of Nonagglomerating Polylactide Particles Obtained through Covalent Surface Grafting with Hydrophilic Polymers. Langmuir 2013, 29, 8873−8881. (28) Merrill, W. W.; Pocius, A. V.; Thakker, B. V.; Tirrell, M. Direct measurement of molecular level adhesion forces between biaxially oriented solid polymer films. Langmuir 1991, 7, 1975−1980.
that the even nonspecific chain−chain interactions could be much restricted in the case of the LbL films. This study demonstrated that the adhesion force due to the selective interaction of PLLA and PDLAbetween PLLA/PDLA films could be directly measured, and depended on the mobility of the outermost polymer chains, suggesting that the stereocomplex formation can be utilized to fabricate the composite materials by simply compressing each other if the mobility of the polymer chains was controlled.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02623. Scheme of adhesion force measurement and Newton’s ring; optical microscope images of contacting surfaces during adhesion force measurement using SFA (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.A.). *E-mail:
[email protected] (K.K.). Present Address †
H. Ajiro: Nara Institute of Science and Technology.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (S) (23225004) and Grant-in-Aid for Challenging Exploratory Research (26620182) from the Ministry of Education, Culture, Sports, Science and Technology. The present study is also partly supported by JST PRESTO “Molecular Technology” with Prof. Takashi Kato.
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REFERENCES
(1) Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S. H. Stereocomplex Formation between Enantiomeric Poly(lactides). Macromolecules 1987, 20, 904−906. (2) Tsuji, H. Poly(lactide) Stereocomplexes: Formation, Structure, Properties, Degradation, and Applications. Macromol. Biosci. 2005, 5, 569−597. (3) Fukushima, K.; Kimura, Y. Stereocomplexed polylactides (NeoPLA) as high-performance bio-based polymers: their formation, properties, and application. Polym. Int. 2006, 55, 626−642. (4) Ouchi, T.; Uchida, T.; Ohya, Y. Synthesis of Poly(L-lactide) with One Terminal D-Glucose Residue and Wettability of Its Film Surface. Macromol. Biosci. 2001, 1, 371−375. (5) Masutani, I.; Kawabata, S.; Aoki, T.; Kimura, Y. Efficient Formation of Stereocomplexes of Poly(L-lactide) and Poly(D-lactide) by Terminal Diels-Alder Coupling. Polym. Int. 2010, 59, 1526−1530. (6) Brzeziński, M.; Bogusławska, M.; Ilčíková, M.; Mosnácě k, J.; Biela, T. Unusual Thermal Properties of Polylactides and Polylactide Stereocomplexes Containing Polylactide Functionalized Multi-Walled Carbon Nanotubes. Macromolecules 2012, 45, 8714−8721. (7) Ajiro, H.; Ito, S.; Kan, K.; Akashi, M. Catechin Modified Polylactide Stereocomplex at Chain End Improved Antibiobacterial Property. Macromol. Biosci. 2016, 16, 694−704. (8) Ajiro, H.; Kuroda, A.; Kan, K.; Akashi, M. Stereocomplex Film Using Triblock Copolymers of Polylactide and Poly(ethylene glycol) Retain Paxlitaxel on Substrates by Aqueous Inkjet System. Langmuir 2015, 31, 10583−10589. 9505
DOI: 10.1021/acs.langmuir.6b02623 Langmuir 2016, 32, 9501−9506
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Langmuir (29) Luengo, G.; Pan, J.; Heuberger, M.; Israelachvili, J. N. Temperature and time effects on the ″Adhesion Dynamics″ of poly(butyl methacrylate) (PBMA) surfaces. Langmuir 1998, 14, 3873−3881. (30) Zeng, H.; Tian, Y.; Zhao, B.; Tirrell, M.; Israelachvili, J. Transient interfacial patterns and instability associated with liquid film adhesion and spreading. Langmuir 2007, 23, 6126−6135. (31) Mizukami, M.; Kurihara, K.; Suzuki, S.; Matsudaira, M.; Yamabe, H.; Andoh, I. Evaluation of Metal-Polymer Adhesion by Surface Forces Apparatus. Shikizai Kyokaishi 2009, 82, 279−283. (32) Mizukami, M.; Sugihara, O. H.; Yamabe, I.; Andoh, S.; Kurokawa, K.; Kurihara. Surface Forces Study on Metal-Polymer Adhesion 2. Shikizai Kyokaishi 2011, 84, 87−91. (33) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. Molecular Mechanisms Associated with Adhesion and Contact Angle Hysteresis of Monolayer Surfaces. J. Phys. Chem. 1991, 95, 10736−10747. (34) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press, Ltd.: New York, 2010. (35) Ajiro, H.; Hsiao, Y. J.; Thi, T. H.; Fujiwara, T.; Akashi, M. Stereocomplex of Poly(lactide)s with Chain End Modification: Simultaneous Resistances to Melting and Thermal Decomposition. Chem. Commun. 2012, 48, 8478−8480. (36) Serizawa, T.; Yamashita, H.; Fujiwara, T.; Kimura, Y.; Akashi, M. Stepwise Assembly of Enantiomeric Poly(lactide)s on Surfaces. Macromolecules 2001, 34, 1996−2001.
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DOI: 10.1021/acs.langmuir.6b02623 Langmuir 2016, 32, 9501−9506
