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Jul 7, 2016 - ABSTRACT: The adhesion and detachment of polymer and solid ... (PS) films and a mica substrate to probe their detachment behaviors...
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Adhesion and Detachment Mechanisms between Polymer and Solid Substrate Surfaces: Using Polystyrene−Mica as a Model System Hongbo Zeng,*,† Jun Huang,† Yu Tian,‡ Lin Li,† Matthew V. Tirrell,§ and Jacob N. Israelachvili∥ †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China § Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States ∥ Department of Chemical Engineering, Materials Department, Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States ‡

S Supporting Information *

ABSTRACT: The adhesion and detachment of polymer and solid substrate surfaces play important roles in many engineering applications such as for designing adhesives, biomedical adhesives, adhesive tapes, robust protective coatings, biomedical scaffolds, prosthetic devices (e.g., artificial joints and implants), and fabrication of micro- and nanoelectromechanical devices. In this work, a surface forces apparatus (SFA) coupled with top-view optical microscopy was employed to measure the adhesion between thin polystyrene (PS) films and a mica substrate to probe their detachment behaviors. Various factors, including molecular weight (MW), contact time, and polarity-enhancing UV/ozone treatment, were examined. The results show that increased chain-end density, chain mobility, and segment polarity can all contribute to enhanced adhesion strength for both the “symmetric” PS−PS and “asymmetric” PS−mica systems but attributed to different adhesion/detachment mechanisms. For the asymmetric PS−mica system, the increased chain-end density (lower MW), increased chain mobility, and increased polarity (induced by UV/ozone treatment) facilitate the rearrangement of the polystyrene chains and the development of mainly “polar” interactions such as dipole−dipole, dipole−induced dipole, and attractive hydrogen bond interactions between the polar groups on the UV-treated PS (the π-electron clouds of the phenyl rings) and the highly polar mica surface. For the symmetric PS−PS system, the enhanced adhesion is mainly due to the interdiffusion, interdigitation, interpenetration, and entanglement of chains across the polymer−polymer contact interface. Importantly, during the separation of a UV/ozone-treated PS surface from mica, “stick−slip” detachment was observed, resulting in a residue of concentric polymer rings left on the mica surface. Our results provide new fundamental and practical insights into the adhesion, detachment, and damage (wear) mechanisms of polymer− polymer and polymer−solid surfaces.

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INTRODUCTION

polymer(s), segment polarity, chain mobility, surface roughness, temperature, contact time, and loading history; in addition, irreversible “adhesion hysteresis” is also widely observed.2,7,11−18 It is generally believed that the population of polymer chain ends at the contact interface and their ability to interdiffuse and interdigitate across the interface contribute significantly to the adhesion between two polymer surfaces.2,5,13,19−24 Previous studies2,19,20 have shown enhanced

Polymer materials and their surface adhesion properties play an important role in many engineering and bioengineering applications such as for designing adhesives, biomedical adhesives, adhesive tapes, robust protective coatings, biomedical scaffolds, prosthetic devices (e.g., artificial joints, implants), and robotics, which have received increasing attention over the past few decades.1−10 Great efforts have been dedicated to elucidating the adhesion and detachment mechanisms between two surfaces of the same or different polymer materials. The adhesion between polymer surfaces generally depends on many parameters such as the molecular weight (MW) of the © XXXX American Chemical Society

Received: May 6, 2016 Revised: July 1, 2016

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Macromolecules adhesion between polymer surfaces of reduced MW due to the higher number of free chain ends at the interface and the enhanced chain mobility attributed to the lower glass transition temperature. It has been reported that the adhesion between polymer surfaces of high MW can be significantly enhanced after UV irradiation due to the exposure of free distal ends of polymer chains (especially at the surface) resulting from chain scission and enhanced polarity.2,20 Despite the extensive studies carried out on the adhesion between two polymer surfaces, the adhesion and especially the detachment mechanisms between (“asymmetric”) polymer and solid substrates have been much less investigated. The interactions and adhesion between a polymer and solids such as inorganic solid substrates are important in many engineering and biological systems.1,8 For examples, the adhesion between the polyimides (e.g., pyromellitic dianhydride−oxidianiline polyimide) and metals (e.g., copper, chromium) is crucial for the performance of microelectronic devices. The adhesion between epoxy resins and inorganic substrates like glass fibers are important for the mechanical properties of composite materials. In the preparation and application of microfluidic and bioengineering devices, the bonding between polymers/ biopolymers and inorganic substrates is critical for the performance of these devices. In many of these applications, the surface damage/wear or component failure is often associated with (and a result of) stick−slip friction at the interfaces.6,25 To enhance adhesion between polymer surfaces and substrates, several studies reported the application of UV/ ozone surface treatment for surface modification of polymers such as poly(dimethylsiloxane).26,27 However, the understanding of the highly diverse adhesion and detachment mechanisms of polymer surfaces on solid substrates remains limited. Tack tests and peeling tests have been widely applied for measuring the adhesion strength and detachment behavior of polymer adhesives, particularly pressure-sensitive adhesives (PSA), against solid substrates.28−30 Contact adhesion tests based on “contact mechanics” models is a common method used to evaluate the adhesion of polymer surfaces.6,31,32 The classic Johnson−Kendall−Roberts (JKR) model has been extensively applied to describe the adhesion and contact mechanics between soft elastic materials, both at the macroscopic and microscopic length scales.26,31,33,34 According to the JKR model,33 a purely elastic sphere of radius R, when pressed by an external load F⊥ against a flat surface of the same material with surface energy γ (adhesion energy W = 2γ), will have a flat contact area of radius r given by eq 1, where K is the elastic modulus: r3 =

R [F⊥ + 3πRW + K

6πRWF⊥ + (3πRW )2 ]

However, for many viscoelastic or hysteretic materials exhibiting irreversible bulk and/or surface properties, the abrupt detachment of the surfaces occurs at much higher adhesion forces than predicted by the JKR equation34−37 (eq 2). This “effective” nonequilibrium adhesion force, Fad, can be used in eq 2 to give the effective adhesion energy:

Weff = 2Fad /3πR

In this work, polystyrene (PS) of varying MW was chosen as a model polymer for investigating the adhesion and detachment behavior with a solid mica substrate (the mica surfaces being chemically inert and molecularly smooth) using a surface forces apparatus (SFA) coupled with visualization of the surfaces through multiple beam interferometry (MBI) using fringes of equal chromatic order (FECO),38 simultaneously with a topview optical microscope. The effects of MW, contact time, and surface treatment with UV/ozone (UV/O) were studied.

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3 πRW 2

MATERIALS AND EXPERIMENTAL METHODS

Materials and Surface Preparation. Polystyrene (PS) of different molecular weightsPS 590 (MW = 590 Da) with polydispersity index (PDI ≤ 1.1), PS 800 (PDI ≤ 1.3), PS 1390 (PDI ≤ 1.1), PS 3700 (PDI ≤ 1.1), PS 8000 (PDI ≤ 1.05), PS 16 000 (PDI ≤ 1.05), PS 280 000 (PDI ≤ 2), PS 1 000 000 (PS 1M, PDI ≤ 1.1)was purchased from Sigma-Aldrich, Pressure Chemical Co., Scientific Polymer Products Inc., or Polymer Source and used as received. Polystyrene solutions were prepared by dissolving PS in toluene (Sigma-Aldrich, HPLC grade). PS films were prepared by spin-coating PS−toluene solution on a mica substrate glued on silica disks, followed by very slow drying in vacuum for >10 h to remove the solvent and leave a film of uniform thickness, before mounting the disks into the SFA chamber. The film thicknesses of the PS films were controlled at 100−120 nm as confirmed by SFA measurements and, independently, by ellipsometry. Surface treatment of PS films was conducted by exposure to UV/ ozone using a T10X10/OES UV/ozone cleaning system (UVOCS Inc., Lansdale, PA) for specified times. The surface elements and functional groups on the PS surfaces before and after UV/ozone treatment were analyzed using X-ray photoelectron spectroscopy (XPS, Kratos AXIS 165), which could be correlated to the surface energy or adhesion strength.39 The morphology of the mica and PS surfaces before and after UV/ozone treatment was investigated using an atomic force microscope (AFM) through tapping-mode imaging. Silicon AFM cantilever (Bruker) with a spring constant of ∼40 N/m was used and operated in the repulsive regime during imaging, with a free amplitude ∼1 V, set point amplitude ratio A/A0 70−85%, and driving frequency ∼5% below the resonance frequency (300−400 kHz). Water contact angle measurements on PS surfaces before and after UV/ozone treatment were performed using a Rame-Hart goniometer, where a drop of deionized water was injected onto films supported on a horizontal, backlit stage. The images were recorded with a CCD camera horizontal to the drop to allow measurements of the water contact angles on the polymer surfaces. Contact Mechanics Measurements. A surface forces apparatus (SFA) coupled with multiple beam interferometry by employing FECO fringes and a top-view optical microscope was used for measuring the surface separation, local (contact) geometry, and adhesion forces between the PS and bare mica surfaces. Details of the SFA setup for such measurements have been reported previously.19,40−42 Briefly, a thin mica sheet of 1−5 μm was glued onto a cylindrical silica disk (radius R ≈ 2 cm). The back surfaces of mica substrates were coated with an ∼50 nm semireflective layer of silver, which is required to obtain sharp FECO fringes to monitor the surface separation, surface shape, deformations, and contact area in real time and in situ. The PS surface and bare mica surface were mounted in the SFA chamber in a crossed-cylinder configuration, which is locally equivalent to a sphere of radius R interacting with a flat surface (based on the Derjaguin approximation34). The SFA chamber was purged

(1)

For the more general case of two dissimilar spheres with radii R1 and R2 of dissimilar materials, the adhesion energy W is related to their surface energies γ1 and γ2, and interfacial energy γ12, by W = γ1 + γ2 − γ12, where now R = R1R2/(R1 + R2). The JKR model predicts reversible loading−unloading paths (cycles) and no hysteresis for purely elastic (i.e., not viscoelastic) materials exhibiting no “surface adhesion hysteresis”. Under these “ideal” conditions the adhesion or “pull-off” force Fad,JKR and adhesion energy are related by Fad,JKR =

(3)

(2) B

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Macromolecules with dry N2 for >30 min before measurements, and a small container filled with P2O5 was also placed in the sealed chamber to ensure the dry atmosphere conditions. As shown in Figure 1, when the two surfaces are in contact, the surface deformation can be monitored from the FECO fringes in real

Figure 1. Schematic of a polymer layer in adhesive (JKR-like) contact with a mica surface under an external load F⊥ showing a contact region of contact diameter 2r (left) and typical corresponding FECO interference fringe pattern (right). The surface radius, R ≈ 2 cm, can be obtained from the shapes of the FECO fringes when the surfaces are separated. time. The contact diameter 2r can be determined from the flat region of the recorded FECO fringes, and its changes as a function of the applied load F⊥ (2r vs F⊥) can also be measured. During the adhesion mechanics tests, a maximum load of F⊥,max = 33.8 mN was applied at an average loading and unloading rate of dF⊥/dt = 0.67 mN/s. The two surfaces were kept at the maximum load F⊥,max for different contact times tc ranging from 10 s to ∼3 h before unloading. The maximum force Fad required to separate the two surfaces from adhesive contact was then recorded as the “pull-off” force at the end of the unloading process, and the corresponding effective adhesion energy, Weff, was calculated using eq 3. All the contact mechanics tests were conducted at room temperature (23 °C).

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RESULTS AND DISCUSSION Figure 2a shows some typical JKR plots of contact diameter 2r as a function of the applied load F⊥, measured for mica vs PS 1390 at room temperature. When untreated PS 1390 and a mica surface are brought slowly toward each other, at some finite separation they “jump” into contact and immediately deform into a flattened circular contact area of diameter 2r ∼ 75 μm due to the van der Waals attraction and contact adhesion force between the two surfaces under zero external load (F⊥ = 0). When the two surfaces are further compressed, the contact diameter increases with increasing F⊥, reaching 2r ∼ 98 μm at F⊥,max = 33.8 mN. The two surfaces were kept under this load for different periods or “contact times”, tc, during which the contact diameter hardly changed. This is because PS 1390 has a glass transition temperature of 46 °C and is therefore mainly in the elastic “glassy state” at the temperature of the experiments (23 °C). Fitting the loading curve (or path) with the JKR equation (eq 1) gives an adhesion energy of W0 ∼ 115 mJ/m2 for the PS−mica interface. On gradually reducing the applied load, F⊥, the contact area, πr2, either decreased (for short contact times of tc = 10 and 720 s) or remained almost unchanged (for long contact times, e.g., tc = 10 800 s) until a critical tensile (i.e., negative) load was applied and the two surfaces abruptly detached (“jumped” apart) at a critical adhesion force, Fad, which, using eq 3, gives the effective adhesion energies Weff, as shown in Figure 2b. The nonreversible loading and unloading processes in Figure 2a clearly indicate strong “adhesion hysteresis” between PS 1390 and mica, and Figure 2b shows that the effective adhesion

Figure 2. Results of JKR adhesion measurements of PS−mica and PS−PS of varying molecular weights and contact times, tc, at the maximum load F⊥,max = 33.8 mN. (a) JKR plots of untreated PS 1390 vs mica for different contact times, tc. (b) Contact time dependence of the adhesion energies for PS 1390, PS 16 000, and PS 1M with mica. (c) Effects of contact time on the effective adhesion energy for PS− mica (asymmetric case: filled data points) and PS−PS (symmetric case: unfilled points). (d, e) Microscope images of (d) the PS1390 surface and (e) the corresponding mica surface, after detachment (pull-off) after a contact time of tc = 2 h at the maximum load. C

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Figure 5. Contact angle of water on PS 1M surfaces after treatment by UV/ozone for different exposure times.

Figure 3. Effect of PS MW on the effective adhesion energy of PS and mica at 23 °C and tc = 10 s.

interface with increasing the contact time. The adhesion of PS 1390 vs PS 1390 increases with contact time more rapidly than that of PS 1M vs PS 1M, which is attributed to the significantly higher density of chain ends at the contact interface and higher chain mobility of the lower MW PS 1390. Interestingly, the adhesion for the asymmetric PS−mica interfaces showed similar time-dependent trends as for the PS− PS interfaces, even though interdigitation and interdiffusion (i.e., interpenetration) of polymer chains into the hard and smooth mica surface is not possible in this case. A previous study characterized the molecular orientation of the phenyl groups at the interface between a polystyrene film and a glass substrate using sum frequency generation vibrational spectroscopy (SFG-VS), where it was found that the adhesion between PS and the hydrophilic glass surface could be stronger than that between PS and hydrophobized glass due to the rearrangement/reorientation of the polystyrene chains, allowing for the development of attractive hydrogen bonds between the surface

energy Weff increases significantly with the contact time. Similar JKR tests were conducted between both PS 16 000 and PS 1M and mica, with qualitatively similar results as also shown in Figure 2b. However, the higher the MW of the PS, the smaller was the magnitude of the increase in the effective adhesion energy, Weff, oralternativelythe rate of increase of Weff with the contact time. Figure 2c shows results for the adhesion of asymmetric contacts between PS and mica compared with those for symmetric contacts between two PS surfaces for PS 1390 vs mica, PS 1390 vs PS 1390, PS 1M vs mica, and PS 1M vs PS 1M. The adhesion between two polystyrene surfaces has been studied previously.2,19,20 The time-dependent adhesion between two PS 1390 or PS 1M surfaces in Figure 2c agree with these previous studies2,19,20 and confirm that the adhesion “mechanism” is mainly due to the enhanced interdigitation and interdiffusion of the PS chains and loops across the contact

Figure 4. (a, b) Topographic AFM images (5 μm × 5 μm, and 1 μm × 1 μm) and (c) corresponding phase AFM image (1 μm × 1 μm) of untreated PS 1M surfaces. (d, e) Topographic AFM images (5 μm × 5 μm, and 1 μm × 1 μm) and (f) corresponding phase AFM image (1 μm × 1 μm) of PS 1M surfaces treated with UV/O for 1 min. D

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Figure 6. XPS spectra of UV/ozone-treated PS films. (a) XPS O 1s peaks of PS 1M treated by UV/ozone for 0, 18, and 66 s. (b−d) Peak fitted highresolution C 1s XPS spectra of PS 1M after UV/ozone treatment for (b) 0, (c) 18, and (d) 66 s.

hydroxyl groups on the glass surface and the π electron clouds of the phenyl rings of PS.43 Therefore, the time-dependent adhesion between polystyrene and mica should be mainly attributed to contact time-enhanced polar interactions such as attractive H-bonds between the rearranging polystyrene chains (mainly the π electron cloud of the phenyl rings) and the polar −Si−OH and −Si−O−Si− groups on the mica surface. Figures 2d and 2e show typical patterns of a low MW polystyrene (PS1390) surface and a mica surface, respectively, after the two (asymmetric) surfaces were separated from adhesive contact after a contact time of tc = 2 h at the maximum load. These images clearly show that “material transfer” had occurred, i.e., that some of the PS chains were transferred to the mica surface during the separation. Material transfer was more pronounced with increasing contact time (as comparing the microscope images of the mica surfaces with transferred PS for tc = 10 s, 20 min, and 1 h at the maximum load shown in Figure S1 in the Supporting Information and Figures 2d and 2e for tc = 2 h), but less so or nonexistent for the higher MW polystyrenes (e.g., PS 1M). The fracture patterns in Figures 2d and 2e suggest a stick−slip or nonuniform separation process that damaged the contacting adhesive surfaces. The effect of molecular weight on the adhesion between polystyrene and mica was also investigated, with the PS MW ranging from 590 to 1 000 000. The adhesion results are summarized in Figure 3, in which the contact time was fixed at tc = 10 s. Figure 3 shows that the effective adhesion energy increases with decreasing MW of PS from 1 000 000, reaching a peak at MW ≈ 800−1000, and then decreasing sharply as the MW is further lowered to ∼590. This dramatic effect of MW on the PS−mica adhesion (at the same tc and experimental

temperature) is attributed the increase in the mobility of the interfacial PS chains at T > Tg. It has been reported that the glass transition temperatures Tg of PS 1M, PS 1390, and PS 800 are ∼107, ∼46, and ∼−3 °C, respectively (as shown in Figure 3).2,19,41,44 The higher mobility of PS of MW < ∼1000 at room temperature would allow more effective polar interactions to be developed (at the fixed contact time) between the π electron cloud of the phenyl rings of the rearranged polystyrene chains and the polar groups on the mica surface. Another important aspect of polymers of low MW where Tg is below the ambient (room) temperature is that these films show viscoelastic/viscous behavior, and the contribution of viscous forces to the adhesion cannot be neglected. In this case, as for PS 800 in Figure 3, surface detachment does not occur at the PS−mica interface; instead, the adhesivea junction (i.e., meniscus neck region) formed between the polymer film and mica during contact narrows during the unloading separation, coupled with the appearance of viscous fibrils due to the “Saffman−Taylor fingering instability”45 at the polymer−air neck interface, as previously reported for such systems.19,41,46,47 Separation or detachment finally occurs when the bridge “snaps”. As shown in Figure 3, the effective adhesion energy for such viscoelastic junctions peaks at a MW where Tg is close to the experimental temperature; this effect is due to the maximum in the viscoelastic energy being dissipated at these conditions of temperature, separation rate, and possibly also the contact time (where the so-called Deborah number is close to 1.0).34 To further investigate the effects of segment polarity, UV/ ozone treatment was applied to increase the polarity of the PS films.26,27 Figure 4 shows typical topographic AFM images of E

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Figure 8. (a−d) Images of FECO patterns upon separation of PS 1M (UV/ozone treatment for 60 s) vs mica: the “ripples” or discontinuities on FECO patterns outside the flat contact regime manifest the formation of fracture patterns of concentric polymer rings; meanwhile, the contact diameter decreases in steps with lowering the load on separation, which indicates “stick−slip” detachment. (e, f) Top-view normal optical microscope images of the polymer residue concentric ring pattern left on the mica (e) and the PS surface (f) after separation.

Figure 7. (a) Effect of contact time tc at maximum load of F⊥,max on the effective adhesion energy for UV/ozone-treated PS 1M vs treated PS 1M, and treated PS 1M vs mica, compared with untreated PS 1M vs PS 1M and PS 1M vs mica. (b) Effect of UV/ozone radiation time on the adhesion of PS 1M vs PS1M and PS 1M vs mica (tc fixed at 10 s).

are shown in Figures 6b, 6c, and 6d, respectively. Sample charging was compensated for by taking the C 1s peak of the background hydrocarbon at 284.8 eV as an internal standard. The C 1s spectrum of pure (untreated) polystyrene shows two features: the peak at 284.8 eV is due to hydrocarbon, and the peak at 288.5−293.5 eV is the π−π* satellite peak. The spectra of UV/ozone treated samples (Figures 6c and 6d) can be resolved into six components: the peaks at 284.8 eV are due to C−C/C−H, and those at 285.2−285.4 eV are due to βjuxtaposition to carbons bonded to oxygen.48−50 The other four peaks at ∼286.3, ∼286.8, ∼287.8, and ∼289.3 eV are due to C−OR, β-shifted C−OR, R2CO, and RO−CO.48−50 The effects of UV/ozone treatment on the effective adhesion energies between PS 1M and mica and between two PS 1M surfaces are shown in Figure 7. Figure 7a shows the effect of contact time for the same UV/ozone treatment time. For adhesion between two PS surfaces, after exposure to UV/ozone for 24 and 60 s, the effective adhesion energy between two treated PS 1M surfaces increased by 100 and 300%, respectively, when the contact time at maximum load was fixed at tc ∼ 10 s. It was also found that Weff increased more dramatically with contact time for the UV/ozone-treated PS surfaces than the untreated PS surfaces: for the adhesion between PS 1M and mica, Weff increased by 800% when the contact time tc was fixed at 10 s after UV/ozone treatment for tUV/o = 24 s, which further increased to ∼2300 mJ/m2 (i.e., by 2200%) for tUV/o = 60 s. It was also found that for the same UV/ozone treatment time of tUV/o = 24 s, Weff increased much more with tc for treated PS 1M and mica than for two treated PS 1M surfaces.

untreated PS 1M surfaces (Figure 4a,b), and PS 1M surfaces (Figure 4e,f) after UV/ozone treatment for 1 min. The untreated PS surfaces were very smooth with a root-meansquare (RMS) roughness of ∼0.2 nm. After UV/ozone treatment for 1 min, the RMS roughness increased to ∼0.3 nm. The phase AFM images (Figure 4c,f) suggest that PS surface might show some surface heterogeneity after UV ozone treatment. The 3D topographic AFM images of PS surfaces before and after UV ozone treatment are also shown in Figure S2 to illustrate the morphology change. Figure 5 shows that with increasing the UV/zone exposure time the water contact angle on a PS 1M surface dropped from 90° (tUV/o = 0) to 20° (tUV/o = 120 s), indicating that the PS surface becomes hydrophilic after UV/ozone treatment, as expected. The increased hydrophilicity is due to the presence of polar functional groups introduced by the UV/ozone treatment.26,27 The surface chemistry of untreated and treated PS films was further characterized by XPS (Figure 6). The XPS survey spectrum of untreated polystyrene shows the presence of carbon only, while UV/ozone treated PS for 18 and 66 s show both carbon and oxygen peaks (Figure S3). As shown in Figure 6a, with increasing UV/ozone treatment time from 0 to 18 and 66 s, the atomic percentage of oxygen on the PS surface increases from 0% to 4.2% and 19.3%, respectively, which indicates an increased amount of oxygen-containing functional groups on the PS surface. The C 1s spectra of the PS 1M, treated PS 1M (tUV/o = 18 s), and treated PS 1M (tUV/o = 66 s) F

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Figure 9. Schematic for adhesive detachment between a mica surface and a polystyrene layer UV-ozone treated (which induces both chain scission and polar functional groups) enhancing the dipole-related molecular-group or segment−segment interactions resulting in chain transfer that gives rise to stick−slip detachment manifested by concentric ring residue patterns.

Figure 7b shows the effective adhesion energy Weff between PS 1M and mica and between two PS 1M surfaces as a function of UV/ozone treatment time tUV/o at a fixed contact time of tc = 10 s at F⊥,max. Weff increased much more dramatically with UV/ ozone treatment time tUV/o for the treated PS 1M and mica than for the treated PS 1M surfaces. The results in Figure 7 demonstrate that the effects of contact time tc and UV/ozone treatment time tUV/o on the adhesion of the asymmetric PS−mica systems show similar trends as the symmetric PS−PS systems. The adhesion enhancement of PS−mica with increasing contact time or UV/ozone treatment is much greater than for the PS−PS systems, which suggests different adhesion enhancement mechanisms. The adhesion between two polymer surfaces largely depends on the interfacial chain ends density and chain mobility (or diffusion rate) at the contact interface which can enhance the interdiffusion, interdigitation, interpenetration, and entanglement of the interfacial chains.7,15,19−21,51 The UV/ ozone-treated surfaces, with their additional polar functional groups, further enhance any dipole- or H-bonding molecular group or segment−segment interactions, thus further enhancing the adhesion. For PS vs mica, interdigitation, interpenetration, and entanglement of interfacial PS chains with the apposing atomically smooth and rigid mica surface are not possible. Thus, the adhesion between PS and mica, which also increased dramatically with increasing contact time, must be attributed to the rearrangement of the polystyrene chains/ segments at the mica surface and the development of attractive hydrogen bonds between the π electron cloud of the phenyl rings (on PS) and the surface hydroxyl groups on the mica, as previously reported for PS and glass using sum frequency generation vibrational spectroscopy.43 UV/ozone treatment can enhance all nondispersive interactions, such as dipole−dipole, dipole induced dipole, and hydrogen bond interactions between oxidized PS chains and mica. When a PS surface or film is UV/ozone treated, chain scission can also occur,26,27 which lowers the average MW of the exposed polymer chains. Material transfer was observed with high MW PS 1M after UV/ozone treatment (cf. the material transfer with untreated low MW PS in Figure 4d,e). Interestingly, on separating this treated (i.e., initially high MW) PS from mica, it was noted that some of the cohesive bonds between the PS chains rather than the interfacial adhesive bonds between the treated PS and mica got fractured. These fractures occurred locally, i.e., at different radial locations within the “contact zone”, leading to “stick−slip” detachment. The surface patterns of the transferred material (or residue) resulting from this mode of detachment were characterized by concentric polymer rings left on the apposing mica surface, which were

visualized both with the FECO fringes (Figure 8a−d) and topview microscope imaging (Figure 8e,f). The JKR detachment process for a curved surface separating from adhesive contact with a flat surface (or two curved surfaces) proceeds via a “peeling” process, where the decreasing contract diameter is described by the JKR equation.31,33,52 To the best of our knowledge, no study has so far reported a JKR detachment process via a stick−slip mode. The stick−slip peeling detachment and energy dissipation behavior observed here resembles commonly observed stick−slip friction during sliding. And advancing or receding liquid fronts on surfaces (the liquid−vapor−solid three-phase contact boundaries) can also proceed via stick−slip motion. Figure 9 is a schematic illustration of how UV-ozone-treated PS (which induces both chain scission and polar functional groups) could result in a chain transfer and detachment mechanism that gives rise to stick−slip detachment and concentric ring residue patterns.53 More concentric ring-like polymer facture patterns on UV/ ozone-treated PS and mica associated with their adhesive detachment are shown in Figure S4. It is worth noting that stick−slip separation, peeling, and/or sliding processes have been shown to be one of the major causes of damage and wear of surfaces.6,25,32,34,54

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CONCLUSIONS In this work, the adhesion and detachment mechanisms of polystyrene films and mica substrates, as well as two PS surfaces, were directly probed using the SFA and other surface imaging and characterization techniques. The effects of molecular weight (MW), contact time (tc), and UV/ozone treatment on the adhesion of PS vs mica (asymmetric case) and PS vs PS (symmetric case) were investigated and compared. It was found that that increasing the chain mobility, polarity, and chain-end density (or lowering the MW) all contribute to enhanced adhesion strength for both the symmetric and symmetric systems. The adhesion enhancement for the asymmetric (PS−mica) system with increasing contact time or UV/ozone treatment is proportionately greater than for the symmetric (PS−PS) system, which is attributed to their very different adhesion mechanisms. For the adhesion between the two polymer surfaces (symmetric PS−PS), the increased interfacial chain ends density and chain mobility at the contact interface enhances the interdiffusion, interdigitation, interpenetration, and entanglements of the interfacial chains, while the UV/ozone treatment induces polar function groups which enhance the polar interactions between the two treated polystyrene surfaces. For asymmetric PS−mica systems, such interdigitations, interpenetrations, and entanglements between the interfacial PS chains and the apposing rigid mica surface G

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Macromolecules

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cannot occur. However, the increased chain end density, chain mobility, and especially polarity (induced by UV/ozone treatment) facilitate the rearrangement of the PS chains/ segments and the development of attractive hydrogen bonds and nondispersive polar interactions between the treated PS chains and mica. Importantly, “stick−slip” detachment behavior was observed during the separation of UV/ozone treated polystyrene from mica, coupled with concentric ring-like polymer residues left on the mica surface after separation. Our results provide new insights into the molecular events and mechanisms occurring during the adhesion, detachment, and damage (wear) of polymer surfaces and solid substrates.

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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.6b00949. AFM images and XPS spectra of untreated and UVozone-treated polystyrene surfaces and microscopic images of mica and UV-ozone-treated polystyrene surfaces after adhesive detachment (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph 780-492-1044, Fax 780492-2881 (H.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) (H. Zeng), and the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-87ER-45331 (J. Israelachvili - SFA adhesion measurements, analysis and interpretations of results).

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ADDITIONAL NOTE Strictly, the detachment is now cohesive failure rather than adhesive failure. a

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00949 Macromolecules XXXX, XXX, XXX−XXX