Letter Cite This: ACS Macro Lett. 2019, 8, 1080−1085
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Uniaxial Extension of Ultrathin Freestanding Polymer Films R. Konane Bay and Alfred J. Crosby* Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States
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S Supporting Information *
ABSTRACT: The mechanical properties of ultrathin polystyrene (PS) films have been shown to change as the thickness approaches the average size of a polymer molecule. Previous measurements of the uniaxial stress−strain relationship for ultrathin polymer films have required the use of liquid-support layers. However, the influence of the liquid support layer, specifically water, on the mechanical properties of PS films has remained an open question. Here, we introduce a method for directly measuring the complete stress−strain response of ultrathin freestanding polymer films. For freestanding PS thin films, we observe a constant elastic modulus and maximum stress with decreasing thickness for film thicknesses as thin as 30 nm, consistent with the liquid supported measurement. From the freestanding measurements, we identify that the liquid supporting layer leads to enhanced craze stability for ultrathin PS films. We compare these results to the previous liquid-supported measurements and provide insights into how the liquid surface interactions can alter polymer behavior in thin polymer films. film.15 One of the keys to TUFF is the development of a method to fabricate freestanding polymer “dog-bone” shaped films. The PS (Mw = 151.5 kDa, PDI = 1.09, Scientific Polymer; Mw = 1007 kDa, PDI = 1.15, Polymer Source) films are spun-coat out of dilute toluene solutions onto freshly cleaved mica. The films are vacuum annealed to remove residual solvent and stress (annealing conditions are provided in the SI). The films are cut into two rectangular shapes (∼4 mm × 24 mm, ∼20 mm × 24 mm) and floated onto the surface of deionized water. The smaller film is picked up with a silicon wafer to measure the film thickness with ellipsometry. The larger film is picked up off the water surface with a metal frame (Figure 1a,b and Movie S1). The frame has a slotted opening where the film is later held between the three edges of the frame, and the open side of the frame allows for a cantilever to be attached. To pick up the film, the frame is held underwater perpendicular to the water surface with the open end of the frame oriented down. One edge of the film aligned to the top edge of the frame with the frame opening centered near the midpoint of the film edge. The frame is lifted vertically out of the water, and the film edge is pinned to the top of the frame. The orientation of the frame is important to overcome the capillary force of the water. If the frame is oriented in any other direction, the breaking of the capillary bridge leads to detachment of the film from the frame. The frame is vertically translated with a nanopositioner at a fixed speed (1000 μm/s). During the pickup process, the film attaches to all three sides
ltrathin films of polystyrene (PS), where the film thickness approaches the average size of a polymer molecule, exhibit different mechanical properties when compared to thicker samples of the same material.1−9 Although a range of methods has been recently introduced to measure the low strain mechanical properties, for example, elastic modulus, only one method currently exists to assess changes in large strain, failure-dictating properties.2,4,10, This method, which we refer to as The Uniaxial Tensile Tester for UltraThin films (TUTTUT), relies on a liquid substrate, such as water, to aid the manipulation of ultrathin films during uniaxial extension.6−9,11−14 These measurements have led to several important observations, including a slight decrease in the elastic modulus and a significant loss in maximum stress as the film thickness decreases below the average configuration size scale of a single polymer molecule.6−8 While these measurements provide insight into ultrathin film mechanical properties, the influence of the liquid substrate, specifically water, has remained an open question. Here, we report the first measurements of the uniaxial mechanical response that capture the failure response of ultrathin freestanding polymer films as thin as 30 nm. Furthermore, these measurements allow us to identify the unique role that a liquid support layer can play in stabilizing deformation failure morphologies, that is, crazes, leading to enhanced ultimate failure strains. We introduce a new technique that directly measures the uniaxial stress−strain response of freestanding polymer films as thin as 30 nm. We refer to this method as the Tensile tester for Ultrathin Freestanding Films (TUFF). To our knowledge, the previous thinnest freestanding glassy polymer film measured by uniaxial tensile testing was a 100 nm thick polycarbonate
U
© XXXX American Chemical Society
Received: May 28, 2019 Accepted: August 7, 2019
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DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
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ACS Macro Letters
Figure 1. Fabrication of freestanding films and the Tensile tester for Ultrathin Freestanding Films (TUFF). (a) Schematic of the pickup process of a polymer film from a water surface on to a support frame. Side view perspective is from the dashed line in the front view. (b) Photo of the pickup of 55 nm thick polystyrene (PS) film from the water surface with a support frame. Film is labeled UMASS with a permanent marker for visualization purposes. (c) Schematic of a laser cut “dog-bone” shaped freestanding film. (d) Photo of a laser cut “dog-bone” shaped 100 nm thick freestanding PS film. (e) Schematic of TUFF. (f) Photo of a 100 nm thick freestanding PS film loaded into TUFF.
of the frame, which also prevents the film from collapsing due to the water capillary forces. The freestanding films are then laser cut (Universal Laser System VSL 3.5, wavelength = 10.6 μm, infrared) in a “dog-bone” shape, leaving the film attached to the frame at the end of the “dog-bone”, as shown in Figure 1c,d (laser cutting parameter are in Table S1 and the geometry is shown in Figure S1). The frame with the “dog-bone” film is rigidly fixed into a clamp that is attached to a linear actuator. A reflective cantilever (aluminum coated cover glass) with an extension piece (allows for the in situ imaging of the films during tensile testing) is brought into contact with the film on the open side of the frame. The extra material at the contact point that connects to the frame is removed (Movie S1). The freestanding film is now only attached to the frame and the cantilever (Figure 1e,f). The linear actuator moves at a fixed speed; therefore, the film stretches at a fixed strain rate (8.0 × 10−3 s−1). The cantilever, which is calibrated for force (force resolution ∼ 6 μN) and displacement (displacement resolution ∼ 90 nm), deflects and using the film geometry, we calculate stress and strain (details are provided in ref 7). Representative stress−strain curves for 100 to 32 nm thick freestanding 151.5 kDa PS films are provided in Figure 2. For all film thicknesses, we observe an initial linear elastic stress− strain behavior followed by a yield response, which we associate with craze formation within the film (Movies S2 and S3). After yield, we observe an “ideal plastic” response,
Figure 2. Stress−strain (σ11 vs ε11) response of four representative 151.5 kDa freestanding PS films with decreasing thickness. σ11 is the stress applied in the x1 direction on the x1 plane, ε11 is the strain applied in the x1 direction on the x1 plane.
constant stress with increasing strain. We measure constant elastic modulus, E and maximum stress, σmax, with decreasing thickness (E calculation is provided in the SI). To examine the effect of water on the mechanical properties of thin films, we compare our results from the new freestanding method 1081
DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
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ACS Macro Letters (TUFF) to our previous results from the water supported method (TUTTUT) by comparing the E, σmax, and nonlinear response of 30 nm thick films. We measure a constant elastic modulus (E ∼ 2.8 ± 0.3 GPa) and maximum stress (σmax ∼ 48.7 ± 8.9 MPa) for the unconfined (hF/Ree > 1, where Ree is the radius end-to-end distance of the polymer molecule) freestanding 151.5 kDa PS films (Figure 3a, 3b). In the water supported method, we previously measured the similar trend for both E and σmax and comparable values (E ∼ 3.5 ± 0.5 GPa, σmax ∼ 49.5 ± 9.9 MPa) for unconfined PS films.7 The E values for both the freestanding and water supported PS films are within the range of bulk PS literature values (E = 2.28−3.5 GPa).16,17 The comparable trend and values for E and σmax are expected since PS thin films have been demonstrated not to swell in water.18 If swelling occurred in the water supported films, we would expect the water to act as a plasticizer leading to lower E and σmax. The similar values for E and σmax confirm our previous hypothesis, which proposed that water supported measurements on the PS films are in the elastic energy dominated regime and the water surface tension has a negligible impact on
Figure 4. Water acts as a craze stabilizer. (a) Stress−strain response of six independent 30 nm thick 151.5 kDa freestanding PS film. Five out of six films show low failure strain and low craze stability and one film has higher craze stability (lime green). (b) Stress−strain response of five independent 30 nm thick 151.5 kDa water supported PS film.
the stress−strain response.6 We note that water immersed PS nanoparticles have shown a reduced surface mobile layer thickness compared to freestanding PS films.19,20 The surface mobile layer has been associated with a reduction in the E in PS films thinner than we measure in this work (hF < 30 nm).3,6,7 Future improvement in the TUFF technique will enable the measurement of thinner freestanding films to investigate if the reduction in E would be greater than the previously measured reduction in E in the supported methods. While we do not observe a difference in E or σmax between water supported and freestanding PS films, we do observe a difference in the nonlinear behavior (Figure 4). In particular, the craze stability, or the extent that the material strains after craze initiation (calculation details are provided in the SI), is found to be larger for water supported films as compared to equivalent films in a freestanding state. For example, for 30 nm 151.5 kDa PS, the average craze stability for freestanding films is 0.03 ± 0.06%, while for water supported films, the average craze stability is 0.17 ± 0.12%. These differences in craze stability are evident in the stress−strain curves (Figure 4, experimental method for TUTTUT measurement is provided in the SI), as water-supported films display a plateau stress, while freestanding films generally behave in a brittle manner. Although this difference in craze stability is subtle, we hypothesize that the water acts a craze stabilizer by infiltrating between the craze fibrils and hindering crack propagation
Figure 3. Elastic modulus, E, and maximum stress, σmax, for freestanding films is consistent with previous results from water supported films. (a) E as a function of confinement (hF/Ree), where the dark gray shading is the bulk PS E values from refs 16 and 17, the black diamonds are 151.5 kDa freestanding PS, the red diamonds are 1007 kDa freestanding PS, and the blue half circles are 137 kDa water supported PS from ref 7. (b) σmax as a function of confinement (hF/ Ree). Error bars denote 5−7 independent films with the same thickness. 1082
DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
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ACS Macro Letters across the craze. We propose the capillary forces of the water between the fibrils effectively blunts a propagating crack and lessens the stress concentrations within the craze. Similarly, Ling and Yang demonstrated enhanced craze stability of PS films by sandwiching a PS film between two poly(phenylene oxide) (PPO) films, where the PPO acts as a craze stabilizer.21 Further studies are necessary to investigate the role of liquids on the fracture mechanics and failure of ultrathin polymer films; however, these initial results suggest that liquids or support layers could be used to enhance the failure properties of ultrathin polymer films. Currently with TUFF, we are limited to measuring films as thin as 30 nm, where 30 nm thick 151.5 kDa freestanding PS are unconfined (hF/Ree > 1). However, we previously observed changes in the mechanical properties in confined water supported PS films (hF/Ree < 1).6,7 To explore the effect of confinement on the mechanical properties of freestanding PS films, we measure the stress−strain behavior of higher molecular weight (1007 kDa, Ree = 67 nm) PS both in the unconfined (hF/Ree = 1.58) and confined (hF/Ree = 0.46) state. We compare the confined σmax and full stress−strain response results to previous confinement results with the water supported methods. We do not observe a transition in σmax (Figure 3b), which was observed with the 137 kDa water supported confined PS film.6,7 We associated the embrittlement or decrease in σmax to a loss of interchain entanglements in which the confined polymer molecule becomes more entangled with itself (intrachain entanglements) than with its neighbors (interchain entanglements).22,23 We hypothesize the nonembrittlement for the confined freestanding 1007 kDa PS is due to the polymer being more entangled than the water supported 137 kDa PS film. The approximate number of the entanglements per chain in unconfined 137 kDa PS film is ∼7, whereas the 1007 kDa PS film has about 59 entanglements per chain. While we hypothesize that 1007 kDa confined PS films experience a loss of interchain entanglements, the loss in number of entanglements has not dropped below a critical value, which controls the onset of severe embrittlement. To further support the nonembrittlement in high molecular weight PS, we do not observe a brittle stress−strain behavior of the confined 1007 kDa freestanding PS films. Instead, our measurements reveal high maximum strains (4.3 ± 1.7%) in the confined 1007 kDa freestanding PS films (hF = 30 nm; Figure 5a). We observe a decrease in stress with increasing strain after yield, which we associate with two tears forming slowly in the film (Figure 5b, Movie S4). The failure mode is different compared to both the water supported and freestanding 30 nm thick low molecular weight PS films, where failure occurs at a fast speed (>10 m/s) at one location in the film. Hence, while failure processes appear to change in these higher molecular weight films, the overall finding from our measurements is that the elastic modulus and maximum stress are not affected significantly in a confined regime. In summary, we introduce the freestanding method (TUFF) and compare the results to our previous water support method (TUTTUT). Both systems measure the full stress−strain response of ultrathin polymer films with a cantilever-based system (comparison of the TUFF and TUTTUT sample preparation and sample loading is provided in the SI). We compare the results for these two methods for 30 nm thick 151.5 kDa PS in Table 1. We measure a similar elastic modulus and maximum stress for both methods and a difference in craze
Figure 5. Confined 1007 kDa PS freestanding films exhibit ductile stress−strain behavior. (a) Stress−strain response of a 30 nm thick 1007 kDa freestanding PS film. (b) Optical micrograph of tearing during uniaxial extension at 5% strain.
Table 1. Comparison between the Freestanding and Water Supported Method for 30 nm Thick 151.5 kDa PS property elastic modulus maximum stress craze stability minimum thickness achieved to date
TUFF
TUTTUT
(freestanding)
(water supported)
similar (∼2.8 ± 0.3 GPa) similar (∼41.4 ± 4.4 MPa) low (∼0.03 ± 0.06%) 30 nm
similar (∼3.0 ± 0.2 GPa) similar (∼43.8 ± 3.6 MPa) high (∼0.17 ± 0.12%) 15 nm
stability. For the water supported method (TUTTUT), we previously measured films as thin as 15 nm, whereas with the freestanding method, we are currently limited to films as thin as 30 nm. It would be interesting to further develop the freestanding method to measure thinner films to observe the effects of the surface mobile layer and continued loss of interchain entanglements on the mechanical properties. In addition, the freestanding method provides opportunities to measure the recoil dynamics of the polymer film and allows for out of plane deformation, that is, bending. The water supported method also provides opportunities to investigate the role of other liquids on the mechanical properties of ultrathin polymer films. 1083
DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
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ACS Macro Letters Notes
We note another freestanding mechanical measurement method, nanobubble inflation, has been previously developed to measure the biaxial stress−strain response of ultrathin freestanding polymer films.24 While both techniques allow low strain mechanical properties to be measured, both approaches differ in terms of stress state as well as the thermal state. The nanobubble technique measures the biaxial stress−strain response near the rubbery regime (e.g., for PS, at temperatures ranging from 60 to 105 °C, changing the temperature to be above the measured glass transition temperature for each film thickness measured),25 whereas TUFF measures the uniaxial stress−strain response in the glassy regime at a constant temperature of approximately 21 °C for all samples. These differences in thermal state/history make a direct comparison difficult, currently. The most important difference between TUFF and the nanobubble technique is the ability for TUFF to quantify the large strain deformation and failure mechanisms. In the nanobubble technique, the films span micron-scale diameter holes, whereas, in the TUFF technique, the film gauge section is on a millimeter scale. This difference is significant since many failure mechanisms for polymer glasses can grow to lateral size scales, larger than micron-scale. The large lateral scale of our films ensures a uniaxial stress distribution within the gauge of the film as well as the ability to quantify growth mechanisms of failure-inducing localizations, such as crazes and shear deformation zones. To our knowledge, these are the first uniaxial stress−strain measurements beyond the low strain regime that capture the failure deformation mechanism of freestanding polymer films thinner than 100 nm. We measured a constant modulus and maximum stress with decreasing thickness for PS thin films, which is consistent with the water-supported method. However, we observe reduced craze stability of the freestanding films compared to previously measured watersupported films. We propose from these observations that the water acts as a craze stabilizer and enhances the ultimate failure strains. This tensile tester for ultrathin freestanding films provides a new technique to measure the mechanical properties of polymer thin film without the presence of a liquid, allowing for the measurement of full stress−strain behavior of water-sensitive polymer films in the future. The technique also provides opportunities to study the mechanical properties of ultrathin films that can deform out of plane and investigate the high-speed failure of the polymer films.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.K.B. would like to thank C. Bukowski, Y. Kim, and M. Ilton for their help during the experiments. We acknowledge financial support from the National Science Foundation (DMR 1608614), Northeast Alliance for Graduate Education and Professoriate at the University of Massachusetts Amherst, and the Spaulding-Smith Fellowship.
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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/acsmacrolett.9b00408. Table S1 and Figure S1 (PDF) Movie S1 (MP4) Movie S2 (MP4) Movie S3 (MP4) Movie S4 (MP4)
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REFERENCES
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R. Konane Bay: 0000-0002-3980-8491 1084
DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
Letter
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DOI: 10.1021/acsmacrolett.9b00408 ACS Macro Lett. 2019, 8, 1080−1085
