Subscriber access provided by Universitätsbibliothek Bern
Article
Debonding mechanisms of soft materials at short contact times Chelsea Simone Davis, Florian Lemoine, Thierry Darnige, David Martina, Costantino Creton, and Anke Lindner Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5023592 • Publication Date (Web): 15 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 24
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Davis, et al.
Debonding mechanisms of soft materials at short contact times Chelsea S. Davis1,2†, Florian Lemoine2, Thierry Darnige1, David Martina2, Costantino Creton2 and Anke Lindner1* 1
Laboratoire de Physique et Mécanique des Milieux Hétérogènes, UMR 7636 CNRS/ESPCI, Université Pierre et Marie Curie, Université Denis Diderot, 10, rue Vauquelin, 75005 Paris, France 2
Laboratoire de Sciences et Ingénierie de la Matière Molle, UMR 7615 CNRS/ESPCI, Université Pierre et Marie Curie, 10, rue Vauquelin, 75005 Paris, France †
Present address: Polymers and Complex Fluids, MSED, NIST, 100 Bureau Drive, MS 8542, Gaithersburg, MD 20899, USA *
Correspondence to: Anke Lindner (E-mail:
[email protected])
1
ABSTRACT
A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked polydimethyl siloxane (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined JohnsonKendall-Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMSGlass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 ms to 10,000 ms. This influence of dwell time is particularly pronounced for polymer-polymer (SBR-SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported. 2
KEYWORDS
Adhesion, rubber, contact time, dwell time, PDMS, SBR, entanglement 3
INTRODUCTION
In a number of industrial applications adhesive contact between two soft surfaces or a soft and a hard surface must be established after very short contact times. These scenarios can involve crosslinked or uncrosslinked polymeric materials. A very well-known example is found in tire manufacturing, where several layers of uncrosslinked or “green” filled rubber are rapidly laminated together for tire assembly.1–4 For obvious reasons, delamination of the layers before final crosslinking is extremely undesirable and a good control of the adhesion between these elastomeric materials at short dwell times is essential. Additional examples in industry where Page 1 of 24
ACS Paragon Plus Environment
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 24
Davis, et al. strong adhesion between soft materials, usually called tackiness, must occur quickly are in the production of athletic footwear, hoses (for industrial and automotive uses), as well as rubber belts reinforced with steel or high stiffness polymeric fibers. For the latter case, the interface between a soft and rigid material is of interest. From a fundamental point of view, contact between two polymeric surfaces above their glass transition temperature is particularly interesting due to the possibility of chain mobility at and across the interface. For several decades, scientists have understood that, in general terms, adhesion strength increases as a function of the amount of time that two polymer surfaces are left in contact.3,5–10 This increase is typically attributed to changes in contact area,6,11 polymer chain relaxation,12,13 and chain interpenetration9,10,13–15 across the interface. Polymer chain relaxation occurs as short portions of the polymer network rearrange and relax to conform to the impenetrable probe surface while chain interdiffusion across the interface can occur between two surfaces composed of mobile polymers.16–19 Many investigations have been conducted for dwell times ranging from hundreds of milliseconds to 105 seconds and have focused on the relationship between dwell time and debonding energy.7,8,20–22 Experiments carried out at very short contact times have typically not considered chain interpenetration and focused on the effects of surface roughness.6,7 Alternatively, long dwell time experiments have focused on times longer than typical chain relaxation timescales;23 as a result there is still very little known about the first moments of contact where changes in contact area and polymer relaxation (for impenetrable interfaces) and polymer interdiffusion (for penetrable interfaces) might be closely coupled. The most notable experimental setup used to probe short dwell times is the swinging pendulum experiment. It was developed by Gent and coworkers7,8 and allows the application of short contacts (dwell time, td ranging from 3 to 100 ms). This test has been employed primarily in the study of elastomer-glass and elastomer-elastomer adhesion scenarios for both crosslinked and uncrosslinked materials. The position of the pendulum (and thus the approach velocity) and the stiffness of the system are varied in this setup to control the duration of the dwell time. The difference between the approach and retraction velocities of the pendulum is used to calculate the total energy dissipated during the contact event, and by subtracting the energy dissipated by compression of the material, the adhesion energy can be estimated. While the swinging pendulum test is an ingenious way to measure polymer adhesion at short dwell times, changing the stiffness of the experimental setup can have complex, dramatic effects on the adhesive response, particularly in geometrically-confined adhesion scenarios.24–27 Additionally, the set-up does not permit the contact area to be imaged over the course of the test. To overcome these key experimental design challenges, our experimental setup has been developed to allow well-defined short contact times and simultaneous imaging of the contact area while holding probe velocity and system stiffness constant. Here, we utilize a newly developed experimental setup which has unprecedented, independent control of td, approach and retraction velocities and the ability to monitor normal force, displacement, and contact area during each test.
Page 2 of 24
ACS Paragon Plus Environment
Page 3 of 24
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Davis, et al. Using this new instrument, the FastTack, we have studied the relationships between td and the debonding energy, wdeb for various material interfaces over a range of dwell times spanning nearly four orders of magnitude. The debonding energy is then related to the visual observation of the debonding mechanisms. The experimental parameters (stiffness, film thickness, velocity, etc.) are held constant for all experiments so that the stress-strain curves and the contact images of the debonding process can be used as probes to measure the effect of td on a given interface. In particular, we studied two different soft materials, a crosslinked (PDMS) material representative of a very elastic rubber, and an uncrosslinked (SBR) material representative of a polymer melt and having roughly the same storage modulus as the PDMS at the strain rates used for our tests. The self-adhesion and adhesion between these materials and a rigid glass substrate was measured. For PDMS, no dependence of adhesive strength or separation mechanism on td is observed (PDMS-Glass). The debonding mechanisms observed for SBR interfaces (SBR-Glass and SBR-SBR) are shown to differ strongly with increasing td. Specific attention is paid to the establishment of molecular contact between the surfaces and it will be shown that this contact formation can be heterogeneous even for very smooth surfaces. 4
EXPERIMENTAL
We chose to focus on two representative soft polymers: crosslinked (PDMS) and uncrosslinked (SBR). The contact experiments occurred between smooth flat films contacted with a spherical probe (radius of curvature, R=16 cm, plano convex, Thor Labs) either uncoated or coated with a much thinner layer of the same material. This particular probe geometry has been selected because the extremely large radius of curvature (relative to film thicknesses, h of approximately 57 µm) allows the desired high confinement ratios of 11.3
