Crosslinking Effect on Polydimethylsiloxane Elastic ...

1 Crosslinking Effect on Polydimethylsiloxane Elastic Modulus Measuredby Custom-Built Compression InstrumentZhixin Wang, Alex A. Volinsky, Nathan D. GallantDepartment of Mechanical Engineering, University of South Florida, Tampa, Florida 33620 Correspondence to: A. A. Volinsky (E-mail: macroscopic compression test utilizing a simple custom-built instrument was employed to measure polydimethylsilox-ane (PDMS) Elastic modulus. PDMS samples with varying Crosslinking density were prepared with the elastomer base to the curingagent ratio ranging from 5 : 1 to 33 : 1. The PDMS network Elastic modulus varied linearly with the amount of crosslinker, rangingfrom MPa to MPa for the samples tested. PDMS Elastic modulus in MPa can be expressed as 20 MPa/PDMS base to curingagent ratio.)

2 This article describes a simple method for measuring Elastic properties of soft polymeric Wiley Periodicals,Inc. J. Appl. Polym. ,131, : Crosslinking ; mechanical properties; properties and characterizationReceived 22 April 2014; accepted 22 May 2014 DOI: (PDMS) is one of the most widely usedsilicone-based organic can be used as thesubstrate to grow mammalian cells, including stem cells,because of its controllable range of Elastic 4 Varyingthe degree of Crosslinking in the polymer network allows tuningits mechanical properties in a range similar to the living lower the degree of PDMS network s Crosslinking , the lowerits , the higher the degree of Crosslinking ,the stiffer the sample will be. To study the PDMS network stiff-ness Effect on the growth and behavior of cells, one would needto characterize the mechanical properties of a series of PDMS network samples cured to different crosslink are several studies focusing on measuring PDMS mechan-ical properties.

3 Some researchers resorted to tensile testing usingstandard testing ,6 However, challenges may arisewhen testing soft polymers, as they exhibit a significant toeregion in the stress strain toe region is oftenobserved at the beginning of the stress strain curve, which is anartifact caused by the slack and specimen misalignment, andmust be compensated for. This region can be quite large interms of the strain range. Ideally, the sample elongation shouldbe measured directly on the sample, and not between the tensilemachine grips, which may be challenging for the softer PDMS samples. The Elastic modulus of the stiffer PDMS samples isbelow 5 MPa, and the softer ones are well below 1 MPa, meas-ured using the tensile is clear that the calculated engi-neering stress strain curves are not linear, especially at thehigher strain ,6 Other challenges may arise trying to man-ufacture the standard specimens per ASTM geometry, whichmay be not feasible for several reasons, including the lack ofmaterial.

4 Additionally, there are also challenges associated withfixing the sample in the grips without local damage and slip-page. For example, it is clearly seen that the slope of the stress strain curves in Figure 1 of Ref. 4 is higher at the lower strainlevels, compared with the larger can also be used to measure PDMS localmechanical properties. However, there are issues associated withdetermining the initial point of contact of the indenter tip, andadvancedin situtests inside the scanning electron microscopemay be required to accurately determine the contact are other high resolution approaches. For example, Lev-ental et al. used a 40 nm resolution hydraulic micromanipulatorand a 1mN resolution tensiometric force probe adapted fromthe surface tension measurement apparatus of a Langmuirmonolayer trough to set up an indentation device for measuringmicrometer-scale tissue stiffness, which potentially could also beused for the validity of the mechanical prop-erties values obtained by nanoindentation using both static anddynamic techniques with the indenter tips of different geometrywill be the subject of another paper.

5 Another advancedapproach utilizes the buckling of a polystyrene thin film sensor,which is attached to a polymer surface with an Elastic modulusbelow 10 mechanical analysis (DMA) is also aviable alternative to test the complex modulus of the PDMS network,11however it was not utilized in this study, whichVC2014 Wiley Periodicals, APPL. POLYM. , DOI: (1 of 4)introduces a simpler method based on the unconfined macro-scopic compression test. Here, a custom-built macroscopic com-pression instrument was used for reliable and repeatablemeasurements of the PDMS network stiffness with varieddegrees of Crosslinking . The reason a custom-built instrumentwas needed is because some of the softer PDMS samples couldnot be tested in compliance with the ASTM AND METHODSS ample PreparationSylgard 184 silicone elastomer base and Sylgard 184 siliconeelastomer curing agent manufactured by Dow Corning (Mid-land, MI) were used to make the PDMS thisstudy, a series of PDMS network samples with different base/curing agent mass ratios were used to explore the relationshipbetween the Elastic modulus and different amount of PDMS network s Crosslinking , whose base/agent mass ratios were 5 : 1,7 : 1, 10 : 1, : 1, 25 : 1, and 33.

6 1, ,14 Sheets of PDMS ( 3 mm thick) were made by thoroughlymixing the corresponding elastomer base and the curing agentmixtures, pouring the mixture into a flat bottom polystyrenedish and degassing the PDMS under vacuum to remove airbubbles. The PDMS was cured in an oven at 65 C for 1 h. Aftercuring, the PDMS network samples were cut out into 3 mmand 4 mm nominal diameter cylinders with the stainless steelcylindrical biopsy punches. The actual diameter of each cut outsample was measured using electronic calipers prior to compres-sion InstrumentA custom-built compression test apparatus is shown schemati-cally in Figure 1. The stage is made from a thick polished gran-ite slab to provide a stable working platform and minimizevibration. The vertical post and the clamp are made from stain-less steel.

7 The Mitutoyo digital depth gauge (Model 547-258,made in the USA) is attached to the stage by the horizontalclamp. The spring inside the Mitutoyo depth gauge wasremoved to minimize the additional force exerted on the sampleby the gauge. A weight supporting platform was added on topof the vertical displacement pin passing through the gauge, anda flat cylindrical punch, 5 mm in diameter, was screwed intothe bottom of the gauge. The displacement resolution of thegauge is 1mm. The displacement gauge calibration was checkedusing thickness standards and digital 1 illustrates how the macroscopic compression test isconducted. The weight placed on the platform attached to thetop of the gauge applied a force to the sample, compressing , the sample was pre-loaded to make sure that the cylin-drical flat punch and the sample are in full contact.

8 Otherwise,the toe-like effects were observed on some samples, similar tothe tensile testing this study, the preload of 50 g wasused. The additional weight of the flat punch with the gaugecenter rod assembly was offset by the small friction in thegauge, as the unsupported unloaded rod would not move atany position because the spring was intentionally removed fromthe displacement gauge. The gauge measured the displacementas more weights were added to the platform, up to the total of250 g, corresponding to about 10 25% maximum strain,depending on the sample. Four samples were tested for eachtype of the PDMS network, two 3 mm diameter samples andtwo 4 mm diameter samples, except for the softer 25 : 1 and 33: 1 samples, for which only two samples were tested.

9 The softer25 : 1 and 33 : 1 samples are quite tacky, making it challengingFigure illustration of the macroscopic compression instru-ment setup. [Color figure can be viewed in the online issue, which isavailable at ]Figure : 1 PDMS network compression test results. [Color figurecan be viewed in the online issue, which is available at ] APPL. POLYM. , DOI: (2 of 4)to cut a cylinder with a smaller 3 mm diameter. Each samplewas tested only once to avoid the Mullins ,16 Compression Data AnalysisThe change in the compressive stress can be calculated as:Dr5Dm 9:834pD2512:477 DmD2;(1)whereDmis the additional mass placed on the platform abovethe gauge andDis the sample diameter (3 mm or 4 mm). Thecorresponding change of the sample strain is:De5 DLL0;(2)whereDLis the change in the sample height under compressiveforce measured by the displacement gauge, andL0is the origi-nal sample height.

10 The original sample height was measured byusing both the displacement gauge and the digital to Hooke s law, the Young s modulus,E, is expressedas:E5 DrDe(3)for an ideal Elastic solid, whereDris the change of the stressandDeis the corresponding change of the strain. The strainrange in this study was 1 25%, accounting for the pre-loadweight. The advantage of this approach is that the change ofthe strain is calculated as a function of the change of theapplied stress, minimizing initial contact 2 shows the results of the 10 : 1 PDMS network com-pression test. A linear fit of the data gives the slope of Pa, meaning that the Elastic modulus of the 10 : 1 PDMS network sample is MPa. It is important to properly deter-mine the necessary pre-loading weight to develop full contactbetween the flat punch and the sample, which depends on thesample geometry and the Elastic modulus.