Polymer

20 Oct 2023

Investigating the effect of temperature on Young's modulus of polymer materials through the application of AFM

Introduction Polymer materials have become increasingly important in various industrial applications due to their unique physical and chemical properties. One of the key mechanical properties of polymers is their Young's modulus, which is a measure of the stiffness or rigidity of a material [1]. Temperature changes can have a significant impact on the mechanical properties of polymers [2]. As the temperature increases, the polymer’s Young's modulus typically decreases, making the material more flexible and less stiff. This is because thermal energy causes the polymer chains to move more freely, reducing their ability to resist deformation under stress [3-5]. Understanding the mechanical properties of polymer materials according to temperature changes is crucial for designing and engineering materials that can withstand various environmental conditions. In this application note, we explore the effects of temperature changes on the mechanical properties of polymer materials, with a particular focus on Young's modulus using Atomic force microscopy (AFM). Among the modes that can measure mechanical properties with AFM, the average value of Young's modulus was measured by mapping local regions of the polymer surface using the PinPoint™ nanomechanical mode. In addition, a temperature-controlled stage (TCS) was coupled to AFM to track the changes in Young's modulus according to temperature while changing the temperature from room temperature (RT, 25 oC) to 170 oC. AFM and TCS The FX40, a newly launched AFM model for research by Park Systems, was used for these measurements. The FX40 is a high-performance AFM model designed for research applications in various fields, including nanotechnology, material science, life science, and semiconductor industries. It offers several advanced features, including high-resolution imaging and a wide range of imaging modes. It also has a high-precision motorized stage that allows for precise sample positioning and scanning. Park Systems also provides TCS that can be connected to an AFM model to measure the changes in various sample properties the respect to temperature. There are three types of stages which have different temperature adjusting range, stage size and hardware setup. TCS type 1 has a fast temperature change due to active heating and cooling. While TCS type 2 and TCS type 3 increase the temperature quickly with active heating, it may take some time for the temperature to decrease due to passive cooling (Figure 1). Depending on the TCS type, it can be applied from -20 °C to 600 °C. In this test, type 1 was used that can be set the temperature from -20 °C to 170 °C .   Figure 1. Three types of TCS modules of Park Systems.     Result First, changes in the surface of polyurethane and collagen film according to temperature variation were measured using Non-contact and Tapping mode, respectively. As shown in Figure 2a, temperature fluctuations can affect the surface morphology of polyurethane. With increasing temperature, the surface becomes rougher, whereas at RT, the surface appears smoother. At higher temperatures, the polyurethane undergoes a transition from a crystalline to an amorphous state or from one crystalline structure to another. This transition can result in changes in the material's crystalline morphology, such as a decrease in crystallinity or alteration in crystal size or arrangement [6]. Collagen is a temperature-sensitive material, and its structure can change significantly as the temperature is varied. In Figure 2b, at higher temperatures, collagen film denatures, leading to changes in its structure and surface morphology and phase transition. Denaturation can cause collagen to lose its structural integrity, resulting in a rougher and more irregular surface [7].   Figure 2. Topographical comparison of materials according to temperature change. Height images of polyurethane at RT and 80 oC (a), Height and phase images of collagen film at RT and 100 oC (b)   Next, Young's modulus measurements by PinPoint™ nanomechanical mode were performed according to temperature change for five polymers. As this is a test to measure changes in mechanical properties depending on temperature, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), and polymethylpentene (PMP) and Polyvinylidene fluoride (PVDF) were selected in consideration of rigidity and melting point with thermoplastic properties (Table 1). Each polymer was prepared in the form of a 2×2×0.5 cm square film and attached to a metal disk using a thermal compound.   Table 1. Properties of polymers   All five polymers have thermoplastic properties, Young's modulus of 0.1 ~ 3 GPa, and a melting point of 115 ~ 260 oC. Before the measurements, the AFM probe was calibrated for accurate and quantitative results. Figure 3 shows Young's modulus images of LDPE (Relatively low Young's modulus and melting point among the five polymers) and PVDF (High Young's modulus and melting point) as a function of temperature. As can be seen in the Young's modulus images and the center line profile of each image, as the temperature increases, the Young's modulus of the polymers decreases. This trend can also be confirmed in the measurements for all polymers in Table 2 and Figure 4.   Figure 3. Young’s modulus images and center line profiles at different temperatures. Young's modulus values at center line profile indicate that the values decrease as the temperature increases.   At higher temperatures, the mobility of polymer chains increases due to increased thermal energy, which causes the polymer to become more flexible and less resistant to deformation. This increased mobility allows the polymer chains to move more easily and rearrange themselves under stress, leading to a lower Young's modulus. Additionally, at higher temperatures, the intermolecular forces holding the polymer chains together become weaker, which also contributes to the lower Young's modulus. Therefore, the combination of increased chain mobility and weakened intermolecular forces at higher temperatures leads to a decrease in Young's modulus of a polymer. Table 2. Young’s modulus of polymers at different temperatures. All measurements were performed by PinPoint nanomechanical mode and analyzed by SmartAnalysis (Image analysis program, Park Systems).   Figure 4. The graph of Young’s modulus-Tempurature of 5 polymers. The graphical information shows a similar trend for all polymers.   Conclusion In this application note, we discuss the effects of temperature changes on the mechanical properties of polymer materials, specifically focusing on Young's modulus. Using an AFM equipped with a TCS, we measured changes in Young's modulus based on temperature for various polymers, including LDPE, HDPE, PS, PMP, and PVDF. As the temperature of a polymer material increases, its Young's modulus typically decreases, making the material more flexible and less stiff. The properties of polymers can vary depending on factors such as their molecular structure, composition, and processing conditions, including temperature. Studying the properties of polymers with respect to temperature is important for various industries, such as materials science, polymer engineering, and polymer processing, as it can help in developing new polymer materials with tailored properties for specific applications, optimizing processing conditions, and ensuring the performance and reliability of polymer-based products in different temperature environments. Also, researchers and engineers can better understand their behavior and performance under different conditions, and design polymers with specific properties for desired applications. References 1. Ward, I. M., & Sweeney, J. (2012). Mechanical properties of solid polymers. John Wiley & Sons. 2. Shahabaz, S. M., Sharma, S., Shetty, N., Shetty, S. D., & Gowrishankar, M. C. (2021). Influence of temperature on mechanical properties and machining of fibre reinforced polymer composites: a review. Engineered Science, 16, 26-46. 3. Huang, P. Y., Guo, Z. S., & Feng, J. M. (2020). General model of temperature-dependent modulus and yield strength of thermoplastic polymers. Chinese Journal of Polymer Science, 38, 382-393. 4. Nasraoui, M., Forquin, P., Siad, L., & Rusinek, A. (2012). Influence of strain rate, temperature and adiabatic heating on the mechanical behaviour of poly-methyl-methacrylate: Experimental and modelling analyses. Materials & Design, 37, 500-509. 5. Kendall, M. J., & Siviour, C. R. (2014). Experimentally simulating high-rate behaviour: rate and temperature effects in polycarbonate and PMMA. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 372(2015), 20130202. 6. Arévalo-Alquichire, S., Morales-Gonzalez, M., Navas-Gómez, K., Diaz, L. E., Gómez-Tejedor, J. A., Serrano, M. A., & Valero, M. F. (2020). Influence of polyol/crosslinker blend composition on phase separation and thermo-mechanical properties of polyurethane thin films. Polymers, 12(3), 666. 7. Shanmugam, G., & Polavarapu, P. L. (2009). Structural transition during thermal denaturation of collagen in the solution and film states. Chirality: The Pharmacological, Biological, and Chemical Consequences of Molecular Asymmetry, 21(1), 152-159.

17 Feb 2022

Characterization of roughness and mechanical properties of a contact lens surface using atomic force microscopy (AFM) with a liquid cell for contact lens

Research Application Technology Center Abstract A soft contact lens is mostly used type of contact lens which is made of gel-type material. This slippery and transparent material with a unique curvature like human eye requires cutting, drying or freezing to fix for a surface characterizing. Considered that most contact lens is used in moist environment inside of human eye, these preparations may result in inaccurate research peroration. We introduce a liquid cell for contact lens which can place a soft contact lens without deformation and provide a stable environment for AFM application. As fully immersed in saline solution, surface of a soft contact lens with color coating was scanned by tapping and pinpoint mode and successfully acquired roughness values, Young’s modulus, and adhesion energy of the lens surface with nano-scale resolution. Introduction As a replacement for glasses, contact lenses are a commonly used tool for vision correction with benefits such as comfort and convenience. Their application has expanded into the cosmetic market and contact lenses are considered as substitute for invasive surgery. A variety studies have focused on the characteristics and material properties of contact lenses, which have led to a versatility in properties and application fields [1, 2]. Technical reviews on contact lenses often name surface roughness and material analysis as key factors that define the comfort for the customer as well as the resistance against bacterial contamination [3, 4, 5]. High tech manufacturing innovations of the lens mold facilitated lens surfaces with roughness in the nanometer-scale. Lens surface takes on distinctive roughness and surface adhesion features depending on their manufacturing process and the after treatment [6]. These features establish guidelines for commercial use of contact lenses and lead to innovations in research industry. Among commonly used microscopy techniques, AFM is best suited for studying surface properties on the nanometer scale. The ability of AFM for quantitative topography analyzation surpasses any other system; features such as pores or surface defects can be mapped in three dimensions. Using force distance spectroscopy, AFM additionally obtains mechanical properties like surface adhesion and Young’s modulus. While this versatility is quite useful, the domed shape of a contact lens contrasts with the need for a flat surface in AFM system to obtain useful surface data. Eventually, lenses are cut or deformed in a way to flatten the surface enough to achieve stable measurement conditions. However, the deformation of the lenses for AFM measurements could entail a loss of information on actual mechanical properties. Here, we introduce a liquid cell for a contact lens measurement. This special cell design has a convex structure with a similar curvature to human eye to hold a contact lens without deformation. A cover with a hole in the middle fixes the sample and enables access for the AFM tip. We demonstrate surface roughness measurements using tapping mode as well as the Park Systems’s PinPointTM nanomechanical mode to determine additional properties such as the difference in Young’s modulus and adhesion energy between the center and the color coated layer. Figure 1.(a) Schematic structure of the liquid cell design for contact lenses, (1) Cell adapter for AFM body (2) Cell Base (3) Cell bracket (4) O-ring between base and bracket (5) 2Ф bolts (6) 2.5Ф bolts. (b) Cell adapter installed on the AFM body (c) Cell base and bracket components connected with soft contact lens inside. Materials and Methods Liquid cell for contact lens The cell design consists of three main parts, the cell adapter, the cell base and the cell bracket (bottom to top in figure 1 (a)). The cell adapter is screwed into AFM body first (Figure 1 (b)), and then the assembled cell is magnetically attached to the adapter. This configuration allows an independent assembly and manipulation of the liquid cell to facilitate an easy preparation of measurement. A dome Experimental Set-up To demonstrate the working principle of the cell, we mounted it to an NX12 AFM from Park Systems. We applied saline solution to the cell and fully immersed the contact lens under this solution to maintain the aqueous environment, in which actual usage of the lens mostly occurs. A Biolever Mini (BL-AC40TS) cantilever from Olympus was mounted on the liquid probe hand enabling inliquid tapping and pinpoint modes on the soft contact lenses with color coating. The resonant frequency of the cantilever is 110 kHz in air and 25 kHz in water. We chose the Biolever Mini cantilever with a small spring constant (k = 0.1 N/m) for its softness due to concerns that deformation by the cantilever could change the surface. Figure 2. (a) Concept drawing of a soft contact lens with a color layer. (b) Optical camera image: Center of the lens surface on the convex top region and (c) color coated area. The red circles mark the AFM scan areas. The optical image scale for both (b) and (c) is 480 µm × 360 µm. Figure 3. All images are scaled to the same color. (a) Topography result of the center region on the top of the lens. Topography on the edge (b) and inside (c) of the color dot. (d) Peak to valley (Rpv), root mean square roughness (Rq), and average roughness (Ra) chart from (a), (b), and (c). Figure 4. Height (a), modulus (b), and adhesion energy images (c) at the center of the lens and at the color dot region (d), (e) and (f), respectively. (g) Modulus and adhesion energy mean values chart; modulus and adhesion energy values in center is from (b) and (c), modulus of the edge and inside of color pigment were partially analyzed from (e), and adhesion energy for mean value of whole image except some high peaked distinctive points from (f). All images were performed using PinPointTM nanomechanical mode. Result and Discussion Using an optical camera with 10× magnification, we examined the sample surface prior to the AFM measurement. The center of this contact lens is transparent and the coating layer covers the rim of the lens as common in colored contact lenses (Figure 2 (a)). Optical images of these two different regions are shown in figure 2; in the center (b) and on the color coated outer ring (c). Each of the optical images are marked with a red dotted circle where the AFM topography was scanned in the following. For the center region of the lens, we simply zoomed into the center of the optical position and scanned a 20 µm × 20 µm region (Figure 3 (a)). Same measurements were conducted on both edge (Figure 3 (b)) and inside (Figure 3 (c)) of the color pigment for comparison. For analysis, three different roughness values were reported; peak to peak difference between the highest and lowest height value (Rpv), root mean square roughness (Rq) and the average roughness value (Ra). All three roughness values (Rpv, Rq, and Ra) displayed the same trend, with the smallest roughness values for the center of the lens and highest roughness in the color pigment, respectively. The color coating increased the surface roughness more than two times compared to the center region without coating layer. Furthermore, topography data obtained at the inside of color pigment showed noticeable differences compared to outside of color pigment. For a comparison of the mechanical properties, we scanned the center of the lens and the edge of the coating with the PinPointTM technique. While the center region showed homogenous roughness and mechanical property characteristics (Figure 4 (a, b, c)), the edge of the color coating clearly indicated a structural change figure 4 (d, e, f). Coating layer edge is scanned for immediate comparison between edge (upper left side region) and inside (bottom right side region) of coating (Figure 4 (e)). As the roughness increases towards the color coating, the mechanical properties reveal that this coating consists of two phases with hard coating pigments embedded in a softer matrix: With respect to the edge of coating, the color pigments are harder and the bottom matrix of the coating is softer. While these Young’s modulus difference is obvious, this feature does not appear in adhesion energy (Figure 4 (e, f)). There were points with high adhesion energy in figure 4 (f) which were mainly distributed on soft matrix. In these distinctive points, the adhesion energy was 70.48 aJ which was over ten times higher than rest of the image region. Since the tip and sample reaction is acted under saline solution, adhesion energy was not following the regional contrast observed in the modulus image in color pigment images due to the cantilever damping in liquid. Conclusions We introduced a liquid cell with a curvature similar to human eye structure. This cell provides a simple and stable measurement environment for soft contact lenses without deforming or destroying the lens and facilitates imaging in saline solution. Roughness measurements were obtained with a high resolution, comparing the color coated layer with the non-coated substrate. We discovered a clear change in topography as well as the Young’s modulus between the two regions of the lens. These results represent a first reference of an AFM study on contact lenses, which can then be used to analyze critical issues such as degradation and bio-fouling. The new innovative cell design will allow the AFM to become a reliable tool in the contact lens industry and revolutionize the way that contact lens can be studied on the nanoscale. References 1. Carnt, N., Wu, Y., & Stapleton, F. (2017) Contact Lenses, Reference Module in Neuroscience and Biobehavioral Psychology. 2. Furqan A. Maulvi, Tejal G. Soni & Dinesh O. Shah (2016) A review on therapeutic contact lenses for ocular drug delivery, Drug Delivery, 23:8, 3017-3026. 3. Gerda M. Bruinsma, Minie Rustema-Abbing, Joop de Vries, Boudewijn Stegenga, Henny C. van der Mei, Matthijs L. van der Linden, Johanna M. M. Hooymans, and Henk J. Busscher (2002) Influence of Wear and Overwear on Surface Properties of Etafilcon A Contact Lenses and Adhesion of Pseudomonas aeruginosa, Investigative Ophthalmology & Visual Science December 2002, Vol.43, 3646-3653. 4. Maria Jesus Giraldez and Eva Yebra-Pimentel (2012) Hydrogel Contact Lenses Surface Roughness and Bacterial Adhesion, Ocular Diseases, Adedayo Adio, IntechOpen. 5. Ţălu, Ştefan & M., Ţǎlu. (2012) Surface roughness of contact lenses investigated with atomic force microscopy, The Scientific Bulletin of VALAHIA University – MATERIALS and MECHANICS, Romania, no. 7 (year 10), p. 107-110. 6. Jones, Lyndon & Subbaraman, Lakshman & Rogers, R. & Dumbleton, Kathryn. (2006). Surface treatment, wetting and modulus of silicone hydrogels. Optician. 232. 28-34.

28 Apr 2020

High Resolution Phase Imaging of Polymer Thin Film Coated on Gold (Au)

John Paul Pineda, Charles Kim, Byong Kim, and Keibock Lee Park Systems Inc., Santa Clara, CA USA INTRODUCTION Functional polymer thin filmshave numerous technological applications ranging from optoelectronics to sensors. Theirtunable properties, flexibility, and low-cost preparation make them attractive to the industry. [1]The properties of polymer thin films are heavily affected by the distribution, shapes, and monomer composition of individual macromolecules. For this reason, designing polymer properties requires a characterization technique that canimage material contrasts with high spatial resolutions to visualize the polymer morphologyon the nanoscale and measuremacromolecule sizes accurately.[2] Scanning Electron Microscopy (SEM) and Energy-filtered transmission electron microscopy (EFTEM) are common methods to characterize polymer thin films. However, SEM has a low spatial resolution when imaging nanostructured organic samples containing light elements only, while EFTEM has high level of knock-on damage due to high accelerating voltages applied during operation. [3, 4]An effective tool to overcome these problems is Atomic Force Microscopy (AFM). AFM is a nondestructive technique that provideshigh-resolution images required for nanoscale characterization. Moreover, this tool can be operated in different measurement modes that image material properties in addition to topological information.Among these, Phase Imaging is a powerful AFM mode used by scientists and engineers to study compositional heterogeneities in polymer materials.This mode provides complementary information to the topography image by revealinglocalvariations of surface properties and compositionsinthe investigated materials. In this study, a Park NX20 was used to characterize polymer thin filmscoated on Gold(Au) using Tapping Mode AFM. A high-resolution phase image acquired simultaneously with the topography imagerevealed that surface changes occurred after heating the sample and applying DC tip bias. The conformation and phase separation of the polymer domains were measured and evaluated. EXPERIMENTAL A polymer thin film coated on Au on a silicon wafer was heated up to 230ıCusing a Park Systems temperature control stage.Additionally a 1V DC voltage was applied between the AFM tip and the sample surface.With aPark NX20, both topography and phase image were obtained simultaneously by using Tapping Mode AFM under ambient conditions. Imageswere acquiredbefore and after the sample had undergonethe heating process and tip bias application. A conductive MikromaschNSC36A Ti-Pt cantilever (nominal spring constant k = 1 N/m and resonant frequency f = 90 kHz) was used in the experiment. RESULTS AND DISCUSSION In Tapping Mode, the tip oscillates near its resonance frequency, which allows the tip to periodically touch the sample surface at the lower turning point of the oscillation.Topography images are obtained by measuring the changes in the vibrational amplitude of the cantilever induced by the attractive Van-der-Waals force as the cantilever is mechanically oscillated near its resonant frequency during the scan. The measured changes are compensated by the AFM’s feedback loop, which maintains the cantilever’s constant amplitude and distance by controlling the Z scanner movement. [5]The phase image is acquired simultaneously by monitoring the phase lag between the signal that drives the cantilever oscillation and its output signal, as shown in Figure 1. Figure 1. Schematic Illustration of Phase Imaging [6]. Table 1 shows the topography and phase images of a polymer thin filmbefore and afterheating and bias application.The topography image acquired before the treatmentreveals asmooth sample surface, while the phase image showsthat the surface was composed of lamellar polymer fibrilsdistributed homogeneously over the surface.On the other hand, the topography acquired after the heat and bias treatment revealsthat two circular protrusions (inner and outer protrusions) formed in the center of the scanned surface. The inner circular protrusionhas a height of around 53 nm and a diameter ranging from 1.4 to 2.0μm. The outside circular protrusionhas a lower height of around 30 nm anda diameter ranging from 2.0 to 4.0μm. Thesurrounding area isrelatively flat with a mean square surface roughness of 0.8 nm.By comparing topography and phase image, it could be observed that the orientation of the lamellar fibrils in the inner and outer circular protrusions is re-arranged and the fibrils are aligned differentlyin both protrusions as well as the flat surface.   Table1:Topography and Phase image before and after heating the sample at 230ıC and applying 1V DC voltage between the sample and the tip. Figure 2a and 2bare the enlarged view and corresponding 3D representation of the area highlighted in red in the phase image of Table 1.Both images clearly show distinct differences in the fibril alignment.As such the upper and lower part of Figure 2a captured periodic fibrils align in-plane, while the dotted area in the center of the Figure corresponds to domains with fibrils that re-arranged inthe vertical direction during the heat and bias treatment. Figure 2c is the corresponding line profile generated using Park XEI image analysis software to evaluate the sizes of the lamellar fibrilsas well as phase separation. The measured width of the lamellar fibrilswas approximately 26 nm, while separation between each fibrils was approximately 22 nm. Figure 2. (a) Enlarged view of the phase image in table 1 (after heat and bias treatment), (b) 3D view of Figure 2a, (c) Corresponding line profileof Figure 2a. SUMMARY This study demonstrates the suitability of phase imaging for the characterization of polymer thin films. The high-resolution phase images obtained in the experiment reveals significant surface changes of the polymer thin films after they underwent heating and bias application. It is observed that during the heat and bias application, some polymer fibrils re-aligned to a vertical orientation and others remained in-plane. Additionally, the in-plane fibrils formed domains of varying orientations. Thus, phase imaging can provide unique insights of how temperature and DC voltage affect the fibril arrangement and orientation of polymer thin films, as required to design customized properties. Since phase imaging operates in AFM tapping mode, this technique allows safe acquisition of high-resolution images without the continuous tip-sample contact. In sum, phase imaging is a promising tool in applications that require high-resolution imaging of material contrasts on the nanoscale. REFERENCES 1. S. Satish, et al., Preparation and characterization of nanoscale PMMA thin films Indian Journal of Pure & Applied Physics. Vol. 52, January 2014, pp. 64-67. 2. V. Korolkov, et al., Ultra-high resolution imaging of thin films and single strands of polythiophene using atomic force microscopy. Nature Communications volume 10, Article number: 1537 (2019) 3. J. Brostin, Compositional Imaging of Polymers Using a Field Emission Scanning Electron Microscope with a Microchannel Plate Backscattered Electron Detector. SCANNING Vol. 17, 327–329 (1995). Received March 13, 1995 and Accepted with revision May 22, 1995. 4. R. Masters, et al., Sub-nanometre resolution imaging of polymer–fullerene photovoltaic blends using energy-filtered scanning electron microscopy. Nature Communications volume 6, Article number: 6928 (2015) 5. https://www.parksystems.com/images/spmmodes/standard/3_Phase-Imaging-and-Phase- Detection-Microscopy-(PDM).pdf 6. J. Gunther, Introduction to Atomic Force Microscopy, March 10, 1998, ACS Group and MENs, Beckman Institute. https://www.slideshare.net/joserabelo/atomic-force-microscopy-37962055

7 Apr 2018

Using AFM PinPoint™ Nanomechanical Mode for Quantification of Elastic Modulus in Materials Two Orders of Magnitude Faster than Force Volume Spectroscopy

John Paul Pineda, Gerald Pascual, Byong Kim, and Keibock Lee Park Systems Inc., Santa Clara, CA USA Introduction Since the invention of atomic force microscopy, AFM has had a revolutionary impact in material sciences and device engineering by providing accurate, reliable, non-destructive imaging at the nanoscale. AFM is used across a wide range of nanotechnology applications such as biomedical implantable actuators to ultrathin cathode material for batteries to photo detectors and switches for memory storage and logic circuits.[1-3]As the dimensions of devices continuously shrink, measurement methods of local properties of materials have become more effective over bulk property in providing accurate nanoscale measurement. Local mechanical properties such as adhesion and elastic modulus are critical parameters in determining the reliability and performances of these devices. [4-6]There were existing AFM-based nanomechanical methods introduced to measure the mechanical properties, examples include force-volume spectroscopy and nanoindentation. However, some of these techniques are extremely time consuming and others are destructive which doesn’t allow in situ and high-throughput monitoring in certain applications. Figure 1 demonstrates the principles of AFMPinPoint™ nanomechanical modedeveloped by Park Systems. The patented PinPoint™ technique is at least two orders of magnitude faster than the conventional force-volume technique which allow users to acquire quantitative mechanical properties and high-resolution topography images of materials simultaneously in a short period of time.During operation, the tip moves in approach-retract manner, ensuring a frictionless operation which eliminates the lateral force due to continuous tip-sample contact and preserves tip and sample condition, ideal in measuring both hard and soft samples such as hard disks and biological samples. At each point of the image, force-distance curve is acquired and used to calculate the mechanical characteristics of the sample being measured.During data acquisition, the XY scanner stops, and the contact time is controlled to give enough time for the scanner to acquire precise and accurate data. In this experiment, 4different materials with various ranges of modulus were successfully quantified. The results acquired for each test are close to the nominal modulus value of each material, proving the superior ability of the PinPoint™ mode in quantification of mechanical properties. In addition, high-resolution images were acquired simultaneously which revealed the surface features of the samples. Fig. 1. Principles of PinPoint™ nanomechanical mode by Park Systems. This diagram demonstrates the feedback-controlled approach and retraction of a probe at multiple sites along a sample’s surface. Feedback control from the AFM systems’ controller allows this technique to acquire both surface topography. The F/D curve demonstrate how Pinpoint extracts mechanical property data. Experimental Sample and Tip An Olympus micro cantilever (OMCL-AC160TS) with nominal spring constant k = 26 N/m and resonant frequency f = 300 kHz was used to measure the Highly Ordered Pyrolytic Graphic (HOPG), Polydimethylsiloxane (PDMS) and Polystyrene – low density polyolefin elastomer (PS-LDPE)in the experiment.The HOPG sample is a highly ordered form of highly pure Pyrolytic Graphite annealed under high temperatures and pressure to obtain a high degree of crystallographic orientation. The PDMS sample is a silicone polymer which is physically soft like rubber or resin mounted on a magnetic sample disk.The PS-LDPE sample is a copolymer composedof PS and PE that were spin-cast onto a silicon substrate creating a film with different modulus properties. PS serves as the matrix while PE is the low density doping component.[8] Pinpoint Imaging Conditions The samples were investigated using Park NX20 AFM system under ambient air condition.Except for scan range, the scanned parameters used in measuring the sampleswereall the same. Results and Discussion Hertzian model The quantification of mechanical properties of Pinpoint mode such as elastic modulus is based on the well-established Hertzian model. In this technique, it is assumed that no other interaction other than elastic deformation can be observed between tip and sample. The calculation is based on the equation E (Elastic modulus) = F (Loading force). This equation is greatly affected by the tip geometries. Assuming that a spherical tip with specific radius (R) was used to indent elastic halfspace to a displacement of d, the applied force can be derived as below: Fig. 2.Illustration of spherical tip and sample interaction showing the displacement created in the sample surface as force is applied. By measuring the corresponding F, R, and d, the E* can be reversed-calculated. However, E* is a function of two material and its equation can be expressed as below: Etip and Esample are the elastic modulus of the two materials and νtip and νsample are their corresponding Poisson's ratio. By knowing E*, Etip, νtip and νsample, Esample can be reversed-calculated. [7] Table 1 shows the topography images andthe correlated maps of elastic modulus acquired during the tests. The images acquired in this experiment were all analyzed using XEI software developed by Park Systems which mapped the acquired signals to a color table. For topography images, the intensity of the shading correlates to the surface height variation with extremely bright and dark areas having the highest and lowest height regions. Same thing applies with the elastic modulus, wherein brighter and darker regions correspond to areas with highest and lowest elastic modulus. The topography image of HOPG revealed that surfaces of the sample are made of atomic terraces with step edges. These terraces demonstrate a homogenous diagonal path and they were measured to have approximately 1.5 nm step height. On the other hand, the elastic modulus map shows only few variations on the mechanical property of thesample surfaces. The diagonal dark lines visible on the elastic modulus imagerepresent the surfaces with low modulus. The topography image of the PDMS revealed the rough surface of the sample. It can be also observed on both topography and modulus image that there are only few variations on height and modulus of the surfaces. The topography image of PS-LDPE confirms that the sample is compose of two different materials. The dark circular dots features are the LDPE (low density polyolefin) materials, while the brighter regions in this image are the PS (polystyrene) materials. As expected, the elastic modulus map of this sample has two domains, wherein LDPE materialsrepresented by the dark circular dots are the domains with lower modulus, while PS represented by the areas with bright color are the domains with higher modulus. Table. 1. Topography image andcorrelated maps of elastic modulus acquired for each material. The quantitative results of elastic modulus are shown in Table 2 and corresponding graph shown in Figure 3. The results were plotted to better analyze the data. To evaluate the performance of Pinpoint mode, the results acquired in this experiment were compared to the nominal values of each sample. And it was found out that the measured modulus values of all the samples are close to their nominal values. For HOPG, the measured modulus is22.09 GPa, close to its nominal value of 18 GPa. For PDMS, the measured modulus is0.0018 GPa, close to its nominalvalue of .003 GPa.While for PS-LDPE, the measured values are1.955 GPa and 0.132 GPa, which are also comparable to their nominalvalues of 2 GPa and 0.1 GParespectively.The standard deviation of the modulus results were also calculated and based on the results, PDMS sample has the lowest standard deviation with Std = 4.03E-04 GPa, while HOPG sample has the highest standard deviation with Std =2.224 GPa. Table. 2. Measuredmean and standard deviation (Std) of elastic modulus. Table. 3. Plotted value of measured mean and standard deviation of elastic modulus.   Conclusion The quantitative and qualitative data of the four materials were successfully acquired by Park NX20 AFM usingPinPoint™ nanomechanical mode.The surface features of the samples such as the atomic terracessurface of the HOPG, the rough surface of the PDMS, and the circular dots matrix of the PS-LDPE were clearly observed on the topography images.The measured elastic modulus of each samplewascloseto theirnominal modulus value, confirming the capability of PinPoint™ nanomechanical modein quantifying a wide range of mechanical properties of various materials.The newPinPoint™ nanomechanical mode asdemonstrated in this experiment provides researchersaccurate material surface References [1] Dr. R. Cook, Nanomechanical Measurements and Tools, Written by AZoNanoJan 17 2010. [2] F. Wu, et al., Ultrathin Spinel Membrane-Encapsulated Layered Lithium-Rich Cathode Material for Advanced Li-Ion Batteries, Nano Lett., 2014, 14 (6), pp 3550–3555. [3] H. Kind, et al., Nanowire Ultraviolet Photodetectors and OpticalSwitches, Advanced Materials. [4] Z. Drira, et al., Nanomechanical measurements of polyethylene glycol hydrogels using atomic force microscopy, Volume 18, February 2013, Pages 20-28. [5] J. Pineda, et al., Optimum Current Distribution Measurement of Zinc Oxide Nanorods via PinPoint™ Conductive AFM. [6] C. Clifford, et al., Quantification issues in the identification of nanoscale regions of homopolymers using modulus measurement via AFM nanoindentation, Applied Surface Science, Volume 252, Issue 5, 15 December 2005, Pages 1915-1933. [7] M. Hong, et al., Collagen Fibrils Imaging in Air and in Liquid Using Atomic Force Microscope-Based Fast Nanomechanical Mode. [8] W. Shi, et al., PinPoint Nanomechanical Mode Using Probes with Different Stiffness

10 Jul 2016

Textile Nanocharacterization: Topography, Phase Imaging, and Nanomechanical Property Investigation of Polyester Yarn Interaction with Silicon Matrix.

The addition of nanotechnology to textile development can yield next-generation fabrics for many new exciting applications. Understanding the characteristics of the nanomaterials being integrated with existing fabric matrices is paramount in predicting how targeted enhanced properties will manifest in novel composites. For example, weaving nanofibers into tear-resistant fabric can significantly further increase the fabric's strength without significantly increasing its weight.

23 Dec 2013

Cross-section of Polymer Film

Imaging a Narrow Area Using a Park AFM Instrument Case study: Cross-section of a Polymer Film   By H.-Y. Nie The University of Western Ontario The surface of a biaxially-oriented polypropylene (BOPP) film is characterized by a fiber-like network, which is determined by the draw ratios at the two orthogonal directions, i.e., machine-direction (MD) and transverse-direction (TD).  The surface morphology of such a film has been studied with atomic force microscopy (AFM). To fully understand and characterize these materials, it would be beneficial to find out whether this structure goes through the entire thickness of the film.  Using a Park Systems’ AFM one can easily address this question by imaging the cross-section of a BOPP film. The probe tip approach to the surface combined with excellent optical views makes such tasks trivial. The Park AFM series have a separate sample stage for the X-Y scanning from the Z-scanner for height measurement.  The lens (features are magnified ~780x on a 19" monitor) for a CCD camera can move together with the z-scanner in the vertical direction; this is used for the cantilever to quickly travel a long distance and become very close to the sample surface (say, a couple of mm).  Once the cantilever gets nearby to the sample surface, the lens can be set to move alone so that the sample surface is focused before the cantilever is set to engage to the working distance from the sample surface.  The ease of landing the tip on a narrow sample surface is well suited for imaging cross-section of a thin sample.  We demonstrate this by imaging cross-section of a BOPP film which is ~30 µm thick. Figure 1. The cross-section images of BOPP film torn in (a) transverse-direction (TD) and (b) machine-direction (MD), respectively.   The cross-section of a BOPP film was achieved by first making a cut along the MD (or TD) followed by precisely tearing the film in that direction.  The specimen was mounted on an adopter by sandwiching the film between the two metal plates of the adopter.  This adopter is then placed on the sample stage such that the BOPP film would be proximately under the cantilever.  Then the Z-scanner and the lens (the cantilever was focused) were lowered together towards the sample surface.  This coarse approach was stopped when the cantilever-sample distance was shortened to a couple of mm.  Then, one lowers the lens alone further towards the sample surface to focus on the sample surface only (viewed through the CCD camera monitor). The sample position may then be optimized such that a good area to scan is achieved. After having located the sample surface, the cantilever was brought into view by lowering the Z-scanner.  This step was well controlled until the cantilever image became clear (i.e., to prevent the contact between the tip and the sample surface).  Then the approach process was started to engage the tip to the working distance from the sample surface; this is regulated by the set point, which is either a contact force in contact mode or a distance corresponding to the damping of the oscillation of the cantilever in True Non-Contact mode. Shown in Figure 1 are 10 µm x 10 µm area scans in True Non-Contact mode for cross-sections of a BOPP film achieved by tearing the film in MD (a) and TD (b), respectively. The edge of the cross-section of the film is also indicated. As easily noticeable, there is a well-defined layered structure along the scan direction. Therefore, it has been directly confirmed that the layered structure goes through the entire thickness of the BOPP film taken under investigation. It is also interesting to note that the structure appears to be different for the two samples made by tearing the film in MD and TD, respectively. Additional investigation is ongoing. Note An application note for PSIA by H.-Y. Nie at Surface Science Western, Room G-1 WSC, The University of Western Ontario, London, Ontario, Canada N6A 5B7,http://www.uwo.ca/ssw

20 Dec 2013

Surface Morphology of Electrospun Fibers

Investigation of Phase Morphology for E-spun Fibers Composed of Polybutadiene and Polycarbonates Figure 1. Topography information in Non-Contact AFM taken by XE-150 (50 μm scan size)     Electrospinning techniques are capable of producing polymer fibers in the nanometer scale. Polymer blends offer the potential to prepare new materials for use in a variety of applications. The focus of this investigation has been primarily on the phase morphology of the fiber surface.  A large-scan (50 μm × 50 μm) view of an e-spun fiber is shown in Figure 1. There were a few bead formations along the length of the fiber. Figure 2 (a) shows the TEM images of polybutadiene (PB) / polycarbonate (PC) blends with weight ratios of 75/:25 after staining. The dark regions in the TEM images are identified as the PB phase and the light regions show the PC phase.  An electro-spun fiber was scanned in Non-Contact AFM and the resulting image is shown in Figure 2 (b). The features on the fiber surface were examined using the high performance Park AFM. Figure 2. (a) TEM image and (b) AFM topography (6x6μm) and profile for e-spun fiber. Courtesy of Dr. Jun Seok Lee, Bongwoo Kang, and Prof. Changmo Sung, Univ. of Mass., Lowell.     Figure 3. (a) Topography, (b) Amplitude, and (c) Phase of Dynamic Force mode AFM (2 μm scan size).     Dynamic Force Microscopy (DFM) can remove the contribution from lateral frictional force. For this circular type of sample, e.g. fiber, DFM is helpful to avoid tip slippage on the sample surface. It produces three types of information: topography, amplitude, and phase, as shown in Figure 3. The phase image in this mode is created by the feedback received from angular shifts as compared with amplitude detection from the magnitude of the cantilever deflection.

19 Dec 2013

Polymer Composite (Phase Separation)

Easy to Detect Phase Information for Composite Materials by Force Modulation Mode   Polymer blends of poly (styrene-isobutylene-styrene) and poly (styrene-co-maleic anhydride) have been investigated using the XE-100. This study examined the morphology and the phase dispersion characteristics of the polymer blend of SIBS and SMA at different compositions. Specifically, the morphology of the blended composition either consisted of a binary phase system of randomly dispersed particles embedded in a matrix or as a co-continuous phase. In both cases, the SMA and SIBS formed their own separate domains. In terms of mechanical properties such as stiffness, there were contrast changes in the FMM images which correlated to the micro-phase changes in the polymer blend.   Fractured samples were embedded in epoxy and polished to produce a smooth flat surface. A non-contact tip, stiffer than contact tips, on a silicon cantilever was used. The cantilever’s length and width was 125 μm and 35 μm, respectively. The cantilever had a resonant frequency of 325 kHz and a force constant of 40 N/m. Choosing the right cantilever is a critical factor in FMM.     Figure 1. Images of microstructures for compounding polymer material (80% SIBS and 20% SMA, 40 μm size scan). (a) Topography, (b) amplitude image, and (c) phase image. Image courtesy of Bongwoo Kang, Dr. Jun Seok Lee and Prof. Changmo Sung, Univ. of Mass., Lowell.   Topographical information was obtained simultaneously with phase and amplitude force modulation data at optimized scanning conditions. Elasticity and stiffness differences on the surface can be distinguished using this method. Figure 1 and 2 show the mapping of these differences with different blend compositions.     Figure 2. Microstructures of composite material (60% SIBS and 40% SMA, 40 x 40 μm). (a) Topography, (b) amplitude image, and (c) phase image.  

19 Dec 2013

Phase Separation in the Co-Extruded Polymer

Instability Flow in the Multi-Layered Copolymer Force Modulation Microscopy (FMM) is one of the unique AFM modes that is gaining more popularity. This technique is useful to identify composite or blended materials that have different phases. Specifically, FMM creates images based on the mechanical properties of the sample surface such as hardness and viscosity. It operates in dynamic contact mode by applying an adjustable frequency oscillation to the probe. As the probe tip contacts the sample surface, the probe deflection is detected by measuring the vertical movement and shift in the phase angle . The oscillation amplitude of the probe is reduced with increasing surface hardness. Surface topography images can simultaneously be obtained along with the phase information. Based on the configuration of FMM mode, we can obtain three images in one scan: topography, amplitude, and phase.  Multilayer structures were prepared by co-extrusion technology using a feedblock with a multiple layered element. Polycarbonate (PC) and polypropylene (PP) were extruded to form alternating structures by repeated layer multiplications using multiplying elements. Microstructure images were taken of copolymers consisting of 80% PP and 20% PC in the form of composites. It is a good sample for identifying the interface of phase instability and composition in the multilayer using the XE-series AFM. Figure 1. AFM image of a co-extruded film taken with the XE-150 (90 μm scan size). This image shows no phase information. Image Courtesy of Dr. Jun Seok Lee and Prof. Changmo Sung, Univ. of Mass., Lowell. Figure 1 shows the topographical information. In FMM mode, the amplitude map shows the elastic properties of the sample which is shown in Figure 2. Figure 3 displays the phase image which indicates energy dissipation. Figure 2. FMM amplitude image of a multi-layered structure of PP and PC taken with the XE-150 (90 μm scan size). Contrast in the phase image is a typical feature of the oscillating cantilever phase shift. This contrast is explained by the dependence of the phase shift on the surface properties. The phase shift is affected by surface geometry, adhesion, and viscosity. The phase image is capable of identifying a surface region with different properties. In the composite material, the lighter regions in Figure 3 correspond to a greater phase shift for the 20% polycarbonate. Figure 3. FMM phase image of a multi-layered sample of PP and PC taken with the XE-150 (90 μm scan size). The thickness of each layer ranges between 5 μm to 9 μm. The XE-series AFM can be easily used to detect these phase separations due to stiffness and energy dissipation. The user-friendly interface enables precise operation and gives you the freedom to control operational parameters.

18 Dec 2013

Green Chemical Polymer

Water-Soluble Polymers are Developed for Environmentally Friendly Polymer Coating Applications. Enzymatic polymerization of self-assembling amphyphilic alkyl tyrosine derivatives from emulsions results in water-soluble polymers which are more easily processable than highly crosslinked polyphenols. Polymers of DELT yield thin films when spread on the surface of water. This surface activity may be exploited to eliminate organic solvents and other toxic reagents currently used in polymer coating applications. Such coatings provide both chemical protection and electrical insulation of gold-covered metallic conductors during subsequent processing. The aqueous based coatings are of interest as environmentally friendly and cost-effective replacements for epoxy-based coatings currently employed in microelectronics. Figure 1. Non-contact AFM image of Green Chemical Polymer taken by the XE-100 (5 μm and 20 μm scan size). Image Courtesy of Dr. Jun Seok Lee and Prof. Changmo Sung, Univ. of Mass., Lowell.   Sub-nano scale polymer samples are investigated without damage to the surface using Non-Contact AFM, Figure 1. Irregular spherical aggregates below and above 1 μm in diameter were observed bound to the gold covered mica substrates. The small features on the bead surface are proof of the micellular structure of DELT, which could only be detected by the high performance of the XE-100 AFM. The three dimensional view given in Figure 2, can be much more helpful than regular electron microscopy in understanding height information of this unique round shaped sample. The coverage of the polymer surface was examined using different scan areas. Using AFM, we can investigate the nano texture on the surface of each bead more effectively than with SEM. Figure 2. Images of microstructures for DELT bead formation on the gold-coated mica by NC-AFM (5 μm scan size). NC-AFM image shows fine details on each small bead.