Observation of Topographical Structures found on Stretched Glassy Polymers

Dawei Chen

This work was done as a project for OPT407-SEM Practicum offered at the Institute of Optics, University of Rochester in Spring 2017.

Introduction

Polymers are susceptible to premature failure due to the formation of crazes and microcracks. These are small deformations and fractures that occur in a material due to the material's imperfections. These microcracks form readily in a stretched polymer. Also, when these microcracks are formed, they encounter little resistance to growth, especially at higher temperatures where there is high chain mobility. This project will serve to view what happens on the surface of a stretched polymer. This project will involve around 4 main microscopic techniques: secondary electron scattering, atomic force microscopy, light microscopy, and sample preparation. It will also use two non-microscopic techniques with ImageJ and electron flight analysis. This project will involve imaging stretched glassy polymer networks to gain insight of the topographical features found during the stretch.

Estimate of Sample Preparation Time

12-15 hours including making the glassy polymer

Estimate of Microscopic Analysis Time

15-18 hours

Estimate of Image Processing Time

2-3 hours

Sample Preparation

Glassy polymer is made by UV-curing acrylate-based monomer and small polymer molecules. The polymer is then stretched to strains of 100%, 200%, and 300% using a mechanical testing system. The sample was then cut to fit inside the stage area of the AFM and fixed onto a stub. After atomic force microscopy, the sample were placed onto SEM stubs and then had gold sputtered on top of it through the Denton sputter system to prepare for SEM imaging. Due to the nonconductive nature of polymers, it is important to coat the polymer with a conductive material. Gold was sputtered for 60 seconds at 15 mA to have samples having roughly 5nm of gold nanoparticles sputtered on top. The gold acts as a thin conductive layer for SEM imaging. Carbon paint was then used along the sides on the sample for grounding purposes.

SEM Imaging

The working distance used in this project was between 5mm and 10mm. The InLens was used primarily for secondary electron imaging and the accelerating voltage was kept between 5kV and 10kV to avoid overcharging. In order to view microcracks in our sample, magnifications were used between 1KX to over 60KX.

ImageJ was used to sharpen and color several images to see surface features more clearly.

It is important to note that only a couple of images were processed and will be mentioned if they were.

The images below show the surface of the polymer at different strains:

Figure 1. (left) unstrained sample (right) 300% strained sample for comparison

Figure 2. images of features found on a 100% strained sample

Figure 3. images of features found on a 200% strained sample, (left) image was slightly sharpened and recolored to more clearly see deformation

Figure 4. images of features found on a 300% strained sample

Atomic Force Microscopy

AFM was used to see the height profiles of the topographical features of the polymer. It was observed that the crevices were found on the surface of the specimen that needed to be furthered explored by the SEM.

Figure 5. (top) primary profile of 300% strained sample (middle) height profile of 300% strained sample (bottom) primary, height, and roughness of 100% sample

Light Microscopy

Light microscopy was used on the samples post sample preparation. Gold cracks are observed as well as other potential features to be further studied via SEM.

Figure 6. (a) 0% strained (b) 100% strained (c) 200% strained (d) 300% strained (e) dark field imaging to see notable features on 300% strained sample
Note: Images (b)-(d) has been slightly recolored to match image (a)

Electron Flight Simulation

Using the monto-carlo electron flight simulation, we can see the electron paths inside the sample and it is observed that most electrons were generated close to the surface of the specimen.


Figure 7. Electron Flight Simulation

Conclusions

The deformations that occur at the surface of polymers after stretching provides a very interesting phenenomena that occurs in polymer systems. Through different microscopy techniques, we can observe and understand the topographical features that form during polymer stretching. SEM allowed us to visually see the surface of the polymer and the AFM allowed us to see the height and roughness profile of the sample. The light microscope allowed us insight on what structures should be furthered examined under the scanning electron microscope and the electron flight simulation gives us an idea on how the electrons are traveling through the specimen. This study concludes that deformations do exist on the surface of polymers when the polymer is stretched to 100%, 200%, and 300% strain.

Acknowledgment

I would like to thank Brian McIntyre for his help and support throughout this project and for teaching a very informative class. I would also like to thank Caleb Whittier for all of the wondrous laboratory experiences we endeavored together.


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