Birefringence of Stressed Polystyrene Reinforced With Clay Rohit Gupta, Harold Metcalf, John Noé Laser Teaching Center, Department of Physics & Astronomy and Miriam Rafailovich Department of Materials Science and Engineering, Stony Brook University ABSTRACT: This was a study of whether blending clay into polystyrene will reduce the material's response to stress, as observed by the intensity and form of birefringence patterns. This type of material modification is a relatively new field of study with important engineering implications. The hypothesis was that blending clay into polystyrene would deflect the stress to the edges and lessen the intensity of the stress. Birefringence is the optical phenomenon that occurs when an object that has different indices of refraction in different directions is placed between two crossed polarizers; these different indices can be inherent in the object or induced by means of applied stress. Thus birefringence literally allows one to "see" the stress produced on an object because of the colored fringes induced. The clay additive used was cloisite 20A at a concentration of 5% clay. Polystyrene samples with clay and samples without clay were molded into the same rectangular shapes (2x12 cm with a thickness of 2 mm). Differing levels of stress were chosen, and the samples with clay and the samples without clay were subjected to these levels of stress. Adjustable stresses were applied to the samples in a Stress Opticon, and pictures of the stress fringes were recorded with a camera. The shape, form, and intensity of the fringes were qualitatively described. This research was supported by the Simons Foundation. INTRODUCTION: Birefringence is the optical phenomenon that occurs when an object that has different indices of refraction in different directions is placed between two crossed polarizers; these different indices can be inherent in the object or induced by means of applied stress. It is the result of a phase difference of the light hitting the object. The phase difference results because of the different directions' of the object (ie length, width) having different indices of refraction. The phase difference equals the thickness of the sample (ie the distance the light will go through the sample) divided by the wavelength of the light, all multiplied by the difference in the indices of refraction of the two other directions (the first direction being the thickness of the sample). Birefringence literally allows one to "see" the stress produced on an object because of the colored fringes. The fringes follow the stress field (the path of stress), and that is why they are curved and not straight. This project involved blending clay into polystyrene and detecting how this blending affected the material's response to stress, as observed by the intensity and form of birefringence patterns. This type of material modification is a relatively new field of study with important engineering implications. Clay blending is very important in the Materials Science arena, and many modern experiments are underway to determine what all the effects of the blending are. A specific concern of experiments involving clay blending is whether or not the clay will increase the mechanical strength of the substance. The hypothesis of this research project was that adding clay to polystyrene would make the material more strong. MATERIALS: Cloisite 20A (clay) Polystyrene Stress Opticon, made by Vishay Measurements Corp. Brabender machine for blending Machine for molding samples Camera Molding plate that fits dimensions 2 cm x 12 cm x 2 mm METHODS: The first step is making the samples. The molding plate, which is a disc with a cutout of the shape of the samples to be made, must be a appropriate for the size of the Stress Opticon. (The Stress Opticon, made by Vishay Measurements Corp., is a device for inducing stress on and seeing birefringence in photoelastic materials. In a shop, the right-sized molding plate must be made out of aluminum/steel (2 cm x 12 cm x 2 mm). Then, in the Brabender machine for blending, the clay is mixed with the polystyrene. To prepare a sample with 5% clay, 47.5 g of polystyrene and 2.5 g of cloisite 20A clay are put in. Then, the pure polystyrene and the polystyrene mixed with clay are taken to the machine for molding. Here, the polystyrene is placed in the slit of the molding plate's, and the samples are made. Samples are made of the polystyrene mixed with clay, also. The next step is to place the samples in the Stress Opticon and put stress on them. The Stress Opticon is as such: a sample is held by screws which not only hold the sample in place but provide stress in adjustable positions. The sample is, of course, placed between two crossed polarizers, which are attached to the device (they come attached to the device readymade). The screws will not be able to hold the sample unless the sample is flat or extremely close to flat. If the samples are not flat, their edges must be sanded. Then, the pure polystyrene samples and the mixed clay-polystyrene samples are subjected to the same amounts of stress. One way of measuring the stress is simply by the number of turns of the screws. A torque wrench can also be used for more accurate values. The torque wrench is attached to the screw, and measurements are read off the wrench. The stress can be raised or lowered by tightening or loosening the screws, respectively, and the same values of stress must be used for the pure polystyrene samples and polystyrene-clay samples The shape and form of the fringes are now noted, and the intensities, if possible, are measured. There were some problems with the methods behind this experiment. For one, it was extremely difficult to produce samples which were homogenous, if not impossible. For this reason, the materials showed birefringence in the imperfect parts of the sample. This added birefringence could not be ignored because at these imperfect parts, the stress is amplified. For example, if I were to produce uniform stress on the entire object (at every point), the stress at the imperfect parts would be higher than we would expect, since already-stressed parts are more vulnerable to stress than ``perfect,'' homogeneous parts. Additionally, there was the problem of data collection. Since the data were not single measurements but an image of birefringence over an entire sample, it was difficult to take data. This fact led to the partial solution of making qualitative observations and taking pictures of the birefringence present in the sample under stress. Quantitative results were impossible unless there were some constant relation between the birefringence at every point in the stressed pure polystyrene and the corresponding point in the stressed clay-polystyrene sample. RESULTS: Some preliminary pictures are as follows, showing birefringence in stressed pure polystyrene. These pictures demonstrate the kind of data this investigation seeks to find. What must be done is the taking of pictures of samples under stress and the comparing of the birefringence of the samples with clay to those without clay. Fig 1 Fig 2 Fig 3 Fig 4 As shown, when the stress was gradually increased from Fig 1 to Fig 4, the birefringence increased. The screws were tightened, and the corollary increase in the fringes present was apparent. The stress applied can be measured a number of ways; one way was by number of turns of the screw. The most difficult data of this project to measure were the results of the birefringence. This problem should be the most important issue for anyone in the future who wants to undertake this project. The samples with clay were slightly too cloudy to show much birefringence. The birefringence was extremely faint and small, and additionally the clay samples were not uniform enough to show good birefringence in the Stress Opticon. RECOMMENDATIONS: One of the recommendations for such future experiments is that a more appropriate apparatus than the Stress Opticon should be used. The possibilities of this experiment were severely limited because of the device. The size of the samples, the number of places for stress to be induced, and the size of the area of the sample under stress are limited. Also, the Stress Opticon's screws, which apply stress and hold the object, have a tendency to be unstable, and it is difficult to get any samples but the most flat to stay aloft. Additionally, the sample must not be too thin, which limits the project again. Instead of such an apparatus, a sample could be simply placed between two crossed polarizers that can be found in any optics lab. An interesting setup that Dr. Metcalf showed me is one in which an overhead projector is used. An object is placed between two crossed polarizers on the projector. In this way, we not only produced the birefringence, but also reflected the image of it on the screen/wall. We can then take pictures of the enlarged image showing excellent birefringence. Some setup that is similar would be better than the Stress Opticon for this study. Another problem was the cloudiness in the sample which results from clay being added. When the sample becomes opaque, no birefringence can be measured because only transparent and translucent materials are photoelastic. Other methods of stress measurement other than birefringence should be used if the concentration of clay is too high to produce a transparent or translucent sample. The 5% clay concentration used in this experiment was slightly high; the samples were not optimal for photoelastic observation. In the future, concentrations should be explored to determine the optimal concentration for photoelastic study, the concentration not being too low as well because an extremely low concentration of clay will have very little effect on the stress fields.