BMEG 311 Homework 2

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West Virginia University *

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311

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Material Science

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Apr 3, 2024

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pdf

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5

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BMEG 311 Biomaterials Homework 2 Due Thursday, September 7 th by 11:59 PM Madison Paris 1. You are examining a copolymer for its potential as a material for a vascular graft. You are trying to determine whether you want a material with a high or low degree of crystallinity. a. What type of structures for copolymers have a higher probability for crystallization? Explain your reasoning. Alternating and block copolymers have a higher affinity for crystallization, due to the repeating structure. This will result in an organized structure. b. You evaluate the crystal structure of two potential materials and obtain the following plots below. What technique was used, how does the technique work, and what information does it provide about the analyzed biomaterials? X-ray diffraction (XRD) was used to obtain the plots. XRD involves shining X-rays onto a crystalline material. We can then observe the pattern produced by the X-rays interacting with the crystal lattice. XRD provides information about the arrangement of atoms in a unit cell, the geometry and size of the unit cell, and can identify a compound. c. Based on the 2 plots below for sample 1 and sample 2, is each material crystalline or amorphous? Explain your reasoning. Sample 1 is amorphous. Amorphous materials have an irregular shape and no sharp diffraction peaks, thus the x-ray diffraction pattern would be difficult to read. Sample 2 is crystalline. Crystalline materials have defined edges and peaks, which would be an easily readable x-ray diffraction pattern.
2. You are given three polymeric materials (A, B, C) to examine for potential as a suture material. You aim to use size-exclusion chromatography to determine the approximate molecular weights of the unknown polymers. You obtain the graph below for 5 known polymer standards. a. Using the graph below, create a standard curve of molecular weight vs. time. Include the graph in your answer, label the axes and provide the estimated trendline equation. b. Using your standard curve, calculate the molecular weight of each unknown polymer sample A, B, and C. Show your work. Unknown A: -13657(7.4) + 142807 = y y = 41745.2 g/mol Unknown B: -13657(8.4) + 142807 = y y = 28088.2 g/mol Unknown C: -13657(9.6) + 142807 = y y = 11699.8 g/mol Standard 1: Molecular weight 50,000 g/mol Standard 2: Molecular weight 40,000 g/mol Standard 3: Molecular weight 35,000 g/mol Standard 4: Molecular weight 20,000 g/mol Standard 5: Molecular weight 10,000 g/mol Unknown A: Peak amount polymer eluted at 7.4 min Unknown B: Peak amount polymer eluted at 8.4 min Unknown C: Peak amount polymer eluted at 9.6 min
3. Draw a Burger’s circuit around the dislocations shown below for silver metal and strontium oxide. Include your drawings in your answer. What is the magnitude and direction of Burger’s vector in each case? 4. Materials that have crystallinity will have defects within the crystal. a. Explain the three types of defects and how they are observed in both a metal and a polymer. The three types of defects are point, line, and area defects. Point defects occur when atoms are missing, thus causing strain on the crystal structure. In metals, specific point defects can include vacancies, interstitial impurities, self-interstitials, substitution impurities, and a Frenkel defect. Polymers can only have vacancies or impurities. Line defects, or dislocations, occur when an extra half plane of atoms is inserted into a crystal. Metals and polymers can have dislocations, although metals are likely to have more of them. Specifically, polymers can have edge or screw dislocations. Area defects, or grain boundaries, occur at different region boundaries of a crystal and have an angle of misalignment. Metals can have grain boundaries, while polymers cannot. Polymers have spherulites, spherical semi crystalline regions, instead of grain boundaries. b. Would a perfect metal crystal with no defects be ductile or brittle? Why? A perfect metal crystal with no defects would be brittle. Metals have a small number of slip planes available for deformation based on their unit cell crystal structure. Materials with many slip planes would be considered ductile. A perfect metal crystal has zero slip planes making it brittle.
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