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Lecture 5 Notes (BME 296)

Lecture Overview

  • Course: BME 296

Metals Overview

  • Completed discussions on:

  • Bonding

  • Structure: cubic cell structure, Miller indices

  • Defects: 0D, 1D, 2D, 3D and their effects on biomaterial properties

Initial Thoughts on Properties

  • Engage in discussions about common properties encountered in materials.

Ceramics

  • Composed of two or more elements.

  • Can have ionic and/or covalent bonds.

  • Characterized by having more ions than atoms.

Properties of Ions

  • Cations (positively charged ions) and anions (negatively charged ions) must be adjacent for stability.

  • Critical for maximizing the number of nearest neighbors that are anions.

  • Understand the sizes of cations and anions and the reasons for size disparities.

Crystal Structures in Ceramics

  • Affected by two parameters:

  • Magnitude of the electrical charge on constituent ions.

  • Physical size of ions (must maintain Rc/Ra < 1 for stability).

  • Example: CaF2 consists of Ca2+ and 2 F- ions.

AX Crystal Structures

  • Composed of equal numbers of cations and anions with the same charge.

  • Stable crystal structure example: Sodium Chloride (NaCl).

AmXp Crystal Structures

  • Compounds where cations and anions have unequal charges (AmXp, where m and/or p≠1).

  • Example: Zirconia (ZrO2)

  • Ceramics may also include multiple types of cations (AmBnXp).

  • Example: Calcium Zirconium silicate: enhances properties for biomedical applications and has apatite forming capability.

Point Defects in Crystal Structures

  • Types of point defects: interstitial and vacancy defects in cations and anions.

  • Importance of maintaining electroneutrality: no individual point defects should occur; group defects sustain neutrality.

  • Examples of group defects: Schottky defect and Frenkel defect.

Schottky and Frenkel Defects

  • Schottky defect: Vacancies in cations and anions in ratios that maintain charge neutrality.

  • Frenkel defect: Occurs when a cation displacement creates a vacancy, thus maintaining electroneutrality (only applicable for cations).

Review of Metals and Ceramics

  • Metals: metallic bonds, arrangements, and properties relevant to orthopedic implants.

  • Ceramics: focus on ionic bonds and electroneutrality, explaining their brittleness and applications in dentistry and orthopedic coatings.

Polymers: Overview and Properties

  • Polymers consist of long chains of monomers.

  • Degree of polymerization signifies number of repeating units.

  • Molecular weight is critical for understanding polymer characteristics.

Molecular Weights of Polymers

  • Example: Calculate molecular weight of poly tetrafluoroethylene (Teflon) ((C2F4)n).

  • Atomic mass of C: 12 g/mole; F: 19 g/mole.

  • Calculation: (2(12)+4(19)=100) g/mole per repeat unit.

  • For a chain of 14 repeat units, total molecular weight = (1402) g/mole.

Weight Distribution and Molecular Weight

  • Individual components can lead to varying molecular weights within a polymer sample.

  • Number-average molecular weight (Mn): based on mole fraction of molecules.

  • Weight-average molecular weight (Mw): based on weight fraction.

  • Polydispersity index (PI): Ratio of Mw to Mn, indicating sample heterogeneity.

Example Problem with Molecular Weight Calculations

  • Tasks include calculating number-average and weight-average molecular weights, determining which average significantly impacts results, and computing the polydispersity index.

Arrangement of Molecules in Polymers

  • Linear structures consist of a backbone made of repeating units.

  • Pendant groups may attach to main chains, influencing properties.

  • Hydrocarbons serve as basic polymer examples, while vinyl polymers contain carbon-carbon double bonds.

Comparison of Organic vs Inorganic Polymers

  • Course focuses on organic polymers, mainly carbon-based.

  • Inorganic polymers may include different backbones like silicon or phosphorus.

Bond Rotation and Configuration of Polymers

  • Carbon-carbon single bonds allow for rotation, creating various configurations (stable under non-crowding).

  • Folded structures play a crucial role in polymer mechanical properties.

Types and Characteristics of Polymers

  • Linear vs branched: branched structures affect density and mechanical properties.

  • Cross-linking increases molecular weight, impacting viscosity and stability.

Thermoplastic vs Thermosetting Polymers

  • Thermoplastics can be molded with heat; thermosets cannot be re-shaped after curing.

Tacticity in Polymers

  • Isotactic: side groups on the same side of the chain.

  • Syndiotactic: alternating side groups.

  • Atactic: random arrangement of side groups.

Crystallinity in Polymers

  • Influences mechanical properties and is determined by the order of molecules.

  • Regular arrangements lead to higher strength, whereas irregularities reduce crystalline stability.

Copolymers

  • Variations include random, alternating, block, and graft copolymers; each has distinct arrangements and properties.

Polymer Synthesis Mechanisms

  • Involves addition, condensation, and genetic engineering techniques for polymer creation.

Addition Polymerization

  • Steps: initiation (activating monomers), propagation (joining monomers), and termination.

Condensation Polymerization

  • Involves loss of small molecules, such as water or methanol during synthesis.

Genetic Engineering in Polymer Production

  • Allows for more control over polymer architecture via manipulation of genetic code within host organisms.

  • Example: Reprogramming bacteria to synthesize polymers.

Example Problem with Copolymer Calculations

  • Determine molecular weights based on provided structures and repeat units for PLGA (poly(lactic-co-glycolic acid)).