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)).