Polymers

Synthetic Polymers

Key Takeaways

  • Understand what polymers are and their manufacturing processes.
  • Distinguish between synthetic and natural polymers, providing examples for each.
  • Comprehend how the chemical and physical properties of a polymer chain influence its function.
  • Identify methods to modify the properties and functions of a polymer network.

Why Use Polymers?

  • Metals and ceramics have limitations in applications, bioactivity, and degradability.
  • Polymers offer greater control over material properties and processing methods.
  • Structure-property (function) relationship: The physical and chemical properties of a macroscopic material are directly related to molecular characteristics, including:
    • Molecular architecture
    • Molecular weight
    • Chemical composition
  • Polymers are naturally more porous, which is beneficial for drug incorporation and controlled release (e.g., coatings for drug-eluting stents).

Polymer Classifications

  • Two Main Classes of Polymers:
    1. Synthetic Polymers: Fully derived through chemical reactions.
    2. Natural Polymers: Fully derived from natural sources.
  • Synthetic polymers offer greater reliability due to better control over raw materials and reduced immunogenicity.

What Are Polymers?

  • Polymers are formed by linking together subunit "monomers."
  • Monomers can either be identical or varied.
  • The polymerization reaction depends on the type of monomer(s) involved.

Chemical Variety in Monomers

  • Monomers are typically carbon-based (used primarily in plastics).
  • They exhibit a variety of structures, including backbones, chemistries, and side groups that influence their properties:
    • Backbone: The main chain of repeating units in the polymer.
    • Pendant or Side Group: Atoms or groups attached to the backbone, affecting physical properties.
  • Example: Silicone as a polymer backbone.

Polymer Synthesis

  • Polymers are synthesized by breaking unstable double or triple bonds in monomers:
    • Example Reaction: Ethane ( ext{C}2 ext{H}6) converts to ethylene ( ext{C}2 ext{H}4), which can polymerize to form polyethylene (CnH{2n}).

Polymer Chains and Molecular Weight

  • The length of the polymer is described by its average molecular weight.
  • Polydispersity Index: Describes the distribution of molecular weights within a given polymer sample:
    • Low: Most uniform distribution.
    • Medium: Moderate uniformity.
    • High: Low uniformity.

Thought Experiment on Molecular Weight

  • Questions raised:
    • How does molecular weight affect mechanical properties?
    • How does polymer structure influence properties?
    • Example Polymers Compared: LDPE (Low-Density Polyethylene), LLDPE (Linear Low-Density Polyethylene), HDPE (High-Density Polyethylene).

Molecular Weight Effects

  • Longer polymer chains exhibit greater entanglement, leading to more solid structures.
  • Increased entanglement correlates with:
    • Greater strength
    • Higher Young’s Modulus
  • Example with Polylactic Acid (PLA): Longer polymers form bends and knots that enhance mechanical properties.

Branching Effects

  • Longer branches in polymers can enhance entanglement, thereby increasing polymer strength.
  • Very short branches may hinder entanglement, preventing effective polymer interaction.

Mechanical Properties of Polymers

  • Overview of selected synthetic polymers and their mechanical properties:
    • Polyamide:
    • Strength: 90 MPa
    • Young’s Modulus: 2800 MPa
    • Low Density PE (branched):
    • Strength: 30 MPa
    • Young’s Modulus: 207 MPa
    • High Density PE (linear):
    • Strength: 22 MPa
    • Young’s Modulus: 850 MPa
    • Polylactic Acid (PLA):
    • Strength: 28-50 MPa
    • Young’s Modulus: 1200-3000 MPa
    • Polycaprolactone (PCL):
    • Strength: 17 MPa
    • Young’s Modulus: 320 MPa
    • PMMA:
    • Strength: 30 MPa
    • Young’s Modulus: 2200 MPa
    • PTFE:
    • Strength: 17-28 MPa
    • Young’s Modulus: 500 MPa
    • Compact Bone:
    • Strength: 50-150 MPa
    • Young’s Modulus: 1100 MPa
  • Notable observation: Generally, as modulus decreases, the strength tends to increase.

Common Applications of Synthetic Polymers

  • Application table of synthetic polymers:
    • Poly(methyl methacrylate) (PMMA): Bone cement, intraocular lenses, hard contact lenses.
    • Poly(vinyl chloride) (PVC): Tubing, blood storage bags, dialysis devices.
    • Polyamide (Nylon): Catheters, sutures, mold parts.
    • Polypropylene: Permanent sutures, hernia repair, vascular grafts.
    • Polyurethane: Artificial hearts, catheters, pacemaker leads, sutures.
    • Poly(tetrafluoroethylene) (PTFE): Heart valves, facial prostheses, shunts, catheters.
    • Poly(ethylene terephthalate) (Dacron): Implantable sutures, mesh, vascular grafts and valves.
    • Poly(dimethyl siloxane) (PDMS): Finger joints, heart valves, breast and facial implants.
    • Poly(lactic-co-glycolic acid) (PLGA): Mesh, orthopedic implants, particles for drug delivery.
  • Classification of applications: Hard, soft or flexible, bioactive.

Plastics Manufacturing

  • Synthetic polymers are commonly processed into:
    • Films (for coatings or membranes) or solid parts
    • Fibers
    • Foams
  • Manufacturing process: Polymer extrusion.

Thermal Effects on Polymers

  • Classification of polymers based on thermal effects:
    • Thermoplastics: Soften when heated (no cross-links between chains).
    • Types include crystalline and amorphous.
    • Thermosets: Remain hard upon heating (strong covalent cross-links).
    • Elastomers: A sub-group of thermoplastics that retain elastic properties.

Thermosets vs Thermoplastics

  • Thermoplastics (Thermosoftening):
    • No cross-links; weak intermolecular forces.
    • Softens when heated.
  • Thermosets (Thermosetting):
    • Strong covalent cross-links bond polymer chains.
    • Do not soften upon heating.

Physical Behaviors of Polymers

  • The physical structural arrangement of polymer chains influences intermolecular interactions:
    • Inter-polymer bonding is affected by morphology:
    • Categories of morphologies include amorphous, crystalline, and semi-crystalline.

Amorphous vs Crystalline Polymers

  • Crystallinity Definition: The degree to which chains bond and pack tightly into a crystalline structure.
  • Characteristics:
    • Crystalline polymers:
    • Strong, rigid, less affected by solvents.
    • Amorphous polymers:
    • Softer, more accessible to solvents.
  • Note: Polymers with branches or irregular side groups cannot pack regularly to form crystals, hence are amorphous.

Polymer Failure Mechanisms

  • Fracture: Failure propagation stemming from an existing defect.
  • Creep: The gradual deformation some polymers undergo under continual mechanical stress.

Copolymers for Modifying Properties

  • Copolymers: Polymers composed of two or more different monomers.
  • Resulting properties represent a blend of the individual polymers.

Varying Crystal Structure with Copolymers

  • The chemistry of Block A and Block B directly influences intermolecular interactions and outcomes on the crystal structure:
    • By varying the size, order, and chemistry of polymer blocks, material properties can be tailored for different characteristics like:
    • Crystallinity
    • Charge
    • Hydrophobicity
    • Elasticity

Modifying Properties with Interpenetrating Networks

  • Involves two polymer networks woven together:
    • The material properties become an amalgam of the two networks involved.

Strengthening by Crosslinking

  • Crosslinking can be categorized into:
    1. Minimal interactions
    2. Non-covalent interactions
    3. Covalent connections
    4. Networked integrations

Summary of Tuning Polymeric Properties

  • Factors influencing the properties of polymeric implants:
    • Polymer molecular weight and structure
    • Side groups (charge, steric interactions)
    • Hydrophobicity
    • Crystallinity
    • Crosslinker size, flexibility, and density
    • Type and ratio of co-polymers