Chapter 19. Synthetic Polymers
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1. Characteristics of Polymers
1. Characteristics of Polymers
⑴ Degree of Polymerization (DP)
① Definition: The number of monomer units in a polymer chain
⑵ Average Molecular Weight
① Number-average molecular weight (Mn): Related to freezing point depression, boiling point elevation, vapor pressure lowering, osmotic pressure, etc.
② Weight-average molecular weight (Mw): Related to viscosity, tensile strength, light scattering, etc.
③ Z-average molecular weight (Mz): Related to melt elasticity, centrifugation, etc.
④ Viscosity-average molecular weight (Mv): Related to viscosity, etc.
○ Mark-Howink equation
○ For PLA solutions dissolved in chloroform (25°C): K = 5.45 × 10-4, α = 0.73
⑤ Mz > Mw > Mv > Mn
⑥ Polydispersity Index (PDI): Polydispersity index in the general sense
○ Definition: PDI = Mw ÷ Mn. PDI = 1 for monodisperse polymers, PDI > 1 for polydisperse polymers
○ Polymer systems have various molecular weights due to different numbers of monomers, leading to PDI > 1
○ Molecular weight distribution graph types: narrow distribution, broad distribution, bimodal distribution (2 peaks)
○ General PDI can be determined using techniques like GFC, HPLC, GC
⑦ Meaning of PDI in DLS, Zetasizer
○ σ: The Standard Deviation of the Particle Size Distribution
○ d: The Average Hydrodynamic Particle Size
○ If 0 < PDI < 0.2, the dispersion of the nanoparticles is very good.
○ If highly polydisperse, PDI can exceed 1
○ Zetasizer sets PDI between 0 and 1
⑶ Crystallinity
① Factors: Molecular structure, functional groups, processing temperature, cooling rate
② Measurement methods: X-ray diffraction, infrared spectroscopy, differential scanning calorimetry (DSC) which measures melting heat change
③ Partial Crystallinity
○ Polymers have a mixture of ordered crystalline and disordered amorphous structures
○ Factor 1: Incomplete crystallinity due to numerous repeating units
○ Factor 2: High branched structures hinder crystallinity
⑷ Stereoregularity
① Random copolymers (e.g., PLGA): Arrangement like ABBAABBBABABAAAABBB
② Alternating copolymers: Arrangement like ABABABABABABABABABA
③ Block copolymers: Arrangement like AAAAAAAAABBBBBBBBBB
④ Graft copolymers: Arrangement like AAAAAAAAAAAAAAAAAAA
⑸ Glass Transition Temperature (Tg)
① Definition: Temperature at which a polymer solid becomes rubbery
② Cause: Transition temperature for irregular motion of main chain. Only present in amorphous polymers
③ T < Tg: Rigid, glass-like state
④ T > Tg: Rubber-like state
2. Polymer Synthesis
⑴ Type 1. Radical Addition Polymerization
① Reaction under catalysts like radical initiators (e.g., benzoyl peroxide) or acids/bases
② Example 1: Polyethylene synthesis
Figure 1. Polyethylene synthesis
③ Example 2: Polyisoprene synthesis (e.g., natural rubber)
Figure 2. Polyisoprene synthesis
④ Example 3: Polystyrene (PS) synthesis
○ Polystyrene has preferred direction in polymerization for maximizing the number of resonance contributors
Figure 3. Polystyrene synthesis
⑤ Example 4: Polymerization of acetone
Figure 4. Polymerization of acetone
⑥ Example 5: Polyethylene oxide synthesis using epoxide ring-opening reaction
Figure 5. Polyethylene oxide synthesis
⑦ Example 6: Polyvinylchloride (PVC) synthesis (CH2CHCl)n
⑧ Example 7: Polyacetic acid vinyl synthesis (CH2CHCOOCH3)n
⑨ Example 8: Nylon synthesis (first synthetic fiber)
Figure 6. Nylon 6 synthesis
○ Step A: Ketone Amination Reaction. Faster under weak acid conditions compared to weak alkaline conditions
○ Step B: Beckmann Rearrangement. Ring expansion reaction through rearrangement
○ Step C: Ring-opening polymerization. Initiated by radicals, acids, or bases
⑵ Type 2. Dehydration Condensation Polymerization
① Capable of hydrolysis reactions
② Example 1: Acetaldehyde polymerization
Figure 7. Acetaldehyde polymerization
③ Example 2: Polymerization via acetal formation reaction
Figure 8. Polymerization via acetal formation reaction
④ Example 3: Polyester synthesis
Figure 9. Polyester synthesis
○ glycolic acid → poly(glycolic acid)
○ lactic acid → poly(lactic acid)
○ 3-hydroxybutyric acid → poly(hydroxybutyrate)
⑤ Example 4: Polylactide (PLA) synthesis
Figure 10. Polylactide synthesis
⑥ Example 5: Polymerization of amides
Figure 11. Polymerization of amides
⑦ Example 6: Polymerization of ethylene glycol and cyanate
Figure 12. Polymerization of ethylene glycol and cyanate
3. Non-degradable Polymers
⑴ Polyethylene (PE)
① Low-density polyethylene: Weak properties. Vulnerable to high temperatures
② High-density polyethylene: Used for tubes and catheters
⑵ Polypropylene (PP)
① Has a linear structure
② Good repeated bending properties
③ Resistant to external shocks
④ Applications: Finger joint replacements, disposable syringe body
⑶ Polyamide (Nylon)
① Strong inter-chain hydrogen bonding and high crystallinity → Excellent fiber forming ability
② High absorbency → Water acts as a plasticizer → Degradation of properties due to water and protein enzymes
③ Applications: Surgical sutures
⑷ Polymethyl Methacrylate (PMMA)
① High light transmittance (92%)
② High refractive index (1.49)
③ Large in size
④ High strength and hardness
⑤ Applications: Hard contact lenses, artificial intraocular lenses, dentures, facial prosthetic materials, bone cement
⑸ Polytetrafluoroethylene (Teflon, PTFE)
① A polymer material with carbon-fluorine bonding: C-F bond is strong. Doesn’t decompose even when heated
② High molecular weight, high crystallinity (> 94%), hydrophobic, high density, low coefficient of friction, high heat resistance, stability, porosity
③ Applications: Coating material for frying pans, small diameter artificial blood vessels, catheters
⑹ Polyvinyl Chloride (PVC)
① Hard but flexible with plasticizers
② Long-term plasticizer leaching, toxicity
③ Applications: Tubes used for blood transfusion and dialysis, blood bags
⑺ Polyurethane (PU)
① Stable in the body, blood-compatible, strong, elastic, tough
② Applications: Artificial blood vessels, artificial heart valves, catheters, burn dressings
⑻ Polycarbonate
① Very hard and lightweight
② Excellent mechanical and thermal properties
③ Applications: Heart assist devices, lung assist devices, body of dialysis machines, heart valve actuators
⑼ Polyacetal
① Very hard and lightweight
② Formed by the polymerization of formaldehyde
③ Applications: Joint connection part of artificial hip joints, heart valve actuators
⑽ Polysulfone
① Very hard and lightweight
② Applications: Porous coating material for metallic orthopedic implants, oxygen barrier film
⑾ Hydrogel
① A polymer that swells (30% or more) in water
② Formed by cross-linking of hydrophilic polymers
③ Hydrogel formed by physical cross-linking: Freely undergoes sol-gel transition
④ Hydrogel formed by chemical cross-linking: Once it becomes a gel, cannot transition back to sol
⑤ Representative example: Polyhydroxyethylmethacrylate (PHEMA, polyhydroxyethylmethacrylate)
○ Water absorption is similar to living human tissue
○ Biologically inert, heat resistant, stable, excellent refractive index, high oxygen permeability
○ Applications: Contact lenses
4. Degradable Polymers
⑴ PLA(poly lactic acid): Aliphatic polyester
Figure 13. Structure of PLA
① Crystallinity: L-PLA is 1.25-1.29 g/㎠
② Melting point: L-PLA is 159 ~ 178 ℃
③ Glass transition temperature
○ L-PLA is 54 ~ 59 ℃, D-PLA is 50 ~ 53 ℃
○ As molecular weight increases, glass transition temperature increases ( because solidification due to increased intermolecular force)
④ Lifespan: 18 ~ 24 months
⑵ PGA(poly(glycolic acid)): Aliphatic polyester
Figure 14. Structure of PGA
① Molecular weight: 20 ~ 145 kg/mol
② Crystallinity: 1.5 ~ 1.64 g/㎠
③ Melting point: 210 ~ 226 ℃
④ Glass transition temperature: 36 ℃. Semi-crystalline
⑤ Has OH group at α position
⑶ PLGA(poly(lactic-co-glycolic acid))
Figure 15. Structure of PLGA
① Definition: Copolymer of PLA and PGA
② Comparison between PLGA and PLA
Table 1. Comparison between PLGA and PLA
③ Physical properties depending on PLGA’s crystallinity
Table 2. Physical and mechanical properties of various biodegradable polymers
Figure 16. Relationship between PGA content and crystallinity
Figure 17. Relationship between PGA content and melting point
Amorphous PLGA has no melting point
Figure 18. Change in half-life depending on PLGA’s crystallinity
○ Lifespan comparison: PLA ＞ PGA ＞ PLGA
④ Decomposition time of PLGA
○ Decomposes into lactic acid and glycolic acid by non-specific hydrolysis
○ The closer the weight ratio of PLA and PGA is to 1:1, the shorter the lifespan
Table 3. Decomposition period of PLGA depending on composition
○ Ability to control lifespan is an advantage of PLGA
○ Most protein enzymes do not participate in decomposition
○ Exception: Decomposition of PLGA is accelerated in vitro experiments by microbial-derived proteinase K or lipase
⑤ Decomposition process of PLGA
Figure 19. Change in device weight and PLGA molecular weight over time
○ 1st. Hydrolysis of ester: Water molecules attack the ester bond in PLGA
○ 2nd. As the polymer breaks, the carboxyl terminus is exposed and molecular weight continues to decrease
○ 3rd. Decomposition rate increases over time: Carboxyl end groups act as a catalyst for ester hydrolysis ( autocatalysis )
○ 4th. Even if the polymer chain breaks, it remains insoluble at high molecular weight and does not diffuse into the solution
○ 5th. After approaching the critical molecular weight (1,000 ~ 1,100), it dissolves → The device weight starts to decrease
○ 6th. As the carboxyl terminus dissolves, pH decreases
⑷ Other biodegradable polymers
① Polycaprolactone(PCL)
② Polyanhydride
③ Polyortho ester
④ Polyamino acid
⑤ Polyhydroxybutyrate(PHB)
⑥ Polyhydroxyvalerate(PHV)
⑦ Polyphosphazene: An inorganic polymer where the main chain is made of nitrogen and phosphorus
Edited: 2020.03.06 18:32