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Properties of Polymers
Physical Pharmacy 2 Properties of Polymers Kausar Ahmad Kulliyyah of Pharmacy, IIUM Physical Pharmacy 2 KBA
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Contents Polymerisation factors Tg, Tm Tensile strength
Diffusion coefficient Permeability Physical Pharmacy 2
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TOP MONOMERS Ethylene Propylene Styrene Physical Pharmacy 2
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TOP Polymers in pharmaceuticals?
Type Application Properties Submit through LMS Physical Pharmacy 2
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Types of Polymers Addition polymers:
break a double bond to make a polymer Condensation polymers: give by-product such as water, methanol, or hydrogen chloride Physical Pharmacy 2
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Reaction mechanisms Addition-> Chain-growth e.g. polyethylene
By free radical or ion e.g. polyethylene polypropylene polystyrene Condensation-> Step-growth By functional groups e.g. polyurethane polyester naturally occurring polymers Physical Pharmacy 2
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Polymerisation Conditions
Bulk polymerisation Suspension polymerisation Emulsion polymerisation Seed polymerisation Physical Pharmacy 2
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Degree of Polymerization
(n)- the number of monomer units that have polymerized together. D.P. values can be as high as 10,000. Exercise: What are the factors that influence D.P.? Submit through LMS Physical Pharmacy 2
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Physical Properties Determined by: molecular weight & polydispersity
strength of intermolecular forces regularity of the polymer structure flexibility of the polymer molecule Physical Pharmacy 2
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Physical and Mechanical Properties
Physical Pharmacy 2 Physical and Mechanical Properties melting point boiling point solubility melt viscosity Tg hardness/ crystallinity diffusion coefficient tensile strength tear strength puncture strength A polymer may have a glass transition temperature (Tg) and a crystalline melt temperature (Tc.) The glass transition temperature refers to a point where there is a change in polymer molecular chain motion which has drastic effects on strength. Tg is sometimes called the "glass-rubber" transition. Glassy rubbery The crystalline melt temperature (Tc) is higher than Tg, at Tc the crystalline domains of a polymer melt to become amorphous. the melting point of a polymer does not occur over a sharp temperature range (1- 2 degrees C) as is observed for small organic molecules. If a polymer becomes a melt, there is usually a range of as much as 50 degrees C over which the viscosity of the polymer slowly changes from that of a solid to that of a liquid. Note that for a polymer to melt, a polymer must be a thermoplastic. The term "softening point" is sometimes used. Physical Pharmacy 2 KBA
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Polymer Crystallinity
The extent of crystallinity in a polymer affects: Rigidity Fluidity Resistance to diffusion of small molecules in the polymer and solubility Water/solvent uptake Extent of leaching Physical Pharmacy 2
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Tensile strength Tensile strength measures how difficult it is to break a substance when stress is applied to pull it apart. Tensile strength generally increases with molecular weight. Physical Pharmacy 2
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Physical Pharmacy 2 [A] Stress at the Yield Point: At this point the force of the tensile pull is sufficient to cause polymer chains to slip past each other, and yield occurs. [C] Stress at Failure: At this level of stress the polymer molecules can no longer maintain cohesive integrity and the sample breaks [D] Elongation at Failure: This is how much the sample can stretch before it breaks C stress A E.g. An Instron grips a sample and pulls it apart. The Instron creates a plot of where the y axis is the force exerted between the two grips, and the x axis is the separation distance between the two clamps. Usually, the electronics cause the Instron to increase the distance between the two clamps (pulling) at a constant rate, and the force or "strength" required is measured. [B] Elongation at the Yield Point: Maximum stretching before yield failure takes place B strain D Physical Pharmacy 2 KBA
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J = -DKC/L Diffusion in a Polymer
The diffusion in a solid although very slow still obeys Fick’s first law: J = -DKC/L J is the flux D is the diffusion coefficient K is distribution coefficient of the permeant towards the polymer C is the difference in concentration at the interface L is the thickness of the polymer Physical Pharmacy 2
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Factors Influencing Diffusion Coefficient
Diffusion coefficient decreases when: MW increase Crosslinking increases Crystallinity increases Fillers increase i.e. reinforcing filler Diluent decreases Size of drug increases Physical Pharmacy 2
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Physical Pharmacy 2 Permeability describes the ability of molecules to diffuse through a layer of polymer for a tightly arranged polymer /high degree of crystallinity, permeability is very low. The crystalline regions act as a barrier to diffusing molecules highest for a completely amorphous polymer Physical Pharmacy 2 KBA
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Natural Permeability of Polymers
Physical Pharmacy 2 Natural Permeability of Polymers important in gas diffusion, water sorption, permeation & dialysis. Water-vapour permeability depends on polarity of polymer Polar polymers are more ordered and less porous; thus less permeable to oxygen Less polar polymers may allow oxygen to permeate However, less polar polymers have lesser affinity for water. Need to control hydrophilicity of polymer or use a copolymer Exercise: Arrange the polymers below in decreasing order of oxygen permeability. Teflon Polyvinyl alcohol Polyvinyl chloride High Density Polyethylene Polystyrene Natural rubber Dimethyl silicone rubber Physical Pharmacy 2 KBA
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Polymer Film Preparation
The way a polymer film is prepared affects the permeability When a polymer film is prepared by casting, gradual drying and the resultant shrinkage gives a less porous film compared to a polymer film prepared by ‘coacervation’ The difference would be in the number of pores/voids, size and density of the film Physical Pharmacy 2
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Phase Separation in Polymer Films
Incompatibility e.g. can be based on solubility parameter. Different polarity Physical Pharmacy 2
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References Aulton, M. E. (1988). Pharmaceutics: The Science of dosage form design. London: Churchill Livingstone. Wise, D. L. (2000). Handbook of Pharmaceutical Controlled Release Technology. New York: Marcel Dekker. Chasin, M & Langer, R (1990). Biodegradable polymers as drug delivery systems. New York: Marcel Dekker. Vyas, S. P & Khar, R. K. (2002). Targeted and controlled drug delivery. New Delhi: CBS. Physical Pharmacy 2
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