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Polymer properties (5.Gelling of polymers (examples (Poloxamer (pluronic),…

- - - - irreversible
      - Na
  - - - The temperature at which gelation occurs
    - - gels are formed from solutions, each system is characterized by a critical gelation concentration, below which a gel is not formed
  - - - Polymers as poly(hydroxyethyl methacrylate) known as polyHEMA when cross linked with ethylene glycol dimethacrylate they can imbibe body fluids and swell to a predetermined volume but don’t dissolve, and hence they can be used as implants
      - Hydrophilic contact lenses can also be made from this polymer type
    - - Polyvinylalcohols gels dry quickly upon application on the skin, leaving a plastic film causing the drug to be in intimate contact with the skin.
    - - a block copolymer of type AAABBBAAA, in which A is water-soluble (polyoxyethylene) and B is water-insoluble (polyoxypropylene).
      - They are used as surfactants
      - They can form micelles in aqueous solutions above the critical micelle concentration in which the hydrophobic central block associates with other like blocks, leaving the hydrophilic poly(oxyethylene) chains outside and protecting the inner core.
      - Gelation occurs due to two possible reasons
        
        -when concentrated solutions are warmed (the solubility of polyoxyethylene decreases by increasing temperature causing them to be more hydrophobic leading to formation of more micelles more closely packed together (Thermoreversible gelation)
        
        -at high solution concentrations the micelles pack so closely that the solution becomes immobile
    - - Conventional eye medication has been modified by viscosity-enhancing agents such as hydroxypropylmethylcellulose HPMC, poly(vinyl alcohol) and silicones. These all prolong contact of drug with the cornea by increasing the viscosity of the medium and retard the drainage of the tear fluid from the eye.
- - - - The extrapolated value of ηsp/c at zero concentration is known as the intrinsic viscosity — [η].
      - unique function of molecular weight (for a given polymer-solvent pair) and measurements of [η] can be used to measure molecular weight.
      - The limiting value of the reduced viscosity or the inherent viscosity at infinite dilution of the polymer.
    - - expresses the incremental viscosity due to the presence of the polymer in the solution
      - Normalizing ηsp to concentration gives ηsp/c which expresses the capacity of a polymer to cause the solution viscosity to increase; i.e., the incremental viscosity per unit concentration of polymer.
      - ηsp/c will depend on c
      - The ratio of the difference between the viscosities of solution and solvent to the viscosity of the solvent.
    - - The ratio of the viscosity of the solution to the viscosity of the solvent.
    - - Like ηsp, ln ηr is zero for pure solvent and increases with increasing concentration, thus ln ηr also expresses the incremental viscosity due to the presence of the polymer in the solution.
      - ln ηr/c gives the inherent viscosity In the limit of zero concentration, ηi extrapolates the same as ηsp/c and becomes equal to the intrinsic viscosity.
      - The ratio of the natural logarithm of the relative viscosity to the mass concentration of the polymer.
  - - - in good solvents
        
        linear molecules will be expanded as the polar gps will be solvated & viscosity inc
      - in poor solvents
        
        the intramolecular attraction bet the segments is greater than the segment -solvent affinity and the molecule tend to coil up
- - - - The molecular weight is thus an average molecular weigh
        
        determined by
        
        osmotic pressure
        
        chemical analysis
        
        lightscattering measurements.
- - - - the larger the molecule, the stronger are the forces holding the chains together and more energy is needed to force the chains apart in the solvent).
      - Choice of appropriate polymer molecular weight controls both
        
        the rate of dissolution
        
        the viscosity of its resulting solution
      - the greater the degree of crystallinity of the polymer, the lower is the rate of dissolution
  - - - surgical dressings
      - membranes for dialysis or filtration
      - film coating materials
      - matrices for enveloping drugs to control their release properties
      - form thin films
      - packaging materials
- - - - Positively charged polymers are more bioadhesive than negatively charged polymers
- - - - The melting point (Tm) is not as well defined as in low molecular weight crystalline solids due to the presence of the poorly crystalline regions which melt over a range of temperatures
      - Polymers may exhibit a glass transition temperature (Tg) (amorphous).
        
        Below the Tg the chains are frozen in position and the polymer is glassy and brittle
        
        while above Tg the chains are mobile and the polymer is more flexible