The mechanism of action of plasticizers is defined as to interpose between every individual strand of polymer and thereby causing breakdown of polymer-polymer interactions. The tertiary structure of the polymer is modified into more porous, flexible and with less cohesive structure.

Plasticizers soften and swell the polymer which aids in overcoming their resistance to deformation. plasticized polymer would deform at a lower tensile force as compared to without plasticizer. This enhances the polymer – plasticizer interaction. This effect in turn enhances the film elongation effect.

The interaction to a greater extend depends upon the glass transition temperature of polymers. Glass transition temperature. Tg is the temperature at which hard glassy polymer is converted into a rubbery material.

All polymers have higher glass transition temperature and addition of plasticizers reduces the glass transition temperature.

Some theories have been proposed to explain the mechanisms of plasticization action.   

The lubrication theory postulates that plasticizers act as internal lubricants by reducing frictional forces by interspersing themselves between polymer chains. 

The gel theory postulates that the rigidity of polymer comes from three-dimensional structures through the center  of force (e.g., hydrogen bonds and Van der Waals or ionic forces), and plasticizers take effect  by breaking polymer-polymer interactions and masking these center of force.  

The free volume theory states plasticization as a study of ways to increase free volume and is useful in explaining the lowering of the Tg by a plasticizer. The free volume or free space of a crystal, glass or liquid may be defined as the difference between the volume observed at absolute zero temperature and the volume measured for the real crystal, glass or liquid at a given temperature.

  In specific volume of material and temperature relationship, the glassy matter becomes rubbery or fluid (obviously increasing specific volume). When the temperature is above Tg, the molecules have enough energy to move, bend or rotate. The Brownian motion of molecules or segments of molecules produces a greater amount of free volume (the torsional or hole free volume). 
 Free volume comes from three principal sources:
  1. the motion of chain ends
  2. the motion of side chains
  3. the motion of the main chain.
The free volume of a resin system may be increased by:
  1. Increasing the number of end groups. (lower the molecular weight).
  2. Increasing the number or length of (proper) side chains. (internal    plasticization)
  3. Increasing the chance for main chain movement by inclusion of    segments of low steric hindrance and low intermolecular attraction    (low polarity and H bonding). (internal plasticization)
  4. Inclusion of a compatible compound of lower molecular weight    that acts as though it does all of 1 through 3 above. (external  plasticization)
  5. Raising the temperature.