How does molecular weight effect glass transition temperature?

1 Answer
May 28, 2016

Answer:

The glass transition temperature #T_g# increases with molecular weight and asymptotically approaches a maximum value.

Explanation:

The glass transition temperature is the temperature range in which a polymer changes from a rigid “glassy” state to a more pliable “rubbery” state.

In polymer chemistry, the Flory–Fox equation relates the number-average molecular mass #M_n# of a polymer to its glass transition temperature #T_g#:

#color(blue)(|bar(ul(color(white)(a/a) T_g = T_(g,∞) -K/M_n color(white)(a/a)|)))" "#

where

  • # T_(g,∞)# is the maximum value of #T_g# that can be achieved at a theoretical infinite molecular mass
  • #K# is an empirical parameter that is related to the free volume present in the polymer sample.

The free volume is a measure of the room a polymer chain has in which to move in relation to the other polymer chains around it.

A polymer with long chains (high molecular mass) has less free volume than one with short chains.

Thus, low molecular mass gives lower values of #T_g#, and higher molecular mass causes #T_g# to approach #T_(g,∞)# asymptotically.

We see this behaviour in a typical Flory-Fox plot:

Flory-Fox

In the above plot, #T_(g,∞) ≈ "380 K"#.