(20 points) You can weld together two pieces of plastic by heating them above th
ID: 1056469 • Letter: #
Question
(20 points) You can weld together two pieces of plastic by heating them above the glass transition temperature (or melting temperature if semicrystalline) and allowing interdiffusion, then cooling back to room temperature. Maximum strength is obtained when the polymer chains have diffused across the weld line to about Rg in distance. For Mn = 105 g/mol polystyrene Rg = 8.5 nm and the diffusion coefficient D @ Don-2 where Do @ 10-5 cm 2 /s at 175 °C. a) Why is Rg the relevant length scale for maximum strength? b) How long should heat be applied to insure a maximum strength joint strength if M = 600,000 g/mol pieces of PS are to be joined at 175 °C? As a rough estimate use the simple solution to the diffusion equation (diffusion length) learned earlier in class. c) What is the minimum time for maximum strength if M is cut in half? d) Airplane glue is basically toluene, a solvent that mixes with PS and reduces the glass transition temperature below room temperature. Describe in words how you think it works.
Explanation / Answer
a) The system at one scale will generally consist of similar properties of itself when
viewed at a smaller scale, with different parameters describing the components of the system.
(that is the reason why Rg is in nm). Maximum strength is obtained at this Rg to find out
the properties at micro level that is at crystal level which are required to study inter-diffusion.
That is the reason why Rg is the relevant length scale for maximum strength.
b) Using the Knudsen diffusion flow equation since it is a means of diffusion that occurs when the
scale length of a system is comparable to or smaller than the mean free path of the particles involved
Do = u / 3
Do = urms / 3t
Do = /3 [ (3RT/M ) / t]
you can calculate by substituting everything in to this equation [ lambda = 8.5nm , M =600000]
and find t
c) same goes for this , put M =600000/2 and find t
d) When the Tg is below the room temperature that is when the polymer is cooled below glass transistion
temperature, the PS mixes with toluene it becomes hard and brittle, like glass. PS is a hard plastic
and all hard plastics are used below their Tg in their glass-like state
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