as i understand by increase the molecular weight of the polymer ,glass transition ( Tg ) increases but
why some polymer have high molecular weight but it is glass transition is lower than another polymer that have lower molecular weight
for example poly (hexyl methacrylate) have lower Tg than poly ( methyl methacrylate)
if we consider molecular weight we will find that poly (hexyl methacrylate) have higher molecular weight than poly ( methyl methacrylate)
Glass transition temperarure may depend on the molecular weight. However in its essence, it is the temoerature at which the chains become mobile which is affected by several factors such as additives, cross linking, crystallinity (although glass transition isnt a phase transition) etc.
Factors Affecting Tg are:
1. Chemical Structure
Molecular Weight: In linear polymers, increase the MW leads to decrease in chain end concentration which results in decreasing free volume at end group region and increase in Tg.
Molecular Structure: Existence of bulky, inflexible side groups increases Tg of material due to decreasing in mobility.
Chemical cross-linking: Increase in cross-linking decreases mobility leads to decrease in free volume and increase in Tg
Polar groups: Presence of polar groups lead to increase intermolecular forces, inter chain attraction and cohesion leading to decrease in free volume resulting in increase in Tg.
2. Addition of Plasticizers
Addition of plasticizer increases the free volume in polymer structure, so the polymer chains can move around at lower temperatures which cause decreasing in Tg.
3. Water or moisture content
Increase in moisture content leads formation of hydrogen bonds with polymeric chains increasing the distance between polymeric chains. And, hence increases the free volume and decreases Tg.
4. Effect of entropy and enthalpy
The value of entropy for amorphous material is higher and low for crystalline material. If value of entropy is high, then value of Tg is also high.
5. Pressure and free volume
Increase in pressure of surrounding leads to decrease in free volume and ultimately high Tg.
Other factors like branching, alkyl chain length, bond interaction, flexibility of polymer chain, film thickness etc. also have significant impact on glass transition temperature of polymers.
The relation between length or number of monomer or molecular weight on glass transition temperature is described according to the Flory-Fox equation Tg= Tg∞ - K/Mn.
Tg∞: the maximum glass transition temperature of the polymer with an infinitive molecular weight
K: an empirical constant relating to the free volume of the end group in the polymer chains
Ref: https://omnexus.specialchem.com/polymer-properties/properties/glass-transition-temperature
Dear Dawood Haydar,
The molecular weight of polymer depends on the degree of polymerization and its monomer. We can not compare two monomer and judge about the molecular weight of their polymers.
Dear Dawood Haydar :
You are mixing to questions. Tg as stated above depends on several characteristics as very well stated by Ms. Mina Karimi-Avargani .
If you want a relationship of Tg regarding only the number average molecular weight (Mn), this is well correlated by the Flory-Fox esquation as:
Tg = Tg(infinite Mn) + K/Mn
K is a constant for each homopolymer
Look at any polymer book.
Increasing the long-range order, resulting from the crystalline structure, decreases the ability of the material to fluctuate easily and thus results in increasing the Tg. Lots of other factors other than molecular weight; affect the Tg; for instance Polar groups, molecular structure, and degree of amorphous-crystalline structure which have to be taken into the account.
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