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How does the glass transition temperature in polymers depend on molecular weight ?
I present a study of the molecular weight dependence, M, of the glass transition Tg(M),
xperimentally and with simple modeling. We find three regimes for T_g(M), roughly corresponding to (1) short inflexible oligomers, (2) oligomers with 2-20 or so Kuhn steps, and (3) polymeric molecules. We identify quasi-universal behaviour across wide classes of polymers, and argue that dynamic facilitation that balances intramolecular and intermolecular cooperatively is necessary in order to describe all three regimes.The coupling of intramolecular degrees of freedom to intermolecular degrees of freedom distinguishes polymer glasses from simpler glass formers such as rigid molecules or colloidal suspensions.