Heat capacity in the 1 K range and close to Tg was studied in a series of poly(n-alkyl methacrylate)s, PnAMA, including some random copolymers. We report on two findings: (i) The glass transition temperature Tg and the cooperativity of the R process that freezes-in at Tg, NR(Tg), decrease smoothly between methyl (PMMA) and hexyl member (PnHMA); the cooperativity from NR(Tg) ) 35 to NR(Tg) ≈ 1. (ii) The tunnel density measured in the 1 K range as c1 constant of the heat capacity function increases between PMMA and the ethyl member (PEMA), shows, after a maximum near PEMA, a sharp drop (factor of 6) to the butyl member (PnBMA), and remains constant at a low level up to the octyl member (PnOMA). The cooperativity of the R process near Tg and the tunnel density will be correlated assuming that freezing-in (vitrification) fixes the dynamic heterogeneity in the self-organized equilibrium liquid near Tg. We try to explain how freezing of dynamic heterogeneities in the PnAMA series is responsible for the trend in the tunnel density including the sharp drop: Freezing-in of cooperativity shells around Glarum defects forms some vaults that enlarge the free volume near these defects in comparison to a situation without vaults. The maximum in c1 is promoted by this additional free volume; the drop in c1 is caused by a vault breakdown effect if the cooperativity shell is too small for vault formation, NR(Tg) < 15.

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