posted on 2016-09-26, 18:34authored byKangsheng Liu, Ele L. de Boer, Yefeng Yao, Dario Romano, Sara Ronca, Sanjay Rastogi
In
the past, studies have been performed to follow chain dynamics
in an equilibrium polymer melt using low molar mass polymers. Here
we show that in linear ultrahigh molecular weight polyethylene entanglements
formed during or after polymerization are influencing differently
the overall chain topology of the polymer melt. When a disentangled
UHMWPE sample is crystallized under isothermal conditions after melting,
two endothermic peaks are observed. The high temperature peak is related
to the melting of crystals obtained on crystallization from the disentangled
domains of the heterogeneous (nonequilibrium) polymer melt, whereas
the low melting temperature peak is related to the melting of crystals
formed from entangled domains of the melt. On increasing the annealing
time in melt, the enthalpy of the lower melting temperature peak increases
at the expense of the high melting temperature peak due to the transformation
of the disentangled nonequilibrium melt into the entangled equilibrium
one. However, independent of the equilibrium or nonequilibrium melt
state, the high melting temperature peak is observed when the disentangled
samples are left to isothermally crystallize at a specific temperature,
although with a decrease in bulk crystallinity. A commercial (entangled)
sample, instead, shows both shift in the position of the melting temperature
peak and drop in crystallinity. To ascertain that entanglements are
the cause for the observed difference, experiments are performed in
the presence of reduced graphene oxide (rGON): the melting response
of disentangled UHMWPE crystallized from its heterogeneous melt state
remains nearly independent of the annealing time in melt. This observation
strengthens the concept that in the presence of a suitable filler,
chain dynamics is arrested to an extent that the nonequilibrium melt
state having lower entanglement density is retained.