posted on 2019-02-06, 19:08authored byXiangning Wen, Yunlan Su, Yudan Shui, Weiwei Zhao, Alejandro J. Müller, Dujin Wang
The interfacial interactions
of polymer–nanoparticles have
dramatical effects on the crystallization behavior of grafted polymers.
In this study, methoxypolyethylene glycol (MPEG) (molecular weights
750, 2000, and 4000 g mol–1) was grafted onto amino-modified
nanosized silica (SiO2-NH2) by the “grafting
to” method. The effects of the grafting density and molecular
weight on the confined crystallization of grafted MPEG (MPEG-g-SiO2) were systematically investigated by differential
scanning calorimetry (DSC), thermogravimetric analysis (TGA), and
wide-angle X-ray scattering (WAXS). It was found that confinement
effects are stronger when lower molecular weights of grafted MPEG
are employed. These grafted MPEG chains are more difficult to stretch
out on SiO2-NH2 surfaces than when they are
free in the bulk polymer. Both crystallization temperature (Tc) and crystallinity of grafted
MPEG chains decrease with reductions of grafting density. Additionally,
covalent bonding effects and interfacial interaction confinement effects
are strengthened by the decrease in grafting density, leading to an
increase in decomposition temperature and to the disappearance of
the self-nucleation Domain (i.e., Domain II), when self-nucleation
experiments are performed by DSC. Overall isothermal crystallization
kinetics was studied by DSC, and the results were analyzed with the
Avrami equation. An Avrami index of n ≈ 3
was obtained for neat MPEG (indicating that instantaneous spherulites
are formed). However, in the case of MPEG-g-SiO2 with the lowest grafting density, the Avrami index of (n) was less than 1 (first-order kinetics or lower), indicating
that nucleation is the determining factor of the overall crystallization
kinetics, a signature for confined crystallization. At the same time,
the crystallization from the melt for this MPEG-g-SiO2 with the lowest grafting density occurs at Tc ≈ −30 °C,
a temperature close to the glass transition temperature (Tg) of MPEG, indicating that this confined
MPEG crystallizes from homogeneous nuclei.