Abstract
We previously showed that nanoparticles (NPs) could be ordered into structures by using the growth rate of polymer crystals as the control variable. In particular, for slow enough spherulitic growth fronts, the NPs grafted with amorphous polymer chains are selectively moved into the interlamellar, interfibrillar, and interspherulitic zones of a lamellar morphology, specifically going from interlamellar to interspherulitic with progressively decreasing crystal growth rates. Here, we examine the effect of NP polymer grafting density on crystallization kinetics. We find that while crystal nucleation is practically unaffected by the presence of the NPs, spherulitic growth, final crystallinity, and melting point values decrease uniformly as the volume fraction of the crystallizable polymer, poly(ethylene oxide) or PEO, ϕPEO, decreases. A surprising aspect here is that these results are apparently unaffected by variations in the relative amounts of the amorphous polymer graft and silica NPs at constant ϕ, implying that chemical details of the amorphous defect apparently only play a secondary role. We therefore propose that the grafted NPs in this size range only provide geometrical confinement effects which serve to set the crystal growth rates and melting point depressions without causing any changes to crystallization mechanisms.
Highlights
The field of polymer nanocomposites (PNCs) has grown significantly since Kojima’s work with nylon-6−clay hybrids in the early 1990s
While we adopt an experimental protocol of fixed φsilica in some cases, we instead find that a more “unifying” behavior occurs when we examine samples at the same volume fraction of poly(ethylene oxide) (PEO), where φPEO = 1 − φNP. (This implies that φsilica varies as we go from the low to the high grafting samples in this protocol.) The relationship between φsilica and φPEO for each system is plotted in Figure 1B
With decreasing φPEO, there is a monotonic depression of the crystallization temperature. These results suggest that the fillers are not capable of nucleating the PEO, since otherwise the crystallization temperatures would increase upon filler addition
Summary
The field of polymer nanocomposites (PNCs) has grown significantly since Kojima’s work with nylon-6−clay hybrids in the early 1990s. Inorganic NP fillers are utilized here to enhance these properties by using a technique that takes advantage of the kinetic processes associated with polymer crystallization to order NPs into desired dispersion states.[5] Recent work has shown that the idea of “ice templating”, where a solidification front expels the particles out to the edge of the growing crystal, can be used to create hierarchically ordered polymer composites.[6] By extending this idea to lamellar semicrystalline polymers, we find that the placement of NPs in the amorphous interlamellar, interfibrillar, and interspherulitic regions can be controlled through changes in the rate of polymer crystallization, which in turn is tuned by varying the isothermal crystallization temperatures, Tc.[7] A balance of the forces on a NP in the presence of the growing crystal (Stokes drag force and the disjoining pressure of incorporating the NP into the crystal) is used to obtain the critical growth velocity, Gc. kBT , 6πηaRH,NP where kB is the Boltzmann constant, T is temperature, η is the polymer viscosity, a is the crystal lattice spacing, and RH,NP is the effective diffusive radius of the NP. These results are explained by the fact that the confinement offered by the NPs is primarily controlled by φPEO
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