We have previously shown that semicrystalline polymers can be reinforced by adding nanoparticles (NPs) and then ordering them into specific motifs using the crystallization process. A key result we have found is that when the spherulite growth rate is slowed below a critical value, then, NPs can order into the amorphous interlamellar regions of the semicrystalline structure. The effects of spherulite growth rate in this context have previously been examined, and here we focus on the role of NP diffusivity. We achieve this goal by changing the poly(ethylene oxide) (PEO) molecular weight as a route to altering the matrix viscosity. In particular, four molecular weights of PEO were employed ranging from 5.4−46 kDa. Each sample was loaded with 10 vol % of bare 14 nm diameter silica NPs. After initially studying spherulite growth rates, experiments were designed to fix the spherulite growth rate across sample molecular weights to study particle ordering, induced by polymer crystallization. We find that, at the fastest growth rate studied (12 μm/s), the lowest molecular weight sample showed the highest order, presumably due to enhanced particle mobility. However, as the spherulite growth rate is slowed, the maximum ordering behavior is observed at intermediate molecular weights. The trend observed at slow growth rates is explained by the large-scale segregation of NPs (presumably into the grain boundaries, i.e., the interspherulitic regions); evidence for this is the observed transition of spherulite growth to diffusion-control at slow growth rates in the lowest molecular weight PEO sample studied.
We present in situ tracking of silica nanoparticle (NP) migration from a poly(ethylene oxide) (PEO) melt into interlamellar region using in situ atomic force microscopy (AFM). Our results confirm the previous hypothesis that NPs migrate into the interlamellar regions at crystallization growth rates smaller than a critical value under isothermal conditions. Under these slow crystallization conditions, bare silica NPs are rejected as defects by the growing crystal of PEO, and the in situ imaging on the large (50 nm) NPs helps track the migration into the amorphous zones. We extend this AFM technique to estimate lamellar growth rates that correlate with spherulite growth rates determined by polarized light optical microscopy (PLOM) but at smaller undercoolings than are typical for PLOM.
X-ray photon correlation spectroscopy measurements were used to quantify the dynamics of bare and bimodal grafted silica nanoparticles mixed with PEO melts of different molecular weights. In dilute polymer nanocomposite (PNC) samples, we find diffusive NP behavior as described by the Stokes–Einstein relationship so long as the adsorbed PEO polymer layer is taken into account in determining both the effective NP size and its role on composite viscosity. The size of this bound layer was found to be approximately 2R g, where R g is the chain radius of gyration. We also expanded our system to investigate how the dynamics of grafted NPs differ from bare NPs with an adsorbed layer. We showed that the dynamics again can be determined by an effective NP radius at a scale smaller than the effective interparticle spacing; however, at larger length scales, the morphology and grafting parameters play a major role in the system dynamics. These results allow us to quantify NP ordering driven by polymer crystallization. It has previously been speculated that behavior is controlled by the relative ratio of time scale of crystal growth and the diffusive time scale of the NPs, a Peclet number. When the former time scale is longer, then the NPs are expected to be segregated into the interlamellar amorphous zones, while the NPs are too slow to be reorganized in the opposite case. We show here that this conjecture is quantitatively correct and the demarcation in behavior occurs for Pe = 1. Thus, we provide a way to estimate a critical spherulite growth rate for any semicrystalline PNC, at which a given NP can be ordered. Together, the results of this study permit us to tunably design PNCs through directed dispersion of NPs in a polymer matrix.
We systematically investigate the effects of adding ungrafted PMMA versus silica-grafted PMMA (PMMA-g-SiO2) on the crystallization kinetics and the mechanical properties of PEO. In the grafted analogue, PMMA chains were grown from the surface of 14 nm diameter silica nanoparticles at a grafting density of ∼0.42 chain/nm2. The molecular weight of the PMMA was varied systematically (20–85 kDa), and the extent of confinement was regulated by varying the PEO fraction. While the PEO crystallinity and the crystal growth rate in the ungrafted and grafted systems track each other, the grafted systems exhibit enhanced crystallization kinetics, most likely because of faster chain dynamics. Additionally, the grafted systems have higher nucleation rates, especially for shorter PMMA chains. These differences in crystallization kinetics are likely due to the dry-brush zone near the silica surface, where graft chains do not interpenetrate with the matrix. In the context of mechanical properties, both additives enhance the PEO modulus and toughness, with the PMMA-g-SiO2 filled materials undergoing their brittle-to-ductile transition at lower loadings. Surprisingly, there is little difference between the two additives in how they affect PEO crystallization and mechanical propertiesthis is likely due to the high grafting densities where the core is effectively shielded from the PEO matrix.
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