Organizing Nanoparticles in Semicrystalline Polymers by Modifying Particle Diffusivity

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.

Quantifying Nanoparticle Ordering Induced by Polymer Crystallization

It has recently been established that polymer crystallization can preferentially place nanoparticles (NPs) into the amorphous domains of a lamellar semicrystalline morphology. The phenomenology of this process is clear: when the time for NP diffusion is shorter than the crystal growth time, then the NPs are rejected by the growing crystals and placed in the amorphous domains. However, since there is no quantitative characterization of this ordered NP state, we develop a correlation function analysis for small-angle X-ray scattering data, inspired by classical methods used for enunciating the local morphology of lamellar semicrystalline polymers. We show that when the spherulitic growth rate is slower than NP diffusion, then all the NPs are expelled from the crystals. As we increase the crystallization temperature, T_c, the long period characterizing the periodically repeating crystal-amorphous polymer structure, r_cc, increases. This results in a smaller number of amorphous domains per unit volume-the number of NPs per amorphous domain thus increases. While the scattering contrast between the pure silica and the polymer is constant, these arguments predict that the apparent contrast between the NP-rich and the polymer-rich domains scale linearly with r_cc, as we confirm from our experiments. These facts allow us to posit that the NPs become more efficiently packed in the interlamellar zone with increasing T_c until they form a fully filled monolayer. Above this temperature, NP multilayers form within each of the NP-rich domains. Our analysis approach, therefore, describes NP ordering that is achieved when driven by polymer crystallization.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.