Effects of Hairy Nanoparticles on Polymer Crystallization Kinetics

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.

Isothermal Crystallization Kinetics of Poly(ethylene oxide)/Poly(ethylene glycol)-g-silica Nanocomposites

“…A convenient method to improve the dispersion of NPs in the polymer matrix is to graft polymer chains on the NPs surface [ 10 , 11 ]. The addition of polymer grafted nanoparticles (PGNPs) into polymer matrix can optimize their mechanical properties [ 12 , 13 ], optical ability [ 14 , 15 ], rheological properties [ 16 , 17 ], and electrical performance [ 18 , 19 ]. Recent studies have shown the spatial distribution of PGNPs can be controlled by varying the grafting density ( σ ), chain length of matrix vs. the grafted polymer ( P / N ), NPs size, and chemical properties of the grafted chains [ 20 , 21 ].…”

“…The aggregation of PEG-g-SiO 2 at higher P / N values and low σ occurs to limit the effectiveness of grafted chains on the nucleation ability of the nanocomposites. Jimenez et al [ 13 ] study the crystallization kinetics of PEO matrix with amorphous poly(methyl methacrylate) chains grafted SiO 2 (PMMA- g -NPs). It was found the crystal nucleation is unaffected by the addition of PMMA- g -NPs, while causing a decrease in spherulitic growth, crystallinity, and melting points.…”

Mechanisms of Directional Polymer Crystallization

“…Scale bars are 100 μm. (D) Isothermal crystal growth rate data from polarized light optical microscopy 20 and Lauritzen–Hoffman analysis.…”

“…The isothermal crystal growth rates of PEO were determined by tracking the rate of spherulitic growth using cross-polarized optical microscopy (CPOM). In particular, we used the published data of Jimenez et al 20 for a variety of PEO systems. To compare to ZA samples, these growth rates were fit to Lauritzen–Hoffman theory: 21 where G 0 is a preexponential constant; U * is an activation energy characteristic of the transport of polymer segments across the melt-crystal front (6280 J mol –1 ); R is the gas constant (8.314 J mol –1 K –1 ); T ∞ is T g – 30 (178.15 K); T g is the glass transition temperature; K g is the rate of surface nucleation; f is a factor defined as that corrects for the temperature dependence of the heat of fusion; Δ T = T m 0 – T c ; and T m 0 is the equilibrium melting temperature (352.15 K).…”

“…The integral under the heat flow curve during the first heating cycle provides the enthalpy of fusion, which when normalized by the enthalpy for 100% crystalline PEO (205.4 J/g) 25 yields φ c,w , the crystal weight fraction. 20 The crystal volume fraction, φ c,v , is then obtained from the relation 26 , where ρ c and ρ a are the crystalline 27 and amorphous 28 densities, respectively. It should be noted that the ZA samples were cooled to room temperature before heating in the DSC.…”