Linear

Article pubs.acs.org/Macromolecules

Anomalous Rheological Behavior of Dendritic Nanoparticle/Linear Polymer Nanocomposites Hadi Goldansaz,‡ Fatemeh Goharpey,§ Faramarz Afshar-Taromi,§ Il Kim,∥ Florian J. Stadler,*,†,⊥,#,% Evelyne van Ruymbeke,*,‡ and Vahid Karimkhani*,§,∥ †

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China Institut de la Matière Condensée et des Nanosciences (IMCN), Bio and Soft Matter Division (BSMA), Université catholique de Louvain, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium § Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran ∥ BK21 PLUS Centre for Advanced Chemical Technology, Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea ⊥ Nanshan District Key Lab for Biopolymers and Safety Evaluation, Shenzhen 518060, P. R. China # Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, P. R. China % Shenzhen Engineering Laboratory for Advanced Technology of Ceramics, Shenzhen 518060, P. R. China ‡

S Supporting Information *

ABSTRACT: We investigated the effects of soft dendritic polyethylene (dPE) nanoparticles on the rheological properties of a linear polystyrene (PS) matrix. The viscosity of PS−dPE nanocomposites is found to exhibit nonmonotonic dependence on the dPE concentration. In particular, with the addition of 1% dPE nanoparticles, we already observe more than 1 order of magnitude reduction in viscosity. The minimum viscosity was observed at 5% nanoparticles. At dPE concentrations higher than 5%, nanocomposite viscosity increases by addition of nanoparticles, yet it remains below the viscosity of PS. Addition of nanoparticles not only influences the terminal relaxation times of the nanocomposites but also affects their whole relaxation spectra. The viscosity of PS−dPE nanocomposites at high temperature is found to reversibly evolve with time, which proves the existence of supramolecular interactions between the PS matrix and the nanoparticles. Atomic force microscopy confirms that dPE nanoparticles are well distributed in the PS matrix, though each component of the nanocomposite exhibits its own glass transition. While the origin of viscosity reduction remains unknown, it cannot be attributed to confinement, free volume effect, change of entanglement density, surface slippage, shear banding, or particle induced shear thinning.

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INTRODUCTION

viscosity should be observed, similarly to the usual colloidal systems. It has been shown by simulation that in the weakly interacting polymer−NP system relaxation time and glass transition temperature (Tg) of the matrix chains decrease in the close vicinity of the NP surfaces.6 Faster dynamics of polymeric chains were also observed in thin films that are controlled by interfacial adhesion energy and relative softness of confined and confining materials.7 It is well accepted now that for glassy polymers the chains in a surface layer can have higher mobility than bulk chains.8−11 Moreover, the ratio of NP size to polymer entanglement mesh size is another important criterion, which should be considered in interpreting dynamic behavior of nanocompo-

The presence of nanosized particles in polymeric systems significantly changes their dynamic behavior at different length and time scales in comparison with bulk state. In some of polymeric nanocomposites, surprising breaking down of classic theory of mixtures viscosity has been observed.1 One of the greatest breakthroughs was Mackay’s seminal work on rheology of athermal nanoparticle/linear polymer nanocomposites.2,3 They showed that for the linear entangled matrices the presence of nanoparticles (NPs) in a matrix with interparticle distances smaller than the equilibrium size of the linear chains will cause a dramatic reduction in system viscosity, while for the unentangled system, Einstein’s classic theory about colloidal suspension is applicable.2−4 Based on their results and also theoretical modeling (albeit only indirectly stated),5 it was concluded that confinement in entangled system is necessary for observing viscosity reduction; otherwise, an increase in © XXXX American Chemical Society

Received: February 23, 2015 Revised: April 25, 2015

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DOI: 10.1021/acs.macromol.5b00390 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

al.24 Linear viscoelastic properties of dPE nanoparticles were measured with a Kinexus Pro (Malvern Instruments, UK) using parallel plate geometries. AFM analyses on PS−dPE nanocomposites were performed in ambient condition in the HarmoniX mode on a Multimode Nanoscope V microscope (Veeco Instruments). The HarmoniX mode allows simultaneous mapping of topography, phase shift, adhesion force, energy dissipation at the tip−surface contact, and elastic modulus.25 In order to obtain quantitative information, the probe, the photodetector sensitivity, and the tip−surface transfer function are calibrated through a thin film of polystyrene (G ≈ 2 GPa) with dispersed low-density polyethylene nodules (G ≈ 50 MPa). For dPE nanoparticles, Multi-Mode IIIa AFM (Bruker/Veeco/ Digital Instruments, Santa Barbara, CA) was used in tapping mode, employing silicon cantilever-tip assemblies (Veeco type MLCT, nominal resonance frequency of 15 Hz and spring constant of 0.03 N/m). Dynamic mechanical analysis (DMA) was performed using a Mettler Toledo DMA/SDTA861 in shear mode at 10 Hz with a shear stress amplitude σ = 1 N and a heating rate q = 3 °C/min in a temperature interval from −110 to 170 °C. DSC analysis performed using a Mettler Toledo DSC821 e calorimeter calibrated against indium. The samples weights were 7− 15 mg. Heating ramps of 10 °C/min were used to determine glass transition temperatures. Broadband dielectric relaxation spectroscopy (DRS) were performed using an Alpha analyzer (Novocontrol, Germany) over the frequency range of 1 MHz−0.1 Hz, in combination with a MCR 301 rheometer (Anton Paar, Germany) as the sample holder. Materials were sandwiched between 20 and 15 mm rheometer parallel plates with ceramic isolations and then heated to 170 °C and compression molded in order to stablish good contact and reach a gap 30 min at 170 °C, DRS frequency sweep measurements were performed at 150, 125, and 115 °C. Temperature was controlled using a convection oven operating under nitrogen. In order to extract mean relaxation time, τHN, and the shape factors describing broadness, a, and asymmetry, b, of the relaxation peaks, complex dielectric permittivity ε*(ω) was fitted by an empirical Havriliak−Negami equation

sites.12−14 Based on thermorheological simplicity, it is expected that in bulk polymers the temperature dependencies of segmental motion and whole chain motion are the same. Nonetheless, thermorheological complexity can happen when nanoscale effects are predominant.8 Mackay et al. showed that with the correct processing strategy, if the radius of the nanoparticle is smaller than the radius of gyration (Rg) of the linear polymer, it is possible to fabricate thermodynamically stable dispersion of NPs in a polymeric matrix.15 They proved that while linear polyethylene−linear polystyrene is a classic example of phaseseparating blend, it is possible to have uniform dispersion of dendritic PE nanoparticles (dPE) in PS matrix with RgPS > RgdPE. Surprisingly, they showed that the presence of dPE ( RgdPE). At NP concentrations ≥8%, DMA and DSC revealed existence of two separate glass transition processes, for PS and dPE phases, respectively (see Figure SI7). At lower concentrations the glass transition of the dPE phase is not found, due to the fact that at such low concentrations, the signal intensity from dPE phase is smaller than the resolution of both techniques. Coexistence of the two glass transitions in the well-dispersed PS-dPE nanocomposites can be rationalized by considering the small length scale of cooperative rearranging region (CRR) (200 s prior to measurement for all the data points reported. Unlike pure PS, the PS−5% dPE nanocomposite at 170 °C demonstrated significant viscosity buildup with time, before eventually reaching the same equilibrium state after