Tuning the mechanical properties in model nanocomposites: Influence of the polymer-filler interfacial interactions (original) (raw)
Related papers
Polymer, 2012
We report on the influence of parameters controlling filler dispersion and mechanical reinforcement in model nanocomposites. We elaborate a series of nanocomposites and present a structural characterization of silica dispersion in polymer matrix for several particle sizes and polymer matrices, at all relevant scales, by coupling Small Angle X-ray Scattering and Transmission Electronic Microscopy. The mechanical properties are investigated in the linear regime by coupling Dynamical Mechanical Analysis and plate/plate rheology. The results show that: (i) for all filler sizes and matrices, a structural transition is observed from non-connected fractal aggregates at low silica concentration to connected network at high particle content. (ii) In the dilute regime, the reinforcement implies a polymer chain contribution with different possible origins: increase of entanglements density for PS and increase of friction coefficient for PMMA. (iii) In the concentrated regime, for a given polymer, the reinforcement amplitude can be tuned by the rigidity of the filler network, which directly depends on the particleeparticle interaction.
Filler aggregation as a reinforcement mechanism in polymer nanocomposites
Mechanics of Materials, 2013
Significant reinforcing effects that are often observed in polymer nanoparticulat e composites are usually attributed to strong interfacial interactions over extended inter faces in these systems. Her e, we study linear low density polyethylene (LLDPE) reinforced with 1-4% fumed silica nanoparticles. Nanocomposi te modulus, evaluated as a function of filler volume fraction, significantly exceeds classical micromechanics predictions. Possible reasons for the observed discrepancy are evaluated experimen tally and theoretically. It is concluded that primary nanop article aggregation rather than polymer-nanoparticle interaction at the interface is mainly responsible for the observed reinforcement effect. A simple micromechan ics-informed model of a composite with primary particle aggregates is presented based on the model of secondary aggregation developed earlier. The model is shown capable of predicting nanocomposites behavior by introducing a single new structural parameter with a straightforward physical interpretation. As nanoparticl es are prone to agglomerate, their primary or secondary aggregates may be present in many nanocomposite systems and the aggregation state and its effects need to be thoroughly evaluated, along with the classical interfacial interactions. The described reinforcing mechanism may be responsible for other anomalous property changes in nanoparticulate comp osites reported in the literature.
Macromolecules, 2009
We are presenting a new method of processing polystyrene-silica nanocomposites, which results in a very well-defined dispersion of small primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the matrix. The process is based on the use of a high boiling point solvent, in which the nanoparticles are well dispersed, and a controlled evaporation procedure. The filler's fine network structure is determined over a wide range of sizes, using a combination of Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy (TEM) experiments. The mechanical response of the nanocomposite material has been investigated both for small (ARES oscillatory shear and Dynamical Mechanical Analysis) and large deformations (uniaxial traction), as a function of the concentration of the particles in the matrix. Our findings show that with a simple tuning parameter, the silica filler volume fraction, we can investigate in the same way the structure-property correlations related to the two main reinforcement effects: the filler network contribution, and a filler-polymer matrix effect. Above a silica volume fraction threshold, we were able to highlight a divergence of the reinforcement factor, which is clearly correlated to the formation of a connected network built up from the finite-size primary aggregates, and is thus a direct illustration of the filler network contribution. For a silica volume fraction lower than this percolation threshold, we obtain a new additional elastic contribution of the material, of longer terminal time than the matrix. This cannot be attributed to the filler network effect, as the filler is well dispersed, each element separated from the next by a mean distance of 60 nm. This new result, which implies the filler-matrix contribution of the reinforcement, must include interfacial contributions. Nevertheless, it cannot be described solely with the concept of glassy layer, i.e. only as a dynamic effect, because its typical length scale extension should be much shorter, of the order of 2nm. This implies a need to reconsider the polymer-filler interaction potential, and to take into account a possible additional polymer conformational contribution due to the existence of indirect long range bridging of the filler by the chains.
Physical Review E, 2010
In this paper we present a direct measurement of stretched chain conformation in polymer nanocomposites in a large range of deformation using a specific contrast-matched small angle neutron scatttering ͑SANS͒ method. Whatever are the filler structure and the chain length the results show a clear identity of chain deformation in pure and reinforced polymer and offer more insight on the polymer chain contribution in the mechanical reinforcement. It suggests that glassy layer or glassy paths, recently proposed, should involve only a small fraction of chains. As a result, the remaining filler contribution appears strikingly constant with deformation as explained by continuous locking-unlocking rearrangement process of the particles.
Filler–filler interactions and viscoelastic behavior of polymer nanocomposites
2004
This work presents the main results obtained within a project on mechanical properties of polymer based nanocomposites. The specific point was how to analyze and model the filler-filler interactions in the description of the viscoelastic behavior of these materials. This paper aims at presenting the general strategy used by the different partners to address this question, together with original experimental results and micro-mechanical modeling. Different nanocomposite materials were fabricated using the latex route, leading to random dispersions of rigid submicronic particles (PS = polystyrene, silica) in a flexible polybutylacrylate matrix at various volume fractions. In addition, encapsulated silica particles in a styrene-acrylate copolymer were produced, leading, after film formation, to a limited number of contacts between silica fillers. The processing route of these encapsulated particles was optimized and the resulting morphology was analyzed by TEM experiments. In the case of random mixtures, a strong effect of reinforcement appears in the rubbery field of the soft phase when the filler content is above a critical fraction (percolation threshold). The reinforcement in the rubbery plateau can be still exacerbated in the case of the PS particles if the material undergoes a heat treatment above the main relaxation of the PS phase. These experimental results illustrate the difference between geometrical percolation (when particles are just in contact) and mechanical percolation (with strong interactions between the fillers). The comparison of the results for PS and silica fillers shows once more that the strength of the interactions plays an important role. To account for the whole set of experimental data, two ways of modeling were explored: (i) homogenization methods based on generalized self-consistent schemes and (ii) a discrete model of spheres assembly which explicitly describes the ability of the contacts to transmit efforts.
Macromolecules, 2011
We investigate the dispersion mechanisms of nanocomposites made of well-defined polymer (polystyrene, PS) grafted-nanoparticles (silica) mixed with free chains of the same polymer using a combination of scattering (SAXS/USAXS) and imaging (TEM) techniques. We show that the relevant parameter of the dispersion, the grafted/free chains mass ratio R tuned with specific synthesis process, enables to manage the arrangement of the grafted nanoparticles inside the matrix either as large and compact aggregates (R < 0.24) or as individual nanoparticles dispersion (R>0.24). From the analysis of the interparticles structure factor, we can extract the thickness of the spherical corona of grafted brushes and correlate it with the dispersion: aggregation of the particles is associated with a significant collapse of the grafted chains, in agreement with the theoretical models describing the free energy as a combination of a mixing entropy term between the free and the grafted chains and an elastic term of deformation of the grafted brushes. At fixed grafting density, the individual dispersion of particles below the theoretical limit of R =1 can be observed, due to interdiffusion between the grafted and the free chains but also to processing kinetics effects, surface curvature and chains poly dispersity. Mechanical analysis of nanocomposites show the appearance of a longer relaxation time at low frequencies, more pronounced in the aggregated case even without direct connectivity between the aggregates. Correlation between the local structure and the rheological behavior suggests that the macroscopic elastic modulus of the nanocomposite could be described mainly by a short-range contribution, at the scale of the interactions between grafted particles, without significant effect of larger scale organizations.
Polymer, 2005
A study of the reinforcement effect of a soft polymer matrix by hard nanometric filler particles is presented. In the main part of this article, the structure of the silica filler in the matrix is studied by Small Angle Neutron Scattering (SANS), and stress-strain isotherms are measured to characterize the rheological properties of the composites. Our analysis allows us to quantify the degree of aggregation of the silica in the matrix, which is studied as a function of pH (4-10), silica volume fraction (3-15%) and silica bead size (average radius 78 Å and 96 Å). Rheological properties of the samples are represented in terms of the strain-dependent reinforcement factor, which highlights the contribution of the filler. Combining the structural information with a quantitative analysis of the reinforcement factor, the aggregate size and compacity (10%-40%) as a function of volume fraction and pH can be deduced. In a second, more explorative study, the grafting of polymer chains on nanosilica beads for future reinforcement applications is followed by SANS. The structure of the silica and the polymer are measured separately by contrast variation, using deuterated material. The aggregation of the silica beads in solution is found to decrease during polymerization, reaching a rather low final aggregation number (less than ten).
2014
A series of polystyrene (PS) nanoparticles were<br> prepared by grafting polystyrene from both aggregated silica and<br> colloidally dispersed silica nanoparticles using atom-transfer radical<br> polymerisation (ATRP). Cross-linking and macroscopic gelation<br> were minimised by using a miniemulsion system. The thermal and<br> mechanical behaviour of the nanocomposites have been examined by<br> differential scanning calorimetry (DSC) and dynamic mechanical<br> thermal analysis (DMTA).
Correlations between mechanical, structural, and dynamical properties of polymer nanocomposites
Physical Review E, 2012
We study the structural and dynamical mechanisms of reinforcement of a polymer nanocomposite (PNC) via coarse-grained molecular dynamics simulations. In a regime of strong polymer-filler interactions, the stress at failure of the PNC is clearly correlated to structural quantities, such as the filler loading, the surface area of the polymer-filler interface, and the network structure. Additionally, we find that small fillers, of the size of the polymer monomers, are the most effective at reinforcing the matrix by surrounding the polymer chains and maximizing the number of strong polymer-filler interactions. Such a structural configuration is correlated to a dynamical feature, namely, the minimization of the relative mobility of the fillers with respect to the polymer matrix.