Interessi principali - Main interests

Theory and simulation of polymeric materials

Conformational statistics of polymers

Stereospecificity of Ziegler-Natta catalysts

Theory and simulation of liquid crystals

See also: Laboratory of Stereoselective Polymerizations (LSP)

Riassunti di pubblicazioni recenti - Abstracts of recent papers

G.Allegra, G.Raos, M.Vacatello, Theories and simulations of polymer-based nanocomposites: from chain statistics to reinforcement, Prog. Polym. Sci, 33,683(2008)[PDF]

A survey of the present understanding of particle-filled polymers is presented, as obtained from either theoretical or computational approaches. We concentrate on composites in which the nanoparticles are either spherical or statistically isotropic aggregates, and the matrix is a homopolymer melt or a cross-linked elastomer. Recent progress has been prompted by the preparation and careful characterization of well-defined model systems, as well as by theoretical developments and the application of computer simulation to increasingly realistic models. After an introduction providing the main motivations (Section 1), an overview of the basic phenomenology and recent experimental results is presented (Section 2), with special emphasis on the Payne effect and related aspects. In Section 3, we discuss results of equilibrium molecular dynamics and Monte Carlo simulations of polymer chains in the presence of nanoparticles. After a concise theoretical description, these are compared with those obtained from integral equation and density functional approaches(Section 4). The molecular origins of the inter-particle-depletion interaction are discussed, as well as the phase-separation diagram of the nanoparticle/polymer system. The related issue of polymer chains and networks compressed between planar surfaces is also dealt with. In Section 5 simulations and theories of polymer dynamics at the interface are discussed, with special emphasis on the effects of surface roughness and on the vicinity of the glass transition. In Section 6 the overall viscoelastic response of polymer nanocomposites is considered, both from the point of view of molecular-level simulations and of continuum mechanics approaches. The concluding remarks (Section 7) discuss some of the open challenges in the field.

M.Vacatello, Phantom chain simulations of realistically sized polymer-based nanocomposites, Macromol. Theory Simul., 15,303(2006) [PDF]

Phantom chain MC simulations have been performed for realistically sized systems of polymer chains filled with solid nanoparticles. The results of the simulations and simple theoretical considerations are used to rationalize a number of parameters relevant to the characterization of these systems. Even when the average number of nanoparticles in contact with a chain is very small (much less than unity), the nanoparticles are nodes of highly interconnected transient networks bridged by the polymer chains.

M.Vacatello, Monte Carlo simulations of polymers in nanoslits, Macromol. Theory Simul., 13,30(2004) [PDF]

Monte Carlo computer simulations have been performed for model polymers confined in slits of thickness comparable to the transverse diameter of the chains. The density of polymer within the slits is allowed to vary with the slit thickness in such a way that the content of the slits is in equilibrium with a large reservoir of bulk polymer. The calculations reveal the presence of polymer-mediated attractive or repulsive interactions between the slit plates, oscillating with the slit thickness in good agreement with experimental results..

M.Vacatello, Chain dimensions in filled polymers: an intriguing problem, Macromolecules, 35,8191(2002) [PDF]

Simulations of realistically dense polymer melts filled with randomly distributed solid nanoparticles and calculations performed with various methods for single chains in the presence of filler do not show the large increase of chain dimensions with respect to the unfilled polymer predicted by previous RIS calculations when the chains are larger than the particles. It is concluded that the increased dimensions found in recent SANS experiments for poly(dimethyl siloxane) filled with relatively small polysilicate particles cannot be explained on the basis of simple excluded volume arguments. It is also shown that simulation studies of polymers in the presence of solid obstacles can only be performed by simulating systems with realistic densities.

M.Vacatello, Order-disorder transitions in model liquids of mesogenic trimers, Macromol. Theory Simul., 11,501(2002) [PDF]

Monte Carlo simulations have been performed for realistically dense liquids of model trimers consisting of rigid cores connected by virtual bonds representing semiflexible spacers. The conformational characteristics of the trimers are approximately regulated in two cases to mimic (CH2)n spacers with n odd or even. All simulated systems undergo reversible isotropic/nematic phase transitions at well defined temperatures, showing odd-even oscillations in good agreement with experiments. The transitions are coupled with a conformational selection favoring extended conformations in the nematic liquids. The odd-even oscillations and the conformational distribution in the nematic liquids are fully explained on the basis of the intrinsic conformational characteristics of the model trimers.