resumo
Experimental results by Fisher et al. [Faraday Discuss. Chem. Soc. 1979, 68, 26] showed the existence of local orientational correlations of segments of the n-alkane molecules in the liquid state. Since the estimated correlation volume is below 10(3) angstrom(3), molecular dynamics simulations appear to be a suitable method for their identification and characterization. We used molecular dynamics to fully characterize short-range order in polyethylene melts. The characterization started by identifying sequences of aligned segments in chains, each one having at least one Kuhn monomer in length. Afterward, a search was made for interactions of a tagged aligned chain segment with others fulfilling the same condition, laying within a limiting separation distance (18 angstrom) and making a limiting orientation angle (40 degrees). When, at least, four interactions are counted, a short-range ordered region is defined. Chain placement in these regions has similarities to that of the unit cell at the solid phase, although with different separation distances and angle. Overall, short-range order resembles a dynamic uniaxial nematic phase, the local order parameter increasing with the chain length in agreement with experimental results. Segments at the ordered regions persist over time, for times longer than the Rouse relaxation time of the chain. Those in random conformational sequences between the ordered regions have a mass distribution in agreement with the Flory distribution, with a number-average value comparable to experimental results for the molecular mass between entanglements.
palavras-chave
MOLECULAR-DYNAMICS SIMULATIONS; CYCLING NMR RELAXOMETRY; MONTE-CARLO-SIMULATION; POLYMER MELTS; RHEOLOGICAL RESPONSE; AMORPHOUS POLYMERS; GLASS-TRANSITION; SIMPLE LIQUID; MOLTEN STATE; CHAINS
categoria
Polymer Science
autores
Martins, JA; Micaelo, NM
nossos autores
José António da Purificação Martins
Professor CatedráticoGrupos
agradecimentos
We acknowledge Vitor Felix for making available computer facilities at the University of Aveiro that were used at the start of this work. We thank discussions with Ralph Colby and Loic Hilliou. This work was in part financially supported by FCT through PEst-C/QUI/UI0686/2011, FCOMP-01-0124-FEDER-022716 and PEst-C/CTM/LA0011/2011 programs. The authors thank the access to the Minho University GRIUM cluster and for contract research grant C2008-UMINHO-CQ-03.