Study of Ethylene−Propylene Copolymer Thermoreversible Gels

Abstract

Molecular structure and rheological properties of ethylene−propylene (EP) random copolymer gels formed in toluene and methylcyclohexane, respectively, have been investigated by small-angle neutron scattering (SANS) and oscillatory shear. In both solvents, SANS results are analyzed by considering a fiberlike structure with a radius cross-section distribution function expressed by w(r) ∼ r -λ. It is shown that a small change in the EP copolymer ethylene content has a large effect on the density of network connecting fibers but a much smaller effect on the diameter of the fibers. Oscillatory shear experiments reveal that the relaxation rate of the elastic modulus increases with decreasing EP copolymer ethylene content. This behavior indicates a decrease in the density of connections between fibers with decreasing ethylene content. The relationship between the elastic modulus and polymer concentration has also been examined. The results suggest, in light of the theory developed by Jones and Marques for rigid networks, that the rigidity of fibers formed in the gel is achieved only locally.

Viscoelastic Relaxation of Cross-Linked, Alternating Copolymers in the Free-Draining Limit

“…In this way the network is built from -AAAAand -BBBB-segments such that an A-segment is connected to a B segment and viceversa. Furthermore, there also exist multifunctional centers (e.g., crystallites 31 ) to which more than two segments are attached; these centers are fundamental in the formation of the network. For simplicity, we assume first that all -AAAAsegments are similar (i.e., have roughly the same length) and that the same holds true for the -BBBBsegments.…”

“…In general, one has two types of networks. First, there are physical (transient) networks of gel-forming copolymer solutions. − The second type of networks are those attained through chemical reactions, leading to the formation of permanent cross-links, − such as the copolymerization of styrene with ethylene glycol dimethacrylate …”

Solvents effects in the formation and viscoelasticity of DBS organogels

“…In another approach, the Jones-Marques model 23 related the elastic modulus to the gelator concentration through a power-law, whose exponent depends upon the fractal dimension D f of the objects connecting at the junctions. For example, Daniel et al 24 studied ethylene-propylene copolymer gels and found the scaling law G 0 $ c 2.85AE0. 20 .…”

“…The deviation of fractal dimension from the ideal value of straight brils suggested that the brils formed in the ethylene-propylene copolymer gel are not straight and the rectilinearity of the brils is only achieved locally. Despite these attempts (the percolation model 12,25,26 and the brillar model 24,27 ) to establish structureproperty relationships, however, the morphological development of the gel network with the solvent polarity and gelator concentration is still not clear. Moreover, little is known about the gelator-solvent interaction on the nonlinear rheological behavior.…”

Artifact Correction in Temperature-Dependent Attenuated Total Reflection Infrared (ATR-IR) Spectra

“…The development of the mechanical properties of the gel is similarly very dependent on the rate and extent of crystallization during the thermal cycling process. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Vibrational spectroscopy is an excellent tool for following the crystallization process in polymers. Both the infrared (IR) and Raman spectra are sensitive to the conformational states.…”