Peroxide Cross-linked Natural Rubber and cis-Polybutadiene

Abstract

Changes in the structure of natural rubber and cispolybutadiene have been observed using solid state carbon-13 NMR. Cis-trans isomerization has been shown to occur in the natural rubber by rearrangement of the allylic free radical. At least four different methyl groups have been detected in the cross-linking of natural rubber by dicumyl peroxide, which indicates that the simple combination of allylic free radical is an oversimplification of the curing process. Quater nary aliphatic carbons have been detected which suggest double bond migration. Polybutadiene showed only methine and methylene carbons in the cross-linked net work with a small amount of methyl end groups. The increase in the line width of the highly cross-linked elastomers has been shown to be due in part to the static dipolar interaction between carbons and protons and chemical shift dispersions. The formation of trans double bonds has been observed in the infrared spectra of cis-polybutadiene. Weak broad bands around 1320 cm -1 observed in the difference spectrum of cured rubbers may be due to carbon-carbon cross-links. From the structural interpretation of spectra obtained from solid state carbon-13 NMR and Fourier transform infra red spectroscopy, models are proposed for the crosslinked networks of natural rubber and cis-1, 4-polybutadiene formed by peroxide vulcanization.The range of end use applications of elastomeric materials is ex tended by their ability to be cross-linked. It is well known that the chemical microstructure of rubber influences its physical properties and the reaction mechanism of the curing process. The use of dicumyl peroxide as a curative for natural rubber and cis-polybutadiene produces a network that contains 0097-6156/84/0243-0205S07.75/0

Entropically Influenced Reconstruction at the PBD-ox/Water Interface: The Role of Physical Cross-Linking and Rubber Elasticity

“…Macromolecules, Vol. 33 stirred overnight. The desired amount of dicumyl peroxide (DCP) 33 was then added, and the solution was stirred for 10-15 min before being poured into a crystallizing dish.…”

“…In a 250 mL beaker covered with aluminum foil, ∼200 mL of chloroform and 0.7 g of PBD were stirred overnight. The desired amount of dicumyl peroxide (DCP) was then added, and the solution was stirred for 10−15 min before being poured into a crystallizing dish. The chloroform was removed using a vacuum desiccator and a water aspirator.…”

“…An ATR-IR spectrum of a cross-linked film pressed under the same conditions contained the usual peaks due to PBD, as well as three small peaks at 1373, 1261, and 801 cm -1 , consistent with the presence of cumyl alcohol, a byproduct of the cross-linking reaction. 33 For comparison, an XPS spectrum (20°takeoff angle) of a sample from the same cross-linked batch after 2 months showed that the amount of oxygen had increased to 5%, consistent with autoxidation of the polymer in air or with contamination.…”