Molecular dynamics of substituted polycarbonates and model monomers by NMR

Young-Hwa Lee, Purdue University

Abstract

Molecular dynamics of the following cycloalkyl substituted polycarbonates and model monomers were studied by solution and solid state NMR: 1,1-cyclopentylidenebisphenol polycarbonate; 1,1-cyclohexylidenebisphenol, its dimethyl ether and polycarbonate; 1,1-(4-t-butylcyclohexylidene)bisphenol, its dimethyl ether and polycarbonate; 1,1-cycloheptylidenebisphenol polycarbonate; triphenyl 1,3,5-benzenetricarboxylate and phenyl 1,3,5-tribenzoate. Two distinct internal motions along the C$\sb2$ axis, small angle diffusion and 180$\sp\circ$ flipping, were observed in the pendant rings of the conjugated esters in solution. Cooperative motion of a few ($\sim$5) repeat units was determined from T$\sb1$($\sp{13}$C) studies of polycarbonates in solution. Differential internal mobilities of axial and equatorial phenylene rings were observed in polycarbonates. Structural variation at the molecular level leads to different CPMAS spectra between the crystalline and glassy monomers. However, the spectrum of glassy 1,1-(4-t-butylcyclohexylidene) bisphenol was found to be similar to its polycarbonate. This indicates that polycarbonates have closely related structures to their glassy monomers at the molecular level. Linewidth variation of $\sp{13}$C rotational spin echo NMR and $\rm T\sb1(\sp{13}C)$ relaxation studies in the solid state revealed that 180$\sp\circ$ phenylene ring flipping occurs only in the polymeric state not in the glassy monomer at room temperature. This may suggest that cooperativity along a chain is dominant over intermolecular interactions for ring flipping. Ring flipping of the equatorial phenylene group in polycarbonates was possible although the axial ring was "locked" in the repeat unit. The conformation of the neighboring aromatic ring did not affect the flipping motion significantly. These show that cooperative motion along the polymer chain is a more important influence for phenylene ring flipping than cooperativity between the adjacent aromatic rings. Substituted cycloalkyl groups strongly influence the mobility of phenylene rings in polycarbonates. Among the polycarbonates investigated in this work, the cyclohexyl group hinders internal phenylene motion the most and the cyclopentyl group, the least. Differential mobility of the phenylene rings was shown to depend on the flexibility of the cycloalkyl group itself. Re-examination of DMS for copolymeric polycarbonates studied by Yee's group showed that at most the cooperative motion of five repeat units may be responsible for the $\gamma$ relaxation in BPA-PC.

Degree

Ph.D.

Advisors

Grutzner, Purdue University.

Subject Area

Chemistry|Polymers

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