Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): A deuteron magnetic resonance study

Abstract

In isotactic poly͑methyl methacrylate͒ ͑PMMA͒, we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance ͑DMR͒ in a deuterated sample. We find that the motion of the CD 3 -group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T ϭ291 K and Tϭ70 K reveal a broad temperature dependent probability distribution of autocorrelation times c for the 2 /3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies (E a ) with variance E a ϭ13.8Ϯ0.5 meV ͑1.33 kJ/mol͒. The line shape transition between Tϭ70 K and Tϭ23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width E a /E a of the E a distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between Tϭ70 K and Tϭ23 K.

Temperature and configurational effects on the Young’s modulus of poly (methyl methacrylate): a molecular dynamics study comparing the DREIDING, AMBER and OPLS force fields

“…However, increasing the chain regularity and isotacticity of polypropylene has been shown to be accompanied by proportionally greater increase in the stiffness [36]. Moreover, the barrier energy of the ester methyl groups for rotation in i-PMMA is greater than in s-PMMA [37] [38], which implies higher local order and stronger non-bonded interactions with neighbouring groups. The current simulations show that the dihedral angle energy for i-PMMA is greater than for s-PMMA e.g.…”

Composition Dependence of Segmental Dynamics of Poly(methyl methacrylate) in Miscible Blends with Poly(ethylene oxide)

“…We first note that blending is unlikely to influence local motions such as rotation and thus consider methyl group rotation in pure PMMA. Rotation of the ester methyl has been extensively studied using inelastic and quasi-elastic neutron scattering. − At 400 K the rotational time is 2.6 pswell before the minimum time of HFBS [242 ps] . We conclude that rotation of the ester methyl has completely decayed prior to HFBS time window at the temperatures of our measurements.…”

Investigation of the Dynamic Rheological Properties of a Polycarbonate Melt Presenting Solid-Like Characteristics and a Departure from Pure Liquid Newtonian Behavior at Long Relaxation Times

“…Vo represents the height of the barrier, and is approximately 2.7 kcal/mole for the C-C bonds, 8.7 kcal/mole for the CO-O bonds of PMMA. [20,28] Other, more sophisticated, internal rotation potential energy profiles have also been described. [22] The potential energy of a bonded conformer accounts for the inter-molecular interactions, Van der Waals and electrostatic in nature, including hydrogen bonding type.…”