Evolution of Clay Morphology in Polypropylene/Montmorillonite Nanocomposites upon Equibiaxial Stretching: A Solid-State NMR and TEM Approach

Abstract

Solid-state NMR and TEM were used to quantitatively examine the evolution of clay morphology upon equibiaxial stretching of polypropylene/montmorillonite (PP−MMT) nanocomposites up to a stretch ratio (λ = final length/initial length) of 3.5. 1H spin−lattice relaxation times were measured by the saturation−recovery sequence. For the nanocomposites, initial portions of the magnetization recovery curves (≤∼20 ms) were found to depend on √t, indicative of diffusion-limited relaxation and in agreement with calculations based on estimates of the spin-diffusion barrier radius surrounding the paramagnetic centers in the clay, the electron−nucleus coupling constant, and the spin-diffusion coefficient. Initial slopes of these magnetization recovery curves directly correlated with the fraction of clay/polymer interface. New clay surface was exposed as a near linear function of strain. Long-time portions of the magnetization recovery curves yielded information on the average interparticle separations, which decreased slowly before reaching a plateau at λ = ∼2.5 as particles aligned. TEM images supported these findings and were used to define and quantify degrees of exfoliation and homogeneity from the NMR data. Exfoliation, defined as (platelets/stack)−1, increased from 0.38 (unstretched) to 0.80 at λ = 3.5 for PP−MMT nanocomposites stretched at 150 °C and 16 s−1. A lower stretch temperature, 145 °C, which is slightly below melting onset, led to an exfoliation degree of 0.87 at λ = 2.8, consistent with the ability of higher melt viscosities to allow for higher shear stress transfer. Exposure of new clay surface is attributed to aggregate breakup and orientation at low strains (λ ≤ ∼2) and to platelets sliding apart at higher strains.