Deformation induced molecular behavior of Cis 1,4-polyisoprene and its nanocomposites monitored by solid-state NMR

Abstract

Proton spin lattice (T1) relaxation time constants were used to monitor changes in the molecular motion and architecture of polyisoprene, polyisoprene-clay composites and polyhedral oligomeric silsesquioxane (POSS) nanofillers. The high frequency relaxations monitored by NMR are sensitivity to changes in the environment of polyisoprene chains as a function of compressive strain. These are the first experiments to use magic angle spinning nuclear magnetic resonance techniques in situ with compression measurements to identify changes in the chain environment during compressive strain. For the polyisoprene composites, in situ compression measurements were made during the acquisition of NMR relaxation measurements. Therefore, the architecture of polyisoprene-clay composites was monitored as a function of strain. Clay aggregates, composed of stacked clay platelets, were identified in the nanocomposites. Increases in strain resulted in an irreversible, increase in interfacial area between clay and the polymer as the aggregate broke apart. This increase in area could be easily quantified by NMR (- 230%) and was verified with optical microscopy (- 150%). The correlation between NMR and optical microscopy indicates, with certainty, that NMR relaxation measurements can be used to quantify differences in interfacial area of nanocomposites. With this being established, the techniques developed in this thesis could be applied to analyze strain induced changes in interfacial area for samples which are not optically clear or samples in which the particle dimensions make microscopy techniques difficult. The quantitative discrepancy between the two techniques suggests that NMR captures changes in the bulk whereas microscopy is confined to surface effects. Finally, these results point to the potential to design a composite in which this mechanism is halted, such as crosslinking the clay directly to the polymer matrix, in hopes of creating a higher energy mechanism of deformation and thereby improving the mechanical properties. Octaethyl POSS was found to exhibit characteristics exemplified by plastic crystals. Phase transitions were identified with NMR and differential scanning calorimetry at -258 K and -253 K for partially deuterated and fully protonated ethyl POSS, respectively. For both derivatives at temperatures above the phase transition, the molecular motions of POSS were found to be on the order of nanoseconds (-30 ± 2 ns) and associated with molecular tumbling of the POSS molecule. After the phase transition, the molecular tumbling of POSS molecules slowed ( -530 ± 15 ns ) and became more asymmetric. These phase changes were characterized by a change in entropy of 20.2 ± 5 Jmol-'K', a typical value for plastic crystals. This plastic crystalline behavior suggests that in the high temperature phase, the POSS molecules will undergo plastic flow at relatively low levels of stress. Therefore, in the high temperature phase POSS will not enhance mechanical behavior in the same manner as other inorganic fillers, such as silica. In addition, the trends in the transition temperature suggest that the derivatives can be chemically altered to achieve the desired phase at a given operation temperature. Finally, a compression device suitable for fitting inside the rotor of a magic angle spinning NMR probe was built. Although in situ NMR compression experiments have previously been investigated, these experiments were the first to be done while magic angle spinning. This allowed for increased specificity for assigning molecular mobility. Additionally, the magic angle spectra doubled the signal to noise, as compared to static proton NMR spectra. Thus, more rapid spectral acquisition was possible, allowing "snapshots" to be acquired under dynamic processes such as mechanical deformation. This led to a more detailed analysis of deformation than that possible from static spectra. For instance, using magic angle spinning the behavior of bulk polymer was separated from that adjacent to the clay. For this reason, the effect of the changes on the architecture of nanocomposites could be monitored through those protons in closest proximity to the clay surface. This enhanced selectivity resulted in an unambiguous determination of change in composite architecture and its time dependence.