Microrheological study on polyethylene/thermotropic liquid crystalline polymer/layered silicates nanocomposites
by Youhong Tang
Ph.D. Chemical Engineering
xxix, 288 leaves : ill. ; 30 cm
The objective of the thesis is investigation microrheology and microstructure behaviors to understanding mechanisms of bulk viscosity reduction in layer silicate (organoclay) / thermotropic liquid crystalline copolyester (TLCP)/ high molecular weight polyethylene (PE) nanocomposites. 0...[ Read more ]
The objective of the thesis is investigation microrheology and microstructure behaviors to understanding mechanisms of bulk viscosity reduction in layer silicate (organoclay) / thermotropic liquid crystalline copolyester (TLCP)/ high molecular weight polyethylene (PE) nanocomposites.
Before illustrate mechanisms of ternary blends, the interactions between layered silicate and liquid crystalline polymer have been characterized by microrheological and microstructural methods first. Four typical kinds of binary composites have been studied with nominal 3.0 wt % organoclay in TLCP, i.e., TC3. (1) TC3 UP is a typical partial intercalated and partial exfoliated nanocomposite. Phase separation occurred during steady shear experiments and observed by POM at 190 0C. Separation of TC3 UP was performed and two hybrids were obtained: TC3 white and TC3 dark. (2) In TC3 white, organoclays formed exfoliated morphologies and well dispersed in TLCP matrix with uniform sizes 15-25 nm. The presence of organoclay did not affect liquid crystallinity of matrix but enhanced the rigidity of TLCP molecules. The molecular level interactions between organoclay and TLCP molecules held molecular orientation in the flow direction during relaxation after steady shear. The bulk viscosity mechanism of this binary filler in PE has been analyzed in detailed. (3) Organoclays in TC3 dark hybrid totally destroyed matrix liquid crystallinity and deteriorated thermal properties. Due to intercalated morphologies, TC3 dark has strong solid-like behaviors with high viscosities, long relaxation time but short linear viscoelastic region. (4) TC3 FS is the nanocomposite having comparable lengths for organoclays and TLCP molecules. Exfoliated structures formed in the nanocomposite with slightly modified matrix liquid crystallinity. It has the similar rheological behavior with TLCP.
PE/TLCP/organoclay blends, i.e., PTC, have been prepared to analyze the concentrations and preparation sequences effects. (1) Increasing TLCP concentration in PE as well as in organoclay/PE matrix can effectively reduce the bulk viscosities, but too much would cause shear-induced agglomeration of TLCP in matrix; (2) The same ratio of TLCP and as-received organoclay (1/1) in PE matrix can cause bulk viscosities reduction but no processing window increasing at the same time. (3) With suitable preparation sequences and optimal TLCP/organoclay ratio, the blend (3 wt% TLCP/PE)/1 wt% clay ((PT3) C1) has even lower viscosities than the corresponding 3wt% TLCP/PE blend (PT3) with the dramatic improved processing window. A binary flow pattern model, originally developed for viscosity reduction in HMMPE/ TLCP blend, successfully predicted both the dramatic viscosity reduction effects and the critical yielding stress for the above blends.
Organoclay enhanced TLCP in PE matrix have been characterized mainly by rheological method with adding 1 wt% of above four kinds TC3 in PE. Relative lower yield stresses and shear rates as well as higher viscosity reduction abilities than 1 wt% TLCP/PE (PT1) blend have been observed. P (TC3 dark 1.0 wt %) and P (TC3 UP 1.0 wt %) rheological behaviors can not be predicted by the model, due to the dramatic impact of organoclay to TLCP in PE at high shear rates regions. P (TC3 FS 1.0 wt %) and P (TC3 white 1.0 wt %) showed highest viscosity reduction ability in the four blends with P (TC3 FS 1.0 wt %) having the same processing window and P (TC3 white 1.0 wt %) holding a relatively broad processing window than PT1. The model has been successfully proposed for these two blends. Shear-induced phase transition with the combination of synergic effect of organoclay with TLCP is the mechanism for bulk viscosity reduction as well as low yielding stress and shear rate in P(TC3 white 1 wt%).
Fibrillation with different fibre lengths, widths and shapes controlled the final rheological properties of PE blends, which were controlled by different organoclay morphologies and interaction types with TLCP. With tailorable organoclay, controllable viscosity modification can be achieved in PE/TLCP/organoclay nanocomposites.