Polyolefin-blend/Inorganic Nanocomposites: Morphology, Rheological and Thermomechanical Properties

Open Access
Author:
Salcedo Galan, Felipe
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 13, 2012
Committee Members:
  • Evangelos Manias, Dissertation Advisor
  • Ralph H Colby, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Gregory Ray Ziegler, Committee Member
Keywords:
  • Polymer nanocomposites
  • rheology
  • polyolefin blends
  • montmorillonite
  • heat-sealants
  • thermomechanical properties
  • polyethylene
Abstract:
A systematic series of polymer nanocomposite systems has been investigated, having polymer matrices of ethyl vinyl acetate (EVA), polyethylenes (PE), and EVA/PE blends. Nanometer-size organically modified fillers of montmorillonite (oMMT), calcium carbonate (CC), or both oMMT and CC were used as fillers, and all composites were produced via conventional melt-blending techniques, namely extrusion, injection molding, compression molding, or extrusion blow-molding for films. The focus of this research was on the role of the different kind of polymer-filler and filler-filler interactions in the composites’ morphology, rheological, and thermomechanical properties. In particular, synergies between fillers and new properties were designed and investigated, as they could relate to macroscopic performances of heat sealants for flexible film packaging applications. Among the new scientific findings from this work, some characteristic examples include: (a) The interactions beween LLDPE-grafted-maleic anhydride (LLDPE-g-MAH) and oMMT can be designed to promote the formation of thermodynamically favored polymer-mediated filler-assemblies that, in turn, yield composites with strong solid-like rheological behaviors at low frequencies. (b) In a different approach, EVA/oMMT combinations can be designed to promote preferential dispersion of the silicate nanofillers in EVA-rich phases, leading to the formation of topologically jammed filler-networks that, in turn, yield composites with less solid-like rheological behavior and with improved mechanical properties at ambient temperatures. (c) Finally, it was also found that there are temperature-associated effects in the filler networks formation and stability for these multi-component systems. Namely, all nanocomposites having LLDPE-g-MAH functional copolymer show deviations from the time-temperature superposition (tTS) principle, whereas for nanocomposites having EVA as the only functional copolymer tTS is nicely satisfied. The most interesting, probably, findings from this thesis are associated with (d) synergies between MMT and CC nanofillers, in dual-filler composites. Specifically, composites with superior filler dispersions and amplified property improvements were designed and prepared for both the above polymer matrices. Investigating the macroscopic (mechanical, rheological) properties of these composites, revealed: (d1) for ambient temperature mechanical properties, there is a simultaneous increase in tensile modulus and in elongation at break when oMMT is introduced in EVA/PE blend matrix nanocomposites. However, further addition of CC, as second filler in the composite, largely removes the toughening effect of the oMMT, decreasing the elongation at break in the two-filler composites. This change in mechanical response, upon addition of CC particles to EVA/PE/oMMT nanocomposites, was traced to non-typical highly-exfoliated clay morphologies. It is apparent that these exfoliated morphologies are due to a synergy between CC and oMMT, and an oMMT edge-modification process, in which the hydroxyl groups at the edges of the oMMT layers are reacting with the alkyl-carboxyl surface-treatements of the CC particles, has been postulated as the possible mechanism. (d2) The CC-induced highly-exfoliated clay morphologies also modify substantially the viscoelastic properties of the composites, leading to a very pronounced low-frequency solid-like character: This response was attributed to the larger populations of highly exfoliated clay layers, which can promote the formation of polymer-mediated filler-networks, more in populations and stronger in association than in the single-filler respective composites (LLDPE-g-MAH/oMMT). In the second part of the thesis, the impact of the above composite fundamentals are studied in view of properties associated with flexible packaging applications, namely peel strength design and caulking ability upon heat-sealing. Specifically: (e) The addition of CC fillers does not significantly alter the mechanisms responsible for peelability in EVA/PE/oMMT nanocomposites, as reported before, so that EVA/PE/oMMT/CC systems exhibited similar ultra-broad temperature-range of peelability as EVA/PE/oMMT sealants before. However, (f) changes in other thermomechanical properties (tensile, seal-strength, caulking-ability and heat capacity) were observed, and all are consistent with the additional clay exfoliation induced by the CC particles as co-fillers. Such property changes depend systematically on the CC content, up to a critical concentration, and exhibiting a percolation reminiscent leveling-off above that concentration. (g) An experimental lab-scale method to comparatively test the caulking-abilities of polymer and composite film sealants was devised and validated, and was subsequently used to quantify caulking of PE/EVA/oMMT/CC films. The systematic study from this viewpoint, led to principles for predicting/explaining caulking performance of heat sealants and, subsequently, led to the development of first-approach design paradigms for improving caulking performance based on fundamental (rheological, viscoelastic) system parameters. Finally, beyond the trends identified in this work, the quantitative dependencies of the materials properties on the systems’ formulation (i.e. VA content in the EVA copolymer, oMMT or CC concentration, etc) offer an attractive avenue towards designing optimized performance EVA/PE-based nanocomposites.