Background of Polymer/Carbon nanotubes Composites
Mixing polymer of various kinds to obtain polymer blends is considered as an easy and efficient way to obtain multiple desirable properties without synthesizing new polymers. However, the reality is much more complex. When mixing semi-compatible or in-compatible Polymer A and Polymer B, the end product is A, B, and C, where C stands for interfaces between A and B. Previous research reveals that although the volume percentage of interface is extremely small, it has a major impact on the mechanical property of polymer blends.
The most common solution to improve the mechanical property of incompatible polymer blends is to add small molecule compatibilizers, which interact with both phases and increasing the interfacial adhesion of the polymer blends. However, due to the small molecular weight of the compatibilizer, adding too much compatibilizers usually reduces the mechanical property of polymer blends.
Compared with small-molecule compatibilizers, carbon nanotubes (CNTs) are much stronger and thus more suitable for toughening the polymer blends. It was reported that crack bridging CNTs can significantly suppress the propagation of cracks, thus toughening the material. Since most of the crack will initiate and then propagate on the interface of the polymer blends, CNTs located on interfaces should be able to efficiently suppress cracking and consequently toughen the polymer blends.
Carbon nanotubes has different affinity with different polymers, and this affinity can be further tailored by refluxing CNTs in concentrated nitric acid. Multiple carboxylic acid and hydroxyl groups can be attached to the defective sites on CNTs surface, introducing strong polarity to the functionalized carbon nanotube (F-CNTs).
This polar f-CNTs has higher affinity with polar polyamide 6 (PA6) and thus less likely to reside in non-polar high density polyethylene (HDPE). Consequently, if f-CNTs was initially within the HDPE phase, a migration of f-CNTs from HDPE to PA6 can be realized.
In my first study, f-CNTs was firstly dispersed in HDPE to make a master batch, which was then melt-mixed with PA6. As a result of higher affinity between PA6 and f-CNTs, most of the f-CNTs migrated into the PA6 phase, leaving some bridging on the interface. However, even with these few interface-bridging f-CNTs, a dramatic increase (106.9%) in elongation at break was achieved.
In order to further improve mechanical property by dispersing more f-CNTs at the interface, maleic anhydride grafted HDPE (HDPE-MA) was used as an compatibilizer to slow down the migration of f-CNTs while they are crossing the interface. The addition of HDPE-MA was found to result in more interfacial dispersion of f-CNTs, leading to a 10x improvement in elongation at break (from 45.5 to 413.7%).
Moreover, I also investigated the influence of f-CNTs content on morphology of polymer blends. f-CNTs had little influence on morphology at lower content (<2 wt%), with HDPE phase dispersed in PA6 matrix. Increasing the content to 5 wt% leading to the establishment of an interconnecting f-CNTs network within the PA6 phase. This network significantly increased the viscosity of PA6 phase, transferring the morphology from sea-island to co-continuous. Further increasing the content of f-CNTs content to 10 wt% led to even higher viscosity and denser f-CNTs network, resulting in phase inversion, where HDPE became the matrix and PA6 existed as dispersed phase.
In all, mechanical and morphological properties of Polymer/CNTs composites is highly tailorable. By controlling the migration of f-CNTs, nanocomposites with greatly enhanced toughness can be obtained. Besides, the ability to induce phase inversion using just f-CNTs is of great interest to the general polymer science community as well.