Synthesis, characterization, and mode I fracture toughness of aligned carbon nanotube polymer matrix nanocomposites

Abstract

In an effort to fully understand the contribution of carbon nanotubes (CNTs) to strength and toughness enhancement in hierarchical nanoengineered composites, particularly steady state Mode I fracture toughness, RTM6 and EPON 862/W epoxy based vertically-aligned carbon nanotube (A-CNT) polymer nanocomposites (A-PNCs) are manufactured. These A-PNCs can be tested to isolate structure-property relationships between the polymer matrix and the A-CNTs without the presence of the micro-scale fibers. Additionally, A-CNT volume fraction can be varied via a densification process to realize 1-30% volume fraction (vf.%) A-PNCs. An investigation of the Mode I initiation fracture toughness via single edge notch beam (SENB) testing of A-PNCs with 1-5 vf.% uniaxially densified A-CNT forests finds that RTM6 baseline and A-PNC samples have a KIc,i of ~ 1 MPa-m¹/², with the exception of 1 vf.% having 1.33 ± 0.09 MPa-m¹/², which needs to be further explored due to process-structure questions of specimen quality. No statistically significant change is observed in EPON 862/W A-PNCs at 1-5 vf.% over baseline specimens having a KIc,i of 1.49 ± 0.06 MPa-m¹/² , indicating that A-CNTs do not offer any toughening at initiation in this system. Scanning electron microscopy of the fracture surface for both A-PNC systems reveals that < 10% of the A-CNTs available are engaged during crack bridging, i.e., in a 5 vf.% A-PNC specimen, at most 0.5 vf.% of the A-CNTs are engaged during fracture, and pull-out from the matrix is less than 1 pm. Thus, the pullout and debonding toughening contribution offered by the A-CNTs is expected and measured to be negligible. It is possible that these A-CNTs may offer steady-state toughening, however, results are not achieved due to limitations in specimen size and geometry. While changes in Mode I initiation toughness are not observed, significant changes in properties from both quasi-static and dynamic nanoindentation testing are observed. The A-CNT alignment confers a non-isotropic mechanical response when quasi-statically tested with A-CNTs parallel or perpendicular to the indentation load. An ~ 270% modulus increase over baseline for the 30 vf.% EPON 862/W A-PNC parallel configuration and a ~ 140% increase in the perpendicular configuration, are observed and dynamic nanoindentation supports this finding with, e.g., a storage modulus increase of ~ 200% in the parallel orientation. RTM6 A-PNCs show less of an increase in indentation modulus (~ 33% in 10 vf.% specimens in the parallel direction) over baseline when compared to those of EPON 862/W, likely due to the relatively stiff RTM6 (modulus is ~ 1.5x larger than EPON 862/W) and therefore there is a relatively smaller A-CNT stiffness contribution. Looking forward, the A-CNTs used in this work are noted to be unmodified/as-grown, and the A-CNT fracture results highlight the need for modifying the A-CNTs towards increased A-CNT strength (defect density reduction), and/or reducing the strength of the CNT-polymer interface, to increase A-CNT toughness contribution further.