Creep Compliance Characterization of Vapor-Grown Carbon Nanofiber/Vinyl Ester Nanocomposites Using a Central Composite Design of Experiments

Abstract

The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic response (creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). The nanocomposite test articles were fabricated by high shear mixing, casting, curing, and postcuring in an open face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at prescribed combinations of temperatures (23.8 -69.2 o C), applied stresses (30.2 -49.8 MPa), and VGCNF weight fractions (0.00 -1.00 parts of VGCNF per hundred parts of resin, phr) determined from the CCD. The response surface model (RSM) for predicting the creep compliance was developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction marginally increases the creep resistance of the VGCNF/VE nanocomposite at low temperatures (i.e., 23.8 -46.5 o C). However, increasing the VGCNF weight fraction for temperatures greater than 50 ºC decreased the creep resistance of these nanocomposites. The latter response may be due to a decrease in the nanofiber-to-matrix adhesion as the temperature is increased. The creep compliance RSM revealed the interactions between the VGCNF weight fraction, stress, and temperature on the creep behavior of thermoset polymer nanocomposites. I. IntroductionNano-reinforced polymer composites have been extensively researched to improve material performance and functionality for a wide variety of applications [1][2][3][4][5][6][7][8][9] . The inclusion of small amounts of nano-fillers can improve such composite properties as interlaminar shear strength, toughness, fatigue life, and corrosion resistance. In particular, nanocomposites with vapor-grown carbon nanofibers (VGCNFs) have been shown to improve the composite mechanical, electrical, and thermal properties 10-13 . However, these efforts have been hindered due to poor VGCNF/matrix adhesion, poor VGCNF dispersion, and VGCNF agglomeration that lead to poor load transfer to the nanofibers 14 . The interfacial adhesion between nanofibers and certain polymers has been improved by oxidizing the nanofiber surfaces 4 .The addition of VGCNFs has also been shown to affect the time-dependent (viscoelastic) response of polymer matrix composites. Nouranian et al. 4 used a full factorial experimental design to compare the effect of the VGCNF type, weight fraction, mixing method, and the use of a dispersing agent on the storage and loss moduli of a VGCNF/vinyl ester (VE) material system. Less than 0.50 phr (parts of VGCNF per hundred parts of resin) increased the storage moduli of the nanocomposites by 20% compared to the storage modulus of the neat VE polymer. Creep, the deformation over time at a constant applied stress, is also influenced by the VGCNF weight fraction. Plaseied and Fatemi 15 performed tensile creep testing and creep modeling of VGCNF/VE nanocomposites at a single weight fraction (0.50 ph...