Characterization and Modeling of Creep Behavior of a Thermoset Nanocomposite M.I. Faraz, N.A.M. Besseling, A.V. Korobko, S.J. Picken Department of Chemical Engineering, Delft University of Technology, Delft 2628 BL, The Netherlands In this article, we report creep and recovery behavior of nanocomposites based on a high-temperature-resistant thermosetting matrix. Nanocomposites with up to 2 wt% of organically modified clay were prepared. The creep and recovery behavior was investigated under various stress levels and at various temperatures. Creep behavior was modeled by a modified Burgers model by introducing a stretched exponential function. This stretched Burgers model satisfactorily describes the creep behavior of the matrix and nanocomposites. The role of filler on the system dynamics has been also dis- cussed and an interesting finding discovered from the stretched Burgers model results. The model results suggest that the dynamics of the filled system is inde- pendent of the filler, which is scientifically quite inter- esting in the field of nanocomposites. The multiple cycle creep and recovery behavior of the matrix were also investigated and the Boltzmann superposition prin- ciple was applied to describe the multistep loading creep response. POLYM. COMPOS., 00:000–000, 2014. V C 2014 Society of Plastics Engineers INTRODUCTION Polymers are viscoelastic materials having time dependent mechanical properties. When a constant load is applied the deformation increases with time which is known as creep. Polymeric materials are subjected to dif- ferent stresses for a period of time during their usage. Therefore, creep is a serious concern for polymers, impairing their service durability and safety. Creep limits their applicability in aviation and automotive industries [1]. Thermoset polymers possess excellent dimensional stability because of their cross-linked network structure. Their creep resistance is much better than that of thermo- plastics [2]. Thermosets are widely used as matrix in composites for various applications including aviation, automobile, sports, and buildings. The use of nano-sized fillers to modify the polymer properties has received considerable attention because the stiffness, strength, and toughness are improved by the nanoscale inclusions. Promising effects of nanoparticles on creep resistance have also been reported in several studies [3–6]. Particularly, the creep resistance has been studied for clay reinforced epoxy network, polystyrene/ fluorohectorite nano- and micro-composites [7–9]. Substantial interest has been given to carbon nanotube composites and their enhanced mechanical properties by fillers. Zhang et al. [10] studied the creep response of car- bon nanotubes (CNT) epoxy composites. The creep strain of epoxy reduced significantly with CNT content at lower weight fraction (up to 0.1–0.25 wt%) and lower stress levels. Upon increasing the CNT contents to 1 wt% fur- ther improvement of the creep resistance was badly affected by poor dispersion. Tehrani et al. [11] reported that the addition of 3 wt% multiwall carbon nanotubes (MWCNT) in epoxy reduced the creep. However, the effect is more pronounced at elevated temperature and high nano-identation creep load. The effect of graphene on creep behavior of epoxy was investigated in Ref. 12. At low stress level and ambient temperature no difference was observed between the creep response of filled and unfilled epoxy. The graphene platelet-filled epoxy creeps significantly less at elevated temperature and higher lev- els of stress, with optimum filler amount of 0.1 wt%. Starkova et al. [13] found that MWCNTs did not influ- ence the creep characteristics of the epoxy matrix, and other mechanical properties also remained unaffected. The viscoelastic properties of nanocomposites have been modeled using both constitutive models and simula- tions. Plaseied and Fatemi [14] used the Findley model for predicting the long-term creep compliance of 0.5 wt% carbon nanofiber filled vinyl ester nanocomposites. Yang et al. [15] explained the creep behavior of polyamide clay nanocomposites by applying the Burgers model and Find- ley law. Jia et al. [5] used the Burgers model to illustrate the creep mechanism involved in polypropylene/MWCNT composites. A few examples can be found, in which the creep behavior of thermoset polymers and thermoset nanocomposites have been modeled by a modified form of Burgers model [16–18]. An important assumption Correspondence to: N.A.M. Besseling; e-mail: N.A.M.Besseling@ tudelft.nl Contract grant sponsor: Higher Education Commission (HEC) of Pakistan. DOI 10.1002/pc.22946 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2014 Society of Plastics Engineers POLYMER COMPOSITES—2014