Creep behavior of polyurethane nanocomposites with carbon nanotubes

Abstract

The polyurethane (PU) nanocomposites containing carbon nanotubes (CNTs) were prepared through in situ polymerization for the creep study. The results show that the presence of CNTs leads to a significant improvement of creep resistance of PU. However, this creep resistance does not increase monotonously with increase of CNT contents because it is highly dependent on the dispersion of CNTs. Several theoretical models were then used to establish the relations between CNT dispersion and final creep and creep–recovery behaviors of nanocomposites. The as-obtained viscoelastic and viscoplastic parameters of PU matrix and structural parameters of CNTs further confirmed the retardation effect by CNTs during creep of the nanocomposite systems. Besides, the time–temperature superposition (TTS) principle was also employed in this work to make a further evaluation on the creep of PU/CNT nanocomposites with long-term time scale.

Introduction

As an important engineering material, polyurethane (PU) exhibits many attractive properties, such as adjustable mechanical strength, unique shape-memory ability and superior chemical resistance, because of its special soft–hard segment structure [1], [2]. In order to further extend its applications, several techniques, including interpenetrating [3], [4], blending [5] and copolymerization [6], have been developed to control hierarchical structures and final properties of the PU material. Besides, hybridation with nanofiller [7], [8] is also an effective strategy to improve the performance of PU.

In recent years, the carbon nanotubes (CNTs) have become the next-generation reinforcements for nano-structured polymeric composites, owing to its extraordinarily high elastic modulus, strength, and resilience [9], [10], [11]. Hitherto, many CNTs based polymer nanocomposites [12], [13], [14] have been successfully prepared via various methods, also including the PU/CNT nanocomposites [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. The structure and properties of PU/CNT systems have hence been extensively studied and the focus of attention are: (1) the dispersion of CNTs and the microstructure of nanocomposites [15], [16], [17], [18], [19]; (2) the thermal and rheological behaviors in the presence of CNTs [20], [21]; (3) the improvement of active shape memory with addition of CNTs [22], [23], [24]; (4) the mechanical properties, especially the reinforcing effect of CNTs [20], [25], [26], [27], [28], [29], [30].

As an interesting time dependent behavior and as an important evaluation standard on the dimensional stability of engineering materials, creep and creep–recovery of the polymer/CNT composite systems have attracted much attention recently [30], [31], [32], [33], [34], [35], [36], [37]. It has been reported that the presence of CNTs restrains the creep deformation of matrix polymer significantly, but the improvement of creep resistance diminishes with the growing stress and increased temperature [34], [35], [36]. The recovery property is also highly improved, especially at high temperature [31], [35]. Such improvement is attributed to two possible reasons: (1) the presence of CNTs leads to the formation of a significant interphase zone with altered polymer mobility, namely chain immobilization, which results in the initial reduction of the creep compliance [33], [34], [35]; (2) good CNTs–matrix interfacial bonding further reduces creep through frustrating chain disentanglement, stretching and fragmentation of the macromolecule [33].

The creep study is also very important to the PU/CNT nanocomposites. There are merely few reports on the creep of PU/CNT nanocomposites found in the literatures, however. Jia et al. [30] performed a preliminary study on the creep properties of the thermoplastic polyurethane (TPU)/CNT nanocomposites and found that the presence of CNTs (pure or functionalized) could improve the creep resistance through their good interfacial adhesion to the TPU matrix. The interesting results reveal a common creep behavior of the PU/CNT systems, similar with that reported on the other CNTs based polymeric composites. To deeply understand how the presence of CNTs affects the time dependent properties of PU, however, it is necessary to further explore the creep and creep–recovery behaviors of PU/CNT nanocomposites. Therefore, in this work, the PU/CNT nanocomposites with different contents of CNTs were prepared through in situ polymerization for the creep and creep–recovery studies at different temperatures and load levels. Some constitutive models were then used to establish the relations between CNT dispersion and creep of nanocomposites. Besides, the creep with long-term time scale was also explored by time–temperature superposition (TTS), aiming at further evaluating the time dependent properties of PU/CNT nanocomposites.