Influence of cross-link density on the properties of ROMP thermosets

Abstract

A norbornene-based cross-linker was synthesized and mixed at different loadings with two separate monomers for self-healing polymer applications: 5-ethylidene-2-norbornene (ENB) and endo-dicyclopentadiene (endo-DCPD). The monomer/cross-linker systems were polymerized by ring-opening metathesis polymerization (ROMP) with Grubbs' catalyst. The thermal–mechanical properties of the polymerized networks were evaluated by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) and the curing process was monitored by parallel plate oscillatory rheometry. The viscosities of the pre-polymer blends are shown to be adequately low for self-healing, and exhibit a high ROMP reactivity to form cross-linked networks with enhanced thermal–mechanical properties. The addition of cross-linker increases the glass transition temperature (T_g) and the storage modulus both above and below T_g. The storage modulus increase above T_g is used to estimate the molecular weight (M_c) between entanglements or cross-link sites for both ENB and endo-DCPD-based networks. The cross-linker also greatly accelerates network formation as defined by the gelation time.

Introduction

Self-healing polymer composites represent a new paradigm in materials design [1], [2]. In these materials, which are inspired by biological systems where damage triggers an autonomic healing response, self-healing is accomplished by embedding liquid healing agent filled microcapsules within a polymer matrix. When damage occurs, matrix microcracks develop and coalesce to rupture the embedded capsules, releasing healing agent into the crack plane. There the released healing agent contacts an embedded chemical trigger (catalyst) and polymerizes, bonding the crack faces back together. Because of its mild reaction conditions and ability to be triggered at room or even lower temperatures without external heating, ring-opening metathesis polymerization (ROMP) has proven to be an excellent polymerization technique for the in situ cure of cyclic olefin healing agents and resulting repair of damage in these self-healing material systems. Specifically, norbornene-based monomer, endo-dicyclopentadiene (endo-DCPD), has been used extensively as the healing agent in self-healing composites with good results [3], [4], [5], [6], [7], [8], [9], [10]. In these works, ROMP of DCPD healing agent was initiated by embedded 1st generation Grubbs' ruthenium catalyst. The catalyst shows high reactivity, functional group tolerance, and air/moisture insensitivity [11], [12], [13]; however, usage of catalyst in self-healing materials is restricted because of its high cost. In order for the self-healing strategy to be successfully realized, the encapsulated healing agent should meet several special requirements: 1) the healing agent should polymerize at room temperature to produce a very strong thermoset with good thermomechanical properties and adhesive strength; 2) the catalyst loading should be as low as possible to eliminate its deleterious effect on the virgin mechanical properties of the polymer matrix and to reduce overall cost; 3) the self-healing agent should have a very low viscosity and low surface energy to completely fill the microcracks before polymerization occurs.

While the previously used endo-DCPD healing agent is capable of forming a cross-linked structure with high toughness and strength, it has a relatively slow in situ polymerization rate and requires high loadings of catalyst, which is undesirable for self-healing applications. Low temperature self-healing applications are also limited because pure endo-DCPD has a melting point (33 °C) just above room temperature.

Another norbornene-based monomer (Fig. 1), 5-ethylidene-2-norbornene (ENB) has a very low melting point of −80 °C, and much higher ROMP reactivity than DCPD [14], [15]. However, ENB forms a linear polymer through ROMP. Also, the strength and glass transition temperature (T_g) for poly-ENB are both lower than the cross-linked poly-DCPD. ENB can be blended with endo-DCPD or with other cross-linking agents (or cross-linkers) to form a cross-linked network and improve its thermal–mechanical properties. Liu et al. [16] reported that blends of ENB and DCPD polymerize much faster than neat DCPD, even at lower catalyst loadings, with increasing ENB content. The ENB/DCPD also shows increased rigidity after cure compared to pure ENB. In our previous work [17], norbornene-based cross-linkers (CL-2 and CL-3) with varying degrees of complexity were synthesized from norbornadiene (CL-1) (Fig. 2). By adding the cross-linker at varying concentrations to the ROMP monomers, the properties of the resulting polymers can be tailored. With addition of CL-2 and CL-3, the glass transition temperature (T_g) of both ENB and endo-DCPD systems increases, while the addition of CL-1 decreases T_g for both systems. In addition, the cross-linkers were shown to decrease the melting point of DCDP-based healing agents and expand their applications to lower temperature ranges.

In this study, we first investigate the influence of CL-2 and CL-3 on the viscosity of the DCPD and ENB healing agents. Based on these results we then select the most promising cross-linker as CL-2 (hence forth referred to as CL) and investigate its effect on gelation kinetics by parallel plate oscillatory rheometry. The thermal–mechanical properties of cured ENB/CL and DCPD/CL systems are characterized by using dynamic mechanical analysis (DMA) and dynamic scanning calorimetry (DSC).