Comparative Study of Shape Memory Bisphenol A and Bisphenol F Cyanate Ester Systems

The demand for shape memory polymers in both terrestrial and space applications has increased in recent years as technologies move towards smart systems. Thermoresponsive shape memory polymers find potential use particularly in space and biomedical applications [1–3]. The literature cites the development of a host of polymers belonging to the class of thermoplastics, and a few thermosets, exhibiting shape memory characteristics. Among thermosets, epoxy resins are the most versatile in view of their ease of synthesis, the numerous ways of effecting cure, and their amenability to copolymerization, compounding and composite processing. In fact, epoxies are best suited for elastic memory composites with good strength characteristics if they possess shape memory properties [4–6]. Many reports refer to the use of proprietary shape memory epoxy formulations, and composites derived therefrom, for the development of several self-deployable systems for potential space applications [7–10]. However, there are no details on their synthesis or processing aspects, or on the structural features leading to the manifestation of shape memory properties. Though epoxy resins are the workhorse matrices for structural composites, they are not the ‘last word’ in thermosets.

Cyanate ester (CE) has emerged as a better alternative to epoxies, with several advantages, as CE is the answer to many issues raised by epoxies in aerospace applications. It forms highly crosslinked, three-dimensional, triazine-linked polycyanurate networks through a polycyclotrimerization reaction in the presence of suitable catalysts. The properties of cyanate ester can be modified by reaction with an epoxy resin. When epoxy is introduced, the moisture resistance of CE increases, whereas its thermal properties are not noticeably sacrificed [11]. The toughness of the CE/epoxy system depends on the molar ratio of CE to epoxy.

Xie et al. studied the shape memory properties of epoxy resin–aliphatic amine systems. They tuned the glass transition temperatures of the epoxy resins by varying the crosslink density of the systems [12]. Merline et al. synthesized a shape memory oxazolidinone polymer by reacting an isocyanate with hydroxy-telechelic poly(tetramethylene oxide) and an epoxy resin. The modulus ratio and shape recovery increased with increasing oxazolidinone content in the polymer [13]. Tong et al. reported a shape memory cyanate ester synthesized from polyols and diols in the presence of a metal catalyst. The glass transition temperature obtained was greater than 150 °C [14].

The present work focuses mainly on the synthesis, characterization and shape memory properties of thermoset epoxy-cyanate ester systems. The research includes detailed investigations of the synthesis of selected shape memory thermoset polymers, viz. epoxies and cyanate esters. An attempt has been made to correlate the shape memory behaviour with the structural features and morphology of the systems. It is hoped that the findings of this study make a modest contribution to the emerging field of shape memory materials, which have tremendous application potential in many areas, especially where manual intervention is impossible, as in space exploration and planetary missions.

PCLOH was synthesized by reaction of poly(ε-caprolactone) (PCL) diol (0.1 mol) with p-hydroxybenzoic acid (PHBA, 0.6 mol) in toluene/diglyme solvent in the presence of p-toluenesulfonic acid (pTSA) as catalyst. The mixture was refluxed for 20 h and the by-product water was removed by azeotropic distillation using toluene in a Dean–Stark apparatus. After completion of the reaction, the solvent was removed by distillation, the resultant viscous fluid was poured into CHCl₃, and the unreacted PHBA was filtered out. The filtrate was washed several times with 5% NaHCO₃ solution to remove the catalyst and unreacted PHBA. The CHCl₃ solution was dried over anhydrous Na₂SO₄ for one day and filtered, and the solvent was removed in a flash evaporator at 60 °C. The resultant resin was characterized by Fourier transform infrared spectroscopy (FTIR) and size exclusion chromatography (SEC). The ester value was estimated by chemical analysis.

Diglycidyl ether of bisphenol A (DGEBA), PCLOH, bisphenol A cyanate ester (BACE) and the catalyst (zinc octoate and nonylphenol, 4 wt% of BACE) were first mixed together, degassed at 80 °C for 30 min and then poured into a steel mould. Blends of DGEBA and PCLOH with bisphenol F cyanate ester (BFCE) were prepared using the same procedure. A DGEBA/PCLOH/DCE (DCE: different cyanate ester) molar ratio of 1/0.07/0.82 was selected for this study. Since the three components of the base resin systems are difunctional and have the same molar mass, their molar ratios equal their equivalent ratios. The structures of the different cyanate esters are shown in Fig. 1.

The method of synthesis of PCLOH is illustrated in Fig. 2, and the FTIR spectrum of PCLOH is shown in Fig. 3. The peak at 3403 cm⁻¹ corresponds to the stretching vibration of –OH groups and that at 1724 cm⁻¹ to the C=O stretching of the ester group. The peak at 1164 cm⁻¹ corresponds to C–O–C stretching of PCLOH. The polymer was further characterized by ester value estimation. The ester value of PCLOH (47 mg KOH/g) agreed closely with the theoretical value (50 mg KOH/g). The similarity in the SEC profiles obtained with both the refractive index (RI) and UV detectors shows that all polymer chains are end-capped with phenolic groups (Fig. 4). PCL diol is insensitive to the UV detector as it does not contain any chromophoric groups; on end-capping with phenol groups, it becomes UV sensitive.

The cure schedules of the DGEBA/PCLOH/BACE (PCLBA) and DGEBA/PCLOH/BFCE (PCLBF) blends were determined by differential scanning calorimetry (DSC) and FTIR. The DSC profile of the PCLBF system (Fig. 6) shows two exotherms, one at a lower temperature and the other at a higher temperature (Table 1). The exotherm at 175 °C is attributed to the cyanate–epoxy reaction leading to the formation of oxazolidinone groups (Fig. 5, pathways 1 and 4) [15], and that at 185 °C to the polymerization of cyanate groups (Fig. 5, pathway 2) [15, 16]. The PCLBA system shows a single exotherm at 185 °C (Fig. 6), due to the formation of the triazine network.

FTIR (Fig. 7) confirmed the completion of the cure reaction of the epoxy and –OCN groups of both systems (PCLBA and PCLBF) in the network, through the absence of the peaks corresponding to the epoxy ring at 916 cm⁻¹ and to –OCN at 2271–2238 cm⁻¹. This was further confirmed by the appearance of triazine peaks at 1363 cm⁻¹ and of isocyanurate absorptions at 1698 and 1459 cm⁻¹ [17, 18]. The oxazolidinone rings formed by the reaction of cyanate and epoxy were observed at 1731 cm⁻¹ [16]. Based on the above studies, all the compositions were cured under the same cure schedule: 100 °C for ½ h, 120 °C for 1 h, 150 °C for 1 h, 180 °C for 1 h and 200 °C for 3 h.

A recent paper reported that cyanate ester polymerization in the non-functionalized PCL blend occurred at a high temperature [19]. The blend containing functionalized PCL showed this exotherm at a lower temperature, owing to the involvement of the phenolic groups of the switching segments in the catalysis of cyanate ester homopolymerization (Fig. 5, pathway 2). It is possible that the phenol groups also react with the epoxy resin. The likely mechanisms of co-reaction of the epoxy resin with cyanate ester involving PCLOH are shown in Fig. 8.

The PCLBA system shows higher flexural strength than PCLBF (Table 2). The switching segments (PCLOH) are expected to give the epoxy-cyanate ester more flexibility, which could help improve its shape memory properties.

Fig. 9(a) and (b) show the SEM images of the fractured surfaces of PCLBA and PCLBF, respectively. In the case of PCLBA and PCLBF (Fig. 9(a) and (b)), the blends exhibited relatively ductile fracture. Comparing the SEM images of PCLBA and PCLBF, the latter shows a more brittle pattern. This is because bisphenol A cyanate ester contains two –CH₃ groups, whereas these groups are absent in bisphenol F cyanate ester; therefore, the crosslink density of PCLBF is higher than that of PCLBA.

Fig. 10 shows a typical TGA trace of PCLBA. The two systems showed thermal stability above 280 °C. The cyanate ester-epoxy system alone normally decomposes at temperatures above 350 °C. Addition of the shape memory components diminishes the thermal stability. PCL made the system thermally less stable: practically all the PCL degraded at around 300 °C. However, both systems are stable well above their service temperature (normally around the transition temperature).

The viscoelastic properties of the samples were investigated by dynamic mechanical thermal analysis (DMTA) in three-point bending mode. Fig. 11 shows the loss tangent (tan δ) of the different systems as a function of temperature. The transition temperature, T_trans, was determined from the tan δ peak. The modulus ratio, E_g/E_r, is taken as E′_Ttrans−20/E′_Ttrans+20, where E′_Ttrans−20 and E′_Ttrans+20 stand for the storage modulus at T_trans − 20 °C and T_trans + 20 °C, respectively. Initially, PCLBF shows a higher storage modulus than PCLBA. While the storage modulus decreases, the E_g/E_r ratio (Table 2) increases in the order PCLBF < PCLBA. The transition temperature shows the same trend as the storage modulus, in the order PCLBA < PCLBF. The higher transition temperature of PCLBF compared with PCLBA is due to the crystalline nature of its segments (absence of –CH₃ groups).

Qualitatively, the shape memory behaviour was evaluated by a bending test between T_trans + 20 °C and T_trans − 20 °C. The shape memory studies were carried out with respect to the transition temperature. The shape memory behaviour of a representative sample, PCLBA, is demonstrated in Fig. 12. The original (permanent) rectangular shape (a) was heated at T_trans + 20 °C, and the sample was deformed into different shapes through bending and twisting. Upon cooling under load, these deformed temporary shapes (b) were fixed. On reheating above T_trans, the sample recovered its original rectangular shape (c). The recovered shape was indistinguishable from the original shape, confirming the excellent shape fixity and recovery.

For quantitative evaluation, bending tests of the two systems (PCLBA and PCLBF) were conducted at T_trans + 20 °C (Table 2). At T_trans + 20 °C, the shape recovery increases in the order PCLBF < PCLBA. This observation is consistent with the modulus ratio (E_g/E_r). The recovery time decreases proportionally. At a high E_g/E_r ratio, the shape recovery is maximal and the recovery time minimal.

On increasing the temperature to T_trans + 30 °C, the two systems showed good shape recovery, as the higher temperature favoured chain relaxation. Though the cyanate ester system showed good thermal and mechanical properties, it exhibited relatively poorer shape memory properties, as it provides a rigid matrix with a low concentration of the shape memory component. Highly crosslinked structures exert a strong restraining force on their segments, which need a large free volume and more energy, and hence a higher temperature, to accomplish shape recovery [20]. The present system possesses shape fixity of 95–98% and shape recovery of 90–95%. It is mechanically sturdy and has good thermal stability for use in composites for developing smart systems for diverse applications. The advantages of this system include a controllable transition temperature and better processability than traditional shape memory thermoset systems.

Novel thermoset polymers showing shape memory properties were synthesized by reacting diglycidyl ether of bisphenol A (DGEBA) and phenol-telechelic poly(ε-caprolactone) (PCLOH) with two cyanate ester resins, namely bisphenol A cyanate ester and bisphenol F cyanate ester. SEM analysis showed that the PCLBA and PCLBF blends exhibited relatively ductile fracture. TG analysis showed that both systems were thermally stable above 280 °C. Initially, PCLBF showed a higher storage modulus than PCLBA. The E_g/E_r ratio increases in the order PCLBF < PCLBA. The higher transition temperature of PCLBF compared with PCLBA is due to the crystalline nature of its segments. The shape recovery and recovery time of the PCLBA system are better than those of PCLBF. The present system possesses shape fixity of 95–98% and shape recovery of 90–95%. The actuation temperature could be tuned by varying the PCLOH content. The polymer possessed good mechanical properties, thermal stability and shape recovery characteristics that could be controlled by varying the composition of the matrix. The resin is suitable for processing good elastic memory composites, as cyanate esters normally promote excellent composite formation. These systems are potentially useful for fabricating smart systems such as actuators.