Characterization and Liquid Crystallinity of DGETP-Me Monomer
The DSC curves of the DGETP-Me during the heating process and cooling process are shown in Figure 4. The DGETP-Me monomer exhibits three endothermic DSC peaks at 178, 208, and 227°C during the heating process. The large peak area for the feature at 178°C suggests that this is the melting temperature. Moreover, the three exothermic peaks were observed at 167, 207, and 226°C during the cooling process.
To elucidate the phase transition behavior, polarized optical micrographs of the DGETP-Me monomer were obtained (Figure 5). At room temperature, the monomer is crystalline, but at temperatures above 179°C, a birefringence pattern emerged and the focal conic texture associated with a smectic A type LC phase was observed. At 208°C, the monomer transitions to a schlieren texture, consistent with the formation of a nematic LC phase (Figure 5). Smectic A and nematic LC phases in this temperature range emerge because of the self-organization of the mesogenic-containing monomers DGETP-Me. At temperatures greater than 227 C, visibility decreased, indicating an isotropic phase; at high temperature, strong thermal molecular motions disrupt the π–π stacking interaction of the mesogenic groups, leading to the isotropic structure. This analysis showed that, depending on the temperature, the DGETP-Me monomer can exist in one of four forms and can exhibit two types of LC phases over a wide temperature range. Even in the cooling process, the LC pattern of the DGETP-Me monomer was almost the same as that during the heating process.
The time-temperature-transformation (TTT) diagram constructed for the DGETP-Me/DDM system is shown in Figure 6. At 170°C, the epoxy monomer and curing agent mixture retains its crystallinity. At 175–225°C, the system adopts an isotropic phase during the initial curing stage, and gradually develops (between 30 and 50 s) into a LC phase with a characteristic birefringence pattern; a locked LC phase structure formed at these curing temperatures. At temperatures greater than 225°C, the systems were isotropic from the entire curing process. This analysis showed that LC and isotropic systems could be prepared by varying the curing temperature.
Preparation of Bulk DGETP-Me/DDM Systems with Variable Phase Structures and Acid Resistance
The physical properties of bulk isotropic and LC cured systems, prepared using the TTT diagram specifications, were evaluated. It is important to note that the structures of the bulk curings are strongly dependent upon the heat of reaction. As a result, the actual curing temperature was determined by considering the temperature increase caused by the exothermic curing reaction.
Polarized optical micrographs of the polished, diglycidyl ether of 3-methyl-terphenyl/4,4′-diaminodiphenylmethane (DGETP-Me/DDM) systems cured at 240, 170, and 120°C are shown in Figure 7. The system cured at 240°C was transparent and was observed as a dark field under the cross-nicols, meaning that the network is composed of isotropic chains. Alternatively, the systems cured at 170 and 120°C were opaque and had birefringence patterns consistent with a LC phase.
Figure 8 shows the XRD patterns of the systems cured at 240, 170, and 120°C. The systems cured at 240 and 170°C exhibited broad halos at ∼20° (4.5 Å) indicative of amorphous structures; the system cured at 170°C was previously shown to exhibit a birefringence LC pattern, and likely consists of many nematic LC domains. For the system cured at 120°C, a high intensity peak at 3.5° (25 Å) was observed, which signifies a smectic LC phase. This peak is representative of a layered smectic structure, and the distance calculated using Bragg's law is the length of the layered phase. Based on this data, two types of networks could be prepared depending on the curing temperature chosen.
The resistance of DGETP-Me to acid was compared to that of the DGETAM using HCl aq. immersion tests (see Experimental section for details). The residual weight of the nematic DGETAM/DDM system decreased drastically as a function of immersion time and reached 0 wt % after 28 days, whereas the nematic DGETP-Me/DDM system maintained its original weight over the course of the experiment (Figure 9).
The appearance and IR spectrum of the DGETP-Me/DDM system, before and after immersion in HCl aq., were also evaluated. The sample became green after immersion in aqueous HCl, due to the absorption of water by DDM. IR measurements showed that the peaks associated with hydroxyl stretching, at 3200–3400 cm−1, increased after the sample was exposed to HCl aq., but the peaks in the range 400–1700 cm−1 did not change, indicating that the DGETP-Me/DDM system is resistant to degradation by acid.
The DGETAM/DDM system decomposed, and the degradation products dissolved in HCl aq. (pH = 0.3). The procedure to reprecipitate the dissolved components is shown in Scheme 2. White (I) and yellow (II) powder extracts were isolated from the separation process. The IR spectra and appearances of the DGETAM/DDM system before and after immersion in HCl (aq) are shown in Figure 10. Before immersion, the CN stretching vibration at 1620 cm−1 was clearly observed (Figure 10).
However, after immersion, extract I (soluble in CHCl3) showed a drastic decrease in the intensity of the peak at 1620 cm−1, whereas a new peak, corresponding to the aldehyde group, appeared at 1690 cm−1. Based on this result, we assume that this component is p-hydroxybenzaldehyde. The IR spectrum for extract II, which was dissolved in oil after neutralization with NaOH, exhibited peaks at 1620, 1690, and between 3200 and 3400 cm−1, assigned as CN, aldehyde, and hydroxyl functional groups, respectively. Thus, it is assumed that extract II contains the amine groups formed by the hydrolyzed CN bond; this extract was soluble in water, likely as a salt, during the CHCl3 extraction.
GPC was performed on extract II to help characterize the degradation products (Figure 11). The GPC data indicates that extract II contains a variety of products over a wide molecular-weight range. Based on this result, it appears that this extract consists of various polyamines containing the partially or fully hydrolyzed DGETAM/DDM. These results suggest that DGETAM, which contains a CN moiety in the backbone, is easily hydrolyzed in HCl. Therefore, synthesized DGETP-Me exhibits superior chemical resistance when compared to DGETAM.