Phase Behavior

The phase transition temperatures of H-acceptor polymers (P1 and P2), H-donor dyes (S1-S4), and H-bonded side-chain polymers (i.e., H-bonded polymer complexes

P1/S1-P1/S4 and P2/S1-P2/S4) are summarized in Table 5.1, which were determined by DSC (under nitrogen) and POM. The weight-average molecular weights (Mw) of H-acceptor polymers P1 and P2 (determined by GPC) are 14400 g/mol (PDI = 1.72) and 38100 g/mol (PDI = 3.24), respectively. The glass transition temperatures (Tg) of H-acceptor polymers P1 and P2 are 63 and 88 °C, respectively.⁷⁹ To elucidate the H-bonding effect of H-bonded pendants on the thermal properties of supramolecular side-chain polymers, H-donor dyes S1-S4 were introduced to be incorporated with H-acceptor side-chain polymers P1 and P2. As shown in Table 5.1, both series of H-bonded complexes containing H-acceptor polymers P1 and P2 showed only a single glass transition, which suggests good miscibilities between H-donor dyes (i.e.,

S1-S4) and H-acceptor polymers (i.e., P1 and P2).

Since no melting and crystallization transitions were observed in the DSC measurements, it suggests that these H-bonded complexes possess amorphous characteristics. However, the Tg values of the H-bonded complexes are notably higher than those of their corresponding H-acceptor polymers P1 and P2. The increases of Tg values in H-bonded complexes are probably due to the larger π-π interactions

originated from the increased rigid-rod lengths of the integrated H-bonded pendants (containing both pyridyl H-acceptor units and H-donor dyes). In contrast to H-acceptor homopolymer P1 and its H-bonded complexes, H-acceptor copolymer P2 and its H-bonded complexes possessed higher Tg values due to the integration of more bulky and rigid CAZ components in copolymer P2. Comparing the H-bonded complexes containing fluorene-linked dyes (S1 and S2) and bithiazole-linked dyes (S3 and S4), owing to the higher rigidity of bithiazole units in H-donor dyes S3 and

S4, the latter H-bonded complexes (P1-P2/S3 and P1-P2/S4) have higher Tg values

than the former H-bonded complexes (P1-P2/S1 and P1-P2/S2), respectively. This obviously indicates that the rigid bithiazole linkers will enhance the aggregation of the pendants in the H-bonded complexes effectively. In contrast to the H-bonded complexes (P1-P2/S1 and P1-P2/S3) containing end-capping triphenylamine dyes (S1 and S3), owing to the higher rigidity and coplanarity of end-capping cabazole units in H-donor dyes (S2 and S4), the analogous H-bonded complexes (P1-P2/S2 and

P1-P2/S4) containing end-capping cabazole dyes (S2 and S4) have higher Tg values.

The isotropization temperatures (Ti) have the similar trends as the glass transition temperatures (Tg) in H-bonded polymer complexes (P1/S1-P1/S4 and

P2/S1-P2/S4). Moreover, comparing analogous H-bonded complexes consisting of the same H-donor dyes, H-bonded complexes containing H-acceptor homopolymer

P1 possess the higher isotropization temperatures (Ti) and the broader mesophasic

ranges than those containing H-acceptor copolymer P2. In addition, compared with H-bonded polymer complexes P1/S1 and P1/S2 bearing fluorene-linked dyes (S1 and

S2), H-bonded polymer complexes P1/S3 and P1/S4 bearing bithiazole-linked dyes

(S3 and S4) have higher Ti values and broader mesophasic ranges. In general, the isotropization temperatures (Ti) and mesophasic ranges of H-bonded side-chain polymers would be enhanced while the H-bonded central cores are longer and more rigid.

As shown in Figure 5.5(a), the mesomorphic behavior of H-bonded polymer complex P2/S4 (cooling at 130 °C) was confirmed as the nematic phase by the schlieren texture of POM, which was further elucidated by X-ray diffraction (XRD) measurements in Figure 5.5(b) that no sharp d-spacing values, i.e., no layered structures of the smectic phase, were observed in the XRD intensity against angle profiles of H-bonded polymer complexes P1/S1 and P2/S4 at 130 °C (in the mesophasic range). According to the POM and XRD measurements, H-acceptor homopolymer P1 and all H-bonded polymer complexes (P1/S1-P1/S4 and

P2/S1-P2/S4) in Table 5.1 were verified to possess the nematic phase, but H-acceptor

copolymer P2 bearing 50% molar ratio of CAZ units did not possess any mesophase.

Hence, the integration of CAZ units in copolymer P2 is detrimental to the formation

of the mesophase, which can be explained by that the CAZ units with non-mesomorphic property may dilute and hinder the molecular packing of the LC arrangements in copolymer P2. However, the nematic phase was introduced to the corresponding H-bonded polymer complexes (P2/S1-P2/S4) of copolymer P2 due to the extended H-bonded mesogens by combination of H-acceptor pedants with H-donor dyes. Moreover, the mesophasic ranges and Ti values of the H-bonded polymer complexes (P2/S1-P2/S4) containing copolymer P2 were apparently reduced by the CAZ units of H-acceptor copolymer P2, which diluted and interfered the LC arrangements of the H-bonded mesogens in their subsequent H-bonded polymer complexes. However, acid-protected dye S1P and physical blend P1/S1P (without H-bonds) have lower phase transition temperatures (including the isotropization temperature Ti) than H-bonded polymer complex P1/S1 due to the dilution effect of the acid-protected dye S1P moieties in the physical blend P1/S1P (see Table 5.1).

Table 5.1 Thermal Properties of H-Acceptor Polymers (P1-P2), H-Donor Dyes (S1-S4), Acid-Protected Dye S1P, H-Bonded Polymer Complexes (P1/S1-P1/S4

and P2/S1-P2/S4), and Physical Blend P1/S1P

Compound Phase transitions, °C^a,b P1 G 63 N 125^c I P2 G 88 K 110^c I

S1 K 156 (10.8) I

S2 K 163 (13.4) I

S3 K 173 (25.9) I

S4 K 180 (26.3) I

S1P K 98 (13.4) I

P1/S1 G 87 N 151^c I P1/S2 G 89 N 155^c I P1/S3 G 96 N 167^c I P1/S4 G 99 N 172^c I P2/S1 G 96 N 141^c I P2/S2 G 98 N 147^c I P2/S3 G 104 N 156^c I P2/S4 G 105 N 162^c I P1/S1P G 67 K 100 (12.4) I

a Phase transition temperatures (°C) and enthalpies (in parentheses, kJ/mol) were determined by DSC at a heating rate of 10 °C/min.

b G, glassy state; K, crystalline; N, nematic; I, isotropic.

c Phase transition temperatures were obtained by POM and confirmed by XRD.

(a)

(b)

Figure 5.5 (a) Optical texture of the nematic phase in H-bonded polymer complex P2/S4 observed by POM at 130 °C (cooling) and (b) XRD intensity against angle profiles obtained from H-bonded polymer complexes P1/S1 and P2/S4 at 130 °C (in the nematic phase).

0 10 20 30

P2/S4 P1/S2

Intensity (a.u.)

2 theta

5.3.3 Optical Properties

The UV-visible absorption spectra of H-acceptor polymers P1-P2 and H-donor dyes S1-S4 (in both THF solutions and solid films), and H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 (in solid films) are displayed in Figures 5.6 and 5.7, and their photophysical properties are demonstrated in Table 5.2. The absorption energy band-gaps of H-bonded polymer complexes (P1/S1-P1/S4 and

P2/S1-P2/S4) could be easily tuned by the introduction of H-donor dyes (S1-S4), and their absorption spectra covered broad wavelength ranges for both solutions and solid films. As shown in Figure 5.6, the maximum absorption wavelength (λabs) of H-acceptor polymers P1-P2 in THF solutions and solid films were 385 and 393 nm, respectively, which were mainly contributed from the PBB units. The maximum absorption wavelength (λabs) of H-donor dyes S1-S4 in THF solutions were in the range of 458-462 nm (in THF solutions) and 471-490 nm (in solid films). Due to the interchain association and π-π stacking of these polymers and dyes in solids, the absorption spectra of all H-acceptor polymers and H-donor dyes in solid films were generally larger than those in dilute solutions (i.e., 8 nm red shifts in polymers and 11-30 nm red shifts in dyes). After complexation (in solid films as shown in Figure 5.7), H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 displayed blue-shifted absorption peaks (at 440-462 nm) in contrast to H-donor dyes S1-S4. The

blue shifted absorption (blue shifted wavelength Δλabs = 19-39 nm) was due to the dilution effect of H-acceptor polymers as solid solvents for dyes (as solutes) in solid H-bonded polymer complexes. Compared with the H-bonded complexes containing fluorene-linked dyes (S1 and S2), the corresponding H-bonded complexes containing bithiazole-linked dyes (S3 and S4) have longer absorption wavelengths and thus to have lower optical band-gaps, which were originated from the smaller optical band-gaps of bithiazole-linked dyes (S3 and S4) in solid films. Therefore, the H-bonded complexes containing bithiazole-linked dyes (S3 and S4) might have the gifts of lower optical band-gaps for further good performance in photovoltaic properties. However, due to the lack of supramolecular interactions in polymer blend

P1/S1P and a larger aggregation of the acid-protected dye S1P, a red-shifted (33 nm)

absorption in the solid film of polymer blend P1/S1P than that of H-bonded polymer complex P1/S1 was observed (see Table 5.2).

The photoluminescence (PL) spectra of H-acceptor polymers P1-P2, H-donor dyes S1-S4, and H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4 (in solid films) are summarized in Table 5.2. Similar to the UV-visible absorption spectra, the PL emission wavelengths of H-bonded polymer complexes P1/S1-P1/S4 and

P2/S1-P2/S4 (at 611-645) nm were all blue-shifted in contrast to those of H-donor

dyes S1-S4 (at 631-677 nm). The PL emission spectra (in solid films) of the

H-acceptor polymers P1 and P2 were dramatically quenched by adding H-donor dyes

S1-S4 in the H-bonded polymer complexes P1/S1-P1/S4 and P2/S1-P2/S4. The corresponding optical quenching properties of these H-bonded complexes in solid films, including the broad optical absorptions and low optical band-gaps, proposed the potential applications in photovoltaic cells.

Table 5.2 Absorption and Photoluminescence Spectral Data of H-Acceptor Polymers (P1-P2), H-Donor Dyes (S1-S4), Acid-Protected Dye S1P, H-Bonded Polymer Complexes (P1/S1-P1/S4 and P2/S1-P2/S4), and Physical Blend P1/S1P

Compound λabs,sola (nm) λabs,filma (nm) λPL,film (nm)

a Absorption and PL emission spectra were recorded in dilute THF solutions.

(a)

(b)

Figure 5.6 UV-visible absorption spectra of H-acceptor polymers P1-P2 and H-donor dyes S1-S4 (a) in THF solutions and (b) in solid films.

300 400 500 600 700

0.0

300 400 500 600 700

0.0

(a)

(b)

Figure 5.7 UV-visible absorption spectra of (a) H-bonded polymer complexes P1/S1-P1/S4 and (b) H-bonded polymer complexes P2/S1-P2/S4 in solid films.

300 400 500 600 700

0.0

300 400 500 600 700

0.0