Phase Characterization

The thermal properties, including glass transition temperatures (Tg) and isotropization temperatures (Ti) with corresponding enthalpies, of these copolymers and H-bonded complexes, determined by DSC and POM, are shown in Table 3.2. The glass transition temperatures of copolymers P1-P6 and H-bonded complexes

PBT1/P7, PBT1/P8, and PBT1/P9 (H-acceptor homopolymer PBT1 blended with

H-donor homopolymers P7-P9 in equal molar ratios) are ca. 22 ~ 57 °C. Moreover, all self-H-bonded copolymers P1-P3 containing H-donors have higher Tg and Ti values than their corresponding non-self-H-bonded copolymers P4-P6, which do not contain H-donors. This is because the self-H-bonded cross-linking polymer structures, in copolymers P1-P3, have a longer rigid core than the non-H-bonded side-chain polymer structures in acid-protected copolymers P4-P6. Both Tg and Ti values of H-bonded complexes PBT1/P7, PBT1/P8, and PBT1/P9 have a similar tendency (i.e.,

PBT1/P7 > PBT1/P8 > PBT1/P9) as those of self-H-bonded copolymers P1-P3

(para-acid-substituted copolymer P1 > meta-acid-substituted copolymer P2 >

ortho-acid-substituted copolymer P3). Overall, the para-substituted copolymer P1

and the H-bonded complex PBT1/P7 exhibit the highest Tg and Ti values because they have the most linear H-bonded (cross-linking) structures.

Because of the linear H-bonded structures of self-H-bonded copolymer P1 and

H-bonded complex PBT1/P7 (containing para-acid-substituted homopolymer P7), the smectic phase was only generated in the para-acid-substituted copolymer P1 with a mesomorphic range of 92 °C and in the H-bonded complex PBT1/P7 with a mesomorphic range of 114 °C. The H-bonded complex PBT1/P7 had a wider mesophase range than self-H-bonded copolymer P1, because the H-bonded complex (PBT1/P7) contains highly ordered H-bonded mesogens (from homopolymer blends) in the smectic arrangement. Due to the same reason of highly ordered H-bonds in H-bonded complexes, the nematic phases observed in nonlinear H-bonded structures (with non-para-acid-substituted H-donors) of H-bonded complexes PBT1/P8 and

PBT1/P9 were wider than those of self-H-bonded copolymers P2-P3. This result suggests that the steric factor related to the isomeric acid-substituted positions in H-donors is decisive for the formation of the mesomorphism in all H-bonded structures. In addition, the acid-protected copolymers P4-P6 (which lack H-bonds) possess the nematic phase adopted from the H-acceptor PBT moieties (with the nematic phase between 42°C and 71°C), which is similar to observations with the homopolymer PBT1. However, non-self-H-bonded copolymers P4-P6 have lower phase transition temperatures (including the isotropization temperature Ti) than homopolymer PBT1 due to the dilution effect of the acid-protected monomer moieties (M4-M6) in copolymers P4-P6.

To further elucidate the mesomorphic behavior in Table 3.2, POM and XRD measurements were performed at the mesophasic ranges of all copolymers and H-bonded complexes. The mesophases of copolymer P1 and H-bonded complex

PBT1/P7 were characterized by POM. To confirm the formation of

supramolecular structures, the smectic layer arrangements for copolymer P1 and H-bonded complex PBT1/P7 were characterized by d-spacing values of XRD measurements. The theoretical value of the fully-extented molecular length of copolymer P1 is 57.8 Å, estimated using CS ChemOffice. The d-spacing values of copolymer P1 and the H-bonded complex PBT1/P7 at 130°C are 50.3 and 52.4 Å, respectively (Figure 3.3(a)). These XRD measurements support the hypothesis that the copolymer P1 and H-bonded complex PBT1/P7 are suitable to be identified as the tilted smectic C phase with tilt angles of 29.6° and 25.0°, respectively. On the other hand, the absence of d-layer spacing peaks, for copolymers P2-P6 and H-bonded complexes PBT1/P8 and PBT1/P9, indicates that these species possess the nematic phase. For instance, Figure 3.3(b) shows the XRD intensity against angle profiles obtained in the nematic phase of copolymer P2 at 70 °C.

Table 3.2 Thermal Properties of Polymers and H-Bonded Homopolymer Complexes

polymer/complex Phase transitions, °C^a PBT1^b glass 51 N 145^c I

P1 glass 57 Sc 149 (3.7) I P2 glass 52 N 80^c I P3 glass 42 N 69^c I P4 glass 22 N 58^c I P5 glass 22 N 47^c I P6 glass 24 N 45^c I PBT1/P7^d glass 53 Sc 167 (4.3) I PBT1/P8^d glass 30 N 94(1.8) I PBT1/P9^d glass 24 N 80^c 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 Phase transitions of PBT (monomer of homopolymer PBT1): K 42^c N 71^c I.

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

d No detectable Tg temperatures (by DSC) were observed in homopolymers P7-P9.

(a)

(b)

Figure 3.3 (a) X-ray diffraction (XRD) patterns of copolymer P1 and H-bonded complex PBT1/P7 in the Sc phase at 130°C. (b) XRD intensity against angle profiles obtained in the nematic phase of copolymer P2 at 70°C.

10 20 30

Intensity (a.u.)

2 theta

10 20 30

PBT1/P7 d₀₀₁=52.4

Å

d₀₀₁=50.3

Å

Intensity (a.u.)

2 theda

P1

3.3.4 Optical Properties

The absorption and PL spectral data of all copolymers P1-P6 (in both THF solutions and solid films) and H-bonded complexes PBT1/P7, PBT1/P8, and

PBT1/P9 (in solid films) are summarized in Table 3.3 and Figure 3.4. As shown in

Table 3.3, the maximum absorption peaks of copolymers P1-P6, near 350 nm (in THF solutions), are mainly attributed to the PBT units. In Table 3.3 and Figure 3.4, PL emissions at 444-447 nm (in THF solutions) and at 515-529 nm (in solids for self-H-bonded copolymers P1-P3) are all red-shifted in contrast to PL emissions of acid-protected copolymers P4-P6 (without H-bonds) at 429-431 nm (in THF solutions) and at 466-472 nm (in solids), respectively. Therefore, compared with PL emissions of non-self-H-bonded copolymers P4-P6, the H-bonding effects on PL emission wavelengths (λmax) of self-H-bonded copolymers P1-P3 are further enhanced in solids (red-shifted 43-63 nm) than in solutions (red-shifted 15-16 nm). However, due to the PL quenching effect by the aggregation of longer self-H-bonded chromophores in copolymers P1-P3, the acid-protected copolymers P4-P6 (without H-bonds) exhibit higher PL quantum yields (in both solutions and solid films) than the self-H-bonded copolymers P1-P3. For all copolymers (P1-P6), it is apparent that more π-π stacking and molecular aggregation occur in solid films than in solutions. The aggregation phenomena are even more pronounced for P1-P3 in solid films due to the

self-H-bonded structures.

On the basis of ourfindings, non-luminescent acids are important for inducing various wavelength shifts in PL emissions of luminescent H-acceptors by tuning the pKa values of H-donors in self-H-bonded copolymers and homopolymer complexes.

The H-donor moieties with different pKa values, i.e., para-H-donor (M1): pKa ~ 4.36;

> meta-H-donor (M2): pKa ~ 4.19; > ortho-H-donor (M3): pKa ~ 4.15,⁹⁵ offer a solid solvent environment in the H-bonded copolymers and complexes. Therefore, the PL emission wavelengths of H-bonded complexes in solid films are related to the pKa values as follows: PBT1/P7 < PBT1/P8 < PBT1/P9. Additionally, in contrast to self-H-bonded copolymers P2-P3, the para-substituted acid has the smallest acidity, and therefore the weakest H-donor effect which induces the smallest red-shifted PL emission in the solid film of copolymer P1. However, both acidic and steric effects of isomeric H-donors play important roles on the PL properties of H-bonded structures.

The molecular π-π stacking in the H-bonded bending structures consisting of meta- and ortho-substituted H-donors were suppressed owing to the steric hindrance of meta- and ortho-substitution. Thus, the PL emission wavelengths of copolymers P2

and P3 in solid films have the following order: meta-P2 (529 nm) > ortho-P3 (523 nm), which is in the reverse order of acidity. In general,the results demonstrate that similar red-shifted PL emissions occur when isomeric H-donors are complexed to

form supramolecular structures in both self-H-bonded copolymers (P1-P3) and H-bonded homopolymer complexes (PBT1/P7-P9). Moreover, the acidities and steric structures of the isomeric H-donors do affect the molecular packing and PL emission wavelengths of the H-bonded supramolecular architectures. Therefore, the λmax values, which correspond to the emission colors, of the photoluminescence in the supramolecular systems can be tuned not only by adjusting theemitting cores but also by changing the isomeric structures of the H-donors.

Table 3.3 Absorption and PL Emission Spectral Data of Polymers and H-Bonded Homopolymer Complexes in THF Solutions and Solid Films

absorption

solution^a film solution^a film

Φ (%)

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

b PL emissions were excited at the maximum absorption peaks.

c PL quantum yield of 9,10-diphenylanthrance in THF (10^-6 M) as the reference quantum yield in solutions.

d PL quantum yield of 9,10-diphenylanthrance blended in PMMA as the reference quantum yield in solid film.

(a)

(b)

Figure 3.4 (a) PL spectra of copolymers P1-P6 in THF solutions (b) normalized PL spectra of copolymers P1-P6 and H-bonded homopolymer complexes PBT1/P7, PBT1/P8, and PBT1/P9 in solid films.

450 500 550 600 650

PL intensity (a.u.)

400 450 500 550 600

P1

3.3.5 Fluorescence Quenching Effects of Copolymers by