Thermal Behavior of Aromatic Polymers Derived from Phenols Related

Chapter 26

Downloaded by EAST CAROLINA UNIV on April 7, 2018 | https://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0489.ch026

Thermal Behavior of Aromatic Polymers Derived from Phenols Related to Lignin S. Hirose , H. Yoshida , T. Hatakeyama , and Hyoe Hatakeyama 1

3

2

1

Industrial Products Research Institute and Research Institute for Polymers and Textiles, 1—1—4 Higashi, Tsukuba, Ibaraki 305, Japan Tokyo Metropolitan University, Fukazawa, Setagaya-ku, Tokyo 158, Japan 1

2

3

Two aromatic polyethers having phosphine oxide groups were synthesized from bisphenols having p-hydroxyphenyl groups. The p-hydroxyphenyl group is known to be a lignin-related structure. The molecular relaxation of amorphous polyethers having 2,2-diphenyl propane units (I) and those having 4,4'-biphenyl units (II) was stud ied by differential scanning calorimetry (DSC) and dy namic mechanical analysis ( D M A ) . Enthalpy relaxation was observed for the samples annealed at temperatures below glass transition temperatures (T 's) in D S C mea surements. The value of relaxation time (τ) of polyether I was shorter than that of polyether II. The activation en ergies (E 's) of enthalpy relaxation were 250 k J / m o l for polyether I and 410 k J / m o l for polyether II. Three relax ations, α-, ß- and γ-relaxations, were observed at around 473K, 273K, and 173K, respectively, i n DMA measure ments. The E for each relaxation was calculated using the relationship between relaxation temperature and fre quency. α-relaxation is attributed to long-range molecu lar relaxation of the main chain. ß-relaxation is related to local mode relaxation of the main chain. γ-relaxation is assumed to be due to the rotation of phenyl groups in the side chain. It was concluded that the mobility of the main chain of polyethers I and II affects enthalpy relaxation and ß-relaxation and that the 2,2-diphenyl propane units in polyether I are more mobile than the 4,4-biphenyl units in polyether II. g

a

a

0097-6156/92AM89-0385$06.00A) © 1^92 American Chemical Society

Glasser and Hatakeyama; Viscoelasticity of Biomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

386

VISCOELASTICITY OF BIOMATERIALS

Aromatic polymers having phenylene groups in their main chain have been recognized as high-performance polymers due to their excellent thermal and mechanical properties. Recently, aromatic polyesters (1,2) were synthesized in our laboratory from bisphenols which were derived from phenols having core structures of lignin such as p-hydroxyphenyl, guaiacyl, and syringyl groups. We have studied these polymers with reference to the relationships between the chemical structures and thermal properties of the polymers. It has been found that the existence of methoxyl groups attached to the phenylene groups does not reduce the starting temperatures of thermal decomposition (TVs) of polymers, and also that the glass transition tem peratures (T 's) of polymers with guaiacyl groups are lower than those of polymers with syringyl groups. We also studied aromatic polyethers (3) having phosphine oxide groups which were synthesized from bisphenols having p-hydroxyphenyl groups. These polyethers were found to have T 's higher than 450K and T 's higher than 760K. Recently, attention has been paid to the relaxation process of aromatic polymers having high T / s in the glassy state. This is because the physical properties of glassy polymers are markedly affected by the molecular relaxation at a temperature lower than T . A t the same time, it has been suggested that the rate of enthalpy change is related to the local-mode relaxation of polymers, especially in the case of polymers hav ing rigid phenylene groups in their main chain. In the present study, the relaxation process of polyethers having p-hydroxyphenylene groups, such as 2,2-diphenyl propane units and 4,4'-biphenyl units, was studied using differential scanning calorimetry (DSC) and dynamic mechanical analysis ( D M A ) in order to establish the relationship between chemical structure and relaxation behavior.


g

g

d

g

Experimental Samples. Aromatic polyethers having phosphine oxide groups were pre pared according to the procedure reported previously (3). The chemical structures of the polyethers are shown in Table I. Measurements. A Perkin Elmer differential scanning calorimeter D S C II and a Seiko thermal analysis system SSC 5000 were used in the measure ments for enthalpy relaxation. The sample weight was ca. 3 mg and the heating rate was 1 0 K / m i n . The sample was sealed in an aluminum vessel and heated to a temperature higher than T -f 50K, quenched to 300K and then reheated at 1 0 K / m i n . The quenched samples were annealed at various temperatures and times in the D S C holder. Heat capacity (C ) (4) and T were measured as reported previously (5). Dynamic mechanical analysis ( D M A ) was carried out using a Seiko Dynamic Mechanical Spectrometer S D M 5600 equipped with a Tension Module D M S 200. Films 20 m m i n length, 3 m m in width, and 0.25 m m in thickness were used for D M A measurements. The initial stress was 3.4 χ 10 P a . The temperature was controlled from 120 to 520K. The measurements were carried out under a nitrogen atmosphere at a heating rate of 2 K / m i n and at a frequency of 1, 2, 5 and 10 Hz. g

p

s


g

26.

Thermal Behavior of Aromatic Polymers

HIROSE ET AL.

387

Table I. The chemical structures of the polyethers


Abbreviation

Chemical Structure

Ο

QH

(Q)

CH

3

3

Results and Discussion Figure 1 shows the C curves of polyethers I and II. A jump in T is clearly observed. Figure 2 shows representative D S C curves of the quenched and annealed samples of poly et her I. The endothermic peak shown in Figure 2 increased with increasing annealing time. The enthalpy of the equilibrium state at a temperature below T was estimated by assuming that the C in the liquid state could be interpolated to T — 50K. Based on the above assumption, the excess enthalpy (ΔΗ ) of the sample can be defined as follows: p

g

g

p

g

0

AH

= /

0

9

C i(Ti)dT

-

p

/

JT

(1)

C (1\)dT

9

pg

JT

a

a

where C i is the C i n the liquid state, C is that at the glassy state of the sample immediately after quenching from the liquid state, T = T-. In the present study, a = 15K was used. Instead of equation 1, the equation AH = AC x a can be used (6), where AC = C j—C at T . The enthalpy difference between the annealed glass and the quenched glass can be obtained from the experimental data as follows: p

p

pg

a

0

P

p

pg

g a

P

g

J

fTg+a

ί

T +a

f

C {Ti)dT-

g

/

pa

T -a

(2)

C {Ti)dT pq

JT -a

g

g

where C is the heat-capacity of the annealed glass and C is that of quenched glass. The total excess enthalpy of an annealed sample, AH can be obtained from equation 3: pa

pq

U

AH

t

= AH

0

-

AH

t


(3)



388


26. fflROSE E T AL.

Thermal Behavior of Aromatic Polymers 389

When the annealing time increases, AH increases and AH decreases; that is, the state of the sample approaches the equilibrium state. The rate of this change and the relaxation time can also be calculated from equation 4: a

AH

t

0

(4)

= AH exp(-t/r) q

where AH is the enthalpy of quenched glass and t is the annealing time. The value of AH is almost the same as that of AH . The C data indicated in Figures 1 and 2 were used in order to cal culate AH suid AHt using equations 2 and 3. r values can be obtained using equation 4. Figure 3 shows the relationship between r and AH of polyether I. Using the relationship shown in Figure 4, the relaxation time at AHt/AH = 0.5 (τχ/2 value) can be evaluated for each annealing tem perature. Figure 4 shows the relationship between Τχ/ and (T — T„) for two polymers. The activation energy (E ) of enthalpy relaxation was also calculated from the relationship between Τχ/ and 1/T by assuming that the enthalpy relaxation will proceed according to Arhenius' kinetics equa tion τχ/2 ~* = Aexp(-E /RT). The plots of r / vs. 1/T are shown in Figure 5. A s shown i n Figure 4, the r i / values for polyether I are smaller than those for polyether II. The calculated E s were 250 k J / m o l for polyether I and 410 k J / m o l for polyether II. It has been suggested that the inter nal rotation of the main chain is related to the enthalpy relaxation of the amorphous chain, since molecular rearrangement of long-range order will not occur at a temperature below T . The difference in Τχ/ and in E values between polyether I and II suggests that the molecular relaxation is accelerated in the presence of 2,2-diphenylpropane units in the main chain. Figures 6 and 7 show the changes of dynamic Young's modulus E ' and dynamic loss t a n j as a function of temperature. These results were obtained by the measurements at 1 H z . The dynamic modulus E ' of polyether I de creased slightly at around 170 and 270K and markedly at around 470K. The change in E ' for polyether II was almost the same as that for polyether I. The t a n i curves show two small peaks and a large peak. The Greek char acters α, /?, and 7 indicate each tano peak from the high temperature side. Table II indicates the temperatures of t a n i peaks measured at 1 Hz and the calculated activation energies (E 's). The calculated Ea& of the enthalpy relaxation are also listed in Table II. Concerning α-relaxation, E ' markedly decreases i n the temperature range of the α-tanJ peak. A t the same time, E of α-relaxation shows a high value of ca. 800 k J / m o l . In addition to this, glass transition was observed i n this temperature region in D S C measurements. Therefore, it is reasonable to consider that the a relaxation is glass transition which is related to the long-range molecular motion of the main chain. 7-relaxation was observed in the t a n i curve at around 170K for polyethers I and II. The calculated E 's are ca. 35 k J / m o l as shown in Table II. It has been reported (7,8) that polymers having phenyl groups in q

q

0

p

a


t

0

g

2

a

2

a

x

2

2

y

a

g

2

a

a

a


a

390


10

1

10° \

438K


T/hr

\

10

-1

\

442

\ 445 10

-2

0

1

1

1

1

2

3

AH / J g t

Figure 3. Relationship between r and Ht of polyether I.

0

10 20 Tg-T / Κ

30

a

Figure 4. Relationship between relaxation time ( Τ Ϊ / ) and (T polyethers I and II. 2

g

—T)


a

of


26.

H I R O S E ET A L .


22

2.3 1000/T/K'

1

Figure 5. Relationship between relaxation time ( r i / and 1/T of poly ethers I and II. 2

120

220 Τ

320 / Κ

420

520

Figure 6. The changes of dynamic Young's modulus E ' and dynamic loss tan6 as a function of temperature of polyether I. The curves were obtained at 1 H z .


391


392


120

220

320 Τ /

420

520

Κ

Figure 7. The changes of dynamic Young's modulus E ' and dynamic loss t a n i as a function of temperature of polyether II. The curves were obtained at 1 H z .


26. fflROSE ET A L


Table II. The t a n i peak temperatures and calculated activation energies

Thermal Behavior of Aromatic Polymers Derived from Phenols Related

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