Hybrid Liquid-Crystalline Block Copolymers - American Chemical

Chapter 19

Hybrid Liquid-Crystalline Block Copolymers 1

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1

2

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M. Laus , M. C. Bignozzi , A. S. Angeloni , G. Galli , E. Chiellini , and O. Francescangeli Downloaded by NORTH CAROLINA STATE UNIV on October 9, 2012 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch019

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Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna, 40136 Bologna, Italy Dipartimento di Chimica e Chimica Industriale, Università di Pisa, 56126 Pisa, Italy Dipartimento di Scienze dei Materiali e della Terra, Università di Ancona, 60131 Ancona, Italy

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The synthesis and some thermal properties of three series of block copolymers comprising both main-chain and side-chain liquid-crystalline (LC) blocks in the same macromolecular structure are described. The former block is a semiflexible LC polyester (block B), and the latter is an LC polymethacrylate (block A) containing a variously substituted mesogenic unit. The two structurally different blocks were partly phase -separated within the glassy and LC states and underwent distinct phase transitions. Significant deviations of the transition enthalpies relative to those of the corresponding homopolymers suggest the occurrence of a more or less diffuse interphase which may depend on the nature of the mesophase formed.

Block copolymers exhibit unique characteristics in that they are able to self-order to multiphase domain structures of submicron scale with various morphologies because of the relative incompatibility of the different blocks. On the other hand, the liquid crystalline (LC) mesophases provide an additional example of a state of matter characterized by non-crystalline order. The combination of these two different aspects into one single macromolecular architecture leads to block copolymers containing L C blocks (7). These materials can be valuable in elucidating specific aspects of polymer physics and, in addition, may be employed as highly versatile interfacially active additives and viscosity improvers potentially capable of providing enhanced optimization of material processing and performance. By a synthetic procedure involving the use of azo macroinitiators (2), we have started to prepare and study new block copolymers consisting of semicrystalline/side-chain L C blocks (3) and amorphous/main-chain L C blocks (4-6).

0097-6156/96/0632-0332$15.00/0 © 1996 American Chemical Society In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Hybrid Liquid-Crystalline Block Copolymers

333

Very recently, the first examples of hybrid block copolymers comprising in the same macromolecule both main-chain and side-chain L C blocks (7) have also been described. To gain a better insight into the effects of the macromolecular architecture on the phase transition behavior of this last class of block copolymers, in the present contribution we report on the synthesis and properties of three series of main-chain and side-chain L C block copolymers 1-3, characterized by the following general structure:

0(CH fcO

- ^ ~ ^ C O O — ^ - 0 ( C H

Downloaded by NORTH CAROLINA STATE UNIV on October 9, 2012 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch019

2

2

)

W

- 0 - ^ ^ C O O ( C H

2

)

C H

3

C=0 • f C-CH CH. 1

2

+ x

ι

—

5

~

O O C - ^ ^

—

LOOC-^^O(CH ) 2

1

0

-

O—Çy~CO -

Block copolymers 1-3 m

3

5

series

Block copolymers 1-3 are constituted by a side-chain L C polymethacrylate block, containing a variously substituted mesogenic unit, and a main-chain L C polyester block made up by two mesogenic p-oxybenzoyl diads alternatively interspaced by aliphatic chains of five and ten methylene groups connected to the mesogenic cores by two ester and ether linking groups, respectively. The side-chain polymethacrylate homopolymers 4, 5 and 6, structurally analogous to the side-chain block, exhibited a nematic mesophase (4), smectic A (Sa) and nematic mesophases (5) and a Sa (6) mesophase.

CH

3

m -E-C-CH

2

-3x

I

series

0

3

5

4

5

6

c=o 0(CH ) O _ ^ ~ ^ - C O O — Q - 0 ( C H ) C H 2

TF

2

M

3

Polymethacrylates 4-6

In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

334

LIQUID-CRYSTALLINE POLYMER SYSTEMS

COO(CH ) OOG 2

L-OOC—I

5

>-0(CH ) 0 2

1()

-O-coy

Homopolyester C C


5

1 0

Polyester homopolymer C5C10, structurally analogous to the main-chain block, formed nematic and smectic C mesophases (8,9). Within each series, the block copolymers are designated with a letter from a to d referring to the different amount of the methacrylate monomer present in the feed mixture. Experimental Part Methacrylate monomers 10-12 were prepared by a literature procedure (10). Macroinitiator M-C5C10 was prepared according to the procedure reported in (7). Block copolymer series 1-3 were prepared following the synthetic route illustrated in Scheme 1. Synthesis of Block Copolymers. In a typical copolymerization reaction, the required amount of the methacrylate monomer and 0.4 g of the macroinitiator M C5C10 were dissolved in 10 mL of anhydrous THF. The reaction mixture was introduced into a Pyrex glass ampoule, freeze-thaw degassed and then sealed under vacuum. After reacting for 20 h at 70 °C, the copolymer was recovered by addition of a ten-fold excess of methanol and purified from oligomers by extraction with boiling methanol in a Kumagawa extractor. The copolymer was then dried in vacuum for 24 h. The copolymerization yield was in the 60-70% range. Four copolymer samples were synthesized according to the above procedure by keeping constant the amount of the macroinitiator (0.4 g) and using different quantities of methacrylate in the feed mixture: 0.3 g, a; 0.7 g, b; 1.0 g, c; 2.0 g, d. 1 3

Physicochemical Characterization. *H N M R and C N M R spectra were recorded with a Varian Gemini 200 spectrometer. The composition of the copolymers was determined from the H N M R spectra. Molar mass characteristics were determined by size exclusion chromatography (SEC) of chloroform solutions with a 590 Waters chromatograph equipped with a Perkin Elmer U V detector using a X

4

1 0 Â Polymer Laboratories column. Differential scanning calorimetry (DSC) analyses were carried out under dry nitrogen flow with a Perkin-Elmer DSC 7 apparatus. X-ray diffraction photographs were taken on a Rigaku-Denki RU300 rotating anod generator equipped with a pin hole flat camera. Ni-filtered C u K radiation was used. a


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Results and Discussion Synthesis. Block copolymer series 1-3 were synthesized by sequential polycondensation and free-radical polymerizations as illustrated in Scheme 1. In the first step, the polyester macroinitiator M-C5C10, containing a 10 mol-% of 8 with respect to the total content of diacid chlorides 7 and 8, is prepared which is then used to initiate the polymerization of methacrylates 10, 11 and 12 through the thermal decomposition of the azo group at 70 °C. Three block copolymer series (Table I), consisting each of four samples, were prepared by reacting the macroinitiator with the three methacrylate monomers, and using within each series, different amounts of methacrylate in the feed mixture. The content of the main-chain block, as evaluated by U N M R , ranges from 19 to 72 wt.-%. In addition, a sample of M-C5Q0 was thermally decomposed at 70 °C in the presence of a large amount l

CH

CH

3

I I

CIC-^-O(CH ) 2

1 0

3

O — 0 - < j CI + C 1 C ( C H ) C - N = N - C ( C H ) C C 1 + 2

II Ο

Οο

ο

2

2

I CN

I CN

2

η υ

8

NaOH M-C C 5

Ο

Ο

9

CH

I

M-C C 5

+

C

1 0

BTBAB

3

70°C =

C

1 0

H

Block copolymers 1-3

2

c=o 0(CH ) 0 2

0

0

0

°(CH ) CH

6

2

10-12

m methacrylate

0

3

10

M

3

5

11 12

SCHEME 1. Synthesis of block copolymers 1-3.

of 2,6-di-ter^butyl-4-methylphenol. The resulting polyester C5C10 was also studied as a model of the main-chain block in the copolymers. The molecular characteristics of the polymers and copolymers were studied by SEC. M = 11500 and M / M = 2.3 for M-C5C10, while M = 4800 and M /M = 1.9 for C5C10 were found using the universal calibration method. Within each series, M increases as the concentration of n

n

w

w

n

n


n

336



Table I. Composition and molar mass data of macroinitiator M-C5C109 polyester Ç5C10, and block copolymers 1-3

a

Sample

C Cio (wt.-%)

M-C5C10 C5C10

100 100

11,500 4,800

2.3 1.9

la lb lc Id

72 49 36 24

19,000 33,000 38,500 50,000

3.5 2.2 2.5 3.0

2a 2b 2c 2d

65 45 32 29

30,000 32,000 43,000 37,000

2.4 2.1 2.3 2.5

3a 3b 3c 3d

72 47 28 19

28,000 36,000 34,500 50,000

2.4 2.2 2.7 3.0

M

a

5

M /M

b

w

n

b n

Polyester block C5C10, by H NMR. B y SEC, in chloroform at 25 °C. l

b

Table Π. Liquid-crystalline properties of polyester C 5 C 1 0 , block copolymers la-d and polymethacrylate 4 3

Sample

C5C10 la lb lc Id 4 a

A,b I-N (K)

T A

A A,b I-N q

1

(Jg- )

377 378 378 377 378

1.3 1.9 1.5 1.6 2.1 1

A

N-Sc (K) 411 408 408 408 408

-

B,b I-N (K) 430 424 426 426 427

T A

-

B

b

N-Sc (Jg- ) 4.7 3.1 2.7 3.3 4.3

AH ' I-N (Jg" ) 2.9 1.9 1.5 1.6 1.2

-

-

A H

1

1

b

B y DSC, at -10 K m h r scanning rate. A : polymethacrylate block; B: polyester block. from 30,000 to 43,000 for series 2 and from 28,000 to 50,000 for series 3. A f / M values comprised between 2.2 and 3.5 were usually observed. It is well known (77,72) that the free-radical polymerization of methacrylate monomers terminates by a disproportionation mechanism. Accordingly, in the present case starting from macroinitiator chains containing one reactive azo group, A B diblock copolymers were w


n

19. LAUS ET AL.


337


formed, but additionally A B A triblock copolymers could also derive from macroinitiator chains containing more than one reactive azo group.

Thermal Behavior. The L C behavior of block copolymer series 1-3 was studied by DSC, polarizing microscopy and X-ray diffraction. The relevant phase transition parameters were taken from the DSC cooling curves, on account of the better resolution of the transition peaks (Tables Π-IV). Polymethacrylates 4, 5 and 6 were amorphous and formed a nematic mesophase, a nematic and a smectic A mesophase and a smectic A mesophase, respectively. Polyester C5C10 was also amorphous and exhibited smectic C (Sc) and nematic (N) mesophases. The DSC traces of all block copolymers showed two enthalpic peaks associated to the isotropic-nematic and nematic-smectic C transitions of the main-chain block and one or two enthalpic peaks relevant to the L C transition of the side-chain block. Two glass transitions, sometimes not well resolved, were also observed in the 20-30°C and 20-60°C regions attributed to the main-chain and side-chain block respectively. The mesophase nature attribution is confirmed by X-ray diffraction analysis. As a typical example, Figure 1 illustrates the DSC cooling curve of block copolymer 2c with two insets. The low temperature inset shows the small angle region of the X-ray diffraction spectrum at temperatures in which the smectic mesophase of the main-chain block (interlayer spacing d =20 Â) coexists with the smectic mesophase of the side-chain block (interlayer spacing d = 29 Â). The high temperature inset represents the X-ray pattern of the smectic mesophase of the polyester block, which coexists with the isotropic phase of the side-chain block. The observation of two distinct signals relevant to both smectic mesophases of the main-chain and side-chain blocks clearly indicates that the chemically different blocks are phase-separated. Figure 2 represents collectively the phase transition temperatures of both the main-chain and the side-chain blocks in the three series as functions of the main-chain block content. The phase transition temperatures of the main-chain block are quite constant, whereas the phase transition temperatures of the side-chain block slightly decrease as the main-chain block content increases. This decrease is probably connected to the parallel decrease of the side-chain block length. The molar mass of the polyester block is constant throughout the series, whereas the molar mass of the polymethacrylate block increases from samples a to samples d in each of the three series. Accordingly, the evolution of the transition parameters of the main-chain block can be taken as a better marker of the level of interaction between the chemically different blocks. Considering the phase transition temperatures of both the chemically different blocks, it should be noted that in specific temperature ranges the smectic and the nematic mesophases of the main-chain block coexist with a nematic and an isotropic phase in series 1, with a smectic, a nematic and an isotropic phase in series 2 and with a smectic and an isotropic phase in series 3 thus producing a variety of different interphases or interfaces. In particular, it appears that the smectic or, at higher temperature, the nematic domains of the main-chain blocks are surrounded by the isotropic domains of the side-chain blocks.



a

C5C10 2a 2b 2c 2d 5 B y DSC, at

Sample

-1

A

1

b

AH ' I-N (Jg- )

b

T A

A,b N-Sa (K) 1

(Jg" )

N-Sc (K) 411 408 410 412 411

A

B,b I-N (K) 430 427 429 428 428

T A A H

a

A,b I-Sa (K)

T A

-1

1

I-Sa (Jg- )

AA1

b

B,b N-Sc (K) 411 408 409 410 410 T A

I-N (K) 430 426 428 427 426

A

1

(jg- ) 4.7 3.2 3.2 3.3 2.7

b

^: sc

B

b

1

I-N (Jg" ) 2.9 1.8 1.7 1.2 0.9

AH >

C5C10 9.8 380 3a 385 15.3 3b 385 13.6 3c 386 15.3 3d 6 388 15.5 B y DSC, at -10 K m i n scanning rate. A : polymethacrylate block; B : polyester block.

Sample

b

1

-

'

-

1

B

I-N (Jg" ) 2.9 2.3 1.9 1.5 1.4

A H

N-Sc (Jg- ) 4.7 3.1 3.2 3.3 3.2

Table IV. Liquid-crystalline properties of polyester C5C10, block copolymers 3a-d and polymethacrylate 6

3

383 2.7 368 1.8 385 3.2 373 2.7 385 3.0 376 3.0 387 3.2 370 3.3 2.8 382 372 3.0 -10 K m i n scanning rate. A : polymethacrylate block; B : polyester block.

-

A,b I-N (K)

T A

21

Table ΠΙ. Liquid-crystalline properties of polyester C 5 C 1 0 , block copolymers 2a-d and polymethacrylate 5



1

FIGURE 1. D S C second cooling curve (10 Kmin" ) for block copolymer 2c and the small angle region of the relevant X-ray diffraction spectra at 340 Κ (low temperature inset) and at 390 Κ (high temperature inset).


340

LIQUID-CRYSTALLINE POLYMER SYSTEMS 440


T/K

20 40 60 80 100 Main-chain block/wt.-%

FIGURE 2. Trends of the isotropic-nematic (open symbols) and nematicsmectic C (full symbols) transition temperatures of the polyester block in block copolymers 1 (ΔM), 2 ( Ο , · ) , and 3 ( • J | ) and of the isotropicnematic (V, copolymers 1; S , copolymers 2), nematic-smectic A (ffl, copolymers 2) and isotropic-smectic A (O, copolymers 3) of the side-chain block as functions of the main-chain block content.

20

40

60

80

Main-chain block/wt.-%

FIGURE 3. Trend of the ratio between the normalized nematic-smectic C and isotropic-nematic transition enthalpies of the polyester block in block copolymers 1 ( A ) , 2 ( · ) , and 3 ( • ) as a function of the main-chain block content.



19. LAUS ET AL.


341

The normalized phase transition enthalpies of the side-chain blocks increase with increasing amount of the side-chain block (see Tables Π-IV). The normalized enthalpy changes associated to the nematic-smectic C and to the isotropic-nematic transition ( A # N - S C and Δ/7Ι-Ν) of the main-chain block as a function of the mainchain block content displays a dual behavior. The former is quite constant, whereas the latter increases regularly as the main-chain block content increases. A better visualization of the distinct behavior of the main-chain block at the smectic C-nematic and nematic-isotropic transitions is obtained by plotting the ratio between the nematic-smectic C and isotropic-nematic normalized phase transition enthalpies of the main-chain block as a function of the main-chain block content for the three block copolymer series as illustrated in Figure 3. In each copolymer series, the ratio Ai/N-Sc/A#i-N decreases as the main-chain block content increases. This behavior strongly points toward a differential effect of the side-chain isotropic melt on the L C mesophase structures, namely the nematic and the smectic C ones, of the main-chain block. As this effect may arise either from the existence of a disordered interface (partial miscibility) between the side-chain and main-chain blocks, or from unfavorable boundary conditions, we conclude that the smectic mesophase generated by the main-chain block results less miscible or less perturbable than the nematic mesophase. In addition, the size of the main-chain domains should depend on the overall block copolymer composition and decrease as the side-chain block content increases. A tentative schematic representation of the interphase boundary situation in the various block copolymer samples is reported in Figure 4.

Conclusion We have synthesized and studied three series of block copolymers comprising both main-chain and side-chain L C blocks within the same polymer structure. Although thermal and X-ray diffraction results show that the two chemically different blocks are at least partly phase-separated, a differential effect of the side-chain isotropic melt on the L C mesophases, namely the nematic and the smectic C ones, of the mainchain block is observed. Analysis of the enthalpic data suggests that the extension of the interdomain boundary region resulting from the coexistence of the isotropic phase of the side-chain block and the nematic phase of the main-chain block is larger than the one relevant to the boundary region resulting from the coexistence of the isotropic phase of the side-chain block and the smectic phase of the main-chain block. In addition, the size of the main-chain domain should depend on the overall block copolymer composition and decrease as the side-chain block content increases.

Acknowledgment. This work was supported by the National Research Council of Italy (Progetto Chimica Fine 2 - Sottoprogetto Material! Polimerici).


342



Main-chain nematic/side-chain isotropic inteiphasic region

Main-chain smectic/side-chain isotropic inteiphasic region

Β FIGURE 4. Schematic representation of the main-chain nematic side-chain isotropic interphasic region (A) and of the main-chain smectic side-chain isotropic interphasic region (B).


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Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12.

Chiellini, E.; Galli, G.; Angeloni, A. S.; Laus, M. Trends Polym. Sci. 1994, 2, 244 and refs. therein. Chiellini, E.; Galli, G.; Angeloni, A. S.; Laus, M.; Bignozzi, M . C.; Yagci, Y.; Serhatli, Ε. I. Makromol. Chem., Makromol. Symp., 1994, 77, 349. Galli, G.; Chiellini, E.; Yagci, Y.; Serhatli, Ε. I.; Laus, M.; Angeloni, A. S.; Bignozzi, M . C. Makromol. Chem., Rapid Commun., 1993, 14, 185. Angeloni, A. S.; Bignozzi, M. C.; Laus, M.; Chiellini, E.; Galli, G. Polym. Bull.(Berlin), 1993, 31, 387. Bignozzi, M . C.; Angeloni, A. S.; Greco, M.; Laus, M.; Chiellini, E.; Galli, G. In Liquid Crystalline Polymers, Carfagna, C., Ed.; Pergamon Press, NY, 1994 p p 61-68. Galli, G.; Chiellini, Ε.; Laus, M.; Angeloni, A. S.; Bignozzi, M . C.; Francescangeli, O. Mol. Cryst. Liq.Cryst.,1994, 254, 429. Galli, G.; Chiellini, E.; Laus, M.; Bignozzi, M. C.; Angeloni, A. S.; Francescangeli, O. Macromol. Chem. Phys., 1994, 195, 2247. Chiellini, E.; Galli, G.; Laus, M.; Angeloni, A. S.; Francescangeli, O.; Yang, B. J. Mater. Chem., 1992, 2, 449. Francescangeli, O.; Yang, B.; Albertini, G.; Angeloni, A. S.; Laus, M.; Chiellini, E.; Galli, G. Liq. Cryst., 1993, 3, 353. Finkelmann, H.; Ringsdorf, H.; Wendorff, J. H. Makromol. Chem., 1978, 179, 273. Heitz, W. Makromol.Chem., Macromol. Symp., 1987, 10/11, 297 Eastmond, G. C. Makromol. Chem., Macromol. Symp., 1987, 10/11, 71.


Hybrid Liquid-Crystalline Block Copolymers - American Chemical

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