Chapter 8
Effect of the Polymer Backbone on the Thermotropic Behavior of Side-Chain Liquid Crystalline Polymers Coleen Pugh and Virgil Percec Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106
Poly(epichlorohydrin) (PECH) and poly(2,6 - dimethyl1,4-phenylene oxide) (PPO) containing pendant mesogenic units separated form the main chain through spacers of zero to ten methylene units were synthesized and characterized in order to test the "spacer concept." Both polymers were modified by phase transfer catalyzed esterifications of the chloromethyl groups (PECH) or the bromobenzyl groups (brominated PPO) with potassium ω-(4-oxybiphenyl) alkanoates and potassium ω-(4-methoxy-4-'oxybiphenyl)alkanoates. While PPO required ten methylene units as a spacer and 4,4'-methoxybiphenyl as mesogen to present thermotropic liquid crystalline mesomorphism, PECH required no spacer. Since Finkelmann and Ringsdorf introduced the spacer concept, i t has been well accepted that a requirement for obtaining thermotropic side-chain l i q u i d c r y s t a l l i n e polymers i s that a f l e x i b l e spacer must be introduced to p a r t i a l l y decouple the mobility of the main chain from that of the mesogenic groups (1-4). Without a spacer, mesophase formation would require that the polymer backbone be s i g n i f i c a n t l y distorted from i t s normal random c o i l conformation. At the same time, the backbone imposes a s t e r i c hindrance on the packing of the mesogens. For this reason, most polymers with the mesogenic groups d i r e c t l y attached to the backbone are amorphous (_5). There are however several exceptions to this rule, most of which are l i s t e d i n Table I, together with their phase transitions. Why i s this? I t must certainly be true that the motions of the mesogen must be decoupled from those of a r i g i d polymer backbone. But what about very f l e x i b l e polymer backbones? We propose that such a backbone may i t s e l f act as a f l e x i b l e spacer. Table I demonstrates that most l i q u i d c r y s t a l l i n e polymers lacking a spacer are formed from a f l e x i b l e polyacrylate backbone. In contrast, the methyl substituent i n polymethacrylate backbones both reduce main chain mobility and imposes additional s t e r i c barriers to mesophase formation. Therefore, successful l i q u i d c r y s t a l l i n e formation of polymethacrylates has been achieved only 0097-6156/88/0364-0097$06.50/0 © 1988 American Chemical Society
98
CHEMICAL REACTIONS ON POLYMERS
Table I . Polymers with Direct Attachment of Mesogens
Polymer
Phase Transitions
-CH -ÇH— 2
-CH -ÇH—
_
2
_
—CH2-CH-
>-cx—
^-°-\>0·
X - H X - H
Κ
R - CN R - C H 5
Χ - CH
3
R - CN
Χ - CH
3
R - c H
n
5
_CH->-CH—
N
R - 0C H 4
S 285 I
7-11
S 205 I
7.8
Κ 81 S 222 I
13
S 270 I S 303 I
12 12
S 240 I
12
S 232 I
12
Κ 88.3 Ν 120.6 I
13
9
R - 0C H 6
1 3
R - CN Χ - H -CH-CX-
ο
X - CH
O-Lo-O-OÎ-r^-OR
™—*
—*
R - C
C H
R
3
3
-CHz-^X—
X - H
°*-0-O-CH=N-O-R w
w
-CHjr-ÇXû4-0-^^t**N-Q-R
Χ - H X - CH
X - CH -CH^X0270 I
5
R - 0C H
n
S >270 I
5
3
5
3
13
Κ 113.8 Ν 140.5 î
S 270 I
R - C00H R - C00H
3
Κ 94.5 S 97.7 Ν 116 I
S 200 I
3 3
9
" 9 19 R - C H η f» IJ R - C H C
R -
χ - CH
H
4
" 3 X - CH \t /»ιι X - CH
X
1 6
R- C H
3
Reference
5
Κ 125.8 I (124.8 C 91 K) 17 Κ 114.8 (111.8 C) 17
8.
P U G H AND PERCEC
Side-Chain Liquid Crystalline Polymers
99
when the anisotropy of the mesogen i s increased by increasing the number of s t i f f and highly polarizable units that i t contains. This in turn enlarges the mesogen, and i s most frequently achieved by attaching a polar substituent para to the polymerizable group of the monomer. A review of the literature demonstrates some trends concerning the e f f e c t of the polymer backbone on the thermotropic behavior of side-chain l i q u i d c r y s t a l l i n e polymers. In comparison to low molar mass l i q u i d c r y s t a l s , the thermal s t a b i l i t y of the mesophase increases upon polymerization (3,5,18). However, due to increasing v i s c o s i t y as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this i s why the isotropization temperature increases up to a c r i t i c a l value as the degree of polymerization increases (18). There are also some trends when looking at the main chain flexibility. Table II demonstrates that when the main chain flexibility decreases from cyanobiphenyl containing polymethacrylates to polysiloxanes, not only does the Tg drop, but the isotropization temperature increases. However, the trend i s the opposite when the mesogen i s methoxyphenyl benzoate (18). Therefore, this e f f e c t of the main chain f l e x i b i l i t y i s s t i l l ambiguous. In order to determine the necessity and/or the length of the spacer that i s required to achieve l i q u i d c r y s t a l l i n e behavior from f l e x i b l e vs. r i g i d polymers, we have introduced mesogenic units to the backbones of a r i g i d [poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)] and a f l e x i b l e [poly(epichlorohydrin) (PECH)] polymer through spacers of from 0 to 10 methylene groups v i a polymer analogous reactions. The synthetic procedure used for the chemical modification of PPO involved i n the f i r s t step the r a d i c a l bromination of PPO methyl groups to provide a polymer containing bromobenzyl groups. The bromobenzyl groups were then esterified under phase-transfer-catalyzed (PTC) reaction conditions with potassium 4-(4-oxybiphenyl)butyrate (Ph3C00K, Ph3C00-PP0), potassium 4- (4-methoxy-4'-oxybiphenyl)butyrate (Me3C00K, Me3C00-PP0), potassium 5-(4-oxybiphenyl)valerate (Ph4C00K, Ph4C00-PP0), potassium 5- (4-methoxy-4*-oxybiphenyl)valerate (Me4C00K, Me4C00-PP0), potassium ll-(4-oxybiphenyl)undecanoate (PhlOCOOK, PhlOCOO-PPO) and potassium ll-(4-methoxy-4'-oxybiphenyl) undecanoate (MelOCOOK, MelOCOO-PPO). The notations between parentheses correspond to the starting potassium s a l t and the resulting functionalized PPO. PECH was modified under similar reaction conditions, except that dimethylformamide (DMF) was used as the reaction solvent. In addition, the phase-transfer-catalyzed e t h e r i f i c a t i o n of the chloromethyl groups of PECH with sodium 4-methoxy -4 -biphenoxide was used to synthesize PECH with d i r e c t attachment of the mesogen to the polymer backbone. Similar notations to those used to describe the functionalized PPO are used for functionalized PECH. In this l a s t case, PPO was replaced with PECH. E s t e r i f i c a t i o n routes of both PPO and PECH are presented i n Scheme I. The attachment of mesogenic units to a polymer backbone v i a polymer analogous reactions i s not a new concept, although they are much less frequently used than the polymerization of mesogen containing monomers. Liquid crystalline polyacrylates, 1
100
CHEMICAL REACTIONS ON POLYMERS
Table I I . Influence o f the Main Chain F l e x i b i l i t y on L i q u i d C r y s t a l l i n e Phase T r a n s i t i o n s f o r Polymers w i t h Cyano-biphenyl as Mesogen
Main
chain
Tg (
o
T i ( C)
Phase
Reference
6
14
166
S
18
9
-1
157
Ν
19
2
50
112
Ν
20
5
40
120
Ν
20
6
35
125
S
18
11
25
145
S
20
2
95
5
60
121
S
20
6
55
100
S
18
11
40
121
S
20
Polysiloxane
Polyacrylate
20
Polymethacrylate
length o f methylenic u n i t s between mesogen and main chain
8.
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101
102
CHEMICAL REACTIONS ON POLYMERS
polymethacrylates, and polyacrylamides have been prepared by conventional e s t e r i f i c a t i o n or amidation of poly(acryloyl chloridej and poly(methacryloyl chloride) with a mesogenic alcohol or amine i n the presence of triethylamine (8,21,22). Similarly, liquid c r y s t a l l i n e polyacrylates, polymethacrylates, and polyitaconates have been prepared by phase-transfer-catalyzed reactions on the sodium s a l t s of the corresponding polycarboxylates (23-25). In addition, alternating poly(methylvinylether-co-maleate) copolymers were prepared by the PTC reactions of poly(methylvinyletherco-disodium maleate) with mesogen containing bromoalkylesters (26). The most important use of this class of reactions, however, i s i n the preparation of l i q u i d c r y s t a l l i n e polysiloxanes, which cannot be obtained by any other method. We have recently summarized the work on the preparation of l i q u i d c r y s t a l l i n e polysiloxanes (19), which involves the platinum catalyzed hydrosilation reaction of v i n y l substituted mesogenic molecules with poly(hydrogen methylsiloxane) or i t s copolymers. Lastly, i t was demonstrated with PPO substituted with a series of a l k y l side-chains as we have here, that the glass t r a n s i t i o n temperature decreases with an increase i n the side-chain length (28). At the same time, the Tg's of the more f l e x i b l e sidechain l i q u i d c r y s t a l l i n e polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (_3). Therefore, i t i s quite l i k e l y that we may obtain side-chain l i q u i d c r y s t a l l i n e polymers of approximately the same Tg from PPO and PECH. Experimental The starting polymers were commercial products purified by p r e c i p i t a t i o n with methanol from chloroform solutions: PECH (B.F. Goodrich, Mn = 873,000, Mw = 10,250,000); PPO (Aldrich, Mn = 19,000m Mw = 49,000). The mesogenic units with methylenic spacers were prepared by reacting the sodium s a l t of either 4-methoxy-4'-hydroxybiphenyl or 4-phenylphenol with a bromoester i n DMF at 82 C for at least 4 hours i n the presence of tetrabutylammonium hydrogen sulfate (TBAH) as phase transfer catalyst. In this way, ethyl 4-(4-oxybiphenyl)butyrate, ethyl 4-(4-methoxy-4 -oxybiphenyl)butyrate, ethyl 4-(4-oxybiphenyl)valerate, ethyl 4-(4-methoxy-4'-oxybiphenyl)valerate, n-propyl 4-(4-oxybiphenyl)undecanoate and n-propyl 4-(4-methoxy-4'-oxybiphenyl)undecanoate were obtained. These esters were hydrolyzed with base and a c i d i f i e d to obtain the carboxylic acids. The corresponding potassium carboxylates were obtained by reaction with approximately stoichiometric amounts of potassium hydroxide. Experimental details of these syntheses were described elsewhere (27). Bromobenzyl groups were introduced into PPO by r a d i c a l bromination of the methyl groups. The PPO bromobenzyl groups and PECH chloromethyl groups were then esterified under phasetransfer-catalyzed reaction conditions with the potassium carboxylates just described. This procedure has been described previously (29). The sodium s a l t of 4-methoxy-4 -hydroxybiphenyl was also reacted with PECH (no spacer). Thermal analysis was performed with a Perkin-Elmer DSC-4 1
1
8.
P U G H AND PERCEC
Side-Chain Liquid Crystalline Polymers
103
d i f f e r e n t i a l scanning calorimeter equipped with a Perkin-Elmer TADS thermal analysis data station. Heating and cooling rates were 20 C/min, and Indium was used as the c a l i b r a t i o n standard. A l l samples were heated to just above Tg and quenched before the f i r s t heating scan was recorded. A Carl-Zeiss o p t i c a l polarizing microscope equipped with a Mettler FP82 hot stage and FP80 central processor was used to analyze the anisotropic textures. Table III summarizes the reaction conditions and the results of the substitution of PPO for a l l reactions performed, while Table IV presents the results for the modification of PECH. Results and Discussion The synthetic route used for the chemical modification of PPO and PECH i s outlined i n Scheme 1. The results of the e s t e r i f i c a t i o n reactions of PPO and of PECH are summarized i n Tables III and IV respectively. I t was previously shown that the only available procedure for the nucleophilic substitution of bromomethylated PPO was by s o l i d - l i q u i d phase-transfer reactions i n aprotic nonpolar solvents (29), since PPO i s not soluble i n aprotic dipolar solvents which are required for conventional nucleophilic substitutions. In both cases, i t was necessary to use elevated temperature (60 C) to obtain good halide displacement. However, i t i s again demonstrated that PECH i s less reactive toward nucleophilic displacement than PPO. Although Me4C00K seem to result i n the most e f f i c i e n t substitution, and although the carboxylate n u c l e o p h i l i c i t i e s would be expected to change with d i f f e r e n t spacers, these results are not completely comparable due to d i f f e r e n t amounts of excess hydroxide present i n each displacement reaction. In addition, we found that sodium 4-phenylphenoxide degraded PECH to a number average molecular weight of approximately 3000, rather than s t r i c t l y displacing halide as was the case with the other nucleophiles. Table V summarizes the thermal characterization of substituted PPO, and demonstrates that although the Tg i s e a s i l y dropped with any of the substituents, l i q u i d c r y s t a l l i n e behavior i s not observed u n t i l 4-methoxy-4'-hydroxybiphenyl i s decoupled from backbone PPO by ten methylenic units. Therefore, l i q u i d c r y s t a l l i n e behavior can be obtained from very r i g i d polymers, provided a long enough spacer i s employed. A smectic l i q u i d c r y s t a l l i n e mesophase was confirmed by polarized o p t i c a l microscopy for MelOCOO-PPO, but the exact smectic phase could not be determined because some thermal crosslinking takes place during extensive annealing. This could be the result of the presence of unreacted bromobenzyl groups. In contrast to MelOCOO-PPO, PhlOCOO-PPO i s not l i q u i d c r y s t a l l i n e , although there i s comparatively l i t t l e substitution i n this case, p-biphenyl i t s e l f would not be expected to act as a mesogen i n such a rigid polymer due to i t s low degree of anisotropy and polarizability. Comparison ot Tables V and VI demonstrates that the thermal behavior of MelOCOO-PPO and MelOCOO-PECH are very similar, with the glass transition temperatures converging with substitution. Therefore, i t appears that when very long spacers are used with the same mesogen, the polymer backbone has l i t t l e e f f e c t on the thermotropic phases formed. This conclusion i s supported by additional unpublished experiments performed i n our laboratory.
CHEMICAL REACTIONS ON POLYMERS Table I I I .
Reaction Conditions and Results of Synthesis of PPO Containing Β1phenyl Groups *
Mole Fraction Moles per Mole -CH Br Structural Units Nucleophlle TBAH Conta1n1ng -CHgBr ?
Nucleophlle
1 2 3
biPhO-(CH ),C00K
Reaction Temp. Time (°C) (hr)
ZCH-Br SubStltuted
0.52 0.74 0.74
2.2 1.8 1.8
0.22 0.24 0.12
25 25 60
45 45 46
25 26 100
4 Me0-blPh0-(CH )-C00K 5 6
0.52 0.74 0.74
2.0 2.0 1.7
0.19 0.19 0.39
25 25 60
40 40 61.5
15 12 100
7 8 9
0.52 0.74 0.74
2.0 1.9 1.9
0.21 0.07 0.14
25 25 60
62 62 46
87 93 100
10 Me0-blPh0-(CM C00K 11 12
0.52 0.74 0.74
2.1 2.1 2.0
0.14 0.10 0.39
25 25 60
62 62 61.5
50 61 100
13 14
0.52 0.74
2.0 1.9
0.12 0.21
25 25
96 96
25 24
15 Me0-blPh0-(CH,) C00K 0.74 16 0.74
0.9 0.9
0.24 0.24
60 60
9
2
3
9
C
J
blPh0-(CH,) C00K A
6
4
A
£
4
blPh0-(CH,) C00K in
2
1 0
in
2
1 0
71 75
54.5 139.5
* Solvent • toluene
Table IV.
Reaction Conditions and Results of Synthesis o f PECH Containing Methoxyblphenyl Groups *
#
Nucleophlle
—
Moles per Mole -CHpCl Nucleophlle
TBAH
Reaction Time (hr)
ZCH C1 Substituted 2
1 2
Me0-blPh0Na
0.76 0.76
0.10 0.10
92 140
29 36
3 4 5 6
Me0-blPh0-(CH ) C00K
0.69 0.75 0.75 0.75
0.13 0.12 0.12 0.12
91 108.5 139.5 158.5
22 26 30 34
7 8 9 10 11 12 13 14
MeO-blPhO-iCHJ-COOK
0.75 0.75 0.75 0.73 0.75 0.75 0.75 0.75
0.12 0.12 0.12 0.11 0.12 0.12 0.12 0.12
19 49 83 91 115.5 140 159.5 181.5
23 42 52 51 62 65 65 65
15
MeO-blPHO-(CH ) CO0K
0.64
0.17
54
17
2
Solvent - DMF; 60°C
3
2
10
8.
P U G H AND PERCEC
Table V.
105
Side-Chain Liquid Crystalline Polymers
Thermal C h a r a c t e r i z a t i o n o f PPO C o n t a i n i n g B1phenyl Groups
Temperature (°C) a # Polymer Sample Tg
1 2 3
0.13 Ph3C00-PP0 0.19 Ph3C00-PP0 0.74 Ph3C0O-PP0
4 5 6
0.08 Me3COO-PP0 0.09 Me3COO-PP0 0.74 Me3C00-PPO
172.5 155.2 69.4
7 8 9
0.43 Ph4C00-PP0 0.69 Ph4C00-PP0 0.74 Ph4C00-PP0
80.8 67.0 59.6
10 11 12
0.26 Me4C00-PP0 0.45 Me4C00-PP0 0.74 Me4C0O-PP0
106.2 89.2 62.5
13 14
0.13 PhlOCOO-PPO 0.18 Phl0C00-PP0
105.5 83.7
15 16
0.53 MelOCOO-PPO 0.56 MelOCOO-PPO
54.2 38.6
*\from T a b l e I I I ; ' b u r l e d 1n peak
b
Heating Endotherms
Tg
Cooling Exotherms
141.7 130.0 69.5
58.4
85.6 53.1
72.4,
114.5 116.8, 129.0
39.2 c
77.5 40.4, 101.4
' mole f r a c t i o n o f mesogen s u b s t i t u t e d s t r u c t u r a l
units;
106
CHEMICAL REACTIONS ON POLYMERS
CM VO rH rH 00 00
rH 00 I HNCO H S H »ί · · · · I . . . . . . .HNO01 I CO rH VO toconooHO rOSOOCOHHHH
ο «± LD ^- oo r«. CSJ CM d -n cô
LocvjroovocNjr^rH
en
CO 00 00 VO
Cf> Ο «ςΓ rH
α rJ- ο 3 Ο •Ρ ο ί.Φ 40->0 0.rH Ε Φ
co Ν η σι en rH vo
ιο ο Ν e\j ι ι ι rH I
I
CM CM
ι
I
CO CM rH
VO CO 3 3 . .Ο Ο CM CM J= SZ U") CO 10 rH rH rH rH CD C O *C M * csj 00* un un un vo vo
00 d CO
CMCO d rH
rH en rH O * *t ο ο C I I
rH r-. VOο COCMΟ rH in νο «dd r*«' d ι rH rH co co COCO
3= ο Lu α. I ο φ Σ
3= Ο LU Ο Ι Ο Ο ο co φ Σ
3= Ο LU Ο Ι Ο Ο Ο
3= Ο Lu α. ι ο ΣΦ
en νο CMco ο ο
3= ο LU Ο Ι ο ο ο co φ Σ
vo" rH m
3= ο Lu CL I ο ο ο co φ Σ
χ ο Lu Ο Ι Ο Ο Ο CO φ Σ
CMνο ο «dCMCSJ COco Ο Ο Ο Ο co *t un νο
Un
d COCOun d
vo" rH vo d vo vo
3= 3= 3Ζ 3= 3= 3= Ο Ο Ο Ο Ο Ο Lu Lu Lu Lu Lu Lu Ο Ο Ο Ο Ο Ο Ι Ι Ι Ι Ι Ι Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο Ο
