Dynamic Mechanical Properties of Poly(tetramethylene ether) Glycol

Chapter 15

Dynamic Mechanical Properties of Poly(tetramethylene ether) Glycol Polyurethanes

Downloaded by UNIV MASSACHUSETTS AMHERST on August 5, 2012 | http://pubs.acs.org Publication Date: May 1, 1990 | doi: 10.1021/bk-1990-0424.ch015

Effect of Diol-Chain Extender Structure James V. Duffy, Gilbert F. Lee, John D. Lee, and Bruce Hartmann Polymer Physics Group, Naval Surface Warfare Center, Silver Spring, MD 20903-5000 The s t e r i c e f f e c t that pendant groups present i n the diol chain extender have on the glass (T ) and melting point (T ) t r a n s i t i o n s and dynamic mechanical properties of some PTMG polyurethanes was studied. A series of poly(tetramethylene ether) glycol/4,4'-diphenylmethane diisocyanate prepolymers were extended with d i o l s that contained either methyl, ethyl, or butyl pendant groups. D i f f e r e n t i a l scanning calorimetry (DSC) was used to determine the T and T of the soft and hard phases. I t was found that d i o l pendant groups increased the amount of phase mixing i n the soft segment while preventing c r y s t a l l i z a t i o n i n both the soft and hard segments. The e f f e c t of d i o l pendant groups on the shear modulus and loss factor of these polyurethanes was determined by a resonance technique. The size of the pendant group had no apparent e f f e c t on either T , T , or the dynamic mechanical properties of these polymers. g

m

g

m

g

m

Polymers are often used i n sound and v i b r a t i o n damping areas. In general, one of the most important polymer properties for these applications i s the glass t r a n s i t i o n . At the glass t r a n s i t i o n , a polymer i s most e f f i c i e n t i n converting sound and mechanical v i b r a t i o n a l energy into heat which results i n absorption. By t a i l o r i n g the polymer structure so that the glass t r a n s i t i o n i s i n the required temperature and frequency range, the polymer becomes an e f f e c t i v e damper. This chapter not subject to U.S. copyright Published 1990 American Chemical Society

In Sound and Vibration Damping with Polymers; Corsaro, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

282

SOUND AND VIBRATION DAMPING WITH POLYMERS

For the work presented here, the polymers considered are i n the general c l a s s of m a t e r i a l s known as polyurethanes. Polyurethanes are p a r t i c u l a r l y a t t r a c t i v e f o r a study of the e f f e c t of chemical s t r u c t u r e on damping s i n c e i t i s p o s s i b l e t o change t h e i r T 's over a wide range of temperatures (>100°C). T h i s corresponds t o a damping peak l o c a t i o n t h a t spans more than 10 decades of frequency. In a d d i t i o n , changes i n polyurethane s t r u c t u r e can be used t o produce a t r a n s i t i o n t h a t can vary from narrow t o broad. To take advantage of these d e s i r a b l e p r o p e r t i e s , one must understand the dependence of T on the chemical s t r u c t u r e of polyurethane polymers. Polyurethanes are a l t e r n a t i n g b l o c k copolymers made of s o f t segments d e r i v e d from p o l y e s t e r or p o l y e t h e r d i o l s and hard segments which come from the d i i s o c y a n a t e and d i o l chain extender (1). Since the s o f t and hard segments are c h e m i c a l l y d i s s i m i l a r , they tend t o be incompatible and separate i n t o d i f f e r e n t phases as shown i n F i g u r e 1. Thus, hard segment domains can separate or be d i s p e r s e d i n a s o f t segment matrix. Separate g l a s s t r a n s i t i o n s can occur i n each phase and e i t h e r one or both of the phases can be c r y s t a l l i n e (2-3). Dynamic mechanical property (DMP) measurements are used t o evaluate the s u i t a b i l i t y of a polymer f o r a p a r t i c u l a r use i n sound and v i b r a t i o n damping. Since the dynamic mechanical p r o p e r t i e s of a polyurethane are known t o be a f f e c t e d by polymer morphology (4), i t i s important t o e s t a b l i s h the c r y s t a l l i z a t i o n and m e l t i n g behavior as w e l l as the g l a s s t r a n s i t i o n temperature of each polymer. D i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) was used t o determine these p r o p e r t i e s and the data used t o i n t e r p r e t the dynamic mechanical property r e s u l t s . The s p e c i f i c o b j e c t i v e of t h i s work was t o determine the e f f e c t t h a t d i o l c h a i n extender s t r u c t u r e had on s o f t / h a r d phase mixing, morphology, and the dynamic mechanical p r o p e r t i e s of a s e r i e s of poly(tetramethylene ether) g l y c o l (PTMG) polyurethanes. Information regarding the i n f l u e n c e of PTMG molecular weight and hard segment content on these p r o p e r t i e s was a l s o obtained.


g

g

Synthesis Prepolymers were prepared from poly(tetramethylene ether) g l y c o l (PTMG) having nominal molecular weights of 650, 1000, 1430, 2000 and 2900, and 4,4 -diphenylmethane d i i s o c y a n a t e (MDI). The p o l y o l , which was added t o MDI at 45-50°C, produced an exothermic r e a c t i o n t h a t r a i s e d the temperature t o 80°C. The mixture was h e l d a t 75-80°C f o r 1-2 hours and the mixture was then degassed, cooled, and s e a l e d under n i t r o g e n . The percent f r e e isocyanate was determined u s i n g ASTM method D1638. The prepolymers, which had MDI/PTMG molar r a t i o s which v a r i e d from 2:1 t o 6:1, were chain extended with e i t h e r 1,4-butanediol (1,4BDO), 1,3-butanediol (1,3-BDO), 2,2-dimethyl-l,31



15. DUFFY ETAL.

Dynamic Properties of Polyurethanes

283

propanediol (DMPD), 2-ethyl-2-methyl-l,3-propanediol (EMPD), 2 , 2 - d i e t h y l - l , 3 - p r o p a n e d i o l (DEPD), o r 2-butyl-2ethy1-1,3-propanediol (BEPD). Figure 2 i s an i d e a l i z e d s t r u c t u r e o f a l i n e a r polymer t h a t was s y n t h e s i z e d u s i n g one mole o f PTMG, three moles o f MDI, and two moles o f 1,4-BDO. At the bottom o f Figure 2, f i v e other c h a i n extender s t r u c t u r e s are shown, namely: 1,3-BDO, DMPD, EMPD, DEPD, and BEPD. Stoichiometry o f the c h a i n extender was adjusted, i n a l l cases, so t h a t the isocyanate index was 1.05, which means t h a t a 5% excess of isocyanate was used beyond the s t o i c h i o m e t r i c r a t i o r e q u i r e d f o r complete r e a c t i o n . T h i s insured t h a t these polymers would be c r o s s l i n k e d through t h e formation o f allophanate bonds during the c u r i n g r e a c t i o n (4). Test specimens were cured a t 100°C f o r 16 hours. The polymers are i d e n t i f i e d i n the f o l l o w i n g manner: a polymer o f 1000 molecular weight PTMG reacted i n a molar r a t i o o f 1:3 with MDI and chain extended with DMPD i s denoted as PTMG 1000/M3/DMPD, and so f o r t h . Experimental Thermal A n a l y s i s . A DuPont 9900 computer/thermal a n a l y z e r was used i n conjunction with a 910 DSC module t o o b t a i n thermograms. Samples (10-15 mg) were c u t from the DMP t e s t bars and placed i n aluminum t e s t pans f o r a n a l y s i s . Measurements were made from -170 t o 250°C a t a scanning r a t e o f 10°C/min, i n a n i t r o g e n atmosphere. Dynamic Mechanical A n a l y s i s Data C o l l e c t i o n . The dynamic mechanical apparatus (5) used i s based on producing resonance i n a bar specimen. T y p i c a l length o f a specimen i s 10-15 cm with square l a t e r a l dimensions o f 0.635 cm. In b r i e f , measurements are made over 1 decade o f frequency i n t h e kHz r e g i o n from -60 t o 70°C a t 5 degree i n t e r v a l s . By a p p l y i n g the time-temperature s u p e r p o s i t i o n p r i n c i p l e , the raw data are s h i f t e d t o generate a reduced frequency p l o t (over as many as 20 decades o f frequency) a t a constant r e f e r e n c e temperature. Frequencies g r e a t e r than 10 Hz a r e not s i g n i f i c a n t i n sound and v i b r a t i o n a p p l i c a t i o n s , but a t these frequencies the e f f e c t o f v a r i a t i o n o f chemical s t r u c t u r e on g l a s s y modulus can be determined. As shown i n Figure 3, an electromagnetic shaker i s used t o d r i v e a t e s t specimen a t one end while t h e other end i s allowed t o move f r e e l y . M i n i a t u r e accelerometers are a d h e s i v e l y bonded on each end t o measure the d r i v i n g p o i n t a c c e l e r a t i o n and the a c c e l e r a t i o n o f t h e f r e e end. The output s i g n a l s from the accelerometers a r e a m p l i f i e d and routed t o a dual channel Fast F o u r i e r Transform spectrum a n a l y z e r . The analyzer d i g i t i z e s and d i s p l a y s the measured s i g n a l s . The s i g n a l s (amplitude and phase of the a c c e l e r a t i o n r a t i o ) can be measured over a frequency range o f three decades (25 t o 25,000 Hz). 7


284


SOFT SEGMENT CRYSTALLINITY


PHASE MIXED

HARD SEGMENT CRYSTALLINITY

-~-SOFT HARD

Figure 1. Microphase

separation i n polyurethanes.

•HARD SEGMENT •

• SOFT SEGMENT PTMG

MDI

MDI

1,4-BDO

3r

MDI

1.4-BDO

HNCOOCH2CH2CH2CH2OOCNH

CH2CH2CH2CH2O 4 — OCNH

CH2

CH

CH

2

2

HNC00

HNCOOCH2CH2CH2CH2OOCNH

3r

DIOL CHAIN EXTENDERS WITH PENDANT GROUPS CH

CH

3

H0CH CCH 0H 2

2

CH

2

2

C H

3

2

DMPD

C H

3

H0CH CCH 0H 5

EMPD

F i g u r e 2. Chemical polyurethane.

2

2

2

C H 2

C H

5

H0CH CCH 0H 5

DEPD

2

2

CH

5

H0CH CCH 0H 2

3

H0CH CH CH0H 2

2

C4H9

BEPD

structure of a t y p i c a l

1,3-BDO

linear



PRINTER

COMPUTER

STORAGE DISC

F i g u r e 3.

FAST FOURIER SPECTRUM ANALYZER

Resonance apparatus.

SUPPORT

^ CONTROLLED TEMPERATURE CHAMBER

THERMOCOUPLE



286


At c e r t a i n frequencies, the amplitude o f the a c c e l e r a t i o n r a t i o goes through l o c a l resonant peaks. The number of resonant peaks that can be measured i s dependent on the l o s s f a c t o r of the m a t e r i a l , but, t y p i c a l l y , there are three t o f i v e peaks. As expected, the resonant peaks appear a t higher frequencies i n the g l a s s y s t a t e than i n the rubbery s t a t e . From the amplitude and frequency o f each measured resonant peak, Young's modulus and l o s s f a c t o r are determined a t the corresponding frequency and temperature. By assuming Poisson's r a t i o o f 0.5, Young's modulus i s converted t o shear modulus. The l o s s f a c t o r i n extension i s assumed to equal the l o s s f a c t o r i n shear. In making measurements over a temperature range o f -60 t o 70°C, the f o l l o w i n g thermal c y c l e was used: (1) c o o l the t e s t specimen, which has been mounted i n the t e s t apparatus, t o -60°C.; (2) allow the specimen t o e q u i l i b r a t e a t -60°C f o r a t l e a s t 12 hours before making a measurement; (3) a f t e r each measurement, r a i s e the temperature by 5 degrees; (4) allow 20 minutes t o elapse between each change i n temperature t o o b t a i n thermal e q u i l i b r i u m before making the next measurement. Generation o f Master Curves. Modulus and l o s s f a c t o r data were processed i n t o a reduced frequency p l o t i n the f o l l o w i n g manner: modulus curves a t d i f f e r e n t temperatures were s h i f t e d along the frequency a x i s u n t i l they p a r t i a l l y overlapped t o o b t a i n a best f i t minimizing the sum o f the squares o f a second order equation ( i n l o g modulus) between two sets o f modulus data a t d i f f e r e n t temperatures. T h i s procedure was completely automated by a computer program. The modulus was chosen t o be s h i f t e d r a t h e r than the l o s s f a c t o r because the modulus i s measured more a c c u r a t e l y and has l e s s s c a t t e r than the l o s s f a c t o r . The f i n a l r e s u l t i s a constant temperature p l o t o r master curve over a wider range o f frequency than a c t u a l l y measured. Master curves showing the overlap o f the s h i f t e d data p o i n t s w i l l not be presented here, but a t y p i c a l one i s found i n another chapter o f t h i s book (Dlubac, J . J . e t a l . , "Comparison o f the Complex Dynamic Modulus as Measured by Three Apparatus"). The amount o f s h i f t one s e t o f data a t a g i v e n temperature has t o move along the l o g frequency a x i s t o overlap another s e t o f data a t a d i f f e r e n t temperature i s represented by the s h i f t f a c t o r , l o g a . The l o g a versus temperature data i s then f i t t e d t o the WLF equation (6) T

log a = - c T

1 o

(T - T ) / ( c 0

2 o

T

+ T - T)

(1)

Q

where c, and c are constants f o r a given polymer and a r e s u b s c r i p t e d t o i n d i c a t e the r e f e r e n c e temperature T a t which t h e equation i s evaluated. Knowing c and c , t h e master curve a t T can be r e p l o t t e d a t a d i f f e r e n t 2

0

1 o

0


2 o

287


15. DUFFY ETAL.

r e f e r e n c e temperature using the procedure d e s c r i b e d by F e r r y (6). The lower l i m i t i n s e l e c t i n g a r e f e r e n c e temperature i s T , while the upper l i m i t i s about T +100°C (6). T h i s upper l i m i t can change f o r d i f f e r e n t polymers. The l i m i t s e x i s t because the WLF equation only a p p l i e s i n the t r a n s i t i o n r e g i o n . A l l the master curves t o be presented here a r e referenced t o 25°C. The WLF constants c and c a r e determined by f i t t i n g the l o g a versus temperature data only i n the temperature range o f the g l a s s t r a n s i t i o n . I t was found t h a t these constants can vary c o n s i d e r a b l y depending on the temperature range s e l e c t e d . To avoid s u b j e c t i v i t y i n the choice o f temperature range, a systematic approach was devised f o r t h i s work using h y p o t h e t i c a l data t o which a small e r r o r was added t o each s h i f t f a c t o r t o simulate experimental data. Larger and l a r g e r temperature ranges were included i n the f i t t i n g and s h i f t constants evaluated f o r each temperature range. I t was found t h a t both s h i f t constants went through a minimum as the temperature range increased, and the v a l u e s a t the minimum were the c o r r e c t values. Using t h i s approach on the experimental data, the minimum values were s e l e c t e d t o be the s h i f t constants o f the m a t e r i a l . g

g

1

2


T

R e s u l t s and D i s c u s s i o n F a c t o r s E f f e c t i n g T . T . and Dynamic Mechanical Properties g

m

D i o l Chain Extender S t r u c t u r e . The DSC r e s u l t s f o r PTMG 650-2900/M3 prepolymers chain extended with 1,4-BDO, 1,3BDO, DMPD, EMPD, DEPD, and BEPD are shown i n Table I . Table I . T and T g

PTMG MW

m

f o r D i o l Extended PTMG 650-2900/M3 Prepolymers by DSC T ss (°C)

-24 -48 -66 -71

32 6 -40 -58

37 31 9 2 -35 -39 -54 -56

ss - s o f t segment

m

m

1,4-BDO DMPD EMPD DEPD BEPD 1,3-BDO 1,4-BDO 650 1000 2000 2900

T hs (°C)

T ss (°C)

35 11 -34 -54

_

_

0

-

-

-56

2 10

1,4-BDO 160 157 184,197 191,198

hs - hard segment

T r a n s i t i o n temperatures were determined from expanded v e r s i o n s o f the o r i g i n a l t r a c e s . For PTMG 650/M3, the s o f t segment T with 1,4-BDO i s -24°C while DMPD, EMPD, DEPD, and BEPD polymers have g l a s s t r a n s i t i o n s a t much g


288


higher temperatures (32, 37, 31, and 35°C). Thus, d i o l s t h a t c o n t a i n pendant groups appear t o promote phase mixing as i n d i c a t e d by higher s o f t segment T 's (7). A w e l l d e f i n e d hard segment melting occurs a t 160°C with 1,4-BDO but no hard segment c r y s t a l l i z a t i o n o r melting occurs i n any d i o l extended polymer t h a t c o n t a i n s pendant groups (DMPD, EMPD, DEPD, o r BEPD). These pendant groups do not f i t i n t o the l a t t i c e because o f t h e i r b u l k i n e s s thereby preventing c r y s t a l l i z a t i o n . The same type o f r e s u l t s are obtained i n the PTMG 1000/M3 s e r i e s o f polymers. S o f t segment T s f o r DMPD, EMPD, DEPD, BEPD, and 1,3-BDO a r e l o c a t e d a t much higher temperatures (6, 9, 2, 11, and 0°C) than the 1,4-BDO extended polymer (-48°C) and hard segment melting i s seen only i n t h e 1,4-BDO polymer (157°C). The PTMG 2000 (Table I) and 2900 polymers (Table I and F i g u r e 4) extended with 1,4-BDO show s o f t segment c r y s t a l l i z a t i o n and melting f o r the f i r s t time (2 and 10°C r e s p e c t i v e l y ) . T h i s i s the r e s u l t o f phase s e p a r a t i o n i n these higher MW PTMG polymers which allows the s o f t segments t o aggregate and then c r y s t a l l i z e . Hard segment melting appears as a s e r i e s o f peaks a t higher temperatures (8). The corresponding DMPD, EMPD, DEPD, BEPD, and 1,3-BDO extended polymers as before e x h i b i t n e i t h e r s o f t nor hard segment c r y s t a l l i z a t i o n o r m e l t i n g . T 's o f the PTMG 2000 and 2900 polymer s o f t segments are again lower with 1,4-BDO (-66 and -71°C) than with the corresponding DMPD (-40 and -58°C), EMPD (-35 and -54°C), DEPD (-39 and -56°C), BEPD (-34 and -54°C), and 1,3-BDO ( -, and -56°C) polymers because they are l e s s phase mixed. T h i s major morphological d i f f e r e n c e between 1,4-BDO extended polymers ( c r y s t a l l i n e hard segment) and those extended with any o f the d i o l s c o n t a i n i n g pendant groups ( n o n - c r y s t a l l i n e hard segment) a l s o y i e l d s s i g n i f i c a n t d i f f e r e n c e s i n the shear moduli and l o s s f a c t o r s o f these polymer systems. Shear modulus curves f o r 1,4-BDO, and DMPD extended polymers a r e shown i n Figures 5 and 6. The EMPD, DEPD, BEPD, and 1,3-BDO extended polymers had modulus curves which were s i m i l a r t o the DMPD r e s u l t s and t h e r e f o r e a r e not presented i n g r a p h i c a l form. An a d d i t i o n a l molecular weight, 1430, i s a l s o a v a i l a b l e f o r t h e 1,4-BDO system. The rubbery modulus f o r 1,4-BDO polymers i s about 1 x 10 Pa whereas DMPD, EMPD, DEPD, BEPD, and 1,3-BDO polymers had moduli t h a t a r e lower by about a f a c t o r 5 (2 x 10 Pa). T h i s d i f f e r e n c e can be explained by the f a c t t h a t the 1,4-BDO polymers are r e i n f o r c e d by c r y s t a l s from the hard segment which a c t as a f i l l e r whereas d i o l s c o n t a i n i n g pendant groups y i e l d polymers t h a t have lower rubbery moduli because they l a c k hard segment crystallinity. At high frequencies, these polymers a r e i n the g l a s s y s t a t e and the moduli f o r a l l systems have approximately the same value, 1 x 10 Pa (3) .


1

g

7

6

9


Dynamic Properties ofPolyurethanes

DUFFY ET AL.

\

DMPD

\

DEPD

o
-

\J

. i l l I i I i I -150 -100 -50 0 50 100 150 200 250 TEMPERATURE (°C) i

F i g u r e 4.

I

i

I

i

DSC thermograms o f PTMG 2900/M3/diols.

0

5

10

15

LOG FREQUENCY (Hz)

F i g u r e 5. polymers.

Shear moduli f o r PTMG 650-2900/M3/1,4-BDO



290


The l o s s f a c t o r shows s i m i l a r dramatic d i f f e r e n c e s among the d i o l chain extenders (Figures 7 and 8) as a r e s u l t of hard segment c r y s t a l l i n i t y . The l o s s f a c t o r curve, which i s a measure of the damping p r o p e r t i e s of a polymer, has a maximum value of 0.3 f o r 1,4-BDO extended PTMG 650, 1000, and 1430 polymers and i s very broad i n nature. The h a l f width i s about 8 decades on the average. The l o s s f a c t o r curves f o r PTMG 2000 and 2900 are only p a r t i a l l y p l o t t e d because the data c o u l d not be s h i f t e d due t o c r y s t a l l i z a t i o n i n the s o f t segment which takes p l a c e as the temperature i s r a i s e d . The DMPD, EMPD, DEPD, BEPD, and 1,3-BDO extended polymers, i n c o n t r a s t , gave very narrow ( h a l f width 3 decades) l o s s f a c t o r curves and at much higher maxima values of 1.0. I t appears t h a t the l a c k of any c r y s t a l l i n i t y i n the d i o l chain extender allows f o r increased molecular m o b i l i t y i n the r e g i o n of the g l a s s t r a n s i t i o n which i n t u r n leads t o higher l o s s f a c t o r maxima. D i o l Chain Extender Blends. Some measurements were made on polymer systems cured with blends of 1,4-BDO and DMPD. I t should be remembered t h a t polymers extended with 1,4-BDO show hard segment c r y s t a l l i z a t i o n while polymers extended with DMPD do not c r y s t a l l i z e . The hard segment content i s maintained constant at a given l e v e l (45-50 percent) while a d j u s t i n g the 1,4-BDO/DMPD r a t i o . Thus PTMG 1000/M3 was chain extended with three d i f f e r e n t blends of 1,4-BDO/DMPD (25/75, 50/50, and 75/25), and the component percentages are shown along with DSC r e s u l t s i n Table I I . Table I I .

Composition Prepolymers

EQ.WT. RATIO 1,4-BDO/DMPD 100/0 75/25 50/50 25/75 0/100

and DSC R e s u l t s f o r PTMG 1000/M3 Chain Extended with 1,4-BDO/DMPD Blends

% hs

%BD0 i n hs

%DMPD i n hs

47.0 48.1 48.3 48.4 48.6

17.7 13.2 8.7 4.3 0

0 5.1 10.1 15.1 20.0

T

ss (°C) -48 -30 -4 -3 6

T

m

hs (°C) 157 160

—

The data shows the s h i f t i n s o f t segment T from -48°C (100 percent 1,4-BDO) t o 6°C (100 percent DMPD) as expected due t o phase mixing. As a n t i c i p a t e d , hard segment c r y s t a l l i z a t i o n occurs when the DMPD content i s l e s s than 50 percent. The shear modulus data (Figure 9) show a steady decrease i n the rubbery modulus as the DMPD content of the blend i n c r e a s e s because the degree of hard segment c r y s t a l l i n i t y i s decreasing, and these r e s u l t s



DUFFY ET AL.


1

650

u 2

0.1

2000 / / 2900

_1_

-JL_

0.01

o

5

10

15

LOG FREQUENCY (Hz)

Figure 7. polymers.

Loss f a c t o r s f o r PTMG 650-2900/M3/1,4-BDO

I 001

Figure 8. polymers.

I o

I

I

5 10 LOG FREQUENCY (Hz)

1

1

15

Loss f a c t o r s f o r PTMG 650-2900/M3/DMPD


291

292


are c o n s i s t e n t with the DSC c r y s t a l l i z a t i o n r e s u l t s . F o r the polymers with DMPD contents o f 50 percent o r more, the l o s s f a c t o r reaches a value o f 1.0 as shown F i g u r e 10, while the polymers with 0 and 25 percent DMPD have broader peaks a t lower values o f 0.3. Molecular Weight o f P o l y e t h e r d i o l . A summary o f the e f f e c t o f PTMG molecular weight on T i s g i v e n i n Table I. As the PTMG molecular weight i n c r e a s e s , the modulus (Figures 5 and 6) and l o s s f a c t o r curves (Figures 7 and 8) s h i f t t o higher frequencies (lower temperatures). These trends are i n agreement with the DSC data f o r T . The p o s i t i o n o f the l o s s f a c t o r peak can be moved w i t h i n a frequency range o f 10" t o 10 Hz by changing the MW o f the PTMG, while the rubbery and g l a s s y moduli remain f a i r l y constant w i t h i n a s e r i e s t h a t uses the same c h a i n extender.


g

3

7

Hard Segment Content. I t was o f i n t e r e s t t o determine the e f f e c t o f hard segment content on DSC and DMP f o r a system where the hard segment c r y s t a l l i z e s and f o r one t h a t does not. The systems are: PTMG 1000/M3/1,4-BDO ( c r y s t a l l i n e hard segment) and PTMG 2000/M3/DMPD (nonc r y s t a l l i n e hard segment) o f v a r y i n g hard segment content. The d i i s o c y a n a t e / p o l y e t h e r d i o l r a t i o , f o r the PTMG 1000/1,4-BDO polymers v a r i e d from 2:1 t o 6:1, which corresponds t o hard segment content from 37 t o 66 percent, r e s p e c t i v e l y . The DSC r e s u l t s show t h a t the s o f t segment T 's (-44, -48, and -43 °C) do not change very much as the hard segment content i n c r e a s e s from 2:1 t o 3:1 t o 4:1 as presented i n Table I I I . Table I I I .

E f f e c t o f Hard Segment Content and D i o l S t r u c t u r e on T r a n s i t i o n P r o p e r t i e s

MDI/1,4-BDO RATIO 2:1 3:1 4:1 6:1

PTMG 1000/M/1,4-BDO hs T ss CONTENT(%) (°C) 37 48 56 66

-44 -48 -43 -18

T hs (C) m

156 155 169 174,196

PTMG 2 000/M/DMPD MDI/DMPD hs T ss RATIO CONTENT(%) (°C) 3:1 4:1 6:1

32 39 50

-40 -21 2



DUFFY ET AL.

Dynamic Properties ofPolyurethanes

25/75

LOG FREQUENCY (Hz)

F i g u r e 10. Loss f a c t o r s f o r PTMG 1000/M3 c h a i n extended with 1,4-BDO/DMPD blends.


294


At a r a t i o o f 6:1, however, T jumps d r a m a t i c a l l y t o -18°C as a r e s u l t o f phase mixing. I n t h i s system (6:1), the hard segment melts a t a higher temperature which i s an i n d i c a t i o n t h a t the p u r i t y o f the phase and c r y s t a l s i z e i s i n c r e a s i n g as the hard segment content i n c r e a s e s . The DMP r e s u l t s c o r r e l a t e w e l l with t h e DSC r e s u l t s . Shear modulus versus frequency f o r the v a r i o u s r a t i o s a r e p l o t t e d i n F i g u r e 11. The rubbery shear modulus i s i n c r e a s i n g due t o the r e i n f o r c i n g e f f e c t o f t h e c r y s t a l l i n e hard segment on the amorphous s o f t segment. Loss f a c t o r versus frequency f o r the v a r i o u s r a t i o s a r e p l o t t e d i n F i g u r e 12. The p o s i t i o n s o f t h e l o s s f a c t o r peaks f o r the 2:1, 3:1, and 4:1 r a t i o s are the same. At a r a t i o o f 6:1, the p o s i t i o n o f the peak s h i f t s t o lower frequency, corresponding t o a high T , which i m p l i e s a g r e a t e r degree o f phase mixing. The peak h e i g h t a l s o decreases from 0.6 t o 0.1 and the peak changes from narrow ( h a l f width 5 decades) t o broad ( h a l f width 11 decades) as t h e hard segment i n c r e a s e s , because the degree o f c r y s t a l l i n i t y i n c r e a s e s . A s i m i l a r r e s u l t has been found f o r s e m i - c r y s t a l l i n e p o l y e s t e r s (9-10). The d i i s o c y a n a t e / p o l y e t h e r d i o l r a t i o , f o r the PTMG 2000/DMPD polymers, was v a r i e d from 3:1, t o 4:1 and 6:1 which r a i s e d the hard segment content from 32 t o 50 percent. The DSC thermograms f o r these polymers c o n t a i n no c r y s t a l l i n i t y i n e i t h e r the s o f t o r hard segments. The s o f t segment T , however, s t e a d i l y i n c r e a s e d (-40, -21, 2°C) as the r a t i o increased, i n d i c a t i n g more phase mixing o f the amorphous phases. The DMP r e s u l t s r e f l e c t e d those obtained i n DSC. The rubbery shear modulus appears t o i n c r e a s e only s l i g h t l y (Figure 13), s i n c e no hard segment c r y s t a l l i n i t y i s apparent i n any o f these polymers. The amorphous hard segment i s not as e f f e c t i v e i n r e i n f o r c i n g t h e modulus as c r y s t a l l i n e hard segment. The l o s s f a c t o r peaks are h i g h (about 1.0) and the h a l f width i s about 3 decades as shown i n F i g u r e 14. The peaks s h i f t t o lower frequencies (higher T s ) as the r a t i o i n c r e a s e s due t o i n c r e a s e phase mixing. I t should be noted t h a t as hard segment content i n c r e a s e s so does the percent DMPD i n t h e polymer and because DMPD contains bulky methyl groups t h a t do not f i t i n t o t h e l a t t i c e , c r y s t a l l i z a t i o n f a i l s t o occur.


g

g

g

f

g

F r a c t i o n a l Free Volume and C o e f f i c i e n t o f Thermal Expansion. The s h i f t constants c-, and c from the WLF equation are not only f i t t i n g parameters t h a t d e s c r i b e the frequency-temperature r e l a t i o n o f a given polymer, but they are a l s o r e l a t e d t o chemical s t r u c t u r e . Ferry has shown (6) t h a t these constants can be r e l a t e d t o t h e f r a c t i o n a l f r e e volume and c o e f f i c i e n t o f thermal expansion o f the f r e e volume, which have p h y s i c a l meaning i n terms o f t h e polymer s t r u c t u r e . One can d e f i n e t h e f r e e volume a t the g l a s s t r a n s i t i o n d i v i d e d by the t o t a l volume as f and the c o e f f i c i e n t o f thermal expansion o f 2

g



15.


DUFFY ETAL.

0.01

i

i

5

10

15

LOG FREQUENCY (Hz)

F i g u r e 12. Loss f a c t o r s f o r PTMG 1000/M/l,4-BDO o f v a r i o u s MDI mole r a t i o s .


295



296


15.

DUFFY ETAL.

297


the f r e e volume as the i n c r e a s e of thermal expansion c o e f f i c i e n t a t the g l a s s t r a n s i t i o n , a . While the s h i f t constants can be determined a t any a r b i t r a r y r e f e r e n c e temperature, i f they are r e f e r e n c e d t o T , then f

g

B / (2.303 c )

(2)

1g


where B i s the D o o l i t t l e constant (11) and can be taken t o be equal t o 1 (6). The c o e f f i c i e n t of thermal expansion of the f r e e volume i s expressed as a

f

= B / (2.303 c

1 g

(3)

c ) 2g

Values of f and a are t a b u l a t e d i n Table IV and are i n the range or values as reported by F e r r y (6) on polymers of widely d i f f e r e n t s t r u c t u r e s . While the u n c e r t a i n t y i n the values of f and a , 10 and 50 percent r e s p e c t i v e l y , i s comparable t o the v a r i a t i o n s due t o changes i n chemical s t r u c t u r e , the systematic procedure used t o evaluate f and a here i s b e l i e v e d t o g i v e r e s u l t s t h a t can at l e a s t i n d i c a t e trends i n the data. As shown i n p a r t A of Table IV, f i n c r e a s e s s l i g h t l y with PTMG MW, while a decreases. A s i m i l a r t r e n d was observed with low molecular weight polymers of p o l y ( v i n y l acetate) and p o l y s t y r e n e (6). I t i s reasoned t h a t the molecular weight dependence of f a r i s e s because longer s o f t segment l e n g t h r e q u i r e s more volume t o accommodate the increased number of conformations a v a i l a b l e t o such a c h a i n . Phase mixing a t the lower molecular weights (650 and 1000) causes higher f v a l u e s . As hard segments phase mix i n t o s o f t segments, both f and a i n c r e a s e (parts B and C) because the hard segment c o n t a i n s bulky, r i g i d MDI phenyl groups as w e l l as pendant groups l i k e methyl and e t h y l from c e r t a i n c h a i n extenders. The s o f t segments are f o r c e d apart t o accommodate these groups, thus c r e a t i n g more f r e e volume. The s i z e or the number of the pendant groups do not i n f l u e n c e e i t h e r f or a (parts B - D). T h i s i s i n agreement with the T and DMP r e s u l t s . Hard segment c r y s t a l l i n i t y tends t o decrease f and a , a t l e a s t f o r the lower molecular weights. As shown i n p a r t s A t h r u C, f and a f o r 650 and 1000 MW polymers of 1-4,BDO are l e s s than f o r DMPD and DEPD, because of c r y s t a l l i n e hard segments. The f r e e volume of a c r y s t a l l i n e m a t e r i a l i s l e s s than the f r e e volume of the same m a t e r i a l when i t i s amorphous. Since the s o f t segments are c h e m i c a l l y bonded t o the hard segments, the s o f t segment chains are drawn toward the hard segment domains when c r y s t a l l i z a t i o n occurs. Thus, the f r e e volume i n the s o f t segment domains i s reduced when c r y s t a l l i z a t i o n occurs. The 2000 MW i s e v i d e n t l y the l i m i t i n g MW where the s o f t segment i s no longer a f f e c t e d f

g

f

f

g

f

g

g

f

g

f

g

g

f

g


298


Table IV. WLF S h i f t Constants, F r a c t i o n a l Free Volume, and Thermal Expansion C o e f f i c i e n t

PTMG MW

T (°C)

a,

c (deg) 2

f x 10

a x 10 (deg ) f

2

4

1


A. 650 1000 1430 2000 2900

-24 -48 -57 -66 -71

650 1000 2000 2900

32 6 -40 -58

650 1000 2000 2900

31 2 -39 -56

1000

0

B.

C.

D.

MDI/PTMG RATIO

1.174 1.139 1.105 1.080 1.056

PTMG 650-2900/M3/DMPD 11.4 36.1 3.8 10.6 13.8 64 3.2 4.9 16.0 54.7 2.7 5.0 21.5 133 2.0 1.5

1.157 1.123 1.074 1.033

PTMG 650-2900/M3/DEPD 11.6 50.6 3.8 11.6 55.3 3.7 13.7 49.3 3.2 15.8 126 2.7

7.4 6.8 6.4 2.2

1.140 1.073 1.072 1.043

PTMG 1000/M3/1,3-BDO 11.7 48.6 3.7

7.7

1.116

Dynamic Mechanical Properties of Poly(tetramethylene ether) Glycol

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