Multiphase Polymers - American Chemical Society

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3 Time Dependence of Mechanical Properties and Domain Formation of Linear and Crosslinked Segmented Polyurethanes

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Z O H A R H . OPHIR Polymer Materials Program, Department of Chemical Engineering, Princeton University, Princeton, NJ 08540 G A R T H L. WILKES Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, V A 24061

A systematic series of segmented polyester-MDI thanes were investigated

polyure-

by SAXS, DSC and stress strain

measurements. The linear polymer as well as three peroxide cured samples of different crosslinking levels were utilized. Correlations

were made between time-dependent

in structure and mechanical

properties following

changes specific

thermal treatments. Higher crosslinking leads to lower final domain formation and to lower modulus.

Also, the cross-

linked samples had their domain texture more easily disrupted at lower temperature than did the linear system. Finally,

the rate of phase separation

upon cooling

was

increased as the degree of crosslinking was decreased.

S e g m e n t e d polyurethanes are thermoplastic materials that generally ^

display elastomeric behavior—the degree of which depends on the

relative amounts of "soft" and "hard" segments.

T h e properties also

depend on the extent of microphase separation and the morphological characteristics of the phases. Because of the basic thermodynamic incompatibility of these two segment types, localized microphase formation occurs leading to the well-recognized "domain" morphology. It is widely accepted that the mechanical properties of the final bulk material are 0-8412-0457-8/79/33-176-053$05.00/0 © 1979 American Chemical Society

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

54

MULTIPHASE POLYMERS

dictated by the nature and extent of the domain formation ( 1 - 7 ) . In light of this, one must not always picture a domain "morphology" as shown in Figure l a where the hard segments form a distinctly dispersed phase i n the matrix comprised of the soft elastomeric segments.

While

this schematic may well be quite realistic, if the hard-segment composition is higher, or through different processing procedures, both the hard and soft segments could form continuous phases (Figure l b ) or finally the hard segment may become the matrix for dispersed soft segments (Figure l c ) . T h e latter two systems are not likely to display conventional

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elastomeric behavior because of the interconnectivity of the hard-segment phase which, by itself, displays stiff, high modulus behavior. In view of the variation in hard-segment content within the broad class of segmented polyurethane materials ( linear and crosslinked ), one must not therefore expect identical structure as to specific time-dependent behavior as addressed within this chapter.

Furthermore, other factors as

variation in chain chemistry (of either soft or hard segment) must be recognized because of its affect on thermodynamic incompatibility as well as molecular symmetry which may lead to crystallization of one or both segment types (2,3,8).

Other basic aspects which also must not be

ignored when comparing segmented systems is the mode of polymerization, i.e., one- or two-step polymerization and molecular weights ( a n d distribution) of the segments (1,3, 8-11 ). In recalling Figure 1, where two-phase behavior exists, the related question is "does the domain texture become instantaneously induced during processing whether it be through thermal or solvent means?" A second related question is "once established, how thermally stable ( i n a structural sense ) is this texture?" F r o m an applications point of view both questions have very obvious ramifications. Recent work

(7,8,11,12,

Figure 1. Molecular schematics of some possible segmented polyurethane morphologies. For convenience, good phase separation is assumed, (a) Hard-segment domains dispersed within a soft-segment matrix; (b) interconnected hard-segment phase giving rise to continuous phases of both hard and soft segments; (c) softsegment phase is dispersed within a continuous matrix of hard segments.

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

3. 13)

OPHIR AND WILKES

55

Linear and Crosslinked Polyurethanes

has shown that upon heating many different types of linear seg­

mented polyurethanes, the domains become unstable and mixing of the soft and hard segments takes place; the temperature

of disruption de­

pends on the type of polymer. U p o n cooling, the hard and soft segments start to phase separate and the domains are again formed. Several methods ( stress-strain, S A X S , D S C , N M R ) have been applied to show that the time-dependent changes in Young's modulus, soft-segment glass transi­ tion, and the degree of phase separation are very similar in all of these experiments.

These results showed that the domain formation is a ther­

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mally reversible process and that its kinetics are directly correlated with the changes in the mechanical properties with time. The current work deals with similar time-dependent phenomena of segmented polyurethanes but focuses on a specific segmented

urethane

where different levels of chemical crosslinking have been induced.

Experimental Materials. T h e materials used in this study were based on a single linear polyester urethane that had been crosslinked by different amounts of peroxide. T h e hard segments contained ρ,ρ'-diphenylmethyl diisocyanate ( M D I ) and 1,4-butanediol. The soft segment was poly(tetramethylene adipate) glycol (mol wt ~ 1100). T h e materials were prepared as follows : the M D I , diol, and glycol were mixed and "melt reacted" ran­ domly to yield a polyurethane containing 30 weight percent of hard segments. Next, commercial organic peroxide was mixed into the poly­ mer, and the films were then compression molded at 2 1 0 ° C for 10 minutes while curing took place. Four samples were prepared this way: E S X 2.0 (contains 2.0% peroxide by weight), E S X 1.0, E S X 0.5, and E S X 0.0 contains no peroxide ). Crosslinking was demonstrated by the lower egree of swelling in D M F as the peroxide level increased ( the exact site of chemical crosslinking is not known ). T h e well-characterized materials were kindly prepared by C . S. Schollenberger and K . Dinsbergs of the B. F . Goodrich Center, Brecksville, Ohio. Equipment and Methods. The domain morphology and its changes with time were detected by a Kratky small angle x-ray camera, operated by a P D P 8/a computer. T h e x-ray source was a Siemens A G Cu40/2 tube operated at 40 k V and 25 m A . A CuK 2

scattered

( E q u a t i o n 1 ) a n d hence a reduction i n the

i n t e n s i t y as m e a s u r e d .

T h e a b o v e statement is d i r e c t l y s u p -

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

58

MULTIPHASE POLYMERS

18i

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!6-

0

0.02 0.04

0.06 0.08 0.10

0.12

0.14

0.16 0.18

1

h«2irSin(28)/X ( X " ) Figure

2.

Besmeared

Table I .

S A X S of well-aged ESX samples at room

Important Characterization Parameters E S X Series of Segmented Urethanes

P e r o x i d e ( w t %) S w e l l i n g i n D M F (%) S o f t - s e g m e n t glasstransition temperature ( ° C ) Young's Modulus (MPa) M e a n square of t h e electron d e n s i t y fluct u a t i o n s ( m o l elect/ c m ) X 10 3

2

0.0 ( E S X 0.0) soluble

0.5 ( E S X 0.5) 275

1.0 ( E S X 1.0) 205

temperature

of the 2.0 ( E S X 2.0) 165

-40.0

-38.5

-37.0

-35.0

12.8

11.5

10.3

7.2

0.605

0.495

0.429

3

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

0.385

3.

OPHIR AND WILKES

59

Linear and Crosslinked Polyurethanes

p o r t e d b y t h e v a l u e s of t h e i n v a r i a n t c a l c u l a t e d f r o m t h e s m e a r e d S A X S d a t a (see T a b l e I ) . V a l u e s of Y o u n g ' s m o d u l u s as f o u n d f o r t h e w e l l - a g e d E S X samples also a r e g i v e n i n T a b l e I.

T y p i c a l l y f o r elastomeric polymers, Young's

m o d u l u s increases as t h e d e g r e e of c r o s s l i n k i n g increases. s t u d i e d h e r e t h e results s h o w a n o p p o s i t e t r e n d . r e p o r t e d b y K i m b a l l a n d F i e l d i n g - R u s s e l l (15) b e r g e r et a l . ( I ) .

I n t h e case

S i m i l a r results

were

as w e l l as b y S c h o l l e n -

T h e e x p l a n a t i o n is as f o l l o w s : t h e m a i n c o n t r i b u t o r

to Y o u n g s m o d u l u s i n these t w o - p h a s e systems is the h a r d d o m a i n s t h a t

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serve b o t h as a filler a n d as p h y s i c a l crosslinks. I n t h e p r e s e n c e of these glassy d o m a i n s , t h e r e l a t i v e c o n t r i b u t i o n of t h e c h e m i c a l c r o s s l i n k i n g ( a t least u p to t h e l e v e l that w a s i n d u c e d ) is s m a l l . T h e r e f o r e ,

increasing

the c r o s s l i n k i n g d e n s i t y reduces t h e extent of final p h a s e s e p a r a t i o n a n d thus decreases Y o u n g s m o d u l u s ( a t r o o m t e m p e r a t u r e ) .

This explana-

t i o n is c l e a r l y c o n f i r m e d b y t h e S A X S results. Time-Dependence Experiments. I n t h e

fixed-angle

ments t h e t i m e - d e p e n d e n t scattered i n t e n s i t y I(t)

J

b

SAXS

experi-

was normalized b y :

is t h e b a c k g r o u n d s c a t t e r i n g w h i c h is a b o u t 1 0 % of t h e t o t a l i n t e n s i t y

a n d is a l m o s t constant.

I

0

is t h e i n i t i a l scattered i n t e n s i t y of t h e s a m p l e

b e f o r e t h e t h e r m a l treatment.

T h e n o r m a l i z a t i o n helps correct for changes

i n x - r a y t u b e i n t e n s i t y a n d f o r differences i n t h e s a m p l e thickness.

Fixed-

a n g l e experim en t s w e r e m a d e o n samples w h i c h h a d b e e n a n n e a l e d at the

temperatures

earlier.

of 1 7 0 ° , 1 4 0 ° , 1 2 0 ° , 1 0 0 ° , a n d 8 0 ° C

as

described

F i g u r e s 3a, 3 b , a n d 3 c s h o w t h e results at t h e t w o extreme

temperatures a n d at 1 2 0 ° C .

F r o m these d a t a o n e observes t h e f o l l o w i n g .

( a ) U p o n h e a t i n g a b o v e 1 4 0 ° C , a l l o f t h e samples a p p a r e n t l y lose m o s t of t h e i r phase s e p a r a t i o n . c a n b e r e l a t e d to l o c a l

T h e s m a l l scattered

fluctuations

i n t e n s i t y that

i n the e l e c t r o n d e n s i t y .

m e a s u r e d at a t e m p e r a t u r e b e l o w t h e soft-segment

remained

A f u l l scan

glass-transition tem-

p e r a t u r e a n d f o l l o w i n g t h e r m a l t r e a t m e n t s u p p o r t s this a s s u m p t i o n (b)

(12).

B e l o w 1 2 0 ° C , t h e h i g h e r t h e d e g r e e of c r o s s l i n k i n g , t h e easier t h e

d o m a i n s are p a r t i a l l y d i s r u p t e d ,

( c ) I n a l l cases t h e r e c o v e r y rate d e -

creases as t h e c r o s s l i n k i n g d e n s i t y increases. T h e f o l l o w i n g e x p l a n a t i o n of this b e h a v i o r is n o w suggested.

A s the

S A X S d a t a s h o w , as t h e c r o s s l i n k i n g d e n s i t y goes u p t h e r e is less p h a s e separation.

T h e soft segments that r e m a i n m i x e d i n t h e h a r d d o m a i n s

a p p a r e n t l y act as p l a s t i c i z e r s a n d w e a k e n the d o m a i n s t r u c t u r e .

Cross-

l i n k i n g w i t h i n t h e glassy phase itself p r o v i d e s o n l y a m i n o r c o n t r i b u t i o n to its s t r e n g t h a n d at the same t i m e prevents o p t i m a l p a c k i n g a n d t h e r e b y

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

MULTIPHASE POLYMERS

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60

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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3.

OPHIR AND WILKES

Linear

o I 0

and

Crosslinked

61

Polyurethanes

È

I

I

I

I

1

10

20

30

40

50

60

1

TIME (min) Figure 3. SAXS at a fixed angle of ESX samples at room temperature as a function of time following a thermal treatment of (a) 170°C, (b) 120°C, and (c) 80°C p o s s i b l y decreases the d e g r e e of h y d r o g e n b o n d i n g .

W h e n the

sample

is h e a t e d , the effect of crosslinks w i t h i n the soft p h a s e w i l l t e n d to i n crease the r e t r a c t i v e elastic forces that h e l p p r o m o t e t h e d i s r u p t i o n of t h e d o m a i n s . A c c o r d i n g to the k i n e t i c t h e o r y of r u b b e r e l a s t i c i t y

σ ^Ν ΚΤ 0

w h e r e σ is the stress, N

ν

. f(c)

(5)

is t h e n u m b e r of n e t w o r k segments i n a u n i t v o l ­

v

u m e ( w h i c h is p r o p o r t i o n a l to t h e c r o s s l i n k i n g d e n s i t y ) , Τ is t h e t e m p e r a ­ ture a n d f(e)

represents a s t r a i n f u n c t i o n t h a t w i l l b e a s s u m e d t o

i n d e p e n d e n t of t e m p e r a t u r e .

be

I t is n o t e d t h a t f o r a finite s t r a i n , i.e., f(c) is

n o n z e r o , stress w i l l increase i n d i r e c t p r o p o r t i o n to t e m p e r a t u r e

a n d to

degree of c r o s s l i n k i n g .

indeed

A c c e p t i n g that t h e soft segments are

p a r t i a l l y s t r a i n e d as a result of d o m a i n f o r m a t i o n (7,11),

then one m a y

w e l l expect that the " i n t e r n a l " stress a r i s i n g f r o m s t r a i n e d soft segments at h i g h temperatures samples.

could be

greater f o r the

chemically

crosslinked

W h i l e this statement is d i f f i c u l t to c o n f i r m e x p e r i m e n t a l l y , i t is

l i k e l y as is the fact that t h e d o m a i n cohesiveness is e x p e c t e d to b e less i n the c r o s s l i n k e d samples b e c a u s e of the l i m i t a t i o n s i m p o s e d o n m o l e c u l a r p a c k i n g b y the crosslink j u n c t i o n s .

T h u s , as the t e m p e r a t u r e goes u p , t h e

d o m a i n s of the c r o s s l i n k e d s a m p l e w i l l l i k e l y start to d i s r u p t at l o w e r temperatures t h a n those i n the u n c r o s s l i n k e d s a m p l e .

U p o n r e t u r n i n g to

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

MULTIPHASE POLYMERS

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62

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

3.

OPHIR AND WILKES

!4h" Ε

0

10

Q Ο

8

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I *

r

c

ESX SERIES ANNEALED K}minAT80*C

-ESX 0.0 ~ 12 b ^-ESX 0.5 o -

E S X L0

i-ESX 2.0

4

0.0

ESX

as -i

ESX

1.0

•i

1

ι 20

14

ESX

ESX 2.0 ι*——

UJ

63

Linear and Crosslinked F oly methanes

• ^0

1 I 40 60 TIME (min)

1 80

1..K>0

d

ESX SERIES ANNEALED 10min AT 60*0

-ESX 0.0 £ 12 I-ESX 0 5 2E *-ESX 1.0 ω ίο

ESX 0.0 ESX 0.5