PTMA Polyurethane

of chain extender, which affects the extent of phase separation. ... fection, due to variation of the chain extender. ... 180. URETHANE CHEMISTRY AND ...
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14 The Structure of the Hard Segments in MDI/diol/PTMA Polyurethane Elastomers

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J. BLACKWELL, M. R. NAGARAJAN, and Τ. Β. HOITINK Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106

Polyurethane elastomers d e r i v e t h e i r e l a s t o m e r i c p r o p e r t i e s from phase s e p a r a t i o n o f the hard and s o f t copolymer segments, such t h a t the hard (urethane) segment domains serve as c r o s s ­ l i n k s between the amorphous s o f t segment domains, which a r e u s u a l l y p o l y e s t e r s o r p o l y e t h e r s . We a r e i n t e r e s t e d i n the systems i n which the hard segments a r e prepared from d i p h e n y l methane 4 , 4 - d i i s o c y a n a t e (MDI) w i t h a l i n e a r d i o l as the c h a i n extender: f

- [ - 0 - CO - NH

C

H

2 ^ ^ - NH - CO - 0 - ( 0 Η ) - ] ~ 2

χ

n

The s o f t segments are p o l y ( t e t r a m e t h y l e n e adipate) (PTMA; ^ * 2089). I n these p r e p a r a t i o n s the hard domains a r e c r y s t a l ­ l i n e and there has been c o n s i d e r a b l e i n t e r e s t i n the way the hydrogen bonding and c h a i n packing c o n t r i b u t e t o the s t r u c t u r a l s t a b i l i t y . The mechanical p r o p e r t i e s a r e dependent on the c h o i c e of c h a i n extender, which a f f e c t s the extent o f phase s e p a r a t i o n . Development o f c r y s t a l l i n i t y i s thought t o be an important f a c t o r c o n t r o l l i n g phase s e p a r a t i o n , and we a r e i n v e s t i g a t i n g the e f f e c t on the molecular conformation, packing, and the c r y s t a l l i t e per­ f e c t i o n , due t o v a r i a t i o n o f the c h a i n extender. In our i n i t i a l s t u d i e s we concentrated on the s t r u c t u r e o f poly(MDI-butandiol). I n i t i a l proposals f o r the s t r u c t u r e o f these hard segments were made by Bonart and co-workers (1-3) and by Wilkes and Yusek. (4) The x-ray p a t t e r n o f the o r i e n t e d an­ nealed elastomer f i l m showed a s i n g l e r e f l e c t i o n which c o u l d be assigned t o the hard segments, a t d ~ 7 . 9 Â , a z i m u t h a l l y i n c l i n e d at ^ 30° t o the m e r i d i a n . This r e f l e c t i o n must o r i g i n a t e from Bragg planes i n c l i n e d a t ^ 60° t o the c h a i n a x i s , and Bonart et a l proposed t h a t these planes a r i s e from a s t a g g e r i n g o f adj a c e n t chains so t h a t i n t e r m o l e c u l a r hydrogen bonds can be formed between the urethane groups. Formation o f C = 0 · · · Η - Ν hydrogen bonds which a r e approximately p e r p e n d i c u l a r t o the c h a i n neces­ s i t a t e s s t a g g e r i n g o f the c h a i n s , as occurs f o r example i n the s t r u c t u r e o f the α-form o f Nylon 66. ( 5 )

0097-6156/81/0172-0179$05.00/0 © 1981 American Chemical Society

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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The above s t u d i e s provided i n s i g h t i n t o the s t r u c t u r e o f the hard segments, but more d e t a i l e d proposals c o u l d not be de­ veloped without a knowledge o f the stereochemistry o f the MDI u n i t . I n a d d i t i o n t o the standard bond lengths and angles f o r the phenyl, urethane, and b u t a n d i o l groups, we need t o determine ( i ) the bond angle a t the c e n t r a l C-CH -C b r i d g e ; ( i i ) the o r i e n t a t i o n o f the phenyl r i n g s w i t h respect to each other and to the c e n t r a l C-CH -C plane; ( i i i ) the degree o f p l a n a r i t y o f the urethane groups and t h e i r o r i e n t a t i o n s w i t h respect t o t h e i r adjacent phenyls; and ( i v ) the conformation o f the - ( C H ) ^ - c h a i n and i t s o r i e n t a ­ t i o n w i t h respect t o the diphenylmethane diurethane. In order t o determine the above parameters we are i n v e s t i ­ g a t i n g the s t r u c t u r e s o f model compounds prepared by capping MDI w i t h a l c o h o l s . The f i r s t o f these s t r u c t u r e s was methanol-capped MDI (6) [MeMMe, CH (C H NHC00CH ) ]. This compound c r y s t a l l i z e s i n two polymorphic s t r u c t u r e s , which w i l l be designated MeMMeI and I I . The s t r u c t u r e o f MeMMeI has been determined i n t h i s l a b o ­ r a t o r y , (6) and r e v e a l s many f e a t u r e s which can be expected t o occur i n the polymer s t r u c t u r e . The conformation o f the molecule i s shown i n f i g . l a and the bc_ p r o j e c t i o n of the c r y s t a l s t r u c ­ ture i s shown i n f i g . l b . The c e n t r a l C-CH -C b r i d g e angle i s 114.5° and the phenyl planes are mutually i n c l i n e d a t 90.0°. Despite the chemical symmetry, the molecule i s not p h y s i c a l l y symmetrical, and the two ends o f the molecule are d i f f e r e n t i a t e d by A and Β l a b e l s i n f i g . 1 (a and b ) ; t h i s asymmetry probably occurs i n order t o o p t i m i z e the hydrogen bonding and packing.* The phenyl groups are i n c l i n e d a t 74.5°(A) and 34.7°(B) to the b r i d g e C-CH -C plane. The urethane groups are p l a n a r and i n ­ c l i n e d a t 39.4°(A) and 10.2°(B) to t h e i r adjacent phenyls; the t e r m i n a l methyl carbons l i e approximately i n the planes o f the urethane groups. The B-urethane groups form C = 0 ··· Η - Ν hydro­ gen bonds approximately i n the b£ plane, as seen i n f i g . l b . The molecules are stacked on top o f each other along the a. a x i s (per­ p e n d i c u l a r to the be p l a n e ) , and the A-urethane groups are hydro­ gen bonded to each other along t h i s s t a c k . In c o n s i d e r i n g the type o f packing p o s s i b l e i n the polymer s t r u c t u r e , i t i s c l e a r t h a t the hydrogen bonding seen f o r the Aurethanes i s l i k e l y to occur f o r the polymer, b u t not that f o r the B-urethanes. Another f e a t u r e o f the MeMMe s t r u c t u r e i s t h a t the molecules have c r y s t a l l i z e d end-to-end: see the shaded molecules i n f i g , l b . A model f o r the polymer c h a i n was d e r i v e d (9)

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2

2

2

2

6

i+

3

2

2

2

* I t i s i n t e r e s t i n g that i n the s t r u c t u r e o f MeMMeII, as d e t e r ­ mined by Born e t a l , (7) the molecules are symmetrical, w i t h the two ends o f the molecule r e l a t e d by a twofold a x i s through the CH group. This twofold molecular symmetry a l s o occurs f o r HO-BMB-OH (butandiol-capped MDI; CH [C H NHC00(CH )^OH], as de­ termined i n t h i s l a b o r a t o r y . (8) However, n e i t h e r "of these two s t r u c t u r e s has i n t e r m o l e c u l a r hydrogen bonding networks t h a t could be analogous to s i t u a t i o n i n the polymers. 2

2

6

i+

2

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Figure 1. Structure of MeMMe (methanol-capped MDI) (6): (a) conformation molecule; (b) be projection of the structure showing the packing and intermole hydrogen bonding.

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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by a d d i t i o n o f a -CH2-CH2- c h a i n t o form the b u t a n d i o l u n i t , which was assumed to have a p l a n a r z i g z a g conformation (as i n p o l y e t h y l e n e ) . The s t a c k i n g o f the molecules along the a. a x i s of the MeMMe s t r u c t u r e was r e t a i n e d f o r the polymer, such t h a t the ac p r o j e c t i o n o f the polymer s t r u c t u r e i s as shown i n f i g . 2, where the hydrogen bonding o f the A-urethane groups can be seen. The B-urethane groups are hydrogen bonded t o the next sheet o f c h a i n s , and thus the s t r u c t u r e i s s t a b i l i z e d by hydrogen bonding i n two (approximately p e r p e n d i c u l a r ) d i r e c t i o n s . This proposed s t r u c t u r e has a t r i c l i n i c u n i t c e l l w i t h dimensions a. = 5.2A, b = 4^81, c = 35.OA, α = 121°, 3 = 116°, and γ =85°. The space group i s P I s i n c e the center o f symmetry between molecules i n the MeMMe s t r u c t u r e r e l a t e s the two monomer u n i t s forming the polymer f i b e r repeat. C o n f i r m a t i o n f o r t h i s s t r u c t u r e was obtained i n l a t e r x-ray work, (10) i n which a more h i g h l y r e s o l v e d x-ray f i b e r diagram was obtained f o r the poly(MDI-butandiol) hard segments. Twelve unique r e f l e c t i o n s were observed and these were indexed by a t r i ­ c l i n i c u n i t c e l l w i t h dimensions a. = 5.05Â, ID = 4.67Â, £ = 37.9Â, α = 116°, 3 = 116°, and γ =83.5°. Given the experimental uncer­ t a i n t y i n the measured d-spacings, and the assumptions i n the model b u i l d i n g , the agreement between the observed and p r e d i c t e d u n i t c e l l s i s s t r i k i n g . The most s i g n i f i c a n t d i f f e r e n c e i s probably i n the f i b e r repeat ( c ) , which i n d i c a t e s a more elonga­ ted conformation than t h a t p r e d i c t e d from the "monomer" s t r u c ­ ture. This paper d e s c r i b e s a c o n t i n u a t i o n o f the above work, f i r s t l y t o apply c o n f o r m a t i o n a l a n a l y s i s t o p r e d i c t the c o n f o r ­ mation o f the poly(MDI-butandiol) c h a i n , and secondly to extend these x-ray d i f f r a c t i o n and model b u i l d i n g techniques t o i n v e s ­ t i g a t e the s t r u c t u r e s formed w i t h other c h a i n extenders, n o t a b l y p r o p a n d i o l and ethylene g l y c o l . More d e t a i l e d accounts o f t h i s work have been p u b l i s h e d elsewhere. (11,12) EXPERIMENTAL Specimens The polyurethane specimens used i n t h i s r e s e a r c h were generously provided by Dr. C.S. Schollenberger o f B.F. Goodrich Co., B r e c k s v i l l e , Ohio. The specimens were prepared from r e actant mixtures c o n t a i n i n g the r a t i o 6 moles MDI:5 moles d i o l : 1 mole PTMA ( M * 2089), corresponding t o approximately 50% hard segment content; the d i o l c h a i n extenders were ethylene g l y c o l (EDO), p r o p a n d i o l (PDO), and b u t a n d i o l (BDO). Oriented f i l m s o f the BDO and PDO polymers were prepared by s l o w l y s t r e t c h i n g 1mm t h i c k unoriented f i l m s t o 700% e x t e n s i o n on a s t a i n l e s s s t e e l rack a t 130°C over a p e r i o d o f 7 days. As w i l l be seen, these specimens showed no evidence f o r s o f t seg­ ment c r y s t a l l i n i t y . This procedure was u n s u c c e s s f u l f o r the EDO n

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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BLACKWELL ET AL.

MDI/diol/PTM A Polyurethane Elastomers

1

Figure 2. The ac projection of the structure of poly (MDI-butandiol) hard segments proposed by Blackwell and Gardner (9).

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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polymer, i n that the f i l m s always broke during s t r e t c h i n g above 60°C. The l a t t e r specimens were o r i e n t e d by s t r e t c h i n g 400% a t room temperature. The x-ray p a t t e r n s obtained showed both hard and s o f t segment c r y s t a l l i n i t y .

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X-ray D i f f r a c t i o n X-ray d i f f r a c t i o n p a t t e r n s were recorded u s i n g a S e a r l e t o r o i d a l f o c u s i n g camera w i t h n i c k e l f i l t e r e d CuKa r a d i a t i o n generated by a Rigaku-Denki r o t a t i n g anode source. Conformational A n a l y s i s The c h a i n conformations o f poly(MDI-BDO), poly(MDI-PDO), and poly(MDI-EDO) were i n v e s t i g a t e d (11) using both s e m i e m p i r i c a l p o t e n t i a l energy f u n c t i o n s and CNDO/2 quantum mechanical methods. (13) Standard i n t e r a t o m i c d i s t a n c e s and angles were used f o r the diphenylmethane and - ( C H ) - u n i t s . S i m i l a r para­ meters f o r the urethane group were obtained from a survey o f 24 c r y s t a l s t r u c t u r e s of low molecular weight urethane compounds. The s e m i e m p i r i c a l c a l c u a l t i o n s determined the t o t a l conforma­ t i o n a l energy by summing non-bonded (van der Waals), t o r s i o n a l , angular deformations, and e l e c t r o s t a t i c e n e r g i e s . P o i n t charges f o r the c a l c u l a t i o n o f the e l e c t r o s t a t i c c o n t r i b u t i o n s were c a l ­ c u l a t e d u s i n g the CNDO/2 method. The c o e f f i c i e n t s used f o r the p o t e n t i a l f u n c t i o n s were due to Sheraga and co-workers. (14) CNDO/2 c a l c u l a t i o n s were a l s o used to estimate the e f f e c t s of i n t e r a c t i o n o f the π-orbitals of the phenyl and urethane groups. 2

X

RESULTS AND DISCUSSION Conformational A n a l y s i s o f Poly(MDI-BDO) The conformation o f the poly(MDI-BDO) c h a i n i s d e f i n e d by s i x t o r s i o n angles as shown below. (CH ) ^ — ^ C 0 N H H K \ 2

/V-àr€H —Λ-^-NHCO.O—^(CH^ 2

The phenyl and urethane groups a r e p l a n a r and a r e assumed t o be r i g i d ; the conformation o f the - ( C H ) - c h a i n must a l s o be de­ f i n e d . The d e f i n i t i o n s o f the o r i g i n s f o r the s i x t o r s i o n angles are given i n reference 11. This polymer has a long chemical repeat and conformational a n a l y s i s would be a formidable task were i t not t h a t the x-ray data i n d i c a t e a h i g h l y extended conformation: two monomer u n i t s repeat i n 37.9Â, i . e . a r i s e o f 1 8 . 9 5 Â per repeat. Consequently, the conformation w i l l be determined by i n t e r a c t i o n s between contiguous groups. Conformational energy c a l c u l a t i o n s have been used to i n v e s t i g a t e the phenyl-phenyl, the pheny1-urethane, and, a f t e r c o n s i d e r a t i o n o f the p o s s i b l e - ( C H ) - conformations, the 2

if

2

i+

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

BLACKWELL

MDI/diol/PTMA

ET AL.

Polyurethane Elastomers

urethane - ( C H ^ ^ i n t e r a c t i o n s . These r e s u l t s have then been combined t o p r e d i c t the conformation o f the polymer c h a i n . For the phenyl-phenyl i n t e r a c t i o n s , the energy o f the d i phenylmethane u n i t was minimized as a f u n c t i o n o f φ} and φ f o r d i f f e r e n t values o f the C-CH -C b r i d g e angle, τ. Minimum energy occurs f o r τ = 110°, the t e t r a h e d r a l angle. However there i s a s u b s i d i a r y minimum a t τ = 118.3°, w i t h energy 1.9 kcal/mole highei than the τ = 110° minimum. The s u b s i d i a r y minimum i s a t Φχ,Φ = (-60°,-60°) f o r which the phenyl r i n g s are mutually p e r p e n d i c u l a i and e x a c t l y gauche t o the c e n t r a l CH group ( c . f . τ=114.5° i n the s t r u c t u r e o f MeMMeI. (6) This s u b s i d i a r y minimum a l s o leads to a more extended conformation and f o r these reasons was s e l e c ­ ted f o r the diphenylmethane s e c t i o n o f the polymer c h a i n . For the phenyl-urethane i n t e r a c t i o n s , minimum energy occurs a t X = ±90°, f o r which the phenyl and urethane planes are perpendicu­ l a r . The most extended conformation i s Χ ,X =-90°,+90°; a monomer repeat o f c_ =18.95Â cannot be achieved f o r the other combinations o f Χχ,Χ f o r any conformation o f the r e s t o f the chain. The above conformation: Χ = -90°, φ = -60°, φ = -60°, X = +90° has a l e n g t h o f 14.0Â measured from the t e r m i n a l oxygens of the urethane groups. Minimum energy f o r the 0-(CH ) -0 sect i o n i s f o r an a l l - t r a n s conformation which i s coplanar w i t h the urethanes, ω , ω = 180°,180°, i . e . the s e c t i o n -NH-C0-0-(CH ) -O-CO-NHi s a p l a n a r z i g z a g . Examination o f s u b s i d i a r y m i n i m a c o n t a i n i n g gauche bonds show t h a t the monomer repeat o f _c = 18.95A cannot be a t t a i n e d w i t h any but the a l l - t r a n s conformation. Thus the p r e d i c t e d polymer conformation i s d e f i n e d by the f o l l o w i n g t o r s i o n angles ω! = 180°, Χ = -90° , φ! = -60°, φ = -60°, χ = +90°, ω = 180°, w i t h an a l l - t r a n s - ( C H ) - u n i t . The monomei repeat f o r our model was 18.95Â which matches the observed value, ( V a r i a t i o n o f the bond lengths and angles i s p o s s i b l e w i t h i n the standard ranges, r e s u l t i n g i n a v a r i a t i o n o f approximately ±0. 5Â i n the monomer repeat ; the exact agreement i s c o n t r i v e d f o r convenience i n l a t e r packing a n a l y s e s . ) A p r o j e c t i o n o f the pred i c t e d conformation i s shown i n f i g . 3. We can be reasonable c o n f i d e n t t h a t the a c t u a l t o r s i o n angles i n the s o l i d s t a t e s t r u c t u r e w i l l be c l o s e t o the p r e d i c ted v a l u e s , except f o r X j and X , which d e f i n e the phenylurethane o r i e n t a t i o n s . The model d i p h e n y l - d i u r e t h a n e s t r u c tures (6-8) have X angles i n the range 10-37°. The d e v i a t i o n s from X = 90° c o u l d be due t o e l e c t r o n d e r e a l i z a t i o n between the phenyl and urethane groups, which would f a v o r the s t e r i c a l l y d i s a l l o w e d X = 0° conformation. Such e l e c t r o n d e r e a l i z a t i o n has been considered elsewhere f o r aromatic polyamides and p o l y e s t e r s . However, a s i m p l e r e x p l a n a t i o n may be t h a t s i g n i f i c a n t space occurs between the molecules i f they are hydrogen bonded i n the X = ±90° conformations, and t h a t t h i s i s e l i m i n a t e d when X i s v a r i e d i . e . the urethanes are t w i s t e d away from a p o s i t i o n 2

2

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18

2

2

χ

2

2

ι

χ

2

2

2

χ

1+

2

2

lf

Q

χ

2

2

2

2

I+

2

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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18.95 Â Figure 3. Projection of the minimum energy conformation of poly(MDI-BDO) (11). The fiber repeat is 37.9 Â and contains two monomers, that is, the monomer repeat along the fiber axis is 18.95 Â.

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 1, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch014

14.

BLACKWELL

ET

AL.

MDI/diol/PTMA

Polyurethane Elastomers

187

p e r p e n d i c u l a r to the phenyl r i n g s . The energy i n c r e a s e f o r the molecular conformation would be more than compensated by t h a t due to the packing e f f e c t s . V a r i a t i o n of X has l i t t l e e f f e c t on the l e n g t h of the monomer repeat, owing to the f a c t t h a t the t e r m i n a l oxygen of the urethane group i s almost c o l i n e a r w i t h the N-C (phenyl) bond. Table 1 shows the monomer repeats f o r centrosymmetric chains w i t h a range of values of Xj and X , where i t i s seen t h a t e/2 does not vary by more than 0.2Â. We are now c o n s i d e r i n g the packing of the poly(MDI-BDO) chains i n the proposed t r i c l i n i c u n i t c e l l , and w i l l r e p o r t on t h i s i n due course. 2

Poly(MDI-PDO) and Poly(MDI-EDO) X-ray d i f f r a c t i o n p a t t e r n s of o r i e n t e d f i l m s of the BDO, PDO, and EDO p r e p a r a t i o n s (12) are shown i n f i g . 4. The p o l y (MDI-BDO) p a t t e r n has been analyzed p r e v i o u s l y and i n d i c a t e s a t r i c l i n i c u n i t c e l l i n which the base plane i s t i l t e d , i . e . i t i s not p e r p e n d i c u l a r to the f i b e r a x i s , such t h a t the chains are i n a staggered hydrogen bonded a r r a y . The conformational a n a l y s i s above shows t h a t the polymer c h a i n i s f u l l y extended, w i t h the a l l - t r a n s - ( C H ^ ^ - u n i t coplanar w i t h the urethane groups. The x-ray p a t t e r n of poly(MDI-PDO) and the EDO p r e p a r a t i o n can be seen to be q u i t e d i f f e r e n t from t h a t of poly(MDI-BDO). Poly(MDI-PDO) shows m e r i d i o n a l r e f l e c t i o n s a t d=16.2Â, 8.1Â, and ^4.OA, and an e q u a t o r i a l a t d - 4.7Â, w i t h a row l i n e at t h a t p o s i t i o n . This p a t t e r n i s c h a r a c t e r i s t i c of a m o n o c l i n i c u n i t c e l l w i t h the base plane p e r p e n d i c u l a r to the f i b e r a x i s , i . e . the chains are not staggered but are arranged i n r e g i s t e r . The poly(MDI-BDO) and poly(MDI-PDO) specimens had been prepared i n the same manner, and the lower q u a l i t y of the x-ray p a t t e r n f o r the l a t t e r i n d i c a t e s a lower degree of c r y s t a l l i n e order i n the poly(MDI-PDO) hard segments. I n the p a t t e r n of the EDO preparat i o n , m e r i d i o n a l r e f l e c t i o n s are seen a t 15. OA and 7.5Â, and there i s a s t r o n g e q u a t o r i a l a t d=4.05Â. This specimen had been prepared at room temperature, and consequently c r y s t a l l i n e r e f l e c t i o n s f o r the s o f t segments are to be expected. The e q u a t o r i a l r e f l e c t i o n matches t h a t a t d=4.05A f o r the α-form of PTMA, (15) the more probable polymorph g i v e n the method of prepa­ r a t i o n . However the m e r i d i o n a l r e f l e c t i o n s are not observed f o r e i t h e r a- or $-PTMA and these can be assigned to the poly(MDIEDO) hard segments. Analogy w i t h the poly(MDI-PDO) p a t t e r n suggests t h a t the PDO and EDO hard segments have s i m i l a r c h a i n conformations, w i t h monomer repeats of 16.2 and 15.OA respec­ t i v e l y , and are packed i n an unstaggered a r r a y . The 1.2Â d i f f e r e n c e i n the monomer repeats f o r poly(MDI-PDO) and poly(MDI-EDO) i s probably due to d e l e t i o n of a trans-CH?group: i n p o l y e t h y l e n e the repeat i s 1.27Â per CH group. However, the d i f f e r e n c e between the poly(MDI-BDO) repeat of 18.95A and t h a t f o r poly(MDI-PDO) of 16.2Â cannot be e x p l a i n e d so simply. 2

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

U R E T H A N E CHEMISTRY A N D

APPLICATIONS

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188

Figure 4. X-ray diffraction patterns of oriented polyurethanefilms:(a) MDI/ BDO/PTMA stretched 700% at 130°C (10); (b) MDI/PDO/PTMA stretched 700% at 130°C (12); (c) MDI/EDO/ΡΊΜΑ stretched 400% at room tempera­ ture (12). In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

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Polyurethane Elastomers

189

From the c o n f o r m a t i o n a l a n a l y s i s poly(MDI-BDO) i s seen t o be f u l l y extended, w i t h an a l l - t r a n s b u t a n d i o l u n i t . Thus i t must be concluded t h a t the poly(MDI-PDO) and poly(MDI-EDO) conforma­ t i o n s a r e c o n t r a c t e d and that there a r e some gauche conformations i n the 0-(CH ) -0 s e c t i o n s o f the c h a i n s . Our procedure i n p r e d i c t i n g poly(MDI-PDO and poly(MDI-EDO) conformations has been t o take the extended minimum energy con­ formation f o r the diphenylmethane diurethane s e c t i o n (Χχ = -90°, Φΐ = -60°, Φ = -60°, Χ = +90°) and to l o o k a t p o s s i b l e conforma­ t i o n s o f the d i o l s e c t i o n s , comparing t h e i r r e l a t i v e energies and f i b e r r e p e a t s . (12) These conformations a r e d e f i n e d by the t o r s i o n angles ω , ω , ω and [ f o r poly(MDI-PDO) o n l y ] ω , as shown below. 2

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MDI/diol/PTMA

ET AL.

x

2

2

1

2

3

4

-O^H ^H -^H --^C0NI^ 2

2

2

ΛΗ:Η -Λ

/)^C00-^H —^Η ^Η ·^

2

2

2

2

The chemical repeats a r e symmetrical and i t i s l i k e l y t h a t t h i s w i l l l e a d t o symmetry i n the conformations on e i t h e r s i d e o f the center o f the d i o l u n i t i . e . ω = ω and ω = ω f o r poly(MDI-PDO) and ω = - ω f o r poly(MDI-EDO). These symmetries l e a d t o monomer repeats. The x-ray data appear to be compatible w i t h m u l t i p l e repeats e.g. where s u c c e s s i v e monomer u n i t s a r e r e l a t e d by a screw a x i s , and we a r e i n the process o f c o n s i d e r i n g these op­ t i o n s . However, there i s no p o s i t i v e x-ray evidence f o r anything more complex than a monomer r e p e a t , and the m u l t i p l e repeat pos­ s i b i l i t i e s w i l l not be d i s c u s s e d f u r t h e r . An energy map was obtained f o r the ω ω conformations f o r poly(MDI-PDO) and the p o s i t i o n s and r e l a t i v e energies o f the minima a r e l i s t e d i n t a b l e 2. Not s u r p r i s i n g l y , the lowest energy i s f o r ω ,ω =180°,180°, corresponding to the f u l l y ex­ tended a l l - t r a n s conformation. This has a monomer repeat o f 17.6A which i s s i g n i f i c a n t l y longer than the observed v a l u e o f 16.2Â. I n a d d i t i o n i t can be seen t h a t such a c h a i n cannot form s t r a i g h t i n t e r m o l e c u l a r hydrogen bonds s i n c e the N-H groups o f urethanes adjacent t o the c e n t r a l - ( C H ) - a r e p o i n t i n g i n the same d i r e c t i o n . The next lowest minimum, 0.9 kcal/mole above the a l l - t r a n s conformation i s f o r ω , ω = 180°,±60° (+60° and -60° correspond to the gauche " and gauche"" conformations f o r the CH groups) . This trans-gauche^-gauche"*"-trans (and trans-gauche"-gauche""-trans) o p t i o n has a f i b e r repeat of 16.20Â. F o l l o w i n g t h i s , the t h i r d minimum i s 1.4 kcal/mole h i g h e r than a l l - t r a n s a t ω , ω = ±80°,180° ( d e v i a t i o n from ±60° i s due to the CH - c a r b o n y l i n t e r a c t i o n ) . This corresponds t o the gauche"*"-trans-trans-gauche"*" (and gauche"trans-trans-gauche ) conformation, and has a f i b e r repeat o f 16.24Â. The second and t h i r d conformations a r e both compatible w i t h the observed monomer repeat. The remaining minima a r e f o r a l l - g a u c h e o p t i o n s , of which the most extended conformation has a repeat of 15.31Â, s i g n i f i c a n t l y s h o r t e r than the observed value. 1

1

4

2

3

3

1 ?

χ

2

2

2

χ

3

2

4

2

1

2

2

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

190

U R E T H A N E CHEMISTRY AND

The two p o s s i b l e conformations of poly(MDI-PDO) are shown i n f i g . 5. We are c u r r e n t l y c o n s i d e r i n g packing models f o r these conformations, which may a l l o w f o r s e l e c t i o n of one or the other as the more l i k e l y . However, evidence i n favor of the lower energy form (trans-gauche -gauche -trans) comes from the s t r u c ­ ture of another model compound determined i n t h i s l a b o r a t o r y : butandiol-capped MDI[CH {C H NHC00(CH ) 0H} ; HO-BMB-OH], which i s shown i n f i g . 6. The t e r m i n a l -C^-0 chains are approximately p l a n a r , but are gauche to the urethane: ω ,ω = 170.5,69.5°. As i n d i c a t e d i n f i g . 6, the C ··· C d i s t a n c e corresponding to the poly(MDI-PDO) repeat i s 16.34Â. I t should be noted t h a t i n the HO-BMB-OH s t r u c t u r e χ = 10.4°, i . e . the phenyl and urethane groups are almost coplanar. However, symmetrical v a r i a t i o n of Xj and X changes the f i b e r repeat by no more than ±0.2Â, i . e . c_ i s r e l a t i v e l y i n s e n s i t i v e to the phenyl-urethane o r i e n t a t i o n s , as was the case f o r poly(MDI-BDO). The same energy c a l c u l a t i o n s can be used to i n v e s t i g a t e the p o s s i b l e poly(MDI-EDO) conformations. The energy minima and monomer repeats are l i s t e d i n t a b l e 3. The a l l - t r a n s and t r a n s gauche"*"- trans conformations have monomer repeats t h a t are s i g n i f i c a n t l y longer than the observed value of 15.0Â, which c o i n c i d e s w i t h t h a t f o r the t h i r d minimum: gauche"*"-trans-gauche". This conformation i s proposed f o r the poly(MDI-EDO) s t r u c t u r e , and i s shown i n f i g . 7. As was the case f o r the other polymers, symmetr i c a l v a r i a t i o n of Xj and X have l i t t l e e f f e c t on the f i b e r repeat. We are c u r r e n t l y s t u d y i n g packing models f o r a l l three polymers, which should throw l i g h t on the more l i k e l y X angles. At t h i s p o i n t the f o l l o w i n g c o n c l u s i o n s can be drawn from these i n v e s t i g a t i o n s . The BDO hard segments have b e t t e r c r y s t a l l i n e order than the PDO hard segments, suggesting more e f f e c t i v e phase s e p a r a t i o n i n the BDO p r e p a r a t i o n . This may have the simple e x p l a n a t i o n t h a t the poly(MDI-BDO) chains can c r y s t a l l i z e more e a s i l y , s i n c e i n the c r y s t a l s t r u c t u r e they have the lowest energy f u l l y extended conformation, which forms s t r a i g h t i n t e r molecular hydrogen bonds i n both dimensions p e r p e n d i c u l a r to the chain a x i s . Poly(MDI-PDO) however cannot form an e q u i v a l e n t hydrogen bonding network i f i t adopts the lowest energy f u l l y extended conformation, and thus w i l l need to rearrange to a h i g h e r energy c o n t r a c t e d conformation. This s t r u c t u r e w i l l then be s t a b i l i z e d by i n t e r m o l e c u l a r hydrogen bonds, but w i l l be of h i g h e r energy than the poly(MDI-BDO) s t r u c t u r e , and thus there w i l l be l e s s of a d r i v i n g f o r c e f o r phase s e p a r a t i o n due to c r y s t a l l i z a t i o n of the hard segments. The r e s u l t s f o r poly(MDIEDO) are more s u r p r i s i n g i n t h a t one might expect t h a t t h i s polymer would adopt a s t r u c t u r e s i m i l a r to poly(MDI-BDO), s i n c e EDO has an even number of CH groups. I t may be however t h a t the ethylene g l y c o l c h a i n i s too s h o r t to a l l o w adequate packing of the diphenylmethane diurethane u n i t s i n the extended conformation, r e s u l t i n g i n adoption of a c o n t r a c t e d conformation more analogous to poly(MDI-PDO). +

2

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APPLICATIONS

+

6

lt

2

lt

2

+

1

2

2

2

2

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

BLACKWELL

MDI/diol/PTMA

ET AL.

Polyurethane Elastomers

Table 1 Minimum Energy Conformations of the MDI-butandiol Fragment

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I n i t i a l Conformation (degrees) φ

Relative Energy (kcal/mole)

Length o f the MDI-butandiol Fragment c_/ 2

No.

ωχ

1

-180

Xl - 90

Φι -60

-60

90

180

0.00

18.95

2

-180

-100

-60

-60

80

180

0.32

19.02

3

-180

-110

-60

-60

70

180

0.88

19.06

4

-180

-120

-60

-60

60

180

1.62

19.09

5

-180

-130

-60

-60

50

180

2.58

19.06

6

-180

-140

-60

-60

40

180

3.92

19.02

7

-180

-150

-60

-60

30

180

5.35

18.95

8

-180

-160

-60

-60

20

180

7.03

18.86

9

-180

-170

-60

-60

10

180

9.12

18.77

Χ

2

2

ω

2

Table 2 Minimum Energy Conformations of MDI-PDO Fragment Conformational Angles (degrees) ωχ

ω

180

φ

Fiber Repeat c(A)

X2

ω

-60

90

180

180

0.00

17.60

-60

-60

90

60

180

0.90

16.20

-90

-60

-60

90

180

80

1.39

16.24

-90

-60

-60

90

80

60

1.64

15.31

180

Xl -90

Φι -60

180

60

-90

80

180

60

80

2

Relative Energy (kcal/mole)

2

3

Table 3 Minimum Energy Conformations of MDI-EDO Fragment Conformational Angles (degrees)

Relative Energy (kcal/mole)

Fiber Repeat c(A)

2

ω

180

180

180

0.00

16.45

90

180

60

180

0.94

15.80

-60

90

60

180

-60

1.28

15.00

-60

90

60

60

-60

1.98

14.16

Xl -90

Φΐ -60

Φ2

X

-60

90

-90

-60

-60

-90

-60

-90

-60

ω 2

1

ω

3

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

191

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192

URETHANE CHEMISTRY AND APPLICATIONS

Figure 5. Projection of predicted conformations of poly (MDI-PDO) (12): (a) trans-gauche -gauche -trans; fiber repeat 16.20 Â; (b) gauche -trans-transgauche ; fiber repeat 16.24 Â. +

+

+

+

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ET

AL,

MDI/ diol/PTMA Polyurethane Elastomers

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BLACKWELL

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

193

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194

URETHANE CHEMISTRY AND APPLICATIONS

15.0

Figure 7. Predicted conformation for poly(MDI-EDO) (12). The ethylene glycol unit has the g a u c h e - t r a n s - g a u c h e conformation; thefiberrepeat is 15.0 A. +

+

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

I

14.

BLACKWELL E T AL.

MDI/diol/PTMA

Polyurethane Elastomers

195

We have r e c e n t l y obtained x-ray d i f f r a c t i o n diagrams o f e q u i v a l e n t p r e p a r a t i o n s using p e n t a n d i o l and h e x a n d i o l and a r e extending the above c o n s i d e r a t i o n s t o these and systems prepared w i t h other c h a i n extenders. Our research i s supported by A.R.O. grant number DAAG29-79G-0070.

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Abstract We are using x-ray methods to determine the structure of the poly(MDI-diol) hard domains in polyurethane elastomers, in order to understand the effect of chemical structure on the crys­ talline order and extent of phase separation. Oriented films containing approximately 50% hard segments have been obtained for preparations using butandiol (BDO), propandiol (PDO), and eth­ ylene glycol (EDO) as the chain extender; the soft segment is polytetramethylene adipate (M =2089). Poly (MDI-BDO) is the most crystalline, and has a triclinic unit cell with a tilted base plane such that there is a staggering of adjacent chains in the crystal structure. In contrast, poly(MDI-PDO) and poly(MDI-EDO) have monoclinic unit cells with adjacent chains in register. In order to develop molecular models for these polymers we have used single crystal x-ray methods to determine the structures of methanol- and butandiol-capped MDI. Based on the data from these structures, conformational analysis has been used to predict the likely chain conformations for the polymers. A model has been developed for poly(MDI-BDO) in which fully extended chains are staggered to give a hydrogen bonding network in two dimensions perpendicular to the chain axis. In contrast poly(MDI-PDO) and poly(MDI-EDO) crystallize in higher energy contracted conforma­ tions, which are necessary for the non-staggered packing of the chains. This difference may explain the higher crystalline per­ fection of the hard segments in the butandiol preparation. n

Literature Cited 1. Bonart, R.J. J. Macromol. Sci. (Β), 1968, 2, 115. 2. Bonart, R.J.; Morbitzer, L . ; Hentze, G. J. Macromol. Sci. (Β), 1969, 3, 337. 3. Bonart, R.J.; Morbitzer, L . ; Muhler, E.H. J. Macromol. Sci. (B), 1974, 9, 447. 4. Wilkes, C.W.; Yusek, C. J. Macromol. Sci. (Β), 1973, 7, 157. 5. Bunn, C.W. and Garner, E.V. Proc. Roy. Soc. (London), 1947, A189, 39. 6. Gardner, K.H. and Blackwell, J. Acta Crystallogr., 1980, B36, 1972. 7. Born, L.; Hocker, J.; Paulus, H.; Wölfel, E. Krist. Struct. Commun., (in press); Hocker, J. and Born, L. J. Polymer Sci.Polymer Letters Edns., 1979, 17, 723. 8. Forcier, P.G. and Blackwell, J. Acta Crystallogr., 1980,B37, 286.

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

URETHANE CHEMISTRY AND APPLICATIONS

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196

9. Blackwell, J. and Gardner, K.H. Polymer, 1979, 20, 13. 10. Blackwell, J. and Ross, M. J. Polymer Sci.-Polymer Letters Edns., 1979, 17, 447. 11. Blackwell, J. and Nagarajan, M.R. Polymer, 1981,22,202. 12. Blackwell, J.; Nagarajan, M.R.; Hoitink, T., submitted to Polymer. 13. Pople, J.A. and Severidge, D.L. "Approximate Molecular Or­ bital Theory"; McGraw-Hill, New York, 1970. 14. Ooi, T.; Scott, R.A.; Vanderkooi, G.; Scheraga, H.A. J. Phys. Chem., 1967, 46, 4410. 15. Minke, R. and Blackwell, J. J. Macromol. Sci.-Phys. (Β), 1979, 16(3), 407. RECEIVED May 11, 1981.

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.