11
Thermally Stimulated Creep
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for the Study of Copolymers and Blends Ph. Demont, L . Fourmaud, D . Chatain, and C. Lacabanne Solid State Physics Laboratory, Paul Sabatier University, 31062 Toulouse Cédex, France
Amorphous phase segregation in polyamide-based copolymers and blends was investigated by thermally stimulated creep. Because of its resolving power, this technique allows complex retardation time spectra to be resolved. A series of poly(ether-amide) block copolymers with constant stoichiometry was studied as a function of mean block length. The thermally stimulated creep values of the polyamide-poly(vinylidene fluoride) dominant phase were compared. In both cases, the dominant phase was found to keep its own amorphous phase structure. Activation entropy-enthalpy compensation diagrams are well suited for determining amorphous phase separation in copolymers and blends.
M
ULTIPHASE MATERIALS S U C H AS B L O C K COPOLYMERS or blends are
widely applied in various fields as functional materials. In this chapter, we will consider, as examples of block copolymers, poly(ether-Mocfc-amide) (PEBA) copolymers, which constitute a new class of thermoplastic elastomer (1). P E B A copolymers consist of alternating linear soft polyether (PE) and hard polyamide (PA) blocks. The soft segments, which possess a relatively low glass transition temperature, are in their rubbery state at use temperature; they impart the elastomeric properties to the copolymers. The hard segments are semicrystalline; they can undergo some kind of intermolecular association with other such hard blocks and thereby form physical crosslinks like hydrogen bonds. The physical cross-linking of the copolymers provides dimensional stability and minimizes cold flow.
0065-2393/90/0227-0191$06.75/0 © 1990 American Chemical Society
Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1990.
POLYMER CHARACTERIZATION
192
T h e i n c o m p a t i b i l i t y o f t h e t w o different c h a i n segments causes m i c r o phase separation w i t h t h e f o r m a t i o n o f h a r d - a n d soft-segment-rich d o m a i n s . T h e extent o f this microphase separation w i l l b e i n f l u e n c e d b y t h e b l o c k l e n g t h , the c r y s t a l l i z a b i l i t y of t h e soft segment, a n d the o v e r a l l h a r d - s e g m e n t content. T h e d e g r e e to w h i c h t h e d i s s i m i l a r blocks segregate i n t o t h e i r respective d o m a i n s w i l l d e t e r m i n e t h e t h e r m a l a n d m e c h a n i c a l p r o p e r t i e s of the b l o c k c o p o l y m e r s (2-4). I n p o l y a m i d e - p o l y ( v i n y l i d e n e fluoride) b l e n d s , b o t h p a r e n t h o m o p o l y m e r s are s e m i c r y s t a l l i n e . D e s p i t e this s i m i l a r i t y , t h e y also s h o w s t r o n g
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differences, s u c h as a glass transition t e m p e r a t u r e that is above r o o m t e m p e r a t u r e for p o l y a m i d e a n d b e l o w r o o m t e m p e r a t u r e for p o l y ( v i n y l i d e n e fluoride). M o r e o v e r , t h e s t r u c t u r e o f the a m o r p h o u s phase is s t a b i l i z e d b y h y d r o g e n bonds i n p o l y a m i d e b u t not i n p o l y ( v i n y l i d e n e fluoride). M o s t l i k e l y , those b l e n d s also w i l l demonstrate m i c r o p h a s e segregation. T h e i n tegrity o f t h e a m o r p h o u s phase w i l l b e strongly r e l a t e d to t h e c h e m i c a l composition. T h e r m a l l y s t i m u l a t e d c r e e p ( T S C r ) analysis has b e e n a p p l i e d to t h e investigation o f phase segregation i n m u l t i p h a s i c p o l y m e r s (5-9). I n d e e d , because o f its v e r y l o w e q u i v a l e n t f r e q u e n c y (10" H z ) a n d its h i g h r e s o l v i n g 3
p o w e r , i t is v e r y w e l l s u i t e d for e x p l o r i n g b r o a d r e t a r d a t i o n m o d e s g e n e r a l l y found i n multiphasic polymers.
Experimental Details Materials. Copolymer. The poly(ether-amide) block copolymers used in this study were synthesized by A T O C H E M (France). A detailed procedure was published by Deleens et al. (1). This kind of block copolymer consists of sequences of soft oiigoether and hard oligoamide segments:
O H - a - ^ - C C H ^ - ^
o
o
Chemical structure of poly(ether—amide) block copolymers The poly(tetramethylene oxide) (PTMO) soft segment has an average degree of polymerization specified by n. In this series of copolymers, n was 28, 14, and 9, giving average soft-segment molecular weights (M„, ) of 2000, 1000, and 650, respectively. The polyamide-12 hard segment was prepared for values of m varying from 7 to 21, giving an average hard-segment molecular weight (M„, ) range of600 to 4000. A summary of the structural parameters is given in Table I. The total molecular weight of the block copolymers was 20,000. The samples for TSCr experiments were prepared by compression molding under a pressure of 100 bar and at a temperature of 20 °C above the melting point of the respective copolymer, followed by quenching in ambient air. Films having dimensions of 60 X 5 X 0.5 mm were obtained and dried under vacuum for 2 h at 380 K. PE
PA
Blends. Polyamide-12 (PA12), poly(vinylidene fluoride) (PVDF), and their blends with or without compatibilizer were obtained from A T O C H E M . The char-
Craver and Provder; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1990.
11.
D E M O N T ET A L .
TSCr To Study Copolymers ir Blends
193
Table I. Structural Parameters of Poly(ether-fefocfc-amide) Copolymers Sample
M ,p
4000-2000 2000-1000 1300-650
4200 2135 1360
a
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b
n
A
M
n > P £
2032 1000 600
m
n
21 10 7
28 14 9
W
H
(%)
a
67 67 67
1.33 1.37 1.30
Weight fraction of the hard segment. Inherent viscosity in m-cresol solvent at 20 °C.
acteristics of homopolymers and blends are reported in Table II. The composition of blends is expressed in weight percent. Compression-molded sheets of homopolymers and blends were made in a press at a 200 °C melt temperature. The samples were then cut to a dimension of 60 X 6 X 0.5 mm for TSCr experiments. Before measurements, samples were dried under vacuum (IO torr, 13.3 mPa) at 400 K for lh. -4
Table II. Weight Fraction of Polyamide (PA), Poly(vinylidene fluoride) (PVDF), and Compatibilizer (A) in Blends Sample
W
PA-PVDF PVDF-PA PA-PVDF-A PVDF-PA-A
85 36 84.5 35.5
W
WpvDF
PA
A
0 0 1 1
15 64 14.5 63.5
NOTE: All values are given in percents. Differential Scanning Calorimetry
Methods.
(DSC).
DSC thermograms over
the temperature range from -140 to about 200 °C were recorded on a Perkin Elmer DSC II. Calibration was performed with indium and mercury as standards. The experiments were carried out at a heating rate of 20 °C/min under a helium purge on 10-mg samples. Thermally
Stimulated Creep (TSCr)
Recovery.
The technique has been de-
scribed elsewhere (5, 6,10). The samples of copolymers were heated to a temperature T , and a shear stress was applied for 2 min. The samples were then quenched from Ta to T
