Environmental-Stress Crazing and Cracking of Poly(methyl

18

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Environmental-Stress Crazing and Cracking of Poly(methyl Methacrylate)Poly(vinylidene Fluoride) Blends S. R. MURFF1, J. W . B A R L O W , and D . R. P A U L Department of Chemical Engineering and Center for Polymer Research, University of Texas, Austin, TX 78712

The tensile properties of miscible blends of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVF2) were determined in air. These results were interpreted in terms of the plasticization caused by adding PVF2 to PMMA and the crystallinity of the blends. Strength exhibits a minimum, and elongation at break shows a maximum as a function of blend composition because of the competition between these two effects. The critical strain for the development of crazes or cracks was determined for blends in the presence of seven different liquids by using a Bergen strain jig. For all liquids, this threshold strain or stress increased dramatically as the PVF2 content of the blend increased.

SCIENTIFIC AND TECHNOLOGC IAL INTEREST

in polymer blends has been intensifying (1,2). This increased interest is driven, in part, by the extra dimension blending of fers for solving problems. A significant portion of the growing scientific literature in this area was devoted to examining and understanding the phase behavior of such mixtures (3) because miscibility is a more common occurrence than originally forecast (4). A n equally important direction is to learn about the problems that can be solved by this new knowledge base and what the trade-offs are. This chapter examines the premature physical mechanical failure that many polymers experience when stressed in the presence of certain fluids. This problem is especially serious for amorphous glassy polymers (5-12) such as poly(methyl methacrylate) ( P M M A ) . O n the other hand, some semicrystalline polymers, such as poly(vinylidene fluoride) ( P V F 2 ) , are extremely resistant to chemicals and are generally the materials of choice when this characteristic is critical. Interestingly, these two polymers, 2

Current address: Catalysts Division, Armak, Pasadena, TX 77507 0065-2393/86/0211/0313$06.00/0 © 1986 American Chemical Society

Paul and Sperling; Multicomponent Polymer Materials Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

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MULT C IOMPONENT POLYMER MATERA ILS

P M M A and P V F 2 , when blended form a miscible amorphous phase. These blends may also have a coexisting crystalline phase of P V F 2 , depending on composition and thermal history. This blend system has been the object of extensive scientific studies since this fact was first reported in 1971. T h e question examined here is, how do mixtures of these distinctly different polymers behave mechanically when exposed to hostile liquids?


Sample Preparation and Characterization The P V F 2 used was supplied by Pennwalt Corporation in the form of pel lets and is commercially designated as Kynar 460, which is a general-pur pose extrusion and molding grade. T h e P M M A was Plexiglas V(811)100 molding pellets (Rohm and Haas Company) and has a number average mo lecular weight (Mn) of 52,900 and a weight average molecular weight (Mw) of 130,000. Blends containing various proportions of P V F 2 and P M M A were fabri cated into sheets 0.015 to 0.030 in. thick by the following procedure. Pellets of the two polymers were mixed in the desired proportions and melt blended in a Brabender single-screw extruder by using barrel and die tem peratures in the range of 200 to 220 ° C . The extrudate was then pelletized. Blend pellets were re-extruded into films approximately 3 in. wide and 0.002 to 0.010 i n . thick. These films were too thin for the subsequent test procedures and were rather highly oriented. Consequently, several layers of film were laminated together in a compression mold at 220 ° C to pro duce sheets essentially free of orientation and with the desired thickness. A l l materials were dried prior to each melt-processing step. T h e blends were characterized by differential scanning calorimetry (DSC). Glass transition temperatures (T g ) and melting points agreed very well with previous work (14), that is, each blend had a single, compositiondependent Τg as seen in Figure 1. First heats in the D S C gave the crystallin ity levels shown in Figure 2. Extruded blends had slightly less crystallinity than compression-molded blends because of the more rapid quenching in this process. Crystallinity goes to zero at about 40% P V F 2 .

Mechanical Properties Mechanical properties were determined for compression-molded sheets by using strips 1 i n . wide with a gauge length of 4 i n . between the grips of an Instron tensile tester. Crosshead speed was 0.2 in./min for determining the modulus and 2 in./min for determining yield and failure behavior. Crosshead travel and a gauge length of 4 in. were used to compute strain. Figure 3 shows typical stress-strain diagrams. Blends containing 40% or more of P V F 2 exhibited ductile behavior, whereas P M M A and the 20 % P V F 2 blend showed typically brittle failure. Modulus, strength, and percent elongation values were averaged for five to six samples, and the results were plotted versus blend composition


18. MURFF ET AL.

Crazing and Cracking of PMMA-PVF2

Blends

315


I2CV

0

20

PMMA

40 WEIGHT

60 (%)

80

100 PVF

2

Figure 1. Glass transition behavior of PMMA-PVF2 blends. Transitions at high PVF2 contents had very small base line shifts and are not shown. Key: A, previous results (14); and ·, this study.

60

r -

40 WEIGHT Figure 2. Crystallinity

60 (%)

of PMMA-PVF2 blends. Key: A , extruded films; and Φ, compression-molded film.



316

MULTC IOMPONENT POLYMER MATERA ILS

% ELONGATION Figure

3. Typical

stress-strain diagrams temperature.

determined

in air at

room

(Figures 4-6). T h e agreement with the results given by Noland et al. (13) is quite good where comparisons are possible. T w o factors are important to the understanding of the variation of these mechanical properties with blend composition. First, the T g decreases continuously with P V F 2 content and becomes less than the testing temperature at high P V F 2 contents (1316). Second, these materials are completely amorphous until the P V F 2 content is about 4 0 % , after which crystallinity increases rapidly. Interestingly, the modulus shows no discernible discontinuity at the composition that divides the amorphous material from the semicrystalline materials. However, the competition between the two factors previously mentioned is clearly seen in the yield and failure characteristics given in Figures 5 and 6. Both the yield and the ultimate strengths decrease initially as P V F 2 is added to P M M A . This response is expected when a liquid or rubbery diluent is added to a glassy polymer (i.e., P V F 2 is effectively a plasticizer). However, these properties go through a minimum and then rise on further addition of P V F 2 . This response is undoubtedly a result of the strengthening effect caused by the increasing crystallinity occurring in the region of high P V F 2 contents. Conversely, the elongation at break initially increases as P V F 2 is added to P M M A because of plasticization or reduction of Tg. It reaches a maximum value at about 60% P V F 2 and then declines as crystallinity becomes a more dominant factor.

Environmental-Stress Cracking and Crazing M a n y polymers, especially glassy ones, develop crazes or possibly cracks when mechanically loaded beyond some threshold stress. This threshold



may be considerably lower than the normal yield or failure stress when the sample is exposed to certain fluids (5-12). F o r wholly amorphous, glassy polymers, the reduction of this threshold stress or strain has been directly related (6-9,12) to the extent particular liquids or vapors swell or are dissolved i n the polymer. This threshold, or critical stress, defines an upper Paul and Sperling; Multicomponent Polymer Materials Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

318


100 80

Ο I 60 ο z

h-

_J


LU

^ 40 20 0 0 PMMA

20

40 60 WEIGHT (%)

Figure 6. Elongation

80 100 PVF

2

at break for blends.

limit on the load a polymer can be expected to bear within a certain envi ronment without damage. Several tests have been devised to measure this limit, and one of the most effective is an elliptical-strain jig first described by Bergen (5) and used extensively by Kambour (6-9,12). This technique was the primary one used in this work. The jigs were machined from aluminum sheet stock 1 in. thick and had the shape of a one-quarter section of an ellipse conforming to the equa tion

where all dimensions are in inches. Sheet specimens 1 i n . wide having thicknesses in the range of 0.015 to 0.030 i n . and fabricated in the manner described earlier were strapped to the curved surface of the jig. Thus, the specimen experiences a maximum surface strain that varies with its position on the jig surface. T h e strain at a given position can be calculated from the relationships given by Bergen (5). T h e clamps prevent cracks in one area from changing the strain in other areas of the sample. T h e jig with a sample so installed was immersed in the liquid of interest overnight, although ap parently all damage to the sample occurred within a few minutes and cer tainly in less than an hour. After removal from the liquid, the specimen was visually inspected by using a low-power magnifying glass with light from


18.

MURFF ET AL.


Blends

varying angles of incidence. T h e distinction between crazing and cracking can be somewhat subjective in difficult samples. Following the usual criteria (17), cracking is associated with visible surface rupture. The critical strain below which no cracking or crazing could be seen was determined with an estimated precision of about ± 10 % based on repeated measurements for each sample and liquid combination. Table I shows a qualitative summary of the observations for seven different liquids and blends ranging from pure P M M A to pure P V F 2 . E a c h Downloaded by CHINESE UNIV OF HONG KONG on March 9, 2016 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch018

liquid caused severe damage to pure P M M A and had no apparent effect on pure P V F 2 . T h e blends showed various effects between these limits. Figure 7 shows a more quantitative picture by giving the measured critical strain as a function of blend composition for four of these liquids. Except for Nmethylformamide, the effect of blend composition on the critical strain is quite similar for each liquid. F o r clarity, data for the other three liquids are not plotted. As a first approximation, the critical-strain values can be multiplied by the tensile moduli measured in air (see Figure 4) to give an estimate of the critical or threshold stress. T h e result is shown in Figure 8. T h e critical stress is the product of a functioji that increases with P V F 2 level (critical strain) and a function that decreases with P V F 2 level (modulus). Consequently, the critical stress varies less with blend composition than does the critical strain. Another more laborious technique for determining the threshold stress for environmental-stress cracking or crazing was used in limited tests for comparison with the results obtained by the Bergen strain jig. In this method, the specimen is loaded to a predetermined stress in an Instron and liquid is applied to an absorbent material clipped to the stressed sample. In the absence of any environmentally active liquid, the sample will presumably support the applied load indefinitely, if it is adequately below the yield stress. However, after applying the appropriate liquid to the wick, the stressed sample breaks catastrophically after a certain time, which is shorter the larger the applied stress. T h e solid points in Figure 9 define the response for 1-propanol and pure P M M A . As seen in this figure, the time to break becomes very long as the stress level is reduced to a level that defines the threshold stress. T h e open point in Figure 9 is the critical stress estimated by the Bergen strain-jig approach. T h e threshold stresses estimated by the two methods agreed very closely.

Summary The strength and elongation at break of miscible blends of P M M A and P V F 2 are influenced by the T g -eomposition relationship, or the plasticizing effect caused by adding P V F 2 to P M M A , and by the development of P V F 2 crystallinity in compositions containing more than about 40 % P V F 2 . The competition between these two opposing effects causes a minimum in strength and a maximum in elongation at break when these quantities are


319


cracking and slight dissolution severe cracking severe cracking

severe cracking

severe cracking

Methanol

Ethanol 2-Propanol

1-Propanol

2-Butanol

Toluene

cracking and swelling crazing cracking and crazing cracking and crazing cracking and crazing

2

20% PVF

cracking and slight swelling crazing cracking and crazing cracking and crazing cracking and crazing

2

40% PVF

slight crazing cracking and slight crazing cracking and crazing slight cracking

cracking

2

60% PVF

cracking and cracking and cracking and crazing crazing crazing cracking and severe cracking and cracking and slight cracking swelling and severe swelling severe swelling and slight dissolution and dissolution and dissolution swelling

n-Methylformamide cracking

100% PMMA

Solvent

very slight cracking no effect

slight cracking

crazing

slight crazing slight crazing

2

80% PVF

no effect

2

1

no effect

no effect

no effect

no effect

no effect no effect

no effect

100 7o PVF-

Table I. Qualitative Observations on the Influence of Various Liquids on PMMA-PVF Blends Under Stress


18. MURFF ET AL.

Blends

321



Figure

8. Calculated critical N-methylformamide;

stress. Key: ·, 1-propanol; ethanol; and A , 2-propanol.


O,

322

MULT C IOMPONENT POLYMER MATERA ILS 2000 f—


Q.

Ο Χ

Ο

Lu

0 0

10

20

30

TIME REQUIRED TO BREAK (min) Figure 9. Time to break as a function of tensile stress for PMMA in contact with 1-propanol (%). The open circle is the critical stress estimated with a Bergen strain jig.

plotted versus blend composition. T h e initial modulus shows a continuous monotonie decline as the P V F 2 content of the blend is increased. No dra matic change occurs at the composition dividing amorphous from semicrystalline mixtures. Interactions between the components that apparently influence the mechanical property relationships for completely amor phous, miscible blends of glassy polymers (18-20) are evidently masked by the more influential issues of plasticization and crystallinity in the P M M A P V F 2 system. Clearly, addition of P V F 2 to P M M A results in materials that are more resistant to environmental-stress cracking or crazing. This result is indi cated by the significant increases in the critical strain and stress obtained by using a Bergen strain jig. Whereas these increases are accelerated as the blends develop crystallinity, significant improvements occur at P V F 2 levels in which crystallinity is absent. Thus, the inherent chemical resistance of P V F 2 is translated into its blends with P M M A by an amount related to the fraction of the blend it occupies. A n interesting way of viewing these results is to superimpose plots of yield strength in air and the critical strength in the presence of a liquid like 1-propanol (see Figure 10). T h e effect of adding P V F 2 to P M M A is to de crease the yield strength in air initially (i.e., plasticization), but to increase the usable stress when in the presence of hostile liquids. One might specu late that plasticization is also responsible for the latter response to some degree. T h e dashed continuation of the critical-stress curve in Figure 10 suggests that the stress required for environmentally induced cracking or crazing of pure P V F 2 is higher than its yield stress.


18. MURFF ET AL.


Crazing

0L_ 0

PMMA Figure

I 20

and Cracking

I 40

of PMMA-PVF2

I 60

WEIGHT (%)

10. Strength in air compared with critical 1-propanol.

I 80

Blends

323

I 100

PVF

2

stress when exposed to

The results presented illustrate one example of the possibilities for us ing miscible blends to solve certain problems and the trade-offs involved. W i t h respect to the latter, acrylic plastics like P M M A or its homologue, poly(ethyl methacrylate), are often used when optical clarity is required. This characteristic will obviously be compromised when P V F 2 is blended to the concentration at which crystallinity is developed.

Acknowledgment This research was supported by the U . S . A r m y Research Office.

Literature Cited 1. B a r l o w , J. W.; P a u l , D . R. Annu. Rev. Mater. Sci. 1981, 11, 299. 2. P a u l , D . R . ; N e w m a n , S., Eds. "Polymer Blends"; Academic: N e w York, 1978; Vols. 1-2. 3. Olabisi, O.; Robeson, L . M.; Shaw, M. T., "Polymer-Polymer M i s c i b i l i t y " ; Academic: N e w York, 1979. 4. D o b r y , Α . ; B o y e r - K a w e n o k i , F . J. Polym. Sci. 1947, 2, 90. 5. Bergen, R. L. SPE J. 1962, 18, 667. 6. Bernier, G . Α . ; Kambour, R. P . Macromolecules 1968, 1, 393. 7. Kambour, R. P.; Romagosa, E. E.; Gruner, C . L. Macromolecules 1972, 5, 335.



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8. Kambour, R. P.; Gruner, C. L . ; Romagosa, Ε . E . Macromolecules 1974, 7, 249. 9. Kambour, R. P.; Gruner, C . L . J. Polym. Sci. Polym. Phys. Ed. 1978, 16, 703. 10. Haven, R. E.; Sung, N . H . SPE Tech. Pap. 1978, 24, 406. 11. Bigg, D . M.; Leininger, R. I.; Lee, C . S. SPE Tech. Pap. 1981, 27, 227. 12. White, S. Α.; Weissman, S. R.; Kambour, R. P. J. Appl. Polym. Sci. 1982, 27, 2675. 13. Noland, J. S.; Hsu, Ν. N . C . ; Saxon, R.; Schmitt, J. M . In "Multicomponent Polymer Systems"; Platzer, N . A . J., E d . ; A D V A N C E S IN CHEMISTRY SE RIES No. 99; ACS: Washington, D . C . , 1971; pp. 15-28. 14. Paul, D . R.; Altamirano, J. O . In "Copolymers, Polyblends, and Composites", Platzer, N . A. J., E d . ; A D V A N C E S IN CHEMISTRY SERIES No. 142; ACS: Washington, D . C . , 1975; pp. 371-85. 15. Paul, D . R.; Barlow, J. W . ; Bernstein, R. E . ; Wahrmund, D. C. Polym. Eng. Sci. 1978, 18, 1225. 16. Nishi, T . ; Wang, T . T . Macromolecules 1975, 8, 909. 17. Beardmore, P.; Rabinowitz, S. Crit. Rev. Macromol. Sci. 1972, 1, 1. 18. Kleiner, L . W . ; Karasz, F. E.; MacKnight, W . J. Polym. Eng. Sci. 1979, 19, 519. 19. Olabisi, O . ; Farnham, A . G . In "Multiphase Polymers"; Cooper, S. L . ; Estes, G . M . , Eds.; A D V A N C E S IN CHEMISTRY SERIES No. 176; ACS: Washing ton, D . C . , 1979; pp. 559-85. 20. Joseph, Ε . Α.; Lorenz, M . D . ; Barlow, J. W . ; Paul, D . R. Polymer 1982, 23, 112. RECEIVED for review November 15, 1984. ACCEPTED February 12, 1985.


Environmental-Stress Crazing and Cracking of Poly(methyl

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