33
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The Influence of Water Concentration on the Mechanical and Rheo-Optical Properties of Poly(methyl methacrylate) R. S. MOORE and J. R. FLICK Eastman Kodak Co., Rochester, N Y 14650
Although several studies have been done of the influence of H O on the dynamic mechanical properties of methacrylate polymers (1, 2, 3), relatively little is generally known about the influ ence of H O content on the rheo-optical and tensile properties of such polymers. This study of poly(methyl methacrylate), PMMA, was undertaken to obtain further understanding of the effect of H O on these properties and to serve as a model system for exam ining experimental methods for use in future studies of the mechanical and rheo-optical properties of other methacrylate polymers. In addition, we hope to gain insight into the poten tial importance of a variation in H O content in general studies on other polymers, how it influences the manner and degree of orientation of the polymer in response to stress, and how it influences the correlation between mechanical and optical proper ties in terms of the stress-optical law. Study of H O in PMMA is also of interest as a model system for studying the effect of a diluent at low concentrations. 2
2
2
2
2
Experimental Material Characterization and Sample Preparation. Polymer. The PMMA was a commercial material, Plexiglas RV 811, made by Rohm and Haas Company. This polymer was especially chosen be cause of its widespread use as an acrylic thermoplastic. It was analyzed by gel permeation chromatography. Molecular weights, determined from the respective polystyrene-equivalent values with appropriate conversion factors, indicate that the polymer has a weight-average molecular weight, Μ , of 1.5 x 10 and a weightto-number average molecular weight ratio, M/M of 2.3. Hence, the polymer has a reasonably narrow molecular 'weight d i s t r i b u 5
w
w
n
tion. Specimen Preparation. Specimens f o r t e n s i l e and b i r e f r i n g ence studies were prepared from sheets molded a t about 200°C between s t a i n l e s s s t e e l platens t o the appropriate t h i c k n e s s . 0-8412-0559-0/ 80/47-127-555S05.00/0 © 1980 American Chemical Society Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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556
WATER
IN P O L Y M E R S
The i n i t i a l p e l l e t s of the polymer were m i l l e d t o a granular powder. The polymer was then d r i e d a t 110°C to remove r e s i d u a l moisture, a p o t e n t i a l source of bubbles during molding. No r e s i d u a l b i r e f r i n g e n c e was observed i n the molded sheets as judged by viewing them between crossed p o l a r i z e r s . Specimens f o r t e n s i l e studies were t y p i c a l l y 3 χ 1 χ 0.03 i n . , and specimens f o r b i r e f r i n g e n c e s t u d i e s were 3 χ 0.5 χ 0.01 i n . The thinner specimens were cut on a Thwing-Albert apparatus; t h i c k e r s p e c i mens were prepared on an e l e c t r i c a l l y d r i v e n saw designed f o r c u t t i n g g l a s s . Samples were stored a t room temperature i n v a r i ous environments, as discussed below. Specimen C o n d i t i o n i n g . Specimens were conditioned i n three d i f f e r e n t environments as f o l l o w s : samples placed i n a wet environment were s t i r r e d i n d i s t i l l e d H^O i n capped b o t t l e s a t room temperature f o r as long as 29 days before use; samples placed i n a dry environment were stored i n a d e s i c c a t o r over f o r a l i k e p e r i o d a t room temperature; samples stored i n a reg u l a r environment were kept a t ambient c o n d i t i o n s f o r a s i m i l a r p e r i o d such that the usual e q u i l i b r i u m H^O content (see below) obtained. F i g u r e 1 shows a p l o t of the percent gain i n weight as a f u n c t i o n of time f o r the t h i n ( b i r e f r i n g e n c e ) and t h i c k ( t e n s i l e ) specimens a f t e r immersion i n H^O. The i n f l u e n c e of specimen thickness on the rate of e q u i l i b r a t i o n i s evident, t h i n specimens imbibing H^O somewhat f a s t e r than t h i c k ones. (This e f f e c t of thickness i s rather common and has i t s o r i g i n i n the more r a p i d attainment of a f i n i t e H^O concentration i n the c e n t r a l core i n the t h i n specimens (4).) The e q u i l i b r i u m value i s n e a r l y reached a f t e r 8-10 days. Results of desorption s t u d i e s i n d i c a t e that a l i k e p e r i o d i s required to remove excess H^O during specimen storage i n a normal room environment. Since specimens were im mersed u n t i l j u s t before t e s t i n g , no H^O l o s s i s considered to occur. F i g u r e 1 a l s o i n d i c a t e s the percent weight l o s s as a func t i o n of time f o r samples under d e s i c c a t i o n . The e f f e c t of sample thickness i s l e s s pronounced. Again, a p e r i o d of 10 days appears to be s u f f i c i e n t to a t t a i n n e a r l y complete e q u i l i b r i u m . Other studies on p r e v i o u s l y d r i e d samples i n d i c a t e that at l e a s t 4 h must elapse before a s i g n i f i c a n t increase i n H^O content i s ob served i n d e s i c c a t e d samples exposed to room a i r . Since these specimens were d e s i c c a t e d u n t i l j u s t before t e s t i n g , we consider that no moisture was present. Results of Figure 1 can be summar i z e d as f o l l o w s : normal samples contained /0.6% H^O, d r i e d samples had /0% H O , and saturated samples had 2.2% H^u. Analy s i s by gas chromatography i n d i c a t e d that a small amount of r e s i d u a l monomer (F 0.6%) was present i n these specimens. The a n a l y s i s a l s o i n d i c a t e d that t h i s monomer was not removed by
Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
33.
Rheo-Optical
MOORE A N D FLICK
desiccation. monomer.
Properties of
PMMA
557
Hence, any weight l o s s r e f l e c t s l o s s of H^O
and
not
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Apparatus Tensile Tests. T e n s i l e t e s t s were done on an Instron ten s i l e t e s t e r unjler ambient c o n d i t i o n s at s t r a i n r a t e s , ε , ranging from 1.0 χ 10 to 5.5 χ 10 sec . Experiments were done i n t r i p l i c a t e at each s t r a i n r a t e . S t r e s s , σ , and s t r a i n , ε , at y i e l d (not shown) were determined using tie 0.2% o f f s e t method (5). S t r e s s , σ^, and s t r a i n , ε ^ , at break and the work to break, W^, (the area under the s t r e s s - s t r a i n curve) were a l s o c a l c u lated. The l a t t e r was evaluated v i a a computer program using a Simpson's Rule method. Birefringence. A separate apparatus using a l a s e r l i g h t source was constructed f o r measuring the absolute r e l a t i v e b i r e fringence | Δ | , the absolute value of the d i f f e r e n c e between the r e f r a c t i v e i n d i c e s p a r a l l e l and perpendicular to the deformation axis, | [ i j ~ % ] | > °f specimens subjected to constant ε deforma t i o n i n t e n s i o n . (The absolute values of Δ would equal (n„ - n^) and (nj_ - n^), where n^ i s the value of η at zero s t r e s s . ) This apparatus i s s i m i l a r to one described p r e v i o u s l y (6) and i s p i c tured s c h e m a t i c a l l y i n F i g u r e 2. The b i r e f r i n g e n c e i s determined from a comparison of changes i n transmitted i n t e n s i t y , I, using crossed p o l a r i z e r s , with the transmitted i n t e n s i t y using p a r a l l e l p o l a r i z e r s , 1^, through the relationship n
I/I
2
Q
= sin (Ô/2)
.
(1)
Here δ i s the r e t a r d a t i o n , defined as δ
= 2πάά/λ
,
(2)
where d i s the specimen thickness and λ i s the wavelength of the incident l i g h t . The appropriate derived f u n c t i o n s of Δ were c a l c u l a t e d by computer. G r a p h i c a l computer outputs of composite graphs of measurements at s e v e r a l ε were a l s o prepared using s p e c i a l l y developed programs. Nominal values of ε were c o r r e c t e d f o r e f f e c t s of specimen load on the drive-motor response f o r the t h i c k e r specimens. Values of ε^ were considered to be small enough to not warrant modifying the values of ε from t h e i r i n i t i a l values f o r both kinds of measurements. The absolute s i g n of the r e l a t i v e b i r e f r i n g e n c e was deter mined as described p r e v i o u s l y (6). The PMMA studied has negative b i r e f r i n g e n c e at room temperature, i n agreement with r e s u l t s of other workers (7). #
#
Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
WATER IN P O L Y M E R S
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^ T h i n sample
Thin sample
Time (days)
Figure 1.
Percent gain or loss in weight for PMMA
as a function of time
F R A M E AND SPECIMEN
Figure 2.
Schematic of the birefringence apparatus
Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
33.
MOORE A N D FLICK
Rheo-Optical
Properties of
PMMA
559
Results Ultimate Response. A p l o t o f as a f u n c t i o n o f ε i s given i n F i g u r e 3 f o r specimens conditioned i n the three environments. Within the experimental u n c e r t a i n t y , no appreciable dependence on ε or on the environment i s apparent a t the two highest r a t e s . The r a t h e r large s c a t t e r i n the data r e f l e c t s i n p a r t the s e n s i t i v i t y o f u l t i m a t e p r o p e r t i e s o f PMMA t o a d v e n t i t i o u s flaws, e s p e c i a l l y i n the m a t e r i a l s o f lower H 0 content. F i g u r e 4 i s a p l o t o f W^ as a t u n c t i o n of ε f o r m a t e r i a l s conditioned i n the three environments. No pronounced dependence on ε i s observed f o r the two m a t e r i a l s o f lower water content, although the maximum i n W^ i s probably a r e a l e f f e c t . The sample with 2.2% H^0 shows a rather dramatic increase i n W^ with i n c r e a s i n g ε . This i s p r i m a r i l y due t o an increase i n ε ^ , about 11% a t the highest r a t e , and r e f l e c t s the enhanced a b i l i t y of the m a t e r i a l to deform. This suggests that the added H^O acts as a p l a s t i c i z e r , i n c r e a s i n g the m a t e r i a l ' s d u c t i l i t y . The p l o t i n Figure 5 shows the e f f e c t of H^O content on and ε ^ a t constant ε . With i n c r e a s i n g H^O content, there i s a decrease i n while ε, i n c r e a s e s , again suggesting that the H^O i s f u n c t i o n i n g as a p l a s t i c i z e r . The range i n measured values decreases markedly as H^O content i n c r e a s e s . Thus, the e f f e c t o f a d v e n t i t i o u s flaws diminishes as the system gains m o b i l i t y .
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9
e
Intermediate Response. F i g u r e 6 i s a double l o g a r i t h m i c p l o t of σ/ε v s . time i n seconds a t three d i f f e r e n t s t r a i n rates f o r the samples as a f u n c t i o n o f H 0 content. To extend the time s c a l e and to c o r r e l a t e r e s u l t s a t v a r i o u s ε , we have used the reduced-variables procedure shown t o be a p p l i c a b l e i n d e s c r i b i n g the v i s c o e l a s t i c response o f rubbery m a t e r i a l s (8) as w e l l as o f s e v e r a l g l a s s y polymers (6). (To compensate f o r the e f f e c t of d i f f e r e n t ε we p l o t σ/ε v s . ε / ε ; the l a t t e r i s simply the time, t.) S u p e r p o s i t i o n over the e n t i r e time s c a l e f o r 0% H^O (upper curve) i s e x c e l l e n t except f o r times c l o s e to the f r a c t u r e times of the m a t e r i a l s t e s t e d a£ ^ i e higher s t r a i n r a t e s . For example, a d e v i a t i o n occurs a t 10 ' sec f o r the m a t e r i a l a t ε = 3.3 χ 10 sec Results f o r samples c o n t a i n i n g 0.6% H 0, the middle curve i n F i g u r e 6, are g e n e r a l l y s i m i l a r to those naving 0% water. How ever, y i e l d i p g ^ i s more p^roigDunced a t 0.6% H^O, d e v i a t i o n s occur r i n g a t 10 * and 10 * s e c . More curvature (and hence a smaller slope) than i s seen i n the upper curve i s evident a t the lowest rate a t long times. Since the magnitude o f σ/ε i s a l s o l e s s , the slope o f the corresponding l i n e a r p l o t of σ/ε v s . t ( a t the same t ) would a l s o be lower. Because ά(σ/ε)/at i s equal t o E ( t ) , the t e n s i l e r e l a x a t i o n modulus, the modulus thus decreases with i n c r e a s i n g H 0 content. A s i m i l a r e t f e c t i s seen i n the lowest curve i n F i g u r e 6 a t the lowest ε f o r samples of highest H 0 content. Generally, 9
β
9
9
?
Rowland; Water in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
WATER IN P O L Y M E R S
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560
0.05
0.10
0.15
Strain rate (
