Polymers as Rheology Modifiers - American Chemical Society

Chapter 17

Rheological Properties of High-Molecular -Weight Poly[(methyl methacrylate)-co -ethylacrylate-co-butylacrylate] Solutions Influence of Polymer—Solvent Interactions

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch017

Wendel J. Shuely and Brian S. Ince U.S. Army, SMCCR-RSC-P, Aberdeen Proving Ground, MD 21010-5423

The effects of polymer-solvent interactions on rheological viscoelastic properties i s being investigated. Relatively small volume fractions, 0.02-0.06, of an u l t r a high molecular weight rheological processing aid, terpolymer poly(methylmethacrylate -co-ethylacrylate-co-butylacrylate), poly (MMA/EA/BA), form polymer solutions in the semidilute regime. Over 30 solutions were formulated to define several interaction categories. Polymer-solvent interactions were characterized by several methods: polymer cohesion phase diagram coordinates, limiting viscosity number, proton donating strength, and s o l u b i l i t y with control homopolymers. Rheological measurements included steady shear f i r s t normal stress difference, apparent viscosity, hysteresis, transient, dynamic viscosity and storage and loss moduli. Relationships between degree or type of interaction and rheological properties have been formulated. The fluid dynamics of liquids can be modified and controlled by the a d d i t i o n o f polymer additives (1-3). U l t r a h i g h , megadalton ( m i l l i o n gm/mole) m o l e c u l a r w e i g h t (MW) p o l y m e r s a r e e f f e c t i v e a t low c o n c e n t r a t i o n s . Most specific i n d u s t r i a l process applications involving the control of f l u i d dynamics are p r o p r i e t a r y ; examples of applications that involve free surface flow are r o l l splatter, bulk liquid spraying, aero-stripping, antimisting of fuels, and spray droplet distribution control

This chapter not subject to U.S. copyright Published 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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POLYMERS AS R H E O L O G Y MODIFIERS

in crop pest management. The process phenomena of interest i n a p p l i c a t i o n s o f t e n are c h a r a c t e r i z e d by l a r g e nonlinear deformations, simultaneous shear and extensional flows, complex geometries and free surface flows. The ideal geometries and flows imposed by rheological evaluation are a useful framework for u n d e r s t a n d i n g the e f f e c t s of m a t e r i a l p r o p e r t i e s on t h e s e complex processes. The large deformations present in the process phenomena o f i n t e r e s t suggest correlations with steady shear measurements instead of the small strains from dynamic mode measurements. However, oscillatory measurements were a l s o r e c o r d e d f o r p o t e n t i a l c o r r e l a t i o n to steady shear rheological properties at low s h e a r rates. Given this selection of steady shear measurements, two r h e o l o g i c a l p r o p e r t i e s were c o n s i d e r e d : first normal s t r e s s d i f f e r e n c e and apparent viscosity, i n c l u d i n g t h e power law coefficient for the shear rate dependence. The most r e l i a b l e q u a n t i t a t i v e measure of solution elasticity under simple steady shear is first normal stress difference. Correlations of both of these rheological properties with solvent effects were investigated, although f i r s t normal s t r e s s d i f f e r e n c e had shown more p r o m i s i n g p r e d i c t i o n o f p r o c e s s f l u i d d y n a m i c s and had allowed d i r e c t comparisons of polymer solutions over a wider v a r i e t y of concentration, MW a n d s o l v e n t regimes. Polymer-Solvent I n t e r a c t i o n s : T h e r h e o l o g i c a l and viscoelastic properties of polymer solutions are i n f l u e n c e d by the polymer MW a n d MW d i s t r i b u t i o n ( M W D ) , the chemical s t r u c t u r a l f e a t u r e s and c o n f i g u r a t i o n , the c o n c e n t r a t i o n , and p o l y m e r - s o l v e n t i n t e r a c t i o n s (4). The quantitative range of influence of polymer-solvent interactions is relatively limited when compared w i t h these other variables. Furthermore, concentration, MW and s t r u c t u r a l p r o p e r t i e s p r o v i d e a c o n t i n u u m o f solution properties, although the values of these v a r i a b l e s might produce solutions in different c o i l density regimes. On the other hand, solvent effects are f i n i t e i n that they are bounded between nominally theta solvents and maximally interacting solvents. Most s t u d i e s o n l y employ theta and 'good' solvents to bracket the extent of solvent effects. One p u r p o s e o f t h i s i n v e s t i g a t i o n was to determine the influence of the full spectrum of solvents between near theta conditions and highly expanded p o l y m e r c o i l s on r h e o l o g i c a l properties and, i n addition, determine the influence of several other qualitatively unique solvent sets on rheological behavior. Among these solvent sets are: (1) solvents that are insoluble with the major comonomer components and o n l y ' d i s s o l v e d ' by p r e f e r e n t i a l i n t e r a c t i o n w i t h t h e m i n o r i t y comonomer c o m p o n e n t s , (2) s o l v e n t s w i t h specific hydrogen b o n d i n t e r a c t i o n as p r o t o n d o n o r , weak a c c e p t o r solvents, a n d (3) solvents that precipitate during shear.


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S e m i d i l u t e Regime:


289

Poly (MMA—EA—BA) Solutions Variables

held

constant

were

polymer composition, MW, MWD, concentration and temperature. The concentration of 4.7 + 0.1 g/dL resulted in coil density based on a concentration χ limiting viscosity number (LVN) p r o d u c t o f ca. 5-20, spanning the semidilute regime. Characterization: S e v e r a l physical, chemical and rheological c h a r a c t e r i z a t i o n methods a r e b e i n g applied. Polymer Cohesion Phase Diagrams (PCPD) o f t h e terpolymer (5) and homopolymer c o n t r o l s were employed to identify solvents with specific solution interactions. The phase diagrams consist of a bounded area on a plot of s o l u b i l i t y or i n s o l u b i l i t y i n terms of a s e l f - c o n s i s t e n t solvent parameter set (Hansen p a r a m e t e r s , ASTM D3132 solvent parameters (6), etc.). The boundary c o o r d i n a t e s approximate t h e t a c o n d i t i o n s or ' p o o r ' s o l v e n t s and the coordinates toward the center provide a selection of 'good' solvents. This semi-quantitative selection of s o l v e n t s was t h e n f u r t h e r quantified by measurement of LVN. LVN o r i n t r i n s i c v i s c o s i t y measurement p r o v i d e d an e s t i m a t e of c o i l expansion and degree of i n t e r a c t i o n (7). Linear S o l v a t i o n E n e r g y R e l a t i o n s h i p (LSER) s c a l e s were used to i d e n t i f y and quantify p r o t o n d o n a t i n g , weak o r nonacceptor solvents (8). The LSER s y s t e m of scaling solvent parameters is more rigorous than the various cohesion parameter systems and i n c l u d e s s p e c i f i c terms for proton donating or acceptor strength. Unique solvents with relatively strong proton donating capability relative to proton acceptor strength (e.g. c h l o r o f o r m , methylene c h l o r i d e ) were i d e n t i f i e d employing LSER d a t a (8). Straightforward solubility determinations (6) were employed to classify solvent/nonsolvent interactions with homopolymer controls f o r each of the terpolymer repeat units. Rheological measurements provided a characterization of v i s c o u s and viscoelastic p r o p e r t i e s : steady shear hysteresis, f i r s t normal stress difference (FNSD), apparent v i s c o s i t y and c r i t i c a l shear r a t e f o r o n s e t o f FNSD a r e u n d e r e v a l u a t i o n . Dynamic and t r a n s i e n t d a t a have a l s o been r e c o r d e d f o r evaluation.

Experimentation Materials,

The s p e c i f i c p o l y m e r i c additive investigated is poly(methylmethacrylate-co-ethylacrylate-co-butylacrylate), (poly(MMA/EA/BA)), Rohm and Haas Acryloid (now Paraloid) K125 (9). The Mw is 1.5 megadalton (million gm/mole) by l i g h t s c a t t e r i n g ( L S ) a n d Mw/Mn = 1.8 (10). Size exclusion chromatography (SEC) a n d L i g h t S c a t t e r i n g (LS) show t h e t a i l e x t e n d i n g f r o m 1 t o c a . 15 megadalton. The structure is that o f a l i n e a r random copolymer produced by aqueous emulsion polymerization ( r e c o v e r e d by s p r a y drying). A lot (3-6326) o f several hundred Kg has been set aside for detailed c h a r a c t e r i z a t i o n and r h e o l o g i c a l p r o c e s s correlations.


290



The structure and molar monomeric ratios were d e t e r m i n e d b y NMR w i t h Mass Spectral (MS) c o n f i r m a t i o n and a r e (MMA:82, E A : 1 2 , BA:6) ( 1 1 ) . A l l monomeric r e p e a t units are dipolar, proton acceptors. The homopolymer control samples were employed only for solubility determinations and were as follows: polymethyl methacrylate) (PMMA), DuPont Elvacite 2041, lot no. 2 2 0 0 , Mw c a . 0.4 m e g a d a l t o n ; poly(ethylacrylate) (PEA), Rohm and Haas, l o t no. P S - 1 , Hw c a . 2 megadalton; and poly(butylacrylate) (PBA), Scientific Polymer Products, lot no. 3, Mw c a . 0.06 megadalton. The commerical q u a l i t y s o l v e n t s were used as received. Procedures. Solubility Determinations: The poly(MMA/ EA/BA) s o l u t i o n s were p r e p a r e d a t a c o n c e n t r a t i o n o f 4.7 +0.1 g/dL. T h e P E A was r e c e i v e d as an aqueous emulsion and p r e c i p i t a t e d u s i n g m e t h a n o l . T h e PBA was r e c e i v e d as a 26% s o l u t i o n i n t o l u e n e and dried to constant weight. A1J p o l y m e r s were d r i e d o v e r n i g h t i n a vacuum oven a t c a . 40 C b e f o r e s o l u t i o n preparation. A l l h o m o p o l y m e r PMMA, PEA, PBA s o l u t i o n concentrations ranged from ca. 5-10 g/dL and a l l s o l u t i o n s were p r e p a r e d a t room t e m p e r a t u r e (ca. 25 C ) . A l l s o l u t i o n s were p l a c e d i n a shaker ca. 1-5 w e e k s b e f o r e f i n a l visual solubility determinations were made. LVN: T h e L V N was e m p l o y e d a s a m e a s u r e o f t h e coil expansion of the polymer coil in the solvent. A poor solvent, approaching a theta solvent, has a low LVN relative to a good solvent, which has a high LVN. Procedures for o b t a i n i n g the LVN from dilute solution e x t r a p o l a t i o n s have been p u b l i s h e d (7). LVN measurements were p e r f o r m e d by S p r i n g b o r n L a b o r a t o r i e s , E n f i e l d , C T . Rheology: R h e o l o g i c a l measurements were performed using t h e R h e o m e t r i c s F l u i d s R h e o m e t e r (RFR) M o d e l 7800 w i t h cone and p l a t e geometry at a temperature of 25.0 + 1.0 C . The i n i t i a l r h e o l o g i c a l data from the steady r a t e sweep e x p e r i m e n t were further analyzed; viscosity data were reduced to o b t a i n power law coefficients. FNSD versus shear rate squared was reduced to o b t a i n FNSD coefficients and z e r o s h e a r FNSD. F o r many s o l v e n t s , the onset of measurable FNSD occurred in or above the t r a n s i t i o n to the n o n l i n e a r r e g i o n which has complicated data analysis. T h e l i n e a r r e g i o n a n a l y z e d was d e f i n e d as occurring above 1% f u l l s c a l e n o r m a l t r a n s d u c e r output and i n c l u d i n g the linear data until the correlation coefficient dropped below 0.98. Results

and

Discussion

Solvent Sets. Preferential polymer-solvent interactions can be viewed i n c o m p l e x i t y as a r a n g e from a m a t r i x o f a homopolymer with a single solvent through a copolymer with cosolvents. The system investigated consists of a terpolymer with single solvents. The i n v e s t i g a t i o n was



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291

structured to q u a l i t a t i v e l y determine solvent-nonsolvent classes for each solvent with the three comonomeric components by independent determination of solubility with homopolymer analogs of the comonomers. Each of the t h r e e h o m o p o l y m e r s , PMMA, P E A a n d P B A , was i n d i v i d u a l l y t e s t e d f o r s o l u b i l i t y i n each s o l v e n t . For a terpolymer, this generates eight hypothetical classes. Based on the qualitative r h e o l o g i c a l r e s u l t s and t h e s m a l l number of solvents in several classes, t h e s e were c o l l a p s e d into two c l a s s e s as shown by t h e d i v i s i o n i n Table I. Note that i n the nomenclature used h e r e i n , the " / " denotes the usual copolymerized polymer, poly(MMA/EA/BA). The " - " i s used for homopolymer solubility classes based on combinations of independent homopolymer solubility experiments; f o r example, PEA-PBA represents a group of solvents i n w h i c h b o t h PEA and PBA a r e s o l u b l e , but not PMMA. Polymer-solvent interactions were further evaluated according to a matrix of nonpolar, p o l a r , and hydrogen-bond interactions. The poly(MMA/EA/BA) is a completely aprotic dipolar structure. The s o l v e n t sets are defined below.

Table

1. 2. 3. 4. 5. 6. 7. 8.

I.

Formation of S o l u b i l i t y C l a s s e s by P e r m u t a t i o n of S o l u b i l i t y (S) a n d I n s o l u b i l i t y (I) of Component Homopolymers o f t h e Terpolymer

Terpolymer Component Homopolymer S o l u b i l i t y PBA Solubility Classes PEA PMMA S PMMA-PEA-PBA Soluble S S PMMA-PEA S o l u b l e S I S D PMMA-PBA Soluble S S I I PMMA S o l u b l e S I PEA-PBA Soluble S S I PEA S o l u b l e S I I 2) S PBA Soluble I I PMMA-PEA-PBA Insoluble I I I (Not a p p l i c a b l e ) 1) S o l u b l e w i t h 82-100% o f comonomer c o n t e n t , e.g. MMA, M M A - B A , M M A - Ε A o r M M A - E A - B A . 2) S o l u b l e w i t h 6-18% o f c o m o n o m e r c o n t e n t , e.g. BA, EA o r E A - B A , l a b e l l e d " N o n s o l v e n t W/0.82+".

Aprotic Solvent Set: The m a j o r i t y of s o l v e n t s were aprotic dipolar solvents, further classified by a nonsolvent/solvent determination for each terpolymer component, as d e s c r i b e d above. Aprotic is used i n the usual sense here as indicating absence of a proton capable of hydrogen bonding. Theta Solvent Set: Solvents were identified that were theta o r n e a r - t h e t a s o l v e n t s and a l s o belonged to the class demonstrating solubility with a l l three


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terpolymer components, that is, controls were insoluble with solvent.


Nonsolvent

W/0.82+

Solvent

none o f t h e homopolymer the terpolymer theta

Set: T h e t e r m " N o n -

solvent W/0.82+" defines an operational subset of solvents i n which the solvent dissolves the terpolymer b u t was a n o n s o l v e n t for at least t h e m a j o r i t y 0 . 8 2 MMA fraction based on a s o l u b i l i t y d e t e r m i n a t i o n w i t h PMMA homopolymer. A comparison of the r h e o l o g i c a l properties of "Nonsolvent W/0.82+" set with theta s o l v e n t s was of interest since the low viscometric coil size estimates were similar but based on different physiochemical phenomena. The t h e t a solvents appear to systematically extend t h e r e l a t i o n s h i p o f i n c r e a s i n g FNSD a n d apparent viscosity with decreasing LVN. The s o l v e n t s t h a t are i n the "Nonsolvent W/0.82+" set display extremely high viscoelasticity as evidenced by FNSD, a p p a r e n t viscosity and dynamic properties. In general, hysteresis e x p e r i m e n t s do not demonstrate structure formation in these s o l v e n t s a l t h o u g h o v e r 82% (PMMA) o f t h e c h a i n is nominally insoluble i n the solvent. Proton Donor Solvent Set: T h e r e a r e a s e l e c t n u m b e r of solvents with low to moderate proton donating strength, but even weaker a c c e p t o r strength (in most media); examples are chloroform, methylene chloride, pentachloroethane, trichloroethylene and pentachlorocyclopropane. These solvents should specifically i n t e r a c t w i t h p r o t o n a c c e p t o r c a r b o n y l and e s t e r m o i e t i e s of the polymer solute. A l l such solvents investigated showed enhanced c o i l expansion evidenced by LVN values clustered higher than any other 'good' solvents and c o r r e s p o n d i n g l y low v a l u e s f o r r h e o l o g i c a l properties. Shear Induced P r e c i p i t a t i o n Solvent S e t : P o l y m e r s o l u t i o n s made f r o m four solvents were discovered to undergo shear induced p r e c i p i t a t i o n during rheological measurements: the solvents were dimethyl methylphosphonate, dimethyl formamide, 1-methy1-2-pyrrolidone and t r i m e t h y l phosphate. Since only 17 instances of nonaqueous shear induced p r e c i p i t a t i o n have been r e p o r t e d (12), the p o s s i b i l i t y of additions to t h i s unique class o f p o l y m e r - s o l v e n t s y s t e m s was f u r t h e r i n v e s t i g a t e d . The s t e a d y s h e a r FNSD and apparent v i s c o s i t y were r e c o r d e d d u r i n g the shear induced p r e c i p i t a t i o n p r o c e s s as l o n g as homogeneous s o l u t i o n was p r e s e n t b u t n e i t h e r t h e FNSD n o r the apparent v i s c o s i t y from the p r e c i p i t a t i o n experiments were used in the regression analyses. Clear viscous solutions had been i n t r o d u c e d i n t o the cone and plate fixture and low viscosity solvent with solid white polymer p i e c e s were r e c o v e r e d . The p r e c i p i t a t i o n s were r e p e a t a b l e and c o u l d n o t be prevented by u t i l i z a t i o n of various sample p r e p a r a t i o n and measurement procedures. The water c o n t e n t o f the s o l v e n t s was a n a l y z e d a n d f o u n d t o be s e v e r a l t i m e s h i g h e r i n t h e p r e c i p i t a t e d solutions.


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The p o l y m e r p a r t i c l e s were found to r e d i s s o l v e i n f r e s h , low water content solvent. The phase diagram boundaries of the p o l y m e r - s o l v e n t system w i t h r e s p e c t to s o r p t i o n of water as a c o s o l v e n t were e v a l u a t e d . The precipitating s o l v e n t s have c o o r d i n a t e s on o r n e a r t h e b o u n d a r y between solubility/insolubility. Thermodynamically, an addition of a s m a l l volume f r a c t i o n of sorbed water as a cosolvent would produce a nonsolvent mixture. The shear then may only i n i t i a t e p r e c i p i t a t i o n or accelerate the k i n e t i c s of precipitation. Previously published polymer-solvent pairs showing shear-induced precipitation will be evaluated f o r s i m i l a r water cosolvent effects.


Correlations,

Solvent

Interaction Effect

range of solvent i n f l u e n c e o n FNSD c a n inspection of Figure 1. FNSD vs shear graphed f o r examples of each s o l v e n t set. o f FNSD d a t a i s p r e s e n t e d including high well into the nonlinear region. Descrip a b b r e v i a t i o n s and s o l u b i l i t y c l a s s e s are II.

Table

No. 1 2 3 4 5 6 7 8

II.

on FNSD: T h e

be viewed by rate squared is The f u l l range shear rate data tions of solvent l i s t e d i n Table

Codes and S o l u b i l i t y C l a s s e s for Representative Solvents ( F i g u r e 1)

Codes CHCL^ ACP DEM 2HP 3HP 4HP DPGMME

Solvent Solubility Class Chloroform P r o t o n d o n a t i n g PMMA-PEA-PBA* Acetophenone Good PMMA-PEA-PBA Moderate PMMA-PEA-PBA D i e t h y l malonate Near t h e t a PMMA-PEA-PBA 2-heptanone N e a r t h e t a PMMA-PEA 3-heptanone 4-heptanone Preferential PEA-PBA Dipropyleneglycol Intramolecular monomethylether PMMA-PEA-PBA TPPO Tripropyl phosphate Preferential PEA-PBA See T a b l e I f o r T e r p o l y m e r S o l u b i l i t y c l a s s codes.

Figure 2 contains a plot of First Normal Stress Difference (FNSD) v s L i m i t i n g V i s c o s i t y Number (LVN) f o r a moderately high shear rate of 400/sec. The LVN v a l u e s provide a quantitative indication of degree of solvent interaction; t h e L V N v a l u e s r a n g e f r o m a b o u t 1.2 t o 4.1 r e p r e s e n t i n g a low t o h i g h d e g r e e o f c o i l e x p a n s i o n . One can note t h a t c e r t a i n s o l v e n t s e t s f a l l i n t o l i m i t e d LVN ranges. Only the proton donating solvents ('+','•') have LVN values i n the h i g h range between 3.3 t o 4 . 1 . Even the lowest values for a proton donating solvent were h i g h e r t h a n the b e s t 'good' s o l v e n t based on n o n - s p e c i f i c interactions. The LVN range from about 1.4 to 3.3 on the regression line in Figure 2 contains three solvent sets.


294


100009000 H


8000 1: 2: 3: 4: 5: 6: 7: 8:

7000υ σ

6000 — 50004000

CHCL flCP DEM 2HP 3HP 4HP DPGMME TPPO 3

3000—L 20001000-

"Ί 0.0

0.5

I

1

1

1.0

1.5

2.0

1

1

1

1

2.5 3.0 3.5 4.0 X10 SHEAR RATE SQUARED. 1/SEC. SCL

1— 4.5

5

Figure 1. F i r s t Normal S t r e s s D i f f e r e n c e (FNSD) vs Shear Rate Squared f o r Examples of A l l Solvent Sets with Poly(MMA/EA/BA), f o r S o l u t i o n s a t 4.7 + 0.1 g/dL a n d 25.0 + 1.0 C: Proton Donating PMMA-PEA-PBA(1), P M M A - P E A - P B A (2,3,4), P M M A - P E A (5), P E A - P B A ( 6 , 8 ) and Intramolecular P M M A - P E A - P B A (7). See T a b l e s I and II for abbreviations.


5.0

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25000

1.0

1.5

2.0

2.5

3.0

3-5

4.0

4.5

LVN, DL/G Figure 2. F i r s t Normal S t r e s s D i f f e r e n c e (FNSD) vs L i m i t i n g V i s c o s i t y Number (LVN) f o r A l l S o l v e n t Sets with Poly(MMA/EA/BA) f o r S o l u t i o n s a t 4.7 + 0.1 g/dL and 2 5 . 0 + 1.0 C : R e g r e s s i o n L i n e = PMMA-PEA-PBA (*) and P r o t o n D o n a t i n g PMMA-PEA-PBA (+ ) . Not i n c l u d e d i n the Regression f i t : I n t r a m o l e c u l a r PMMA-PEA-PBA ( ο ) , PMMA-PEA (Ο), Proton D o n a t i n g PMMA-PBA ( £ ) / P E A - P B A (Δ) and P r o t o n Donating PEA-PBA (•). See T a b l e I for abbreviations.


296


The m a j o r i t y o f the solvents are contained i n the set d e f i n e d as s o l u b l e w i t h a l l t e r p o l y m e r components (Code = PMMA-PEA-PBA; * ) . The solvent set consisting of solvents soluble w i t h b o t h PMMA a n d P E A a r e contained w i t h i n t h i s r a n g e (Code = PMMA-PEA; ' 0 ' ) · Qualitatively, the PMMA-PEA set, s o l u b l e w i t h 94% o f the terpolymer, a p p e a r s t o be following the general trend of the 100% soluble PMMA-PEA-PBA set, however, due t o the limited data set, t h e s e have not been included in the regression i n F i g u r e 2. The t h i r d s o l v e n t s e t i n t h e 1.4 t o 3 . 3 range has been d e f i n e d as i n t r a m o l e c u l a r hydrogen bonded solvents. These solvents form both c y c l i c (usually 5-7 member rings) intramolecular hydrogen bonds and acyclic i n t e r m o l e c u l a r hydrogen bonds. A l t h o u g h t h e i r LVN v a l u e s fall within the usual range of 1.4 to 3.3, their viscoelasticity, a s m e a s u r e d b y FNSD v a l u e s , are quite scattered, do n o t f a l l on the regression l i n e and, i n some cases, are not w i t h i n the Figure 2 a x i s . In fact, o n l y one i n t r a m o l e c u l a r hydrogen bond s o l v e n t f a l l s into the plot axis (Code = PMMA-PEA-PBA; Ό ' ) , methyl salicylate, w i t h an ortho hydroxy p r o t o n bonded to the adjacent carbonyl moiety. The other intramolecular hydrogen bonded solvent, dipropyleneglycolmonomethylether, with a six-member i n t r a m o l e c u l a r r i n g c o n t a i n i n g a hydroxy proton hydrogen bonded to the e t h e r moiety, is off-scale i n F i g u r e 2, a t an LVN v a l u e o f 1.64 a n d FNSD value of 28,000. From t h e l i m i t e d d a t a i n t h i s s e t , one cannot yet determine if this set will form a unique regression equation or will be generally anomalous. T r a c e i m p u r i t i e s , such as water o r o t h e r p r o t i c solvents, can i n f l u e n c e the e q u i l i b r i u m between i n t r a m o l e c u l a r and i n t e r m o l e c u l a r hydrogen bonding s o l v e n t conformations and thereby c o n t r i b u t e to the appearance of anomalous solvent effects. The LVN range between 1.5 to 2.0 appears quite complex, at first. T h e FNSD v a l u e s s e e m t o be rather randomly d i s t r i b u t e d between 3,000 and 25,000 d y n e s / s q . cm., a l t h o u g h some s o l v e n t s l i e u p o n t h e r e g r e s s i o n line. Actually, FNSD values range off-scale up to 900,000 dynes/sq. cm. (not p l o t t e d ) w i t h i n t h i s 1.5 t o 2.0 LVN range. One c a n b e g i n to unravel the apparent random nature of the v i s c o e l a s t i c FNSD m e a s u r e m e n t s by noting the solubility classifications and solvent data sets associated with the data points. F o r t h e 1.5 t o 2.0 LVN r a n g e , no i n s t a n c e o f a n o m a l o u s l y h i g h ( d e f i n e d a s : above the regression line 95% confidence interval) FNSD measurements were r e c o r d e d for solvents soluble with the 82% t o 94% m a j o r i t y c o m o n o m e r c o n t e n t . A l l solvents that were s o l u b l e w i t h the 18% comonomer fraction (Code = PEA-PBA; 'Δ') h a v e FNSD v a l u e s a t o r a b o v e t h e regression 95% c o n f i d e n c e interval; this includes several PEA-PBA solvents that are off-scale a t v e r y h i g h FNSD v a l u e s .


/

/


17.

297

Poly (MMA-EA-BA) Solutions

SHUELY & INCE

FNSD coefficient can be independent of solvent effects for solutions of high MW-concentration products within the concentrated, network entangled regime (4). Within the semidilute regime f o r C χ LVN = 5-20, and C χ Mw = 7.1 g/dL megadalton, the FNSD increases with d e c r e a s i n g L V N as shown i n Figure 2. T h e FNSD d a t a h a s been a n a l y z e d as a f u n c t i o n of s p e c i f i c shear rates of 100, 400, 1000/sec and as a f u n c t i o n o f FNSD coefficient at the onset of s i g n i f i c a n t (>1% n o r m a l f o r c e transducer f u l l scale) normal force response. The r e l a t i o n s h i p s at v a r i o u s shear r a t e s are s i m i l a r and data o b t a i n e d to date can be summarized by t h e f o l l o w i n g equation.


FNSD

(@ 4 0 0 / s e c )

=

8900

-

(2000

X LVN)

The e q u a t i o n r e p r e s e n t s a l l o f t h e 17 P M M A - P E A - P B A soluble solvents i n c l u d i n g the proton donors ('*' and '+'). E x c l u d e d a r e t h e 11 o f 28 s o l v e n t s c o m p r i s i n g t h e intramolecular PMMA-PEA-PBA ( Ο ) , PMMA—PEA s o l u b l e (O), p r o t o n d o n a t i n g P M M A - P B A fà) , P E A - P B A s o l u b l e (Δ)/ proton donating E B (•) and s h e a r - i n d u c e d p r e c i p i t a t i o n solvent set (not shown on p l o t ) . Solvent Influence on Apparent V i s c o s i t y : T h e s o l v e n t influence as estimated by LVN i s summarized i n the equation.

Apparent

Viscosity

(@ 1 0 / s e c )

= 8 . 1 x e

*

^

ν Γ ,

>

The r e s u l t s i n c l u d e o v e r 16 s o l v e n t s s o l u b l e i n at l e a s t 82 m o l e p e r c e n t o f t h e copolymer content and cover all solubility classes. The slope shows a nominal decrease of apparent v i s c o s i t y w i t h i n c r e a s i n g LVN but is not statistically different from a zero slope at a polymer concentration of 4.7 g/dL. Limited experiments at lower c o n c e n t r a t i o n s o f 2-3 g/dL found a slightly increasing apparent viscosity with increasing LVN; at higher c o n c e n t r a t i o n s o f 6.0 g / d L a s l i g h t l y decreasing apparent viscosity with increasing L V N was found. Therefore, the i n c i p i e n t s l o p e change from positive to n e g a t i v e o c c u r s b e t w e e n C χ Mw v a l u e s o f 4 . 5 a n d 7 . 1 g / d L megadalton. Overall, ten additional solvents were studied to better define the FNSD vs LVN r e l a t i o n s h i p ; additional apparent viscosity vs LVN d a t a did not s i g n i f i c a n t l y influence the slope.

Conclusions Range o f Solvent

Effects. It would be d e s i r a b l e t o be a b l e to summarize the magnitude of the solvent e f f e c t on rheological viscoelastic properties i n the same manner t h a t MW a n d concentration have been presented i n the past. T h e e f f e c t o f MW a n d c o n c e n t r a t i o n o n r h e o l o g i c a l p r o p e r t i e s has been a p p r o p r i a t e l y summarized i n terms of



298

P O L Y M E R S AS R H E O L O G Y MODIFIERS

the slope of the property-variable relationship, for e x a m p l e , c h a n g e i n F N S D p e r d e c a d e c h a n g e i n Mw ( 4 ) . S o l v e n t e f f e c t s a r e more complex i n that the slope and even the d i r e c t i o n of the r e l a t i o n s h i p depend on the coil density regime; in addition, there is increased complexity due to the various s p e c i a l solvent effects considered here (preferential, proton donating). One approach to summarizing the solvent effect data takes advantage of the finite extent of solvent interaction between t h e two e x t r e m e s o f i n s o l u b l e o r t h e t a conditions and maximal coil expansion. One can then r a t i o the rheological properties a t o r n e a r t h e s e two limits in solvent interaction. One can also view the range or r a t i o of r h e o l o g i c a l properties with respect to the range i n LVN or i n t r i n s i c v i s c o s i t y over the s o l v e n t set. These LVN, apparent viscosity a n d FNSD v a l u e s , and the r a t i o s for the l i m i t s of the solvent set are listed in Table III. Note t h a t these r a t i o s apply to a specific semi-dilute concentration-MW value and that solvent e f f e c t s d i s a p p e a r a t h i g h v a l u e s o f FNSD a t t h e h i g h c o i l densities within the entangled regime. The ratio of high/low rheological values are listed in Table III for the example of a moderate coil density within the semidilute regime of C χ Mw = 4.7 χ 1 . 5 = 7 . 1 g/dL megadalton.

Table

III.

S o l v e n t E f f e c t Ranges and R a t i o s f o r Rheological P r o p e r t i e s of Polymer Solutions at a Moderate C o i l Density (Concentration χ Mw = 7 . 1 g / d L m e g a d a l t o n ) (See T a b l e I f o r Abbreviations) Solvent Set Within Within Within Between PMMA-PEA-PBA PEA-PBA P r o t o n Donor Soluble P r o t o n Donor Soluble and PEA-PBA (3.03/1.27) (4.08/3.36) (2.12/1.27) (4.08/1.27) = 2.4 = 1.2 = 1.7 = 3.2

Property LVN (dL/g) Apparent V i s c o s i t y (11.7/4.05) (poise) = 2.9 @ 10/sec F i r s t Normal Stress (6800/1420) Difference =4.8 (dynes/sq. cm.) é 400/sec

(16.1/2.52) = 6.4

(29.2/5.18) = 5.6

(29.2/2.52) = 11.6

(6660/100.) =67.

(900K/9400) =96.

(900K/100.) = 9000.

Two types of ranges are l i s t e d ; 'within' a solvent class refers to the ratio between the highest/lowest values for a single class. 'Between' solvent classes



17.

SHUELY & INCE


299

refers to the ratio between the highest value of one c l a s s and the lowest of the other c l a s s . The properties l i s t e d i n the f i r s t column are LVN, apparent v i s c o s i t y at low s h e a r ( 1 0 / s e c ) , and f i r s t normal s t r e s s d i f f e r e n c e at a moderate shear rate (400/sec). The solvent sets d e f i n e d i n a d j a c e n t columns i n c l u d e : w i t h i n PMMA—PEA-PBA s o l u b l e , w i t h i n p r o t o n donor, w i t h i n PEA-PBA s o l u b l e , and between p r o t o n donor and PEA-PBA soluble. The PMMA-PEA-PBA s o l u b l e s o l v e n t s e t p r o v i d e d a LVN r a t i o of 2.4, r a n g i n g from near t h e t a s o l v e n t s to maximal values from non-specific interactions. The range of apparent viscosity and first normal stress values measured from this range is about 3X and 5X, respectively. The PMMA-PEA-PBA soluble solvent set of nonspecific interactions is bracketed at higher LVN v a l u e s by t h e p r o t o n donor s e t and a t lower LVN v a l u e s by the preferential PEA-PBA soluble set. Both of these solvent sets have LVN ranges of about half of the 'regular' solution range, although the preferentially soluble solvent set and near theta s o l v e n t LVN v a l u e s overlap. Both of these solvent s e t s have wider ranges and l a r g e r r a t i o s of apparent viscosity ( a b o u t 6X) a n d first normal stress difference (67X-96X) than the PMMA-PEA-PBA s o l u b l e set. The l a s t column r e c o r d s the measurement ranges and their ratios between the extremely high LVN and low rheological property values of the proton donor set and the low LVN and extremely high rheological property values of the preferentially PEA-PBA s o l u b l e s e t . The LVN range has been extended to 3.2X, the apparent viscosity ratio to 11.6X, and first normal stress difference to 9000X. Therefore, although the normal range of solvent influence on rheological properties between 'good' and theta solvents is limited, the s e l e c t i o n of polymer-solvent p a i r s can extend the viscous properties by about one decade and viscoelastic p r o p e r t i e s by over t h r e e decades. Molecular Interactions. At the molecular level, the interactions can be interpreted in the u s u a l manner, whereby the s p e c i f i c proton donor i n t e r a c t i o n s further minimize polymer-polymer interactions and thereby m i n i m i z e the f r i c t i o n f a c t o r i n f l u e n c i n g v i s c o u s flow and t r a n s i e n t entanglements influencing viscoelasticity. The s e t o f s o l v e n t s s o l u b l e w i t h o n l y 6-18% o f t h e comonomer can be interpreted in a general way as containing substantial s e q u e n c e l e n g t h s o f t h e i n s o l u b l e PMMA; b o t h i n t r a c o i l and i n t e r c o i l polymer-polymer contacts would occur and high frictional interaction and transient entanglement d e n s i t i e s would be present. Rheo-optical studies might detect any non-statistical, ordered structure in these solutions.


300



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Polymers as Rheology Modifiers - American Chemical Society

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