New Polymeric Materials

Chapter 21

Downloaded by OHIO STATE UNIV LIBRARIES on October 14, 2014 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch021

Viscometry and Light Scattering of Polymer Blend Solutions 1

2

1,3,

M . J. K. Chee , C. Kummerlöwe , and H. W. Kammer * 1

School of Chemical Sciences, University Sains Malaysia, 11800 Minden Penang, Malaysia University of Applied Sciences Osnabrück, Albrechtstrasse 30, D-49076 Osnabrück, Germany Current address: Mansfelder Strasse 28, D-01309 Dresden, Germany 2

3

Polymer blends comprising PHB in combination with PEO and PCL, respectively, were dissolved in common good solvents. Viscometry and light scattering served studying interactions between unlike polymers in dilute solutions of the blends. Specific viscosities obey the Huggins equation to an excellent approximation in the concentration range studied. Huggins coefficients display nonlinear dependencies on blend composition. - Apparent quantities of the ternary systems, molecular mass, radius of gyration and second virial coefficient, determined by light scattering, show also non linear variation with blend composition. The second virial coefficients versus blend composition display positive and negative deviations from additivity those are indicative of repulsion and attractions, respectively, between the polymer constituents.

282 In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

283


Introduction We report on viscosity and light scattering studies of ternary polymer-blend solutions to reveal dominant interactions in polymer blends. Knowledge about interactions between constituents in polymer blends is important for understanding of their phase behavior. Evaluation of polymer-polymer miscibility is especially complicated in polymer blends comprising crystallizable constituents. Solution viscometry and light scattering might then be powerful tools in assessing miscibility of the components in the amorphous state. The viscometric method utilizes the fact that polymer-polymer interactions are reflected in the viscosity of polymer-blend solutions consisting of chemically different polymers in a common solvent. In recent years, several attempts have been made using dilute-solution viscometry to predict polymer-polymer miscibility (/-/2). Starting point is the equation for the viscosity of a polymer solution proposed by Huggins (13). According to Huggins* equation, the secondorder term in the viscosity as a function of polymer concentration represents interactions between constituents of the system. This equation is extended to a polymer-blend solution consisting of two polymers in a common solvent. There are polymer-solvent and polymer-polymer interactions in the system. The interactions, including polymer-polymer interactions, are reflected in Huggins' equation via hydrodynamic interactions. In a common solvent, coils have a certain hydrodynamic volume. This volume will change owing to attractions or repulsions between chemically different chains, which in turn causes alteration of the Huggins coefficient. It means, we can deduce information about polymerpolymer interactions from the Huggins coefficient. Given the two polymers are dissolved in a common good solvent, at sufficiently low concentration of polymer they form separate swollen coils that behave like hard spheres and do not interpenetrate. Hence, one expects additive behavior of intrinsic viscosities in ternary solutions. When the concentration increases, the coils will interpenetrate and the Huggins coefficient reflects not only polymer-solvent but also polymer-polymer interactions. In that way, one can determine easily parameters from measurements of viscosity that are suitable to evaluate phase behavior of a polymer blend. There are a number of light scattering studies on ternary solutions chiefly of immiscible polymers in a common solvent (14 - 21). Light scattering data yield the second virial coefficient. In Flory-Huggins approximation, the second virial coefficient for polymer blend solutions can be represented by an additivity term and an excess term with respect to blend composition (22). The excess term is proportional to the interaction parameter of the two polymer constituents. Positive deviation from additivity is caused by a positive interaction parameter and indicates immiscibility of the two polymers. Miscibility can be inferred

In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


284

when the second virial coefficient as a function of composition displays negative deviation from additivity. Determination of the excess term of the second virial coefficient in ternary solutions becomes especially simple under so-called optical Θ condition (16, 17, 23). This condition is characterized by refractive index increments of opposite sign for the two polymer components. Unfortunately, for the blends under discussion no common good solvent could be found that obeys this condition. In this paper we present quantities provided by viscosity and light scattering measurements on polymer blend solutions comprising poly(3-hydroxy butyrate) (PHB) in combination with po!y(ethylene oxide) (PEO) and poly(fcaprolactone) (PCL), respectively, in chloroform and trifluoroethanol (TFE). We focus especially on the composition dependence of Huggins coefficient and second virial coefficient for the blend solutions.

Theoretical Background Viscosity. We start with Huggins' equation for the viscosity of polymer solutions. Formulated for a polymer blend solution, it reads

IspecJb = Mb b + & Hb Mb b c

c

1

()

where c is the mass concentration of the macromolecules in the solvent and K is the Huggins coefficient, subscript b refers to polymer blend. Specific viscosity, j? , and intrinsic viscosity, [η], are defined as usual. Quantity [η] of eq 1 comprises dependence of the terms on molecular mass. Hence, the Huggins coefficient is independent of molecular mass to a good approximation. Equation 1 provides a linear relationship between reduced viscosity, % /c, and concentration c with intercept [η], and the slope yields the dimensionless Huggins coefficient K . Quantities [η^ and K that follow from eq 1 refer to the blend solution. These quantities might be represented by superposition of quantities of the respective binaries. With total polymer concentration c = cj + c , it follows for intrinsic viscosity and K of the blend solution H

spec

pec

H

b

H b

2

Hb

[7lb*[7]iW| + [/7]2W2

(2)

with massfractionw, = cjc (i = 1,2). Eq 2 is straightforward since one expects additivity of the intrinsic viscosities for dilute solutions. The Huggins coefficient b


285 can be expressed as a second order equation of the individual quantities of the constituents plus an excess term, which is ruled by parameter *· defined by *^[KHI2-(KHIW ]

(4)

/ 2

We define a solution as ideal if the excess term of eq 3 disappears, i.e., **= 0 or ( # Η ΐ # Η 2 ) · Parameter κ reveals deviations from ideal behavior that are, to a good approximation, chiefly due to thermodynamic interactions between the different macromolecular species. At sufficiently high polymer concentration, coils will interpenetrate. Then, attractions between chemically different molecules will cause swelling of the coils leading to an excess increase in viscosity as compared to perfect behavior. Hence, positive deviations from ideal behavior are indicative of attractions between the different polymer species whereas negative deviations result from repulsions. Expressing these findings in terms of coefficient κ, it follows, κ > 0 reflects miscibility whereas κ < 0 stands for immiseibility. Light scattering. Light scattering data are organized in Zimm plots (24). Extrapolations of the reduced scattering intensity to scattering angle (9=0 and concentration c = 0 yield the quantities of interest, the molecular mass, A/ , the second virial coefficient, A , and the form factor, Ρ(Θ). The latter quantity depends on the mean square radius of gyration, . Moreover, the optical constant Κ of the scattering equations depends on the refractive index increment, which is symbolized by n . The scattering equations, applied to ternary blend solutions, yield apparent quantities of the ternary system (14, 22). These quantities can be expressed analogously to eqs 2 and 3 in terms of the corresponding binaries. It follows K\

=

, / 2


H 2

w

2

2

c

where subscript t refers to the ternary system. For the apparent molecular mass and the apparent radius of gyration, eq 5, we get simply additivity relations since these quantities are coefficients of first order terms in concentration. The second virial coefficient of the ternary system can be represented by a perfect and an excess part. The excess term characterizes deviations of the real blend solution from the perfect blend solution. Parameter Xof eq 5 might be seen as an excess second virial coefficient. It is defined in a similar way as parameter *rof eq 4


286 (7) Quantities G and V are the derivative of Gibbs free energy per NkT with respect to concentrations of polymer components 1 and 2 in the solution and the molar volume of the solvent, respectively. Experimental results of light scattering are discussed chiefly in terms of eqs 5 and 6. In Flory-Huggins approximation and for small total volume fraction of polymer in the solution, quantities in eq 7 read P\p>G = I + Χ\ι - Zs\ - Zsi and 2V p , ^ 2 I - 2χ$ι where ρ j is the density of the amorphous component /, and % is the interaction parameter between components / and j. It follows for the interaction parameter Xi2fromeq 7 l2

s


2

xl

( / )

=

s

l}

(8) Equation 8 demonstrates that negative excess X of the second virial coefficient leads to χ < 0, which is indicative of miscibility of the two polymers 1 and 2. η

Experimental Materials. Molecular characteristics of the polymers used in this study are given in Table I. Polymers were purified by dissolution and precipitation in chloroform and methanol, respectively, and finally dried in vacuum at 50 °C for 48 h. Polymer blend solutions for viscometry and light scattering were prepared in chloroform and trifluoroethanol (TFE), respectively, as solvents. Solvents were used without further purification. Viscosity measurement. Ternary solutions of PHB with PEO and PCL, respectively, were prepared by dissolving polymer mixtures having different weight ratios in chloroform at 50 °C, The solutions were diluted to the designated volume at room temperature to adjust desired concentrations. Concentrations ranged between 1 and 4g/l. Purification of solutions was done by filtration using nylon membrane filter prior to viscosity measurement. Ubbelohde viscometers of appropriate sizes were employed to determine the relative viscosities, η of the blend solutions. In any case, the flow time of pure solvent exceeded 200 seconds so as to minimize experimental errors. Values of η fall within the range of 1.2 - 2.0. Under these experimental conditions the kinetic energy and shear corrections were negligible. Viscosity measurements were carried out at 298 K. Temperature control was recorded to ± 0.1 K. η

τ


287 Light scattering experiments. Ternary solutions of PHB and PEO or PCL, respectively, were prepared in TFE in the same way as described under viscosity measurement. Solutions of five different total polymer concentrations, ranging from 1 to 5 g/1, were studied. Solutions werefilteredprior to the light scattering experiments.


Table L Characteristics of the polymer samples and the solvent Sample

a )

h)

c)

AC'/ kgmol"' 102 102 130

h)

MJM

n

3e )

p/gcm"

PHB 2.32 1.15 (25) PEO 1.91 1.13 (26) PCL 1.08 (27) 1.95 TFE 1.373 (28) obtained from light scattering in this study obtained from GPC analysis amorphous density at 25 °C

1

A/o/gmol" 86 44 114 100

Source Aldrich Acros Union Carbide Aldrich

Refractive index increments were measured directly in a differential refractometer (Brice-Phoenix). Unfortunately, polymer blend solutions in chloroform could not be used in light scattering experiments since the refractive index increment of PHB solutions in chloroform turned out to be very small. A photometer, equipped with a Helium-Neon Laser from Spectra-Physics (λ 632.8 nm), a Thorn-Emi photo multiplier, Model 9863/100KB, which is attached to a goniometer SP-81 from ALV-Laser GmbH, Langen (Germany), allowed to monitor the scattering intensity. 0

Results and Discussion Viscometry. Data of specific viscosity over blend concentration, plotted versus concentration c, for the blend solutions in chloroform fit Huggins equation 1 with high correlation; typically, correlation coefficients amount to r = 0.9998. Regression analysis shows that intrinsic viscosities have errors of around 0.2 %. Selected results for intrinsic viscosities and Huggins coefficients are compiled in Table II. We note, both PCL and PEO display higher intrinsic viscosities than that of PHB. Moreover, intrinsic viscosities, [17], vary with blend composition according to eq 2 in excellent approximation. The respective regression functions are also listed in Table II.


288


Table II. Selected intrinsic viscosities and Huggins coefficients for blends with PHB in chloroform Sample [/71/cmV KH PEO/PHB 100/0 0.3347 ± 0.0035 173.2 ±0.2 0.3373 70/30 0.3451 ±0.0045 151.1 ±0.2 0.3395 50/50 0.3529 ±0.0092 140.4 ±0.4 0.3424 30/70 0.364 ±0.013 112.5 ±0.4 0/100 0.3487 ± 0.0082 91.2 ±0.2 7fc(PHB/PEOVcm ig = 173.5 (1 - w ) + 92.9 w ; correlation: 0.992 PCL/PHB 100/0 0.3180 ±0.0033 207.6 ±0.2 0.3228 70/30 0.3146 ± 0.0065 162.3 ±0.3 0.3272 50/50 0.3131 ±0.0043 142.2 ±0.2 0.3334 30/70 119.4 ±0.2 ifciPHB/PCD/cmV = 196.8 (1 -ννρ ) + 89.5 »ν ; correlation: 0.992 3

l

PHB

PHB

ΗΒ

ΡΗΒ

Experimental values of the Huggins coefficients, K were determined according to eq 1 from the slopes of functions η ^ versus c. Values of K were calculated after the first term of eq 3. Optical inspection of data in Table II shows that we observe outside experimental errors (K -K d>0 for PEO/PHB blends and (K - K ) < 0 for PCL/PHB blends. These results indicate miscibility of the two constituents in blends of PHB and PEO whereas immiscibility in blends with PCL. This can be seen more clearly in Figure I where experimental values of /C b are depicted versus blend composition. Positive and negative deviations from ideal behavior become evident. Relevant results are summarized in Table III. Parameters κ are the result of regression calculations. For the blend PEO/PHB, it becomes obvious that the value of AT i2 exceeds significantly the values of K for the parent constituents. The opposite is true for blends of PCL and PHB. This fact points towards miscibility of the components in the former case and immiscibility in the latter case. These conclusions are consistent with results published in references (29 - 31). Martuscelli et al. inferred miscibility of PHB and PEO from melting point depression (29). Measurements of glass transition temperatures supported this conclusion (30). On the other hand, detection of glass transition temperatures and morphological studies provided evidence for immiscibility of PHB and PCL (31). H

$ρκ

H

H

H

H

Ht

New Polymeric Materials

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