Gas Permeability of Latex Interpenetrating Polymer Network Films

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22 Gas Permeability of Latex Interpenetrating Polymer Network Films A. K. Holdsworth and D. J. Hourston The Polymer Centre, Lancaster University, Lancaster LA1 4YA, United Kingdom

The study of the transport of gases through membranes based on polymer blends is increasingly prevalent. These investigations are generally designed either to assess the ability of such membranes to separate gas mixtures or to use the gas molecules as morphology probes. Thisfieldis briefly reviewed. The polymers involved in this study are poly(ethyl acrylate) and poly(ethyl methacrylate) combined in three distinct ways: as a latex blend, as a poly(ethyl acrylate)poly(ethylmethacrylate) latex interpenetrating polymer network, and as a poly(ethyl methacrylate)-poly(ethyl acrylate) latex interpenetrating polymer network. Carbon dioxide acted as a plasticizer for both homopolymers. The measured permeabilities were modeled using the relations proposed by Robeson, Maxwell, and Bottcher that attempt to relate permeability to morphology. The Robeson approach worked well for the latex blends, whereas the Bottcher equation was the most appropriate for interpenetrating polymer networks.

THE SORPTION AND TRANSPORT OF GASES IN POLYMER BLENDS has been increasingly studied over the last 15 years because it is possible to use the penetrant as a probe to investigate the phase morphology of blends (1). The majority of the work has been done in the area of miscible polymer blends with respect to the effect of blend composition on permeability, diffusivity, and solubility coefficients of these blends. For miscible blends, a simple logarithmic relationship has been postulated (2) between permeability ( P ) 0065-2393/94/0239-0449S06.00/0 © 1994 American Chemical Society

In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

450

N ITERPENETRATN IG POLYMER NETWORKS

and blend composition:

In Ρ = φ 1η P + φ In P χ

(1)

2

P and P are the permeability coefficients of components 1 and 2, respec tively, and φ and φ are the volume fractions of the components in the blend. This relationship, however, is a special case of more general mixing rules and is applicable (3) only under certain conditions. It has been shown (4) that gas transport in polymer films follows an Arrhenius-type relation, which describes the temperature dependence of the diffusion coefficient. In general, the activation energy of diffusion ( E ) may be written (3) as in eq 2, where ΔΕ is a deviation term: 1

2

χ


2

x

2

D

12

E

D

= φ Ε 1

+ φ Ε

0 χ

2

Empirical observations (5) indicate that E are related in the following way: In

D

= αΕ

D

+ ΔΕ

ϋ 2

(2)

12

and D (the diffusion coefficient)

+ b

Ό

(3)

where a and b are constants. From eqs 2 and 3, (aRT - 1) ΔΕ In D = «h In D, + φ In D + — 2

12

(4)

2

where R is the gas constant and Τ is the absolute temperature. Similarly, the solubility coefficient ( K ) for the blend may be related to the components by D

Βν φ φ 3

In K

1

= φ In K + φ In K +

D

2

1

ι

2

2

(5)

fly

Β is the binary interaction parameter for components 1 and 2 and V is the molar volume of the small molecule penetrant (component 3). Combining the preceding two equations and remembering that Ρ = K D, it follows that 3

O

J

(aRT - 1) ΔΕ

In Ρ = φ, In Ρ + φ In Ρ + ^ λ

2

2

12

Βν φ φ

+ —

3

1

^


2

(6)

22.

HOLDSWORTH AND HOURSTON Gas Permeability of Latex IPN Films

451

For weakly interacting components, the thermodynamic term Β is zero, and when the transport interaction energy term, Δ £ , is also zero, eq 6 reduces to the special case, eq 1. Sorption and transport of small molecules in miscible blends are often complicated by several factors.


1 2

1. The experimental temperature may be below the glass-transi tion temperature of one component of a blend and above the glass-transition temperature for the other component. The result for a miscible blend is that the mixture is glassy over a portion of the composition range and rubbery over the remain der. Of course, rubbers and glasses behave rather differently with respect to sorption and diffusion of gases. 2. One or both of the blend components may be partially crys talline with the result that the fraction of the mixture that is amorphous varies with blend composition. 3. The permeability may depend on concentration in a variety of ways, and care must be exercised to select a common basis for comparison of one blend composition with another. In the case of immiscible blends, as S-shaped relationship between Ρ and φ is usually observed (6-12). This contrast readily allows a miscible blend system to be distinguished from an immiscible blend system. Shur and Ranby were early investigators (8-12). The early work was centered on the study of mechanical blends of poly(vinyl chloride) (PVC) with ethylene-vinyl acetate copolymers (EVA) (13) and with PVC-acrylonitrile-butadiene copolymers (NBR) blends (14). The E V A copolymers contained 45 and 65% vinyl acetate, respectively. The effect of variation of vinyl acetate content on miscibility and, hence, on transport behavior of the blends was the focus of the study. Higher vinyl acetate content in the E V A and a higher milling temperature was found to coincide with decreased rate of permeation, decreased rate of diffusion, and increased activation energy, E . The data were interpreted as the result of denser packing of polymer molecules because of increased P V C - E V A interaction at the higher vinyl acetate content and with higher milling temperature. A n S-shaped plot obtained from Ρ versus E V A weight percent for the blends containing 45% vinyl acetate is indicative of a two-phase system because of phase inversion. Blends milled at lower temperatures gave curves concave toward the composition axis, which indicates the possibility of poor mixing. Similar results were obtained (14) with P V C - N B R blends: increased acrylonitrile content in the N B R gave rise to lower values of Ρ and D , but an increase in E was D

D


452


observed. The blends also showed increased volume contraction with in creased acrylonitrile content. These effects were caused by an increased degree of polymer-polymer interaction that resulted in reduced segmental mobility and increased compatibility of the two polymers. Work by K i m and co-workers (15) using poly(methyl methacrylate)poly(vinylidene fluoride) ( P M M A - P V F ) membranes and some hydrolyzed P M M A - P V F films showed interesting results. At 30% P V F composition the blend was completely amorphous and homogeneous. However, a blend that underwent hydrolysis with sulfuric acid for 30 min showed a crystalline melting point and became cloudy—indications that phase separation had occurred. These phenomena were reflected in permeability experiments where it was shown that unmodified P M M A - P V F membranes gave a convex shape when plotted against composition, whereas the partially hydro lyzed materials gave S-shaped curves. The blend of poly(2,6-dimethyl-l,4-phenylene ether) (PPE) and polysty rene (PS) has been studied (16) extensively in terms of it sorption and transport behavior with gaseous molecules. The authors studied the transport behavior of neon, argon, and krypton in solution-cast, annealed, glassy P P E - P S blends and observed that the permeabilities of argon and krypton increased monotonically with respect to increasing P P E content. The perme ability of neon was not monotonie and, hence, could not be explained in terms of a homogeneous system. The authors also observed that a plot of diffusivity versus composition was nonadditive and there was a minimum at the 50:50 composition whose magnitude decreased with increasing penetrant size. It was concluded that the nonadditivity behavior was the result of a change in the relative contribution of the two sorption mechanisms (16) operative in glasses that was not taken into account when the diffusion coefficients were calculated. Paul and co-workers also studied (17, 18) this blend system and deduced that the results could be described by the dual sorption model (17) for glassy polymers. A n interaction parameter that quantified the exothermic heat of mixing for the system was also obtained. Transport properties of the blends were found to lie well below predictions based on simple additivity; this finding is consistent with the strong interaction between the two polymers. Paul and co-workers investigated (19, 20) several other miscible, glassy blends that include blends based on copolyesters and polycarbonates. The results were again described well by the dual sorption model and the fact that negative deviations of permeability and diffusion coefficients from simple additivity relations could be explained as a result of decrease in volume on mixing for the blends. The effect of crystallinity was also studied, and, although the relative rates of permeation of one gas with respect to another varied greatly with blend composition, it was shown that the effect of crystallinity was not large. 2


2

2

2



22.

HOLDSWORTH AND HOURSTON Gas Permeability of Latex IPN Films

453

Two papers by Chiou and Paul (21, 22) on the gas permeation of homopolymer-eopolymer blends compared the differences between blends containing P M M A and styrene-co-acrylonitrile (SAN) copolymer and tetramethyl bisphenol A polycarbonate ( T M P C ) and S A N copolymer. The study of the P M M A - S A N system showed that gas transport was different from many other miscible blends in several aspects. The gas permeability and diffusion coefficients for the miscible blends were somewhat higher than the values calculated from the semilogarithmic additivity rale (2), whereas most other miscible blends show either a negative deviation or no deviation at all. O n the other hand, the ideal gas separation factors for P M M A - S A N blends followed this rule well, whereas the other miscible T M P C - S A N blend gave higher separation factors than predicted. The observed behavior of the P M M A - S A N blend was attributed to the very weak interactions between P M M A and S A N , whereas the negative deviations in the T M P C - S A N materials were indicative of strong polymer-polymer interactions. This behavior also has been ob served in other blends (23-25). The gas transport behavior of phase-sep arated P M M A - S A N blends was described well by a two-phase intercon nected model proposed by Kraus and Rollmann (26). Chiou and Paul (27, 28) also investigated gas permeation and sorption in P V F - P M M A blends and found that C 0 caused significant plasticization for all blends; in some blends, further crystallization of P V F was induced. Also, permeabilities increased with increased upstream pressure for all blend compositions and high separation factors were achieved for the C 0 ~ C H gas pair. Kang et al. (29) studied polymer blend membranes of PS and poly[l,l,l-tris(trimethylsiloxy)methacrylate propylsilane] ( P T M P S ) and showed that the permeability for an immiscible polymer blend could be represented by 2

2

2

2

P = (1 - α ) ? ! + αΡ + P 2

4

(7)

h

Ρ and P are the permeation coefficients of PS and P T M P S , a is the fraction of the discontinuous phase (PTMPS in this case), and P represents the permeation through the interstices at phase boundaries. A dramatic decrease in Ρ was observed if a graft copolymer that was miscible with both PS and P T M P S was incorporated. The explanation for this was an increase in the miscibility in the two components that gave rise to better mixing and a reduction in the number of phase boundaries and interstices. Because these interstices are the major pathway for gaseous flux, a reduction in their number led to a decrease in permeability. When 10% of graft copolymer (that contained 30% by weight of P T M P S ) was added to the blend, no visible interstices were observed using light microscopy. λ

2

h



454


A study by E l - H i b r i and Paul (30) reported on the gas sorption and transport properties of a multiphase acrylic polymer before and after subjec tion to mechanical drawing operations. A number of previously published reports have demonstrated (31-34) that molecular orientation of amorphous polymer films by mechanical drawing causes significant improvements in the gas barrier properties. In this chapter, permeation and dynamic mechanical analysis data are reported for latex interpenetrating polymer networks (IPNs) and a latex blend based on poly(ethyl acrylate) (PEA) and poly(ethyl methacrylate) (ΡΕΜΑ). The permeation data are modeled using the equations of Maxwell (35), Robeson et al. (36), and Bottcher (37): Maxwell equation:

Ρ =Ρ

P +

2P -2^(P -P )

d

m

m

à

(8)

P + 2P + c|> (P -P ) d

m

d

m

d

P is the permeability of the domains, P is the matrix permeability, and φ is the volume fraction of polymer in the domains. Equations used by Robeson et al. (36) are as follows: d

m

P = P Ρ /(Φπ,+ m

m

+ φ P d

m

series model

Φ

Gas Permeability of Latex Interpenetrating Polymer Network Films

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