Polystyrene And In Miscisl Le N Of Surface Pol Adsopion (u

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Polystyrene And In Miscisl Le N Of Surface Pol Adsopion (u

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PPORLEYFSETRYERNETNIEAL ANSDURPFAOCLE.. A(UD)SOCOPNINOENCTICIUNTMIUSNCIVISLSTORRLSEN OF 0 S IHATIA ET AL. 02 FEB 00 TR-4 ARO-24i65. 4-NS

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1. REPORT NUMUER

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In IQ 96 q&f i+-fS

44

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5. TYPE OF REPORT & PERIOD COVERED

Preferential Surface Adsorption in Miscible Blends of Polystyrene and Poly(vinyl methyl ether)

Technical

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6. PERFORMING ORG. REPORT NUMBER

7. AUTNOR(a)

S. CONTRACT OR GRANT NUMBER(.)

Q.S. Bhatia,P. H. Pan, and J. T. Koberstein

DAAL03-86-K-0133

9. PERFORMING ORGANIZATION NAME AND ADDRESS
University of Connecticut Storrs, CT 06268

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I1. CONTROLLING OFFICE NAME AND ADDRESS
U. S. Army Research Office Post Office Box 12211 Resenrt,h Tri~nnpl PairtlIt'2"7709o
14. MONITORING AGENCY'IiAME & AOLRESft1-i iorle't from ControllingOflce)

12. REPORT DATE
Feb. 2, 1988 13. NUMBE4RO3FPAGES
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Unclassified

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Approved for public release; distribution unlimited.

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The view, opinions, and/or findings contained in this report are

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those of the author(s) and should not be construed as an official

Department of the Army position, policy, or decision, unless so

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v_ other dtirtmentat~rinn

S. KEY WOROS (Cnt nuLe on reveree slid Itnecesarynd identify by block number)

Miscible polymer blends; x-ray photoelectron spectroscopy, Surface Adsorption; surface tension; polystyrene; poly(vinyl methyl ether)

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Preferential Surface Adsorption in Miscible Blends of Polystyrene and Poly(vinyl Methyl ether)
Technical Report No. 4
Q. S. Bhatia, D. H. Pan, and J. T. Koberstein (accepted for Macromolecules)
February 1988
U.S. Army Research Office Contract Number: DAALO3-86-K-0133
University of Connecticut Storrs, CT 06268
Approved for public release Distribution Unlimited
88

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Preferential Surface Adsorption in Miscible Blends of Polystyrene and Poly(vinyl methyl ether)
Qamardeep S. Bhatia Department of Chemical Engineering
Princeton University Princeton, NJ 08544'l
David H. Pan Xerox Webster Research Center
800 Phillips Road 0114/39D Webster, NY 11580 and
Jeffrey T. Koberstein* Institute of Materials Science and Department of Chemical Engineering
University of Connecticut Storrs, CT 06268

*to whom correspondence should be addressed.

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ABSTRACT
The surface structure and properties of miscible blends of polystyrene (PS) with poly(vinyl methyl ether) (PVME) have been studied as a function of the blend composition and constituent molecular weights. The lower surface tension of hte PVME compared to that of PS results in preferential adsorption of PVME at the surface. The surface PVME enrichment is characterized by measurements of the surface tension as a function of the temperature, accomplished with an automated pendant drop apparatus, and by x-ray photoelecron spectroscopy (XPS). Angledependent XPS has been used to determine the surface concentraiton profiles of the blend constituents. The results of these measurements demonstrate that: 1) the PVME surface conce tration is elevated substantially from that in the bulk; 2) the integrated surface co centration gradient detemrined from XPS measurements can be modeled as a cotl 2 (z/ +a) profile where is the screening length; and 3) the degree of surface enrihment depends strongly on the blend composition and molecular weight of the onstituents, correlating well with the surface energy difference between PS and 'VME.

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I. INTRODUCTION
Current technologies frequently employ multiconstituent polymer systems in order to tailor the material's bulk physical and mechanical properties. Although much emphasis has been placed on understanding the bulk phase relationships and properties of multicomponent polymeric materials, comparitively little is known about their surface structure and properties.
In small molecule systems, such as metallic alloys1 and liquid mixtures 2, it is well known that the surface composition differs from that of the bulk due to preferential surface adsorption of one constituent. This process is driven, in part, by differences in surface energies and can be expressed classically through the Gibbs adsorption isotherm3

-dy = i ri dpi

(1)

where ri is the surface excess (i - ni/A) of component i, dA the fractional surface
area, and pi is the chemical potential of species i for ni moles of that component. From (1) it is apparent that a surface concentration gradient exists in multiconstituent systems where the surface is enriched in the component of lower surface energy (i.e., surface tension y).

*blend

Preferential surface adsorption has been documented by surface tension, contact angle and x-ray photoelectron spectroscopy (XPS) measurements on several multicomponent polymeric systems. In immiscible binary homopolymer blends, for example, the surface behavior is generally dominated by adsorption of the lower surface energy component.4, 5 This phenomena also occurs for many
additives. 6 Since equilibrium bulk thermodynamics favor complete demixing of the two homopolymers, the "equilibrium" surface should be occupied exclusively by the constituent of lower surface energy. In actuality, macroscopic

equlibrium is usually not attained in immiscible polymer blends, such that the surface structure obtained is dependent on intrinsic factors such as the relative wettabilities of the two constituents and the degree of phase separation; as well as extrinsic factors including the procedure for sample preparation and blend morphology.
A number of investigations of copolymer surfaces have also appeared. Early studies of the surface tensions of block copolymer melts7 ,8 illustrated significant surface activity by the sequence of lower surface energy. Surface activity increased with block length, and complete surface coverage by the low surface energy constituent was observed for copolymers of sufficient length. XPS investigations have reported similar results for a number of diblock and triblock copolymer systems.9 -15 Surface enrichment has also been demonstrated in random copolymers of hexamethyl sebacate with dimethyl siloxane and ethylene oxide with propylene oxide.7 Complete domination of the surface by lower energy constituent does not occur however, reflecting the influence of configurational constraints which limit migration to the surface.
Block copolymers exhibit similar behavior when added to homopolymers. 16 18 A practical example is the reduction of poly(propylene glycol) surface tension in the manufacture of polyurethane foams.1 9 The addition of a few tenths of a percent of certain polyether-polysiloxane block copolymers reduces the surface tension of the blend to that corresponding to pure polysiloxane.
The surface topology of block copolymers has also been investigated by XPS measurements. The results for a number of block copolymers containing dimethyl siloxane (PDMS) sequences9 ,13 showed that the surface is comprised of a homogeneous PDMS-rich overlayer, the composition and thickness of which are dependent on the composition and block lengths of the copolymer. Under certain conditions, this overlayer consisted of essentially pure PDMS. In contrast, similar

studies on other block copolymer systems have concluded that, although the surface is dominated by the species of lower surface energy, the topology is heterogeneous.10-12 14 ,15 That is, the species dominating the surface resided in either lens-shaped, cylindrical, or lamellar microdomains protruding from the surface.
Gaines 2 0 has attributed the two types of behavior observed for block copolymers to differences in spreading or wetting for the two systems. In the siloxane systems, the surface tension difference between components is large enough to favor surface wetting by the siloxane sequences. In cases where the surface energy difference is small, one sequence cannot wet the other, resulting in a heterogeneous surface as has been found in ethylene oxide block copolymers.lO,11
More recently, Fredrickson 21 has proposed a theory for surface ordering in block copolymers. Even in the disordered state, block copolymers are shown to possess ordered surfaces with periodic surface composition profiles. The initial theory is derived for systems close to the order-disorder transition (i.e., in the weak segregation limit) and does not consider directly the effects associated with preferential wetting.
There is a large body of experimental data and theory pertaining to the surface properties of polymer solutions, especially concerning their surface tensions.2 2-2 6 The success of these theories in representing the experimental data has been discussed in a recent review.2 7 Most polymer-solvent systems show adsorptive behavior for the solute wherein a large initial reduction in surface tension (3-5 mN/m) is seen upon polymer addition. Repulsive behavior (i.e., surface enrichment of solvent) has also been observed for several polymer systems. In this case the surface tension increases almost linearly with polymer
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concentration until it jumps suddenly to the pure homopolymer value as the polymer concentration approaches unity.
Miscible homopolymer blends are similar to polymer solutions and also exhibit pronounced surface-excess behavior. 7,2 8 ,2 9 Measurements on several oligomeric mixtures reveal that the surface excess is accentuated by increasing the molecular weight. LeGrand and Gaines 2 9 modelled the surface tension data for compatible oligomers of PDMS and polyisobutylene by a theory which combined the Flory-Huggins lattice model for polymer solutions 3 0 with the Prigogine-Marechal parallel-layer model. 2 In general, however, careful examination of theories for the surface tensions of miscible blends has been hampered by the lack of knowledge regarding the polymer-polymer interaction parameters.
Recently, Pan and Prest,3 1 presented initial results of studies on the surface structure of the miscible polymer blend system polystyrene/poly(vinyl methyl ether). X-ray photoelectron spectroscopy were employed to measure both the effective surface composition and surface concentration gradient in the blends. The results demonstrated a pronounced surface enrichment of the poly(vinyl methyl ether). The bulk thermodynamic phase relationships, interaction parameters, and phase separation mechanisms have already been studied extensively for this miscible blend system.3 2-3 7 The observed phase diagrams exhibit a lower critical solution temperature wherein phase separation occurs upon heating. Miscibility is manifest over a wide range of experimental conditions, making this blend an excellent model system for the study of
preferential surface adsorption in polymer systems. In this communication, we
extend the initial study and report the effects of blend composition and constituent molecular weights on surface enrichment in these miscible polymer blends. The surface structure is characterized by XPS measurements on thin films, while the thermodynamic character of the surface is assessed by determining the
SurfaceSurface AdsorptionPolystyreneSurface TensionSurface Structure