1450
Langmuir 2000, 16, 1450-1453
Relations between Surface Energy and Surface Potentials of a Nitrogen Plasma-Modified Polypropylene F. Poncin-Epaillard*,† and You-Im Chang‡ Laboratoire de Physique et Chimie des Mate´ riaux Polyme` res, Universite´ du Maine, Avenue O. Messiaen, 72000 Le Mans, France, and Department of Chemical Engineering, Tunghai University, Taichung 40704, Taiwan, Republic of China Received May 18, 1999. In Final Form: September 20, 1999
Introduction The cold plasma treatment of materials, which corresponds to both chemical and physical treatment of the surface, was recently developed for polymer surface modification with the idea of creating better adhesionsor nonadhesionsand printing properties.1-3 The interactions between such an ionized fluid and a polymer surface lead to different reactions on the surface: cross-linking, degradation, functionalization, and radical formation. Whatever the natures of the plasma or the polymer, are all these reactions take place on the surface but in a relative proportion depending on the structures of plasma and polymer. Therefore, the surface treatment is efficiently controlled by the choice of plasma parameters. The surface modification of polypropylene in a nitrogen plasma has been described in our previous papers,4-5 which show that cross-linking and functionalization are the major reaction mechanisms. In such a treatment, the degradation is less important. The functionalization corresponds mostly to the attachment of amino groups. A chemical titration of attached groups was developed and allows the quantitative determination of the superficial concentration of amino groups per square nanometer. The lower the plasma parameters (mostly time and discharge power), the higher the functionalization. The functionalization yield of the amino groups, depending on the plasma parameters, is between 0 and 6.4 NH2/nm2. The present paper deals with a new specific application of the plasma surface functionalization: the relationship between the surface energy of a polypropylene film treated in a nitrogen plasma and the calculated surface potential.
When a study was carried out with respect to one of these parameters, the other parameters were kept constant. Before treatment, the following conditions were applied: a primary pumping stage to 10-2 mbar, followed by a secondary pumping stage at 10-5 mbar for 30 min. N2 introduction was run over 5 min; then the discharge power was switched on for 5 min, and the plasma purity was checked by optical emission spectroscopy. The discharge power was switched off, the slide valve was closed, and the sample was introduced into the reactor chamber. A secondary pumping stage was run for 7.5 min. After the nitrogen introduction, the treatment takes place. The polypropylene (PP, Mn ) 50 000, DPn ) 1200, Tm ) 160 °C, crystallinity yield 62% (through differential thermal analysis) and 52% (through X-ray diffraction)), supplied by ITF Lyon, was synthesized by Ziegler-Natta catalysis in a heterogeneous phase. The PP was a film of 100 ( 10 µm thickness without additives. Each sample had a surface area of 15 cm2 and a mass of about 100 mg. They were washed with pure acetone before plasma treatment, and the treated samples were kept under reduced nitrogen pressure before analyses. The purity of the different gases (N2 and H2) (grade U, Air Liquide) is >99.998%. The surface energy was calculated from the Dupre´ equation and the contact angle was measured by the liquid drop method. So, during measurement, the water molecular phase only touches the top layer of the modified surface. The dispersive (d) and nondispersive (nd) components of the surface energies of the different liquids adopted in the present experiment are as follows:
distilled H2O: γ ) 72.8 mJ‚m-2, γd ) 21.8 mJ‚m-2, γnd ) 51.0 mJ‚m-2 diiodomethane (Aldrich, spectrophotometer grade): γ ) 50.8 mJ‚m-2, γd ) 49.5 mJ‚m-2, γnd ) 1.3 mJ‚m-2 All of these liquids are inert to polypropylene. Surface amines were titrated separately by using Ponceau 2R (3-hydroxy-4-[2,4-dimethylphenylazo]-2,7-naphthalenedisulfonic acid, disodium salt) reagent acidified with 0.1 N HCl solution (Fluka). The concentration of amino groups on the surfaces of polypropylene is titrated according to the following reactions:
Experimental Section The conditions for plasma treatment were selected as follows incident power Pi ) 20-150 W; reflected power Pr < 2 × 10-2 W; nitrogen flow Q ) 10-60 sccm; nitrogen purity 99.985%; distance between sample and excitator d ) 10 cm; duration 1-12 min; pressure during plasma treatment 0.3 mbar; system ultimate pressure 10-5 mbar; volume of the reactor 2.7 L; volume of the plasma 1 L. * To whom correspondence should be addressed. E-mail:
[email protected]. † Universite du Maine. ‡ Tunghai University. (1) D’Agostino, R. Plasma deposition, treatment, and etching of polymers, Academic Press: New York, 1990. (2) Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem Ed. 1998, 16, 911. (3) Relley, C. N.; Everhart, D. S. Surf. Interface Anal. 1981, 3, 176. (4) Poncin-Epaillard, F.; Chevet, B.; Brosse, J. C. J. Adh. Sci. Technol. 1994, 8 (4), 455-68. (5) Poncin-Epaillard, F.; Brosse, J. C.; Falher, T. Macromol. Chem. Phys. 1998, 199, 1613.
Immediately after treatment and without contact with the air atmosphere, the PP sample was dipped into a 10 mL solution of reagent (Ponceau 2R concentration in water: 2.0 × 10-6 mol‚L-1). The polymer and the analysis set were degassed before polymer immersion, and the solution was degassed just after. Then, the solution was heated to 70 °C for 12 h while stirring. Then a back-titration of the solution, giving a high sensitivity (
