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Plasma and Silane Treatment for the Improvement of Polymer-Polymer Interfaces in Nanocomposites

E. Jason Robinette1, Steven H. McKnight2, and Giuseppe R. Palmese1. (1) Drexel University, Dept of Chemical and Biological Engineering, 3141 Chestnut St, Philadelphia, PA 19104, (2) Weapons and Materials Division, Army Research Laboratory, Attn: AMSRD-ARL-WM-MA, Building 4600, Aberdeen Proving Ground, MD 21005

The creation of ordered structures using dissimilar polymer materials is particularly difficult when such order is desired at the nanoscopic length scale. Small domain sizes require high specific interfacial areas and therefore these structures are thermodynamically unfavorable. In order to overcome such limitations, a step-by-step building approach is required where a polymer template is joined to a dissimilar polymer material via a reactive process. In this work, we prepare nanocomposites using electrospinning to obtain polysulfone (PSU) fiber mats with sub-micron fiber diameters onto which organofunctional polysiloxane layers of controlled thickness are grafted. A sequential technique was adopted whereby the template material is subjected to plasma treatment followed by a silane treatment to establish an ordered polysiloxane structure. These modified mats may then be encapsulated in a third polymer, thus forming a polymer–polymer composite that possesses a continuous, finely dispersed phase of the templating polymer within a structurally stable polymer matrix.

In previous work, we encapsulated PSU fibers in a vinyl ester (VE) matrix comprising a VE monomer and styrene diluent. Poor interfacial behavior between the fiber and matrix resulted in the formation of voids around the fiber. This phenomenon has also been reported for rubber-toughened vinyl esters and can be attributed to the presence of two chemical components in the matrix [1]. Generally, styrene has a greater potential to swell thermoplastic phases than the VE monomer. At the fiber-matrix interface, styrene diffuses into and swells the thermoplastic fibers. During matrix cure, gelation occurs that locks the second phase morphology into place. Although gelled, the matrix remains reactive and draws styrene from the swollen fibers. Voids form due to the contraction of the fibers during styrene diffusion back into the matrix. As shown in rubber-toughened VEs, reactive second phases can eliminate voids by reacting with the matrix, thus limiting the diffusion of styrene out of the second phase. Therefore, the focus of this work is to improve the interfacial behavior of polymer-polymer nanocomposites by controlling the reactive nature of the second phase modifier through a combination of plasma and silane treatment. This paper describes the details of the fiber modification and characterizes the interfacial behavior of the fibers with the matrix.

Plasma treatment has been an effective technique for altering surface characteristics of many polymers. By selecting an appropriate plasma gas, one can affix desired moieties to a polymer surface. For example, ammonia plasma can incorporate amine groups to a polymer surface. In this work, oxygen plasma was used to attach oxygen moieties to the fiber surface for potential grafting sites. First, we studied the effects of plasma medium (Ar, Ar-O2, and O2) and treatment time on surface modification. In these studies, low pressure plasma was used due to its superior uniformity over atmospheric systems. XPS analysis was used to determine that the bonds most susceptible to cleavage from Ar plasma were the aromatic, ether, and sulfone groups. Oxygen addition to the plasma induced photo-oxidative degradation of the non-aromatic carbons and ether groups. All plasmas yielded the addition of hydroxyl, peroxide, carbonyl, and carboxyl groups. However, the presence of oxygen in the plasma resulted in higher oxygen uptake and more highly-oxidized moieties. The second part of this work investigated the effects of atmospheric dielectric barrier discharge (DBD) O2 plasma on PSU surface modficiation and molecular weight. DBD plasma was effective in adding the same oxygen moieties as low pressure plasma, although losing some efficiency. Ozone generation during DBD treatment proved to reduce PSU molecular weight and glass transition temperature (Tg).

Silane coupling agents are typically used for the surface treatment of fibers in polymer matrix composites with the desired purpose of tailoring fiber-matrix behavior. Organofunctional silanes can undergo hydrolysis and subsequent condensation with hydroxyl groups on a polymer surface to yield a desired chemical reactivity. After O2 plasma treatment, a significant amount of hydroxyl and carboxyl groups are present on the PSU fiber surface. Once hydrolyzed, the silanol reacts with the surface hydroxyl groups. After drying at an elevated temperature, further condensation occurs that crosslinks the silanols and yields a crosslinked polysiloxane containing a desired functionality. The organofunctional silanes studied in this work include methyltrimethoxysilane (MTMS) and vinyltrimethoxysilane (VTMS). MTMS was chosen to determine the effects of a non-reactive polysiloxane structure at the fiber-matrix interface. VTMS was selected because it can react with the free-radically curing matrix. Thermogravimetric analysis (TGA) and XPS were used to ascertain the grafting yield of the polysiloxane as a function of plasma treatment time and silane concentration. In general, the grafting yield of polysiloxane increases with increased plasma treatment time. For silane concentration, a maximum in grafting yield is reached between 0.3 and 0.5 wt. %. The maximum grafting yield was 11.62 wt. %, achieved at 2 minutes of plasma treatment and 0.5 wt. % VTMS in solution.

Analysis of the fiber-matrix interfacial behavior was performed by using an ESEM to examine fracture surfaces. Comparisons were made between PSU fibers that were untreated, MTMS-grafted, and VTMS-grafted. Untreated fibers showed the formation of voids around the fibers, thus indicating the swelling of the fibers with styrene and subsequent removal during cure. The MTMS-grafted fibers also showed voids around the fibers. The presence of polysiloxane at the interface does not impede the diffusion of styrene from the VE matrix into the PSU fibers. VTMS-grafted fibers exhibited very little to no void formation around the fibers. As shown by rubber-modified VEs, a reactive second phase can limit the driving force for styrene diffusion and removal. Therefore by grafting a reactive structure at the surface of the fibers, the interface between PSU and VE is greatly improved.

References

1. Robinette EJ, Ziaee S, Palmese GR. “Toughening of vinyl ester resin using acrylonitrile-butadiene rubber modifiers”, Polymer 45 (2004) 6143–6154.