ARTICLE pubs.acs.org/Langmuir
Bioinspired Material Based on Femtosecond Laser Machining of Cast Sheet Micromolding as a Pattern Transfer Process B. Sarrat,† C. Pecheyran,‡ S. Bourrigaud,§ and L. Billon*,† †
Equipe de Physico-Chimie des Polymeres, et ‡Equipe de Chimie Analytique Bio-Inorganique et Environnement, Institut Pluridisciplinaire de Recherche sur l’Environnement et les Materiaux (IPREM), UMR 5254 CNRS/Universite de Pau et Pays de l’Adour, 2 Avenue du President Angot, 64053 Pau Cedex 9, France § Groupement de Recherche de Lacq, Arkema, BP 34, 64170 Lacq, France
bS Supporting Information ABSTRACT: We present herein a simple, fast, and easy-to-use process to replicate poly(methyl methacrylate) (PMMA) plates generating surface effects. First, a femtosecond laser has been used to fabricate, with sufficient accuracy, a periodic pattern on a glass plate at the microscale. This glass plate, used as a master, can be structured on a large distance with a good control of its roughness. Then, the polymer plates were obtained by bulk polymerization without any solvents with a good replication from the cast sheet process, which has been industrially performed for years. Thus, the modification of this process, environmentally friendly, lets us foresee new applications for commodity polymers by introducing visual iridescent properties and hydrophobicity exaltation.
’ INTRODUCTION Nature provides us a wide pallet of fascinating physical phenomena, such as nacre, interferential effect, or superhydrophobicity, which can be observed in insect wings, shellfish, or leaves. The origin of such an effect is usually attributed to the presence of highly ordered arrangements in natural materials. The physical understanding of such phenomena can lead to original ideas for the creation of new materials exhibiting these specific properties, i.e., the bioinspired materials. Thanks to their properties, one challenge in modern material science is to replicate these highly ordered structured materials exhibiting, as in nature, such physical phenomena.1-3 Recent advancements in patterning technologies have considerably enhanced the ability to control both surface chemistry and topography of various materials at the micro- and nanoscale, thus allowing material properties to be tailored. Many methods have been developed to fabricate complex two- and three-dimensional structures using top-down and bottom-up approaches. Bottom-up approaches, including macromolecular selfassembly and colloidal assembly, have been recently used by our group.4,5 Even if a hierarchically self-organized materiel at the nano- and microscale was easily elaborated by fast solvent process evaporation, the bottom-up approach seems to present several disadvantages especially because of limitations in the geometry of the structures that can be formed. On the other hand, the past decade has attended the rise of new lithographic techniques in particular with major advances in resolution through the use of shorter wavelengths of light [UV-based nanoimprint lithography (UV-NIL)]. These new patterning methods have proven to achieve well-designed nanostructures with high accuracy in shapes and alignments of the objects.6-11 r 2011 American Chemical Society
The lithographic technique consists of a molding process, i.e., pattern transfer, in which the mold surface template determines the patterns imprinted on the substrate. Among the numerous methods used to create identical replicas of the same master, the trend is nowadays to manufacture well-shaped nanoscale structures12,13 with the hot embossing lithography (HEL) or the recent nanoimprint lithography (NIL).14 All of these methods stay focused on the precision and the definition of the pattern, and they give rise to an increasing interest in new microelectronic developments, such as microarrays, microchips, and bioelectronics.15,16 The photolithographic technique can also be obtained using the soft lithography (SL) based on siloxane elastomer, particularly, for the patterning of large-area features. In this case, SL involves three separate processes, i.e., (1) fabrication of a master mold, (2) replication of the mold, and (3) pattern transfer to another surface. Until recently, the master molds were exclusively fabricated by photolithographic techniques. Nevertheless, the fabrication of ordered nanostructured arrays using poly(dimethylsiloxane) (PDMS) replica mold based on three-dimensional colloidal crystals has been described by Choi et al.17 The colloidal crystal was achieved by self-assembling of polystyrene (PS) monodisperse colloidal particles using a dip-coating technique. From an identical concept, Losic et al. described the rapid fabrication of bioinspired micro- and nanoscale patterns. Indeed, they used replica molding from diatom biosilica and demonstrate the ability to generate multiple copies of the diatom frustules.18 Received: November 1, 2010 Revised: January 19, 2011 Published: February 10, 2011 3174
dx.doi.org/10.1021/la104364n | Langmuir 2011, 27, 3174–3179
Langmuir Scheme 1. Description of the CSμM Process
Another approach is based on customization of polymeric materials by micromachining using a femtosecond pulsed laser.19,20 This concept has been recently reviewed by Gattass et al. as the most efficient micromachining of bulk transparent materials to remove or change the properties of the material.21 Indeed, the femtosecond laser process presents unique advantages in favor of micromachining of transparent materials over other photonic-device fabrication techniques. First, the nonlinear nature of the absorption confines any induced changes to the focal volume. This spatial confinement makes it possible to micromachine rapidly, directly, and geometrically complex structures in three dimensions. Finally, the high power density (typically 1013 W/cm2) delivered by femtosecond pulses allows for high-quality ablation with a wide range of materials (minerals, polymers, metals, etc.), enabling optical devices to be fabricated in compound substrates of different materials. Herein, the femtosecond laser machining was used for its simplicity, speed, and reproducibility of the process to synthesize bioinspired materials mimicking natural physical effects, such as light diffraction, also called iridescence and hydrophobicity enhancement.4,5 We present an alternative method derived from an industrial process, the so-called cast sheet micromolding (CSμM), producing industrially poly(methyl methacrylate) (PMMA) sheets with high transparency and hydrophilicity. Indeed, this new concept is developed to transfer micrometric wafer patterns, i.e., diffraction grating, on a polymer surface directly during the in situ polymerization step. This CSμM system, composed of two glass plates, a polyvinyl chloride (PVC) joint, and few clips, allows us to create numerous replicas exhibiting the same physical properties without the use of organic solvents or residual chemical products (Scheme 1). Moreover, the sustainable development and the preservation of the environment becomes more and more sensitive, and a friendly method is highlighted here to prepare few identical imprinted polymer plates. This concept with no polluting constraints represents a simple and “green” pattern transfer process, to generate functional organic glasses presenting new physical surface properties, as hydrophobicity exaltation and iridescence.
’ EXPERIMENTAL DETAILS Materials. PMMA solutions [mixture of methyl methacrylate (MMA, 92%) and PMMA (8%; Mn = 100 000 g mol-1; Ip = 1.9)],
ARTICLE
azobisisobutyronitrile (AIBN), terpinolene, glass plates (100 100 10 mm), and PVC joints were provided by Arkema and used as received. Micromanufacturing. The micromachining of the master glass mold was carried out with a high repetition rate, infrared femtosecond laser (ALFAMET, Novalase Sa, Amplitude Systemes, France). This laser system included a diode-pumped KGW-Yb crystal delivering 360 fs pulses at 1030 nm. The laser can operate at a high repetition rate (1-10 000 Hz) with a precise adjustment of the energy delivered to the sample within 0.5-70 μJ and pulse-pulse stability. The ablated zone on the sample was located precisely with dedicated micro (200) and macro (10) video cameras. The laser system was also fitted with a galvanometric scanner installed between the laser source and the objective, allowing the fast movement of the laser beam at the surface of the sample in two dimensions (up to 280 mm s-1, with a precision of 1 μm). In addition, a high precision (
