Poly(Acrylic Acid) Surface Grafted Polypropylene Films: Near Surface and Bulk Mechanical Response

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CONICET Digital eXPRESS Polymer Letters Vol.2, No.11 (2008) 779–790 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2008.91 Poly(acrylic acid) surface grafted polypropylene films: Near surface and bulk mechanical response L. A. Fasce1, V. Costamagna2, V. Pettarin1, M. Strumia2, P. M. Frontini1* 1Universidad Nacional de Mar del Plata, Instituto de Investigaciones en Ciencia y Tecnología de Materiales, INTEMA, J.B. Justo 4302 - B7608 FDQ - Mar del Plata, Argentina 2Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Argentina Received 19 August 2008; accepted in revised form 20 September 2008 Abstract. Radical photo-grafting polymerization constitutes a promising technique for introducing functional groups onto surfaces of polypropylene films. According to their final use, surface grafting should be done without affecting overall mechanical properties. In this work the tensile drawing, fracture and biaxial impact response of biaxially oriented polypropylene commercial films grafted with poly(acrylic acid) (PAA) were investigated in terms of film orientation and surface modification. The variations of surface roughness, elastic modulus, hardness and resistance to permanent deformation induced by the chemical treatment were assessed by depth sensing indentation. As a consequence of chemical modification the optical, transport and wettability properties of the films were successfully varied. The introduced chains generated a PAA-grafted layer, which is stiffer and harder than the neat polypropylene surface. Regardless of the surface changes, it was proven that this kind of grafting procedure does not detriment bulk mechanical properties of the PP film. Keywords: mechanical properties, depth sensing indentation, surface photo-grafting, polypropylene, films 1. Introduction increasing the polarity introducing special chemical Owing to its useful properties, easy workability and functionalities like carboxylic groups, or by coating low manufacturing costs, polypropylene (PP) is the surfaces with polar thin films, for example, widely used for the preparation of films, fibers, poly(acrylic acid) (PAA). Surface grafting tech- plates, injection-molded as well as blow molded niques including photo-initiated grafting [10–14], parts [1]. Often, insufficient surface properties pre- living radical grafting [15], ceric ion induced graft- clude its use in an application to which bulk ing [16], layer-by-layer ionic grafting [17] and mechanical properties may be well-suited. For plasma polymerization [18, 19] have proven to be example dyeability, printability, paintability, adhe- effective to graft poly-functional monomers or sion, biocompatibility, antifogging, and gas perme- polymers onto PP surfaces. ability of PP parts can be improved by surface Among the methods to modify polymers, radical modification [2–9]. photo-grafting polymerisation appears as an attrac- Efforts have been made to develop polymer modifi- tive method to impart a variety of functional groups cations processes which allow the surface proper- to a polymer [20, 21]. The advantages of photo- ties to be tailored to meet a specific requirement grafting over other available methods are: i) easy while retaining beneficial mechanical properties. and controllable introduction of chains, ii) high Surface modification can be accomplished by density and exact localization of chains onto the *Corresponding author, e-mail: [email protected] © BME-PT and GTE 779 Fasce et al. – eXPRESS Polymer Letters Vol.2, No.11 (2008) 779–790 surface, iii) covalent attachment of chains which The present paper is focused on determining the avoids delamination and assures long-term chemi- mechanical properties of a bi-oriented PP film cal stability of introduced chains, in contrast to before and after surface photo-grafting with PAA physically attached layers [1]. chains. To this aim the bulk mechanical behavior Low energy radiation, such as UV light, is fre- was characterized by evaluating uniaxial tensile quently used to initiate surface modification. The deformation, quasi-static fracture and dart impact ability of graft co-polymerization to quickly change properties. In addition depth sensing indentation the surface properties in a tunable fashion allows experiments [31–36] were carried out in order to for the development of a product that can be gain a deeper insight into differences in surface adapted on-demand to a particular application. mechanical characteristics of PAA-grafted and neat Modification of PP films through grafting at rela- films. tively low amounts were previously demonstrated to be an effective approach to functionalize PP pro- 2. Experimental section viding a defined, stable, organic foundation for 2.1. Film grafting procedure subsequent surface modification of various types [22]. Costamagna et al. were able to modify exclu- A commercial polypropylene film (PP) (Conver- sively the surface of bi-oriented PP films by graft- flex S.A., Buenos Aires, Argentina), was surface ing reaction of poly(acrylic acid) (PAA) [23]. They modified by radical grafting polymerisation initi- showed that values of the permeability coefficient ated by UV light. The appropriate reaction condi- of oxygen, nitrogen, carbon dioxide, carbon tions were previously investigated [23]. The monoxide, argon, methane, ethane, ethylene, and procedure was as follows: Acrylic acid was used as propane across the grafted films undergo a marked grafting co-monomer. The initiator benzophenone drop. Their morphology investigations by Atomic (Fluka AG, Buchs, Switzerland) was dissolved in Force Microscopy (AFM) suggested the initial for- the acrylic acid (BASF, New Jersey, USA) and dis- mation of PAA brushes attached to the PP surface tilled water was added. The solution in contact with at short time of reaction followed by the generation the PP film was enclosed into a photo-reactor and of a network by cross reaction between grafted irradiated with UV light (medium pressure UV growing chains, brushes entanglements and strong lamp Engenlhard-Hanovia, Slough, England) under hydrogen bonds [24]. The improvement in barrier nitrogen atmosphere at room temperature for properties was attributed to the formation of such a 10 min. The grafted film was extensively washed rigid PAA layer onto the PP surface [24]. with a NaOH solution (pH = 8) in order to remove Despite it has been claimed that surface grafting traces of un-reacted monomer and formed theoretically occurs without detriment of the bulk homopolymer as well as to extract rests of the ini- mechanical properties of the polymer [25], undesir- tiator. able changes in the mechanical performance of dif- Before characterization, surface modified films ferent polymeric films modified by photo-grafting were left in a pH 8 solution for 24 h in order to dis- techniques have been reported (see for example in rupt the hydrogen bonds. [22, 26–28]). To a large extent, the tendency and The thickness (t) of the neat PP film determined level of concomitant changes in the material prop- with a micrometer was 34±1 μm. The difference erties depend on whether crosslinking or degrada- between the thickness values of the neat and the tion dominates during the irradiation of the PA-grafted films was determined with a Mitutoyo polymer [29, 30]. UV irradiation may induce the coordinate measuring machine. Measurements formation of free radicals, which promotes made on at least ten different locations for each crosslinking. This in turn minimizes the anisotropic sample yielded an averaged value of 4.5 μm for the character of the polymer leading to an increased PAA layer. degree of molecular disorder. If this occurs, bulk Hereafter, unmodified films and surface modified mechanical properties of a polymeric film would be films are designated as PP and smPP films, respec- impaired since they are highly dependent on molec- tively. ular orientation. 780 Fasce et al. – eXPRESS Polymer Letters Vol.2, No.11 (2008) 779–790 2.2. Physical and chemical characterization in hydrophilicity of the film surface. Five measure- 2.2.1. Chemical structure ments were recorded and an average contact angle was calculated for each material [39]. Fourier transform infrared spectroscopy (FTIR) measurements of the surface modified films were obtained from a Nicolet 5 SXC FTIR spectropho- 2.2.4. Transparency tometer operating in the transmission mode in order Light transmittance measurements were carried out to characterize the chemical modification. Each by using a Leica DMLB microscope incorporating spectrum was collected by cumulating 32 scans at a a photo-detector in its optical path. The light trans- resolution of 8 cm–1. mittance signal (IT) measured for PP and smPP Gravimetric measurements of films before and samples were used to estimate the loss of film after grafting reactions were carried out to deter- transparency according to the following definition mine the grafting percentage by means of the Equa- (Equation (2)): tion (1) [37, 38]: − = IT (smPP) IT (PP) %Grafting= Loss of transparency ·100 (2) I (PP) ⎡ ⎤ T weight of smPP sample−weight of PP sample 100·⎢ ⎥ ⎣⎢ weight of PP sample ⎦⎥ (1) 2.2.5. Microstructure and orientation The weight of grafted layer was also estimated by The main factors that determine the mechanical volumetric titration of the COOH groups grafted properties are the molecular orientation and the onto the PP surface with

Load more