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Grafting of Poly(N-Isopropylacrylamide) Onto Nylon and Polystyrene Surfaces by Atmospheric Plasma Treatment Followed with Free Radical Graft Copolymerization

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Grafting of Poly(N-Isopropylacrylamide) Onto Nylon and Polystyrene Surfaces by Atmospheric Plasma Treatment Followed with Free Radical Graft Copolymerization Grafting of Poly(N-isopropylacrylamide) Onto Nylon and Polystyrene Surfaces by Atmospheric Plasma Treatment Followed with Free Radical Graft Copolymerization Xiaoling Wang,1,2 Marian G. McCord1,2 1Joint Department of Biomedical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7115 2Textile Engineering, Chemistry, and Science Department, North Carolina State University, Raleigh, North Carolina 27695-8301 Received 24 September 2005; accepted 16 December 2006 DOI 10.1002/app.26081 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Stimuli-responsive polymer materials (SRPs) Fourier transform infrared spectroscopy and atomic force mi- have potential uses in drug delivery, tissue engineering, bio- croscopy. Dramatic water contact angle increase was found reactors, and cell-surface adhesion control. Temperature-re- for PNIPAM-grafted polymers at about 328C, indicating the sponsive surfaces were fabricated by grafting poly(N-isopro- temperature sensitivity of the grafted surface, i.e., the change pylacrylamide) (PNIPAM) onto nylon and polystyrene surfa- of surface from hydrophilic to hydrophobic when tempera- ces via a new procedure, i.e., He atmospheric plasma ture increases above the lower critical solution temperature treatment followed by free radical graft copolymerization. (LCST). The addition of Mohr’s salt enhances the grafting The atmospheric plasma exhibits the activation capability to reaction and the magnitude of temperature sensitivity. initiate graft copolymerization. The procedure is suitable for Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3614–3621, 2007 integration into a continuous manufacturing process. To reduce homopolymerization and enhance graft yield, Mohr’s Key words: polystyrene; hydrogels; biomaterials; graft salt was added. The graft of PNIPAM was confirmed by copolymers; plasma polymerization INTRODUCTION cases, their water-solubility, in response to an external stimulus. The environmental stimulus can be pH, Polymer materials have been widely used in the tis- temperature, ions, solvents, electrical field, magnetic sue engineering and biomaterials fields because of field, light, pressure, or chemical/biochemical com- their good resilience, low density, and low cost. How- pounds.2 Poly(N-isopropylacrylamide) (PNIPAM) is ever, due to problems including nonselective protein one kind of thermoresponsive polymer among SRPs. It adsorption and nonselective cell adhesion, polymer shows remarkable changes in aqueous swelling with a biomaterials are limited in their applications. Hence, change in temperature. The thermoresponsive poly- surface modification of polymers to improve their mer shows fully hydrated and extended conformation biocompatibility has been a significant issue in this below 328C. Above 328C, however, it extensively dehy- field.1 It is desirable for biomaterials to direct or par- drates and exhibits a compact chain conformation.3 ticipate in specific biomaterial/biological tissue inter- Grafting thermoresponsive polymers, such as PNI- facial responses. Biomaterials with responsive surfa- PAM, onto a surface endows the surface with con- ces can meet this requirement, since they may exhibit siderable thermoresponsive properties—hydrophilic remarkable property changes in response to stimuli, below the LCST and hydrophobic above it.4 Novel such as temperature, pH, liquid composition, photo, applications that have been indicated for these surfa- electric stimulation, etc. This response can be used to ces include smart and thermally responsive cell culture regulate the activity of biological tissues, for example, substrates that control the attachment and detachment the automatic cell detachment from the substrate. of cells,13 temperature responsive membranes,27 con- A responsive biomaterial surface can be made by trolled release of drugs and growth factor,28 and tem- grafting stimuli-responsive polymers (SRPs) onto an- perature responsive chromatography.25,26 Thus, PNI- other polymer surface. SRPs can react, adjust, or mod- PAM-grafting of surfaces is an enabling technology ulate their physicochemical characters, i.e., in most that can facilitate experiments and applications that were previously difficult or impossible.5 Because of the wide range of biomedical applications Correspondence to: M. G. McCord ([email protected]). of PNIPAM-grafted surfaces, grafting of PNIPAM on Journal of Applied Polymer Science, Vol. 104, 3614–3621 (2007) polymer biomaterial surfaces has been the subject of VC 2007 Wiley Periodicals, Inc. numerous recent experimental studies. Methods GRAFTING OF PNIPAM ONTO NYLON AND POLYSTYRENE SURFACES 3615 reported for surface grafting of PNIPAM include ul- pheric Plasma Laboratory in the College of Textiles at traviolet (UV) irradiation,6–8 vacuum plasma treat- North Carolina State University. It is an atmospheric ment,5,8–10 ozone treatment,11,12 electron-beam3,13,14 pressure glow discharge (APGD) device. Figure 1 and g-irradiation,15 and chemical treatment.16 Irradi- shows a schematic drawing of the experimental facil- ation results in the formation of active species on the ity. It is a capacitively-coupled chamber and contains substrate surface, which are capable of initiating the two horizontal parallel copper electrodes. The radio copolymerization of NIPAM monomer. In one study, frequency power is coupled to the plasma via electro- PNIPAM was grafted onto a polypropylene mem- des through an oscillating electric field. Each elec- brane surface using a vacuum plasma technique to trode is covered with a dielectric material to limit the activate the surface, resulting in thermoresponsive current in the discharge and force the charge to membrane permeability.10 Polyamide has also been spread out over a large area instead of constricting to activated by ozone treatment and then grafted with an arc. The dielectric barrier prevented the transition PNIPAM.12 of the discharge to an arc when the acoustic fre- Among the many irradiation methods, vacuum quency voltage signal was applied. plasma treatment has been widely used because of its The device has two chambers: an inner plasma commercial advantages. It offers flexibility, effective- chamber is for batch treatment and an outer chamber ness, safety, and environmental friendliness. The vac- equipped with a fabric rolling system for continuous uum plasma is effective at near-ambient temperature fabric modification treatments. Therefore, the device without damage to most heat-sensitive biomaterials. is capable of batch treatment of fabric/film pieces Vacuum plasma treatment modifies only the near sur- using a test cell, as well as continuous operation using face of treated substrates and does not change the the roller feed system for large fabric rolls or continu- bulk material properties. It can be used to modify any ous filaments and yarns. The device produces uni- kind of substrate geometry. In spite of its advantages, form plasma at atmospheric pressure. costs associated with maintaining a vacuum and the In this investigation, PS and nylon were treated for constraint of batch processing have limited the accep- 1 min in the batch chamber with atmospheric plasma tance of this technique.17 generated from 100% He. The distance between sam- In contrast, atmospheric plasmas maintain the posi- ple and electrode was 1 in. The power level used was tive aspects of vacuum plasma treatment, without the 4.8 kW. The frequency was 5 kHz. The flow rate of limitations. Compared with conventional vacuum He was 10.18 L/min. plasma treatment, atmospheric plasma treatment costs less and may be integrated into a continuous 18 Graft polymerization of NIPAM manufacturing process. In this investigation, atmos- onto Nylon and PS pheric plasma treatment with subsequent graft copolymerization was used to graft PNIPAM onto ny- After He plasma treatment, the samples were imme- lon and polystyrene (PS) surfaces. The PNIPAM- diately immersed into a NIPAM aqueous solution grafted surfaces were investigated by FTIR, contact (5% wt) in a reaction kettle. The monomer solution angle, and AFM. was degassed with N2 for 30 min to remove the exist- ing O2. The kettle was then sealed under N2 and placed in a 608C shaking water bath to begin the graft EXPERIMENTAL copolymerization. After 24 h, the PNIPAM-grafted Materials films were washed by agitating in ultrapure water at Nylon 6,6 (McMaster-Carr) film and 60-mm nontissue culture-treated polystyrene (PS) plates (Corning, NY) were used as the substrates for graft polymerization. Nylon and PS were cut into 3 Â 3cm2 samples, washed with acetone, dried in an oven at 608C, and weighed. N-isopropylacrylamide (NIPAM) was gen- erously provided by Kohjin, Tokyo, Japan. It was recrystallized using hexane prior to grafting. Mohr’s salt [Fe(NH4)2(SO4)2Á6H2O, EM Science] and hexane (95%, Acros Organic) were used without further puri- fication after purchase. Atmospheric plasma treatment The atmospheric pressure plasma treatment system Figure 1 A Schematic of the atmospheric pressure plasma used in these experiments is located in the Atmos- system. Journal of Applied Polymer Science DOI 10.1002/app 3616 WANG AND McCORD room temperature for 24 h to remove unreacted goes a reversible phase transition in response to tem- monomer and ungrafted homopolymer. The samples perature; hence, graft copolymerization of PNIPAM were then dried at 608C for 3 h and weighed. Mohr’s makes nylon and PS surfaces smart and temperature salt was added into the monomer solution before sensitive. The PNIPAM-grafted
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