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中国科技论文在线 Influence of Hydrophobicity of Substrates on The 中国科技论文在线 http://www.paper.edu.cn Influence of Hydrophobicity of Substrates on the Adsorption of Nonionic Block Copolymers# 5 SONG Junlong* (Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, 210037) Abstract: In this investigation the adsorption of nonionic polymers on model surfaces (cellulose, polypropylene, nylon and polyester) were studied using Quartz Crystal Microbalance (QCM) technique. 10 The underlying driving force for the adsorption of nonionic lubricants onto these surfaces was discussed and our experimental observations also confirmed expected behaviors: A greater affinity of the more hydrophobic polymer species with the hydrophobic surfaces and vice versa. Hydrophobic interactions are concluded as being a predominant factor in adsorption of nonionic polymer [polyalkylene glycols (PAGs) and co-polymer of ethylene oxide and propylene oxide (Pluronic)] on 15 textile-relevant surfaces. Key words: Adsorption, Nonionic Polymers, Cellulose, Polypropylene, Nylon, PET, Hydrophobic forces 0 Introduction 20 Polymer adsorption at the solid/liquid interface plays a crucial role in different technologies involving paintings, coatings, lubricant formulations, ceramics additives, and adhesives.[1] Nonionic block copolymers are commonly used in textile finishes as a lubricant component of the formulation to facilitate the processing of synthetic and natural fibers in various operations and the research on nonionic block copolymers has gained rapid progress in recently years.[2-9] Nonionic 25 block copolymers trend to form micelles in dilute aqueous solution and adsorb extensively to a large variety of interfaces due to amphoteric nature of different blocks. The adsorption of nonionic block copolymers on highly hydrophobic surfaces[4, 9, 10] and highly hydrophilic surfaces3 and the influence factors such as architectures of polymer,[6] temperature[5, 11] and salt[12] on adsorption were investigated extensively. 30 In a previous work done in our group the influence of nature of block nonionic copolymer, such as molecular size, length of hydrophobic portion in the chain and hydrophobic number on the adsorption behaviors of nonionic block polymers were discussed. We found that the adsorption isotherms of the nonionic polymers onto PET surfaces are Langmuirian type and the chain length of the hydrophobic group of the adsorbing polymers, i.e. hydrophobic number strongly correlates 35 with their affinity with the surface. In this paper, we followed up to study the influence of nature of substrates, especially the hydrophobicity of substrates on adsorption of nonionic block copolymers. This is a very import topic for textile processing since textile fibers include various materials: synthetic polymers (polyester fiber, nylon/aramid fiber, olefins, acrylic/modacrylic fibers), as well as generated cellulose, and natural fibers (cotton, wool, and silk fibers), and some 40 specialty fibers. These fibers are of different properties and specifically, different hydrophobicity. Therefore in this investigation, cellulose, polypropylene (PP), nylon and polyester polyethylene terephthalate (PET) were used as representatives of most relevant textile fibers. Foundations: Specialized Research Fund for the Doctoral Program of Higher Education of China (20103204120005); National Natural Science Foundation of China (31270613); Scientific Research Foundation for the Returned Overseas Chinese Scholars (lot 43); the Priority Academic Program Development of Jiangsu Higher Education Institutions (NJFU). Brief author introduction:Junlong Song (1974-),Male, Associate Professor,Research interest includes cellulose chemistry and materials, colloid and surface chemistry. E-mail: [email protected] - 1 - 中国科技论文在线 http://www.paper.edu.cn Quartz crystal microbalance (QCM), one of noninvasive methods which have been used 45 extensively to monitor the adsorption of polymers and surfactants on different surfaces, was used here to inquire on the adsorption on fiber-relevant surfaces. In most studies, typical surfaces used for QCM involve metal, (e.g., gold, stainless steel, etc.) and silica substrates. In order to employ the QCM technique to investigate the effects of hydrophobicity of surfaces on the adsorption behaviors of nonionic surfactants, modification has to be made to each QCM sensors with 50 respective ultrathin organic polymer. Ultrathin films of cellulose, PP, nylon and PET developed by our group in a previous study[13] were used in the present work. 1 Experimental All experiments were performed with deionized water from an ion-exchange system (Pureflow, Inc.) which was further processed in a Milli-Q® Gradient unit to ensure ultrapure 55 water with resistivity greater than18MΩ. 1.1 Materials The nonionic lubricants used in this investigation included two sets of polymers with known structures: polyalkylene glycols (PAGs) from Dow Chemical Co. (Midland, MI), and triblock copolymers based on ethylene oxide and propylene oxide (Pluronics) from BASF Corporation 60 (Florham Park, NJ). Generic chemical structures of PAGs and Pluronics are shown in Fig. 1. PAG and Pluronic lubricants with different structural parameters m and n (see Fig. 1) were used in this study, as listed in Tab. 1. It is worth noting that the supplied samples not only have different monomer composition but also different molecular weights. Nevertheless, these polymers are comparatively pure and better defined than commercial nonionic mixtures 65 commonly used in industry. CH3 CH3 RO-[CH CHO] [CH CH O] -H H-[CHCH O] -[CH CHO] [CH CH O] -H 2 n 2 2 m 2 2 m 2 n 2 2 m Fig. 1. Chemical structure of polyalkylene glycols (PAGs) (left) and co-polymer of polyethylene oxide and polypropylene oxide (Pluronics) (right). 70 Tab.1 Structural information of polyalkylene glycols (PAGs) and Pluronics Symbol* Commercial name** MW n (PO) m (EO) HLB*** RP10E13 UCON 50-HB-400 1,230 10 13 11.92 RP13E17 UCON 50-HB-660 1,590 13 17 12.0 RP33E44 UCON 50-HB-5100 3,930 33 44 8.51 E19P29E19 Pluronic P65 3,400 29 19 21.67 E76P29E76 Pluronic F68 8,400 29 76 27.97 E37P56E37 Pluronic P105 6,500 56 37 17.34 *R, P and E stand for butyl, r propylene oxide and ethylene oxide groups, respectively. * *UCON polymers are from Dow Co. and Pluronic polymers are from BASF Co. ** *HLB values were calculated by the method described by Guo et al. [14] 75 Micro-crystalline cellulose was obtained from Avicel® PH-101 (Fluka Chemical Corporation). Medium density polyethylene (PE, received as a powder), polypropylene (PP, syndiotactic, with Mn and Mw of 54,000 and 127,000, respectively) and nylon 6 (with a Tg of 62.5 °C and particle size of 3 mm) were purchased from Sigma-Aldrich. Poly (ethylene terephthalate) (PET) was provided by Goulston Inc. (Monroe, NC). Polyvinylamine (PVAm) was - 2 - 中国科技论文在线 http://www.paper.edu.cn 80 donated by BASF Corporation. Xylene (HPLC grade), hexafluoroisopropanol (HFIP, 99.5+ %), 50% N-methylmorpholine-N-Oxide (NMMO) and Dimethyl Sulfoxide (DMSO) were purchased from Fisher Scientific. 1.2 Methods 1.2.1 Preparation of model films. 85 The substrates used in this investigation were circular Q-Sense gold sensors (Q-sensor Co, diameter is 12 mm). All substrates were cleaned with Piranha solution (H2SO4(98%): H2O2(30%) = 70: 30 v/v) for one hour and then subjected to Ultra-violet Ozone (UVO) radiation for 10 minutes immediately before spin-coating with textile material solutions. Fiber precursor materials were dissolved in diluted solution and then employed in the spin 90 coater to cast a uniform ultrathin film onto the QCM sensors. Details on the development of cellulose and synthetic films can be found in another paper. [13] 1.2.2 QCM-D technique A QCM-E4 (Q-sense Inc.) was used in this investigation to measure the adsorption of the nonionic polymers on ultrathin films. The principle of the QCM technique involves the monitoring 95 of the resonant frequency (f) of a gold-coated piezoelectric material (quartz crystal) which depends on the total oscillating mass.[15-17] If the film is thin and rigid, the decrease in frequency is proportional to the mass of the film, as stated by Sauerbrey equation: [18] cf m (Equation 1) n 100 where C = 17.7 ng Hz-1 cm-2 for a 5 MHz quartz crystal. n = 1,3,5,7 is the overtone number. Since frequency can be detected very accurately, the QCM operates as a very sensitive balance which can reach to ng/cm2 resolution. Since the density of the given polymer film are easy 105 to find in respective databases, the thickness can be obtained. Besides frequency, QCM-D can also monitor the energy dissipation for the adsorbed films at the same time. Energy dissipation relates to the conformation of adsorbed films. Therefore, one of the advantages of QCM-D is that it can obtain both adsorption amount and conformation for the adsorbed film in one measurement. The QCM performs at constant temperature of 25 ºC and run at constant flow rate at 0.1 ml/min. 110 1.2.3 Contact angle measurements Tab.2 Contact angles of model films Model film WCA (deg) PP 102.9±1.7 Cellulose 28.6±3.4 Nylon 61.9±1.0 PET 64.8±1.1 Contact angle is very sensitive to the surface chemistry change. The contact angle of untreated and polymer-coated surfaces against pure water was measured on a manual Rame-Hart 115 goniometer in this investigation. After 1 min of placing a droplet of Mini-Q water (10 µL) onto the surface the outline of the droplet was detected with an optical magnifier. The tangent line on the - 3 - 中国科技论文在线 http://www.paper.edu.cn droplet in the three-phase zone was traced with a protractor within the optics and therefore provided a measurement of the contact angle. The contact angles of untreated model surfaces were listed in Tab. 2. 120 2 Results and discussion 2.1 Adsorption experiments In this investigation, sample nonionic polymers with five different concentrations (from 0.0001% to 1%) in aqueous media were used in the adsorption experiments with cellulose, polypropylene (PP), nylon, and polyester polyethylene terephthalate (PET).
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