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عنوان فارسی مقاله:
سینتیک وزن مولکولی و مدل بریدگی زنجیره برای پلیمر دکستران در طول تنزل اولتراسونیک
عنوان انگلیسی مقاله:
Molecular weight kinetics and chain scission models for dextran polymers during ultrasonic degradation
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Dextran is a homo-polysaccharide of d-glucose, consisting of more than 50% of -(1→6) linkages in the main chain and other linkages in the branch (Petronijevic, Ristic, Pesic, & Smelcerovic, 2007; Purama, Goswami, Khan, & Goyal, 2009). Dextran is a biopolymer produced via microbial fermentation, particularly from Leuconostoc mestenteroides strains (Naessens, Cerdobbel, Soetaert, & Vandamme, 2005). The exact structure of dextran depends on specific microbial strains (Robyt, Yoon, & Mukerjea, 2008). As a polymer, degrees of polymerization of d-glucose determine the molecular weight of dextran. Dextran polymers with different molecular weights have different applications. On the one hand, macromolecular dextran (Mw > 2 × 107 Da) can be used as gel filtration media in the fine chemical industry (Majumder, Purama, & Goyal, 2007). Addition of a small amount of high molecular weight dextran improves quality of bakery products such as yeast-raised doughs (Bohn, 1961). On the other, dextran with a certain low molecular weight (Mw < 8 × 104 Da) and a narrow molecular weight distribution is often used for preparation of clinical grade dextran which can be used as blood-plasma substitutes in the pharmaceutical industry (Lakshmi Bhavani & Nisha, 2010; Zdolsek et al., 2011). And this clinical product is in a growing demand in recent years. Currently, low molecular weight dextrans were obtained from acid hydrolysis (Guimaraes, Costa, Rodrigues, & Maugeri, 1999) or enzymatic degradation (Kim & Day, 1994) of macromolecular dextrans. However, quality of the degradation products is far from satisfactory, as it has a relatively wide molecular weight distribution which might induce clinical side effects in application. Therefore, a rapid and efficient approach for producing dextrans with a molecular weight range that meets clinical requirements is important. Ultrasonic treatment, one of the most promising methods of polymers degradation, has received considerable attention for being able to irreversibly lower polymer chain length without causing any chemical change (Suslick & Price, 1999; Taghizadeh & Bahadori, 2009). The use of ultrasonic degradation for producing low molecular weight dextrans has been investigated in the last decades. However, previous studies focused on investigating the effects of several parameters on the reduction of molecular weight. Various factors, including ultrasound parameters (frequency and power), solvent properties (concentration and temperature), and operating parameters (i.e., ultrasonic time, the depth of horn) have been studied (Koda, Taguchi, & Futamura, 2011; Zou et al., 2012), suggesting that the decrease in molecular weight was remarkable at higher power, lower temperature and lower concentration. The interests of studying polymers degradation kinetics and chain scission mechanism is growing rapidly. Theoretical andempirical models relating to polymer degradation have been proposed by others (Barambo˘ım, Moseley, & Watson, 1964; Malhotra, 1986; Ovenall, Hastings, & Allen, 1958), which described the changes of molecular weight as a function of ultrasonic time. Though the exact mechanism of polymer scission during ultrasonic process is doubtable, it is generally believed that the cavitation effect caused by ultrasound waves is mainly responsible for the chain rupture (Kardos & Luche, 2001). It is assumed that the degradation is a mechanical effect caused by the rapid growth and collapse of microbubbles when the polymer solution was exposed to ultrasound (Basedow & Ebert, 1977; Price, 1990; Price, Daw, Newcombe, & Smith, 1990). Near the collapsing microbubbles, polymer chains are captured in a high-gradient shear field, causing the polymer segments to move at a higher velocity than those farther away from the collapsing cavity in this shear field. It is the stress generated from relative motions between the polymer segments and the solvent that cause chain rupture (Madras & McCoy, 2001). Regarding the location of chain scission,two simplified models have been proposed and summarized in detail by researchers (Aarthi, Shaama, & Madras, 2007; Bose & Git, 2004; Wu, Zivanovic, Hayes, & Weiss, 2008). One is the midpoint scission model, which assumes that the chain rupture occurs at the center of the chain backbone space (Price & Smith, 1991); the other is the random scission model, which assumes that polymers can be degraded randomly and any chain connection has an equal chance of rupture (Jellinek, 1955). The model of midpoint chain scission (P(x)→2P(x/2)) can be expressed as shown in Eq. (1),
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کلمات کلیدی:
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