Material BehaviourEffect of silica nanofillers on isothermal crystallization of poly(vinyl alcohol): In-situ ATR-FTIR study
Introduction
Semicrystalline polymers have been extensively investigated from the viewpoint of fundamental research and practical applications. Generally, semicrystalline polymers consist of crystalline and amorphous domains, and their major characteristics including mechanical, thermal, electrical, optical, and chemical properties are largely dependent on both crystalline structure and chain orientation. Compared with amorphous polymers, semicrystalline polymers possess higher strength and stiffness, leading to desirable properties in the materials. Semicrystalline polymers include polyethylene [1], [2], [3], [4], polypropylene [5], [6], [7], [8], polystyrene [9], [10], polyamide [11], [12], poly(ethylene terephthalate) [13], [14], [15], [16], poly(trimethylene terephthalate) [17], [18], [19], [20], [21], and poly(vinyl alcohol) (PVA) [22], [23]. Of these, PVA has been widely utilized in a wide range of applications, for example, emulsifiers, coating agents, packaging films, membranes, and adhesives due to its particular advantages, such as high chemical and dimensional stability, transparency, gas barrier property, and electrical insulation. It is known that PVA includes small, dense, and closely packed ‘monoclinic’ crystallites [23].
In recent years, various kinds of semicrystalline polymer nanocomposites have been fabricated in order to improve the physical properties of pristine polymers [24]. Versatile inorganic fillers have been employed, including montmorillonite (MMT) [25], [26], [27], Fe oxide [28], titanium(IV) oxide (TiO2) [29], calcium carbonate (CaCO3) [30], [31], and silica [32], [33], [34], [35]. Notably, silica nanofillers are optically transparent, and their size and surface characteristics are relatively easy to control. In addition, they can be used to enhance the toughness and tensile property of polymers. There has also been considerable interest in the crystallization behavior and kinetics of semicrystalline polymers, because the degree and rate of crystallization considerably affect their main physical properties. Importantly, it has been recently found that inorganic nanofillers play an important role in the crystallization behavior of semicrystalline polymers. In most cases, the crystallization behavior of semicrystalline polymers has been investigated by using differential scanning calorimetry (DSC) [36], [37], polarizing optical microscopy (POM) [38], [39], and X-ray diffraction (XRD) [40], [41], [42]. However, unfortunately, most of these analytical methods [36], [37], [38], [39], [40], [41], [42], [43] are not readily adaptable to in-situ analysis and real-time monitoring, and it is also difficult to provide in-depth insights into the crystallization behavior on a molecular scale.
Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy is a versatile, non-destructive spectroscopic technique. This analytical method has advantages such as high sensitivity, fast analysis, and in-situ monitoring. Moreover, it is possible to analyze various types of samples such as films, fabrics, glass, rubber, and hard polymer sheets [44], [45], [46], [47], [48]. ATR-FTIR spectroscopy employs the phenomenon of total internal reflection, providing useful pieces of information on the molecular behavior of polymers [44]. In this work, we describe the effects of silica nanoparticles on crystallization behavior of PVA/silica nanocomposites by using in-situ ATR-FTIR spectroscopy. The size, content, and surface characteristic of silica nanoparticles were systematically considered as main factors affecting the crystallization behavior, and the influences of annealing time and temperature on the crystallization behavior were also investigated in detail [49], [50], [51], [52], [53], [54], [55].
Section snippets
Materials
PVA (98–99% hydrolyzed, MW: 31,000–50,000) was selected as a semicrystalline polymer. Silica nanoparticles with average diameters of 7, 12, 22, 50, and 100 nm were used as nanofillers. PVA and colloidal silica (7 nm: SM-30, 12 nm: HS-40, 22 nm: TM-40) were purchased from Aldrich Co. and used as received. The larger diameter silica nanoparticles (50, 100 nm) were prepared by the Stöber method [56], [57]. Chloro(dimethyl)vinylsilane (CDVS) was employed to modify the surface characteristic of silica
Results and discussion
The experimental setup for multiple ATR-FTIR measurements is schematically illustrated in Fig. 1. When infrared radiation enters the interface between an ATR cell component of high reflective index (ZnSe: 2.43) and a polymer film of low reflective index (PVA: 1.5), it is totally reflected at the cell/polymer interface [44], [45], [46]. The multiple ATR technique involves the accumulation of radiation reflected from the interface between the polymer and the ATR cell, in which the evanescent wave
Conclusions
The isothermal crystallization behavior of pristine PVA and PVA/silica nanocomposites was investigated using a spectroscopic technique. The silica nanoparticles act affectively as a nucleating agent at contents of less than 0.5 wt%. In contrast, higher weight fractions of silica nanoparticles disturbed the crystal growth during the crystallization process. When 0.5 wt% 22-nm silica nanoparticles were used as nucleating agent, the degree of crystallization was enhanced with increasing annealing
Acknowledgment
This work was supported by the Fundamental R&D Program for Core Technology of Materials of the Ministry of Commerce, Industry and Energy.
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