Thermal restraint on PEG-EG mixtures by FTIR investigations and wavelet cross-correlation analysis

Abstract

The aim of the present work is to investigate the thermal response of PolyEthylene Glycol 1000 (PEG1000) and of its mixtures with the monomer Ethylene Glycol (EG). On purpose Attenuated Total Reflectance Infra-Red (ATR-IR) spectra were collected, in the spectral range spanning from 400 cm⁻¹ to 4000 cm⁻¹, on PEG1000 and on its mixtures with EG, as a function of concentration and temperature, through positive thermal scans, i.e. by increasing temperature. It will be shown that ATR-IR technique reveals a powerful tool for the characterization of the thermal response in polymeric systems. The registered spectra have been analyzed both on the whole investigated spectral range, as well as, separately, on the restricted intramolecular OH stretching vibrational contribution region. In the first case the displacement of the spectral features from the spectrum at the lowest temperature, taken as reference spectrum, shows a lower dependence for the mixture. As far as the intramolecular OH vibrational contribution is concerned, besides a conventional analysis in terms of band components, three different data analysis procedures have been applied, i.e. the characterization of the temperature dependence of the intramolecular OH stretching center frequency, of the spectral distance and of the wavelet cross correlation coefficient. The three applied data analysis approaches indicates that the addition of a small amount of pure EG to PEG1000 significantly influences the OH vibrational properties of the PEG1000 polymeric matrix. The three different methods furnish a unique coherent interpretative picture which supports the validity of the applied approaches. Furthermore, the analyses show the presence of a higher thermal restraint for the PEG + EG mixture which confirms that, within the three-dimensional networks of hydrogen bonded EG-PEG1000 mixtures, a key role is played by EG in determining an increase of the hydrogen bond network density.

Introduction

Nowadays, polymers and polymer blend are very widely used with different applications in various industries such as food, packaging, cosmetics and medical devices. It's well known that polymers and mixtures of polymers with different Molecular Weights (M_w) show different physical properties due to the creation of a new system obtained blending together two or more polymers [1], [2], [3], [4], [5]. In particular, in the case of mixtures of hydrogen-bonded species, the component system interaction allows the formation of intermolecular complexes governed by hydrogen bonds (HBs) [6], [7], [8]. Ethylene Glycol (EG) is the most important glycol commercially and industrially available. It is a colorless, odorless, viscous dihydroxyl alcohol, can be combined with many polar and non-polar solvents, such as water, alcohols, acetone, and, although marginally, benzene, toluene and chloroform. Since EG is difficult to crystallize it is used as an antifreeze and coolant, by transforming aqueous solutions of EG in a glassy state, so preventing ice crystallization below the freezing temperature of water [2], [9], [10], [11], [12], [13], [14]. The mixture with water reduces the toxicity of EG, a property of fundamental importance in cryopreservation of living cells and tissues [1], [6], [15], [16], [17], [18], [19], [20]. The configurations of EG molecule OH-CH₂-CH₂-OH has been widely studied [21], [22], [23], [24], [25], [26], [27], [28]. With reference to some H-bond properties, EG is considered as a water analogue since it is able to generate three-dimensional HB networks; this is due to the fact that EG molecule has two proton donor OH groups and two O atoms that act as proton acceptors; as a consequence both liquid H₂O and EG have four HBs per molecule [29], [30], [31], [32]. The EG hydrogen bonded network mainly consists of linear chains of H-bonded molecules. Poly(Ethylene Glycols), or PEGs, are hydrophilic synthetic polymers, synthesized from EG, whose chemical structure, H–(O–CH₂–CH₂)_m–OH, m being the polymerization degree, includes both hydrophobic ethylene units (CH₂–CH₂) and hydrophilic Oxygens [33], [34], [35], [36], [37]. PEGs represent good model systems in the study of the properties of more complex biomolecules, and of the relaxational properties of complex hydrogen bonded systems [8], [18], [38], [39], [40]. For these reasons PEGs and their mixtures with water or with low molecular weight molecules, have been investigated by means of different experimental techniques [12], [41], [42], [43].

PEGs have various derivatives with different functions; in particular, since many PEGs are hydrophilic, they are extensively used as penetration enhancers, especially in topical dermatological preparations. In addition PEGs, together with their typically nonionic derivatives, are utilized in cosmetic products as surfactants, emulsifiers, cleansing agents, humectants, and skin conditioners. Furthermore they play a key role from packaging to Drug Delivery Systems (DDS) in pharmaceuticals. United States Food and Drug Administration (US FDA) approved PEG in oral, topical and intravenous formulations [44], [45], [46].

Fourier Transform Infrared (FT-IR) spectroscopy is a powerful technique based on measurement of vibrational and rotational motions of condensed matter systems excited by IR radiation at a specific wavelength range. IR spectroscopy covers the range of 14000-10 cm⁻¹of the electromagnetic spectrum. Such a region can be partitioned into three different ranges: Near-IR (NIR) approximately 14000–4000 cm⁻¹, Mid-IR (MIR) 4000–400 cm⁻¹, and Far-IR 400–10 cm⁻¹. In synthesis, when IR radiation passes through a sample, spectral peaks are derived from the absorption of bond vibrational energy changes in the IR region [47], and there is a correlation between IR band positions and chemical structures in the molecule. Spectra are measured by calculating the intensity of the IR radiation before and after it passes through a sample. IR is complementary to Raman scattering, inelastic neutron scattering and density function simulations, these techniques furnishing valuable information on the degree of coupling between the various motions as well as on the systems structural properties [48], [49], [50], [51], [52], [53], [54], [55].

In this study, the polymeric mixture is composed by mixtures of the monomer EG, and PEG1000, i.e. the polymer with a nominal molecular weight M_w = 1000 and a polymerization degree of m = 22.

Such mixtures are of great applicative relevance because the addition to PEG of EG or of other low molecular weight molecules as for example trehalose, increases the rigidity of the polymeric matrix and, in turn, the bioprotective effectiveness [56], [57], [58], [59], [60], [61].

In this framework, in order to characterize the thermal properties of these polymeric systems, we apply three different approaches, and specifically an analysis of the temperature dependence of the OH stretching center frequency, of the Spectral Distance (SD) and of the wavelet cross correlation (XWT) [62], [63], [64], [65].

More precisely, the shift of the OH stretching center frequency, as a function of temperature, allows to follow the changes of spectral band position in the absorbance of the polymeric systems, as far as the SD is concerned, the thermal behavior take into account all the structural changes of the investigated system, finally the XWT allows to evaluate the cross correlation coefficient among couple of spectra.

The three approaches in a coherent way show that the addition of a relatively low amount of a EG to pure PEG1000 leads to a stiffening of the three-dimensional network of hydrogen bonds. On this concern, previously, has been showed that the joint employment of Raman, PCS and ultrasonic velocity measurements it is possible to provide detailed information on the conformational properties of polymeric systems, highlighting that PEO in water solution tends to assume a conformation, closer to the crystalline one [66], [67], [68], [69], [70], [71], [72], [73].