Analysis of the retrograde behavior in PMMA-CO2 systems by measuring the (effective) glass transition temperature using refractive index variations

https://doi.org/10.1016/j.supflu.2020.105159Get rights and content

Highlights

  • A new optical-based method allows determining the effective Tg of PMMA-CO2 systems.

  • Retrograde behavior was experimentally confirmed at low pressures and temperatures.

  • The physical properties of the PMMA strongly influence the effective Tg evolution.

  • The obtained results clarified previous discrepancies found in the literature.

  • PMMA industrial processing could be performed at mild conditions introducing CO2.

Abstract

The physical behavior of polymethyl methacrylate-carbon dioxide (PMMA-CO2) system was studied in a wide range of temperatures (0–100 °C) and pressures (0.1–10 MPa). The glass transition temperature of the systems was determined by a new optical observation methodology. The developed procedure is based on the refraction index variations between rubbery and glassy states, which allowed to differentiate both PMMA states for each pair of temperature and pressure conditions. Following this procedure, retrograde behavior was confirmed for three different grades of PMMA, finding significant differences in their behavior. In addition, the study of the glass transition temperature of the PMMA-CO2 systems included the low-pressure and low-temperature region, scarcely studied before. The new results obtained in the low pressure-temperature region are critical to design advanced foaming approaches and manufacturing processes at low temperatures.

Introduction

Supercritical fluids (SCFs) are widely used in several industrial processes, such as dye impregnation [1], [2], aerogel impregnation [3], drug loading [4], [5], or in cellular polymers production [6], [7], among others. These different techniques take advantage of both the solubility of several compounds in SCFs and the solubility of SCFs in polymers. In particular, gas dissolution foaming using SCFs is a commonly employed technique in the fabrication of cellular polymers from several polymer precursors due to its versatility in controlling the cellular structure and the possibility of using it in a wide variety of polymers, being also considered a green and pollutant-free process.

In the last decades, gas dissolution foaming has become one of the preferred processes for obtaining micro and nanocellular polymers [6], [8], [9]. The research in this field has focused on reducing the cell size to achieve nanocellular structures, often employing amorphous polymers (e.g., polymethyl methacrylate (PMMA)) [9], [10], [11], with the aim to obtain materials with enhanced physical properties. In the last years, nanocellular polymers have shown higher mechanical properties and lower thermal conductivities than conventional cellular or even microcellular polymers, as well as the possibility of creating transparent cellular materials [9], [12], [13]. However, obtaining a fine control over the cellular structure requires understanding several mechanisms that are involved in the gas dissolution foaming process, such as the gas solubility, the gas diffusivity, or how the solved gas affects the properties of the polymer (for instance its glass transition temperature) during the process [8].

Thermoplastic amorphous polymers are characterized by the glass transition temperature (Tg) [14], [15], [16]. It is known that Tg defines the transition between glassy and rubbery states, and it has been demonstrated that a significant change in the polymer physical properties occurs between both states [14], [17], [18], [19]. Based on thermodynamics, glass transition is determined by a second order transition at a fixed pressure, where the entropy of the mixture at equilibrium becomes zero, according to the Gibbs-Di Marzio criterion [20], [21]. Glassy state (T < Tg) presents much higher hardness and elastic modulus, because of all large molecular motions are restricted, just bonds rotations and chains vibrations may be exhibited. On the other hand, the polymer in the rubber state (T > Tg) shows an increase in molecular segment motions, both rotational and vibrational, promoting higher viscosity than the glassy one and being the polymer easily moldable [17]. In fact, this feature has a lot of interest in industrial manufacturing because it is the basic requirement for polymer processing. There are several approaches to reach the rubber-state of a polymer and take advantage of the rubbery properties during its processing. The simplest approach to reach the rubbery state is by heating the polymer over its Tg at atmospheric pressure. However, several polymers present Tg values higher than 100 °C or even there are thermoplastic polymers with Tg values higher than 200 °C, and the energy, setup, and safety requirements needed to process the polymers at these temperatures raise the manufacturing costs. An alternative to achieve lower temperatures is the incorporation of a plasticizer agent, which enhances the chain mobility, leading to a depression on the Tg. In this way, the glass transition takes place at lower temperatures, and thus the production costs can be reduced [22], [23].

Several investigations have predicted and experimentally proved the behavior of the glass transition of polymers in which gas molecules have been solved. The polymer-gas molecules interactions and the molecular size of the diluent are the main features responsible for the plasticization suffered by the polymer. The presence of the gas molecules into the polymer induces higher mobility of the molecular segments, as well as an increase of the free volume. Both facts lead to a Tg reduction, and the new temperature at which the glass transition takes place is called effective glass transition temperature (Tg, eff) of the polymer-gas system [24], [25], that is also mentioned in some works as depressed glass transition temperature [15], [26].

On a first approach, the Tg depression is directly related to the amount of gas solved into the polymer, being directly related to the gas pressure used to dissolve the gas [8], [27]. In addition, high polymer plasticization is usually achieved using SCFs due to their high solubility and diffusivity into polymers [25]. Accordingly, the first models studying the effective glass transition evolution of polymer-gas systems proposed a linear decrease with the gas solubility (i.e., with the saturation pressure). Chow et al. [28] developed a model based on experimental data about the Tg depression of polystyrene by introducing several gases. Also, Yoon et al. [23] proposed a more complex model that takes into account the molecular weight of the polymer and the gas weight gain to predict the Tg reduction. Although these models are accurately related to the experimental data for some amorphous polymers such as polystyrene (PS), polycarbonate (PC) or glycol-modified polyethylene terephthalate (PETG) [16], [23], [29], they fail to predict the non-linear behavior of other polymers such as PMMA, polyethyl methacrylate (PEMA), or acrylonitrile–butadiene–styrene copolymer (ABS) [14], [17]. The atypical behavior of these polymers is called retrograde behavior, and it is characterized by presenting a double Tg for some pressures, resulting in a curve with a maximum in pressure. Thus, the Tg, eff curve could be divided into three different regions in a pressure-temperature diagram. First, the polymer-gas system shows the common Tg, eff decrease when increasing the pressure. Second, the Tg, eff curve presents a maximum in pressure above which the polymer is in the rubbery state independently of the processing temperature. Finally, a second depression on the Tg, eff with the pressure reduction takes place, completing the typical curve of the retrograde behavior. In this way, two glass transition temperatures are observed at the same pressure (Fig. 1). Retrograde behavior is a special capability just seen in some polymer-gas systems, opposing to the usual behavior. When temperature decrease, the molecular segments motion is sharply reduced even though the presence of the diluent. However, the solubility enhancement in some systems when the temperature decreases, allows counterbalancing the reduction on molecular segments motion and achieve the rubbery state. In brief, the decrease in molecular segments motion, which appears by reducing the temperature, is overcome by increasing the gas concentration in the polymer.

The retrograde behavior was only observed for a few polymer-CO2 systems [14], [17], being PMMA one of them. This polymer, and its processing employing SCFs fluids, has a remarkable relevance on the development of ultramicrocellular and nanocellular foams. In fact, the PMMA-CO2 system has revealed as one of the most promising systems to produce and study the enhanced physical properties of such materials [22], [30], [31]. However, the mechanisms involved in the nanocellular foam's production cannot be modeled with high accuracy because of the intrinsic difficulties to understand this kind of systems presenting the retrograde behavior. This non-standard behavior of PMMA could be associated with its remarkable CO2 solubility, particularly at low temperatures. For instance, Martin-de Leon et al. reached a CO2 solubility in PMMA up to 48 wt% at 20 MPa and − 32 °C [13].

Condo et al. [32] developed the first model to predict the retrograde behavior, based on Gibbs-Di Marzio criterion and using randomness lattice fluid contacts approach. However, subsequent theoretical propositions, which involve non-randomness lattice fluid contacts based on Non-Random Hydrogen Bonding model (NRHB), provide better interpretation of thermodynamic sorption kinetics [33], [34], [35], [36]. Retrograde behavior has been confirmed by few experimental methods, such as creep compliance [17], specific heat [18], or solubility and diffusivity measurements [37]. However, most of the experimental data available only confirm this behavior over room temperature, with few data available at lower temperatures. On the one hand, Handa et al. [37] provided indirectly estimated values from foaming experiments showing Tg, eff values as low as 0 °C for CO2 at 3.4 MPa. On the other hand, Dutriez et al. [38] investigated the Tg of PMMA at high pressures (from 10 to 22.5 MPa), finding a constant Tg, eff about 10 °C, which seems to indicate that at high pressures would not be possible to achieve the rubbery state below this temperature threshold. However, this result was obtained for a PMMA thin film (below 2 µm) growth on a quartz crystal resonator (QCR), and it has not been confirmed in thick samples. Besides, the surface interactions between the film and QCR could have a significant impact on the Tg, eff behavior [39], [40], [41], being different the CO2 sorption mechanisms in thin films than in bulk samples [42]. A comparison of the previous results in the literature summarizing the retrograde behavior of PMMA/CO2 system is shown in Fig. 1.

Thus, further work and experimental data are required to provide a complete understanding of the retrograde behavior of PMMA-CO2 systems and determine their Tg, eff curves. In particular, the behavior at temperatures below room temperature and pressures over 10 MPa is still unclear. Herein, a new and direct method based on optical observations to identify the transition between glassy and rubbery states in PMMA is presented and validated in this work. This method takes advantage of the remarkable variation in the refraction index between both states [43], [44]. Moreover, using this new procedure, this work aims to expand the knowledge about the retrograde behavior of the PMMA-CO2 system by analyzing the low-temperature region, which is interesting in the cellular polymer field as well as in polymer manufacturing. The retrograde behavior has been analyzed for three PMMA-CO2 systems with different molecular weights, providing the first direct experimental results in the low-temperature and pressure region, as well as in the high-pressure region for thick samples with different intrinsic properties.

Section snippets

Materials

The polymer selected to carry out this study was PMMA. Three different grades of PMMA were studied in this work. ALTUGLAS V825T was kindly supplied by ARKEMA (Colombes, France), while PLEXIGLAS 6N and 8N were purchased from EVONIK (Essen, Germany). All the PMMA grades were received in the form of pellets. Their main properties were measured and then presented in the Results section (Table 1). A medical-grade CO2 (99.9% purity) was used as a plasticizer agent in the pressurization tests.

PMMA

Results

First, the physical properties (density, Mw, MFI, and Tg, 0) of the three grades of PMMA were determined (Table 1). All of them present a similar density of about 1.19 g/cm3, but significant differences in the other parameters were found. Mw, MFI, and Tg, 0 respectively range from 77,000 to 89,000 g/mol, from 1.65 to 8.20 g/10 min, and from 98° to 114°C. Thus, they are appropriate to study the potential relationship between the retrograde behavior and the PMMA's initial properties.

Conclusions

A new method to measure the transition from glassy to rubbery state in PMMA-CO2 systems based on optical observations has been presented and validated in this work. Taking advantage of the refraction index variation between both states, the glass transition temperature of the PMMA-CO2 systems at different pressures was studied. As a result, the retrograde behavior of the PMMA-CO2 system was confirmed using three different grades of PMMA. Although several differences between the three PMMA

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Financial assistance from Ministerio de Ciencia e Innovación (Spain) (PRE2019-088820) is gratefully acknowledged.

Financial assistance from Ministerio de Ciencia, Investigación y Universidades (MCIU) (Spain), Agencia Estatal de Investigacion (AEI), FEDER, (EU) (RTI2018–098749-B-I00), and (((RTI2018–097367-A-I00) is gratefully acknowledged.

Financial assistance from Ente Regional de la Energía de Castilla y León (EREN, Spain) (EREN_2019_L4_UVA) is gratefully acknowledged.

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