Thermo-analytical study on transitions in styrene–maleic anhydride copolymers with low- and high-molecular weights
Graphical abstract
Introduction
The thermal stability of polymers closely relates to molecular structures, and a detailed analysis of thermal transitions provides additional insight in the molecular arrangement of complex copolymers. Amorphous polymers may have considerable thermal stability, but multiple transitions occur upon heating due to the thermodynamic non-equilibrium of the glassy state. The enthalpy changes are respectively related to progressive structural relaxations of the main polymer chain or local polymer chain segments. From a practical point of view, the use of copolymers as encapsulating agents requires precise control over thermal release mechanisms that primarily depend on the intrinsic properties of the polymer wall [1], [2], [3]. In order to get better insight in the thermal transitions thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are traditionally used, while temperature modulated differential scanning calorimetry (TMDSC) provides more detailed information on local transitions.
Amorphous copolymers of styrene–maleic anhydride (SMA) received considerable industrial interest due to their versatile molecular structure and functionality of the maleic anhydride (MA) groups. The copolymerization of styrene with maleic anhydride provides good physiochemical properties with high polarity, rigidity, glass transition temperature, and functionality. The SMA has been used as adhesive, surfactant, compatibilizer, interface modifier, or surface-sizing agent in textile or paper industry to enhance printability. Alternating SMA copolymers spontaneously form by free radical copolymerization of maleic anhydride and styrene [4], [5]. On the other hand, non-equimolar styrene–maleic anhydride copolymers with a non-alternating molecular structure can be synthesized by reducing the concentration of MA and adding it under strictly controlled conditions [6]. The copolymers can be further modified by chemical ring-opening reaction of the reactive anhydride groups in presence of a nucleophile [7]. The hydrolyzed SMA copolymers became of interest as they show polyelectrolyte behaviour with various molecular conformations depending on pH [8]. After dissociation of the dicarboxylate groups, the copolymer has amphiphilic properties [9], [10] with the ability to form intramolecular or intermolecular associates. Such interesting properties of SMA copolymers are attributed to the presence of both non-polar and polar groups that interact through hydrophobic interactions, hydrogen bonding and ionic interactions [11]. As a result, some SMA copolymer structures can be applied to form micro- and nanostructured materials through self-assembly [12], [13]. Nano-sized latex particles or micelles could be created by emulsion polymerization, using an amphiphilic SMA block copolymer as a surfactant [14], [15].
The thermal degradation of specific MA and SMA copolymers has been studied by pyrolosis-gas chromatography [16]. The predominant degradation mechanism involves reactions in the main polymer chain at 200–600 °C [17], yielding styrene, benzene, toluene and ethylbenzene as main products in variable proportions depending on the temperature and copolymer type. The analysis of different SMA copolymers with 50 and 75 mol% styrene yields a linear relationship between styrene contents in the copolymer and pyrolytic products [18]. The relatively low amount of styrene in pyrolysis products of equimolar SMA was assigned to good stability of the alternating structure [19]. Comparative data on the thermogravimetric analysis of some SMA copolymers indicated a decrease in the apparent thermal stability for alternating, random and block copolymers [20]. However, the SMA copolymers show also important enthalpy relaxations below the glass transition temperature, which is not a simple function of the molecular structure [21]: the relaxation rate seems to increase up to 24 mol% MA and then further decreases as the MA content increased to 52 mol%. This was generally ascribed to a variation in free volume [22], due to conformational changes from random into more alternating molecular distributions of the comonomers by physical ageing.
The thermal degradation of alternating SMA copolymers after hydrolysis was evaluated in more detail, showing a three-step degradation within a temperature range of 23–500 °C [23]. However, the decomposition does not involve the simple regeneration of the maleic anhydride units. The heating of hydrolyzed SMA up to 250 °C causes an increase of Tg compared with the parent copolymer. In parallel, its solubility in polar solvents lowers presumably due to crosslinking. This behaviour depends on the stereoregularity of the carboxylic groups [24]. By esterification of the maleic anhydride groups with long-chain aliphatic alcohols, two relaxation processes were observed [25]: the first α-relaxation is associated with the glass-rubber transition and characterized by the Vogel–Fulcher–Tammann temperature dependence, and the latter β-relaxation is related to the local motion of the ester side groups attached to the polymer backbone. The apparent activation energy for the β-relaxation highly depends on the alkyl chain length. The thermal properties were modified for SMA copolymers with maleic anhydride olefins [26], aliphatic alcohols [27], amino-diethylaniline [28], cross-linked ionomeric materials from poly(styrene-alt-maleic anhydride) and poly(ethylene glycol) [29]. After blending SMA with polygluterimide, the positive deviations in Tg from the ideal (linear) behaviour indicate the presence of significant specific interactions between both polymers [30].
From this overview, it is clear that the molecular conformation and amount of maleic anhydride influences the thermal transitions of SMA copolymers. The variety in industrially available SMA grades offers a broad selection template of materials for specific applications. In our previous work [31], we compared the molecular structure of low- and high-molecular weight SMA copolymers and described intrinsic differences from vibrational spectroscopy: the high-molecular weight SMA has a more heterogeneous molecular structure with styrene- and maleic anhydride rich polymer segments statistically distributed over the copolymer chain. This specific molecular structure allows for conversion of specific SMA grades into spherical nanoparticles during imidization reactions [32]. As the latter nanoparticle formation failed with low-molecular SMA, we further focused on the intrinsic molecular characteristics of various SMA copolymer grades. In this work, we provide further evidence for the molecular structures of low- and high-molecular weight SMA by thermo-analytical evaluation.
Section snippets
Materials
In this study, nine grades of styrene–maleic anhydride (SMA) copolymers were used, covering a broad range of low molecular weights (SMA-L1 to SMA-L4: Mw = 5500–9500 g/mol) to high molecular weights (SMA-H1 to SMA-H6: Mw = 80,000–180,000 g/mol) and different amounts of maleic anhydride (MA = 22–50 mol%). The materials were provided by Sartomer (Colombes, France) and Polyscope (Geleen, The Netherlands), respectively. As reference materials, a series of polystyrene (PS) with well-determined molecular
Thermogravimetric analysis
The TGA curves for low-molecular weight SMA were measured in air (Fig. 1a) and nitrogen (Fig. 1b), together with MA and PS reference materials. The MA degrades fully over the temperature range 145–300 °C in air, while it degrades at a higher rate over the temperature range 155–210 °C in nitrogen. The progressive degradation in air relates to an oxidative polymerization reaction of the functional MA groups that enhances the thermal stability at high temperatures through the formation of a network
Discussion
Thermal data from DSC and TMDSC for high- and low-molecular weight SMA is compared in Fig. 9, Fig. 10, Fig. 11. Here, the effects of molecular weight and molar composition (mol% MA) are further considered, in order to relate the thermal data to the molecular structure of SMA.
Conclusions
The thermal properties and transitions for a variety of SMA grades with low-molecular weight (Mw = 5500–9500 g/mol) and high-molecular weight (Mw = 80,000–180,000 g/mol) with different amounts of maleic anhydride (26–50 mol%) were evaluated by TGA, DSC and modulated DSC in order to detect intrinsic variations in molecular structure.
The data from TGA analysis provides evidence for intrinsic differences in thermal stability between low- and high-molecular weight copolymers, especially during the first
Acknowledgements
G. Schoukens would like to thank the Institute for the Promotion of Innovation by Science and Technology in Flanders for a funding program ‘SNAP’ (contract grant IWT-080213). We thank H. Van den Abbeele and D. Stanssens (Topchim N.V., Belgium) for kindly providing materials. P. Samyn acknowledges the Robert Bosch Foundation for support in the Juniorprofessorprogram.
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