Thermo-analytical study on transitions in styrene–maleic anhydride copolymers with low- and high-molecular weights

Thermochimica Acta 580 (2014) 28–37

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Thermo-analytical study on transitions in styrene–maleic anhydride copolymers with low- and high-molecular weights Gustaaf Schoukens a , Pieter Samyn b,∗ a

Ghent University, Department of Textiles, Technologiepark 907, B-9052 Zwijnaarde, Belgium Albert-Lüdwigs-University Freiburg, Faculty of Environment and Natural Resources, Chair of Bio-based Materials Engineering, Werthmannstrasse 6, D-79085 Freiburg, Germany b

a r t i c l e

i n f o

Article history: Received 7 October 2013 Received in revised form 20 January 2014 Accepted 27 January 2014 Available online 10 February 2014 Keywords: Styrene–maleic anhydride Thermal analysis Molecular structure Composition

a b s t r a c t The thermal behaviour near the glass transition temperature Tg was studied for poly(styrene-co-maleic anhydride) or (SMA) copolymers with a broad range of molecular weight (Mw = 5500–180,000 g/mol) and amount of maleic anhydride (22–50 mol%). The influences of molecular structure for low- and highmolecular weight copolymers were detailed by using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC). Data from TGA provide evidence for variations in thermal stability, mainly over the first degradation step up to 350 ◦ C. Most interesting data results from TMDSC for high-molecular weight SMA, where the transition in reversible heat flow and cp remains almost constant, while the transition in non-reversible heat flow and cp gradually increases with amount MA. Comparing low- and high-molecular weight SMA, a linear model for Tg cannot be applied for the global amount of maleic anhydride, while it can be successfully implemented if a more detailed molecular structure with two constituents is considered. As such, further evidence is provided for the intrinsic heterogeneous molecular structure of high-molecular weight SMA with “styrene-rich” and “maleic-anhydride-rich” polymer segments. © 2014 Elsevier B.V. All rights reserved.

1. 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 [1–3]. 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

∗ Corresponding author. Tel.: +49 761 203 9239; fax: +49 761 203 3673. E-mail address: [email protected] (P. Samyn). 0040-6031/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2014.01.021

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

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Table 1 Characterization of low- and high-molecular weight SMA grades. Mn [g/mol] PS-1 PS-2 PS-3 PS-4 PS-5 PS-6 PS-7 PS-8 SMA-L1 SMA-L2 SMA-L3 SMA-L4a SMA-H1 SMA-H2 SMA-H3 SMA-H4 SMA-H5 SMA-H6 a

1690 3570 5200 12,000 30,500 63,700 258,000 925,000 1800 2870 5340 5340 35,185 54,620 65,640 47,050 41,000 31,100

Mw [g/mol] 1940 4380 5480 12,600 35,100 65,000 275,000 950,000 5800 7800 11,000 11,000 80,200 120,500 180,000 110,500 112,000 80,500

Mz [g/mol] – – – – – – – – 11,025 16,000 18,000 18,000 181,200 260,900 328,400 256,600 236,800 150,200

Mw /Mn

Amount MA [mol%]

1.15 1.23 1.04 1.05 1.15 1.02 1.07 1.03 3.22 2.72 2.06 2.06 2.28 2.21 2.76 2.35 2.73 2.59

0 0 0 0 0 0 0 0 50 33 25 25 26 26 26 22 28 34

All SMA copolymers were available under pellet form, except SMA-L4 was delivered as flakes.

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 ␣-relaxation is associated with the glass-rubber transition and characterized by the Vogel–Fulcher–Tammann temperature dependence, and the latter ␤-relaxation is related to the local motion of the ester side groups attached to the polymer backbone. The apparent activation energy for the ␤-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. 2. Experimental details 2.1. 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 weights and narrow weight distributions was used (American Polymer Standards), together with one grade of maleic anhydride (99%, Sigma–Aldrich). The SMA copolymers were previously characterized by gel-permeation chromatography to determine the weight-average molecular weight Mw and by 1 H NMR/13 C NMR to determine the MA contents, as summarized in Table 1 [31]. Before any further thermal analysis, the SMA pellets were pulverized and dried for 6 h at 100 ◦ C in a hot-air oven. In parallel, a film of high-molecular weight SMA-H1 was prepared by casting from an acetone solution. The solution was prepared by stirring the SMA powder for 1 h at 23 ◦ C until a homogeneous solution was obtained of 10 mg/ml SMA dissolved in acetone

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(pH = 7). After evaporation of the acetone, the film was dried under vacuum. One type of ammonolyzed high-molecular weight SMA-H1 (aSMA-H1) was prepared by stirring SMA-H1 for 1 h in a 25 wt.% solution of ammonium hydroxide at 90 ◦ C. The ammonolyzed product was precipitated at pH < 4 by adding HCl, filtered and washed several times with cold water, and then dried under vacuum. 2.2. Thermal analysis Thermogravimetric analysis (TGA, Mettler Toledo SDTA851) was done on a sample of 12 mg that was loaded in a ceramic alumina crucible and heated at 20 ◦ C/min from 25 to 1050 ◦ C. The measurements were repeated both in air and nitrogen atmosphere with two runs under each condition. The represented curves are an average curve from both runs. Differential scanning calorimetry (DSC) was done under nitrogen flow by loading 5.0 ± 0.1 mg of the sample into an aluminium pan (Q2000 equipment, TA Instruments V3.9A, Zellik, Belgium). Two heating cycles were applied, with a fixed heating and cooling rate of 10 ◦ C/min from 0 to 250 ◦ C and an intermediate isothermal period of 5 min at the highest and lowest temperature. An intermediate scanning rate was selected in order to optimize the sensitivity and resolution. Before testing, the DSC was calibrated with indium (Tm = 156.6 ◦ C, H = 28.45 J/g) and gallium (Tm = 29.8 ◦ C). The measurements were referred against an empty aluminium pan for baseline corrections. For temperature modulated differential scanning calorimetry (TMDSC), smaller sample sizes of 2.0 ± 0.1 mg were used to improve the sensitivity. The samples were heated over two cycles from 0 to 250 ◦ C at 2 ◦ C/min with a temperature modulation amplitude of ±2 ◦ C every 60 s. The cooling rate for TMDSC measurements was similar to the heating rate. The presented results are averaged from two separate scans. There was a relatively good repeatability with a variation on peak transition temperatures within ± 0.5 ◦ C. Automated functions of the TA Universal Analysis Software were used for analysis, with exoherm peaks upwards. 3. Test results 3.1. 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 by oxidative crosslinking. Similar complex oxidation reactions of MA, which are governed by intermediate radical formation, are known for grafted MA-polymers and broaden the decomposition interval [33]. The absence of intermediate oxidizing products in nitrogen is manifested as a more narrow thermal degradation step of MA. For PS, in contrast, the oxidation reactions in air deteriorate the thermal stability and cause fast degradation at 250–400 ◦ C, while the thermal stability in nitrogen is enhanced with degradation at 340–450 ◦ C. There is a slight influence of the molecular weight on a different thermal stability, as the degradation of PS-1 (lowest Mw ) happens before degradation of PS-6 (higher Mw ). The other reference materials PS-2 to PS-5 were in between. The oxidative stability for SMA-L1, L2, L3 in air is better than for MA and PS references. However, the decomposition proceeds in multiple steps with a significant weight loss of 5% at 250–300 ◦ C (see inset Fig. 1a). The first decomposition step might be due to volatiles and residual solvents releasing small molecules (CO2 ).

Fig. 1. Thermogravimetric analysis (TGA) of low-molecular weight SMA copolymers and reference materials, (a) in air atmosphere, (b) in nitrogen atmosphere, (axis and symbols in inset figures correspond to the main figure): samples SMA-L1 to SMA-L3.

The thermal stability for low-molecular weight SMA at 200–300 ◦ C agrees with the properties of alternating SMA [20], while the decomposition is more pronounced for block or random copolymers with weight loss up to 10% at 300 ◦ C. The main decomposition occurs at 310–380 ◦ C, where lower amounts of MA in the copolymer slightly decrease the decomposition temperature. The decomposition at 400–500 ◦ C is characterized by a plateau level through the formation of an oxidative network, which is present in highest amount for L1 (50 mol% MA) and lowest amount for L3 (25 mol% MA). The stabilization in oxidative degradation of MA is found in SMA as it occurs for the reference MA: the presence of MA in SMA enhances the oxidative thermal stability. Finally, the carbonization zone starts above 520 ◦ C when fusion, disproportionation and gasification reactions occur. The thermal stability of SMA-L1, L2, L3 in nitrogen is worse than for PS (Fig. 1b), while lower amounts of MA also reduce the thermal decomposition temperature in parallel with the measurements in air. This behaviour agrees with calculated activation energies for the degradation processes, resulting in a lower degradation temperature for SMA relatively to PS [34]. The thermal degradation in nitrogen for low-molecular weight SMA similarly shows a two-step degradation at 265–340 ◦ C (see inset Fig. 1b) and at 340–400 ◦ C, characteristic for alternating SMA without formation of an oxidative network in nitrogen. The TGA curves for high-molecular weight SMA in air are shown in Fig. 2, with detailed influences of molecular weight for constant mol% MA (see inset Fig. 2), and mol% MA for constant molecular

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Fig. 2. Thermogravimetric analysis (TGA) of high-molecular weight SMA copolymers in air atmosphere, (axis and symbols in inset figures correspond to the main figure): samples SMA-H1 to SMA-H6.

weight (see inset Fig. 2). The high-molecular weight SMA has a single degradation step at 340–420 ◦ C without significant weight loss below 300 ◦ C, in contrast with low-molecular weight SMA. The better thermal stability at 200–300 ◦ C points towards different molecular structures for high-molecular weight copolymers, as detailed in the discussion section. The main decomposition step at 340–420 ◦ C depends on the SMA type: (i) with increasing molecular weight in SMA-H1, H2, H3, the thermal stability increases; (ii) with increasing amount of MA in SMA-H4, H1, H5 and H6, the thermal stability decreases. The latter relations are better illustrated by derivative thermogravimetric signals (DTG curves) in Fig. 3, with maximum degradation rate at following temperatures: 410 ◦ C (H1), 417 ◦ C (H2), 422 ◦ C (H3) at higher molecular weight, and 418 ◦ C (H4), 410 ◦ C (H1), 403 ◦ C (H5), 395 ◦ C (H6) at higher amounts of MA. Comparing our results for high-molecular weight SMA with previous literature data for specifically synthesized low-molecular weight SMA [20], the thermal stability is in between fully alternating and block copolymers. 3.2. Differential scanning calorimetry The results from DSC measurements on PS reference materials are summarized in Table 2, with an indication of the temperatures Table 2 Summary of DSC analysis of different low- and high-molecular weight SMA grades.

PS-1 PS-2 PS-3 PS-4 PS-5 PS-6 PS-7 PS-8 SMA-L1 SMA-L2 SMA-L3 SMA-L4 SMA-H1 SMA-H2 SMA-H3 SMA-H4 SMA-H5 SMA-H6

Tonset [◦ C]

Tg [◦ C]

Tend [◦ C]

T [◦ C]

cp [J/g ◦ C]

56.8 76.2 77.0 89.6 100.2 100.6 103.5 103.9 150.23 130.34 120.54 122.70 153.43 155.02 154.59 144.08 156.74 172.30

60.4 79.9 82.4 93.4 103.1 104.0 106.4 106.7 157.27 135.27 124.38 127.53 158.19 158.32 158.52 147.53 160.34 175.81

64.3 83.7 87.8 96.7 105.9 107.3 109.4 109.7 160.18 138.20 124.47 136.02 159.20 160.12 159.35 148.81 161.87 178.85

7.50 7.50 10.8 7.10 5.70 6.70 5.90 5.80 9.95 7.86 3.93 13.32 5.77 5.10 4.76 4.73 5.13 6.55

– – – – – – – – 0.4248 0.3427 0.2811 0.3016 0.3303 0.3329 0.3043 0.2911 0.3459 0.4909

Fig. 3. Derivative thermogravimetric (D-TGA) curves for the main decomposition step in high-molecular weight SMA copolymers, (a) influence of different molecular weight for samples SMA-H1, H2, H3, (b) influence of different mol-% MA for samples SMA-H1, H4, H5, H6.

at onset Tonset , end Tend of the transition interval (T = Tend − Tonset ) and the glass transition temperature Tg . The standard variation on Tg was about ±0.2 ◦ C averaged from two test runs. In general, Tg shifts towards higher temperatures and the width of the transition becomes smaller at higher molecular weight of PS. The DSC thermographs for low-molecular weight SMA are presented in Fig. 4. By comparing Tg in two subsequent heating cycles, the variation is not significant for most materials except for SMA-L3 and L4 (Fig. 4a). Both materials have similar chemical composition with 25 mol% MA, but were produced in either pellet form (L3) or flake form (L4). The transition during a first heating cycle at around 150 ◦ C for SMA-L3, L4 corresponds to the transition for SMA-L1 and may be due to the formation of an inhomogneous phase at lowest amounts of MA, which stabilizes after short heating at 250 ◦ C. The latter and indicates an original phase with 50 wt.% MA that has not fully stabilized after synthesis. The second heating step shows regular behaviour with only one single transition temperature at 124.4 ◦ C. The difference in Tg in the first and second heating cycle is about 2.8 ◦ C. All following results are reported from the second heating cycle. The DSC thermographs for different low-molecular weight SMA is given in Fig. 4b: the Tg gradually decreases from 157.3 to 124.4 ◦ C at lower amounts of MA, in parallel with a lower change in heat capacity cp from 0.42 to 0.28 (Table 2). The slight differences in the DSC cooling curves also points towards a different production quality of the pulverized pellets and flakes SMA-L3 and

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Fig. 4. DSC heat flow curves for low molecular weight SMA copolymers, (a) two subsequent heating cycles for SMA-L3, (b) second heating cycle for SMA-L1, L2, L3, L4.

L4. From an earlier survey study by Raman and FTIR spectroscopy [31], no significant differences in the SMA-L3 and SMA-L4 were noticed, yielding the same amount of MA and very slight differences in styrene-related absorption bands which might eventually be attributed to a different orientation of the styrene groups. The DSC measurements for high-molecular weight SMA are detailed in Fig. 5. First, the measuring protocol was optimized by using different sample weights for SMA-H1 between 1 and 10 mg (Fig. 5a). The weight may influence the structural relaxations with a shift in Tg and broadness of the response due to internal temperature gradients [35]. From these measurements, we selected a sample of 5 mg providing optimum background-to-signal ratio while the baseline remains parallel for higher weights. Meanwhile, there is good statistical repeatability for all samples with an average value of Tg = 158.19 ± 0.53 ◦ C and cp = 0.33 ± 0.01. The SMA-H1 that was dissolved in acetone and dried as a film has a slightly higher Tg = 159.74 ◦ C and cp = 0.30 J/(g ◦ C) than bulk SMA26. The dissolution in acetone does not affect any degradation of the amorphous material. The SMA-H1 that was first molten under atmospheric conditions in a hot-air oven for 6 h at 220 ◦ C, has a lower Tg = 156.81 ◦ C and cp = 0.18 J/(g ◦ C), likely due to oxidative cross-linking in the amorphous phase as described earlier from TGA. By comparing the high-molecular SMA copolymers (Fig. 5b, Table 2), the Tg mainly depends on the amount of MA and increases with higher amount MA (26 towards 34 mol%, compare SMA-H4, H1, H5, H6) from 147.5 to 175.8 ◦ C, while the Tg less depends on the

Fig. 5. DSC heat flow curves for high molecular weight SMA copolymers, (a) second heating cycle under different measuring conditions for SMA-H1, (b) second heating cycle for SMA-H1, H2, H3, H4, H5, H6.

molecular weight (80,000 towards 180,000 g/mol, compare SMAH1, H2, H3) and remains at 158.34 ± 0.15 ◦ C. From DSC-measurements, the heat capacity in the temperature range 0–250 ◦ C was determined (Fig. 6) and the change in heat capacity cp at the glass transition was calculated (Table 2). As a reference point for absolute values, the heat capacity at 100 ◦ C can be used. For both low- and high-molecular weight SMA, the heat capacity values increase with lower amounts of MA (compare the increasing value in the order SMA-L1, L2, L3; and SMA-H6, H5, H4, H1), while there is also an increase in heat capacity value with increasing molecular weight (compare SMA-H1, H2, H3). Both lowand high molecular weight SMA show a similar trend for lower heat capacity change cp as the amount of MA decreases, demonstrating that molecular interactions become less pronounced and are mainly controlled by presence of MA. 3.3. Temperature modulated differential scanning calorimetry The superposition of a sinusoidal heat flow may increase the sensibility for thermal events. The instantaneously high heating rates allow to differentiate between phenomena related to changes in heat capacity (reversing heat flow: glass transition, melting) and interfering kinetic processes (non-reversing heat flow: crystallization, structural perfection, decomposition, evaporation). As such, more details on the endothermic transitions around the glass temperature are revealed. From TMDSC measurements (second heating

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Fig. 7. Temperature modulated DSC (TMDSC) analysis of SMA copolymers, (a) low molecular weight SMA-L1, (b) and (c) high-molecular weight SMA-H1. Fig. 6. DSC heat capacity for SMA copolymers, (a) low-molecular weight SMA-L1 L2, L3, (b) high-molecular weight SMA-H4, H1, H5, H6, (c) high-molecular weight SMA-H1, H2, H3.

cycle), the reversible and non-reversible heat flows were separated from the total heat-flow signal as shown in a detail around Tg for SMA-L1 (Fig. 7a) and SMA-H2 (Fig. 7b): the molecular transitions related to the non-reversible heat flow are much more pronounced in high-molecular SMA-H1 compared to SMA-L1, and similar observations were made for the other SMA. The heat capacity change related to the reversible heat flow of SMA-H1 is detailed in Fig. 7c, and the change near the glass transition is characterized by an increase in heat capacity. The Tg and heat capacity change cp related to the nonreversible and reversible parts of the glass transition for high-molecular weight SMA are summarized in Table 3. Due to the lower overall heating rate (2 ◦ C/min) in TMDSC, the values for Tg are somewhat lower than from DSC. In parallel, the enthalpy change related to the non-reversible heat flow is measured. From this analysis, we conclude that the transition related to the reversible

heat flow is almost constant (0.134 ± 0.008 = J/(g ◦ C)) for all high-molecular weight SMA, while there is a significant increase in the heat capacity change related to the non-reversible transition for SMA with higher amounts of MA. In parallel, the enthalpy related to the non-reversible heat flow increases at higher amounts of MA as an indication that molecular interactions become more important. 3.4. Comparative study with ammonolyzed SMA The full ammonolysis of SMA-H1 (a-SMA-H1) was characterized by a ring-opening reaction of the MA and formation of the ammonium salt [36,37], as here confirmed by FTIR spectroscopy: the MA-related bands at 1855 and 1781 cm−1 disappear in favour of a peak at 1707 cm−1 due to acid formation (spectral results are here not further detailed). Following the TGA and DTG curves in air (Fig. 8a), the ammonolyzed copolymer degrades with three maximum degradation rates at 100 ◦ C, 336 ◦ C and 380 ◦ C. The TGA curves in nitrogen show a more drastic degradation with principally also three steps. The thermal stability of a-SMA-H1 is lower

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Table 3 Summary of TMDSC analysis of high-molecular weight SMA grades.

SMA-H4 SMA-H1 SMA-H2 SMA-H3 SMA-H5 SMA-H6

Amount MA [mol%]

Tg [◦ C]

Reversible cp [J/(g ◦ C)]

Non-reversible cp [J/(g ◦ C)]

Enthalpy non-reversible transition [J/g]

22 26 26 26 28 33

145.12 155.85 155.63 155.52 159.83 172.13

0.1356 0.1254 0.1312 0.1285 0.1415 0.1320

0.1032 0.1306 0.1365 0.1382 0.1723 0.2412

3.41 4.28 4.38 4.35 4.82 4.95

Fig. 9. Comparative thermo-analytical data for high-molecular weight SMA copolymers () and PS reference samples () as a function of molecular weight. Note: the trendlines are only applied as a guide for the eye.

represent the effects of cross-linking. This may be confirmed by a Tg = 160.4 ± 0.3 ◦ C that is a little higher than for SMA-H1 while the change in heat capacity cp = 0.2805 J/(g ◦ C) is significantly lower for the a-SMA-H1. Also additional electron-interactions between the charged ammonolyzed moieties (and eventually slight curing during 5 min at 250 ◦ C) possibly stabilize the SMA and reduce the molecular mobility in the amorphous phase. 4. Discussion Fig. 8. Thermo-analytical data for ammonolyzed a-SMA-H1, (a) TGA in air (full line) and nitrogen (dotted line) accompanied by DTG in air, (b) DSC heat flow curves in first (i) and second (ii) heating cycle.

than SMA-H1, with the main degradation step in air occurring at 380 ◦ C (a-SMA-H1) instead of 410 ◦ C (SMA-H1). Compared with studies of thermal degradation for low-molecular weight hydrolyzed SMA by other researchers [23,24], also three degradation steps were noticed but at different temperatures of 150, 260 and 355 ◦ C. However, the exact mechanisms for degradation and intermediate crosslinking are not clear: based on spectroscopic observations, it may be related to formation of C O or intermolecular anhydride. The carbonyl groups in anhydrides, which are not part of a strained ring, absorb at lower frequencies than those in five-membered ring. Thus, the presence of small amount of intermolecular anhydride could be a reason for broadening of the carbonyl band observed in the spectrum of the degraded copolymer. The DSC-measurements of a-SMA-H1 (Fig. 8b) show significant features during the first heating cycle, which agree with the TGA curves. The transitions at around 100–150 ◦ C relate to endotherm reactions, and likely represent interactions between the carbonyl groups. The latter reactions do not occur during the second heating step, as the sample is stabilized after 5 min at 250 ◦ C, and

Thermal data from DSC and TMDSC for high- and low-molecular weight SMA is compared in Figs. 9–11. 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. 4.1. Influence molecular weight It is known that Tg becomes constant for PS at above Mw = 25,000 g/mol [38–40], and a similar trend can presently be seen for SMA-H1, H2 and H3 copolymers (Fig. 9). Only in the nanometer range for polymer nanospheres less than 100 nm in diameter, size-effects may become important [41]. Based on a rough extrapolation, the limiting molecular weight providing independent Tg is almost similar for PS and SMA. The limitation in Tg upon higher molecular weight is mainly attributed to the formation of molecular entanglements, and it can be understood that these entanglements for high-molecular weight SMA are more complex than for PS polymers due to presence of MA. 4.2. Influence global molar ratio maleic anhydride When considering the global amount of MA in low- and highmolecular weight SMA, the Tg significantly increases with amount

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(a)

35

200

Temperature Tg (°C)

180 160

y = 1,2428x + 100 R² = 0,8977

140 120 100 80 60 40 20 0 0

10

20

30

40

50

60

Local amount MA (mol -%)

Heat capacity

cp (J/(g°C))

(b)

0.6 0.5 0.4 0.3 0.2 0.1

extrapolation 0 0

10

20

30

40

50

60

Local amount MA (mol -%) Fig. 11. Comparative thermo-analytical data for high-molecular weight SMA (䊉) and low-molecular weight SMA () against local amount MA for heterogeneous molecular structure of high-molecular weight SMA, (a) Tg Fox equation, (b) total heat capacity (䊉,) non-reversible heat capacity () and reversible heat capacity (). Note: the trendlines are only applied as a guide for the eye.

concluded that the thermal transition of high-molecular weight SMA becomes more complicated at higher mol% MA and the amount of MA has a strong influence on the molecular structure of SMA. Therefore, a more detailed analysis is necessary as explained below. 4.3. Influence local molar ratio maleic anhydride

Fig. 10. Comparative thermo-analytical data for high-molecular weight SMA (䊉) and low-molecular weight SMA () against global amount MA, (a) Tg , (b) T, (c) total heat capacity (䊉,) and reversible heat capacity for high-molecular weight SMA (). Note: the trendlines are only applied as a guide for the eye.

of MA. However, a different linear relationship is followed for high- and low molecular weight SMA (Fig. 10a). The extrapolation of Tg towards 0 mol% consistently corresponds with Tg = 100 ◦ C for PS. The Tg for high-molecular weight SMA is higher over the total range of MA contents, as related to their specific structures. However, the temperature interval T also increases almost linearly with the amount of MA (Fig. 10b), which might indicate the complexity of the copolymer structure causing a more progressive glass transition. The changes in total and non-reversible heat capacity cp increase with amount of MA along a different trend for lowand high-molecular weight SMA (Fig. 10c). It can therefore be

In parallel with spectroscopy [31], the molecular structure of high-molecular weight SMA-H1 to SMA-H6 should be considered in more detail. Previous analysis of FTIR, Raman and 13 C NMR spectra revealed differences in the global and local molecular structure for high-molecular weight SMA. The main results are repeated in Table 4, detailing the local copolymer structure. It can be concluded that high-molecular weight SMA contains a heterogeneous structure with “styrene-rich” and “maleic anhydride-rich” segments that are statistically distributed over the copolymer chain. The fraction of each segment depends on the type of SMA-H, with less styrenerich and more of maleic anhydride-rich segments as the global mol% MA increases. The intrinsic complex molecular structure is different from regular low-molecular weight SMA, and can indeed be further confirmed by more detailed analysis of the thermal data. Qualitatively, the heterogeneous structure of high-molecular weight SMA is reflected by broadening of the DTGA curves at 300–350 ◦ C (Fig. 3b), which is most pronounced for the SMA-H6 as it contains highest fraction of polymer segments with 45 mol% MA.

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G. Schoukens, P. Samyn / Thermochimica Acta 580 (2014) 28–37

Table 4 Heterogeneous composition of high-molecular weight SMA copolymer grades according to 13 C NMR [31]. High-molecular weight SMA

Global amount MA [mol%]

Local “styrene-rich” segment

Local “maleic anhydride-rich” segment

Fraction within copolymer

Local copolymer composition

Fraction within copolymer

Local copolymer composition

8 mol% MA 92 mol% St 5 mol% MA 95 mol% St 3 mol% MA 97 mol% St 2 mol% MA 98 mol% St

50%

40 mol% MA 60 mol% St 43 mol% MA 57 mol% St 46 mol% MA 54 mol% St 53 mol% MA 47 mol% St

SMA-H4

22

50%

SMA-H1

26

40%

SMA-H5

28

30%

SMA-H6

34

20%

Moreover, the thermal degradation of the complex heterogeneous structure for high-molecular weight SMA does not correspond with the relatively fast thermal degradation of a traditional SMA block copolymer structure [20], and reflects the more complex structure with different polymer segments. These may induce more complex interactions between the molecular chains that are expressed as structural reordering and perfection. Quantitatively, the Tg of low- and high-molecular weight SMA can be described by the Flory–Fox theory [42], according to the equation (1) with wi the weight fraction of comonomer i and Tgi glass transition temperature of the homopolymer structure corresponding to that of comonomer i: 1 w1 w2 = + Tg Tg1 Tg2

(1)

Theoretically, the equation implies a linear relationship between the Tg and fraction MA but cannot be applied when considering the global SMA composition. However, by reconsidering the detailed heterogeneous structure of the high-molecular weight SMA in Table 4, a different relationship can be formulated. When considering the local molar ratio of MA, then it contains different segments with a styrene-rich fraction (only 2–8 mol% MA) and a maleic anhydride-rich fraction (40–53 mol% MA). As such, the first segment can be considered as an almost pure polystyrene comonomer with Tg = 100 ◦ C; and the latter fraction can be considered as a maleic-anhydride and styrene copolymer almost close to the regular SMA structure with 50 mol% MA (Tg = 157 ◦ C). By implementing both segments as the “constituents” of highmolecular weight SMA, a linear relationship exists between Tg and local amount of MA according to the Flory–Fox theory (statistical variation R2 = 0.90), as illustrated in Fig. 11a. This finding interestingly confirms quantitatively the heterogeneous structure of SMA and demonstrates that the maleic anhydride-rich segment mainly determines Tg , while the other styrene-rich segment has an almost constant contribution close to polystyrene. The linear relationship between cp and local amount of MA is more complicated, as illustrated in Fig. 11b. For high-molecular weight SMA, the total and non-reversible cp can be related to the local amount of MA and maleic anhydride-rich segments. The constant contribution from reversible transitions reflects the presence of polystyrenerich segments with cp corresponding to the extrapolated value of low-molecular weight SMA. The extrapolation of cp for highmolecular weight SMA yields a local fraction of 15 mol% MA: this value can be explained by spectroscopy [31], where a certain fraction of 15–17 mol% MA was not visible by infrared spectroscopy while it was detected by FT-Raman spectroscopy. These variations were attributed to the local arrangements of MA structures and are due to inter- and intramolecular interactions in high-molecular weight SMA.

60% 70% 80%

In conclusion, the two graphs in Fig. 11 confirm that the proposed heterogeneous molecular structure for high-molecular weight SMA is also reflected in the thermo-analytical data, and the local amount of MA in separate polymer chain segments should be considered in comparison with low-molecular weight SMA. 5. 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 degradation steps up to 350 ◦ C. While the low-molecular weight SMA shows a tendency for behaviour as alternating or block copolymers (as expected), the present high-molecular weight SMA provides different thermal stability that is in between the expected thermal stability for fully alternating and block copolymers. According to DSC measurements, the Tg gradually decreases in parallel with a reduction of the heat capacity change cp with decreasing amount of MA. Most interesting thermal data is obtained from TMDSC measurements on high-molecular weight SMA, where the transition in reversible heat flow and corresponding cp remains almost constant, while the transition in non-reversible heat flow and non-reversible cp increases with amount of MA and slightly increases with molecular weight. The TMDSC measurements on low-molecular weight SMA only show a sharp transition in reversible heat flow and do not show a pronounced transition in non-reversible heat flow. The comparative thermal data for ammonolyzed SMA confirms that the molecular interactions near the maleic anhydride groups importantly influence the glass transition. The thermo-analytical data provides evidence for a heterogeneous molecular structure of high-molecular weight SMA. The general Fox model can be applied for comparing glass transition temperatures between low- and high-molecular weight SMA, if a detailed molecular structure is considered with two constituents, including “styrene-rich” and “maleic-anhydride rich” polymer segments. The model for a complex structure for SMA copolymers agrees with our previous calculations based on spectroscopic analysis, and provides intrinsic possibilities molecular self-assembly. 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

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