Mechanical and Electrical Properties of Sulfur-Containing Polymeric Materials Prepared Via Inverse Vulcanization †

polymers

Article Mechanical and Electrical Properties of Sulfur-Containing Polymeric Materials Prepared via Inverse Vulcanization †

Sergej Diez, Alexander Hoeﬂing, Patrick Theato and Werner Pauer * Technical and Macromolecular Chemistry, University of Hamburg, Hamburg D-20146, Germany; [email protected] (S.D.); alexander.hoeﬂ[email protected] (A.H.); [email protected] (P.T.) * Correspondence: [email protected]; Tel.: +49-428-38-6007 † This work is dedicated to Prof. Dr. Hans-Ulrich Moritz. On the occasion of his 65th birthday, the authors wish to thank H.-U. Moritz for his excellent support and inspiration through his enriching ideas.

Academic Editor: Do-Hoon Hwang Received: 5 December 2016; Accepted: 26 January 2017; Published: 15 February 2017

Abstract: Recently, new methods have been developed for the utilization of elemental sulfur as a feedstock for novel polymeric materials. One promising method is the inverse vulcanization, which is used to prepare polymeric structures derived from sulfur and divinyl comonomers. However, the mechanical and electrical properties of the products are virtually unexplored. Hence, in the present study, we synthesized a 200 g scale of amorphous, hydrophobic as well as translucent, hyperbranched polymeric sulfur networks that provide a high thermal resistance (>220 ◦C). The polymeric material properties of these sulfur copolymers can be controlled signiﬁcantly by varying the monomers as well as the feed content. The investigated comonomers are divinylbenzene (DVB) and 1,3-diisopropenylbenzene (DIB). Plastomers with low elastic content and high shape retention containing 12.5%–30% DVB as well as low viscose waxy plastomers with a high ﬂow behavior containing a high DVB content of 30%–35% were obtained. Copolymers with 15%–30% DIB act, on the one hand, as thermoplastics and, on the other hand, as vitreous thermosets with a DIB of 30%–35%. Results of the thermogravimetric analysis (TGA), the dynamic scanning calorimetry (DSC) and mechanical characterization, such as stress–strain experiments and dynamic mechanical thermal analysis, are discussed with the outcome that they support the assumption of a polymeric cross-linked network structure in the form of hyper-branched polymers.

Keywords: sulfur; inverse vulcanization; polymeric materials; crosslinking; divinylbenzene; 1,3-diisopropenylbenzene; mechanical properties; electrical properties; bulk polymerization

1. Introduction Elemental sulfur is produced on a million ton scale. It is mainly generated as waste by hydrodesulfurization of petroleum in crude oil reﬁneries (>90% recovered in the whole, Reston, VA, USA) [1]. Despite many applications, sulfur world production is projected to reach a surplus of around 70,000 million tons annually, leading to overground storage of sulfur mountains (Figure1) with unpredictable environmental risks [1]. Its easy availability, low cost and limited applications make sulfur an attractive feedstock for novel materials with a high sulfur content. Currently, elemental sulfur is mainly used for the production of sulfuric acid, which is the resource for many different ﬁne chemical products (battery acid, phosphate fertilizers, ammonium sulfate, disintegrating agents, surfactants for the detergent industry, dyes, etc.) [2].

Polymers 2017, 9, 59; doi:10.3390/polym9020059 www.mdpi.com/journal/polymers Polymers 2017, 9, 59 2 of 16 Polymers 2017, 9, 59 2 of 16

Figure 1. Oil sand sulfur stacks by Syncrude, Athabasca, AB, Canada. Figure 1. Oil sand sulfur stacks by Syncrude, Athabasca, AB, Canada.

Sulfur occurs naturally in its thermodynamically most stable modification: orthorhombic sulfur Sulfur occurs naturally in its thermodynamically most stable modification: orthorhombic Sα which forms crown-shaped S8 rings. Small amounts of S7-rings and tiny amounts of other rings α sulfuralso occur. S which Overall, forms there crown-shaped are 14 well-known S8 rings. sulfur Small allotropes amounts whose of S formation7-rings and is tinytemperature-de- amounts of otherpendent rings [3]. alsoThe allotrope occur. Overall, polymeric there sulfur are occupies 14 well-known a special sulfur position allotropes with its diradical whose formation chain-char- is temperature-dependentacter. The formation takes [3 ].place The by allotrope a temperature-initiated polymeric sulfur ring-opening occupies a reaction special positionwith subsequent with its diradicaladdition of chain-character. diradical chains The (polymerization) formation takes [4]. place Polymeric by a temperature-initiated sulfur is formed above ring-opening its floor reactiontemper- withature subsequentof 159 °C [5]. addition Upon offurther diradical heating, chains sulfur (polymerization) readily converts [4]. Polymericto a metastable, sulfur one-dimensional is formed above itselastomer. floor temperature When exceeding of 159 200◦C[ °C,5]. the Upon amount further of po heating,lymeric sulfurspecies readily increases converts but the to chain a metastable, length is one-dimensional elastomer. When exceeding 200 C, the amount of polymeric species increases but reduced [3]. After cooling down to room temperature,◦ the polymeric sulfur and other rings (cyclo-Sn) the chain length is reduced [3]. After cooling down to room temperature, the polymeric sulfur and degrade back to the cyclic S8 form (depolymerization) [6,7]. otherRecently, rings (cyclo-S it wasn) described degrade back that tosulfur the cyclicserves S as8 form an inexpensive (depolymerization) and promising [6,7]. new feedstock for the synthesisRecently, of it wasnovel described polymeric that materials. sulfur serves Upon as copolymerization an inexpensiveand with promising vinyl group new containing feedstock formonomers, the synthesis the polymeric of novel polymericsulfur is stabilized materials. via Upon a cross-linked copolymerization structure. with Polymeric vinyl groupmaterials containing formed monomers,by so-called the “inverse polymeric vulcanization” sulfur is stabilized possess via magnificent a cross-linked thermomechanical structure. Polymeric and materials electrochemical formed byproperties so-called [8]. “inverse Because vulcanization”of their high sulfur possess conten magnificentt, the sulfur thermomechanical comonomers are used and electrochemicalas electroactive propertiescathode materials [8]. Because in Li-S of batteries their high (1005 sulfur mA content,∙h∙g−1 at the100 sulfurcycles comonomers[9]). Suitable comonomers are used as electroactive for the Li-S cathode materials in Li-S batteries (1005 mA h g 1 at 100 cycles [9]). Suitable comonomers for the Li-S battery technology are 1,3-diisopropenylbenzene· · − [8–10], divinylbenzene [11,12], styrene (STY) [13], battery1,4-diphenylbutadiyne technology are [14], 1,3-diisopropenylbenzene etc. Oleylamine-based [su8–lfur10], copolymers divinylbenzene have [ 11been,12 ],investigated styrene (STY) regard- [13], 1,4-diphenylbutadiyneing their usage as chalcogenide-semiconductor [14], etc. Oleylamine-based nano sulfurcrystals copolymers in the have area beenof nanomaterial investigated regardingsynthesis their usage as chalcogenide-semiconductor nanocrystals in the area of nanomaterial synthesis [15,16]. [15,16]. In addition, the use of the nanoparticles (NP) PbS and Au has been extended by Bear et al. to In addition, the use of the nanoparticles (NP) PbS and Au has been extended by Bear et al. to InP/ZnS InP/ZnS quantum dots, Fe3O4 and CoO. The sulfur copolymer with 1,3-diisopropenylbenzene (pS- quantumDIB) is used dots, as Fea 3matrixO4 and for CoO. the Theinorganic sulfur NPs. copolymer The resulting with 1,3-diisopropenylbenzene sulfur copolymer NP composites (pS-DIB) is show used astuneable a matrix physical for the and inorganic optical properties, NPs. The resultingmaking them sulfur particularly copolymer suitable NP composites for application show as tuneable optical physicalfilters [17]. and The optical incorporation properties, of making sulfur moieties them particularly into polymers suitable is performed for application to prepare as optical organic filters films [17]. Thewith incorporation high refractive of indices sulfur moietiesfor use in into optical polymers and op istoelectronic performed technologies to prepare organic [18,19]. films The goal with is high the refractiveattainment indices of high for refractive use in optical indices and for optoelectronic use as waveguiding technologies materials [18,19 for]. The future goal optical is the attainmentfiber com- ofmunication high refractive [19]. Applications indices for use in as co waveguidingncrete production materials or spraying for future op opticalerations fiber are communicationalso conceivable [19 as]. Applicationswell as their use in concrete in civil productionengineering or regarding spraying the operations extension are to also asphalt conceivable in road as pavements well as their or useas inin civilsulating engineering material regarding[20]. In this the case, extension sulfur-containing to asphalt in materials road pavements are very or advantageous as insulating materialdue to their [20]. Inresistance this case, towards sulfur-containing aqueous acids materials and areconcentrated very advantageous salt solutions due to [20]. their Recently, resistance high towards porous aqueous poly- acids and concentrated salt solutions [20]. Recently, high porous polymers have been generated from mers have been generated from high-sulfur inverse vulcanized copolymers by supercritical CO2 com- high-sulfur inverse vulcanized copolymers by supercritical CO compression (scCO ). These foams pression (scCO2). These foams (sulfur copolymers based on DIB,2 limonene, etc.) have 2the potential to (sulfuract as adsorbent copolymers material based in on the DIB, area limonene, of gas storage etc.) have and the separation potential [21] to act and as to adsorbent absorb toxic material pollutants in the areafrom ofdrinking gas storage water and (mer separationcury capture) [21] and[22,23]. to absorb toxic pollutants from drinking water (mercury capture)Among [22,23 the]. first vinyl monomers used for copolymerization with elemental sulfur were dicycio- pentadieneAmong and the styrene. first vinyl Blight monomers investigated used the forstructural copolymerization elucidation [24]. with The elemental reaction sulfurwas carried were dicyciopentadieneout at a temperature and of 140 styrene. °C with Blight varying investigated reaction times. the structural Kim et al. elucidationworked out [a24 general]. The synthetic reaction wasstrategy carried to prepare out ata polymer temperature networks of 140 of◦ Cpoly(OLA- with varyingr-S) copolymers reaction times. containing Kim et PbS al. workednanoparticles out a general[15]. These synthetic well-defined strategy PbS/poly(OLA- to prepare polymerr-S) nanocomposites networks of poly(OLA- were synther-S)sized copolymers in a one-pot containing synthesis PbS

Polymers 2017, 9, 59 3 of 16 nanoparticles [15]. These well-defined PbS/poly(OLA-r-S) nanocomposites were synthesized in a one-pot synthesis containing sulfur, oleylamine and PbCl2 in 1,2-dichlorobenzene. In addition, free radical copolymerizations of cyclic aryl disulfides and elemental sulfur were carried out in solution and in bulk polymerization [25]. This reaction represents a ring-opening polymerization of cyclic disulfide oligomers prepared by oxidative coupling of aromatic dithiols which form high molecular and linear poly(arylene sulfane)s with a high sulfur content. Equally, sulfur polymers can be synthesized by anionic copolymerization of cyclic sulfides at high temperatures (159 ◦C) [26,27]. In addition, with the aim of extending the green chemistry to the cross-linker, Parker et al. described the use of low cost and renewable cross-linking monomers with several unsaturated bonds, such as limonene, farnesol, farnesene and myrcene. These alternative cross-linkers have a comparatively lower molecular weight and lower Tg concerning conventional cross-linkers (DIB, DVB, dicylopentadiene) [23]. By varying the vinyl monomers and the sulfur content, the physical and chemical properties of the produced polymer can be tuned. The target is the synthesis of processable polymers with high chemical stability and with good mechanical and electrical properties. From this perspective, it would be economically and ecologically sustainable to use the excess of elemental sulfur of the hydrodesulfurization for the synthesis of advanced materials.

2. Materials and Methods

2.1. General Procedure for Bulk Copolymerization in a Pressure Vessel The reaction was performed in a pressure vessel equipped with a pressure gauge, a safety valve (<10 bar) and a ball valve. During the copolymerization, a constant reaction mass of 200 g in the Polytetraﬂuoroethylene (PTFE)-inlay was used. Both comonomers (S8 and DVB/DIB) were incorporated into the PTFE-inlay with a Komet™ magnetic stir bar (VWR International GmbH, Darmstadt, Germany). The content of used vinyl monomers varied between a content of 10 and 35 wt % in 5 wt % steps. Inverse vulcanization occurred in a closed system without intake of air. The vessel heated to 160 ◦C and then started the time. The reaction time was 90 min at a stirring speed of 350 rpm. The temperature was approx. 160 5 C and was controlled by an oil bath (Marlotherm ± ◦ SH, Sasol Germany GmbH, Hamburg, Germany). The product was immediately removed after 90 min reaction.

2.2. General Procedure for the Preparation of the Polymers for Analytical Measurements After the reaction, the sulfur copolymer was crushed and pulverized with an agate mortar to homogenize the product for analytical measurements, such as TGA, DSC and Powder X-ray Diffraction (PXRD). To enable the comminution, liquid nitrogen was used to cool down the polymer to a brittle state. The powder also served as source material for further processing with the hot press.

2.3. General Procedure for the Processing of the Sulfur Copolymer with the Hot Press The processing of the material is performed by means of a hot press. To ensure better removal, PTFE plates and molds with a rectangular cut were used. The powdered sample (about 35 g) of sulfur copolymers was placed in the rectangular form (150 150 mm2) with a thickness of 2 mm. Afterwards, × a temperature of 150 C and a pressure of 40 10 bar for 20 min was applied to prepare ﬁlms by ◦ ± compression moulding for different analytical measurements (tensile and electrical tests).

2.4. Chemicals and Characterization Methods Sulfur (colloidal powder, 99.5%, Carl Roth GmbH&Co. KG, Karlsruhe, Germany) and the vinyl monomers 1,3-diisopropenyl benzene (97%, abcr GmbH, Karlsruhe, Germany) and/or divinylbenzene (DVB, 80%, technical grade, Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) and styrene (freshly distilled, HRC chemicals, Purmerend, The Netherlands) were used as received without further puriﬁcation. Polymers 2017, 9, 59 4 of 16

Gas Chromatography (GC) measurements were conducted by a Agilent 7820A gas chromatograph (Sim GmbH, Oberhausen, Germany) using tetrahydrofuran (HPLC grade) as a solvent and toluene as internal standard with the column CP7778 (50 m length, CP Wax 58 FFAP, Agilent, J&W GC Columns, Frankfurt, Germany) at a column pressure of 50 kPa with a flame ionization detector (FID). Hydrogen was used as mobile phase. Differential Scanning Calorimetry (DSC) was performed on a DSC 1 thermal analysis system (Mettler-Toledo GmbH, Gießen, Germany) at a heating and cooling rate of 10 C min 1 over the range ◦ · − of 50 to 150 C under nitrogen atmosphere and the mid-points of the transitions were taken as T . − ◦ g Thermal Gravimetric Analysis (TGA) was carried out by a TGA 7 (PerkinElmer LAS GmbH, Rodgau, Germany) at a heating rate of 10 C min 1 under nitrogen atmosphere up to 800 C. ◦ · − ◦ Elemental Analysis (Euro EA) was performed using an inductively coupled plasma optical emission spectrometry (ICP-OES) (HEKAtech GmbH, Wegberg, Germany) with Carbon (C), Hydrogen (H), Nitrogen (N) and Sulfur (S), CHNS–Porapack PQS columns. PXRD measurements were obtained using an X’Pert Pro MPD model (PANalytical GmbH, Kassel, Germany) at room temperature with a Cu-Kα radiation source at 40 mA. Scanning electron microscopy (SEM) images were recorded on a Field Emission Scanning Electron Microscope (LEO 1525 FEG SEM, Carl Zeiss Microscopy GmbH, Jena, Germany) with an In-lens detector. An EHT of 5 kV and a WD of 5 mm was used. The samples for SEM were attached on a carbon surface with a carbon adhesive and vaporized with carbon. Tensile Experiments were performed on a materials testing machine from Zwick (Zwick GmbH&Co., Ulm, Germany) at a temperature of 25 ◦C. Samples were clamped tightly between the two jaws. A camera was fixed in front of the two jaws, to record the elongation process. The tensile test was carried out according to the international norm DIN EN ISO 527-2 with a multipurpose test specimen 5A. Dynamic Mechanical Analysis (DMA) was carried out on a HAAKE CTC from Thermo Fisher Scientific (Thermo Fisher Scientific GmbH, Karlsruhe, Germany). The storage moduli (E’), loss moduli (E”), and loss tangents (tan δ) were obtained in the dual cantilever bending mode as a function of temperature over a range of 50–150 C. The heating rate was 2 C min 1 and the frequencies were − ◦ ◦ · − 0.3, 1, and 10 Hz. The test was performed according to DIN EN ISO 6721 with a multipurpose test specimen 5A without grip section. The specific contact resistance (Ω cm) and the dielectric strength (kV mm 1) were measured by · · − the company ELANTAS Europe GmbH (Hamburg, Germany). For the electrical measurements, blanks with a size of 10 10 mm2 are required for the first method and blanks with a size of 40 40 mm2 for × × the second method.

3. Results and Discussion This work reports on the properties of sulfur copolymers with different divinyl monomers as cross-linker. The vinyl monomers are based on an aryl scaffolding. The general synthetic approach is to use bulk polymerization without any solvent or additive. The temperature is set to the floor temperature of sulfur (159 ◦C), because too high temperature would promote the autoacceleration. The time is set to 90 min at a scale of 200 g; this setting is also not chosen too high because it could lead to an expediting of the depolymerization process [10]. In contrast to Pyun, the polymerization occurs in a hermetically sealed system with PTFE inlay (see the Experimental section).This simple process yields colored (yellow-amber and red) hydrophobic and translucent materials with different ductilities or malleabilities. Depending on the comonomer type or feed, hyper-branched sulfur-containing polymers are obtained which are predominantly thermoplastics with more or less good shape retention. There are, however, influencing factors, such as the gel effect (chain reduction), yielding highly viscous plastomers of the sulfur copolymer with divinylbenzene (pS-DVB), or the impact of intensive heat treatment by the hot press (post cross-linking) leading to brittle thermosettings (pS-DIB). Only high Polymers 2017, 9, 59 5 of 16 comonomer contents of 30%–35% are affected by this. pS-DIB can form at high temperatures above 100 ◦C, a highly viscous melt that enables self-healing properties, while pS-DVB exhibits a permanent Polymersnetwork, 2017 and, 9, 59 it can only soften on the surface and is more difficult to process. 5 of 16

3.1.3.1. Processing Processing TheThe processing processing of of hyper-branched hyper-branched copolymers copolymers is is challenging challenging and and requires requires more more than than just just heat. heat. TheThe copolymer copolymer lumps lumps can can be be milled milled into into a a fine fine powder powder using using an an agate agate mortar mortar and and afterwards afterwards pressed pressed intointo films films by by a a hot hot press. press. Upon Upon subsequent subsequent processing processing with with heat heat and and pressure pressure of of the the hyper-branched hyper-branched copolymer,copolymer, glassy thermosets are formed at pS-DIB with higher incorporated DIBDIB ((≥3030 wt wt %) %) by by post post ≥ cross-linking.cross-linking. ThisThis processprocess strengthens strengthens the the material. materi However,al. However, sulfur sulfur copolymers copolymers with awith higher a contenthigher contentof incorporated of incorporated vinyl monomers vinyl monomers facilitate facilitate the formation the formation of the materials of the materials into thermosets into thermosets upon thermal upon thermaltreatment. treatment. The produced The produced films (Figure films2 (Figure) can be 2) used can forbe used further for analytical further analytical methods. methods.

15 wt % 20 wt % 25 wt % 30 wt % 35 wt % pS-DVB pS-DVB

pS-DIB

FigureFigure 2. 2. IllustrationsIllustrations of of pS-DVB- pS-DVB- ( (aboveabove)) and and pS-DIB-films pS-DIB-ﬁlms ( (belowbelow)) obtained obtained by by hot hot press press treatment treatment withwith varying varying incorporated incorporated monomer content.

3.2.3.2. Structure Structure and and Surface Surface SulfurSulfur copolymers copolymers are are completely amorphous materials. Nevertheless, Nevertheless, the the polymer polymer networks cancan show show slight slight crystalline crystalline reflections reflections in in the the PXRD PXRD.. The The detected detected attenuated attenuated crystalline crystalline reflections reflections are are inin agreement agreement with with the the powder powder X-ray X-ray diffractogram diffractogram of of crystalline crystalline elemental elemental sulfur. sulfur. In In the the case case of of low low residualresidual elemental elemental sulfur sulfur in in all all sulfur sulfur copolymers copolymers,, the the respective respective monomers monomers have have not not been been converted converted completelycompletely and and the the depolymerization depolymerization occurs occurs with with a a temporal dependence. The The small small quantities quantities of of sulfur have no influence on thermal or mechanical properties. S8 can be qualitatively detected in the sulfur have no influence on thermal or mechanical properties. S8 can be qualitatively detected in firstthe firstheating heating phase phase of the of theDSC DSC measurement measurement and, and, in invery very small small amounts, amounts, in in the the SEM SEM images images and and PXRDPXRD diffractograms. diffractograms. SEM-imagesSEM-images of of sulfur sulfur copolymers copolymers show show sulfur sulfur mi microcrystalscrocrystals (Figure (Figure 33b).b). These are characterized byby an an angular angular shape shape (Figure (Figure 3b)3b) with with a aplain plain surface surface showing showing little little spherical spherical dots dots (Figure (Figure 3c).3 Thec). microcrystals form agglomerates (Figure 3a). The image shows exemplarily a section with an S8 that The microcrystals form agglomerates (Figure3a). The image shows exemplarily a section with an S 8 remainsthat remains on the on polymer the polymer and andis not is not distributed distributed ov overer the the entire entire surface surface of of the the copolymers. copolymers. This This observationobservation is is confirmed confirmed by Salman et al. [28]. [28]. The sulfur copolymer films exhibit a plain surface, partly with unevenly distributed holes to rod-shaped notches (Figure4a,d). The holes are probably the remainders of sulfur-occupied gaps (Figure4b,e). After usage of the hot press, elemental sulfur almost disappears, probably by post cross-linking (virtually full conversion of sulfur). The sulfur copolymers show thin cracks when using the hot press (Figure4c,f).

Figure 3. SEM-image of pS-DVB with 30 wt % DVB: (a) section with sulfur crystals and agglomerates; (b) sulfur crystals with spherical dots; (c) spherical dots on sulfur crystals and copolymer surface.

Polymers 2017, 9, 59 5 of 16

3.1. Processing The processing of hyper-branched copolymers is challenging and requires more than just heat. The copolymer lumps can be milled into a fine powder using an agate mortar and afterwards pressed into films by a hot press. Upon subsequent processing with heat and pressure of the hyper-branched copolymer, glassy thermosets are formed at pS-DIB with higher incorporated DIB (≥30 wt %) by post cross-linking. This process strengthens the material. However, sulfur copolymers with a higher content of incorporated vinyl monomers facilitate the formation of the materials into thermosets upon thermal treatment. The produced films (Figure 2) can be used for further analytical methods.

15 wt % 20 wt % 25 wt % 30 wt % 35 wt % pS-DVB pS-DVB

pS-DIB

Figure 2. Illustrations of pS-DVB- (above) and pS-DIB-films (below) obtained by hot press treatment with varying incorporated monomer content.

3.2. Structure and Surface Sulfur copolymers are completely amorphous materials. Nevertheless, the polymer networks can show slight crystalline reflections in the PXRD. The detected attenuated crystalline reflections are in agreement with the powder X-ray diffractogram of crystalline elemental sulfur. In the case of low residual elemental sulfur in all sulfur copolymers, the respective monomers have not been converted completely and the depolymerization occurs with a temporal dependence. The small quantities of sulfur have no influence on thermal or mechanical properties. S8 can be qualitatively detected in the first heating phase of the DSC measurement and, in very small amounts, in the SEM images and PXRD diffractograms. SEM-images of sulfur copolymers show sulfur microcrystals (Figure 3b). These are characterized by an angular shape (Figure 3b) with a plain surface showing little spherical dots (Figure 3c). The microcrystals form agglomerates (Figure 3a). The image shows exemplarily a section with an S8 that Polymersremains2017, 9 ,on 59 the polymer and is not distributed over the entire surface of the copolymers. This6 of 16 observation is confirmed by Salman et al. [28].

Polymers 2017, 9, 59 6 of 16

The sulfur copolymer films exhibit a plain surface, partly with unevenly distributed holes to rod-shapedFigure 3. notches SEM-image (Figure of pS-DVB 4a,d). with The 30 holes wt % areDVB: pr (obablya) section the with remainders sulfur crystals of sulfur-occupiedand agglomerates; gaps Figure 3. SEM-image of pS-DVB with 30 wt % DVB: (a) section with sulfur crystals and agglomerates; (Figure(b) 4b,e).sulfur After crystals usage with of spherical the hot dots;press, (c )elemental spherical dots sulfur on almostsulfur crystals disappears, and copolymer probably surface. by post cross- (b) sulfur crystals with spherical dots; (c) spherical dots on sulfur crystals and copolymer surface. linking (virtually full conversion of sulfur). The sulfur copolymers show thin cracks when using the hot press (Figure 4c,f).

Figure 4. SEM-image of a crude sulfur copolymer with holes and plain surface (a,d); crude sulfur Figure 4. SEM-image of a crude sulfur copolymer with holes and plain surface (a,d); crude sulfur copolymer with sulfur crystals (b,e) and heated copolymer with cracks; treated with the hot press copolymer with sulfur crystals (b,e) and heated copolymer with cracks; treated with the hot press (c,f). (c,f). Above pS-DVB is shown, and below pS-DIB is shown with 15 wt % vinyl monomer. Above pS-DVB is shown, and below pS-DIB is shown with 15 wt % vinyl monomer. Consequently, the structure appears slightly inhomogeneous due to residual elemental sulfur, Consequently,holes, pores and thecracks. structure The linking appears agents slightly DVB an inhomogeneousd DIB seem to be due incorporated to residual completely elemental into sulfur, holes,the pores polymer and network cracks. Thebecause linking no re agentssidual monomer DVB and is DIB detected seem in to the be gas incorporated chromatography. completely However, into the polymerlow networkdouble bonds because are available no residual (solid-state monomer NMR), is detected which show in the thegas presence chromatography. of a few non-converted However, low double bonds, so probably only one functional group of the divinyl monomers is affected. This double bonds are available (solid-state NMR), which show the presence of a few non-converted double observation of incomplete conversion of double bonds of the vinyl monomers agrees with the bonds,investigations so probably ofonly Pyun one using functional larger reaction group scales of the [10]. divinyl Despite monomers the same conditions is affected. for each This reaction observation of incompleteand thus similar conversion conversion of double values bondsof double of bonds, the vinyl other monomers effects (gel agreeseffect, post with cross-linking) the investigations have of Pyunan using additional larger influence reaction on scales the properti [10]. Despitees, possibly the samewith no conditions linear correlation. for each reaction and thus similar conversion values of double bonds, other effects (gel effect, post cross-linking) have an additional inﬂuence3.3. Thermal on the Properties properties, possibly with no linear correlation. The thermogravimetric analysis (Figure 5) shows thermal resistance up to 220 °C of all types of 3.3. Thermalsynthesized Properties sulfur copolymers. Between 200–300 °C, there is a steep, massive weight% loss which is related to the decomposition of the sulfur. By increasing the temperature, no further mass loss occurs. The thermogravimetric analysis (Figure5) shows thermal resistance up to 220 ◦C of all types of The residues of the pS-DVBs correlate with the respective incorporated monomer content. The synthesized sulfur copolymers. Between 200–300 ◦C, there is a steep, massive weight% loss which is residue of pS-DIBs seems to be independent of the incorporated monomer content. The mass residue related to the decomposition of the sulfur. By increasing the temperature, no further mass loss occurs. has a constant value of 17% ± 2%. The high residue after the decomposition indicates that presumably The residues of the pS-DVBs correlate with the respective incorporated monomer content. The residue a cross-linking network is present. of pS-DIBs seems to be independent of the incorporated monomer content. The mass residue has a constant value of 17% 2%. The high residue after the decomposition indicates that presumably a ± cross-linking network is present.

Polymers 2017, 9, 59 7 of 16 Polymers 2017, 9, 59 7 of 16

(a) (b) Figure 5. TGA-thermograms with varying incorporated monomer content: (a) pS-DVB; (b) pS-DIB. Figure 5. TGA-thermograms with varying incorporated monomer content: (a) pS-DVB; (b) pS-DIB. In comparison with the results of Pyun, the decomposition curve of pS-DVB is shifted to a higher temperatureIn comparison (+20 °C), with which the resultsindicates of Pyun,a higher the decompositiontemperature stability curve of[11]. pS-DVB The course is shifted of tothe a decompositionhigher temperature curve (+20 of the◦C), pS-DIB which indicatesis neither a highercomparable temperature nor the stability uniform [11 residue]. The courseexplainable of the withoutdecomposition any dependence curve of the of pS-DIBthe monomer is neither content. comparable The decomposition nor the uniform temperature residue explainable corresponds without to 5%any weight dependence loss of ofthe the initial monomer mass, content.when the The decomposition decomposition starts. temperature This value corresponds occurred in to all 5% samples weight atloss an of average the initial temperature mass, when of the 222 decomposition ± 9 °C (Table starts. 1). Different This value publications occurred in report all samples on similar at an averageresults abovetemperature 200 °C of the 222 decomposition9 C (Table1 ).temperature Different publications [8,11,14]. report on similar results above 200 C of ± ◦ ◦ the decomposition temperature [8,11,14]. Table 1. Decomposition temperature at 5% weight loss (TGA).

Table 1. Decomposition temperatureTerpolymer at 5% weight (monomer loss (TGA). content: 20 wt %) Monomer (wt %) pS-DVB (°C) pS-DIB (°C) Material Tg (°C) Cross-linker ratio Terpolymer (monomer content: 20 wt %) Monomer (wt %) pS-DVB (◦C) pS-DIB (◦C) 10 212 - S8 216 ± 4◦ - Material Tg ( C) Cross-linker ratio 15 231 ± 21 226 ± 11 - 216 DVB/DIB 5:15 10 212 - S 216 4 - 8 ± 1520 231 22621 ± 8 226 21511 ± 7 pS-DVB-DIB - 224 216 DVB/DIB DVB/DIB 10:10 5:15 ± ± 20 226 8 215 7 pS-DVB-DIB 224 DVB/DIB 10:10 25 ± 227 ± 8 224± ± 14 - 234 DVB/DIB 15:5 25 227 8 224 14 - 234 DVB/DIB 15:5 ± ± 3030 241 2414 ± 4 221 2217 ± 7 216216 DVB/STY DVB/STY 10:10 10:10 ± ± pS-DVB-STYpS-DVB-STY 3535 231 2313 ± 3 212 212 213 213 ± 4 4 DVB/STY DVB/STY 15:5 15:5 ± ±

Bear et al. have analyzed the leaching out in the annealing process between 300–400 °C by means Bear et al. have analyzed the leaching out in the annealing process between 300–400 ◦C by ofmeans EDS of(Energy-dispersive EDS (Energy-dispersive X-ray X-rayspectroscopy) spectroscopy) and have and have attributed attributed it to it the to the decrease decrease of of sulfur. sulfur. IndependentIndependent of of the the DIB DIB content, content, the the decay decay contains contains approximately approximately 90 90 wt wt % % sulfur. sulfur. By By means means of of the the supportedsupported quantitative quantitative XPS XPS analysis analysis (X-ray (X-ray photoele photoelectronctron spectroscopy), spectroscopy), it it was was determined determined that that the the remainingremaining sulfur sulfur does does not not alter alter significantly significantly after after annealing. annealing. In In the the remaining remaining compound, compound, organic organic sulfatesulfate groups groups (168.0 (168.0 eV) eV) were were identified, identified, including including such such species species such such as as thioethers thioethers or or disulfides disulfides (164.0 eV), C–S (163.98 eV), SOx and R–SOx–R′ (167.88 eV) as well as C=S (161.5 eV, low intensity). (164.0 eV), C–S (163.98 eV), SOx and R–SOx–R0 (167.88 eV) as well as C=S (161.5 eV, low intensity). WithWith 80% 80% of of the the sulfur sulfur signals, signals, the the C–S C–S bonds bonds have have the the highest highest quantity, quantity, even even though though they they represent represent onlyonly an an average average of of 8% 8% of of the the total total remaining remaining stru structure.cture. This This indicates indicates an an “inverse “inverse vulcanization” vulcanization” architecturearchitecture of of the the copolymer copolymer by by retention retention after after the the annealing annealing process. process. Independent Independent of of the the sample, sample, therethere is is a acritical critical amount amount of of sulfur sulfur between between 7.4%–14.1%, 7.4%–14.1%, which which remains remains after after annealing. annealing. However, However, the carbonthe carbon composition composition fluctuates fluctuates very verystrongly. strongly. Probably, Probably, more more stable stable mono- mono- or disulfide or disulfide linkages linkages are formed,are formed, which which stabilize stabilize the theremaining remaining sulfur sulfur cont content,ent, and and a ahigher higher initial initial sulfur sulfur concentration concentration preferentially forms longer polysulfide chains (S–Sn–S) between the –C–S linkages. The longer chains preferentially forms longer polysulfide chains (S–Sn–S) between the –C–S linkages. The longer chains containingcontaining S–S S–S bonds bonds tend tend to to decompose decompose at at high high temperatures temperatures under under sublimation. sublimation. TheThe observed meltingmelting point point in thein the first first heating heatin stepg instep the DSC-thermogramin the DSC-thermogram of pS-DVB of (<15pS-DVB wt % (<15DVB, wt Figure % DVB,6a) andFigure pS-DIB 6a) and (15–25 pS-DIB wt % (15–25 DIB, Figure wt %6 b)DIB, is caused Figure by 6b) small is caused amounts by ofsmall non- amounts converted of non- converted elemental sulfur. This residue does not crystallize in the cooling step, so neither Tc nor Tm appear afterwards, which shows that a complete S8-conversion has taken place after the

Polymers 2017, 9, 59 8 of 16

Polymers 2017, 9, 59 8 of 16 Polymerselemental 2017 sulfur., 9, 59 This residue does not crystallize in the cooling step, so neither Tc nor Tm appear8 of 16 thermalafterwards, treatment. which shows Tg and that other a complete transitions S8-conversion are determined has taken for all place sulfur after copolymers the thermal in treatment. the secondTg heatingthermaland other steptreatment. transitions (Figure T 6).g are and determined other transitions for all sulfurare determined copolymers for in all the sulfur second copolymers heating step in (Figurethe second6). heating step (Figure 6).

(a) (b) (a) (b) Figure 6. DSC-thermogram: (a) pS-DVB10–35; (b) pS-DIB15–35. Representation of both heating steps and Figure 6. DSC-thermogram: (a) pS-DVB10–35;(b) pS-DIB15–35. Representation of both heating steps Figure 6. DSC-thermogram: (a) pS-DVB10–35; (b) pS-DIB15–35−1 . Representation of both heating steps and ofand the of cooling the cooling step. step.Heating Heating and cooling and cooling rate: rate:10 °C 10∙minC min. 1. of the cooling step. Heating and cooling rate: 10 °C∙min◦ −·1. −

The more pronounced first step, the Tg, shows the transition from glass to plastic in the pS-DVB The more pronounced ﬁrst step, the Tg, shows the transition from glass to plastic in the pS-DVB and fromThe more brittle pronounced to thermoplastic first step, in the the pS-DIB. Tg, shows The the T gtransition increases fromby higher glass cross-linkingto plastic in the density pS-DVB or and from brittle to thermoplastic in the pS-DIB. The T increases by higher cross-linking density or molecularand from brittleweight to as thermoplastic shown in pS-DIB in the (Figure pS-DIB. 7b). The pS-DVB Tg increases (Figure by 7a) higher does notcross-linking behave accordingly. density or molecular weight as shown in pS-DIB (Figure7b). pS-DVB (Figure7a) does not behave accordingly. Frommolecular an amount weight of as 17.5 shown wt % in DVB, pS-DIB the (Figure Tg tends 7b). to pS-DVBdecrease (Figure with an 7a) in creasedoes not of addedbehave DVB accordingly. content. From an amount of 17.5 wt % DVB, the Tg tends to decrease with an increase of added DVB content. At AtFrom a lower an amount scale, anof 17.5increase wt % in DVB, Tg with the increasingTg tends to monomerdecrease withcontent an in[11]crease can ofbe added observed DVB in content. the pS- a lower scale, an increase in T with increasing monomer content [11] can be observed in the pS-DVB. DVB.At a lower This divergence,scale, an increase which ing occurs Tg with in increasingcomparison monomer to a 200 contentg scale, [11]can canbe explained be observed by inthe the chain pS- This divergence, which occurs in comparison to a 200 g scale, can be explained by the chain reduction reductionDVB. This of divergence, pS-DVB (≥ which30 wt %occurs DVB) in caused comparison by exceeding to a 200 a gte scale,mperature can be of explained 200 °C (gel by effect) the chain [3]. of pS-DVB ( 30 wt % DVB) caused by exceeding a temperature of 200 ◦C (gel effect) [3]. Likewise, in Likewise,reduction inof ≥ thepS-DVB case of (≥ a30 higher wt % DIBDVB) content, caused the by T exceedingg is higher aat te amperature lower scale of [8,10]. 200 °C (gel effect) [3]. Likewise,the case of in a the higher case DIB of a content, higher DIB the Tcontent,g is higher the atTg ais lower higher scale at a [ lower8,10]. scale [8,10].

(a) (b) (a) (b) Figure 7. Glass transition Tg of the sulfur copolymers. (a) pS-DVB; (b) pS-DIB. Figure 7. Glass transition Tg of the sulfur copolymers. (a) pS-DVB; (b) pS-DIB. Figure 7. Glass transition T of the sulfur copolymers. (a) pS-DVB; (b) pS-DIB. The Tg of the terpolymer tends to gincrease when the added styrene content is decreased. The decreaseThe ofTg theof theDIB terpolymer content in tendsthe DVB-ba to increasesed terpolymers when the added leads tostyrene an increase content of is T gdecreased. due to higher The cross-linkingdecreaseThe ofTg the ofabilities DIB the content terpolymer of DVB in in the comparison tends DVB-ba to increasesed with terpolymers DIB when monomers the leads added (Figureto an styrene increase 8). Consequently, content of Tg due is decreased. to a higher cross-linkingThe decrease densityabilities of the DIBleads of DVB content to ain higher comparison in the Tg [29]. DVB-based with The workDIB terpolymers monomers of Parker confirmed(Figure leads to 8). an thatConsequently, increase styrene of reducesT ga duehigher the to Tcross-linkinghigherg (similar cross-linking to densityDIB) due abilitiesleads to its to relatively a of higher DVB in Tlowg comparison[29]. Tg Theand workviscous, with of ParkerDIBflexible monomers confirmed ductility. (Figure thatConsequently, styrene8). Consequently, reduces it shows the propertiesTag higher (similar cross-linking ofto aDIB) plasticizer due densityto [13].its relatively leads to low a higher Tg andT gviscous,[29]. The flexible work ductility. of Parker Consequently, conﬁrmed that it styrene shows propertiesreduces the ofT ga (similarplasticizer to DIB)[13]. due to its relatively low Tg and viscous, ﬂexible ductility. Consequently, it shows properties of a plasticizer [13].

Polymers 2017,, 9,, 59 59 9 of 16 Polymers 2017, 9, 59 9 of 16

Figure 8. Glass transition of terpolymer based on pS-DVB with STY and DIB. The monomer content Figure 8. Glass transition of terpolymer based on pS-DVB with STY and DIB. The monomer content amounts to 20 wt %. amounts to 20 wt %.

The second lower transition could not be assigned, but its presence can be confirmed by the The second lower transition could not be assigned, but its presence can be confirmed by the thermogramThe second in the lower Dynamic transition Mechanical could notAnalysis be assigned, (DMA). but This its deformation presence can transition be confirmed occursby in thepS- thermogram in the Dynamic Mechanical Analysis (DMA). This deformation transition occurs in thermogramDVB as a softening in the point Dynamic and in Mechanical pS-DIB as a Analysis transition (DMA). to a viscous This deformationmelt. This occurs transition for all occurspolymers in pS-DVB as a softening point and in pS-DIB as a transition to a viscous melt. This occurs for all pS-DVBat a temperature as a softening of approx. point 131.5 and ± in1 °C. pS-DIB as a transition to a viscous melt. This occurs for all polymers at a temperature of approx. 131.5 1 ◦C. To become a viscous melt, despite the cross-linked± ◦ structure, allows pS-DIB to act as a self-heal- To become a viscous melt, despite the cross-linked structure, allows pS-DIB to act as a self-healing ing material.To become This a viscous behavior melt, occurs despite duethe to the cross-linked lower dissociation structure, energy allows pS-DIBof S–S bonds to act as(dynamic a self-healing cova- material. This behavior occurs due to the lower dissociation energy of S–S bonds (dynamic covalent material.lent bonds) This in behaviorlonger S− occursS chains due (33 to kcal the∙mol lower−1) dissociation[30]. Upon thermal energy activati of S–S bondson, the (dynamic microstructure covalent of 1 bonds) in longer S S chains (33 kcal mol− )[)[30].]. Upon Upon thermal thermal activation, activation, the the microstructure microstructure of of the the the sulfur copolymer− backbone cleaves·· and− reorganizes itself into a different macromolecular frame- sulfur copolymer backbone cleaves and reorganizes itself into a different macromolecular framework. sulfurwork. copolymerpS-DVBs exhibit backbone only cleaves a slight and softening reorganizes point itselfat high into temperatures a different macromolecular (>130 °C) without framework. a molten pS-DVBs exhibit only a slight softening point at high temperatures (>130 C) without a molten state. pS-DVBsstate. Thus, exhibit pS-DVBs only aprovide slight softeningexcellent pointproperties at high in temperaturesterms of temperature (>130 ◦C) resistance without aand molten shape state. re- Thus, pS-DVBs provide excellent properties in terms of temperature resistance and shape retention. Thus,tention. pS-DVBs The properties provide range excellent from properties thermosetting in terms regarding of temperature the cross-linking resistance to plastic and shape regarding retention. the The properties range from thermosetting regarding the cross-linking to plastic regarding the ductility. Theductility. properties range from thermosetting regarding the cross-linking to plastic regarding the ductility. 3.4. Electrical Properties 3.4. Electrical Properties All synthesized types of sulfur copolymers have exceptionally good insulating properties. All synthesized types of sulfur copolymers have exceptionally good insulating properties. The The specific contact resistance ranges from 1015–1016 Ω cm (Figure9).). Thereby, Thereby, they they provide provide similarly similarly specific contact resistance ranges from 1015–1016 Ω∙cm (Figure·· 9). Thereby, they provide similarly good good properties as previously known insulating materials, such as conventional hydrocarbons like properties as previously known insulating materials, such as conventional hydrocarbons like PMMA PMMA with 1015 Ω cm or PE, PP and PB with 1016 Ω cm [31,,32].]. The The specific specific contact contact resistance resistance of of with 1015 Ω∙cm or PE,·· PP and PB with 1016 Ω∙cm [31,32].·· The specific contact resistance of sulfur sulfur copolymers is one order of magnitude higher than that of elemental sulfur with 1015 Ω cm. copolymers is one order of magnitude higher than that of elemental sulfur with 1015 Ω∙··cm. Furthermore, sulfur copolymer materials have a much higher resistivity than the brittle elemental Furthermore, sulfur copolymer materials have a much higher resistivity than the brittle elemental sulfur that does not have any ductility. Thus, sulfur copolymers would be a good alternative for sulfur that does not have any ductility. Thus, sulfur copolymers would be a good alternative for insulatinginsulating materials materials with with high high amounts amounts of of inexpensive inexpensive elemental elemental sulfur. sulfur. Only Only materials materials such such as as PTFE PTFE insulating materials with high amounts of inexpensive elemental sulfur. Only materials such as PTFE have a higher specific contact resistance of 1017 Ω cm [33].]. have a higher specific contact resistance of 1017 Ω∙··cm [33]. The specific contact resistance of pS-DVB increases with increasing cross-linker DVB (Figure9a). The specific contact resistance of pS-DVB increases with increasing cross-linker DVB (Figure 9a). Thus, a higher DVB content increases the insulating properties. The pS-DIB does not show any Thus, a higher DVB content increases the insulating properties. The pS-DIB does not show any correlation with added DIB content and lies in the same order of magnitude (Figure9a). In this correlation with added DIB content and lies in the same order of magnitude (Figure 9a). In this connection, the terpolymer pS-DVB-DIB elucidates the correlation of the increasing specific contact connection, the terpolymer pS-DVB-DIB elucidates the correlation of the increasing specific contact resistance with an increasing DVB content (Figure9b). In contrast to this, the terpolymer pS-DVB-STY resistance with an increasing DVB content (Figure 9b). In contrast to this, the terpolymer pS-DVB- (Figure(Figure9b) refutes the insight that there is an influence on the insulating properties. Certainly, its value STY (Figure 9b) refutes the insight that there is an influence on the insulating properties. Certainly, isis similar similar and and also also the the influence influence of of the the styrene styrene (STY) (STY) could could be be higher higher than than that that of of DVB. DVB. its value is similar and also the influence of the styrene (STY) could be higher than that of DVB. The resistance of insulating sulfur copolymers against high voltage or dielectric strength is very 1 low.low. The The value value of of dielectric dielectric strength strength of of all all test test specimens specimens is is about about 9 9 kV kV mm− ,, similar similar to to the the value value 1 ·· of glass (10 kV mm− ).). Conventional Conventional polymers polymers (hydrocarbons) (hydrocarbons) have have a a dielectric dielectric strength strength of of one one to to ·· − twotwo orders orders of of magnitude magnitude higher higher than than sulfur sulfur copolymers copolymers [ [31].]. This This test test is is very very sensitive sensitive towards towards air air inclusionsinclusions and and inhomogeneity. inhomogeneity. Whether Whether these these low low values values are are due due to to the the material material itself itself or or caused caused

Polymers 2017, 9, 59 10 of 16

by the imperfections of the test specimens could not be resolved deﬁnitively. It indicates that the presence of elemental sulfur in the copolymer strongly affects its stability and the reproducibility of the results obtained by different analytical methods. If these negative side effects could be eliminated, the sulfur copolymers with STY and DVB contents in particular would have the potential to be well

Polymersinsulating 2017 materials., 9, 59 10 of 16

(a) (b)

Figure 9. Specific contact resistance of sulfur materials: (a) copolymers; (b) terpolymers. Measured after 0–3 months of synthesis.

The resistance of insulating sulfur copolymers against high voltage or dielectric strength is very low. The value of dielectric strength of all test specimens is about 9 kV∙mm−1, similar to the value of glass (10 kV∙mm−1). Conventional polymers (hydrocarbons) have a dielectric strength of one to two orders of magnitude higher than sulfur copolymers [31]. This test is very sensitive towards air inclusions and inhomogeneity. Whether these low values are due to the material itself or caused by the imperfections of the test specimens could not be resolved definitively. It indicates that the presence of elemental sulfur(a) in the copolymer strongly affects its stability( band) the reproducibility of the resultsFigure obtained9. Specific by contact different resistance analytical of sulfur methods. materials: If these (a) copolymers; negative side (b) effectsterpolymers. could Measuredbe eliminated, Figure 9. Speciﬁc contact resistance of sulfur materials: (a) copolymers; (b) terpolymers. Measured the sulfurafter 0–3 copolymers months of synthesis.with STY and DVB contents in particular would have the potential to be well insulatingafter 0–3 materials. months of synthesis. The resistance of insulating sulfur copolymers against high voltage or dielectric strength is very 3.5. Mechanical Properties low.3.5. Mechanical The value Propertiesof dielectric strength of all test specimens is about 9 kV∙mm−1, similar to the value of glass The(10 mechanicalkVmechanical∙mm−1). Conventional tests tests are are very very importantpolymers important to(hydrocarbons) characterize to characterize the ha virtuallyve athe dielectric virtually unexplored strength unexplored ductile of one properties toductile two ordersofproperties the productsof magnitude of the synthesizedproducts higher synthesized than by inverse sulfur by vulcanization. copolymersinverse vulcanization. [31]. Consequently, This Consequently,test is very it is possiblesensitive it is possible to towards tune to these tune air inclusionsmaterialsthese materials forand speciﬁc inhomogeneity. for specific applications. applications. Whether All mechanical theseAll mechanical low measurementsvalues measurements are due wereto the donewere materialwith done itself hot-pressed with or hot-pressed caused sulfur by thecopolymerssulfur imperfections copolymers because ofbecause itthe is oftentest it nearlyisspecimens often impossible nearly could impossible tonot reprocess be resolved to reprocess the crudedefinitively. the polymers crude It intopolymersindicates test specimens. thatinto thetest presenceDuctilespecimens. experiments of elementalDuctile experiments indicate sulfur in that the indicate hot-pressed copolymer that strong sulfur hot-pressedly copolymers affects sulfur its stability have copolymers better and ductile the have reproducibility properties better ductile than of thecrudeproperties results copolymers obtainedthan crude (Figure by differentcopolymers 10). They analytical show(Figure amethods. two10). toTh threeey If theseshow times negative a highertwo to elongation.side three effects times could Hence, higher be the eliminated,elongation. remolded thematerialHence, sulfur the has copolymers remolded a higher stability materialwith STY for has and different a DVBhigher contents applications. stabilit iny forparticular Attentiondifferent would shouldapplications. have be paidthe Attentionpotential to the heating toshould be well time be insulatingtopaid prevent to the materials. theheating formation time to of prevent thermosets the dueformat toion post of cross-linking. thermosets due to post cross-linking.

3.5. Mechanical Properties The mechanical tests are very important to characterize the virtually unexplored ductile properties of the products synthesized by inverse vulcanization. Consequently, it is possible to tune these materials for specific applications. All mechanical measurements were done with hot-pressed sulfur copolymers because it is often nearly impossible to reprocess the crude polymers into test specimens. Ductile experiments indicate that hot-pressed sulfur copolymers have better ductile properties than crude copolymers (Figure 10). They show a two to three times higher elongation. Hence, the remolded material has a higher stability for different applications. Attention should be paid to the heating time to prevent the formation of thermosets due to post cross-linking.

(a) (b)

Figure 10. Comparison of ductility experiments of crude and hot-pressed sulfur copolymers. The Figure 10. Comparison of ductility experiments of crude and hot-pressed sulfur copolymers. diagrams show copolymers with 15 wt % incorporated monomer content: (a) pS-DVB; (b) pS-DIB. The diagrams show copolymers with 15 wt % incorporated monomer content: (a) pS-DVB; (b) pS-DIB.

(a) (b)

Figure 10. Comparison of ductility experiments of crude and hot-pressed sulfur copolymers. The diagrams show copolymers with 15 wt % incorporated monomer content: (a) pS-DVB; (b) pS-DIB.

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TheThe ShoreShore hardnesshardness of of all sulfurall sulfur copolymers copolymers (reprocessed (reprocessed with awith hot press) a hot has press) no clear has correlation no clear correlationwith the incorporated with the incorporated monomer contentmonomer or cannotcontent be or detectedcannot be due detected to the largedue to standard the large deviations. standard deviations.The deviations The deviations of measurements of measurements are caused are by caused inhomogeneities by inhomogeneities of the sulfur of the copolymers sulfur copolymers through throughair inclusions air inclusions or the unsuccessful or the unsuccessful uniform uniform reprocessing reprocessing with the with hot the press. hot Thepress. values The values of the of Shore the Shorehardness hardness correspond correspond to those to of those soft plasticsof soft plastics or elastomers or elastomers (Figure 11(Figure). Films 11). of pS-DIBFilms of are pS-DIB relatively are relativelysofter than softer those than of pS-DVB those of (S pS-DVBHORE D (S=HORE 13–67) D below= 13–67) an below incorporated an incorporated monomer monomer content ofcontent 30 wt of %. 30Regarding wt %. Regarding the Shore the hardness, Shore thehardness, sulfur copolymerthe sulfur copolymer materials are materials softer than are conventional softer than conventional hydrocarbon hydrocarbonpolymers like polymers polystyrene like ( Dpolystyrene= 80), polymethylmethacrylate (D = 80), polymethylmethacrylate (PMMA) (D = (PMMA) 87–88), polycarbonate (D = 87–88), polycarbonate(D = 82–85), polypropylene (D = 82–85), polypropylene (D = 65–75), etc. (D [ 34= 65–75),]. etc. [34].

(a) (b)

Figure 11. Shore hardness of sulfur copolymers: (a) pS-DVB; (b) pS-DIB. Figure 11. Shore hardness of sulfur copolymers: (a) pS-DVB; (b) pS-DIB. Depending on the amount of styrene and DIB added to the DVB-based copolymer to form a terpolymer,Depending a softer on thematerial amount is produced of styrene in and cont DIBrast added to conventional to the DVB-based pS-DVB. copolymer Results of toductility form a experimentsterpolymer, ashow softer that material test isspecimens produced of in pS-DVB contrast toact conventional as ductile material pS-DVB. with Results properties of ductility of amorphousexperiments plastomers. show that test The specimens curves of of ductility pS-DVB expe act asriments ductile (Figure material 12a) with have properties a smallof elastic amorphous range andplastomers. a large plastic The curves range ofwith ductility no necking experiments area. The (Figure ductile 12 stressa) have to astretch small the elastic polymer range by and the a same large factorplastic increases range with with no the necking decrease area. of The the ductile incorporat stressed to monomer stretch the content polymer of bythe the sulfur same copolymer. factor increases This implieswith the that decrease a polymer of the incorporatedwith a lower monomerincorporated content monomer of the sulfurcontent copolymer. has a higher This stability. implies thatAt a monomerpolymer with content a lower of 30–35 incorporated wt %, the monomer polymers contentresult in has a sticky a higher and stability. viscous Atresin a monomer because acontent higher monomerof 30–35 wt content %, the implies polymers an autoacceleration result in a sticky (gel and ef viscousfect) that resin causes because a temperature a higher monomerrise above content200 °C. Theimplies released an autoacceleration exothermic energy (gel effect)decreases that the causes viscosity a temperature by reducing rise the above chain 200 length◦C. The when released the temperatureexothermic energy limit is decreases exceeded the to viscositya value above by reducing 200 °C the[3]. chainThese length materials when cannot the temperature be processed limit into is testexceeded specimens to a value to carry above out 200 mechanical◦C[3]. These measurements. materials cannot be processed into test specimens to carry out mechanicalThe E-modulus measurements. tends to decrease with an increase of added monomer content. The hardness of the exceptionalThe E-modulus copolymer tends with to decrease 15 wt % with monomer an increase content of addedcannot monomerbe explained. content. Consequently, The hardness at a monomerof the exceptional content of copolymer 12.5 wt %, with the hardness 15 wt % monomeris highest in content the plastic cannot material. be explained. The stretching Consequently, area of pS-DVBsat a monomer extends content up to of 75%. 12.5 Apparently, wt %, the hardness the fracture is highest point decreases in the plastic first material. with an increase The stretching of the incorporatedarea of pS-DVBs monomer extends content, up to 75%. and Apparently,then increases the again fracture at an point incorporated decreases monomer ﬁrst with ancontent increase of 25 of wtthe %, incorporated when the viscosity monomer of content,the products and thendecreases increases (Figure again 12b). at an incorporated monomer content of 25 wt %, when the viscosity of the products decreases (Figure 12b).

Polymers 2017, 9, 59 12 of 16 Polymers 2017, 9, 59 12 of 16

Polymers 2017, 9, 59 12 of 16

(a) (b)

Figure 12. Ductility experiments(a) of the pS-DVB specimens: (a) stress–strain(b diagram;) (b) ductility Figure 12. Ductility experiments of the pS-DVB specimens: (a) stress–strain diagram; (b) ductility parameters (E-Modulus E, Ultimate Stress σF and Fracture Point εF). Figureparameters 12. Ductility (E-Modulus experimentsE, Ultimate of Stressthe pS-DVBσF and Fracturespecimens: Point (a) εstress–strainF). diagram; (b) ductility Theparameters material (E-Modulus of pS-DIB E, Ultimate acts as Stress amorphous σF and Fracture plastomers Point ε F).with thermoplastic to duromeric properties,The materialmaterial depending of of pS-DIB onpS-DIB the acts incorporated acts as amorphous as amorphous monome plastomers rplastomers content. with thermoplasticThe with stress–strain thermo toplastic duromeric diagram to duromericof properties, pS-DIB showsproperties,depending a plastic depending on theregion incorporated withon the a necking incorporated monomer area (Figure content.monome 13a).r The content. The stress–strain material The stress–strain has diagram no elastic ofdiagram properties, pS-DIB of shows pS-DIB since a thereshowsplastic is a regionno plastic linear with region elastic a necking with range a neckingin area the (Figurestress–strain area (Figure13a). curve. The 13a). material TheThe elongation material has no has elastic increases no elastic properties, with properties, an increase since theresince of thethereis no monomer linearis no linear elastic content elastic range added range in the toin stress–strain the stress–strainsulfur copoly curve. curve.mer The (Figure elongationThe elongation 13b). increases At increases an incorporated with with an increase an increasemonomer of the of contentthemonomer monomer of 30–35 content content wt added %, mostadded tothe polymers to sulfur the copolymersulfurbecome copoly brittl (Figuremere thermosets (Figure13b). At 13b).(post an incorporated Atcross-linking an incorporated monomer at the hot monomer content press), of whichcontent30–35 wtbreak of %, 30–35 mostunder wt polymers %,mechanical most become polymers stress, brittle become for thermosets instance brittl,e during (postthermosets cross-linking the process(post cross-linking at of the making hot press), atspecimens. the which hot press), break The stretchingwhichunder mechanicalbreak area under of stress,pS-DIB mechanical for extend instance, stress,s up during to for 432%. instance the The process, fractureduring of making the point process specimens.increases of making continuously The stretching specimens. with area The an of increasestretchingpS-DIBextends of areaincorporated upof pS-DIB to 432%. monomer extend The fractures contentup to point 432%. until increases thermosettingThe fracture continuously point cond itionsincreases with anareincrease reached.continuously of incorporated with an increasemonomer of content incorporated until thermosetting monomer content conditions until thermosetting are reached. conditions are reached.

(a) (b)

Figure 13. Ductility experiments(a) of the pS-DIB specimens: (a) stress–strain( bdiagram;) (b) ductility

parameters (E-Modulus E, Ultimate Stress σF and Fracture Point εF). Figure 13. Ductility experiments of the pS-DIB specimens: (a) stress–strain diagram; (b) ductility Figure 13. Ductility experiments of the pS-DIB specimens: (a) stress–strain diagram; (b) ductility Pyunparameters et al. (performedE-Modulus Etensile, Ultimate tests Stress based σF and on FracturepS-DIB withPoint 20εF). and 30 wt % monomer content. If parameters (E-Modulus E, Ultimate Stress σF and Fracture Point εF). their Pyunresults et are al. comparedperformed to tensile the results tests based obtained on pS-DIBin this work,with 20 the and required 30 wt %stress monomer is two content. orders of If magnitude higher and the E-modulus is one order of magnitude higher than in the tensile their Pyunresults et are al. performedcompared tensileto the testsresults based obtained on pS-DIB in this with work, 20 and the 30 required wt % monomer stress is content. two orders If their of experiments of pS-DIB in this work (Figure 13). These values indicate a material with higher stiffness, magnituderesults are compared higher and to the the results E-modulus obtained is in one this work,order theof requiredmagnitude stress higher is two than orders in of the magnitude tensile and, consequently, a harder material. A monomer content of 20 wt % shows a comparable elongation experimentshigher and the of EpS-DIB-modulus in this is one work order (Figure of magnitude 13). These higher values than indicate in the a material tensile experiments with higher of stiffness, pS-DIB (100%–250%). However, it is worth mentioning that the elongation of pS-DIB in this work is two and,in this consequently, work (Figure a harder13). These material. values A indicatemonomer a content material of with 20 wt higher % shows stiffness, a comparable and, consequently, elongation orders of magnitude higher for a monomer content of 30 wt % [35] in the event that it is not cross- (100%–250%).a harder material. However, A monomer it is worth content mentioning of 20 wt that % showsthe elongation a comparable of pS-DIB elongation in this (100%–250%). work is two linked to a thermoset by the processing with the hot press. The curve shape of pS-DIB with 20 wt % ordersHowever, of magnitude it is worth mentioninghigher for a that monomer the elongation content of of pS-DIB 30 wt in% this[35] workin the is event two orders that it of is magnitudenot cross- corresponds to thermoplastics and the copolymer with 30 wt % DIB corresponds to thermosets [35]. linkedhigher forto a a thermoset monomer contentby the proces of 30 wtsing % [with35] in the the hot event press. that The it is curve not cross-linked shape of pS-DIB to a thermoset with 20 bywt the % correspondsIn comparison, to thermoplastics the pS-DIB andis more the copolymerductile than wi theth pS-DVB,30 wt % DIBsince corresponds DIB-based copolymers to thermosets require [35]. 10 timesIn comparison, less ductility the stress pS-DIB for theis more same ductile elongati thanon thethan pS-DVB, DVB-based since copolymers. DIB-based copolymers An increase require of the 10 times less ductility stress for the same elongation than DVB-based copolymers. An increase of the

Polymers 2017, 9, 59 13 of 16

processing with the hot press. The curve shape of pS-DIB with 20 wt % corresponds to thermoplastics and the copolymer with 30 wt % DIB corresponds to thermosets [35]. In comparison, the pS-DIB is more ductile than the pS-DVB, since DIB-based copolymers require 10 times less ductility stress for the same elongation than DVB-based copolymers. An increase Polymers 2017, 9, 59 13 of 16 of the DIB content in the poly(S-r-DVB-r-DIB) terpolymer (pS-DVB-DIB) results in an increase of Polymers 2017, 9, 59 13 of 16 DIBthe elongationcontent in andthe poly(S- of the ultimater-DVB-r-DIB) stress terpolymer as well as of(pS-DVB-DIB) the E-modulus results (Figure in an 14 b).increase STY actsof the as a plasticizer (like DIB) and increases the elongation (Figure 14a) when added to the DVB-based elongationDIB content and in of the the poly(S-ultimater-DVB- stressr as-DIB) well terpolymer as of the E-modulus (pS-DVB-DIB) (Figure results 14b). STY in actsan increase as a plasticizer of the terpolymer. The terpolymers contain a total of 20 wt % of incorporated vinyl comonomers with (likeelongation DIB) and and increases of the ultimate the elongation stress as well(Figure as of 1 4a)the whenE-modulus added (Figure to the 14b).DVB-based STY acts terpolymer. as a plasticizer The varying ratios. terpolymers(like DIB) and contain increases a total the of elongation 20 wt % of (Figure incorporated 14a) when vinyl added comonomers to the DVB-based with varying terpolymer. ratios. The terpolymers contain a total of 20 wt % of incorporated vinyl comonomers with varying ratios.

(a) (b)

Figure 14. Ductility (experimentsa) of terpolymer specimens: (a) stress–strain(b diagram;) (b) ductility Figure 14. Ductility experiments of terpolymer specimens: (a) stress–strain diagram; (b) ductility parameters (E-Modulus E, Ultimate Stress σF and Fracture Point εF). Constant total incorporated Figureparameters 14. Ductility (E-Modulus experimentsE, Ultimate of terpolymer Stress σF and specimens: Fracture Point(a) stress–strainεF). Constant diagram; total incorporated(b) ductility monomer content: 20 wt %. parametersmonomer content: (E-Modulus 20 wt %.E, Ultimate Stress σF and Fracture Point εF). Constant total incorporated Bymonomer the incorporation content: 20 wt %.of a divinylbenzene network, the mechanical strength values are significantlyBy thethe incorporation increasedincorporation and of the aof divinylbenzene ductilitya divinylbenzene is decreased. network, network, theRegarding mechanical the themechanical strengthmechanical valuesstrength properties, are signiﬁcantlyvalues sulfur are copolymersignificantlyincreased andmaterials increased the ductility are and softer the is than decreased. ductility conventional is Regardingdecreased. polymers. theRegarding mechanical They are the characteristic mechanical properties, forproperties, sulfur low copolymertoughness sulfur andcopolymermaterials strength are materials and softer high are than ductility softer conventional than [31]. conventional polymers. polymers. They are They characteristic are characteristic for low for toughnesslow toughness and andstrength strengthThe DMA and and high plots high ductility of ductilitypS-DVB [31]. and[31]. pS-DIB (Figure 15) hardly differ in a sulfur copolymer. Only the three Therespective DMA plots graphs of pS-DVB are shifted and pS-DIB in the (Figure respec tive15)15) hardly direction differ (lower in a sulfur or higher copolymer. temperature), Only the dependingthree respectiverespective on the graphs graphs Tg. Moreover, are are shifted shifted inthe the interminal respective the respec plateau directiontive ofdirection (lowerG’ increases or(lower higher with or temperature), higherhigher temperature),incorporated depending comonomer content, which implies a higher cross-linking degree in the form of hyper-branching. dependingon the Tg. Moreover, on the T theg. Moreover, terminal plateau the terminal of G’ increases plateau with of G’ higher increases incorporated with higher comonomer incorporated content, comonomerwhich implies content, a higher which cross-linking implies a degreehigher incross-linking the form of degree hyper-branching. in the form of hyper-branching.

(a) (b)

Figure (15.a) DMA diagrams of sulfur copolymers: (a) pS-DIB; (b() bpS-DVB.) Figure 15. DMA diagrams of sulfur copolymers: (a) pS-DIB; (b) pS-DVB. Above a temperatureFigure 15. ofDMA approximately diagrams of sulfur −10 °C, copolymers: the glassy (a) state pS-DIB; of (theb) pS-DVB. materials ends and the glass Abovetransition a temperature range begins of withapproximately a sharp increase −10 °C, of the the glassy loss modulus state of Gthe”. Here,materials the copolymerends and the is increasinglyglass transition losing range its elasticbegins propertieswith a sharp and increase instead ofa greaterthe loss conversion modulus G of”. mechanicalHere, the copolymer energy into is heatincreasingly energy takes losing place. its elastic This resultsproperties in a andsimultaneous instead a greaterdecrease conversion in the storage of mechanical modulus Genergy’. The lossinto modulusheat energy reaches takes its place. maximum This results in the inglass a simultaneous transition range. decrease This maximumin the storage results modulus from the G’. required The loss energymodulus to reachesincrease its the maximum chain mobility in the (molecularglass transition friction range. processes), This maximum which is results irreversible. from the required energyThe to maxima increase of the G” chain can be mobility used as (molecular reference points friction to processes), determine whichrelaxations. is irreversible. This applies also to the maximumThe maxima of tanof G δ” can(loss be factor). used as Thus, reference the first points maximum to determine of G” relaxations. is exhibited This in appliesthe Tg, whichalso to the maximum of tan δ (loss factor). Thus, the first maximum of G” is exhibited in the Tg, which

Polymers 2017, 9, 59 14 of 16

Above a temperature of approximately 10 C, the glassy state of the materials ends and the − ◦ glass transition range begins with a sharp increase of the loss modulus G”. Here, the copolymer is increasingly losing its elastic properties and instead a greater conversion of mechanical energy into heat energy takes place. This results in a simultaneous decrease in the storage modulus G’. The loss modulus reaches its maximum in the glass transition range. This maximum results from the required energy to increase the chain mobility (molecular friction processes), which is irreversible. PolymersThe 2017 maxima, 9, 59 of G” can be used as reference points to determine relaxations. This applies also14 to of the 16 maximum of tan δ (loss factor). Thus, the ﬁrst maximum of G” is exhibited in the Tg, which corresponds tocorresponds the results to of the results DSC. The of the second DSC. weak The maximumsecond weak of G” maximumdeviates of approximately G” deviates approximately by 30–40 ◦C from by the30–40 secondary °C from transitionthe secondary (β-relaxation) transition in ( theβ-relaxation) DSC thermogram. in the DSC However, thermogram. the β-relaxation However, (Gthe”) ofβ- therelaxation pS-DIBs (G cannot”) of the be pS-DIBs determined cannot due be to determined impending du softeninge to impending of the material softening (viscous of the polymermaterial melt).(viscous The polymer results obtainedmelt). The by results DSC are obtained in agreement by DSC with are those in agreement of the DMA with regarding those of the theTg .DMAG’ is steadilyregarding decreasing the Tg. G’ (sigmoidal is steadily decrease) decreasing throughout (sigmoidal the decrea test periodse) throughout and has a turningthe test period point in and the has glass a transitionturning point range. in Thethe glass course transition of G’ in the range. terminal The course plateau of suggests G’ in the a lowterminal altitude, plateau and, consequently,suggests a low a lowaltitude, cross-linking and, consequently, polymer network, a low cross-linking and is conﬁrmed polymer by thenetwork, results and of the is confirmed stress–strain by experiments.the results of the stress–strainFinally, the experiments. sulfur copolymers can be subdivided into different classes regarding their thermomechanicalFinally, the sulfur behavior. copolymers The most can noticeable be subdivided behavior caninto bedifferent observed classes above anregarding incorporated their monomerthermomechanical content ofbehavior. 30 wt %,The in mo whichst noticeable case a dramatic behavior changecan be observed of the mechanical above an propertiesincorporated of bothmonomer sulfur content materials of 30 occurs wt %, (Figure in which 16 ).case In a the dramatic pS-DVB, change the high of the exothermicity, mechanical properties which is released of both atsulfur a high materials monomer occurs content (Figure ( 16).30 In wt the %), pS-DVB, is responsible the high for exothermicity, the property which change. is released In the at pS-DIB, a high ≥ however,monomer cross-linkingcontent (≥30 wt continues %), is responsible upon thermal for the treatment property inchange. the hot In press,the pS-DIB, whereupon however, a glassycross- thermosettinglinking continues occurs. upon thermal treatment in the hot press, whereupon a glassy thermosetting occurs.

Figure 16. Scheme of the dependence between polymerpolymer type and incorporated monomer content.

4. Conclusions This report showsshows processableprocessable sulfursulfur copolymerscopolymers withwith exceptionalexceptional mechanicalmechanical and insulatinginsulating properties. The The copolymers copolymers exhibit exhibit different different degrees degrees of ofductility ductility with with an adjustable an adjustable Tg betweenTg between −22– 22–22 C as well as a specific contact resistance of more than one order of magnitude in comparison −22 °C as ◦well as a specific contact resistance of more than one order of magnitude in comparison with withelemental elemental sulfur. sulfur. The Theproduct product can canbe beobtained obtained via via a astraight-forward straight-forward synthetic synthetic route route (bulk polymerization) underunder simple simple conditions, conditions, called called inverse inverse vulcanization. vulcanization. Furthermore, Furthermore, all of the all products of the areproducts translucent are translucent and hydrophobic. and hydrophobic. A wide range of polymer properties is covered. Poly(S- r-DVB) copolymers with 12.5%–30% DVB possess amorphous and and plastomeric plastomeric pr propertiesoperties with with slight slight elasticity elasticity and and a high a high shape shape retention. retention. A Ahigher higher DVB DVB content content between between 30%–35% 30%–35% results results in lo inw lowviscose, viscose, waxy waxy plastome plastomersrs with witha high a highflow flowbehavior. behavior. The properties The properties of the ofpoly(S- the poly(S-r-DIB)r -DIB)copolymers copolymers range rangefrom flexible from flexible thermoplastic thermoplastic (15%– (15%–30%30% DIB) to DIB) brittle to brittlethermosetting thermosetting (30%–35% (30%–35% DIB). Th DIB).e divergent The divergent properties properties of the respective of the respective sulfur sulfurcopolymers copolymers with a withhigh across-linker high cross-linker content content (≥30%) ( are30%) evoked are evoked by effects, by effects, such as such the gel as theeffect gel (chain effect ≥ (chainreduction reduction by high by highexothermicity) exothermicity) in the in thepoly(S- poly(S-r-DVB)r-DVB) copolymer copolymer and and thermal thermal post-crosslinking post-crosslinking (processing(processing at thethe hothot press)press) inin thethe poly(S-poly(S-rr-DIB)-DIB) copolymer.copolymer. By additionaddition of a secondsecond cross-linker to the comonomers,comonomers, the ductility can be influenced.influenced. A higher DVB content leads to a higherhigher strengthstrength and shape retention. The The addition addition of of DIB DIB or or STY STY so softensftens up up the the material or or makes makes it it more more flexible. flexible. The material properties dependdepend largelylargely onon thethe contentcontent andand onon thethe typetype ofof incorporatedincorporated monomer.monomer. The processing of the crude sulfur copolymer with the hot press improves the material properties significantly regarding mechanical deformation, and it induces post cross-linking of the remaining elemental sulfur. Therefore, it does not have to be removed separately. After the treatment, the shape retention of the sulfur material was clearly improved. In all polymers, an incorporated monomer content of 15 wt % yields the best results concerning the visual properties, the ductility and the processing. These novel polymers based on sulfur are potentially very useful materials, resulting in a multitude of new possibilities for the application of sulfur, like thermal or electrical insulation materials for underground cable systems or primary and secondary windings in transformers, optical lenses, seals in civil engineering, etc.

Polymers 2017, 9, 59 15 of 16

The processing of the crude sulfur copolymer with the hot press improves the material properties signiﬁcantly regarding mechanical deformation, and it induces post cross-linking of the remaining elemental sulfur. Therefore, it does not have to be removed separately. After the treatment, the shape retention of the sulfur material was clearly improved. In all polymers, an incorporated monomer content of 15 wt % yields the best results concerning the visual properties, the ductility and the processing. These novel polymers based on sulfur are potentially very useful materials, resulting in a multitude of new possibilities for the application of sulfur, like thermal or electrical insulation materials for underground cable systems or primary and secondary windings in transformers, optical lenses, seals in civil engineering, etc.

Acknowledgments: The authors thank G. Baumgarten of ELANTAS Europe GmbH (Hamburg, Germany). The authors appreciate permission to use Figure1 from David Dodge (The Pembina Institute, Calgary, AB, Canada, http://www.pembina.org/user/david-dodge). Author Contributions: Sergej Diez and Alexander Hoefling designed the experiments, performed the experiments and analyzed the data. Sergej Diez and Werner Pauer wrote the manuscript. Werner Pauer and Patrick Theato contributed to the discussion. Conflicts of Interest: The authors declare no conflict of interest.

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