Inductive Heating Using a High-Magnetic-Field Pulse to Initiate Chemical Reactions to Generate Composite Materials

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Article Inductive Heating Using a High-Magnetic-Field Pulse to Initiate Chemical Reactions to Generate Composite Materials

Cordelia Zimmerer 1,* , Catalina Salazar Mejia 2, Toni Utech 1, Kerstin Arnhold 1, Andreas Janke 1 and Joachim Wosnitza 2,3

1 Leibniz Institute of Polymer Research Dresden e.V., Polymer Materials, Reactive Processing, 01069 Dresden, Germany; [email protected] (T.U.); [email protected] (K.A.); [email protected] (A.J.) 2 Hochfeld-Magnetlabor Dresden (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany; [email protected] (C.S.M.); [email protected] (J.W.) 3 Institute of Solid State and Materials Physics, Electronically Correlated Matter, Dresden University of Technology, 01062 Dresden, Germany * Correspondence: [email protected]

Received: 5 February 2019; Accepted: 8 March 2019; Published: 21 March 2019

Abstract: Induction heating is efficient, precise, cost-effective, and clean. The heating process is coupled to an electrically conducting material, usually a metal. As most polymers are dielectric and non-conducting, induction heating is not applicable. In order to transfer energy from an electromagnetic field into polymer induction structures, conducting materials or materials that absorb the radiation are required. This report gives a brief overview of induction heating processes used in polymer technology. In contrast to metals, most polymer materials are not affected by electromagnetic fields. However, an unwanted temperature rise of the polymer can occur when a radio frequency field is applied. The now available high-field magnetic sources provide a new platform for induction heating at very low frequencies, avoiding unwanted thermal effects within the material. Using polycarbonate and octadecylamine as an example, it is demonstrated that induction heating performed by a magnetic-field pulse with a maximum flux density of 59 T can be used to initiate chemical reactions. A 50 nm thick Ag loop, with a mean diameter of 7 mm, placed in the polymer-polymer interface acts as susceptor and a resistive heating element. The formation of urethane as a linker compound was examined by infrared spectroscopic imaging and differential scanning calorimetry.

Keywords: induction heating; high-magnetic-field; polycarbonate; bonding polymers; susceptor material

1. Introduction

1.1. Induction Heating for Materials Processing Induction heating (IH) is used in industry, medicine, household, et cetera [1–40]. As non-ionizing radiation it is applied to trigger or to control physical processes and chemical reactions (see Figure1). IH provides a high efﬁciency because electrically conductive materials are internally heated by induced currents.

Polymers 2019, 11, 535; doi:10.3390/polym11030535 www.mdpi.com/journal/polymers Polymers 2019, 11 FOR PEER REVIEW 2 of 17 Polymers 2019, 11, 535 2 of 17

Figure 1. Different applications of induction heating for various materials and their processing temperatures.Figure 1. Different applications of induction heating for various materials and their processing temperatures. In applying IH, no direct contact, e.g., to heating plates, is required [11]. IH can be considered as an alternativeIn applying technology IH, no direct to thermal contact, or e.g., radiation to heating processing plates, is of required plastics [11]. and polymersIH can be [considered16]. Further as advantages,an alternative in technology polymer chemistry to thermal in particular,or radiation are pr environmentalocessing of plastics sustainability, and polymers the reduction[16]. Further or completeadvantages, exclusion in polymer of solvents chemistry needed in for particular, the curing ar ofe environmental coatings [16,28 ],sustainabi and axinglity, the the use reduction of a catalyst. or Thiscomplete is caused exclusion by reaching of solvents a high needed temperature for the very curi quickly,ng of coatings a high productivity, [16,28], and andaxing a reductionthe use of of a cyclecatalyst. times This [24 ].is IHcaused provides by reaching great process a high variability, temperature it allows very quickly, heating ofa high opaque productivity, composites and [16 ],a forreduction joining complexof cycle componentstimes [24]. [IH34 ],provides and the jointsgreat are process ready-to-use variability, after processing.it allows heating In comparison of opaque to conventionalcomposites heating[16], for methods joining appliedcomplex in polymercomponents synthesis, [34], IHand leads the to joints similar are results ready-to-use for conversion after rates,processing. polymerization In comparison degree, to andconventional glass transition heating temperature methods applied of the formedin polymer polymer synthesis, and theIH finalleads adhesionto similar of results joints for [20 ,conversion24]. rates, polymerization degree, and glass transition temperature of the formedThe polymer rate of heating and the depends final adhesion upon the of frequencyjoints [20,24]. of the induced current. According to Faraday’s law ofThe induction, rate of heating the induced depends current upon is the correlated frequency to of the the frequency. induced current. Consequently, According high-frequency to Faraday’s electromagneticlaw of induction, fields, the suchinduced as microwaves, current is correlat are idealed forto the induction frequency. processes. Consequently, On the one high-frequency hand, in [41] aelectromagnetic microwave-based fields, rapid such polymer as microwaves, processing techniqueare ideal for is demonstrated induction processes. to initiate On chemical the one reactions hand, in and[41] acceleratea microwave-based reaction kinetics, rapid polymer to change processing the diffusional technique behavior, is demonstrated and to generate to initiate gradients chemical in polymerreactions layers. and accelerate On the other reaction hand, thekinetics, selective to degradationchange the ofdiffusional polymers wasbehavior, successfully and to applied generate to generategradients ordered in polymer crystalline layers. frameworks On the byother microwave hand, the heating selective [42]. degradation of polymers was successfullyOther IH applied approaches to generate to plastics ordered and crysta compositelline frameworks materials are by basedmicrowave on thermoset heating [42]. curing and thermoplasticOther IH bonding approaches or welding. to plastics Other and types composite of on-command materials reactionsare based comprise on thermoset polymerizations curing and andthermoplastic reactions bonding in heterogenic or welding. systems. Other types Thus, of the on-command spatiotemporal reactions control comprise of IH polymerizations makes it an interestingand reactions technology in heterogenic for traditionally systems. Thus, engineered the spat materials,iotemporal especially control of forIH temperature-sensitivemakes it an interesting substratestechnology [9 ,for21, 28traditionally], e.g., paper, engineered wood, human materials, tissue, hydrogels,especially plastics,for temperature-sensitive and natural fiber reinforcedsubstrates composites.[9,21,28], e.g., The paper, ability wood, for internally humanapplied tissue, andhydr localizedogels, plastics, heat, with and high natural gradients, fiber allowsreinforced for processingcomposites. at The room ability temperature for internally and alsoapplied for thickand localized samples heat, by avoiding with high large gradients, thermal allows stresses, for degradation,processing at or room deformation temperature resulting and inalso adhesive for thick failures. samples Another by avoiding benefit large of IH thermal is based stresses, on the possibilitydegradation, to detach or deformation and reassemble resulting hybrid in adhesive joints at thefailures. end of Another their product benefit lifetime of IH [is29 ,based34]. on the possibilityThe aim to ofdetach this study and reassemble is to demonstrate hybrid joints the potential at the end of electromagneticof their product lifetime field pulses [29,34]. with very high magneticThe aim of flux this densities study is to to join demonstrate different polymers the pote withoutntial of inducedelectromagnetic thermal field effects pulses in the with polymer very bulkhigh phases.magnetic As flux most densities applications to join of electromagneticdifferent polymers fields without in polymer induced technology thermal areeffects based in onthe high-frequencypolymer bulk fields,phases. general As most physical applications relations of and electromagnetic selected examples fields of in established polymer approachestechnology areare firstbased highlighted on high-frequency in the following fields, two general sub sections. physical relations and selected examples of established approaches are first highlighted in the following two sub sections.

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1.2. Interaction of Polymers with Electromagnetic Radiation Common polymers are dielectric materials as they do not have any free charges. All electrons are bound or associated with atoms. However, when an electromagnetic field is applied, a slight displacement of the bound electrons is induced. The displacement creates small electric dipoles within the dielectric material. This process is called polarization where negative charges are displaced to the opposite direction of positive charges. Peter Debye developed a model describing the interaction of an electromagnetic field with dielectric media. According to the Debye model, polar entities of the dipoles tend to align parallel to the direction of the applied electromagnetic field. At lower frequencies, the dipoles have ample time to follow the vector of the electromagnetic field, generating a new electromagnetic wave. When the frequency increases, the dipoles become more and more unable to fully restore their original orientation. Thus, energy is absorbed by the dielectric material, as long as the electromagnetic field enforced motions of the polar entities take place. As charges are displaced from their equilibrium position within the molecule, polarization is related to the transfer of energy. How the dielectric material is polarized can be measured by the electric susceptibility (Xe). The ability of the material to polarize reduces the electric field inside the material. The permittivity (ε) refers to the ability of the material to permit an electromagnetic field. The complex permittivity, also called dielectric constant, characterizes the interaction of an electromagnetic field with the material and is expressed as follows: ε = ε0 − iε00 . (1)

While the real part of the dielectric constant (ε0) describes the propagation of the field, the imaginary part (ε”) relates to the energy absorbed by the material. The imaginary part is also often called “loss factor”. Polar polymers contain a relatively high number of permanent dipoles leading to a high loss factor. Non-polar polymers have a low loss factor. However, it should be noted that these polarization effects are relevant at frequencies between 0 Hz (steady field) and approximately 1012 Hz. At higher frequencies the permanent dipoles can no longer follow the frequency of the electromagnetic field and only electron distortion polarization is possible, referred to as infrared absorption. For most polymer materials, ε0 decreases monotonically from a high value at 0 Hz to lower values towards 1012 Hz. The loss factor (ε”) is zero at 0 Hz and at far infrared frequencies (>1012 Hz), showing a maximum at an intermediate frequency governed by the relaxation time. The behavior is the reason for heating when a high frequency (f = 106–109 Hz) electromagnetic field interacts with a polymer material. Materials that are used to absorb electromagnetic energy are often denoted as susceptors. Usually, susceptors convert the electromagnetic energy in thermal energy and are often used as a heating component for microwaves. Susceptors are an ideal transducer if a small volume within a polymer material has to be heated, e.g., to trigger a reaction or to modify specific material properties. A selection of the most common susceptors, used in the different fields of polymer synthesis and processing, are represented in Figure2. For high-frequency applications, metallic additives, metal meshes, ferro- or ferrimagnetic (nano) particles, and semiconducting carbon fiber fabrics [9] are applied. In all susceptors an external electromagnetic field induces a current, resulting in resistance heating. The frequencies are in the kHz and MHz range in order to induce energy. Recently, a low frequency high-magnetic-field (HF) pulse (59 T) was also successfully applied to induce a current in a thin metallic loop and to trigger a chemical reaction between Bisphenol-A based polycarbonate (PC) and a polyvinylamine [21,35]. Polymers 2019, 11, 535 4 of 17 Polymers 2019, 11 FOR PEER REVIEW 4 of 17

Figure 2. Different designs and materials used as susceptors (not(not toto scale).scale).

For particulate susceptors forming a percolating network, the contact resistance at the particle For particulate susceptors forming a percolating network, the contact resistance at the particle or fiber junctions can be the dominating heating effect [9]. Important properties of the susceptor or fiber junctions can be the dominating heating effect [9]. Important properties of the susceptor are are the (a) electrical and thermal conductivity, (b) heat capacity, (c) form and dimension, and (d) the (a) electrical and thermal conductivity, (b) heat capacity, (c) form and dimension, and (d) surface surface morphology. Graphene might be a well-suited candidate as particulate susceptor, especially morphology. Graphene might be a well-suited candidate as particulate susceptor, especially for for structures with a very low thickness and the need for transparency [43,44]. However, currently structures with a very low thickness and the need for transparency [43,44]. However, currently available graphene structures are not conductive enough to induce sufficient energy [45]. The overall available graphene structures are not conductive enough to induce sufficient energy [45]. The overall heating properties of particulate embedded susceptors are difficult to predict because these various heating properties of particulate embedded susceptors are difficult to predict because these various factors, (a) to (d), are related to each other by complex behavior [2]. A poor adhesion with the factors, (a) to (d), are related to each other by complex behavior [2]. A poor adhesion with the surrounding polymer can create defects within the polymer and lead to residual stress at the interfaces surrounding polymer can create defects within the polymer and lead to residual stress at the and, probably, excessive deformation [32]. interfaces and, probably, excessive deformation [32]. Table1 summarizes susceptor materials, energy conversion in electromagnetic fields, and thermal Table 1 summarizes susceptor materials, energy conversion in electromagnetic fields, and processes. The listed heating effects depend on the applied frequency of the electromagnetic field. thermal processes. The listed heating effects depend on the applied frequency of the electromagnetic Some effects, e.g., the ring effect, depend on the experimental set-up, the distance between sample and field. Some effects, e.g., the ring effect, depend on the experimental set-up, the distance between the type of coil. On the other hand, the skin effect has not been considered for low frequencies. For HF sample and the type of coil. On the other hand, the skin effect has not been considered for low IH applications, loops are applied as susceptors and placed into the homogeneous field of the coil bore frequencies. For HF IH applications, loops are applied as susceptors and placed into the volume. This sample-coil set-up results in negligible proximity and ring effect. homogeneous field of the coil bore volume. This sample-coil set-up results in negligible proximity and ring effect. Table 1. Thermal processes in susceptor materials.

Heating EffectTable 1. Thermal processes Susceptor in Material susceptor materials. Remarks Literature

resistive heatingHeating Effect eddy current Susceptor electrically Material conductive Remarks Literature [2,6,21 ] local dielectric particulate, electrically various factors in junctionresistive heating [2] eddy currentheating electrically conductiveconductive complex relationship [2,6,21] heating particulate, ferro- and various factors in magnetic polarization hysteresis heating [3] junction local dielectric particulate,ferrimagnetic electrically variouscomplex factors relationship in [2] heating heatingcurrent flow at conductive complex relationship skin effect electrically conductive increases with frequency [5,46] magnetic hysteresisouter surfaceparticulate, ferro- and various factors in [3] polarization heating ferrimagnetic complexincreases relationship with frequency, proximity effect current crowding electrically conductive thermal hot spot [47] current flow at skin effect electrically conductive increases withformation frequency [5,46] outer surface defined by susceptors edge effect diffraction on arras electrically conductive [9] increases withgeometry frequency, proximity current electrically conductive thermalinhomogeneous hot spot field, [47] variation in field ringeffect effect crowding all susceptor materials which depends on the [2,47] line density formation diffraction on definedcoupling by susceptors distance edge effect electrically conductive [9] arras geometry 1.3. Chemical Reactionsvariation at Polymer-Polymer in Interfaces inhomogeneous field, ring effect field line all susceptor materials which depends on the [2,47] The term reaction promoter is used if IH is utilized to initiate chemical reactions surrounding the density coupling distance susceptor [2]. To the best of our knowledge, polymerization reactions for curing processes are the most

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1.3. Chemical Reactions at Polymer-Polymer Interfaces

PolymersThe2019 term, 11, reaction 535 promoter is used if IH is utilized to initiate chemical reactions surrounding5 of 17 the susceptor [2]. To the best of our knowledge, polymerization reactions for curing processes are the most common fields to initiate a chemical reaction by IH. The polymerization reaction can be common ﬁelds to initiate a chemical reaction by IH. The polymerization reaction can be performed performed to cure high-temperature thermosets [11,16] as well as for polymerization reactions at to cure high-temperature thermosets [11,16] as well as for polymerization reactions at relatively low relatively low temperatures [15–17,26,27]. A further application was published about the temperatures [15–17,26,27]. A further application was published about the decomposition of an organic decomposition of an organic compound to release nitrogen gas [18]. A reactive joining to form a compound to release nitrogen gas [18]. A reactive joining to form a polymer hybrid component [48] is polymer hybrid component [48] is based on the triggering of a chemical reaction by IH in HF. The based on the triggering of a chemical reaction by IH in HF. The reactive joining and hybrid formation reactive joining and hybrid formation can be divided into the following five process steps: (i) can be divided into the following ﬁve process steps: (i) Susceptor placement, (ii) electromagnetic Susceptor placement, (ii) electromagnetic induction, (iii) heating, (iv) chemical reaction, and (v) induction, (iii) heating, (iv) chemical reaction, and (v) cooling. A contact between the polymers is cooling. A contact between the polymers is required [34]. An interpenetration of polymer chains can required [34]. An interpenetration of polymer chains can support the formation of a compact interphase. support the formation of a compact interphase. Another aspect concerning the triggering of chemical Another aspect concerning the triggering of chemical reactions is realized in the debonding processes reactions is realized in the debonding processes or a reassembling to detach joined materials and or a reassembling to detach joined materials and composites by a thermal degradation reaction [29,49]. composites by a thermal degradation reaction [29,49]. A common thermoplastic, PC, has found wide-spread applications as a cheap, transparent, A common thermoplastic, PC, has found wide-spread applications as a cheap, transparent, and and nonpolar plastic material. Its molecular structure makes it a well-suited candidate for reactive nonpolar plastic material. Its molecular structure makes it a well-suited candidate for reactive joining. The carbonate groups can act as a nucleophilic reaction center for the attack by different joining. The carbonate groups can act as a nucleophilic reaction center for the attack by different functional and electrophilic groups. Selected compound classes, which are reactive to PC, and newly functional and electrophilic groups. Selected compound classes, which are reactive to PC, and newly formed coupling groups of products are shown in Figure3. formed coupling groups of products are shown in Figure 3.

H R O O N O R R N O R O O O +R-OH +R-NH2 + R-NH-R +R-ONa

O O

HO +NH2-NH2 +H2O n H O N O +CO2 H2N Polycarbonate O

+R-SH +NH3 + R-NH-OH R S O NH2 O R N O O OH O O Figure 3. Scheme of chemical reactions for PC with different components. Figure 3. Scheme of chemical reactions for PC with different components. A former published experiment [21,48] described the chemical reaction of polycarbonate A former published experiment [21,48] described the chemical reaction of polycarbonate (PC) (PC) with a polyelectrolyte polyvinylamine (PVAm) and proved for the ﬁrst time the new bond with a polyelectrolyte polyvinylamine (PVAm) and proved for the first time the new bond formation. The reactivity is based on the presence and density of functional groups and their steric formation. The reactivity is based on the presence and density of functional groups and their steric proximity. The requirement to realize an appropriate distance and orientation is the mobility of proximity. The requirement to realize an appropriate distance and orientation is the mobility of reactive groups or the entire molecule. In the solid state, mobility is reduced by intermolecular reactive groups or the entire molecule. In the solid state, mobility is reduced by intermolecular interactions and is smaller by orders of magnitude than in the liquid or dissolved state. For solids, interactions and is smaller by orders of magnitude than in the liquid or dissolved state. For solids, the thermodynamically preferred crystalline state is less reactive through stronger short and long-range the thermodynamically preferred crystalline state is less reactive through stronger short and interactions. The amorphous state is marked by larger distances between chains and weaker interaction. long-range interactions. The amorphous state is marked by larger distances between chains and Both PC and PVAm are both amorphous polymers and PVAm has the highest density of the primary weaker interaction. Both PC and PVAm are both amorphous polymers and PVAm has the highest amino groups. density of the primary amino groups. The understanding of the reaction mechanism of PC with primary amines was experimentally investigated by IR spectroscopy. In general, the 2-reactive-component system leads to a copolymerization reaction. Urethane groups as linker structures are formed, leading to a substance to Polymers 2019, 11, 535 6 of 17 substance bond which results in a stable and permanent hybrid material. In order to gain a deeper insight into the solid-state chemical reaction mechanism taking place in the interfacial layer, a further system consisting of PC with the crystalline octadecylamine (ODA) was investigated [50]. As a low molecular weight compound, ODA possesses different steric demands, higher movability, but clearly reduced amino group density.

2. Materials and Methods

2.1. Chemicals PC Makrolon® 2245 was received from Bayer AG (Leverkusen, Germany). Octadecylamine (ODA) with 97% purity and silver were supplied by Sigma Aldrich (Munich, Germany). Chloroform (99% purity) and toluene were received from Acros Organics (Geel, Belgium). Ethanol absolute was received from VWR International (Dramstadt, Germany).

2.2. Sample Preparation Polymer samples were prepared on an IR transparent calcium fluoride (CaF2) window with a thickness of 1 mm and 10 mm in diameter (Korth Kristalle GmbH, Altenholz, Germany). The pure CaF2 window was rinsed with chloroform, absolute-ethanol, and dried afterwards with air. Subsequently, a PC solution of 10 g/L, solved in a 1:1 mixture of chloroform and toluene, was spin coated on the pure surface of the substrate. The resulting PC film thickness amounts to approximately 46 µm. Afterwards, a 50 nm thick silver ring with an inner diameter of 6 mm and an outer diameter of 8 mm was vapor deposited onto the PC layer, induced in a high vacuum chamber (p = 1 × 10−6 mbar) Creamet 300V2 from Creavac GmbH (Dresden, Germany), at a constant rate of 1 nm/s until the quartz crystal microbalance registered the desired thickness. The Ag ring acts as susceptor. The sample had an 18 cm distance to the Ag source and was kept at room temperature. ODA was solved in ethanol-absolute. The concentration of the ODA solution was adjusted to 30 mmol/L. The solution was used to spin-coat an ODA layer onto the PC-layer/silver ring at 500 rpm with a ramp of 1000 rpm/s for 30 s time at final speed.

2.3. High-Magnetic-Field Induced Heating Experiment The pulsed magnets are energized using a 50 MJ/24 kV modular capacitor bank [51,52]. The magnets are installed in individual pulse cells and electrically separated from each other. Liquid nitrogen is used to cool the magnets down to 77 K to reduce the ohmic resistance. Different pulsed-field magnets are available at the Dresden High Magnetic Field Laboratory (HLD). The magnet used is made out of soft copper wire with Zylon® fiber reinforcement. Information on magnet details can be found elsewhere [53]. In this study, a so-called KS21 magnet with a 20 mm bore was used (coil parameters are listed in Figure4D). In this specific magnet, a voltage of 22 kV corresponds to a maximum magnetic field of Bmax = 59 T with a total amount of energy of 1.2 MJ. The pulse duration is approximately 25 ms. Figure4 shows a schematic drawing of the experimental set-up. A home-built sample holder made out of Polyetheretherketone was used to place the sample (Photograph in Figure4A) in the center of the magnetic field, Figure4C. The sample was kept at room temperature during the pulse and temperature was controlled and recorded permanently (Figure4B). The temperature was controlled using a Lakeshore 340 temperature controller. The value of the magnetic field was determined by measuring the induced voltage in a calibrated pick-up coil. The induced voltage was recorded by a digital oscilloscope, Yokogawa DL750 (or DL850). Data was stored and later integrated numerically to determine the magnetic-field intensity as a function of time. PolymersPolymers2019 2019,,11 11, 535FOR PEER REVIEW 77 of 1717

FigureFigure 4.4. SchematicSchematic drawing drawing of of experimental experimental set-up set-up and and parameters parameters for formagnetic-field magnetic-field pulse. pulse. (A) A B (Photograph) Photograph of a ofsample a sample with with silver silver ring ringon a onspectroscopic a spectroscopic substrate. substrate. (B) Schematic ( ) Schematic drawing drawing of the of the thermo- and magneto-sensors near the sample surface. (C) Position of the sample and the pulse thermo- and magneto-sensors near the sample surface. (C) Position of the sample and the pulse setup. (D) Specifications and coil parameters. setup. (D) Specifications and coil parameters. 2.4. Atomic Force Microscopy 2.4. Atomic Force Microscopy The measurements in the atomic force microscopy were taken in the peak force tapping mode by a DimensionThe measurements FASTSCAN in (Bruker-Nano the atomic force Surfaces microscopy Division, were Santa taken Barbara, in the CA, peak USA). force A tapping silicon nitride mode sensor,by a Dimension FASTSCAN-B FASTSCAN (Bruker, USA),(Bruker-Nano with a nominal Surfaces spring Division, constant Santa of 1.8 Barbara, N/m and CA, tip USA). radius A of silicon 5 nm wasnitride used. sensor, The set-point FASTSCAN-B was 0.015 (Bruker, V. The offlineUSA), analysiswith a nominal was performed spring byconstant using NanoScopeof 1.8 N/m Analysis and tip v1.90radius (Bruker-Nano of 5 nm was Surfaces used. The Division) set-point software. was 0.015 V. The offline analysis was performed by using NanoScope Analysis v1.90 (Bruker-Nano Surfaces Division) software. 2.5. Resistance Measurements 2.5. Resistance Measurements A digital multimeter DMM 2001, from Keithley Instruments (Germering, Germany), was used to measureA digital the multimeter resistance ofDMM the susceptor 2001, from loop. Keithley The resistance Instruments was (Germering, measured by Germany), a constant was current. used Figureto measure5A illustrates the resistance the measurement of the susceptor set-up. loop. In Th ordere resistance to eliminate was measured line and transitionby a constant resistances current. fromFigure measurement 5A illustrates objects the measurement with low resistance set-up. In values, order 4-wireto eliminate measurement line and technology transition resistances was used, i.e.,from constant measurement current objects supply with and voltagelow resistance measurement values, are 4-wire carried measurement out via separate technology lines. Thewas DMM used, 2001i.e., constant covers a current total measuring supply and range voltage between measurement 1 µΩ and 1 are GΩ carried. For the out low-impedance via separate lines. loops, The only DMM the lowest2001 covers measuring a total range measuring 1 µΩ torange 20 Ω betweenis of interest, 1 µΩ and in which 1 GΩ the. For measurement the low-impedance objects loops, are fed only with the a constantlowest measuring current of range 9.2 mA. 1 µ ThisΩ to set-up 20 Ω is allows of interest, two half in which rings tothe be measurement measured in parallel.objects are Assuming fed with aa homogeneousconstant current structure, of 9.2 mA. the This ring set-up resistance allowsRRing twois half calculated rings to from be measured the measured in parallel. resistance AssumingRmess as a follows:homogeneous Under structure, the assumption the ring that resistance the contact-resistances RRing is calculated are from equal, the the measured total resistance resistance of the Rmess loop as hasfollows: the fourfold Under the value assumption of the measured that the resistance.contact-resistances For the thinnest are equal, Ag the loops total of resistance 10 nm no of resistance the loop washas measurable.the fourfold value of the measured resistance. For the thinnest Ag loops of 10 nm no resistance was measurable.

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A) B)

FigureFigure 5.5.( A(A)) Experimental Experimental set-upset-up forfor determinationdetermination ofof thethe resistanceresistance ofof the the Ag Ag loop. loop. (B(B)) Measured Measured speciﬁcspecific electric electric resistance resistance depending depending on Ag on loop Ag thickness.loop thickness. Variation Variation of evaporation of evaporation process contained process staticcontained and rotating static and samples rotating during samples Ag deposition.during Ag deposition.

2.6.2.6. InfraredInfrared SpectroscopicSpectroscopic Imaging Imaging FT-IRFT-IR spectroscopic spectroscopic images images were were collected collected in transmission in transmission mode mode using using a FT-IR a spectrometerFT-IR spectrometer Vertex 70Vertex coupled 70 coupled with an infraredwith an microscope,infrared microscope, Hyperion Hyperion 3000 (both 3000 from (both Bruker from Optics Bruker GmbH, Optics Ettlingen, GmbH, Germany),Ettlingen, andGermany), a 64 × 64and MCT a 64 focal × 64 plane MCT array focal detector. plane Thearray 15-fold detector. Cassegrainian The 15-fold objective, Cassegrainian with a numericalobjective, aperturewith a numerical of 0.4, imaged aperture a sample of 0.4, areaimaged of approximately a sample area 175of approximately× 175 µm2. 175 × 175 µm2. SpectraSpectra collection,collection, data data treatment, treatment, and and evaluation evaluation are are described described in in detail detail elsewhere elsewhere [ 21[21].].

2.7.2.7. DifferentialDifferential ScanningScanning CalorimetryCalorimetry DifferentialDifferential ScanningScanning CalorimetryCalorimetry (DSC)(DSC) waswas usedused toto studystudy thethe thermalthermal behaviorbehavior ofof PCPC andand ODA.ODA. DSCDSC curvescurves were recorded by a a Q2000 Q2000 DSC DSC de devicevice (TA-Instruments, (TA-Instruments, New New Castle, Castle, NJ, NJ, USA) USA) in ◦ ◦ inthe the temperature temperature range range between between −80− °C80 andC and 200 200°C, atC, a atheating a heating rate rateof 2 K/min of 2 K/min modulated modulated with ± with 0.31 ±K/400.31 s. K/40 The s. neat The neatcomponents components PC and PC andODA ODA were were milled milled to topowder. powder. Each Each DSC DSC measurement waswas carriedcarried out out in in TzeroTzero aluminum aluminum crucibles crucibles with with a a sample sample massmass ofof approximatelyapproximately 1212 mgmg underunder nitrogennitrogen ﬂow.flow. MeasurementsMeasurements ofof sevenseven mixturesmixtures werewere performedperformed withwith varyingvarying massmass ratiosratios ofof PCPC andand ODA.ODA. ForFor mixtures,mixtures, thethe diligentdiligent milledmilled samplessamples werewere weighedweighed directlydirectly intointo thethe DSCDSC panspans accordingaccording toto thethe desireddesired mixing mixing ratio. ratio. After After closing closing the the pans, pans, they they were were shaken shaken manually manually to obtain to aobtain thorough a thorough mixing. mixing. 3. Results and Discussions

3.1.3. Results Chemical and Reaction Discussions between PC and ODA

3.1. ChemicalThe interfacial Reaction reaction between between PC and PC ODA and ODA indicates a controlled mechanism for the formation of urethane linkages (Figure6) and directly influences the final joint strength. It is controlled by the The interfacial reaction between PC and ODA indicates a controlled mechanism for the temperature gradient of susceptors during IH and heat dissipation between the newly generated formation of urethane linkages (Figure 6) and directly influences the final joint strength. It is interphase and both bulk-hybrid components, PC and ODA. controlled by the temperature gradient of susceptors during IH and heat dissipation between the Since ODA is a non-polymeric amine, its reactivity is expected to be higher than for polymeric newly generated interphase and both bulk-hybrid components, PC and ODA. amines, such as polyvinylamine. The thermal-reactive behaviors of PC and ODA were investigated for Since ODA is a non-polymeric amine, its reactivity is expected to be higher than for polymeric different mixtures and compared with the neat PC and neat ODA. amines, such as polyvinylamine. The thermal-reactive behaviors of PC and ODA were investigated For neat ODA, Figure7A shows the heat flow depending on temperature. Within the temperature for different mixtures and compared with the neat PC and neat ODA. range, the melting process is clearly revealed. The glass transition of the neat PC was determined For neat ODA, Figure 7A shows the heat flow depending on temperature. Within the at 147 ◦C. If mixtures of PC and ODA are heated (Figure7B), immediately after the melting of temperature range, the melting process is clearly revealed. The glass transition of the neat PC was ODA, an endothermic reaction starts. The first-heating signal above 100 ◦C is assigned to secondary, determined at 147 °C. If mixtures of PC and ODA are heated (Figure 7B), immediately after the exothermic chemical reactions. The measurement of the mixture with a mass ratio of PC/ODA 20/80 melting of ODA, an endothermic reaction starts. The first-heating signal above 100 °C is assigned to results in the only DSC curve (light green), showing four signals as follows: Melting of the ODA, the secondary, exothermic chemical reactions. The measurement of the mixture with a mass ratio of PC/ODA 20/80 results in the only DSC curve (light green), showing four signals as follows: Melting of the ODA, the endothermic reaction between PC and ODA, the exothermic reaction, and the glass

Polymers 2019, 11, 535 9 of 17 Polymers 2019, 11 FOR PEER REVIEW 9 of 17 Polymers 2019, 11 FOR PEER REVIEW 9 of 17 endothermictransition of reaction PC. This between points PCto the and presence ODA, the of exothermic the two educt reaction, phases and and the glasstwo different transition product of PC. transition of PC. This points to the presence of the two educt phases and two different product Thisphases. points In toall theother presence mixtures, of the PC two is completely educt phases converted and two or different the remaining product PC phases. fragments In all cannot other phases. In all other mixtures, PC is completely converted or the remaining PC fragments cannot mixtures,form a stable PC is phase. completely converted or the remaining PC fragments cannot form a stable phase. form a stable phase.

FigureFigure 6. 6.First First step step of of chemical chemical reaction reaction between between polycarbonate polycarbonate and and octadecylamine, octadecylamine, which which shows shows the Figurethe initial 6. First formation step of ofchemical urethane reaction and the between remain poinglycarbonate chain fragment and octadecylamine, of polycarbonate. which Secondary shows initialthe initial formation formation of urethane of urethane and the and remaining the remain chaining fragment chain fragment of polycarbonate. of polycarbonate. Secondary Secondary reactions arereactions described are elsewheredescribed [elsewhere48]. [48]. reactions are described elsewhere [48].

Figure 7. (A) DSC curves of neat ODA and neat PC. (B) DSC curves of seven mixtures from PC and Figure 7. (A) DSC curves of neat ODA and neat PC. (B) DSC curves of seven mixtures from PC and ODAFigure with 7. (A mass) DSC ratios curves PC/ODA, of neat asODA denoted and neat in the PC. legend. (B) DSC curves of seven mixtures from PC and ODA with mass ratios PC/ODA, as denoted in the legend. 3.2. EstimationODA with ofmass the ratios Induced PC/ODA, Temperature as denoted Change in the legend. 3.2. Estimation of the Induced Temperature Change 3.2. EstimationThe induced of the voltage Induced (U) Temperature in a ring of radiusChange (r ) according to Faraday’s law of induction is as follows: The induced voltage (U) in a ring of radius (r) according to Faraday’s law of induction is as The induced voltage (U) in a ring of radius (r) accordingdB to Faraday’s law of induction is as follows: U(t) = − πr2 , (2) follows: dt 𝑈𝑡 =−𝜋𝑟 , (2) 𝑈𝑡 =−𝜋𝑟 , (2) where dB/dt is the temporal change of the magnetic ﬁeld. The energy dissipated in the metal ring is aswhere follows: dB/dt is the temporal change of the magnetic field. The energy dissipated in the metal ring is 2 whereas follows: dB/dt is the temporal change of the magneticZ U( tfield) . The energy dissipated in the metal ring is E = dt, (3) as follows: R 𝐸= 𝑑𝑡, (3) where R is the resistance of the ring, which can𝐸= be expressed 𝑑𝑡, as follows: (3) where R is the resistance of the ring, which can be2π expressedr as follows: where R is the resistance of the ring, which canR = be expressedρ, as follows: (4) A 𝑅= 𝜌, (4) 𝑅= 𝜌, (4) where ρ is the electrical resistivity. Assuming that the energy completely converts into thermal whereenergy ρ ( Eis), thethe followingelectrical resistivity.relation can Assuming be made: that the energy completely converts into thermal energy (E), the following relation can be made:

Polymers 2019, 11, 535 10 of 17

where ρ is the electrical resistivity. Assuming that the energy completely converts into thermal energy Polymers 2019, 11 FOR PEER REVIEW 10 of 17 (E), the following relation can be made: E = cm∆T, (5) 𝐸=𝑐𝑚∆𝑇, (5) where c is the speciﬁc heat, m is the mass, and ∆T is the change in the temperature. Combining the whereequations, c is the we specific get an induced heat, m temperatureis the mass, changeand ∆T asis follows:the change in the temperature. Combining the equations, we get an induced temperature change as follows: 2 Z 2 r dB(t) ∆T ∆𝑇= = 𝑑𝑡dt, ,(6) (6) 4ρcD dt withwith the the density density ( (DD)) D𝐷=𝑚/2𝜋𝑟𝐴= m/(2πrA).. (7) (7) AccordingAccording to thethe valuesvalues reportedreported in in Table Table2, the2, the induced induced temperature temperature change change on an on Ag an loop Ag withloop withan average an average radius radius of r =of 3.5 r = mm 3.5 mm (inner (inner diameter diameter 6 mm, 6 mm, outer outer diameter diameter 8 mm) 8 mm) due due to a to 59 a T/25 59 T/25 ms mspulse, pulse, is estimated is estimated to beto aboutbe about 67.5 67.5 K, asK, shownas shown in Figure in Figure8B. 8B.

TableTable 2. 2. PhysicalPhysical properties properties of of silver silver (Ag) (Ag) from from [54]. [54].

RingRing Material Specific Speciﬁc Heat Heat (J/k (J/kgg K)K) Electrical Electrical Resistivity ( Ω(Ωm) m) DensityDensity (kg/m (kg/m33) Ag 235 235 1.6 1.6× × 1010−−8 10.510.5 × 101033

Figure 8 shows the magnetic-flux density vs. the time for the field pulse applied in this Figure8 shows the magnetic-ﬂux density vs. the time for the ﬁeld pulse applied in this experiment, experiment, together with the calculated temperature of the Ag ring due to electromagnetic together with the calculated temperature of the Ag ring due to electromagnetic induction. induction.

Figure 8. (A) Temporal magnetic-flux profile of the field pulse. (B) Heating of the Ag loop (50 nm Figure 8. (A) Temporal magnetic-flux profile of the field pulse. (B) Heating of the Ag loop (50 nm thickness) with bulk metal properties caused by IH. thickness) with bulk metal properties caused by IH. The Ag susceptor loop has a thickness of 50 nm and a resistance of 15 Ohm. This is a very The Ag susceptor loop has a thickness of 50 nm and a resistance of 15 Ohm. This is a very good good compromise and also the lower limit of the thickness. A larger thickness would be more compromise and also the lower limit of the thickness. A larger thickness would be more disadvantageous in respect to reactions across an ultrathin interface. A thickness of less than 50 nm disadvantageous in respect to reactions across an ultrathin interface. A thickness of less than 50 nm leads to a higher resistance of the ring and the energy dissipation is too small. The final temperature of leads to a higher resistance of the ring and the energy dissipation is too small. The final temperature 67.5 ◦C is sufficient to trigger the reaction between PC and ODA. of 67.5 °C is sufficient to trigger the reaction between PC and ODA. 3.3. Morphology of the Ag Loops 3.3. Morphology of the Ag Loops The Ag layer shows a granular structure which also controls the electrical resistance. The morphologyThe Ag layer is furthershows a influenced granular bystructure the deposition which process.also controls Figure the9 shows electrical the AFMresistance. images The for morphologydifferent Ag ringis further thicknesses. influenced The granular by the deposition structures, shownprocess. in FigureFigure9 9column shows A, the were AFM captured images from for different Ag ring thicknesses. The granular structures, shown in Figure 9 column A, were captured from samples without rotation during the evaporation process. Images in Figure 9 column B were recorded from samples with rotation during the evaporation process. The mean roughness Sa (mean value from all 5 images) for the samples evaporated without rotation is 1.51 ± 0.01 nm and 0.81 ± 0.06 nm for the samples evaporated with rotation.

Polymers 2019, 11, 535 11 of 17 samples without rotation during the evaporation process. Images in Figure9 column B were recorded from samples with rotation during the evaporation process. The mean roughness Sa (mean value from all 5 images) for the samples evaporated without rotation is 1.51 ± 0.01 nm and 0.81 ± 0.06 nm for the Polymers 2019, 11 FOR PEER REVIEW 11 of 17 samples evaporated with rotation.

Figure 9. AFM topography images (1 µm × 1 µm) of evaporated Ag layers with different thickness onFigure PC. 9. Column AFM topography (A) shows images evaporation (1 µm with × 1 µm) no sampleof evapor movementated Ag layers during with deposition different andthickness column on (PC.B) showsColumn evaporation (A) shows with evaporation rotating thewith samples no sample during movement deposition. during The deposition z-scales correspond and column to the(B) imagesshows evaporation in the (A) or inwith the rotating (B) column, the respectively.samples during deposition. The z-scales correspond to the images in the (A) or in the (B) column, respectively. The AFM (z-scale) records a smoother surface for the evaporation with sample rotation. For 10 nm Ag layers,The AFM individual (z-scale) spherical records clusters a smoother dominate surface the morphology.for the evaporation For thicker with layers, sample morphology rotation. For looks 10 morenm Ag like regularlayers, layersindividual with sinkholesspherical and clusters trenches. dominate Comparing the both morphology. evaporation For methods, thicker there layers, is no clearmorphology difference looks in the more size or like shape regular of the clusterslayers forwith each sinkholes individual and thickness. trenches. The morphologyComparing ofboth the Agevaporation layers does methods, not change there signiﬁcantly is no clear when difference the deposited in the Agsize layer or shape thickness of the increases. clusters All for images each showindividual a clear thickness. percolation The of themorphology Ag clusters of and the pointAg layers to a conductive does not layerchange formation. significantly For a when smoother the layerdeposited a better Ag currentlayer thickness distribution increases. is expected. All images show a clear percolation of the Ag clusters and point to a conductive layer formation. For a smoother layer a better current distribution is expected. The resistance measurements (see Figure 5B) reveal a decrease in resistivity by increasing thickness of the Ag layers. The thicker the Ag loops the higher is their IH efficiency. The sample rotation further reduces the electrical resistance of the granular structures. However, the chemical reaction can only be initiated at the interface. As known from literature, the thickness of polymer interfaces is often in the range of a few nanometers. For thicker susceptor structures the bulk material of the components will be proportionally more heated. This heat is no longer available to trigger the interfacial reaction.

Polymers 2019, 11, 535 12 of 17

The resistance measurements (see Figure5B) reveal a decrease in resistivity by increasing thickness of the Ag layers. The thicker the Ag loops the higher is their IH efficiency. The sample rotation further reduces the electrical resistance of the granular structures. However, the chemical reaction can only be initiated at the interface. As known from literature, the thickness of polymer interfaces is often in the range of a few nanometers. For thicker susceptor structures the bulk material of the components will be proportionally more heated. This heat is no Polymers 2019, 11 FOR PEER REVIEW 12 of 17 longer available to trigger the interfacial reaction. 3.4. Verification of Chemical Reaction by FTIR Spectroscopic Imaging 3.4. Verification of Chemical Reaction by FTIR Spectroscopic Imaging Reaction between PC and ODA due to electromagnetic induced heating of the Ag loops was Reaction between PC and ODA due to electromagnetic induced heating of the Ag loops was spectroscopically evaluated as described for the polymeric system in [22]. spectroscopically evaluated as described for the polymeric system in [22]. Figure 10 shows the microscopic and IR spectroscopic image of the sample. The Ag ring Figure 10 shows the microscopic and IR spectroscopic image of the sample. The Ag ring appears appears in the microscopic image (Figure 10A) as a black area. Contrast of the IR spectroscopic in the microscopic image (Figure 10A) as a black area. Contrast of the IR spectroscopic image was image was calculated by the integration of absorbance values (Figure 10B). As the Ag ring reflects IR calculated by the integration of absorbance values (Figure 10B). As the Ag ring reflects IR radiation it radiation it appears in red and brown colors, while the PC-ODA layer system is dominated by blue appears in red and brown colors, while the PC-ODA layer system is dominated by blue pixels. Spectra pixels. Spectra that correspond to the Ag ring were eliminated from the data set. The IR that correspond to the Ag ring were eliminated from the data set. The IR spectroscopic brightfield spectroscopic brightfield image IR (Figure 10C) reveals an inhomogeneous formation of the image IR (Figure 10C) reveals an inhomogeneous formation of the PC-ODA layer. The spectra of the PC-ODA layer. The spectra of the pre-processed data set show a variation across the whole sample. pre-processed data set show a variation across the whole sample. This is also illustrated by the plot of This is also illustrated by the plot of the mean spectra and the standard deviation, represented in the mean spectra and the standard deviation, represented in Figure 11. The spectral profile exhibits the Figure 11. The spectral profile exhibits the typical bands of PC and ODA, assigned and discussed in typical bands of PC and ODA, assigned and discussed in the previous paper [48,50]. the previous paper [48,50].

Figure 10. (A) Light microscopic image of the sample. The Ag ring appears as a dark area on the Figureleft side 10. of (A the) Light sample. microscopic The dotted image rectangle of the indicatessample. The the Ag sample ring appears area measured as a dark by area FT-IR on imaging. the left side(B) FT-IRof the spectroscopic sample. The image.dotted Therectangle contrast indicates is calculated the sample by integration area measured of absorbance by FT-IR values imaging. between (B) FT-IR950 and spectroscopic 2000 cm−1 .(image.C) Selected The contrast FT-IR imagesis calculat of theed by inspected integration area. of White absorbance pixels values indicate between spectra −1 950eliminated and 2000 from cm the. ( dataC) Selected set. FT-IR images of the inspected area. White pixels indicate spectra eliminated from the data set.

Polymers 2019, 11, 535 13 of 17 Polymers 2019, 11 FOR PEER REVIEW 13 of 17 Absorbance (normalized)

0 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Wavenumber (cm-1)

Figure 11. Mean spectrum (black) and standard deviation (gray) calculated from the data set of selectedFigure spectra. 11. Mean spectrum (black) and standard deviation (gray) calculated from the data set of selected spectra. The results of principal component analysis (PCA) are represented in Figure 12. As the PCA was The results of principal component analysis (PCA) are represented in Figure 12. As the PCA performed on the raw matrix the loading plot of the 1st principal component (pc) represents the mean was performed on the raw matrix the loading plot of the 1st principal component (pc) represents the spectrum,mean showingspectrum, a showing number ofa number specific of and specific characteristic and characteristic signals of PCsignals and of ODA PC (seeand SupplementaryODA (see −1 Materialssupplementary Tables S1–S4). materials The Tables strongest S1–S4). signal The between strongest 1400 signal and between 1500 cm 1400 isand a composition 1500 cm−1 is ofa CHx deformationcomposition and of ring CHx vibrationdeformation modes and ring assigned vibration to modes ODA andassigned PC, to respectively. ODA and PC, ODA respectively. has another strongODA signal has another at 1560 strong cm−1 ,signal associated at 1560 withcm−1, vibrationsassociated with of amino vibrations groups. of amino The groups. triplet The between triplet 1150 and 1250between cm −11501 arises and from1250 C–O–Ccm−1 arises and from O–C(O)–C C–O–C vibrationsand O–C(O)–C of PC. vibrations Finally, of the PC. vibration Finally, modethe of the carbonylvibration groupmode of PCthe appearscarbonyl atgroup 1770 cmof −PC1. Theappears score at map1770 showscm−1. The predominantly score map shows red colors, indicatingpredominantly an overall red homogeneous colors, indicating formation an overall of thehomogeneous PC/ODA layer.formation The secondof the PC/ODA pc points layer. to variations The second pc points to variations within the PC and the ODA layer. Spectral features of PC appear as within the PC and the ODA layer. Spectral features of PC appear as negative signals in the loading negative signals in the loading plot. While the pattern of negative signals in the loading plot of the plot. While the pattern of negative signals in the loading plot of the 2nd pc corresponds very well with 2nd pc corresponds very well with the spectrum of PC, positive signals indicate ODA. In the the spectrumcorresponding of PC, score positive map, signals yellow indicateand orange ODA. pixels, In the as correspondingwell as blue and score green map, pixels, yellow form andlarger orange pixels,areas as welland point as blue to variations and green between pixels, formthe PC larger and ODA areas layers. and point The score to variations map of the between 3rd pc shows the PC a and ODAsmall layers. strip The of dark score green map and of the blue 3rd pixels pc shows on the aAg small ring. strip The corresponding of dark green loading and blue plot pixels exhibits on thea Ag ring.number The corresponding of negative signals loading which plot are exhibits all associated a number with the of negativeexpected reaction signals product which are of urethane. all associated withThe the most expected prominent reaction signals product are located of urethane. at 1470 cm The−1 and most 1700 prominent cm−1. The signalsfirst relatively are located broad atsignal 1470 is cm−1 a composition−1 of NH, CN, and CHx vibration modes. The signal around 1700 cm−1 is assigned to C=O and 1700 cm . The first relatively broad signal is a composition of NH, CN, and CHx vibration modes. The signalvibration around of the 1700 urethane cm−1 group.is assigned In addition, to C=O positive vibration signals of the represent urethane PC group. and InODA. addition, Negative positive values (blue and green pixels) in the score map indicate their conversion to the product. The signals represent PC and ODA. Negative values (blue and green pixels) in the score map indicate their formation of urethane is underlined by the 4th principal component. Besides the signals at 1700 cm−1 conversion to the product. The formation of urethane is underlined by the 4th principal component. and 1220 cm−1, the relative strong signals centered at 1396 cm−1 (which is associated with CHx −1 −1 −1 Besidesgroups), the signalspoint again at 1700 to the cm formationand 1220of urethane. cm , Blue the relativepixels in the strong infinity signals of the centered Ag ring reveal at 1396 the cm (whichconversion is associated of PC and with ODA. CHx However,groups), the point overall again variance to the is formation small and some of urethane. other variations Blue pixels across in the infinitythe ofinspected the Ag area ring are reveal present, the making conversion a full ofinterpretation PC and ODA. of the However, 4th principal the component overall variance difficult. is small and some other variations across the inspected area are present, making a full interpretation of the 4th principal component difficult. PolymersPolymers 20192019,, 1111 ,FOR 535 PEER REVIEW 1414 of of 17 17

Figure 12. Principal component (pc) analysis of FT-IR Images recorded from a PC-ODA layer Figuresample 12. after Principal exposure component to a pulsed (pc) high-magneticanalysis of FT-IR field. Images (A) recorded Represents from score a PC-ODA maps and layer (B )sample shows afterthe corresponding exposure to a loading pulsed plots high-magnetic for each individual field. (A principle) Represents component. score maps and (B) shows the corresponding loading plots for each individual principle component. 4. Conclusions 4. Conclusions High-magnetic-field sources enable the development of novel techniques for polymer engineering. HighHigh-magnetic-field energies can be transferred sources into enable a limited the volume,development such as anof interfacenovel techniques layer, within for a verypolymer short engineering.period of time, High while energies avoiding can thermal be transferred effects in the into polymer a limited bulk volume, phases. Strongsuch as electromagnetic an interface layer, fields withinare an a outstanding very short period platform of time for the, while development avoiding thermal of new effects polymer in the composite polymer and bulk hybrid phases. materials. Strong electromagneticIn contrast to electromagnetic fields are an outstanding fields with radioplatform frequency, for the the development magnetic pulse of new does polymer not cause composite thermal andeffects hybrid in the materials. polymer In materials. contrast to electromagnetic fields with radio frequency, the magnetic pulse does notIn this cause report thermal it is demonstrated effects in the polymer that a chemical materials. reaction between PC and ODA can be initiated by electromagneticIn this report induction it is demonstrated when a 50 that nm a thin chemical Ag loop reaction is placed between at the PC interface and ODA of both cancomponents. be initiated byJust electromagnetic a single magnetic induction pulse of approximately when a 50 nm 59 Tthin leads Ag to aloop local is heating placed of theat the Ag susceptor,interface resultingof both components.in the formation Just ofa single urethane magnetic groups pulse from of PC approximately and ODA. The 59 linkerT leads structures to a local wereheating probed of the by Ag IR susceptor,spectroscopic resulting imaging. in the formation of urethane groups from PC and ODA. The linker structures were Thisprobed study by demonstratesIR spectroscopic the applicationimaging. potential of low-frequency but high-magnetic-field pulses in polymerThis study technology demonstrates and offers, the in application particular, newpotential applications of low-frequency for the development but high-magnetic-field of novel polymer pulsescomposites in polymer and hybrids. technology The and scalability offers, ofin thepart experimentsicular, new isapplications coupled to for the the availability development of high of novelmagnetic polymer fields. composites Basically, the and new hybrids. techniques The can scalability be scaled upof tothe samples experiments several is centimeters coupled to in size.the availability of high magnetic fields. Basically, the new techniques can be scaled up to samples severalSupplementary centimeters Materials: in size.The following are available online at http://www.mdpi.com/2073-4360/11/3/535/s1, Table S1–S4: FTIR band assignments. SupplementaryAuthor Contributions: Materials:Conceptualization, The following are writing, available reviewing, online editing,at www.mdpi.com/xxx/s1, project administration, Table and S1–S3: supervision FTIR bandwere assignments. realized by C.Z. DSC and AFM methodology were performed by K.A. and A.J. Resistance measurements and data validation were provided by T.U. Investigation of electromagnetic pulses and induction heating experiments Authorin pulsed Contributions: fields were performed Conceptualization, by C.S.M., aswriting, well as reviewing, the calculation editing, of the project expectable administration, temperature and increase supervision based were realized by C.Z. DSC and AFM methodology were performed by K.A. and A.J. Resistance measurements and data validation were provided by T.U. Investigation of electromagnetic pulses and induction heating

Polymers 2019, 11, 535 15 of 17 on induction with a high magnetic field pulse for different susceptor structures. J.W. is head of the High Magnetic Field Laboratory Dresden and supervised all experiments, conceptualized, and reviewed the manuscript. Acknowledgments: We acknowledge Wolfgang Jenschke and Mathias Ullrich, both are from Leibniz Institute of Polymer Research Dresden, for the construction of the resistance measurement unit. We acknowledge the support by HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL). Conflicts of Interest: The authors declare no conflict of interest.

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