Thermoplastic Polyurethanes As A Function Of

Polymers 2019, 11, 19103 of 21Polymers 2019, 11, 19103 of 21diisocyanate (MDI) and 1,4-butanediol (BD) as the chain extender, which were obtained from SigmaPolymers 2019, 11, 19103 of -butanediol (BD) as the chain extender, which were obtained from Sigma 3 of 19Polymers2019,Polymers2019, 11,191011, 19103 of 21Polymers2019, 11, 19103 of 21Theflexiblesegmentsor macrodiolswithmolar massof 10002000, ,11,19103 of ragechain extender,whichwereandobtainedfrom Sigmaare Theas l extender,Eternacoll (UH100UH200),flexiblesegmentsor ).diisocyanate(MDI)1,4-butanediol(BD) as thechainextender, MethodsPolymers11, 19103 iolEternacoll )carbonatediolEternacoll aAldrichSpain).or macrodiols with an average molar mass of 1000 and 2000, e)carbonateEternacoll ne)carbonatediolEternacoll r macrodiolsTheflexiblesegmentswith andiolaveragemolarmassofUH1000and carbonatediolEternacoll by prolactone)carbonatediolEternacoll rodiolswith anaveragemolarmassofUH1000and ediolEternacoll e)carbonatediolEternacoll ternacoll apa l ene)carbonatediolEternacoll Eternacoll UHCPH tsormacrodiolswithan averagemolarmass ylene–pentamethylene)carbonatediolEternacoll 0)andCapa ;andpolycaprolactonepolyesterdiolCapa iolEternacoll lactone)carbonatediolEternacoll ylene)carbonatediolEternacoll UH(UH100andUH200),carbonatediolEternacoll m e;molecularweight;andmelting(PCL100)andUBECapa olEternacoll ctonepolyesterdiolCapa natediolEternacoll ).m andTg, aresummarizedin ;polycaprolactonepolyesterCapa glass2100(PCL100)andCapa coll ich;andpolycaprolactonepolyesterdiolCapa 2100The edand keptinthea dryboxto avoidhumidity.transitiontemperatures,TmbyandTg, din Table1.and erdiolCapa glass2100(PCL100)andCapa 100(PCL100)andCapa den).respectively,areas follows:poly(hexamethylene)carbonatediolEternacoll UH(UH100andAllmaterialswereusedasreceivedand keptinthea dryto avoidhumidity.(PCL100)and Capa andSweden).meltingandglasstransitionTmbyandTg, izedin Table1. actonepolyesterdiolmeltingCapa re(Tg), ne)carbonatediolEternacoll uchmolecularstructure;molecularweight;and meltingand glasstransitiontemperatures,Tm andTg, asrespectively,ofinthepureboxmaterialsare summarizedin Table1.Allwere2200usedasreceivedand keptdryto avoid(PCL100)andCapabyPerstorpHoldingAB ialsg, suppliedrespectively,ofathepurematerialsare summarizedin andTable1.temperature(Tm ) oftheused.UH,poly(hexamethylene)carbonatePH, onatediolEternacoll ll materials were used as received and kept in a dry box to avoid em) ofChemicalthe dkept inaSpain);dryboxto avoidhumidity.Table1. ratures,Tm andTSigmag, respectively,of thepurematerialsare summarizedin Tablecarbonatediol; aprolactonepolyesterdiolCapa 21001.(PCL100)Table1. Symbol,structure,moleculartransition temperature(Tg), anddiol;meltingtemperature(Tm) ofthe purematerialsweight,used. glassUH, ),andmeltingAllmaterialsusedas receivedandina glycol;MDI,andCapa lding(Malmö,Sweden).Table1. ture(Tg), andmeltingtemperature(Tm) of the pure materialsused. UH,carbonate diol; ol; poly(hexamethylene)UHC, poly(hexamethylene–caprolactone)temperature (Tm) ofthe pure BD,materialsused. UH, poly(hexamethylene) carbonate diol; olecular sofpolycaprolactonethe pure PH, and natediol;UHC, poly(hexamethylene–caprolactone)Table1. Symbol, structure, molecular weight,glasstransitiontemperature(Tg), andTg fthepurematerialsaresummarizedin Table l;UHC,poly(hexamethylene–caprolactone)m Structureg 1,4-butanediol.Symbolcarbonatediol; PCL, polycaprolactonepolyesterdiol; PTMG, poly(tetramethylene ether) glycol; ;Molecularcarbonate lycol;MDI,temperature(Tm) PCL,of thepure materialsused. diol;UH, PTMG,poly(hexamethylene)carbonatePH, 1ether)( C)( C)) glycol;Weight(g·molcarbonate diol; PCL, polycaprolactonepolyesterdiol; PTMG, ’-diphenylmethane diisocyanate; ene)carbonatediol; UHC, poly(hexamethylene–caprolactone)TmeltingmTg g), and ( C)Molecular( C)Weight(g·mol 1) (TTable 1. Symbol,structure,BD,molecularweight, glass anate;1,4-butanediol.SymbolStructurecarbonate diol; PCL, polycaprolactonepolyester diol; PTMG, poly(tetramethyleneether) glycol;MDI,–gTmTMoleculartemperature (Tm ) of the pure materials used. UH, poly(hexamethylene)( C)Tg diol;( C)TmPH,Weight(g·mol 1) thanediisocyanate;BD, 1,4-butanediol.TmT–gMolecular e)carbonate diol; UHC,poly(hexamethylene–caprolactone)100042.8( C)( C)Weight(g·mol ) 1 69.34SymbolStructure( C)( C))Weight(g·molpoly(tetramethyleneether)glycol;( C)( C)Weight(g·mol 1)UH carbonate diol; PCL, polycaprolactone polyester diol; PTMG,100069.3442.8TmT–gMoleculardiisocyanate; BD, 1,4-butanediol.SymbolStructure200050.4–UH MDI, 4,4’-diphenylmethane100069.3442.8( C) –( C)Weight(g·mol ructureT g 69.34( C)T m ( C)100042.8 0–1000 69.34 50.4 42.852.30PH UHUH1000–56.452.302000 00–52.11000–66.0PH1000 56.4 11.3PH2000–52.1UHCPH1000–66.02000 52.12000–52.1 71.132 11.31000 66.0 11.3UHCUHCUHC2000 59.12000–59.114PCL1000–71.132 14 142000–59.1 59.1141000–71.1321000 71.11000–75.820.9–71.132 322000–63.951.7PCLPCL2000 00–63.9 0–75.820.9PCL1000 75.8 24.4 000–75.8 51.720.92000 TMGPTMGPTMG2000–76.224.490.12-20.1 20.1BD BD90.122000–76.2 50.1290.12-- 20.1BD 250.1240MDI All materials were used as received and kept in a dry box to avoid250.12 humidity.40MDI250.12-40

Polymers 2019, 11, 19104 of 192.2. Polyurethane SynthesisThermoplastic polyurethanes were obtained via a two-step, prepolymer synthesis method [16].In the first step, the macrodiol and an excess of diisocyanate were poured in a reactor at a temperatureof 70 C over 1 h in an argon atmosphere to form a prepolymer of polyol endcapped with diisocyanategroups. In a second step, butanodiol at a molar ratio of NCO/OH 1.03 was added to the prepolymerin a SpeedMixerTM Dac 600.1 FVZ mixer (Landrum, SC, USA) at room temperature for 1 min at2250 r.p.m. The subsequent PU solution was cast on aluminium moulds at 90 C, and was compressionmoulded at a pressure of 50 bars and temperature of 100 C for 24 h using a water-cooled hydraulicCarver press model 4128CE S/N 4128-220 (Wabash, IN, USA). The cooling procedure was kept uniformby carefully controlling the water flow rate. PU plaques were 2 mm thick.A molar ratio of 1:3:2 (polyol/MDI/BD) was used for the synthesis of all polyurethanes. Sampleswere tested in the as-moulded condition only. The experimental results are the mean value of at leastthree independent tests for every system.2.3. Characterization Techniques2.3.1. Differential Scanning Calorimetry (DSC)DSC scans were performed using a TA Instrument Q20 (New Castle, DE, USA) equipped with arefrigerated cooling system and nitrogen purge. Calibration was performed with indium according tothe manufacturer’s recommended procedures. The uncertainty associated with each temperature isapproximately 2 C. About 4–6 mg of sample was sealed in an aluminium pan for every test. Thermalbehavior was investigated by scanning the samples from 80 to 220 C at a heating rate of 20 C·min 1 .Previous thermogravimetric analysis results show that these samples are stable until 250 C [16].After the first scan samples were cooled with liquid nitrogen, a second scan was immediately recorded.The midpoint of the heat capacity change was chosen to represent Tg , Tm refers to the endotherm peaktemperature, and the area of the endotherm peak to the melting enthalpy is H.2.3.2. Fourier Transform Infrared-Attenuated Total Reflection Spectroscopy (FTIR-ATR)FTIR-ATR measurements were performed with a Thermo Nicolet Nexus FTIR spectrometer(Waltham, MA, USA) equipped with a multiple internal reflection accessory ATR single bounce.Samples were pressed against ATR accessory diamond crystal by means of the fixing screw using a flattip. Single beam spectra of the samples were obtained after averaging 128 scans between 4000 and400 cm 1 with a resolution of 4 cm 1 . All spectra were obtained in the transmittance mode.2.3.3. Dynamic Mechanical Analysis (DMA)DMA was performed on a 2980 Dynamic Mechanic Analyzer (TA instruments) equippedwith tensile head and reducing force option using the Custom Test and single cantilever geometry.Calibration was performed as per the manufacturer’s recommendations included in TA software,version 4.5A. The experiments were carried out on rectangular samples of dimensions close to(18.000 6.000 2.000) mm3 . The experimental conditions employed were frequency of 1 Hz andamplitude of 30 µm (linear viscoelastic region) with a temperature ramp of 3 C·min 1 and a scanningtemperature range from 100 to 180 C. These experiments yield the storage modulus (E0 ), the lossmodulus (E00 ), and the damping factor tan δ ( E00 /E0 ). The glass transition temperature was determinedfrom the peak of the tan δ curve.2.3.4. Synchrotron Small-Angle X-Ray Scattering (SAXS)SAXS experiments (samples with 10 mm of diameter and 1 mm thickness) were done on the SAXS1beamline of the Brazilian Synchrotron Light Laboratory (LNLS). The X-ray was monitored with aphotomultiplier and detected on a Pilatus (300 k, 84 mm 107 mm) positioned at 1000 mm, generating

Polymers 2019, 11, 19105 of 19scattering wave vectors, q, from 0.12 to 4.0 nm 1 . The wavelength of the incident X-ray beam, λ,was 0.155 nm. Silver behenate (AgBH) was used to calibrate the diffraction angle. Polyurethanesamples were placed in perpendicular position regarding the X-ray beam at room temperature. Thebackground and parasitic scattering were determined by separate measurements on an empty holderand subtracted.The PU morphology can be explained through the use of a pseudo two-phase system consideringthat the copolymer structure is composed of periodical stacks of alternate lamellar crystals andamorphous layers [18,19]. The long period (Lp ), amorphous thickness (La ), and crystalline thickness(Lc ) were determined using Lorentz-corrected plots and the one-dimensional correlation function,γ(r) [19–21]. The γ(r) function was calculated according to a procedure given in the literature [18,22]with the following equation:R γ(r) I (q)q2 cos(qr)dq0R q2 I (q)dq1 Q Zq2 I (q) cos(qr)dq(1)00where r is the real space direction perpendicular to the lamellar surfaces, and Q is the invariantand represents the electron density difference between the hard and soft phases. In this work, theinterdomain distance (b Lp ) obtained by the correlation function corresponds to the r value of the firstmaximum of the γ(r) data. The r value at the first zero (r0 ) is defined to be r0 H(1–V h ), where H is thethickness of hard domain and Vh corresponds to its volume fraction. The H value is determined basedon the right triangle whose hypotenuse passes through at γ(r) 1 and γ(r) 0, and whose baselineis tangent to the γ(r) curve at its minimum [23]. By the combination of r0 and H data, it is possibleto calculate V h 1–(r0 /H). The soft domain thickness, S, is defined as the difference between H andinterdomain distance: S b Lp –H. Another important structure parameter in the PU copolymer is theaverage interface thickness between rigid and flexible segments, IT, which is obtained from the ratio ofthe hard thickness to the first minimum long period: IT H2 /Lpmin [22].2.3.5. Shore D HardnessShore D hardness was measured at room temperature using a Zwick Roell (Ulm, Germany)analogical hardness testing apparatus following “UNE-EN ISO 868:1998: Plastics and ebonite.Determination of indentation hardness by means of a durometer (Shore hardness)” standard procedureat (23 2) C and 50% relative humidity.2.3.6. Tensile PropertiesTensile properties were measured at 23 C on five replicates of each material with an InstronModel 5582 Universal Testing machine (Grove City, PA, USA) according to “ISO 527-3 Testing methodfor thermoplastic polyurethane elastomers”. A 100 kN load cell was used and the cross head speedwas 200 mm/min. Pneumatic grips were required to hold the test specimens.2.3.7. Durability TestsThe polyurethanes were subjected to different durability tests according to a method based on theinternational standard ISO 13206, ‘Thermoplastic covering films for use in agriculture and horticulture.’These studies are as follows: (a) heating resistance: test pieces were heated in Gear oven P Selecta at120 C for 15, 30, and 50 days; (b) hydrolytic resistance: test pieces were immersed in water at 80 Cfor 20, 40, and 60 days; (c) oil resistance: test pieces were immersed in BP Oil CS 150 at 100 C for10, 20, and 30 days; (d) weather resistance: test pieces were exposed in sunshine weatherometer for200 h. Weather conditions: λ 340 nm borosilicate filters, radiation of 35 Wm2 nm, T (65 3) C;

Polymers 2019, 11, 19106 of 19and relative humidity of 65% 5%. A dried cycle of 102 min was followed by 1 min of spray water(raining simulation).After the degradations, the material retention of tensile properties was measured following theequation:value a f ter degradation testproperty retention (%) 100(2)value be f ore degradation test3. Results and DiscussionThe current study gives valuable information about the influence of the flexible segment molecularstructure and chain length on the morphology, thermal, and mechanical properties and resistance toexternal agents like weather, water, oil, and heat of thermoplastic polyurethanes. The main task is toevaluate the influence of the soft phase in order to tailor a polyurethane with selected properties for aspecific application. So, segmented thermoplastic polyurethanes with five different flexible segmentmolecular structures and two molar masses (1000 and 2000 g/mol) were synthesized without solventby the two-step method. Butanediol (BD) was used as chain extender and 4,4’-diphenylmethanediisocyanate (MDI) is the diisocyanate that was used to react with the OH groups of the polyols.The notation used in this article is PU-XY. The letter X denotes the flexible segment type, that is, X UH,PH, UHC, PCL, or PTMG (see Table 1), and the letter Y 100 or 200 refers to the macrodiol molarmass, that is, 1000 or 2000 g/mol. Different techniques were used to determine the degree of phasemixing rigid-flexible segments that determine polyurethanes properties, thus relating with mechanicalproperties and resistance to degradation under different agents.Differential scanning calorimetry (DSC) curves were obtained from the as-casted systems todetermine the behavior under heat flow. Figure 1 depicts the first and the second scans of all thesamples assayed as a function of temperature. All the systems show two temperature regions, with theone at a low temperature showing a glass transition temperature and the one at a high temperature asan endothermic peak [24–27]. The glass transition temperature observed at low temperature valuesis related to the amorphous part of the flexible segment. Values of Tg of the polyurethane stronglydepend on the type of macrodiol employed in the synthesis. This value is indicative of the soft andrigid segment mixing degree. The higher the difference between the pure macrodiol or flexible segment,Tg,s (see Table 1), and the Tg of the final polyurethane, the higher miscibility or compatibility

1000 and 2000 g/mol, and 4,4'-diphenylmethane diisocyanate (MDI) and 1,4-butanediol as a rigid segment. The polyurethanes obtained reveal a wide variation of microphase separation degree that is correlated with mechanical properties and retention of tensile properties under degradation by heat, oil, weather, and water.