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Biomaterials based on new polyurethane and hydrolyzed collagen, k-elastin, hyaluronic acid and chondroitin sulfate

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In this paper biomaterials based on various polyurethane formulations have been physically characterized by FT-IR spectroscopy, contact angle measurements, DSC, TG/DTG and SEM methods. It has been established that the transition temperatures of soft
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  International Journal of Biological Macromolecules 47 (2010) 646–653 Contents lists available at ScienceDirect InternationalJournalofBiologicalMacromolecules  journal homepage: www.elsevier.com/locate/ijbiomac Biomaterials based on new polyurethane and hydrolyzed collagen, k-elastin,hyaluronic acid and chondroitin sulfate Maria Cristina Popescu a , ∗ , Cornelia Vasile a , Doina Macocinschi a , Maria Lungu b , Oana Craciunescu c a “Petru Poni” Institute of Macromolecular Chemistry, Department of Physical Chemistry of Polymers, 41A Grigore Ghica Voda Alley, Iasi 700487, Romania b SC INCERPLAST SA, Bucharest, Romania c RTD National Institute for Biological Science, Bucharest, Romania a r t i c l e i n f o  Article history: Received 14 July 2010Received in revised form 10 August 2010Accepted 18 August 2010 Available online 26 August 2010 Keywords: PolyurethaneBiomaterialsNatural polymers a b s t r a c t In this paper biomaterials based on various polyurethane formulations have been physically character-ized by FT-IR spectroscopy, contact angle measurements, DSC, TG/DTG and SEM methods. It has beenestablished that the transition temperatures of soft and hard segments of polyurethane (glass transitionor melting) depend on the blend composition. The melting temperature varies from 54.2 to 81.9 ◦ C forsoftsegments,andfrom220to235 ◦ Cforhardsegments.FT-IRspectrometryallowsidentifyingthefunc-tional groups involved in interactions and consequently the changes in polymer chain mobility. FromSEM images, is evident that polyurethanic film is porous and spongious. By adding of the others compo-nentssuchashydrolyzedcollagen,elastin,chondroitinsulfateorhyaluronicacid,areductionofporosityof films was obtained. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Polyurethanes (PU) are versatile materials used in a largenumber of applications [1,2]. Their properties depend on sev- eral factors and can be varied in a wide range by the properselection of components, composition and preparation condi-tions[1,3,4].Biocompatibility,uniquechemistryandprocessability makepolyurethanesidealfornumerousmedicalapplications[5,6].They are used as encapsulants for hollow-fiber devices, dip-molded gloves and balloons, asymmetric membranes, functionalcoatings, and as extruded profiles for catheters. Bio- and haemo-compatibility, stability, appropriate mechanical properties andother conditions can be satisfied by the appropriate selection of polyurethanes composition [7,8] or by blending with other poly- mers.Polyurethanes can be synthesized in a wide variety of struc-tures suitable for different applications thus being possible toobtain even two total different groups of biomaterials such as:bio-stable and bio-degradable. However there are some studieswhich have demonstrated that these biodegradable polyurethanematerials could be transformed in products which are suspectfor carcinogenity. From these reasons it is necessary to knowthe mechanism of interaction between blood and material and todevelopnewtestingmethods[4].Inourpreviousworkswedemon- ∗ Corresponding author. Tel.: +40 746 029204; fax: +40 232 211299. E-mail address:  cpopescu@icmpp.ro (M.C. Popescu). strated that the polyurethane mixed with extracellular matrixmolecules (hydrolyzed collagen, elastin, chondroitin sulfate andhyaluronic acid) allowed cell attachment and growth over theculture period and did not interfere with morphological and func-tionalcharacteristicsofthecellsevidencingahighbiocompatibility[23]. However, the proper adjustment of properties to meet thedemands of a specific application requires the extensive knowl-edgeofstructure–propertycorrelationsandthefactorsinfluencingthem.The biocompatibility of the polymer blends, that is necessaryin biomedical applications, can be increased by the incorpora-tion of biologic polymers, that will conferee superior propertiesand will reduce the failure of implants and adverse reactions. Theblendsbetweenbiopolymersandsyntheticpolymersareofpartic-ular significance because they can combine biocompatibility withgood processability and mechanical resistance and can be used asbiomedical and biodegradable materials [9]. In order to improve the biological characteristics of the materials used in contact withblood and tissues for long periods, natural polymers like collagen,fibrin and glycosaminoglycans were more often used than othernatural products [10]. Recent studies showed that elastin assure a better contact surface than collagen due to its antithrombogenicproperties [11].Collagen is the most widely used tissue-derived natural poly-mer, and it is a main component of extracellular matrices of mammalian tissues including skin, bone, cartilage, tendon, andligament.Thecombinationwithnon-collageneousstructuralcom-ponents results in modification of the ionic exchange function, as 0141-8130/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ijbiomac.2010.08.013  M.C. Popescu et al. / International Journal of Biological Macromolecules 47 (2010) 646–653 647 well the reactivity with cells and biodegradability. Collagen meetsmanyofthebiologicaldesignparameters,asitiscomposedofspe-cific combinations of amino acid sequences that are recognized bycells and degraded by enzymes secreted from the cells (i.e., colla-genase).Collagenandelastinaretwoofthekeystructuralproteinsfound in the extracellular matrices of many tissues [12,13]. These proteins are important modulators for the physical properties of any engineered scaffold, affecting cellular attachment, growth andresponses to mechanical stimuli [14,15].Chondroitin sulfate (CS) is a glycosaminoglycan (GAG) thatplays an important role in regulating the expression of the chon-drocyte phenotype. It comprises alternating units of    -1,3-linkedglucuronicacidandN-acetyl-galactosaminewithsulfationateitherthe 4 or the 6 position of the N-acetyl-galactosamine residues.Chondroitin sulfate is also involved in intracellular signaling, cellrecognition,andtheconnectionofextracellularmatrixcomponentsto cell-surface glycoproteins [16]. Chondroitin sulfate is an impor- tant structural component of cartilage and provides much of itsresistance to compression [17].Hyaluronate is one of the glycosaminoglycan components innaturalextracellularmatricesandplaysasignificantroleinwoundhealing. It can be degraded by hyaluronidase, which exists in cellsandserum[18];canformhydrogelsbycovalentcross-linking.Ithas shown also, excellent potential for tissue engineering applicationssuch as artificial skin [19], f acial intradermal implants [20], wound healing [21], and soft tissue augmentation. However, hyaluronate may potentially transmit disease or act as an adjuvant in elicitingan immune response [22].The purpose of this study is to establish by physico-chemical methods the compatibility of complex blendsPU/HC/ELS/HA/chondrotin sulfate (CS) and the influence of glycosaminoglycans on biocompatibility. 2. Experimental  2.1. Materials The materials used were diphenylmethane 4,4  -diisocyanate(MDI) freshly distilled, from Merck, poly (ethylene glycol) adi-pate(PEGA,purity97%,MW=2000g/mol)andethyleneglycol(EG,purity 95%) from Fibrex SA Savinesti-Romania. PEGA was dehy-drated at 120 ◦ C, for 3h to remove residual water.Briefly in polyurethane synthesis, PEGA reacted with MDI at90 ◦ C, under nitrogen atmosphere, for 1h. In the second stage, theisocyanategroupsoftheprepolymerreactedwiththechainexten-der (EG) at 60 ◦ C. The molar ratio PEGA:MDI:EG was 1:5:4. Theobtained polyurethane was precipitated in water and dried undervacuum for several days.Obtained polyurethane is a white powder, is soluble indimethylformamide (DMF) and dimethylacetamide, with a molarmass Mn=109,613 and it has the following formulae:[PEGA − MDI − (EG − MDI) 4 ] n  2.1.1. Natural components As natural components were used hydrolyzed collagen (HC), k-elastin, hyaluronic acid (HA) and chondroitin sulfate (CS).Elastin (kEL) was prepared from insoluble elastin powder (calf ligament, Sigma) by stirring in KOH 1M (in ethanol, 1:4, v/v), at30 ◦ C, for 48h, neutralizing with acetic acid and dialysis againsttwice distilled water. The final solution has 1.3% dry weight and68.7% protein content.Hyaluronic acid (HA, Sigma) was obtained from human umbil-ical cord and chondroitin sulfate (CS, Sigma) was obtained frombovine tracheal cartilage.  Table 1 Surface tension parameters (mN/m) of the liquids used for contact anglemeasurements.Liquid    LV    dLV    pLV    − LV    ∗ LV Water 72.8 21.8 51.0 25.5 25.5Ethylene glycol 48.0 29.0 19.0 47.0 1.92Diiodomethane 50.8 50.85 0 0 0.72  2.1.2. The obtaining of polyurethane blends With new synthesised polyurethane and natural componentswere prepared films with the following compositions: (1) PU,(2) 90% PU+10% HC, (3) 90% PU+9% HC+1% kEL, (4) 90 PU+9%HC+0.9% kEL+0.1% HA, (5) 90 PU+9% HC+0.9% kEL+0.1% CS.The mixing of these components was realized in DMF solutionsat 60 ◦ C temperature. After solutions homogenization, films wereobtained by DMF evaporating. These were dried a few days in ahigh vacuum chamber until a full evaporation of the solvent wasobtained, which was tested by FT-IR spectroscopy.  2.2. Methods Fourier Transform Infrared Spectroscopy (FT-IR)FT-IR spectra were recorded on thin films by ATR method bymeansofaFT-IRDIGILAB,ScimitarSeries(USA)spectrometerwitha resolution of 4cm − 1 . A crystal from SeZn with refraction indexof 2.4 was used. Processing of the spectra was done by means of Grams/32 program (Galactic Industry Corp.).The contact angles for the polymer films were determined bythe sessile drop method, at 20 ◦ C, within 30s after placing 1  L drops of liquids on the film surface [24] by means of a KSV CAM 200 instrument. The components of the free surface energy for thestudied blends were determined using three pure liquids (water,formamide and diiodomethane) and the geometric mean (GM)method [25]:1 + cos    2  ·   LV     dLV =     pSV ·     pLV   dLV +     dSV   SV  =   dSV +   pSV where   isthecontactanglebetweensolidsurfaceandliquid,deter-minedwiththreeliquidsnamelytwicedistilledwater,formamide,diiodomethane whose surface tension parameters are known –Table1,subscripts‘LV’and‘SV’denotetheinterfacialliquid–vapourand surface–vapour tensions, respectively, while superscripts ‘p’and ‘d’ denote the polar and disperse components, of the total sur-face tension,    SV .For electron donor    − and electron acceptor    + components of the free surface energy the following relations have been used:1 + cos     = 2   LV     LWSV  ·   LWLV  +     + SV ·   − LV +     − SV ·   + LV    ABSV  = 2     + SV ·   − SV   LW/ABSV  =   LWSV  +   ABSV wheresuperscripts‘LW’and‘AB’indicatethedisperseandthepolarcomponent obtained from the    sv  electron donor and the    + sv  elec-tron acceptor interactions, while superscript ‘LW/AB’ indicates thetotal surface tension.Using polar-dispersive theory for the surface energy, the inter-facial tension between blood and the film surface (   SL  ) is:   SL   =  (   pL  ) 1 / 2 − (   pS ) 1 / 2  2 +  (   dL  ) 1 / 2 − (   dS ) 1 / 2  2  648  M.C. Popescu et al. / International Journal of Biological Macromolecules 47 (2010) 646–653 Fig. 1.  FT-IR–ATR spectra of PU and blends of PU with HC, kEL, HA, and CS. where    p and    d are the polar and, respectively, dispersive com-ponents of the free surface energy; L and S stand for liquid andsolid.The medium–cell interfacial tension is    SL  =1–3mNm − 1 . Thus,it seems reasonable to consider that good compatibility with aforeign surface and mechanical stability of this interface can beassured when the blood–biomaterial interfacial tension is in thisrange [26]. To obtain reproducible results for contact angle deter- minations, several conditions have to be met, such as constanttemperature during the determinations; the same volume of sol-vent drops (not higher than 1  L); evaluation of the contact anglesat different points of the studied surface (at least 10 determina-tions), the final result being the average of the obtained values.Precision in evaluating the components of the free surface energyisgivenbytheprecisioninreadingthecontactanglesbetweenthepolymer surface and the used pure liquids. The produced errorsin contact angle determination are mainly caused by the surfaceroughness and by the chemical heterogeneity of the polymericsystems [27]. The highest accepted variation in the values of the contact angles was ± 1–2 ◦ , in the case of the most heterogenic sur-faces.  2.2.1. DSC measurements DSC measurements were performed by means of a Pyris Dia-mond(PerkinElmer)instrument.Thesamplesmassof6–8mgwereplaced in aluminum pans. DSC curves were recorded in nitrogenatmosphere(20mL/minflow)withaheatingrateof20 ◦ C/minfrom − 60 to 245 ◦ C temperature range. The inflexion point of DSC curvewas took as glass transition temperature ( T  g ). Two runs were per-formed for each sample. As reference was used high purity (98%)indium which has melting temperature at 156.68 ◦ C and meltingenthalpy of 28.4J/g.  2.2.2. Thermogravimetry (TG/DTG) The thermogravimetric (TG) and derivative thermogravimet-ric (DTG) curves were recorded on a Paulik-Paulik–Erdey typeDerivatograph, MOM-Budapest, under the following operationalconditions: film samples, heating rate (  ) 12 ◦ Cmin − 1 , tempera-ture range 25–600 ◦ C, sample mass 50mg, in platinum crucibles,air flow 30cm 3 min − 1 . Two curves were recorded for each sam-ple. Actual ( ˇ ) values were evaluated from the temperature–timecurve and the calculated ( ˇ ) values were further employed in theevaluation of the kinetic parameters.For each TG stage, the following thermal characteristics havebeen determined: onset temperature ( T  i ); temperature corre-sponding to the maximum mass loss ( T  m ); and to the end of stage( T  f  ),respectively(errorsintemperaturedeterminationareof  ± 2 ◦ C),massloss(  w ,error ± 1%)andoverallkineticparametersasactivationenergy, E  a  (errorofdetermination ± 10–15kJ/mol)usinga VERSATILE computer program.  2.2.3. Scanning electron microscopy (SEM) Thesampleswereinitiallycuttedinliquidnitrogen.Toimprovethe conductivity of the samples and the quality of the SEM images,the samples were coated with one carbon and three silver thinlayers using a covering SEM device.Microscopical examination was made using a SEM tip VEGA 2SBH Tescan microscope, the magnification being indicated on thefigures. 3. Results and discussions  3.1. FT-IR spectroscopy Infrared spectroscopy is an important tool used to characterizethe interactions between two or more components in a polymerblend.Polyurethanes(PU)areatypeofpolymerswhichconsistofalter-natingsoftandhardsegmentunits.IthasbeenacceptedthatmanyoftheunusualpropertiesofthesematerialsareprimarilyduetothetwophasestructurewhichiscloselyrelatedtotherelatingH-bond.Itisacceptedthat  NHand  C ObandsinFT-IRarethetwotypicalbands used to judge if the H-bonds have been formed in PU. Theirfrequencies are usually used to analyze the H-bond strength. Both  NH and   C O bands will shift to lower frequency when H-bond(NH ··· O C)isformed.  C Obandfromurethaneisalsosometimescalled amide I band, which usually appears at 1700–1740cm − 1 .The amide bands are sensitive to states and conformations. Theyare composed of different vibrational modes and have compli-cated structures. Therefore, other four bands except amide I bandhave seldom been deeply studied on the relation with H-bonds.Sometimes, amide II and III bands at about 1530–1540cm − 1 and1220–1230cm − 1 areusedtobringadditionalproofsfortheforma-tion of urethane structure and H-bond in PU.In Fig. 1 are presented the IR spectra of PU and PU blends with HC, kEL, CS and HA. The IR spectrum of PU confirm the presenceof N–H stretching vibration at 3320cm − 1 ; the absorption of C–Hstretching of methyl or methylene group at 2925 and 2855cm − 1 .Theabsorptionbandat1726and1704cm − 1 correspondstothefreeand H-bonded C O groups; the absorption bands at 1648, 1528and 1218cm − 1 corresponds to the vibration of amide I, II and IIIgroups; the band at 1598cm − 1 corresponds to the symmetricalstretchvibrationabsorptionofaminogroup;thebandat1380cm − 1 corresponds to stretching vibrations of CN bond and the stretchvibrations of C–O which are found at 1061 and 1020cm − 1 .The amide and carbonyl regions of the spectra provided infor-mation about the intermolecular attraction by hydrogen bonding(H-bonding). The PU spectrum displayed a broad peak centered at3320cm − 1 (H-bonded NH groups) and strong bands at 1703cm − 1 (assigned to carbonyl groups involved in H-bonding with the NHgroups of the urethane fragments). This is related to the high sym-metryofthechemicalstructureandthepresenceofanevennumberofcarbonatoms,whichallowedaclosechainassociationbyhydro-gen bonding.By adding of the other components in the blends was possi-ble to evidence spectral modifications in 1500–1800cm − 1 region.Comparison of the IR spectra of the PU blends studied shows thatthe nature of absorption of the carbonyl groups strongly dependson the component type and ratio. For example, in the spectrumof the PU − HC blend (Fig. 1, spectrum 2), the intensity of the band at 1729cm − 1 increase as compared to that of PU; a new band is  M.C. Popescu et al. / International Journal of Biological Macromolecules 47 (2010) 646–653 649 found at 1644cm − 1 ; and the intensities of the bands at 1600 and1615cm − 1 decrease.By adding of kEL in PU − HC blend a significant increase of theband at 1645cm − 1 (by contribution of the amide I groups) wasobserved and this band merges with that from 1600cm − 1 ; the lastone being visible as a shoulder in the spectrum.InthecaseofPU − HC − kEL  − HAblend,wasobservedthesepara-tion of these two bands, as distinct ones, having almost the sameintensity.By adding of CS in PU − HC–kEL blend these two bands appearas individual bands, the intensity of that at 1655cm − 1 being muchlower than that corresponding to 1600cm − 1 .The appeared spectral modifications, evidence the existence of specificinteractionsbetweenblendcomponentsatthelevelofcar-bonyl groups.For all samples, the FT-IR spectra revealed sharp peaks at1729cm − 1 corresponding to the free urethane and/or ester car-bonyl groups and at 1706cm − 1 corresponding to the H-bondedurethaneand/orestercarbonylgroups.Tomakethespectralmodi-ficationsofthesebandsclearerthespectraweredeconvolutedwithGaussiantypefunction.Afterdeconvolutionwasobservedthattheintegral absorption of the first one increases and the second onedecrease by adding the blends components. This fact suggests thatthe nature of the intermolecular interactions changes on passingfrom one system to another.These observations are supported also by the variation of theband position corresponding to the H-bonded NH groups from3320cm − 1 .ByaddingoftheHCandkELthisbandisshiftedtolowerwanenumbers (3316 and 3304cm − 1 ), while by adding the HA andCS this band is shifted to 3311 and 3313cm − 1 .Using the value of wavenumbers corresponding to the max-imum of absorption bands for hydrogen bonds, the energy andhydrogen bonding distance can be estimated. The energy of thehydrogen bonds has been evaluated using the following formulae[28]: E  H  = 1 k   0 −  0  where   0  is the standard frequency of the NH (3474cm − 1 )monomers observed in the gas phase while    is the stretchingfrequency of NH groups observed in the infrared spectrum of thesample, and  k  is a constant equal to 0.38 × 10 − 2 kJ − 1 .The hydrogen bonding distances are obtained by using theSederholm equation [29], namely:  (cm − 1 ) = 0 . 548 × 10 3 (3 . 21 − R )where   =  −  0  (stretching frequency of the NH monomerobserved in gas phase)—   (stretching frequency observed in theinfraredspectrumofthesample),and  0 (monomericNHstretchingfrequency) is taken to be 3474cm − 1 [29].The obtained values for H-bonding energy and H-bondingdistance are: 11.6kJ and 2.93 ˚A for PU, 11.9kJ and 2.92 ˚A forPU+HC, 12.8kJ and 2.89 ˚A for PU+HC+kEL, 12.3kJ and 2.91 ˚A forPU+HC+kEL+HA and 12.0kJ and 2.92 ˚A for PU+HC+kEL+CS.ThisindicatesthatthefunctionalgroupsinvolvedinH-bondingwould bond in blends in a more efficient manner. The occurredchanges in the energy of H-bonds suggest the role of these bondsin formation and stabilization of the blends structures.  3.2. Contact angle measurements The adding of the natural polymer increases the polar compo-nents and total surface tension of the blends (Fig. 2).Glycosaminoglycans are very hydrophilic due to the negativecharge on their chains. Both electron donor and electron acceptor Fig. 2.   S  and    ABS  variation for the systems based on PU. components of the surface tension vary similarly with the polarcomponent of the surface tension. From Fig. 3 can be remarkedthat the polar term    ABS  gives a large contribution to    S , due to thelarge electron donor    − LV  interactions The added natural extracel-lularmatrixpolymersdetermineanincreaseofthesurfacetensionvalue for all the blends in comparison with polyurethane.Free energy of hydration and interfacial tension blood mate-rial are very important in that they determines the interactionforce between two different media and controls the different pro-cesses: stability of the colloidal aqueous suspensions; dynamic of themolecularself-assembling;wettabilityofthesurface;spacedis-tributionandadhesiveness.Thebiologicalandchemicalprocesses,which take place at the surface level of the biomaterial, depend onthe interfacial interactions between solid and liquid (water). Thevariation of the interfacial tension by incorporating of the biologi-cal molecules into polyurethane is not essentially changed – Fig. 4– it varies between 5 and 8mN/m.It is well known that the “biomaterials” are non-living sub-stances selected to have predictable interactions with contactingbiological phases, in applications ranging from medical/dentalimplantstofoodprocessingorcontrolofbiofoulinginthesea.Morethan 30 years of empirical observations of the surface behaviorsof various materials in biological settings support the definitionof the “theta surface”, when correlated with the contact-angle-determinedcriticalsurfacetensions(CST)forthesesamematerials.The “theta surface” is that characteristic expression of outermostatomic features least retentive of depositing proteins, and iden-tified by the bioengineering criterion of having measured CSTbetween20and30mN/m[30].Biomaterialsapplicationsrequiring strong bioadhesion must avoid this range, while those requiringeasy release of accumulating biomass should have “theta sur-face” qualities. Selection of blood-compatible materials is a main Fig.3.  ThecomponentsvariationofthefreesurfaceenergywithcompositionofthePU-based blends.  650  M.C. Popescu et al. / International Journal of Biological Macromolecules 47 (2010) 646–653 Fig. 4.  Interfacial tension blood/material for systems based on polyurethane. example. It is forecast that future biomaterials will be safely andeffectivelytranslateddirectlytoclinicaluse,withoutrequiringani-maltesting,basedonlaboratorydataforCST,proteindenaturation,and cell spreading alone. The main concepts for over thirty yearshave been control of (a) surface charge, (b) surface texture, and(c) surface energy. Judging each concept by the practical productsthat have resulted and continue to benefit personal, public andenvironmental health, there is a good case for surface energy con-trol as the dominant factor in modulating biological responses tosynthetic materials. Contact angle measurements with evaluationof the blood–material interfacial tension could be considered andin vitro laboratory tests for biomaterials. According to the resultsobtained the studied samples proved to have good characteristicsas biomaterials [31].The obtained results demonstrate that even incorporatedproteins (collagen and elastin) in polyurethane blends assuresimprovement of the biocompatibility characteristics in respectwith polysaccharides whose effect is less significant.However, in some applications the surface of the biomaterialmustreducetominimumtheblood–biomaterialinterfacialtensionsuch as the modification of the adsorbed proteins to be little. It isgenerallyconsideredthattheblood–biomaterialinterfacialtension Fig. 5.  DSC curves of the PU/HC blends with addition of elastin, hyaluronic acid orchondroitin sulfate. Fig. 6.  TG/DTG curves of PU and blends of PU with HC, kEL, HA, and CS. shouldbe1–3mN/mforagoodblood–biomaterialcompatibility,aswell as a good mechanical stability of the interface. In this purposefurther treatments should be applied as it was demonstrated forother polyurethane materials [32,33].  3.3. DSC resultsT  g  is one of the most important characteristics of the polymericmaterials. The DSC curves obtained on heating of PU and blendsusedinthisstudyareshowninFig.5andthethermalpropertiesare summarizedinTable2.InPU,phaseseparationofsoftandhardseg- mentscantakeplacedependingontheirrelativecontent,structuralregularity and thermodynamic incompatibility.Depending on the composition/structure, PUs show soft andhard segments which can act as independent structures, thus canappear both second order transitions ( T  g ) and melting of soft seg-ments ( T  ms ) or melting of hard segments ( T  mh ).  Table 2 Thermal properties of studied blends determined by DSC.Sample  T  g  ( ◦ C)   c  p  (J/g K)  T  ms  ( ◦ C)   H  ms  (J/g)  T  mh  ( ◦ C)   H  mh  (J/g)PU  − 24.6 0,18 54.2 2.20 220,1 2,61PU+HC  − 25.1 0.17 73,5 1,84 222,4 5,29PU+HC+kEL   − 25.6 0,23 80,1 2.89 219,8  − PU+HC+HA  − 24.7 0.23 79,7 6,24 219,9 8,73PU+HC+CS  − 26.3 0.22 81,9 8,18 223,9 9,28
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