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Composites: Part B 58 (2014) 496–501
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Composites: Part B
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Aliphatic polycarbonate-based polyurethane nanostructured materials.The influence of the composition on thermal stability and degradation
1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.11.006
⇑ Corresponding author. Address: University of Novi Sad, Faculty of Technology,Bulevar cara Lazara 1, 21000 Novi Sad, Serbia. Tel.: +381 214853628; fax: +38121450413.
E-mail addresses: [email protected] (R. Poreba), [email protected] (M.Špirková), [email protected] (J. Pavlicevic), [email protected] (J.Budinski-Simendic), [email protected] (K. Mészáros Szécsényi), [email protected] (B.Holló).
Rafal Poreba a, Milena Špirková a, Jelena Pavlicevic b,⇑, Jaroslava Budinski-Simendic b,Katalin Mészáros Szécsényi c, Berta Holló c
a Institute of Macromolecular Chemistry AS CR v.v.i., Prague, Czech Republicb University of Novi Sad, Faculty of Technology, Novi Sad, Serbiac University of Novi Sad, Faculty of Sciences, Novi Sad, Serbia
a r t i c l e i n f o a b s t r a c t
Article history:Received 28 August 2013Received in revised form 21 October 2013Accepted 4 November 2013Available online 12 November 2013
Keywords:A. Polymer–matrix composites (PMCs)B. Thermal propertiesD. Thermal analysis
Thermal properties of new materials usually determine their applicability to special purpose. In order tofollow the influence of the composition on the thermal decomposition pattern of segmented polycarbon-ate-polyurethanes (PC-PUs), a series of unfilled and filled elastomers based on different polycarbonatediols and their mixtures, hexamethylene diisocyanate and 1,4-butane diol as chain extender were synthe-sized. Hard segment (HS) content in final materials was in the range from 8 to 35 wt.%. Nanocompositeswere obtained by dispersion of organically modified bentonite additive (1 wt.%). Thermogravimetricanalysis coupled with differential scanning calorimetry (TG–DSC) was used to study the influence of mac-rodiol chain constitution, HS content and bentonite addition on the decomposition of PC-PUs underdynamic temperature conditions. It was found that the highest thermal stability belongs to the elasto-mers prepared without chain extender. The decomposition temperatures (To) of PC-PUs increase withdecreasing HS content. The significant enhancement of thermal stability is achieved in polyurethaneswith higher HS content and well dispersed nano-scale layered bentonite particles. The regularity inthermal decomposition pattern and the hard segment content is used to determine the composition ofpolyurethane materials.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
In the last decade, thermoplastic segmented polyurethanes, asspecifically multi-block copolymers, have been expansively inves-tigated to provide a unique design of elastomeric materials for thebroad range of application. In industry, due to their improvedelastic and thermal performances, low temperature flexibility,chemical and heat resistance, compared to the traditional polyure-thanes (based on polyether or polyester), they can be used asbuilding materials, industrial parts and sports goods [1,2].Segmented polyurethanes (PUs) are thermoplastics with rubber-like behavior, consisting of an alternating flexible soft macrodiol,and a hard segment formed from diisocyanate component andchain extender. Such micro-separated structure is influenced bythe incompatibility of both phases, caused by segmental lengths,
hydrogen bonding and crystallization extent [3,4]. Soft segment(macrodiol) is usually in the amorphous state and affects the PUsproperties at low temperature. Hard domains act as thermo-reversible physical crosslinks providing the formation of thermo-plastic and elastomeric properties of polyurethane materials[5,6]. The most used polycarbonate diols (PCD) are based onpoly(hexamethylene carbonate) diol, or on PCD copolymers con-taining 3–10 methylene units. Polyurethanes with hard segmentsderived from aliphatic diisocyanates possess a better light stability,a better resistance to hydrolysis and do not release toxic aminesduring their thermal decomposition unlike those containing corre-sponding aromatic derivatives [7]. In the hard segments, hydrogenbridging results in different interactions between the two phases,leading to hard and soft segments domain formation.
Since the interest for development of medical devices has beenincreased, segmented polyurethanes, due to their biostability andbiocompatibility, are noticed as possible candidates for the appli-cations as vascular prostheses, soft tissue adhesives, pericardialpatches, and even in tissue engineering [8–11]. For medical device,the research on biodegradability of segmented thermoplastic poly-urethanes has attracted growing interest recently [12] and hence,the thermal stability and decomposition of these materials have
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been intensively studied [13,14]. Soft segment, chain extender anddiisocyanate are typical impact factors on decomposition proper-ties of polymers. Minor difference in the component of eachsegment is amplified because of the repetitive nature of polymers,resulting in drastic changes in property [15]. The polyurethanedecomposition has been investigated by many researchers. The re-sults indicate that several decomposition processes simultaneouslytakes place which can be assigned to urethane bond decompositionand polyol depolycondensation [16–18]. A number of polyure-thane/clay nanocomposites have been prepared. The studies referto the enhancement of thermal stability by reinforcement ofpolymer matrix with organoclay particles (bentonite or montmo-rillonite) [19–24].
In our previous publication, which was focused on the investi-gation of polyurethanes based on polycarbonate diols with molec-ular mass ca. 1000 (type 5651 or 4671), it was shown that thethermal decomposition of elastomers is a complex process consist-ing of more or less overlapped steps [23]. Also, it was found thatthe thermal stability can be improved by addition of organicallymodified nanoparticles with layered structure (bentonite or mont-morillonite). Aliphatic polyurethanes based on polycarbonate diolswith higher molecular mass of 2000 (types 5652 and 4672) werethe subject of research. The influence of hard segment content,the macrodiol composition and bentonite addition on bottom-upself assembly [25], as well on dynamical, thermomechanical andbarrier properties of elastomers and their nanocomposites [26],was studied. The aim of the present study is to determine the cor-relation between the composition and the decomposition patternof the new materials using simultaneous TG/DSC measurements.The influence of macrodiol chain constitution, its molecular mass,the hard segment content and bentonite addition on the decompo-sition was analyzed. Samples with the highest and lowest hardsegment content were chosen to determine the effect of nanofilleron the decomposition. The relationship found between the calcu-lated DTG height ratio and the NHCOO or HS content can be usedto determine the composition even in polyurethane materials withsuperposed decomposition steps.
2. Experimental part
2.1. Materials and preparation
The detailed specification of all aliphatic components used forthe synthesis of polycarbonate-based polyurethanes and their hy-brid materials is described in the previous paper [25]. Two polycar-bonate diols with molecular mass of 2000 (T5652 and T4672), andtwo polycarbonate diols with molecular mass of 1000 (T5651 andT4671), kindly provided from Asahi Kasei Chemical Corporation)were used, differing in OH value, water content and viscosity at50 �C [27]. The macrodiol nomenclature is following: the firsttwo digits are numbers of methylene units in the copolymer (C5and C6 resp. C4 and C6), the third digit is the molar ratio of the bu-tane (resp. pentane) units, and the last digit indicates the molecu-lar mass of PC in thousands. Hexamethylene-diisocyanate (HDI),1,4-butane diol (1,4-BD) and the catalyst, dibutyltin dilaurate(DBTDL), were all purchased from Fluka. Organically modified ben-tonite (Bentonite for organic systems, BO, Fluka) was used for thepreparation of nanocomposites (1 wt.%). The detailed descriptionof applied one-step technique to obtain polyurethane sheets canbe found in our previous study [25]. The ratio R of OH groupsbelonging to the macrodiol and the chain extender (CHE) was var-ied from 0.3 to 10. Some samples based on the mixture of polycar-bonate diols differing in molecular mass (1000 and 2000, 50/50 mol) and without chain extender were also prepared. Thesample codes and compositions of all investigated polyurethaneunfilled and filled materials are given in Table 1 (for example, sam-
ple code 5652/10/1/9 means PU prepared from macrodiol type5652, with R = 10, containing 1 wt.% of bentonite and 9 wt.% ofhard segments).
2.2. Thermogravimetry coupled with differential scanning calorimetry(TG–DSC analysis)
The thermal decomposition of the polyurethane materials wasstudied by simultaneous TG/DSC measurements using TA Instru-ments SDT Q600 thermal analyzer. The measurements wereperformed from room temperature to 450 �C in flowing nitrogenatmosphere (100 cm3/min). Sample masses were about 3 mg. Allexperiments were done with a heating rate of 20 �C/min andemploying open alumina crucible and a corresponding emptyreferent crucible.
3. Results and discussion
The micro-phase structure of prepared PC-PUs depends on hardsegment content (Fig. 1). Hard segments are composed of HDI andBD, while soft segments are formed from polycarbonate diol (type5652 or 4672). It was found that different regularity and flexibilityof polycarbonate chains influence the end-use properties of PC-PUsmainly in the region of low HS content [25]. It is supposed that atlow hard segment content a uniform physical crosslink PU struc-ture is formed (Fig. 1a), while in samples with higher HS contentextended cross-linking between the hard segment building units(HDI-BD) is expected (Fig. 1b).
3.1. Thermal stability of starting components
The thermal behavior of starting compounds was studied first.The decomposition of polycarbonate diols is completed in arelatively narrow temperature range between 300 and 400 �C.However, a slow decomposition starts above 200 �C. This initialpart of the decomposition most probably belongs to the breakingof the terminal groups’ bonds. No significant difference was ob-served in the thermal stability of diols type 5652 and 4672. TheDTG onset as well as peak temperatures of both macrodiol typesare very close, in the temperature range from 323 to 325 �C, andfrom 374 to 378 �C, respectively. Similar decomposition patternis observed for diols with molecular mass of about 1000 (types5651 and 4671). It means that the length of the aliphatic chain inmacrodiols very little affects the decomposition mechanism. Onlythe onset temperature shifts slightly to higher values with increas-ing molecular mass, as is expected [28]. DSC curves of both macro-diols show a small endothermic effect around 80 �C which is mostprobably related to the relaxation of the chain segments. Thedecomposition is endothermic in the whole temperature range.The soft segments (5652 and 4672 diols) show significantly betterthermal stability compared to that of the hard segment buildingunits. The evaporation of 1,4-BD and the decomposition of HDI ispractically completed up to 250 �C, with the onset temperaturesat 105 �C and 130 �C and the corresponding peak temperatures at170 �C and at 190 �C, respectively.
3.2. The influence of macrodiol molecular mass on thermaldecomposition of PC-PU elastomers
The assessment of the macrodiol molecular mass effect on thedecomposition was done by comparison of elastomers preparedwithout chain extender based on diol with molecular mass of2000 and the mixture of diols differing in chain length (2000 and1000, mole ratio 50/50). Fig. 2 shows that the thermal stability ofpolyurethane based on 5652–5651 polycarbonate diol mixture is
Table 1Formulation and names of filled and unfilled polyurethane samples prepared withdifferent ratio R.
Sample code PC diol Bentonite,wt.%
RatioR
Hard segmentcontent, wt.%
5652/inf/0/8 5652 0 No CHE 85652/10/0/9 5652 0 10 95652/10/1/9 5652 1 10 95652–5651/inf/0/11 5652/5651 0 No CHE 115652/2/0/14 5652 0 2 145652/1/0/19 5652 0 1 195652/0.5/0/27 5652 0 0.5 275652/0.5/1/27 5652 1 0.5 275652/0.3/0/35 5652 0 0.3 354672/inf./0/8 4672 0 No CHE 84672/10/0/9 4672 0 10 94672/10/1/9 4672 1 10 94672–4671/inf/0/10 4672/4671 0 No CHE 104672/2/0/13 4672 0 2 134672/1/0/18 4672 0 1 184672/0.5/0/26 4672 0 0.5 264672/0.5/1/26 4672 1 0.5 264672/0.3/0/34 4672 0 0.3 34
Fig. 2. DTG curves of polyurethanes based on polycarbonate diol with molecularmass of 2000 (type 5652 or 4672), and on the mixture of diols with molecularmasses of 2000 and 1000 (5652–5651 and 4672–4671).
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significantly lower than that of the sample synthesized using diol5652 only. DTG curves (Fig. 2) refer to better separated processesin elastomers prepared using the mixture of diols differing in size(DT = 16 �C). The thermal stability of polyurethane based on themacrodiol mixture 4672–4671 is practically independent on themacrodiol molecular mass.
3.3. Thermal decomposition of unfilled PC-PU elastomers
Typical simultaneous TG/DSC measurements data presented bythe corresponding TG, DTG and DSC curves of unfilled polyure-thane elastomer (with 27 wt.% of hard segments and based on dioltype 5652) are given in Fig. 3.
The curves show that the elastomer is chemically stable toabout 250 �C, with the onset temperature at To = 297 �C. Up tothe onset temperature only a small mass loss (2.7%) is observed.All decomposition processes are accompanied by endothermiceffects. In the temperature range to 200 �C two endothermic peaks,related to solid-phase structural changes, are detected. As thesimultaneous processes are better detected in DTG curves,corresponding DTG curves were used to determine the influenceof the elastomers’ constituents on the decomposition pattern.
DTG curves for elastomers with hard segments varying from8 wt.% to 35 wt.% are shown in Figs. 4 and 5. It can be seen thatthe thermal decomposition of polyurethanes consists of highly
Fig. 1. Schematic illustration of PC-PU structu
overlapped processes. Based on the DTG results of starting compo-nents, the first step in DTG curves is most probably assigned to thedecomposition of hard segments and is related to the scission ofurethane links [29,30]. The second step could be ascribed to thethermal decomposition of soft segments (i.e. polycarbonate units).According to the shape of DTG curves, the decomposition mecha-nism depends on the macrodiol chain constitution and the macro-diol/chain extender ratio which varied in the range from 0.3 to 10.The influence of soft segments building units is more pronouncedfor elastomers based on diol type 4672 (with different ratio of bu-tane, C4, and hexane, C6, units), than for those based on the type5652 containing pentane (C5) and hexane (C6) units in equal molarratio (see Figs. 4 and 5). Namely, the DTG peak separation is betterin case of polyurethanes with more asymmetrical building units. Inboth series the highest thermal stability belongs to the elastomerswithout chain extender. In addition, the decomposition mecha-nism in samples without chain extender differs from that insamples with similar hard segment content. This observation isin accordance with the higher regularity of the structure and theuniformity of cross-linking between the building units.
The thermal stability in both series increases with decreasinghard segment content (see Figs. 4 and 5). The stability ofpolyurethanes based on diol type 5652 is significantly higher for
re with (a) low and (b) high HS content.
Fig. 3. TG, DTG and DSC curves of unfilled 5652-based polyurethane sample containing 27% of hard segments (sample code is 5652/0.5/0/27).
Fig. 4. DTG decomposition curves of segmented polyurethanes based on polycar-bonate diol type 5652, for different ratio R (different HS content).
Fig. 5. DTG decomposition curves of segmented polyurethanes based on polycar-bonate diol type 4672, for different ratio R (different HS content).
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the elastomers with low hard segment content. The reason of thedecrease of the thermal stability with increasing the hard segmentcontent might be related to the restricted molecular motions ofsoft segments due to urethane link formation. Namely, the short1,4-BD and HDI components increase the rigidity of PUs, promot-ing thus the bond breaking between the soft and hard segments.This effect is more pronounced in the case of macrodiols with high-er molecular mass [23].
In order to determine the relationship between the thermalstability and the composition of polyurethanes, onset temperaturevalues are presented in function of HS content (Fig. 6). To in sam-ples based on diol type 4672 linearly decreases with increasinghard segment content. The relationship is not so straightforwardfor the samples containing diol type 5652 though a decrease of on-set temperatures with increasing HS content is also observed. Thisdifference can explained by the different flexibility of 5652 and4672 polycarbonate chains which is more significant in PU systemswith very low isocyanate and hence less than 10 wt.% HS content.
3.4. Determination of PC-PU composition
The determination of the composition of the polymers is neveran easy task. In this study the regularity observed in polyurethanedecomposition pattern was used for quantitative determination ofhard segment and NHCOO content of the polymer. Namely, theasymmetry of the main decomposition peak with increasing hardsegment content is inverted. For samples with higher HS content(from 18 wt.% to 35 wt.%) the first part of DTG peak has a higherintensity. In samples with lower HS content (8–14 wt.%) thehigher intensity belongs to the second part of the DTG curve.Different mechanism of thermal decomposition might be causedby different ordering of polyurethanes structure (samples withHS content less than 15 wt.% are amorphous and with higherHS content are crystalline [25]). As the peak separation is poor,instead of the peak and the shoulder temperatures, the heightof the peaks (marked as h1 and h2) at the inflexion points weredetermined (Fig. 7).
Fig. 6. The dependence of PC-PU thermal stability (To) on HS content.
Fig. 8. The linear dependence of DTG height ratio hrel on HS content of PC-PUs.
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The calculated DTG height ratio hrel = h1/h2 when presented as afunction of the hard segment content gives a linear relationship(Fig. 8). In this way, HS content of the PC-PUs with unknown com-position can be calculated using the corresponding equations(Fig. 8). The linearity in PUs containing diol type 4672 is somewhatlimited and is valid for the compositions with HS content higherthan 10% only. (The anomalous behavior of the same PU systemswas also found for the tensile property determination).
3.5. The influence of bentonite on thermal stability of polyurethanenanocomposites
The corresponding DTG curves of filled elastomers based onboth macrodiols with low and high HS content are presented inFig. 9. As can be seen, the influence of bentonite addition dependson the hard segment content. For elastomers based on 4672 or
Fig. 7. The height determination (h1 and h2) of two consecutive DTG inflectionpoints for 5652-based polyurethanes with different HS content (9, 19 and 35 wt.%).
Fig. 9. DTG curves of unfilled and bentonite reinforced polyurethane elastomersbased on polycarbonate diol type 5652 or 4672 and with different HS content.
5652 diol with low hard segment content (9 wt.%) bentonite addi-tion slightly shifts the onset temperatures and the correspondingDTG peak maxima to higher values by about 6 �C. It is assumed thatthe number of hydrogen bonds in samples with less than 9 wt.% ofHS is low, forming uniform physical crosslink PU structure. Addi-tion of bentonite does affect the already established hydrogenbond network, therefore no significant changes in thermal stabilityis caused. On the contrary, addition of inorganic particles topolyurethanes containing high HS content significantly shifts theonset temperatures to higher values (by about 17 �C).
4. Conclusions
All aliphatic polycarbonate-based polyurethanes and their hy-brid materials (with HS content in the range from 8 to 35 wt.%)
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were prepared by one-step synthesis method. Thermogravimetricanalysis coupled with differential scanning calorimetry (TG–DSC)was used to investigate the influence of polycarbonate diol, HScontent and bentonite addition (1 wt.%) on the PC-PU decomposi-tion processes. 5652-based PU samples are characterized by higherthermal stability compared to 4672-based ones. This phenomenoncould be explained by better flexibility and hence easier arrange-ment of 5652-based polycarbonates into the physical network,though the regularity of carbonate units in polycarbonate chainis higher in 4672 macrodiol than in 5652 analogue. The highestthermal stability was found for PU elastomer prepared withoutany chain extender. The onset temperatures of PC-PUs increasewith the decrease of HS content. The linear relationship, existingbetween the DTG height ratio and HS content can be used to deter-mine the composition of PC-PU samples. The addition of bentonitedid not improve thermal stability of the samples with low HS con-tent. A significant enhancement of thermal stability was achievedfor the PU elastomer with higher HS content, where formed harddomains act like defects and interact strongly with well dispersedorganoclay layered nanoparticles.
Acknowledgements
The authors from Prague would like to acknowledge to theGrant Agency of the Czech Republic (Czech Science Foundation,Project No. P108/10/0195). The authors from Serbia thank to forthe financial support to the Ministry of Education, Science andTechnological Development of the Republic of Serbia (ProjectsNos. III45022 and ON172014) and to Provincial Secretariat for Sci-ence and Technological Development (Project No. 114-451-2396/2011-01).
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