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Bio-based polyamide 11: Synthesis, rheology and solid-state properties of star structures

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Bio-based polyamide 11: Synthesis, rheology and solid-state properties of star structures
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  Bio-based polyamide 11: Synthesis, rheology and solid-stateproperties of star structures Lucrezia Martino a , Luca Basilissi b , Hermes Farina b , Marco Aldo Ortenzi b , Elisa Zini a ,Giuseppe Di Silvestro b, ⇑ , Mariastella Scandola a, ⇑ a University of Bologna, Department of Chemistry ‘‘G. Ciamician’’ and INSTM UdR Bologna, via Selmi 2, 40126 Bologna, Italy b Dept. of Chemistry, University of Milano and INSTM UdR Milano, Via Golgi 19, 20133 Milano, Italy a r t i c l e i n f o  Article history: Received 6 February 2014Received in revised form 18 June 2014Accepted 12 July 2014Available online 23 July 2014 Keywords: Polyamide 11Star polymersSolid-state propertiesMelt viscosityCrystal structure a b s t r a c t Polyamide 11 (PA11) is a bio-based technopolymer synthesized using 11-aminoundeca-noic acid from castor oil. In this work star-shaped PA11 structures are obtained via one-pot copolycondensation of 11-aminoundecanoic acid with a multifunctional agent, either3-functional bis hexamethylentriamine or tetra-functional 2,2,6,6-tetra[ b -carboxyethylcy-clohexanone]. A theoretical model predicting the macromolecular complexity of the starshaped polycondensates that result from the reaction is used to interpret the rheologicalbehavior of the synthesized samples, that is found to be strongly affected by the presenceof branching. It is shown that melt properties can be modulated, for easier processability,by simply controlling monomer feed ratio and conversion of the copolycondensation reac-tion. In the range of comonomer functionality and content explored, thermal properties donotsignificantlyvarywithchangingmacromolecular architecture. All star-shapedpolyam-idesampleseasilycrystallizeandtheirstructuralcharacterizationaboveroomtemperatureshows that upon heating they reversibly undergo the alpha-gamma crystal phase transi-tionthatistypicaloflinearPA11.Investigationoftheeffectofbranchingonthemechanicalproperties shows that careful selection of chain controller type and amount allows toobtain star-shaped PA11 samples with  ad-hoc   modulated melt viscosity and preservedmaterial’s mechanical performance.   2014 Elsevier Ltd. All rights reserved. 1. Introduction Polyamides have been widely investigated for decades.They are engineering polymers applied in several indus-trialfieldssuchasautomotive,packaging,electricandelec-tronics and oil, due to their excellent performance relatedto high-temperature and chemical resistance, toughnessand easy processability [1]. Regarding the last feature, inrecent years there has been an increasing interest [2–9]in star shaped polyamides, consisting of linear chainslinkedto a central core, whichpossess interestingrheolog-ical properties and offer significant advantages from anindustrial point of view [9].Star-shapedpolyamidescanbesynthesizedbypolycon-densation of AB type monomers in the presence of amultifunctional compound RA  f  , where  f   is the number of A-type functional groups (in order to obtain star-shapedpolymers,  f  P 3) and R is the core of the compound. Theproduct of such reacting system is a mixture of linearchains and star polymers (Scheme 1) in which  f   chains, atmost, are attached to the central branch point [4].Itisknownthatmeltviscosityoflinearpolymerscanbemodifiedbytheintroductionofstarstructuresinthemacro-molecule [10–14]. The change of rheological properties http://dx.doi.org/10.1016/j.eurpolymj.2014.07.0120014-3057/   2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses:  Giuseppe.disilvestro@unimi.it (G. Di Silvestro),Mariastella.scandola@unibo.it (M. Scandola).European Polymer Journal 59 (2014) 69–77 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj  dependsnotonlyonthefunctionalityofthebranchingagentbutalsoonthelengthofeverybranchofthestarmacromol-ecules.Inparticular,onlywhenthebranchlengthexceedsacriticalvaluewhichenablesarmstoentangle,meltviscosityincreases with respect to that of the linear polymer withcomparable molecular weight. On the other hand, if thestar-shapedmacromoleculespossessshortarms,theyformcompact structures exhibiting lower melt viscosity thantheir linear counterparts [11,15]. The opportunity to easilymodulate melt viscosity is desirable for industrial applica-tions; in injection molding easy mold filling requires lowmeltviscosity,thatalsoallowstolowerprocessingtemper-atureandpressure,whereashighviscosityisdesiredinpro-cessessuchasblowmolding[8,16].Many studies attempt to theoretically elaborate thecomplexity of polycondensate systems leading to starstructures, including Flory’s pioneering work [17–19] andlately the kinetic model developed by Yan et al. [20] inwhich molecular weight distribution and other molecularparameters like dispersity index and average degree of branching can be theoretically calculated.Astatisticalmodel wasdevelopedbyYuanandcowork-ers [4] where both molecular weight and content of allmacromolecular species inthe final product – whetherlin-ear or star-shaped – can be simply derived from two datainputs: functional RA  f   agent concentration in the initialfeed and conversion of the polymerization reaction, givenfor example in a polyamide by the amount of amino andacid terminal groups in the final product. The model wasearlier successfully applied to describe the structural com-plexity and molecular weight distribution of star-shapedpolyamide 6 [9] and polyamide 12 [3]. Polyamide 11 is synthesized starting from castor oil; itis therefore 100% derived from renewable sources and thisfeature, together with its good mechanical properties,makes it very attractive from an industrial point of viewas an environmentally friendly engineering polymer. Itfinds a wide range of applications as a technopolymer(e.g.pipesfornaturalgastransportation,hosesforvehicles,bearings for windshield wiper arms and other car parts,precision moldings for engineering uses, electrical cables,sports articles, travel bags, etc. [1,21]). Despite the highinterest towards this polyamide, star shaped structuresbased on 11-aminoundecanoic acid monomer have notyet been described.Thepresentworkreportstheone-potsynthesisofnovelstar-shaped PA11 polymers obtained using differentamountsofatri-functional (i.e. bishexamethylentriamine)or tetra-functional agent (i.e. 2,2,6,6-tetra[ b -carboxyethyl-cyclohexanone]) as RA  f   comonomer. Several importantsolid-state properties, in particular mechanical properties,may be affected by the presence of branching, to slight orgreat extent depending on the concentration of themultifunctional agent used as comonomer and on itsfunctionality [6,8].This paper investigates both rheological and solid-stateproperties of the synthesized PA11 star samples. Theresults of melt viscosity, thermal, mechanical, dynamic-mechanical and X-ray diffraction measurements are dis-cussed in terms of the polymeric species that the Yuanmodel[4]predictstobepresentintheappliedpolymeriza-tion conditions. It is concluded that, as long as the armlength of the stars is high enough, solid-state propertiesof star PA11 do not appreciably change although meltviscosity remarkably varies. 2. Experimental section  2.1. Materials and sample preparation Polyamide 11 (PA11) samples were synthesized using11-aminoundecanoic acid (99% purity, from Arkema) asmonomer and bis hexamethylentriamine (T3) or 2,2,6,6-tetra[ b -carboxyethylcyclohexanone] (T4), as tri- andtetra-functional agents respectively: T3 was purchasedfrom Sigma Aldrich, T4 was synthesised according to liter-ature[22]. Benzene-1,3,5-tricarboxylicacid(trimesicacid),used for the synthesis of a ‘‘model’’ star-PA11, chloroformand trifluoracetic anhydride were also provided by SigmaAldrich. All chemicals were used without furtherpurification.Out of clarity, Fig. 1 reports the actual molecularstructure of the multifunctional comonomers used for thiswork:bishexamethylentriamine(T3), 2,2,6,6-tetra[ b -carb-oxyethylcyclohexanone] (T4) and trimesic acid.PA11 samples investigated in this work were synthe-sized in bulk at 240  C for 4h under nitrogen flow.Synthesis of star-shaped PA11 was carried out in one-potby adding the multifunctional comonomer during thepolymerization step. Seven additional linear PA11 sampleswere synthesized specifically for SEC calibration in bulkunder nitrogen flow for different times (0.5, 1, 2, 3, 4, 5or 6h respectively) using only 11-aminoundecanoic acidas monomer in the feed.  2.2. Instrumental methods SEC analyses were conducted with a Waters IsocraticHPLC Pump 1515, a Waters 2487 Dual  k  absorbance detec-tor set at 244nm and a Shodex KF 804-803-803-803-805-802.5 six column set using anhydrous CH 2 Cl 2 ; before SEC Scheme 1.  Macromolecular species that may form using a multifunc-tional comonomer RA  f  .70  L. Martino et al./European Polymer Journal 59 (2014) 69–77   analysis PA11 samples were N-trifluoroacetylated. O-dichlorobenezene was used as internal reference for theanalyses.AsstatedinSection2.1,linearPA11samplesusedfor SEC calibration were synthesized with controlledmolecularmassesthroughkineticallycontrolledprocesses.Solid-state  15 NNMRwereobtainedusinganNMR spec-trometer AvanceTM 500 (Bruker BioSpin S.r.l) geared witha superconductingultrashieldmagnet of 11.7T; the exper-iments were acquired at 50.68MHz frequency, with theCPMAS probe facility (cross polarization with Magic AngleSpinning), able to acquire NMR spectra directly from solidsamples, without the need of any additional solvent. 15 N CPMAS runs were carried out under Hartmann–Hahn conditions (cross-polarization CP NMR sequence),with  1 H high power decoupling, at 5kHz MAS spinningrate, with a contact Time (CT)=2ms, a relaxation delayD1=5s and the temperature was around 29  C.Full spectral widths of 600ppm were acquired, withexpansions and plot of the regions of interest. Degree of polymerization was determined by amino end group anal-ysis. Before such analysis, samples were washed in hotwater for 6h to eliminate unreacted monomer and thendried overnight at 60  C under vacuum. Titration was per-formedon samples dissolvedat reflux in a mixture of met-acresol/methanol/water (83/12/5). Amino terminal groupswere titrated with a 0.01M HCl solution in methanol.Table 1 shows the results of titration for the linear PA11samples used in SEC calibration. The polydispersity ( D ) of the samples listed in Table 1 was set at 2, as the standardvalue for linear polycondensation polymers.Thermogravimetric (TGA) measurements were carriedout using a TA-TGA 2950 instrument. The analyses wereperformed at 10  C/min from room temperature to 700  Cunder air flow. Differential Scanning Calorimetry (DSC)measurements were performed using a TA-InstrumentsQ100DSCequippedwithaLiquidNitrogenCoolingSystem(LNCS) accessory. DSC scans were run at 20  C/min in thetemperature range   80  C/210  C in helium atmosphere.Controlled cooling at 10  C/min was applied betweenscans. The melting temperature ( T  m ) was taken at the peakof the melting endotherm. When multiple endothermswere present, the highest temperature peak was taken as T  m . Temperature modulated DSC measurements (TMDSC)werealsoperformedfrom10  Cto210  C. Theheatingrateof the TMDSC scan was 3  C/min, the oscillation amplitude0.32  C and oscillation period 40s. Dynamic-mechanicalmeasurements were performed with a DMTA (PolymerLaboratories Ltd.) operated in the dual cantilever bendingmode, at a frequency of 3Hz and a heating rate 3  C/min,on samples in the form of injection molded bars(30  7.5  2mm 3 ). The temperature range investigatedwas   150 to 160  C. Stress–strain measurements wereperformed on rectangular specimens (5mm wide) die-cut fromcompression-molded sheets (0.2mmthick) usingan INSTRON 4465 tensile testing machine. At least fivespecimens of each sample were tested at 3mmmin  1 cross head speed and 30mm initial gage length and theresults are reported as average values with standard devi-ation. The rheological behavior was investigated using aPhysica MCR 300 rotational rheometer. Frequency sweepcurves (from 100Hz to 0.1Hz) were obtained at 200  C,using a 25mm plate–plate geometry with 5% constantdeformation, in order to be in the linear viscoelastic rangeof the polymer. Wide angle X-ray diffraction measure-ments (WAXS) were carried at and above room tempera-ture (RT) by using an Anton Paar TTK-450 heating deviceinstalled on a PANalytical X’PertPro diffractometerequipped with an X’Celerator detector (for ultrafast datacollection). A Cu anode was used as X-ray source (K a  radi-ation:  k  =0.15418nm), and 1   divergence slit was used tocollect the data in 2 h  range from 5   to 35  . Heating runswere performed from RT to 160  C at 3  C/min, with 50sisothermal steps every 10  C to allow data collection. 3. Results and discussion  3.1. Synthesis In the present work, linear and star branched PA11samples were synthesized through polycondensation of agiven amount of 11-aminoundecanoic acid monomer inthe presence of different amounts of bis hexamethylentri-amine, T3, with three amine functionalities or 2,2,6,6-tet-ra[ b -carboxyethylcyclohexanone], T4, with four acidicfunctionalities. Alinearpolyamide11wasalsosynthesizedin the same conditions in the absence of the multifunc-tional agents.The statistical model developed by Yuan and coworkers[4] for star-branched polymers synthesized via polycon-densation of AB type monomers in the presence of a H 2 NHNNH 2 O COOHCOOHHOOCHOOC COOHCOOHHOOC Fig. 1.  Molecular structure, from left to right, of bis-hexamethylenetriamine (T3), of 2,2,6,6-tetra[ b -carboxyethylcyclohexanone] (T4) and of trimesic acid.  Table 1 Amino groups concentration and molecular weight ( M  n ) of linear PA11samples used for SEC calibration, obtained by titration analysis. Sample Reaction time (h) –NH 2  groups (meq/kg)  M  n  (Da)SEC1 0.5 413.73 2417SEC2 1.0 118.76 8798SEC3 2.0 87.59 11,416SEC4 3.0 77.80 12,853SEC5 4.0 60.57 16,510SEC6 5.0 52.01 19,227SEC7 6.0 23.61 42,355 L. Martino et al./European Polymer Journal 59 (2014) 69–77   71  polyfunctional agent was applied to PA11 samples in thiswork in order to predict the macromolecular complexityof star shaped polycondensates resulting fromthe reaction(see Scheme 1). The model quantifies the content, averagemolecular weight ( M  n ) and dispersity index ( D ) of linearchains and of macromolecules with different arm number(at most equivalent to comonomer functionality) in thefinal product, assuming that all reactive groups have thesame reactivity.While the reactivity of T4 in bulk polymerization of polyamideshasalreadybeenstudiedinscientificliterature[5], given the issues related to the presence both of pri-mary and secondary amino groups in T3 comonomer, thatcould result in different reactivity in the polymerizationconditions, a check was performed to be sure that in poly-merizationconditionsallthefunctionalgroupsofthismul-tifunctional comonomer can effectively react leading to astar polymer.Two ‘‘model’’ polyamides 11 were synthesised using 3%m/m of trimesic acid, that has already proved to be effec-tive as three-functional comonomer in polyamides poly-merization [5], and 3% m/m of T3; these ‘‘model’’ polymers contain higher quantities of multifunctionalcomonomer in comparison to the samples studied in thepaper in order to magnify potential differences betweenthe behavior of the two comonomers.SEC curves of the two model polymers are almost iden-tical (Fig. 2), showing that reactivityof T3 is comparable tothereactivityofatrifunctionalcomonomerandthat there-fore T3 acts as a trifunctional comonomer itself.To further check the quantitative reaction of T3 duringthe polymerizations, solid state  15 N NMR was used to ana-lyze pure T3 and sample 3, containing 1.45% m/m of T3(see Table 2), in order to verify that no free amino groupsof T3 are present in the polymer. The obtained spectraare shown in Fig. 3. T3 spectrum shows the presence of both primary and secondary amino groups, as expected.In the same region of sample 3 spectrum, T3 amino groupsare not visible, confirming that the comonomer has quan-titatively reacted.Table 2 collects the  M  n  and  D  values calculated consid-ering a high conversion of the polymerization reactionaccordingtotheequationsproposedinthemodel[4], from the initial feed of AB monomer and of multifunctionalagent and from the experimental concentration of endgroups, obtained by titration; in the table, the experimen-tal amino group concentration obtained via end-grouptitrationisreported. It isseenthat,withincreasingcontentof a given multifunctional comonomer (either T3 or T4),the overall average molar mass ( M  n ) of the PA11 samplesdecreases and the molecular weight distribution narrows.The values reached at the highest amount of multifunc-tional agent tested (1.45mol%) are lower than those of the linear polymer. Moreover, at a given comonomer feed,molecular weight decreases with increasing functionality(compare sample 1 with sample 4 and sample 3 withsam-ple 5, in Table 2).The table also reports the experimental  M  n  obtainedviaSEC, using the standard linear PA11 calibration previouslydescribed.Fig. 4 shows in a graphic form the distribution of thedegreeofpolymerization(DP n )ofthevariousmacromolec-ular species present, according to the model, in sample 5(taken as an example). Curve a describes the overall DP n distribution,whilecurvedregardslinearchainsandcurvesb and c are related to stars with different arm number (4and 3 arms respectively, refer to Scheme 1). It is clear thatsample5, withthehighestamountoftetrafunctional agentinvestigated, contains a prevalence of four arm species(curve b), whereas the content of two and one arm macro-molecules is very low and hence their curves (dashed) arealmost undetectable in Fig. 2.Application of the model to samples polymerized withdifferent amounts of RA  f   comonomer (either T3 or T4),shows that the content of star species with arm numberequal to the functionality increases with multifunctionalcomonomer content, at the expenses of linear chains.Fig. 4 illustrates the macromolecular complexity obtainedin an equilibrium polycondensation of AB monomers inthe presence of multifunctional agents. It shows that it ispractically impossible to obtain homogeneous polymersin terms of both degree of polymerization and number of arms possessed by each macromolecule, even at very highconversion such as in the example shown in Fig. 4.Experimental molecular weight data of the star shapedpolyamidessynthesizedinthis workwereobtainedbySECanalysis. SECcalibrationwas performedusing‘adhoc’ syn-thesized linear PA11 samples in order to obtain more reli-able results than those provided by common polystyrenestandards. The curves of star branched samples polymer-izedusingdifferentamountsofT3(samples1–3of Table2)are reported in Fig. 5, where it is clearly shown that thehydrodynamic volume decreases with increasing multi-functionalagentcontent;sincethisparameteriscorrelatedto molecular weights and their distributions, the experi-mental data are in agreement with the trend of model cal-culations (Table 2).Worth noting is that the values obtained by SEC arelower than the molecular weights calculated according tothe model (Table 2). This is due to the reduction of hydro-dynamicvolumeofstarbranchedpolymerswithrespectto Fig. 2.  SECcurvesofPA11with3%m/mT3(solid)andPA11withtrimesicacid (short-dash).72  L. Martino et al./European Polymer Journal 59 (2014) 69–77   that of the linear macromolecules with the same DP n  usedfor column calibration [17] and the difference gets higheras quantity and functionality of the multifunctional como-nomer increase. Hence, notwithstanding calibration withstandardsof thesame chemicalnature, it is concludedthatSEC analysis does not provide a precise estimate of molec-ular weight, especiallyat highconcentrations of T3andT4.Furthermore, the macromolecular complexity of starshaped polymers illustrated by the results frommodel cal-culations in Fig. 4 is not evidenced by the SEC curves  Table 2 Number average molecular weight ( M  n ) of linear and of star species in PA11 samples, evaluated by model calculations [4] according to the feed and the endgroup concentration. Sample Multi-functional agent  M  n b (SEC)  D b (SEC) –NH 2  (meq/kg) Overall  M  n c D c M  n c 1-arm  M  n c 2-arm  M  n c 3-arm  M  n c 4-armContent (mol%)  f  a Linear 0 _ 14,126 1.8 76.79 1.81 0.125 3 22,423 2.0 57.47 22,794 2.1 17,446 34,893 52,339 _2 0.45 3 21,672 1.6 93.75 22,246 1.7 10,767 21,533 32,300 _3 1.45 3 9204 1.4 251.65 10,368 1.5 4093 8187 12,280 _4 0.125 4 18,257 2.0 39.53 21,532 2.2 15,015 30,030 45,044 60,0575 1.45 4 5045 1.3 31.50 8966 1.5 3029 6059 9088 12,118 a Functionality. b Experimental value from SEC. c Values from the mathematical model Ref. [4]. Fig. 3.  Solid-state  15 N NMR of sample 3 (top) and pure T3 (bottom). Fig. 4.  Distribution of the polymerization degree of linear and star-branched species in sample 5, calculated according to reference [4]: (a) overall DP n  distribution; (b) four-arm macromolecules; (c) three-armmacromolecules; (d) linear PA11 chains. The dashed curves are related toone-/two-arm molecules. Fig. 5.  SEC curves of: Linear PA11 (solid), sample 1 (long-dash), sample 2(dot) and sample 3 (short-dash). L. Martino et al./European Polymer Journal 59 (2014) 69–77   73
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