Properties of poly(ethylene terephthalate) containing epoxy-functionalized polyhedral oligomeric silsesquioxane
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Polymer International Polym Int 54:4753 (2005)DOI: 10.1002/pi.1661
Properties of poly(ethylene terephthalate)containing epoxy-functionalized polyhedraloligomeric silsesquioxaneKwan Han Yoon,1 Malcolm B Polk,2 Jae Hong Park,1 Byung G Min1 andDavid A Schiraldi31School of Advanced Material & System Engineering, Kumoh National Institute of Technology, 188 Shinpyung-dong, Kumi, Kyungbuk,730-701, Korea2School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, 801 Ferst Dr, Atlanta GA 30332, USA3Department of Macromolecular Science & Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland OH 44106, USA
Abstract: Poly(ethylene terephthalate) (PET) containing epoxy-functionalized polyhedral oligomericsilsesquioxane (POSS) was prepared by melt-mixing and in situ polymerization methods. The melt-mixed composite showed phase separation while the in situ polymerized composite did not, based onSEM characterization. During melt mixing, the reaction between the epoxy groups of POSS and hydroxylgroups of PET occurred, based on DSC results. DSC results on the in situ polymerization product showedformation of a lower-melting component compared with PET. The tensile strength and modulus of themelt-mixed composite fiber decreased compared with those properties of PET, whereas those of the insitu polymerized composite showed slightly higher values than PET despite the relatively small amounts(1wt%) of POSS used. Dynamic mechanical analysis results showed an increase in storage modulus forthe in situ polymerized composite of POSS and PET compared with PET over the temperature range of40 C to 140 C. 2004 Society of Chemical Industry
Keywords: PET; POSS; melt mixing; in situ polymerization; nanocomposite
INTRODUCTIONOrganic/inorganic hybrid materials have recentlyreceived considerable attention because they offerthe opportunity to develop composite materialswith properties that differ significantly from thosepredicted applying the rule of mixtures to thecomponent phases. One family of inorganic phaseswhich is being combined with an increasingly broadrange of polymers is the polyhedral oligomericsilsesquioxanes, (RSiO1.5)n, where R may be awide variety of functional groups.15 Octamericpolyhedral silsesquioxanes have a SiO cage structurevery similar to that of silica particles with eightvertices. Each vertex may be functionalized for specificapplications.
The incorporation of polyhedral oligomeric silses-quioxane (POSS) reagents that contain reac-tive groups into organic polymer systems cre-ates the possibility of nanoscale reinforcement withPOSS bound to the polymer chain with chemi-cal bonds. There are a number of recent papers
that describe polymer/POSS covalent composites,including epoxy/POSS,6,7 polypropylene/POSS8 andstyryl-based POSS.9 Recently, based on WAXS andSAXS studies, Hsiao et al10,11 have reported the pres-ence of nanoscale POSS crystals in the polyurethanematrix.
In this study, we prepared and characterizedcomposite materials of poly(ethylene terephthalate)(PET) and epoxy-functionalized POSS by melt mixingand in situ polymerization methods.
EXPERIMENTALMaterialsTextile grade PET, supplied by KoSa Co, with anintrinsic viscosity of 0.64 dl g1 was used. Epoxy-functionalized POSS in the form of a high-viscosity liq-uid and titanium isopropoxide used as a catalyst wereobtained from the Aldrich Chemical Co. Materialswere used as received. Its structure is given as follows:
Correspondence to: Kwan Han Yoon, School of Advanced Material & System Engineering, Kumoh National Institute of Technology, 188Shinpyung-dong, Kumi, Kyungbuk, 730-701, KoreaE-mail: email@example.comContract/grant sponsor: US Department of Commerce National Textile Center(Received 20 October 2003; revised version received 27 April 2004; accepted 6 May 2004)Published online 16 November 2004
2004 Society of Chemical Industry. Polym Int 09598103/2004 /$30.00 47
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Preparation of compositesMelt mixing of PET and triepoxy-POSS (5 wt%) wasperformed in a Haake Mixer (Rheomix 600) for5 min at 280 C. In situ polymerization of PET andtriepoxy-POSS was done by melt polycondensation.Dimethyl terephthalate was reacted in a nitrogenenvironment with ethylene glycol having dispersedtriepoxy-POSS (1 wt%) in the presence of titaniumisopropoxide (0.05 wt%) as a catalyst in a small scalebatch reactor (500 mL). Transesterification reactionwas carried out for 2 h at 190 C and another 2 hat 210 C. Condensation reaction was performed byprogressively increasing the temperature to 280 Cfor 50 min under reduced pressure. The presumedreaction between the hydroxyl groups of PET andepoxy groups of POSS is described in Scheme 1.
CharacterizationsThermal properties were measured using a TA Instru-ments Q100 differential scanning calorimeter with aheating and cooling rate of 20 C min1 under nitro-gen flow. Thermogravimetric analysis (TGA) wascarried out with a TA Instrument TGA 2950 thermo-gravimetric analyzer at a heating rate of 1 C min1
from room temperature to 700 C under nitrogen. Inorder to follow chemical reactions, samples obtainedby melt mixing were heated to 280 C at a heatingrate of 20 C min1, and then maintained in the meltfor 5 min. The samples were then cooled to 100 Cat a rate of 20 C min1. This cycle was repeatedseveral times. The morphology of the fractured sur-faces of the samples was investigated using a HitachiS-2400 scanning electron microscope. Wide-angle X-ray diffraction (XRD) measurements were performedat room temperature on a Rigaku (D/Max-IIIB) X-ray diffractometer, using Ni-filtered Co-K radiation.The scanning rate was 2 min1 over a range of2 = 540 . The tensile properties were studied ona Rheometric Scientific Solids Analyzer (RSA III).Tensile samples were prepared by fourfold drawing at100 C of fibers which were taken up onto a bobbin at100 rev min1 and measured with the guage length setat 25 mm and at a crosshead speed of 50 mm min1.The diameters of fibers were in the range of 1020 m.An average of at least fifteen individual determina-tions was used. The temperature dependence of themechanical properties such as storage modulus andtan was examined using RSA III. A frequency sweep
OPET O PETCHCH2
PET O PETCHOCH2
CH CH2O O PET
Scheme 1. The reaction between the hydroxyl groups of PET and epoxy groups of POSS.
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PET containing epoxy-functionalized polyhedral oligomeric silsesquioxane
temperature ramp test was performed at a frequencyof 1 Hz from 25 C to 210 C at a heating rate of2 C min1 using the tensile mode. The specimen wasca 25 mm in length and 0.1 mm in diameter.
RESULTS AND DISCUSSIONMorphology of the compositesFigure 1 shows the SEM fracture morphology ofPET (a), PET/POSS melt-mixed (b) and in situpolymerized (c) composites. It was expected thatthe liquid state of POSS would disappear when areaction occurs between the epoxy groups of POSSand hydroxyl groups of PET during the melt mixingprocedure. However, the result seen in Fig 1(b) showsa definite phase separation of POSS in the PET matrix,which indicates that there was little reaction duringmelt mixing, so that the poor interfacial adhesionpermits the pull-out of a POSS domain from thePET matrix. Contrary to the case of the melt-mixed composites, the in situ polymerized compositeshows homogeneous morphology. The result canbe explained as follow: first, there was a sufficientreaction between the epoxy groups of POSS andhydroxyl groups of PET due to the long reaction time;second, there was a good dispersion of ethylene glycol(EG)the monomer precursor of PETat the initial
polymerization stage; and thirdly, there was a lack ofan adequate amount for detection and reinforcementbecause 1 wt% of POSS represents about 0.2 mol%(the melt-mixed sample contained ca 5 % POSS).
Figure 2 shows the X-ray scattering profiles of PETas well as melt-mixed and in situ polymerized com-posites. A strong diffraction (2) of 8.2 (10.8 A) wasobserved in the PET/POSS-melt mixed composite,
Figure 2. XRD patterns of PET, melt-mixed and in situ polymerizedcomposites.
Figure 1. SEM photographs of (a) PET, (b) melt-mixed composite (5wt% of POSS), and (c) in situ polymerized composite (1wt% of POSS).
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which is a typical peak of POSS9,12 having cyclopentylor cyclohexyl groups. It is clear that the melt-mixedcomposite shows crystalline features that are charac-teristic of the two separate components. The POSSused in this study was a liquid which should not pro-duce an X-ray diffraction peak. Therefore the peakshown at 8.2 implies that the POSS has changed tosolid in the composite, presumably by degradation ofepoxy groups at the high temperature of melt mixing orreaction with PET. In order to check the occurrence ofdegradation, POSS was heated in a differential scan-ning calorimeter pan at 280 C for 10 min. POSS wasnot changed into a solid by degradation. Therefore wesurmise that the peak at 8.2 results from the partialreaction of epoxy-POSS with PET even though phaseseparation was observed in SEM. This is evidence thattriepoxy-POSS is chemically incorporated into PETto form a PET/POSS composite. Contrary to the caseof the melt-mixed composite, the PET/POSS in situcomposite does not show a peak at 8.2 . For thePET/POSS in situ composite case, the incorporationof small amounts of a branching agent may lead to theformation of long-chain branches. If the amount of thebranching agent exceeds about 1 wt%, a crosslinkedstructure may be obtained. The branches would prob-ably be associated with crystalline distortions thatprevent better packing in the material, and this wouldexplain why a crystalline exotherm which will bedescribed later is decreased in the in situ composite.
Thermal propertiesTGA plots for the POSS macromer and compos-ites are shown in Fig 3. In a comparison of thethermal stabilities of the composites, 5 % mass losstemperatures were used as standards. The decompo-sition temperatures of POSS, PET, melt-mixed and insitu polymerized composites were observed at 294 C,355 C, 354 C and 346 C, respectively. No differ-ences in decomposition temperatures of PET and themelt-mixed composite were observed. That indicatesthat melt mixing at 280 C for 5 min did not cause thedegradation of POSS. Unlike the melt-mixed sample,
Figure 3. TGA overlay for POSS, PET, melt mixed, and in situpolymerized composites.
the decomposition temperature of the in situ poly-merized composite was lower than that of PET eventhough a relatively small amount of POSS was incorpo-rated during polymerization. This might be the resultof the degradation of POSS because it took 50 min topolymerize the composite at 280 C.
Figure 4 shows the second heating and cooling dif-ferential scanning calorimetry (DSC) curves of PETand the composites. The glass transition tempera-tures of PET, melt-mixed and in situ polymerizedcomposites were observed at 82 C, 77 C and 80 C,respectively. The relatively low glass transition of themelt-mixed composite might have resulted from theunreacted POSS acting as a plasticizer. The meltingtemperature of the melt-mixed composite observedat 254 C was in accord with that of PET, whereasthat of the in situ composite was observed at 249 C.The expected melting temperature depression is dueto incorporation of POSS into the PET crystal struc-ture. It is interesting to observe the cold crystallizationtemperatures of the composites. Unlike PET, the com-posites showed two crystallization peak temperatureswhich differ from each other and show broadeningcompared with that of PET. It is seen that POSSprevents the best packing of PET segments for crystal-lization and decreases the crystallization rate. Gener-ally, the cold crystallization temperature and enthalpyof crystallization decrease with melting time becausecrystallization from the melt depends on the presenceof small crystals which are affected by melting time.A similar situation was observed with regard to theextent of reaction of the two components. A shoulder
Figure 4. The second heating and cooling differential scanningcalorimetry curves of PET, melt-mixed, and in situ polymerizedcomposites.
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Figure 5. Cooling differential scanning calorimetry curves of in situand melt-mixed composite with melting time at 280 C.
was observed in the melt-mixed composite adjacentto the expected PET cold crystallization peak. It isthought that this peak results from the reaction ofPET and POSS.
In order to investigate the fact that the thermalbehavior, especially the cold crystallization tempera-ture, of the melt-mixed composite with reaction timecomes close to that of the in situ composite at a longtime, the cooling DSC curves of the melt-mixed com-posite were measured versus melting time in a DSCinstrument pan and compared with the PET/POSS insitu crystallization exotherm in Fig 5. As seen in Fig 5,the crystallization temperature and enthalpy of crystal-lization decreased with melting time. After 50 min ofmelting time, the peak temperature of the mixed sam-ple decreased until it was approximately the same asthat of the in situ composite, but the enthalpy of crys-tallization was still large compared to that of the in situcomposite. It can be concluded, as expected, that thereaction of the melt mixed composite was not as greatas that of the in situ composite even though largeramounts of POSS were used. This is well demon-strated in Fig 6 which shows the SEM data for themelt-mixed composite with melting time. The dropsizes as well as the number are decreased as a result
PET/POSS for 5 min PET/POSS for 10 min
PET/POSS for 20 min PET/POSS for 30 min
Figure 6. SEM photographs of melt-mixed composite with melting time.
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of the increase of the extent of reaction with melt-ing time. The reaction in the case of the melt-mixedcomposite could be confirmed from the melt viscos-ity results seen in Fig 7. The melt viscosity dependson the molecular weight. However, PET/POSS melt-mixed as well as in situ composites were not solublein conventional PET solvents so that their molecularweights or intrinsic viscosities could not be measured.In this experimental frequency range, as shown inFig 7, melt-mixed composite showed a viscosity inter-mediate between that of PET and the in situ composite.If no reaction occurred in forming the melt-mixedcomposite, its viscosity would be expected to be lowdue to the slippage of the matrix, which is composedof the low viscosity POSS and highly viscous PET.However, the viscosity of melt-mixed composite washigher than that of PET, indicating that the reactionbetween POSS and PET might have occurred.
Mechanical propertiesThe mechanical properties of PET and melt-mixedcomposite fiber are listed in T...