Polyisobutylene: A Most Unusual Polymer

1 polyisobutylene : A most Unusual PolymerK. KUNAL,1M. PALUCH,2C. M. ROLAND,3J. E. PUSKAS,1Y. CHEN,1A. P. SOKOLOV11 Department of Polymer Science, The University of Akron, Akron, Ohio 44325-39092 Institute of Physics, Silesian University, ulica Uniwersytecka 4, 40-007 Poland3 Naval Research Laboratory, Chemical Division, Code 6120, Washington, District of Columbia 20375-5342 Received 22 February 2008; revised 7 April 2008; accepted 10 April 2008 DOI: online in Wiley InterScience ( ).ABSTRACT:The influence of molecular weight,M, on the fragility and fast dynamicsin polyisobutylene (PIB) was studied using dielectric and mechanical relaxation spec-troscopies, calorimetry, and Raman spectroscopy. The measurements indicate adecrease in fragility with increasingMfor shorter chains, in the range ofMwhereTgisM-dependent. Such behavior is not observed for other polymers and is at oddswith traditional theoretical models that predict an increase in fragility with chainlength.

2 These results confirm the Unusual character of PIB, as evident in variousproperties including extremely low gas permeability, a low fragility, and a segmentalrelaxation spectrum much broader than expected for a low-fragility material. Thereason for this anomalous behavior remains unclear, but might be related to the sym-metric structure of the PIB repeat unit, together with comparable flexibility of bothstructural components, the backbone and side Wiley Periodicals, Polym Sci Part B: Polym Phys 46: 1390 1399, 2008 Keywords:dielectric relaxation; fragility; glass transition; light scattering; mechani-cal spectroscopy; polyisobutylene ; Raman spectroscopy; structure-property relationshipINTRODUCTIONP olyisobutylene (PIB) is a commercially impor-tant Polymer that finds a great number of appli-cations exploiting its thermal stability, good flex-ibility at ambient temperature, and imperme-ability to gases.

3 Reflecting this importance, in1949 the National Bureau of Standards (cur-rently the National Institute of Standards andTechnology) chose PIB as a standard Polymer toestablish usual polymeric properties. Twenty-seven laboratories worldwide collaborated in astudy of a PIB sample, with the accumulatedresults used to define what was assumed to bethe standard viscoelastic properties of ,2 This early work on PIB played a centralrole in establishing general polymeric proper-ties, for example, the molecular weight depend-ence of the viscosity of entangled polymers andthe time temperature superposition , it appears that PIB has many prop-erties that differentiate it from most of the otherpolymers: (i) a markedly low permeability tosmall molecule penetrants3; (ii) one of the weak-est temperature dependences of structural relax-ation and viscosity ( , low fragility)4,5and, cor-respondingly, strong inelastic scattering (bosonpeak) even at temperatures much above theglass transition6.

4 (iii) a very small differencebetween the temperature dependences of seg-mental and chain relaxations,4,5which can leadto an apparent conformance to time tempera-ture superpositioning, unlike the obvious break-down seen in other polymers such as polysty-rene, polyvinyl acetate, and polypropylene5,7,8;Correspondence to:A. P. Sokolov (E-mail: of Polymer Science: Part B: Polymer Physics, Vol. 46, 1390 1399 (2008)VVC2008 Wiley Periodicals, from from (iv) a mechanical segmental dispersion muchbroader than expected for such a low fragilitymaterial8; and (v) a very Unusual spectrum ofthe fast dynamics; that is, a constant loss re-gime where the susceptibility changes negligiblywith examples illustrate thatPIB can be viewed as a very Unusual polymerwith properties deviating from behavior commonfor many other glass transition in supercooled liquids isassociated with rapid changes of the rate ofstructural relaxation (segmental relaxation pro-cess in polymers).)

5 The segmental relaxationtime,sa, increases by many orders of magnitudeupon cooling only a few degrees at temperaturesclose to the glass transition temperature, polymers exhibits a strongly non-Arrheniustemperature dependence, which is well-approxi-mated by the Vogel Fulcher Tamman (VFT)equation as follows:sa s0exp B= T T0 1 wheres0andBare material constants andT0isthe temperature of an ideal glass concept of fragility has been introduced tocharacterize the steepness of the temperaturevariations ofsaclose toTg. Essentially, fragilitycharacterizes the degree of deviation of the tem-perature dependence ofsafrom Arrhenius-likebehavior ( , deviation from thermally acti-vated behavior with a constant activationenergy). A strong material has a near Arrhe-nius temperature dependence ofsa, whereas a fragile material exhibits strongly non-Arrhe-nius behavior.

6 To quantify the fragility, Boehmeret the fragility (steepness) indexmas follows:m @logsa@ Tg=T T Tg: 2 In general, polymers tend to be more fragilethan molecular liquids. It has been observedthat polymers with a rigid backbone or bulkypendant groups, such as polystyrene (PS) andpoly(methyl methacrylate) (PMMA), exhibit astrong increase in fragility with increase in mo-lecular ,12On the other hand, flexiblepolymer chains such as poly(dimethyl siloxane)(PDMS) and poly(methyl phenyl siloxane)(PMPS) show less or no increase in fragilitywith molecular results wereexplained in the framework of the couplingmodel12and more recently in the model of Dudo-wicz et common idea of bothapproaches is that fragility relates to the sever-ity of steric constraints from neighboring seg-ments, which impede segmental motions12and/or frustrate chain packing idealeads to the expectation that packing of rigidchains or chains with bulky side groups will bemore frustrated asMincreases.

7 As a result,such polymers should exhibit a strong increasein fragility with increasing molecular chains should pack better and, accord-ing to the model,14should show a much weakerincrease in fragility withM. No theory predictsa decrease of fragility with molecular , preliminary studies of PIB pre-sented in ref. 15 indicated a decrease in fragilitywith increase of the chain length. This wouldsuggest that PIB again exhibits a trend oppositeto the pattern of other polymers and to the theo-retical expectation. We present herein a detailedstudy of the dynamics of PIB using mechanicaland calorimetry measurements, dielectric relax-ation, and Raman scattering spectroscopy. Theresults demonstrate a decrease of fragility withincrease in molecular weight for PIB. Possiblereasons for the Unusual behavior of PIB are samples were precision synthesized by liv-ing carbocationic polymerization in the Depart-ment of Polymer Science at the University ofAkron.

8 The polymerization of isobutylene wasinitiated by 2-chloro-2,4,4-trimethyl-pentane/TiCl416 and terminated using trimethyl alumi-num (Aldrich)17to obtain PIBs with only C andH NMR analysis of the resultingPIBs showed that they were exclusively termi-nated by CH3groups (synthesis details will bereported elsewhere). This is important for ourstudy to exclude any chain-end different molecular weight PIB samples(Table 1) were synthesized using this Mechanical AnalysisDynamic mechanical data were obtained using aBohlin VOR with a parallel plate geometry. Isother-mal measurements were made over a 10 128 CPROPERTIES OF POLYISOBUTYLENE1391 Journal of Polymer Science: Part B: Polymer PhysicsDOI of temperatures, corresponding typicallyto Tg/T , with K control. Thefrequency was varied according to the tempera-ture within the range 2 Hz to obtain thepeak in the loss modulus,G00, within the mea-surement window.

9 Strains were between Figure 1(a) presentsG00spectra forthe sample with smallest molecular Dependent DSCD ifferential scanning calorimetry (DSC) was car-ried out using TA Instruments Q100 with liquidnitrogen cooling. Samples were cooled from theliquid state to 508C. belowTg, at rates,qc, to 10 K/min. After 5 min, this was followedby heating at 10 K/min throughTg. Fictive tem-peratures were calculated from the SpectroscopyThe dielectric measurements in the frequencyrange Hz 10 MHz were carried out on twospectrometers, both using the Novocontrol Alphaanalyzer, with the complex dielectric permittiv-itye*(x) e0(x) e00(x) obtained from samplesin a parallel-plate capacitor geometry. For oneset of experiments, temperature was controlledusing a Novocontrol Quattro temperature con-trol unit; the other set utilized a closed-cycle he-lium cryostat.

10 In both cases, the stability was atleast K. Measurements below Hz werenot attempted because PIB has a very low dipolemoment and thus weak dielectric signal. Thisproblem becomes significantly worse withincreasing molecular weight. For this reason,dielectric relaxation times were determined onlyup tosa 1 s. Figure 1(b) presents the dielectricloss spectra for the PIB sample with the small-est molecular SpectroscopyDepolarized Raman scattering spectra weremeasured in back-scattering geometry using aJobin Yvon T64000 triple monochromator, anAr1ion laser with a wavelength of nmand laser power 15 mW at the sample. Sam-ples were kept in silanized glass ampoules,placed into an optical cryostat (Oxford Instru-ments, Optistat). Measurements were carriedout at 150 K, that is, belowTgfor all light scattering spectra of PIB sam-Table Summary of Results for Different Molecular Weights of PIBS ampleVFT ParametersTgFragilityFast DynamicsMn(g/mol)a(Mn/Mw)bB(K)T0(K)DMA(K )DS(K)DMADSDS(DMATg)DSCmBP(cm 1)R1(Imin/Imax) ( ) ( ) ( ) VFT parameters were obtained from the fitting of the dielectric relaxation ET of Polymer Science: Part B: Polymer PhysicsDOI with various molecular weights are pre-sented in Figure 2, as the spectral densityIn I/[m(n(m)11)], wheren(m) [exp(hm/kT) 1] 1is the temperature Bose factor.