Polymer analysis by pyrolysis gas chromatography, Artykuły naukowe, Polimery i ich analiza
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//-->Journal of Chromatography A, 843 (1999) 413–423ReviewPolymer analysis by pyrolysis gas chromatographyFrank Cheng-Yu Wang*Analytical Sciences Laboratory,Michigan Division,Building1897B,The Dow Chemical Company,Midland,MI48667,USAAbstractPyrolysis gas chromatography (Py–GC) has been an important technique in the qualitative and quantitative analysis ofpolymers for more than thirty years. Recent developments in Py–GC technology are mainly focused on pyrolysis operationand applications. In the former category, the focus is on the development of flexible pyrolysis instrumentation, ‘‘pre-’’ and‘‘post-’’ pyrolysis derivatization and database creation / maintenance. In the application field, the development focused onkinetics of thermal degradation, structure determination and integrate techniques associated with pyrolysis to performqualitative and quantitative analysis of low level additives in polymers.©1999 Elsevier Science B.V. All rights reserved.Keywords:Pyrolysis; Derivatization, GC; PolymersContents1. Introduction ............................................................................................................................................................................2. Pyrolysis technique .................................................................................................................................................................2.1. Instrument configuration..................................................................................................................................................2.2. Pyrolysis with derivatization ............................................................................................................................................2.2.1. Pre-pyrolysis derivatization..................................................................................................................................2.2.2. Post-pyrolysis derivatization ................................................................................................................................2.3. Database and library........................................................................................................................................................3. Applications ...........................................................................................................................................................................3.1. Kinetics of thermal degradation .......................................................................................................................................3.2. Structure determination ...................................................................................................................................................3.3. Qualitative and quantitative analysis.................................................................................................................................4. Conclusions ............................................................................................................................................................................References ..................................................................................................................................................................................4134144144154154164164174174184214224221. IntroductionPyrolysis is an important technique for the studyof synthetic polymers. For more than thirty yearsdevelopment of this technique, the applications of*Tel.:11-517-636-0565;fax:11-517-638-6999.E-mail address:wangfc@dow.com (F.C.-Y. Wang)pyrolysis to synthetic polymers focused on quali-tative and quantitative composition analysis as wellas structure exploration. Because of advances ininstrumentation, computer technology, and labora-tory sample preparation procedures, several methodsassociated with pyrolysis and gas chromatography(GC) have been re-visited in order to capture theadvantages of this progress.0021-9673 / 99 / $ – see front matter©1999 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 98 )01051-6414F.C.-Y.Wang/J.Chromatogr.A843 (1999) 413–423Based on the goal of the analysis and the ex-perimental approach, this review has been dividedinto different sections. All methods discussed areassociated with pyrolysis gas chromatography (Py–GC) or pyrolysis-gas chromatography–mass spec-trometry (Py–GC–MS). The applications are con-fined to the area of synthetic polymers. Methodssuch as the structure determination of copolymersand pre-pyrolysis derivatization are discussed indetail because recent developments differ from thetraditional approach. There are also methods wherenot too much change has occurred, such as the studyof the kinetics of thermal degradation as well asqualitative and quantitative analysis. In other ways,some methods associated with pyrolysis have beenimproved in order to increase sensitivity of detectionto meet the requirements of certain applications.2. Pyrolysis technique2.1.Instrument configurationPy–GC–MS remains the most convenient methodto qualitatively analyze polymers. The heated fila-ment, Curie-Point, and furnace are three major typesof pyrolyzer used in the experiments. The standardconfiguration is the pyrolyzer mounted on top of theGC injection port. Once the pyrolyzer is attached, theGC is exclusively used for pyrolysis experiments. Onthe detection side of the GC, in addition to flameionization detector (FID) and mass selective de-tection (MSD), there are other GC detection methodssuch as atomic emission detection (AED), flamephotoionization detection (FPD), nitrogen–phosphor-us detection (NPD) etc., which have been used.Besides using different types of detector, instrumentconfiguration may be varied or other thermal analysisequipment may be converted to meet specific appli-cations.Different configurations include utilizing a pro-grammable temperatures vaporization (PTV) injectorto conduct the multi-step thermal desorption andprogrammed Py–GC experiments. Polymers havebeen studied by using a high-temperature program-mable temperatures vaporization (PTV) injector tothermally treat the polymer sample at differenttemperatures [1]. The GC system is used to separatethe complex mixtures of several polymers andadditives. The individual chromatograms of thevarious constituents of the polymeric sample werecorrelated with those of the final material in order toidentify additives (thermal desorption) and degra-dation products (pyrolysis). The advantage of usingthe PTV injector for this purpose is that no heatedtransfer line and switching values are needed. Thiswill eliminate the risk of losses of high-molecular-mass components. Other advantages of the proposedtechnique are simplicity, versatility and low cost.Another configuration approach is the creation of adual inlet (pyrolysis and autosampler) system, suffi-ciently flexible to use both kinds of injection system.In a conventional Py–GC system, the pyrolyzer wasinterfaced on top of the GC system which blocks thenormal sample injection port. In this study, a differ-ent approach has been developed [2]. The pyrolyzeris mounted differently so it can coexist with thetraditional sample injection device, namely an auto-sampler. The advantages of this configuration arethat the pyrolyzer attachment does not interfere withsample introduction through the injection port; theGC system can be easily converted to a Py–GCsystem without mounting or dismounting of theequipment; and when operated as a Py–GC unit, theconventional sample injection port can be used as anauxiliary sample introduction route to greatly en-hance the capability of Py–GC data handling inqualitative and quantitative analysis.The third development in the instrument configu-ration is the development of a Py–GC system with amovable reaction zone [3]. The device enables thethermal degradation of polymers inside a capillarypre-column and transfer of the reaction zone into acolumn oven. The pyrolysis procedure describedprotects the thermally sensitive compounds prior topyrolysis, prevents the process of irreversible con-densation of high-boiling pyrolysis products duringthe chromatographic process and eliminates extra-column effects on peak broadening.The use of a thermal extraction unit for a furnace-type pyrolysis interface has been studied for theanalysis of polymers [4]. Pyrolysis is achieved byaccurate temperature programming of the pyrolysiscell from ambient to very high temperatures. Thesuitability of the thermal extraction unit for use as apyrolyzer was evaluated by analyzing several modelF.C.-Y.Wang/J.Chromatogr.A843 (1999) 413–423415polymers. The results obtained demonstrated that thisunit can be used as a pyrolyzer. The main advantagesof the technique are good reproducibility, minimumsecondary reactions, capability for quantitative anal-ysis, and minimum sample handling.Pyrolysis is merely the mechanism to decompose thepolymer to fragments.2.2.Pyrolysis with derivatizationDerivatization is a well-known technique in chro-matographic analysis used to enhance chromato-graphic separation and / or detection for those com-pounds not suitable for separation / detection. Thesame concept has been adapted to Py–GC or Py–GC–MS analysis.However, the scope of the derivatization reactionshould be expanded from the conventional chromato-graphic analysis point of view to include thepyrolysis process. The derivatization reaction servesnot only to enhance the chromatographic separationand / or detection, but also to allow thermal degra-dation pathway re-selection to improve the pyrolysisprocess for qualitative and quantitative analysis. InPy–GC, the derivatization reaction does not have tobe limited to the ‘‘pre-column’’ and ‘‘post-column’’situation. The derivatization reaction may be dividedinto ‘‘pre-pyrolysis’’ and ‘‘post-pyrolysis’’.In ‘‘post-pyrolysis’’, the derivatization techniqueshave experienced additional development inpyrolysis analysis of polymers. The major portion ofthe derivatizations involve the methylation of al-cohols and acids. The most popular methylationregents are tetramethyl ammonium hydroxide(TMAH) and trimethyl sulfate (TMS). The purposeof derivatization is to modify the pyrolysates in orderto obtain better separation and detection results.There is another development in derivatizationwhich can be called ‘‘pre-pyrolysis’’ derivatization.The purpose of this type of derivatization is toconvert the functional group in the polymer to obtaina favorable thermal degradation pathway duringpyrolysis. The term favorable pathway means amajor monomer or monomer related fragment isproduced to allow for easier qualitative and quantita-tive analysis. The major difference between the‘‘pre-pyrolysis’’ derivatization and ‘‘post-pyrolysis’’derivatization is the polymer backbone must bestable enough to resist the attack from the deri-vatization reagent in ‘‘pre-pyrolysis’’ derivatization.2.2.1.Pre-pyrolysis derivatizationIn ‘‘pre-pyrolysis’’ derivatization, the main con-siderations of this modification or derivatization arefour-fold: (1) Ease and convenience. The derivatiza-tion reaction takes place without rigorous conditionssuch as heating the sample to high temperature orreacting in high pressure. (2) Reduced interferencefrom surrounding materials. Most derivatization re-agents for primary amines are hydrogen sensitive,which means these reagents cannot be used inaqueous solutions or with chemicals with alcohol oracid functional groups. (3) The final product mustdegrade with a favorable thermal degradation path-way. The derivatization reaction will produce astable functional group. Under thermal degradation,this derivatized polymer will depolymerize witheither an unzipping pathway or a major fragment-producing pathway. (4) The major monomer orfragment produced either by the unzipping reactionor by monomer-related chain cleavage should besuitable for GC separation and detection. For exam-ple, the monomer or fragment should be a nonpolarcompound when using a nonpolar capillary sepa-ration column. The elution time of the fragmentshould not be so fast as to lose the resolution fromother pyrolysates in the separation. At the same time,the elution time should not be too long, which wouldwaste analysis time.A thermal degradation pathway re-selection studyof polymethacrylic acid through derivatization [5]has demonstrated the value of pre-pyrolysis deri-vatization in qualitative and quantitative Py–GCanalysis. The pyrolysis of polymethacrylic acid willproduce a number of pyrolysates which reflect theunzipping degradation as well as the random chainsession and recombination. If the polymer has beenderivatized by TMAH to convert the acid functionalgroup to methyl ester, the thermal degradation ofpoly(methyl methacrylate) only produces one majorfragment which is the methyl methacrylate mono-mer. In the study, several copolymers containing lowlevel of methacrylic acid have been successfullyidentified by this ‘‘pre-pyrolysis’’ derivatization tech-nique.416F.C.-Y.Wang/J.Chromatogr.A843 (1999) 413–4232.2.2.Post-pyrolysis derivatizationThe ‘‘post-pyrolysis’’ derivatization technique inthe Py–GC study of polymers has been developedfor a long time. For example, the pyrolysates can bederivatized ‘‘simultaneously,’’ ‘‘in-situ,’’ or ‘‘on-col-umn’’ to reduce the difficulties of polar pyrolysatesbeing separated in a nonpolar capillary column [6].In another example, unsaturated aliphatic alkenes canbe derivatized by hydrogenation to simplify thenumber of fragments and increase the possibility ofstructural exploration [7]. In general, these tech-niques modify the pyrolysates produced by thepyrolysis process. Post-pyrolysis derivatization im-plies that during the course of pyrolysis, there is nointention of altering the thermal degradation pathwaythrough derivatization.In-situ hydrolysis / methylation pyrolysis gas chro-matography for the characterization of polyaramidshas been studied [8]. Some parameters which in-fluenced in-situ methylation by TMAH duringpyrolysis were explored. Both pyrolysis temperatureand excess TMAH (pH effect) influenced the meth-ylation of carboxy-, aromatic, amino-, and hydroxyl-functional groups. The solvent of TMAH, i.e.,methanol or water, significantly affected the meth-ylation for the polyaramids but hardly influencedother model compounds studied. The explanationgiven assumed a transesterification mechanism ratherthan hydrolysis / methylation. However,n-methyla-tion prior to the decomposition of polyaramids maynot be excluded.A study of characterization of copolymer typepolycarbonates by reactive Py–GC in the presence ofTMAH has been reported [9]. In this study, Py–GCin the presence of TMAH was successfully appliedto the determination of chemical composition andend group content of two kinds of polycarbonate(PC) copolymers, thermally and light stabilized PCs.The Py–GC with post-derivatization of these PCcopolymers enabled almost quantitative detection ofthe constituents of the polymer sample as theirmethyl ethers on the resulting pyrograms. On thebasis of these peak intensities, the compositions aswell as the number-average molecular masses wereaccurately estimated without using any referencepolymer.The reagent used in the post-pyrolysis does nothave to be limited to organic alkali, there are othertypes of reagent that have been used for this purpose.The sequence distribution study of polyacetals bypost-pyrolysis Py–GC in the presence of cobaltsulfate is an example [10]. In this study, post-pyrolysis of copolymer polyacetals in the presence ofcobalt sulfate incorporated with Py–GC was appliedto the study of sequence distribution. The ethyleneoxide content and the distribution of ethylene oxidesequences up to seven monomer units in the polymerchain were evaluated on the basis of peak intensitiesof cyclic ethers in the pyrogram. These values werein good agreement with those obtained by hydrol-ysis.Another polymer composition study through de-rivatization is the study of alternating olefin–carbonmonoxide copolymers and their derivatives by Py–GC–MS [11]. Alternating copolymers of carbonmonoxide and olefins (ethylene, styrene and norbor-nadiene) and their modified polymers with primaryamines, P2S5and P2O5were prepared. These poly-mers were then pyrolyzed at 5508C for 10 s in aPy–GC–MS system. In each pyrolysis, hydrocarbonsarising from the corresponding olefin comonomerwere detected as the volatile products. Py–GC–MSconfirmed that these 1,4- arrangements of the ketonicgroups in the alternating copolymers were convertedinto pyrrole, thiophene, or furan-containing chainswith primary amines, P2S5or P2O5, respectively. Theoxygen-containing groups, which remained intactduring the modification with P2S5, were detected insmall amounts in the pyrolysates.2.3.Database and libraryChromatography databases such as for GC andliquid chromatography (LC), based on the retentiontime have been available for many years [12–14].Because chromatography techniques focus on sepa-ration, the peaks in a chromatogram are representa-tive of the number of components in that mixture andrelative elution / retention order of that specific mix-ture under that experimental condition. The elution /retention time of peaks can be affected by manyfactors. These factors include type of mobile phase,type of stationary phase, thickness of stationaryphase, mobile phase flow-rate and elution tempera-ture.F.C.-Y.Wang/J.Chromatogr.A843 (1999) 413–423417Because the retention time is highly dependent onexperimental conditions, it may not be suitable as asearchable parameter in a database. To solve thisproblem, there have been different types of retentionindices developed [15–17]. The most commonlyaccepted indices is the Kovats retention indices.More recently, the concept of retention indices hasbeen further developed in the chromatography field(especially in GC) to accommodate various ex-perimental conditions [18,19]. Even with retentionindices, there are still difficulties when comparingchromatograms obtained from different types ofstationary and mobile phases. This is one of themajor reasons why there is no universal chromatog-raphy database widely available. However, there aresmall chromatography databases for specific groupsof compounds for specific purposes [20,21].As mentioned above, chromatography focuses onthe separation of the mixture. The mobile phase,stationary phase and experimental conditions areoptimized to obtain the best separation. These pa-rameters should not be standardized to obtain aconsistent retention time. The reference chromato-gram in the database should be used mainly todetermine the separation phases and experimentalconditions, the elution / separation order, the com-ponents in the mixture, and the relative abundance ofcomponents. If these are the most important parame-ters in the chromatography database, the databasedesign should concentrate on how to find the desiredreference chromatograms and the information aboutthose components with minimum effort.The creation and maintenance of a Py–GC data-base of polymers has been described previously.However, these databases are in book [22] form as acollection of chromatograms (pyrograms) or in anelectronic [23] or hard copy format [24]. The depthof information included is limited. The search capa-bility is restricted to a polymer index in the databaseor library. The users have to carry the electronic filesor book to have the database available. In order tocreate and maintain a chromatography database inthe electronic form, there are issues which have to beaddressed. The issues include inputting data fromdifferent sources, maintaining (add, delete, andmodify the entries) the database, access / distributionmanagement, compatibility with other similar data-bases, potential to integrate with other databases,access from different computer platforms, and thecost of software.A hyper text markup language (HTML) baseddatabase for chromatographic database creation andmaintenance has been developed [25]. In this de-velopment, a Py–GC database has been used as anexample to demonstrate the structure and the ar-chitecture of a chromatographic database. The data-base program, the users and database interface, aswell as the distribution issues of a database havebeen discussed.3. ApplicationsAnalysis of synthetic polymers is one of the majorareas for the application of pyrolysis technology.From another perspective, pyrolysis technology isfrequently used to characterize synthetic polymers.Pyrolysis retains its important role in compositionidentification as well as in thermal degradationkinetic studies. The major advantage of pyrolysiscompared with other technologies is the simplesample preparation. In addition to compositionidentification, pyrolysis was able to explore thecopolymer structure with statistical theory applied.Pyrolysis can also study the stereoregularity of thehomopolymers through tetramers or higher oligo-mers.3.1.Kinetics of thermal degradationPyrolysis experiments have continued to play animportant role in the evaluation of the thermalstability, thermal degradation mechanisms, as well askinetic measurements of degradation of polymericmaterials. Different types of polymers (thermoplasticand thermoset) are continuously being pyrolyzed toinvestigate their thermal behavior and degradationmechanism under different temperature conditions.In thermoplastic polymers, sulfur containing poly-mers such as poly(p-phenyleneether-sulfone) (PES)resin and a polysulfone resin (PSR) were studied byPy–GC with FID, FPD and MSD [26]. This studywas to evaluate the kinetics of SO2formation fromPES and PSR by sequential pyrolysis. A similarstudy of the thermal degradation of four poly-thiophenes has also been reported [27]. The different
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