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APPLICATION NOTE

AuthorsAuthorsAuthorsAuthorsAuthorsJan P. Plog1 and Matthew Meyer2

1Thermo Fisher Scientific, Karlsruhe, Germany2Thermo Fisher Scientific, Madison, USA

Rheology-Raman Spectroscopy: Trackingpolymer crystallization with the combinationof a rheometer and Raman spectrometer

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IntroductionIntroductionIntroductionIntroductionIntroductionThe use of a coupled rheometer and Raman spectrometerfor obtaining comprehensive insight into a materialsbehavior is presented in this application report.

Rheology is the analytical method of choice to correlatethe absolute flow and deformation characteristics of agiven product with its behavior towards a certain pro-cessing or application step. However, Rheology as anintegral method only yields answers on the bulk of theinvestigated sample. It does not give any insights intowhat is actually happening on the molecular level duringa certain processing step.

Raman spectroscopy has shown its ability as a powerful,effective and non-invasive method for chemical analysis.Coupling a rheometer with a Raman spectrometer providesdirect information about the molecular structure and themechanical properties. This is extremely useful forstudying the crystallization behavior of polymer meltsduring processing. It can also provide insight for in-situcharacterization and monitoring which can be challengingwhen working with on-line techniques as only relativeflow fields are characterized.

In this application note, we present results obtainedon an HDPE melt in cooperation with NIST publishedpreviously [1].The results shown can be obtained with the brand newcombination of a Thermo Scientific™ HAAKE™ MARS™rheometer with a Thermo Scientific™ iXR Raman spectro-meter (Thermo Scientific™ HAAKE MARSXR RheoRamanSystem) as shown in Fig. 1.

Fig. 1:Fig. 1:Fig. 1:Fig. 1:Fig. 1: The Thermo Scientific HAAKE MARSXR RheoRaman system.

Key wordsKey wordsKey wordsKey wordsKey wordsRheology, Raman spectroscopy, Polymer crystalli-zation, Combined methods

Result and discussionResult and discussionResult and discussionResult and discussionResult and discussionMelting and crystallization are two common phase tran-sitions that are critical to the flow properties of variouscomplex fluids. These temperature-sensitive transitionsare often indicated via changes in molecular conforma-tion, while optical measurements provide direct observationof structural characteristics. However, measurementsperformed on separate instruments are often challengingto correlate due to variations between samples, process-ing history, and temperature control. To demonstrate thecapabilities of the MARSXR, we provide simultaneousRaman and rheological measurements on high densitypolyethylene (SRM 1475, National Institute of Standardsand Technology, Gaithersburg, MD) during crystalli-zation.

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Fig. 2: Fig. 2: Fig. 2: Fig. 2: Fig. 2: Raman spectra of polyethylene at temperatures correspond-ing to the semi-crystalline state (22 °C) and the amorphous state(170 °C).

Fig. 3: Fig. 3: Fig. 3: Fig. 3: Fig. 3: Complex modulus and crystallinity of PE versus time duringisothermal crystallization at 124 °C.

The experimental setup shown in Fig. 1 represents anovel integration of commercial instrumentation: a Ramanspectrometer (Thermo Scientific iXR) and rotational rheo-meter (HAAKE MARS) are coupled through an opticallytransparent base modified from the Thermo ScientificTM

RheoScope Module.To monitor crystallinity one has to have a look at theRaman spectra for HDPE as shown in Fig. 2.

The spectrum at room temperature shows sharp peakscorresponding to the C-C stretch, CH2 twist, and CH2bend. At 170 °C HDPE is in the melt state, and the sharppeaks are replaced with broader spectral features.

The Raman spectra of HDPE can be analyzed to quantifythe crystallinity of the sample. Specifically, the areaunder the peak at 1416 cm-1 in the HDPE spectra isdirectly proportional to the mass fraction of crystallinityin the sample. In order to calculate the crystallinity, theintegrated peak area I1416 is normalized by the total areaunder the peaks in the CH2 twist region and a scalefactor Nc

(1)

The scale factor Nc is a ratio of I1416/(I1296 + I1303) for anHDPE sample to the crystallinity of that samplemeasured via DSC.For HDPE on the MARSXR, the measured scale factoris Nc = 0.80 ± 0.03. Although this is larger than calcu-lated values of Nc from our prior measurements [2], theRaman peak intensities of HDPE (and therefore the scalefactor values) are strongly dependent upon the polari-zation state of the incoming and collected light as wellas the scattering angle [3]. The crystallinity for the roomtemperature sample in Fig. 2 is (73 ± 4)% which agreeswith the crystallinity value of (74 ± 5)% measured viaDSC.

Raman spectra measured during the crystallizationprocess are used to calculate the crystallinity of thesample via Equation 1.

Fig. 3 shows the instantaneous mass fraction of crystallinematerial, which first exceeds the measurement noise atapproximately 800 s and increases over time. Theappearance and increase in crystallinity as measuredby Raman spectroscopy correlates with the increase inthe complex modulus near the crossover point. Phasetransition in the SRM 1475 is thus clearly driven bycrystallization.

SummarySummarySummarySummarySummaryRelating Raman and rheological measurements wouldbe difficult on multiple instruments due to the sensitivityof the crystallization process on temperature, but usingthe MARSXR we can clearly correlate changes in thecomplex modulus with structural and conformationalchanges in the crystallizing HDPE melt.

The structure-property relationships during polymercrystallization are of critical interest and can be studiedsimultaneously using the MARSXR. An HDPE sampleof thickness 750 μm was heated for 5 minutes at 155 °C,cooled at 10 °C/min to 134 °C, and then cooled at aslower rate of 2 °C/min to 124 °C and held at temperatureto crystallize.

Fig. 3 shows simultaneous rheological and Ramanmeasurements during HDPE crystallization. The complexmodulus is measured during small-amplitude oscillatoryshear using a fixed strain amplitude of 0.01 and oscillationfrequency of 2π rad/s. Fig. 3 shows that early times inthe crystallization process are characterized by G’< G’’,but over time a crossover occurs in the modulus as thevalues of G’ and G’’ increase over 2 orders of magnitude.The plateau in G’ and G’’ at later times indicates thecessation of crystallization as measured by thecomplex modulus.

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ReferencesReferencesReferencesReferencesReferences[1] A. P. Kotula et. al., Review of Scientific Instruments 87, 105105 (2016).[2] K. B. Migler, A. P. Kotula and A. R. Hight Walker, Macromolecules 48, (2015).[3] P. J. Hendra, H. P. Jobic, E. P. Marsden and D. Bloor, Spectrochim. Acta, Part A 33, (1977).