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An Alternative Method for Long Chain Branching Determination by Triple-Detector Gel Permeation Chromatography

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An Alternative Method for Long Chain Branching Determination by Triple-Detector Gel Permeation Chromatography Polymer 107 (2016) 122e129 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer An alternative method for long chain branching determination by triple-detector gel permeation chromatography * Thipphaya Pathaweeisariyakul a, , Kanyanut Narkchamnan a, Boonyakeat Thitisak a, Wonchalerm Rungswang a, Wallace W. Yau b a SCG Chemicals Co., Ltd., 1 Siam Cement Rd., Bangsue, Bangkok 10800, Thailand b Polymer Characterization Consultant, 7755 Tim Tam Ave., Las Vegas, NV 89178, USA article info abstract Article history: Gel permeation chromatography with refractive index (RI) or infrared detector (IR), viscometer for Received 8 August 2016 intrinsic viscosity (IV) and/or light scattering (LS) has become a prominent technique for quantitative Received in revised form study of polymer long chain branching (LCB). A quantity known as LCB frequency (LCBf) derived from g- 14 October 2016 factor of Zimm-Stockmayer (ZS) model was here studied and found its problems in reproducibility and Accepted 5 November 2016 accuracy. In this work, an alternative high precision method of the gpcBR approach was used to estimate Available online 8 November 2016 a more reliable g-factor, called gest. The branching index, B-factor for number of LCB per molecule was found to be more relevant than LCBf in relating to polymer properties. A proposal is made that enables Keywords: Long chain branch the distinction between two LCB categories, i.e. branch on main chain and branch on branch. This allows GPC the analysis of these two LCB types on their relative importance to polymer processing and end-use Light Scattering properties. NMR results were obtained in support of the LCB data. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction calculation, it is possible also to obtain LCB distribution along the polymer molecular weight by using the Zimm-Stockmayer random In polyethylene technology, long chain branching (LCB) is an branching model [7,9,11,16,17]. With the same molecular weight, important microstructure that affects many processing properties, the root-mean-square radius (Rg) of a branched polymer is smaller such as melt viscosity, melt strength, die swell ratio, etc. [1,2] For than that of a linear polymer. The branching index g-factor is example, in LDPE extrusion coating process, long chain branching defined as the ratio of the Rg square of the branched polymer over structure can improve neck-in properties [3,4]. To support the that of a linear polymer of same molecular weight, as shown in polymer design development, a well-defined characterization Equation (1). Similarly, the branching index from intrinsic viscosity, technique for LCB is required. g’-factor, is defined as the ratio of intrinsic viscosity ([h]) of a Gel permeation chromatography (GPC) or size exclusion chro- branched polymer over the [h] of linear polymer of same molecular matography (SEC) has become a prominent technique to determine weight, as shown in Equation (2). These two indices are related long chain branching of polymer, in addition to its traditional use with an exponent power ε, so called structure factor, as shown in for the determination of molecular weight (MW) and molecular Equation (3). weight distribution. The well-known MW-sensitive online de- D E.D E tectors coupling with GPC to analyze LCB are multi-angle light g ¼ R2 R2 (1) scattering (MALS) and intrinsic viscometer (IV), in cooperation with g branch g linear a conventional concentration detector, like refractive index detec- tor (RI) or infrared detector (IR) [2,5e15]. GPC with RI or IR, MALS, 0 ¼½h ½h g branch linear (2) and IV, often called 3D-GPC or GPC-3D (GPC with triple detectors), can be used to study polymer LCB. Based on slice by slice 0 ε g ¼ g (3) To estimate g from g’, a value of this epsilon (ε) parameter must * Corresponding author. be assumed. The problem is that, this structure factor can vary from E-mail address: [email protected] (T. Pathaweeisariyakul). 0.5 to 1.5, depending on molecular architecture [5] and recently it http://dx.doi.org/10.1016/j.polymer.2016.11.007 0032-3861/© 2016 Elsevier Ltd. All rights reserved. T. Pathaweeisariyakul et al. / Polymer 107 (2016) 122e129 123 was found to be also depending on degree of polymerization 2. Experiment [18,19]. It is even more challenging for LDPE which has very com- plex branching architectures. A commercial low density polyethylene (LDPE), SCG Chemicals, From g value, the number average LCB per molecule (Bn) can be Thailand, was selected to study the precision of MW,ABS and Rg estimated from the Zimm-Stockmayer equation for a mono- calculation from GPC with online multi-angle light scattering (GPC- disperse, tri-functionally branched polymer, Equation (4) [16]. MALS) measurement. Around 24 mg of sample was dissolved in 8 ml of 1,2,4-trichlorobenzene at 160 C for 60 min. Then the sample solution, 200 ml, was injected into the high temperature " #À = 1=2 1 2 GPC with triple detectors (Polymer Char, Spain) with flow rate of Bn 4Bn g ¼ 1 þ þ (4) 0.5 ml/min at 145 C in column zone and 160 C in all 3 detectors. 7 9p The online detectors consist of IR5 MCT detector, viscometer, and an 8-angle light scattering (Wyatt Technology, USA, assembled by From the Bn value obtained from Equation (4), the LCBf value for fi the number of LCB per 1000 carbon can be calculated by using Polymer Char) or MALS. IR5 MCT is a lter-based infrared detector, Equation (5). designed for GPC to determine online concentration and chemical composition in polyolefins. Infrared detector is useful as a con- centration detector only when applied to polyolefins with aliphatic 1000 CH band. The mobile phase must be selected carefully, in order to LCBf ¼ l ¼ R$B $ ¼ number of LCB per 1000 carbon n M minimize background signal. The light scattering model is DAWN (5) HELEOS II, the static light scattering with 8 angles, and the 8 photodetector positions are at 32, 44, 57, 72, 90, 108, 126, and 141, where, M is molecular weight, and R is a factor of the repeating respectively. However, because of detector noise, the smallest angle molecular weight unit. For example, polyethylene, R ¼ (14 þ 14)/ was generally neglected in all calculations in this report. Laser 2 ¼ 14, or for PVC, R ¼ (14 þ 13þ35)/2 ¼ 31. wavelength (l0) is 662 nm, and the value of 0.104 ml/mg for A new branching approach, gpcBR, has been recently introduced refractive index difference (dn/dc) of the PE in TCB solution was to analyze LCB by 3D-GPC by Yau and co-workers [15,20]. used. The second virial coefficient was negligible (A2 ¼ 0) due to Comparing to the traditional g or g’ and LCBf parameter, this new very dilute solution condition of GPC. The absolute molecular gpcBR index provides a measurement of polymer branching level weight (MW,ABS) and radius of gyration (Rg) were determined from with a much improved precision. It is the combination of intrinsic the intercept and slope at low angle of the Debye plot [22]. The data ® viscosity [h] and absolute weight average molecular weight was processed by GPC One software, Polymer Char, Spain. To fi (MW,ABS) measured from MALS. The expression of gpcBR is the evaluate the LCB quanti cation method in section 3.3, various following: grades of in-house commercial LDPE were selected to obtain the branching parameters and relate them to the processing property. Another model sample used in this work is a linear HDPE, which is a ½h M ; passed several times through the twin screw extruder at 200 C, gpcBR ¼ CC $ W ABS À 1 ½h M ; Hake, Germany, to generate long chain branching from a combi- " ! W CC # a a nation of thermal degradation and chain extension LCB processes. kM ; M ; ¼ W ABS $ V CC À The samples at even extrusion rounds (2, 4, 6, and 8) were collected ½h 1 (6) MW;CC for further measured by 3D-GPC. Die swell ratio was measured by passing the material through a where MW,CC and [h]CC are the weight average molecular weight single screw extruder (Thermo Scientific, HAAKE Rheomex OS, and intrinsic viscosity from conventional GPC calculation assuming Germany) at 190 C, connected with Capillary Circular Die 2 Â 30 polymer is linear with no LCB. The [h] term is the actual intrinsic Taper and Laser measurement unit. The measurement was per- viscosity, which is the measured value from the online viscometer, formed at the distance of 10 mm from die exit with 5 repetitions. 13 calculated by the viscometer peak area method. MW,ABS is the C-NMR spectra were recorded by 500 MHz ASCEND™, Bruker, weight average of absolute molecular weight from LS detector, also with cryogenic 10 mm probe. TCB was used as a major solvent with calculated by the LS peak area method for high precision. The high TCE-d2 as a locking agent with the ratio of 4:1 by volume. The precision quality of gpcBR can be credited to the three key attri- experiments were done at 120 C, and the inverse gate 13C (zgig) of butes in the gpcBR formulation, as can be explained by the two pulse program with 90 for pulse angle were used. The delay time terms in the gpcBR expression in Equation (6). First, all four pa- (D1) was set to 40 s for full-spin recovery. The LCB number for LDPE rameters, [h], MW,ABS, MV, CC, MW,CC are the most reliable measure- was done by the concept of primitive core [21]. ments from the 3D-GPC technique, [h] and MW,ABS is not the function of the conventional GPC MW or IV calibration or the GPC 3.
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