Polymer Characterization by High Temperature Size Exclusion

Chapter 5

Polymer Characterization by High Temperature Size Exclusion Chromatography Employing Molecular Weight Sensitive Detectors Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 18, 2016 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0731.ch005

S. J. O'Donohue and E . Meehan Polymer Laboratories Ltd., Essex Road, Church Stretton, Shropshire SY6 6AX, United Kingdom

The use of molecular weight sensitive detectors, in particular light scattering and viscometry, has become commonplace in the application of size exclusion chromatography (SEC) to the characterization of polymers. Both detectors can facilitate accurate determination of molecular weight distribution when coupled to SEC. High temperature SEC is used to analyse a range of engineering polymers whose properties are such that dissolution can only be achieved in aggressive solvents at temperatures in excess of 135°C. SEC equipment for this type of analysis is generally very specialised and demands a number of design features which are necessary to achieve acceptable chromatography. The incorporation of molecular weight sensitive detectors into such equipment is a vital tool in molecular characterization experiments. Typical applications include polyolefin polymers whose analysis is illustrated in trichlorobenzene at temperatures in the range 135-180°C. A more demanding application, the characterization of poly(phenylene sulfide), in o -chloronaphthalene at 210°C is studied in detail.

Instrumentation for high temperature SEC experiments is generally purpose built to encompass some basic design requirements including a constant flow rate solvent delivery system, a temperature controlled oven compartment in which the SEC columns and detection systems can be housed and an automated high temperature sample injection system.

The use of on-line molecular weight sensitive detectors, light scattering and viscometry, has become commonplace in SEC because not only do they facilitate the determination of molecular weight distribution, they can also provide information on polymer conformation e.g. branching. Ideally these detectors also need to be housed

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© 1999 American Chemical Society Provder; Chromatography of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 18, 2016 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0731.ch005

in the main SEC instrument in order to avoid the spurious effects of temperature variation throughout the system which can occur when individually temperature controlled modules are employed (I). Figure 1 illustrates the arrangement of the three detectors in a modern integrated high temperature SEC system (PL-GPC210, Polymer Laboratories, UK). The injection system and oven compartment can be temperature controlled up to 220°C to accommodate demanding applications. Commercial instrumentation for high temperature SEC has until recently been limited in temperature capability to around 145°C. Consequently very little work has been published on polymer characterization by SEC at temperatures in excess of 145°C, despite the fact that the increase in temperature may offer significant benefits in the analysis of very high melting point polyolefins. However analysis of polymers at temperatures in excess of 145°C has been reported using a home built system for the characterization of poly(phenylene sulfide), although severe restrictions in the instrument design could not be overcome and compromises in detector choice and injection of the samples had to be made (2). This paper describes the use of a modern, commercial high temperature SEC instrument applied to polyolefin analysis at higher temperatures employing both viscometry and light scattering detection. A case study describing the characterization of poly(phenylene sulfide) at 210°C is discussed in detail. Poly(phenylene sulphide) (PPS) is a semi-crystalline polymer possessing a combination of properties desirable to a designer making it an important engineering thermoplastic. There are no known solvents for PPS below 200°C and pioneering work has been published (2) describing the use of 1-chloronaphthalene as a suitable solvent for the determination of solution properties at 208°C. The characterisation of PPS samples by SEC-viscometry is presented here. Experimental The PL-GPC210 high temperature SEC system contains a differential refractive index (DRI) detector as standard. The instrument used extensively in this study was also fitted with a four capillary bridge viscometer (Model 21 OR, Viscotek, USA). A second system in which a viscometer and a light scattering detector (model PD2040, Precision Detectors, USA) were installed was also used. In both systems the viscometer was connected in parallel with the DRI detector to give a flow split between the two detectors of 55:45 (viscometenDRI). In the triple detector system the light scattering detector was connected in series before the other two detectors as illustrated in Figure 2. SEC separations were performed using a column bank of three PLgel ΙΟμπι MDŒDB 300 χ 7.5 mm columns and the eluent flow rate was maintained at 1 ml/min. A flushed full loop injection of 200μ1 was employed throughout the study. The eluents and temperatures studied are summarised in Table I. A l l solvents were analytical reagents and were used without any further purification.

Provder; Chromatography of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


54

Figure 1.

Triple Detector Arrangement in a PL-GPC210

Figure 2.

Detector Configuration


55 Table I. SEC Eluents and Temperatures of Operation Solvent Temperatures (°C) 40 Tetrahydrofuran (THF) 135,145,150,160,180 1,2,4-trichlorobenzene (TCB) 180,210 o-chloronaphthalene (CN)


Stabilised THF was used as received (250ppm BHT) and BHT was added to the TCB at a concentration of 0.00125%. Narrow polydispersity polystyrene standards (Polymer Laboratories, UK) were used for column and instrument calibration. Solutions were prepared and allowed to dissolve at ambient temperature overnight before use. Polyethylene standards (NIST, USA) were dissolved in TCB and C N at 160°C for 1-2 hours. The commercial poly(phenylene sulphide) sample solutions were prepared in C N at 230°C for 1 hour. In all cases the polymer concentration was known accurately and was of the order of 0.5-2.0 mg/ml depending on molecular weight. A l l data manipulation was performed using PL Caliber SEC software (Polymer Laboratories, UK). The volumetric offset or interdetector delay (HDD) between the various detectors was calculated from the retention time offset of the narrow polystyrene standards and verified by analysing a broad polymer standard. Results SEC-Viscometry. Figure 3 illustrates typical DRI raw data chromatograms for a pair of polystyrene narrow standards (Mp=520,000 g/mol and Mp=l,320 g/mol) in three different solvents. The most striking feature of this comparison is the magnitude of the DRI response which of course reflects the specific refractive index increment, v, for each polymer/solvent combination. Assuming a value of v=0.185 cm /g for polystyrene in THF (3), values for polystyrene in TCB and C N were interpolated and are summarised in Table Π. Results for polystyrene in TCB as a function of temperature are summarised in Table ΙΠ. The results for the higher molecular weight polystyrene standards in TCB agree well with literature values quoted at similar temperatures (4). The results also show a significant reduction in ν for lower molecular weight polystyrene standards which is well documented in the literature (5) and a slight reduction in ν for polystyrene generally at higher temperatures. The extremely small value of ν for polystyrene in C N implies that for routine calibration procedures using these standards the DRI detector must exhibit very high sensitivity. 3

Table Π. Polystyrene ν as a Function of Solvent Polymer CN(210°C) TCB (145°C) THF(40°C) 0.003 0.051 Mp 520000 g/mol 0.185 0.001 Mp 1320 g/mol 0.033 0.172 units cm /g



56

PS Standards Mp = 520000g/mol and Mp = 1320g/mol

A

A

THF@40°C

r.

i \

\j TCB@145°C \ /

~\ 1

^

(M

CN@210°C ^

Retention Time (min)

Figure 3.

DRI Detector Response as a Function of Solvent


35

57 The peak retention time of the polystyrene standards was also found to vary but since the volumetric flow rate was measured in each case to be precisely 1.0 ml/min, this reduction in retention time with increasing temperature must be associated with changes in the column characteristics. Table III. Polystyrene ν as a Function of Tern perature Polymer

TCB


(145°C)

TCB

TCB

(160°C)

(180°C)

0.040 0.013

0.046 0.051 0.019 0.015 units cm /g

Mp 210500 g/mol Mp 580 g/mol

From the combined DRI and viscosity detector responses, the intrinsic viscosity, [n], of each of the polystyrene and polyethylene standards was calculated. These values, together with the vendor molecular weight (M) values of the standards, were used to produce a Mark-Houwink-Sakurada plot of log [n] versus log M . According to the Mark-Houwink-Sakurada relationship, [nJ^KM , the slope of this plot equates to α and the intercept to log K. Figure 4 illustrates typical plots for polystyrene in the three solvents studied and Figure 5 shows similar plots for polystyrene and polyethylene in TCB at different temperatures. The Κ and α values calculated from these plots for polymers with molecular weight greater than 10,000 g/mol are summarised in Table IV. The Mark-Houwink-Sakurada parameters for polystyrene in THF and for polystyrene and polyethylene in TCB at 145°C agree well with literature values (6, 7, 8) and increasing temperature had a relatively small effect on the values determined. The values for polystyrene in C N at 210°C differed somewhat to those reported by Stacey (2) but were fairly consistent with the values determined at 180°C. 01

Table IV. Mark-Houwink-Sakurada Parameters Determined Literature

Experimental Polymer

PS/THF/40°C PS/TCB/135°C 145°C 150°C 160°C 180°C PE/TCB/135°C 145°C 150°C 160°C 180°C PS/CN/180°C 210°C PE/CN/180°C 210°C

Κ

(xl(f)

a

13.9 12.8 8.7 9.6 8.3 9.8 39.0 32.0 53.0 41.0

0.714 0.690 0.704 0.690 0.704 0.690 0.729 0.746 0.703 0.725

1.6 3.3 15.0 2.4

0.755 0.713 0.741 0.863

5

KfxlO )

a

14.1 12.1

0.700 0.707

40.6

0.725

18.6

0.657

64.0

0.671



58

Log M

Figure 4. Mark-Houwink-Sakurada Plots for Polystyrene in the Three Different Solvents Studied

Log M

Figure 5. Mark-Houwink-Sakurada Plots for PS and PE in T C B at Different Temperatures



59 In order to characterize polymers by SEC-viscometry the molecular weight and intrinsic viscosity data for the polystyrene standards was used to produce a Universal Calibration (9) where log M[n] was plotted against peak retention time for each standard. Figure 6 shows Universal Calibration plots for polystyrene under three solvent/temperature conditions. In each case the data has been fitted using a first order polynomial which reflects the column characteristics, mixed gel columns are intended to provide a linear conventional log M versus retention time calibration plot. This linear fit is useful as some degree of extrapolation is normally required when employing molecular weight sensitive detectors in order to compensate for the lack of detector sensitivity at the extreme tails of the polymer distribution. Characterization of Poly(phenylene sulfide) (PPS) Figure 7 shows typical DRI raw data chromatograms for three PPS samples contrasted with the DRI trace for two polystyrene standards. The PPS samples, prepared nominally at 2 mg/ml, exhibit very good DRI detector response compared to the polystyrene standards and retention time differences indicate variation in molecular weight between the PPS samples, sample A lowest and sample C highest in molecular weight. Typical viscometer raw data chromatograms for the same set of samples is shown in Figure 8. By contrast the polystyrene shows very good response and for the PPS samples both retention time and response height varies in accordance with the molecular weight (and intrinsic viscosity) differences between the samples. For a set of six PPS samples the weight average molecular weight (Mw) was determined by two methods : 1. Employing DRI response only together with Mark-Houwink-Sakurada parameters for PS and PPS as reported by Stacey (2) 2. Employing SEC-viscometry with a Universal Calibration plot generated using polystyrene standards. The results for the six samples, designated A to F, are summarised in Table V. For the higher molecular weight samples, C through to F, there was reasonably good correlation between the results obtained by the two methods. The two lower molecular weight samples (A and B) exhibited more variation in molecular weight by the two methods. This could be associated with the fact that the Mark-HouwinkSakurada parameters for PPS reported by Stacey were calculated over a relatively narrow molecular weight range with no data below around 20,000 g/mol. The molecular weight distributions calculated for the six samples by SEC-viscometry are compared in Figures 9 and 10. The molecular weight differences between samples A, Β and C can clearly be seen in Figure 9 but another notable feature was the difference in polydispersity with sample Β exhibiting a significant low molecular weight tail. In Figure 10 it can be seen that samples D and Ε have very similar distributions and that sample F has an overall higher molecular weight but again all



60

Figure 7.

Typical DRI Rawdata Chromatograms of PPS


61


520000

1320 Polystyrene

PPSC PPS Β PPS A

^ —

5

30

Retention Time (min)

Figure 8.

Typical Viscometer Raw Data Chromatograms for PPS

C

A Β

f

/

/

/

\

/

/' V \ / Λ V Λ /\ /

/ \ / /

2.0

y

log M

\ \ \ \ \ \ \

5.5

Figure 9. GPC-Viscometry Molecular Weight Distribution Overlays for the PPS Samples A , Β and C



62

Figure 10. GPC-Viscometry Molecular Weight Distribution Overlays for the PPS Samples D, Ε and F


63 three samples show the presence of a low molecular weight tail on the distribution.

Table V . Summary of Results for PPS Samples Sample

Mw GPC,

determined applying


determined

A Β C D Ε F

by Κ and

by

conventional a

values

Mw

from

Stacey

6851 22209 51850 42422 43742 46213

determined

(Universal PS

by

GPC-Viscometry

Calibration

generated

standards)

3100 16300 47500 37000 37740 42600

SEC-Light Scattering-Viscometry The results quoted in Table III indicated that as the temperature was increased, ν for polystyrene in TCB decreased slightly. In light scattering experiments the scattered light intensity is proportional to the square of the ν term and therefore detector sensitivity will be of ultimate concern as the temperature of SEC operation is increased. Work with the triple detector system was more limited than with the SECviscometry system but experiments have been carried out to study the feasibility of the technique. Figures 11 and 12 shows the detector responses from the triple detection system for a pair of polystyrene standards (Mp=520,000 g/mol and Mp=9680 g/mol) and for a polyethylene standard (NBS 1475) in TCB at 160°C. The concentrations prepared for these sample solutions were 0.76 mg/ml, 1.72 mg/ml and 2.29 mg/ml. NBS 1475 is quoted as having Mw=52,000 g/mol and for linear polyethylene in TCB at 135°C the published ν value is 0.107 cm /g (4). The results obtained so far would suggest that the triple detector system is well suited to the characterization of commercial, higher molecular weight polyolefins in TCB at temperatures of 160±20°C. It is also suggested that for PPS characterization in C N at 210°C where, although the polymers have typically low M w values (20,000-60,000 g/mol), the ν is quoted as 0.137 cm /g (2), this triple detector system could offer significant advances in the determination of molecular weight distribution. 3

3

Conclusions The Mark-Houwink-Sakurada parameters have been determined for polystyrene and polyethylene in a range of solvents and at various temperatures, some at temperatures well in excess of values previously reported in literature. This work has been performed using a commercial high temperature SEC instrument fitted with DRI and viscometry detectors. The parameters for polystyrene and polyethylene in TCB did not appear to vary significantly over the temperature range 135°C to 180°C. This suggests that similar methodology can be applied routinely for the characterization of polyolefins by SEC-viscometry using higher temperatures.


64


Polystyrene standards Mp = 520000 and Mp = 9680

Retention time (min)

Figure 11.

SEC-LS-viscometry in the PL-GPC210

Polyethylene NBS 1475 (2.29mg/ml, 200ul)

Retention time (min)

Figure 12.

SEC-LS-viscometry in the PL-GPC210


65


The application of SEC-viscometry to the characterisation of PPS has been shown to be sensitive to changes in both molecular weight and molecular weight distribution. Molecular weight values obtained by this online, multi-detector technique were in good agreement with values calculated using literature quoted parameters which have been generated using off-line techniques. Based on the work presented here, the DRI response for PPS suggests that the ν in C N at 210°C is relatively large and usable for on-line SEC-light scattering experiments. The potential of SEC-LS for the characterisation of PPS will be the subject of future work. Literature Cited 1. Lesec, J.; Millequant, M. International GPC Symposium Proceedings. 1996, 87115. 2. Stacy, C. J. J.Appl.Polym. Sci. 1986, 32, 3959-3969. 3. Zigon, M.; The, Ν. K.; Shuyao, C.; Grubisic-Gallot, Z. J. Liq Chrom. & Rel. Technol. 1997, 20(14), 2155-2167. 4. Horska, J.; Stejskal, J.; Kratochvil, P. J. Appl. Polym. Sci. 1983, 28, 3873-3874. 5. Margerison, D.; Bain, D. R.; Kiely, B. Polymer. 1973, 14, 133-136. 6. Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Liquid Chromatography; John Wiley & Sons: New York, NY, 1979; 337. 7. Lehtenin, Α . ; Vainikken, R. International GPC Symposium Proceedings. 1989 612 8. Scholte, T.G.; Meijerink, N. L. J.; Schoffeleers, Η. M.; Brands, A. M. G. J. Appl. Polym. Sci. 1984, 29, 3763-3782. 9. Ζ Grubisic, Z.; Rempp, P.; Benoit, H. J. Polym. Sci., Part B, Polym. Lett. 1967, 5, 753.


Polymer Characterization by High Temperature Size Exclusion

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