Narrow molecular weight distribution polystyrene standards

Determinarion of MW and MWD by SEC using commercial narrow molecular weight distribution polystyrene as calibration standards is an ASTM-D5296 standard method for polystyrene (11). However, no data on precision are included in the 1997 edition of the ASTM method. In the ASTM-D3536 method for gel-permeation chromatography from seven replicates, the M of a polystyrene is 263,000 30,000 (11.4%) for a single determination within the 95% confidence level (12). A relative standard deviation of 3.9% was reported for a cooperative determination of of polystyrene by SEC (7). In another cooperative study, a 11.3% relative standard deviation in M, of polystyrene by GPC was reported (13). 

Since the calibration of a GPC is dependent upon the effective size in solution of the sample molecules, the type (structure) of molecules used for the calibration is important. The ideal case is to calibrate with a standard sample(s) of the material of interest. However, this is not always possible. In those instances, arbitrary standards are chosen. The arbitrary standards are used to construct a size calibration where the molecular size is calculated from the standard. For polymer analysis, these standards are often polystyrene of narrow molecular weight distribution. These standards may be purchased from a variety of suppliers. 

Polystyrene standards used were narrow molecular weight distribution sample produced by anionic polymerization and available from Pressure Chemical Co. Also sample NBS7C from the National Bureau of Standards was used. The sample of poly n-butyl methacrylate was obtained from Aldrich Chemical. It was produced by free radici polymerization with an Mw of 320,(XK) and an Mn of 73,500 (Cat. No. 18,153-6). 

Novolac molecular weights were measured in THF at 35°C by high pressure size exclusion chromatography using a Waters Model 510 pump (flow rate=1.0 ml/min), 401 differential viscometer detector and a set of Dupont PSM 60 silanized columns. A universal calibration curve was obtained with a kit of 10 narrow molecular weight distribution, linear polystyrene standards from Toya Soda Company. Data acquisition and analysis were performed on an AT T 6312 computer using ASYST Unical 3.02 software supplied with the Viscotek instrument. 

Table III shows some surprising results obtained from the NBS (National Bureau of Standards) polystyrene sample 705. This material has a very narrow molecular weight distribution since it is a specially prepared...

Standard Reference Material 705, Polystyrene (Narrow Molecular-Weight Distribution)... 

The most straightforward method for calibrating the relationship between D and M is to measure both D and M for a set of monodisperse samples with different molecular weights. In reality, the monodisperse samples have to be replaced by narrowly distributed standards made available either by relevant living polymerization or by fractionation of a broadly distributed sample. However, only a few kinds of polymers, e.g. polystyrene and poly(methyl methylacrylate), can actually be prepared so as to have a sufficiently narrow molecular weight distribution (Mw/Mn 1.1), and the fractionation is very time consuming. Thus, the straightforward calibration of the D vs M relation is not always practical. 

Three 7.5 mm i.d. X 300 mm PLgel 5 )iim Mixed-C columns (Polymer Laboratories, Amherst, MA) were thermostated to 30.0°C. Uninhibited tet-rahydrofuran (THF) at a nominal flowrate of 1.0 mL/min was used as the eluent. Narrow distribution polystyrene standards (Polymer Laboratories) were injected in a volume of 100 ijlL with concentrations ranging from 0.1 to 2.5 mg/mL, depending on molecular weight. Broad molecular-weight-distribution polystyrene 18,242-7 was obtained from Aldrich Chemical Company (Milwaukee, WI). Acetone, added to each sample at a concentration of 0.1%, was used as a flow marker. 

The ratio M IMn approaches unity asymptotically as increases. Narrow molecular weight distributions should thus be obtained in living ionic polymerizations with fast initiation in the absence of depropagation, termination, and chain transfer reactions. Values of polydispersity index (PDI) below 1.1 -1.2 are indeed found for many living polymerizations. Molecular weight-standards for polystyrene, poly-isoprene, poly(a-methylstyrene), and poly (methyl methacrylate) are thus synthesized by living anionic polymerizations. However, the termination reactions in methyl methacrylate polymerizations and depropagation in or-methylstyrene polymerizations tend to broaden the PDI in these systems. 

The SEC columns were calibrated using narrow molecular weight distribution water soluble polymer standards sodium polystyrene sulfonate, NaPSS (Pressure Chemical Company, Pittsburgh, PA) and dextrans (Pharmacia Fine Chemicals, Piscataway, NJ). The universal calibration curve proposed by Grubisic et al. (13) was found not to be applicable for these polymer standards. The alternative calibration procedure of Coll and Prusinowski (J ) which incorporates excluded volume effects in the numerical value of the Flory parameter was found to be applicable for this system ( ). The calibration curve for the two-column network system described above is shown in Figure 3. The Kyyy parameter used on the abscissa of Figure 3 is a normalized elution volume parameter defined by Equation 1,... 

The degree of purity of dendrons and dendrimers can be checked by size-exclusion chromatography. As an example, the chromatograms of the dendron 15c-[G j] show that its dispersity is comparable to or better than that of narrow distribution polystyrene standards (polydispersity M /Mn = 1.03). Comparison of the SEC trace for ISc-lG j] and that for 21-[G 3]-[G 3]2 indicates that the latter one has a narrow molecular weight distribution. Interestingly, the elimination of the excess of dendron ISc-lG j] is easily controlled by SEC, as can be seen when comparing Fig. 7 a (21-[G 3]-[G 3]2 partially purified, contaminated by traces of ISc-lG j]) and Fig. 7b (21-[G 3]-[G 3]2 purified). Moreover, the retention time of 21-[G 3]-[G 3]2 is smaller than that of the dendron, corroborating the increase of... 

To date, the standard samples of narrow molecular weight distribution polymers available commercially are mainly polystyrenes. These samples have polydispersity indexes that are close to unity and are available over a wide range of molecular weights. For determining molecular weights of polymers other than polystyrene, however, the molecular weights obtained from these samples... 

There is an interest in forming polystyrenes with vary narrow molecular-weight distributions, because of practical applications, and from purely academic interests. Several preparations of virtually monodisperse polystyrenes of Mw/Afn = 1.06 by anionic polymerizations were developed. The materials are available commercially in small quantities, as standards for GPC. 

As is well known, a SEC system should be calibrated by plotting the elution volume of the peak maxima of a series of calibrants with narrow molecular weight distribution against the log molecular weight Mbefore SEC analysis is made. Commonly, standard polystyrenes are used for the calibrants. The cali-... 

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