Assumptions in Molecular Weight Determination and their impact on MW determination
DSC measures the quantity of required heat necessary to melt a sample where the heat of fusion is calculated based on a percentage of crystallinity. Therefore, the methods used to measure crystallinity, weight determination such as DSC must be detailed to retrieve valuable results. However, in most of the molecular weight determination methods, there are certain assumptions undertaken but those assumptions also have a certain impact on results. In DSC profile, three types of distinct probabilities of uncertainty exist. The first is natural uncertainty that may be linked to equipment such as uncertainty of weight measurements of the sample. The natural uncertainty may be random or systematic because the analytical balance utilised in this technique causes the main reason of measurement related uncertainty the other uncertainties of this category exist in heat flow and temperature measurements while performing experiments. It is necessary to assume these uncertainties while evaluating the DSC results. The second assumption might be based on the fact that the provided polymer was not perfect at industrial scale that indicates the impurities or the imperfections that could result in the uncertain thermal profiles. Moreover, the last assumption can be taken through assuming that the provided polymer was a pure substance and does not belong to any secondary variety of polymer (Crompton, 2006).
The assumptions would have a major impact on the molecular weight determination as the results could be erroneous. Also, the random uncertainties might not result in accurate results for the molecular weight determination.
Other Experimental Techniques for Detection of Polymer End Group Concentrations
There are a number of valuable and relevant techniques for polymer end group terminations and detections. Some of the prominent ones are tracer technique, ultraviolet absorption spectroscopy and infra-red absorption. Each of the different techniques is suitable for different types of polymer concentrations and impurity levels and imperfections (Ghosh, 2002). Two most prominent techniques for polymer end group detection are discussed below.
The Dye Partition Technique
Ghosh and co-workers developed a biphasic dye partition technique to rapidly detect the polymer end groups and their concentrations in favourable cases. The technique works on the concept of mixing a Benzene or Chloroform solution of a particular polymer (acid or basic) with an aqueous solution of a suitable ionic dye (basic or acid) in equal volumes. After shaking the mixture, the dye gets partitioned into an organic layer rendering the organic layer into a proportionate intensity of coloured layer equivalent to the concentration of the ionic end group present. Thereby the experiment in a controlled manner indicates presence and concentration of polymer end group by the intensity of the colour and when the aqueous solution remains colourless it determines that that the adsorption of the polymer end group is negligible. Some of the end groups that can be studied using this technique are –OH, –COOH, –NH2 and halogen atom with end groups (-Br,-Cl, and related anionic sulfoxy end groups). A variation of this technique is the Dye interaction technique where the technique utilises homogeneous benzene solution is used for extracting and detecting the presence of polymer end group concentrations. The technique works by treating with a dilute benzene solution of polymers that makes the dye change colours to highly sensitive benzene extracts and further the analysis of colour change is performed using Spectrophotometric or Colorimetric analysis to determine the polymer end group concentrations. Some simple compounds like sodium lauryl sulphate, fatty amine, strong organic acids, or quaternary ammonium compounds are used for the estimations in this technique (Ghosh, 2002).
Infrared End Group Analysis
Infrared End group analysis is a helpful technique for determining the concentration of polymer end groups in a given sample specifically when the sample is insoluble in normal solvents at ambient temperature that hampers the measuring of colligative characteristics of solutions. For example Zhang (1996), presented an infrared analysis for carboxyl and hydroxyl end groups for poly (ethylene naphthalene-2, 6-dicarboxilate) that provided a precise and clear results regarding number average molecular weight and absorbance. The technique works by limiting the polymer end groups to one or two per molecule. The end group analysis is appropriate for determining the polymers synthesised through the condensation process where there is less possibility of branching. In such polymers, the end groups are not attached spectroscopically to the chain. The low molecular weight analogues may be employed for calibration (Zhang, Rankin, and Ward, 1996).
References
Crompton, Thomas Roy. Polymer reference book. iSmithers Rapra Publishing, 2006.
Ghosh, P., 2002. Polymer Science and Technology – Plastics Rubbers, Blends and Composites. 2nd Ed. Tata McGraw-Hill.
Zhang, H., Rankin, A. and Ward, I.M., 1996. Determination of the end-group concentration and molecular weight of poly (ethylene naphthalene-2, 6-dicarboxylate) using infra-red spectroscopy. Polymer, 37(7), pp.1079-1085.
