What Is Polymer?

Polymer came from the Greek word 'poly' and 'meros' which means many parts. Polymers can be natural like DNA or can be synthetic like polystyrene. Polymers are huge molecule or macromolecule made of many repeating units which can be the same or different depending upon the desired characteristics.

How are polymers made?

The polymers are made by a process called by polymerization no matter if it is a natural or synthetic one. In this process the repeating units are called monomers.

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Polymers are also of different kinds:

• Polymer type Characteristics
• Intra-polymer Soft
• Stiff
• In-between
• Inter-polymer Weak forces between their chains
  strong force between the chains
• Van der Waals forces decide this inter-polymer force.

What is the chemical structure of the polymer?

The chemical structure of a polymer depends upon the composition of the polymer i.e. which kind of monomers it is made of, amount of elements present in it. The structure of polymers is also dependent upon the chemicals bonds with which monomers are linked and also the size of the molecules that are linked.

Polymerization

Polymerization is a process in which monomers are the input and a 3 dimensional or linked chain of polymer came as output. This process is a chemical reaction.

Polymerization is again of 2 types:

Polymerization type What are these?

Example

• Step-growth polymerization Polymers formed by the stepwise reaction between functional groups of monomers Polyurethanes
• Chain growth or Addition polymerization The linking together of molecules incorporating double or triple carbon-carbon bonds Polyvinyl Chloride (PVC)

Polymer molecular weight

Polymer are long chains of monomers and the length of these chains vary from each another so because of this kind of variations finding the exact molecular weight of each polymer is quite difficult so instead of calculating the exact weight average weight is calculated.

The number average molecular weight is the total weight of the sample divided by the number of molecules in the sample i.e. multiply the weight of the molecule by the number of molecules. The units of average molecular weight are kg/mol.

Properties and Characteristics of polymers

Polymer properties Architecture It is basically the structure and shape of the polymer chains that are linked together to give a specific 2D shape. These shapes can be star, comb, brush, dendronized, ladder and dendrimers. These all exist in branched polymers

• Chain length

This a physical property of a polymer sand this property also influence many other properties like melting point, boiling point, resistance, viscosity, molten states, etc.

• Morphology

It describes the arrangement and microscale ordering of polymer chains

• Tacticity

It describes the stereochemistry of chiral carbons in the polymer chain.

It can be :

1. Isotactic
2. Atactic
3. Syndiotactic

• Transport properties

It describes the diffusivity rate of how rapidly molecules move through the polymer matrix

Characteristics

The characteristics of the polymer can be determined by various techniques such as:

1. Side angle X-ray scattering, small angle X-ray scattering Small angle neutron scattering
2. Gel permeation chromatography
3. Polydispersity.
4. FTIRRaman and NMR
5. Differential scanning calorimetry and dynamic mechanical

These techniques are used to know the crystalline structure, chain lengths, molecular weight, composition, glass transition temperature, melting points, etc.

https://en.wikipedia.org/wiki/Polymer#/media/File:Polymer_Branch.svg

Gel Permeation Chromatography (GPC):

GPC is an analytical technique which makes use of a pore network of the cross-linked polymer as stationary phase, forming a molecular sieve, for the separation of components of a polymer molecule in the solution form (mobile phase) based on their size or molecular weight. In GPC, columns with small porous particles packed in the form of the gel are used to allow permeation of a solvent, hence gel permeation chromatography. There are no chemical interactions between the solvent particles and the gel in GPC, rather solvent particles are separated based on the size.

The physical and chemical parameters of a cross linked polymer in the GPC column is vital for the overall performance of a GPC in terms of separation. Such few properties are hardness, tensile strength, melt viscosity, brittleness, impact resistance, tear strength, friction etc. GPC can determine various important parameters of a polymer, like a number average (Mn), polydispersity index, volume average (Mv), weight average (Mw), Z average and the most important molecular weight distribution, by determining the chain lengths.

Components of GPC:

Instrumentation of GPC is comprised of several components working together to yield a system with an optimal separation of components in a given sample. Different components of GPC are as follows:

Components of GPC system [1]

• Pump – The pump in the GPC system, push the solvent at a constant flow rate to all other components such that there would be no interferences in the analysis because of the viscosity differences of a polymer solution. To avoid reactions of solvent molecules with the pump, it is made up of inert materials.
• Injector – The injector delivers the polymer solution into the mobile phase without disturbing the flow of the mobile phase. In the process of determining molecular weight the injector should be capable of injecting small volume. Usually, the injection volume is in the range 20 – 200 microliters.
• Column– Column is comprised of a dense network of the porous cross-linked polymer of a well-defined size which separates the components in the sample solely based on their molecular weight. Silica beads are commonly used to design the cross-linked polymer of a GPC column. The size of the column varies from smaller ( 7.5mm length X 50mm diameter ) to larger ( 25mm length X 300mm diameter ) based on the purpose for which it is being used. Nature and size of the bead in the 2784591367549column is dependent on the type of components need to separate. Columns are usually used in sets or groups to enhance the resolution of the GPC system. Highly efficient columns will provide robust and reproducible data over a long period of time.GPC Column [2]
• Detector – Detector in the GPC system identify the separated components of a sample after they percolated through the gel membrane based on their size under different measurement parameters. GPC detectors can be broadly divided into two classes based on the type of measurement parameter:

1. Detectors measuring concentration – They are sensitive enough to determine a very small change in the mobile phase by determining the difference in the refractive index of the solvent and mobile phase. They are called differential refractive index (DRI) detectors and are the most common and widely used GPC detector. In this case, the response at the detector end corresponds to the sample concentration.
2. Detectors measuring scattered light – When the response at the detector end proportional to the scattered light can be measured then such detectors came into the role. Few examples are multi-angle laser light scattering (MALLS), low angle laser light scattering (LALLS) and right-angle laser light scattering (RALLS).

• Data processing equipment -After a detector, detects the change in the refractive index, the data processing software in the processing equipment calculate the important parameters like Mn, Mv, Mw, molecular weight distribution etc. pertaining to a sample polymer discussed in the introduction section. The updated GPC software offers multiple detections, calibration checks, and corrections in band broadening which makes the GPC system more efficient overall.

GPC principle: The components of a sample would be separated based on their molecular weight or size. Larger particles traverse less volume since they cannot enter the small pores of a cross-linked network of polymer in the column and hence elute sooner while smaller particles enter the pores, traverse large volume and elute later.

Larger particles in red elute sooner than smaller particles in pink [3]

GPC chromatogram with retention time [3]

GPC working: First, a sample is prepared by dissolving resin (sample) into an appropriate solvent such as toluene or aqueous buffers. Sample preparation is a crucial step, therefore unless the sample does not get fully dissolved in the solvent further steps shouldn't be considered.

After sample preparation, it is injected via injector into the flowing stream of solvent in the GPC system, where sample components get separated, while permeating through the porous cross-linked polymer, based on their size or molecular weight. The detector then detects the sample components based on the difference in the refractive index of the sample and the mobile phase.

The GPC processing software at the end then generates a molecular weight distribution curve which elicits information about the molecular weight of the individual components of the sample along with several other important parameters.

Working of GPC [4]

GPC application:

GPC is widely used to:

1. Characterize different polymers.
2. Differentiate mixtures into small fractions.
3. Routinely clean samples like organic acids, sugars, environmental samples, free fatty acids etc.
4. Utilized in the paint industry.
5. Purify proteins, nucleic acid and polysaccharides.

POLYMER ANALYSIS BY MALDI

• Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analysis
• Every molecule in the sample of the polymer has different molecular weight, we have a distribution of weights.
• The other method used is SEC which is not really helpful as it measures hydrodynamic volume i.e., how big is the polymer chain in the solution. This gives a relative measure of molecular weight, therefore higher the molecular weight higher is the hydrodynamic volume.
  (http://www.pslc.ws/macrog/images/mal01.gif)
• Therefore as a result matrix-assisted laser desorption/ionization mass spectroscopy or MALDI mass spectrometry is used to determine the weight of different polymers.
• The first step is the sample preparation that needs to be run in the instrument.
• The polymer of interest is dissolved using a solvent which can be water but some of the proteins are soluble in it, therefore another solvent used is usually a 70:30 ratio of water: acetonitrile. Again the solvent selection depends on the kind of solvent dissolves the polymer of interest.
• Addition of chemical with the sample which will act as a special agent which will absorb ultra-violet light. Add about 104 times more of UV absorber compared to polymer
• Example of ultraviolet absorber used :trans-cinnamic acid 2,5-dihydroxybenzoic acid. (http://www.pslc.ws/macrog/images/mal05.gif)

PROCESS: Add the sample mixture in an airtight chamber, on the sample probe as shown in the diagram.

The vacuum will help suck the air out of the chamber, when we do this the sample evaporates and only a layer of UV absorbing compound is left behind(with very little sample)/

At this time shoot a laser to the compound, use UV laser in the 330-360 nm range. So the UV absorbing compound absorbs all the energy it can from the laser, it also passes some of that energy along to the polymer molecules. (https://goo.gl/images/2fPKVo)

The polymer is now dispersed in a matrix of the UV-absorbing compound – that's why it is called matrix-assisted laser desorption/ionization mass spectrometry. The matrix material react with the sample polymer in such a way that the polymer gets charged, when the polymers absorb this energy they get evaporated, it's not possible for heavy polymers but this happens under the high temperature and low pressure conditions.  (https://goo.gl/images/qEDDjc)

The polymer is vaporized right between the electrodes. Most of the time there is a single charge on the polymer, therefore the same electrical force is applied to the polymer when accelerated in the electric field between the two electrodes. Heaver the polymer slower is its acceleration, the big heavy polymer will take time to reach the detector at the end of the chamber.(https://goo.gl/images/LRR2v7)

The smaller polymer will hit the detector first followed by the heavier ones- order by mass.(http://www.pslc.ws/macrog/images/mal08.gif)
When the polymer hits the detector, we get the peak. The peak is proportional to the number of molecules that hit at one time.

References

• http://www.waters.com/waters/en_GB/Size-exclusion-chromatography-%28SEC%29-Gel-Permeation-Chromatography-%28GPC%29-Guide/nav.htm?cid=10167568;locale=en_GBhttps://www.agilent.com/store/en_US/Prod-PL1110-6400/PL1110-6400https://slideplayer.es/slide/1661278/A. E. Hamielec, S. T, Balke, B. P. Leclair ; S. L. Pearce; Ind. Eng.Chem.,Prod. Res. Dev., 8,54 (1969).
• Z. Grubistic, R. Rempp ; H. Benoit; J. Polymer Sci., Part B, 5, 753(1967).
  https://www.agilent.com/cs/library/primers/Public/59906969EN%20GPC%20SEC%20Chrom%20Guide.pdf.
• Creel, Howard, Trends in Polymer Science, Elsevier, 1993, vol.1, no.11, pp.336-342 "Prospects for the Analysis of High Molar Mass Polymers Using MALDI Mass Spectrometry".
• Y Brun J. Liq. Chrom ; Rel. Technology, 1979 - 2015 (1998), , 21 (13), "Data Reduction in Multidetector Size Exclusion Chromatography".
• J. Brandrup, E.H. Immersut:John Wiley ; Sons Publisher, "Polymer Hardbook"
• W.W. Yau, J.J. Kirkland, D.D. BlyJohn Wiley ; Sons, Inc., New York, 1979, "Modern Size-Exclusion Liquid Chromatography"
• Rasmussen, W. (2010)," The Use of Gel Permeation: Chromatography for the Cleanup of Samples in the Analytical Laboratory". American laboratory, 42(2).
• Malvern Panalytical, 29 July 2015, Measuring Molecular Weight, Size and Branching of Polymers.
  https://en.wikipedia.org/wiki/Polymer

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