Suppose you are given a new polymer made by a typical chain-growth polymerizatio

Question

Suppose you are given a new polymer made by a typical chain-growth polymerization in solution with free-radical initiation and are told that it's structure -(-CH2 - CH__X -)n- * don't select X= H or CH3

1. How would you go about confirming its complete structure, ie., its detailed micro-structure? (consider all possible microstructural elements at the monomer triad level)

2. Confident in your microstructural analysis, what would you do next to begin establishing its structure-property relationships, and how would you accomplish this?

3. Describe those properties (solution and bulk) that could most likely be connected to the microstructure of your polymer and why and how these structure-property relation could be established?

4. Suppose instead that you are given a polymer made by the step-growth polymerization of monomers X-R-X and Y-R'-Y, where the X,Y-functional groups react to form a link -Z-(-X+Y- --> -Z-). re-examine questions 1.) through 3.) and indicate if, how, and why your answers might, or might not, be altered for this step-growth polymer, i.e,. X-(-R-Z-R'-Z-)n-Y

Explanation / Answer

1. How would you go about confirming its complete structure, ie., its detailed micro-structure? (Consider all possible microstructural elements at the monomer triad level)

Ans: A variety of laboratory techniques can be used to characterize the polymers. Techniques such as wide angle X-ray scattering & small angle X-ray scattering, and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the average molecular weight and polydispersity. FTIR, Raman, and NMR can be used to determine composition. Pyrolysis followed by analysis of the fragments is another technique for determining the possible structure of the polymer.

The preliminary analysis such as solubility and viscosity in suitable solvent can give an idea about the linear (if soluble) or crosslinked (if insoluble) structure of the polymer. The viscosity of the polymer solution can reveal information about the molecular weight (high viscosity refers to the high molecular weight and vice versa) of the polymer. The micro-structural characterization can be carried out with help of spectroscopic techniques such as XRD, Raman, FTIR, 1H-NMR and 13C-NMR characterization tools. XRD can give information about the crystallinity of the polymer. Here X is a pendant group or atom attached to the –C-C-C- backbone of the polymer chain. X may be -Cl, -Br, -Ph –CH=CH2 etc. The presence of –Cl, -Br atom can be detected with the help of elemental analysis. The 1H-NMR and 13C-NMR can reveal the different types (different chemical environment) of proton and carbon present in the polymer chain. NMR can also provide about the tacticity of the polymer (atactic, isotactic or syndiotactic). Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) can be employed to determine the aggregate formation and type of chain packing in the semi dilute solution of the polymer, respectively. light scattering techniques can also be used to determine the microstructure of the polymer. The fine structure of the solutions of polymers can be demonstrated by distribution of relaxation times obtained from dynamic light scattering.

2. Confident in your microstructural analysis, what would you do next to begin establishing its structure-property relationships, and how would you accomplish this?

Ans: Structure-property relationship of a polymer is very important in the study of polymers. The structure of a polymer can dictate the property of the polymer as a whole. Since the given polymer is having -C-C-C- backbone with a branching group (X) attached to the backbone, the nature of the branching group plays a significant role in the overall property of the said polymer. If X is bulky (eg. –Ph group) and/or polar (eg. –Cl group) in nature then there would be rise in Tg (glass transition temperature) of the polymer, eventually that increases crystallinity leading to the increase in mechanical property of the polymer. Tacticity of the polymer can also give an idea about the Tg value and its mechanical properties. Isotactic structure are highly crystalline in nature followed by syandiotactic structure. The actactic is generally amorphous in nature with low Tg value and hence poor in properties. High molecular weight of the polymer give rise to high mechanical properties of the polymer and vice versa.

3. Describe those properties (solution and bulk) that could most likely be connected to the microstructure of your polymer and why and how these structure-property relation could be established?

Ans: The properties of the polymeric solution are also influenced by its microstructure and presence of branching, polar side group etc. in the backbone of the polymer chain. Highly branched structure and lower molecular weight polymers can be easily solubilized in a suitable solvent. Molecular weight and molecular weight distribution can also give an idea about the solution properties of the polymer. Viscoelastic properties of polymers depend on the molecular weight and weight distribution, degree of branching and crystallinity, types of functionality attached to the polymer backbone. The bulk properties of the polymers like Tg, Td, Thermo-stability, mechanical properties etc. are governed by the microstructure of the polymer like its molecular weight and weight distribution, degree of branching, crystallinity and types of functionality present in the polymer chain. Generally high molecular weight with narrow molecular weight distribution give rise to increase in Tg value and crystallinity with high mechanical properties, superior thermal properties. The nature of the branch side chain can also provide information about its thermos-stability and mechanical properties. Linear polymeric chain (with polar groups) are more crystalline than that of the branched polymers (eg. High density polyethylene, HDPE and Low density polyethylene, LDPE).

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