Characterization of Polymer Networks

Infrared (IR) spectroscopy is one of the most commonly used techniques for the study and characterization of polymers (Koenig, 1992). The goal of such characterization is to relate the structure of polymers to their performance properties. IR has been used to characterize not only the resulting polymers but also the polymerization processes leading to the production of polymer systems (Scranton et al, 2003). The aim of the following sections is to summarize the use of IR in the characterization of polymer network structure with particular attention paid to intermolecular hydrogen bonding that occurs in such systems. 

Arndt, K. F. Zander, P., Characterization of Polymer Networks by Measurements of the Freezing Point Depression. Coll. Polym. Sci. 1990,268 (9), 806-813. 

The higher the average valence of a system, the greater its density of crosslink-ing. The description and the characterization of polymer networks will be described in Section 3.5. 

All of the studied polymers are described by the network scheme in Fig. 2.1. Consequently, the composition of the polymers is uniform but the molecular chains between crosslinks differ in length. The molecular mass between crosslinks is therefore a dominant parameter for the characterization of the networks. 

A detailed theoretical characterization of the network structure of the polymer carrier in terms of the correlation length, , in combination with... 

Polymer molecules in a solution undergo random thermal motions, which give rise to space and time fluctuations of the polymer concentration. If the concentration of the polymer solution is dilute enough, the interaction between individual polymer molecules is negligible. Then the random motions of the polymer can be described as a three dimensional random walk, which is characterized by the diffusion coefficient D. Light is scattered by the density fluctuations of the polymer solution. The propagation of phonons is overdamped in water and becomes a simple diffusion process. In the case of polymer networks, however, such a situation can never be attained because the interaction between chains (in... 

Kraus, G. Quantitative characterization of polybutadiene networks. J. Appl. Polymer Sci. 7, 1257-1263 (1963). 

Polymer networks depending on the structure of initial monomers and oligomers are characterized by different framing at atom of silicon and different regulated distance between network points. For the purpose of studying some physical and chemical properties, formation of a network structure directly during film formation from the solution is the unique method of obtaining film samples of polymer networks. 

Fig. 1 Torsion vs. axial analyzers The PerkinElmer Diamond DMA (A) is an axial analyzer while the ATS Rheo (B) is a torsional instrument. Both are controlled stress but can act as strain controlled because of the feedback programmed in. Photos taken by the author of Equipment at the University of North Texas (C) shows the TBA. (Courtesy of Dr. John Enns of Polymer Network Characterizations, Inc.) (View this art in color at www.dekker.com.)...

In addition, there has been an increasing interest in new synthetic methods for the preparation of well-defined polymers with controlled chain-end functional groups [23], such as telechelic polymers, which are characterized by the presence of reactive functional groups placed at both chain ends. These materials can then be used as precursors in the synthesis of block copolymers, as modifiers of the thermal and mechanical properties of condensation polymers, as precursors in the preparation of polymer networks, and as compatibilizers in polymer blends [24]. 

A detailed theoretical characterization of the network structure of the polymer carrier in terms of the correlation length, in combination with diffusion studies of model drugs and proteins provide an invaluable insight into the very complex structure of polymer networks and aid in the design of drug delivery carriers (Narasimhan and Peppas, 1997a). 

Zhang Y, Won CY, Chu CC. Synthesis and characterization of biodegradable network hydrogels having both hydrophobic and hydrophilic components with controlled swelling behavior. J Polym Sci Part A. Polym Chem 1999 37 4554 569. 

Sellergren, B. Hall, A.J. Fundamental aspects on the synthesis and characterization of imprinted network polymers. In Molecularly Imprinted Polymers. Elsevier Amsterdam, the Netherlands, 2001 p. 21. 

A new class of liquid crystal/polymer network composite with very small amounts of polymer network (3 Wt%) is described. These composites are formed by photopolymerization of the monomers in-situ from a solution of monomer dissolved in low-molar-mass liquid crystals. Several techniques have proven useful to characterize these polymer networks. This review describes polymer network structure and its influence on electro-optic behavior of liquid crystals. Structural formation in these composites begins with the phase separation of polymer micronetworks, which aggregate initially by reaction-limited, and then by diffusion-limited modes. The morphology can be manipulated advantageously by controlling the crossover condition between such modes, the order of the monomer solution prior to photopolymerization, and the molecular structure of monomers or comonomers. 

Spectroscopic methods are also useful. IR and NMR have been used to characterize the polymerization reaction (52), as previously mentioned. In addition, dichroic measurements using polarized IR have measured the orientation of polymer networks (72). IR and NMR have also measured the order parameter of the LC component, which is effected by polymerization (72, 52). NMR and dielectric spectroscopies have been used to determine relaxation times for the polymer and LC solvent (P, 52). 

Millar, J. A., Smith, D. G., Marr, W. E., and Kresmarm, T. R. E. Solvent modified polymer networks. Part 1. The preparation and characterization of expanded-networks and macro-porous styrene-DVB copolymers and their sulfonates. J Cherw Soc, pp. 218-225 (1963). Kun KA, Kunin R. Macroreticular resins III. Formation of macroreticular styrene-divinyl-benzene copolymers. JPolym Sci, PartAl, Polym Chem, 6,2689-2701 (1968). 

The molecular characterization of polymers is most effectively carried out by taking the polymer into solution. It is much more difficult to make analytical measurements on polymers which do not dissolve. Dissolution is not thermodynamically or kinetically favoured and polymers which are crosslinked to form a three-dimensional network cannot dissolve. 

Kloczkowski, A. Mark, J. E. Sharaf, M. A. Erman, B., Theory of Strain-Induced Crystallization in Real Elastomeric Networks. In Synthesis, Characterization, and Theory of Polymer Networks and Gels, Aharoni, S. M., Ed. Plenum Press New York, 1992 pp 227-241. 

Jaramillo-Botero A et al (2010) First-principles based approaches to nano-mechanical and biomimetic characterization of polymer-based hydrogel networks for cartilage scaffold-supported therapies. J Comput Theor Nanosci 7(7) 1238—1256... 

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