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Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 112 where Wf Wb and Wnf are the freezing the freezing-bound and the nonfreezing water contents respectively. Determination of the exact proportions of these water species in a hydrated polymer is an important step in understanding the physicochemical processes which govern the behaviour of the system. 5.12.1 Experimental Procedure DSC is commonly used to determine the proportions of the various water fractions present in hydrated polymers. Sample vessels which can be sealed hermetically are required. If aluminium sample vessels are to be used they should first be placed in an autoclave with a small amount of pure water at 373 K for 3-5 h to eliminate the formation of aluminium hydroxide on the inner surfaces of the sample vessel during the measuring cycle. Almost all polymers contain a small amount of water which is absorbed during synthesis processing or storage. When closely associated with the polymer matrix this water can remain in the matrix even after heating the polymer to 373 K under reduced pressure. It is important to establish the concentration of this water species so that the total amount of water present in the sample after hydration is precisely known. To determine this intrinsic water content the sample should be weighed as accurately as possible noting that the sample will absorb water from the atmosphere during weighing. A microbalance with sensitivity 0.001 mg is necessary. The sample vessel is pierced quickly placed in the DSC at room temperature and heated at 10 K min. An endothermic deviation in the sample baseline due to the vaporization of water is observed. The heat of vaporization of water is high 2257 J g and the presence of very small amounts of water can be detected by this procedure. The sample is heated until no deviation in the sample baseline is observed. The dried sample is then quickly reweighed and the intrinsic water content determined. The following procedure is recommended to obtain a uniformly hydrated sample. A .
