Change in Physical Structure

The availability of data on the change in physical structure of carbons after different degrees of bumoff at different temperatures can aid in the understanding of gas-carbon reactions. In the broadest sense, use of profile data after fractional bumoff enables a clear determination to be made of the temperature zone in which the reaction has occurred, as follows ... 

Dr. Mitchell No, surface area measurements were not compared in this way. However, in view of the extremely mild acetylation conditions used (brief exposure to acetic anhydride vapor), it is difficult to believe that any material change in physical structure resulted. 

As the actual material properties are anchored in the chemical and physical structure of the material, all intrinsic properties relate to a material with a distinct processing history. Usually the change in chemical structure during processing is small compared with the change in physical structure. 

The evaporation of milk has been known for many years, even as early as 1200 when Marco Polo described the production of a paste-like milk concentrate in Mongolia [6.4.1.1]. Approximately 600 years later the concentration of milk and of other liquid food products, for example extracts such as coffee, was taken-up as an industrial technology, eventually ending in the production of a dry powder. During water removal, pronounced changes in physical structure and appearance take place. Since the process starts with a thin, water-like liquid and ends with a dry powder, it was found that one method of liquid removal is not optimal for all conditions In the food and dairy industry, the methods listed in Tab. 6.4-2 have been adopted for liquid removal. 

Crystallization can be accounted for by the moisture dependence of Tg according to Roos and Karel (1991b). Crystallization releases water, which in closed containers is absorbed by the amorphous portion of the food. As a result when Tg drops, T-Tg increases and rapid crystallization follows. Products, which have high moisture transfer rates in the environment, will lose water, but the moisture content in the amorphous part remains fairly constant. Crystallization proceeds at a rate defined by a constant T-Tg. Crystallization leads to a complete change in physical structure. It may considerably decrease stability. Lactose crystallization in milk powders... 

Alkaline Degradations - Change in Physical Structure. The hydroxyl accessibility of the fibrous hydrocellulose was initially 51.4 0.8%. In contrast, the amorphous substrate had an accessibility of 99.2 1.0%. Exposure of the fibrous hydrocellulose to the alkaline media caused the accessibility to decrease slightly to 50.7 1.0% and 49.1 1.2% at 60 and 80°C, respectively, but accessibility did not change significantly during the reaction periods (0-168 hr). 

The solid-state l C-NMR spectra of the fibrous hydrocellulose also demonstrate the predominance of the cellulose I allomorph (Figure 6). All three spectra contain the sharp resonances associated with the cellulose I conformation and the broader C-4 and C-6 resonances indicative of regions of three-dimensional disorder and crystallite surfaces (16,17). The relative intensities of the sharp and broad resonances of the three spectra are similar, again demonstrating the lack of change in physical structure during degradation. 

Often, the concept of physical aging is not used in this strictly limited sense, but rather generally, describing changes in physical structure that occur during the life span of plastics materials. 

Organic molecules may behave as indirect-gap semiconductors in that their minimum energy excitation may involve a change in physical structure. Transitions involving triplet states are forbidden. Only singlet states may recombine efficiently. 

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