химический каталог




Компьютерное материаловедение полимеров

Автор А.А.Аскадский, В.И.Кондращенко

of molecular mass distribution in the polymer.

Chapter VI Melting Point of Polymers

A polymer melting point T„ is defined as a temperature at which the polymer comes from its cryslaline stale into a viscous flow state. Due to polydispersity of polymeric chains, different numberof links in them and imperfection of crystallites formed, tin's process is smoothwithina certain temperature range, rather than abrupt There exist two approaches to calculation of T„ based on the chemical structure of

^ Summary 503

a polymer repeating unit. In the first approach Equation (171) connecting the glass transition temperatures and melting point of polymer has been obtained from the experimental fact that both melting of a crystaline polymer and transition from the glassy slate to the rubbery state of a polymer with the same chemical structure take place at reaching an approximately identical percentage of empty volume. In order lo increase the accuracy of Tm calculation it is expedient to take into consideration the experimental values of parameters 8 and у for different atoms and types of intermolecular interaction (Table 21); during which these parameters are related lo the partial packing coefficients for different atoms and constants of polymers. The use of those parameters ensures an acceptable accuracy of melting point calculation (Table 23) The other approach derives Equation (175) connecting Tm to the polymer chemical structure from consideration of a polymer repeating unit as a set of anhar-monic oscillators. In spite of a sufficiently good fitting of calculated and exTxtrimental values of T„ (Table 23), we should mention a limited applicability of this approach.

Chapter VII

Temperature of Onset of Intense Thermal Degradation of Polymers

The most important characteristic of polymer thermal stability is the temperature of onset of intense thermal degradation Td determined at a crossing of tangents to two branches of a thermogravimetric curve (Fig.68). As in calculation of the glass transition temperature Tg and melting point Tm Equation (177) was obtained from consideration of valence-connected atoms of a polymer repeating unit as a set of anliarmomc oscillators, however the energy of chemical bonding is taken into consideration rather than the energy of intermolecular interaction, though the latter may substantially influence (lie dissociation energy of chemical bonds. The last circumstance is considered in Equation (181) with the help of coefficients Kh their numerical values being different for the same atom depending upon its presence or absence in a polar group (Table 25). The calculated values of Td as specified in Table 26 for representatives of various polymer classes satisfactorily fil their experimental values. The use of Equation (181) confirmed a possibility for polymer thermal degradation taking place at temperatures below their respective temperatures of glass transition or melting point. This found an experimental and calculated verification for polymethylidenephthalide (55). Such a peculiarity in polymer behaviour can be explained by presence of voluminous lateral subsitituents containing polar groups, as distinct from, say, weakly polar polymers whose temperatures of glass transition and melting point are much lower than their thermal degradation temperature. A situation is also possible (as for polyheteroarylenes) when the temperatures of glass transition and thermal degradation actually coincide.

Summary

A peculiar case takes place when a polymer degradation starts with its decomposition at end groups. If such a decomposition does not lead to formation of the same end groups, the value of Td is calculated from chemical structure of a repeating unit; otherwise the value of Tj is calculated only from the structure of end groups. The consideration of these two cases in analysis of polyfonnaldehyde thermal decomposition lias shown a good fit between calculated and experimental values of temperature of onset of intense thermal degradation for such a polymer (Fig.70).

Chapter VIII

Optical and Optico-Mechanical Properties of Polymers

The most important optical property of polymers is their refraction index n, which is closely connected with the matter dielectric constant e. In the general case the static dielectric permittivity originates from orienlational, atomic and electronic polarization. In optical range of frequencies the contribution from orientational and atomic polarization can be neglected. In this case die square of refraction index n will be equal to e provided both values have been determined at an identical frequency. Using the Lorenz-Lorentz"s equation (188), Equations (189) and (190) were obtained for calculation of the refraction index of polymers and copolymers respectively on the basis of their chemical structure. Atomic refractions R used in (hose relations are specified in Table 27 whereas Table 28 gives calculated and experimental values of refraction index.

страница 122
< К СПИСКУ КНИГ > 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141

Скачать книгу "Компьютерное материаловедение полимеров" (8.44Mb)


[каталог]  [статьи]  [доска объявлений]  [прайс-листы]  [форум]  [обратная связь]

 

 

Реклама
кровати орматекс
крокус сити холл афиша руки вверх
архивные стеллажи производство
самые красивые овощные витрины

Рекомендуемые книги

Введение в химию окружающей среды.

Книга известных английских ученых раскрывает основные принципы химии окружающей среды и их действие в локальных и глобальных масштабах. Важный аспект книги заключается в раскрытии механизма действия природных геохимических процессов в разных масштабах времени и влияния на них человеческой деятельности. Показываются химический состав, происхождение и эволюция земной коры, океанов и атмосферы. Детально рассматриваются процессы выветривания и их влияние на химический состав осадочных образований, почв и поверхностных вод на континентах. Для студентов и преподавателей факультетов биологии, географии и химии университетов и преподавателей средних школ, а также для широкого круга читателей.

Химия и технология редких и рассеянных элементов.

Книга представляет собой учебное пособие по специальным курсам для студентов химико-технологических вузов. В первой части изложены основы химии и технологии лития, рубидия, цезия, бериллия, галлия, индия, таллия. Во второй части книги изложены основы химии и технологии скандия, натрия, лантана, лантаноидов, германия, титана, циркония, гафния. В третьей части книги изложены основы химии и технологии ванадия, ниобия, тантала, селена, теллура, молибдена, вольфрама, рения. Наибольшее внимание уделено свойствам соединений элементов, имеющих значение в технологии. В технологии каждого элемента описаны важнейшие области применения, характеристика рудного сырья и его обогащение, получение соединений из концентратов и отходов производства, современные методы разделения и очистки элементов. Пособие составлено по материалам, опубликованным из советской и зарубежной печати по 1972 год включительно.

 

 



Рейтинг@Mail.ru Rambler's Top100

Copyright © 2001-2012
(25.04.2017)