fundamentals of polymer science：高分子科学基础

1、POLYMER SCIENCEFUNDAMENTALS OF POLYMER SCIENCE Thermal Transitions in Polymers Prof. Premamoy GhoshPolymer Study Centre“Arghya” 3, kabi Mohitlal RoadP.P. Haltu, Kolkata- 700078(21.09.2006)CONTENTSIntroductionGlass transition and Melting TransitionMelting Point or First Order TransitionGlass Transition or Second Order TransitionBrittle PointDevelopment of Crystallinity in PolymersCrystalllization of Rubber on CoolingMechanism of CrystallizationStress-induced Crystallization of RubberMelting of Ru

2、bberPolymer Single CrystalsStructure of Bulk PolymersSpherulitesThermal AnalysisKey Words Morphology, crystallization, crystallinity, crystalline zones, crystallizability, glassy state, rubbery state, glass transition, melting, first order transition, second order transition, brittle point, volume change, specific volume, polymer single crystals, spherulites, thermal analysis, dilatometer, folded chain theory, stress-induced crystallization. Introduction: Polymer MorphologyTwo different states o

3、r forms can be identified in which a polymer can display the mechanical or thermomechanical properties that can be associated with solids, viz., the form of a crystal or the form of a glass. It is not really the case that all polymers are able to crystallize. As a matter of fact, a high degree of molecular symmetry and microstructural regularity within the polymer chains are a prerequisite for crystallization to occur. Even in those polymers, which do crystallize in any rate, the ultimate degree

4、 of crystallinity developed is mostly less than 100%.Studies of physical form, arrangement and structure of the molecules or the molecular aggregates of a material system relates to what is known as its morphology. Polymer morpho-logy covers the study of the arrangement of macromolecules over the crystalline, amorphous and the overlapping regions and the overall physical clustering of the molecular aggregates. When cooled from, the molten states, different polymers exhibit different tendencies t

5、o crystallize at different rates depending on many factors including prevailing physical conditions, chemical nature of the repeat units and of the polymer as a whole, their molecular or segmental symmetry and structural regularity or irregularity, as referred to above. Bulky pendent groups or chain branches of different lengths hinder molecular packing and hence crystallization. The nature of the crystalline state of polymers is not simple and it should not be confused with the regular geometry

6、 of the crystals of low molecular weight compounds such as sodium chloride or benzoic acid. There are polymers, which are by and large amorphous, and they have very poor tendency to get transformed into ordered or oriented structures on cooling to near or even below room temperature. Natural or synthetic rubbers and glassy polymers such as polystyrene, acrylate and methacrylate polymers belong to this class. In a crystalline polymer, a given polymer chain exists in or passes through several crys

7、talline and amorphous zones. The crystalline zones are made up of intermolecular and intramolecular alignment or orderly and hence closely packed arrangement of molecules or chain segments, and a lack of it results in the formation of amorphous zones.Glass Transition and Melting Transition On the basis of following the changes in a mechanical property parameter such as shear modulus with changes (rise) in the temperature of observation for polymer material systems, one can readily observe succes

8、sively (i) glass transition and (ii) melting transition phenomena, more easily from a graphical plot , and may also have a measure of the glass transition temperature, Tg and the melting temperature, Tm.The glass transition and the melting transition may also be observed and ascertained from a plot of specific volume ( Vsp ) versus temperature. Let us consider the various possibilities as a melt is cooled from the position A at a high temperature that corresponds to a relatively high Vsp value a

9、s well, fig. 1. The path ABDG shows how the specific volume drops down as a low molecular weight compound is frozen. As the melting temperature Tm is reached at the point B, a sharp discontinuity in Vsp is observed (BD). The slopes AB and DG give measures of coefficients of thermal expansion of the liquid and the solid respectively. The thermal expansion coefficient also suffers a discontinuity at Tm. Fig.1:Schematic diagram highlighting possible changes in the specific volume (Vsp)of a polymer with change in temperature .We may however, start with a molten polymer material at A and observe volume change as described by the path ABHI and there is no discontinuity notable at Tm. The liquid line AB gets further extended beyond Tm with lowering of temperature and it is seen to suffer a change in slope at a much lower temperature, Tg and finally, turns into a di

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