高分子物理”课程资料4

Polymer physics

Shuang Cui Department of Mechanical Engineering Imperial College London

Crystallinity in polymers

Polymer molecules possess the ability to crystallise; the extent to which this occurs varies with the type of polymer and its molecular microstructure. Polymer are normally semi-crystalline. The degree of crystallinity and the size and arrangement of the crystallites in a semi-crystalline polymer have a profound effect upon the physical and mechanical properties of the polymer.

AFM image: growth patterns of a semi-

crystalline polymer on a solid surface.

Regular and irregular chains

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How can long molecules give rise to small crystallites?•

What are the sizes and shapes of polymer crystallites?•

How are the crystallites disposed with respect to each other to the non-crystalline materials?•What is the nature of the non-crystalline materials? In some polymers, the crystallite dimensions are only of the order of tens of nanometers, which is very small compared with the lengths of polymer chains. Therefore questions arise.

Degree of crystallinity Definitions:

Volume crystallinity: x v= V c/ (V a+ V c)

Mass crystallinity: x m= W c/ (W a+ W c)

Determination of degree of crystallinity:•Density methods (flotation gradient column): x v= (ρs–ρa)/ (ρc-ρa)

•Wide-angle X-ray scattering

•Differential scanning calorimetry (DSC)

X-ray scattering

x c= A c/(A a+ A c)

WAXS CURVES for semi-crystalline polymer

DSC curve

The DSC technique determines the change in enthalpy during the melting of a semi-crystalline polymer. The degree of crystallinity can be calculated by comparing with enthalpy for a 100% crystalline sample.

Crystalline structure

Crystalline structure of PE

Semi-crystalline polymer

Single spherulite (球晶)

Morphological hierarchy exhibited

by block copolymers crystallizing

from single-phase melts. Left to

right: spherulites; block copolymer

microdomains; lamellar crystals;

crystalline block unit cell.

Electron microscopic image of

spherulite

Factors determining crystal structure

•Polymer chain must be linear. A limited number of branches on a polymer chain tend to limit the extent of crystallisation but dot stop it completely.

•Copolymerisation affects the perfection of a polymer chain, but it can be tolerated to a limited extent.

•Tacticity is a important factor to determine the extent of crystallisation and crystal structure. In general, isotactic and syndiotactic polymers will crystallise; whereas atactic polymers are non-crystalline.