PLASTICS 



finest structures, however, become visible Ohsinicted Spherulitcs. The structure 



only with an electron microscope. is still rej2;ular, t»iil (lie purel}^ spherical or 



We can distinguish these crystal structures ellipsoid form is no loiijicr attained. This is 



microscopically according to (1) their type usually because the individual spherulites 



and (2) their shape. The most common types have impeded one another's growth, 

 are described below. Misfornied Spln'i-iililes. If there are 



Single Spherulites. The polarizing mi- very many nuclei in the melt, the spherulites 



croscope show^s these as round to ellipsoid may obstruct one another to such an extent 



particles with a distinct polarization cross, during growth that no regular structures 



the arms of which are not uniformly thick, can arise. In this case it is often very difficult 



If the spherulite is rotated this cross often to recognize the above-mentioned types, 

 splits into two hyperbolas. If these crystal 



structures are elhpsoidal the bigger axis is Melting Point Tests with High Poh- 

 in the direction of elongation. mers 



Dendrite Spherulites. Spherulites with a To determine the melting points of various 

 pronounced radial main structm-e. The ra- plastics the ordinary melting point micro- 

 dial fibrils have pronounced featherwise scope can be used. It should be borne in 

 branches. This branching is also clearly vis- mind that pieces of polymer are usually very 

 ible w^th an electron microscope. poor conductors of heat. Only very small 



"Peacock"-Spherulites. Spherulites are granules, 10-15 m, are therefore used as 



sometimes found that show an alternation specimens. If bigger pieces are used it is 



of concentric bands. These bands have alter- often found that the granules are already 



nate positive and negative birefringence, melted on the outside, while the inside does 



Electron microscopic examination suggests not melt until a higher temperature is 



that this is a mixed structure of the single reached, as shown by the thermometer in 



and dendrite types. The negative bands are the microscope heating plate, 

 said to be formed by the branched fibrils Even with very small granules the tem- 



and the positive ones by the unbranched perature must be allowed to rise only very 



pieces with their purely radial orientation. slowly. It is advisable not to increase it by 



A pattern resembling the preceding one is more than 2°C a minute, 



fomid with polyethylene, in which spheru- ^ . „ . . 



lites occur with alternate negatively hire- Surface Examination 

 fringent rings and isotropic bands. This Examination of the surface of fractures 



gives a "peacock" effect under the polarizing causes little difficulty on the whole. With 



microscope. transparent plastics it may happen that 



Irregular Spherulites. These are under- cracks continuing to several microns below- 

 stood to be structures without any of the the surface cause interference colors, 

 above-mentioned regularly geometrical fibril If it is desired to examine filler disp(Msion 

 orientations. Sometimes zig-zag structures from fracture surfaces, it is often adxisable 

 are found, while in other cases there is no to use polarizetl light. For many plastics a 

 regularity at all (e.g., Terylene). specific angle of incidence can be determined 



A classification of spherulites according to at which polarized light depolarizes when 



their shape includes the following: reflected. As (lci)()larization does not occur 



Regular Spherulites. Hound to ellip- with the liller particles or occurs at a differ- 



soid in shape these have originated independ- ent angle of incidence, filler particle disper- 



ently and have not been obstructed in their sion can be effectively studied with an ana- 



o-rowth. lyzing filter. It should be remembered that 



393 



