Chemistry Reference
In-Depth Information
films, however, are hard to measure. Recently, Chan et al. [
27
] reported that the viscoelasticity can be
measured through thermal wrinkling. Thermally induced instability develops when polymer films are
compressed between rigid, stiffer layers. This is due to differences in coefficients of thermal
expansion between the polymer and the inorganic layers. A net compressive stress develops at the
polymer-substrate interface when the composite layers are heated to temperatures that promote
mobility of the polymer layer. Such wrinkling substrate surface is characterized by an isotropic
morphology that can be approximated as a sinusoidal profile. Chan et al. utilized the thermal
wrinkling to measure the rubbery modulus and shear viscosity of polystyrene thin films as a function
of temperature. They used surface laser-light scattering to characterize the wrinkled surface in real-
time in order to monitor the changes in morphology as a function of annealing time at fixed annealing
temperatures. The results were compared with a theoretical model, from which the viscoelastic
properties of the PS thin film are extracted.
2.3 The Crystalline State
Mendelkern [
5
] pointed out that essentially all properties of polymers are controlled by the molecular
morphology. In contrast to the amorphous or liquid state, the crystalline state is relatively inelastic and
rigid. In the crystalline state, the bonds adopt a set of successive preferred orientations, while in the
liquid state the bond orientation is such that the chains adopt statistical conformations. In the
crystalline state the properties of the polymers differ considerably. If a polymer molecule is coiled
randomly, then it cannot fit readily into a geometrically arranged, regular crystalline lattice. So the
molecules must change into a uniform shape to fit into a crystal pattern. In many cases they assume
either a helix or a zigzag conformation. Such arrangements are more regular than in a random coil
[
29
-
40
]. This can even be detected by spectral and thermodynamic studies. During crystallization,
polymers with bulky substituents that are spaced close to each other on the polymeric chains tend to
form helical conformations that remain in the crystalline phase. The arrangement allows close packing
of the substituents without much distortion of the chain bonds. That is particularly true of many
isotactic polymers that crystallize in helical conformations, taking on
gauche
and
trans
positions. For
the
position steric hindrance always forces the rotation to be such as to place the substituents
into a juxtaposition generating either a right-hand or a left-had helix. The helical conformations of
isotactic vinyl polymers were illustrated by Gaylord and Mark [
41
] as shown in Fig.
2.9
:
When macromolecules possess a certain amount of symmetry, then there is a strong accompanying
tendency to form ordered domains, or crystalline regions.
Crystallinity
, however, in polymers differs
in nature from that of small molecules. When the small molecules crystallize, each crystal that forms
is made up of molecules that totally participate in its makeup. But, when polymers crystallize from a
melt, which means that certain elements of the polymeric system or segments of the polymeric chains
have attained a form of a three-dimensional order. Complete crystallization, from the melt, however,
is seldom if ever achieved The ordered conformations may be fully extended or may be in one of the
helical forms as shown above. This resembles orderly arrangement of small molecules in crystals.
The crystalline domains, however, are much smaller than the crystals of small molecules and possess
many more imperfections.
gauche
2.3.1 Crystallization from the Melt
Certain basic information was established about the
crystallization from the melt
[
5
]: The process is a
first-order phase transition and follows the general mathematical formulation for the kinetics of a
