Modern Polymer Spect..

156 3 Vibratioiicil Sr,ectru CIS a Probe of Stnictiiid Order
It has been found that the outer skin of the film is much more crystalline (-SO(%)
and the crystallinity decreases when probing inside the film reaching a concentration
of -60%) crystallinity approximately 5-8 pm inside the film [130].
The above analysis combined with defect mode spectroscopy carried out in infrared
(absorption in polarized and unpolarized light, and multiple internal reflection)
and Raman has been applied to the structural characterization at the molecular
level of ultra-drawn films of high-density polyethylene (UD-HDPE) [ 1331. The
comparative study has allowed to describe the structural situation of UD-HDPE
films as follows:
(i) The skin of the film within - 10 pin is highly crystalline and orthorhombic.
(ii) UD-HDPE film is a multiphase system with the topological distribution of
the phases changing in going from the surface into the core.
(iii) The orthorhoinbic phase exists both on the surface and in the core, while the
monoclinic phase starts at a penetration depth d, > - 10 pm.
(iv) Both the orthorhombic and the monoclinic phases are highly anisotropic and
are oriented along the draw direction.
(v) An additional phase is identified from d, > 10 pni into the core. This phase
consists of sequences of CH2 groups in tvrrrzs conformation. The orientation of
the chains is not parallel to the draw direction. This additional phase exists at
low draw ratios and decreases its concentration qualitatively at higher draw
ratios with respect to the orthorhoinbic component.
(vi) GTG’ kinks are organized in anisotropic domains, seemingly as a result of
collective phenomena of orientation during stretching.
(vii) The chain end methyl groups find themselves in a disordered environment
similar to that found in liquid n-alkanes. The existence of liquid-like droplets
or of disordered domains around the CH3 groups may be envisaged.
(viii) GG defects are observed and may originate from intrinsic liquid-like structures,
from the disorder around the CH3 heads and from some kind of chain
folding between crystallites or within polymer lainellae.
(ix) The concentration of conformational defects is comparatively small when
normal solid polyethylene is considered. The concentration of defects decreases
when the draw ratio increases, thus showing that conforniationally
disordered and coiled chains unwind towards a better alignment and perfection
of the tmns-planar molecules.
All these information have been compared with other physical data whch help
in the understanding of the molecular structure of such technologically relevant
materials.
3.14.2.5 Case 5: Moving Towards More Complex Polymethylene Systems
The role of long alkyl chains in natural phenomena and in industrially relevant
systems is well recognized. Due to the flexibility of the polymethylene segments
these systems show many phase transitions which are not yet clearly understood at