Research Interests
Determination of the solid state structure and morphology
of polymers. X-ray analysis of the structure of thermotropic
copolyesters, copolyimides, polyurethanes, polysaccharides;
supramolecular assemblies, fluoropolymers; molecular
modeling of semi-crystalline and liquid crystalline
polymers; rheological properties of polysaccharides and
glycoproteins.
Overview of Research
A principal area of interest is directed to elucidation of
the solid state structure and morphology of polymers,
primarily via x-ray diffraction and electron microscopy and
associated molecular modeling. In 1974, the first and now
generally-accepted determination of the crystal structure of
the world's most common polymer, cellulose, by x-ray
diffraction was achieved. Pioneering studies have been
carried out to establish the solid state structures of
polyurethanes, thermotropic coplyesters and copolyimides,
and supramolecular assemblies. Diffraction methods and
computer modeling techniques were developed for the first
time to characterise and simulate the structures of the
ordered arrays formed by wholly aromatic copolyesters. The
results show that, despite their random comonomer sequences,
the chains can be packed without stereochemical overlap
according to the observed dimensions, and the simulated
diffraction patterns are in good agreement with the observed
data. The same diffraction methods were then applied
successfully to wholly aromatic copolyimides prepared for
application as high strength, high modulus fibers. Depending
on the monomer chemistry, copolyimides with blocky and
random sequences can be differentiated. Correlations were
established between copolymer composition, the
conformational order (along the chain axis direction) and
the drawability of the copolymers. Definitive analyses of
the structures of the hard domains in polyurethane
elastomers were obtained which identify the hydrogen bonding
networks and hydrophobic domains which influence the chain
conformation and chain packing.
Current Activity
We are interested in the formation of order in random
copolymers, and have focused on a serrieds of polyethylenes
with well characterized branching. X-ray data point to a
three-phase structure in isotropic and drawn specimens of a
homogenous poly(ethylene-co-octene) (7.3 mol-% 1-octene): in
addition to the amorphous and orthorhombic crystalline phase
there is a hexagonal mesophase. At 800% extension, the
degree of crystallinity increases from ~25% to ~50%, due
mainly to the formation of smaller hexagonal domains, while
the proportions of the orthorhombic and amorphous phases
decline. These high crystallinities suggest that the C6 side
chains can be incorporated in the hexagonal mesophase. The
work is now being extended to a range of polyethylenes with
well characterized branching. Molecular modeling is being
used to simulate the disorder in cellulose microfibrils.
These are known to contain mixtures of two structures that
differ in terms of their mutual chain stagger: all-+0.25c in
the a-form, and alternately +0.25c and –0.25c in the b-form.
We have modeled the structures of cellulose crystallites
that contain mixtures of both types of stagger, and have
found that some of the diffraction characteristics explained
by the mixture of two phases are predicted for the distorted
structure. Efforts are proceeding to develop software to
average over all possible assemblies of staggered sheets of
chains, looking to simulate not only the observed Bragg
reflections but also the diffuse scatter assigned to the
amorphous regions.
Recent Publications
“Structure of a poly(2,5-benzimidazole)/phosphoric acid
complex,” J. Cho, J. Blackwell, S.N. Chvalun, M. Litt, and
Y. Wang, JPSA, Part B: Polym. Phys., 42(13),
2576-85 (2004).
“Crystal structure of poly(2-cyano-1,4-phenylene
terephthalamide),” S.Y. Park, S.W. Lee, T.J. Oh, and J.
Blackwell, Macromolecules, 38(9), 3713-18
(2005).
“Preparation and characterization of alpha-chitin
whisker-reinforced poly(vinyl alcohol) nanocomposite films
with or without heat treatment,” J. Sriupayo, P. Supaphol,
J. Blackwell, and R. Rujiravanit, Polymer, 46(15),
5637-44 (2005).
“Preparation and characterization of alpha-chitin
whisker-reinforced poly(vinyl alcohol) nanocomposite films
with or without heat treatment,” J. Sriupayo, P. Supaphol,
J. Blackwell, and R. Rujiravanit, Carbohydrate Polymers,
62(2), 130-36 (2005).
Awards
John Blackwell received the 1999 Anselme-Payen Award of the
Cellulose, Paper and Textile Division of the American
Chemical Society for his definitive achievements in
elucidating the crystal structure of cellulose.
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