Resumen:
Polymer matrices with well defined structure and pore sizes are widely used in
several areas of chemistry such as catalysis, enzyme immobilization, HPLC,
adsorbents or droug controled release. These polymers have pores in its structure both
in the dry and swollen state. Although it is well known that the structures and properties
greatly differ between these two states, only a few methods provide information about
the swollen, even though most of the applications involve the matrices in this situation.
Nuclear Magnetic Resonance (NMR) is a suitable tool for the study of the molecular
dynamics of different liquids spatially confined in macro, meso and nanopores through
changes in relaxation times. In transverse relaxation experiments, either diffusion
inside the pore, or relaxation induced by mobility restriction of the liquid near the wall,
are additional sources of relaxation, which are extremely useful in the determination of
structural and functional properties. In particular the use of
1
H-CPMG allows the
determination of the transverse relaxation time (T2) of the molecules of liquid, and this
can be related to the pore size at which it has bound. Hence through relaxometry data
it is not only possible to perform the characterization of the material in the swollen
state, but also to monitor the behavior of the matrix during evaporation.
For this work we synthesized polymers of 2-hydroxyethyl methacrylate (HEMA)
cross-linked with ethylene glycol dimethacrylate (EGDMA), varying the proportion of
crosslinking between 6 and 33%. The amount of crosslinking has a marked effect on
the final properties of the material. Given the polar characteristics of the matrices,
heptane was used to characterize the dry state, since the molecules enter in the pores
but do not solvate the polymer chains, so the network morphology is not altered. On
the other hand, water was used to study the properties of the matrix in the swollen
state. Analizing the distribution of T
2
valuesis possible to determine parameters that are
useful in the characterization of the material, such as pore size distribution, porosity
and the contribution of each pore population to the total porosity. Furthermore, this
technique allows the observation of the variation in the distribution of pore populations
as the swelling liquid evaporates. This enables to infer specific properties of each
organic matrices under studied.
Sumarizing, NMR relaxometry provides information of the pore structure of different
polymeric matrices. These parameters are useful for the characterization of porous
materials and, unlike most conventional techniques, this information can be obtained in
both the dry and the swollen state. The swollen state study also provides information
on the structure of the mesh network. This technique represents a direct, simple, quick
and non-destructive approach to determine the porperties and architecture of organic
porous materials.