| dc.description.abstract | Crystalline phases are extremely relevant for properties and applications of many polymeric
materials. In fact, their amount, structure and morphology constitute the main factors controlling
physical properties of fibers, films and thermoplastics and can be also relevant for properties of
rubbers and gels.
It is also well known that processing and physical properties of polymer-based materials are
strongly affected by the occurrence of polymorphism (i.e. the possibility for a given polymer to
crystallize in different crystalline forms) and mesomorphism (i.e. the occurrence of “disordered”
crystalline phases, characterized by a degree of structural organization that is intermediate between
those identifying crystalline and amorphous phases).
Different has been the destiny of polymeric co-crystalline forms, i.e. structures were a polymeric
host and a low-molecular-mass guest are co-crystallized.
Systems composed of solid polymers and of low molecular mass molecules find several practical
applications, including advanced applications. In several cases, additives (often improperly referred
as guest molecules) are simply dispersed at molecular level in polymeric amorphous phases,
although frequently, to reduce their diffusivity, the active molecules are covalently attached to the
polymer backbone, either by polymerization of suitable monomeric units or by grafting the active
species onto preformed polymers.
A more simple alternative method to reduce diffusivity of active molecules in solid polymers and
to prevent their self-aggregation consists in the formation of co-crystals with suitable polymer hosts.
Polymeric co-crystalline forms are quite common for several regular and stereoregular polymers,
like e.g. isotactic and syndiotactic polystyrene (s-PS), syndiotactic poly-p-methyl-styrene,
syndiotactic poly-m-methyl-styrene, syndiotactic poly-p-chloro-styrene, syndiotactic poly-p-fluorostyrene,
polyethyleneoxide, poly(muconic acid), polyoxacyclobutane, poly(vinylidene fluoride),
syndiotactic polymethylmethacrylate.
The removal of the low-molecular-mass guest molecules from co-crystals can generate
nanoporous-crystalline phases. In this respect, it is worth noting that nanoporous crystalline
structures can be achieved for a large variety of chemical compounds: inorganic (e.g., zeolites),
metal-organic as well as organic. These materials, often referred as inorganic, metal-organic and
organic “frameworks” are relevant for molecular storage, recognition and separation techniques.
The removal of the low-molecular-mass guest molecules from polymer co-crystalline forms
generates host chain rearrangements, generally leading to crystalline forms that, as usual for
polymers, exhibit a density higher than that one of the corresponding amorphous phase. However,
in few cases (to our knowledge, up to now only for s-PS), by using suitable guest removal
conditions, nanoporous crystalline forms, exhibiting a density definitely lower than that of the
corresponding amorphous phases are obtained.
Poly-4-methyl-1-pentene isotactic (i-P4MP1) is a polymer characterized by a complex
polymorphism and 4 different crystalline forms, some of which are obtainable only by
crystallization with solvent, have been described in the literature.
Monolithic and highly crystalline aerogels of isotactic poly(4-methyl-pentene-1) (i-P4MP1) have
been prepared by sudden solvent extraction with supercritical carbon dioxide from thermoreversible
gels. The cross-link junctions of i-P4MP1 gels, depending on the solvent, can be constituted by pure
polymer crystalline phases (I or III or IV) or by polymer-solvent co-crystalline phases (for
cyclohexane and carbon tetrachloride gels). Gels with co-crystalline phases lead to aerogels
exhibiting the denser crystalline form II while all the other considered gels lead to aerogels
exhibiting the thermodynamically stable form I. The effect of solvent on the aerogels pore structure
and morphology has been also investigated by scanning electron microscopy and N2 sorption
measurements. In all cases the areogels present highly porous interconnected structures with
macropores and a large heterogeneity of mesopore size but without micro-sized pores.
Poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) is a linear regular polymer, which as s-PS has the
advantage to be a commercial thermoplastic polymer. PPO exhibits a high free volume or
ultrapermeable amorphous phase and has been recognized as a membrane material with high
permeation parameters. Although few papers have recognized that PPO crystalline phases can play
a role in gas sorption and transport processes, no correlation between the amount or nature of the
crystalline phase and guest sorption properties has been reported. This is mainly due to the scarce
information available in the literature relative to the crystalline phases of PPO.
Crystalline modifications, exhibiting largely different X-ray diffraction patterns, have been
obtained for poly(2,6-dimethyl-1,4-phenylene)oxide (PPO), by gel desiccation procedures as well
as by solvent-induced crystallization of amorphous films. The choice of the solvent allows
controlling the nature of the crystalline phase. Both amorphous and semicrystalline samples of this
commercial thermoplastic polymer exhibit a high uptake of large guest molecules (like, e.g.,
benzene or carbon tetrachloride), both from vapor phases and from diluted aqueous solutions.
Surprisingly, the semicrystalline PPO samples present guest solubility much higher than fully
amorphous PPO samples. These sorption experiments, as well as density measurements and
classical BET experiments, clearly indicate that the obtained PPO crystalline phases are
nanoporous. For these thermally stable PPO-based materials exhibiting nanoporous crystalline and
amorphous phases, many applications are predictable.
Finally, the preparation procedures and the thermal stability of the co-crystalline phase and FTIR
and VCD analysis are presented. In particular co-crystalline phases with racemic and non-racemic
guest molecules have been prepared and characterized. The experimental data indicates that the
PPO/a-pinene co-crystalline form is chiral, i.e. the unit cell includes all right or left handed polymer
helices and (1S-(–) or (1R)-(+) a-pinene guest molucules, respectively. [edited by author] | en_US |
