Crystallization of Polymers Under 1D Confinement
225
Focusing on the structural process, the intensity of the polarization process is
proportional to the number of dipole moments fluctuating at the time of the experiment [35]. Because of this link, the intensity of the structural peak, ε, is a probe
of the volume fraction of the amorphous phase. The reduction in the solid angle
over which dipole moments are allowed to fluctuate when they are incorporated
into crystals allows monitor crystallization via BDS. The timescale of the reduction in ε is thus associated with the conversion of the amorphous phase into the
ordered structures, and to glass stability. Monitoring the changes in the dielectric
response of a material while it crystallizes, hence, permits measuring the timescale
of crystallization together with the changes in the structural relaxation time.
3 Slowing Down in the Crystallization Kinetics of 1D
Confined Polymers
The crystallization kinetics in ultrathin polymer films has been investigated by several
techniques in both isothermal and nonisothermal conditions [1]. The large experimental data set collected so far shows that the rate of crystallization strongly decreases
upon reducing the thickness, leading to an increase of the crystallization time t cry up
to several orders of magnitude, compared to the value measured in bulk melts [4, 24,
36–39]. In some case, below a given thickness, crystallization seems suppressed, no
detectable change is observed within timescales exceeding months [39, 40]. These
extreme conditions have attracted a large number of studies. The inhibition of crystallization upon confinement has attracted a considerable technological interest. Being
able to suppress, or at least reduce, the crystallization rate would yield, for example,
a tremendous increase in safety of those amorphous drugs where the crystalline form
has non-negligible toxicity [2], and a neat improvement in the lifetime of polymerbased nanodevices, as coatings for optics and display applications where the presence
of crystals affects materials properties [1], e.g., opacity.
Understanding the reasons yielding these tremendous changes in crystallization
rate upon confinement is thus of fundamental importance. One of the first hypotheses
considered was an increase in glass transition temperature upon confinement [39].
Such change in T g would imply an increase in segmental relaxation time in isothermal
conditions and consequently lower self-diffusion coefficient. The latter reduction
corresponds to a neat drop in mass transport towards the crystalline growth front,
which would justify the observed confinement effect on crystallization. For example,
in the case of poly(ethylene oxide), PEO, [41, 42] it was argued that a reduction in
crystal growth factor by 2 orders of magnitude was imputable to an increase of 30 K
in the glass transition temperature.
These hypotheses, however, were not supported by experimental evidence,
because of the difficulties in monitoring crystallization kinetics and probing
molecular mobility within the same experiment.
225
Focusing on the structural process, the intensity of the polarization process is
proportional to the number of dipole moments fluctuating at the time of the experiment [35]. Because of this link, the intensity of the structural peak, ε, is a probe
of the volume fraction of the amorphous phase. The reduction in the solid angle
over which dipole moments are allowed to fluctuate when they are incorporated
into crystals allows monitor crystallization via BDS. The timescale of the reduction in ε is thus associated with the conversion of the amorphous phase into the
ordered structures, and to glass stability. Monitoring the changes in the dielectric
response of a material while it crystallizes, hence, permits measuring the timescale
of crystallization together with the changes in the structural relaxation time.
3 Slowing Down in the Crystallization Kinetics of 1D
Confined Polymers
The crystallization kinetics in ultrathin polymer films has been investigated by several
techniques in both isothermal and nonisothermal conditions [1]. The large experimental data set collected so far shows that the rate of crystallization strongly decreases
upon reducing the thickness, leading to an increase of the crystallization time t cry up
to several orders of magnitude, compared to the value measured in bulk melts [4, 24,
36–39]. In some case, below a given thickness, crystallization seems suppressed, no
detectable change is observed within timescales exceeding months [39, 40]. These
extreme conditions have attracted a large number of studies. The inhibition of crystallization upon confinement has attracted a considerable technological interest. Being
able to suppress, or at least reduce, the crystallization rate would yield, for example,
a tremendous increase in safety of those amorphous drugs where the crystalline form
has non-negligible toxicity [2], and a neat improvement in the lifetime of polymerbased nanodevices, as coatings for optics and display applications where the presence
of crystals affects materials properties [1], e.g., opacity.
Understanding the reasons yielding these tremendous changes in crystallization
rate upon confinement is thus of fundamental importance. One of the first hypotheses
considered was an increase in glass transition temperature upon confinement [39].
Such change in T g would imply an increase in segmental relaxation time in isothermal
conditions and consequently lower self-diffusion coefficient. The latter reduction
corresponds to a neat drop in mass transport towards the crystalline growth front,
which would justify the observed confinement effect on crystallization. For example,
in the case of poly(ethylene oxide), PEO, [41, 42] it was argued that a reduction in
crystal growth factor by 2 orders of magnitude was imputable to an increase of 30 K
in the glass transition temperature.
These hypotheses, however, were not supported by experimental evidence,
because of the difficulties in monitoring crystallization kinetics and probing
molecular mobility within the same experiment.