Article pubs.acs.org/Macromolecules
Crystallization of Poly(L‑lactide) Confined in Ultrathin Films: Competition between Finite Size Effects and Irreversible Chain Adsorption D. E. Martínez-Tong,†,* B. Vanroy,‡ M. Wübbenhorst,‡ A. Nogales,† and S. Napolitano§,* †
Instituto de Estructura de la Materia, IEM-CSIC, C/Serrano 121, Madrid 28006, Spain Laboratory of Acoustics and Thermal Physics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, Leuven,3001, Belgium § Laboratory of Polymer and Soft Matter Dynamics, Faculté des Sciences, Université Libre de Bruxelles (ULB), Boulevard du Triomphe, Bâtiment NO, Bruxelles 1050, Belgium ‡
ABSTRACT: Confined at the nanoscale level, polymers crystallize much slower than in bulk, and in some cases the formation of ordered structures results inhibited for extremely long experimental time scales. Here, we report on the thickness dependence of the cold crystallization of thin poly(L-lactide) (PLLA) films ( Tg permitted us to investigate the competition between formation of ordered crystalline structure and irreversible chain adsorption of the polymer onto the solid substrates.
* (ω) = ε∞ + εHN
Δε (1 + (iωτHN)b )c
(1)
Here ω is the angular frequency (ω = 2πf) and τHN is the central relaxation time. Exponents b and c are the shape parameters, which describe the symmetric and asymmetric broadening of the relaxation process, respectively. It holds that 0 < b, bc ≤ 1. The central relaxation time τHN is related to the time at maximum loss τMAX of the relaxation time distribution by:27 −1/ b 1/ b ⎡ ⎡ bπ ⎤ bcπ ⎤ τmax = τHN⎢sin sin ⎥ ⎢ ⎥ ⎣ 2 + 2c ⎦ ⎣ 2 + 2c ⎦
(2)
The mean relaxation time τMAX can be obtained experimentally also through the frequency of the maximum of in ε″, f max, by τMAX = 1/ (2πf max). For symmetric relaxation processes (c = 1) τHN = τMAX.
3. RESULTS AND DISCUSSION Results of the DRS isothermal experiments are presented and discussed in three sections. Section A presents a description of the effect of nanoscopic confinement on the α-relaxation of amorphous thin films of PLLA. In section B, we discuss the impact of crystallization on the structural peak as a function of the film thicknesses. Finally, in section C, we analyze the interplay between crystallization kinetics and irreversible chain adsorption. A. Shape of the Relaxation for Amorphous Thin Films. Figure 1 shows the dielectric loss data as a function of
2. EXPERIMENTAL PART Samples and Techniques. Polymer nanocapacitors for dielectric measurements were prepared in the form of thin dielectric films capped between conductive layers by a previously described procedure.11,26 A first aluminum (Al) layer (thickness 50 nm) was evaporated over a glass substrate (aluminum 99%, pressure x(nm) >1 Δε decreases approaching the solid wall down to a final plateau where the dielectric strength is reduced to 17% of its bulk value. Therefore, the interfacial region for x < 10 nm can be considered as an indication for the length scale where the adsorption of the polymer into one interface occurs. Finally the fact that Δε/Δεbulk extrapolates to a finite value at x = 0 is the manifestation of a residual dielectric activity in the interfacial volume in high interaction with the metals.1 Following this interpretation the thickness of this socalled reduced mobility layer (RML) can be estimated from the x value after the plateau where Δε/ΔεBULK starts increasing. In the present case, the thickness of the RML is estimated to be around 1 nm, and is in good agreement with the literature.1,11 From the obtained value of penetration depth φ, we estimate that the influence of the interfacial interaction should be relevant up to thicknesses on the order of 2φ = (14 ± 4) nm. Previously, studies of crystallization of PET confined into thin films showed that this value can be related to the thickness where δ in eq 3 starts deviating from zero,11 indicating the upper limit for interfacial interactions. However, this is not the case for PLLA since we observed a nonzero value of δ even at thicknesses as high as 150 nm. This qualitative difference between the two polymers as well as the low value obtained for the RML might be related to the degree of flexibility, which is much higher for PLLA than for PET. This trend can be rationalized considering that chain rigidity increases the correlation between segments and thus the length scale over which confinement effects take place. Moreover, the value of 2φ = (14 ± 4) nm can be seamlessly related to the point in which the HN bc parameters starts deviating. This confirms the idea proposed in section A, where changes in bc were related to impact of interfacial interactions.
JAE-Pre fellowship and Fondo Social Europeo (FSE) for cofinancing the JAE Program. B.V. and M.W. acknowledge financial support from the Research Council of the KU Leuven, Project No. OT/11/065, and financial support from FWO (Fonds Wetenschappelijk Onderzoeks−Vlaanderen). S.N. acknowledges financial support from the funds FER of the Université Libre de Bruxelles.
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4. CONCLUSIONS Thickness dependence of the crystallization behavior of thin poly(L-lactide) films was studied by means of dielectric relaxation spectroscopy. Relaxation spectra were analyzed under the Havriliak−Negami formalism and the parameters of the relaxation function were followed during annealing in isothermal experiments. An analytical method assessing the impact of irreversible chain adsorption was also followed. This permitted to disentangle finite size and interfacial effects in the thin capped films. We have found that crystallization was inhibited in films thinner than 10 nm. Moreover, we analyzed the thickness dependence of the dielectric strength and obtained the gradient in segmental mobility inside our capped films. We conclude that irreversible adsorption of chains onto the Al electrodes ultimately leads to a reduction in molecular mobility and to slower crystallization kinetics compared to the bulk.
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AUTHOR INFORMATION
Corresponding Authors
*(D.E.M.-T.) E-mail:
[email protected]. *(S.N.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation (MICINN) through MAT 2011-23455. D.E.M.-T. thanks CSIC for the tenure of the 2359
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