Intrinsic friction analysis—Novel nanoscopic access to molecular mobility in constrained organic systems
Introduction
Reflecting on effective material design concepts, a bottom-up molecular approach comes to mind, where, with the appropriate synthesis and under suitable processing conditions, materials are designed with desired properties and functionalities. The challenge is to anticipate bulk- or mesophase material properties from the molecular structure. This is generally approached in a reversed fashion by first analyzing the product, i.e. the condensed phase, and then pondering the possible molecular origin for specific properties. Depending on the specificity of the information gained experimentally, either a molecular understanding is directly obtained, a molecular model is developed, or a trial-and-error procedure is put in place. Once a molecular understanding (or a molecular model) for a specific class of materials is obtained, computer-assisted molecular engineering can take place, and a wide variety of molecular structures and functionalities directed towards the material design objective can be tested. Unfortunately, most experiments lack specificity, and thus provide only phenomenological models instead of a molecular understanding. This results in molecular designs being reduced to a mere trial-and-error procedure.
An example of a multifunctional organic system that would profit significantly from direct insight into molecular scale mobilities are second-order non-linear optical (NLO) materials. The properties that are of foremost importance for organic NLO materials are the electro-optical (EO) activity that demands acentric ordering of NLO chromophores, and the temporal phase stability. One main approach to generating acentric order within NLO systems is to utilize electric field poling, in which the randomly dispersed chromophores are aligned by applying an external field. For electric field poling, the glass transition temperature (Tg) is a key parameter with regard to both the poling efficiency, requiring high chromophore mobility often only available near Tg, and the temporal stability of the acentric order, demanding minimal chromophore mobility. These two contradicting factors make the molecular design of NLO systems challenging, requiring many levels of subtle adjustments for device optimization.
The glass transition temperature has been a primary parameter in discussing molecular mobility due to the characteristic relaxation in which the change in modulus exceeds three orders of magnitude. Secondary relaxations in the glassy state (i.e., below Tg) are characterized by local segmental mobilities or interactions, and thus do not have a strong effect on mechanical properties like the modulus. Control over secondary relaxations can provide the possibility of modifying functional properties, such as the electrical and opto-electrical properties of organic materials. In this regard, the segmental mobility of a dendronized NLO side-chain polymer was exploited to achieve a high electro-optical (EO) activity [1]. In the aforementioned study, an enhancement of poling-induced acentric ordering was achieved above the critical side-chain transition temperature, but significantly below the detrimental primary backbone relaxation (glass transition) temperature, [1] an aspect discussed in greater detail below in Section 4.1.
The challenge in integrating secondary relaxations, or segmental mobilities, in molecular designs of organic materials is situated in a submolecular scale understanding of the relaxation phenomena of the condensed phase. Segmental mobilities are governed by a variety of interrelated factors in complex organic systems. For example, in self-assembling molecular glasses, electrostatic interactions provide stability, while in polymers the size of the side-chain, the flexibility of the backbone and the length the linkage in between collectively contribute to the side-chain mobility. Intricate molecular structures are generally in self-constrained metastable states, such that molecular dynamic descriptions based on equilibrium thermodynamics hardly portray the molecular picture. Most instrumental tools, such as differential scanning calorimetry, are to a great degree insensitive to secondary relaxations in thin films, and due to the amorphous nature of organic NLO systems, structure analysis methods are inadequate [2], [3].
Progress has been made with the application of novel SFM techniques related to mechanical properties [4]. In a recent study [2] using SFM-based techniques, in particular intrinsic friction analysis (IFA), the dynamics of self-assembling glassy chromophores confined in thin films were explored. Molecular electrostatic π-interactions were found to be crucial towards a fundamental understanding of the phase behavior during electric field poling, and for long-term material phase stability. In another study involving more complex material composed of side-chain dendronized NLO polymers, low-temperature transitions in the glassy state, identified by another SFM-based technique, namely, shear modulation force microscopy (SM-FM), turned out to be vital for the local dipole mobility [1]. Although the SM-FM study was significant for the understanding of the enhancement of the EO activity, the exact relaxation mechanisms have been left unanswered, and are here illuminated by IFA.
In this paper, our focus is two-fold. First, we will critically assess the use of IFA after a brief review of the method, and, second, we discuss its application towards the local mobility analysis of the two previously mentioned NLO material systems, i.e., self-assembling glassy chromophores and side-chain dendronized NLO polymers. Thereby, ADFD, a molecular glassy chromophore which self-assembles due to anthryl-pentafluorophenyl interactions, will serve as the material for the assessment of the IFA methodology. This study will also be contrasted to a prior IFA study on a similar but segmentally different self-assembling molecular glassy system [2]. To elucidate the effect of local submolecular mobility and its effect on electric field poling in side-chain dendronized NLO polymers, we will investigate by SM-FM and IFA a series of polymer systems with small but very effective modifications of side-chain linker groups.
Section snippets
Intrinsic friction analysis (IFA)
IFA is based on the well-established lateral force microscopy [5], [6], which was shown to elucidate molecular modes of dissipation in poly(methyl-methacrylate) [7] and polystyrene [8]. Thereby, IFA provides molecular descriptions of relaxation processes that are associated with structural transitions. A nano-scaled probe provides the means to conduct perturbation experiments by transferring kinetic energy very specifically to internal modes on a molecular or even submolecular level, as shown
Materials
ADFD, a self-assembling molecular glassy chromophore (Fig. 7), was synthesized in a manner analogous to that described previously [18] except that anthryl groups rather than phenyl groups were used in the non-fluorinated dendrimers. The glass transition temperature of the material was measured to be 125 °C using SM-FM, a well-established SFM technique for measuring structural transitions [19], [20].
Three dendronized NLO side-chain polymers, PS-CLD1, PM-FTC1, and PM-FTC2 (see Fig. 9 below), were
IFA and relaxations in molecular glasses
In this subsection, we present a critical analysis of the IFA methodology, illustrated on the self-assembling molecular glassy chromophore system, ADFD. In IFA, temperatures are randomized to eliminate time bias in the experiment and it is therefore possible to gage the overall consistency of a run by monitoring various parameters at each temperature. For example, out-of-contact vertical (FT−B) and lateral (FL−R) photodiode signals were monitored to determine if substantial changes in the laser
Summary and outlook
We have presented a critical assessment of IFA as a technique for elucidating molecular interactions and mobilities in various systems. Using ADFD as an illustration, we provided step-by-step insight into a successful energy mobility analysis involving IFA, and showed that ADFD exhibited non-cooperative Arrhenius-type behavior below Tg due to intermolecular interactions rather than intramolecular rotations. The change from phenyl (HDFD) to anthryl groups (ADFD) is subtle, but resulted in a
Acknowledgements
We would like to thank Alex K.-Y. Jen and his group, in particular Tae-Dong Kim, Jingdong Luo and Zhengwei Shi for providing the sample materials. The authors also acknowledge a UIF graduate fellowship through the Center for Nanotechnology at the University of Washington and funding from the National Science Foundation (CBET 0729849).
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