Polymers for electronics, sensors, and photonics

Nonpolar polymers for electronic packaging and
miniaturized electret devices
Smart piezoelectric foams and hybrids: Piezo-, pyro- and ferroelectrets
Ferroelectric polymers and polymer-ceramic nanocomposites 
Nonlinear optical polymers for photonics 



Electret application areas 
Polymers are no longer simply inexpensive disposable materials, they have emerged as a new class of materials for various high technology applications. We investigate polymer electrets, e.g. dielectrics with quasipermanently stored charge (either on the surface or in the bulk), or with oriented dipoles (frozen-in or ferroelectric). These are nonpolar polymers for electronic packaging and charge electret devices (microphones etc.), internally charged cellular polymers displaying strong piezoelectric effects, polar photonics polymers with optically nonlinear chromophores and ferroelectric polymers for piezo- and pyroelectric sensors and actuators. 

Nonpolar polymers for electronic packaging and miniaturized electret devices 

Nonpolar polymers 
Polymers for electronic packaging should possess low dielectric constant and dielectric losses for high switching speeds with low levels of electrical crosstalk. In order to be compatible with high temperature process steps in the semiconductor industry, electronic polymers should also have high glass transition temperature, and good thermal and mechanical properties to prevent delamination between the polymer and adjacent materials. 

Among all nonpolar polymers, fluorinated systems show the lowest dielectric constants and losses. We currently investigate several fluoropolymers, plasma polymerized fluorocarbons, polytetrafluoroethylene Teflon PTFE, pulsed laser deposited PTFE-like polymers, amorphous Teflon AF, and perfluorinated benzocyclobutane PFCB.

Teflon PTFE - structural transitions 
A precise measurement of the real part of the dielectric function of nonpolar polymers allows for the investigation of structural phase- and glass transitions via dielectric dilatometry. Dielectric dilatometry is very sensitive, for illustration thickness changes on the order of 1A in 1µm thick nonpolar polymer films can be easily resolved.

Teflon PTFE - thermal expansion 
Teflon AF - glass transition dynamics 
Besides the investigation of the dielectric properties, we also determine the coefficient of thermal expansion (CTE) over wide ranges of temperatures, essential for the estimation of residual stress in electronic packaging process steps. Furthermore, the CTE is important not only for practical applications, but also for fundamental studies of the glass transition dynamics in thin polymer films on substrates.
Charge stability of PLD-PTFE 
Charge stability of low-k polymers 

Polymers for charge electret applications should have exceptional stability of both positive and negative charges trapped on the surface and/or in the bulk and should be processible in thin film form on underlying semiconductor substrates. We currently investigate the charge stability of several new electret materials. Excellent charge stability has been for example identified in pulsed laser deposited Teflon PTFE . 

The charge stability of low dielectric constant polymers is excellent in comparison to that of "classical" electrets like PTFE. Applications are anticipated in miniaturized electret devices.

Smart piezoelectric foams and hybrids:
Piezo-, pyro- and ferroelectrets


Low-densitiy piezoelectrics are interesting for a variety of applications, such as 
 - audio systems (flat-panel "paper" loadspeakers driven by conventional amplifiers)
 - large-area medical diagnostic systems (pedobarography, patient mattress monitors)
 - security systems
 - pedestrian and car sensors
 - airborne ultrasound transmitters and receivers for non-destructive testing and object recognition in robotics.

Most promising are cellular polymers, with closed lens-like cells. Such polymers are easily produced by biaxial stretching and inflation of filler loaded polymers. The materials are internally charged by dielectric barrier microdischarges and display huge longitudinal piezoelectric effects. Surprisingly they display also features typical of ferroelectric materials, like hysteresis and switching. Hence these materials are termed piezo-, pyro- and ferroelectrets.

Charged, heterogeneous polymer electrets 
Piezoelectricity relates electrical and mechanical properties and is described by a third-rank tensor. The d-tensor is defined by 

D: dielectric displacement; E: electric field; T: mechanical stress; S: mechanical strain.

Piezoelectricity is found only in noncentrosymmetric materials. Charged foams or hybrid electrets offer nonconventional routes for symmetry breaking 

- in foams electrically, by nonsymmetric charge distributions
- in hybrids geometrically by combination of hard and soft layers


Ferroelectric polymers and polymer-ceramic nanocomposites 

Piezo- and pyroelectric polymers 
Piezo- and pyroelectricity is the electrical response of a material to a change in pressure and temperature. The piezo-and pyroelectric effect is the base for numerous applications in sensors, for example hydrophones, infrared detectors etc. The most efficient piezo- and pyroelectric polymers are ferroelectric, such as polyvinylidenefluoride (PVDF)
P(VDF-TrFE)-piezo- and pyroelectric signal 
An efficient means of determining the piezo-and pyroelectric response of ferroelectric polymers is by thermal excitation of the sample with a short light pulse while measuring the electrical signal response (electrothermal technique) . Piezoelectric signals arise from thermally induced bending and thickness vibration modes, pyroelectric signals from the induced increase in the sample temperature. Advantageous is the possibility of determining electrical, thermal and acoustical properties of polymer films in one experiment.
P(VDF-TrFE)- polarization distribution 
Ferroelectric polymers must be electrically poled in order to show the piezo- and pyroelectric effect. During poling the electric field is by no means uniform across the film thickness and so is the ferroelectric polarization. Even films poled under optimal conditions typically show near-surface depolarized layers. The electrothermal technique enables the measurement of nonuniform polarization distributions with high spatial resolution near the thermally excited electrode. 

Nonlinear optical polymers for photonics

NLO polymers 
Amorphous photonics polymers with incorporated functional chromophores. possess large nonresonant, purely electronical nonlinear-optical susceptibilities, with potential for applications in photonics devices (electro-optical modulators, switches, frequency converters, etc.). For second-order nonlinearities, the chromophores must be oriented noncentrosymmetrically in a poling process. We currently investigate side-chain and crosslinking nonlinear optical polymers. 

Dielectric function of a NLO polymer 
The dielectric function of photonics polymers shows several distinct relaxation processes, associated with the glass transition (a) and sub-glass (b, g) transitions. In high-Tg polymers chromophore degradation is a severe problem in the determination of dielectric properties via broadband dielectric spectroscopy. Temperature-dependent dielectric relaxation spectroscopy (TDRS) enables a quick characterization of the relaxation processes (relaxation strength, temperature-dependent mean relaxation time and distribution).

Relaxation of the electro-optic response of an NLO polymer 
The long term stability of the dipole orientation in photonics polymers requires high glass transition temperatures, coupled with thermally stable chromophores. The initial, fast decay of the nonlinear optical response after poling is related not only to the primary a relaxation (glass-transition), but also to the strength of secondary sub-glass relaxation processes. Photonics polymers should therefore possess a small sub-glass relaxation strength for good temporal stability of the nonlinear-optical effects.

Simona Bauer-Gogonea
last modified: 30.08.2007