13.2 Experiment
13.2.1 Polymer Sample Preparation
The key functional group of the PDMS polymers is –SiO(CH 3 ) 2 , which can be
considered as an organically modified quartz; i.e. two oxygen atoms attached to
every silicon atom have been replaced by methyl groups. By this, the
three-dimensional backbone structure of quartz is changed to give linear polymer
molecules. Here, the methyl groups are free to rotate around the –Si–O–Si– chain.
As derivatives, other groups instead of the methyl subunit can be attached to the
silicon-oxygen backbone. The linear silicone polymers can be cross-linked to each
other by different extents, i.e. covalently via groups containing different atoms,
which explains the wide range of their optical, thermal, mechanical and chemical
properties.
For this work, a two-component commercial PDMS polymer called RT 601
(Wacker Chemie, Burghausen, Germany; http://www.wacker.com) was used as
cladding material. In close cooperation with our group, the PDMS core material
was especially developed by Wacker Chemie (provisional product name SLM
77522), with a few methyl groups substituted by phenyl groups for obtaining a
higher refractive index as required for waveguiding. Both materials were prepared
from two components by means of a cross-linking procedure through a hydrosilylation reaction by thermal curing using a platinum catalyst. The addition reaction
occurs mainly on the terminal carbon and is catalysed by organo-metallic Pt colloid
complexes. The mechanism, as illustrated in Fig. 13.1, has been proposed after the
oxidative addition of the SiH groups in the coupling agent by means of the Pt
catalyst with the following hydrogen transfer onto the double bond and a reductive
elimination in the product (for simplification, other Pt ligands and Si substituents
are omitted).
To study the optical properties of amorphous PDMS cladding and core polymers, it was necessary to purify the raw materials before the curing process to avoid
extrinsic losses, e.g. by particle scattering. The base material and coupling agent
were both filtered using a cellulose-mixester membrane filter of pore size of
0.2 µm. After purification, they were mixed in the ratio of 19:1 for the cladding and
9:1 for core materials, respectively, and deaerated through vacuum application.
Subsequently, the mixed polymers were cured in a clean oven for 2 h at 70 °C to
obtain samples for the optical characterization. All experiments were performed
under clean room (class 10) conditions.
13.2.2 Waveguide Fabrication
For waveguide fabrication using the PDMS materials, casting is adopted as the
suitable technology [5, 10, 11]. The first basic prerequisite for the replication
13 Spectroscopic Aspects of Polydimethylsiloxane (PDMS) Used for …
403
13.2.1 Polymer Sample Preparation
The key functional group of the PDMS polymers is –SiO(CH 3 ) 2 , which can be
considered as an organically modified quartz; i.e. two oxygen atoms attached to
every silicon atom have been replaced by methyl groups. By this, the
three-dimensional backbone structure of quartz is changed to give linear polymer
molecules. Here, the methyl groups are free to rotate around the –Si–O–Si– chain.
As derivatives, other groups instead of the methyl subunit can be attached to the
silicon-oxygen backbone. The linear silicone polymers can be cross-linked to each
other by different extents, i.e. covalently via groups containing different atoms,
which explains the wide range of their optical, thermal, mechanical and chemical
properties.
For this work, a two-component commercial PDMS polymer called RT 601
(Wacker Chemie, Burghausen, Germany; http://www.wacker.com) was used as
cladding material. In close cooperation with our group, the PDMS core material
was especially developed by Wacker Chemie (provisional product name SLM
77522), with a few methyl groups substituted by phenyl groups for obtaining a
higher refractive index as required for waveguiding. Both materials were prepared
from two components by means of a cross-linking procedure through a hydrosilylation reaction by thermal curing using a platinum catalyst. The addition reaction
occurs mainly on the terminal carbon and is catalysed by organo-metallic Pt colloid
complexes. The mechanism, as illustrated in Fig. 13.1, has been proposed after the
oxidative addition of the SiH groups in the coupling agent by means of the Pt
catalyst with the following hydrogen transfer onto the double bond and a reductive
elimination in the product (for simplification, other Pt ligands and Si substituents
are omitted).
To study the optical properties of amorphous PDMS cladding and core polymers, it was necessary to purify the raw materials before the curing process to avoid
extrinsic losses, e.g. by particle scattering. The base material and coupling agent
were both filtered using a cellulose-mixester membrane filter of pore size of
0.2 µm. After purification, they were mixed in the ratio of 19:1 for the cladding and
9:1 for core materials, respectively, and deaerated through vacuum application.
Subsequently, the mixed polymers were cured in a clean oven for 2 h at 70 °C to
obtain samples for the optical characterization. All experiments were performed
under clean room (class 10) conditions.
13.2.2 Waveguide Fabrication
For waveguide fabrication using the PDMS materials, casting is adopted as the
suitable technology [5, 10, 11]. The first basic prerequisite for the replication
13 Spectroscopic Aspects of Polydimethylsiloxane (PDMS) Used for …
403