Decomposition of poly(propylene carbonate) with UV sensitive iodonium salts
Todd J. Spencer
*
, Paul A. Kohl
Georgia Institute of Technology, School of Chemical and Biomolecular Engineering, Atlanta, GA 30332-0100, USA
article info
Article history:
Received 2 March 2010
Received in revised form
19 November 2010
Accepted 8 December 2010
Available online 24 December 2010
Keywords:
Poly(propylene carbonate)
Acid degradation
Thermal degradation
Diphenyliodonium
Microelectronics
Sacrificial polymer
abstract
The decomposition characteristics of poly(propylene carbonate) containing a photoacid generator have
been studied. The influence of casting solvent, photoacid concentration and type, UV exposure dose,
substrate surface, and ambient gas were included in this study. Dynamic thermogravimetric analysis was
used to analyze the decomposition characteristics. Kinetic parameters were extracted using the Kissinger
method and the CoatseRedfern method. Fourier Transform Infrared Spectroscopy was used to analyze
effects of casting solvent. The highest thermal stability was found to occur in high molecular weight,
high-purity poly(propylene carbonate) samples. Cyclohexanone and trichloroethylene solvents were
found to increase the thermal stability. Photoacid generators based on diphenyliodonium salts lowered
the onset decomposition temperature and activation energy.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Poly(propylene carbonate) (PPC) is a soft aliphatic thermoplastic
formed by the copolymerization of propylene oxide and carbon
dioxide, as first reported by Inoue et al [1]. The glass transition
temperature of PPC is near room temperature (T
g
z 25
Cto45
C)
[2,3] and depends on molecular weight and backbone structural
arrangement [4]. PPC has a relatively low decomposition temper-
ature, with the five percent mass loss (T
dÀ5%
) widely reported to be
approximately 180
C [5]. This has limited its commercial applica-
tions as a replacement for conventional plastics but makes it
attractive as a sacrificial material. Numerous efforts have been
made to increase the thermal stability and mechanical properties of
PPC [3,5e14]. Thermal decomposition in inert environments
proceeds via unzipping into cyclic propylene carbonate (4-methyl-
1,3-dioxolan-2-one) at lower temperatures (ca. 180
C) followed by
random chain scission at higher temperatures (ca. 250
C) [15]. The
decomposition mechanism depends on molecular weight [3],
temperature, ambient gaseous environment [16,17], and additives
[15]. Techniques to increase the thermal stability have been
reported [5,6,11e13]. Methods to lower the decomposition
temperature for use as a sacrificial material has also been reported
[18e21].
The low decomposition temperature and volatile products make
PPC an ideal sacrificial material for microelectronic applications
which are constrained to temperatures between ca. 100
C and
250
C. This is well within the stability range of epoxy-based
products. Gaseous cavities can be created by forming spatial PPC
patterns followed by overcoating with a dielectric material. The PPC
decomposition products can diffuse through the overcoat leaving
an embedded gas cavity within the overcoat whose shape is
determined by the original shape of the PPC pattern. Embedded
cavities are potentially valuable in the packaging of microchips and
microelectromechanical systems (MEMS).
Air cavities or other gas-filled spaces are valuable in a variety of
semiconductor manufacturing and microchip packaging technolo-
gies including microfluidic channels for microprocessor cooling
[22], lab-on-a-chip applications [23], air-insulated electrical signal
lines [24], and vacuum packaging of MEMS resonators [25]. Air
cavities surrounding a core waveguide in fiber optical cables have
shown low losses [26]. Similar low losses were demonstrated in
porous fiber structures with subwavelength holes [27]. Thermally
decomposed PPC loaded with aluminum nitride particles has been
studied for use in tape automated bonding [17,28].
A variety of polycarbonates including PPC [18e20] and poly-
norbornene formulations [29], have been previously studied for use
as sacrificial materials. Air cavities in a variety of dimensions have
been reported using polynorbornene [24] and SiO
2
overcoats [30].
A small quantity of a photoacid generator (PAG), which creates
a catalytic amount of an acid upon exposure to ultraviolet (UV)
radiation, reduces the decomposition temperature, enabling direct
*
Corresponding author. Tel.: þ1 404 805 3207.
E-mail address: tspencer@gatech.edu (T.J. Spencer).
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2010.12.003
Polymer Degradation and Stability 96 (2011) 686e702