EFFECT OF FLUORINATED
HYPERBRANCHED POLYMER ON
FOAMING OF PMMA
Masataka Sugimoto*, Tomohiro Asakawa,
Sathish K. Sukumaran*, Kiyohito Koyama,
Yamagata University, Japan
Masaaki Ozawa and Motonobu Matsuyama,
Nissan Chemical Industries, LTD., Japan

Abstract
The effect of fluorinated hyperbranched polymer on
the foaming of PMMA using supercritical carbon
dioxide (scCO2) was investigated. In order to enhancecell nucleation we used the fluorinated hyperbranchedpolymer. The foaming behavior was determined in
dependence upon an amount of the fluorinated polymer.
The introduction of the fluorinated polymer remarkably
reduced the size of foaming cells of PMMA.
Furthermore, the cell number density was significantly
changed by a presence of the fluorinated polymer. Thisfinding can be explained by the enhanced nucleation
generated from the well-dispersed fluorinatedhyperbranched polymer as nucleating agent.

sense, the utilization of fillers is beneficial. Fillers
reduce activation barrier of free energy for nucleationand enhance inhomogeneous nucleations at interface
between fillers and matrix polymer. The dissolved gas isdistributed into the fillers, leading to the small cells andhigh cell number density with narrow size distribution.
The cell size and density depends on the filler size, thesize distribution and surface chemistry. Chiu et al. [3]reported that addition of calcium carbonate led to thereduced cell size and increased foam number density ofpolystyrene. Chang et al. [4] reported the effect ofnanoclays on the cellular structure of polyethyleneoctane copolymer. Their results indicated the loweredaverage cell size and increased cell density. In adifferent study, Goren et al. [5] studied the effect ofsurface chemistry on the cellular structure of PMMA.
They showed that surface treatment by fluorination led
to decrease of the cell size due to the reduced freeenergy barrier for nucleation.

Introduction

There are only few studies reporting that use offluorinated polymer for polymeric foams. Yokoyama etal. [6] reported the foaming of polystyrene-blockpoly(perfluorooctylethyl methacrylate) diblock
copolymer. Their results suggest that nanocellularstructure was generated in the phase separated spherical
domains which had high solubility in CO2. In this casethe temperature of foaming is crucial. CO2 soluble
domains form cellular structure, while the anotherglassy block phase suppresses the cell growth. The
temperature should be between glass temperatures of
two block phases in order to prevent over-expanding.

Polymeric foams are used in many applications
because of thermal conductivity, light weight, lowdielectric constant, sound damping and opticalproperties [1, 2]. Supercritical carbon dioxide (scCO2)
is used as environmentally benign physical blowing
agent. CO2 has many advantage such as non-toxicity,non-flammability, low cost and chemical inertness.
scCO2 has high solubility, low interfacial tension andexcellent surface wettability. It has been reported thatthe use of scCO2 as a physical blowing agent enables
creation of well-distributed micron size closed cellular
structure that is beneficial in mechanical strengthcompared with heterogeneous or open cell structure.

In the current study, we focused on the use of
hyperbranched polymer in which the end groups arefluorinated. The effect of the fluorinated hyperbranchpolymer on the foaming behavior of poly(methylmethacrylate) (PMMA) was studied by using scCO2 interms of the fluorinated polymer concentration andfoaming temperature. It is expected that the fluorinatedpolymer has high dispersion due to the compact
dendritic architecture and the effect as nucleation agent
due to end group fluorination. The cellular structuresuch as size, size distribution and number of cell density
was remarkably changed due to a presence of thehyperbranch polymer.

The final properties of the polymeric foams dependon the cellular diameter, diameter distribution, porosityand cell number density. Inorganic fillers are used tocontrol the cell size and the distribution. According to
the concept of cellular nucleation, during nucleation
process gas molecules overcome an energy barrier toform stable nuclei. In order to generate fine pores withsmall diameter distribution, nucleation at the same timeis required following rapid depressurization. In thisMaterials and Methods
A commercial PMMA (“PARAPET G”, KurarayCo., Ltd.) was used. The melt flow rate at 230oC is 8.0
g/10min and the density is 1.13 g/cm3 at roomtemperature. We used fluorinated hyperbranchedpolymer HBP-1 which has compact spherical molecularsize and high compatibility to PMMA as an additive toPMMA. The weight-average molecular weight isFoams 2013, Seattle, USA September 11- September 12, 2013

16,000 g/mol (conventional GPC column calibration byusing polystyrene standard) and the glass transition
temperature is 81oC. PMMA and HBP-1 was blended asweight ratio of 0.01, 0.03, 0.1, 0.3, 1 and 3 wt% byinternal mixer (Labo Plastmill, Toyo Seiki Seisaku-sho,Ltd.) at 190oC for 5 min. The blended samples werepress molded into sheets of thickness of 1~2 mm at
190oC.
The film samples prepared in the described above
were inserted into autoclave (Taiatsu Technology Co.).
The whole setup was heated to the final temperature of
40~100oC by a heating jacket. After reaching theconstant temperature CO2 was loaded up to the foamingpressure of 15 MPa until saturation of CO2 in thesample films. The foaming was carried out by a rapid
release of the pressure of CO2 by the supersaturation of
the polymer films with CO2. The saturation time was 8hr. We confirmed that the time was sufficient to absorb
CO2 until saturation state by the gravimetric method.
The depressurization from 15 MPa to ambient wascompleted in about 2 s.
The density of the foamed film was determinedfrom the sample with diameter of 20 mm by the meansof buoyancy method. The porosity V was calculated
from the unfoamed and foamed density, ρ0 and ρ f ,respectively:
V = 1−

ρf
ρ0(1)The morphology of cell structure was visualized byfield emission scanning electron microscopy (FE-SEM,JEM-7600F, JEOL Ltd.). The average cell diameter D
and the distribution were determined by using the image
analysis software (Mac-view, Mountec Co., Ltd.) from1000~3000 cells. The cell number density Nf wasobtained by the density of unformed and foamed
samples and average cell diameter:6( ρ f ρ0 − 1)
Nf =(2)
πd3

Results and Discussion
Effect of Additive Amount on the Foaming
Figure 1 shows SEM image of cross section ofPMMA/HBP-1 films foamed at 50oC. Here the neat
PMMA and PMMA contained HBP-1 of 0.3 wt% arejust shown. Surprisingly, an introduction of a small
amount of fluorinated dendritic polymer caused
remarkable reduction of cell diameter as shown in
Figure 1. Furthermore, an apparently increased numberof cell can be observed in comparison with neat
PMMA.
The quantitative analysis of the foaming
morphology is plotted in Figure 2 (a and b) for thecellular structure of PMMA incorporated with HBP-1 of0, 0.01, 0.03, 0.1, 0.3, 1.0 and 3.0 wt%. It is clear that avery small amount of HBP-1 even at 0.01 wt% has a(a)(a)

1µm(b)(b)

Figure 1. Scanning electron micrographs of the crosssections of (a)PMMA and (b) PMMA blended withfluorinated hyperbranched polymer of 0.3 wt%
foamed at 50oC.

Figure 2. Effect of fluorinated hyperbranch polymer
content on (a) average cell diameter and (b) celldensity foamed for PMMA/HBP-1 foamed at 50oC.

Foams 2013, Seattle, USA September 11- September 12, 2013

beneficial impact on the foamed cell structure. The
decrease of cell diameter was found for whole range ofcontent of the fluorinated hyperbranch polymer(HBP-1), starting with 2.4 µm for neat PMMA to 0.34µm at 0.3 wt%. With increasing the content of HBP-1,
number of the large cell with a diameter of about 1 µm
slightly decreased and distribution of the cell size
became narrow, while the average diameter was not thatdifferent within 0.3 wt%. Above 0.3 wt%, the diameter
showed a tendency to increase. On the other hand, thenumber density of cell Nf showed jump from 3.8 × 1010
to 4.6 × 1012 by an addition of the HBP-1 of 0.01 wt%
as shown in Figure 2b. Nf reached a peak value3.6 × 1013 at 0.3 wt% of HBP-1. Above 0.3 wt%, Nf
decreased as increasing the content. Although we will
discuss the increased diameter and the decreased cellnumber density above a critical concentration asincreasing HBP-1, this is open question at present.
Figure 3 shows cell size distribution of PMMA and
PMMA incorporated with HBP-1 of 0.01, 0.03 and 0.3wt%. Note that x-axis of this column plot is not equally
spaced intervals due to a limitation of the analysis
software. For example, the values of x-axis at 0.1 and 1µm represents the frequency of cells with 0.1≤d<0.2
and 1.0≤d<2.0 µm, respectively. As shown in the figureneat PMMA shows broad cell diameter distribution.
When adding HBP-1 of 0.01 wt%, the diameter
distribution suddenly shifted towards small size. Thismeans that HBP-1 well dispersed into PMMA andplayed an important role as nucleating agent, in spite ofvery small amount of additive amount. Here we shouldnote that the distribution had a bimodal shape with longtail, while main peak was present in the low diameter.
With increasing HBP-1, the tail disappeared and thedistribution converged into single distribution, resultingin Gaussian-type distribution. There was little change in

Figure 3. Effect of fluorinated hyperbranch polymer
content on (a) cell diameter and (b) cell density
foamed for PMMA/HBP-1 foamed at 50oC.

(a)

1µm(b)

Figure 4. Scanning electron micrographs of the crosssections of (a)PMMA and (b) PMMA blended withfluorinated hyperbranched polymer of 0.3 wt%
foamed at 80oC.
the peak of the frequency in spite of the content. We
consider that the dispersed fluorinated hyperbranchedpolymer affected the number of nucleation sites, but
0.01 wt% was insufficient to cover whole the volume
and to generate heterogeneous nucleation throughout
the sample.

Effect of Temperature on the Foaming
One of the dominant parameters to control thecellular structure and void fraction is the foaming
temperature. It strongly affects not only the viscoelastic
behavior of the polymer but also the solubility anddiffusion coefficient of the gas. The solubility of the gas
generally decreases with the elevated temperature due
to negative value of solution enthalpy of the gas intopolymer. Therefore, the reduced gas dissolved inpolymer gives rise to a small amount of nucleation sites.
Furthermore, the increased temperature enhances thediffusion coefficient of the gas, resulting in small
number of nuclei. The higher temperature brings lesselastic behavior and low viscosity of polymer. This
leads to the large deformation of the softened cell
membrane under the low stress. Figure 4 shows theSEM micrographs of PMMA and PMMA containing
HBP-1 of 0.3 wt% foamed at 80oC. Comparing with
Figure 1, the cell diameter of PMMA became large andthe cell number decreased apparently. For PMMA/HBP-1 blend, the cell structure was larger and numberof cell was smaller than those of the foams obtained at50oC. However, it is clear that the fluorinatedhyperbranch polymer still influenced the cellularstructure at the temperature, though the difference

Foams 2013, Seattle, USA September 11- September 12, 2013

between neat PMMA and PMMA/HBP-1 seems to besmall comparing with that at 50oC.

Effect of Viscoelastic Behavior on Foaming ofPMMA/HBP-1
As shown in Figures 1-4, the fluorinatedhyperbranch polymer has remarkable effect on thefoaming behavior of PMMA. Here we will discuss theeffect of the additive on the viscoelastic behavior ofPMMA. The foaming was carried out at 40~100oC. Thisis below or close to the glass transition temperature (Tg)of PMMA. However, the adsorption of gas provide theplasticization of polymer. It enhances the mobility ofpolymer chain: depression of Tg and the melting
temperature. Therefore, we examined the rheologicalmeasurements of PMMA melts above Tg. The dynamicviscoelastic measurements were carried out at
150~230oC. The time-temperature superposition wasvalid for all the samples including PMMA/HBP-1
blends. Figure 5 indicates mastercurves of storage
modulus (G’) and loss modulus (G”) of PMMA and
PMMA blended with fluorinated hyperbranchedpolymer at a reference temperature of 190oC. Neat
PMMA showed typical viscoelastic behavior of flexible
polymer with moderate molecular weight distribution.
At high frequencies the clear plateau modulus which
can be relevant with the entanglement of polymer
chains was invisible. The terminal flow behavior wasseen at low frequencies: G ' ∝ ω 2 , G " ∝ ω 1 . All thecurves of PMMA/HBP-1 were superimposed on that of
the neat PMMA irrespective of the content. In Figure 2,
high concentration of HBP-1 above 0.3 wt% gave riseto increase of cell diameter. The molecular weight of
HBP-1 is much lower than that of PMMA used here.

Figure 6. Effect of concentration of HBP-1 on thesolubility of PMMA/HBP-1 blends at 15 MPa of CO2and 50oC.
However, it is not the case. The variation of therheological behavior over the wide range of frequencies
with addition of HBP-1 was not confirmed, so that we
can say that the foaming behavior cannot correlate withthe rheological change.

Effect of Solubility of Gas on the Foaming
As discussed already, the solubility of gas has
strong effect on the polymer foaming. It has beenreported that fluorinated block copolymer provided
nanocells generated in the segregated fluorinateddomains due to the high affinity to CO2 [6]. In thisstudy, the solubility of CO2 was examined by variationof weight with the amount of HBP-1 after saturation ofgas at 15 MPa and 50oC. Figure 6 shows weight ratio ofdissolved CO2 to PMMA/HBP-1 blends. The solubilityof PMMA was 0.24, which agreed with literature [7].
There was little change below 1 wt% of HBP-1. Evenfor excess addition of HBP-1 a slight increase ofsolubility was observed at 10 wt%, followed bysaturation above it. PMMA has high affinity with CO2
due to the interaction of carbonyl group with CO2. We
consider that the reason why the difference of solubility
was observed attributes to the original properties of highsolubility of CO2 of PMMA.

Effect of Fluorination of Hyperbranched
Polymer

Figure 5. Mastercurves of storage modulus (G’) andloss modulus (G”) of PMMA and PMMA blended
with fluorinated hyperbranched polymer at reference
temperature of 190oC.

The apparent increase of CO2 solubility of PMMA/
HBP-1 blend was not observed even at highconcentration of HBP-1 as shown in Figure 6. In order
to discuss the effect of the fluoride group in thehyperbranched polymer, we prepared the nonfluorinated hyperbranched polymer (HBP-2) which had
similar chemical structure and molecular architecture as
HBP-1 except no fluoride groups. Figure 7 shows SEM
images of PMMA blended with HBP-2 of 0.1 and 0.3

Foams 2013, Seattle, USA September 11- September 12, 2013

(a)

(b)

1µmFigure 7. Scanning electron micrographs of the crosssections of (a)PMMA/HBP-2 (0.1 wt%) and (b)PMMA/HBP-2 (0.3 wt%) at 50oC. As for neat
PMMA, see Figure 1(a).

wt%. As for neat PMMA, see Figure 1 (a). By
comparing Figure 1 and 7, the difference betweenHBP-1 and HBP-2 is distinctly obvious. For PMMA/HBP-2 system, the diameter of cell became large andnumber of cell density was decreased, though theimpact was not so remarkable comparing with theinfluence of HBP-1. From the results of foaming
PMMA with non-fluorinated polymer, we can say thatthe fluorinated group existed on near the surface ofdendritic polymer plays important role as a nucleationsites. Fluoride compounds generally have high affinity
to CO2. We observed cryogenic fractured surface ofPMMA/HBP-1 blends. In case of incompatible blends,
domains can be observed due to segregated weakboundary. However, for PMMA/HBP-1 voids due to thedomains was not observed in the fractured surface. Thismeans the high compatibility between them and gooddispersion of fluorinated hyperbranched polymer. This
provides the high efficiency to generate numerous
nuclei. We can say that the dispersed hyperbranchedpolymer reduced the free energy barrier to form nuclei
and enhanced the nucleation rate. Although there waslittle change in solubility of PMMA/HBP-1 systems,
the fluorinated group which has high affinity to CO2
plays a key role in PMMA foaming.

clear effect on the cellular structure of PMMA even at avery low amount of 0.01 wt%. At the low loading, a tailin the size distribution was observed. Increasing content
lowered large cell fraction leading to the small averagecell size (0.34 µm at 0.3 wt%) and 3 decade increase ofcell number density. At 0.3 wt% the cell size and celldensity showed minimum and maximum, respectively.
There was little change in the dynamic viscoelasticity ofPMMA/HBP-1. Variation of foaming behavior could
not be related to the rheology of PMMA/HBP-1.
Furthermore, change of the solubility of PMMA/HBP-1
was not confirmed below 1 wt% of HBP-1. The nonfluorinated HBP-2 consisting of similar structure toHBP-1 contributed not at all to the small cell and highcell density. Fluorination of hyperbranched polymer
decreases the free energy and reduces the critical
nucleus radius. HBP-1 showed high compatibility withPMMA up to several weight percentages. These factors
resulted in the small cell size and increased cell density.

References
1. A.H. Landrock, Handbook of Plastic Foams, NewJersey (1995).
2. S. Siripurapu, J.M. DeSimone, S.A. Khan, and R.J.
Spontak, Macromolecules, 38, 2271 (2005).
3. F.C. Chiu, S.M. Lai, C.M. Wong, and C.H. Chang,
J. Appl. Polym. Sci., 102, 2276 (2006).
4. Y.W. Chang, D. Lee, and S.Y. Bae, Polym. Int., 55,184 (2006).
5. K. Goren, L.M. Chen, L.S. Schadler, and R. Ozisik,
J. Supercrit. Fluids, 51, 420 (2010).
6. H. Yokoyama, L. Li, T. Nemoto, and K. Sugiyama,Adv. Mater., 16, 1542 (2004).
7. M.L. O'Neill, Q. Cao, R. Fang, K.P. Johnston, S.P.
Wilkinson, C.D. Smith, J.L. Kerschner, and S.H.
Jureller, Ind. Eng. Chem. Res, 37, 3067 (1998).
K e y Wo r d s : F o a m , P M M A , F l u o r i n a t i o n ,Hyperbranched Polymer, Nucleation agent.

Conclusions
We studied foaming of PMMA incorporated withvarious concentration of the hyperbranched polymer inwhich end groups were fluorinated were foamed byusing scCO2. Fluorinated HBP-1 exerted remarkablyFoams 2013, Seattle, USA September 11- September 12, 2013