Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide

Effects of Crystallization on Cell Morphology in Microcellular
Polyphenylene Sulfide
by Masayasu Itoh * and Akira Kabumoto *2
Microcellular plastic (MCP) is a polymeric foam with an average cell size of
10 µm or less. The basic process for MCP was proposed in 1981 by Professor
Suh at the Department of Mechanical Engineering of MIT in the U.S. 1), and theprocess has been applied to various plastic materials. In this work, the effects of crystallization of
microcellular polyphenylene sulfide (PPS) on its morphology are investigated. When PPS was
foamed below its crystallization temperature, a unimodal distribution was observed with a cellsize of about 10 µm. When foamed above its crystallization temperature, on the other hand, abimodal distribution was observed with two peaks at about 10 µm and the submicron order incell size. It was shown that these submicron-sized cells were generated by cell nucleationinduced by crystallization of PPS.

ABSTRACT1.

INTRODUCTION

Microcellular plastic (hereinafter called MCP) is a polymeric foam with an average cell size of the order of 10 µm
or less and a cell density of the 109~1015 cells/cm3 order.
The foam features improved mechanical properties 2)~4),
reduced material consumption, decreased dielectric constant 5) and increased thermal insulation capability, in
addition to enhanced light reflecting capability that havebeen investigated by the authors 6). The MCP process was
originally proposed in 1981 by Professor Suh at the
Department of Mechanical Engineering of MIT in the
U.S. 1)
There are many R&D activities aimed at industrial applications. In this connection, Trexel Inc. in the U.S. hasdeveloped the extrusion molding and injection molding
processes for MCPs using CO2 in its supercritical state,
and they are promoting licensing of these technologies.
With regard to microcellular foaming behavior using the
MCP processes, a broad range of study paper have beenpresented about the process factors that influence microcellular foaming 6)~10). In cases of crystalline polymers inparticular, interesting results have been reported about
the correlation between their crystallization and microcellular foaming 6)~10).
In this work, we focused our attention on polyphenylene
sulfide (hereinafter called PPS), one of the crystalline
polymers provided with superior sliding property and heat
resistance, and investigated the effects of its crystallization on the microcellular foaming behavior.

2.

BASIC CONCEPTThe basic concept of the MCP process is described indetail in the papers by Professor Suh et al. 12), and it isshown in Figure 1 schematically.
The process A is characterized by effecting gas saturation at room temperature 13). More specifically, a solid
polymer sheet is saturated with a physical blowing agentof high-pressure inert gas (N2 or CO2) at room temperature. To that end, the polymer sheet is kept at rest in theinert gas under a high pressure for sufficient dissolution
until a saturated concentration is reached. Then, the gaspressure within the high-pressure vessel is rapidly lowered to atmospheric pressure. Due to this depressurization, the gas dissolved in the solid polymer sheet reaches
a super-saturation state thus achieving a thermodynamic
instability in the sheet. When the polymer sheet saturated
with gas is heated and quenched, microcellular foaming
Injection of high-pressure gas Rapid release of gas• Gas dissolution up to saturation at room temperature(A)
Martini et al.

MCP

Heating(B)
Colton et al.

MCP

Injection of high-pressure gas* Polymer Engineering Center, Ecology and Energy Lab., R&D• Gas dissolution up to saturation at high temperatureDiv.

* 2 Production Dept., Foam Products Div., Energy and Industrial
Products Co.

Heating/cooling• Super-saturation of gas• Heating above Tg• Thermodynamic instability • Cell growth (foaming)• Cell nucleation• Cell stabilization (cooling)

Figure 1• Heating below Tg• Gas desorption

Rapid release of gas and cooling• Super-saturation of gas• Thermodynamic instability• Cell nucleation• Cell growth (foaming)• Cell stabilization (cooling)

Process for microcellular foaming.

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide

occurs, whereby cell nucleation is induced when the temperature reaches just above its glass transition temperature (Tg). Because the dissolved gas induces plasticization of the polymer, however, the cell nucleation is actuallyinduced at a Tg lower than that intrinsic to the polymer.
Meanwhile, the plasticization phenomenon becomes themore prominent the higher the dissolved gas concentration, and this phenomenon is accordingly applied in the
extrusion of ultra-high-molecular weight polyethylene thathas been considered very difficult to be extruded 14).
In the process B 15), on the other hand, after the inert
gas is dissolved up to a saturated concentration at higher
temperatures, the gas is rapidly removed to simultaneously promote gas super-saturation, cell nucleation andcell growth, thereby obtaining MCP through cooling.
The process B aims at solving the problems that constitute the impediments to practical application of the basic
process of batch type such as shortening the gas dissolution time (i.e. increasing the gas diffusion rate by hightemperatures) and promoting the dissolution of gas into
crystalline polymers. However, from the industrial view
point, this process has some problems including thedecreased gas uptake and the repetition of heating andcooling cycles of large high-pressure vessels of industrial
scale.
In this report, we used the process A above mentioned.

3.

EXPERIMENTAL

3.1 Sample
Figure 2 shows the details of the sample used here. The
PPS sample is 0.2 mm in thickness, 1.31 g/cm3 in density
and about 11 % in crystallinity, i.e. low-crystallinity type.
3.2 Experimental Procedure
The experiments were conducted through the process A
shown in Figure 1. More specifically, the sample was
loaded in a high-pressure vessel, and pressurized by CO2
for a specified time at room temperature in order to saturate the PPS sample with CO2. After the specified time
elapsed, the pressure was released, and the polymer
sample saturated with CO2 was heated in an air oven of
constant temperature to effect foaming, thereby obtaininga polymeric form.
3.3 Evaluation Method of Foamed Polymer
The foamed sample was evaluated as follows.
● Polyphenylene sulfide (PPS)Sn● Sample sheet used
• Crystallinity: about 11 %Peak endothermic heat due to 100 % crystal dissolutionof PPS : 146.2 J/g• Thickness: 0.2 mm• Density: 1.31 g/cm3

Figure 2

Polyphenylene sulfide.

3.3.1. Weight Gain due to Gas Saturation
The weight gain due to gas saturation was calculated by
the equation below, using electronic balance measurements of the sample weight before loading into the highpressure vessel for gas saturation W0 and the sample
weight immediately after gas saturation W1.

Weight gain (wt%) =(W1 − W0 ) ×100
W0

3.3.2. Expansion Ratio
The expansion ratio was calculated by the equation
below, using the density of foamed polymer ρf measured
by the water substitution method and that of non-foamed
polymer sheet ρ0, i.e. 1.31 g/cm3.
ρ
Expansion ratio = ρ 0
f3.3.3. Cell Morphology
The foamed polymer sample was broken in liquid nitrogen, and the cross-sectional surface was observed usinga scanning electron microscope (SEM) for cell morphology.
3.3.4. Crystallinity
A differential scanning calorimeter (DSC) was used tomeasure the exothermic peak resulting from crystallization and the endothermic peak resulting from crystal dissolution, whereby the crystallinity was calculated by theequation below. The heating rate was set at 10°C/min,and the endothermic peak due to 100 % crystal dissolution of PPS was taken as 146.2 J/g 16).
Crystallinity{(Peak endothermic heat due to crystal dissolution)-(Peak exothermic heat due to crystallization)}=
Peak endothermic heat due to 100 %crystal dissolution (146.2 J/g)
× 100

4.

EXPERIMENTAL RESULTS

4.1

Basic Study of Microcellular Foaming Behaviorof PPS
4.1.1. Gas Saturation Behavior
Figure 3 shows the gas saturation curve of PPS/CO2 system, where three levels of 2.0, 4.0 and 6.5 MPa wereselected for the gas pressure. It can be seen that theweight gain of CO2 gas in the polymer monotonously
increased over time for every gas pressure, reaching anequilibrium value in 20 to 30 hr. From these results therelationship between the equilibrium CO2 concentrationand saturation pressure is reduced as shown in Figure 4,
whereby it is indicated that the equilibrium gas concentration is in proportion to the saturation gas pressure inaccordance with Henry’s Law. Based on the results above
mentioned, experiments hereafter were conducted while
fixing the conditions to 6.5 MPa for gas saturation pres-Furukawa Review, No. 28

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide
4.0
3.5

Weight gain (%)

3.0
2.5
2.0
1.5
1.0

2.0 MPa
4.0 MPa
6.5 MPa

0.5
0.0

0

20

40

60

80

100

120

Saturation time (h)Figure 3

Weight gain for PPS as a function of saturation time.

Accordingly, we proceeded to observe the changes incell morphology at each foaming temperature. Figure 7
shows the cross-sectional SEM photographs of foamed
PPS for each foaming temperature. As can be seen, whileat low foaming temperatures foamed polymers having asingle cell size ranging from several to 10 µ m are
obtained, at higher temperatures of 393 K and beyond abimodal cell morphology is observed, where cells about
10 µ m in size are surrounded by microcells that areorders of magnitudes smaller, i.e. the submicron order.
Moreover, as the foaming temperature increases, the volume fraction of the submicron-sized cells increases.
In order to understand such changes in cell morphology, the crystallinity of foamed PPS samples was measured using DSC, and the results are also shown in
5

4

4
Expansion ratio (times)

Equilibrium CO2 concentration (%)

5

3
2
1
0

0

2

4

6

8

Saturated CO2 concentration as a function of saturation pressure.

373 K
473 K

1

0

10

20

30

40

50

60

70

80

Foaming time (s)Figure 5

sure and 48 hr for gas saturation time, i.e. sufficient forreaching an equilibrium.

Expansion ratio for PPS as a function of foaming
time.

6
5
Expansion ratio (times)

4.1.2. Foaming Behavior
Figure 5 shows the dependence of expansion ratio on
foaming time for two foaming temperature levels of 373 K
and 473 K. The cell growth is seen to be completed within
60 sec for either foaming temperature. Based on thisresult, while fixing the foaming time to 60 sec, experiments were carried out to see the dependence of expansion ratio on foaming temperature, and the results areshown in Figure 6. It is observed that, despite the fact thatthe expansion ratio increases as the foaming temperaturerises, it begins to decrease in the vicinity of 410 K in foaming temperature.

20

Saturation pressure (MPa)Figure 4

3

4
3
2
1
0

300

350

400

450

500

550

600

Foaming temperature (K)Figure 6

Expansion ratio for PPS as a function of foaming temperature.
20 µm

Foaming temperature
Crystallinity of
foamed polymer

353 K

373 K

393 K

413 K

423 K

473 K

523 K

10.2%

11.6%

27.5%

27.5%

26.2%

26.5%

26.9%

Gas concentration: 3.6 wt%, Saturation pressure: 6.5 MPa, Foaming time: 60 sec

Figure 7

Dependence of PPS cell morphology on foaming temperature.

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide

DISCUSSIONSThe experimental results suggest that there is a relation ofsome kind between the foaming behavior and crystallinityof PPS. We investigated therefore the relationshipbetween the crystallization behavior and the cell morphology of microcellular PPS foams.
Figure 8 shows the change in crystallinity associatedwith gas saturation. It can be seen that in this case of
PPS, unlike in the case of polyethylene terephthalate(PET) 6), crystallization induced by CO2 does not occur at
the time of gas saturation. On the other hand, from the
DSC curve of as-received PPS sheet shown in Figure 9,
the thermal crystallization of PPS is seen to begin at about
120°C.
It is thought from these results that the change in cellmorphology shown in Figure 7 is affected by the crystallization of PPS induced by heating at the time of foaming,so that we proceeded on further investigations.

4

40

3

30

2

Weight gain
Crystallinity

1

0

20

Crystallinity (%)

Weight gain (%)

5.1 Cell Morphology Change in the Foaming Process
Figure 10 shows the cell morphology in the initiation and

100

Figure 820

40
Saturation time (h)

60

80

0

Effect of gas saturation time on the change in crystallinity of PPS sheet.

30
404.3

DDSC

5

DSC

0

357.3

0

38.2 mJ/mg-30

365.7 -24.7 mJ/mg
398.7
-5

DDSC (mW/min)

5.

10

DSC (mW)

Figure 7. It is seen that the crystallinity of the foamed
polymers begins to rise at a temperature where the cellmorphology shows a change from single-sized cells to
two different-sized cells.

-60
554.5

-10
273

353

433

-90
593

513

Temperature (K)Figure 9

DSC chart for as-received PPS sheet.

completion of the foaming process at various foaming
temperatures. In this experiment, in order to carry out amore accurate observation, the gas saturation pressure
was set to 4.0 MPa while lowering the gas concentration
before foaming, and the samples for observation of theinitial stage of foaming was prepared in such a way thatafter the sample saturated with gas was heated for 10 sec
it was quenched to prevent cell growth. The results show
that, while at 373 K and 383 K secondary microcells of thesubmicron scale were not generated in both the initiationand completion of foaming, at 393 K and higher such secondary microcells were generated during the cell growth
process, and that at 423 K the secondary microcells werealready generated in 10 sec.
Accordingly, we proceeded to confirm the cell nucleation and growth process in more detail selecting thefoaming temperature as 393 K and 403 K. Figure 11
shows the changes in cell morphology as the foaming
time was changed from 6 sec through 60 sec. Each sample was prepared by heating for the specified foaming
time followed by quenching after foaming. The results
show that the secondary microcells were generated during the cell growth process at both 393 K and 403 K, andthat as compared with the sample foamed at 393 K, theone foamed at 403 K indicates a shorter time in having
the secondary microcells.

20 µm

Foamingtime : 10 s

Foamingtime : 60 s

Foaming temperature

373 K

383 K

393 K

403 K

413 K

423 K

Gas concentration : 3.1 %Saturation pressure : 4.0 MPa

Figure 10 Cell morphology in the initiation and completion of foaming process at various foaming temperatures.

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide

5.2

5.3

Hypothesis about the Microcellular Foaming
Behavior of PPS
A hypothesis about the microcellular foaming behavior of
PPS is made from the results mentioned heretofore,
which is illustrated in Figure 14 with foaming time repre-

40
35
30
Crystallinity(%)

Relationship between the Cell Morphology and
Crystallinity Changes
The experimental results above mentioned demonstratethat the secondary microcells are generated during thecell growth process of microcellular foaming, and suggest
that, as shown in Figure 7, the secondary microcellularfoaming is based on crystallization of PPS. Accordingly, inorder to confirm this phenomenon in more detail, we
measured using DSC the crystallinity of the foamed samples shown in Figure 11 having different foaming times tounderstand its change in crystallinity during the cell
growth process. The results are shown in Figure 12, andas can be clearly seen, crystallization is induced by heating in the foaming process, and the secondary microcells
are generated due to the crystallization.
We investigated the cell growth process and the change
in crystallinity for a wider range of foaming temperatures.
Three temperatures of below the crystallization temperature of PPS (373 K), near the crystallization temperature(473 K) and near the melting temperature (473 K) wereselected as the foaming temperature, and the results areshown in Figure 13. When foamed below the crystallization temperature crystallization does not occur during thecell growth resulting in a unimodal cell distribution, while
when foamed near the crystallization temperature crystallization occurs during the cell growth process accompanied by secondary microcells. When foaming is carriedout at a higher temperature, it can be confirmed that crystallization proceeds in a very short time and that it isaccompanied by immediate secondary microcellularfoaming.
From these results, it may be concluded that the generation of secondary microcells is caused by the PPS crystals generated during the cell growth process.

25
20
15
10

393 K
403 K

5

Gas concentraion:3.1 %

0

0

10

20

30

40

50

60

Foaming time(s)Figure 12 Relationship between the crystallinity of PPS and cellmorphology (I).

senting the cell growth process on the X-axis and foaming
temperature on the Y-axis. It is thought that, below thecrystallization temperature, cell nucleation occurs when
the temperature of the gas saturated sample goes beyond
the glass transition temperature, and that the cells growas the gas around the cell nuclei diffuses toward thenuclei. When foaming proceeds above the crystallization
temperature, while, just as is the case with the foaming
process below the crystallization temperature, cell nucleation occurs when the temperature goes beyond theglass transition temperature whereby the cells grow as
the gas around the cell nuclei diffuses toward the nuclei,
there occurs crystallization when the temperature goesbeyond the crystallization temperature. At this time, it is
20 µm

Foaming
temperature :393 K

Foaming
temperature :403 K

Foaming time

6s

10 s

20 s

30 s

60 s

Gas concentration : 3.1 %Saturation pressure : 4.0 MPa

Figure 11 Cell morphology changes during the cell growth process at various foaming times.

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide

40
35

Crystallinity (%)

30
25
20

373 K
393 K
473 K

15
10

Gas concentraion:3.6 %

5
0

0

10

20

30
40
Foaming time (s)5060Figure 13 Relationship between the crystallinity of PPS and cell morphology (II).

Foaming temperature

C1＞C2
C1 C2

6.
Foaming near
the melting temperature
Foaming near
the crystallization
temperatureAftercrystallization

Tc
Beforecrystallization

Foaming below
the crystallization
temperature

Tg

Concentration
change due tocrystallizationof PPS

Initiation of foaming

Completion of foaming
Foaming time (cell growth process)Figure 14 Hypothesis about the microcellular foaming behaviorof PPS.

thought, secondary cell nucleation due to the crystallization occurs in the region that is free from cell nucleation,
whereby the cells initially generated grow into the cells
about 10µm in size while those due to the secondary cell
nucleation become submicron-sized cells.
At a higher foaming temperature, secondary cell nucleation occurs in a shorter time due to faster crystallization,
thus resulting in the generation of submicron-sized cellswith a higher ratio.
With regard to the reason that the secondary cell nucleation occurs accompanied by the crystallization of PPS, it
may be that the gas in the vicinity of crystallized areas ispushed out into the surroundings as crystallization proceeds resulting in a higher gas concentration in the surroundings thus enhancing cell nucleation.

CONCLUSIONS

The microcellular foaming behavior of PPS has been
investigated, and it has been observed that the foaming
behavior below the crystallization temperature is different
from that above the crystallization temperature. In particular, it has been shown that the submicron-sized cells
observed are generated by the cell nucleation induced by
crystallization of PPS. The present study is insufficient interms of such issues as discussions from the chemical
kinetics standpoint and the relationship between crystalline morphology and cell shape. We intend to addressthese issues in quantitative terms in the future, ultimately
promoting this study toward the ultra-microcellular foaming technology.

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Effects of Crystallization on Cell Morphology in Microcellular Polyphenylene Sulfide11) T. Mizumoto et al.: Seikei Kako Symposia, B208 (2000) 211. (inJapanese)12) J. S. Colton and N. P. Suh: Polym., Eng., Sci., 27 (1987) 485.
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