Plasma treatment of polymer powders

Surfaces of fine polystyrene (PS) and polymethyl methacrylate (PMMA) powders were modified by exposure to the downstream products of a nitrogen or oxygen microwave plasma. The effects of nitrogen and indium incorporation in the powder surface were studied with emphasis on variations in the triboelectric properties of the powder. X-ray photoelectron spectroscopy (XPS) was utilized to determine the changes in surface elemental composition. After nitrogen plasma treatment, the C 1s peak profiles suggested the formation of amines in the case of PS, and the formation of imines and amides in the case of PMMA. Oxygen plasma treatment suggested the formation of hydroxyl and carbonyl groups on the surfaces of both PS and PMMA. After treatment with a nitrogen or oxygen plasma, the charge-to-mass ratio (Q/M) of PS and PMMA powders in contact with carrier particles was measured using the cage blowoff method. The surface charge density (Q/A) was calculated from Q/M. The Q/A of nitrogen plasma-treated PS powder was seen to shift towards positive charge with small increases in the nitrogen concentration. The Q/A of oxygen plasma treated PS powder initially shifted toward negative charge, but changed towards positive charge with higher oxygen concentrations. Plasma-treated PMMA powder showed a different behaviour and the variation of Q/A on PMMA was much less than that of PS. Results suggest that triboelectrification of the polymer powder may be related to changes in the electrical surface states, and that nitrogen may act as a group V dopant within the PS surface.

BACKGROUND OF THE INVENTION 
This invention relates to the charging of materials by 
triboelectrification, especially the fine polymer powders used as toners 
in electrophotographic systems. 
The charging of materials by triboelectrification has been applied to a 
number of industrial products for some time. Since the invention of the 
electrophotographic technique used for copiers and non-impact printers by 
Carlson in 1938 (see U.S. Pat. No. 2,297,691 (Carlson, 1942)), the field 
has developed into a large commercial market. Recently, electrophotography 
has required higher resolution images than was necessary previously, for 
application in colour image systems (see E. Czech, W. Ostertag, SPIE 1253, 
Hard Copy and Printing Products, 64, (1990)). For this purpose it is very 
important to control accurately the electrical charge of the fine polymer 
powders used as toners in electrophotographic systems (see S. Kume, The 
Institute of Electrostatics of Japan 10(5), 306 (1986)). 
Toners are fine polymer particles typically about 10 .mu.m in diameter 
mixed with various additives and usually include a coloured dye. In a 
two-component development system, the toner particles are charged by 
making contact with larger metal beads known as carriers [see L. B. 
Schein, "Electrophotography and Development Physics", ISBN 3-see 
540-18902-5, Springer Verlag (1988)]. The toner is transferred to the 
photoreceptor due to an attractive electric field to form a real image 
(development). For a high quality image, it is important to control the 
charge-to-mass ratio (Q/M) of the toner within predetermined limits. The 
Q/M varies with changes in environmental conditions and surface properties 
of the toner (see N. Matsui, K. Oka and Y. Inaba, J. Electrophotographics 
30(3), 282 (1991)). Much work has been done to investigate the 
triboelectrification of fine polymer powders (see J. Henniker, Nature 196, 
474 (1962); C. B. Duke and T. J. Fabish, J. Appl. Phys. 49, 315 (1978); 
and L. B. Schein and M. Latta, J. Appl. Phys. 69 (10), 6817 (1991)), but 
the mechanisms are still not fully understood. Some investigations suggest 
that the nature of the chemical species on the surface is the most 
important aspect for controlling the triboelectric charge of the particle 
(see I. Shinohara, F. Yamamoto, H. Anzai, and S. Endo, J. Electrost. 2, 99 
(1976), and H. W. Gibson, Polymer 25, 3 (1984)). 
SUMMARY OF THE INVENTION 
There is a need to improve the understanding of the relationship between 
the surface elemental composition and the mechanism of surface 
electrification on polymer powders, and the present invention has improved 
that understanding. Nitrogen or indium have been incorporated into the 
surface structure of polymer powders using a downstream microwave plasma 
reactor. After this incorporation each of the elements was found to 
increase and stabilize the Q/M. A model to explain the changes in the 
electrification properties is suggested. 
Moreover, there is a need to improve the triboelectric properties of 
polymer powder, and the invention provides a method for doing so, 
comprising the step of positioning the polymer powder in the afterglow 
region of a gas plasma having a main region and an afterglow region, where 
the gas plasma is in a low-pressure stream of a gas selected from the 
group consisting of oxygen, nitrogen, and gases containing oxygen or 
nitrogen, whereby low concentrations of oxygen or nitrogen as the case may 
be are incorporated into the surface of the powder. 
Preferably, the polymer powder is of any conjugated polymer, or a polymer 
bearing aromatic constituents. 
Preferably, the gas is nitrogen, and the polymer is polystyrene or 
co-polymers of polystyrene. 
Alternatively, the method comprises the step of positioning the polymer 
powder in the afterglow region of a gas plasma having a main region and an 
afterglow region, where the gas plasma is in a low-pressure stream of a 
gas selected from the group consisting of oxygen, nitrogen, and gases 
containing oxygen or nitrogen, and where indium as a metal foil is 
suspended in the gas stream adjacent the gas plasma, thereby generating 
indium vapour, whereby low concentrations of indium are incorporated into 
the surface of the powder.

DETAILED DESCRIPTION OF THE INVENTION 
1. EXPERIMENTS 
Polystyrene (PS) and polymethyl methacrylate (PMMA) powders in the form of 
small spherical beads were obtained from Sekisuikaseihin Co., (Tokyo, 
Japan). The XPS analysis was performed using an SSX-100 X-ray 
photoelectron spectrometer which utilizes monochromatized Al K.alpha. 
X-rays for excitation of the sample. For analysis, the polymer sample was 
fixed on indium foil (5.times.5 mm.sup.2, 0.5 mm thickness) in a sample 
holder. For the elemental broad scan analysis, the X-ray spot size was set 
to 600 .mu.m. The elemental broad scan was measured at the three different 
spots to ensure even and overall treatment of the powder. For high 
resolution analysis of the C 1s peak, the X-ray spot size was reduced to 
150 .mu.m. The charging of the polymer surface during X-ray exposure was 
controlled using the flood gun/screen technique (see C. B. Bryson, Surface 
Sci. 189/190, 50 (1987)). Binding energies have been corrected for the 
shifts observed due to sample charging and spectra are referenced to the C 
1s hydrocarbon component which was assigned the value of 284.8 eV (see 
ASTM Standard, E1015, Vol. 03.06 (1984)). 
Atomic percentages of oxygen and nitrogen were calculated using Scofield 
cross-sections correlated for differences in inelastic mean free path due 
to electron kinetic energy. The inelastic mean free path for C 1s 
electrons in PS was taken to be 2.9 nm (see R. F. Roberts, D. L. Allara, 
C. A. Pryde, D. N. E. Buchanan, and N. D. Hobbins, Surface and Interface 
analysis 2(1), 5 (1980)). The atomic percentages of nitrogen and oxygen 
were converted into relative quantities as N/C (nitrogen/carbon atom 
ratio) or O/C (oxygen/carbon atom ratio). 
XPS showed the original PS powder (mass mean diameters of 8, 15, and 20 
.mu.m) to contain some oxygen O/C=0.03.+-.0.01 on the surface. This oxygen 
is thought to be the result of oxidation during processing. PMMA powder 
(mass mean diameters of 8, 12, and 20 .mu.m) showed oxygen 
O/C=0.35.+-.0.01 on the surface. Since the properties of the powder 
surfaces change with atmospheric conditions (see Y. Nurata, Hyomen 23 (9), 
528 (1985); K. P. Homewood, J. Electrostat. 10, 299 (1981)), it is 
important to control these conditions. 
Surface treatment of the powders was carried out in a vortex reactor 
located downstream from a microwave plasma discharge. The vortex reactor 
used during this work (FIG. 1) consisted of a 100 ml pyrex flask 1 with 
upper and side necks 2 and 3 respectively. The powder samples 4 were 
placed in the bottom in the flask along with a pyrex covered magnetic 
stirrer 5. The amount of powder sample used for each experiment was 
0.3-1.2 g. The reactor was connected to the plasma by a 1.5 cm diameter 
quartz tube 6 fitted into the upper neck 2 of the flask. The upper part of 
this tube was surrounded by an Evenson microwave cavity 7 connected to a 
120 W, 2.45 GHz microwave generator 8. The side neck 3 of the flask was 
connected to a high volume one stage rotary pump 9 with a pumping speed of 
.apprxeq.1.1.times.10.sup.6 sccm (standard cubic centimeters per minute). 
A fine cloth filter 10 was placed between the reactor and pump to prevent 
loss of powder to the pump. 
Pure nitrogen gas (99.99%) and pure oxygen gas (99.99%) were used as plasma 
source gases, introduced via a gas inlet 11. After the reactor was pumped 
to an initial base pressure of 5.times.10.sup.-2 torr, the gas flow was 
typically set at a low flow rate (40 sccm) or at a high flow rate (O.sub.2 
; 1600 sccm, N.sub.2 ; 2000 sccm) for the experiments. The gas flow rates 
were controlled by a mass flow controller (not shown). The pressures in 
the reactor were 1.0 torr (at 40 sccm), 3.5 torr (at 1600 sccm), and 5.0 
torr (at 2000 sccm). The net microwave power for the experiments was set 
at 40 W. Treatment times were controlled and varied between 5 min and 30 
min. 
In the above described configuration the sample is located .apprxeq.16 cm 
downstream from the plasma, and is therefore exposed only to the longer 
lived species in the plasma afterglow region 12. Downstream plasma 
treatment of polymer surfaces has previously been studied [see R. Foerch, 
J. Izawa, and N. S. McIntyre, J. Polymer Sci.: Appl. Polymer Symposium 46, 
415 (1990)] and it has been shown that the efficiency of the treatment is 
dependent on the gas flow rate, the microwave power applied and the 
distance of the sample from the plasma (see R. Foerch, N. S. McIntyre, R. 
N. S. Sodhi, and D. H. Hunter, J. Appl. Polymer Sci. 40, 1903 (1990), and 
R. Foerch, N. S. McIntyre, and D. H. Hunter, J. Polymer Sci., Part A, 
Polymer Chemistry 28, 193 (1990)). 
In downstream plasma treatment, the reactive species of a gas plasma are 
reacted with a sample positioned a distance beyond the main plasma region, 
i.e. downstream from the main plasma region in terms of the direction of 
gas flow. The actual distance could vary from installation to 
installation. This is known as downstream or remote plasma modification, 
the main application of which to date has been in the electronics and 
semiconductor industry for the purpose of deposition of dielectric 
coatings on semi conductor devices or the cleaning of polymers (see 
Bachman U.S. Pat. No. 4,946,549). 
The downstream or remote plasma treatment in the present invention provides 
a much less destructive method for polymer treatment in comparison to 
direct plasma modification and other more commonly used methods such as 
corona discharge, ozone and flame treatment. In addition, the remote 
plasma treatment enables greater control over the reactive species 
interacting with the polymer, such that only the longer lived species, 
i.e. N atoms, reach the sample, rather than a whole range of electrons, 
ions and other excited species. 
The cage blowoff method is a simple and reproducible method for measuring 
the charge-to-mass ratio (Q/M) of polymer powders in contact with carrier 
particles (see L. B. Schein and J. Cranch, J. Appl. Phys 46, 5140 (1975). 
The carrier particles were made of polymer-coated ferrimagnetic particles, 
with a diameter of 120 .mu.m. The plasma treated samples were mixed with 
the carriers in a 30 ml glass bottle. The ratio of the quantity of sample 
to carrier was changed with the diameter of the sample powders to ensure 
the same initial ratio of sample surface area to carrier surface area 
(0.5:1) (see N. Hoshi and M. Anzai, J. Electrophotographics 25 (4), 269 
(1986). Thus, for a typical powder diameter of 8 .mu.m, the mixing ratio 
of sample to carriers was 2.0 wt %, while for 20 .mu.m the ratio was 4.5 
wt %. A mixing time of 30 minutes at 120 rpm was utilized to electrify the 
samples sufficiently. 
After the mixing, the sample/carrier mixture was transferred to a 
double-walled aluminum Faraday cage (the "blowoff cage") with 44 .mu.m 
metal mesh covering both ends of the inner container. The smaller powder 
particles were blown through the screen using a strong jet of air. The 
charge remaining on the carrier beads was measured using a Keithley Model 
602 electrometer. This charge measured is equal and opposite to the charge 
on the powder. The change in mass before and after the blowoff was 
measured using a balance with an accuracy of .+-.0.1 mg. This data allowed 
Q/M to be calculated. Q/M measurements were repeated three times on 
different samples from the same batch of materials within 30 min. Two 
types of carriers were used to electrify the samples either with a 
negative charge or a positive charge. The difference between the two types 
of carriers lies in the surface polymer compositions of each of the 
surfaces. 
The charge density Q/A was calculated using the following equation to 
compare the samples of different diameters. 
##EQU1## 
where: 
Q/A is the charge density [.mu.C/m.sup.2 ], 
Q/M is the-charge-to-mass ratio [.mu.C/g], 
d.sub.0 is the density of polymer [g/m.sup.3 ], 
r.sub.0 is the radius of polymer [m]. 
The surface elemental composition and the electrical properties of the 
starting materials were continuously verified by XPS and measurement of 
Q/M. 
The results of these experiments are discussed below. 
2. RESULTS 
a) Plasma treatment of powders 
FIG. 2(a) shows the relationship between plasma exposure time and N/C on PS 
powder for nitrogen gas flow rates of 40 and 2000 sccm. N/C increased with 
treatment time, reaching a maximum of 12% after 20 min at 2000 sccm and 5% 
after 30 min at 40 sccm. FIG. 2(b) shows the relationship between the 
treatment time and O/C for PS powder using oxygen gas flow rates of 40 and 
1600 sccm (the highest stable flow rate for oxygen gas). At 1600 sccm the 
O/C was seen to change from 0.03 to 0.34 within 10 minutes. Decreasing the 
gas flow rate to 40 sccm showed a similar reaction rate to that at 1600 
sccm with a maximum O/C of 0.28 within 10 minutes. It thus appears that 
the oxygen plasma treatment is very efficient for PS powder. 
FIG. 3(a) shows the rate of reaction of a nitrogen plasma on PMMA powder. 
At 2000 sccm the maximum N/C was 0.15 after 10 minutes. Unlike PS, the 
rate of reaction was seen to change significantly with changes in flow 
rate. FIG. 3(b) shows that with oxygen plasma treatment, the O/C of PMMA 
changed from 0.35 to 0.56 within 10 minutes. Decreasing the flow rate to 
40 sccm showed an increase in O/C from 0.35 to 0.45 after 30 minutes of 
exposure. For ease of comparison .DELTA.O/C will be used for PMMA, where 
.DELTA.O/C indicated O/C of treated PMMA minus O/C of untreated PMMA 
(O/C=0.35). 
The mass of sample used during plasma treatment was changed to investigate 
the effects of the sample quantity in the reactor. FIGS. 4(a)(b) shows the 
relationship between mass of PS(a) and PMMA(b) powder and nitrogen or 
oxygen concentration. The treatment time was 10 min, using a high gas flow 
rate. When increasing the mass of samples in the reactor, N/C and O/C 
decreased for both polymers. These results show that less quantity of 
sample is more effective for achieving higher concentrations of nitrogen 
and oxygen. 
FIGS. 5(a)(b) shows the high resolution C1s peaks of PS powder with 
nitrogen and oxygen plasma treatment. The C1s peak for untreated PS powder 
was resolved into two components. The component of greatest intensity at a 
binding energy of 284.8 eV represents the hydrocarbon component. The 
feature shifted by 6.7 eV represents the .pi..fwdarw..pi.* shake up 
satellite characteristic for aromatic or conjugated species (see D. T. 
Clark and A. Dilks, J. Polym. Sci., Polym. Chem. Ed. 14,533 (1976)). 
After exposure to a remote nitrogen plasma (N/C=0.04, O/C=0.06), the peak 
shape was seen to alter with the appearance of new functional groups (FIG. 
5(a)). Since the binding energy shifts for these peaks are small (&lt;2 eV), 
it is believed that the nitrogen adds to the polymer as amine functional 
groups (C--NH.sub.2, C--NHR, C--NR.sub.2) and imines (C.dbd.N). Oxygen 
appears to add as hydroxyl or ether groups (shift of 1.5 eV). With longer 
exposure time to a nitrogen plasma (N/C=0.12, O/C=0.07), the intensity of 
these peaks was seen to increase and a low intensity peak at a binding 
energy shift of 3.6 eV was observed suggesting the formation of C.dbd.O 
and RCO--NHR groups. A decrease in the .pi..fwdarw..pi.* shake up 
satellite intensity was seen with remote nitrogen plasma treatment 
suggesting some disruption of the PS conjugated structure. 
With exposure to a remote oxygen plasma (FIG. 5(b)) the C 1s peak shape of 
PS was seen to change and suggested the formation of hydroxyl groups 
(shift of 1.5 eV) and carbonyl groups (shift of 3.0 eV) at an O/C=0.10. 
With higher oxygen concentration (O/C=0.32), the spectrum was seen to 
change further with the appearance of additional peak components 
suggesting the formation of carboxyl groups (shift of 4.0 -4.5 eV). It was 
also noted that the .pi..fwdarw..pi.* shake up satellite disappeared with 
longer exposure to the plasma. 
The C 1s peak for PMMA (FIGS. 6(a)(b) was also seen to change with plasma 
treatment. The original material showed four components within the peak 
envelope. These can be associated with the hydrocarbon component; 
C--CO.sub.2 at a binding energy shift of 0.8 eV, C--O at a shift of 1.5 
eV, and the ester carbon RO--C.dbd.O at 3.9 eV. With nitrogen plasma 
treatment (N/C=0.07, .DELTA.O/C=0.07), small changes are observed the C1s 
spectrum with an increase in the peaks at 3.9 eV and 0.8 eV and the 
appearance of a small feature at 3.0 eV (FIG. 6(a)). With further nitrogen 
plasma treatment (N/C=0.16, O/C=0.05), an increase in the intensity of all 
high binding energy peaks was observed. Since O/C did not change, it can 
be assumed that this represents the formation of further nitrogen 
functional groups such as amines (0.8 eV), amides (3.0 eV), and urea type 
functional groups (4.0-4.5 eV) on the surface [see D. T. Clark and A. 
Harrison, J. Polym. Sci., Polym. Chem. Ed. 19, 1945 (1981)]. 
With oxygen plasma treatment of PMMA, similar changes in the C 1s envelope 
were observed (FIG. 6(b)). After a change in O/C of 0.10, the original 
peak shape changed significantly, with an intensity increase of the C--O 
(shift of 1.5 eV) and RO--C.dbd.O (shift of 3.9 eV) components. With 
longer exposure times (.DELTA.O/C=0.22) an extra feature appeared at 3.0 
eV indicating the formation of C.dbd.O groups on the surface. This 
appeared to be accompanied by an intensity decrease of the C--O and 
RO--C.dbd.O groups, which may suggest disruption of the PMMA polymer 
structure. 
b) Electrical characteristics 
Polystyrene: 
FIG. 7(a) shows the relationship between the relative concentration of 
nitrogen (N/C) or oxygen (O/C) and the charge density (Q/A) on the PS 
powder electrified using a negative carrier. Powder samples of diameter 8 
.mu.m (.DELTA.), 15 .mu.m (.quadrature.), and 20 .mu.m (.largecircle.) 
were used. Since all measured points fit on the same curve it is evident 
that the charge density is independent of the diameter of the powder. 
Untreated PS powder showed a Q/A of about -120 .mu.C/m.sup.2. Even after 
very brief nitrogen plasma treatment a significant change towards positive 
charge was observed. Q/A reached a maximum (0 .mu.C/m.sup.2) when XPS 
indicated N/C to be 0.08-0.09. No further change in Q/A was observed with 
higher nitrogen content. This effect was found to be even more rapid when 
a positive carrier was used (FIG. 7(b)). Untreated PS powder did not 
accumulate charge when in contact with the positive carrier (Q/A=0 
.mu.C/m.sup.2). However, after nitrogen plasma treatment to N/C=0.02, Q/A 
shifted toward a maximum positive charge of +110 .mu.C/m.sup.2. The 
difference in Q/A before and after nitrogen plasma treatment, .DELTA.Q/A, 
was similar for both negative and positive carriers, but the effect of low 
nitrogen surface concentrations was much more pronounced with a positive 
carrier. 
In contrast to the changes caused by the nitrogen plasma treatments, oxygen 
plasma treatment of polystyrene resulted in markedly different charging 
behaviour. In FIG. 1 electrification of oxygen plasma treated PS with a 
negative carrier caused its Q/A to decrease under conditions where the 
surface oxygen content is relatively low. However, with higher oxygen 
concentrations a minimum in Q/A is reached. Then, at still higher oxygen 
concentrations (O/C=0.1) the Q/A increases. Electrification of oxygen 
plasma treated PS particles with a positive carrier was found to respond 
to oxygen surface concentrations in a manner qualitatively similar to 
electrification with a negative carrier. 
Polymethylmethacrylate: 
Similar experiments using PMMA powder showed a very different behaviour 
than that of PS. FIGS. 8(a)(b) show the relationships between the N/C or 
.DELTA.O/C and the Q/A of PMMA, electrified by negative (a) and positive 
(b) carriers. In this graph again several powder diameters were plotted 
together (diameter 8, 12, 20 .mu.m). The value of Q/M of untreated PMMA 
was found to be about +85 .mu.C/m.sup.2 using a positive carrier. Q/A was 
found to increase only slightly (+100 .mu.C/m.sup.2) for low nitrogen 
concentration (N/C&lt;0.07) and then actually decreased with higher values of 
N/C. Q/M decreased with an increase in .DELTA.O/C. A very rapid change in 
Q/A was observed near .DELTA.O/C=0.10 for both carriers suggesting 
significant changes in the surface properties at that point. 
The data observed for PMMA in fact showed no resemblance to that of PS 
suggesting very different charging mechanisms for the two polymers. 
c) Surface aging 
Q/A on the plasma treated PS and PMMA was measured for several weeks after 
the plasma treatment in order to investigate the effect of aging on the 
ability of the particle to accumulate charge. FIG. 9 shows the change in 
Q/A for PS powder using negative carrier. For high nitrogen concentration 
(N/C=0.12) Q/A did not appear to change over a period of 120 days. For low 
nitrogen concentration PS (N/C=0.03) Q/A actually increased to a more 
positive value (20%) over 130 days. The Q/A of oxygen plasma treatment PS 
was seen to decrease to a more negative value (about 2-5%) over 140 days. 
The concentration of nitrogen (N/C) on the PS and PMMA was also measured 
for several weeks after the plasma treatment to investigate the aging 
phenomemon. FIG. 10 shows the change in the N/C ratio on PS powder as a 
function of time. The nitrogen concentration was seen to remain within 
experimental error over the 120 days for samples with low surface nitrogen 
concentrations. These results suggest that while the concentration of 
oxygen and nitrogen changes slightly with time, the surfaces retain their 
electrical properties. 
d) Photoemission Studies 
The photoelectron yields of nitrogen-treated and untreated polystyrene have 
been measured using a dedicated instrument. In FIGS. 11(a) and 11(b), the 
photoelectron yields are compared. Both samples have work functions close 
to 5.05 eV, but the electron yields above this energy is greater than ten 
times higher for the nitrogen-treated surface. 
e) Effect of Indium 
Other elements have also been found to affect the charging properties of 
polystyrene when introduced in low concentrations into the surface. For 
example, indium added to the surface in quantities of 1 atomic % caused 
the Q/A to increase to similar values as was found for nitrogen. 
Indium as a metal foil was suspended in the gas flow just below the 
microwave cavity. A gas flow of 1000 sccm was used to transfer indium 
vapor to the surface of polystyrene particles in the normal place in the 
reactor shown in FIG. 1. XPS showed that a small concentration of indium 
was present on the surface as In.sup.+3. The presence on the surface of 
low concentrations of indium caused a major increase in the electrical 
charge retained, as shown in FIG. 12. 
3. DISCUSSION 
These results have shown that, by using a downstream nitrogen plasma, it is 
possible to make major and long-lasting changes to the triboelectric 
properties of PS powder. Q/A measurements of nitrogen plasma-treated PS 
have shown dramatic changes in charging properties with very low 
incorporation of nitrogen into PS. In order to understand this phenomenon, 
Q/A data must be carefully compared to the available XPS data. XPS 
analysis has suggested that during nitrogen plasma treatment the major 
species formed are amines, even at higher N/C (FIG. 5(a)). Q/A 
measurements have shown a very rapid shift towards positive charge during 
the initial stages of nitrogen incorporation. The charge reaches a plateau 
well before the surface amine concentration has reached its maximum. The 
effect is particularly marked when using a positive carrier for 
electrification. 
This has led the inventors to believe that the change in Q/A is not 
entirely controlled by surface chemistry and interfacial adhesion forces, 
but may also be affected by changes in the surface electronic properties. 
Polystyrene, on the basis of calculations of band gap [see C. B. Duke and 
T. J. Fabish, J. Appl. Phys. 49,315 (1979)], is believed to have few 
surface states near its Fermi level (FIGS. 13 (a)(b)). A small 
concentration of nitrogen (N/C&lt;0.03), acting as a donor, could create 
additional unoccupied surface states in this region (FIG. 14(a)) which may 
stabilize charges. The effect of the nitrogen on the photoemission yield 
provides evidence for an alteration of electronic band structure. The 
differences in the effectiveness of positive and negative carriers may, 
however, be governed by surface chemistry. For a positive carrier even low 
concentrations of nitrogen impart the maximum effect on the Q/A of 
polystyrene particles. The less dramatic effect achieved with the negative 
particle may be due to a difficulty in achieving good contact between 
carrier and toner until the toner surface has sufficient hydrophylicity 
brought about by extensive nitrogen treatment. 
XPS results have shown that the main structure of PS has remained, even at 
high nitrogen concentration, but with some loss of aromatic character. 
Loss of the conjugated structure could alter significantly the charging 
properties by altering the density of occupied and unoccupied states. 
Oxygen plasma treatment results for PS suggest that rather different 
mechanisms are involved. Q/A increased towards a small negative charge at 
low O/C and slowly decreased towards positive charge at O/C&gt;0.05 or 0.10 
for negative and positive carriers respectively. The oxygen added may 
create both occupied and unoccupied surface states which could change the 
Fermi level (FIG. 14(b)). 
In contrast to PS, PMMA has many surface states in the band gap. The two 
band gaps are compared in FIG. 13. The band gap states are therefore less 
sensitive to the addition of potential donor atoms such as nitrogen. The 
effect of nitrogen addition on Q/A for PMMA may only be the result of 
changes in surface charging. The change of Q/A on the oxygen 
plasma-treated PMMA is also much less than that for PS. A rapid change 
near O/C=0.10 of Q/A on oxygen plasma-treatment PMMA may be caused by the 
disruption of the PMMA polymer structure (FIG. 6(b)). 
In conclusion, the surfaces of PS and PMMA powders were modified by 
treating them with downstream nitrogen and oxygen plasmas. Q/A of nitrogen 
plasma-treated PS powder has shown a very rapid change towards positive 
charge with small increases in N/C. It is believed that nitrogen atoms 
could act as a donor and increase the unoccupied surface states in the 
surface of PS. The variation of Q/A of PMMA has been much less than that 
of PS, perhaps because of the larger number of surface states in the band 
gap.