VOL. 15, 1409-1425 (1977)
JOURNAL OF POLYMER SCIENCE: Polymer Physics Edition
Properties of Polyethylene Modified with Phosphonate
Side Groups. I. Thermal and Mechanical Properties
R. A. WEISS,* R. W. LENZ, and W. J. MACKNIGHT, Departments of
Chemical Engineering and Polymer Science and Engineering, University of
Massachusetts, Amherst, Massachusetts 01002
Synopsis
Low-density polyethylene was modified by the inclusion of phosphonate ester pendent groups
by using an oxidative chlorophosphonylation reaction followed by esterification of the polyethylene
poly(phosphony1chloride) with an alcohol. Two different types of phosphonate esters were prepared
dimethyl phosphonate from the reaction with methanol and a phosphonate graft copolymer from
the reaction with hydroxy-terminated poly(ethy1eneoxide) (PEO). For the latter, oligomers with
molecular weights of 350 and 750 were used. For each type of phosphonate, a series of polymers
were prepared with pendent group concentrations ranging from 0 to 9.1 substituents per 100 carbon
atoms. The modified polymers were characterized by infrared spectroscopy, differential scanning
calorimetry, and by measurement of the tensile modulus. Infrared spectroscopy proved to be useful
for determining if the polymer modification reaction resulted in entirely phosphonate ester pendent
group substitutions or if unesterified phosphonic acid groups were also present. The polymers
prepared in this investigation exhibited no infrared absorbancesarising from phosphonic acid groups.
The presence of phosphonate ester groups resulted in a decrease of crystallinity with increasing
phosphonate concentration and with the exception of the polymers containing 9.1 PEO-phosphonate
grafts per 100 carbon atoms, the effect of phosphonylationon the melting temperature of the polymers
was consistent with Flory’s theory for the melting point depression of random copolymers. The
tensile modulus measured from a constant uniaxial elongation experiment decreased with increasing
phosphonylation. The behavior of all three phosphonate series was identical and could be attributed
to the effect of decreasing polymer crystallinity.
INTRODUCTION
Phosphonic acid groups and their derivatives can be readily incorporated into
hydrocarbon polymers by oxidative chlorophosphonylation,1>2which involves
the reaction of the polymer with phosphorus trichloride, PC13, and oxygen at
relatively mild temperatures. The reaction is free radical in mechanism and
proceeds by the abstraction of a labile hydrogen atom from the polymer followed
by the formation of a carbon-phosphorus covalent bond3 according to the following stoichiometry:
R-H
+ 2PC13 + O2
-
R-POC12
+ POC13 + HC1
The primary product of this reaction is a polymeric phosphonyl dichloride,
RPOC12, which is highly reactive and can be easily converted either to the
*Present address: Exxon Research and Engineering Company, Corporate Research Laboratories,
Linden, New Jersey 07036.
1409
0 1977 by John Wiley & Sons, Inc.
1410
WEISS, LENZ, AND MACKNIGHT
polymeric phosphonic acid by hydrolysis:
R-POC12
2Hz0 R--PO(OH)2
2 HCl
or to the polymeric phosphonate by esterification:
R-POC12
2R’OH 4R-PO(OR’)2
2HC1
Most of the investigations involving the chlorophosphonylationof hydrocarbon
polymers have been based on low-density polyethylene, because the reaction
proceeds rapidly a t low temperatures (60-70°C) and incorporates phosphonyl
groups randomly and without degradation of the main polymer ~ h a i n . ~ s
Therefore, a well characterized series of random phosphonylated copolymers
can be prepared from a single starting polyethylene, and this is desirable for
studying the relationship between polymer structure and properties of such
systems.
The preparation and characterization of the phosphonic acid derivative of
phosphonylated low-density polyethylene have recently received considerable
attention. Schroeder and Sopchak4 studied the kinetics of the reaction, and
Rafikov and c o - w o r k e r ~and
~ * ~MacKnight and co-workers7-11 studied the
properties of polyethylene phosphonic acids. MacKnight et al. found that the
phosphonic acid pendent group disrupted the polymer crystallinity in a predictable manner7y8and microphase separation was observed in the form of hydrogen-bonded phosphonic acid
Many of the resulting polymer
properties were attributed to the existence of this two-phase structure.
Several attempts have been made to synthesize phosphonate ester derivatives
of phosphonylated polyethylene.12J3 In these investigations, however, the resulting polymers contained measurable amounts of phosphonic acid groups,
which significantly affected the polymer properties. As a result, all previous
conclusions concerning the effect of the phosphonate ester groups on the behavior
of polyethylene must be considered somewhat speculative because of the effect
of hydrogen-bonded crosslinks which can occur between phosphonic acid groups.
The difficulty in preparing pure polymeric phosphonate esters arises from the
highly reactive nature of the intermediate phosphonyl dichloride and the necessity for maintaining completely anhydrous conditions in order to avoid hydrolysis.
In this paper, the preparation of the pure dimethyl phosphonate derivatives
of high-pressure polyethylene is described. The absence of hydrolysis was
confirmed by infrared spectroscopy and calorimetric analysis. In addition the
preparation of grafted phosphonate esters derived from poly(ethy1ene oxide)
(PEO) oligomers is also described, and the thermal and mechanical properties
of these are also reported. Subsequent publications will deal with the dynamic
mechanical properties and the hydration characteristics of these polymers.
+
4
+
+
+
EXPERIMENTAL
Low-density polyethylene was obtained from the Cities Service Research and
Development Co. It had a density of 0.923 g/cm3, a number average molecular
weight of 20,000, and a weight average molecular weight of 80,000. The polyethylene was purified by precipitation from chlorobenzene, washing with refluxing methanol for 24 hr in a Soxhlet extractor, and subsequently drying at
4OoC in a vacuum oven.
PHOSPHONATE MODIFIED POLYETHYLENE.
I
1411
Phosphorus trichloride of 99% purity was obtained from the Research Inorganic Chemicals Co. and distilled under a dry nitrogen atmosphere directly into
a specially built glass container which was designed so pure samples could be
removed under a nitrogen blanket.
Poly(ethy1ene oxide) oligomers with number average molecular weights of 350
and 750 were obtained from Polysciences Inc. Both polymers were monomethyl
ether derivatives so that they had only one reactive hydroxyl end-group per
molecule. These oligomers were used as received.
Chlorobenzene was dried with phosphorus pentoxide and distilled under a
nitrogen atmosphere. Reagent-grade methanol was used for polymer recovery,
but the methanol used in the esterification reactions was first distilled under
a nitrogen atmosphere and then stored over molecule sieves.
Preparation of Phosphonylated Polymers
The optimum reaction ratio4 of 2O:l (w/w) of phosphorus trichloride to lowdensity polyethylene was charged to a 1liter round-bottom flask equipped with
a gas inlet tube, thermometer, mechanical stirrer, and a dry ice-acetone cooled
reflux condenser. Nitrogen, dried by passing through a column packed with
calcium sulfate (Drierite), molecular sieves, and potassium hydroxide, was
bubbled through the mixture while the temperature was raised to 70°C. Once
the polyethylene dissolved in the phosphorus trichloride, the temperature was
lowered to 6OoC, and the reaction was started by replacing the nitrogen flow with
oxygen. The oxygen was dried by passing it through the same drying column
as the nitrogen. The effluent gases were passed through the condenser, in which
unreacted phosphorus trichloride and phosphorus oxychloride product vapors
were returned to the reaction mixture. From the reflux condenser, the gases
passed through a cold trap immersed in a dry ice-acetone mixture and into a
dilute aqueous sodium hydroxide solution to neutralize the evolved HC1.
The sodium hydroxide solution was periodically titrated to a bromothymol
blue end point with a standardized HCl solution to follow the progress of the
reaction. Tygon tubing, rather than rubber tubing, was used for all flexible
connections because of the inhibiting effect of sulfur and sulfur compounds on
the chlorophosphonylation reaction.14 A poly(trifluorochloroethy1ene) grease
was used instead of a silicone-based grease on all ground glass joints because of
the reactivity of PC13, POCl3, and HC1 with the latter.
The amount of phosphonylation was controlled by the time of the reaction,
and at the completion of the reaction, the oxygen flow was replaced with nitrogen
and the reaction solution was allowed to cool to 30°C. Unreacted phosphorus
trichloride and all of the phosphorus oxychloride product were removed by
distillation at a reduced pressure of 50 mm Hg while dry chlorobenzene was added
to keep the polymer in solution. Distillation was continued until the temperature
of the solution increased to 55°C and remained so for 30 min, at which point only
pure chlorobenzene was being removed.
The chlorophosphonylated polyethylene-chlorobenzene solution was divided
into three aliquots. To aliquot A, a large excess of dry methanol was added
dropwise and the effluent gases were trapped in an aqueous sodium hydroxide
solution. The two poly(ethy1ene oxide) monomethyl ether oligomers were dissolved in chlorobenzene, and the oligomer with a molecular weight of 350 was
1412
WEISS, LENZ, AND MACKNIGHT
TABLE I
Phosphonylated Polymers Prepared from Low-Density Polyethylene
Dimethyl phosphonates
Sample
DSb
A1
A2
A3
A4
A5
A6
A7
0.09
0.35
0.44
2.7
4.5
5.8
9.1
PEOa (350) phosphonates
B1
B2
B4
B5
B6
0.09
0.35
2.7
4.5
5.8
9.1
€37
PEOa (750) phosphonates
c1
0.09
0.35
4.5
5.8
c2
c5
C6
C7
a
9.1
Poly(ethy1ene oxide) derivatives.
Degree of substitution equals the number of phosphonate groups per 100 carbon atoms.
added to aliquot B while the oligomer with a molecular weight of 750 was added
to aliquot C. In all cases, the esterification reaction was allowed to proceed under
a dry nitrogen atmosphere for 48 hr. The polyethylene phosphonates were
precipitated with methanol and washed for 24 hr with refluxing methanol in a
Soxhlet extractor, dried in a vacuum oven at 4OoC, and stored in a dessicator over
calcium chloride. The polymeric phosphonate ester samples prepared in this
investigation are listed in Table I. The degree of phosphonate substitution, DS,
is defined as the number of phosphonate groups per 100 carbon atoms.
Property Measurements
Preparation of Test Films
Film samples used for the characterization of the polymeric phosphonates were
prepared by compression molding. The polymers were heated between layers
of Teflon in a mold cut from either 4 or 10 mil aluminum shim stock and sandwiched between two l/8 in. stainless steel plates. After 10 min at 18OoC, the
sandwich was compressed under 10,000 psi for 30 sec. The sample was then
quenched to room temperature in a waterbath and dried in a vacuum oven at
room temperature. Annealed samples were prepared by heating the polymer
for 24 hr at 20°C below its melting temperature.
PHOSPHONATE MODIFIED POLYETHYLENE. I
1413
In.frared Spectroscopy
Infrared measurements were made using a Perkin-Elmer Spectrometer, model
257. Polymer samples were in the form of films 2 to 4 mil thick, and the scanning
range was from 4000 to 625 cm-l. A normal grid setting and a medium scanning
speed were used.
Differential Scanning Calorimetry ( D S C )
The thermal behavior of the polyethylene phosphonates was studied using
a Perkin-Elmer DSC, model 2. Samples were prepared from polymer films, and
the temperature range scanned was from -160 to 130OC. All measurements were
made at heating and cooling rates of 10°C/min.
The melting temperatures reported were defined as the point of maximum
excursion of the melting endotherm from the base line. For samples which exhibited two distinct endotherms, the melting point was taken as the higher
temperature transition. Similarly, the recrystallization temperature was defined
as the point of maximum excursion of the recrystallization exotherm from the
base line. The apparent heats of fusion A H j of the polymers were calculated
from the areas under the melting endotherms using indium as a reference, and
the area was measured by planimetry. Polymer crystallinity was calculated
relative to the crystallinity of the annealed sample of the unmodified low-density
polyethylene (LDPE) using the following equation:
percent relative crystallinity = ( A H j ) s / (AHj)LDpE
(1)
Tensile Mod u 1us
The engineering stress-strain behavior of the phosphonated polymers was
measured using a Tensilon, Universal Tensile Testing Instrument, model
UTM-11. Samples approximately 100 mil wide and 10 mil thick were cut from
polymer films. The elongation rate for all experiments was 40 mm/min, and the
tensile modulus was defined as the initial slope of the stress-strain curve. A t
least five samples were measured for each polymer, and those samples which
ruptured in the grips of the apparatus or at some obvious defect in the film were
discarded.
RESULTS AND DISCUSSION
Infrared Spectroscopy
The infrared spectra of unmodified low-density polyethylene and samples A3
(D.S. = 0.44), A4 (D.S. = 2.7), and A6 (D.S. = 5.8) are presented in Figure 1. The
absorption bands associated with the phosphorus substituents and their functionality assignments are listed in Table I1 for the methyl phosphonate polymers
(series A).
The presence of the phosphonyl group was confirmed by the P=O stretching
doublet at 1255and 1210 cm-'. The origin of this doublet has not been resolved,
but it is known that both bands are associated with the phosphonyl g r 0 ~ p . l ~
Several investigators have suggested that doubling may be caused by interactions
WEISS, LENZ, AND MACKNIGHT
1414
100
60
20
100
f
60
20
T
100 (70)
60
20
100
u
60
!O
3350
I
2500
1800
1400
1000
625
WAVE NUMBER ( c m - 1 )
Fig. 1. Infrared spectra of (top to bottom): low-density polyethylene, A3, A4, and A6.
with neighboring CH groups.l6 Comparison of the phosphonylated polymers
with model compounds such as methyl phosphonic acid and its monomethyl
ester, Table 111, indicated that the frequency of the main phosphonyl vibration
increased as hydroxyl groups were replaced by methoxy groups. In methyl
phosphonic acid, the P=O function absorbed a t 1120 cm-l which is indicative
of strong hydrogen bonding effects, while in the monomethyl ester derivative,
the same group absorbed at 1200 cm-1, indicating much weaker hydrogen
bonding. The phosphonyl vibration assignment at 1255 cm-l for the polymeric
phosphonate esters is consistent with those of earlier investigations of alkyl
p h o ~ p h o n a t e s , ~and
~ - lthe
~ high frequency of the assignment precludes the existence of hydrogen bonding in these groups. Similarly, it can be seen from Table
I1 that the frequency of the phosphonyl vibrations was independent of the
phosphorus concentration in the polymer, and this result can be interpreted as
evidence of the absence of significant amounts of phosphonic acid concentration
in these polymers.
PHOSPHONATE MODIFIED POLYETHYLENE. I
1415
TABLE I1
Infrared Absorption Peaks and Assignments for Series A Polymers”
A1
A2b
A3
A4
A5
820 (w)
980 (w)
1040 (m)
820 (w)
980 (m)
1040
-(s)
1045 (s)
820 (m)
980 (w)
820
-(m)
980 (m)
810(s)
1060 (m)
1190 (w)
a
940 (vs)
one band
1045(vs)
1055 (s)
1185 (m) 1185 (m)
”
,,
A6
A7b
790 (vs)
980 (sh) 920(vs)
one band
1045(vs)
”
820 (vs)
-
Functionalitv
?
P-0-C
P-0-CH3
P-O-CH3/
P-0-c
P-0-c
I,
1200(vs) 1180(vs) 1190(vs) 1210(vs)
P-O-CH3
Bar denotes broad band (w) weak; (m) moderate; (s) strong; (vs) very strong; (sh) shoulder.
Very thick samples (>6 mil).
Infrared Spectra-Structure
CH3PO(OH)2
TABLE I11
Correlations for Methyl Phosphonic Acid and its Monomethyl
Ester
Wave number (cm-l)
CH3PO(OH)(OCH3)
2850
2350
1650
1320
1180
1120
2660
2300
1710
1320
1200
1150
1050
1015
955
885
770
1000
905
815
735
Assignment
P-0-H
P-0-H
P-0-H
P-0-H/P=O
combination
P-CH3
P=O
P=O
either P=O
or P-O-CHB
P-O-CH3
P-OH
P-OH
P-0-CP-OH
P-OH
P-c
P-0-c
Additional evidence that the number of acid groups in these polymers was
negligible was found by studying the characteristic frequencies of the P-0-H
group. This group normally has four specific absorption bands between 4000
and 600 cm-1:20 2525 to 2725 cm-l, 2080 to 2350 cm-l, 1600 to 1740 cm-l, and
917 to 1040 cm-l. Although several bands were observed within the latter frequency range, notably a t 980,1040, and 1060 cm-l, no absorptions were observed
above 1600 cm-l. Therefore, the vibrations between 980 and 1060 cm-l are
group.
probably not associated with the P-0-H
Absorptions in the region of 1030to 1060cm-l can be assigned to the stretching
linkage.19 In the polymers of low
vibration associated with the P-0-C!
phosphorus concentrations (Al, A2, and A3), two distinct bands were observed.
1416
WEISS, LENZ, AND MACKNIGHT
I 1
i600
I
I
1400
1200
I
I
7
I
I
I
L
I
1000
800
621
WAVE NUMBER (cm-')
Fig. 2. Infrared spectra of (a) poly(ethy1ene oxide) and (b) sample A6.
I '
I
I
1600
1400
1200
I
1000
I
800
I
62
WAVE NUMBER (cm-1)
Fig. 3. Infrared spectra of (a) sample A6 and (b) sample C6.
The exact origin of this doublet is unknown, but it may be due to symmetric and
asymmetric stretching modes of the
9
-c-0-P-0-c-
substituent. At higher phosphorus concentrations, this doublet became a single
broad absorption centered about 1045 cm-l. As in the case of the phosphonyl
vibration, the constancy of the frequency of this absorption indicated that it was
not associated with phosphonic acid groups.
PHOSPHONATE MODIFIED POLYETHYLENE.
I
1417
The presence of the methoxy group in the phosphonated polymers was confirmed by a strong band between 1180 and 1200 cm-1. Previous investigatorsl8>21
have found this frequency range to be specific for the P-O-CH3
linkage.
Many organophosphorus compounds contain an intense absorption near 980
cm-l, but the origin of this vibration has not been determined. Bellamy and
Beecher22suggested that for pentavalent phosphorus compounds the 1030 cm-'
band arises from stretching of the 0-C portion of the P-0-C
group and the
980 cm-l band from the P-0 bond. In a study of the infrared spectrum of
O-ethyl phosphorodichloridate, Nyquist et al.23observed bands at 1013and 1041
cm-l and assigned these absorptions to the C-0 bond. In the present investigation infrared bands were observed at 980 and 1040-1045 cm-l for the phosphonylated polymers (Table 11)and the intensities of these bands increased with
increasing phosphonylation. Although it is apparent that these absorptions are
a direct consequence of the phosphonate group, the bond origins of these bands
are not obvious.
All of the phosphonylated polymers exhibited a distinct absorption at 820
cm-', but the origin of this band is not clear. Investigations of low-molecularweight phosphorus compounds have assigned vibrations between 650 and 770
cm-' to the P-C bond, and the 820 cm-l absorption observed for the phosphonylated polyethylenes may be attributable to stretching of the P-C bond.
It has also been suggested,23however, that an absorption near 800 cm-l may arise
from the P-0 bond.
The infrared spectra of the phosphonates derived from poly(ethy1ene oxide)
did not prove to be as informative as for the methyl phosphonates. The spectra
of a polyethylene-dimethyl phosphonate containing 5.8 phosphonate groups
per 100 carbon atoms (A6) and poly(ethy1ene oxide) are compared in Figure 2.
With the exception of the absorption at 1140 cm-l, the characteristic absorption
bands for poly(ethy1ene oxide) are shielded by the intense phosphorus bands
in the phosphonylated polymer. In Figure 3, the infrared spectra of A6 and C6,
a dimethyl phosphonate and a phosphonate ester graft of poly(ethy1ene oxide)
with comparable phosphorus concentrations, are compared. The spectrum of
A6 showed distinct absorptions at 1250,1190, and 1045 cm-l, while the spectrum
of C6 exhibited only a continuous absorption between 950 and 1250 cm-l. Such
a result would be expected if polymer C6 absorbed strongly between 1100 and
1200 cm-l, which is the region characteristic of C-0-C
~ t r e t c h i n g .Similar
~~
results were found in polymers containing 2.7 and 9.1 phosphonates per 100
carbon atoms indicating the presence of poly(ethy1ene oxide) in the phosphonated polymers with high phosphorus concentrations.
No indication of an absorption near 1140 cm-l was observed for the poly
(ethylene oxide) grafts with degrees of substitution less than 1%. Nuclear
magnetic resonance (NMR) measurements were made for these polymers in
dichlorobenzene a t elevated temperatures using a Perkin-Elmer R-32,90 MHz
NMR. The results were strictly qualitative, but indicated the presence of a small
amount of poly(ethy1ene oxide) in these materials. Apparently a high concentration of poly(ethy1ene oxide) is necessary in order to be detected by infrared
spectroscopy. This is probably a result of the broadening of the polymer spectra
and the loss of resolution due to the phosphorus absorptions.
Empirical correlations between the intensities of the infrared absorptions and
phosphorus concentrations in phosphonylated polymers were not found in the
WEISS, LENZ, AND MACKNIGHT
1418
(CPsO/CCH2)"
XB
Fig. 4. Ratio of infrared absorbances at 1250 and 1465 cm-' vs. phosphonate concentration for
dimethyl phosphonates.
literature. One difficulty in determining such a relationship is the characteristic
broadening of the infrared spectra due to the incorporation of phosphoruscontaining substituents.
Recently, MacKnight and AzumaZ5found a Beer's Law relationship between
the concentrations of phosphonate groups and carbon-carbon double bonds in
phosphonated polypentenamer. Their investigation involved the comparison
of the intensities of the 820 cm-1 infrared band, corresponding to the P-C
stretching mode, and the 1350 cm-l C=C stretching vibration, which served as
an internal standard.
In the present investigation, a linear relationship was found (Fig. 4) between
the ratio of the absorbances at 1250 cm-l (P=O stretching) and 1465 cm-l (CHz
bending) and the ratio of phosphonate and methylene concentrations as deterTABLE IV
Calorimetric Data for Series A Polymers
Annealed samples
Sample
Melting
point
X B ~ ("C)
LDPE
A1
A2
A3
A4
A5
A6
A7
0
0.091
0.35
0.44
2.71
4.50
5.80
9.13
112.6
111.7
108.6
104.4
92.1
76.1
59.6
44.6
Quenched samples
Heat of
fusion
(cal/d
Melting
point
20.0
20.9
15.1
18.5
11.0
4.0
4.8
0.7
112.8
111.8
109.1
104.9
93.6
75.1
59.2
36.9
("0
aNumberof phosphonate groups per 100 carbon atoms.
Heat of
fusion
(caW
Recrystallization
temp.
("C)
12.9
12.2
7.4
8.7
3.3
1.8
1.8
0.2
99.6
95.8
94.5
89
71.9
42.0
37.5
26.0
PHOSPHONATE MODIFIED POLYETHYLENE. I
1419
mined by elemental analysis. The data can be represented by the following
equation:
A125dA1465
(2)
= 0*2o(CP=O/cCH,)
where A represents absorbances and C represents mole fractions. The coefficient
0.20 in eq. (2) is the ratio of the extinction coefficients of the 1250 and 1465 cm-l
absorption bands. The deviation of the highest phosphonylated polymer from
the correlation in Figure 4 is most likely a result of the low transmittance of infrared radiation by this sample, which can introduce significant error in the
calculation of the absorbance ratio. In contrast, attempts to find a relationship
between the 1050 cm-1 (P-OCH3 stretching), 820 cm-l (P-C stretching), and
720 cm-l (CH2 rocking) bands were unsuccessful. This was most likely due to
interference of other infrared absorptions in the neighborhood of these
bands.
DSC Analysis
Several investigators have reported the effect of bulky substituent groups,
The
other than chain branches, on the thermal behavior of p0lyethylene.8,~~+~~
TABLE V
Calorimetric Data for Series B Polvmers
Annealed samples
Sample
X#
LDPE
B1
B2
B4
B5
B6
B7
0
0.091
0.35
2.71
4.50
5.80
9.13
Quenched samples
Melting
point
("C)
Heat of
fusion
(calk)
Melting
point
("C)
Heat of
fusion
(calk)
Recrystallization
temp.
("C)
112.6
112.3
107.4
95.6
76.4
61.3
57.4
20.0
19.5
14.6
10.9
4.4
1.7
0.4
112.8
109.3
105.9
91.8
75.6
61.9
-
12.9
12.7
10.4
3.4
2.2
0.2
-
99.6
95.6
82.9
63.9
40.3
28.2
-
Number of phosphonate groups per 100 carbon atoms.
a
TABLE VI
Calorimetric Data for Series C Polymers
Annealed samples
Sample
Xea
Melting
point
("C)
LDPE
c1
c2
0
0.091
0.35
4.50
5.80
9.13
112.6
111.9
107.4
74.1
61.5
55.8
c5
C6
c7
a
Heat of
fusion .
(cal/g)
20.0
22.7
13.3
5.2
2.2
0.6
Quenched samples
Melting
point
("0
Heat of
fusion
(calk)
Recrystallization
temp.
("C)
112.8
112.3
107.3
75.1
61.2
-
12.9
16.2
9.2
1.7
0.2
-
99.6
96.3
85.5
41.1
28.1
-
Number of phosphonate groups per 100 carbon atoms.
1420
WEISS, LENZ, AND MACKNIGHT
I
I
1
I
I
Fig. 5. Relative crystallinities of dimethyl phosphonates vs. percent phosphonate substitution:
(v)quenched samples.
( 0 )annealed samples,
1201
I
I
I
I
Fig. 6. Relative crystallinities of PEO (350) phosphonates: ( 0 )annealed, (v)quenched; and
( 0 )annealed, (V)quenched.
PEO (750) phosphonates:
most closely analogous study to the present work is that of MacKnight et aL8on
low-density polyethylene modified with phosphonic acid pendent groups. They
found that both the melting point and the degree of crystallinity decreased as
the phosphonic acid content of the polymers increased.
The thermal behavior of the phosphonylated polymers prepared in the present
study are given in Tables IV-VI. Incorporation of the phosphonate groups re-
PHOSPHONATE MODIFIED POLYETHYLENE. I
2.5
0
2
6
4
8
1421
10
- In XA lo2
*
Fig. 7. Reciprocal melting temperatures of phosphonated LDPE vs. -In X, of the mole fraction
of ethylene units (0)
dimethyl phosphonates, ( 0 )PEO (350) phosphonates, and (v)PEO (750)
phosphonates.
sulted in lower melting points and heats of fusion. Polymer crystallinities calculated from eq. (1) are plotted against phosphonate concentration in Figures
5 and 6. As was previously suggested? the decrease in crystallinity with increasing phosphonate substitution was most likely due primarily to the inability
of the bulky pendent groups to cocrystallize with the polyethylene segments.
However, both the melting point and the degree of crystallinity were dependent
upon the thermal history of the polymer, as evidenced by the differences in these
values for the annealed and quenched samples in Tables IV-VI. Therefore, the
decrease in crystallinity is not only caused by the disruption of the crystallizable
polyethylene sequences by the presence of phosphonate groups, but it can also
be attributed to the restrictive effect of these groups on the diffusional motion
of the ethylene segments. The latter effect can be overcome by prolonged annealing below the melting temperature.
With the exception of the highest phosphonylated samples (A7, B7, and C7),
the melting temperatures of the polymers with the same phosphonate concentrations were identical within the uncertainty of the experiment. For the highest
phosphonylated samples (9.1 phosphonate groups per 100 carbon atoms), however, the melting temperatures for samples B7 and C7 were 11 to 15 degrees
higher than for sample AS.
The phosphonylated polymers may be regarded, in a sense, as copolymers of
ethylene and vinyl phosphonate in which only the ethylene segments can crystallize. If, as has been previously reported: the chlorophosphonylationreaction
proceeds randomly, the polyethylene-phosphonates may be considered as statistical copolymers, which should obey the following dependence of melting
temperature on the concentration of crystallizable units:28
1 R
_
---1
(3)
T,
Tm0 ( A H u ) l n X a
where T, is the melting point of the copolymer, T,O is the melting point of the
WEISS, LENZ, AND MACKNIGHT
1422
crystalline homopolymer, X, is the mole fraction of crystallizable units in the
copolymer, and AH, is the heat of fusion of the 100%crystalline homopolymer.
In Figure 7, the reciprocal melting temperatures of the phosphonylated
polymers are plotted as a function of -In X,,where X, = 1 - (DW100). With
the exception of samples B7 and C7, the data for all three phosphonate series
are represented by a common straight line, and the intercept corresponds to the
melting temperature of unmodified polyethylene.
The linear relationship confirmed that these polymers behaved as random
copolymers. The deviation of samples B7 and C7 from this correlation is
probably due to the presence of intermolecular interactions in these polymers,
similar to the occurrence of microphase separation in the systems of low-density
polyethylene modified with phosphonic acid groups in which the associated regions were stabilized through hydrogen b0nding.~9~In this investigation the
microphases could be clusters of poly(ethy1ene oxide) graft segments, which may
be stabilized by crystallization. That microphase separation does occur in the
phosphonates derived from poly(ethy1ene oxide) will be discussed in a subsequent
paper.
The slope of the line in Figure 7 corresponded to a value of AHu of 330 cal/mole
of CH2 units. This result is appreciably lower than the value of 560 cal/mole
of CH2 units determined by the effect of a diluent on the melting temperature
of the low-density polyethylene. It is a common result for polymers, however,
that the value of AH, determined by the copolymer method is less than that
determined by the diluent method.29 Several reasons have been advanced for
this observed discrepancy, and it is generally agreed that it can be attributed
primarily to experimental inadequacies rather than to deficiencies in the theory.
The most significant error is caused by the inability to measure the final small,
but significant traces of crystallinity as the polymer melts. As the crystallinity
of the polymer decreases, the temperature interval over which this small amount
of crystallinity exists increases, and an abnormally large melting point depression
is recorded experimentally. Because the difference between the measured and
the equilibrium melting temperatures increases with decreasing crystallinity,
a low value of AH, results.30
Another source of error in the AHu determination involves the crystallization
technique. One requirement of the Flory theory is that the crystallites be
TABLE VII
Tensile Moduli of Series A Polymers
E
Sample
XBa
(kg/cm2)
LDPE
0
0.09
0.35
0.44
2.7
1000
A1
A2
A3
A4
A5
A6
A7
a
4.5
5.8
9.1
Phosphonate substitution (number/100 carbon atoms).
9ao
780
750
480
210
40
160
PHOSPHONATE MODIFIED POLYETHYLENE. I
1423
composed of long sequences of ethylene units whose lateral development is restricted only by thermodynamic considerations. This morphology can be
achieved only by adopting stringent crystallization techniques (i.e., by slow
crystallization just below the melting temperature), because various imperfections in the crystalline phase can easily be developed by normal crystallization
procedures. The departure from equilibrium becomes more severe as the concentration of noncrystallizable units increase, and this leads to a low value for
m u .
Tensile Modulus
The moduli of elasticity of the phosphonylated polymers are given in Tables
VII-IX. Since the effects of the test rate on tensile properties are complex and
often difficult to interpret, no comparisons were made between the moduli
measured in this investigation and those of other modified polyethylenes reported
in the literature. Instead, comparisons were made only between the properties
of the phosphonylated polymers and the parent polyethylene determined under
identical conditions.
In Figure 8, the elastic moduli for the three polymer series are plotted against
the degree of phosphonate substitution. With the exception of samples A7, B7,
and C7, the data fit onto one curve. Samples A7, B7, and C7 deviated from the
extrapolated curve in an identical manner, which suggested that the high tensile
moduli of these polymers could be attributed to the occurrence of crosslinking
TABLE VIII
Tensile Moduli of Series B Polymers
a
Sample
XBa
E
(ka/cm2)
LDPE
B1
B2
B4
B5
36
B7
0
0.09
0.35
2.7
4.5
5.8
9.1
1000
980
970
480
220
40
100
Phosphonate substitution (number/100 carbon atoms).
TABLE IX
Tensile Moduli of Series C Polymers
E
Sample
XB
(kg/cm2)
LDPE
0
0.09
0.35
4.5
5.8
9.1
1000
920
980
230
40
120
c1
c2
c5
C6
c7
a
Phosphonate substitution (number/100 carbon atoms).
1424
WEISS, LENZ, AND MACKNIGHT
XB
(%I
Fig. 8. Elastic modulus of phosphonated polymers vs. percent substitution: (0)dimethyl
phosphonates, (v)PEO (350) phosphonates, and ( 0 ) PEO (750) phosphonates.
during the chlorophosphonylation reaction. This argument was supported by
the fact that although these polymers could be compression molded into films,
they could not be extruded.
The single-curve behavior indicated that the tensile modulus was independent
of the chemical nature of the phosphonate group. This suggests that the major
resistance to deformation in these polymers is the crystalline polyethylene segments, and the decrease in crystallinity with increasing phosphonylation accounted for the systematic decline of the elastic moduli.
Reding and Faucher31reported that for polyethylene, it is the free volume and
not the crystallinity which determines the tensile modulus. For a series of
polymers prepared from the same parent polymer and subjected to the identical
thermal history, the degree of crystallinity should, however, reflect the free
volume influence. Therefore, a significant, although not unexpected, result of
this investigation was that the free volume of the polymers increased with increasing phosphonate concentration. This suggested that the glass transition
temperatures of these polymers should be affected by the phosphonate content,
and this will be discussed in detail in a subsequent paper.
CONCLUSION
Phosphonate pendent groups can be inserted into low-density polyethylene
in a well controlled manner by free radical chlorophosphonylation followed by
esterification under anhydrous conditions. The absence of phosphonic acid
groups can be confirmed from the infrared spectrum. The result of the inclusion
of the phosphonate groups is to decrease the melting temperature, the crystallinity, and the tensile modulus of the polymers in a predictable way.
The mechanical behavior of these materials can be explained by the disruption
of crystallinity. The thermal behavior of the dimethyl phosphonates and of the
poly(ethy1ene oxide)-graft phosphonates, with degrees of substitution below
9.1, indicate that these materials can be considered as random copolymers. At
PHOSPHONATE MODIFIED POLYETHYLENE. I
1425
a concentration of 9.1 phosphonate groups per 100 carbon atoms, the thermal
behavior of the poly(ethy1ene oxide)-graft phosphonates deviated from that
predicted for a random copolymer. This suggests that strong intermolecular
interactions were present in these systems, although the origin of these interactions was not determined.
The authors are indebted to Mrs. Mary Kaye Angel, Mrs. Amy Currie, and Mrs. Stephanie
Dembroski for the final preparation of the manuscript. This work was supported in part by a grant
from the Diamond Shamrock Corporation.
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Received October 11,1976
Revised March 3,1977