Studies of polymerization and crosslinking of aqueous acrylamide

Radiat. Phys. Chem.Vol. 32, No. 6, pp. 793-796, 1988 Int. J. Radiat. Appl. lnstrum.Part C

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STUDIES OF POLYMERIZATION AND CROSSLINKING OF AQUEOUS ACRYLAMIDE J. ROSIAK,I K. BURCZAK,l W. PI~KALA,N. PI~LEWSKI,2 S. IDZIAK2 and A. CHARLESBY3 qnstitute of Applied Radiation Chemistry, Wr6blewskiego 15, 93-590 lf~Sd~,Poland 2Institute of Molecular Physics of the Polish Academy of Science, Smoluchowskiego 17/19, 60-179 Poznafi, Poland 3Silverspring, Eagle Lane, Watch_field,Swindon SN6 8TF, England

(Received 10 October 1987) Abstract--The two radiation-induced reactions of polymerization and crosslinking have been followed by various techniques including T2 pulsed NMR, solubility (conversion rate) and intrinsic viscosity [r/]. The monomer is acrylamide (20%) in water (D20). Polymerization is largely completed at the very low dose of 0.03 kGy, and the molecular weight then further increases as shown by T2 and [q] measurements. The indirect effect due to water is deafly seen. The dose for incipient network formation is greater (Dg = 0.18 kGy) and the soluble fraction then decreases according to the Charlesby-Pinner formula. The T2 relaxation time shows at first a rise due to the reduction in non-polymerized monomer, and then falls as the Mc between successive crosslinks diminishes with increasing dose. This behaviour is also shown in irradiated aqueous polyethylene oxide samples. INTRODUCTION Investigations of the spin-spin nuclear magnetic relaxation times, T2, in the polymer systems provide much information concerning their supramolecular structure, i.e. the degree of crystallinity(j) and crosslinking densityc2) as well as polymer chain entanglements 33) The increase in temperature causing a change in the mobility of some fragments or whole macromolecules is reflected by an increase in the spin-spin relaxation time /'2. The relaxation time T2 for polymers may be determined experimentally using the analysis of the decay of free precession signal or the decay of the spin echo sequence. Depending on polymer morphology, different modes of spin-spin relaxation are observed. For rigid systems or polymers of a high degree of crystailinity, the corresponding curves are described by Gaussian equation:

[

A(t)= A(O)exp - ~

,

(1)

where .4 (0) denotes the amplitude of the free precession signal after switching off the field of a high frequency; and T2 is the spin-spin relaxation time. For polymers of low molecular weight and great flexibility of polymer chains, the shape of the signal of the free precession can have an exponential character:

A(t)=A(O)eIp(--T2 ),

The times T2 found in polymers of a high degree of crystallinity are of the order of some tens of microseconds, but for polymers having a low molecular weight they increase up to milliseconds and even to seconds at high temperatures. Under these conditions, for their estimation the pulse sequence as suggested by Carr-Purcell-Maiboom-Gill (CPMG) (41 is used. In this method a sequence of spin echoes gradually decaying is formed, being characterized by a decay constant equal to the relaxation time T2. Polymer systems treated with ionizing radiation show a number of new physical properties, due to the significant modification of their supramolecular structure caused by the degradation and/or crosslinking, and these can be traced with the T2 relaxation shown by the specimen. For the polymers modified in this way to give a partly crosslinked network, a complex course of a C P M G echo sequence is observed which is quite well described by the formula with several distinct relaxation times:

(2)

where A (0) and T2 have the same meaning as in equation (1).

A (t) = Af(O)exp(- ~-~L) + A, _ f(0)exp(-- ~2s),

(3)

where Af(0) denotes the amplitude dependent on the number of protons assigned to the mobile fraction, of longer spin-spin relaxation time, T2L; A1_f(0) denotes the amplitude connected with the network forming fraction (through the chain entangiements or permanent crosslinks), of shorter spin-spin relaxation time, T2s; A (t) is the amplitude of the consecutive spin echoes; and T2L and T2s are the corresponding spin-spin relaxation times, and 793

794

J.

ROSIAKel al.

often differ by a factor of at least 10. When no network due to crosslinks or entanglements is present, only a single T2 relaxation is observed. Information on the N M R / ' 2 relaxation of aqueous solutions of water-soluble polymer is sparse. In the literature there is only one report on the N M R behaviour of water molecules (self-diffusion coefficient and T2 time) in polyacrylamide gels.(5) In 1980, Charlesby and Folland (6) reported on the dependence of relaxation time/'2 for aqueous solutions of polyethylene oxide (PEO) following a series of doses of ionizing radiation (y from 6°Co) up to 12 kGy. These studies were carried out in the concentration range 0.2-10% (w/w) of PEO in heavy water, at a temperature of 283K, for a polymer of J(-/,=3.4 x 104. These authors observed that the characteristic relaxation time 7"2 was reduced inversely in proportion to the radiation dose; but was always long. There did not appear any very marked transition in 7"2 at the gel point, nor the presence of two components corresponding to the occurrence of both a network and of a non-network component. This paper presents similar but more detailed results of N M R studies on the spin-spin relaxation of polyacrylamide solutions in heavy water obtained by 6°Co 7-irradiation of a series of aqueous (D20) solutions of acrylamide monomer, as well as of polymer formed. These two processes of polymerization and crosslinking of the polymer form two very distinct sections of this research programme and are discussed separately. It must be noted that the spin-spin relaxation of the network has a T2 relaxation time far longer than those observed in most flexible polymer networks, and this might be ascribed to the aqueous environment. Moreover, there is little evidence for a double-exponential decay [equation (3)] typical of a partial-network polymer, in which the shorter time decay T2s corresponds to a network, while the much longer time decay T2L characterizes the non-network molecules. EXPERIMENTAL

For these studies, acrylamide obtained from Fluka A G (Chemische Fabrik, 9470 Buchs) was used without any additional purification. Acrylamide solutions were made by dissolving in heavy water (99.7% D20) obtained from the Center of Reactors and Isotope Production (Swierk, near Warsaw). Aqueous 20*/0 acrylamide solutions were irradiated after previous saturation with gaseous Ar purified from 02 at a dose rate of 0.117 Gy/s. The measurements of T2 relaxation times were carried out by means of a Brucker pulse N M R spectrometer (type SXP 4/100) operating in the magnetic field Bo = 2.1 T. F o r these studies the method of C P M G was applied. The sequence of spin echoes was registered in the memory of the computer and then plotted on recorder paper. The amplitudes

O.05

1 ooo

O.O4

8OO

a03

600

0.02

400

\A Dg

O.Ol

0.0

1

I

I

0.05

0.10

0.15

E kN

/,0.2O

.~00

Dose (kGy)

Fig. 1. Relation between reciprocal intrinsic viscosity [r/]-1 and /'2 with respect to absorbed dose. 20% aqueous acrylamide solution saturated with Ar.

of appropriate spin echoes were then replotted on a semi-logarithmic scale according to formula (2). The temperature of the samples was adjusted and stabilized with accuracy _+ l K by means of a device with a flow of N 2 gas. The content of the crosslinked fraction (gel), g, was determined by a long-term (>>24h) extraction of samples with hot distilled water. The gel fraction in the samples was defined as the ratio of weight of extracted dry gel to total initial monomer weight. Intrinsic viscosity [r/] of the polyacrylamide in irradiated samples was determined using a modified Ubellohde viscometer (H20, at 298 K). Dynamic viscosity of polyacrylamide solutions [r/.p] was estimated by means of an U N I P A N ultrasonic viscometer, type 508. RESULTS

Results up to the gelation dose (O-O.18kGy) In this first dose range, the relaxation curves of the irradiated monomer are exponential in character, and the characteristic T2 relaxation time decreases rapidly with radiation dose once a fair amount of polymer has been formed (Fig. 1). Measurements of inverse intrinsic viscosity [q]-t show a somewhat similar fall (Fig. 1). The molecular weight M corresponding to [r/] = 6.8 x 10 - 4 ~ / ,0 6 6 can be calculated for a range of doses up to 0.18kGy, and the product T~.M is approximately constant (Table 1). A similar relation has been reported in other (non-aqueous) irradiated polymeric systems, and may be taken to mean that T 2 measures the mobility (i.e. the ease of motion) of molecules in this regime, in the same way as does the intrinsic viscosity. Whether this change in T2 is due to the higher molecular weight of each polymer molecule formed, or to their number, or to the smaller monomer fraction has not been settled. The degree of conversion to polymer is shown in Fig. 2. It is seen that for a dose as low as 0.05 kGy, 75% of the initial monomer has been polymerized. It

Polymerization and crosslinking of aqueous acrylamide

795

Table 1. Dose, T, and intrinsic viscosity [7] up to the gel point (kGy)

r2 (ms)

(g/ml)

( x 10-')

r~ x g/'n

0.06 0.07 0.09 0.14 0.155

1080 1080 1020 725 600

0.040 0.041 0.040 0.019 0.015

8.3 8.0 8.3 25.5 36.5

9.6 × 1012 9.3 x 10]2 8.6 × 1012 13.4 x 1012 13.2 x 1012

Dose

[~]-'

~.

must be assumed that higher doses are largely required to link these molecules together to form an incipient network, which occurs at 0.18 kGy. It is in this region (0.05-0.15kGy) that the T2 value falls most rapidly and [~/] rises. The dynamic viscosity also rises rapidly from this dose of 0.05 kGy, to reach a substantial limit (taken as 100%) near the gelation dose Dg.

T~ x [7] 29.2 x 28.5 x 26.0 x 27.7 x 24.0 x

=E .E lOO

1oo ls

E

80 A

¢L

o

8o

60

/

Results on T 2 beyond the gelation dose (>O.18kGy) It is at the gelation dose D s that the network structure first begin to appear, denoted by an insoluble fraction (gel), and theoretically corresponding to an average 6 of one crosslinked unit per weight average molecule. F o r a r a n d o m distribution of crosslinks between the polyacrylamide molecules formed from the polymerized system, the relation between sol fraction s ( = 1 - network) and dose D s can be written in the well-known form

s + ~ s = 2Ds/D.

(4)

This relationship is accurately followed for the acrylamide polymer formed by the prior polymerization reaction, outlined above, and taking D s = 0.18 kGy if

0.2 I

0.3 I

I06 I06 106 l06 l06

•

I I I

u

20 "~

c

8 0.0

~--'~°~° I o.1

:f/'~g 0.2

I 0.3

Dose (kGy)

Fig. 2. Dose dependence of degree of conversion and of dynamic viscosity [r/.p] of 20% aqueous acrylamide solutions.

the very small dose needed for polymerization is ignored (Fig. 3). The table above the figure shows the dose and corresponding value of 2Ds/D(calculated); also the experimental values of g = l - s, as well as s + x/~ deduced therefrom. The close agreement between 2Ds/D(calculated) and s + ~/s (experimental)

0.4 I

0.5 0.6 I I

0.8 1.0 I I

0.325 1.108

0.55 0.854

0.80 0.45

0.44 0.58 1.308 1.068

0.77 0.710

0.85 0.537

2.0 I

D (kGy) 0.275 1.309

0.2 1.8 0.18 1.726

2.0 ~

1.75 d o n D (kGy) 0.2(~ 2Dg/D (DO. 0.10 kGy) 0.90 gel. I - $ experimental 0.416 s + ~

from g obove

[~ 1.8 + 1.6 1.4 1.2 1.0 0.8 0.6 0.4

e ~ , ~

0.2 2.0

I

I

I

I

i

~.o

~.e

~.4

~.2

~.o

I o.0

I ae

I 0.4

I 0.2

I ao

2DolD Fig. 3. Relation between sol fraction and absorbed dose for 20% aqueous solution of acrylamide (saturated with Ar) assuming D s = 0.18 (O) and 0.19kGy (O).

796

J. ROSIAK et al.

1000

.v.PEO

lOO

99%

that T2 is therefore a measure of Me, i.e. of the reciprocal of the crosslink density. As such it could be used to characterize the network structure, as made or later modified by radiation. The explanation of the much higher /'2 values in these systems might be ascribed to the presence of water, greatly increasing chain mobility or proton reorientation.

•

CONCLUSIONS

E

I.

og lO

II ~2 I -1.2

II I I I Ill 0.4Q5 1 2 4 8~12 D(kGy) I I I I I I I I I -1.6 0 0.4 ~8 LogD

Fig. 4. Dose dependence of T: for polymers obtained from 20*/, aqueous acrylamide solution (PAAm) and 2% aqueous solution of polyethylene oxide(s) above gel point. Percentages show calculated gel fraction in PAAm. The rapid decrease of T2 in PEO occurs when little soluble fraction remains.

shows that the random distribution of crosslinked units, in proportion to the dose absorbed and a random molecular weight distribution of polymer molecules, are valid assumptions. Figure 4 shows the T2 values for the series of samples from the gelation dose onwards, both for the polymer formed from the 20% monomer solution of acrylamide, and from the polyethylene oxide. These data differ from the T2 usually observed in irradiated polymer systems in several ways. In the first place, the T2 curves form a single exponential curve vs time and show no evidence of the two distinct components (sol and gel) seen in most systems. In the second place, the /'2 times are of the order of hundreds of milliseconds, typical of single highly mobile macromolecules, and not a few milliseconds of the usual Mc network structure. This must mean that the chain motion in the network is very different and more akin to that of separate molecules. Nevertheless there is an effect of the sol-gel distribution, in that/'2 increases in time as the sol fraction diminishes up to 0.4 kGy. Beyond the dose at which the soluble fraction plays a major role, it may well be

In the range of 6°Co 7-radiation doses involved in acrylamide polymerization in an oxygen-free atmosphere, the spin-spin relaxation time T2 decreases with dose, corresponding to an increased molecular weight and an increased polymer fraction. T2 varies approximately inversely with the molecular weight of polymer, and the product T~.[~] is approximately constant, where [q] is intrinsic viscosity. 2. Once the gelation dose has been passed, but there is still a fair amount of monomer left, the relaxation time at first increases with dose. This is interpreted as primarily due to a reduction in monomer concentration, together with an increase in the network fraction. At higher doses, when there is almost no monomer, any higher dose results in a decrease in Me, a stiffer polymer network, and a reduction in T2--as is seen in other crosslinking polymers. 3. Spin-spin relaxation times of the order of many milliseconds determined for crosslinked aqueous polyacrylamide solutions (in D20 ) show that their structure is composed of flexible and relatively long polymer chains. Acknowledgement--This work has been carried out within the framework of problems CPBP 01.19 and CPBP 01.12.

REFERENCES

1. B. J. Bridges, A. Charlesby and R. Folland, Proc. Roy. Soc. 1979, A367, 343. 2. A. Charlesby, 5th Symposium on Radiation Chemistry. Akademiai Kiado, Budapest, 1982. 3. A. Charlesby and B. J. Bridges, Radiat. Phys. Chem. 1982, 19(2), 155. 4. T. C. Farrar and E. D. Becker, Pulse and Fourier Transform NMR. Academic Press, New York, 1971. 5. B. Nystr6m, M. E. Moseley, W. Brown and J. Roots, J. Appl. Polym. Sci. 1981, 26, 3385. 6. A. Charlesby and R. Folland, Radiat. Phys. Chem. 1980, 15, 393.