Biomacromolecules 2001, 2, 518-525
518
Modification of Natural and Artificial Polymer Colloids by “Topology-Controlled” Emulsion Polymerization David J. Lamb, James F. Anstey, Christopher M. Fellows, Michael J. Monteiro,† and Robert G. Gilbert* Key Centre for Polymer ColloidssF11, School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia Received December 26, 2000; Revised Manuscript Received March 27, 2001
A diffuse layer of water-soluble polymer chemically grafted to the surface of a hydrophobic polymer colloid has been created by the second-stage polymerization of dimethylaminoethyl methacrylate (DMAEMA) onto the biomacromolecule polyisoprene in natural rubber latex and also onto synthetic polybutadiene and polystyrene latexes. To control the locus of radical formation, the process was initiated by a redox couple wherein one component (e.g., cumene hydroperoxide) is hydrophobic and the other (e.g., tetraethylenepentamine) is hydrophilic. The modified latexes displayed a dramatic increase in colloidal stability at low pH which is attributed to grafted hydrophilic polymer acting as an electrosteric stabilizer; the effect is particularly remarkable in natural rubber latex, which usually has poor colloidal stability for pH j 8. 13C NMR was performed to verify the existence of the grafted copolymer and to quantify yield. The mechanism by which such a novel morphology can be generated is postulated to be via a process of radical formation at the particle surface followed by the subsequent grafting to the hydrophobic polymer backbone and polymerization of hydrophilic monomer in the aqueous phase. This technique is potentially useful for creating novel materials from natural rubber latex. I. Introduction The design of novel copolymers with specific and controlled molecular architectures is both an intellectual challenge (equivalent to the synthesis of unusual molecules by organic chemists) and a procedure with considerable technical potential. Presented here is a study for the development of a new process to produce novel copolymer structures that utilizes the topology of a polymer colloid specifically for the purpose of chemically grafting hydrophilic polymer onto hydrophobic polymer backbones. It is believed that this copolymer is present as a hydrophilic polymer chain extending into the aqueous phase from the surface of a hydrophobic polymer colloid onto which it is grafted. One of the primary targets for this chemical modification is the biomacromolecule cis-polyisoprene from natural rubber latex (NRL). This polymer in its native form has many special properties (such as being essentially 100% cispolyisoprene, which gives a mechanical response that makes it well suited in applications such as surgical gloves). However, it also has limitations, such as a colloidal stability which is very sensitive to pH changes, and the presence of proteins which can cause allergenic reactions.1 There are clearly potential advantages in being able to carry out grafting reactions using NRL in a seeded emulsion polymerization * To whom correspondence may be addressed. E-mail: gilbert@ chem.usyd.edu.au. † Present address: Department of Polymer Chemistry & Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MP Eindhoven, The Netherlands.
to yield a synthetically modified latex, and indeed there are a number of such modifications reported in the literature (e.g., refs 2-15). The “topology-controlled” process reported here combines the particular topology in an emulsion polymerization with the design made possible by free-radical chemistry, in a way that should be applicable to any latex with appropriate graft sites on the seed polymer. The grafting requires an extractable hydrogen, such as present on the backbone of polyisoprene, polybutadiene, and polystyrene. The objective is to create a graft site at the particle-water interface, so that efficient subsequent propagation from this graft site with a hydrophilic monomer is then possible. In addition, a transfer reaction leading to homopolymer in the water phase, and cross-linking of the seed polymer, may also occur. This is shown schematically in Figure 1. To control the locus of radical formation, a redox initiation system was selected consisting of a hydrophilic reducing agent and a hydrophobic oxidizing agent. It was proposed that a couple such as cumene hydrogen peroxide (CHP) and tetraethylenepentamine (TEPA), hydrophobic and hydrophilic species, respectively, will generate radicals mainly at the polymer/water interface due to the partitioning of CHP and TEPA primarily into the organic phase and aqueous phases, respectively (as first adumbrated by Cockbain et al. in the context of graft copolymers between NRL and methyl methacrylate10,11). The grafting process can occur either through abstraction or addition, as shown in Figure 2. Bulky initiator radicals, such as the cumyloxy radical from the present system, favor abstraction compared to addition.16,17
10.1021/bm005654e CCC: $20.00 © 2001 American Chemical Society Published on Web 04/27/2001
Modification of Natural and Artificial Polymer Colloids
Biomacromolecules, Vol. 2, No. 2, 2001 519
Figure 1. Schematic of topology-controlled emulsion polymerization leading to graft copolymer. Water-phase polymerization to form homopolymer of the water-soluble monomer, W, and cross-linking of the graftable seed polymer within the particle phase are also possible.
Figure 3. Structure of DMAEMA.
Figure 2. Abstraction and addition reactions which can lead to grafting in systems with a labile hydrogen such as polyisoprene (shown here), polybutadiene, and polystyrene. I ) initiating radical (e.g., cumyloxy radical).
The primary cumyloxy radical will readily abstract protons from the polymer backbone, creating a grafting site accessible to aqueous monomer. Further addition of hydrophilic monomer will extend the grafted chain into the aqueous phase. A second criterion for effective grafting is that the monomer to be used for the second-stage graft be sufficiently reactive to propagate readily with the radical resulting from the abstraction step. In the present case, that radical is allylic (Figure 2), and hence it is essential to choose the secondstage monomer to be sufficiently reactive: the stability of the allylic radical may mean that it could have a very slow
reinitiation rate coefficient with some monomers. Thus monomers such as vinyl esters, where the radical is reactive but the monomer is not, would be inappropriate, while acrylates and, to a lesser extent, methacrylates are more reactive monomers. The monomer chosen for this study was dimethylaminoethyl methacrylate (DMAEMA, Figure 3), although the process should work with any hydrophilic monomer which is sufficiently reactive with the backbone radical resulting from the grafting reaction. The resulting graft block copolymer could be a simple branch or a comb. In addition to any polyDMAEMA-graft-polyalkenylene chains that may have formed, the initiation process is likely to lead to cross-linking in a polyalkylene seed polymer, and hence an increase in gel fraction is to be expected from the procedure, whether or not the anticipated grafting occurs. Three polymer systems have been chosen for study. Polyisoprene was chosen due to the established ability of
520
Biomacromolecules, Vol. 2, No. 2, 2001
tert-butoxy and benzoyloxy radicals to abstract protons17,18 from its backbone in preference to the addition of such radicals across residual double bonds. As the source of polyisoprene in this work was natural rubber latex rather than a synthetic polyisoprene latex, polybutadiene was selected as a second target to provide a system that avoids the complexities inherent with this natural polymer (high polydispersity, protein groups, phospholipid content, etc.19-21) while displaying analogous chemistry. Polystyrene was also used as it affords the study of a system with slower diffusion rates, particularly with respect to primary radical diffusion. 22 It is noted that the topology-controlled technique discussed here can produce grafts of the hydrophilic monomer at any point along the backbone of the hydrophobic substrate polymer and, hence, is likely to result in a comb copolymer (although the number of side units may well be small, depending on the radical flux and grafting chemistry). It is thus qualitatively different from the well-known emulsion polymerization technique used to produce an electrosteric stabilizer,23,24 which is a (largely) block copolymer formed when hydrophobic and hydrophilic monomers are copolymerized in an emulsion polymerization (rather than the grafting of hydrophilic monomer onto hydrophobic polymer of the present technique). The electrosteric stabilizer is formed by the hydrophilic monomer undergoing homopolymerization in the water phase, until it copolymerizes with sufficient units of the (sparingly soluble) hydrophobic monomer in the water phase that the radical end becomes hydrophobic, enters the lipophilic region of a latex particle, and continues polymerization with the large amount of hydrophobic monomer in the particle interior. Having noted the qualitative difference in synthetic procedures and final molecular architecture, it should also be noted that the present topology-controlled technique will result in a copolymer which can also function as an electrosteric stabilizer. Enhanced latex stability at low pH (conditions under which the unmodified polybutadiene and natural rubber latexes used in this study coagulate) was used as one of the means to test for successful grafting. To verify that changes in latex properties observed after the grafting reaction were due to the presence of chemically grafted poly(DMAEMA), rather that the adsorption of poly(DMAEMA) onto the latex particles surfaces (steric stabilization) or free poly(DMAEMA) in solution (depletion stabilization),24 a series of natural rubber latex samples with varying weight fractions of added poly(DMAEMA), i.e., ungrafted homopolymer, was prepared and acidified, so that the colloidal stability of these mixtures could be compared to those of an analogous supposedly grafted system. II. Experimental Section A. Materials. Butadiene, sodium persulfate, sodium dodecyl sulfate, sodium hydrogencarbonate, and dodecyl mercaptan used in the preparation of polybutadiene were used without further purification as supplied as reagent grade by Aldrich. cis-Polyisoprene was obtained as ammoniated natural rubber latex (“high-ammonia latex”, 60% solids) and used as supplied by RLA Polymers. DMAEMA, CHP, and
Lamb et al. Table 1. Preparation of Polybutadiene compound
amount (g)
compound
amount (g)
butadiene Na2S2O8 SDS
45.08 0.563 6.022
NaHCO3 dodecyl mercaptan water
1.705 0.558 175.01
Table 2. Preparation of Polystyrene Latex compound
amount (g)
compound
amount (g)
AMA-80 NaHCO3 styrene
2.36 0.25 18.32
KPS H 2O
0.26 126.00
TEPA were also used as received without further purification as reagent grade chemicals from Aldrich. B. Preparation of Synthetic Latexes. Because of the difficulties of analyzing for grafts onto natural rubber latex (given the high polydispersity and large number of species present), the topology-controlled methodology was also applied to two synthetic latexes, where analysis of grafting is easier. 1. Polybutadiene Latex. Polybutadiene seed latex was prepared by free radical emulsion polymerization in a modification of the method of Weerts et al.25,26 Sodium persulfate (Na2S2O8) and sodium dodecyl sulfate (SDS) were used as initiator and surfactant, with NaHCO3 as buffer. The use of a chain transfer agent has been found to be essential in the emulsion polymerizations of dienes both for initiation and for control of the degree of branching and crosslinking;25,26 dodecyl mercaptan was used for this purpose. Materials were combined as indicated in Table 1, with the addition of butadiene after the rest of the system was cooled to approximately -60 °C in an acetone/dry ice bath. The reaction vessel was then sealed and polymerization performed in a bottle polymerizer with a rotation speed of 50 rpm and a reaction temperature of 60 °C. This reaction was allowed to proceed for a period of 20 h. The resulting latex was found to have a solid content of 20% and a gel fraction of 47.5%. The gel fraction was measured by allowing the dried polymer to dissolve in an excess of tetrahydrofuran for a period of 2 weeks, before the residual solid and solvent were separated and the solid weighed. 2. Polystyrene Latex. The polystyrene seed used in the present study was prepared using styrene, surfactant (AMA80, sodium dihexyl sulfosuccinate), and buffer (NaHCO3) in the amounts shown in Table 2 and was left for 45 min at 90 °C. The potassium persulfate initiator was added as a water solution and the system allowed to react in a bottle polymerizer at 90 °C for 12 h. Although not measured in this study, this formulation has been shown to reproducibly form particles of approximately 50 nm in diameter; the latex was measured to have a solid content of 14%. (It is noted that, although the polystyrene is glassy under the conditions of the experiment, there is more than enough time for the CHP to diffuse throughout the polystyrene particles in the 1-h settling time and ensuing 2-h feed time used here. This can be verified using the relation between average distance, time, and diffusion coefficient, 〈r2〉 ) 6Dt, with D ) 10-18 m2 s-1 (typical for a monomeric-sized molecule in a glassy
Biomacromolecules, Vol. 2, No. 2, 2001 521
Modification of Natural and Artificial Polymer Colloids Table 3. Modification of Natural Rubber, Poly(butadiene), and Poly(styrene) Latexes amount (g) unmodified latex DMAEMA CHP TEPA NH3 (2.5%)
NRL
poly(butadiene)
poly(styrene)
155.78 15.7 0.60 0.80a 200
75.13 9.585 0.259 0.1627b 6.91d
52.74 5.06 0.1521 0.1688c 50.36
a Made up to a 10% solution with distilled H O. b Diluted to 2.01 g with 2 distilled H2O. c Diluted to 14.5258 g with distilled H2O. d Diluted to 77.86 g with distilled H2O.
polymer27). One thus finds for a radius of ∼25 nm that the diffusion time is ∼100 s, much less than the time over which the experiment is performed. C. Grafting Procedures. 1. Grafting Procedure for Natural Rubber Latex. Natural rubber latex, DMAEMA, CHP, and 2.5% ammonia solution were combined in the amounts indicated in Table 3. The reaction mixture was agitated with a low shear impellor at approximately 100 rpm under a nitrogen atmosphere over a period of 1 h to allow the partitioning of CHP into the rubber particles, before injection of the TEPA solution. The TEPA solution was then added shotwise over 1 h, at 5 min intervals, to initiate polymerization. As with the polybutadiene modification, the system was cooled in an ice/water bath during this process and held at low temperature for the first 8 h of reaction before gradual warming to room temperature and allowed to continue to react for another 16 h. 2. Grafting Procedure for Polybutadiene Latex. The seed polybutadiene latex, monomer (DMAEMA), 25% ammonia solution, and CHP were added in the amounts indicated in Table 3 under a nitrogen atmosphere and agitated in the same manner as for the NRL preparation for a period of 1 h to allow the CHP to partition inside the latex particles. During this period the reaction was cooled to ∼4 °C using an ice/water bath. TEPA was injected continuously using a syringe pump into the system over a period of 1 h. The temperature was held constant during the first 8 h of reaction, after which the system was allowed to warm gradually to room temperature and the reaction allowed to proceed for a further 16 h. In a separate experiment, the reaction was carried out in the same manner as above except the entire reaction was performed at room temperature. 3. Grafting Procedure for Polystyrene Latex. The seed polystyrene latex, DMAEMA, and CHP were added in the amounts indicated in Table 3, and the mixture was agitated under a nitrogen atmosphere at room temperature for 1 h. TEPA was fed in using a syringe pump over 2 h to initiate grafting, and the system was allowed to react for 24 h, again at room temperature. D. Analysis Techniques. 1. Poly(DMAEMA) Adsorption: Colloidal Stability Test. One indication that grafting had occurred as predicted would be to see if the latex has increased colloidal stability at low pH, since grafted poly(DMAEMA) chains should act as electrosteric stabilizers. Since it is likely that some ungrafted poly(DMAEMA) will also form, it is necessary to ensure that any enhanced colloidal stability is not simply due to ungrafted homopoly-
Table 4. Preparation of Poly(DMAEMA) compound
amount (g)
compound
amount (g)
oxalic acid water V50
28 100 40a
DMAEMA thioglycolic acid
50 0.38
a
As a 1% w/w solution in H2O.
mer. Poly(DMAEMA) was prepared using DMAEMA monomer, thioglycolic acid (a chain transfer agent), oxalic acid (for pH control), and V50 (2,2′-azobis(2-amidinopropane) dihydrochloride) as initiator in the amounts indicated in Table 4. The reaction was carried out over 2 h at 55 °C. Addition of this poly(DMAEMA) to natural rubber latex as a representative sample was carried out with the concentrations of poly(DMAEMA) varied between 0 and 5 wt %. Each NRL sample contained 15 g of latex with 30% solids content. The acidification was carried out by dropwise addition of 1% HCl solution to the samples with agitation; the pH was monitored throughout. The amount of coagulation can be measured quantitatively in a number of ways, such as turbidimetry. However, for the present purpose simple visual inspection to see if coagulation had occurred was deemed sufficient, since it was found that dramatic coagulation occurred over a small range of pH. 2. Extraction and Analysis of Grafted Materials. Separation of reaction products for analysis was performed using Soxhlet extraction. The protocol for this procedure was similar for each system. The latex was acidified by dropwise addition of 6% HCl solution and dried thoroughly (in air at 60 °C over 24 h), and a two-stage extraction process was carried out: first an extraction with acidified water and second, after drying the latex, an extraction with toluene. The use of acidic conditions in the extraction has been shown28 not to cause hydrolysis of poly(DMAEMA) under the conditions used here. The time for the extraction was varied from 24 h for modified polybutadiene and NRL to 72 h for polystyrene. Gravimetry was carried out with the modified NRL to see if an increase in water-soluble materials was observed, as expected if significant amounts of ungrafted poly(DMAEMA) were to be formed. There are thus three separate phases for analysis: the water-soluble extraction, which will contain any ungrafted poly(DMAEMA) and any graft material which largely comprises poly(DMAEMA); the toluene-soluble extraction, which will contain any ungrafted polybutadiene and any graft material which largely comprises polybutadiene; the insoluble gel, which by the double extraction process can contain no ungrafted poly(DMAEMA). Even though this homopolymer will be partitioned between the water and organic phases, the double extraction process will remove any poly(DMAEMA) that is not grafted. 1H and 13C solution NMR studies were performed using a 200 MHz Bruker spectrometer on the extraction products of the modified polybutadiene and polyisoprene latexes using D2O and CDCl3 as solvents. Solid-state NMR studies were performed (by Dr. Andrew Whittaker at the Centre for Magnetic Resonance, University of Queensland) using a MSL300 spectrometer. For modified polybutadiene and modified polyisoprene, this was on the fraction of polymer remaining after the extractions had been completed.
522
Biomacromolecules, Vol. 2, No. 2, 2001
Lamb et al.
For the modified polystyrene, the reaction product was first acidified and dried and then subjected to Soxhlet extraction with acidified water for 3 days. The organic Soxhlet extraction product was found to be largely insoluble in CDCl3, in C6D6, and even in styrene monomer. Solidstate NMR was performed on this organic fraction (after the aqueous extraction). III. Results and Discussion A. Latex Properties. The modified polybutadiene and polyisoprene latexes displayed dramatically enhanced colloidal stability on acidification as compared to the unmodified latexes: the modified latexes were colloidally stable at pH 2. This is in contrast to the instability exhibited by the unmodified samples, whereby the addition of just a few drops of 6% HCl solution resulted in coagulation (i.e., unmodified NRL is colloidally unstable when acidified to below pH 10). It should, however, be noted that the modified polybutadiene latex exhibited some flocculation several days after the acidification had been performed. This might possibly be attributed to bridging flocculation29 caused by high molecular weight aqueous poly(DMAEMA) or simply to a decrease in the stability ratio caused by the modification process. It would be interesting to quantify the change in colloidal stability by measuring the stability ratios of the NRL. However, work in this group (by Mr. Fugang Li) has shown that measuring these by the usual technique of plotting the slope of the turbidity vs time curves30,31 at different electrolyte concentrations cannot be used for NRL: reproducible data cannot be obtained, presumably because of the extremely broad polydispersity of NRL. Moreover, it is extremely difficult to separate high molecular weight aqueous poly(DMAEMA) from (modified) small rubbery particles to test the bridging-flocculation hypothesis. This is because separation by dialysis or by ultrafiltration will not completely separate small particles from high molecular weight watersoluble polymer, while centrifugation and subsequent redispersion of the washed centrifugate is impossible because rubbery particles flocculate (irreversibly) when compacted in the centrifugal field. Varying the way in which the TEPA solution was injected into the reaction (in shots or as a continuous feed) vessel did not appear to have an effect on the stability of the end latex to acidification, although increasing the period of time over which the TEPA was injected did appear to result in a larger degree of polymerization of the DMAEMA, based qualitatively on the end viscosity. This would be expected if the TEPA generates radicals that, while unable to polymerize monomer, could be involved in termination reactions. Increasing the time over which TEPA is injected is equivalent to lowering the concentration of this terminating radical species which would enable the poly(DMAEMA) chains to increase in their degree of polymerization before a termination event. It should be noted that the viscosity of a polymer solution is increased not only by an increase in the amount of polymer but also by its molecular weight. Similarly to changing the injection technique, varying the experimental temperature from the initial ice/water bath
Figure 4. 13C NMR spectrum of the modified polybutadiene organic extraction product. Poly(DMAEMA) peaks indicated by the arrow.
temperature to room temperature yielded no observable difference in the stability of the modified latex but did result in a higher viscosity at the end of the 24 h reaction period. This indicates that the formation of poly(DMAEMA) was able to proceed to larger conversions at the higher temperature simply by an acceleration of the reaction processes. With the modified NRL, gravimetry results from the aqueous extraction showed a 4% increase in the amount of water-soluble material. This however is an inconclusive result in terms of solubilizing polyisoprene into the water phase by a grafting reaction, as (a) the nature of such gravimetric measurements is subject to large uncertainties and (b) there is an abundance of extraneous species in the natural rubber latex which may have also been extracted. B. Grafting or Adsorbed Polymer? The unmodified NRL and polybutadiene latex samples before acidification had a pH of 10, and for all cases (0, 1, 2.5, and 5% weight fraction added poly(DMAEMA)), only a few drops of dilute HCl solution (1%) to each small (15 mL) sample was required before coagulum was observed and any noticeable change in pH had occurred. Above 5%, the samples became too viscous to perform the acidification. This indicates that the increase in stability following the latex modification is not due to poly(DMAEMA) adsorbing onto particles or acting as a depletion stabilizer and, thus, provides evidence for the presence of a grafted polymeric stabilizer. C. NMR Studies. 1. 1H and 13C NMR Spectroscopy of Soluble Fractions. 1H and 13C NMR spectra were obtained for the aqueous and organic fractions of the extraction process with the modified polybutadiene and polyisoprene latexes. The aqueous fractions in both cases failed to show any peaks other than those characteristic of poly(DMAEMA). In the case of modified NRL, useful spectra of the organic fraction could not be obtained due to difficulties in achieving sufficient dissolution of polymer samples after the latex had been dried. Figure 4 shows the spectrum obtained for the organic fraction of the organic extraction from the modified polybutadiene. In addition to the peaks expected from cisand trans-polybutadiene, this exhibits peaks at 63 and 65
Modification of Natural and Artificial Polymer Colloids
Biomacromolecules, Vol. 2, No. 2, 2001 523
Figure 5. Solid-state 13C NMR spectrum of insoluble polybutadiene extraction product from single pulse excitation. Arrows indicate poly(DMAEMA) aliphatic carbon peaks.
ppm that are assigned to carbon environments adjacent to nitrogen groups, as expected with grafted poly(DMAEMA). However, the intensities are extremely low and other possibilities such as TEPA-derived radicals terminating on the particle surfaces cannot be ruled out. Spectra were unable to be obtained for the organic extractions from the modified polystyrene latex due to the dried latex exhibiting extremely low solubility in a wide range of solvents. 2. Solid-State NMR Spectroscopy. 13C solid-state NMR was performed for all three modified polymer systems. Single-pulse 13C spectra were measured with a magic-angle spinning speed of 2.5 or 4 kHz with a recycle-delay time of 4 s; cross-polarization 13C spectra were also measured, with a contact time of 2 ms. It is repeated that any peaks corresponding to poly(DMAEMA) in the gel fraction can only arise from grafting having occurred with the seed polymer. Evidence of poly(DMAEMA) was found in all three polymer samples. Figure 5 shows the single pulse excitation 13C spectrum of modified polybutadiene and gives evidence for the presence of both polybutadiene and poly(DMAEMA). The large peaks in the spectrum are characteristic of polybutadiene but the smaller peaks between 50 and 65 ppm and those between 20 and 30 ppm are characteristic of the aliphatic carbons of a methacrylate-type polymer. Upon the basis of the number of aliphatic carbons that each type of repeating unit possesses, this area was integrated to give the relative proportion of grafted poly(DMAEMA) to polybutadiene. This integration indicated that the grafted material comprised approximately 12% of the total material present. The cross-polarization experiment of the gel fraction left after double extraction of the modified polybutadiene, Figure 6, shows the poly(DMAEMA) carbonyl peak at 178 ppm and the quaternary carbon at 45 ppm, further verifying the presence of grafted polymer. The single-pulse 13C spectrum of the modified natural rubber latex sample showed little conclusive evidence of poly(DMAEMA) presence in the sample. However, the cross-polarization spectrum (Figure 7) shows the quaternary carbon as a small peak at 45 ppm and does provide firm evidence for successful grafting. It was also apparent that
Figure 6. Cross-polarization 13C NMR spectrum of modified polybutadiene: (a) carbonyl carbon; (b) quaternary carbon for poly(DMAEMA).
Figure 7. Cross-polarization 13C NMR spectrum of insoluble portion remaining after double extraction from modified NRL. Arrow indicates poly(DMAEMA) quaternary carbon peak.
considerably less poly(DMAEMA) was present in this case than in the modified polybutadiene sample. On the basis of the size of the quaternary carbon peak, observed to be an order of magnitude smaller than that in the modified polybutadiene, the amount of grafted poly(DMAEMA) in NRL is estimated at 1-2%. The amount of grafted material in the modified natural rubber sample as measured by solid-state NMR spectroscopy is a factor of ∼10 less than that for polybutadiene. This result may be rationalized by noting that the amount of grafting onto the latex should be a function of the latexes particle size distributions and proportional to (area/mass) ∼ (radius)-1. The average particle radius in a typical natural rubber latex is ∼1 µm,21,32 while that of the polybutadiene latex prepared with this technique is ∼50 nm. Although NRL is highly polydisperse and contains many small particles, it is the more massive particles that will largely determine the observed amount of grafting. Indeed, evaluating the average of r-1 for the NRL particle size distribution determined experimentally by Li et al.32 gives (250 nm)-1, compared with (50
524
Biomacromolecules, Vol. 2, No. 2, 2001
Figure 8. Cross-polarization solid state 13C NMR spectra of modified polystyrene. Arrows indicate polystyrene aliphatic peaks.
nm)-1 for the polybutadiene latex. This difference of a factor of 5 is consistent with the observed amounts of grafted polymer. Figure 8 shows the 13NMR cross-polarization spectrum of the product from modified polystyrene following aqueous extraction. This spectrum indicates that grafting had been achieved for polystyrene with the poly(DMAEMA) peaks evident at 0-30 and 49-80 ppm. The aliphatic region of the spectra was integrated in the same manner as modified polybutadiene and polyisoprene samples to yield a content of poly(DMAEMA) present in the sample of 35%. The larger amounts of apparent graft polymer in this styrene system compared to the modified polybutadiene and polyisoprene systems can be attributed in part to slower diffusion of primary cumyloxy radicals away from the particle surface to result in an increase in the amount of graft sites accessible to the DMAEMA monomer. Another explanation is a higher rate coefficient for the abstraction and/or subsequent propagation steps. D. Mechanistic Interpretation. The proposed mechanism for the grafting process in given in Figure 1. Control of the loci of radical formation and of subsequent polymerization is considered to be the crucial step in this procedure. To graft a hydrophobic monomer successfully onto a hydrophilic polymer colloid, a grafting site must be created in an area that is accessible to monomer. We postulate that this is achieved by using a redox couple with a highly hydrophilic component and a highly hydrophobic component. Under these circumstances, the majority of primary radicals should be formed at the surface of the particles where the two phases meet and the peroxy radical can create a graft site by hydrogen abstraction from a polymer backbone. Monomer can be grafted by subsequent propagation from this site. The data are certainly consistent with this postulate as a major mechanism. The fact that the polystyrene exhibits more grafting than polybutadiene or polyisoprene suggests (although does not prove) that diffusion of primary cumyloxy radicals away from the particle surface into areas low in concentration in hydrophilic monomer is also an important factor. Polystyrene is glassy at the experimental conditions (swelling by DMAEMA and CHP is unlikely to lower its Tg from 100 °C to the reaction temperature range of 4-25 °C); hence
Lamb et al.
diffusion of radical species in the polymer matrix should be significantly slower than that for the other rubbery systems. The grafting efficiency is of course dependent on rate coefficients as well as on topology. The primary radical species derived from TEPA on reaction with an acyl peroxide is not known, and its effect on the overall reaction scheme is thus hard to quantify. It is postulated here that there is the formation of a nitrogencentered radical, a species known to have slow rates of addition to alkenes.33 This radical is therefore expected to be water-soluble and to act primarily as a terminating species, as has been proposed previously34,35 along with the redox reaction of TEPA with solvated metal ions.36 However it has also been postulated37,38 that a carbon-centered radical may be formed in the R position for similar redox systems such as dimethylaniline with benzoyl peroxide, which may lead to polymerization. Moreover, the role of metal ions in the aqueous phase and their effect on the reaction kinetics have not been elucidated, and it is possible that the species arising from hydrophobic/hydrophilic redox-couple initiation which causes the abstraction may be different from that suggested by the foregoing considerations. IV. Conclusions A technique has been devised which should result in the formation of graft, block, or comb copolymers between a hydrophobic polymeric seed and a hydrophilic second-stage polymer, using the topology of a seeded emulsion polymerization. The latex formed when natural rubber and polybutadiene latexes are modified by seeded emulsion polymerization with a hydrophilic monomer (DMAEMA) is very colloidally stable in acidic conditions; this is good evidence that a steric or electrosteric stabilizer has been attached to the surface of the particles and hence that a graft polymer has been formed. The solid-state 13C NMR spectra of insoluble products left after extraction with water and toluene in sequence, and the inability of free aqueous poly(DMAEMA) to enhance latex stability, show that a significant amount of this is a grafted hydrophilic polymer layer, not adsorbed poly(DMAEMA), and further it is likely that a significant fraction of this grafting occurs at the particle/water interface. It also appears that the diffusion of the initiating species from the particle surface is an important factor in this process. These results support the proposed mechanism for block copolymer formation by radical production at particle surfaces and subsequent polymerization of aqueous monomer into the water phase. This opens the way to novel modifications of polymer colloids, in particular those from natural rubber latex, to make new materials previously unobtainable with current techniques. Optimization of mechanical properties will require control of processes such as the number and size of grafts, and of side reactions such as extraparticle homopolymerization and intraparticle cross-linking, both of which can also arise from the reactions shown schematically in Figure 1. Acknowledgment. Dr. H. De Bruyn is thanked for synthesizing the polystyrene latexes. We greatly appreciate
Modification of Natural and Artificial Polymer Colloids
the collaboration of Dr. Andrew Whittaker for the collection and interpretation of the solid-state NMR spectra. The Key Centre for Polymer Colloids is established and supported under the Australian Research Council’s Research Centres Program. References and Notes (1) Yip, E.; Turjanmaa, K.; Ng, K. P.; Mok, K. L. J. Nat. Rubber Res. 1994, 9, 79. (2) Schneider, M.; Pith, T.; Lambla, M. J. Appl. Polym. Sci. 1996, 62, 273. (3) Lehrle, R. S.; Willis, S. L. Polymer 1997, 38, 5937. (4) Subramaniam, N.; Balic, R.; Taylor, J. R.; Griffiths, M.; Monteiro, M. J.; Gilbert, R. G.; Ho, C. C.; Abdullah, I.; Cacioli, P. J. Nat. Rubber Res. 1997, 12, 223. (5) Fukushima, Y.; Kawahara, S.; Tanaka, Y. J. Rubber Res. 1998, 1, 154. (6) Hourston, D. J.; Romaine, J. J. Appl. Polym. Sci. 1990, 39, 1587. (7) Hourston, D. J.; Romaine, J. J. Appl. Polym. Sci. 1991, 43, 2207. (8) Hourston, D. J.; Romanie, J. J. Appl. Polym. Sci. 1989, 25, 695. (9) Bloomfield, G. F. Rubber DeV. 1952, 5, 34. (10) Cockbain, E. G.; Pendle, T. D.; Turner, D. T. J. Polym. Sci. 1959, 39, 419. (11) Cockbain, E. G.; Pendle, T. D.; Turner, D. T. Chem. Ind. 1958, 759. (12) Blackley, D. C. Polymer Latices. 1. Fundamental Principles; Chapman & Hall: London, 1997. (13) Cooper, W.; Vaughan, G.; Miller, S.; Fielden, M. J. Polym. Sci. 1959, 34, 651. (14) Cooper, W.; Vaughan, G. J. Polym. Sci. 1959, 37, 241. (15) Cooper, W.; Sewell, P. R.; Vaughan, G. J. Polym. Sci. 1959, 41, 167. (16) Moad, G.; Solomon, D. H. The Chemistry of Free Radical Polymerization; Pergamon: Oxford, 1995. (17) Anstey, J. F.; Subramaniam, N.; Pham, B. T. T.; Lu, X.; Monteiro, M. J.; Gilbert, R. G. Macromol. Symp. 2000, 150/151, 73. (18) Elson, I. H.; Mao, S. W.; Kochi, J. K. J. Am. Chem. Soc. 1975, 97, 335.
Biomacromolecules, Vol. 2, No. 2, 2001 525 (19) Ho, C. C.; Kondo, T.; Muramatu, N.; Ohshima, H. J. Colloid Interface Sci. 1996, 178, 442. (20) Chen, S. F. Latex Rubber Anal. 1979, 44. (21) Yeang, H. Y.; Esah, Y.; Hamzah, S. J. Nat. Rubber. Res 1995, 10, 108. (22) Griffiths, M. C.; Strauch, J.; Monteiro, M. J.; Gilbert, R. G. Macromolecules 1998, 31, 7835. (23) Bassett, D. R.; Hoy, K. L. In Polymer Colloids II; Fitch, R. M., Ed.; Plenum: New York, 1980; p 1. (24) Napper, D. H. Polymeric stabilization of colloidal dispersions; Academic: London, 1983. (25) Weerts, P. A.; Van der Loos, J. L. M.; German, A. L. Polym. Commun. 1988, 29, 278. (26) Weerts, P. A.; German, A. L.; Gilbert, R. G. Macromolecules 1991, 24, 1622. (27) Tonge, M. P.; Stubbs, J. M.; Sundberg, D. C.; Gilbert, R. G. Polymer 2000, 41, 3659. (28) Vandewetering, P.; Zuidam, N. J.; Vansteenbergen, M. J.; Vanderhouwen, O. A. G. J.; Underberg, W. J. M.; Hennink, W. E. Macromolecules 1998, 31, 8063. (29) Otsubo, Y. Heterog. Chem. ReV. 1996, 3, 327. (30) Overbeek, J. T. G. AdV. Colloid Interface Sci. 1982, 16, 17. (31) Jayasuriya, R. M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2819. (32) Li, F.; Fellows, C. M.; Gilbert, R. G.; Hunter, R. J. J. Rubber Res. 2000, 3, 70. (33) Maxwell, B. J.; Tsanaktsidis, J., in N-Centered Radicals; Alfassi, Z. B., Ed.; John Wiley & Sons Ltd.: 1998; p 663. (34) Orr, R. J.; Williams, H. L. J. Am. Chem. Soc. 1955, 77, 3715. (35) Minato, H.; Hisada, R. J. Polym. Sci., Part A-1 1969, 7, 735. (36) Beaunez, P.; Helary, G.; Sauvet, G. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1459. (37) Feng, X.-D. Makromol. Chem., Macromol. Symp. 1992, 63, 1. (38) Vazquez, B.; Elvira, C.; San Roman, J.; Levenfeld, B. Polymer 1997, 38, 4365.
BM005654E
