Characterization of Cross-Linked Two-Phase Acrylic Polymers R. P. KUSY, B. J. LYTWYN, and D. T. TURNER Dental Research Center, University of North Carolina, Chapel Hill, North Carolina 27514, USA

Dental materials were made from mixtures of acrylic particles and a cross-linking monomer. The resulting two-phase polymers were tharacterized by quantitative microscopy. Components were analyzed by measurements of sol-gel partition, solution viscosity, and equilibrium swelling. A cross-linked network formed in the particulate phase only after slow polymerization.

Two-phase acrylic polymers are widely used in dentistry and orthopedics because of their convenience of processing in the clinic. This convenience is due to the use of a polymeric powder and a monomer system that, after polymerization, make major contributions to a disperse phase and a matrix, respectively. Progress has been made recently in the characterization of these materials at the microstructural level.' At the molecular level, an important question concerns molecular size. When the monomer yields a soluble polymer, this question can be answered by studies of molecular weight distribution, as exemplified by the recent work of Brauer, Dickson, and Haas.2 However, in many instances the monomer includes a cross-linking agent and consequently yields an insoluble cross-linked network. The objective of the present work is to characterize systems of this type.

Materials and Methods Two commercial materials were used that polymerized rapidly, within five minutes, without external heating. Caulk Repair ResThis investigation was supported, in part, by National Institutes of Health Research Grant DE 02668 from the National Institute of Dental Research and, in part, by National Institutes of Health Grant RR 05333 from the Division of Research Facilities and Resources, Bethesda, Md. Received for publication March 5, 1975. Accepted for publication November 25, 1975.

452

in,a pink fibered (CR) was molded, according to manufacturer's instructions, in the form of a plate 65x62x5 mm from a powder (20 gm) and monomer (10 ml) which was placed under pressure in a denture flask lined with aluminum foil. Sevriton Simplified direct filling resinb (S) was molded in glass tubes (diameter, eu3 mm). A slurry, based on 12 drops of monomer, was used according to a clinical procedure for injecting this material into cavities.3 The third material, a denture base, Lucitone, pinka (L), was molded at 70 C for 16 hours. This procedure was otherwise similar to that used for CR: powder, 21 gm; monomer, 9 ml. The gross initial composition of the materials was determined gravimetrically. In the case of material S, the weight of powder was obtained indirectly by subtraction of the weight of 12 drops of monomer from that of the total mixture. Volume fractions were calculated from densities; p (polymer) = 1.22 gm/cm3 and p (monomer) = 0.94 gm/cm3. The chemical composition of polymer in the powders was investigated by infrared spectroscopy,c after washing the powder with diethyl ether to remove any benzoyl peroxide, by examination of degassed thin films. Inorganic pigments in the powder were estimated gravimetrically as residues insoluble in chloroform. SOLUBILITY AND SWELLING.-Although the S samples were retained in their original cylindrical form, the polymerized specimen plates (CR and L) were machined into 1)rick-shaped samples. The initial weight, w1 a L. D. Caulk Co., Milford, Del. b Amalgamated Dental Tract Distributors, Ltd., London, Eng. e Perkin-Elmer Model 600 Spectrophotometer, PerkinElmer Corp., Norwalk, Conn.

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CROSS-LINKED TWO-PHASE POLYMERS

(0.1 gm) and volume, ¢pl (measured using a traveling microscope) were determined on at least two samples. After immersion in chloroform (50 ml) for one week at room temperature, each sample was weighed (w2) and measured ((P2) in this swollen state. After slowly drying the swollen samples to prevent damaging the insoluble portion (a), the material was pumped in vacuum at 80 C to a constant weight (W3) and volume (fpt). The following were calculated: % insoluble (a) = 100 (w3/w1), (1) % volume increase (gravimetric) = 100 [Pp (W2 - W3) /PSW3]' (2) = volume increase (volumetric) %0 1 00 [ ((P21(Pl)- '](3) The density of the polymer was taken as pp = 1.22 gm/cm3 and the density of solvent as p,= 1.47 gM/cm3. The percent of soluble involatile material (b) was estimated by weighing the residual solid from the extracted chloroform solution. The quantity 100 -(a + b) is a measure of the percent of soluble volatile material such as residual monomer, decomposition products of benzoyl peroxide, and others. NETWORK CHARACTERIZATION.-The number of moles of effective network chains (between cross-links) per gram, Ve, was calculated from Flory's equation4: - [ln (1-v2) + V2 + XlV22] =

(VlvelD) (V21/3-V2/2)

where v2 = (1 + % volume increase/l00)-1. (5) Values of v2 were obtained from tabulated experimental data using equation 5. V1 is the molar volume of solvent = 80.7 cm3 (chloroform); vU, the specific volume of polymer (0.82 cm3 gm-1); and X1 = 0.370, the polymer-solvent interaction parameter.5 MOLECULAR WEIGHT.-Limiting viscosity numbers [X7], dlg-1, were estimated from capillary viscometer data obtained using chloroform solutions of polymer at 20 C. A viscosity average molecular weight, M, was calculated from equation 6, which was established empirically for poly (methyl methacrylate) (PMMA) :6 [-q] = 6 x 10-5 MO.7s (6) MICROSTRUCTURE.-Samples were examined by reflected light (brightfield) with a Zeiss Universal Mlicrocsope.d Where halation was d Carl Zeiss Corp., Oberkochen, W Ger.

453

a problem, surface opacity was increased by vacuum deposition of a Au-Pd layer. The volume fraction of the disperse phase was estimated by standard quantitative microscopy of polished surfaces7 etched by exposure to the fumes of nitric acid.18 Optimum etching time for the materials increased in the order L > CR > S.

Results The particles in the powders are mostly "pearls" of polymer (Fig 1, left-hand frames). Infrared analysis indicates that the polymeric component of the powders CR and S is PMMA (> 97%). Material L is predominantly PMMA (> 85%) but included other chemical groups that were not analyzed. The molecular weights of these polymers are given in Table 1. The value for material L is an approximation because the viscositymolecular weight relationship used (equation 6) was established for pure PMMA. Material L also includes a small fraction of much larger polymer pearls (about 1,000 micrometers [Ltm]) that are not seen in the field of view in Figure 1. Material CR includes pigmented fibers not shown in Figure 1. Material S can be seen to include small particles of pigment that adhere to the surface of the pearls (Fig 1). Amounts of these minor, insoluble constituents are given in Table 1. After polymerization, the etched microstructures of the resulting two-phase materials appear as shown (Fig 1, center frames) . A particularly sharp contrast between the phases is evident in the case of material S. The more complex details apparent in material L are believed to be caused by the longer exposure to acid required to effect contrast by etching. Confirmatory information about microstructure was obtained, without the need for etching, by examination of fracture surfaces (Fig 1, right-hand frames). The fracture mode is not very critical. For example, similar results were obtained either by breaking samples by hand or with a constant strain testing machine.e Details observed on the fracture surfaces of materials CR and L have been discussed previously.9 Estimates of the volume percent of the disperse phase in the final product (v1) e Instron Universal Testing Machine, Instron Corp., Canton, Mass.

KUSY, LYTWYN, AND TURNERD J Dent Res

454

May-June 1976

CR i

LI

Fic.

1.-Microsti-uctuLres

of dental materials. Left-hand frames, powder; center, polished and

etchedl surfaces after polymerization; and s-ight-hiand frames, fracture surfaces after polymerization. wet e

obtained from phiotomicrograplhs

sim-

ilar to those slhown in Figure 1 (center ft ames) by standardl proce(Ittres7 of qutan-titative microscopy. These estimates are com-

pared wiith volume

percents

calculated by

reference to the initial components (vI) in Table 2. The percent of inxolatile two-plhase mate-

rial solulle in clhloroform is given in Table 3 along with its molecular weighit. The percent of insoluble material is also given along witlh estimates of equilibrium swelling alues, expressetl as a percent volume increase, calculated fronms both volumetric anti gi aviimietric dlata. There is a large dliscrepancy between thiese estimates. It is believed that

Vol 33 No. 3

CROSS-LINKED TWO-PHASE POLYMERS

TABLE 1 CHARACTERISTICS OF POWDERS BEFORE MIXING Material

CR

M, Polymer

2.75

+

% Insoluble (by weight)

0.14 X 105 (2)

xxx

2.4 ±+0.6 (2)

(with fibers present)

3

0.3 (1) (with fibers re-

+1

+1 +1

0

00

e

os

moved) 0.04 x 105(2) 0.02 x 105 (2)

2.73

L S

1.22

0.7±0.9(3) 6.8±2.2 (4)

Note: The data are reported as a mean (x) ± standard deviation (SD) with the number of tests specified in parenthesis. * Reflects range for a variety of shades provided by manufacturer.

~

00

00

+

z

>

in the volumetric method the bulk expansion represents the true volume expansion despite the partial dissolution of the specimen, whereas in the gravimetric method, the results yield an overestimate on account of the solvent trapped in the uncollapsed holes left after extraction of the sol. Microscopic evidence for the formation of holes is presented for materials CR and S. Examination of the microstructure of extracted samples, having prepolished surfaces, after complete removal of solvent indicates the presence of holes in materials CR and S. These holes correspond in size and shape to the particles originally present in the disperse phase (Fig 1, center sections, and Fig 2, left-hand sections). In order to achieve good resolution with material L, it was necessary to repolish and etch the dry extracted samples. In this case, the results show retention of a disperse phase, the volume of which is composed of -60% fine voids that are dispersed throughout the composite. This is in contrast to CR and S in which -60%

+I +I +I t1( rto

cc-co O

+0 E

+I +I +I

y

~+I1+I1+ I

TABLE 2 COMPOSITION OF Two-PHASE MATERIALS

0

Volume

Material

CR L S

% Powder in Initial Mixture, (v )

62 66

0.5 (4) 1 (4)

54

1

(5)

cc

Volume % Disperse Phase

(vi) 45 1 (3) 71 4 (4) 42 9 (67)

Volume % Change (100 (vfv) /Vi )

-27 +8

-22

Note: Data are reported as a mean (x) ± standard deviation (SD) with the number of tests specified in parenthesis.

11

cc; o6

455

I_ I1 l|t10Mm KUSY, LYTWYN, AND TURNER

456

J Dent Res May-June 1976

C R

i

L

I -Ii--_

-4

E

~~~~~~~~Om

I oom

hb&¶4ASt',lOom

FIG 2.-Microstructure after cold extraction and removal of solvent. Left-hand frames, free surface, initially; right-hand frames, free surface, after polishing and etching. (Note: S coated with Au-Pd to reduce lhalation.) (Polishing artifact caused the "bowling-out" of holes in CR).

gross voids are occluded in the regions in whiich the particles once were (Fig 2, righlthand frames, and Table 3). Further evidence that the disperse phase is extractedl in materials CR and S leaving holes but retained in material L was obtained by similar experiments ou- samples with a fracture surface (Fig 3) It is interesting to note that details of fracture markings are retained despite pi olonged extraction andl dirying. Rib) markings are visible on the matrix of material CR, and parabolic markings can be seen on the disperse phase particles in material L. The definition and

significanice of these types of markings in the fractography of two-phase materials have 1cen dliscussecd previously.9 In a final experiment, exti-eme measures were takeni to rule out any objection that the retention of a disperse phase in material L mighit be (dtue to inadequate extraction. SamIles were continuously extractedl with hot benzene in a Soxhlet apparatus for 30 (lays. T his drastic treatment cauised disintegration of the samlles. Notwitthstanding, the soluble fractioni (100-at) was similar to that measuredl by the stand(lard cold extractiornmethod: % soluble: L, 61.7 --4 1.8 (6) (cold) and 64.0

Vo l 55 No 3

CROSS-LINKED TWO-PHASE POLYMERS

457

isotropic specimens, but, its use can be extended to includle materials witlh pores, such as CR and S, using volumetric estimates of swelling. Estinmates of concentrations of effective chains are compared in Table 4 witlh valutes reported previously for samples of cross-linked PMMA. Evaluating ve over the range of the "mean % volume increase ± 1 SD"; the resuilts show that the mole % of cross-linking agent in CR and S appears to he somewlhat higher than 1%. Such a value accounts for the ease with whichi the insolulle network could be handled despite the large soluble fraction since all the cross-links were concentrated in -5935% of tlhe volume (Table 3).

50u m _z(~ Fic. 3.-Microstructure of fracture surface aftea coldi extraction andl removal of solvent. ToP, CR; center, L; bottom, S; R, riblike features; H, holes. (Note: S coated with Au- Pd to seduce halation.)

1.1 (2), (hlot) ; CR, 66.9 + 0.7 (5) (cold) andl 68.4 -+- 1.3 (2) (hot). Moreover, retenition of a disperse plhase could again he demonstratedl by repolishlinig and etclhitng fragments of the extractedl material after (Irying (Fig 4). 1lie insoluible netwoi-k left after extraction can

he

clhatacterizeI

partially at the molec-

Lular level from swellin-g data by use of Flory's equlbation 4. Thlis celuation was derined for

Discussion The disperse plhase is formed from the particles originally present in the powder. Neglectinig minlor constittuents such as pigments, thiis phase is formed from polymer pearls. It was shiown previously, in a preliminary study of the material CR,' that these pearls are partially dissolved, and this find3ing is confirmed in the present work and also extended to include material S. The evidence is tlhat the volume fraction of the splherical (lisperse phase is less than that of thie polymer pearls in the initial mixture of powder and monomer (Table 2). In contrast, in the case of material L it is necessary to account for an effect in just the opposite sense, that is, for an increase in the disperse phase. It is suggested that partial dissolution also occurs in this instance but that it is more than offset by an increase in the voluime of the pearls because of imbibition of monomomer. This explanation is reasonable b)y referenice to the mtihli slowet rate of polymeriizationI (lutring the formation of L an-d, lhence, to the more prolonged contact of its pearls vwitli monomer. Flls imbibition of moniomer into the particulate plase accounts for the lower chemical reactivity between pltases whiclh results in longer exposure times to effect contrast by etching. In considering the distribution of crosslinked (gel) andl soluble (sol) polymer betweeni the two )hases, there is one simplifying featture in that the sol appears to be due maitnly to polymer originally present in the pearls. Conyversely, the gel is formed by polymerization of the monomer cross-linking system. The evidence for this statement is, first, that the soluTble fraction (1.00-a) (0.67,

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Characterization of cross-linked two-phase acrylic polymers.

A cross-linked network was formed in the particulate disperse phase carried over from the soluble pearls only in one material tested (L). The preparat...
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