Light scattering studies of DGEBA-anhydride epoxy resin heterogeneity

V0lumc

number 1

7 I,

CHEMICAL

LIGHT SCATTERING

PHYSICS LETTERS

STUDIES OF DGEBA-ANHYDRIDE

1 April 1980

EPOXY RESIN HETEROGENEITY

G.C. STEVENS Cerrtral Elccrrrc~r_v Research

Laboratones.

Leatherhead.

Surrey

UA*

and J.V. CHAMPION,

P. LIDDELL

Pli~srcs Deparrmerrr.

CII_I of London

Recewcd 12 November

1979.

and A DANDRIDGE Po~~vechnrc.

III fmal form

London

18 January

EC31V ZEY.

UK

1980

Ra>lclgh scattermg. Brllloum spectroscopy and infrared spectroscopy oian unreacted and reacting tiglycldyl ether of blsphenol A-phrhaix nnhldnde epox) resm system has been undertaken to assess bulk heterogeneity and its reconabatlon with SO-200 nm surface heterogenerty. h¶olccular aggregates earst m the unreacted resm prowdmg a possible mhomogeneous reactton mechanism alth lmphcatlons for bulk propertles

1. Introduction Local order and structure m amorphous glassy polymers remain controversial topics Bulk scattering mvestigatlons, mcludmg neutron. small-angle X-ray (SAXS) and hght scattering, in a variety of polymers Indicate an absence of significant nonthermodynamlc density fluctuations [I] . However eiectron and optlcal microscopy of the same polymers support the existence of surface heterogeneity wth dlmenslons ranging from 10 nm to microns [ 1,2] . Extensive muzroscopic investlgatlons of a number of glassy crosslinked epoxy resm systems also mdicate surface heterogeneity [3-6]_ Indirect evidence from ddferential scannmg calorimetry (DSC) and thermomechamcal analysis [7,8] IS also clalmed to support bulk heterogeneny but the endothermic peak observational evidence of the former techmque may be due to excess enthalpy effects [l] . Similar evidence exists for hnear polymeric glasses A dlstmct lack of bulk scattermg analysis of epoxy resm systems is apparent. However, a recent SAXS study [9] compared an epoxy system with polymethylmethacrylate and polystyrene and suggested that fully cured epoxy resms do not display different scattenng characteristics to other common glassy polymers. The need to elucidate chemical structure-morphology-physical property relatlonshps in this and other technologically important glassy crucial and WC need to apply several techmques to 011s problem. In tlus letter prehmmary refractive mdex, Raylelgh scattermg, Brllloum spectroscopy and infrared absorption spectroscopy studies of a multi4igomer epoxy resin system are reported whch were almed at assessmg the chemical and physical nature of the unreacted components. the reaction mechamsm and the tinal matrix structure. Mlcroscopy, DSC, SAXS, wide-angle X-ray scattering and small-angle hght scattermg are also under study. The former mdlcates that fracture surface heterogenelty IS present m the system reported here and increases from 50 nm to 200 nm m progressmg from partially to fully cured materials [IO]. The material considered here IS the Cuba-Geigy CT200 (resin) plus HT901 (hardener) system. CT200 IS a particular dlglycidyl ether of blsphenol A type (DGEBA) epoxy resin which exhibits a molecular weight distribution typlcal of a taffy process resin [ 1 l] . poiymers

104

IS

Volume 71, number 1

1 April 1980

CHEMICAL PHYSICS LETTERS

/O\

CH2-CH-CH2 k

CH3

CH3

contaming srmdar k = O-4 &epoxrde oligomer mass fractions and a hydroxyl group content of 2.1-2.2 e.g. kg-’ and an epoxide group content of 2.3-2.5 e g. kg-‘. The HT901 crosslinkmg component is phthahc anhydride (PA). Both components were repeatedly faltered through 0.2 pm PTFE filters at elevated temperatures in dry nitrogen to remove dust and particulate lmpurltres mcludmg dtcarboxylic acid impurities in PA. PA was recrystalhsed between filtrations CT200 was subsequently exhaustively stured and evacuated at 393 K to degas and remove voiattle rmpuntres which were prmcipally water and smaller quantities of mrxed oxygenated solvents and residues. A CT200 HT90 1 mass ratio of 100-30 was used givmg an epoxrde-anhydride reaction stoichiometry of 1:0.85, au optimum stoicmometry for this system [ 121. Component mrxmg was carried out m the liquid phase at the reactron temperature of 398 K and was followed by rapid stunng and partral evacuation. Prior to and during reaction Abbe refractometer, Sofica photometer, Brdloum spectrometer (single pass Fabry-Perot etalon) and infixed absorptron spectroscopy observatrons were undertaken.

2. Unreacted

DGEBA observations

A DSC glass transrtron temperature of 297 f 2 K was determined for CT200 so liquid phase variable temperature photometer and Brllloum spectral observatrons between 298 and 443 K were undertaken to assess local order_ Also, room temperature tissolutron of CT200 m chloroform was undertaken. Fig. 1 dustrates the temperature dependence of the lsotroprc and amsotropic contrrbutions to the Rayleigh ratio (R9uo corrected for refractive index and cahbrated agamst benzene) obtained from photometer data and the Landau-Placzek ratio IR/2Lu where 1, is the Brillouin peak intensity and I, is the Rayleigh peak intensity corrected for depolarized contnbutions. En contrast to a non-associated liquid whose isotropic Rayleigh rarlo RgO~,tS and fR/21B would increase with increasing temperature due to increased thermodynamic density and concentration fluctuations all three parameters a iow molecular weight liquid vahe. In con(R900. IS3 R90°, AN and fR/21B) decrease wrth IR/21B approaching IX ld)

8 ‘RR’6 6

(A)

4

I 20

1 40

1 60

I 80

I 100

I 120

I 140

I 160

I 180

T ‘C

FIN. 1. CT200 uetroplc and anisouoprc

Raylergh ratio and Landau-Placzek

ratio temperature dependence.

105

Volume

7 1. number

I

CHEMICAL

PHYSICS LETTERS

lApnl1980

O-J I

100

1

80

8

I

60

40

% DCEBA

Flp;. 2

Isotropic and amsotroplc

behawour

of the dlssolutron

1

8

20

0

MASS

of CT200

m chloroform.

Dotted

lme suggested

by another

resm

system.

data, remained constant trast the scattermg envelope dissymmetry, Z = 1450/lt350, obtained from the photometer at 1.28 and the depolarisatlon ratio was also constant at 0.47. These results Indicated the presence of RaylelghCans scatterers wth &mensions greater than 30 nm whose size dlstrlbution or total number decreases with increasing temperature but retam a larger scatterer fraction dominatmg dissymmetry. In the Brilloti spectra 1~ is almost independent of temperature and IR dominates the Landau-Placzek ratio behavIour. Also, the phonon shift (obtamed from the Brdfouin spectrum) decreases, initially linearly, from about 12 GHz at 298 K to about 6 GHz at 443 K whereas the phonon attenuation increases with mcreasmg temperatures approachmg a relaxation peak. Tlus mdlcates that the longitudinal phonon velocity is essentially directly proportional to temperature which agrees with the Gruneisen approximation of Huang et al. 113) and phonon behaviour is msen=tlve to changes m indivrdual scatterer nature. At 50% CT200 dissolution in chloroform the dissymmetry IS unity and on further dilution the rsotropic and anisotroplc Raylelgh ratios varied as shown in fig. 2. RI5 behaves m a manner consistent with the dissolutron of large scattermg entItles to a larger number of Rayleigh scatterers. The contmuous fall of RAN with increasmg dilution would agree with the gradual dissolution of entities whose polarizablllty tensor was also gradually changmg. Following Sicotte and Rmfret [ 141 plots of the CT200-CHC13 excess depolanzation versus solvent volume fraction show an mdial slope change at low CHC13 concentrations in a positive excess depolanzation regune, consistent with scattering entity breakup. followed by a negative excess depolarlsatlon regime above 65% CHQ3 and a slope inversion at hgh dllutlons suggesting a further change in CTZOO-CHCI, local order. Other anomalous behaviour occurs m the dissolution behaviour ofthe refractive index. These results suggest the presence of RGEBA molecular aggregates exhibltmg a range of sizes up to a dissymmetry inferred [I 5f size of about 34 nm. Infrared absorption spectroscopy of urueacted but prepared CT200 indicates a vO-H broadened doublet with components at 3500 cm-1 and 3450 cm-1 consistent with free hydroxyl group hydrogen-bonding. DGEBA chemical structure suggests that hydroxyl-epoxlde group hydrogen-bonding and dispersion forces are responsible for the formation of these molecular aggregates.

3. Reaction

observations

The reaction of anhydrides with DGEBA k > 0 resins is usua.Uy described by the F~sch and Hofman [12,15-181 of eqs. (l)-(3) below: 106

scheme

Volume 71, number 0

3

\\

H:-oH + R2

R1 H:-OH

1

CHEMICAL

y-Loo Ii

lo, //O

c c * u

0

C-OH

HC-O-C

1

0

CH2-CH-R3

II

8Q

+

Hf-O-C

anhydride

/\ + CH2-CH-R3

HC-O-CH2-CH-II3

+

I

carboxylic acid

R2

LETTERS

H:-o-cC-oH’ R2

+

PHYSICS

R

.

(3)

2

Eqs. (1) and (2) are consecutive step addrtion esterificatron reactions_ Initial DGEBA hydroxylgroup ring opening of the anhydrrde produces a monoester branch containing a carboxylic acid group available to react with a DGEBA epoxrde-group forming an aromatic diester cross-link and a secondary hydroxyl group capable of further reaction. In the presence of anhydnde or carboxylic acid a simultaneous DGEBA hydroxyl-epoxide addition etherification reaction (3) was invoked to account for an epoxide consumption greater than that required by eq. (2). Etherification was considered negligrble by Tanaka and Kakiuchi [ 191 in their work on accelerated DGEBA (k = 0, I)-anhydride systems wluch led them to propose a termolecular actrvated complex reaction scheme. Infrared studies [201 mdicate that in CT200-HT901 the Fisch and Hofman scheme applies but the detailed behaviour is not consistent with a homogeneous reaction. During reaction a number of chemical and optical property changes were observed some of which could be COTrelated in time in terms of their reaction extents, P(t) = 1 - [C(t) - C(w)] /[C(O) - C(a)], or extent of change. The key optical parameter kinetics and correlations are summarised in table 1. Addition of PA to CT200 is accompanied by a rapid appearance of associated carboxylic acid groups and aromatic ester involving an initial rapid reac-

Table 1 OptIcal parameter-chemical

Cnenkal

group correlatron

and behavlour of CTZCIO-HT901

durmg cure at 398 K

Optical parameter

change

lmmcdiate anhydrIde consumption on ad&tion accompamed by assorted carboxyhc group formatlon

rapid

immediate

increase in system

dissymmetry anhydride

consumption’

aromatic

ester formatron:

branched

ether formatlon:

first-order firstarder &der

change

decrease:

dissymmetry

fnstqrder

kinetics

-

kinetics

refractive

kmetics kinetics slow

index increase:

fnstqrder

kinetics

phonon velocity increase and phonon attenuation decrease: $*rder kinetics

Volume 7 1, number 1

CHEMICAL PHYSICS LETTERS

1 Apr.111980

tron of about 8% of the available free hydroxyl ncrease m the scattering envelope dissymmetry

groups (assuming no drester formation). This is accompamed by an from 1.28 to 2-l_ Subsequent reactron progresses with a contmuous behavrour wrth carboxyhc acrd formation displaying a peaked behaviour consistent wrth the consecutrve step reactions of eqs. (1) and (2) The most rapid extent of change behaviour subsequent to the uutral rapid changes 1s shown by the dissymmetry which decreases from 2.1 to 1.65 with first-order kmetics and a corresponding rate constant of 2 2 x 10-4 s---1_ Anhydnde consumption 1s the most rapid chemical reactron showmg first-order kmetic behaviour and a rate constant of 8.6 X IO-5 s-1. Total aromatic ester formation parallels the refractive index increase in exhrbrtmg first-order behaviour wrth rate constants of 6 5 X 10B5 S-I and 5.6 X 10m5 s-l respectrvely. In contrast phonon shrft decrease and phonon attenuation increase m paraliel with branched ether formatron and display f order extent of change behavrour. These chemrcal group-optical parameter extent of change correlations are Illustrated in fig. 3. These observations suggest an mhomogeneous reaction model for DGEBA (k > 0)-anhydrrde systems arrsmg from the presence of molecular aggregates m the DGEBA resm pnor to reactron. This IS illustrated in fig. 4. Unreacted resm observations suggest the exrstence of a drstrrbution of molecular aggregates formed by hydroxylepoxide hydrogen bondmg and drspersron forces and surrounded by largely k = 0 drepoxide oligomer solvent such that the refractive index of the respectwe regions follows tzc > tzr = 12,. These aggregates and their surfaces are hydroxyl group rrch in comparison wrth the solvent. Addition of the anhydride should result in rapid aggregate surface reactions producmg temporary refractive Index contrasting such that tzc > nr > rz, revealmg the true extent of the aggregates, which IS indicated n-r fig. 4 by correiated regons around a central core. This srtuatron would lead to the imtial rapid reaction and dramatic increase m drssymmetry. For CTZOO-HT901 the concentratrons of hydroxyl. epoxide and anhydride groups are approxrmately equal whrch for a homogeneous Frsch and Hofman reactron scheme would, m the early phases of reaction, require a secondorder reactron for anhydride consumption. The termolecular scheme of Tanaka and Kakmchr requires a thrrd-order reactron. The former second-order behavrour IS stdi expected m a highly VISCOUSmedmm unless hydroxyl-group limrtation or collordal A + B + B

reactions occur rn which case first-order kinetrcs are expected [?I]. Raprd aggregate surface reactions may be followed by further anhydnde ddfusron into and reaction m the aggregates accounting for the most rapid dissymmetry decrease rate of change. Secondary hydroxyl groups formed from diester reactrons wrll allow further shell like reactions to occur around the aggregate nucler where both esterrficatron and ethenfication reactrons compete and are limited by hydroxyl and epoxrde group supply. In this respect the chemrcal-optrcal parameter correlatrons mdrcate that refractive mdex and hence densrty parallels esterrfication suggestmg that bulk nucler reaction and growth controls density. Phonon shift and longrtudmal phonon velocrty and hence longrtudmal elastrc modulus

0

I

I

I

I

02

04

06

08

‘AROMATIC ESTER I%.

108

3.

I 10

0

,

02

I

I

I

04

06

08

I 10

PflRANCHEOETHER

CTZOO-HT901 chsmrcal group-optical parameter extent of change (P) durmg reactron correlatron plots.

Volume 71, number 1

! Apa

CHEMICAL PHYSICS LETTERS

1980

0 OGEBA . ANHYDRIDE

OGEBA

Fig. 4 Suggested mhomogeneous

reaction

mechanism

for X-> 0 DGEBA-anhydrrde

epoxy resms.

etherificatlon suggesting in the nuclei context that nuclei interconnections are relatively ether crosslink rich and act to control elastic modulus. llus model is sunilar to that proposed by Solomon and others [Xl for alkyd resin systems. Its consequence OR final matnx morphology and properties are being studied further. Clearly questions of nuclei growth and interconparallels

nectlon,

phase stability,

the importance

of functional

group reactivity

and network

statistics

require

further

elucida-

tlon.

Acknowledgement

Thus work was supported by an SRC grant.

References G. Allen and S E.B Pctne. eds , PhysIcal Structure of the Amorphous State, .I. Maaomol Sci. B12 (1976). G.S.Y. Yeh, CRC CrItical Reviews m Macromolecular Science (Aprd 1973) 173. E.H. Erath and M. Robinson, J. Polym. Sci. C 3 (1963) 65. R.E Cuthrell,J Appl. Polym. Sci 12 (1968) 1263. J.L. Raach and J.A. Koutsky. J. AppL Polym. Sn. 20 (1976) 211. 161R J. Morgan and J-E. O’Neal, J. Mater. Sci. 12 (1977) 1966. 171 UT. Kreiblch and R. Schoud. J. Polym. Sci Polym. Symp. 53 (1975) 177. 181 H. Batzer, F. Lohse and R. Schmrd, Angew. hYakromo1. Chem. 29/30 (1973) 349. [91 K. Dusek, J. Plestd. F. Lednicky and S. Lunak. Polymer 19 (1978) 393. [lOI G.C Stevens, to be pubhshed. 1111H. Batzer and S A Zahu, J. Appl Polym. Sa 19 (1975) 585.21 (1977) 1843. 1121H. Lee and K. NewlIe, Handbook of epoxy resins (McGraw-Hill, New York, 1967). I131 Y.Y. Huang, E A. Friedman, R D. Andrews and T.R. Hart, IJI- Light scattering in sohds. cd. M. BalkanskiWlammuion. Paris. 1971). IllI Y. Sxotte and hl Rmfret, Trans Faraday Sot. 58 (1962) 1090. 1151 H.C. van der Hulst, Light scattering from small partxles (Whey, New York, 1957). 1161 W. Flsch and W. Hofman, J. Polym Sci. 12 (1954) 497. 1171 W. Flsch, W. Hofman and J. Koskikalho, J. Appl. Chem. (1965) 429. [ISI Y. Tanaka and T.F. M&a, in Epoxy resms. chemistry and technology, eds. CA. May and Y. Tanaka (Dekker. New York. 1973). t191 Y. Tanaka and H. Kakiuchi. J Appl. Polym. Sci. 7 (1963) 1063; J. Polym. Sci. A2 (1964) 3405. PO1 G.C. Stevens, to be published. t211 T.R. Wrote. Phys. Rev. 107 (1957) 463; J. Chem. Phys. 28 (1958) 103. WI D-H Solomon, J. MacromoL !%I. Cl (1967) 179. [I] [2] [3] [4] [S]